Investigation of Mutual Solubility in the NH4VO3−NH4NO3−H2O system

Apr 7, 2009 - Mieczysław Trypuc* and Sebastian Druz˙ynski. Department of Chemical Technology, Faculty of Chemistry, Nicolaus Copernicus UniVersity,...
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Ind. Eng. Chem. Res. 2009, 48, 5058–5063

Investigation of Mutual Solubility in the NH4VO3-NH4NO3-H2O system Mieczysław Trypuc´* and Sebastian Druz˙yn´ski Department of Chemical Technology, Faculty of Chemistry, Nicolaus Copernicus UniVersity, Gagarina 7, 87-100 Torun´, Poland

The solubilities of ammonium vanadate and ammonium nitrate in the NH4VO3-NH4NO3-H2O system were investigated at 293, 303, 313, and 323 K by the method of isothermal saturation of solutions. A section of the solubility isotherm for the investigated system was plotted based on the obtained data. Knowledge of that dependence is necessary for developing the new method of using the postfiltration liquor from the soda-chlorine-saltpeter (SCS) method of sodium carbonate production, based on an equilibrium plot for the quaternary system of pairs of the exchange salts NH4NO3-NaVO3-H2O. 1. Introduction Currently, sodium carbonate is produced mainly by the classic Solvay method. This process is characterized by a low energy and material efficiency and noxiousness for the natural environment caused by large amounts of liquid and solid waste. The liquid waste containing all of the chloride ions involved in the process is directed to water reservoirs, and the solid waste is stored in so-called white seas.1 Therefore, many modifications and improvements in the method have been developed, among which the sodachlorine-saltpeter (SCS) process is one of the most important. This method is based on carbonization of the ammoniated brine of sodium nitrate according to the equation2,3

In practice, after filtration of the precipitate of sparingly soluble ammonium vanadate, the liquor contains only sodium nitrate. This liquor is subsequently concentrated and either subjected to crystallization of the salt, which might be used in agriculture as a nitrogenous fertilizer, or redirected to the process of ammonization and carbonization according to eq 1. Figure 1 presents the chemical flow diagram for the process of ammonium nitrate conversion to ammonium vanadate. Selection of the optimal parameters for the precipitation of ammonium vanadate from the postfiltration liquor requires

NaNO3 + NH3 + CO2 + H2O T NaHCO3 + NH4NO3 (1) The main components of the liquor obtained after filtration of the precipitated sodium bicarbonate are ammonium nitrate, unreacted sodium nitrate, and a small amount of sodium chloride from the production of sodium nitrate. This liquor is a material for the production of nitrogenous fertilizer in the form of ammonium-sodium saltpeter.2,3 In contrast to the Solvay method, in such an approach to the production of sodium carbonate, the energy-demanding process of ammonia regeneration with using calcium hydroxide is not required, which eliminates the waste from the method. Despite these advantages, the SCS method is not broadly used because of its significant defect. The production and storage of ammonium-sodium saltpeter creates a serious explosion danger. Ammonium nitrate is a strong and thermally unstable oxidant, which, in the presence of reducing agents, undergoes an uncontrolled decomposition at elevated temperatures.3,4 The postfiltration liquor contains small amounts of chloride ions remaining after the process of oxidation of sodium chloride with nitric acid in the presence of air. Chloride ions are reducing agents and catalyze the complicated process of ammonium nitrate decomposition.4,5 The problem can be solved by transforming the ammonium nitrate into sodium nitrate, which has significantly greater thermal stability. This transformation is based on the doubleexchange reaction between ammonium nitrate and sodium vanadate according to the equation6-9 NH4NO3 + NaVO3 T NH4VO3 + NaNO3

(2)

* To whom correspondence should be addressed. Tel.: (+48) 566114537. Fax: (+48) 566542477. E-mail: [email protected].

Figure 1. Chemical flow diagram of conversion of ammonium nitrate into ammonium vanadate.

10.1021/ie801341j CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

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Figure 2. Hypothetical equilibrium plot for pairs of exchange salts in the planar projection (E, eutonic points for the ternary systems; P1 and P2, ternary points of the quaternary system). Table 1. Comparison of Water Solubilities of Pure NH4VO3 and NH4NO3 solubility of NH4VO3 (g/100 g of H2O)

solubility of NH4NO3 (g/100 g of H2O)

T (K)

this work

ref 10

this work

ref 11

293 303 313 323

0.62 0.86 1.21 1.70

0.59 0.87 1.24 1.74

182 223 275 318

183 227 281 344

precise knowledge of the equilibrium plot in a planar projection prepared according to the Ja¨necke method for the systems of exchange salts NH4NO3-NaVO3-H2O and four ternary sysNH4VO3-NaVO3-H2O, tems: NaVO3-NaNO3-H2O, NaNO3-NH4NO3-H2O, and NH4NO3-NH4VO3-H2O. These are subsystems of the quaternary system mentioned previously, and they are located on the respective edges of the square on the equilibrium plot prepared with the Ja¨necke method. A hypothetical equilibrium plot in a planar projection is presented in Figure 2. Literature reports concerning the ternary systems mentioned above are not complete, and no data are available for the quaternary system. Complete reports have been published for the NaVO3-NH4VO3-H2O,10 NaNO3-NH4NO3-H2O,11-16 and NaVO3-NaNO3-H2O8 systems for the temperature range 293-323 K. Also, no report is available on the mutual solubility in the NH4NO3-NH4VO3-H2O system. Therefore, we decided to perform equilibrium research on that system. 2. Experimental Section 2.1. Reagents. In the reported research, analytical-grade reagents were used, namely, NH4VO3 (g99%, Aldrich) and NH4NO3 (POCh Gliwice), with no further purification. 2.2. Methods. Determination of the mutual solubility of salts in the NH4VO3-NH4NO3-H2O system was performed at 293, 303, 313, and 323 K by the method of isothermal saturation of solutions.6-11 The required amounts of components, with one of them used in excess relative to its water solubility, were placed in an Erlenmeyer flask, and an appropriate amount of redistilled water was added. The solutions were thermostatted and stirred until equilibrium between the phases was reached. The experimentally determined equilibration time was approximately 120 h. After that time, stirring was stopped, and after a 50-h sedimentation period, the clear equilibrated solution was sampled into a precalibrated Ostwald pycnometer at the

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required temperature for a density measurement with the precision of (0.002 g cm-3. Sample collection from the flasks was performed at elevated pressure with the use of the micrometering pump, which prevented the crystallization of the solution components, especially at higher temperatures. The contents of the pycnometer were subsequently transferred quantitatively into a 500 cm3 measuring flask, which was then filled with distilled water. The composition of the equilibrated solutions was determined based on the results of chemical analyses of the collected samples. Analyses were performed for the concentrations of ammonium ions, the sum of nitrate and ammonium ions as total ammonia, and vanadate ions. For selected experimental points, XRD analyses of the solid phase were also performed. 2.3. Analytical Methods. The concentration of ammonium ions was determined by the formalin method, which is based on a reaction of formaldehyde with ammonium ions. The products of this reaction are hexamethylenetetramine and acid in an amount equivalent to the amount of ammonium ions, according to eq 3. The amount of acid was determined by titration with a standard solution of sodium hydroxide.17 4NH4NO3 + 6HCHO f (CH2)6N4 + 4HNO3 + 6H2O (3) The total amount of ammonium and nitrate ions (NH3,tot) was determined by the distillation method. The nitrate ions were reduced with a Devard alloy in concentrated sodium hydroxide. The distilled ammonia was absorbed in a standard solution of sulfuric acid, the excess of which was titrated with a standard solution of sodium hydroxide.17,18 The concentration of vanadate ions was determined spectrophotometrically as a complex with hydrogen peroxide. The analyses were performed with a double-beam Hitachi U-2000 UV/vis spectrophotometer, using 10-mm quartz absorption cells. In the presence of concentrated sulfuric acid and a 3% solution of hydrogen peroxide, vanadium forms the compounds V(O2)X3 and/or V(O2)X52-, where X is a monoanion, according to the equation18 (VO)2(SO4)3 + 2H2O2 T [V(O2)]2(SO4)3 + 2H2O

(4)

This compound has a maximum absorbance at wavelength 450 nm, and the molar absorption coefficient is 300 dm3 mol-1 cm-1.18 For samples of solutions with low vanadium concentrations below 2 × 10-2 mol dm-3, the spectrophotometric method with 2-(4-pyridylazo)resorcinol (PAR) was applied. Experiments were again performed with a double-beam Hitachi U-2000 UV/vis spectrophotometer, using 10-mm quartz absorption cells. Vanadium compounds at pH 5-6 form a complex with PAR that has a maximum absorbance at a wavelength of 540 nm, and the molar absorption coefficient is 3.6 × 104 dm3 mol-1 cm-1.18 For selected experimental points, analysis of the solid phase was performed with an X-ray diffractometer (Philips X-Pert PRO System). To determine the composition of the solid phase, the obtained diffractograms were compared to the diffraction patterns of the respective standards.19 For all collected samples, three independent analyses of the ions mentioned above were performed. The accuracy of the analyses, determined as the relative error for all of the analytical methods used, did not exceed 1%; the relative standard deviation (RSD) for all analyses also did not exceed 1%. The equilibrium concentrations of ammonium nitrate and ammonium vanadate were calculated from the following dependencies: [NH4NO3] ) [NH4+] - [VO3-], [NH4VO3] ) [VO3-].

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Table 2. Solubility in the NH4VO3-NH4NO3-H2O System c (mol/1000 g of H2O) no.

d (g cm-3)

NH4VO3

NH4NO3

xNH4VO3

solid-phase composition

0 4 × 10-6 5 × 10-6 7 × 10-6 8 × 10-6 9 × 10-6 1.2 × 10-5 2.1 × 10-5 3.5 × 10-5 9.2 × 10-5 1.32 × 10-3 5.92 × 10-3 0.0320 0.1399 0.3835 1

NH4NO3 NH4NO3 + NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3

0 6 × 10-6 9 × 10-6 1.2 × 10-5 2.0 × 10-5 5.5 × 10-5 2.82 × 10-4 6.56 × 10-4 3.24 × 10-3 0.0169 0.0935 0.3115 1

NH4NO3 NH4NO3 + NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3

0 1.0 × 10-5 1.1 × 10-5 1.4 × 10-5 2.7 × 10-5 6.3 × 10-5 2.67 × 10-4 1.26 × 10-3 7.62 × 10-3 0.0397 0.1768 0.4811 1

NH4NO3 NH4NO3 + NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3

0 1.4 × 10-5 1.9 × 10-5 2.5 × 10-5 3.5 × 10-5 6.1 × 10-5 1.91 × 10-4 2.87 × 10-3 0.0166 0.0864 0.3018 0.5672 0.7492 1

NH4NO3 NH4NO3 + NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3 NH4VO3

T ) 293 K 1 2 (E) 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1.307 1.305 1.274 1.232 1.191 1.165 1.145 1.117 1.088 1.057 1.012 1.005 1.000 0.999 0.999 1.001

0 3.5 × 10-5 3.7 × 10-5 4.2 × 10-5 4.5 × 10-5 4.7 × 10-5 5.0 × 10-5 6.6 × 10-5 9.4 × 10-5 1.59 × 10-4 6.38 × 10-4 1.51 × 10-3 4.53 × 10-3 0.0130 0.0284 0.0532

1 2 (E) 3 4 5 6 7 8 9 10 11 12 13

1.325 1.325 1.269 1.219 1.163 1.104 1.043 1.028 1.011 1.004 1.001 1.000 1.000

0 5.5 × 10-5 6.6 × 10-5 7.7 × 10-5 9.6 × 10-5 1.76 × 10-4 4.06 × 10-4 6.34 × 10-4 1.55 × 10-3 4.22 × 10-3 0.0127 0.0310 0.0733

1 2 (E) 3 4 5 6 7 8 9 10 11 12 13

1.344 1.343 1.248 1.212 1.165 1.113 1.055 1.023 1.007 1.000 0.998 0.998 1.000

0 9.1 × 10-5 1.12 × 10-4 1.17 × 10-4 1.35 × 10-4 2.27 × 10-4 5.16 × 10-4 1.24 × 10-3 3.62 × 10-3 9.94 × 10-3 0.0270 0.0553 0.1035

22.70 22.79 17.88 13.18 9.388 7.631 6.315 4.933 3.441 2.011 0.503 0.257 0.139 0.080 0.046 0 T ) 303 K 27.81 27.83 18.16 12.36 7.612 4.296 1.630 1.047 0.496 0.251 0.124 0.069 0 T ) 313 K 34.37 34.39 15.85 12.34 8.294 5.088 2.286 1.066 0.490 0.245 0.127 0.059 0 T ) 323 K

1 2 (E) 3 4 5 6 7 8 9 10 11 12 13 14

1.363 1.362 1.294 1.243 1.191 1.137 1.082 1.019 1.005 0.999 0.998 0.998 0.998 0.999

0 1.30 × 10-4 1.57 × 10-4 1.73 × 10-4 1.97 × 10-4 2.61 × 10-4 5.27 × 10-4 2.82 × 10-3 8.13 × 10-3 0.0236 0.0572 0.0924 0.1240 0.1456

3. Results and Discussion The experimental data on the water solubilities of the pure salts were compared with the data obtained by other authors (Table 1). The data obtained for ammonium vanadate are consistent with the literature reports. The measured solubilities of ammonium nitrate at 313 and 323 K differ significantly from the literature data. This discrepancy might be explained by crystallization of the salt during the collection of samples of

39.74 39.78 23.44 15.96 10.50 6.560 3.532 1.064 0.500 0.254 0.133 0.070 0.041 0

the equilibrated solution, which would cause the literature values of ammonium nitrate solubility to be overestimated. Therefore, the solubility data reported here are more reliable then those quoted in Table 1. The experimental data on the mutual solubility of ammonium nitrate and ammonium vanadate in water are presented in Table 2. This table presents the equilibrium concentrations of the salts, the densities (d) of the equilibrated solutions, the mole fractions

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Figure 3. Solubility isotherms for solutions saturated with ammonium nitrate: (9) 293, (() 303, (b) 313, (2) 323 K.

Figure 4. Solubility isotherms for solutions saturated with ammonium vanadate.

(x) of ammonium vanadate without taking into account the solvent, and the compositions of the solid phase for the four temperatures at which the experiments were conducted. The mole fractions of ammonium vanadate without taking into account the solvent wree calculated from the equation xNH4VO3 )

[NH4VO3] [NH4VO3] + [NH4NO3]

The data presented in Table 2 constituted the basis for plotting a section of the solubility polytherm for the investigated system (Figures 3 and 4). The presented solubility isotherms obtained for 293, 303, 313, and 323 K are composed of two branches: one for solutions saturated with ammonium nitrate (Figure 3) and one for solutions saturated with ammonium vanadate (Figure 4). The eutonic points marked as (E) in Figures 3 and 4 correspond to solutions saturated with both salts. The data in Table 2 corresponding to the solutions saturated with ammonium nitrate indicate that the concentration of this salt has a strong salting-out effect on ammonium vanadate. For that reason, the eutonic points (E) are positioned close to Y axis. For a clearer representation of that dependency, the polytherm branches corresponding to solutions saturated with ammonium nitrate are presented separately in Figure 3. The isotherms corresponding to solutions saturated with ammonium nitrate appear as straight

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Figure 5. Changes in the density of equilibrated solutions for solubility isotherms corresponding to solutions saturated with ammonium vanadate.

lines, and the concentration of ammonium nitrate is practically constant. The hyperbolic course of the isotherms corresponding to solutions saturated with ammonium vanadate (presented in Figure 4) is caused by the very strong salting-out effect of ammonium nitrate on ammonium vanadate. Starting from the eutonic point, for high concentrations of NH4NO3, the amount of NH4VO3 in the equilibrated solutions does not increase. The increase of ammonium vanadate solubility is evident only for concentrations of ammonium nitrate below 0.1 mol/1000 g of H2O. The salting-out effect decreases with increasing temperature, and for higher temperatures, an increase in the ammonium vanadate concentration is observed for high concentrations of ammonium nitrate. Detection of the formation of double salts or addition compounds in the investigated system is possible with equilibrium plots of the property-composition type.6-10,12,20 In the case of formation of a new solid phase in the system, inflection points or breaks would appear in curves presenting the dependence between the solution density and the component concentration in mole fractions, at positions corresponding to the formation of a new compound. Based on the data in Table 2, the dependency of the solution density on the mole fraction of ammonium vanadate was plotted for solutions saturated with ammonium vanadate (Figure 5). Because of the strong saltingout effect of NH4NO3 on NH4VO3, the isotherms representing solutions saturated with ammonium nitrate are not visible on plots prepared at that scale. For that reason, they are presented in Figure 6. Figure 5 does not reveal any characteristic points indicating the formation of a new compound in the solid phase. Therefore, it can be concluded that, in the investigated temperature range, the solid phase for the analyzed branches of isotherms contains only ammonium vanadate and, at the eutonic points (E), the solid phase is a mixture of the two salts. Isotherms corresponding to solutions saturated with ammonium nitrate also do not reveal the presence of the characteristic points (Figure 6), which indicates that the solid phase contains only ammonium nitrate. These conclusions were confirmed by X-ray diffraction experiments, which revealed the lack of compounds other than ammonium vanadate and ammonium nitrate for points on the branches of isotherms corresponding to solutions saturated with these salts. The typical diffraction patterns are shown in Figures 7-9.

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Figure 6. Changes in the density of equilibrated solutions for solubility isotherms corresponding to solutions saturated with ammonium nitrate: (9) 293, (() 303, (b) 313, (2) 323 K.

Figure 9. Diffraction pattern of the solid phase at the eutonic point (T ) 303 K).

ternary systems mentioned in the Introduction enable the prediction that the area of crystallization of ammonium vanadate in an equilibrium plot for the quaternary system NaVO3-NaNO3-NH4VO3-NH4NO3-H2O would dominate over the areas of crystallization of the other system components. In the investigated system, a very low solubility of ammonium vanadate was observed. A strong salting-out effect of ammonium nitrate on ammonium vanadate occurs because of the commonion effect. This is advantageous because of the equilibrium and efficiency of the reaction described by eq 2. Acknowledgment This work was supported by Grant 378-Ch from Nicolaus Copernicus University in Torun´, Poland. Figure 7. Diffraction pattern for the solid phase remaining in equilibrium with solution saturated with ammonium vanadate (T ) 303 K).

Figure 8. Diffraction pattern for the solid phase remaining in equilibrium with solution saturated with ammonium nitrate (T ) 303 K).

4. Conclusions The conducted research on the mutual solubility in the NH4VO3-NH4NO3-H2O system and the research on other

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Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 (14) Karnaukov, S. The solubility isotherm of the ternary system sodium nitrate-ammonium nitrate-water at 25°. Dokl. Akad. Nauk 1951, 81, 593– 595. (15) Nikitina, E. A. Equilibrium in the system NH4NO3-NaNO3-H2O. Zh. Obshch. Kim 1933, 3, 513–518. (16) Shenkin, Y. S.; Ruchnova, S. A.; Rodionova, N. A. Sodium nitrateammonium nitrate-water system. Zh. Neorg. Khim. 1975, 20, 2852–2854. (17) Struszyn´ski, M. QuantitatiVe and Technical Analysis; Polish Scientific Publishers: Warsaw, Poland, 1954; Vol. II. (18) Williams, W. J. Handbook of Anion Determination; Butterworth and Co Ltd.: London, 1979.

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(19) Powder Diffraction File; Joint Committee on Powder Diffraction Standards; Swarthmore, PA, 1976. (20) Sulaimankulow, K. S. Carbamide Compounds with Inorganic Salts; ILIM: Frunze, U.S.S.R., 1971.

ReceiVed for reView September 5, 2008 ReVised manuscript receiVed January 13, 2009 Accepted March 10, 2009 IE801341J