Ind. Eng. Chem. Res. 2010, 49, 12379–12382
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Investigation of the Dissolution Kinetics of Meta-Kaolin in H2SO4 Solution Mehmet Rıza Altıokka,* Handan Akalın, Nergis Melek, and Sema Akyalc¸ın Anadolu UniVersity, Department of Chemical Engineering, 26555, Eskisehir, Turkey
Kaolin with a given size distribution was calcined at 1023 K for 2 h then digested in 3.0, 4.5, and 6.0 N sulfuric acid at constant temperatures within the range 328-358 K. The samples taken from the solution at predetermined time intervals were analyzed by using a complexometric method and the amount of aluminum passed into the solution was determined. The results showed that the dissolution kinetics follow the shrinking core model for spherical particles of unchanging size, t/τ ) 1 - (1 - xB)1/3, with the apparent activation energy of 98.4 kJ/mol. The leaching reaction was found to be the 0.75th order with respect to acid concentration and controlled by reaction step. 1. Introduction
2. Experimental Section
Kaolinitic clay, being abundant in nature, is considered as an alternative raw material in the production of alumina. The kaolinitic clay, after purification, can be easily rendered acid soluble. At the end of the leaching process, aluminum in solution can be precipitated as Al(OH)3 by adjusting the pH of solution as neutral or slightly alkaline.1 Thus, the high-grade Al2O3 suitable for electrolysis is obtained by the calcination of the resulting Al(OH)3. Studies show that the calcination of kaolin at 1023 K for 2 h prior to leaching is necessary to solubilize aluminum in acid solutions.2 Although the effect of calcination on solubility is not clearly understood, the general opinion is that calcination increases the reactivity of the particles besides removing the physically and chemically combined water and any organic materials in pores.3 It was also reported that kaolinite, Al2Si2O5(OH)4, can be converted to meta-kaolin, Al2Si2O7, during the calcination step and that aluminum can be extracted much more readily from meta-kaolin than from kaolinite.4 Although the leaching of kaolin in acidic solution has been studied by many researchers, there is no general agreement on the dissolution mechanism and reaction kinetics. Ford reported that, while the kinetics of sulfuric acid leaching of fine particles of meta-kaolin was first order with respect to the unleached aluminum, the leaching of pelletized meta-kaolin under the same conditions followed the shrinking core model with chemical reaction control.2 However, the leaching of metakaolin in boiling HCl acid solution was reported to be a zero order with diffusional control.5 Mitra et al. carried out another study, in which they illustrated that the leaching rate of aluminum in both acid and alkali solutions can be given by the equation Qa ) KT, where Q is the amount of aluminum extracted from the clay in time T, a, and K are constants depending on temperature and concentration of solution.6
Before the leaching, the kaolinitic clay must be purified according to the literature method.6 However, purified kaolin obtained from Sigma Chemical Co. was used in this study. Prior to the leaching, the sample was calcined in a muffle furnace at 1023 K for 2 h. The XRD patterns of kaolin before and after calcination were obtained using Rigaku Rint 2000 diffractometer, by CuKR radiation at a scanning rate of 1°/min in the range of 5-70° 2θ. The chemical analysis of the sample was determined by XRF analysis using Rigaku ZSX Primus spectrometer. The result is given in Table 1. Structural quantitative analyses from XRD patterns were also made by the Rietveld method8 using MAUD software (Material Analysis Using Diffraction) and it was found that the sample is (98.81 ( 0.62) % kaolinite and (1.19 ( 0.21) % gibbsite. From these results, the sample can readily be considered to be pure kaolinite. Pure kaolinite, Al2O3 · 2SiO2 · 2H2O, contains 39.5% Al2O3 which increases to 45.9% after calcination. A known amount of sample was digested with 10 N H2SO4 at its boiling point for 5 h under reflux. By the determination of Al2O3 in the solution, it was confirmed that all of the Al2O3 in the sample is soluble. Thus, the conversion of Al2O3 into the reaction products was based on this ratio. In the digestion process, 3.0, 4.5, and 6.0 N H2SO4 solutions, prepared from Merck product, were used. For each experiment, 100 mL solution at known concentration was placed in a flatbottomed Pyrex flask connected with a reflux condenser. Water was circulated between the jacket around the flask and a thermostat at constant temperature. After thermal equilibrium was established, ∼1 g of calcined kaolin sample was loaded into the flask and the reaction mixture was magnetically stirred at about 300 rpm. Thereafter, 5 mL of samples were withdrawn at the predetermined time interval for the complexometric analysis.9 Thus, the amount of aluminum passed into the solution was determined. The particle size distribution analysis of the sample given in Table 2 was determined using a Malvern 2600C Laser Diffraction Particle Size Analyzer.
In the previous study, it was found that the dissolution kinetics, in HCl solution, follow the shrinking core model for spherical particles of unchanging size.7 Therefore, the aim of this study is to investigate the dissolution kinetics of kaolin in sulfuric acid. The effects of the parameters such as particle size, concentration of the acid solution, and the leaching temperature on the dissolution rate are to be determined. * To whom correspondence should be addressed. Tel: +90-222-321 35 50/6502. Fax: +90-222-323 95 01. E-mail: mraltiokka@ anadolu.edu.tr.
3. Results and Discussion The sample was calcined at 1023 K for 2 h to transform kaolinite into meta-kaolin since aluminum extraction from metakaolin is much easier than kaolinite.4 This transformation is seen in XRD patterns given in Figure 1.
10.1021/ie101147n 2010 American Chemical Society Published on Web 11/03/2010
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Table 1. Chemical Composition of Kaolin (% w/w)
a
component
SiO2
Al2O3
CaO
MgO
Na2O
K2O
TiO2
Fe2O3
L.O.Ia
result
44.32
38.71
0.12
0.14
0.14
0.19
1.43
0.99
13.98
Loss on ignition.
Table 2. Particle Size Distribution of the Tested Kaolin Sample range of particle size (µm)
mean particle size (µm)
weight (%)
34.674
0.32 0.73 0.96 1.27 1.68 2.21 2.91 3.84 5.06 6.67 8.79 11.59 15.28 20.14 28.79
1.83 4.68 7.62 11.05 14.23 15.49 14.21 11.31 8.06 5.28 3.22 1.789 0.84 0.31 0.08 0.00
-rB′′ ) -
1 dNB ) bksCAn S dt
(2)
According to this model, the time for complete conversion of a particle, τ, is proportional to the size, R.12 Let τ(2.5) is the time required for the complete conversion of particle of size 2.5 µm, which is the highest percentage in the sample. Then the time for complete conversion of particle of size Ri will be as follows: τ(Ri) )
Ri τ(2.5) 2.5
(3)
where Ri is in µm. The relationship between the conversion of particle of size Ri and time is given by the following:
Kaolin used in this study is a mixture of different size particles which are readily considered to be in spherical shape since, as seen in Table 2, more than 80% of them are under 4 µm in size.7,10 Each particle in the reactor has its own conversion XB(Ri). Although, XB(Ri) cannot be determined individually, the j B can be measured. The mean mean conversion of the solid, X conversion data, which are the arithmetic mean of two parallel measurements, at different acid concentrations and temperatures are given in Table 3. The values in this table indicate that the reaction rate is highly temperature-sensitive. This implies that the reaction is controlled by chemical reaction rather than diffusion.11,12 Therefore, it is reasonable to test the shrinking core model for spherical particles of unchanging size with chemical reaction control. This conclusion is also confirmed by Figure 2, which indicates that the size distribution of the calcined clay before and after leaching 80% of Al2O3 is similar. The reaction between aluminum oxide and sulfuric acid is written as follows: H2SO4 + 1/3 Al2O3 f 1/3 Al2(SO4)3 + H2O
The dissolution rate expression based on the shrinking core model for spherical particles of unchanging size with chemical reaction control is given as follows:
(1)
[
1 - xB(Ri) ) 1 -
t τ(Ri)
]
3
(4)
Inserting eq 3 into eq 4, the equation becomes
[
1 - xB(Ri) ) 1 -
2.5t τRi
]
3
(5)
The conversion for individual particles can be related to the mean conversion by the equation: Rm
1 - xB )
∑
[1 - xB(Ri)]w(Ri)
(6)
R(t)τ)
where R(t ) τ) is the radius of the largest particle completely converted in the reactor and w(Ri) is the weight fraction of particle of size Ri.12 Combining eqs 5 and 6 with the values given in Table 2, gives the following equation:
(
1 - xB ) 1 -
2.5t 3 2.5t 3 0.0183 + 1 0.0468 + 0.32τ 0.73τ 2.5t 3 10.0762 + ... 0.96τ
)
(
)
(
)
(7)
From eq 7 the values of τ can be calculated by a computer program using the mean conversion and time values in Table 3. The average values of τ, for each reaction conditions, are calculated from eq 8 and the results are given in Table 4. τave )
∑tτ ∑t
i i
(8)
i
where τave is the average time for complete conversion of particle size of 2.5 µm. Alternatively, the time, τ for complete conversion, in the tested model, is given by eq 9: Figure 1. X-ray diffractograms of kaolin and meta-kaolin (G: Gibbsite, K:Kaolinite).
τ)
FBR0 bksCAn
(9)
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Table 3. Mean Conversion of Al2O3 at Different Temperatures and Acid Concentrations 3.0 N H2SO4, 328 K t (min) jB X 3.0 N H2SO4, 338 K t (min) jB X 4.5 N H2SO4, 338 K t (min) jB X 6.0 N H2SO4, 338 K t (min) jB X 3.0 N H2SO4, 348 K t (min) jB X 4.5 N H2SO4, 348 K t (min) jB X 6.0 N H2SO4, 348 K t (min) jB X 1.5 N H2SO4, 358 K t (min) jB X
90 0.1438 45 0.186 105 0.476 25 0.189 45 0.363 30 0.333 30 0.398 30 0.420
210 0.2988 90 0.332 150 0.621 50 0.288 75 0.540 60 0.611 45 0.531 50 0.608
330 0.4758 135 0.476 195 0.714 80 0.455 95 0.650 120 0.833 60 0.664 80 0.829
450 0.5976 180 0.576 240 0.776 110 0.610 115 0.727 150 0.889 75 0.730 100 0.895
570 690 0.6529 0.7082 420 0.819
155 175 195 0.793 0.848 0.892
90 0.863
Taking the molar density of the calcined kaolin is 13 300 mol Al2O3/m3.13 R0 is 2.5 × 10-6 m and b is 1/3 from eq 1; then, eq 9 can be arranged as follows: ln τ ) ln(0.100/ks) - nln CA
Figure 3. ln CA vs ln τ at 338 and 348 K. Table 5. Calculated ks, Independent of Acid Concentration, at Different Temperatures
(10)
where CA is in mol/m3 and τ is in s. Figure 3 was derived by using the values in Table 4 with eq 10 and shows a good straight line relationship between ln CA vs lnτ. From the slope of these lines, n, the order of the reaction was found to be 0.7452 ( 6.31 × 10-3with 95% confidence interval which can be rounded to 0.75. From the intercept of the lines, ks values, at 338 and 348 K were determined to be 8.5 × 10-9 mol0.25 m0.25s-1 and 2.33 × 10-8 mol0.25 m0.25s-1, respectively. By using the data given in Table 4, the ks values at 328 and 358 K were also calculated from eq 10 as shown in Table 5. Applying the Arrhenius equation to the values given in Table 5, the temperature dependency of the dissolution rate constant was found to be:
T, K 9
ks*10 , mol
0.25
0.25 -1
m
s
328
338
348
358
3.4
8.5
23.3
70.4
(
ks ) 1.51*107exp -
98440 mol0.25m0.25s-1 RT
)
(11)
with the correlation coefficient, R2 ) 0.9951, where T is absolute temperature in Kelvin. The activation energy of the reaction as seen in eq 11 is 98.4 kJ/mol. 4. Conclusions The leaching kinetics of pure kaolin, calcined at 1023 K for 2 h, in sulfuric acid solutions follow the shrinking core model for spherical particles of unchanging size with chemical reaction control. It was determined that the order of the reaction is 0.75 with respect to acid concentration. It was also shown that the temperature dependency of the rate constant, within the temperature range of 328-358 K, can be given as follows:
(
ks ) 1.51 × 107exp -
98440 mol0.25m0.25s-1 RT
)
where T is absolute temperature in Kelvin. Nomenclature
Figure 2. Particle size distributions before and after leaching 80% of Al2O3 is dissolved. Table 4. Calculated τave with 95% Confidence Interval at Different Reaction Conditions acid concentration T (K)
(N)
(mol/m3*10-3)
τave*10-4 (s)
328 338
3.0 3.0 4.5 6.0 3.0 4.5 6.0 1.5
1.5 1.5 2.25 3.0 1.5 2.25 3.0 0.75
12.15 ( 0.97 4.99 ( 0.20 3.46 ( 0.16 2.99 ( 0.32 1.93 ( 0.17 1.38 ( 0.19 1.16 ( 0.16 0.99 ( 0.2
348
358
b ) stoichiometric coefficient of solid reactant when that of the fluid reactant is unity CA ) bulk concentration of the H2SO4 (mol/m3) ks ) the dissolution rate constant NB ) moles of solid reactant S ) unchanging exterior surface of a particle w(Ri) ) the weight fraction of particle of size Ri xB ) conversion of Al2O3 in the sample FB ) molar density of the particle (mol/m3) τ ) time for complete conversion Subscripts ave ) average A ) sulfuric acid B ) Al2O3 Literature Cited (1) Pauling, L. General Chemistry; Freeman: San Francisco, 1959. (2) Ford, K. J. R. Leaching of fine and pelletised Natal kaolin using sulfuric acid. Hydrometallurgy 1992, 29, 109.
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(3) Gajam, S. Y.; Raghavan, S. A kinetic model for the hydrochloric acid leaching of kaolinite clay in the presence of fluoride ions. Hydrometallurgy 1985, 15, 143. (4) Phillips, C. V.; Wills, K. J. A laboratory study of the extraction of alumina of smelter grade from China clay micaceous residue by a nitric acid route. Hydrometallurgy 1982, 9, 15. (5) Olsen, R. S.; Bullard, S. J.; Gruzensky, W. G.; Mazrek, R. V.; Henry, J. L. Leaching rates for the HCl extraction of aluminium from the calcined kaolinitic clay. U.S. Bur. Mines RI, 8744, 1983. (6) Mitra, N. K.; Dastidar, R. G.; Mandal, R. K.; Basumajumdar, A. Kinetics of the leaching process of clay minerals in acid and alkali medium. J. Indian Chem. Soc. LXIII (August) 1986, 747–751. (7) Altiokka, M. R.; Hos¸gu¨n, H. L. Investigation of the dissolution kinetics of kaolin in HCl solution. Hydrometallurgy 2003, 68, 77. (8) Young, R. A. The RietVeld Method; Oxford University Press: New York, 1993.
(9) Merck, E. Complexometric Assay Methods with Titriplex;: Darmstadt: West Germany, 1979. (10) Tang, A.; Su, L.; Li, C.; Wei, W. Effect of mechanical activation on acid-leaching of kaolin residue. Appl. Clay Sci. 2010, 48, 296. (11) Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall PTR: NJ, 1999. (12) Levenspiel, O. Chemical Reaction Engineering; Wiley Press: New York, 1972. (13) Weast, R. CRC Handbook; CRC Press: OH, 1975.
ReceiVed for reView May 24, 2010 ReVised manuscript receiVed September 26, 2010 Accepted October 17, 2010 IE101147N
