| Summary: | Residues of the CESL Cu and Vale Ni sulphide medium temperature pressure leach contain elevated levels of the interest Cu and Ni metals. Metal losses are disproportionately found within a poorly formed amorphous and semi-crystalline iron oxide phase analogous to ferrihydrite. This phase results from incomplete ferric hydrolysis and precipitation that occurs simultaneously with the leaching of the sulphide minerals. An investigation was conducted by ageing the residues at 95°C in water to assess the extraction of lost Cu and Ni and the transformation of ferrihydrite to the more crystalline goethite and hematite species. It was found that 22% of the Cu could be removed from the CESL residue in 24 hours. Repeated ageing was only capable of extracting 38% of the Cu. Extractions of 22% of the Ni and 6% of the Cu were achieved in 24 hours from the Vale residue. Recovery of the lost Cu and Ni was mainly attributed to the washing of entrained and adsorbed Cu and Ni sulphates evenly distributed throughout the residues. A small amount of Cu and Ni was gained from ferrihydrite dissolution during the ageing process. Further investigations by SEM-EDS revealed the presence of unleached Cu and Ni sulphides that also contribute notable metal losses. EMP analysis showed that all hematite precipitated in the Vale residue contained Cu, Ni and SO₄²−. The average hematite particle consisted of 66.1% Fe, 0.85% Ni, 0.73% Cu and 1.19% SO₄²− for an equivalent of 30% Ni and 25% Cu losses to the residue. QXRD analysis of the aged residue showed little evidence of ferrihydrite transformation though ageing stabilised the residues by reducing the amount of X-ray amorphous material present. Improved stability was confirmed by a reduction in mass loss following treatment by a sequential extraction procedure utilising acidified hydroxylamine hydrochloride. This treatment dissolves the ferrihydrite present in the residues and is used as a proxy to assess residue stability and losses of Cu and Ni to the non-crystalline iron oxide phase. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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