As mentioned above, the crystallization process is not finished with the
crystallization step itself. The created suspension still has to be separated,
the crystals have to be dried and packed. The vapours are to be condensed and
the non-condensable gases taken off by vacuum pumps. Below shows a simplified
flow sheet of such a complete crystallization plant operated for vacuum
evaporation crystallization.
Simplified flow sheet
Instead of the FC-type crystallizer any other type of crystallizer can be
installed. Also multiple-effect units can be taken. Or, instead of using steam
for heating it could be decided for mechanical or thermal vapour
re-compression. In this example the vapours from the (last) crystallizer are
condensed in a surface condenser. Also a mixing condenser could be chosen. The
suspension can be withdrawn by overflow, as shown. The suspension still is too
low concentrated regarding the best operating conditions for the centrifuge.
The suspension densities are between 15 to 25 wt.-% whereas centrifuges are
operated best at 50 to 60 wt.-%. Therefore, the suspension at first is
pre-concentrated in pre-thickeners or hydrocyclones. The clear liquor overflow
is recycled. Part of it always has to be taken as purge liquor for bringing out
the system impurities. The underflow is guided to the centrifuge for
separation. Depending on the CSD there is the choice between approx. 4 types of
centrifuges, the decanting-type and peeler-type for the finer products, the
sieve-worm-type and the pusher-type for normal products. The final drying of
the product in most of the cases is proceeded in draft-tube or vibrating
fluidized-bed dryers.