How does the Bayer process refine bauxite into alumina?

The Bayer process is the primary industrial method for refining bauxite into alumina, consisting of four distinct stages:1

The Four Stages of the Bayer Process

• Digestion: Ground bauxite is mixed with a sodium hydroxide solution and heated under pressure in digesters. This reaction creates a saturated solution of sodium aluminate, while insoluble impurities remain suspended.1
• Clarification: The mixture is depressurized, and insoluble residues—known as red mud—are separated from the sodium aluminate solution using sand traps, thickeners, and filters.1
• Precipitation: The clarified solution is cooled and pumped into precipitators. Aluminum hydroxide crystals are added as "seed" to encourage the formation of larger crystal agglomerates, which are then filtered and washed.1
• Calcination: The crystals are heated in kilns at temperatures exceeding 960 °C (1,750 °F). This process removes all water, resulting in commercially pure alumina, a fine white powder.1

This refined alumina is highly pure and serves as the essential raw material for producing metallic aluminum.1