Comments on Reference Document on BAT
in the Non Ferrous Metals Industries
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“Techniques to consider in the determination of BAT”
Circulating fluid bed technology for calcination of aluminium trihydrate as final step in the production
of alumina
Description:
The circulating fluid bed technology ensures minimum fuel consumption by using the counter flow principle
of gas and solids. Wet aluminium trihydrate (Al2O3 * 3 H2O) is conveyed to the calcination plant (see Figure
1), passes a hydrate feed bin and is then delivered (1) into the first venturi preheater (2). There the solids are
intensively mixed with the waste gas, which leaves the cyclone (3) of the second drying stage. The pre-dried
hydrate is entrained with the waste gas and conveyed into the electrostatic precipitator (4), where the waste gas
is cleaned and released to the atmosphere. The solids precipitated are conveyed pneumatically to the airlift
cyclone, and then to a second venturi preheater (5). Part of the solid stream coming from the first drying stage
can be bypassed to the mixing pot (6). The hydrate entering the second venturi preheater (5) of the second
drying stage is mixed and dehydrated with the hot waste gas leaving the recycling cyclone (7). From cyclone
(3), the pre-calcined alumina is delivered into the fluid bed furnace. The waste gas leaving the cyclone of the
second drying stage is conveyed to the first venturi preheater. The final calcination of the preheated and partly
dehydrated hydrate takes place in the fluid bed furnace (8). The heat required for calcination is generated by
direct combustion of fuel in the fluid bed. As fuel coal gas, natural gas or oil can be used. Part of the required
combustion air is introduced through the nozzle grate as primary air, the remaining air is added above the grate
as secondary air. The hot alumina, which is separated in the recycling cyclone (7) passes through the seal pot
(9) and re-enters the fluid bed furnace. The alumina, which is discharged from the calcining stage, is cooled
down to the required outlet temperature in two liftducts (10, 11) of the secondary air cyclones (12, 13) and in a
fluid bed cooler (14). The cooler is separated in an air-cooled section heating-up the primary combustion air
and in a water-cooled section for the final cooling of the alumina. The remaining heat in the water-cooled part
of the fluid bed cooler can be utilised for heating-up of boiler feed water or filter wash water for the hydrate
filtration.
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Figure 1: Calcination of aluminium hydrate in a circulating fluid bed calciner
EG, 30.05.2006
Print 12.02.2016
Comments on Reference Document on BAT
in the Non Ferrous Metals Industries
Main environmental benefits:
2/2
Reduction of fuel consumption due to recovery of product
and waste gas heat.
Operational data (depending on flowsheet details and feed and product properties):
Fuel consumption:
2600 to 2900 kJ / kg Al2O3
Electric power consumption:
15 - 20 kWh / t Al2O3
Emission level of dust using an ESP:
typically < 50 mg/mn3 (wet basis)
Emission level of dust using a hybrid filter:
typically < 20 mg/mn3 (wet basis)
Cross media effects:
No cross media effects
Economics:
Circulating fluid bed calciner has lower operating costs than
other systems
Applicability
Applicable to grass root and brown field
Reference literature
Schmidt, H.W., and W. Stockhausen. 2002. Latest Developments in Circulating Fluid Bed Calcination based
on Operating Experience of Large Calciners, Proceedings of 6th International Alumina Quality Workshop, 813 September 2002, Brisbane, Australia.
Brodie, D. J and H.W. Schmidt. 1999. Custom Designed Fluid Bed Calciner for Nabalco Pty Ltd.
Proceedings of 5th International Alumina Quality Workshop, 21-26 March 1999, Bunbury, Australia.
Schmidt, H.W., Stockhausen, W. and A. N. Silberberg. 1996. Alumina Calcination with the Advanced
Circulating Fluid Bed Technology, in Light Metals 1996, edited by Wayne Hale. TMS, Pennsylvania (USA)
EG, 30.05.2006
Print 12.02.2016