Effect of sodium pyrophosphate on the flotation separation of calcite from apatite

[Display omitted] •Sodium pyrophosphate depresses apatite at pH 6.5–11.0.•Sodium pyrophosphate depresses calcite at pH 9.3–11.0 but not at 6.5–9.3.•Sodium pyrophosphate adsorbs on the surface of calcite but not apatite.•The depression of pyrophosphate is related to the selective dissolution of Ca.•Whether pyrophosphate promotes or inhibits Ca dissolution depends on the dosage. In this study, the separation of calcite and apatite was achieved by the depressant sodium pyrophosphate (SPP). Microflotation results showed that when using 20 mg/l sodium oleate (NaOL) as the collector, apatite could be significantly depressed by 120 mg/l SPP, and calcite remained floatable at pH 6.5–9.3. Relatively more Ca2+ was selectively dissolved from the surface of apatite by pyrophosphate, and the dissolution of calcium on the surface of calcite was inhibited. For calcite, the dissolution of Ca2+ decreased first but then increased with increasing SPP dose, and the adsorption amount of SPP illustrated an opposite tendency. With respect to apatite, the dissolution of Ca2+ was similar to that of calcite, but pyrophosphate failed to adsorb onto apatite. The solution chemistry of pyrophosphate at different pH values and in the presence of Ca2+ was discussed. The XPS results revealed that both SPP and NaOL could be adsorbed on the surface of calcite, while NaOL was rarely adsorbed on the surface of apatite. Moreover, the Ca/P ratio of apatite decreased after adding SPP. Zeta potential measurements showed that NaOL could still be adsorbed on the negatively charged calcite surface by chemical adsorption with surface calcium. Regarding apatite, the adsorption of NaOL was hindered by the decrease of active calcium sites and negative surface charge. The differences in polyphosphate concentration and pH should be responsible for the divergent existing theories of the interaction of polyphosphate and calcium minerals.