Why is sodium hydroxide used to process bauxite?

In the Bayer process, sodium hydroxide—also known as caustic soda—is used to refine bauxite ore into alumina.1

During the digestion step, the bauxite is ground and mixed with a sodium hydroxide solution under high heat and pressure. The sodium hydroxide reacts specifically with the aluminous minerals in the bauxite to form a saturated solution of sodium aluminate. This chemical reaction is essential because it allows the aluminum-bearing components to dissolve into the solution, while the insoluble impurities—collectively known as red mud—remain in suspension.1

By separating these impurities from the dissolved sodium aluminate, the process ensures that the final alumina product is of sufficient purity for commercial use, preventing contaminants like iron oxide, silica, and titania from interfering with the subsequent smelting process.12