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

The interaction of an efficient deflocculant, sodium hexametaphosphate (NaHMP), with illite and montmorillonite (Mt) samples of different origins was investigated analyzing the HMP uptake (adsorption on mineral surface and/or intercalation inside the interlayer space) and the ion release mechanisms. HMP adsorption isotherms on a standard Na-saturated illite were determined at different temperatures which yielded 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 with respect to that on kaolinite and is consistent with adsorption of HMP on the aluminol edge sites, forming Al–O–P linkages. The effect of HMP on the natural illite and Mt samples was analyzed using different techniques. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to detect the concentration of P, Si, Al, Ca, Mg and K in deflocculant solutions wetting the clay minerals in order to identify the immobilization, dissolution and exchange reactions associated to the deflocculant activity. Thermal analysis and XRD measurements were used to gain information on the properties of the HMP-treated clay minerals. The peculiar ability of Mt samples to immobilize HMP is proposed to be due to intercalation processes involving the formation of Ca2 +/HMP complexes inside the interlayer space. These processes could be responsible of the well known decrease in the deflocculant efficiency of HMP in ceramic slurries rich in Mt.