Boric Acid Extraction from Calcined Colemanite with Ammonium

Article pubs.acs.org/IECR

Boric Acid Extraction from Calcined Colemanite with Ammonium Carbonate Solutions Asım Künkül,† Nezahat Ezgi Aslan,‡ Ahmet Ekmekyapar,† and Nizamettin Demirkıran†,* †

Chemical Engineering Department, Faculty of Engineering, Inonu University, Malatya 44280, Turkey General Directorate of Mineral Research and Exploration, Ankara 06800, Turkey

‡

ABSTRACT: Colemanite, a calcium-borate hydrate, is found in significant amounts in Turkey. Colemanite is used as a raw material in the production of boric acid. In this study, the leaching kinetics of calcined colemanite in ammonium carbonate solutions was investigated in a batch reactor. The effects of calcination temperature, solution concentration, solid-to-liquid ratio, stirring speed, and reaction temperature on the dissolution rate were determined and evaluated. It was found that the dissolution rate of calcined samples was higher than that of the uncalcined sample. It was observed that the leaching rate of calcined colemanite enhanced with increasing calcination temperature up to 450 °C, and did not change above this temperature. Thus, it was found that the applied calcination temperature had a significant effect on the leaching of colemanite. It was determined that the leaching rate raised with an increasing ammonium carbonate concentration, increasing reaction temperature, and decreasing solid/liquid ratio. It was detected that calcium carbonate was not formed on the particle surface. The dissolution kinetics of calcined colemanite was examined using both heterogeneous and homogeneous reaction models, and it was determined that the reaction rate can be described by a first-order pseudohomogeneous reaction model. The activation energy for this process was calculated to be 59.03 kJ/mol.

1. INTRODUCTION Boron is found naturally in various metal borate forms, of which the predominant metals are sodium, calcium, or magnesium.1,2 Turkey has tremendous potential with regard to high-quality boron mineral reserves. Most of the boron minerals in Turkey are colemanite, ulexite, and tincal, and these are found in the western part of the country. Bigadiç borate deposits represent the largest colemanite reserves in the world.2−4 The most important compound obtained from boron minerals is boric acid, but boric anhydride, borax pentahydrate, borax decahydrate, and anhydrous borax are also available as refined borates.5 Boric acid is used in many branches of industry and as the starting material in the preparations of many boron chemicals.6,7 Colemanite has a monoclinic crystal structure with a chemical formula of 2CaO·3B2O3·5H2O, and it is used in the production of boric acid.8,9 Boric acid is obtained from a reaction involving colemanite and sulfuric acid in a heterogeneous solid−liquid reaction. In this process, the ground colemanite reacts with an excess amount of sulfuric acid at 85−90 °C. Gypsum is formed as a byproduct and precipitates in the reactor, while boric acid, which is highly soluble in water, remains in the liquid phase throughout the reaction. Gypsum is removed by filtration, and boric acid is crystallized by cooling the filtrate to about 40 °C.2,9,10 The yield of this process is not good because of difficulties that are encountered in the filtration of the reaction mixture and crystallization. However, one of the byproduct forms of gypsum, borogypsum, is discharged into the environment and causes environmental pollution.11 Therefore, much research has been carried out to clarify the kinetics and mechanism of colemanite dissolution in various inorganic and organic leach solutions. © 2012 American Chemical Society

Dissolution kinetics of colemanite in water saturated with carbon dioxide and sulfur dioxide have previously been examined, and it was determined that both dissolution reactions were chemically controlled.12,13 The dissolution kinetics of calcined colemanite in ammonium chloride solution has been investigated by Kum et al.,14 and it was found that the reaction rate could be expressed using the homogeneous reaction model. In that study, it was observed that the dissolution rate was extremely sensitive to calcination temperature, and the rate of dissolution increased with increasing calcination temperature. Kurtbaş et al.11 and Kücu̧ ̈k et al.15 examined the dissolution of colemanite in leach solution saturated with SO2. Some inorganic acids and their salts have been used in the dissolution of mineral colemanite.16−18 Organic acids, such as acetic acid, oxalic acid, citric acid, and propionic acid have been also used in the leaching studies of colemanite ore by researchers.9,19−22 The majority of boron minerals, including ulexite, colemanite, tincal, and pandermite, exist in the hydrated form and lose their water of crystallization upon heating at a certain temperature. When substances containing water are heated to certain temperatures, they lose mass as they give off H2O, a process known as dehydration. Thus, under heat treatment, hydrated boron minerals undergo a reduction in their original weight due to water loss.23 Dehydration process can be applied for technological and economical reasons. For example, it may be a necessary step to decrease the weight of a material to reduce transportation costs in cases where hydrated water Received: Revised: Accepted: Published: 3612

October 18, 2011 January 29, 2012 February 11, 2012 February 12, 2012 dx.doi.org/10.1021/ie202388x | Ind. Eng. Chem. Res. 2012, 51, 3612−3618

Industrial & Engineering Chemistry Research

Article

given temperature. Following this procedure, the sample was cooled and weighed. Thus, calcination data of samples at various temperatures were obtained. Figure 1 shows X-ray

constitutes a significant fraction of the material or it may be used to obtain a porous solid for increasing the reaction rate of a solid−liquid reaction.24−27 Colemanite is a hydrated-boron mineral. The calcination process can be applied to colemanite to remove impurities, increase the efficiency of boric acid extraction, and obtain the anhydrous mineral. The thermal treatment of colemanite leads to certain structural modifications. When colemanite is heated to its decomposition temperature, it breaks to a fine powder. This phenomenon is called decrepitation. Decrepitation of colemanite occurs as a result of the sudden release of confined water vapor within micropores during the thermal treatment. During heating to the temperature of decrepitation, the original weight of the minerals decreases, the grade increases, and the calcined product become more chemically active.23,26,28−30 Davies et al.31 have reported that the calcination temperature for colemanite is around 600 °C. The calcined product at this temperature has a very porous structure, and boric acid production from the calcined colemanite increases. At higher temperatures, the leaching rate decreases because of structural modifications. The objective of this study was to investigate the leaching of calcined colemanite in ammonium carbonate solutions. When ammonium carbonate solution is used as an extractive agent, it provides ammonium ions, thus furnishing the protons required for the dissolution reaction. In this process, calcium carbonate, which has industrial importance, is formed as a byproduct. Calcium carbonate is used in large amounts in the pulp and paper industry as paper filler, as well as in coatings to provide opacity, high brightness, and improved printability due to its good ink receptivity. Additionally, calcium oxide can be obtained by calcining calcium carbonate.32,33 Several studies on the use of ammonium carbonate in the leaching of some ores and wastes have previously been performed.34−38 As mentioned above, the B2O3 grade of colemanite is increased upon calcination, a porous solid can be obtained, and the material becomes more chemically active. Therefore, the leaching rate of the mineral also increases. In this study, the leaching kinetics of calcined colemanite at different calcination temperatures was investigated in ammonium carbonate solutions. The effects of calcination temperature, solution concentration, reaction temperature, and solid/liquid ratio were examined, and kinetic parameters are reported.

Figure 1. X-ray diffractogram of the original (uncalcined) and calcined samples at various temperatures (Sample amount: 2 g. Particle size: −2 mm. Calcination time: 6 h).

diffractograms of both the original and calcined samples, which were obtained using a Rigaku RadB-DMAX II X-ray diffractometer. 2.2. Leaching Procedure. The dissolution experiments were carried out in a 500 mL cylindrical glass reactor equipped with a mechanical stirrer, a reaction temperature control unit, and a cooler to avoid loss of solution from evaporation. The experimental procedure was initiated by adding 250 mL of an ammonium carbonate solution into the glass reactor and bringing it to the desired reaction temperature. A given amount of calcined solid sample was then added to the solution. The dissolution process was carried out for various reaction times. Aliquots of 5 mL each were withdrawn at regular intervals during the reaction, and these were immediately filtered. The amount of B2O3 in the solution was then determined titrimetrically using mannitol.39 Because aqueous solutions of boric acid are too weakly acidic, the amount of B2O3 cannot be determined directly by titration with a basic solution. For this reason, mannitol was added to the solution to impart mildly acidic character to boric acid. Only then, can boric acid be analyzed by titration with a basic solution like sodium hydroxide. The conversion fraction of colemanite was calculated as follows: XB2O3 = amount of B2O3 passing to the solution/amount of B2O3 in the ore sample. In the leaching experiments of calcined colemanite, the effects of calcination temperature, solution concentration, reaction temperature, solid/liquid ratio, and stirring speed were investigated. The kinetic parameters and their ranges used in this work are given in Table 1.

2. MATERIALS AND METHODS Colemanite ore samples were obtained from Boric Acid Factories in Bandırma, Turkey. The ore was cleaned from impurities by hand, crushed with a laboratory crusher, ground with a laboratory mill, and then sieved to obtain different particle size fractions with ASTM standard sieves. A fraction of −2 mm size was used in the calcination process. The original ore sample (uncalcined) was analyzed, and it was determined that the mineral contained 44.65% B2O3, 26.75% CaO, 22.20% H2O, 4.96% other oxides (SrO, As2O3, SiO2, MgO, Na2O, and Fe2O3), and 1.44% insoluble matter. 2.1. Calcination Process. The aim of this process essentially was to obtain calcined colemanite samples for use in the determination of the relationship between solubility and calcination. Colemanite calcination was performed isothermally in an oven at constant temperatures between 350 and 600 °C for 6 h until the mass was practically constant. After putting 2 g of the sample with particle size of −2 mm in a ceramic crucible furnished with a cover, the sample was subjected to the

3. RESULTS AND DISCUSSION 3.1. Calcination and Dissolution Reactions. During the thermal dehydration process, colemanite loses some fraction of its hydrate water content, depending upon the dehydration 3613

dx.doi.org/10.1021/ie202388x | Ind. Eng. Chem. Res. 2012, 51, 3612−3618


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3.2. Effects of Parameters. To observe the effect of dehydration temperature on the leaching rate of calcined colemanite, experiments were performed on the calcined colemanite at dehydration temperatures of 350, 375, 400, 450, 500, and 600 °C. In these tests, the reaction temperature, solution concentration, liquid/solid ratio, and stirring speed were kept constant at 30 °C, 0.5 mol/L, 0.01 g/mL, and 400 rpm, respectively. The results plotted in Figure 2 show that the

Table 1. Parameters and Their Ranges Used in the Dissolution Experiments param.

param. values

calcination temps., °C concn, mol/L solid/liquid ratio, g/mL stirring speed, rpm reaction temp., °C

350, 375, 400, 450, 500, 600 0.25, 0.50, 1.00, 2.00 0.005, 0.010, 0.020, 0.030 200, 300, 400, 500 25, 30, 35, 40, 45

temperature. The dehydration reaction of colemanite is as follows: 2CaO·3B2O3 ·5H2O(s) → 2CaO· 3B2O3 ·nH2O(s) + (5 − n)H2O(g)

(1)

where n is the number of moles of water remaining after dehydration. The chemical compositions of the calcined samples at various temperatures are given in Table 2. It can Table 2. Chemical Compositions of Calcined Samples at Various Temperatures content, % sample

weight loss

B2O3

CaO

H2O

impurities

original (uncalcined) calcined at 350 °C calcined at 375 °C calcined at 400 °C calcined at 450 °C

1.35 4.08 19.69 21.28

44.65 45.26 46.55 55.60 56.72

26.75 27.12 27.89 33.31 33.98

22.20 21.13 18.88 3.12 1.17

6.40 6.49 6.66 7.97 8.13

Figure 2. Effect of calcination temperature on the dissolution of colemanite (Reaction temperature: 30 °C. Solution concentration: 0.5 mol/L. Solid/liquid ratio: 0.01 g/mL. Stirring speed: 400 rpm).

dissolution rate increased with increasing dehydration temperature up to 450 °C, and it did not change above this temperature. The effect of ammonium carbonate concentration on the dissolution rate of calcined colemanite was investigated in concentrations of 0.25, 0.50, 1.00, and 2.00 mol/L, while a calcination temperature of 400 °C, solid/liquid ratio of 0.01 g/ mL, reaction temperature of 30 °C, and stirring speed of 400 rpm were kept constant. Figure 3 shows the experimental

be seen from Table 2 that the B2O3 content increased with increasing weight loss. At temperatures higher than 450 °C, the weight loss was not significant. According to the following reaction, ammonium carbonate ionizes in an aqueous medium: (NH 4)2 CO3(s) ⇆ 2NH 4+(aq) + CO32 −(aq)

(2)

Ammonium and carbonate ions both undergo hydrolysis, as shown in the following equations: NH 4+(aq) + H2O(l) ⇆ NH3(aq) + H3O+(aq)

(3)

CO32 −(aq) + H2O(l) ⇆ HCO3−(aq) + OH−(aq)

(4)

The ammonium ions furnish the hydronium ions that are required for the dissolution reaction. When the calcined colemanite is added into ammonium carbonate solution, the overall reaction occurring during the dissolution process is probably as follows: 2CaO· 3B2O3 ·nH2O(s) + 2(NH 4)2 CO3(aq)

Figure 3. Effect of ammonium carbonate concentration on the dissolution of calcined colemanite (Reaction temperature: 30 °C. Solid/liquid ratio: 0.01 g/mL. Stirring speed: 400 rpm).

+ (7 − n)H2O(l) → 2CaCO3(s) + 4NH3(aq) + 6H3BO3(aq)

(5)

2+

As the mineral dissolves, Ca ions pass into the solution medium. Initially, the concentration of Ca2+ ions in the solution increases until [Ca2+][ CO32−] ≥ Ksp(CaCO3), and then, CaCO3 begins to precipitate. Thus, the following precipitation reaction occurs: Ca 2 +(aq) + CO32 −(aq) ⇆ CaCO3(s)

results concerning the effect of solution concentration. As can be seen in Figure 3, as the concentration increased, the dissolution rate also increased. Hydronium ion concentration increases with an increase in the concentration of ammonium carbonate. Thus, the leaching rate of calcined colemanite increases with an increase in ammonium carbonate concentration.

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Experiments were carried out at stirring speeds of 300, 400, and 500 rpm to observe the effect of the stirring speed on the reaction rate. In these experiments, the calcination temperature, solution concentration, solid/liquid ratio, and reaction temperature were fixed at 400 °C, 0.50 mol/L, 0.01 g/mL, and 30 °C, respectively. The experimental results showed that, after 45 min of leaching, 95.80% B2O3 at 300 rpm, 96.11% B2O3 at 400 rpm, and 96.40% B2O3 at 500 rpm were extracted. According to these results, it can be concluded that the effect of stirring speed on the dissolution rate of colemanite can be practically neglected. The effect of the solid/liquid ratio on the dissolution rate was studied in the range 0.005−0.030 g/mL. During experiments, the calcination temperature, solution concentration, stirring speed, and reaction temperature were kept constant at 400 °C, 0.50 mol/L, 400 rpm, and 30 °C, respectively. The variations of the dissolution rate for various solid/liquid ratios are given in Figure 4. This figure shows that the dissolution rate decreased

Figure 5. Effect of reaction temperature on the dissolution of calcined colemanite (Solution concentration: 0.5 mol/L. Solid/liquid ratio: 0.01 g/mL. Stirring speed: 400 rpm).

The experimental data obtained were analyzed using both the heterogeneous and homogeneous models, and it was determined that the data did not fit the heterogeneous models. Thus, pseudohomogeneous models were applied to derive the rate equation for this process. When pseudohomogeneous kinetic models were used, it was determined that the conversion process can be expressed by the following first-order pseudohomogeneous reaction model: −ln(1 − X B2O3) = kt

(7)

If the dissolution follows this model, the plot of the left side of eq 7 versus time must be a straight line. Using the first-order pseudohomogeneous reaction model, the left side of eq 7 was plotted against reaction time for different solution concentrations, solid/liquid ratios, and reaction temperatures. As can be seen from the plots shown in Figures 6−8, we obtained straight lines passing

Figure 4. Effect of the solid/liquid ratio on the dissolution of calcined colemanite (Reaction temperature: 30 °C. Solution concentration: 0.5 mol/L. Stirring speed: 400 rpm).

as the solid/liquid ratio increased. This can be explained by the increase in the amount of solid per amount of reagent in the reaction mixture. To determine the effect of the reaction temperature on the dissolution rate, the experiments were performed with five different reaction temperatures in the range 25−45 °C. The calcination temperature, concentration of solution, solid/liquid ratio, and stirring speed were kept constant at 400 °C, 0.50 mol/L, 0.01 g/mL, and 400 rpm, respectively. The results obtained are plotted in Figure 5. According to the results given in Figure 5, increasing the reaction temperature also increased the dissolution rate, as expected from the exponential dependence of the rate constant in the Arrhenius equation. 3.3. Kinetic Analysis. The heterogeneous and homogeneous reaction models can be used to analyze the kinetics of noncatalytic liquid−solid reactions. According to the heterogeneous reaction models, the reaction rate between a solid particle and the leaching reagent can be controlled by diffusion through the fluid film, diffusion through the product layer, or a chemical reaction at the particle surface. The particle size can stay constant or decrease during the reaction. Rate equations for these models have already been introduced in the literature.40−42 In the homogeneous model, the liquid reactant is assumed to enter the solid particle and react at all times throughout the particle. Under these conditions, the reaction rate can be described by pseudohomogeneous models, such as first or second order models.41

Figure 6. Agreement of experimental data with a first-order kinetic model for various ammonium carbonate concentrations.

through the origin. Therefore, the equation representing the kinetics of this process can be expressed as follows: ln(1 − X B2O3) = k 0C a(S /L)b exp( −Ea /RT )t

(8)

The constants a and b in eq 8 were calculated from the apparent rate constants for the concentration and solid/liquid ratio. The values of these constants are 0.58 and −0.38, respectively. To determine the activation energy of the dissolution reaction, an Arrhenius plot was constructed. Using 3615



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calcination process, which evidently enhances the leaching rate. Therefore, the applied calcination temperature is important. The surface areas were determined for the original and some calcined samples at various temperatures. The obtained results are given in Table 3. As can be seen in Table 3, Table 3. Surface Areas for the Original (Uncalcined) and Some Calcined Samples

Figure 7. Agreement of experimental data with a first-order kinetic model for various solid/liquid ratios.

sample

surface area (SBET, m2/g)

original calcined at 375 °C calcined at 400 °C calcined at 450 °C

0.2679 0.3614 1.1410 1.2990

as the calcination temperature increased, the colemanite surface area also increased. SEM images concerning the original and calcined (at 400 °C) samples are shown in Figure 10. As can be

Figure 8. Agreement of experimental data with a first-order kinetic model for various reaction temperatures.

the rate constants obtained from Figure 8, the plot of ln k versus 1/T was constructed, which is shown in Figure 9. The

Figure 10. SEM images of colemanite samples: (a) original (uncalcined) colemanite; (b) calcined colemanite at 400 °C.

Figure 9. Arrhenius plot for the dissolution process.

seen, a porous structure came into existence at 400 °C. Together, the surface area data and the SEM images confirm that a porous structure appeared following calcination. This allows the leaching reagent to penetrate the colemanite more easily, and the reaction rate increase. Colemanite contains calcium, and as it dissolves, calcium ions enter into the solution. When the amount of calcium ions in solution increases to a certain point, solid calcium carbonate precipitates according to eq 6. X-ray diffractograms of the

activation energy of calcined colemanite dissolution in ammonium carbonate solutions was calculated to be 59 kJ/ mol. As a result, the following kinetic expression, including the parameters used in this process, can be written as ln(1 − X B2O3) = 1.43109C 0.58(S /L)−0.38 exp( − 7100/T )t

(9)

3.4. Discussion on the Effects of Parameters. It is known that a porous structure is obtained during the 3616



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obtained precipitate after 15 and 45 min of leaching are given in Figure 11. This diffractograms indicate that calcium carbonate

Figure 11. X-ray diffractograms of the precipitate (calcite) obtained after 15 and 45 min of leaching.

Figure 13. X-ray diffractogram of the crystallized product (boric acid) from the filtered leach solution.

is formed. A SEM image obtained after a 45 min reaction time is given in Figure 12, and this photograph shows the formation

4. CONCLUSIONS In this experimental work, the conversion of calcined colemanite to boric acid in ammonium carbonate solutions was investigated. It was determined that the leaching rate increased with increasing calcination temperature up to 450 °C, and it did not change above this temperature. The effects of various parameters on the reaction rate were also examined. According to the experimental findings obtained from this work, it was concluded that the most effective parameters on the leaching of colemanite were calcination temperature and reaction temperature. It was detected that the leaching rate was enhanced with an increasing solution concentration and reaction temperature, and decreasing solid/liquid ratio. It was observed that the reaction rate of calcined samples was higher than that of the uncalcined sample. It was found that solid calcium carbonate has not formed on the surface of calcined colemanite particles. It was determined that this dissolution process could be described by a first-order pseudohomogeneous reaction model, and the rate-controlling step was the chemical reaction. The activation energy for this leaching system was calculated to be 59 kJ/mol. Boric acid was recovered as the main product, and calcium carbonate, which has industrial importance, was formed as a byproduct.

Figure 12. SEM image of the precipitate (calcite) obtained after 45 min of leaching.

of calcium carbonate. It is apparent from Figure 12 that solid calcium carbonate has not formed a layer on the surface of the colemanite particles. This event is important because, if calcium carbonate forms on the surface of the particles, then this solid product layer would offer resistance to the diffusion of the leaching fluid. In this case, the rate-controlling step of the reaction becomes the diffusion control through ash or a product layer. The dissolution process is not controlled by diffusion, which is indicated by the fact that a product layer is not formed on the surface of the colemanite particles, the stirring speed is ineffective on the reaction rate, and the activation energy is high. In the literature, processes having activation energies above 40 kJ/mol are reported to be chemically controlled.43 The activation energy in our work was determined to be 59 kJ/mol. A leach solution obtained after 45 min leaching was filtered and the filtered solution was evaporated. The obtained crystal product was analyzed by X-ray diffraction and found to be boric acid. Figure 13 shows an X-ray diffractogram of the crystallized product from the filtered leach solution.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +904223774760. E-mail: nizamettin.demirkiran@inonu. edu.tr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Inonu University Research Fund (Project No: 2006/44).

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NOMENCLATURE C = concentration of ammonium carbonate, (mol/L). Ea = activation energy, (J/mol). Ksp = solubility constant. S/L = solid/liquid ratio, (g/mL). R = universal gas constant, (J/molK). dx.doi.org/10.1021/ie202388x | Ind. Eng. Chem. Res. 2012, 51, 3612−3618


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T = temperature, (K). t = reaction time, (s). XB2O3 = converted fraction of B2O3. k = apparent rate constant of the reaction, (s−1). k0 = frequency or pre-exponential factor, (s−1). n = number of moles of water remaining after dehydration, mol. a, b = constants in eq 8.

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REFERENCES

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Boric Acid Extraction from Calcined Colemanite with Ammonium

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