Oxidizing MoO2: ultrasound-assisted two-step recovery of molybdenum from industrial ammonia leaching residue.

The production of ammonium molybdate generates large amounts of industrial ammonia leaching residue (IALR), from which conventional ammonia leaching gives a low molybdenum recovery. Quantitative phase analysis identified MoO2 in IALR, which must be oxidized before it can dissolve in ammonia. Depth-resolved XPS detected only Mo(VI) before etching, while the Mo(IV) signal increased with etching depth. In the one-step process, H2O2 is rapidly consumed in an alkaline ammonia solution. This study therefore proposes an ultrasound-assisted two-step process, in which oxidation and leaching are separated in time and the oxidation is carried out in a neutral medium, keeping the ORP above 480 mV throughout. COMSOL modeling predicted a 4.6-fold increase in average particle velocity under ultrasound, while particle-size analysis showed a shift toward smaller sizes. In the ultrasound-treated residues, no Mo(IV) signal was detected at the examined depths up to 50 nm. Radical-scavenging experiments indicated that the ·OH contribution to Mo extraction was 4.43 percentage points, about 13 times the conventional value. The two-step process finally achieved Mo extraction of 37.75%, higher than the 34.43% of the conventional process and corresponding to 99.13% of the route-specific theoretical limit of 38.08% for Mo hosted in MoO3 and MoO2. Under mild conditions, this process returns molybdenum from a difficult residue to reuse, providing a viable route for the recycling of this critical metal resource.