Stawell gold mine in co-operation with Outotec Services completed a flotation circuit upgrade on time and on budget last year that, instead of the projected 3.5% improvement, resulted in an increase of 4.5% since the project was completed. Payback was also impressive, occurring within less than four months.
Flotation has been at the heart of the mineral processing industry for over 100 years, addressing the sulphide problem of the early 1900s, and continues to provide one of the most important tools in mineral separation today. The realisation of the effect of a minerals hydrophobicity on flotation all those years ago has allowed us to treat oxides, sulphides and carbonates, coals and industrial minerals economically, and will continue to do so in the future.
There have been a number of important changes in the industry over the years as flotation technology and equipment have advanced. Xstrata Technology considers the most noticeable has been the increase in sizes of the flotation machines, from the multiple small square cells that were initially used, to the 300 m round cells used today that are the norm in large scale plants.
Other changes have been more subtle, but equally as important. One of these has been the design of the flotation circuit to make the most of the liberation and surface chemistry effects of the minerals. In a lot of these situations it is not a matter of bigger is better, that will make the process work, but being smarter in the application of flotation technology.
Xstrata Technology is one company that believes the smarter use of flotation machines can deliver big improvements in plant performance. Through its use of the naturally aspirated Jameson Cell, Xstrata Technology has been making inroads into the processing of more complex ores. Having a small footprint, and using the high intensity mixing environment of slurry and naturally induced air in a simple downcomer, the Jameson Cell provides an ideal environment for the separation of hydrophobic particles and gangue, it says. The small footprint of the cells also makes them ideal to retrofit into a circuit especially where space is tight.
While the cell has been included in some flotation applications as the only flotation technology such as coal and SX-EW, the main applications in base metals have seen the cell operating in conjunction with conventional cells. The combination of the two technologies enables the Jameson Cell to target the quicker floating material, while the conventional cells target the slower floating material. Such a combination provides a superior overall grade recovery response for the whole circuit, than just one technology type on its own, Xstrata Technology says. Below are some of the duties for which the Jameson Cell can be used.
Jameson Cells in a scalping operation target fast floating liberated minerals, and produce a final grade concentrate from them. The wash water added to the Jameson Cell assists in obtaining the required concentrate grade due to washing out the entrained gangue. Scalping can be done at the head of the cleaner (also known as pre cleaning), or at the head of the rougher (also known as pre roughing), and minimises the downstream flotation capacity using conventional cells needed to recover the slower floating minerals.
Sometimes deleterious elements found in the orebody are naturally highly hydrophobic, and need to be removed at the start of flotation, otherwise they will report with the valuable minerals to the concentrate and effect concentrate grade. Mineral species such as talc, carbon and carbon associated minerals, such as carbonaceous pyrite, can all be difficult to depress in a flotation circuit. On the other hand, floating them off in a prefloat circuit before the rougher is an easier way to handle them. Jameson Cells acting as a prefloat cell at the head of a rougher circuit, or treating the hydrophobic gangue as a prefloat rougher cleaner, is an ideal way to produce a throw away product before flotation of the valuable minerals, minimising reagent use and circulating loads.
Jameson Cells can be used in cleaning circuits to produce consistent final grade concentrates. The ability of the cell to keep a constant pulp level, even with up stream disturbances or loss of feed, enables a constant grade to be obtained.
Xstrata Technology concludes: Importantly in a lot of these circuits, it is not the selection of one type of technology that produces therequired grade and recovery, but the selection of several technologies to get the best results. The interaction of slow floating and fast floating minerals, entrainment, hydrophobic gangue and a myriad of other variables make it rare for just one type of technology to prevail, but the combination of different flotation machines can achieve the required outcome more efficiently, as well as make the circuit robust enough to handle variations in feed quality.
Clariant Mining Solutions service engineers develop custom formulated reagents for each ore to be processed, while collectors and frothers are carefully selected for mutual compatibility. Clariant is investing considerably in mining chemicals and in support services for its customers all over the world.
The Jameson Cell has benefitted from over 20 years of continuous development. Early this year, the 300th cell was sold into Capcoals Lake Lindsay coal operation in the Bowen Basin of Australia. Around this time there were a number of coal projects taking in new Jameson Cells, including expansion projects for Wesfarmers Curragh and Gloucester Coals Stratford operations (both in Australia), Riversdales Benga project in Mozambique and Energy Resources Ukhaa Khudag coking coal project in Mongolia.
Le Huynh, Jameson Cell Manager, said the interest for coal preparation plants has remained strong, where operators needed dependable and reliable technology to treat fine coal, an important source of revenue. During 2010, the Jameson Cell business also found success in other applications, including recovering organic from a copper raffinate stream at Xstrata-Anglo Americans Collahuasi copper SX-EW plant in Chile.
Le said the consistent generation of very fine bubbles and the high intensity mixing in the Jameson Cell, was ideal for recovering very low concentrations of organic from raffinate streams, typically less than several hundred ppm. High throughput in a small footprint, simple operation and extremely low maintenance due to no moving parts in the cell are distinct advantages in this application.
The cell is designed with features specific to suit such hydrometallurgy applications including specialist materials, a flat-bottomed flotation tank with integrated pump box and tailings recycle system, and large downcomers. The Collahuasi cell was the first of its type in Chile, though there are many other large cells installed in SX-EW plants in Mexico, USA and Australia to treat both raffinate and electrolyte streams.
Dominic Fragomeni, Manager Process Mineralogy, Xstrata Process Support (XPS), notes that accurate, rapid development of a milling and flotation flowsheet for a new orebody is key to successful mine development. Time honoured conventional practice has typically favoured the extraction of a bulk sample of up to several hundred tonnes for conventional pilot plant campaigns which could operate at several hundred kilograms per hour. Where sample extraction is limited, much reliance has been placed on locked cycle tests alone to produce design basis criteria. These approaches can be lengthy, expensive, carry scale up risk, and have seen a wide range of successes and failures at commissioning and during life of a mine.
XPS has miniaturised the pilot plant process. At the same time, it has improved the representativeness of results from the pilot plant campaign by using exploration drill core to formulate the pilot plant sample. This Flotation Mini Pilot Plant (MPP) was developed in collaboration with Eriez subsidiary Canadian Processing Technologies (CPT) and operates in fully continuous mode either around the clock or can be made to demonstrate unit operations on a shift basis. The feed samples are in the range of 0.5-5 t and can consist of exploration NQ drill core which improves the sample representativeness. The MPP operates in the range of 7-20 kg/h, an order of magnitude lower in sample mass and typically at a lower cost when compared to conventional pilot plants.
XPS has developed and validated a representative sampling strategy, an appropriate quality control model for metallurgical results and has accurately demonstrated operations results using Raglan and Strathcona ores and flowsheets. These validation campaigns, in scale down mode from the full scale plants, have produced actual mill recoveries to within 0.5% at the same concentrate grade with internal material balance consistent with the plant.
When designing a plant to recover copper, Scott Kay, Process Engineer with METS suggests (in METS Gazette, issue 32, October 2011) it would be prudent to perform some mineralogical analysis test work such as QEMSCAN (Quantitative Evaluation of Mineral by Scanning electron microscopy) to provide some knowledge on the proportion of sulphide and oxide minerals present, the grain sizes of each mineral and a suggested grind size before jumping into the bulk of the beneficiation test work.
Ideally, the characteristics of the copper bearing minerals should suggest an appropriate grinding circuit P80 of between 100 and 200 m (0.1 and 0.2 mm), which can be controlled by cyclones, or in some cases fine screens.
The Delkor BQR flotation machine (formally Bateman BQR) here at Messina Mowana Copper mine in Botswana. 15 x 50BQR and 16 x 200BQR flotation cells for Copper roughing, cleaning & re-cleaning. Oxide / Sulphide
Flotation reagent selection is paramount and test work is necessary to ensure the optimum reagent suite is utilised. If the ore contains a low amount of iron sulphides, xanthate collectors are often suitable to float copper sulphideminerals. If native gold is present, dithiophosphates can be used which are less selective to iron sulphides. Increasing and controlling the pH within the flotation vessel to between 10 and 12 causes the process to become more selective, away from iron sulphide gangue minerals such as pyrite to produce a cleaner copper mineral concentrate. Depending on the ore mineralogy, activators and depressants may be required to achieve the optimum reagent suite.
Recovery of copper oxide minerals can be achieved with flotation by sulphidising the ore. In essence, this creates a thin layer of copper sulphide (chalcocite) on the oxide grains which can then be activated and collected in the froth. When employed, this occurs after the sulphide flotation stage, however, this is not commonly used as other beneficiation processes, such as leaching and SX-EW are often more cost effective for copper oxide minerals.
A common flotation circuit usually includes a rougher/scavenger and a cleaner stage. As most copper orebodies exhibit an in-situ grade of less than 1% Cu, the mass pull to the rougher froth is often low. This means that the throughput of the cleaner stage is significantly less than the throughput of the rougher stage which imparts a relatively low capital and operating cost to the flotation circuit.
To counteract the possible absence of a scavenger stage, a slightly higher mass pull to the rougher froth is targeted (although still low overall) to increase overall copper recovery. The rougher froth can then be reground to increase the liberation of the copper sulphides from the iron sulphides before being fed to the cleaner flotation vessels. This results in a significant upgrade in copper in the cleaner froth whilst still achieving a high copper recovery. The final flotation concentrate usually contains between 25 and 40% Cu.
Alain Kabemba, Flotation Process Specialist at Delkor notes the major trend to treating lower-grade and more finely disseminated ores and lately the re-treatment of tailings. He also points to the broad applicability of size to below 10 m.
Real systems do not fulfil ideal conditions, mainly because of feed variation or disturbances. Before considering disturbances to flotation specifically, Kabemba says it is important to emphasise the interlock between grinding and flotation, not only with respect to particle size effects, but equally to flotation feed rate variations. The grinding circuit is usually designed to produce the optimum size distribution established in testing and given in the design criteria. When the product size alters from this optimum, control requires either changing feed tonnage to the circuit or changing product volume, with either causing changes in flotation feed rates.
While grindability changes due to the variation in ore properties are disturbances to the grinding circuit, they generate feed rate changes as disturbances to the flotation circuit. The variations in ore properties which affect flotation from those assumed in the design criteria must therefore necessarily include grindability changes.
This reflects important differences in flotation machine characteristics between the two processes. Grinding circuits are built and designed with fixed total mill volumes and energy input, so the grinding intensity is not a controllable variable, instead grinding retention time is changed by variation of feed rates. In contrast, the flotation circuit is provided both with adjustable froth and pulp volume for variation of flotation intensity by aeration rate or hydrodynamic adjustment. Reagent levels and dosages provide a further means for intensity control.
One recent trend has been towards larger, metallurgical efficient and more cost effective machines. These depart from the simpler tank/mechanism combination towards design which segregates and directs flow and towards providing an external air supply for types which had been self aerating and towards the application of hydrodynamic principles to cell design, like the Delkor BQR range of flotation machines, initially the Bateman BQR Float Cells.
Bateman has steadily developed the BQR flotation cells which have been in application for the past 30 years, and with its acquisition of Delkor in 2008, decided to rebrand the equipment into the Delkor equipment range. Kabemba explains that BQR cell capacities range from 0.5 to 150 m3 currently installed, and can be used in any application as roughers, scavengers and in cleaning and re-cleaning circuits.
Provide good contact between solid particles and air bubbles Maintain a stable froth/pulp interface Adequately suspend the solid particles in the slurry Provide sufficient froth removal capacity Provide adequate retention time to allow the desired recovery of valuable constituent.
Highest possible effective volume and reduced the froth travel distance Improved metallurgical performances in terms of grade recovery and reduced capital and operating costs based on reduced fabrication material and ease of maintenance
Kabemba says there are not many differences in terms of design between BQR Flotation cells; however, from the BQR1000 upwards, the flotation cells have internal launders to maintain the design objectives and benefits highlighted.
Operating variables, such as impeller speed, air rate, pulp and froth depths have to be adjustable over a sufficient range to provide optimum results with a given ore, grind and chemical treatment, but adjustment should not extend beyond the hydrodynamic regime in which good flotation is possible.
The largest current BQR flotation machine is shown in the table. In the near future the BQR2000 (200 m3) and BQR3000 (300 m3) will be available to the market. Kabemba also explained that circular cells reduce the amount of dead volume when compared to square cells. This enables a much higher effective pulp volume, hence increasing the effective energy input into the flotation cell. In addition tank type cells offer enhanced froth removal due to the uniform shape of the circular launders. He concluded that fully automated flotation cells are becoming more and more common with the aid of smart control and advances in software in the marketplace.
FLSmidths flotation team notes that fundamental flotation models suggest that a relationship exists between fine particle recovery and turbulent dissipation energy1. Conversely, increased turbulence in the rotorstator region is theoretically related to higher detachment rates of the coarser size range2. Conceptually, the suggested modes of recovery for the extreme size distribution regions appear to be diametrically opposed.
Industrial applications have previously confirmed that imparting greater power to flotation slurries yields significant improvements in fine particle recovery. However, recovery of the coarser size class favours an opposing approach, the FLSmidthteam believe. An improvement in the kinetics of the fine and coarse size classes, provided there is no adverse metallurgical influence on the intermediate size ranges, is obviously beneficial to the overall recovery response. Managing the local energy dissipation, and hence the power imparted to the slurry, offers the benefit of targeting the particle size ranges exhibiting slower kinetics.
New concept, Hybrid Energy FlotationTM (HEFTM),was recently introduced by FLSmidth. In principle it decouples regimes where fine and coarse particles are preferentially floated. HEF includes three sections:
This subject will be expanded upon at the 5th International Flotation Conference (Flotation 11) in Cape Town, South Africa. The fundamental parameters that influence fine and coarse particle recovery will be reviewed. The potential dual recovery benefit is then presented in terms of its practical implementation in a scavenging application. HEF is proposed as the preferred methodology of recovering these slow-floating size ranges; a method that opposes the traditional approach of residence time compensation.
StackCell offers column-like performance in a substantially smaller footprint than conventional cells. These compact, stackable units provide considerable savings for new installations and are ideal for expanding capacity in an existing plant
Eriez Flotation Group introduced the StackCell flotation concept in 2009. This innovative technology recovers fine particles more efficiently than mechanical flotation cells. Weve taken the inherent advantages of mechanical flotation and adapted them to a new design that is significantly smaller and requires less energy, explained Eriez Vice President Mike Mankosa. We focused on reducing the retention time and energy consumption by implementing a completely different approach to the flotation process. This new approach provides all the performance advantages of column flotation while greatly reducing capital, installation and operation costs.
At the core of the StackCell technology is a proprietary feed aeration system that concentrates the energy used to generate bubbles and provides bubble/particle contacting in a relatively small volume. An impeller in the aeration chamber located in the centre of the cell shears the air into extremely fine bubbles in the presence of feed slurry, thereby promoting bubble/particle contact. Unlike conventional, mechanically agitated flotation cells, the energy imparted to the slurry is used solely to generate bubbles rather than to maintain particles in suspension. This leads to reduced mixing in the cell and shorter residence time requirements.
The StackCell sparging system operates with low pressure, energy efficient blowers that decrease power consumption by 50% compared to air compressors or multi-stage blowers used in other flotation devices.
The low-profile StackCell design features an adjustable water system for froth washing and also takes advantage of a cell-to-cell configuration to minimise short-circuiting and improve recovery rates. Space requirements for the StackCell design are approximately half of equivalent column circuits, with corresponding reductions in weight leading to reductions in installation costs. Units can be shipped fully assembled and lifted into place without the need for field fabrication.
This technology can provide recoveries and product qualities comparable to column flotation systems while using a low profile design. Not intended to replace the need forcolumn flotation, it does provide an alternative method to column-like performance where space and/or capital is limited. The small size and low weight of the new StackCell makes possible lower cost upgrades where a single cell or series of cells may be placed into a currently overloaded flotation circuit with minimal retrofit costs.
Steve Flatman, General Manager of Maelgwyn Mineral Services (MMS) also comments on the trend of moving towards a finer grind to improve mineral liberation. Unfortunately conventional tank flotation cells are relatively inefficient in recovering these metal fines below 30 m and very inefficient at the ultra fine grind sizes below 15 m. The incorporation of regrind mills on rougher concentrates has further exacerbated this problem. To date the conventional flotation tank cell manufacturers have attempted to counter this fall off in recovery of fine particles by inputting increasing amounts of energy (bigger agitation motors) into the system to improve bubble particle contact. Unfortunately this tends to compromise coarse particle recovery.
He says the solution is MMSs Imhoflot pneumatic flotation technology and specifically the Imhoflot G-Cell. Recent pilot plant test work at a nickel operation with a three stage Imhoflot G-Cell pilot plant enabled an additional 30% nickel to be recovered from the conventional flotation tank cell final plant tails. The recovery was predominantly associated with the minus-11 m fraction indicating that this improved recovery was not just related to additional residence time. The above results are in line with an earlier pilot plant trial using G-Cells on a zinc operation where an additional 10-20% zinc was recovered from cleaner tailings this time being associated with minus 7 m material.
It is postulated that the above improvements are related to the order of magnitude increase in terms of air rate (m/min/m pulp)for the G-Cells due to their principle of operation where forced bubble particle contact takes place in the aeration chamber rather than the cell itself with the cell merely acting as a froth separation chamber. Typically in percentage terms the G-Cell air rates are five to ten times that of conventional flotation although the overall or total air usage is approximately half.
When this additional targeted energy input is combined with the centrifugal action of the GCell and small bubbles benefits are obtained in both the flotation rate (kinetics) and overall recovery. The improved kinetics results in a much lower residence time than conventional flotation facilitating a double benefit of both reduced footprint and improved recovery.
Metso notes a main drawback of column cells being low recovery performance, typically resulting in bigger circulating loads. Its CISA sparger is derived from the patented MicrocelTM technology and enhances metallurgical performance by allowing flexibility on the graderecovery curve. Metso Cisa says the main advantages of its column technology include:
At the bottom of the column, the sparger system raises mineral recovery by increased carrying capacity due to finer bubble sizes. This maximises the bubble surface area flux which is a standard parameter in evaluating flotation device performance. It also provides maximum particle-bubble contacts within the static mixers and effective reagent activation from the mechanical operation of the pump.
It is well known that coarse particles behave poorly in a conventional flotation cell and were previously regarded as non-floatable. However, recent laboratory work demonstrates that Fluidised Bed Froth (FBF) flotation extends the upper size limit of flotation recovery by a factor of 2-3 resulting in significant concentrator performance benefits. AMIRAs P1047 project, Improved Coarse Particle Recovery by FBF Flotation, is expected to commence in 2012, and will be structured in two phases.
Early rejection of gangue with minimum mineral loss. Potential for significant increase in concentrator throughput or significant improvement in capital efficiency Reduced energy consumption. Independent modelling predicts that if particles of 1 mm can be floated, comminution energy consumption will be lowered by at least 20%. Better management of water requirement. FBF cells can take product straight from the milling circuit without dilution, and the feed to the FBF cell could be up to 80% w/w solids, which could lead to significant savings in process water demand. Improve recovery of metallic and other dense minerals. In a continuous FBF Cell, dense mineral particles will tend to sink to the bottom and accumulate in the cell, thus they can be recovered in a concentrated form by emptying the cell periodically. This could be a significant benefit where the concentration of the heavy metallic material is too low to warrant a separate treatment plant to recover them.
In Australia, Northgate Minerals Stawell gold mine recently completed a project through which it aimed to increase recoveries by 3.5% by upgrading the flotation plant. This upgrade was implemented after Stawell changed its production profile to process lower grade ore at higher throughput rates.
Instead of the projected 3.5% improvement, the upgrade from Outotec Services has resulted in an increase of 4.5% since the project was completed on time and on budget last year, despite the wettest seasonal weather in recorded history. Payback was also impressive, occurring within less than four months. The projected payback was 5.5 months, so it was a pleasant surprise when it happened so soon explains Jodie Hendy, senior metallurgist at Stawell.
The project has also achieved payback in less than four months and has delivered further ongoing benefits, including easier operation and reduced maintenance costs, says Outotec Services, which worked in close partnership with Stawell Gold to ensure the site remained fully operational during
The mine, which has produced more than 2 Moz in its 26-year history, previously employed a flotation circuit consisting of a bank of eight mechanical trough cells in the rougher circuit, followed by two banks of 2 x OK3 Outotec cells as cleaners. The feed rate to the cells was between 90-105 t/h, at 50-55% solids. The overall flotation circuit was not performing at optimal rate due to entrainment problems in the rougher cells when feed density increased from 45% to 55% solids, typically at 105 t/h.
In anticipation of future production levels and as part of Stawells focus on operational excellence, it was decided to upgrade the flotation circuit. Following a site audit from Outotec Services, a 2 x TankCell -20 configuration equipped with larger TankCell -30 mechanisms was proposed to help optimise flotation. The larger mechanisms would allow operation at very high percent solids (50% and over).
The TankCell design also allows a much deeper froth depth and better concentrate grade through optimised launder lip length and surface area. These cells known for great performance, ease of operation and reduced power and air consumption. Outotec Services was commissioned to handle the complete turnkey solution of the new rougher circuit, including design, supply, installation and commissioning.
The schedule was demanding but achievable, in just 30 weeks. It was decided to adopt the partnering approach between Stawell and Outotec Services, because this collaborative method ensured open communication, with all parties having greater ownership of the project and its aims. This close teamwork resulted in meticulous planning and site remaining fully operational at all times. Pipework and electrical easement ducts, for example, were rerouted early in the project. Tie-in points for new cells and rerouting of pipework were also planned upfront in the project and all disruptive work was completed during shutdowns.
The project overcame a number of challenges, including an extremely limited footprint, which was adjacent to a gabion wall, close to the runof-mine pad and also close to a reagents shed, which could not be moved. Additionally, existing process requirements at Stawell required specific elevations for the new TankCells. Structural stability was the main issue when designing the tank support structure due to the height of the tanks and the limited footprint. Sufficient stiffness was required such that the operation frequencies of the TankCells would not interfere with the natural frequency of the tank support structure. Through FE modelling of the structure, section sizes and bracing orientations were optimised to produce the required stiffness.
Despite the challenges, the turnkey installation of the new rougher circuit, along with blowers for the complete flotation circuit, was completed within deadlines. Because all tie-in points had been already carefully planned upfront, commissioning was a seamless exercise.
Designed to cope with projected increases in production and considerably more operator friendly than its predecessor, the new TankCell 20 cells have quickly proved their worth at site. The air demand for the old rougher cells, for example, was estimated at over 3,000 Am3/h, whereas the estimated air demand on the Outotec TankCells is a maximum of 992 Am3/h.
The Outotec FloatForce rotor-stator mechanism, with its unique design, delivers enhanced flotation cell hydrodynamics and improved wear life and maintenance. Maintenance on the Outotec TankCells has also been minimal since the upgrade, Hendy commented. Basically we check the cells during shutdowns but there has been no maintenance required in the nine months since commissioning. The TankCells have really delivered on their reputation. Basically, they do exactly what they are supposed to do.
Turning to flotation reagents, Frank Cappuccitti, President of Flottec explains that Flottec and Cidra are working very hard jointly on developing instruments that will measure hydrodynamics in the flotation cell and circuit in a bid for better flotation control. This would be a great step forward in using a combination of reagents and sensors to optimise flotation systems. It brings together the knowledge we have developed in both how reagents effect hydrodynamics and measuring the hydrodynamics to maintain optimum conditions. He explains that back in the 1990s, when he worked at a well-known mining chemicals supplier, we spent most of our research on trying to find the best collectors. The thinking was that we could try to develop collectors with absolute specificity. In other words, we could develop a collector that would float only specific minerals and provide clients with an almost perfect flotation separation. This was our approach to flotation optimisation. Unfortunately, we discovered that there was no such thing as absolute specificity. In fact, we had trouble measuring any improvements in the circuits because they were multi-variant and highly complex. Every change made was always a trade off between grade, recovery and cost. Changing one thing in the circuit seemed to improve something but always got a negative response in some other variable. It was also very hard to measure the performance of the flotation circuit because the only real parameters you could measure on line were concentrate grades and tails of the circuits, which were always after the fact. There was little ability and understanding about what real time measurements we could take other than air rates, cell levels and flow rates. So even if we got an improvement or a response to a change, we never knew if that was a response to a change or a natural variation in the system. Every test needed long term statistical trials to get some confidence in any real change.
So, I wrote a paper in the 1990s that basically said that until we could measure the real time variables in a flotation system and learned to really understand and control the system, we were limited in our ability to work on continuous improvement in reagent optimisation. We needed new sensors that could measure the performance of the flotation circuit so we could learn to control it. Once we got this, then we could actually measure improvements and use this to develop reagents.
Fortunately, with the advent of strong computing power and software, we have moved forward tremendously in the last decade in understanding the flotation circuit. Froth cameras that tried to measure froth grade and velocity were one of the first new sensors developed to assist in optimising circuits. Through the work of universities such as McGill and organisations like JKtech, new sensors have been developed that could actually measure reliably and in real time the hydrodynamic parameters in the flotation cell. Flotation cell hydrodynamics (gas dispersion parameters) is critical to the performance of the cell. When we talk about these parameters, we are talking about measuring what is happening in a flotation cell. Flotation is really about making bubbles and using the surface area of the bubble to do the work of transporting hydrophobic minerals to the froth. In flotation cells, we add air, create bubbles of a certain size and speed that provide the surface area to do the flotation. The more bubbles and the smaller the bubble, the more surface area we have to do the work. This surface area we create is known as the surface bubble flux (Sb) and controls the kinetics of flotation. Now that we have instruments that can measure the air into a cell (known as Jg), measure the size of the bubble diameter (Db) and the gas hold up (Eg), we can figure out how the relationship between these parameters and how they affect the Sb and flotation circuit performance. We can also now do research on how reagents can be used to control these parameters as well.
Research of the last few years has shown that frothers actually play a much more important role in flotation hydrodynamics than once thought. Frothers perform two major functions. They create and maintain small bubbles in the pulp to transport the minerals and they create the froth on top of the cell to hold the minerals until they can be recovered. The froth is created because frothers allow a film of water to form on the bubbles which makes them stable enough not to break when they reach the surface of the cell. Fortunately, the water drains over a short period of time and the froth will eventually break down. Froth breakdown is essential for cleaning and transporting the concentrates. Small bubbles are essential in making flotation efficient. For the same volume of air in a cell, smaller bubbles give much higher surface area, which in turn gives much higher kinetics.
We now know that as you increase the concentration of frothers to the cell, the bubble size gets smaller, and the film of water on the bubble gets bigger. But bubble size does not keep getting smaller forever. The frother will reduce the bubble down to a certain size, which is about the same for all frothers in the same set of conditions. The concentration of frother where the bubble is at a minimum is known as the critical coalescence concentration or CCC.
Each frother has a different CCC. Each frother also has a different ability to add water to the bubble and hence provides different froth stability. This also changes with concentration. We have learned in the last few years that each frother has a hydrodynamic curve which relates the bubble size with the froth stability. Strong frothers give very high froth stability at the CCC, while weak frothers give very low stability of the froth at the CCC.
This new understanding of how frothers affect flotation cell hydrodynamics has lead to new methodologies to optimise flotation circuits. Flottec has worked on an optimisation system where a frother is added to a circuit at the CCC (which guarantees maximum kinetics or maximum Sb) and the performance is measured. Then frothers of different strength are added (always at the CCC) until the right strength for maximum performance is determined. Adding the frother at the CCC is the critical optimisation difference. By doing this you are always guaranteed to have maximum kinetics. If the frother used is too strong, the dosage will have to be cut back below the CCC or the froth will be too persistent. This lowers flotation kinetics. If the frother is too weak, too much has to be added to get the froth strength and this increases cost and likely reduces recovery. Flottec has been conducting research withMcGill University to develop the hydrodynamic curves and CCC for all families of frothers in order to implement the new methodology of frother optimisation in plants.
The next step in this research is to be able to use new sensor technology to measure and control the flotation system by controlling the hydrodynamics in the cell. With our current knowledge of how air rate, cell levels, and frother addition affect bubble size, water recovery and gas hold up, we can use these control variables to maintain the optimum hydrodynamics in the cell resulting in the optimum flotation circuit performance. Flottec is working with companies like Cidra to develop new sensors that can provide real time information on cell hydrodynamics (gas dispersion parameters) and on froth stability properties in order for us to optimise the reagents and operating strategies used in a plant. This will bring flotation performance to the next level.
Clariant Mining Solutions business is investing considerably in mining chemicals. It has opened a new laboratory at its US headquarters in Houston, Texas, dedicated to the development and optimisation of chemical solutions for North American customers. The laboratory is part of a planned multi-million dollar investment into Clariants global Mining Solutions business, which includes establishing several new Mining Solutions laboratories around the world. This network is intended to enable the business to better support customer needs and address regional challenges. Most recently, Clariant has opened new mining labs in South Africa (Johannesburg) and in China (Guangzhou). The new laboratories will complement existing facilities in Europe and Latin America.
As part of Clariant Mining Solutions global investment, new mining labs have been established in South Africa and China, to complement those already operating in Brazil, Chile, Peru, Australia, North America, Russia and Germany
Mining is a strategic focus area for Clariant, said Christopher Oversby, Global Head of Clariants Oil & Mining Services business unit. This investment further demonstrates Clariants ongoing commitment to providing innovative technologies and solutions for our mining customers around the world.
The Houston laboratory will process ore samples from customers in the USA and Canada. These samples were previously handled in Clariants mining laboratories located in South America and at the companys global research facility in Frankfurt, Germany. We are very excited about the new mining laboratory and the opportunity it provides us for offering our North American mineral processing customers even more localised services and attention, said Paul Gould, Global Head of Marketing and Application Development for Clariant Mining Solutions. The Houston lab will allow Clariant technicians to more efficiently develop optimised reagent solutions for our US and Canadian customers.
Additionally, Clariant is in the process of developing a new Innovation Center in Frankfurt at a cost of 50 million. Employing nearly 500 people and covering 30,000 m2, the facility will focus on customers using an integrated multidisciplinary approach to problem solving. Clariant says an open innovation approach on joint ventures with external partners will ensure the acceleration of the idea-to-market process. Mining research and development will also be part of this facility.
Axis House has been developing reagent technologies for the past 10 years, at its flotation laboratory in Cape Town, South Africa and more recently at it metallurgical labs in Sydney and Melbourne. These were acquired when Axis House bought the oxide flotation reagent technology from Ausmelt Chemicals. A practical application technology strategy was followed with Axis House providing a complimentary suite selection and optimisation service to its clients, who were then mainly interested in the Axis developed technology of combining fatty acids, hydroxamates and sulphidisation suites to effectively and economically float oxide minerals.
Early on the focus was on developing reagents to float complex ores which contained multiple minerals with varying flotation kinetics. Often the limiting factor was not only the sluggish flotation kinetics of the minerals but the process plants own equipment limitations, like flotation and conditioning times. Developing a reagent that floated a certain mineral was simply not enough. The solution was to develop suites of reagents which could function synergistically. By altering the types of collectors and the dosages, the company could optimise both the use of the processing equipment and the collecting power. It says this approach has successfully been applied to various types of base metal oxide ores.
It is now taking this innovative approach into the field of rare earth element (REE) flotation. This fits into the Axis House business plan as the chemistries are quite similar to what is in existence at Axis already. Of course some tweaks will have to be made to the reagents as well as the laboratories this process has already started, with the first batch of REE test material having arrived at Cape Town, and new reagent samples at the ready. There are a large number of REE projects coming online in the next few years. Most of these orebodies have not been previously treated at industrial level and so will face difficulties when scaling up. REO (Rare Earth Oxides) are often difficult to float and the development of multiple collector systems for these ore types would help increase the viability of these projects.
Jerry Sullivan, Global Marketing Manager-Mineral Processing, Cytec Industries Inc, discussed collectors, which contain mineralselective functional groups. They have a hydrophobic hydrocarbon tail. Changing the molecules functional group changes the preference for what mineral it will adsorb on to. Changing the length of the hydrocarbon chain changes the hydrophobicity of the molecule. This is related to the strength of the collector.
Within the collector molecule, there are donor atoms whose goal is to form bonds with acceptor atoms within the ore. Nitrogen, oxygen, and sulphur are the most important donor atoms in all reagent chemistry. Sulphur is the most important donor in sulphide collectors. Nitrogen and oxygen are additional donor atoms. Phosphorous and carbon are central atoms carrying the donors. They only have indirect participation in interactions. He noted the general characteristics of sulphide collectors to be:
Ionic collectors are stronger and less selective Neutral, oily collectors are weaker, more selective Higher homologues (more carbons) are stronger than lower homologues (fewer carbons) Cytecs NCPs are very selective collectors
There is a strong case for formulated products (or blends), he continued That is because mineralogy is complex. Plant performance is also inherently variable. Mineralogy changes routinely. In addition, different minerals have different affinities for reagents. Various minerals will compete for a given reagent. Modifiers used will also influence reagent partitioning. Particle size distribution will also affect recoveries (recovery losses in coarse and fine size range). A single collector will not be sufficiently robust. Indeed, most plants use two or more collectors. The goal is to pick reagents that will get to the right minerals. Utilising a collector blend can balance cost and performance.
Cytec has multiple collectors and collector blends that are continuously being developed to tailor to the customers application. A few of the collector families that have recently been introduced to the market include the new XR Series Xanthate Replacement Collectors, developed to meet the desire to replace xanthates. This new series of collectors are cost competitive with xanthates and are strong collectors but with high selectivity. In addition, they are safer and vastly improves handling and level of toxic exposure of the personnel to product, stock safety management and simplifies plant operations.
The XD 5002 blends were developed to operate in a broad pH range 8-12 and be highly selective in Cu/Mo, Cu/Au sulphide ores, enhance Mo recovery in Cu/Mo bulk float and enhance Au recovery in Cu/Au ores. The MAXGOLDTM blends were introduced to float primary Au ores; auriferous pyrite, arsenopyrite, and tellurides and are also capable of enhancing recovery in Cu/Au ores.
It is now possible to use measurement devices based on impedance tomography to create realtime 3D images. The technology opens up entirely new possibilities in controlling flotation processes. With Flotation Watch the operator can see what takes place underneath the surface. Flotation Watch measures several parameters at the same time, on-line. The sensor can measure the stiffness of the froth, the thickness of the froth, analyse the interface area between the froth and the slurry and it can analyse the slurry too depending on the customer needs, says Jukka Hakola, Numcores Vice President of Sales and Marketing.
With Numcore measurement devices, the size and quantity of air bubbles and the solid matter content of the froth bed can be monitored by means of electric conductivity distribution. With Flotation Watch the stiffness of the flotation froth can be measured and this helps to keep the recovery in higher level. The signals for the production failures, such as hardening and collapse of the froth bed, can be seen beforehand and avoided. This way we can help to minimise the losses in the flotation process, says Hakola.
Real-time characteristics are a key in this technology; in other words, the system continuously provides the operator with factual data on what is happening in the flotation cells, for example the location of minerals and the bottom surface of the froth bed. Because it has not been possible to look inside tanks, controlling a mineral concentration process has largely been based on experience-derived knowhow. Now that operators can look inside the process, it is possible for them to maintain an optimal mix all the time, says Hakola.
Numcore has, in close co-operation with a few key customers, developed measurement technology to better serve everyday work. We have now delivered several Flotation Watch sensors to flotation cells in several markets and for different metals such as copper, zinc and gold. One of the main benefits is that contamination of the probe is taken into account in mathematical formula and the measurement probe does not need to be cleaned. Our sensor has been in a zinc rougher flotation cell for nine months and is giving accurate results to the operator. We can now offer automated control for flotation process with Flotation Watch and see that this can bring new benefits for our customers, he promises.
Numcores measurement technology is currently in test use at Inmets Pyhsalmi copper-zinc mine (IM, April 2010, pp10-18), among others. According to Seppo Lhteenmki, Processing Mill Manager, the system has provided accurate information on the condition of the froth bed, and the technology has functioned reliably. We have tested the device for a few months, and it has provided clear benefits for those operators who have received operator training for it and actively monitored the data provided by the system. The device appears to be so useful, in fact, that we are seriously considering buying it after the test period, he says.
Mettler Toledo notes that pH greatly determines the efficiency of the flotation, which minerals will float, or even if there will be any flotation at all. The critical pH value for efficient flotation depends on the mineral and the collector. Below this value the mineral will float, above it, it will not (or, in some cases, vice versa).
In a recent white paper www.mt.com/pro-phflotation, the company says in order to overcome difficulties with the hostile environment in flotation cells, sensor manufacturers are very creative in their choice of sensor design. It is possible to find pH electrodes with a ceramic, plastic, rubber or even a wood reference diaphragm. Still, their performance can be severely limited as the colloidal particles and sulphides interfere almost instantly with the reference system. The sensors maintenance requirement is therefore high, requiring very frequent cleaning and calibration, and usually sensor life is short.
Mettler Toledo has acknowledged this issue and to combat it has designed the InPro 4260i pH electrode with Xerolyt Extra solid polymer electrolyte. The InPro 4260i does not have a diaphragm and instead features an open junction, which is an opening that allows direct contact between the process medium and the electrolyte. Contrary to the miniscule capillaries of any other type of diaphragm in conventional pH electrodes, the diameter of the open junction is extremely large and much less susceptible to clogging or fouling. Another significant difference is in the choice of polymer electrolyte. Xerolyt Extra was designed specifically for service in tough environments to provide a strong and lasting barrier against sulphide poisoning.
The companys Intelligent Sensor Management (ISM) is a platform based on sensors with embedded digital technology for better pH management. The integrated system consists of a digital sensor and ISM-compatible transmitter. The key to the technology is a microprocessor which is contained within the sensor head and is powered by and read through the transmitter. Critical sensor information such as identification, calibration data, time in operation and process environment exposure are all recorded and used to continuously monitor the health of the sensor.
By constantly keeping track of process pH value, temperature and operating hours, ISM calculates when sensor calibration, cleaning or replacement will be needed. Any need for maintenance is recognised at an early stage.
In recent years, researchers at Imperial College have been focusing on measuring air recovery in industrial flotation cells and have found that a peak in metallurgical performance (improvements in both grade and recovery) corresponds well with a peak in air recovery. Major platinum and copper operations have already observed the benefits of using this methodology as developed by the researchers. JKTech is now licensed by Imperial Innovations to commercially provide this methodology and associated benefits to the global minerals industry.
The PAR technique comprises two stages evaluation and implementation. The evaluation stage involves determining the effect of the technology at a mine site, typically determining the peak air recovery for a bank (or banks) of flotation cells and evaluating the resultant metallurgical performance. The implementation stage involves setting the air rates to those that maximise the air and/or metal recovery, and support and training of site personnel including operating manuals. The implementation stage requires an end-user license to be obtained by the sites through Imperial Innovations.
GIW Industries has launched its new High Volume Froth (HVF) pump. Unlike any other pump on the market, GIW says, the HVF pump can pump froth without airlocks. It provides continuous operation without shutdown or operator intervention. The new hydraulic design actually removes air from the impeller eye while the pump is running, so you can keep
Designed for air-entrained slurries, the pump can be used in phosphate mining, hard rock mining and oil sands. The pump offers improved efficiency and is environmentally friendly and cost effective, GIW reports
The GIW HVF can be retrofit into many existing froth applications. The pumps deaeration system includes a patent-pending vented impeller and airlock venting. This helps to eliminate sump overflow due to pump airlock; reduce downtime; and allow water use to be restricted to the bare minimum. Fewer pumps are required for less capital expense, requiring less water and power usage.
The HVF pump has been fully tested on froth and viscous liquids. The pump exceeded expectations at a large phosphate company in Finland. The companys existing pumps were not able to provide the required flow and were airlocking at only one-third of process design capacity. After installing an HVF pump, the company achieved a flow of 415 m3/h.
Traditional slurry pumps are prone to airlock when working with slurries that incorporate froth. A pump works by pulling in a liquid at a certain pressure and adding mechanical force to expel the liquid at a higher pressure. The air in the froth does not want to move to a higherpressure zone, and it is prone to build up at the lower-pressure pump entrance. The accumulation of air can eventually block the pump entrance completely, leading to airlock, which requires pump shutdown or operator intervention to avoid sump overflow.
How is GIWs HVF pump different? The main innovation is in the impeller design. Typically, air bubbles gather at the centre of the impeller as the heavier fluids are spun to the outer edges. The HVF pumps de-aeration system includes the vented impeller and airlock venting. In the HVF pump, small holes in the centre of the impeller allow air bubbles to pass through to a separate port. The port vents air up and out of the pump to normal atmospheric pressure.
Flotation circuits are a common technology for the concentration of a broad range of minerals and wastewater treatments. Froth flotation is based on differences in the ability of air bubbles to adhere to specific mineral surfaces in a solid/liquid slurry. Particles with attached air bubbles are carried to the surface and removed, while the particles that are not attached to air bubbles remain in the liquid phase. The concept is simple, but the phenomena are complex because the results depend on what happens in the two phases (froth and pulp phases) and other phenomena, such as particle entrainment. In flotation, several parameters are interconnected, which can be classified into chemical (e.g., collectors, frothers, pH, activators, and depressants), operation (e.g., particle size, pulp density, temperature, feed rate and composition, and pulp potential), equipment (e.g., cell design, agitation, and air flow), and circuit (e.g., number of stages and configuration). If any of these factors is changed, it causes (or demands) changes in other parts, and studying all of the parameters simultaneously is impossible. Conversely, there is not a model that includes all variables; most of the models are empirical and use only a few variables.
In the literature, various methodologies for flotation circuit design have been proposed, with most using optimization techniques. In these methodologies, the alternatives are presented using a superstructure, a mathematical model is developed, and an algorithm is used to find the best option based on an objective function. The differences between these methodologies depend on the superstructure, the mathematical representation of the problem, and the optimization algorithm. However, one problem with these methods is that the recovery of each stage must be modeled, and because the recovery of each stage is a function of many variables and is difficult to model, the results are usually debatable.
Flotation circuits are commonly used for the concentration of a broad range of minerals; such circuits also used in wastewater treatment (Rubio et al. 2002). Froth flotation is based on the differences in the ability of air bubbles to adhere to specific mineral surfaces in a solid/water slurry. Then particles with or without air bubbles attached are either carried to the surface and removed or left in the liquid phase. The current method used to design these circuits is based on seven steps (Harris et al., 2002): 1) conduct a mineralogical examination in conjunction with a range of grinding tests; 2) conduct a range of laboratory scale batch tests and locked cycle tests; 3) create a circuit design based on scale-up laboratory kinetics; 4) perform a preliminary economic evaluation of the ore body; 5) pilot-plant test the circuit design; 6) evaluate the economy; 7) design a full-scale plant. This procedure has several problems: 1) the design of the circuit is made in step three based on rule-of-thumb scale-up from laboratory data; therefore, the design depends heavily on the designer's experience; 2) building laboratory and pilot plants are costly and time consuming; therefore, the designed circuit cannot be analyzed in depth; 3) other aspects, such as system dynamics, are not considered in the design process.
Froth flotation design and operation are complex tasks because various important parameters are interconnected. The parameters can be classified into four types of components, as shown in Fig. 1 If any of these factors is changed, other parts of the design will also be changed. It is impossible to study all of the parameters at the same time. For example, if six parameters are selected for study in a four-stage circuit, over eight million tests are required for a two-level fractional experiment design. In addition, for any given number of stages, there are several potential circuit configurations. For example, if six flotation stages are considered, more than 1,400 potential circuit configurations will be possible. Therefore, the design will not be analyzed in depth.
In this work, a new methodology that integrates the first five design steps outlined previously is presented. The objectives are 1) to better orient the goals of the laboratory tests; 2) to reduce the number of laboratory and pilot-plant testing, and achieving lower cost and execution times; 3) to design the flotation circuit systematically, and 4) to expedite the design process. The methodology, inspired by the work of d'Anterroches and Gani (2005), has three design steps: 1) definition and analysis; 2) process synthesis; and 3) final design. Four tools are used in this procedure: 1) laboratory testing, 2) process group contribution, 3) sensitivity analysis, and 4) reverse simulation (Fig. 2).
The example of flotation circuit without grinding considered the concentration of copper ore. The feed to the circuit corresponded to 6 t/h of chalcopyrite (33% of copper), 12 t/h of chalcopyrite slow (16% of copper), and 300 t/h of gangue. The superstructure considers five flotation stages. If all interconnection was allowed, there were over 3 million circuit structure alternatives. However, if origin-destination matrices were used to eliminate nonsense and redundant alternatives, the number of feasible flotation circuits was 6912. The procedure utilized for the postulation of a superstructure and the formulation of the mathematical programming model was the one utilized by Calisaya et al. (2016), which corresponded to a MINLP. The variables with uncertainties corresponded to the stage recoveries of the chalcopyrite, chalcopyrite slow, and gangue. The stage recoveries were difficult to model as there is not a model that can be used under all flotation circuit structures included in the superstructure. Here, the stage recoveries were represented by values obtained from the uniform distribution. Under these conditions, the design problem is a MILP.
After the optimal flotation circuit configuration is determined, the water integration problem is addressed. This problem has not been analyzed before because the main concern in mineral processing has been the recovery and the product grade. Water can be recycled from tail or concentrate dewatering operations. However, only some of the water recovered in these operations can be recycled because it affects the flotation behavior.
Recently, El-Halwagi et al. (2004) have used the concept of clustering for process design based on property integration. Property integration is defined as a functionality-based holistic approach to the allocation and manipulation of streams and processing units, which is based on tracking, adjusting, assigning, and matching functionalities throughout the process. Here the methodology developed by El-Halwagi et al. (2004) was used to design the water integration problem.
The overall problem definition given by El-Halwagi et al. (2004) can be adapted as follows: "Given a flotation circuit with certain sources (process streams and water streams) and flotation units along with their properties (pH, solid concentration, oxygen concentration) and constraints, it is desired to develop graphical techniques that identify optimum strategies for allocation and interception that integrate the properties of sources, sinks, and interceptors so as to optimize a desirable process objective (minimum usage of fresh water, maximum utilization of water recycled from tail and concentrate dewatering operations, minimum cost of slaked lime while satisfying the constraints on properties and flow rate for the sinks".
The objective of the flotation circuit used by the mining concentration plant in El Salvador is to obtain a concentrate rich in Cu and Mo. The influence of the input factors on the variability of the grade and recovery of Cu and Mo in the final concentrate is analyzed. The input factors to consider are the kinetic constants of each species in each stage, maximum recoveries of each species in each stage, residence time of the pulp in each stage, number of cells in the R and RS stages, solids concentration in each stage, froth depth height on the columns, and superficial air and water rates in the column. In total, there are 47 input factors, and uncertainty ranges were taken from Yianatos et al. (2005) and Yianatos and Henriquez (2006). Based on the current operating conditions of the El Salvador plant, variations of 15% and 3% for kinetic constants and maximum recoveries, respectively, were considered. In the case of the solids concentration and the froth depth, variations of 10% were considered. In the case of superficial air and water rates, variations of 11% and 12%, respectively, were considered. The variation in the number of cells in R and CS were 7-9 and 8-10, respectively. The variation of the residence time in the R and CS stages was 10%. Finally, the residence time in C varied by 7%.
Figure 2 shows that input factors 10 (kmax molybdenite in C), 11 (kmax of pyrite in C), 25 (surface flow), 27 (Rmax chalcopyrite in R), 30 (Rmax molybdenite in R), and 42 (Rmax molybdenite in CS) are the main sources of variability of Cu and Mo recoveries and grades. Input factor 10 affects the Mo grade and recovery, input factor 11 affects the Mo and Cu grades, input factor 25 affects both the Cu and Mo grades and recoveries, input factor 27 affects the Cu recovery, and input factors 30 and 42 affect the Mo grade and recovery. From these results, it is clear that six of the 47 input factors are the key variables for the Cu-Mo grade and recovery.
There are various methods available for cleaning fine coal, of which froth flotation has become the most common practice. Froth flotation depends on differences in surface properties between coal and shale. Air bubbles are generated within an aqueous suspension of fine raw coal with a solids concentration of less than 10%. The hydrophobic coal particles attach to the air bubbles and are buoyed to the top of the froth flotation cell where they are removed as froth. The hydrophilic shale particles remain as a suspension and are removed over the tailings weir. The property of hydrophobicity is imparted to coal particles by the addition of a collector like diesel oil. This facilitates the attachment of coal to air bubbles in preference to gangue particles. The aircoal attachment is made stable by the addition of a frothing agent like pine oil. Successful flotation is governed by different factors like oxidation and rank of coal, flotation reagents, agitation and aeration, particle size and pulp density, flotation machine, conditioning time, and pH of the pulp.
Conventional mechanically agitated flotation machines use relatively shallow rectangular tanks, whereas column cells are usually tall vessels with heights normally varying from 7 to 16m as per requirement. Column cells do not use mechanical agitation and are typically characterised by an external sparging system, which injects air into the bottom of the column cell. The absence of intense agitation promotes higher degrees of selectivity. Modern flotation machines are high-intensity equipment designed to create very small bubbles and higher flotation rates. Smaller bubbles are generated by intensive mixing of pulps with air so that fast collisions between particles and bubbles take place. Microcel machines work with forced air, whereas the Jameson cell works with induced air. These machines are particularly suitable for coal flotation (Lynch et al., 2010).
Hydrodynamic analyses have shown that the use of air bubbles smaller than typically generated by conventional flotation machines can improve fine coal recovery. The selectivity also increases as smaller bubbles rise more slowly through the pulp, leaving the high ash impurities at the bottom. One disadvantage of flotation is that efficiency reduces for size range below 100m. To overcome this constraint, a new stirrerless cone-shaped flotation cell was developed in Germany (Bahr, 1982), now called the Pneuflot. This cell uses a novel aeration technique in which minute bubbles are introduced into the slurry before it reaches the cell. The upper particle size is restricted to 300m.
Flotation circuits are simple for Indian coals. Concentrates can be produced in one stage of flotation, and recleaning of the products may not be generally necessary. In the case of highly oxidised coal, two-stage flotation may be required to be incorporated, with rougher cells and cleaner cells. The circuit consists of a number of banks depending upon the total quantity to be handled, whereas each bank can have four to eight cells. For smooth operation of the system, proper operation and control are necessary.
It is observed that the existing flotation circuits in India are not working well. There is substantial loss of coal substance along with tailings. The causes of poor performance can be attributed to the following major factors as stated by Haldar (2007):
In addition, the quality of coal fines has deteriorated and other parameters have changed. Lower seam coals of inferior quality are now supplied. The concept of treating fine coal may need changes. The floatability tests are illustrated in Fig. 8.8; it is possible to produce clean coal with ash% of 17%18% ash with sufficient yield.
A model for the design of flotation circuits under uncertainty has been presented. Uncertainty is represented by scenarios that include changes in the feed grade and in the metal price. The model allows the operating conditions (residence time and mass flows of each stream) and flow structure (tail and concentrate stream of cleaner and scavenger stage) to be changed for each scenario while the fixed design (size of cells in flotation stages) for all scenarios is maintained. The model can be modified to include other uncertainties and other adaptive variables.
To solve the two-stage stochastic model, two solution strategies were proposed. The results show that the use of average values for the stochastic parameters leads to an inefficient design and hence a decrease in the profits made in the process.
Finally, it can be concluded that the use of stochastic programming can be a beneficial tool in the design of a metallurgical process, specifically the copper flotation process. The optimal configuration is capable of adapting to uncertainty, leading to an increase in the company profits
The simplest way of smoothing out grade fluctuations and of providing a smooth flow to the flotation plant is by interposing a large agitated storage tank (agitator) between the grinding section and the flotation plant:
Any minor variations in grade and tonnage are smoothed out by the agitator, from which material is pumped at a controlled rate to the flotation plant. The agitator can also be used as a conditioning tank, reagents being fed directly into it. It is essential to precondition the pulp sufficiently with the reagents (including sometimes air, Section 12.8) before feeding to the flotation banks, otherwise the first few cells in the bank act as an extension of the conditioning system, and poor recoveries result.
Provision must be made to accommodate any major changes in flowrate that may occur; for example, grinding mills may have to be shut down for maintenance. This is achieved by splitting the feed into parallel banks of cells (Figure 12.53). Major reductions in flowrate below the design target can then be accommodated by shutting off the feed to the required number of banks. The optimum number of banks required will depend on the ease of control of the particular circuit. More flexibility is built into the circuit by increasing the number of banks, but the problems of controlling large numbers of banks must be taken into account. The move to very large unit processes, such as grinding mills, flotation machines, etc., in order to reduce costs and facilitate automatic control, has reduced the need for many parallel banks.
Some theoretical considerations have been introduced (Section 12.11.2), but there is a practical aspect as well: if a small cell in a bank containing many such cells has to be shut down, then its effect on production and efficiency is not as large as that of shutting down a large cell in a bank consisting of only a few such cells.
Flexibility can include having extra cells in a bank. It is often suggested that the last cell in the bank normally should not be producing much overflow, thus representing reserve capacity for any increase in flowrate or grade of bank feed. This reserve capacity would have to be factored in when selecting the length of the bank (number of cells) and how to operate it, for example, trying to take advantage of recovery or mass pull profiling. If the ore grade decreases, it may be necessary to reduce the number of cells producing rougher concentrate, in order to feed the cleaners with the required grade of material. A method of adjusting the cell split on a bank is shown in Figure 12.54. If the bank shown has, say, 20 cells (an old-style plant), each successive four cells feeding a common launder, then by plugging outlet B, 12 cells produce rougher concentrate, the remainder producing scavenger concentrate (assuming a R-S-C type circuit). Similarly, by plugging outlet A, only eight cells produce rougher concentrate, and by leaving both outlets free, a 1010 cell split is produced. This approach is less attractive on the shorter modern banks. Older plants may also employ double launders, and by use of froth diverter trays cells can send concentrate to either launder, and hence direct concentrate to different parts of the flowsheet. An example is at the North Broken Hill concentrator (Watters and Sandy, 1983).
Rather than changing the number of cells, it may be possible to adjust air (or level) to compensate for changes in mass flowrate of floatable mineral to the bank. To maintain the bank profile at Brunswick Mine, total air to the bank was tied to incoming mass flowrate of floatable mineral so that changes would trigger changes in total air to the bank, while maintaining the air distribution profile (Cooper et al., 2004).
Consider that the circuit discussed in section 2 corresponds to a flotation circuit in which a material composed of gangue (specie 2) and a valuable species (species 1). The operating conditions of this circuit are such that the transfer functions for species 1 are, TR1=0.75, TS1=0.85, TC1 =0.73, and for specie 2 are TR2=0.25, TS2=0.40, TC2=0.30. Each stage corresponds to flotation bank with 5, 8 and 7 cells in the stages R, S and C, respectively. The transfer function for each stage is given by (Cisternas et al., 2006)
Under the conditions of Figure 2, we have thatR1 ( TR1, 0. 86 ,0.73 ) TR1 and R2 ( TR2, 0 40, 030) TR2 . Also, as TR1=0.75 for species 1 and TR1 =0.25 for species 2, we have the following overall recoveriesR1(0.75, 0.86, 0.73)=0.94 and R2(0.25, 0.40, 0.30 =0.143 (i.e., 94% for species 1 and 14.3% for species 2.) A sensitivity analysis can help us to improve the operational conditions of the stages involved in the process, and, therefore, reduce the percentage of gangue recovery without affecting too much the percentage recovery of useful material. The recovery and sensitivity of the above circuit are given by the equations 1 to 4. Then, setting the transfer functions TSj and TCj in the eq.2, the behavior of global recovery sensitivity with respect to the transfer function TRjcan be studied as shown in figure 3. Figure 3a shows Rj/TRj versus TRj for values of (TSj,TCj) close to the species 2. It can be seen that sensitivity increases as the value of TRjincreases. The opposite behavior is shown in Figure 3b which corresponds to values of (TSj,TCj) near species 1. This means that the behavior of the sensitivity is inversely between the valuable specie and the gangue.
Figure 4a shows RjTijversus Tijfor values of (TRj,TSj,TCj)close to species 2.We can observe that for species 2, the highest sensitivity to the global recovery is given in ( 0.25 , 0.4, TC2) and ( TR, 0.40, 0.3 ), which indicates that it is more sensitive to the transfer functions in the stage C (TC2 ) and R (TR2 ). Looking at the graphs in Figure 4b, we can observe that for species 1, the highest sensitivity in the global recovery is given in (0.75,TS1, 0.73), which indicates that it is more sensitive to the transfer function in the stage S (TS1). The same analysis can be performed for the other derivatives in equations 3 and 4. Performing the sensitivity analysis, it is possible to reach the following conclusions: 1) for species 1, the highest sensitivity to the global recovery for the stages R, S and C are given in ( 0.75 , TS1, 0.73 ),( 0.75 ,TS1, 0.73 )-( 0.75 , 0.86 , TC)and (0.75,TS 1, 0.73)-(0.75,0.86,TS 1), respectively. On the other hand, for species 2, the highest sensitivity to the global recovery for the stages R, S and C are given in (0.25, 0.4,TC2)-(TR2, 0.4,0.3),(0.25, 0.4,TC2)-(TR2, 0.4,0.3) and (TR2, 0.4, 0.3), respectively.
With this information about the behavior of global recovery, the sensitivity with respect to the transfer functions, we can intuitively change the values of the transfer functions of species 1 and 2 in the stages where they are most influential. Then, by reverse simulation with equation 5, it is possible to determine new designs (N) and/or operating conditions(kijand/or ) to achieve a better system performance. Thus, by changing the number of cells at 11 and 9 for stages S and C respectively, it is possible to obtain the following results: R1(0.75, 0.80, 0.68)=0.91 and R2(0.25, 0.40, 0.20)=0.077.
The self-tuning control algorithm has been developed and applied on crusher circuits and flotation circuits  where PID controllers seem to be less effective due to immeasurable change in parameters such as the hardness of the ore and wear in crusher linings. STC is applicable to non-linear time-varying systems. It however permits the inclusion of feed forward compensation when a disturbance can be measured at different times. The STC control system is therefore attractive. The basis of the system is
The disadvantage of the set-up is that it is not very stable and therefore in the control model a balance has to be selected between stability and performance. A control law is adopted. It includes a cost function CF, and penalty on control action. The control law has been defined as
A block diagram showing the self-tuning set-up is illustrated in Figure20.26. The disadvantage of STC controllers is that they are less stable and therefore in its application, a balance has to be derived between stability and performance.
Since several types of flotation circuits can generally be employed in conjunction with the various processes for the flotation of different classes of minerals, an outline of the standard circuits in common use is best given before the processes to which they are applicable are described. The flow sheets illustrating them are diagrammatic, but, in cases where the circuit includes two or more machines, the latter are shown in their approximate relative positions. The diagrams must not, however, be considered to represent exact plant layouts; any standardised arrangement willusually need a certain amount of alteration in minor details to suit a particular ore, not only to meet the requirements of the flotation process but also to conform to the design of the plant as a whole.
is taken off the first few cells and the remainder are run as scavengers, the froth from them being returned to the head of the machine through a middling pipe, as described in the previous chapter in connection with M.S. Sub-aeration and Denver Sub-A Machines. This method can be operated successfully when the gangue is relatively unfloatable, but it requires rather more careful control than when a separate cleaning circuit is used.
Circuit No. 2 (Fig. 49) is, as a rule, preferable to the preceding one, and it is advisable to employ it when a very clean concentrate is desired. The froth from the primary or roughing cells, termed the rougher concentrate, is diluted to a W/S ratio between 6/1 and 10/1 and transferred through a middling pipe to the cleaning section. This consists of a series of similar cells, varying in number from one-half of those in the roughing section for the treatment of a bulky low-grade rougher concentrate to one-quarter of the number when it is required to raise a small quantity of comparatively rich primary concentrate to the highest possible grade. It is usual, though not necessary, for the cleaning cells to be placed at the head of the machine. The tailing from the last cell is then mixed, as it overflows the discharge weir, with the incoming primary feed, so that both enter the roughing section through the transfer passage or pipe.
Circuit No. 2 is easier to regulate than No. 1 because the final concentrate is made in a pulp containing a much larger proportion of mineral than of gangue ; there is therefore less likelihood of gangue entering the froth and lowering the grade of the final concentrate. Moreover, variations in the character and rate of the feed, such as are encounteredperiodically, especially in small plants, may cause considerable alteration in the grade of the rougher concentrate before the change is noticed and the machine readjusted. Variations of this sort are evened out to a large extent in cleaning cells, so that the grade of the finished concentrate is affected very much less than would be the case if circuit No. 1 were in use.
Circuit No. 3 (Fig. 50) is developed from the preceding one. Here the primary cells are divided into roughing and scavenging sections. The concentrate from the first section is re-treated in cleaning cells as before, but that from the latter part of the machine is returned to the first roughing cell through a middling pipe. Thus the cleaning cells receive a comparatively high-grade feed, whilst the scavenging section can be run with an excess of air so as to obtain the maximum recovery of the valuable minerals ; the resulting low-grade froth, being usually little richer in mineral than the normal run of ore, is best returned to the circuit with the primary feed. This circuit, besides being useful for ores that need the maximum amount of aeration at the end of the machine to give a profitable recovery of the valuable minerals, is often employedwhen the gangue is floatable and difficult to separate from the mineral. In the latter case, if one cleaning operation does not yield a concentrate of high enough grade, a re-cleaning section becomes necessary, such as is shown in circuit No. 4 (Fig. 51), in which the cleaner concentrate is diluted and re-treated in a separate series of cells. In this circuit the use of middling return pipes eliminates the necessity for pumping three separate concentrate products.
It should be noted that the diluting water of the cleaning cells in circuits 2 and 3 passes to the roughing cells and dilutes the primary feed, which ought therefore to contain a correspondingly smaller proportionof water as it leaves the grinding section in order that the dilution of the cleaner tailing may bring it to the correct pulp ratio in the roughing machine. Little further dilution of the primary pulp is involved when recleaning is practised, since the water added passes with the recleaner tailing to the cleaning cells where it constitutes the bulk of the diluting water, very little extra being normally required. Thus practically the same amount of water enters the roughing section in the cleaner tailing whether recleaning is practised or not. This also applies to the cleaning operations in the air-lift and pneumatic machines.
An air-lift machine can often be run as a rougher-cleaner cell, as in circuit No. 5 (Fig. 52), by taking a finished concentrate off the first few feet and returning the froth from the rest of the machine to the feed end. This method is also applicable to pneumatic cells, but its use is not common. As with mechanically agitated machines, careful control is needed if the required grade of concentrate is to be consistently maintained, and it is more usual, and generally more satisfactory, to employ a circuit such as No. 6 or 7, in which a separate stage of cleaning is included. Here the primary(rougher) concentrate is diluted as before to a W/S ratio between 6/1 and 10/1 before being sent to the cleaning section, and the cleaner tailing is returned to the head of the primary machine.
Circuit No. 6 (Fig. 53) is suitable when the feed flows by gravity to the rougher ; it is generallyadvisable to adopt it also in cases where the cleaning operation requires the addition of extra reagents, since the pump provides a convenient method of mixing them thoroughly with the pulp, but when it is necessary for the feed to be pumped to the rougher, circuit No. 7 (Fig. 54) has
the advantage that the cleaner tailing can be elevated in the same pump, the primary concentrate flowing by gravity to the cleaning section. When vigorous aeration is needed to obtain a maximum recovery of
the valuable minerals, it is preferable to use separate roughing and scavenging machines, the scavenger froth being returned with the cleaner tailing to the head of the rougher. Of the two usual arrangements, circuit No. 8 (Fig. 55) is the simpler, requiring only one pump to handle both products, but, should the layout of the plant make it advisable to place the cleaning section above the rougher, circuit No. 9 (Fig. 56) withtwo pumps will be necessary. The second method has the advantage that the pumps provide points of agitation ahead of both the roughing and the cleaning sections for the addition of reagents, should they be required. It is usually possible to place the two pumps close together with a third between them so connected that it can be used as a spare for either of the other two. By separating the roughing and scavenging machines in this way a rougher concentrate without an undue proportion of gangue can be sent to the cleaning section.
It will then be correspondingly easy to produce a final concentrate of the highest possible grade, while still maintaining maximum aeration in the scavenging section, the froth from which is returned to the point where the comparatively large amount of gangue in it does the least harm.
Even with separate roughing and scavenging sections it is sometimes found impossible to make a concentrate of high enough grade in the cleaner without allowing too much mineral to return in the tailing to the rougher, with consequent overloading of the latter section. In such a case recleaning is necessary, the cleaner concentrate being diluted with water and re-treated in a separate machine. Circuits 10 and 11 (Fig. 57 and 58) are the two most usual arrangements, but various modifications of the layout are possible to suit special cases. In fact, although the flotation method to be adopted is the primary consideration, the actual positions of the flotation machines in relation to each other and to the pumps handling the products must necessarily depend to a great extent on the requirements of the rest of the plant.
As in the case of mechanically agitated machines, a recleaning circuit involves very little more dilution of the pulp in the rougher than does one cleaning operation, since the diluting water in the second stage passes to the first stage with the recleaner tailing and serves the same purpose there.
It occasionally happens that, although two cleaning operations are necessary to obtain a final concentrate of the required grade, the tailing from neither stage contains much mineral. In snch cases the circuit can be simplified by dispensing with one pump and sending the tailings from both stages to the head of the rougher, as is shown by the dotted line in circuit No. 11 (Fig. 58). The method has the disadvantage, however, that the diluting water from both cleaning sections enters the roughing machine and may make the pulp too thin. It is therefore preferable to pump one or both tailings to the classifiers in the grinding section, especially if they contain imperfectly ground material. Not only is the mineral then given another chance of entering the ball mill and becoming liberated from the gangue, but the water of the two tailings serves todilute the pulp in the classifiers and does not then interfere with the density of the pulp in the roughing machine. The tailing from a single- stage cleaning operation is often returned in the same way to the grinding circuit, if it contains mineral which has not been completely liberated from the gangue.
When a large tonnage is to be handled, the flotation section of the plant is divided up into a number of parallel units, each consisting of one or more lines of roughing and scavenging machines with the appropriate cleaners. Occasionally, if the reagent mixture and control is simple, the grinding section is also designed in separate units, from each of which the pulp is discharged directly to the corresponding flotation unit. It is more usual, however, to send the discharge of the grinding section to a central distributor which splits the pulp up evenly amongst the flotation units ; the number of the latter need not then correspond to the number of grinding units.
The MAC flotation cell was developed by Kadant-Lamort Inc. It can save energy comparedto conventional flotation systems. The MAC flotation cell is mainly used in the flotation section of waste paper deinking pulping, for removal of hydrophobic impurities such as filler, ash,ink particles, etc. It can increase pulp whiteness and meet the requirements of final paper appearance quality. Table11.11 shows the features of MAC flotation cell. Kadants MAC flotation cell deinking system uses air bubbles to float ink particles to the cell surface for removal from the recycled material. The latest generation of the MAC cell deinking system incorporates a patented bubble-washing process to reduce power consumption and also fiber loss. It combines small, new, auto-clean, low-pressure injectors with a flotation cell. The function of injectors is to aerate the stock before it is pumped and sent tangentially to the top of the cell. The air bubbles collect ink particles in the cell and rise up to the top to create a thick foam mat that is evacuated because of the slight pressurization of the cell. The partially deinked stock then goes to a deaeration chamber and is pumped to the next stage. Here, the operation is exactly the same as for the first stage. This stage also has the same number of injectors and same flow (Kadant,2011). This operation is repeated up to five times for a high ink removal rate. Remixing of the air coming from downstream stages of the process helps the upstream stages and improves the overall cell efficiency. Adjustable and selective losses of fiberdepend on the application and technical requirements inks, or inks and fillers. The use of low-pressure injectors in the MAC flotation cell could save about 2530% of the energy used in conventional flotation systems (ECOTARGET,2009). The benefits of the MAC flotation cell are summarized in Table11.12.
Agitated flotation cells are widely used in the mineral processing industry for separating, recovering, and concentrating valuable particulate material from undesired gangue. Their performance is lowered, however, when part of the particulate system consists of fines, with particle diameters typically in the range from 30 to 100m. For example, it was observed difficult to float fine particles because of the reduction of middle particles (of wolframite) as carriers and the poor collision and attachment between fine particles and air bubbles; a new kinetic model was proposed .
As an alternative to agitated cells, bubble columnsused in chemical engineering practice as chemical reactorswere proposed for the treatment of fine particle systems. Flotation columns, as they came to be known, were invented back in the 1960s in Canada . The main feature that differentiates the column from the mechanical flotation cell (of Denver type) is wash water, added at the top of the froth. It was thought to be beneficial to overall column performance since it helps clean the froth from any entrained gangue, while at the same time preventing water from the pulp flowing into the concentrate. In this way, it was hoped that certain cleaning flotation stages could be gained.
Let us note that the perhaps insistence here on mineral processing is only due to the fact that most of the available literature on flotation is from this area, where the process was originated and being widely practiced. The effect of particle size on flotation recovery is significant; it was shown that there exists a certain size range in which optimum results may be obtained in mineral processing. This range varies with the mineral properties such as density, liberation, and so on, but was said to be of the order of 10100m .
Regulating the oxidation state of pyrite (FeS2) and arsenopyrite (FeAsS), by the addition of an oxidation or reduction chemical agent and due to the application of a short-chain xanthate as collector (such as potassium ethyl xanthate, KEX), was the key to selective separation of the two sulfide minerals, pyrite and arsenopyrite . Strong oxidizing agents can depress previously floated arsenopyrite. Various reagents were examined separately as modifiers and among them were sodium metabisulfite, hydrazinium sulfate, and magnesia mixture. The laboratory experiments were carried out in a modified Hallimond tube, assisted by zeta-potential measurements and, in certain cases, by contact angle measurements.
This conventional bench-scale flotation cell provides a fast, convenient, and low-cost method, based on small samples (around 2g), usually of pure minerals and also artificial mixtures, for determining the general conditions under which minerals may be rendered floatableoften in the absence of a frother (to collect the concentrate in the side tube) . This idea was later further modified in the lab replacing the diaphragm, in order to conduct dissolved air or electroflotation testssee Section 3.
Pyrite concentrates sometimes contain considerable amounts of arsenic. Since they are usually used for the production of sulfuric acid, this is undesirable from the environmental point of view. However, gold is often associated with arsenopyrite, often exhibiting a direct relationship between Au content and As grade. There is, therefore, some scope for concentrating arsenopyrite since the ore itself is otherwise of little value (see Fig.2.2). Note that previous work on pyrites usually concentrated on the problem of floating pyrite .
In the aforementioned figure (shown as example), the following conditions were applied: (1) collector [2-coco 2-methyl ammonium chloride] 42mg/L, frother (EtOH) 0.15% (v/v), superficial liquid velocity uL=1.02cm/s, superficial gas velocity uG=0.65cm/s, superficial wash water velocity uw=0.53cm/s; (2) hexadecylamine, 45mg/L; pine oil, 50mg/L; EtOH, 0.025%; uL=0.84cm/s; uG=0.72cm/s; uw=0.66cm/s; (3) Armoflot 43, 50mg/L; pine oil, 50mg/L; EtOH, 0.025%; uL=0.84cm/s; uG=0.71cm/s; uw=0.66cm/s . The pyrite (with a relatively important Au content of 21g/ton) was a xanthate-floated concentrate. The presence of xanthates, however, might cause problems in the subsequent cyanidation of pyrites when recovering their Au value, which perhaps justified the need to find alternative collectors. In general, the amines exhibited a behavior similar to that of the xanthates (O-alkyl dithiocarbonates). The benefit of the amine was in its lower consumption, as compared with the xanthate systems.
The arsenic content of the pyrite was approximately 9% (from an initial 3.5% of the mixed sulfide ore). The material was sieved and the75m fraction was used for the laboratory-scale cylindrical column experiments. The effect on metallurgical characteristics of the flotation concentrate of varying the amount of ferric sulfate added to the pulp was studied; three collectors were used and their performance was compared (in Fig.2.2). Both hexadecylamine and Armoflot 43 (manufactured by Akzo) exhibited an increased recovery but a very low enrichment, whereas 2-coco 2-methyl ammonium chloride (Arquad-2C) showed a considerable enrichment; a compromise had to be made, therefore, between a high-grade and a low recovery.
Electroflotation (electrolytic flotation) is an unconventional separation process owing its name to the bubbles generation method it uses, i.e., electrolysis of the aqueous medium. In the bottom of the microcell, the two horizontal electrodes were made from stainless steel, the upper one being perforated. The current density applied was 300 Am2. It was observed that with lime used to control pH, different behavior was observed (see Fig.2.3). Pyrite, with permanganate (a known depressant) also as modifier, remained activated from pH 5.0 to 8.0at 80% recovery, while it was depressed at the pH range from 9.0 to 12.0. A conditioning of 30min was applied in the presence of modifier alone and further 15min after the addition of xanthate. The pure mineral sample, previously hand collected, crushed, and pulverized in the laboratory, was separated by wet sieving to the45 to+25m particle size range.
Pyrite due to its very heterogeneous surface, consisting of a mosaic of anodic and cathodic areas, presents a strong electrocatalytic activity in the anodic oxidation of xanthate to dixanthogen. It is also possible that the presence of the electric field, during electroflotation, affected the reactions taking place. In order to explain this difference in flotation behavior thermodynamic calculations for the system Fe-EX-H2O have been done . It was concluded that electroflotation was capable of removing fine pyrite particles from a dilute dispersion, under controlled conditions. Nevertheless, dispersed air and electroflotation presented apparent differences for the same application.
The size of the gas bubbles produced was of the order of 50m, in diameter . Similar measurements were later carried out at Newcastle, Australia ; where it was also noted that a feature of electroflotation is the ability to create very fine bubbles, which are known to improve flotation performance of fine particles.
In fact, the two electrodes of a horizontal electrodes set, usually applied in electroflotation, could be separated by a cation exchange membrane, as only one of the produced gases is often necessary . In the lower part/separated electrode, an electrolyte was circulated to remove the created gas, and in the meantime, increase the conductivity; hence having power savings (as the electric field is built up between the electrodes through the use of the suspension conductivity). Attention should be paid in this case to anode corrosion, mainly by the chloride ion (i.e., seawater).
Microorganisms have a tremendous influence on their environment through the transfer of energy, charge, and materials across a complex biotic mineralsolution interface; the biomodification of mineral surfaces involves the complex action of microorganism on the mineral surface . Mixed cationic/anionic surfactants are also generating increasing attention as effective collectors during the flotation of valuable minerals (i.e., muscovite, feldspar, and spodumene ores); the depression mechanisms on gangue minerals, such as quartz, were focused .
Another design of a flotation cell which applies ultrasound during the flotation process has been developed by Vargas-Hernndez et al. (2002). The design consists of a Denver cell (Koh and Schwarz, 2006) equipped with ultrasonic capabilities of performing ultrasound-assisted flotation experiments. This cell is universally accepted as a standard cell for laboratory flotation experiments. In Figure 35.25, a schematic of the Denver cell equipped with two power transducers is shown operating at 20kHz. The ultrasonic transducers are in acoustic contact with the body of the flotation cell but are not immersed in the same cell. Instead, they are submerged in distilled water and in a thin membrane that separates the radiant head of the transducer from the chamber body. The floatation chamber has a capacity of 2.7l and is also equipped with conventional systems to introduce air and mechanical agitation able to maintain the suspension of metallurgical pulp. In the upper part of the cell there is an area in which the foam is recovered for analysis by a process called skimming. The block diagram of Figure 35.25 further shows that the experimental system was developed to do ultrasonic-assisted flotation experiments. The transducers operate at 20kHz and can handle power up to 400W. In the Denver cell an acoustic probe, calibrated through a nonlinear system and capable of measuring high-intensity acoustic fields, is placed (Gaete-Garretn et al., 1993, 1998). This is done in order to determine the different acoustic field intensities with a spatial scanner during the experimentation. Figure 35.26 shows the distribution of ultrasonic field intensity obtained by a spatial scanner in the central area of the flotation chamber. The Denver cell with ultrasonic capabilities, as described, is shown in Figure 35.27. The obtained results were fairly positive. For example, for fine particle recovery it worked with metallurgical pulp under 325mesh, indicating floating particles of less than 45m, and the recovery curves are almost identical to those of an appropriate size mineral for flotation. This is shown in Figure 35.28, where a comparison between typical copper recovery curves for fine and normal particles is presented. The most interesting part of the flotation curves is the increase in recovery of molybdenum with ultrasonic power, as shown in Figure 35.29. The increase in recovery of iron is not good news for copper mines because the more iron floating the lower grade of recovery. This may be because the iron becomes more hydrophobic with ultrasonic action. According to the experts, this situation could be remedied by looking for specific additives to avoid this effect. Flotation kinetics shown in Figure 35.30 with 5 and 10W of acoustic power applied also show an excellent performance. It should be noted that the acoustic powers used to vary the flotation kinetics have been quite low and could clearly be expanded.
Figure 35.28. Compared recovering percent versus applied power in an ultrasonic-assisted flotation process in a Denver cell: (a) fine and ultrafine particles recovering and (b) normal particles recovering.
These experiments confirm the potential of power ultrasound in flotation. Research on assisted flotation with power ultrasound has been also carried out by Ozkan (2002), who has conducted experiments by pretreating pulp with ultrasound during flotation. Ozkhans objective was to recover magnesite from magnesite silts with particles smaller than 38m. Their results show that under ultrasonic fields the flotation foam bubbles are smaller, improving magnesite recovery rates. When Ozkhan treated magnesite mineral with a conventional treatment the beneficial effect of ultrasound was only manifested for mineral pretreatment. The flotation performed under ultrasonic field did not show improvement. This was because power ultrasound improves the buoyancy of clay iron and this has the effect of lowering the recovery of magnesite.
Kyllnen et al. (2004) employed a cell similar to Jordan to float heavy metals from contaminated soils in a continuous process. In their experiments they obtained a high recovery of heavy metals, improving the soil treatment process. Alp et al. (2004) have employed ultrasonic waves in the flotation of tincal minerals (borax Na O710 B4 H2O), finding the same effects as described above, i.e., that power ultrasound helps in the depression of clay. However, the beneficial effect of ultrasound is weakened when working with pulps with high mineral concentration (high density), probably due to an increase in the attenuation of the ultrasonic field. Safak and Halit (2006) investigated the action mechanisms of ultrasound under different flotation conditions. A cleaning effect on the floating particles was attributed to the ultrasonic energy, making the particles more reactive to the additives put in the metallurgical pulp. Furthermore due to the fact that the solid liquid interface is weaker than the cohesive forces of the metallurgic pulp liquids, it results in a medium favorable to creation of cavitation bubbles. The unstable conditions of a cavitation environment can produce changes in the collectors and even form emulsions when entering the surfactant additives. In general, many good properties are attributed to the application of ultrasound in flotation. For example, there is a more uniform distribution of the additives (reagents) and an increase in their activity. In fact in the case of carbon flotation it has been found that the floating times are shortened by the action of ultrasound, the bubble sizes are more stable, and the consumption of the reagents is drastically lowered.
Abrego Lpez (2006) studied a water recovery process of sludge from industrial plants. For this purpose he employed a flotation cell assisted by power ultrasound. In the first stage he made a flotation to recover heavy metals in the metallurgical pulp, obtaining a high level of recovery. In the second stage he added eucalyptus wood cones to the metallurgical pulp to act as an accumulator of copper, lead, nickel, iron, and aluminum. The author patented the method, claiming that it obtained an excellent recovery of all elements needing to be extracted. zkan and Kuyumcu (2007) showed some design principles for experimental flotation cells, proposing to equip a Denver flotation cell with four power transducers. Tests performed with this equipment consisted of evaluating the possible effects that high-intensity ultrasonic fields generated in the cell may have on the flotation. The author provides three-dimensional curves of ultrasonic cavitation fields in a Denver cell filled with water at frequencies between 25 and 40kHz. A warming effect was found, as expected. However, he states that this effect does not disturb the carbon recovery processes because carbon flotation rarely exceeds 5min. They also found that the pH of tap water increases with the power and time of application of ultrasound, while the pH of the carbonwaterreagentsludge mixture decreases. The conductivity of the metallurgical pulp grows with the power and time of application of ultrasound, but this does not affect flotation. The carbon quality obtained does not fall due to the application of ultrasound and the consumption of lowered reagents. They did not find an influence from the ultrasound frequency used in the process, between 25 and 40kHz. They affirmed that ultrasound is beneficial at all stages of concentration.
Kang et al. (2009) studied the effects of preconditioning of carbon mineral pulp in nature by ultrasound with a lot of sulfur content. They found that the nascent oxygen caused by cavitation produces pyrite over oxidation, lowering its hydrophobicity, with the same effect on the change of pH induced by ultrasonic treatment. Additionally, ultrasound decreases the liquid gas interfacial tension by increasing the number of bubbles. Similar effects occur in carbon particles. The perfect flotation index increases 25% with ultrasonic treatment. Kang et al. (2008) continued their efforts to understand the mechanism that causes effects in ultrasonic flotation, analyzing the floating particles under an ultrasonic field by different techniques like X-ray diffraction, electron microscopy, and scanning electron microscopy techniques. In carbon flotation it is estimated that ultrasonic preconditioning may contribute to desulfurization and ash removal (deashing) in carbon minerals. Zhou et al. (2009) have investigated the role of cavitation bubbles created by hydrodynamic cavitation in a flotation process, finding similar results to those reported for ultrasonic cavitation flotation. Finally, Ozkan (2012) has conducted flotation experiments with the presence of hard carbon sludge cavitation (slimes), encountering many of the effects that have been reported for the case of metallurgical pulp with ultrasound pretreatment. This includes improved flotation, drastic reduction in reagent consumption, and the possible prevention of oxidation of the surface of carbon sludge. A decrease in the ash content in floating carbon was not detected. However, tailings do not seem to contain carbon particles. All these effects can be attributed to acoustic cavitation. However, according to the author, there is a need to examine the contribution of ultrasound to the probability of particlebubble collision and the likelihood of getting the bubbles to connect to the particles. The latter effects have been proposed as causes for improvements in flotation processes in many of the publications reviewed, but there is no systematic study of this aspect.
In summary, power ultrasound assistance with flotation processes shows promising results in all versions of this technique, including conditioning metallurgical pulp before floating it, assisting the continuous flotation process, and improving the yields in conventional flotation cells. The results of ultrasonic floating invariably show a better selectivity and an increase, sometimes considerable, in the recovery of fine particles. Paradoxically, in many experiments an increase has been recorded in recovering particles suitable for normal flotation. These facts show the need for further research in the flotation process in almost all cases, with the exception perhaps of carbon flotation. For this last case, in light of the existing data the research should be directed toward scale-up of the technology.
The concentrate obtained from a batch flotation cell changes in character with time as the particles floating change in size, grade and quantity. In the same way, the concentrate from the last few cells in a continuous bank is different from that removed from the earlier cells. Particles of the same mineral float at different rates due to different particle characteristics and cell conditions.
The recovery of any particular mineral rises to an asymptotic value R which is generally less than 100%. The rate of recovery at time t is given by the slope of the tangent to the curve at t, and the rate of recovery at time t1 is clearly greater than the rate at time t2. There is a direct relationship between the rate of flotation and the amount of floatable material remaining in the cell, that is:
The process is carried out in a flotation cell or tank, of which there are two basic types, mechanical and pneumatic. Within each of these categories, there are two subtypes, those that operate as a single cell, and those that are operated as a series or bank of cells. A bank of cells (Fig. 8) is preferred because this makes the overall residence times more uniform (i.e., more like plug flow), rather than the highly diverse residence times that occur in a single (perfectly mixed) tank.
FIGURE 8. Flotation section of a 80,000t/d concentrating plant, showing the arrangement of the flotation cells into banks. A small part of the grinding section can be seen through the gap in the wall. [Courtesy Joy Manufacturing Co.]
The purpose of the flotation cell is to attach hydrophobic particles to air bubbles, so that they can float to the surface, form a froth, and can be removed. To do this, a flotation machine must maintain the particles in suspension, generate and disperse air bubbles, promote bubbleparticle collision, minimize bypass and dead spaces, minimize mechanical passage of particles to the froth, and have sufficient froth depth to allow nonhydrophobic (hydrophilic) particles to return to the suspension.
Pneumatic cells have no mechanical components in the cell. Agitation is generally by the inflow of air and/or slurry, and air bubbles are usually introduced by an injector. Until comparatively recently, their use was very restricted. However, the development of column flotation has seen a resurgence of this type of cell in a wider, but still restricted, range of applications. While the total volume of cell is still of the same order as that of a conventional mechanical cell, the floor space and energy requirements are substantially reduced. But the main advantage is that the cell provides superior countercurrent flow to that obtained in a traditional circuit (see Fig. 11), and so they are now often used as cleaning units.
Mechanical cells usually consist of long troughs with a series of mechanisms. Although the design details of the mechanisms vary from manufacturer to manufacturer, all consist of an impeller that rotates within baffles. Air is drawn or pumped down a central shaft and is dispersed by the impeller. Cells also vary in profile, degree of baffling, the extent of walling between mechanisms, and the discharge of froth from the top of the cell.
Selection of equipment is based on performance (represented by grade and recovery), capacity (metric tons per hour per cubic meter); costs (including capital, power, maintenance), and subjective factors.
Among all processing industries, only in the ore and mining industries is the accent more on wear resistance than corrosion. In mining industries, the process concerns material handling more than any physical or chemical conversions that take place during the refining operations. For example, in the excavation process of iron ore, conventional conveyer systems and sophisticated fluidized systems are both used [16,17]. In all these industries, cost and safety are the governing factors. In a fluidized system, the particles are transported as slurry using screw pumps through large pipes. These pipes and connected fittings are subjected to constant wear by the slurry containing hard minerals. Sometimes, depending on the accessibility of the mineral source, elaborate piping systems will be laid. As a high-output industry any disruption in the work will result in heavy budgetary deficiency. Antiabrasive rubber linings greatly enhance the life of equipment and reduce the maintenance cost. The scope for antiabrasive rubber lining is tremendous and the demand is ever increasing in these industries.
Different rubber compounds are used in the manufacture of flotation cell rubber components for various corrosion and abrasion duty conditions. Flotation as applied to mineral processing is a process of concentration of finely divided ores in which the valuable and worthless minerals are completely separated from each other. Concentration takes place from the adhesion of some species of solids to air bubbles and wetting of the other series of solids by water. The solids adhering to air bubbles float on the surface of the pulp because of a decrease in effective density caused by such adhesion, whereas those solids that are wetted by water in the pulp remain separated in the pulp. This method is probably the more widely used separation technique in the processing of ores. It is extensively used in the copper, zinc, nickel, cobalt, and molybdenum sections of the mineral treatment industry and is used to a lesser extent in gold and iron production. The various rubber compounds used in the lining of flotation cells and in the manufacture of their components for corrosive and abrasive duties are:
Operating above the maximum capacity can cause the performance of flotation cells to be poor even when adequate slurry residence time is available (Lynch et al., 1981). For example, Fig. 11.21 shows the impact of increasing volumetric feed flow rate on cell performance (Luttrell et al., 1999). The test data obtained at 2% solids correlates well with the theoretical performance curve predicted using a mixed reactor model (Levenspiel, 1972). Under this loading, coal recovery steadily decreased as feed rate increased due to a reduction in residence time. However, as the solids content was increased to 10% solids, the recovery dropped sharply and deviated substantially from the theoretical curve due to froth overloading. This problem can be particularly severe in coal flotation due to the high concentration of fast floating solids in the flotation feed and the presence of large particles in the flotation froth. Flotation columns are particularly sensitive to froth loading due to the small specific surface area (ratio of cross-sectional area to volume) for these units.
Theoretical studies indicate that loading capacity (i.e., carrying capacity) of the froth, which is normally reported in terms of the rate of dry solids floated per unit cross-sectional area, is strongly dependent on the size of particles in the froth (Sastri, 1996). Studies and extensive test work conducted by Eriez personnel also support this finding. As seen in Fig. 11.22, a direct correlation exists between capacity and both the mean size (d50) and ultrafines content of the flotation feedstock. The true loading capacity may be estimated from laboratory and pilot-scale flotation tests by conducting experiments as a function of feed solids content (Finch and Dobby, 1990). Field surveys indicate that conventional flotation machines can be operated with loading capacities of up to 1.52.0t/h/m2 for finer (0.150mm) feeds and 56t/h/m2 or more for coarser (0.600mm) feeds. Most of the full-scale columns in the coal industry operate at froth loading capacities less than 1.5t/h/m2 for material finer than 0.150mm and as high as 3.0t/h/m2 for flotation feed having a top size of 0.300mm feeds.
Froth handling is a major problem in coal flotation. Concentrates containing large amounts of ultrafine (<0.045mm) coal generally become excessively stable, creating serious problems related to backup in launders and downstream handling. Bethell and Luttrell (2005) demonstrated that coarser deslime froths readily collapsed, but finer froths had the tendency to remain stable for an indefinite period of time. Attempts made to overcome this problem by selecting weaker frothers or reducing frother dosage have not been successful and have generally led to lower circuit recoveries. Therefore, several circuit modifications have been adopted by the coal industry to deal with the froth stability problem. For example, froth launders need to be considerably oversized with steep slopes to reduce backup. Adequate vertical head must also be provided between the launder and downstream dewatering operations. In addition, piping and chute work must be designed such that the air can escape as the froth travels from the flotation circuit to the next unit operation.
Figure 11.23 shows how small changes in piping arrangements can result in better process performance. Shown in Fig. 11.23 is a column whose performance suffered due to the inability to move the froth product from the column launder although a large discharge nozzle (11m) had been provided. In this example, the froth built up in the launder and overflowed when the operators increased air rates. To prevent this problem, the air rates were lowered, which resulted in less than optimum coal recovery. It was determined that the downstream discharge piping was air-locking and preventing the launders from properly draining. The piping was replaced with larger chute work that allowed the froth to flow freely and the air to escape. As a result, higher aeration rates were possible and recoveries were significantly improved.
Some installations have resorted to using defoaming agents or high-pressure launder sprays to deal with froth stability. However, newer column installations eliminate this problem by including large de-aeration tanks to allow time for the froth to collapse (Fig. 11.24a). Special provisions may also be required to ensure that downstream dewatering units can accept the large froth volumes. For example, standard screen-bowl centrifuges equipped with 100mm inlets may need to be retrofitted with 200mm or larger inlets to minimize flow restrictions. In addition, while the use of screen-bowl centrifuges provides low product moistures, there are typically fine coal losses, as a large portion of the float product finer than 0.045mm is lost as main effluent. This material is highly hydrophobic and will typically accumulate on top of the thickener as a very stable froth layer, which increases the probability that the process water quality will become contaminated (i.e., black water).
This phenomenon is more prevalent in by-zero circuits, especially when the screen-bowl screen effluent is recycled back through the flotation circuit, either directly or through convoluted plant circuitry. Reintroducing material that has already been floated to the flotation circuit can result in a circulating load of very fine and highly floatable material. As a result, the capacity of the flotation equipment can be significantly reduced, which results in losses of valuable coal. Most installations will combat this by ensuring that the screen-bowl screen effluent is routed directly back to the screen bowl so that it does not return to the flotation circuit. The accumulation of froth on the thickener, which tends to be especially problematic in by-zero circuitry, is also reduced by utilizing reverse-weirs and taller center wells, as this approach helps to limit the amount of froth that can enter into the process water supply. Froth that does form on top of the clarifier can be eliminated by employing a floating boom that is placed directly in the thickener (Fig. 11.24b) and used in conjunction with water sprays. The floating boom can be constructed out of inexpensive PVC piping, and is typically attached to the rotating rakes. The boom floats on the water interface and drags any froth around to the walkway that extends over the thickener, where it is eliminated by the sprays.
Column cells have been developed over the past 30 years as an alternative to mechanically agitated flotation cells. The major operating difference between column and mechanical cells is the lack of agitation in column cells that reduces energy and maintenance costs. Also, it has been reported that the cost of installing a column flotation circuit is approximately 2540% less than an equivalent mechanical flotation circuit (Murdock et al., 1991). Improved metallurgical performance of column cells in iron ore flotation is reported and attributed to froth washing, which reduces the loss of fine iron minerals entrained into the froth phase (Dobby, 2002).
The Brazilian iron ore industry has embraced the use of column flotation cells for reducing the silica content of iron concentrates. Several companies, including Samarco Minerao S.A., Companhia Vale do Rio Doce (CRVD), Companhia Siderrgica Nacional (CSN), and Mineraes Brasileiras (MBR), are using column cells at present (Peres et al., 2007). Samarco Minerao, the first Brazilian producer to use column cells, installed column cells as part of a plant expansion program in the early 1990s (Viana et al., 1991). Pilot plant tests showed that utilization of a column recleaner circuit led to a 4% increase in iron recovery in the direct reduction concentrate and an increase in primary mill capacity when compared to a conventional mechanical circuit.
There are also some negative reports of the use of column cells in the literature. According to Dobby (2002), there were several failures in the application of column cells in the iron ore industry primarily due to issues related to scale-up. At CVRD's Samitri concentrator, after three column flotation stages, namely, rougher, cleaner, and recleaner, a secondary circuit of mechanical cells was still required to produce the final concentrate.
Imhof et al. (2005) detailed the use of pneumatic flotation cells to treat a magnetic separation stream of a magnetite ore by reverse flotation to reduce the silica content of the concentrate to below 1.5%. From laboratory testing, they claimed that the pneumatic cells performed better than either conventional mechanical cells or column cells. The pneumatic cells have successfully been implemented at the Compaia Minera Huasco's iron ore pellet plant.
This chapter presents a novel approach to establish the relationship between collector properties and the flotation behavior of goal in various flotation cells. Coal flotation selectivity can be improved if collector selection is primarily based on information obtained from prior contact angle and zeta potential measurements. In a study described in the chapter, this approach was applied to develop specific collectors for particular coals. A good correlation was obtained between laboratory batches and large-scale conventional flotation cells. This is not the case when these results are correlated with pneumatic cell trial data. The study described in the chapter was aimed at identifying reasons for the noncorrelation. Two collectors having different chemical compositions were selected for this investigation. A considerable reduction in coal recovery occurred at lower rotor speeds when comparing results of oxidized and virgin coal. The degree to which a collector enhances flocculation in both medium- and low-shear applications and also the stronger bubble-coal particle adherence required for high-shear cells must, therefore, all be taken into consideration when formulating a collector for coal flotation.
More ores are treated using froth flotation cells than by any other single machines or process. Non-metallics as well as metallics now being commercially recovered include gold, silver, copper, lead, zinc, iron, manganese, nickel, cobalt, molybdenum, graphite, phosphate, fluorspar, barite, feldspar and coal. Recent flotation research indicates that any two substances physically different, but associated, can be separated by flotation under proper conditions and with the correct machine and reagents. The DRflotation machine competes with Wemco and Outotec (post-outokumpu) flotation cells but are all similar is design. How do flotation cells and machinework for themineral processing industry will be better understood after you read on.
While many types of agitators and aerators will make a flotation froth and cause some separation, it is necessary to have flotation cells with the correct fundamental principles to attain high recoveries and produce a high grade concentrate. The Sub-A (Fahrenwald) Flotation Machines have continuously demonstrated their superiority through successful performance. The reliability and adaptations to all types of flotation problems account for the thousands of Sub-A Cells in plants treating many different materials in all parts of the world.
The design of Denver Sub-A flotation cells incorporates all of the basic principles and requirements of the art, in addition to those of the ideal flotation cell. Its design and construction are proved by universal acceptance and its supremacy is acknowledged by world-wide recognition and use.
1) Mixing and Aeration Zone:The pulp flows into the cell by gravity through the feed pipe, dropping directly on top of the rotating impeller below the stationary hood. As the pulp cascades over the impeller blades it is thrown outward and upward by the centrifugal force of the impeller. The space between the rotating blades of the impeller and the stationary hood permits part of the pulp to cascade over the impeller blades. This creates a positive suction through the ejector principle, drawing large and controlled quantities of air down the standpipe into the heart of the cell. This action thoroughly mixes the pulp and air, producing a live pulp thoroughly aerated with very small air bubbles. These exceedingly small, intimately diffused air bubbles support the largest number of mineral particles.
This thorough mixing of air, pulp and reagents accounts for the high metallurgical efficiency of the Sub-A (Fahrenwald) Flotation Machine, and its correct design, with precision manufacture, brings low horsepower and high capacity. Blowers are not needed, for sufficient air is introduced and controlled by the rotating impeller of the Denver Sub-A. In locating impeller below the stationary hood at the bottom of the cell, agitating and mixing is confined to this zone.
2) Separation Zone:In the central or separation zone the action is quite and cross currents are eliminated, thus preventing the dropping or knocking of the mineral load from the supporting air bubble, which is very important. In this zone, the mineral-laden air bubbles separate from the worthless gangue, and the middling product finds its way back into the agitation zone through the recirculation holes in the top of the stationary hood.
3) Concentrate Zone:In the concentrate or top zone, the material being enriched is partially separated by a baffle from the spitz or concentrate discharge side of the machine. The cell action at this point is very quiet and the mineral-laden concentrate moves forward and is quickly removed by the paddle shaft (note direct path of mineral). The final result is an unusually high grade concentrate, distinctive of the Sub-A Cell.
A flotation machine must not only float out the mineral value in a mixture of ground ore and water, but also must keep the pulp in circulation continuously from the feed end to the discharge end for the removal of the froth, and must give the maximum treatment positively to each particle.
It is an established fact that the mechanical method of circulating material is the most positive and economical, particularly where the impeller is below the pulp. A flotation machine must not only be able to circulate coarse material (encountered in every mill circuit), but also must recirculate and retreat the difficult middling products.
In the Denver Sub-A due to the distinctive gravity flow method of circulation, the rotating impeller thoroughly agitates and aerates the pulp and at the same time circulates this pulp upward in a straight line, removing the mineral froth and sending the remaining portion to the next cell in series. No short circuiting through the machine can thus occur, and this is most important, for the more treatments a particle gets, the greater the chances of its recovery. The gravity flow principle of circulation of Denver Sub-A Flotation Cell is clearly shown in the illustration below.
There are three distinctive advantages of theSub-A Fahrenwald Flotation Machines are found in no other machines. All of these advantages are needed to obtain successful flotation results, and these are:
Coarse Material Handled:Positive circulation from cell to cell is assured by the distinctive gravity flow principle of the Denver Sub-A. No short circuiting can occur. Even though the ore is ground fine to free the minerals, coarse materials occasionally gets into the circuit, and if the flotation machine does not have a positive gravity flow, choke-ups will occur.
In instances where successful metallurgy demands the handling of a dense pulp containing an unusually large amount of coarse material, a sand relief opening aids in the operation by removing from the lower part of the cell the coarser functions, directing these into the feed pipe and through the impeller of the flowing cell. The finer fraction pass over the weir overflow and thus receive a greater treatment time. In this manner short-circuiting is eliminated as the material which is bled through the sand relief opening again receives the positive action of the impeller and is subjected to the intense aeration and optimum flotation condition of each successive cell, floating out both fine and coarse mineral.
No Choke-Ups or Lost Time:A Sub-A flotation cell will not choke-up, even when material as coarse as is circulated, due to the feed and pulp always being on top of the impeller. After the shutdown it is not necessary to drain the machine. The stationary hood and the air standpipe during a shutdown protects the impeller from sanding-up and this keeps the feed and air pipes always open. Denver Sub-A flotation operators value its 24-hour per day service and its freedom from shutdowns.
This gravity flow principle of circulation has made possible the widespread phenomenal success of a flotation cell between the ball mill and classifier. The recovery of the mineral as coarse and as soon as possible in a high grade concentrate is now highly proclaimed and considered essential by all flotation operators.
Middlings Returned Without Pumps:Middling products can be returned by gravity from any cell to any other cell. This flexibility is possible without the aid of pumps or elevators. The pulp flows through a return feed pipe into any cell and falls directly on top of the impeller, assuring positive treatment and aeration of the middling product without impairing the action of the cell. The initial feed can also enter into the front or back of any cell through the return feed pipe.
Results : It is a positive fact that the application of these three exclusive Denver Sub-A advantages has increased profits from milling plants for many years by increasing recoveries, reducing reagent costs, making a higher grade concentrate, lowering tailings, increasing filter capacities, lowering moisture of filtered concentrate and giving the smelter a better product to handle.
Changes in mineralized ore bodies and in types of minerals quickly demonstrate the need of these distinctive and flexible Denver Sub-A advantages. They enable the treatment of either a fine or a coarse feed. The flowsheet can be changed so that any cell can be used as a rougher, cleaner, or recleaner cell, making a simplified flowsheet with the best extraction of mineral values.
The world-wide use of the Denver Sub-A (Fahrenwald) Flotation Machine and the constant repeat orders are the best testimonial of Denver Sub-A acceptance. There are now over 20,000 Denver Sub-A Cells in operation throughout the world.
There is no unit so rugged, nor so well built to meet the demands of the process, as the Denver Sub-A (Fahrenwald) Flotation Machine. The ruggedness of each cell is necessary to give long life and to meet the requirements of the process. Numerous competitive tests all over the world have conclusively proved the real worth of these cells to many mining operators who demand maximum result at the lower cost.
The location of the feed pipe and the stationary hood over the rotating impeller account for the simplicity of the Denver Sub-A cell construction. These parts eliminates swirling around the shaft and top of the impeller, reduce power load, and improve metallurgical results.
TheSub-A Operates in three zones: in bottom zone, impeller thoroughly mixes and aerates the pulp, the central zone separates the mineral laden particles from the worthless gangue, and in top zone the mineral laden concentrate high in grade, is quickly removed by the paddle of a Denver Sub-A Cell.
A Positive Cell Circulation is always present in theSub-A (Fahrenwald) Flotation Machine, the gravity flour method of circulating pulp is distinctive. There is no short circulating through the machine. Every Cell must give maximum treatment, as pulp falls on top of impeller and is aerated in each cell repeatedly. Note gravity flow from cell to cell.
Choke-Ups Are Eliminated in theSub-A Cell, even when material as coarse as is handled, due to the gravity flow principle of circulation. After shutdown it is not necessary to drain the machine, as the stationary hood protects impeller from sanding up. See illustration at left showing cell when shut down.
No Bowlers, noair under pressure is required as sufficient air is drawn down the standpipe. The expense and complication of blowers, air pipes and valves are thus eliminated. The standpipe is a vertical air to the heart of the Cell, the impeller. Blower air can be added if desired.
The Sub-A Flexibility allows it tobe used as a rougher, cleaner or recleaner. Rougher or middling product can be returned to the front or back of any cell by gravity without the use of pumps or elevators. Cells can be easily added when required. This flexibility is most important in operating flotation MILLS.
Pulp Level Is Controlled in each Sub-A Flotation Cell as it has an individual machine with its own pulp level control. Correct flotation requires this positive pulp level control to give best results in these Cells weir blocks are used, but handwheel controls can be furnished at a slight increase in cost. Note the weir control in each cell.
High Grade Concentrate caused by thequick removal of the mineral forth in the form of a concentrate increases the recovery. By having an adjustment paddle for each Sub-A Cell, quick removal of concentrate is assured, Note unit bearing housing for the impeller Shaft and Speed reducer drive which operates the paddle for each cell
Has Fewer Wearing Parts because Sub-A Cells are built for long, hard service, and parts subject to wear are easily replaced at low cost. Molded rubber wearing plates and impellers are light in weight give extra long life, and lower horsepower. These parts are made under exact Specifications and patented by Denver Equipment Co.
TheRugged Construction of theSub-A tank is made of heavy steel, and joints are welded both inside and out. The shaft assemblies are bolted to a heavy steel beam which is securely connected to the tank. Partition plates can be changed in the field for right or left hand machine. Right hand machine is standard.
The Minerals Separation or M.S. Sub-aeration cells, a section of which is shown in Fig. 32, consists essentially of a series of square cells with an impeller rotating on a vertical shaft in the bottom of each. In some machines the impeller is cruciform with the blades inclined at 45, the top being covered with a flat circular plate which is an integral part of the casting, but frequently an enclosed pump impeller is used with curved blades set at an angle of 45 and with a central intake on the underside ; both patterns are rotated so as to throw the pulp upwards. Two baffles are placed diagonally in each cell above the impeller to break up the swirl of the pulp and to confine the agitation to the lower zone. Sometimes the baffles are covered with a grid consisting of two or three layers each composed of narrow wood or iron strips spaced about an inch apart. The sides and bottom of the cells in the lower or agitation zone are protected from wear by liners, which are usually made of hard wood, but which can, if desired, consist of plates of cast-iron or hard rubber. The section directly under the impeller is covered with a circular cast-iron plate with a hole in the middle for the admission of pulp and air. The hole communicates with a horizontal transfer passage under the bottom liner, through which the pulp reaches the cell. Air is introduced into each cell through a pipe passing through the bottom and delivering its supply directly under the impeller. A low-pressure blower is provided with all machines except the smallest, of which the impeller speed is fast enough to draw in sufficient air by suction for normal requirements.
The pulp is fed to the first cell through a feed opening communicating with the transfer passage, along which it passes, until, at the far end, it is drawn up through the hole in the bottom liner by the suction of the impeller and is thrown outwards by its rotation into the lower zone. The square shape of the cell in conjunction with the baffles converts the swirl into a movement of intense agitation, which breaks up the air entering at the same time into a cloud of small bubbles, disseminating them through the pulp. The amount of aeration can be accurately regulated to suit the requirements of each cell by adjustment of the valve on its air pipe.
Contact between the bubbles and the mineral particles probably takes place chiefly in the lower zone. The pumping action of the impeller forces the aerated pulp continuously past the baffles into the upper and quieter part of the cell. Here the bubbles, loaded with mineral, rise more or less undisturbed, dropping out gangue particles mechanically entangled between them and catching on the way up a certain amount of mineral that has previously escaped contact. The recovery of the mineral in this way can be increased at the expense of the elimination of the gangue by increasing the amount of aeration. The froth collects at the top of the cell and is scraped by a revolving paddle over the lipat the side into the concentrate launder. The pulp, containing the gangue and any mineral particles not yet attached to bubbles, circulates to some extent through the zone of agitation, but eventually passes out through a slot situated at the back of the cell above the baffles and flows thence over the discharge weir. The height of the latter is regulated by strips of wood or iron and governs the level of the pulp in the cell. The discharge of each weir falls by gravity into the transfer passage under the next cell and is drawn up as before by the impeller. The pulp passes in this way through the whole machine until it is finally discharged as a tailing, the froth from each cell being drawn off into the appropriate concentrate launder.
No pipes are normally fitted for the transference of froth or other middling product back to the head of the machine or to any intermediate point. Should this be necessary, however, the material can be taken by gravity to the required cell through a pipe, which is bent at its lower end to pass under the bottom liner and project into the transfer passage, thus delivering its product into the stream of pulp that is being drawn up by the impeller
Particulars of the various sizes of M.S. Machines are given in Table 21. It should be noted that the size of a machine is usually defined by the diameter of its impeller ; for instance, the largest one would be described as a 24-inch machine.
The Sub-A Machine, invented by A. W. Fahrenwald and developed in many respects as an improvement in the Minerals Separation Machine, from which it differs considerably in detail, particularly in the method of aerating the pulp, although the principle of its action is essentially the same. Its construction can be seen from Figs. 33 and 34.
In common with the M.S. type of machine, it consists of a series of square cells fitted with rotating impellers. Each cell, however, is of unit construction, a complete machine being built up by mounting the required number of units on a common foundation and connecting up the pipes which transfer the pulp from one cell to the next. The cells are constructed of welded steel. The impeller, which can be rubber-lined,if required, carries six blades set upright on a circular dished disc, and is securely fixed to the lower end of the vertical driving shaft. It is covered with a stationary hood, to which are attached a stand-pipe, a feed pipe, and the middling return pipes. The underside of the hood is fitted with a renewable liner of rubber or cast-iron. The pulp, entering the first cell through the feed pipe and sometimes through the middling pipes, falls on to the impeller, the rotation of which throws it outwards into the bottom zone of agitation. The suction effect due to the rotationof the impeller draws enough air down the standpipe to supply the aeration necessary for normal operation. A portion of the pulp, cascading over the open tops of the impeller blades, entraps and breaks up the entrained air, the resulting spray-like mixture being then thrown out into the lower zone of agitation, where it is disseminated through the pulp as a cloud of fine bubbles. Should this amount of aeration be insufficient, air can be blown in under slight pressure through a hole near the top of the stand-pipe, in which case a rubber bonnet is fastenedto the lower bearing and clamped round the top of the stand-pipe so as to seal the supply from the atmosphere.
The bottom part of the cell is protected from wear by renewable cast-iron or rubber liners. Four vertical baffles, placed diagonally on the top of the hood, break up the swirl of the pulp and intensify theagitation in the lower zone. The pumping action of the impeller combined with the rising current of air bubbles carries the pulp to the quieter upper zone, where the bubbles, already coated with mineral, travel upwards, drop out many of the gangue particles which may have become entangled with them, and finally collect on the surface of the pulp as a mineralizedfroth. One side of the cell is sloped outwards so as to form, in conjunction with a vertical baffle, a spitzkasten-shaped zone of quiet settlement, where any remaining particles of gangue that have been caught and held between the bubbles are shaken out of the froth as it flows to the overflow lip at the front of the cell. The baffle prevents rising bubbles from entering the outer zone, thus enabling the gangue material released from the froth to drop down unhindered into the lower zone. A revolving paddle scrapes the froth past the overflow lip into the concentrate launder.
Should the machine be required to handle more than the normal volume of froth, it is built with a spitzkasten zone on both sides of the cell. For the flotation of ores containing very little mineral the spitzkasten is omitted so as to crowd the froth into the smallest possible space, the front of the cell being made vertical for the purpose.
Circulation of the pulp through the lower zone of agitation is maintained by means of extra holes at the base of the stand-pipe on a level with the middling return pipes. An adjustable weir provides for the discharge of the pulp to the next cell, which it enters through a feed-pipe as before. Below the weir on a level with the hood is a small sand holeand pipe through which coarse material can pass direct to the next cell without having to be forced up over the weir. The same process is repeated in each cell of the series, the froth being scraped over the lip of the machine, while the pulp passes from cell to cell until it is finally discharged as a tailing from the last one. The middling pipes make it an easy matter for froth from any section of the machine to be returned if necessary to any cell without the use of pumps.
Table 22 gives particulars of the sizes and power requirements of Denver Sub-A Machines and Table 23 is an approximate guide to their capacities under different conditions. The number of cells needed
Onemethod of driving the vertical impeller shafts of M.S. Subaeration or Denver Sub-A Machines is by quarter-twist belts from a horizontal lineshaft at the back of the machine, the lineshaft being driven in turn by a belt from a motor on the ground. This method is not very satisfactory according to modern standards, firstly, because the belts are liable to stretch and slip off, and, secondly, because adequate protection againstaccidents due to the belts breaking is difficult to provide without making the belts themselves inaccessible. A more satisfactory drive, with which most M.S. Machines are equipped, consists of a lineshaft over the top of the cells from which each impeller is driven through bevel gears. The lineshaft can be driven by a belt from a motor on the ground, by Tex- ropes from one mounted on the frame work of the machine, or by direct coupling to a slow-speed motor. This overhead gear drive needs careful adjustment and maintenance. Although it may run satisfactorily for years, trouble has been experienced at times, generally in plants where skilled mechanics have not been available. The demand for something more easily adjusted led to the development of a special form of Tex-rope drive which is shown in Fig. 35. Every impeller shaft is fitted at the top with a grooved pulley, which is driven by Tex-ropes from a vertical motor. This method is standard on Denver Sub-A Machines, and M.S. Machines are frequently equipped with it as well, but the former type are not made with the overhead gear drive except to special order.
The great advantage of mechanically agitated machines is that every cell can be regulated separately, and that reagents can be added when necessary at any one of them. Since, as a general rule, the most highly flocculated mineral will become attached to a bubble in preference to a less floatable particle, in normal operation the aeration in the first few cells of a machine should not be excessive ; theoretically there should be no more bubbles in the pulp than are needed to bring up the valuable minerals. By careful control of aeration it should be possible for the bulk of the minerals to be taken off the first few cells at the feed end of the machine in a concentrate rich enough to be easily cleaned, and sometimes of high enough grade to be sent straight to the filtering section as a finished product. The level of the pulp in these cells is usually kept comparatively low in order to provide a layer of froth deep enough to give entangled particles of gangue every chance of dropping out, but it must not be so low that the paddles are prevented from skimming off the whole of the top layer of rich mineral. Towards the end of the machine a scavenging action is necessary to make certain that the least possible amount of valuable mineral escapes in the tailing, for which purpose the gates of the discharge weirs are raised higher than at the feed end, and the amountof aeration may have to be increased. The froth from the scavenging cells is usually returned to the head of the machine, the middling pipes of the Denver Sub-A Machine being specially designed for such a purpose. The regulation of the cleaning cells is much the same as that of the first few cells of the primary or roughing machine, to the head of which the tailing from the last of the cleaning cells is usually returned.
A blower is sometimes required with the M.S. Subaeration Machine. Since each cell is fitted with an air pipe and valve, accurate regulation of aeration is a simple matter. The Denver Sub-A, Kraut, and Fagergren Machines, however, are run without blowers, enough air being drawn into the machines by suction.
In the Geco New-Cell Flotation Cellthe pneumatic principle is utilized in conjunction with an agitating device. The machine, which is illustrated in Fig. 44, consists of a trough or cell made of steel or wood, whichever is more convenient, through the bottom of which projects a series of air pipes fitted with circular mats of perforated rubber. The method of securing the mat to the air pipe can be seen from Fig. 45. Over each mat rotates a moulded rubber disc of slightlylarger diameter at a peripheral speed of 2,500 ft. per minute. It is mounted on a driving spindle as shown in Fig. 46. Each spindle is supported and aligned by ball-bearings contained in a single dust- and dirt-proof casting, and each pair is driven from a vertical motor through Tex-ropes and grooved pulleys, a rigid steel structure supporting the whole series of spindles with their driving mechanism. The machine can be supplied, if required, however, with a quarter-twist drive from a lineshaft over flat pulleys.
The air inlet pipes are connected to a main through a valve by which the amount of air admitted to each mat can be accurately controlled. The air is supplied by a low-pressure blower working at about 2 lb. per square inch. It enters the cell through the perforations in the rubber mat and is split up into a stream of minute bubbles, which are distributed evenly throughout the pulp by the action of the revolving disc. By this means a large volume of finely-dispersed air is introduced withoutexcessive agitation. There is sufficient agitation, however, to produce a proper circulation in the cell, but not enough to cause any tendency to surge or to disturb the froth on the surface of the pulp. All swirling movement is checked by the liner-baffles with which the sides of the cell are lined ; their construction can be seen in Fig. 44. They are constructed of white cast iron and are designed to last the life of the machine, the absence of violent agitation making this possible.The pulp must be properly conditioned before entering the machine. It is admitted through a feed box at one end at a point above the first disc, and passes along the length of the cell to the discharge weir without being made to pass over intermediate weirs between the discs. The height of the weir at the discharge end thus controls the level of the pulp in the machine. The froth that forms on the surface overflows the froth lip in a continuous stream without the aid of scrapers, its depth being controlled at any point by means of adjustable lip strips combined with regulation of the air.The Geco New-Cell is made in four sizesviz., 18-, 24-, 36-, and 48-in. machines, the figure representing the length of the side of the squarecell. Particulars of the three smallest sizes are given in Table 27. Figures are not available for the largest size.
The difficulty in process plants comes from the constantly changing feed characteristics, stringent product quality requirements and the economic need to maximize the recovery of a finite resource. A key part of successful plant control is the operation of the flotation circuit.
Flotation cells have three main control parameters (1) reagent dosing rate (2) froth depth and (3) air addition rate. Many other parameters may vary such as feed rate, particle size distribution and head grade, however these are the output of upstream processes and are not controlled in the flotation circuit itself.
The selection of reagent type and dosing rate is critical to successful processing of a given ore. It offers a coarse control mechanism as it is difficult to determine the impact of changes in either dosing rate or reagent type unless significant change in flotation performance is observed. In a relatively stable operation, the addition rate of reagents does not vary greatly. The operator seeks to ensure that a slight excess of reagent is available for the flotation process. Too much reagent, however, results in wastage and economic loss whilst too little results in either reduced grade or recovery and again economic loss. Thus, in a situation where the ore changes and marginally less reagent could be used, the operator generally should not chase this small reduction as it is difficult and time-consuming to optimize. The exception to this is where the ore change is expected to last for a long time.
Froth depth is fundamentally used to provide concentrate grade control. This occurs in two ways firstly, the depth determines the residence time in the froth phase and thus the time available for froth drainage.
Generally the greater the froth depth, the more drainage of entrained gangue (waste) and the richer the concentrate grade. There is a limit to the froth depth that a given flotation situation will support. If the froth gets too deep it begins to collapse on itself. The depth at which collapse begins is determined by the structure of the froth. Froth structure is driven by factors such as reagent type, reagent dose rate and the quality/level of mineral in the ore. Froth depth also plays a role in the recovery rate of the concentrate from the cell. As the froth gets deeper, the rate of froth removal reduces at a constant air addition rate. It is important to note that froth depth relationships are not linear in nature.
Once a froth depth has been established for a particular flotation duty (i.e rougher, cleaner etc), changes are generally small and infrequent. A flotation circuit where the slurry level is subjected to large or frequent changes is usually going to be in a constant state of flux as the changes in one cell will impact other cells in the circuit. Pump hoppers overflowing and flotation cell pulping are common symptoms of this.
Air addition rate offers the finest control of flotation cells. Small changes in concentrate recovery rate and grade can be achieved via changes in air addition rate. The impacts of changes in air addition rate are observed quickly in the plant providing a good source of operator feedback. Changes to air addition rate may be made several times in a normal shift as operators seek to optimise concentrator performance. As air addition represents a fine control method, changes should be small and one needs to wait several minutes before these results can be seen. Sudden large changes in air addition rate can create issues with level control as the pulp in the flotation cell will experience a rapid expansion and may overflow the cell launders. The ability to make regular changes to air addition rate in a convenient manner has led to automatic air control being the norm in modern concentrators. Changes in the concentrate grade that result from changes in air addition rate can be observed rapidly by utilising an on stream analysis system.
Leading-edge minerals processing plants incorporate automatic process control through some form of PID-driven system. In the case of flotation plants, the ideal system uses the three parameters discussed above to control a single parameter such as froth speed. Instruments such as FrothMaster use vision technologies to measure the speed of the froth over the lip. The desired froth speed can then be determined by monitoring the concentrate grade via an on stream analysis system. This type of control system automates the minute-to-minute running of the flotation circuit, which is driven by the desired concentrate grade. In plant trials, this approach has seen a significant improvement in recovery when compared to a manually monitored plant.
In figure 1, above, the variation between automatic control (Line 1) and manual control (Line 2) can easily be seen. In Line 2, the air addition rate is manually set so has to regularly monitor concentrate grade and recovery rate. This is time-consuming and is also not necessarily the best means of achieving the targeted set point. In Line 1, where the circuit was completely automatic, the control system constantly monitors and responds accordingly to any variations from the set point goal. In this particular trial, fully automated control brought real dollar benefits increasing overall recoveries, substantially reducing deviations from targets and reducing use of reagents in the cell.
Being able to successfully manage the different control parameters in a flotation circuit is a critical exercise in minerals recovery. Whilst the air addition rate offers the finest means of control, other parameters such as reagent type, reagent dosage rate and froth depth are also important controls for an operator to understand. It is also vital for an operator to understand the cause and effect relationships of these controls. Automated technologies such as on-stream analysers, along with newer developments such as FrothMaster or froth imaging systems, take this control to the next level. Not only do these automated systems monitor and analyse froth characteristics highly efficiently, but they can also optimise recovery, reduce reagent use and free up operator-time.
Flotation columns work on the same basic principle as mechanical flotation cells mineral separation takes place in an agitated and/or aerated water mineral slurry where the surfaces of selected minerals are made hydrophobic (water-repellent) by conditioning with selective flotation reagents. However, in column flotation, there is no mechanical mechanism causing agitation.As well, separation takes place in a vessel (known as a column) that is much taller than the width (or cross-section) of the cell. Air is introduced into the slurry in the column through spargers, which create a countercurrent flow of air bubbles.
The contact cell is mainly used in rougher flotation circuits. It is compact, highly efficient, simple to operate, and has low energy and maintenance costs. There are a number of other benefits and capital savings that can be obtained when installing contact cells including:
SGS is the most experienced and trusted organization in the mineral processing industry. We have conducted flowsheet design and pilot plant testing on thousands of flotation projects. Let our experts improve your flotation performance with our proven column and contact cell technologies.
From its beginnings in the first decade of this century, flotation has gradually moved to a predominant role in mineral separation. Alain Kabemba, flotation process specialist at Delkor notes the major trend to treating lower- grade and more finely disseminated ores and lately the re-treatment of tailings. He also points to the broad applicability of size to below 10 m.
Real systems do not fulfill ideal conditions, mainly because of feed variation or disturbances. "Before considering disturbances to flotation specifically," Kabemba says. "It is important to emphasise the interlock between grinding and flotation, not only with respect to particle size effects, but equally to flotation feed rate variations. The grinding circuit is usually designed to produce the optimum size distribution established in testing and given in the design criteria. When the product size alters from this optimum, control requires either changing feed tonnage to the circuit or changing product volume, with either causing changes in flotation feed rates."While grindability changes due to the variation in ore properties are disturbances to the grinding circuit, they generate feed rate changes as disturbances to the flotation circuit. The variations in ore properties which affect flotation from those assumed in the design criteria must therefore necessarily include grindability changes.
"This reflects important differences in flotation machine characteristics between the two processes. Grinding circuits are built and designed with fixed total mill volumes and energy input, so the grinding intensity is not a controllable variable, instead grinding retention time is changed by variation of feed rates. In contrast, the flotation circuit is provided both with adjustable froth and pulp volume for variation of flotation intensity by aeration rate or hydrodynamic adjustment. Reagent levels and dosages provide a further means for intensity control."
One recent trend has been towards larger, metallurgical efficient and more cost effective machines. These depart from the simpler tank/mechanism combination towards design which segregates and directs flow and towards providing an external air supply for types which had been self aerating and towards the application of hydrodynamic principles to cell design, like the Delkor BQR range of flotation machines, initially the Bateman BQR Float Cells.
Bateman has steadily developed the BQR flotation cells which have been in application for the past 30 years, and with its acquisition of Delkor in 2008, decided to rebrand the equipment into the Delkor equipment range. Kabemba explains that BQR cell capacities range from 0.5 to 150 m3 currently installed and can be used in any application as roughers, scavengers and in cleaning and re- cleaning circuits.
Provide good contact between solid particles and air bubblesMaintain a stable froth/pulp interfaceAdequately suspend the solid particles in the slurryProvide sufficient froth removal capacityProvide adequate retention time to allow the desired recovery of valuable constituent. This led to the following benefitsHighest possible effective volume and reduced the froth travel distance
Improved metallurgical performances in terms of grade recovery and reduced capital and operating costs based on reduced fabrication material and ease of maintenance. Kabemba says: "there are not many differences in terms of design between BQR Flotation cells; however, from the BQR1000 upwards, the flotation cells have internal launders to maintain the design objectives and benefits highlighted.
"Operating variables, such as impeller speed, air rate, pulp and froth depths have to be adjustable over a sufficient range to provide optimum results with a given ore, grind and chemical treatment, but adjustment should not extend beyond the hydrodynamic regime in which good flotation is possible."
In the near future the BQR2000 (200 m3) and BQR3000 (300 m3) will be available to the market.Kabemba also explained: "Circular cells reduce the amount of dead volume when compared to square cells. This enables a much higher effective pulp volume, hence increasing the effective energy input into the flotation cell. In addition tank type cells offer enhanced froth removal due to the uniform shape of the circular launders."