Selective recovery of Cr from stainless steel slag by alkaline roasting followed by water leaching

• The mineralogical Cr speciation in a stainless steel slag was analyzed after selective dissolution of matrix material.
• At the optimum condition of alkali roasting most of the Cr present in spinel phases in the SS slag is extracted.
• Cr is locked in the alkali roasted and water leached residue in FeCr alloy particles by an Fe-oxide passivation layer.
• Mechanical activation treatment enhanced the leaching efficiency and decreased alkali consumption by 40%.
• The mineralogy of the Cr-leached residue is suitable for further carbonation treatment to produce construction materials.

Selective chromium (Cr) leaching from stainless steel slag (SS slag) by alkali roasting followed by water leaching was investigated. The efficiency of the alkali roasting process for Cr leaching was increased by optimizing the mass ratio of alkaline agents (NaOH, and NaOH–NaNO3) to the slag, roasting temperature and time. At the optimum condition (0.67 mass ratio of NaOH to SS slag, 400 °C, 2 h) of NaOH roasting, chromium leaching was around 83%, while the matrix material was dissolved only to a limited extent (Si 8.0%). Mechanical activation of the SS slag prior to roasting reduced the optimum NaOH to SS slag mass ratio to 0.4. The addition of NaNO3 as an oxidant to the NaOH salt increased Cr leaching to 89% after roasting at 400 °C for 2 h. The remaining Cr phases in the residue were almost exclusively FeCr alloys. Further chromium dissolution from these alloys is prevented by a passivation layer of Fe oxides as shown by SEM/EDS images. Based on these results, a SS slag recycling process is suggested in which roasting–water leaching followed by water washing to remove Cr yields a residue which has potential for application as a construction material.

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