flotation cell kinetics

flotation kinetics - an overview | sciencedirect topics

In order to get a good separation, the solids present must be liberated: that is, not physically or chemically attached, be suspended in a liquid medium and the flotation kinetics of the materials must be different. One or more stages of separation may be needed, depending on the kinetics and chemistry of the separation. To achieve sharper separation when difference in flotation rate of components is not high and/or material is not completely liberated, complicated flowsheets including multiple recycle lines and regrinding are used. Regrinding operations for middlings are used to avoid over-grinding of the bulk of material as it would cause reduction in flotation rate and selectivity for fine particles. For finely disseminated ores, entrainment is a substantial factor reducing sharpness of separation. Entrainment is a process of particle transfer to froth without their attachment on to bubble surfaces. This phenomenon can be explained by movement of small particles in the wake behind the rising bubble or within the static layer of liquid surrounding it. In machines with intensive mixing (impeller cells) the entrainment can also be caused by local upward slurry flows. These flows are not present in columns therefore reducing overall entrainment intensity and improving separation efficiency. A classical flotation flowsheet includes several cleaning stages generally linked by recycle of the cleaner tailings to previous stages. When more than one material is floatable and separation depends only on degrees of hydrophobicity (molybdenite-chalcopyrite), four to six stages may be required. If insufficient recovery is achieved in the primary vessel (rougher flotation), scavenger cells may be used. In general, all stages do have a common separation goal. For example, silica (impurity) is floated away from hematite in a four stage iron ore circuit in Figure 4. This circuit, or variations of it, is common when the valuable product is hydrophilic or an underflow product of the column. The example gives four stages of separation; however, in many cases fewer stages are required.

The circuit for a hydrophobic product is shown in Figure 5. The second cleaner stage of this circuit is generally not needed unless the separation is between hydrophobic materials with similar flotation rates. As an example, this configuration or variations of it can be used in phosphate, copper, zinc and plastics separations, or for soil remediation.

Frothers are added to create a stable dispersion of bubbles in the pulp which will subsequently create a reasonably stable froth and which will allow selective drainage from the froth of entrained gangue and improve the flotation selectivity. The frother also affects the flotation kinetics. They are nonionic heteropolar molecules and, unlike collectors, are not associated with particular categories of minerals. The frothing ability of a compound is associated with hydroxyl (OH), ester (COOR) and carbonyl (CO) chemical groups, and commercial frothers can be divided into three main categories: alcohols, alkoxyparaffins, polyglycols and polyglycol ethers. The polar end of the frother molecule forms hydrogen bonds with the water and no mineralfrother bonds are formed. The nonpolar end is hydrophobic so that the frother concentrates at the airwater interface and is thus described as being surface-active. This affects the surface tension, which indicates the difference between the surface activity of frothers and causes a stable froth to form. In general, increased surface activity results in increased floatability and froth stability.

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.

Typical dependence of the flotation recovery on particle size is shown in Figure 25. The recovery of minerals by flotation is most successful in the 10200m size range. Major problems in flotation are the relatively poor response in many cases of fine and coarse particle fractions. The recovery suffers a decline for both small and large particles. The reasons for the drop off in recovery and flotation rate for the fine and the coarse ends of the particle size distribution are different. The relatively slow flotation rate of fine particles is generally attributed to the decrease in efficiency of the particle-bubble collision and attachment. The very poor recovery of coarse particles is thought to be due to disruption of particle-bubble aggregates in excessively turbulent zones in flotation cells, leading to the increase in efficiency of the particle-bubble detachment.

It appears that flotation kinetics depend on a balance among the collision, attachment and detachment processes which are strong functions of the particle size, density and hydrophobicity, the bubble size and the turbulent kinetic energy. The particle physical properties are the input into the flotation system and are, therefore, difficult to manipulate, but the latter are the manipulated variables, which define the best flotation cell design and operating conditions for a given particle size distribution.

The existence of an optimum field of operation for various particle sizes has been appreciated. For fine particles an increased rotational speed of rotor above the minimum (1scriterion) value required for particle suspension often results in an increased recovery. On the other hand, the recovery of coarse particles has a maximum at the minimum suspension speed of rotor, and decreases with further agitation. Therefore, the optimum hydrodynamic conditions for fine and coarse particles are different. Fine particle flotation is controlled by the bubble-particle collision and attachment processes and should be operated at higher power-input, which produces smaller bubbles at moderate to high agitation. Coarse particle flotation is controlled by the bubble-particle detachment process and requires a turbulence intensity which just ensures the complete suspension of all particles. For coarse or high-density particles the advantage of using smaller bubbles is reduced and a high turbulence is detrimental.

The requirements of the different particle sizes are schematically shown in Figure 26. These requirements need not necessarily involve different cell designs for fine and coarse particles, but the bubble size is, to some extent, controlled by the impeller design and the mechanical entrainment of fine particles can be reduced by a proper baffle system.

With flotation feeds containing a wide range of particle sizes, it is unlikely that a single flotation cell can be devised to give optimum recovery over the whole particle size range. The feed should be split into fine and coarse particles so that they can be handled separately in the flotation circuits. Fine particles should be floated with moderate to high agitation, whereas the coarse fraction would respond best to a stirring speed just above the minimum speed for complete suspension. Alternatively, a wide bubble size distribution can be generated to float particles of a size distribution at high rates of recovery.

There is also a very pronounced chemical effect on the relationship between particle size and floatability. Both the reagent regime and the hydrodynamic conditions are equally important in flotation and should be optimized in parallel. In the past, the hydrodynamic conditions were often neglected in the flotation research. Regarding the reagent regime, the chemical conditions for fine and coarse particle flotation are different. It is not easy to optimize the reagent regime for both fine and coarse particles in the one flotation cell.

The recovery of valuable metal being lost in slimes is of paramount importance. It has been estimated that recovering 15% of the metal lost in the less than 10m fines in Canada would increase revenue by approximately $100m.

High intensity conditioning of slimes in the presence of collector and frother substantially improves the flotation kinetics of pentlandite slimes. The floatability of fine particles by high intensity conditioning, however, largely depends on the nature of the ore and the power input per unit volume of pulp treated. Maximum recoveries are obtained at a power input exceeding 1.5 kWh m3 of pulp. High intensity conditioning of slurry adopted at the Trojan mill in Zimbabwe has improved nickel recoveries by 24%, owing to the increased flotation response of oxidized and tarnished nickel-bearing sulfides.

Many hydrophobic silicate minerals such as talc, chlorite, kaolinite and serpentinite activate other minerals by forming slime coatings. It is also known that the serpentine slimes, containing chrysotile and lizardite, form slime coatings on unoxidized pentlandite and such coatings depress nickel flotation. The formation of these slime coatings is directly related to the magnitude and sign of surface charges of the slimes and sulfide particles.

Crysotile is known to depress nickel sulfide more than lizardite. Addition of small amounts of chrysotile to the flotation pulp (0.05g L1) dramatically decreases pentlandite flotation recovery, from 90% to 5%.

The reagents generally known to modify the slime surface charge and reduce their adverse influence on pentlandite flotation are dextrin, sodium pyrophosphate, sodium silicate, sodium carbonate, guargum and CMC. Among these reagents CMC has been found to be the best, despite the high concentration required (more than 2kg t1). CMC treatment of an Australian ore containing a number of magnesium silicate minerals improved the flotation rate of pentlandite when using amyl xanthate as a collector. The carbonate ions derived from soda ash enhanced dispersion in the pulp, while CMC facilitated the removal of slime particles from pentlandite, thereby allowing the xanthate to coat the surface. The pentlandite fines flotation was found to be improved with either soda ash or CMC but the synergistic effect of both proved beneficial for intermediate sizes.

The extent of slime coating is related to the zeta potential, being greatest when slime and particles have a high zeta potential of opposite sign. The zeta potential of the particles can be decreased by increasing the concentration of counterions. A highly saline pulp produces a high concentration of counterions, Na+, and Cl, which influence the slime dispersion, besides enhancing hydrophobicity due to modification of the hydration layer around the mineral particles and air bubbles.

In the presence of 10% saline solution, fine nickel sulfide flotation is promoted, whereas flotation with 1kg t1 CMC does not result in upgrading the nickel sulfides. Flotation tests on the whole feed and cyclone overflow of a transition ore, from Western Australia, in saline pulp gave better results (Figure 8) than the addition of 1kg t1 of CMC.

The separation of chalcocite (Cu2S) and heazlewoodite (Ni3S2) from high nickel matte can be achieved by maintaining the proper pH and pulp potential while using xanthates as collectors. The flotation recovery of Cu2S and Ni3S2 has been studied by adjusting the oxidationreduction potential of the flotation pulp with (NH4)2S2O8 and KMnO4. The recovery of Ni3S2 is almost zero either at pH 8.5 where Eh>500mV or at pH 11.2 with Eh>400mV, whereas the potential has little effect on Cu2S at pH 8.5. The depression of Ni3S2 can be improved by employing Ca(ClO)2, as modifier of potential, and Ca2+ ions. As the potential of Ni3S2 is increased, with increase in dosage of Ca(ClO)2 the recovery decreased, but the potential appears to have little effect on the flotation behaviour of Cu2S (Figure 9).

Figure 9. Flotation recovery of Cu2S and Ni3S2 and Eh as a function of Ca(ClO)2 in the presence of a mixture of butyl xanthate and ethyl xanthate (104 mol L1). (Reprinted with permission from International Academic Press.)

The rate of flotation can be defined as a measure of the efficiency of flotation expressed per unit time. The rate of flotation accounts for the amount of floated particles with regard to time (d/dt). However, there are several problems related to the rate of flotation, some of which include (a) an experimental method determining the rate of flotation, (b) the effect of flotation variables and (c) equations denoting the rate of flotation. In spite of these, the rate of flotation is the most reliable source of describing the flotation kinetics. There are numerous methods by which the kinetics of flotation in determined. Some of the most prominent kinetic models are listed in Table 6.3.

As can be seen from these data, it is not possible to develop a universal model that would describe the flotation process as a whole. In the mini-models listed, the authors have used simple ores (e.g. copper porphyry ores), which would correlate well with experimental data (e.g. Klimpel models). In the case of complex ores, such a model cannot be applied. In general, the approach in model selection would be the selection of criteria based on the process that has to be described (effect of reagents, pulp density, etc). Some criteria on the methodology in the determination of the rate of flotation is described below.

The most simple example of the process kinetics are the rate tflh, concentrate production tc/h), recovery per given unit time, etc. As an example of how to use these criteria, let us examine the floatability of sphalerite as a function of aeration assuming that aeration had a positive effect on sphalerite flotation. It is of interest to analyze the kinetics of sphalerite flotation with and without aeration. These results are plotted in Figure 6.12, indicating that with aeration, flotation of zinc improves in the initial stage (i.e. first 4 min) and also produces improved selectivity. In practice, from this example, it may be possible to increase froth discharge rate, which would result in the improvement of overall circuit performance. Of more importance is the relationship between rate of flotation of floated minerals and gangue particles expressed as selectivity index L.

In assessing the kinetics of flotation, two steps are usually followed. In the first step, using experimental data, recovery e versus time t is plotted in increments , and in the second step, the rate of flotation determined using one of the formulas. The simplest kinetic formula is the so-called first-order chemical reaction.

It should be noted that it cannot be assumed that experimental data will fit the theoretical curve. For example, curve a can be similar to curve 1 eKt, in which case curve (t) would satisfy the first order. This corresponds to flotability of slow floating fractions where recovery increase per unit time is small. Giving different values for K, it is possible to fit several curves that would satisfy eq. (6.8) and best fit experimental data that represent the kinetic constant. This method is considered to be inaccurate. A more simple and accurate way is the linear representation of the kinetic constant. The eq. (6.8) in logarithmic form is as follows:

where the curve coordinates should also be a linear curve from which the kinetic constant can be graphically determined. The graphical method of determining the kinetic constant is not accurate and can be used only to give some idea of the kinetics of a process. For a more accurate determination of the kinetic constant, a statistical method is used and usually accounts for experimental errors, as shown in the example below:

Assuming that (R) is the experimental error for recovery then the corrected recovery would be E(c) = E (R) including corrected recovery in the first-order equation, the kinetic constant is determined from the following relations:

This method would be sufficient for simple study case (i.e. simple copper ore). In more complex cases (ore), the flotability of minerals is not always uniform; so calculating the rate constant using recovery for short intervals of time usually gives higher rate constants and for longer intervals the rate constant is lower. To avoid this problem, the least squares method is used. For example,

In reality, kinetic curves derived from experimental data often do not fit values calculated from the first-order equation. This may be due to a number of factors, including differences in flotability. This is where different kinetic models come into play. The following example of fit model development illustrates the model development process to give some idea about how various models from Table 6.3 are derived and what they mean. In the majority of model developments, it is assumed that there is a maximum value for recovery Emax such that

If the flotation time is sufficiently long, then it can be assumed that Emax is equal to the recovery achieved in the experiment. On this basis, the rate constant calculation is derived from the following relations:

The above equation is a linear function of Er 1 = a + bEr. To define this relationship, the experimental data can be approximated to obtain a curve that is near Emax. At relatively long flotation time, it is always assumed that recovery is maximum therefore Er and Er 1 equal the maximum. The graphical determination of Emax involves determining curve intercepts from Er 1 (Er) and curve Er 1 = Er. By including the graphical value of Emax in eq. (6.13), the kinetic constant is calculated. The accuracy of this method depends on the accuracy of the curves and the angle of intercept. For smaller angles of intercept, the error is large.

The shape and associated surface chemistry on the flotation kinetics of chalcopyrite were investigated with and without the addition of potassium amyl xanthate (PAX) as collector [57]. In the absence of collector, angular chalcopyrite particles can be floated faster than round ones, but the difference in flotation rate between angular and round particles becomes not obvious in the presence of PAX as collector [57]. The PAX can absorb on chalcopyrite surface and significantly enhance the floatability of chalcopyrite particles. The high degree of hydrophobicity would conceal the effects of shape on flotation. It should be noted that the flotation of gangue particles may be enhanced if they obtain angular shape properties. Hence, the grade of concentrate may be affected due to the increasing recovery of gangue particles in the concentrate. Grinding processes should be controlled and the chosen type of mill is very important prior to the flotation of minerals.

Pyrite is an important resource for the production of sulphuric acid and solar cells [58]. Pyrite commonly has its natural hydrophobicity, which cause the difficulty in the flotation desulfurization of coal from fine pyrite gangues [59]. Liberation is also required prior to pyrite flotation. Therefore, grinding processes using different types of mills have various effects on the flotation of pyrite because the shape properties and surface properties of pyrite particles will be governed by the grinding processes.

It was found that ball milled pyrite product had lower acuteness than autogenously milled products [60]. As shown in Fig. 2 [60], the flotation recovery of pyrite increases with the increase of acuteness. In other words, ball mill creates products with lower acuteness increasing the floatability of pyrite. It is understandable that ball mill can produce rounder particles. However, it is also a contradictory, because rounder pyrite particles have higher floatability than acute ones while the rounder chalcopyrite particles have lower floatability than the particles with higher elongation ratio [57]. The contradictory may be attributed to the difference in mineral types. Each mineral has its own grinding and surface characteristics, and the flotation of various minerals use various flotation reagents [61,62]. The adsorption of flotation reagents on mineral surface would depend on mineral types. Therefore, the flotation response of minerals of various types would be different.

Molybdenite (MoS2) is a naturally hydrophobic mineral and can be floated using common oily collectors [52,63]. It is reported that molybdenite particles of higher aspect ratio are floated more quickly than those of lower aspect ratio because the surface hydrophobicity of the plate-like molybdenite structure particles is much higher than that of round particles [64]. The flattened molybdenite particles can obtain more hydrophobic sulfur species on its plate surface than the round particles. In addition, the adsorption of collector on the plate surface of flattened molybdenite particles will be higher than that of round ones. Therefore, the surface hydrophobicity of flattened molybdenite particles will be greatly enhanced in the flotation pulp, compared to round particles.

A reduction in bubble coalescence in the collection zone of the flotation cell can be seen as a potential benefit since the flotation kinetics increase with a decrease in bubble size (Ahmed and Jameson, 1985; Castro and Laskowski, 2011; Diaz-Penafiel and Dobby, 1994). Effectively, the flotation rate constant in the collection zone of a flotation cell kc was shown to be directly proportional to the superficial surface area rate of bubbles Sb and the probability of collection P, following the relationship (Yoon and Mao, 1996)

Consequently, in a system where the relationship between ionic strength of the solution and the bubble size was established, this specific electrolyte effect on the flotation kinetics can be directly calculated using Eqs. (4) and (5). Finally, knowing that the ion pairs preventing bubble coalescence can be relatively well predicted from thermodynamics, it appears possible to quantifiably predict whether or not a specific modification of the ionic concentrations in the process water will affect the flotation kinetics through this prevention of bubble coalescence.

There is a large similarity between the particle capture processes [27,94,95] that occur in both flotation and filtration [146150]. Thus, the SRHI theory regarding particle capture during filtration is of interest to flotation kinetics. This theory is acknowledged in fundamental colloid science [149]. Spielman and Goren [139,140,146148] introduced a local co-ordinate system for the description of the local hydrodynamic field around the small particle and in the film between the particle and the collector, obtaining equations for the short-range hydrodynamic forces. These results are an important component of the theory of Deijaguin, Dukhin and Rulyov (DDR) on the SRHI in flotation [27].

When a particle moves along a bubble surface the conditions for the particlebubble interaction change and the short-range interaction becomes unsteady. However, in terms of the local co-ordinates linked to the particle, the interaction can be considered as quasi-steady. The system of cylindrical co-ordinates can be used with its centre on the bubble surface and a z-axis crossing the centre of the moving particle. Such cylindrical co-ordinates simplify the description of hydrodynamic interaction. The equation for the normal Vr=dzdt and tangential V=abddt components of the particle velocity are given by

where Fpr is the pressing force, z is distance from the centre of the bubble and coefficients f1 and f2 take into account the resistance force in the thin layer between particle and bubble (f1) and delay in particle tangential velocity with respect to the liquid (f2).

The equations describe that stage of the particle movement when the liquid interlayer is thin and the distance between the centres of the particle and the bubble is almost equal to ab+ap. The rates of interlayer thinning and particle movement are identical and are controlled by the action of the pressing force [product of Stokes drag coefficient and the dimensionless function f1(H), H=zapap=hap]. The equation for the particle trajectory follows from Eqs. (4.1) and (4.2), after replacement of z and H and the exclusion of time,

A semi-analytical solution of Eq. (4.3) is possible if the radius of action of the surface forces is small compared with the particle size, Brownian motion is not taken into account and a sufficiently high electrolyte concentration is present. Gravity will be neglected because in this section, potential hydrodynamic flow is considered. The hydrodynamic pressing force is proportional to the local value of the normal component of the hydrodynamic velocity

The functions f1, f2, f3 are determined [139,140,146148] for collision with a solid sphere with the use of Stokes hydrodynamic field. A similar problem for potential flow was solved [36,151] which enables a generalization of the theory of the short-range hydrodynamic interaction for a rising bubble with a non-retarded surface.

flotation '21

The 10th International Flotation Conference (Flotation '21) is organised by MEI in consultation with Prof. Jim Finch and is sponsored by Promet101, Maelgwyn Mineral Services, Magotteaux, Gold Ore, CiDRA Minerals Processing, Hudbay Minerals, Senmin, Clariant, BASF, Eriez, Nouryon, Festo, Newmont,Cancha, Zeiss,FLSmidthand Kemtec-Africa.

advanced flotation technology | eriez flotation division

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The rotary slurry-powered distributor (RSP) is used to accurately and evenly split a slurry stream into two or more parts, without creating differences based on flow, percent solids, particle size or density. Applications include Splitting streams for feeding parallel lines for any mineral processing application

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Applications forEriez Flotation equipment and systems include metallic and non-metallic minerals, bitumen recovery, fine coal recovery, organic recovery (solvent extraction and electrowinning) and gold/silver cyanidation. The company's product line encompasses flotation cells, gas spargers, slurry distributors and flotation test equipment.Eriez Flotation has designed, supplied and commissioned more than 1,000 flotation systems worldwide for cleaning, roughing and scavenging applications in metallic and non-metallic processing operations. And it is a leading producer of modular column flotation systems for recovering bitumen from oil sands.

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Eriez Flotation provides advanced engineering, metallurgical testing and innovative flotation technology for the mining and minerals processing industries. Strengths in process engineering, equipment design and fabrication positionEriez Flotation as a leader in minerals flotation systems around the world. Read More

flotation kinetics | springerlink

Flotation kinetics is briefly introduced and its history is reviewed. Main theoretical approaches are discussed, and the kinetic models are presented in detail. The application of flotation kinetics to modeling and simulation of the circuits is shortly surveyed, then some industrial results are used to show how the models fit to experimental results.

flsmidth's flotation reflux revolution - international mining

IM Editorial Director Paul Moore recently spoke to FLSmidths Lance Christodoulou, Global Product Manager Flotation, about the revolutionary REFLUX Flotation Cell (RFC) which is being developed together with the University of Newcastle & brings a unique design that is capable of achieving higher grades, recoveries and throughputs than any other flotation cell currently available.

Development of the all new REFLUX Flotation Cell (RFC) has been an extension of the already established development partnership with the University of Newcastle, working closely with Professor Kevin Galvin, who is Director ARC Centre of Excellence for Enabling Eco-Efficient Beneficiation of Minerals. This partnership has already seen the successful development and commercialisation of the REFLUX Classifier (RC) with a similar commercialisation path being envisioned for the new REFLUX Flotation Cell.

Christodoulou says there is already an expansive database of supporting pilot-scale data and the company is working with clients to investigate suitability of the implementation of the technology into their flowsheets. These efforts can typically start off with lab scale testing with their ore followed by on-site pilot testing ultimately leading, in principle, to full scale installation. The lab scale work is possible either at the University of Newcastle or at FLSmidth in Salt Lake City. If that testing goes well then we would recommend onsite trials with our containerised pilot system which receives diverted continuous feed from the main process route. We have a number of these pilot units, which are available for on-site testing and can be shipped to test sites globally. We already have a fairly well-developed set of models that will help predict how the RFC will fit into a flotation circuit in design and scale terms. We have existing data sufficient for high level technical studies such as feasibility work.

The RFC joins the wider suite of REFLUX technologies along with the REFLUX Classifier, which is a hydraulic classifier that separates particles using gravity, based on a difference in density or particle size. It combines a conventional fluidised bed separator with a set of parallel inclined plates that form lamella channels, and this lamella technology is what is implemented in the RFC. That said, the REFLUX Flotation Cell utilises flotation principles instead of gravity principles as with the RC.

Christodoulou told IM: You are essentially dealing with a form of staged flotation in that we take the feed and it makes contact with air in an environment where there is high energy density and high shear rates, effectively increasing the probability of collision and attachment to promote the kinetics of the system. The whole system we believe is unique in its design and mechanics. Once you have the contact occurring with the feed and air supply, this bubbly mixture is then passed down into a chamber which operates at a very high air fraction, much higher than anything currently available in the market and approaching what we would call flooding conditions.

He adds: We control the operation of the equipment with a positive bias flow which means a generally downward volumetric flow of material but because you are operating in these conditions, you start dragging this bubbly mixture down into the bottom of the device. In traditional open top flotation cells, this mixture would simply be pulled out of the bottom of the cell. With the RFC the lamella plates come into play which, due to the Boycott effect, allows this bubbly mixture to segregate very efficiently, preventing any loss of air bubbles to underflow. This enhanced segregation capability allows us to operate at a very high air fraction and control what is coming out of the overflow in terms of volumetric flow rate it allows us to control the bias of the system. Then, with the application of wash water, we are able to displace any material that is coming up in the remaining water present in the bubbly mixture to maximise control of the grade so we can achieve a very high quality product coming out of the overflow.

FLSmidth says it has seen a good technology transfer of the RC in terms of the lamella plates, which enhances segregation, to implementation in flotation in the form of the RFC. In the RC the lamella plates allow for much higher separation efficiencies, effectively producing a much steeper partition curve when considering density and sizing applications. In the RFC having this allows for advanced bubble slurry segregation, allowing the RFC to operate in higher air fractions with higher air flux and higher throughput.

You have improved kinetics, improved grade and improved recovery all at the same time. In a normal flotation cell as you increase the gas flux, you start entering an operating condition where you lose the interface so there is no discernible difference between the pulp and the froth so called flooded conditions. For example if you run a conventional cell at the same gas fractions as we run the RFC, you start operating at flooded conditions which results in bubbles and pulp that contains valuable mineral being discharged out of the tails, so you arent effecting a very good separation or separation efficiency. In most open cell systems, you have an upper limit in terms of the bubble surface area flux that you can realistically operate at, even if you increase the gas flux or reduce the bubble diameter. The RFC lamella plates allow you to achieve the segregation and therefore operate with a smaller bubble diameter as well as higher superficial bubble surface area flux which then allows for better recoveries.

The chamber operates at extremely high gas flux so is a really high air fraction environment. We have enclosed the system and apply fluidisation wash water which allows us to control the underflow and overflow; and that allows us to operate with very high wash water fluxes and very high gas fluxes, pushing us out into operating ranges that are not possible in open cell systems, when considering air and wash water fluxes, achieving simultaneously higher grade and higher recovery shifting the grade recovery curve along with increasing the throughput of the equipment. A typical scale up factor for conventional equipment in benchtop kinetics tests is 2 to 2.5 times. For the RFC this is a number less than 1, meaning you have a reduction in the flotation volume needed.

There is a big potential market across all the major flotation markets including, but not limited to, coal, gold tailings, graphite, iron ore, copper roughing, moly cleaning where in every case the RFC is operating on the left hand side of the kinetics curve. We are seeing recoveries beyond what is possible in a conventional benchtop testing. In coal for example, the RFC performance is being predicted utilising a coal grain analysis method which gives the true ultimate flotation response. The RFC is operating on this curve, which is to the left of a traditional tree curve that is typically used to predict conventional flotation performance. That means in coal we are able to produce much lower ash product with very similar combustible recovery numbers than you would see with traditional coal flotation technologies. We are recovering much more coal meaning the RFC has huge potential in recovering coal fines from tailings streams or retreatment of already stacked tailings for example. Beyond this, we can also offload overloaded coal flotation circuits as the RFC is a high capacity machine with a small footprint.

Reference: Cole, M.J., Dickinson, J.E., and Galvin, K.P., Recovery and Cleaning of Fine Hydrophobic Particles using the RefluxTM Flotation Cell, Separation and Purification Technology, 240 116641 (10 pages) 2020

While coal has historically been the initial focus of the technology development recent testing in copper, moly, gold, iron ore, and graphite applications have all produced similar results in terms of improved grade and recovery along with a reduction in required flotation retention time. In some cases we are producing not only a higher grade product at a higher recovery rate but also in a shorter timeframe.

Comparing it with other hydraulic technologies like column flotation, in that case you have two types of sparging systems one is air introductory only which is based at the bottom of the cell; which has many similarities with conventional flotation. The other column flotation sparging system is a recirculating pump which draws material from the base of the column and passes it through the contact area in this respect there is a similarity with our RFC, except that the RFC sees the contact with the feed material where the concentration of the floatable material is at its highest in terms of the feed slurry, whereas in column flotation, the contact is usually with the tails of the column, where the floatable material concentration is at its lowest. The RFCs method of contacting with the feed promotes the flotation kinetics better than with the tails.

A comparison to the Imhoflot cell shows similarities in the initial first high intensity precontact step, but beyond this FLSmidth says the unique nature of the RFC segregation comes into play to bring performance well beyond what is possible with any other cell. All other flotation technologies also have froth recovery limitations the RFC doesnt really have a defined froth-pulp interface further promoting recovery.

Christodoulou adds: The unique nature of the design and use of the lamella plates means this isnt something you can retrofit onto existing conventional cells. To start with we see RFC flotation being able to work to offload overloaded circuits in coal and base metal cleaner circuits, then start to replace column flotation circuits, and ultimately replace banks of open conventional cells, though there is still some way to go for full market acceptance before we get to that point.

A number of trials have already been carried out with a pilot scale system that is similar in scale to the lab scale system except that the pilot testing has been on a continuous basis using a continuous feed. In addition to the pilot, FLSmidth and the University of Newcastle are currently trialling a two metre RFC machine in an Australian coal application. This full-scale evaluation is set to be completed early 2021.

The pilot site trials use a containerised test skid housed in a standard 20 ft shipping container. The test skid can be deployed quickly and can be commissioned in less than a week, with typical test programs wrapping up in 4 weeks or less. Utility and feed requirements are minimal with typical tests requiring less than 100 litres a minute of feed slurry.

minerals | free full-text | adsorption kinetics of various frothers on rising bubbles of different sizes under flotation conditions | html

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minerals | free full-text | hydrodynamic and flotation kinetic analysis of a large scale mechanical agitated flotation cell with the typical impeller and the arc impeller | html

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editors Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

leachox process for flotation concentrates | maelgwyn mineral services

The majority of Aachen reactors are employed on Run of Mine (ROM) feed applications primarily to increase gold recovery through increasing the kinetics. In the Leachox process the role of the Aachen reactor is somewhat different in that the reactor is used to facilitate partial oxidation of the sulphide minerals encapsulating gold particles and is therefore normally used to improve the gold recovery on sulphide flotation concentrates.

Processes such as roasting, pressure oxidation, and bacterial oxidation are all aimed at breaking down the sulphide matrix to liberate gold. Ultra-fine grinding performs the same function particularly where gold is locked in silicates or other minerals. Many of these processes are well developed and can yield very high gold recoveries but unfortunately all tend to have inherent issues.

Roasting for instance is an environmentally unfriendly process and presents permitting issues in many countries; pressure oxidation requires a fairly high degree of operator skill and control not to mention often exotic materials of construction. Bacteria used in bacterial oxidation whether heap of tank leaching are susceptible to changes in environmental conditions and require careful control. Pressure oxidation works well but requires high levels of technical expertise and exotic materials of construction

Whilst it is not possible here to go into the relative merits and demerits of each process the biggest issue that they all have in common to some degree is the very high associated capital and operating costs limiting their application to large high grade deposits which can accommodate the high capex and opex

For many deposits where the resource base is too small to justify Pressure oxidation etc then Leachox can be used. Leachox is a partial oxidation process for refractory ores centred around the Aachen reactors. Whilst it does not yield as high a gold recovery as POX and Roasting it is order of magnitudes cheaper resulting in an overall improved project economics

Often when gold is locked in sulphides and silicates it is necessary to reduce the particle size to a size where gold is liberated or partially liberated. For refractory ores this generally translates into grinding below 10 microns and often to as low as 3-4 microns. Historically this was cost prohibitive as the only mills that were available to do this were tumbling mills which become highly inefficient at these low sizes particularly below 20 microns. The last 10-15 years however, have seen the development of a number of grinding mills specifically designed for ultra-fine grinding in the minerals industry. This has lead to a commensurate reduction in cost to grind fine and has been the catalyst for the development of many refractory ore treatment processes.

One of the drawbacks of grinding finer is that in addition to liberating the desired mineral it also increases the surface area of other host minerals .This can result in order of magnitude increases in reagent consumption particularly cyanide for the subsequent leaching process unless cognisance of this is taken in the final process design.

Whilst the installation is similar to that used for pre-oxidation or Aachen assisted leaching in the Leachox process the flotation concentrate is pumped through the reactor multiple times perhaps as many as 30 passes in contrast to 1-2 passes for the pre-oxidation role .Depending upon the mineralogy ultrafine grinding of the flotation concentrate may be required prior to cyanidation

In Leachox rather than just raising the DO level and providing shear to remove passivating species to enhance cyanidation kinetics the sulphide matrix is partially oxidised (60-70%) resulting in the liberation of gold particles from the host sulphide matrix.

Unfortunately the breakdown of the sulphide matrix along is associated with a significant increase in the surface area of the various cyanide consuming minerals and can result in order of magnitude increases in cyanide consumption often as high as 20-30kg/t which in itself is uneconomic. In addition, significant passivation also can occur rendering the mineral surface refractory to cyanide

The Aachen reactor solves both of these problems by accelerating the leach kinetics such that gold is able to dissolve into solution prior to the cyanide being consumed and also continuously removing the passivating layers forming on the gold particles (See information on Aachen reactors for pre-oxygenation and Aachen assisted Leaching)

Whilst as mentioned previously oxygen lances might be suitable for readily cyanidable oxide ores their limitations become apparent with the more demanding refractory leaching applications and their use is associated with very high cyanide and oxygen consumptions and poor gold recoveries

Imhoflot G-Cells are used to produce the flotation concentrate. The reason for this is that the patented G-Cell is able to produce a higher grade flotation concentrate than conventional mechanical flotation cells resulting in a small concentrate mass and so reduced treatment costs (see under Imhoflot flotation for more information)

As previously mentioned ultra fine grinding is often required on the flotation concentrates. Grinding is by its very nature an expensive process and more so for fine grinding where particle size reduction is through abrasion rather than impact. Whilst there are a number of commercial ultrafine grinding mills available Maelgwyn specifically designed its own mill the Ro-Star mill to reduce the capex and opex costs for ultra- fine grinding where this is required

The cyanide consumption in refractory gold leaching is generally much higher than in ROM ore cyanidation circuits and so efficient cyanide destruction is important. The Aachen reactors can be used to enhance the well-known sulphur dioxide based cyanide detox process