Mechanistic Insights into the Selective Depression of Calcium Bearing Gangue in Scheelite Flotation Using a Green and Biodegradable Depressant

Excellent physicochemical properties and high-tech production applications have led to tungsten (W) and its associated minerals being classified as critical raw materials. Currently, scheelite (CaWO 4 ) has become the main global source of tungsten following the depletion of wolframite ((Fe,Mn)WO 4 ) reserves. In flotation systems, the selective concentration of scheelite from associated gangue minerals, particularly Ca-bearing minerals, remains a major challenge owing to their similar surface characteristics and exposed active Ca 2+ sites. In this work, the mechanistic role of quebracho as a green and biodegradable reagent in the selective depression of calcite (CaCO 3 ) and fluorite (CaF 2 ) during scheelite flotation was investigated. Various surface characterization techniques including contact angle, zeta potential, and UV–Vis spectroscopy were applied to reveal the adsorption and interaction behavior of quebracho on mineral surfaces. Results demonstrated that quebracho preferentially adsorbs onto fluorite and calcite, leading to significant surface negativity and hydrophilicity, while showing minimal interaction with scheelite. This selective adsorption is mainly attributed to high availability and reactivity of surface Ca 2+ sites on gangue minerals. Continuous flotation experiments (rougher and scavenger) on a real low-grade scheelite sample, conducted in the presence of quebracho, revealed enhanced separation efficiencies (S.E.) across the examined pH range of 9–10, with the most favorable performance observed at pH 10, where scheelite recovery and S.E. exceeded 80 and 70%, respectively. From the findings, it can be confirmed that quebracho acts as a selective depressant through surface complexation, thereby offering an environmentally friendly reagent for successful scheelite flotation.