Alcoa combination process

The Alcoa combination process is a chemical method developed during World War II to process lower-grade bauxite ores that contain higher percentages of silica. This process reclaims alumina that has combined with silica and been filtered out with red mud. The red mud is heated with limestone and soda ash to create a sintered product containing leachable sodium aluminate. This product is then processed similarly to bauxite in the Bayer process to extract the sodium aluminate, leaving behind insoluble materials. The remaining slurry then follows the standard Bayer process steps. The waste residue from this combination process is known as brown mud.1