Abstract
Reducing agents can be applied to leach metal values from natural and secondary resources such as laterite ores and deep sea manganese nodules and recycled materials such as spent batteries/catalysts, which contain high-valent metal oxides of manganese, cobalt, and iron. Selective leaching of some metals can be achieved by selecting conditions to control the saturated solubility and dissolution rates of metals from oxides or mixed metal/oxide matrices. The leaching results are rationalized on the basis of chemical species, Eh–pH diagrams and heterogeneous kinetic models. The role of reducing agents, such as hydrogen peroxide, sulfur dioxide, and iron(II) sulfate, in acid, alkali, and in the presence of complexing ligands such as ammonia for the improvement of the leaching of metal oxides and metals is discussed. Previous studies on acid leaching of metals from nickel laterites, manganese nodules, and spent catalysts/batteries in the absence or presence of reducing agents show that the rate controlling step involves the diffusion of protons through a thickening product layer on the particle being leached. It is shown that the leaching of nickel and cobalt from manganese nodules and zinc–carbon batteries in ammoniacal sulfur dioxide solutions also obey shrinking core kinetics.