Separate Crystallization of Lanthanide Oxalates and Calcium Oxalates from Nitric Acid Solutions

The separation of lanthanides from calcium compounds in the form of oxalates from hot nitric acid solutions of Ln(NO₃)₃ and Ca(NO₃)₂ with the insertion of oxalic acid and a Ln₂(C₂O₄)₃ · nH₂O crystal seed was studied by mass-spectrometric, atomic emission, microscopic, X-ray diffraction, and fluorescence analyses. The produced single-phase precipitate was found to contain an isomorphic impurity of La–Sm oxalates, while calcium oxalate remained in the hot nitric acid solution (95°С) saturated with oxalic acid. This facile and efficient method provides Ln₂(C₂O₄)₃ · nH₂O (n = 9.5 mol) in one step in a 80.1 rel. % yield, with the major phase being at least 99.4 wt %. The unit cell parameters were determined for the crystals of the isomorphic lanthanide oxalate mixture: a = 11.243(2) Å, b = 9.591(2) Å, c = 10.306(2) Å; α = γ = 90°, β = 114.12(1)°; Z = 2; V = 1013.7(5) ų.