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15

PRAYON PROCESSES

THE CENTRAL-PRAYON PROCESS

The Central-Prayon Process was developed

to produce a calcium sulphate (phospho-

gypsum) which could replace the natural

gypsum in different applications.

This process is a dihydrate-hemihydrate

process. During the first stage, slurry con-

taining dihydrate crystals is produced. From

that flow, the quantity corresponding to the

product acid is sent to storage,the remaining

quantity being sent with the solids to the

conversion tank. In this reactor, sulphuric

acid and steam are added to transform the

dihydrate solids into hemihydrate liberating

most of insoluble losses.The slurryproduced

is then filtered and the cake washed. All

filtrates from the second filter are recycled

to the reaction tank, while solids can be

neutralised and used as a raw material for

gypsum products.

THE MA IN CHARACTER I ST I CS OF

THIS PROCESS ARE

+

it produces a higher acid strength than

DPP with much higher P

2

O

5

recovery

+

it can process both types of phosphates:

sedimentary and igneous

+

it produces CaSO

4

.1/2H

2

O which is self-

drying due to its chemical properties

and is purer than dihydrate, and can

therefore be used directly as a merchant

grade raw material (phosphate selec-

tion may be important)

RECOMMENDED FOR LOCATIONS

+

with medium - high-cost rock

+

with high-cost energy

+

with disposal limitation and/or with a

potential market for this purer gypsum

Phosphate

Flash Cooler

Steam

Water

Phosphoric Acid 32 to 36% P

2

O

5

Pure Calcium

Sulphate

Hemihydrate

H

2

SO

4

Reaction

Recycled Acid

Digestion

DIHYDRATE

HEMIHYDRATE

Vacuum

Separation filter

Hemihydrate conversion

Hemihydrate

Filter