From waste to wealth: Valuable metals recovered from waste tungsten leaching residue by a smelting reduction process using CaO-Fe capture agent

Tungsten leaching residue (W-residue), a hazardous waste generated from the industrial recovery of the spent tungsten carbides, poses a threat to the environment. The W-residue contains high grades of SiO2 and TiO2 oxides, and low grades of high-value metal oxides such as WO3, CoO, Ta2O5, and Nb2O5. Herein, a smelting reduction process was proposed to effectively recover the high-value metals from the W-residue and obtained a medium-entropy carbide (MEC) composite. During the smelting process, the valuable metals were recovered as alloy consisting of (Fe, Co)-rich silicide, (Fe, Cr, W)-rich silicide, and (W, Ta, Nb, Ti)C, respectively. The addition of flux CaO lowered the liquidus temperature of the molten slag, and effective alloy-slag phase separation was achieved by adding Fe as a metal collector to capture the alloy droplets entrapped in the slag. Under the optimal conditions, the total recovery rate reached 86.8%. Moreover, the obtained (W, Ta, Nb, Ti)C as MEC exhibited a Vickers hardness of over 19 GPa and can be used as a wear-resistant alloy. This study proposed a strategy for extracting high-value metals from hazardous tungsten residue, which could complement existing recycling processes towards a closed-loop process in the tungsten industry.