Dissolution of Gold in Ferric and Cupric Chloride Solutions

M.Sc. Sipi Seisko, M.Sc. Otto Forsström, Dr. Jari Aromaa, Prof. Mari Lundström

Currently, cyanide is the dominant lixiviant used in gold leaching. However, there is demand for noncyanide lixiviants due to several environmental disasters, e.g. in Baia Mare in Romania in 2000 [1, 2]. The use of cyanide has been limited or forbid by legislation in many countries and/or territories [1].

Therefore, alternative leaching solutions have been investigated, and chlorides are proposed as one of the most promising lixiviants for gold due to the high dissolution rates [3]. In order to dissolve gold, strong oxidant is required [4]. When using ferric and cupric ions and/or complexes as oxidants in gold leaching, Fe(III) and Cu(II) are reduced into Fe(II) and Cu(I), respectively. These ferrous and cuprous ions can be further oxidized in the presence of oxygen, which is the major advantage of chloride leaching compared to cyanide and thiourea [5, 6].

In this study, gold dissolution in chloride solutions was investigated by quartz crystal microbalance (QCM) with ferric and cupric ions and/or chloride complexes as oxidants originating from FeCl3, Fe2(SO4)3, CuCl2 and CuSO4 (0.02 – 1.0 M). The source of oxidant and the effect of its concentration as well as the redox potential during leaching were studied, while the chloride concentration (3 M), pH (1.0) and temperature (95 °C) were fixed. With ferric ions, higher dissolution rates were achieved compared to cupric with all investigated oxidant sources. Ferric and cupric ions originating from chloride salts (FeCl3 and CuCl2) were shown to result in higher gold dissolution rates compared to corresponding sulfate salts as oxidant source. The redox potentials of the solutions were shown to increase with increasing oxidant concentration, from 683 to 740 mV vs. SCE with ferric and from 586 to 674 mV vs. SCE with cupric. The best gold dissolution rates, achieved with 0.5 M oxidant concentration, were 1.34·10-4 and 1.66·10-5 mol s-1 m-2 with ferric and cupric ions, respectively.

Additionally, QCM results were compared to the results of rotating disc electrode (RDE) measurements in order to investigate the oxidation state of gold in chloride leaching. It can be suggested that with both oxidants, the oxidation state of gold is +1 instead of +3.