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Dehydration Kinetics of S¸ ebinkarahisar (Gedehor) Alunite Ore in a Fluidized-Bed Reactor A. Ku 1 c¸ u 1 k,*,† M. S¸ . Gu 1 labogˇ lu,‡ and S. Bayrakc¸ eken§ Department of Metallurgical Engineering, Atatu¨ rk University, 25240 Erzurum, Turkey, Department of Chemical Engineering, Atatu¨ rk University, 25240 Erzurum, Turkey, and Department of Chemical Education, Atatu¨ rk University, 25240 Erzurum, Turkey
The dehydration kinetics of S¸ ebinkarahisar (Gedehor) alunite ore in a fluidized bed was investigated. It was observed that the dehydration reaction rate increased with increasing temperature and decreasing water vapor pressure. However, the particle size and air flow rate did not have a significant influence on the dehydration rate. On the basis of these conversiontime relations, the dehydration process was assumed to be controlled by chemical reaction. By nonlinear regression analysis, the Avrami model was found to fit the kinetic data best. n ∼ 1, the high activation energy value (209 kJ mol-1), and scanning electron microscopic photographs of the calcined ore supported this control mechanism. Table 1. Chemical Analysis of S¸ ebinkarahisar (Gedehor) Alunite Ore
1. Introduction Alunite is a naturally occurring, hydrated double sulfate of aluminum and potassium of the approximate composition represented by KAl3(SO4)2(OH)6. Impurities such as salts of iron and magnesium are usually present in small and varying amounts. Alunite is insoluble in acid solutions even when the ore is finely ground. The solubility of the ore in acid at the industrial scale can be made possible when the alunite ore is dehydrated at temperatures of up to 600 °C.1-4 The objective of the dehydration is to eliminate the bulk of the combined water without the loss of appreciable sulfur trioxide and consequently to obtain alum as a commercial raw material for the chemical industry. Dehydration of alunite begins at about 450 °C and proceeds with an appreciable velocity at 550 °C. Sulfur trioxide is evolved above 600 °C and, at an appreciable rate, as sulfur dioxide at about 650 °C.5-14 According to Morgan,5 removal of combined water from alunite is not completed at the decomposition temperature of Al2(SO4)3. Hereby, SO2 evolution causes air pollution and an increase in the acid consumption needed for dissolving the dehydrated material. To prevent this undesirable case, the definite temperature limitations creating the need for furnace-temperature control must be given. The calcination temperature can be easily held in a stable value in a fluidized bed. The literature survey shows that some scientists15-17 have focused their attention on dehydration of alunite ore by using a fluidized bed. However, these works have been performed in reductive environments such as natural gas and water vapor. Prior to this research,13 no reference studying the kinetics and mechanisms controlling the decomposition reactions of alunite could be found in the literature. It was considered that an attempt should be made to determine the kinetic parameters and the rate-controlling steps for the de* To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Metallurgical Engineering. ‡ Department of Chemical Engineering. § Department of Chemical Education.
component
wt %
component
wt %
SiO2 Al2O3 SO3
31.39 25.34 26.55
Na2O K2O H2O
0.60 6.89 9.23
composition reactions of alunite ore because the kinetic data are very important for industrial applications. In the present study, evaluation of the dehydration kinetics rather than the desulfurization kinetics of S¸ ebinkarahisar (Gedehor) alunite (the richest alunite deposit with 20 million tons in Turkey) in air by using a fluidized bed was first considered. As a result, the present work will be useful for understanding of the dehydration process of the alunite ore. The independent system parameters investigated include the gas flow rate, temperature, particle size, and water vapor pressure. 2. Experimental Section 2.1. Material. S¸ ebinkarahisar (Gedehor) alunite ore used in the present study was provided from the mineral deposits located in Giresun, Turkey. The alunite ore was crushed, ground, and then sieved to -1.19 + 0.81, -0.81 + 0.59, and -0.59 + 0.42 mm size fractions using ASTM standard sieves. Chemical analysis of the alunite ore was carried out by standard gravimetric, volumetric, and spectrometric methods. The analytical results of this analysis are given in Table 1. Differential thermal analysis (DTA) was performed using a Linseis L 1040 DTA apparatus. A Shimadzu model 50 TGA apparatus was employed for thermogravimetric analysis (TG). The DTA curve in Figure 1 shows two large endothermic peaks and one small exothermic peak. The first endothermic peak at 548 °C is strong and sharp. The exothermic peak at 710 °C appears immediately before the second endothermic peak at 810 °C. The TG curve in Figure 2 shows two steps of weight losses corresponding to the two endothermic peaks. The weight losses are about 9.23% for the first endotherm and 29.75% as the total weight loss. The weight loss in the second step, 20.52%, refers to a
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Figure 1. DTA curve for S¸ ebinkarahisar (Gedehor) alunite ore.
Figure 2. TG curve for S¸ ebinkarahisar (Gedehor) alunite ore.
loss of 3/4 of the sulfate present. The sulfate from K2SO4 does not decompose up to 900 °C. An X-ray powder diffractometer (Rigaku 2200D/Max) was used to identify the crystalline compounds in the
ore. The X-ray pattern in Figure 3 shows that the principal constituents in the ore are alunite and quartz. The cross section of the alunite particles was analyzed using a scanning electron microscopy (JEOL 6400) with an energy-dispersive spectrometry (SEM-EDS). Before analysis the particles were embedded in epoxy, ground, and polished to expose the cross section. From the image in Figure 4, the raw alunite particle has a highly compact structure but the mineral quartz does not show how it disperses in this compact structure. However, the results of the EDS analyses in the two selected areas with 10 × 10 µm2 (A and B) show that the quartz quantity in the A area in Figure 5 is much higher than that in the B area in Figure 6. It can be said that the quartz is dispersed heterogeneously in the ore. 2.2. Apparatus. A quartz fluidized-bed reactor with an inner diameter of 30 mm placed in an electrically heated vertical tube furnace was used for the dehydration tests of the S¸ ebinkarahisar (Gedehor) alunite ore. The schematic experimental setup is shown in Figure 7. A temperature controller programs the temperature of the furnace. A thermocouple, K type, is used to measure the bed temperature. The compressed air as the fluidizing gas is dried by passing through it a molecular sieve. In the tests performed with humidified air, the system saturated with water vapor was added to the experimental setup to form an air-water vapor atmosphere. This system was located between the exit of air from the flowmeter and the entrance into the furnace and consists of a water column with a thermostat. 2.3. Procedure. The experiments were carried out as follows. The inert material, approximately 15 g of quartz sand with a median diameter of 0.30 mm, was first charged into the reactor. The temperature of the bed was raised to the desired level, and after it was maintained at that level for 10 min, 1 g of the alunite sample was charged on top of the sand bed. The velocity of the fluidizing gas was maintained at a higher value than the minimum value to ensure the fluidizing
Figure 3. X-ray powder diffraction pattern of the following: Q, quartz (JCPDS Card No. 461045); A, alunite (JCPDS Card No. 140136).
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rates at the studied temperatures were computed by using these experimental Umf values from the following equation.18
Umf(T) ) Umf(25 °C)
Figure 4. Backscattered electron image of a cross-sectioned raw S¸ ebinkarahisar (Gedehor) alunite sample.
conditions in the calcination of the ore. The sample in the reactor, which was pulled with a vacuum at the end of the reaction, was collected in the other vessel. The sand particles were separated from the alunite particles by sieving. The partially reacted alunite sample was weighed. The full calcination of this sample was performed in a muffle furnace for 12 h at 510 °C, and then the weight of the calcined sample was noted again. The conversion ratio of the calcined sample in the fluidized bed was calculated on the basis of the weight losses. The tests were repeated three times, and the total standard deviation was calculated as 1.1%. 3. Results and Discussion 3.1. Effect of the Air Flow Rate. For all of the present size fractions, the minimum fluidizing air flow rates (Umf) were measured experimentally at room temperature. Later, the minimum fluidizing air flow
Figure 5. EDS analysis result of the area A with 10 × 10 µm2.
µ(25 °C) µ(T)
(1)
The Uf/Umf ratio as the air flow rate was used so that fluidizing air flow rates (Uf) can be independent and dimensionless from external impacts.18 The dependence on the air flow rate of the dehydration reaction extent of the alunite ore was studied using four different Uf/ Umf ratios. Typical rate curves are shown in Figure 8. The reaction rate seems not to be influenced by the air flow rate because these rate curves are placed close together. For the other kinetic experiments, the optimum air flow rate chosen was Uf/Umf ) 3. 3.2. Effect of the Temperature. A series of experiments were carried out at temperatures of 520, 540, 570, and 600 °C. The results are shown in Figure 9. As seen from this figure, it is clear that temperature has a strong influence on the reaction rate especially at the early stages of the reaction. 3.3. Effect of the Particle Size. The dependence of the dehydration reaction extent of the alunite ore studied using three different particle sizes is shown in Figure 10. From this figure, the decrease in the particle sizes does not have a significant effect on the dehydration rate of the alunite ore. 3.4. Effect of the Water Vapor Content. A series of experiments were carried out at five different water vapor contents of the reaction system, namely, dried air and 4%, 8%, 15%, and 30% water vapor contained air. The results show that the reaction rate decreases with an increase in the water vapor content, which is one of the products of the reacting system (Figure 11). In other words, it has a significant influence on the reaction kinetics.
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Figure 6. EDS analysis result of the area B with 10 × 10 µm2.
Figure 8. Effect of the air flow rate on the dehydration process. Temperature: 540 °C. Particle size: -1.19 + 0.84 mm.
Figure 7. Scheme of the experimental setup.
3.5. Kinetic Analysis. As noted in previous studies,6-11 the following reaction accompanies the dehydration of alunite:
KAl3(SO4)2(OH)6 / KAl(SO4)2 + Al2O3 + 3H2O
(2)
In the present study, an attempt was made to determine the kinetic parameters and rate-controlling step for S¸ ebinkarahisar (Gedehor) alunite ore in a fluidized bed. In the result of experimental data, the dehydration rate was found to increase with increasing temperature and decreasing water vapor pressure. In contrast, the particle size and air flow rate did not have a significant influence on the dehydration rate. Therefore, the mass-transfer effects were negligible at the air flow rates studied. It is known that, in the fluid film diffusion-controlled solid-fluid reactions, the
Figure 9. Effect of the temperature on the dehydration process. Uf/Umf ) 3. Particle size: -0.84 + 0.59 mm.
reaction rate increases as the gas flow rate increases.19-21 It is also known that the reaction rate on the solid surface increases with increasing temperature.22 However, the surface reactions are markedly sensitive to the
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Figure 10. Effect of the particle size on the dehydration process. Uf/Umf ) 3. Temperature: 540 °C.
Figure 12. SEM photograph of the ore calcined at 540 °C.
Figure 11. Effect of the water vapor content on the dehydration process. Uf/Umf ) 3. Temperature: 520 °C. Particle size: -0.59 + 0.42 mm.
particle size or surface area.23-25 The increase in the water vapor pressure as a reaction product, of course, causes a decrease in the reaction rate and, therefore, this reversible reaction proceeds toward reactants.22 It is apparent that the diffusion of water vapor out of fluidized particles under these conditions is rapid and the process is predominantly chemical reaction controlled.26,27 In addition to the result presented above, the SEM photograph of the ore calcined at 540 °C is presented in Figure 12. From this image, any shell formation in the particle is not observed and the reaction proceeds ubiquitously in the particle homogeneously. This photograph has also supported the preliminary findings. Considering these findings, it was concluded that the homogeneous reaction models could be employed to explain the dehydration process. The rate expression for the first-order homogeneous reaction22 is expressed as follows:
-ln(1 - x) ) kt
(3)
where x is the reacted fraction of the solid, t time, and k the apparent rate constant. Using the above rate expression, the linear regression analysis was performed for the kinetic data in Figure 9. The values of the regression coefficients illustrated in Table 2 decreased as the temperature was increased. Increasing the temperature causes the duration needed to reach the maximum conversion to decrease. After about 4 min, all of the fractional conversions reach approximately the
same value, which is the maximum fractional conversion, at higher temperatures than 520 °C. Thus, the linear regression analysis was again performed irrespective of the conversion values after reaching the maximum values. The new values of the regression coefficients were very high, but the results were not reliable enough to use only a few kinetic data. Kinetic modeling of noncatalytic solid-fluid reactions is accomplished usually by means of well-established theoretical models. For this reason, the three models generally used for the decomposition reactionssthe sharp interface model, the crackling core model, and the Avrami model, which is the one of nucleation modelss were considered for eq 2.21,28 The most suitable one for all data was the the following Avrami expression because the others were not good correlations with the experimental data for the overall conversion range.
x ) 1 - e-k′t
n
(4)
where x is fractional conversion, t is time, and k′ and n are model parameters in eq 4. All of the experimental data from 1 to 95% conversion fit the model well; hence, the range taken here is assumed to be an acceptable test of the model validity. The experimental and modelestimated conversion values are depicted in Figure 13. The Avrami equation was first derived theoretically by Avrami21,28 for the kinetic modeling of a new phase nucleation during solid-state phase changes and later applied to other heterogeneous reaction systems. n in eq 4 may indicate the nature of the controlling step. Kabai21 reported that, according to the value of n, for n < 1 the initial rate is infinite and the rate continually decreases with increasing time, for n ) 1 the initial rate is finite, and for n > 1 the reaction exhibits an initial rate approaching zero. Bamford and Tipper29 pointed out that n > 1 corresponds to a solid transformation, which takes place by nucleation, and n < 0.7 indicates that the reaction rate is influenced by diffuse mechanisms. In the kinetic analysis of the present experimental data with eq 4, the estimated values of the parameter k′ and n were obtained by nonlinear regression analysis with the Levenberg-Marquardt method. These calculated k′ values are illustrated in Table 3. The value of n was found to be equal to 0.96 with a mean deviation of (0.05. It is interesting to note that n is approximately
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Figure 13. Comparison between experimental data and model prediction. Uf/Umf ) 3. Particle size: -0.84 + 0.59 mm. Table 2. Regression Coefficients from Linear Analysis of the Kinetic Data in Figure 9 regression coefficient regression coefficient temp temp (°C) raw data modified data (°C) raw data modified data 520 540
0.9924 0.8526
0.9924 0.9943
570 600
0.6019 0.4464
0.9884 0.9988
Table 3. k′ Values at Various Temperatures, Particle Sizes, Air Flow Rates (Uf/Umf), and Water Vapor Contents k′
T (K)
θ, Uf/Umf
r (cm)
y
0.123 0.268 0.807 2.250 0.193 0.231 0.256 0.257 0.268 0.279 0.128 0.117 0.107 0.090 0.060
793 813 843 873 813 813 813 813 813 813 793 793 793 793 793
3 3 3 3 1 2 3 3 3 3 3 3 3 3 3
0.070 0.070 0.070 0.070 0.100 0.100 0.100 0.100 0.070 0.050 0.050 0.050 0.050 0.050 0.050
0 0 0 0 0 0 0 0 0 0 0 0.04 0.08 0.15 0.30
equal to 1 and the two rate expressions, eqs 3 and 4, are equal at n ) 1. This result supports chemical reaction control. k′ depends on some factors such as temperature, T, particle size, r, air flow rate, θ, and water vapor content. In this case, it can be represented as follows:
k′ ) ae-Ea/RTrbθc(1 - y)d
Figure 14. Arrhenius plot.
as 209 kJ mol-1. The high value of the activation energy for the process implies that the reaction rate is influenced by chemical reaction control.22,30-32 The data given in Table 3 are used in nonlinear regression analysis, and unknown parameters are estimated. The final for the Avrami constant k′ is shown as follows:
k′ ) 41200 × 108e-25200/Tr-0.12θ0.258(1 - y)2.143
Finally, the rate expression may be summarized as follows:
x ) 1 - e-41200×10 (5)
The activation energy of the reaction is estimated from the Arrhenius plot [ln(k′) versus 1/T] in Figure 14
(6)
8e-25200/Tr-0.12θ0.258(1-y)2.143t0.96
(7)
4. Conclusions In the present study, the dehydration kinetics of S¸ ebinkarahisar (Gedehor) alunite ore in a fluidized bed
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Figure 15. Theoretical x versus experimental x to test eq 7.
was investigated. It is concluded that the dehydration rate increases with increasing temperature and decreasing water vapor pressure. The particle size and air flow rate do not show a significant influence on the dehydration rate. In addition, from the SEM photograph of the calcined ore, it is observed that any shell formation in the particle is not present and the reaction proceeds in internal homogeneous dehydration of the particle. Different kinetic models were analyzed, and the best kinetic data fitting was obtained using the Avrami model. With the high value of the activation energy, 209 kJ mol-1, and the n value obtained around 1, it can be said that the dehydration reaction of S¸ ebinkarahisar (Gedehor) is chemically controlled. These results also support the preliminary findings. The validity of the model is shown in Figure 15 by plotting the theoretical conversion values with the standard deviation of 2.4% from eq 7 against experimental values. The fit is sufficiently good to confirm eq 7. Nomenclature k′ ) model parameter in eq 4 t ) time, min T ) temperature, K x ) fractional conversion n ) model parameter in eq 4 θ ) air flow rate (Uf/Umf) r ) particle size, cm y ) mole fraction of water vapor in air Ea ) activation energy, kJ mol-1 µ(25 °C) ) viscosity of air at 25 °C µ(T) ) viscosity of air at the dehydration temperature
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Received for review February 3, 2003 Revised manuscript received November 21, 2003 Accepted December 2, 2003 IE0300979
