Gold extraction

After dissolving the gold by a cyanide solution the metal can be extracted from the liquor by cementation or adsorption.

Cementation of dissolved gold from the leaching solution is performed by pulverised zinc (Merrill-Crowe process).

2[Au(CN)₂]^- + Zn → 2Au + [Zn(CN)₄] ^2-

As soon as the zinc dust is added almost the complete gold content of the liquor is immediately precipitated and the cementate can be filtrated and melted to a gold bullion. Before smelting the cementate is washed with acid in order to remove the remainder of zinc.

The adsorption of gold from cyanide solution makes use of the natural affinity of gold for carbon. During adsorption the entire gold cyanide complex becomes attached to the carbon. The gold attached to the carbon is then desorbed by a cyanide solution from which it can be recovered by electrolysis. The desorbed or “stripped” carbon can be regenerated by heating it at approx. 600 °C for 30 minutes in absence of air. From a head value of approx. 2g of gold per tonne of solution a carbon loading of up to 15 kg gold per tonne of carbon is achieved leaving as little as 0,01g of gold per tonne of solution in the tailings.

There are essentially three process varieties that use carbon adsorption:

- The carbon in pulp (CIP) process,

- The carbon in leach (CIL) process and

- The carbon in column (CIC) process.

During the CIP process activated carbon particles are added to the leached pulp in a counter current circuit. The counter current principle is important because the amount of gold adsorbed on the carbon is in equilibrium with the residual gold concentration in the solution, and therefore the low gold concentrations in the final stage can be adsorbed only if the pulp in this stage is contacted with unloaded carbon. The considerable difference in grain size between carbon and pulp particles permits an easy separation of the two by sieving. Whereas pulp particles generally do not exceed 100 microns, the carbon granules appear in fractions between 1 and 3 mm.

The CIL process and the CIC process are essentially variations of the carbon in pulp technique that are used under special conditions.

In principle adsorption can also be accomplished by ion exchange technology with artificial resins. The disadvantages of the existing resins, however, prevented their wider spread use (they are expensive, most of them are non selective for gold and difficult to strip, and if they are selective and easy to strip their loading capacity is very low).