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Ind. Eng. Chem. Res. 2005, 44, 1728-1733
Dissolution of Ulexite-Containing Clay Minerals in Sulfur Dioxide-Saturated Water O 2 zkan Ku 1 c¸ u 1 k* and M. Muhtar Kocakerim Department of Chemical Engineering, Faculty of Engineering, Atatu¨ rk University, 25240 Erzurum, Turkey
Turkey has approximately 60% of the boron ores of the world. Boron in general is found as calcium or sodium borates or both in nature. Ulexite, which is one of the most widely available boron minerals, has the chemical formula of Na2O‚2CaO‚5B2O3‚16H2O and triclinic crystal structure. In addition, it is generally found to contain some clay minerals, which cause trouble in filtration processes. The dissolution characteristics of ulexite in water saturated with SO2 was investigated, and a discussion about whether this new process can provide an alternative in terms of the products obtained was made in the current study. The parameters selected for the study are particle size, reaction temperature, solid-to-liquid ratio, and stirring speed. The ulexite dissolution rate was found to increase with decreasing particle size and solid-to-liquid ratio and with increasing reaction temperature, while it was not affected by stirring speed. The activation energy of the dissolution process was 49.87 kJ‚mol-1. A model equation was constructed for the process. Evaluation of the experimental data and the mathematical model revealed the dissolution process to be controlled by chemical reaction. Furthermore, the solid waste of the process was found to be easily and quickly filterable, and the boron lost was less when compared to sulfuric acid and other similar processes. The byproduct of the proposed process, CaSO3, can be used in many different industries, as an antichlor in the textile industry, as a disinfectant in the sugar industry, in brewing, in biological cleansing, in preservation, and as a discoloration retarder in the food industry. In addition, this process can be said to be an environmentally friendly one. Introduction Borates are widely used in chemical, metallurgical, and nuclear engineering and in many related industries. The most common boron compounds commercially used are boric acid, anhydrous borax, borax hydrates, and sodium perborates, which are generally obtained from colemanite, ulexite, and tincal. Turkey has the largest fraction of the world’s boron reserves. Ulexite together with other borates is mainly found in Bigadic¸ town in Balıkesir province and Emet town in Ku¨tahya province in Turkey. There have been many investigations on the kinetics and mechanisms of the dissolution of boron minerals in aqueous solutions. Imamutdinova investigated the dissolution of boron minerals such as inyoite, ulexite, colemanite, and hydroboracite by using various mineral acids.1 She reported that the dissolution curves of the borates were the same type with respect to all the mineral acids used, and the maximal dissolution changed according to the nature of the mineral acid as well as the medium temperature. In the studies in which the dissolution kinetics of some boron minerals was investigated in basic media, films of metal oxides or metal hydroxides have been observed to form on the surface of product crystals at base concentrations above certain values, and these films appeared to limit the dissolution.2-4 Many investigations, most of which have been patented, have been conducted to produce various boron compounds from boron minerals.5-8 In another study in which borax production from ulexite ore was inves* To whom correspondence should be addressed. Tel.: +90 442 231 45 76. Fax: +90 442 236 09 57. E-mail: okucuk@ atauni.edu.tr.
tigated, the dissolution of natural and calcinated ulexite in sodium carbonate-sodium bicarbonate solutions was investigated comparatively.9 The investigators found that the ore achieved activity and the B2O3 concentration was higher when it was calcinated. Imamutdinova and Bikchurova10 studied the dissolution of inyoite, ulexite, colemanite, and hydroboracite minerals in HNO3 and proved the mechanism proposed earlier by Imamutdinova1 to be true. According to the mechanism, a H3BO3 product film forms on the crystals to be dissolved, which limits the dissolution. They also reported the dissolution process to be diffusional-type based on the calculated rate constants and activation energies. In another study in which the dissolution kinetics of ulexite in ammonia solutions saturated with CO2 was investigated, the investigators concluded that the dissolution rate of ulexite can be defined according to pseudo-first-order kinetics.11 They reported the activation energy to be 55 kJ‚mol-1. O ¨ zmetin et al.,12 investigating dissolution of colemanite in acetic acid solutions, determined that the dissolution rate of colemanite obeyed the first-order pseudo-homogeneous reaction model in the form of -ln(1 - X) ) kt. 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.13 Yapıcı et al. studied the optimum conditions for the dissolution of ulexite in water saturated with CO2 to obtain boric acid.14 Kocakerim et al. investigated the effect of particle size and temperature on dissolution rate in another study in which water saturated with CO2 was used to dissolve ulexite.15 Alkan and Kocakerim conducted research to determine the dissolution kinetics of pure ulexite in water saturated with SO216 and found that
10.1021/ie049271z CCC: $30.25 © 2005 American Chemical Society Published on Web 02/04/2005
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Figure 1. X-ray diffractogram of the original sample.
the process was controlled by chemical reaction and the activation energy was 58.01 kJ‚mol-1. Ku¨c¸ u¨k et al. studied the dissolution kinetics of Kestelek’s colemanite containing clay in water saturated with SO217 and found that the dissolution rate was controlled by chemical reaction and the activation energy for the process was 39.53 kJ‚mol-1. Ulexite in natural form generally contains clay minerals and other impurities. The clay poses filtration difficulties. In the current study, the dissolution kinetics of original ulexite containing various clay minerals in SO2-saturated water was investigated, and the effects of particle size, reaction temperature, solid-to-liquid ratio, and stirring speed on dissolution were determined. In addition, monosodium pentaborate was produced from the solution obtained from the dissolution of ulexite. Experimental Method The ulexite mineral used in the study was obtained from Bigadic¸ town of Balıkesir Province in Turkey. The sample mineral was first broken into small pieces, ground, and sieved with ASTM standard sieves to have three samples with the particle sizes of -1.700 + 0.850, -0.850 + 0.425, and -0.300 + 0.212 mm. The original sample was tested for chemical composition and found to have 35.85% B2O3, 15.22% CaO, 6.38% Na2O, 29.67% H2O, 5.38% MgO, and other components containing 7.5% SiO2 and clay minerals. The X-ray diffractogram of the original sample was taken and is given in Figure 1. Dissolution experiments were performed in a 500 mL jacketed glass reactor equipped with gas inlet and outlet tubing. Reactor content was stirred with a mechanical stirrer with tachometer, and the temperature was controlled with a constant temperature circulator. In all the experiments, first 100 mL of distilled water was saturated with SO2, and then the sample was transferred into the reactor. In addition, SO2 gas was passed through the reaction mixture, and the mixture was stirred at a certain rate during all reaction period. The amount of B2O3 passing into solution was determined by titration at the end of the reaction period.18
Results and Discussion Dissolution Reactions. The reactions taking place during the dissolution process were as follows:
SO2(aq) + H2O(l) T H2SO3(aq)
(1)
H2SO3(aq) + H2O(l) T H3O+(aq) + HSO3-(aq)
(2)
Na2O‚2CaO‚5B2O3‚16H2O(s) + 6H3O+(aq) f 2Na+(aq) + 2Ca2+(aq) + 10H3BO3(aq) + 10H2O(l) (3) Thus, the overall reaction is
Na2O‚2CaO‚5B2O3‚16H2O(s) + 6SO2(aq) + 2H2O(l) f 2Na+(aq) + 2Ca2+(aq) + 6HSO3-(aq) + 10H3BO3(aq) (4) When the solution obtained was boiled, the following reaction occurs.
2Na+(aq) + 2Ca2+(aq) + 6HSO3-(aq) + 10H3BO3(aq) + (2x - 18)H2O(l) f 2CaSO3‚xH2O(s) + 2Na+(aq) + 2B5O8-(aq) + 4SO2(g) (5) The Effects of Parameters. While the effect of one of the parameters was being determined, the values of all the others were kept constant. The data obtained were plotted as time vs conversion fraction (the amount of B2O3 taken into solution/the amount of B2O3 in the original sample). The effect of particle size on dissolution rate was investigated by using three different particle sizes (1.700 ( 0.850, 0.850 ( 0.425, and 0.300 ( 0.212 mm). In these experiments, the reaction temperature was kept constant as 18 °C, solid/liquid ratio as 0.04 g/mL, and stirring speed as 400 min-1. The results, as depicted in Figure 2, show that the dissolution rate increases as particle size decreases.
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Figure 2. The effect of particle size on the dissolution rate of ulexite in SO2-saturated water (T ) 18 °C; S/L ) 0.04 g‚mL-1; W ) 400 min-1).
Figure 3. The effect of reaction temperature on the dissolution rate of ulexite in SO2-saturated water (D ) 1.275 mm; S/L ) 0.04 g.mL-1; W ) 400 min-1).
The effect of reaction temperature on dissolution rate was investigated by using the reaction temperatures 4, 11, 18, 25, and 30 °C and keeping the other parameters constant: particle size as 1.700 ( 0.850 mm, stirring speed as 400 min-1, and solid/liquid ratio as 0.04 g/mL. According to the results given in Figure 3, the dissolution rate of ulexite increased as the temperature was increased, though the solubility of SO2 decreases by increasing temperature. This finding was attributed to the positive effect of reaction temperature on dissolution rate. The effect of stirring speed on dissolution rate was investigated by using the stirring speeds of 300, 400, and 500 rpm and keeping the other parameters constant: particle size as 1.700 ( 0.850 mm, reaction temperature as 18 °C, and solid/liquid ratio as 0.04 g/mL. The results of these experiments shows that the stirring speeds do not have any effect on dissolution rate. To investigate the effect of solid/liquid ratio on dissolution rate, the solid/liquid ratios of 0.01, 0.02, 0.04, and 0.06 g/mL were employed. During these experi-
ments, the particle size was kept constant at 1.700 ( 0.850 mm, temperature at 18 °C, and stirring speed at 400 min-1. The results, as depicted in Figure 4, show that dissolution rate decreased as solid/liquid ratio increased. The X-ray diffractogram of the solid residue obtained at the end of the dissolution process showed that it contained calcium sulfite, magnesium sulfite, cliniptilolite (montmorillonite), quartz, Al2O3, and illite. Kinetic Analysis. The reaction kinetics of a noncatalytically progressing reaction between a solid and a liquid such as A(fluid) + bB(solid) f Products can be explained by one of the following two models defined: progressive conversion model and shrinking core model.19 In the progressive conversion model, the liquid reactant is assumed to enter into the solid particle and react at all instances all around it. Under these conditions, the reaction rate can be defined by pseudo-first-order models. With the shrinking sphere model, integrated rate equations have been derived to show reaction rates. Here the particle size can stay constant or decrease. If the particle size stays constant, the integrated rate
Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1731
Figure 4. The effect of solid-to-liquid ratio on the dissolution rate of ulexite in SO2-saturated water (D ) 1.275 mm; T ) 18 °C ; W ) 400 min-1).
equations derived are as follows:
t/t* ) X, (diffusion-controlled from liquid film) (6) t/t* ) 1 - 3(1 - X)2/3 + 2(1 - X), (diffusion-controlled from ash or product film) (7) t/t* ) 1 - (1 - X)1/3, (surface chemical reaction-controlled) (8) If particle size decreases during reaction, there will be no ash or product layer and the integrated rate equations will be as follows:
t/t* ) 1 - (1 - X)2/3, (diffusion-controlled from liquid film for small particles) (9) 1/2
t/t* ) 1 - (1 - X) , (diffusion-controlled from liquid film for large particles) (10) t/t* ) 1 - (1 - X)1/3, (surface chemical reaction-controlled) (11) The suitability of the experimental data and the abovementioned models were investigated with statistical and graphical methods in the current study, and the chemical reaction-controlled model was found to be suitable. The graphical lines seen in Figure 5 for 1 - (1 - X)1/3 vs t at the reaction temperatures of 4, 11, 18, and 25 °C confirm the suitability of the surface chemical reactioncontrolled model. Similar analyses were performed similarly for solid/liquid ratio and particle size. The mathematical model to represent the kinetics of this process can be expressed as
1 - (1 - X)1/3 ) kt
(12)
The relationship of k in this equation with reaction temperature, solid/liquid ratio, gas concentration, and particle size can be given as
k ) k0(D)a(S/L)b e-E/(RT)
(13)
where the values of k0, a, b, c, and E/R were found to be 2.3 × 1010, -0.877, -0.363, and 5998. In addition,
the activation energy of the dissolution process was determined as 49.87 kJ‚mol-1. In an investigation (Alkan 1987),16 for the dissolution of pure ulexite in water saturated with SO2, the activation energy has been found as 58.01 kJ‚mol-1. To investigate the difference between the activation energies of pure and original ulexite, the initial reaction rates were obtained from Figure 3 and plotted against 1/T values. The activation energy was determined from this graph to be 58.45 kJ‚mol-1, which was same as the value obtained with pure ulexite. This finding was attributed to the composition of concentrated ulexite containing 7.5% clay and quartz and 9.07% calcite and dolomite. Calcite and dolomite are readily dissolved in SO2-H2O systems according to the following reactions:
CaCO3(s) + 2SO2(aq) + H2O(l) f Ca2+(aq) + 2HSO3-(aq) + CO2(g) (14) CaCO3‚MgCO3(s) + 4SO2(aq) + 2H2O(l) f Ca2+(aq) + Mg2+(aq) + 4HSO3-(aq) + 2CO2(g) (15) Clay and quartz minerals, because of their insolubilities in this system, form a thickening layer around the ulexite particle. However, no such layer is observed when pure ulexite is dissolved. The formation of this clay layer makes the dissolution of concentrated ulexite relatively difficult compared to pure ulexite. Thus, the lower activation energy is not unexpected with concentrated ulexite in comparison to pure ulexite. Conclusion The ulexite content of the original ulexite was totally dissolved in SO2-saturated water. The dissolution rate was found to increase with decreasing particle size and solid/liquid ratio and with increasing reaction temperature and to be unaffected by stirring speed. The semiempirical model for the dissolution process was obtained as follows:
1 - (1 - X)1/3 ) 2.3 × 1010(D)-0.877 (S/L)-0.363 e-5998/T T (16)
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Figure 5. The dissolution of ulexite in SO2-saturated water (D ) 1.275 mm; S/L ) 0.04 g‚mL-1; W ) 400 min-1).
Figure 6. The experimental and theoretical dissolution fraction data for all the parameters.
The theoretical values of XB2O3 for various parameters for all time values were calculated and plotted against the corresponding experimental XB2O3 values (Figure 6). The activation energy of the current dissolution was determined to be 49.87 kJ‚mol-1 from the mathematical equation defining the process. The increasing effect of reaction temperature and ineffectiveness of stirring rate on dissolution process, the straight lines obtained when t vs 1 - (1 - X)1/3 plots are drawn, the relatively better regression constants obtained with the chemical reaction-controlled model, and the attribution of the activation energies of and over 40 kJ‚mol-1 in processes to their chemically controlled nature20 all show the current process to be controlled by chemical reaction on the particle surface. The difference in activation energy obtained in the current study in comparison to an earlier study conducted with pure ulexite was attributed to various impurities existing in the current sample. By this process, after ulexite was dissolved and undissolved parts were filtered, monosodium pentaborate can be crystallized from filtrate. Byproduct, CaSO3‚ xH2O mixed with clay minerals, can be separated from clay by dissolving with SO2 in aqueous medium and obtained as pure.
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) a,b ) constants in eq 13 E ) activation energy (kJ‚mol-1) k ) reaction rate constant
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Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1733 (4) Strezhneva, I. I.; Tkachev, K. V. Kinetics of the Reaction of Some Borates with Soda in a Solution. Tr. Ural’sk. N.-I˙ . Khim. In-ta, 40, 52-59; Chem Abs. 1977, 88 (12), 79782e. (5) Wiseman, J. Process for the Manifacture 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. Rev. Fac. Sci. Istanbul 1952, 17, 191-195. (7) Meixner, H. New Turkish Borate Deposits. Hachschule Leoben. 98, 86-92; Chem. Abs. 1952, 47, 10413f. (8) Meixner, H. Mineralogical Observation of Colemanite, Inyoite, Meyerhofferite, Tertschite and Ulexite from Turkish Borate Deposits. Petros 1955, 3, 445-455; Chem. Abs. 49-787 g. (9) Demirciogˇlu, A.; Gu¨lensoy, H. The Yield Studies in the production of Borax from the Turkish Ulexite Ores, Chim. Acta Turc. 1977, 5 (1), 83-91. (10) Imamutdinova, V. M.; Bikchurova, A. Kh. Kinetics of dissolution of borates in HNO3 solutions. Zh. Prikl. Khim. 1967, 40 (7), 1616-1618. (11) Ku¨nku¨l, A.; Yapıcı, S.; Kocakerim, M. M.; C¸ opur, M. Dissolution kinetics of ulexite in ammonia solutions saturated with CO2. Hydrometallurgy 1977, 5, 145. (12) O ¨ zmetin, C., Kocakerim, M. M., Yapıcı, S.; Yartas¸ ı, A. A Semiemprical Kinetic Model for Dissolution of Colemanite in Aqueous CH3COOH Solutions. Ind. Eng. Chem. Res. 1996, 35 (7), 2355-2359. (13) 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.
(14) Yapıcı, S.; Kocakerim, M. M.; Ku¨nku¨l, A. Optimization of production of H3BO3 from ulexite. Turkish J. Eng. Env. Sci. 1994, 18, 91-94. (15) Kocakerim, M. M.; C¸ olak, S.; Davies, T.; Alkan, M. Dissolution kinetics of ulexite in CO2-saturated water. Can. Metall. Q. 1993, 32 (4), 393-396. (16) Alkan, M.; Kocakerim, M. M. Dissolution Kinetics of Ulexite in Water Saturated by Sulphur Dioxide. J. Chem. Technol. Biotechnol. 1987, 40, 215-222. (17) 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 Sulphur Dioxide. Ind. Eng. Chem. Res. 2002, 41, 2853-2857. (18) Gu¨lensoy, H. Kompleksometrinin Esasları ve Kompleksometrik Titrasyonlar, I˙ stanbul U ¨ niversitesi Yayıları: I˙ stanbul, 1984; p 250. (19) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley and Sons: New York, 1999; pp 566-586. (20) Jackson, E., Hyrometallurgical Extraction and Reclamation; Ellis Horwood Ltd.: Chichester, U.K., 1986; p 46.
Received for review August 11, 2004 Revised manuscript received December 15, 2004 Accepted December 17, 2004 IE049271Z
