MECHANISM OF OLEATE INTERACTION ON SALT-TYPE MINERALS .3. ADSORPTION, ZETA POTENTIAL AND DIFFUSE REFLECTANCE FT-IR STUDIES OF SCHEELITE IN THE PRESENCE OF SODIUM OLEATE

The mechanism of oleate adsorption on scheelite has been examined through adsorption isotherms, zeta-potential measurements and diffuse reflectance FT-IR studies. The form of the adsorption isotherms for alkaline pH values indicated a bilayered structure of oleate, preceeding the precipitation of calcium oleate. The precipitation of calcium oleate is characterized by a steep increase in the slope of the adsorption isotherms after 10-mu-mol m-2 adsorption density. This adsorption density corresponds to a condensed state of oleate molecule for a bilayer formation with a molecular coverage area of 33 angstrom 2 (liquid-crystal state). In the monolayer filling, monocoordination of oleate through counter sodium and calcium ions is suggested from the IR studies. A linear relationship between the amount adsorbed and the Kubelka-Munk function of diffuse reflectance is obtained. The zeta potentials are correlated to a successive oleate adsorption up to a bilayer formation. At high oleate concentrations, the constant negative zeta potentials (-52 mV) corresponds to that of calcium oleate, suggesting that the solids are covered with a layer of calcium oleate precipitate. Monocoordination of oleate through counter sodium and calcium ions in the monolayer range followed by a bilayer formation through hydrophobic association of hydrocarbon chains, prior to the growth of calcium oleate precipitate on the substrate, is suggested for the adsorption mechanism of oleate on scheelite.