The electrochemistry of the leaching of pre-reduced nickel laterites in ammonia-ammonium carbonate solution

Abstract

A fundamental study was undertaken in order to establish the mechanisms of the leaching of pre-reduced nickel laterites in ammonia-ammonium carbonate solution. Although the process has undergone various improvements since it was first introduced, the total recoveries are still relatively low, averaging at 80% for nickel and 45-50% for cobalt. The lack of sound fundamental information regarding the mechanisms and the kinetics of the dissolution of nickel and cobalt from iron-alloy grains produced by the reduction roasting of lateritic ores is seen as the main obstacle to establishing the reasons for the lack of adequate recovery and to defining alternative processing strategies which would lead to improved extraction and better process optimisation. Based on fimdamental electrochemical studies and bench scale leaching tests, as well as on-site measurements at an industrial plant where this process is applied, the work presented in the thesis introduces significant new evidence and sheds more light on the understanding of the mechanism of the dissolution process and on the reasons for the low extractions of nickel and cobalt. It has been established that the oxidative dissolution of the iron-alloy grains formed during the reduction roasting takes place primarily via a reaction involving reduction of dissolved cobalt (111) to cobalt (11), and also that the reduction of water is not a significant component of the reactions involved in the dissolution process. Open-circuit potential measurements with various metals and iron-alloys selected to model the reduced material in various solutions which simulated the leaching conditions were also conducted and revealed that passivation takes place during the leaching process. This brings to light a possible new reason for the low recoveries of nickel and cobalt which has not been previously considered. In addition, a kinetic study was conducted, in which the dissolution rates for the various metals, iron-alloys and solutions were measured as a function of time, and the results of which shed more light on the development of the process of passivation. The passivation itself is attributed to formation of iron-oxide on the surface of the dissolving iron-alloy grains, which restricts the transfer of electrons to the reduced metal and brings the oxidative leaching reaction to an end. It has been shown that the passivation can occur in two ways. On the one hand, particularly high concentration of dispersed and dissolved oxygen in the leaching reactors causes passivation by overcoming the limiting current density for the oxidation of iron to divalent iron ions and taking the potential to the region where the direct oxidation to trivalent ions becomes favourable resulting in the formation of an iron-oxide film on the surface. On the other hand, in normally aerated solutions, the passivation of the dissolving ironalloy grains occurs due to the formation of a cobalt and nickel sulphide layer, as a byproduct of the reduction of thiosulphate on the metal surface, which in turn gradually shrinks the area available for the oxidation of iron, again leading to a situation where the oxidising agents present in the solution under standard aerated conditions become sufficient to shift the mixed potential to the region where the iron-oxide forming reaction becomes favourable. The presence of thiosulphate has a decisive role in the second type of passivation. What is more, besides preventing further dissolution of the nickel and cobalt locked in the iron-alloy matrix, the loss of nickel and cobalt by precipitation in the presence of thiosulphate can further reduce the recovery of these metals. For this reason, a method was developed for the oxidation and removal of thiosulphate fkom the actual plant liquor