With the rapid development of new energy vehicles, electronic devices, and energy storage technologies, lithium's application in new energy materials has attracted significant attention, and it is hailed as the "21st-century energy metal" and "white petroleum." Global lithium resources are mainly found in salt lake brines and ores; in currently explored lithium resources, salt lake brines account for 58%, and lithium concentrates account for 26%.

According to the USGS, as of 2021, the world's explored lithium reserves totaled approximately 89 million tons, mainly concentrated in Bolivia, Argentina, Chile, the United States, Australia, and China.

China's explored lithium resources (metal equivalent) reserves are approximately 5.34 million tons, accounting for 6% of the global reserves, mainly distributed in Qinghai, Tibet, Hubei, and Sichuan.

Salt lake resources account for approximately 82% of the national total reserves, while ore resources account for about 18%. Lithium-containing salt lakes are mainly distributed in Qinghai, Tibet, and Hubei; spodumene and lepidolite are mainly distributed in Sichuan, Jiangxi, and Hunan. With the rapid development of the new energy vehicle industry, the demand for lithium resources is increasing, but due to environmental protection, transportation, and technological factors, domestic lithium ore and salt lake lithium extraction cannot meet market demand. In 2020, China's self-sufficiency rate of lithium raw materials was only 32%.

The rapid development of new energy vehicles is driving demand for upstream power batteries and metallic lithium. From the demand side, metallic lithium is widely used in the new energy industry, traditional industries, 3C electronic digital consumer products, and energy storage batteries, with significant growth in new energy vehicles and energy storage batteries.

In 2021, the sales volume of new energy vehicles in the Chinese market reached 3.521 million units. Based on sales forecasts from major auto brands and under-construction production lines, the sales volume of new energy vehicles in China is expected to reach 11.68 million units in 2025, with a CAGR of 35% from 2021 to 2025. In the global market, the sales volume of new energy vehicles in 2021 was 6.5 million units. According to forecasts from institutions such as EVTank, the global sales volume of new energy vehicles is expected to reach 18 million units in 2025, with a CAGR of 29% from 2021 to 2025. Assuming that a pure electric vehicle has a power capacity of approximately 60 kWh and the current lithium consumption per kilowatt-hour of battery is 0.8 kg/kWh, the demand for lithium carbonate in China's new energy vehicles in 2021 was 169,000 tons, and the global demand was 312,000 tons; in 2025, the demand for lithium carbonate in China's new energy vehicles will reach 561,000 tons, and the global demand will reach 864,000 tons.

Although more than 80% of China's lithium resources come from salt lakes, compared to the South American lithium triangle, China's salt lake resources have low lithium content and high impurity levels, especially magnesium and lithium, which are difficult to separate due to their similar chemical properties. In addition, the natural environment in the areas where China's salt lakes are distributed is relatively harsh, and transportation is inconvenient, making expansion difficult and the domestic expansion rate slow.

As of 2021, the annual production capacity of salt lakes in Qinghai province was only 117,000 tons/year, with an output of 59,000 tons, and a capacity utilization rate of 50.4%; only the Zabuye Salt Lake in Tibet has achieved industrial production, with a designed production capacity of 5,000 tons/year. Qinghai has relatively well-developed infrastructure and is the earliest region in China to implement salt lake lithium extraction projects. The proportion of salt lake lithium resources in Qinghai province accounts for approximately 50% of the national total, mainly concentrated in the Chaerhan Salt Lake, East Taijiner Salt Lake, West Taijiner Salt Lake, Yiliping Salt Lake, and Dachaidan Salt Lake.

Compared with foreign salt lakes, Qinghai salt lake brine has low lithium content and a high magnesium-to-lithium ratio (the magnesium-to-lithium ratio is generally higher than 60, and the lithium content is 0.02-0.085%), and the brine contains many associated elements such as boron, potassium, magnesium, and sodium, making the composition more complex and the separation of lithium from the brine more difficult. Therefore, despite abundant resources and good sun-drying conditions, traditional solar pond methods cannot be directly used, and more advanced technologies are needed to solve problems such as lithium ion concentration and impurity separation.

In 2018, the first 10,000-ton adsorption + membrane method project in China, the 10,000-ton high-purity, high-quality lithium carbonate project of Lanko Lithium in the Chaerhan Salt Lake, achieved its target output, marking the official breakthrough of large-scale production of salt lake lithium extraction technology in China, and thus initiating the development of salt lake lithium extraction in Qinghai.

Salt lakes in Tibet are mainly distributed in the northwest of Tibet, and most of them are carbonate-type salt lakes with high lithium and boron content and low magnesium content, and a relatively high average lithium grade. However, due to the fact that most salt lakes in Tibet are located in remote, harsh, high-altitude, and cold regions, limited by strict environmental protection requirements and lack of infrastructure, the overall development level is currently low, and project progress is slow; only the Zabuye Salt Lake has achieved industrial development of 5,000 tons, while the Chaka and Bajiancuo Salt Lakes have small experimental production capacities of lithium carbonate.

The development of salt lakes in China is more difficult, giving rise to a variety of lithium extraction processes. Strict environmental protection requirements in Qinghai and Tibet restrict the application of traditional solar pond methods, and the huge differences in grade between salt lakes also make it difficult for salt lake lithium extraction processes to have the same universality as ore lithium extraction.

In order to develop processes suitable for China's salt lakes, many domestic scientific research institutions and salt lake enterprises have invested for many years in the field of salt lake lithium extraction, and have explored and developed various routes such as adsorption, membrane, extraction, and electrochemical methods, using technological innovation to compensate for the shortcomings of China's salt lake natural endowments. Currently, the domestic salt lake lithium extraction field has formed a situation of "one lake, one policy" and multiple processes running in parallel.

The imbalance between supply and demand has driven a surge in lithium carbonate prices, and the short, flat, and fast equipment-based salt lake lithium extraction method has a greater advantage. With the rapid growth of downstream demand, the imbalance between supply and demand of lithium resources has driven a surge in lithium prices. As of December 25, 2022, the price of lithium carbonate was 565,000 yuan/ton, and the price of lithium hydroxide was 568,000 yuan/ton. According to Baichuan Yingfu, at a lithium carbonate price of 565,000 yuan/ton, the gross profit margin of salt lake lithium extraction (adsorption method) is 83.4%, and the gross profit margin of ore lithium extraction (sulfuric acid method) is 21.0%.

Due to the relatively stable price of brine, the high lithium price brings substantial profits to upstream lithium companies that adopt salt lake lithium extraction processes. Currently, major lithium companies have announced plans to expand their production of lithium carbonate. For owners, how to develop existing lithium resources faster under high lithium prices has become a top priority. Since the development cycle of lithium concentrates is 3-5 years, and the traditional solar pond method of building salt fields and drying original brine also takes at least 4 years; in contrast, the equipment-based salt lake lithium extraction process with a construction period of 1-2 years is undoubtedly the fastest lithium extraction option.

We believe, Years of technological breakthroughs have continuously improved the equipment and technology suppliers in China's salt lake lithium extraction industry; with the wave of industry expansion, Chinese companies, with their more cost-effective processes and technical services, have the full potential to stand out and establish themselves at the forefront of the industry.

The solar pond method is the most traditional and mature process in the field of lithium extraction from salt lakes, widely used in lithium extraction projects in South American salt lakes. Based on the negative temperature effect of lithium carbonate solubility, the solar pond method uses multi-stage salt fields to gradually sun-dry and concentrate brine from salt lakes. After obtaining high-lithium brine that meets the requirements, the brine is heated in a salt gradient solar pond, causing lithium to crystallize out in the form of lithium carbonate. After further processing using methods such as causticization or carbonization, industrial-grade or battery-grade lithium carbonate products can be obtained.

The core of the solar pond lithium extraction process lies in the salt gradient solar pond, which consists of three layers from top to bottom:

The upper layer is the upper convection layer (freshwater layer) It is composed of freshwater, its temperature is close to the ambient temperature, and its main function is to form and protect the middle salt gradient layer;

The middle layer is the non-convection layer (salt gradient layer) Its salt concentration increases continuously with depth. On the one hand, it prevents heat loss from the surface of the pond, and on the other hand, it utilizes the difference in refractive index between freshwater and brine to store heat energy in the brine at the bottom of the pond. Due to the existence of the salt gradient layer, the evaporation of brine in the lower convection layer is slow, other salts are difficult to precipitate, and lithium carbonate is easily precipitated in large quantities at the bottom of the pond, improving the grade.

The lower layer is the lower convection layer (energy storage area) It is composed of a saturated salt solution, and its temperature is much higher than that of the upper convection layer. Its main function is to collect and store solar energy and increase the brine temperature. The salt gradient solar pond can store heat across seasons. Even in winter, the bottom of the pond can still maintain a certain temperature. Combined with the higher lithium concentration in the brine in winter, lithium carbonate can still precipitate in the salt gradient solar pond, achieving continuous production throughout the year.

The advantages of the solar pond method are its simple operation and low cost; however, its disadvantages are that it is only suitable for lithium extraction from carbonate-type salt lake brine with extremely low Mg/Li ratio, and it is easily affected by climatic conditions, with high freshwater consumption, and is not suitable for China's salt lakes.

South American lithium salt lake resources have a low Mg/Li ratio, and the salt lake sun-drying conditions are excellent, so the solar pond method is mainly used, and the technical route consists of three stages: salt field evaporation, lithium separation from other ions, and purification precipitation.

Currently, the only application of the solar pond method in China is in the Zabuye Salt Lake in Tibet. The Zabuye Salt Lake brine is of the carbonate type, with a mass concentration of 100 mg/L, which is close to the saturation point of lithium carbonate. Therefore, the first-phase 0.5-ton production capacity uses the salt gradient solar pond method (SGSP method).

In addition, the Zabuye Salt Lake is located on a plateau, with no electricity, no mineral energy, and inconvenient transportation, lacking fuel energy supply; this process route fully utilizes the abundant solar energy on the Qinghai-Tibet Plateau and the geographical conditions suitable for building salt fields, adding no chemical reagents, and can extract lithium carbonate concentrate products with a grade of 50%~80% locally. After purification, the purity of lithium carbonate reaches more than 99.5%. However, from an environmental protection perspective, the solar pond method in China has the potential threat of damaging the natural ecosystem, and very few new projects are planned to use this method in the future.

0 2 Adsorption coupled membrane method

The adsorption coupled membrane method is one of the most mainstream and most mature industrialized processes in the field of salt lake lithium extraction in China, widely used in the Qinghai region.

This process is divided into an adsorption section and a membrane section. The adsorption section uses an adsorbent to selectively adsorb lithium ions from brine, and then elutes to achieve lithium ion concentration and separation from other ions (mainly magnesium ions); the membrane section uses a series of organic membrane gradient couplings to further concentrate and purify brine: ultrafiltration membrane (UF) is mainly used to filter suspended particles in the qualified liquid to reduce pollution and consumption of subsequent nanofiltration membranes, nanofiltration membrane (NF) is mainly used to achieve separation of monovalent and divalent ions, and reverse osmosis membrane (RO) is mainly used for concentration of lithium solution in the later stage of the process.

The main advantages of the adsorption + membrane method are lithium extraction from salt lakes with low lithium ion concentration and high Mg/Li ratio. Compared with salt lakes in other parts of the world, low lithium ion concentration and high Mg/Li ratio are the most significant characteristics of Chinese salt lakes. Because lithium and magnesium have similar properties and hydration radii, the higher the Mg/Li ratio in brine, the more difficult and costly lithium extraction becomes; the advent of this process effectively solves the problem of lithium extraction from salt lakes with high Mg/Li ratio.

In the adsorption section, raw brine/old brine is used as the lithium extraction raw material. First, an adsorbent with selectivity for lithium is used to adsorb lithium ions from the brine, and then freshwater/acid is used to elute the lithium ions from the adsorbent, achieving separation of lithium ions from other impurity ions. The core is the separation of magnesium and lithium elements. The adsorbents used in the adsorption section are mainly divided into inorganic adsorbents and organic adsorbents.

Organic adsorbents are generally ion exchange resins, which have poor selectivity and are difficult to elute;

Inorganic adsorbents mainly include aluminum-based adsorbents, manganese-based and titanium-based spinel-type oxide adsorbents, etc., which have high selectivity for lithium, large adsorption capacity, and high elution rate, and are widely used adsorption materials for lithium extraction from salt lake brine.

Aluminum-based adsorbents are currently the most mature and the only adsorbents that have been industrialized. Aluminum-based adsorbents belong to layered double hydroxides (LDHs), formed by the ordered stacking of positively charged layers (positively charged aluminum-oxygen octahedra and Li in the holes) and interlayer anions, and the overall structure of the adsorbent is electrically neutral. ⁺ After freshwater washing, Li ⁺ is removed and holes are formed in the corresponding positions; these holes can only accommodate cations roughly equivalent to Li ⁺ Approximately equivalent cations.

For brine with a high Mg/Li ratio, although Mg ²⁺ The radius of (0.065nm) and Li ⁺ (0.060nm) close, but Mg ²⁺ standard hydration free energy is much greater than Li ⁺ Entering the cavity requires more energy, so aluminum-based adsorbents have good separation properties for other cations in salt lakes, effectively solving the problem of lithium extraction from brine with a high Mg/Li ratio.

Based on the high selectivity of aluminum-based adsorbents for lithium ions, simple preparation process, and the advantage of damage-free lithium desorption through water washing, the product has been industrialized in Qinghai Salt Lake. However, the main problems of aluminum-based adsorbents are their small adsorption capacity and their suitability for chloride-type and magnesium sulfate subtype neutral salt lakes; alkaline or acidic environments will destroy the structure of aluminum-based adsorbents.

In view of the defects of aluminum-based adsorbents, new adsorbents have been studied at home and abroad; among them, titanium-based and manganese-based adsorbents with more promising industrial application prospects are ion-sieve type. Titanium-based and manganese-based ion sieves are prepared by mixing lithium source with titanium dioxide, manganese oxide and other titanium and manganese sources to form an ion sieve precursor, and using acid to remove Li ⁺ The ion sieve can be obtained by elution, and the ion sieve can be placed in lithium-containing brine for adsorption to form lithium titanium and lithium manganese composite oxides again.

Titanium-based ion sieve for Li ⁺ The theoretical adsorption capacity can reach 39.8mg/g, and the capacity after granulation is 3~5mg/g, which is comparable to that of aluminum-based adsorbents. It has the advantages of low damage rate, high lithium elution rate, and stable performance, and compensates for the high damage rate of manganese-based adsorbents. Under the action of the lithium desorption agent, the manganese-based ion sieve can almost completely remove the lithium ions in the structure, thus having a higher adsorption capacity, and the maximum theoretical adsorption capacity for Li ⁺ can reach up to 82.3mg/g.

In addition, aluminum-based adsorbents are usually more suitable for neutral brine (not acid-resistant or alkali-resistant), manganese-based adsorbents are suitable for neutral and slightly acidic brine (up to 7.5-8, acid-resistant but not alkali-resistant), and titanium-based adsorbents are suitable for carbonate-type salt lake brine and strongly alkaline liquid minerals (acid and alkali resistant).

Although titanium-based and manganese-based adsorbents have shown superior adsorption performance in experiments, there are still many problems that are difficult to overcome in the industrialization process:

1) There is a huge gap between the actual adsorption capacity and the theoretical adsorption capacity, mainly because Li ⁺ is not completely desorbed during the elution of the adsorbent precursor, and the adsorption channels are blocked during the cycle, resulting in a reduction in the number of effective vacancies.

2) During the elution process, the core skeleton of the powder adsorbent is damaged, cracked, and collapsed in the solution; especially for manganese-based adsorbents, trivalent and tetravalent manganese ions are prone to disproportionation reaction damage during the cycle, resulting in partial dissolution of the ion sieve framework, which seriously affects the cycle performance of the ion sieve and greatly limits its industrial application.

3) In industrial production, processes such as granulation, film formation, foaming, fiber formation, and magnetization can easily lead to blockage and coverage of the adsorption sites of the adsorbent, reducing the adsorption capacity of the industrial adsorbent.

4) Both titanium-based and manganese-based ion sieves require acidic eluents to elute Li ⁺ , and industrial applications can easily cause environmental problems.