Calcite, isoelectric point

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

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. 

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