Page 509 - Industrial Wastewater Treatment, Recycling and Reuse

Simulation, Control, and Optimization of Water Systems in Industrial Plants 479

Figure 12.5 Bayer process.

In the Bayer process, bauxite is mixed with a side stream of returning
spent caustic liquor and ground down to a size appropriate for extraction.
The slurry is then preheated and held in storage tanks to facilitate silica
removal (desilication). The main spent liquor stream is also preheated and
mixed with the slurry stream. The combined stream is further heated,
and the slurry is digested at this elevated temperature under pressure to
extract the available alumina from the bauxite. After digestion, the slurry
of green liquor (rich in alumina) and inert solids is cooled in a series of flash
tanks and reduced to atmospheric pressure. The heating medium for slurry
and liquor is a combination of direct and indirect steam, or flash vapor. Thus
condensate is produced in this area. The cooled slurry is clarified to remove
the unextractable solids from the circuit, which are washed to recover the
associated soda.
Further cooling of the green liquor takes place on its way to precipita-
tion, which is favored at lower temperatures, by heat interchange with the
spent liquor returning from precipitation. This is again done in a series of
flash tanks and heat exchangers, producing more condensate.