Mechanism Of Pollutant Control And Glass Ceramics Made From MSWI Fly Ash

In 2020,the amount of Municipal solid waste incinerated fly ash(fly ash for short)in China was 10 million tons.Fly ash is classified as hazardous solid waste(HW18)because it contains dioxins and heavy metals.The traditional methods such as landfill and cement solidification pose the risk of environmental pollution,and the melting solidification method cannot achieve the resource utilization.It is urgent to develop technologies to realize the harmless disposal and resource utilization of fly ash.With the heavy metals as nucleating agent and silicon oxygen tetrahedron as glass network structure,the glass-ceramics were prepared by smelting the stainless-steel pickling sludge and fly ash.The one-step formation mechanism of glass-ceramics was elucidated,and the migration,transformation and solidification of heavy metals and chloride salts were revealed.The nucleation and crystallization processes of fly ash glass-ceramics are described,and the one-step process is developed.The main conclusions are as follows:(1)The mechanism of nucleation and crystallization at the same temperature was elucidated,and the one-step preparation of glass-ceramics was realized.Fe2O3,CaF2 and Cr2O3 in stainless-steel pickling sludge can be used as nucleating agents of glass-ceramics.The free oxygen provided by Cr2O3 and non-bridging oxygen from Si-O bond in the glass network caused by the substitution of F-for O2-in CaF2,will reduce the stability of the glass and enhance the crystallization ability.When the stainless-steel pickling sludge is added to 22 wt.%,the crystallization temperature Tp and glass transition temperature Tg of the basic glass are close(?T(Tp-Tg)<177?),and the stability is poor.The Avrami index is 3.5 and the activation energy of nucleation is 264.29 kJ/mol.The crystallization mechanism can be described by a three-dimensional nucleation growth mode,and nucleation and crystallization processes take place at the same temperature.(2)The mechanism of synergetic solidification of heavy metals by microcrystalline and glass was elucidated.Heavy metals in glass-ceramics form stable microcrystalline phase or are physically coated by glass phase.Cr and Ni mainly exist in forms of spinel NiCr2O4 and Fe0.99Ni0.01Fe1.97Cr0.03O4,and a small amount of Cr is dissolved in diopside phase;Pb,Zn and Cu are uniformly distributed in the glass matrix by physical coating.The solidification effect of heavy metals was positively correlated with crystallinity.With the increase of crystallinity,heavy metals change from unstable acid extractable state,reduced state or oxidized state to stable residual state.The solidification effect of heavy metals in glass ceramics is enhanced.(3)The mechanism of solid solution curing of chlorine in glass-ceramics was revealed.Part of the Cl volatilizes during the heating process.Residual Cl dispersed in the glass phase and the calcium chlorite phase as solid solution,which shows excellent chemical stability in glass-ceramics.(4)The technology of one-step preparation of glass ceramics with fly ash,acid washing sludge and waste glass was developed.40 wt.%fly ash,22 wt.%stainless-steel pickling sludge and 38 wt.%waste glass were melted at 1400? for 3 h,and the obtained parent glass was nucleated and crystallized at 800? for 0.5 h.Glass-ceramics were successfully prepared by the one-step process.The hardness and flexural strength are 13.11 GPa and 135.84 MPa,respectively,and the acid and alkali resistance are 98.65%and 99.88%respectively.The TCLP leaching concentrations of heavy metals(Pb,Cr,Ni,Zn,Cu)are 0.15,0.20,0.12,0.01,0.11 mg/L,respectively,which are far below the US EPA TCLP threshold.The performances meet requirements of industrial glass-ceramic plates(JC/T 2097-2011).(5)The maximum absorption of fly ash in glass-ceramics was studied by mechanism of chemical composition deployment.The results show that the upper limit of fly ash is 50 wt.%.The glass-ceramics were prepared from 50 wt.%fly ash,28 wt.%waste glass and 22 wt.%stainless-steel pickling sludge,whose density,water absorption,hardness,bending strength,acid resistance,and alkali resistance are 3.13 g/cm3,0.07%,7.97 GPa,114.86 MPa,99.29%,and 99.79%,respectively.The TCLP leaching concentrations of Cr,Cu,Ni,Pb,and Zn are 0.12,0.1,<0.01,<0.01 and 2.56 mg/L,respectively,which are far below the US EPA TCLP threshold.The HJ 557-2010 leaching concentration of heavy metals is lower than the maximum allowable discharge concentration limit specified in the comprehensive wastewater discharge standard(GB8978-2017).The performances meet requirements of industrial glass-ceramic plates(JC/T 2097-2011).Chlorine shows good stability in glass-ceramics.When the pH of the leaching solution is 3,6,10,13,the leaching concentration of chlorine is 0.When the pH is 1,the leaching concentration of chlorine is 0.1 mg/L.This research provides a new idea for the harmless disposal of fly ash as a resource,which not only avoids environmental pollution,but also prepares it into non-toxic,high-value glass-ceramics.