MECHANISM OF ACTION OF SODIUM-SILICATE IN THE FLOTATION OF APATITE FROM HEMATITE

The mechanism of the action of sodium silicate in the flotation of pho sphate 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 minera l (apatite) and the gangue mineral (hematite). Two models of equilibri a were used to calculate the distribution of silicate species in solut ion. The first was based on monomers and tetramers and the second, mor e sophisticated model, was based on Si-29 NMR studies and potentiometr ic titrations (Sjoberg et al., 1985) which includes a variety of polym eric silicate species in solution. The latter is supported by our atte nuated total reflection-Fourier transform infrared (ATR-FTIR) spectros copic studies of the silicate solutions. X-ray photoelectron spectrosc opic (XPS) analyses have also provided strong evidence for the adsorpt ion 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 depressi ng effect than monomeric silicate species and colloidal amorphous sili ca particles. Selective depression of flotation of hematite from apati te can be achieved under conditions of moderate depression (Gong et al ., 1992b) and this is shown to correspond to silicate solutions contai ning moderately polymerized silicates and/or very small colloidal sili ca particles. It is proposed that under such conditions of moderate po lymerization the adsorbed silicate on hematite reduces oleate adsorpti on whereas on apatite, oleate can displace adsorbed silicate, and rend er the surface hydrophobic.