Synergistic adsorptions of Na(2)CO(3) and Na(2)SiO(3) on calcium minerals revealed by spectroscopic and ab initio molecular dynamics studies

The synergistic effects between sodium silicate (Na(2)SiO(3)) and sodium carbonate (Na(2)CO(3)) adsorbed on mineral surfaces are not yet understood, making it impossible to finely tune their respective amounts in various industrial processes. In order to unravel this phenomenon, diffuse reflectance infrared Fourier transform and X-ray photoelectron spectroscopies were combined with ab initio molecular dynamics to investigate the adsorption of Na(2)SiO(3) onto bare and carbonated fluorite (CaF(2)), an archetypal calcium mineral. Both experimental and theoretical results proved that Na(2)CO(3) adsorbs onto CaF(2) with a high affinity and forms a layer of Na(2)CO(3) on the surface. Besides, at low Na(2)SiO(3) concentration, silica mainly physisorbs in a monomeric protonated form, Si(OH)(4), while at larger concentration, significant amounts of polymerised and deprotonated forms are identified. Prior surface carbonation induces an acid–base reaction on the surface, which results in the formation of the basic forms of the monomers and the dimers, i.e. SiO(OH)(3)(–) and Si(2)O(3)(OH)(4)(2–), even at low coverage. Their adsorption is highly favoured compared to the acid forms, which explains the synergistic effects observed when Na(2)SiO(3) is used after Na(2)CO(3). The formation of the basic form on the bare surface is observed only by increasing the surface coverage to 100%. Hence, when Na(2)CO(3) is used during a separation process, lower Na(2)SiO(3) concentrations are needed to obtain the same effect as with lone Na(2)SiO(3) in the separation process.