E-pH diagrams for the metal-water system at 150 °C: Thermodynamic analysis and application for extraction and separation of target metals from saprolitic laterite

Hydrometallurgical processes of laterite ores mainly occur in the aqueous solution of acid, alkali or salt. The ionization of water and change of pH are of great importance during the extraction and separation of target components. Based on the empirical equations of potential (E), pH and ionic activity terms of various species, the thermodynamics of metal-water system is summarized in the form of potential—pH (E-pH) diagrams. In the Ni + Co + Mg-H2O potential-pH diagram, the predominant area of Ni2+, Co2+ and Mg2+ is contained in the region soluble Co2+ as the pH lower than 3.8 and potential higher than −0.2 V. After superimposing the Fe-H2O potential diagram, the stable area of Ni2+, Co2+, Mg2+ and Fe2O3 is critical. The thermodynamic calculation results indicate that the extracting of Ni, Co and Mg is easy and the separation of Fe can be achieved by controlling final pH of nitric acid pressure leaching. The experiments of nitric acid pressure leaching display that more than 95% Ni and Mg and nearly 90% Co can be extracted and more than 98% Fe is separated into leach residue at initial nitric acid concentration of 120 g/L and a final pH of 0.8; leaching temperature of 150 °C; leaching time of 60 min and liquid to solid ratio of 4–8 ml/g. A new flowsheet is proposed to treat saprolitic laterite, and target components are recovered efficiently. The experimental results are in agreement with the results of the thermodynamic calculation. The process is environmentally friendly and has high economic efficiency, which can provide possibilities for commercial application of saprolitic laterite by hydrometallurgical processes.