Experimental Determination and Modeling of the Solubility Phase

Oct 16, 2013 - Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt ... After multistep evaporation by solar ponds, the salt...
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Experimental Determination and Modeling of the Solubility Phase Diagram of the Quaternary System MgCl2+LiCl+NH4Cl+H2O at 298.15 K and Its Applications in Industry Haitang Yang,† Dewen Zeng,*,†,‡ Tengyu Liang,† Xia Yin,§ and Qiyuan Chen† †

College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, P. R. China Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810007, P. R. China § College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, P. R. China ‡

S Supporting Information *

ABSTRACT: Solubility isotherms as well as the corresponding solid phases of the quaternary system MgCl2+LiCl+NH4Cl +H2O at 298.15 K have been elaborately measured by an isothermal equilibrium method. Four crystallization fields including two double salts (LiCl·MgCl2·7H2O(s) and NH4Cl·MgCl2·6H2O(s)), one hydrate salt (MgCl2·6H2O(s)), and one solid-solution (LiCl· H2O+NH4Cl)(ss) were detected in this system. A Pitzer-Simoson-Clegg (PSC) thermodynamic model was used to simulate and predict the thermodynamic properties of this quaternary system and its subsystems. The water activities of the ternary systems MgCl2+LiCl+H2O and LiCl+NH4Cl+H2O, as well as the solubility of the quaternary system MgCl2+LiCl+NH4Cl+H2O, were predicted by the PSC model, the results of which were compared with available literature data and the experimental results in this work. The excellent agreement between the predicted and experimental results indicates that the solubility results obtained in this work are reliable. On the basis of the quaternary phase diagram calculated by the model, several examples are provided for industrial applications.

1. INTRODUCTION After multistep evaporation by solar ponds, the salt-lake brine containing LiCl can be roughly treated as the system of MgCl 2 +LiCl+H 2 O, where the mass ratio of Mg:Li is approximately 20−25. A method for economically separating magnesium and lithium in the salt brine with such a high Mg/Li ratio is a challenge for all chemical engineers. Among the separation methods proposed is the method suggested by Xu and Xu,1 in which NH3 gas was added to precipitate MgCl2 as Mg(OH)2 and the NH3 gas was reproduced by reacting CaO with NH4Cl at high temperatures to guarantee its economy. Engineers reported that several types of unexpected salts were crystallized on the walls of reactors or pipelines and harm the precipitation process of Mg(OH)2. It is well-known that the brine system gradually converts from MgCl2+LiCl+H2O into MgCl2+LiCl+NH4Cl+H2O and finally into LiCl+NH4Cl+H2O in the process of adding NH3 gas to precipitate Mg2+. To gain a profound understanding of the crystallization phenomena, information on the solubility phase diagram of these systems is required. To date, the phase diagrams of the ternary systems MgCl2+LiCl+H2O, MgCl2+NH4Cl+H2O, and LiCl+NH4Cl +H2O have been reported both in the literature2 and in our previous paper;3 however, the phase diagram for the quaternary system has never been reported. In this paper, the solubility isotherms and their corresponding solid phases of the quaternary system MgCl2+LiCl+NH4Cl +H2O were determined experimentally at T = 298.15 K. Next, a proper thermodynamic model was chosen to simulate the properties of the binary and ternary subsystems and to predict the solubility phase diagram of the quaternary title system. On © 2013 American Chemical Society

the basis of the model calculation, a profound understanding of the crystallization mechanism in the precipitation process of Mg(OH)2 by NH3 gas can be obtained.

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. Lithium chloride was prepared by neutralizing lithium carbonate (purity in mass fraction >0.999, Shanghai China-Lithium Industry Co. Ltd.) with hydrochloric acid (G. R., Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). Magnesium chloride and ammonium chloride were A. R. reagents also purchased from Sinopharm Chemical Reagent Co. Ltd. These salts were then purified by double crystallization with 50% salt recovery each time before use. The main impurity elements Na, K, Ca, Fe, and Ba in the prepared reagents were analyzed by inductively coupled plasma (ICP) emission spectrometry (US Perkin-Elmer Corporation, Optima 5300DV), and the content of each element was determined to be less than 50 ppm. Doubly distilled water (S ≤ 1.2 × 10−4 S/m) and silver nitrate (purity in mass fraction greater than >0.999) were also used in the experiment. A vertical section of the apparatus is presented in Figure 1a. The thermostat (LAUDA E219, Germany) consists of a stainless steel tank (A), digital heating controller (B), heating rings (C), and a stirring pump (D), exhibiting temperature stability of ±0.01 K. The bath temperature was controlled by the Received: Revised: Accepted: Published: 17057

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(ii) The concentration of MgCl 2 in this system was determined by titrating the Mg2+ ions in bottle (2) with EDTA, as described in the literature.5 (iii) The total concentration of MgCl2, LiCl, and NH4Cl in the quaternary system was determined by precipitating Cl− ions in bottle (3) with AgNO3, as described in the literature.5 The difference between the total amount (iii) and the sum of the other two components (i+ii) gives the concentration of LiCl. 2.3. Accuracy Analysis. In each run of an equilibrium experiment, the equilibration time was set to be 72 h for the ternary systems and 200 h for the quaternary system. The relative analysis accuracies for the concentrations of NH4+, Mg2+, and Cl− in the solution were 0.15, 0.1, and 0.05 wt %, respectively. Considering the errors from other procedures, e.g., the purities of the chemical substances and equilibrium time, we can reasonably evaluate that the maximal relative uncertainty of the experimental solubility results in this work is less than 0.4%.

3. EXPERIMENTAL RESULTS AND DISCUSSIONS The experimental solubility isotherms of the quaternary system at T = 298.15 K are presented in Table S1, Supporting Information, and plotted in Figure 2. The solid phases in equilibrium with the saturated solution were determined exclusively by powder X-ray diffraction. According to Figure 2, the phase diagram of the quaternary system MgCl2+LiCl+NH4Cl+H2O consists of four crystallization fields corresponding to two double salts (LiCl·MgCl2· 7H2O(s) and NH4Cl·MgCl2·6H2O(s)), one hydrate salt (MgCl2· 6H2O(s)), and one solid solution (LiCl·H2O+NH4Cl)(ss). The main crystallization fields are the solid solution (LiCl·H2O +NH4Cl)(ss) and the double salt NH4Cl·MgCl2·6H2O(s), whereas the fields of MgCl2·6H2O(s) and LiCl·MgCl2·7H2O(s) are extremely narrow. The crystallization field of the double salt NH4Cl·MgCl2·6H2O(s) is so large that it extends to the area where the LiCl contents (in dry-salt figure) is greater than 0.9. As shown from the water content figure of Figure 2, the lowest water content point of this system located on E00 eutectic point. To verify the reliability of the experimental data, we compared the eutectic points in the ternary systems determined in this work (filled circles in Figure 2) and those reported in the literature2 (unfilled triangles in Figure 2) and found that they are in good agreement. Furthermore, the solubility isotherms of the quaternary system were determined by two types of experimental methods. One method involves the gradual addition of a third salts into the ternary eutectic solution, as shown at points 1, 7, 12, and 15 in Figure 2 and then stirred until equilibrium was reached. We analyzed the saturated solution and obtained the composition points, e.g., points 2, 3, 4, 5, 6, 8, 9, 10, 11, 13, and 14 in Figure 2. The connection of the points forms the solubility isotherms () in Figure 2. The other method involves a step-by-step evaporation of a four-composition solution equilibrated by one type of solid phase. According to the phase rule, the composition points of the solutions that equilibrate with one specific solid phase, as in points NH4Cl and NH4Cl·MgCl2· 6H2O in Figure 2, should fall in a straight line that, once extrapolated, should go through the composition point of the solid phase. Once a second solid phase forms, the composition point of the saturated solution will deviate from the previous straight line. After the second solid phase forms, the composition point of the saturated solution will follow the solubility isotherm. When the solubility isotherms determined by the two methods

Figure 1. (a) Vertical section of the apparatus for sample equilibrium. A, stainless steel thermostat; B, digital heating controller; C, heating rings; D, circulation pump (LAUDA E219, Germany); E, glycol−water bath; F, plastic plate of sealed cover; G, glass equilibrium flask; H, equilibrium sample; I, multimagnetic blender; J, plastic pad. (b) Top view of the apparatus.

cooperation of the heating systems and the stirring pump (D) which drives the mixed solvent through a cooler outside the thermostat. The temperature was determined using a calibrated glass thermometer with an accuracy of ±0.01 K. A sealing plate (F) was used to separate the water vapor from the mouths of the glass equilibrium flasks (G). The plastic pads with holes (J) were used to make the temperature of the samples more uniform. A Sartorius (CPA225D) balance was used for weighing with an error of ±0.1 mg. 2.2. Experimental Procedures. Solid−liquid equilibrium experiments were carried out in a ground 250 cm3 glass equilibrium flask (G) that was immersed in the thermostat filled with a glycol−water mixture (E). The solution and solid in the flask were stirred with a magnetic stirrer (I) outside the thermostat. Each sample was placed and stirred in the thermostat for approximately 200 h and then kept static for approximately 12 h. The sample of the saturated solution was then removed using a pipet, with the tip covered in glass cloth as a filter, and transferred to two weighed 40 cm3 quartz bottles (1, 2) and a weighed 30 cm3 quartz bottle (3). The wet solid was transferred using a glass scoop into a 40 cm3 sintered glass filtering crucible and then rapidly washed several times by alcohol to remove the aqueous solution from the surface of the solid phase. The solid was analyzed by X-ray diffraction, and the solution was analyzed according to the following procedures: (i) The concentration of NH4Cl in bottle (1) in this system was determined by the method of distillation combined with acid−base titration, as described in our previous paper.4 17058

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represent the properties of this quaternary system and its ternary systems. 4.1. Modeling Methodology. The Pitzer-Simonson-Clegg model (PSC model),6−9 which expresses the excess Gibbs energy of an aqueous electrolyte system with a long-range electrostatic term and a short-range Margules expansion, is applicable for both dilute and concentrated solutions and was applied in our previous work.10−14 Considering that the solubility of the NH4Cl solute is relatively low, while the solubility of LiCl and MgCl2 solutes are high, the PSC model is chosen to describe and predict the phase equilibrium of the systems considered in this work. 4.2. Simulation of the Model Parameters. 4.2.1. Binary PSC Model Parameters. The binary PSC parameters for the systems LiCl+H2O and NH4Cl+H2O and the solubility products KLiCl·H2O and KNH4Cl were determined in our previous work.3 These values were directly used in this work without any modification. The binary model parameters for the system MgCl2+H2O system and solubility product KMgCl2·6H2O were taken directly from the literature.9 All of the parameters are presented in Tables 1 and 2. Table 1. PSC Model Parameters for the Binary Systems at 298.15 K binary parameters system

Bmx

Wmx

Umx

Vmx

source

LiCl+H2O NH4Cl+H2O MgCl2+H2O

230.01 11.31 102.33

0.0305 2.50 −35.36

29.10 10.06 −42.56

−33.73 −6.78 15.37

3 3 9

Table 2. Solubility Products lnK of Solid Phases in the Quaternary MgCl2+LiCl+NH4Cl+H2O System at 298.15 K Figure 2. Phase diagram of the quaternary system MgCl2+LiCl+NH4Cl +H2O at 298.15 K (dry-salt and water content figures). ○, ◊, ⧫, □, ▲, ▼, ▶, and ◀, experimental values located on solubility isotherms or single salt crystallization field; ●, eutectic points in ternary systems determined in this work; △, eutectic points in ternary systems reported in literature;2 ■, the composition of double salts in the quaternary system; , experimental isotherms; - - -, evaporation path. Crystallization fields: A = solid solution (LiCl·H2O+NH4Cl)(ss), B = NH4Cl·MgCl2·6H2O(s), C = MgCl2·6H2O(s), and D = LiCl·MgCl2· 7H2O(s). E0: eutectic point (A + B + D) in this quaternary system; E00: eutectic point (B + C + D) in this quaternary system.

solid phase

lnK

source

LiCl·H2O(s) NH4Cl(s) MgCl2·6H2O(s) LiCl·MgCl2·7H2O(s) NH4Cl·MgCl2·6H2O(s)

4.8702 −5.1095 −0.7229 1.6500 −11.1962

3 3 9 this work this work

4.2.2. Ternary PSC Model Parameters. By applying the binary parameters in Tables 1 and 2, we predicted the solubility isotherms of the ternary systems MgCl2+LiCl+H2O and MgCl2+NH4Cl+H2O at 298.15 K, as represented by the dashed lines in Figures 3 and 4, respectively. The large deviations between the predicted results and the experimental data15,16 indicate that the ternary model parameters are necessary for describing the properties of these ternary systems. In addition, we predicted the water activities in the ternary system MgCl2+LiCl+H2O at 298.15 K and compared them with the experimental values.17 The remarkable deviations between these values can be observed in Figure 5a. Thus, we fitted the mixing model parameters KLiCl·MgCl2·H2O and KNH4Cl·MgCl2·H2O to the experimental solubility data15,16 and obtained their values which are listed in Tables 2 and 3, respectively. The recalculated solubility isotherms, represented by the solid lines in Figures 3 and 4, agree with the experimental data very well. Correspondingly, the water activity in the ternary system MgCl2+LiCl+H2O predicted by the binary and mixing model parameters also agrees with the experimental values,17 as shown in Figure 5b. The agreement in solubility isotherms and water activity indicates the thermodynamic consistency of the model

overlap, they should be judged as reliable. In this work, the series of points 1, 2, 3, 4, 5, and 6 in Figure 2 were obtained by the first method and the series of points (a, a′, a″, a‴), (b, b′, b″), (c, c′), (d, d′), (e, e′, e″), (f, f′, f″, f‴, f⁗), and (g, g′, g″, g‴, g⁗) were determined by the second method. The solubility isotherm 1 → 2 → 3 → 4 → 5 → 6 in Figure 2 determined by the first method overlaps with that (the line a′ → a″ → a‴ in Figure 2) determined by the second method. Thus, the results should be regarded as reliable. Because the prolonged lines c−c′, d−d′, e−e′−e″, f−f′− f″−f‴, and g−g′−g″−g‴ goes through the composition point of the solid phase NH4Cl·MgCl2·6H2O, the area where these points were located should be assigned as the formation field of the solid phase NH4Cl·MgCl2·6H2O.

4. MODELING To obtain more information on the solid−liquid equilibrium of the titled quaternary system and to further verify the reliability of the experimental results, a thermodynamic model is necessary to 17059

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Figure 3. Solubility phase diagram of the MgCl2+LiCl+H2O system at 298.15 K. ○●, experimental data;15 - - -,calculated by the binary parameters only; , calculated by the binary and mixing parameters.

Figure 5. Deviation of water activity in the ternary system LiCl +MgCl2+H2O at 298.15 K. (a) Deviation of the experimental17 and model values calculated by the binary parameters only; (b) deviation of the experimental17 and model values calculated by the binary and mixing parameters. Figure 4. Solubility phase diagram of the MgCl2+NH4Cl+H2O system at 298.15 K. ○, experimental data;16 - - -, calculated by the binary parameters only; , calculated by the binary and mixing parameters.

Table 3. Mixing PSC Model Parameters of the Ternary Systems at 298.15 K mixture parameters

parameters. The mixing parameters for the LiCl+NH4Cl+H2O system were taken from our previous work.3 4.3. Solubility Prediction of the Quaternary System MgCl2+LiCl+NH4Cl+H2O. By applying all of the binary and ternary model parameters in Tables 1, 2, and 3, we predicted the phase diagram of the quaternary system MgCl2+LiCl+NH4Cl +H2O at 298.15 K without further fitting and compared it with the experimental data determined in this work, as shown in Figure 6. In the calculation process, the solid solution (LiCl·H2O +NH4Cl)(ss) was treated as a regular solution, with Ω = 1000 J· mol−1 in the mixing enthalpy equation (ΔH = Ωx1x2), as performed in our previous work.3 As shown in Figure 6, the predicted solubility isotherms agree well with the experimental data. This not only delineates the large formation area of the solid phases, NH4Cl·MgCl2·7H2O(s) and (LiCl·H2O+NH4Cl)(ss), but also the narrow formation area of the solid phases LiCl·MgCl2·7H2O and MgCl2·6H2O. Both the experimental and the predicted results reveal that the formation area of the solid phase NH4Cl·MgCl2·7H2O(s) occupies a large field from the MgCl2−NH4Cl side to the LiCl side. Furthermore, the ratio of NH4Cl to LiCl·H2O in the solid solution phase in equilibrium with any specific solution can be accurately calculated by the model. According to the calculation, when the LiCl content (mass fraction in dry salt base) in saturated aqueous solution is more than 0.7, the LiCl content in

system

Wmnx

Qmnx

Umnx

source

MgCl2+LiCl+H2O MgCl2+NH4Cl+H2O LiCl+NH4Cl+H2O

−53.00 −80.10 −11.50

17.40 22.30 3.66

−3.80 5.70 5.499

this work this work 3

the equilibrated solid solution is greater than 0.02 in mass fraction, the lowest content detectable by the XRD technique. In addition, the content of H2O on the isothermal surface of the quaternary system was also predicted and compared with the experimental values, as shown in Table 4. The relative average deviation of the prediction from the experimental data is 1.69%. These agreements further indicate the reliability of the predicted and experimental results.

5. INDUSTRIAL APPLICATION On the basis of the model calculation, profound understanding can be obtained on the crystallization phenomena in the magnesium removal process from the brine containing MgCl2 and LiCl using the NH3 gas. For example, a typical brine equilibrated with solid phase MgCl2·6H2O(s) after evaporation in a solar pool contains 5.69 mol·kg−1 MgCl2 and 0.284 mol·kg−1 LiCl, as shown in point M in Figure 6. When the NH3 gas is used to precipitate MgCl2, the reaction equation can be written as 17060

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Figure 6. Dry-salt phase diagram of the quaternary system MgCl2+LiCl+NH4Cl+H2O at 298.15 K. ●, experimental values with two solid phases; ○, the quaternary eutectic points; , predicted eutectic lines with PSC model; ■: the predicted mass fraction of NH4Cl and LiCl·H2O in solid-solution solid phase; −, the crystallization behavior of a mixed brine solution (MgCl2 5.69 mol·kg−1; LiCl 0.284 mol·kg−1) while adding NH3. Crystallization fields: A = solid solution (LiCl·H2O+NH4Cl)(ss); B = NH4Cl·MgCl2·6H2O(s); C = MgCl2·6H2O(s); D = LiCl·MgCl2·7H2O(s).

precipitation process. The comparison results are presented in Figure 7a. With increasing moles n of NH3 gas, the calculated activity products lnQ of the solid phases LiCl·H2O(s), LiCl· MgCl2·7H2O(s), and MgCl2·6H2O(s) are always lower than their solubility products; thus, these solid phases never form in the precipitation process. In contrast, the lnQ value of the ammonium carnallite NH4Cl·MgCl2·7H2O(s) is higher than its solubility product even after a small amount of the NH3 gas is added into the solution. In the later period of the precipitation process, the activity product lnQ of the carnallite becomes lower than its solubility product. This indicates that the formed carnallite in the early period will redissolve in the later period of the precipitation process. The comparison of the activity and solubility products of NH4Cl(s) indicates that the carnallite gets lower than it, which means the formed carnallite in the early period will dissolve again in the late period of the precipitation process. Comparison of the activity and solubility product of NH4Cl(s) indicates the possibility of formation of NH4Cl(s) in the later period of the precipitation process. To avoid the formation of ammonium-carnallite, 450 g of H2O should be added to a brine containing 5.69 mol of MgCl2, 0.284 mol of LiCl, and 1000 g of H2O, as shown in Figure 7b. In this case, the solid phase NH4Cl(s) may still form. When 800 g of H2O is added to the brine, no solid phase will form in the precipitation process, as shown in Figure 7c. Moreover, a series of equal-scale lines of water activity were calculated by the thermodynamic model and are presented in Figure 8. With the help of the equal-scale lines of water activity, one can intuitively formulate how a solution composition changes as it is evaporated at 298.15 K. For instance, when a solution with an initial composition at point “a” in Figure 8 is evaporated at 298.15 K, the solution composition will proceed along the direction a → b → c → d → e → f → g, as illustrated in Figure 8, and the final liquid phase will disappear at point g.

Table 4. Comparison of Predicted and Experimental Water Content on the Isothermal Surface of the Quaternary System MgCl2+LiCl+NH4Cl+H2O at 298.15 K salt contents (gram)

a

water content (gram)

no.a

MgCl2

LiCl

NH4Cl

H2Oa

H2Ob

1 2 3 4 5 6 10 11 12 13 14 average

0.7005 0.6261 0.5355 0.2861 0.1980 0.1061 0.3553 0.3452 0.3343 0.2585 0.1903

0 0.0820 0.1846 0.4740 0.5829 0.7105 0 0.0120 0.0243 0.1134 0.2042

0.2995 0.2919 0.2799 0.2399 0.2191 0.1834 0.0009 0.0006 0.0006 0.0006 0.0009

2.3345 2.3225 2.3076 2.2198 2.1515 2.0269 1.8072 1.7944 1.7836 1.6844 1.5290

2.3125 2.2940 2.2741 2.1821 2.1162 1.9710 1.7868 1.7664 1.7522 1.6493 1.4928

relative deviations 0.0094 0.0123 0.0145 0.0170 0.0164 0.0277 0.0113 0.0156 0.0176 0.0208 0.0237 0.0169

The experimental values. bThe model values.

0.284 LiCl + 5.69 MgCl2 + 55.51 H 2O + n NH3 = 0.284 LiCl + (5.69 − n/2) MgCl2 + n NH4Cl + (55.51 − n)H 2O + n/2Mg(OH)2 ↓

(1)

During the precipitation process, the concentration of each salt as a function of n can be calculated: LiCl:

0.284 × 55.51/(55.51 − n) mol · kg −1

MgCl2:

(5.69 − n/2) × 55.51/(55.51 − n) mol ·kg −1

NH4Cl:

n × 55.51/(55.51 − n) mol ·kg −1

6. CONCLUSIONS The solubility data in the quaternary system MgCl2+LiCl +NH4Cl+H2O at 298.15 K was elaborately measured with a relative accuracy of 0.4%. It was revealed that the phase diagram of the quaternary system at 298.15 K consists of four

By calculating the activity products lnQ of the solid phases NH4Cl(s), LiCl·H2O(s), MgCl2·6H2O(s), NH4Cl·MgCl2·6H2O(s), and LiCl·MgCl2·7H2O(s) and comparing them with their solubility products (constant values), one can predict which one of the concerned solid phases will crystallize during the 17061

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Figure 8. Equal-scale lines of water activity in the quaternary system MgCl2+LiCl+NH4Cl+H2O at 298.15 K; () predicted eutectic lines and (---) predicted equal-scale lines of water activity. The numerals in the plot indicate the water activities.

mined in the binary and ternary systems. It was demonstrated that the model could reproduce the solubility isotherms and water activity of each ternary system and predict the phase behavior in quaternary system accurately. Moreover, the H2O contents on the isothermal surface of the quaternary system were also predicted and compared with the experimental data. The relative average deviation of the prediction from the experimental data is 1.69%, which supports the reliability of the experimental data in this work. On the basis of the model calculations, application examples were given to understand the crystallization phenomena in the Mg-removal process from the brine containing MgCl2 and LiCl by NH3 gas. Moreover, a series of equal-scale lines of water activity of the titled quaternary system has also been calculated. The calculated results help to elucidate the crystallization route in isothermally evaporating the brine containing LiCl, NH4Cl, and MgCl2.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.:+86 13618496806. Fax:+86 731 88879616. Notes

The authors declare no competing financial interest.

Figure 7. Solubility products and activity product (lnQ) of various salts as a function of n moles NH3 added into the solution: (a) 5.69 mol of MgCl2, 0.284 mol of LiCl, and 1000 g of H2O; (b) 5.69 mol of MgCl2, 0.284 mol of LiCl, and 1450 g of H2O; (c) 5.69 mol of MgCl2, 0.284 mol of LiCl, and 1800 g of H2O.

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ACKNOWLEDGMENTS This work was supported by Hunan Provincial Innovation Foundation for Postgraduate and Scholarship Award for Excellent Doctoral Student granted by the Ministry of Education of China.

crystallization fields: two double salts (LiCl·MgCl2·7H2O(s) and NH4Cl·MgCl2·6H2O(s)), one hydrate salt (MgCl2·6H2O(s)), and one solid solution (LiCl·H2O+NH4Cl)(ss). The largest two crystallization fields are found to be of NH4Cl·MgCl2·6H2O(s) and (LiCl·H2O+NH4Cl)(ss). A Pitzer-Simonson-Clegg model was used to describe the solubility behavior and component activity in the ternary systems MgCl2 +LiCl+H 2 O and MgCl2+NH4Cl+H2O at 298.15 K. The phase diagram of the quaternary system was predicted with the parameters deter-

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NOMENCLATURE aw = water activity Bmx, Wmx, Umx, Vmx = binary parameters of model PSC for electrolyte MX lnK = solubility product of electrolyte MiXj·nH2O lnQ = activity product of electrolyte MiXj·nH2O m = molality, mol·kg−1 dx.doi.org/10.1021/ie4027555 | Ind. Eng. Chem. Res. 2013, 52, 17057−17063

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n = amount of substance, mol T = temperature, K Wmnx, Qmnx, Umnx = ternary parameters of model PSC for system MX-NX-H2O

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REFERENCES

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dx.doi.org/10.1021/ie4027555 | Ind. Eng. Chem. Res. 2013, 52, 17057−17063