Ind. Eng. Chem. Res. 2004, 43, 8403-8406
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Solubility of KVO3 in Water + Ammonia Solutions from 293 to 323 K Mieczysław Trypuc´ , Katarzyna Białowicz,* and Krzysztof Mazurek Faculty of Chemistry, Nicolaus Copernicus University, 7 Gagarin Street, 87-100 Torun´ , Poland
The solubility of KVO3 in water + ammonia solutions was determined within the temperature range of 293-323 K by employing the isothermal solution saturation method. The initial concentration of NH3 in solution varied between 1 and 6 mol‚dm-3. The density of the equilibrated solutions was also determined. The data presented herein are essential for the assessment of optimum operating conditions for the production process of potassium carbonate introducing the modified Solvay method. 1. Introduction Modern methods of potassium carbonate production have a low material and energy efficiency and significant environmental hazards. Therefore, the research on the wasteless method of the production was initiated. The proposed method allows the simultaneous production of potassium carbonate with chlorine or hydrogen chloride and with the use of V2O5 as an additional reagent. The reaction of vanadium(V) oxide with KCl gives a product not containing undesirable chloride ion and easily converted to K2CO3. Moreover, V2O5 is completely recovered and utilized in the process.1 The proposed technological process consists of four major steps. In the first step, vanadium(V) oxide reacts with KCl in the presence of oxygen or steam, producing potassium metavanadate(V) and Cl2 or HCl, respectively. Our results showed that under optimal conditions the yield of the KVO3 synthesis in the presence of atmospheric oxygen is 89%, while with the use of steam, it is 90%.2-7 In the second step, the produced potassium metavanadate(V) is subjected to the unit processes analogous to the Solvay method, i.e., absorption of ammonia and carbonization. These processes result in the precipitation of insoluble ammonium metavanadate(V) and simultaneous formation of potassium bicarbonate and carbonate, which, in contrast to the Solvay method, remain in solution.8-10 After filtration of precipitated NH4VO3, the solution is concentrated. Crystals of KHCO3, formed at the end of this process, are washed and calcined to form a final product. The important advantage of this method is that NH4VO3, separated from the mother liquor and subsequently washed and dried, is thermally degraded into vanadium(V) oxide, ammonia, and water, which are recovered and recycled in adequate stages of the process.11 Also, research was initiated on the use of the waste vanadium catalyst from the production of sulfuric(VI) acid in the contact process as a source of V2O5 necessary for the synthesis of potassium metavanadate(V). Such a possibility makes the method reported here very attractive for ecological reasons.12 * To whom correspondence should be addressed. Tel.: (0048) 566114537. Fax: (0048) 566542477. E-mail: fiodor@ chem.uni.torun.pl.
A very important step of the vanadate method of K2CO3 production is the process of carbonization of a water + ammonia solution of KVO3, consisting of polythermic-polybaric chemisorption of carbon dioxide and crystallization of ammonium metavanadate according to the equations
KVO3 + NH3 + CO2 + H2O T NH4VO3V + KHCO3 (1) 2KVO3 + 2NH3 + CO2 + H2O T 2NH4VO3V + K2CO3 (2) Efficacy of that process would determine the basic material and energetic indicators of the described method. The detailed research on the carbonization process of water + ammonia solutions of potassium metavanadate(V) should focus on the preparation of the KVO3 water + ammonia solutions of a required composition, which would subsequently be carbonized to reach the desired yield relative to the ammonium cation. This requires determination of the solubility in the KVO3 + H2O9,13,14 and KVO3 + NH3 + H2O systems. In the available literature, we have not found any data on the solubility of KVO3 in the aqueous ammonia solutions. Therefore, it was necessary to determine the dependence of the potassium metavanadate(V) solubility on temperature and the ammonia concentration as well as to identify the solid phases. 2. Experimental Section 2.1. Chemicals. Analytically pure KVO3 (purity of 98%, Aldrich Chemical Co., Inc.) and NH3 (Nitrogen Works Tarno´w-Mos´cice SA, Poland) were used without further purification. 2.2. Experimental Procedure. The solubility measurements of potassium vanadate(V) in water + ammonia solutions were carried out at the temperature range of 293-323 K using the isothermal solution saturation method. The starting concentration of NH3 varied from 1 to 6 mol‚dm-3. The required amount of salt and water + ammonia solutions of the required concentration in the 100-cm3 capacity Erlenmeyer flasks were thermostated at a selected temperature for 120 h with constant stirring to ensure the equilibration between the solid phase and
10.1021/ie040128f CCC: $27.50 © 2004 American Chemical Society Published on Web 11/30/2004
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solution. For each initial water + ammonia solution, a constant volume of 50 cm3 was designed to provide repeatability of the measurements and consistency for data interpretation. The temperature was kept constant with a Bioblock Scientific thermorelay with an accuracy of (0.02 K. Additionally, the temperature was controlled with a mercury thermometer with an accuracy of (0.05 K. After equilibration, the clear solution was sampled into the Ostwald pycnometer calibrated for the temperature range of 293-323 K to determine the solution density. The accuracy of that determination was (0.002 g‚cm-3. Next, the solution from the pycnometer was transferred to the flask, diluted, and analyzed to determine the concentrations of different system components. 2.3. Analytical Methods. The concentration of vanadium(V) in the equilibrated solutions was determined by a spectrophotometric analysis using the peroxide method. The analysis was performed with Hitachi U-2000 a double-beam UV-vis spectrophotometer. A detailed description of the techniques used can be found in earlier papers reported by the authors.13-16 The error of the vanadium determination is approximately 2%. The potassium ion concentration was determined by weight with sodium tetraphenylborate (Na[B(C6H5)4]) as a precipitating agent. 14 The precision of the measurements was estimated within 2%. The concentrations of the water + ammonia solutions were determined with the automatic distillation apparatus VAPODEST 30.17,18 The precision of the measurements was estimated within 2%. The identification of the solid phases equilibrated with the mother solution was carried out by an X-ray method.19 The analysis was performed for the precipitates separated from the solution by a fast vacuum filtration with a sintered-glass crucible. 3. Results and Discussion The results of the chemical analyses of the solutions at different temperatures are presented in Table 1. Each value presented in Table 1 was determined upon the arithmetical average of results obtained from the analysis of three simultaneously prepared equilibrium solutions. Data on the KVO3 solubility in water13,14 are also given in Table 1 as a reference point. We have found that substrates in the investigated system react according to the equation
KVO3 + NH3 + H2O T NH4VO3V + KOH
(3)
and the insoluble ammonium metavanadate(V) is produced. This is the reason that the concentrations of K+ and VO3- ions given in Table 1 are different because some of the vanadate ions introduced to the system as KVO3 precipitate as NH4VO3. A similar effect was also observed in the research on the NaVO3 solubility in the aqueous solutions of ammonia.18 As long as KVO3 is the only salt remaining in an equilibrium with the solution, [K+] ) [VO3-], and the investigated system is a ternary system. When the first ammonium metavanadate(V) crystals are formed, the system becomes a quaternary KVO3 + NH4VO3 + NH3 + H2O system, and the solution contains six components: K+, NH4+, VO3-, OH-, NH3, and H2O. Their
Table 1. Solubility in the KVO3 + NH3 + H2O System c/mol‚dm-3 F/g‚cm-3
NH3 init
K+
VO3-
K+-VO3-
solid phase
1.064 1.160 1.212 1.269 1.260 1.213
0.00 1.16 2.36 3.60 4.81 6.05
T ) 293 K 0.604 0.604 1.71 1.54 2.57 2.24 3.10 2.47 3.51 2.37 3.59 1.99
1.085 1.182 1.238 1.278 1.240 1.195
0.00 1.16 2.36 3.60 4.81 6.05
0.842 2.08 2.88 3.62 3.97 4.07
T ) 303 K 0.842 1.74 2.32 2.64 2.54 2.09
0.00 0.34 0.56 0.98 1.43 1.98
KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3
1.121 1.206 1.254 1.270 1.247 1.196
0.00 1.36 2.49 3.66 4.96 6.09
1.18 2.85 3.70 4.52 5.02 5.08
T ) 313 K 1.18 2.30 2.81 2.91 2.71 2.19
0.00 0.55 0.89 1.61 2.31 2.89
KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3
1.171 1.233 1.273 1.277 1.248 1.185
0.00 1.36 2.49 3.66 4.96 6.09
1.77 3.18 4.01 4.79 5.20 5.36
T ) 323 K 1.77 2.50 2.90 2.94 2.63 2.12
0.00 0.68 1.11 1.85 2.57 3.24
KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3
0.00 0.17 0.33 0.63 1.14 1.60
KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3 NH4VO3, KVO3
concentrations are related by two relationships: the ammonia dissociation constant and the following equation
[K+] + [H+] + [NH4+] ) [VO3-] + [OH-]
(4)
Because of the small value of the ammonia dissociation constant in water, relatively high concentrations of ammonia and OH- ions in the equilibrated solutions, and the low solubility of NH4VO3,8 the [NH4+] and [H+] terms in eq 4 are negligible, and consequently
[K+] = [VO3-] + [OH-]
(5)
[OH-] . [H+] + [NH4+]
(6)
where
The concentrations of hydroxyl ions in the equilibrated solutions, calculated according to eq 5, are presented in Table 1. Analyses for K+, VO3-, and NH3 are necessary to characterize the system. Assuming that there is no change in volume upon the addition of salt to the ammonia solution, material balance equations for K+, VO3-, and NH3 give
[NH3]init ) [NH3] + [NH4+] + ([K+] - [VO3-]) (7) where subscript init indicates the initial concentration before the addition of salt. We can utilize the approximate form
[NH3]init ) [NH3] + ([K+] - [VO3-])
(8)
[NH3] . [NH4+]
(9)
where
Ind. Eng. Chem. Res., Vol. 43, No. 26, 2004 8405
Figure 1. K+ ions concentration dependence on the initial NH3 concentration and temperature: (b) T ) 293 K; (O) T ) 303 K; (1) T ) 313 K; (3) T ) 323 K.
Thus, the ammonia concentration in the equilibrated solution can be calculated approximately from the initial concentration. The KVO3 solubility in the water + ammonia solutions was determined based on the concentration of potassium ions in the solution. The K+ ions are introduced into the system exclusively by the dissolved KVO3, and therefore their concentration represents the solubility of that salt in the water + ammonia solutions. Comparison of the KVO3 solubility in the water + ammonia solutions and its solubility in the water at the same temperature (Table 1) clearly indicates that the presence of NH3 results in a significant increase of the KVO3 solubility. We found that the concentration of potassium ions in the equilibrated solutions rises with increasing NH3 concentration at the investigated temperature range. It should be emphasized that the salt effect of ammonia on the solubility of potassium metavanadate(V) is useful for the industrial implementation of the vanadate method of potassium carbonate production. Analysis of the data listed in Table 1 also reveals that the KVO3 solubility increases with the temperature elevation at the whole range of NH3 concentrations in the solution. The graphical interpretation of these results is presented in Figure 1, which illustrates the changes of the K+ concentration as a function of temperature and the initial concentration of water + ammonia solutions. On the basis of the data in Table 1, the ratio of KVO3 conversion into NH4VO3 might be calculated according to the following definition:
[K+] - [VO3-]/[K+]
(10)
The dependence of the conversion ratio of potassium metavanadate(V) into ammonium metavanadate(V) calculated from eq 10 on the initial NH3 concentration and temperature is presented in Figure 2. The curves indicate that the amount of NH4VO3 synthesized in the reaction rises with increasing NH3 concentration, reaching a maximum of 0.605 at 323 K and an ammonia
Figure 2. Dependence of the conversion ratio of KVO3 into NH4VO3 on the initial NH3 concentration and temperature: (b) T ) 293 K; (O) T ) 303 K; (1) T ) 313 K; (3) T ) 323 K.
concentration of approximately 6 mol‚dm-3. The KVO3 conversion ratio also significantly rises with a temperature increase at a constant initial concentration of ammonia, which indicates the endothermic character of reaction (3). It has to be stated that the chemical reaction in the investigated system differs from the classical Solvay system of sodium carbonate production in the process of carbonization of NaCl water + ammonia solutions because precipitation of NH4VO3 occurs partly before carbonization, which totally changes its course. The density changes of the equilibrated solutions result from two effects. For each investigated temperature, the solution density rises with increasing NH3 concentration to 3.6 mol‚dm-3. This is a consequence of the significant increase of the KVO3 solubility at this range of NH3 concentrations. Above this concentration, the solution density decreases with the rise of the initial concentration of NH3. The changes of the equilibrated solution density for NH3 concentrations between 3.6 and approximately 6 mol‚dm-3 are no longer determined by the increasing solubility of potassium metavanadate(V) but rather the increasing initial concentration of ammonia. This phenomenon is consistent with the general tendency observed for water + ammonia solutions, for which the density decreases with an increase of the NH3 concentration. The X-ray analysis of the solid phase revealed that, for all investigated samples, the precipitate remaining in equilibrium with the water + ammonia solution is a mixture of NH4VO3 and KVO3. This is a confirmation that the reaction in the system is described by eq 3. In the aqueous solutions of KVO3 with no ammonia added, the precipitate in the equilibrated systems contains only KVO3.13,14 4. Conclusions (1) The precipitated NH4VO3 is a product of chemical reaction in the KVO3 + NH3 + H2O system. (2) The conversion ratio of KVO3 into NH4VO3 is limited by the initial concentration of NH3 and temperature.
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(3) The precipitate in equilibrium with the water + ammonia mother liquor is a mixture of KVO3 and NH4VO3. Literature Cited (1) Trypuc´, M.; Stefanowicz, D.; Kiełkowska, U.; -Lyjak, G.; Torski, Z. Sposoby wytwarzania we¸ glanu potasu. Zgłoszenie patentowe Rp/4242/98, 1998. (2) Trypuc´, M.; Torski, Z.; Kiełkowska, U.; Białowicz, K. Zastosowanie V2O5 przy opracowaniu bezodpadowej metody wytwarzania potaz˘ u. Pr. Nauk. Politech. Wrocław. 1999, 48, 228. (3) Trypuc´, M.; Torski, Z.; Białowicz, K.; Kiełkowska, U. Badania nad stopniem przemiany KCl w KVO3 w obecnos´ci V2O5 i pary wodnej. Zesz. Nauk. Politech. SÄ la¸ sk., Chem. 2001, 142, 227. (4) Trypuc´, M.; Torski, Z.; Białowicz, K. Investigations on the influence of silicon dioxide introduced as a neutral carrier on V2O5 conversion into KVO3. Pol. J. Chem. Technol. 2001, 3 (1), 28. (5) Trypuc´, M.; Białowicz, K.; Kiełkowska, U.; Torski, Z.; Mazurek, K. Sythesis of KVO3 from KCl and V2O5 in Presence of Oxygen. Pol. J. Chem. Technol. 2001, 3 (1), 33. (6) Trypuc´, M.; Białowicz, K.; Mazurek, K.; Kiełkowska, U. Eksperymentalne wyznaczenie optymalnych parametro´w syntezy KVO3 z V2O5 i KCl w obecnos´ci tlenu z powietrza. Przem. Chem. 2004, accepted for publication. (7) Trypuc´, M.; Białowicz, K.; Mazurek, K. Investigations on the synthesis of KVO3 and Cl2 from KCl and V2O5 in presence of oxygen. Chem. Eng. Sci. 2004, 59 (6), 1241. (8) Trypuc´, M.; Białowicz, K. Solubility of NH4VO3 in Water + Ammonia. J. Chem. Eng. Data 1997, 42, 318. (9) Trypuc´, M.; Stefanowicz, D. I. Solubility in the KVO3 + NH4VO3 + H2O System. J. Chem. Eng. Data 1997, 42, 1140.
(10) Trypuc´, M.; Kiełkowska, U.; Stefanowicz, D. I. Solubility investigations in the KHCO3 + NH4HCO3 + H2O system. J. Chem. Eng. Data 2001, 46, 800. (11) Shimizu, A.; Watanabe, T.; Inagaki, M. Single-crystal Study of Topotatic Changes between NH4VO3 and V2O5. J. Mater. Chem. 1994, 4 (9), 1475. (12) Trypuc´, M.; Białowicz, K.; Mazurek, K. Wykorzystanie odpadowego katalizatora wanadowego do syntezy NaVO3 i KVO3 metoda¸ bezodpadowa¸ . Monografia Kwas siarkowy-nowe wyzwania; Instytut Ochrony Ros´lin: Poznan´, 2003; p 67; ISBN 83-9162049-2. (13) Trypuc´, M.; Białowicz, K.; Mazurek, K. Solubility in the KVO3 + KCl + H2O System from 293 to 323 K. Ind. Eng. Chem. Res. 2002, 41 (17), 4174. (14) Trypuc´, M.; Kiełkowska, U. Solubility diagram for the system KHCO3 + KVO3 + H2O at 293-323 K. Fluid Phase Equilib. 2003, 213, 81. (15) Williams, W. J. Oznaczanie aniono´ w; PWN: Warsaw, Poland, 1985. (16) Sandell, G. Calorimetric metal analysis; Interscience Publishers: New York, 1959. (17) Trypuc´, M.; Białowicz, K. Solubility of NH4VO3 in Water + Ammonia. J. Chem. Eng. Data 1997, 42, 318. (18) Trypuc´, M.; Białowicz, K. Solubility in the System NaVO3 + NH4VO3 + NH3 + H2O from 293 to 323 K. J. Chem. Eng. Data 2000, 45, 492. (19) Powder Diffraction File; Joint Committee on Powder Diffraction Standards: Washington, DC, 1976.
Received for review April 19, 2004 Accepted November 12, 2004 IE040128F
