Efficient recovery of chromium from electroplating sludge by a two-step roasting process: Mechanisms and kinetics

Electroplating sludge (ES), a hazardous industrial waste, poses environmental challenges due to its complex composition, high chromium (Cr) content, environmental toxicity, and its efficient treatment has become a pressing necessity. To address this issue, a novel two-step roasting process—Na₂CO₃ roasting with water leaching for initial dechroming, followed by NaNO₃ roasting of the residue—was used to recover chromium from ES. Given the compositional complexity of ES, FactSage (Facility for the Analysis of Chemical Thermodynamics) was utilized to predict the thermodynamic behavior and phase transformation of the roasting process. Based on the prediction, roasting experiments were carried out at temperatures ranging from 500 to 1000 °C, and the phase transformation mechanisms during the roasting processes were verified by mineral liberation analysis (MLA) and X-ray diffraction (XRD) analysis. Results reveal that the total recovery efficiency of Cr reached 99 % under the optimal conditions of the two-step roasting process. Moreover, this study elucidated a systematic Cr-phase transition mechanism in ES, beginning with hydroxide formation, followed by transformation into oxides, spinel phases (FeCr₂O₄, MgCr₂O₄, ZnCr₂O₄, and CaCr₂O₄), and finally conversion into soluble Na₂CrO₄. Kinetic analysis further indicated that Cr(III) oxidation during Na₂CO₃ roasting followed a product-layer diffusion mechanism with an apparent activation energy of 44.5 kJ/mol, supported by a strong linear correlation to the shrinking core model (R² > 0.99) between 500–800 °C, while sintering-induced diffusion limitations emerged at higher temperatures. Overall, this work systematically clarified the Cr phase transformation mechanisms during the two-step roasting, offering theoretical insights and practical guidance for high-efficient and environmentally sustainable recovery of Cr from hazardous industrial wastes.