Characterization of adsorbed oleate at calcite and fluorite surfaces by infrared and Raman spectroscopy

In-situ Fourier-transform infrared internal reflection spectroscopy (FT-IR/IRS) with reactive internal reflection elements (IREs) has been used to study oleate adsorption at a calcite (CaCO{dollar}sb3{dollar}) surface. Birefringent and transmission limitations of calcite were overcome with appropriate optics and near-infrared (NIR) spectroscopy. The FT-IR/IRS adsorption density equation was used and spectral data were corrected to establish adsorption isotherms at pH 9.2 for temperatures of 20{dollar}spcirc{dollar}C and 60{dollar}spcirc{dollar}C. At low equilibrium oleate concentrations, the extent of adsorption at the calcite surface increased slightly with increase in temperature and was attributed to chemisorption. At higher oleate concentrations, surface precipitation of calcium dioleate predominated but was observed to decrease with increasing temperature. This effect was associated with surface-passivation resulting from increased chemisorption and with increased solubility of calcium dioleate. Ex-situ FT-IR/IRS spectra of calcite particles were obtained in the mid-infrared (MIR) region and found to support the in-situ FT-NIR/IRS results. Research efforts reported in the literature using other sampling methods were unsuccessful in obtaining spectra due to the strong infrared absorbance of carbonate vibrations.; Oleate adsorption isotherms were found to be similar to those determined previously for fluorite (CaF{dollar}sb2{dollar}). However, the extent of chemisorption at the calcite surface was two to five times less than that at the fluorite surface whereas the extent of calcium dioleate surface precipitation on calcite was four to five times that on fluorite. Contact angle measurements and flotation recoveries showed the hydrophobicity of fluorite to be consistently greater than calcite. Significant differences were noted in the chemisorption region where only fluorite hydrophobicity was sensitive to the presence of oxygen. MLRS showed this phenomenon was due to double-bond reactivity of chemisorbed oleate and subsequent formation of an epoxide/polyether complex at the fluorite surface. Such polymerization products were not detected on calcite. These differences in adsorption density, hydrophobicity, and surface polymerization were ascribed to phase transitions exhibited by chemisorbed oleate as well as to intrinsic surface properties of the minerals related to hydration and number of surface calcium sites.