The 288.2 K Isothermal Evaporation Experiment of Potassium Precipitation Brine in West Taijinair Salt Lake

Abstract

Rubidium and cesium are important strategic resources, and West Taijinar Salt Lake is rich in rubidium and cesium reserves, while the concentration is low and the relationship with coexisting potassium and magnesium ions is complex. In order to understand the evaporative enrichment and salt precipitation patterns of rare elements such as lithium, rubidium, cesium, and boron of the brine after potassium precipitation in West Taijinar Salt Lake, the 288.2 K isothermal evaporation experiment was carried out. The experimental results show that during the evaporation process at 288.2 K, the following salts precipitate from the brine after potassium crystallization: halite (NaCl), bischofite (MgCl₂·6H₂O), carnallite (KCl·MgCl₂·6H₂O), hexahydrite (MgSO₄·6H₂O), epsomite (MgSO₄·7H₂O), boric acid (H₃BO₃), and lithium sulfate monohydrate (Li₂SO₄·H₂O). The concentrations of lithium and boron are significantly enriched, the content of Li⁺ was enriched from 1.7 g/L to 5.63 g/L, and the B₂O₃ content was enriched from 6.72 g/L to 50.78 g/L. The isomorphism phenomenon of Rb⁺, Cs⁺, and K⁺ makes Rb⁺ and Cs⁺ enter potassium ore to form solid solution-type carnallite ((K, Rb)MgCl₃·6H₂O, (K, Cs)MgCl₃·6H₂O)) and reduce the content of brine. This study provides data support for the development and comprehensive utilization of lithium, boron, rubidium, and cesium resources in West Taijinar Salt Lake.

1. Introduction

The western regions of China, particularly Qinghai, Xizang, Xinjiang, and Inner Mongolia, are home to a vast number of salt lakes [1,2]. Salt lakes contain abundant resources such as lithium, potassium, magnesium, rubidium, cesium, boron, etc., and hold significant value for development and utilization [3].

Typically, most salt lakes are distributed in arid regions, which are characterized by extended periods of sunlight, high evaporation rates, and scant rainfall. This geographical setting indicates that the development and utilization of salt lake mineral resources can capitalize on local solar and wind energy to concentrate valuable elements. Therefore, the salt field process is the key to the enrichment of valuable elements in salt lake brines [4]. The isothermal evaporation of salt lake brine is one of the foundations of salt field process application. The isothermal evaporation experiment facilitates the determination of ion variation patterns within brine and the precipitation sequence of salt minerals. Through the obtained evaporation data, reasonable brine segmentation can be used to preliminarily separate valuable elements to obtain high-grade salt ore [5].

Using West Taijinar Salt Lake as a case study, the lake is notably rich in sodium, potassium, magnesium, and rare metals including lithium, rubidium, and cesium [6]. The process of brine development and utilization in West Taijinar Salt Lake involves the sequential removal of sodium, precipitation of potassium, and extraction of lithium from the salt field, ultimately aiming to produce lithium and potassium products [7]. After potassium precipitation, the content of Na⁺ and K⁺ in the brine decreases, and the original low-concentration Li⁺ and B³⁺ are further enriched. The brine after potassium precipitation also contains Rb⁺ and Cs⁺. The majority of existing studies in the literature concentrate on the evaporation of brine prior to lithium enrichment. Guo [8] carried out the natural evaporation of the brine in West Taijinaier Salt Lake and obtained the separation point of solid and liquid phases and the distribution relationship of materials in each stage of the brine. Lithium does not precipitate in a solid form. Zhang [9] studied the rules of the evaporation and salt precipitation of Zabuye Salt Lake brine at 288 K. It was observed that the lithium carbonate precipitation was not obvious during the brine concentration process, which made it impossible to obtain high-grade lithium salts. Li [10] conducted an experimental study on the evaporation of Uyuni brine at 298 K and found that lithium was dispersed and precipitated in the form of Li₂SO₄·H₂O in the later stage of evaporation, which was not conducive to the separation and extraction of lithium. Zheng [11] carried out 298 K isothermal evaporation of the autumn brine of Zabuye Salt Lake and found that a small amount of rubidium in the solution entered the potassium chloride mineral during the evaporation process. For the crystallization of lithium, sodium, and potassium in salt lakes, the phase diagrams of Li⁺, Na⁺, K⁺ // SO₄²⁻-H₂O at 273 K [12], 288 K [13], and 298 K [14] were studied. The results showed that the crystallization region of Li₂SO₄·H₂O and LiKSO₄ increased with the increase in temperature, which made it easier for lithium to precipitate. Therefore, temperature has an effect on the rules of the salt precipitation of ions in salt lake brine. As for rubidium, there have been some reports on the enrichment rule of rubidium in salt fields. Wang [15] conducted isothermal evaporation experiments at 278 K on the brine of Zhabei Salt Lake during the autumn season and found that rubidium can substitute for potassium in potash minerals through isomorphous substitution. Li [16] researched the mechanism of rubidium loss during the potash crystallization process of the Jiangling brine and found that the loss of rubidium was mainly due to the isomorphic substitution of rubidium and potassium, with this part of the rubidium loss rate reaching 9.66%. Gao [17] studied the fate of rubidium during the evaporation process in the salt fields of Chaka Salt Lake. This study determined that the potassium-rich carnallite-based solid solution is the key mineral controlling the entry of trace rubidium from the brine into the solid phase. The formation of potassium-rich carnallite causes the concentration of rubidium in the brine to decrease rapidly to a deficient level. Wang [18] conducted a systematic sampling and analysis of the brine and solids in the salt fields of Chaka Salt Lake. This study found that the enrichment patterns of rubidium and cesium are correlated with the precipitation of carnallite.

West Taijinar Salt Lake, situated in Geermu, experiences an annual average temperature of approximately 288 K. Therefore, an experimental study on the 288 K isothermal evaporation of brine after potassium precipitation in West Taijinar Salt Lake was carried out to obtain the enrichment rules of each element and the order of salt precipitation, which provided a theoretical basis for the development and utilization of rubidium and cesium in West Taijinar Salt Lake brine.

2. Materials and Methods

2.1. Experimental Materials

The 10 L experimental brine, with a density of 1.2767 g/cm³, was taken from the salt field brine after potassium precipitation in West Taijinar Salt Lake, and the composition is detailed in

Table 1. Chemical composition of brine after potassium precipitation (g/L).

Element	Li+	Na+	K+	Mg2+	* Rb+	* Cs+	Cl−	SO42−	B2O3
content	1.70	5.03	2.34	98.27	3.2650	3.8150	276.52	20.31	6.72

* concentration units (mg/L).

.

It can be seen from Table 1 that the main components of the brine are Li⁺, Na⁺, K⁺, Mg²⁺, Cl⁻, and SO₄²⁻, which constitute a complex six-element system. In order to observe the rules of the salt precipitation of potassium and lithium, the simplified phase diagram of Na⁺, K⁺, Mg²⁺ // Cl⁻, SO₄²⁻-H₂O at 15 °C and the phase diagram of Li⁺, Mg²⁺ // Cl⁻, SO₄²⁻-H₂O at 15 °C were used to analyze the evaporation process of brine.

Ion analysis method: Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺: Inductively Coupled Plasma Optical Emission Spectrometry method (ICP-OES); Mg²⁺: the EDTA complexity method; Cl⁻: silver nitrate volumetric titration; SO₄²⁻: weighing titration analysis; B₂O₃: mannitol volumetric method [19]. The density (ρ) of the liquid phase was measured using a gravimetric method.

The X-ray powder diffraction (DX-2700, Dandong Fangyuan Instrument Co., Ltd., Dandong, China) was used to identify the component of the solid phase

2.2. Experimental Method

In this experiment, the isothermal evaporation method is used to evaporate the brine after potassium precipitation in West Taijinar Salt Lake. Firstly, A plastic basin with 10 L brine is placed in a constant temperature box with a temperature of 15 °C, and the watch glass vessel is placed in the plastic basin to observe the precipitation of new minerals. When a new mineral is observed or after evaporating for 1–2 days, liquid and solid phases in the brine are separated, the evaporated brine is weighed, and the evaporation rate is calculated. At the same time, the solid and liquid are separated, and the chemical composition is analyzed. The chemical composition is tested to obtain the enrichment rules of each ion. The solid precipitated at the segmented point is analyzed by X-ray diffraction to determine the mineral type [20,21].

The evaporation rate formula is as follows:

evaporation rate = (1 − m_r/m_o) × 100%

m_r is the mass of the residual brine, and m_o is the mass of the original brine.

3. Results and Discussion

3.1. Salting Out the Rules of Crystallization

During the evaporation process, a total of 20 solid/liquid separations were performed; the corresponding data are presented in

Table 2. The liquid phase composition of brine by isothermal evaporation at 288 K.

No	Evaporation Rate/%	Chemical Composition of Liquid Phase (g/L)
		Li+	Na+	K+	Mg2+	* Rb+	* Cs+	Cl−	SO42−	B2O3
L0	0.00	1.70	5.03	2.34	98.27	3.2650	3.8150	276.52	20.31	6.72
L1	5.35	1.76	4.21	2.30	103.68	2.8450	3.3450	290.14	20.57	6.83
L2	10.94	1.85	2.46	1.37	114.38	1.8400	0.1800	315.84	22.31	7.35
L3	16.35	1.96	1.90	0.82	114.47	1.0825	0.2550	330.72	23.18	7.94
L4	20.01	2.05	1.51	0.55	115.57	1.0475	0.3625	336.17	24.5	8.09
L5	23.27	2.16	1.54	0.58	114.90	0.5900	0.4025	334.31	25.34	8.58
L6	37.71	2.45	1.57	0.57	114.93	0.7150	0.4675	333.35	27.48	9.49
L7	46.87	2.77	1.52	0.56	115.26	0.7425	0.5150	332.39	33.38	10.73
L8	53.55	3.00	1.58	0.54	116.50	0.7525	0.4900	336.22	35.01	11.89
L9	56.30	3.14	1.61	0.53	115.96	0.5525	0.5875	335.84	35.44	12.20
L10	61.39	3.35	1.60	0.56	116.25	0.5275	0.6675	332.78	38.82	12.94
L11	64.07	3.54	1.57	0.55	116.97	0.5400	0.875	333.92	42.73	13.51
L12	67.05	3.62	1.63	0.56	116.03	0.7075	0.8900	331.06	42.62	14.39
L13	69.99	3.94	1.65	0.52	116.66	0.5250	0.8025	332.20	44.59	15.63
L14	72.59	4.17	1.63	0.55	116.72	0.2750	0.8000	332.20	47.69	15.77
L15	81.72	5.11	1.62	0.46	114.33	0.2725	1.7750	331.06	46.45	20.11
L16	83.26	5.16	1.59	0.49	113.66	0.2925	1.7575	328.76	46.18	20.22
L17	88.80	5.31	1.63	0.48	112.24	0.0300	2.0225	327.62	43.27	25.59
L18	93.70	5.31	1.56	0.42	113.17	0.1575	2.1025	328.37	45.51	34.64
L19	95.55	5.49	1.59	0.44	114.11	0.2250	2.0600	330.82	46.79	42.62
L20	97.32	5.15	1.64	0.44	113.03	0.5075	2.1100	326.41	47.40	50.78

* concentration units (mg/L).

and

Table 3. Chemical compositions of solid phase precipitated by 288 K isothermal evaporation system.

No	Evaporation Rate/%	Solid Phase Chemical Components (%)
		Li+	Na+	K+	Mg2+	Rb+	Cs+	Cl−	SO42−	B2O3
S1	5.35	0.02	32.28	0.09	1.48	0.0006	0.0006	54.26	0.62	0.09
S2	10.94	0.02	13.86	6.99	5.61	0.0094	0.0020	64.73	0.11	0.07
S3	16.35	0.02	9.34	7.98	6.31	0.0091	0.0019	33.58	1.46	0.08
S4	20.01	0.02	1.29	0.77	10.64	0.0091	0.0012	27.36	0.15	0.08
S5	23.27	0.04	0.14	0.05	10.84	0.0079	0.0005	32.57	0.57	0.12
S6	37.71	0.03	0.09	0.02	10.79	0.0096	0.0008	32.83	0.75	0.11
S7	46.87	0.02	0.17	0.08	10.85	0.0085	0.0002	32.00	0.44	0.08
S8	53.55	0.04	0.06	0.03	10.74	0.0096	0.0006	33.05	0.73	0.12
S9	56.30	0.04	0.12	0.06	10.74	0.0080	0.0005	32.98	0.17	0.13
S10	61.39	0.05	0.15	0.03	10.68	0.0093	0.0003	32.64	1.36	0.16
S11	64.07	0.06	0.06	0.02	10.61	0.0064	0.0005	32.42	1.99	0.21
S12	67.05	0.04	0.24	0.09	10.76	0.0059	0.0005	33.28	0.81	0.13
S13	69.99	0.05	0.11	0.06	10.88	0.0093	0.0004	32.95	2.31	0.16
S14	72.59	0.05	0.15	0.02	10.85	0.0107	0.0004	38.15	4.95	0.17
S15	81.72	0.06	0.18	0.14	10.52	0.0078	0.0001	28.79	8.94	0.12
S16	83.26	0.10	0.12	0.03	10.62	0.0075	0.0001	31.02	5.06	0.27
S17	88.80	0.77	0.17	0.13	9.99	0.0031	0.0002	29.95	4.83	0.29
S18	93.70	0.74	0.14	0.06	10.19	0.0087	0.0005	31.54	4.19	0.23
S19	95.55	0.18	0.15	0.07	10.57	0.0060	0.0002	33.01	4.16	0.35
S20	97.32	0.63	0.12	0.04	9.97	0.0070	0.0001	30.66	15.25	0.60

, respectively. The evaporation crystallization path of the brine is listed in

Table 4. Salt precipitation rule of brine.

Evaporation Rate /%	NaCl	MgSO4·7H2O	MgSO4·6H2O	KCl·MgCl2·6H2O	MgCl2·6H2O	H3BO3	Li2SO4·H2O
5.35	√		√	√	√
20.01	√		√		√
23.27			√		√
64.07		√			√	√
67.05		√			√
88.80		√			√	√	√

and plotted in the phase diagrams of Na⁺, K⁺, Mg²⁺ // Cl⁻, SO₄²⁻-H₂O (saturated with NaCl) at 288 K (Figure 1a) [22] and Li⁺, Mg²⁺ // Cl⁻, SO₄²⁻-H₂O at 288 K (Figure 1b). Under the saturation of NaCl, the phase diagram of Na⁺, K⁺, Mg²⁺ // Cl⁻, SO₄²⁻-H₂O at 288 K is characterized by eight distinct phase regions: thenardite (Na₂SO₄), glaserite (Na₂SO₄·3K₂SO₄), sylvite (KCl), picromerite (K₂SO₄·MgSO₄·6H₂O), epsomite (MgSO₄·7H₂O), hexahydrite (MgSO₄·6H₂O), carnallite (KCl·MgCl₂·6H₂O), and bischofite (MgCl₂·6H₂O), respectively.

Point L0 is the composition of the original brine and is situated within the crystallization region of halite, carnallite, and bischofite. From point L0 to point L1, halite, carnallite, hexahydrite, and bischofite precipitated out (Figure 2a). When the evaporation rate reaches 20.01% at point L4, halite, hexahydrite, and bischofite precipitated out (Figure 2b). At the evaporation rate of 20.01% (L4), the content of lithium is higher than that of sodium and potassium. To accurately describe the crystallization behavior of lithium during evaporation, the phase diagram of Li⁺, Mg²⁺ // Cl⁻, SO₄²⁻-H₂O at 288 K was used to illustrate the changes in the behavior of lithium in the brine as evaporation progresses. At point L5, with an evaporation rate of 23.27%, only hexahydrite and bischofite precipitate out (Figure 2c). From the crystallization path of the phase diagram of Li⁺, Mg²⁺ // Cl⁻, SO₄²⁻-H₂O at 288 K, it is observed that the system enters the epsomite region at an evaporation rate of 64.07% (L11) with the precipitation of H₃BO₃ (Figure 2d). Subsequently, epsomite and bischofite (Figure 2e) were precipitated out until the evaporation rate reached 88.80%. After this, epsomite, bischofite, H₃BO₃, and Li₂SO₄·H₂O (Figure 2f) precipitated out until evaporation was complete.

3.2. Change in Ion Content

Data from Table 2 and Table 3 were used to establish the correlation between the ion content and evaporation rate, as shown in Figure 3, Figure 4 and Figure 5. From Figure 3a,c, it is observed that during evaporation, the concentrations of Na⁺ and K⁺ in the liquid phase decrease sharply before the evaporation rate reaches 20.01%, with the Na⁺ and K⁺ concentrations being 1.51 g/L and 0.55 g/L, respectively, and then they level off. The concentrations of Mg²⁺ and Cl⁻ first increase and then stabilize, while the concentration of SO²⁻ remains on an increasing trend. Mg²⁺ reaches its maximum value of 116.97 g/L at an evaporation rate of 64.07%, after which it slowly declines. Cl⁻ reaches its maximum value of 336.22 g/L at an evaporation rate of 53.55%, and then it levels off. The concentration of SO²⁻ continues to increase gradually, reaching a maximum of 47.69 g/L. Figure 1b,d indicate that before the evaporation rate of 20.01%, the solid phase contains a large amount of Na⁺, K⁺, and Cl⁻, and subsequently, it contains a large amount of Mg²⁺, Cl⁻, and SO²⁻. Figure 2 shows that Na⁺ precipitates in the form of NaCl, and potassium precipitates in the form of KCl·MgCl₂·6H₂O.

Figure 4a shows that during the evaporation process, the enrichment rates of B₂O₃ and Li⁺ have exceeded their precipitation rates, leading to an increase in their concentrations in the liquid phases. Specifically, B₂O₃ increases from 6.72 g/L to 50.78 g/L, and Li⁺ increases from 1.7 g/L to 5.31 g/L. Figure 4b indicates that the enrichment rate of boron and lithium in the solid phase significantly increases at evaporation rates of 64.07% and 88.8%, respectively. Subsequent XRD analysis of the solid phase at these points revealed that boron and lithium precipitate as H₃BO₃ and Li₂SO₄·H₂O. After the evaporation rate reached 64.07%, the solid phase that precipitated was subjected to XRD analysis, which revealed that boron precipitates in a phased manner. Therefore, it is difficult to obtain high-grade H₃BO₃ by an evaporation separation operation, but the enrichment rate of B₂O₃ in the evaporation process is much larger than the precipitation rate, which makes B₂O₃ appear to have a high enrichment rate in the subsequent evaporation process. Throughout the entire evaporation process, the loss rate of boron is 13.44%. When the evaporation rate reaches 88.8%, Li⁺ increases from 1.7 g /L to 5.31 g /L and then begins to precipitate in the form of Li₂SO₄·H₂O. The concentration of Li⁺ then fluctuates between 5.31 g/L and 5.63 g/L, preventing the further enrichment of lithium in the liquid phase. The overall loss rate of Li⁺ throughout the entire evaporation process is 43.89%.

Figure 5a shows that during the evaporation process, the concentrations of K⁺ and Rb⁺ in the liquid phase follow similar trends, initially increasing and then gradually decreasing. Before the evaporation rate reaches 20.01%, the concentrations of Cs⁺ and K⁺ exhibit similar patterns, which are followed by a subsequent slow increase. The concentration of Rb⁺ decreased from 3.265 mg/L to 0.03 mg/L, reaching its minimum value at an evaporation rate of 88.8%, with a precipitation rate of the element reaching 99.08% compared to the original brine content. Subsequently, the concentration of Rb⁺ rose to 0.508 mg/L as evaporation progressed. In the evaporation process, the content of Cs⁺ decreased from 3.815 mg/L to 0.18 mg/L, and it reached its minimum value when the evaporation rate was 10.94%. The precipitation rate of the element reached 95.28% relative to the original halogen content. The content of Cs⁺ in the subsequent evaporation was enriched to 2.11 mg/L, which was enriched by 11.72 times in this process.

From Figure 5b, it is evident that prior to an evaporation rate of 20.01%, the solid phase contains high concentrations of K⁺, Rb⁺, and Cs⁺, with K⁺ predominantly precipitating as KCl·MgCl₂·6H₂O. The ionic radii of rubidium (0.149 nm), cesium (0.169 nm), and potassium (0.133 nm) are similar. The (R₁-R₂)/R₂ of rubidium and potassium is 10.74% < 15%, which is prone to complete isomorphism. The (R₁-R₂)/R₂ of cesium and potassium is 21.3% < 30%, which will form incomplete isomorphism [23]. Therefore, when K⁺ precipitated in the form of carnallite, the K⁺ from KCl·MgCl₂·6H₂O that precipitated during evaporation is replaced by Rb⁺ and Cs⁺, leading to the precipitation of Rb⁺ and Cs⁺ as (K, Rb) MgCl₃·6H₂O and (K, Cs) MgCl₃·6H₂O, respectively. Consequently, the contents of Rb⁺ and Cs⁺ decrease significantly during carnallite precipitation. The development and utilization of rubidium and cesium resources in sulfate-type salt lakes can involve dissolving carnallite ore to extract rubidium and cesium resources, potentially reducing production costs.

4. Conclusions

The sequence of the 288 K isothermal evaporation of brine after potassium precipitation in West Taijinar Salt Lake is as follows: NaCl, MgCl₂·6H₂O, KCl·MgCl₂·6H₂O, (K, Rb) MgCl₃·6H₂O, (K, Cs) MgCl₃·6H₂O, MgSO₄·6H₂O, H₃BO₃, MgSO₄·7H₂O, and Li₂SO₄·H₂O. During the experiment, high concentrations of Li⁺ and B₂O₃ were observed to be enriched. Li⁺ was enriched from 1.7 g/L to 5.63 g/L, a concentration increase by a factor of 3.31. At an evaporation rate of 88.8%, lithium precipitated as Li₂SO₄·H₂O, preventing the further effective enrichment of Li⁺ in the subsequent evaporation process. The enrichment of B₂O₃ increased from 6.72 g/L to 50.78 g/L, a 7.56-fold increase, significantly reducing the cost of boron extraction. Rb⁺ and Cs⁺, being isomorphic to K⁺, form (K, Rb) MgCl₃·6H₂O and (K, Cs) MgCl₃·6H₂O with the precipitation of carnallite-type salts. The findings of this study offer insights for the comprehensive development and utilization of lithium, boron, rubidium, and cesium resources in West Taijinar Salt Lake.