What happens during digestion in the Bayer process?

In the Bayer process, digestion is the initial stage used to extract alumina from bauxite ore. During this step, the bauxite is ground and mixed into a slurry with a sodium hydroxide (caustic soda) solution.1

This mixture is then pumped into large pressure tanks known as digesters. Inside these tanks, the ore is subjected to high levels of steam heat and pressure. This environment causes the sodium hydroxide to react with the aluminous minerals present in the bauxite, resulting in the formation of a saturated solution of sodium aluminate. Meanwhile, insoluble impurities—collectively referred to as red mud—remain in suspension to be separated during the subsequent clarification stage.1