Treating waste with waste: Metals recovery from electroplating sludge using spent cathode carbon combustion dust and copper refining slag

Electroplating sludge is a hazardous waste and secondary metal resource because of its heavy metal content, which poses a huge threat to environmental safety if not properly disposed. An innovative process of oxidizing roasting followed by water leaching and smelting reduction to recover Cr, Cu, and Ni from electroplating sludge was proposed in this research, in which other two hazardous wastes of spent cathode carbon combustion dust and copper refining slag were co-treated. The NaF from spent cathode carbon combustion dust could convert Cr2O3 to Na2CrO4 using the oxidizing roasting process, resulting in a Cr recovery through the subsequent water leaching. The Na2CrO4 formation was promoted by CaO owing to it transferring the Cr spinel phase of FeCr2O4 [1+] to CaCrO4 and then to Na2CrO4. Under optimal conditions, the Cr recovery reached 97.1 %, and most 'F' was solidified into CaF2. In the next smelting reduction of the leaching residue, the Cu and Ni were recovered mainly in the form of Cu-Ni alloy. The addition of copper refining slag promoted their recovery, due to it modifying the molten slag and alloy structures and increasing the Cu-Ni alloy separation from molten slag. Some generated high-melting-point Cu-Ni-Fe and Ni-Fe alloys were converted to a Cu-Ni alloy with a low melting point in presence of Co from the copper refining slag, simultaneously with which the Fe was transferred out from Cu-Ni-Fe and Ni-Fe alloys and combined with Co to form a Fe-Co alloy. It increased Cu-Ni alloy droplets aggregation from molten slag and decreased their contents in the residual slag. Under optimized conditions, the Cu and Ni contents in the residual slag decreased to 0.37 and 0.06 wt%, respectively. Besides, the residual slag mainly composed of CaO, CaF2 and SiO2 could be used to prepare building materials rendering it harmless.