Researchers develop a new process to get lithium out of rocks

Source: Ars Technica

Process overview

The key chemical in the process is ammonium fluoride (NH₄F). While it can be used directly in a molten form, heating it typically generates hydrogen fluoride (HF), which is extremely hazardous. To avoid this, the researchers dissolved NH₄F in water; the aqueous environment suppresses the formation of HF. Heating the solution to about 70 °C converts the salt to NH₄F₂⁻ ions and releases ammonia gas, which is later reused in the process.

The NH₄F₂⁻ ion donates a fluorine atom to lithium, producing an aqueous solution of lithium fluoride (LiF). Silicon forms the soluble ion (NH₄)₂SiF₆, while aluminum forms (NH₄)₃AlF₆, which remains as a solid. Each of these streams is processed separately.

Using everything

Aluminum chemistry

1. 300 °C – Heating (NH₄)₃AlF₆ yields aluminum trifluoride (AlF₃) and releases ammonia (NH₃) and hydrogen fluoride (HF).
2. 700 °C – AlF₃ reacts with water, producing aluminum oxide (Al₂O₃) and additional HF.

The generated HF is hazardous and must be handled with care. It can be reacted with the released NH₃ to regenerate NH₄F, the original reagent, minimizing losses. The resulting Al₂O₃ is of high purity (over 98 %) and can serve as a feedstock for aluminum metal production.

Note: The high temperatures required for these steps represent a significant energy demand and safety challenge.

Silicon purification

Adding extra ammonia to the solution causes (NH₄)₂SiF₆ to react with water, producing silicon dioxide (SiO₂) and regenerating NH₄F. The SiO₂ precipitates out of the solution and can be collected. The team demonstrated that the recovered silicon dioxide can be used to strengthen concrete, among other applications.