Thermal Process And Mechanism Of Functional Glass-ceramics From Asbestos Tailings By Phase Reconstruction In High Temperature

Asbestos tailings are hazardous solid wastes from the residue of asbestos mine,containing fibrous chrysotile.The accumulation of asbestos tailings not only occupies a large amount of land,leading to waste of resources,but also causes great pollution to the environment and endangers people’s health due to the asbestos fibers and heavy metal chromium ions contained in it.Based on the mineral and chemical composition of asbestos tailings,this paper took asbestos tailings in Shimian County as the research object.The asbestos tailings was firstly pre-calcined then added small amounts of auxiliary materials to prepare refractory ceramics,foamed glass-ceramics and glass-ceramics with lattice solidification to Cr³⁺by direct sintering method,which would achieve the harmless,low cost and high value-added utilization of asbestos tailings.Mineralogical characteristics of asbestos tailings and pre-calcined samples,effects of additive amount and thermal process parameters on the phase composition,microstructure and physico-chemical properties of functional glass-ceramics were systematically investigated by means of X-ray diffraction（XRD）,X-ray fluorescence（XRF）,transmission electron microscopy-energy dispersive spectroscopy（TEM-EDS）,thermogravimetry-differential scanning calorimetry analysis（TG-DSC）,scanning electron microscopy（SEM-EDS）,inductively coupled plasma-atomic emission spectrometry（ICP-OES）and physico-chemical properties tests.The thermodynamics and dynamics mechanism of thermal process for the preparation of glass-ceramics from asbestos tailings were studied by thermodynamics and dynamics models.The distribution of temperature field and stress field during the thermal process of preparing glass-ceramics from asbestos tailing were simulated by finite element analysis,which provides the theoretical basis for making the optimized thermal condition.Results showed that:（1）the mineral composition of asbestos tailings in Shimian County mainly contained serpentine,with a small amount of brucite,magnetite,dolomite,etc.;the chemical composition were mainly consist of SiO₂,MgO and Fe₂O₃,with small amounts of CaO,Al₂O₃,etc.After calcination at 700℃,the asbestos tailings basically removed hydroxyl water and volatile components,and the harmful component chrysotile was transformed into forsterite,etc..（2）The pre-calcined asbestos tailings was used as the total raw materials to prepare glass-ceramics with forsterite and enstatite as the main crystalline phases,magnesioferrite and diopside as the minor crystalline phases.The glass-ceramics achieved optimum compactness at the sintering temperature of 1200°C,and the crystal morphology were intertwined bodies of spherulites,ellipse and round bar-like shape.Moreover,the water absorption rate of glass-ceramics was 0.026%,the bulk density was 3.164g/cm³,and the flexural strength was 88MPa,which reached the standard of glass ceramics for construction.（3）The pre-calcined asbestos tailings was mixed with different contents of MgO and sintered to develop refractory ceramics containig phases of forsterite,magnesioferrite and merwinite.The addition of MgO in raw materials promoted the conversion of low melting phases（enstatite and diopside）into refractory phases through solid state reaction,which would significantly increase the optimal temperature of refractory ceramics.（4）Foamed glass-ceramics without using foaming agents have been fabricated in the system of SiO₂-MgO-CaO-Fe₂O₃-Na₂O by self-expansion during the process of powder sintering,using pre-calcined asbestos tailings and waste glass as raw materials.At high temperature,the fusible waste glass melted and formed glass melt network among the aggregate of refractory forsterite,and encapsulated gas in the gaps of green body.The pore size gradually increased due to the thermal expansion of gas,which eventually led to the formation of foamed glass-ceramics.The samples prepared with 67wt.%of waste glass and sintered at 1180℃possessed bulk density of 0.96g·cm^-3,volume expansion rate of 93%,porosity of 64%and compressive strength of 14Mpa,respectively.（5）The immobilization of chromium in asbestos tailings have been realized through formation of ceramics containing the phases of Cr-spinel,forsterite,enstatite,and diopside.Cr³⁺was incorporated into spinel phase,with the formula of MgFe_xCr_2-xO₄,and had significant isomorphic substitution with Fe³⁺.The increase in Cr₂O₃ content promoted the incorporation of Cr into spinel phase,resulting in higher spinel content.The immobilization ratio of Cr in spinel was about 62.45%for 0 wt.%of Cr₂O₃,which increased to 95.58%for 10 wt.%of Cr₂O₃.Leaching tests showed that Cr³⁺concentration in leachates was lower than the criteria of the toxic characteristic leaching standard TCLP,moreover,the leaching ratio of Cr³⁺decreased with Cr₂O₃ content in the raw material.（6）The pre-calcined asbestos tailings mixed with different contents of waste glass was heat-treated to prepare glass-ceramics through the processes of crystallization and sintering.With the increase in waste glass content（from 10wt.%to 30wt.%）,the activation energy of forsterite phase formation decreased gradually from 254.73 kJ?mol^-1 to 140.4 kJ?mol^-1,which indicated that the increase in waste glass content was conducive to the solid-phase reaction.Moreover,the activation energy of sintering decreased gradually from 69.935 kJ?mol^-1 to 27.156 kJ?mol^-1,which indicated that the increase in waste glass content promoted the densification and sintering of samples.（7）The results of the finite element simulation showed that,under the same heating rate,the maximum temperature difference between the inner and outer of green body,and the maximum stress of green body increased with the size.In order to optimize the heating rate and the effect of heating rate on the green body,the higher heating rate should be used in the low temperature region,while the lower heating rate should be used in the high temperature region.