Effect of grinding and alkali pretreatment in before cyanidation of oxidized refractory Au/Ag ores

Niğde–Ulukışla/Bolkardağ Au (8,6g/t) and Ag (176g/t) ore is an oxidized ore composed of calcite, barite, quartz, clay minerals and mostly limonite, goethite, cerrusite, smithsonite, hydrozincite, hemimorphite and jarosite minerals. The ore is defined as refractory due to recoveries of about 30% Ag and 50% Au by direct cyanidation. Mineralogical investigations have showed that gold was in native form and in the form of electrum as fine particles (<24 µm) with mostly Fe–oxide minerals and silver was in the form of electrum, acantite/arjantite (Ag2S) and Ag phases within jarosite minerals like beudantite (PbFe3(AsO4)(SO4)(OH)6). In this study, usability of alkalies such as CaO, NaOH and KOH before cyanidation has been investigated with continuous and discontinuous tests in detail. As a result, it has determined that Au/Ag recoveries on cyanidation increases directly proportional with alkali concentration, temperature and time of pretreatment. Generally, while it has reached 90% Au and 80% Ag cyanidation recoveries at the alkali CaO pretreatment process, it has been showed to be obtained similarly higher recoveries (97% Au and 93% Ag) at the NaOH and KOH. It has been found that this increases in Ag recovery is caused by decomposition of Ag–jarosite minerals in ore. It is determined that particle size and conditions of cyanidation process aren't significant effect on recoveries. It has observed that cyanide consumptions are realized as a maximum of approximately 100 kg/tons of ore for all alkalies and increased proportionally with cyanide concentration. As a result, it can be said that alkali pre-treatment can be successfully used before cyanidation in oxidized refractory Au / Ag ore.