Study On Dechlorination Pretreatment Of MSWI Fly Ash With The Purpose Of Promoting Its Resource Utilization

With rapid urbanization and improvement of people’s living standard,the amount of municipal solid waste（MSW）is increasing year after year.In China,the MSW are mainly disposed of by landfilling and incineration.In comparison with landfilling,incineration has gained more attention in recent years as it could reduce the volume of waste by approximately 85-90%,the mass by 60-90%,while also having the benefit of energy recovery.The main by-products from MSWI include MSWI bottom ash and MSWI fly ash.It should be noted that the term“fly ash”in this study refers to a combination of boiler ash and air pollution control（APC）residues unless stated otherwise.The MSWI fly ash usually accounts for about 3-15%of MSW amount depending on different MSWI furnaces.MSWI fly ash is generally regarded as hazardous waste due to its high contents of heavy metals,organic contaminants（dioxins and furans）,and salts（sulphate,chloride etc.）.The"Thirteenth Five-Year Plan for the Construction of Harmless Treatment Facilities for Urban Domestic Waste"points out that by 2020,the production of fly ash from MSWI is expected to reach 10 million tons in China.Although the"Technical specification for pollution control of fly-ash from municipal solid waste incineration"（HJ 1134-2020）standard stipulates that MSWI fly ash can be disposed by landfill after safe treatment,however,the MSWI fly ash disposed of by landfill is not proportional to its huge amount of production.In addition to the current shortage of landfill sites in many cities,it is already difficult to dispose of fly ash by landfill alone.As a substance with Ca,Al,and Si as the main structure,MSWI fly ash has considerable resource utilization potential,and the high chlorine content in fly ash has always been the main factor restricting its resource utilization.As a consequence,although the major mineralogical composition of the fly ash makes it suitable as raw materials for cement clinker production,the allowable incorporation content of MSWI fly ash was poor when chlorides are constrained according to the standard for pollution control on co-processing of solid wastes in Cement kiln（GB 30485-2013）.Therefore,finding a more effective and convenient in-depth dechlorination pretreatment method to dispose of fly ash and maximize its resource utilization is not only vital,but also imminent.For this purpose,this study collected MSWI fly ash in Chongqing,Qingdao and Tianjin.SEM microscopic morphology analysis,particle size determination,XRF element analysis,XRD mineral phase analysis,thermal gravimetric analysis and the determination of leaching toxicity have been carried out to gain an in-depth understanding of the structure and properties of fly ash from many perspectives.The results show that the three fly ash samples have low water content,approximately normal particle size distribution,and high porosity.The p H of Chongqing and Qingdao fly ash is about 12,which is strongly alkaline.The chlorine occurrence forms mainly include Na Cl,KCl,calcium hydroxychloride（Ca Cl OH）,Friedel salt（3Ca O·Al₂O₃·Ca Cl₂·10H₂O）and Ca₆（CO₃）₂（OH）₇Cl and other chlorine-containing salts.Tianjin fly ash has a p H of about 10.5,which is relative low and contains a large amount of Na Cl and KCl.The content of calcium hydroxychloride（Ca Cl OH）and insoluble chloride salts is showed low.The main elements contained in the three fly ashes is Ca,which is above 50%,followed by Cl.The Cl content of Chongqing,Qingdao and Tianjin fly ash is 16.95%,20.52%and 10.51%,respectively.In addition,there are a small amount of S,Fe,Al,Si and other elements in the three kinds of fly ash.Based on the analysis of the basic physical and chemical properties of MSWI fly ash,this study compared four pretreatments:water washing,sodium carbonate salt washing,water flushing with CO₂（primary carbonation）and water flushing with CO₂+Na₂CO₃washing（secondary carbonation）.The results show that the highest chlorine removal ratio of Chongqing,Qingdao and Tianjin fly ashes after washing with solid-liquid ratio of 1:10 at room temperature for 2 hours is 87.54%,90.12%and 95.60%,respectively.Increasing washing temperature does not contribute significantly to chlorine removal.Salt washing will significantly reduce the chlorine removal ratio in fly ash.When the concentration of sodium carbonate is 30g/L,the chlorine removal ratio of the three fly ashes drops to82.87%,86.19%and 90.76%,respectively.For Chongqing and Qingdao fly ash,water washing technology is difficult to achieve the desired dechlorination effect.By introducing CO₂during the washing process,the p H value of the reaction system is significantly reduced,the conversion and dissolution of insoluble salts are promoted,and the removal ratio of the fly ash chloride salt can be improved.This study shows that adding CO₂water washing can significantly improve the chlorine removal ratio of Chongqing and Qingdao fly ash.When the CO₂aeration ratio is 60ml/min and the aeration time is 30min,the chlorine removal ratio of the two fly ashes is 93.69%and 99.19%,respectively.The dechlorination effect is significantly higher than that of water eluting chlorine,while at the same time,the chlorine removal ratio of Tianjin fly ash is slightly reduced（93.42%）.Although the addition of water flushing with CO₂increased the removal ratio of chlorine in the two fly ashes in Chongqing and Qingdao,it still resulted in high concentrations of calcium ions remaining in the liquid phase.In this study,adding sodium carbonate to the reaction system on the basis of water flushing with CO₂,proved that it can effectively reduce the content of soluble calcium ions.For Tianjin fly ash,the chlorine element mainly exists in the form of soluble chloride salt,and the proportion of insoluble salt is very small.Therefore,the water washing technology can achieve a high chlorine removal ratio of95.60%,and the chlorine ion removal after carbonation is reduced.The main reason for analysis may be related to the entrapment effect of the re-precipitation of calcium carbonate and other substances on the chloride salt during the accelerated carbonation process.In addition,the SO₄^2-content in fly ash is also included in the"Technical specification for pollution control of fly-ash from municipal solid waste incineration"（HJ 1134-2020）and"Environmental protection technical specification for co-processing of solid waste in cement iln"（HJ 662-2013）.It is clearly required that the sulfate removal ratio is analyzed at the same time during the dechlorination process.The results show that the sulfate removal ratios of Chongqing and Qingdao fly ash in water washing are 25.96%and 25.40%,respectively.The three methods of salt washing,washing with CO₂and washing with CO₂+Na₂CO₃washing will increase the SO₄^2-removal ratio.But the effect is not significant,and the SO₄^2-removal ratio of the two fly ash does not exceed 30%.After the second carbonation of Tianjin fly ash,the removal ratio of SO₄^2-increased from 27.53%to more than 35%.It is inferred that the main reason may be related to the displacement reaction of CO₃^2-.In this study,the MSWI fly ash pretreated by three methods（water washing,water washing with CO₂and water washing with CO₂+Na₂CO₃washing）were characterized and the leaching toxicity was determined.By analyzing its element composition,particle size distribution,leaching toxicity and calculating the maximum mixing ratio of its cement kiln,the resource utilization potential after treatment was comprehensively evaluated.The results show that the three methods are conducive to the conversion of fly ash to the direction of recycling,Cl,S,Na,K and other elements are removed,and Ca,Si,Al and other elements are enriched.Chongqing and Qingdao fly ash after washing with CO₂treatment have the lowest chlorine content（0.58%and 0.51%,respectively）,and their cement kiln mixing ratio reaches the maximum（3.44%and 5.37%,respectively）.Tianjin fly ash is pretreated by water washing,in which the chlorine content reaches the lowest（0.45%）,and the cement kiln reaches the highest blending ratio（5.40%）at this time.The liquid phase after pretreatment also has a certain recovery value,and can be purified to obtain salts such as Na Cl and KCl.In addition,in the process of washing with CO_2,the three types of fly ash will absorb 6%,6%,and 3%of their own mass,respectively,to achieve a certain degree of carbon neutralization.In this study,the difference in the dechlorination effect of different pretreatments on different fly ash was compared,and different pretreatment methods were used to treat fly ash with different properties.The best working conditions of each method were explored,and different pretreatments were compared.Compatibility of treatment methods and fly ash with different properties,and analysis of economic and environmental benefits,in order to maximize cost savings and improve resource efficiency,so as to provide a theoretical basis for further resource utilization of fly ash that is difficult to dispose.