The effect of metal chloride additions upon the extraction of nickel from lateritic ores by hydrochloric acid

Although lateritic nickel ores are efficiently leached by pure dilute hydrochloric acid at atmospheric pressure and temperature between ambient and 90oC, the degree of extraction of nickel, as well as iron and magnesium is considerably decreased in the presence of moderate concentrations of the respective cations in the leach liquor. In the case of nickel itself extraction falls to zero at concentrations of Ni2+ of about 20 g/l for most laterite ores. Increase in free acid concentration or temperature does not improve the extraction. It has been shown that the poor extraction is not due to cessation of the reaction between the acid and the ore but to the formation of a layer of insoluble reaction product which prevents further dissolution of the cations. The problem may be solved by the application of washing with pure dilute HC1. The washing parameters have been studied and a leach-washing flowsheet is proposed that would permit leach liquor containing at least 20 g/l nickel to be produced with 92-95% extraction from the ore. The leaching of the cations Ni, Mg and Fe was found to obey a kinetic model of the form 1- 2/3 R - (1 - R)2/3 = kt, where R is the fraction of cation extracted, up to a certain fraction of extraction after which a change to another mechanism of leaching occurred. This fraction depended on both the cation involved and the temperature. The kinetics were chemically controlled up to 50-60°C and diffusion controlled at higher temperatures.