Aachen Reactors – Application 2. Cyanide leaching of gold where sulphides are present

A process for the treatment of refractory mineral ores has also been developed with the first successful commercial application already in operation.

Oxygen is essential for the gold leach reaction, as illustrated in Elsner’s Equation previous. With no pre-oxidation iron-sulphide minerals will react with cyanide to produce ferrocyanide and thiocyanate, and so consume cyanide that should be leaching gold. The following equation describes this interaction:

FeS + 1/2O2 + 7CN- + H20 = Fe(CN)64– + SCN- + 2OH-

In the case of ore containing sulphides pre-oxidation of the pulp before cyanide addition results in the production of a surface film of ferric hydroxide which prevents further reaction of the sulphides and so ensures that the cyanide added is consumed for gold leaching purposes only. There are various other species that also act as cyanicides, but often they also consume oxygen, so it’s logical to satisfy these consumers with oxygen first, bearing in mind that oxygen is a lot cheaper than cyanide. If the ore treated results in low dissolved oxygen levels then the use of an Aachen Reactor can reduce O2 consumption, reduce cyanide consumption and increase leach kinetics with an associated increase in gold recovery.