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9

PRAYON PROCESSES

In the late 1970s, the increase of oil prices

put pressure on Prayon to develop higher

strength processes, and two alternatives

were considered. First, the development

of a process with hemihydrate as the first

stage, to produce 43-46% P

2

O

5

acid, or

changing the operating conditions of the

existing Central-Prayon plant to achieve

a product acid strength higher than the

34-36% obtained at that time. This solu-

tion was intended to produce merchant

grade gypsum with similar qualities to

that obtained previously.

A single-stage HH process could not

achieve the gypsum quality required by

the downstream plaster producer, nor a

high enough efficiency to produce phos-

phoric acid economically at the inland

European site.

Although a two-stage hemihydrate-di-

hydrate process would have been more

efficient, the drying stage would have

been costly, compared to the self-drying

characteristics of hemihydrate which

cause the free water to be absorbed

during hydration.

A novel process was then developed, a

Hemi-Di-Hemi process with two recrystal-

lisation stages and two filtration stages.

This process was named the ‘Prayon

Hemihydrate 3-crystal process” or PH3,

and can produce a 46% P

2

O

5

acid with

over 98.5% process efficiency and high

quality calcium sulphate.

Under less stringent conditions, where

gypsum quality is not so critical, the final

stage of the PH3 process can be deleted,

leading to a hemihydrate-dihydrate pro-

cess. This type of process was developed

for licensing purposes and is known as

the “Prayon Hemihydrate 2-crystal pro-

cess” or PH2. This process can yield a

43-46% P

2

O

5

acid and has a process ef-

ficiency of over 98.5%.

In cases where lower efficiencies are ac-

ceptable, the removal of the dihydrate

stage leads to a single-stage hemihydrate

process known as PH1. This can yield acid

39-45% P

2

O

5

, with a process efficiency of

up to 95%. If lower strengths are accept-

able, efficiency can be increased. Alterna-

tively, higher strengths can be achieved

at the expense of efficiency.

COMPARISON OF PRAYON PROCESSES (TYPICAL VALUES)

Characteristics of the process

Mark 4

CPP

DA-HF

PH3

PH2

PH1

Solids product type

Dihydrate

Hemihydrate

Hemihydrate

Hemihydrate

Dihydrate

Hemihydrate

Product acid: % P

2

O

5

28.5

34 to 36

32 to 36

43 to 46

43 to 46

39 to 45

Product acid: % SO

3

1.5

0.6 to 1.2

2 to 3%

0.6 to 1.2

0.6 to 1.2

0.6 to 1.2

Efficiency %

95 to 96

> 98.5

97 to 98

> 98.5

> 98.5

92 to 95

Analysis of the calcium sulphate

(On dry basis 50°C)

Free H

2

O %

18 to 20

14 to 20

14 to 20

14 to 20

16 to 20

14 to 20

(On dry basis 250°C)

Crystal H

2

O %

20.5

6.2 to 6.5

6.2 to 6.5

5.4 to 5.8

18 to 19

6.5

Total P

2

O

5

%

0.8 to 1

0.25 to 0.35

0.35 to 0.65

0.15 to 0.25

0.25 to 0.35

1.2 to 1.8

W.S. P

2

O

5

%

0.2 to 0.3

0.1 to 0.15

0.1 to 0.2

0.05 to 0.1

0.05 to 0.1

0.2 to 0.3

UNR. P

2

O

5

%

0.05 to 0.1

0.05 to 0.1

0.05 to 0.1

0.05 to 0.1

0.05 to 0.1

0.05 to 0.1

Cocryst P

2

O

5

%

0.5 to 0.6

0.1 to 0.2

0.1 to 0.2

0.05 to 0.1

0.15 to 0.25

0.8 to 1.4

CaO %

39.8

39.7

39.7

40.6

39.8

39

SO

3

%

56.9

57

57

58

57.4

54.9

F%

0.5 to 0.8

0.4 to 0.6

0.4 to 0.6

0.1 to 0.2

0.5 to 0.7

1 to 1.2

Na

2

O%

0.4 to 0.8

0.3 to 0.6

0.3 to 0.6

0.1 to 0.2

0.3 to 0.6

1 to 1.2