Scandium extraction from sulfate media with di-(2-ethylhexyl) phosphoric acid in decane or toluene mixed with proton-donor additives

In this work, the effect of proton-donor additives (monocarboxylic acids, aliphatic alcohols, and phenol derivatives) on scandium extraction from sulfate solutions with di-(2-ethylhexyl)phosphoric acid (D2EHPA, HR) in decane or toluene as the diluent was investigated. It was shown that addition of these additives into the organic phase leads to an antagonistic effect, i.e., to a decrease in the extraction of scandium, and thus improves the stripping efficiency. The antagonistic effect decreases in the series octanol >4-t-butylphenol > > octanoic acid. The low activity of octanoic acid is due to self-association, which significantly prevents the formation of intermolecular associates between D2EHPA and the monocarboxylic acid. Alcohols and phenols form stable associates with D2EHPA, which leads to a decrease in scandium extraction. Analysis of the interphase scandium distribution as well as the nuclear magnetic resonance (NMR) and infrared (IR) spectra of the loaded organic phases showed that the composition of the extracted scandium compound in the mixed extractant system containing D2EHPA and octyl alcohol is ScR₃. In the case of D2EHPA in inert diluents (such as decane and toluene), the composition of the extracted scandium compound in excess extractant is Sc(HR₂)₃. Scandium stripping from the D2EHPA and octyl alcohol mixtures with a sulfuric acid solution is almost impossible, but it can be successfully achieved with an alkaline solution containing sorbitol. In this case, no precipitation occurs in the organic phase which is an advantage. Extraction systems containing D2EHPA and higher alcohols, such as 2-ethylhexanol, can be used to recover scandium from various industrial solutions.