Thermal stabilization of chromium slag by sewage sludge: effects of sludge quantity and temperature

To investigate the feasibility of detoxifying chromium slag by sewage sludge, synthetic chromium slag containing 3% of Cr(VI) was mixed with sewage sludge followed by thermal treatment in nitrogen gas for stabilizing chromium. The effects of slag to sludge ratio (0.5, 1 and 2) and temperature (200, 300, 500, 700 and 900 degrees C) on treatment efficiency were investigated. During the mixing process before thermal treatment, 59.8%-99.7% of Cr(VI) was reduced, but Cr could be easily leached from the reduction product. Increasing heating temperature and decreasing slag to sludge ratio strengthened the reduction and stabilization of Cr(VI). When the slag to sludge ratio was 0.5 and thermal treatment temperature was 300 degrees C, the total leached Cr and Cr(VI) declined to 0.55 mg/L and 0.17 mg/L respectively, and 45.5% of Cr in the thermally treated residue existed as residual fraction. A two-stage mechanism was proposed for the reduction and stabilization of Cr.