Crystallisation of strontium sulphates from Si-bearing aqueous solutions

The crystallisation behaviour in the SrCl2–Na2SO4–H2O system at room temperature is strongly modified by the presence of dissolved silicon. Homogeneous nucleation experiments show that silicon inhibits the formation of celestite while promoting the precipitation of SrSO4.0.5H2O and an amorphous phase. Interfacial free energies for celestite have been calculated for increasing silicon concentrations from measurements of induction times for homogeneous nucleation. The slight increase in the interfacial free energies confirms that dissolved silicon is an inhibitor of celestite nucleation. In addition, dissolved silicon has striking morphological effects. Celestite grown in the presence of silicon typically shows rounded and peanut-like morphologies formed by numerous disoriented crystals. The anomalous celestite morphologies and the increase in both induction times and interfacial free energies reveal a complex interaction between silicic acids and celestite surfaces. In situ atomic force microscopy (AFM) was used to study in detail the effect of dissolved silicon on the growth of celestite (001) faces. AFM observations show that the presence of silicon increases the growth velocity of a first monolayer on the celestite (001) face. However, once such a first monolayer is formed, no further multilayer growth is observed. Height and friction AFM images show clear differences in contrast between the first monolayer and the celestite (001) substrate, revealing differences in composition and/or structure. High-resolution AFM images of the first monolayer show patterns consistent with the celestite (001) surface lattice, indicating that a limited amount of silicon can be incorporated into the celestite structure. Therefore, both the observed inhibition of nucleation and growth of celestite and the changes in crystal morphologies can be partially related to the formation of self-limiting Si-bearing celestite monolayers