Ind. Eng. Chem. Res. 2002, 41, 2853-2857
2853
Dissolution of Kestelek’s Colemanite Containing Clay Minerals in Water Saturated with Sulfur Dioxide O 2 zkan Ku 1 c¸ u 1 k, M. Muhtar Kocakerim, Ahmet Yartas¸ ı,* and Mehmet C ¸ opur Engineering Faculty, Chemical Engineering Department, Atatu¨ rk University, Erzurum, Turkey
Colemanite is one of the most important underground riches of Turkey, having approximately 60% of the world boron deposits, and it has a large portion in the deposits. When colemanite having a 2CaO‚3B2O3‚5H2O formula is mined naturally, it contains various clay minerals. During boric acid production by a sulfuric acid process, some kinds of clay minerals cause difficulties in the filtration process. In this study, the dissolution of Kestelek colemanite has been investigated in SO2-saturated water as an alternative method to the sulfuric acid process to remove the difficulties faced in filtration. The particle size, solid-to-liquid ratio, stirring speed, and reaction temperature have been chosen as parameters in the experiments. It was determined that the dissolution rate of colemanite increased with decreasing particle size and solid-to-liquid ratio and increasing reaction temperature but was unaffected by the stirring speed. The activation energy of the dissolution process was estimated to be 39.53 kJ‚mol-1. A semiempirical model was found by using experimental data and package programs, as follows: 1 - (1 - X)1/3 ) 3.423 × 104D-0.70(S/L)-0.65e-4754/Tt. Evaluation of the experimental data and semiempirical model shows that the dissolution process is controlled by a chemical reaction. Introduction Turkey has approximately 60% of the extent boron ores in the world. Boron is found as borates of metals, especially of calcium and sodium, in nature. Colemanite, one of the most common boron minerals, has a monoclinic crystal structure with a chemical formula of 2CaO‚ 3B2O3‚5H2O and is used usually in the production of boric acid.1 Borates are used commonly in many industries, such as chemical, metallurgy, nuclear engineering, etc. Commercially, extensively used boron compounds are boric acid, borax, borax hydrates, and sodium perborates. There are many studies on the dissolution, dissolution kinetics, and mechanism of boron minerals in various aqueous media. Kum et al.2 studied the leaching kinetics of calcinated colemanite in ammonium chloride solutions and found that the dissolution rate fit the homogeneous reaction model with 89 kJ‚mol-1 activation energy. O ¨ zmetin et al.3 investigating dissolution of colemanite in acetic acid solutions determined that the dissolution rate of colemanite obeyed the first-order pseudohomogeneous reaction model in the form of -ln(1 - X) ) kt. Gu¨lensoy and Kocakerim examined the dissolution of colemanite in CO2-saturated waters and proposed an opinion on the geological formation of this mineral.4 Kocakerim and Alkan studied the kinetics of dissolution colemanite in water saturated with SO2 and found that the dissolution rate was chemically controlled.5 Karago¨lge et al.6 investigated the leaching kinetics of colemanite in aqueous dissodium ethylenediaminetetraacetic acid (EDTA) solutions and calculated the activation energy as 50.6 kJ‚mol-1 and the preexponential factor as 5.14 × 107 m‚s-1. Temur et al. carried out the dissolution of colemanite in phosphoric acid solutions and concluded that the dissolution rate was a chemical reaction controlled process.7 Yartas¸ i et al. investigated the dissolution of colemanite in boric acid solutions and found it to be controlled by diffusion
through the product layer around the unreacted core of colemanite particles.8 Davies et al. examined boric acid production by leaching calcined colemanite in water saturated with CO2, and they demonstrated that precalcination of powdered colemanite can have a strongly beneficial effect on the extraction of boric acid during subsequential leaching of the calcine.9 In addition to these, there are many studies on colemanite and other boron minerals in different aqueous media containing various acids.10-19 When colemanite is mined naturally, it contains various clay minerals and impurities. The clay minerals and other impurities of Kestelek’s colemanite ore are of different properties in accordance with colemanite ores from other regions, and production of boric acid from it by the sulfuric acid process is a problem because of filtration difficulties. This situation affects the process negatively. In this study, the dissolution kinetics of Kestelek’s original colemanite containing various clay minerals was investigated in water saturated with SO2, and the effects of the parameters such as the particle size, reaction temperature, solid-to-liquid ratio, and stirring speed on the dissolution rate were determined. Experimental Section The colemanite mineral used in the study was supplied from Kestelek, Bursa in Turkey. After breaking and grinding, the colemanite was sieved to give 1.4001.000, 1.000-0.710, 0.710-0.355, 0.355-0.212, and 0.212-0.150 mm size fractions with ASTM standard sieves. Chemical analysis of the original sample gave a composition of 28.28% CaO, 39.80% B2O3, 17.15% H2O, and 14.77% others containing clay minerals. An X-ray diffractogram of the original sample obtained by a Rigaku DMAX 2000 series X-ray diffractometer is given in Figure 1. It is seen that the sample contains colemanite, quartz, dolomite, illite, calcite, and aluminum oxide.
10.1021/ie010356z CCC: $22.00 © 2002 American Chemical Society Published on Web 05/18/2002
2854
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002
Figure 1. X-ray diffractogram of the original sample.
Dissolution experiments were carried out in a 500 mL jacketed glass reactor, equipped with gas inlet and outlet tubes. The reactor contents was mixed by a mechanical stirrer with a tachometer, and its temperature was controlled by a constant-temperature circulator. In the experiments, 250 mL distilled water was saturated previously with SO2 at the desired experimental temperature. After the sample was added to the reactor, during the desired time period, SO2 was passed through the reaction mixture, and the mixture was stirred at a fixed speed. At the end of the dissolution period, the amounts of B2O3 passing to the solution during the reaction were determined by the spectrophotometrical method, which is known as the Carmine method.20 Results and Discussion Dissolution Reactions. The following reactions occur during the dissolving process:
4SO2(aq) + 4H2O(l) S 4H2SO3(aq)
(1)
4H2SO3(aq) + 4H2O(l) S 4H3O+(aq) + 4HSO3-(aq) (2) 2CaO‚3B2O3‚5H2O(s) + 4H3O+(aq) f 2Ca2+(aq) + 6H3BO3(aq) + 2H2O(l) (3) Hence, the overall reaction is
2CaO‚3B2O3‚5H2O(s) + 4SO2(aq) + 6H2O(l) f 2Ca2+(aq) + 4HSO3-(aq) + 6H3BO3(aq) (4) Effects of Parameters. In the experiments, while the effect of one parameter was examined, the values of other parameters were kept constant. The data obtained were plotted in the form of a conversion fraction, described as X ) amount of dissolved B2O3 in the sample/amount of B2O3 in original sample, versus time. To observe the effect of the particle size on the dissolution rate, experiments were carried out using four particle sizes of the sample (1.400-1.000, 1.0000.710, 0.710-0.355, 0.355-0.212, and 0.212-0.150 mm)
Figure 2. Effect of colemanite particle size on dissolution rates in SO2-saturated water (T, 30 °C; S/L, 0.04 g‚mL-1; W, 300 min-1).
at a reaction temperature of 30 °C, a solid-to-liquid ratio of 0.04 g‚mL-1, and a stirring speed of 300 min-1. As seen in Figure 2, the dissolution rate increases as the particle size decreases. The effect of temperature on the dissolution rate was studied using four reaction temperatures (18, 24, 30, and 40 °C) at particle sizes of 0.710-0.355 mm, a stirring speed of 300 min-1, and a solid-to-liquid ratio of 0.04 g‚mL-1. As seen in Figure 3, the dissolution rate of colemanite increases with increasing reaction temperature, although the solubility of SO2 decreases with temperature. This shows that the reaction temperature affects the dissolution rate positively. To observe the effect of the stirring speed on the dissolution rate, experiments were investigated using three stirring speeds (300, 400, and 500 min-1) at particle sizes of 0.710-0.355 mm, a reaction temperature of 30 °C, and a solid-to-liquid ratio of 0.04 g‚mL-1. The dissolution rate was observed to be independent of the stirring speed (Figure 4). To investigate the effect of the solid-to-liquid ratio on the dissolution rate, experiments were carried out using three solid-to-liquid ratios (0.02, 0.04, and 0.06 g‚mL-1) at particle sizes of 0.710-0.355 mm, a stirring speed of 300 min-1, and a reaction temperature of 30 °C. As seen in Figure 5, the dissolution rate decreases as the solidto-liquid ratio increases. Also, the residue from the
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2855
A(fluid) + bB(solid) f products, can be represented by one of two ideal models: the progressive-conversion model and the shrinking-core model.21 In the progressive-conversion model, it is visualized that a reactant liquid enters the particle and reacts throughout the particle at all times. In these cases, the reaction rate can be defined by pseudohomogeneous models, and in the shrinking-core model, it is derived integrated rate equations to show the reaction rate. In such a case, the particle size can remain unchanged or shrink. If it remains unchanged, the derived integrated rate equations are
t/t* ) X
Figure 3. Effect of temperature on colemanite reaction rates in SO2-saturated water (D, 0.710-0.355 mm; S/L, 0.04 g‚mL-1; W, 300 min-1).
(for diffusion control through a liquid film)
t/t* ) 1 - 3(1 - X)2/3 + 2(1 - X) (for diffusion control through ash or a product layer) t/t* ) 1 - (1 - X)1/3
(for surface chemical reaction control)
If it shrinks during the reaction, the diffusion through ash or a product layer is absent and the integrated rate equations are
(the diffusion control t/t* ) 1 - (1 - X)2/3 through a liquid film for small particles) t/t* ) 1 - (1 - X)1/2 (the diffusion control through a liquid film for large particles) t/t* ) 1 - (1 - X)1/3
Figure 4. Effect of stirring speed on the dissolution rate of colemanite in SO2-saturated water (D, 0.710-0.355 mm; T, 30 °C; S/L, 0.04 g‚mL-1).
Figure 5. Effect of solid-to-liquid ratio on the dissolution rate of colemanite in SO2-saturated water (D, 0.710-0.355 mm; T, 30 °C; W, 300 min-1).
dissolution process was analyzed by an X-ray diffractometer, and it was found that the residue consisted of montmorillonite, quartz, aluminum oxide, and illite (Figure 6). Kinetics Analysis. The kinetics of a noncatalytic reaction between a solid and a liquid, represented by
(for surface chemical reaction control)
In the presented study, it has been examined by statistical and graphical methods which of the above models fits with experimental data and it has been observed that the most appropriate model is surface chemical reaction control. Lines of 1 - (1 - X)1/3 versus t in Figure 7 for different reaction temperatures (18, 24, 30, and 40 °C) confirm that the surface chemical reaction control model is fitting for this process. Similar lines can be obtained for other parameters (solid-toliquid ratio and particle size). In accordance with these results, the semiempirical equation representing the kinetics of this process can be expressed as 1 - (1 X)1/3 ) kt. The dependence of k on the particle size, solid-toliquid ratio, and reaction temperature has been given as k ) koDa(S/L)be-E/RT, and the values of ko, a, b, and E/R have been found to be 3.423 × 104, -0.70, -0.65, and 4754, respectively. The activation energy was evaluated as 39.53 kJ‚mol-1. In a previous study,5 pure colemanite mineral was dissolved in SO2-saturated water, and the activation energy of this dissolution process was found to be 53.97 kJ‚mol-1. However, in this study carried out by Kestelek’s colemanite concentrate, the apparent activation energy was calculated as 39.53 kJ‚mol-1. When an Arrhenius plot is established by initial reaction rates from Figure 3, the activation energy of the process was found to be 53.76 kJ‚mol-1 (Figure 8), and this value is the same of that of pure colemanite. This result has been attributed to the composition of the concentrated colemanite containing 14.77% clay and quartz and 12.59% calcite
2856
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002
Figure 6. X-ray diffractogram of the undissolved solid part at the end of the reaction period.
Figure 7. Dissolution of colemanite in SO2-saturated water. Plots of 1 - (1 - X)1/3 versus different temperatures (D, 0.710-0.355 mm; S/L, 0.04 g‚mL-1; W, 300 min-1).
and dolomite. Calcite and dolomite dissolve rapidly in the SO2/H2O system as follows:
CaCO3(s) + 2 SO2(aq) + H2O(l) f Ca2+(aq) + 2HSO3-(aq) + CO2(g) (5) CaCO3‚MgCO3(s) + 4SO2(aq) + 2H2O(l) f Ca2+ (aq) + Mg2+(aq) + 4HSO3-(aq) + 2CO2(g) (6) Because quartz and clay minerals are not dissolved in this system, they gradually form a thick layer around the unreacted core of concentrated colemanite, but such a layer does not form in the case of the dissolution of pure colemanite. The clay layer occurring around the concentrated colemanite core gradually makes the dissolution of colemanite difficult by comparison to pure colemanite. However, when it is compared to the results of pure colemanite, it is seen that the dissolution of colemanite in the concentrated ore is more rapid. This case has been attributed to the fact that when it is
Figure 8. Arrhenius plot in accordance with the initial reaction rates from Figure 3.
studied by the same solid-to-liquid ratio, the solid-toliquid ratio of concentrated colemanite (as the colemanite content) is less than that of pure colemanite because concentrated ore contains impurities of about 27.36%. Because the smaller the solid-to-liquid ratio is, the more rapid the dissolution rate, the two opposite effects above cause the slope of graph 1 - (1 - X)1/3 versus t for the concentrated colemanite to be smaller than that of pure colemanite. As a result, a smaller apparent activation energy for concentrated colemanite than that of pure colemanite is not an unexpected case. Conclusions In the present technology, colemanite ore is reacted with sulfuric acid at 95 °C, and boric acid is crystallized from a saturated hot boric acid solution obtained by filtering the reaction mixture. Although H3BO3 is produced using colemanite from other mines of Turkey by this process, the colemanite ore from the Kestelek mine cannot be used because its impurities cannot be filtered. Using the SO2/H2O system instead of H2SO4 removes the problems faced in filtration and gives a
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2857
residue which can be filtered very easily. In addition to this, SO2 is cheaper than H2SO4. On the other hand, gypsum (CaSO4‚2H2O), a byproduct of the H2SO4 process, is unvaluable, but CaSO3, a byproduct of the SO2/ H2O system, can be used in various industries. For this reason, the SO2/H2O process offers a clean technology. Kestelek’s colemanite sample used in this study was not pure. An X-ray diffractogram showed that it contains aluminum oxide, quartz, dolomite, calcite, and some silicate minerals such as illite in addition to colemanite. Colemanite and dolomite dissolved during the dissolution process, but the other components did not. Also, montmorillonite has been seen in an X-ray diffractogram of the undissolved part. Untracing of montmorillonite in the original sample has been attributed to the fact that it has a small percent. Kestelek’s colemanite ore contains 72.64% colemanite, 14.77% clay minerals and quartz, and 12.59% calcite and dolomite. When the same solid-to-liquid ratio is used with pure colemanite, the actual solid-to-liquid ratio of Kestelek’s colemanite ore as colemanite is 0.7264 times the solid-to-liquid ratio of pure colemanite. For this reason, the dissolution rate of ore is bigger than that of pure colemanite in the same solid-to-liquid ratio. Also, when the ore is dissolved with SO2-saturated water, clay minerals disperse as the colloidal and particle size decreased. This case, also, causes an increase of the dissolution rate for all of the studied parameters. The colemanite content of Kestelek colemanite could be completely dissolved by SO2-saturated water. It was found that the rate of dissolution increases with decreasing particle size and solid-to-liquid ratio and with increasing reaction temperature but is unaffected by the stirring speed. For this dissolution process, a semiempirical kinetic model was obtained as follows:
1 - (1 - X)1/3 ) 3.423 × 104D-0.70(S/L)-0.65e-4754/Tt In the literature, the processes having activation energies above 40 kJ‚mol-1 were reported to be chemically controlled.23 According to this, also the activation energy calculated as 39.53 kJ‚mol-1 confirms that this process is chemically controlled. On the other hand, the kinetics of the dissolution of pure colemanite in water saturated with SO2 was studied by Kocakerim et al.,5 and they found that the dissolution process was chemically controlled and was calculated to be the activation energy of 53.97 kJ‚mol-1. In this study, obtaining a different activation energy in accordance with the dissolution of pure colemanite has been attributed to the fact that the original Kestelek colemanite ore contains various impurities and the solidto-liquid ratio of colemanite is lower than that of pure colemanite because of causes stated above. 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 ) particle size (mm) T ) reaction temperature (°C) S/L ) solid-to-liquid ratio (g‚mL-1)
Literature Cited (1) Garret, D. E. Borates; Academic Press Ltd.: New York, 1998. (2) Kum, C.; Alkan, M.; Kocakerim, M. M. Dissolution kinetics of calcined colemanite in ammonium chloride solution. Hydrometallurgy 1994, 36, 259-268. (3) O ¨ zmetin, C.; Kocakerim, M. M.; Yapıcı, S.; Yartas¸ ı, A. A semi-emprical kinetic model for dissolution of colemanite in aqueous CH3COOH solution. Ind. Eng. Chem. Res. 1996, 35 (7), 2355-2359. (4) Gu¨lensoy, H.; Kocakerim, M. M. Solubility of colemanite mineral in CO2-containing water and geological formation of this mineral. Bull. Miner. Res. Explor. Inst. Turk. 1979, 90, 1. (5) Kocakerim, M. M.; Alkan, M. Dissolution kinetics of colemanite in SO2-saturated water. Hydrometallurgy 1988, 19, 385392. (6) Karago¨lge, Z.; Alkan, M.; Kocakerim, M. M. Leaching kinetics of colemanite by aqueous EDTA solution. Metall. Trans. B 1992, 23B, 409-413. (7) Temur, H.; Yartas¸ ı, A.; C¸ opur, M.; Kocakerim, M. M. The kinetics of dissolution of colemanite in H3BO3 solution. Ind. Eng. Chem. Res. 2000, 39, 4114-4119. (8) Yartas¸ ı, A.; O ¨ zmetin, C.; Kocakerim, M. M.; Demirhan, M. H. Kinetics and mechanism of leaching colemanite in boric acid solution. Chim. Acta Turc. 1998, 26 (2), 7-13. (9) Davies, T. W.; C¸ olak, S.; Hooper, R. M. Boric acid production by the calcination and leaching of powdered colemanite. Powder Technol. 1991, 65, 433-440. (10) Imamutdinova, V. M. Rate of borate dissolution in acetic acid solution Zh. Prikl. Khim. 1970, 42 (2), 425-428. (11) Kononova, G. N.; Nozhko, E. S. Nature of the sulphuric acid dissolution of magnesium borates. Zh. Prikl. Khim. 1981, 54 (2), 397-399. (12) Alkan, M.; Kocakerim, M. M.; C ¸ olak, S. Dissolution kinetics of colemanite in water saturated by carbon dioxide. J. Chem. Technol. Biotechnol. 1985, 35A, 382-386. (13) Tunc¸ , M.; Kocakerim, M. M.; Gu¨r, A.; Yartas¸ ı, A. A semiempirical kinetic model for dissolution of ulexite in aqueous acetic acid solution. Energy, Educ., Sci. Technol. 1999, 3 (1), 1-10. (14) Tunc¸ , M.; Kocakerim, M. M; Gu¨r, A.; Yartas¸ ı, A. Dissolution of ulexite in sulphuric acid solution for high solid -to- liquid ratios. Energy, Educ., Sci. Technol. 1999, 3 (1), 32-41. (15) C¸ olak, S.; Wragg, A. A.; Davies, T. W.; Ekmekyapar, A. Effect of temperature and CO2 partial pressure on the dissolution of colemanite in CO2-saturated water. The 13th Mining Congress of Turkey, The Chamber of Mining Engineers of Turkey, Istanbul, May 10-14, 1993. (16) Morales, G. V.; Capretto, M. E.; Fuentes, L. M.; Quiraga, O. D. Dissolution kinetics of hydroboracite in water saturated with carbon dioxide. Hydrometallurgy 2000, 58, 127-133. (17) Demirbas¸ , A. Recycling of lithium from borogypsum by leaching with water and leaching kinetics. Resourc., Conserv. Recycl. 1999, 25, 125-131. (18) Erdogˇan, Y.; Aksu, M.; Demirbas¸ , A.; Abalı, Y. Analyses of boronic ores and sludges and solubilities of boron minerals in CO2-saturated water, Resourc., Conserv. Recycl. 1998, 24, 275283. (19) Ata, O. N.; C¸ olak, S.; C¸ opur, M.; C¸ elik, C. Determination of optimum conditions for boric acid extraction with carbon dioxide gas in aqueous media from colemanite containing arsenic. Ind. Eng. Chem. Res. 2000, 39, 488-493. (20) Greenberg, A. E.; Trussel, R. R.; Clesceri, L. S. Standard Methods for the Examination of Water and Wastewater, 16th ed.; American Public Health Association: Washington, 1985; pp 276277. (21) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley and Sons: New York, 1999; pp 566-586. (22) Bronikowski, T. Model selection for aqueous slurry coal desulphurization. Fuel 1984, 63 (1), 116-120. (23) Jackson, E. Hyrometallurgical Extraction and Reclamation; Ellis Horwood Ltd.: Chichester, U.K., 1986; p 46.
Received for review April 20, 2001 Revised manuscript received March 15, 2002 Accepted March 16, 2002 IE010356Z
