Zabuye Salt Lake, a carbonate-type saline lake, is located in Zhongba County, Ngari Prefecture, Tibet, with an arid plateau climate. The lake covers an area of 247 km², and is divided into the northern part and the southern part (Figure 1). Zabuye Salt Lake was formed in the later stage of salt lake evolution, and its southern part has become a semi-dry salt lake with large sedimentary deposit containing dominantly lithium, potassium, boron, rubidium, cesium, bromine and other mineral elements. Among them, the reserves of lithium, boron, potassium and cesium have reached a super-high level, and the reserves of lithium carbonate has reached 1.84 million tons.

The process of extracting lithium carmine in Zabuye Salt Lake is generally divided into two steps, i.e. brine production and crystallization. In the brine production stage, lithium is rapidly enriched in salt fields to obtain a high-concentration lithium-containing brine. The crystallization stage mainly adopts the salt gradient solar pond technology (Ding et al., 2021), with the production process shown in Figure 2. The salt gradient solar pond consists of three functional regions. The uppermost region, called the upper troposphere, is a layer of fresh water. The middle region is called the concentration gradient layer, and the lowest region is called the energy-storage layer. In the concentration gradient layer, the specific gravity of the salt solution gradually increases with the depth of the pond, so that there is no convective movement in this layer. Therefore, the concentration gradient layer acts to prevent the upward heat of the lower layer from dissipating, so that the solar energy can be stored in the energy storage area at the bottom of the pond. The brine could be heated to 40∼60°C in the solar pond for high-temperature crystallization, in which the precipitation was promoted to produce lithium carbonate with a grade of about 75%.

The data for the experiment was collected from the Zabuye Continuous-observation Station, which was originally established in 1980 on the Zabuye Island in the middle of Zabuye Salt Lake, and later moved to the southern salt pan. The altitude of the observation station is 4,422.8 m, and the altitude of the sensing part of the barometer is about 4,423 m. The instrument shelter and the anemometer are 1.5 and 5 m from the ground, respectively. The station could record 11 items including air temperature and ground temperature, and provide dry bulbs, wet bulbs, maximum, minimum and self-recording thermometers in the instrument shelter. The daily evaporation can be detected by a small evaporator (φ20 cm) installed about 0.75 m from the ground. The precipitation can be measured with a rain gauge and a snow gauge. There are automatic recording devices for detecting sunshine hours, sunshine intensity and wind speed.

Inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin-Elmer-7000DV, USA) was employed to determine the concentrations of metal ions in the salt lake brine. To investigate brine phase chemistry, isothermal evaporation experiments on the brine of Zabuye Salt Lake were carried out at temperatures of 15 and 25°C in May and October, respectively. Table 1 for main chemical composition of Zabuye salt lake brines details. The experiments were carried out in a incubator with incandescent lamps as the heat sources. The mercury contact thermometers were applied to control the evaporation temperature (15 ± 1°C and 25 ± 1°C) to analyze the enrichment of elements and the precipitation characteristics of minerals during the brine evaporation process.

As can be seen from Figure 3, the annual average temperature of Zabuye Salt Lake showed an upward trend from 1991 to 2020, with the lowest annual average temperature of 1.3°C in 1992, and the highest annual average temperature of 5.5°C in 2020. The average heating rates reached 0.127°C/yr from 1991 to 2020, 0.051°C/yr from 1991 to 2000, and 0.178°C/yr from 2011 to 2020. reached. The above results are consistent with the reported studies (Chen et al., 2014; Guo et al., 2018; Ye et al., 2021), in which the temperature of the Qinghai-Tibet Plateau was observed to be rising and the heating rate in the past 10 years was much higher than that in the period from 1991 to 2000. The months with the monthly average temperature below zero are January, February, March, November and December in Zabuye Salt Lake. In rare cases, the average temperature in April may be lower than 0°C. The lowest temperature of each year generally appears in January, and the highest temperature occurs in July or August, as shown in Figure 4. The monthly average maximum temperature was 22.8°C in August 2020 and the single-day maximum temperature was 28.5°C on 28 August 1992.

Based on the monitoring data of precipitation and evaporation shown in Figure 5, the average annual evaporation of Zabuye Salt Lake from 1991 to 2020 was 2,579.07 mm/yr, and the average annual precipitation was 168.65 mm/yr. The average annual evaporation is 15.3 times the average annual precipitation. It can be seen from Figure 6 that the precipitation months in Zabuye Salt Lake were concentrated in the rainy season from early July to mid-September. The precipitation during the rainy season accounts for more than 90% of the total annual precipitation. Therefore, the precipitation process of Zabuye Salt Lake is dominated by rainfall, with less snow and hail. The monthly variation in evaporation is similar to that of temperature, with low points appearing in January, February and December each year, and high points in June and July. Despite of the high temperature, the evaporation curve showed a distinct trough in August and September because the high air humidity greatly limited the amount of evaporation.

Zabuye Salt Lake is rich in solar energy resources with an average annual sunshine duration of 3,122.4 h from 2018 to 2020. It can be seen from Figure 7 that the monthly sunshine duration changes little, with the longest sunshine duration of 297.7 h in June. The weather in the lake district is mainly sunny or less cloudy, and rarely cloudy. The rainfall in the rainy season is mostly in the form of short-term thunderstorms. As can be seen from Figure 8, the water level of the North Lake of the Zabuye Lake shows an upward trend, corresponding to the trend of climate change (Wang et al., 2019). During the period from 1991 to 2020, the lowest annual average water level was 4,420.79 m in 1997, and the highest annual average water level was 4,421.61 m in 2020. As shown in Figure 9, the highest point of the water level in the past 10 years appears in April, August and September, while the lowest point generally appears in January, February and December. Owing to the obvious and short rainy season in Zabuye Salt Lake, the water level always rises significantly in August and September. The ratio of evaporation to rainfall reached the maximum in May and the minimum in August. Thus, evaporation is also one of the crucial factors influencing the variance of water level. In addition, the melting of glaciers, one of the main supply of the Zabuye North Lake, is also affected by climate change. The snow and ice in the mountainous cannot melt after October, resulting in a great reduction in the water supply of the North Lake. Therefore, the water level of the salt lake displayed a slow decline after October, and reached the lowest point in January of the following year. With the rising temperature in February, the snow and ice began to melt, leading to an gradual enhancement in the water level.

The isothermal evaporation experiments at 15 and 25°C revealed the crystallization process of salt minerals in the brine. Figures 10, 11 show the route diagrams of isothermal evaporation for brine taken from Zabuye Salt Lake at of 15°C brine in spring and 25°C brine in autumn which are taken from Zabuye Salt Lake. The metastable phase diagram of the quinary water-salt system Na⁺, K⁺//Cl⁻, CO₃ ²⁻, SO₄ ²⁻ -H₂O was applied to describe the brine evaporation route. The original composition of brine in spring in Zabuye Salt Lake is different from that in autumn, and its composition in the system is located in the sylvite region. Therefore, the sylvite is precipitated earlier during the isothermal evaporation at 15°C. The quinary phase diagram could be equivalent to a simplification of complex Li⁺, Na⁺, K⁺//Cl⁻, CO₃ ²⁻, SO₄ ²⁻, B₄O₇ ²⁻, HCO₃ ⁻ -H₂O eight system of Zabuye Salt Lake (Nie et al., 2010), and it is hard to determine the precipitation order of lithium carbonate and borax. The precipitation sequence and combination of salt minerals are relatively complicated during the experiment, as listed in Table 2. During the isothermal evaporation process at 25°C, the precipitation sequence from the brine was halite, glaserite, lithium carbonate, trona, thermonatrite, sylvine and borax. During the isothermal evaporation process at 15°C, the precipitation sequence of salt minerals was halite, sylvine, trona, borax, lithium carbonate, thermonatrite, natronite and glaserite.

Figure 12 shows the relationship between Li₂CO₃ and brine formation rate during isothermal evaporation at 15 and 25°C. It can be seen that Li₂CO₃ was precipitated from the metaphase. In addition, lithium precipitation is a continuous process that occurred throughout subsequent evaporation process. During the period, the Li₂CO₃ grade in the mixed salt was relatively low, only 4% at 25°C and 2% at 15°C. Two peaks for lithium carbonate precipitation clearly appeared at 25°C, while no obvious precipitation peak was observed at 15°C. Thus, high-grade lithium carbonate mixed salt could be obtained at a higher temperature, with more concentrated precipitation. Since Zabuye Salt Lake brine is dominated by carbonate type, the large amount of CO₃ ²⁻ limited the concentration range of Mg²⁺ and Ca²⁺ in brine, resulting in a very small ratio of magnesium to lithium in Zabuye Salt Lake brine. However, the enrichment of Li⁺ in brine was also restricted, resulting in the precipitation of lithium carbonate in most stages of the evaporation experiment. As described in Figure 13, Li⁺ in the liquid phase could be enriched to a higher concentration at 15°C than at 25°C. Therefore, it can be concluded that a low temperature was more conducive to the enrichment of lithium in Zabuye Salt Lake brine, and a high temperature favored the precipitation of lithium carbonate in Zabuye Salt Lake.

As can be seen from the temporal and spatial distribution of lithium ion concentration in the North Lake of Zabuye Salt Lake (Figure 14), the lithium ion concentration has shown a downward trend in the past 30 years from 1991 to 2020, and a significant decreasing trend from 2001 to 2010. This was because the lithium ore resources in Zabuye Salt Lake have been developed and utilized since 2002, and the ion concentration in the salt lake in winter is higher than that in summer. These results corresponded to the phase chemistry experiment of the brine in Zabuye Salt Lake. The lithium in Zabuye Salt Lake brine was prone to be enriched at a low temperature and predicated to be lithium carbonate at a high temperature. Thus, the lithium ions in the lake displayed a low concentration in summer.

The phase diagram experiment indicated that a high temperature is conductive to the precipitation of lithium carbonate in the salt lake, while a low temperature is favorable for the enrichment of lithium ions in the salt lake. Since Zabuye Salt Lake is located in the plateau sub-arctic climate zone and has an obvious upward trend in the annual average temperature in the past 30 years, the best period for brine storage is from November to April each year, owing to the low temperature, basically no precipitation and relatively small evaporation. All the climatic conditions mentioned above are suitable for the enrichment of lithium ions in brine. The temperature between May to October began to rise and the evaporation increased under sufficient sunshine duration. Thus, this period was more suitable for the precipitation and recovery of lithium carbonate. Since July and August are rainy seasons, precautionary preparations shall be made to prevent the entrainment and loss of the precipitated lithium carbonate mixed salt.