High-Temperature Reaction of Kaolin with Sulfuric Acid - Industrial

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APPLIED CHEMISTRY High-Temperature Reaction of Kaolin with Sulfuric Acid Fernando G. Colina,* Santiago Esplugas, and Jose Costa

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Department of Chemical Engineering and Metallurgy, Universitat de Barcelona, Marti i Franques 1-6, E-08028 Barcelona, Spain

A new procedure is described in which kaolin and a small amount of concentrated sulfuric acid are heated in a furnace at temperatures between 150 and 1000 °C. Parameters studied were reaction temperature and time, proton to alumina molar ratio, calcination temperature and time, and amount of water in the reaction medium. Al yield grew sharply until reaching a maximum at a reaction temperature of 700 °C and decreasing sharply beyond this temperature. Ti reaction yield values were lower than the Al reaction yield, showing a maximum at 200 °C and decreasing steadily as the reaction temperature was increased. Fe reaction yield showed a maximum in the same range of reaction temperature as Al. Previous calcination of kaolin produced only slight increases in reaction yields. Brunauer-Emmett-Teller (BET) surface area tests indicated an increase of the BET surface area for short reaction times, reaching values above 100 m2/g. Products of the reaction were mostly Al2(SO4)3. Introduction Kaolin is an industrial mineral used in many applications.1 It is also an abundant and widespread resource of aluminum. A large number of works describe processes to produce aluminum from kaolin using different reactions: (i) suspension of kaolin in aqueous solutions of inorganic acids (HCl, H2SO4, HNO3);2-4 (ii) reaction with inorganic salts such as (NH4)2SO4, NH4HSO4,5-12 or NaHSO4;13-18 and (iii) reaction with gaseous mixtures of Cl2 plus CO in the presence of NaCl19,20 or KAlCl4.21-23 A side effect is that reaction between kaolin and the above-mentioned reactants also solubilizes some impurities, namely, Ti24,25 and Fe.26 A new procedure was described elsewhere27 in which raw materials prepared by mixing kaolin and concentrated sulfuric acid are heated in a furnace at temperatures between 150 and 1000 °C. Depending on the temperature, the reaction between H2SO4 and kaolin could take place following eqs 1 and/or 2:

3H2SO4 + Al2O3 f Al2(SO4)3 + 3H2Ov

(1)

3SO3 + Al2O3 f Al2(SO4)3

(2)

In eq 1, it has been assumed that all of the Al present in kaolin is Al2O3, but it is actually in forms such as Al(OH)3 and others. It is likely that reaction (1) will be dominant at temperatures below the boiling point of H2SO4 (317 °C), whereas reaction (2) will be the most important above this temperature. However, slight changes could take place in the boiling point of H2SO4 because of the presence of kaolin. Si does not suffer any change by the action of H2SO4. * To whom correspondence must be sent. Tel.: (34-93) 4021296. Fax: (34-93) 4021291. E-mail: [email protected].

According to eqs 1 and 2, the reaction yield may be defined as

XM )

nM(kaolin0) - nM(kaolin) nM(kaolin0)

(3)

where nM(kaolin0) is the initial number of moles of M in unreacted kaolin, expressed as moles of the corresponding oxide, where M ) Al, and nM(kaolin) is the number of moles of M in reacted kaolin, also expressed as the oxide. Equation 3 may be used with M ) Ti, Fe following reactions described elsewhere.24-26 From reaction (1), it can be concluded that aluminum and H+ are the most relevant species for the present study. Given that reaction takes place between a fluid and a solid and concentrations are therefore difficult to measure, a ratio relating the amounts of both species was defined as follows:

rH ) nH+/nAl2O3

(4)

where nH+ is the total number of moles of H+ supplied by the acid and nAl2O3 is the total number of moles of aluminum in unreacted kaolin, expressed as aluminum oxide. Experimental Procedure Kaolin (Remblend grade) from St. Austell (Cornwall, U.K.) was supplied by English China Clay International Europe Ltd. (St. Austell, U.K.). Mineralogical analysis28 of the kaolin was performed on a D-500 Siemens X-ray diffractometer. Diffraction patterns gave a result of 85 wt % kaolinite, 12 wt % mica, 2 wt % feldspar, and 1 wt % quartz for unreacted kaolin. The Hinckley index29 (HI) for unreacted kaolin was 1.07. Products of the reaction were also analyzed using the same method. Major elements on unreacted kaolin were determined

10.1021/ie010886v CCC: $22.00 © 2002 American Chemical Society Published on Web 07/18/2002

Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002 4169 Table 1. Chemical Analysis of Kaolin major elements component

wt %

component

ppm

SiO2 Al2O3 Fe2O3 TiO2 K2O MgO P2O5

47.02 36.81 1.05 0.11 1.90 0.28 0.15

LOIa total

11.96 100.7

Na2O C S CaO Ba Rb Sr Zr MnO Y Th Pb Nb Cr

960 700 600 600 267 241 153 104 100 32 28 18 16 7

moisture a

trace elements

8

Loss on ignition.

by X-ray fluorescence analysis of fused material using lithium tetraborate as the flux on a Philips PW 1400 spectrophotometer. Trace elements on unreacted kaolin were determined by induced coupled plasma-optical emission spectroscopy (ICP-OES) using a Polyscan 61E Thermo Jarrell Ash spectrophotometer. Products of the reaction were removed from unreacted kaolin by leaching using a method described elsewhere.30 Chemical analysis of metals present in lixiviates was also performed by ICP-OES. All of the reagents were Baker Analyzed, analytical grade (J. T. Baker Chemicals N.V., Deventer, Holland). Results of the chemical analysis of unreacted kaolin are given in Table 1. Typical standard deviations for Al, Ti, and Fe reaction yields were obtained by repeating one experiment five times at average experimental conditions (Tr ) 500 °C; rH ) 3; Tcalc ) 0 °C; tcalc ) 0 h). The particle size distribution of unreacted kaolin was measured by light scattering using a Microtrac SRA150 from Leeds & Northrup. The analysis showed an average diameter of 15 µm. A standard analytical grade quality 36 N H2SO4 acid supplied by Probus (Barcelona, Spain) was used for the reaction with kaolin. Chemical analysis gave 0.0002 wt % in Fe and 0.0005 wt % in heavy metals (expressed as Pb). Samples for the reaction were prepared by mixing uncalcined kaolin and concentrated sulfuric acid at molar ratios (rH) between 0.375 and 6.000, putting the amount of concentrated H2SO4 in close contact with 5 g of dry kaolin. Aliquots of the mixture were placed in open quartz crucibles, which were heated in a 10-PR/ 300 Heron furnace to the reaction temperature (Tr) using a flash procedure. Temperatures ranged from 150 ( 5 to 1000 ( 5 °C. The heating rate of the sample was 1-5 °C/s. Once the reaction temperature was reached, the crucibles were left in the furnace for a certain time (tr) ranging from 2 to 360 min. After the reaction time elapsed, the crucibles were removed from the furnace and cooled in air and the samples were removed and ground using a mortar and pestle. Additional experiments were carried out using previously calcined kaolin at a temperature (Tcalc) between 500 and 1000 °C for a time (tr) between 1 and 5 h. Finally, experiments adding a certain amount of water on the reaction medium were carried out by adding 2.3-36 N sulfuric acid to the kaolin and following the experiment as described above. For the analysis of the Brunauer-Emmett-Teller (BET) specific surface area, samples were washed with water until all of the soluble products of the reaction

Figure 1. Reaction temperature vs Al, Ti, and Fe reaction yields ([, XAl; 2, XFe; 9, XTi; tr ) 1 h, rH ) 6).

were removed. The BET specific surface area was measured on an ASAP 2010 from Micromeritics.31 The specific surface area for unreacted kaolin was 9.9 m2/g. Results and Discussion It was shown elsewhere27 that the reaction between kaolin and H2SO4 is fast and renders soluble aluminum in very short reaction times. To study this reaction, six parameters were considered in the present work: (i) reaction temperature (Tr); (ii) reaction time (tr); (iii) proton to alumina molar ratio (rH); (iv) calcination temperature (Tcalc); (v) calcination time (tcalc); and (vi) amount of water in the reaction medium. The results are described below. Reaction Temperature. Experiments were carried out in the interval 150-1000 °C. An important effect of the reaction temperature on reaction yields may be appreciated in Figure 1: the aluminum yield grows sharply until it reaches a maximum at a reaction temperature of 700 °C. Beyond this temperature, the aluminum yield decreases sharply. This is not due to the lack of reaction between kaolin and H2SO4 but to the formation of insoluble products of reaction that cannot be leached. This was confirmed by the appearance of Al2O3 peaks on the X-ray diffraction (XRD) patterns corresponding to these samples. Al2O3 might be formed by thermal decomposition of Al2(SO4)3. On the other hand, a fast loss of acid reactant due to its evaporation could be an additional reason for the low reaction yield at temperatures above 700 °C. Similar results for the dependence between Al reaction yield and calcination temperature were found in previous works for the reaction between kaolin and inorganic acids in aqueous solutions. Experiments found in the bibliography2-4 using kaolin and sulfuric acid in an aqueous suspension showed that the maximum Al reaction yield was achieved for kaolin samples calcined at temperatures between 800 and 900 °C. Because no previous calcination is needed in the procedure described herein to achieve high values of reaction yields, this suggests that a dehydroxylation process could take place simultaneously during the reaction between kaolin and

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Table 2. Al, Ti, and Fe Reaction Yields (Results Calculated as wt % of the Corresponding Oxide; rH ) 6) Tr (°C) tr (min)

200

300

400

500

600

700

800

900

1000

typical std.

0.338 0.349 0.325 0.317 0.264

0.328 0.321 0.140 0.047 0.052

0.137 0.051 0.056 0.052 0.052

0.015 0.018 0.014 0.013 0.014

0.009 0.004 0.010 0.011 0.008

15 30 60 120 180

0.260 0.248 0.254 0.286 0.275

0.284 0.295 0.286 0.303 0.299

0.287 0.281 0.266 0.291 0.294

0.323 0.340 0.351 0.344 0.338

XAl 0.330 0.335 0.335 0.336 0.334

15 30 60 120 180

0.082 0.080 0.090 0.086 0.084

0.074 0.072 0.070 0.069 0.068

0.066 0.069 0.058 0.057 0.060

0.044 0.043 0.037 0.027 0.036

XTi 0.038 0.041 0.036 0.039 0.034

0.034 0.041 0.032 0.036 0.032

0.043 0.049 0.016 0.007 0.007

0.014 0.007 0.005 0.004 0.004

0.002 0.002 0.002 0.004 0.002

0.003 0.003 0.002 0.003 0.001

15 30 60 120 180

0.201 0.207 0.203 0.220 0.223

0.217 0.254 0.214 0.233 0.229

0.208 0.209 0.200 0.211 0.212

0.218 0.224 0.231 0.222 0.223

XFe 0.212 0.215 0.217 0.219 0.209

0.215 0.226 0.206 0.196 0.171

0.212 0.210 0.058 0.019 0.019

0.057 0.018 0.022 0.020 0.018

0.007 0.009 0.007 0.006 0.007

0.005 0.001 0.006 0.002 0.002

H2SO4 at high temperature by a process resembling to the one described for thermal dehydroxylation of kaolin.34 The lower interval for the maximum Al reaction yields obtained with H2SO4 at high temperature could be explained in terms of an easier dehydroxylation thanks to the action of the acid. However, while the decrease in the reaction yield at calcination temperatures near 1000 °C for the reaction in an aqueous solution is due to the formation of mullite, the decrease in the reaction yield in the reaction under study could be mostly due to the formation of insoluble reaction products such as Al2O3 (as discussed before in this paper). Regarding the Ti reaction yield, its value is lower than the Al reaction yield. However, it is in the same range as the Ti reaction yield obtained for the reaction in an aqueous solution. The difference in the reaction under study is that the Ti reaction yield reaches a maximum at 200 °C and then decreases steadily (Figure 1). The Fe reaction yield shows a behavior similar to that shown by aluminum, with a maximum in the same range of reaction temperature. However, the Fe reaction yield is much higher in the reaction between kaolin and H2SO4 at high temperatures than it is in the reaction in an aqueous solution. Experiments found in the bibliography2-4 on the reaction between the same kaolin and sulfuric acid in an aqueous solution showed that the Fe reaction yield achieves a maximum value for calcination temperatures around 800 °C, but the Fe reaction yield values are much lower, around 0.040. Given that no Fe mineral was found in the mineralogical analysis of the kaolin used in the present work, this could indicate that the attack of H2SO4 over the kaolinite particle takes place in a way different from when the reaction takes place between kaolin and an inorganic acid in an aqueous solution regarding the way aluminum is removed from the kaolinite particle. The reaction in an aqueous solution could deal with the formation of deep pores by removing more Al than Fe, while the reaction with H2SO4 at high temperature could be removal of Al in an homogeneous way, thus removing equal parts of Al and Fe. This behavior would give Al and Fe reaction yields in the same range, as is the case (see Figure 1). Reaction Time. This variable is of paramount importance regarding the conversion of some components

Figure 2. rH molar ratio vs Al, Ti, and Fe reaction yields. The dotted line shows values for eq 5 ([, XAl; 2, XFe; 9, XTi; Tr ) 400 °C, tr ) 1 h).

of kaolin into water-soluble aluminum salts. As can be seen in Table 2, there is a fast increase of the reaction yield during the initial time of reaction regardless of the reaction temperature or rH molar ratio. The maximum value of the reaction yield is achieved at very low reaction times, only several minutes. Once this point has been reached, reaction stops because of the consumption of the acid. This behavior is very useful in determining the end point of the reaction. Proton to Aluminum Molar Ratio (rH). Experiments were carried out in the rH interval 0.375-6.000. Results are shown in Figure 2. All reaction yields, for aluminum, titanium, and iron, show steady increases when the rH molar ratio is increased regardless of the reaction temperature. No maximum is observed in the rank of reaction temperatures considered in this study. This behavior could mean that the product layer over the surface of the kaolin particle is not an obstacle to the continuity of the reaction. For aluminum, a good correlation between the yield and rH molar ratio was

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Figure 4. XRD patterns showing samples of reaction products from the reaction between kaolin and H2SO4 (tr ) 15 min, rH ) 6). Table 3. BET Specific Surface Area Values for Samples Reacted at Different Tr (rH ) 6) Tr (m2/g)

Figure 3. Calcination temperature vs reaction yield ([, XAl; 2, XFe; 9, XTi; tr ) 1 h, rH ) 3, tcalc ) 1 h).

found. Experimental data fit very well to the regression equation:

XAl ) -0.003rH2 + 0.064rH + 0.009

(5)

The coefficient r2 found was 0.996. Calcination Temperature. It has been reported in many works that a previous calcination of kaolin increases the aluminum yield when the reaction is carried out in an aqueous solution. In this study the previous calcination of kaolin, prior to the reaction at high temperature, was also considered. For this purpose, kaolin was calcined at different temperatures in the rank 500-1000 °C. Once kaolin had been previously calcined, it was mixed with H2SO4, and the reaction took place at 500 °C. Results show that previous calcination of kaolin only produces a slight increase on the aluminum yield. The titanium yield also changes slightly and so does the iron yield (Figure 3). Calcination Time. Three different calcination times were tested: 1, 3, and 5 h. The calcination temperature was kept constant at 800 °C. Once kaolin had been previously calcined, it was mixed with H2SO4, and the reaction took place at 500 °C. It was observed that the calcination time increases only slightly the aluminum yield (XAl ranged from 0.188 to 0.212 for tr ) 1 h and tcalc from 1 to 5 h, respectively), having no effect on Ti and Fe reaction yields. The reason for the above results regarding the previous calcination of kaolin could be justified by the fact that H2SO4, together with the heat provided to achieve the reaction temperature, produces the dehydroxylation of kaolin (as discussed above), making the previous calcination unnecessary. Amount of Water on the Reaction Medium. To test the relevance of water on the reaction of kaolin with H2SO4 at high temperature, several experiments were carried out by adding small amounts of water to H2SO4, thus decreasing its concentration to 2.3 N. There was practically no effect in the addition of water on Al, Ti, and Fe reaction yields (results not shown). Only for

tr (min)

200

300

400

500

600

30 60 180

57.0 54.5 54.4

42.5 30.3 37.9

15.2 72.7 99.4

78.6 68.5 91.5

104.2 93.6 83.5

low values of the rH ratio were slightly higher reaction yields obtained. This result could be explained in terms of contact between the acid and surface of kaolin particles: the addition of water, at a given rH value, improves the contact while keeping constant the amount of available H+ ions, but the reaction yield does not change. This means that reaction yields depend mainly on the reaction temperature and time and on the rH molar ratio, whereas contact is not important either because the amount of acid is high enough or the reaction takes place between the aluminum and SO3 in the gas phase. Physical Properties of Products of the Reaction. BET surface area tests were run on a number of samples of reacted kaolin. Results show an increase of BET surface area for short reaction times (Table 3). Once the reaction has reached a certain reaction time, BET specific surface area values become stable. This behavior is very similar from one reaction temperature to another and has already been reported for the reaction of kaolin with H2SO4 or HCl in an aqueous solution.2,32 According to these works, the increase of the BET specific surface area would be related to the extraction of Al during the course of the reaction, giving tube-shaped pores. An extended extraction of aluminum would give way to the interconnection of these pores, thus stabilizing or even decreasing the value of BET specific surface area. Indeed, samples of the reaction of kaolin with H2SO4 by the new method showed a sensitive increase in the BET surface area, reaching values slightly above 100 m2/g, whereas reported values for the reaction between kaolin and sulfuric acid in an aqueous solution2 showed values above 200 m2/g. This difference could be explained in terms of the environment surrounding the kaolin particles: while in the reaction of kaolin with H2SO4 at high temperature, products of the reaction stay at the particle surface, forming a layer; in the reaction of kaolin with sulfuric acid in an aqueous solution, products of the reaction dissolve in water leaving the reaction surface and, thus, allow new H+ ions to contact the alumina surface. This could be the

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Figure 5. Continuation of Figure 4.

reason for higher values of BET surface areas in kaolin particles reacted with acid in an aqueous solution. Regarding the qualitative analysis of products of the reaction, on XRD patterns performed for the samples (Figures 4 and 5), the presence of Al2(SO4)3 was seen for all reaction temperatures below 1000 °C. Other products of the reaction are present depending on the reaction temperature, such as Al(HSO4)3. High-temperature products of the reaction can also be observed: such is the case of Al2O3 and mullite. Al2O3 could be formed by thermal decomposition of Al2(SO4)3. Mullite, on the other hand, could be formed from metakaolinite, as usual. Though mullite peaks appear at reaction temperatures below 1000 °C, as reported,33 there could be some reason to think that mullite formation could be promoted in some way by the presence of aluminum salts, as has been proposed for other metallic ions such as Cu.34,35 Conclusions From the results discussed above, it can be concluded that the reaction between kaolin and H2SO4 could take place according to the following steps: (i) H2SO4 present in the mixture heats to reach its boiling point (317 °C); (ii) boiling H2SO4 decomposes to SO3 (probably around 300-400 °C); (iii) gaseous SO3 ascends through the kaolin bed, and then reaction takes place. Water released by the dehydroxylation of kaolinite could help the reaction between kaolin and SO3 to take place, but this is not clear and further work will be needed. On the other hand, though the reaction temperature is achieved soon, there could also be some reaction between kaolin and H2SO4 at room temperature, as was already reported for the reaction in an aqueous solution.2-4 Products of the reaction were mostly Al2(SO4)3, though reaction temperatures above 800-900 °C yield insoluble Al2O3. Authors truly believe that the procedure shown in this paper to react kaolin with sulfuric acid could be an improvement to the process using aqueous solutions widely reported by others. Significant benefits could derive mainly from an improved control over the reaction. However, gaseous emissions could become important if no abatement measure is implemented (a mass flow rate of 0.26 kg of SO3/kg of kaolin has been estimated for Tr ) 700 °C and rH ) 3 or 0.59 kg of SO3/kg of kaolin for Tr ) 700 °C and rH ) 6). Literature Cited (1) Kirk, R. E.; Othmer, D. F. Encyclopedia of Chemical Technology; Wiley: New York, 1991.

(2) Ford, K. J. R. Leaching of Fine and Pelletised Natal Kaolin using Sulphuric Acid. Hydrometallurgy 1992, 29, 109. (3) Hulbert, S. F.; Huff, D. E. Kinetics of Alumina Removal from a Calcined Kaolin with Nitric, Sulphuric and Hydrochloric Acids. Clay Miner. 1970, 8 (3), 337. (4) Ziegenbalg, S.; Haake, G. Investigations into the Alumina Extraction from Clay by Hydrochloric and Sulphuric Acid Leaching. Light Met. 1983, 1119. (5) Bayer, G.; Kahr, G.; Mueller-Vonmoos, M. Reactions of Ammonium Sulphates with Kaolinite and other Silicate and Oxide Minerals. Clay Miner. 1982, 17, 271. (6) Davies, H.; Dering, H. O.; Parker, T. W. Al2O3 from Clay by an NH4 Alum-NH3 Process. U.S. Patent 2,375,977, 1945. (7) Fetterman, J. W.; Sun, S. C. Alumina Extraction from a Pennsylvania Diaspore Clay by an Ammonium Sulfate Process. Alumina 1963, 1, 333. (8) Fouda, M. F. R.; Amin, R. S.; Abd-Elzaher, M. M. Characterization of Products of Interaction between Kaolin Ore and Ammonium Sulphate. J. Chem. Technol. Biotechnol. 1993, 56, 195. (9) Nagaishi, T.; Ishiyama, S.; Yoshimura, J.; Matsumoto, M.; Yoshinaga, S. Reaction of Ammonium Sulphate with Aluminium Oxide. J. Therm. Anal. 1982, 23, 201. (10) Peters, F. A.; Johnson, P. W.; Kirby, R. C. Methods for Producing Alumina from Clay: an Evaluation of Two Ammonium Alum Process. U.S. Department of the Interior, Bureau of Mines. Report of Investigations RI 6573, 1965. (11) Seyfried, W. R. The Ammonium Sulfate Process for the Extraction of Alumina from Clay and its Application in a Plant in Salem, Oregon. Trans. AIME 1949, 182, 39. (12) St. Clair, H. W.; Ravitz, S. F.; Sweet, A. T.; Plummer, C. E. The Ammonium Sulfate Process for Production of Alumina from Western Clays. Trans. AIME 1944, 159, 255. (13) Garcia-Clavel, M. E.; Martı´nez-Lope, M. J.; Casais-Alvarez, M. T. Me´todo de Obtencio´n de Alu´mina a partir de Arcillas y Silicatos Alumı´nicos en general. Spanish Patent 482,881, 1979. (14) Garcia-Clavel, M. E.; Martı´nez-Lope, M. J.; Casais-Alvarez, M. T. Method for Obtaining Alumina from Clays. U.S. Patent 4,342,729, 1982. (15) Garcia-Clavel, M. E.; Martı´nez-Lope, M. J.; Casais-Alvarez, M. T. Procedimiento Continuo de Obtencio´n de Compuestos de Aluminio a partir de Silicatos Alumı´nicos y otros Minerales de Aluminio. Spanish Patent 522,398, 1983. (16) Garcia-Clavel, M. E.; Martı´nez-Lope, M. J.; Casais-Alvarez, M. T. Me´todo de Solubilizacio´n de los Componentes Meta´licos de los Filosilicatos. Spanish Patent 545,690, 1985. (17) Martı´nez-Lope, M.; Garcı´a-Clavel, M. E.; Casais-Alvarez, M. T. Solubilization Reaction of the Alumina from Kaolin by Solid State Reaction. Thermochim. Acta 1991, 177, 77. (18) Solano, E.; Juan, D. Obtencio´n de Alu´mina a partir de Arcillas utilizando como Agente Disgregante el Bisulfato So´dico. Quı´m. Ind. 1995, 14, 82. (19) Grob, B.; Richarz, W. Chlorination of Alumina in Kaolinitic Clay. Metall. Trans. B 1984, 15, 529. (20) Grob, B. Selektive Chlorierung von Aluminiumoxid in Kaolinit. Dissertation. Eidgenoessiche Technische Hochschule Zurich, Zurich, Switzerland, 1983. (21) Martin, E. S.; Wefers, K. Microscopic Investigation of Clay Chlorination. Light Met. 1984, 619. (22) Martin, E. S.; Wohleber, D. A. Production of Aluminum Chloride from Raw Materials Containing Aluminum Compounds and Silicon Compounds. U.S. Patent 4,086,320, 1978. (23) Martin, E. S.; Wohleber, D. A. Production of Anhydrous Aluminum Chloride from Clay using Catalyst and Recycling of Silicon Chloride. U.S. Patent 4,096,234, 1978b. (24) Dolcater, D. L.; Syers, J. K.; Jackson, M. L. Titanium as Free Oxide and Substituted Forms in Kaolinites and other Soil Minerals. Clays Clay Miner. 1970, 18, 71. (25) Maynard, R. N.; Millman, N.; Iannicelli, J. A Method for Removing Titanium Dioxide Impurities from Kaolin. Clays Clay Miner. 1969, 17, 59. (26) Malden, P. J.; Meads, R. E. Substitution by Iron in Kaolinite. Nature 1967, 215, 844. (27) Colina, F. G.; Esplugas, S.; Costa, J. High-Temperature Reaction of Kaolin with Inorganic Acids. Br. Ceram. Trans. 2001, 100, 203.

Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002 4173 (28) Chung, F. H. Quantitative Interpretation of X-Ray Diffraction Patterns of Mixtures. I. Matrix Flushing Method for Quantitative Multicomponent Analysis. J. Appl. Crystallogr. 1974, 7, 519. (29) Hinckley, D. N. Variability in Cristalinity Values among the Kaolin Deposits of the Coastal Plain of Georgia and South Carolina. Clays Clay Miner. 1963, 11, 229. (30) Colina, F. G.; Esplugas, S.; Costa, J. A New Extraction Procedure for Simultaneous Quantitative Determination of WaterSoluble Metals in Products of Reaction of Clays with Inorganic Salts. Clays Clay Miner. 2002, 50, 403. (31) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309. (32) Gajam, S. Y.; Raghavan, S. A Kinetic Model for the Hydrochloric Acid Leaching of Kaolinite Clay. Trans. Inst. Miner. Metall., Sect. C 1985, 94, 115.

(33) Lemaitre, J.; Bulens, M.; Delmon, B. Influence of Mineralizers on the 950 °C Exothermic Reaction of Metakaolinite. Proc. Int. Clay Conf., Mexico D.F. 1975, 1, 539. (34) Lemaitre, J.; Leonard, A.; Delmon, B. Le Me´canisme de la Transformation Thermique de la Me´takaolinite. Bull. Mineral. 1982, 105, 501. (35) Bachiorrini, A.; Murat, M. Spectroscopie d’Absortion Infrarouge Applique´e a` la Caracte´risation de l’E Ä tat d’Amorphisation de la Me´takaolinite. C.R. Acad. Sci. Paris, Se´ r. II 1986, 303 (20), 1783.

Received for review October 29, 2001 Revised manuscript received May 28, 2002 Accepted June 9, 2002 IE010886V