Sulphuric acid pressure leaching of laterites - speciation and prediction of metal solubilities "at temperature"

Sulphuric acid pressure leaching of nickeliferous laterites has attracted considerable attention from the nickel industry during the past 5 years. The process is especially advantageous for limonitic laterites that mostly contain goethite, because iron precipitates releasing acid and thus rendering low acid consumption. It is also applicable to mixtures of limonites and saprolites, although at a higher acid consumption due to magnesium. Effective process design requires the solubility of metals that precipitate during the process to be known. In the present work, determination of metal solubilities is based on a simple speciation program that assumes the presence of only one dominant complex for each metal. The thermodynamic data for the precipitation reactions are extracted from high-temperature experiments with monometallic systems published previously. The validity of the approach is then tested against mixed bimetallic systems, and finally applied to calculate the solubility of aluminium, iron and magnesium in laterite leaching effluents "at temperature". In both cases of limonitic feed and limonitic/saprolitic blends, the prediction closely follows metal solubilities measured experimentally at temperatures from 230 to 270 degrees C and at terminal-free acidities ranging from 10 to 75 g/l. (C) 2000 Elsevier Science B.V. All rights reserved.