Immobilization Of Heavy Metals In MSWI Fly Ash By Magnesium Oxychloride Cement

With the rapid development of society,the rapid urbanization has led to an increase in the generation of urban domestic waste in my country.There are a large number of toxic and harmful heavy metal substances in the incineration fly ash generated after garbage incineration must be cured/stabilized.Magnesium oxychloride cement is a Mg O-Mg Cl₂-H₂O ternary system mineral cement,which has the advantages of high impermeability,high strength and low alkalinity.This paper uses magnesium oxychloride cement to solidify the heavy metals Cu,Zn,Cd,Cr and Pb in the fly ash.Experiments show that magnesium oxychloride cement has a good curing effect on heavy metals in fly ash from waste incineration.In this paper,the physical and chemical properties of waste incineration fly ash and the leaching toxicity of heavy metals were studied.The study found that the waste incineration fly ash sample is mainly composed of four elements of O,Ca,Si and Al,and the main mineral phases are Ca CO₃,Si O₂and Al₂O₃,belonging to the ternary system of Ca O-Si O-Al O.Comparing HJ/T 300-2007 acetic acid leaching method,HVT557-2009 water leaching method and HJ/T 299-2007 sulfuric acid nitric acid leaching method,it is found that HJ/T 300-2007 acetic acid leaching method has the highest heavy metal leaching rate.And the heavy metals Pb and Cr in the three leaching methods all exceed the stipulated value of the pollution control standard of domestic waste landfill?GB 16889-2008?.From the BCR form distribution of heavy metals in waste incineration fly ash,it can be seen that the proportions of metal Cr and Pb residues are relatively high,62.5%and 54.7%,respectively,which are more stable under natural conditions.The residual state content of heavy metal Cd is only 3.8%,which is easy to migrate to the natural environment.In this paper,fly ash curing experiments under different magnesium oxychloride cement mixing ratios and different Mg O/Mg Cl₂molar ratios were designed.The experiment found that the cured body had good compressive strength.When the molar ratio of Mg O/Mg Cl₂is 6:1 and the mixing amount is 45%,the compressive strength of cured body curing 28d can reach 36.1Mpa;magnesium oxychloride cement has a significant curing/stabilizing effect on heavy metals in waste incineration fly ash The leaching concentrations of Cu,Zn,Cd,Cr and Pb in all groups under different conditions are far lower than those specified in the pollution control standard for domestic waste landfill?GB 16889-2008?.The XRD and FTIR were used to study the hydration products of magnesium oxychloride cement solidification/stabilization waste incineration fly ash,and the results showed that the mechanical strength of the solidified body was mainly provided by the needle rod-like phases 518?5Mg?OH?₂·Mg Cl₂·8H₂O?and 318?3Mg?OH?₂·Mg Cl₂·8H₂O?.Under the condition that the molar ratio of Mg O/Mg Cl₂is 6:1,the increase of the doping amount helps the conversion of 318 phase to 518 phase.When the dosing amount is 30%and the Mg O/Mg Cl₂molar ratio is less than 6:1,the 518 phase in the solidified body gradually transforms to the 318 phase as the molar ratio decreases.When the molar ratio of Mg O/Mg Cl₂is greater than 6:1,excess Mg O is hydrated to form Mg?OH?₂,which reduces the compressive strength of the cured body.SEM showed that the Mg?OH?2 in the shape of strips and blocks closely adhered to the surface of the fly ash particles and the cross-wrapping of the fly ash particles by the 518 phase and 318 phase whiskers helped to suppress the migration of heavy metals.According to the analysis of BCR heavy metal morphology,it is found that after fly ash solidified by magnesium oxychloride cement,the stable state?residue state and oxidizable state?ratio of heavy metals Cu,Zn,Cr,Cd,Pb are significantly increased,in the Mg O/Mg Cl₂mole When the ratio is 6:1,the steady state ratio increases with the increase of the cement content;under the condition of 30%,the steady state ratio shows a positive correlation with the increase of the Mg O/Mg Cl₂molar ratio;chlorine oxygen Magnesium cement has relatively poor curing/stabilizing effect on heavy metal Cd.The ratio of acid-extracted Cd is 8.3%when the dosage is 45%,and the increase of the dosage has little effect on the change of the acid-extracted Cd ratio,but increasing Mg O/The molar ratio of Mg Cl₂is favorable for the conversion of acid-extracted Cd to stable state.