Waste Management 38 (2015) 185–193
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Development of a sintering process for recycling oil shale fly ash and municipal solid waste incineration bottom ash into glass ceramic composite Zhikun Zhang, Lei Zhang, Aimin Li ⇑ Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science & Technology, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2014 Accepted 30 December 2014 Available online 31 January 2015 Keywords: Glass ceramic composite Heavy metals MSWI bottom ash Oil shale fly ash Sintering Vitrification
a b s t r a c t Oil shale fly ash and municipal solid waste incineration bottom ash are industrial and municipal by-products that require further treatment before disposal to avoid polluting the environment. In the study, they were mixed and vitrified into the slag by the melt-quench process. The obtained vitrified slag was then mixed with various percentages of oil shale fly ash and converted into glass ceramic composites by the subsequent sintering process. Differential thermal analysis was used to study the thermal characteristics and determine the sintering temperatures. X-ray diffraction analysis was used to analyze the crystalline phase compositions. Sintering shrinkage, weight loss on ignition, density and compressive strength were tested to determine the optimum preparation condition and study the co-sintering mechanism of vitrified amorphous slag and oil shale fly ash. The results showed the product performances increased with the increase of sintering temperatures and the proportion of vitrified slag to oil shale fly ash. Glass ceramic composite (vitrified slag content of 80%, oil shale fly ash content of 20%, sintering temperature of 1000 °C and sintering time of 2 h) showed the properties of density of 1.92 ± 0.05 g/cm3, weight loss on ignition of 6.14 ± 0.18%, sintering shrinkage of 22.06 ± 0.6% and compressive strength of 67 ± 14 MPa. The results indicated that it was a comparable waste-based material compared to previous researches. In particular, the energy consumption in the production process was reduced compared to conventional vitrification and sintering method. Chemical resistance and heavy metals leaching results of glass ceramic composites further confirmed the possibility of its engineering applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction As one of the most important crude oil substitute resources, oil shale has been widely used to produce liquid shale oil due to the increase in the price of crude oil (Guo et al., 2014a; Jiang et al., 2007; Reinik et al., 2011). It is reported that shale oil (calculated based on the in situ oil shale) accounted for about 400 billion tons of oil that is higher than worldwide total for traditional crude oil (about 300 billion tons) (Han et al., 2014; Qian et al., 2008). However, the resulting oil shale fly ash (OSFA) is considered environmentally hazardous due to high alkalinity and heavy metals concentrations (e.g. Cr, Cd, Zn, Pb, Cu) (Blinova et al., 2012; Reinik et al., 2011). In order to reduce the hazards of oil shale fly Abbreviations: MSWI, municipal solid waste incineration; OSFA, oil shale fly ash; BA, bottom ash; VS, vitrified slag; RAM, raw ash mixture of 80 wt.% oil shale fly ash and 20 wt.% bottom ash; SCS, slow cooling slag. ⇑ Corresponding author. E-mail address: [email protected] (A. Li). http://dx.doi.org/10.1016/j.wasman.2014.12.028 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
ash and achieve its resource utilization, the production of glass ceramics was studied in our previous study (Luan et al., 2010) and the results showed that it would be suitable raw material to synthesize glass ceramics by adding analytic reagent calcium oxides. Due to unbalance components in some solid wastes, in practice, the supplementing of some oxides or natural resources was necessary measurement to obtain desired crystals and good performances (Kang and Kang, 2012; Wang et al., 2010; Wu et al., 2013). From the viewpoint of natural resources conservation and maximal (100%) waste utilization, a cost-effective method involving the cheaper raw materials and economical process will be more popular for the recycling of large scale wastes. Therefore, the production of glass ceramic composites by utilizing various wastes with complementary components seems to be an attractive strategy. On the other hand, increasing incineration plants in China exhaust large quantities of wastes in the form of bottom ash (BA), leading to disposal, economical, and environmental problems. The generated volume of bottom ash usually ranged between 10% and
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12% of the initial volume of the wastes and weight between 20% and 35% of the initial wastes (Andreola et al., 2008; Barbieri et al., 2002; Monteiro et al., 2008). It is also identified as a hazardous waste due to the release of hazardous heavy metals and chlorides into the environment. Municipal solid waste incineration bottom ash generally contains high level of calcium content because the calcium hydroxide is injected into the waste gas stream to neutralize acid gas. Based on the analysis above, the comprehensive utilization of oil shale fly ash with high silica content and bottom ash with high calcium oxide content might be an attractive strategy to synthesize glass ceramics in which silica and calcium oxide are glass network former and modifier. Glass ceramics as a kind of promising material have attracted much attention due to a variety of unique properties. Recent years, the conversion of wastes into glass ceramics has gradually become an important method to improve the recycling of hazardous inorganic wastes into value-added materials while simultaneously reducing the leaching concentrations of heavy metals. Various hazardous industrial (Erol et al., 2008; Kavouras et al., 2007; Zhao et al., 2012) and municipal wastes (Andreola et al., 2008; Cheng et al., 2011; Cyr et al., 2012; Yang et al., 2008) have been reported to synthesize glass ceramic materials by devitrifying a glass by single or two-stage heat treatment. Among these methods, the hightemperature melting as one of the most widely used methods could greatly reduce the volume of wastes and stabilize the heavy metals into the glass matrix, but the process (1400–1600 °C) is general energy-intensive and therefore expensive, which does not meet the economic strategy in terms of the recycling of wastes. Another common option for glass ceramics production is a direct heat treatment process. Although the raw materials are not melted, it need to be shaped firstly by compacting the mixed powder and then sinter the compact at targeted heat treatment temperatures (Aloisi et al., 2006; Appendino et al., 2004; Barbieri et al., 2002; Bernardo et al., 2009; Tang et al., 2013). It is also a cost in producing the powder and there are some limitations on the size and shape of compositions that may be compacted. Therefore, it is important to develop a new processing method in order to implement widespread availability of these hazardous industrial and municipal wastes. Vitrification of hazardous wastes has been proved to be an attractive method for a safe immobilization of heavy metals in the glass matrix and the obtained vitrified slag (VS) can convert into glass ceramic materials through a two-stage heat treatment (Cheng, 2004; Lin et al., 2006; Luan et al., 2010; Kavouras et al., 2007). From the viewpoint of energy consumption reduction, if a certain amount of raw materials can directly reuse without melting, the energy consumption in the process can be reduced compared to common vitrification and sintering method. Based on this consideration, a more economical and simple method of co-sintering of vitrified amorphous slag and oil shale fly ash is proposed for the recycling of these two kinds of fly ash in the study. Oil shale fly ash and bottom ash were vitrified into the slag and then converted into the glass ceramic composite with the addition of various percentages of oil shale fly ash by the subsequent sintering process. The objective of our research is to explore the possibility of producing waste-based glass ceramic composites by the proposed method. To validate this hypothesis, the present study has been conducted to (i) produce glass ceramic composites using vitrified slag with various percentages of oil shale fly ash; (ii) characterize some important indexes of the products (density, weight loss on ignition, sintering shrinkage, compressive strength, chemical resistance and heavy metal leaching); (iii) study the phase transformation process and co-sintering mechanism of vitrified amorphous slag and oil shale fly ash by thermal analysis and XRD analysis. The results of the study will provide fundamental knowledge for the development of
waste-based glass ceramic composites through a simple and low energy consumption process. 2. Materials and methods 2.1. Raw material OSFA used in this study was obtained from the thermal power plant, Jilin, China; and MSWI BA was obtained from municipal solid waste incinerator, Dalian, China. OSFA and MSWI BA that removed any coarse impurities were dried at 105 °C for 24 h in an electric dry oven, and then were graded to pass sieve NO. 150 (the diameter of mesh is 106 lm) for subsequent experiments. As a result of preliminary experiments, the raw ash mixture of 80% OSFA and 20% BA (marked as RAM) by weight were used in this study. 2.2. The preparation of glass ceramic composites A total amount of 80 g RAM were separately put in a corundum crucible and melted at 1500 °C for 1 h to ensure complete melting. The melt was rapidly poured into water to obtain the vitrified slag. Meanwhile, the slow cooling slag (SCS) was obtained at a cooling rate of 4 °C/min from the melting temperature to room temperature. Then they were dried, ground and sieved to powders below 106 lm for subsequent experiments. The vitrified slag (VS) with the addition of 10, 20 and 30 wt.% OSFA, denoting to VS10, VS20 and VS30, were homogenized in ball-mill for 1 h. The mixed powder was filled in the corundum crucible and a single sintering scheme with 2 h dwelling time was used for the targeted temperatures ranged from 850 to 1000 °C. The sintered samples were then cooled inside the furnace to room temperature to obtain the glass ceramic composites. Meanwhile, SCS and RAM samples with the addition of 10, 20 and 30 wt.% OSFA, labelled SCS10, SCS20, SCS30, RAM10, RAM20 and RAM30, were sintered at 1000 °C for 2 h, respectively. 2.3. Analysis and methods 2.3.1. Chemical analysis The chemical compositions of OSFA and BA were determined by X-ray fluorescence (XRF) (PDA-5500II, Shimadzu, Japan). The heavy metal concentrations of OSFA and BA were determined by inductively coupled plasma–optical emission spectrometry (ICP–OES). 2.3.2. Thermal analysis Thermal behavior of OSFA, BA and VS were tested by thermogravimetric (TG) and differential thermal analysis (DTA) (TG/DTA 6300, NSK, Japan) to determine the sintering temperatures. About 20 mg of samples was heated to 1100 °C at the rate of 10 °C/min in nitrogen atmosphere. 2.3.3. X-ray diffraction (XRD) analysis Crystalline phase components of raw materials and glass ceramic composites were analyzed by XRD (D/MAX-2400, Rigaku, Japan). They were analyzed over a range of 2h angles from 10° to 60° with Cu Ka radiation at 4° min1 scanning speed. The measured results were studied by the standard powder diffraction database of International Centre for Diffraction Data (ICDD PDF-2 Release 2004). 2.3.4. Leaching toxicity Toxicity assessment of selected glass ceramic composites were tested in a leaching experiment modified from the U.S. EPA SW-846 Method 1311-TCLP, using a pH 4.93 extraction fluid as
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the leaching fluid. Each leaching vial was filled with 20 mL of TCLP extraction fluid and 1 g of sample and the soaking time was 20 days. The concentrations of heavy metals in the leaching solution were determined by ICP–OES. 2.3.5. Characterization of glass ceramic composites The density, weight loss on ignition, sintering shrinkage, compressive strength, chemical resistance and heavy metal leaching of glass ceramic composites were tested to determine the optimum condition and evaluate the environmental risk. The density and water absorption were tested according to Archimedes method. Weight loss on ignition was calculated from Eq. (1), and the sintering shrinkage was calculated from Eq. (2). The compression strength tests were done on the smooth surface of the products at a loading speed of 1 mm/min using a universal test machine (CSS-2220, Changchun Testing Machine Institute, China). Bulk samples were treated at 100 °C for 1 h in 5% acid/alkali solutions, and the chemical resistance was evaluated by the mass loss of the samples from Eq. (3). For each test, three samples were tested in order to get accurate results.
WL ¼
WBS WAS WBS
ð1Þ
where WL is the weight loss; WBS is the weight of samples before sintering; WAS is the weight of samples after sintering.
SS ¼
VBS VAS VBS
ð2Þ
where SS is the sintering shrinkage; VBS is the volume of samples before sintering; VAS is the volume of samples after sintering.
CR ¼
WAB WBB WAB
ð3Þ
where CR is the chemical resistance; WAB is the weight of samples before boiling; WBB is the weight of samples after boiling. 3. Results and discussion 3.1. Chemical compositions and XRD analysis of raw materials The chemical compositions and total heavy metals content of OSFA and BA are given in Table 1. It can be seen that SiO2, Al2O3, Fe2O3 and CaO were the main compositions for oil shale fly ash,
while CaO and SiO2 were major in bottom ash. It indicated that the mixture of OSFA and BA was suitable raw material to produce glass ceramic materials. It will be a double-win choice, because it not only can turn OSFA and BA into resource, but also can reduce the cost of raw materials, achieving the maximal (100%) waste utilization. XRD analysis of OSFA and BA are shown in Fig. 1. The crystals components of OSFA were mainly quartz SiO2 (PDF#79-1906), and there were a little anorthite [Ca,Na][Al,Si]2Si2O8 (PDF#090465) and Augite Ca[Mg,Fe]Si2O6 (PDF#24-0203). The main crystalline phases of BA were rankinite Ca3Si2O7 (PDF#22-0539) and quartz SiO2, and the coexisting crystalline phases were brianite Na2CaMg[PO4]2 (PDF#29-1192) and cliniferrosilite FeSiO3 (PDF#17-0548). It can be seen that the XRD results were associated to the chemical components of OSFA and BA as shown in Table 1.
3.2. Thermal analysis of OSFA, BA and VS As Fig. 2(A) shown, there was a continuous weight loss in TG curves of OSFA and BA. The maximum weight loss (19.08%) of BA was higher than that (4.56%) of OSFA, indicating there were more decomposition of components in BA. The little weight loss of 4.56% of OSFA was mainly due to the decomposition of some crystals, and thus it can be considered as an inert component below 1100 °C. The weight loss of 6.84% of BA at 20–426.65 °C was attributed to the removal of adsorbed water (He et al., 2012), and the weight loss of 8.24% at 426.65–740.10 °C was caused by some volatile components. Between 740.10 and 1100 °C, the trend of weight loss of BA decreased and the loss of 4.00% was mainly due to the decomposition of some crystals. For VS, there was almost no weight loss or increase from room temperature to 1100 °C. DTA investigations were carried out to determine the sintering temperatures used in producing the glass ceramic composites. For the DTA curve of OSFA shown in Fig. 2(B), it was comparative smooth and no remarkable endothermic or exothermal peaks were observed. However, the phase transformation temperatures at around 418.89 and 729.07 °C were observed in the DTA curve of BA, which were related to the main weight loss temperatures in the corresponding TG curve. As for VS, three exothermal peaks indicated that the phase transformation process may occur in the temperature interval 661.23–990.21 °C. Based on the results, the temperature interval 850–1000 °C was selected as the tested sintering temperatures for subsequent experiments.
Table 1 The chemical compositions (wt.%) and total heavy metals content (mg/kg) of OSFA and BA. Compositions
OSFA
BA
SiO2 CaO Al2O3 Fe2O3 MgO SO3 K2O P2O5 Na2O MnO Cl TiO2 Others
66.23 6.18 11.89 8.20 1.45 1.83 2.02 0.13 0.59 0.22 – 0.97 0.29
22.00 42.90 5.95 5.23 3.96 5.30 1.47 3.73 1.78 – 2.33 1.08 4.27
Element (mg/kg) Cd Cr Cu Zn Ba
20 314 2560 267 1640
146 67 9290 6510 2450
187
Fig. 1. XRD patterns of OSFA and BA.
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stable, leading to higher phase transition temperatures, and thus SiO2 phase was still the main crystalline phase in the RAM samples. In addition, the XRD patterns of SCS samples were similar to those of VS samples, but the corresponding intensities of diffraction peaks were weaker than those of VS samples. It is because that the excess energy was fixed in the vitrified amorphous slag due to the quenching process and it remained metastable state. As for the slow cooling slag, the excess energy was released in the slow cooling process and thus it was stable state. From the viewpoint of thermodynamics analysis, the vitrified amorphous slag was more inclined to release excess energy by crystallization. Therefore, the intensities of diffraction peaks of crystals in VS samples were stronger than those of SCS samples under the same thermal treatment condition.
3.4. The effect of sintering temperature and OSFA content on the properties of glass ceramic composites
Fig. 2. TG and DTA curves of VS, OSFA and BA.
3.3. XRD analysis of glass ceramic composites Fig. 3 shows the phase transformation process of vitrified slag (VS) with various percentages of OSFA content for 2 h at temperatures ranging from 850 to 1000 °C. For the sample without the addition of OSFA, there were almost no diffraction peaks in crystals when sintered at 850 °C (Fig. 3(A)), indicating the sintering temperature was not high enough for new phase formation and growth. However, the signal of CaAl2Si2O8 phase was detected when sintering temperature increased to 900–1000 °C (Fig. 3(B–D)). It is because when the sample was sintered at higher temperatures, the sintering densification of vitrified slag and oil shale fly ash was promoted and all the ions were arranged to form CaAl2Si2O8 phase. In the case of adding OSFA, the CaAl2Si2O8 phase and SiO2 phase were detected in all tested samples, and the relative intensity of SiO2 phase increased and that of CaAl2Si2O8 phase decreased, indicating that excess oil shale fly ash hindered the viscous flow of the glass and the diffusion of atoms. Moreover, SiO2 phase only existed in the samples that were added oil shale fly ash. Combining with the results of XRD shown in Fig. 1, it indicated that the detected SiO2 phase was derived from oil shale fly ash. XRD results of SCS and RAM samples sintered at 1000 °C for 2 h are shown in Fig. 4. It can be seen that SiO2 phase was the main crystalline phase and CaAl2Si2O8 phase was the minor phase in all RAM samples. However, the main crystalline phase of sintered SCS samples was identified as CaAl2Si2O8 phase, and a small amount of SiO2 phase was detected with the addition of OSFA. Combining with the results of TG–DTA curves (Fig. 2), it indicated that the crystalline structure of components of raw materials was
Four groups of samples (VS, VS10, VS20 and VS30) were sintered at 850, 900, 950 and 1000 °C for 2 h, respectively. The important parameters of density, sintering shrinkage, weight loss on ignition and compressive strength are shown in Fig. 5. It can be seen that the sample without the addition of OSFA showed the best physical and mechanical properties for the same heat treatment condition. For the same sample, the values of four performance indexes increased with the variation of sintering temperature, indicating that sintering temperature was an important factor in the production of glass ceramic composite from vitrified slag and oil shale fly ash. In the study, the densification of sintering body was achieved by viscous sintering and grains rearrangement, which was controlled by the viscosity of parent glass during the heat treatment. Therefore, a higher sintering temperature was beneficial to the generation of liquid phase and crystals growth. Independent of the sintering temperature, the variation tendency of sintering shrinkage, density and compressive strength observably decreased with the increase of OSFA content, but the variation tendency of weight loss was just the opposite. The densification of sintering body is attributed to viscous sintering, but the oil shale fly ash was considered as an inert substance below 1000 °C by analyzing the DTA curve (Fig. 2B). Therefore, with the proportion of oil shale fly ash to vitrified slag increased, the densification caused by the viscous flow of parent glass was hindered by the oil shale fly ash and then affected the mechanical properties of the products. The phenomenon that the weight loss increased with the increase of OSFA content can be explained by the TG curves (Fig. 2), which showed that the weight loss of oil shale fly ash was higher than that of vitrified slag. Previous researches (Bernardo et al., 2009; Cheng et al., 2011) have shown that an increase in the strength of glass ceramic composites was a result of more crystals. In the study, with the increase of the proportion of oil shale fly ash to vitrified slag, the product performances decreased, indicating a decreased amount of crystals in the products. The product (vitrified slag content of 80%, oil shale fly ash content of 20%, sintering temperature of 1000 °C and sintering time of 2 h) showed the properties of density of 1.92 ± 0.05 g/ cm3, weight loss on ignition of 6.14 ± 0.18%, sintering shrinkage of 22.06 ± 0.6% and compressive strength of 67 ± 14 MPa, indicating it was a applicable waste-based material compared to some other materials as reported in previous researches (e.g. glass ceramics, bricks, ceramics and granite) and confirmed the possibility of engineering applications (Cheng, 2004; Cheng et al., 2011; Guo et al., 2014b; Karamanov et al., 2003). Specially, the energy consumption in the production of the glass ceramic composites by the modified method can be reduced a lot compared to conventional vitrification and sintering process, which is important for the
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Fig. 3. XRD patterns of the glass ceramic composites VS, VS10, VS20 and VS30 at various sintering temperatures; (A) 850 °C, (B) 900 °C, (C) 950 °C, and (D) 1000 °C.
more inclined to release excess energy by crystallization, so there were more crystals in VS samples than SCS and RAM samples for the same sintering temperature and the same addition of oil shale fly ash. The speculation can also be identified by XRD analysis (Figs. 3 and 4) and performance comparison results (Fig. 5). On the other hand, the values of sintering shrinkage, density and compressive strength decreased with the increase of OSFA content, indicating that the increasing dosages of oil shale fly ash resulted in an increasing temperature of liquid phase formation and thus hindered the densification of sintering body. The variation tendency of weight loss on ignition in Fig. 6(B) indicated that there was more decomposition of components in raw materials than molten slag, which was agreed with the results of TG curves (Fig. 2(A)). 3.5. The effect of sintering temperature and OSFA content on the chemical resistances of glass ceramic composites
Fig. 4. XRD results of SCS and RAM samples sintered at 1000 °C for 2 h.
implement widespread availability of large-scale industrial and municipal wastes. Fig. 6 compares the physical and mechanical properties of these three kinds of materials (VS, SCS and RAM) sintered at 1000 °C for 2 h. The values of sintering shrinkage, density and compressive strength of the products from vitrified slag (VS) with various percentages of OSFA contents were the highest for the same test conditions. The above analysis showed that an increase in the product performances was a result of more crystals. Excess energy was fixed in the vitrified slag due to the quench process and it was
In Fig. 7, three acid and alkaline solutions of H2SO4, HAC and NaOH were selected to evaluate the chemical resistances of glass ceramic composites (VS, VS10, VS20 and VS30) sintered at various temperatures. All the samples had better chemical resistance when soaked in weak acid (HAC) and alkaline (NaOH) than when soaked in strong acid (H2SO4). The weight loss of the products decreased for the same sample with the rising of the sintering temperature. However, when the samples were sintered at the same temperature, the weight loss of the products increased with the increase of the proportion of oil shale fly ash to vitrified slag. This is because that large amount of acid and alkaline components (e.g. SiO2, CaO, Al2O3, Fe2O3) were in raw materials, leading a high tendency to react with strong acids, thus leading to the high weight loss with
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Fig. 5. The characteristics of sintered VS samples; (A) sintering shrinkage, (B) weight loss on ignition, (C) density, and (D) compressive strength.
Fig. 6. The characteristics of VS, SCS and RAM samples sintered at 1000 °C for 2 h; (A) sintering shrinkage, (B) weight loss on ignition, (C) density, and (D) compressive strength.
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Fig. 7. The chemical resistances of sintered VS samples in different solutions; (A) H2SO4, (B) NaOH, and (C) HAC.
Fig. 8. The chemical resistances of VS, SCS and RAM samples sintered at 1000 °C for 2 h in different solutions; (A) H2SO4, (B) NaOH, and (C) HAC.
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Table 2 Concentrations of selected toxic heavy metals in the leachates of the glass ceramic composites sintered at 1000 °C for 2 h (mg L1). Toxicity metal elements
Cd
Cr
Cu
Zn
Ba
VS VS10 VS30 SCS SCS10 SCS30 RAM RAM10 RAM30 Toxicity thresholds
N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 1
0.02 ± 0.001 0.05 ± 0.003 0.04 ± 0.001 0.07 ± 0.002 0.10 ± 0.004 0.10 ± 0.006 0.48 ± 0.021 0.47 ± 0.030 1.03 ± 0.052 15
0.04 ± 0.002 0.07 ± 0.001 0.08 ± 0.002 0.06 ± 0.001 0.09 ± 0.003 0.10 ± 0.005 0.04 ± 0.002 0.05 ± 0.003 0.05 ± 0.002 100
0.13 ± 0.009 0.15 ± 0.020 0.19 ± 0.021 0.15 ± 0.008 0.26 ± 0.010 0.29 ± 0.032 0.28 ± 0.019 0.34 ± 0.022 0.18 ± 0.010 100
0.02 ± 0.001 0.06 ± 0.001 0.08 ± 0.003 0.06 ± 0.002 0.10 ± 0.008 0.13 ± 0.010 0.09 ± 0.007 0.10 ± 0.010 0.18 ± 0.022 100
N.D. Indicates not detected.
the increase of OSFA content (Cheng et al., 2011; Vu et al., 2012). The decrease of the weight loss of glass ceramic composites may be a result of more crystals, because the corrosion resistance of the crystalline phases was generally much higher than that of glass phases. Although the glass ceramic composites VS10, VS20 and VS30 showed higher weight loss than that of glass ceramic composite VS, they still showed acceptable chemical resistances compared to previous researches (Cheng et al., 2002; Cheng, 2004; Vu et al., 2012). Fig. 8 shows the variation of chemical resistances of VS, SCS and RAM samples sintered at 1000 °C for 2 h, respectively. The chemical stability of glass ceramic materials is directly associated with the composition and quantity of crystalline phases. Glass ceramic composite VS showed lower weight loss and better chemical resistance in both acid and alkaline solutions, indicating that anorthite phase was more resist to the chemical corrosion. The chemical resistances of SCS samples were lower than those of VS samples, indicating that there were fewer crystals in SCS samples, which was consistent with the analysis results of XRD and physical– mechanical properties. There was almost no phase transition process occurred in the RAM samples, so the maximum weight loss for RAM samples indicated that the main components of raw materials reacted with acids and alkaline solutions and thus leading to higher weight loss. According to above analysis, the stronger bonding function in well crystallized glass ceramic network was more resist to chemical corrosion. 3.6. Leaching toxicity The leaching performances of glass ceramic composites were evaluated using US EPA TCLP Method 1311 (EPA, 1992) and all the results are summarized in Table 2. The vitrified slag without and with the addition of 10% and 30% of oil shale fly ash were selected for the leaching tests, meanwhile the SCS and RAM samples without and with the addition of 10% and 30% of oil shale fly ash were carried out as the contrast experiments. It can be seen that the leaching concentrations of heavy metals in the tested samples were much lower than the regulatory standard limits required by US EPA. Under the condition of the same addition of oil shale fly ash and same sintering temperature, the concentrations of heavy metals in the leachates of VS samples were lower than those of SCS and RAM samples, indicating that the proposed method had good immobilization effect on the heavy metals. It could be ascribed to more crystals and higher bonding energy of crystals in the products. In the process, the heavy metals ions could replace with other ions and successfully fixed into the crystal structures of glass ceramics and were strongly bonded inside the structure of the crystalline phases. Therefore, anorthite phase was identified as an ideal crystalline phase for the immobilization of heavy metals. In short, the results showed that the glass ceramic composites produced were sufficiently stable and the leaching risk of heavy metal was low.
4. Conclusions The study highlighted a modified method for recycling oil shale fly ash and MSWI bottom ash into a low-cost glass ceramic composite. They were pre-mixed, vitrified into slag and converted into the glass ceramic composites with the addition of various percentages of oil shale fly ash. XRD results revealed that the crystalline phases of glass ceramic composites were anorthite (CaAl2Si2O8) and quartz (SiO2). Sintering shrinkage, weight loss on ignition, density and compressive strength of glass ceramic materials were tested to determine the optimum preparation condition and study the co-sintering mechanism of vitrified slag and oil shale fly ash. With the proportion of oil shale fly ash to vitrified slag increased, it hindered the densification of sintering body that caused by the viscous flow of parent glass and thus affected the mechanical properties of the product. However, the glass ceramic composite (vitrified slag content of 80%, oil shale fly ash content of 20%, sintering temperature of 1000 °C and sintering time of 2 h) showed applicable mechanical properties compared to previous researches. The chemical resistances and heavy metals leaching results confirmed the possibility of engineering applications. In particular, the energy consumption in the production of glass ceramic composite was reduced a lot compared to conventional vitrification and sintering method, which is important for the implement widespread availability of large-scale industrial and municipal wastes. The research demonstrated the feasibility of recycling oil shale fly ash and MSWI bottom ash into a low-cost glass ceramic composite by a modified method and it will play an important role to eliminate the storage and pollution of these large-scale wastes and generate economic benefits.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Development of a sintering process for recycling oil shale fly ash and municipal solid waste incineration bottom ash into glass ceramic composite Zhikun Zhang, Lei Zhang, Aimin Li ⇑ Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science & Technology, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2014 Accepted 30 December 2014 Available online 31 January 2015 Keywords: Glass ceramic composite Heavy metals MSWI bottom ash Oil shale fly ash Sintering Vitrification
a b s t r a c t Oil shale fly ash and municipal solid waste incineration bottom ash are industrial and municipal by-products that require further treatment before disposal to avoid polluting the environment. In the study, they were mixed and vitrified into the slag by the melt-quench process. The obtained vitrified slag was then mixed with various percentages of oil shale fly ash and converted into glass ceramic composites by the subsequent sintering process. Differential thermal analysis was used to study the thermal characteristics and determine the sintering temperatures. X-ray diffraction analysis was used to analyze the crystalline phase compositions. Sintering shrinkage, weight loss on ignition, density and compressive strength were tested to determine the optimum preparation condition and study the co-sintering mechanism of vitrified amorphous slag and oil shale fly ash. The results showed the product performances increased with the increase of sintering temperatures and the proportion of vitrified slag to oil shale fly ash. Glass ceramic composite (vitrified slag content of 80%, oil shale fly ash content of 20%, sintering temperature of 1000 °C and sintering time of 2 h) showed the properties of density of 1.92 ± 0.05 g/cm3, weight loss on ignition of 6.14 ± 0.18%, sintering shrinkage of 22.06 ± 0.6% and compressive strength of 67 ± 14 MPa. The results indicated that it was a comparable waste-based material compared to previous researches. In particular, the energy consumption in the production process was reduced compared to conventional vitrification and sintering method. Chemical resistance and heavy metals leaching results of glass ceramic composites further confirmed the possibility of its engineering applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction As one of the most important crude oil substitute resources, oil shale has been widely used to produce liquid shale oil due to the increase in the price of crude oil (Guo et al., 2014a; Jiang et al., 2007; Reinik et al., 2011). It is reported that shale oil (calculated based on the in situ oil shale) accounted for about 400 billion tons of oil that is higher than worldwide total for traditional crude oil (about 300 billion tons) (Han et al., 2014; Qian et al., 2008). However, the resulting oil shale fly ash (OSFA) is considered environmentally hazardous due to high alkalinity and heavy metals concentrations (e.g. Cr, Cd, Zn, Pb, Cu) (Blinova et al., 2012; Reinik et al., 2011). In order to reduce the hazards of oil shale fly Abbreviations: MSWI, municipal solid waste incineration; OSFA, oil shale fly ash; BA, bottom ash; VS, vitrified slag; RAM, raw ash mixture of 80 wt.% oil shale fly ash and 20 wt.% bottom ash; SCS, slow cooling slag. ⇑ Corresponding author. E-mail address: [email protected] (A. Li). http://dx.doi.org/10.1016/j.wasman.2014.12.028 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
ash and achieve its resource utilization, the production of glass ceramics was studied in our previous study (Luan et al., 2010) and the results showed that it would be suitable raw material to synthesize glass ceramics by adding analytic reagent calcium oxides. Due to unbalance components in some solid wastes, in practice, the supplementing of some oxides or natural resources was necessary measurement to obtain desired crystals and good performances (Kang and Kang, 2012; Wang et al., 2010; Wu et al., 2013). From the viewpoint of natural resources conservation and maximal (100%) waste utilization, a cost-effective method involving the cheaper raw materials and economical process will be more popular for the recycling of large scale wastes. Therefore, the production of glass ceramic composites by utilizing various wastes with complementary components seems to be an attractive strategy. On the other hand, increasing incineration plants in China exhaust large quantities of wastes in the form of bottom ash (BA), leading to disposal, economical, and environmental problems. The generated volume of bottom ash usually ranged between 10% and
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12% of the initial volume of the wastes and weight between 20% and 35% of the initial wastes (Andreola et al., 2008; Barbieri et al., 2002; Monteiro et al., 2008). It is also identified as a hazardous waste due to the release of hazardous heavy metals and chlorides into the environment. Municipal solid waste incineration bottom ash generally contains high level of calcium content because the calcium hydroxide is injected into the waste gas stream to neutralize acid gas. Based on the analysis above, the comprehensive utilization of oil shale fly ash with high silica content and bottom ash with high calcium oxide content might be an attractive strategy to synthesize glass ceramics in which silica and calcium oxide are glass network former and modifier. Glass ceramics as a kind of promising material have attracted much attention due to a variety of unique properties. Recent years, the conversion of wastes into glass ceramics has gradually become an important method to improve the recycling of hazardous inorganic wastes into value-added materials while simultaneously reducing the leaching concentrations of heavy metals. Various hazardous industrial (Erol et al., 2008; Kavouras et al., 2007; Zhao et al., 2012) and municipal wastes (Andreola et al., 2008; Cheng et al., 2011; Cyr et al., 2012; Yang et al., 2008) have been reported to synthesize glass ceramic materials by devitrifying a glass by single or two-stage heat treatment. Among these methods, the hightemperature melting as one of the most widely used methods could greatly reduce the volume of wastes and stabilize the heavy metals into the glass matrix, but the process (1400–1600 °C) is general energy-intensive and therefore expensive, which does not meet the economic strategy in terms of the recycling of wastes. Another common option for glass ceramics production is a direct heat treatment process. Although the raw materials are not melted, it need to be shaped firstly by compacting the mixed powder and then sinter the compact at targeted heat treatment temperatures (Aloisi et al., 2006; Appendino et al., 2004; Barbieri et al., 2002; Bernardo et al., 2009; Tang et al., 2013). It is also a cost in producing the powder and there are some limitations on the size and shape of compositions that may be compacted. Therefore, it is important to develop a new processing method in order to implement widespread availability of these hazardous industrial and municipal wastes. Vitrification of hazardous wastes has been proved to be an attractive method for a safe immobilization of heavy metals in the glass matrix and the obtained vitrified slag (VS) can convert into glass ceramic materials through a two-stage heat treatment (Cheng, 2004; Lin et al., 2006; Luan et al., 2010; Kavouras et al., 2007). From the viewpoint of energy consumption reduction, if a certain amount of raw materials can directly reuse without melting, the energy consumption in the process can be reduced compared to common vitrification and sintering method. Based on this consideration, a more economical and simple method of co-sintering of vitrified amorphous slag and oil shale fly ash is proposed for the recycling of these two kinds of fly ash in the study. Oil shale fly ash and bottom ash were vitrified into the slag and then converted into the glass ceramic composite with the addition of various percentages of oil shale fly ash by the subsequent sintering process. The objective of our research is to explore the possibility of producing waste-based glass ceramic composites by the proposed method. To validate this hypothesis, the present study has been conducted to (i) produce glass ceramic composites using vitrified slag with various percentages of oil shale fly ash; (ii) characterize some important indexes of the products (density, weight loss on ignition, sintering shrinkage, compressive strength, chemical resistance and heavy metal leaching); (iii) study the phase transformation process and co-sintering mechanism of vitrified amorphous slag and oil shale fly ash by thermal analysis and XRD analysis. The results of the study will provide fundamental knowledge for the development of
waste-based glass ceramic composites through a simple and low energy consumption process. 2. Materials and methods 2.1. Raw material OSFA used in this study was obtained from the thermal power plant, Jilin, China; and MSWI BA was obtained from municipal solid waste incinerator, Dalian, China. OSFA and MSWI BA that removed any coarse impurities were dried at 105 °C for 24 h in an electric dry oven, and then were graded to pass sieve NO. 150 (the diameter of mesh is 106 lm) for subsequent experiments. As a result of preliminary experiments, the raw ash mixture of 80% OSFA and 20% BA (marked as RAM) by weight were used in this study. 2.2. The preparation of glass ceramic composites A total amount of 80 g RAM were separately put in a corundum crucible and melted at 1500 °C for 1 h to ensure complete melting. The melt was rapidly poured into water to obtain the vitrified slag. Meanwhile, the slow cooling slag (SCS) was obtained at a cooling rate of 4 °C/min from the melting temperature to room temperature. Then they were dried, ground and sieved to powders below 106 lm for subsequent experiments. The vitrified slag (VS) with the addition of 10, 20 and 30 wt.% OSFA, denoting to VS10, VS20 and VS30, were homogenized in ball-mill for 1 h. The mixed powder was filled in the corundum crucible and a single sintering scheme with 2 h dwelling time was used for the targeted temperatures ranged from 850 to 1000 °C. The sintered samples were then cooled inside the furnace to room temperature to obtain the glass ceramic composites. Meanwhile, SCS and RAM samples with the addition of 10, 20 and 30 wt.% OSFA, labelled SCS10, SCS20, SCS30, RAM10, RAM20 and RAM30, were sintered at 1000 °C for 2 h, respectively. 2.3. Analysis and methods 2.3.1. Chemical analysis The chemical compositions of OSFA and BA were determined by X-ray fluorescence (XRF) (PDA-5500II, Shimadzu, Japan). The heavy metal concentrations of OSFA and BA were determined by inductively coupled plasma–optical emission spectrometry (ICP–OES). 2.3.2. Thermal analysis Thermal behavior of OSFA, BA and VS were tested by thermogravimetric (TG) and differential thermal analysis (DTA) (TG/DTA 6300, NSK, Japan) to determine the sintering temperatures. About 20 mg of samples was heated to 1100 °C at the rate of 10 °C/min in nitrogen atmosphere. 2.3.3. X-ray diffraction (XRD) analysis Crystalline phase components of raw materials and glass ceramic composites were analyzed by XRD (D/MAX-2400, Rigaku, Japan). They were analyzed over a range of 2h angles from 10° to 60° with Cu Ka radiation at 4° min1 scanning speed. The measured results were studied by the standard powder diffraction database of International Centre for Diffraction Data (ICDD PDF-2 Release 2004). 2.3.4. Leaching toxicity Toxicity assessment of selected glass ceramic composites were tested in a leaching experiment modified from the U.S. EPA SW-846 Method 1311-TCLP, using a pH 4.93 extraction fluid as
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the leaching fluid. Each leaching vial was filled with 20 mL of TCLP extraction fluid and 1 g of sample and the soaking time was 20 days. The concentrations of heavy metals in the leaching solution were determined by ICP–OES. 2.3.5. Characterization of glass ceramic composites The density, weight loss on ignition, sintering shrinkage, compressive strength, chemical resistance and heavy metal leaching of glass ceramic composites were tested to determine the optimum condition and evaluate the environmental risk. The density and water absorption were tested according to Archimedes method. Weight loss on ignition was calculated from Eq. (1), and the sintering shrinkage was calculated from Eq. (2). The compression strength tests were done on the smooth surface of the products at a loading speed of 1 mm/min using a universal test machine (CSS-2220, Changchun Testing Machine Institute, China). Bulk samples were treated at 100 °C for 1 h in 5% acid/alkali solutions, and the chemical resistance was evaluated by the mass loss of the samples from Eq. (3). For each test, three samples were tested in order to get accurate results.
WL ¼
WBS WAS WBS
ð1Þ
where WL is the weight loss; WBS is the weight of samples before sintering; WAS is the weight of samples after sintering.
SS ¼
VBS VAS VBS
ð2Þ
where SS is the sintering shrinkage; VBS is the volume of samples before sintering; VAS is the volume of samples after sintering.
CR ¼
WAB WBB WAB
ð3Þ
where CR is the chemical resistance; WAB is the weight of samples before boiling; WBB is the weight of samples after boiling. 3. Results and discussion 3.1. Chemical compositions and XRD analysis of raw materials The chemical compositions and total heavy metals content of OSFA and BA are given in Table 1. It can be seen that SiO2, Al2O3, Fe2O3 and CaO were the main compositions for oil shale fly ash,
while CaO and SiO2 were major in bottom ash. It indicated that the mixture of OSFA and BA was suitable raw material to produce glass ceramic materials. It will be a double-win choice, because it not only can turn OSFA and BA into resource, but also can reduce the cost of raw materials, achieving the maximal (100%) waste utilization. XRD analysis of OSFA and BA are shown in Fig. 1. The crystals components of OSFA were mainly quartz SiO2 (PDF#79-1906), and there were a little anorthite [Ca,Na][Al,Si]2Si2O8 (PDF#090465) and Augite Ca[Mg,Fe]Si2O6 (PDF#24-0203). The main crystalline phases of BA were rankinite Ca3Si2O7 (PDF#22-0539) and quartz SiO2, and the coexisting crystalline phases were brianite Na2CaMg[PO4]2 (PDF#29-1192) and cliniferrosilite FeSiO3 (PDF#17-0548). It can be seen that the XRD results were associated to the chemical components of OSFA and BA as shown in Table 1.
3.2. Thermal analysis of OSFA, BA and VS As Fig. 2(A) shown, there was a continuous weight loss in TG curves of OSFA and BA. The maximum weight loss (19.08%) of BA was higher than that (4.56%) of OSFA, indicating there were more decomposition of components in BA. The little weight loss of 4.56% of OSFA was mainly due to the decomposition of some crystals, and thus it can be considered as an inert component below 1100 °C. The weight loss of 6.84% of BA at 20–426.65 °C was attributed to the removal of adsorbed water (He et al., 2012), and the weight loss of 8.24% at 426.65–740.10 °C was caused by some volatile components. Between 740.10 and 1100 °C, the trend of weight loss of BA decreased and the loss of 4.00% was mainly due to the decomposition of some crystals. For VS, there was almost no weight loss or increase from room temperature to 1100 °C. DTA investigations were carried out to determine the sintering temperatures used in producing the glass ceramic composites. For the DTA curve of OSFA shown in Fig. 2(B), it was comparative smooth and no remarkable endothermic or exothermal peaks were observed. However, the phase transformation temperatures at around 418.89 and 729.07 °C were observed in the DTA curve of BA, which were related to the main weight loss temperatures in the corresponding TG curve. As for VS, three exothermal peaks indicated that the phase transformation process may occur in the temperature interval 661.23–990.21 °C. Based on the results, the temperature interval 850–1000 °C was selected as the tested sintering temperatures for subsequent experiments.
Table 1 The chemical compositions (wt.%) and total heavy metals content (mg/kg) of OSFA and BA. Compositions
OSFA
BA
SiO2 CaO Al2O3 Fe2O3 MgO SO3 K2O P2O5 Na2O MnO Cl TiO2 Others
66.23 6.18 11.89 8.20 1.45 1.83 2.02 0.13 0.59 0.22 – 0.97 0.29
22.00 42.90 5.95 5.23 3.96 5.30 1.47 3.73 1.78 – 2.33 1.08 4.27
Element (mg/kg) Cd Cr Cu Zn Ba
20 314 2560 267 1640
146 67 9290 6510 2450
187
Fig. 1. XRD patterns of OSFA and BA.
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stable, leading to higher phase transition temperatures, and thus SiO2 phase was still the main crystalline phase in the RAM samples. In addition, the XRD patterns of SCS samples were similar to those of VS samples, but the corresponding intensities of diffraction peaks were weaker than those of VS samples. It is because that the excess energy was fixed in the vitrified amorphous slag due to the quenching process and it remained metastable state. As for the slow cooling slag, the excess energy was released in the slow cooling process and thus it was stable state. From the viewpoint of thermodynamics analysis, the vitrified amorphous slag was more inclined to release excess energy by crystallization. Therefore, the intensities of diffraction peaks of crystals in VS samples were stronger than those of SCS samples under the same thermal treatment condition.
3.4. The effect of sintering temperature and OSFA content on the properties of glass ceramic composites
Fig. 2. TG and DTA curves of VS, OSFA and BA.
3.3. XRD analysis of glass ceramic composites Fig. 3 shows the phase transformation process of vitrified slag (VS) with various percentages of OSFA content for 2 h at temperatures ranging from 850 to 1000 °C. For the sample without the addition of OSFA, there were almost no diffraction peaks in crystals when sintered at 850 °C (Fig. 3(A)), indicating the sintering temperature was not high enough for new phase formation and growth. However, the signal of CaAl2Si2O8 phase was detected when sintering temperature increased to 900–1000 °C (Fig. 3(B–D)). It is because when the sample was sintered at higher temperatures, the sintering densification of vitrified slag and oil shale fly ash was promoted and all the ions were arranged to form CaAl2Si2O8 phase. In the case of adding OSFA, the CaAl2Si2O8 phase and SiO2 phase were detected in all tested samples, and the relative intensity of SiO2 phase increased and that of CaAl2Si2O8 phase decreased, indicating that excess oil shale fly ash hindered the viscous flow of the glass and the diffusion of atoms. Moreover, SiO2 phase only existed in the samples that were added oil shale fly ash. Combining with the results of XRD shown in Fig. 1, it indicated that the detected SiO2 phase was derived from oil shale fly ash. XRD results of SCS and RAM samples sintered at 1000 °C for 2 h are shown in Fig. 4. It can be seen that SiO2 phase was the main crystalline phase and CaAl2Si2O8 phase was the minor phase in all RAM samples. However, the main crystalline phase of sintered SCS samples was identified as CaAl2Si2O8 phase, and a small amount of SiO2 phase was detected with the addition of OSFA. Combining with the results of TG–DTA curves (Fig. 2), it indicated that the crystalline structure of components of raw materials was
Four groups of samples (VS, VS10, VS20 and VS30) were sintered at 850, 900, 950 and 1000 °C for 2 h, respectively. The important parameters of density, sintering shrinkage, weight loss on ignition and compressive strength are shown in Fig. 5. It can be seen that the sample without the addition of OSFA showed the best physical and mechanical properties for the same heat treatment condition. For the same sample, the values of four performance indexes increased with the variation of sintering temperature, indicating that sintering temperature was an important factor in the production of glass ceramic composite from vitrified slag and oil shale fly ash. In the study, the densification of sintering body was achieved by viscous sintering and grains rearrangement, which was controlled by the viscosity of parent glass during the heat treatment. Therefore, a higher sintering temperature was beneficial to the generation of liquid phase and crystals growth. Independent of the sintering temperature, the variation tendency of sintering shrinkage, density and compressive strength observably decreased with the increase of OSFA content, but the variation tendency of weight loss was just the opposite. The densification of sintering body is attributed to viscous sintering, but the oil shale fly ash was considered as an inert substance below 1000 °C by analyzing the DTA curve (Fig. 2B). Therefore, with the proportion of oil shale fly ash to vitrified slag increased, the densification caused by the viscous flow of parent glass was hindered by the oil shale fly ash and then affected the mechanical properties of the products. The phenomenon that the weight loss increased with the increase of OSFA content can be explained by the TG curves (Fig. 2), which showed that the weight loss of oil shale fly ash was higher than that of vitrified slag. Previous researches (Bernardo et al., 2009; Cheng et al., 2011) have shown that an increase in the strength of glass ceramic composites was a result of more crystals. In the study, with the increase of the proportion of oil shale fly ash to vitrified slag, the product performances decreased, indicating a decreased amount of crystals in the products. The product (vitrified slag content of 80%, oil shale fly ash content of 20%, sintering temperature of 1000 °C and sintering time of 2 h) showed the properties of density of 1.92 ± 0.05 g/ cm3, weight loss on ignition of 6.14 ± 0.18%, sintering shrinkage of 22.06 ± 0.6% and compressive strength of 67 ± 14 MPa, indicating it was a applicable waste-based material compared to some other materials as reported in previous researches (e.g. glass ceramics, bricks, ceramics and granite) and confirmed the possibility of engineering applications (Cheng, 2004; Cheng et al., 2011; Guo et al., 2014b; Karamanov et al., 2003). Specially, the energy consumption in the production of the glass ceramic composites by the modified method can be reduced a lot compared to conventional vitrification and sintering process, which is important for the
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Fig. 3. XRD patterns of the glass ceramic composites VS, VS10, VS20 and VS30 at various sintering temperatures; (A) 850 °C, (B) 900 °C, (C) 950 °C, and (D) 1000 °C.
more inclined to release excess energy by crystallization, so there were more crystals in VS samples than SCS and RAM samples for the same sintering temperature and the same addition of oil shale fly ash. The speculation can also be identified by XRD analysis (Figs. 3 and 4) and performance comparison results (Fig. 5). On the other hand, the values of sintering shrinkage, density and compressive strength decreased with the increase of OSFA content, indicating that the increasing dosages of oil shale fly ash resulted in an increasing temperature of liquid phase formation and thus hindered the densification of sintering body. The variation tendency of weight loss on ignition in Fig. 6(B) indicated that there was more decomposition of components in raw materials than molten slag, which was agreed with the results of TG curves (Fig. 2(A)). 3.5. The effect of sintering temperature and OSFA content on the chemical resistances of glass ceramic composites
Fig. 4. XRD results of SCS and RAM samples sintered at 1000 °C for 2 h.
implement widespread availability of large-scale industrial and municipal wastes. Fig. 6 compares the physical and mechanical properties of these three kinds of materials (VS, SCS and RAM) sintered at 1000 °C for 2 h. The values of sintering shrinkage, density and compressive strength of the products from vitrified slag (VS) with various percentages of OSFA contents were the highest for the same test conditions. The above analysis showed that an increase in the product performances was a result of more crystals. Excess energy was fixed in the vitrified slag due to the quench process and it was
In Fig. 7, three acid and alkaline solutions of H2SO4, HAC and NaOH were selected to evaluate the chemical resistances of glass ceramic composites (VS, VS10, VS20 and VS30) sintered at various temperatures. All the samples had better chemical resistance when soaked in weak acid (HAC) and alkaline (NaOH) than when soaked in strong acid (H2SO4). The weight loss of the products decreased for the same sample with the rising of the sintering temperature. However, when the samples were sintered at the same temperature, the weight loss of the products increased with the increase of the proportion of oil shale fly ash to vitrified slag. This is because that large amount of acid and alkaline components (e.g. SiO2, CaO, Al2O3, Fe2O3) were in raw materials, leading a high tendency to react with strong acids, thus leading to the high weight loss with
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Fig. 5. The characteristics of sintered VS samples; (A) sintering shrinkage, (B) weight loss on ignition, (C) density, and (D) compressive strength.
Fig. 6. The characteristics of VS, SCS and RAM samples sintered at 1000 °C for 2 h; (A) sintering shrinkage, (B) weight loss on ignition, (C) density, and (D) compressive strength.
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Fig. 7. The chemical resistances of sintered VS samples in different solutions; (A) H2SO4, (B) NaOH, and (C) HAC.
Fig. 8. The chemical resistances of VS, SCS and RAM samples sintered at 1000 °C for 2 h in different solutions; (A) H2SO4, (B) NaOH, and (C) HAC.
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Table 2 Concentrations of selected toxic heavy metals in the leachates of the glass ceramic composites sintered at 1000 °C for 2 h (mg L1). Toxicity metal elements
Cd
Cr
Cu
Zn
Ba
VS VS10 VS30 SCS SCS10 SCS30 RAM RAM10 RAM30 Toxicity thresholds
N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 1
0.02 ± 0.001 0.05 ± 0.003 0.04 ± 0.001 0.07 ± 0.002 0.10 ± 0.004 0.10 ± 0.006 0.48 ± 0.021 0.47 ± 0.030 1.03 ± 0.052 15
0.04 ± 0.002 0.07 ± 0.001 0.08 ± 0.002 0.06 ± 0.001 0.09 ± 0.003 0.10 ± 0.005 0.04 ± 0.002 0.05 ± 0.003 0.05 ± 0.002 100
0.13 ± 0.009 0.15 ± 0.020 0.19 ± 0.021 0.15 ± 0.008 0.26 ± 0.010 0.29 ± 0.032 0.28 ± 0.019 0.34 ± 0.022 0.18 ± 0.010 100
0.02 ± 0.001 0.06 ± 0.001 0.08 ± 0.003 0.06 ± 0.002 0.10 ± 0.008 0.13 ± 0.010 0.09 ± 0.007 0.10 ± 0.010 0.18 ± 0.022 100
N.D. Indicates not detected.
the increase of OSFA content (Cheng et al., 2011; Vu et al., 2012). The decrease of the weight loss of glass ceramic composites may be a result of more crystals, because the corrosion resistance of the crystalline phases was generally much higher than that of glass phases. Although the glass ceramic composites VS10, VS20 and VS30 showed higher weight loss than that of glass ceramic composite VS, they still showed acceptable chemical resistances compared to previous researches (Cheng et al., 2002; Cheng, 2004; Vu et al., 2012). Fig. 8 shows the variation of chemical resistances of VS, SCS and RAM samples sintered at 1000 °C for 2 h, respectively. The chemical stability of glass ceramic materials is directly associated with the composition and quantity of crystalline phases. Glass ceramic composite VS showed lower weight loss and better chemical resistance in both acid and alkaline solutions, indicating that anorthite phase was more resist to the chemical corrosion. The chemical resistances of SCS samples were lower than those of VS samples, indicating that there were fewer crystals in SCS samples, which was consistent with the analysis results of XRD and physical– mechanical properties. There was almost no phase transition process occurred in the RAM samples, so the maximum weight loss for RAM samples indicated that the main components of raw materials reacted with acids and alkaline solutions and thus leading to higher weight loss. According to above analysis, the stronger bonding function in well crystallized glass ceramic network was more resist to chemical corrosion. 3.6. Leaching toxicity The leaching performances of glass ceramic composites were evaluated using US EPA TCLP Method 1311 (EPA, 1992) and all the results are summarized in Table 2. The vitrified slag without and with the addition of 10% and 30% of oil shale fly ash were selected for the leaching tests, meanwhile the SCS and RAM samples without and with the addition of 10% and 30% of oil shale fly ash were carried out as the contrast experiments. It can be seen that the leaching concentrations of heavy metals in the tested samples were much lower than the regulatory standard limits required by US EPA. Under the condition of the same addition of oil shale fly ash and same sintering temperature, the concentrations of heavy metals in the leachates of VS samples were lower than those of SCS and RAM samples, indicating that the proposed method had good immobilization effect on the heavy metals. It could be ascribed to more crystals and higher bonding energy of crystals in the products. In the process, the heavy metals ions could replace with other ions and successfully fixed into the crystal structures of glass ceramics and were strongly bonded inside the structure of the crystalline phases. Therefore, anorthite phase was identified as an ideal crystalline phase for the immobilization of heavy metals. In short, the results showed that the glass ceramic composites produced were sufficiently stable and the leaching risk of heavy metal was low.
4. Conclusions The study highlighted a modified method for recycling oil shale fly ash and MSWI bottom ash into a low-cost glass ceramic composite. They were pre-mixed, vitrified into slag and converted into the glass ceramic composites with the addition of various percentages of oil shale fly ash. XRD results revealed that the crystalline phases of glass ceramic composites were anorthite (CaAl2Si2O8) and quartz (SiO2). Sintering shrinkage, weight loss on ignition, density and compressive strength of glass ceramic materials were tested to determine the optimum preparation condition and study the co-sintering mechanism of vitrified slag and oil shale fly ash. With the proportion of oil shale fly ash to vitrified slag increased, it hindered the densification of sintering body that caused by the viscous flow of parent glass and thus affected the mechanical properties of the product. However, the glass ceramic composite (vitrified slag content of 80%, oil shale fly ash content of 20%, sintering temperature of 1000 °C and sintering time of 2 h) showed applicable mechanical properties compared to previous researches. The chemical resistances and heavy metals leaching results confirmed the possibility of engineering applications. In particular, the energy consumption in the production of glass ceramic composite was reduced a lot compared to conventional vitrification and sintering method, which is important for the implement widespread availability of large-scale industrial and municipal wastes. The research demonstrated the feasibility of recycling oil shale fly ash and MSWI bottom ash into a low-cost glass ceramic composite by a modified method and it will play an important role to eliminate the storage and pollution of these large-scale wastes and generate economic benefits.
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