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ALUMINIUM SMELTING INDUSTRY
Fig. 7: Basic flow
chart for seawater
neutralisation of
red mud [4]
Fig. 6: Tube digestion [3]
It goes without saying that the aim of every
refinery is to become a low-cost producer, and
so must examine the major factors influencing
the first-quartile costs:
• Proximity to the bauxite mine
• Proximity to a deepwater harbour
• Low energy cost
• Low soda cost
• Low energy, caustic and lime
consumptions.
The first four factors depend on the location
selected as well as on available infrastructure.
The final factor depends on the bauxite mineralogy
and on selection of the technology
to be applied. Fig. 4 shows the bauxite entering
the grinding step at ambient temperature,
where it is then mixed with the spent liquor.
The ground suspension is heated to its digestion
temperature, which is determined by the
bauxite’s composition: gibbsitic bauxites need
low temperatures, while boemitic bauxites require
higher temperatures. The digested slurry
is then rapidly cooled and the green liquor
(alumina-rich solution) is separated from the
red mud. In the same figure, heat recovery is
maximised so as to minimise the addition of
live steam needed to bring the fresh slurry to
the required digestion temperature. A similar
principle can be seen when the incoming green
liquor is indirectly cooled with outgoing spent
liquor prior to precipitating alumina tri-hydrate
in the next stage. The hydrate is then fed
to the calcination stage. The figure also makes
it easy to distinguish between the white and
red sides of an alumina refinery production
site.
venture with Hatch – HOT (Hatch
Outotec) – can supply the entire
digestion tool box. The joint venture focuses
on designing and developing integrated tube
digestion and evaporation solutions using single-stream
heating in jacketed pipe technology.
This allows you to efficiently recover the
heat in the digested slurry. Fig. 6 shows the
slurry as it is heated to the digestion temperature
and then held for a time in tube reactors
to meet the reaction requirements. Next, the
slurry is flash cooled in stages to near- ambient
pressure, while using the heat from vapour to
the pre-heat incoming slurry in counterflow.
The water vapour transfers its heat to the
slurry as it condenses on the outside shell of
the jacketed pipes.
Red mud disposal: sea
water neutralisation
The residue from the Bayer process – more
commonly known as red mud – still has a
high alkaline content and must be neutralised.
Fig. 7 illustrates the basic flow
chart for neutralisation by magnesium in the
sea water. The mud / seawater mixture is held
in a reactor so that the caustic is chemically
neutralised. The hydrotalcite-rich mud and
magnesium-deficient seawater are then decanted
using a conventional clarifier.
The supernatant magnesium-deficient seawater
with the correct permits is now suitable
for environmental discharge.
The small facility shown in Fig. 8 can treat
the entire rate of red mud production from
a refinery producing four million tonnes of
product per year to environmentally acceptable
levels. The magnesium actively reacts
with the liquor phase components of red mud;
namely aluminum and hydroxide ions. During
the neutralisation process, the dissolved
magnesium levels drop from approx. 1,200
mg/l to around 300 mg/l and the aluminum
level drops to less than 5 mg/l. These net reductions
of magnesium and aluminum are the
Digestion and evaporation
The appropriate equipment for digestion must
be selected based on the kinetics of the digestion
reactions, bauxite type and target heat
recovery. Available technology for the digestion
process includes autoclaves and tube
digestors, which are selected according to
throughput, number and size. Outotec’s joint
Fig. 8: Neutralisation facility [4]
36 ALUMINIUM · 1-2/2013