Gold Leaching with THIOSULPHATE

The oxidation of gold in the presence of thiosulphate ions can be expected to produce the gold complex Au(S₂O₃)₂^3-, but because thiosulphate does not work in acid solutions, alkaline solutions must be used. Few oxidants are suitable for use in alkaline solutions. Although oxygen is an effective reagent for cyanidation, it is not electrochemically active at the required potential for thiosulphate leaching. The reaction is;

4Au + 8S₂O₃^2- + O₂ + 2H₂O = 4Au(S₂O₃)₂^3- + 4OH^-

The rate of dissolution is dependent on thiosulphate and dissolved oxygen concentrations and temperature. Leaching rate is improved by copper in ammonium solution.

4Cu(S₂O₃)₂^3- + 16NH₃ + O₂ + 2H₂O = 4Cu(NH₃)₄²⁺ + 8S₂O₃^2- + 4OH^-

4Au + 4S₂O₃^2- + Cu(NH₃)₄²⁺ = 4Au(S₂O₃)₂^3- + 4NH₃ + Cu(S₂O₃)₂^3-

Silver chloride and silver sulphide are easily leached. The thiosulphate compounds are consumed by several oxidations and association reactions.

5S₂O₃^2- + S₂O₆^2- + 3H₂O = 4S₄O₆^2- + 6OH^-

Thiosulphate can be stabilized by the addition of small amounts of sulphite ions which react with sulphide sulphur and regenerate thiosulphate. This prevents the precipitation of silver as the insoluble sulphide.

In the anodic area gold is oxidized and complexed with ammonia. This complex can be replaced by a more stable gold thiosulphate complex. In the cathodic are, the cupric amine complex is reduced and the oxygen present in the ammonia solution oxidizes the cuprous complex to a cupric compound. Ammonia and copper are recycled in the system.

It has been reported that the thiosulphate consumption is very high such as 29 kg/t, but it can be reduced by adding reducing agents as chelates which can dive a consumption of 13 kg/t. obviously, there are certain factors that influence thiosulphate consumption. Some of these factors are: sulphide minerals present in the ore react with thiosulphate; oxidant compounds trends to consume more thiosulphate, solid percentage.

Thiosulphate does not present a good stability when is exposed to the presence of ultraviolet light, to the presence of heavy metals, or if exist a big amount of cations in the water.

Gold recoveries by leaching can be higher than 90%, but as was mentioned with a high consumption of thiosulphate. The pH can be in the range 10.0 to 10.5 and the pulp density can be in the range 40 to 45% solids. Pregnant solutions obtained with thiosulphate don’t give a good recovery with activated carbon or resins. The best results have obtained by cementation with zinc, copper or iron.