Mechanically activated and microbubble-assisted alkaline leaching of chromium from chromite

Chromite exhibits sluggish oxidation leaching kinetics due to its dense spinel structure and strong Cr-O bonding. Here, a mechanistically informed strategy integrating mechanical activation, NaOH roasting, and O 2 microbubble enhanced reactive oxygen species (ROS) generation is developed to promote chromium extraction. Mechanical activation induces lattice distortion and grain refinement, while alkali roasting at 700°C with 75 wt% NaOH generates abundant Fe vacancies that facilitate cation migration and accelerate the oxidation of Cr 3+ to soluble Cr 6+ . During alkaline leaching, O 2 microbubbles markedly enhance in situ ROS production, with ·OH identified as the dominant oxidative species. Under the selected operating conditions, temperature 70°C, liquid-to-solid ratio of 10:1, the chromium leaching efficiency increases from 50.83% to 95.22%. Kinetic analysis reveals a transition from diffusion-controlled to mixed-controlled behavior, with the Avrami exponent increasing from 0.29 to 0.35, indicating enhanced interfacial reaction and mass transfer. Spectroscopic characterization combined with density functional theory (DFT) calculations shows that ·OH preferentially adsorbs on Cr sites with an adsorption energy of −5.12 eV, driving cooperative oxidation via electron density redistribution and progressive weakening of Cr-O bonds. This work provides quantitative mechanistic insights into ROS-mediated oxidation and offers an effective route for chromium extraction from refractory chromite.