Effect of hydrolysed metal cations on the liquid–liquid extraction of silica fines with cetyltrimethylammonium chloride

Abstract

Silica fines of <5 μm were liquid–liquid extracted from aqueous suspension containing cationic surfactant cetyltrimethylammonium chloride (CTAC) to the isooctane–water interface region. The silica depression on addition of hydrolysable metal cations Fe(III), Al(III) and La(III) was investigated to clarify the role of such cations in the hydrophile–lipophile transition of the silica fines together with CTAC. At an addition of only 5 μM CTAC, the silica fines were completely recovered in the dense emulsion phase from metal salt-free suspension in the entire pH range investigated. In contrast, addition of 1-mM total metal cations caused silica depression in a certain pH range, specific for the metal salt used. Speciation distribution diagrams for the respective metal–H 2O systems indicate that decreases in percentage recoveries of the silica in the lower pH range are in line with increases in concentrations of the metal hydroxo complexes. Furthermore, the silica depression disappears when the pH approaches the point-of-zero charge of the hydroxide precipitate. In these pH ranges, the neutral to negative metal precipitates are also extracted into the oil–water interface during the liquid–liquid extraction. It seems that the lipophilic-to-hydrophobic transition, that is, depression by metal-salt addition of silica fines in cationic liquid–liquid extraction is controlled by the presence of metal-hydroxide species and their charges, attributable to the hydroxide coating on the silica surface.