Synergistic adsorptions of Na2CO3 and Na2SiO3 on calcium minerals revealed by spectroscopic and ab initio molecular dynamics studies
The synergistic effects between sodium silicate (Na2SiO3) and sodium carbonate (Na2CO3) adsorbed on mineral surfaces are not yet understood, making impossible to finely tune their respective amount 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 Na2SiO3 onto bare and carbonated fluorite (CaF2), an archetypal calcium mineral. Both experimental and theoretical results proved that Na2CO3 adsorbs onto CaF2 with a high affinity and forms a layer of Na2CO3 on the surface. Besides, at low Na2SiO3 concentration, silica mainly physisorbs under a monomeric protonated form, Si(OH)4, while at larger concentration, significant amounts of polymerized and deprotonated forms are identified. A 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 Si2O3(OH)42-, even at low coverage. Their adsorption is highly favoured compared to the acid forms, which explains the synergistic effects observed when Na2SiO3 is used after Na2CO3. The formation of the basic form on the bare surface is observed only by increasing the surface coverage to 100%. Hence, when Na2CO3 is used during a separation process, lower Na2SiO3 concentrations are needed to obtain the same effect as with lone Na2SiO3.