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Case studies on the performance and characterisation of the froth phase in industrial flotation circuits
Minerals Engineering 19 (2006) 774–783 This article is also available online at: www.elsevier.com/locate/mineng
Case studies on the performance and characterisation of the froth phase in industrial flotation circuits G. Tsatouhas c, S.R. Grano a
b
a,*
, M. Vera
b
Ian Wark Research Institute, University of South Australia (The ARC Special Research Centre for Particle and Material Interfaces), Mawson Lakes Campus, Adelaide, South Australia 5095, Australia Oil Sands Technology Group—Extraction and Separation, Calgary Research Centre, Shell Canada Limited, 3655-36th St, NW Calgary, Alberta, Canada 2L 1Y8 c Huntsman Australia, Performance Products, Gate 2 Newsom Street, Ascot Vale Victoria 3032, Australia Received 15 July 2005; accepted 9 September 2005 Available online 26 October 2005
Abstract This paper deals with two separate case studies investigating the froth phase performance and characterisation of two industrial rougher/scavenger flotation circuits. Froth phase performance was quantified using a mass balance approach to estimate froth zone recovery. Measured characteristics of the froth phase included frother solution concentration determined by gas chromatography, and the time taken for an equilibrium froth sample to decay to one-half of its original froth height. The latter measurement is referred to as the Ôfroth half-lifeÕ and is strongly linked to froth stability. Special methods and techniques developed to preserve frother in solution and to measure froth half-life are briefly described. The frother type in the first case study was a mixture of straight and branched alcohols, whilst the frother type in the second case study was a mixture of alcohols, aldehydes and triethoxybutane. The first case study focussed on a flotation circuit treating a low grade ore containing only a small fraction of floatable copper sulphide minerals, while the second case study focussed on a flotation circuit treating a higher grade complex sulphide ore containing significant quantities of chalcopyrite, galena, sphalerite and pyrite. It was found that froth zone recovery of valuable mineral generally decreased down-the-bank of the two industrial rougher/scavenger circuits. Moreover, decreases in froth zone recovery significantly limit the overall cell recovery of valuable mineral achievable from the plant scavenger cells. However, the decrease in froth zone recovery could not be linked to the removal of frother from the pulp solution to the concentrate product in the preceding rougher flotation stages. Measurements of residual frother in solution suggested that, approximately, only 5–10% of the added frother was removed into the rougher/scavenger concentrate, with the remainder appearing in the scavenger tailings. This finding suggested there was apparently adequate frother in solution in the scavenger stages. There was, however, a correlation to the froth half-life, with the froth half-life also generally decreasing down-the-bank. A simple, empirical model, based on the froth half-life and froth residence time of gas, is proposed here to predict froth zone recovery. Further, it is proposed that the froth stability, as measured by the froth half-life, is strongly linked to the presence of particles in the froth, with poorly mineralised scavenger froth characterised by a short half-life and, potentially, a low froth zone recovery. The importance of particles on froth stability was confirmed in separately conducted laboratory experiments. These experiments also demonstrated the wide variation in froth stability behaviour between different frother types. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation froths; Flotation frothers; Sulphide ores
*
Corresponding author. E-mail addresses: [email protected] (G. Tsatouhas), [email protected] (S.R. Grano), [email protected] (M. Vera). 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.033
1. Introduction In flotation, the transport of hydrophobic particles attached to bubbles from the collection zone of the cell,
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
where particle collection by bubbles occurs, to the concentrate launder is of key importance. Transport takes place via a froth phase in froth flotation. Frother surfactants are principally added to a flotation pulp so that a transient, mineralised froth is formed at the top of the flotation cell, forming the so-called froth phase. A transient froth is a prerequisite for the successful transport of mineral laden bubbles from the pulp zone/froth zone interface to the concentrate launder. It is desirable that the mineralised froth breaks down (i.e., the bubbles collapse or coalesce), once the froth reports to the concentrate launder. A froth which is overly unstable may cause significant loss of valuable mineral from the froth to the pulp phase and, subsequently, to the cell tailing stream. Conversely, an overly stable froth will cause problems in pumping, and may adversely impact mineral separation in subsequent flotation stages. Our research here is focussed on controlling froth stability to allow particle transport to take place, while also ensuring the froth breaks down once it enters the concentrate launder. The recovery of particles attached to bubbles across the froth phase is referred to as froth recovery. Froth recovery can be defined in several ways. However, it is now accepted that the recovery of attached particles across the froth zone is best expressed in terms of the first order rate constants (Savassi et al., 1997): Rf ¼
k kc
ð1Þ
In Eq. (1), Rf is the fractional froth recovery, k is the overall flotation cell first order rate constant, treating the collection and froth zones as a single phase, and kc is the collection zone rate constant. It is noted that entrained particles are included in this particular definition. Entrainment is the phenomena of the recovery of mineral particles, which are not attached to air bubbles, by their unselective recovery with water (Johnson et al., 1974). However, in the context of this current paper, which principally focuses on hydrophobic value minerals, the contribution by entrainment is expected to be only minor. This definition also provides an experimental approach to the measurement of froth recovery in which the overall flotation cell rate constant is determined as a function of froth height, or the froth residence time (FRT) of gas (Laplante et al., 1983; Vera et al., 1999). The froth residence time of gas is defined as FRT ¼
FD Jg
ð2Þ
In Eq. (2), FD is the froth depth (cm) and Jg is the superficial gas velocity (cm/s), which is the gas flow per unit cross-sectional area of the flotation cell. In this approach, the overall flotation cell first order rate constant is measured as a function of froth residence time, with the collection zone rate constant, kc, determined by extrapolation to zero froth depth. The ratio of the overall flotation cell first order rate constant, at any froth depth (FD), to the collection zone rate constant defines the froth recovery at that
775
froth depth according to Eq. (1). This particular method, termed the mass balance approach, is used in the current study to determine froth recovery of the plant cells. It is recognised that in this approach, the froth recovery at zero froth height is assumed to be 100%. This may not be the case due to bubble bursting at the top of the froth, or inefficiencies in the lateral transport of particles in the froth, even at very shallow froth depths. However, for industrial cells it is an expedient approach, in the first instance, to obtain an estimate of froth recovery. Both the physical scale of the froth (defined here by FRT) and froth type, exemplified by the froths stability, may control the froth recovery of attached particles. Froth residence time and froth height characterise the physical scale of the froth, whilst the frother type, frother concentration as well as the particle characteristics in the froth (e.g., particle size, particle contact angle, particle loading in the froth) may all control froth stability (Schwarz et al., 2002). A decrease in the carrying capacity of the froth, and the possibility that some mineral particles may become detached and be lost from the froth, has been observed from froth recovery measurements conducted at plant scale where there is continuous removal of froth in successive flotation cells (Savassi et al., 1997). The decrease in froth recovery seems particularly important in the scavenger flotation stages, where the froth has apparently low stability. In contrast, froth recovery in the initial stages of rougher flotation of a continuous separator is usually higher, and the froths apparently more stable. This particular aspect of the operation of flotation froths is the principal focus of this paper. It is the purpose of this paper to determine if decreases in froth recovery in a continuous, industrial flotation separator may be linked to changes in froth stability, and to determine reasons for the change in froth stability with flotation cell number. Froth stability is used here to describe the general phenomena of thin film rupture, bubble coalescence and loss of froth volume. In this work, both the effect of particles contained within the froth, as well as frother type and concentration on froth stability has been investigated. In recent years there has been strong interest in modelling froth zone performance in laboratory flotation cells, in scale-up to a continuous flotation process, and in froth phase assessment in plant cells. Froth phase performance is better understood here in terms of froth recovery, Rf, and hence a specific objective of this work is to identify how physical factors, such as froth height, and chemical factors, such as frother concentration in solution and froth solids loading, control froth recovery. The exponential decay of froth recovery with an increase in froth retention (residence) time is well known and useful in terms of froth modelling (Gorain et al., 1998): Rf ¼ 100 expðbFRTÞ
ð3Þ
In Eq. (3), b is a parameter, which is arguably related to the rate at which bubbles coalesce and burst, or simply a parameter, which could depend on the physical and chemical froth factors. An expansion of the Rf–FRT relationship to account
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for the influence of froth stability is used here to describe a semi-empirical model to predict froth phase performance in plant cells. 2. Experimental 2.1. Sampling and froth recovery measurements A series of comprehensive sampling surveys were performed at two different major sulphide mineral concentrators located in Australia (Cu circuit—Circuit A) (Cu– Pb–Zn circuit—Circuit B). In the feed to Circuit A, copper occurred in bornite and chalcopyrite while there was only minor pyrite (Table 1). In the feed to Circuit B, copper occurred in chalcopyrite, lead in galena and zinc in sphalerite. The two flotation circuits treat ores with significantly different head mineralogy (Table 1), which has implications for the stability of the froth in both plants. It is noted that copper recovery and rate constant were used to define froth recovery in Circuit A, whilst lead recovery and rate constant were used to define froth recovery in Circuit B. In each case, the objective was to assess froth zone performance, with the sampling conducted in Circuit A in 2001 and Circuit B in 2002, respectively. Briefly, plant flotation cells in the rougher and scavenger banks were operated at different froth depths classified as shallow, intermediate (baseline) and deep, corresponding to short, intermediate and long froth residence times, respectively, through Eq. (2). In some cases, and at specific froth depths, the flotation cells were also operated at different airflow rates. However, the flotation rate constant and froth recovery data presented here are at a fixed airflow rate for simplicity sake. In addition to the comprehensive sampling surveys, the metallurgical performance of a single flotation cell in the lead/copper rougher flotation bank of Circuit B was also investigated. Three different froth depths, three different air flow rates and three different frother concentrations were employed in the flotation bank in these investigations. The frother concentration in Circuit B was varied from the baseline plant frother addition rate to high and low additions, in order to determine the overall effect of frother concentration on froth recovery. The flotation rate constant, k (min1), and froth recovery, Rf, were calculated from the recovery of copper (Circuit A) and lead (Circuit B) for each of the cell operating conditions used in the test programme, assuming perfect mixing in the flotation cell: k¼
R t ð1 RÞ
ð4Þ
In Eq. (4), R is the fractional recovery of copper or lead, and t is the effective mean residence time (minutes) of the pulp in the flotation cell. The objective of the flotation bank surveys was to characterise froth performance down-the-bank under standard operating conditions, and to obtain a data set for a single flotation cell under different operating conditions in Circuit B only. The data is used to explore relationships between performance and conditions, i.e., between froth recovery and froth depth, air flow rate and frother concentration. The surveys involved collection of samples from all the major streams down the respective rougher and scavenger flotation banks. Sample collection was performed over a 2-h period, during which four increments were taken at each sampling point. The samples collected from each cell during each survey, included: (a) the feed to the flotation bank (i.e., feed to rougher cell 1), (b) the combined concentrate of the flotation bank, (c) the tailings from the flotation bank, (d) timed lip concentrates from individual cells in the bank, and (e) tailings from each cell in the tested bank. Pulp chemical surveys were also performed in parallel with the main metallurgical surveys to determine frother concentrations in solution in major streams, as well as the distribution (or recovery) of frother down-the-bank. This information is used to complement the metallurgical assessment. Surveys involved collection of samples from all the major streams, which was also extended to both the rougher and cleaner banks of both circuits. Frother in solution in aqueous samples was measured using gas chromatography with flame ionisation detection. Frother balances were made using all the measured chemical assay data, % solids, measured frother assays and, taking as the reference, the feed to the flotation bank. Flowrates of the various slurry streams were obtained by computation. Individual adjustment of the standard deviations of frother assays was necessary to aid the balance and to check the quality of the data. Preliminary work showed that the relevant frothers were stable in solution after sampling, provided the samples were stored in the absence of oxygen and at temperatures less than 3 °C. This study used three different frothers—plant frother A (Circuit A), a mixture of low molecular weight alcohols, aldehydes and esters; plant frother B (Circuit B), a triethoxybutane and alcohol mixture, and MIBC (methyl isobutyl carbinol). All frothers were used as received. 2.2. Froth stability methods Froth stability generally refers to the foam decay process, during which time there is no further generation of
Table 1 Head mineralogy of the feeds to Circuits A and B Circuit
A B
Mineral Chalcopyrite
Bornite
Galena
Sphalerite
Pyrite
Non-sulphide gangue
1.2 0.7
1.8 –
– 1.3
– 21.7
0.9 29.4
96.1 46.9
Note: Non-sulphide gangue includes silicates and alumino-silicates.
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
gas bubbles. Froth decay is indicated by some visible occurrence such as froth volume variation, bubble collapse, or change in the bubble size. In order to quantify the global change of the froth, froth volume variation is recorded as a function of time. The faster the change, the less stable the foam. The simplest method of estimating the stability of a foam is to measure the change in volume (or height, Hf), for constant cross-sectional area with time, t. The initial time is taken when there is negligible further increase in froth volume, and the gas flow is then discontinued. The froth formed was then allowed to decay and the change in froth height noted as a function of decay time. A differential equation was then fitted to describe the rate of foam collapse. According to Iglesias et al. (1995): dH f k ¼ t dt
ð5Þ
In Eq. (5), k is an empirical constant. Iglesias et al. (1995) noted that plotting the froth height against log (t) yielded a series of straight lines for a range of surfactant types and foam stabilities, justifying in part Eq. (5). Furthermore, Iglesias et al. (1995) plotted HH of against log
t t1=2
, which nec-
essarily caused all the straight-line relationships to converge to HH of ¼ 0:5 at t = t1/2. In the current study, a similar approach was adopted using Eq. (6): Hf t ¼ a log ð6Þ t1=2 Ho In Eq. (6) a is dimensionless empirical constant. The halflife is based on the height value and the reference is taken as the time, t1/2, for the foam to decrease to one-half of its original height, Ho. A series of experiments was designed in both plant and laboratory flotation cells. Plant tests were performed in situ of different rougher cells (cells 1 and 4) of the lead/copper rougher flotation bank in Circuit B to highlight differences in froth stability, due to differences in particle type and solids loading in the froth. A 500-mL cylindrical column (1.5 m long, 80 mm diameter) was immersed in each cell between the froth surface and just below the pulp/froth interface and the froth collected over 2 min using plant air conditions. The equilibrium foam height was recorded and the gas
777
flow discontinued by means of a sliding partition at the base of the column. The froth was then allowed to decay and the change in froth height noted as a function of time. Froth decay experiments were also carried out in a graduated multi-holed sparger column (1.5 m long, 2.5 cm i.d.) (Schwarz et al., 2002), using plant flotation feed slurry for Circuit B only. The slurry sample was collected prior to reagent addition in the plant. The particle surface chemistry was controlled using reagent (collector and frother) additions in the laboratory with the slurry entering the column diluted to 10% solids weight/slurry weight. The experimental approach described previously was used to investigate the decay process at controlled air (400 mL/ min) and feed flow rates, which were adjusted to optimise the froth residence time. Dynamic measurements were made, both with and without particles, at 8, 20 and 40 ppm of the plant frother concentration, as well as with MIBC used as the reference surfactant. 3. Results 3.1. Plant froth zone recovery 3.1.1. Circuit A Table 2 presents a summary of parameters measured in individual cells of Circuit A, under the same reagent addition rates and during the survey period. The overall cell rate constant decreased down-the-bank due to both decreases in the collection zone rate constant and froth zone recovery. While our attention here is principally on froth zone recovery, it is worthwhile discussing the significance of the collection zone rate constant at this point. The collection zone rate constant decreases down-the-bank due to the removal of fast floating components in the initial stages of flotation, the rate constant being an undistributed rate constant of all floating components. In the scavenger stages only the slow floating components remain, being the less hydrophobic value mineral such as coarse composites. This is reflected in the decreasing copper assay of the froth down-the-bank (Table 2). Both the decreasing grade (hydrophobicity) of the particles entering the froth, as well as the decreasing solids loading in the froth (Table 2), have consequences on the stability of the froth as discussed further below.
Table 2 Cell characterisation summary and froth parameters measured in Circuit A Plant cell
Superficial gas velocity (cm/s)
Rate constant at FD = 13 cm (min1)
Froth recovery at FD = 13 cm (%)
Froth (gas) residence time at FD = 13 cm (s)
Solids loading in froth (%)
Copper grade in froth (%)
Roughers
1 2 3 4
0.99 1.19 0.84 0.96
0.39 0.44 0.18 0.20
62 54 53 52
13.1 10.9 15.5 14.1
48.2 49.4 48.0 40.0
42.5 39.7 31.1 18.8
Scavengers
1 2 3 4
0.64 0.72 0.56 0.68
0.06 0.12 0.09 0.06
29 25 28 23
20.3 18.8 24.1 21.3
26.8 26.5 27.9 23.0
24.1 7.4 3.3 2.6
Note: Circuit A consisted of four rougher cells, followed by four scavenger cells. Rate constant and froth recovery at 13 cm froth depth are only shown.
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Flotation Rate Constant (min-1)
1.2 Rougher Cell 1
1.0
Scavenger Cell 3
Froth Recovery (%)
80 Scavenger Cells
60
Rf ~ 20-35% ~27% solids in froth ~8 % Cu in froth
Rougher Cells
40
Rf ~ 50-65% ~48% solids in froth ~42% Cu in froth
20
(a)
0 Ro1 Ro2
Ro3
Ro4 Scv 1 Scv 2 Scv 3 Scv 4
Cell
100
Froth Recovery (%)
The superficial gas velocity values are also shown for reference in Table 2, and it is expected that for higher air rates in the cell there would be higher froth zone recovery for a given froth stability. Higher air rates will reduce the residence time of the gas in the froth before reaching the froth surface. Note that changes in superficial gas velocity from cell to cell may be accounted for by considering froth residence time, FRT. The froth residence time values presented in Table 2 refer to values calculated at a fixed froth depth, however, during the campaign in Circuit A, the froth depth was varied under different survey conditions in order to assess the effect of froth depth on froth recovery. The copper flotation rate is plotted as a function of froth depth for rougher cell 1 and scavenger cell 3 in Fig. 1, from which the value of the collection zone rate constant, kc, may be determined by extrapolation to zero froth depth. This allows calculation of froth recovery, Rf, at the experimental conditions and at any froth depth, using the definition of Rf as the ratio of overall rate constant, k, and the collection zone constant, kc. The froth recovery values shown in Table 2 and Fig. 2a are at a froth depth of 13 cm. At the head of the rougher bank (in cells 1 and 2), the froth recovery is approximately 50–65%, decreasing down-the-bank to the last scavenger cells where the froth recovery is estimated to be only 23%. This trend confirms other measurements of froth recovery in industrial flotation cells (Savassi et al., 1997). The decrease in froth recovery may be linked to the particle type and concentration that reports to the froth phase. There is a decrease in solids loading in the froth from 48% w/w in the rougher cells to 27% w/w in the scavenger cells, i.e., particle floatability decreases down-the-bank (Table 2). There is also a decrease in the copper composition of the particles (i.e., the proportion of hydrophobic particles) in the froth with increasing cell number (Table 2). To further illustrate the importance of the particles that report to the froth phase on froth recovery, the results of a simple, empirically based, regression is shown in Fig. 3. Here a predicted froth recovery is calculated based on the % of solids in the slurry in the concentrate, as well as the
Rougher Cells Scavenger Cells
80 60
Ro2
Ro1 Ro4 Ro3
40
Scv 1 20
Scv 2 Scv 4
Scv 3 (b)
0 0
5
10
15
20
25
30
35
Froth (Gas) Residence Time (s) Fig. 2. Froth zone characterisation for the rougher (Ro cells 1–4) and scavenger (Scv cells 1–4) cells in Circuit A: (a) froth recovery of copper as a function of cell number, and (b) froth recovery of copper as a function of froth residence time, FRT. The continuous lines shown in b are calculated using Eq. (7) and the parameters shown in Table 4.
Predicted Froth Recovery (%)
778
70 60 50 40
Rougher Cells
Scavenger Cells
30 20
Predicted Froth Recovery % = %Solids in Conc. + 0.2*%Cu of Solids in Conc.
10 0 0
10
220
300
40
50 5
60
70
Measured Froth Recovery (%)
0.8 Fig. 3. Predicted froth recovery of copper as a function of measured froth recovery for cells in Circuit A. The predicted froth recovery is calculated using the regression equation shown.
kc = 0.65
0.6 0.4
kc = 0.32
0.2 0.0 0
20
40
60 80 100 120 140 160 180 Froth Depth (mm)
Fig. 1. Copper flotation rate constant as a function of froth depth for rougher cell 1 and scavenger cell 3 at a fixed aeration rate in Circuit A.
% of copper in the concentrate solids. Using the same regression coefficients for each cell number shows remarkable agreement between the measured and predicted froth recovery, calculated using the regression equation shown in Fig. 3. Evidently, the % of solids in the slurry in the concentrate is an apparently more important determinant on froth recovery than % copper in the concentrate.
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
3.1.2. Circuit B In a similar approach, the froth recovery–froth residence time relationship was explored in the lead/copper rougher flotation bank of Circuit B. In this circuit, the focus was on assessing the effect of frother B concentration and froth depth on lead recovery in a set of surveys in a single cell (cell 3) of the first rougher bank. Table 3 gives a summary of the measured parameters from surveys conducted in Circuit B. Values of froth recovery of lead were found to vary from 38% at low frother concentration (5.4 ppm in the solution phase) and intermediate froth depth (15 cm), to 75% at intermediate frother concentration (7.5 ppm) and shallow froth depth (10 cm). Fig. 4 shows the froth recovery of lead plotted against froth residence time (FRT), under the different survey conditions. While there is a lack of data points for tests conducted at different frother concentrations, it is clear that froth recovery is not only a func-
10 100 Decrease FD Survey 7
80 Froth Recovery (%)
Froth recovery (Rf) is plotted as a function of froth residence time, calculated according to Eq. (2), in Fig. 2b. The nature of the froth changes down-the-bank, i.e., the composition and nature of the particles, as well as the froth structure. Visual observation confirmed that the bubbles in the froth of the scavenger cells were larger and bursted more frequently in comparison to the bubbles in the froth of the rougher cells. Fig. 2b suggests an exponential decrease in froth recovery with an increase in froth retention time. A longer froth residence time leads to a higher probability of particle drop-back within the froth, resulting in low froth recovery. This is the case for the weakly stable froths encountered in the scavenger cells, compared to the rougher cells in Circuit A. In this case, bubble coalescence causes the particles to be detached from the bubbles. The data for the rougher and scavenger cells apparently fall on different fitted lines of Eq. (3), with different values of b apparently applicable. Most likely, there is a significant difference in froth stability between the two stages, and the large decrease in froth recovery in the scavenger cells is due to the weakly structured and, hence, very different froth compared with the rougher cells. It is also noted that cell 1 of the scavengers shows a marked decrease in froth recovery that is most strongly correlated to the decreased solids loading in the froth, rather than to a decrease in copper composition of the particles (Table 2). The significance of solids loading in the froth on froth stability is discussed further below.
779
7.5 ppm Frother B 10 ppm pp mFrother F rother B B B 5.4 ppm ppm Frother F rot he rB Survey 4
60 Survey 3
40 Survey 5
20
Survey 6 Increase FD
0 0
10
20 30 40 50 Froth (Gas) Retention Time (sec)
60
Fig. 4. Froth zone recovery of lead as a function of froth residence time, FRT, and frother B concentration, measured in cell 3 of the rougher bank in Circuit B. The continuous lines are calculated using Eq. (7) and the parameters shown in Table 4.
tion of froth residence time but also froth type, which may be controlled by frother addition. Changes in froth depth, but without an accompanying change in frother concentration (in surveys 3, 6, and 7), gave changes in froth recovery that were explicable in terms of froth residence time changes only (Fig. 4). This conclusion could be drawn because these data points fit a Rf–FRT relationship defined by Eq. (3), using a single value of b. The general trend for the change in froth depth tests follows the expected exponential decay of Rf with an increase in froth residence time. The results demonstrate that other factors, such as froth stability, altered through changes in frother concentration, also controls froth recovery in addition to froth residence time. In contrast, changes in frother concentration in surveys 4 and 5 gave changes in froth recovery that could not be accounted for by simple changes in froth residence time (Fig. 4). In these cases, different values of the parameter b where required to fit the experimental data points indicating that the stability of the froth had changed (Fig. 4). Unfortunately, the stability of the plant froth was not measured in parallel with the changes in frother concentration in these particular tests. However, it was measured in separate tests on the rougher flotation bank in Circuit B, enabling a correlation to be made between froth stability, as measured by the froth half-life, to froth recovery. This correlation is examined further below.
Table 3 Cell characterisation summary and froth parameters measured in Circuit B Survey no.
Cell feed Pb grade (%)
Frother concentration (ppm)
Superficial gas velocity (cm/s)
Froth depth (cm)
Rate constant (min1)
Solids loading in froth (%)
Lead grade in froth (%)
Overall cell recovery (%)
Froth recovery (%)
Froth (gas) residence time (s)
3 4 5 6 7
0.60 0.64 0.67 0.64 0.74
7.5 10 5.4 7.5 7.5
0.61 0.63 0.63 0.63 0.63
18 25 15 25 10
0.14 0.23 0.11 0.12 0.17
45.6 46.0 46.1 37.4 54.5
15.7 15.5 17.3 17.3 21.5
22.7 31.3 27.3 23.0 22.9
60 54 38 49 75
29.5 39.7 23.8 39.7 15.9
780
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
3.2. Plant froth stability measurements The preceding discussion implies that both chemical (e.g., frother type and concentration) and physical conditions (froth depth, aeration rate, etc.) may be manipulated in flotation systems to achieve a desired froth zone recovery. However, the particle characteristics and their influence on drainage, stability and frothing phenomena, need also be considered as they play a significant role in controlling froth zone recovery. The stability of froths generated in cells 1 and 4 of the rougher bank in Circuit B is shown in Fig. 5. The froth in cell 1 in Circuit B was heavily mineralised (Table 3), and the froth formed in the rougher stages was relatively stable. Froth phase stability behaviour is quantified here in terms of the froth half-life parameter, t1/2, estimated from the froth decay profiles. The froths generated in the two cells are significantly different, both in terms of their different froth half-lives and their different solids loading. While the froth formed in cell 1 is significantly more stable than the froth in cell 4, both froths are relatively stable compared to the more unstable and dynamic froths produced in the rougher/scavenger cells of Circuit A. Differences in froth phase behaviour are likely to be influenced by the composition (i.e., hydrophobicity) and solids loading of particles in the froth, with Circuit A froth having much less particles contained within it due to its lower head grade of valuable and gangue sulphide minerals (Table 1). Reasons for low froth recovery, particularly in the scavenger stages of Circuit A, are discussed further below in terms of the distribution of frother amongst the products of flotation. 3.3. Plant frother in solution measurements Frother type and concentration are likely to control the foaming properties in the presence of particles through a 35 Rougher Cell 4 Rougher Cell 1
Froth Height (cm)
30 25 20 15 10 5 0
0
10
20
30
40
50
60
70
80
90
number of mechanisms. In cases of low froth recovery and stability, it had not, to this point, been established whether there was sufficient frother in solution to create a stable froth and effectively transport particles from the collection zone to the concentrate product. A pulp chemical survey was performed in both Circuits A and B under normal operating conditions (i.e., at normal plant frother concentration), to determine the distribution of frother across the plant cells. Chemical sampling of plant cells was made from key sample points, with feed, concentrate and tailings samples collected from each major unit stream. The deportment of frother to the flotation products was subsequently determined using the mass balanced flows of water. A summary of the mass balance of residual frother in solution as a percentage of the frother recovered in the flotation products of each unit is shown in Fig. 6. In each circuit, as frother is consumed down-the-bank, the majority of frother presented to each unit is found to deport to the respective tailing product. In Circuit A, for example, only 4% of frother A added to the plant rougher feed reported to the combined rougher and scavenger concentrate stream (froth product), the remainder reporting to the tailing stream. This behaviour in frother deportment was also observed for laboratory scale batch tests (not reported here) treating Circuit A rougher feed. A key finding was that frother recovery was linked strongly to the recovery of water, and clearly the largest amount of frother predominantly deports with the largest water flow, i.e., the tailings product. An important practical finding was that the frother in Circuit B largely decomposed in the water recovery circuit designed to recycle water from the tailings stream to the process (Fig. 6b). The percentage of water that reports to the concentrate may be calculated from Fig. 6(a) in the case of Circuit A. The water recovery is 3.1% in the roughers and 1.4% in the scavengers, compared with 2.6% and 1.5%, respectively, for the frother itself. It could be expected that frother recovery may be similar to water recovery for a froth in which the main mechanism of frother recovery to the concentrate is in the bulk liquid phase. This appears to be the case for the scavengers, suggesting that little of the frother in the scavenger concentrate is reporting as frother adsorbed at the gas/liquid interface. Further experimental work would be required to verify this statement unequivocally. The decrease in froth zone recovery could not be linked to the removal of frother from the pulp solution to the concentrate product. This is particularly pertinent to the situation that apparently exists in the scavenger froths of Circuit A. In this case, it appears that froth stability is poor and froth recovery low in spite of the continuing presence of frother in solution in the scavenger stages.
100
Time (sec) Fig. 5. In situ decay measurements of plant froths generated at plant reagent and gas conditions in the Rougher 1 bank in Circuit B. Rougher cell 1 froth contained 66.8% solids, 48.7% galena, t1/2 130 s; Rougher cell 4 froth contained 45.6% solids, 18% galena, t1/2 = 35 s.
3.4. Effect of particles and frother concentration on froth stability at laboratory scale Fig. 7 shows the effect of frother concentration, frother type (MIBC and frother B) and hydrophobic particles (i.e.,
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
h 573 tph 40 % w/w 3.3 ppm 97.4% Fr A
Frother A
Feed Rougher 590 tph 40% w/w 40 3.25 ppm 100% Fr A
18.1 tph 33% w/w 2.8 ppm 2..6% Fr A Concentrate
781
Tailing
Scavenger
Key (tph) Water flow %) % Solids w/w (% Frother Concentration (ppm) Frother Recovery (%)
8.4 tph 22% w/w 3.4 ppm 1.5% Fr A Concentrate (a) Circuit A Recycled Water 2 ppm Fr B
Frother B Feed
Rougher 1 7.. 6 ppm 100%Fr B
7..1 ppm 97.1% Fr B
Rougher 2
6.7 ppm 98..8 %Fr B Tailing
9.. 0 ppm 1.2% B 1.2%Fr
Concentrate
8.0 ppm Fr B
Cleaners
Key
7.9 ppm Fr B
Frother Concentration (ppm) Frother Recovery (%)
Thickener
4.4 ppm Fr B (b) Circuit B
Final Concentrate
Fig. 6. Mass balance of residual frother in solution, shown as the percent recovery of frother in the flotation products from down-the-bank chemical surveys: (a) Circuit A, frother A; (b) Circuit B, frother B. Note: Frother addition points, measured frother concentrations, water flows and frother recoveries (distribution) are also shown for reference.
45
i.e., greater t1/2 values. Hydrophobic particles appear to give added stability to the froth, especially in the case of MIBC, where the increase in concentration from 10 to 40 ppm shows a greater change in froth stability in the presence of particles than that observed with frother B. MIBC gives lower t1/2 values and, hence, weaker froths. These tests have highlighted important differences between frother types and particle effects on froth stability, and how the froth half-life may be used to represent froth stability. This has implications for its significance and usefulness in froth phase modelling, which is explored further below.
40
Half Life (sec)
35 30 Frother B no particles Frother B with particles MIBC no particles MIBC with particles
25 20 15 10 5 0
0
10
20
30
40
50
60
70
Frother Concentration (ppm) Fig. 7. Froth half-life (t1/2), as a function of frother concentration determined from froth decay measurements in a laboratory sparger column. Notes: Froths were generated in the absence and presence of particles (rougher feed—Circuit B) using frother B (plant frother) and MIBC.
treating Circuit B rougher feed) on froth stability, as defined by the froth half-life in a laboratory sparger column. The surfactant type clearly gives differences in froth structure, with weaker frothers such as MIBC requiring higher concentrations to give moderately stable froths,
3.5. Froth recovery and froth residence time correlation at plant scale The exponential decay of froth recovery with an increase in froth retention (residence) time is well known and useful in terms of froth modelling (Gorain et al., 1998). Based on the data shown in Figs. 2b and 4, it is reasonable to suppose that the froth zone recovery of particles that arrive at the pulp–froth interface may be described in terms of froth residence time by a simple exponential function such as Eq. (3). An alternative interpretation to the b parameter in Eq. (3) is proposed here, which attempts to represent the interaction between particle transport in the froth transfer
782
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Table 4 Froth parameters useful for describing froth zone performance measured during surveys in Circuits A and B Plant conditions
Rf (range) (%)
FRT (range) (s)
Froth half-life t1/2 (s)
Recovery of frother to tailing stream (%)
Circuit A—rougher cells 3.5 ppm frother A
65–50
10–30
35
97.4
Circuit A—scavenger cells 3.5 ppm frother A
30–15
30–50
15
95.9
Circuit B—rougher cells 5.5 ppm frother B 7.5 ppm frother B 10 ppm frother B
40–35 75–50 50
25 15–30 40
25 54 65
96.4 95.9 96.5
and froth stability in the Rf–FRT relationship through consideration of the froth half-life. FRT Rf ¼ 100 exp ð7Þ t1=2 By taking into consideration differences in froth stability it is possible that different frother types and/or concentrations may show similar dependencies of froth recovery on the ratio of the froth residence time to froth half-life. For each survey condition, the exponential model according to Eq. (7) was fitted to the data points in Fig. 2b (Circuit A) and Fig. 4 (Circuit B), respectively. Table 4 shows the froth half-life values that were used to fit lines using Eq. (7) to the plant data shown in Figs. 2b and 4, respectively. Table 4 also summarises useful parameters for describing froth zone performance in the flotation systems. In particular, it shows that the frother predominantly reports to the tailing stream over the range of frother concentrations examined in Circuit B, in agreement with previous comments made in relation to Fig. 6. It should be noted that Eq. (7) is an empirical relationship only and has no direct physical meaning. It simply states that the decrease in froth recovery with froth residence time may also be related to the stability of the froth. For a more stable froth, the froth recovery decreases to a lesser extent for the same froth residence time. It is apparent from the different values of froth half-life in Table 4 that the relationship, according to Eq. (7), is strongly dependent on frother concentration and feed composition. While the data points show scatter, the interaction of froth recovery with froth stability and froth retention time appear to be useful for interpreting froth performance in flotation at an industrial scale. Froth stability appears to serve as a good indication of the significance of the sub-processes occurring within the froth. To ensure a froth zone recovery greater than 70%, the FRT ratio should t1=2 be <0.3. 4. Conclusions This work has shown that the froth phase has a pronounced influence on the overall flotation rate in industrial flotation circuits. The overall cell rate constant can be
reduced to less than 25% of the collection zone rate constant due to inefficiencies of the froth phase. This is particularly pronounced for scavenger cell froths in which the low stability of the froth apparently limits froth recovery and the overall cell rate constant. Because of the complexity and interactive nature of processes occurring within the froth phase, an important first step towards optimising the froth phase recovery depends upon determining practically useful froth parameters, such as the froth residence time and froth stability, which may then be correlated with froth recovery. Two separate industrial case studies were targeted to investigate froth phase performance and quantify the measured characteristics of the froth phase. Froth phase performance was defined by the froth zone recovery and the measured characteristics of the froth phase including frother concentration and froth stability. Froth zone recovery was found to generally decrease down-the-bank in Circuit A, with decreases in froth zone recovery limiting the recovery achievable in the plant scavenger cells. The decrease in froth zone recovery could not be linked to the removal of frother from the pulp solution to the concentrate product. However, there was a correlation of froth recovery to froth stability, as measured by the froth halflife. This parameter is strongly linked to the presence of particles in the froth, with a wide variation in froth stability between cells and feed types, which may help to explain the low froth recovery of poorly mineralised scavenger froths. These particular froths are characterised by short half-lives and low froth recovery. A simple exponential model, based on the froth half-life and froth residence time of gas, is proposed to predict froth zone recovery, however its significance or usefulness has not yet been completely defined. Further research is continuing to determine if observations regarding the relationship between froth recovery and froth stability may be applied generically to all plants. It is suspected that froth instability contributes to low recovery in scavenger stages generally, particularly for feeds containing low quantities of the mineral to be floated. Methods to counter froth instability are also being researched. These aspects are being studied in the new AMIRA International project, P541B ‘‘Optimising Froth Zone Performance in Mineral Flotation’’.
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Acknowledgements The authors wish to gratefully acknowledge the sponsors of the AMIRA International project P541A ‘‘Particle and Frother Interactions in Flotation Froths’’ including Anglo Platinum, Cytec, Newmont (Golden Grove Operations), Ok Tedi Mining, Rio Tinto, Teck Cominco and the Australian Research Council. The authors also wish to thank the P9 project ‘‘Mineral Processing’’ for the froth recovery measurements in Circuit A. This paper was first published in Proceedings Centenary of Flotation Symposium, Brisbane, Australia, 5–9 June 2005, by The Australasian Institute of Mining and Metallurgy. References Gorain, B.K., Napier-Munn, T.J., Franzidis, J.-P., Manlapig, E.V., 1998. Studies on impeller type, impeller speed and air flow rate in an
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industrial scale flotation cell, Part 5: Validation of k–Sb relationship and effect of froth depth. Miner. Eng. 11, 615. Iglesias, E., Anderez, J., Forgiarini, A., Salager, J., 1995. A new method to estimate the stability of short life-time foams. Colloids Surf. 98, 167. Johnson, N.W., McKee, D.J., Lynch, A.J., 1974. Flotation rates of nonsulphide minerals in chalcopyrite flotation processes. Trans. AIME 256. Laplante, A.R., Toguri, J.M., Smith, H.W., 1983. The effect of airflow rate on the kinetics of flotation—Part 1: The transfer of material from the slurry to the froth. Int. J. Miner. Process 11, 203. Savassi, O.N., Alexander, D.J., Johnson, N.W., Manlapig, E.V., Franzidis, J.-P., 1997. Measurement of froth recovery of attached particles in industrial cells. In: Lauder, D. (Ed.), Proceedings of the Sixth Mill Operators Conference. Aust. Inst. Min. Metall. Publ., Melbourne, pp. 149–155. Schwarz, S., Grano, S., Fornasiero, D., Ralston, J., 2002. Froth behaviour in the presence and absence of model quartz particles. In: Proceedings of WCPT4—World Congress on Particle Technology, Sydney, Paper 290. Vera, M.A., Franzidis, J.-P., Manlapig, E.V., 1999. Simultaneous determination of collection zone rate constant and froth zone recovery in a mechanical flotation environment. Miner. Eng. 12, 1163.
Case studies on the performance and characterisation of the froth phase in industrial flotation circuits G. Tsatouhas c, S.R. Grano a
b
a,*
, M. Vera
b
Ian Wark Research Institute, University of South Australia (The ARC Special Research Centre for Particle and Material Interfaces), Mawson Lakes Campus, Adelaide, South Australia 5095, Australia Oil Sands Technology Group—Extraction and Separation, Calgary Research Centre, Shell Canada Limited, 3655-36th St, NW Calgary, Alberta, Canada 2L 1Y8 c Huntsman Australia, Performance Products, Gate 2 Newsom Street, Ascot Vale Victoria 3032, Australia Received 15 July 2005; accepted 9 September 2005 Available online 26 October 2005
Abstract This paper deals with two separate case studies investigating the froth phase performance and characterisation of two industrial rougher/scavenger flotation circuits. Froth phase performance was quantified using a mass balance approach to estimate froth zone recovery. Measured characteristics of the froth phase included frother solution concentration determined by gas chromatography, and the time taken for an equilibrium froth sample to decay to one-half of its original froth height. The latter measurement is referred to as the Ôfroth half-lifeÕ and is strongly linked to froth stability. Special methods and techniques developed to preserve frother in solution and to measure froth half-life are briefly described. The frother type in the first case study was a mixture of straight and branched alcohols, whilst the frother type in the second case study was a mixture of alcohols, aldehydes and triethoxybutane. The first case study focussed on a flotation circuit treating a low grade ore containing only a small fraction of floatable copper sulphide minerals, while the second case study focussed on a flotation circuit treating a higher grade complex sulphide ore containing significant quantities of chalcopyrite, galena, sphalerite and pyrite. It was found that froth zone recovery of valuable mineral generally decreased down-the-bank of the two industrial rougher/scavenger circuits. Moreover, decreases in froth zone recovery significantly limit the overall cell recovery of valuable mineral achievable from the plant scavenger cells. However, the decrease in froth zone recovery could not be linked to the removal of frother from the pulp solution to the concentrate product in the preceding rougher flotation stages. Measurements of residual frother in solution suggested that, approximately, only 5–10% of the added frother was removed into the rougher/scavenger concentrate, with the remainder appearing in the scavenger tailings. This finding suggested there was apparently adequate frother in solution in the scavenger stages. There was, however, a correlation to the froth half-life, with the froth half-life also generally decreasing down-the-bank. A simple, empirical model, based on the froth half-life and froth residence time of gas, is proposed here to predict froth zone recovery. Further, it is proposed that the froth stability, as measured by the froth half-life, is strongly linked to the presence of particles in the froth, with poorly mineralised scavenger froth characterised by a short half-life and, potentially, a low froth zone recovery. The importance of particles on froth stability was confirmed in separately conducted laboratory experiments. These experiments also demonstrated the wide variation in froth stability behaviour between different frother types. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation froths; Flotation frothers; Sulphide ores
*
Corresponding author. E-mail addresses: [email protected] (G. Tsatouhas), [email protected] (S.R. Grano), [email protected] (M. Vera). 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.033
1. Introduction In flotation, the transport of hydrophobic particles attached to bubbles from the collection zone of the cell,
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
where particle collection by bubbles occurs, to the concentrate launder is of key importance. Transport takes place via a froth phase in froth flotation. Frother surfactants are principally added to a flotation pulp so that a transient, mineralised froth is formed at the top of the flotation cell, forming the so-called froth phase. A transient froth is a prerequisite for the successful transport of mineral laden bubbles from the pulp zone/froth zone interface to the concentrate launder. It is desirable that the mineralised froth breaks down (i.e., the bubbles collapse or coalesce), once the froth reports to the concentrate launder. A froth which is overly unstable may cause significant loss of valuable mineral from the froth to the pulp phase and, subsequently, to the cell tailing stream. Conversely, an overly stable froth will cause problems in pumping, and may adversely impact mineral separation in subsequent flotation stages. Our research here is focussed on controlling froth stability to allow particle transport to take place, while also ensuring the froth breaks down once it enters the concentrate launder. The recovery of particles attached to bubbles across the froth phase is referred to as froth recovery. Froth recovery can be defined in several ways. However, it is now accepted that the recovery of attached particles across the froth zone is best expressed in terms of the first order rate constants (Savassi et al., 1997): Rf ¼
k kc
ð1Þ
In Eq. (1), Rf is the fractional froth recovery, k is the overall flotation cell first order rate constant, treating the collection and froth zones as a single phase, and kc is the collection zone rate constant. It is noted that entrained particles are included in this particular definition. Entrainment is the phenomena of the recovery of mineral particles, which are not attached to air bubbles, by their unselective recovery with water (Johnson et al., 1974). However, in the context of this current paper, which principally focuses on hydrophobic value minerals, the contribution by entrainment is expected to be only minor. This definition also provides an experimental approach to the measurement of froth recovery in which the overall flotation cell rate constant is determined as a function of froth height, or the froth residence time (FRT) of gas (Laplante et al., 1983; Vera et al., 1999). The froth residence time of gas is defined as FRT ¼
FD Jg
ð2Þ
In Eq. (2), FD is the froth depth (cm) and Jg is the superficial gas velocity (cm/s), which is the gas flow per unit cross-sectional area of the flotation cell. In this approach, the overall flotation cell first order rate constant is measured as a function of froth residence time, with the collection zone rate constant, kc, determined by extrapolation to zero froth depth. The ratio of the overall flotation cell first order rate constant, at any froth depth (FD), to the collection zone rate constant defines the froth recovery at that
775
froth depth according to Eq. (1). This particular method, termed the mass balance approach, is used in the current study to determine froth recovery of the plant cells. It is recognised that in this approach, the froth recovery at zero froth height is assumed to be 100%. This may not be the case due to bubble bursting at the top of the froth, or inefficiencies in the lateral transport of particles in the froth, even at very shallow froth depths. However, for industrial cells it is an expedient approach, in the first instance, to obtain an estimate of froth recovery. Both the physical scale of the froth (defined here by FRT) and froth type, exemplified by the froths stability, may control the froth recovery of attached particles. Froth residence time and froth height characterise the physical scale of the froth, whilst the frother type, frother concentration as well as the particle characteristics in the froth (e.g., particle size, particle contact angle, particle loading in the froth) may all control froth stability (Schwarz et al., 2002). A decrease in the carrying capacity of the froth, and the possibility that some mineral particles may become detached and be lost from the froth, has been observed from froth recovery measurements conducted at plant scale where there is continuous removal of froth in successive flotation cells (Savassi et al., 1997). The decrease in froth recovery seems particularly important in the scavenger flotation stages, where the froth has apparently low stability. In contrast, froth recovery in the initial stages of rougher flotation of a continuous separator is usually higher, and the froths apparently more stable. This particular aspect of the operation of flotation froths is the principal focus of this paper. It is the purpose of this paper to determine if decreases in froth recovery in a continuous, industrial flotation separator may be linked to changes in froth stability, and to determine reasons for the change in froth stability with flotation cell number. Froth stability is used here to describe the general phenomena of thin film rupture, bubble coalescence and loss of froth volume. In this work, both the effect of particles contained within the froth, as well as frother type and concentration on froth stability has been investigated. In recent years there has been strong interest in modelling froth zone performance in laboratory flotation cells, in scale-up to a continuous flotation process, and in froth phase assessment in plant cells. Froth phase performance is better understood here in terms of froth recovery, Rf, and hence a specific objective of this work is to identify how physical factors, such as froth height, and chemical factors, such as frother concentration in solution and froth solids loading, control froth recovery. The exponential decay of froth recovery with an increase in froth retention (residence) time is well known and useful in terms of froth modelling (Gorain et al., 1998): Rf ¼ 100 expðbFRTÞ
ð3Þ
In Eq. (3), b is a parameter, which is arguably related to the rate at which bubbles coalesce and burst, or simply a parameter, which could depend on the physical and chemical froth factors. An expansion of the Rf–FRT relationship to account
776
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for the influence of froth stability is used here to describe a semi-empirical model to predict froth phase performance in plant cells. 2. Experimental 2.1. Sampling and froth recovery measurements A series of comprehensive sampling surveys were performed at two different major sulphide mineral concentrators located in Australia (Cu circuit—Circuit A) (Cu– Pb–Zn circuit—Circuit B). In the feed to Circuit A, copper occurred in bornite and chalcopyrite while there was only minor pyrite (Table 1). In the feed to Circuit B, copper occurred in chalcopyrite, lead in galena and zinc in sphalerite. The two flotation circuits treat ores with significantly different head mineralogy (Table 1), which has implications for the stability of the froth in both plants. It is noted that copper recovery and rate constant were used to define froth recovery in Circuit A, whilst lead recovery and rate constant were used to define froth recovery in Circuit B. In each case, the objective was to assess froth zone performance, with the sampling conducted in Circuit A in 2001 and Circuit B in 2002, respectively. Briefly, plant flotation cells in the rougher and scavenger banks were operated at different froth depths classified as shallow, intermediate (baseline) and deep, corresponding to short, intermediate and long froth residence times, respectively, through Eq. (2). In some cases, and at specific froth depths, the flotation cells were also operated at different airflow rates. However, the flotation rate constant and froth recovery data presented here are at a fixed airflow rate for simplicity sake. In addition to the comprehensive sampling surveys, the metallurgical performance of a single flotation cell in the lead/copper rougher flotation bank of Circuit B was also investigated. Three different froth depths, three different air flow rates and three different frother concentrations were employed in the flotation bank in these investigations. The frother concentration in Circuit B was varied from the baseline plant frother addition rate to high and low additions, in order to determine the overall effect of frother concentration on froth recovery. The flotation rate constant, k (min1), and froth recovery, Rf, were calculated from the recovery of copper (Circuit A) and lead (Circuit B) for each of the cell operating conditions used in the test programme, assuming perfect mixing in the flotation cell: k¼
R t ð1 RÞ
ð4Þ
In Eq. (4), R is the fractional recovery of copper or lead, and t is the effective mean residence time (minutes) of the pulp in the flotation cell. The objective of the flotation bank surveys was to characterise froth performance down-the-bank under standard operating conditions, and to obtain a data set for a single flotation cell under different operating conditions in Circuit B only. The data is used to explore relationships between performance and conditions, i.e., between froth recovery and froth depth, air flow rate and frother concentration. The surveys involved collection of samples from all the major streams down the respective rougher and scavenger flotation banks. Sample collection was performed over a 2-h period, during which four increments were taken at each sampling point. The samples collected from each cell during each survey, included: (a) the feed to the flotation bank (i.e., feed to rougher cell 1), (b) the combined concentrate of the flotation bank, (c) the tailings from the flotation bank, (d) timed lip concentrates from individual cells in the bank, and (e) tailings from each cell in the tested bank. Pulp chemical surveys were also performed in parallel with the main metallurgical surveys to determine frother concentrations in solution in major streams, as well as the distribution (or recovery) of frother down-the-bank. This information is used to complement the metallurgical assessment. Surveys involved collection of samples from all the major streams, which was also extended to both the rougher and cleaner banks of both circuits. Frother in solution in aqueous samples was measured using gas chromatography with flame ionisation detection. Frother balances were made using all the measured chemical assay data, % solids, measured frother assays and, taking as the reference, the feed to the flotation bank. Flowrates of the various slurry streams were obtained by computation. Individual adjustment of the standard deviations of frother assays was necessary to aid the balance and to check the quality of the data. Preliminary work showed that the relevant frothers were stable in solution after sampling, provided the samples were stored in the absence of oxygen and at temperatures less than 3 °C. This study used three different frothers—plant frother A (Circuit A), a mixture of low molecular weight alcohols, aldehydes and esters; plant frother B (Circuit B), a triethoxybutane and alcohol mixture, and MIBC (methyl isobutyl carbinol). All frothers were used as received. 2.2. Froth stability methods Froth stability generally refers to the foam decay process, during which time there is no further generation of
Table 1 Head mineralogy of the feeds to Circuits A and B Circuit
A B
Mineral Chalcopyrite
Bornite
Galena
Sphalerite
Pyrite
Non-sulphide gangue
1.2 0.7
1.8 –
– 1.3
– 21.7
0.9 29.4
96.1 46.9
Note: Non-sulphide gangue includes silicates and alumino-silicates.
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
gas bubbles. Froth decay is indicated by some visible occurrence such as froth volume variation, bubble collapse, or change in the bubble size. In order to quantify the global change of the froth, froth volume variation is recorded as a function of time. The faster the change, the less stable the foam. The simplest method of estimating the stability of a foam is to measure the change in volume (or height, Hf), for constant cross-sectional area with time, t. The initial time is taken when there is negligible further increase in froth volume, and the gas flow is then discontinued. The froth formed was then allowed to decay and the change in froth height noted as a function of decay time. A differential equation was then fitted to describe the rate of foam collapse. According to Iglesias et al. (1995): dH f k ¼ t dt
ð5Þ
In Eq. (5), k is an empirical constant. Iglesias et al. (1995) noted that plotting the froth height against log (t) yielded a series of straight lines for a range of surfactant types and foam stabilities, justifying in part Eq. (5). Furthermore, Iglesias et al. (1995) plotted HH of against log
t t1=2
, which nec-
essarily caused all the straight-line relationships to converge to HH of ¼ 0:5 at t = t1/2. In the current study, a similar approach was adopted using Eq. (6): Hf t ¼ a log ð6Þ t1=2 Ho In Eq. (6) a is dimensionless empirical constant. The halflife is based on the height value and the reference is taken as the time, t1/2, for the foam to decrease to one-half of its original height, Ho. A series of experiments was designed in both plant and laboratory flotation cells. Plant tests were performed in situ of different rougher cells (cells 1 and 4) of the lead/copper rougher flotation bank in Circuit B to highlight differences in froth stability, due to differences in particle type and solids loading in the froth. A 500-mL cylindrical column (1.5 m long, 80 mm diameter) was immersed in each cell between the froth surface and just below the pulp/froth interface and the froth collected over 2 min using plant air conditions. The equilibrium foam height was recorded and the gas
777
flow discontinued by means of a sliding partition at the base of the column. The froth was then allowed to decay and the change in froth height noted as a function of time. Froth decay experiments were also carried out in a graduated multi-holed sparger column (1.5 m long, 2.5 cm i.d.) (Schwarz et al., 2002), using plant flotation feed slurry for Circuit B only. The slurry sample was collected prior to reagent addition in the plant. The particle surface chemistry was controlled using reagent (collector and frother) additions in the laboratory with the slurry entering the column diluted to 10% solids weight/slurry weight. The experimental approach described previously was used to investigate the decay process at controlled air (400 mL/ min) and feed flow rates, which were adjusted to optimise the froth residence time. Dynamic measurements were made, both with and without particles, at 8, 20 and 40 ppm of the plant frother concentration, as well as with MIBC used as the reference surfactant. 3. Results 3.1. Plant froth zone recovery 3.1.1. Circuit A Table 2 presents a summary of parameters measured in individual cells of Circuit A, under the same reagent addition rates and during the survey period. The overall cell rate constant decreased down-the-bank due to both decreases in the collection zone rate constant and froth zone recovery. While our attention here is principally on froth zone recovery, it is worthwhile discussing the significance of the collection zone rate constant at this point. The collection zone rate constant decreases down-the-bank due to the removal of fast floating components in the initial stages of flotation, the rate constant being an undistributed rate constant of all floating components. In the scavenger stages only the slow floating components remain, being the less hydrophobic value mineral such as coarse composites. This is reflected in the decreasing copper assay of the froth down-the-bank (Table 2). Both the decreasing grade (hydrophobicity) of the particles entering the froth, as well as the decreasing solids loading in the froth (Table 2), have consequences on the stability of the froth as discussed further below.
Table 2 Cell characterisation summary and froth parameters measured in Circuit A Plant cell
Superficial gas velocity (cm/s)
Rate constant at FD = 13 cm (min1)
Froth recovery at FD = 13 cm (%)
Froth (gas) residence time at FD = 13 cm (s)
Solids loading in froth (%)
Copper grade in froth (%)
Roughers
1 2 3 4
0.99 1.19 0.84 0.96
0.39 0.44 0.18 0.20
62 54 53 52
13.1 10.9 15.5 14.1
48.2 49.4 48.0 40.0
42.5 39.7 31.1 18.8
Scavengers
1 2 3 4
0.64 0.72 0.56 0.68
0.06 0.12 0.09 0.06
29 25 28 23
20.3 18.8 24.1 21.3
26.8 26.5 27.9 23.0
24.1 7.4 3.3 2.6
Note: Circuit A consisted of four rougher cells, followed by four scavenger cells. Rate constant and froth recovery at 13 cm froth depth are only shown.
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
Flotation Rate Constant (min-1)
1.2 Rougher Cell 1
1.0
Scavenger Cell 3
Froth Recovery (%)
80 Scavenger Cells
60
Rf ~ 20-35% ~27% solids in froth ~8 % Cu in froth
Rougher Cells
40
Rf ~ 50-65% ~48% solids in froth ~42% Cu in froth
20
(a)
0 Ro1 Ro2
Ro3
Ro4 Scv 1 Scv 2 Scv 3 Scv 4
Cell
100
Froth Recovery (%)
The superficial gas velocity values are also shown for reference in Table 2, and it is expected that for higher air rates in the cell there would be higher froth zone recovery for a given froth stability. Higher air rates will reduce the residence time of the gas in the froth before reaching the froth surface. Note that changes in superficial gas velocity from cell to cell may be accounted for by considering froth residence time, FRT. The froth residence time values presented in Table 2 refer to values calculated at a fixed froth depth, however, during the campaign in Circuit A, the froth depth was varied under different survey conditions in order to assess the effect of froth depth on froth recovery. The copper flotation rate is plotted as a function of froth depth for rougher cell 1 and scavenger cell 3 in Fig. 1, from which the value of the collection zone rate constant, kc, may be determined by extrapolation to zero froth depth. This allows calculation of froth recovery, Rf, at the experimental conditions and at any froth depth, using the definition of Rf as the ratio of overall rate constant, k, and the collection zone constant, kc. The froth recovery values shown in Table 2 and Fig. 2a are at a froth depth of 13 cm. At the head of the rougher bank (in cells 1 and 2), the froth recovery is approximately 50–65%, decreasing down-the-bank to the last scavenger cells where the froth recovery is estimated to be only 23%. This trend confirms other measurements of froth recovery in industrial flotation cells (Savassi et al., 1997). The decrease in froth recovery may be linked to the particle type and concentration that reports to the froth phase. There is a decrease in solids loading in the froth from 48% w/w in the rougher cells to 27% w/w in the scavenger cells, i.e., particle floatability decreases down-the-bank (Table 2). There is also a decrease in the copper composition of the particles (i.e., the proportion of hydrophobic particles) in the froth with increasing cell number (Table 2). To further illustrate the importance of the particles that report to the froth phase on froth recovery, the results of a simple, empirically based, regression is shown in Fig. 3. Here a predicted froth recovery is calculated based on the % of solids in the slurry in the concentrate, as well as the
Rougher Cells Scavenger Cells
80 60
Ro2
Ro1 Ro4 Ro3
40
Scv 1 20
Scv 2 Scv 4
Scv 3 (b)
0 0
5
10
15
20
25
30
35
Froth (Gas) Residence Time (s) Fig. 2. Froth zone characterisation for the rougher (Ro cells 1–4) and scavenger (Scv cells 1–4) cells in Circuit A: (a) froth recovery of copper as a function of cell number, and (b) froth recovery of copper as a function of froth residence time, FRT. The continuous lines shown in b are calculated using Eq. (7) and the parameters shown in Table 4.
Predicted Froth Recovery (%)
778
70 60 50 40
Rougher Cells
Scavenger Cells
30 20
Predicted Froth Recovery % = %Solids in Conc. + 0.2*%Cu of Solids in Conc.
10 0 0
10
220
300
40
50 5
60
70
Measured Froth Recovery (%)
0.8 Fig. 3. Predicted froth recovery of copper as a function of measured froth recovery for cells in Circuit A. The predicted froth recovery is calculated using the regression equation shown.
kc = 0.65
0.6 0.4
kc = 0.32
0.2 0.0 0
20
40
60 80 100 120 140 160 180 Froth Depth (mm)
Fig. 1. Copper flotation rate constant as a function of froth depth for rougher cell 1 and scavenger cell 3 at a fixed aeration rate in Circuit A.
% of copper in the concentrate solids. Using the same regression coefficients for each cell number shows remarkable agreement between the measured and predicted froth recovery, calculated using the regression equation shown in Fig. 3. Evidently, the % of solids in the slurry in the concentrate is an apparently more important determinant on froth recovery than % copper in the concentrate.
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
3.1.2. Circuit B In a similar approach, the froth recovery–froth residence time relationship was explored in the lead/copper rougher flotation bank of Circuit B. In this circuit, the focus was on assessing the effect of frother B concentration and froth depth on lead recovery in a set of surveys in a single cell (cell 3) of the first rougher bank. Table 3 gives a summary of the measured parameters from surveys conducted in Circuit B. Values of froth recovery of lead were found to vary from 38% at low frother concentration (5.4 ppm in the solution phase) and intermediate froth depth (15 cm), to 75% at intermediate frother concentration (7.5 ppm) and shallow froth depth (10 cm). Fig. 4 shows the froth recovery of lead plotted against froth residence time (FRT), under the different survey conditions. While there is a lack of data points for tests conducted at different frother concentrations, it is clear that froth recovery is not only a func-
10 100 Decrease FD Survey 7
80 Froth Recovery (%)
Froth recovery (Rf) is plotted as a function of froth residence time, calculated according to Eq. (2), in Fig. 2b. The nature of the froth changes down-the-bank, i.e., the composition and nature of the particles, as well as the froth structure. Visual observation confirmed that the bubbles in the froth of the scavenger cells were larger and bursted more frequently in comparison to the bubbles in the froth of the rougher cells. Fig. 2b suggests an exponential decrease in froth recovery with an increase in froth retention time. A longer froth residence time leads to a higher probability of particle drop-back within the froth, resulting in low froth recovery. This is the case for the weakly stable froths encountered in the scavenger cells, compared to the rougher cells in Circuit A. In this case, bubble coalescence causes the particles to be detached from the bubbles. The data for the rougher and scavenger cells apparently fall on different fitted lines of Eq. (3), with different values of b apparently applicable. Most likely, there is a significant difference in froth stability between the two stages, and the large decrease in froth recovery in the scavenger cells is due to the weakly structured and, hence, very different froth compared with the rougher cells. It is also noted that cell 1 of the scavengers shows a marked decrease in froth recovery that is most strongly correlated to the decreased solids loading in the froth, rather than to a decrease in copper composition of the particles (Table 2). The significance of solids loading in the froth on froth stability is discussed further below.
779
7.5 ppm Frother B 10 ppm pp mFrother F rother B B B 5.4 ppm ppm Frother F rot he rB Survey 4
60 Survey 3
40 Survey 5
20
Survey 6 Increase FD
0 0
10
20 30 40 50 Froth (Gas) Retention Time (sec)
60
Fig. 4. Froth zone recovery of lead as a function of froth residence time, FRT, and frother B concentration, measured in cell 3 of the rougher bank in Circuit B. The continuous lines are calculated using Eq. (7) and the parameters shown in Table 4.
tion of froth residence time but also froth type, which may be controlled by frother addition. Changes in froth depth, but without an accompanying change in frother concentration (in surveys 3, 6, and 7), gave changes in froth recovery that were explicable in terms of froth residence time changes only (Fig. 4). This conclusion could be drawn because these data points fit a Rf–FRT relationship defined by Eq. (3), using a single value of b. The general trend for the change in froth depth tests follows the expected exponential decay of Rf with an increase in froth residence time. The results demonstrate that other factors, such as froth stability, altered through changes in frother concentration, also controls froth recovery in addition to froth residence time. In contrast, changes in frother concentration in surveys 4 and 5 gave changes in froth recovery that could not be accounted for by simple changes in froth residence time (Fig. 4). In these cases, different values of the parameter b where required to fit the experimental data points indicating that the stability of the froth had changed (Fig. 4). Unfortunately, the stability of the plant froth was not measured in parallel with the changes in frother concentration in these particular tests. However, it was measured in separate tests on the rougher flotation bank in Circuit B, enabling a correlation to be made between froth stability, as measured by the froth half-life, to froth recovery. This correlation is examined further below.
Table 3 Cell characterisation summary and froth parameters measured in Circuit B Survey no.
Cell feed Pb grade (%)
Frother concentration (ppm)
Superficial gas velocity (cm/s)
Froth depth (cm)
Rate constant (min1)
Solids loading in froth (%)
Lead grade in froth (%)
Overall cell recovery (%)
Froth recovery (%)
Froth (gas) residence time (s)
3 4 5 6 7
0.60 0.64 0.67 0.64 0.74
7.5 10 5.4 7.5 7.5
0.61 0.63 0.63 0.63 0.63
18 25 15 25 10
0.14 0.23 0.11 0.12 0.17
45.6 46.0 46.1 37.4 54.5
15.7 15.5 17.3 17.3 21.5
22.7 31.3 27.3 23.0 22.9
60 54 38 49 75
29.5 39.7 23.8 39.7 15.9
780
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
3.2. Plant froth stability measurements The preceding discussion implies that both chemical (e.g., frother type and concentration) and physical conditions (froth depth, aeration rate, etc.) may be manipulated in flotation systems to achieve a desired froth zone recovery. However, the particle characteristics and their influence on drainage, stability and frothing phenomena, need also be considered as they play a significant role in controlling froth zone recovery. The stability of froths generated in cells 1 and 4 of the rougher bank in Circuit B is shown in Fig. 5. The froth in cell 1 in Circuit B was heavily mineralised (Table 3), and the froth formed in the rougher stages was relatively stable. Froth phase stability behaviour is quantified here in terms of the froth half-life parameter, t1/2, estimated from the froth decay profiles. The froths generated in the two cells are significantly different, both in terms of their different froth half-lives and their different solids loading. While the froth formed in cell 1 is significantly more stable than the froth in cell 4, both froths are relatively stable compared to the more unstable and dynamic froths produced in the rougher/scavenger cells of Circuit A. Differences in froth phase behaviour are likely to be influenced by the composition (i.e., hydrophobicity) and solids loading of particles in the froth, with Circuit A froth having much less particles contained within it due to its lower head grade of valuable and gangue sulphide minerals (Table 1). Reasons for low froth recovery, particularly in the scavenger stages of Circuit A, are discussed further below in terms of the distribution of frother amongst the products of flotation. 3.3. Plant frother in solution measurements Frother type and concentration are likely to control the foaming properties in the presence of particles through a 35 Rougher Cell 4 Rougher Cell 1
Froth Height (cm)
30 25 20 15 10 5 0
0
10
20
30
40
50
60
70
80
90
number of mechanisms. In cases of low froth recovery and stability, it had not, to this point, been established whether there was sufficient frother in solution to create a stable froth and effectively transport particles from the collection zone to the concentrate product. A pulp chemical survey was performed in both Circuits A and B under normal operating conditions (i.e., at normal plant frother concentration), to determine the distribution of frother across the plant cells. Chemical sampling of plant cells was made from key sample points, with feed, concentrate and tailings samples collected from each major unit stream. The deportment of frother to the flotation products was subsequently determined using the mass balanced flows of water. A summary of the mass balance of residual frother in solution as a percentage of the frother recovered in the flotation products of each unit is shown in Fig. 6. In each circuit, as frother is consumed down-the-bank, the majority of frother presented to each unit is found to deport to the respective tailing product. In Circuit A, for example, only 4% of frother A added to the plant rougher feed reported to the combined rougher and scavenger concentrate stream (froth product), the remainder reporting to the tailing stream. This behaviour in frother deportment was also observed for laboratory scale batch tests (not reported here) treating Circuit A rougher feed. A key finding was that frother recovery was linked strongly to the recovery of water, and clearly the largest amount of frother predominantly deports with the largest water flow, i.e., the tailings product. An important practical finding was that the frother in Circuit B largely decomposed in the water recovery circuit designed to recycle water from the tailings stream to the process (Fig. 6b). The percentage of water that reports to the concentrate may be calculated from Fig. 6(a) in the case of Circuit A. The water recovery is 3.1% in the roughers and 1.4% in the scavengers, compared with 2.6% and 1.5%, respectively, for the frother itself. It could be expected that frother recovery may be similar to water recovery for a froth in which the main mechanism of frother recovery to the concentrate is in the bulk liquid phase. This appears to be the case for the scavengers, suggesting that little of the frother in the scavenger concentrate is reporting as frother adsorbed at the gas/liquid interface. Further experimental work would be required to verify this statement unequivocally. The decrease in froth zone recovery could not be linked to the removal of frother from the pulp solution to the concentrate product. This is particularly pertinent to the situation that apparently exists in the scavenger froths of Circuit A. In this case, it appears that froth stability is poor and froth recovery low in spite of the continuing presence of frother in solution in the scavenger stages.
100
Time (sec) Fig. 5. In situ decay measurements of plant froths generated at plant reagent and gas conditions in the Rougher 1 bank in Circuit B. Rougher cell 1 froth contained 66.8% solids, 48.7% galena, t1/2 130 s; Rougher cell 4 froth contained 45.6% solids, 18% galena, t1/2 = 35 s.
3.4. Effect of particles and frother concentration on froth stability at laboratory scale Fig. 7 shows the effect of frother concentration, frother type (MIBC and frother B) and hydrophobic particles (i.e.,
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
h 573 tph 40 % w/w 3.3 ppm 97.4% Fr A
Frother A
Feed Rougher 590 tph 40% w/w 40 3.25 ppm 100% Fr A
18.1 tph 33% w/w 2.8 ppm 2..6% Fr A Concentrate
781
Tailing
Scavenger
Key (tph) Water flow %) % Solids w/w (% Frother Concentration (ppm) Frother Recovery (%)
8.4 tph 22% w/w 3.4 ppm 1.5% Fr A Concentrate (a) Circuit A Recycled Water 2 ppm Fr B
Frother B Feed
Rougher 1 7.. 6 ppm 100%Fr B
7..1 ppm 97.1% Fr B
Rougher 2
6.7 ppm 98..8 %Fr B Tailing
9.. 0 ppm 1.2% B 1.2%Fr
Concentrate
8.0 ppm Fr B
Cleaners
Key
7.9 ppm Fr B
Frother Concentration (ppm) Frother Recovery (%)
Thickener
4.4 ppm Fr B (b) Circuit B
Final Concentrate
Fig. 6. Mass balance of residual frother in solution, shown as the percent recovery of frother in the flotation products from down-the-bank chemical surveys: (a) Circuit A, frother A; (b) Circuit B, frother B. Note: Frother addition points, measured frother concentrations, water flows and frother recoveries (distribution) are also shown for reference.
45
i.e., greater t1/2 values. Hydrophobic particles appear to give added stability to the froth, especially in the case of MIBC, where the increase in concentration from 10 to 40 ppm shows a greater change in froth stability in the presence of particles than that observed with frother B. MIBC gives lower t1/2 values and, hence, weaker froths. These tests have highlighted important differences between frother types and particle effects on froth stability, and how the froth half-life may be used to represent froth stability. This has implications for its significance and usefulness in froth phase modelling, which is explored further below.
40
Half Life (sec)
35 30 Frother B no particles Frother B with particles MIBC no particles MIBC with particles
25 20 15 10 5 0
0
10
20
30
40
50
60
70
Frother Concentration (ppm) Fig. 7. Froth half-life (t1/2), as a function of frother concentration determined from froth decay measurements in a laboratory sparger column. Notes: Froths were generated in the absence and presence of particles (rougher feed—Circuit B) using frother B (plant frother) and MIBC.
treating Circuit B rougher feed) on froth stability, as defined by the froth half-life in a laboratory sparger column. The surfactant type clearly gives differences in froth structure, with weaker frothers such as MIBC requiring higher concentrations to give moderately stable froths,
3.5. Froth recovery and froth residence time correlation at plant scale The exponential decay of froth recovery with an increase in froth retention (residence) time is well known and useful in terms of froth modelling (Gorain et al., 1998). Based on the data shown in Figs. 2b and 4, it is reasonable to suppose that the froth zone recovery of particles that arrive at the pulp–froth interface may be described in terms of froth residence time by a simple exponential function such as Eq. (3). An alternative interpretation to the b parameter in Eq. (3) is proposed here, which attempts to represent the interaction between particle transport in the froth transfer
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Table 4 Froth parameters useful for describing froth zone performance measured during surveys in Circuits A and B Plant conditions
Rf (range) (%)
FRT (range) (s)
Froth half-life t1/2 (s)
Recovery of frother to tailing stream (%)
Circuit A—rougher cells 3.5 ppm frother A
65–50
10–30
35
97.4
Circuit A—scavenger cells 3.5 ppm frother A
30–15
30–50
15
95.9
Circuit B—rougher cells 5.5 ppm frother B 7.5 ppm frother B 10 ppm frother B
40–35 75–50 50
25 15–30 40
25 54 65
96.4 95.9 96.5
and froth stability in the Rf–FRT relationship through consideration of the froth half-life. FRT Rf ¼ 100 exp ð7Þ t1=2 By taking into consideration differences in froth stability it is possible that different frother types and/or concentrations may show similar dependencies of froth recovery on the ratio of the froth residence time to froth half-life. For each survey condition, the exponential model according to Eq. (7) was fitted to the data points in Fig. 2b (Circuit A) and Fig. 4 (Circuit B), respectively. Table 4 shows the froth half-life values that were used to fit lines using Eq. (7) to the plant data shown in Figs. 2b and 4, respectively. Table 4 also summarises useful parameters for describing froth zone performance in the flotation systems. In particular, it shows that the frother predominantly reports to the tailing stream over the range of frother concentrations examined in Circuit B, in agreement with previous comments made in relation to Fig. 6. It should be noted that Eq. (7) is an empirical relationship only and has no direct physical meaning. It simply states that the decrease in froth recovery with froth residence time may also be related to the stability of the froth. For a more stable froth, the froth recovery decreases to a lesser extent for the same froth residence time. It is apparent from the different values of froth half-life in Table 4 that the relationship, according to Eq. (7), is strongly dependent on frother concentration and feed composition. While the data points show scatter, the interaction of froth recovery with froth stability and froth retention time appear to be useful for interpreting froth performance in flotation at an industrial scale. Froth stability appears to serve as a good indication of the significance of the sub-processes occurring within the froth. To ensure a froth zone recovery greater than 70%, the FRT ratio should t1=2 be <0.3. 4. Conclusions This work has shown that the froth phase has a pronounced influence on the overall flotation rate in industrial flotation circuits. The overall cell rate constant can be
reduced to less than 25% of the collection zone rate constant due to inefficiencies of the froth phase. This is particularly pronounced for scavenger cell froths in which the low stability of the froth apparently limits froth recovery and the overall cell rate constant. Because of the complexity and interactive nature of processes occurring within the froth phase, an important first step towards optimising the froth phase recovery depends upon determining practically useful froth parameters, such as the froth residence time and froth stability, which may then be correlated with froth recovery. Two separate industrial case studies were targeted to investigate froth phase performance and quantify the measured characteristics of the froth phase. Froth phase performance was defined by the froth zone recovery and the measured characteristics of the froth phase including frother concentration and froth stability. Froth zone recovery was found to generally decrease down-the-bank in Circuit A, with decreases in froth zone recovery limiting the recovery achievable in the plant scavenger cells. The decrease in froth zone recovery could not be linked to the removal of frother from the pulp solution to the concentrate product. However, there was a correlation of froth recovery to froth stability, as measured by the froth halflife. This parameter is strongly linked to the presence of particles in the froth, with a wide variation in froth stability between cells and feed types, which may help to explain the low froth recovery of poorly mineralised scavenger froths. These particular froths are characterised by short half-lives and low froth recovery. A simple exponential model, based on the froth half-life and froth residence time of gas, is proposed to predict froth zone recovery, however its significance or usefulness has not yet been completely defined. Further research is continuing to determine if observations regarding the relationship between froth recovery and froth stability may be applied generically to all plants. It is suspected that froth instability contributes to low recovery in scavenger stages generally, particularly for feeds containing low quantities of the mineral to be floated. Methods to counter froth instability are also being researched. These aspects are being studied in the new AMIRA International project, P541B ‘‘Optimising Froth Zone Performance in Mineral Flotation’’.
G. Tsatouhas et al. / Minerals Engineering 19 (2006) 774–783
Acknowledgements The authors wish to gratefully acknowledge the sponsors of the AMIRA International project P541A ‘‘Particle and Frother Interactions in Flotation Froths’’ including Anglo Platinum, Cytec, Newmont (Golden Grove Operations), Ok Tedi Mining, Rio Tinto, Teck Cominco and the Australian Research Council. The authors also wish to thank the P9 project ‘‘Mineral Processing’’ for the froth recovery measurements in Circuit A. This paper was first published in Proceedings Centenary of Flotation Symposium, Brisbane, Australia, 5–9 June 2005, by The Australasian Institute of Mining and Metallurgy. References Gorain, B.K., Napier-Munn, T.J., Franzidis, J.-P., Manlapig, E.V., 1998. Studies on impeller type, impeller speed and air flow rate in an
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industrial scale flotation cell, Part 5: Validation of k–Sb relationship and effect of froth depth. Miner. Eng. 11, 615. Iglesias, E., Anderez, J., Forgiarini, A., Salager, J., 1995. A new method to estimate the stability of short life-time foams. Colloids Surf. 98, 167. Johnson, N.W., McKee, D.J., Lynch, A.J., 1974. Flotation rates of nonsulphide minerals in chalcopyrite flotation processes. Trans. AIME 256. Laplante, A.R., Toguri, J.M., Smith, H.W., 1983. The effect of airflow rate on the kinetics of flotation—Part 1: The transfer of material from the slurry to the froth. Int. J. Miner. Process 11, 203. Savassi, O.N., Alexander, D.J., Johnson, N.W., Manlapig, E.V., Franzidis, J.-P., 1997. Measurement of froth recovery of attached particles in industrial cells. In: Lauder, D. (Ed.), Proceedings of the Sixth Mill Operators Conference. Aust. Inst. Min. Metall. Publ., Melbourne, pp. 149–155. Schwarz, S., Grano, S., Fornasiero, D., Ralston, J., 2002. Froth behaviour in the presence and absence of model quartz particles. In: Proceedings of WCPT4—World Congress on Particle Technology, Sydney, Paper 290. Vera, M.A., Franzidis, J.-P., Manlapig, E.V., 1999. Simultaneous determination of collection zone rate constant and froth zone recovery in a mechanical flotation environment. Miner. Eng. 12, 1163.