Solid–Liquid Phase Equilibria in the Quinary System Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O and Its Subsystems at 258 K

Abstract

The phase equilibrium of the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O and the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O were investigated at 258 K using the isothermal dissolution equilibrium method. The phase diagrams were obtained based on the measured data. Double salt KCl·MgCl₂·6H₂O was found in the quaternary and the quinary systems at 258 K. In the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O, there are two invariant points, five univariant curves, and four crystallization fields. The quinary system saturated with NaCl·2H₂O contains four invariant points, nine univariant curves, and six crystallization fields corresponding to KCl, NaNO₃, KNO₃, Mg(NO₃)₂·6H₂O, KCl·MgCl₂·6H₂O and MgCl₂·8H₂O. Mg(NO₃)₂·6H₂O and MgCl₂·8H₂O that have higher concentrations and stronger salting-out effect on other salts. Therefore, low-temperature pretreatment can offer an alternative treatment of brines in accordance with the necessity for current brine treatment processes to reduce the presence of double salts and be crucial for purer products to be separated.

1 Introduction

Nitrate brines are found in the area close to Lop Nur in Xinjiang Province of China, containing Na⁺, K⁺, Mg²⁺, Cl⁻, ${\text{NO}}_{3}^{ - }$, ${\text{SO}}_{4}^{2 - }$, which can be used to produce potassium nitrate or sodium nitrate, and play important roles in many industrial fields [1, 2]. However, the brine treatment process is difficult because the separation processes are extremely complex at normal temperature and cannot even produce pure products, due to the characteristics of multicomponent system and large numbers of complex salts, such as NaNO₃·Na₂SO₄·H₂O, Na₂SO₄·3K₂SO₄, KCl·MgCl₂·6H₂O, etc. [3,4,5,6,7,8,9].

The solid–liquid phase equilibrium of a salt-water system is closely affected by temperature, especially for normal temperatures, and the phase diagrams are complex because of the presence of various double salts. For example, at 298 K, six, two and four types of double salts have been found in the Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{SO}}_{4}^{2 - }$–H₂O system [6, 10], the Na⁺, K⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$, ${\text{SO}}_{4}^{2 - }$–H₂O system [6, 11], and the Na⁺, K⁺, Li⁺//Cl⁻, ${\text{SO}}_{4}^{2 - }$–H₂O system [12], respectively. However, at a higher operating temperature, the crystallization regions of the double salts involving nitrate or magnesium chloride, such as KCl·MgCl₂·6H₂O and NaNO₃·Na₂SO₄·H₂O, decrease or even disappear; at a lower temperature, a similar tendency could also be observed that crystallization zones of sulfate-type double salts, such as Na₂SO₄·3K₂SO₄ and K₂SO₄·MgSO₄·H₂O, decrease or even disappear. All of the crystallization zones of sulfate-type double salts in these systems disappear when the temperature is below 298 K [13,14,15]. This indicates that the temperature has a strong influence on the crystallization behavior and the solubility of salts. The low-temperature treatments have some advantages in terms of fewer species of complex salts, high separation efficiency and purer final quality of salts products. Thus, the phase equilibria at low temperature are suitable for applications associated with crystallization and purification processes.

Currently, there are few reports in the aspect of the brine associated with Na⁺, K⁺, Mg²⁺, Cl⁻, ${\text{NO}}_{3}^{ - }$, ${\text{SO}}_{4}^{2 - }$, H₂O at low temperature. However, it can be speculated that the types of double salts involved in this multicomponent system diminish at low temperature, which could simplify the phase diagram relationship. Thus, an alternative route in terms of dealing with the brine, that is, a relatively simple low-temperature processing method, can be promoted. The cooling crystallization processes have proved to play an efficient role in some fields [16,17,18,19,20,21,22]. Accurate knowledge of the phase diagrams is thus essential at low temperatures.

To exploit the valuable brine resources economically, it is important to adopt the local natural conditions including low temperatures. The winter in salt lake areas is both long and cold, with an average temperature of 258 K, providing large cold capacity. So, the study of phase equilibrium at this temperature would have more practical significance to better guide salt lake production in winter.

The Na⁺, K⁺, Mg²⁺//Cl⁺, ${\text{NO}}_{3}^{ - }$, ${\text{SO}}_{4}^{2 - }$–H₂O system saturated with NaCl·2H₂O has four quinary subsystems at 258 K, which are necessary for studying the solid–liquid equilibrium of the complex six-component system. Among them, the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O is an important system of natural brine type associated with nitrate deposits, also involved in other chemical processes or some industrial wastewater treatments, but the phase equilibrium of this system has not been reported so far. This quinary system saturated with NaCl contains three quaternary subsystems and eight ternary subsystems. Except for the solubilities of the quaternary subsystems Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O, Na⁺ and Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O, those of other subsystems can be found in some compilations. The results show that the subsystems Na⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$, ${\text{SO}}_{4}^{2 - }$–H₂O [13] and Na⁺, K⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$, ${\text{SO}}_{4}^{2 - }$–H₂O [14] are both of simple types without double salts, whereas the quinary subsystem Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{SO}}_{4}^{2 - }$–H₂O [15] is of a complex type with the double salt KCl·MgCl₂·6H₂O formed at 258 K.

In this paper, we will supply the solubility data of the Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O and Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O systems; these data can be used as a theoretical reference for the design and optimize the process and is the basis for the utilization of brine resource.

2 Experimental Section

2.1 Reagents and Apparatus

The experimental materials were of analytical grade with purities not less than 98.0% mass fraction and all of them were used without any further purification. As tabulated in Table 1. NaCl, KCl, NaNO₃ and KNO₃ were purchased from the Chemical Reagent Factory of Tianjin Kermel, China, and were dried for 5–8 h at 105 °C prior to their use, the MgCl₂·6H₂O and Mg(NO₃)₂·6H₂O supplied by Tianjin First Reagent Corporation. Doubly distilled water (DDW) with an electrical conductivity less than 1.0 × 10⁻⁴ s·m⁻¹ and pH of about 6.6 at room temperature was used.

Table 1 Chemical sample specifications

A magnetically stirred thermostatic bath (DHJF-4010A, Zhengzhou Changcheng Instrument Co., Ltd., China), maintaining the temperature range from –40 to 99 °C with an accuracy of ± 0.1 °C, was employed for the equilibrium experiments, and its temperature control accuracy was calibrated using a precise thermometer. As presented in Fig. 1, the apparatus mainly included four parts: a stainless steel tank, the heating tube, the cooling coils and a stirring motor. An X-ray diffraction analyzer (D8 Advance, Bruker, Germany) was employed for solid phase characterization. Raman spectra(LabRAM HR Evolution, HORIBA Scientific, France) was recorded from 1000 to 4000 cm⁻¹ using the 532 nm line from an argon ion laser at a power of 100 mW and am integration time of 30 s at the sample.

Fig. 1

Schematic illustration of experimental apparatus: 1. sample, 2. refrigerant, 3. stirring motor, 4. cooling coils, 5. heating tube, 6. stainless steel tank

2.2 Experimental Procedure

The solubilities data of the systems were determined by means of the isothermal solution method in this study. The other third or fourth salt was added gradually to the initial samples until the quaternary or quinary system’s invariant point was seen at 258 K. The appropriate salts and distilled water were mixed together and sealed in a 250 cm³ glass bottle. Then the bottle was placed in the magnetic stirring thermostatic bath, which was set to the desired temperature. The magnetic stirrer in the bottom of flask worked at a fixed speed. Samples of the liquid phase were taken periodically for chemical analysis. It was assumed that equilibrium was achieved when the concentration of the solution remained constant. The experimental results showed that the equilibrium time for the three systems was at least 16 h. After equilibration, stirring was stopped, and the equilibrated system was allowed to rest for 3 h to ensure that the solid settled and the liquid cleared.

The liquid phases were taken out with a pre-cooled pipette at 258 K and transferred to a 250 mL volumetric flask and diluted for the measurement of the compositions by chemical or instrument analysis. The solid phases were identified by powder X-ray diffraction. By changing the amount of the salts and deionized water added, other points in the liquid and their equilibrium solids can be determined, thus allowing the phase diagram to be plotted. All of these were applied to the above systems.

2.3 Analytical Methods

The concentration of (Cl⁻) was analyzed by 0.1 mol·L⁻¹ AgNO₃ volumetric method with a precision of 0.3%; The (Mg²⁺) ion concentration was determined by titration with an EDTA at pH = 9.0–10.0 (ammonia buffer) with the indicator Eriochrome Black-T with a standard uncertainty of less than 0.5%; The (K⁺) ion concentration was measured by a gravimetric method (uncertainty of 0.2%), The (${\text{NO}}_{3}^{ - }$) ion concentration was analyzed by potassium dichromate oxidization (uncertainty of 0.6%). The (Na⁺) ion concentration was evaluated from an ion balance.

3 Results and Discussion

3.1 Quaternary Homo-Ion System Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O

The experimental data on the solubilities of the equilibrated solution and equilibrated solid phases in the quaternary system determined experimentally are given in Table 2.

Table 2 Solubilities data of the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O at T = 258 K and 0.1MPa

As a quaternary conjugate salt system, the phase diagram of the quaternary system with a common anion can be represented by a trigonal prism as shown in Fig. 2. In this diagram, its three coordinates are J(2Na⁺), J(2 K⁺) and J(H₂O), respectively, which are respective Jänecke index values, and can be calculated according to the following correlations. The Jänecke index values were listed in Table 2.

Fig. 2

Stereodiagram of the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O at 258 K (the region near point A₃, where the KCl and ice are crystallized simultaneously at 258 K because of the low solubility and high eutectic temperature of the solution, was not investigated in this paper, therefore not plotted in proportion)

The Jänecke indices were calculated using the following equations. Letting

$$[b]\; = \;\frac{1}{2} \times \frac{{w({{\text{K}}^ + })}}{{39.10}}\;{\text{ + }}\;\frac{1}{2} \times \frac{{w({\text{N}}{{\text{a}}^ + })}}{{22.99}}\;{\text{ + }}\;\frac{{w({\text{M}}{{\text{g}}^{2 + }})}}{{24.30}}$$

(1)

$$J(2{\text{Na}}^{ + } ) = \frac{1}{2} \times \frac{{w({\text{Na}}^{ + } )}}{22.99[b]} \times 100$$

(2)

$$J(2{{\text{K}}^ + })\;{\text{ = }}\;\frac{1}{2} \times \frac{{w({{\text{K}}^ + })}}{{39.10[b]}} \times 100$$

(3)

$$J({{\text{H}}_2}{\text{O}})\;{\text{ = }}\;\frac{{w({{\text{H}}_2}{\text{O}})}}{{18.01[b]}} \times 100$$

(4)

where w(ion) or w(H₂O) is the mass of the ion or water in g per 100 g of solution, respectively. J(ion) or J(H₂O) are the Jänecke index values of the ion or water, respectively.

On the basis of the Jänecke index values in Table 2, the stereodiagram of the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O at 258 K was plotted as shown in Fig. 2 and the planar projection of the phase diagram and the diagram of water contents are plotted in Figs. 3, 4 and 5.

Fig. 3

Planar projection of the quaternary system Na⁺, K⁺, Mg²⁺// Cl⁻–H₂O at 258 K

Fig. 4

Partial enlargement diagram of Fig. 3

Fig. 5

Water-content diagram of the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O at 258 K

The phase diagram is composed of two invariant points, five univariant curves, and six crystallized regions of single salts. Three vertices A₁, A₂, A₃ of the triangle represent salts of NaCl, MgCl₂ and KCl. B₁ and B₂ are the invariant points of ternary systems Na⁺, K⁺//Cl⁻–H₂O, Na⁺, Mg²⁺//Cl⁻–H₂O, at 258 K, respectively. The K⁺, Mg²⁺//Cl⁻–H₂O ternary system at 258 K is a complex type with two invariant points of point B₃ and B₄. The invariant points of the quinary system are labeled as C₁ and C₂. The point C₁ is co-saturated with three salts NaCl·2H₂O + KCl + KCl·MgCl₂·6H₂O and an equilibrium solution with w(Cl⁻) = 20.29 wt%, w(Mg²⁺) = 6.28 wt%, and w(K⁺) = 0.85 wt%. The point C₂ is co-saturated with three salts NaCl·2H₂O + MgCl₂·8H₂O + KCl·MgCl₂·6H₂O and an equilibrium solution with w(Cl⁻) = 24.29 wt%, w(Mg²⁺) = 8.26 wt%, and w(K⁺) = 0.04 wt%.

As shown in Fig. 6, the solids at two invariant points C₁ and C₂ were analyzed by X-ray diffraction, allowing the determination of the natures of the solid phases in equilibrium. The existence of the double salt KCl·MgCl₂·6H₂O can be determined. The salts can form a hydrated phase, which is only stable in a particular temperature range, and melt with changes of the conditions. Magnesium chloride can be formed from various hydrated salts, including MgCl₂·12H₂O, MgCl₂·8H₂O and MgCl₂·6H₂O. The literature demonstrated the existence of the hydrate MgCl₂·12H₂O in the temperature range 239–256 K, MgCl₂·8H₂O at 256–269 K and MgCl₂·6H₂O at 273–389 K. The research on this system shows that MgCl₂·8H₂O is the crystallization form of magnesium chloride [5, 6, 23,24,25,26]. In addition, the literature [5, 6, 25,26,27] shows that at 258 K, sodium chloride can form hydrated (NaCl·2H₂O) in binary or ternary systems. Besides, Raman microscopy makes it possible to determine precisely the salt hydrates that crystallize upon cooling [28, 29]. We measured the Raman spectra of various aqueous solutions at different temperatures and showed that NaCl·2H₂O exists in the systems at 258 K, consistent with the literature.

Fig. 6

X-ray diffraction patterns of the solid phases in the invariant point of a C₁, b C₂ of the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O at 258 K

It is worth mentioning that, compared to the phase studies at higher temperature, studies at 258 K are more difficult, the salts can form various hydrated salts and the solutions have greater viscosity, which increases the complexity of the study on the phase equilibrium. Besides, because hydrate phases are only stable in special temperature and pressure ranges, they melt with the change of conditions. The limitation of experimental and equipment conditions, MgCl₂·8H₂O and NaCl·2H₂O were difficult to detect by X-ray diffraction. So, in Figs. 7 and 12, the X-ray diffraction patterns of the invariant points consist of MgCl₂·6H₂O and NaCl, instead of MgCl₂·8H₂O and NaCl·2H₂O.

Fig. 7

The crystallization region of NaCl·2H₂O in the phase diagram of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, NO₃⁻–H₂O at 258 K (only invariant points)

The salts corresponding to the five-univariant curves B₁C₁, B₄C₁, C₁C₂, B₃C₂ and B₂C₂ are NaCl·2H₂O + KCl, KCl·MgCl₂·6H₂O + KCl, KCl·MgCl₂·6H₂O + NaCl·2H₂O, KCl·MgCl₂·6H₂O + MgCl₂·8H₂O and MgCl₂·8H₂O + NaCl·2H₂O, respectively.

The four crystallization fields correspond to three single salts NaCl·2H₂O, KCl, MgCl₂·8H₂O, and one double salts KCl·MgCl₂·6H₂O. The crystallization area of MgCl₂·8H₂O occupies the smallest part because of its high solubility. Whereas the crystallization areas of KCl and NaCl·2H₂O are larger, which indicates that they are more easily saturated and crystallize out from the solution in this system as a result of the strong salting-out effect of other salts.

The water-content diagram is essential for the design and calculation of crystallization processes. As shown in Fig. 5, with J(2Na⁺) as the abscissa, and J(H₂O) as the ordinate, we can see that at the univariant curves B₁C₁, B₂C₁ and C₁C₂, the water content changes obviously, while at the other univariant curves, the water content changes slightly, which can be explained by the increase of the mass fraction of MgCl₂. On the univariant curve B₁C₁, J(H₂O) increases to a maximum value with the decreasing of J(2Na⁺) and then decreases with further reduction of J(2Na⁺).

4 Quinary System Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O

Findings obtained from our previous research show that, NaCl should crystallize first from the solution during the cooling process because of the high concentration of NaCl in actual nitrate brine. So, the phase diagrams on the condition saturated with NaCl·2H₂O at low temperatures have more practical value. Besides, the equilibrium of the quinary system is beneficial to the construction of the phase diagrams of the complex six-component system.

The measured solubilities data of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O saturated with NaCl·2H₂O are presented in Tables 3 and 4. The concentration of each ion is expressed in mass fraction.

Table 3 Solubilities data of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O saturated with NaCl·2H₂O at T = 258 K and 0.1 MPa

Table 4 Jänecke index J [mol·(100 mol)⁻¹ (2 K⁺ + ${\text{NO}}_{3}^{ - }$ + Mg²⁺)] data of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O saturated with NaCl·2H₂O at T = 258 K

Na⁺, K⁺, Mg²⁺//Cl ⁻, ${\text{NO}}_{3}^{ - }$–H₂O is a complicated quinary system, the isothermal phase behavior of which has four dimensions. The solvent-less projection (the dry salt phase diagram) obtained by elimination of the solvent coordinate decreases the dimensionality to three and can be represented in form of a trigonal prism as shown in Fig. 8, the coordinates of which are the Jänecke index J(2 K⁺), J(Mg²⁺) and J(2Cl⁻) respectively. These Jänecke index can be calculated according to the following equations and listed in Table 3.

Fig. 8

Planar projection diagram of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O at 258 K saturated with NaCl·2H₂O

Letting

$$\left[ {\text{M}} \right] = \frac{1}{2} \times \left( {\frac{{w\left( {{\text{Cl}}^{ - } } \right)}}{35.45} + \frac{{w\left( {{\text{NO}}_{3}^{ - } } \right)}}{62.00}} \right)$$

(5)

$$J(2{{\text{K}}^{\text{ + }}})\;{\text{ = }}\;\frac{1}{2} \times \frac{{w({{\text{K}}^{\text{ + }}})}}{{39.10[{\text{M}}]}} \times 100$$

(6)

$$J({\text{M}}{{\text{g}}^{2 + }})\; = \;\frac{{w({\text{M}}{{\text{g}}^{2 + }})}}{{24.31[{\text{M}}]}}\; \times 100$$

(7)

$$J(2{\text{C}}{{\text{l}}^ - })\; = \;\frac{1}{2} \times \frac{{w(2{\text{C}}{{\text{l}}^ - })}}{{35.45[{\text{M}}]}} \times 100$$

(8)

$$J({{\rm{H}}_2}{\rm{O}})\; = \;\frac{{w({{\rm{H}}_2}{\rm{O}})}}{{18.01[{\rm{M}}]}} \times 100$$

(9)

According to the Jänecke index in Table 3, the crystallization space of NaCl·2H₂O can be plotted in Fig. 7, which has four invariant points saturated with NaCl·2H₂O and other three salts, nine univariant curves saturated with NaCl·2H₂O and other two salts, and six crystallization fields saturated with NaCl·2H₂O and the other salts.

Figure 7 is a stereodiagram and not easy to apply. The Cl-less projection (planar projection diagram) obtained by deducting the variable of Cl can reduce the dimension to two, and be plotted in a 2-D figure as shown in Figs. 8 and  9, the two coordinates of which are J(Mg²⁺) and J(2 ${\text{NO}}_{3}^{ - }$); The water and contents of Cl can be indicated by the water-content diagram and chlorine-content diagrams respectively as shown in Figs. 10 and  11, the two coordinates of which are J(Mg²⁺) and J(H₂O), J(Mg²⁺) and J(Cl⁻). These Jänecke indices are listed in Table 4.

Fig. 9

Partial enlargement diagram of Fig. 8

Fig. 10

Water-content diagram of the quinary Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O at 258 K saturated with NaCl·2H₂O

Fig. 11

Chlorine-content diagram of the quinary Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O at 258 K saturated with NaCl·2H₂O

As shown in Table 4 and Figs. 8, 9, 10 and 11, the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O is of a complex type, with the double salt KCl·MgCl₂·6H₂O formed at 258 K, the phase diagram of which contains four invariant points, nine univariant curves, and six crystallization fields.

Each vertex B₁, B₂ and B₅ represents the invariant points of NaCl–KCl, MgCl₂–NaCl, and NaNO₃–NaCl salts in water at 258 K, respectively. The points C₁, C₂, C₃, C₄, C₅, and C₆ on the sides correspond to the invariant points of quarternary subsystems Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O, Na⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O, and Na⁺, K⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O respectively.

The points D₁, D₂, D₃ and D₄ are invariant points of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O. The mass fraction compositions of the corresponding equilibrated solution and equilibrated solids of D₁, D₂, D₃ and D₄ are listed as follows.

Point D₁ is analyzed as 9.36% mass (${\text{NO}}_{3}^{ - }$), 16.64% mass (Cl⁻), 0.71% mass (K⁺), 6.63% mass (Mg²⁺), and co-saturated with salts NaCl·2H₂O, NaNO₃, KNO₃ and Mg(NO₃)₂·6H₂O.

Point D₂ is analyzed as 2.82% mass (${\text{NO}}_{3}^{ - }$), 18.78% mass (Cl⁻), 1.37% mass (K⁺), 5.91% mass (Mg²⁺), and co-saturated with salts NaCl·2H₂O, KCl, KNO₃ and KCl·MgCl₂·6H₂O.

Point D₃ is analyzed as 9.67% mass (${\text{NO}}_{3}^{ - }$), 18.03% mass (Cl⁻), 0.42% mass (K⁺), 7.72% mass (Mg²⁺), and co-saturated with salts NaCl·2H₂O, KNO₃, KCl·MgCl₂·6H₂O and Mg(NO₃)₂·6H₂O.

Point D₄ is analyzed as 8.52% mass (${\text{NO}}_{3}^{ - }$), 20.34% mass (Cl⁻), 0.15% mass (K⁺), 8.36% mass (Mg²⁺), and co-saturated with salts NaCl·2H₂O, KCl·MgCl₂·6H₂O, MgCl₂·8H₂O, and Mg(NO₃)₂·6H₂O.

Figure 12 shows the X-ray diffraction patterns corresponding to the equilibrium solid phases of the invariant points D₁, D₂, D₃ and D₄, respectively. The XRD spectra in Fig. 12 can only determine that MgCl₂·6H₂O and NaCl, in the solid phases, as a result of the limitations of sample and testing conditions. It is very difficult to find MgCl₂·8H₂O and NaCl·2H₂O.

Fig. 12

X-ray diffraction patterns of the solid phases in the invariant point of a D₁, b D₂, c D₃ and d D₄ of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O at 258 K saturated with NaCl·2H₂O

The nine univariant curves, C₅D₁, C₃D₁, D₁D₃, C₄D₄, D₃D₄, D₂D₃, C₆D₂, C₁D₂ and C₂D₄, are co-saturated with NaCl·2H₂O and two other salts, respectively.

The quinary phase diagram has six crystallization fields saturated with NaCl·2H₂O and another salt, the second salts being NaNO₃ (B₅C₅D₁C₃ field), KNO₃ (C₅C₆D₂D₃D₁ field), KCl (C₆B₁C₁D₂ field), Mg(NO₃)₂·6H₂O (C₃D₁D₃D₄C₄ field), KCl·MgCl₂·6H₂O (C₁D₂D₃D₄C₂ field), and MgCl₂·8H₂O (C₂ D₄C₄ B₂ field).

It can be seen that the areas of the crystallization zones saturated with NaCl·2H₂O decrease in the order of KCl, KNO_3, NaNO₃, KCl·MgCl₂·6H₂O, MgCl₂·8H₂O, Mg (NO₃)₂·6H₂O. Among them, KNO₃ and KCl have larger crystallization areas and lower solubility than the other salts. This means that separation processes from the mixture solutions for the two salts are feasible due to their relatively large areas of crystallization. The fields of Mg(NO₃)₂·6H₂O and MgCl₂·8H₂O are extremely small, which are the result of their high solubility.

Figure 10 shows that the water-contents for the invariant points decrease apparently from D₂–D₁–D₃ to D₄, reaching the J(H₂O) values of 832.19 mol·(100 mol)⁻¹ (2 K⁺ + 2${\text{NO}}_{3}^{ - }$ + Mg²⁺) at the invariant point D₄. It can be concluded that the point D₄ has the highest concentration in the quinary system. The water content gradually decreases at the univariant curves C₆D₂ and it is almost unchanged at the univariant curves C₁D₂, C₂D₄ with increasing of the Jänecke index values of J(Mg²⁺). The high concentration of Mg²⁺ ion in the quinary system implies that Mg²⁺ has a strong salting-out effect on other salts, which can decrease the solubilities of the other salts.

Figure 11 indicates that the chlorine-content of the points C₃, C₄, D₁, D₃ and D₄ are similar, about 65–70 mol·(100 mol)⁻¹ (2 K⁺ + NO₃⁻ + Mg²⁺); The lowest contents of Cl is at point D₃; The concentration of magnesium and nitrate ion significantly influence the chlorine content, especially on univariant curves C₆D₂, C₁D₂D₃ and C₂D₄. It clearly shows that the regularity in the variation of the chlorine content related to the J(Mg²⁺) is a feature similar to that of the water content. The point D₄ possess the lowest water content, which is because of the higher solubility of MgCl₂ and Mg(NO₃)₂ compared to other salts at 258 K.

5 Conclusions

The phase equilibria of the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O and the quaternary system Na⁺, K⁺, Mg²⁺//Cl⁻–H₂O have been investigated by the method of isothermal solution saturation. The results demonstrated that the quinary system Na⁺, K⁺, Mg²⁺//Cl⁻, ${\text{NO}}_{3}^{ - }$–H₂O saturated with NaCl·2H₂O at 258 K belongs to a complex type, which contains four invariant points, nine univariant curves, and six crystallization fields. The salts Mg(NO₃)₂·6H₂O and MgCl₂·8H₂O have smaller crystallization areas and higher solubility than the other salts, so they are more difficult to separate from the solution in this system at the research temperature. The concentrations of Mg²⁺ and ${\text{NO}}_{3}^{ - }$ in the solution are the main factors affecting the water content and solution density. The findings of this study can be applicable in various industrial salts recovering units.