Ind. Eng. Chem. Res. 1996, 35, 4379-4385
4379
Manufacture of Low-Soda Alumina from Clay Kyun Young Park* Department of Chemical Engineering, Kongju National University, 182 Shinkwandong, Kongju, Chungnam 314-701, Korea
Jinki Jeong Department of Materials Development, Korea Institute of Geology, Mining, and Materials, P.O. Box 14, Taedok Science Town, Taejon, Korea
The manufacture of low-soda alumina from clay was studied with a major concern in morphological changes during thermal decomposition of the aluminum chloride hexahydrate (AlCl3‚6H2O) derived from clay and calcination of the resultant amorphous alumina. The temperature was varied from 300 to 900 °C for decomposition and from 1100 to 1400 °C for calcination. Changes in the BET surface area, porosity, and scanning electron microscopic image of the powder surface were observed under the varying conditions of decomposition and calcination. These transitional behaviors then were compared with those of aluminum hydroxide, which has long been used for the commercial production of alumina. A low-soda alumina could be made directly by the thermal decomposition of aluminum chloride hexahydrate derived from clay, whereas an extra step is required to reduce the sodium content with the Bayer process. I. Introduction The Bayer process, in which bauxite is treated with NaOH to produce an intermediate product of aluminum hydroxide (Al(OH)3) and the aluminum hydroxide then thermally decomposed and calcined, until now has been the major route to commercial production of alumina. Meanwhile, non-bauxite aluminum ores, including kaolinic clay, have been considered as alternative sources for alumina. Among various methods proposed for producing alumina from non-bauxite sources, the HCl method is known to be the most promising (Peters and Johnson, 1974). By the HCl method, aluminum in the ore is extracted by hydrochloric acid leaching and recovered in the form of aluminum chloride hexahydrate. The chloride is then thermally decomposed into hydrochloric acid vapor and alumina. The acid is recycled to the leaching step. The Bureau of Mines performed an extensive research for the production of alumina from clay using the HCl method (Shanks et al., 1986). Eisele et al. (1983) reported experimental results on the recovery of alumina from clay by a HCl process. A 25 ton/day pilot plant was proposed by the Bureau of Mines (Bengtson et al., 1979) but not built. All of the previous works were directed to the production of metallurgical-grade alumina, which appears to have no apparent economic advantage over the Bayer process, although the comparison between the two processes is difficult because the Bayer process is well refined and the HCl process is not. In the present work, the manufacture of lowsoda aluminas used in fine-ceramic industries was undertaken by taking advantage of the nature of the HCl process in which no sodium-containing agent is used. The leaching agent of hydrochloric acid used in the HCl process contains no sodium, which must be beneficial to the manufacture of low-soda alumina. In the Bayer process, NaOH is used as the leaching agent and the alumina produced therefrom is usually contaminated by sodium; therefore, an extra step may be required to produce the low-soda alumina. The low-soda alumina requires a soda content of less than 0.1 wt %, with an alumina content of about 99.7 wt %. The alumina from clay by the HCl process could S0888-5885(95)00716-0 CCC: $12.00
meet this requirement; a soda content as low as 0.01 wt % was reported (Marchessaux et al., 1979). The R alumina content should be nearly 100% for the low-soda alumina, while 20% may be enough for metallurgicalgrade alumina. Additional characteristics of R crystal size, surface area, and morphology become involved in the specification of low-soda alumina. Thermal decomposition of the aluminum chloride hexahydrate, the intermediate product of the HCl process, produces a solid product, Al2O3, and two gaseous products, HCl and H2O.
2AlCl3‚6H2O ) Al2O3 + 6HCl + 9H2O
(1)
The decomposition nears completion within an hour at a temperature of 900 °C. Complete decomposition reduces the chloride charged to 21.1% of its initial weight. The alumina resulting from the decomposition is in amorphous phase. The phase is reported to be transformed to R at a temperature of 1100-1150 °C (Petzold and Naumann, 1981; Sato et al., 1978). Most of previous works on the decomposition were limited to identification of evolved gases and investigation of the transition mechanism of the crystal structure of alumina. The present work treats different aspects of transition behavior during thermal decomposition of the chloride and calcination of the resultant alumina: evolution and growth of R crystals were investigated by scanning electron microscopy, and changes in the BET surface area and the porosity were measured. These changes then were interpreted in comparison with those observed for the decomposition of aluminum hydroxide, presently the major source for low-soda alumina. II. Experimental Section The aluminum chloride hexahydrate used for decomposition was made from the clay obtained from Sancheong, Korea, the composition of which is shown in Table 1, by following the procedure of the Bureau of Mines (Eisele, 1980). The chemical composition of the aluminum chloride is shown in Table 2, and that of the aluminum hydroxide used for comparison is shown in Table 3. As seen from the tables, the sodium content © 1996 American Chemical Society
4380 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 Table 1. Composition of Clay compound
wt %
compound
wt %
Al2O3 SiO2 Fe2O3 CaO Na2O
37.86 44.03 1.46 1.85 0.33
K2O MgO TiO2 MnO H2O
1.53 0.78 0.20 0.02 11.32
Table 2. Chemical Analysis of Impurities in AlCl3‚6H2O component
content (wt ppm)
component
content (wt ppm)
Si Na Fe Mg
30 180 5
