Selective Chromium Recovery from Electroplating Sludge During Alkaline Roasting and Water Leaching

Electroplating sludge is a hazardous industrial waste and a potential secondary resource containing valuable metals such as Cr, Ni, and Cu. Selective Cr removal is important for reducing the chromium impurity load while retaining Ni and Cu for subsequent recovery. In this study, selective chromium recovery from electroplating sludge was investigated through alkaline roasting followed by water leaching. Thermodynamic analysis indicated that Cr(OH) 3 to Na 2 CrO 4 is feasible at the solid–gas–alkali interface. The effects of roasting time, NaOH/sludge mass ratio, and roasting temperature on chromium conversion were evaluated. Chromium leaching efficiency increased with roasting time, while further extension after 4 hours provided limited improvement. Increasing the NaOH/sludge mass ratio to 1.0 promoted chromate formation, whereas further alkali addition enhanced competitive reduction and NiCr 2 O 4 formation. At 300 °C to 400 °C, chromium was preferentially converted into water-soluble Na 2 CrO 4 , whereas temperatures above 500 °C promoted insoluble Cr containing phases. The highest chromium leaching efficiency of 96.8 pct was obtained at 400 °C for 4 hours with a NaOH/sludge mass ratio of 1.0. This process selectively removed Cr while retaining more than 99 pct of Ni and Cu in the solid residue, thereby producing a Ni and Cu containing intermediate feedstock for downstream recovery.