- Email: [email protected]
Dissolution kinetics of ulexite in ammonium nitrate solutions
Hydrometallurgy 95 (2009) 198–202
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Dissolution kinetics of ulexite in ammonium nitrate solutions Nizamettin Demirkıran ⁎ Department of Chemical Engineering, Faculty of Engineering, Inonu University, Malatya, Turkey
A R T I C L E
I N F O
Article history: Received 6 July 2007 Received in revised form 12 May 2008 Accepted 27 May 2008 Available online 1 June 2008 Keywords: Ulexite Dissolution kinetics Ammonium nitrate
A B S T R A C T In this study the dissolution kinetics of the boron mineral ulexite, a sodium-calcium-borate hydrate (Na2O·2CaO·5B2O3·16H2O), in ammonium nitrate solutions was investigated in a batch reactor. The effects of solution concentration, particle size, solid/liquid ratio, and reaction temperature on the dissolution rate were determined. It was found that the dissolution rate increased with increase in ammonium nitrate concentration and reaction temperature, and with decrease in particle size and solid/liquid ratio. No effect of stirring speed was observed on the conversion. It was determined that the dissolution rate fit to the chemical reaction control model. The activation energy of this dissolution process was found to be 58.2 kJ/mol. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Boric acid, which is an important boron compound, is used in many branches of industry, including nuclear technology, production of heat-resistant materials (such as refractors and ceramics), heatresistant polymers, catalysts, and in the medical and pharmaceutical, and electronic sectors, among others. Boric acid is also used as the starting material in the preparation of many boron chemicals, including synthetic organic borate salts, boron phosphate, fluoroborates, boron tri-halides, borate esters, boron carbide, and metal alloys such as ferroboron. Ulexite and colemanite ores are used as raw materials in the production of boric acid (Temur et al., 2000; Çetin et al., 2001; Künkül et al., 2003). Boric acid is commercially produced in Turkey by the heterogeneous solid–liquid reaction of sulfuric acid and colemanite. In this process, colemanite ore is reacted with sulfuric acid at 95 °C. Gypsum forms as a by-product and precipitates in the reactor while boric acid remains in the liquid phase through the reaction. After gypsum is removed by filtration, boric acid is crystallized by cooling the solution. Filtration of gypsum has an important role in boric acid production because it affects the efficiency, purity, and crystallization of boric acid. This present process has some disadvantages, such as sulfate contamination in the final product and environmental pollution (Temur et al., 2000; Çetin et al., 2001; Taylan et al., 2007). Because of these disadvantages, various studies have been performed by many investigators on the kinetics and mechanism of dissolution of boron minerals in aqueous solutions. The dissolution kinetics of inderite and inyoite in water saturated with carbon dioxide was investigated by Alkan et al. (1991). The dissolution of colemanite ⁎ Tel.: +90 422 3410010; fax: +90 422 3410046. E-mail address: [email protected]. 0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2008.05.041
was examined in different solutions (Kocakerim and Alkan, 1988; Temur et al., 2000; Okur et al., 2002; Kurtbaş et al., 2006). Morales et al. (2000) studied the dissolution kinetics of hydroboracite in water saturated with carbon dioxide. The dissolution kinetics of ulexite in diverse solutions was investigated by various researchers (Kocakerim et al., 1993; Künkül et al., 1997a; Tunç et al., 1999; Alkan et al., 2004; Demirkıran and Künkül, 2007). In the hydrometallurgical processes, inorganic acids are generally used as the leaching agents. However, the acid methods are uneconomical because basic ores consume excessive acid during the leaching. Furthermore, some undesired impurities can pass into the leaching media when inoganic acids are used. Therefore, more basic lixiviants than inorganic acids (or weakly acidic reagents) may be more favorable in the leaching processes, allowing impurities in the solution to be separated more effectively. Some ammonium salts have been employed by researchers as the lixiviant. The dissolution kinetics of magnesite (Ranjitham and Khangaonkar, 1990; Raschman, 2000), colemanite (Kum et al., 1994), ulexite (Tekin et al., 1998), malachite (Ekmekyapar et al., 2003), and smithsonite (Ju et al., 2005) were investigated in ammonium chloride solutions. Ammonia/ammonium carbonate solution (Bingöl et al., 2005) was used as a leachant in the leaching of malachite ore. The dissolution kinetics of ulexite in ammonia solutions saturated with carbon dioxide was studied by Künkül et al. (1997b). In this study, it was determined that the dissolution rate of ulexite fit the first-order pseudo-homogeneous reaction model. Künkül and Demirkıran (2007) examined the dissolution kinetics of calcined ulexite in ammonium carbonate solutions. Künkül et al. (2003) studied the dissolution kinetics of ulexite in ammonium sulfate solutions. They determined that the dissolution rate was controlled by diffusion through the product layer. The dissolution kinetics of colemanite in ammonium sulfate solutions was examined by Tunç et al. (2007).
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202
199
showed that after a 60 min leaching time, 94.17% of B2O3 at 300 rpm, 95.08% of B2O3 at 400 rpm, and 95.90% of B2O3 at 500 rpm were extracted. This meant that the particles were fully suspended in the solution. Thus, it can be concluded that the effect of stirring speed on the dissolution rate can be practically neglected. For this reason, all subsequent experiments were carried out at a stirring speed of 400 rpm. 3. Results and discussion The dissolution rate of ulexite was determined as a function of time by changing the ammonium nitrate concentration, particle size, solid/ liquid ratio, and reaction temperature. The data obtained were plotted as a function of conversion, described as x = amount of dissolved B2O3 in the solution / amount of B2O3 in the original sample, versus time. Fig. 1. Effect of concentration on dissolution of ulexite (temperature: 308 K; solid/liquid ratio: 0.5/100 g/mL; particle size: 0.84–0.42 mm; stirring speed: 400 rpm).
In the literature, no study was found that used ammonium nitrate as the leaching reactant. Thus, the aims of this study were to investigate the dissolution of ulexite using ammonium nitrate solution and to determine the effects of the experimental parameters, including solution concentration, solid-to-liquid ratio, particle size, and temperature. Ammonium nitrate, a salt of ammonia and nitric acid, is a colorless crystalline substance that is highly soluble in water. Ammonium nitrate solutions have a weakly acidic character. When ammonium nitrate ionizes in aqueous media, it yields ammonium and nitrate ions. When the ammonium ions hydrolyze, they provide the protons required for the dissolution reaction. The solutions obtained from the dissolution of ulexite in the ammonium nitrate include calcium, sodium, nitrate, dissolved boric acid, and ammonia/ammonium. Various products, such as boric acid, sodium borates, and ammonium borates may be obtained from this solution. Sodium and ammonium borates are notable boron compounds. In addition, calcium nitrate and sodium nitrate can form as by-products, and are used in fertilizers. Ammonium-containing systems have been reported previously in the literature (Künkül et al., 2003; Künkül and Demirkıran, 2007). In these studies, the solid waste products form, and the dissolution kinetics and mechanisms are different from the present study. In the reported process, no waste products form, and the side product obtained is more valuable than the one obtained in the commercial sulfuric acid process. In light of this, the present study can have an important environmental impact. 2. Experimental The ulexite samples used in this study were obtained from Kırka, Eskişehir, Turkey. The original ore sample was analyzed, and the mineral content was determined to be 42.08% B2O3, 13.94% CaO, 7.85% Na2O, 35.96%H2O, and 0.17% insoluble matter. In the leaching studies, variables such as reaction temperature, concentration of solution, solid-to-liquid ratio, particle size, and stirring speed are generally chosen as the experimental parameters. The dissolution data obtained are plotted as a function of conversion fraction, and the kinetic analysis is performed. In the present study, the experimental procedure is the same as the one reported in our previous papers (Künkül et al., 2003; Künkül and Demirkıran, 2007; Demirkıran and Künkül, 2007). Before studying the effects of other parameters that may influence the dissolution rate, the effect of stirring speed was examined. The experiments were carried out at stirring speeds of 300, 400, and 500 rpm to observe the effect of the stirring speed on the dissolution rate. In these experiments, ammonium nitrate concentration, particle size, solid/liquid ratio, and reaction temperature were fixed at 1.00 M, 0.840–0.420 mm, 0.50/100 g/mL, and 308 K, respectively. The results
3.1. Dissolution reactions Ammonium nitrate ionizes in an aqueous medium according to the following reaction: − NH4 NO3ðsÞ ↔NHþ 4 ðaqÞ þ NO3 ðaqÞ:
The hydrolysis of
NH+4
ð1Þ
ions gives:
þ NHþ 4 ðaqÞ þ H2 OðlÞ ↔NH3ðaqÞ þ H3 OðaqÞ :
ð2Þ
The reaction of ulexite in aqueous medium is: 2þ Na2 Od2CaOd5B2 O3 d16H2 OðsÞ þ 2H2 OðlÞ →2Naþ ðaqÞ þ 2CaðaqÞ − þ 4H3 BO3ðaqÞ þ 6BðOHÞ4 ðaqÞ:
ð3Þ
The neutralization reaction between H3O+ and B(OH)−4 occurs according to: BðOHÞ−4 ðaqÞ þ H3 Oþ ðaqÞ ↔H3 BO3ðaqÞ þ 2H2 OðlÞ :
ð4Þ
Consequently, when ulexite is added into ammonium nitrate solution, the overall reaction taking place in the medium can be written as follows: Na2 Od2CaOd5B2 O3 d16H2 OðsÞ þ 6NH4 NO3ðaqÞ →2Naþ þ 2Ca2þ ðaqÞ þ 6NH3ðaqÞ þ 6NO−3 ðaqÞ þ 10H3 BO3ðaqÞ þ 4H2 OðlÞ :
ð5Þ
3.2. Effects of parameters To investigate the effect of the ammonium nitrate concentration on dissolution rate, the experiments were carried out in concentrations of 0.25, 0.50, 1.00, and 2.00 M. The results are plotted in Fig. 1. H3O+ concentration increases with an increase in the concentration of ammonium nitrate. Thus, the dissolution rate of ulexite increase with an increase in concentration. The pH values of ammonium nitrate solutions are given Table 1. The effect of solid-to-liquid ratio on the dissolution of ulexite was studied in the range of 0.25/100–1.5/100 g/mL. Fig. 2 shows the effect of this parameter. As can be seen in Fig. 2, the dissolution rate of ulexite decreases with an increase in solid-to-liquid ratio. When the solid-to-liquid ratio increased, the amount of solid per amount of reagent in the reaction mixture increased. The results of the effect of particle size on the dissolution rate are given in Fig. 3. The experiments on particle size were carried out
Table 1 The pH values of ammonium nitrate solutions at different concentrations Concentration, M pH
0.010 5.86
0.025 5.65
0.050 5.50
0.100 5.34
0.250 5.20
0.500 5.03
1.000 4.87
2.000 4.66
200
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202 Table 2 The apparent rate constant and correlation coefficient values
Fig. 2. Effect of solid-to-liquid ratio on dissolution of ulexite (temperature: 308 K; concentration: 1.00 M; particle size: 0.84–0.42 mm; stirring speed: 400 rpm).
Surface chemical reaction 1 − (1 − x)1/3
Diffusion through the product layer 1 − 3(1 − x)2/3 + 2(1 − x)
kr (min− 1)
R2
kd (min− 1)
R2
0.9921 0.9918 0.9937 0.9919
0.0036 0.0057 0.0107 0.0148
0.9358 0.9502 0.9672 0.9723
0.9937 0.9959 0.9904 0.9930
0.0107 0.0164 0.0254 0.0372
0.9672 0.9490 0.9737 0.9656
0.9917 0.9937 0.9927 0.9972
0.0177 0.0107 0.0054 0.0033
0.9622 0.9672 0.9547 0.9351
0.9952 0.9954 0.9937 0.9933 0.9954
0.0034 0.0059 0.0107 0.0160 0.0234
0.9396 0.9496 0.9672 0.9569 0.9662
Concentration (M) 0.25 0.0055 0.50 0.0073 1.00 0.0109 2.00 0.0144 Particle size (mm) 0.84–0.42 0.0109 0.42–0.25 0.0168 0.25–0.18 0.0250 0.18–0.15 0.0368 Solid-to-liquid ratio (g/mL) 0.25/100 0.0178 0.50/100 0.0109 1.00/100 0.0071 1.50/100 0.0053 Temperature (K) 298 0.0054 303 0.0075 308 0.0109 313 0.0165 318 0.0231
The effect of the reaction temperature on the dissolution rate under certain reaction conditions was investigated in the range of 298–318 K, where the results are presented in Fig. 4. According to results given in Fig. 4, the reaction temperature has a noticeable effect on the dissolution of ulexite. It can be seen that the reaction rate increases with an increase in the reaction temperature. 3.3. Kinetic analysis
Fig. 3. Effect of particle size on dissolution of ulexite (temperature: 308 K; concentration: 1.00 M; solid/liquid ratio: 0.5/100 g/mL; stirring speed: 400 rpm).
using the following size fractions: 0.840–0.420, 0.420–0.250, 0.250– 0.180, and 0.180–0.150 mm. The results shown in Fig. 3 indicate that the particle size has a significant effect on the dissolution of ulexite. It is seen that the dissolution rate increases with a decrease in particle size.
Fig. 4. Effect of reaction temperature on dissolution of ulexite (concentration: 1.00 M; particle size: 0.84–0.42 mm; solid/liquid ratio: 0.5/100 g/mL; stirring speed: 400 rpm).
In order to determine the kinetic parameters and the ratecontrolling step of the dissolution of ulexite in ammonium nitrate solutions, the non-catalytic heterogeneous reaction models were performed on the experimental data. According to these models, the rate of reaction between solid particle and the leaching reagent may be controlled by one of the following steps: diffusion through the fluid film, diffusion through the product layer, or the chemical reaction at the surface. The particle size can stay constant or decrease during the reaction. The rate equations for these models have been introduced in the literature (Wen, 1968; Levenspiel, 1972; Mazet, 1992). After the experimental data were analyzed, it was determined that the chemical reaction controlled model was suitable for this leaching reaction. The
Fig. 5. Plot of 1 − (1 − x)1/3 versus time for different particle size.
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202
201
Fig. 8. Arrhenius plot for dissolution of ulexite. Fig. 6. Plot of 1 − (1 − x)1/3 versus time for different reaction temperature.
rate constant values and their correlation coefficients are given for the diffusion through the product layer and the chemical reaction controlled models in Table 2. Furthermore, the linear relation between 1 − (1 − x)1/3 and the reaction time can be seen in Figs. 5 and 6 for the particle size and the reaction temperature, respectively. In accordance with these results, the equation representing the kinetics of this process was determined to obey the chemical reaction model. Therefore, the rate expression for this process can be written as follows: 1−ð1−xÞ1=3 ¼ kr t:
ð6Þ
In addition, if the surface chemical reaction controls the reaction rate, the relation between the apparent rate constant and the inverse of the particle radius must be linear. This relationship is given in Fig. 7. This figure confirms that the chemical reaction model can be the ratecontrolling step for this process. To include the effect of the reaction parameters on the rate constant of reaction, a semi-empirical model can be written as follows: a
b
c −Ea =RT
kr ¼ k0 C D ðS=LÞ e
:
ð8Þ
The constants a, b, and c were estimated from the apparent rate constants given in Table 2. The values of these constants were
Fig. 7. Plot of rate constants versus 1/R0 for different particle sizes.
1− 1−xB2 O3
1=3
¼ 6 104 C 0:47 D−0:81 ðS=LÞ−0:67 expð−58200=RT Þt:
ð9Þ
ð7Þ
Combining Eqs. (6) and (7), the following equation is obtained: 1−ð1−xÞ1=3 ¼ k0 C a Db ðS=LÞc e−Ea =RT t:
calculated as 0.47, −0.81, and −0.67, respectively. The activation energy of the dissolution process was determined from the Arrhenius equation. The Arrhenius plot of the process is shown in Fig. 8. From the slope of the straight line in this figure, the activation energy of the process was calculated to be 58.2 kJ/mol. Diffusion controlled heterogeneous processes are slightly dependent on temperature, while chemically controlled processes are strongly dependent on temperature. Therefore, the value of the activation energy of a dissolution reaction may be used to predict the rate-controlling step. The activation energy of a diffusion controlled process is typically between 4 and 12 kJ/mol, while, for a chemically controlled process, the value is usually greater than 40 kJ/mol (Abdel-Aal, 2000; Ashraf et al., 2005). The value of the activation energy obtained in the dissolution process confirms that this leaching process is controlled by the chemical reaction. As a result, the following kinetic expression including the parameters used in this dissolution process can be written:
To test the agreement between the experimental conversion and the values calculated from the semi-empirical model, the graph of xexp versus xcal was plotted, as shown in Fig. 9. It is observed that the agreement between the experimental and the calculated values is very good.
Fig. 9. Agreement between experimental and calculated conversion values.
202
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202
4. Conclusion In this work, the dissolution kinetics of ulexite in ammonium nitrate solutions was investigated. The effect of the several parameters on the dissolution rate was examined and evaluated. It was determined that the conversion rate increased with increases in ammonium nitrate concentration and reaction temperature and with decreases in solid-to-liquid ratio and particle size. It was observed that the dissolution rate was not affected by stirring speed. It was determined that the dissolution process was controlled by the chemical reaction. The activation energy of the dissolution reaction was found to be approximately 58.2 kJ/mol. According to the experimental results, ammonium nitrate solutions can be used as a leachant for the mineral ulexite. References Abdel-Aal, E.A., 2000. Kinetics of sulfuric acid leaching of low-grade zinc silicate ore. Hydrometallurgy 55, 247–254. Alkan, M., Oktay, M., Kocakerim, M.M., Karagölge, Z., 1991. Dissolution kinetics of some borate minerals in CO2-saturated water. Hydrometallurgy 26, 255–262. Alkan, M., Doğan, M., Namlı, H., 2004. Dissolution kinetics and mechanism of ulexite in oxalic acid solutions. Ind. Eng. Chem. Res. 43, 1591–1598. Ashraf, M., Zafar, Z.I., Ansari, T.M., 2005. Selective leaching kinetics and upgrading of low-grade calcareous phosphate rock in succinic acid. Hydrometallurgy 80, 286–292. Bingöl, D., Canbazoğlu, M., Aydoğan, S., 2005. Dissolution kinetics of malachite in ammonia/ammonium carbonate leaching. Hydrometallurgy 76, 55–62. Çetin, E., Eroğlu, İ., Özkar, S., 2001. Kinetics of gypsum formation and growth during the dissolution of colemanite in sulfuric acid. J. Crystal Growth 231, 559–567. Demirkıran, N., Künkül, A., 2007. Dissolution kinetics of ulexite in perchloric acid solutions. Int. J. Miner. Process. 83, 76–80. Ekmekyapar, A., Oya, R., Künkül, A., 2003. Dissolution kinetics of an oxidized copper ore in ammonium chloride solution. Chem. Biochem. Eng. Q. 17, 261–266. Ju, S., Motang, T., Shenghai, Y., Yingnian, L., 2005. Dissolution kinetics of smithsonite ore in ammonium chloride solution. Hydrometallurgy 80, 67–74. Kocakerim, M.M., Alkan, M., 1988. Dissolution kinetics of colemanite in SO2-saturated water. Hydrometallurgy 19, 385–392. Kocakerim, M.M., Çolak, S., Davies, T., Alkan, M., 1993. Dissolution kinetics of ulexite in CO2-saturated water. Can. Metall. Q. 32, 393–396. Kum, C., Alkan, M., Kocakerim, M.M., 1994. Dissolution kinetics of calcined colemanite in ammonium chloride solution. Hydrometallurgy 36, 259–268. Kurtbaş, A., Kocakerim, M.M., Küçük, Ö., Yartaşı, A., 2006. Dissolution of colemanite in aqueous solution saturated with both sulfur dioxide (SO2) gas and boric acid. Ind. Eng. Chem. Res. 45, 1857–1862. Künkül, A., Yapıcı, S., Kocakerim, M.M., Çopur, M., 1997a. Dissolution of thermally dehydrated ulexite in sulfuric acid solution. Ind. Eng. Chem. Res. 36, 4847–4851.
Künkül, A., Yapıcı, S., Kocakerim, M.M., Çopur, M., 1997b. Dissolution kinetics of ulexite in ammonia solutions saturated with CO2. Hydrometallurgy 44, 135–145. Künkül, A., Demirkıran, N., Baysar, A., 2003. Dissolution kinetics of ulexite in ammonium sulfate solutions. Ind. Eng. Chem. Res. 42, 982–986. Künkül, A., Demirkıran, N., 2007. Dissolution kinetics of calcined ulexite in ammonium carbonate solutions. Korean J. Chem. Eng. 24, 947–952. Levenspiel, O., 1972. Chemical Reaction Engineering. John Wiley, New York. Mazet, N., 1992. Modeling of gas–solid reactions 1. Nonporous solids. Int. Chem. Eng. 32, 271–284. Morales, G.V., Capretto, M.E., Fuentes, L.M., Quiroga, O.D., 2000. Dissolution kinetics of hydroboracite in water saturated with carbon dioxide. Hydrometallurgy 58, 127–133. Okur, H., Tekin, T., Ozer, A.K., Bayramoglu, M., 2002. Effect of ultrasound on the dissolution of colemanite in H2SO4. Hydrometallurgy 67, 79–86. Ranjitham, A.M., Khangaonkar, P.R., 1990. Leaching behaviour of calcined magnesite with ammonium chloride solutions. Hydrometallurgy 23, 177–189. Raschman, P., 2000. Leaching of calcined magnesite using ammonium chloride at constant pH. Hydrometallurgy 56, 109–123. Taylan, N., Gürbüz, H., Bulutçu, A.N., 2007. Effects of ultrasound on the reaction step of boric acid production process from colemanite. Ultrason. Sonochem. 14, 633–638. Tekin, G., Onganer, Y., Alkan, M., 1998. Dissolution kinetics of ulexite in ammonium chloride solution. Can. Metall. Quart. 37, 91–97. Temur, H., Yartaşı, A., Çopur, M., Kocakerim, M.M., 2000. The kinetics of dissolution of colemanite in H3PO4. Ind. Eng. Chem. Res. 39, 4114–4119. Tunç, M., Kocakerim, M.M., Yapici, S., Bayrakçeken, S., 1999. Dissolution mechanism of ulexite in H2SO4 solution. Hydrometallurgy 51, 359–370. Tunç, M., Kocakerim, M.M., Küçük, Ö., Aluz, M., 2007. Dissolution of colemanite in (NH4)2SO4 solutions. Korean J. Chem. Eng. 24, 55–59. Wen, C.Y., 1968. Noncatalytic heterogeneous solid–fluid reaction models. Ind. Eng. Chem. 60, 34–54.
Glossary C: concentration of ammonium nitrate, (mol/cm3) D: particle diameter, (cm) Ea: activation energy, (J/mol) S/L: solid-to-liquid ratio, (g/mL) R0: average radius of solid particle, (cm) R: universal gas constant, (J/molK) T: temperature, (K) t: reaction time, (s) x: converted fraction of B2O3 kd: apparent rate constant for diffusion through the product layer, (s− 1) kr: apparent rate constant for the surface chemical reaction, (s− 1) k0: frequency or pre-exponential factor, (s− 1) a, b, c: constants in Eq. (7)
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Dissolution kinetics of ulexite in ammonium nitrate solutions Nizamettin Demirkıran ⁎ Department of Chemical Engineering, Faculty of Engineering, Inonu University, Malatya, Turkey
A R T I C L E
I N F O
Article history: Received 6 July 2007 Received in revised form 12 May 2008 Accepted 27 May 2008 Available online 1 June 2008 Keywords: Ulexite Dissolution kinetics Ammonium nitrate
A B S T R A C T In this study the dissolution kinetics of the boron mineral ulexite, a sodium-calcium-borate hydrate (Na2O·2CaO·5B2O3·16H2O), in ammonium nitrate solutions was investigated in a batch reactor. The effects of solution concentration, particle size, solid/liquid ratio, and reaction temperature on the dissolution rate were determined. It was found that the dissolution rate increased with increase in ammonium nitrate concentration and reaction temperature, and with decrease in particle size and solid/liquid ratio. No effect of stirring speed was observed on the conversion. It was determined that the dissolution rate fit to the chemical reaction control model. The activation energy of this dissolution process was found to be 58.2 kJ/mol. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Boric acid, which is an important boron compound, is used in many branches of industry, including nuclear technology, production of heat-resistant materials (such as refractors and ceramics), heatresistant polymers, catalysts, and in the medical and pharmaceutical, and electronic sectors, among others. Boric acid is also used as the starting material in the preparation of many boron chemicals, including synthetic organic borate salts, boron phosphate, fluoroborates, boron tri-halides, borate esters, boron carbide, and metal alloys such as ferroboron. Ulexite and colemanite ores are used as raw materials in the production of boric acid (Temur et al., 2000; Çetin et al., 2001; Künkül et al., 2003). Boric acid is commercially produced in Turkey by the heterogeneous solid–liquid reaction of sulfuric acid and colemanite. In this process, colemanite ore is reacted with sulfuric acid at 95 °C. Gypsum forms as a by-product and precipitates in the reactor while boric acid remains in the liquid phase through the reaction. After gypsum is removed by filtration, boric acid is crystallized by cooling the solution. Filtration of gypsum has an important role in boric acid production because it affects the efficiency, purity, and crystallization of boric acid. This present process has some disadvantages, such as sulfate contamination in the final product and environmental pollution (Temur et al., 2000; Çetin et al., 2001; Taylan et al., 2007). Because of these disadvantages, various studies have been performed by many investigators on the kinetics and mechanism of dissolution of boron minerals in aqueous solutions. The dissolution kinetics of inderite and inyoite in water saturated with carbon dioxide was investigated by Alkan et al. (1991). The dissolution of colemanite ⁎ Tel.: +90 422 3410010; fax: +90 422 3410046. E-mail address: [email protected]. 0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2008.05.041
was examined in different solutions (Kocakerim and Alkan, 1988; Temur et al., 2000; Okur et al., 2002; Kurtbaş et al., 2006). Morales et al. (2000) studied the dissolution kinetics of hydroboracite in water saturated with carbon dioxide. The dissolution kinetics of ulexite in diverse solutions was investigated by various researchers (Kocakerim et al., 1993; Künkül et al., 1997a; Tunç et al., 1999; Alkan et al., 2004; Demirkıran and Künkül, 2007). In the hydrometallurgical processes, inorganic acids are generally used as the leaching agents. However, the acid methods are uneconomical because basic ores consume excessive acid during the leaching. Furthermore, some undesired impurities can pass into the leaching media when inoganic acids are used. Therefore, more basic lixiviants than inorganic acids (or weakly acidic reagents) may be more favorable in the leaching processes, allowing impurities in the solution to be separated more effectively. Some ammonium salts have been employed by researchers as the lixiviant. The dissolution kinetics of magnesite (Ranjitham and Khangaonkar, 1990; Raschman, 2000), colemanite (Kum et al., 1994), ulexite (Tekin et al., 1998), malachite (Ekmekyapar et al., 2003), and smithsonite (Ju et al., 2005) were investigated in ammonium chloride solutions. Ammonia/ammonium carbonate solution (Bingöl et al., 2005) was used as a leachant in the leaching of malachite ore. The dissolution kinetics of ulexite in ammonia solutions saturated with carbon dioxide was studied by Künkül et al. (1997b). In this study, it was determined that the dissolution rate of ulexite fit the first-order pseudo-homogeneous reaction model. Künkül and Demirkıran (2007) examined the dissolution kinetics of calcined ulexite in ammonium carbonate solutions. Künkül et al. (2003) studied the dissolution kinetics of ulexite in ammonium sulfate solutions. They determined that the dissolution rate was controlled by diffusion through the product layer. The dissolution kinetics of colemanite in ammonium sulfate solutions was examined by Tunç et al. (2007).
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202
199
showed that after a 60 min leaching time, 94.17% of B2O3 at 300 rpm, 95.08% of B2O3 at 400 rpm, and 95.90% of B2O3 at 500 rpm were extracted. This meant that the particles were fully suspended in the solution. Thus, it can be concluded that the effect of stirring speed on the dissolution rate can be practically neglected. For this reason, all subsequent experiments were carried out at a stirring speed of 400 rpm. 3. Results and discussion The dissolution rate of ulexite was determined as a function of time by changing the ammonium nitrate concentration, particle size, solid/ liquid ratio, and reaction temperature. The data obtained were plotted as a function of conversion, described as x = amount of dissolved B2O3 in the solution / amount of B2O3 in the original sample, versus time. Fig. 1. Effect of concentration on dissolution of ulexite (temperature: 308 K; solid/liquid ratio: 0.5/100 g/mL; particle size: 0.84–0.42 mm; stirring speed: 400 rpm).
In the literature, no study was found that used ammonium nitrate as the leaching reactant. Thus, the aims of this study were to investigate the dissolution of ulexite using ammonium nitrate solution and to determine the effects of the experimental parameters, including solution concentration, solid-to-liquid ratio, particle size, and temperature. Ammonium nitrate, a salt of ammonia and nitric acid, is a colorless crystalline substance that is highly soluble in water. Ammonium nitrate solutions have a weakly acidic character. When ammonium nitrate ionizes in aqueous media, it yields ammonium and nitrate ions. When the ammonium ions hydrolyze, they provide the protons required for the dissolution reaction. The solutions obtained from the dissolution of ulexite in the ammonium nitrate include calcium, sodium, nitrate, dissolved boric acid, and ammonia/ammonium. Various products, such as boric acid, sodium borates, and ammonium borates may be obtained from this solution. Sodium and ammonium borates are notable boron compounds. In addition, calcium nitrate and sodium nitrate can form as by-products, and are used in fertilizers. Ammonium-containing systems have been reported previously in the literature (Künkül et al., 2003; Künkül and Demirkıran, 2007). In these studies, the solid waste products form, and the dissolution kinetics and mechanisms are different from the present study. In the reported process, no waste products form, and the side product obtained is more valuable than the one obtained in the commercial sulfuric acid process. In light of this, the present study can have an important environmental impact. 2. Experimental The ulexite samples used in this study were obtained from Kırka, Eskişehir, Turkey. The original ore sample was analyzed, and the mineral content was determined to be 42.08% B2O3, 13.94% CaO, 7.85% Na2O, 35.96%H2O, and 0.17% insoluble matter. In the leaching studies, variables such as reaction temperature, concentration of solution, solid-to-liquid ratio, particle size, and stirring speed are generally chosen as the experimental parameters. The dissolution data obtained are plotted as a function of conversion fraction, and the kinetic analysis is performed. In the present study, the experimental procedure is the same as the one reported in our previous papers (Künkül et al., 2003; Künkül and Demirkıran, 2007; Demirkıran and Künkül, 2007). Before studying the effects of other parameters that may influence the dissolution rate, the effect of stirring speed was examined. The experiments were carried out at stirring speeds of 300, 400, and 500 rpm to observe the effect of the stirring speed on the dissolution rate. In these experiments, ammonium nitrate concentration, particle size, solid/liquid ratio, and reaction temperature were fixed at 1.00 M, 0.840–0.420 mm, 0.50/100 g/mL, and 308 K, respectively. The results
3.1. Dissolution reactions Ammonium nitrate ionizes in an aqueous medium according to the following reaction: − NH4 NO3ðsÞ ↔NHþ 4 ðaqÞ þ NO3 ðaqÞ:
The hydrolysis of
NH+4
ð1Þ
ions gives:
þ NHþ 4 ðaqÞ þ H2 OðlÞ ↔NH3ðaqÞ þ H3 OðaqÞ :
ð2Þ
The reaction of ulexite in aqueous medium is: 2þ Na2 Od2CaOd5B2 O3 d16H2 OðsÞ þ 2H2 OðlÞ →2Naþ ðaqÞ þ 2CaðaqÞ − þ 4H3 BO3ðaqÞ þ 6BðOHÞ4 ðaqÞ:
ð3Þ
The neutralization reaction between H3O+ and B(OH)−4 occurs according to: BðOHÞ−4 ðaqÞ þ H3 Oþ ðaqÞ ↔H3 BO3ðaqÞ þ 2H2 OðlÞ :
ð4Þ
Consequently, when ulexite is added into ammonium nitrate solution, the overall reaction taking place in the medium can be written as follows: Na2 Od2CaOd5B2 O3 d16H2 OðsÞ þ 6NH4 NO3ðaqÞ →2Naþ þ 2Ca2þ ðaqÞ þ 6NH3ðaqÞ þ 6NO−3 ðaqÞ þ 10H3 BO3ðaqÞ þ 4H2 OðlÞ :
ð5Þ
3.2. Effects of parameters To investigate the effect of the ammonium nitrate concentration on dissolution rate, the experiments were carried out in concentrations of 0.25, 0.50, 1.00, and 2.00 M. The results are plotted in Fig. 1. H3O+ concentration increases with an increase in the concentration of ammonium nitrate. Thus, the dissolution rate of ulexite increase with an increase in concentration. The pH values of ammonium nitrate solutions are given Table 1. The effect of solid-to-liquid ratio on the dissolution of ulexite was studied in the range of 0.25/100–1.5/100 g/mL. Fig. 2 shows the effect of this parameter. As can be seen in Fig. 2, the dissolution rate of ulexite decreases with an increase in solid-to-liquid ratio. When the solid-to-liquid ratio increased, the amount of solid per amount of reagent in the reaction mixture increased. The results of the effect of particle size on the dissolution rate are given in Fig. 3. The experiments on particle size were carried out
Table 1 The pH values of ammonium nitrate solutions at different concentrations Concentration, M pH
0.010 5.86
0.025 5.65
0.050 5.50
0.100 5.34
0.250 5.20
0.500 5.03
1.000 4.87
2.000 4.66
200
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202 Table 2 The apparent rate constant and correlation coefficient values
Fig. 2. Effect of solid-to-liquid ratio on dissolution of ulexite (temperature: 308 K; concentration: 1.00 M; particle size: 0.84–0.42 mm; stirring speed: 400 rpm).
Surface chemical reaction 1 − (1 − x)1/3
Diffusion through the product layer 1 − 3(1 − x)2/3 + 2(1 − x)
kr (min− 1)
R2
kd (min− 1)
R2
0.9921 0.9918 0.9937 0.9919
0.0036 0.0057 0.0107 0.0148
0.9358 0.9502 0.9672 0.9723
0.9937 0.9959 0.9904 0.9930
0.0107 0.0164 0.0254 0.0372
0.9672 0.9490 0.9737 0.9656
0.9917 0.9937 0.9927 0.9972
0.0177 0.0107 0.0054 0.0033
0.9622 0.9672 0.9547 0.9351
0.9952 0.9954 0.9937 0.9933 0.9954
0.0034 0.0059 0.0107 0.0160 0.0234
0.9396 0.9496 0.9672 0.9569 0.9662
Concentration (M) 0.25 0.0055 0.50 0.0073 1.00 0.0109 2.00 0.0144 Particle size (mm) 0.84–0.42 0.0109 0.42–0.25 0.0168 0.25–0.18 0.0250 0.18–0.15 0.0368 Solid-to-liquid ratio (g/mL) 0.25/100 0.0178 0.50/100 0.0109 1.00/100 0.0071 1.50/100 0.0053 Temperature (K) 298 0.0054 303 0.0075 308 0.0109 313 0.0165 318 0.0231
The effect of the reaction temperature on the dissolution rate under certain reaction conditions was investigated in the range of 298–318 K, where the results are presented in Fig. 4. According to results given in Fig. 4, the reaction temperature has a noticeable effect on the dissolution of ulexite. It can be seen that the reaction rate increases with an increase in the reaction temperature. 3.3. Kinetic analysis
Fig. 3. Effect of particle size on dissolution of ulexite (temperature: 308 K; concentration: 1.00 M; solid/liquid ratio: 0.5/100 g/mL; stirring speed: 400 rpm).
using the following size fractions: 0.840–0.420, 0.420–0.250, 0.250– 0.180, and 0.180–0.150 mm. The results shown in Fig. 3 indicate that the particle size has a significant effect on the dissolution of ulexite. It is seen that the dissolution rate increases with a decrease in particle size.
Fig. 4. Effect of reaction temperature on dissolution of ulexite (concentration: 1.00 M; particle size: 0.84–0.42 mm; solid/liquid ratio: 0.5/100 g/mL; stirring speed: 400 rpm).
In order to determine the kinetic parameters and the ratecontrolling step of the dissolution of ulexite in ammonium nitrate solutions, the non-catalytic heterogeneous reaction models were performed on the experimental data. According to these models, the rate of reaction between solid particle and the leaching reagent may be controlled by one of the following steps: diffusion through the fluid film, diffusion through the product layer, or the chemical reaction at the surface. The particle size can stay constant or decrease during the reaction. The rate equations for these models have been introduced in the literature (Wen, 1968; Levenspiel, 1972; Mazet, 1992). After the experimental data were analyzed, it was determined that the chemical reaction controlled model was suitable for this leaching reaction. The
Fig. 5. Plot of 1 − (1 − x)1/3 versus time for different particle size.
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202
201
Fig. 8. Arrhenius plot for dissolution of ulexite. Fig. 6. Plot of 1 − (1 − x)1/3 versus time for different reaction temperature.
rate constant values and their correlation coefficients are given for the diffusion through the product layer and the chemical reaction controlled models in Table 2. Furthermore, the linear relation between 1 − (1 − x)1/3 and the reaction time can be seen in Figs. 5 and 6 for the particle size and the reaction temperature, respectively. In accordance with these results, the equation representing the kinetics of this process was determined to obey the chemical reaction model. Therefore, the rate expression for this process can be written as follows: 1−ð1−xÞ1=3 ¼ kr t:
ð6Þ
In addition, if the surface chemical reaction controls the reaction rate, the relation between the apparent rate constant and the inverse of the particle radius must be linear. This relationship is given in Fig. 7. This figure confirms that the chemical reaction model can be the ratecontrolling step for this process. To include the effect of the reaction parameters on the rate constant of reaction, a semi-empirical model can be written as follows: a
b
c −Ea =RT
kr ¼ k0 C D ðS=LÞ e
:
ð8Þ
The constants a, b, and c were estimated from the apparent rate constants given in Table 2. The values of these constants were
Fig. 7. Plot of rate constants versus 1/R0 for different particle sizes.
1− 1−xB2 O3
1=3
¼ 6 104 C 0:47 D−0:81 ðS=LÞ−0:67 expð−58200=RT Þt:
ð9Þ
ð7Þ
Combining Eqs. (6) and (7), the following equation is obtained: 1−ð1−xÞ1=3 ¼ k0 C a Db ðS=LÞc e−Ea =RT t:
calculated as 0.47, −0.81, and −0.67, respectively. The activation energy of the dissolution process was determined from the Arrhenius equation. The Arrhenius plot of the process is shown in Fig. 8. From the slope of the straight line in this figure, the activation energy of the process was calculated to be 58.2 kJ/mol. Diffusion controlled heterogeneous processes are slightly dependent on temperature, while chemically controlled processes are strongly dependent on temperature. Therefore, the value of the activation energy of a dissolution reaction may be used to predict the rate-controlling step. The activation energy of a diffusion controlled process is typically between 4 and 12 kJ/mol, while, for a chemically controlled process, the value is usually greater than 40 kJ/mol (Abdel-Aal, 2000; Ashraf et al., 2005). The value of the activation energy obtained in the dissolution process confirms that this leaching process is controlled by the chemical reaction. As a result, the following kinetic expression including the parameters used in this dissolution process can be written:
To test the agreement between the experimental conversion and the values calculated from the semi-empirical model, the graph of xexp versus xcal was plotted, as shown in Fig. 9. It is observed that the agreement between the experimental and the calculated values is very good.
Fig. 9. Agreement between experimental and calculated conversion values.
202
N. Demirkıran / Hydrometallurgy 95 (2009) 198–202
4. Conclusion In this work, the dissolution kinetics of ulexite in ammonium nitrate solutions was investigated. The effect of the several parameters on the dissolution rate was examined and evaluated. It was determined that the conversion rate increased with increases in ammonium nitrate concentration and reaction temperature and with decreases in solid-to-liquid ratio and particle size. It was observed that the dissolution rate was not affected by stirring speed. It was determined that the dissolution process was controlled by the chemical reaction. The activation energy of the dissolution reaction was found to be approximately 58.2 kJ/mol. According to the experimental results, ammonium nitrate solutions can be used as a leachant for the mineral ulexite. References Abdel-Aal, E.A., 2000. Kinetics of sulfuric acid leaching of low-grade zinc silicate ore. Hydrometallurgy 55, 247–254. Alkan, M., Oktay, M., Kocakerim, M.M., Karagölge, Z., 1991. Dissolution kinetics of some borate minerals in CO2-saturated water. Hydrometallurgy 26, 255–262. Alkan, M., Doğan, M., Namlı, H., 2004. Dissolution kinetics and mechanism of ulexite in oxalic acid solutions. Ind. Eng. Chem. Res. 43, 1591–1598. Ashraf, M., Zafar, Z.I., Ansari, T.M., 2005. Selective leaching kinetics and upgrading of low-grade calcareous phosphate rock in succinic acid. Hydrometallurgy 80, 286–292. Bingöl, D., Canbazoğlu, M., Aydoğan, S., 2005. Dissolution kinetics of malachite in ammonia/ammonium carbonate leaching. Hydrometallurgy 76, 55–62. Çetin, E., Eroğlu, İ., Özkar, S., 2001. Kinetics of gypsum formation and growth during the dissolution of colemanite in sulfuric acid. J. Crystal Growth 231, 559–567. Demirkıran, N., Künkül, A., 2007. Dissolution kinetics of ulexite in perchloric acid solutions. Int. J. Miner. Process. 83, 76–80. Ekmekyapar, A., Oya, R., Künkül, A., 2003. Dissolution kinetics of an oxidized copper ore in ammonium chloride solution. Chem. Biochem. Eng. Q. 17, 261–266. Ju, S., Motang, T., Shenghai, Y., Yingnian, L., 2005. Dissolution kinetics of smithsonite ore in ammonium chloride solution. Hydrometallurgy 80, 67–74. Kocakerim, M.M., Alkan, M., 1988. Dissolution kinetics of colemanite in SO2-saturated water. Hydrometallurgy 19, 385–392. Kocakerim, M.M., Çolak, S., Davies, T., Alkan, M., 1993. Dissolution kinetics of ulexite in CO2-saturated water. Can. Metall. Q. 32, 393–396. Kum, C., Alkan, M., Kocakerim, M.M., 1994. Dissolution kinetics of calcined colemanite in ammonium chloride solution. Hydrometallurgy 36, 259–268. Kurtbaş, A., Kocakerim, M.M., Küçük, Ö., Yartaşı, A., 2006. Dissolution of colemanite in aqueous solution saturated with both sulfur dioxide (SO2) gas and boric acid. Ind. Eng. Chem. Res. 45, 1857–1862. Künkül, A., Yapıcı, S., Kocakerim, M.M., Çopur, M., 1997a. Dissolution of thermally dehydrated ulexite in sulfuric acid solution. Ind. Eng. Chem. Res. 36, 4847–4851.
Künkül, A., Yapıcı, S., Kocakerim, M.M., Çopur, M., 1997b. Dissolution kinetics of ulexite in ammonia solutions saturated with CO2. Hydrometallurgy 44, 135–145. Künkül, A., Demirkıran, N., Baysar, A., 2003. Dissolution kinetics of ulexite in ammonium sulfate solutions. Ind. Eng. Chem. Res. 42, 982–986. Künkül, A., Demirkıran, N., 2007. Dissolution kinetics of calcined ulexite in ammonium carbonate solutions. Korean J. Chem. Eng. 24, 947–952. Levenspiel, O., 1972. Chemical Reaction Engineering. John Wiley, New York. Mazet, N., 1992. Modeling of gas–solid reactions 1. Nonporous solids. Int. Chem. Eng. 32, 271–284. Morales, G.V., Capretto, M.E., Fuentes, L.M., Quiroga, O.D., 2000. Dissolution kinetics of hydroboracite in water saturated with carbon dioxide. Hydrometallurgy 58, 127–133. Okur, H., Tekin, T., Ozer, A.K., Bayramoglu, M., 2002. Effect of ultrasound on the dissolution of colemanite in H2SO4. Hydrometallurgy 67, 79–86. Ranjitham, A.M., Khangaonkar, P.R., 1990. Leaching behaviour of calcined magnesite with ammonium chloride solutions. Hydrometallurgy 23, 177–189. Raschman, P., 2000. Leaching of calcined magnesite using ammonium chloride at constant pH. Hydrometallurgy 56, 109–123. Taylan, N., Gürbüz, H., Bulutçu, A.N., 2007. Effects of ultrasound on the reaction step of boric acid production process from colemanite. Ultrason. Sonochem. 14, 633–638. Tekin, G., Onganer, Y., Alkan, M., 1998. Dissolution kinetics of ulexite in ammonium chloride solution. Can. Metall. Quart. 37, 91–97. Temur, H., Yartaşı, A., Çopur, M., Kocakerim, M.M., 2000. The kinetics of dissolution of colemanite in H3PO4. Ind. Eng. Chem. Res. 39, 4114–4119. Tunç, M., Kocakerim, M.M., Yapici, S., Bayrakçeken, S., 1999. Dissolution mechanism of ulexite in H2SO4 solution. Hydrometallurgy 51, 359–370. Tunç, M., Kocakerim, M.M., Küçük, Ö., Aluz, M., 2007. Dissolution of colemanite in (NH4)2SO4 solutions. Korean J. Chem. Eng. 24, 55–59. Wen, C.Y., 1968. Noncatalytic heterogeneous solid–fluid reaction models. Ind. Eng. Chem. 60, 34–54.
Glossary C: concentration of ammonium nitrate, (mol/cm3) D: particle diameter, (cm) Ea: activation energy, (J/mol) S/L: solid-to-liquid ratio, (g/mL) R0: average radius of solid particle, (cm) R: universal gas constant, (J/molK) T: temperature, (K) t: reaction time, (s) x: converted fraction of B2O3 kd: apparent rate constant for diffusion through the product layer, (s− 1) kr: apparent rate constant for the surface chemical reaction, (s− 1) k0: frequency or pre-exponential factor, (s− 1) a, b, c: constants in Eq. (7)