Mechanism Study Of Mechanochemistry On PCDD/Fs Degradation And On Heavy Metals Stabilization In MSWI Fly Ash

Municipal solid waste incineration?MSWI?fly ash?FA?is classified as hazardous waste in most countries,including China,because of its high load of leachable heavy metals and of polychlorinated dibenzo-p-dioxins and dibenzofurans?PCDD/Fs or dioxins?.The industry of MSWI is developing rapidly in China,generating a huge amount of FA.The production of FA exceeds 5 million ton in 2017,and lacking proper treatment technology is a major concern of limiting the development of waste-to-energy industry.In the present study,two types of FA including circulating fluidized bed?CFB?and grate FA are treated with mechanochemical?MC?method,which is a non-thermal method featured with simple-operation and mild reaction conditions.Special attentions are paid on the studies of the physicochemical characteristics of FA,of the contaminants content in FA,of the water washing technology of FA,of the MC degradation mechanism of PCDD/Fs in FA,of the MC stabilization mechanism of heavy metals in FA,and of the utilization of FA in cementitious materials,to form an integrated technology roadmap of MC detoxification and recycling pretreatment of FA.The major contents and conclusions are as follows:?1?The physicochemical characteristics and contaminants content of 5 CFB and 4 grate FA are studied in detail,and the water washing technology is optimized.The results are as follows:the type of waste incineration furnace greatly impacts the composition of FA.For CFB and grate FA,the former has a higher content of chlorides,while the latter is richer in elements of Al,Si,Mg,and Fe.The leaching concentration of several heavy metals from CFB FA is higher than that from grate FA,except for Pb.The PCDD/Fs contents in two types of FA are largely different.Water washing removes elements of Cl,Na,and K from FA effectively.The two-stage counter-current water washing is proposed,which reduces the water consumption of reaching the same chlorine removal efficiency as co-current water washing does.?2?To obtain an additive for the MC degradation of PCDD/Fs in FA,five candidates are firstly selected out of nine additives;the acting mechanism of CaO-Al is then revealed;an innovative additive SiO₂-Al featured with high-efficiency,low-price,and good-practicality is finally developed.The results show:for CaO-Al during MC degradation,CaO releases O^2-,transferring electrons to attach C-Cl bond;Al serves as the electrons supplier of CaO,significantly accelerating the release of O^2-.With 10 wt.%addition of SiO₂-Al,the total and I-TEQ concentration of PCDD/Fs in FA after 14 h of milling decrease from 443 to 40.8 ng/g?90.8%?and6.75 to 0.64 ng I-TEQ/g?90.5%?.The efficiency of SiO₂-Al is comparable to CaO-Al,but much more practical.?3?An energy transfer model in horizontal ball mill is established to quantitatively compare with that in planetary ball mill,and the PCDD/Fs degradation experiments are performed within a horizontal ball mill.The results show:the energy utilization efficiency and the treatment capacity of horizontal ball mill is much higher than those of planetary ball mill.The impact energy per hit of ball is comparable for both two types of mill,but the effective impact power of planetary ball mill is much higher due to its higher ball-hitting frequency.With the same adding ratio of additive and total supplied energy,the degradation efficiency of PCDD/Fs in horizontal ball mill is obviously lower than that in planetary ball mill.Selecting horizontal ball mill with big radius and ensuring an appropriate filling ratio can retain the reactivity of Al powder and facilitate the energy transfer,respectively,resulting in a higher degradation efficiency.?4?The evolution of PCDD/F-signatures during MC degradation is systematically investigated at the level of individual congeners,to reveal the acting mechanism of additives and the specific degradation pathways of PCDD/Fs.The results show:the evolution of individual congeners almost follow the same rule with the immune to additive and FA types.The acting mechanisms of all six additives are similar:additives transfer electrons to attack the C-Cl bond and then expulse chlorine.Two dechlorination pathways respectively for PCDD and PCDF are proposed based on the signature analysis of CP-route congeners.MC dechlorination can be less favored for the chlorine on?-position?2,3,7,8-position?.The oxygen with stronger electronegativity in PCDD/Fs negatively influences the adjacent C-Cl to accept the donated electrons.?5?MC treatment activates the crystals within FA to increase their solubility,enhancing the acid buffering capacity of FA and raising pH value of the leachate,reducing the leaching concentration of heavy metals which are sensitive to the variation of pH value.The mechanism of heavy metals stabilization during MC treatment is revealed as:additives and soluble compounds containing heavy metals are activated to react with each other,forming insoluble or stable compounds to stabilize heavy metals.Among five additives,Ca₃?PO₄?₂ exhibits the best performance of stabilizing heavy metals.Compared to the raw FA,the leaching concentration of many heavy metals?except for Ni?from the treated FA significantly decreases at the same end-point pH value of leachate.Nemerow index is employed to assess the environmental risk of treated FA.MC treatment lowers the Nemerow index from 9.45?heavy pollution?to 0.72?slightly over the safety standard of 0.7?.Extending milling time or increasing the dosage of additives can further promote the stabilization of heavy metals to meet the safety standard.?6?The FA is added into mortar to study its recycling performance.The wet MC treatment modified the properties of FA,greatly improving its recycling quality.Wet MC treatment promotes the dissolution of insoluble chlorides by converting crystals from crystalline to amorphous state.During the curing of mortars,traces of aluminum metal in CFB FA are oxidized,liberating lots of hydrogen gas;this would cause expansion problems and significantly reduces the flexural and compressive strength.Wet MC treatment of CFB FA solves such expansion problems by exhausting these reactions in advance,dramatically enhancing flexural and compressive strength of mortar.The grate FA exhibits a poor performance in mortars,because of its low content of Si and Al.An innovative pre-treatment technique of CFB FA,combining wet ball milling and counter-current two-stage water washing,is proposed for facilitating its recycling.Also,an integrated technology roadmap of MC detoxification and recycling pretreatment of FA is proposed.