Apatite, isoelectric point

For the above sparingly soluble minerals, the effect of dissolved species on interfacial properties can be marked. Results obtained for the zeta-potential of apatite and calcite in water and in 2 x 10 M KNO3 solutions are given in Fig. 3.7. It can be seen that the isoelectric points of calcite and apatite in both water and KNO3 solutions are about 10.5 and 7.4, respectively. The effect of the supernatant of calcite on the zeta-potential of apatite is also shown in Fig. 3.8. 

Fig. 3.8. Effect of mineral supernatants on the isoelectric points of calcite and apatite (Amankonah et al., 1985 Somasundaran et al., 1985).

In other words, these results show calcite and apatite to interchange approximately their isoelectric points in the supernatants of each other. This suggests surface conversion of apatite to calcite and calcite to apatite respectively. Implications of such conversion on selective flotation is to be noted. In flotation, the presence of flotation reagents such as oleate or other collectors actually complicates the system behavior even further and this aspect is discussed in a subsequent section. 

The caseins exist in milk as polydisperse aggregates ranging in size from ca. 40 to 220nm (3), but the size distribution of micelles depends upon the method of measurement. These casein micelles scatter light and are responsible for the whitish, opaque nature of skim milk. The casein micelles are also associated with a colloidal apatite comprised of calcium-phosphate-citrate (CPC) which has a stabilizing influence on the micelle structure. The colloidal CPC is in equilibrium with soluble CPC in the milk serum phase and is solubilized as the pH is reduced. Thus, as the pH is reduced to the isoelectric point of the caseins (4.6), the colloidal CPC solubilizes, and the caseins precipitate (143). This phenomenon should be kept in mind during some of the following discussions. 

However, the possibilities of achieving an adequate selectivity based on electrostatic interactions alone are limited since the particulates involved may have similar isoelectric points (e.g. the apatite-dolomite system) or there may be specific interactions between the polymer and mineral surface. 

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