EXTRACTION OF TITANIUM(IV) BY MIXTURES OF MONO-(2-ETHYLHEXYL) 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 HCl 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 HCl) 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 HCl) 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=1:1:2 complex is realized. This complex is yellow. Titanium is effectively extracted by DEHPA at low (less-than-or-equal-to 0.1 mole/Kg HCl) and high (greater-than-or-equal-to 7.5 mole/Kg HCl) aqueous feed acidities. At low acidities formation of six- (or eight-) coordinate hydrated monodentate complex: Ti:4DEHPA:2H2O (or 4H2O) is suggested. Aging of loaded, colorless organic samples during 2 months leads to dehydration and to yellowing. Formation of six- (or eight-) coordinate bidentate complexes: Ti:4DEHPA is suggested. At high acidities formation of yellow six- (or eight-) coordinate bidentate complexes is attained during mixing of the phases. At acidities of 0.5 - 7.0 mole/Kg HCl the dominating mechanism of extraction is considered to be only cation-exchange with formation of Ti:4DEHPA compound. Prolonged mixing of the phases (30-50 hrs, depending on Ti:DEHPA ratio and acidity) or aging during 2 months leads to coordination of titanium central atom with formation of bidentate, yellow complexes. Extraction by mixtures of MEHPA and DEHPA with Trioctylphosphine oxide (TOPO) showed an untisinergistic effect of TOPO at high aqueous feed acidities (greater-than-or-equal-to 7 mole/Kg HCl). Distribution coefficient decreased about two orders of magnitude for MEHPA-TOPO and an order for DEHPA-TOPO.