A computer simulation study of sorption of model flotation reagents to planar and stepped {111} surfaces of calcium fluoride

Abstract

Atomistic computer simulation techniques were employed to investigate the interaction of a selection of organic surfactant molecules with different surface features of the calcium fluoride {111} surface. The adsorbates coordinate mainly to the surface through interaction between their oxygen (or nitrogen) atoms to surface calcium ions, followed by hydrogen-bonded interactions to surface fluoride ions. Bridging between two surface calcium ions is the preferred mode of adsorption, but a bidentate interaction by two adsorbate oxygen ions to the same surface calcium ion is also a stable configuration and multiple interactions between surfaces and adsorbate molecules lead to the largest adsorption energies. All adsorbates containing carbonyl and hydroxy groups interact strongly with the surfaces, releasing energies between approximately 60 and 190 kJ mol⁻¹, but methylamine containing only the –NH₂ functional group adsorbs to the surfaces to a much lesser extent (60–80 kJ mol⁻¹). Both hydroxy methanamide and hydroxy ethanal adsorb to some surfaces in an eclipsed conformation, which is a requisite for these functional groups. Sorption of the organic material by replacement of pre-adsorbed water at different surface features is calculated to be mainly exothermic for the O and –OH functional groups, but less so for the –NH₂ group. Alkyl hydroxamates are judged to be more suitable than carboxylic acids for use in flotation reagents, as the hydroxy amide functional group adsorbs most strongly to the predominant terrace sites. The efficacy of the surfactant molecules is hence calculated to be alkyl hydroxamates > carboxylic acids > hydroxy aldehydes > alkyl amines. The results from this study suggest that computer simulations may provide a route to the identification or even design of particular organic surfactants for use in mineral separation processes.