J. Cent. South Univ. Technol. (2011) 18: 1945-1952

DOI: 10.1007/s11771-011-0927-8

Glass-ceramics made from arc-melting slag of waste incineration fly ash

LIU Han-qiao(刘汉桥), WEI Guo-xia(魏国侠), LIANG yin(梁茵), DONG Fei-ying(董飞英)

Department of Energy and Mechanics Engineering, Tianjin Institute of Urban Construction, Tianjin 300384, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: Grate fly ash and fluidized bed fly ash mixed with glass cullet additive respectively were melted in the electronic arc-furnace. The product, arc-melting slag, was further treated by crushing, pressing and heat treatment in order to make the glass-ceramics. The crystallization behaviors of the produced glass-ceramics were examined by differential thermal analysis (DTA), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Results show that main crystalline phase of the glass-ceramics from grate fly ash is wollastonite (CaSiO₃) with small amount of diopside (Ca(Mg,Al)(Si,Al)₂O₆), and that from fluidized bed fly ash is diopside (Ca(Mg,Al)(Si,Al)₂O₆). It is found that the glass-ceramics sintered at 850 °C and 1 000 °C from grate fly ash and fluidized bed fly ash respectively have the optimal physical, mechanical and chemical characteristics. Glass-ceramics samples, produced from incinerator fly ash with desirable properties and the low leaching concentration of heavy metals, can be the substitute of nature materials such as marble, granite and porcelain tiles.

Key words: waste incineration fly ash; arc-melting; glass-ceramics; crystallization behavior

1 Introduction

Incineration has become an important treating method for municipal solid waste (MSW) due to the increasing difficulty to get suitable sites for landfill in China. However, the incineration processes of MSW will generate fly ash (3%-5% of the original mass) unavoidably [1] which contains a considerable number of hazardous materials such as leachable heavy metals  and PCDD/Fs. MSW incinerator fly ash is usually classified as hazardous waste and may cause an immensely potential risk on the environment if it is directly dumped into waste landfill sites according to traditional means. Melting can not only decompose dioxins in the incineration fly ash completely by high temperature, but also solidify the majority of heavy metals in the molten slag safely [1]. The molten slag can be reused for aggregate, permeable brick, cement substitute and so on [2-4], but the added values of these products are relatively low. Incineration ?y ash, containing large amounts of glass forming oxides (SiO₂, Al₂O₃, and CaO) and minor nucleating agents (P₂O₅, TiO₂, and Fe₂O₃), can be expected to produce the glass-ceramic by controlling the initial composition through suitable heat treatment [5-6]. Recent works demonstrated that it was possible to vitrify fly ash from incinerators with some additives such as silica, feldspar and bottom ash [7-10].

Meanwhile, the metallurgical electric arc furnace can produce high temperature arc plasma up to 1 600 °C [11]. Therefore, the electric arc steel making technique may be a suitable treatment to immobilize toxic substances. It is necessary to reuse molten slag in a high value-added way to reduce running cost of MSW fly ash treated by metallurgical electric arc furnace. Fly ash usually has high content of CaO as a network modifying oxide because of sprayed lime in purification of the acid gases. Thus, the addition of quantitative additives rich in SiO₂ as a glass network former is required to obtain inert vitreous molten slag in order to facilitate the immobilization of heavy metal ions in the network of the glass matrix [12]. Moreover, if fly ash was melted in electric arc furnace with the proper amount of glass cullet additive, it is favorable for not only improving flowing property of the melt and inhibiting splash of fly ash in melting process, but also lightening the corrosion of refractory liner and the burning loss of graphite electrode generated from impurities such as chloride and activated carbon in fly ash [13]. In recent years, the research on using electric arc furnace to treat MSW incinerator fly ash has just started in China. In this work, the effects of heat treatment conditions on crystalline phases and final properties of glass-ceramics from two kinds of typical waste incineration fly ash were investigated.

2 Experimental

2.1 Raw materials and parent glass samples

The used incineration fly ash samples (named as FA1 and FA2) were respectively obtained from two refuse incineration power plants with a stoker-type incinerator and a fluidized bed furnace incinerator in the northern China. The air pollution control districts (APCDs) of the former incinerator compose of a semi- dry scrubber, activated carbon sprayer device and a bag filter. The APCDs of the latter incinerator are composed of a water spray tower, activated carbon sprayer device, an electrostatic precipitator and a bag filter. The fly ash samples were collected over a 7 d period. The ash samples were homogenized after collecting, passed through a sieve of 850 μm, and then dried at 105 °C for 24 h for further analysis. Additional glass cullet made from ordinary plate glass was crushed and ground in an agate mortar until they passed through a sieve of 100 mesh. Chemical compositions of incinerator fly ash and glass cullet were listed in Table 1. The fly ash and glass cullet were mixed at a ratio of 3:1 with 3% (mass fraction) TiO₂ of chemical grade which was added as a nucleating agent additionally. After mixing, the samples (each of  2 kg) were placed in DC lab-scale electric arc furnace (the flow diagrams of those have been shown in Ref.[13]). The samples were melted for 30 min to ensure completely homogeneous melting. Subsequently, the melting was rapidly quenched by pouring it into water, followed by drying and grinding to less than 106 μm. The ground samples were uniaxially pressed into 7 mm × 7 mm × 30 mm stainless mold using a laboratory oil hydraulic press (p=150 MPa, holding time 30 s) with adding 5% (mass fraction) polyvinyl alcohol (PVA) solution. The green bars as-formed were then dried at 383 K for 3 h. Post heat treatments were performed in a chamber furnace at temperatures of 750, 800, 850, 900, 950 and 1 050 °C at a rate of 5 °C/min. Samples were held at these temperatures for 2 h, and then cooled in the furnace naturally.

2.2 Analytical methodology

Differential thermal analysis (DTA) was carried out to evaluate the crystallization properties of as-received glass samples. Measurements were performed in the TG/DTA6300 by heating preliminarily ground glass powders in a Pt-crucible up to 1 200 °C at a heating rate of 10 °C/min. Al₂O₃ was used as the reference material. SEM investigation was performed by a FEI NANOSEM 430 apparatus. For the SEM investigations, specimens were polished after being etched in HF solution (10%) for 30 s, immediately rinsed with excess distilled water and then cleaned in ethanol for 2 min. XRD analysis was conducted with a Rigaku Ultima IV diffractometer. Three-point bending tests were carried out with a XXW-20 kN universal test machine with the span of   20 mm at the loading speed of 0.05 mm/min. Bulk density and water absorption capacity of samples were measured by the Archimedes method. Chemical resistance of heat-treated glass-ceramic samples was tested by measuring the mass loss after etching in acidic and alkali solutions. Crushed samples (5 g) with particle sizes of 0.5-1.0 mm were boiled in HCl (20%, mass fraction) and NaOH (20%, mass fraction) solutions for  1 h separately. Leaching tests could be referred to the Chinese solid waste extraction procedure for leaching toxicity-sulfuric acid and nitric acid method (HJ/T299—2007).

3 Results and discussion

3.1 Thermal analysis of parent glass

Figure 1 shows the DTA thermograms of the parent glass samples produced from FA1 and FA2. It can be seen that two samples exhibit shallow endothermic peaks at 663 °C and 693 °C, respectively, which show the glass transition temperature (T_g). FA1 sample has two exothermic peaks (T_p) at 769 °C and 868 °C, indicating possible appearance of two or above kinds of crystalline phases when it is heat treated. FA2 sample has one exothermic peak at 1 017 °C corresponding to the crystallization temperature. Peak crystallization temperature of FA1 sample is lower than that of FA2, which is related to higher CaO (network modifying  oxide) content in FA1. The endothermic peak at about  1 100 °C on two curves represents either re-solution of a crystalline phase or the formation of a liquid phase. Nucleation temperature usually lies at 50-100 °C above T_g [14]. Therefore, the temperature range selected for heat treatment is between 750 and 950°C for FA1 sample, and 850-1 050 °C for FA2 sample, respectively.

Table 1 Chemical compositions of incinerator fly ash and glass cullet

Fig.1 DTA curve of parent glasses

3.2 Microstructure of glass-ceramic

Figures 2 and 3 present XRD patterns of FA1 and FA2 after heat treatment of 2 h at different temperatures, respectively. Figure 2 shows that the FA1 sample is amorphous at 750 °C, and at 800 °C some crystalline phases appear, indicating the transformation from glass to glass-ceramic. Major phases of glass-ceramic observed are wollastonite (CaSiO₃), with smaller amounts of diopside (Ca(Mg,Al)(Si,Al)₂O₆). These results are in good agreement with DTA results and the intensity of the peaks increases with heat treatment temperature increasing. Results show that diopside (Ca(Mg,Al)- (Si,Al)₂O₆) appears at  850 °C in the FA2 sample, but diffraction peaks are not intensive. The diffraction peaks become more intensive with heat-treating temperature increasing, indicating the increasing numbers of crystalline phases. Distinction in crystalline phases of FA1 and FA2 is in connection with their chemical composition of parent glass. High content of Ca and Si in FA1 composition is in favor of composition theoretically shifting to the eutectic point of the wollastonite region in the CaO-SiO₂-Al₂O₃ ternary [15]. Thus, CaSiO₃ makes up the main crystalline phases on the XDR patterns of FA1.

SEM micrographs of samples FA1 and FA2 at different heat-treating temperatures are shown in Figs.4 and 5. Figure 4 shows that at 750 °C the bulk of the sample FA1 apparently remains glassy. At 800 °C，a lot of crystals with very small size are formed and isolate each other in residual glassy phase. The proportions of amorphous glassy phase remain more, and the crystal size increases with the heat-treatment temperature increasing. At 850 °C and 900 °C, the morphology of the crystalline phase is composed of spherical crystal of size ranging from 0.1 μm to 0.3 μm, forming a fully ordered micro-crystalline mosaic. The crystal size increases about 1 μm after the heat treatment temperature reaches 950 °C. This is due to the higher driving force and the increasing crystal growth rate at higher temperature than those of lower heat treatment temperature.

Fig.2 XRD patterns of glass-ceramics from FA1 samples at various heat treatment temperatures for 2 h: (a) 750 °C;      (b) 800 °C; (c) 850 °C; (d) 900 °C; (e) 950 °C

Fig.3 XRD patterns of glass-ceramics from FA2 samples at various heat treatment temperatures for 2 h: (a) 750 °C;      (b) 800 °C; (c) 850 °C; (d) 900 °C; (e) 950 °C

Fig.4 SEM micrographs of glass-ceramics from FA1 at various heat treatment temperatures: (a) 750 °C, (b) 800 °C,  (c) 850 °C; (d) 900 °C; (e) 950 °C

Figure 5 shows that a small number of spherical crystals of 50-200 nm in size are embedded in a glassy matrix for the FA2 sample heat-treated at lower temperature of 850 °C. At 950 °C and 1 000 °C, the spherical crystals grow up and tend to coalesce as strip, which is interlocked in the material. At 1 050 °C, the crystals aggregate and coarsen, having a wide range of size, and reaching as large as 2-3 μm in size. Aggregation of crystals presumably results from reduced surface energy during the process [15]. No diopside (Ca(Mg,Al)(Si,Al)₂O₆) with leaf-shaped crystals occurring in the glass-ceramic samples can be observed in contrast to other work [16] because of their initial chemical composition difference.

Generally, the crystallization process at a given temperature is controlled by the nucleation and growth rate. Since both nucleation and growth rates vary oppositely with the temperature, it is expected that those two curves for nucleation and growth rates will overlap in a certain temperature range [17]. Accordingly, in this work, the optimal temperature for the heat treatment is identified to achieve better microstructure. When the samples are heat treated at exorbitant temperature, the crystal growth rate is improved whereas the crystallization is far from optimization due to limited nucleation. While the heat treatment at low temperature such as 850 °C or 900 °C for FA1, 950 °C or 1 000 °C for FA2, the samples yield more nuclei as well as increased nucleation rate, thus producing a better and refined crystallization in material microstructure.

Fig.5 SEM micrographs of glass-ceramics from FA2 at various heat treatment temperatures:  (a) 850 °C; (b) 900 °C; (c) 950°C; (d) 1 000 °C; (e) 1 050 °C

3.3 Physical mechanical and chemical properties of glass-ceramic

The factors regulating physical, mechanical and chemical properties of glass-ceramics are crystalline phase, crystallization degree, size of the crystallites and homogeneity of crystal size [18]. Physical, mechanical and chemical properties obtained from FA1 and FA2 with post-heat treatment at different temperatures are shown in Figs.6-11. The results are mean values calculated from five measurements.

Devitrification of glass means structure ordering process amorphous glassy. It is expected that the densities of the samples increase with the increasing in crystalline phase occurring in the glass-ceramic samples [19]. Therefore, the densities of the samples reach the peak values of 2.63 g/cm³ and 2.55 g/cm³ at 850 °C and      1 000 °C for FA1 and FA2, respectively. The micro- structure of glass-ceramic, heat-treated at 750 °C for FA1 or 850 °C for FA2, contains small amount of crystalline phases, and bending strength mainly depends on glassy phase. Therefore, bending strength of the sample is relatively low. However, the structure changes to a dense glass-ceramic with high crystallinity and small size crystals as the heat-treatment temperature increases up to 850 °C for FA1 or 1 000 °C for FA2. The bending strengths of the samples are the maximum, 90.44 MPa and 79.44 MPa. The density and bending strength are comparable to or superior to natural materials such as granite and marble, which might be applied in the construction sector. The density and bending strength drop when the heat treatment temperature is above   850 °C for FA1 or 1 000 °C for FA2.

Fig.6 Volumetric density of glass-ceramics obtained under different temperatures

Fig.7 Bending strength of glass-ceramics obtained under different temperatures

Fig.8 Porosity of glass-ceramics obtained under different temperatures

Fig.9 Water absorption of glass-ceramics obtained under different temperatures

Fig.10 Chemical resistance to HCl of glass-ceramics obtained under different temperatures

Fig.11 Chemical resistance to NaOH of glass-ceramics obtained under different temperatures

Porosity and water absorption values correlate well with each other and have the same trend that first decreases and then increases with the increase of heat-treatment temperature. The amount of nucleus increases when the heat treatment temperature reaches 850 °C for FA1 and 1 000 °C for FA2. This allows the fine grains to form and thus causes an increase of the grain boundary area. Subsequently, better characteristics of physical properties (density, porosity, water absorption) and mechanical properties (three-point bending strength) are yielded [15]. However, at high treatment temperatures, the amount of nucleus decreases, and the crystal growth rate increases, resulting in an increase in crystal size. Physical and mechanical properties are declined. Therefore, physical and mechanical properties of FA1 and FA2 are strongly influenced by their respective crystalline phases.

Figures 10 and 11 show the chemical durability of samples at different heat-treating temperatures. Samples have relatively high mass losses for the acid attacks compared to the alkali solutions. It is known that the glassy matrix is more easily leached in the acidic solutions. Consequently, an increase in the content of the crystallize phase results in the higher chemical resistance in the sintered glass-ceramic materials. So, the mass loss goes on a downward trend with the increase of heat treatment temperature.

3.4 Leaching characteristics of glass-ceramics

According to HJ/T299—2007, the leaching concen- trations of fly ash, molten slag and glass-ceramics were tested, and the test results are listed in Table 2. As shown in Table 2, the leaching concentrations of Pb in FA1 and FA2 are 8.34 mg/L and 6.47mg/L, respectively, which exceed Chinese National Toxicity Identification Standard. Except minute quantity of volatile heavy metals, most of heavy metals are enveloped in the Si-O network structures of glassy slag after melting fly ash. The heavy metals in glass-ceramic samples heated at 850 °C for FA1 and 1 000 °C for FA2 are sufficiently stabilized, especially part of those are less than the detection limit. The produced glass-ceramic samples are non-toxic materials.

Table 2 Leachate concentrations of heavy metals in incinerator fly ash, molten slag and glass-ceramic

4 Conclusions

1) Glass-ceramics can be produced from the arc-melting slag of the mixture of the fly ash and glass cullet at a ratio of 3:1 with 3% (mass fraction) TiO₂ additionally. Main crystalline phase of the glass-ceramics from grate fly ash is wollastonite (CaSiO₃) with small amount of diopside (Ca(Mg,Al)(Si,Al)₂O₆), and that from fluidized bed fly ash is diopside (Ca(Mg,Al)- Si,Al)₂O₆).

2) The optimal heat treatment temperature for grate fly ash and fluidized bed fly ash are 850 °C and 1 000 °C, respectively, while the morphology of the crystalline phase is composed of spherical crystal or interlocked strip crystal, forming a fully ordered microcrystalline mosaic. The bending strengths of two kinds of glass- ceramics are 90.44 MPa and 79.44 MPa respectively.

3) Glass-ceramics samples produced from incinerator fly ash at optimal heat treatment temperature have desirable properties, and can be the substitute of nature materials. Melting treatment of MSWI fly ash using the electric arc steel making technique can simultaneously achieve the diplex effect of detoxification and resource recycling.

References

[1] IBANEZ R, ANDRES A, VGURI J R, ORTIZ I, IRABIEN J A. Characterization and management of incinerator wastes [J]. Journal of Hazardous Materials, 2000, A79(3): 215-227.

[2] KATSUNORI N, YOSHIKAZU N, HITOSHI O, HIDEKAZU N. Melting and stone production using MSW incinerated ash [J]. Waste Management, 2001, 21(5): 443-449.

[3] LIN K L, WANG K S, TZENG B Y. The hydration characteristics and utilization of slag obtained by the vitrification of MSWI fly ash [J]. Waste Management, 2004, 24(2): 199-205.

[4] LIU Han-qiao, WEI Guo-xia, ZHANG Shu-guang, ZHANG Shu-ting, ZHANG Yu-feng. Hydration characteristics of the slag from the electric arc melting furnace of hospital waste incineration ash [J]. Journal of Tianjin University, 2010, 43(4): 339-343 (in Chinese)

[5] CHENG T W. Effect of additional materials on the properties of glass-ceramic produced from incinerator fly ash [J]. Chemoshere, 2004, 56(2): 127-131.

[6] KIM J M, KIM H S. Glass-ceramic produced from a municipal waste incinerator fly ash with high Cl content [J]. Journal of the European Ceramic Society, 2004, 24(8): 2373-2382.

[7] ROETHER J A, DANIEL D J,RANI D, DEEGAM D E, CHEESEMAN C R, BOCCACCINI A R. Properties of sintered glass-ceramics prepared from plasma vitrified air pollution control residues[J]. Journal of Hazardous Materials, 2010, 173(1): 563-569.

[8] CHENG T W, HUANG M Z, TZENG C C. Production of colored glass–ceramics from incinerator ash using thermal plasma technology [J]. Chemosphere, 2007, 68(10): 1937-1945.

[9] EROL M, KUCUBAYRAKS A, ERSOY M B. The influence of the binder on the properties of sintered glass-ceramics produced from industrial wastes [J]. Ceramics International, 2009, 35(7): 2609- 2617.

[10] YANG Jia-kuan, BO Xiao, BOCCACCINI A. Preparation of low melting temperature glass-ceramics from municipal waste incineration fly ash [J]. Fuel, 2009, 88(7): 1275-1280.

[11] WANG Yan, MAO Zhi-zhong, TIAN Hui-xin. Modeling of electrode system for three-phase electric arc furnace [J]. Journal of Central South University of Technology, 2010, 17(3): 560-565. (in Chinese)

[12] LIU Han-qiao, WEI Guo-xia, LIANG Yin, YANG Jun-lan. Influence of additive on crystallization behaviour of arc-molten slag from incinerator fly ash [J]. Journal of Shenzhen University Science and Engineering, 2011, 28(6): 559-564. (in Chinese)

[13] CAI Jiu-ju, LIU Han-qiao, QI Peng-fei, TIAN Dong-qing. Experimental study on melting of hospital waste incineration ash by electric arc furnace [J]. The Chinese Journal of Process Engineering, 2007, 7(2): 337-341. (in Chinese)

[14] YOUNG J P, JONG H. Conversion to glass-ceramics from glasses made by MSW incinerator fly ash for recycling [J]. Ceramics International, 2002, 28(6): 689-694.

[15] KAROLY Z, MOHAI I, TOTH M. Production of glass-ceramics from fly ash using arc plasma [J]. Journal of the European Ceramic Society, 2007, 27(2): 1721-1725

[16] CHENG T W, CHEN Y S. On formation of CaO-Al₂O₃-SiO₂ glass-ceramics by vitrification of incinerator fly ash [J]. Chemosphere, 2003, 51(9): 817-824.

[17] EROL M, KUCUBAYRAKS A, ERSOY M B. Production of glass-ceramics obtained from industrial wastes by means of controlled nucleation and crystallization [J]. Chemical Engineering Journal, 2007, 132(1/2/3): 335-343.

[18] CHENG T W, CHU C, TZENG C C, CHEN Y S. Treatment and recycling of incinerated ash using thermal plasma technology [J]. Waste Management, 2002, 22(5): 485-490.

[19] CHEN Jin-shu, LI Hong, TANG Li-yin. Glass-ceramics [M]. Beijing: Chemical Industry Press, 2007: 145-146. (in Chinese)

(Edited by DENG Lü-xiang)

Foundation item: Project(20806051) supported by the National Natural Science Foundation of China; Project(20080440680) supported by China Postdoctoral Science Foundation

Received date: 2011-03-01; Accepted date: 2011-05-03

Corresponding author: LIU Han-qiao, PhD; Tel: +86-22-23085107; E-mail: lhqlkx@yahoo.com.cn