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«An overview of Southern African PGM Smelting, Nickel and Cobalt 2005: Challenges in Extraction and Production, 44th Annual Conference of ...»

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R.T. Jones, An overview of Southern African PGM Smelting, Nickel and Cobalt 2005: Challenges in Extraction and Production,

44th Annual Conference of Metallurgists, Calgary, Alberta, Canada, 21-24 August 2005, pp.147-178.

An overview of Southern African PGM smelting

R.T. Jones

Mintek

Private Bag X3015, Randburg, 2125, South Africa

ABSTRACT

The largest known platinum group metal (PGM) deposit in the world is the Bushveld Complex in South Africa, with the Great Dyke in Zimbabwe also being one of the biggest. It is therefore not surprising that the majority (just over 5 million troy ounces or 156 metric tons per annum) of the world’s platinum is produced in Southern Africa. Primary smelting of ore concentrates is carried out in that region by five companies, namely Anglo Platinum, Impala Platinum, Lonmin Platinum, Northam Platinum, all of South Africa, and Makwiro Platinum in Zimbabwe. The only other primary smelter of platinum group metals is Stillwater Mining of Montana, USA, although very significant quantities of PGMs are produced as co-products by Norilsk Nickel of Russia. Smaller (but still significant) quantities of PGMs are produced by Falconbridge and Inco of Canada, also as co-products from nickel sulphide smelting.

There are many similarities between PGM smelting and nickel sulphide smelting, and the range of technologies in use includes six-in-line rectangular electric furnaces, three- electrode circular AC furnaces, Peirce-Smith converters, and Anglo’s ACP (based on Ausmelt technology). PGMs are also recovered from residue materials using DC arc furnace technology.

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INTRODUCTION

The platinum group metals (PGMs) are a family of six greyish to silver-white metals with close chemical and physical affinities. The three heavier metals platinum (Pt), iridium (Ir), and osmium (Os), have densities of about 22 g/cm3; and the three somewhat lighter metals palladium (Pd), rhodium (Rh), and ruthenium (Ru), have densities of about 12 g/cm3. The PGMs belong to the transition metals of Group VIII in the Periodic Table, as do iron (Fe), nickel (Ni), and cobalt (Co). These metals have similar geochemical behaviour and tend to be concentrated together geologically. The PGMs, along with gold (Au) and silver (Ag), are classified as noble metals because of their high resistance to oxidation and corrosion.

Their great scarcity classifies them as precious metals. Only about one thirteenth as much platinum is produced as gold, itself a very rare metal. (By contrast, about 5 million times as much iron as platinum is produced in the world.) As precious and noble metals, PGMs are chemically more versatile than gold, and have found numerous industrial applications. They are also the only competitors for gold as investment metals and for jewellery purposes.

Platinum was originally called “platina” or “little silver” in Spanish, as it was considered a poor-quality by-product of silver mining operations 400 years ago in Colombia. Platinum was formally discovered only in 1751, although it (possibly mistaken for silver at that time) was used as far back as the 7th century BC in Egypt, when the ‘Thebes casket’ was produced. (This was made for Shepenupet, daughter of the King of Thebes, and has gold hieroglyphics on one side and platinum on the other.) The catalytic properties of PGMs were described in the period 1823 – 1838.

Interestingly, jewellery and catalysis remain the most important applications of these metals today.

PGMs have extraordinary physical and chemical properties that have made them indispensable to the modern industrial world. The PGMs have very high melting points, and are chemically inert to a wide variety of substances (even at very high temperatures), and thus resist corrosion. They also have excellent catalytic properties, and are widely used in the chemical industry and in automobile catalytic converters. Commercial substitution by cheaper metals has rarely been successful, although an individual platinum group metal may readily be replaced by another.

The most economically important of the PGMs are platinum, palladium, and rhodium, with ruthenium, iridium, and osmium being less prevalent and less in demand.

Gold, though it is a precious metal, is not one of the PGMs although it is often lumped together with the PGM content when talking about the valuable products from PGM smelting. The base metals nickel, copper, and cobalt commonly occur together with the PGMs and are produced as co-products in the smelters and refineries. The PGM market is fundamentally strong, particularly in platinum, where recent years have shown good 2 of 32 growth in jewellery demand, and in autocatalysts, especially for the diesel vehicles that now make up 50 per cent of new car sales in Europe.

Platinum group elements are generally associated with nickel-copper sulphides in magmatic rocks. Depending on the relative concentrations (and market prices) of the precious and base metals, the PGMs are produced either as the primary products, or as by-products of the nickel and copper. The primary PGM-rich deposits include the Bushveld Complex in South Africa (the largest known layered igneous complex of its type in the world, extending some 350 km from west to east, and some 250 km from north to south, containing more than two thirds of the world’s reserves of PGMs), the Great Dyke in Zimbabwe (the second largest known deposit of platinum in the world), the Stillwater deposit of the USA, and the Lac des Isles deposit of Canada. PGMs are produced in significant quantities as by-products from the Norilsk-Talnakh area of Russia and the Sudbury deposit of Canada. Other deposits occur in Finland, the Jinchuan deposits of northwest China, the Duluth complex of the USA, and in numerous smaller deposits. PGMs are also produced in small quantities as by-products from the nickelcopper industry in Australia and Japan (1).





Table I shows the geographical distribution of the most important PGMs, and Table II shows the demand by market sector for Pt, Pd, and Rh, courtesy of Johnson Matthey (2).

–  –  –

South Africa has more than three quarters of the world’s platinum reserves, and is the world’s largest producer of platinum group metals (PGMs). These vast resources occur together with the world’s largest reserves of chromium and vanadium ore in the unique Bushveld Complex geological formation. South Africa’s PGM output is derived almost exclusively from the Bushveld Complex, with only about 0.1 per cent coming from the gold deposits of the Witwatersrand and Free State, and the Phalaborwa copper deposit.

Platinum group metals now constitute South Africa’s largest mining export earner, accounting for a third of all sales last year. Revenue from PGMs surpassed those from gold for the first time in 2001, although gold dominated briefly again in 2002.

Since the identification of economic deposits of platinum in South Africa in 1924 by Hans Merensky, a number of platinum mines have come and gone, and some have merely changed identity (3-5). South Africa currently has four integrated primary platinum producers, namely Anglo American Platinum Corporation Ltd (6) (formerly Rustenburg Platinum Holdings Ltd), Impala Platinum (7), Lonmin Platinum (8) (which includes Western Platinum), and Northam Platinum (9). Their range of operations includes open-cast and underground mining, milling, flotation, drying, smelting, converting, refining, and marketing. Anglo Platinum, Impala Platinum, and Lonmin Platinum are the three largest producers of platinum in the world. Since 1971, these operations have established South Africa as the world’s largest producer of PGMs. The precious metals are the most valuable products in South African platinum ores, unlike the situation in many other countries where smaller quantities of platinum are produced as by-products or co-products of base-metal production, particularly of nickel. Apart from South Africa’s platinum mines, only Makwiro Platinum (10) (formerly Hartley Platinum) in Zimbabwe, and Stillwater Mining (11) in Montana, USA are major primary producers of PGMs. For comparative purposes, mention should also be made of Norilsk Nickel (12), which, although strictly not a primary PGM smelter, is the world's largest producer of nickel and palladium, and is also one of the leading producers of copper, platinum and gold. Other significant producers of PGMs include Falconbridge and Inco of Canada.

Table III shows the production by individual companies of the economically most important PGMs, namely platinum, palladium, and rhodium. Most company production figures are taken from their most recent annual reports (for 2003 in most cases) that are mostly available from their company websites. Note that Russian law was amended only this year to allow the release of official production figures for PGMs, and it is still too early for this information to be available. However, estimated production figures produced by Johnson Matthey (13) for the Polar Division (which produces the bulk of Norilsk’s PGMs) are believed to be fairly reliable.

–  –  –

South Africa has the greatest concentration of primary PGM producing companies (14,15), each of which has their own approach to smelting, governed to a large extent by the type of ore that they process.

The currently exploitable South African reserves of platinum-group metals are concentrated in narrow but extensive strata known as the Merensky Reef, the Platreef, and the UG2 chromitite layer. These three layers in the Bushveld Complex each have their own distinctive associated mineralogy, and have been well described mineralogically (16-18). The Merensky Reef has the PGMs occurring in conjunction with base metal sulphides; the Platreef has an even greater quantity of base metal sulphides present; and the UG2 chromitite layer has a high chromite content together with relatively low quantities of base metal sulphides. Platreef is currently smelted only by Anglo Platinum, but Merensky and UG2 ores are processed by all the producers.

These ores are quite different from each other, and require different approaches to metallurgical processing. For example, UG2 ore has a much lower content of nickel and copper sulphides, and contains much more chromite than Merensky ore. The Platreef can be considered as metallurgically similar to Merensky ore, although somewhat enriched in palladium.

The extent to which UG2 ore is processed has a major influence on the smelting behaviour, as traditional six-in-line furnaces are susceptible to build-ups of high-melting chromite spinels if the Cr2O3 content of the feed is too high.

With the change in political dispensation in South Africa, and the change in mineral rights legislation (bringing South Africa closer in line to common international practice), there has been a great deal of new mine development, especially on the Eastern Limb of the Bushveld Complex, with much of the new capacity centred around UG2 5 of 32 ores. Most of these new mines have concentrate offtake agreements with existing smelters and refiners.

Ore from the Merensky Reef contains up to 3% base-metal sulphide minerals, distributed as follows: pyrrhotite (45%), pentlandite (32%), chalcopyrite (16%), and pyrite (2 to 4%). The majority of the PGMs in the Merensky ore are associated with pentlandite, occurring either in pentlandite grains or at the pentlandite-gangue grain boundaries. To a lesser extent, the PGMs are associated with other base-metal sulphides or occur in the form of minerals such as braggite, cooperite, laurite, or ferroplatinum.

The major gangue minerals are pyroxene, plagioclase feldspar, and biotite.

The principal constituents of UG2 ore are chromitite (60-90%), orthopyroxene, and plagioclase, together with minor amounts of talc, chlorite, and phlogopite, as well as smaller amounts of base-metal and other sulphides and platinum-group minerals. The base-metal sulphides are predominantly pentlandite, chalcopyrite, pyrrhotite, pyrite, and to a lesser extent millerite. The sulphide grains of UG2 ore are generally finer than those of the Merensky Reef.

Merensky ore contains much more sulphide than does the UG2 ore, and the minerals are found in a silicate substrate, while UG2 ore has a chromite matrix. The Cr2O3 content of the UG2 ore presents major challenges in processing. In Merensky ores, the ratio of nickel to copper is fairly constant at about 1.7, but the PGM to base metals ratio is not constant (19).

The Merensky and UG2 reefs are situated in close proximity to each other. The UG2 reef lies anywhere between 20 and 330 metres below the Merensky horizon, and varies in thickness between 0.15 to 2.5 metres. Reserves of PGMs plus gold are estimated (20) at 547 million ounces in the Merensky Reef, and more than 1000 million ounces in the UG2 reef. Another estimate (21) says the UG2 reef contains about 800 million ounces of PGMs.

The PGM content of the UG2 reef is comparable with, and sometimes higher than, that of the Merensky Reef. The PGM content in the Merensky Reef ranges between about 4 and 10 g/t, whereas the UG2 reef contains between 4.4 and 10.6 g/t. UG2 ore is by far the richest source of rhodium, which is currently the highest-priced PGM and an important constituent of the catalysts used in motor car exhaust systems. The copper and nickel contents of UG2 ore are generally less than a tenth of those found in the Merensky Reef. The Cr2O3 content of UG2 ore is about 30%, as opposed to about 0.1% for Merensky ore. The low-grade chromite produced as a by-product during the treatment of UG2 ore is also sold, and there is no reason why it could not be used for the production of ferrochromium (22,23). The high demand for palladium also makes the processing of UG2 concentrates very attractive.

Average grades and current values of the individual precious metals in Merensky, UG2, and Platreef ores are shown in Table IV. Further detail regarding the distributions

–  –  –

It is evident from the data above that Pt, Pd, and Rh make up a remarkably constant 96% of the value of all the precious metals, for all three ore types. In the case of Merensky ore, these three dominant PGMs make up about 80% of the value of all the metals produced (i.e. PGMs plus base metals). For UG2 ore, the fraction is closer to 90%. The situation is quite different for Platreef ore, where the contained base metals may be worth even more than the precious metals (if base metals prices are high, as they are at present).



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