Study on the Extraction Mechanism and the Phase Equilibrium of Fluorosilicic Acid with Trioctylamine

Solvent extraction technology was applied to extract fluorosilicic acid from aqueous solution with trioctylamine (TOA) as extractant. The extraction mechanism of fluorosilicic acid with TOA was studied via stoichiometry methods combined with spectroscopic methods. The results of saturation capacity method and equimolar methods showed that there were two kinds of organic extracts produced with molar ratio of 1: 1 and 2: 1 in the system, whose chemical formulas were (R₃N · H₂SiF₆)₃ and [(R₃N)₂ · H₂SiF₆]₂, respectively. Fuorosilicic acid was extracted with TOA via hydrogen bonds. The liquid-liquid phase equilibria of the ternary system fluorosilicic acid (1)-water (2)-TOA (3) from 283.15 K to 323.15 K were determined. The equilibrium phase diagram of the ternary system is obtained. The partition coefficient and separation factor were calculated to evaluate the extraction capacity of TOA for fluorosilicic acid, which showed that TOA exhibited high selectivity.