Study On Preparation Of Glass-ceramics From Solid Waste And Migration And Solidification Of Heavy Metals

The social demand for resources is increasing with the development of the economy and the acceleration of urbanization.In the process of developing and using resources,however,due to low utilization efficiency,a large number of solid wastes are produced.If it is not handled properly,solid waste will cause a great burden to human society and the natural environment.Glass-ceramics,as a new building material with a high tolerance for raw materials,can digest a large amount of solid waste and effectively solve the problem of harmless treatment and resource utilization of solid waste.However,at present,the research and reports mostly focus on one or two kinds of solid wastes,and the preparation system of multi-solid waste glass-ceramics has not yet been formed.Furthermore,the components of solid waste are complex and often contain a variety of heavy metal elements.But at present,there is little research on the migration and solidification law of heavy metals during high-temperature reconstruction,and the coupling mechanism between heavy metals is still unclear.In this paper,based on the above situation,municipal solid waste incineration fly ash,waste glass,and lead-zinc tailings were used as raw materials to design five groups of ratios.Moreover,on this basis,four kinds of heavy metals were added with 5 wt%each to prepare glass-ceramics.The crystallization kinetics and thermodynamics of samples with different basicity and sintering temperature were analyzed by differential scanning calorimetry（DSC）and Fact Sage software.The properties of solid waste glass-ceramics samples were characterized by testing density,water absorption,and shrinkage.X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,and energy dispersive spectroscopy（EDS）were used to analyze the crystal phase composition and microstructure,the valence change and distribution of elements,and to explore the effects of basicity and sintering temperature on the crystallization of samples and the solidification and migration of heavy metals.In the end,the solidification effect of glass-ceramics on heavy metals was explored by toxicity characteristic leaching procedure（TCLP）and BCR sequential extraction method.The research shows that:（1）With the increase of basicity,the crystallization activation energy E_c and Avrami index n of the samples first decreased and then increased.The nucleation of the sample changed from surface nuclear to volume nucleation,and from one-dimensional space growth to two-dimensional space growth.The increase of basicity promoted the fracture of non-bridging oxygen and the formation of bridging oxygen bonds in the glass matrix.Simultaneously,ion channels can be formed to improve the driving force of crystallization.However,excessive Ca would increase the viscosity and inhibit crystallization.（2）In the thermodynamic simulation,the liquid phase and various crystalline phases appeared in the sample.Si O₂ was the main component of the liquid phase,and pyroxene and spinel were the main crystalline phases.Among them,the pyroxene included diopside Ca Mg Si₂O₆,enstatite Mg₂Si₂O₆,and hedenbergite Ca Fe Si₂O₆,and the spinel mainly included magnetite Fe₃O₄ and zinc-iron spinel Zn Fe₂O₄.With the increase of basicity,the content of clinopyroxene raised.While the increase of sintering temperature led to the appearance of the liquid phase in the sample,and the content of the liquid phase kept rising.（3）With the increase of basicity,the density and shrinkage of glass-ceramics samples increased by 16.93%and 47.36%respectively,and the water absorption decreased by 98.42%.The properties of the sample were greatly improved when the basicity was 0.752.With the increase of sintering temperature,the density and shrinkage of the samples increased by 30.41%and 360.48%respectively,and the water absorption decreased by 99.42%.1000℃was the critical point of sintering temperature,and at this temperature,the properties of the samples achieved a leap.（4）The glass-ceramics samples with different basicity contained pyroxene phase（Ca,Na）（Mg,Fe,Al）（Si,Al）₂O₆ and spinel phase（Mg,Fe,Zn,Cu）Fe₂O₄.At low basicity,the content of the spinel phase in the sample was high.Along with the increase of basicity,the diffraction peak of pyroxene increased and occupied a dominant position.Moreover,the increase of basicity could make the internal structure of the sample more compact,the grain size more uniform and the arrangement more regular.（5）In the glass-ceramics samples with different sintering temperatures,pyroxene（Ca,Na）（Mg,Fe,Al）（Si,Al）₂O₆ was the main crystalline phase,and magnetite spinel（Mg,Fe,Zn,Cu）Fe₂O₄ was the secondary crystalline phase.At low sintering temperatures,the content of pyroxene in the sample was high.With the increase of temperature,the isomorphic substitution phenomenon intensified,and a large number of spinel phases were precipitated.In addition,the increase of sintering temperature could accelerate the internal reaction of the sample,provide the driving force for the growth of the crystal,increase the grain size,and make the internal structure more compact.（6）Fe played a leading role in the migration and solidification of heavy metals.The replacement capacity and abundance of Fe were higher than that of Zn and Cu,so Fe was solidified in the form of hedenbergite Ca Fe Si₂O₆ and magnetite Fe₃O₄.While Zn and Cu entered the Fe₃O₄ lattice through isomorphic substitution,forming zinc-iron spinel Zn Fe₂O₄ and copper-iron spinel Cu Fe₂O₄.However,Pb was finally solidified in the glass matrix in the form of ions because its oxide was unstable.（7）After high-temperature calcination,heavy metals in solid waste glass-ceramics mostly existed in the form of oxidizable state and residual state which were more stable.The order of solidification effect of the four heavy metals was Fe>Zn>Cu>Pb.In addition,the crystalline phase and amorphous phase in the sample formed a double barrier,which could effectively inhibit the leaching of heavy metals,and the leaching concentrations of the four heavy metals were all below the standard threshold.