The dissolution of gold in acidic solutions of thiourea

Abstract

In order to evaluate the potential of acidic thiourea as a reagent for leaching gold, a study was made of the dissolution of gold in acidic solutions of thiourea containing various oxidants. Experiments were conducted on rotating disks of pure gold and on ground gold ores. The chemical oxidants used included iron(III), hydrogen peroxide, oxygen and formamidine disulphide; the latter reagent was formed in situ by the action of both hydrogen peroxide and dissolved oxygen on thiourea. Gold was observed to dissolve in these solutions at rates which approached the limiting diffusion controlled rate. Iron(III) as the oxidant caused the most rapid initial rate of dissolution of gold, but this rate soon decreased because of the reaction between iron(III) and thiourea; this resulted in the consumption of an excessive amount of thiourea which made the use of iron(III) as the oxidant unattractive in any ore leaching system based upon the use of thiourea as leaching agent. The results observed in the rotating disk study were applied to the leaching of crushed ores. A large proportion of the oxidant necessary for the extraction of the gold was derived from the ore itself; the remainder of the oxidant required could be supplied as hydrogen peroxide during preparation of the leach liquor, and by agitation of the slurry by a flow of air. When solutions containing 1.2 M thiourea were used it was possible to extract the gold from an ore within one hour; under these conditions the consumption of thiourea was about 1.4 kg thiourea per ton of ore treated. This figure could be reduced to 0.4 kg thiourea/ton if 0.1 M thiourea was used; complete extraction of the gold then occurred within eight hours. Gold can be leached at a much greater rate by acidic solutions of thiourea than is possible by conventional cyanidation techniques. However on economic grounds the latter technique must be preferred unless a really rapid rate of dissolution of gold is required.