简介概要

Integrated utilization of municipal solid waste incineration fly ash and bottom ash for preparation of foam glass-ceramics

来源期刊：Rare Metals2019年第10期

论文作者：Bo Liu Qiang-Wei Yang Shen-Gen Zhang

文章页码：914 - 921

摘要：For the purpose of solid waste co-disposal and heavy metal stabilization,foam glass—ceramics were produced by using municipal solid waste incineration（MWSI）bottom ash and fly ash as main raw materials,calcium carbonate（CaCO₃） as foamer and sodium phosphate（Na₃PO₄） as foam stabilizer.The influences of the raw material composition,foaming temperature and foaming time on the properties were investigated.Porosity,bulk density,mechanical property and leaching of heavy metals were analyzed accordingly.The product,foamed at 1150℃ for 30 min with 14% fly ash and 74% bottom ash,exhibits excellent comprehensive properties,such as high porosity（76.03%）,low bulk density(0.67 g·cm^-3) and high compressive strength（10.56 MPa）.Moreover,the amount of leaching heavy metals,including Cr,Pb,Cu,Cd and Ni,in foam glass-ceramics is significantly lower than that of the US EPA hazardous waste thresholds.This study not only realizes the integrated utilization of bottom ash and fly ash,but also addresses a new strategy for obtaining foam glass-ceramics.

稀有金属(英文版) 2019,38(10),914-921

Integrated utilization of municipal solid waste incineration fly ash and bottom ash for preparation of foam glass-ceramics

Bo Liu Qiang-Wei Yang Shen-Gen Zhang

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

作者简介：*Shen-Gen Zhang e-mail:zhangshengen@mater.ustb.edu.cn;

收稿日期：12 June 2019

基金：financially supported by the National Natural Science Foundation of China (Nos.51672024 and 51502014);the National Key Research and Development Program of China (No.2017YFB0702304);the Program of China Scholarships Council (No.201806465040);the Fundamental Research Funds for the Central Universities (No.FRF-IC-18-008);

Integrated utilization of municipal solid waste incineration fly ash and bottom ash for preparation of foam glass-ceramics

Bo Liu Qiang-Wei Yang Shen-Gen Zhang

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

Abstract：

For the purpose of solid waste co-disposal and heavy metal stabilization,foam glass—ceramics were produced by using municipal solid waste incineration(MWSI)bottom ash and fly ash as main raw materials,calcium carbonate(CaCO₃) as foamer and sodium phosphate(Na₃PO₄) as foam stabilizer.The influences of the raw material composition,foaming temperature and foaming time on the properties were investigated.Porosity,bulk density,mechanical property and leaching of heavy metals were analyzed accordingly.The product,foamed at 1150℃ for 30 min with 14% fly ash and 74% bottom ash,exhibits excellent comprehensive properties,such as high porosity(76.03%),low bulk density(0.67 g·cm^-3) and high compressive strength(10.56 MPa).Moreover,the amount of leaching heavy metals,including Cr,Pb,Cu,Cd and Ni,in foam glass-ceramics is significantly lower than that of the US EPA hazardous waste thresholds.This study not only realizes the integrated utilization of bottom ash and fly ash,but also addresses a new strategy for obtaining foam glass-ceramics.

Keyword：

Municipal solid waste incineration ash; Foam glass-ceramics; Mechanical properties; Stabilization;

Received： 12 June 2019

1 Introduction

In order to cope with the challenges brought by the surge of municipal solid waste,municipal solid waste incineration(MSWI) technology has been widely used [ 1, 2] .However,MSWI ashes,including MSWI fly ash and MSWI bottom ash,which account for 20%-30%of the total weight of incineration,are produced during the incineration process [ 3, 4] .MSWI fly ash is mainly collected in flue gas purification and waste heat recovery system,while MSWI bottom ash is mainly collected at the bottom of combustion chambers [ 5] .MSWI ashes,especially fly ash,contain a considerable amount of salts,leachable heavy metals and toxic organics [ 6] .Improper disposal of MSWI ash will result in environmental pollution and waste of secondary resources.Therefore,the disposal of MSWI ash has become a hot issue of global concern.

Various disposal methods of MSWI ash have been developed,including landfill [ 7, 8] ,hydrometallurgical treatment [ 9] ,production of Portland cement and related products [ 10, 11] .Landfill can not only occupy a large amount of lands,but also cause a loss of valuable metals.Hydrometallurgical process has been proved to be effective in obtaining various metals [ 6, 9] ,such as copper,zinc,lead and cadmium.However,in this process,a large number of acid,alkali and organic solvent will be consumed,which increases the risk of secondary pollution.Owing to their huge market demand and simple production process,Portland cement and its products have become one of the most important target products for the utilization of MSWI ash [ 10] .However,Portland cement and its products have low added value.In addition,recent research has shown that heavy metals in cement and its products can leach again under certain conditions,which may cause secondary pollution to the environment [ 12] .So,looking for a new recycling method of MSWI ash to reduce environmental pollution risks and improve product's additive value are of great significance to the research.

Foam glass-ceramic is an environment-friendly porous material with outstanding properties [ 13, 14, 15] ,such as excellent heat insulation and chemical stability,low density and high specific strength.Owing to these advantages,foam glass-ceramic has been widely used in building,road and other fields [ 16, 17, 18, 19] .From the perspective of composition,MSWI ashes contain valuable mineral resources,such as SiO₂,Al₂O₃ and CaO,which are precisely necessary for the preparation of foam glass-ceramic.Moreover,heavy metals can be solidified in glass-ceramics during preparation,thus eliminating potential environmental pollution [ 20, 21] .Considering these reasons,preparation of foam glass-ceramic from MSWI ash is reported frequently [ 22, 23, 24] .However,in most studies,MSWI ash needs to be mixed with other solid wastes with relatively simple components as main raw materials.Few studies have involved the preparation of foam glass-ceramics only using MSWI ash as main raw materials.This may result in relatively low MSWI ash disposal capacity.

Therefore,the aim of the present study was to evaluate the feasibility of using MSWI fly ash and bottom ash in production of foam glass-ceramic.The effects of raw material composition,foaming temperature and foaming time on the physical-mechanical properties of foam glassceramics were investigated.Meanwhile,the heavy metal leaching toxicity test was also conducted on the prepared foam glass-ceramics to evaluate its environmental safety.The results can provide the basis of integrated utilization of MSWI ashes.

2 Experimental

2.1 Materials

CaCO₃ (analytical reagent (AR),Sinopharm Chemical Reagent Co.,Ltd.,Shanghai,China) with 99.0%purity was used as foamer,and Na₃PO₄ (chemically pure (CP),Sinopharm Chemical Reagent Co.,Ltd.,Shanghai,China)with 98%purity was used as foam stabilizer.The MSWI fly ash and bottom ash used in this study were obtained from Lujiashan Waste Incineration Plant in Beijing,China.The chemical compositions of MSWI bottom ash and fly ash determined by X-ray fluorescence spectrometer (XRF)are shown in Table 1.

2.2 Preparation of foam glass-ceramics

In this study,foam glass-ceramics were prepared by multistep processes involving mixing,grinding,compacting,sintering,foaming and annealing treatment.The experimental flowchart is shown in Fig.1.First,MSWI bottom ash and fly ash were mixed in different proportions.Subsequently,CaCO₃ and Na₃PO₄ were added to the mixed ashes to obtain raw materials.By adjusting the dosage,the proportions of CaCO₃ and Na₃PO₄ in the raw materials were fixed at 8 wt%and 4 wt%,respectively.The alkalinity of raw materials is expressed by Eq.(1):

where A is the alkalinity of raw materials and C is the molar concentration of corresponding components.The raw materials with different alkalinities were named A08,A09,A10 and A11,and their components and compositions are shown in Table 2.

The raw materials were dry-milled for 2 h in a vibrating ball mill and then filtrated through 200 mesh.Then,the batches were prepared by uniaxial dry-pressing into cuboid samples with size of 10 mm×10 mm×20 mm,using a pressure of 20 MPa.These cuboid samples were heated at5℃·min^-1 to 600℃for 20 min in air in a muffle furnace.Subsequently,these samples were heated to a certain temperature for different times with a heating rate of10℃·min^-1 and then annealed at 500℃for 30 min in a heat-treatment furnace.Finally,the foam glass-ceramics were obtained after the samples were cooled down to room temperature with a natural cooling rate.

Fig.1 Process flowchart of foam glass-ceramics obtained from MSWI bottom ash and fly ash

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Table 1 Chemical composition of MSWI fly ash and bottom ash (wt%)

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Table 2 Components and compositions of raw materials with different alkalinities (wt%)

2.3 Materials characteristics

The chemical compositions of MSWI fly ash and bottom ash were analyzed by XRF (XRF-1800,SHIMADZU,Japan).The crystalline phases present in samples were identified by X-ray diffractometer (XRD,D/max-RB,Rigaku,Japan).The morphology of the samples was observed by scanning electron microscope (SEM,Zeiss EVO-18,Germany).Thermogravimetric and differential scanning calorimetry (TG-DSC,DSC204F1,Netzsch,Germany) measurements were recorded at a heating rate of10℃·min^-1 in air.

According to the United States Environmental Protection Agency toxicity characteristic leaching procedure method 1311 (USEPA,1992),leaching toxicity of prepared foam glass ceramics was carried out.Inductively coupled plasma (ICP,OPTIMA 7000DV,PerkinElmer,USA) was used to analyze the content of heavy metals in leachate.The bulk density (ρ) and powder density (ρ0) were measured by Archimedes method and the pycnometer method,respectively.The total porosity (P) is evaluated using the following Eq.(2):

The water absorption was measured after immersion in distilled water at (20±2)℃for 48 h according to the GB/T 9966.3-2001.Water absorption (W) of the sample was calculated from the weights of dry samples (m₀) and soaked samples (m₁) by Eq.(3):

Fig.2 TG-DSC curves of A10 material

The compressive strength was measured with a universal electronic tester (DDL50,Sinotest Equipment Co.,Ltd.,China) at a speed of 1 mm·min^-1.

3 Results and discussion

3.1 TG-DSC analysis

In order to study the foaming process,A10 with alkalinity of～1.0 was selected for TG-DSC analysis and the results are shown in Fig.2.Three distinct changes occur in the sample weight,more specifically,a small decrease below49.2℃,a significant decrease in the range of 582.2-672.4℃and a small decrease above 672.4℃.The weight loss below 49.2℃is mainly attributed to the evaporation of adsorbed moisture.In view of the decomposition temperature of CaCO₃ between 590 and 950℃ [ 22, 23] ,the significant weight loss at 582.2-672.4℃is mainly caused by the decomposition of CaCO_3.When the temperature increases to 885.8℃,an exothermic peak appears on the DSC curve.This may be due to the reaction of CaO produced by decomposition with SiO₂ in the raw materials [ 22] .The processes can be expressed as following:

When the temperature rises from 885.8 to 1088.8℃,the exothermic perk appears again,which may be due to the crystallization of raw material.Based on the above results,in order to meet the needs of foaming and crystallization simultaneously,the foaming temperature should be higher than 1088.8℃.

3.2 Effect of raw material composition

A series of experiments were performed with the foaming temperature ranging from 1100 to 1200℃,while the foaming time was fixed at 0.5 h.The foaming conditions of samples were observed,and the observations are summarized in Table 3.At the same temperature,with the alkalinity of raw material increasing,the obtained foam glassceramics sample shows worse foaming effects,including smaller pore size and/or uneven pore distribution.This result will be discussed later in the basis of XRD analysis.Moreover,the melting and collapse of samples can be observed at high foaming temperatures.

XRD patterns of foam glass-ceramics obtained at1150℃are shown in Fig.3.The main and subordinate crystalline phases of as-synthesized foam glass-ceramics are gehlenite (Ca₂Al[AlSiO₇]) and akermanite (Ca₂Mg[Si₂O₇]),respectively.It can also be seen that the content of akermanite phase in the sample decreases with alkalinity increasing.This can be explained by the fact that the sample with higher alkalinity has a lower MgO content (Table 2).Because the melting-point temperature of gehlenite (～1593℃) is higher than that of akermanite (～1450℃),it is more difficult for samples with higher gehlenite phase content to foam.This explains why the foaming effect of sample gets worse with the alkalinity increasing at the same foaming temperature.Among the four samples (Table 3),the sample from A08 is the easiest to foam,while the sample from A10 has the best foaming effect.Based on this fact,A08 and Al0 are considered to have optimized compositions.

3.3 Effect of foaming temperature

SEM images and physical photographs of foam glass-ceramics prepared from A10 at different foaming temperatures are shown in Fig.4.From the appearance,the sample volume increases and then decreases slightly with foaming temperature increasing.And the sample obtained at1170℃has the largest volume.According to SEM images,from 1150 to 1170℃,the pore size of product increases significantly with the increase in the foaming temperature.At 1170℃,foam glass ceramic with good cellular structure is obtained.When the temperature rises to1180℃,pores with larger sizes appear in the product,but pore distribution is extremely uneven.From the physical photograph,the pores with big size mainly distribute on the surface of the sample,while the pore size inside the

Fig.3 XRD patterns of foam glass-ceramics obtained from different raw materials at 1150℃

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Table 3 Foaming conditions of samples prepared at different annealing temperatures

N.B.no bubbles,S.S.small-size bubbles,M.S.medium-sized bubbles,L.S.large-size bubbles,uniform uniform distribution of bubbles,Uneven:uneven distribution of bubbles,×melting and collapse

Fig.4 SEM images and physical photographs (inserted in upper-right corner) of A10 obtained at different foaming temperatures:a 1150℃,b 1160℃,c 1170℃and d 1180℃

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Table 4 Physical properties and leaching toxicity of foam glass ceramic obtained at different foaming temperatures

product is very small.This is due to the significant decrease in melt strength caused by excessive foaming temperature [ 24, 25, 26] .From above discussion,appropriate foaming temperature for A10 is 1170℃.

The physical properties and leaching toxicity of sample obtained at different foaming temperatures are summarized in Tables 4 and 5.With foaming temperature increasing,the porosity of product first increases and then decreases,while the compressive strength decreases.The type of porous structure and pore distribution play an important role in the resulting mechanical strength [ 27, 28, 29, 30, 31] .Thus,the decrease in compressive strength of products is mainly due to the increase in porosity.The only exception is the product obtained at 1180℃.Compared with product obtained at 1170℃,product obtained at 1180℃has lower porosity but lower compressive strength.As shown in Fig.4,there are a lot ofuneven pores with large size,which may lead to thedecrease in compressive strength.

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Table 5 Leaching toxicity of foam glass ceramic obtained at dif-ferent foaming temperatures (mg-L^-1)

From the leaching toxicity of products (Table 5),the leaching concentrations of Cr,Pb,Cu,Cd and Ni in all foam glass-ceramics samples are significantly lower than that of the US EPA hazardous waste thresholds.Furthermore,of all products,the product obtained at 1170℃possesses the largest porosity of 71.75%,the lowest bulk density of 0.79 g·cm^-3 and a relatively high compressive strength of 12.26 MPa.Based on this,appropriate foaming temperature for A10 is 1170℃.

Fig.5 XRD patterns of foam glass-ceramic prepared at different foaming times

3.4 Effect of foaming time

A08 was heated to 1150℃and foamed for different times from 10 to 360 min.From XRD patterns of as-synthesized foam glass ceramic (Fig.5),the main and subordinate crystalline phases are Ca₂Al[AlSiO₇]and Ca₂Mg[Si₂O₇],respectively.With the foaming time prolonging,the content of Ca₂Mg[Si₂O₇]phase in the product increases obviously.

With the foaming time prolonging,the color of products changes from yellow,yellow-brown to brown (Fig.6).In addition,the pore size increases significantly with the prolongation of foaming time.The color change indicates the change of phase composition of the products.Based on XRD results,this is mainly due to the content changes of Ca₂Al[AlSiO₇]and Ca₂Mg[Si₂O₇]phases.Prolonging foaming time can make the decomposition of CaCO₃ more complete,resulting in more CO₂ bubbles in the sample.Thus,the probabilities of bubble collision and coalescence are greatly increased.Therefore,the product obtained at longer foaming time possesses more pores with larger size and good cellular structure.

From the properties of products (Table 6),with foaming time increasing,the porosity increases dramatically to 76%and stabilizes at about 80%,which is consistent with the results of SEM analysis.The compressive strength and bulk density show similar tendency with foaming time increasing,more specifically,they all decrease by more than 50%from 10 to 30 min and decrease slightly with any further increase in foaming time.Moreover,the leaching toxicities of products (Table 7) are significantly lower than that of the US EPA hazardous waste thresholds.Taking into account all these results,along with the economic factors,the optimal foaming time for A08 is 30 min.

Fig.6 SEM images and physical photographs (inserted in upper-right comer) of products obtained at different foaming times:a 10 min,b 30 min,c 180 min and d 360 min

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Table 6 Physical properties and leaching toxicity of foam glass ceramic obtained at different foaming times

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Table 7 Leaching toxicity of foam glass ceramic obtained at dif-ferent foaming times (mg·L^-1)

4 Conclusion

In this study,foam glass-ceramics were produced successfully by using MWSI bottom ash and fly ash as main raw materials,CaCO₃ as foamer and Na₃PO₄ as foam stabilizer.The main and subordinate crystalline phases of synthesized foam glass-ceramics are Ca₂Al[AlSiO₇]and Ca₂Mg[Si₂O₇],respectively.Lower raw material alkalinity can result in more Ca₂Mg[Si₂O₇]phases,which are beneficial for the foaming of samples.Appropriate increase in foaming temperature or prolongation of foaming time is beneficial to increase the porosity and decrease the bulk density.The foam glass-ceramics,foamed at 1150℃for30 min with 14%fly ash and 74%bottom ash,exhibit excellent comprehensive properties,such as high porosity(76.03%),low bulk density (0.67 g·cm^-3) and high compressive strength (10.56 MPa).Moreover,the leaching concentrations of Cr,Pb,Cu,Cd and Ni in obtained foam glass-ceramics are significantly lower than that of the US EPA hazardous waste thresholds.This study not only realizes the integrated utilization of MSWI ash,but also addresses a new strategy for obtaining foam glassceramics.