Dissolution behavior of the associated rare-earth elements in scheelite using a mixture of sulfuric and phosphoric acids

Abstract

In scheelite, the associated rare-earth elements (REEs) can exist in the form of isomorphous RE2(WO4)3 or monazite (REPO4). To extract REEs from scheelite using the mixture of H2SO4 and H3PO4 acids, the dissolution behavior of isomorphic REEs and monazite was investigated in this study. The results show that the isomorphic REEs were easily decomposed using mixed acid, that is, H2SO4–H3PO4. Owing to the eutectic and sorption of gypsum, a part of the leached REE enters gypsum residue in the form of RE phosphate with increasing reaction temperature and volume ratios of H2SO4–H3PO4. After washing with a diluted acid solution, the precipitated REE was completely leached again into the washing liquor until 100% digestion of REE was obtained. However, only 1.3% leaching of the REE was attained for the monazite REE due to inefficient dissolution from using the mixed acid (H2SO4–H3PO4). However, considering mechanical activation of the associated REE in scheelite, the REE leaching could be improved to 35% in the presence of mixed acid.