Multifunctional Green Depressant for Selective Calcite Depression in Cassiterite Flotation: Interfacial Adsorption and Hydration Mechanism

Abstract

Cassiterite (SnO2), the primary mineral resource for tin production, is commonly associated with calcium-bearing gangue minerals, such as calcite, and their similar flotation responses remain a major challenge for selective separation. In this study, chondroitin sulfate A sodium salt (CSA) was evaluated as a green selective depressant for cassiterite/calcite flotation separation, and its interfacial depression mechanism was systematically investigated through surface characterization and molecular simulations. The flotation tests were conducted under the optimal conditions of pH 9, 30 mg/L CSA, and 80 mg/L NaOL. Artificial mixed-mineral flotation tests produced a tin concentrate with a Sn grade of 64.51% and a recovery of 85.30%, demonstrating the effective separation performance of CSA. AFM observations revealed that CSA exhibited only weak adsorption on the cassiterite surface, whereas a much denser adsorption layer was formed on calcite. FTIR, XPS, and ToF-SIMS analyses further confirmed that CSA preferentially interacted with the Ca-active sites on calcite through its oxygen-containing functional groups, leading to enhanced surface hydrophilicity and reduced floatability. Molecular dynamics simulations showed that CSA strengthened the interfacial hydration structure of calcite, as evidenced by the increased ordering and distribution of interfacial water molecules. DFT calculations further verified the formation of stable Ca–O coordination interactions between CSA and the calcite surface. These results reveal the selective adsorption and interfacial regulation mechanism of CSA at the mineral–water interface, providing both mechanistic insight and a practical strategy for the efficient flotation separation of cassiterite from calcite.