Co-stabilization Mechanism Of Heavy Metals In MSWI Fly Ash And Electrolytic Manganese Residue And Ceramisite Reutilization Research

Municipal solid waste incineration(MSWI)power generation become the main disposal method for municipal solid waste in China.MSWI fly ash has been in explosive increase and electrolytic metal manganese(EMR)was discharged for 10million tons each year.MSWI fly ash is listed in National Hazardous Waste List,as HW18,and EMR contained massive soluble manganese and ammoniums.Therefore,MSWI fly ash and EMR can be innovative to co-dispose of due to the same as inorganic CaO-MgO-Fe₂O₃-Al₂O₃-SiO₂ system under the condition of zero-waste city construction.The morphology,chemical and phase composition,thermodynamic characteristics and heavy metals accumulation characteristics of MSWI fly ash and EMR were systematically analyzed by SEM-EDS,XRF,XRD,FT-IR,TG-DTG-DSC,microwave digestion,and AAS.Different cooperative disposal schemes as follows:exploring the effect of EMR as a sulphoaluminate cement on stabilizing heavy metals in MSWI fly ash;co-disposing of MSWI fly ash and EMR based on geopolymic system,exploring stabilization/solidification mechanism of heavy metals;preparing lightweight ceramisite of MSWI fly ash,EMR and coal fly ash with increasing the proportion of MSWI fly ash and elaborating the migration and transformation of heavy metals in ceramsite and the mechanism of ceramification;analyzing the environmental and economical impacts of these three co-disposal solutions.The results were shown as follows:(1)MSWI fly ash(FA)and EMR contained considerable heavy metals(Pb,Zn,Cd,Cu,Cr and Mn).H₂NCSNH₂ and NaH₂PO₄ has a synergic effect on stabilizing FA,which can be explained by that more phosphate ions participated in the stabilization reaction of heavy metals in the presence of thiourea.The free manganese ions of original EMR had a competition with heavy metals and stabilizers in the original MSWI fly ash,which can be inhibited by calcining EM.By using 20%calcined pre-treated EMR(800?),composite stabilizer(6%H₂NCSNH₂ and 6%Na H₂PO₄)and 80%MSWI fly ash with water/solid 0.5 m L/g to form a solidified body,the leaching concentration of heavy metals meet the standard(GB16889-2008).(2)Alkaline surroundings of FA-EMR geopolymers can inhibit the releasing of soluble heavy metals.The Fe-rich polysilicon lithium mica appearing in the FA-EMR-based geopolymer has strong adsorption characteristics,which was benefical for geopolymer to solidify heavy metals.The FA-EMR-based geopolymer had high structural stability due to the high ratio of SiO₂/Al₂O₃.The leaching ratios of Zn,Cr,Pb and Cu in FE-7.5S-2(75%FA and 25%EMR,cured 28d)was lesser 38.04%,83.33%,25.92%and 58.70%than FC-2,respectively.The total leaching ratios of heavy metals in FE-7.5S-2 was less than 2%and the compressive strength reached 1.47MPa.(3)The FE-SP(activated by Na OH and KOH)geopolymer performed the best since the leaching concentrations of Zn,Cr,Pb,Cu,Cd,and Mn were 59.90%,20.00%,65%,100%,100%,and 99.88%lower than FE-7.5S-2.Cement,and calcium hydroxide addition can improve the geopolymer performances,indicating that calcium ions played a crucial role in geopolymerization.Calcining EMR as pretreatment and alkaline complexes can further improve the curing efficiency of heavy metals in FA-EMR geopolymers,enhancing the proportions of residual fractions of heavy metal(Zn,Pb,Cu,Cr,Mn and Cd).The main curing mechanism of heavy metals in FA-EMR geopolymer was the formation of an alkaline environment,followed by physical encapsulation of geopolymer gel.The optimal mixture of the geoplymeric system consisted of 75 wt%MSWI fly ash and 25 wt%EMR with a KOH/NaOH(1:1)complex solution(7.5M OH^-)/solid of 0.5mL/g.(4)For maximizing the recycling proportion of FA,the mixture of 34.5%EMR,24.1%CFA,20.7%FA and 20.7%WFA sintered at 1160?for 12 minutes with a procedural heating rate was the optimal.The granule strength of ceramsite was 958N,the bulk density 792m³/kg,1^-h water absorption rate 8.12%,and porosity 45.3%(distributing in 1-20?m),which belongs to Class 800 lightweight aggregate.The residual fraction of Pb,Cu,Zn,Mn and Cr in ceramisite were 42.7,53.7,79.4,82.7 and96.7%,respectively.Cu,Zn and Cr in ceramisite were mainly ion-exchanged and charge balanced with the Ca-bearing minerals(diopside and wollastonite).However,the transformation and migration of Pb and Mn were mainly by chlorides and sulfates respectively.Cd behavior of ceramisite was attributed to the co-existence of chlorides and sulfates.(5)The sintering mechanism of FA-EMR ceramsite was divided into six stages:blank forming,blank preheating(25-384?),blank pre-activation(384-710?),blank activation(710-1012?),pottery forming(1068-1160?and at 1160?for 12 min)and pottery cooling,forming diopside(Ca(Mg,Al)(Si,Al)₂O₆),anorthite(CaAl₂Si₂O₈),wollastonite(1A,CaSiO₃),and a large amount of amorphous silica-alumina minerals(SiO₂ diffraction peaks disappear,and glassy Si-O infrared peaks appear).The Hakanson ecological index RI of the FA-EMR ceramisite was 23.87,and lesser than FA and EMR 99.82%and 94.29%,respectively.The human risk assessment of FA-EMR ceramsite is acceptable,and it had a low potential ecological risk.