Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 266

Crushing and Grinding

Ore is commonly fed to a crusher for initial grain size reduction. Crushed ore is then
directed to the mill which operates in closed circuit with a cluster of cyclones to grind the
ore so that the product passes a pre-defined design grain diameter (typically in the order of
100 microns).

Leach and CIL

Milled ore from the cyclone overflow gravitates through a vibrating trash screen into the
leach and CIL circuit, where it passes through leach tanks and CIL tanks over a total
period of about 48 hours. Each tank is agitated. Screens are installed at the outlets of each
CIL tank to retain carbon. Carbon in the CIL circuit is transferred counter-current to the
flow, by air lifts.

Acid Washing and Elution

From the CIL circuit, loaded carbon is pumped to a vibrating screen where it is washed
in water before gravitating to an acid wash column. Here loaded carbon is soaked for 30
to 60 minutes in a hydrochloric acid solution, washed in water again to remove the acid,
and transferred to the elution column. The elution circuit process uses staged additions of
different solutions, including NaCN 3 , NaOH, and water to strip gold from carbon. After
stripping, the carbon is regenerated in a rotary kiln, for reuse. The pregnant solution is
directed to the electrowinning process.

Electrowinning

In electrowinning, the stripped and 'pregnant' solution circulates through electrowinning
cells. Sludge containing gold and silver collects at cathodes from which it is washed into a
collection vessel. The sludge is then dewatered and oven dried.

Smelting

The oven-dried material is smelted in a furnace, located within a secured gold room. It is
only then that gold emerges from the process in visible form. Molten metal from the fur-
nace solidifies to produce bars of gold-silver doré. The doré is stored in a safe within the
gold room until it is transported off-site for final refining.

Detoxification of Tailings

Detoxifi cation needs to be
carried out to decompose
cyanide, which may be present
in the thickener underfl ow at
concentrations up to 300 mg/L.

Tailings emerge from the CIL circuit as fine-grinded cyanide-containing material.
Detoxification needs to be carried out to decompose cyanide, which may be present in the
thickener underflow at concentrations up to 300 mg/L. Detoxification commonly uses the
well-proven INCO SO 2 Air System, in which sodium metabisulphite is added, together