Effects of Metal Ions on the Flotation of Fluorite and Barite: An Experimental and Mechanistic Investigation

Fluorite (CaF2) and barite (BaSO4) commonly occur together in the same deposits. Due to their similar surface chemical properties, their flotation separation is often challenging. In flotation pulps, dissolved metal ions can further interfere with separation and exert a pronounced influence on the flotation behavior of these minerals. This study investigated the effects of metal ions frequently encountered in industrial pulps (Fe3+, Al3+, Mg2+, Ca2+, and Zn2+) on the floatability of fluorite and barite in a sodium oleate (NaOL) collector system. The aims were to clarify how metal ions affect flotation behavior and to evaluate the feasibility of enhancing fluorite–barite separation via metal-ion regulation. Flotation results showed that, in the NaOL system, the largest floatability difference between fluorite and barite occurred at pH 10. Al3+ exhibited the strongest depression on barite while only weakly affecting fluorite flotation. Fe3+ and Mg2+ caused slight depression of barite, whereas Ca2+ and high concentrations of Zn2+ (>20 mg/L) promoted barite flotation. Overall, these metal ions had little influence on fluorite flotation. Adsorption measurements indicated that Al3+ reduced NaOL adsorption by more than 40% and decreased the contact angle from 35.6° to 23.1°, resulting in a sharp loss of surface hydrophobicity. ICP adsorption tests revealed that Al3+ showed the highest uptake on barite surfaces. Density functional theory (DFT) calculations further confirmed that surface SO42− groups on barite form strong chemisorption with hydrolyzed Al species (adsorption energy: −436.19 kJ/mol), whereas only weak physisorption occurs on hydroxylated fluorite surfaces (adsorption energy: −43.73 kJ/mol). This study provides insights into the flotation separation of non-metallic minerals dominated by polar ionic bonding and offers practical guidance for efficient fluorite–barite separation under complex ionic environments.