Lithium Extraction in Aqueous Solutions through the Use of Heterosite FePO4

The global resources of lithium, essential for electric vehicle batteries, are a subject of some debate. Lithium resources are mainly from two types of deposit; brine (21.6 Mt of Li) and pegmatite (3.9 Mt of Li)¹. The largest deposit of lithium reserves are the brine deposits  in South America. Although existing lithium resources may be sufficient to support demand until the year 2100 (assuming that lithium batteries are recycled ^{1, 2)}, lithium extraction is currently too complex, slow and inefficient to be economical ^{3, 4}. 

In current lithium industries, one of the most economical ways to extract lithium is by lime soda evaporation; a solar evaporation and chemical plant process, which takes between 12 to 24 months. After the solar evaporation, the concentrated brine is pre-treated to a pH of 2 in order to remove Boron by a solvent extraction. Lime (CaO) is then added to remove magnesium, sulphate, and borate. Calcium left in the brine is removed by a small amount of soda ash or sodium carbonate (Na₂CO₃). Then, the brine is heated to about 90°C and more soda ash is added to precipitate lithium carbonate ^{4, 5}. This lime soda evaporationis considered as a long-process of lithium extraction. Therefore, the objective of this work is to develop a new, fast and inexpensive approach to recover lithium chemically, from the lithium sources that contain other metal cations.

Heterosite iron phosphate (FePO₄) is a discharged product of lithium iron phosphate (LiFePO₄). There are a few studies about the structure of heterosite FePO₄ that report that it is more selective for lithium ions (Li⁺) over sodium ions (Na⁺). This structure displays  excellent reversibility charge/discharge properties ^6,7. There small potential differences of the redox couple and the stability of LiFePO₄ over a wide range of pH in an aqueous solution are the main advantages of this structure to this application ⁶. 

This work investigates a novel process that may prove to be more economical than the lime soda evaporation process for lithium extraction. Heterosite FePO₄ was used to selectively remove Li⁺ from the brine with the aid of a reducing agent (sodium thiosulfate; Na₂S₂O₃). The resulting LiFePO₄can be directly sent to lithium battery industries. In principle, the other cations could be retrieved back into the brine as shown in figure 1.

We have examined and demonstrated the process of lithium insertion into a heterosite FePO₄ framework in aqueous salt solutions. In this process, the amount of Li⁺ uptake can take up to 46 mg_Li⁺/g_solid and other cations (i.e. sodium, potassium, and magnesium) can take less than 3 mg/g_solid. Furthermore, the re-lithiated FePO₄ performed satisfactorily as a positive electrode. This work could also be developed for future lithium recycling processes. 

References

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