Studies on hydrometallurgical recovery of rare earth elements as mixed fluoride compound from coal fly ash using environmentally friendly organic acid

Coal fly ash (CFA) emerged as one of the potential secondary source of rare earth elements (REEs). This paper discusses development of a process scheme for the recovery of REE from a CFA sample, assaying total REE of about 0.2% using environmentally benign lactic acid (LA) as leachant. The CFA is generated at a lignite coal-based thermal power plant. The REE occurs dominantly in an ion-exchangeable form and also as bound to iron oxides. Nearly 25% of total REE is constituted of heavy rare earth elements (HREEs). About 89% of REE values were found leachable with LA at 80 & DEG;C in 2 h of reaction time at 10% solids content. Kinetic analysis of leaching process shows that the reaction between CFA and LA is chemical reaction controlled with activation energy of 42-45 kJ/mol. The dissolved REE values were extracted quantitatively from leachate using D2EHPA solvent diluted in kerosene and stripped back with 6 N HCl. The REE in strip liquor was precipitated as mixed REE-fluoride concentrate of purity 86% with precipitation efficiency of 98%. A scheme for quantitative recovery of LA from barren stream was developed for its recycle in leaching circuit.