W and Mo separation from waste W-Mo alloy by an integrated ultrasound acid leaching − alkali leaching – Solvent extraction process

Tungsten-molybdenum (W-Mo) alloy are critical materials and widely used in aerospace and defense industries. During the manufacture, mechanical processing, and scrapping of W-Mo alloy products, a substantial quantity of waste W-Mo alloy is generated, which holds considerable importance for sustainable critical metals recycling and environmental protection. Hydrometallurgy treatment is demonstrated to be effective to address the multi-component metallic solid waste. However, the alloyed structure and strong chemical inertness of waste W-Mo alloys give rise to low recovery efficiency in conventional leaching processes. This study proposes an integrated ultrasound acid leaching - alkali leaching - solvent extraction process for the W and Mo separation from waste W-Mo alloy. First, the ionic form differences of W and Mo in acidic and alkaline media were investigated to lay the leaching and separation foundations. Thermodynamic calculation indicates that HNO₃ can realize the dissolution of W and Mo, while W will hydrolyze to form oxide precipitates, realizing the pre-separation of W and Mo. However, results demonstrate that the dense alloy structure and solid product layer give rise to the poor extraction efficiency in conventional process. The ultrasound-assisted leaching can shorten the leaching time from conventional 4 h to 3 h. It is found that about 97.44 % W and 50.07 % Mo are retained in the residue throughout the acid leaching stage, and a relatively pure Mo leaching solution with 17.51 g/L and 0.32 g/L W was obtained. After that, the acid leaching residue was conducted to alkali leaching to transfer all the W and Mo into the solution as the form of ionic states. Eventually, four-stage N1923 solvent extraction delivers thorough W-Mo separation, and the W and Mo concentrations in raffinate are 0.02 g/L and 20.39 g/L, respectively. Electrochemical analysis and material characterization techniques were employed to highlight the ultrasound-assisted intensification mechanisms. Ultrasound destroys the passivation layer by generating cavitation effects, improving the solid-liquid contact interface and optimizing reaction kinetics. The stepwise separation mechanisms and element migration of W and Mo from the scrap W-Mo alloy during the acid leaching, alkali leaching, and solvent extraction were also discussed. This study proposes an integrated strategy for the efficient separation and recovery of W and Mo from scrap W-Mo alloy.