In the complex production process required to convert spodumene concentrate into lithium compounds (lithium carbonate or lithium hydroxide), calcination plays a key role in not just one process, but two, making it An essential technology for developing this increasingly important lithium deposit.
While extracting lithium from brine has been the main focus of commercial efforts, spodumene is becoming increasingly popular due to its high lithium content and surging demand for lithium-ion batteries.
Calcination, often called roasting in this context, is one of the most commonly used processing techniques throughout the process of extracting lithium from spodumene.
Calcination is a heat treatment technique widely used in various industries to induce chemical reactions or physical changes in materials. Most commonly, it refers to the chemical dissociation of components in a material, such as the separation of calcium carbonate to form calcium oxide and carbon dioxide. Calcination typically takes place in an inert environment.
In the production of lithium compounds from spodumene concentrate, calcination is used to perform two separate steps in the process: decrepitation and acid roasting.
Spodumene ore naturally has a monoclinic α-form (α-form) crystal structure. However, in order to extract lithium from an ore through the leaching process, the crystal structure of the ore must be tetragonal β-type (β-type). This transformation is achieved by dehiscence, or shattering of the crystal structure.
Detonation, facilitated by calcination, expands the crystal structure of spodumene by about 30%, ultimately allowing the chemically inert ore to react with sulfuric acid.
When processing spodumene concentrates, calcination at temperatures of 1075 – 1100 C causes delamination. Temperature control during calcination is critical to the success of the process; if temperatures are allowed to approach 1400°C, undesirable structures (called eutectics) can form between alpha-spodumene and other silicates.
Since α-spodumene ore is not sensitive to combustion products, direct fired rotary kilns are used in this case to process the material. Direct fired kilns utilize direct contact between the material and the process gas to heat treat ore. The kiln is set at a slight angle to allow gravity to help move the material through the drum.
Once the material detonates or transforms into beta-phase spodumene (β-type), an additional calcination step is required. The concentrate, now in beta form, is first mixed with sulfuric acid (a step called sulfuric acid digestion) before being sent to a separate rotary kiln. This acid roasting step uses a kiln that operates at a much lower temperature – usually around 250 degrees Celsius.
The purpose of the acid roasting step is to allow lithium to be extracted from the mixture in the form of water-soluble lithium sulfate, which is easily leached. Lithium sulfate can be converted by leaching into the marketable compounds lithium carbonate or lithium hydroxide.
Unlike kilns used for blasting, this type of kiln is configured indirectly and is often called a calciner. The indirect configuration is used because beta-phase spodumene cannot be exposed to combustion products.
Indirect fired kilns use external heating to keep materials and combustion products separate. Material heated by direct contact with the shell is rolled along the inside of the drum, exposing fresh material as it tumbles to promote even heat distribution throughout the bed, sometimes with the help of bed turbulence.
Calcination is an important tool for converting spodumene concentrates into lithium compounds, with two key roles in the process. Through calcination, detonation and acid roasting can be achieved to produce lithium carbonate and lithium hydroxide for lithium-ion batteries or other applications.
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