One other substance that would be in the 'red mud' is sodium hydroxide (NaOH). Sodium hydroxide is commonly used in the Bayer process, which is used to extract alumina from bauxite ore. When bauxite ore is processed using the Bayer process, sodium hydroxide is used to dissolve the alumina and create a solution of sodium aluminate. This solution is then treated to precipitate the alumina, leaving behind a residue known as 'red mud'.