Bayer alumina calciners produce large flows of flue gas at typically 150°C to 200°C and 45% to 50% by volume of water vapour. The flue gas also contains traces of fine electrostatically charged alumina dust making it undesirable for direct contact with heat transfer surfaces.
Alcoa have developed a process to recover a large fraction of the sensible and latent heat in the flue gas, and applying the recovered energy to evaporate Bayer spent liquor. This evaporation is normally performed in the Bayer process using custom built evaporators, and using steam produced in powerhouse boilers.
The process which has been developed uses counter current contact of water with the calciner flue gas, heating the water to the flue gas dew point. The water is then cooled in the shell of a falling film evaporator, exchanging heat with evaporating Bayer spent liquor on the tube side.
In addition to the spent liquor evaporation benefit, the process also removes the dust and some volatile organics from the flue gas, and has the potential to eliminate the visual plume, which is characteristic of Bayer calciners. By using air cooled heat exchangers to condense the water evaporated from the spent liquor, significant water recovery is also enabled.
Details of the process are described in patent number WO 2008/034196 A1.