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OPTIMISING DIGESTION FLASH TANK DESIGN FOR THE ALUMINA INDUSTRY

Haneman, B. and Wang, A.

The digestion unit of the alumina refinery incorporates a number of flash tanks to depressurise to atmospheric conditions a boiling three phase slurry from pressures of approximately 30 to 40 Barg for a high temperature digestion unit or 1.5 to 2 Barg for a low temperature unit. The steam released from the flash tanks is then used to preheat the incoming liquor or slurry on its passage to the digestion conditions.

Various equipment designs have been utilised for the digestion flash tank over the last half century. These can be generally categorised into top or side entry, and bottom entry. Each design, and variation thereof, seeks to attain a controlled depressurisation of the slurry such that a relatively quiescent disentrainment of the liberated steam from the boiling liquor and solids is achieved. This serves to maintain good thermal performance of the recuperative heat transfer equipment.

This paper provides a brief review of some of these equipment designs and their inherent strengths and weaknesses. A new design of flash tank developed by Hatch Associates is also presented. This new flash tank design offers the industry the collective benefits of those designs historically used without their inherent weaknesses.