Influence of dispersants on the solubility of calcined kaolin

The solubility of calcined kaolin relative to different aqueous solution chemistries is important to its industrial application. When used in paper-making, a calcined kaolin product is required to have good flow characteristics and viscosity stability, both of which are largely controlled by the type and level of soluble salts in the kaolin suspension. In this study, five common dispersing agents including tetrasodium pyrophosphate, sodium hexametaphosphate, sodium polyacrylate, sodium silicate, and sodium carbonate and one flocculant aluminum sulfate were investigated in terms of their relationship with the concentration of dissolved ions (Al3+, Si4+, Mg2+, and Ca2+) in calcined kaolin suspensions. Controlled variables included dispersant dosage, suspension pH, and aging. Results from the study showed that the calcined clays dispersed with tetrasodiumpyrophosphate and sodium hexametaphosphate yielded higher levels of soluble aluminum than those with sodium polyacrylate, sodium carbonate, and sodium silicate at equivalent chemical dosage and pH condition. The variations of other soluble salts (Si4+, Mg2+, and Ca2+) were not as significant. Mg2+, and Ca2+ ion concentrations were generally low compared to aluminum and silicon. Higher dosage of any dispersant generally raised the concentration of soluble aluminum. Higher pH favored soluble aluminum but lowered soluble silicon. Addition of aluminum sulfate in calcined clay suspension did not raise the soluble aluminum level. The effect of aging on soluble salts concentration was not significant. Overall, the solubility of calcined kaolin seemed largely related to the pH condition in addition to the type and level of dispersant/flocculant.