A process for increasing the particle size of rare earth oxides is disclosed comprising: forming an aqueous mineral acid solution containing a rare earth source dissolved therein, reacting the rare earths in said solution with oxalic acid to form an insoluble rare earth oxalate, separating the rare earth oxalate from the reaction medium, contacting the rare earth oxalate with an aqueous oxalic acid solution having a concentration of oxalic acid of from about 1 to about 40 percent by weight and heating the rare earth oxalate to a sufficient temperature and time to convert said rare earth oxalates into rare earth oxides having a particle size of at least 3 microns.
Particulate and coarse ammonium rare earth double oxalate crystals, readily converted into rare earth oxide particles by simple calcination, e.g., yttrium oxide particles, are prepared by (a) intimately admixing, in an aqueous medium, (i) at least one rare earth neutral oxalate with (ii) at least one precursor compound adapted to liberate ammonium ions in water and (iii) at least one precursor compound adapted to liberate oxalate ions in water, (b) separating the double oxalate precipitate thus formed, and (c), optionally, washing and drying such double oxalate precipitate.
Disclosed is an improvement in the method for the preparation of a rare earth oxide powder particularly suitable as a base material of luminescence phosphors. The improvement comprises, in a method comprising the steps of mixing aqueous solutions of a rare earth salt and oxalic acid to precipitate the rare earth oxalate, collecting the precipitates by filtration, drying the precipitates and calcining the dried precipitates, conducting the oxalate-precipitating reaction at a temperature at 15.degree. C. or lower and freezing the wet precipitates at -25.degree. C. or lower prior to vacuum drying to give dried particles of the rare earth oxalate containing 20% by weight or less of water including the water of crystallization. The particles of the thus obtained rare earth oxide powder are characterized by the small pore volume and average crystallite diameter.
A new improved process of producing a rare earth metal alkoxide is now provided, which comprises reacting a rare earth metal carboxylate with an alkali metal alkoxide in an inert organic solvent under anhydrous conditions, and which can be conducted in a facile way and give the desired rare earth metal alkoxide of a high purity in a high yield.
A process for low temperature sintering of rare earth and actinide oxides which have a cubic to monoclinic transformation by heating a powder compact at a temperature just above the transformation temperature.
Y.sub.2 O.sub.3 is recovered from investment casting shell molds by introducing used mold materials into a dissolution reactor preferably consisting of a rotating vessel having an interior lining of acid and abrasion resistant material. An acid is combined with the materials in the reactor and agitated for a selected time period. Thereafter, the liquid remaining in the reactor is separated from the residual solids and removed. The liquid contains dissolved Y.sub.2 O.sub.3 which is precipitated, preferably using oxalic acid to produce yttrium oxalate [Y.sub.2 (C.sub.2 O.sub.4).sub.3.9H.sub.2 O]. The Y.sub.2 (C.sub.2 O.sub.4).sub.3.9H.sub.2 O is ultimately removed from the solution and calcined to produce high purity Y.sub.2 O.sub.3.
