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Selective transport of attached particles across the pulp–froth interface
Minerals Engineering 19 (2006) 841–851 This article is also available online at: www.elsevier.com/locate/mineng
Selective transport of attached particles across the pulp–froth interface D.R. Seaman *, E.V. Manlapig, J.-P. Franzidis Julius Kruttschnitt Mineral Research Centre, University of Queensland, Isles Road, Indooroopilly, Queensland, Australia Received 13 July 2005; accepted 25 October 2005 Available online 20 December 2005
Abstract A technique for determining the recovery of attached particles across the froth phase in flotation that relies on measuring the rate at which bubble–particle aggregates enter the froth is used to investigate the selectivity of attached particles across the froth phase. Combining these measurements with those of other techniques for determining the froth recovery of attached particles provides an insight into the different sub-processes of particle rejection in the froth phase. The results of experiments conducted in a 3 m3 Outokumpu tank cell show that the detachment of particles from aggregates in the froth phase occurs largely at the pulp–froth interface. In particular it is shown that the pulp–froth interface selectively detaches particles from aggregates according to their physical attributes. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation; Selectivity; Detachment; Froth recovery
1. Introduction In the field of mineral flotation, flotation machines perform the task of separating (typically valuable) hydrophobic minerals from the (typically less valuable) hydrophilic particles. The froth phase separates bubble–particle aggregates from surrounding suspended material as well as upgrading the attached material on the bubble surfaces either by selective rejection of less hydrophobic mineral from the bubble surfaces or the selective re-attachment and/or displacement by more strongly hydrophobic minerals. Within froth flotation there is much literature on the topic of rejection of suspended/entrained material within the froth (Johnson et al., 1974; Bisshop and White, 1976; Warren, 1985; Savassi et al., 1998; Smith and Warren, 1989). However, the selectivity of the froth phase towards attached particles on the basis of their hydrophobicity is an area which has received little attention until now. *
Corresponding author. E-mail address: [email protected] (D.R. Seaman).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.020
An important measure of froth performance is the recovery of attached particles across the froth, namely froth recovery, defined as the fraction of particles entering pulp phase attached to air bubbles that are transferred to the concentrate (Cutting et al., 1981; Feteris et al., 1987; Finch and Dobby, 1990; Woodburn et al., 1994; Vera et al., 1999a; King, 2001). Several techniques are available to determine froth recovery, falling predominantly into two categories: (a) the use of empirical relationships observed between operating conditions and metallurgical performance (Feteris et al., 1987; Vera et al., 1999a), and (b) through the estimation or direct measurement of bubble loading (mass of attached particles per volume of air bubbles) below the pulp–froth interface (Seaman et al., 2004; Alexander et al., 2003; Savassi et al., 1997; Falutsu and Dobby, 1989). This paper presents a methodology in which results obtained using these different froth recovery measurement techniques are compared to investigate the selective nature of the froth phase with respect to mineralogy, and to provide an insight as to where in the froth phase this selectivity occurs.
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2. Attached particle selectivity Many researchers have tried to address the question of selectivity of attached particles across the froth phase either by analysis of the sub-processes occurring across the froth phase or through experimentation. In both cases, one needs to analyse the sub-processes occurring in the froth in order to determine which of these could result in the selective rejection of attached material or the re-attachment of particles previously detached in the froth phase. A froth that selectively rejects attached (or previously attached) particles on the basis of size, density or hydrophobicity is regarded as being selective with respect to attached material; this also implies that the mineral content of aggregates overflowing into the concentrate stream is different from those entering the froth. Selectivity of this nature has been observed in both industrial (Young, 1982; Seaman et al., 2004) and laboratory scale (Ata et al., 2002) flotation studies. However, in several other systems no apparent selectivity with respect to mineralogy has been found (Alexander et al., 2003; Savassi et al., 1997; Vera et al., 1999a), but only an apparent selectivity with respect to particle size. Four distinct sub-processes affect the detachment and re-attachment of particles in the froth phase: 1. 2. 3. 4.
bubble coalescence, particle detachment, particle drainage (of previously attached particles), and particle re-attachment.
2.1. Bubble coalescence In the upper regions of the froth there is a reduction in total available surface area of bubbles as they coalesce. When a specific lamella breaks on coalescence, the particles and fluid of the boundary ‘‘burst’’ and fall to the base of the new larger bubble. From here, the particles and water could drain back into the pulp zone, or remain entrained in the froth and be recovered to the concentrate; or, alternatively, some of the particles could re-attach to bubble surfaces. The likely fate of the now detached particles is discussed below in the sections dealing with drainage and re-attachment in the froth phase. The process of detachment upon coalescence is very unlikely to be selective towards particle type, as the rupture of bubble lamellae is instantaneous and all the particles attached to the bursting lamellae will fall to the base of the coalesced bubble. Gourram-Badri et al. (1997) conducted experiments that involved creating two bubbles in water, attaching particles of different hydrophobicity (sphalerite, pyrite and chalcopyrite) to them and then allowing them to coalesce. They found that the mineral content of the particles attached to the bubbles before and after coalescence of the two bubbles was different. In each case, the less hydrophobic mineral in the system, sphalerite, was preferentially detached
upon coalescence. It is highly unlikely that the detachment occurred at the ‘‘actual moment’’ of coalescence, but rather that the coalescence initiated bubble oscillation which has been reported to cause selective detachment (Cheng and Holtham, 1995; Schulze, 1984; Falutsu, 1994). This concept of bubble oscillation is discussed below in the section on froth selectivity. In any event, these experiments were conducted under conditions that more approximately represent the pulp phase than the current focus of the selective detachment in the froth. 2.2. Particle detachment Particles detach from bubble surfaces when sufficient force is exerted to separate the particle from the aggregate. The equilibrium forces acting on a suspended spherical particle attached at a fluid interface were investigated by Schulze (1984) to determine the amount of energy required to detach particles from aggregates. The equilibrium forces can be resolved for a variety of particle sizes, densities, hydrophobicities and interfacial surface tension. Schulze (1984) calculated the energy required to detach a particle from a bubble surface for a limited range of physical and chemical conditions. Fig. 1 uses this information to calculate the conditions under which particles are likely to detach from bubble surfaces. Using the balance of equilibrium forces on a spherical particle attached to a single bubble, the energy required to remove the particle completely from the bubble surface has been estimated for a range of particle diameters (0–160 lm) and hydrophobicities (0° 6 h 6 60°) typical in flotation. The calculation provides a range of detachment energies required to detach particles from bubble surfaces. It is clear from this figure that larger and more hydrophobic particles are more difficult to remove from bubble surfaces. However, this figure is only true for fully liberated minerals of spherical shape and should thus be used with caution in its application. Any deviation from these ideal spherical particles (non-spherical, not fully liberated and/or surface
Fig. 1. Variation of detachment energy of spherical particles with particle size and hydrophobicity; density of particles, qp = 7.5 g/cm3; fluid/slurry density, qfl = 1.1 g/cm3; bubble radius, Rb P 0.1 cm; interfacial surface tension, r = 72 dyne/cm.
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imperfections) will result in less energy being required to detach from a bubble surface. Klassen and Mokrousov (1963) identified six mechanisms by which energy can be supplied to bubble–particle aggregates in order to separate the particle from the bubble. These six scenarios are shown in Fig. 2. With reference to detachment occurring around the froth phase in flotation, the only mechanism shown in Fig. 2 likely to cause detachment is the impact of an aggregate with a stationary (or nearly stationary) object, namely the pulp–froth interface. The other mechanisms are more likely to occur within the pulp phase where there is more turbulence. A seventh event known to detach mineral particles from bubble surfaces is bubble oscillation. When an aggregate strikes another object (stationary or moving), the energy absorbed from the impact causes the bubble to oscillate. Klassen and Mokrousov (1963) show photographic evidence of these oscillations which were first observed by Spedden and Hannan (1948). Cheng and Holtham (1995) studied the effect of bubble-oscillation on particle detachment in an artificial flotation system by creating single bubble–particle aggregates that were then oscillated by means of a loud speaker. They found that the amount of detachment occurring was related to the frequency and amplitude of the loud speaker. It is hard to determine how the kinetic energy of an aggregate is released upon striking the pulp–froth interface. This energy will be dissipated in oscillation of the bubble as well as heat. The overall change in kinetic energy can, however, be calculated by considering the change in momentum of an aggregate as it decelerates upon reaching the pulp–
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froth interface; i.e. from a velocity of u1 in the pulp phase to u2 in the froth phase (where magg is the mass of the bubble–particle aggregate): 1 1 DE ¼ magg u21 magg u22 . 2 2
ð1Þ
The mass of the aggregate depends on the bubble loading in the pulp phase. The velocity of the aggregate in the pulp phase can be estimated through a drift flux analysis approach. King (2001, Chapter 9) provides an approach to estimate the terminal rise velocity of an aggregate, based on the bubble diameter, the mass loading of solids on the bubble, and the conditions in the slurry. Combined with an estimate of the rise velocity of bubbles above the pulp–froth interface (0.5 cm s1), the change in kinetic energy of aggregates as a function of bubble size and bubble loading is displayed in Fig. 3. Fig. 3 shows that the energy released when an aggregate decelerates is in the order of 0–0.02 erg, which is more than adequate to detach particles from bubble surfaces (see Fig. 1). The energy dissipation increases for larger bubble sizes and for more loaded bubbles. This energy can be dissipated in many forms, e.g. it can be absorbed by bubble oscillation, converted to heat, or it can detach particles from the bubble surface by accelerating the attached particles. It is impossible to determine in what way this energy will be dissipated with this type of collision, but there is a strong likelihood that a portion of this energy will be dissipated in a form that results in the selective detachment of particles from aggregates.
(a)
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(d)
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Fig. 2. Mechanisms of particle detachment (after, Klassen and Mokrousov, 1963). Forces tending to separate the mineral particle from the bubble: (a) during rise (accelerated or equilibrium) of a mineralised bubble surface; (b) under the action of liquid streams; (c) during sliding of particle along bubble; (d) with change in motion of a mineralised bubble; (e) during impact and attrition of particles in the pulp against a mineralised bubble surface and (f) during impact of a bubble with an obstacle.
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Fig. 3. Change in energy of an aggregate decelerating at the pulp–froth interface.
2.3. Particle drainage When particles detach in the froth phase, a portion of them drain back into the pulp phase. The drainage of previously attached particles back into the pulp phase is selective with respect to particle size and density, much like the entrainment process. The particles that were previously attached to bubbles that are recovered by this entrainment process in the froth phase are included in the calculation of froth recovery (which does not distinguish between attached and previously attached particles). Since the drainage of these particles back into the pulp phase is selective, the resulting froth recovery must be selective on a size and density basis. Following the same principles governing entrainment, the mechanism of particle drainage of previously attached particles results in the froth recovery of finer, less dense minerals being greater than that of coarser, more dense minerals. Moys (1989) acknowledged the selective drainage of previously attached particles in the froth phase, and proposed that the drainage of these particles is proportional to their concentration in each size class. Mathe et al. (2000) as well as Vera et al. (2002) went on to provide an empirical model for froth recovery incorporating the concept of froth retention time, FRT proposed by Gorain et al. (1998). 1 Rf ðiÞ ¼ expðbFRTÞ þ ½1 expðbFRTÞ . 1 þ wi FRT ð2Þ The parameter b is related to the rate of detachment of attached particles in the froth phase, and xi is the rate of drainage of detached particles of size class i. Eq. (2) shows that larger froth retention times result in a lower froth recovery, because the aggregates have more chance to detach through loss of surface area by coalescence, and also more time to drain from the froth phase back in to the pulp. 2.4. Re-attachment in the froth Particles that are removed from bubble surfaces in the froth phase through any of the detachment mechanisms (e.g. bubble coalescence, bubble oscillation etc.) may re-
attach to bubbles lower down in the froth phase. They could re-attach either by displacing less strongly attached particles or by attaching onto uncovered bubble surfaces in the lower regions of the froth (Ata et al., 2002) where the particle loading per bubble surface area is lower. The residence time in the froth phase (particularly of columns) is relatively high in comparison to the pulp phase. This allows for much longer times for unattached particles to be in contact with fresh bubble surface. Ata et al. (2002) added particles of known hydrophobicity and size to the surface of a column froth. They showed that the more hydrophobic particles attached themselves to bubbles in the froth and were collected in the concentrate stream. It is unlikely that these particles were recovered by entrainment due to the high water bias velocity employed in the froth phase. In addition, they showed that the more surface area available for attachment in the froth, the greater the recovery of the particles introduced to the top layers of the froth. This selective attachment of (hydrophobic) particles in the froth phase was well known in the former Soviet Union where froth separation was introduced in the early 1970s (Young, 1982; Ata et al., 2002; Malinovskii et al., 1973). Young (1982) showed that much larger particles can be floated by this technique—up to 3000 lm—than by conventional flotation. 2.5. Summary of sub-processes in the froth phase From the above analysis, it seems very likely that the froth recovery of attached particles in a flotation cell is selective based on the particle size, density and hydrophobicity (encompassing liberation characteristics as well as surface chemistry). It is hard to estimate which of the sub-processes has the largest impact on froth recovery as little work has been done to date to isolate these subprocesses in an industrial environment. A summary of the identified sub-processes and their impact on the selectivity of attached particles is presented below: 1. Bubble coalescence is unlikely to be selective in terms of detaching particles attached to bubbles in the froth phase. However, coalescence is likely to initiate other forms of detachment which may in fact be selective (e.g. bubble oscillation, see below). 2. Particle detachment has been shown to be a function of particle size, density and hydrophobicity as well as operating conditions in the flotation cell. In particular, it is likely that selective particle detachment occurs at the pulp–froth interface (due to the change in momentum of aggregates striking the interface) as well as during bubble oscillations initiated either by the aggregates striking the pulp–froth interface or when bubbles coalesce. It is hard to determine whether smaller or larger particles would be selectively detached, but all else being equal, less strongly attached particles will be preferentially detached.
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3. Particle drainage of previously attached particles is certainly a selective process, particular with respect to size and density. In general, larger and more dense particles will drain faster than finer and less dense particles. It is thus expected that through this mechanism of selective froth recovery, larger particles of the same mineral type will have a lower froth recovery. 4. Particle re-attachment has been shown to occur in flotation froths. More hydrophobic particles have a greater probability of being re-attached in the froth phase. Like the pulp phase of flotation cells, it is hard to predict what size of particles is more likely to be collected. Some researchers (Vera et al., 1999a; Savassi et al., 1997) in the past have investigated (either directly or indirectly) the selective nature of the froth phase using different techniques for determining the froth recovery of attached particles. They found no significant selectivity of attached particles across the froth phase. Fig. 4 compares the froth recovery of chalcopyrite with that of pyrite in the JKMRC 16L High Sb cell (Vera et al., 1999b) treating a copper rougher ore under a range of operating conditions. Fig. 4 clearly shows no selectivity between these two minerals in terms of froth recovery; the conclusion made by Vera et al. (1999a) is that froth recovery is a non-selective process. However, evidence for froth selectivity on a size basis has been found: Fig. 5 shows froth recovery determined on a size by size basis for various flotation cells down a rougher bank at Mount Isa (Savassi, 1998). It is clear that the froth phase is exhibiting selective froth recovery in terms of particle size. The most probable mechanism for the decrease in froth recovery with increasing particle size is the drainage of previously attached particles. A similar result was shown by Vera (2002) on secondary analysis of the work of Contini et al. (1988). The present authors (Seaman et al., 2004) have found evidence of froth selectivity of attached particles with the use of a technique to measure directly the bubble loading
Fig. 5. Froth recovery on a size basis for different flotation cells in the rougher bank at Mount Isa (after Savassi, 1998).
of particles in the pulp phase, by comparing the grade of these particles with those found in the top layer of the froth phase (Sadr-Kazemi and Cilliers, 2000; Vera and Franzidis, 2003). This measurement technique is used below to provide a further evaluation of the sub-processes occurring across the pulp–froth interface. In summary, the selective nature of the sub-processes occurring across the froth phase cannot be ignored. For most of the sub-processes, the selectivity appears to be towards more valuable minerals (generally having a higher hydrophobicity). A deeper understanding of these mechanisms may lead to improved machine operation and/or design which take these processes into account. 3. Experimental details Experiments were carried out on a portable pilot scale Outokumpu 3 m3 flotation cell operated at Newmont Golden Grove Operation. The cell was fed with the rougher feed stream to the flotation circuit. Two different ore types, a copper/lead/zinc rich ore (prior to sphalerite activation) and a copper rich ore, were investigated. The purpose of conducting these tests was to compare two different froth recovery measurement techniques and to investigate whether any difference between the two measurements would allow conclusions to be made about the sub-processes occurring in the froth phase. 3.1. Froth recovery measurement
Fig. 4. Froth recovery of chalcopyrite versus that of pyrite for a copper rougher ore in a 16L continuous flotation cell (after Vera et al., 1999a).
3.1.1. Changing froth depths The first froth recovery measurement technique employed was the changing froth depths method, initially proposed by Feteris et al. (1987) for flotation columns and later adapted to mechanical flotation cells by Vera et al. (1999a). This approach requires a flotation cell to be operated continuously at different froth depths and surveyed at steady state. The overall first order rate constant
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collection whilst still maintaining a positive water bias flow in the sampling tube (to prevent the introduction of entrained material). The set-up of this chamber in a mechanical flotation cell is shown schematically in Fig. 7. The calculation of froth recovery using these bubble load techniques includes the transfer of aggregates across the pulp–froth interface. This distinction makes it possible to isolate effects occurring across the pulp–froth interface, which forms the remaining focus this paper. 3.2. Survey details and data analysis
Fig. 6. Changing froth depth technique for determining froth zone recovery (after Vera et al., 1999a).
of attached particles, k, is determined for each condition, and plotted against froth depth as shown in Fig. 6. It is assumed at a froth depth of zero that the overall rate constant is equal to the collection zone rate constant, kc, due to the absence of a froth zone. Froth zone recovery, Rf, is calculated as the ratio between the collection zone rate constant and overall rate constant (Vera et al., 1999a): Rf ¼
k . kc
ð3Þ
For each ore type, the OK 3 m3 cell was operated at four different froth depths (in a random order) with the aeration rate to the cell kept constant. After each froth depth change, the cell was allowed four residence times to reach steady state. For each condition, the major streams to the cell were sampled over a 30 min period with three cuts of each stream composited. Feed and concentrate flowrate estimates were made by timing the rate at which the cell initially filled and taking timed samples of the entire concentrate stream. At each froth depth, the bubble load device (see Fig. 7 above) was used to measure the bubble loading of aggregates and hence the rate at which attached material rises to the pulp–froth interface.
This approach considers only events occurring in the dryer regions of the froth phase as the changes to froth depth are made above the pulp–froth interface. 3.1.2. Bubble loading in the pulp zone The second froth recovery measurement technique employed is through the direct measurement of bubble loading in the pulp zone. Savassi et al. (1997) developed special measuring devices to capture samples from the pulp zone of flotation cells containing different proportions of bubble–particle aggregates and suspended material. A mass balance across these samples and a sample of the concentrate was used to determine the bubble loading of aggregates in the pulp zone as well as their mineral content, and froth recovery. Seaman et al. (2004) followed the approach of Falutsu and Dobby (1992) in developing a new technique to isolate aggregates from the suspended pulp and hence determine the bubble loading of the aggregates. The technique requires the use of a bubble load collection chamber suspended above the froth phase. The chamber has a sampling tube which is inserted into the quiescent area of the pulp zone, filled initially with a water–frother mixture (to prevent coalescence in the sampling tube). Aggregates are then allowed to rise up the tube and into the chamber where they are dispersed to the outside of the chamber by a deflector cone. Bubbles burst at the air/liquid interface and particles are collected in the base of the chamber. A vacuum pump may be used to prolong the collection time by removing some of the air entering the chamber during
Fig. 7. Schematic representation of the bubble load chamber used to isolate bubble–particle aggregates (Seaman et al., 2004).
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Because froth recovery deals only with ‘‘true floating’’ (attached) material, the contribution to recovery by entrainment needs to be discounted. This was done through the use of an entrainment factor, ENTi, based on the slowest/non-floating mineral species identified in the concentrate stream. The approach is described in more detail by Johnson et al. (1974) and Savassi et al. (1998). Briefly, the slow or non-floating mineral is assumed to be recovered only by entrainment, and the calculated ENTi values are applied to all other minerals on a size by size basis. All samples were weighed wet and dry and analysed for Cu, Pb, Zn, Fe, Ag and Au from which average mineral compositions were determined. One of the sets of data was sized and assayed for the same elements.
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sulphide gangue with fast floating sulphide minerals than that of sphalerite or pyrite. It can be seen that the froth recoveries determined using the changing froth depth technique are similar for all the minerals (save for the mineral used to calculate entrainment). The froth recoveries determined using the bubble load measurement technique are however very different. This implies that most of the selectivity occurs across the pulp–froth interface where there is a significant amount of loss of attached particles. This difference observed between the froth recovery measurement techniques can be manipulated to provide more detailed information on events occurring across the pulp–froth interface. 4. Three zone flotation cell description
3.3. Froth recovery results
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In light of the possible selective froth recovery of attached particles, a three zone flotation cell description is proposed (pulp zone, pulp–froth interface and froth zone) to provide insight into the different mechanisms occurring across the froth phase as a whole (Fig. 10). This three zone description has been proposed previously by Hanumanth and Williams (1992) for the purpose of developing a more descriptive flotation model. In the discussion below, the description is used in conjunction with different froth recovery measurement techniques to resolve the flow of material between the three zones. The representation of material flows within and around the flotation cell is similar to that presented by Wilson and Stratton-Crawley (1991) for a two-zone flotation cell model. It is important to note in Fig. 10 that material dropping out from the pulp–froth interface returns to the pulp zone, essentially as an additional feed stream. In the same way, attached material dropping out of the froth zone does not return directly into the pulp zone, but rather as an additional feed to the pulp–froth interface. This description allows for the possibility of previously attached material to become re-attached or entrained within the pulp–froth interface and return to the froth zone. The model has an
Froth Recovery of Chalcopyrite (%)
Froth Recovery of Chalcopyrite (%)
Fig. 8 shows the froth recoveries of chalcopyrite determined using the changing froth depths technique (CFD) and the bubble load measurement technique (BLM) for (a) the copper/lead/zinc ore and (b) the copper rich ore in the Outokumpu 3 m3 cell. As has been observed previously (Seaman et al., 2004), the froth recovery determined using the changing froth depth technique is much greater than that determined using the bubble load measurement technique in the same situation. This is because the changing froth depth technique does not take the pulp–froth interface into account, whereas the bubble load measurement technique does. Fig. 9 shows the froth recoveries for all of the minerals determined using the two techniques on the different ore types. The zero froth recoveries shown in the figure for sphalerite (for the copper/lead/zinc ore) and pyrite (for the copper ore) are an artefact of using the slowest floating mineral to determine the contribution to recovery by entrainment (discussed previously in this section). The sphalerite and pyrite presented slower kinetics than the non-sulphide gangue. This is most likely due to a greater association of non-
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Fig. 8. Froth recovery of chalcopyrite determined using two different measurement techniques: CFD—changing froth depths, and BLM—bubble load measurement, for two different ore types in the Outokumpu 3 m3 flotation cell. (a) Copper/lead/zinc rich ore and (b) copper rich ore.
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Fig. 9. Comparison of froth recovery measurement techniques using different feed ores in a per mineral basis. (a) Changing froth depths—copper/lead/ zinc ore, (b) changing froth depths—copper rich ore, (c) bubble load measurement—copper/lead/zinc rich ore and (d) bubble load measurement—copper rich ore.
all recovery of particles by true flotation can be determined as: Rc Rpfi Rf . ð4Þ R¼ 1 Rc þ Rpfi ðRf þ Rc 1Þ Based on the measurement techniques used to determine froth recovery, and an appreciation for whether or not the pulp–froth interface is included in the measurement, the recoveries across each zone can be determined. The froth recovery determined using the bubble load measurement technique, Rf,h, includes the pulp–froth interface in the determination of froth recovery and can therefore be expressed in terms of the streams shown in Fig. 10 as: xRpfi Rf Rpfi Rf . ð5Þ ¼ Rf;h ¼ Rc 1 Rpfi ð1 Rf Þ
Fig. 10. Three zone description of a flotation cell showing the flow of attached and previously attached material.
additional reflux stage in the flotation system than the more traditional two-zone descriptions. A simple mass balance allows for the determination of all flows of material based on the recoveries of each zone: Rc, the collection zone recovery; Rpfi, the pulp–froth interface recovery; and Rf the froth zone recovery of material above the pulp–froth interface. From this figure, the over-
Since the froth recovery determined using the changing froth depths approach, Rf,cfd does not incorporate the pulp–froth interface, it can be assumed that this measurement is equivalent to the froth recovery, Rf shown in the figure (i.e. Rf,cfd = Rf). The pulp–froth interface recovery, Rpfi can then be determined from Eqs. (4) and (5): Rpfi ¼
Rf;h . Rf;h ð1 Rf;cfd Þ þ Rf;cfd
ð6Þ
The pulp–froth interface recovery on a per mineral basis was determined through Eq. (6) for the two systems shown previously in Fig. 9. Fig. 11 shows the results.
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Fig. 11. Pulp–froth interface recovery per mineral for the two different feed ore types. (a) Copper/lead/zinc ore and (b) copper rich ore.
For both feed types, it is apparent that the pulp–froth interface recovery is certainly significant. In each case, the more hydrophobic (faster floating) minerals appears to have a higher Rpfi than the less hydrophobic minerals.
CP Gal Sphal Py NSG
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Samples collected from the copper/lead/zinc rich ore were sized and assayed in order to determine the froth recovery and pulp–froth interface recovery on a size-bysize basis. The froth recoveries determined using both techniques were used to determine the pulp–froth interface recovery shown in the previous section. Fig. 12 shows the froth recovery by size for all minerals at one of the test conditions with the copper/lead/zinc rich ore. Like other authors previously (Contini et al., 1988; Vera et al., 1999a; Savassi et al., 1998) a decrease in froth recovery is observed with an increase in particle size. In order to evaluate the recovery of attached particles across the pulp– froth interface, the pulp–froth interface recovery on a size basis for the same test as in Fig. 12 is shown in Fig. 13.
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Particle size (μm) Fig. 12. Froth recovery determined using the bubble load measurement technique on a size by mineral basis for the copper/lead/zinc rich ore at a froth depth of 132 mm in the OK 3 m3 cell.
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Rf (%)
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Particle size (μm) Fig. 13. Pulp–froth interface recovery on a size by mineral basis for the copper/lead/zinc rich ore at a froth depth of 132 mm in the OK 3 m3 cell.
As with the overall froth recovery results shown in Fig. 12, the pulp–froth interface recovery of weaker hydrophobic minerals is lower than that of the more strongly hydrophobic minerals. In addition, the pulp–froth interface recovery decreases with increasing particle size. These observations are consistent with the expectation that less strongly attached particles will detach preferentially to those that are more strongly attached. The review of detachment energy showed that larger particles require more energy to detach from bubbles due to their higher contact area with the bubble surface. This determination of detachment energy makes the assumption that (a) the particles are spherical, and (b) that the particles are fully liberated. Neither of these assumptions are likely to be true in a real system. Furthermore, the pulp–froth interface recovery includes the recovery of previously detached particles draining out of the froth interface. Larger particles will drain faster through the pulp–froth interface, this is probably the more significant sub-process with relation to particle size. It is impossible to identify which of the sub-processes is dominant in the pulp–froth interface (selective detachment
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or selective re-attachment). It is likely that sub-processes occur with an overall effect of the pulp–froth interface selectively rejecting larger and less hydrophobic particles. 6. Conclusions This paper shows how different froth recovery measurement techniques can be combined to successfully model a flotation cell as three distinct zones. This model can be used to assess the performance of the froth phase and the pulp– froth interface independently of each other. The model is also useful in the qualitative understanding of particle detachment and could be utilised in the analysis of performance of flotation machines to investigate where particles belonging to a specific class are rejected. Selective transfer of attached particles across the froth phase in terms of their size and mineral content has been identified. It has been shown that the pulp–froth interface is responsible for a large degree of upgrading of the particles attached to bubble surfaces and also for a significant proportion of the recovery loss across the froth phase as a whole. Acknowledgements The authors of this paper wish to express their gratitude to the people involved in the testwork, Brigitte Comley and Michael Wortley. They also express their thanks to the site at which the testwork was conducted, Newmont Golden Grove Operations and to the personnel involved, Ashley Kidd and Kathryn Ellis. Peter Bourke from Outokumpu provided the OK 3 m3 cell required to complete this testwork for which the authors are grateful. Finally, the authors wish to acknowledge the financial support of the sponsors of the AMIRA P9 project, without which this work would not have been possible. References Alexander, D., Franzidis, J.-P., Manlapig, E., 2003. Froth recovery measurement in plant scale flotation cells. Minerals Engineering 16, 1197–1203. Ata, S., Ahmed, N., Jameson, G., 2002. Collection of hydrophobic particles in the froth phase. International Journal of Mineral Processing 64, 101–122. Bisshop, J., White, M., 1976. Study of particle entrainment in flotation froths. Transactions of the Institute of Mining and Metallurgy 85, C191–C194. Cheng, T., Holtham, P., 1995. The particle detachment process in flotation. Minerals Engineering 8 (8), 883–891. Contini, N., Wilson, S., Dobby, G., 1988. Measurement of rate data in flotation columns. In: Sastry, K. (Ed.), Column Õ88. AIME, Littleton, USA, pp. 81–89 (Chapter 10). Cutting, G., Watson, D., Whitehead, A., Barber, S., 1981. Froth structure in continuous flotation cells: relation to the prediction of plant performance from laboratory data using process models. International Journal of Mineral Processing 7, 347–369. Falutsu, M., 1994. Column flotation froth characteristics—stability of the bubble–particle system. International Journal of Mineral Processing 40, 225–243.
Falutsu, M., Dobby, G., 1989. Direct measurement of froth drop back and collection zone recovery in a laboratory flotation column. Minerals Engineering 2, 377–386. Falutsu, M., Dobby, G., 1992. Froth performance in commercial sized flotation columns. Minerals Engineering 5, 1207–1223. Feteris, S., Frew, J., Jowett, A., 1987. Modelling the effect of froth depth in flotation. International Journal of Mineral Processing 20, 121– 135. Finch, J., Dobby, G., 1990. Column Flotation. Pergamon Press (Chapter 3). Gorain, B., Harris, M., Franzidis, J.-P., Manlapig, E., 1998. The effect of froth residence time on the kinetics of flotation. Minerals Engineering 11 (7), 627–638. Gourram-Badri, F., Conil, P., Morizot, G., 1997. Measurements of selectivity due to coalescence between two mineralized bubbles and characterization of MIBC action on froth flotation. International Journal of Mineral Processing 51, 197–208. Hanumanth, G., Williams, D., 1992. A three phase model of froth flotation. International Journal of Mineral Processing 34, 261–273. Johnson, N., McKee, D., Lynch, A., 1974. Flotation rates of nonsulfide minerals in chalcopyrite flotation processes. Transactions of Society of Mining Engineers, AIME 256, 204–209. King, R., 2001. Modelling and Simulation of Mineral Processing Systems. Butterworth-Heinemann, Oxford (Chapter 9). Klassen, V., Mokrousov, V., 1963. An Introduction to the Theory of Flotation. Butterworths, London (Chapter 5). Malinovskii, V., Matveenko, N., Knaus, O., Uvarov, Y., Teterina, N., Boiko, N., 1973. Technology of froth separation and its industrial application. In: Tenth International Mineral Processing Congress. Institute of Mining and Metallurgy, London, pp. 717–726. Mathe, Z., Harris, M., OÕConnor, C., 2000. A review of methods to model the froth phase in non-steady state flotation systems. Minerals Engineering 13 (2), 127–140. Moys, M., 1989. Mass transport in flotation froths. Mineral Processing and Extractive Metallurgy Review 5 (1–4), 203–228. Sadr-Kazemi, N., Cilliers, J., 2000. Technical note: a technique for measuring flotation bubble shell thickness and concentration. Minerals Engineering 13 (7), 773–776. Savassi, O., 1998. Direct estimation of the degree of entrainment and the froth recovery of attached particles in industrial flotation cells. Ph.D. Thesis., University of Queensland. Savassi, O., Alexander, D., Johnson, N., Franzidis, J.-P., Manlapig, E., 1997. Measurement of froth recovery of attached particles in industrial floatation cells. In: Sixth Mill OperatorsÕ Conference. Madang, Papua New Guinea, pp. 149–156. Savassi, O., Alexander, D., Franzidis, J.-P., Manlapig, E., 1998. An empirical model for entrainment in industrial flotation plants. Minerals Engineering 11 (3), 243–256. Schulze, H., 1984. Physico-Chemical Elementary Processes in Flotation. Elsevier, Amsterdam. Seaman, D., Franzidis, J., Manlapig, E., 2004. Bubble load measurement in the pulp zone of industrial flotation machines—a new device for determining the froth recovery of attached particles. International Journal of Minerals Processing 74 (1–2), 1–13. Smith, P., Warren, L., 1989. Entrainment of particles into flotation froths. Mineral Processing and Extractive Metallurgy Review 5, 123–145. Spedden, H., Hannan, W., 1948. Attachment of mineral particles to air bubbles in flotation. AIME Technical Publication, p. 2354. Vera, M., 2002. A touch of froth: how bubble–particle aggregates take the strain. Ph.D. Thesis, University of Queensland. Vera, M., Franzidis, J., 2003. A touch of froth: how bubble–particle aggregates take the strain. In: 35th Annual Meeting of the Canadian Mineral Processors, Canadian Institute of Mining, Metallurgy and Petroleum, pp. 621–638. Vera, M., Franzidis, J.-P., Manlapig, E., 1999a. Simultaneous determination of collection zone rate constant and froth zone recovery in a mechanical flotation environment. Minerals Engineering 12 (10), 1163– 1176.
D.R. Seaman et al. / Minerals Engineering 19 (2006) 841–851 Vera, M., Franzidis, J.-P., Manlapig, E., 1999b. The JKMRC high bubble surface area flux flotation cell. Minerals Engineering 12 (5), 477– 484. Vera, M., Mathe, Z., Franzidis, J., Harris, M., Manlapig, E., OÕConnor, C., 2002. The modelling of froth zone recovery in batch and continuously operated laboratory flotation cells. International Journal of Mineral Processing 64, 135–151. Warren, L., 1985. Determination of the contributions of true flotation and entrainment in batch flotation tests. International Journal of Mineral Processing 14, 33–44.
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Wilson, S., Stratton-Crawley, R., 1991. Design of production scale flotation columns using a first order kinetic model. In: Agar, G.E., Huls, B.J., Hyma, D.B. (Eds.), Column Õ91, Proceedings of an International Conference on Column Flotation, Sudbury, Ont., Canada, pp. 165–179. Woodburn, E., Austin, L., Stockton, J., 1994. A froth based flotation kinetic model. Transactions of the Institution of Chemical Engineers 72 (Part A), 211–226. Young, P., 1982. Flotation machines. Mineralogical Magazine 145 (1), 35– 39.
Selective transport of attached particles across the pulp–froth interface D.R. Seaman *, E.V. Manlapig, J.-P. Franzidis Julius Kruttschnitt Mineral Research Centre, University of Queensland, Isles Road, Indooroopilly, Queensland, Australia Received 13 July 2005; accepted 25 October 2005 Available online 20 December 2005
Abstract A technique for determining the recovery of attached particles across the froth phase in flotation that relies on measuring the rate at which bubble–particle aggregates enter the froth is used to investigate the selectivity of attached particles across the froth phase. Combining these measurements with those of other techniques for determining the froth recovery of attached particles provides an insight into the different sub-processes of particle rejection in the froth phase. The results of experiments conducted in a 3 m3 Outokumpu tank cell show that the detachment of particles from aggregates in the froth phase occurs largely at the pulp–froth interface. In particular it is shown that the pulp–froth interface selectively detaches particles from aggregates according to their physical attributes. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation; Selectivity; Detachment; Froth recovery
1. Introduction In the field of mineral flotation, flotation machines perform the task of separating (typically valuable) hydrophobic minerals from the (typically less valuable) hydrophilic particles. The froth phase separates bubble–particle aggregates from surrounding suspended material as well as upgrading the attached material on the bubble surfaces either by selective rejection of less hydrophobic mineral from the bubble surfaces or the selective re-attachment and/or displacement by more strongly hydrophobic minerals. Within froth flotation there is much literature on the topic of rejection of suspended/entrained material within the froth (Johnson et al., 1974; Bisshop and White, 1976; Warren, 1985; Savassi et al., 1998; Smith and Warren, 1989). However, the selectivity of the froth phase towards attached particles on the basis of their hydrophobicity is an area which has received little attention until now. *
Corresponding author. E-mail address: [email protected] (D.R. Seaman).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.020
An important measure of froth performance is the recovery of attached particles across the froth, namely froth recovery, defined as the fraction of particles entering pulp phase attached to air bubbles that are transferred to the concentrate (Cutting et al., 1981; Feteris et al., 1987; Finch and Dobby, 1990; Woodburn et al., 1994; Vera et al., 1999a; King, 2001). Several techniques are available to determine froth recovery, falling predominantly into two categories: (a) the use of empirical relationships observed between operating conditions and metallurgical performance (Feteris et al., 1987; Vera et al., 1999a), and (b) through the estimation or direct measurement of bubble loading (mass of attached particles per volume of air bubbles) below the pulp–froth interface (Seaman et al., 2004; Alexander et al., 2003; Savassi et al., 1997; Falutsu and Dobby, 1989). This paper presents a methodology in which results obtained using these different froth recovery measurement techniques are compared to investigate the selective nature of the froth phase with respect to mineralogy, and to provide an insight as to where in the froth phase this selectivity occurs.
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2. Attached particle selectivity Many researchers have tried to address the question of selectivity of attached particles across the froth phase either by analysis of the sub-processes occurring across the froth phase or through experimentation. In both cases, one needs to analyse the sub-processes occurring in the froth in order to determine which of these could result in the selective rejection of attached material or the re-attachment of particles previously detached in the froth phase. A froth that selectively rejects attached (or previously attached) particles on the basis of size, density or hydrophobicity is regarded as being selective with respect to attached material; this also implies that the mineral content of aggregates overflowing into the concentrate stream is different from those entering the froth. Selectivity of this nature has been observed in both industrial (Young, 1982; Seaman et al., 2004) and laboratory scale (Ata et al., 2002) flotation studies. However, in several other systems no apparent selectivity with respect to mineralogy has been found (Alexander et al., 2003; Savassi et al., 1997; Vera et al., 1999a), but only an apparent selectivity with respect to particle size. Four distinct sub-processes affect the detachment and re-attachment of particles in the froth phase: 1. 2. 3. 4.
bubble coalescence, particle detachment, particle drainage (of previously attached particles), and particle re-attachment.
2.1. Bubble coalescence In the upper regions of the froth there is a reduction in total available surface area of bubbles as they coalesce. When a specific lamella breaks on coalescence, the particles and fluid of the boundary ‘‘burst’’ and fall to the base of the new larger bubble. From here, the particles and water could drain back into the pulp zone, or remain entrained in the froth and be recovered to the concentrate; or, alternatively, some of the particles could re-attach to bubble surfaces. The likely fate of the now detached particles is discussed below in the sections dealing with drainage and re-attachment in the froth phase. The process of detachment upon coalescence is very unlikely to be selective towards particle type, as the rupture of bubble lamellae is instantaneous and all the particles attached to the bursting lamellae will fall to the base of the coalesced bubble. Gourram-Badri et al. (1997) conducted experiments that involved creating two bubbles in water, attaching particles of different hydrophobicity (sphalerite, pyrite and chalcopyrite) to them and then allowing them to coalesce. They found that the mineral content of the particles attached to the bubbles before and after coalescence of the two bubbles was different. In each case, the less hydrophobic mineral in the system, sphalerite, was preferentially detached
upon coalescence. It is highly unlikely that the detachment occurred at the ‘‘actual moment’’ of coalescence, but rather that the coalescence initiated bubble oscillation which has been reported to cause selective detachment (Cheng and Holtham, 1995; Schulze, 1984; Falutsu, 1994). This concept of bubble oscillation is discussed below in the section on froth selectivity. In any event, these experiments were conducted under conditions that more approximately represent the pulp phase than the current focus of the selective detachment in the froth. 2.2. Particle detachment Particles detach from bubble surfaces when sufficient force is exerted to separate the particle from the aggregate. The equilibrium forces acting on a suspended spherical particle attached at a fluid interface were investigated by Schulze (1984) to determine the amount of energy required to detach particles from aggregates. The equilibrium forces can be resolved for a variety of particle sizes, densities, hydrophobicities and interfacial surface tension. Schulze (1984) calculated the energy required to detach a particle from a bubble surface for a limited range of physical and chemical conditions. Fig. 1 uses this information to calculate the conditions under which particles are likely to detach from bubble surfaces. Using the balance of equilibrium forces on a spherical particle attached to a single bubble, the energy required to remove the particle completely from the bubble surface has been estimated for a range of particle diameters (0–160 lm) and hydrophobicities (0° 6 h 6 60°) typical in flotation. The calculation provides a range of detachment energies required to detach particles from bubble surfaces. It is clear from this figure that larger and more hydrophobic particles are more difficult to remove from bubble surfaces. However, this figure is only true for fully liberated minerals of spherical shape and should thus be used with caution in its application. Any deviation from these ideal spherical particles (non-spherical, not fully liberated and/or surface
Fig. 1. Variation of detachment energy of spherical particles with particle size and hydrophobicity; density of particles, qp = 7.5 g/cm3; fluid/slurry density, qfl = 1.1 g/cm3; bubble radius, Rb P 0.1 cm; interfacial surface tension, r = 72 dyne/cm.
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imperfections) will result in less energy being required to detach from a bubble surface. Klassen and Mokrousov (1963) identified six mechanisms by which energy can be supplied to bubble–particle aggregates in order to separate the particle from the bubble. These six scenarios are shown in Fig. 2. With reference to detachment occurring around the froth phase in flotation, the only mechanism shown in Fig. 2 likely to cause detachment is the impact of an aggregate with a stationary (or nearly stationary) object, namely the pulp–froth interface. The other mechanisms are more likely to occur within the pulp phase where there is more turbulence. A seventh event known to detach mineral particles from bubble surfaces is bubble oscillation. When an aggregate strikes another object (stationary or moving), the energy absorbed from the impact causes the bubble to oscillate. Klassen and Mokrousov (1963) show photographic evidence of these oscillations which were first observed by Spedden and Hannan (1948). Cheng and Holtham (1995) studied the effect of bubble-oscillation on particle detachment in an artificial flotation system by creating single bubble–particle aggregates that were then oscillated by means of a loud speaker. They found that the amount of detachment occurring was related to the frequency and amplitude of the loud speaker. It is hard to determine how the kinetic energy of an aggregate is released upon striking the pulp–froth interface. This energy will be dissipated in oscillation of the bubble as well as heat. The overall change in kinetic energy can, however, be calculated by considering the change in momentum of an aggregate as it decelerates upon reaching the pulp–
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froth interface; i.e. from a velocity of u1 in the pulp phase to u2 in the froth phase (where magg is the mass of the bubble–particle aggregate): 1 1 DE ¼ magg u21 magg u22 . 2 2
ð1Þ
The mass of the aggregate depends on the bubble loading in the pulp phase. The velocity of the aggregate in the pulp phase can be estimated through a drift flux analysis approach. King (2001, Chapter 9) provides an approach to estimate the terminal rise velocity of an aggregate, based on the bubble diameter, the mass loading of solids on the bubble, and the conditions in the slurry. Combined with an estimate of the rise velocity of bubbles above the pulp–froth interface (0.5 cm s1), the change in kinetic energy of aggregates as a function of bubble size and bubble loading is displayed in Fig. 3. Fig. 3 shows that the energy released when an aggregate decelerates is in the order of 0–0.02 erg, which is more than adequate to detach particles from bubble surfaces (see Fig. 1). The energy dissipation increases for larger bubble sizes and for more loaded bubbles. This energy can be dissipated in many forms, e.g. it can be absorbed by bubble oscillation, converted to heat, or it can detach particles from the bubble surface by accelerating the attached particles. It is impossible to determine in what way this energy will be dissipated with this type of collision, but there is a strong likelihood that a portion of this energy will be dissipated in a form that results in the selective detachment of particles from aggregates.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 2. Mechanisms of particle detachment (after, Klassen and Mokrousov, 1963). Forces tending to separate the mineral particle from the bubble: (a) during rise (accelerated or equilibrium) of a mineralised bubble surface; (b) under the action of liquid streams; (c) during sliding of particle along bubble; (d) with change in motion of a mineralised bubble; (e) during impact and attrition of particles in the pulp against a mineralised bubble surface and (f) during impact of a bubble with an obstacle.
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Fig. 3. Change in energy of an aggregate decelerating at the pulp–froth interface.
2.3. Particle drainage When particles detach in the froth phase, a portion of them drain back into the pulp phase. The drainage of previously attached particles back into the pulp phase is selective with respect to particle size and density, much like the entrainment process. The particles that were previously attached to bubbles that are recovered by this entrainment process in the froth phase are included in the calculation of froth recovery (which does not distinguish between attached and previously attached particles). Since the drainage of these particles back into the pulp phase is selective, the resulting froth recovery must be selective on a size and density basis. Following the same principles governing entrainment, the mechanism of particle drainage of previously attached particles results in the froth recovery of finer, less dense minerals being greater than that of coarser, more dense minerals. Moys (1989) acknowledged the selective drainage of previously attached particles in the froth phase, and proposed that the drainage of these particles is proportional to their concentration in each size class. Mathe et al. (2000) as well as Vera et al. (2002) went on to provide an empirical model for froth recovery incorporating the concept of froth retention time, FRT proposed by Gorain et al. (1998). 1 Rf ðiÞ ¼ expðbFRTÞ þ ½1 expðbFRTÞ . 1 þ wi FRT ð2Þ The parameter b is related to the rate of detachment of attached particles in the froth phase, and xi is the rate of drainage of detached particles of size class i. Eq. (2) shows that larger froth retention times result in a lower froth recovery, because the aggregates have more chance to detach through loss of surface area by coalescence, and also more time to drain from the froth phase back in to the pulp. 2.4. Re-attachment in the froth Particles that are removed from bubble surfaces in the froth phase through any of the detachment mechanisms (e.g. bubble coalescence, bubble oscillation etc.) may re-
attach to bubbles lower down in the froth phase. They could re-attach either by displacing less strongly attached particles or by attaching onto uncovered bubble surfaces in the lower regions of the froth (Ata et al., 2002) where the particle loading per bubble surface area is lower. The residence time in the froth phase (particularly of columns) is relatively high in comparison to the pulp phase. This allows for much longer times for unattached particles to be in contact with fresh bubble surface. Ata et al. (2002) added particles of known hydrophobicity and size to the surface of a column froth. They showed that the more hydrophobic particles attached themselves to bubbles in the froth and were collected in the concentrate stream. It is unlikely that these particles were recovered by entrainment due to the high water bias velocity employed in the froth phase. In addition, they showed that the more surface area available for attachment in the froth, the greater the recovery of the particles introduced to the top layers of the froth. This selective attachment of (hydrophobic) particles in the froth phase was well known in the former Soviet Union where froth separation was introduced in the early 1970s (Young, 1982; Ata et al., 2002; Malinovskii et al., 1973). Young (1982) showed that much larger particles can be floated by this technique—up to 3000 lm—than by conventional flotation. 2.5. Summary of sub-processes in the froth phase From the above analysis, it seems very likely that the froth recovery of attached particles in a flotation cell is selective based on the particle size, density and hydrophobicity (encompassing liberation characteristics as well as surface chemistry). It is hard to estimate which of the sub-processes has the largest impact on froth recovery as little work has been done to date to isolate these subprocesses in an industrial environment. A summary of the identified sub-processes and their impact on the selectivity of attached particles is presented below: 1. Bubble coalescence is unlikely to be selective in terms of detaching particles attached to bubbles in the froth phase. However, coalescence is likely to initiate other forms of detachment which may in fact be selective (e.g. bubble oscillation, see below). 2. Particle detachment has been shown to be a function of particle size, density and hydrophobicity as well as operating conditions in the flotation cell. In particular, it is likely that selective particle detachment occurs at the pulp–froth interface (due to the change in momentum of aggregates striking the interface) as well as during bubble oscillations initiated either by the aggregates striking the pulp–froth interface or when bubbles coalesce. It is hard to determine whether smaller or larger particles would be selectively detached, but all else being equal, less strongly attached particles will be preferentially detached.
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3. Particle drainage of previously attached particles is certainly a selective process, particular with respect to size and density. In general, larger and more dense particles will drain faster than finer and less dense particles. It is thus expected that through this mechanism of selective froth recovery, larger particles of the same mineral type will have a lower froth recovery. 4. Particle re-attachment has been shown to occur in flotation froths. More hydrophobic particles have a greater probability of being re-attached in the froth phase. Like the pulp phase of flotation cells, it is hard to predict what size of particles is more likely to be collected. Some researchers (Vera et al., 1999a; Savassi et al., 1997) in the past have investigated (either directly or indirectly) the selective nature of the froth phase using different techniques for determining the froth recovery of attached particles. They found no significant selectivity of attached particles across the froth phase. Fig. 4 compares the froth recovery of chalcopyrite with that of pyrite in the JKMRC 16L High Sb cell (Vera et al., 1999b) treating a copper rougher ore under a range of operating conditions. Fig. 4 clearly shows no selectivity between these two minerals in terms of froth recovery; the conclusion made by Vera et al. (1999a) is that froth recovery is a non-selective process. However, evidence for froth selectivity on a size basis has been found: Fig. 5 shows froth recovery determined on a size by size basis for various flotation cells down a rougher bank at Mount Isa (Savassi, 1998). It is clear that the froth phase is exhibiting selective froth recovery in terms of particle size. The most probable mechanism for the decrease in froth recovery with increasing particle size is the drainage of previously attached particles. A similar result was shown by Vera (2002) on secondary analysis of the work of Contini et al. (1988). The present authors (Seaman et al., 2004) have found evidence of froth selectivity of attached particles with the use of a technique to measure directly the bubble loading
Fig. 5. Froth recovery on a size basis for different flotation cells in the rougher bank at Mount Isa (after Savassi, 1998).
of particles in the pulp phase, by comparing the grade of these particles with those found in the top layer of the froth phase (Sadr-Kazemi and Cilliers, 2000; Vera and Franzidis, 2003). This measurement technique is used below to provide a further evaluation of the sub-processes occurring across the pulp–froth interface. In summary, the selective nature of the sub-processes occurring across the froth phase cannot be ignored. For most of the sub-processes, the selectivity appears to be towards more valuable minerals (generally having a higher hydrophobicity). A deeper understanding of these mechanisms may lead to improved machine operation and/or design which take these processes into account. 3. Experimental details Experiments were carried out on a portable pilot scale Outokumpu 3 m3 flotation cell operated at Newmont Golden Grove Operation. The cell was fed with the rougher feed stream to the flotation circuit. Two different ore types, a copper/lead/zinc rich ore (prior to sphalerite activation) and a copper rich ore, were investigated. The purpose of conducting these tests was to compare two different froth recovery measurement techniques and to investigate whether any difference between the two measurements would allow conclusions to be made about the sub-processes occurring in the froth phase. 3.1. Froth recovery measurement
Fig. 4. Froth recovery of chalcopyrite versus that of pyrite for a copper rougher ore in a 16L continuous flotation cell (after Vera et al., 1999a).
3.1.1. Changing froth depths The first froth recovery measurement technique employed was the changing froth depths method, initially proposed by Feteris et al. (1987) for flotation columns and later adapted to mechanical flotation cells by Vera et al. (1999a). This approach requires a flotation cell to be operated continuously at different froth depths and surveyed at steady state. The overall first order rate constant
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collection whilst still maintaining a positive water bias flow in the sampling tube (to prevent the introduction of entrained material). The set-up of this chamber in a mechanical flotation cell is shown schematically in Fig. 7. The calculation of froth recovery using these bubble load techniques includes the transfer of aggregates across the pulp–froth interface. This distinction makes it possible to isolate effects occurring across the pulp–froth interface, which forms the remaining focus this paper. 3.2. Survey details and data analysis
Fig. 6. Changing froth depth technique for determining froth zone recovery (after Vera et al., 1999a).
of attached particles, k, is determined for each condition, and plotted against froth depth as shown in Fig. 6. It is assumed at a froth depth of zero that the overall rate constant is equal to the collection zone rate constant, kc, due to the absence of a froth zone. Froth zone recovery, Rf, is calculated as the ratio between the collection zone rate constant and overall rate constant (Vera et al., 1999a): Rf ¼
k . kc
ð3Þ
For each ore type, the OK 3 m3 cell was operated at four different froth depths (in a random order) with the aeration rate to the cell kept constant. After each froth depth change, the cell was allowed four residence times to reach steady state. For each condition, the major streams to the cell were sampled over a 30 min period with three cuts of each stream composited. Feed and concentrate flowrate estimates were made by timing the rate at which the cell initially filled and taking timed samples of the entire concentrate stream. At each froth depth, the bubble load device (see Fig. 7 above) was used to measure the bubble loading of aggregates and hence the rate at which attached material rises to the pulp–froth interface.
This approach considers only events occurring in the dryer regions of the froth phase as the changes to froth depth are made above the pulp–froth interface. 3.1.2. Bubble loading in the pulp zone The second froth recovery measurement technique employed is through the direct measurement of bubble loading in the pulp zone. Savassi et al. (1997) developed special measuring devices to capture samples from the pulp zone of flotation cells containing different proportions of bubble–particle aggregates and suspended material. A mass balance across these samples and a sample of the concentrate was used to determine the bubble loading of aggregates in the pulp zone as well as their mineral content, and froth recovery. Seaman et al. (2004) followed the approach of Falutsu and Dobby (1992) in developing a new technique to isolate aggregates from the suspended pulp and hence determine the bubble loading of the aggregates. The technique requires the use of a bubble load collection chamber suspended above the froth phase. The chamber has a sampling tube which is inserted into the quiescent area of the pulp zone, filled initially with a water–frother mixture (to prevent coalescence in the sampling tube). Aggregates are then allowed to rise up the tube and into the chamber where they are dispersed to the outside of the chamber by a deflector cone. Bubbles burst at the air/liquid interface and particles are collected in the base of the chamber. A vacuum pump may be used to prolong the collection time by removing some of the air entering the chamber during
Fig. 7. Schematic representation of the bubble load chamber used to isolate bubble–particle aggregates (Seaman et al., 2004).
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Because froth recovery deals only with ‘‘true floating’’ (attached) material, the contribution to recovery by entrainment needs to be discounted. This was done through the use of an entrainment factor, ENTi, based on the slowest/non-floating mineral species identified in the concentrate stream. The approach is described in more detail by Johnson et al. (1974) and Savassi et al. (1998). Briefly, the slow or non-floating mineral is assumed to be recovered only by entrainment, and the calculated ENTi values are applied to all other minerals on a size by size basis. All samples were weighed wet and dry and analysed for Cu, Pb, Zn, Fe, Ag and Au from which average mineral compositions were determined. One of the sets of data was sized and assayed for the same elements.
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sulphide gangue with fast floating sulphide minerals than that of sphalerite or pyrite. It can be seen that the froth recoveries determined using the changing froth depth technique are similar for all the minerals (save for the mineral used to calculate entrainment). The froth recoveries determined using the bubble load measurement technique are however very different. This implies that most of the selectivity occurs across the pulp–froth interface where there is a significant amount of loss of attached particles. This difference observed between the froth recovery measurement techniques can be manipulated to provide more detailed information on events occurring across the pulp–froth interface. 4. Three zone flotation cell description
3.3. Froth recovery results
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In light of the possible selective froth recovery of attached particles, a three zone flotation cell description is proposed (pulp zone, pulp–froth interface and froth zone) to provide insight into the different mechanisms occurring across the froth phase as a whole (Fig. 10). This three zone description has been proposed previously by Hanumanth and Williams (1992) for the purpose of developing a more descriptive flotation model. In the discussion below, the description is used in conjunction with different froth recovery measurement techniques to resolve the flow of material between the three zones. The representation of material flows within and around the flotation cell is similar to that presented by Wilson and Stratton-Crawley (1991) for a two-zone flotation cell model. It is important to note in Fig. 10 that material dropping out from the pulp–froth interface returns to the pulp zone, essentially as an additional feed stream. In the same way, attached material dropping out of the froth zone does not return directly into the pulp zone, but rather as an additional feed to the pulp–froth interface. This description allows for the possibility of previously attached material to become re-attached or entrained within the pulp–froth interface and return to the froth zone. The model has an
Froth Recovery of Chalcopyrite (%)
Froth Recovery of Chalcopyrite (%)
Fig. 8 shows the froth recoveries of chalcopyrite determined using the changing froth depths technique (CFD) and the bubble load measurement technique (BLM) for (a) the copper/lead/zinc ore and (b) the copper rich ore in the Outokumpu 3 m3 cell. As has been observed previously (Seaman et al., 2004), the froth recovery determined using the changing froth depth technique is much greater than that determined using the bubble load measurement technique in the same situation. This is because the changing froth depth technique does not take the pulp–froth interface into account, whereas the bubble load measurement technique does. Fig. 9 shows the froth recoveries for all of the minerals determined using the two techniques on the different ore types. The zero froth recoveries shown in the figure for sphalerite (for the copper/lead/zinc ore) and pyrite (for the copper ore) are an artefact of using the slowest floating mineral to determine the contribution to recovery by entrainment (discussed previously in this section). The sphalerite and pyrite presented slower kinetics than the non-sulphide gangue. This is most likely due to a greater association of non-
CFD BLM
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Fig. 8. Froth recovery of chalcopyrite determined using two different measurement techniques: CFD—changing froth depths, and BLM—bubble load measurement, for two different ore types in the Outokumpu 3 m3 flotation cell. (a) Copper/lead/zinc rich ore and (b) copper rich ore.
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Fig. 9. Comparison of froth recovery measurement techniques using different feed ores in a per mineral basis. (a) Changing froth depths—copper/lead/ zinc ore, (b) changing froth depths—copper rich ore, (c) bubble load measurement—copper/lead/zinc rich ore and (d) bubble load measurement—copper rich ore.
all recovery of particles by true flotation can be determined as: Rc Rpfi Rf . ð4Þ R¼ 1 Rc þ Rpfi ðRf þ Rc 1Þ Based on the measurement techniques used to determine froth recovery, and an appreciation for whether or not the pulp–froth interface is included in the measurement, the recoveries across each zone can be determined. The froth recovery determined using the bubble load measurement technique, Rf,h, includes the pulp–froth interface in the determination of froth recovery and can therefore be expressed in terms of the streams shown in Fig. 10 as: xRpfi Rf Rpfi Rf . ð5Þ ¼ Rf;h ¼ Rc 1 Rpfi ð1 Rf Þ
Fig. 10. Three zone description of a flotation cell showing the flow of attached and previously attached material.
additional reflux stage in the flotation system than the more traditional two-zone descriptions. A simple mass balance allows for the determination of all flows of material based on the recoveries of each zone: Rc, the collection zone recovery; Rpfi, the pulp–froth interface recovery; and Rf the froth zone recovery of material above the pulp–froth interface. From this figure, the over-
Since the froth recovery determined using the changing froth depths approach, Rf,cfd does not incorporate the pulp–froth interface, it can be assumed that this measurement is equivalent to the froth recovery, Rf shown in the figure (i.e. Rf,cfd = Rf). The pulp–froth interface recovery, Rpfi can then be determined from Eqs. (4) and (5): Rpfi ¼
Rf;h . Rf;h ð1 Rf;cfd Þ þ Rf;cfd
ð6Þ
The pulp–froth interface recovery on a per mineral basis was determined through Eq. (6) for the two systems shown previously in Fig. 9. Fig. 11 shows the results.
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Fig. 11. Pulp–froth interface recovery per mineral for the two different feed ore types. (a) Copper/lead/zinc ore and (b) copper rich ore.
For both feed types, it is apparent that the pulp–froth interface recovery is certainly significant. In each case, the more hydrophobic (faster floating) minerals appears to have a higher Rpfi than the less hydrophobic minerals.
CP Gal Sphal Py NSG
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Samples collected from the copper/lead/zinc rich ore were sized and assayed in order to determine the froth recovery and pulp–froth interface recovery on a size-bysize basis. The froth recoveries determined using both techniques were used to determine the pulp–froth interface recovery shown in the previous section. Fig. 12 shows the froth recovery by size for all minerals at one of the test conditions with the copper/lead/zinc rich ore. Like other authors previously (Contini et al., 1988; Vera et al., 1999a; Savassi et al., 1998) a decrease in froth recovery is observed with an increase in particle size. In order to evaluate the recovery of attached particles across the pulp– froth interface, the pulp–froth interface recovery on a size basis for the same test as in Fig. 12 is shown in Fig. 13.
20 CP Gal Sphal Py NSG
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Particle size (μm) Fig. 12. Froth recovery determined using the bubble load measurement technique on a size by mineral basis for the copper/lead/zinc rich ore at a froth depth of 132 mm in the OK 3 m3 cell.
Rpfi (%)
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5. Froth recovery by size
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As with the overall froth recovery results shown in Fig. 12, the pulp–froth interface recovery of weaker hydrophobic minerals is lower than that of the more strongly hydrophobic minerals. In addition, the pulp–froth interface recovery decreases with increasing particle size. These observations are consistent with the expectation that less strongly attached particles will detach preferentially to those that are more strongly attached. The review of detachment energy showed that larger particles require more energy to detach from bubbles due to their higher contact area with the bubble surface. This determination of detachment energy makes the assumption that (a) the particles are spherical, and (b) that the particles are fully liberated. Neither of these assumptions are likely to be true in a real system. Furthermore, the pulp–froth interface recovery includes the recovery of previously detached particles draining out of the froth interface. Larger particles will drain faster through the pulp–froth interface, this is probably the more significant sub-process with relation to particle size. It is impossible to identify which of the sub-processes is dominant in the pulp–froth interface (selective detachment
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or selective re-attachment). It is likely that sub-processes occur with an overall effect of the pulp–froth interface selectively rejecting larger and less hydrophobic particles. 6. Conclusions This paper shows how different froth recovery measurement techniques can be combined to successfully model a flotation cell as three distinct zones. This model can be used to assess the performance of the froth phase and the pulp– froth interface independently of each other. The model is also useful in the qualitative understanding of particle detachment and could be utilised in the analysis of performance of flotation machines to investigate where particles belonging to a specific class are rejected. Selective transfer of attached particles across the froth phase in terms of their size and mineral content has been identified. It has been shown that the pulp–froth interface is responsible for a large degree of upgrading of the particles attached to bubble surfaces and also for a significant proportion of the recovery loss across the froth phase as a whole. Acknowledgements The authors of this paper wish to express their gratitude to the people involved in the testwork, Brigitte Comley and Michael Wortley. They also express their thanks to the site at which the testwork was conducted, Newmont Golden Grove Operations and to the personnel involved, Ashley Kidd and Kathryn Ellis. Peter Bourke from Outokumpu provided the OK 3 m3 cell required to complete this testwork for which the authors are grateful. Finally, the authors wish to acknowledge the financial support of the sponsors of the AMIRA P9 project, without which this work would not have been possible. References Alexander, D., Franzidis, J.-P., Manlapig, E., 2003. Froth recovery measurement in plant scale flotation cells. Minerals Engineering 16, 1197–1203. Ata, S., Ahmed, N., Jameson, G., 2002. Collection of hydrophobic particles in the froth phase. International Journal of Mineral Processing 64, 101–122. Bisshop, J., White, M., 1976. Study of particle entrainment in flotation froths. Transactions of the Institute of Mining and Metallurgy 85, C191–C194. Cheng, T., Holtham, P., 1995. The particle detachment process in flotation. Minerals Engineering 8 (8), 883–891. Contini, N., Wilson, S., Dobby, G., 1988. Measurement of rate data in flotation columns. In: Sastry, K. (Ed.), Column Õ88. AIME, Littleton, USA, pp. 81–89 (Chapter 10). Cutting, G., Watson, D., Whitehead, A., Barber, S., 1981. Froth structure in continuous flotation cells: relation to the prediction of plant performance from laboratory data using process models. International Journal of Mineral Processing 7, 347–369. Falutsu, M., 1994. Column flotation froth characteristics—stability of the bubble–particle system. International Journal of Mineral Processing 40, 225–243.
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