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11

PRAYON PROCESSES

LOW LEVEL FLASH COOLER (LLFC)

The reaction of phosphate with sulphuric

acid and the dilution of sulphuric acid are

exothermic. To avoid boiling inside the

reactor and to obtain the desired calcium

sulphate crystals (gypsum or hemihy-

drate), the reaction slurry must be cooled.

From the beginning, flash cooling has

been a part of the Prayon process. This

has proved to be more accurately control-

lable than air cooling, especially when

the cooling rate must exceed nominal

capacity.

The principle of operation is the follow-

ing: the LLFC is a vacuum chamber into

which the hot slurry is pumped. The water

in the slurry begins to boil, causing water

to evaporate from the slurry and thus

cooling the slurry. In order to reduce scal-

ing, the temperature difference between

the inlet and the outlet of the LLFC is

low (about 2°C or 4°F). This means that

for efficient cooling the flow through the

LLFC must be very large. This is ensured

by a high flow rate axial flow pump with

a low head and a low power consumption.

REACTOR AND LOW LEVEL FLASH COOLER