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19

PRAYON PROCESSES

This process is divided into three stages:

Stage 1

α-

hemihydrate attack-filtration:

The phosphate rock is digested in con-

ditions which ensure the production of

43-46% P

2

O

5

acid with low SO

3

(± 1%).

The control of the operating conditions

allows the production of hemihydrate

which will filter rapidly and lend itself to

full rehydration during the second stage.

Stage 2

Conversion of

α-

hemihydrate to dihydrate:

By changing the operating conditions (tem-

perature, % P

2

O

5

, % SO

3

),

α-

hemihydrate

is converted into dihydrate in order to

achieve a first purification, especially of

unreacted and cocrystallised P

2

O

5

.

Stage 3

Conversion of dihydrate to

α-

hemihydrate

and final filtration:

Operating conditions aremodified to trans-

form the dihydrate into

α-

hemihydrate.

The slurry produced is then filtered. This

step is similar to the second part of the

Central-Prayon Process and aims at produc-

ing a high purity calcium sulphate which

is separated by filtration and washed with

water. If requested, the hemihydrate can be

processed further (neutralisation, rehydra-

tion to produce a dry solid) to produce a

substitute for natural gypsum.

THE MAIN CHARACTERISTICS OF

THE PROCESS ARE

+

high P

2

O

5

recovery (> 98.5%)

+

high-strength phosacid

+

self-drying gypsum

The last two points reduce energy con-

sumption.

RECOMMENDED FOR LOCATIONS

+

with medium - high-cost rock

+

with high-cost energy

+

without easy gypsum disposal