Heterogeneous electrochemical reactions taking place on metallic iron in ammoniacal-carbonate solutions containing dissolved nickel, cobalt, copper and thiosulfate ions

In the Caron process, nickeliferous laterite ores undergo a pyrometallurgical pre-treatment step, in which their nickel and cobalt content is reduced to the metallic state. This is followed by leaching in ammoniacal-carbonate solutions, where metallic nickel and cobalt dissolve by forming complexes with ammonia. Since the reductive pre-treatment step also results in some metallic iron being formed, nickel and cobalt are present in the pre-reduced ore mainly as ferro-alloys. As a result, the dissolution behaviour of metallic nickel and cobalt is strongly influenced by the behaviour of metallic iron. In particular, the passivation of iron has been identified as a potential factor contributing to the relatively low metal value recoveries suffered by the process (80-82% nickel and 50-60% cobalt). The present study consists of an investigation into various heterogeneous reactions involving metallic iron and ions commonly found in Caron leach liquors, namely nickel(II), cobalt(II), copper(II) and thiosulfate, in order to identify interactions which may adversely affect the extraction efficiency. The study was carried out using a combination of electrochemical and surface characterisation techniques, as well as thermodynamic calculations. Metallic iron was found to generally dissolve in ammoniacal-carbonate solutions by forming ferrous ammine complexes, but a few specific conditions were found to promote its spontaneous passivation: the presence of copper(II) ions and the presence of both cobalt(II) and thiosulfate ions. In the presence of copper(II) ions, the passivation process was promoted by the cementation and subsequent redissolution of metallic copper, as confirmed by surface characterisation studies. However, when thiosulfate ions were also present, passivation was not observed and the formation of a partially adherent layer containing metallic copper dendrites and cuprous sulfide took place on the iron surface. In the presence of both cobalt(II) and thiosulfate ions, the passivation process, which was not observed when either species was present on its own, was promoted by the formation of an amorphous cobalt sulfide or polysulfide species. The loss of cobalt from the leach solution into this layer was identified as another potential factor contributing to the poor cobalt extractions suffered by the process