Electrokinetic properties of hydroxyapatite under flotation conditions

The effect of calcite supernatant, calcium, and carbonate ions on the hydroxyapatite (HA) zeta potential without and in the presence of sodium oleate (1 × 10−4 mol L−1) was examined within the pH range from 4 to 12. The interpretation of results was based on the HA surface and oleate solution chemistry, and on some floatability tests. HA, with different positive and negative surface sites formed depending on its solubility and pH, had a negative zeta potential over the whole pH range. This mineral is not naturally floatable (flotation recovery, 5% < R < 18%). The oleate ions (Ol−), present in a very low concentration in an acidic medium (pH from 4.8 to 6), chemisorb individually on HA surface centers Ca+, HPO4Ca+, and OH2+, increasing the negative zeta potential of the mineral. Within the pH range from 7 to 9, the dominant oleate species Ol− ion and ion–molecule complex, H(Ol)2-, adsorbed on HA by head groups toward the solid and associated due to chain–chain interaction in hemimicelles, made the HA surface with zeta potential about −22/−23 mV, and more floatable (R = 80–100%) than in 4 < pH < 7 (R = 15–35%) or in pH > 9.3. The HA surface is less negatively charged in calcite supernatant than in water from pH 6.6 to 9.2 due to the adsorption on HA negative surface active centers (HPO4-andPO42-) of the Ca2+, CaHCO3+, and CaOH+ ions (present in the calcite supernatant), producing more surface sites HPO4Ca+, PO4Ca, HPO4CaOH, and PO4- CaOH, and new centers HPO4CaHCO3 and PO4- CaHCO3. In the presence of 1 × 10−3 mol L−1 CaCl2, the HA sample has positive zeta potential, the same as calcite from the same deposit, up to IEP at pH 11.25. Carbonate ions (1 × 10−3 mol L−1 Na2CO3) do not affect the HA zeta potential. However, a possible process can be the ion-exchange reaction between bicarbonate (or carbonate) and some anion from the surface sites formed on HA. The obtained values of the HA zeta potential with the collector (1 × 10−4 mol L−1 Na-oleate) added into hydroxyapatite/calcite supernatant suspensions corroborate the weak chemisorption of Ol− and H(Ol)2-. The likely processes in this system also are the ion-exchange reactions on HPO4CaOH and PO4- CaOH, HPO4CaHCO3 and PO4- CaHCO3 between oleate ion and surface hydroxyl and bicarbonate ions, surface and bulk precipitations of calcium oleate, Ca(Ol)2, and the surface and bulk precipitations of Ca[H(Ol)2-]2 over the pH range from 7 to 9. Calcite supernatant does not influence natural floatability of the mineral. However, calcite supernatant depresses the hydroxyapatite flotation in the presence of 1 × 10−4 mol L−1 Na-oleate (pH 9, R ∼ 50%), a likely result of the weak chemisorption due to the steric effect of heterogeneous HA surface formed in calcite supernatant, Ca(Ol)2 and Ca[H(Ol)2-]2 surface and bulk precipitations.

Graphical abstractProposed mechanisms of interactions of hydroxyapatite surface with oleate species present in the solution at different pH (concentration of Na-oleate ⩽1 × 10−4 mol L−1).Figure optionsDownload full-size imageDownload high-quality image (56 K)Download as PowerPoint slide