Selective alteration mechanisms of sodium tripolyphosphate towards serpentine: Implications for flotation of pyrite from serpentine

Abstract

Since the preferential dissolution of hydroxyl ion, serpentine is positively charged under weak alkaline conditions, and easily covers on sulfide minerals through electrostatic attraction in flotation process. To address this problem, sodium tripolyphosphate (STPP) was adopted as a novel depressant for serpentine in this work. Micro-flotation results demonstrated that compared with SHMP and CA, STPP possessed a superior effect for removing the adverse impact of serpentine on pyrite flotation with a maximum increase about 75% in pyrite recovery at pH 9. Results of ICP-OES tests showed that STPP could selectively adsorb on serpentine and facilitate the dissolution of magnesium cations from bulk serpentine at pH 9, while had a little influence on pyrite. The synergetic actions overcompensated the positive surface charge on serpentine and its zeta potential was rendered negative. Hence, the electrostatic repulsion between the negatively charged pyrite and serpentine destroyed their hetero-coagulation, which was verified by the DLVO calculations. DFT calculations and XPS analyses indicated that the oxygen anions in STPP could chelate with the magnesium cations exposed on the magnesium surface of serpentine (001) plane with the generation of P-O-Mg bonds, which led to the adsorption of STPP on serpentine. Besides, XPS results also confirmed the stepped-up dissolution of magnesium cations from serpentine with a decrease in the Mg 1s signal of serpentine after adding STPP. In general, STPP may be applied as an efficient dispersant in the flotation separation of sulfide ores from serpentine.