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The effect of increased frother dosage on froth stability at high depressant dosages
Minerals Engineering 23 (2010) 1010–1017
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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
The effect of increased frother dosage on froth stability at high depressant dosages J.G. Wiese a,*, P.J. Harris a, D.J. Bradshaw b a b
Centre for Minerals Research, University of Cape Town, South Africa Julius Krutschnitt Mineral Research Centre, University of Queensland, Australia
a r t i c l e
i n f o
Article history: Available online 9 June 2010 Keywords: Precious metal ores Froth flotation Flotation depressants Flotation frothers
a b s t r a c t High energy costs required to smelt low grade concentrates could be alleviated by the production of high grade concentrates. Obtaining maximum PGM recovery by the use of high dosages of polysaccharide depressants may be problematic in that a significant decrease in the stability of the froth, particularly with CMC, results. These highly unstable froths may result in restricted mass pulls and decreased valuable mineral recovery. There are a number of ways of counteracting unstable froths, such as increasing airflow rate, reducing froth height or increasing frother dosage. Although necessary to maximise PGM recovery, all of these are likely to lead to increased water recovery and dilution of the concentrate by entrained material. This work examines the effect of increasing frother dosage on the recovery of sulphide minerals and floatable gangue from a Merensky ore at varying dosages of guar gum and CMC, as well as on the recovery of entrained gangue and its dependence on the physical nature of the flotation pulp. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The presence of naturally floating gangue (NFG) in both the Merensky and UG2 ore types of the Bushveld Complex, South Africa, presents two processing problems. In the flotation of the sulphides and discrete Platinum Group Elements (PGE) after suitable comminution, the NFG interferes with the successful flotation of the slower-floating species such as pyrrhotite and ultrafine pentlandite because it enters the flotation hierarchy ahead of these species. In so doing it stabilises the froth and is recovered into the final concentrate partially at the expense of the aforementioned slow-floating species. Small amounts of NFG, including talc, in the order of 0.5–5% by mass in the ore, are sufficient to cause these undesirable effects.It has been shown that these naturally floatable minerals are due to the presence of talc along pyroxene grain boundaries (Becker et al., 2006; Jasieniak and Smart, 2009). The electricity supply problem in South Africa has led the reassessment of the strategies to optimise the recovery of PGE in order to reduce the load to the smelter. As a consequence, the use of high depressant dosages to increase the grade of the concentrate is being investigated. One of the major problems of using high depressant dosages is the marked decrease in froth stability due to the removal of the froth stabilising NFG (Bradshaw et al., 2005; Martinovic et al., 2005; Wiese, 2009). This may severely restrict mass pulls and decrease valuable mineral recovery. Muinonen
* Corresponding author. Tel.: +27 21 650 2517; fax: +27 21 650 5501. E-mail address: [email protected] (J.G. Wiese). 0892-6875/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2010.04.011
(2006) reported successful depression of Magnesium Oxide minerals at the Thompson, Manitoba nickel concentrator by the use of CMC and additional cleaner circuit capacity. The investigation into the effects on valuable mineral recovery is beyond the scope of the current paper. Although the two polysaccharide depressants appear to be almost interchangeable in their efficiency as depressants, their structures are significantly different. The CMC molecule has a high negative charge density in that, at high concentrations the adsorption of CMC results in a strong negative charge on the particles causing significant dispersion, while the guar gum molecule has a very low charge density which, even at high concentrations would not result in dispersed pulps in the presence of the high ionic strength waters found in the concentrators. Comparative adsorption studies of CMC, guar gum and starch depressants on talc were reported by Steenberg and Harris (1984). The measurements made of these reagents individually adsorbed on talc showed that the starch and guar gum produced a much higher level of adsorption on talc than did the CMC for any equilibrium concentration of that reagent. Adsorption of CMC on talc was reported by Parolis et al. (2008) to be enhanced by the presence of Ca++, Mg++ and K+, which are commonly found in stabilised concentrations in the recycle process water of Merensky flotation circuits. Low mass pulls can, to some extent, be overcome by an increase in frother dosage, a decrease in froth height or an increase in air flow rates. This work investigates the use of increased frother dosage on the froth stability and the recovery of valuable minerals in a Merensky ore at zero and high concentrations of guar gum and CMC depressants.
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xanthate (SIBX), supplied by Senmin was added to the mill prior to grinding at a dosage of 150 g/t. The polymeric depressants used in the batch flotation tests were Depramin 267, a carboxymethylcellulose (CMC) supplied by AKZO Nobel Functional Chemicals and Stypres 504, a modified guar gum supplied by Chemquest. The depressants were added at dosages of 250 and 500 g/t. Depressants were made up every second day at dosages corrected for active content. Tests were also conducted in the absence of a depressant. The frother, DOW 200, was supplied by Betachem and the chemical salts used in making up the synthetic plant water, were obtained from Merck. The air supply to the flotation cell was maintained at a flow rate of 7 L/min in all tests and the froth height was kept constant at 2 cm throughout. Four concentrates were collected at 2, 6, 12 and 20 min of flotation time by scraping the froth into a collecting pan every 15 s. A feed sample was taken before and a tailings sample was taken after each test. Water recoveries were measured for each test. Feeds, concentrates and tails were filtered, dried and weighed before analysis. All batch flotation tests were conducted in duplicate. Error bars showing standard error between duplicate tests are included in figures. Copper and total nickel analysis of all samples was done using a Bruker XRF. Sulphur analysis was carried out using a LECO sulphur analyser.
Recent testwork in the Reagent Research Facility at UCT has shown (Wiese, 2009) that at high depressant dosages (300 g/t) almost all the floatable gangue is depressed irrespective of the amount of NFG present in the ore. It has also been shown (Wiese, 2009) that increasing the depressant dosage to 500 g/t ensures that all NFG is depressed and that the only gangue reporting to the concentrate is in the form of entrained gangue. Under these conditions an entrainment factor for the particular ore, grind and frother dosage can be established. This factor can be used at lower depressant dosages to determine the amount of NFG reporting to the concentrate, and the effect of increasing frother dosage on NFG and the sulphides. For this work a Merensky ore containing a significant amount of NFG (Becker et al., 2010; Wiese, 2009) was selected and since the PGE in this type of ore are strongly associated with the sulphide minerals the effect of the increased frother dosages on the recovery of copper and nickel representing the sulphide minerals, chalcopyrite and pentlandite may be assessed. 2. Experimental details An ore from the southern section of the Merensky Reef in the Bushveld Igneous Complex, South Africa was prepared at UCT (Wiese et al., 2007) for use in batch flotation tests. All batch flotation tests were conducted using synthetic plant water, whereby distilled water is modified by the addition of various chemical salts to contain total dissolved solids of 1023 ppm (Wiese, 2009). Batch flotation tests were conducted using a method developed at UCT (Wiese, 2009). One kilogram portions of the ore sample were milled at 66% solids in synthetic plant water to achieve a grind of 60% passing 75 lm using a laboratory scale stainless steel rod mill. The milled slurry was transferred to a modified 3 L Leeds flotation cell. The volume in the cell was made up to produce 35% solids using synthetic plant water. The flotation cell was fitted with a variable speed drive and the pulp level was controlled manually by the addition of synthetic plant water. The impeller speed was set at 1200 rpm. The frother used for all the batch flotation tests was DOW 200 at dosages varying from 40 to 70 g/t. The collector, sodium isobutyl
3. Results and discussion For all previous testwork using this ore, DOW 200, at a fixed dosage of 40 g/t was used as the frother in order to limit the variables in the study of depressant action. In this study DOW 200 was again used as the frother, but the dosage was varied from 40 to 70 g/t. The dosage of both guar gum and CMC as depressant was set at 0, 250 and 500 g/t to examine the effect that the presence or absence of NFG may have on the changes in frothability brought about by the changes in frother dosage. It has been shown that all the NFG is totally depressed by the addition of 500 g/t depressant (Wiese, 2009) and therefore the effect that increasing frother dosage would have on the entrainment of gangue to the concentrate could be evaluated. At a depressant dosage of 250 g/t a small
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Fig. 1. Total mass and water recovered for all conditions evaluated.
40 50 60 70 500 g/t guar gum
Cumulative mass, g
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significant amount of NFG in the froth in tests conducted in the absence of depressant resulted in a much greater increase in the water recovered than that observed when either 250 or 500 g/t depressant was added and indicates the important role of NFG in enhancing the stability of the froth. The amount of entrained gangue (or entrainability) in the concentrate as a function of the water recovered is shown in Fig. 2 for 500 g/t guar gum addition and Fig. 3 for 500 g/t CMC addition. Fig. 2 indicates a slight decrease in the entrainment factor (gradient of the line) for guar gum as frother concentration was increased whereas, for CMC (Fig. 3), the entrainability factor remained constant. It has been previously shown (Wiese, 2009) that, at a CMC dosage of 500 g/t, due to the strong negative charge nature of the CMC molecule, the pulp is strongly dispersed whereas a dosage of 500 g/t guar gum would not result in dispersed pulps. The difference in the behaviour of the two polysaccharides suggests that, at higher
amount of NFG would still be reporting to the concentrate and from previous testwork (Wiese, 2009) the amount of NFG reporting to the concentrate per unit water should be of the same order as the amount of gangue reporting to the concentrate by entrainment. As in previous work the measure of froth stability has been taken as the mass of water reporting to the concentrate at a fixed froth height. Total mass and water recovered for the various conditions evaluated are shown in Fig. 1. For all depressant dosages it is shown that an increase in frother dosage resulted in an increase in water recovery and a corresponding increase in the mass reporting to the concentrate. This indicates that, whatever the nature of the froth stability (stabilised or destabilised) an increase in frother dosage resulted in an increase in froth stability and a greater mass recovery. The addition of depressant, either guar gum or CMC, significantly reduced froth stability and the mass of water recovered. The presence of a
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Total gangue, g
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12 slope = 0.0231 slope = 0.0266
slope= 0.0195
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Water, g 40 g/t DOW 200
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Fig. 2. Total gangue versus water recovered using guar gum at a dosage of 500 g/t.
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500 g/t CMC 16
Total gangue, g
slope = 0.0269
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slope = 0.0280
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Water, g 40 g/t DOW 200
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Fig. 3. Total gangue versus water recovered using CMC at a dosage of 500 g/t.
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and the linear relationship between water recovered and entrained gangue. The addition of depressant to the flotation system resulted in a major decrease in the amount of NFG reporting to the concentrate. In the absence of a depressant, NFG comprises the major mass reporting to the concentrate. For the 1 kg feed mass used in each batch flotation test and the frother dosages used, 60–80 g of NFG were recovered compared to 8 g of sulphide minerals and 10–20 g of entrained gangue. This results in the very low grades obtained in the absence of depressant. The amount of gangue entrained has been included in Figs. 4 and 5, and it can be seen that, at a dosage of 250 g/t guar gum or CMC, the amount of floatable gangue is of the same order as the amount of gangue recovered by entrainment. At depressant dosages of 500 g/t the amount of floatable gangue, by definition, is zero. The cumulative mass of water recovered as a function of time is shown in Figs. 6 and 7, at depressant additions of 250 and 500 g/t, for guar gum and CMC respectively. Fig. 6 shows an almost linear
frother dosages and the resultant higher water recoveries, some coagulation of the entrained pulp in the froth may be occurring in the more voluminous froths obtained when using guar gum. The strongly dispersing nature of the CMC, however, ensures that the entrainment factor remains constant. The resultant changes in the amount of entrained gangue reporting to the concentrate are, however, minor compared to the changes in the mass of NFG recovered, resulting from the increased depressant dosage. The effect of increasing frother dosage on the recovery of NFG to the concentrate for the selected depressant dosages is shown in Fig. 4 for guar gum and Fig. 5 for CMC, as a function of water recovered. These figures indicate that the increased frother dosage had no measurable influence on the hydrophobicity of the floatable gangue at any depressant dosage evaluated confirming that frother does not adsorb on or alter the nature of NFG. However, it should be noted that an increase in froth stability would invariably lead to a lowering of the grade due to the slow floating nature of the NFG
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Fig. 4. Floating gangue versus water recovered for all conditions evaluated in the presence of guar gum.
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Fig. 5. Floating gangue versus water recovered for all conditions evaluated in the presence of CMC.
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800 250 g/t guar gum - broken 500 g/t guar gum - solid
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Fig. 6. Water recovered versus time for all tests conducted in the presence of guar gum.
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Water, g
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Fig. 7. Water recovered versus time for all tests conducted in the presence of CMC.
increase in the cumulative amount of water reporting to the concentrate during the batch flotation test in the presence of guar gum. Increasing frother concentration led to an increase in the rate of water recovery and invariably the addition of 500 g/t depressant reduced the rate of water recovery when compared to a dosage of 250 g/t. The lower water recoveries suggest that, when the amount of floatable gangue is equivalent to the amount of entrained gangue, some stabilisation of the froth by the NFG was occurring at a guar gum dosage of 250 g/t. Since NFG floats relatively slowly the stabilisation occurs in all four concentrates. Upon examination of the data displayed in Fig. 7, it can be seen that the rate of water recovery observed at a CMC dosage of 250 g/t is linear and increasing the frother dosage increased the rate of water recovery. However, water recoveries at 500 g/t CMC addition are not linear and show a distinct reduction in the first two concen-
trates followed by an increased rate of recovery in the final two concentrates. These results indicate that there was significant destabilisation of the froth during the collection of the first two concentrates. At this dosage of CMC, the pulp is in a strongly dispersed state and, since previous postulations (Wiese, 2009) have indicated that the sulphide minerals (chalcopyrite and pentlandite) themselves can play a significant role in the destabilisation of the froth, it is evident that the strong dispersion has led to an enhancement in the destabilisation of the froth since it occurs during the collection of the first two concentrates when the majority of the copper and nickel sulphides are recovered. It should be noted that at the lowest dosage of 40 g/t frother the water recovery remains low (destabilised) for all concentrates. This restricted recovery of mass inhibits the recovery of sulphide minerals and for tests using 40 g/t frother there was still significant recovery of sulphide min-
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Merensky ore being used is not extreme, the assumption that total nickel recoveries are sufficient to monitor pentlandite behaviour is workable within certain limits. In this work total nickel recoveries are thus sufficient to monitor pentlandite behaviour (Brough, 2008; Wiese, 2009). The figures are of particular interest since there was no NFG reporting to the concentrate and the grade and recovery of the sulphides is totally dependent on their rate of flotation and the amount of gangue being recovered by entrainment, i.e. the stability of the froth. For guar gum addition at 500 g/t the increase in water recovery resulting from the increased frother dosage led to a decrease in nickel grade with an equivalent but smaller change in the copper grade. Very different behaviour was, however, observed at 500 g/t CMC addition (Fig. 9). For 40 and 50 g/t frother addition there was a marked increase in the initial copper grades while, particularly at 40 g/t, there was a marked reduction in the initial grade
erals in the final two concentrates and destabilisation of the froth was occurring. The enhancement in the destabilising ability of the sulphides was caused by the dispersing action of the CMC presumably by slime-cleaning the sulphide particles. The grade versus recovery curves for copper and nickel at 500 g/t guar gum and CMC addition are shown in Figs. 8 and 9, respectively. The analysis of the response of the sulphide minerals to increased frother dosage was determined by following the recovery of copper and nickel. Some of the nickel present in Merensky ore is associated with the gangue minerals due to serpentinisation, and total nickel recoveries obtained are lower than the sulphide nickel recoveries would be expected to be. This is because serpentine carries nickel in solid solution to varying degrees but in an unfloatable mineral form. A measure of this degree of serpentinisation was proposed to be the total of talc, chlorite and serpentine (Peyerl, 1983). Provided that the degree of serpentinisation in the
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Fig. 8. Copper and nickel grade versus recovery for tests conducted in the presence of 500 g/t guar gum.
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Fig. 9. Copper and nickel grade versus recovery for tests conducted in the presence of 500 g/t CMC.
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and final recovery of nickel. At the low water recoveries obtained for these conditions it is apparent that the enhancement of the copper grade was due to competitive retention (froth crowding) occurring in the highly unstable froth where the faster floating copper was recovered at the expense of the slower floating nickel. The final grades and recoveries of copper and nickel for all conditions are shown in Fig. 10 for copper and Fig. 11 for nickel. The highest recoveries of copper and nickel were obtained in the absence of any depressant addition and increasing the frother dosage always resulted in a slight increase in both copper and nickel recoveries. As expected the grades observed were extremely low and impractical, indicating the reasons for the addition of depressant. Increasing the depressant dosage to 250 g/t (guar gum and
CMC) resulted in much higher copper and nickel grades. An improvement in froth stability by increasing the frother dosage resulted in a slight improvement in copper and nickel recoveries, but at the expense of reducing the grade. However, there was a reduction in the recovery of both copper and nickel when compared to the zero depressant tests indicating that some loss of valuable minerals was occurring presumably due to the depression of sulphide/ gangue associated particles. It would appear that, at 250 g/t depressant dosage higher sulphide mineral recoveries were obtained with guar gum than with CMC at the higher frother dosages. However, the fact that higher froth stabilities were obtained with guar gum than with CMC may account for the increased recovery (see Fig. 1).
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Final copper grade, %
Final copper recovery, %
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40 50 60 70 500 g/t CMC
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40 50 60 70 500 g/t guar gum
Frother Dosage, g/t Cu recovery
Cu grade
Fig. 10. Final copper grades and recoveries for all conditions evaluated at varying frother dosages.
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Final nickel recovery, %
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Ni grade
Fig. 11. Final nickel grades and recoveries for all conditions evaluated at varying frother dosages.
Final nickel grade, %
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By increasing the depressant dosage to 500 g/t, copper and nickel grades were almost double those obtained when using a depressant dosage of 250 g/t. This was expected since the mass of NFG which was depressed was similar to the entrained mass. Again, the further increase in depressant dosage to 500 g/t led to a slight reduction in both copper and nickel recoveries. The froth crowding nature of the destabilised froths is apparent in the reduced recovery of nickel compared to copper at the two lower frother dosages. Since all NFG had been depressed at 500 g/t depressant addition it is very unlikely that using even higher dosages of depressant would lead to further improvement in grades and may even lead to further depression of composite and possibly liberated particles. Grades obtained would always depend on the stability of the froth and the resultant water recovery. 4. Conclusions Increased frother dosage, from 40 to 70 g/t, resulted in increased froth stability (as indicated by water recovery) for all conditions evaluated. Highly destabilised froths, due to the dispersing action of high dosages of CMC depressant, were rendered more stable by the use of increased frother dosage. An increase in frother dosage improved the recovery of copper and nickel, but at the expense of grade. At high depressant dosages (>300 g/t) the copper and nickel grades were controlled by the water recovery and the associated entrained gangue. High dosages of depressant resulted in an increase in copper and nickel grades but always at the expense of recovery, presumably by the depression of partially liberated sulphide/gangue particles. However, it must always be borne in mind that increased frother dosages may lead to downstream problems in a concentrator. This testwork was carried out using one frother. Further work should involve the use of different frothers to determine whether the same effects are noted.
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Acknowledgements Lonmin Platinum for supplying the Merensky ore used in this study Members of the Reagent Research Facility for providing the financial support to enable this work to take place. References Becker, M., Harris, P., Wiese, J., Bradshaw, D., 2006. The use of quantitative mineralogical data to interpret the behaviour of gangue minerals in the flotation of Merensky Reef ores. In: Proceedings of Automated Mineralogy 06, Brisbane, Australia. Becker, M., Harris, P., Wiese, J., Corin, K., Bradshaw, D., The use of automated mineralogy to interpret the batch flotation performance of Merensky Reef ore. Accepted for presentation at XXV IMPC 2010 Brisbane, Australia. Bradshaw, D.J., Harris, P.J., O’Connor, C.T., 2005. The effect of collectors and their interactions with depressants on the behaviour of the froth phase in flotation. In: Proceedings of Centenary of Flotation Symposium Brisbane 2005. Brough, C., 2008. An investigation into the process mineralogy of the Merensky Reef at Northam Platinum Limited. MSc Thesis, Faculty of Engineering and the Built Environment, University of Cape Town. Jasieniak, M., Smart, R. St C., 2009. Collectorless flotation of pyroxene in Merensky ore: residual layer identification using statistical ToF-SIMS analysis. International Journal of Mineral Processing 92, 169–176. Martinovic, J., Bradshaw, D.J., Harris, P.J., 2005. Investigation of surface properties of gangue minerals in platinum bearing ores. Journal of the South African Institute of Mining and Metallurgy 105, 1–7. Muinonen, J., 2006. Thompson mill MgO rejection. In: Proc. Canadian Mineral Processors, Ottawa, Paper 30, pp. 481–499. Parolis, L.A.S., van der Merwe, R., Groenmeyer, G., Harris, P.J., 2008. The influence of metal cations on carboxymethylcelluloses as talc depressants. Journal of Colloids and Surfaces, A: Physiochemical and Engineering Aspects 317 (1–3), 109–115. Peyerl, W., 1983. The metallurgical implications of the mode of occurrence of platinum-group metals in the Meresnsky Reef and UG2 chromitite of the bushveld igneous complex. Special Publication of the Geological Society of South Africa 7, 295–300. Steenberg, E., Harris, P.J., 1984. Adsorption of carboxymethyl cellulose, gur gum and starch onto talc, sulphides, oxides and salt type minerals. South African Journal of Chemistry 37 (3). Wiese, J., 2009. Investigating depressant behaviour in the flotation of selected Merensky ores. MSc Thesis Faculty of Engineering and the Built Environment, University of Cape Town. Wiese, J., Harris, P., Bradshaw, D., 2007. The response of sulphide and gangue minerals in selected Merensky ores to increased depressant dosages. Minerals Engineering 20, 986–995.
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
The effect of increased frother dosage on froth stability at high depressant dosages J.G. Wiese a,*, P.J. Harris a, D.J. Bradshaw b a b
Centre for Minerals Research, University of Cape Town, South Africa Julius Krutschnitt Mineral Research Centre, University of Queensland, Australia
a r t i c l e
i n f o
Article history: Available online 9 June 2010 Keywords: Precious metal ores Froth flotation Flotation depressants Flotation frothers
a b s t r a c t High energy costs required to smelt low grade concentrates could be alleviated by the production of high grade concentrates. Obtaining maximum PGM recovery by the use of high dosages of polysaccharide depressants may be problematic in that a significant decrease in the stability of the froth, particularly with CMC, results. These highly unstable froths may result in restricted mass pulls and decreased valuable mineral recovery. There are a number of ways of counteracting unstable froths, such as increasing airflow rate, reducing froth height or increasing frother dosage. Although necessary to maximise PGM recovery, all of these are likely to lead to increased water recovery and dilution of the concentrate by entrained material. This work examines the effect of increasing frother dosage on the recovery of sulphide minerals and floatable gangue from a Merensky ore at varying dosages of guar gum and CMC, as well as on the recovery of entrained gangue and its dependence on the physical nature of the flotation pulp. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The presence of naturally floating gangue (NFG) in both the Merensky and UG2 ore types of the Bushveld Complex, South Africa, presents two processing problems. In the flotation of the sulphides and discrete Platinum Group Elements (PGE) after suitable comminution, the NFG interferes with the successful flotation of the slower-floating species such as pyrrhotite and ultrafine pentlandite because it enters the flotation hierarchy ahead of these species. In so doing it stabilises the froth and is recovered into the final concentrate partially at the expense of the aforementioned slow-floating species. Small amounts of NFG, including talc, in the order of 0.5–5% by mass in the ore, are sufficient to cause these undesirable effects.It has been shown that these naturally floatable minerals are due to the presence of talc along pyroxene grain boundaries (Becker et al., 2006; Jasieniak and Smart, 2009). The electricity supply problem in South Africa has led the reassessment of the strategies to optimise the recovery of PGE in order to reduce the load to the smelter. As a consequence, the use of high depressant dosages to increase the grade of the concentrate is being investigated. One of the major problems of using high depressant dosages is the marked decrease in froth stability due to the removal of the froth stabilising NFG (Bradshaw et al., 2005; Martinovic et al., 2005; Wiese, 2009). This may severely restrict mass pulls and decrease valuable mineral recovery. Muinonen
* Corresponding author. Tel.: +27 21 650 2517; fax: +27 21 650 5501. E-mail address: [email protected] (J.G. Wiese). 0892-6875/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2010.04.011
(2006) reported successful depression of Magnesium Oxide minerals at the Thompson, Manitoba nickel concentrator by the use of CMC and additional cleaner circuit capacity. The investigation into the effects on valuable mineral recovery is beyond the scope of the current paper. Although the two polysaccharide depressants appear to be almost interchangeable in their efficiency as depressants, their structures are significantly different. The CMC molecule has a high negative charge density in that, at high concentrations the adsorption of CMC results in a strong negative charge on the particles causing significant dispersion, while the guar gum molecule has a very low charge density which, even at high concentrations would not result in dispersed pulps in the presence of the high ionic strength waters found in the concentrators. Comparative adsorption studies of CMC, guar gum and starch depressants on talc were reported by Steenberg and Harris (1984). The measurements made of these reagents individually adsorbed on talc showed that the starch and guar gum produced a much higher level of adsorption on talc than did the CMC for any equilibrium concentration of that reagent. Adsorption of CMC on talc was reported by Parolis et al. (2008) to be enhanced by the presence of Ca++, Mg++ and K+, which are commonly found in stabilised concentrations in the recycle process water of Merensky flotation circuits. Low mass pulls can, to some extent, be overcome by an increase in frother dosage, a decrease in froth height or an increase in air flow rates. This work investigates the use of increased frother dosage on the froth stability and the recovery of valuable minerals in a Merensky ore at zero and high concentrations of guar gum and CMC depressants.
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xanthate (SIBX), supplied by Senmin was added to the mill prior to grinding at a dosage of 150 g/t. The polymeric depressants used in the batch flotation tests were Depramin 267, a carboxymethylcellulose (CMC) supplied by AKZO Nobel Functional Chemicals and Stypres 504, a modified guar gum supplied by Chemquest. The depressants were added at dosages of 250 and 500 g/t. Depressants were made up every second day at dosages corrected for active content. Tests were also conducted in the absence of a depressant. The frother, DOW 200, was supplied by Betachem and the chemical salts used in making up the synthetic plant water, were obtained from Merck. The air supply to the flotation cell was maintained at a flow rate of 7 L/min in all tests and the froth height was kept constant at 2 cm throughout. Four concentrates were collected at 2, 6, 12 and 20 min of flotation time by scraping the froth into a collecting pan every 15 s. A feed sample was taken before and a tailings sample was taken after each test. Water recoveries were measured for each test. Feeds, concentrates and tails were filtered, dried and weighed before analysis. All batch flotation tests were conducted in duplicate. Error bars showing standard error between duplicate tests are included in figures. Copper and total nickel analysis of all samples was done using a Bruker XRF. Sulphur analysis was carried out using a LECO sulphur analyser.
Recent testwork in the Reagent Research Facility at UCT has shown (Wiese, 2009) that at high depressant dosages (300 g/t) almost all the floatable gangue is depressed irrespective of the amount of NFG present in the ore. It has also been shown (Wiese, 2009) that increasing the depressant dosage to 500 g/t ensures that all NFG is depressed and that the only gangue reporting to the concentrate is in the form of entrained gangue. Under these conditions an entrainment factor for the particular ore, grind and frother dosage can be established. This factor can be used at lower depressant dosages to determine the amount of NFG reporting to the concentrate, and the effect of increasing frother dosage on NFG and the sulphides. For this work a Merensky ore containing a significant amount of NFG (Becker et al., 2010; Wiese, 2009) was selected and since the PGE in this type of ore are strongly associated with the sulphide minerals the effect of the increased frother dosages on the recovery of copper and nickel representing the sulphide minerals, chalcopyrite and pentlandite may be assessed. 2. Experimental details An ore from the southern section of the Merensky Reef in the Bushveld Igneous Complex, South Africa was prepared at UCT (Wiese et al., 2007) for use in batch flotation tests. All batch flotation tests were conducted using synthetic plant water, whereby distilled water is modified by the addition of various chemical salts to contain total dissolved solids of 1023 ppm (Wiese, 2009). Batch flotation tests were conducted using a method developed at UCT (Wiese, 2009). One kilogram portions of the ore sample were milled at 66% solids in synthetic plant water to achieve a grind of 60% passing 75 lm using a laboratory scale stainless steel rod mill. The milled slurry was transferred to a modified 3 L Leeds flotation cell. The volume in the cell was made up to produce 35% solids using synthetic plant water. The flotation cell was fitted with a variable speed drive and the pulp level was controlled manually by the addition of synthetic plant water. The impeller speed was set at 1200 rpm. The frother used for all the batch flotation tests was DOW 200 at dosages varying from 40 to 70 g/t. The collector, sodium isobutyl
3. Results and discussion For all previous testwork using this ore, DOW 200, at a fixed dosage of 40 g/t was used as the frother in order to limit the variables in the study of depressant action. In this study DOW 200 was again used as the frother, but the dosage was varied from 40 to 70 g/t. The dosage of both guar gum and CMC as depressant was set at 0, 250 and 500 g/t to examine the effect that the presence or absence of NFG may have on the changes in frothability brought about by the changes in frother dosage. It has been shown that all the NFG is totally depressed by the addition of 500 g/t depressant (Wiese, 2009) and therefore the effect that increasing frother dosage would have on the entrainment of gangue to the concentrate could be evaluated. At a depressant dosage of 250 g/t a small
1000
120
900
Cumulative water, g
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600 500
60
400 40
300 200
20 100 0
0 40 50 60 70 no depressant
40 50 60 70 250 g/t CMC
40 50 60 70 500 g/t CMC
40 50 60 70 250 g/t guar gum
Frother dosage, g/t Water
Mass
Fig. 1. Total mass and water recovered for all conditions evaluated.
40 50 60 70 500 g/t guar gum
Cumulative mass, g
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significant amount of NFG in the froth in tests conducted in the absence of depressant resulted in a much greater increase in the water recovered than that observed when either 250 or 500 g/t depressant was added and indicates the important role of NFG in enhancing the stability of the froth. The amount of entrained gangue (or entrainability) in the concentrate as a function of the water recovered is shown in Fig. 2 for 500 g/t guar gum addition and Fig. 3 for 500 g/t CMC addition. Fig. 2 indicates a slight decrease in the entrainment factor (gradient of the line) for guar gum as frother concentration was increased whereas, for CMC (Fig. 3), the entrainability factor remained constant. It has been previously shown (Wiese, 2009) that, at a CMC dosage of 500 g/t, due to the strong negative charge nature of the CMC molecule, the pulp is strongly dispersed whereas a dosage of 500 g/t guar gum would not result in dispersed pulps. The difference in the behaviour of the two polysaccharides suggests that, at higher
amount of NFG would still be reporting to the concentrate and from previous testwork (Wiese, 2009) the amount of NFG reporting to the concentrate per unit water should be of the same order as the amount of gangue reporting to the concentrate by entrainment. As in previous work the measure of froth stability has been taken as the mass of water reporting to the concentrate at a fixed froth height. Total mass and water recovered for the various conditions evaluated are shown in Fig. 1. For all depressant dosages it is shown that an increase in frother dosage resulted in an increase in water recovery and a corresponding increase in the mass reporting to the concentrate. This indicates that, whatever the nature of the froth stability (stabilised or destabilised) an increase in frother dosage resulted in an increase in froth stability and a greater mass recovery. The addition of depressant, either guar gum or CMC, significantly reduced froth stability and the mass of water recovered. The presence of a
20
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Total gangue, g
16
12 slope = 0.0231 slope = 0.0266
slope= 0.0195
8 slope = 0.0269
4
0 0
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200
300
400
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600
Water, g 40 g/t DOW 200
50 g/t DOW 200
60 g/t DOW 200
70 g/t DOW 200
Fig. 2. Total gangue versus water recovered using guar gum at a dosage of 500 g/t.
20
500 g/t CMC 16
Total gangue, g
slope = 0.0269
12 slope = 0.0263
8
slope = 0.0280
4
slope = 0.0353
0 0
100
200
300
400
500
Water, g 40 g/t DOW 200
50 g/t DOW 200
60 g/t DOW 200
70 g/t DOW 200
Fig. 3. Total gangue versus water recovered using CMC at a dosage of 500 g/t.
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and the linear relationship between water recovered and entrained gangue. The addition of depressant to the flotation system resulted in a major decrease in the amount of NFG reporting to the concentrate. In the absence of a depressant, NFG comprises the major mass reporting to the concentrate. For the 1 kg feed mass used in each batch flotation test and the frother dosages used, 60–80 g of NFG were recovered compared to 8 g of sulphide minerals and 10–20 g of entrained gangue. This results in the very low grades obtained in the absence of depressant. The amount of gangue entrained has been included in Figs. 4 and 5, and it can be seen that, at a dosage of 250 g/t guar gum or CMC, the amount of floatable gangue is of the same order as the amount of gangue recovered by entrainment. At depressant dosages of 500 g/t the amount of floatable gangue, by definition, is zero. The cumulative mass of water recovered as a function of time is shown in Figs. 6 and 7, at depressant additions of 250 and 500 g/t, for guar gum and CMC respectively. Fig. 6 shows an almost linear
frother dosages and the resultant higher water recoveries, some coagulation of the entrained pulp in the froth may be occurring in the more voluminous froths obtained when using guar gum. The strongly dispersing nature of the CMC, however, ensures that the entrainment factor remains constant. The resultant changes in the amount of entrained gangue reporting to the concentrate are, however, minor compared to the changes in the mass of NFG recovered, resulting from the increased depressant dosage. The effect of increasing frother dosage on the recovery of NFG to the concentrate for the selected depressant dosages is shown in Fig. 4 for guar gum and Fig. 5 for CMC, as a function of water recovered. These figures indicate that the increased frother dosage had no measurable influence on the hydrophobicity of the floatable gangue at any depressant dosage evaluated confirming that frother does not adsorb on or alter the nature of NFG. However, it should be noted that an increase in froth stability would invariably lead to a lowering of the grade due to the slow floating nature of the NFG
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Floating gangue, g
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entrained
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250 g/t guar gum 20 10
500 g/t guar gum 0 0
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400
600
800
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Water, g 40 g/t DOW 200
50 g/t DOW 200
60 g/t DOW 200
70 g/t DOW 200
entrain
Fig. 4. Floating gangue versus water recovered for all conditions evaluated in the presence of guar gum.
100 90
Floating gangue, g
80
No depressant 70 60 50 40
entrained gangue
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250 g/t CMC
20 10
500 g/t CMC 0 0
200
400
600
800
1000
1200
Water, g 40 g/t DOW 200
50 g/t DOW 200
60 g/t DOW 200
70 g/t DOW 200
Fig. 5. Floating gangue versus water recovered for all conditions evaluated in the presence of CMC.
entrain
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800 250 g/t guar gum - broken 500 g/t guar gum - solid
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Water, g
500 400 300 200 100 0 0
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Time, min 40 g/t DOW 200 40 g/t DOW 200
50 g/t DOW 200 50 g/t DOW 200
60 g/t DOW 200 60 g/t DOW 200
70 g/t DOW 200 70 g/t DOW 200
Fig. 6. Water recovered versus time for all tests conducted in the presence of guar gum.
700 250 g/t CMC - broken 500 g/t CMC - solid
600
Water, g
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400
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0 0
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Time, min 40 g/t DOW 200 40 g/t DOW 200
50 g/t DOW 200 50 g/t DOW 200
60 g/t DOW 200 60 g/t DOW 200
70 g/t DOW 200 70 g/t DOW 200
Fig. 7. Water recovered versus time for all tests conducted in the presence of CMC.
increase in the cumulative amount of water reporting to the concentrate during the batch flotation test in the presence of guar gum. Increasing frother concentration led to an increase in the rate of water recovery and invariably the addition of 500 g/t depressant reduced the rate of water recovery when compared to a dosage of 250 g/t. The lower water recoveries suggest that, when the amount of floatable gangue is equivalent to the amount of entrained gangue, some stabilisation of the froth by the NFG was occurring at a guar gum dosage of 250 g/t. Since NFG floats relatively slowly the stabilisation occurs in all four concentrates. Upon examination of the data displayed in Fig. 7, it can be seen that the rate of water recovery observed at a CMC dosage of 250 g/t is linear and increasing the frother dosage increased the rate of water recovery. However, water recoveries at 500 g/t CMC addition are not linear and show a distinct reduction in the first two concen-
trates followed by an increased rate of recovery in the final two concentrates. These results indicate that there was significant destabilisation of the froth during the collection of the first two concentrates. At this dosage of CMC, the pulp is in a strongly dispersed state and, since previous postulations (Wiese, 2009) have indicated that the sulphide minerals (chalcopyrite and pentlandite) themselves can play a significant role in the destabilisation of the froth, it is evident that the strong dispersion has led to an enhancement in the destabilisation of the froth since it occurs during the collection of the first two concentrates when the majority of the copper and nickel sulphides are recovered. It should be noted that at the lowest dosage of 40 g/t frother the water recovery remains low (destabilised) for all concentrates. This restricted recovery of mass inhibits the recovery of sulphide minerals and for tests using 40 g/t frother there was still significant recovery of sulphide min-
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Merensky ore being used is not extreme, the assumption that total nickel recoveries are sufficient to monitor pentlandite behaviour is workable within certain limits. In this work total nickel recoveries are thus sufficient to monitor pentlandite behaviour (Brough, 2008; Wiese, 2009). The figures are of particular interest since there was no NFG reporting to the concentrate and the grade and recovery of the sulphides is totally dependent on their rate of flotation and the amount of gangue being recovered by entrainment, i.e. the stability of the froth. For guar gum addition at 500 g/t the increase in water recovery resulting from the increased frother dosage led to a decrease in nickel grade with an equivalent but smaller change in the copper grade. Very different behaviour was, however, observed at 500 g/t CMC addition (Fig. 9). For 40 and 50 g/t frother addition there was a marked increase in the initial copper grades while, particularly at 40 g/t, there was a marked reduction in the initial grade
erals in the final two concentrates and destabilisation of the froth was occurring. The enhancement in the destabilising ability of the sulphides was caused by the dispersing action of the CMC presumably by slime-cleaning the sulphide particles. The grade versus recovery curves for copper and nickel at 500 g/t guar gum and CMC addition are shown in Figs. 8 and 9, respectively. The analysis of the response of the sulphide minerals to increased frother dosage was determined by following the recovery of copper and nickel. Some of the nickel present in Merensky ore is associated with the gangue minerals due to serpentinisation, and total nickel recoveries obtained are lower than the sulphide nickel recoveries would be expected to be. This is because serpentine carries nickel in solid solution to varying degrees but in an unfloatable mineral form. A measure of this degree of serpentinisation was proposed to be the total of talc, chlorite and serpentine (Peyerl, 1983). Provided that the degree of serpentinisation in the
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500 g/t guar gum
Cu, Ni grade, %
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12
Copper 8
Nickel 4
0 0
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50
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Cu, Ni recovery, % 40 g/t DOW 200
50 g/t DOW 200
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70 g/t DOW 200
Fig. 8. Copper and nickel grade versus recovery for tests conducted in the presence of 500 g/t guar gum.
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500 g/t CMC 16
Cu, Ni grade, %
Nickel
Copper
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8
4
0 0
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30
40
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60
70
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Cu, Ni recovery, % 40 g/t DOW 200
50 g/t DOW 200
60 g/t DOW 200
70 g/t DOW 200
Fig. 9. Copper and nickel grade versus recovery for tests conducted in the presence of 500 g/t CMC.
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and final recovery of nickel. At the low water recoveries obtained for these conditions it is apparent that the enhancement of the copper grade was due to competitive retention (froth crowding) occurring in the highly unstable froth where the faster floating copper was recovered at the expense of the slower floating nickel. The final grades and recoveries of copper and nickel for all conditions are shown in Fig. 10 for copper and Fig. 11 for nickel. The highest recoveries of copper and nickel were obtained in the absence of any depressant addition and increasing the frother dosage always resulted in a slight increase in both copper and nickel recoveries. As expected the grades observed were extremely low and impractical, indicating the reasons for the addition of depressant. Increasing the depressant dosage to 250 g/t (guar gum and
CMC) resulted in much higher copper and nickel grades. An improvement in froth stability by increasing the frother dosage resulted in a slight improvement in copper and nickel recoveries, but at the expense of reducing the grade. However, there was a reduction in the recovery of both copper and nickel when compared to the zero depressant tests indicating that some loss of valuable minerals was occurring presumably due to the depression of sulphide/ gangue associated particles. It would appear that, at 250 g/t depressant dosage higher sulphide mineral recoveries were obtained with guar gum than with CMC at the higher frother dosages. However, the fact that higher froth stabilities were obtained with guar gum than with CMC may account for the increased recovery (see Fig. 1).
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8 7
90 6 85 5
80 75
4
70
3
65
Final copper grade, %
Final copper recovery, %
95
2 60 1
55 50
0 40 50 60 70 No depressant
40 50 60 70 250 g/t CMC
40 50 60 70 500 g/t CMC
40 50 60 70 250 g/t guar gum
40 50 60 70 500 g/t guar gum
Frother Dosage, g/t Cu recovery
Cu grade
Fig. 10. Final copper grades and recoveries for all conditions evaluated at varying frother dosages.
70
12
65
Final nickel recovery, %
55
8
50 45
6
40 4
35 30
2
25 20
0 40 50 60 70 No depressant
40 50 60 70 250 g/t CMC
40 50 60 70 500 g/t CMC
40 50 60 70 250 g/t guar gum
40 50 60 70 500 g/t guar gum
Frother dosage, g/t Ni recovery
Ni grade
Fig. 11. Final nickel grades and recoveries for all conditions evaluated at varying frother dosages.
Final nickel grade, %
10
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By increasing the depressant dosage to 500 g/t, copper and nickel grades were almost double those obtained when using a depressant dosage of 250 g/t. This was expected since the mass of NFG which was depressed was similar to the entrained mass. Again, the further increase in depressant dosage to 500 g/t led to a slight reduction in both copper and nickel recoveries. The froth crowding nature of the destabilised froths is apparent in the reduced recovery of nickel compared to copper at the two lower frother dosages. Since all NFG had been depressed at 500 g/t depressant addition it is very unlikely that using even higher dosages of depressant would lead to further improvement in grades and may even lead to further depression of composite and possibly liberated particles. Grades obtained would always depend on the stability of the froth and the resultant water recovery. 4. Conclusions Increased frother dosage, from 40 to 70 g/t, resulted in increased froth stability (as indicated by water recovery) for all conditions evaluated. Highly destabilised froths, due to the dispersing action of high dosages of CMC depressant, were rendered more stable by the use of increased frother dosage. An increase in frother dosage improved the recovery of copper and nickel, but at the expense of grade. At high depressant dosages (>300 g/t) the copper and nickel grades were controlled by the water recovery and the associated entrained gangue. High dosages of depressant resulted in an increase in copper and nickel grades but always at the expense of recovery, presumably by the depression of partially liberated sulphide/gangue particles. However, it must always be borne in mind that increased frother dosages may lead to downstream problems in a concentrator. This testwork was carried out using one frother. Further work should involve the use of different frothers to determine whether the same effects are noted.
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Acknowledgements Lonmin Platinum for supplying the Merensky ore used in this study Members of the Reagent Research Facility for providing the financial support to enable this work to take place. References Becker, M., Harris, P., Wiese, J., Bradshaw, D., 2006. The use of quantitative mineralogical data to interpret the behaviour of gangue minerals in the flotation of Merensky Reef ores. In: Proceedings of Automated Mineralogy 06, Brisbane, Australia. Becker, M., Harris, P., Wiese, J., Corin, K., Bradshaw, D., The use of automated mineralogy to interpret the batch flotation performance of Merensky Reef ore. Accepted for presentation at XXV IMPC 2010 Brisbane, Australia. Bradshaw, D.J., Harris, P.J., O’Connor, C.T., 2005. The effect of collectors and their interactions with depressants on the behaviour of the froth phase in flotation. In: Proceedings of Centenary of Flotation Symposium Brisbane 2005. Brough, C., 2008. An investigation into the process mineralogy of the Merensky Reef at Northam Platinum Limited. MSc Thesis, Faculty of Engineering and the Built Environment, University of Cape Town. Jasieniak, M., Smart, R. St C., 2009. Collectorless flotation of pyroxene in Merensky ore: residual layer identification using statistical ToF-SIMS analysis. International Journal of Mineral Processing 92, 169–176. Martinovic, J., Bradshaw, D.J., Harris, P.J., 2005. Investigation of surface properties of gangue minerals in platinum bearing ores. Journal of the South African Institute of Mining and Metallurgy 105, 1–7. Muinonen, J., 2006. Thompson mill MgO rejection. In: Proc. Canadian Mineral Processors, Ottawa, Paper 30, pp. 481–499. Parolis, L.A.S., van der Merwe, R., Groenmeyer, G., Harris, P.J., 2008. The influence of metal cations on carboxymethylcelluloses as talc depressants. Journal of Colloids and Surfaces, A: Physiochemical and Engineering Aspects 317 (1–3), 109–115. Peyerl, W., 1983. The metallurgical implications of the mode of occurrence of platinum-group metals in the Meresnsky Reef and UG2 chromitite of the bushveld igneous complex. Special Publication of the Geological Society of South Africa 7, 295–300. Steenberg, E., Harris, P.J., 1984. Adsorption of carboxymethyl cellulose, gur gum and starch onto talc, sulphides, oxides and salt type minerals. South African Journal of Chemistry 37 (3). Wiese, J., 2009. Investigating depressant behaviour in the flotation of selected Merensky ores. MSc Thesis Faculty of Engineering and the Built Environment, University of Cape Town. Wiese, J., Harris, P., Bradshaw, D., 2007. The response of sulphide and gangue minerals in selected Merensky ores to increased depressant dosages. Minerals Engineering 20, 986–995.