The Bushveld Complex
Formed about 2,000 million years ago, the Bushveld Igneous Complex (BIC) can be compared to an enormous, irregularly-shaped saucer 370 kilometres across, with its centre buried deep underground but its rim exposed. It has a series of distinct layers, three of which contain economic concentrations of platinum group metals (pgm). The principal pgm-bearing reefs are the Merensky Reef and the Upper Group 2 (UG2) Reef, which occur around the Eastern and Western sides ("limbs") of the BIC. A third pgm-rich layer, the Platreef, is found only on the Potgietersrus limb at the north-eastern edge. See Map of the Bushveld Complex 2008 below.
The Merensky Reef was the principal source of pgm from the time that it was first worked in 1925 until the end of the 20th century. However, extraction of other reefs has grown in importance, and by 2011 the Merensky Reef accounted for only 22 per cent of all the platinum ore processed in South Africa. Exploitation of the UG2 began in the 1970s and has steadily increased; in 2011, it was the source of 63 per cent of ore processed. The Platreef, briefly mined in the 1920s, was not exploited on a large scale until 1993 but by 2011 accounted for 15 per cent of ore treated by South African platinum mines.
The Merensky and UG2 reefs are narrow - typically less than a metre thick. Traditionally, both reefs were mined using narrow reef methods, and many operations continue to use these methods today. Miners use hand-held pneumatic drills to bore holes which are then filled with explosive. After blasting, ore is removed from the stope using scrapers attached to winches. It is then transported through a series of ore passes to the bottom of the shaft, and hauled to the surface.
Mechanical and hybrid methods are increasingly being adopted by both new and existing mines. Drilling may be carried out either using conventional hand-held pneumatic drills, or via low-profile machines equipped with specialised drilling equipment. Ore is subsequently cleaned from the stopes using low-profile LHD (load-haul-dump) vehicles. The mining width must be greater in order to allow the use of machinery - typically around 1.8 metres - although some mines are experimenting with ultra-low profile equipment which can operate in stopes little more than a metre high.
Open-pit methods are used to mine the Platreef, which is much wider than the other reefs, varying between 5 and 90 metres in thickness. Open casting is also used on a smaller scale to exploit the UG2 and Merensky reef where it outcrops.
Mill-head grades of BIC ore (a measure of the ore's pgm content as it enters the first stage of processing) are typically between 2 and 6 grams of combined platinum, palladium, rhodium and gold per tonne. Allowing for losses which occur during refining, and the varying platinum contents of the different Bushveld ores, this means that between 10 and 40 tonnes of ore must be processed to obtain a single ounce of platinum.
The ore is crushed and milled to reduce the size of the rock particles and to expose the minerals which contain the pgm. The particles are mixed with water and special reagents and air is pumped through the liquid, creating bubbles to which the pgm-containing particles adhere. These float to the surface and are removed as a soapy froth. The pgm content of this flotation concentrate varies between 100 and 1,000 grams per tonne.
After being dried, the concentrate is smelted in an electric furnace at temperatures which can exceed 1,500ºC. During this process, a matte containing the valuable metals is separated from the unwanted minerals, which form a slag and are discarded. The matte is transferred to converters, where air is blown through it in order to remove iron and sulphur. The pgm content of the "converter matte" now exceeds 1,400 grams per tonne.
The next step is to separate base metals from the pgm. The final stage is the separation and purification of the six pgm, plus gold and small amounts of silver. The soluble metals - gold, palladium and platinum, which dissolve in hydrochloric acid and chlorine gas - are generally the first to be extracted, in that order. The insoluble pgm come out next, with rhodium usually last.
The refined pgm have a purity of over 99.95 per cent, and can be produced in a number of forms: ingot, grain or a fine powder known as "sponge". The time between mining of the ore and production of pure metal typically ranges from around 6 weeks for palladium to up to 20 weeks for rhodium.