March/April 2009

Precious metals, pioneering processes

An overview of recent developments in South African platinum and rhodium production

By B. Davenport and N. Stubina

Platinum bars and grains

South Africa is far and away the world’s largest primary (from ore) producer of platinum and rhodium. It produces 75 per cent of the world’s platinum and over 80 per cent of the world’s rhodium, the global primary output of that stood at about six million and 0.75 million troy ounces, respectively. It is also a large producer of palladium, ruthenium and iridium, the other industrial platinum group metals.

Dearer than gold

Two-thirds of the world’s platinum production and four-fifths of the world’s rhodium are used in automobile and truck engine emission reduction catalysts. These metals are therefore critical to the efficient functioning of our world as we know it.

Rhodium is one the most valuable platinum group metals. Its price reached a record high of US$10,000 per troy ounce in June 2008 and stood at about $1,200 in early 2009. This makes rhodium more expensive than gold, the price of which stood at around $825 in January 2009. The high price of rhodium is due to its rarity (especially outside South Africa), its low concentration in ores and, on the demand side, its utility in reducing vehicle emissions.

Platinum, too, is among the most valuable platinum group metals. Its price was about $930 in early 2009. This high price can be ascribed to the extreme utility of platinum for reducing emissions and to its rarity, especially outside of South Africa.

Getting at the treasure

Platinum and rhodium occur mainly in the feldspathic pyroxinite Merensky Reef, the chromitite Upper Group 2 (UG2) Reef and, to a lesser extent, the mixed-rock Platreef — all near Rustenburg, South Africa. Platinum occurs in small platinum mineral grains, e.g. isoferroplatinum (Pt3Fe) and cooperite (PtS) in association with base metal sulphide minerals such as pentlandite (Ni,Fe)9S8, chalcopyrite (CuFeS2) and pyrrhotite (Fe8S9). Rhodium, on the other hand, is found, like palladium, mainly dissolved in pentlandite.

Mineral processing must therefore aim to maximize the recovery of platinum group element minerals and base metal minerals, especially pentlandite. Furthermore, the ore must be ground very fine to liberate platinum group mineral grains as small as 10 micrometres across.

Figure 1 shows the steps by which platinum group metals are produced from South African ores. The platinum (Pt) concentration in each step’s product is shown in table 1. Rhodium concentrations are typically about one-tenth those of platinum concentrations. The major steps are:

  • concentrate production (crushing, grinding, froth flotation);
  • matte production (smelting, converting);
  • platinum group element residue production; and
  • platinum group metal production.

The process yields 99.9 per cent platinum and rhodium from 0.00025 per cent platinum and 0.00003 per cent rhodium ore.

Table 1 shows the platinum enrichments, losses and costs for each of the steps shown in Figure 1. It is clear from this table that mining operations should do everything possible to reduce costs. Mineral processing should receive the most attention to improve platinum recovery. It is painful to lose metal after so much effort has gone into mining it. Not shown in the table is the need for efficient sulphur dioxide capture during smelting/converting. This is discussed later.

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