The photo is sourced from Harward International Review
As a rule, natural brines containing lithium have a high concentration of foreign substances, including sodium and potassium. One method for extracting lithium from such brines is solar concentration with subsequent carbonate precipitation. A natural solution with a lithium concentration of 20 milligrams to 1 gram per litre is kept in the sun for several years, as a result of which the water evaporates and the lithium concentration rises to 10 grams per liter. First, chemical methods are used to remove alkaline earth metals, i.e., magnesium and calcium, from the substance, followed by potassium. Sodium carbonate is added to the solution in order to separate lithium from sodium: the exchange reaction leads to the formation of precipitate – poorly soluble lithium carbonate, which is the target compound. However, due to the length of the cycle, this method is not very cost-effective.
An alternative is the extraction of lithium based on a varied distribution of lithium, sodium and potassium between two immiscible liquids. The role of extractors is played by foreign substances that are capable of selectively extracting lithium from solutions with a low lithium content, as well as a high concentration of sodium and potassium. In 2022, scientists from the Institute of Physical Chemistry and Electrochemistry RAS found that these extractors include alkyl esters of salicylic acid, as well as trioctylphosphine oxide (TOPO), a white solid phosphorus-containing compound that easily reacts with metal ions. In 2024, the scientists discovered similar properties in phosphorus-containing ortho-substituted phenols – equivalents of alkyl esters, in which the carbonyl group is replaced by a phosphoryl group.
“In order for a substance to extract lithium well, the complex compound it forms with lithium must be, firstly, more stable than complex compounds with other alkali metals and, secondly, it must dissolve well in organic matter. We carried out quantum chemical calculations and found that the energy gain from the formation of a complex of 2-hydroxyphenylphosphonates with lithium is higher than from a complex with sodium or potassium. Later, we synthesised the required compounds and studied the extraction systems based on these compounds in detail. It turned out that they do, in fact, exhibit high lithium selectivity,” Alexey Bezdomnikov, candidate of chemical sciences, is quoted as saying by the Institute of Physical Chemistry and Electrochemistry RAS.
It had previously been thought that 2-hydroxyphenylphosphonates cannot be used to extract lithium, as they form insoluble complex compounds with alkali metal cations. This problem was solved by using a solvating additive that increases their solubility. The scientists used the aforementioned trioctylphosphine oxide (TOPO) for that purpose, which became one of the innovations of the study.