Stable Phase Equilibria of Ternary Systems Li⁺,Rb⁺//SO₄^2––H₂O and Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K

Abstract

The stable phase equilibria of ternary systems Li⁺,Rb⁺//SO₄^2––H₂O and Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K are studied herein by the isothermal dissolution equilibrium method. The solubility and density of the systems are measured experimentally, and the related phase diagrams, density versus composition diagrams, are plotted. Results show that the ternary systems Li⁺,Rb⁺//SO₄^2––H₂O and Li⁺,Cs⁺//SO₄^2––H₂O are both complex systems with two kinds of double salts formed at 273.2 K. The stable phase diagrams of these two ternary systems all consist of one unsaturated solution region, three co-crystallization regions, three invariant points, four invariant curves, and four crystallization regions. By comparing the phase diagrams of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 and 298.2 K, it is found that the crystallization regions of the two double salts (3Li₂SO₄·Rb₂SO₄·2H₂O and Li₂SO₄·Rb₂SO₄) and the single salt Li₂SO₄·H₂O all decrease, while that of Rb₂SO₄ increases when the temperature drops. Also, it can be seen from the phase diagram of the ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 and 298.2 K that the crystallization regions of the double salt 3Li₂SO₄·Cs₂SO₄·2H₂O and the single salt Cs₂SO₄ increase, while those of the double salt Li₂SO₄·Cs₂SO₄ and the single salt Li₂SO₄·H₂O decrease as the temperature drops.

1. Introduction

In recent years, with the rapid development of high-tech industries such as electronic materials and new energy, the demand for lithium, rubidium, cesium, and other mineral resources is increasing.¹⁻³ The Qaidam Basin of the Qinghai–Tibet Plateau is one of the most concentrated areas of modern salt lake brine resources in China.⁴ There are more than 30 large and medium-sized salt lakes, most of which are sulfate-type salt lakes, which store a lot of lithium, rubidium, cesium, and other resources.⁵ Among them, the reserves of lithium resources (calculated by LiCl) are up to more than 15 million tons, accounting for 83% of the total reserves in China,⁶ and the average contents of rubidium and cesium are 10.8^— and 0.034 mg·L^–1, respectively,⁷ which have great development value.

The theoretical and experimental study on the phase equilibrium of the water–salt system is the basis for the comprehensive utilization of brine resources.^8,9 It is extremely difficult and unrealistic to obtain the phase diagram covering all components of brine, and it is common practice to approximate practical problem guidance using simplified phase diagrams covering the main components of different regions and different types of salt lakes.¹⁰ For example, for chloride-type and borate-type salt lakes containing lithium, rubidium, and cesium in different regions, a series of phase equilibria of related systems have been studied by many scholars. Namely, Na⁺,Rb⁺//Cl^––H₂O at T = 288.15, 298.15, and 318.15 K;¹¹ Na⁺,Cs⁺//Cl^––H₂O at 298.15, 308.15, and 318.15 K;¹² Rb⁺,Cs⁺//Cl^––H₂O at 323 K;¹³ Li⁺,K⁺,Rb⁺//Cl^––H₂O at 298.15 K;¹⁴ Rb⁺,Mg²⁺//Cl^–,borate–H₂O at 323 K;¹⁵ Li⁺,K⁺,Rb⁺,Mg²⁺//borate–H₂O at 348.15 K;¹⁶ and K⁺,Rb⁺,Mg²⁺//borate–H₂O at 348 K.¹⁷ According to the main element types, the sulfate salt lake brine in Qaidam Basin can be simplified to a complex septenary system Li⁺,Na⁺,K⁺,Rb⁺,Cs⁺,Mg²⁺//SO₄^2––H₂O.¹⁸ For the old brine with potassium, sodium, and magnesium being extracted, it can be simplified to a quaternary system Li⁺,Rb⁺,Cs⁺//SO₄^2––H₂O according to its main element composition.^19,20 The ternary systems Li⁺,Rb⁺//SO₄^2––H₂O and Li⁺,Cs⁺//SO₄^2––H₂O are not only the two basic systems of the above complex seven-element system but also the two important sub-systems of the old brine; however, there are few research works on these systems and related systems and are concentrated in the temperature range of room temperature and above. For example, Li⁺,Rb⁺//SO₄^2––H₂O at 298.2 K;²¹ Li⁺,Cs⁺//SO₄^2––H₂O at 298.2 K;²² Li⁺,Na⁺,Cs⁺//SO₄^2––H₂O at 298.2 K;²³ Li⁺,Na⁺,Rb⁺//SO₄^2––H₂O at 298.2 K;²⁴ and Mg²⁺,Rb⁺,Cs⁺//SO₄^2––H₂O at 298.2 K.²⁵ The existing studies show that the interaction between various salts containing lithium, rubidium, and cesium is complex, and most of them are generated by double salts or solid solutions,²⁶ which is extremely unfavorable to the separation and extraction of lithium, rubidium, and cesium. It is necessary to further study the phase equilibrium of related systems in order to find out the interactive relationship and the law of salting-out between ions at different temperatures and comprehensively utilize resources such as lithium, rubidium, and cesium in the salt lake of Qaidam Basin.

In view of the above factors and the natural conditions of a long winter and an annual average temperature close to 0 °C in the Qaidam Basin of the Qinghai–Tibet Plateau,^27,28 the study of stable phase equilibrium of ternary systems Li⁺,Rb⁺//SO₄^2––H₂O and Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K was carried out in this work which can not only provide a basis for the extraction of lithium, rubidium, and cesium resources from old brine but also provide basic data for complex systems.

2. Experimental Section

2.1. Reagents and Instruments

The main reagents used in the experiment were recrystallized or dried, and their specific information is listed in Table 1. Deionized water (κ ≤ 1.0 × 10^–4 S·m^–1), produced by the ultrapure water manufacturing system, was used to prepare the experimental samples and for chemical analysis. The main instruments used in the experiment are listed in Table 2.

Table 1. Main Experimental Reagents’ Description.

reagent	recrystallization	CAS no.	initial purity w (wt %)	final purity w (wt %)	source
lithium sulfate monohydrate (Li2SO4·H2O)		10102-25-7	≥99.0		Chengdu Kelong Chemical Reagent Plant, China
rubidium sulfate (Rb2SO4)	dried in an oven at 393.15 K	7488-54-2	≥99.0	≥99.5	Shanghai Dingli Chemical Co., Ltd., China
cesium sulfate (Cs2SO4)	dried in an oven at 393.15 K	10294-54-9	≥99.0	≥99.5	Shanghai Dingli Chemical Co., Ltd., China

Table 2. Main Instruments and Equipment Description.

instruments	type	source
standard analytical balance	BSA124S	Sartorius Scientific Instruments (Beijing) Co., Ltd., China
flame atomic absorption spectroscopy	iCE-3300	Thermo Fisher Scientific Instrument Co., Ltd., China
thermostat	SDH-001	Chongqing InBev Experimental Instrument Co., Ltd., China
oscillator	HY-5	Jiangsu Kexi Instrument Co., Ltd., China
X-ray diffractometer	DX-2700	Dandong Fangyuan Instrument Co., Ltd., China
ultrapure water system	UPR-I-15T	Sichuan ULUPURE Ultrapure Technology Co., Ltd., China
electric thermostatic drying oven	HG-9240A	Shanghai Yiheng Technology Instrument Co., Ltd., China

2.2. Experimental Methods

The isothermal dissolution method was applied in this experiment. At the temperature studied, a series of ternary system samples were prepared by adding another new salt in a gradient to the initial samples on the basis of the secondary invariant point. The samples were placed in the SDH-001 thermostat (273 ± 0.2 K) and oscillated continuously through the HY-5 oscillator. When the mixed solution reached equilibrium, the liquid phase composition and physicochemical properties are basically unchanged as the sign of the solution equilibrium of the system, and the phase equilibrium time was about 4 weeks. After equilibration, the solid and liquid of the sample were separated. The composition of the liquid phase was determined by chemical analysis, and the density of the liquid phase was determined by a gravity bottle method.²⁹ The equilibrium solid phase was identified by Schreinemakers’ wet slag method and X-ray powder crystal diffraction method.

2.3. Analysis Method

The content of Li⁺, Rb⁺, and Cs⁺ was determined by the Thermo Fisher iCE-3300 atomic absorption spectrometer (precision ±0.5%). The concentration of SO₄^2– was determined using alizarin red volumetric titration (precision ±0.6%).

3. Results and Discussion

3.1. Stable Phase Equilibrium of the Ternary System Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 K

The solubility data of the ternary system measured and the density data of the equilibrium solution are listed in Table 3, and the stable phase diagram of the system is drawn in Figure 1 according to these data. As shown in Figure 1, the stable phase diagram of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 K consists of one unsaturated solution region (L₁), three invariant points and three co-crystallization regions (S_E1, S_E2, S_E3), four invariant curves and four crystallization regions. The system is a complex system composed of two double salts (3Li₂SO₄·Rb₂SO₄·2H₂O and Li₂SO₄·Rb₂SO₄).

Table 3. Equilibrium Solid–Liquid Composition and Density of the Ternary System Li⁺,Rb⁺(Cs⁺)//SO₄^2––H₂O at T = 273.2 K and Pressure p = 94.77 kPa^a.

		composition of the solution w(M) × 102		composition of the solid w(M) × 102
no.	density (g·cm–3)	w1 × 102	w2 × 102	w1 × 102	w2 × 102	equilibrated solid phase
Li2SO4 (1) + Rb2SO4 (2) + H2O
1, A	1.2470	26.88	0.00			Li2SO4·H2O
2	1.2408	25.10	0.69	46.57	1.23	Li2SO4·H2O
3, E1	1.2414	23.06	1.36	28.40	11.62	Li2SO4·H2O + 3Li2SO4·Rb2SO4·2H2O
4	1.2542	22.60	4.21	38.29	25.77	3Li2SO4·Rb2SO4·2H2O
5	1.2696	21.87	6.14	32.60	19.20	3Li2SO4·Rb2SO4·2H2O
6	1.2862	20.49	7.98	36.26	24.40	3Li2SO4·Rb2SO4·2H2O
7	1.2926	18.35	10.56	28.39	19.18	3Li2SO4·Rb2SO4·2H2O
8, E2	1.3039	17.72	11.94	22.79	16.15	3Li2SO4·Rb2SO4·2H2O + Li2SO4·Rb2SO4
9	1.3189	14.79	15.14	19.83	34.91	Li2SO4·Rb2SO4
10	1.3358	12.69	18.55	21.16	47.35	Li2SO4·Rb2SO4
11, E3	1.3755	9.67	24.67	18.73	46.42	Li2SO4·Rb2SO4 + Rb2SO4
12	1.3629	6.56	25.70	4.12	57.18	Rb2SO4
13	1.3393	2.85	26.06	1.84	53.86	Rb2SO4
14	1.2968	1.04	26.48	0.23	69.05	Rb2SO4
15, B	1.2801	0.00	27.36			Rb2SO4
Li2SO4 (1) + Cs2SO4 (2) + H2O
16, C	1.2470	26.88	0.00			Li2SO4·H2O
17	1.2496	25.45	0.16	43.73	0.73	Li2SO4·H2O
18, F1	1.2395	23.90	1.11	54.01	9.58	Li2SO4·H2O + 3Li2SO4·Cs2SO4·2H2O
19	1.2365	22.26	1.36	32.43	22.70	3Li2SO4·Cs2SO4·2H2O
20	1.2282	20.53	2.47	32.40	24.59	3Li2SO4·Cs2SO4·2H2O
21	1.2270	18.90	4.17	29.92	23.71	3Li2SO4·Cs2SO4·2H2O
22	1.2443	17.23	8.16	33.74	33.18	3Li2SO4·Cs2SO4·2H2O
23	1.2595	16.97	13.72	30.01	29.89	3Li2SO4·Cs2SO4·2H2O
24	1.3308	12.63	18.87	34.86	39.98	3Li2SO4·Cs2SO4·2H2O
25	1.3756	12.28	23.89	31.76	38.98	3Li2SO4·Cs2SO4·2H2O
26	1.4174	11.84	25.67	38.44	45.34	3Li2SO4·Cs2SO4·2H2O
27, F2	1.4233	11.75	27.92	20.45	60.08	3Li2SO4·Cs2SO4·2H2O + Li2SO4·Cs2SO4
28	1.4342	10.45	28.78	19.08	63.26	Li2SO4·Cs2SO4
29	1.4560	9.26	32.16	19.61	62.81	Li2SO4·Cs2SO4
30	1.4899	8.38	33.14	18.69	61.80	Li2SO4·Cs2SO4
31	1.5100	7.08	34.91	16.27	57.41	Li2SO4·Cs2SO4
32	1.5738	6.00	39.71	17.46	63.60	Li2SO4·Cs2SO4
33	1.5961	5.42	48.43	14.41	63.91	Li2SO4·Cs2SO4
34	1.6365	4.65	53.45	14.32	65.51	Li2SO4·Cs2SO4
35	1.6865	3.94	54.67	17.68	70.33	Li2SO4·Cs2SO4
36	1.8630	2.16	59.70	12.56	63.58	Li2SO4·Cs2SO4
37, F3	1.9493	1.50	62.07	15.18	69.92	Li2SO4·Cs2SO4 + Cs2SO4
38	1.9588	1.33	62.53	1.23	82.34	Cs2SO4
39, D	1.9946	0.00	62.88			Cs2SO4

^aStandard uncertainties u and the relative standard uncertainty u_r are u(T) = 0.10 K; u(p) = 0.50 kPa; u(ρ) = 0.0002 g·cm^–3; u_r[w(Li⁺)] = 0.005; u_r[w(Rb⁺)] = 0.0061; u_r[w(Cs⁺)] = 0.0058; u_r[w(SO₄^2–)] = 0.006; w(M) is the mass fraction of M.

Figure 1.

Stable phase diagram of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 K.

In the phase diagram of a ternary water–salt system, the triangle composed of two equilibrium solid phases corresponding to the invariant point and the vertices corresponding to water is called the corresponding triangle of the invariant point. An invariant point is called a commensurate invariant point when it is in the triangle or on the edge of the triangle, and an invariant point is called an incommensurate invariant point when it is outside the corresponding triangle. As can be seen from Figure 1, the invariant point points E₁ and E₃ are within their corresponding triangles JOI and HOG, respectively. Therefore, the invariant points E₁ and E₃ belong to commensurate invariant points, while the invariant point E₂ belongs to the incommensurate invariant point because it is outside the corresponding triangle IOH.

The XRD patterns corresponding to the solid phase composition of invariant points E₁, E₂, and E₃ are shown in Figures 2–4. As shown in Figures 2–4, the XRD patterns of the samples were compared and analyzed with the PDF standard cards, and it was found that the XRD patterns of the samples were in good agreement with the PDF standard cards. In combination with Table 3 and Figures 1 and 2–4, the solid–liquid composition of the three invariant points (E₁, E₂, and E₃) of the system was determined. At invariant point E₁, the equilibrium liquid phase composition is w(Li₂SO₄) = 23.06%, w(Rb₂SO₄) = 1.36%, the equilibrium solid phase consists of 3Li₂SO₄·Rb₂SO₄·2H₂O and Li₂SO₄·H₂O; at the invariant point E₂, the equilibrium liquid phase composition is w(Li₂SO₄) = 17.72%, w(Rb₂SO₄) = 11.94%, the equilibrium solid phase consists of 3Li₂SO₄·Rb₂SO₄·2H₂O and Li₂SO₄·Rb₂SO₄; at the invariant point E₃, the equilibrium liquid phase composition is w(Li₂SO₄) = 9.67%, w(Rb₂SO₄) = 24.67%, the equilibrium solid phase consists of Li₂SO₄·Rb₂SO₄ and Rb₂SO₄. The invariant curves AE₁, E₁E₂, E₂E₃, and E₃B represent the saturated solubility curves of Li₂SO₄·H₂O, 3Li₂SO₄·Rb₂SO₄·2H₂O, Li₂SO₄·Rb₂SO₄, and Rb₂SO₄, respectively. The four crystallization regions AE₁J, GBE₃, E₁IE₂, and E₂HE₃ correspond to two single salts Li₂SO₄·H₂O, Rb₂SO₄·2H₂O and two double salts 3Li₂SO₄·Rb₂SO₄·2H₂O, Li₂SO₄·Rb₂SO₄, respectively. The crystallization region of Li₂SO₄·H₂O is the smallest, indicating that it is difficult for the hydrated salt Li₂SO₄·H₂O to crystallize directly from the system at 273.2 K. The co-crystallization regions S_E1, S_E2, and S_E3 in the stable phase diagram (Figure 1) are respectively represented as follows:

• S_E1: Li₂SO₄·H₂O and 3Li₂SO₄·Rb₂SO₄·2H₂O coexist;

• S_E2: 3Li₂SO₄·Rb₂SO₄·2H₂O and Li₂SO₄·Rb₂SO₄ coexist;

• S_E3: Li₂SO₄·Rb₂SO₄ and Rb₂SO₄ coexist.

Figure 2.

XRD pattern of the point E₁ of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 K.

Figure 4.

XRD pattern of the point E₃ of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 K.

Figure 3.

XRD pattern of the point E₂ of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 K.

The chemical reaction equations for the formation of double salt are shown in eqs 1 and 2. According to eqs 1 and 2, the double salt Li₂SO₄·Rb₂SO₄ will be formed when the coefficient ratio of Li₂SO₄·H₂O and Rb₂SO₄ is 1, and the double salt 3Li₂SO₄·Rb₂SO₄·2H₂O will be formed when the coefficient ratio reaches 3. It can be concluded that the double salt Li₂SO₄·Rb₂SO₄ is easily formed when Li₂SO₄ and Rb₂SO₄ coexist. This is consistent with the phase region rule shown in Figure 1, which shows that the crystallization region of the double salt Li₂SO₄·Rb₂SO₄ is larger than that of the double salt 3Li₂SO₄·Rb₂SO₄·2H₂O, that is, the double salt Li₂SO₄·Rb₂SO₄ is more likely to precipitate.

1

2

The density versus composition diagram of the system is drawn in Figure 5 according to the data in Table 3. As can be seen from Figure 5, the density of an equilibrium solution varies with its composition according to the following regularity: the density of the equilibrium solution decreases first, then increases rapidly, and finally decreases as w(Rb₂SO₄) goes up. The solution density is minimum at the invariant point E₁, and reaches maximum at the invariant point E₃. The reason why the density of the equilibrium solution appears to follow such a rule may be that curve AE₁ is the saturated solubility curve of Li₂SO₄, and the content of Li₂SO₄ is the main factor affecting the density of equilibrium solution in this section, so the density of equilibrium solution decreases with the decrease of w(Li₂SO₄). However, the density of the equilibrium solution increases rapidly on curve E₁E₃, which may be because the relative molecular weight of Rb₂SO₄ is much higher than that of Li₂SO₄. Therefore, the solution density increases rapidly with the increase of the content of Rb₂SO₄ in the solution. On curve E₃B, the solution density decreases again, possibly because curve E₃B is the saturated solubility curve of Rb₂SO₄. At this time, Rb₂SO₄ reaches saturation and w(Rb₂SO₄) has a small variation range. The content of Li₂SO₄ becomes a key factor affecting the solution density, so the solution density decreases with the decrease of w(Li₂SO₄).

Figure 5.

The density vs composition diagram of ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 K.

In order to more intuitively analyze the influence of temperature on the salt crystallization form of the system, the research data of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 298.2 K was compared, and the contrast phase diagram of the ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 and 298.2 K was drawn in Figure 6.

Figure 6.

Contrast phase diagram of ternary system Li⁺,Rb⁺//SO₄^2––H₂O at 273.2 and 298.2 K.

As can be seen from Figure 6, the ternary system Li⁺,Rb⁺//SO₄^2––H₂O is a complex system when the temperature is 273.2 and 298.2 K, and the crystallization form of salts in the system does not change due to temperature change. The phase diagram configuration at different temperatures is basically the same, which consists of three saturation points, four invariant curves and four solid phase crystallization regions. With the decrease in temperature, the crystallization regions of double salt (3Li₂SO₄·Rb₂SO₄·2H₂O and Li₂SO₄·Rb₂SO₄) and single salt (Li₂SO₄·H₂O) decreased, while the crystallization region of Rb₂SO₄ increased. Therefore, within a certain range, Li₂SO₄·H₂O is more easily enriched with increasing temperature. After the separation and extraction of lithium, Rb₂SO₄ can be extracted at a lower temperature so as to achieve the purpose of comprehensive utilization of resources.

3.2. Stable Phase Equilibrium of the Ternary System Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K

The solubility data of the ternary system measured and the density data of the equilibrium solution are listed in Table 3, and the stable phase diagram (Figure 7) of the system is drawn according to these data. As shown in Figure 7, the stable phase diagram of the ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K mainly consists of three invariant points, four invariant curves, and four crystallization regions. In addition, the phase diagram also includes one unsaturated solution region (L₂) and three co-crystallization regions (S_F1, S_F2, and S_F3). The system is a complex system composed of two double salts (3Li₂SO₄·Cs₂SO₄·2H₂O, Li₂SO₄·Cs₂SO₄).

Figure 7.

Stable phase diagram of ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K.

As shown in Figure 7, the three invariant points (F₁, F₂, and F₃) all belong to the commensurate type because the three invariant points are located in their corresponding triangles PQR, PNR, and KNR, respectively. The corresponding equilibrium solid-phase XRD patterns at the invariant points are shown in Figures 8–10. In combination with Table 3, Figures 7 and 8–10, the solid–liquid composition of the three invariant points (F₁, F₂, F₃) of the system was determined. The composition of equilibrium liquid at the invariant point F₁ is w(Li₂SO₄) = 23.90%, w(Cs₂SO₄) = 1.11%; the equilibrium solid phase consists of 3Li₂SO₄·Cs₂SO₄·2H₂O and Li₂SO₄·H₂O; the composition of equilibrium liquid at the invariant point F₂ is w(Li₂SO₄) = 11.75% and w(Cs₂SO₄) = 27.92%, the equilibrium solid phase consists of 3Li₂SO₄·Cs₂SO₄·2H₂O and Li₂SO₄·Cs₂SO₄; the composition of equilibrium liquid at the invariant point F₃ is w(Li₂SO₄) = 1.50% and w(Cs₂SO₄) = 62.07%, the equilibrium solid phase consists of Li₂SO₄·Cs₂SO₄ and Cs₂SO₄. The invariant curves CF₁, F₁F₂, F₂F₃, and F₃D represent the saturated solubility curves of Li₂SO₄·H₂O, 3Li₂SO₄·Cs₂SO₄·2H₂O, Li₂SO₄·Cs₂SO₄, and Cs₂SO₄, respectively. The four solid crystallization regions CF₁Q, KDF₃, F₁PF₂, and F₂NF₃ represent the crystallization regions of single salt Li₂SO₄·H₂O and Cs₂SO₄ and double salt 3Li₂SO₄·Cs₂SO₄·2H₂O and Li₂SO₄·Cs₂SO₄, respectively. The two double salts (3Li₂SO₄·Cs₂SO₄·2H₂O and Li₂SO₄·Cs₂SO₄) almost occupy the entire solid phase crystallization region at 273.2 K, and the crystallization region of the two single salts is small. The existence of a large amount of double salts will cause lithium to precipitate prematurely in the process of salt enrichment, resulting in lithium entrainment loss and not effective enrichment. The three co-crystallization regions (S_E1, S_E2, and S_E3) in the stable phase diagram (Figure 7) are respectively represented as follows:

• S_F1: Li₂SO₄·H₂O and 3Li₂SO₄·Cs₂SO₄·2H₂O coexist;

• S_F2: 3Li₂SO₄·Cs₂SO₄·2H₂O and Li₂SO₄·Cs₂SO₄ coexist;

• S_F3: Li₂SO₄·Cs₂SO₄ and Cs₂SO₄ coexist.

Figure 8.

XRD pattern of the point F₁ of the ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K.

Figure 10.

XRD pattern of the point F₃ of the ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K.

Figure 9.

XRD pattern of the point F₂ of the ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K.

The chemical reaction equations for the formation of double salt are shown in eqs 3 and 4. According to eqs 3 and 4, the double salt Li₂SO₄·Cs₂SO₄ will be formed when the coefficient ratio of Li₂SO₄·H₂O and Cs₂SO₄ is 1, while the double salt 3Li₂SO₄·Cs₂SO₄·2H₂O will be formed when the coefficient ratio reaches 3. Thus, when Li₂SO₄ and Cs₂SO₄ coexist, the double salt Li₂SO₄·Cs₂SO₄ is easily generated. This is consistent with the rules shown in Table 3 and Figure 7. It can be seen from Table 3 and Figure 1 that the crystal of the double salt Li₂SO₄·Cs₂SO₄ occurs when the mass fraction of lithium sulfate is low [w(Li₂SO₄) = 1.50%]. When the mass fraction of Li₂SO₄ reaches a high level [w(Li₂SO₄) = 11.75%], the double salt 3Li₂SO₄·Cs₂SO₄·2H₂O is generated.

3

4

The density versus composition diagram (Figure 11) of the ternary system at 273.2 K is constructed with w(Cs₂SO₄) as the abscissa and the density of the equilibrium solution as the ordinate according to the data in Table 3. As can be seen from Figure 11, with the increase of w(Cs₂SO₄), the density of the equilibrium solution showed a trend of first slightly decreasing and then gradually increasing. The reason for the decrease in the density of the equilibrium solution may be that w(Li₂SO₄) is relatively large and w(Cs₂SO₄) is relatively small at this stage. The content of Li₂SO₄ is the main factor affecting the density of the equilibrium solution. Therefore, with the decrease of w(Li₂SO₄), the density of the equilibrium solution decreases. Then the density of the equilibrium solution increases continuously, which may be because the relative molecular weight of Cs₂SO₄ is much higher than that of Li₂SO₄. When the content of Cs₂SO₄ reaches a certain value, it becomes the determinant factor affecting the density of the equilibrium solution of this system.

Figure 11.

Density vs composition diagram of the ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 K.

In order to explore the influence of temperature on the salt crystallization form of the system, the contrast phase diagram of the ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 and 298.2 K was drawn in Figure 12.

Figure 12.

Contrast phase diagram of ternary system Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 and 298.2 K.

As can be seen from Figure 12, the ternary system Li⁺,Cs⁺//SO₄^2––H₂O is a complex system at two different temperatures (273.2 and 298.2 K). The crystallization forms of salts in the system did not change with temperature change, and the phase diagrams at different temperatures were basically the same, consisting of three saturation points, four univariate curves, and four solid phase crystallization regions. It can also be seen that with the decrease of temperature, the crystallization region of double salt 3Li₂SO₄·Cs₂SO₄·2H₂O and single salt Cs₂SO₄ increases, while the crystallization region of double salt Li₂SO₄·Cs₂SO₄ and Li₂SO₄·H₂O decreases. Therefore, lithium is more likely to be enriched at room temperature than at low temperatures in the sulfate system, where lithium and cesium coexist.

4. Conclusions

• (1)The ternary system Li⁺,Rb⁺//SO₄^2––H₂O contains two double salts (Li₂SO₄·Rb₂SO₄ and 3Li₂SO₄·Rb₂SO₄·2H₂O) at 273.2 K, which is a complex system. The stable phase diagram mainly consists of one unsaturated solution region, three co-crystallization regions, three invariant points, four invariant curves, and four crystallization regions. The crystallization regions of double salt (Li₂SO₄·Rb₂SO₄ and 3Li₂SO₄·Rb₂SO₄·2H₂O) and single salt (Rb₂SO₄) are larger, and that of Li₂SO₄·H₂O is the smallest.
• (2)The ternary system Li⁺,Cs⁺//SO₄^2––H₂O is also a complex system formed by two double salts (3Li₂SO₄·Cs₂SO₄·2H₂O and Li₂SO₄·Cs₂SO₄) at 273.2 K. The stable phase diagrams of the ternary system consist of one unsaturated solution region, three co-crystallization regions, three invariant points, four invariant curves, and four crystallization regions. The crystallization regions of the double salt 3Li₂SO₄·Cs₂SO₄·2H₂O are the largest and that of the single salt Cs₂SO₄ is the smallest.
• (3)It can be seen from the contrast phase diagrams of the ternary systems Li⁺,Rb⁺//SO₄^2––H₂O and Li⁺,Cs⁺//SO₄^2––H₂O at 273.2 and 298.2 K, respectively, that both crystallization regions of the single salt Li₂SO₄·H₂O decrease in two contrast phase diagrams, while that of Rb₂SO₄ and Cs₂SO₄ increase as the temperature drops. It is indicated that the single salt Li₂SO₄·H₂O is precipitated more easily at high temperatures, while Rb₂SO₄ and Cs₂SO₄ are precipitated more easily at low temperatures.

The authors declare no competing financial interest.