Technospheric mining of scandium from hydrometallurgical tailings of nickel laterite processing: Selection of lixiviant and optimisation of leaching variables

In this study, the potential of tailings generated from hydrometallurgical processing of nickel laterite ores as a source of scandium was investigated. The use of an atmospheric leaching process was proposed and five different lixiviants, namely sulphuric, phosphoric, oxalic, citric and ascorbic acids, were examined systematically to extract the scandium. Characterisation results showed that the nickel laterite residue sample contained about 148 ppm scandium and that the major crystalline phases of the sample were quartz, chromite, magnetite and akaganeite. Results of the leaching experiments suggested that most of the scandium was hosted within the refractory iron-bearing minerals, namely magnetite and chromite, based on the apparent correlation between the scandium dissolution and iron and chromium dissolutions and the composition of crystalline phases in the leached residues. Oxalic acid was found to be the most effective lixiviant to overcome the refractoriness of these minerals and render the scandium accessible for dissolution. To further improve the leaching performance with oxalic acid, the effects of acid concentration, stirring speed, pulp density, temperature and leaching duration on the leaching efficiency were studied. It was found that the scandium dissolution can reach up to 94% in 0.75 mol/L of the acid at a temperature of 95 °C, pulp density of 100 g/L, leaching time of 24 h and stirring speed of 600 rpm. These results demonstrated that the simple, atmospheric leaching process with an organic acid is feasible to extract scandium from nickel laterite residues.