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THE CENTRAL PRAYON PROCESS CENTRAL PRAYON PROCESS FEATURE INTRODUCTION The Central-Prayon process for the production of phosphoric acid by sulphuric attack of natural phosphates is a m

THE CENTRAL PRAYON PROCESS

CENTRAL PRAYON PROCESS FEATURE

INTRODUCTION

The Central-Prayon process for the production of phosphoric acid by sulphuric attack of natural phosphates is a modern process giving a high P2O5 yield and due to this, a calcium sulphate at low P2O5 content which can compete with the natural calcium sulphate for many applications in function of Phosphate origin, such as the manufacture of plaster products, the production of ammonium sulphate, clinker and sulphuric acid, the addition to clinker as cement retarder, etc

Like most of the highly efficient processes of the chemical and metallurgical industries, the Central-Prayon process is a two stages process; in a first stage, the raw materials are fed to the reaction under such conditions that the main reaction product is obtained in a form offering the finest range of qualities, even if the by-products obtained are not completely exhausted; the latter are then separated from the main product and treated again during a second stage under more aggressive conditions so as to completely extract the main product The resulting liquors are recycled together with the raw materials to the reaction of the first stage, whereas the by-products, which are now free from the main product, may be commercialised

As a matter of fact, the Central-Prayon process originates from the utilisation of the calcium sulphate, by-product of phosphoric acid manufacture Soon after 1960, Société de PRAYON has developed for Japanese users of the classical Prayon process, a purification process for the residually dihydrate calcium sulphate which was difficult to utilise just as it was On the other side, the company was conducting research to improve the P2O5 extraction efficiency and both goals were reached simultaneously by a two-stage process

At the same time, the Japanese Company, Central Glass Company, starting with a classical Prayon process plant has directed their research towards maximum valorisation of the calcium sulphate in order to meet the requirements of the Japanese market short of natural gypsum Their research work also resulted in a two-stage di-hemihydrate process

Thus, without consulting each other, both companies have developed a process with very similar characteristics

After some laboratory and pilot plant tests, the existing classical Prayon plant in Engis has been transformed and started production, in 1965, per the new process, with a capacity of 30 – 50 T/d P2O5, while today the revamped plant produces 500/550 T/d P2O5

On their side, Central Glass Company have started in 1967 a 75 T/d P2O5 plant which presently turns out about 200 T/d P2O5

Both companies decided to valorise the results of their respective research work and signed a co-operation agreement, by which the commercialisation of what is called the Central Prayon process is made exclusively by Prayon and its general licensees

This agreement is thus based upon patents taken out by the two companies in most of the industrial and agricultural countries

In 1980, there were, over the world, nine commercial plants based on the Central-Prayon process Nowadays, five are still active: one in Belgium and four in Japan

• Acid storage of weak and concentrated phosphoric acid

• Storage of fluosilisic acid

• Gas scrubbing

• Gypsum handling and grinding Gypsum quality is suitable for cement

PROCESS FEATURES

Easy and Reliable Operation

Part of the unit run with a dihydrate process It gives relatively low temperatures and concentrations which reduce the demands for high grade materials of construction, reducing maintenance load significantly

Only the hemihydrate portion need special material but this section have a limited size

Compact Design and High Reliability

Compact design and high reliability due to ample experience in the engineering, construction and operation of phosphoric acid plants

Process Flexibility

The Central Prayon Process has operated on various type of phosphate available world-wide along with a unrivalled experience of the construction of plants based on pilot-plant operation of unknown or little known phosphates

Optimum Construction Materials

Optimum construction materials for phosphoric acid and slurry duties have been developed and tried in industrial plants and in pilot-plant studies

Operational Stability

The control of the plant is well proven and the reaction section operates automatically with minimum operator input

Better Acid Quality and Filtration

A digestion section is used to mature the slurry before filtration This de-supersaturation ensures minimum scaling on the filter It also reduces to a minimum the down time required for the washing of the dihydrate filter circuit normally a weekly wash is applied

Hemihydrate filter is specially designed to reduce down time for washing Its piping arrangement and the design of the cells dramatically reduce the washing cycles Design of hemihydrate piping allows washing without plant stop

Process Specific Agitator Designs

Prayon's experience in agitator design has enabled them to design plants to work on the most difficult foaming phosphates with very little or no use of defoamer This is due to the Prayon GTA surface agitation impellers

The other impellers are designed to give the optimum shear and flow for the reactional mass, a Prayon PPT or PPR pitched blade turbine can be used This gives good homogenisation and keeps the tank bottom clean

Slurry Cooling

The patented Low Level Flash Cooler, LLFC, has been developed to enable a high flow rate to

be attained with a low power consumption This enables the LLFC to operate at a low delta T, about 2°C, reducing the scaling by sodium and potassium fluosilicates in the cooler and its slurry circuits The axial-flow pump has a very low wear rate

Self drying gypsum

The hemihydrate leaving the hemihydrate filter naturally reverts back to dihydrate, transforming free water into crystal water This effect allows the production of a natural gypsum having a physical behaviour close to the natural gypsum It also avoids strongly the drying cost of the gypsum

Proven Acid Concentration

This type of concentration section is well established and has been operated on many licensed plants and has run up to two weeks between wash cycles Scaling is minimised by the use of an axial flow pump with a low power input and high recirculation rate giving a small temperature gradient across the heat exchanger

The layout of the concentration section is such that no boiling occurs in the heat exchanger All the boiling occurs within the flash chamber, any entrained spray is collected in a droplet separator and returned to the process ensuring a high efficiency

Improved Fluorine Recovery System

Fluorine is removed from the vapour ex-concentration flash chamber by a specially designed Prayon scrubbing system Fluosilicic acid containing up to 18-20 % w/w H2SiF6 and less than

200 ppm P205 is produced with a fluorine recovery efficiency close to 85%, with respect to the recoverable fluorine

The Prayon design of fluorine recovery unit has been progressively modified to improve the efficiency and reduce the investment cost The towers used by the old "Swift" process had to be made of an even larger diameter than the evaporator flash chamber to avoid droplet entrainment The irrigation of this large cross-sectional area requires a very large recirculation and thus a high power consumption The multiple nozzles of small diameter have a tendency to plug

The latest Prayon design is co-current and uses a very small cross-sectional area and a single large non-plugging nozzle It uses gravity to help separation of droplets followed by a special high efficiency FSA droplet separator

CENTRAL PRAYON PROCESS DESCRIPTION

BRIEF DESCRIPTION

typical flowsheet

First stage : production and separation of the phosphoric acid

In the first stage of the Central Prayon process, the phosphate is first reacted with sulphuric acid and recycled sulpho-phosphoric acid (coming from the exhaustion of the by-product which

in this case is calcium sulphate) The conditions of this reaction are controlled so as to precipitate the calcium sulphate as dihydrate and to obtain phosphoric acid in the most advantageous form, i.e :

• as concentrated as possible in P2O5, taking into account the limits imposed by the water balance of the unit and the necessity of precipitating well crystallised gypsum in view of its subsequent separation

• with the lowest possible free sulphuric acid content, as a function of the subsequent utilisation's of the acid produced

The resulting slurry of phosphoric acid and gypsum undergoes a separation operation in order

to extract the phosphoric acid obtained by decomposition of the phosphate, whereas the thickened gypsum slurry is directed to the second process stage

H2SO4 98%

PhosphateAdditive

Conversion tank

Steam

H2O

Hemi Filter Pure hemi cake

First stage Second stage

Second stage : exhaustion and separation of calcium sulphate

During this second stage, the thickened slurry from the first stage is first retreated under far more aggressive conditions obtained by a strong increase of sulphuric acid concentration and of temperature Under these conditions, gypsum is no longer stable; it decomposes very quickly, thus allowing the very thorough dissolution of the phosphates it contains, whereas the calcium sulphate reprecipitates as hemihydrate with a low P2O5 content

The conditions of this treatment can be controlled to obtain the maximum exhaustion of the P2O5

contained in the calcium sulphate and the development of quick filtering hemihydrate crystals During this operation quality requirements of the liquid phase will not interfere as the latter is not a finished product and is meant for recycling

The hemihydrate-sulphophosphoric acid slurry is then submitted to a counter-current washes filtration to separate the low P2O5 content hemihydrate whereas the filtrates (mother liquors combined with the product of the washes) are recycled to the first process stage

The hemihydrate discharged from the filter is treated according to the utilisation it is intended for

IMPORTANT CHARACTERISTICS

The production of phosphoric acid and of calcium sulphate with a low P2O5 content by the Central-Prayon process comprises five successive operations, which constitute the two process stages, i.e :

First operation :

Reaction of phosphate, sulphuric acid and recycled sulphophosphoric acid, with production of a slurry made up of phosphoric acid and calcium sulphate in the form of gypsum

Second operation :

Separation of this slurry into 2 fractions :

• on one hand, the phosphoric acid corresponding to the phosphate treated during the first stage,

• on the other hand, the thickened slurry, i.e with a higher gypsum concentration

Third operation :

Conversion of the gypsum of this thickened slurry into hemihydrate, essentially by adding sulphuric acid and by raising the temperature, with production of a slurry made up of hemihydrate in suspension in a sulphophosphoric acid

Fourth operation :

Separation of this slurry into 2 fractions :

• on one hand, the hemihydrate from which the sulphophosphoric acid has been extracted by counter-current wash,

• on the other hand, the liquids which have accumulated all the acids contained in the slurry and which are recycled to the first operation

Fifth operation :

Treatment of the hemihydrate with a view to its utilisation or its disposal to a dumping area

FIRST OPERATION : REACTION

Description

The chemical reactions taking place in the reactor are extremely complex due to the diversity of minerals and elements in the phosphate fed However, the following principle reactions take place in the attack tank simultaneously

Main reaction

Ca3(PO4)2 + 3H2SO4 + 6H2O
2H3PO4 + 3CaSO4.2H2O

Secondary Reactions

Ca3(PO4)2 + H3PO4 + 6H20
3CaHPO4.2H2O CaHPO4.2H2O + H2SO4
H3PO4 + CaSO4.2H2O CaCO3 + H2SO4 + 2H2O
CaSO4.2H2O + CO2 + H2O

H2SO4 98%

Phosphate Additive

Conversion tank

Steam

H2O

Hemi Filter Pure hemi cake

First stage Second stage

As the Central-Prayon process comprises two successive stages, only the main product – phosphoric acid – is obtained in its final condition during this stage

Consequently, phosphoric acid has to be produced in its most advantageous form, i.e essentially with a P2O5 content as high as possible and with a minimum free sulphuric acidity

The pursuit of these aims is facilitated by the fact that one may, to a certain extent, disregard the properties of the calcium sulphate precipitated in the form of gypsum

In fact, to produce a phosphoric acid with as high as possible P2O5 concentration, concentrated sulphuric acid (H2SO4 content between 93 and 98 %) is used without dilution On the other hand, the free sulphuric acidity at the end of the reaction is limited to very low values

The combination of these factors : utilisation of concentrated sulphuric acid, production of phosphoric acid with high P2O5 content and low free sulphuric acidity content impedes the reaction to be completed: the gypsum still contains at lot of P2O5 cocrystallised

Although the latter matters less than in a dihydrate process, one nevertheless has had to perfect the operation techniques of the reaction section – agitation, cooling and circulation of the slurry – to obtain a gypsum of sufficient quality with a view to subsequent operations On the other hand, operation at low free sulphuric acidity has allowed to treat more coarsely ground phosphates and this all the more as the process allows to leave in the gypsum an important amount of co-crystallized attacked P2O5

In practice, the limits vary according to the phosphates treated, but the possibilities of the process are very interesting To this day, the following results have been reached:

• in the laboratory :35 to 38 % P2O5 with 1 % of free H2SO4

• in industrial operation : 32 to 37 % of P2O5 with 0,8 to 1,5 % of free H2SO4

Main devices and important flows

Attack tank

The attack tank is made off three compartments operating in time The attacking tank is constructed of reinforced concrete, lined with a protective rubber lining which in turn is covered with a layer of carbon brick (walls and floor) The reactor top is constructed of reinforced concrete with a special acid fume resistant coating applied to the underside

Specially designed openings are made in the attacking tank partitions so that the flow of slurry from one compartment to another is through the centre of each compartment The partition is designed in such a manner as to give each attacking tank compartment four well defined corners The corners act as baffles for each of compartments and promote better agitation of the slurry

One agitator is placed in each of the compartments of the attacking tank These agitators have three impellers

The top impeller extends the slurry surface and serves three functions:

• to break the foam (if any) generated during reaction, reducing the need for defoamer,

• to disperse slurry above the surface to further dilute the sulphuric acid and to mix the rock as it enters the attack tank,

• to improve the removal of heat through the ventilation system

The middle impeller receives slurry from the top impeller and pumps the slurry down wards, thus providing proper homogenisation throughout the tank height

The bottom impeller blades are close to the tank floor to aid agitation and ensure a clean floor

A high rate of ventilation is provided in these compartments to avoid escape of fluorine compound fumes This also results in considerable cooling of the slurry

Removable covers are provided around the agitator shaft for the proper operation of the ventilation system

The clean-out access points are provided for the attacking compartments

Digestion Section

Slurry overflows from Compartment 4 to the digestion tank The additional retention time in a desaturated slurry medium ensures that crystallisation of the gypsum and fluosilicates is as complete as possible This reduces supersaturation and scaling tendencies and thereby promotes increased filtration

The attack and digestion configuration ensures the prevention of short circuiting of unreacted rock from the reactor to the filter

The digestion tank agitator has two impellers ensuring gentle agitation in the digestion tank to allow maximum crystal growth

Sulphuric Acid Feed System

The sulphuric acid 98 % H2SO4 is pumped from the sulphuric acid storage tank and is metered into the attack system The sulphuric acid flow rate is controlled within 0.5% by

a magnetic flow meter-recorder controller

It is vital to have accurate flowrate control of the phosphate rock feed (± 0.5 %) and area and the sulphuric acid feed (± 0.5 %) in order to control the free sulphuric acid concentration in the reaction slurry

The sulphuric acid feed, rather than rock feed, is adjusted to maintain the desired attacking tank conditions The sulphuric acid control system is easily adjusted and does not change the plant P2O5 production rate

The concentrated sulphuric acid is mixed with recycle acid from the filter in a specially designed mixing tee before contacting the slurry Sulphuric acid can also be added separately to the digestion tank, if desired

Phosphoric Acid Recycle

Phosphoric acid filtrate, coming from the hemihydrate filter, containing approximately 20

% P2O5 is collected from the first wash of the Prayon tilting pan filter and returned to the reaction system This recycle acid is mixed with the sulphuric acid described above

Control of the recycle stream is important to maintain the correct solids content and

P2O5 concentration in the attack system High solids levels could result in crystallisation and viscosity problems which would limit filtration rate High solids content also makes mixing more difficult to achieve In some situation air content of the slurry can increase driven to process instability

Low solids content have a negative impact on gypsum crystal formation It results in a reduction of the filtration efficiency Also when solids content is low, residential time in the attack section is low which may give scaling on the filter

of the sulphuric acid by evaporation of water from the slurry under vacuum

The temperature is maintained by regulation of vacuum in the flash cooler The cooled slurry returns to Compartment 1

The flash cooler pump is designed to circulate a large flow of slurry at a small liquid head thus producing a low slurry temperature gradient with a minimum energy requirement This high circulation and low slurry temperature gradient minimises the