The global energy transition, driven by the need for high-capacity batteries, has made lithium a commodity of major importance. While a significant portion of the world’s lithium comes from brines, spodumene, a hard-rock mineral, is a primary source for the remainder, holding a theoretical lithium oxide content of over eight percent. Extracting lithium from this dense, natural mineral structure is a complex, multi-stage process. The method combines extreme heat treatment (pyrometallurgy) to make the lithium accessible, followed by chemical stages (hydrometallurgy) of acid leaching, impurity removal, and final precipitation to yield a battery-grade product.
Pre-treatment and Thermal Conversion of Spodumene
The journey to extract lithium begins with the raw ore, which must first undergo physical beneficiation to produce a concentrated product. This pre-treatment involves crushing and grinding the mined ore to increase the lithium oxide content to a commercially viable level, typically between five and seven percent. This initial concentration step removes many of the unwanted gangue minerals before the energy-intensive chemical processing begins.
The core challenge of hard-rock extraction is that natural spodumene, known as alpha-spodumene, is chemically inert and highly resistant to acid attack. The mineral’s dense, monoclinic crystal structure locks the lithium atoms firmly in place, preventing their reaction with leaching agents. To overcome this, the concentrated ore is subjected to a process called calcination, or thermal conversion, which is the most energy-intensive step.
During calcination, the spodumene concentrate is heated in a kiln to extremely high temperatures, typically ranging from 1000°C to 1100°C. This intense heat causes a permanent phase change, transforming the inert alpha-spodumene into a more reactive allotrope called beta-spodumene. This structural rearrangement, known as decrepitation, is accompanied by a significant expansion of the crystal lattice, increasing its volume by nearly one-third.
This volumetric expansion and the resulting open, tetragonal crystal structure are precisely what make the lithium chemically accessible for the subsequent steps. The beta-spodumene, which has a lower density than its alpha counterpart, is then ready to react with chemical agents. This high-temperature conversion is a prerequisite for efficient extraction, fundamentally distinguishing the hard-rock process from the simpler treatment of lithium brines.
Acid Leaching and Initial Impurity Removal
Once the beta-spodumene is formed, the next major stage, acid leaching, begins the actual chemical separation of lithium from the bulk material. The cooled, thermally activated concentrate is mixed and roasted with concentrated sulfuric acid. This “sulfation roasting” step is where the lithium is selectively converted into a soluble compound.
The hydrogen ions in the sulfuric acid exchange with the lithium ions within the open lattice structure of the beta-spodumene. This reaction forms lithium sulfate, which is highly soluble in water, while the aluminum and silicon components of the mineral remain largely insoluble. The roasted material is then leached with water, dissolving the soluble lithium sulfate and creating a pregnant leach solution (PLS).
This liquid solution, however, is not yet pure, as minor amounts of other elements like iron and aluminum are also dissolved by the strong acid. The solid residue, mostly insoluble silicates, is separated from the PLS through filtration, leaving a liquid stream that still requires significant purification. The first purification step involves neutralizing the highly acidic PLS to raise the pH.
This controlled increase in pH causes the bulk metallic impurities, primarily aluminum and iron, to precipitate out of the solution as insoluble hydroxides. Adjusting the pH to around 6.5 is sufficient to remove most of the dissolved aluminum and iron(III). The removal of these major contaminants is crucial, as it protects downstream equipment and ensures the final product meets purity requirements for battery manufacturing.
Final Purification and Lithium Compound Precipitation
After the bulk impurities are removed, the cleaned lithium sulfate solution enters secondary purification stages to eliminate remaining minor contaminants. Elements such as calcium and magnesium, which can interfere with the final product quality, are selectively precipitated out of the solution. This is typically achieved by adding specific reagents in a controlled sequence.
The solution is often treated sequentially to raise the pH to around 12, causing magnesium to precipitate as magnesium hydroxide. Following this, the highly purified lithium sulfate solution is concentrated through evaporation before the final product is precipitated. The choice of final product, either lithium carbonate or lithium hydroxide, depends on the prevailing market demand.
To produce lithium carbonate, the concentrated lithium sulfate solution is reacted with sodium carbonate, which precipitates the lithium as solid lithium carbonate. This reaction is often performed at high temperatures, near the boiling point of the solution, to maximize the yield and purity. Alternatively, the solution can be treated with sodium hydroxide to produce lithium hydroxide.
Lithium hydroxide is increasingly preferred for manufacturing high-nickel content cathodes used in modern electric vehicle batteries. Both the carbonate and hydroxide products are then filtered, washed with hot deionized water to remove any residual impurities, and dried to yield the final battery-grade material.