Efficient extraction of refractory Cr from stainless steel dust by sodium persulfate oxidation roasting

Stainless steel dust (SSD) contains a high content of Cr and is listed as hazardous solid waste. However, SSD is mainly stockpiled in legacy waste sites due to its complexity and lack of investigation. Herein we comprehensively investigates the microscopic interaction between heavy metals and coexisting phases of SSD from the electric arc furnace (EAF). Furthermore, a new approach for extracting Cr from SSD was developed for the first time. It was found that SSD is enriched with Cr, Ni, Mn, and Zn. Most of these heavy metals are bound to stable spinel (e.g., FeCr₂O₄) and encapsulated by silicate. Using Na₂S₂O₈ as the flux (1-24 mmol), under roasting at 350-750 °C in the air for 0.25-6 h, efficient heavy metal extraction can be achieved (Cr-80 %, Ni-86 %, Mn-93 %, Zn-94 %) at 650 °C for 1h. During the roasting processes, the silicate shell was broken, which facilitated the mass transfer of the oxidant. Fe(II) was oxidized to Fe(III) and collapsed the spinel structure, releasing the heavy metals during the following acid leaching. The total mass of SSD is reduced by 90-95 %, and detoxification of SSD is achieved after processing. In conclusion, efficient extraction refractory metals in SSD by Na₂S₂O₈ roasting and the mechanism was elucidated for the first time. This method might also be applied for the treatment of other slags/ash waste that have a stable spinel structure and are encapsulated by silicate.