Kinetics of Direct Leaching of Natural Alunite in KOH - Industrial

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Kinetics of direct leaching of natural alunite in KOH Mirzaagha mohammadi, and Mohammad Mehdi Salarirad Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401549w • Publication Date (Web): 15 Sep 2013 Downloaded from http://pubs.acs.org on September 16, 2013

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Industrial & Engineering Chemistry Research

Kinetics of Direct Leaching of Natural Alunite in KOH Mirzaagha Mohammadi* and Mohammad Mahdi Salarirad* Department of Mining and Metallurgical Eng., Amirkabir University of Technology, 424 Hafez Ave., Tehran, Iran *

E-mail addresses: [email protected]; [email protected].

Abstract Alunite is a potential resource for production of alumina and potassium sulfate. Hydrometallurgy is the conventional process employed for this purpose and direct leaching in KOH is suitable as one of the process steps because it does not require prior calcination. In this paper dissolution kinetics of pure natural alunite in KOH is described. Kinetics of dissolution of alunite in potassium hydroxide follows the shrinking core model. The rate of reaction is controlled by surface chemical reaction step and the order of reaction with respect to KOH is 1.327. The activation energy of this reaction was estimated as 94.18kJ/mol and the following kinetic model for alunite dissolution in potassium hydroxide was obtained: [

(

)]

1. Introduction The primary resource of aluminum production is bauxite that is processed with the Bayer-Hall process. Meanwhile, the quantity and quality of reserves appropriate for the 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 countries in the world have no reserves of bauxite suitable for the Bayer process. Therefore, in recent years attention to other sources 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 that 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 monovalent cations, such as K+, Na+, NH4+, H3O+, or divalent cations, such as Ca2+, Ba2+, Sr2+, Pb2+, or trivalent cations, for example, Bi3+. G is occupied by either Al3+ or

1 ACS Paragon Plus Environment


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Fe3+, and S6+, As5+, or P5+ could be present 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.9-14Natural alunites contain Al, K, and Na, which can be recovered as K2SO4, Na2SO4, Al2O3, Al2(SO4)3, or potassium alum KAl(SO4)2.6 The main method of alunite processing and recovering its valuable products is hydrometallurgy. Several processes have been suggested for leaching of alunite which can be divided to acid15−18 and basic19−23 processes. Dissolution of alunite in different reagents was investigated by Adams (1935).24 In this study only the effect of calcination at various temperatures were studied. The kinetics of dissolution of calcined alunite by sulfuric and hydrochloric acid has been studied and the controlling mechanism was found to be ash layer diffusion process.25 The kinetics of dissolution of calcined alunite by NaOH was also found to have the same mechanism.26 In all the acidic leaching methods of alunite, calcination is a prerequisite step. However, in basic leaching, alunite can be dissolve directly. In comparison with other basic leachates, potassium hydroxide possesses the advantage of having common cation with alunite. However, basic data on leaching kinetics of alunite in KOH are not commonly seen in published literature. Therefore, the kinetics of direct leaching of alunite in KOH was investigated in the present study. 2. Materials and Methods 2.1. Material and Sample Preparation The alunite mineral sample used in this study was supplied by Mineralogical Research Co. from Willow springs, near Mojave, Kern country, California. The sample was crushed, ground, and wet screened into size fractions. Five size fractions (425−500, 149−177, 125−149, 105−125 and 74−88 µm) were used in this study. Each size fraction was dried at 110 °C for 24 h. Trace iron oxide present in the samples was removed by dry High Intensity Induced Roll Magnetic Separation with an approximate intensity of 1.5 Tesla.


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Merck analytical-grade chemicals were used throughout, and the solutions were made up in distilled water. 2.2. Sample Characterization and Analysis Mineralogical analysis of the sample was performed by XRD by using Bruker AXS GmbH-D4 ENDEAVOR diffractometer. The determination of SiO2, Al2O3, Fe2O3, CaO in the alunite sample was carried out by atomic absorption spectroscopy (AAS). The amount of K2O and Na2O were determined by flame photometry. Total sulfur was determined with the LECO SC432DR instrument. Aluminum in solution was analyzed with AAS. 2.3. Experimental Set up and Procedure The leaching experiments were performed in a 500 ml flat-bottom three necked Pyrex flask which was equipped with a thermometer and a water-cooled condenser. The stirring, heating and control of temperature were conducted by a magnetic stirring hotplate (MR Hei-Tec) equipped with EKT Hei-Con stainless steel sensor manufactured by Heidolph. The accuracy of temperature measurement was ± 1 °C. Figure 1 illustrates the experimental set up used in this work. Temperature Controller

Condenser Stainless steel sensor Sampling Port Magnet bar Hot plate stirrer

Figure1. Schematic drawing of leaching set up. All the experiments were conducted in batch mode. For each run, 250 ml KOH solution with known concentration was first charged into the reactor. Next the system was heated to the required temperature under continuous stirring. When the temperature



reached the pre-set value and remained stable, 1.25 g of alunite sample was added to the reactor through the sampling port. The mixture of all the reactants was intensively stirred and 2 ml samples of solution were withdrawn through the sampling port at appropriate time intervals during a run. The pH of the sample was adjusted to below 3 and the volume was made up to 25 ml. Finally, the prepared solution was centrifuged and analyzed for Al using atomic absorption spectrophotometer (AAS). For calculation of the leached fraction of Al, the following equation was used:27 ∑

(

∑

)

(1)

3. Results 3.1. Sample Characterization The result of XRD analysis of the alunite sample is shown in Figure 2. The XRD pattern indicates that the only constituent of the sample is alunite. The chemical composition of the alunite sample is presented in Table 1 and the aluminum and iron contents of the size fractions prepared from it are shown in Table 2 (see section 2.1). According to the chemical composition given in Table 1 the chemical formula for the alunite sample used in this study is K0.63Na0.21(H3O)0.16Al3(SO4)2(OH)6. d=2.98798

70

60

50

d=4.95590

40

d=1.90714

d=1.49509

d=1.75181

d=2.86619

10

d=2.21878

20

d=2.27558

d=3.50230

30

d=5.71823

Lin (Counts)

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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0 4

10

20

30

40

50

60

2-Theta - Scale File: 90-84.raw - Type: 2Th/Th locked - Start: 4.000 ° - End: 70.000 ° - Step: 0.020 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 41 s - 2-Theta: 4.000 ° - Theta: 2.000 ° - Chi: 0.00 ° - Phi: 0.00 ° - X: 0.0 mm - Y Operations: Smooth 0.150 | Import

Figure 2. XRD pattern of alunite sample.


70

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Table 1. Chemical Composition of Tested Alunite

*

Al2O3

SO3

K2O

Na2O

Fe2O3

SiO2

P2O5

CaO

TiO2

L.O.I*

(%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)*

36.90

38.7

7.1

2.15

0.2

0.58

0.12

0.07

0.08

13.95

L.O.I was measured at 550 ˚C for 24 h.

Table 2. Aluminum and Iron Contents of Different prepared Size Fractions size fractions(µm) Al2O3 (%) Fe2O3(%)

74−88 37.16 0.05

105−125 37.24 0.04

125−149 37.26 0.05

149−177 37.12 0.05

425−500 37.31 0.07

3.2. Effect of the Agitation Speed Figure 3 shows the effect of agitation speed on dissolution of alunite in KOH. Increasing the agitation speed up to 1000 rpm increases the rate of leaching due to decreasing thickness of the liquid boundary layer on the surface of the alunite particles. Further increase has no effect on dissolution. These observations indicate that diffusion of KOH across the liquid boundary layer could be avoided by maintaining the speed at more than 1000 rpm. Hence, all the experiments were carried out at an agitation speed greater than 1000 rpm. In these experiments, dissolution of potassium was also studied in addition to leaching of aluminum. As shown in Figure 3, the rates of potassium and aluminum dissolutions are similar. Hence, in the rest of the experiments, only aluminum content was analyzed in order to determine the rate of alunite dissolution.



1 0.9 0.8 0.7

Leached fraction

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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0.6

Al- 500 rpm

0.5

Al-1000 rpm

0.4

Al-1400 rpm

0.3

K - 500 rpm

0.2 0.1 0 0

20

40

60

80

time, min Figure 3. Plot of conversion vs. time at various agitation speeds (temperature: 80 °C, particle size: 425−500 µm , KOH concentration: 2 mol/l, pulp density: 0.5 % wt/vol).

3.3. Effect of KOH Concentration The effect of KOH concentration on the alunite dissolution was investigated by varying the KOH concentration in the 1−4 mol/l range. The experimental results are summarized in Figure 4. It can be observed that the conversion rate increases greatly with increasing KOH concentration. The approximate time for completion of reaction for 1, 2 ,3, and 4 mol/l KOH were 150, 80, 40, and 25 minutes respectively. 3.4. Effect of Particle Size The effect of particle size on the rate of extraction was investigated by using four size fractions. The results are illustrated in Figure 5. It can be observed that the conversion rate increases with reducing particle size.


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1 0.9 0.8 1M 2M 3M 4M

0.7

Leached fraction

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


0.6 0.5 0.4 0.3 0.2 0.1 0 0

50

100

150

200

time, min Figure 4. The effect of different KOH concentration on the conversion rate of alunite (temperature: 80 °C; particle size: 105−125 µm, agitation speed: 1400 rpm, pulp density: 0.5 % wt/vol). 3.5. Effect of Temperature Experiments were performed to investigate the temperature dependence of the reaction and the results are summarized in Figure 6. It can be seen that the conversion rate increases rapidly with increasing temperature, which is attributed to the fact that raising the temperature increases the surface reaction rate and diffusion rate of KOH. From Figure 6 it can be seen that an increase in temperature from 60 to 95 °C in 20 minutes increases the dissolution from 8 to 100 %.



1 0.9

Leached fraction

0.8

Particle Size: µm 149-177 125-149 105-125 74-88

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

20

40

60

80

100

time, min Figure 5. The effect of particle size on the conversion rate of alunite (KOH concentration: 2 mol/l, temperature: 80 °C, agitation speed: 1400 rpm, pulp density: 0.5 % wt/vol). 1 0.9 0.8 0.7

Leached fraction

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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0.6

Temparature: ˚C

0.5

60 70 80 95

0.4 0.3 0.2 0.1 0 0

50

100

150

200

250

Time, min Figure 6. The effect of temperature on the conversion rate of alunite (KOH concentration: 2 mol/l, particle size: 105−125 µm, agitation speed: 1400 rpm, pulp density: 0.5 % wt/vol).


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4. Discussion 4.1. Determination of the Reaction Controlling Mechanism For a liquid-solid reaction system, the reaction rate is generally controlled by one of the following steps: diffusion through the liquid film, diffusion through the ash/product layer or the chemical reaction at the surface of the solid particles. The rate of the process would be controlled by the slowest of these sequential steps. The 13 rate equations of these three steps have been presented by Levenspiel (1999).29 For determination of the controlling mechanism of reaction two mathematical methods were used as follows:29 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 rate constant for the overall reaction. 2- Comparison of shapes of α−reduced time plots of experimental data with curves calculated for various functions. Analysis of experimental data by using these two methods showed that the equation relevant to surface chemical reaction has the best fit to the experimental data. Figure 7 and 8 show the result of calculation for determination of effect of concentration on alunite leaching. These plots have been obtained from data given in Figure 4. 1

y = 0.03647x y = 0.02118x R² = 0.97424 R² = 0.98564

0.9

y = 0.01182x R² = 0.99728 y = 0.00559x R² = 0.99229

0.8 0.7 1M 2M 3M 4M

0.6

F(α)

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


0.5 0.4 0.3 0.2 0.1 0 0

50

100

150

Time, min Figure 7. F(a)−t plot for different KOH concentration.


200


Another method to determine the controlling stage is observation of the reaction product. The reaction of alunite with KOH is as follows: (

) (

)

( )

( )

( )

(2) ΔrGo of the reaction was calculated by HSC−5 software.28 According to this reaction there are no solid products. In this study pure alunite was used and after the completion of reaction no residue or ash was observed. These evidences show diffusion from ash/product layer is not applicable. Figure 3 shows that beyond 1000 rpm, conversion ratio is independent of agitation speed which implies diffusion through the liquid film is not the controlling stage. As in all the experiments agitation speed was 1400 rpm, the only remaining possible mechanism is surface reaction. Therefore, the experimental kinetic data was analyzed on the basis of the shrinking core model assuming that (1) no product layer covers or hinders diffusion to the unreacted core as the reaction proceeds and (2) a surface chemical reaction controls the rate. The integrated rate equation of surface chemical reaction is as follows:29 ( )

(

)

(3)

1

0.8

Leached fraction

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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1M 2M 3M 4M

0.6

0.4

0.2

0 0

1

2

3

4

t/t50 Figure 8. α vs.

Plot for different KOH concentration.


5

Page 11 of 18

4.2. Determination of Reaction Order In order to determine the reaction order, the relation between K and concentration should be determined. First, the magnitude of K was determined for different concentrations from the slopes of the F(α)=Kt plots as shown in Figure 7. These plots have been obtained from data given in Figure 4. The reaction order was obtained by plotting ln(K) against lnCKOH; the slope of the line indicates the reaction order. According to Figure 9 the order of reaction with respect to KOH concentration equals 1.327. 4.3. Determination of Particle Size Effect The effect of particle size on the apparent kinetics constant is shown in Figure 10. This figure has been drawn from data given in Figure 5. If K values are plotted against 1/r a linear curve is obtained as shown in Figure 11. This shows that the reaction rate is controlled by surface chemical reaction.29 This conclusion supports the results of section 4.1.

-3

-3.5

ln(K), min-1

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


y = 1.327x - 5.252 R² = 0.985

-4

-4.5

-5

-5.5 0

0.5

1

ln(CKOH), mol/l Fig 9 ln(K) vs. ln(C) for determination of reaction order.


1.5


1 0.9

Particle size: µm

0.8

74-88 105-125 125-149 149-177

𝟏−(𝟏−𝛂) 1/3

0.7 0.6 0.5

y = 0.0142x R² = 0.9962 y = 0.0125x R² = 0.9951 y = 0.0115x R² = 0.9833 y = 0.0103x R² = 0.9872

0.4 0.3 0.2 0.1 0 0

10

20

30

40

50

60

70

Time, min Figure 10. F(a)−t plot for different particle size. 0.015

y = 0.5994x + 0.0069 R² = 0.9665

0.014

K, min-1

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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0.013

0.012

0.011

0.01 0.006

0.008

0.01

0.012

1/r , µm-1 Figure 11. Dependence of K on particle size.


0.014

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4.4. Determination of Activation Energy Temperature dependence can be used to estimate the apparent activation energy. It is widely accepted that reactions with activation energy higher than 40kJ/mol are controlled by a chemical reaction, while those with an activation energy less than 40 kJ/mol are controlled by a transport process, whether in the product layer or a boundary fluid.29 The relation between K and temperature follows the Arrhenius Equation as shown in Eq. 4 K = A e –Ea/RT

(4)

Ea and A could be determined by plotting ln(K) versus 1/T. Figure 12 shows temperature dependence of K. This plot has been drawn from the data given in Figure 6. If data from these curves are used to plot ln(K) vs. 1/T, Figure 13 is obtained. On the basis of this figure, the activation energy for leaching of alunite in KOH is 94.18kJ/mol. The high value of activation energy indicates that the reaction rate is controlled by the chemical reaction at the surface. 4.5 Kinetics Model The following model describes the progress of reaction of alunite with potassium hydroxide: (

)

(5)

k0 was calculated by fitting the data to this model by the solver program in Excel environment and found to be 5.04×109. By placing this value in the above equation and rearranging, the following relation is obtained: [

(

)]

(6)

Experimental values have been plotted against the calculated values for validation of model. As shown in Figure 14 the agreement of the model with the experimental values is good.



1 y = 0.0522x R² = 0.9932

0.9

y = 0.0108x R² = 0.9907

y = 0.0056x R² = 0.996

0.8

𝟏−(𝟏−𝛂) 1/3

0.7

Temparature: ˚C

0.6

60 70 80 95

0.5 0.4 0.3

y = 0.0019x R² = 0.9907

0.2 0.1 0 0

50

100

150

200

Time, min Figure 12. Dependence of K on temperature.

-2.00 -2.50 -3.00 -3.50

-ln(K), min-1

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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-4.00

y = -11.328x + 27.75 R² = 0.9907

-4.50 -5.00 -5.50 -6.00 -6.50 2.70

2.75

2.80

2.85

2.90

2.95

3.00

3.05

1/T*1000, K-1 Figure 13. Arrhenius plot for Alunite dissolution in 2 m KOH solution.


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1

0.8 α(predicted Value)

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


0.6 y = 0.9815x R² = 0.9826 0.4

0.2

0 0

0.2

0.4

0.6

0.8

1

α( Eperimental value)

Figure 14. Predicted vs. experimental values of leached fraction.

5. Conclusions 

On the basis of experiments carried out, temperature, KOH concentration and particle size affect the dissolution of alunite in KOH.



Temperature plays the more important role in the process. A temperature higher than 80 °C is essential for dissolution of alunite in KOH. The optimum temperature from kinetics point of view (short reaction time) and the industrial point of view (low reactor volume and no requirement for pressure reactor) is 95°C. At this temperature the reaction is completed in less than 20 minutes under the experimental conditions employed. Therefore this leaching procedure is very suitable since it does not require calcination.



The shrinking core model aptly describes the dissolution of alunite in potassium hydroxide. On this basis, the rate of process is controlled by a surface chemical reaction step and the order of reaction with respect to KOH is 1.327.



The activation energy of alunite dissolution in KOH was found to be 94.18kJ/mol.



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Nomenclature A frequency factor b stoichiometric coefficient of the reagent CAl wt % of Al in different prepared size fraction of alunite sample Ci concentration of Al in sample i (g/l) CKOH concentration of KOH in solution (mol/l) F(α) rate function K apparent kinetics constant (min−1) K intrinsic rate constant of the surface chemical reaction (cm s−1) M initial weight of alunite sample added into the reactor R universal gas constant (J k−1mol−1) r particle radius (cm) T temperature (°K) t retention time (min) t50 time required for leached fraction to equal 0.5 (min) V0 initial volume of the leaching solution in the reactor (ml) Vi volume of sample i withdrawn from the reactor (ml) Greek Letters αi leached fraction of Al corresponding to sample i ρ molar density of solid (mol cm−3)

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Kinetics of Direct Leaching of Natural Alunite in KOH - Industrial

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