The leaching of metal sulfides in ammoniacal carbonate solutions

The leaching of metal sulfides in ammoniacal solutions is of industrial importance for the production of copper, nickel and cobalt. In some processes, such as the Sherritt Gordon process, they are introduced as feed; while in others, such as the Caron process, they form in-situ, as intermediate products. The focus of this study is to understand the leaching behaviour of the in situ formed copper, nickel and cobalt sulfides, but the findings are expected to be relevant to the other related systems. The Caron process involves an oxidative dissolution of pre-reduced iron-based nickel and cobalt alloy grains in an aqueous solution containing ammonia and ammonium carbonate. Under the process conditions, these iron-based alloy grains undergo passivation because of the formation of an iron oxide surface layer, as nickel and cobalt become trapped within the passive iron matrix, resulting in reduced extractions. Thiosulfate ions have been reported to facilitate the leaching process. The presence of metal ammines and thiosulfate ions in the same system, however, has been observed to result in a formation of a metal sulfide layer on the surface of the dissolving iron-based alloy grains. This, on the other hand, results in a partial loss of nickel and cobalt from the solution, while also further increasing the tendency for iron passivation. It has been postulated, that after iron passivates, nickel and cobalt from the sulfide layer redissolve, however, there is little evidence of this, or the effect of process parameters on the rate and extent of such re-dissolution. The present study is aiming to shed some light on these processes. The dissolution of copper, nickel and cobalt sulfides was examined using three forms of these compounds - metal sulfides formed electrochemically (EFMS) under simulated Caron process leach conditions, synthetic monometallic metal sulfides (SMMS) and real metal sulfides, naturally found in ore (RMS). In the first instance, electrochemically formed metal sulfides were studied by techniques which included open circuit potential and cyclic voltammetry measurements. The dissolution rate and extent of the nickel, cobalt and copper from the metal sulfide was monitored. The leaching of the synthetic monometallic metal sulfides was carried out under similar conditions to those of the Caron process. The effects of key process parameters such as concentration of ammonia, concentration of ammonium carbonate and temperature were investigated. The pH and Eh of the studied systems were monitored and solution samples were analysed for Ni, Co and Cu. It was observed that the highest extraction of nickel and copper from their respective metal sulfides was at 3M [NH3]T + 1M [CO2]T at 60 °C, whereas the maximum extraction of cobalt from cobalt sulfide was observed at 5M [NH3]T + 1M [CO2]T at 45 °C. The kinetic data shows that leaching of nickel follows the ash layer diffusion model, with the activation energy for the process reported as ‘different’ at the different tested temperatures (69.6 kJ/mol at 25 °C and 14.7 kJ/mol at 60 °C). An attempt was made to derive the reaction orders with respect to ammonia (0.18) and ammonium carbonate (0.09). The available data is possibly not very reliable, however. The rate of copper leaching shows that the surface reaction model applies best to this system. The activation energy was 11.5 kJ/mol at 25 °C and 57.6 kJ/mol at 60 °C. The reaction order with respect to ammonia was 1.9 and for ammonium carbonate it was 4.0. The leaching kinetic data for cobalt from monometallic cobalt sulfide, showed that the process follows the surface reaction model similar to copper, with a steady activation energy of 6.5 kJ/mol. In contrast to nickel, the leaching of cobalt from cobalt sulfide had a higher order with respect to ammonium carbonate of 1.8 than with respect to ammonia of 0.9. The observed leaching behaviour of the electrochemically formed metal sulfides (EFMS) and the synthetic monometallic metal sulfides (SMMS) was compared to leaching of real metal sulfides naturally found in ore (RMS) by testing flotation concentrate under the conditions which gave the highest extractions for the former. Tested were two types of flotation concentrate, one containing high nickel sulfide and another containing high copper sulfide. A sample of cobalt sulfide was not available. In these experiments, the effect of adding 0.25M sodium sulfite (Na2SO3) was also examined. The results have shown that the addition of sulfite improved both rate and extent of the extraction of nickel. It significantly hindered the extraction of copper, however. Namely, the rate of nickel extraction with sodium sulfite present was approximately 1.31 mg/L/min vs. 0.07 mg/L/min without it. The rate of copper extraction on the other hand, decreased from 4.09 mg/L/min without sodium sulfite, to just 0.89 mg/L/min when it was added. On the basis of this data, and assuming similar behaviour between copper and cobalt sulfides, it does not appear that addition of sodium sulfite could reduce the losses of copper and cobalt from the system