Energy & Fuels 2008, 22, 3057–3064
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Diagnostic of Novel Atmospheric Plasma Source and Its Application to Vitrification of Waste Incinerator Fly Ash Xin Tu,*,†,‡ Qin Wang,† Liang Yu,† Bruno Che´ron,‡ Jianhua Yan,† and Kefa Cen† State Key Laboratory of Clean Energy Utilization, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and UMR 6614 CNRS CORIA, Saint Etienne du RouVray 76801, France ReceiVed February 26, 2008. ReVised Manuscript ReceiVed May 30, 2008
The characteristics of a novel atmospheric plasma source generated by a dc double-anode plasma torch have been investigated by measuring its emission spectra and heat flux. The original plasma torch design has been proven to be effective in enhancing the aerodynamic stability and luminous intensity of the plasma arc, as well as the jet length. As an application, a vitrification process based on the double-arc argon plasma has been employed to convert toxic incinerator fly ash into a harmless, stable, and chemical durable product. The vitrified slag exhibits a homogeneous, highly dense, and amorphous glassy structure, which is further confirmed by scanning electron microscopy and X-ray diffraction. The leaching concentrations of the target heavy metals in the slag are decreased significantly and are much lower than the current regulatory thresholds of China and U.S., which reveals that the contaminants have been sufficiently immobilized in the silicate network matrix. It is also determined that the vitrification contributes to the predominant reduction of the specific surface area of the fly ash and to the improvement of the slag hardness. Furthermore, the total concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are also appreciably reduced through the plasma vitrification process with a high decomposition rate of 99.97% in PCDD/Fs and 99.94% in I-TEQ. These results indicate that the vitrified product with good quality has great potential to be used as a viable alternative for construction or geotechnical applications. The vitrification process based on the double-arc plasma technology has been demonstrated to be effective and reliable for the stabilization, detoxification, and recycling of waste incinerator fly ash.
As the amount of municipal solid waste (MSW) increases significantly because of the growing urbanization and industrialization, its appropriate treatment has been paid great attention to reduce the negative impacts on humans and environments. In 2005, more than 156 million tons of MSW were generated in China, and this figure is increasing annually by 6-8%.1 The treatment of MSW by incineration has been adopted gradually in China over the past 20 years. Incineration has been technically proved as an effective waste treatment approach because of its prominent advantages, including volume and weight reduction of the original waste, waste detoxification, as well as the possibility of energy recovery. This technology has been chosen as the priority disposal method for several major metropolitan areas in China and a great number of MSW incinerators (MSWIs) have been planned and built recently. Up to Dec 2005, there are 66 constructed MSWIs, with a total capacity of 32 200 tons waste per day.1 Nevertheless, a large number of residues resulting from MSW incineration, known as bottom ash and fly ash, must be concerned carefully, because they contain significant concentrations of hazardous constituents, such as leachable heavy metals, dioxins, and furans. It is planned by the Chinese government that 88 incinerators with a total processing capability of 66 600 tons waste per day will be
constructed in the future 5 years.2 When these new incinerators are all put into operation, they will produce more than 6 million tons of incinerator ash every year, including 9 × 105 tons of hazardous fly ash. If these hazardous materials are improperly treated or disposed of, they could cause detrimental secondary contamination. Various technologies have been performed for the treatment of MSWI fly ash, such as cement-based solidification/stabilization (S/S), chemical process, acid extraction, sintering, and vitrification.3 Vitrification has been identified as a promising alternative to the treatment and recycling of incinerator fly ash. It is a process that uses a heat source (electricity or combustion of fuel) to melt incinerator residues at high temperature (1200-1600°C),producingahomogeneousglassorglass-ceramic phase slag, into which toxic heavy metals can be immobilized to become an integral part of the slag. Furthermore, this process for vitrification of fly ash can lead to a significant volume reduction and destruction of 99% or higher of organic compounds, such as PCDD/Fs. After vitrification, the leachability of heavy metals is substantially reduced and the produced product with sufficient strength could be potentially used, for example, as road pavement, concrete aggregate, building materials, etc.4 Previous studies on the vitrification of radioactive waste, medical waste, metal-rich sludge, and incinerator fly ash have
* To whom correspondence should be addressed. Telephone: +86-57187952443. Fax: +86-571-87952438. E-mail:
[email protected]. † Zhejiang University. ‡ UMR 6614 CNRS CORIA. (1) China Statistical Yearbook; China Statistics Press: Beijing, China, 2006.
(2) The eleventh five-year plan for national municipal solid waste management. National Development and Reform Commission, 2005. (3) Ecke, H.; Sakanakura, H.; Matsuto, T.; Tanaka, N.; Lagerkvist, A. Waste Manage. Res. 2005, 18, 41–51. (4) Ferreira, C.; Ribeiro, A.; Ottosen, L. J. Hazard. Mater. 2003, 96, 201–216.
1. Introduction
10.1021/ef800141b CCC: $40.75 2008 American Chemical Society Published on Web 07/22/2008
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shown that thermal plasma is a promising technology and particularly well suitable for hazardous waste treatment.5–12 It offers the predominant characteristics of high temperature and high energy density, which allows for effective heat transfer at the reactor boundaries and corresponding fast reaction times during the waste processing.13,14 In the past decade, plasmabased vitrification processes for incineration residues have been extensively developed at laboratory and pilot scales.15–20 The most widely used arc plasma sources in these processes are transferred arc or nontransferred arc plasma torch. One example of the demonstrated vitrification system for incinerator fly ash immobilization is provided by Europlasma.18 The plasma reactor process up to 10 tons/day of fly ash using a 700 kW torch operated with air. Recently, the rising demand toward highly reliable and more desirable plasma processing has also promoted the research and development of novel plasma sources, such as three-electrode plasma torch,5 twin torch reactor,19,20 and steam plasma torch,21 to adapt for better waste processing. In the frame of present studies devoted to the hazardous waste destruction, an original dc double-anode plasma torch has been designed and tested, which produces a long lifetime and highly stable argon plasma jet at atmospheric pressure. The experimental works of plasma diagnostic are carried out to optimize and improve the performance of this novel plasma source and its processing. In addition, a laboratory-scale vitrification system based on the double-arc plasma technology has been developed to demonstrate the feasibility and effectiveness of the treatment and recycling of MSWI fly ash. In this work, MSWI fly ash is melted without any additives by using the double-anode plasma torch. The characteristics of the fly ash and the vitrified slag including their chemical composition, microstructure, and leachability of heavy metals are investigated by various analytical techniques. The physicomechanical properties (porosity and microhardness) of the slag are also examined. Moreover, the decomposition effect of dioxins and furans in the vitrification (5) Tendler, M.; Rutberg, P.; Van Oost, G. Plasma Phys. Controlled Fusion 2005, 47, A219–A230. (6) Sakai, S. I.; Hiraoka, M. Waste Manage. 2000, 20, 249–258. (7) Iwao, T.; Yumoto, M. IEEE Trans. Electron. Electron. Eng. 2006, 1, 163–170. (8) Torres-Reyes, C. E.; Pacheco-Pacheco, M.; Pacheco-Sotelo, J. O.; Lopez-Callejas, R.; Benitez- Read, J. S.; Estrada-Martinez, N.; CotaSanchez, G. IEEE Trans. Plasma Sci. 2007, 35, 1758–1765. (9) Min, B. Y.; Kang, Y.; Song, P. S.; Choi, W. K.; Jung, C. H.; Oh, W. Z. J. Ind. Eng. Chem. 2007, 13, 57–61. (10) Chu, J. P.; Hwang, I. J.; Tzeng, C. C.; Kuo, Y. Y.; Yu, Y. J. J. Hazard. Mater. 1998, 58, 179–194. (11) Ramachandran, K.; Kikukawa, N. IEEE Trans. Plasma Sci. 2002, 30, 310–317. (12) Park, K.; Hyun, J.; Maken, S.; Jang, S.; Park, J. W. Energy Fuels 2005, 19, 258–262. (13) Pfender, E. Plasma Chem. Plasma Process. 1999, 19, 1–31. (14) Heberlein, J.; Murphy, A. B. J. Phys. D: Appl. Phys. 2008, 41, 053001. (15) Cortez, R.; Zaghloul, H. H.; Stephenson, L. D.; Smith, E. D. J. Air Waste Manage. Assoc. 1996, 46, 1075–1080. (16) Cheng, T. W.; Chu, J. P.; Tzeng, C. C.; Chen, Y. S. Waste Manage. 2002, 22, 485–490. (17) Haugsten, K. E.; Gustavson, B. Waste Manage. 2000, 20, 167– 176. (18) www.europlasma.com. (19) www.tetronics.com. (20) Rani, D. A.; Gomez, E.; Boccacini, A. R.; Hao, L.; Deegan, D.; Cheeseman, C. R. Waste Manage. 2008, in press, doi: 10.1016/j.wasman.2007.06.008. (21) Hrabovsky, M.; Kopecky, V.; Sember, V.; Kavka, T.; Chumak, O.; Konrad, M. IEEE Trans. Plasma Sci. 2006, 34, 1566–1575.
Tu et al.
Figure 1. Schematic diagram of the plasma diagnostic setup.
treatment is further evaluated by measurement of PCDD/Fs in the fly ash and slag. 2. Experimental Section 2.1. Plasma Source and Diagnostic System. Figure 1 shows a schematic diagram of the plasma diagnostic setup. The core of the plasma system is a specially designed homemade dc nontransferred plasma torch, as described in refs 22–24 In comparison to the conventional plasma torch, this one consists of a cone-shaped tungsten cathode (2 wt % Th) and two nozzle-shaped copper anodes set at different axial distance from the cathode tip. All of the electrodes and the plasma torch body are water-cooled independently. The torch is supplied through two identical triphase rectified power sources, with different unloaded voltages of 140 and 210 V. In the present study, argon is used as a working gas at a flow rate of 10-20 L/min and injected axially into the arc chamber through a gas diffuser. The plasma is created as follows: a first arc is initiated between the cathode tip and the throat (5 mm in diameter) of the first anode, and then the plasma is reheated by the second arc, which is ignited between the cathode tip and the throat (10 mm in diameter) of the second anode. Thus, the torch is operated in a double-arc mode, generating a long lifetime, weak fluctuation, and reproducible argon plasma jet at atmospheric pressure; meanwhile, the plasma is sustained between one cathode spot and two independent arc root attachments along the anode walls. The aerodynamic stability and luminous intensity of the plasma arc as well as the jet length are greatly enhanced by the latter ignition. The same phenomenon has also been observed when the plasma is released under low pressure using a similar double-anode torch.25,26 The electrical operating parameters are typically 25-30 V/100 A for the first arc and 50-60 V/100 A for the second arc. It suggests that such double-anode configuration also provides the possibility to maintain a much higher arc voltage and higher torch electrical power with reduced fluctuation amplitude. In our studies, the thermal efficiency of this torch fed with pure argon at the double-arc mode is around 40%. This value is comparable to or greater than those of long laminar argon plasma (22) Tu, X.; Che´ron, B. G.; Yan, J. H.; Cen, K. F. Plasma Sources Sci. Technol. 2007, 16, 803–812. (23) Tu, X.; Yan, J. H.; Yu, L.; Cen, K. F.; Che´ron, B. G. Appl. Phys. Lett. 2007, 91, 131501. (24) Tu, X.; Che´ron, B. G.; Yan, J. H.; Yu, L.; Cen, K. F. Phys. Plasmas 2008, 15, 053504. (25) Delair, L.; Tu, X.; Bultel, A.; Che´ron, B. G. High Temp. Mater. Processes 2005, 9, 583–596. (26) Che´ron, B. G.; Bultel, A.; Delair, L. IEEE Trans. Plasma Sci. 2007, 35, 498–508.
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Figure 2. Schematic diagram of the MSWI fly ash vitrification system based on the plasma arc technology.
jets under similar power conditions.27,28 At constant gas flow rate, this efficiency is appreciably enhanced by increasing the torch power. In most of the plasma torches, the heat transfer through the anode wall occupies the greatest part of the total heat losses. In this case, at an argon flow rate of 20 L/min, the heat losses on the first and second anode contribute to 47 and 36%, respectively, of the total power released by the cooling circuit, in contrast with the cathode power loss of about 4%. When the plasma torch is operated at the single-arc mode or adding a few percentages of nitrogen into argon as plasma-forming gas, the torch thermal efficiency is improved by about 25-35%. To compare the plasma properties along the torch axis inside and outside the arc chamber, two VUV optical fibers with 600 µm in diameter are used to record the emission spectra of the plasma jet. One is located at 30 mm downstream from the first anode throat (divergent part of the first anode), while the other is disposed at 10 mm downstream from the torch exit. Both fibers are connected to an Acton 758i spectrometer set for a 0.03 nm resolution within a wide spectral range from 190 to 1000 nm. The detailed principles for the determination of plasma electronic parameters are explained in ref 22. The arc voltages (U1 and U2) and currents (I1 and I2) are measured synchronously using a four-channel digital oscilloscope (9314AL LeCroy, 400 MHz) connected to a computer via GPIB. Each signal is recorded for a duration of 0.1 s, with a sampling rate of 107 points/s. The heat flux of a plasma source can be derived from the temperature difference across a thermally conductive block placed between the heat source (plasma jet) and a heat sink (cooling water circuit). When the stationary state is attained, the Fourier’s law leads to a linear relation between the thermocouple voltage ∆V and heat flux density φ
∆V )
lR φ k
(1)
where k and l are respectively the thermal conductivity and the thickness of the block and R is the Seebeck coefficient of the thermocouple. In this study, the measurements have been performed using a commercial thin foil heat flux sensor with water cooling. Constantan and copper are used as the thermoelectric materials, with constantan as the center block and copper as the heat sink. These materials produce an output voltage that is directly proportional to the absorbed heat flux. The calibration coefficient of this device is equal to 0.335 µV W-1 m2. As shown in Figure 1, the distance of the heat flux meter to the torch exit can be changed from d ) 13 to 35 cm by vertically moving the plasma torch through a stepping motor. The output voltage of the transducer is recorded by the digital oscilloscope and then converted to the heat flux value. 2.2. Plasma Arc Vitrification Reactor. Figure 2 shows a schematic diagram of the fly ash vitrification system based on the (27) Osaki, K.; Fukumasa, O.; Kobayashi, A. Vacuum 2000, 59, 47– 54. (28) Li, G.; Pan, W. X.; Meng, X.; Wu, C. K. Plasma Sources Sci. Technol. 2005, 14, 219–225.
plasma arc technology. The plasma vitrification reactor is a cylindrical stainless-steel vessel and has several view ports for easy access of in situ process-monitoring tools. The plasma torch is mounted on the top of the vitrification reactor, and the generated plasma jet is adjusted to be localized in the center of the reaction chamber, close above the substrate holder. The incinerator fly ash (150 g) contained in an alumina crucible is placed in the center of the reactor and melted by the double-arc argon plasma jet (∼8 kW). The molten sample is kept at 1500-1600 °C for 30 min to ensure complete melting. Subsequently, the melt liquid is rapidly quenched by cold air, followed by drying and grinding to small particles for further analysis. The produced slag is homogeneous, vitreous in nature, with a dark color appearance. A small amount of exhaust acid gases produced in the process are absorbed by alkaline liquid. 2.3. Material and Analysis Method. The fly ash used in this study is collected from a mechanical stoke MSW incinerator with oil as an auxiliary fuel located in the eastern part of China. This incinerator is capable of processing 350 tons of local MSW/day and is equipped with an air pollution control device (APCD) consisting of a semidry scrubber and a fabric baghouse filter, from where the fly ash is taken. The fly ash sample is preliminarily homogenized and oven-dried at 105 °C for 24 h until a constant weight is reached. The major chemical composition of the fly ash and slag are analyzed by EDAX Genesis energy-dispersive X-ray spectrometry (EDX), while the heavy metal concentrations (Cd, Cr, Pb, Cu, Zn, and Ni) are determined by an inductive coupled plasma-atomic emission spectrometry (ICP-AES, IRIS Intrepid II XSP) after sample acid dissolution according to U.S. Environmental Protection Agency (EPA) 3050 procedure. The morphology of the fly ash and produced slag is examined using a Nation SX-570 scanning electron microscope (SEM). In addition, the X-ray diffraction (XRD) investigations are carried out by a Rigaku Model D/max-rA using Cu KR radiation at 40 kV and 150 mA settings in the 2θ range from 5° to 70°. The crystallized phases are identified by comparing the positions and intensities of Bragg peaks to those in the Joint Committee on Powder Diffraction Standards (JCPDS) data files. The leaching behavior of heavy metals in the fly ash and produced slag is investigated according to U.S. EPA TCLP 1311 method. The leachability for the target heavy metals is analyzed using ICP-AES. The physical and mechanical properties are investigated by several techniques. The porosity characteristics are analyzed by Autosorb-1-C/TCD (Quantachrome Instrument), while the hardness of the slag is measured using a HV-1000 Vickers microhardness tester with a load of 300 g and loading time of 10 s. The exhaust gas is measured by a GASMET Dx4000 Fourier transform infrared spectroscopy (FTIR). The plasma vitrification processing leads to generation of low-molecular-weight flue gases (SO2, CO, HCl, and NOx) with low concentrations in the range of 3-42 ppm. The analysis of PCDD/Fs in the fly ash and slag is performed by HRGC/HRMS on a 6890 Series gas chromatograph (Agilent, Santa Clara, CA) and coupled to a JMS-800D mass spectrometer (JEOL, Japan). The detailed quantitative determination of PCDD/Fs is referred to U.S. EPA method 1613.29
3. Results and Discussion 3.1. Characteristics of Plasma Source. 3.1.1. Heat Flux Characteristic of the Plasma. Figure 3 shows the axial distribution of the heat flux exchanged between the double-arc argon plasma jet and the flat plate set along its flow direction at different current intensities. The measured heat flux value ranges between 20 and 70 kW m-2 and shows its variations versus the distance to the plasma torch exit. It can be seen clearly that the heat flux is decreasing with the increase of the distance d at a constant arc current. The value is roughly divided by a factor 2 (29) United States Environmental Protection Agency (U.S. EPA). Method 1613. Revision B: Tetra- through octa-chlorinated dioxins and furans by isotope dilution HRGC/HRMS, 1994.
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Tu et al. Table 1. Major Components of MSWI Fly Ash and Vitrified Slag (in wt %)
Figure 3. Axial evolution of the heat flux exchange between the doublearc argon plasma jet and the plate surface (qAr ) 10 L/min, and I1 ) 100 A).
when the distance changes from d ) 13.9 to 23 cm. A similar decreasing axial profile has been reported in earlier studies in which the net heat flux between a low-pressure nitrogen plasma jet and a flat plate was evaluated.30 At a given distance, the heat flux is enhanced about 25% when the current intensity is 20 A and higher. Previous studies by Meng et al. have reported the distribution of heat flux for an argon laminar plasma jet impinging on a flat plate at atmospheric pressure.31 The estimated heat flux is several times higher than our data because of the different distance between the plasma torch and the heat flux meter. However, the obtained values in this study are quite comparable to the results (30-70 kW m-2) in the previous literature.30,32 3.1.2. Spectroscopic Diagnostic. The optical emission spectroscopy (OES) technique is a powerful tool in the plasma diagnostics to determine critical parameters of plasma source, such as the temperature, particle densities, as well as the thermodynamic state.33,34 In this study, OES is employed to measure the emission spectra of the double-arc argon plasma jet between 350 and 750 nm inside and outside the arc chamber. The spectral scans at the first and second stage of the plasma source are very much alike. Both spectra are clearly dominated by the numerous intensive argon atomic lines emitted in the near-UV and visible range. Some argon ionic lines are observed in the divergent part of the first anode inside the arc chamber, while these lines become very weak at the torch exit. In addition, owing to the engulfment of the ambient air, three nitrogen atomic lines (742.364, 744.229, and 746.831 nm) emitted from the same energy state (Eu ) 11.99 eV) are observed at the torch exit, where the N2+ first negative system is hardly detected. In our experiments, the excitation temperatures of the plasma arc are determined using the Boltzmann plot method and are expected to be close to the electronic ones, while the electron number densities are derived from the Stark broadening of two argon atomic lines (415.86 and 430.01 nm). The accuracies of these two methods can be estimated as better than 5 and 10%, (30) Che´ron, B. G.; Robin, L.; Vervisch, P. Meas. Sci. Technol. 1992, 3, 58–61. (31) Meng, X.; Pan, W. X.; Zhang, W. H.; Wu, C. K. Plasma Sci. Technol. 2001, 3, 953–958. (32) Tahara, H.; Ando, Y.; Yoshikawa, T. IEEE Trans. Plasma Sci. 2003, 31, 281–288. (33) Nassar, H.; Pellerin, S.; Musiol, K.; Martinie, O.; Pellerin, N.; Cormier, J. M. J. Phys. D: Appl. Phys. 2004, 37, 1904–1916. (34) Bourg, F.; Pellerin, S.; Morvan, D.; Armouroux, J.; Chapelle, J. J. Phys. D: Appl. Phys. 2002, 35, 2281–2290.
compostition
fly ash
slag
CaO Al2O3 SiO2 Na2O K2 O TiO2 MgO Fe2O3
26.53 12.32 27.81 4.21 2.72 1.03 2.23 3.96
27.29 16.81 40.39 3.11 2.93 1.18 3.30 4.60
respectively. In the double-arc mode, the electron temperature and number density of the argon plasma drop from 12 300 K and 7.6 × 1022 m-3 inside the arc chamber to 10 500 K and 3.1 × 1022 m-3 at the torch exit. However, these electron parameters are not significantly changed from the first arc creation zone to the torch exit for the case of argon plasma in single-arc mode. Therefore, the special torch configuration with a second anode effectively reduces the arc instabilities and prevents a strong decay of the plasma excitation between these lines of sight. Furthermore, we have developed a simple LTE model involving four species (Ar, Ar+, Ar2+, and e-) to calculate the argon plasma composition at atmospheric pressure, which are bound by the Dalton’s law, electroneutrality condition, and the mass action laws (Saha equation). In the arc chamber and at the torch exit, the electron densities derived from the LTE model are 7.9 × 1022 and 2.4 × 1022 m-3, respectively. These calculated electron parameters are quite compatible to the measured ones, which indicates that the atmospheric doublearc argon plasma is close to the LTE state under our experimental conditions. 3.2. Plasma Vitrification. 3.2.1. Chemical Composition. The major components of the fly ash and slag are presented in Table 1. It is shown that the fly ash primarily consisted of SiO2 (27.81 wt %), CaO (26.53 wt %), and A12O3 (12.32 wt %), accompanied by lesser amounts of other oxides (Na2O, Fe2O3, K2O, MgO, and TiO2). The high CaO content in the fly ash can be ascribed to the APCD involved using lime as an additive to remove acidic pollutants in the flue gas. The composition profile of the fly ash is similar to those reported in the literature.35,36 During the vitrification process, SiO2 plays an important role in forming a structured matrix, known as network former, while components, such as CaO, Na2O, and K2O, act as networkmodifying elements and could cause problems for the chemical stability of the resultant matrix when their content is high. Al2O3 may be a network former or a modifier depending upon the ratio of aluminum to alkali and alkaline earth ions. Therefore, the relative content of CaO/SiO2/Al2O3 in specimens is of great importance for the effectiveness of the vitrification process. In this study, the comparable content of SiO2 and CaO leads to the fly ash basicity (defined as a mass ratio of CaO/SiO2) of about 0.95, while the other basicity value is defined as the ratio of basic oxides to the acid ones K )
Fe2O3 + CaO + MgO + K2O + Na2O SiO2 + Al2O3 + TiO2
(2)
is also around 0.97. The latter value is much lower as compared to the previous result from Li et al.37 Both basicity values (35) Karamanov, A.; Pelino, P.; Hreglich, A. J. Eur. Ceram. Soc. 2003, 23, 827–832. (36) Wan, X.; Wang, W.; Ye, T.; Guo, Y.; Gao, X. J. Hazard. Mater. 2006, B134, 197–201. (37) Li, R. D.; Wang, L.; Yang, T. H.; Raninger, B. Waste Manage. 2007, 27, 1383–1392.
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Table 2. Heavy Metal Contents of MSWI Fly Ash and Vitrified Slag (in mg/kg) heavy metal
fly ash
slag
Pb Ni Cd Cr Cu Zn
428.9 122.2 11.9 317.9 469.1 2697
46.6 141.3 0.6 386.8 293.5 245.2
indicate that the composition of the fly ash examined in this study could be suitable for the vitrification process without any additives. When examining the vitrified slag, it is also found that the major composition of the slag is SiO2 (40.39 wt %), CaO (27.29 wt %), and Al2O3 (16.81 wt %). However, the mass percentages of these oxides increase obviously, and their contents in the slag are much higher than those in the fly ash sample. The total contents are contributed to about 84.5% of the slag in comparison to 66.6% in the original ash. During the plasma vitrification process, the heavy metals with a low boiling point are volatilized at high temperature, which leads to both volume (83.2%) and mass reduction (67.5%) of the slag. Moreover, the total amount of these oxides is not obviously changed. Both of these effects contribute to the increase in the mass percentage of these oxides. The contents of hazardous heavy metals in the fly ash and produced slag are presented in Table 2. The major heavy metals of environmental concern are zinc, copper, and lead, accompanied by lesser amounts of Cd, Cr, and Ni. For the case of elements with low boiling point, such as Cd, Pb, and Zn, their mass percentages in the slag are significantly low as compared to the fly ash, even though the total weight of the slag is reduced after the vitrification, whereas the metals Ni and Cr exhibit the inverse evolution, as similar to most oxides. The residual fractions of heavy metals in the vitrified slag are presented in Figure 4. The metals Cr and Ni show a high solidification rate because of their low volatility and reach about 82 and 78% in the slag. The major chromium species found in the fly ash are chromium chloride (bp 1200-1500 °C) and chromium oxide (bp 1900 °C), both of which have high boiling and melting points, leading to the low vaporization rate of Cr. The proportion of Cu retained in the slag is about 42%. It maybe explained that some of this metal has been converted to Cu2O, which has a relatively high boiling point (>1800 °C), so that the volatile fraction of Cu is relatively less. It can be seen that the solidification rates of metal species, such as Cd, Pb, and Zn remain at very low values of about 3.4, 7.3, and 6.1%, respectively. The vaporization rate of these metals from the molten ash is kept at a high level of 92-97%. It is due to the fact that these heavy metals with high vapor pressure (>1600 mmHg) and low boiling point (
