Characterisation of iron-rich precipitates from synthetic atmospheric nickel laterite leach solutions

Abstract

Iron-rich precipitates from atmospheric nickel laterite leach solutions normally contain large amounts of poorly defined phases such as schwertmannite and ferrihydrite. This complicates mineralogical identification using routine X-ray Diffraction (XRD) technique. In the present study, the iron-rich precipitates from synthetic nickel laterite leach solutions were characterised by a combination of several techniques that include selective Acidified Ammonium Oxalate (AAO) dissolution, Differential X-ray Diffraction (DXRD), Scanning Electron Microscopy (SEM) and Fourier Transform Infra-Red (FTIR) spectroscopy. These techniques in combination allowed reliable mineralogical identification for samples containing high proportions of schwertmannite and ferrihydrite. The effects of foreign metallic cations on the crystallization, dissolution behaviour and surface sulphate coordination were investigated. The results suggest that selective AAO dissolution is a good method to distinguish between poorly and highly structurallyordered phases in a mixed assemblage. The presence of goethite in the iron-rich precipitates was only determined after removing the schwertmannite and/or ferrihydrite. Nickel, aluminium and chromium retarded the transformations of schwertmannite and/or ferrihydrite to goethite, but aluminium and chromium supressed the formation of 6-line ferrihydrite. Also, aluminium and chromium influenced the absorbed sulphate symmetry of iron-rich precipitates. The structural order of the phases became less pronounced with the presence of foreign metallic cations, particularly aluminium and chromium. Aluminium and chromium can strongly stabilize iron-rich precipitates making these resistant to leaching by AAO solution. FTIR analysis confirmed the presence of goethite in the bi-metallic precipitates and suggested that the sulphate is present to a greater extent in lower symmetry environments.