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Ind. Eng. Chem. Res. 2007, 46, 2688-2692
Investigation of the Solubility in the NaVO3-NaNO3-H2O System Mieczysław Trypuc´ * and Sebastian Druz3 yn´ ski Faculty of Chemistry, Nicolaus Copernicus UniVersity, 7 Gagarin Street, 87-100 Torun´ , Poland
The solubility of sodium vanadate and sodium nitrate was investigated in the NaVO3-NaNO3-H2O system at the temperature range 293-323 K, with the method of isothermal saturation of a solution. The obtained results gave a basis for the plot of a fragment of the solubility isotherm for the investigated system. That system is a part of the five-component NH4NO3-NaVO3-NH4VO3-NaNO3-H2O system and is necessary for plotting the solubility isotherm in the projection according to Janecke method. Knowledge of the course of that isotherm for the five-component system is necessary for determination of the optimal conditions for utilization of the postfiltration liquor from the soda production from NaNO3 by the Solvay method with the use of NaVO3. presence of oxygen or air, according to eq 4:
1. Introduction More than half of the sodium carbonate is produced by the Solvay method, which in general is a carbonization of the ammoniated NaCl brine. Formed NaHCO3 precipitate is filtered and calcinated. The process can be described by the reaction eqs 1 and 2:
NaCl + NH3 + CO2 + H2O T NaHCO3 + NH4Cl (1) Q
2NaHCO3 98 Na2CO3 + CO2 + H2O
(2)
The use of that method is caused by the availability of the necessary substrates (sodium chloride, limestone, and ammonia) and their relatively low costs.1 That method has numerous disadvantages, among them are high energy demands, low efficiency in the use of raw materials, and high environmental noxiousness. In the industrial practices, the achieved efficiency related to the sodium ion is about 70%, while the levels of limestone and coke use are 68% and 53%, respectively. The environment noxiousness of that method is caused by the complete waste of chloride ions. These two aspects gave the inspiration for numerous modifications of the Solvay method, aiming to utilize the large amount of waste and improve the process efficiency of substrate and energy use. The liquor from the ammonia distillation node (DS), containing mainly CaCl2 and unreacted NaCl finds only limited use in a production of the precipitated calcium carbonate and solid calcium chloride.2 E. Pischinger and co-workers developed the practically wasteless method of the sodium carbonate production based on carbonization of the ammoniated NaNO3 brine with the so-called SCS method (Soda-Chlorine-Saltpeter).3 In its first step, analogous to the Solvay method, the bicarbonate is produced according to eq 3:
NaNO3 + NH3 + CO2 + H2O T NaHCO3 + NH4NO3 (3) Sodium nitrate used in carbonization is obtained in a reaction of solid sodium chloride with the nitric acid in the * To whom correspondence should be addressed. Tel.: (+48) 566114537. Fax: (+48) 566542477. E-mail address: sebdru@ uni.torun.pl.
4NaCl + 4HNO3 + O2 T 4NaNO3 + 2Cl2 + 2H2O
(4)
That reaction is more complicated and goes in three steps described by eqs 5-7:3
NaCl + HNO3 f NaNO3 + HCl
(5)
3HCl + HNO3 f Cl2 + NOCl + 2H2O
(6)
NOCl + 2HNO3 f 0.5Cl2 + 3NO2 + H2O
(7)
The advantage of that method over the Solvay method is the complete utilization of chloride ions from NaCl as the produced chlorine. The yield of carbonization related to the sodium ions WNa+ is larger than that in the classic Solvay method, in which NaCl brine is a substrate. For the triple point P1 at 303 K and the NaNO3 concentration of 20.79 mol‚1000 g-1 H2O, it reaches a maximum of 89.4%.3 The liquor after precipitation of NaHCO3 crystals is a mixture of NH4NO3, NaNO3, and unreacted NaCl from the oxidation process, the approximate composition being 80%, 18%, and 2%, respectively. That mixture is a raw material in the concentration process for obtaining mixed sodiumammonium nitrate, which can be utilized as a nitrogenous fertilizer. In such a procedure, the energy-demanding part of the post-filtration liquor regeneration with the lime milk is eliminated from the process, while it is present in the Solvay method of the soda production from NaCl. The use of the post-filtration liquor for obtaining the mixed sodium-ammonium nitrate is made difficult due to its explosiveness during the concentration process, especially in a presence of chloride ions.3,4 Investigations of different additives to ammonium nitrate revealed that the presence of chloride ions accelerates 50 times its decomposition at 180 °C.4 The research on the mixed sodium-ammonium nitrate revealed that the presence of sodium nitrate does not affect significantly the mechanism of decomposition, compared to the pure ammonium nitrate, and the decomposition products of the mixed nitrate are the same as for ammonium nitrate: nitrous oxide, water, nitrogen and oxygen, and other nitrogen oxides, while NH3 and HNO3 are present only in small amounts. The presence of unreacted NaCl (2%) in the post-filtration liquor from the oxidation process destabilizes the process of obtaining the mixed
10.1021/ie0614125 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007
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sodium-ammonium nitrate by the solution concentration. It was observed that NaCl accelerates the thermal decomposition by shortening the induction time (i.e., time of heating until the mixture decomposes) and lowering the temperature at which the process becomes exothermic.5 That phenomenon causes the losses of N2 in production and creates a risk of explosion or fire. The mechanism of the catalytic role of chloride ions on the nitrate decomposition is not well explained. According to research by Rozman, chlorine, which is a byproduct, oxidizes ammonia and is reduced to hydrochloride5 according to the following equations:
3NO2 + 6HCl T 3NO + 3Cl2 + 3H2O
(8)
2NH3 + 3Cl2 T N2 + 6HCl
(9)
3NO2 + 2NH3 T 3NO + N2 + 3H2O
(10)
It was found that urea can be used for stabilization of sodium-ammonium nitrate during its production, and an addition of 1% elongates the induction time.5 That could be explained by the thermal decomposition of urea, in which the produced ammonia reverses the thermal dissociation of ammonium nitrate5 according to eq 11:
NH4NO3 T NH3 + HNO3
(11)
It has to be stated that the addition of urea does not cancel the catalytic action of chloride ions but only delays the decomposition of ammonium nitrate. That indicated the necessity for determination of a more safe method for processing the post-filtration liquor from the SCS method into a nitrogenous fertilizer. That problem might be solved by a conversion of ammonium nitrate from the post-filtration liquor into a more stable compound. The research on the utilization of the post-filtration solution from the classic Solvay method indicates such a possibility based on the use of sodium metavanadate.6,7 The postfiltration solution contains NH4Cl, in an equimolar amount to that of sedimented NaHCO3, as well as unreacted sodium chloride from reaction 1. Utilization of that liquor includes the reaction of NaVO3 with NH4Cl, resulting in a formation of insoluble NH4VO3 in equilibrium with the NaCl solution, according to equation:
NH4Cl + NaVO3 T NH4VO3 + NaCl
(12)
After NH4VO3 filtration, the NaCl solution is recycled into the step of preparation of the saturated brine which is subsequently saturated with NH3, while wet NH4VO3 is subjected to calcination according to the reaction equation: Q
2NH4VO3 98 2NH3 + V2O5 + H2O
(13)
The products of reaction 13 are recycled to the appropriate stages of the process of soda production, i.e., NH3 into ammonization of NaCl brine and V2O5 into the NaVO3 synthesis from NaCl in the presence of an oxygen or water steam. The post-filtration liquor from the SCS method contains NH4NO3 in the equimolar amount to the sediments of NaHCO3, as well as the unreacted NaNO3 from reaction 3. Therefore, utilization of that liquor would be based on a reaction of NaVO3 with NH4NO3, with a formation of insoluble
NH4VO3, while the equivalent amount of NaNO3 remains in the solution, as described by eq 14:
NH4NO3 + NaVO3 T NH4VO3 + NaNO3
(14)
After filtration of NH4VO3, the NaNO3 solution would be subjected to a concentration process to obtain sodium nitrate exclusively, that would eliminate the dangerous process of sodium-ammonium nitrate isolation. The obtained wet sediments of NH4VO3 would be calcinated, analogously to the utilization of the post-filtration liquor from the Solvay method (eq 13). From that point of view, the SCS method is both economical and ecologically fitted. The economical control of the utilization of the post-filtration liquor from the SCS method with the use of NaVO3 requires the precise knowledge of the equilibrium plot for the NH4NO3NaVO3-NH4VO3-NaNO3-H2O system with salt pairs involved in the substitution reaction, projected onto the plane by the Janecke method. Such an isotherm plot requires the equilibrium research for four ternary systems: NaNO3-NaVO3H2O, NH4NO3-NH4VO3-H2O, NaNO3-NH4NO3-H2O, and NaVO3-NH4VO3-H2O, these being components of the quaternary system and are the isotherm projections on the corresponding edges of the square equilibrium plot, as well as on the plot area, to determine the lines separating the areas of cocrystallization of salts and the triple points. The literature referring to the ternary systems mentioned above is not complete, and there are no reports on the quaternary system. We have found the reports on the NaVO3-NH4VO3H2O system at 293-323 K8 and partial data for the NaNO3NH4NO3-H2O system.9,10 Therefore, we decided to investigate the mutual solubility of NaVO3 and NaNO3 in water at the temperature range of 293-323 K. 2. Experimental 2.1. Reagents. In the research, analytical grade reagents were used: NaVO3 (g98% Fluka) and NaNO3 (pure for analysis POCh Gliwice). 2.2. Method. Determination of the salt mutual solubility in the NaNO3-NaVO3-H2O system was performed at 293, 303, 313, and 323 K by the method of isothermal saturation of solutions. The adequate amounts of components, one of them used in excess relative to its solubility in the pure water, were diluted with 80 cm3 redistilled water in the 100 cm3 Erlenmeyer flasks. The solutions were heated in the water thermostat and stirred magnetically. The required temperature was maintained with the Polystat CC1 thermostat with a precision of (0.02 K, controlled with a mercury thermometer with an accuracy of (0.1 K. The time of equilibration of the solution with the solid phase was experimentally determined to be 100 h. After that time, the stirring was stopped and precipitate was sedimented over 1 h. The sample of the clear equilibrated solution was transferred to the precalibrated Ostwald pycnometer for the density measurement. Density was determined with an accuracy of (0.002 g‚cm-3. Sample collecting was performed under the increased pressure, with the use of micrometering pump, which prevented crystallization of the solution components, especially at higher temperatures. The pycnometer contents were subsequently transferred quantitatively into a measuring flask and diluted to 500 cm3 with the redistilled water. The obtained solutions were subjected to chemical analyses. The dried solid phase was analyzed with the X-ray diffraction (XRD) method.
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2.3. Analytical Methods. The concentration of sodium ions in the equilibrated solutions was determined with the gravimetric method of Kolthoff and Barber by a formation of sodiumzinc-uranyl acetate. That precipitate has a relatively high solubility in water -5.85 g in 100 cm3 water at 294 K.11 To prevent the analyte losses, a vast volume excess (10:1) of the concentrated solution of the precipitating agent was used, presaturated with the precipitate. The precipitate was obtained from the smallest possible sample, which therefore was previously concentrated to a volume less than 1 cm3. Vanadium(V) was determined spectrophotometrically as a complex with the hydrogen peroxide. The analyses were performed with the double-beam UV-vis Hitachi U-2000 spectrophotometer in the quartz cells with the 10 mm optical path. Vanadium(V) in the concentrated sulfuric acid (1:3) exists in a form of VO2+ ions. In a presence of 3% aqueous solution of H2O2, the compounds V(O2)X3 and/or V(O2)X52- are formed, where X is the monovalent anion. That reaction proceeds in an equimolar ratio according to eq 15:12
(VO)2(SO4)3 + 2H2O2 T [V(O2)]2(SO4)3 + 2H2O
(15)
These compounds reveal maximum absorbency at 450 nm, and the molar absorbency is 300 dm3‚mol-1‚cm-1. The system obeys the Beer law in a broad range of concentrations, from 8 × 10-3 to 2 mol‚dm-3.12 The relative standard deviation (RSD) of the analyses did not exceeded 1%. For solution samples with very low vanadium concentration, below 2 × 10-2 mol‚dm-3, the spectrophotometric method with 4-(2-pyridylazo)resorcinol (PAR) was used. The analyses were performed with the double-beam UV-vis Hitachi U-2000 spectrophotometer in the quartz cells with the 10 mm optical path. At pH 5-6, vanadium(V) compounds react with PAR forming a complex with maximum absorbency at 540 nm. The maximum color saturation appears after 30 min and stays for 2 h. The molar absorbency is 3.6 × 104 dm3‚mol-1‚cm-1.12 The RSD of the vanadium concentration determination with that method did not exceeded 2%. Determination of the NO3- concentration was performed with the distillation method. For that, the nitrates in the sample were reduced to NH3 with the Deward alloy in 30% NaOH. Ammonia was absorbed in the excess of the standard solution of sulfuric acid. Subsequently, the excess of acid was titrated with NaOH in the presence of methyl red.11 The RSD had not exceeded 2%. For the selected experimental points, the analyses of the solid phase were performed with the X-ray powder diffractometer Philips X-Pert PRO system. To determine the solid-phase composition, the diffraction data were compared to the standards included in the “Powder Diffraction File”.13 3. Results and Discussion The experimental data on the mutual solubility of NaVO3 and NaNO3 in the NaVO3-NaNO3-H2O system are presented in Table 1. The equilibrium concentrations of salts, density of the equilibrated solutions, and the molar ratio calculated without taking into consideration a solvent or a solid phase are shown for four temperatures. The equilibrium concentrations of the system components, presented in Table 1, were determined from analyses of three independent samples. The concentrations of sodium metavanadate(V) in the equilibrated solutions were determined based on the spectrophotometric determination of vanadium(V), while the sodium nitrate concentration was calculated from the difference
Table 1. Solubility in the NaVO3- NaNO3- H2O System c (mol‚1000 g-1 H2O) NaNO3 NaVO3
no.
d (g‚cm-3)
1 2 3 4 (E)
1.392 1.390 1.392 1.392
0 0.002 0.006 0.009
5 6 7 8 9 10 11 12 13 14
1.254 1.195 1.154 1.112 1.069 1.063 1.060 1.088 1.108 1.118
0.012 0.023 0.024 0.060 0.194 0.228 0.393 0.780 0.993 1.427
1 2 3 4 (E)
1.412 1.412 1.412 1.417
0 0.002 0.006 0.013
5 6 7 8 9 10 11 12 13 14
1.307 1.253 1.155 1.108 1.092 1.084 1.090 1.110 1.114 1.135
0.020 0.027 0.080 0.199 0.302 0.484 0.633 1.068 1.215 1.753
1 2 3 4 (E)
1.418 1.418 1.418 1.419
0 0.010 0.017 0.026
5 6 7 8 9 10 11 12 13 14 15
1.270 1.220 1.204 1.141 1.114 1.098 1.107 1.107 1.121 1.130 1.150
0.032 0.048 0.081 0.139 0.237 0.423 0.715 0.885 1.136 1.417 1.944
1 2 3 4 (E)
1.431 1.431 1.431 1.431
0 0.007 0.024 0.037
5 6 7 8 9 10 11 12 13 14 15
1.268 1.204 1.181 1.144 1.125 1.112 1.110 1.110 1.114 1.130 1.164
0.050 0.068 0.100 0.197 0.318 0.489 0.666 0.832 1.050 1.341 2.155
x solid-phase NaVO3 NaNO3 composition
T ) 293 K 9.502 0 9.376 0.0002 9.178 0.0007 9.003 0.0010 5.872 4.571 3.171 2.092 0.969 0.783 0.427 0.285 0.118 0
1 0.9998 0.9993 0.9990
0.0021 0.0049 0.0075 0.0277 0.1668 0.2254 0.4797 0.7325 0.8937 1
0.9979 0.9951 0.9925 0.9723 0.8332 0.7746 0.5203 0.2675 0.1063 0
T ) 303 K 10.639 0 10.604 0.0002 10.574 0.0005 10.404 0.0013
1 0.9998 0.9995 0.9987
7.870 5.561 3.123 1.647 1.204 0.788 0.518 0.297 0.221 0
0.0026 0.0048 0.0251 0.1080 0.2003 0.3807 0.5499 0.7825 0.8462 1
0.9974 0.9952 0.9749 0.8920 0.7997 0.6193 0.4501 0.2175 0.1538 0
T ) 313 K 11.640 0 11.552 0.0009 11.366 0.0015 11.301 0.0023
1 0.9991 0.9985 0.9977
9.119 6.497 4.289 2.544 1.817 1.122 0.773 0.666 0.390 0.206 0
0.0035 0.0074 0.0186 0.0517 0.1152 0.2739 0.4808 0.5704 0.7442 0.8729 1
0.9965 0.9926 0.9814 0.9483 0.8848 0.7261 0.5192 0.4296 0.2558 0.1271 0
T ) 323 K 13.387 0 13.208 0.0006 13.002 0.0019 12.945 0.0028
1 0.9994 0.9981 0.9972
6.784 4.722 3.926 2.868 2.190 1.534 1.134 0.800 0.587 0.384 0
0.0073 0.0143 0.0248 0.0643 0.1268 0.2418 0.3701 0.5100 0.6415 0.7772 1
0.9927 0.9857 0.9752 0.9357 0.8732 0.7582 0.6299 0.4900 0.3585 0.2228 0
NaNO3 NaNO3 NaNO3 NaNO3 + NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaNO3 NaNO3 NaNO3 NaNO3 + NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaNO3 NaNO3 NaNO3 NaNO3 + NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaNO3 NaNO3 NaNO3 NaNO3 + NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3
between the determined concentrations of sodium and vanadate ions, according to the following equation:
[NaNO3] ) [Na+] - [VO3-]
Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007 2691
Figure 1. Branches II of the solubility isotherms for the NaVO3-NaNO3H2O system.
The precision of the analyses, determined as the relative error for all the analytical methods used, had not exceeded 2%; the relative standard deviation (RSD) for all analyses also had not exceeded 2%. Data in Table 1 gave the basis for plotting the fragment of the solubility polytherm of the investigated system (Figure 1). The presented solubility isotherms for 293, 303, 313, and 323 K consist of isotherm branches denoted I and corresponding to the solutions saturated with NaNO3 and branches denoted II corresponding to solutions saturated with NaVO3. The eutonic points indicated as E reflect the solutions saturated with both salts. Data in Table 1 for branches I of the solubility isotherms indicate that at the investigated temperature range NaNO3 has a strong salting out effect on NaVO3. Therefore, points E are positioned very close to the Y-axis, on which the NaNO3 concentration is marked. For better clarity, the isotherm branches corresponding to the saturated NaNO3 solutions relative to NaVO3 are presented in Figure 2. The hyperbolic character of branches II on Figure 1 is caused by the strong salting out effect of sodium metavanadate. Starting from the eutonic point E, the concentration of NaVO3 increases very slowly at high NaNO3 concentrations. For NaNO3 concentrations smaller than 3 mol‚1000 g-1 H2O, the NaVO3 concentration increases rapidly. The salting out effect decreases with an increase of temperatures, and for high temperatures, the rapid increase of the NaVO3 concentration occurs at high concentrations of NaNO3. For example, at 303 K, the increase of the NaVO3 concentration is observed for ∼ 3 mol‚1000 g-1 H2O NaNO3 in the solution, while at 293 K it is found for ∼2 mol‚1000 g-1 H2O NaNO3. The branches I of isotherms presented in Figure 2, are linear, and small decrease of the NaNO3 solubility with an increasing concentration of NaVO3 is observed. In ref 14, for identification of the double salts or the additive compounds, the equilibrium plots of the propertycomposition type are used. In the case of the formation of a new phase, the curves of solution density-component concentration reveal the inflection or brake points corresponding a new compound formation. Data from Table 1 were used to plot the changes in the solution density as a function of the NaVO3 molar ratio (Figure 3) only for equilibrium solutions described with branches II of the isotherms. Due to the strong NaNO3 salting out effect on NaVO3, branches I at the scale used on Figure 3 are not visible. Therefore, they are plotted in Figure 4. The lack
Figure 2. Branches I of the solubility isotherms: ([) 293, (9) 303, (2) 313, (×) 323 K.
Figure 3. Changes in the density of equilibrium solutions for branches II of the solubility isotherms at temperatures of 293-323 K.
of break points in Figure 3 indicates that, at the investigated temperature range, the solid phase for branches II is composed only of NaVO3 and in the eutonic points E it is composed of both salts. The lack of changes in the density of equilibrium solutions in Figure 4 proves that for branches I the solid phase contains only NaNO3. These conclusions are confirmed by the X-ray diffraction results, not revealing any compounds other than NaNO3 for branches I and NaVO3 for branches II. The molar ratios for salts, without taking into consideration a solvent, were calculated according to the following equations:
XNaVO3 )
[NaVO3]
,
[NaVO3] + [NaNO3]
XNaNO3 )
[NaNO3] [NaVO3] + [NaNO3]
4. Conclusions Both the experimental data on the mutual solubility of salts in the investigated system and the literature reports8 allow the conclusion that both the strong salting out effect of sodium
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Literature Cited
Figure 4. Density changes of the equilibrium solutions for branches I of the solubility isotherms at temperatures of ([) 293, (9) 303, (2) 313, and (×) 323 K.
nitrate on sodium metavanadate and the fact that the NaVO3 solubility is 5 times larger than that of NH4VO3 at the eutonic points are advantageous for the reaction equilibrium (eq 14). According to the obtained results and literature data,8 the area of crystallization of sodium nitrate and sodium metavanadate on the Janecke equilibrium plot would be small compared to that for ammonium metavanadate. That indicates that the reaction equilibrium (eq 14) is strongly shifted toward the reaction products and reaching high efficiency for NH4VO3 is possible.
(1) Niederlin´ski, A; Bukowski, A.; Koneczny H. Soda and accompanying products; WNT: Warsaw, 1978. (2) Trypuc´, M.; Kiełkowska, U.; Torski, Z. Research on a production of calcium carbonate from the liquid waste of the soda industry; scientific papers of IChN: Gliwice, 1995. (3) Pischinger, E.; et al. Research on the New method of soda production; Nicolaus Copernicus University: Torun´, 1969. (4) Kołaczkowski, A. Spontaneous decomposition of ammonium nitrate; scientific papers of the Institute of Inorganic Technology and Mineral Fertilizers, Wrocłlaw Technical University: Wrocłlaw, 1980. (5) Bobrownicki, W.; Biskupski, A.; Kołlaczkowski, A. On the thermal decomposition of ammonium-sodium nitrate. Appl. Chem. 1977 21, 3-18. (6) Trypuc´, M.; Łlyjak, G. Application of NaVO3 for the utilization of the after-filtration liquor from Solvay’s process. Ind. Eng. Chem. Res. 2000 40, 2188-2192. (7) Trypuc´, M.; Łlyjak, G. Solubility investigations in the NH4Cl+NaVO3+NH4VO3+NaCl+H2O system at 303K. J. Chem. Eng. Data 2000 45, 872-875. (8) Trypuc´, M.; Kiełkowska, U. Solubility in the NaVO3 + NH4VO3 + H2O system. J. Chem. Eng. Data 1997, 42, 523-525. (9) Nikitina, E. A. J. Gen. Chem. (Russia) 1933, 3, 513. (10) Koneczny, H.; Lango, M. Research on the ternary system NaNO3NH4NO3-H2O. Research on a new method of soda production; UMK: Torun´, 1967. (11) Struszyn´ski, M. QuantitatiVe and technical analysis; PWN: Warszawa, 1954; Vol. II. (12) Williams, W. J. Handbook of anion determination; Butterworth and Co. Ltd.: London, 1979; (polish translation Anion analysis; PWN: Warszawa, 1985). (13) Joint Committee on Powder Diffraction Standards. Powder Diffraction File; U.S.A., 1976. (14) Sułlajmankułlow, K. ILIM, Frunze, Russia, 1971.
ReceiVed for reView November 3, 2006 ReVised manuscript receiVed March 1, 2007 Accepted March 3, 2007 IE0614125
