mineral detection in ball mill

mill

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It is the key equipment for mineral materials secondary grinding after crushing, widely applied in the industries of cement, silicate products, new building materials, refractory materials, fertilizer, etc. Also suitable for nonferrous metals, ferrous metals, and nonmetal separation.

grinding circuit control strategy

The reduction of the mill feed size could be reduced by modification of the tertiary crusher system by increasing the power draught and proper screen sizing. Detailed scale up experiments indicated that the ball charge size should be reduced due to the small contribution to the grinding efficiency.

The translation of raw measurements into adequate indices (process indicators) permits the operator to concentrate into a few valuable variables after prechecking by the computer system. This concept contributed to the definition and development of the ball mill load index, the grind index and the transport index calculation obtained from raw measurements filtering and mass balance calculations. The circulating load flowmeter and density gauge were not considered vital measurements for implementation of the control strategies since these values can be obtained from calculations from other measurements and where not used by the controls.

A simplified overall function block schematic for the new grinding controls is shown in Figure 5. At the left side the ball mill load constraint and transport indices are shown. At the right side of the figure the infered grind index and the sump level controllers manipulating the mill discharge water are depicted. Notice that there are seven controllers with ten indices and only three manipulated variables.

The ball mill load control system: The main control scheme is the ball mill load controller. Any additional capacity of the ball mill depending on the grind settings is sensed by the ball mill load controller and the feed rate is increased (if desired to bring the ball mill its capacity). It was implemented as a cascade controller to set the new feed rate based on a constraint control with a long term horizon control provided by a 20 minutes mill load calculated variable and limits on pumps amperage and sump level rate of change. The operator enters the constraint and alarm settings from a special panel. The rate of change is provided on a seven minute basis to identify large disturbances or a change in mill grinding characteristics. The pump amperage is also closely monitored as the second constraint and the sump level rate of change as the third constraint.

The success of the implementation of this control strategy resides in the adaptive nature of it. The system is at all times monitoring the state of the ball mill transport and loading thus advising to the operator where to set an alarm to start reducing feed. The gain in mill throughput by working closer to active constraint is depicted in Figure 6. This strategy works very well with difficult to grind ores and very well with softer ores. Throughputs of up 400 TPH can be achieved as depicted in Figure 7 when running regular ores from a regular tonnage of 250 TPH, while maintaining the metallurgical performance indexes in range.

This scheme uses the reasoning followed by the operators in interpreting the power draught chart with the earlier electronic controllers. A technique was developed to infer the ball mill loading and to set a constraint procedure using the new system. The developed algorithm uses the computer memory to store the history of several variables and to calculate process variables rate of change and moving averages. Adequate signal processing techniques were used to interpret the raw power draught signal from the ball mill motor. A first order filtered raw measurement was processed by a least square filter to obtain the rate of change and a reasonable base value for a typical process time constant period.

The new feed rate controller is the secondary controller. It takes the cascaded signal from the ball mill load controller or the operator feed rate set point when in automatic. This controller maintains the feed rate selected by the ball mill controller by raising or lowering the speed of a variable speed feeder.

Various techniques are available to measure on stream particle size. Usually these expensive instruments require considerable maintenance and periodic tedious calibrations. For the type of circuit in study it was not economically justified the installation of a particle size analyzer for all the grinding lines. The inferential size prediction was chosen due to the simplicity in implementation with current state of the art control systems. The calculation used resulted from the evolution of a rigorous inferred particle size-estimator developed for the dynamic simulation of grinding circuits.

The grind index is calculated performing a mass balance around the ball mill circuit to estimate the cyclone overflow density (viscosity effect) and a correction factor due to specific power consumption (size effect).

The grind cut index is maintained by controlling the addition of water to the cyclone feed. This water control method sets the cyclone feed viscosity at the proper value to give a split at the desired cyclone overflow particle size distribution. Because viscosity is principally a function of density and surface area, controlling to a fixed size by adjusting of hydrocyclone water continuously compensates for viscosity changes in the cyclone feed that occur because of particle size changes in the mill discharge slurry.

The grind index controller is implemented as a regular PI controller cascading a water setpoint to sump water addition controller when the pump condition is normal. A switch was implemented for the ball mills without a variable speed pump to maintain the sump level within reasonable limits to control the sump level by manipulating the sump water. In this case the grind index controller is left in manual. The implementation of this loop uses the water flow instead of the valve controller direct to prevent the adverse effect on the grind cut index and to be able to maintain a reasonable amount of water under critical conditions.

A simple ratio controller was implemented to regulate the solids content in the mill by manipulating the feed water. A calculated percent solid index is used to monitor the transport in the ball mill. A separate study was initiated to develop a measurement of this property (Bascur, 1985). The large volume (capacitance) of circulating load versus feed water involved load precludes a good control. The historical inventory tracking of the ball mill power draught decouples this water addition by adding in proportion to the feed rate set by the ball mill load controller.

When a variable speed pump is available in the grinding line the sump level is controlled by setting the pump speed. If the variable speed pump is not available a dead band controller was implemented with an automatic switch coordinated with the grind cut index controller. When the pump is new the sump level will be lower so water is added to keep the pump from cavitating and the grind index is kept in manual. At other times, the water is adjusted according to the grind index except when a high level sump is indicated due to an old pump. The logic involved in implementing such switch strategy is relatively simple using a current state of the art distributed control system.

This controller accepts the output from either the sump level controller or the inferred grind index as a water flow set point (cascaded). The controller then maintains the automatic valve in the required position. An hysteresis compensation lookup table was implemented to avoid over controlling due to valve wear. All actuators were implemented with output clamping and hysteresis compensation.

grinding mill control - grinding & classification circuits - metallurgist & mineral processing engineer

The required acoustic level is entered by the control room operator as a set point into the acoustic controller. The acoustics of the mill charge motion is measured by a device mounted adjacent to the SAG Mill shell. The resulting signal is transmitted to the acoustic indicating controller which, in turn, sends a remote set point to the SAG mill speed controller. The speed of the SAG mill is then automatically adjusted (within set limits) to maintain the required acoustic level.

How did you measure effectiveness? Did you bog the mill after a liner change out because somebody didn't reset the microphone correctly? I've heard of that happening a number of times with load cells. Are you comparing to some planned capacity rate? Is the controller cycling the tonnes and letting the mill internals get smashed up?

Mills are integrators and to control them you need to measure the inventory. But, there are a lot of details to the control scheme other than the sensor that is used to measure the inventory. Could you share anymore information?

The problem we had was that the mill never seemed to fill up enough to carry out efficient grinding. As our target was a certain percentage micron passing P80 we could never meet the target. Once going back to a more traditional way (mill weight and speed) we were able to meet targets.

It seems to be a Hardinge Electric Ear revisited. This device was patented by Hardinge in the thirties to be used in a ball mill with fixed speed and an almost constant volumetric filling level, given the appropriate ball make up. The load remains approximately at the same place all the time, such that with only one sensor you cope all conditions. The only one varying condition is the amount of slurry in the interstices between balls. The rms sound level is inversely proportional to the amount of damping exerted by the slurry. High level, low noise. Low level, high noise.

In a SAG mill, most of these conditions are not valid. Speed changes, such that you may get confused by the noise coming from grinding media impacting mill shell superimposed to the standard load noise. As the global volumetric filling level is also ever changing, your only sensor may easily be fooled by the change of toe position. The actual change of mill filling is now superimposed to the change of relative position of your sensor with respect to load toe. As these factors add up, the feedback to your controller may turn crazy!

I have seen many sound control inputs over the years and the science seems valid in most cases when the ore body is consistent. Load bearing pads and power draw still usually is the primary data for adjustment, with sound evaluation being another check.

As a standard check we do listen to the mill first hand at the feed and also the discharge end to get some idea of impact levels. Also if the media is targeting the toe of the charge or there is too much ball on liner/lifter direct hits. After experiencing this "listening" over time you can determine these different sounds such as ball on ball, ball on ore (toe or center of charge), and ball on lifter/liner impacts.

Over the years our philosophy has changed, with the one exception of Africa where mills were commonly running at 90% of critical speed to be gentle with incompetent brittle media. The change is to not discuss mill changes in operation, but to design a purpose built media for the equipment and meet the existing operational work practices.

Just a quick story - Whilst on a trip to South Africa after a mill tour I returned to the Mill Managers office for a short debrief. His office was directly over the SAG mill, it had to be one of the noisiest offices I have ever been in with obvious vibration emanating from the SAG. I told him he should move - he said NO! "If anything changes in that mill I feel it immediately through my "seat".

Yes, back in my early days in milling you could easily tell if the mill was bogging even if you were working up at the crusher because of the muffled sound emanating from the mill. Back then the Mill was totally steel lined and sounded like a freight train. I place a lot of emphasis on sound when operating.

The sound signal can often be processed to get better control. I've used a 1/3 octave simple band pass filter at 2kHz but I know that others have used more complex approaches. I inherited a good system at Parkes and couldn't find a reason to change it.

In the example you've given how was the mill feed-rate controlled? Overall, it seems that the control system ended up controlling the speed, density and feed-rate to the wrong set-points. I'd call that an optimisation problem (but I might just be being pedantic).

This is all a commissioning exercise and one of the things we wanted to do was - incorporate an additional loop whereby mill speed is automatically adjusted to control mill weight/load. This acoustic control was used when running the Mill in cascade, when in Auto the required speed of the SAG mill was entered by the control room operator as a set-point to the speed controller. And you are correct it was an optimisation issue which we were trying to address.

Good quality well maintained electronic ears work quite well in indicating load LEVEL in the mill (as does load measurement but this is not a true indication of load level because it is prone to ore SG changes). If you are controlling mill speed based on sound you will never achieve steady state because all this does is allow the mill to fill and empty repeatedly. Best result is achieved by aiming for highest possible speed and controlling feed rate to allowable sound levels, in other words if the sound goes up instead of slowing the mill down you increase the feed-rate, for this to work effectively you must remove load as an operating constraint and use it only as an alarm situation. The other advantage of aiming to operate at the highest permissible speed levels is maximum throughput.

In one mill we used sound to adjust mill speed, without much success. As discussed above key issues were generally the ability to effectively differentiate sound when media contacts media or grinding media hits toe of charge (at high ball / rock charge), or media contacts liner or large ore particle contacts charge. There were some good correlations, but not reliable enough to use mill control. In one 36 ft SAG we used load cells for weight control (to adjust speed), with feed rate control and power control were used in conduction with the speed control. Worked excellent. There was an AMIRA project to develop some instrumentation to analyse sound generated by the operation of mills and use it for mill control. I am not sure how far they went.

I have seen several locations try to run a control strategy with acoustics and many were unsuccessful. They can work, but they have to be set up right and maintained properly. The way they are used in a control strategy is also very important. How are you using the information in your strategy? Is it used to control feed rate, speed, both?

Remember tight control is the key to optimizing throughput. Consider recovery as well, it is likely much easier for the controller on the recovery side to react to small steps in tonnage then large swings. You want to keep your system running as tight as possible.

From all I have read in these comments, the problem with achieving the proper load in the SAG mill is not the 'sound' controller but the 'retention' of the ore in the mill. I suggest that you look at the discharge size of the Pebble Ports. I had a client with a similar problem and easily solved it by this means, successfully achieving the design throughput.

I agree with you totally. Plus there are other additional benefits that must be mentioned. Port discharge should have magnetic separation of the rejected media after screening but before cyclone - this saves cyclone wear/damage and with the newer through hard media designs competent/spherical rejected media can be reloaded into the secondary milling process.

Magnet can be before the screen. This done in SAG applications but I can't see why it wouldn't work. I suppose that the chute that takes the susceptible material might need replacing each shutdown if its getting hit by 70mm media that passed through the pebble ports. But when you have to reline the whole mill anyway what is the problem with replacing a small chute as well?

An interesting concept to use a magnet. Any ideas whether it would be a permanent or electro magnet? Perhaps an electro-magnet would be better since the field could be turned off to discharge the ball load?

There are a few magnets that I have seen, but the best is one that is over the ported ore stream after screening and before any cyclone. These magnets are electro powered with a belt that takes tramp steel off the belt and "flicks" it to a collector bin.

As I explained before, mill sound in a SAG mill may depend either on mill speed or mill filling. We did develop an instrument called "impact meter", which has been marketed by FLSmidth. The device is intended to recognize the specific sound pattern of balls hitting mill liners, in order to avoid such operating condition. We did use three microphones to overcome the effect of changing load level, having always one of them near the toe. We did not develop a specific control product, just made recommendations based on the mentioned findings. You may use the RMS noise level as an estimator of mineral load inside the mill, given that the speed is below the threshold for direct liner hitting and that you have multiple microphones to adapt to variable toe position.

This does make a lot of sense to have tuned sound collection with the important pinpointed sounds of "interest" being flagged. My hearing is really taking a hit from doing this first hand over the years!

Sound when used correctly is a good process variable to help protect your mill liners/lifters, minimize steel depletion, and increase throughput. I'll email you some detailed strategy info for and some info about our Mill Scanner listening device.

Mill Ears - that brings back memories. Love them. But my favourite was the initial work in South Africa with "Instrumented Liner Bolts" (Kloof Gold Mine SA, Mintek) measuring impact and conductivity (indication of viscosity).

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ball mill for sale | grinding machine - jxsc mining

Ball mill is the key equipment for grinding materials. those grinding mills are widely used in the mining process, and it has a wide range of usage in grinding mineral or material into fine powder, such as gold, ironzinc ore, copper, etc.

JXSC Mining produce reliable effective ball mill for long life and minimum maintenance, incorporate many of the qualities which have made us being professional in the mineral processing industry since 1985. Various types of ball mill designs are available to suit different applications. These could include but not be restricted to coal mining grate discharge, dry type grinding, wet mineral grinding, high-temperature milling operations, stone & pebble milling.

A ball mill grinds ores to an end product size of thirty-five mesh or finer. The feeding material to a ball mill is treated by: Single or multistage crushing and screening Crushing, screening, and/or rod milling Primary crushing and autogenous/semi-autogenous grinding.

Normal feed sizes: eighty percent of six millimeters or finer for hard rocker eighty percent of twenty-five millimeters or finer for fragile rocks (Larger feed sizes can be tolerated depending on the requirements).

The ratio of machine length to the cylinder diameter of cylindrical type ball mills range from one to three through three to one. When the length to diameter ratio is two to one or even bigger, we should better choose the mill of a Tube Mill.

Grinding circuit design Grinding circuit design is available, we experienced engineers expect the chance to help you with ore material grinding mill plant of grinding circuit design, installation, operation, and optimization. The automatic operation has the advantage of saving energy consumption, grinding media, and reducing body liner wear while increasing grinding capacity. In addition, by using a software system to control the ore grinding process meet the requirements of different ore milling task.

The ball mill is a typical material grinder machine which widely used in the mineral processing plant, ball mill performs well in different material conditions either wet type grinding or dry type, and to grind the ores to a fine size.

Main ball mill components: cylinder, motor drive, grinding medium, shaft. The cylinder cavity is partial filling with the material to be ground and the metal grinding balls. When the large cylinder rotating and creating centrifugal force, the inner metal grinding mediums will be lifted to the predetermined height and then fall, the rock material will be ground under the gravity force and squeeze force of moving mediums. Feed material to be ground enters the cylinder through a hopper feeder on one end and after being crushed by the grinding medium is discharged at the other end.

Mining Equipment Manufacturers, Our Main Products: Gold Trommel, Gold Wash Plant, Dense Media Separation System, CIP, CIL, Ball Mill, Trommel Scrubber, Shaker Table, Jig Concentrator, Spiral Separator, Slurry Pump, Trommel Screen.

ball mills | industry grinder for mineral processing - jxsc machine

Max Feeding size <25mm Discharge size0.075-0.4mm Typesoverflow ball mills, grate discharge ball mills Service 24hrs quotation, custom made parts, processing flow design & optimization, one year warranty, on-site installation.

Ball mill, also known as ball grinding machine, a well-known ore grinding machine, widely used in the mining, construction, aggregate application. JXSC start the ball mill business since 1985, supply globally service includes design, manufacturing, installation, and free operation training. Type according to the discharge type, overflow ball mill, grate discharge ball mill; according to the grinding conditions, wet milling, dry grinding; according to the ball mill media. Wet grinding gold, chrome, tin, coltan, tantalite, silica sand, lead, pebble, and the like mining application. Dry grinding cement, building stone, power, etc. Grinding media ball steel ball, manganese, chrome, ceramic ball, etc. Common steel ball sizes 40mm, 60mm, 80mm, 100mm, 120mm. Ball mill liner Natural rubber plate, manganese steel plate, 50-130mm custom thickness. Features 1. Effective grinding technology for diverse applications 2. Long life and minimum maintenance 3. Automatization 4. Working Continuously 5. Quality guarantee, safe operation, energy-saving. The ball grinding mill machine usually coordinates with other rock crusher machines, like jaw crusher, cone crusher, to reduce the ore particle into fine and superfine size. Ball mills grinding tasks can be done under dry or wet conditions. Get to know more details of rock crushers, ore grinders, contact us!

Ball mill parts feed, discharge, barrel, gear, motor, reducer, bearing, bearing seat, frame, liner plate, steel ball, etc. Contact our overseas office for buying ball mill components, wear parts, and your mine site visits. Ball mill working principle High energy ball milling is a type of powder grinding mill used to grind ores and other materials to 25 mesh or extremely fine powders, mainly used in the mineral processing industry, both in open or closed circuits. Ball milling is a grinding method that reduces the product into a controlled final grind and a uniform size, usually, the manganese, iron, steel balls or ceramic are used in the collision container. The ball milling process prepared by rod mill, sag mill (autogenous / semi autogenous grinding mill), jaw crusher, cone crusher, and other single or multistage crushing and screening. Ball mill manufacturer With more than 35 years of experience in grinding balls mill technology, JXSC design and produce heavy-duty scientific ball mill with long life minimum maintenance among industrial use, laboratory use. Besides, portable ball mills are designed for the mobile mineral processing plant. How much the ball mill, and how much invest a crushing plant? contact us today! Find more ball mill diagram at ball mill PDF ServiceBall mill design, Testing of the material, grinding circuit design, on site installation. The ball grinding mill machine usually coordinates with other rock crusher machines, like jaw crusher, cone crusher, get to know more details of rock crushers, ore grinders, contact us! sag mill vs ball mill, rod mill vs ball mill

How many types of ball mill 1. Based on the axial orientation a. Horizontal ball mill. It is the most common type supplied from ball mill manufacturers in China. Although the capacity, specification, and structure may vary from every supplier, they are basically shaped like a cylinder with a drum inside its chamber. As the name implies, it comes in a longer and thinner shape form that vertical ball mills. Most horizontal ball mills have timers that shut down automatically when the material is fully processed. b. Vertical ball mills are not very commonly used in industries owing to its capacity limitation and specific structure. Vertical roller mill comes in the form of an erect cylinder rather than a horizontal type like a detachable drum, that is the vertical grinding mill only produced base on custom requirements by vertical ball mill manufacturers. 2. Base on the loading capacity Ball mill manufacturers in China design different ball mill sizes to meet the customers from various sectors of the public administration, such as colleges and universities, metallurgical institutes, and mines. a. Industrial ball mills. They are applied in the manufacturing factories, where they need them to grind a huge amount of material into specific particles, and alway interlink with other equipment like feeder, vibrating screen. Such as ball mill for mining, ceramic industry, cement grinding. b. Planetary Ball Mills, small ball mill. They are intended for usage in the testing laboratory, usually come in the form of vertical structure, has a small chamber and small loading capacity. Ball mill for sale In all the ore mining beneficiation and concentrating processes, including gravity separation, chemical, froth flotation, the working principle is to prepare fine size ores by crushing and grinding often with rock crushers, rod mill, and ball mils for the subsequent treatment. Over a period of many years development, the fine grinding fineness have been reduced many times, and the ball mill machine has become the widest used grinding machine in various applications due to solid structure, and low operation cost. The ball miller machine is a tumbling mill that uses steel milling balls as the grinding media, applied in either primary grinding or secondary grinding applications. The feed can be dry or wet, as for dry materials process, the shell dustproof to minimize the dust pollution. Gear drive mill barrel tumbles iron or steel balls with the ore at a speed. Usually, the balls filling rate about 40%, the mill balls size are initially 3080 cm diameter but gradually wore away as the ore was ground. In general, ball mill grinder can be fed either wet or dry, the ball mill machine is classed by electric power rather than diameter and capacity. JXSC ball mill manufacturer has industrial ball mill and small ball mill for sale, power range 18.5-800KW. During the production process, the ball grinding machine may be called cement mill, limestone ball mill, sand mill, coal mill, pebble mill, rotary ball mill, wet grinding mill, etc. JXSC ball mills are designed for high capacity long service, good quality match Metso ball mill. Grinding media Grinding balls for mining usually adopt wet grinding ball mills, mostly manganese, steel, lead balls. Ceramic balls for ball mill often seen in the laboratory. Types of ball mill: wet grinding ball mill, dry grinding ball mill, horizontal ball mill, vibration mill, large ball mill, coal mill, stone mill grinder, tumbling ball mill, etc. The ball mill barrel is filled with powder and milling media, the powder can reduce the balls falling impact, but if the power too much that may cause balls to stick to the container side. Along with the rotational force, the crushing action mill the power, so, it is essential to ensure that there is enough space for media to tumble effectively. How does ball mill work The material fed into the drum through the hopper, motor drive cylinder rotates, causing grinding balls rises and falls follow the drum rotation direction, the grinding media be lifted to a certain height and then fall back into the cylinder and onto the material to be ground. The rotation speed is a key point related to the ball mill efficiency, rotation speed too great or too small, neither bring good grinding result. Based on experience, the rotat

ion is usually set between 4-20/minute, if the speed too great, may create centrifuge force thus the grinding balls stay with the mill perimeter and dont fall. In summary, it depends on the mill diameter, the larger the diameter, the slower the rotation (the suitable rotation speed adjusted before delivery). What is critical speed of ball mill? The critical speed of the ball mill is the speed at which the centrifugal force is equal to the gravity on the inner surface of the mill so that no ball falls from its position onto the mill shell. Ball mill machines usually operates at 65-75% of critical speed. What is the ball mill price? There are many factors affects the ball mill cost, for quicker quotations, kindly let me know the following basic information. (1) Application, what is the grinding material? (2) required capacity, feeding and discharge size (3) dry or wet grinding (4) single machine or complete processing plant, etc.

modelling mineral size reduction in the closed-circuit ball mill at the pine point mines concentrator - sciencedirect

Using plug flow material transport and a cumulative-basis rate-of-breakage parameter, overall size reduction through the closed-circuit ball mills at the Pine Point and Gibraltar concentrators was simulated over a wide range of operating conditions. The rate-of-brakage parameter was related to particle size by a power law, the exponent (n) being: Pine Point, n = 1.043 0.026, and Gibraltar, n = 0.747 0.020. The success of this approach probably stems from the high (> 1.5) circulating load ratios encountered.

By analogy individual mineral size reduction at Pine Point was examined. A similar rate-of-breakage parameter versus size relationship was found. Pyrite was the hardest mineral, but fine galena was equally resistant. However, the approximation that mineral and overall rates of breakage were the same gave an adequate fit to the mineral size reduction. This was emphasized by combining with a cyclone model to simulate cyclone overflow mineral size distribution. A more accurate cyclone model is shown to be more important in simulating mineral deportment at Pine Point.

Complementary laboratory batch grinding tests were conducted on rod mill discharge and ball mill feed samples. Sufficient agreement with the first-order hypothesis was observed to analyse the rate-of-breakage parameter. The kinetics was similar for both samples and in turn similar to the plant-derived kinetics in terms of relative mineral rates-of-breakage and the relationship of the rate-of-breakage parameter with particle size.

small ball mill | mini ball mill for small scale mineral grinding

Applications: It can be used in production industries such as cement, refractory materials, fertilizers, ferrous and non-ferrous metal beneficiation and glass ceramics, as well as schools, scientific research units and laboratories.

The small ball mill is a small-capacity grinding equipment, which is defined relative tolarge ball mill. It is generally suitable for small-scale production in the trial production stage. Due to its small size and easy movement, small ball mill is sometimes referred to as mobile ball mill. The mobile ball mill can be easily moved to the location of the material for on-site grinding and threshing, which greatly reduces the transportation cost of the material.

Small ball mill is another new type of energy-saving ball mill equipment developed by industry experts in combination with new ball mill technology. Although the output of small ball mill is limited, there are many kinds of materials that can be processed, such assmall ball mill for ceramics, copper ore, potassium feldspar, etc.

The small ball mill for sale in our company is composed of a horizontal barrel, a hollow shaft for feeding and discharging materials, and a grinding head. The barrel is a long cylinder with grinding medium in the barrel. The grinding medium is generally steel balls or steel segments, and are loaded into the barrel according to different diameters and a certain proportion. The barrel is made of steel plate, with steel lining plate fixed on it.

When the small ball mill is running, the materials enter the first chamber of the small ball mill evenly through the hollow shaft at the feeding end of the small ball mill. There areball mill ladder linersor corrugated liners in the chamber, which are equipped with steel balls of different specifications.

The centrifugal force generated by the rotation of the cylinder will bring the steel balls to a certain height and then they will fall down, which will have a heavy impact and grinding effect on the material. After the material reaches rough grinding in the first chamber, it enters the second chamber through the single-layer partition board. The second chamber is lined with flat liner and contains steel balls for further grinding of materials. The fine powder material is discharged from the small ball mill through the discharging grate to complete the grinding process.

To meet the needs of the market, we have smaller mini ball mill for sale. This mini ball mill can also be called laboratory ball mill, which is a special experimental equipment for scientific research and teaching.

The mini ball mill used in the experiment has the characteristics of beautiful appearance, compact structure, simple operation, uniform grinding particle size and high working efficiency. The mini ball mill has been widely used in the teaching of mineral processing engineering, mining engineering, metallurgical engineering and environmental engineering, and also in scientific research institutions.

As a ball mills supplier with 22 years of experience in the grinding industry, we can provide customers with types of ball mill, vertical mill, rod mill and AG/SAG mill for grinding in a variety of industries and materials.

monitoring the fill level of a ball mill using vibration sensing and artificial neural network | springerlink

Ball mills are extensively used in the size reduction process of different ores and minerals. The fill level inside a ball mill is a crucial parameter which needs to be monitored regularly for optimal operation of the ball mill. In this paper, a vibration monitoring-based method is proposed and tested for estimating the fill level inside a laboratory-scale ball mill. A vibration signal is captured from the base of a laboratory-scale ball mill by using a5g accelerometer. Features are extracted from the vibration signal by using different transforms such as fast Fourier transform, discrete wavelet transform, wavelet packet decomposition, and empirical mode decomposition. These features are given as input to an artificial neural network which is used to predict the percentage fill level inside the ball mill. In this paper, the predicted fill level obtained by using different features are compared. It is found that the predicted fill level due to features obtained after fast Fourier transform outperforms other transforms.

Gonzalez GD, Miranda D, Casali A, Vallebuona G (2008) Detection and identification of ore grindability in a semiautogenous grinding circuit model using wavelet transform variances of measured variables. Int J Miner Process 89:5359

Huang C, Zhang H, Huang L (2019) Almost periodicity analysis for a delayed Nicholsons blowflies model with nonlinear density-dependent mortality term. Commun Pure Appl Anal 18(6):33373349. https://doi.org/10.3934/cpaa.2019150

Huang C, Zhang H (2019) Periodicity of non-autonomous inertial neural networks involving proportional delays and non-reduced order method. Int J Biomath 12(2):1950016. https://doi.org/10.1142/S 00165

Labate D, Foresta FL, Occhiuto G (2013) Empirical mode decomposition Vs. Wavelet decomposition for the extraction of respiratory signal from single-channel ECG: a comparison. IEEE Sensors J 13(7):26662674

Huang NE, Shen Z, Long SR, Wu MC, Shih HH, Zheng Q, Yen NC, Tung CC, Liu HH (1998) The empirical mode decomposition, and the Hilbert spectrum for nonlinear and nonstationary time series analysis. Proc R Soc Lond A 454:903995

Nayak, D.K., Das, D.P., Behera, S.K. et al. Monitoring the fill level of a ball mill using vibration sensing and artificial neural network. Neural Comput & Applic 32, 15011511 (2020). https://doi.org/10.1007/s00521-019-04555-5

measurement system of the mill charge in grinding ball mill circuits - sciencedirect

Investigation of a dry fine grinding circuit has shown significant influence of the mill load (powder filling) on the production capacity. To improve the circuit performance at industrial scale, alternative ways of mill load measurement were investigated. Detection of strain changes in the mill shell during mill rotation, by using a piezoelectric strain transducer, provided very interesting results, allowing evaluation of the weight of the mill charge and control of the powder filling to obtain an optimal level. Power draw has thus been increased by about 5% compared to the old configuration where mill motor power input was used to control the mill charge. By measuring mechanical vibration with the transducer, additional useful information has been obtained about the behavior of the cataracting and cascading balls inside the mill shell. Finally an important factor was simplicity and low investment cost of the total installation, as many fine grinding mills operate in relatively small circuits that do not warrant large investment for alternative measurement methods.

energy use of fine grinding in mineral processing | springerlink

Fine grinding, to P80 sizes as low as 7m, is becoming increasingly important as mines treat ores with smaller liberation sizes. This grinding is typically done using stirred mills such as the Isamill or Stirred Media Detritor. While fine grinding consumes less energy than primary grinding, it can still account for a substantial part of a mills energy budget. Overall energy use and media use are strongly related to stress intensity, as well as to media size and quality. Optimization of grinding media size and quality, as well as of other operational factors, can reduce energy use by a factor of two or more. The stirred mills used to perform fine grinding have additional process benefits, such as polishing the mineral surface, which can enhance recovery.

Fine grinding is becoming an increasingly common unit operation in mineral processing. While fine grinding can liberate ores that would otherwise be considered untreatable, it can entail high costs in terms of energy consumption and media use. These costs can be minimized by performing adequate test work and selecting appropriate operating conditions. This paper reviews fine grinding technology, research, and plant experience and seeks to shed light on ways in which operators can reduce both operating costs and the environmental footprint of their fine grinding circuit.

This paper will begin by giving an overview of fine grinding and the equipment used. It will then discuss energyproduct size relationships and modeling efforts for stirred mills in particular. The paper will go on to cover typical test work requirements, the effect of media size, and the contained energy in media. In closing, specific case studies will be reviewed.

Grinding activities in general (including coarse, intermediate, and fine grinding) account for 0.5pct of U.S. primary energy use, 3.8pct of total U.S. electricity consumption, and 40pct of total U.S. mining industry energy use. Large energy saving opportunities have been identified in grinding in particular.[1]

TableI shows a very large disparity between the theoretical minimum energy used in grinding and the actual energy used. More interestingly, a fairly large difference remains even between Best Practice grinding energy use and current energy use. This suggests that large savings in grinding energy (and associated savings in maintenance, consumables, and capital equipment needed) could be obtained by improving grinding operations.

As fine grinding is typically used on regrind applications, the feed tonnages to fine grinding circuits are small compared to head tonnages, typically 10 to 30tph. However, the specific energies are often much larger than those encountered in intermediate milling and can be as high as 60kWh/t. Total installed power in a fine grinding circuit can range from several hundred kW to several MW; for example, the largest installed Isamill has 3MW installed power.[3] This quantity is small compared to the power used by a semi-autogenous mill and a ball mill in a primary grinding circuit; a ball mill can have an installed power of up to 15MW, while installed power for a SAG mill can go up to 25MW. However, the energy used for fine grinding is still significant. Moreover, as this paper seeks to demonstrate, large energy reduction opportunities are frequently found in fine grinding.

Grinding can be classified into coarse, intermediate, and fine grinding processes. These differ in the equipment used, the product sizes attained, and the comminution mechanisms used. The boundaries between these size classes must always be drawn somewhat arbitrarily; for this paper, the boundaries are as given in TableII. As shown in the table, coarse grinding typically corresponds to using an AG or SAG mill, intermediate grinding to a ball mill or tower mill, and fine grinding to a stirred mill such as an Isamill or Stirred Media Detritor (SMD). Of course, various exceptions to these typical values can be found.

In fine grinding, a material with an F80 of less than 100m is comminuted to a P80 of 7 to 30m. (P80s of 2m are at least claimed by equipment manufacturers.) The feed is typically a flotation concentrate, which is reground to liberate fine particles of the value mineral.

The three modes of particle breakage are impact; abrasion, in which two particles shear against each other; and attrition, in which a small particle is sheared between two larger particles or media moving at different velocities. In fine grinding, breakage is dominated by attrition alone.[4] In stirred mills, this is accomplished by creating a gradient in the angular velocity of the grinding media along the mills radius.

Fine grinding is usually performed in high-intensity stirred mills; several manufacturers of these stirred mills exist. Two frequently used stirred mills include the Isamill, produced by Xstrata Technology, and the SMD, produced by Metso (Figure1). A third mill, the KnelsonDeswik mill (now the FLS stirred mill), is a relative newcomer to the stirred milling scene, having been developed through the 1990s and the early 2000s.[5] In all these mills, a bed of ceramic or sand is stirred at high speed. Ceramic media sizes in use range from 1 to 6.5mm.

The Isamill and the SMD have very similar grinding performance. Grinding the same feed using the same media, Nesset et al.[7] found that the Isamill and SMD had very similar specific energy use. Gao et al.[8] observed that an Isamill and SMD, grinding the same feed with the same media, produced very similar product particle size distributions (PSDs). This similarity in performance has also been observed in other operations.

Nevertheless, there are important differences. In the Isamill, the shaft is horizontal and the media are stirred by disks, while in the SMD, the stirring is performed by pins mounted on a vertical shaft. In an SMD, the product is separated from the media by a screen; the Isamill uses an internal centrifugation system. This means that the screens in an SMD constitute a wear part that must be replaced, while for the Isamill, the seals between the shaft and body constitute important wear parts. Liner changes and other maintenance are claimed by Xstrata Technology to be much easier than in an SMD: While an SMDs liner is removed in eight parts, the Isamills liner can be removed in two pieces, with the shell sliding off easily.[3] The KnelsonDeswik mill is top stirred and can therefore be considered to be similar to an SMD.[5]

An important difference among the Isamill, the SMD, and the KnelsonDeswik mill is that of scale. The largest Isamill installed at time of writing had 3MW of installed power; an 8MW Isamill is available, but appears not to have yet been installed.[3] The largest SMD available has 1.1MW of installed power; one 1.1-MW SMD has been installed. The next largest size SMD has 355kW of installed power.[6] Thus, several SMDs are often installed for a fine grinding circuit, while the same duty would be performed by a single Isamill. SMDs are typically arranged in series, with the product of one becoming the feed for the other. This has the advantage that each SMD in the line can have its media and operating conditions optimized to the particle size of its particular feed. The largest installed power in a KnelsonDeswik mill is 699kW[5]; this places it in an intermediate position between the 355-kW and 1.1-MW SMDs.

In 2012, FLSmidth reported that it had acquired the KnelsonDeswik mill; the mill is now known as the FLSmidth stirred mill. An FLSmidth stirred mill will be installed to perform a copper concentrate regrind in Mongolia.[9] It is speculated that the mill will continue to be scaled up under its new owners to allow it to effectively compete against the SMD and Isamill.

Gravity-induced stirred (GIS) mills include the Tower mill, produced by Nippon Eirich, and the Vertimill, produced by Metso. Grinding to below 40m in GIS mills or ball mills is usually not recommended. In their product literature, Metso give 40m as the lower end of the optimal P80 range for Vertimills.[6] At lower product sizes, both tower mills and ball mills will overgrind fines. At Mt. Isa Mines, a GIS mill fed with material of F80 approximately 50m lowered the P80 size by only 5 to 10m, at the same time producing a large amount of fines.[10] Similarly, in ball mills, it is known that grinding finer than approximately 40m will result in overgrinding of fines as well as high media consumption. However, it must be noted that the product size to which a mill can efficiently grind depends on the feed material, the F80, and media type and size. A Vertimill has been used to grind to sizes below 10m.[11]

The phenomenon of overgrinding is largely the result of using media that are too large for the product size generated. The smallest ball size typically charged into ball mills and tower mills is inch (12.5mm), although media diameters as small as 6mm have been used industrially in Vertimills.[11]

In a laboratory study by Nesset et al.,[7] a GIS mill charged with 5-mm steel shot, and with other operating conditions similarly optimized, achieved high energy efficiencies when grinding to less than 20m. This appears to qualitatively confirm the notion that fine grinding requires smaller media sizes. In the case of the Nesset study, the power intensity applied to the laboratory tower mill was lowthat is, the shaft was rotated slowly in order to obtain this high efficiency, leading to low throughput. This suggests that charging GIS mills with small media may not be practicable in plant operation.

Millpebs have been used as grinding media to achieve fine grinding in ball mills. These are 5- to 12-mm spherical or oblong cast steel pellets, charged into ball mills as a replacement of, or in addition to, balls. While Millpebs can give significantly lower energy use when grinding to finer sizes, they also can lead to high fines production and high media use.

Millpebs were tested for fine grinding at the Brunswick concentrator. The regrind ball mills at the concentrator used 25-mm slugs to produce a P80 of 28m. In one of the regrind mills, the slugs were replaced by Millpebs; these were able to consistently maintain a P80 of 22m while decreasing the power draw by 20pct. However, media use increased by 50pct and the production of fines of less than 16m diameter increased by a factor of 5.[12] The observed drop in specific energy may be due to the fact that Millpebs had smaller average diameters than the slugs and so were more efficient at grinding to the relatively small product sizes required. It is therefore unclear whether the performance of Millpebs would be better than that of conventional 12-mm steel balls. To the best of the authors knowledge, no performance comparison between Millpebs and similarly sized balls has been performed.

A host of other technologies exist to produce fine grinding, including jet mills, vibrating mills, roller mills, etc. However, none of these technologies has reached the same unit installed power as stirred mills. For example, one of the largest vibrating mills has an installed power of 160kW.[13] Therefore, these mills are considered as filling niche roles and are not treated further in this review. A fuller discussion of other fine grinding technologies can be found in a review by Orumwense and Forssberg.[14]

Neese et al.[15] subjected 50- to 150-m sand contaminated with oil to cleaning in a stirred mill in the laboratory. The mill operated at low stress intensities: A low speed and small-size media (200- to 400-m quartz or steel beads) were used. These conditions allowed the particles to be attrited without being broken. As a result, a large part of the oil contaminants was moved to the 5-m portion of the product. This treatment may hold promise as an alternative means of processing bituminous sands, for example, in northern Alberta.

The Albion process uses ultrafine grinding to enhance the oxidation of sulfide concentrates in treating refractory gold ores.[16] In the process, the flotation concentrate is ground to a P80 of 10 to 12m. The product slurry is reacted with oxygen in a leach tank at atmospheric pressure; limestone is added to maintain the pH at 5 to 5.5. The leach reaction is autothermal and is maintained near the slurry boiling point. Without the fine grinding step, an autoclave would be required for the oxygen leaching process. It is hypothesized that the fine grinding enhances leach kinetics by increasing the surface area of the particles, as well as by deforming the crystal lattices of the particles.

Numerous researchers, for example, Buys et al.,[17] report that stirred milling increases downstream flotation recoveries by cleaning the surface of the particles. The grinding media used in stirred mills are inert, and therefore corrosion reactions, which occur with steel media in ball mills, are not encountered. Corrosion reactions change the surface chemistry of particles, especially with sulfide feeds, and hamper downstream flotation.

Further increases in flotation recoveries are obtained by limiting the amount of ultrafine particles formed; stirred mills can selectively grind the larger particles in the feed with little increase in ultrafines production. Ultrafine particles are difficult to recover in flotation.

In intermediate grinding to approximately 75m, the Bond equation (Eq. [1]) is used to relate feed size, product size, and mechanical energy applied. Below 75m, correction factors can be applied to extend its range of validity.[4]

No general work index formula governing energy use over a range of conditions, like the Bond equation for intermediate grinding, has yet been found for the fine grinding regime. Instead, the work-to-P80 curve is determined in the laboratory for each case. The energy use usually fits an equation of the form

Signature plot (specific energy vs P80 curve) for Brunswick concentrator Zn circuit ball mill cyclone underflow; F80=63m. The plots give results for grinding the same feed using different mills and media. After Nesset et al.[7]

Values for the exponent k have been found in the range 0.7 to 3.5, meaning that the work to grind increases more rapidly as grind size decreases than in intermediate grinding. The specific energy vs product size curve has a much steeper slope in this region than in intermediate grinding.

The values of k and A are specific to the grinding conditions used in the laboratory tests. Changes in feed size, media size distribution, and in other properties such as media sphericity and hardness can change both k and A, often by very large amounts. Media size and F80 appear to be the most important determinants of the signature plot equation.

The connections (if any) between k and A and various operating conditions remain unknown. Because of the relatively recent advent of stirred milling in mineral processing, fine grinding has not been studied to the same extent as grinding in ball mills (which of course entail much larger capital and energy expenditures in any case). One of the research priorities in the field of stirred milling should be the investigation of the effects of F80 and media size on the position of the signature plots. If analogous formulas to the Bond ball mill work formula and the Bond top ball size formula can be found, the amount of test work required for stirred milling would be greatly reduced.

Larson et al.[19] found that when specific energy is plotted against the square of the percent particles in the product passing a given size (a proxy for particle surface area), a straight line is obtained. This is demonstrated in Figure3.

In contrast to the conventional signature plot, this function gives zero energy at the mill feed. It is therefore hypothesized that if a squared function plot is obtained by test work for one feed particle size, the plot for another feed particle size can be obtained simply by changing the intercept of the line while keeping the slope the same. Therefore, the squared function plot allows the effect of changes in both F80 and P80 to be modeled.

While the Squared Function Plot is intriguing, experimental validation of its applicability has not yet been published. It nevertheless remains an interesting topic for further investigation and if validated may be used in the future as an alternative measure of specific energy.

A similar analysis has been performed by Musa and Morrison,[21] who developed a model to determine the surface area within each size fraction of mill product. They defined a marker size below which 70 to 80pct of the product surface area was contained; the marker size thus served as a proxy for surface area production. Specific energy use was then defined as kWh of power per the tonne of new material generated below the marker size. Musa and Morrison found that by defining specific energy in this way, it was possible to accurately predict the performance of full-scale Vertimills and Isamills from laboratory tests.

Blecher and coworkers[22,23] found that stress intensity combines the most important variables determining milling performance. Stress intensity for a horizontal stirred mill, with media much harder than the mineral to be ground, is defined as in Eq. [4].

Note that the stress intensity is strongly sensitive to changes in media diameter (to the third power), is less sensitive to stirrer tip speed (to the second power), and is relatively insensitive to media and slurry density.

For vertical stirred mills such as the SMD and tower mill, both SIs and SIg are non-zero. For horizontal stirred mills such as the Isamill, net gravitational SI is zero due to symmetry along the horizontal axis. Therefore, for horizontal stirred mills, only SIs need be taken into consideration.

Kwade and coworkers noted that, at a given specific energy input, the product P80 obtainable varies with stress intensity and passes through a minimum. Product size at a given energy input can be viewed as a measure of milling efficiency; therefore, milling efficiency reaches a maximum at a single given stress intensity. This idea was experimentally validated by Jankovic and Valery (Figure 4).[25]

The stress intensity is defined by parameters that are independent of mill size or type. According to Jankovic and Valery,[25] once the optimum SI has been determined in one mill for a given feed, the same SI should also be the point of optimum efficiency in any other mill treating that feed. Therefore, the optimum SI need only be determined in one mill (e.g., a small test mill); the operating parameters of a full-scale mill need only be adjusted to produce the optimum SI.

Stress frequency multiplied by stress intensity is equal to mill power; therefore, stress intensity could in theory be used to predict mill specific energy. However, to the authors knowledge, a comprehensive model linking stress intensity, stress frequency, and specific energy has not yet been developed. Therefore, there is not yet any direct link between stress intensity and specific energy.

The definition of SIs as given in Eq. [4] is valid only for cases where the grinding media are much harder than that of the material ground (for example, the grinding of limestone with glass beads). Becker and Schwedes[26] determined that, in a collision between media and a mineral particle, the fraction of energy transferred to the product is given by Eq. [6]:

To maintain high efficiency in milling, the media must be chosen so as to be much harder (higher Youngs modulus) than the product material, keeping E p,rel close to unity. Where the Youngs modulus of the product is similar to that of the media, much of the applied energy goes into deformation of the media instead of that of the particle to be ground. The energy used to deform the media is lost, lowering the amount of energy transferred to the product. This fact explains why steel media, with a relatively low Youngs modulus, tend to perform poorly in stirred milling, even though the media are much more dense than silica or alumina media.

The previous sections indicated that stress intensity is independent from individual millsi.e., the optimal stress intensity when using Mill A will also be the optimal stress intensity when using Mill B. However, this does not seem to be the case when actually scaling up mills.

Four-liter Isamills are commonly used for grindability test work. It can be assumed that operating parameters of the test mill (including media type, media size, and slurry density) are adjusted so far as possible to give the optimum SI. These parameters are then used in the full-scale mill as well. However, the 4-L test mills have a tip speed of approximately 8m/s, while full-scale Isamills have tip speeds close to 20m/s.[27] If the same media size, media density, and slurry density are used in the test mill as in the full-scale mill, the stress intensity of the full-scale mill will be approximately 6.25 times larger than that of the test mill. This implies that the full-scale mill is operating outside of the optimum SI and will be grinding less efficiently. That is to say that the operating point of the full-scale mill will be above the signature plot determined by test work.

In reality, however, the operating points of full-scale stirred mills are generally found to lie on the signature plots generated in test work.[19] Therefore, the full-scale mills and test mills have the same milling efficiency, even though the full-scale mill operates at a different stress intensity than the test mill.

This question remains unresolved. One possible answer arises from the observation that two of the P80 vs SI curves in Figure4 appear to have broad troughs, covering almost an order of magnitude change in SI. In this case, even a sixfold increase in SI might not create a noticeable difference in performance, considering experimental and measurement error.

Product size vs stress intensity at three different specific energies for a zinc regrind. Note optimum stress intensity at which the lowest product size is reached. Figure used with permission from Jankovic and Valery[25]

The SMD test unit appears from photographs to have a bed depth of around 30cm, while the full-scale SMD355 has a bed depth of approximately one meter. This represents a change in the gravitational stress intensity of almost two orders of magnitude. As has been previously noted, however, laboratory and full-scale SMDs scale-up with a scale-up factor of approximately unity, with no apparent change in the optimum stress intensity. This observation suggests that the gravitational stress intensity, SIg, is unimportant in SMDs compared to the stirring stress intensity, SIs. By contrast, in GIS mills, where full-size units have bed depths of ten meters or more, gravitational stress intensity can be expected to be much more important in full-size units than in test units, adding a complicating factor to GIS mill scale-up.

Factorial design experiments were performed by Gao et al.[28] and Tuzun and Loveday[29] to determine the effect of various operating parameters on the power use of laboratory mills. Power models were determined giving the impact of different parameters as power equations with linear and nonlinear terms. The derived models did not appear to be applicable to mills other than the particular laboratory units being studied.

In ball milling, the Bond ball mill work index can be used to determine specific energy at a range of feed and product sizes. The Bond top size ball formula can be used to estimate the media size required. No such standard formulas exist in fine grinding. Energy and media parameters must instead be determined in the laboratory for every new combination of operating conditions such as feed size, media size, and media type.

For the Isamill, test work is usually performed with a 4-L bench-scale Isamill. Approximately 15kg of the material to be ground is slurried to 20pct solid density by volume. The slurry is then fed through the mill and mill power is measured. The products PSD is measured, additional water is added if needed, and the material is sent through the mill again. This continues until the target P80 is reached; typically, there will be 5 to 10 passes through the mill. The test work will produce a signature plot and media consumption data as the deliverables.

In contrast to laboratory-scale testing for ball mills and AG/SAG mills, test work results for stirred mills can be used for sizing full-size equipment with a scale-up factor close to one. Larson et al.[19,20] found a scale-up factor for the Isamill of exactly 1, while Gao et al.[8] imply that the scale-up factor for SMDs is 1.25.

A common error in test work is using monosize media (e.g., fresh 2-mm media loaded into in the mill) as opposed to aged media with a distribution of particle sizes. The aged media will grind the smaller feed particles more efficiently. Therefore, using fresh media will give a higher specific energy than in reality.[30]

Another pitfall is coarse holdup in the mill. If the mill is not sufficiently flushed, coarse particles will be kept inside the mill. The mill product then appears finer than it in reality is. This leads to lower estimates of specific energy than reality.[19]

In ball milling, the product particle size distribution (PSD) can usually be modeled as being parallel to the feed PSD on a log-linear plot.[4] When grinding to finer sizes in ball mills, the parallel PSDs mean that large amounts of ultrafine particles are produced. This consumes a large amount of grinding energy while producing particles which are difficult to recover in subsequent processing steps such as flotation.

As shown in the figure, at the left end of the graph, the product PSD is very close to the feed PSD; at the right, the two PSDs are widely spaced. This indicates that the mill is efficiently using its energy to break the top size particles and is spending very little energy on further grinding of fine particles. Thus, the overall energy efficiency of the fine grinding can be expected to be good. As a bonus, the tighter PSD makes control of downstream processes such as flotation easier.

In an experimental study, Jankovic and Sinclair subjected calcite and silica to fine grinding in a laboratory pin stirred mill, a Sala agitated mill (SAM), and a pilot tower mill. The authors found that for each mill, the PSD of the product was narrower (steeper) than that of the feed. In addition, when grinding to P80s below approximately 20m in any of the three mills tested, the PSD became more narrow (as measured by P80/P20 ratio) as the P80 decreased. (When the width of the PSD was calculated using an alternative formula, the PSD was only observed to narrow with decreasing P80 when using the pin stirred mill.) The authors concluded that the width of the PSD was strongly affected by the material properties of the feed, while not being significantly affected by the media size used.[32]

In stirred milling, the most commonly used media are ceramic balls of 1 to 5mm diameter. The ceramic is usually composed of alumina, an alumina/zirconia blend, or zirconium silicate. Ceramic media exist over a wide range of quality and cost, with the lower quality/cost ceramic having a higher wear rate than higher quality/cost ceramic. Other operations have used sand as media, but at the time of writing, only two operations continue to use sand.[8,27,33] Mt Isa Mines has used lead smelter slag as media; however, it is now using sand media.[10,27] Mt Isa is an exception in its use of slag, as a vast majority of operations do not have a smelter on-site to provide a limitless supply of free grinding media. However, in locations where slag is available, it should be considered as another source of media.

Media use in fine grinding is considered to be proportional to the mechanical energy applied. Typical wear rates and costs are given in TableIII and Figure6; these figures can of course vary significantly from operation to operation.

Contained energy refers to the energy required to produce and transport the media, and is distinct from the mechanical (electrical) energy used to drive the mill. Hammond and Jones estimated the contained energy in household ceramics (not taking account of transportation).[39] Hammond and Jones estimates range from 2.5 to 29.1MJ/kg, with 10MJ/kg for general ceramics and 29MJ/kg for sanitary ceramics. Given that ceramic grinding media require very good hardness and strength, especially compared to household ceramics, it is appropriate to estimate its contained energy at the top end of Hammond and Jones range, at 29MJ/kg.

Using 29MJ/kg for the contained energy of ceramic media and a wear rate of 35g/kWh of mechanical energy gives a contained energy consumption of 0.28kWh contained per kWh of mechanical energy applied. A wear rate of 7g/kWh gives a contained energy consumption of 0.06kWh contained per kWh of mechanical energy applied. Therefore, 6 to 20pct of the energy use in fine grinding using ceramic media can be represented by contained energy in the grinding media itself.

Sand media have much lower contained energy than ceramic media as the media must simply be mined or quarried rather than manufactured. Hammond and Jones report a contained energy of 0.1MJ/kg. Blake et al.[36] reported that switching a stirred mills media from sand to ceramic results in a mechanical energy savings of 20pct. Therefore, using sand rather than ceramic media would produce savings in contained energy, but would cost more in mechanical energy. Likewise, Davey[40] suggests that poor-quality media will increase mechanical energy use in stirred milling. It is speculated that this is due to the lower sphericity of sand media. On the other hand, the work of Nesset et al.[7] suggests that the energy use between ceramic and sand media of the same size is the same. Slag media, where a smelter is on-site, would probably have the lowest contained energy consumption of the different media types. There is very little transportation, and for accounting purposes, almost no energy has gone into creating the media as the granulated slag is a by-product of smelter operation.

Becker and Schwedes[41] point out that with poor-quality media, a significant part of the product will consist of broken pieces of media, which will affect the measured product PSD. Clearly, more information on the relationships between contained energy in media and media wear rates is desirable.

Of the different operating parameters for stirred mills, media size probably has the biggest influence on overall energy consumption. The appropriate media size for a mill appears to be a function of the F80 and P80 required. The grinding media must be large enough to break up the largest particles fed to the mill and small enough to grind the material to the product fineness desired. As demonstrated by the experience of Century mine, an inappropriate media size choice can result in energy consumption double that of optimum operation.[8]

In their laboratory study, Nesset et al.[7] varied a number of operating parameters for stirred mills and identified media size as having the largest impact on energy use. It was also noted that the trials which produced the sharpest product PSD were also the ones which resulted in the lowest specific energy use.

Gao et al.[8] report that at Century mine, the grinding media in SMDs performing regrind duty were changed from 1 to 3mm. This resulted in a drop in energy use of approximately 50pct; the signature plot shifted significantly downward (Figure7).

Figure8 shows the product PSD for laboratory SMD tests using 1- and 3-mm media. The PSD for the test using 1-mm media shows that the SMD produced a significant amount of fines (20pct below 4m). The mill also had difficulty breaking the top size particlesthe 100pct passing size appears to be almost the same for both the feed and the product. In contrast, the PSD using 3-mm media shows less fines production (20pct below 9m) and effective top size breakage, with all the particles above 90m broken. This is in line with the observation of Nesset et al.[7] that low energy use is associated with tight product size distributions.

Gao et al.[38] tested copper reverberatory furnace slag (CRFS, SG 3.8) and heavy media plant rejects (HMPR, SG 2.4) in a laboratory stirred mill at two sizes: 0.8/+0.3mm, and 1.7/+0.4mm. For both CRFS and HMPR, the smaller size media gave a lower specific energy than the larger size media. At the same size, both CRFS and HMPR had similar specific energy use. However, the CRFS ground the material much faster than HMPR. Possibly, this was due to its higher density.

Data on F80, P80, and media size were compiled from the literature in order to allow benchmarking against existing operations. The sources are listed in Table IV. F80 and P80 were plotted against media size; the results are given in Figure9.

F80 plotted against media size (blue diamonds); P80 plotted against media size (red crosses). Century UFG=Century ultrafine grind; Century Regr.=Century regrind. Data are taken from Case studies table (Color figure online)

It can be seen from the figure that as the P80 achieved decreases, the media size does as well, from 3mm to achieve 45m to 1mm to achieve under 10m. The F80 decreases with media size in a similar way, from 90m at 3mm to 45m at 1mm. Dotted lines have been added to Figure7 to define the region of operation of mills; these delimit a zone in which the stirred mill can be expected to operate efficiently.

In general, for a particular media size, limits on both F80 and P80 must be respected. For example, the figure suggests that a mill operating with an F80 of 100m should use 3-mm media, while a mill grinding to below 10m would need to use 1-mm media. To reduce a feed of 90m F80 to 10m P80, Figure9 suggests that comminution be done in two stages (two Isamills or SMDs in series) for optimal efficiency. The first stage would grind the feed from 90m to perhaps 45m using 3-mm media, while the second would grind from 45 to 10m using 1- or 2-mm media.

A number of opportunities exist to reduce the energy footprint of fine grinding mills. There are no general formulas, such as the Bond work formula and Bond top size ball formula in ball milling, to describe the performance of stirred mills. Therefore, improvement opportunities must be quantified by performing appropriate test work.

In addition to obtaining the signature plot, the specific energy as a function of new surface area should be determined during test work. This could be done either by the method of Larsen or by that of Musa and Morrison. Defining specific energy as a function of new surface area may constitute a superior means of predicting the performance of full-scale mills, as opposed to defining specific energy as a function of feed tonnage.

Media size should be chosen with care. It is recommended that test work be done with several media sizes in order to locate the stress intensity optimum. Media size can be benchmarked against other operations using Figure9.

There are indications that lower-quality media, apart from degrading faster, require more mechanical energy for grinding due to factors such as lower sphericity. It is recommended to perform test work using media of different quality to determine the effect of media quality on energy use. Slag and sand media may also be considered. Subsequently, a trade-off study involving media cost, electricity cost, improvement in energy efficiency, and contained energy in media should be performed to identify the best media from an economic and energy footprint standpoint.

D. Rahal, D. Erasmus, and K. Major: KnelsonDeswick Milling Technology: Bridging the Gap Between Low and High Speed Stirred Mills, Paper presented at the 43rd Canadian Mineral Processors Meeting, Ottawa, 2011.

Metso: Stirred milling: Vertimill grinding mills and Stirred Media Detritor (product brochure), 2013, available at http://www.metso.com/miningandconstruction/MaTobox7.nsf/DocsByID/F58680427E2A748F852576C4005210AC/$File/Stirred_Mills_Brochure-2011_LR.pdf, accessed April 21, 2013.

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how to do material and water balance to develop mineral processing flowsheets - minerallurgy

Material balance in the mineral processing industry is required for two primary purposes; One is to take stock of inputs and outputs and to identify the losses. Second, develop the flowsheet showing the flow of material (the amount of material flowing from one unit to another).

In several mineral processing operations, we got two products from a single feed. The plant output consists of concentrate and tailing. The middling does not form the output; because they are further circulated. They can join either concentrate or tailing, but the plant outputs are concentrate and tailing. The concentrate is sent to the metal extraction plant, where tailing is dumped.

A mineral processing flowsheet is a path that shows how the material is flowing, from the feed to the end separated products. It consists of a series of units with a specific combination (subjected to study and tests). It contains all the information about the amount of material solid and liquid flowing in every unit.

Usually, we have some data about the feed. We do tests, and we determine how much water should be used. Also, we can measure slurry density and slurry flow rate using instruments. However, to design, optimize, and control (after plant running) the processing circuits, you should perform material and water balance.

We could equally use the mineral grade or the commodity-grade, but we usually use the metal grade in metallurgy. In the case of metallic ores, we are interested in the extraction of metals, thus analyzing the feed concentrate and tailing in terms of metal has a significant value because everything other than the metal is the gangue.

Be careful with the units! 1000 is the density or volumetric mass of water in kg/m3. You have to substitute all values in kilogram per meter cube if the density of water used is in kilogram per meter cube.

For example, suppose you want to make a slurry density of 1700 kilogram per meter cube from a solid, which has a density equal to 2800 kilogram per meter cube. You can find out by putting both these values into the formula on percent solid, you get 64 percent. Therefore to make this slurry you have to use 64% solis and 36% water.

In the hydrocyclone, the oversize is a useful product and it is sent for concentration operation. It means we attend a good mineral liberation degree in the oversize (of the hydrocyclone), thus it can go for further concentrating operation whereas the undersize size is still bigger than the required. It is fed into a ball mill and the product of the ball mill goes further to the hydrocyclone in a closed circuit

On another circuit which consists of only a ball mill, we have the feed which can directly go into the ball mill (water is added in the ball mill discharge), then it goes to the hydrocyclone, then you have the undersize and the oversize.

The water balance in the cyclone is based on; The weight of water entering the cyclone should be equal to weight of water leaving the cyclone; %water (in the feed)=%water(underflow)+%water(in the overflow)