To overcome the low recovery, generally obtained for the heavy and middle rare earth elements (HREE and MREE) by sodium double sulfate precipitation, the recovery of the MREE and HREE as phosphate salts was investigated. High recovery yields (≥ 95%) were obtained even for sulfate solutions containing low concentrations of REE and a high concentration of iron(II). The influence of many industry-relevant parameters (temperature, pH, stoichiometric excess of phosphate, aluminum contamination, residence time) is discussed in order to optimize the REE recovery and to minimize the precipitation of iron. The developed process was also validated in continuous operation at a pilot scale. Finally, the REE phosphate concentrate could be converted into REE hydroxides which allows its dissolution into diluted HCl. Taking into account the phosphate precipitation, the conversion into hydroxides and the dissolution in HCl(aq), high recovery yields were obtained for Y, Gd and La (68%, 77% and 86% respectively). The results show that it is possible to economically obtain purified REE solutions starting from industrial solutions which are usually discarded.

The current hydrometallurgical processes used for niobium and tantalum recovery are operated in strongly acidic and fluoride-containing solutions. To avoid the use of these highly toxic media, a fluoride-free process was developed to recover Nb and Ta from low-grade industrial concentrates. The process is based on the caustic conversion of the raw material with NaOH(aq) at atmospheric pressure and at relatively low temperature which then allows the selective dissolution of sodium hexaniobates. Finally, Nb is recovered as purified hydrous oxide by acidification of the hexaniobate solution. The influence of many industry-relevant parameters (temperature, initial NaOH concentration, residence time, impurity content in the initial concentrate, pH of precipitation) was studied in order to optimize the recovery and purification of the valuable metals. Finally, the process was validated in continuous operation at a pilot scale. High recovery yields for Nb and Ta (65%) were obtained as well as high separation factors toward Ti, Fe, P, S, Th and U. The results demonstrate that it is possible to recover and purify Nb and Ta from industrial concentrates without using fluoride solutions.

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.