Mechanism Of Chlorine And Heavy Metals Removing From Municipal Solid Waste Incineration Fly Ash By Electric Field Enhanced Washing

In 2020,the national waste removal and transportation volume has reached 235 million tons,and the urban domestic waste will increase at a rate of 6%per year.Due to its remarkable volume reduction effect,the proportion of waste treated by incineration is increasing,However,a kind of hazardous waste-fly ash will be produced in this process.There are chlorine salts and heavy metals（HM_S）in the fly ash.These pollutants seriously limit the subsequent treatment and resource utilization of fly ash.Paying attention to the effective removal of pollutants in fly ash has become a focus of academia and society.Electric field enhanced removal technology can remove soluble chlorine and heavy metals,moreover the insoluble chlorine and insoluble heavy metals can be removed at the same time.Therefore,based on the basic characteristics of fly ash,this paper constructs electric field enhanced formic acid（CH₂O₂）system,electric field enhanced oxalic acid（C₂H₂O₄）system and mixed acid system,systematically studies the removal law of chlorine and heavy metals under each system,and explores the removal mechanism.The conclusions are as follows:1.Characteristics of untreated fly ash:the main elements are Ca,Cl,Na,Si,K,S,Mg,etc,The phases include Na Cl,KCl,Ca CO₃,Ca（OH）₂,Ca SO₄ and Si O₂.The content of chloride ion is15.6%,soluble chlorine is 15.29%,insoluble chlorine is 0.31%.Under electron microscope,the fly ash mainly exists in irregular shape such as ball,flake and block.The content of HM_Sin fly ash is high,in which Zn,Ba,Pb and Mn are 3317.78,957.33,486.84 and 411.33 mg/kg respectively.In the leaching experiment,the leaching concentration of Cd and Pb is 1.05 and1.59 times of the standard of identification standard of hazardous waste-leaching toxicity（GB5085.3-2007）.Speciation analysis shows that most of the HMs are distributed in the residue state,which is difficult to remove effectively.2.The optimal treatment system of electric field CH₂O₂:electrode exchange frequency 20 Hz,current density 50 m A/cm~2,formic acid concentration 2.0 mol/L,reaction time 10 h,residual chlorine in fly ash after treatment is only 0.17%.The removal rates of total chlorine,soluble chlorine and insoluble chlorine are 98.89%,99.37%and 75.06%respectively;The removal rates of Zn,Cd,Ba,As,Sb,Ni and Cr are 86.36%,81.02%,77.12%,62.22%,58.46%,49.55%and 49.36%respectively.After the reaction,the residue state is transformed into acid soluble state and oxidizable state,which promotes the removal of HMs,and the leaching concentration of HMs in the samples after the reaction is within the specified threshold.3.Electric field enhanced C₂H₂O₄ removal system:when the electrode exchange frequency is40 Hz,the current density is 50 m A/cm~2,the oxalic acid concentration is 0.5 mol/L,after 4 h of reaction,the residual chlorine content in fly ash is only 0.14%,and 99.10%of total chlorine,99.18%of soluble chlorine and 95.31%of insoluble chlorine can be removed.However,the overall removal rate of HM_S decreased under this system.Except that the removal rates of As,Ni,Cu and Mn are 79.08%,75.43%,55.68%and 52.81%,which are higher than that of electric field enhanced formic acid system,the removal rates of Zn,Cd,Sb and Cr,are lower,which are 69.44%,50.04%,46.97%and 44.43%respectively.Compared with water washing,the removal effect of chlorine,especially insoluble chlorine,was significantly improved by63.07%.The removal efficiency of HMs increased by 41.62%-67.51%（except Ba and Pb）.Calcium compounds in fly ash react with C₂H₂O₄ to form Ca C₂O₄.Ca C₂O₄attached to the surface of fly ash will block the"escape"of pollutants in the absence of an electric field.When the electric field exists,some Ca C₂O₄on the surface of fly ash will fall off under the stimulation of changing the direction of electrons.At the same time,electrons enter the internal structure of fly ash to open up the pollutant channel,so that the removal effect is enhanced.4.Optimal treatment system for electric field mixed acid:When n（CH₂O₂）:n（C₂H₂O₄）=1:1（0.5 mol/L）,20 Hz,50 m A/cm~2 and reaction time 16 h,the removal rates of total chlorine,soluble chlorine and insoluble chlorine are 98.95%,98.98%and 96.70%respectively;The removal efficiency of HMs is better than that of single acid solution.The removal rates of Ni,Zn,Ba,Cd,As,Sb,Mn and Cr are 83.64%,78.58%,76.66%,71.07%,68.44%,67.79%,62.20%and 54.5%,respectively.Moreover the removal effect of electric field-enhanced mixed acid system is better than that of previous studies.From the analysis of removal effect,The electric field-enhanced mixed acid system is the best choice.