Solubility of di-(2-ethylhexyl)phosphoric acid (D2EHPA) in aqueous electrolyte solutions: Studies relevant to liquid-liquid extraction

Di-(2-ethylhexyl)phosphoric acid (D2EHPA) is an acidic extractant frequently used in solvent extraction for metal refining. Only limited information is available on the amount and type of impurities present in commercially available D2EHPA and their effect on liquid-liquid extraction. In this paper, these impurities were identified and the importance of pre-conditioning the extractant before carrying out extractions is demonstrated. Furthermore, the solubility of D2EHPA in aqueous solutions was investigated as a function of pH, concentration of extractant and anion present in the aqueous phase (chloride, bromide, iodide, sulfate, methanesulfonate, thiocyanate, and nitrate). The density of the organic phases and the sodium and water uptake in the organic phase were determined as a function of the different anions present in the aqueous phase. The type of anion was found not to have a large influence on the solubility of D2EHPA in the aqueous phases. Low concentrations of salt in the aqueous phase or high pH values resulted in significant losses of D2EHPA to the aqueous phase. A very sharp increase in water content in the organic phase was observed around pH 5 (pH 6 for the nitrate system), accompanied by a significant increase in extraction of sodium ions, which explains the observed increase in density of the organic phase, and the tendency to third-phase formation. The results from this fundamental study help to better understand the behavior of liquid-liquid extraction systems in which D2EHPA is used as extractant and can help to prevent third-phase formation and losses of extractant to the aqueous phase. Moreover, the impurities present in commercially available D2EHPA and their impact on liquid-liquid extraction processes are determined.