1. Introduction
It is well known that Russia and South Africa are the major producers of platinum group metals and hold the world’s largest reserves of minerals bearing these metals. Cawthorn [
1] has estimated that South Africa has reserves and resources of platinum group elements (PGEs) of about 85,000 metric tons. In Russia, Norilsk alone has Pt and Pd reserves of about 2500 metric tons [
2]. Platinum group metals or elements commonly refer to ruthenium, rhodium, palladium, osmium, iridium, and platinum and will be referred to as PGEs in the present paper. These metals are present in a very wide range of platinum group minerals, which will be referred to as PGMs.
Table 1 shows the 2018 production figures for platinum, palladium, and rhodium illustrating the dominant global position held by these two countries. Together, they produce 84% of the world’s total production of platinum and 65% of the total gross demand. The equivalent figures for palladium are 78% and 53%, and for rhodium, they are 87% and 61%, respectively [
3].
Recently, there has been a decrease in the demand for platinum, which is ascribed to the decline in its use as an autocatalyst. Palladium, on the other hand, is the preferred component in exhaust catalysts for gasoline engines and is currently in great demand. In this context, it is interesting to note the LME (London Metal Exchange) price trends for these two metals over the past three decades. In the 1990s, palladium prices were ≈USD100/oz and platinum ≈USD400/oz, whereas in 2020, palladium prices had increased almost 20-fold to ≈USD 2000/oz, and platinum only doubled to ≈USD 800/oz. Although the demand for rhodium is much less than that for Pt or Pd, its 2020 price of almost USD7000/oz makes it an economically attractive product for companies processing PGMs.
The aim of this paper is to review at a high level the geological occurrence, mineralogy, and flotation processing practice of treating PGMs in South Africa and Russia the world’s leading producers of PGEs.
3. Processing of the Ores
The dominant processing route for ore bodies in South Africa and Russia is comminution followed by flotation and downstream processing includes smelting and converting to produce a matte that is treated hydrometallurgically. In the present paper, the focus will be on comparing the flotation processes. Comminution practice has been extensively reviewed in numerous publications [
7,
9,
17,
18,
19,
20]. Generally, such comminution circuits consist of two or three stages of autogenous milling in primary milling circuits, while ball mills are employed in secondary milling stages. Each comminution stage is followed by flotation in the so-called MF2 or MF3 circuits [
9].
In South Africa, a variety of flotation circuits are currently used across the industry, and
Figure 6 illustrates two such typical circuits. Concentrate grades for the Merensky Reef concentrate are typically 300–600 g/t compared to about 200 g/t for the treatment of UG2 ore. Overall recoveries are typically between 80 and 85%.
Fine grinding is also extensively practised using IsaMills in the so-called mainstream inert grinding (MIG) and ultra-fine grinding (UFG) mode of operation [
17,
20].
Figure 7 shows such applications of IsaMills on a South African PGM concentrator. The main purpose of MIGs is to increase recovery in the mainstream flotation circuit. The main purpose of UFGs is to increase grade without losing recovery in the cleaner circuit. Typical particle size distributions targeted for MIG is reduction from p80 of 75–150 microns to p80 of 45–60 microns; and for UFG, reduction from p80 of 40–80 microns to p80 of 15–25 microns.
In Russia, in the Talnakh and Norilsk concentrators, different process flowsheets are used for various types of ores, e.g., Norilsk and Chernogorskoye operations. In the processing of disseminated and cuprous ores at the Norilsk processing plant, the mass pull in the roughers is about 10–15%. After this, the focus of the circuit is to selectively recover the copper and nickel. In the processing of disseminated ores contained in the Norilsk-1 deposit, the process additionally includes the use of gravity concentration methods such as in the Knelson concentrator, which increases the recovery of noble metals [
21]. Regarding gravity concentration, it is worth noting that in South Africa, PGMs found in flotation tailings are usually associated with silicates (60%) or are free PGM particles (10%) and can be separated from oxide and sulphide minerals using gravity concentration [
7].
In the case of the low-grade sulphide ores in Russia, the combined gravity–flotation concentration scheme is very effective for the recovery of extremely low base metal grade ores, which are relatively rich in PGMs, with up to 89% of platinoids represented by native forms and intermetallic compounds. Losses of PGMs into the tailings of processing plants depend on the type of ores being treated and on the processing flowsheets being used. At the Talnakh Plant, processing of the sulphide ores is carried out using differential flotation, and the tailings represent only about 25–30% of the feed solids. The main difference in this processing technology is that the low nickel grade ore is a separate product, which contributes to a significant reduction in costs in the metallurgical processing due to the presence of high-grade concentrates of non-ferrous metals [
22].
A flowsheet that incorporates the application of flash flotation methods is used to achieve an increase in the recovery of PGMs. A typical flowsheet for the processing of sulphide-type ores using this approach is shown in
Figure 8. The numbers refer to the reagent that is added at a particular point in the circuit. The flotation feed is usually about 65% passing 75 µm with highest recoveries of Pt and Pd occurring at about 45% pulp density. It is interesting to note the similarity to the South African MF2 concept. However, the major differences are the use of gravity concentration immediately after the primary grind followed by flash flotation of the gravity tailings to produce a final concentrate. Four stages of flash flotation follow using Outotec Skim-Air flash flotation technology and result in overall recoveries of platinum and palladium of ≈92% and ≈87%, respectively. It is important to note that the grades of Pd and Pt in the gravity concentrate are as high as 184 and 114 g/t. By comparison, the equivalent grades from the cleaner concentrate are 50 and 15 g/t, respectively. Flash flotation tails are fed to a secondary ball mill followed by rougher and scavenger flotation. Reagent addition sequences are described below.
At the Chernogorskoye operation, ore preparation is carried out in a 9.8 m semi-autogenous mill to reduce losses due to over-grinding of the PGMs. Flash flotation is used to recover coarse grains of PGMs and non-ferrous metals. This is followed by gravity concentration using centrifugal concentrators. The combination of these operations extracts about 30% of the PGEs into a collective concentrate. This is followed by rougher/scavenger flotation and two stages of cleaner flotation. A feature of the flotation process for the Chernogorskoye operation is the use of nitrogen produced at the metallurgical plant, although the benefit of using nitrogen is still controversial.
In South Africa, flotation is typically carried out at a pH of between 8 and 9, which is the natural pH of the Bushveld Complex ores. Various collectors, depressants, and frothers are used. Typically, the most widely used collector in both countries is sodium isobutyl xanthate. Although information in the public domain on dosages is limited, it is understood that for the xanthate, the dosages may be as high as 100–300 g/t in the case of Platreef possibly because of the low sulphide content of the ore and the need to use high dosages to recover tellurides and arsenides. In Russia, where recently isobutyl xanthate has replaced n-butyl xanthate, dosages are in the range 30–50 g/t. In South Africa, Dowfroth 250 is widely used as a frother (40–60 g/t), and carboxymethyl cellulose is used as a depressant (200–300 g/t) [
23]. In both countries, copper sulphate is often used as an activator, although the dosages are quite different (South Africa ≈40 g/t, Russia ≈10 g/t). However, recently, there has been a reduction in the use of copper sulphate on flotation plants. In South Africa, sodium ethyl dithiophosphate is often used at a dosage of ≈60 g/t. The effect of water quality is gaining ever greater importance both in terms of its quantity used as well as its quality. For example, it has been shown that an increase in the ionic strength of the system results in an increase in froth stability, leading to increased mass pulls and water recoveries [
24].
In Russia, specific reagents have been designed for the processing of Norilsk ores that take into account their chemical and mineralogical composition. One such example is the introduction of the DP-4 reagent, which was originally developed as an additive to machine oil. However, later, its properties as an effective flotation reagent were observed. DP-4 is an apolar collector obtained from a heavy fraction of oil refinery waste. Due to its insolubility in water, it is used as a 5% emulsion in diesel fuel. The implementation of this reagent, together with other measures, has made it possible to significantly improve the enrichment of the disseminated ores of Norilsk and Talnakh at the Norilsk Processing Plant for both non-ferrous and precious metals. Another reagent known as T-80, which is a blend of monohydric alcohols of dioxane and pyran, is added at a dosage of ≈30 g/t overall.
With respect to the South African operations, the role of the dithiophosphate collector remains contentious, since it appears to be playing a role as a froth modifier rather than that of a classical collector [
25,
26]. It has also been shown that the dosage of frother and depressant needs to be carefully balanced. Depressants reduce the solids content of the froth, which may destabilise it, but this can be compensated for by a greater frother addition [
27]. The use of copper sulphate remains contentious. It is widely speculated that copper sulphate activates the base metal sulphides and PGMs as in its traditional usage. However, the addition of copper sulphate can also contribute to the recovery of gangue minerals due to the inadvertent activation of these minerals [
28,
29]. It also appears to be playing a role in affecting the stability of the froth and flotation kinetics [
30]. A further complicating factor on South African operations is to ensure that the chromite concentration in the concentrate is less than ≈3% so that it does not negatively impact on the downstream smelting process. This problem can also be addressed by the use of spirals [
8]. In Russia, sodium dimethyl dithiocarbamate is also frequently used as an additional collector, and lime is used to control alkalinity to about 10.5 [
31].
Jones [
32] has provided a comprehensive review of the downstream processing in South Africa of flotation concentrates. Although this is not a focus area of the present paper, it is worth noting that the flotation concentrates undergo smelting and converting to produce a PGM-containing nickel-copper matte. The matte is treated hydrometallurgically to separate the base metals from the precious metals. Finally, the PGM concentrate is refined to separate the individual precious metals (PGE) into their pure forms. As a rough guide, the PGE contents during the various stages are typically as follows: ore 0.0005% (5 g/t), flotation concentrate 0.0150% (100–400 g/t), converter matte 0.20%, PGM concentrate 30–65%, refined metals 99.90% for Rh, Ru, Os, and 99.95% for Pt, Pd, and Au [
33]. In Russia, nickel and pyrrhotite concentrates from the Talnakh Enrichment Plant are sent to the Nadezhda Smelter, while the Norilsk concentrator’s output is sent to the nickel smelter. Copper concentrates from both plants are processed at the copper smelter. The resulting product, rich in PGEs, is sent for toll refining at independent precious metals refineries, primarily the Krastsvetmet refinery in Krasnoyarsk [
10].
4. Challenges and Opportunities
At the beginning of the 21st century, platinum-bearing ore zones similar to those found in the Kamchatka region (referred to above) were identified in the dunites in the Middle Urals. The platinum-metal mineralisation in the former contains more than 80% of platinum-bearing mineral grains in the size class <100 µm. It has been proposed that these ore bodies could be processed using primary gravity concentration, and results of laboratory experiments on flotation of the combined gravity tailings have shown that up to 65% of platinum can be recovered into the flotation concentrate and 18–20% can be recovered by magnetic separation from flotation tailings [
34].
In recent years, there has been considerable interest in treating the noble metal deposits contained in black shale strata. All the major six PGEs are found together with gold in the black shale formation, and clearly, they have a potentially important role to play in terms of future sources of PGMs in Russia. Platinum-bearing black shales are considered to be widespread in the Riphean-Palaeozoic region of the Siberian Platform, in the Selemdjino-Kerbinskaya and Nileskaya structural formations of the Amur-Okhotsk folded system, and in the marginal troughs of the Bureya massif. Studies carried out on samples of the black shale formation of the Bureya massif showed that 11.9% of the platinum, which is likely to be graphite-bound or ultra-dispersed, could be extracted using gravity concentration, and a recovery of 88.1% was obtained by flotation of the gravity tailings. Significant quantities of gold were also recovered [
34,
35].
Significant amounts of PGMs may be associated with vanadium-iron-titanium-sulphide oxide formations in basites, primarily in the titanium-magnetite ores of the Pudozhgorsk and Kochkarskoe deposits in Karelia in which the combined grade of (Pt + Pd + Au) is ≈1.4 g/t. In the Pudogorskoye deposit, the equivalent combined grade is ≈0.9 g/t. It is proposed that the processing of this deposit will include magnetic separation to produce a ferrotitanium concentrate, and the tailings could be treated by sulphide flotation. An alternative processing route may involve flotation to yield a copper-rich concentrate with a high Au and PGE content. Then, the flotation tailings can be treated by magnetic separation to yield a ferrotitanium concentrate resulting in a platinum recovery in the copper concentrate of between 70 and 90% [
3].
Currently, the main source of PGEs in Russia is the layered ultrabasite-basite blocks containing unique deposits of copper-nickel sulphide ores such as occur in the Norilsk ore body. However, in some stratified ultrabasite–basite arrays, localised in the folded frame of the Siberian ore body, significant concentrations of PGEs are present in relatively low content sulphide ores. These are typical of the Chineyskoye field, which is located in the northeast of the Trans-Baikal Territory (Siberia) (660 km from Chita) in the Kalarsky District. The Yoko-Dovyrensky and Chineiskoye plutons belong to the peridotite-pyroxenite-anorthozite-gabbroic type of magmatic formations and can be compared with the uniquely platinum-bearing stratified arrays of the ancient platform regions such as Stillwater and Platreef (cf.
Table 2). The mineralisation of the Yoko-Dovyrensky massif belongs to the low-sulphide platinum-metal massif, and the Chineisky Chinea deposit belongs to the platinum-containing titanomagnetite formation [
16]. To date, as a result of much geological research, general regularities and typomorphic features of unique platinum deposits have been identified within the massifs, which may have major significance [
36]. However, the economic viability of extracting PGEs as by-products from non-conventional ores such as black shale, polymetallics, uranium ores, etc. remains to be demonstrated.
In South Africa, chromite is one of the main gangue constituents of especially UG2 ore, making up to between 20 and 60% by mass of the ore [
37] (cf.
Figure 3). Although the main minerals targeted in flotation are generally the base metal sulphides, viz. pyrrhotite, pentlandite, and chalcopyrite, a certain amount of chromite reports to the flotation concentrate. This is problematic, as chromite is a spinel mineral and forms species that are stable up to 2000 °C, and these stable species can severely affect the efficiency of the smelting process. Furthermore, the maximum chromite content that can be tolerated in flotation concentrates reporting to a typical smelter is considered to be in the region of 3%, although recently, higher grades appear to be acceptable. Chromite is naturally hydrophilic and probably reports to the concentrate by entrainment. It is possible to reduce chromite in the concentrate by the use of gravity separation prior to secondary grinding [
38] or by reducing entrainment through reduced water recovery. However, it is worth noting that recently, chrome has become an economically important by-product. Another challenge in South Africa is that it has been shown that although tellurides and arsenides of Pt and Pd minerals float readily when treated with sodium isobutyl xanthate (SIBX), the addition of copper sulphate can result in a decrease in the recoveries of PtTe
2 and PtAs
2, which are key component minerals in the Platreef ore body [
39,
40].
As already explained, the use of IsaMills is aimed at promoting the liberation of PGMs, but it may also result in problems associated with fine particle flotation, although it is frequently observed that the flotation of <10 µm particles is not a major problem. However, fine particles are prone to oxidation, but this may be reduced by using inert grinding media in the IsaMill. Moreover, inert grinding of cleaner feed (MIG) can increase the flotation rate and performance by removing surface layers of talc, oxidation products, and iron hydroxides. Corin et al. [
25] have also shown that dithiophosphate plays a major role in promoting the recovery of PGM particles in the <25 μm range.
One of the major challenges in the flotation of PGMs 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 then makes them naturally floatable. Apart from the problems mentioned above, copper sulphate may also inadvertently activate the gangue minerals [
28].
5. Conclusions
This paper presents, firstly, a general overview of the geological occurrences of the ore bodies containing the PGMs in Russia and South Africa, the world’s major producers of such minerals, as well as the typical mineralogy of the ores in which the PGMs occur. Then, the processing of these ores in each country using flotation is compared. Whereas in Russia, the major host minerals for the PGEs tend to be generally sulphide deposits, particularly pentlandite as well as pyrrhotite and chalcopyrite, in South Africa, this is only the case in the Merensky and UG2 reefs. In the Platreef, which contains most of that country’s PGM reserves, the dominant host minerals are tellurides, arsenides, and alloys. The UG2 deposit also contains large amounts of chromite, which, although being potential problematic in the downstream smelting process, is currently of significant economic value. It has been shown that in both countries, a mill-float circuit sequence is commonly used. However, in Russia, there is significant use of gravity concentration. Challenges and opportunities in both countries aimed at increasing the production of PGEs are briefly summarised. In South Africa, this is often related to managing gangue recovery and improving the recovery of fines as well as of tellurides, arsenides, and alloys. In Russia, there is much interest in processing in the future of unconventional ore deposits such as dunites and “black shale” as well as the treatment of titanium–magnetite ores and low-sulphide content ores. Given the dominant global position of both countries in the production of the PGEs, it was thought to be useful for the various operations to be compared with a view ultimately to increasing the global production of these key commodities.
