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Dissolution Kinetics of Ulexite in Ammonium Sulfate Solutions Asım Ku 1 nku 1 l, Nizamettin Demirkıran, and Ahmet Baysar* Department of Chemical Engineering, Faculty of Engineering, I˙ no¨ nu¨ University, 44069 Malatya, Turkey
The dissolution kinetics of ulexite in ammonium sulfate solutions was investigated. The effects of ammonium sulfate concentration, particle size, solid/liquid ratio, stirring speed, and reaction temperature on the dissolution rate have been evaluated. It was found that the dissolution rate increased with increasing ammonium sulfate concentration, stirring speed, and reaction temperature. However, increasing the particle size and solid/liquid ratio decreased the dissolution rate. Experimental data were examined by the heterogeneous and homogeneous models. The heterogeneous diffusion-controlled ash or product layer may describe the dissolution rate. The following mathematical model was used to represent the reaction kinetics: 1 - 3(1 - X)2/3 + 2(1 - X) ) 3.9 × 107C0.64D-0.85(S/L)-1.06ω0.40e-10050/Tt, where X is the fractional conversion, C the ammonium sulfate concentration, D the particle size, S/L the solid-to-liquid ratio, ω the stirring speed, T the reaction temperature, and t the reaction time. Introduction
Table 1. Parameters and Their Range Used in the Experiments
Boron is found in nature in the form of metal borates, mostly as sodium, calcium, and magnesium borates. Boron compounds are widely used in many branches of industry such as cosmetic, leather, textile, rubber, paint, and glass industries as well as medicine. Recently, the use of boron compounds has increased greatly because of its increasing demand in nuclear technology, in the glass and ceramic industries, as abrasives and refractors, in the production of heat-resistant polymers, as catalysts, etc. Among boron minerals, ulexite is one of the most common and is found in great amounts in Turkey. Ulexite is a sodium-calcium borate hydrate mineral Na2O‚2CaO‚5B2O3‚16H2O. Commercially, boric acid, boron oxides, and sodium perborate are the most utilized compounds of boron. Ulexite is one of the boron compounds used for the production of these compounds as a raw material.1-3 The dissolution kinetics of boron minerals in various solutions has been studied extensively.3-5 The dissolution kinetics of ulexite in ammonia solutions saturated with carbon dioxide,2 in aqueous ethylenediaminetetraacetic acid,6 in ammonium chloride solutions,7 in H2SO4 solutions,8,9 in water saturated with carbon dioxide,10 in water saturated with sulfur dioxide,11 and in water12 has been reported. Pocovi et al.13 have studied the dissolution of ulexite, hydroboracite, and colemanite in sulfuric and hydrochloric acids. In this study, we report the effect of five parameters, namely, concentration, particle size, solid/liquid ratio, stirring speed, and reaction temperature, on the dissolution rate of ulexite in ammonium sulfate solutions. Experimental Section The ulexite ore used in this work was obtained from Kırka, Eskis¸ ehir, Turkey. The material was first cleaned of visible impurities, crushed, and ground. After grinding, it was sieved to obtain different particle size fractions. The ore sample was analyzed, and it was determined that the mineral contained 42.08% B2O3, * To whom correspondence should be addressed. E-mail:
[email protected].
parameter concn of ammonium sulfate (mol L-1) particle size (mm) solid/liquid ratio (g mL-1) stirring speed (s-1) temperature (°C)
values 0.1, 0.25, 0.5, 1.0a 0.850-0.425, 0.425-0.250,a 0.250-0.180, 0.180-0.150, 0.150-0.120 0.01,a 0.02, 0.04 2.500, 4.167, 7.500,a 10.833 20, 25, 30,a 35, 40, 45
a The values used when the effect of the other parameters was investigated.
13.98% CaO, 7.95% Na2O, 35.82% H2O, and 0.17% insoluble matter. The parameters that were expected to affect the dissolution rate were chosen as the ammonium sulfate concentration, particle size, solid/liquid ratio, stirring speed, and reaction temperature. The ranges of parameters are given in Table 1. The experiments were carried out in a 500 mL spherical glass reactor equipped with a mechanical stirrer, a reaction temperature control unit, and a cooler to avoid loss of solution by evaporation. The experimental procedure was as follows: A total of 225 mL of an ammonium sulfate solution was placed in the glass reactor. The reactor jacket was heated to the desired temperature, and the stirring speed was set. A given amount of solid sample was added to the solution. The dissolution process was carried out for various reaction times. At the end of reaction, the contents were filtered and the amount of B2O3 was determined in the solution.14 Each experiment was repeated twice, and the arithmetic average of the results was used in kinetic modeling. The results could be repeated with a maximum of about (2% variation in terms of the dissolution fraction. Results and Discussion The dissolution rate of ulexite was measured as a function of time by changing the ammonium sulfate concentration, particle size, solid/liquid ratio, stirring speed, and reaction temperature. In the experiments, the effect of one parameter was examined while the other parameters were kept constant. The results were
10.1021/ie020666x CCC: $25.00 © 2003 American Chemical Society Published on Web 01/30/2003
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Figure 1. Ammonium sulfate concentration effect on the dissolution rate.
Figure 4. Stirring speed effect on the dissolution rate.
Figure 2. Particle size effect on the dissolution rate.
Figure 5. Temperature effect on the dissolution rate.
Figure 3. Solid/liquid ratio effect on the dissolution rate.
plotted as a function of conversion (X ) amount of dissolved B2O3 in the solution/amount of B2O3 in the original sample) versus time. To determine the effect of the ammonium sulfate concentration on the dissolution rate, experiments were carried out at 0.1, 0.25, 0.5, and 1.0 mol L-1 concentrations while the particle size, solid/liquid ratio, stirring speed, and reaction temperature were kept constant at 0.425-0.250 mm, 1/100 g mL-1, 7.5 s-1, and 30 °C, respectively. The results plotted in Figure 1 show that
Figure 6. X-ray diffractogram of leached ulexite up to about 15% conversion.
the dissolution rate increases with an increase in the ammonium sulfate concentration. The effect of the particle size on the dissolution rate was determined by using 0.850-0.425, 0.425-0.250, 0.250-0.180, 0.180-0.150, and 0.150-0.125 mm fractions while the ammonium sulfate concentration, solid/ liquid ratio, stirring speed, and reaction temperature were kept constant at 1.0 mol L-1, 1/100 g mL-1, 7.5 s-1, and 30 °C, respectively. Figure 2 shows that as the particle size decreases the dissolution rate increases because of increased surface area.
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Figure 7. Variation of t/t* versus reaction time for (a) various ammonium sulfate concentrations, (b) different particle sizes, (c) different solid/liquid ratios, (d) different stirring speeds, and (e) different reaction temperatures.
The effect of the solid/liquid ratio was determined by carrying out experiments at 1/100, 2/100, and 4/100 g mL-1 while the ammonium sulfate concentration, particle size, stirring speed, and reaction temperature were kept constant at 1.0 mol L-1, 0.425-0.250 mm, 7.5 s-1, and 30 °C, respectively. As expected, Figure 3 shows that the dissolution rate decreases with increased solid/liquid ratio.
To observe the effect of the stirring speed on the dissolution rate, tests were carried out at 2.5, 4.167, 7.5, and 10.833 s-1 while the ammonium sulfate concentration, particle size, solid/liquid ratio, and reaction temperature were kept constant at 1.0 mol L-1, 0.425-0.250 mm, 1/100 g mL-1, and 30 °C, respectively. The results in Figure 4 show that the stirring speed increases the dissolution rate.
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The effect of the reaction temperature on the dissolution rate was also determined. Reaction temperatures at 20, 25, 30, 35, 40, and 45 °C were tested while the ammonium sulfate concentration, particle size, solid/ liquid ratio, and stirring speed were kept constant at 1.0 mol L-1, 0.425-0.250 mm, 1/100 g mL-1, and 7.5 s-1, respectively. Figure 5 shows that as the solution temperature is increased the dissolution rate also increases. This is expected from the exponential dependence of the rate constant in the Arrhenius equation. Kinetic Analysis The rate of a reaction between a solid and a fluid such as the system considered here can be expressed by homogeneous and heterogeneous models.15 The data obtained in this work were analyzed for the homogeneous model, but the homogeneous model was found inappropriate. The heterogeneous reaction model assumes that the rate may be controlled by diffusion through a fluid film, by diffusion through the ash or product layer, or by a surface chemical reaction. Levenspiel15 gives rate equations for each of the above control mechanisms. For spherical particles, the fractional conversion, X, as a function of the reaction time, t, is given by
t/t* ) 1 - (1 - X)1/3
Figure 8. Arrehenius plot for the dissolution process.
(1)
for surface chemical reaction control,
t/t* ) X
(2)
for film diffusion control, and
t/t* ) 1 - 3(1 - X)2/3 + 2(1 - X)
(3)
for diffusion control through the ash or product layer. The application of the above models to the experimental data will help to find the kinetics of the dissolution process. Experimental data did not fit to any heterogeneous rate models except the diffusion through the ash or product layer model. The evidence for this proposal is that the constituents forming ulexite are completely soluble in an aqueous solution, but Ca2+ ions as the dissolution continues form CaSO4‚2H2O precipitate. This precipitate forms an insoluble layer, which covers the surface of the particles. After 15% of ulexite was dissolved, the solid part was separated from the solution and the X-ray diffractogram (Rigaku X-ray diffractometer using Cu KR radiation) was taken. Figure 6 shows that the solid phase consists of ulexite and CaSO4‚2H2O phases. Figure 4 shows that the dissolution rate increases with increased stirring speed. If the dissolution were controlled by the chemical reaction, the stirring speed would not have much effect on the dissolution rate. Therefore, the dissolution process is taught to be controlled by diffusion through the ash or product layer. Using the diffusion control through the ash or product layer model, the right-hand side of eq 3 or the t/t* value is plotted against the reaction time. Figure 7 shows the plots of t/t* versus reaction time. As seen, all of the plots in Figure 7 are linear and pass through the origin, indicating that the assumed model appropriately represents the dissolution process. Parts a-e of Figure 7 represent the graphs for the ammonium sulfate concentration, particle size, solid/liquid ratio, stirring speed, and reaction temperature, respectively.
Figure 9. Plot of theoretical conversion rates against experimental values.
The dependence of the rate constant on the concentration, particle size, solid/liquid ratio, stirring speed, and reaction temperature may be given by
k ) k0CaDb(S/L)cωde-E/RT
(4)
Rate constant values obtained from Figure 7a-d were used to calculate constants a, b, c, and d in eq 4. Average calculated values were a ) 0.64, b ) -0.85, c ) -1.06, and d ) 0.40. To determine the activation energy of the dissolution reaction, a ln k versus 1/T plot was constructed. Figure 8 shows a straight line. The slope of this line gives the E/R value, and the intercept is k0. The values of E/R and k0 were found to be 10 050 and 3.9 × 107, respectively. To determine the compatibility of the model with the experimental data, a plot of theoretical conversion versus experimental conversion is given in Figure 9. The agreement between theoretical and experimental values is found to be very good. Conclusions In this work, the dissolution kinetics of ulexite in ammonium sulfate solutions was investigated. It was determined that the dissolution rate increased with increased concentration, stirring speed, and reaction temperature. The rate decreased with higher solid/liquid ratio and particle size. The dissolution process was described by the heterogeneous diffusion control through the ash layer or product layer model. The mathematical
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form of the model is 1 - 3(1 - X)2/3 + 2(1 - X) ) 3.9 × 107C0.64D-0.85(S/L)-1.06ω0.40e-10050/Tt. The activation energy of the reaction was found to be about 83.5 kJ mol-1. Acknowledgment This work was supported by I˙ no¨nu¨ University Research Fund (Project No. 2001/69). Nomenclature X ) fractional conversion of B2O3 C ) ammonium sulfate concentration (mol L-1) D ) particle size (mm) S/L ) solid/liquid ratio (g mL-1) ω ) stirring speed (s-1) T ) temperature (K) t ) reaction time (s) t* ) reaction time for complete conversion
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(5) Imamutdinova, V. M. Rates of Dissolution of native borates in H3PO4 Solutions. Zh. Prikl. Khim. 1967, 40 (11), 2596. (6) Alkan, M.; C¸ ifc¸ i, C.; Ayaz, F.; Dogˇan, M. Dissolution kinetics of ulexite in aqueous EDTA solutions. Can. Metall. Q. 2000, 39, 433. (7) Tekin, G.; Onganer, Y.; Alkan, M.; Dissolution kinetics of ulexite in ammonium chloride solution. Can. Metall. Q. 1998, 37 (2), 91-97. (8) Tunc¸ , M.; Kocakerim, M. M.; Yapıcı, S.; Bayrakc¸ eken, S. Dissolution mechanism of ulexite in H2SO4 solution. Hydrometallurgy 1999, 51, 359-370. (9) Ku¨nku¨l, A.; Tunc¸ , M.; Yapıcı, S.; Ers¸ ahan, H.; Kocakerim, M. M.; Dissolution of thermally dehydrated ulexite in H2SO4 solution. Ind. Eng. Chem. Res. 1997, 36, 4847. (10) Kocakerim, M. M.; C¸ olak, S.; Davies, T.; Alkan, M. Dissolution kinetics of ulexite in carbon dioxide saturated water. Can. Metall. Q. 1993, 32, 393. (11) Alkan, M.; Kocakerim, M. M. Dissolution kinetics of ulexite in water saturated by sulphur dioxide. J. Chem. Technol. Biotechnol. 1987, 40, 215. (12) Cheng, R. Y.; Xia, S. P.; Feng, S. H.; Gao, S. Y. Study on dissolution and transformation ulexite in water. Chin. J. Inorg. Chem. 1999, 15 (1), 125. (13) Pocovi, R. E.; Latre, A. A.; Skaf, O. A. Lixiviacio´n de ulexita, hydroboracita y colemanita con a´cidos sulfu´rico y clorhı´drico. Instituto de Beneficios de Minerales, Universidat Nacional de Salta, 1992. (14) Scott, W. W. Standard Methods of Chemical Analysis; Van Nostrand: New York, 1963. (15) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley & Sons: New York, 1972.
Received for review August 26, 2002 Revised manuscript received December 7, 2002 Accepted December 7, 2002 IE020666X
