In an alumina refinery the spent liquor from the precipitation stage has to be re-concentrated before it can be recycled to the bauxite digestion. Different evaporative techniques are utilised for this concentration, whereby each system has its own advantages as well as limitations.
Today, the choice is essentially between multiple effect flash or falling film evaporator systems. Rising film evaporators, which found ready acceptance in the past, have been essentially discontinued due to their lack of operating flexibility, whilst forced circulation evaporators are normally only used when salts precipitate during the concentration process.
The aim of this presentation is to examine the flash and falling film systems based upon a spent liquor feed concentration of Na2O total 185 g/l (equivalent Na2CO3 316 g/l), which is typical for the spent liquor feed to many evaporation plants and then analyse the heat transfer areas required in order to achieve a range of green liquor concentrations in the above mentioned range.
This article looks at the most important differences between the flash and falling film systems and the highlights the advantages and disadvantages on the basis of the main operating characteristics, which are can be summarised under the following main headings, heat transfer area required, steam economy achievable, concentration gain needed and electrical power consumption.