Two systems of recovery are in general use, which deserve a brief notice:—
Direct Evaporation.—The liquors may be evaporated to a small bulk ready for incineration by treatment in long shallow pans or furnaces, the heat necessary for the process being obtained mainly from the combustion of the thick concentrated liquor. The most familiar type of this form of apparatus is the Porion evaporator.
Fig. 23.—Scott's Multiple Effect Evaporator.
The combustion of the concentrated liquor is started by a coal furnace at one end of the apparatus. The thick viscous mass catches fire and burns with a fierce flame, and the heat is utilised in evaporating the weaker liquors which flow continuously through shallow brick troughs, the surface of which is freely exposed to the heat and flames from the hearth where the organic soda compounds produced in the boiling of esparto are being incinerated and converted into soda ash.
Under suitable conditions this evaporator is most economical in its results. It can be erected cheaply, and when all the heat is fully used in every possible direction it can be worked at a low cost compared with the more modern multiple effect evaporators.
Vacuum Multiple Effect Evaporation.—Advantage is taken of the fact that water boils at a lower temperature in a vacuum than at the ordinary pressure of the atmosphere. There are many forms of apparatus based on this principle, amongst which the most recent is Scott's evaporator. The black liquor from the boilers is pumped through tubes heated externally by high-pressure steam. The liquor is passed into a chamber in which a slight vacuum is maintained, so that immediately on entering, the liquor parts with a good deal of water in the shape of steam. The steam liberated is utilised in producing further evaporation of the partially concentrated liquor, and this operation is repeated several times until the concentration is effected to the desired point.
In most cases the actual incineration of the thick liquor is carried out in a rotary furnace when such an apparatus as this is used.