Experimental Study On Removal Of Chlorine By Water-washing And Dioxin Degradation Of Grate MSWI Fly Ash

Due to high leaching toxicity of heavy metals and the enrichment of various pollutants,such as dioxins and heavy metals,municipal solid waste incineration?MSWI?fly ash has been seriously listed in the National Hazardous List.With the strict requirements of harmless disposal of fly ash and the urgent need for resource utilization,the washing process of fly ash has attracted more and more attention.The washing pretreatment of fly ash can remove most of the chlorine salts and other soluble salts in the fly ash,which helps to reduce the corrosion of kiln wall in the process of cement kiln co-treatment,improve the solidification effect of heavy metals and degradation of dioxins.In this paper,studies mainly focus about the multi-stage countercurrent washing process and mechanochemistry treatment of washed fly ash.The main research aspects and conclusions are as follows:1)Studies on multi-stage countercurrent washing of fly ash.The solution of soluble salt stabilizes in 20 minutes.After water washing,the moisture content of fly ash is about 55%.And the quality loss of is about 33%after primary washing,and 37%after secondary washing.The proportion of CaClOH,NaCl and KCl in fly ash decreased,but CaCO3 increased.The change of liquid-solid ratio has little influence on the composition of crystal particles.When the solid-liquid ratio of water washing solution is 2⁶ mL/g,the removal rate of chlorine in fly ash by secondary countercurrent water washing is 15.4%⁶1.15%.Under the same liquid-solid ratio,the removal rate of chlorine is higher by secondary countercurrent water washing than that by primary water washing.The lower the liquid-solid ratio,the better the water-saving effect and economic effect are.The calculated values obtained from the theoretical models have good correlation with the measured values and the root-mean-square-error is small.When the liquid-solid ratio of three-stage countercurrent water wash was 3 mL/g,the chlorine in fly ash was 94.0%lower than that of the original fly ash,and the chlorine dissolution of fly ash increases litter when the liquid-solid ratio is increased.After three-stage countercurrent washing,Pb leaching concentration increases to 80 times of the original fly ash,which exceeds the landfill standard,Cr increases to about 26 times of the original.The leaching concentration of Zn,Cu,Cd,Ni and Se all decrease significantly,while the leaching concentration of As do not change compared with the original fly ash.2)Quality analysis of washing water.The main ions in the washing water are Cl^-,Ca²⁺,Na⁺and K⁺.Under the increase of liquid/solid ratio,the iron concentration decreases,but the decreasing amplitude gradually decreases.The pH of washing water increases and is generally stable around 11.45.The ammonia nitrogen in the water wash exceeds the standard and decreases with the increase of the liquid-solid ratio.There is little difference in the ammonia nitrogen concentration by the three processes.The COD is stable at 800 mg/L,and exceeds the standard.The dioxins concentration of washing water is low.The concentration of dioxins under 6 mL/g liquid-solid ratio is 41.92 pg/L and the toxic equivalent concentration is 1.5579pg I-Teq/L.The main heavy metals are mainly Pb,Cu and Zn.,The concentrations of most heavy metals in the washing water are reduced if the fly ash is stored for a period of time before washing3)Degradation of dioxins in washed fly ash by mechanochemistry.When the molar ratio of the additive Al/SiO2 is 1:1 and the addition amount is 15%,the dioxin concentrations of the original fly ash and washed fly ash are reduced by 54.3%and 66.9%,and the dioxin toxic equivalent concentration is reduced by 60.9%and 77.8%respectively.When the molar ratio of Al/SiO2 is 2:1,the degradation rates of dioxin concentration and toxic equivalent concentration are as high as 87.3%and 87.1%respectively.After the mechanochemical treatment of washed fly ash,its chlorination degree decreased to a certain extent,and the higher the molar proportion of Al in the additive,the more obvious the dechlorination behavior of fly ash is.Inorganic chlorine salts in fly ash mainly promote the formation of homologues of high chlorinated dioxins.