Dual-Ligand Hydroxamic Acid Collector: Performance and Mechanism in the Depressant-Free Flotation of Fluorite from Calcite

Because of the similar surface properties of fluorite and calcite, their separation remains quite challenging. In this study, a novel hydroxamic collector, phthaloyl hydroxamic acid (PHA), was synthesized and applied for the flotation separation of fluorite from calcite for the first time. Microflotation results demonstrated that, at pH 7 with 20 mg/L PHA, fluorite recovery reached 92.5%, while calcite recovery was only 15.5%. In contrast, sodium oleate (NaOL) exhibited poor selectivity under identical conditions, recovering both minerals at similar levels (82% for fluorite and 86% for calcite). Compared with benzohydroxamic acid (BHA), which required an elevated pH of 9 to achieve selective separation (91% fluorite vs 20% calcite), PHA delivered comparable or superior selectivity at a milder pH 7. Contact angle, FTIR, ζ-potential, and XPS analyses collectively confirmed the selective chemisorption of PHA onto fluorite surfaces via chelation between -CONHOH groups and surface Ca atoms. Adsorption experiments showed that the adsorption density of PHA on fluorite (0.045 mg/g) was approximately six times higher than that on calcite (0.007 mg/g). Density functional theory (DFT) calculations further revealed a substantially stronger adsorption energy on fluorite (-239.87 kJ/mol) than on calcite (-62.12 kJ/mol), with the optimized geometry confirming five-membered chelate ring formation. Collectively, PHA is a highly promising collector for the efficient and depressant-free flotation separation of fluorite from calcite.