Sodium hexametaphosphate interaction with 2:1 clay minerals illite and montmorillonite

The interaction of an efficient deflocculant, sodium hexametaphosphate (NaHMP), with illite and montmorillonite samples of different origins was investigated analyzing the HMP uptake (adsorption on mineral surface and/or intercalation inside the mineral interlayer) and the ion release mechanisms. HMP adsorption isotherms on a standard Na-saturated illite were determined at different temperatures providing the thermodynamic parameters of the process and the maximum adsorbed amount of the ion on this mineral. The data indicate that the adsorption process is hindered compared to that on kaolinite and are consistent with the hypothesis that HMP is chemiadsorbed on the aluminol edge sites, forming Al-O-P linkages. The effect of HMP on the natural illite and montmorillonite samples was analyzed using different techniques. Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were used to detect the concentration of P, Si, Al, Ca, Mg and K in deflocculant solutions wetting the clay minerals in order to detail the immobilization, dissolution and exchange reactions associated to the deflocculant activity. Thermal analysis and XRD measurements were used as well to gain information on the properties of the HMP-treated clay mineral samples. Montmorillonite samples show a peculiar ability to immobilize HMP. Intercalation processes involving the formation of Ca2+/HMP complexes inside the mineral interlayer are proposed to explain this result and could be the reason of the well known decrease in the deflocculant efficiency of HMP in ceramic slurries rich in montmorillonite.