Dehydroxylation Kinetics of Alunite - American Chemical Society

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Dehydroxylation kinetics of alunite mirzaagha mohammadi, and Mohammad Mahdi Salarirad Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400127q • Publication Date (Web): 11 May 2013 Downloaded from http://pubs.acs.org on May 20, 2013

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Industrial & Engineering Chemistry Research

Dehydroxylation kinetics of alunite Mirzaagha Mohammadia, Mohammad Mahdi Salariradb a

Department of Mining and Metallurgical Eng., Amirkabir University of Technology, 424 Hafez Ave., Tehran, Iran, [email protected].

b

Department of Mining and Metallurgical Eng., Amirkabir University of Technology, 424 Hafez Ave., Tehran, Iran, [email protected]

Abstract In this study dehydroxylation of alunite was studied. On the basis of thermogravimetric result and SO2 loss, dehydroxylation of alunite occurs from 508 to 577C. In this temperature range, temperature and time as well as their interaction have the greatest effect on alunite conversion (dehydroxylation) with 99.85% contribution, while the contribution of particle size was only 0.11 %. Dehydroxylation increased with time up to a maximum and reached a plateau beyond that time. The time required (tmax) for maximum dehydroxylation (i.e. maximum conversion) for various temperatures was different and showed decreasing trend with increasing temperature. Kinetic studies below tmax showed that dehydroxylation of alunite follows Avrami-Erofeev models with different reaction indexes. In the temperature range of 508-534C, dehydroxylation occurs according to the following model:

 e

(( 1.771011 e

23314 T

1

)(t 0.9r 3 ))1.5

While in the 534-575 C temperature range the following model is applicable:

 e

9924 (( 11065 .64 e T

1 )(t 0.213r 3 ))4

Key words: Alunite, Dehydroxylation, Experimental Design, Kinetics. 1. Introduction The primary source of aluminum production is bauxite that is processed with the BayerHall process. Meanwhile, the quantity and quality of reserves appropriate for Bayer process are being depleted. Assuming an annual production growth rate of 5% per capita, the currently known world reserves will be exhausted within the next few decades.1 Furthermore, many 1

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countries in the world have no reserves of bauxite suitable for the Bayer process. Therefore, in recent years attention to other resources of aluminum such as clays, aluminosilicates and alunite has increased.2-5 In countries where potash reserves are scarce, alunite reserves are considered as an important potential resource for both alumina and potash.2,6 Alunite is a group of minerals, which forms part of the alunite Super Group. The general formula is given by DG3(TO4)2(OH,H2O)6 where the D sites are occupied by either monovalent cations such as K+, Na+, NH4+, H3O+ and others, or divalent cations such as Ca2+, Ba2+, Sr2+, Pb2+, or trivalent cations e.g. Bi3+. G is occupied by either Al3+ or Fe3+ and S6+, As5+ or P5+ could be at the T site.7-8 Potassium alunite (KAl3(SO4)2(OH)6), natroalunite (NaAl3(SO4)2(OH)6) and solid solutions between them are the more abundant minerals of alunite deposits. The K2O/Na2O ratio is different for various deposits.6 A minor replacement of K and Na by H3O+is reported in natural alunite but its magnitude is considerably higher in synthetic alunite. The extent of this replacement depends on temperature and solution composition.9-14 Potassium alunite and natroalunite species contain Al, K and Na which can be recovered as K2SO4, Na2SO4, Al2O3, Al2(SO4)3 or potassium alum KAl(SO4)2.6 Hydrometallurgical processing (acidic6,15-20 or basic20-26 process) is the only option for extraction of values from alunite. However, as alunite is almost insoluble in acid, calcination is essential prior to acid leaching.6,15-20 In the basic leaching, calcination improves the kinetics of process.21-26 Many researchers have investigated the calcination of alunite by thermogravimetric methods (TG, DTG, DTA and CRTA).27-33 According to these studies, dehydration, dehydroxylation and desulfurization are the three steps during thermal decomposition of alunite that occur at 50240C, 450-550C and 600-830C respectively. Dehydroxylation and desulfurization can occur in multiple steps. 27-29 Phase transformations and other phenomena have been well summarized by Küçük & Gülaboğlu.32 The thermal decomposition reactions of alunite are given in equations 1-5. KAl3 (SO4 )2 (OH)6 x KAl3 (SO4 )2 (OH)6  x

Dehydration

(50-240°C)

KAl 3(SO4 ) 2 (OH) 6  KAl(SO 4) 2  κ, γAl 2O 3 3H 2O

Dehydroxylation

(450-550°C) (2)

KAl(SO4 )2  1/ 3K3Al(SO4 )3  1/ 3Al2 (SO4 )3

Recrystallization

1/3K 3Al(SO4 )3 1/ 2K2 (SO4 )  1/ 6Al2 (SO4 )3

Recrystallization 2


(700-750°C) (700-750°C)

(1)

(3) (4)

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1/2Al 2(SO4 )3 1/ 2 ,Al2O3  3 / 2SO3

Desulfurization

(600-830°C)

(5)

The decomposition temperatures of alunite with different origins vary because of variation in alunite composition, conditions of formation and the type of associated gangue minerals and should be determined experimentally. In most methods proposed for leaching of alunite, calcination of alunite up to the dehydroxylation step has been suggested for the following reasons: 

Alunite dissolution Increases at low temperatures and consequently energy consumption is reduced.



Acid consumption in producing alum is reduced because of diminishing loss of sulfur by reaction (5)34



Environmental risk is reduced because of diminishing emission of SO3



Aluminum recovery is improved because of non-transformation of κ-Al2O3 and γ-Al2O3 to α-Al2O313,20

Hence dehydroxylation is usually one of the principal stages for the beneficiation of alunite. The kinetics of the dehydroxylation is quite important with respect to identification of types and extent of factors affecting and controlling the process as well as unit operation design. Küçük et al.34 studied the kinetics of dehydration of alunite by isothermal method in a fluidized-bed furnace, where both dehydration and dehydroxylation were studied simultaneously. Based on their studies, dehydration of alunite is controlled by chemical reaction and follows a first order homogeneous reaction. Later study by Küçük and Yildiz35 by using non-isothermal methods showed that the activation energy varies in relation to temperature and degree of conversion. This shows that the mechanism of dehydroxylation changes with respect to these two factors. The aim of the present study is to investigate the effect of particle size, temperature and time on the dehydroxylation of alunite. In addition, the kinetics of dehydroxylation, independent of two steps (i.e., dehydration and desulfurization), was studied and modeled 2. Materials and Methods 2.1. Sample and sample preparation The alunite sample used in this study was from Taikand deposit in the Takestan district of the Qazvin province of Iran. The sample was crushed to -6.3 mm size and further grinding to -2.36 3



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mm was carried out by a pin mill. The product of the pin mill was screened into 11 size fractions. For purpose of this study only four size fractions viz. 2360-1700 µm, 1180-850 µm, 600-425 µm and 300-212 µm were used. The samples were dried at 200C for 24 hours for dehydration and after cooling in a desiccator, stored in a container with silica gel. 2.2. Sample characterization An X-ray powder diffractometer (Bruker AXS GmbH-D4 ENDEAVOR) was used to identify the crystalline phases in the ore. Simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) was performed using NETZSCH STA 409 PC/PG TG-DTA apparatus. The alunite sample was analyzed by EDS method by using WEGA/TESCAN instrument (15 keV with count times less than 60 Seconds). The analysis of SiO2 and Al2O3 the head sample and the prepared size fractions were carried out by UV-Spectrophotometery and atomic absorption Spectrophotometery (AAS) methods respectively. Fe2O3 and CaO were analyzed by complexometry with EDTA and K2O and N2O were analyzed by flame photometry. Sulfur was determined with the LECO SC432DR instrument and rechecked by BaSO4 gravimetric method. 2.3. Dehydroxylation Procedure A muffle Furnace (AZ100) manufactured by Azar Kureh Co., was used for the calcination of alunite samples. The thermocouple used in the furnace is of the K-type. A Shinko temperature controller (Model PCD-300) with PID Auto tune program was employed. The base of the furnace chamber was elevated with a false bottom made of steel in order to improve the heat transfer and to align it with the sensor to reduce the difference between the sample temperature and the temperature indicated by the sensor (Fig. 1). Temperature deviation from the set point was reduced to 1°C under these conditions.

4


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Fig. 1. Arrangement of crucibles inside the muffle furnace (dimensions are in mm)

Each calcination run included four small alumina crucibles containing four different size fractions (Table 1). The crucibles were placed in the furnace by using Inconel palettes which are resistant to oxidation at high temperatures. The arrangement of crucibles is shown in Fig. 1. For performing dehydroxylation tests, the furnace was maintained at the selected temperature for one hour to stabilize the interior temperature of the furnace. Next, the palette along with the preweighed samples was placed inside the furnace and the door was quickly closed to prevent heat loss. After placing the samples in the furnace it took 300 ± 15 seconds to reach the furnace temperature to the set point value in the temperature range of 510 -575°C. After calcination, the palette and the samples were transferred to a desiccator, cooled for two hours and weighed to obtain the weight loss. The weight of samples in the crucibles was about 5 grams with a bed thickness of less than 6 mm. Preliminary tests had shown that the bed thickness had no effect on the extent of conversion within the examined experimental range. 3. Results and discussion 3.1 Sample characterization

XRD analysis showed that the main constituents of the sample are alunite and quartz (Fig. 2). Chemical analysis of head sample gives average 5.6% K2O, 0.65 % Na2O, 15.1 %, SO3, 22.4 % Al2O3 and 6.7% L.O.I which is equivalent to approximately 49.5% alunite with molecular formula of K0.84Na0.13(H3O)0.03Al3.11(SO4)2(OH)6. 5



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Fig.2. XRD of Taikand head sample

For determination of alunite composition ten selected points were analysised SEM-EDS method. Fig. 3 shows a typical result. The chemical composition of alunite calculated for the average of these data was found to be K0.85Na0.13(H3O)0.02Al2.95(SO4)2(OH)6. This result agrees with the formula obtained from the chemical analysis. Chemical analyses of the size fractions are shown in Table 1.

Fig.3. SEM-EDS results

3.2. Dehydroxylation reaction interval

It is very important to delineate the dehydroxylation reaction interval to avoid the overlap between dehydroxylation and desulfurization reactions, which could lead to the erroneous evaluation of results. Two methods were used for this purpose viz. DTA curves and direct 6


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determination of sulfur loss in calcined samples. Table 1. Chemical composition of size fractions of alunite sample

Al2O3

SO3

K2O

Na2O

-2360+1700

Particle SiO2 a size(µm) (%) 2003 49.00

22.00

15.50

5.80

0.68

L.O.I (at 510 C) 6.58±0.1

-1180+850

1001

51.30

21.30

14.80

5.50

0.60

6.27±0.1

-600+425

505

49.60

22.40

15.40

5.40

0.65

6.42±0.1

-300+212

252

47.00

22.90

15.98

5.97

0.69

6.99±0.1

Size fractions (µm)

a

Geometric means of two successive screen aperture

The DTA curve in Fig. 4 shows two large endothermic peaks and one small exothermic peak. The two endothermic peaks appear at 548.6°C and 809.1°C that refer to dehydroxylation and desulphurization of alunite respectively. The exothermic peak at 735.2°C before the second endothermic peak is related to phase transformation. Furthermore, the weight losses for the dehydroxylation and desulfurization steps are 6.59% and 12.54% respectively. The dehydroxylation reaction interval was determined from the DTA curve. As shown in Fig. 5, dehydroxylation reaction interval was 508.6 to 577.3°C. 0.4 100 735.2 ◦C 6.59 %

0.2

0.0 90

TG

-0.2

DTA 12.54 % 85

809.1 ◦C

-0.4

548.6 ◦C

-0.6

80 0

200

400

600

800

1000

Temprature(◦C)

Fig. 4. DTA and TGA curves for the Taikand alunite sample

7


DTA (mW/mg)

95

TG(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fig.5. Dehydroxylation temperature interval from the DTA curve (Fig. 3)

Four samples were calcined for two hours at different temperatures in the furnace and sulfur loss was determined in the calcined samples with the LECO SC432DR instrument. According to Table 2, sulfur losses increase beyond 575°C which indicates the onset of desulfurization step. This agrees well with the DTA results as shown in Fig.4. Table 2. Sulfur losses in the +150 – 212 micron fraction of Taikand alunite sample

Temperature °C 510 530°C 550°C 575°C 600°C 650°C Head

Weight Losses % 6.65 6.62 6.77 6.71 7.23 9.25 0

Sulfur content % 7.89 7.95 7.98 7.90 7.16 6.53 7.47

Sulfur loss % 0.48 0.58 0.40 1.30 11.08 20.59 0

3.3. Maximum weight loss in the dehydroxylation stage In order to determine the conversion ratio at various times, it is necessary to determine the final weight loss of complete dehydroxylation for different size fractions, so that the following relation can be used to determine the conversion ratio34:



Wt Wf

For determination of maximum weight loss for full dehydroxylation, four samples of different size fractions were calcined for 24 hours at 510°C and the results were analyzed using the 8


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“Design Expert 8” software.36 The results of ANOVA as given in Fig. 6 showed that differences in weight loss of the different size fractions were significant. 3.4. Effect of factors on dehydroxylation In this study effects of particle size, temperature and time have been investigated. Table 3 shows the factors and their levels. As per this design and including 2 replications of each experiment, totally 328 experiments were performed.

Fig.6. Effect of particle size on weight loss of Taikand alunite at 510 °C for 24 hours Table 3. Factors and levels of dehydroxylation experiments

Factors Particle Size (μm) Temperature (°C)a

Levels 2003, 1001, 505, 252 510, 530,550, 575 5,7.5b,10, 12.5b, 13.5c, 15, Time ( minutes) 17.5c 20, 35,50, 65,95,125 a These temperatures are near the DTA and DDTA peaks b Only 550 °C for kinetics study, c Only 575 °C for kinetics study The analysis of the experimental data was carried out by the Design-Expert 8(DX8) software which included the following stages of data analysis: data transformation, determination of effects, setting up the ANOVA table and the significance test, validation of ANOVA result with diagnostic tests and plotting of effects graphs.36 The final ANOVA table is shown as Table 4 which shows that time, temperature and the interaction of these two factors have the maximum influence on the dehydroxylation of Taikand alunite (99.85% of total effects) and their contributions are 79.72%, 9.74%, 10.39% of the total 9



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effect respectively. The total effect of particle size and its interaction with other factors is extremely small (0.11%). Table 4. ANOVA table

Source Model A-Particle Size B-Time C-Temperature AB AC BC ABC Pure Error Cor Total

Sum of Mean F Df Squares Square Value 20850.05 143 145.8 2525.62 0.84 3 0.28 4.86 16628.63 8 2078.58 36005.04 2030.82 3 676.94 11725.93 10.21 24 0.43 7.37 0.54 9 0.059 1.03 2166.71 24 90.28 1563.82 12.3 72 0.17 2.96 8.31 144 0.058 20858.36 287

p-value Contribution Prob > F (%) < 0.0001 0.003 0.004 < 0.0001 79.721 < 0.0001 9.736 < 0.0001 0.049 0.4187 0.003 < 0.0001 10.388 < 0.0001 0.059 000 0.04

Fig.7 shows the effect of time, temperature and interaction of these two factors for the -2360+1700 µm size fraction. As shown by these curves, conversion ratio increases with increase in time up to a maximum and thereafter remains approximately constant. This trend holds true for all particle sizes. The average value of maximum conversion (αMax) at 95% confidence level for all particle sizes is 91.74±0.49 %. Similarly, the time required to reach maximum conversion (tMax) decreases with increase in temperature as shown below: Temperature °C

510

530

550

575

Time (minutes)

95

50

35

20

Again, this trend holds true for all particle sizes. 3.5. Dehydroxylation kinetics Several kinetic models have been developed and reported for solid state reactions37-39. These models are divided into three types based on the shapes of the curves: ascending, descending and sigmoidal. Most of these models are valid and applicable in the range of 0.1≤α≤0.9.37-38 Our investigation in section 3.4 has shown that the α vs. t curve after α≥0.91 becomes plateau shaped (Fig. 7). Therefore kinetic studies were performed in the range of α≤0.91. Elimination of data beyond α≥0.91 for 550 and 757 °C leaves only 3 data points. In order to ensure better fit to the models, additional experiments were carried out as indicated in Table 3. The data from these experiments and previous data were analyzed with DX8 and conversion ratios were estimated. 10


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These estimated values have been used in the kinetic studies.

1 0.9 0.8 Temprature: °C 0.7

Conversion ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


510 530 550 575

0.6 0.5 0.4 0.3 0.2 0.1 0 0

20

40

60 80 Time(min)

100

120

140

Fig. 7. Effect of temperature and retention time on the conversion ratio

Preliminary examination of the shapes of the curves showed that dehydroxylation of Taikand alunite follows the sigmoidal model. Kinetic models based on the sigmoidal model are based on the assumption of nucleation and growth; and include the Avrami-Erofeev and Prout-Tompkins models. These models are described by the following equations:37-38 1

[-ln(1 -  )]n = K(t - t d ) (n = [ln(

3 4 , , 2, 3, 4 2 3

Avrami-Erofeev (6)

 )] = K(t) 1-

Prout-Tompkins (7)

In order to determine the most suitable kinetic model the following two methods were used:37 1- Testing the linearity of a plot of F(α) versus time; the best fitting equation is the most suitable and the slope is the apparent kinetics constant for the overall reaction. 2- Comparison of shapes of α-reduced time plots with lines calculated for various functions and determining the best model by using the following equation:  t   t   SS         t50  E  t50 T 

2

(8)

The equation with the minimum value for SS is the most suitable. 11



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For this purpose, t50 and the delay time td should be obtained. Value of t50 was obtained by linear regression and the delay time was obtained from the curve of F(α)= Kt+C and the following equation: td  C . K

The most suitable model is the one which fulfills both the abovementioned conditions. The results of the calculations are shown in Table 5 and Fig. 8, 9 and 10. According to these results within the temperature range of 510-530°C dehydroxylation of alunite follows the AvramiErofeev model with an index of 1.5 and for the temperature range of 550-575°C the same model with an index of 4 is applicable. After finding the most appropriate kinetic model the relationship between K and td with the other variables of the process should be determined. According to the results of the experimental design, particle size has a negligible effect on the dehydroxylation of alunite which can be related to the delay time. Fig. 8 shows the amount of conversion at different temperatures for different particle sizes. It can be seen that particle size has no effect on the dehydroxylation of alunite. The relation between K and temperature follows the Arrhenius Equation as shown in Eq. 9.37-38 K = A e –Ea/RT

(9)

Ea and A could be determined by plotting ln(K) versus 1/T. Fig. 11 shows the Arrhenius relation for four different particle sizes. According to the results obtained from this curve the activation energies Ea and A for the temperature ranges 510-530°C and 550-575°C are 194.75kJ, 2.04x1011 and 82.51 kJ and 11065.64 respectively. As shown in Table 5 no relation exists between K and the particle size. However, the delay time depends on the particle size. The delay time is related to the particle size by Eq.10 as follows:37 1

td  C  r 3

(10)

Therefore, the kinetic model for the dehydroxylation for the different temperature ranges is described by the following equations:

 e

(( 1.771011 e

 e

(( 11065 .64 e





23314 T

1

)(t Cr 3 ))1.5

(11)

1 9924 T )(t Cr 3 ))4

(12) 12


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The value of C was obtained by placing geometric means of size fractions in r and minimizing the relation SS = (αP – αE) 2 using the Solver in the Excel environment. Thus, the value of C was obtained as 0.951 for Eq.11 and 0.217 for Eq.12. Table 5. Summary of calculations for the best fit kinetic model

Temperature

510

530

550

575

Particle Size 1001.5 500.5 252.5 126 1001.5 500.5 252.5 126 1001.5 500.5 252.5 126 1001.5 500.5 252.5 126

Kinetics model A-E A-E A-E A-E A-E A-E A-E A-E A-E A-E A-E A-E A-E A-E A-E A-E

n

K

SS

t50

td

1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 4 4 4 4 4 4 4 4

0.0215 0.0205 0.0203 0.0206 0.0435 0.0440 0.0436 0.0435 0.0641 0.0655 0.0644 0.0626 0.0907 0.0925 0.0917 0.0912

0.002 0.016 0.006 0.027 0.008 0.035 0.040 0.073 0.002 0.002 0.002 0.003 0.005 0.006 0.010 0.020

36.2 37.9 38.7 39.9 18.0 16.5 17.9 18 14.5 14.2 14.1 14.9 10.2 10 9.9 10.1

8.77 7.32 7.39 6.87 7.94 7.78 7.11 6.44 1.97 2.04 1.75 0.98 1.73 1.78 1.56 1.22

1 0.9 0.8 Particle size: μm 0.7 Conversion ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2003 1001 505 252

0.6 0.5

Temperature: °C 0.4

510 530 550 575

0.3 0.2 0.1 0 0

20

40

60

80

100

Time(min)

Fig. 8. α-(t-td) for different particle sizes and temperatures

13



1.0 0.9 Temperature(°C) 510-530 550-575

0.8

Conversion ratio

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2 t/t50

3

4

Fig. 9. α-(t-td)/t50 for different particle sizes and temperatures

2 1.8 1.6 1.4

Particle size: μm 2003 1001 505 252

1.2 f (a)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

0.8

Temperature: °C

0.4

510 530 550

0.2

575

0.6

0 0

20

40

t(min)

60

80

100

Fig. 10. f(α)-(t-td) for different particle sizes and temperatures

3.6. Validation of the Model For the validation of the model the measured experimental values were plotted versus the estimates as shown in Fig 10. More experiments are needed for validation between the data points and especially in the range 540-550°C. As shown in Fig. 11 the intersection of two lines occurs at 807° K (534°C). Hence, Eq.1 should be used for estimation of transformations occurring below 533°C while Eq. 2 should be used for higher temperatures. 14


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-2 510-530 (four size fractions)

-2.4

550-575(four size fractions)

-2.8 ln(K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3.2

-3.6

-4 1.16

1.18

1.2

1.22

1.24

1.26

1.28

1.3

1000/T

Fig. 11. Arrhenius plot for dehydroxylation of alunite

Experiments for validation were carried out as shown in Table 6 and the relevant results are shown in Fig. 12. These results also show that the model gives good estimates of dehydroxylation of alunite in the investigated temperature range. Table 6. Conditions of validation tests

Temperature (°C) 520 540 560

Particle Size (μm) 2003,1001,505 2003,1001,505 2003,1001,505

Time (min) 25,45, 65 10,20, 30 5,10, 15

4. Conclusions On the basis of TG and DTA Studies, alunite decomposes in two stages. In the first stage, alunite loses its molecular water with a weight loss of 6.59 %. This stage is known as the dehydroxylation. In the second stage, known as desulfurization, the weight loss is 12.54 %. On the basis of the present study, dehydroxylation of alunite occurs between 508-577 C. Sulfur loss determinations also corroborated the TG results. DTA and DDTA curves of alunite show several peaks indicating several steps of dehydroxylation which have been also observed by other researcher.27-29 These steps might have different mechanisms and kinetics model. Hence kinetic studies were carried out in the region near these peaks.

15



1.2 1 Pridicted Values

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0.8 0.6 0.4 Model Devlopment Tests Validation Tests

0.2 0 0

0.2

0.4

0.6

0.8

1

1.2

Experimental Values

Fig. 12. Predicted vs. experimental values of conversion ratio

According to the experimental design results, temperature, time and interaction of these factors have 99.85% of total effect on the dehydroxylation of alunite, while the contribution of particle size was negligible (0.11 %). This minor contribution was related to the time delay. The experimental results showed that the dehydroxylation increased with time up to a maximum (αMax=91.74±0.49) and remained almost constant beyond that time (tMax). The time needed to reach maximum conversion decreases with increasing temperature so that these times are 95, 50, 35 and 20 minutes at 510°C, 530°C, 550°C and 575°C respectively. Kinetic studies at less than tMax shows that dehydroxylation of alunite within 508-575C range followed the Avrami-Erofeev models, but the index of reaction (n) depends on the temperatures i.e. its value is 1.5 in the 508-534 C range while in the 534-575C range its value is 4. Nomenclature

A C Ea F(α) K n R r SS T t t50 td tMax

frequency factor (min-1) constant activation energy (J mol-1) rate function apparent kinetics constant (min-1) reaction index universal gas constant (J k-1mol-1) particle radius (cm) Sum of squares temperature (°K) retention time (min) time for 50 % conversion (min) delay time (min) time required to reach maximum conversion(min) 16


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Wf Wt

maximum weight loss for full dehydroxylation (%) weight loss at different times (%)

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(15) Mc Cullough, W. E. Cyclic Process Treating Alunite. U.S. Patent 2,120,840, August 20, 1934. (16) Haff, R. C. Process for Treating Alunite Ore and The Like. U.S. Patent 2,398,425, Aprile 27, 1943. (17) Loast, Kent W. Recovery of Aluminum form Alunite Using Acid Leaching to Purify the Residue for Bayer Leach. U.S. Patent 4,031,182. March 24, 1977 (18) Froisland, L.J. Wouden, M.L., Harbuck, D.D. Acid Sulfation of Alunite. U.S. Bureau of Mines Report, RI 9222, 1989. (19) Ozdemir, M., Cetisli H. Extraction Kinetics of Alunite in Sulfuric Acid and Hydrochloric Acid. Hydrometallurgy, 2005, 76, 217–224. (20) Adams J.R., Effect of Roasting on Solubility of Alunite. Ind. Eng. Chem. 1935, Vol. 27, No. 7, pp 780-783. (21) Tanaka H., Procees of Producing Potassium Sulfate, Ammonium Sulfate and Alumina from Alunite. U.S. Patent 1,850,038, January 2, 1932 (22) Stevens, D.; Forberg, H.O.; Jennings, L.D.; Stephens, F.M.; Bowen, F.J; Thompson, D.L.; Copenhaver, J.V. Redox Treatment of Alunite Ore. U.S. Patent 3,890,425, June 17, 1975. (23) Stevens, D.; Forberg, H.O.; Jennings, L.D.; Stephens, F.M.; Bowen, F.J. Method of Treating Alunite Ore. U.S. Patent 3,890,426, June 17, 1975. (24) Stevens, D.; Forberg, H.O.; Jennings, L.D.; Thompson, D.L.; Copenhaver, J.V. Redox Treatment of Alunite Ore. U.S. Patent 4,029,737, June 14, 1977 (25) Nasyrov G. Z.; Zemlyanskaya, E.I.; Ravndonikas, I.V. Process for Alunite Treatment., U.S. Patent 4,117,077, September 26,1978 (26) Ozacar, M. and Sengil, I. Optimum Conditions for Leaching Calcined Alunite Ore in Strong NaOH. Can. Metall. Quart. 1999, 38,. 4, 249-255. (27) (11) Kristóf, J.; Frost, R. L.; Palmer, S. J.; Horváth, E.; Jakab, E. Thermoanalytical Studies of Natural Potassium, Sodium and Ammonium Alunites. J Therm Anal Calorim J Therm Anal Calorim. 2010, 100(3), 961-966. (28) Frost, R.L.; Wain, D. A Thermogravimetric and Infrared Emission Spectroscopic Study of Alunite. J. Therm. Anal. Calorim. 2008, 91(1), 267-274. (29) Frost, R.L.; Wain, D.L.; Wills, R.A.; Musemeci A.; Martens, W. A Thermogravimetric Study of the Alunites of Sodium, Potassium and Ammonium. Thermochim. Acta. 2006, 443, 56–61. (30) Bayliss, N. S.; Cowley, J. M.; Farrant, J. L.; Miles, G. L. The Thermal Decomposition of Synthetic and Natural Alunite: An Investigation by X-Ray Diffraction, Electron Diffraction and Electron Microscope Methods. Aust. J. Sci. Res. 1948, A1, 343-350. (31) Fink, W. L.; Van Horn, K. R.; Pazour, H.A. Thermal Decomposition of Alunite. Ind. Eng. Chem. 1931, 23 (11), 1248-1250. (32) Küçük, A.; Gülaboğlu, M.S. Thermal Decomposition of Şaphane Alunite Ore. Ind. Eng. Chem. Res. 2002, 41, 6028-6032. (33) Ogburn, S. C.; Stere, H. B. Thermal Decomposition of Alunite. Ind. Eng. Chem. 1932, 24 (3), 288-290. (34) Küçük, A.; Gülaboğlu, M.S.; Bayrakcüeken, S. Dehydration Kinetics of Şebinkarahisar (Gedehor) Alunite Ore in a Fluidized-Bed Reactor. Ind. Eng. Chem. Res. 2004, 43, 962968. (35) Küçük, F.; Yildiz, K. The Decomposition Kinetics of Mechanically Activated Alunite. 18


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Thermochim. Acta. 2006, 448, 107–110. (36) Anderson, M.; Kraber, S.; Hansel, H.; Klick, S.; Beckenbach, R.; Cianca-Betancourt, H. Design-Expert 8 manual. from Stat-Ease: http://www.statease.com/dx8_man.html. (Accessed August 10, 2010) (37) Bamford, C.H.; Tipper, C.F.H. Reactions in the Solid State. In Comprehensive Chemical Kinetic; Vol. 22; Elsevier: Amsterdam, 1980. (38) House, J. E. Principles of Chemical Kinetics; Elsevier: San Diego, 2007. (39) Schmalzried, H. Chemical kinetics of solids, VCH: New york, 1994.

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Dehydroxylation Kinetics of Alunite - American Chemical Society

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