Researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia have developed a continuous electrically-driven membrane process which successfully enriches lithium from seawater samples of the Red Sea by 43,000 times (i.e., from 0.21 to 9013.43 ppm) with a nominal Li/Mg selectivity >45 million.

They precipitated lithium phosphate with a purity of 99.94% directly from the enriched solution, thereby meeting the purity requirements for application in the lithium battery industry. Preliminary economic analysis shows that the process can be made profitable when coupled with the Chlor-alkali industry.

An open-access paper on their work is published in the RSC journal Energy & Environmental Science.

Seawater contains significant quantities of lithium—approximately 5,000 times more than is found on land—potentially providing an almost unlimited resource of lithium for meeting the rapid growth in demand for lithium batteries. However, lithium extraction from seawater is exceptionally challenging because of its low concentration (~0.1–0.2 ppm) and an abundance of interfering ions (i.e., >13000 ppm of sodium, magnesium, calcium, and potassium ions, among others).

The presence of monovalent ions, such as sodium and potassium, is not a significant issue in the conventional precipitation method since their salts are highly soluble. Instead, the lithium concentration and the ratio of lithium to other multivalent ions, such as Mg²⁺ and Ca²⁺, are the key factors to consider.

In terms of separation, the membrane process is one of the most energy-efficient methods, with a potential to save up to 90% energy in many industrially important separation processes. In addition, this process runs continuously, and is easy to scale up. Unlike conventional membrane processes where the transport proceeds down the concentration gradient, the electrically-driven membrane process can up-grade the concentration; this system has been commercialised for use in the purification of hydrogen. As lithium possesses one of the smallest ionic sizes, we considered that it could be technically feasible to use a molecular sieving membrane to enrich lithium and to remove multivalent ions at an affordable energy cost. After enrichment, the lithium can be readily extracted using the conventional precipitation method.

—Li et al.

The electrical cell used in the paper was divided into three compartments: cathode, feed and anode. The cathode and the feed compartments were separated by a dense glass-type Li_0.33La_0.56TiO₃ (LLTO) membrane with a diameter of ~20 mm and a thickness of ~55 μm.

LLTO is one of the superior solid-state lithium ion superconductors. The LLTO membrane proved to provide efficient separation between lithium and other interfering ions, in addition to a high lithium permeation rate.

Schematic illustration of the three-compartment electrical cell to continuously enrich lithium from the feed solution to the cathode compartment and simultaneously generate H₂ and Cl₂ at the cathode and anode, respectively. Li et al.

The feed compartment and the anode compartment were separated by an anion exchange membrane (AEM) that allows the transport of anions only. The use of the membrane and and the use of a saturated NaCl solution in the anode compartment allow the release of Cl₂—necessary to prevent the dissolution of the highly soluble Cl₂ in the large volume of feed stream.

A CO₂ and phosphate buffer solution stabilises the pH and prolongs the lifetime of the membrane. The team found that the LLTO membrane could be used for more than 2000 hours with a negligible decay in performance.

Although a rigorous economic analysis will be still necessary to include other capital and operating expenses, it is arguable that the energy cost is the major expenditure in this process. Furthermore, the process possesses further potential for optimisation, and for its combination with seawater desalination to create innovative designs under the energy-water nexus scheme, which will further improve the process profitability. Hence, it is expected that our approach will lead to the development of a promising process to secure the supply of lithium for future energy uses.

—Li et al.

Resources

• Zhen Li, Chunyang Li, Xiao-Wei Liu, Li Cao, Peipei Li, Ruicong Wei, Xiang Li, Dong Guo, Kuo-Wei Huang and Zhiping Lai (2021) “Continuous Electrical Pumping Membrane Process for Seawater Lithium Mining” Energy Environ. Sci. doi: 10.1039/D1EE00354B