Description of the Extraction of Rare Earth Element Nitrates from Weakly Acidic Solutions with Concentrated Tributyl Phosphate Solutions

A mathematical model based on the published data was developed for the extraction of REE (from La to Lu, including also Y) nitrates in a wide concentration range in weakly acidic ([H+] ≪ [REE]) systems with TBP, taking into account the dilution of TBP with paraffins and including systems with salting-out agents. Parallel occurrence of several complexation reactions in the aqueous and organic phases and of nonstoichiometric physicochemical interaction of the components is assumed. The formation of REE trisolvate, tetrasolvate, and hydrate solvate, the coextraction of cerium- and yttrtium-group REEs in the form of an ion pair, and the self-salting-out effect caused by the apparent hydration of Ln3+ in the aqueous phase are taken into account. The model considers the effect of ammonium, sodium, and aluminum nitrates as salting-out agents. The influence of temperature on the REE extraction in these systems is also described. The error of calculating the distribution ratios is ≤10%.