Appeal No. 94-2858
Application 07/873,715
by digesting bauxite with alkali solution and precipitating
aluminum hydroxide from the digestion solution, comprising the
steps of:
(a) combining bauxite and an alkali solution
having a caustic concentration of at least 180 grams per liter
expressed as sodium carbonate and partially digesting the bauxite
in a first digestion step at a first digestion temperature of
from 80 to 155EC. to yield a first digestion product,
(b) separating said first digestion product of
step (a) into a liquid phase and a solid/liquid slurry, said
liquid phase having a reduced free caustic concentration in
relation to that of said alkali solution, and recovering said
liquid phase and said solid/liquid slurry as separate streams,
(c) preheating at least a portion of said liquid
phase stream having said reduced free caustic concentration and
combining said preheated portion of said liquid phase stream and
said stream recovered in step (b),
(d) digesting the resulting admixture of step (c)
comprised of said liquid phase formed in step (b) in a second
digestion step at an elevated second digestion temperature higher
than the first digestion temperature of step (a) of from 140 to
320EC. to yield a second digestion product,
(e) cooling said second digestion product by
recovering heat therefrom, said liquid phase stream being
preheated in step (c) with heat recovered from said second
digestion product,
(f) separating the second digestion product of
step (d) after being cooled in step (e) into a supersaturated
sodium aluminate solution and undissolved solids material;
(g) precipitating aluminum hydroxide from the
supersaturated sodium aluminate solution obtained in step (f) and
separating aluminum hydroxide from the resulting spent liquor;
and
(h) recycling said spent liquor to step (a) for
use as said alkali solution.
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Last modified: November 3, 2007