Extraction of titanium(IV) by mixtures of mono- and di-(2-ethylhexyl) phosphoric acid esters

Solvent extraction of titanium(IV) from hydrochloric acid solutions by mixtures of mono- and di-2-ethylhexyl phosphoric acid esters (MEHPA and DEHPA) has been investigated as a function of HC1 concentration in the aqueous phase and extractants concentration in the organic phase. It was found that MEHPA extracts Ti, 3 orders of magnitude more efficiently than DEHPA. Efficiency of extraction by MEHPA does not depend on acid concentration in the aqueous phase in the range of 0, 1 - 8.8 mole/Kg. As a rule Ti/MEHPA ratio in the complex is 1/2, At low aqueous phase acidities (0.1-1.0 mole/Kg HC1) formation of six- (or eight-) coordinate bidentate hydrated Ti(IV)-2MEHPA complexes is suggested. At acidities above 7.0 mole/Kg HCl titanium forms tridentate six- (or eight-) coordinate complexes. At medium acidities (2.0-6.0 mole/Kg HC1) mixtures of these complexes are formed. Prolonged mixing of the phases or aging of the organic phase leads to dehydration and to transformation of bidentate to tridentate Ti-2MEHPA complexes. Ti-2MEHPA complexes are colorless. At ratios Ti/MEHPA>0.5 formation of Ti-MEHPA hydrated or solvated complex ions is suggested which form emulsion in the aqueous phase. These species slowly react with DEHPA and formation of Ti:MEHPA:DEHPA=l:l:2 complex is realized. This complex is yellow.