Leaching of niobium- and REE-bearing iron ores: Significant reduction of H2SO4 consumption using SO2 and activated carbon

An innovative process was developed for the leaching of niobium- and rare earth elements-bearing pyrochlores ores and the subsequent separation of Nb from the REE. Compared to the conventional Nb hydrometallurgical methods, the proposed leaching process is based on a triphasic system: H2SO4(aq)-SO2(g)-activated carbon/ore. The implementation of this leaching step allows the selective dissolution of the non-valuable phases while significantly reducing the H2SO4 consumption. The results obtained in continuous operation, at the pilot scale, show a reduction of the sulfur consumption by 48% when compared to the classical H2SO4 pasting-roasting process. The presence of activated carbon, at concentration as low as 0.4% (w/w), was found to considerably fasten the dissolution reactions and allows optimizing the SO2 utilization to a nearly quantitative rate. The triphasic leaching also affords concentrating the Nb stream which decreases the energy consumption of the down-stream operations and especially for the subsequent Nb-REE-bearing pyrochlores roasting step. The process was optimized at the laboratory scale and then tested in continuous operation for 15 days at a flow of 15 kg h−1 of dry equivalent of non-magnetic Nb-REE pyrochlore ore from the Gabonese Mabounié deposit and the equivalent of 8.25 kg h−1 of H2SO4 and 0.15 kg h−1 of activated carbon.