Selective precipitation of rare earth from non-purified and purified sulfate liquors using sodium sulfate and disodium hydrogen phosphate

Abstract

The conventional hydrometallurgical process routes for the recovery of rare earth elements (REEs) from ores employs sulfuric acid. After being leached, the rare earth sulfate liquor presents impurities that need to be removed before they can be used in the production of marketable products. In this sense, a non-purified rare earth sulfate liquor obtained from a rare earth mineral processing and containing low rare earth oxides (REOs) concentration (∼4.6 g/L) and high Ca2+, Mg2+, Mn2+, Fe3+, Al3+, Th4+, UO22+, PO43− and SO42− content were used in this study. In order to remove all the Fe3+, Th4+ and PO4−3 ions and reduce Al3+, Ca2+, UO22+ and SO42− concentrations, the liquor was purified in two consecutive steps: first by addition of limestone to pH of 3.5 and then with a dosage of lime until it reached a pH of 5.0. Only about 7% of the REEs mass feed in the liquor was lost in the purification step. The REEs precipitation experiments were carried out with two liquors, non-purified and purified, using Na2SO4 and Na2HPO4 as reagents. The rare earth products with the highest purity were obtained with purified rare earth sulfate liquor. In the best condition, the consumption ratio was 0.31 ton of Na per ton of REEs and it resulted in the precipitation of 100% of the REEs feed in the liquor by addition of Na2HPO4. In the experiments in which Na2SO4 was used, the consumption was 0.82 ton of Na per ton of REEs and only 88% of the REEs feed was precipitated. The temperature to precipitate the REEs using Na2HPO4 ranged from 20 °C to 70 °C, whereas precipitation with Na2SO4, only occurred at 70 °C. The rare earth products obtained with Na2SO4 (sodium and rare earth double sulfate) and with Na2HPO4 (a mixture of rare earth phosphate and sodium and rare earth double sulfate), containing respectively 41.4% w/w and 42.9% w/w of REOs, seemed to have enough purity to be processed in downstream steps. As a result, a flowsheet for a process route to recover the rare earth elements from a non-purified rare earth liquor was proposed.