Study On Migration And Transformation Of Chromium And Other Heavy Metals During Sewage Sludge Incineration

With the wastewater discharge amount increasing and the development of wastewater treatment, the quantity of sludge is growing quickly in China. Sludge incineration has broad application prospects due to the advantages of making sludge decrement rate increase, killing organic micro-organisms and energy-saving power generation. The emission of heavy metals which cause serious harm to humans and the environment restricted the development and popularization of sludge incineration technology in China. In this paper, experimental method analysis is used to predict speciation migration and transformation of heavy metal during sludge incineration.This paper simulated a sewage sludge system based on an actual measured data. Setting up the sludge incineration experiment system that including distribution system, tube furnace burning system and flue gas absorption system. Measure the content and status of heavy metal in ash that collecting after sludge incinerating. Measurement methods include inductively coupled plasma mass spectrometry (ICP-MS), ultraviolet-visible spectrophotometry, X-ray diffraction (XRD) and energy spectrometer (EDS).This paper studied on incineration conditions which contain burning temperature, residence time and combustion atmosphere influence on transformation and migration of chromium and other heavy metals. Burning temperature is set to500℃,600℃,700℃,800℃and900℃. With the rising of temperature, chromium evaporation rate increase, and the quality percentage of Cr(Ⅵ) in ash is from6.87%to3.60%. Chromium compounds in low temperature are Ca(CrO4), Cr2O3, MgCr2O4, by contrast the compounds in high temperature are Cr2O3, MgCr2O4make the compounds with Cr(Ⅵ) reduce. With the temperature rise, the volatilization rate of Cd, Pb and Zn increases, and that of the Ni change a little. The retention time range set at10min to100min, time interval is for10min. The volatilization rate Chromium increases over time, meanwhile the quality percentage of Cr(VI) in ash fell slightly after increase and then being stable, the residue rate of Cd, Pb and Zn is reduced, Ni slightly increased., the volatilization rate of chromium increase when Oxygen content in gas increase, because oxygen promote Cr(III) transfer to Cr(VI). Excessive oxygen give rise to Cr(VI) increase, emitting into the air with fly ash, result in the ash content is lower.This paper also studied on the influence of solid additives on the transformation and migration chromium and other heavy metals. This experiment selects CaCl2, CaO, SiO2, A12O3, NH4Cl,(NH4)2SO4and (NH4)3PO4as solid additives. Experimental data show that CaCl2and CaO are beneficial to turn Cr2O3to Ca (CrO4), chromium can be fixed in the ash by SiO2. NH4C1,(NH4)2SO4and (NH4)3PO4promote the volatilization of chromium so that the content of Cr(VI) in the ash become lower. When sludge incinerate with solid additives, CaCl2and CaO make the evaporation rate of As, Cd, Ni, Pb and Zn decrease.