Selective extraction of Ti(IV) from acidic sulfate media by phosphonium-based ionic liquids and stripping with sulfuric acid

Abstract

The Ti(IV) liquid-liquid extraction from acidic sulfate media has been investigated by the ionic liquids trihexyl(tetradecyl)phosphonium chloride [P66614+][Cl−] and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate [P66614+][BTMPP−], diluted in 5% (v/v) 1-decanol/kerosene. The Room Temperature Ionic Liquids (RTIL) showed fast extraction kinetics reaching equilibration after 2 min, requiring a 450 rpm stirring rate. For [P66614+][Cl−] at 0.01 mol L−1, the maximum extraction efficiency was 30% from H2SO4 solution of 11 mol L−1. The IL [P66614+][BTMPP−] shows better Ti(IV) extraction than [P66614+][Cl−], and two maximum extraction conditions were observed at 0.1 and 7.3 mol L−1 H2SO4 due to the dual mechanism; both cationic and anionic exchange reactions, reaching 60% and 49% extraction efficiency, respectively.The increase in [P66614+][BTMPP−] concentration to 0.06 mol L−1, increases the extraction yield to 95.9%. The Fourier transform infrared FTIR characterization confirmed the presence of Ti(IV) in the organic phases, and the modification in the phosphonium bonds was correlated to the role of the ionic liquids in the Ti(IV) extraction. The organic phase containing 0.02 mol L−1 [P66614+][BTMPP−] can be loaded with 0.197 g/L of Ti(IV) from 7.3 mol L−1 H2SO4 media. In contrast, at 0.1 mol L−1 H2SO4, an increase in the Ti(IV) concentration promotes the polymerization in the form Ti-O-Ti-O-Ti inhibiting the extraction efficiency. The H2SO4 solution at 2 mol L−1 was the best stripping agent to achieve a high Ti(IV) recovery of 97%. The organic phase was reused for four cycles, and the extraction and stripping efficiencies remained unchanged after the third cycle. The selective separation of Ti(IV) from a mixture containing Fe(III) was reached with a separation factor of 78.4, with a 2.51% Fe(III) extraction and containing only 0.25 mg L−1 of Fe(III) in the stripping phase. The proposed system avoids the main problems associated with Ti(IV) recovery from acidic sulfate media.