Selective separation mechanism of a metal inhibitor complex for scheelite and calcite: The structural stability of Fe3+-pyrogallol complex

The flotation separation of scheelite and calcite remains an industrial challenge due to their similar surface properties and active sites, making the development of efficient novel depressants crucial for achieving separation. This study investigates the effect of Fe 3+ -pyrogallol (Fe-PA) complex depressants on the flotation separation of scheelite and calcite, exploring the mechanism by which complex structural stability governs the selective depression of minerals. Flotation results demonstrate that, compared to PA alone, Fe-PA complexes exhibit significant depression of calcite flotation while minimally affecting scheelite flotation; with increasing molar ratio of Fe 3+ to PA in the complexes, the grade and recovery of scheelite initially increase then decrease, reaching optimum values at a Fe 3+ :PA molar ratio of 1:2, thereby enabling effective separation of scheelite from calcite. UV–Vis absorption spectroscopy, automatic potentiometric titration, and DFT calculations collectively reveal that the molar ratio of Fe 3+ to PA critically influences complex stability: at the 1:2 ratio, complex absorbance plateaus, stepwise stability constants maximize, the O-Fe bond order peaks, the HOMO-LUMO energy gap widens maximally, and overall Fe-PA complex stability is highest. This enhanced stability directly impacts flotation separation, evidenced by negative adsorption energies of Fe-PA complexes on calcite versus positive values on scheelite; specifically, the 1:2 complex achieves the most negative adsorption energy on calcite, primarily due to its formation of a hydrophilic surface via an octahedral coordination configuration where two O atoms from surface CO 3 2– groups bond with O-Fe in the complex, thereby depressing calcite.