Bauxite / Alumina / Aluminium
Global Bauxite Resources and Production
Global bauxite resources are estimated between 55–75 billion tonnes, distributed as follows:
Formation And Occurrence
Bauxite is formed over millions of years through the weathering and leaching of pre-existing rocks or deposits. It develops in high-rainfall tropical and sub-tropical regions, leading to the accumulation of aluminium hydroxide minerals—gibbsite, boehmite, or diaspore—along with iron, titanium, silica, and, less commonly, phosphorus minerals. The appearance and texture of bauxite vary significantly, sometimes preserving the structure of the original rock.
Most of the world’s bauxite deposits are found near the surface and are extracted using open-pit, open-cut, or strip-mining methods. In Guinea, home to the world’s largest bauxite reserves, deposits are found on plateaus stretching over several square kilometers, containing tens of millions of tonnes of ore. In contrast, Jamaica’s bauxite deposits are located in karst depressions or sinkholes, which are smaller and typically yield tens of thousands of tonnes.
Bauxite Production in 2023
Bauxite to Alumina Conversion
Converting bauxite to alumina requires approximately 2.5 tonnes of dry bauxite to produce 1 tonne of alumina. Additionally, producing 1 tonne of aluminium metal requires 2 tonnes of alumina. Therefore, a total of 4–5 tonnes of bauxite is needed to produce 1 tonne of aluminium.
While most of the world’s bauxite is refined into alumina (Al₂O₃) via the Bayer Process and later smelted into aluminium metal using the Hall-Héroult Process, a small portion is used in manufacturing abrasives and refractory products.
The Bayer Process
The Bayer Process is the primary industrial method for refining bauxite into alumina (aluminum oxide). Developed by Carl Josef Bayer in the late 19th century,
This process is crucial in aluminum production, as alumina serves as the intermediate product before being further refined into pure aluminum.
Alumina is highly versatile, with industrial applications due to its high thermal conductivity, hardness, and resistance to chemical attack. The Bayer Process involves the following stages:
Digestion
Bauxite ore is crushed and mixed with a hot sodium hydroxide solution, dissolving aluminum oxide to form sodium aluminate. Depending on the bauxite composition, digestion occurs at either 145°C or 250°C.
Clarification
The mixture is allowed to settle, separating undissolved impurities.
Precipitation
The cooled sodium aluminate solution is seeded with aluminum hydroxide crystals, triggering aluminum hydroxide precipitation.
Calcination
The aluminum hydroxide is heated to 1000°C in stationary calciners, removing chemically bound water and producing alumina.
Chemical Grade Alumina (CGA)
While more than 95% of alumina is used for aluminum metal production via electrolysis, some grades Alumina Trihydrate (ATH) and Calcined Alumina are used in other industries. These variants are collectively termed Chemical Grade Alumina (CGA) and are divided into:
- Chemical Grade Hydrate (CGH).
- Chemical Grade Calcined Alumina (CGCA)
Also known as Specialty Hydrates and Specialty Aluminas, global CGA production is approximately 6 million tonnes annually, with around 50% used as CGH.
Applications of CGA
Uses of Chemical Grade Hydrate (CGH):
- Water treatment chemicals (Alum, Poly Aluminum Chloride) – 50%.
- Cost-effective fillers in plastics, sheet, or dough molding compounds.
- Fire retardant & smoke suppressant fillers in cables, rubber, and carpet backing
Uses of Chemical Grade Calcined Alumina (CGCA):
- Refractory applications (50%) – Shaped and monolithic, with medium to high alumina content, used both as an additive and base aggregate.
- Ceramics – High-tension insulators, spark plug insulators, alumina grinding media.
- Glass products – Including high-end Gorilla Glass.
- Abrasives – Grinding wheels, polishing compounds for steel and granite.
- Activated alumina – Used for fluoride control, catalyst carriers, and more.
Aluminium Reduction - Smelting
Hall–Héroult Process
The Hall–Héroult process is the primary industrial method for producing aluminum metal. It involves dissolving aluminum oxide (alumina) in molten cryolite and electrolyzing the molten salt bath in a specialized pot cell. These pots are electrically connected in series, forming a pot line.
At an industrial scale, this process operates at 940–980°C (1700–1800°F) and yields 99.5–99.8% pure aluminum.
Additionally, aluminum metal can be melted and recycled without requiring electrolysis. This recycling method is less energy-intensive and more cost-effective but does not remove alloys, meaning it cannot achieve the same purity as electrolysis.
Anode Technology
The Hall–Héroult process employs two primary anode technologies: Söderberg anodes and Prebaked anodes.
Söderberg Anodes (Self-Baking Anodes)
- Each electrolysis cell contains a single, self-baking anode held within a frame.
- The bottom of the anode reacts during electrolysis, forming CO₂, causing it to gradually sink as it loses mass.
- Fresh material (coke and pitch briquettes) is continuously added at the top, baking into carbon using heat from the smelting process.
- This method releases higher levels of PAHs (carcinogenic pollutants), leading to the decline of Söderberg anodes in favor of prebaked anodes.
- Alumina is added from the sides after breaking the crust on top of the electrolyte.
Prebaked Anodes
- These anodes are pre-baked in gas-fired ovens before use.
- Each electrolysis cell typically contains 24 anodes arranged in two rows.
- Computer-controlled adjustments lower each anode individually, ensuring optimal distance from the molten aluminum.
- This reduces resistance, improving efficiency over Söderberg anodes.
- However, prebaked anodes are costlier to implement and require more labor for removal and replacement.
- Alumina is added through point feeding between the anodes.
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