Effect and mechanism of phosphoric acid in the apatite/dolomite flotation system

Abstract

To elucidate how phosphoric acid (H3PO4) as a depressant achieves selective separation of apatite and dolomite, the flotation response of these minerals was investigated using both single and mixed mineral flotation. Microflotation results demonstrated that H3PO4 exhibits a selective depressive effect on apatite. Flotation kinetics analyses also revealed that the presence of H3PO4 increases the dolomite/apatite selectivity index significantly. The adsorption mechanisms were examined using zeta potential tests, X-ray photoelectron spectroscopy, solution chemistry analysis, and thermodynamic analysis. H2PO4− was found to be the pivotal ion for depressing apatite under slightly acidic conditions (pH=5.0–6.0), where the separation of these minerals most effective. H2PO4− reacts with free Ca2+ to depress the acidolysis of apatite and promote the formation of aqueous CaHPO4. H3PO4 chemisorbs onto apatite surface by the formation of aqueous CaHPO4 and Ca(H2PO4)2, with the former being the major species to depress apatite. Moreover, dissolution of Mg2+ and Ca2+ from the dolomite surface is thermodynamically more favorable than that of Ca2+ from the apatite surface. As Mg(H2PO4)2 is soluble, H3PO4 only exhibits a weak depressive effect on dolomite. Further, the collector (sodium oleate) maintains excellent dolomite floatability in the presence of H3PO4, because chemisorption of the collector on the dolomite surface is enhanced by reaction with abundant Mg2+. Therefore, H3PO4 selectively depresses apatite in the apatite/dolomite flotation system.