Synergistic adsorptions of NaCO and NaSiO on calcium minerals revealed by spectroscopic and molecular dynamics studies.

The synergistic effects between sodium silicate (NaSiO) and sodium carbonate (NaCO) 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 molecular dynamics to investigate the adsorption of NaSiO onto bare and carbonated fluorite (CaF), an archetypal calcium mineral. Both experimental and theoretical results proved that NaCO adsorbs onto CaF with a high affinity and forms a layer of NaCO on the surface. Besides, at low NaSiO concentration, silica mainly physisorbs in a monomeric protonated form, Si(OH), 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, SiO(OH) and SiO(OH) , even at low coverage. Their adsorption is highly favoured compared to the acid forms, which explains the synergistic effects observed when NaSiO is used after NaCO. The formation of the basic form on the bare surface is observed only by increasing the surface coverage to 100%. Hence, when NaCO is used during a separation process, lower NaSiO concentrations are needed to obtain the same effect as with lone NaSiO in the separation process.