High-Temperature Reaction of Kaolin with Sodium Hydrogen Sulfate

A procedure is described in which kaolin and NaHSO4 are heated in a furnace at temperatures between 200 and 1000 °C. Parameters studied were the reac...
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Ind. Eng. Chem. Res. 2005, 44, 4495-4500

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APPLIED CHEMISTRY High-Temperature Reaction of Kaolin with Sodium Hydrogen Sulfate Fernando G. Colina* and Jose Costa Department of Chemical Engineering and Metallurgy, Universitat de Barcelona, Marti i Franques 1-6, E-08028 Barcelona, Spain

A procedure is described in which kaolin and NaHSO4 are heated in a furnace at temperatures between 200 and 1000 °C. Parameters studied were the reaction temperature and time, proton to alumina molar ratio, calcination temperature and time, and reaction atmosphere. The Al reaction yield grew until reaching a maximum at a reaction temperature of 700 °C and decreasing sharply beyond this temperature. The Ti reaction yield values were lower than the Al reaction yield, showing a maximum at 400 °C and decreasing smoothly as the reaction temperature was increased. The Fe reaction yield showed a maximum in the same range of reaction temperature as Al. Precalcination of kaolin produced only slight increases in reaction yields. BrunauerEmmett-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 Na3Al(SO4)3, NaAl(SO4)2, NaAl(SO4)2‚xH2O (x ) 6 and 12), Na3H(SO4)2, Al2O3, Na2SO4, and mullite. Introduction Kaolin is an abundant and widespread resource of aluminum. A large number of works describe processes to produce Al from kaolin using different reactions: (i) suspension of kaolin in aqueous solutions of inorganic acids (HCl, H2SO4, and HNO3);1-3 (ii) reaction with inorganic salts such as (NH4)2SO4, NH4HSO4,4-11 or NaHSO4;12-17 (iii) reaction with gaseous mixtures of Cl2 plus CO in the presence of NaCl18,19 or KAlCl4;20-22 and (iv) reaction with H2SO4-SO323,24 at high temperature. A side effect is that reaction between kaolin and the above-mentioned reactants also solubilizes some impurities, namely, Ti25,26 and Fe.27 Sodium hydrogen sulfate is formed as an intermediate in the production of HCl by the Mannheim process and is a byproduct of the manufacture of chromium(VI) oxide. It can be also obtained by mixing H2SO4 and Na2SO4.28 The use of either NaHSO4 or (NH4)2SO4 would depend on the availability of these salts and their cost. In principle, one or the other can be used satisfactorily to solubilize Al from kaolin. The advantage of NaHSO4 over (NH4)2SO4, if any, is that no NH3 is released during the reaction. For the purpose of this study, the reaction yield may be defined as

XM )

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

(1)

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 * To whom correspondence should be addressed. Tel.: (3493) 4021296. Fax: (34-93) 4021291. E-mail: [email protected].

moles of M in reacted kaolin, also expressed as the oxide. Equation 1 may be used with M ) Ti and Fe following reactions described elsewhere.25-27 Si does not suffer any change by the action of NaHSO4. As will be shown below, Al and H+ are the most relevant species for the present study. A ratio relating the amounts of both species was defined as follows:

rH ) nH+/nAl2O3

(2)

where nH+ is the total number of moles of H+ supplied by the acid and nAl2O3 is the total number of moles of Al in unreacted kaolin, expressed as Al2O3. Martı´nez-Lope et al.16 have reported on the reaction of NaHSO4 with kaolin. Experiments on a thermobalance and the analysis of products of the reaction by X-ray diffraction (XRD) analysis suggested that the following reaction took place:

2SiO2‚Al2O3‚2H2O + 6NaHSO4‚H2O f 2Na3Al(SO4)3 + 2SiO2 + 11H2O (3) Martinez-Lope et al.16 also said that there were three differentiated steps in the reaction process, the first one corresponding to the loss of hydration water of kaolin and NaHSO4, the second one due to decomposition of unreacted NaHSO4, and the third one corresponding to the decomposition of the Al-Na double salt formed in the reaction. According to results obtained by the same authors, the double salt formed is Na3Al(SO4)3. This salt first appears at 280 °C, and the reaction is complete when the temperature reaches 500 °C. At temperatures above 700 °C, the double salt begins to decompose. According to results reported by Martinez-Lope et al.,16 the Al reaction yield grows from approximately 0.10 for a reaction temperature of 200 °C, to reach a

10.1021/ie030273d CCC: $30.25 © 2005 American Chemical Society Published on Web 05/24/2005

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maximum of approximately 0.70 for a reaction temperature of 500 °C, and then decreases to values of approximately 0.30 for a reaction temperature of 700 °C. The values reported by these authors were obtained at a reaction time of 75 min. Regarding the effect of the reaction time, it was only studied for a reaction temperature of 160 °C, showing that the Al reaction yield grows for reaction time values below 6 h. Results in the work of Martinez-Lope et al.16 were only obtained for a value of rH ) 6. A study conducted by Solano and Juan17 for the reaction of NaHSO4 and two mineral wastes containing mixtures of kaolinite, muscovite, jarosite, natrojarosite, quartz, galena, blende, and pyrite (14-18 wt % SiO2, 33-43 wt % Al2O3, and 5-9 wt % Fe2O3) showed that, when the reaction temperature was considered, the Al reaction yield grew until approximately 0.90 for a reaction temperature of 400 °C. Above this temperature, no appreciable rise in the reaction yield was observed as a result of the decomposition of the aluminum sulfate formed to give Al2O3. Regarding the reaction time, the Al reaction yield grew, reaching a maximum at 3 h. Finally, values of rH above 1.4 did not allow an additional rise in the Al reaction yield values. Given that neither the work of Martinez-Lope et al.16 nor the work of Solano and Juan17 describes the reaction of kaolin with NaHSO4 for a wide range of reaction temperatures, reaction times, and rH values, we decided to undertake this study. Results obtained in this work were also compared to those obtained by Colina et al.24 for the reaction of kaolin with H2SO4-SO3. 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 analysis29 of kaolin was performed on a D-500 Siemens X-ray diffractometer. XRD patterns gave a result of 85 wt % kaolinite, 12 wt % mica, 2 wt % feldspar, and 1 wt % quartz for unreacted kaolin. The Hinckley index30 for unreacted kaolinite was 1.07. Products of the reaction were also analyzed using the same method. Major elements on unreacted kaolin were determined 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 reacted kaolin by leaching using a method described elsewhere.31 Chemical analysis of metals present in lixiviates was also performed by ICPOES. All of the reagents were Baker analyzed, analytical grade (J. T. Baker Chemicals NV, Deventer, Holland). Results of the chemical analysis of unreacted kaolin are given in Table 1. 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 NaHSO4 salt of analytical grade supplied by Probus (Barcelona, Spain) was used as a reagent. Qualitative XRD analysis showed that it was mostly NaHSO4‚H2O, with minor amounts of the anhydrous salt. NaHSO4 was ground before reacting with kaolin to obtain salt particles of the same size as kaolin particles. Chemical analysis of NaHSO4 after grinding

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.

gave 0.002 wt % Fe and 0.001 wt % heavy metals (expressed as Pb), indicating that no significant amounts of the species came from the salt. Samples for the reaction were prepared by mixing uncalcined kaolin and freshly molten NaHSO4 at molar ratios (rH) between 0.750 and 18.000, according to eq 2, putting the amount of NaHSO4 in close contact with 5 g of dry kaolin. Aliquots of the mixture were placed in quartz crucibles, which were heated in a 10-PR/300 Heron furnace to the reaction temperature (Tr) using a flash procedure. Temperatures ranged from 200 ( 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. Intensive grinding was avoided to limit metal extraction of the clay itself, which can solubilize some metals by mechanical activation.32,33 Additional experiments were carried out using previously calcined kaolin at a temperature (Tcalc) between 500 and 1000 °C for a time (tcalc) between 1 and 5 h. Also, some experiments were run in the same experimental conditions but covering crucibles with caps to study the effect of atmosphere on the reaction. XRD patterns of reacted samples showed unreacted kaolin and soluble Al salts, such as Na3Al(SO4)3, as previously reported by Martinez-Lope et al.16 and Colina.34 Qualitative analysis of mineral phases of products of the reaction was performed using the International Centre for Diffraction Data database. Powder diffraction file (PDF) numbers used to identify phases were as follows: kaolinite (14-0164), mica (260911), quartz (33-1161), feldspar (19-0932), turmaline (33-1261), NaHSO4‚2H2O (25-0834), NaHSO4 (25-0833), Na2S2O7 (01-0834), Na3H(SO4)2 (32-1090), NaAl(SO4)2 (27-0631), NaAl(SO4)2‚6H2O (19-1186), NaAl(SO4)2‚ 12H2O (01-0397), Na3Al(SO4)3 (03-0546), NaAlSiO4 (110221), Na2SO4 (01-0990), Al(HSO4)3 (28-0023), Al(HSO4)3‚ 6H2O (28-0104), Al2H2(SO4)4‚8H2O (22-0005), Al2(SO4)3 (30-0043), Al2(SO4)3‚17H2O (22-0022), R-Al2O3 (26-0031), δ-Al2O3 (04-0877), γ-Al2O3 (10-0425), η-Al2O3 (04-0875), θ-Al2O3 (11-0517), mullite (15-0776), and nephelin (350424). Typical standard deviations for Al, Ti, and Fe reaction yields were obtained by repeating one experiment five times at average experimental conditions (Tr ) 400 °C; tr ) 15-180 min; rH ) 3; Tcalc ) 0 °C; tcalc ) 0 h).

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Figure 1. Reaction temperature vs Al, Ti, and Fe reaction yields ([, XAl; 2, XFe; 9, XTi). tr ) 1 h; rH ) 3.

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 were removed. The BET specific surface area was measured on an ASAP 2010 from Micromeritics.35 The specific surface area for unreacted kaolin was 9.9 m2/g. Results and Discussion 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); (vi) reaction atmosphere. The results are described below. Reaction Temperature. Experiments were carried out in the interval 200-1000 °C. The effect of the reaction temperature on reaction yields may be appreciated in Figure 1: the Al reaction yield grows until it reaches a maximum at a reaction temperature of 700 °C. Beyond this temperature, the Al reaction yield decreases. The decrease at very high temperatures, 800 °C and above, could be due to the formation of insoluble products of the reaction that cannot be leached. This was confirmed by the appearance of Al2O3 peaks on the XRD patterns corresponding to these samples (see Figure 5). Al2O3 might be formed by thermal decomposition of Na3Al(SO4)3. Similar results for the dependence between the Al reaction yield and reaction temperature were found in previous works for the reaction between kaolin and H2SO4 at high temperature.24 As in the case of the reaction of kaolin with H2SO4 at high temperature, 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 NaHSO4 at high temperature by a process resembling the one described for thermal dehydroxylation of kaolin36 and also for the reaction of kaolin with H2SO4 at high temperature.24 The lower interval for the maximum Al reaction yields obtained with NaHSO4 at high temperature could be explained in terms of an easier dehydroxylation thanks to the action of the salt. 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). Results, on the other hand, agree quite well with those presented by Mar-

tinez-Lope et al.16 regarding the interval of the reaction temperature, but the Al reaction yield values found in the present work are lower. This fact could be due to the different sources of kaolin used. Regarding the Ti reaction yield, its value is lower than that of the Al reaction yield. However, it is in the same range as the Ti reaction yield obtained for the reaction with H2SO4 at high temperature.24 The difference in the reaction under study is that the Ti reaction yield reaches a maximum at 400 °C and then decreases smoothly (Figure 1). The Fe reaction yield shows a behavior similar to that shown by Al, with a maximum at 700 °C. The Fe reaction yield values achieved are in the same range as those obtained on the reaction between kaolin and H2SO4 at high temperatures. Experiments found in the bibliography1-3 on the reaction between kaolin and H2SO4 in an aqueous solution showed that the Fe reaction yield achieves a maximum value for calcination temperatures of 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 kaolin used in the present work, this could indicate that the attack of NaHSO4 over the kaolinite particle takes place in a manner different from that when reaction takes place between kaolin and an inorganic acid in an aqueous solution regarding the way Al 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 NaHSO4 at high temperature would remove Al in a 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). This conclusion suggests that the removal of Al from kaolin in the reaction with NaHSO4 is accomplished by a mechanism resembling that described for the reaction of kaolin with H2SO4 at high temperature. Reaction Time. This variable is of paramount importance regarding the conversion of some components of kaolin into water-soluble Al salts. As can be seen in Table 2, there is a relatively fast increase of the reaction yield during the initial time of reaction, and that increase is faster for reaction temperatures above 300 °C. This agrees with results obtained by Solano and Juan17 for mineral wastes. The increase in the reaction yield is affected by the increase in the rH molar ratio: the increase in the Al reaction yield values is faster for rH values of 3-6. Only for reaction temperatures of 700 °C and above does the Al reaction yield increase extremely fast for low reaction times, resembling the behavior described for the reaction of kaolin with H2SO4 at high temperature.24 However, for reaction temperatures in the range of 700-1000 °C, there is a decrease in the reaction yield for reaction times above 15 min. Proton to Aluminum Molar Ratio (rH). Experiments were carried out in the rH interval 0.750-18.000. Results are shown in Figure 2. All reaction yields, for Al, Ti, and Fe, show steady increases when the rH molar ratio is increased regardless of the reaction temperature. In contrast with results obtained for the reaction of kaolin with H2SO4,24 a maximum is observed for rH values of 3-6. For rH values of 6-18, all reaction yields decrease. This behavior could mean that the product layer over the surface of the kaolin particle is an obstacle to the continuity of the reaction. Results

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Figure 2. rH molar ratio vs Al, Ti, and Fe reaction yields ([, XAl; 2, XFe; 9, XTi). Tr ) 400 °C; tr ) 1 h.

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

obtained are, on the other hand, much higher than the maximum at 1.4 reported by Solano and Juan17 for mineral wastes, probably because of the higher Fe content of their samples. Fe minerals also react with NaHSO4, therefore reducing the amount of NaHSO4 available for the extraction of Al. Calcination Temperature. It has been reported in many works that a previous calcination of kaolin increases the Al reaction 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 of 500-1000 °C. Once kaolin had been previously calcined, it was mixed with NaHSO4, and the reaction took place at 400 °C. Results show that previous calcination of kaolin only produced a slight increase on the Al reaction yield. The Ti reaction yield also changed slightly and so did the Fe reaction yield (Figure 3). These results agree very well with those described for the reaction of kaolin and H2SO4 at high temperature.24 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 NaHSO4, and the

reaction took place at 400 °C. It was observed that the calcination time increases only slightly the Al reaction yield (XAl ranged from 0.150 to 0.180 for tr ) 1 h and tcalc from 1 to 5 h, respectively), having also a slight effect on the Ti and Fe reaction yields. The reason for the above results regarding the previous calcination of kaolin could be justified by the fact that NaHSO4, together with the heat provided to achieve the reaction temperature, produces the dehydroxylation of kaolin (as discussed above), making the previous calcination unnecessary. Reaction Atmosphere. To test the relevance of the reaction atmosphere on the reaction of kaolin with NaHSO4 at high temperature, several experiments were carried out by covering crucibles with caps. Contrary to the results reported by others,16 there was practically no effect on the Al, Ti, and Fe reaction yields (results not shown). Only for values of the rH ratio above 6 were slightly higher reaction yields obtained. The same thing happened for the reaction of kaolin with H2SO4 at high temperature; this means that reaction yields depend mainly on the reaction temperature and time and on the rH molar ratio, whereas the concentration of gaseous compounds on the reaction atmosphere generated by NaHSO4 and kaolin is not important.

Table 2. Al, Ti, and Fe Reaction Yields (Results Are Calculated as wt % of the Corresponding Oxide; rH ) 3) Tr (°C) tr (min)

200

300

400

500

600

700

800

900

1000

typical std.

0.250 0.245 0.213 0.185 0.159

0.204 0.147 0.089 0.050 0.027

0.117 0.050 0.024 0.014 0.014

0.020 0.013 0.021 0.029 0.038

0.005 0.010 0.004 0.016 0.004

15 30 60 120 180

0.003 0.009 0.010 0.013 0.022

0.011 0.040 0.132 0.140 0.139

0.115 0.186 0.187 0.188 0.199

0.184 0.186 0.207 0.176 0.187

XAl 0.199 0.196 0.200 0.202 0.195

15 30 60 120 180

0.002 0.006 0.006 0.008 0.018

0.009 0.021 0.032 0.033 0.033

0.035 0.049 0.046 0.064 0.068

0.028 0.021 0.032 0.036 0.028

XTi 0.043 0.035 0.034 0.038 0.033

0.034 0.028 0.036 0.034 0.039

0.038 0.046 0.031 0.020 0.008

0.026 0.024 0.020 0.008 0.007

0.026 0.007 0.006 0.008 0.011

0.005 0.002 0.001 0.012 0.006

15 30 60 120 180

0.027 0.032 0.028 0.035 0.043

0.030 0.049 0.087 0.099 0.107

0.094 0.144 0.144 0.153 0.161

0.128 0.125 0.141 0.127 0.121

XFe 0.154 0.146 0.164 0.169 0.161

0.175 0.180 0.251 0.245 0.270

0.232 0.292 0.239 0.189 0.200

0.221 0.230 0.186 0.038 0.030

0.230 0.062 0.028 0.027 0.037

0.003 0.004 0.002 0.010 0.006

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Figure 4. XRD patterns showing samples of reaction products from the reaction between kaolin and NaHSO4. tr ) 15 min; rH ) 3. Table 3. BET Specific Surface Area Values (m2/g) for Samples Reacted at Different Tr Values (rH ) 3) Tr (°C) tr (min)

200

300

400

500

600

30 60 180

14.2 13.9 19.7

22.9 77.1 74.7

101.5 99.4 98.1

95.0 23.5 54.6

37.5 21.4 23.1

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 the BET surface area for short reaction times (Table 3). Once the reaction has reached a certain reaction time, BET specific surface area values become more or less stable. This behavior is very similar from one reaction temperature to another and has already been reported for the reaction of kaolin with H2SO4 at high temperature24 and for the reaction of kaolin with H2SO4 or HCl in an aqueous solution.1,37 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 Al would give way to the interconnection of these pores, thus stabilizing or even decreasing the value of the BET specific surface area. Indeed, samples of the reaction of kaolin with NaHSO4 showed a sensitive increase in the BET surface area, reaching values as high as 100 m2/g, whereas reported values for the reaction between kaolin and H2SO4 in an aqueous solution1 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 NaHSO4 at high temperature, products of the reaction stay at the particle surface, forming a layer, in the reaction of kaolin with H2SO4 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 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 Na3Al(SO4)3 was seen for reaction temperatures in the range of 400-600 °C. However, other products of the reaction not reported in previous works,16 such as NaAl(SO4)2 or NaAl(SO4)2‚ xH2O (where x ) 6 and 12; reaction temperatures in the range of 500 °C) and Na3H(SO4)2 (reaction temper-

Figure 5. XRD patterns showing samples of reaction products from the reaction between kaolin and NaHSO4. tr ) 15 min; rH ) 3.

atures below 500 °C), are present depending on the reaction temperature. High-temperature products of the reaction can also be observed: such is the case of Al2O3, Na2SO4, and mullite. Al2O3 and Na2SO4 could be formed by thermal decomposition of Na3Al(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,38 there could be some reason to think that mullite formation could be promoted in some way by the presence of Al salts, as has been proposed for other metallic ions such as Cu.36,39 Phases such as Na nepheline could appear as a result of the reaction of Na3Al(SO4)3 with SiO2 from kaolin, but their presence cannot be guaranteed because their XRD peaks appear at the same two θ values of other products of the reaction. Conclusions From the results discussed above, it can be concluded that the reaction between kaolin and NaHSO4 leads to the formation of Na3Al(SO4)3, NaAl(SO4)2, NaAl(SO4)2‚ xH2O (where x ) 6 and 12), Na3H(SO4)2, Al2O3, Na2SO4, and mullite, depending on the reaction conditions. The Al reaction yield grows until it reaches a maximum at a reaction temperature of 700 °C. Beyond this temperature, the Al reaction yield decreases because of the formation of insoluble products of the reaction that cannot be leached. There is a relatively fast increase of the reaction yield during the initial time of reaction, and that increase is faster for reaction temperatures above 300 °C. All reaction yields, for Al, Ti, and Fe, show steady increases when the rH molar ratio is increased regardless of the reaction temperature. A maximum is observed for rH values of 3-6. This behavior could mean that the product layer over the surface of the kaolin particle is an obstacle to the continuity of the reaction. Previous calcination of kaolin only produces a slight increase on the Al reaction yield. These results agree very well with those described for the reaction of kaolin and H2SO4 at high temperature. The calcination time increases only slightly the Al reaction yield, having also a slight effect on the Ti and Fe reaction yields. Samples of the reaction of kaolin with NaHSO4 showed a sensitive increase in the BET surface area, reaching values as high as 100 m2/g. In the reaction of kaolin with NaHSO4 at high temperature, products of the reaction stay at the particle surface, forming a layer and giving lower values of the BET surface area.

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Received for review March 31, 2003 Accepted April 20, 2005 IE030273D