On the Management of Gangue Minerals in the Flotation of Platinum Group Minerals
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The Bushveld Complex of South Africa contains almost 90% of the world’s reserves of platinum group minerals (PGMs). In the flotation of PGMs, there are significant challenges arising from the need to treat ever-decreasing grades of the relevant ore deposits. The major challenge in the flotation of these ore bodies is the control and management of the gangue minerals, particularly silicates such as orthopyroxene, plagioclase feldspar, and pyroxene which are often rimmed with talc which makes them naturally floatable. It has been shown that various polysaccharide depressants such as CMC and guar have different properties in terms of depressing the gangue minerals. Since the PGMs are often associated with sulphides, copper sulphate is widely used as an activator in PGM flotation but can inadvertently activate the gangue minerals as well as reduce the recovery of PtTe2 which accounts for up to 40% of the Pt in the Platreef ore body. Depressants also reduce the mass of solids reporting to the froth and can thus destabilise the froth. This effect on the froth can be mitigated by using higher frother dosages or water of higher ionic strength. Reducing chromite recovery is of critical importance since high levels may negatively affect the downstream smelting process. Chromite recoveries can however be reduced through the application of gravity separation or reducing entrainment through reduced water recovery. In summary, due care needs to be taken to carry out site test work to develop an optimum ratio of collector, frother, activator, and depressant to ensure that the highest grades and recoveries of the PGEs are obtained while reducing depressant dosage as much as possible.
KeywordsPlatinum group minerals Depressants Copper sulphate Chromite
The Bushveld Complex located in the north of South Africa contains the world’s largest deposit of platinum group minerals (PGMs). The so-called upper critical zone of the complex hosts the largest concentration of platinum group elements (PGEs) in the world. The zone hosts the Upper Group Chromitite No.2 (UG2) and Merensky Reef as well as the Platreef mineralisation of the northern limb of the Bushveld Complex. The Merensky pegmatoid Zone contains the base metal sulphide grains and associated PGMs. Of particular importance to the topic of this paper is the occurrence in these reefs of various gangue minerals.
The Merensky Reef consists predominantly of orthopyroxene (~ 60%), plagioclase feldspar (~ 20%), pyroxene (~ 15%), phlogopite (5%), and occasional olivine. Secondary minerals such as talc, serpentine, chlorite, and magnetite have widespread occurrence. Of these, talc is the only mineral to show significant natural floatability [1, 2]. Although present in small amounts, talc is closely associated with pyroxene and imparts significant floatability to the high concentrations of pyroxene present in these ores . The base metal sulphides consist predominantly of pyrrhotite (~ 40%), pentlandite (~ 30%), and chalcopyrite (~ 15%). The major platinum group minerals are cooperite (PtS), braggite [(Pt,Pd)NiS], sperrylite (PtAs2), and PGE alloys.
The UG2 consists predominantly of chromite (between 60 and 90% by volume) with lesser silicate minerals. Other minerals present in minor concentrations can include silicates, oxides, and base metal sulphides. Total PGE varies from locality to locality, but on average range between 4 and 7 g/t. The PGMs present in the UG2 Reef are highly variable, but generally the UG2 is characterised by PGE sulphides such as laurite (RuS2), cooperite (PtS), and braggite [(Pt,Pd)NiS]. The base metal sulphides consist predominantly of chalcopyrite, pentlandite, and pyrrhotite. The Platreef is located in the northern limb of the Bushveld Complex and is extremely rich in PGMs. This reef consists of a complex assemblage of rock types, with pyroxenites, serpentinites, and calc-silicates being the most abundant. Base metal mineralisation and platinum group element (PGE) concentrations are found to be highly irregular, both in value as well as in distribution.
The predominant platinum group minerals (PGM) in the mined area of the Platreef are the PGE tellurides, alloys, arsenides, and sulphides. The Pt and Pd tellurides are the most important and contribute to around 20–30% and up to 45% of the PGMs present in the Platreef ore followed by the alloys (26%), arsenides (21%), and sulphides (19%) .
Flotation is used for concentrating sulphide and PGE minerals at PGM concentrators and it has become increasingly necessary to treat low-grade complex polymetallic ores. Moreover, due to the need to enhance liberation, there is a concomitant need to process fine particles which, as is well known, may be relatively more difficult to float. Given the mineralogy of the ore body, it is thus self-evident that the major challenge in the flotation of these ore bodies is the control of the gangue minerals, both the silicates and the chromitite.
This paper presents results obtained in an extensive program of test work investigating the influence of various factors on the behaviour and control of the gangue minerals during flotation. These factors include the use of different types of depressants, the interactive effects between various collectors, frothers, and depressants, the effect of different ions present in the process water on the depression of the gangue minerals, and the effect of using depressants on the behaviour of the froth.
2 Materials and Methods
As indicated above, the PGMs occur in different zones of the Bushveld Complex. In the experimental programmes described in this paper, the ores used were from the Merensky, UG2 and Platreef bodies. In the results section, the type of ore used in the various tests is indicated.
2.2 Flotation Procedure
Composition of the synthetic plant water used in these investigations (TDS = 1023 ppm; ionic strength = 0.0213 M)
After milling in a laboratory scale rod mill, the slurry was transferred to a 3-L Barker flotation cell, where the volume was made up using synthetic plant water to produce 35% solids. The cell was fitted with a variable speed drive, and the pulp level was controlled manually by the addition of synthetic plant water keeping the froth height constant at 2 cm. The impeller speed was set at 1200 rpm. The air was maintained at a flow rate of 7 L/min in all the tests. Four concentrates were collected at 2, 6, 12, and 20 min of flotation time. Water and solids recoveries were measured for each test. Feeds, concentrates, and tails were filtered, dried, and weighed before analysis. All samples were pulverised before sub-sampling for analytical work. Elemental analysis of samples was done after acid digestion using an atomic absorption spectrophotometer. Sulphur analysis was carried out using a LECO sulphur analyser. PGM assays were carried out in a commercial laboratory. Due care was taken, through repeat tests, to check on reproducibility of all experiments and analyses.
3 Results and Discussion
3.1 Comparing the Performance of Guar and CMC as Depressants
Depressants are used to reduce the extent to which naturally floating gangue (NFG) present in the ore reports to the concentrate. The most commonly used gangue depressants are polysaccharides such as modified guar gum and carboxymethyl cellulose (CMC). Polysaccharides are made up of sugar monomers bonded together differently . Guar gum belongs to the galactomannan group and is a branched polysaccharide. The structure of this polymer is such that it is soluble without the introduction of charged sites into the molecule and therefore the modified guars have very low degrees of substitution (DS) and on adsorption reduce the charge on the particles. CMC is a cellulose derivative. The DS of the carboxymethyl groups is high (~ 0.7) and on adsorption can lead to the development of a high negative charge on the particles and result in strong dispersion. It is important to note that the cost associated with the use of depressants often exceeds that of any other reagents used in the flotation process. Because of its natural hydrophobicity, most of the fundamental studies on the use of depressants to reduce the recovery of naturally floating gangue (NFG) minerals have been carried out on talc. The adsorption of depressants onto talc has been extensively reported in literature (e.g. [7, 8]). Steenberg and Harris  and Shortridge et al.  have shown that guar adsorbs more strongly onto talc than does CMC.
McFadzean et al.  have studied the relative effectiveness of CMC and guar in depressing gangue minerals at different dosages and with different average molecular weights using a Merensky ore. The high molecular weight depressants ranged between 600,000 and 700,000 g/mol and the low molecular weights between 40,000 and 70,000 g/mol. The depressants were tested using microflotation, batch flotation, and equilibrium adsorption studies. The investigation showed that at essentially starvation dosages (e.g. ~ 100 g/t), the high molecular weight polymers did not depress naturally floating gangue (NFG), whereas the low molecular weight depressants did. At relatively higher dosages (e.g. ~ 300 g/t), both high and low molecular weight polymers essentially depressed all NFG, without reducing sulphide recovery. High molecular weight depressants appeared to be relatively more selective towards hydrophilic gangue minerals such as pyroxene and feldspar at starvation dosages in batch flotation experiments. This was inferred from the fact that even though all the polymer was adsorbed from solution, the naturally floating gangue was not depressed by high molecular weight polymers at 100 g/t dosages. In addition, there was a marked reduction in froth stability when using the high molecular weight polymers, which was attributed to the more selective adsorption of the polymer on the fine pyroxene and feldspar, resulting in slime cleaning of the sulphides. At 300 g/t dosages, all polymers act as good depressants of the naturally floating gangue. None of the polymers depressed the sulphides. It was shown that at depressant dosages of 100–300 g/t, adsorption densities range from 20 to 50% pseudo-monolayer coverage. In another study, it was shown that the shorter chain lengths within a distribution in large molecular weight depressants were preferentially adsorbed. The low molecular weight depressants were adsorbed across the entire molecular weight range, but still showed some preferential adsorption of the shorter chains . It has also been shown that no benefit is gained from the blending of the depressants guar and CMC .
With respect to the relative adsorption of depressants onto various gangue minerals, Mhlanga et al.  observed that in a buffered solution, talc had the highest affinity for guar, followed by chromite, pyroxene, and then plagioclase. This progression in the affinity for guar in a buffered solution correlated with the differences observed in the surface charge of each mineral at pH 9 as determined in zeta potential measurements. These observations are also consistent with the acid/base interaction theory of Laskowski et al.  since talc, pyroxene, and chromite had the most basic surfaces and hence, the highest affinity for guar, and plagioclase, which had the most acidic surface, had the weakest interaction with guar. This study also showed that in a binary mixture of each mineral with talc, guar adsorbed preferentially onto pyroxene, chromite, and talc, with limited adsorption onto plagioclase. This means that, in industrial applications, large quantities of depressant may be wasted in adsorption onto minerals such as pyroxene and chromite, which are naturally hydrophilic.
Although the amount of NFG still floating was small, it is speculated that this is may be due to the fact that these gangue minerals are present as composites with sulphide minerals. An important ramification of this investigation was that it was now possible to determine the amount of entrained gangue, viz. those minerals which do not respond to depressant treatment and report to the concentrate through entrainment, by using depressant dosages of about 300 g/t.
3.2 Effect of Depressants on Froth Behaviour
Since increasing ionic strength is also known to enhance froth stability, Corin and Wiese  studied the effect of both ionic strength and frother dosages on solids and water recoveries as well as on copper and nickel grades and recoveries using a Merensky Reef ore sample. It was observed that at low ionic strength (i.e. deionised water), increased frother dosages increased the amount of NFG and entrained gangue recovered and this is consistent with the increased water and solids recoveries observed. Clearly it is critical to carefully balance the relative effects of frother and depressant dosages. Although the depressants may reduce froth stability through reduced mass of solids reporting to the froth, Schreithofer et al.  in a study of the foaming properties of the frother, Dowfroth 200, in the presence of a modified guar and a CMC, has shown that depressants can in fact enhance froth formation, but the mechanism of action is different. The CMC depressant appeared to act more through increasing bulk viscosity while the guar has more influence on the surface rheology. These phenomena may be attributed to formation of surfactant-polymer complexes in the solution. Clearly there are two competing influences, viz. the loss of froth stability due to the reduced amount of solids present in the froth and increased froth stability due to a frother-depressant interaction. It is likely that the loss of solids will have the overriding influence on froth stability, since reduction in froth stability was observed in the presence of both frother and depressant.
3.3 Interaction Between Collectors and Depressants
3.4 Effect of Ions in the Water on the Depression of Gangue Minerals
Laskowski et al.  have shown that non-hydrolyzable metal cations can affect the adsorption of polysaccharides such as CMC or guar onto a strongly acidic surface such as quartz. For example, water structure breakers such as K+ and Cs+ will significantly increase their adsorption while water structure makers such as Na+ and Li+ do not affect the adsorption. With respect to the ions present in a flotation pulp, apart from naturally occurring ions present in the process water, one of the most important of such species is copper sulphate. Many PGM concentrators use copper sulphate as an activator since it will most likely promote the recovery of PGMs which are associated with sulphide minerals.
Wiese et al.  have shown that the recoveries of NFG at different dosages of CMC or guar were always higher when copper sulphate was present. These results are consistent with the findings of Shackleton et al. , in an investigation using XPS and ToF SIMS that copper sulphate resulted in significant inadvertent activation of gangue minerals such as pyroxene. It was shown in the latter study that it was possible to reverse this effect by using an amine such as ethylenediamine and it was postulated that this was because of the amine complexing with copper ions in solution thus affecting the equilibrium between copper ions on the surface of the pyroxene and in solution. Apart from the effect of inadvertent activation of gangue minerals as a consequence of adding copper sulphate, Shackleton et al. [5, 23] have shown that there can be another unintended negative consequences resulting from the use of copper sulphate. In the case of the Platreef ore, in which tellurides account for between 30 and 40% of the platinum, the addition of copper sulphate reduced recoveries of the tellurides from > 90% to about 50%. Interestingly, this was not observed in the case of Pd As2.
Nyabeze and McFadzean  carried out a comprehensive study of the effect of copper sulphate on the froth behaviour and flotation kinetics of both a Merensky and a UG2 ore both using laboratory tests and plant investigations. Notwithstanding the observations referred to above regarding copper sulphate being able to inadvertently activate gangue minerals, they showed that for the UG2 ore, froth stability decreased with addition of copper sulphate. This was attributed to the formation of hydrophobic copper–xanthate species, which have a destabilising effect in the sparsely mineralised UG2 froth. The results of the surveys on a UG2 concentrator conducted with and without the addition of copper sulphate supported the results of the laboratory tests. The amount of solids entrained was reduced upon addition of copper sulphate and this resulted in an improvement in grade and a slight decrease in recovery presumably due to a less stable froth.
3.5 Entrainment of Gangue Such as Chromite
As shown above, it is possible to completely depress the naturally floatable gangue by using depressants at dosages in excess of about 300 g/t. However, hydrophilic gangue will still report to the concentrate through entrainment. One of the most significant of those gangue minerals is chromite which, as already referred to, can constitute between 60 and 90% of the content by volume of the UG-2 ore. Chromite is of particular importance since when present in the concentrate at concentrations in excess of about 5% can have major negative effects on the downstream smelting process.
It is also possible to reduce chromite in the concentrate by prior use of gravity separation prior to secondary grinding . It is also well known that entrainment decreases with increasing particle size  and hence, fine grinding which is widely used in the processing of PGM ores will exacerbate the problem of managing chromite entrainment. Recently, it has been observed that the entrainment of chromite is not only affected by the size and mass of the particles but also their shape with significant entrainment being observed for relatively coarse chromite particles with a high aspect ratio. These effects may have some important ramifications for the comminution procedures and devices currently used on concentrators since these may affect the particle morphology .
This paper has presented results from an extensive body of research carried out by the authors investigating the role of depressants in the flotation of major PGM-bearing ore bodies. It has been shown that at commonly used dosages of about 300 g/t, they fully depress the naturally floatable gangue but do not affect the recovery of sulphides. It is worth noting that there is a diminishing use of guar in practice mainly as a result of its relatively high cost compared to CMC. It is also shown that dosages of depressants at levels greater than 300 g/t is arguably not necessary unless there is a very high talc content in the ore. Given that depressants reduce the solids concentration in the froth and hence reduce the froth stability, it is necessary to carefully balance the dosages of depressant and frother so as to mitigate any negative effects, this may have on either recoveries or grades. High ionic strengths of water can increase froth stability, and hence solids and water recovery, but that at high depressant dosages (400–500 g/t), this effect is negated by depressing all of the naturally floatable gangue. However, copper sulphate, which is widely used as an activator in sulphide flotation, can inadvertently activate the gangue minerals and, in the case of the Platreef ore, reduce the recovery of PtTe2 which accounts for up to 40% of the Pt in that ore body. Chromite generally does not respond to the use of depressants unless it is associated with a naturally floatable gangue mineral such as talc. In general, it is useful to carry out mineralogical analysis of the ore being treated using techniques such as Qemscan to gain a better insight into the nature of the gangue minerals present in the ore so as to be able to optimise the methods used, especially with respect to reagent type and dosage, to manage the behaviour of gangue minerals in flotation.
This study was financially supported by the following companies: Impala Platinum, Anglo American, Lonmin.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that they have no conflict of interest.
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