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The mechanism of the action of sodium silicate in the flotation of phosphate was investigated by studying the nature of the silicate species both in solution and on the mineral surfaces and the influence of the silicate species on the flotation performances of the valuable mineral (apatite) and the gangue mineral (hematite). Two models of equilibria were used to calculate the distribution of silicate species in solution. The first was based on monomers and tetramers and the second, more sophisticated model, was based on 29 Si NMR studies and potentiometric titrations (Sjoberg et al., 1985) which includes a variety of polymeric silicate species in solution. The latter is supported by our attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopic studies of the silicate solutions. X-ray photoelectron spectroscopic (XPS) analyses have also provided strong evidence for the adsorption of polymeric silicate species on hematite from silicate solutions with concentrations typical of those used in flotation, viz. 10 −3 to 10 −2 mol L −1 . Polymeric silicate species have a stronger depressing effect than monomeric silicate species and colloidal amorphous silica particles. Selective depression of flotation of hematite from apatite can be achieved under conditions of moderate depression (Gong et al., 1992b) and this is shown to correspond to silicate solutions containing moderately polymerized silicates and/or very small colloidal silica particles. It is proposed that under such conditions of moderate polymerization the adsorbed silicate on hematite reduces oleate adsorption whereas on apatite, oleate can displace adsorbed silicate, and render the surface hydrophobic. |
