Almost every aluminium can, window frame and aircraft panel began as bauxite. Two industrial processes, both invented in the 1880s, still carry it there: the Bayer process, which makes alumina, and the Hall–Héroult process, which turns alumina into metal.
A commonly cited rule of thumb: roughly four to five tonnes of bauxite make about two tonnes of alumina, which make about one tonne of aluminium. The exact ratio depends heavily on the ore, which is why refiners are so particular about what they buy.
Stage one: the Bayer process
Patented by Carl Josef Bayer in 1888, the Bayer process dissolves the aluminium minerals in bauxite and leaves the rest behind.
- Grinding. Bauxite is crushed and wet-ground with caustic soda liquor into a slurry.
- Desilication. The slurry is held at temperature so that reactive silica, mainly from kaolinite clay, precipitates early as a sodium aluminosilicate rather than contaminating the product.
- Digestion. In pressure vessels, hot caustic soda dissolves the alumina as sodium aluminate. Gibbsitic ore digests at around 140–150 °C; boehmitic ore needs considerably higher temperatures.
- Clarification. The undissolved solids, mostly iron oxides, titania and the desilication product, settle out as bauxite residue, often called red mud.
- Precipitation. The cooled, clear liquor is seeded with fine crystals, and aluminium hydroxide grows out of solution.
- Calcination. The hydroxide is heated to around 1,000 °C or more, driving off water and leaving white, powdery alumina, Al₂O₃.
The caustic liquor is recycled back to the start. Anything that consumes it, above all reactive silica, adds directly to cost.
Stage two: the Hall–Héroult process
Discovered independently by Charles Martin Hall and Paul Héroult in 1886, this process uses electricity to split alumina into aluminium and oxygen.
- Alumina is dissolved in molten cryolite at roughly 950–960 °C inside a carbon-lined steel cell, or "pot".
- A large direct current passes from carbon anodes through the bath.
- Molten aluminium collects at the bottom of the cell and is siphoned off; oxygen reacts with the carbon anodes.
Smelting is extremely electricity-intensive, which is why smelters are built near reliable power. But smelters only see alumina. It is the refinery that feels the quality of the bauxite.
Which ore properties change the economics
| Ore property | Effect at the refinery |
|---|---|
| Available alumina | More product per tonne of ore mined, shipped and ground |
| Reactive silica | Lost caustic soda and lost alumina in the residue |
| Mineral form (gibbsite vs boehmite) | Digestion temperature, and so energy use |
| Iron and titania | More residue to handle and store |
| Organic carbon | Liquor contamination and lower precipitation yield |
| Lot consistency | Steadier digestion and fewer upsets |
This is why metallurgical grade bauxite is bought on detailed specifications, and why gibbsitic ore with low reactive silica is so valued. Our posts on reactive silica and on gibbsite, boehmite and diaspore go deeper into both.
What happens to the residue
Bauxite residue is the largest by-product of the industry, often a tonne or more per tonne of alumina. Modern refineries increasingly use dry stacking rather than wet ponds, and research continues into using residue in cement, bricks and the recovery of iron and other metals. Lower-impurity bauxite produces less of it in the first place.
The takeaway
The smelter makes the headlines, but the refinery makes the margin, and its margin starts with the ore. Bauxite that is gibbsitic, low in reactive silica and consistent from lot to lot travels more cheaply through every stage between the mine and the metal.