A process for manufacturing alumina, Al₂O₃. The raw material, such as clay or anorthosite, is sintered with limestone and soda ash to form sodium aluminate and calcium silicate. This sinter is then leached with water, caustic soda solution, or sodium aluminate liquor to dissolve the soluble sodium aluminate. The resulting slurry is then filtered, and the liquor is decomposed as in the Bayer process or is treated with carbon dioxide to precipitate hydrated alumina. When operated in conjunction with the Bayer process to recover alumina and soda from red mud, it is called the combination process.