Employing sulfur–phosphorus mixed acid as a depressant: a novel investigation in flotation of collophanite

Inorganic acids (sulfuric acid (H2SO4) or phosphoric acid (H3PO4)) are generally employed as depressants in the reverse flotation of collophanite to separate apatite from dolomite. In this study, the flotation performance of a collophanite ore was studied, employing H2SO4, H3PO4, and a sulfur–phosphorus mixed acid as the depressants. The flotation tests revealed that the sulfur–phosphorus mixed acid could depress the apatite effectively. A high-quality concentrate with P2O5 content of 28.4 wt% and MgO content of 0.95 wt% was obtained, the corresponding P2O5 recovery of which was 85.0%. Furthermore, the separation mechanism of apatite and dolomite was clarified by zeta potential measurements, solution chemistry analysis and X-ray photoelectron spectroscopy (XPS). Within the pH range of 4.0–7.0, the zeta potentials of apatite were negative while those of dolomite were positive in the presence of all the three acid-depressants. The addition of sodium oleate made the potentials of both minerals to a more negative position. However, the potential shifts of dolomite were much more significantly negative than apatite when H2SO4 or H3PO4 was used. It indicated a greater absorption of sodium oleate on the surface of dolomite than apatite, which contributed to a more widen hydrophobicity gap between the surfaces of apatite and dolomite. SO42− and H2PO4−, not H+ or Cl−, were founded to be the pivotal ions for depressing. XPS analyses illustrated that the simultaneous additions of H2SO4 and H3PO4 as depressants were beneficial to the formation of CaHPO4 and CaSO4 on the surface of the apatite, which were harmful for the adsorption of sodium oleate, making the depressing effect more remarkable and result in a better separation of dolomite from apatite. It performed much more efficiently in collophane ore flotation when H3PO4 and H2SO4 were used corporately as a combined depressant than when they were used separately.