The recovery of rare earth oxides from a phosphoric acid by-product. Part 2: The preparation of high-purity cerium dioxide and recovery of a heavy rare earth oxide concentrate

Abstract

The development of a solvent-extraction process for the recovery of high-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 Shellsol K with high selectivity over the remaining trivalent rare earths. In continuous counter-current trials, four extraction stages were used, followed by four stages of scrubbing with 3 M nitric acid. Stripping of the organic phase was accomplished by reduction of the cerium(IV) with dilute hydrogen peroxide in two stages, giving solutions containing up to 90 g 1 −1 of cerium(III). Addition of oxalic acid to the strip liquors, followed by calcination of the precipitated oxalate, gave cerium dioxide of at least 99.98% purity in about 70% yield. Continuous counter-current extraction of the raffinate from the cerium recovery 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 an organic-to-aqueous phase ratio of 10:1 gave strip liquors containing up to 8 g −1 of yttrium and 6 g l −1 of dysprosium, together with smaller amounts of heavier rare earths. Recovery to the strip liquors was 98–99% for yttrium and erbium, 50–75% for dysprosium, and 65–75% for holmium. Losses of the principal middle rare earths (samarium, europium and gadolinium) to the strip liquors were only 1–3%. Precipitation and calcination of the oxalate gave a heavy rare earth oxide containing yttrium (41–63% Y 2O 3), dysprosium (17–31% Dy 2O 3), holmium (4% Ho 2O 3) and erbium (4–5% Er 2O 3), together with smaller amounts of other rare earths (total rare earth oxide content 99.4–99.8%).