Neutralisation of bauxite residue by carbon dioxide prior to acidic leaching for metal recovery

The present work considers the neutralisation of bauxite residue (red mud) with CO2 as a potential technology for reducing the acid consumption in the acidic leaching step for metal recovery. The pH of bauxite residue was reduced during neutralisation by the transformation of hydroxide ions to (hydrogen) carbonate ions. Neutralisation at high CO2 partial pressures and high temperatures reduces the alkalinity of the bauxite residue, but it leads to the stabilisation of silicate compounds such as cancrinite and grossular. After acidic leaching of the neutralised product with sulfuric acid, a decrease by 20% in the dissolution yield of Al, Fe, and Ti was observed, due to an insufficient amount of acid devoted to leaching as the transformation of calcite into bassanite and the high concentration of silicate compounds consumed part of the acid. Sc recovery by leaching of the highly neutralised bauxite residue was about 35 wt%, which depends on Fe and Ti recovery. A positive correlation between La/Nd with Sc indicates that the recovery of La/Nd indirectly depends on the extraction of Fe/Ti, as Sc is chemically associated to these major metals.