High-Temperature Reaction of Kaolin with Ammonium Sulfate

The products of the reaction were triammonium hydrogen disulfate [(NH4)3H(SO4)2], ammonium aluminum sulfate [NH4Al(SO4)2], aluminum sulfate [Al2(SO4)3...
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Ind. Eng. Chem. Res. 2006, 45, 495-502

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High-Temperature Reaction of Kaolin with Ammonium Sulfate Fernando G. Colina,* Maria N. Abellan, and Ivan Caballero Department of Chemical Engineering, UniVersitat de Barcelona, Marti i Franques 1-6, E-08028 Barcelona, Spain

Reactions between kaolin and inorganic salts, such as sodium hydrogen sulfate and ammonium sulfate, or inorganic acids, such as hydrochloric acid and sulfuric acid, can be focused either on the production of Al or on the adjustment of the silica-to-alumina ratio of the kaolin. On the basis of adjusting the silica-to-alumina ratio, kaolin and ammonium sulfate [(NH4)2SO4] are mixed and heated in a furnace at temperatures between 200 and 1000 °C. The parameters studied were the reaction temperature and time, the proton-to-alumina molar ratio, the calcination temperature and time, and the reaction atmosphere. The products of the reaction between kaolin and (NH4)2SO4 were characterized by means of X-ray diffraction (XRD). The Al reaction yield grew until reaching a maximum at 600 °C and started to decrease sharply beyond this temperature. The Ti reaction yield values were lower than the Al reaction yield values, showing a maximum at 600 °C and decreasing smoothly as the reaction temperature was increased. The Fe reaction yield exhibited the same tendencies as the Al reaction yield. With regard to the proton-to-alumina molar ratio, a maximum in Al, Ti, and Fe reaction yields was observed at rH ) 3. Precalcination of kaolin produced only slight decreases in the Al and Ti reaction yields and a slight increase in the Fe reaction yield. The products of the reaction were triammonium hydrogen disulfate [(NH4)3H(SO4)2], ammonium aluminum sulfate [NH4Al(SO4)2], aluminum sulfate [Al2(SO4)3], and alumina [Al2O3]. Brunauer-Emmett-Teller (BET) specific surface area tests indicated an increase of the BET specific surface area for short reaction times, reaching values above 200 m2/g. Introduction Kaolin is a silicate of hydrated aluminum, formed mainly of kaolinite [Al4(Si4O10)(OH)2]. The Al exists in nature not as a raw material, but in combination. Among the silicates, the most common minerals containing Al are bauxite, feldspar, mica, and clays. Kaolin can be considered, therefore, as a source of Al. 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, HNO3);1-3 (ii) reaction with inorganic salts such as (NH4)2SO4, NH4HSO4,4-12 or NaHSO4;13-19 (iii) reaction with gaseous mixtures of Cl2 plus CO in the presence of NaCl20,21 or KAlCl4;22-24 and (iv) reaction with H2SO4-SO325,26 at high temperature. A side effect is that reaction between kaolin and the abovementioned reactants also solubilizes some impurities, namely, Ti27,28 and Fe.29 The reaction between kaolin and (NH4)2SO4 was first reported at the beginning of the 20th century,30-33 and presently, several applications are being proposed.34,35 The reactions between the kaolin and inorganic salts or acids can be focused either on the production of Al or on the adjustment of the silica-to-alumina ratio of the kaolin to a subsequently synthesized zeolite, for example. On a large scale, (NH4)2SO4 is produced mainly by the reaction of H2SO4 with NH3 and is also formed by the reaction of both anhydrite and gypsum with NH3 and CO2 (Merseburg process), which is only economically interesting if S is in short supply. It is a coproduct in the production of synthetic fiber intermediates such as caprolactam and can also be recovered as a byproduct in coal coaking.36,37 For the purposes of this study, the reaction yield can be defined as * To whom correspondence should be addressed. Tel.: (34-93) 4021296. Fax: (34-93) 4021291. E-mail: [email protected].

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 moles of M in reacted kaolin, also expressed as the oxide. Equation 1 can be used with M ) Ti and Fe following reactions described elsewhere.7,27-29 Si does not suffer any change by the action of (NH4)2SO4. Al and H+ are the most relevant reactive species for the present study, and therefore, a ratio relating the amounts of the two species was defined as

rH )

n H+ 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. The proton action has been assumed to be the principal agent responsible for the extraction of the species Al, Ti, and Fe. Fetterman and Sun6 have reported on the reaction of (NH4)2SO4 with kaolin, suggesting that the following reaction takes place

2SiO2‚Al2O3‚2H2O + 4(NH4)2SO4 f Al2(SO4)3‚(NH4)2SO4 + 6NH3 + 5H2O + 2SiO2 (3) During the reaction of (NH4)2SO4 with kaolin, thermal decomposition of the salt occurs.38,39 Analysis by X-ray diffraction (XRD) of the products of the thermal decomposition suggests that the following mechanism might take place: (NH4)2SO4 decomposes into (NH4)3H(SO4)2 or NH4HSO4, which decom-

10.1021/ie050872f CCC: $33.50 © 2006 American Chemical Society Published on Web 12/14/2005

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Figure 1. Diagram of (NH4)2SO4 and NH4HSO4 thermal decomposition (based on the work of Kiyoura and Urano38).

poses into (NH4)2S2O7 or NH2SO3H, and finally into several gases, namely, N2, H2O, NH3, SO2, and SO3 (Figure 1). Works found in the literature report on the reaction of (NH4)2SO4 with alumina,6,8 on one hand, and (NH4)2SO4 with kaolin4,7 on the other. According to these reports, the two reactions occur under similar reaction conditions and by pathways. Fouda et al.7 studied the reaction of (NH4)2SO4 with kaolin, finding NH4Al(SO4)2 as the only product of reaction for an rH value of 4 at 340-380 °C and a reaction time of 2 h. Experiments performed with kaolin and NH4HSO4 for an rH value of 6 gave slightly different results: The reaction started at 150 °C and led directly to NH4Al(SO4)2, which decomposed into Al2(SO4)3 at 430 °C (Figure 2). Bayer et al.4 characterized the products of the reaction between kaolinite and (NH4)2SO4 for an rH value of 8. The authors reported that (NH4)3H(SO4)2 was obtained at 230 °C, with partial formation of (NH4)3Al(SO4)3, which is very unstable and decomposed above 250 °C to NH4Al(SO4)2. This sulfate changed to Al2(SO4)3, which remained stable up to 650 °C, when it decomposed to Al2O3. Other works in the literature on the reaction between kaolin and (NH4)2SO4 showed that there is a reaction temperature at which a maximum of extraction of soluble Al compounds occurs.6-9 Results reported show disagreement about the value of the reaction temperature at which the maximum is achieved, ranging between 450 and 600 °C. According to the results reported by Fouda et al.,7 the extraction of soluble Al compounds at a reaction temperature of 350 °C increased with increasing rH molar ratio up to 4 and reaction time up to 2 h, whereas the extraction of soluble Al compounds at temperatures ranging from 450 to 550 °C decreased at reaction times longer than 2 h. A maximum extraction of soluble Al compounds of ap-

proximately XAl ) 0.65 was obtained at 350 °C and a reaction time of 2 h. The same results were reached at 450 °C and a reaction time of 0.5 h for an rH value of 4. Fetterman and Sun6 also reported on the reaction of kaolin with (NH4)2SO4 and the influence of particle size, reaction temperature and time, and amount of (NH4)2SO4 on Al extraction. Increasing particle size produces a decrease in the extraction of soluble Al compounds. The optimum reaction temperature was 500 °C, because above this temperature, monohydrated minerals are converted into insoluble species. With regard to the reaction time, a minimum of 4 h was required to achieve a maximum of Al extraction. According to the Fetterman and Sun,6 the extraction of soluble Al compounds increased with increasing rH and started to decrease only above an rH value of 5. The present work undertakes the study of the reaction of kaolin with (NH4)2SO4, for a wider range of reaction temperatures, times, and rH values (molar ratios) Previous calcination and reaction atmosphere are also studied. Experimental Procedure Kaolin (Remblend grade) from St. Austell (Cornwall, U.K.), supplied by Imerys (St. Austell, U.K.) was used for this study. Mineralogical analysis of the kaolin40 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 index41 (HI) 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.42 Chemical analysis of the 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 reported in Table 1. The particle size distribution of unreacted kaolin was measured by light scattering using a Microtrac SRA150 instrument from Leeds & Northrup. The analysis showed an average diameter of 15 µm. A standard (NH4)2SO4 salt supplied by Sigma-Aldrich (Barcelona, Spain) was used as a reagent. Qualitative XRD analysis showed that it was mostly (NH4)2SO4. The (NH4)2SO4 was ground before being reacted with kaolin to obtain salt particles of the same size as the kaolin particles. Chemical analysis of (NH4)2SO4 after grinding gave 0.001 wt % Fe and 0.001 wt %

Figure 2. Reaction between (NH4)2SO4 and kaolin (based on the work by Fouda et al.7).

Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 497 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.

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 (NH4)2SO4 at molar ratios (rH) between 0.75 and 18.00, according to eq 2, homogenizing mixtures of (NH4)2SO4 with kaolin (5.00 g). 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. Once the reaction temperature had been reached, the crucibles were left in the furnace for a certain time (tr) ranging from 15 to 180 min. After the reaction time had 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.43,44 Calcination of the kaolin was carried out in the temperature interval of 500-1000 °C, for a time (tcalc) between 1 and 5 h. The reaction atmosphere was also studied, covering the crucibles with caps, to study the effect of this variable on the reaction. Qualitative analysis of the mineral phases of the products of reaction was performed using the International Centre for Diffraction Data (ICDD) database. Powder diffraction file (PDF) numbers used to identify phases were as follows: kaolinite, 140164; mica, 26-0911; quartz, 33-1161; Al2(SO4)3, 42-1428; δ-Al2O3, 16-0394; Al2O3, 73-2294; NH4Al(SO4)2, 23-0001; (NH4)3H(SO4)2, 35-1500; (NH4)2SO4, 76-0579. 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, rH ) 3, Tcalc ) 0 °C, tcalc ) 0 h). Thermal analysis of the (NH4)2SO4-kaolin mixture by thermogravimetric analysis (TGA) was carried out using a Mettler TGA/SDTA 851 E. The rate of heating was 10 °C/ min. For the analysis of Brunauer-Emmett-Teller (BET) specific surface areas, samples were washed with water until all of the soluble products of the reaction were removed. BET specific surface areas were measured on an ASAP 2010 instrument from Micromeritics.45 The specific surface area for unreacted kaolin was 8.6 m2/g. Results and Discussion To study this reaction, the following parameters were considered: reaction temperature (Tr), reaction time (tr), protonto-alumina molar ratio (rH), calcination temperature (Tcalc),

calcination time (tcalc), and reaction atmosphere. The results are described below. Products of Reaction. The products obtained from the reaction between kaolin and (NH4)2SO4 (rH ) 3) were characterized by means of XRD. As observed in Figure 3, (NH4)3H(SO4)2 has already formed at 300 °C. Above this temperature, (NH4)3H(SO4)2 gives place to the formation of NH4Al(SO4)2. This double sulfate changes gradually to Al2(SO4)3, which decomposes between 800 and 900 °C into Al2O3, which is insoluble and therefore cannot be leached (Figure 4). The results reported in this work agree very well with those obtained by Fouda et al.,7 although they studied the reaction only below 550 °C. Bayer et al.4 characterized the products of the reaction between kaolinite and (NH4)2SO4 (rH ) 8). According to their results, the reaction started immediately after melting of (NH4)3H(SO4)2 at 230 °C, with partial formation of (NH4)3Al(SO4)3. According to their results, the latter compound is very unstable and decomposed above 250 °C to NH4Al(SO4)2. This sulfate changed to Al2(SO4)3, which remained stable up to 650 °C, when it decomposed to Al2O3. The results reported by Bayer et al.4 agree very well with those found in our research. However, there is a significant difference in the transition temperature from Al2(SO4)3 to Al2O3 reported by Bayer et al.4 (650 °C) and the value reported in this work (800-900 °C), which can be explained only in terms of a different rH value used in the experiments. On the contrary, the transition temperature reported in this work agrees very well with those obtained by the same authors for the reactions of kaolin with H2SO426 and NaHSO4.18 The results of the TGA suggest four important steps (see Figure 5). The first one (from 210 to 380 °C) corresponds to the loss of the hydration water of the kaolin, accompanied by the appearance of (NH4)3H(SO4)2, which comes from the decomposition of (NH4)2SO4. The second step (from 380 to 500 °C) is due to the formation of NH4Al(SO4)2, which changes gradually to Al2(SO4)3 in the third step (from 500 to 650 °C). In the last step (from 650 to 950 °C), Al2(SO4)3 decomposes into Al2O3. These results agree very well with those obtained in the characterization of the reaction products by XRD. Reaction Temperature. Experiments were carried out in the interval 200-1000 °C, as the reaction does not take place at temperatures below 200 °C and insoluble Al2O3 would be obtained at temperatures above 1000 °C.4 As observed in Figure 6, the Al reaction yield grows until it reaches a maximum at a reaction temperature of 600 °C. Above this temperature, the Al reaction yield decreases. The significant decrease at high temperature could be caused by the formation of insoluble products of reaction that cannot be leached. This was confirmed by the appearance of Al2O3 peaks in the XRD patterns corresponding to these samples (see Figure 4). Al2O3 might be formed by the thermal decomposition of (NH4)3Al(SO4)3. The Al, Ti, and Fe reaction yields are slightly higher than those obtained for the reaction with H2SO426 and significantly higher than those obtained for the reaction with NaHSO4.18 Similar results for the dependence between the Al reaction yield and the reaction temperature were found in previous works for the reaction between kaolin and H2SO426 or NaHSO418 at high temperature. As in the case of the reaction of kaolin with H2SO4 or NaHSO4 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 (NH4)2SO4 at high temperature

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Figure 3. XRD patterns showing samples of reaction products of the reaction between kaolin and (NH4)2SO4 (tr ) 60 min, rH ) 3).

by a process resembling the one described for the thermal dehydroxylation of kaolin8 and also for the reaction of kaolin with H2SO426 or NaHSO418 at high temperature. The lower interval for the maximum Al reaction yields obtained with (NH4)2SO4 at high temperature can be explained in terms of an easier dehydroxylation as a result of the action of the salt. The decrease in all reaction yields in the reaction under study could be mostly due to the formation of insoluble reaction products such as Al2O3 (as discussed before). In the present work, a maximum Al reaction yield of 0.52 was obtained (Tr ) 600 °C, tr ) 1 h, and rH ) 3), which is similar to that reported by Fouda et al.7 (XAl ) 0.60, Tr ) 600 °C, tr ) 1 h, and rH ) 3) who studied the reaction between Dehisa kaolin ore and (NH4)2SO4. On the other hand, Fetterman and Sun6 reported an Al reaction yield of 0.75 when the reaction took place at 500 °C, but their raw material was a high-alumina refractory clay from central Pennsylvania, a mixture of kaolinite plus boehmite, diaspore, and siderite (56.13 wt % Al2O3). As refers to the Ti reaction yield, the influence of reaction temperature is less significant than that observed for Al and Fe: The Ti reaction yield reaches a maximum at 500 °C and

then decreases smoothly (Figure 6). The Fe reaction yield shows a behavior similar to that shown by Al, with a maximum at 600 °C, although the maximum values obtained are lower than those obtained for Al. The Fe reaction yield values achieved are higher than those obtained from the reaction between kaolin and H2SO4 at high temperatures.26 The reaction of Dehisa kaolin ore with (NH4)2SO4, exhibited an Fe reaction yield similar to that reported in this work, achieving a maximum at 350 °C. On the other hand, the values for the Ti reaction yield were lower than those obtained in the present work. However, the raw material used in the work by Fouda et al.7 had a much higher content of TiO2 present not as isomorphic substitution in kaolinite but as anatase. Reaction Time. There is a relatively rapid increase of the reaction yield during the initial time of reaction, followed by stabilization at higher reaction times (Table 2). If rH is lower than 3, a stable value is achieved at a reaction time of 60 min. However, working at higher rH values, the Al reaction yield increases gradually, achieving a stable value at higher reaction times (120-180 min). The Al, Fe, and Ti reaction yields show the same trend. This behavior was also observed for the reaction

Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 499

Figure 4. XRD patterns showing samples of reaction products of the reaction between kaolin and (NH4)2SO4 (tr ) 60 min, rH ) 3) (continued).

Figure 5. TGA analysis of a mixture (NH4)2SO4-kaolin (rH ) 3).

with NaHSO4,18 although the rH values needed to achieve equal trends for the reaction with (NH4)2SO4 were lower. Regarding the reaction with H2SO4,26 the reaction time needed to achieve equal reaction yields was lower than in the case of (NH4)2SO4. The results do not agree with those obtained by Fouda et al.7 who achieved a stable value at 180 min. Fetterman and Sun6 reported that the stabilization time was 240 min. The reason

Figure 6. Reaction temperature vs Al, Ti, and Fe reaction yields ([, XAl; 2, XTi; 9, XFe) (tr ) 1 h, rH ) 3).

for these different results might be the source of the raw materials used in their works, as discussed before.

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Table 2. Al, Ti, and Fe Reaction Yieldsa (rH ) 3) tr (min)

a

Tr (°C) 200

300

400

500

600

700

800

900

1000

41.592 32.632 30.559 28.365 28.484

34.169 17.860 21.101 18.699 18.525

3.918 2.670 2.383 3.241 2.138

15 30 60 120 180

0.246 0.307 0.341 0.458 0.371

0.832 2.000 2.633 6.867 13.700

9.055 20.570 38.346 34.032 37.074

XAl (typical standard deviation ) 0.0004) 34.130 42.775 45.040 33.784 46.842 47.961 38.032 51.990 47.280 42.503 48.269 44.095 39.844 51.173 45.593

15 30 60 120 180

0.000 0.146 0.604 0.904 0.664

2.060 3.689 5.177 8.425 9.497

8.069 9.050 11.225 12.207 10.395

XTi (typical standard deviation ) 0.0025) 11.820 14.828 12.965 9.837 12.869 9.807 11.465 13.813 11.886 12.059 9.725 12.914 12.537 13.226 12.265

2.382 7.189 9.215 7.293 7.862

5.159 2.822 3.898 7.293 2.350

1.271 0.804 1.199 2.979 0.371

15 30 60 120 180

3.533 5.316 5.632 3.952 4.563

5.623 7.076 8.592 10.395 13.651

9.809 16.387 26.031 27.580 25.833

XFe (typical standard deviation ) 0.0102) 22.062 26.459 28.581 24.236 33.686 30.716 25.709 34.890 30.713 29.935 27.417 26.399 28.089 33.129 28.307

12.191 17.917 17.290 16.441 15.790

18.417 10.260 12.450 11.300 11.526

4.613 4.834 5.019 4.441 4.890

Results calculated as weight percentages of the corresponding oxide.

Figure 7. rH molar ratio vs Al, Ti, and Fe reaction yields ([, XAl; 2, XTi; 9, XFe) (Tr ) 400 °C, tr ) 1 h).

Proton-to-Alumina Molar Ratio. Experiments were carried out with proton-to-alumina molar ratios in the interval 0.7518.00, and the results are shown in Figure 7. 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 to the results obtained for the reaction of kaolin with H2SO4,26 a maximum is observed for the rH value of 3. This trend, however, resembles that observed for the reaction with NaHSO4.18 As in the case of NaHSO4, this behavior could mean that the product layer over the surface of the kaolin particle is an obstacle to the continuation of the reaction. Fouda et al.7 obtained an increase in Al and Fe reaction yield trends at rH values up to 3.00, which agrees very well with the results obtained in the present work. Fetterman and Sun,6 on the other hand, described an increasing trend of the Al reaction yield until a value of rH ) 6 is reached. The reason for these different results might be the source of the raw material used in their work, as discussed before. Calcination Temperature. It has been reported in many works that the 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

Figure 8. Calcination temperature vs reaction yield ([, XAl; 2, XTi; 9, XFe) (tr ) 1 h, rH ) 3, tcalc ) 1h).

to the reaction at high temperature, was also considered. For this purpose, kaolin was calcined at different temperatures in the range of 500-1000 °C. Once kaolin had been previously calcined, it was mixed with (NH4)2SO4, and the reaction took place. The results showed that the precalcination of kaolin produces only a slight decrease of the Al and Ti reaction yields and a slight increase of the Fe reaction yield (Figure 8), which are lower than the typical experimental standard deviation (see Table 2). The same trend was observed for the reactions with H2SO426 and NaHSO4.18 These results agree very well with those reported by Fouda et al.,7 who worked only with calcined kaolin. 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 (NH4)2SO4. The reaction took place at 400 °C. It was observed that the calcination time increased the Al reaction yield only slightly, also having a slight effect on Ti and Fe reaction yields. The same trend was observed for the reactions with H2SO426 and NaHSO4.18 The reason for the above results regarding the previous calcination of kaolin can be justified by the fact that (NH4)2-

Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 501 Table 3. BET Specific Surface Areas (m2/g) for Samples Reacted at Different Tr’s (rH ) 3) Tr

tr (min)

200

300

400

500

600

30 60 180

9.1 9.4 10.7

17.3 14.3 65.7

216.1 195.6 215.0

179.6 184.4 176.1

152.4 149.7 133.9

SO4, together with the heat provided to achieve the reaction temperature, causes the dehydroxylation of kaolin (as discussed above), making the previous calcination unnecessary. Reaction Atmosphere. To test the effect of the reaction atmosphere on the reaction of kaolin with (NH4)2SO4 at high temperature, several experiments were carried out by covering the crucibles with caps. Contrary to results reported elsewhere,17 there was practically no effect on the Al, Ti, and Fe reaction yields. As happened for the reactions of kaolin with H2SO426 and NaHSO418 at high temperature, this means that the reaction yields depend mainly on the reaction temperature and time and on the rH molar ratio, whereas the concentration of gaseous compounds in the reaction atmosphere generated by (NH4)2SO4 and kaolin is not important. Physical Properties of the Products of the Reaction. BET specific surface area tests were run on a number of samples of reacted kaolin. The results show an increase of BET specific surface area for short reaction times (Table 3). Once the reaction reaches 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 reactions of kaolin with H2SO426 and NaHSO418 at high temperature, as well as for the reaction of kaolin with H2SO4 or HCl in an aqueous solution.1 According to these works, the increase of the BET specific surface area is 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, as can be observed in Table 3, where the values corresponding to 600 °C are lower to those obtained at 400 °C, whereas the maximum extraction was achieved at 600 °C. Indeed, samples of the reaction of kaolin with (NH4)2SO4 showed a sensitive increase in the BET specific surface area, reaching values as high as 200 m2/g, that agrees with the results reported for the reaction between kaolin and H2SO4 in an aqueous solution1. However, the maximum BET specific surface areas obtained for the reactions with H2SO426 and NaHSO418 are significantly lower, being around 100 m2/g in both cases. Conclusions From the results discussed above, it can be concluded that the reaction between kaolin and (NH4)2SO4 gives rise to the formation of NH4Al(SO4)2, (NH4)3HSO4, Al2(SO4)2, and Al2O3. The Al reaction yield grows until it reaches a maximum of 0.52 at a reaction temperature of 600 °C. The maximum Ti and Fe reaction yields were also observed at 600 °C, and the values were 0.15 and 0.35, respectively. Above this temperature, the Al reaction yield decreases as a result of the formation of insoluble products of reaction that cannot be leached. There is a relatively rapid increase of the reaction yield during the initial time of the 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 an rH value of 3. 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 causes only a slight increase of the Al reaction yield. These results agree very well with those described for the reactions of kaolin with H2SO4 and NaHSO4 at high temperature. Calcination time increases the Al reaction yield only slightly, also having a slight effect on the Ti and Fe reaction yields. Samples of the reaction of kaolin with (NH4)2SO4 showed a sensitive increase in the BET specific surface area, reaching values as high as 200 m2/g. The maximum value of BET specific surface area was achieved at 400 °C, whereas the maximum extraction was taken place at 600 °C. An extended extraction of Al would give way to the interconnection of the pores created during the extraction, thus stabilizing or even decreasing the value of the BET specific surface area. Future work on this reaction should include an in-depth study of its reaction mechanism for a wide range of reaction conditions. Literature Cited (1) Ford, K. J. R. Leaching of Fine and Pelletised Natal Kaolin Using Sulphuric Acid. Hydrometallurgy 1992, 29, 109. (2) 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. (3) Ziegenbalg, S.; Haake, G. Investigations into the Alumina Extraction from Clay by Hydrochloric and Sulphuric Acid Leaching. Light Met. 1983, 1119. (4) Bayer, G.; Kahr, G.; Mueller-Vonmoos, M. Reactions of Ammonium Sulphates with Kaolinite and other Silicate and Oxide Minerals. Clay Miner. 1982, 17, 271. (5) Davies, H.; Dering, H. O.; Parker, T. W. Al2O3 from Clay by an NH4 Alum-NH3 Process. U.S. Patent 2,375,977, 1945. (6) Fetterman, J. W.; Sun, S. C. Alumina Extraction from a Pennsylvania Diaspore Clay by an Ammonium Sulfate Process. Alumina 1963, 1, 333. (7) 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. (8) Nagaishi, T.; Ishiyama, S.; Yoshimura, J.; Matsumoto, M.; Yoshinaga, S. Reaction of Ammonium Sulphate with Aluminium Oxide. J. Therm. Anal. 1982, 23, 201. (9) Badyoczek, H. Phase Transformations of Kaolin Minerals under the Effect of Heat Treatment with Ammonium Sulfate. Sprechsaal 1978, 111, 565. (10) Peters, F. A.; Johnson, P. W.; Kirby, R. C. Methods for Producing Alumina from Clay: An EValuation of Two Ammonium Alum Process; Report of Investigations RI 6573; Bureau of Mines, U.S. Department of the Interior: Washington, DC, 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.; Martinez-Lope, M. J.; Casais-Alvarez, M. T. Metodo de Obtencion de Alumina a partir de Arcillas y Silicatos Aluminicos en general. Spanish Patent 482,881, 1979. (14) Garcia-Clavel, M. E.; Martinez-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.; Martinez-Lope, M. J.; Casais-Alvarez, M. T. Procedimiento Continuo de Obtencion de Compuestos de Aluminio a partir de Silicatos Aluminicos y otros Minerales de Aluminio. Spanish Patent 522,398, 1983. (16) Garcia-Clavel, M. E.; Martinez-Lope, M. J.; Casais-Alvarez, M. T. Metodo de Solubilizacion de los Componentes Metalicos de los Filosilicatos. Spanish Patent 545,690, 1985. (17) Martinez-Lope, M.; Garcia-Clavel, M. E.; Casais-Alvarez, M. T. Solubilization Reaction of the Alumina from Kaolin by Solid State Reaction. Termochim. Acta 1991, 177, 77. (18) Colina, F. G.; Costa, J. High-Temperature Reaction of Kaolin with Sodium Hydrogen Sulfate. Ind. Eng. Chem. Res. 2005, 44, 4495.

502

Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006

(19) Solano, E.; Juan, D. Obtencion de Alumina a partir de Arcillas utilizando como Agente Disgregante el Bisulfato Sodico. Quı´m. Ind. 1995, 14, 82. (20) Grob, B.; Richarz, W. Chlorination of Alumina in Kaolinitic Clay. Metall. Trans. B 1984, 15, 529. (21) Grob, B. Selektive Chlorierung von Aluminiumoxid in Kaolinit. Dissertation, Eidgenoessiche Technische Hochschule Zurich, Zurich, Switzerland, 1983. (22) Martin, E. S.; Wefers, K. Microscopic Investigation of Clay Chlorination. Light Met. 1984, 619. (23) 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. (24) 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, 1978. (25) Colina, F. G.; Esplugas, S.; Costa, J. High-Temperature Reaction of Kaolin with Inorganic Acids. Brit. Ceram. Trans. 2001, 100 (5), 203. (26) Colina, F. G.; Esplugas, S.; Costa, J. High-Temperature Reaction of Kaolin with Sulfuric Acid-SO3. Ind. Eng. Chem. Res. 2002, 41, 4168. (27) 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. (28) Maynard, R. N.; Millman, N.; Iannicelli, J. A Method for Removing Titanium Dioxide Impurities from Kaolin. Clays Clay Miner. 1969, 17, 59. (29) Malden, P. J.; Meads, R. E. Substitution by Iron in Kaolinite. Nature 1967, 215, 844. (30) Rinman, E. L. U.S. Patent 914,187, 1909. (31) Hultman, G. H. Swedish Patent 41,884, 1917. (32) Whittington, J. A. U.S. Patent 1,549,398, 1925. (33) Buchner, M. U.S. Patent 1,493,320, 1924. (34) Zhand, Q.; Ye, Q. Utilization of Hard Kaolin with High Titanium Content. Feijinshukuang. 2001, 24 (4), 20. (35) Yan, S.; Chen, Z.; Li, C. Method for Modification of Kaolin, CN Patent 97-116,832 19970829, 1999.

(36) Kirk, R. E.; Othmer, D. F. Encyclopedia of Chemical Technology, 4th ed.; John Wiley & Sons: New York, 1991. (37) Ullmann, F. Ed. Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; VCH cop: Weinheim, Germany, 2003. (38) Kiyoura, R.; Urano, K.; Mechanism, Kinetics and Equilibrium of Thermal Decomposition of Ammonium Sulfate. Ind. Eng. Chem. Process Des. DeV. 1970, 9, 489. (39) Halstead, W. D. Thermal Decomposition of Ammonium Sulphate. J. Appl. Chem. 1978, 129. (40) 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. (41) 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. (42) Ruiz, M. T. Obtencion de Alumina por Ataque Acido de Materiales no Bauxiticos Espan˜oles Activados por Aportacion de Energia Mecanica. Ph.D. Dissertation, Universidad de Sevilla, Sevilla, Spain, 1988. (43) Colina, F. G.; Esplugas, S.; Costa, J. A New Extraction Procedure for Simultaneous Quantitative Determination of Water-Soluble Metals in Products of Reaction of Clays with Inorganic Salts. Clays Clay Miner. 2002, 50, 403. (44) Klevtsov, D. P.; Logvinenko, V. A.; Zolotovskii, B. P.; Krivoruchko, O. P.; Buyanov, R. A. Kinetics of Kaolinite Dehydration and its Dependence on Mechanochemical Activation. J. Therm. Anal. 1988, 33 (2), 531. (45) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309.

ReceiVed for reView July 26, 2005 ReVised manuscript receiVed November 10, 2005 Accepted November 10, 2005 IE050872F