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Ind. Eng. Chem. Res. 1997, 36, 2646-2650
Production of Poly(aluminum chloride) and Sodium Silicate from Clay Kyun Young Park* Department of Chemical Engineering, Kongju National University, 182 Shinkwandong, Chungnam 314-701, Korea
Jin-Kwon Kim Department of Chemistry, Kongju National University, 182 Shinkwandong, Chungnam 314-701, Korea
Jinki Jeong and Young Youn Choi Department of Materials Development, Korea Institute of Geology, Mining and Materials, P.O. Box 14, Taedok Science Town, Taejon, Korea
A novel process is proposed for the extraction of aluminum from clay, by which the aluminum is recovered as poly(aluminum chloride) solution requiring energy much less than that for the recovery as alumina, and the volume of the siliceous residue left after the aluminum was extracted from the clay can be reduced by 70% by extracting the silicon from the residue as sodium silicate solution. The poly(aluminum chloride) solution was made by partially decomposing the aluminum chloride hexahydrate, an intermediate product leading to alumina on complete decomposition, and subsequently dissolving the resultant basic chloride in water at 90 °C. About 80% of the silicon in the siliceous residue could be recovered as sodium silicate by leaching the residue with 20 wt % NaOH for 1 h at 100 °C under atmospheric pressure. The maximum molar ratio of SiO2:Na2O of the sodium silicate solution thus obtained was 3.0:1. Introduction 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, the ore is treated by hydrochloric acid leaching. The resulting solution, which contains iron and aluminum, is subjected to a purification step in which iron is removed by solvent extraction. Aluminum values in the purified liquor is recovered as aluminum chloride hexahydrate (AlCl3‚6H2O) by crystallization. The chloride is then thermally decomposed and calcined at about 1100 °C to produce alumina. The process of alumina production from clay based on the HCl method, however, 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. The decomposition of aluminum chloride hexahydrate is highly endothermic, requiring a supply of considerable heat volumes. In comparison with the Al2O3 calcination from aluminum hydroxide according to the Bayer process, the specific heat requirement and the specific waste gas volume are four times as high (Marchessaux et al., 1979). This must be a problem of the HCl process. Another problem associated with the HCl process is that bulky residues are generated; about two-thirds by weight of the clay charged for the acid leaching remains undissolved, the major component of which is silica. Utilization of this residue should be important, but has received little attention. A new process is proposed for the recovery of aluminum from clay, which would reduce the energy con* Author to whom correspondence should be addressed. Tel, 82-416-50-8637; FAX, 82-416-50-8589. S0888-5885(96)00786-5 CCC: $14.00
sumption and the waste volume considerably. By this process, as shown in Figure 1, the aluminum is recovered in the form of poly(aluminum chloride) used as a flocculating agent in water treatments, and the volume of the solid waste is reduced by extracting the silicon from the residue in the form of sodium silicate solution used as adhesives and for the production of precipitated silicas and synthetic zeolites. The poly(aluminum chloride) is produced by partially decomposing the aluminum chloride hexahydrate at a temperature of 200 °C, much lower than the decomposition temperature required for alumina, and subsequently dissolving the resulting basic chloride in water. This is a new method for the production of poly(aluminum chloride). The sodium silicate solution is produced by leaching the siliceous residue with NaOH under atmospheric pressure. Experimental Section The clay samples were obtained from Sancheong, Korea. The chemical analysis of the clay is shown in Table 1. Mineralogical X-ray diffraction analyses were made using Cu KR radiation. The principal mineral was halloysite. Minor minerals found were: kaolinite, feldspar, quartz, and montmorillonite. Preparation of Samples. The clay was calcined at 750 °C for 2 h, then leached at 105 °C for an hour with 105% of stoichiometric requirement of 26 wt % hydrochloric acid in a closed 1-L flask with mechanical stirring and refluxing. The clay charged for leaching weighed about 200 g. Following the leaching, the slurry was filtered and the residue washed three times with water and dried. The filtrate was passed to a solvent extraction step, in which iron is removed with an organic phase containing 15 vol % Alamine 336 (a tertiary amine), 10 vol % decyl alcohol, and 75 vol % kerosene. The extraction was carried out at a phase ratio of 4:1, aqueous/organic. By this solvent extraction, © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2647
Figure 2. Experimental apparatus for partial decomposition of AlCl3‚6H2O. Table 3. Chemical Analysis of Impurities in AlCl3‚6H2O
Figure 1. Block diagram for production of poly(aluminum chloride) and sodium silicate from clay. 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 H 2O
1.53 0.78 0.20 0.02 11.32
Table 2. Composition of the Siliceous Residue compound
wt %
compound
wt %
SiO2 Al2O3 CaO Fe2O3
79.58 11.48 0.15 0.33
MgO Na2O TiO2 ignition loss
0.095 0.099 0.16 7.62
the iron content was reduced from 0.4 wt % to less than 5 ppm. The purified pregnant solution was sparged with HCl gas to 26% to produce aluminum chloride hexahydrate crystals. The crystals were separated from the mother liquor by vacuum filtering, washing with 36% HCl, and vacuum drying. This procedure for the preparation of aluminum chloride hexahydrate is the same as that used by the Bureau of Mines (Eisele, 1980). Chemical analyses for the siliceous residue and the aluminum chloride hexahydrate are shown in Tables 2 and 3, respectively. Production of Poly(aluminum chloride). The aluminum chloride hexahydrate was thermally decomposed in a 50-cc flask immersed in a silicon oil bath. The schematic drawing of the experimental apparatus is shown in Figure 2. About 5 g of the chloride was spread on the bottom of the flask and then heated to a temperature of 150-200 °C with a flow of nitrogen gas through the flask at a rate of 40 cc/min. Hydrogen chloride gas and water vapor evolved with decomposition. They were absorbed by water in a 1-L flask
component
content (wt ppm)
Si Na Fe Mg Ca K Ti
30 180 5
