Particulates in hydrometallurgy: Part I. Characterization of laterite acid leach residues

Two different laterite samples, designated as Ore No. 1 and Ore. No. 2, respectively, were leached with sulfuric acid in the temperature range of 25°C to 275°C, in order to evaluate the effects of ore type and leaching conditions (e.g., temperature and solid/liquid ratio) on the settling behavior of the leach residues. The initial solids, as well as the leach residues, were characterized by chemical and physical techniques such as X-ray diffraction, electrophoretic mobility, particle size, and chemical analyses. Spectrochemical and X-ray analyses determined that Ore No. 1 was a limonitic-type (iron oxide-rich ore) material, whereas Ore No. 2 was a transitional laterite ore (silicate-rich, iron oxidecontaining material). By means of X-ray diffraction analysis, it was found that the conversion of goethite to hematite was enhanced as the temperature and/or leaching time was increased. Moreover, it was determined by chemical analysis that as the leaching temperature increased, the concentrations of Ni and Co progressively increased in solution, whereas the impurities (Fe and Al) rose through a maximum and then decreased. This behavior is attributed to the reprecipitation of iron and aluminum from solution as hydrolysis products of hematite and aluminum sulfates. The particle size studies revealed a particle size growth with temperature increase, which is attributed to the deposition of reprecipitation products onto the surface of the solid residues. Investigations of the surface chemistry of the coated particles via the electrophoretic mobility technique revealed that the 275°C leach residues (Ore No. 2, pzc ∼2) exhibited a point of zero charge (pzc) near pH 2, which is probably due to the redeposition of the dissolved silica. In contrast, the 250°C leach residues (Ore No. 1, iron oxide-rich ore, pzc∼2) had a pzc value of about 3.8. This shift in mobility to less acidic values suggests that in this case, the insoluble iron and aluminum oxide products on the particles dominate the surface chemistry.