(3) The alumina processing flow is described in the following paragraphs.
Refer to Figure 2-9 as you read the following steps in the production flow.
(a) Bauxite is unloaded onto open stockpiles or stored in bins inside a
large, long, low shed. A conveyor transports the ore to the crushing and grinding
building where It is reduced to a fine powder.
(b) The powdered bauxite is conveyed to the digester building where it is
mixed with concentrated caustic soda and cooked under steam pressure in an
autoclave building.
The concentrated caustic soda is stored in closed tanks
adjacent to the digester building, and steam is supplied by a boiler house located
nearby.
Under the influence of heat and pressure the alumina dissolves from the
ore and chemically bonds with water to form aluminum hydroxide.
This then goes
into solution with the caustic soda to create a sodium aluminate liquor. The waste
impurities, primarily silica, and ferrous oxide, stay in solid suspension.
(c) The liquid is piped to the filter press building where porous metal
filters remove the suspended impurities.
(d) The impurities are piped into settling tanks where water and caustic
soda are recovered for reuse.
The waste (red mud) is pumped to a waste pond and
discarded.
(e) The liquid is passed to the precipitator building comprised of a series
of large size and tall silo tanks and a large rectangular structure which is
usually covered with a sawtooth roof.
Aluminum hydroxide is precipitated out of
the sodium aluminate liquid inside these tall silo tanks.
Small granules of
aluminum hydroxide (seed crystals) are introduced into the tank at the top and as
they drift down, cause aluminum hydroxide to crystallize out of the solution and
settle at the bottom.
The seed crystals are supplied from seed tanks located
adjacent to the precipitation building. Regularly, precipitated aluminum hydroxide
crystals are drawn from the silo tanks and washed in settling tanks prior to being
sent to the rotary kilns for drying.
The caustic soda solution remains in the
precipitator tank until it is needed for reuse.
(f) After being washed in settling tanks, the aluminum hydroxide crystals
are conveyed to the rotary kilns. These are long, pipe-like furnaces mounted on an
incline and slowly rotated on their long axis. Aluminum hydroxide is introduced at
the upper end and is slowly tumbled down its length, drying, and in the process
changes from a hydroxide to an oxide by the loss of oxygen and hydrogen. Alumina
is taken out of the bottom of the rotary Kiln and sorted in silos or in an alumina
storage building.
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IT 0673
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