The recovery of rare earth oxides from a phosphoric acid by-product. Part 3. The separation of the middle and light rare earth fractions and the preparation of pure europium oxide

Abstract

The development and operation of a continuous solvent extraction process for the separation of the middle (Sm, Eu, Gd, Tb) and light rare earth fractions (La, Ce, Pr, Nd) from a nitrate feed liquor is described. The process consists of extraction of the middle rare earths into a 15 vol% solution of D2EHPA in Shellsol AB in 8 counter-current stages, followed by scrubbing with 1 M nitric acid in 2–4 stages, and stripping with 1.5 M hydrochloric acid in 6–8 stages. Residual rare earth values in the organic phase (mainly Dy, with some Tb and Gd) were removed in a secondary stripping circuit using 2.5 M hydrochloric acid in 4 stages. More than 1000 1 of feed liquor were processed in two continuous counter-current trials lasting a total of 630 h. From a feed containing Sm 3.5, Gd 2.4, Eu 0.8 and Nd 20 g l −1 (together with 4–8 g l −1 each of the lighter rare earths), strip liquors containing Sm 35, Gd 20 and Eu 8 g l −1 were obtained, with neodymium (5 g l −1) as the main impurity. Recoveries of the middle rare earths to the strip liquors were high (95–100%), whereas losses of the light rare earths were low (0–4%). Addition of oxalic acid to the strip liquors, followed by calcination of the precipitated oxalate, gave a middle rare earth oxide containing 45% Sm 2O 3, 29% Gd 2O 3, 13% Eu 2O 3 and 6% Nd 2O 3. This oxide was dissolved in acetic acid and electrolysed in 1 M potassium citrate solution, using a mercury cathode and a platinum anode, to give a europium amalgam containing a small amount of samarium (Eu:Sm ≈ 66:1). The amalgam was treated with acetic acid, and the resulting acetate solution was again electrolysed to give a purified amalgam from which was recovered a final europium oxide product containing only traces of other rare earths (< 25 mg kg −1 Sm, <6 mg kg −1 Nd and <3 mg kg −1 each of Gd, Pr, Ce and La).