Processing of xenotime concentrate by sulphuric acid digestion and selective thorium precipitation for separation of rare earths

摘要

A process is described for the recovery of rare earths from xenotime concentrate by digestion with sulphuric acid. A low grade xenotime concentrate (typically assaying 57 percent light rare earths, 27 percent Y_2O_3 and 15.6 percent heavy rare earth) and a high grade xenotime (assaying 38 percent light rare earths, 41.8 percent Y_2O_3 and 20.55 percent heavy rare earths) is routinely being produced in India. Both types of materials have been tested and > 98 percent solubilisation of metal values has been achieved. Xenotime sulphate leach liquor has been processed for removal of thorium by selective precipitation with ammonia and sodium pyrophosphate. Although in both systems thorium removal was almost complete, the ammonia precipitation was found suitable since it facilitates the preferential precipitation of lighter rare earths along with thorium. Consequently, the heavy rare earths have been enriched in the filtrate. Other impurities such as iron, uranium, sulphate and phosphate have been effectively removed by precipitation of rare earths with oxalic acid. It has been shown possible to recover rare earths along with yttrium (approx 99 percent recovery) by these processes.