Aqueous Partition Mechanism of Organophosphorus Extractants in Rare Earths Extraction

The aqueous partition mechanism of traditional extractants and ionic liquid for the extraction of rare earths (REs) is explored in this work. To investigate the aqueous partition of extractants, the aqueous solubility of di(2-ethylhexyl) phosphoric acid (P204), 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester (P507), di(2-ethylhexyl) phosphinic acid (P227), bis (2,4,4-trimethylpentyl) phosphinic acid (C272), and [trialkylmethylammonium][di(2-ethylhexyl)orthophosphinate] ([A336][P507]) was analyzed systemically. The results demonstrated that the solubility of extractants decreased with increase of aqueous acidity, RE loading, and electrolyte concentration. Especially, the solubility of P204, P507, P227, and C272 decreased with the increase of RE complex, indicating that aqueous partition of the extractants was accompanied by the RE coordination reaction. The electrical double layer theory and the Pitzer equation was used to explain the inhibition of electrolyte on the aqueous partition of extractants. In addition, a solubility of [A336][P507] lower than that of saponified P507 illustrated that the partition behavior was related to extractant property.