South Africa
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.
Ore extraction
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.
Concentration
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.
Refining
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.