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Bench-Scale Decomposition of Aluminum Chloride Hexahydrate to Produce Poly(aluminum chloride) Kyun Young Park,* Yong-Woon Park, Sung-Ho Youn, and Sun-Young Choi Department of Chemical Engineering, Kongju National University, 182 Shinkwandong, Chungnam 314-701, Korea
Partial thermal decomposition of aluminum chloride hexahydrate was carried out in a rotary glass tube at temperatures of 230-260 °C. The decomposition solid product was dissolved in water to produce a ploy(aluminum chloride) (PAC) solution. The effects of the decomposition temperature, the rotation speed of the tube, and the feed rate of aluminum chloride hexahydrate on the properties and morphology of the basic chloride resulting from the decomposition were investigated. The basicity of the PAC obtained by dissolving the decomposition solid product in water was related to the extent of decomposition by the linear equation Y ) 2.025 + 1.063X, where Y is the basicity (%) and X is the extent of decomposition (%). The PAC solution prepared by the present method showed a flocculation capacity comparable to that of an existing PAC made from aluminum hydroxide and hydrochloric acid. Introduction. Poly(aluminum chloride), PAC, has been widely used in water treatments as a flocculating agent, along with the alum referring to a commercial aluminum sulfate hydrate. It is a partially hydrolyzed aluminum chloride solution, the aluminum content of which is about 10 wt %, expressed as Al2O3. PAC is known to provide faster flocculation and stronger flocs than alum in some applications.1 It has been produced commercially by dissolving aluminum hydroxide in hydrochloric acid in autoclaves at elevated temperatures and pressures. Recently, we reported a new method of PAC production,2 in which aluminum chloride hexahydrate (AlCl3‚6H2O) is partially decomposed at about 200 °C under atmospheric pressure and the resulting basic chloride in solid form is dissolved in water. The dissolution of the basic chloride is simple and can be performed locally at the place where the flocculant is needed. This is an advantage from the viewpoint of transportation cost. With the conventional method, in which PAC is produced as a solution containing more than 90 wt % of water, an extra evaporation step is required to obtain PAC in solid form. There have been other studies on thermal decomposition of aluminum chloride hexahydrate.3-5 All of these studies are, however, associated with the production of alumina by complete decomposition of the chloride and focus on the transition mechanism of morphology and crystal structure of the alumina at temepratures of 400-1400 °C. For the production of PAC, the aluminum chloride is not completely decomposed, but partially decomposed at a lower temperature of about 200 °C. In our previous work,2 the partial decomposition of aluminum chloride hexahydrate was performed in a 50cm3 flask immersed in a silicon oil bath. The formation of a basic chloride was identified by IR spectrometry, the decomposition kinetics were studied in the temperature range of 140-200 °C, and the effect of the * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: 82-41-858-2575. Tel.: 82-41-850-8637.
decomposition degree on the basicity of the PAC solution was investigated. In the present work, the decomposition was carried out continuously in a rotary glass tube 8 cm in diameter and 100 cm in length, with the feed rate of aluminum chloride varying between 250 and 650 g/h. The effects of the decomposition temperature, the rotation speed of the tube, and the feed rate of aluminum chloride hexahydrate on the properties and morpholgy of the basic chloride resulting from the decomposition were investigated. The PAC solution obtained by dissolving the basic chloride in water was tested on the water taken from a river using a jar tester. Experimental Section A schematic drawing of the experimental apparatus is shown in Figure 1. The aluminum chloride hexahydrate is charged by a screw feeder into a rotating glass tube 8 cm in diameter and 100 cm in length. The tube is heated by an electrical heater over the length of 50 cm, between 30 and 80 cm from the inlet. The tube slopes down to the outlet by 1.8°. Three glass rods, each 1 cm in diameter, equivalent to the flights in commercial rotary kilns, are located on the interior of the tube to lift and shower the solids through the gas space. The temperature in the gas space was measured by a thermocouple travelling in the axial direction. Another thermocouple was located very close to the outside wall of the cylinder. The decomposition products leaving the decomposition cylinder are routed to a solid-gas separator equipped with a filter. The solid product or the basic chloride drops by gravity from the separator to a graduated cylinder on a balance. At the same time, the gaseous products containing the hydrochloric acid and water vapors evolved during the decomposition are passed through a condenser to condense the acid vapors. Any acid vapors remaining uncondensed are finally removed by caustic soda in two packed columns in series. The aluminum chloride hexahydrate used is of reagent grade from Junsei Chemical Co., and the particle shape is cylindrical with a aspect ratio of about 3.0. The extent of decomposition of the chloride was determined
10.1021/ie000029b CCC: $19.00 © 2000 American Chemical Society Published on Web 10/06/2000
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Figure 1. Schematic drawing of the experimental apparatus.
by subjecting the solids (or basic chlorides) leaving the decomposer to complete decomposition in a separate furnace at 1000 °C. From the weight loss on the further decomposition and from the fact that complete decomposition of aluminum chloride hexahydrate (2AlCl3‚ 6H2O ) Al2O3 + 6HCl + 9H2O) reduces the virgin chloride to 21.1% of its initial weight, the extent of decomposition can be determined as follows:
Extent of decomposition (%) ) W - Wf 1× 100 (1) (Wf/0.211 - Wf)
[
]
where W is the weight of the basic chloride charged to the furnace and Wf is the weight after it was completely decomposed. The basic aluminum chloride was dissolved in water at 80 °C to yield a poly(aluminum chloride) solution. The volume of water was controlled so that the aluminum content of the solution was about 10 wt % on the Al2O3 basis. The basicity of the PAC solution, which is defined as the equivalent ratio in percentage of OH to Al present in the solution, was determined by following the procedure described in KS (Korean Standard) M 1510-1981. For the flocculation test, the PAC solution of 10 wt % Al2O3 was diluted by 100 times. A predetermined volume of the diluted solution was added to 1 L of the water to be tested, with vigorous agitation for 1 min. Then, about 20 min of agitation followed at a reduced rpm of 20. After the flocs settled, the turbidity of the water was measured by a turbidity meter. Results and Discussion Effects of Decomposition Variables on the Extent of Decomposition. The effect of the temperature on the extent of decomposition was studied over a range of 230 to 260 °C, with the feed rate of aluminum chloride hexahydrate at 350 g/min. and the rotation speed of the tube at 3 rpm. The temperature is the wall temperature mid-way between two ends of the heated zone. At the
Figure 2. Extent of decomposition vs cumulative weight of solid product at various temperatures (feed rate, 350 g/h; rotation speed, 3 rpm).
wall temperature of 250 °C, the temperature in the gas space was measured between the two ends. The maximum gas temperature was about 200 °C. This temperature level was maintained in the zone of 20 cm in length. Actual temperature of the chlorides being decomposed must be between the wall temperature and the gas temperature. Figure 2 shows the extents of decomposition of solid product leaving the decomposer against cumulative weight of the solid produced, at the three temperatures of 230, 250 and 260 °C. In every case, the extent of decomposition showed a transient behavior in the initial period, then reached a steady value. The extents of decomposition at the steady state were 32, 47, and 55%, respectively. At a steady state, the fraction of cross section occupied by the solids was measured at 8%. The volume of the solids in the constant-temperature zone of 20 cm in length is calculated at 80.4 cm3. Assuming that this is an approximate volume for the decomposition and the solid flow pattern is plug flow axially,6 the residence time is roughly calculated at 11 min. Using the activation energy of 55.5 kJ/mol and the preexponential factor of 297.6 s-1, which
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Figure 3. Effect of rotation speed of the decomposer on the extent of decomposition (feed rate, 350 g/h; wall temp, 250 °C).
Figure 4. Effect of feed rate of Chloride on the extent of decomposition (wall temp, 250 °C; rotation speed, 3 rpm).
were determined previously in the laboratory-scale experments,2 the extents of decomposition were estimated by the following simple model to be 28.9, 43.2, and 51.3%, respectively, for 230, 250 and 260 °C.
dX ) Ae-E/RT(1 - X) dt
(2)
where X is the fractional extent of decomposition, t is the time, A is the preexponential factor, E is the activation energy and T is the temperature. Considering that the decomposition should have also occurred to some extent outside the zone assumed for the estimation, model prediction based on the kinetic data is in good agreement with experimental data. The rotation speed of the tube was varied from 1.5 to 6 rpm, holding the feed rate at 350 g/h and the wall temperature at 250 °C. As shown in Figure 3, the extent of decomposition decreased as the speed is increased from 1.5 to 3 rpm, probably due to the residence time becoming shorter with the speed increase. A further increase of the rotation speed to 6 rpm, however, had little effect on the extent of decomposition despite the reduction in residence time. A possible explanation for the reason is that the increase of the speed may have increased the frequency of the solids touching the hotter wall, resulting in an increase of the solids temperature on the average and consequently the rate constant increased so as to compensate for the influence of the reduction in residence time. Figure 4 shows the effect of the feed rate on the extent of decomposition. The feed rate was varied from 260 to 650 g/h, with the rotation speed of the tube kept constant at 3 rpm and the wall temperature at 250 °C. The extent of decomposition decreased gradually with increasing the feed rate. The data showed a considerable scattering; the extent of decomposition differed by as high as 10% at the same feed rate. Such a scattering of data may be due to the formation of agglomerates of chloride particles. Many agglomerates were found in the entrance of the decomposing tube. Some of the water vapors formed during the decomposition may have diffused back toward the inlet of the decomposer and condensed on the incoming aluminum chloride hexahydrate particles, thereby forming agglomerates. The decomposition behavior of the agglomerates should be different from those of individual free particles, because of internal resistances to mass and heat transfers. The influence of the agglomeration on the extent of decomposition will depend on the number, size and shape of
Figure 5. Cross-sectional images of the decomposition product.
the agglomerates. The agglomeration was irregular, which might have brought about the scattering of the data. SEM Investigation of the Basic Chloride. Figure 5 shows the scanning electron microscopic images of the cross sections of basic chloride particles produced at two decomposition temperatures of 230 and 260 °C, with the rotation speed of the decomposer kept at 6 rpm. By
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Figure 6. Correlation between extent of decomposition and basicity.
investigation of the images, the decomposition may have occurred from the outside toward the center, not uniformly throughout the cross section. At the decomposition temperature of 230 °C, an annular layer, the thickness of which is about one-third the radius of the particle, existed outside (Figure 5a). By eye inspection, the outer layer appears to have undergone more severe decomposition than the inner zone. To verify this conjecture, local weight ratios of Cl to Al were measured using energy-dispersive X-ray (EDX). The ratio was 3.36 in the center of the section and 1.57 at a point close to the edge. The ratio of virgin aluminum chloride hexahydrate is 3.94 by stoichiometry. In the course of the decomposition, the chlorine is removed as HCl, while the aluminum remains unremoved: therefore, the ratio would decrease with an increase of the decomposition. The lower ratio measured near the edge indicates that the decomposition was more severe than in the center. With the increase of the temperature to 260 °C, the outer layer became thicker and bigger cracks were formed, as shown in Figure 5b. The weight ratios of Cl to Al were measured at 3.52 in the center and 1.40 near the edge. The increase of the temperature increased the decomposition near the edge, but affected little the decomposition in the center. The ratio in the center even became a little higher with the temperature increase. The increment, however, may be within the error limits of analysis. It is more reasonable to say that the temperature increase had no effect on the extent of decomposition in the center. The average ratio of Cl to Al over the cross section was 3.0 for the wall temperature of 230 °C and 1.7 for 260 °C, respectively. The decomposition was considered to have proceeded like a shrinking core. Evaluation of the PAC Solution. The basicity of a PAC solution is an important specification. The basicity of commercial-grade PAC is around 50%. In Figure 6, the correlation between the basicity and the extent of decomposition is indicated. The basicity increases with increasing the extent of decomposition. The correlation can be represented by the linear equation Y ) 2.025 + 1.063X, where Y is the basicity (%) and X is the extent of decomposition (%), with a correlation coefficient of 0.77. The PAC solution prepared by the present method was tested on some water taken from the Kum river using a jar tester. The turbidity of the water was 55.3 NTU, the pH was 7.35, and the temperature was 26 °C.
Figure 7. Comparison of flocculation capacity between present and conventional PACs.
The aluminum content of the PAC solution was 10 wt % on the Al2O3 basis, and its basicity was 50.0%. For comparison, a PAC solution of the same aluminum content and basicity, but made by the existing method from aluminum hydroxide and hydrochloric acid, was obtained from a manufacturer. One gram of each PAC solution was dissolved in 100 mL of distilled water. Figure 7 shows the changes in the pH and turbidity of the treated water with variations in the volume of the diluted PAC solution added to 1 L of the raw water. With a dosage of 5 mL, the turbidity of the water was dropped to 3.3 NTU with the present PAC and 4.7 NTU with the conventional PAC. A further increase of the dosage to 10 mL increased the turbidity to 27.2 and 48.8 NTU, respectively; over-dosage of a flocculant leads to deflocculation.7 The pH of the treated water was a little lower with the conventional PAC. We heard from the manufacturer that a small amount of hydrosulfuric acid was added in the manufacture of that PAC. This may be the reason for the pH being lower. This level of flocculation test is sufficient for the purpose of this paper. A more elaborate evaluation of the flocculating capacity of the PAC may be necessary in the future. Conclusion The partial decomposition of aluminum chloride hexahydrate was performed successfully on a bench scale. The effects of various operating variables on the extent of decomposition were studied. Increasing the decomposition temperature from 230 to 260 °C increased the extent of decomposition from 32 to 55%, at a feed rate of 350 g/h and a rotation speed of 3 rpm. By SEM and EDX analyses, the decomposition appeared to have proceeded from the outside toward the center of a particle, like a shrinking core. The extent of decomposition decreased as the rotation speed was increased from 1.5 to 3 rpm, probably because of a decrease in the residence time. A further increase of the rotation speed to 6 rpm, however, had little effect on the extent of decomposition. A possible explanation for this observation is that the increase in the speed may have increased the frequency with which the solids touched the hotter wall, resulting in an increase in the average temperature of the solidsand, consequently, an increase in the rate constant so as to compensate for the influence of the reduction in residence time. The correlation between
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the basicity of the PAC obtained by dissolving the decomposition solid product in water and the extent of decomposition could be represented by the linear equation Y ) 2.025 + 1.063X, where Y is the basicity (%) and X is the extent of decomposition (%), with a correlation coefficient of 0.77. The PAC solution prepared by the present method showed a flocculation capacity comparable to that of an existing PAC made from aluminum hydroxide and hydrochloric acid. Acknowledgment The authors acknowledge the financial support of the R&D Management Center for Energy and Resources. Literature Cited (1) Heitner, H. I. Flocculating Agents. In Encyclopedia of Chemical Technology; Jacqueline, I. K., Howe-grant, M., Eds.; John Wiley & Sons: New York, 1994.
(2) Park, K. Y.: Kim, J.; Jeong, J.: Choi, Y. Y. Production of Poly(aluminum chloride) and Sodium Silicate from Clay. Ind. Eng. Chem. Res. 1997, 36, 2646. (3) Petzold, D.; Naumann, R. J. Thermoanalytical Studies on the Decomposition of Aluminum Chloride Hexahydrate. Thermal Anal. 1981, 20, 71. (4) Marchessaux, P.; Plass, L.; Reh, L. Thermal Decomposition of Aluminum Chloride Hexahydrate for Alumina Production. Light Met. 1979, 189. (5) Park, K. Y.; Jeong, J. Manufacture of Low-Soda Alumina from Clay. Ind. Eng. Chem. Res. 1996, 35, 4379. (6) Walas, S. M. Chemical Reactors. In Perry’s Chemical Engineers’ Handbook, 7th ed.; Green, D. W., Maloney, J. O., Eds.; McGraw-Hill: New York, 1980. (7) Hughes, M. A. Coagulation and Flocculation. In SolidLiquid Separation, 3rd ed.; Svarovsky, L., Ed.; ButterworthHeinemann, Ltd.: Cambridge, U.K., 1990.
Received for review January 6, 2000 Revised manuscript received August 3, 2000 Accepted August 4, 2000 IE000029B
