How alumina is made

Alumina is white aluminum oxide pulled out of red bauxite with hot caustic soda, and the red leftovers stay behind.

stations: 6
things upstream: 6
raw sources: 4
steps from the ground: 2

The short version

Alumina is refined from bauxite by the Bayer process. Crushed ore is cooked under pressure with a hot sodium hydroxide solution, which dissolves the aluminum and leaves iron-rich red residue behind. The clear liquid is cooled and seeded so aluminum hydroxide crystals grow, and a kiln at about 1,200 °C then bakes the crystals into dry alumina powder.

Grind the bauxite

Bauxite contains only 30 to 60 percent aluminum oxide, with the rest a mix of silica, iron oxides and titanium dioxide. It is ground so the hot liquor can reach the aluminum minerals.

Goes in
Bauxite
Digest it in hot caustic

The ground bauxite is heated in a pressure vessel with a strong sodium hydroxide solution. The aluminum dissolves as sodium aluminate. Silica dissolves too, and lime is sometimes added to help knock the silica back out.

150 to 200 °C

Goes in
Caustic soda Quicklime
Filter out the red mud

What does not dissolve, mostly iron oxide, silica and titania, is filtered from the liquid with the help of a flocculant such as starch. This residue is called bauxite tailings, or red mud.
Seed the liquor to crystallize

The clear solution is cooled and fine crystals of aluminum hydroxide are added as seeds. New crystals grow on them. Without seeds, precipitation can take several days.
Wash and filter the crystals

The aluminum hydroxide is separated from the liquor, which is recycled to digest more bauxite. Recycling lets gallium and vanadium build up in the liquor until they are worth extracting.

Goes in
Water
Bake it in a kiln

About 90 percent of the aluminum hydroxide goes into a rotary kiln or flash calciner heated to around 1,200 °C. The water leaves and fine white alumina powder remains.

about 1,200 °C

End of the lineAlumina Next it goes into aluminum, smartphone cover glass, laminate flooring and 1 more.

Worth repeating

The Bayer process is also the main source of the metal gallium, as a byproduct.
Some of the aluminum hydroxide never goes to the kiln, and is used for water-treatment chemicals and as a fire-retardant filler in rubber and plastics.
Red mud dries into a fine powder of iron, aluminum, calcium and sodium, and refiners keep looking for uses for it. Ceramics is one.