Low Temperature Decomposition of Chrysotile ...

Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

Low Temperature Decomposition of Chrysotile Asbestos by Freon-Decomposed Acidic Gas Takahiro Kozawa1, Ayumu Onda1, Koji Kajiyoshi1, Kazumichi Yanagisawa1, Junichi Shinohara2, Tetsuro Takanami2, Masatsugu Shiraishi2 and Masazumi Kanazawa2 1. Research Laboratory of Hydrothermal Chemistry, Kochi University, Kochi, Japan 2. Division of Environmental Engineering, Daioh Construction Inc., Kochi, Japan Abstract: The simultaneous decomposition technique of Freon and asbestos has been newly developed. Chrysotile asbestos with formula Mg3Si2O5(OH)4 was successfully decomposed at 150˚C by contacting Freon-decomposed acidic gas such as HF and HCl generated by superheated steam method. Chrysotile fibers were decomposed and transformed into MgF2 and MgSiF6·6H2O at 150˚C for 30 min. Chrysotile-containing slates as asbestos-containing waste (ACW) were also decomposed at 150˚C for 60 min. The present study presents a lower temperature decomposition technique of asbestos with Freon-decomposed acidic gas, in comparison with the traditional melting method. Keywords: Decomposition of chrysotile, Asbestos, Low temperature, Freon-decomposed acidic gas 1. INTRODUCTION Asbestos minerals were extensively used in numerous building materials in the past because of their excellent properties for insulation and fireproofing. Most of these products contain chrysotile asbestos (Mg3Si2O5(OH)4), which supplied over 90% in the worldwide asbestos applications. However, it has been clarified that inhalation of asbestos fibers causes serious and fatal damage to the respiratory system, namely, mesothelioma and asbestosis [1]. Due to this problem, asbestos use has been banned in Japan. It is estimated that the total amount of asbestos-containing building materials exists over 40 million tons in Japan [2]. In the future, these materials will be asbestos-containing waste (ACW) by the deterioration of buildings. The current major treatment technique for ACW is the melting method at about 1500˚C in Japan. In view of ecological ethics, it is necessary to develop an inexpensive decomposition technique for ACW with low energy consumption. Lowering temperature for the decomposition of chrysotile asbestos has been attempted in the thermal treatment. Kojima et al. [3] carried out the thermal decomposition of chrysotile mixed with the thermal decomposed matters derived from chlorofluorocarbons, which are called Freon. The harmful chrysotile fiber form was disappeared at 700˚C. They suggested that the trace component of thermal decomposed matters derived from Freon, mainly CaCl2, contributes to the lowering of decomposition temperature of chrysotile. In response to this, Fujishige et al. [4-6] reported the thermal decomposition of chrysotile in sprayed-on asbestos using calcium salts. It was revealed that the decomposition temperature of chrysotile mixed with CaCO3 is lowered by adding CaCl2 [5]. Nevertheless, the decomposition temperature was 700˚C and decomposition procedure was multistep. Therefore, lower decomposition temperatures and more concise procedure are required. On the other hand, Freon causes destruction of the ozone layer and global warming. They should be decomposed and can be decomposed by the reaction with su-

perheated steam [7]. The decomposition plants of Freon with a continuous pipeline-system have been already marketed [8]. In this method, high reactive acidic gas such as HF and HCl is formed by the decomposition of Freon. We presumed that chrysotile asbestos would be decomposed by Freon-decomposed acidic gas, because chrysotile asbestos has a hydroxyl group and their resistance to acids is relatively weaker than that to basis [9]. The decomposition of ACW will proceed effectively with high velocities due to gas-solid reaction in a continuous pipeline-system. Utilization of Freon-decomposed acidic gas to decompose of ACW opens new opportunities to decrease of neutralization process in the decomposition of Freon. Furthermore, the decomposition can be carried out in existing Freon decomposition plants without production of new plants. Low temperature decomposition of chrysotile asbestos with Freon-decomposed acidic gas formed by superheated steam method is reported in this paper. 2. EXPERIMENTAL 2.1. Samples Decomposition experiments used two chrysotile samples, chrysotile fibers (Wako Pure Chemical Industries, Japan) and chrysotile-containing slates (Daioh Construction Inc., Japan). Chrysotile fibers contained brucite (Mg(OH)2) as impurity. The chrysotile minerals are formed from forsterite (Mg2SiO4) and water in earth crust, and brucite is produced as a byproduct [10]. Chrysotile fibers were used without preparation. Chrysotile-containing slates were commercial products and widely used in numerous buildings. Chrysotile-containing slates used in this study contained 5 mass% chrysotile and mainly consisted of quartz (SiO2), calcite (CaCO3) and calcium silicate hydrates (C-S-H, unidentified). Slates were ground below 2 mm in diameter. 2.2. Apparatus A schematic diagram of experimental apparatus is

Corresponding author: K. Yanagisawa, [email protected] 830


Window

Freon decomposition reactor tube (800˚C)

Sample (10 g)

Asbestos setting tube (φ 35 mm × 150 mm, SUS304)

Blower

Electric furnace

Water pump Heater (600˚C)

Flow meter

Neutralization tank (20% NaOH aq.) Evaporator (400˚C)

Air-drive pump Air (8.6 L/min)

Distilled water (2.43 mL/min)

Fig. 1

CHClF2 (3 L/min)

Schematic diagram of the experimental apparatus.

shown in Fig. 1. This apparatus consists of the Freon decomposition plant by superheated steam and the asbestos decomposition reactor tube connected between the Freon-decomposition reactor tube and the neutralization tank, where ACW contact the Freon-decomposed acidic gas. Chlorodifluoromethane (CHClF2; HCFC-22) supplied by Daioh Construction Inc. was used as Freon. The decomposition of CHClF2 was carried out with this apparatus under following conditions that gave high decomposition ratio over 99.9%; reaction temperature 800˚C, and constant flow rates of CHClF2, distilled water and air 3L/min, 2.43 mL/min and 8.6 L/min, respectively. The decomposition may proceed by following proposed chemical equation to produce acidic gas (HF and HCl) and carbon dioxide: CHClF2 + H2O + 1/2O2 → 2HF + HCl + CO2 (1)

were identified by powder X-ray diffractions (XRD; Rigaku Rotaflex RAD-RC) with Cu Kα radiation (40 kV and 100 mA). XRD patterns were recorded in the scan range 2θ = 5-70˚, at a scan rate of 4˚/min. A scanning electron microscope (SEM; Hitachi S-530) was used to observe the morphological changes of chrysotile samples. For the determination of asbestos in building material products, the use of phase-contrast microscope (PCM) is specified in Japan [11]. In this study, PCM (OLYMPUS BX51N-DPH) observation by using the immersion liquid with nD25˚C = 1.550 (Cargille, USA) was carried out to verify the presence or absence of chrysotile asbestos. PCM analytic procedure was based in Japanese Industrial Standard (JIS A 1481: 2006) [11]. The samples for characterization by XRD, SEM and PCM were prepared by grounding to a powder in a pestle.

2.3. Decomposition procedure First, the evaporator, heater and Freon reactor shown in Fig. 1 were heated to 400˚C, 600˚C and 800˚C, respectively. Simultaneously, the asbestos reactor tube was also heated to 150-400˚C. After the temperature was held constant at predetermined one for 5 min with supplying distilled water and air with each constant flow rates but not CHClF2, about 10 g of chrysotile fibers or chrysotile-containing slates were placed and sealed in the asbestos reactor tube supplying water and air. Then, CHClF2 was immediately supplied for 5-60 min and the decomposition of CHClF2 with superheated steam was carried out continuously. After stopped supplying CHClF2, the sample was taken out of asbestos reactor tube.

3. RESULTS AND DISCUSSION 3.1. Chrysotile fibers According to the XRD analysis, chrysotile fibers were completely decomposed and transformed into MgF2 and MgSiF6·6H2O by contacting CHClF2-decomposed acidic gas formed by superheated steam method at 150˚C for 30 min. MgSiF6·6H2O changed to MgF2 and Mg(OH)2 with increasing heating temperature, but any silicate compounds were not detected from the treated products. At high temperatures, silica components might evaporate in the form of SiF4 by the reaction with HF gas. Mg(OH)2 also transformed into MgF2 at high temperatures over 400˚C. At this high temperature, Fe2O3 which might be generated from asbestos setting tube by the oxidation corrosion was identified as impurity. Consequently, MgF2 was only the decomposition product from chrysotile fibers.

2.4. Analytical methods The before and after treatment of chrysotile samples

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Though the decomposition of CHClF2 with superheated steam generates HCl gas as shown in Eq. (1), possible chloride compounds such as MgCl2 were not detected in treated products. In the reaction system of this study, a small amount of water might exist in the decomposition gas. Therefore, chloride compounds might not be stable under these conditions and replaced by the fluoride compounds. Thus, the reactions of chrysotile fibers with CHClF2-decomposed acidic gas can be explained by following chemical equations with the increase in temperatures. Mg3Si2O5(OH)4 + 14HF + 3H2O → MgF2 + 2MgSiF6·6H2O (2) 2MgSiF6 + 2H2O → Mg(OH)2 + MgF2 + 2SiF4 + 2HF (3) (4) Mg(OH)2 + 2HF → MgF2 + 2H2O SEM and PCM observation were carried out to evaluate the decomposition products. In the treated products at 150˚C for 30 min with CHClF2-decomposed acidic gas, chrysotile fibers were not detected and disintegrated to form bulky particles from fibrous texture.

4. CONCLUSIONS Chrysotile fibers and chrysotile-containing slates as ACW were completely decomposed by contacting acidic gas formed by decomposition of CHClF2 by superheated steam method at 150˚C for 30 and 60 min, respectively. The decomposition of both chrysotile samples were accomplished by this technique at much lower temperatures, in comparison with the traditional melting method. REFERENCES 1. B. T. Mossman, J. Bignon, M. Corn, A. Seaton and J. B. L. Gee, Science, 247 (1990), 99. 294-301. 2. Japan Asbestos Association, Investigation Report of Predictive Waste Amount of Asbestos-Containing Building Materials, (2003) (in Japanese). 3. A. Kojima, M. Fujishige and R. Sato, J. Mater. Sci. Soc. Japan, 42 (2005), pp. 41-46 (in Japanese). 4. M. Fujishige, H. Obuchi, R. Sato and A. Kojima, J. Ceram. Soc. Japan, 114 (2006), pp. 355-358 (in Japanese). 5. M. Fujishige, R. Sato, A. Kuribara, I. Karasawa and A. Kojima, J. Ceram. Soc. Japan, 114 (2006), pp. 844-848. 6. M. Fujishige, R. Sato, A. Kuribara, I. Karasawa and A. Kojima, J. Ceram. Soc. Japan, 114 (2006), pp. 1133-1137. 7. T. Moriya and M. Kanazawa, Shigen Kankyo Taisaku, 33 (1997), pp. 932-937 (in Japanese). 8. Y. Saito, T. Moriya and M. Kanazawa, Jpn. Kokai Tokkyo Koho 3,219,689 (1998) (on Japanese). 9. Virta, R. L.; Open-File Report 02-149; U.S. GEOLOGICAL SURVEY, (2002) 10. A. M. Langer and R. P. Nolan, Ann. Occup. Hyg, 38 (1994), pp. 427-451. 11. Japanese Standards Association, Determination of asbestos in building material products, JIS A 1481: 2006 (2006) (in Japanese)

3.2. Chrysotile-containing slates In general, ACW such as chrysotile-containing slate contains cement components and the surface of asbestos is coated by them. We predicted that Freon-decomposed acidic gas might be easily penetrated into inside because the cement is a basic material and chrysotile in the slates would be decomposed under the same conditions for the decomposition of chrysotile fibers. The ground slate particles with 5 mass% content of chrysotile contacted CHClF2-decomposed acidic gas at 150˚C. We confirmed that X-ray diffraction peak of chrysotile in the slates was disappeared at 150˚C for 60 min by the reactions with CHClF2-decomposed acidic gas. The decomposition products mainly consisted of CaF2 and quartz but decomposed products of chrysotile origin such as MgSiF6·6H2O and MgF2 were not detected. The SEM and PCM observation was helpful to reveal the decomposition of chrysotile in the slates. The treated products at 150˚C for 60 min with CHClF2-decomposed acidic gas were powdery materials without chrysotile fibers. That is, this treated product was no longer defined as chrysotile-containing.

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