RARE-EARTH AND YTTRIUM PHOSPHATE SOLUBILITIES IN AQUEOUS-SOLUTION

Rare earth and yttrium phosphate solubility products range over more t han 1 order of magnitude. Minimum solubilities are observed for light rare earths between Ce and Sm. For the elements Ce, Pr, Nd, and Sm sol ubility products (log K-sp(0)(M) = log ([M-i(3+)][PO43-])) at zero ion ic strength and 25 degrees C can be approximated as log K-sp(0) (M) = -26.3 +/- 0.2. Rare earth phosphate solubility products for well-aged, coarse precipitates increase substantially between Sm and Lu, with lo g K-sp(0)(Lu) estimated as -24.7. The solubility product of Y is simil ar to that of Ho (log K-sp(0)(Y) = -25.0) and is much higher than thos e of all light rare earths. The solubility product of La is substantia lly larger than that of Ce (log K-sp(0)(La) - log K-sp(0)(Ce) approxim ate to 0.5). Solubility products are strongly dependent on the conditi ons of solid phase formation. Fresh precipitates are much more soluble than slowly formed, well-aged, coarse precipitates. The pattern of ra re earth and yttrium phosphate solubility products is generally simila r to the fractionation patterns which are developed during phosphate c oprecipitation. Copyright (C) 1997 Elsevier Science Ltd.