Reduction roasting of limonite ores: effect of dehydroxylation

The effect of pre-calcination in the reducibility of tropical limonite ore, obtained from New Caledonia, was investigated in this study. A series of reduction tests were carried out with pre-calcined and raw limonite ores. The extent of metallisation was determined from nickel and iron which dissolved in ammoniacal solution. Mineralogical transformations that occurred during reduction were analysed using thermal gravimetric, differential thermal analysis (TGA/DTA) and in situ high temperature synchrotron based X-ray diffraction. Bulk mineral analysis of the raw ore was also obtained using neutron diffraction. This study suggests that there is benefit in the pre-calcination of limonite ores. Pre-calcination of the ores which opens the main gangue mineral structure, goethite, allows a rapid interaction between the reductant gases and the nickel species during reduction. The greater nickel metallisation achieved in reduction of pre-calcined ores resulted in higher dissolved nickel whilst reduced raw limonite showed poorer nickel recovery. Reduction of metal minerals was also accompanied by diffusion of nickel into the reduced iron matrix that resulted in the formation of ferro-nickel alloy (Ni₃Fe). The formation of this solid solution was monitored by in situ synchrotron based X-ray diffraction and was also reflected by the lower nickel leachability in ammoniacal solution. It was found that incorporation of nickel into the reduced iron oxide resulting from pre-calcination is significantly less in comparison to the diffusion occurring in reduced raw or uncalcined limonite ores. This effect is prevalent in prolonged reduction periods (greater than 20 min), at higher temperatures (>500 °C) and highly reducing conditions (H₂/CO₂=1:1). This study showed that optimal nickel recovery can be achieved by controlling the incorporation of nickel into the reduced iron structure by pre-calcination of the limonite ore.