Characterisation, concentration and acidic reductive leaching of limonitic laterites from New Caledonia

This study aimed at determining Ni and Co leaching kinetics from a New Caledonian laterite in an acidic medium (H 2 SO 4 pH 1.5) and in a reductive environment (addition of SO 3 2- or Fe(II)) at 46 °C. The mineralogical study revealed that Co was mainly carried by Mn oxyhydroxides in the limonite sample. Conversely, Ni was hosted by both Fe and Mn oxyhydroxides. In the presence of a reductive reagent, Mn oxyhydroxides dissolved rapidly compared to goethite, the main Fe oxyhydroxide in the sample. Co, Mn and Ni reductive leaching yields reached 79%, 83% and 9% respectively after 2 days. Based on these results, a Mn oxides concentrate was produced in order to efficiently leach Co while limiting Fe oxyhydroxide dissolution. This concentrate resulted from a combination of particle size and gravity separation steps. The volume/mass of sample was drastically decreased since the mass of the final sample was only 3.3% of the initial one. Co content increased from 0.16 wt% in the limonite to 2.3 wt% in the concentrate, representing an enrichment factor of 13.8 and recovery yield of 60%. Co, Mn and Ni leaching yields reached 87%, 95% and 80% respectively in the Mn oxides concentrate leaching experiment. The difference in Ni behaviour was consistent with the mineralogical composition: Ni was mainly carried by the goethite in the laterite, while it was hosted mainly by the Mn oxyhydroxides in the Mn oxides concentrate. This study gives a proof of concept for the development a robust pre-concentration process to recover Co. • Both Fe and Mn oxyhydroxides host Ni, while Co is mainly hosted by the latter • Mineral processing enabled an increase in the Co content from 0.16 to 2.3 wt% • High Mn and Co dissolution kinetics were obtained during reductive leaching • Goethite dissolution was limited in both acid and reductive conditions • Ni leaching amenability was strongly dependent on its host mineral