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 Ca^2 + 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 Mg^2 + and Ca^2 + from the dolomite surface is thermodynamically more favorable than that of Ca^2 + 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 Mg^2 +. Therefore, H₃PO₄ selectively depresses apatite in the apatite/dolomite flotation system.