Ind. Eng. Chem. Res. 2006, 45, 1857-1862
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Dissolution of Colemanite in Aqueous Solutions Saturated with Both Sulfur Dioxide (SO2) Gas and Boric Acid Abdusselam Kurtbas¸ , M. Muhtar Kocakerim,* O 2 zkan Ku1 c¸ u1 k, and Ahmet Yartas¸ ı Department of Chemical Engineering, Faculty of Engineering, Atatu¨rk UniVersity, 25240, Erzurum, Turkey
Boron compounds that are produced from boron ores are important industrial materials. In this report, an alternative method for the formation of boric acid obtained via the reaction between colemanite and sulfuric acid in aqueous media was studied. For this purpose, the kinetics and mechanism of the dissolution of colemanite were investigated in saturated boric acid solutions that were saturated with SO2 gas. In the experiments, the particle size, solid-to-liquid ratio, stirring speed, and temperature were chosen as parameters. As a result, the conversion rate increased as the particle size and solid-to-liquid ratio each decreased and the temperature increased; however, the effect of stirring speed was very minimal. In addition, the conversion rate was determined to fit the Avrami model, and the activation energy of the process was estimated to be 50.15 kJ/mol. The integrated rate equation for this conversion was determined to be as follows: -ln(1 - X) ) (1.26 × 105)[SO2]D-1.52(S/L)-0.14 e-6031/Tt0.73. Introduction Boron, which comprises 0.001% of the Earth’s crust, is present in the nature in its compounds. Most of the boron minerals that are encountered are in the form of boron salts that contain sodium, calcium, or magnesium. These minerals are named in according to the water contents and crystal structure. For example, colemanite, which has been used in this study, is a boron mineral that contains calcium, has the formula of 2CaO‚3B2O3‚5H2O, and is one of most important boron minerals industrially. It is used in the production of boric acid and other boron compounds that are used in various industries, especially glass and ceramics. In this respect, the production of boron compounds from boron ores is important and various technologies exist that have generally been patented in this area. However, much research is involved in developing new technologies each year. The dissolution kinetics and mechanisms of boron minerals have been studied in various acid solutions.1-7 Many authors have stated that the reaction products that form a film on the surface of the mineral decrease the reaction rate. In one research paper that has focused on colemanite, the dissolution of colemanite was investigated in CO2-saturated water, and it was determined that the reaction rate was controlled by chemical reaction.8 In another dissolution study that also used SO2saturated water, the dissolution kinetics of colemanite was modeled and it was determined that the rate of dissolution was controlled by chemical reaction.9 In a kinetic investigation of colemanite leaching by TitriplexIII, it was found that the dissolution rate increased by decreasing the particle size and pH and increasing the Titriplex-III concentration and temperature.10 Yartas¸ ı et al.11 studied the kinetics and mechanism of colemanite dissolution in boric acid solutions. The authors determined that the reaction rate was controlled by diffusion through the product layer around the * To whom correspondence should be addressed. Tel.: +90 442 231 45 52. Fax: +90 442 236 09 57. E-mail:
[email protected].
unreacted core of colemanite particles. O ¨ zmetin et al.12 examined the dissolution kinetics of colemanite in acetic acid solutions and determined that the reaction rate fit a homogeneous firstorder model with an activation energy of 51.49 kJ/mol. Temur et al.13 performed a dissolution of colemanite in phosphoric acid solutions and concluded that the dissolution was a chemicalreaction-controlled process. Kum et al.14 studied the leaching kinetics of calcined colemanite in ammonium chloride solutions and determined that the dissolution rate fit a homogeneous reaction model with an activation energy of 89 kJ/mol. Ku¨c¸ u¨k et al.15 examined the dissolution of Kestelek colemanite in water that was saturated with SO2 and determined that the dissolution process was chemically reaction controlled and the activation energy was 39.53 kJ/mol. Currently, boric acid is produced by dissolving colemanite ore in H2SO4 solution, as given in the following reaction:
2CaO‚3B2O3‚5H2O(s) + 2H2SO4(aq) + 6H2O f 2CaSO4‚2H2O(s) + 6H3BO3(aq) (1) After the reaction mixture is filtered, boric acid is crystallized from the solution. However, the process yield is not good, because of difficulties that are encountered in filtration of the reaction mixture and crystallization. On the other hand, a form of gypsum that is a byproduct of the process (borogypsum) is discharged into the environment and causes environmental pollution. The aim of this study is to investigate the dissolution kinetics of colemanite in saturated boric acid solutions that have been saturated with SO2, as a basis for improving environmentally sensitive boric acid production technology. Therefore, it is believed that crystallized boric acid is produced, and calcium, in the form of Ca(HSO3)2, remains in the solution. The solution that contains Ca(HSO3)2 can be directly recycled in the process or used as saturated H3BO3 solution after Ca2+ cations are precipitated as CaSO3‚xH2O by boiling the solution. The SO2 gas that forms during the boiling can be recycled in the process.
10.1021/ie050050i CCC: $33.50 © 2006 American Chemical Society Published on Web 02/21/2006
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Figure 1. X-ray diffractogram of the original sample.
This knowledge showed that CaSO3‚xH2O did not form under the studied conditions. The runs were conducted in a 250-mL glass reactor that was equipped with a constant temperature circulator (to set the temperature) and a tachometer-stirrer assembly (to set the stirring speed). After 150 mL of the saturated H3BO3 solution was taken to the reactor and saturated with SO2 gas at a constant volumetric rate, a defined amount of colemanite was added to the reactor and the reactor content was mixed for a certain reaction period; in addition, SO2 gas was passed through the reaction mixture continuously during the experiment. After the completion of each experiment, the reaction mixture was filtered and the Ca2+ content in the filtrate was determined volumetrically. The conversion fraction of colemanite was calculated as follows:
XCaO ) Figure 2. Dependence of pH and solubility of SO2 in water on temperature under an atmospheric pressure of 610 mm Hg in Erzurum.
Materials and Methods Colemanite ore was taken from Borax and Acid Factories in Bandırma, Turkey. The ore was cleaned mechanically with hand, crushed with a laboratory crusher, ground with a laboratory mill, and then, sieved with ASTM standard sieves (the fractions were -2000 µm/+1000 µm, -710 µm/+600 µm, -600 µm/+425 µm, -425 µm/+300 µm, -300 µm/+250 µm, and -250 µm/ +212 µm). These fractions were determined to contain, on average, 50.61% B2O3, 27.16% CaO, 21.82% H2O, and 0.39% other compounds. An X-ray diffractogram of the original sample, which was obtained using a Rigaku DMAX 2000 Series X-ray diffractometer, is given in Figure 1. The SO2 gas used in these studies was supplied from Akkim Corp. in Gemlik, Turkey. The dependence of the solubility of SO2 in water on temperature at atmospheric pressure (610 mm Hg in Erzurum), and pH changes of these solutions, are given in Figure 2. To determine whether CaSO3‚xH2O formed under the studied conditions or not, Ca(HSO3)2 solutions saturated with H3BO3 were prepared, and it was determined that, after the Ca2+ concentration reached 0.6 M, CaSO3‚xH2O began to precipitate.
amount of CaO passing to the solution amount of CaO in the original sample
(2)
In the experiments, the particle size, temperature, solid-to-liquid ratio, and stirring speed were used as parameters, the ranges of which are given in Table 1. Results and Discussion The effects of the particle size, stirring speed, solid-to-liquid ratio, and temperature on conversion rate of colemanite in saturated H3BO3 solutions saturated with SO2 gas were investigated. Reactions. When colemanite was mixed with saturated H3BO3 solutions that were saturated with SO2 gas, the reactions that were believed to occur were as follows:
SO2(g) + 2H2O T HSO3-(aq) + H3O+(aq)
(3)
HSO3- + H2O T SO32- + H3O+
(4)
2CaO‚3B2O3‚5H2O(s) + 2H3O+(aq) f 2Ca2+ + H2B6O112-(aq) + 7H2O (5) H2B6O112-(aq) + 2H3O+(aq) + 5H2O f 6H3BO3(s)
(6)
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Figure 3. Schematic representation of the dissolution process. Table 1. Parameters and Their Ranges in the Experiments parameter
values
particle size (µm) solid-to-liquid ratio (g/mL) reaction temperature (K) stirring speed (rpm) time (min)
-2000/+1000, -710/+600, -600/+425, -425/+300, -300/+250, -250/+212 1/150, 2/150, 4/150, 8/150 285, 293, 298, 303 200, 300, 400, 500, 600, 700 3, 5, 10, 20, 30, 45, 60
Figure 4. Effect of particle size on the dissolution rate of colemanite in SO2-saturated water (temperature, T ) 298 K; solid-to-liquid ratio, S/L ) 2 g/150 mL; stirring speed, SS ) 500 rpm).
Equations 3-6 show that the reaction medium contains the species H2B6O112-, Ca2+, HSO3-, SO32-, H3O+, and H3BO3(aq), and H3BO3(s) (see Figure 3). When the SO32- and Ca2+ concentrations reached sufficient values, CaSO3‚xH2O precipitated, according to the equations
[Ca2+][SO32-][H2O]x ) Ksp
(7)
Ca2+(aq) + SO32-(aq) + xH2O T CaSO3‚xH2O(aq) (8) After filtration of the reaction mixture, if the filtrate is boiled, the following reaction and physical conversion occur:
Ca(HSO3)2(aq) + (x - 1)H2O T CaSO3‚xH2O(s) + SO2(g) (9) H3BO3(s) T H3BO3(aq)
Figure 5. Effect of the solid-to-liquid ratio on the dissolution rate of colemanite in SO2-saturated water (R ) -425 µm/+300 µm; T ) 298 K; SS ) 500 rpm).
(10)
The SO2(g) that is formed can be recycled in the process; in addition, after CaSO3‚xH2O(s) in the reaction medium was filtrated, H3BO3 in the filtrate can be crystallized by cooling to the dissolution temperature of the ore. Effects of Parameters. The effect of particle size was investigated using the -2000 µm/+1000 µm,-710 µm/+600 µm, -600 µm/+425 µm, -425 µm/+300 µm, -300 µm/+250 µm, and -250 µm/+212 µm fractions. In these experiments, the solid-to-liquid ratio was assumed to be 2 g of ore/150 mL of solution, the stirring speed was 500 rpm, and the temperature was 298 K. The feed rate of SO2 gas passing through the solution to saturate it with SO2 gas was 15 mL/min before each experiment, as well as during each experiment. The results are given in Figure 4. As the figure indicates, the conversion rate increases as the particle size decreases. The effect of the solid-to-liquid ratio on the conversion rate was studied using solid-to-liquid ratios of 1 g/150 mL, 2 g/150 mL, 4 g/150 mL, and 8 g/150 mL. The temperature was assumed to be 298 K, the particle size distribution was -425 µm/+300 µm, and the stirring speed was 500 rpm. The results are given in Figure 5. This figure shows that the conversion rate decreases as the solid-to-liquid ratio increases.
Figure 6. Effect of the stirring speed on the dissolution rate of colemanite in SO2-saturated water (T ) 298 K; S/L ) 2 g/150 mL; R ) -425 µm/ +300 µm).
The effect of the stirring speed on the conversion rate was identified, using stirring speeds of 200, 300, 400, 500, 600, and 700 rpm. In these experiments, the particle size was -425 µm/ +300 µm, the solid-to-liquid ratio was 2 g/150 mL, and the temperature was 298 K. The results given in Figure 6 showed that the effect of stirring speed on the conversion rate is very small.
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The effect of temperature on the conversion rate was studied by conducting the experiments at 285, 293, 298, and 303 K. In these experiments, the particle size was assumed to be -425 µm/+300 µm, the solid-to-liquid ratio was 2 g/150 mL, and the stirring speed was 500 rpm. The results in Figure 7 show that the conversion rate increases slightly as the reaction temperature increases. Kinetics. If the noncatalytic reaction between a fluid and a solid, given as
A(fluid) + bB(solid) ) product (fluid and/or solid) is taken into account, all or some of the following physical and chemical phenomenon steps occur: (1) Diffusion of a fluid reactant through a fluid film on a solid product (2) Diffusion of a fluid reactant through a solid product on the surface of a solid reactant (3) Reaction between a solid reactant and a fluid reactant (4) Diffusion of fluid products through a solid product film to a fluid film (5) Diffusion of fluid products through a fluid film to a bulk fluid The resistance of each of these described steps is different than that of another. In such situations, it is accepted that the step with the biggest resistance controls the rate. To derive the rate equations of such reactions, the resistance that corresponds to steps 1, 2, and 3 are taken into consideration and the others are omitted. Therefore, noncatalytic fluid-solid reactions may be presented with one of three control mechanisms; gas-film diffusion control, chemical reaction control, and product film diffusion control. There is a different integrated rate equation for each control and particle geometry. For a spherical particle, the following integrated rate equations are given:
X ) k[Cg]t
Figure 7. Effect of the reaction temperature on the dissolution rate of colemanite in SO2-saturated water (R ) -425 µm/+300 µm; S/L ) 2 g/150 mL; SS ) 500 rpm).
(for diffusion through fluid film control) (11)
1 - 3(1 - X)2/3 + 2(1 - X) ) k[Cg]t (for diffusion through product film control) (12) 1 - (1 - X)1/3 ) k[Cg]t
Figure 8. Experimental and theoretical dissolution fraction data for all the parameters.
(for chemical reaction control) (13) 1 t* ) [Cg] k
(14)
In addition to these models, pseudo-homogeneous models may be also used to derive the rate equations in such systems, for example,
-ln(1 - X) ) kt (for the first-order pseudo-homogeneous model) (15) X ) kt 1-X (for the second-order pseudo-homogeneous model) (16) -ln(1 - X) ) kt
m
(for the Avrami model)
(17)
In deriving a rate equation for the conversion between colemanite and solutions saturated with both SO2 gas and boric acid, experimental data were applied to the aforementioned integrated rate equations, using multiple regression, and the regression coefficients were determined to be R ) 0.728 for eq 11, R ) 0.977 for eq 12, R ) 0.911 for eq 13, R ) 0.934 for
Figure 9. Dissolution of colemanite in SO2-saturated water; plots of ln(-ln(1 - X))/[SO2(g)] versus ln(t) for different temperatures (R, -425 µm/+300 µm; S/L, 2 g/150 mL; SS, 500 rpm).
eq 15, R ) 0.894 for eq 16, and R ) 0.964 for eq 17. In addition, the acceptability of these models was discussed, by accepting that the reaction is first order in accordance with the SO2 concentration in the solution.16 Because of the fact that the stirring speed was ineffective, it was not taken into consideration to derive the model. In the evaluation that has been conducted, chemical control, the first-order pseudo-homogeneous model, and the second-order pseudo-homogeneous model were eliminated, because of its low regression coefficients. High activation
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Figure 10. X-ray diffractogram of the solid residue at the end of the reaction period.
energies and ineffectiveness of the stirring speed eliminated diffusion through product film control and film diffusion control models. As a result, it was decided that the most proper control model was the Avrami model:
-ln(1 - X) ) 1.26 × 105[SO2]D-1.52
-0.14
(LS)
e-6031/Tt0.73 (18)
For this conversion, the activation energy was 50.15 kJ/mol. The values from the aforementioned model equation were plotted versus the experimental values in Figure 8, and it was observed that the model was fitted with experimental data. On the other hand, using the integrated rate equations that correspond to each control model and taking into account the change in solubility of SO2 in water, relative to temperature, graphs of f(x)/[SO2(g)] versus t, to determine the effect of temperature, were prepared, and it was determined that the models that had the biggest correlation coefficients were chemical reaction control (in eq 13) and the Avrami model (in eqs 17 and 18). A plot of ln(f(X)/[SO2(g)]) versus t, to determine the effect of temperature according to the Avrami model, is observed in Figure 9. By taking into account the model obtained by multiple regression, the graphs of ln(f(X)/[SO2(g)]) versus t (for the Avrami model) and f(X)/[SO2(g)] versus t (for all other models) to determine the effect of temperature, an activation energy of 50.15 kJ/mol, the ineffectiveness of the stirring speed, as well as the formation of H3BO3 on the colemanite particles during the process, it was thought that the most reliable model was the Avrami model.17 Conclusion In this study, the conversion of colemanite to boric acid in solutions that were saturated with both SO2 gas and boric acid was investigated. The chosen parameters were the particle size, stirring speed, solid-to-liquid ratio, and temperature. A mathematical model for this conversion was derived (as shown in eq 18). The conversion rate was determined to fit Avrami model. Solid residue of the conversion contained solid H3BO3 and unreacted colemanite. After the reactor content was filtered, boric acid could be recovered by leaching the solid portion of
the conversion products with hot water and then crystallizing it. The liquid portion of the conversion products can be reused in the conversion. The phosphoric acid process mentioned in ref 13 is more advantageous than the classical sulfuric acid process, because there is no borogypsum byproduct. Calcium phosphates can be separated and used as fertilizer, or phosphoric acid can be gained and reused in the process. In the process investigated in this paper, the byproducts are CaSO3(s), CaSO3‚1/2H2O(s), CaSO4.2H2O(s), and Ca(HSO3)2(aq); these compounds were confirmed by repeated and careful examination of the X-ray analysis of the solid residue (see Figure 10). Until the Ca2+ concentration in the solution reached a value of 0.6 M, only aqueous Ca(HSO3)2 was present. When this concentration was greater than this value, excessive Ca2+ cations precipitated as various sulfites. The boric acid present in this process is in the solid form. When the solution was boiled, it dissolved, Ca(HSO3)2 decomposed, and sulfites precipitated. The SO2 gas that forms can be sent to the dissolving reactor and reused as a reactant. Precipitated sulfites can be used in various industries such as paper and textile industries. Therefore, there is no waste in this process. The advantage of this process, in comparison to the phosphoric acid process, is that boric acid solution is separated easily from the byproducts and is not contaminated by ions such as phosphate and calcium. Repeated examination of the X-ray analysis results showed that the solid waste contained CaSO3(s), CaSO3‚1/2H2O(s), and CaSO4‚2H2O(s), in addition to clay minerals that cannot be detected, because of their low concentration. List of Symbols X ) fractional conversion t ) time (min) t* ) time for complete conversion of a single solid particle (min) b ) stoichiometric coefficient of B (solid) reacting with each mole of A (fluid) D ) geometrical average of lower and upper sieves ranges (mm) [CSO2] ) SO2 concentration in the solution (mol/L) f(X) ) left side of eqs 11-17 T ) reaction temperature (K) S/L ) solid-to-liquid ratio (g/mL)
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SS ) stirring speed (rpm) m ) constant in eq 17 [Cg] ) gas concentration in the solution (mol/L) Literature Cited (1) Imamutdinova, V. M. Kinetics of dissolution of borates in mineral acid solutions. Zh. Prikl. Khim. (S. Petersburg, Russ. Fed.) 1967, 40 (11), 2593-2596. (2) Mardanenko, V. K.; Karazhanov, N. A.; Kalacheva, V. G. Kinetics of dissolution of borates in sodium hydroxide solutions. Zh. Prikl. Khim. (S. Petersburg, Russ. Fed.) 1974, 47 (2), 439-441. (3) Zdanovskii, A. B.; Strezhneva, I. I.; Tkacvhev, K. V. Kinetics of decomposition of certain borates by sodium carbonate solutions at 90 °C. Zh. Prikl. Khim. (S. Petersburg, Russ. Fed.) 1973, 46 (10), 2303-2305. (4) Strezhneva, I. I.; Tkachev, K. V. Kinetics of the Reaction of Some Borates with Soda in a Solution. Tr. Ural’. Nauchno.-Issled. Khim. Inst. 1977, 40, 52-59; Chem. Abstr. 1977, 88 (12), 79782e. (5) Wiseman, J. Process for the Manufacture of Boric Acid. U.S. Patent No. 2,531,182, 1950. (6) Constable, I. H.; Tugˇtepe, M. The Water Solubility of the Precipiated Borates of Calcium, Strontium and Barium. Istanbul Fen Fak. Mecm., Seri A 1952, 17, 191-195. (7) Meixner, H. New Turkish Borate Deposits (in Turk.). Berg. Hu¨ttenma¨n. Monatsh. Montan. Hochsch. Leoben 1953, 98, 86-92; Chem. Abstr. 1952, 47, 10413f. (8) Alkan, M.; Kocakerim, M. M.; C¸ olak, S. Dissolution Kinetics of Colemanite in Water Saturated by CO2. J. Chem. Technol. Biotechnol. 1985, 35A, 382-386.
(9) Kocakerim, M. M.; Alkan, M. Dissolution Kinetics of Colemanite in SO2 Saturated Water. Hydrometallurgy 1988, 19, 385-392. (10) Karago¨lge, Z.; Alkan, M.; Kocakerim, M. M. Leaching Kinetics of Colemanite by Aqueous EDTA Solutions. Metall. Trans. B 1992, 23B, 409-413. (11) Yartas¸ ı, A.; O ¨ zmetin, C.; Kocakerim, M. M.; Demirhan, M. H. Kinetics and Mechanism of Leaching Colemanite in Boric Acid Solutions. Chim. Acta Turc. 1998, 26, 7-13. (12) O ¨ zmetin, C.; Kocakerim, M. M.; Yapıcı, S.; Yartas¸ ı, A. A semiempirical kinetic model for dissolution of colemanite in aqueous CH3COOH solutions. Ind. Eng. Chem. Res. 1996, 35, 2355-2359. (13) Temur, H.; Yartas¸ ı, A.; C¸ opur, M.; Kocakerim, M. M. The Kinetics of Dissolution of Colemanite in H3PO4 Solution. Ind. Eng. Chem. Res. 2000, 39, 4114-4119. (14) Kum, C.; Alkan, M.; Kocakerim, M. M., Dissolution kinetics of calcined colemanite in ammonium chloride solution. Hydrometallurgy 1994, 36, 259-268. (15) Ku¨c¸ u¨k, O ¨ .; Kocakerim. M. M.; Yartas¸ ı, A.; C¸ opur, M. Dissolution of Kestelek’s Colemanite Containing Clay Minerals in Water Saturated with Sulfur Dioxide. Ind. Eng. Chem. Res. 2002, 41, 2853-2857. (16) Levenspiel, O. Chemical Reaction Engineering, 3rd Edition; Wiley: New York, 1999; pp 566-586. (17) Avrami, M. Kinetics of Phase Change. J. Chem. Phys. 1939, 7, 1103.
ReceiVed for reView January 14, 2005 ReVised manuscript receiVed October 27, 2005 Accepted December 29, 2005 IE050050I
