Phase Diagram for the Na - American Chemical Society

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Phase Diagram for the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O System at 303.15 K and 323.15 K Krzysztof Mazurek* and Sebastian Druzẏ ński Faculty of Chemistry, Nicolaus Copernicus University in Toruń, Ulica Gagarina 7, 87-100 Toruń, Poland ABSTRACT: The solubility isotherms were determined for the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at (303.15 and 323.15) K. On the basis of these isotherms, the maximum yields have been determined for the conversion of sodium sulfate into metavanadate salt. The location of the P1 and P2 triple points has been determined on the equilibrium plots prepared as the planar projection according to the Jänecke method. The results are necessary for determining the optimal conditions for recovery of sodium metavanadate from leaching solution obtained from utilization process of spent vanadium catalyst using sodium hydroxide solution.

1. INTRODUCTION Sulfuric acid is used in many chemical industry branches. Most of the sulfuric acid produced is used in the manufacture of phosphoric acid and phosphate fertilizers, and it is also applied in the production of chemicals, paper, dyes, and synthetic fibers.1,2 A contact method using vanadium catalysts is currently the technology most widely used for producing sulfuric acid. The production process consists of three basic steps: the formation of the SO2 gas, catalytic oxidation of SO2 to SO3, and SO3 absorption in concentrated sulfuric acid. Sulfide ores of iron, copper, and zinc and elemental sulfur are the raw materials that are most used to produce SO2.1−3 The SO2 to SO3 oxidation stage takes place on a catalyst whose active phase containing V2O5 and promoters are on the surface of silica.1,2 Working in a contact apparatus, the vanadium catalyst gradually loses its catalytic properties and it needs to be replaced. Inactive catalyst weight is toxic waste for the environment. The waste from landfills had high concentrations of vanadium and other heavy metals such as Fe, Pb, Zn, Hg, Cd, and As.4 An example of the chemical composition of spent vanadium catalysts is presented in Table 1.3−5

There are a number of methods for the recovery of vanadium pentoxide form a spent vanadium catalyst. These include a method of sintering a spent catalyst with different fluxes (sodium chloride, sodium carbonate, sodium nitrate, sodium sulfate, sodium hydroxide) and then leaching the resulting alloys with water or acid. Vanadium compounds transferred to the solution in subsequent technological operations are isolated as precipitate that is purified and calcined to obtain the final product in the form of V2O5. This method is energy-intensive and can transfer only about 70 % of the compounds to the solution. Direct leaching of vanadium compounds from a spent catalyst with water, acid, or alkali solutions is much better than the sintering methods. One way to use this annoying waste is to recover vanadium compounds through leaching with different media such as NaOH, KOH, H2SO4, CO(NH2)2.3,5−10 A method for recovering vanadium compounds by extraction with concentrated solutions of sodium hydroxide or sodium carbonate is known from the Polish patent no. 177 606. This method does not prevent the extraction of soluble compounds of aluminum and silica, and it leads to a substantial increase in the content of sodium oxide in the resulting vanadium pentoxide.11,12 One way of recovering V2O5 from a spent vanadium catalyst is leaching with a 8 % to 10 % solution of ammonium carbonate with the addition of 3 % hydrogen peroxide. In a next step, after separation of the precipitate, the solution is acidified with sulfuric acid to pH 6.5 and V(V) is reduced to V(IV) with gaseous sulfur dioxide. Vanadium(IV) is isolated from the solution in the form of V2O4·H2O with gaseous ammonia. After separation of the precipitate, V(IV) is oxidized to V(V) with hydrogen peroxide, and then the precipitate is calcined with the

Table 1. Chemical Composition of Spent Vanadium Catalysts Used for the Oxidation of SO2 to SO3 content wt % component

ref 3

ref 4

ref 5

V2O5 K2O Fe2O3 SiO2

4.80 5.80 2.10 49.50

4.40 7.40 1.50 38.90

4.68 8.70 2.59 57.31

© XXXX American Chemical Society

Received: November 30, 2014 Accepted: April 13, 2015

A

DOI: 10.1021/je501087m J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

The fully automated, multielement, true double beam with all-reflective optics atomic absorption spectrometer Savant AA Sigma GBC was employed to determine the concentrations of Na and K in solution. Experimental Procedure. The equilibrium studies of the Na2SO4 + KVO3 + H2O system were performed using the method of isothermal saturation solution at temperatures 303.15 K and 323.15 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 sulfate were gradually added. A diagram showing the preparation of the mixtures is presented in Table 2. Because

addition of ammonium nitrate and ammonium carbonate at a temperature of 663 K to give the crude V2O5. The procedure makes it possible to recover 70 % to 74 % of V2O5 relative to the amount contained in the catalyst. These methods have not found any practical application due to the difficulties in obtaining pure, solid V2O5, a small degree of vanadium recovery and difficulties with recycling the resulting solutions and solid waste.11,12 Effective recovery of vanadium and potassium compounds by leaching spent vanadium catalyst is obtained using sodium hydroxide solution.9,10 Under optimum conditions, the extraction is carried out for 4 h at a temperature of 313 K, using 15 %wt NaOH at a ratio of 1:10 wt/vol, and recovering potassium and vanadium compounds in the amounts of 99 % and 82 %, respectively.9 The introduction of 30 % H2O2 at a ratio of 0.175 catalyst weight as an oxidizing agent to the extraction solution increases the recovery of vanadium up to 87 % and potassium compounds up to 99.9 %.10 To isolate pure vanadium and potassium compounds from the extraction solution, it is necessary to neutralize in a subsequent step the excess of sodium hydroxide using sulfuric acid to pH = 6.5 to 7.5. After the neutralization process, the major components of the solution are Na+, K+, SO42−, and VO3−, which determines the conversion according to eq 1

Table 2. Preparation of Solutions Corresponding to Different Sections of the Isotherm

m Na 2SO4 + (2m + n)KVO3 ↔ Na 2mK n(VO3 )n + 2m ↓ +mK 2SO4

(1)

The discussed conversion process (eq 1) is a dual ion exchange reaction in aqueous environment. Values which determine the equilibrium position of this reaction are mutual solubility of the components in the system and temperature. Detailed knowledge of the system under study, based on observation of qualitative and quantitative correlations, requires equilibrium studies of the mutual solubility of salts in the system of interchangeable salt pairs Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O. The obtained data enable the construction of equilibrium diagrams for the system that are necessary to determine the process parameters such as temperature, brine composition subjected to the conversion process and the efficiency of the process.13−16

section of isotherm

initial point

solid phase

additive

E3−P1 E2−P2 P1−P3 P2−P3 E6−P4 E5−P3 E1−P2 E4−P1

E3 E2 P1 P2 E6 E5 E1 E4

KVO3, NaVO3 NaVO3, Na2SO4 K2SO4, NaVO3 NaK3(SO4)2, Na2mKn(VO3)n+2m NaVO3, Na2mKn(VO3)n+2m NaK3(SO4)2, K2SO4 NaK3(SO4)2, Na2SO4 K2SO4, KVO3

K2SO4 K2SO4 Na2SO4 K2SO4 K2SO4 NaVO3 NaVO3 NaVO3

of the very low solubility of sodium metavanadate in saturated solutions of sodium and potassium sulfates, and in saturated solutions of potassium sulfate and potassium metavanadate, the preparation of solutions corresponding to isothermal sections E1−P2, E4−P1, and 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 volume 100 cm3. Magnetic stir bars were placed in the flasks and an appropriate amount of deionized water was added. The mixtures were thermostated 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. In the next step, the pycnometer content was quantitatively transferred to a volumetric flask of volume 500 cm3. Concentrations of potassium, sulfate, sodium, and vanadate ions were determined in the resulting solutions. Vanadate and sulfate ions were determined by means of Xray fluorescence (ED-XRF). For samples of solutions containing low concentrations of vanadate ions below 2·10−2 M, a spectrophotometric method with 4-(2-pyridylazo)resorcinol (PAR) was applied. At pH 5 to 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−1.17

2. MATERIALS AND METHODS Reagents and Apparatus. Reagents used in this study were KVO3 (AR grade, ≥ 98 wt %), NaVO3 (AR grade, ≥ wt 98 %), K2SO4 (AR grade, ≥ 98 wt %), Na2SO4 (AR grade, ≥ 99 wt %), and deionized water (0.06 μS·cm−1). KVO3 and NaVO3 were purchased from Sigma-Aldrich Co. LLC, and Na2SO4 and K2SO4 was purchased from Avantor Performance Materials Poland. All chemicals were used without further purification. Temperature in water bath was maintained constant applying the Polystat CC1 thermorelay with an accuracy of ± 0.02 K. Additionally, the temperature was controlled using a mercury thermometer with accuracy of ± 0.1 K. The PANalytical MiniPal 4 EDXRF spectrometer, equipped with a 30 kV rhodium anode tube, five filters, a helium purge facility, a high-resolution Silicon Drift Detector was used to determine the concentrations of V and S in solutions. The LABOMED Inc. high performance UV−vis double beam automatic scanning spectrophotometer UVD-3000 was employed to determine the low vanadium concentrations in solutions. B



Article

Table 3. Composition of the Equilibrium Solutions and the Solid Phase in the System Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O at 303.15 K and Pressure 0.1 MPa ρ g·cm

a

mole fractiona

c −3

mol·kg

solid phase composition

−1

Na+

K+

V(V)

SO42−

1.284 1.296

5.633 5.564

0 0.1055

0.0765 0.0615

2.778 2.804

1.296 1.312 1.347

5.564 4.890 4.176

0.1055 0.3199 0.6241

0.0615 0.0415 0.0046

2.804 2.584 2.395

1.347 1.303 1.254 1.184

4.176 2.356 1.760 0.9770

0.6241 0.5854 0.5938 0.7443

0.0046 0.0043 0.0044 0.0046

2.395 1.466 1.173 0.8558

1.184 1.150 1.125 1.120 1.120

0.9770 0.4974 0.3005 0.1443 0.1433

0.7443 0.9136 1.011 1.199 1.659

0.0046 0.0095 0.0308 0.0707 0.3057

0.8558 0.6960 0.6249 0.6012 0.6155

1.120 1.110 1.106 1.097 1.093 1.089 1.093

0.1433 0.1282 0.1194 0.1038 0.0886 0.0758 0.0651

1.659 1.494 1.413 1.261 1.127 1.047 0.9704

0.3057 0.3446 0.4047 0.5029 0.6097 0.7561 1.012

0.6155 0.6173 0.5378 0.4050 0.2688 0.1560 0.0000

1.120 1.107

0.1433 0.0000

1.659 1.618

0.3057 0.2334

0.6155 0.6925

1.347 1.342

4.176 4.473

0.6241 0.6837

0.0046 0.0000

2.395 2.578

1.144 1.145 1.149 1.150

1.340 1.254 0.9817 0.8134

1.749 1.715 1.607 1.537

0.0000 0.0029 0.0069 0.0112

1.544 1.483 1.291 1.169

1.158 1.143 1.156 1.159 1.172 1.198 1.241 1.296

1.854 2.177 2.382 2.588 2.999 3.577 4.451 5.564

0.0913 0.0741 0.0612 0.0601 0.0610 0.0709 0.0822 0.1055

1.854 1.316 1.019 0.6567 0.4399 0.2546 0.1720 0.0615

0.0000 0.4679 0.7121 0.9954 1.309 1.697 2.181 2.804

V(V) Curve E2−P4 0.0522 0.0420 Curve P4−P2 0.0420 0.0311 0.0038 Curve P2−P3 0.0038 0.0059 0.0074 0.0105 Curve P3−P1 0.0105 0.0266 0.0897 0.1903 0.4983 Curve P1−E3 0.4983 0.5275 0.6009 0.7130 0.8194 0.9065 1.0000 Curve P1−E4 0.4983 0.4026 Curve P2−E1 0.0038 0.0000 Curve E5−P3 0.0000 0.0040 0.0107 0.0188 Curve E6−P4 1.0000 0.8490 0.7410 0.5689 0.4019 0.2308 0.1362 0.0420

K+ 0.0000 0.0186

Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4, NaVO3

0.0186 0.0614 0.1300

Na2mKn(VO3)n+2m, Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2

0.1300 0.1990 0.2523 0.4325

Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4

0.4325 0.6475 0.7708 0.8926 0.9205

Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m,

0.9205 0.9210 0.9221 0.9240 0.9271 0.9324 0.9371

Na2mKn(VO3)n+2m, K2SO4, KVO3 KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m

0.9205 1.0000

KVO3, Na2mKn(VO3)n+2m, K2SO4 KVO3, K2SO4

0.1300 0.1326

NaK3(SO4)2, Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, NaK3(SO4)2

0.5662 0.5776 0.6207 0.6539

NaK3(SO4)2, NaK3(SO4)2, NaK3(SO4)2, NaK3(SO4)2,

0.0469 0.0329 0.0250 0.0227 0.0199 0.0194 0.0181 0.0186

Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m,

NaK3(SO4)2, K2SO4 K2SO4 K2SO4 K2SO4 K2SO4, KVO3

K2SO4 K2SO4 K2SO4 K2SO4, Na2mKn(VO3)n+2m NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3, Na2SO4, NaVO3

Mole fraction of salts in solution on a water free basis, c = mol·kg−1 of H2O. The uncertainty of T is ± 0.02 K, ρ is ± 0.002 g·cm−3, c is ± 1.0 %.

The concentration of sodium and potassium ions in the equilibrium solutions was determined by atomic absorption spectrometry.

shown in Tables 3 and 4. For each temperature, density of the solution and concentrations of sodium, potassium, sulfate, and vanadate ions as well as 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.

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 303.15 K and 323.15 K are

x K+ = C

[K+] [Na +] + [K+]

(2) DOI: 10.1021/je501087m J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Table 4. Composition of the Equilibrium Solutions and the Solid Phase in the System Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O at 323.15 K and Pressure 0.1 MPa ρ g·cm

a

mole fractiona

c −3

mol·kg

solid phase composition

−1

Na+

K+

V(V)

SO42−

1.308 1.316

6.566 6.849

0.0000 0.1404

0.0888 0.0925

3.239 3.351

1.316 1.322 1.343

6.849 6.543 6.382

0.1404 0.3678 0.9649

0.0925 0.0539 0.0029

1.343 1.298 1.251 1.185 1.178 1.172 1.163

6.382 5.034 3.592 2.382 1.583 0.9436 0.9897

0.9649 1.116 1.294 1.637 1.779 1.816 1.949

0.0029 0.0039 0.0054 0.0090 0.0134 0.0231 0.0227

1.163 1.170 1.166 1.167

0.9897 0.5441 0.3004 0.1509

1.949 1.886 2.106 2.453

0.0227 0.0696 0.3164 0.9172

1.167 1.166 1.166 1.166 1.167 1.167 1.172 1.177

0.1509 0.1304 0.1178 0.1117 0.1099 0.1009 0.1009 0.1009

2.453 2.287 2.155 2.104 2.108 1.978 2.049 2.086

0.9172 1.054 1.209 1.324 1.408 1.512 1.933 2.099

1.167 1.164

0.1509 0.0000

2.453 2.212

0.9172 0.7105

1.322 1.345

6.543 6.397

0.3678 0.9637

0.0539 0.0000

1.160 1.161 1.163 1.163

0.9116 0.9390 0.9624 0.9897

1.934 1.940 1.943 1.949

0.0000 0.0093 0.0177 0.0227

1.199 1.173 1.180 1.175 1.179 1.192 1.198 1.221 1.233 1.316

2.417 2.711 2.901 2.950 3.156 3.592 3.947 4.339 4.575 6.849

0.1369 0.1277 0.1185 0.1034 0.0905 0.1164 0.1189 0.1329 0.1339 0.1404

2.554 2.071 1.826 1.333 0.9832 0.5031 0.4346 0.2946 0.2723 0.0925

V(V)

Curve E2−P4 0.0520 0.0523 Curve P4−P2 3.351 0.0523 3.428 0.0305 3.672 0.0016 Curve P2−P3 3.672 0.0016 3.072 0.0026 2.440 0.0044 2.006 0.0088 1.673 0.0157 1.368 0.0327 1.458 0.0302 Curve P3−P1 1.458 0.0302 1.180 0.1056 1.045 0.3771 0.7843 0.7005 Curve P1−E3 0.7843 0.7005 0.6585 0.7619 0.4937 0.8305 0.4094 0.8661 0.3436 0.8912 0.2413 0.9261 0.1000 0.9748 0.0000 1.0000 Curve P1−E4 0.7843 0.7005 0.7506 0.6544 Curve P2−E1 3.428 0.0305 3.680 0.0000 Curve E5−P3 1.422 0.0000 1.434 0.0128 1.444 0.0239 1.458 0.0302 Curve E6−P4 0.0000 1 0.3835 0.9152 0.5969 0.8595 0.8598 0.7562 1.131 0.6347 1.527 0.3972 1.707 0.3374 2.034 0.2247 2.168 0.2007 3.351 0.0523

K+ 0.0000 0.0201

Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4, NaVO3

0.0201 0.0532 0.1313

Na2mKn(VO3)n+2m, Na2SO4, NaVO3 Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2

0.1313 0.1815 0.2649 0.4074 0.5292 0.6581 0.6632

Na2SO4, Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2 Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4

0.6632 0.7761 0.8752 0.9420

Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m,

0.9420 0.9461 0.9482 0.9496 0.9504 0.9515 0.9531 0.9539

Na2mKn(VO3)n+2m, K2SO4, KVO3 KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m KVO3, Na2mKn(VO3)n+2m

0.9420 1.0000

KVO3, Na2mKn(VO3)n+2m, K2SO4 KVO3, K2SO4

0.0532 0.1309

NaK3(SO4)2, Na2mKn(VO3)n+2m, Na2SO4 Na2SO4, NaK3(SO4)2

0.6797 0.6738 0.6687 0.6632

NaK3(SO4)2, NaK3(SO4)2, NaK3(SO4)2, NaK3(SO4)2,

0.0536 0.0450 0.0392 0.0339 0.0279 0.0314 0.0292 0.0297 0.0284 0.0201

Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m, Na2mKn(VO3)n+2m,

NaK3(SO4)2, K2SO4 K2SO4 K2SO4 K2SO4, KVO3

K2SO4 K2SO4 K2SO4 K2SO4, Na2mKn(VO3)n+2m NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 NaVO3 Na2SO4, NaVO3

Mole fraction of salts in solution on a water free basis, c = mol·kg−1H2O. The uncertainty of T is ± 0.02 K, ρ is ± 0.002 g·cm−3, c is ± 1.0 %.

x VO−3 =

[VO−3 ] [VO−3 ]

+

0.5· [SO24 −]

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 +

(3)

D



Article

KVO3; P2 = Na2SO4 (Na2SO4·10H2O), Na2mKn(VO3)n+2m, NaK3(SO4)2; P3 = Na2mKn(VO3)n+2m, NaK3(SO4)2, K2SO4; P4 = Na2mKn(VO3)n+2m, Na2SO4 (Na2SO4·10H2O), NaVO3. Characteristic points of the test system (E1−E6, P1−P4) designate nine sections of solubility isotherms (Figure 1 and 2). The curve E1−P2 corresponds to solutions saturated with respect to the solid phase consisting of K2SO4 and NaK3(SO4)2. The curve E4−P1 corresponds to solutions saturated with respect to K2SO4 and KVO3. The curve 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 sulfate. 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 (Na2SO4·10H2O) 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 wich are saturated with respect to NaVO3 and Na2mKn(VO3)n+2m. These curves divide the area of the equilibrium plots (Figure 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 Figures 1 and 2 shows that potassium sulfate 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 sulfate and potassium metavanadate are a chemically unstable pair of salts in the system under study. In Figures 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. The position of triple points P1−P4 in Figures 1 and 2 indicates 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 The obtained and literature data13−16 show that sodium metavanadate added to solution E4, which is saturated with respect to potassium sulfate and metavanadate, causes a chemical reaction opposite to the conversion of sodium sulfate to sodium metavanadate (eq 1). This phenomenon continues until the solution is saturated with respect to Na2mKn(VO3)n+2m, or until the solution has composition P1. Therefore, the maximum conversion efficiency (eq 1) is reached at triple point P1 of the exchanging salt pairs under study. On the basis of the calculated mole fractions of potassium ion, the maximum conversion efficiencies were calculated according to eq 4. For 303.15 K and 323.15 K, these yields are, respectively, 92.05 % and 94.20 %.

KVO3 + NaVO3 + K2SO4 + H2O salt system at temperatures 303.15 K and 323.15 K are presented in Figure 1 and 2.

Figure 1. Equilibrium plot for the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at 303.15 K.

Figure 2. Equilibrium plot for the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at 323.15 K.

Designations E1 to E6 represent eutonic points of the corresponding ternary systems: • E1 and E5: Na2SO4 + K2SO4 + H2O • E2: Na2SO4 + NaVO3 + H2O • E3 and E6: NaVO3 + KVO3 + H2O • E4: K2SO4 + KVO3 + H2O The solutions represented by triple points P1 to P4 are saturated with three salts: P1 = Na2mKn(VO3)n+2m, K2SO4,

WNa+ = x K+·100%

(4)

Another important information on the physical chemistry of the system under study is the effect of temperature changes on the size of the salt crystallization fields in the equilibrium diagrams presented in Figures 1 and 2. The sizes of the individual crystallization fields are shown in Table 5. It should E



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in the solid phase, the formation new mixed salt Na2mKn(VO3)n+2m was detected. Studies on the stoichiometry and physicochemical properties of the new compound are not yet completed.

Table 5. Area of the Crystallization Fields of Components Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O System in Temperature 303.15 K and 323.15 K component

area of the crystallization fields

4. CONCLUSIONS Based on the experimental results, phase diagrams of Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at temperatures of 303.15 K and 323.15 K have been plotted (Figure 1 and 2). The positions of the triple points in the equilibrium diagrams (Figure 1 and 2) shows that all are congruently saturated with respect to the solid phase. The results have confirmed that potassium sulfate and new compound Na2mKn(VO3)n+2m are a chemically stable pair of salts in the system under study, whereas potassium metavanadate and sodium sulfate are an unstable pair. Mixed salt Na2mKn(VO3)n+2m is the least soluble compound in the system. In Figures 1 and 2, the surface of the salt crystallization field dominates the fields of the other components. Low solubility of Na2mKn(VO3)n+2m at point P1 determines the high yield of the salt precipitation from the leach after the extraction of spent vanadium catalyst. The designated maximum conversion efficiencies (eq 1) at temperatures of 303.15 K and 323.15 K are, respectively, 92.05 % and 94.20 %.

% Na2mKn(VO3)n+2m K2SO4 NaVO3 KVO3 NaK3(SO4)2 Na2SO4

303.15 K

323.15 K

83.11 6.41 2.56 6.82 0.59 0.51

83.60 10.29 3.10 2.20 0.79 0.35

be noted that the size of Na2mKn(VO3)n+2m crystallization field dominates the fields of the other components at 303.15 K and 323.15 K. With the increase in temperature, the crystallization field of the salt is slightly expanded. The dominant size on the crystallization field of Na2mKn(VO3)n+2m with respect to the other components indicates that it is the least soluble salt in the test system. In the reaction (eq 1), Na2mKn(VO3)n+2m will precipitate from the solution, and the potassium sulfate will remain in the solution. The data gathered in Table 5 show a significant increase in the crystallization field of potassium sulfate with growing temperature. It should be noted that the increase of the field takes place mainly at the expense of potassium metavanadate crystallization field. The growth of potassium sulfate crystallization field at the expense of the potassium metavanadate crystallization field can be explained with a significant increase in the solubility of the salt with increasing temperature. The data on the composition of the equilibrium solutions corresponding to points P1 at temperatures of 303.15 K and 323.15 K (Tables 3 and 4) show that at a temperature of 303.15 K, the obtained concentration of vanadium ions in the solution is lower by 65 % when compared to that at 323.15 K, with practically constant concentration of sulfate ions at both temperatures. Given the slight difference between the reaction yields (eq 1) obtained at 303.15 K and 323.15 K, the conversion should be preferably carried out at the lower temperature. The process carried out at 303.15 K gives a solution with significantly lower concentration of vanadate ions, as compared to 323.15 K, which will result in lower levels of contamination with vanadium compounds separated from the potassium sulfate solution. The composition of the solid phase for solutions defined by points E2−P4−E6 (Figure 1 and 2) it has been found that mainly NaVO3·2H2O and small amounts of β-NaVO3 are formed in the solid phase. This observation is consistent with previous works on systems containing sodium metavanadate.18 In NaVO3 + NH4VO3 + H2O, the main component of the solid phase obtained at the temperature range of 293.15 K to 303.15 K is NaVO3·2H2O, which with the passage of time turns into βNaVO3. The main component of the solid phase obtained at higher temperatures 313.15 K to 323.15 K is β-NaVO3. The quantitative analysis of the solid phase remaining in the equilibrium with saturation solution in the area defined by points E2−P4−P2−E1 taken show that at a temperature of 303.15 K, in the solid phase decahydrate (Na2SO4·10H2O) is present. At a temperature of 323.15 K, in the solid phase sodium sulfate in an anhydrous form is present, which is consistent with literature data.19−24 In the investigated system

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (0048)566114309. Fax: (0048)566542477. Funding

This work was financed by the National Science Centre (Poland), Project No. NN209760640. Notes

The authors declare no competing financial interest.

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REFERENCES

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Phase Diagram for the Na - American Chemical Society

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