Study On Migration And Distribution Of Heavy Metals During Co-combustion Of Pickling Sludge And Coal

Currently, the technology of co-combustion of hazardous waste is increasingly developing due to its advantage of reduction, reuse and recycle. In the process of co-combusting hazardous waste that contains high-level heavy metals, the heavy metals transported into flue gas, fly ash or bottom ash will pose a threat to human health and environment. In this paper, the migration and distribution of 10 kinds of heavy metals was investigated during the co-combustion of pickling sludge and coal by experimental and Thermodynamic approaches. In addition, the energy spectrum, X-Ray diffraction and leaching characteristics analysis of bottom ash in molten state were carried out.Firstly, the effect of combustion temperature on the partitioning behavior of heavy metals was studied through experimental approach. The results showed that Cd, Se and Zn were volatile heavy metals which predominated in gas phase. As, Cu, Pb and Sb were semi-volatile heavy metals. When the temperature increased, the partitioning of these metals in flue gas, fly ash and bottom ash was different form each other. Cr, Mn and Ni were non-volatile heavy metals, the majority of which were retained in bottom ash, and few were transited into flue gas and fly ash.Next, the thermodynamic equilibrium simulation of the effect of temperature on the heavy metal migration and distribution was investigated by using FactSage softerware. The result showed that As existed in the form of Ca3(AsO4)2 and AS4O6, while Cd, Sb and Se existed completely in gaseous form. CuO combined with Fe2O3 to form CuO·Fe2O3(s), which could be decomposed into CuCl(g), (CuCl)3(g) and Cu(g) with the temperature increasing. Pb existed mainly in the form of Pb2Fe2Si2O9(s), PbO(g) and PbCl2(g). Zn was enriched in the bottom ash in the form of ZnAl2O4 when the temperature varied from 600 to 1000 ℃. As the temperature reached 1200 ℃, ZnAl2O4 would be completely decomposed into Zn(g). In the bottom ash, Cr and Ni existed as the form of FeCr2O4 and FeNi2O4, while Mn reacted with Al2O3 and SiO2 thus forming aluminosilicate.Then, the effect of sludge ratio on the partitioning behavior of heavy metals was discussed. The results indicated that the content of As, Cd, Cu, Pb, Se, Sb and Zn in flue gas was negatively correlated with sludge ratio, while that in bottom ash gradually increased. But the partitioning of these metals in fly ash seemed slight difference. The content of Cr, Mn and Ni in flue gas and fly ash was little, and its partitioning in bottom ash was positively correlated with the sludge ratio.At last, the energy spectrum, X-Ray diffraction and leaching characteristics analysis of bottom ash in molten state were performed. It turned out that the main species of elements in the energy spectrum stayed generally unchanged in different experiment conditions. But the content of oxygen increased and the content of Cr and Ni, on the contrary, descended. Through analyzing the XRD pattern, it was concluded that Cr and Ni mainly existed in bottom ash in the form of NiO、NiFe2O4、NiCr2O4、 Cr2O3 at the temperature of 1300 ℃. However, only SiO2 and Fe2O3 were detected at the temperature of 1400 ℃ and 1500 ℃. In general, the concentration of 10 heavy metals in leachate generally descended with the increasement of sludge ratio.