Study On Redox Reaction And Leaching Risk Of Chromium During The Thermal Treatment Of Electroplating Sludge

Thermally treatment is a mian treatment strategy for the disposal of solid waste containing chromium such as electroplating sludge,due to the thermal treatment with advantages of reducing quality and volume.However,Cr（Ⅲ）will be oxidized into Cr（Ⅵ）during the process of heat treatment and high temperature co-processing,Cr（Ⅵ）has great toxicity and solubility,which posed a serious threat to the environment and biology,and limited the application and extension of thermally treatment technology of electroplating sludge.The objectives of this study were to explore the oxidation and reduction mechanism of Cr in electroplating sludge at high temperature,and evaluate the leaching safety of heavy metals during the co-processing of electroplating sludge in the cement kiln,in order to provide a theoretical basis for thermally treatment technology of electroplating sludge.The contents and conclusions of this study are as follows:（1）The Cr（Ⅵ）reduction behavior in electroplating sludge during heat treatment was investigated.It was found that Ca Cr O₄ reacted with Ca O and formed a new product Ca₅（Cr O₄）₃O_0.5 at temperature range of 800 and 1000℃.The valence state of Cr in Ca₅（Cr O₄）₃O_0.5 is determined to be+5 by XPS analysis,and the color for new formed Cr（V）is observed in green,similar to Cr（Ⅲ）compounds.The temperature and Ca O are two keys to arouse the reduction reaction of Cr（Ⅵ）into Cr（V）.In particular,the reduction of Cr（Ⅵ）into Cr（V）is strongly depended on temperature（800-1000℃）,this reaction can be balanced within 10 min,while prolonging sintering time has little help for promoting the reduction of Cr（Ⅵ）to Cr（V）.Additionally,it was found Cr（V）can keep stable and not be re-oxidized into Cr（Ⅵ）at 800-1000℃.（2）The oxidation and reduction behavior of spinels containing chromium in electroplating sludge during heat treatment was analyzed.Ni Cr₂O₄,Zn Cr₂O₄and Cu Cr₂O₄ were prepared.The reaction behavior of spinels containing chromium during heat treatment was analyzed.Meanwhile,the products were characterized by XRD,TEM and XPS.Ni Cr₂O₄,Zn Cr₂O₄ and Cu Cr₂O₄ reacted with Ca O at high temperature to form Ca Cr O₄ and corresponding oxides（Ni O,Zn O and Cu O）.The feasibility of the reaction in the experimental temperature range is confirmed by thermodynamic calculation.The oxidation rate of Cr（Ⅲ）in the three calcined products reaches the maximum value of about 90%at700℃,and the oxidation of Cr（Ⅲ）in Zn Cr₂O₄ can be completed at a lower temperature.The Cr（Ⅲ）oxidation rate of Cu Cr₂O₄ decreases to 13.7%at1300℃,which is much lower than that of Ni Cr₂O₄ and Zn Cr₂O₄.The decrease of Cr（Ⅲ）oxidation rate is mainly caused by the reduction of Ca Cr O₄ to Cr（Ⅲ）compound during the heating process.（3）The oxidation and reduction reaction behavior of chromium oxide solid solution in the heat treatment process of electroplating sludge was investigated.Six kinds of solid solution materials which are easy to form in the heat treatment process of electroplating sludge were prepared,which are(Al_0.5,Cr_1.5)O₃,（Al,Cr）₂O₃,(Al_1.5,Cr_0.5)O₃,(Fe_0.5,Cr_1.5)O₃,（Fe,Cr）₂O₃ and(Fe_1.5,Cr_0.5)O₃,respectively.The oxidation behavior of Cr（Ⅲ）was investigated.Cr（Ⅲ）in Al-Cr solid solution and Fe-Cr solid solution can be oxidized to Cr（Ⅵ）at high temperature.Temperature and Ca O are the key factors to initiate Cr（Ⅲ）oxidation.The Cr（Ⅲ）oxidation rates of the two kinds of chromium containing solid solutions increase and decrease in the temperature range of 300-800℃and 1000-1300℃,respectively.After 1000℃,part of the Cr（Ⅵ）formed will be reduced into Cr（Ⅲ）.The higher the content of Al and Fe in Al-Cr solid solution and Fe-Cr solid solution,the better the inhibition effect on Cr（Ⅲ）oxidation.（4）The leaching safety of chromium from cement clinker prepared with electroplating sludge was evaluated.Cement clinker was prepared by mixing electroplating sludge with different mass ratio.The phase of cement clinker was characterized by XRD,and the sample was leached by acetic acid buffer.The results show that the formation of C₂S and C₃S in cement may be inhibited by the addition of electroplating sludge,and the concentration of Cr（Ⅵ）in leaching solution is higher than the concentration limit of 5mg/L specified in the code（GB 5085.3-2007）,so there is a risk of environmental pollution.The technology of burning cement with chromium containing electroplating sludge needs to be further improved.In conclusion,during the heat treatment of electroplating sludge,Cr（Ⅲ）in chromium bearing spinel and solid solution will also be oxidized to Cr（Ⅵ）,and Cr（Ⅵ）will be reduced to Cr（V）in alkaline condition.Measures should be taken to inhibit the oxidation of Cr（Ⅲ）during the process of thermally treatment and co-processing of electroplating sludge in the cement kiln,so as to prevent the leaching of Cr（Ⅵ）in the disposal products from exceeding the standard.