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Solubility in the reciprocal quaternary K+-Na+-SO42−-VO3−-H2O system at (293.15 and 313.15) K
Fluid Phase Equilibria 404 (2015) 75–80
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Solubility in the reciprocal quaternary K+-Na+-SO42-VO3-H2O system at (293.15 and 313.15) K ski* , Krzysztof Mazurek, Urszula Kiełkowska, Aleksandra Szalla, _ n Sebastian Druzy Adriana Wróbel , Poland Nicolaus Copernicus University, Faculty of Chemistry, Department of Chemical Technology, Gagarina St. 7, 87-100 Torun
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 April 2015 Received in revised form 22 June 2015 Accepted 23 June 2015 Available online 3 July 2015
The equilibrium solubility studies of the Na2SO4 + KVO3 + H2O system at 293.15 and 313.15 K have been performed. Based on the results, the equilibrium plots for the system in a planar projection have been obtained according to the Jänecke method. The plots enable the determination of the optimal conditions for the conversion reaction of potassium metavanadate into complex salt sodium, potassium metavanadate with the use of sodium sulfate. The aim of the process is the recovery of vanadium compounds from solutions obtained after leaching spent vanadium catalyst. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Spent vanadium catalyst Recycling Solubility isotherms Equilibrium state Double salt
1. Introduction The main step of sulfuric(VI) acid production process is the oxidation of sulfur(IV) oxide to a sulfur(VI) oxide in the present of vanadium catalysts. The vanadium catalyst becomes inactive after certain time and it is troublesome waste. The waste of the sulphuric acid(VI) production process is divided into: waste sieving generated during screening of catalyst, the catalyst withdrawn from the process due to loss of its catalytic properties, and deposit of the catalyst from industrial installations which have been stopped due to economic conditions [1–3]. Utilization of spent vanadium catalyst is one of the fundamental problems of inorganic industry. This problem, in spite of many attempts, has not been fully resolved due to the complexity of the structure of the catalyst impeding its recycling. The annual amount of new catalyst needed to fill a medium-sized production installation is about 100–140 tons (with the production capacity of approximately 300,000 tons of sulphuric(VI) acid). These data show the use of the catalyst, and indicate the necessity for research into methods of waste vanadium mass recycling [1–3]. Recycling of the used vanadium catalyst is very important, not only for environmental reasons, but also from the economical point of view. The economic indicators of production can be improved by
* Corresponding author. Fax: +48 566542477. ski). _ n E-mail address: [email protected] (S. Druzy http://dx.doi.org/10.1016/j.fluid.2015.06.036 0378-3812/ ã 2015 Elsevier B.V. All rights reserved.
the recovery of vanadium oxide, which is a valuable resource material. The literature describes many recycling methods of the spent vanadium catalyst, or ways to recover vanadium pentoxide [4–10]. The most commonly used methods include: adding of certain amount of a spent catalyst to the new product, ferrovanadium production, recovery of vanadium compounds using leaching solutions (chemical methods). Leaching of the catalyst components using different solutions is an interesting ways of chemical method. Literature provides a number of patents and descriptions of separations of vanadium compounds with alkaline and acidic solutions, but they relate to solutions without pollution. Research indicate that the separation of vanadium is more difficult when the concentration of this element is too low in the solution. The optimal concentration is 30–60 g of V2O5/dm3 [4,5,8]. Effective recovery of vanadium and potassium compounds on the way of leaching of spent vanadium catalyst was obtained by means of sodium hydroxide solutions [11,12]. Under optimum conditions, the extraction is proceeded for 4 h at 313.15 K using a 15% wt. sodium hydroxide solution, in a proportion of solid phase to the extracting agent of 1:10 w/v. Under these conditions recovery of potassium and vanadium compounds are 99 and 82% respectively [11]. Increasing the recovery of the potassium and vanadium compounds (up to 99.9 and 87% respectively) is
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obtained by adding the 30% wt. H2O2 solution to the extracting agent in a proportion of 0.175 catalyst mass [12]. In order to isolate pure potassium and vanadium compounds from the extraction solution, neutralization of excess sodium hydroxide with sulfuric(VI) acid to 6.5–7.5 pH range is necessary in further step. After the neutralization process, the main components of the solution are the Na+, SO42, K+, VO43 ions. These ions present in the solution determine the course of the conversion reaction according to the Eq. (1). mNa2SO4 + (2m + n)KVO3 $ Na2mKn(VO3)n+2m# + mK2SO4+
(1)
The conversion process mentioned above (Eq. (1)) is a reaction of double ion exchange in the water medium, and the factors determining the equilibrium of that reaction are the mutual solubility of the system components and temperature. The detailed knowledge of the investigated system, i.e. the qualitative and qualitative relationships, requires the detailed equilibrium research of the mutual solubility of salts for the pairs of exchanging salts Na2SO4 + KVO3 + K2SO4 + NaVO3 + H2O. The obtained experimental data enable to build the equilibrium plots for the investigated system of pairs of the exchange salts, which are required for determination of the process parameters such as temperature, composition of the brine subjected to conversion and the process yield [13–16]. 2. Materials and methods 2.1. Apparatus and reagents Water thermostat Polystat CC1 with a precision 0.02 K was used in the studies. The set temperature was controlled using a mercury thermometer with a precision of 0.1 K. Analytically pure reagents: KVO3 (purity 98%, Aldrich), NaVO3 (purity 98%, Aldrich), K2SO4 (purity 98%, Fluka), Na2SO4 (purity 99% POCH) and deionized water (0.06 mS cm1) were used in the studies. 2.2. Measurement methods The equilibrium studies of the Na2SO4 + KVO3 + H2O system were performed using the method of isothermal saturation solution at temperatures 293.15 and 313.15 K. To prepare the solutions, the composition of which is determined by the course of respective isotherm branches, a suitable eutonic solution (isothermally stable) saturated with two salts was used, and then increasing amounts of the third salt were added to it until a solution saturated with three salts was obtained (P1, P2, P3 and P4). The isothermal section between points P2 and P1 was determined in a similar manner. The difference was that the starting solution had composition P1 to which increasing amounts of sodium sulphate were gradually added. A diagram showing the preparation of the mixtures is presented in Table 1. Due to the very low solubility of sodium metavanadate in saturated solutions of sodium and potassium sulphates, and in Table 1 Preparation of solutions corresponding to different sections of the isotherm.
saturated solutions of potassium sulphate and potassium metavanadate, the preparation of solutions corresponding to isothermal sections E1–P2, E4–P1, E2–P4 did not make sense because of the very short lengths of these isothermal branches. Appropriate amounts of salts were weighed and placed in ground-glass Erlenmeyer flasks of 100 cm3. Magnetic stir bars were placed in the flasks and an appropriate amount of deionized water was added. The mixtures were thermostatted at the predetermined temperature and constantly stirred for 150 h. The time of settling the equilibrium between the solid phase and the solution was determined experimentally and it was 100 h. After 150 h the stirring was turned off and after the sludge sedimentation (24 h) the equilibrium solution was sampled to an Ostwald pycnometer to determine its density (0.0002 g cm3). In the next step, the pycnometer content was quantitatively transferred to a volumetric flask of 500 cm3. Concentrations of potassium, sulphate, sodium and vanadate ions were determined in the resulting solutions. Vanadate, potassium and sulphate ions were determined by means of X-ray fluorescence (ED-XRF), using X-ray spectrometer Minipal4 PANalytical. For samples of solutions containing low concentrations of vanadate ions below 2 102 M, a spectrophotometric method with 4-(2-pyridylazo) resorcinol (PAR) was applied, using double beam spectrophotometer UV–vis UVD–3000 LABOMED. At pH 5–6 vanadium(V) compounds react with PAR forming a complex with maximum absorbancy at 540 nm. The maximum color saturation appears after 30 min, and stays for 2 h. The molar absorbancy is 3.6 104 dm3 mol1 cm1 [17]. The concentration of sodium ions in the equilibrium solutions was determined by atomic absorption spectrometry using Savant AA Sigma GBC. 3. Results and discussion The obtained experimental data concerning the mutual solubility of salts in the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at temperatures of 293.15 and 313.15 K are shown in Tables 2 and 3. For each temperature, density of the solution and concentrations of sodium, potassium, sulphate, vanadate ions and molar ratios for potassium and vanadate ions were presented. The molar ratios for potassium and vanadate ions, without taking into consideration a solvent, were calculated according to the Eqs. (2) and (3) respectively. xKþ ¼
½Kþ ½Na þ ½Kþ
xVO3 ¼
þ
½VO 3
½VO 3 þ 0:5 ½SO2 4
(2)
(3)
Concentrations expressed in molar fractions of potassium and vanadate ions have been used for preparation of the equilibrium plots on a planar projection according to Jäneke method. The solubility isotherms for the investigated Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O salt system at temperatures 293.15 and 313.15 K in Figs. 1 and 2 were presented. Designations E1–E6 represent eutonic points of the corresponding ternary systems:
Section of isotherm
Initial point
Solid phase
Additive
E3–P1 E2–P2 P1–P3 P2–P3 E6–P4 E5–P3 E1–P2
E3 E2 P1 P2 E6 E5 E1
KVO3, Na2mKn(VO3)n+2m
K2SO4 K2SO4 Na2SO4 K2SO4 K2SO4 NaVO3 NaVO3
E4–P1
E4
NaVO3
The solutions represented by triple points P1–P4 are saturated with three salts: P1 – Na2mKn(VO3)n+2m, K2SO4, KVO3, P2 – Na2SO4
K2SO4, Na2mKn(VO3)n+2m NaK3(SO4)2, Na2mKn(VO3)n+2m NaVO3, Na2mKn(VO3)n+2m NaK3(SO4)2, K2SO4 NaK3(SO4)2,Na2SO410H2O (293.15 K) NaK3(SO4)2, Na2SO4 (313.15 K) K2SO4, KVO3
E1 and E5 Na2SO4 + K2SO4 + H2O, E2 Na2SO4 + NaVO3 + H2O, E3 and E6 NaVO3 + KVO3 + H2O, E4 K2SO4 + KVO3 + H2O.
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Table 2 Composition of the equilibrium solutions and the solid phase in the system NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O at 293.15 K.
ra /g cm-3
cb mol kg1 H2O +
xc
Na
K
Curve E2–P4 1.1536 1.1650
2.789 2.375
Curve P4–P2 1.1650 1.1974 1.2297
+
V(V)
SO4
0 0.0682
0.1320 0.0963
2.375 2.764 3.175
0.0682 0.5223 1.002
Curve P2–P3 1.2297 1.1989 1.1785 1.1374
3.175 2.631 2.232 1.524
Curve P3–P1 1.1374 1.1376 1.1381 1.1371 1.1325 1.1217 1.1121 1.1059 1.1012 1.1003 1.0995
Solid phase composition 2–
+
V(V)
K
1.329 1.159
0.1658 0.1424
0 0.0279
Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4, NaVO3
0.0963 0.0516 0.0044
1.159 1.610 2.086
0.1424 0.0602 0.0042
0.0279 0.1589 0.2398
Na2mKn(VO3)n+2m, Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2
1.002 1.146 1.240 1.437
0.0044 0.0049 0.0053 0.0054
2.086 1.886 1.734 1.478
0.0042 0.0052 0.0060 0.0073
0.2398 0.3034 0.3571 0.4854
Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2
1.524 1.432 0.9539 0.9559 0.9602 0.6288 0.4603 0.3861 0.3289 0.1887 0.1239
1.437 1.429 1.363 1.373 1.373 1.342 1.319 1.303 1.303 1.367 1.149
0.0054 0.0054 0.0060 0.0061 0.0061 0.0080 0.0120 0.0149 0.0213 0.0844 0.1488
1.478 1.428 1.156 1.161 1.164 0.9812 0.8835 0.8372 0.8055 0.7384 0.5367
0.0073 0.0075 0.0103 0.0103 0.0103 0.0160 0.0264 0.0344 0.0501 0.1861 0.3567
0.4854 0.4994 0.5884 0.5895 0.5885 0.6809 0.7413 0.7715 0.7985 0.8787 0.9027
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4 Na2mKn(VO3)n+2m, K2SO4
Curve P1–E3 1.0995 1.0923 1.0810 1.0700 1.0667 1.0621 1.0643
0.1239 0.1225 0.1052 0.0865 0.0747 0.0645 0.0596
1.149 1.156 1.013 0.8612 0.7655 0.6760 0.6345
0.1488 0.2143 0.2370 0.3171 0.3985 0.4981 0.6727
0.5367 0.5308 0.4410 0.3185 0.2182 0.1191 0.000
0.3567 0.4467 0.5180 0.6657 0.7851 0.8932 1
0.9027 0.9042 0.9059 0.9088 0.9111 0.9129 0.9141
Na2mKn(VO3)n+2m, K2SO4, KVO3 KVO3, Na2mKn(VO3)n+2m
Curve P1–E4 1.0995 1.0910
0.1239 0
1.149 1.388
0.1488 0.1469
0.5367 0.6206
0.3567 0.3213
0.9027 1
KVO3, Na2mKn(VO3)n+2m, K2SO4 KVO3, K2SO4
Curve P2–E1 1.2297 1.2279
3.175 3.132
1.002 1.008
0.0044 0
2.086 2.070
0.0042 0
0.2398 0.2435
NaK3(SO4)2, Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, NaK3(SO4)2
Curve E5–P3 1.1371 1.1373 1.1374 1.1374
1.606 1.575 1.539 1.524
1.444 1.441 1.439 1.437
0 0.0018 0.0041 0.0054
1.525 1.507 1.487 1.478
0 0.0024 0.0054 0.0073
0.4735 0.4779 0.4832 0.4854
NaK3(SO4)2, K2SO4
Curve E6–P4 1.1420 1.0849 1.0860 1.0970 1.1253 1.1450 1.1650
1.690 1.181 1.283 1.594 2.056 2.395 2.375
0.0585 0.0380 0.0399 0.0472 0.0593 0.0688 0.0682
1.748 0.8207 0.6548 0.4080 0.2371 0.1747 0.0963
0.000 0.2203 0.3339 0.5868 0.9297 1.164 1.159
1 0.8817 0.7968 0.5817 0.3377 0.2308 0.1424
0.0334 0.0312 0.0301 0.0288 0.0280 0.0279 0.0279
Na2mKn(VO3)n+2m, NaVO3
a b c
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4
Na2mKn(VO3)n+2m, K2SO4, KVO3
NaK3(SO4)2, K2SO4, Na2mKn(VO3)n+2m
Na2mKn(VO3)n+2m, Na2SO4, NaVO3
3
u(r) = 0.0018 g cm . u(c) = 0.0015 mol kg1 H2O. Jänecke index for reciprocal quaternary system, u(T) = 0.02 K.
(Na2SO410H2O), Na2mKn(VO3)n+2m, NaK3(SO4)2, P3 – Na2mKn (VO3)n+2m, NaK3(SO4)2, K2SO4, P4 – Na2mKn(VO3)n+2m, Na2SO4 (Na2SO410H2O), NaVO3. Earlier research on ternary K2SO4 + Na2SO4 + H2O + and NaVO3 + KVO3 + H2O systems indicate that complex salts or hydrates were formed. In the sulphate system, it was found that at 293.15 and 303.15 K Na2SO4 occurs as the decahydrate. It also confirmed the presence of complex salt NaK3(SO4)2 in the
solid phase, which is formed in the temperature range (293.15–323.15 K) [18–20]. Studies of the mutual solubility in ternary salt systems containing sodium metavanadate indicate that the temperature significantly influences the crystalline form of sodium metavanadate which is in equilibrium with the solutions. At 293.15 K sodium metavanadate is in the form of NaVO32H2O, while at temperatures of 303.15, 313.15 and 323.15 K the solid phase is a mixture of NaVO32H2O and
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Table 3 Composition of the equilibrium solutions and the solid phase in the system NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O at 313.15 K.
ra/g cm-3
cb mol kg1 H2O +
xc +
Na
K
V(V)
SO4
Curve E2–P4 1.2972 1.3074
4.756 6.133
0 0.1226
0.0802 0.0949
Curve P4–P2 1.3074 1.3101 1.3219
6.133 5.622 4.535
0.1226 0.4442 0.9831
Curve P2–P3 1.3219 1.2502 1.2122 1.1612 1.1528
4.535 3.817 3.012 1.493 1.110
Curve P3–P1 1.1528 1.1524 1.1494 1.1463 1.1428 1.1405 1.1394 1.1392
Solid phase composition 2
+
V(V)
K
2.341 3.080
0.0641 0.0580
0 0.0196
Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4, NaVO3
0.0949 0.0645 0.0053
3.080 3.001 2.756
0.0580 0.0412 0.0038
0.0196 0.0732 0.1782
Na2mKn(VO3)n+2m, Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2
0.9831 1.2032 1.2588 1.5769 1.8056
0.0053 0.0074 0.0078 0.0110 0.0132
2.756 2.507 2.133 1.530 1.451
0.0038 0.0059 0.0073 0.0142 0.0178
0.1782 0.2396 0.2948 0.5136 0.6193
Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2
1.110 1.004 0.860 0.676 0.496 0.330 0.175 0.159
1.8056 1.7975 1.7985 1.8065 1.8150 1.8230 1.8258 1.8711
0.0132 0.0152 0.0173 0.0290 0.0546 0.1099 0.2256 0.5602
1.451 1.372 1.302 1.225 1.129 1.021 0.888 0.707
0.0178 0.0217 0.0260 0.0451 0.0882 0.1771 0.3369 0.6133
0.6193 0.6416 0.6766 0.7277 0.7853 0.8468 0.9126 0.9218
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4 Na2mKn(VO3)n+2m, K2SO4
Curve P1–E3 1.1392 1.1381 1.1356 1.1333 1.1310 1.1283 1.1274 1.1250
0.159 0.153 0.134 0.121 0.113 0.104 0.099 0.097
1.8711 1.8171 1.6046 1.4688 1.4031 1.3574 1.3293 1.3417
0.5602 0.5843 0.6786 0.7776 0.8964 1.0516 1.1739 1.4385
0.707 0.670 0.525 0.390 0.307 0.194 0.109 0.000
0.6133 0.6357 0.7209 0.7993 0.8536 0.9156 0.9556 1
0.9218 0.9221 0.9230 0.9240 0.9257 0.9289 0.9304 0.9327
Na2mKn(VO3)n+2m, K2SO4, KVO3 KVO3, Na2mKn(VO3)n+2m
Curve P1–E4 1.1392 1.1460
0.159 0.000
1.8711 1.8378
0.5602 0.4018
0.707 0.718
0.6133 0.5281
0.9218 1
KVO3, Na2mKn(VO3)n+2m, K2SO4 KVO3, K2SO4
Curve P2–E1 1.2919 1.3017
4.685 4.987
1.0156 0.9704
0.0055 0.0000
2.847 2.977
0.0038 0
0.1782 0.1629
NaK3(SO4)2, Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, NaK3(SO4)2
Curve E5–P3 1.1530 1.1529 1.1527 1.1525
1.127 1.123 1.117 1.110
1.8011 1.8031 1.8040 1.8061
0.0000 0.0055 0.0099 0.0132
1.464 1.461 1.456 1.452
0 0.0074 0.0135 0.0178
0.6151 0.6163 0.6176 0.6193
NaK3(SO4)2, K2SO4
Curve E6–P4 1.1760 1.1741 1.1711 1.1693 1.1690 1.1779 1.1895 1.1927 1.2159 1.2236 1.3074
2.056 2.287 2.463 2.522 2.773 2.952 3.271 3.343 4.037 4.238 6.133
0.1152 0.1161 0.1165 0.1151 0.1158 0.1154 0.1162 0.1165 0.1184 0.1189 0.1226
2.1713 1.9513 1.6878 1.2338 0.8474 0.5944 0.4411 0.3901 0.2601 0.2341 0.0949
0.000 0.226 0.456 0.681 1.003 1.226 1.467 1.537 1.949 2.055 3.080
1 0.9453 0.8809 0.7838 0.6282 0.4922 0.3755 0.3366 0.2107 0.1855 0.0580
0.0531 0.0483 0.0452 0.0436 0.0401 0.0376 0.0343 0.0337 0.0285 0.0273 0.0196
Na2mKn(VO3)n+2m, NaVO3
a b c
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4
Na2mKn(VO3)n+2m, K2SO4, KVO3
NaK3(SO4)2, K2SO4, Na2mKn(VO3)n+2m
Na2mKn(VO3)n+2m, Na2SO4, NaVO3
u(r) = 0.0018 g cm3. u(c) = 0.0015 mol kg1 H2O. Jänecke index for reciprocal quaternary system, u(T) = 0.02 K.
b-NaVO3 [13–16,21,22]. Obtained experimental data show clearly that in the investigated quaternary KVO3 + Na2SO4 + NaVO3 + K2SO4 + H2O system, in addition to solid phases listed earlier, creates the new complex salt of the general molecular formula Na2mKn(VO3)n+2m. The exact determination of the stoichiometry of this compound requires further structural studies.
Characteristic points of the test system (E1–E6, P1–P4) designate nine sections of solubility isotherms (Figs. 1 and 2). E1–P2 curve corresponds to solutions saturated with respect to Na2SO410H2O and NaK3(SO4)2 and Na2SO4 and NaK3(SO4)2, respectively at temperatures of 293.15 and 313.15 K. The curve E4–P1 corresponds to solutions saturated with respect to K2SO4 and KVO3. The curve
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Fig. 1. Equilibrium plot for the NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O system at 293.15 K.
P1–P3 represents solutions which are saturated with respect to K2SO4 and Na2mKn(VO3)n+2m. The curve E2–P4 corresponds to solutions saturated with respect to sodium metavanadate and sulphate. The curve E3–P1 represents solutions saturated with respect to KVO3 and Na2mKn(VO3)n+2m. The curve P4–P2 corresponds to solutions saturated with respect to Na2SO4 (Na2SO410H2O) and Na2mKn(VO3)n+2m. The section P2–P3 corresponds to solutions saturated with respect to two complex salts Na2mKn(VO3)n+2m and NaK3(SO4)2. E6–P4 section represents solutions which are saturated with respect to NaVO3 and Na2mKn(VO3)n+2m.
79
These curves divide the area of the equilibrium plots (Figs. 1 and 2) into six parts representing crystallization fields of the individual components of the system. A position analysis of each salt crystallization field presented in Figs. 1 and 2 show that potassium sulphate and complex salt Na2mKn(VO3)n+2m are a chemically stable pair of salts without a common ion in the system under study. The crystallization fields of these components are adjacent to each other, and therefore, it is possible to create solutions mutually saturated with respect to Na2mKn(VO3)n+2m and K2SO4. However, sodium sulphate and potassium metavanadate are a chemically unstable pair of salts in the system under study. In Figs. 1 and 2, the crystallization fields of these salts are not adjacent to each other, which indicates that there is no possibility of making solutions mutually saturated with these components. These curves divide the equilibrium plot into four parts representing the areas of crystallization of different components of the system. The size of the area of crystallization increases in the row Na2SO410H2O < NaK3(SO4)2 < NaVO3 < K2SO4 < KVO3 << Na2mKn(VO3)n+2m at 293.15 K and Na2SO4 < NaK3(SO4)2 < KVO3 < NaVO3 < K2SO4 << Na2mKn(VO3)n+2m at 313.15 K. It should be noted that the area of crystallization of Na2mKn(VO3)n+2m is larger than those corresponding to all other components of the system, what indicates that the solubility of Na2mKn(VO3)n+2m is the smallest in the investigated system. The position of triple points P1–P4 in Figs. 1 and 2 indicate that the solutions represented by the points are congruently saturated with respect to the solid phase. A solution is convergently saturated when its composition reflects qualitatively the composition of the solid phase remaining in equilibrium. The solution is inconvergently saturated when its equilibrium composition does not correspond qualitatively to the composition of the solid phase [13–16]. Data in Tables 1 and 2 indicate that the addition of sodium metavanadate into the solution E4, which is saturated with potassium sulfate and potassium metavanadate, results in a reaction opposite to the reaction of conversion of potassium sulfate into mixt salt Na2mKn(VO3)n+2m. That phenomenon occurs until the solution is saturated with Na2mKn(VO3)n+2m, i.e. until it reaches the composition corresponding to P1. At point P1, the minimum of the mole fraction of potassium ions is observed for solutions saturated with Na2mKn(VO3)n+2m. For that reason the maximum yield of conversion is reached at the P1 ternary point for the investigated system of the pairs of the exchange salts. The yield of precipitation of Na2mKn(VO3)n+2m is determined graphically based on the equilibrium plots (Figs. 1 and 2). The straight line is plotted between the corner corresponding to potassium metavanadate and the P1 point. That line intersects the axis on which the mole fractions of potassium ions are marked. The mole fraction of potassium ions at the intersection of the plotted line with the axis, multiplied by 100% gives the maximal yield of the reaction of double exchange. The yields of the conversion determined in that way at 293.15 and 313.15 K are 84.9 and 86.8%, respectively. Given the slight difference between the reaction yields (Eq. (1)) obtained at (293.15 and 313.15) K, the conversion should be preferably carried out at the lower temperature. The process carried out at 293.15 K gives a solution with significantly lower concentration of vanadate ions, as compared to 313.15 K, which will result in lower levels of contamination with vanadium compounds separated from the potassium sulphate solution. 4. Conclusions
Fig. 2. Equilibrium plot for the NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O system at 313.15 K.
Based on the experimental data, the phase diagrams for the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at 293.15 and 313.15 K were plotted. The obtained results allowed to conclude that the chemically stable pair of salts in the investigated system is
80
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K2SO4 and Na2mKn(VO3)n+2m, while the unstable pair is formed by potassium metavanadate and sodium sulfate. The mix salt Na2mKn(VO3)n+2m is the compound revealing the smallest solubility in the investigated system, the area of its crystallization is larger than the areas corresponding to other components of the investigated system. Low solubility of Na2mKn(VO3)n+2m at point P1 determines the high yield of precipitation of that salt from the leaching solution obtained from utilization process of spent vanadium catalyst using sodium hydroxide solution. The determined maximum yields of conversion at 293.15 and 313.15 K are 84.9% and 86.8%, respectively.
[7] [8] [9] [10]
[11]
[12]
[13]
Acknowledgement [14]
This work was financed by the National Science Centre (Poland)—Project No. NN209760640.
[15]
References
[16]
[1] M. Trypu c, P. Grzesiak, K., Mazurek, M. Grobela, Kompleksowe zagospodarowanie szkodliwych odpadów katalizatora wanadowego stosowanego do utleniania SO2. Tom 1. Wydawnictwo naukowe IOR, 2007. Poznan [2] P. Grzesiak, Utylizacja odpadów przemysłowych z produkcji kwasu siarkowego(VI), Przem. Chem. 8–9 (2006) 1015. [3] P. Grzesiak, Katalizatory wanadowe do utleniania SO2, wydawnictwo IOR, 2005. Poznan [4] S. Anioł, T. Korolewicz, J. Kubala, Investigation concerning the recovery of V2O5 from the spent vanadium catalyst for the production of sulphuric acid, Pol. J. Appl. Chem. 41 (1997) 25. [5] S. Anioł, T. Korolewicz, J. Kubala, Attempts to recover V2O5 from spent vanadium catalysts, Pol. J. Appl. Chem. 3 (1997) 193. [6] S. Anioł, T. Korolewicz, J. Kubala, Investigation on V2O5 recovery from spent vanadium catalysts, Pol. J. Appl. Chem. 3 (1997) 193.
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Polish Patent 67 525 (1997). Polish Patent 135 966 (1987). Polish Patent 27 552 (1938). S. Khorfan, A. Wahoud, Y. Reda, Recovery of vanadium pentoxide from spent catalyst in the manufacture of sulfuric acid, Period. Polytech. Ser. Chem. Eng. 45 (2) (2001) 131. U. Kiełkowska, K. Białowicz, M. Trypu c, K. Mazurek, P. Grzesiak, Sulfuric acid – New opportunities – Extraction of vanadium compounds from the spent vanadium catalyst using NaOH solution, Institute of Plant Protection, Poznan 2008. ISBN 978-83-89867-30-8. U. Kiełkowska, K. Białowicz, M. Trypu c, K. Mazurek, R. Motała, Sulfuric acid – New opportunities – Effects of oxidizing agent for the extraction of spent vanadium catalyst component with a sodium hydroxide solution, Institute of Plant Protection, Poznan 2008. ISBN 978-83-89867-30-8. ski, _ n M. Trypu c, S. Druzy Phase diagram for the NH4NO3 + NaVO3 + NH4VO3 + NaNO3 + H2O system at 293 and 303 K, Ind. Eng. Chem. Res. 48 (2009) 6937. ski, Solubility in the reciprocal quaternary NH4+–Na+– _ n M. Trypu c, S. Druzy NO3–VO3–H2O system at (313 and 323) K, J. Chem. Eng. Data 56 (2011) 2919. ski, M. Trypu _ n S. Druzy c, U. Kiełkowska, K. Mazurek, Utilization of the postfiltration lye from the soda–chlorine–saltpetre method of soda production, Pol. J. Chem. Technol. 13 (2011) 53. ski, K. Mazurek, Plotting of the solubility isotherm for the _ n M. Trypu c, S. Druzy NH4NO3 + NaVO3 + H2O system, Pol. J. Chem. Technol. 10 (2008) 11. W.J. Williams, Handbook of Anion Determination, Butterworth and Co Ltd., London, 1979. M. Trypu c, M. Chałat, K. Mazurek, Solubility investigations in the Na2SO4 + V2O5 + H2O system from 293 K to 323 K, J. Chem. Eng. Data 51 (2006) 322. Y. Zeng, X. Lin, X. Yu, Study on the solubility of the aqueous quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O at 273.15–K, J. Chem. Eng. Data 57 (2012) 3672. F. Sirbu, O. Iulian, I.A. Catrinel, I. Ion, Activity coefficients of electrolytes in the NaCl + Na2SO4 + H2O ternary system from potential difference measurements at (298.15, 303.15, and 308.15) K, J. Chem. Eng. Data 56 (2011) 4935. M. Trypu c, U. Kiełkowska, Solubility in the NaVO3 + NH4VO3 + H2O system, J. Chem. Eng. Data 42 (1997) 523. M.H. Kuok, S.H. Tang, Z.X. Shen, C.W. Ong, Raman spectroscopic studies of b-NaVO3, a-NaVO3 and NaVO32H2O, J. Raman Spectrosc. 26 (1995) 301.
Contents lists available at ScienceDirect
Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Solubility in the reciprocal quaternary K+-Na+-SO42-VO3-H2O system at (293.15 and 313.15) K ski* , Krzysztof Mazurek, Urszula Kiełkowska, Aleksandra Szalla, _ n Sebastian Druzy Adriana Wróbel , Poland Nicolaus Copernicus University, Faculty of Chemistry, Department of Chemical Technology, Gagarina St. 7, 87-100 Torun
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 April 2015 Received in revised form 22 June 2015 Accepted 23 June 2015 Available online 3 July 2015
The equilibrium solubility studies of the Na2SO4 + KVO3 + H2O system at 293.15 and 313.15 K have been performed. Based on the results, the equilibrium plots for the system in a planar projection have been obtained according to the Jänecke method. The plots enable the determination of the optimal conditions for the conversion reaction of potassium metavanadate into complex salt sodium, potassium metavanadate with the use of sodium sulfate. The aim of the process is the recovery of vanadium compounds from solutions obtained after leaching spent vanadium catalyst. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Spent vanadium catalyst Recycling Solubility isotherms Equilibrium state Double salt
1. Introduction The main step of sulfuric(VI) acid production process is the oxidation of sulfur(IV) oxide to a sulfur(VI) oxide in the present of vanadium catalysts. The vanadium catalyst becomes inactive after certain time and it is troublesome waste. The waste of the sulphuric acid(VI) production process is divided into: waste sieving generated during screening of catalyst, the catalyst withdrawn from the process due to loss of its catalytic properties, and deposit of the catalyst from industrial installations which have been stopped due to economic conditions [1–3]. Utilization of spent vanadium catalyst is one of the fundamental problems of inorganic industry. This problem, in spite of many attempts, has not been fully resolved due to the complexity of the structure of the catalyst impeding its recycling. The annual amount of new catalyst needed to fill a medium-sized production installation is about 100–140 tons (with the production capacity of approximately 300,000 tons of sulphuric(VI) acid). These data show the use of the catalyst, and indicate the necessity for research into methods of waste vanadium mass recycling [1–3]. Recycling of the used vanadium catalyst is very important, not only for environmental reasons, but also from the economical point of view. The economic indicators of production can be improved by
* Corresponding author. Fax: +48 566542477. ski). _ n E-mail address: [email protected] (S. Druzy http://dx.doi.org/10.1016/j.fluid.2015.06.036 0378-3812/ ã 2015 Elsevier B.V. All rights reserved.
the recovery of vanadium oxide, which is a valuable resource material. The literature describes many recycling methods of the spent vanadium catalyst, or ways to recover vanadium pentoxide [4–10]. The most commonly used methods include: adding of certain amount of a spent catalyst to the new product, ferrovanadium production, recovery of vanadium compounds using leaching solutions (chemical methods). Leaching of the catalyst components using different solutions is an interesting ways of chemical method. Literature provides a number of patents and descriptions of separations of vanadium compounds with alkaline and acidic solutions, but they relate to solutions without pollution. Research indicate that the separation of vanadium is more difficult when the concentration of this element is too low in the solution. The optimal concentration is 30–60 g of V2O5/dm3 [4,5,8]. Effective recovery of vanadium and potassium compounds on the way of leaching of spent vanadium catalyst was obtained by means of sodium hydroxide solutions [11,12]. Under optimum conditions, the extraction is proceeded for 4 h at 313.15 K using a 15% wt. sodium hydroxide solution, in a proportion of solid phase to the extracting agent of 1:10 w/v. Under these conditions recovery of potassium and vanadium compounds are 99 and 82% respectively [11]. Increasing the recovery of the potassium and vanadium compounds (up to 99.9 and 87% respectively) is
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obtained by adding the 30% wt. H2O2 solution to the extracting agent in a proportion of 0.175 catalyst mass [12]. In order to isolate pure potassium and vanadium compounds from the extraction solution, neutralization of excess sodium hydroxide with sulfuric(VI) acid to 6.5–7.5 pH range is necessary in further step. After the neutralization process, the main components of the solution are the Na+, SO42, K+, VO43 ions. These ions present in the solution determine the course of the conversion reaction according to the Eq. (1). mNa2SO4 + (2m + n)KVO3 $ Na2mKn(VO3)n+2m# + mK2SO4+
(1)
The conversion process mentioned above (Eq. (1)) is a reaction of double ion exchange in the water medium, and the factors determining the equilibrium of that reaction are the mutual solubility of the system components and temperature. The detailed knowledge of the investigated system, i.e. the qualitative and qualitative relationships, requires the detailed equilibrium research of the mutual solubility of salts for the pairs of exchanging salts Na2SO4 + KVO3 + K2SO4 + NaVO3 + H2O. The obtained experimental data enable to build the equilibrium plots for the investigated system of pairs of the exchange salts, which are required for determination of the process parameters such as temperature, composition of the brine subjected to conversion and the process yield [13–16]. 2. Materials and methods 2.1. Apparatus and reagents Water thermostat Polystat CC1 with a precision 0.02 K was used in the studies. The set temperature was controlled using a mercury thermometer with a precision of 0.1 K. Analytically pure reagents: KVO3 (purity 98%, Aldrich), NaVO3 (purity 98%, Aldrich), K2SO4 (purity 98%, Fluka), Na2SO4 (purity 99% POCH) and deionized water (0.06 mS cm1) were used in the studies. 2.2. Measurement methods The equilibrium studies of the Na2SO4 + KVO3 + H2O system were performed using the method of isothermal saturation solution at temperatures 293.15 and 313.15 K. To prepare the solutions, the composition of which is determined by the course of respective isotherm branches, a suitable eutonic solution (isothermally stable) saturated with two salts was used, and then increasing amounts of the third salt were added to it until a solution saturated with three salts was obtained (P1, P2, P3 and P4). The isothermal section between points P2 and P1 was determined in a similar manner. The difference was that the starting solution had composition P1 to which increasing amounts of sodium sulphate were gradually added. A diagram showing the preparation of the mixtures is presented in Table 1. Due to the very low solubility of sodium metavanadate in saturated solutions of sodium and potassium sulphates, and in Table 1 Preparation of solutions corresponding to different sections of the isotherm.
saturated solutions of potassium sulphate and potassium metavanadate, the preparation of solutions corresponding to isothermal sections E1–P2, E4–P1, E2–P4 did not make sense because of the very short lengths of these isothermal branches. Appropriate amounts of salts were weighed and placed in ground-glass Erlenmeyer flasks of 100 cm3. Magnetic stir bars were placed in the flasks and an appropriate amount of deionized water was added. The mixtures were thermostatted at the predetermined temperature and constantly stirred for 150 h. The time of settling the equilibrium between the solid phase and the solution was determined experimentally and it was 100 h. After 150 h the stirring was turned off and after the sludge sedimentation (24 h) the equilibrium solution was sampled to an Ostwald pycnometer to determine its density (0.0002 g cm3). In the next step, the pycnometer content was quantitatively transferred to a volumetric flask of 500 cm3. Concentrations of potassium, sulphate, sodium and vanadate ions were determined in the resulting solutions. Vanadate, potassium and sulphate ions were determined by means of X-ray fluorescence (ED-XRF), using X-ray spectrometer Minipal4 PANalytical. For samples of solutions containing low concentrations of vanadate ions below 2 102 M, a spectrophotometric method with 4-(2-pyridylazo) resorcinol (PAR) was applied, using double beam spectrophotometer UV–vis UVD–3000 LABOMED. At pH 5–6 vanadium(V) compounds react with PAR forming a complex with maximum absorbancy at 540 nm. The maximum color saturation appears after 30 min, and stays for 2 h. The molar absorbancy is 3.6 104 dm3 mol1 cm1 [17]. The concentration of sodium ions in the equilibrium solutions was determined by atomic absorption spectrometry using Savant AA Sigma GBC. 3. Results and discussion The obtained experimental data concerning the mutual solubility of salts in the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at temperatures of 293.15 and 313.15 K are shown in Tables 2 and 3. For each temperature, density of the solution and concentrations of sodium, potassium, sulphate, vanadate ions and molar ratios for potassium and vanadate ions were presented. The molar ratios for potassium and vanadate ions, without taking into consideration a solvent, were calculated according to the Eqs. (2) and (3) respectively. xKþ ¼
½Kþ ½Na þ ½Kþ
xVO3 ¼
þ
½VO 3
½VO 3 þ 0:5 ½SO2 4
(2)
(3)
Concentrations expressed in molar fractions of potassium and vanadate ions have been used for preparation of the equilibrium plots on a planar projection according to Jäneke method. The solubility isotherms for the investigated Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O salt system at temperatures 293.15 and 313.15 K in Figs. 1 and 2 were presented. Designations E1–E6 represent eutonic points of the corresponding ternary systems:
Section of isotherm
Initial point
Solid phase
Additive
E3–P1 E2–P2 P1–P3 P2–P3 E6–P4 E5–P3 E1–P2
E3 E2 P1 P2 E6 E5 E1
KVO3, Na2mKn(VO3)n+2m
K2SO4 K2SO4 Na2SO4 K2SO4 K2SO4 NaVO3 NaVO3
E4–P1
E4
NaVO3
The solutions represented by triple points P1–P4 are saturated with three salts: P1 – Na2mKn(VO3)n+2m, K2SO4, KVO3, P2 – Na2SO4
K2SO4, Na2mKn(VO3)n+2m NaK3(SO4)2, Na2mKn(VO3)n+2m NaVO3, Na2mKn(VO3)n+2m NaK3(SO4)2, K2SO4 NaK3(SO4)2,Na2SO410H2O (293.15 K) NaK3(SO4)2, Na2SO4 (313.15 K) K2SO4, KVO3
E1 and E5 Na2SO4 + K2SO4 + H2O, E2 Na2SO4 + NaVO3 + H2O, E3 and E6 NaVO3 + KVO3 + H2O, E4 K2SO4 + KVO3 + H2O.
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Table 2 Composition of the equilibrium solutions and the solid phase in the system NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O at 293.15 K.
ra /g cm-3
cb mol kg1 H2O +
xc
Na
K
Curve E2–P4 1.1536 1.1650
2.789 2.375
Curve P4–P2 1.1650 1.1974 1.2297
+
V(V)
SO4
0 0.0682
0.1320 0.0963
2.375 2.764 3.175
0.0682 0.5223 1.002
Curve P2–P3 1.2297 1.1989 1.1785 1.1374
3.175 2.631 2.232 1.524
Curve P3–P1 1.1374 1.1376 1.1381 1.1371 1.1325 1.1217 1.1121 1.1059 1.1012 1.1003 1.0995
Solid phase composition 2–
+
V(V)
K
1.329 1.159
0.1658 0.1424
0 0.0279
Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4, NaVO3
0.0963 0.0516 0.0044
1.159 1.610 2.086
0.1424 0.0602 0.0042
0.0279 0.1589 0.2398
Na2mKn(VO3)n+2m, Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2
1.002 1.146 1.240 1.437
0.0044 0.0049 0.0053 0.0054
2.086 1.886 1.734 1.478
0.0042 0.0052 0.0060 0.0073
0.2398 0.3034 0.3571 0.4854
Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2
1.524 1.432 0.9539 0.9559 0.9602 0.6288 0.4603 0.3861 0.3289 0.1887 0.1239
1.437 1.429 1.363 1.373 1.373 1.342 1.319 1.303 1.303 1.367 1.149
0.0054 0.0054 0.0060 0.0061 0.0061 0.0080 0.0120 0.0149 0.0213 0.0844 0.1488
1.478 1.428 1.156 1.161 1.164 0.9812 0.8835 0.8372 0.8055 0.7384 0.5367
0.0073 0.0075 0.0103 0.0103 0.0103 0.0160 0.0264 0.0344 0.0501 0.1861 0.3567
0.4854 0.4994 0.5884 0.5895 0.5885 0.6809 0.7413 0.7715 0.7985 0.8787 0.9027
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4 Na2mKn(VO3)n+2m, K2SO4
Curve P1–E3 1.0995 1.0923 1.0810 1.0700 1.0667 1.0621 1.0643
0.1239 0.1225 0.1052 0.0865 0.0747 0.0645 0.0596
1.149 1.156 1.013 0.8612 0.7655 0.6760 0.6345
0.1488 0.2143 0.2370 0.3171 0.3985 0.4981 0.6727
0.5367 0.5308 0.4410 0.3185 0.2182 0.1191 0.000
0.3567 0.4467 0.5180 0.6657 0.7851 0.8932 1
0.9027 0.9042 0.9059 0.9088 0.9111 0.9129 0.9141
Na2mKn(VO3)n+2m, K2SO4, KVO3 KVO3, Na2mKn(VO3)n+2m
Curve P1–E4 1.0995 1.0910
0.1239 0
1.149 1.388
0.1488 0.1469
0.5367 0.6206
0.3567 0.3213
0.9027 1
KVO3, Na2mKn(VO3)n+2m, K2SO4 KVO3, K2SO4
Curve P2–E1 1.2297 1.2279
3.175 3.132
1.002 1.008
0.0044 0
2.086 2.070
0.0042 0
0.2398 0.2435
NaK3(SO4)2, Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, NaK3(SO4)2
Curve E5–P3 1.1371 1.1373 1.1374 1.1374
1.606 1.575 1.539 1.524
1.444 1.441 1.439 1.437
0 0.0018 0.0041 0.0054
1.525 1.507 1.487 1.478
0 0.0024 0.0054 0.0073
0.4735 0.4779 0.4832 0.4854
NaK3(SO4)2, K2SO4
Curve E6–P4 1.1420 1.0849 1.0860 1.0970 1.1253 1.1450 1.1650
1.690 1.181 1.283 1.594 2.056 2.395 2.375
0.0585 0.0380 0.0399 0.0472 0.0593 0.0688 0.0682
1.748 0.8207 0.6548 0.4080 0.2371 0.1747 0.0963
0.000 0.2203 0.3339 0.5868 0.9297 1.164 1.159
1 0.8817 0.7968 0.5817 0.3377 0.2308 0.1424
0.0334 0.0312 0.0301 0.0288 0.0280 0.0279 0.0279
Na2mKn(VO3)n+2m, NaVO3
a b c
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4
Na2mKn(VO3)n+2m, K2SO4, KVO3
NaK3(SO4)2, K2SO4, Na2mKn(VO3)n+2m
Na2mKn(VO3)n+2m, Na2SO4, NaVO3
3
u(r) = 0.0018 g cm . u(c) = 0.0015 mol kg1 H2O. Jänecke index for reciprocal quaternary system, u(T) = 0.02 K.
(Na2SO410H2O), Na2mKn(VO3)n+2m, NaK3(SO4)2, P3 – Na2mKn (VO3)n+2m, NaK3(SO4)2, K2SO4, P4 – Na2mKn(VO3)n+2m, Na2SO4 (Na2SO410H2O), NaVO3. Earlier research on ternary K2SO4 + Na2SO4 + H2O + and NaVO3 + KVO3 + H2O systems indicate that complex salts or hydrates were formed. In the sulphate system, it was found that at 293.15 and 303.15 K Na2SO4 occurs as the decahydrate. It also confirmed the presence of complex salt NaK3(SO4)2 in the
solid phase, which is formed in the temperature range (293.15–323.15 K) [18–20]. Studies of the mutual solubility in ternary salt systems containing sodium metavanadate indicate that the temperature significantly influences the crystalline form of sodium metavanadate which is in equilibrium with the solutions. At 293.15 K sodium metavanadate is in the form of NaVO32H2O, while at temperatures of 303.15, 313.15 and 323.15 K the solid phase is a mixture of NaVO32H2O and
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Table 3 Composition of the equilibrium solutions and the solid phase in the system NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O at 313.15 K.
ra/g cm-3
cb mol kg1 H2O +
xc +
Na
K
V(V)
SO4
Curve E2–P4 1.2972 1.3074
4.756 6.133
0 0.1226
0.0802 0.0949
Curve P4–P2 1.3074 1.3101 1.3219
6.133 5.622 4.535
0.1226 0.4442 0.9831
Curve P2–P3 1.3219 1.2502 1.2122 1.1612 1.1528
4.535 3.817 3.012 1.493 1.110
Curve P3–P1 1.1528 1.1524 1.1494 1.1463 1.1428 1.1405 1.1394 1.1392
Solid phase composition 2
+
V(V)
K
2.341 3.080
0.0641 0.0580
0 0.0196
Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4, NaVO3
0.0949 0.0645 0.0053
3.080 3.001 2.756
0.0580 0.0412 0.0038
0.0196 0.0732 0.1782
Na2mKn(VO3)n+2m, Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2
0.9831 1.2032 1.2588 1.5769 1.8056
0.0053 0.0074 0.0078 0.0110 0.0132
2.756 2.507 2.133 1.530 1.451
0.0038 0.0059 0.0073 0.0142 0.0178
0.1782 0.2396 0.2948 0.5136 0.6193
Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2
1.110 1.004 0.860 0.676 0.496 0.330 0.175 0.159
1.8056 1.7975 1.7985 1.8065 1.8150 1.8230 1.8258 1.8711
0.0132 0.0152 0.0173 0.0290 0.0546 0.1099 0.2256 0.5602
1.451 1.372 1.302 1.225 1.129 1.021 0.888 0.707
0.0178 0.0217 0.0260 0.0451 0.0882 0.1771 0.3369 0.6133
0.6193 0.6416 0.6766 0.7277 0.7853 0.8468 0.9126 0.9218
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4 Na2mKn(VO3)n+2m, K2SO4
Curve P1–E3 1.1392 1.1381 1.1356 1.1333 1.1310 1.1283 1.1274 1.1250
0.159 0.153 0.134 0.121 0.113 0.104 0.099 0.097
1.8711 1.8171 1.6046 1.4688 1.4031 1.3574 1.3293 1.3417
0.5602 0.5843 0.6786 0.7776 0.8964 1.0516 1.1739 1.4385
0.707 0.670 0.525 0.390 0.307 0.194 0.109 0.000
0.6133 0.6357 0.7209 0.7993 0.8536 0.9156 0.9556 1
0.9218 0.9221 0.9230 0.9240 0.9257 0.9289 0.9304 0.9327
Na2mKn(VO3)n+2m, K2SO4, KVO3 KVO3, Na2mKn(VO3)n+2m
Curve P1–E4 1.1392 1.1460
0.159 0.000
1.8711 1.8378
0.5602 0.4018
0.707 0.718
0.6133 0.5281
0.9218 1
KVO3, Na2mKn(VO3)n+2m, K2SO4 KVO3, K2SO4
Curve P2–E1 1.2919 1.3017
4.685 4.987
1.0156 0.9704
0.0055 0.0000
2.847 2.977
0.0038 0
0.1782 0.1629
NaK3(SO4)2, Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, NaK3(SO4)2
Curve E5–P3 1.1530 1.1529 1.1527 1.1525
1.127 1.123 1.117 1.110
1.8011 1.8031 1.8040 1.8061
0.0000 0.0055 0.0099 0.0132
1.464 1.461 1.456 1.452
0 0.0074 0.0135 0.0178
0.6151 0.6163 0.6176 0.6193
NaK3(SO4)2, K2SO4
Curve E6–P4 1.1760 1.1741 1.1711 1.1693 1.1690 1.1779 1.1895 1.1927 1.2159 1.2236 1.3074
2.056 2.287 2.463 2.522 2.773 2.952 3.271 3.343 4.037 4.238 6.133
0.1152 0.1161 0.1165 0.1151 0.1158 0.1154 0.1162 0.1165 0.1184 0.1189 0.1226
2.1713 1.9513 1.6878 1.2338 0.8474 0.5944 0.4411 0.3901 0.2601 0.2341 0.0949
0.000 0.226 0.456 0.681 1.003 1.226 1.467 1.537 1.949 2.055 3.080
1 0.9453 0.8809 0.7838 0.6282 0.4922 0.3755 0.3366 0.2107 0.1855 0.0580
0.0531 0.0483 0.0452 0.0436 0.0401 0.0376 0.0343 0.0337 0.0285 0.0273 0.0196
Na2mKn(VO3)n+2m, NaVO3
a b c
Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4
Na2mKn(VO3)n+2m, K2SO4, KVO3
NaK3(SO4)2, K2SO4, Na2mKn(VO3)n+2m
Na2mKn(VO3)n+2m, Na2SO4, NaVO3
u(r) = 0.0018 g cm3. u(c) = 0.0015 mol kg1 H2O. Jänecke index for reciprocal quaternary system, u(T) = 0.02 K.
b-NaVO3 [13–16,21,22]. Obtained experimental data show clearly that in the investigated quaternary KVO3 + Na2SO4 + NaVO3 + K2SO4 + H2O system, in addition to solid phases listed earlier, creates the new complex salt of the general molecular formula Na2mKn(VO3)n+2m. The exact determination of the stoichiometry of this compound requires further structural studies.
Characteristic points of the test system (E1–E6, P1–P4) designate nine sections of solubility isotherms (Figs. 1 and 2). E1–P2 curve corresponds to solutions saturated with respect to Na2SO410H2O and NaK3(SO4)2 and Na2SO4 and NaK3(SO4)2, respectively at temperatures of 293.15 and 313.15 K. The curve E4–P1 corresponds to solutions saturated with respect to K2SO4 and KVO3. The curve
ski et al. / Fluid Phase Equilibria 404 (2015) 75–80 S. Druz_ yn
Fig. 1. Equilibrium plot for the NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O system at 293.15 K.
P1–P3 represents solutions which are saturated with respect to K2SO4 and Na2mKn(VO3)n+2m. The curve E2–P4 corresponds to solutions saturated with respect to sodium metavanadate and sulphate. The curve E3–P1 represents solutions saturated with respect to KVO3 and Na2mKn(VO3)n+2m. The curve P4–P2 corresponds to solutions saturated with respect to Na2SO4 (Na2SO410H2O) and Na2mKn(VO3)n+2m. The section P2–P3 corresponds to solutions saturated with respect to two complex salts Na2mKn(VO3)n+2m and NaK3(SO4)2. E6–P4 section represents solutions which are saturated with respect to NaVO3 and Na2mKn(VO3)n+2m.
79
These curves divide the area of the equilibrium plots (Figs. 1 and 2) into six parts representing crystallization fields of the individual components of the system. A position analysis of each salt crystallization field presented in Figs. 1 and 2 show that potassium sulphate and complex salt Na2mKn(VO3)n+2m are a chemically stable pair of salts without a common ion in the system under study. The crystallization fields of these components are adjacent to each other, and therefore, it is possible to create solutions mutually saturated with respect to Na2mKn(VO3)n+2m and K2SO4. However, sodium sulphate and potassium metavanadate are a chemically unstable pair of salts in the system under study. In Figs. 1 and 2, the crystallization fields of these salts are not adjacent to each other, which indicates that there is no possibility of making solutions mutually saturated with these components. These curves divide the equilibrium plot into four parts representing the areas of crystallization of different components of the system. The size of the area of crystallization increases in the row Na2SO410H2O < NaK3(SO4)2 < NaVO3 < K2SO4 < KVO3 << Na2mKn(VO3)n+2m at 293.15 K and Na2SO4 < NaK3(SO4)2 < KVO3 < NaVO3 < K2SO4 << Na2mKn(VO3)n+2m at 313.15 K. It should be noted that the area of crystallization of Na2mKn(VO3)n+2m is larger than those corresponding to all other components of the system, what indicates that the solubility of Na2mKn(VO3)n+2m is the smallest in the investigated system. The position of triple points P1–P4 in Figs. 1 and 2 indicate that the solutions represented by the points are congruently saturated with respect to the solid phase. A solution is convergently saturated when its composition reflects qualitatively the composition of the solid phase remaining in equilibrium. The solution is inconvergently saturated when its equilibrium composition does not correspond qualitatively to the composition of the solid phase [13–16]. Data in Tables 1 and 2 indicate that the addition of sodium metavanadate into the solution E4, which is saturated with potassium sulfate and potassium metavanadate, results in a reaction opposite to the reaction of conversion of potassium sulfate into mixt salt Na2mKn(VO3)n+2m. That phenomenon occurs until the solution is saturated with Na2mKn(VO3)n+2m, i.e. until it reaches the composition corresponding to P1. At point P1, the minimum of the mole fraction of potassium ions is observed for solutions saturated with Na2mKn(VO3)n+2m. For that reason the maximum yield of conversion is reached at the P1 ternary point for the investigated system of the pairs of the exchange salts. The yield of precipitation of Na2mKn(VO3)n+2m is determined graphically based on the equilibrium plots (Figs. 1 and 2). The straight line is plotted between the corner corresponding to potassium metavanadate and the P1 point. That line intersects the axis on which the mole fractions of potassium ions are marked. The mole fraction of potassium ions at the intersection of the plotted line with the axis, multiplied by 100% gives the maximal yield of the reaction of double exchange. The yields of the conversion determined in that way at 293.15 and 313.15 K are 84.9 and 86.8%, respectively. Given the slight difference between the reaction yields (Eq. (1)) obtained at (293.15 and 313.15) K, the conversion should be preferably carried out at the lower temperature. The process carried out at 293.15 K gives a solution with significantly lower concentration of vanadate ions, as compared to 313.15 K, which will result in lower levels of contamination with vanadium compounds separated from the potassium sulphate solution. 4. Conclusions
Fig. 2. Equilibrium plot for the NaVO3 + K2SO4 + Na2SO4 + KVO3 + H2O system at 313.15 K.
Based on the experimental data, the phase diagrams for the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at 293.15 and 313.15 K were plotted. The obtained results allowed to conclude that the chemically stable pair of salts in the investigated system is
80
ski et al. / Fluid Phase Equilibria 404 (2015) 75–80 S. Druz_ yn
K2SO4 and Na2mKn(VO3)n+2m, while the unstable pair is formed by potassium metavanadate and sodium sulfate. The mix salt Na2mKn(VO3)n+2m is the compound revealing the smallest solubility in the investigated system, the area of its crystallization is larger than the areas corresponding to other components of the investigated system. Low solubility of Na2mKn(VO3)n+2m at point P1 determines the high yield of precipitation of that salt from the leaching solution obtained from utilization process of spent vanadium catalyst using sodium hydroxide solution. The determined maximum yields of conversion at 293.15 and 313.15 K are 84.9% and 86.8%, respectively.
[7] [8] [9] [10]
[11]
[12]
[13]
Acknowledgement [14]
This work was financed by the National Science Centre (Poland)—Project No. NN209760640.
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