Gold Recovery from cyanide liquors

Gold Recovery
From cyanide liquors

Most of the gold is extracted by the cyanide leaching process wherein finely ground ore is agitated in a diluted solution of sodium cyanide and calcium hydroxide (slaked lime). The pregnant solution is commonly contacted with beds of activated-carbon or anionic ion exchange resin that adsorb the gold. The precious metal loaded on the activated carbon or the resin is recovered by elution or simply by incineration of the carrier at high temperature.

Ion exchange resins are more versatile substrates than activated carbons because specific functional groups can be fixed on the resin matrix during the synthesis. Therefore the resins can be tailor-made to improve the carrier features such as selectivity, resistance to poisoning by calcium or organic materials and loading capacity. In addition, activated carbons require much higher temperatures for elution and require thermal reactivation which is unnecessary for resins.

The gold extracted by cyanidation is in the form of anionic aurocyanide complex Au(CN)2- and can be recovered from the pregnant solution by ionic exchange with strong or weak base anion exchange resins, such as Amberjet™ 4400, Amberlite™ IRA900RF and Amberlite™ IRA96RF.

The strong base anion resins contain quaternary ammonium functionality which, when contacted with the high pH leachate stream, adsorb the aurocyanide anion via exchange for the resin's chloride counter-ion:

Elution of the adsorbed gold complex can be accomplished by displacement with a more strongly adsorbed anion or by chemical reaction. Competitive displacement is typically done with either zinc cyanide or thiocyanate:

In either case, the resin must then be converted back to the sulfate form prior to return to adsorption service, typically with sulfuric acid or ferric sulfate. In the former Soviet Union, however, the preferred method of elution is by chemical reaction with thiourea:

An alternative approach to the above is the use of weak base anion exchange resins which, because they are normally supplied in the "free base" form, need to be treated with acid prior to contact with the leachate stream. A typical cycle is conceptually as follows:

Because of the high capacity available with ion exchange resins and the high loading that can therefore be obtained, in most small scale operations it is often an option to consider incineration of the loaded resin rather than elution.