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The Kinetics of Dissolution of Colemanite in H3PO4 Solutions Hakan Temur, Ahmet Yartas¸ ı, Mehmet C ¸ opur,* and M. Muhtar Kocakerim Department of Chemical Engineering, Engineering Faculty, Atatu¨ rk University, Erzurum, Turkey
The dissolution kinetics of colemanite in phosphoric acid solutions was studied. The effects of particle size, temperature, acid concentration, solid-to-liquid ratio, and stirring speed on the dissolution rate were determined. It was observed that the dissolution rate increased with decreasing particle size and solid-to-liquid ratio and with increasing temperature, but stirring speed had no effect on it. The dissolution rate increased up to an acid concentration of 19.52% (by wt) and then decreased with increasing acid concentration. The dissolution kinetics of colemanite were examined according to the heterogeneous and homogeneous reaction models for the acid concentration range of 1.43-19.52% (by wt) of acid concentration, and it was found that the dissolution rate was controlled by surface chemical reaction. The activation energy of this process was determined to be 53.91 kJ mol-1. Introduction
Table 1. Chemical Analysis of Colemanite Mineral Used in This Study
Boron is present in the form of metal borates, mostly in sodium, calcium, magnesium, and sodium calcium borates, but no free boron is found in nature. Colemanite, a calcium borate hydrate, has a chemical formula of 2CaO‚3B2O3‚5H2O. Boron compounds are very commonly used in a number of industrial applications in a variety of ways. The production of boron compounds has substantially increased recently because of increasing demand for them in nuclear technology, in the glass and ceramic industry, as abrasives and refractors, in agriculture, in the production of heat-resistant polymers, as catalysts, etc.1 In the production of boric acid by the classical method, gypsum forms as a byproduct through the reaction between colemanite and sulfuric acid. When it is allowed to escape into the environment, it causes soil and water pollution because of its boron content, as heavy metals in soil and water react with boron and form toxic compounds. These toxic compounds pollute soil and water, especially groundwater. To avoid this pollution, the reaction between colemanite and phosphoric acid was considered. Because the Ca(H2PO4)2, which is a nutrient for plants, that is formed as a byproduct of this reaction will include less boron, its fertilizer value will increase. Many studies related to dissolution of colemanite mineral in various media have been performed. O ¨ zmetin et al.2 studied the dissolution of colemanite in aqueous acetic acid solutions and found that the reaction fit a model of the form -ln(1 - X) ) kt. They also calculated the activation energy of process was to be 51.49 kJ mol-1. Imamutdinova3 performed the dissolution of four native borates (colemanite, ulexite, inyoite, and hydroboracite) in phosphoric acid solutions. He found that the dissolution of borates in phosphoric acid is diffusional in character. The dissolution kinetics of colemanite were investigated in CO2-saturated water by Alkan et al.4 and in SO2-saturated water by Kocakerim et al.,5 and they found that the dissolution of colemanite fit chemicalreaction-controlled kinetics with activation energies of 57.7 and 53.97 kJ mol-1, respectively. In another study, the dissolution kinetics of colemanite were examined in aqueous EDTA solutions.6 It was found that the dissolution rate of colemanite increased with decreasing
component
%
CaO B2O3 H2O SiO2 As2S3 others
27.73 48.33 22.28 0.26 0.0045 1.3955
Table 2. Particle Sizes Used in the Experiments and Amounts of B2O3 particle size (µm)
B2O3 (%)
1400-1000 1000-710 300-250
50.37 48.54 48.33
particle size and pH and with increasing temperature and solution concentration, and the activation energy of process was evaluated to be 50.6 kJ mol-1. The dissolution kinetics of colemanite in boric acid solutions were investigated by Yartas¸ ı et al.,7 and it was determined that the dissolution of colemanite was controlled by diffusion through the product layer around the unreacted core of colemanite particles with an activation energy 28.61 kJ mol-1. Some studies concerning the dissolution kinetics of boron minerals in solutions of acetic acid8 and in CO2-saturated water were also performed.9 In the present study, the dissolution kinetics of colemanite in phosphoric acid solutions were studied, and an attempt was made to derive a mathematical expression representing the dissolution process for chosen parameters. Experimental Section The colemanite used in the study was obtained from the region of Emet-Ku¨tahya, Turkey. The colemanite was crushed, ground, and then sieved by ASTM standard sieves to obtain particle size fractions of 1400-1000, 1000-710, and 300-250 µm in diameter. The chemical analysis of original sample is seen in Table 1, the B2O3 contents of the size fractions in Table 2, and the X-ray diffraction analysis of the sample in Figure 1. The X-ray diffractogram shows that the sample consists of colemanite (2CaO‚3B2O3‚5H2O).
10.1021/ie990647w CCC: $19.00 © 2000 American Chemical Society Published on Web 09/19/2000
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Figure 1. X- ray diffractogram of original sample.
The dissolution process was carried out in a 250-mL glass reactor equipped with a stirrer motor for mixing and a thermostat for controlling the reaction temperature. Also, the reactor was fit with a condenser to prevent losses by evaporation. After the reactor containing 100 mL of phosphoric acid solution was heated to the reaction temperature, a given amount of sample was added into it while stirring was maintained. The content of reactor was filtered as soon as the process finished, and B2O3 in solution was analyzed by the spectrophotometric method.10 Results and Discussion Dissolution Reactions. When colemanite is added into the phosphoric acid solution, the reactions taking place in the medium can be written as follows:2,11
6H3PO4(aq) S 6H2 PO-4(aq) + 6H+(aq)
(1)
6H2PO-4(aq) S 6H PO2-4(aq) + 6H+(aq)
(2)
2(2CaO‚3B2O3‚5H2O)(s) f 4Ca2+(aq) + 3B4O-27(aq) + 9H2O + 2OH-(aq) (3) 3OH-(aq) + 3H+(aq) S 3H2O 3B4O-27(aq) + 15H2O + 6H+(aq) S 12H3 BO3(aq)
(4) (5)
and the overall reaction is
2CaO‚3B2O3‚5H2O(s) + 4H3PO4(aq) + 2H2O f 2Ca2+ + 4H2PO-4(aq) + 6H3BO3(aq) (6) and/or
2CaO‚3B2O3‚5H2O(s) + 2H3PO4(aq) + 2H2O f 2CaH PO4(s) + 6H3BO3(aq) (7) After the colemanite was dissolved by the phosphoric acid solutions, the remaining solid portion was analyzed
by X-ray diffractometer (Figure 2), and it was found that the solid consists of CaHPO4 and undissolved colemanite (2CaO‚3B2O3‚5H2O). Effects of Parameters. The effects of parameters on the dissolution process were investigated using the values given in Table 3 for each parameter. In the experiments, while the effect of one parameter was studied, the values of other parameters shown with asterisks in Table 3 were kept constant. The data obtained were plotted in the form of time versus conversion fraction, decribed as XB2O3 ) the amount of dissolved B2O3 in the mineral/the amount of B2O3 in original mineral. As seen in Figure 3, the dissolution rate increases as the particle size decreases. This situation can be attributed to the increasing contact surface of the samples as the particle size decreases. The variation of the dissolution rate for various solid-to-liquid ratios is seen in Figure 4. This figure shows that decreasing solid-toliquid ratios favor the dissolution process, which can be explained by the decrease in the amount of solid per amount of reagent in the reaction mixture. Figure 5 shows that increasing reaction temperature has an increasing effect on the dissolution rate, as expected from the exponential dependence of the rate constant in the Arrhenius equation. The experimental results for the effect of stirring speed on the dissolution process are shown in Figure 6. It is evident that the dissolution rate is practically independent of the stirring speed. The experiments for observing the effect of phosphoric acid concentration on the dissolution process showed that the dissolution rate increases with increasing acid concentration up to an acid concentration of 19.52% (by wt) and more increase in acid concentration caused decreasing dissolution rate as seen in Figures 7 and 8. This situation can be attributed to the fact that more boric acid forms with increasing acid concentration, but it cannot diffuse into bulk solution at a fast enough rate. This causes a solid boric acid film to develop around the unreacted particle. The boric acid precipitation probably affects a rate-controlling mechanism after 19.52% (by
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Figure 2. X-ray diffractogram of undissolved solid part during the reaction. Table 3. Parameters Used in the Experiments and Their Ranges parameter
values
particle size (µm) acid concentration (wt %) temperature (°C) stirring speed (min-1) solid-to-liquid ratio (g mL-1)
1400-1000, 1000-710, 300-250* 1.43, 4.22, 6.94, 13.43, 19.52* 2.5, 12, 16, 25,* 35 300, 450,* 600 0.02,* 0.04, 0.08
*Values kept constant.
In these models, the fractional conversion, X, is given as a function of the reaction time. This function differs for each control mechanism. The three sets of integrated equations for spherical solids with radius Ro are
t ) t*[1 - (1 - X)1/3] t* )
FBRo bksCA
(8) (9)
for surface chemical reaction control
t ) t*X t* )
(10)
FBRo 3bkdCA
(11)
for film diffusion control, and
t ) t*[1 - 3(1 - X)2/3 + 2(1 - X)]
(12)
2
t* )
Figure 3. Effect of particle size on the dissolution of colemanite [C, 19.52% (by wt); T, 25 °C; S/L, 0.02 g mL-1; and W, 450 min-1)].
wt) acid concentration. Therefore, the kinetic analysis was carried out in the range of 1.43-19.52% (by wt) acid concentration. Kinetics Analysis The rate of a reaction between a solid and a fluid can be expressed using the heterogeneous or homogeneous reaction models.12 According to the heterogeneous reaction model, the rate may be controlled by diffusion through fluid film, by diffusion through a product (ash) layer, or by a surface chemical reaction. The application of this model to the experimental data may enable one to find the kinetics of the dissolution process.
FBRo 6bDeCA
(13)
for product layer (or ash layer) diffusion control. The kinetics of the reaction between colemanite and phosphoric acid were statistically studied by using heterogeneous and homogeneous reaction models. It was found that the dissolution process fit well the chemicalreaction-controlled model described by eqs 8 and 9. In the range of 1.43-19.52% (wt) acid concentration, it can be assumed that the overall rate constant of the dissolution process depends on the reaction parameters as follows
k ) k0(D)a(S/L)c(C)de-E/RT
(14)
and that the overall rate equation can be written as
1 - (1 - X)1/3 ) k0(D)a(S/L)c(C)de-E/RTt
(15)
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Figure 4. Effect of solid-to-liquid ratio on the dissolution of colemanite [C, 19.52% (by wt); T, 25 °C; D, -300 + 250 µm; and W, 450 min-1].
Figure 5. Effect of temperature on the dissolution of colemanite [C, 19.52% (by wt); D, -300 + 250 µm; S/L: 0.02 g mL-1; and W, 450 min-1].
Figure 6. Effect of stirring speed on the dissolution of colemanite [C, 19.52% (by wt); T, 25 °C; S/L, 0.02 g mL-1; and D, -300 + 250 µm].
Statistical calculations by simultaneous multiple regression gave the results k0 ) 5.87 × 109, a ) -0.744, c ) -0.453, d ) -0.328, and E ) 53.91 kJ mol-1 for the constants in eq 15. Inserting these estimated values into eq 15 gives the following kinetic model
1 - (1 - X)1/3 ) (5.87 × 109)(D)-0.744(S/L)-0.453(C)0.328e-53.91/RTt (16)
Figure 7. Effect of acid concentration on the dissolution of colemanite (D, -300 + 250 µm; T, 25 °C; S/L, 0.02 g mL-1; and W, 450 min-1).
Figure 8. Plot of fractional conversion against phosphoric acid concentration for various reaction times (D, -300 + 250 µm; T, 25 °C; S/L, 0.02 g mL-1; and W, 450 min-1).
Figure 9. Plot of t/t* against t for various particle sizes.
It is seen from eq 16 that the most effective parameter was the reaction temperature, with the others following it, and that the activation energy of the dissolution process is 53.91 kJ mol-1. On the other hand, when values of (t/t*) versus t were plotted in according to eqs 8, 10, and 12 for various parameters, straight lines were obtained only for eq 8. The lines for particle size and reaction temperature are shown in Figures 9 and 10, respectively. These results confirm the validity of the model obtained from the statistical analysis. Also, the fact that the stirring speed
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and gives Ca2+ and H2PO4- ions in solution. If the solution is treated with a strong acid-cation exchanger, Ca2+ will be kept, and H3PO4 will be produced and will be able to be reused in the process, according to the following reactions.
2R-SO3H + Ca2+ f (R-SO3)2 Ca + 2H+ (17)
Figure 10. Plot of t/t* against t for various reaction temperatures.
2H2PO4- + 2H+ f 2H3PO4
(18)
H3PO4 + CaHPO4 f Ca2+ + 2H2 PO4-
(19)
CaHPO4 is insoluble in water. It is used as a fertilizer, in the plastics, food, and glass industries, and in medicine. On the other hand, this substance can be dissolved according to reaction 19 in the dissolution, containing phosphoric acid, formed through reactions 17 and 18. The last solution containing Ca2+ and H2PO4- can be treated with a ion exchanger to produce phosphoric acid according to reaction 17 for reuse in the process. For these reasons, this process is more advantageous than the classical sulfuric acid process. Nomenclature
Figure 11. Agreement between observed conversion values and predicted values from the semiempirical expression.
has approximately no effect on the dissolution process for this range also shows that the process is not controlled by diffusion through the fluid film around the particle. Furthermore, the high activation energy of the process, 53.91 kJ mol-1, confirms that the dissolution rate can be a chemical-reaction-controlled process, as it has been reported that the surface chemical-reactioncontrolling processes have an activation energy above 40 kJ mol-1.13,14 To test the agreement between the experimental conversion values and the values calculated from eq 16, a plot of Xobs vs Xprd for 1.43-19.52% (by wt) acid concentration was drawn in Figure 11. The agreement between the experimental and calculated conversion values was found to be very good.
XB2O3 ) Fractional conversion t* ) Time for complete conversion (min) t ) Reaction time (min) FB ) Molar-density of solid reactant (mol m-3) Ro ) Radius of a sphere (mm) b ) Stoichiometric coefficient of the solid ks ) Rate constant for surface reaction (mol min-1) CA ) Concentration of A in the bulk solution (mol m-3) kd ) Mass transfer coefficient (m min-1) De ) Effective diffusion coefficient (m2 min-1) a ) Constant in eq 15 c ) Constant in eq 15 d ) Constant in eq 15 k0 ) Constant in eq 15 C ) Concentration of H3PO4 solution (mol dm-3) S ) Amount of solid (g) L ) Amount of liquid (mL) D ) Mean particle size (µm) W ) Stirring speed (min-1) E ) Activation energy (kJ mol-1) R ) Universal gas constant (kJ K-1 mol-1) T ) Temperature (K)
Literature Cited Conclusions Dissolution of colemanite was studied in 1.43-35.69% (by wt) phosphoric acid solutions. However, for the investigation of dissolution kinetics, 1.43-19.52% (by wt) acid concentrations were used because dissolution rate had a maximum value at 19.52% (by wt) acid concentration. It was observed that the dissolution rate increased with decreasing particle size and solid-toliquid ratio and with increasing temperature and acid concentration, but it was not affected by stirring speed. It was determined that the conversion rate was controlled by surface chemical reaction. The effective parameters on the dissolution rate are reaction temperature, particle size, solid-to-liquid ratio, and acid concentration. Ca(H2 PO4)2 and CaH PO4 were produced as byproducts in the studies. Ca(H2PO4)2 is very soluble in water
(1) Garrett, D. E. Borates; Academic Press: New York, 1998. (2) 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 (7), 2355-59. (3) Imamutdinova, V. M. Rates of dissolution of native borates in H3PO4 solutions. Zh. Prikl. Khim. 1967, 40 (11), 2596-2598. (4) 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. (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 solutions. Metall. Mater. Trans. B 1992, 23B, 409-413. (7) 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 (2).
Ind. Eng. Chem. Res., Vol. 39, No. 11, 2000 4119 (8) Imamutdinova, V. M.; Abdrashitova, N. Rates of dissolution of borates in acetic acid solutions. Zh. Prikl. Khim. 1970, 43 (2), 452-455. (9) Alkan, M.; Oktay, M.; Kocakerim, M. M.; Karago¨lge, Z. Dissolution kinetics of some borate minerals in CO2-saturated water. Hydrometallurgy 1991, 26, 255-262. (10) Greenberg, A. E.; Trussell, R. R.; Clesceri, L. S. Standard Methods For the Examination of Water and Wastewater, 16th ed.; American Public Health Association: Washington, D.C., 1985; pp 276-277. (11) Mortimer, C. E. Chemistry, 4th ed.; Van Nostrand Company: New York, 1979; pp 434-437.
(12) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley and Sons: New York, 1972; pp 357-377. (13) Jackson, E. Hydrometallurgical Extraction and Reclamation; Ellis Horwood Ltd.: Chichester, U.K., 1986; p 46. (14) Habashi, F. Principles of Extractive Metallurgy, General Principles; Gordon and Breach Science Publishers:, New York, 1980; Vol 1, pp 143-144.
Received for review August 30, 1999 Revised manuscript received July 7, 2000 Accepted July 11, 2000 IE990647W
