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Extracting gold from the mined rock is a complex process. From the time that the ore hoisted from underground and enters the processing plant, it takes up to five days before the process of smelting into gold bars can commence.
South Deep’s gold plant has the capacity to process around 330,000 tonnes of ore per month and the capability to treat more underground ore than most other plants in the Far West Rand Region of South Africa. The amount of rock processed per month (at full capacity) would be the equivalent of a pile 125m in diameter and almost 50m high, and would fill approximately 80 Olympic-sized swimming pools.
When ore enters the plant from underground it first needs to be milled to reduce its size so that the gold is liberated from the host rock. South Deep’s milling circuit consists of a single-stage semi-autogenous grinding mill (SAG) and a pebble crusher for primary milling, followed by secondary milling using two overflow ball mills. These mills are essentially rotating drums that have steel balls inside which grind the rock into particles that are finer than 0.075mm in size.
Classification of the milled ore is achieved using cyclones, which separate and sort fine particles in a liquid suspension for downstream processing, and return coarse material to the ball mill for further grinding. Gravity recoverable gold (or free gold) is processed using centrifugal concentrators, which significantly shortens the processing time for gold recovered. About 15% of South Deep’s gold is recovered using the gravity concentrator circuit.
Classified milled product is very dilute and is de-watered in a thickener to produce a dense slurry, a mixture of fine ore particles in water. The slurry is pumped to the leaching circuit which consists of a series of agitated tanks. The agitation ensures that the slurry is mixed thoroughly with the reagents (oxygen, cyanide and lime) so that the gold is liberated effectively.
Dissolved gold in the slurry is then adsorbed onto granular activated carbon in the adsorption or carbon-in-pulp circuit. Activated carbon is simply carbon (or charcoal) that has small pores that increase the surface area available for adsorption. Adsorption is the process through which molecules ‘stick’ to solid material.
Carbon loaded with gold is then withdrawn from the circuit and processed in an acid column. This removes impurities from the carbon, before the material is transferred to the elution column where it is soaked in a heated caustic cyanide solution. Hot water is then pumped through the column to remove the gold as a high-grade eluate solution. The stripped carbon is then heated to regenerate it and returned to the circuit. The barren slurry (from which the gold has been removed) is discarded onto the Tailings Storage Facility (TSF) or sent to the Backfill Plant to produce the backfill product used underground.
The eluate solution is pumped through electro-winning reactors where gold is plated onto cathode steel mesh or wool. (Electrowinning is a process of recovering metals from a solution by passing a current through it). The plated gold is then washed off and dried in cylinders called calciners, inside heated furnaces. It is then smelted in an induction furnace to produce bullion bars that are transported to the Rand Refinery, to produce 99.99% pure gold bars for sale all over the world.
Concentrate from the centrifugal concentrators in stage 2 is enriched on a shaking table to produce a high-grade gold concentrate which is dried in a calciner and smelted to produce bullion bars which are also dispatched to the Rand Refinery.