I. INTRODUCTION

Halons (bromofluorocarbons) have been used for material of extinguisher for the places of water prohibited, for example, computer room. Abilities for extinguishing of halons are high, while they were determined for ozone depletion substance and were forbidden to be produced at the end of 1993 in the world. Halons also have high global warming potential (GWP). The ozone depleting potential (ODP), GWP of halon (halon 1301) and reference substances are summarized in Table 1. Halon has 10 times larger ODP than that of chlorofluorocarbon-11 (CFC-11) and 5600 times larger GWP than that of CO2 [1]. Halon has been exchanged with substitutes. Decomposition methods of halon, however, have not been developed.

Halons are difficult to be decomposed with technically and economically. A few works of decompositions of halons have studied. Kennedey et al decomposed halon 1301 with equal molecular of methane [2]. Babushok et al studied the effect on halon 1301 to depress methane combustion [3]. Battin-Leclerc et al considered the effect on halon 1301 to oxidize methane [4]. Yamamoto et al researched halons for extinguishing [5]. Urano studied the decomposition gases of halon 1301 [6].

Typical exhaust gases by halon decomposition are toxic and corrosible, i.e. fluorine, chlorine, bromine, fluorohydride, chlorohydride and bromohydride. These gases are usually treated with liquid alkaline solution, i.e. calcium hydroxide solution, to collect halogen compounds. The system for exhaust treatment is needed besides decomposition equipment. Disadvantages of the equipments are large scale and complicate to be controlled. A new dry treatment method of halon decomposition and simultaneously adsorbing fluorine and bromine with solid alkaline material (reactant), i.e. calcium oxide, has been developed [1]. The purpose of this study is to develop the reactants to adsorb fluorine and bromine effectively. Light burnt dolomite with changing the burning conditions of temperature and time were prepared for the reactants. The influence of the reactants on the decomposition amount of halon, the recovery, the conversion, and the distributions of F and Br in the reactants were investigated with the pore distributions of the reactants in this study.

Table 1 ODP and GWP of halon and the reference substances.

CONSIDERATION OF HIGH TEMPERATURE REACTION BETWEEN HALON AND SOLID

ALKALINE REACTANTS

A.TAKEUCHI, S. KATO, T.WATANABE and Y.AKATSUKA*

Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology 2-12-1 O-okayama Meguro-ku, Tokyo 152-8550, Japan

*Chubu Electric Power Co., Inc. 20-1, Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan

Carbon halides, especially halons (bromofluorocarbons), are known for the material of both ozone depleting substances and global warming substances. Appropriate decomposition treatments of waste halons are important for global environment. A dry process to decompose halon was developed. Solid alkaline materials (reactants) of CaO with MgO heated over the temperature of 1000 K were used to decompose halon and to react with fluorine and bromine produced by the halon decomposition. The reactants with changing the burning temperature and time were prepared to react with the gasses from halon decomposition. The decomposition amounts of halon and the conversion were correlated with the pore distributions in the reactants.

Substance ODP GWPCO2 �| 1CFC-11 1 4000Halon1301 10.0 5600

6th World Congress of Chemical Engineering Melbourne, Australia 23-27 September 2001

II. EXPERIMENTAL 1. REACTANTS

Six kinds of reactants were prepared from dolomite with changing burning conditions. The burning conditions of dolomite are summarized in Table 2. Chemical compositions and chemical species analyzed with XRD are summarized in Table 3. Large amount of ignition losses indicate that large amounts of CaCO3 remain in the reactants. Three reactants with the burning at 1073 K for 1 and 3 h and the burning temperature at 1173 K for 1 h compose mainly CaCO3. The other reactants compose approximately 60 mass % of CaO and 30 mass % of MgO.

Table 2 Burning conditions of dolomite.

Burning temperature (K) 1073, 1173, 1273 Burning time (h) 1, 3

Table 3 Chemical compositions and species of the reactants.

2. EXPERIMENTAL APPARATUS

The Experimental apparatus is shown in Fig. 1. A ceramic tube (muraite, 24 mm inner diameter) was set in an electric furnace (Advantec KT-1153R). Reactants were set in the tube. Halon 1301 was introduced into the tube. Halon flow rate was controlled with a mass flow controller (Estec SEC-E440J). Exhaust after the treatment was analyzed for Br2 with a gastic reactotube (Gastec GV-100S) and for the composition with a gas chromatography (Hitachi GC 263-50), and neutralized with a scrubber (Dulton SCD-III-10). 3. EXPERIMENTAL METHOD

Halon flow rate was controlled at 32 ml/min. The ceramic tube with reactant (41.8 g, 2-4 mm diameter) was controlled at the temperature of 1173 K by the electric furnace. Halon reacts with the reactants as shown by equation (1).

CBrF3 + 2CaO -> 1/2CaBr2 + 3/2CaF2 + CO2 (1)

The experiments were finished after Br2 was detected. The amount of halon introduced till the end of the experiments defined as the decomposition amount of halon. F2 was not observed during the experiments.

Fig. 1 Experimental apparatus.

Halon 1301 Reactant

(Interior of the tube)Temperature controller

Halon 1301Ceramic tube

Electric furnace

Mass flow controller

pH meter

Scrubber

Sampling bag (GC)

gastic reactotube

or

(mass %)Burning

temperature (K)Burningtime (h) CaO MgO SiO2 Fe2O3 Al2O3 S P Ig. Loss Chemical species

1073 1 43.6 21.7 1.54 0.09 0.09 0.004 0.011 32.5 CaCO3�C(CaMg)CO3

1073 3 48.4 18.7 1.00 0.08 0.07 0.005 0.022 31.3 CaO�CMgO�CCaCO3

1173 1 51.3 28.0 0.63 0.07 0.07 0.005 0.015 19.7 CaO�CMgO�CCaCO3

1173 3 63.1 33.7 1.03 0.11 0.08 0.003 0.017 1.45 CaO�CMgO1273 1 63.2 35.4 0.62 0.09 0.08 0.005 0.018 0.52 CaO�CMgO1273 3 66.6 31.5 1.24 0.08 0.08 0.006 0.032 0.27 CaO�CMgO

6th World Congress of Chemical Engineering Melbourne, Australia 23-27 September 2001

4. ANALYSIS

Reactants before and after the treatments were analyzed by SEM (Scanning Electron Microscopy, Nikon ESEM-2700), EDX (Energy Dispersive X-ray Spectroscope, Horiba EMAX-5770W), XRD (X-ray diffractometry, Riga RINT-2000).

Fluorine and bromine concentrations in the reactants after the treatments were analyzed as follow. The reactants were crushed and stirred. With 3 g of NaOH flux, 0.3 mg of the reactants were stirred again. The reactants added a few drops of H2O were fused with the alkali flux for 30 min without vaporization of bromine and fluorine compounds. The samples after the alkali fusion were diluted with water until 100 ml. Na ions in the sample solutions were removed by cation exchange method (Unfertile ICR76 action exchange resin). Fluorine and bromine in the solutions were analyzed with IC (Ion Chromatography, JEOL PU-980; Shodex ICI-524A column). The reactants before and after the treatments were analyzed for their surface area with mercury porosimeter (Shimadzu Micromeritics Autopore III). III. RESULTS AND DISCUSSIONS 1. EXTERNAL VIEW OF REACTANT SEM photographs of the reactants before and after the halon decomposition treatment are shown in Fig. 3. The reactant before the treatment consists of grains with the diameter of 1-2 µm and pores under 1 µm. of The reactant after the treatment mainly consists of particles with the diameter over than 15µm. The reactant after the treatment partially melted owing to the production of CaBr2 with low melting point of 1003 K.

Fig. 3 Photographs with SEM of the reactant (a) before and (b) after the treatments. The burning condition at 1273 K and 1 h.

2. SPECIFIC SURFACE AREA OF THE REACTANTS

The specific surface areas of the reactants before and after the treatments are summarized in Table 4. The surface areas of the reactants increase with the burning temperature. The increase of the surface areas results from the decarboxylation of CaCO3. The burning temperature over the dissociation temperature of CaCO3, 1171 K, is needed to completely decarboxylate the reactants. All the surface areas with the same burning temperature decrease with the burning time. The decrease results from the growth of the grains in the reactants. The surface areas after the treatments decrease to 3-12 % of those before the treatments. The grains grow after reacting with fluorine and bromine. The growth of grains leads to reduce the surface areas.

Table 4 Specific surface area of the reactants. (m2/g)

Before After Burning time (h) Burning time (h) Burning temperature

(K) 1 3 1 3 1073 11.9 8.2 0.440 0.507 1173 25.0 15.0 0.757 0.891 1273 29.6 16.2 3.94 1.10

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6th World Congress of Chemical Engineering Melbourne, Australia 23-27 September 2001

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Fig. 5 Relationship between the burning conditions of the reactants and the decomposition amounts of halon.

The distributions of the surface areas are shown in Fig. 4. The distributions indicate that the reactant with higher burning temperature has larger pores. The pore distribution curves indicate that the reactants at 1073, 1173 and 1273 K and at 1 h have the maximum numbers of the pores of about 0.03, 0.06 and 0.12 µm, respectively. The reactants at 1073, 1173 and 1273 K and at 3 h also have the maximum number of the pores of about 0.05, 0.08 and 0.12 µm, respectively. Fig. 4 Relationship between the pore diameter and the specific pore areas of the reactants with burning time at (a) 1h and (b) 3h. 3. CONVERSION The relationships between the burning temperature and the decomposition amounts of halon are shown in Fig. 5. The decomposition amounts represent the capacity of the reactants to adsorb fluorine and bromine. The decomposition amounts of halon are almost constant at the different times. The decomposition amounts increase with the burning temperature of the reactants. Conversions of F and Br after the treatments are shown in Fig. 6 (a) and (b) respectively. Conversions of F and Br are calculated as follow. [Freact]/2 XF = (2) [CaOInitial + MgOInitial] [Brreact]/2 XBr = (3) [CaOInitial + MgOInitial] XM : Conversion of atom M. [MInitial] : Number of moles of molecular M before the treatments. [Mreact] : Number of moles of molecular M in the reactants after the treatments.

These equations indicate that the conversions are correlated with the decomposition amount of halon. The conversions increase with the burning temperatures and are almost constant with the burning time. The increases in the decomposition amounts of halon and the conversion can be correlated with the pore distributions in the reactants. Large pores are effective on the reactions of grains with fluorine and bromine. The grains grow by the reactions with fluorine and bromine. The growth of grains leads to reduce the pore areas. Small pores are reduced in the areas compared with large pores. Pores over 0.06 µm are effective on the reactions of fluorine and bromine with being considered in the correlation between the

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6th World Congress of Chemical Engineering Melbourne, Australia 23-27 September 2001

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Fig. 7 Recovery of (a) F, (b)Br after the halon decomposition treatments.

distributions of the specific surface areas and the decomposition amount of halon. A similar result was reported for the desulfulization [7]. The desulfulization amounts increased using CaO with pores of large diameter. Another factor affecting the decomposition would be the physical and chemical properties of the reactants changed by the burning temperature and time [8], however this factor is not clarified.

4. RECOVERY Recoveries of F and Br after the treatments are shown in Fig. 6 (a) and (b) respectively. Recovery is

calculated as follow.

[Freact]/2 RF = (4) [CBrF3 Initial] x 3 [Brreact]/2 RBr = (5) [CBrF3 Initial] RM : Recovery of atom M.

The recoveries of Br are almost constant at the different temperatures, and also almost constant at the

different times. The high recovery of Br indicates that halon is decomposed completely and bromine is recovered at all the conditions.

The recoveries of F are almost constant at the different times and slightly increase with the burning temperature. The reactants at 1073 K have smaller pores and are more influenced on being decreased in the effective surface areas by the growth of the grains than the reactants at 1273 K. Analysis of exhaust gases

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6th World Congress of Chemical Engineering Melbourne, Australia 23-27 September 2001

with gas chromatography observed by-products of mainly perfluorocaobons. The rest components with fluorine result from the by-products of perfluorocarbons. 5. DISTRIBUTION OF FLUORINE AND BROMINE IN THE REACTANTS The results of line analysis of the cross section of the reactants with EDX and SEM photograph of the reactant at 1273 K and 1 h are shown in Fig. 8 (a) and Fig. 8 (b), respectively. Figure 9 (a) and (b) are those of the reactant at 1073 K and 1 h. The reactants were cut to get the cross section without polishing, preventing from elution of fluorine and bromine. Functions of F/Br and Br/F are used to avoid the influence of the roughness. The distributions of F/Br and Br/F of the reactant at 1273 K and 1 h are shown in Fig. 10 (a) and (b), respectively. Figure 11 (a) and (b) are those of the reactant at 1073 K and 1 h. The distances from the surfaces of the left sides of the reactants are shown on the abscissa.

F in the reactant at 1273 K distributes within 300 µm from the surfaces. The amount of F around the center of the reactant is small. Large amount of F/Br also distributes near the both surfaces. These distributions indicate that fluorine reacts with CaO near the surfaces. On the other hand, large amount of Br distributes around the center of the reactant and small amount in the surface area. Br/F also distributes interior of the reactant. The reactivity of fluorine-CaO is higher than that of bromine-CaO. Fluorine reacts with CaO and produces CaF2. On the other hand, bromine reacting with CaO near the surface is smaller than fluorine since the reactivity of bromine-CaO is lower than that of fluorine-CaO. Bromine not reacting with CaO diffuses toward the interior of the reactants. The distributions of F and Br are influenced by the difference in the reactivities of fluorine-CaO and bromine-CaO. The result of the line analysis of the reactant at 1073 K shows that F distributes near the both surfaces of the reactant. The result also shows that Br distributes around the center of the reactant. The amounts of F and Br are smaller than those at 1273 K with the consideration of the small amount of decomposition of halon at 1073 K. The small amounts of F and Br are attributed to the small amount of the pores over 0.06 µm which are effective on the reaction. The amount of the effective pores is reduced by the growth of the grains containing CaBr2 with low melting point. The decrease of the effective pores results in the depression of diffusions of F and Br toward the interior. The decrease leads to the smaller amount of the decomposition of halon than that at 1273 K.

450 µm 450 µm

Fig. 8 (a)Photographs of SEM and (b) resultsof line analysis of the reactant with the burningtemperature at 1273 K after the treatment.

F Br

Ca

Mg

FBr

CaMg

(a) (a)

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Fig. 9 (a)Photographs of SEM and (b) results of line analysis of the reactant with the burning temperature at 1073 K after the treatment.

Inte

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6th World Congress of Chemical Engineering Melbourne, Australia 23-27 September 2001

IV. SUMMARY

Six kinds of the reactants made from dolomite with changing the burning conditions; the burning temperatures of 1073, 1173 and 1273 K, and the burning times of 1 and 3 h. The influence of the reactants on the decomposition amount of halon, the recovery, the conversion, and the distributions of F and Br in the reactants were investigated with the pore distributions of the reactants. (1) The reactants consisted of small grains with a diameter of 1-2 µm. The grains in the reactants after the

treatments grew over the diameter of 15 µm. (2) The specific surface areas of the reactants increased with the burning temperature because of

decarboxylation of CaCO3. The surface areas decreased with the burning time because of the growth of the grains in the reactants. The surface areas decreased to 3-12 % of that before the treatments.

(3) The capacity to decompose halon and to react with fluorine and bromine increased with the burning temperatures. The capacity is influenced by the distributions of the pores over 0.06 µm in the reactants.

(4) F distributed near the surfaces of the reactant. Br distributed interior of the reactant. The difference of the distributions can be correlated with the reactivities of fluorine-CaO and bromine-CaO.

REFERENCES [1] A. Takeuchi, Y. Akatsuka, S. Kato and T. Watanabe : J. Soc. of Inorg. Mater. Japan, in press. [2] E.M.Kennedy, K.Li, B.Moghtaderi, B.Z.Dlugogorski, Chem. Eng. Comm., 176, 195 (1999). [3] V.Babushok, T.Noto, D.R.F.Burgess, A.Hamins, W.Tsang, Combustion and Flame, 107, 351 (1996). [4] F.Battin-Leclerc, G.M.Come, F.Baronnet, Combustion and Flame, 99, 644 (1994). [5] T.Yamamoto, A.Yasuhara, F.Shiraishi, K.Kaya, T.Abe, Chemosphere, 35, 643 (1997). [6] K. Urano : NEDO Report “The trend for a counterplan toward the global warming of fluorocarbon substitutes”, 156 (1998). [7] M. Ueda, Y. Fujita, K. Nakai, T. Toshimori and Z. Morita : Tetsu-to-Hagane, 58,75 (1989). [8] Society of Inorganic Material : Plaster and lime handbook, 127 (1972).

Fig. 10 Distributions of the functions of (a) F/Br, (b) Br/F of the reactant with the burning temperature at 1273 K.

Fig. 11 Distributions of the functions of (a) F/Br, (b) Br/F of the reactant with the burning temperature at 1073 K.

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