Process and mechanism of strengthening chlorination cascade recovery of valuable metals from electroplating sludge

Abstract

Electroplating sludge (ES) is a kind of man-made ore-rich hazardous waste with dual characteristics of resource and hazard. In this paper, through thermodynamic calculation and comparison of the chlorine supply capacity of different chlorinating agents, NH4Cl and CaCl2 are selected as the best synergistic chlorine donors, and FeSO4•7H2O is introduced as the green strengthening agent. The results show that the decomposition products SO2 and O2 of FeSO4•7H2O at approximately 700 °C can promote the decomposition of CaCl2 to Cl2 and transform sulfur into CaSO4. The enhanced chlorination system can prolong chlorination, and realize the efficient stepwise chlorination volatilization of Ni, Cu and Cr in ES by adjusting the temperature field. The mass ratios are controlled of M(ES): M(NH4Cl): M(CaCl2) = 10:12:3, M(CaCl2): M(FeSO4•7H2O) = 1:2, and roast at 1100 °C for 90 min, Cr, Ni and Cu recovery rates reach 31.23%, 93.86% and 98.75%, respectively. The content of metals such as nickel and copper in the roasted product is significantly reduced, and the leaching toxicity reaches the standard, which can be used as a secondary resource. The stepwise separation mechanism of Cr, Ni and Cu is further explored through XRD, thermodynamic analysis, XPS, etc. The Me–O and Me–S (Me = Ni, Cu, Cr, etc.) bonds in NiO, CuO and Cr2S3 are easily destroyed by active-[Cl] in HCl and Cl2 to combine to form CrCl3, NiCl2 and CuCl3, which are gradually volatilized in steps at 500 °C, 800 °C and 900 °C, respectively. This study proposed a new chlorination system and explored the stepwise chlorination volatilization process of multi-metal, which provided a new method for metal recovery from ES and polymetallic solid waste.