Leaching kinetics of colemanite in potassium hydrogen sulphate solutions

Journal of Industrial and Engineering Chemistry 18 (2012) 38–44

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Leaching kinetics of colemanite in potassium hydrogen sulphate solutions Ro¨vs¸en Guliyev, Soner Kus¸lu *, Turan C¸alban, Sabri C¸olak Atatu¨rk University, Engineering Faculty, Department of Chemical Engineering, 25240 Erzurum, Turkey

A R T I C L E I N F O

A B S T R A C T

Article history: Received 9 November 2010 Accepted 10 January 2011 Available online 9 November 2011

The aim of the study was to investigate the dissolution kinetics of colemanite in potassium hydrogen sulphate solutions in a mechanical agitation system and to declare an alternative reactant to produce boric acid. Reaction temperature, concentration of potassium hydrogen sulphate, stirring speed, solid/ liquid ratio and colemanite particle size were selected as parameters on the dissolution rate of colemanite. The experimental results were successfully correlated by linear regression using Statistica Package Program. Dissolution curves were evaluated in order to test shrinking core models for solid– fluid systems. It was observed that increase in the reaction temperature and decrease in the solid/liquid ratio causes an increase the dissolution rate of colemanite. The dissolution extent is highly increased with increase the stirring speed rate between 100 and 500 rpm and the dissolution extent is slowly increased with increase the stirring speed between 500 and 700 rpm in experimental conditions. The activation energy was found to be 26.34 kJ/mol. The leaching of colemanite was controlled by diffusion through the product (or ash) layer. The rate expression associated with the dissolution rate of colemanite depending on the parameters chosen may be summarized as follows:

Keywords: Colemanite Potassium hydrogen sulphate Leaching kinetics

1  3ð1  XÞ2=3 þ 2ð1  XÞ ¼ 10:41  C 1:01  W 1:55  D1:43  ðS=LÞ0:60  eð26:34=RTÞ  t: ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Boron is one of the most important underground richnesses of Turkey having 72% of the World reserves. It has more industrial and strategic importance. It occurs in traces in most soils and plants, but is only found in a concentrated form in a few places [1]. In recent years the production of boron and its compounds has increased greatly, as it can be used in nuclear engineering, as fuel for rocket motors, in hard and refractory alloys, in high quality steels, in the production of heat resistant polymers, for the production of optic and chemically stable glass in the glass industry. The compounds are also used in cosmetic, leather, ceramics, rubber, paint, textile and agricultural area and also as catalysts [2]. They also find application in the wood-processing industry as a protection against moulds. Colemanite has a monoclinic crystal structure and a density of 2.40 g cm3. Its chemical formula is 2CaO3B2O35H2O. It is used to produce boric acid. Boric acid is used as a source of B2O3 in many fused products and as starting material in the preparation of many boron chemicals such as boron phosphate, boron tri halides, boron esters, boron carbide, organic boron salts and fluoroborates [3,4].

* Corresponding author. Tel.: +90 442 231 45 86; fax: +90 442 231 45 44. E-mail address: [email protected] (S. Kus¸lu).

It has been known that the investigation of the dissolution of colemanite ore in various solutions have been studied for production of boron compounds. There are many studies in the literature connected with the dissolution kinetics of colemanite in various solutions. The leach solutions, the value of activation energy, the rate controlling step and the references of these studies are summarized in Table 1 [5–18]. The boric acid is industrially produced with a reaction between colemanite and sulphuric acid solution. As sulphuric acid is a strongly acid, the impurities in boron ore are dissolved. This case causes impurities in boric acid solutions. The quality of boric acid is reduced. Therefore, weak acid solutions should be used for production of boric acid. The aim of our study is to investigate the dissolution kinetics of colemanite in potassium hydrogen sulphate solutions in a mechanical agitation system and also to declare an alternative reactant to produce the boric acid. There is no study reported in the literature about such a procedure. Investigation on the dissolution conditions and the dissolution kinetics of colemanite in potassium hydrogen sulphate solutions will be beneficial to the solution of some problems appeared during boric acid production. The kinetic data for the reaction of colemanite with potassium hydrogen sulphate are very important for industrial application.The dissolution kinetics of colemanite in potassium hydrogen sulphate solutions were examined according to the heterogeneous reaction

1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.082

R. Guliyev et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 38–44

2.1. Experimental procedure

Nomenclature b C CAg D De EA kd ks ko L n r R R S T t t* X W

rB

39

stoichiometric coefficient concentration of borax decahydrate solution (mol m3) concentration of A in the bulk solution (mol m3) mean particle size (m) diffusion coefficient (m2 min1) activation energy (kJ kmol1) mass transfer coefficient (m min1) reaction rate constant for surface reaction (mol min1) frequency or pre-exponential factor (min) amount of liquid (mL) mol number (mol) correlation coefficient universal gas constant (kJ kmol1) initial radius of a solid particle (m) amount of solid (g) reaction temperature (K) reaction time (min) reaction time for complete conversion (min) fractional conversion of B2O3 stirring speed (rpm) molar density of solid reactant (mol cm3)

A typical experiment conducted was as follows: 200 mL of distilled water was poured into the flask. The solution was heated to the desired temperature, at which it was kept constant. All experiments were carried out using 425 + 250 mm size fractions, except in experiments where the effect of particle size on the reaction rate was investigated. After this, large qualities of solid colemanite were added to the solutions. Stirring of the solution was started immediately thereafter. The duration of the treatment depended on the experimental conditions. At definite time intervals, 1 mL samples of the reacted solution were taken for the assay of B2O3 and analyzed by potentiometric and titrimetric methods [18,19]. Based on the B2O3 estimated, the degree of dissolution of colemanite was determined as a function of time. Colemanite samples used in the experiments were obtained from Bandırma Borax Corporation, TURKEY. The colemanite ore samples were crushed, dried under vacuum and sieved with ASTM standard sieves to give fractions of average sizes 638, 338, 215 and 153 mm for dissolution experiments. The chemical analysis of colemanite samples used in the experiments is shown in Table 2. Each experiment was repeated twice, and the arithmetic average of the results of the two experiments was used in the kinetic analysis. 3. Result and analysis 3.1. Dissolution reactions The reaction taking place in the solution can be written as follows [19]:

models. In our study, reaction temperature, concentration of potassium hydrogen sulphate, stirring speed, solid/liquid ratio and colemanite particle size were chosen as process parameters. 2. Methods and materials Leaching experiments were conducted under atmospheric pressure conditions. All reagents used in the experiments were prepared from analytical grade chemicals (Merck) and distilled water. A constant temperature water circulator was used in combination with the reactor to maintain the mixture in the reactor at a constant temperature. The experiments were carried out in a 500 mL spherical glass reactor. The reactor was equipped with a reflux condenser to prevent evaporation during heating and a mechanical stirrer to obtain a homogeneous suspension in the reactor. The mechanical agitation experimental system is fairly common, so no illustration of it appears in this paper.

4KHSO4ðaqÞ ! 4Kþ ðaqÞ þ 4HSO4 1 ðaqÞ

(1)

4HSO4 1 ðaqÞ þ 4H2 OðaqÞ $ 4H3 Oþ ðaqÞ þ 4SO4 2 ðaqÞ

(2)

When colemanite is added to the potassium hydrogen sulphate solutions, the reaction taking place in the solution can be written as follows [19,20]: 2CaO  3B2 O3 5H2 OðsÞ þ 4H3 Oþ ðaqÞ ! 2Caþ ðaqÞ þ 6H3 BO3ðaqÞ þ 2H2 OðlÞ (3) 2Ca2þ ðaqÞ þ 2SO4 2 ðaqÞ ! 2ðCaSO4 2H2 OÞðsÞ

(4)

The total reaction is as follows: 2CaO  3B2 O3 5H2 OðsÞ þ 4KHSO4ðaqÞ þ 6H2 OðaqÞ ! 2ðCaSO4 2H2 OÞðsÞ þ 6H3 BO3ðaqÞ þ 2K2 SO4ðaqÞ

(5)

Table 1 Summary of dissolution kinetics of colemanite in different leaching solutions. Leach solution

Activation energy (kJ/mol)

Rate controlling step

Reference

Water saturated with CO2 Water saturated with SO2 Etilen diamin tetraacetic acid (EDTA) Ammonium chloride Acetic acid Boric acid Phosphoric acid Sulphuric acid Water saturated with SO2 Oxalic acid Citric acid Amonnioum sulphate Perchloric acid Perchloric acid Amonnioum nitrate

57.70 53.97 50.60 89.00 51.49 28.61 53.91 34.00  2 39.53 39.70 28.65 40.46 46.47 41.07 41.40

Chemically reaction Chemically reaction Chemically reaction Chemically reaction The first order pseudo homogeneous reaction model Diffusion through product film around unreacted core of colemanite particles Surface chemically reaction process Second order with respect to saturation level Chemically reaction Product layer diffusion process Diffusion through the product layer Chemically reaction Chemically reaction Heterogeneous chemical reaction Chemically reaction

[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [1] [16] [17] [18]

R. Guliyev et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 38–44

40 Table 2 Chemical analysis of colemanite minerals.

Table 3 Parameters chosen and their ranges.

Component

CaO

B2O3

H2O

SiO2 and others

Parameter

Values

%

24.42

43.52

18.9

13.16

Stirring speed (rpm) Reaction temperature (K) Solid/liquid ratio (g/mL) Particle size (mm) Concentration of potassium hydrogen sulphate (mol L1)

100, 300, 500a, 700 323, 328, 333, 338, 343, 348, 353a, 358 1/5, 1/10, 1/25, 1/50a, 1/100 638, 338a, 215, 153 1.0, 1.5a, 2.0, 2.5, 3.0

3.2. Effect of parameters Reaction temperature, concentration of potassium hydrogen sulphate, stirring speed, solid/liquid ratio and colemanite particle size were selected as process variables to investigate their effects on the dissolution level of colemanite. Parameters chosen and their ranges can be seen in Table 3. In the experiments, while the effect of one parameter was studied, the values of other parameters shown with asterisks in Table 3 were kept constant. XRD diffractogram of borogibbs materials may be shown from Fig. 1. A quantity of 200 mL of potassium hydrogen sulphate solutions was used and kept constant in all experiments. Homogeneity of suspension in the reactor was obtained with a stirring speed of 500 rpm, kept constant in all experiments. The data obtained were plotted in the form of time versus fractional conversion as appearing in Figs. 2–6 In these figures, the fractional conversion X (%) is defined: X ð%Þ ¼

amount of dissolved B2 O3 in the solution  100 amount of B2 O3 in the original sample

(6)

a While the effect of one parameter was studied, the values of the other parameters were kept constant.

3.5. Effect of stirring speed The effect of the stirring speed on the dissolution rate of colemanite was investigated at 100, 300, 500 and 700 rpm. The change between stirring speed and conversion can be seen in Fig. 4. It can be seen from Fig. 4 that the dissolution level of the process increases with increase in the stirring speed rate until about 500 rpm. The dissolution rate of colemanite remained almost constant at stirring speed of between 500 and 700 rpm. Homogeneity of the suspension was exactly obtained at a stirring speed of 500 rpm. Because of this, the stirring speed rate of 500 rpm was as constant value in all experiments to get guaranteed to obtain homogeneity in the batch reactor. 3.6. Effect of solid/liquid ratio

3.3. Effect of reaction temperature The temperature is a factor of great importance for the leaching kinetics. The effect of reaction temperature was examined at 323, 328, 333, 338, 343, 348, 353 and 358 K. The dissolution curves obtained are shown in Fig. 2. Fig. 2 also shows that the quantity of colemanite dissolved increases with increasing reaction temperature. The reaction rate constant is exponentially dependent on reaction temperature.

The effect of solid/liquid ratio on the dissolution rate of colemanite was investigated by varying ratio to 1/5, 1/10, 1/25, 1/ 50 and 1/100 g/mL. The dissolution curves are given in Fig. 5. It can be seen from Fig. 5 that, the dissolution rate decreases with increasing solid/liquid ratio. This situation can be explained by the decrease in the number of colemanite particles per amount of solutions. 3.7. Effect of colemanite particle size

3.4. Effect of concentration of potassium hydrogen sulphate In general, the leaching rate increases with increased concentration of reagent, but only up to a certain maximum level. The effect of concentration of potassium hydrogen sulphate was studied by varying to 1.0, 1.5, 2.0, 2.5 and 3.0 M. The dissolution curves are given in Fig. 3. It can be seen from Fig. 3 that the dissolution level of the process increases with increase in the concentration of potassium hydrogen sulphate until about 1.5 M. The dissolution rate of colemanite remained almost constant at concentration of potassium hydrogen sulphate of between 1.5 M and 3.0 M.

The effect of particle size was studied by treating five sizes of fractions of this mineral, namely 638, 338, 215 and 153 mm. The dissolution curves are presented in Fig. 6. As can be seen from Fig. 6, as the particle size decreases the dissolution rates increased because of increasing surface area. 4. Kinetics analysis The solid–fluid heterogeneous reaction rate can be obtained from the heterogeneous reaction model. The experimental data

Fig. 1. XRD diffractogram of borogibbs materials.

R. Guliyev et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 38–44

1,1

1,1

1,0

1,0

0,9

0,8

X (%B2O 3)

X (%B2O 3)

0,9

0,7 0,6

323 K 328 K 333 K 338 K 343 K 348 K 353 K 358 K

0,5 0,4 0,3 0

5

10

15

20

25

0,8

0,7

0,6

1/100 g/mL 1/50 g/mL 1/25 g/mL 1/10 g/mL 1/5 g/mL

0,5

0,4

30

0

5

10

t (min.)

1,1 1,0

X (%B2O 3)

0,9 0,8 0,7 0,6

C =1,0 M C =1,5 M C =2,0 M C =2,5 M C =3,0 M

0,5

2

4

6

8

10

t (min.) Fig. 3. Effect of concentration of potassium hydrogen sulphate on dissolution rate of colemanite.

30

1,1

1,0

1,0

0,9

0,9

0,8

0,8

X (B2O 3)

X (%B2O 3)

25

The model has been used for solid–liquid heterogeneous systems in both analytical and numerical methods. Integrated rate equations for the un-reacted shrinking-core model and the other models are shown in Table 4. According to the model, the kinetic data were treated by equations in Table 4. The application of the above models to the experimental data will help in to determining the dissolution kinetics of the process. In the cases in which the chemical reaction is much faster than the diffusion the leaching is said to be diffusion-controlled. The leaching mechanism often becomes diffusion controlled when, during the leaching, a porous product layer forms on the surface of the particle to be leached. The mechanism of diffusion controlled leaching of spherical particle is often called the shrinking core model [22]. Experimental data that fits the heterogeneous diffusion controlled ash or product layer in the form of t/t* = 1  3(1  X)2/3 + 2(1  X). The regression coefficients for the all models obtained the study can be shown in Table 5. Diffusion coefficients (De) through product films for the system were obtained from Eq. (9). Time for complete conversion (t*) and the diffusion coefficients (De) obtained in the experimental system can be seen in Table 6. The evidence for this proposal is as follows: Regression analysis has shown that experimental data correlate well with Eq. (9) in Table 3, which means that the dissolution is diffusion controlled ash or product layer. During the reaction, CaSO42H2O precipitates. Therefore, it may appear that the process

1,1

0,7 0,6 1.min. 2.min. 3.min. 4.min. 5.min. 7.min. 10.min.

0,5 0,4 0,3

20

Fig. 5. Effect of solid/liquid ratio on dissolution rate of colemanite.

were analyzed based on the un-reacted shrinking core model to evaluate the rate-controlling step [21,22]. The heterogeneous reaction model gives rate equations for each control mechanisms. The step with the highest resistance is the rate-controlling step.

0

15

t (min.)

Fig. 2. Effect of reaction temperature on dissolution rate of colemanite.

0,4

41

0

100

200

300

400

500

600

700

W (rpm) Fig. 4. The change between stirring speed and conversion.

800

0,7

0,6

0,5

638 µm 338 µm 215 µm 153 µm

0,4

0,3

0

5

10

15

20

25

t (min.) Fig. 6. Effect of particle size on dissolution rate of colemanite.

30

R. Guliyev et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 38–44

42

1,2

Table 4 Integrated rate equations for the un-reacted shrinking core model and the other models. Rate equation

Surface chemical reaction

t t

¼ ½1  ð1  X B Þ1=3 

The film diffusion control

t t

¼ XB

Diffusion control through the ash or product layer For the first-order pseudo-homogeneous model For the second-order pseudo-homogeneous model Avrami model

t t

¼ ½1  3ð1  X B Þ2=3 þ 2ð1  X B Þ

r R

B t ¼ bksC

Ag

r R

B t ¼ 3bkgC

Ag 2

rB R t ¼ 6bDeC

Ag

lnð1  XÞ ¼ kt

lnð1  XÞ1 ¼ kt

1 - 3(1-X) 2/3 + 2(1-X)

1,0

Rate-controlling step

0,8

0,6 50 oC r 2 = 0,9805 55 oC r 2 = 0,9935 60 oC r 2 = 0,9907 65 oC r 2 = 0,9852 70 oC r 2 = 0,9853 75 oC r 2 = 0,9756 80 oC r 2 = 0,9787 85 oC r 2 = 0,9835

0,4

0,2 lnð1  XÞ1 ¼ kt m

0,0

0

2

4

6

8

10

Table 5 The regression coefficients for the models shown in Table 3 obtained the study. Model

Equation

r

Surface chemical reaction model Film diffusion control model Diffusion control through the ash or product layer First-order pseudo-homogeneous model Second-order pseudo-homogeneous model Avrami model

(7) (8) (9) (10) (11) (12)

0.9654 0.9791 0.9993 0.9547 0.9456 0.9214

is controlled diffusion product or ash film. The regression coefficients were found to be 0.9935 as higher linearity. The variation of 1  3(1  X)2/3 + 2(1  X) with time (t) is plotted for reaction temperature, concentration of potassium hydrogen sulphate, and colemanite particle size in Figs. 7–9, respectively. Eq. (9) in Table 3 is the expression for diffusion controlled leaching according to the shrinking core model. As is evident from the equation, the reaction time for complete conversion is proportional to the square of the radius of the particle. For diffusion controlled leaching, the reaction time for complete conversion is proportional to R2. Using the heterogeneous diffusion controlled ash or product layer, the t* values were plotted versus R2. The high linearity between t* and R2 is seen in Fig. 10. The regression coefficient (r2) was found to be 0.9993. The Arrhenius plots of ln ks versus 1/T were

14

16

18

20

22

24

26

t(min.) Fig. 7. Variation of 1  3(1  X)

2

12

2/3

+ 2(1  X) with time for reaction temperatures.

drawn for to found the activation energy of the reaction [21–24]. Arrhenius plots of ln ks versus 1/T are shown in Fig. 11. From the slopes of the straight lines the activation energy of the reaction is found to be 26.34 kJ/mol. It has been reported that the activation energy of the reaction controlled by surface chemical reactions is above 40 kJ/mol [25]. Similar results were found in the literature [10,14]. Further, this value indicates the dissolution rate of colemanite is a diffusion controlled product or ash layer. The fact that the dissolution rate of colemanite is dependent of the stirring speed is shown by the fact that the control mechanism is diffusion controlled product or ash layer. The values were found by non-linear regression analyses (Statistica 7.0, non-linear estimation model, user-specified regression-least squares, security value of 95%, comparison value of 1  exp(6), and maximum iteration values of 1000) and the analyses gave the mathematically model as follows: 1  3ð1  XÞ2=3 þ 2ð1  XÞ ¼ 10:41  C 1:01  W 1:55  D1:43  ðS=LÞ0:60  eð26:34=RTÞ  t

(13)

Table 6 Values of t* and De obtained in the experimental system.

323 328 333 338 343 348 353 358 353 353 353 353 353 353 353 353 353 353 353 353 353 353

C (mol L1) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.0 2.0 2.5 3.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

W (rpm) 500 500 500 500 500 500 500 500 500 500 500 500 100 300 700 500 500 500 500 500 500 500

S/L (g/mL) 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/50 1/100 1/25 1/10 1/5 1/50 1/50 1/50

D (mm) 338 338 338 338 338 338 338 338 338 338 338 338 338 338 338 338 338 338 338 638 215 153

t* (min.) 33.0 27.1 24.7 22.3 22.0 21.9 11.3 10.3 20.5 8.5 7.3 6.6 51.8 53.7 9.8 10.2 15.2 43.6 90.9 51.0 8.5 6.8

1,0

De (m2 s1) 11

8.12  10 9.86  1011 1.08  1010 1.19  1010 1.21  1010 1.22  1010 2.36  1010 2.59  1010 1.95  1010 2.35  1010 2.19  1010 2.02  1010 5.16  1011 4.97  1011 2.72  1010 2.62  1010 1.75  1010 6.13  1011 2.94  1011 1.87  1010 1.27  1010 8.02  1011

0,9 0,8

1-3(1-X) 2/3+2(1-X)

T (K)

0,7 0,6 0,5 0,4 0,3 C=1.0 M r2=0,9519 C=1.5 M r2=0,9768 C=2.0 M r2=0,9843 C=2.5 M r2=0,9548 C=3.0 M r2=0,9481

0,2 0,1 0,0

0

2

4

6

8

10

12

t (min.) Fig. 8. Variation of 1  3(1  X)2/3 + 2(1  X) with time for concentration of potassium hydrogen sulphate.

R. Guliyev et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 38–44

43

1,2

1-3(1-X) 2/3+2(1-X)

1,0

0,8

0,6

0,4 638 µm r 2=0,9971 338 µm r 2=0,9768 215 µm r 2=0,9928 153 µm r2=0,9148

0,2

0,0

0

5

10

15

20

25

30

t (min.) Fig. 9. Variation of 1  3(1  X)2/3 + 2(1  X) with time for particle sizes.

55 50 45

t* (min.)

40 35 30 25 20 15 10 5

0

50000

1E5

1,5E5

2E5

2,5E5

3E5

3,5E5

4E5

4,5E5

2

R2 (µm ) Fig. 10. Linearity between t* and R2.

-2,0

5. Discussion and conclusion

-2,2

The aim of the study was to investigate the dissolution kinetics of colemanite in potassium hydrogen sulphate solutions in a mechanical agitation system and to declare an alternative reactant to produce boric acid. Based on the results obtained in this research, the following conclusion may be drawn:

-2,4

ln k

-2,6 -2,8 -3,0 -3,2 -3,4 -3,6 0,00275 0,00280 0,00285 0,00290 0,00295 0,00300 0,00305 0,00310 0,00315

1/T Fig. 11. Arrhenius plot of the dissolution process.

 The dissolution rate of colemanite increased with increase in reaction temperature and decrease in the solid/liquid ratio.  The dissolution extent is highly increased with increase the stirring speed rate between 100 and 500 rpm and the dissolution extent is slowly increased with increase the stirring speed between 500 and 700 rpm in experimental conditions.  The dissolution process follows a shrinking core model with the heterogeneous diffusion controlled product (or ash) layer as the rate controlling step.  The activation energy was found to be 26.34 kJ/mol.

44

R. Guliyev et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 38–44

 Weak acid solutions such as potassium hydrogen sulphate solutions should be used for production of boric acid. So that the impurities in boric acid produced was reduced.  The mathematical form of the model depended on the parameters chosen was found as follows: 1  3ð1  XÞ2=3 þ 2ð1  XÞ ¼ 10:41  C 1:01  W 1:55  D1:43  ðS=LÞ0:60  eð26:34=RTÞ  t: References ¨ . Ku¨c¸u¨k, M. Aluz, Korean J. Chem. Eng. 24 (2007) 55–59. [1] M. Tunc¸, M.M. Kocakerim, O [2] T.W. Davies, S. C¸olak, R.M. Hooper, Powder Technol. 65 (1991) 433–440. [3] H.P. Kemp, The Chemistry of Borates: Part I, Borax Consolidated Ltd., London, 1956. [4] A. Gur, Korean J. Chem. Eng. 24 (4) (2007) 588–591. [5] M. Alkan, M.M. Kocakerim, S. C¸olak, J. Chem. Technol. Biotechnol. (1985) 382–386, 35/A. [6] M.M. Kocakerim, M. Alkan, Hydrometallurgy 19 (1988) 385–392. [7] Z. Karago¨lge, M. Alkan, M.M. Kocakerim, Metall. Mater. Trans. B 23B (1992) 409–413. [8] C. Kum, M. Alkan, A. Yapıcı, M.M. Kocakerim, Hydrometallurgy 36 (1994) 259–268.

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