Kinetics and mechanism of desilication reaction in sodium chromate solution using sodium aluminate

It is essential to control silica levels in sodium chromate solution during the process of electrolytic synthesis of sodium dichromate. On the basis of previous work, the desilication conditions were systematically studied and pH was found to have the most serious impact on the desilication process. The desilication rate under optimal conditions obtained from the orthogonal experiment results was up to 99.3%. Kinetic experiments showed that the desilication reaction was first-order in SiO₂ concentration and the apparent activation energy was calculated to be 22 kJ mol^-1, which is far less than the reported values. The morphology and structure of the desilication reaction product (DSP) were studied by scanning electron microscope (SEM) and X-ray diffraction (XRD). SEM showed that DSP had a loose etched structure. The pattern of XRD illustrated the crystallinity of DSP increased along the duration of the experiment. The phase of DSP was mainly Na₆(AlSiO₄)₆. Based on the experimental data, the desilication mechanism has been discussed. The hexameric aluminosilicate ions were produced by the reaction of monomeric aluminosilicate ion, through hydrogen bond interaction, which was formed by SiO₂(OH)₂^2- and Al(OH)₄^-.