Reaction of Tricalcium Aluminate with Hydrogen Chloride under

May 2, 2012 - Tricalcium aluminate (3CaO·Al2O3, C3A) is considered to be a precursor of complex oxychlorides, and its reaction with HCl was investiga...
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Reaction of Tricalcium Aluminate with Hydrogen Chloride under Simulated Bag Filter Conditions Kouki Kasuya, Naomi Onodera, Atsushi Iizuka,* Etsuro Shibata, and Takashi Nakamura Research Center for Sustainable Science and Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Sendai, Miyagi, 980-8577, Japan ABSTRACT: The influence of metals present in fly ash on the dechlorination reaction that occurs at bag filters installed in municipal waste incinerators and melting furnaces has not been investigated previously. Tricalcium aluminate (3CaO·Al2O3, C3A) is considered to be a precursor of complex oxychlorides, and its reaction with HCl was investigated in a gas flow reactor under simulated bag filter conditions (473 K, 1000 ppm HCl, 30 vol% H2O, 5 vol% O2). The molar chlorination ratio of C3A leveled off at about 1 after 24 h. The chlorination reaction rate of C3A was similar to that of calcium hydroxide (Ca(OH)2). C3A chlorination products had a mainly amorphous structure. Elemental mapping images of the surface of chlorinated C3A indicated the likely generation of complex oxychlorides of calcium and aluminum. It is possible that complex oxychlorides are produced at bag filters from complex oxides generated in high-temperature incinerators.

1. INTRODUCTION Hydrogen chloride (HCl) gas is generated in municipal waste incinerators and melting furnaces from the chlorine present in waste. Calcium hydroxide [Ca(OH)2] powder is generally used as a dechlorination agent to remove HCl from flue gas. Recently, dry powder blowing systems have been applied for this purpose. In these systems, Ca(OH)2 powder is blown into the flue gas before the bag filter where it reacts with HCl gas in a gas−solid reaction. This gas−solid reaction has been investigated in many studies.1−15 Ideally, the chlorine is removed from the flue gas as calcium chloride (CaCl2): Ca(OH)2 + 2HCl → CaCl 2 + 2H 2O

on tricalcium aluminate (3CaO·Al2O3), which is thought to be a precursor of Friedel’s salt and other complex oxychlorides. The results were compared with those for Ca(OH)2.

2. MATERIALS AND METHODS 2.1. Materials. Tricalcium aluminate (C3A) was synthesized from reagent-grade calcium carbonate (CaCO3) and α-alumina (Al2O3) using a previously described process.15 Both reagents were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). CaCO3 was mixed with Al2O3 at a molar ratio of 3 to 1 and then maintained at 1723 K for 3 h. C3A was formed by the following reaction: 3CaCO3 + Al 2O3 → 3CaO·Al 2O3 + 3CO2 (3)

(1)

The CaCl2 can then be easily removed from ash by washing with water, which allows the ash to be reused for cement production. However, it has been reported that the major product of the gas− solid reaction at the bag filter is calcium oxychloride (CaClOH):8 Ca(OH)2 + HCl → CaClOH + H 2O

The product was analyzed using an X-ray diffractometer (XRD) (RINT2200, Rigaku Corp., Tokyo, Japan), and this confirmed it to be C3A. The C3A was crushed to form a powder using an agate mortar. Reagent-grade Ca(OH)2 was purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan) and was used in the experiments without further processing. The particle sizes of the crushed C3A and Ca(OH)2 powder were measured with a laser light scattering particle size analyzer (MT3300, Nikkiso Co. Ltd., Tokyo, Japan) and were found to be in the ranges 2−300 μm and 1−10 μm, respectively. Figure 1 shows scanning electron microscope (SEM, S-4800, Hitachi Ltd., Tokyo, Japan) images of the surfaces of C3A and Ca(OH)2 samples. The C3A particles appeared to have an angular shape, whereas smaller Ca(OH)2 particles formed aggregates. The BET surface areas of the crushed samples were 4.62 and 6.29 m2/g for C3A and Ca(OH)2, respectively. The BET surface area was measured with a surface area and pore size analyzer (BELSORP-mini II, Bell Japan, Inc., Osaka, Japan)

(2)

Furthermore, other constituents of flue gas and metal elements in fly ash may affect the gas−solid reaction. The solubility of chlorination products can strongly affect the efficiency of the washing treatment to remove chlorine from collected fly ash. The influences of coexisting gases such as water vapor,2,4,7−9,11 sulfur dioxide (SO2),4 and carbon dioxide (CO2)12 on the dechlorination reactions have been reported. However, the effect on the chlorination reaction of metals such as Al, Fe, and Pb contained in fly ash has not been reported despite its great importance. Complex oxychloride compounds could be generated in the presence of metals in fly ash. It has been reported that insoluble complex oxychlorides such as Friedel’s salt (3CaO·Al2O3·CaCl2·10H2O) were present in fly ash collected from bag filters.14 A plausible reaction path to generate these complex oxychlorides is the generation of complex oxides in an incinerator at high temperature and subsequent chlorination reactions at the bag filter. In this study, we conducted chlorination experiments using a gas flow reactor © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6987

March 16, 2012 April 28, 2012 May 2, 2012 May 2, 2012 dx.doi.org/10.1021/ie300707e | Ind. Eng. Chem. Res. 2012, 51, 6987−6990

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syringe (81201, Hamilton, Reno, NV, USA) using a syringe pump (IC3100, Kd Scientific, Holliston, MA, USA). The temperature of the narrow glass tube was sufficient to vaporize the water, and the injected water was supplied in the gaseous phase to the reaction tube. The total gas flow rate of the mixed reaction gas was 200 cm3/min (measured at standard ambient temperature and pressure). The linear flow rate of the reaction gas corresponded to 0.58 m/min in the reaction tube; this value lies within the range of values reported for linear flow rates of flue gas at bag filters (0.5− 1.3 m/min).16 The outlet gas from the reaction tube was treated with 100 mL of 0.1 mol/L sodium hydroxide (NaOH) solution to collect unreacted HCl. At predetermined times, samples were recovered from the reaction tube and were analyzed. 2.2.3. Analytical Methods. The amounts of chlorine, calcium, and aluminum present in the samples after the chlorination experiments were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Optima 3300XL, PerkinElmer, Waltham, MA, USA). The longest experiment period was set at 24 h because the cleaning interval of bag filter depends on operation conditions, but could be 1 day. Chlorinated sample material on the surface of the aluminum balls was leached using a 3.2 mmol/L NaOH solution followed by dilute sulfuric acid (0.10 mmol/L). The leaching solutions obtained were fed to the ICP−AES system for analysis. The molar chlorination ratios in the samples (Cl/Ca) were then determined. The crystalline forms of the chlorinated samples were analyzed using an XRD (RINT2200, Rigaku Ltd., Tokyo, Japan). The surfaces of the chlorinated powder samples were observed by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX, S-4800, Hitachi, Tokyo, Japan).

Figure 1. SEM images of C3A (left) and Ca(OH)2 powder (right).

2.2. Methods. 2.2.1. Sample Preparation for Chlorination Experiments. The sample powder [220 mg of C3A or Ca(OH)2] was shaken with 35 g of high-purity aluminum oxide balls (AL9−1, φ = 1 mm, Hira Ceramics Co. Ltd., Toyota, Japan) for 20 min. In this way, the surfaces of the aluminum oxide balls were thinly coated by the sample powder. These coated aluminum oxide balls were then used in the chlorination experiments. 2.2.2. Chlorination Experiments. A gas flow reactor (Figure 2) was fabricated using quartz glass and Teflon parts. A quartz tube

3. RESULTS AND DISCUSSION Figure 3 shows the time course of the chlorination ratio (Cl/Ca) of C3A samples during the reaction with HCl under simulated bag Figure 2. Schematic of the experimental apparatus: (1) Control valve; (2) mass flow controller; (3) gate valve; (4) pressure gauge; (5) heat furnace; (6) reaction tube (quartz); (7) sample-coated alumina oxide balls; (8) porous quartz plate; (9) syringe containing water; (10) syringe pump; (11) vacuum pump; (12) 0.1 mol/L NaOH(aq); (13) gas mixture; (14) glass beads; (15) 1% HCl gas cylinder (argon balance); (16) oxygen gas cylinder; (17) argon gas cylinder.

(outer diameter, 27 mm; inner diameter, 21 mm; length, 50 cm) was used as the reaction tube. A porous quartz plate (pore size, 10−20 μm) was placed at the center of the reaction tube and was held in place by a smaller quartz tube (outer diameter, 20 mm; inner diameter, 17 mm; length, 20 cm). Aluminum oxide balls (35 g) coated with C3A or Ca(OH)2 powder were placed on the quartz plate and exposed to gaseous HCl under simulated bag filter conditions. Air in the reaction tube was first evacuated using a vacuum pump (V-700, Buchi, Flawil, Switzerland). The reaction tube was then heated to 473 K in a tube furnace (KTF035N1, Koyo Thermo Systems Co. Ltd., Nara, Japan). After the furnace temperature had stabilized, the reaction tube was filled with a mixture of HCl and O2 in argon. The concentrations of HCl and O2 were set at 1000 ppm and 5 vol %, respectively. The gas flow rate of each gas was controlled by a mass flow controller (3660, FCC-3000-G1, Kofloc, Kyoto, Japan). The absolute humidity inside the reactor was maintained at 30 vol % by injecting water into a narrow glass tube (inner diameter, 1 mm). Water was injected at a constant rate from a

Figure 3. The time course of molar chlorination ratios (Cl/Ca) of C3A and Ca(OH)2 samples.

filter conditions. The chlorination ratio of the sample was calculated using the following equation: chlorination ratio (Cl/Ca) [mol/mol] = Cl in the sample [mol]/Ca in the sample [mol]

(4)

The chlorination ratios of C3A and Ca(OH)2 samples increased with HCl exposure time but leveled off after about 24 h. The time course of the Cl/Ca ratio for C3A was almost 6988

dx.doi.org/10.1021/ie300707e | Ind. Eng. Chem. Res. 2012, 51, 6987−6990

Industrial & Engineering Chemistry Research

Research Note

the same as that for Ca(OH)2, which is generally used as a dechlorination agent. The chlorination ratio of C3A powder after 24 h was 0.98. If the chlorination product was Friedel’s salt (3CaO·Al2O3·CaCl2·10H2O), the Cl/Ca ratio would have been 0.5, thus, other chlorides with a higher Cl/Ca ratio were generated after 24 h by the chlorination reaction of C3A. The chlorination ratio of Ca(OH)2 powder after 24 h was also 0.98. Thus, considering this chlorination ratio, it is speculated that the chlorination product of Ca(OH)2 is mainly CaClOH. However, it should be noted that the Ca(OH)2 sample after more than 12 h of chlorination exhibited hydroscopic properties. This fact indicates that CaCl2 was partially produced at the sample surfaces; however, its production rate was very low under the current experimental conditions. Figure 4 shows XRD patterns of C3A and Ca(OH)2 samples before and after the chlorination experiments. After 24 h of

Figure 5. SEM and elemental mapping images of C3A after 24 h of chlorination.

atoms were homogeneously distributed on the surface of the sample. These findings indicate that the chlorination reaction of C3A produces complex oxychlorides of calcium and aluminum under simulated bag filter conditions.

4. CONCLUSIONS The reaction of tricalcium aluminate (C3A) with HCl was investigated in a fixed-bed reactor under simulated bag filter conditions (473 K, 1000 ppm HCl, 30 vol% H2O, 5 vol% O2). The molar chlorination ratio of C3A had leveled off at about 1 after 24 h. This value was higher than the Cl/Ca ratio for Friedel’s salt. The chlorination reaction rate of C3A was almost the same as that of Ca(OH)2. Thus, once C3A is generated in an incinerator at high temperature and is transferred to the bag filter, it can react with HCl at a reaction rate almost the same as that for Ca(OH)2. The chlorination product of C3A has a mainly amorphous structure, but the product was not positively identified. However, from the elemental mapping images of the surface of chlorinated C3A, the generation of complex oxychlorides of calcium and aluminum was strongly suspected. These results show that complex oxychlorides are probably generated at bag filters from the complex oxides generated in high-temperature incinerators.

Figure 4. The XRD patterns of C3A and Ca(OH)2 samples before and after chlorination experiments. (a) C3A, (b) chlorinated C3A after 24 h, (c) Ca(OH)2, and (d) chlorinated Ca(OH)2 after 1 h.

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exposure to HCl, the crystallinity of the C3A sample had reduced dramatically. The weak peaks observed could be assigned only to crystalline C3A. Thus the chlorination products of C3A under simulated bag filter conditions consisted mainly of amorphous compounds and contained some crystalline phases consisting mainly of unreacted C3A. Even though the chlorination ratio (Cl/Ca) reached almost 1.0, XRD peaks for Friedel’s salt (e.g., 11.19°) or other complex oxychlorides were not observed. Even after heat curing (523 K for 30 min, or 973 K for 30 min) of the chlorinated C3A, the crystallinity of the sample was still very low, and the exact nature of the chlorination products was unclear (data not shown). In the XRD pattern of chlorinated Ca(OH)2 powder, peaks assigned to Ca(OH)2 and CaClOH were observed. Because of its hydroscopic properties, a powder sample of the Ca(OH)2 reaction products after 24 h of chlorination could not be recovered from the surfaces of the aluminum oxide balls. However, after 1 h of exposure of the Ca(OH)2 sample to HCl (Cl/Ca = 0.22), a powder sample could be collected and an XRD pattern was obtained. This result also suggested that the chlorination product of Ca(OH)2 at bag filters is mainly CaClOH. Figure 5 shows the typical SEM and elemental mapping images of C3A after 24 h of chlorination. These images demonstrate that calcium, chlorine, aluminum, and oxygen

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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