Gold sorption by silicates in acidic and alkaline chloride media

The ability of framework and layer silicates to adsorb gold in a chloride medium is examined. Experiments are conducted on quartz, feldspar, kaolinite and pyrophyllite powders (0.1-2.5 mu m) under acidic and alkaline conditions (pH 2.5-8). The adsorption of gold (supplied as HAuCl4) in acidic media is strong for all silicates evaluated. The maximum extent of adsorption was 100% for the layer silicates kaolinite and pyrophyllite, 97% for quartz and 51% for feldspar, after 7 h of moderate agitation at 50 g/L solids and 5 ppm initial gold concentration. The adsorption is rapid, and most of the gold is adsorbed from the solution within the first 2 h. In alkaline solutions, the gold adsorption was much lower than in acidic media, only reaching significant levels for the system containing pyrophyllite, which reached a maximum of 92% sorption after 5 h. Reversibility analysis at pH values of 0-1 does not show any gold trapped or physisorbed on silicate surfaces, indicating that chemisorption is the primary mechanism of gold loss from solution. The amount of sorbed gold in acidic environments is more closely related to the exposed surface area of the silicate solids than to the type of silicate. A much lower extent of adsorption by the same concentration of silicates at elevated pH further indicates that gold is not simply physisorbed to the surface, but rather that there is a chemical bonding between the silicate surface and gold complexes at low pH. The adsorption of gold in alkaline media may be the result of the development of colloidal gold particles associated with the silicate minerals. The correlation between dissolution of silicon and gold adsorption can also indicate that newly-formed silica gels are adsorbing gold from the solution. (C) 2011 Elsevier B.V. All rights reserved.