Heavy metal leaching and distribution in glass products from the co-melting treatment of electroplating sludge and MSWI fly ash

Journal of Environmental Management 232 (2019) 226–235

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Research article

Heavy metal leaching and distribution in glass products from the co-melting treatment of electroplating sludge and MSWI fly ash

T

Yang Yuea, Jia Zhanga, Fucheng Sunb, Simiao Wua, Yun Pana, Jizhi Zhoua,∗, Guangren Qiana,∗∗ a b

School of Environmental and Chemical Engineering, Shanghai University, No. 99, Shangda Road, Shanghai 200444, PR China Zhejiang Environmental Science & Design Institute, No. 109, Tianmushan Road, Hangzhou 310000, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Electroplating sludge MSWI fly ash Co-melting Heavy metals Vitrification

Melting is a common solidification treatment that concentrates and encapsulates heavy metals into a glass matrix for waste containing heavy metals (Chae et al., 2016). To control the risk of heavy metal leaching into the glass product, a reduction in the amount of heavy metal was achieved in a pilot-scale furnace by co-melting electroplating sludge (EPS) and municipal solid waste incineration fly ash (MSWI FA). Through the melting process, the chloride from MSWI FA led to heavy metals volatilization in the form of chlorine salts. The fly ash additionally increased heavy metals volatilization by 4%–91%. The highest volatilization ratios of Zn, Pb, Cu, Cd, Cr and Ni were 33%, 96%, 33%, 79%, 81% and 31%, respectively. The concentrations of Pb and Zn in the secondary fly ash were close to the Pb and Zn concentrations in lead-zinc ore that are required in smelting industry. Moreover, glass sand was produced after the melting treatment. With an increase in the fly ash mixing ratio, the leaching concentration of Zn and Cu decreased to 3.8 mg/L and 2.1 mg/L. The leaching concentrations of other heavy metals stayed below 1 mg/L in all cases. When the ratio of MSWI FA reached 10%, the glass sand contained the least amount of impurities and a large amount of phosphate and silicate, which were probably responsible for the stability of the heavy metals. Therefore, our results provided a promising approach to the stability of the waste by the recovery of heavy metals in the co-treatment of heavy metal-bearing wastes.

1. Introduction

melting process does not result in a safe application of the glass matrix, as the long-time leaching of the heavy metal is suspended. Therefore, the removal of heavy metals from the melting process of EPS is essential. On the other hand, municipal solid waste incineration fly ash (MSWI FA) was classified as hazardous waste because of high contents and leachability of heavy metals (Fujii et al., 2018; Raclavska et al., 2017; Wang et al., 2018; Yakubu et al., 2018). Traditional solidification can only reduce the dissolution, but not the total amount of heavy metals (Shiota et al., 2017). The melting process of MSWI FA was developed for the stabilization/solidification of heavy metal in FA. The leaching of heavy metals in MSWI FA is significantly reduced during the melting process (Jiang et al., 2009). It is noted that heavy metal volatilization is promoted during high-temperature treatment (Chiang and Hu, 2010a). At 1000 °C, approximately 95% of Pb and Cd and almost 60% of Zn are removed, and the evaporation ratio increases with an increase in temperature (Wu et al., 2015). This dynamic is attributed to the chloride in MSWI FA. On average, MSWI FA contains 10–30% of chlorine, which leads to the formation of heavy metal chloride. Due to the lower

Metal finishing and electroplating are frequently used in modern manufacturing. However, these industrial processes result in several tons of electroplating sludge (EPS) from the wastewater treatment (Tian et al., 2013). EPS is classified as hazardous waste as it contains a high amount of heavy metals (Ministry of environmental protection of China, 2016; de Souza et al., 2006; Li et al., 2010; Liu and Wang, 2008). In China, more than 100 000 tons of EPS are generated each year (Wang, 2006). Some common treatment methods for EPS include solidification (Chen et al., 2009; Lyu et al., 2016; Qian et al., 2009), heavy metals recycling (de Souza et al., 2006; Li et al., 2010; Zhuang et al., 2012), and new materials regeneration (Zhang et al., 2013, 2014). Recently, the melting process was applied to address EPS. During the melting process, inorganic mass, such as SiO2, CaO, and Al2O3 forms a glass sand for construction application. At the same time, the heavy metals are solidified in the glass sand via the encapsulation in the glass phase, leading to a decreased leaching risk (Chae et al., 2016; Idris and Saed, 2002). However, the solidification of the heavy metals after the

∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Zhou), [email protected] (G. Qian).

∗∗

https://doi.org/10.1016/j.jenvman.2018.11.053 Received 27 July 2018; Received in revised form 12 November 2018; Accepted 15 November 2018 0301-4797/ © 2018 Elsevier Ltd. All rights reserved.

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volatilization point of heavy metal chlorides compared to its oxides, the evaporation of heavy metal is avoided in the thermal treatment process of MSWI FA (Chan and Kirk, 1999; Chiang and Hu, 2010b,a; Wey et al., 2006; Zhang et al., 2016). Despite, the evaporation process of heavy metals is used to collect heavy metals via chloride addition during thermal treatment. Zn, Cd, Cu, etc. concentrated in the secondary fly ash. Based on this principal, it seems possible to obtain vitrified products with a good performance and a low content of heavy metals and low leachability by co-melting EPS and MSWI FA. Although the high cost in maintenance and energy, the supposed benefits of co-melting of hazardous waste include (1) improving the volatilization of heavy metal by the addition of chloride in MSWI FA to concentrate metal recovery from secondary fly ash, in which heavy metal levels were similar to those ore in smelting industry (Mineral resources and reserves department of China, 2013); (2) reducing the heavy metal leaching in the glass product; (3) introducing Ca into the waste mixture to decrease the melting temperature and increase the density of the glass matrix (Cheng and Chen, 2004), and (4) producing glass sand with a low heavy metal to achieve the sustainable utilization of solid wastes. Therefore, based on results in many researches, the pilot-scale industrial furnace of the co-melting of MSWI FA and EPS is essential to investigate the process of heavy metal stability. In this study, a pilot-scale furnace is used as reaction equipment to provide an example for co-melting EPS and MSWI FA. The vitrification characteristics and heavy metal behaviors during the co-melting process are analyzed. The fate of the heavy metals and the characteristics of the vitrified products are discussed. Compared with the experiment in the laboratory, the pilot plant test has more uncertainties due to the large treatment capacity, complex equipment, etc. Therefore, our results provide insight into the co-treatment of EPS and MSWI FA.

Table 1 Blending ratios of EPS/MSWI FA and auxiliary material. Serial number

EPS 1

S1-1 S1-2 S1-3 S1-4 S1-5 S2-1 S2-2 S2-3 S2-4 S2-5 S3-1 S3-2 S3-3 S3-4 S3-5 M0 M1 M2 M3 M4 M5 M6

90 80 70 60 50

EPS 2

EPS 3

MSWI FA

SiO2

Al2O3

70 0 2 5 10 20 30

9 18 27 36 45 9 18 27 36 45 9 18 27 36 45 27 27 27 27 27 27 27

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 3 3 3 3 3 3 3

90 80 70 60 50 90 80 70 60 50 0 70 68 65 60 50 40

glass phase, EPS and MSWI FA were mixed in different proportions before co-melting. Furthermore, a suitable additive agent proportion could lower the co-melting temperature and ensure the material melted completely (Hu et al., 2012). The ratio of the additive agent was set as 9:1 for co-melting the EPS and FA process because Al2O3 was only a fluxing agent; SiO2 was the important factor in the formation of a glass material to encapsulate the heavy metals.

2. Materials and methods

2.2.2. Melting process In this study, liquefied petroleum gas was used as a fuel in the melting furnace (Fig. 1). Before the melting process, the melting furnace should be preheated for nearly 2 h (In the first half hour, the heating rate was about 15 °C. In the next one and half hours, the increasing rate was about 10 °C.) to raise the temperature above 1350 °C. Considering with the affordability of equipment and energy conservation, 1300–1500 °C was thought to be a more appropriate melting temperature. After preheating the furnace, samples were added into the melting furnace through the feeding hole above the stove. After 20–30 min, the samples were melted into a liquid and allowed to flow into the water quench tank, with a glass material generated through rapid cooling. The temperature of the water in the water quench tank should be kept below 50 °C by the water circulation system. The melting furnace flue gas was emitted after treatment of the waste heat recovery system and the flue gas purification system. The flue gas was treated by semidry method and bag-filtering dust precipitator. CaO was used for flue gas treatment to ensure the contaminants concentrations met the emission regulation of hazardous waste combustion. In this case, heavy metals volatilized in the gas could be trapped and enriched in the secondary ash. The processing capacity of the melting furnace was 50 kg per hour.

2.1. Sampling of EPS and MSWI FA EPS was sampled from three plants in Shanghai and each of them was derived from the treatment of a wastewater mixture with several types of electroplating streams: Cu, Cr Zn Ni bearing wastewater. Through simple dewatering via plate-frame pressure filtration, the moisture content of the raw EPS was approximately 82%. To further reduce the moisture content, microwave heating treatment was used to ensure the moisture content was below 10%. The MSWI FA sample was collected from Shanghai Yuqiao Wastes Incineration Plant. Semidry method and bag-filtering dust precipitator were used to treat the flue gas. CaO and activated carbon were used during the flue gas treatment process. 2.2. Melting process 2.2.1. Ratio of mixture samples Mass ratios of EPS/MSWI FA and an auxiliary material are shown in Table 1. The auxiliary material was applied in this study to completely melt and produce a uniform glass material. Al2O3 is one of the most helpful and common fluxing agents and is widely used in metallurgy and glass manufacturing. In addition, enough SiO2 could ensure the product in the glass sand form and could promote the process of melting. To find the optimal content of the fluxing agent, different proportions of fluxing agent were added into three kinds of EPS. According to the estimated phase changing in melting and the preliminary laboratory test (not shown), the additive agent was prepared as a mixture of SiO2 and Al2O3 at a ratio of 9:1 to provide the low melting point of waste. The total quantity of samples with different ratios of additive agents for further melting was 100 kg. To explore the relationship between the concentration of chlorine and the volatilization rate of heavy metals, as well as the homogeneous

2.3. Analysis of samples 2.3.1. Leachability and total amount of heavy metals The leaching test for heavy metals in the glass state was the US EPA toxicity characteristics leaching procedure (TCLP) (US EPA., 1992). The total amount of heavy metals was tested by ICP-MS after digestion by HNO3 and HF in a microwave digestion instrument (Krishna et al., 2017; Lu et al., 2018; Riisom et al., 2018; Zhou et al., 2015). The data was shown in the average value and standard deviations from triplicate experiment. 227

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Fig. 1. The schematic diagram and photo of melting device.

The X-ray diffraction (XRD) patterns of the glass sand were collected on an X-ray diffractometer (3KW D/MAX2200V). The crystalline phase of the glass sand was analyzed over a range of 2θ angles from 10° to 80° at a scanning speed of 6°/min. The microstructures of the glass sand were examined by scanning electron micrographs (NAVA NANOSEM 430). All the glass sand samples were adhered to a metallic plate, and then, a thin film of Au was coated on the surface before observation.

Table 2 Main chemical composition and total amount of heavy metals in raw materials. EPS 1 Main component (wt%) 5.29 Al2O3 CaO 54.5 Cl 0.21 Fe2O3 5.57 MgO 0.32 Na2O 0.78 P2O5 10.5 SiO2 4.19 Heavy metal (mg/kg) Zn 7524 Pb 87.2 Cu 15126 Cd 64.8 Cr 41129 Ni 1858

EPS 2

EPS 3

MSWI FA

5.52 55.5 0.22 5.68 0.49 0.67 11.1 4.37

5.43 56.9 0.19 4.35 0.38 0.62 11.1 4.39

0.30 43.6 31.20 0.47 1.56 5.73 0.10 1.57

8104 103 14417 45.3 37623 1967

5296 84.8 12328 77.2 31506 1180

12917 3522 937 162 87.2 97.3

2.3.3. Thermal analysis of mixture materials The thermal stability of the mixture materials was characterized from room temperature (25 °C) up to 1400 °C by thermal analysis equipment (NETZSCH STA 449 F5). The data were obtained at a heating rate of 10 °C/min and a cooling rate of 20 °C/min. 2.3.4. Flue gas analysis The pollutants in the flue gas were examined by a professional smoke testing company. An incision of 5 cm was cut on the flue pipe for the gas sampling port and the flue gas test was conducted three times every 10 min. The air pressure was 100.9 kPa, and the temperature of the flue gas was 58 °C. 2.3.5. Extra volatilization rate To investigate the promoting effect on the volatilization of heavy metals of the mixing sample, a simple calculation was needed to demonstrate how much volatilization amount had been added. The simple calculation process was as follows:

R=

TVA − FATA × FAVR × FAR − EPSTA × EPSVR × EPSR TVA R: Extra volatilization rate TVA: Total volatilization amount of heavy metal in mixing sample FATA: Total amount of heavy metal in MSWI FA FAVR: Individual melting volatilization rate of heavy metal in MSWI FA FAR: Mixing ratio of MSWI FA EPSTA: Total amount of heavy metal in EPS EPSVR: Individual melting volatilization rate of heavy metal in EPS EPSR: Mixing ratio of EPS

Fig. 2. Ternary phase diagram analysis of MSWI FA and mixture samples.

2.3.2. The characteristic of the microstructure and crystalline phase of glass sand To distinguish whether the co-melting products form the homogeneous glass, each sample was ground into a chip before being glued between two layers of glass slides with cement and other glue. Then, all the samples were observed using a polarizing microscope (LEICA DM750P).

3. Results and discussion 3.1. Feasibility analysis of melting system Table 2 lists the main components and heavy metals content of the EPSs and MSWI FAs in this work. The main components of three EPSs 228

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Fig. 4. Volatilization ratio of heavy metals in ESPs and MSWI FA.

Fig. 5. Volatilization ratio of heavy metals in mixture materials during melting.

Fig. 3. Thermal analysis of M0, M1 and M4 (A: endothermic process, B: cooling process).

were similar to one another. The amounts of CaO were all more than 50%, and the amounts of Al2O3 and SiO2 were approximately 5%. Imaginary lines in Fig. 2 show the melting points of industrial glass with various contents of Al, Si, and Ca (Feng et al., 2016; Tianjin university press, 2010; Shaw et al., 2018). It is noted that the low melting point of the solid mixture is relative to the high Si content with a proper mass ratio of Ca and Al. According to the Ca, Si, and Al contents in the EPS in Fig. 2, the melting point of the EPS sample was estimated between 2200 °C and 2400 °C because the content of CaO was much higher than that of SiO2 and Al2O3. In comparison, the ratio of CaO in MSWI FA was higher than EPS, indicating that the melting point of MSWI FA was

Fig. 6. Volatilization amount of heavy metals in mixture samples.

higher than 2400 °C. Therefore, the energy consumption of melting EPS or MSWI FA may be very high. It was necessary to add auxiliary materials to reduce the melting point. The melting point of samples S1-1 to S1-5 in Fig. 2 became low with a rising ratio of auxiliary material. For sample S1-3 in Fig. 2, the ratio of 229

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Table 3 Extra volatilization rate of heavy metals (%).

M2 M3 M4 M5 M6

Zn

Cu

4.72 ± 0.42 14.5 ± 0.59 11.0 ± 0.44 −7.88 ± 0.15 −22.0 ± 1.26

71.5 82.6 88.7 88.9 86.7

Cr ± ± ± ± ±

2.36 2.59 3.10 2.12 3.15

24.9 67.0 80.2 83.5 84.8

Pb ± ± ± ± ±

1.49 2.45 2.16 2.26 2.99

0.71 7.71 9.85 3.84 1.25

Cd ± ± ± ± ±

0.09 0.15 0.22 0.16 0.12

9.58 15.4 15.4 11.5 8.32

Ni ± ± ± ± ±

0.55 0.26 0.44 0.25 0.26

76.2 91.6 93.5 92.5 90.0

± ± ± ± ±

0.86 1.26 0.22 0.16 1.21

Fig. 7. Leaching concentrations of Zn, Cu, Cr, Ni in EPS melting products (Pb and Cd were not detected).

auxiliary material reached 30%, and the melting point was lower than 1500 °C. To keep the melting points of the mixture materials in the low melting point region (under 1500 °C), 30% of the auxiliary material was deemed necessary. In this case, the mixture samples M0 to M6 were all prepared according to this scheme. To verify the universal applicability of this scheme, hundreds of the components of MSWI FA in articles were summarized (these articles were listed in the supporting material). These samples were marked as hollow circles in Fig. 2. Approximately half of the MSWI FA was in the low melting point region (under 1500 °C). The remainder of the MSWI FA showed a relatively higher melting point due to its high CaO (> 60%) content. As is shown in Fig. 2, the content ratios of EPS and MSWI FA studied in this paper were almost the most extreme. Even so, the melting point could be reduced to a low temperature region by the abovementioned compounding scheme (sample M0 to M6). Accordingly, the reduction of the melting point of the remainder of the MSWI FA can be achieved. The melting of the mixture samples was investigated using a thermal analyzer to describe the endothermic and exothermic processes and measure the mass loss during the heat treatment process. Curve A exhibits the heating process, and the downward peak of A represents the endothermic process. Curve B corresponds to the cooling process, and the upward peak of B represents the exothermic process.

Fig. 8. Leaching concentration of heavy metals from co-melting products.

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between 200 °C and 400 °C correspond the decomposition of Cr(OH)3. The mass loss around 600 °C correspond the decomposition of Ca(OH)2. The last mass loss near 900 °C correspond the decomposition of CaCO3. The melting reaction of M1 began at 1000 °C with the endothermic peak at 1240 °C, higher that of M0 (1150 °C). This suggests that the EPS was more difficult to melt than fly ash. Compared to low content of Na and Mg in M1, the Na and Mg oxides in fly ash is probably responsible for the shift forward of the melting temperature of M0, as these alkane (earth) metals improved in the melting behavior of ceramic or glass industry (Ibrahim et al., 2018). The P content in EPS at about 10% is also relative to the high melting temperature, as P is widely used in the phosphate glass production (Ehrt and Flugel, 2018). During the cooling process, several small exothermic peaks emerged between 900 °C and 1400 °C, indicating crystals formation. In M4, two endothermic peaks of A were clearly revealed at approximately 100–1000 °C and 1000–1400 °C. The former was due to the decomposition of various hydroxides in the sample such as Cr(OH)3, Ca (OH)2 etc., as well as the evaporation of heavy metals chloride and the decomposition of CaCO3. This corresponded to the mass loss of TG at 100–400 °C, 400–800 °C, 800–1000 °C. The latter was due to the heat absorption during the melting process. During the cooling process, no obvious exothermic peak was found on curve B, meaning that no crystals were produced in the process. In the thermal process, the behavior of sample M4 was similar to that of sample M0, as both of them were generated an amorphous glassy form. 3.2. Volatilization of heavy metals To understand the volatilization of heavy metals from EPS and MSWI FA, EPS and MSWI FA were treated at a temperature of 1300 °C. In Fig. 4, the volatilization characteristics of three types of EPS were essentially similar for the heavy metals. The volatilization rate of Cd was the highest, with Zn and Cr following. The volatilization rate of Pb, Cu and Ni of the EPS samples were all below 5%. The large amount of chlorine in MSWI FA could generate a low boiling point of chloride. These chlorine salts were easily volatilized during the heat treatment process (Vogel and Adam, 2011). Over 95% of Pb and Cd, almost 80% of Cr, 70% of Zn and Ni, and 60% of Cu were removed. The difference in volatilization ratio between heavy metals was because the proportions of each heavy metal in the form of chlorine, oxide or mineral were different with each other. This finding indicates that the chlorine in fly ash improved the heavy metal evaporation, consistent with the results of our previous work (Wu et al., 2015). In Fig. 5, the concentrations of heavy metals were volatilized to a different degree during the melting at approximately 1300 °C. The volatilization ratios of mixed samples might increase with a rise in the MSWI FA proportion. The component with the most obvious change in volatility was Pb. With the proportion of MSWI FA increasing from 0% (M1) to 10% (M4), the volatilization ratio of Pb increased rapidly and reached to 96%. This finding suggests that most of the Pb in the mixed material can easily form lead chloride, whose melting point is lower than that of most inorganic Pb compounds. The addition proportion of the MSWI FA in the mixed samples would also affect the volatility of Cu, Cr and Ni. When the percentage of MSWI FA was less than 10%, the volatilization ratios increased rapidly. In contrast, the volatility of the heavy metals increased more slowly than when the rates were more than 10%. This was mainly due to the decrease of EPS which contained large amount of Cu, Cr and Ni. On the other hand, the volatilization ratio of Zn and Cd increased smoothly during when the ratio of MSWI FA increased from 0% to 30%. The highest volatilization ratios of Zn, Pb, Cu, Cd, Cr and Ni were approximately 41%, 96%, 41%, 94%, 62% and 42% respectively. Adding a 10% dosage of MSWI FA was probably the most suitable. The volatilization amount of heavy metals in the mixture samples are shown in Fig. 6. In the initial period, the volatilization amount of

Fig. 9. XRD analysis of co-melting glass sand.

In M0 (Fig. 3), there was an obvious endothermic peak near 600 °C, which was caused by the decomposition of Ca(OH)2 (decomposition temperature: 580 °C). This phenomenon also appeared in samples M1 and M4. In addition, there was another endothermic peak between 700 °C and 800 °C. This was likely due to the release of ZnCl2 (boiling point: 732 °C) from the fly ash. Some other small endothermic peaks between 900 °C and 1000 °C were caused by the volatilization of PbCl2 (boiling point: 950 °C), CuCl2 (boiling point: 993 °C), etc. Before 1000 °C, TG had three obvious mass loss. The mass loss around 600 °C correspond the decomposition of Ca(OH)2. The mass loss between 700 °C to 800 °C means the volatilization of ZnCl2 (boiling point: 732 °C). The last mass loss near 900 °C correspond the volatilization of PbCl2 (boiling point: 950 °C) and CuCl2 (boiling point: 993 °C) and the decomposition of CaCO3 (decomposition temperature: 897 °C). All the above estimated reactions under 1000 °C was corresponded the mass loss of TG. The melting reaction began at 1000 °C when the glassy materials began to melt. At this stage, there was a significant weight loss. It mainly contributed to the volatilization of the unburned organic residues, sulfates, and salinity. Curve B recorded the exothermic reaction during the cooling process. No obvious exothermic peaks indicated that no crystal was generated. Curve B shows a trend of slow decline at the subsequent stage. This means the molten sample was transformed to an amorphous glassy state. As shown in Fig. 3 (M1), there was an obvious endothermic peak between 200 °C and 400 °C. This was mainly due to the decomposition of other hydroxide contained in EPS such as Cr(OH)3. Moreover, the endothermic peak at ∼600 °C was contributed to the decomposition of Ca(OH)2. The endothermic peak between 900 °C and 1000 °C was contributed to the volatilization of Pb and Cu chloride and the decomposition of CaCO3, respectively. All the above estimated reactions under 1000 °C was corresponded the mass loss of TG. The mass loss 231

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Fig. 10. Glass phase analysis of co-melting glass sand.

heavy metals increased with a rise in the fly ash proportion, proving the promotion of the volatilization of heavy metals was affected by the addition of MSWI FA. However, as mentioned above, the volatilization of heavy metals hardly increased or was even slightly inhibited when the ratio of MSWI FA was increased to more than 10%, leading to the inevitable enrichment of heavy metals in the molting product. The extra volatilization rate of heavy metals was shown in Table 3. The extra volatilization rate of Pb, Cd, and Ni were higher than the others when the ratio of FA and EPS reached 10% (M4). A ratio of 20% (M5) was the best condition for the volatilization of Cu and Cr, whereas the best effect to accelerate the volatilization of Zn appeared in sample M3. With the ratio of FA and ESP rising, the additional volatilization amount of Zn dropped quickly. It was reported that MgO and Na2O improved the reduction of melting point, resulting in the quick softening and melting of sample (Ibrahim et al., 2018). This may lead to the encapsulation of element that was subject to evaporate. Accordingly, the drop in extra volatilization ratios of Zn for M5 and M6 was probably contributed the high content of fly ash in the sample which led to the increasing of MgO and Na2O. This feature was investigated further in our next work. Therefore, adding 10% of MSWI FA might be the suitable scheme to promote the volatilization of heavy metals.

internal partition and fall into the tank. There was no raw material can be seen in the glass. With the ratio of auxiliary materials rising, the samples melted more completely. When the ratio of auxiliary materials reached 30%, the mixed materials could melt completely and form a uniform glass material. The relationship between the melting performance and leaching is shown in Fig. 7. With the proportion of auxiliary materials rising, the leaching concentration clearly reduced. When the samples melted completely, the leaching concentrations of Zn, Cu, Cr and Ni were approximately 3 mg/L, 5 mg/L, 0.2 mg/L and 0.4 mg/L, respectively. The leaching concentrations of various heavy metals were different due to their different contents and existence forms. The method for distinguishing hazardous material through the leaching concentration of heavy metals is well accepted throughout the world. The leaching concentrations of Zn, Pb, Cu, Cd, Cr and Ni of comelting samples (M1-M6) are shown in Fig. 8. The leaching concentration of Cu was as high as 6.75 mg/L in sample M1 (adding no MSWI FA) and reduced rapidly to 4 mg/L in sample M6. The performance of Zn was similar to that of Cu. However, the leaching concentration of Cr and Ni hovered between 0 and 1 mg/L and showed no close relationship with the proportion of MSWI FA. Furthermore, there was no leaching of Pb and Cd in all the samples, owing to the low total concentration of them in the co-melting samples. The leaching concentration of all the heavy metals was much lower than the standard of US EPA, which limit the concentration of Zn, Pb, Cu, Cd, Cr and Ni should keep below 50 mg/L, 5 mg/L, 50 mg/L, 1 mg/L, 5 mg/L and 10 mg/L, respectively.

3.3. Leaching concentrations of heavy metals in co-melting product To verify the optimal dosage of the auxiliary materials, different ratios of auxiliary material were added into the EPS samples. After the co-melting process, EPSs mixed with different ratios of auxiliary materials displayed diverse results. “No melting” means the sample cannot flow out of the internal partition and fall into the tank. “Partial melting” means the sample could flow out of the internal partition and fall into the tank. However, a small amount of raw material can be seen in the glass. “Completely melting” means the sample could flow out of the

3.4. Microstructure and crystalline phase of co-melting product Fig. 9 presents the XRD patterns of the crystals (glass sand) of comelting samples (M1-M6). The bulge between 20 and 35° appeared in 232

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Fig. 11. SEM micrographs of co-melting glass sand.

microscope photos of samples M1-M6 (Fig. 10). The number of spots decreased from M1 to M4 due to an increasing amount of chlorine with the rise of the MSWI FA proportion. Heavy metal chloride salt, formed from the abundant chlorine in the FA, volatilized easily during the comelting process, whereas the number of spots increased from M4 to M6 because of the superabundant heavy metals coming from high proportion of MSWI FA. The heavy metals impurities scattered in the glass lead to excessive leaching during the reuse process if the glass sand was fully broken or the impurities were exposed on the surface. SEM micrographs of glass sand samples (M1-M6) are shown in Fig. 11. The surface of the glass sand of M1 was very uneven and covered with several tiny particles. The surface of glass sand became smoother as the proportion of MSWI FA rose to 2% or 5%. In the meantime, several globular particles appeared on the surface. Some of them were embedded inside of the glass sand surface and the others were exposed but were stripped readily when contact with the water. When the proportion of MSWI FA reached 10% (M4), all the globular particles had been removed completely. The holes caused by the globular particles were padded with other materials. The surface became smoother with an increased proportion of MSWI FA, and the holes almost disappeared when the proportion of MSWI FA reached 30%. The

all melting samples, meaning the amorphous glass state generated after the melting process. The major phases of Ca2SiO4 and 3CaO·2SiO2 were observed in all crystals except sample M6. Some other constituents appeared in the glass sand samples with the rise of the MSWI FA proportion, such as quartz in sample M2, Ca3(PO4)2 in sample M3, and SiO2 and MgSiO3 in sample M4. With a large amount of phosphate and silicate existing in the samples, the heavy metals had the opportunity to be encapsulated. The photos of six co-melting samples (M1-M6) taken by a polarizing microscope are shown in Fig. 10. The black background of each sample indicates that all the co-melting samples formed homogeneous glass. Several bright spots scattered in the glass were observed in these photos. These spots could be divided into two categories: boundary clear spots and fuzzy spots. The boundary and bright spots were the impurities mixed in the glass, such as the heavy metal particles that melted incompletely. The fuzzy spots were impurities that came from cement and other adhesives used during the preparation process. The spots appeared blurry when the polarization microscope was focusing on the inside of the glass samples and taking photos through the impurities adsorption on the surface of sample. Several clear spots were found in the glass though the polarizing 233

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smooth surface could reduce the leaching concentration of heavy metals due to few heavy metal particles exposed on the surface. 3.5. Implementation With the FA addition, a large proportion of heavy metals in EPS/FA formed heavy metal chloride with a low boiling point during co-melting process. Heavy metal chloride was easily volatilized and separated from the particles at high temperatures. The remainder of the heavy metals were stabilized by encasing themselves in the amorphous glass states. Therefore, the co-melting technique has the potential to be used for treating other solid waste, such as wastewater sludge, spent catalyst, coal fly ash, medical waste, etc. A total of 3%–5% of the by-product in co-melting process, referred to as secondary fly ash, was produced by the flue gas cleaning system. In this study, the total amount of Zn, Pb, Cu, Cr and Ni in secondary fly ash was 66.4 g/kg, 6.18 g/kg, 15.05 g/kg, 38.16 g/kg, and 27.6 g/kg, respectively. The concentration of Pb and Zn in the secondary fly ash almost reached the grade requirement of some lead-zinc ore, which can be directly used in the smelting industry (Mineral resources and reserves department of China, 2013). The metal recycling is the highly valued treatment of the secondary fly ash. 4. Conclusions Mixing waste and auxiliary material together to adjust their composition could reduce their melting point significantly. A total of 30% of auxiliary material may be the best scheme that could ensure that the mixture material melts completely and forms a uniform glass material. During high-temperature treatment, adding an appropriate amount of chlorine to the waste system can promote the volatilization of heavy metals in the waste. In this study, 10% of MSWI FA showed the best performance for promoting the volatilization of heavy metals and generating well-performing glass products. Compared to other researchers, the surface of the glass material generated from the comelting of MSWI FA and EPS was more smoothly than MSWI FA melting glass material (Wang et al., 2017; Yang et al., 2009). Metal recovery was better than sludge melting alone or adding other agents to melt (Chou et al., 2012). However, the application of glass material and the pollution control of using process need further study. Acknowledgments The research was funded by the National Natural Science Foundation of China (No. 91543123, No. 41402311) and the Shanghai Science and Technology Commission Project (No. 15DZ0501401, No. 15DZ1205905). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2018.11.053. References Chae, J.S., Choi, S.A., Kim, Y.H., Oh, S.C., Ryu, C.K., Ohm, T.I., 2016. Experimental study of fry-drying and melting system for industrial wastewater sludge. J. Hazard. Mater. 313, 78–84. Chan, C.C., Kirk, D.W., 1999. Behaviour of metals under the conditions of roasting MSW incinerator fly ash with chlorinating agents. J. Hazard. Mater. 64, 75–89. Chen, Q., Zhang, L., Ke, Y., Hills, C., Kang, Y., 2009. Influence of carbonation on the acid neutralization capacity of cements and cement-solidified/stabilized electroplating sludge. Chemosphere 74, 758–764. Cheng, T.W., Chen, Y.S., 2004. Characterisation of glass ceramics made from incinerator fly ash. Ceram. Int. 30, 343–349. Chiang, K.Y., Hu, Y.H., 2010a. Water washing effects on metals emission reduction during municipal solid waste incinerator (MSWI) fly ash melting process. Waste Manag. 30, 831–838. Chiang, K.Y., Hu, Y.H., 2010b. Water washing effects on metals emission reduction during

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