Selective separation of cassiterite from fluorite using a green depressant: Experimental and DFT insights

To address the technical challenge of flotation separation between cassiterite and fluorite, this study introduces a biodegradable green chelating agent, tetrasodium glutamate diacetate (AYGLDA), as a novel depressant. The selective depression mechanism of AYGLDA is systematically elucidated through multi-scale research methods. Flotation experiments conducted on single minerals and artificial mixed ores demonstrate that, under optimal conditions of pH 8.0, an AYGLDA concentration of 60–90 mg/L, and a sodium oleate (NaOL) concentration of 80 mg/L, efficient separation of cassiterite and fluorite can be achieved. Under these conditions, cassiterite recovery remains high, while fluorite is significantly depressed. Advanced characterization techniques reveal that AYGLDA exhibits markedly different adsorption behaviors on the surfaces of the two minerals. Specifically, the adsorption capacity on fluorite is greater, with the Ca 2p binding energy on its surface positively shifted by 0.8 eV due to strong chemical chelation. This interaction results in the formation of a dense and rough hydrophilic adsorption layer Ra = 6.60 nm) on the fluorite surface, reducing the contact angle from 78° to 33° and enhancing surface electronegativity, thereby significantly depressing fluorite flotation by hindering NaOL adsorption. On the surface of cassiterite, the adsorption of AYGLDA is weak, resulting in the formation of a thin and flat adsorption layer that minimally impacts the electronic structure of Sn 3d. Consequently, this weak adsorption has a limited effect on the floatability of cassiterite. Analyses of solution chemistry and density functional theory (DFT) calculations further demonstrate that AYGLDA exhibits a significantly stronger complexation ability for Ca 2 + compared to Sn 4+ , leading to its preferential and stable adsorption on the surface of fluorite. This study presents an efficient and environmentally friendly depressant for the flotation separation of cassiterite and fluorite, while elucidating the selective depression mechanism based on atomic-scale differences in adsorption.