| Summary: | Thermodynamic modelling of a leaching system that involves concurrent precipitation depends on an understanding of how the metals distribute into the precipitate before an assessment of solubility can be made. It has been suggested in the past that a pair of rare earths (A and B) in solution will separate from each other by oxalate precipitation according to a logarithmic distribution coefficient (<inline-formula><math display="inline"><semantics><mi mathvariant="sans-serif">λ</mi></semantics></math></inline-formula>), determined by the kinetics of the precipitation. By contrast, the present study hypothesises that <inline-formula><math display="inline"><semantics><mi mathvariant="sans-serif">λ</mi></semantics></math></inline-formula> may be approximated from thermodynamic terms, including the solubility product (K<sub>Sp</sub>) of each rare earth oxalate and the stability constant (<inline-formula><math display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">β</mi><mn>1</mn></msub><mo stretchy="false">)</mo></mrow></semantics></math></inline-formula> for the mono-oxalato complex of each rare earth. The proposed model was used to calculate <inline-formula><math display="inline"><semantics><mi mathvariant="sans-serif">λ</mi></semantics></math></inline-formula> between pairs of rare earths. An experimental study was conducted to determine <inline-formula><math display="inline"><semantics><mi mathvariant="sans-serif">λ</mi></semantics></math></inline-formula> between selected pairs using homogenous precipitation through the hydrolysis of an oxalic acid ester, with fairly close agreement to the values under the proposed model. Though this model requires more thorough testing, as well as application to other organic salts, it may provide insight into distribution factors of a precipitate formed by a sequence of organic complexes.
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