Lithium is currently extracted from hard rock and from brines. As the demand for lithium has begun to rise due to the development of electric vehicles, new deposits will be needed to fulfil such demand. This will require the development of new extraction methods to unlock ‘new’ deposit types of lithium that haven’t previously been utilised.
Hard rock mining entails drilling and blasting solid rock, before it is collected up, crushed and processed to extract the lithium bearing minerals from the rest of the rock. Lithium extraction from minerals such as spodumene and petalite requires a wide range of specialist processes in order to get lithium into the correct chemical composition and concentration required by the battery industry. Such chemicals are currently either lithium hydroxide or lithium carbonate.
Suitable extraction methods have been developed for many hard-rock deposits, but scientists are currently developing new extraction methods for clay minerals and other potential sources of lithium. Advances in processing techniques for hard rock sources have now made it possible to process other lithium-enriched minerals in hard rock such as zinnwaldite and other micas. The science of suitable extraction methods is still at an early stage but will be driven by the urgent need to commercialise different sources for lithium, especially in Europe.
Lithium-enriched brines form in a variety of geological settings and are largely derived from the contact between fluids and lithium enriched rocks. Lithium is a highly soluble ion and hence tends to leach out of rock into saline water very easily.
“Salar” Brine Deposits
Brines from closed sedimentary basins, such as ‘salar’ brines in South America, contain 58% of the world’s known lithium resources and for many years lithium has been extracted from such brines high in the Chilean Andes. This method requires the salt-rich waters to be pumped to the surface and then in to a series of evaporation ponds where solar evaporation concentrates the lithium brine over a period of around 18 months. Once the brine has become concentrated enough, the solution is pumped in to a recovery plant to first remove unwanted elements such as magnesium and boron, before sodium carbonate is added to precipitate out the lithium carbonate product.
Whilst this is currently an important source of lithium the extraction process relies on solar evaporation, given that the area is extremely arid and remote. Solar evaporation techniques are highly inefficient, generate significant volumes of waste and use large amounts of scarce, highly valuable water.
Figure 4. Solar evaporation brine processing (Lithium Americas)
Geothermal fluids are saline waters that circulate through hot crustal rocks and dissolve elements such as lithium from the rocks into solution. Such fluids were first identified in Cornwall in 1864 when miners looking for tin and copper hit large geological structures containing hot saline water. Given that this was deemed a scientific curiosity these fluids were analysed and found to contain high levels of lithium. Such occurrences were continually monitored and recorded until mining ended in Cornwall in 1998 and the mines where these hot springs had been accessed were flooded.
The presence of lithium enriched geothermal fluids in Cornwall relates to the fact that significant areas of the county are underlain by a very large body of lithium-rich granite. This granite remains hot at depth and has reacted with water circulating in the crust to produce lithium-enriched geothermal fluids which circulate to great depths below ground. Cornish Lithium believes that access to these fluids can be gained via extraction boreholes and that lithium can be extracted at surface in a small processing plant using advanced technologies such as ion-exchange membranes or reverse osmosis. A number of similar proprietary processes have been developed by companies such as Posco, Rincon (formerly Enirgi Group), Veolia, Tenova, PurLucid and Eramet. A typical flowsheet may be as seen in the following figure:
Figure 5. Typical processing flow sheet for direct extraction of lithium from brines (Cornish Lithium)
Cornish Lithium are still at an early stage of exploration, so the exact process that will be used has not yet been decided. The company intends to investigate several options and to select the optimal process flowsheet. Direct extraction of lithium from fluids allows products such as high-purity lithium carbonate or lithium hydroxide to be produced, which may then be used directly in battery production. Cornish Lithium has collected a large number of historic assays of these saline fluids and is encouraged by the chemical composition of those encountered.