稀有金属(英文版) 2019,38(03),245-251

Production of glass-ceramics using Municipal solid waste incineration fly ash

Wen-Di Fan Bo Liu Xun Luo Jian Yang Bin Guo Shen-Gen Zhang

Institute for Advanced Materials and Technology,University of Science and Technology Beijing

作者简介：*Bo Liu,e-mail: liubo@ustb.edu.cn;

收稿日期：20 October 2016

基金：financially supported by the National Natural Science Foundation of China (Nos. U1360202, 51472030, 51672024 and 51502014);the Fundamental Research Funds for the Central Universities (No. FRF-TP-16-027A3);the Innovation Project of Yunnan Province New Material Preparation and Processing Key Laboratory (No. 2016cx05);

Production of glass-ceramics using Municipal solid waste incineration fly ash

Wen-Di Fan Bo Liu Xun Luo Jian Yang Bin Guo Shen-Gen Zhang

Institute for Advanced Materials and Technology,University of Science and Technology Beijing

Abstract：

Municipal solid waste incineration(MSWI) fly ash is a by-product from municipal waste incineration.According to incomplete statistics, each year more than one million tons MSWI fly ash was produced in China. Owing to high heavy elements content, widely used disposal methods of landfill are not suitable for MSWI fly ash treatment. In this study, by using MSWI fly ash as raw materials, glassceramics was synthesized for the solidification of heavy metals and waste recycle. Process parameters, including composition, heat treatment temperature and time, were studied and optimized. Under optimizing conditions, the product has good properties of density of 3.42 g·cm^-3 and Vickers hardness of 6.91 GPa. Moreover, the leaching concentration of heavy metal elements meets allowable values of toxicity characteristic leaching procedure(TCLP).This study offers an alternative for MSWI fly ash recycle.

Keyword：

MSWI fly ash; Glass-ceramics; Heavy metal; Solidification; Recycling;

Received： 20 October 2016

1 Introduction

Municipal solid waste incineration (MSWI) generates large quantities of fly ash annually.In 2014,the delivering quantity of municipal solid waste (MSW) in China was178,602,000 t,and about 1066,000 t MSWI fly ash was produced [ 1, 2] .Due to high heavy elements content in MSWI fly ash,it has been classified as a hazardous waste in China [ 3, 4] .The most commonly used methods for disposing MSWI fly ash include:(1) placed in a well-designed landfill,(2) treated as raw materials for cement and then placed in landfills or reuse,(3) separation of heavy metals by physical/chemical treatment and (4) thermal treatment [ 5, 6, 7, 8, 9] .Landfill and converting into cement exists risk of secondary pollution of heavy metals [ 10] .Separation of heavy metals by physical/chemical treatment may need additives and generate secondary pollutants [ 8] .Thus,we need more researches on MSWI fly ash.

The glass-ceramics have unique physical and mechanical properties,such as high hardness,high density and good corrosion resistance [ 11, 12] .Glass-ceramics has been demonstrated to have a good solidification effect on the heavy metal.Utilizing exhaust metallurgical dusts(containing Cu,Pb,Zn and Ni) as raw materials,Mymrin et al. [ 13, 14] synthesized glass-ceramics.The results showed that the leaching values of heavy metal were hundreds of times less than those permitted the national standards of Brazil.In our group,pickling sludge,a hazardous waste,was used to synthesize glass-ceramics.The results showed that the leaching values of all samples can meet the requirement of US EPA [ 15] .MSWI fly ash contains many available compositions (SiO₂,MgO,CaO and A1₂0₃),which can be utilized as raw materials to synthesize glass-ceramics [ 16, 17, 18] .So,if glass-ceramics were synthesized from MSWI fly ash,a good solidification effect can be achieved.However,there are few published literature related to this.

In this study,using MSWI fly ash as raw materials,glass-ceramics was synthesized for the purpose of the solidification of heavy metals and waste recycle.Process parameters,including composition,heat treatment temperature and time,were studied and optimized.This study can offer an alternative for MSWI fly ash recycle.

2 Experimental

Fly ash was collected from MSWI plant in Wuhan,China.Pickling sludge was collected from a steel plant in Shandong,China.The chemical compositions of these raw materials are listed in Table 1.MSWI fly ash is the main source of CaO.The pickling sludge contains a high content of CaF₂.The SiO₂ in glass-ceramics comes from the cullet.In order to study the effect of composition on the glassceramics,there are four samples designed,as listed in Table 2.

The mixed raw materials listed in Table 2 were packed in a corundum crucible and melted at 1450℃for 1 h.After that,the melts were poured into an iron mold,which was preheated at 600℃,and soaked for 30 min.In this way,the parent glasses with different compositions could be obtained (PG-1,PG-2,PG-3 and PG-4).Subsequently,the parent glasses were performed on one-stage heat-treated to obtain glass-ceramics (GC-1,GC-2,etc.).A part of the sample was quenched in cold water after melting.Then,the thermal property was measured by the as-quenched sample,which was less than 10 mg,put into an Al₂O₃crucible and then heated at 10℃·min^-1 from room temperature to 1200℃through differential scanning calorimetry (DSC,STA409C).The exothermic peaks were guided the heating process of each parent glass [ 19] .

The crystalline phase was confirmed by X-ray diffractometer (XRD,DMAX-RB 12 KW) using Cu Kαradiation(λ=0.15418 nm).The scan rate was set as 10 (ο)·min^-1.The microstructure was investigated by scanning electron microscope (SEM,LEO-1450) after the surface polished and subsequently etched for 30 s in 1 vol%HF solution.Five points of the microhardness were measured by the microhardness tester (MH-6) with maximum load of 300 g and loading time of 10 s.The bulk density was tested by the Archimedes’method.Toxicity assessments of produced glass-ceramics were tested in a leaching experiment according to the US EPA SW-846 Method 1311-TCLP.The leaching solution was analyzed by inductively coupled plasma (ICP) [ 20] .The chemical resistances of produced glass-ceramics were texted by 20 vol%H₂SO₄ and 20vol%NaOH solutions.

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Table 2 Chemical compositions of different examinations (wt%)

3 Results and discussion

3.1 Composition design

Figure 1 shows DSC curves of the parent glasses.There is a significant exothermic peak in each DSC curve,which suggests that at least one crystalline phase is formed at that temperature.Based on the endothermic peaks,the parent glasses of PG-1,PG-2,PG-3 and PG-4 are one-stage heattreated for 1 h at 940,914,938 and 927℃,respectively.It can be seen that PG-2 sample has the lowest crystallization peak temperature.This may be caused by the different compositions of parent glasses.

Figure 2a shows XRD patterns of the four parent glasses.It shows that all the parent glasses are almost amorphous.Figure 2b shows XRD patterns of the four obtained glass-ceramics,indicating that the main phase changes from augite (Ca(Mg,Fe³⁺)(Si,Al)₂0_6,PDF#24-0203) to augite (Ca(Mg,Fe³⁺,Al)(Si,Al)₂0_6,PDF#41-1483),with the addition of MSWI fly ash increasing.The secondary phase is cuspidine (PDF No.11-0066).As shown in Fig.2c,the middle peaks of augite move to a lower angle and combine the left peak with the increase in fly ash.The reason of this phenomenon may be that more Fe³⁺ions getinto the augite phase which causes the increase in lattice parameters.

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Table 1 Chemical compositions of raw materials (wt%)

Others:Ti0_2,K₂O,ZnO,BaO,CuO,MnO,BaO,Co₂O₃

Fig.1 DSC curves of PG-1,PG-2,PG-3 and PG-4 samples

In Fig.3a,there are a large quantities of residue glass phase located in PG-1.This indicates that PG-1 is hard to crystallize,which may be due to the insufficiency of glass network modifier such as CaO [ 21, 22] .When the MSWI fly ash content is 40 wt%(Fig.3b),the granular crystals appear and are distributed homogeneously in the sample.As shown in Fig.3c and d,the particle size of microcrystal phase increases with the increase in fly ash content.This result shows that the crystal-growing ability increases with more CaO added in.The increase of Ca2+leads to forming a large number of non-bridged oxygen [ 23, 24] .Thus,the glass structure becomes more“open” [ 25] .

The microhardness,density and chemical resistance of the four glass-ceramics samples are listed in Table 3.The Vickers hardness and density increase at first and then decrease with the increase in MSWI fly ash.The water absorption shows the same trend.PG-2 has the highest Vickers hardness (6.68 GPa) and density (3.04 g·cm^-3),and the water absorption is 0.11 wt%.The increase in Vickers hardness and density may be due to the increase in crystallinity [ 9] .Besides,more Fe³⁺and Al³⁺have come into the augite phase with the increase in MSWI fly ash as discussed above.Al³⁺is generally considered as network former [ 21, 26, 27] .In this paper,the increase in aluminum ions content is very small (1.58 wt%-2.11 wt%).With the increase in aluminum ion content,the content of Ca ²⁺also increases a lot (17.44 wt%-24.00 wt%).When the content of calcium ion is low,the repairing effect of aluminum ion on glass network is obvious.With the calcium ion content increasing further,the calcium ions play a leading role,resulting in the increase in imperfectness of the glass network and the decrease in hardness.At the same time,the increase in the incomplete degree of the glass network also causes the diffusion rate of the particles to increase.The fast diffusion of the particle makes the crystal be more likely to grow,which in turn hinders the densification of the glass-ceramic.As shown in Fig.3,the particle size increases and the content of residue glass phase decreases with the increase in MSWI fly ash,which makes the surface roughness increase,and leads to the increase in water absorption.The chemical resistance of glass-ceramics is related to its chemical composition and network structure.As shown in Table 3,the acid resistance decreases and the alkali resistance increases with the increase in MSWI fly ash.The decrease in acid resistance is because more network modifiers such as CaO are added into the glass-ceramics,which makes the glass network be more open.Meanwhile,the increase in alkali resistance may be caused by the increase in basicity of the glass-ceramics.In a word,PG-2 has the best properties.Thus,the optimal formula of MSWI fly ash glass-ceramics is 40 wt%fly ash,40 wt%glass cullet and 20 wt%pickling sludge.

Fig.2 XRD patterns of PG-1,PG-2,PG-3 and PG-4:a parent glass of PG-1,PG-2,PG-3 and PG-4 and b PG-1,PG-2,PG-3 and PG-4 sintered at each target temperature for 1 h;c part of XRD data

Fig.3 SEM images of a PG-1,b PG-2,c PG-3 and d PG-4

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Table 3 Properties of produced samples

3.2 Treatment condition

The composition of PG-2 provides a good usability performance than other samples.The next experiment is to get produced glass-ceramics with better properties in appropriate treatment condition.The applied heat treatment schemes of the parent glass of PG-2 are listed in Table 4.

Figure 4 shows XRD pattern of PG-2 heat-treated at different temperatures for 1 h.The results show that the main crystal phase of glass-ceramics is augite (PDF No.24-0202).Cuspidine is also precipitated as a secondary phase.It should be noted that the main crystal phase of the glass-ceramics does not change at different crystallization temperatures.XRD peak intensity of GC-1 is relatively low.With the increase in heat treatment temperature,the diffraction peak intensity increases gradually.B ased on this phenomenon,the crystallinity of the samples increases with heat treatment temperature rising.

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Table 4 Applied heat treatment of each samples

Fig.4 XRD patterns of GC samples sintered at different tempera-tures for 1 h

Figure 5 shows SEM images of the glass-ceramics (PG-2 heat-treated at different temperatures for 1 h).The glassceramics heat-treated at 700℃for 1 h is shown in Fig.5a,in which a few grains appear in glass phase.Figure 5b is a SEM image of glass-ceramics sintered at 800℃,in which many large grains can be observed.According to Fig.1,800℃is lower than the maximum nucleation rate temperature.Insufficient nucleation sites lead to poor crystallization ability and abnormal crystal growth;hence,the grains are large.In Fig.5c,more grains with smaller size appear,with few rod-like grains attached.Figure 5d-f shows SEM images of the samples sintered at 900℃and at higher temperatures.As heated at this temperature range,the nucleation numbers are similar,so the grain size is in relation to the crystal growth ability which increases with the heating temperature rising.In Fig.5f,a large number of heterogeneous coarse grains appear,with the crystallites approximately flake-like and rod-like.

Fig.5 SEM images of GC samples sintered at different temperatures for 1 h:a heated at 700℃,b GC-1,heated at 800℃,c GC-3,heated at850℃,d GC-4,heated at 900℃,e GC-5,heated at 95℃,and f GC-7,heated at 1050℃

Figure 6 shows SEM images of GC-3 heated at 850℃for 15 min (Fig.6a),30 min (Fig.6b) and 120 min(Fig.6c),respectively.With the extension of heat-treating time,the rod-like crystallites appear in glass-ceramics gradually.

Table 5 shows the physical and chemical properties of the samples heat-treated at different temperatures for 1 h.The sample has the highest density and hardness value when the heating temperature is 850℃,and the density value is 3.42 g cm^-3.The density increases with the heat treatment temperature rising before 850℃.As shown in Fig.4,the crystal morphology influences the hardness of glass-ceramics.GC-2 has coarse grains,which results in a poor performance on hardness.As grains get smaller,GC-3shows the highest hardness.In Fig.4d-f,with the grains size increasing,the hardness decreases.The increase in temperature leads to the increase in lattice defects,which may be the reason for the improvement in mechanical.With the increase in heating temperature,the water absorption decreases.These phenomena are related to the growth of the crystal.The density of the sample increases with the heat treatment temperature.With the increase in the crystallization temperature,the diffusion rate of the particle is accelerated,and the pores of the sample are filled in.When the temperature is higher,the density of the sample decreases.This is due to that the fast diffusion of the particle makes the crystal be more likely to grow,which in turn hinders the densification of the glass-ceramic.The variation of the pores of sample can also be seen from Fig.4.This phenomenon gives a positive correlation between density and water absorption.The chemical resistances are less affected by heating time.

The effects of holding time on GC-3's physical and chemical properties are also listed in Table 5.At the same heat treatment temperature,the property of density goes with the heat treatment time.The rod-like crystallites occur when the heat treatment time was set as 120 min.This phenomenon leads to the decrease in hardness.In a word,GC-3 heat-treated at 850℃for 1 h exhibits good properties.

TCLP results are shown in Table 6.The leaching concentrations of toxic heavy metals are below the US EPA limits [ 20] .It can be noticed that the concentrations of the heavy metals decrease with the heat-treating time increasing.This phenomenon may be ascribed to that the heavy metals are bonded within the lattice of the crystalline phase [ 28, 29, 30] or solidified in the glass network.The experimental results prove that the produced glass-ceramics are safe for the usability as construction materials.

Fig.6 SEM images of GC-3 sample sintered at 850℃for different time:a 15 min,b 30 min and c 120 min

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Table 5 Properties of glass-ceramics

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Table 6 TCLP leaching concentrations of heavy metals in produced glass-ceramics (mg·L^-1)

4 Conclusion

In this study,by using MSWI fly ash as raw materials,glass-ceramics was synthesized for the solidification of heavy metals and waste recycle.Process parameters,including composition,heat treatment temperature and time,were studied and optimized.The optimized formula is 40 wt%fly ash,40 wt%cullet and 20 wt%pickling sludge.The optimized heat treatment process is heating at850℃for 1 h.The produced glass-ceramics exhibits good properties (density of 3.42 g·cm^-3 and Vickers hardness of6.91 GPa).Moreover,the leaching concentration of heavy metal elements meets allowable values of TCLP.This study offers a potential way for MSWI fly ash recycle.

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