THE RECOVERY OF RARE-EARTH-OXIDES FROM A PHOSPHORIC-ACID BY-PRODUCT .2. THE PREPARATION OF HIGH-PURITY CERIUM DIOXIDE AND RECOVERY OF A HEAVY RARE-EARTH-OXIDE CONCENTRATE

The development of a solvent-extraction process for the recovery of hi gh-purity cerium dioxide and a heavy rare earth oxide concentrate from a mixed rare earth oxide is described. Dissolution of the mixed oxide in concentrated nitric acid afforded solutions in which up to 95% of the cerium is present as cerium(IV) which, after dilution with water, can be extracted into a 15 vol% solution of tri-n-butyl phosphate in S hellsol K with high selectivity over the remaining trivalent rare eart hs. In continuous counter-current trials, four extraction stages were used, followed by four stages of scrubbing with 3 M nitric acid. Strip ping of the organic phase was accomplished by reduction of the cerium( IV) with dilute hydrogen peroxide in two stages, giving solutions cont aining up to 90 g l(-1) of cerium(III). Addition of oxalic acid to the strip liquors, followed by calcination of the precipitated oxalate, g ave cerium dioxide of at least 99.98% purity in about 70% yield. Conti nuous counter-current extraction of the raffinate from the cerium reco very process in six stages with a 5 vol% solution of di(2-ethylhexyl) phosphoric acid in Shellsol AB at an organic-to-aqueous phase ratio of 3:2, followed by stripping in four stages with 1.3 M nitric acid at a n organic-to-aqueous phase ratio of 10 : 1 gave strip liquors containi ng up to 8 gl(-1) of yttrium and 6 g l(-1) of dysprosium, together wit h smaller amounts of heavier rare earths. Recovery to the strip liquor s was 98-99% for yttrium and erbium, 50-75% for dysprosium, and 65-75% for holmium. Losses of the principal middle rare earths (samarium, eu ropium and gadolinium) to the strip liquors were only 1-3%. Precipitat ion and calcination of the oxalate gave a heavy rare earth oxide conta ining yttrium (41-63% Y2O3), dysprosium (17-31% Dy2O3), holmium (4% Ho 2O3) and erbium (4-5% Er2O3), together with smaller amounts of other r are earths (total rare earth oxide content 99.4-99.8%).