Phase Diagrams of the Quaternary System NaClâMgCl2âNH4Cl in

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Phase Diagrams of the Quaternary System NaCl−MgCl2−NH4Cl in Water at 0 and 25 °C and Their Application Shuai-Yong Dou, Hong-Fei Guo, Bin Zhao,* Chun-Yan Xue, and Ji-Lin Cao* Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, College of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China ABSTRACT: On the basis of the comprehensive utilization of the Na−Mg salt deposit, a new process to produce NaCl and ammonium carnallite (MgCl2·NH4Cl·6H2O) by using NH4Cl and the solution left after the preparation of potassium salt as raw materials was proposed. For this, the phase equilibrium of the quaternary system NaCl−MgCl2−NH4Cl−H2O at 25 and 0 °C was studied. The solubilities of the quaternary system NaCl−MgCl2−NH4Cl−H2O were measured by an isothermal method, and the corresponding phase diagrams were drawn out. From the phase diagrams: there are four solid phase crystalline zones, which correspond to NaCl, MgCl2·6H2O, NH4Cl, and MgCl2·NH4Cl·6H2O respectively. NaCl and MgCl2·NH4Cl·6H2O have larger crystalline zones at 25 °C than at 0 °C, and NH4Cl has a smaller crystalline zone at 25 °C than at 0 °C. It indicates that NaCl and MgCl2·NH4Cl·6H2O are easy to crystallize out. On the basis of the analysis and calculation of the phase diagrams, the appropriate conditions for the process were identified. In the process, the NaCl could be prepared at 25 °C, and the ammonium carnallite could crystallize out at 0 °C when NH4Cl was added into the remaining solution. The solubilities of the studied system were calculated based on the extended Pitzer model, and the results showed that the calculated values were closed to the experimental results.

1. INTRODUCTION Qarhan Salt Lake (QSL) of Qaidam Basin is the second largest salt lake in the world, and it is also China’s leading salt lake in potassium chloride deposits.1 Qarhan Salt Lake belongs to chloride type salt lakes, and potassium chloride is the main product developed in this salt lake. However, in the current production of potassium chloride by using the intermediate product carnallite as raw material, 40 m3 of the old brine was wasted for per ton of potassium chloride.2 It not only resulted in the waste of resources but also posed a great threat to the local environment and species conservation. The mixed system after extracting potassium chloride contains about 5% magnesium chloride and 80% sodium chloride (mass fraction),3 which can be simplified as ternary system of NaCl−MgCl2− H2O. Through the analysis of this phase diagram,4 it can be seen that MgCl2·6H2O has a very small crystalline zone, which indicates that pure MgCl2·6H2O cannot be obtained by evaporation crystallization method. As is known, magnesium chloride is a kind of common inorganic chemical products, which is widely used in metallurgy, building materials, chemical, food, medicine, and agriculture. However, high-purity anhydrous magnesium chloride is a kind of essential raw material for the electrolytic production of magnesium metal, so it has important application value.5,6 The high-purity anhydrous magnesium chloride can be prepared by two main methods.7,8 It can be obtained from hydrated magnesium chloride through thermal dehydration in an atmosphere of anhydrous hydrogen chloride or chlorine, and the disadvantage here is that the process need a great amount © XXXX American Chemical Society

of HCl gas, which is liable to corrode equipment under high temperature, and its energy consumption is high. Also, the highpurity anhydrous magnesium chloride can be obtained from the process of dehydration and deamination of ammonium carnallite, and the ammonium carnallite can be prepared by reaction of the MgCl2 and NH4Cl in water solution. This method can inhibit hydrolysis of magnesium chloride effectively. Based on this, when NH4Cl is added into the mixed system which contains magnesium chloride and sodium chloride after extracting potassium fertilizer, magnesium chloride can be converted to ammonium carnallite through the reaction mentioned above. The ammonium carnallite has lower solubility and is easier to crystallize out, which indicates that high-purity sodium chloride and high-purity ammonium carnallite products can be obtained under suitable operating conditions, and the rational utilization of wasted old brine with high additional value can be realized. The phase diagram of NaCl−MgCl2−NH4Cl−H2O system provides a theoretical guidance for the new process, but no study on phase equilibrium of this system was reported. In order to realize the new process for producing NaCl and ammonium carnallite by using NH4Cl and the solution left after the preparation of potassium chloride in salt lake as raw materials, this paper focused on the phase equilibrium of NaCl−MgCl2− NH4Cl−H2O system at 25 and 0 °C. Received: July 24, 2015 Accepted: December 22, 2015

A

DOI: 10.1021/acs.jced.5b00639 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

2.2. Procedure. The experimental apparatus was shown in ref 9. The test tube was filled with a certain percentage of MgCl2·6H2O, NaCl, NH4Cl, and water. Then the agitator began to stir. It could be determined that the phase equilibrium time was 12 h at 25 °C and 16 h at 0 °C. When the mixture system reached phase equilibrium, the agitator was stopped, and the test tube was stood for 30 min vertically, when liquid phase and solid phase separated clearly. Then, the test tube was pulled out 1/3 of its length from water and wiped. Finally, the rubber was opened; some clear liquid was taken out by a rubber buret, and the wet solids were directly poured out to analyze, respectively. 2.3. Analysis. The content of magnesium ion was measured by the EDTA chelatometry.10 The content of ammonium ion was measured by the formol titration method. The content of chloride ion was measured by the method of Mohr.11 The sodium ion was calculated by subtraction method subtracting magnesium ion concentration and ammonium ion concentration from chlorine ion concentration. The water content was obtained by subtraction. The content of solid phase was identified by wet-residue method with the analysis of XRD.

2. EXPERIMENTAL SECTION 2.1. Materials. The chemical reagents of MgCl2·6H2O, NH4Cl, and NaCl were of analytical grade, and their purity was not less than 99.0%. All of the chemicals were supplied by Tianjin First Reagent Corporation (see Table 1). The water used in this work to prepare solutions was twice-distilled water (conductivity < 5 μS/cm). Table 1. Production Factories and Purity of the Experimental Materialsa compounds

purity (mass fraction)

CAS No.

production factories

NaCl

7647-14-5

MgCl2·6H2O

7791-18-6

NH4Cl

12125-02-9

Tianjin First Reagent Corporation Tianjin First Reagent Corporation Tianjin First Reagent Corporation

0.99 0.99 0.99

a

Note: the reported purities were stated by the supplier and that the reagents were used without further purification.

Table 2. Phase Equilibrium Data of NaCl−MgCl2−NH4Cl−H2O−H2O System at 25 °C and 1 atma,b composition of liquid phase/% (wt) NH4Cl

MgCl2

NaCl

H2O

NH4Cl

MgCl2

NaCl

H2O

equilibrium solid phase

E

8.69 9.65 8.42 0.08 0.14 0.13 0.10 2.55 4.24 11.23 21.43 15.86 0.22 0.26 0.00

21.08 19.79 18.54 35.55 36.31 36.33 36.21 24.95 22.53 11.88 8.52 0.00 36.32 35.48 35.68

0.00 5.69 9.76 0.00 4.57 6.44 5.32 8.59 8.97 13.92 26.09 17.28 9.32 7.49 0.48

70.23 64.87 63.28 64.37 58.98 57.1 58.37 63.91 64.26 62.97 43.96 66.86 54.14 56.77 63.84

29.16 27.46 22.93 0.00 0.34 0.30 0.24 7.06 11.87 30.32 38.25 47.86 0.48 0.61 0.00

70.81 56.35 50.49 100.00 88.52 84.68 86.98 69.14 63.03 32.08 15.20 0.00 79.20 82.07 98.67

0.00 16.19 26.58 0.00 11.14 15.02 12.78 23.79 25.10 37.60 46.55 52.14 20.32 17.32 1.33

235.91 184.66 172.33 180.66 143.78 133.10 140.21 177.09 179.80 170.05 78.44 201.75 118.05 131.32 176.55

BA + Ac BA + Ac BA + Ac + Ha Bis + BA Bis + BA BA + Ha Bis + BA + Ha BA + Ha BA + Ha Ac + Ha Ac + Ha Ac + Ha BA + Ha BA + Ha Bis + Ha

H2 F

H1

D

G a

composition of liquid phase [g/100 g dry salt]

point in phase diagram

Note: Bis−MgCl2·6H2O; Ac−NH4Cl; BA−MgCl2·NH4Cl·6H2O; Ha−NaCl. bStandard uncertainties u are u(T) = 0.1 °C, ur(w) = 2.0%, and ur(p) = 1.0%.

Table 3. Phase Equilibrium Data of NaCl−MgCl2−NH4Cl−H2O−H2O System at 0 °C and 1 atma,b composition of liquid phase/% (wt) point in phase diagram K1

K2

E1 F1 G1 D1 a

composition of liquid phase [g/100 g dry salt]

NH4Cl

MgCl2

NaCl

H2O

NH4Cl

MgCl2

NaCl

H2O


0.05 0.03 0.04 0.96 2.22 3.99 5.65 5.05 6.77 8.91 5.93 0.09 0.00 10.26

33.90 33.83 34.04 25.24 21.67 20.02 18.75 14.88 13.25 5.52 21.41 34.43 8.91 0.00

3.24 6.82 7.60 9.05 10.35 10.69 9.68 11.35 11.99 16.92 0.00 0.00 015 19.78

62.81 59.32 58.32 64.75 65.76 65.30 65.92 68.72 67.99 68.65 72.66 65.48 76.09 69.96

0.13 0.07 0.09 2.73 6.48 11.51 16.58 16.15 21.14 28.42 21.69 0.26 0.00 34.15

91.16 83.15 81.67 71.59 63.29 57.69 55.02 47.57 41.39 17.60 78.31 99.74 98.34 0.00

8.70 16.77 18.24 25.68 30.22 30.80 28.40 36.28 37.47 53.98 0.00 0.00 1.66 65.85

168.89 145.82 139.92 183.69 192.06 188.18 193.43 219.69 212.40 218.98 265.76 189.69 318.24 232.89

Bis + Ac Bis + BA + Ha BA + Ha BA + Ha BA + Ha BA + Ha BA + Ac BA + Ac + Ha Ac + Ha Ac + Ha Ac + BA Bis + BA Bis + Ha Ac + Ha

Note: Bis−MgCl2·6H2O; Ac−NH4Cl; BA−MgCl2·NH4Cl·6H2O; Ha−NaCl. bStandard uncertainties u are u(T) = 0.1 °C, ur(w) = 2.0% and ur(p) = 1.0%. B



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XRD patterns were collected using a Rigaku D/MaxII-2500VB2+/ PC X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm) and the scan mode with a speed of 0.25° min−1. The relative standard deviation of the mass fraction of liquid phase is approximately 2.0%. The uncertainty may be caused by inaccuracies in sampling process and titration procedure.

3. RESULTS AND DISCUSSION 3.1. Solubilities and Phase Diagram of NaCl-MgCl2− NH4Cl−H2O System at 25 and 0 °C. The solubilities of the NaCl−MgCl2−NH4Cl−H2O system at 25 and 0 °C are given in Tables 2 and 3. The dry salt composition corresponds to mass fraction of each composition per 100 g of the sum of dry NaCl, MgCl2, and NH4Cl. On the basis of the data in Tables 2 and 3, the phase diagrams of NaCl-MgCl2−NH4Cl−H2O at 25 and 0 °C are plotted in Figures 1 and 2. The black solid line

Figure 1. Phase diagram of the NaCl−MgCl2−NH4Cl−H2O system at 25 and 0 °C.

and red solid line represent the borderline of crystallizing field at 25 and 0 °C, and the dashed line represents the operation line from phase diagram analysis and calculation. It can be seen from Figure 1 and Figure 2 that there exist two invariant points H1 and H2 at 25 °C or K1 and K2 at 0 °C. H1 and K1 correspond to the coexistence of solids NaCl, MgCl2·NH4Cl·6H2O, and MgCl2·6H2O. Point H2 and K2 correspond to the coexistence of solids MgCl2·NH4Cl·6H2O, NaCl, and NH4Cl. There are four crystallization fields which correspond to NH4Cl, MgCl2·NH4Cl·6H2O, MgCl2·6H2O, and NaCl, respectively. Fields BEH2DB and BE1K2D1B correspond to the crystallization area of NH4Cl; fields EFH1H2E and E1F1K1K2E1 correspond to the crystallization area of MgCl2· NH4Cl·6H2O; fields FCGH1F and F1CG1K1F1 correspond to the crystallization area of MgCl2·6H2O; fields ADH2H1GA and AD1K2K1G1A correspond to the crystallization area of NaCl. The XRD patterns of the solid phases of the four isothermal invariant points are shown in Figure 3. It could be seen that point H1 and K1 correspond to the coexistence of solids NaCl, MgCl2·NH4Cl·6H2O, and MgCl2·6H2O; point H2 and K2 correspond to the coexistence of solids MgCl2·NH4Cl·6H2O, NaCl, and NH4Cl.

Figure 2. Partial enlarged phase diagram of NaCl−MgCl2−NH4Cl− H2O system at 25 and 0 °C. Note: Bis−MgCl2·6H2O; Ac−NH4Cl; BA−MgCl2·NH4Cl·6H2O; Ha−NaCl.

3.2. Applications of the Phase Diagram of the NaCl− MgCl2−NH4Cl−H2O System. By the literature data,4,23 the phase diagram of NaCl−MgCl2−H2O system at 25 °C was plotted in Figure 4. The deviations between the points in C



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Figure 3. XRD characterization of the saturation point solid.

crystallization area of MgCl2·6H2O. It can be seen that the crystalline region of MgCl2·6H2O is very small, suggesting that the separation of MgCl2·6H2O from the mixed solution of NaCl and MgCl2 is very difficult. The mass composition of NaCl and MgCl2 of the mixed system are 80.70 and 4.28, respectively, which is represented by using point H. Point H located in the region ADEA, which indicated that the mixture system was supersaturated solution, and NaCl would be crystallized, the compositional point of the corresponding residual solution was point N, which was the cross point of line DE with the extension line of AH. As shown in Figure 1, when NH4Cl was added into the ternary system NaCl−MgCl2−H2O, ammonium carnallite (MgCl2·NH4Cl·6H2O) can be obtained by reaction of ammonium chloride and magnesium chloride in water solution, and it has a larger crystalline region EFH1H2E, which indicates the ammonium carnallite is easier to crystallize out in the quaternary system NaCl−MgCl2−NH4Cl−H2O than MgCl2· 6H2O in the ternary system NaCl−MgCl2−H2O. Point N was plotted in Figure 1, when NH4Cl was put into the mixtures, the mixture compositional point in Figure 1 would move along NB from N to B, as the addition of NH4Cl kept increasing. When the mixture compositional point located in the region E1F1K1K2E1, MgCl2·NH4Cl·6H2O would crystallize out. We chose points M1, M2, and M3 as operating points and found that MgCl2·NH4Cl·6H2O production for the point M1 was at a maximum through the lever principle analysis. After MgCl2· NH4Cl·6H2O crystallized out, the compositional points of the remaining solution were points T1, T2, and T3 separately, which were the cross points of line K1K2 with the extension line of BAM1, BAM2, and BAM3 separately.

Figure 4. Phase diagram of NaCl−Mg2Cl−H2O system at 25 °C.4

refs 23 and4 may be caused by inaccuracies in sampling process and titration procedure. Besides, the points in ref 23 plotted were selected from 19.31 to 29.42 °C, and it would cause the deviations too. The points in ref 4 were used in this work. Points A, B, C, and F represent the compositions of NaCl, H2O, MgCl2, and MgCl2·6H2O separately; points D and G represent the solubilities of NaCl and MgCl2·6H2O in aqueous solution separately; invariant point E represents the coexistence of solids NaCl and MgCl2·6H2O; field ADEA represents the crystallization area of NaCl; field AEFA represents crystallization area of NaCl and MgCl2·6H2O; field GEFG represents D



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Figure 5. Technological flowchart of producing MgCl2·NH4Cl·6H2O and NaCl.

From the partial derivative of eq 1 to ni and nj, respectively, we can calculate out the activity coefficient of each ion in the mixed electrolyte solution. Then the generalized computing equation is as shown in eq 2.

To separate the NaCl crystal out in the solutions T1, T2, and T3, the system points T1, T2, and T3 were analyzed in the phase diagram of NaCl−NH4Cl−MgCl2−H2O system at 25 °C, which was represented by the black solid line. In Figure 1, point T1, T2, and T3 located in the field ADH2H1GA corresponding to the NaCl with saturated solution. After NaCl was separated out, the compositional points of the remaining solution were points S1, S2, and S3. When the raw materials (mixed solution of NaCl and MgCl2) and NH4Cl were added into solution S1, S2, and S3 by a certain percentage, the compositional point of the mixed solution could lie on the line of BAM1/ T1, BAM2/ T2, and BA T3. Then NaCl would crystallize out, and the remaining solution would go back to the points T1, T2, and T3 respectively, and the solution could be reused in next cycle. Therefore, the production of MgCl2·NH4Cl·6H2O and NaCl could be achieved, when we chose M1, M2, or M3 as the initial proportioning point. By analyzing the phase diagrams of NaCl-MgCl2−NH4Cl− H2O system at 25 and 0 °C and the phase diagram of NaCl-MgCl2−H2O system at 25 °C, the Figure 5 shows the technological flowchart of producing MgCl2·NH4Cl·6H2O and NaCl. Taking 100 kg mixed solution after extracting potassium chloride as the original material, the calculations to produce MgCl2·NH4Cl·6H2O and NaCl were fulfilled based on the material conversation law by using the phase diagrams. The composition and amount of key proportioning points and products were calculated and given in Table 4. 3.3. Calculation of Solubility. The Pitzer theory equation is the most effective model to calculate the solubilities of various substances in salt water system so far. It is available to the calculation of solubilities for about 300 kinds of highly concentrated electrolyte solutions, and it can be also used for the calculation of solubility of mixed electrolyte.12−14 So, the calculation of solubility in this paper is carried out by using Pitzer theory equation. Assume there is a solution containing nw kg of water, ni moles of i solute ions, and nj moles of j solute ions, then the excess Gibbs function GE for nw kg of water15 could be calculated by eq 1 GE /RT = (GE /RT )ii = n w f (i) + + ∑ ∑ ∑ μijk ninjnk /n2 w

ln γMX =

1 |z Mz X|f ′(I ) 2 ⎛ 2ν ⎞ + ⎜ M ⎟ ∑ ma [BMa + (∑ mz)CMa] ⎝ ν ⎠ a +

⎡ ⎤ ⎛ν ⎞ ⎛ 2νX ⎞ ⎜ ⎟ ∑ m ⎢B + (∑ mz)CcX + ⎜ M ⎟θMc(I )⎥ c cX ⎝ ν ⎠ c ⎢⎣ ⎥⎦ ⎝ νX ⎠

+

∑ ∑ mcma⎢⎣|z Mz X|Bca′ +

⎡

c

a

⎡⎛ νX ⎞ ⎤ ⎟ψ + |z Mz X|θcc′ ′(I )⎥ cc X ⎠ ⎦ ′ ν

1 + 2

∑ ∑ mcmc′⎢⎣⎝⎜

1 + 2

∑ ∑ mama′⎢⎣⎝⎜

c

c

⎤ 1 (2νMz MCca + νMψMca + νXψcaX)⎥ ⎦ ν

c′

⎡⎛ νM ⎞ ⎤ ⎟ψ + |z Mz X|θaa′ ′(I )⎥ ⎦ ν ⎠ Maa ′

c′

(2)

Here, m refers to molal concentration of each ion, subscript a and c represent anions and cations, respectively, ∑mz = ∑cmczc = ∑ama|za| = ; f ′(I) represents long-range electrostatic contribution; BMX, BMX ′ and CMX are the Pitzer parameters of (1) (2) ϕ the electrolyte dependeding on β(0) MX, βMX, βMX and CMX; θ and ψ stand for the ion interaction parameters. In order to achieve accurate results, Cao expresses the ion interaction parameters θ as follows.16 (0) (1) θMc(I ) = θMc (I ) + θMc (I )F (I )

(3)

F(I) can be calculated by ionic strength I. F (I ) =

2 [1 − (1 + αI 0.5)exp(−αI 0.5)] 2 αI

(α = 2)

Among them: ⎡ I1/2 ⎤ 2 1/2 ⎥ + ln(1 + bI ) f ′(I ) = −2Aϕ⎢ b ⎣ 1 + bI1/2 ⎦

(4)

For the 2:1 electrolyte: (0) BMX = βMX +

(1) 2βMX

α 2I

[1 − (1 + αI1/2)exp( −αI1/2)] (5a)

∑ ∑ λijninj /n w ′ = BMX

(1) E

(1) 2βMX ⎡ −1 2 2 ⎢ ⎣

αI

⎤ ⎛ 1 ⎞ + ⎜1 + αI1/2 + α 2I ⎟exp(− αI1/2)⎥ ⎝ ⎦ 2 ⎠

(5b)



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Table 4. Calculated Results Based on the Phase Diagrams To Produce MgCl2·NH4Cl·6H2O and NaCla composition % (wt) step in the process

point in the diagram

mixed H → NaCl + N

H

N + NH4Cl → BA + H2O + T1(T2, T3)

M1(M2, M3)

system

NaCl

MgCl2

NH4Cl

H2O

amount/kg

T/°C

mixed H solution N NaCl precipitated solution N NH4Cl

80.70 7.80 100 7.80 0

4.28 20.44 0 20.44 0

0 0 0 0 100

15.02 71.76 0 71.76 0

100 20.93 79.07 20.93 1.07 0.83 0.44 3.75 3.67 3.72 15.36 15.77 16.80 2.89 2.32 0.85 15.36 15.77 16.80 1.42 1.62 1.04 13.47 13.69 15.70 0.47 0.48 0.06 13.47 13.69 15.70 0.58 0.57 0.01 0.01 0.01 0.00 1.36 1.56 1.09 15.36 15.77 16.80 0.06 0.06 0.00

25

H2O

0

solution T1(T2, T3)

BA

T1(T2, T3) → NaCl + H2O + S1(S2, S3)

T1(T2, T3)

solution T1(T2, T3)

H2O

solution S1(S2, S3)

NaCl precipitated

S1(S2, S3) + H2O + mixed H + NH4Cl → BA + T1(T2, T3)

M1/, (M2/, T3)

solution S1(S2, S3)

mixed H

0

100

10.63 10.35 9.72 0

20.88 21.67 23.59 37.08

3.07 2.22 1.57 20.83

65.42 65.76 65.12 42.09

10.63 10.35 9.72 0

20.88 21.67 23.59 0

3.07 2.22 1.57 0

65.42 65.76 65.12 100

8.64 8.59 10.36 100

23.81 24.95 25.24 0

3.50 2.56 1.68 0

64.05 63.90 62.72 0

8.64 8.59 10.36 80.70

23.81 24.95 25.24 4.28

3.50 2.56 1.68 0

64.05 63.90 62.72 15.02

NH4Cl

0

0

100

0

H2O

0

0

0

100

solution T1(T2, T3)

BA

a

0

10.63 10.35 9.72 0

20.88 21.67 23.59 37.08

3.07 2.22 1.57 20.83

65.42 65.76 65.12 42.09

0

25

0

Standard uncertainties u are u(T) = 0.1 °C, ur(w) = 2.0%, and ur(p) = 1.0%.

CMX =

ϕ CMX

2 |z Mz X|1/2

′ = BMX

(5c)

For 2:2 electrolyte: (0) BMX = βMX +

+

α22I

+

(1) 2βMX

(2) 2βMX

(1) 2βMX ⎤ ⎡ ⎛ 1 ⎞ −1 + ⎜1 + αI1/2 + α 2I ⎟exp( −αI1/2)⎥ 2 2 ⎢ ⎝ ⎦ 2 ⎠ αI ⎣

α12I

(1) 2βMX ⎡ ⎤ ⎛ 1 ⎞ −1 + ⎜1 + αI1/2 + α 2I ⎟exp( −αI1/2)⎥ 2 2 ⎢ ⎝ ⎦ 2 ⎠ αI ⎣

[1 − (1 + α1I1/2)exp(−α1I1/2)]

[1 − (1 + α2I1/2)exp(−α2I1/2)]

(6b)

CMX =

(6a) F

ϕ CMX

2 |z Mz X|1/2

(6c) DOI: 10.1021/acs.jced.5b00639 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

From the phase equilibrium theory it is known that the substance’s chemical activity remain unchanged at the same temperature. Based on this, the unknown variables ϕ and ψ can be calculated by using the equilibrium data of the related subsystems Mg2+, NH4+//Cl−H2O,4,18,19 Na+, NH4+//Cl−− H2O,4,20−22 Na+, and Mg2+//Cl−−H2O.4,23,24The calculated values are presented in Table 5. When the θ and ψ have been obtained, the solubilities of the studied system NaCl−MgCl2−NH4Cl−H2O at 25 and 0 °C can be calculated through the eq 2−8 on the basis of the phase equilibrium theory. The calculated results are given in Table 6 and Table 7 and close to the experimental values.

Table 5. Interaction Parameters in the System of NaCl− MgCl2−NH4Cl−H2O−H2O at 25 and 0 °C17 values at 25 °C

interaction parameters θ(0)Mg−NH4 θ(1)Mg−NH4

values at 0 °C −0.3283

2.1409 −26.741

−16.414

ψMg−NH4−Cl

−0.1770

0.2109

θ(0)Mg−Na θ(1)Mg−Na ψMg−Na−Cl θ(0)Na−NH4

−0.0337 1.9023 0.0258 −0.1462

2.6415 −29.998 −0.1377 0.1042

θ(1)Na−NH4

1.8390

0.8972

ψNa−NH4−Cl

0.0214

−0.0097

4. CONCLUSIONS (1) The solubilities of the NaCl−MgCl2−NH4Cl−H2O system at 25 and 0 °C have been measured, and the phase diagrams have been plotted. It can be seen from this phase diagram that there exist four solid crystalline regions corresponding to MgCl2·6H2O, MgCl2·NH4Cl·6H2O, NaCl, and NH4Cl separately. (2) By analyzing the phase diagram of NaCl−MgCl2−H2O at 25 °C and the phase diagrams of NaCl−MgCl2−NH4Cl−H2O at 25 and 0 °C, a new process of concentrating and separating sodium chloride and magnesium chloride in salt lakes has been designed. The appropriate process conditions to produce sodium chloride and ammonium carnallite (MgCl2·NH4Cl· 6H2O) has also been determined. In the process, the NaCl could be prepared at 25 °C, and the ammonium carnallite could crystallize out at 0 °C when NH4Cl was added into the remaining solution. (3) The solubilities of NaCl−MgCl2−NH4Cl−H2O at 25 and 0 °C have been calculated on the basis of the Pitzer

Here, α1, α2, Aϕ, b, and α are all constants obtained from (1) (2) ϕ literature.17 β(0) MX, βMX, βMX, and CMX are determined by the Pitzer parameters at 25 °C and the temperature coefficient for NaCl, MgCl2, and NH4Cl.17 The water activity is with relation to the osmotic coefficient ϕ, and it can be obtained from the following formula.

ln a H2O = −

∑i mi 55.51

ϕ

(7)

where mi refers to molal concentration of each ion and ∑mi refers to summation of all ions besides the nonelectrolyte. ϕ can be calculated by eq 8: ϕ−1=

1 [If ′(I ) − f (I ) ∑i mi + 2 ∑ ∑ mc ma[Bca + IBca′ + 2(∑ mz)Cca]] c

a

(8)

Table 6. Calculated and Experimental Solubilities for the NaCl−MgCl4−NH4Cl−H2O System at 25 °Ca mexp/mol·kg−1

a

mcal/mol·kg−1

relative error/%

NH4Cl

MgCl2

NaCl

NH4Cl

MgCl2

NaCl

NH4Cl

MgCl2

NaCl


2.781 2.488 0.044 0.043 0.032 0.746 1.234 3.334 9.114 0.076 0.086

3.204 3.077 6.466 6.683 6.516 4.100 3.682 1.982 2.036 7.046 6.564

1.501 2.639 1.326 1.930 1.560 2.300 2.389 3.783 10.16 2.946 2.258

2.781 2.488 0.044 0.043 0.032 0.746 1.234 3.334 9.114 0.076 0.086

3.212 3.111 6.556 6.765 6.777 4.232 3.551 1.951 2.051 7.111 6.498

1.564 2.765 1.344 1.938 1.573 2.216 2.278 3.657 10.24 2.899 2.251

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.250 1.105 1.392 1.227 4.006 3.220 3.558 1.564 0.737 0.923 1.005

4.197 4.775 1.357 0.415 0.833 3.652 4.646 3.331 0.787 1.595 0.310

BA + Ac Bis + Ac + Ha Bis + BA BA + Ha Bis + BA + Ha BA + Ha BA + Ha Ac + Ha Ac + Ha BA + Ha BA + Ha

Standard uncertainties u are u(T) = 0.1 °C, ur(w) = 2.0%, and ur(p) = 1.0%.

Table 7. Calculated and Experimental Solubilities for NaCl−MgCl4−NH4Cl−H2O System at 0 °Ca mexp/mol·kg−1

a

mcal/mol·kg−1

relative error/%

NH4Cl

MgCl2

NaCl

NH4Cl

MgCl2

NaCl

NH4Cl

MgCl2

NaCl


0.015 0.009 0.013 0.277 0.631 1.142 1.602 1.374 1.862 2.426

5.669 5.990 6.130 4.094 3.461 3.220 2.987 2.274 2.047 0.845

0.883 1.967 2.230 2.392 2.693 2.801 2.513 2.826 3.018 4.217

0.015 0.009 0.013 0.277 0.631 1.142 1.602 1.374 1.862 2.426

5.712 6.011 6.233 4.212 3.500 3.149 3.001 2.301 2.066 0.901

0.991 1.970 2.413 2.299 2.732 2.777 2.514 2.852 3.121 4.555

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.758 0.351 1.680 2.882 1.127 2.205 0.469 1.187 0.928 6.627

12.23 0.153 8.206 3.888 1.448 0.857 0.040 0.920 3.413 8.015

Bis + Ac Bis + BA + Ha BA + Ha BA + Ha BA + Ha BA + Ha BA + Ac BA + Ac + Ha Ac + Ha Ac + Ha

Standard uncertainties u are u(T) = 0.1 °C, ur(w) = 2.0%, and ur(p) = 1.0%. G



Article

(19) Zhou, H.; Huangfu, L.; Bao, Y.; Bai, X.; Ma, R.; Sha, Z. Solubility of ammonium chloride in a MgCl2-NH4Cl-NH3-H2O system at 298 K: Experiments, modeling and prediction. Fluid Phase Equilib. 2014, 383, 174−181. (20) Farelo, F.; Fernandes, C.; Avelino, A. Solubilities for Six Ternary Systems: NaCl + NH4Cl + H2O, KCl + NH4Cl + H2O, NaCl + LiCl + H2O, KCl + LiCl + H2O, NaCl + AlCl3 + H2O, and KCl + AlCl3 + H2O at T = (298 to 333) K. J. Chem. Eng. Data 2005, 50, 1470−1477. (21) Zeng, Y.; Li, Z. Solubility Measurement and Modeling for the NaCl-NH4Cl-Monoethylene Glycol-H2O System from (278 to 353) K. J. Chem. Eng. Data 2015, 60, 2248−2255. (22) Yang, B.; Li, J.; Jin, Y.; Mo, S.; Pan, H. Solid-liquid equilibria of the quaternary system Na+, NH4+//Cl−, H2PO4−-H2O at 298.15 and 323.15 K. Fluid Phase Equilib. 2015, 404, 55−60. (23) Clynne, M. A.; Potter, R. W. I.; Haas, J. L. Solubility of NaCl in Aqueous Electrolyte Solutions from 10 to 100 degrees C. J. Chem. Eng. Data 1981, 26, 396−398. (24) Mun, A. I.; Darer, R. S. Crioscoption of salts water solutions. Zh. Neorg. Khim. 1957, 2, 1658−1661.

theory equation. The deviations between the calculated values and the experimental data are acceptable, indicating that the Pitzer theory equation is suitable for the calculation of the solubilities studied in this work.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 22 60204291. *Phone: +86 13702093697. Funding

All the authors received funding from the National Natural Science Foundation of China Grant 21576066 and Program for Changjiang Scholars and Innovative Research Team in University Grant IRT14R14. Notes

The authors declare no competing financial interest.

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REFERENCES

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