Anaerobic thiosulfate leaching: Development of in situ gold leaching systems

Ferric EDTA and ferric oxalate complexes are both effective oxidants for the aerobic and anaerobic dissolution of gold in thiosulfate solutions, and therefore are potential candidates for the development of an in situ leaching system. The thiosulfate and polythionates were quantified during leaching using HPLC with perchlorate eluent and an anion exchange column, and it was found that both the iron EDTA and oxalate complexes have a low reactivity with thiosulfate, and they do not react with thiourea when it is added as a leaching catalyst. Anaerobic leaching experiments showed that both systems were still active after seven days leaching, and when 1 mM thiourea was present, there was significant gold dissolution. However in the absence of thiourea, the gold leaching was very slow, and hence the addition of thiourea as a gold oxidation catalyst is required for the iron(III) leaching systems. When anaerobic leaching was carried out in the presence of finely ground pyrite, the iron(III) complex was rapidly reduced to iron(II) as a result of the pyrite catalysed oxidation of thiosulfate. Pyrrhotite was also found to be problematic as it directly reduced the iron(III) complex, and therefore the quantity of gold leached was significantly lower in the presence of both these sulfide minerals. These problems associated with the presence of sulfide mineral need to be overcome if such a system is to be used in an in situ leach environment.