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Ind. Eng. Chem. Res. 1998, 37, 894-900
Characteristics of Vaporization of Coal Ash Minerals Chlorinated by Gaseous Hydrogen Chloride Taihei Shimada,† Takaharu Kajinami,† Takehiko Kumagai,‡ Shohei Takeda,§ Jun-ichiro Hayashi,† and Tadatoshi Chiba*,† Center for Advanced Research of Energy Technology and Division of Material Science and Engineering, Graduate School of Engineering, Hokkaido University, N-13 W-8, Kita-ku, Sapporo 060, Japan, and Hokkaido National Industrial Research Institute, 2-17 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062, Japan
When heated in contact with PVC under argon gas flow, minerals in coal ash were chlorinated by HCl evolved from pyrolyzing PVC at 550-600 K and formed chlorides were vaporized above 900 K. The volatilization characteristics were thus systematically measured by heating coal ash at a rate of 10 K/min up to 1273 K in a thermogravimetric reactor under a continuous flow of HCl. The results indicated that Fe in ash was extracted nearly completely at 1000-1273 K. Complete chlorination of Ca compounds occurred above 600 K, but vaporization of CaCl2 hardly proceeded. Vaporization of chlorides of Si, Al, and Mg was also found but was much less extensive. The volatilization characteristics were explained by considering interactions among minerals, i.e., enhanced chlorination of Fe2O3 and CaSO4 by Al2O3. Fe2O3 volatilization resulted in a similarity of the mineral composition for the residual solids derived from bituminous coal ashes. Introduction Incineration and pyrolysis of solid refuse containing organochlorine compounds such as poly(vinyl chloride) (PVC) and polychlorinated biphenyls cause emission of chlorine as hydrogen chloride (HCl). This gives rise to some environmental problems due to the highly soluble and corrosive nature of HCl. Hence, it is useful to find methods to fix chlorine as a disposable or useful material. In literature, the removal of HCl from flue gases in municipal waste incineration, coal combustion, and gasification has been performed using calcium and/or sodium sorbents to fix chlorine by forming chlorides. Uchida et al. (1979), Daoudi and Walters (1991), and Weinell et al. (1992) employed a limestone, a calcined limestone, and/or a slaked lime for the removal. They reported that these sorbents were promising for the removal of HCl. Duo et al. (1996) attempted to use sodium sorbents, namely, Na2CO3, Na2CO3‚10H2O, and NaHCO3, together with the above calcium sorbents. They suggested advantages of the Na sorbents to the Ca sorbents because CO2 suppressed HCl sorption into the latter sorbents at 573-873 K. In contrast to these sorbents, only a few reports have been published on the reaction between iron oxides and HCl for extraction of iron from ores in a form of ferric chloride, which could be a possible way to fix chlorine. Reeve (1955) treated iron ores with gaseous HCl to extract iron as pure ferric chloride which can be easily converted to iron oxides or metallic iron. Khundkar and Ahmad (1995) carried out selective removal of iron from a ferruginous bauxite with HCl to concentrate alumina. Coal ash, which is inevitably discharged from coal utilization, contains various kinds of minerals, such as * Author to whom all correspondence should be addressed. Telephone: +81-11-706-6841. Fax: +81-11-726-0731. Email:
[email protected]. † Center for Advanced Research of Energy Technology. ‡ Division of Material Science and Engineering, Graduate School of Engineering. § Hokkaido National Industrial Research Institute.
oxides of iron, alkali metals (Na, K), and alkaline-earth metals (Ca, Mg) as well as silica and alumina (Vassilev et al., 1995). The mineral compositions of the coal ash generally differ depending on the sites where the parent coal is mined. This leads to difficulty in utilizing the coal ashes as a raw material. The present authors (Shimada et al., 1997) previously studied vaporization of coal ash minerals chlorinated by atmospheric chlorine gas in a temperature range up to 1273 K and found that iron, calcium, and magnesium oxides are easily converted to chlorides above 500 K even without an oxygen sink and complete vaporization takes place for iron and magnesium chlorides while hardly for calcium chloride. Among the mineral chlorides, those vaporized to the gas phase like FeCl3 and MgCl2 can either be deposited on the surface of a solid particle at lower temperature or be dissolved into a liquid, while those left in the residual solid like CaCl2 can easily be recovered by washing with water. The volatilization characteristics of minerals were well explained on the basis of those of pure Fe2O3, CaO, and MgO. It was also found that the extraction of those oxides resulted in uniform mineral compositions of the residual solids. This in turn gave rise to a significant reduction of difference in their melting point temperatures as well as acid-base indices, suggesting homogenization of the ashes. In Figure 1 the standard Gibbs free-energy change (Kubaschewski et al., 1967), ∆G°, is plotted versus temperature for reactions of Fe2O3, CaO, and MgO with Cl2 or HCl. It is seen from the figure that chlorination of these oxides is thermodynamically more favorable by HCl than Cl2 below 850 K. The above literature survey leads to fixation of HClderived chlorine on coal ashes. The present experimental study thus aims to make clear characteristics of vaporization of coal ash minerals chlorinated by gaseous HCl. Experiments were performed for ashes having different mineral compositions. They were heated in a thermogravimetric reactor at a rate of 10 K/min up to a holding temperature of 1273 K under HCl gas flow. Analyses were conducted for elemental and mineral compositions of the residual
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Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 895
Figure 2. Schematic diagram of the experimental apparatus.
Figure 1. Changes in ∆G° with temperature for reactions of metal oxides with Cl2 or HCl. Table 1. Proximate and Ultimate Analyses of Parent Coals proximate analyses, wt % coal
Mois
ash
Yallourn Montana Taiheiyo Illinois Datong
8.4 5.0 3.8 7.0
11.8 13.8 11.2 8.5
a
V.M.b F.C.c 37.3 42.7 39.1 29.5
42.5 38.5 45.9 55.0
ultimate analysis, d.a.f.a wt % C
H
O
N
S
72.9 74.7 80.2 84.4
5.1 6.1 6.0 4.4
23.6 17.7 12.3 10.8
0.7 0.7 0.7 0.4
0.9 0.4 3.4 0.3
Dry ash free. b Volatile matter. c Fixed carbon.
solids. A treatment of coal ash with PVC as a chlorine source was examined to an extent. Experimental Section Ash Samples, Model Compounds, and PVC. Three bituminous coals (Taiheiyo, Illinois, and Datong coals), a lignite (Montana coal), and a brown coal (Yallourn coal) were employed as the parent ash samples having widely different compositions of elements and minerals. The features of elemental and mineral compositions of the ash samples presently employed are mentioned below. Analytical data of the coals are listed in Table 1. High-temperature ash (HTA) samples were prepared by burning the individual coals except for Yallourn coal with air in a muffle furnace at 1088 K for 1 h. The ash sample from Yallourn coal was prepared by treating as above Yallourn coal ash supplied from a company. All HTAs were pulverized and sieved to sizes finer than 74 µm. In addition to these samples, four metal oxides (CaO, MgO, Fe2O3, and SiO2-13% Al2O3), a hydroxide (Ca(OH)2), and a sulfate (CaSO4), which are representa-
tive components in the ashes used, were employed as the model compounds. They had purities of 99-99.99 wt % and were used after being heated under argon gas flow to remove absorbed moisture. Powdered PVC with a polymerization degree of 1020 was employed as a chlorine source. Apparatus and Procedure. Experiments were conducted in a thermogravimetric reactor shown in Figure 2. The reactor was made of a quartz-glass tube with an inner diameter of 30 mm and a length of 1.35 m. At each experimental run, a sample was loaded in a 10 mm i.d. and 10 mm deep quartz-glass basket, which was hung in the reactor by fine quartz threads connected in series to an iron rod with a diameter of 2 mm and a length of 30 mm joined to a quartz-glass spring. The rod was enclosed in a quartz-glass tube to keep it from being exposed to HCl. The thermal expansion coefficient of the spring was determined prior to use. Displacement of the rod from its initial position was measured continuously with a linear variable differential transformer which can detect a weight difference of 0.0001 mg. The output was stored in a personal computer and then converted to the weight change. The sample was heated in an electric furnace at a rate of 10 K/min to 1273 K where the temperature was held for 60 min. The temperature was measured with a thermocouple inserted from the reactor bottom to a distance of about 15 mm below the basket bottom through a 4 mm o.d. quartz tube. Gases were fed through mass flow controllers at prescribed flow rates after filling the reactor with argon. HTAs were subjected to HCl treatments in which HCl or that formed from PVC was used as a chlorine source. In the treatment with PVC, 0.03 g of PVC was loaded in the basket in the form of a layer over which 0.05 or 0.10 g of the ash sample was placed. They were heated in an atmospheric flow of argon gas at a rate of 200 cm3/ min. The effluent gas was bubbled into 200 cm3 of distilled water stirred at 500 rpm. A cumulative amount of HCl was continuously determined from the concentration of proton which was measured with a pH meter. For the treatment with HCl, the ash sample was heated in an atmospheric flow of mixed gases of HCl and argon (10/90 v/v) at a rate of 100 cm3/min. Analyses. Elemental compositions of the original and HCl-treated ashes (H-HTAs) were determined by atomic absorption spectroscopy. A total of 0.05 g of each
896 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Table 2. Elemental Composition of Ash Samples element, wt % Yallourn Montana Taiheiyo Illinois Datong Si Al Fe Ca Mg K Na Ti S ig. lossa a
1.5 1.1 21.7 11.9 27.3 0.2 4.5 0.1 6.2 0
11.4 7.9 1.2 22.2 2.4 0.2 1.0 0.4 4.5 0
25.2 13.9 2.6 4.4 1.6 1.2 0.9 1.0 0.7 0
21.9 9.8 8.8 3.1 1.1 1.4 0.6 0.7 1.1 0
32.0 7.4 6.3 1.3 0.7 0.8 0.1 0.4 0.6 0
Ignition loss.
Figure 4. Changes in w during HCl treatment using PVC.
residual solid weight fraction, w, based on the initial weight of PVC loaded is plotted against temperature for the heating-up period and time for the succeeding period at a holding temperature of 1273 K. Here, w is defined as
w)
Figure 3. X-ray diffractograms of HTAs.
sample was completely dissolved at 388 K in a mixture of 20% HCl, 60% HNO3, and 48% HF aqueous solutions with the respective volumes of 0.75, 0.25, and 1.0 cm3. Boric acid was then added to the solution to mask the fluorine ion. The solution was finally diluted for analysis. Table 2 summarizes the elemental compositions of HTAs. Major differences in the elemental compositions of HTAs are as follows: (1) Si and Al contents are 30-40 wt % for Taiheiyo, Illinois, and Datong HTAs, while those for Yallourn HTA are significantly lower, (2) Fe is abundant in Yallourn, Illinois, and Datong HTAs, and (3) alkaline-earth metals, i.e., Ca and Mg, are abundant particularly in Yallourn and Montana HTAs. Ignition loss of HTA was determined by burning it at 1173 K for 60 min. It is apparent from the table that ignition losses of all HTAs are zero, suggesting no residual carbon in them. Mineral compositions of the original and HCl-treated ashes were analyzed by X-ray diffractometry. The diffractograms of HTAs are shown in Figure 3. It is seen that the major form of Fe is hematite. Further, Ca and Mg are found respectively as CaO, Ca(OH)2, and CaSO4 and as MgO. Though not shown here, there were a few types of silicates of Mg and Ca. Results and Discussion HCl Treatment of Ash Using PVC. The treatment employing PVC as a chlorine source was carried out for 0.05 g of Montana or Illinois HTA. In Figure 4 the
WPVC - (WASH,0 - WASH) WPVC,0
(1)
where WPVC, WASH, WPVC,0, and WASH,0 are the weights of residual PVC and ash and their initial weights, respectively. When PVC is heated alone, i.e., WASH,0 and WASH are both equal to zero, w seems to decrease with temperature in two stages, the first stage from 550 to 600 K and the second from 650 to 800 K. The decrements of w are about 0.60 and 0.25 in the first and second stages, respectively. It is also noted that w remains constant above 800 K at w of 0.13. The result well agrees with that by Wu et al. (1994), who found that the weight fraction of residual solid falls down to around 0.15 above 813 K. They further indicated that the weight losses of PVC in the first and second stages are due to evolution of HCl and hydrocarbons, respectively, leaving the nonvolatile carbonized solid. The above tendency is also observed for PVC heated in contact with HTA (PVC-HTA) in the same temperature ranges. However, the decrement of w in the first stage is smaller for PVC-HTA, especially for PVC-Montana HTA, than that for PVC alone. This would be due to HCl adsorption on ash and/or subsequent chlorination of metal oxides in ash. The decrement in the second stage is equivalent to that for PVC alone. In addition, the decrease of w above 1000 K for PVC-HTAs suggests vaporization of metal chlorides formed in the lower temperature range. By assuming that the residual carbonaceous solid from PVC is inert, WASH/WASH,0 was calculated from eq 1 as 0.88 and 0.93 for Montana and Illinois HTAs, respectively, at 1273 K and a holding time of 60 min. Figure 5 shows temperature- and time-dependent changes in the weight fraction of effluent HCl, wHCl, based on WPVC,0. For PVC alone, wHCl starts to increase
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 897
Figure 6. Difference between weight fractions before and after HCl treatment for elements in HTAs.
Figure 5. Changes in wHCl during HCl treatment using PVC.
above 550 K and levels off at 600 K, indicating completion of HCl evolution. The final value of wHCl, 0.60, well agrees with the value calculated by assuming that chlorine in PVC is completely converted to HCl, i.e., 0.584. For PVC-HTAs, wHCl also begins to increase at 550 K, whereas it reaches 0.36 and 0.50 at 600 K for Montana and Illinois HTAs, respectively. Since the difference of wHCl for PVC-HTA from that for PVC alone indicates the weight fraction of HCl captured by HTA at any temperatures and times, about 60 and 15% of HCl evolved from PVC are captured by Montana and Illinois HTAs, respectively. This is consistent with the above-described w for PVC-HTAs larger than that for PVC alone in the range from 550 to 600 K. The increase in wHCl above 900 K observed for PVC-Montana HTA is probably due to desorption of HCl and/or further reactions of metal chlorides with hydrogen in char to form HCl. In the figure, changes of wHCl in the case of 0.10 g of Montana HTA are also drawn. In this case, the increase of wHCl initiates around 550 K. Then wHCl remains constant at 0.04 up to 900 K, which is followed by a rapid increase from 1000 K. The increase deteriorates above 1100 K, and finally wHCl reaches 0.35. This indicates that a substantial portion (93%) of HCl evolved in the first stage is captured by the HTA. This portion is much larger than that for the case in which 0.05 g of the HTA was used (60%). It should also be noted that the final values of wHCl between those for the above cases are close to each other in spite of the dependence of wHCl on WASH,0 in the first stage. A larger fraction, i.e., 55%, of the captured HCl is released for the latter than for the former (28%). Though not shown, the same dependency as above was observed for Illinois HTA. From these results, it was shown that minerals in HTAs were chlorinated by HCl and formed chlorides were vaporized. This does not immediately mean in practical processes that the mixture of ash and PVC particles should be treated in a reactor because the temperature for HCl evolution from PVC is much lower than that for volatilization of minerals. Here, a twostage reactor system is conceptually assumed, where a reactor is for PVC pyrolysis at low temperature and the other is for volatilization of minerals at high tempera-
ture. In this case HCl gas evolved in the former is supplied to the latter. For systematic evaluation of the effect of concentration of HCl and temperature on chlorination/vaporization, HTAs were heated in continuous flow of HCl and the results are described below. Treatment of Ash with Flowing Hydrogen Chloride Gas. As expected, the treatment with HCl gas brings about appreciable changes in elemental and mineral compositions of HTA. Figure 6 compares the elemental distributions in the initial HTA with those in H-HTA. Here, the bars on the left- and right-hand sides for each ash indicate the weight fractions of elements in HTA and H-HTA, respectively. The extensive decrease of the total weight fractions of the metallic elements confirms the occurrence of volatilization of minerals through chlorination. In particular, the weight fraction of Fe decreases dramatically for all HTAs. More than 83% of initially contained Fe is extracted from HTAs other than Yallourn HTA which exhibits an extent of extraction of 40%. The limited extraction is probably due to formation of molten chlorides of alkali and alkaline-earth metals which might become a diffusion barrier to inhibit the reaction of iron oxide with HCl. Volatilization of minerals containing Mg, Ca, Si, or Al is also evidenced but much less significant. Figure 7 illustrates XRD diffractograms of H-HTAs. Comparison of the diffractograms with those of the original HTAs shown in Figure 3 reveals that mineral compositions are varied by the HCl treatment. Peaks attributed to CaCl2 are clearly seen only for Montana HTA, suggesting its vaporization is slower than its formation. It should be noted that the disappearance of peaks ascribed to hematite in the diffractograms of HTAs results in a similarity among those of H-HTAs from the three bituminous coals. Such a similarity was also found for these HTAs treated with gaseous Cl2 due to the disappearance of hematite (Shimada et al., 1997). In order to evaluate characteristics of volatilization of minerals in HTAs by the HCl treatment, it would be important to know those of individual constituents. Hence, the model compounds were subjected to the treatment. Figure 8 shows temperature- and timedependent changes in the weight fraction of residual solid, w, for Fe2O3, MgO, and SiO2-13% Al2O3 on the basis of their initial weights. The horizontal lines drawn in the figure indicate the stoichiometric weight fraction for each compound at its complete conversion
898 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 7. X-ray diffractograms of H-HTAs. Figure 9. Changes in w during HCl treatment for CaO, Ca(OH)2, and CaSO4.
Figure 8. Changes in w during HCl treatment for Fe2O3, MgO, and SiO2-13% Al2O3.
to the chloride without vaporization of the chloride. For Fe2O3, though not shown in the figure, w increased even at ambient temperature, probably due to adsorption of HCl. The increase continued for about 300 min and terminated at w of 1.7. It is seen that w decreases with increasing temperature rapidly in the ranges from 450 to 500 K and from 600 to 750 K. As shown in Figure 1, chlorination of Fe2O3 by HCl is thermodynamically possible at 300 K. By assuming that the increase in w of 1.7 resulted from FeCl3 formation, the weight fraction of unreacted Fe2O3 would be about 0.3. However, the rapid decrease in w from 450 to 750 K terminates at w of 0.8. This indicates that all of the increase cannot be attributed to FeCl3 formation. Since appreciable vaporization of FeCl3 commences at about 500 K (Shimada et al., unpublished results), the decreases in w from 450 to 500 K and from 600 to 750 K are probably due to desorption of HCl and vaporization of FeCl3 formed at lower temperature, respectively. As the temperature
further increases, w decreases moderately and continues to decrease for the temperature holding period at 1273 K. The volatilization was completed at a holding time of 220 min. For MgO, w increases above 400 K, reaches a plateau over the range of 800-1250 K, and then decreases at higher temperature. The decrease terminated at a holding time of 120 min, leaving the residue at w of 0.5. Since, for MgCl2, vaporization occurs above 1200 K (Shimada et al., unpublished results), the stationary value of w from 800 to 1250 K indicates that chlorination of MgO ceases at about 60% of its conversion to MgCl2 in the temperature range. For SiO2-13% Al2O3, w slightly decreases above 400 K without exhibiting any increase and attains 0.92 at 60 min passed the holding temperature. Because of the low vaporization temperatures of SiCl4 (330.6 K) and AlCl3 (455.7 K) (Liley et al., 1984), chlorination of SiO2-13% Al2O3 is considered to be fairly slow. In Figure 9, w for the calcium compounds is plotted in the same manner as in Figure 8. For CaO and Ca(OH)2, w increases above 600 K and levels off respectively at ca. 1000 and 900 K. The increment of w being nearly stoichiometric indicates their complete conversion to CaCl2. Furthermore, no vaporization of CaCl2 is also noted from the constant values of w above 1100 and 900 K for CaO and Ca(OH)2, respectively. On the other hand, w for CaSO4 slightly decreases above 700 K. Considering little vaporization of CaCl2 as described above, the observed decrease of w is ascribed to an incomplete conversion of CaSO4 to CaCl2 under the present conditions. In Figure 10 the weight fraction, w, for HTA is plotted against temperature and holding time at 1273 K. For Illinois, Datong, and Taiheiyo HTAs, w slightly increases below 700 K, significantly decreases above 1000 K and falls down to a steady value depending on the initial composition for the temperature holding period. From the results shown in Figure 6, the decrease in w for these HTAs can be attributed to volatilization of Fe2O3. The order of decrement of w for these HTAs seems to correspond to that of their Fe contents. However, the above tendency is not consistent with that observed for pure Fe2O3 shown in Figure 8. On the
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 899
Figure 10. Changes in w during HCl treatment for HTAs.
other hand, Yallourn and Montana HTAs give changes in w different from those of the other HTAs. w increases above 300 K, decreases via a maximum at around 900 K, and continues to decrease for Yallourn HTA, while it becomes invariable for Montana HTA at the holding temperature. This can be explained by the contents of CaO, Ca(OH)2, and MgO in Yallourn and Montana HTAs much higher than those in the other HTAs and by the volatilization characteristics of these compounds (Figures 8 and 9). For Yallourn HTA the decrease of w seems to take place in two steps before and after reaching the holding temperature where chlorides of Fe and Mg are respectively vaporized. On the other hand, the decrease of w at 1100-1273 K observed for Montana HTA cannot be simply explained from the volatilization characteristics of the model compounds (Figures 8 and 9) as well as from the change in elemental distribution (Figure 6). Thus, presuming effects of interactions among different minerals on chlorination/vaporization, the treatment was applied for mixtures of model compounds. Figure 11 depicts the results for the mixtures of SiO2-Al2O3-CaSO4-Ca(OH)2 and SiO2-CaSO4Ca(OH)2 which were prepared by a mechanical mixing at the prescribed weight ratios listed in Table 3. From the figure, it is clear that a drastic decrease of w occurs in the temperature range from 1100 to 1273 K when CaSO4 coexists with Al2O3. The temperature range well coincides with that where the decrease of w is observed for Montana HTA (Figure 10). Differences between elemental distributions in the mixtures of SiO2-Al2O3CaSO4 and SiO2-Al2O3-Ca(OH)2 before and after the HCl treatment are shown in Figure 12. For each mixture, the bars on the left- and right-hand sides indicate the weight fractions for the initial mixture (MIX) and HCl-treated mixture (H-MIX), respectively. It can be seen that weight fractions of Ca and Al for both mixtures remain unchanged by the HCl treatment while about 10% of initially contained Si is extracted. This indicates that extraction of metallic elements does not cause the drastic decrease of w for SiO2-Al2O3CaSO4 as shown in Figure 11. Hence, it can be concluded that the conversion of CaSO4 to CaCl2 is enhanced in the presence of Al2O3 at around 1100 K,
Figure 11. Changes in w during HCl treatment for mixtures of SiO2-Al2O3-CaSO4-Ca(OH)2 and SiO2-CaSO4-Ca(OH)2. Table 3. Mass Ratios of Mixtures mixture
SiO2
SiO2-Ca(OH)2 SiO2-Al2O3-Ca(OH)2 SiO2-CaSO4 SiO2-Al2O3-CaSO4 SiO2-Al2O3-Ca(OH)2 -CaSO4 Ca(OH)2-CaSO4 SiO2-Al2O3-Fe2O3 (10%) SiO2-Al2O3-Fe2O3 (25%) SiO2-Al2O3-Fe2O3 (50%)
1
SiO2-13% Ca(OH)2 CaSO4 Fe2O3 Al2O3 1
1 1
1 1 1 9 3 1
1
1 1 1
1
1 1 1 1
Figure 12. Difference between weight fractions before and after HCl treatment for elements in mixtures of SiO2-Al2O3-CaSO4 and SiO2-Al2O3-Ca(OH)2.
whereas Ca(OH)2 is almost completely converted to CaCl2 independently of the presence of Al2O3. In Figure 13, changes in w for the mixtures of SiO2-Al2O3-Fe2O3 are plotted against increasing temperature and holding time at 1273 K. For all mixtures, w starts to decrease from around 400 K after increasing at ambient temperature depending on Fe content and falls down to a
900 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
temperature ash (HTA) prepared from five different coals. HTA was heated at a rate of 10 K/min up to 1273 K, which was maintained for 60 min. When heated in contact with PVC, minerals in HTA were chlorinated by HCl evolved from PVC at around 550 K, followed by the vaporization of metal chlorides formed. HCl captured by HTA was not completely consumed for chlorination and was partly released as the temperature further increased. Vaporization characteristics of minerals chlorinated by HCl gas were investigated by heating HTAs under a continuous flow of HCl gas diluted by argon. Elemental analyses of the HTAs and HCl-treated ashes (H-HTA) revealed that Fe in HTAs was extracted nearly completely. Vaporization of chlorides of Si, Al, Ca, and Mg was also found but was much less extensive. Volatilization characteristics of HTAs were not simply explained from those of the individual model compounds but done from interactions between Fe2O3 or CaSO4 and SiO2Al2O3. Volatilization of Fe2O3 resulted in similar mineral compositions of H-HTA derived from bituminous coal HTAs with different initial compositions. Figure 13. Changes in w during HCl treatment for mixtures of SiO2-Al2O3-Fe2O3.
Acknowledgment The authors thank the Research Institute of Innovative Technology for the Earth (RITE), who financially supported the research in part. Literature Cited
Figure 14. Difference between weight fractions before and after HCl treatment for elements in mixtures of SiO2-Al2O3-Fe2O3.
steady value for the mixtures containing 10 and 25% of Fe2O3 while it continues to decrease for that containing 50% of Fe2O3. The decrement is larger for mixtures containing more Fe2O3. Figure 14 exhibits the differences of elemental distributions of MIX from those of H-MIX for the mixtures of SiO2-Al2O3-Fe2O3. It is evident that vaporization of FeCl3 proceeds completely for the mixtures containing 10 and 25% of Fe2O3, whereas about 13% of initially contained Fe is left for that containing 50% of Fe2O3. It should also be noted that about 65, 25, and 14% of initially contained Al are extracted by the treatment for the mixtures containing 10, 25, and 50% of Fe2O3, respectively. The extent of SiO2 volatilization is around 12% for all mixtures. These indicate that Fe2O3 is completely chlorinated at fairly low temperatures below 1273 K when it is substantially diluted by SiO2-Al2O3. Conclusions HCl treatment using HCl or PVC as a chlorine source was conducted in a thermogravimetric reactor for high-
Daoudi, M.; Walters, J. K. A Thermogravimetric Study of The Reaction of Hydrogen Chloride Gas with Calcined Limestone: Determination of Kinetic Parameters. Chem. Eng. J. 1991, 47, 1. Duo, W.; Kirkby, N. F.; Seville, J. P. K.; Kiel, J. H. A.; Bos, A.; Uil, H. D. Kinetics of HCl Reactions with Calcium and Sodium Sorbents for IGCC Fuel Gas Cleaning. Chem. Eng. Sci. 1996, 51, 2541. Khundkar, M. H.; Ahmad, N. Selective Removal of Iron from Ferruginous Bauxite. J. Indian Chem. Soc., Ind. News Ed. 1955, 18, 109. Kubaschewski, O.; Evans, E. LL.; Alcock, C. B. Metallurgical Thermochemistry, 4th ed.; Pergamon: Oxford, U.K., 1967. Liley, P. E.; Reid, R. C.; Buck, E. Physical and Chemical Data. In Perry’s Chemical Engineers’ Handbook, 6th ed.; Perry, R. H., Green, D., Eds.; McGraw-Hill Book Co.: New York, 1984. Reeve, L. Development of Chemical Treatment of Low-Grade Iron Ores at Appleby-Frodingham. J. Iron Steel Inst. 1955, 181, 26. Shimada, T.; Kumagai, T.; Tsurue, T.; Nakata, Y.; Okutani, T.; Takeda, S.; Hayashi, J.-i.; Chiba, T. A Preliminary Experiment of Volatilization of Minerals in Coal Ash by Chlorination Treatment. J. Jpn. Inst. Energy 1997, 76, 134. Shimada, T.; Hayashi, J.-i.; Chiba, T. (Hokkaido University). Unpublished results. Uchida, S.; Kageyama, S.; Nogi, M.; Karakida, H.; Kakizaki, T.; Tsukagoshi, K. Reaction Kinetics of HCl and Limestone. J. Chin. Inst. Chem. Eng. 1979, 10, 45. Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Influence of Mineral and Chemical Composition of Coal Ashes on Their Fusibility. Fuel Process. Technol. 1995, 45, 27. Weinell, C. E.; Jensen, P. I.; Dam-Johansen, K.; Livbjerg, H. Hydrogen Chloride Reaction with Lime and Limestone: Kinetics and Sorption Capacity. Ind. Eng. Chem. Res. 1992, 31, 164. Wu, C. H.; Chang, C. Y.; Hor, J. L.; Shin, S. M.; Chen, L. W.; Chang, F. W. Two- Stage Pyrolysis Model of PVC. Can. J. Chem. Eng. 1994, 72, 644.
Received for review July 14, 1997 Revised manuscript received November 14, 1997 Accepted November 25, 1997 IE970495+
