Double digestion system to reduce formation of ferrous iron

摘要

Bauxite is reacted with spent caustic soda solution at 113° C. to 205° C. to produce a first pregnant liquor stream, a granular residue stream and a muddy substance stream wherein the granular residue stream is discarded, the muddy substance stream is reacted with or without a small portion of bauxite, but with a spent caustic soda stream at 206° C. to 350° C. to produce a second pregnant liquor stream and a red mud stream which is discarded; the first and second pregnant liquor streams are filtered, cooled to 50° C. to 87. degree. C. and then seeded to precipitate alumina hydrate containing less than 0.017% Fe. sub.2 O.sub.3 by weight.