công nghệ sản xuất axit photphoric

PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 byproducts, 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 1/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 GENERAL The Phosphoric Acid Plant is designed to produce 510 mtpd P2O5 of concentrated Phosphoric acid at +/-52% P2O5 from a feed stock of phosphorite and apatite phosphates Prayon Central Prayon process (CPP) is applied, comprising the following sections: • Reaction • Dihydrate Filtration • Conversion • Hemihydrate Filtration • Hemihydrate neutralisation • Concentration • Fluorine Absorption • Acid storage of weak and concentrated phosphoric acid • Storage of fluosilisic acid • Gas scrubbing • Gypsum handling and grinding Gypsum quality is suitable for cement 2/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 Grinding System In CPP process, the phosphate rock fed to reactor has to be a grain granulometry under 1000 microns Water Balance The phosphate rock with an average water content of % max, weight basis, can be fed to the Plant as is without drying and as high strength sulphuric acid is used, thus there is sufficient water available to ensure a proper washing of the final hemihydrate filter cake 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 3/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 4/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 5/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP CENTRAL PRAYON PROCESS DESCRIPTION BRIEF DESCRIPTION typical flowsheet DH filter Dihydrate Hemihydrate Steam H2O H2SO4 98% Phosphate Additive Hemi Conversion Filter tank Pure Product acid =33 - 35% hemi cake Attack Digestion First stage Second stage 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 6/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 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 7/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Fourth operation : Separation of this slurry into 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 DH filter Dihydrate Hemihydrate Steam H 2O H2SO4 98% Phosphate Additive Hemi Conversion Filter tank Pure Product acid =33 - 35% hemi cake Attack Digestion First stage Second stage 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 8/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 % 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 9/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 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 10/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP 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 Temperature Control The patented Low Level Flash Cooler, LLFC, makes it practical to operate with a reaction slurry temperature gradient of only 2°C at a very low pumping cost Slurry is pumped by an axial flow pump from Compartment of the attack tank into the low level flash cooler The flash cooler removes the heat of reaction and heat of dilution 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 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 11/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP amount of super saturation in the slurry This improves the control of the process and reduces the amount of scale formation Close temperature control, extremely important in any process, is obtained In addition, all the slurry is degassed as it passes through the flash cooler The water vapours and fluorine gases from the flash cooler are partially condensed in the precondenser using recycle water The remaining water vapours is condensed in a barometric condenser using process water, non-condensable gases are removed by a rotary water ring vacuum pump Warm water from the pre-condenser is sent to the cake wash tank Fume A horizontal cross-flow fume scrubbers provided to remove fluorine from the vapours vented frorn the reaction, filtrations and conversion systems The system is designed to achieve a fluorine content of the exhaust vapours of below mg/Nm3 gas The scrubbed gases are discharged to the atmosphere through a stack SECOND OPERATION: SEPARATION OF THE PRODUCT ACID DH filter Dihydrate Hemihydrate Steam H2 O H2SO4 98% Phosphate Additive Hemi Conversion Filter tank Pure Product acid =33 - 35% hemi cake Attack Digestion First stage Second stage Description The phosphoric acid and gypsum slurry obtained during the first operation are submitted to a separation in order to extract the quantity of phosphoric acid corresponding to the P2O5 contained in the phosphate introduced in the system and also some acid to have a accurate control of the solids content in the attack tank It is done by filtration (without washing) 12/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Main devices and important flows Dihydrate Filter System The dihydrate filter system consists of a horizontally travelling belt filter plus ancillary equipment including vacuum system, filtrate pumps, cloth wash tank and pump Only part of the slurry pumped from the digestion is fed on the filter It corresponds to the quantity that need to be filtered to ensure the acid production and to control the solids content in the attack tank The remaining quantity by-pass the filter and is directed to the conversion tank The centrifugal pump supplies the weak acid slurry from the digestion tank to the filter feed distributor The filter slurry is allowed to settle briefly on the cloth before vacuum is applied This allows larger particles to cover the cloth giving a pre-coating effect As the first drops of acid contain a too high solids content, they are send back to the attack tank The section of the filter ensuring this function is the cloudy port The cake on the DH filter is not washed Once it has been filtered it is discharged in the conversion tank Vacuum System A vacuum system, comprising a rotary water ring vacuum pump, acid trap and filter condenser, is provided The system maintains a vacuum on all the necessary zones of the filter Miscellaneous One monorail with an electric hoist is provided over the filter This feature results in easy maintenance and faster turnarounds Concrete Floor and Acid Brick: Concrete floors should be used in all areas of possible spillage to prevent drip to lower levels Anti acid lining is applied in areas subject to potential acid attack including floor trenches Pump foundations are covered with acid proof mortar 13/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP THIRD OPERATION: CONVERSION OF GYPSUM INTO HEMIHYDRATE DH filter Dihydrate Hemihydrate Steam H2O H2SO4 98% Phosphate Additive Hemi Conversion Filter tank Pure Product acid =33 - 35% hemi cake Attack Digestion First stage Second stage Description The second stage, which starts with this operation, aims essentially at extracting the P2O5 from the by-product – calcium sulphate – and at producing the latter with the best possible properties The pursuit of these aims is facilitated by the fact that the working conditions can be set without any subjection to the quality of the main product, as the latter has been separated previously The thickened slurry from the second operation is attacked under far more aggressive conditions obtained by adding sulphuric acid and raising the temperature Due to that modification of the physico-chemical condition of the reactor the calcium sulphate reprecipitates in the form of pure hemihydrate During the transformation, almost all the P2O5 contained in the gypsum lattice is freed and is recovered in the liquid phase The recovery is thus greatly improved Under the resulting physical and chemical conditions, the dissolution of gypsum and of P2O5 is inevitable and immediate and the operation is thus mainly aimed at producing quick filtering hemihydrate crystals Although the crystallisation of hemihydrate is still influenced by the origin of the phosphate treated, it will be obtained in a form that filters more quickly than the gypsum coming from the same phosphate In general, the filterability of hemihydrate is 20% to over 60 % better than that of the corresponding gypsum, the improvement being most important with respect to low filtration speed gypsum 14/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Main devices and important flows Conversion tank The conversion tank is an agitated circular tank made of rubber and carbon bricks lined concrete In this tank to transform the dihydrate into hemihydrate (CaSO4 ½ H2O), steam and sulphuric acid are added FOURTH OPERATION : FILTRATION OF HEMIHYDRATE DH filter Dihydrate Hemihydrate Steam H 2O H2SO4 98% Phosphate Additive Hemi Conversion Filter tank Pure Product acid =33 - 35% hemi cake Attack Digestion First stage Second stage Description Thanks to the quality of the hemihydrate crystallisation and the possibility of controlling it, filtration and exhaustion by counter-current washes are easy and quick And yet, it is this operation which has set the problems most difficult to solve These are related to the very nature of hemihydrate; in fact, the latter is in no case a stable form of calcium sulphate and if it can be kept during several hours in the medium where it has been obtained during the third operation, if it can also be filtered and washed without any problem, the wet cake obtained, after the final wash on the filter, is merely what, in current language, is called plaster Everybody knows that wet plaster is well-known and utilised for its setting properties This is a physical property inherent in the material and which has to be coped with Therefore, filter-washing techniques had to be improved in order to ensure the removal of any hemihydrate trace after each filtration cycle 15/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Thanks to its good filterability, hemihydrate obtained at the end of filtration has a moisture content of 17 - 22 % Consequently, rehydratation towards a solid state, will produce a low moisture gypsum, which can generally be utilised as it is, without subsequent drying Main devices and important flows Hemihydrate filter The filter system consists of a Prayon tilting pan filter plus ancillary equipment including vacuum system, filtrate tanks and pumps and cake and cloth wash tanks and pumps The slurry is pumped on the filter feed distributor by an air lift The filter slurry is allowed to settle briefly on the cloth before vacuum is applied This allows larger particles to cover the cloth giving a precoating effect The filtrate obtained is pumped to attack tank by the recycle acid pump to control the reactor P2O5 concentration and solids content The cake is given two counter-current washes The first wash uses water recycled from the second wash The second wash is a mixture of clean process water heated in the precondenser and water coming from the cake wash tank The clean process water is added after cake wash water tank Such position reduce the impurities content of the cake allowing a down stream use of the gypsum After last wash has been completely drained from the cake, the cake dumps into a cake discharge hopper, is carried by belt conveyors to the gypsum disposal area and if necessary is neutralised by a lime addition as cement manufacturer request a pH higher than for their “cement retarder” gypsum Vacuum System A vacuum system, comprising a rotary water ring vacuum pump, acid trap and filter condenser, is provided The system maintains a vacuum on all the necessary zones of the filter and draws air through the cake before discharge Miscellaneous One monorail with an electric hoist is provided over the filter This feature results in easy maintenance and faster turnarounds 16/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Concrete Floor and Acid Brick: Concrete floors should be used in all areas of possible spillage to prevent drip to lower levels Anti acid lining is applied in areas subject to potential acid attack including floor trenches Pump foundations are covered with acid proof mortar FIFTH OPERATION : TREATMENT OF THE HEMIHYDRATE DH filter Dihydrate Hemihydrate Steam H2O H2SO4 98% Phosphate Additive Hemi Conversion Filter tank Pure Product acid =33 - 35% hemi cake Lime Attack Digestion First stage Second stage Description Whatever the final use of hemihydrate, the cake discharged from the filter is moist plaster and it is generally necessary to provide for an immediate treatment, based on partial or total rehydration, in order to make it easier to handle Commercial product may be dried, calcined, rehydrated in a solid state, with or without granulation, or rehydrated in suspension in water These treatments must be accompanied by neutralisation by a base, in a solid state or dissolved in water In the case of rehydration, this neutralisation allows to precipitate all the remaining P2O5 and to obtain a perfectly neutral or even basic product, with properties equivalent to those of natural gypsum At this stage, the hemihydrate contains less than 0,5 % of completely insoluble P2O5 and, in practice, one obtains values ranging from 0,2 to 0,3 % are obtained with most commercial phosphates After rehydratation of the hemihydrate to gypsum and neutralisation of the P2O5 it contains, it is crushed and sieved to cope with cement industry specification 17/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Main devices and important flows Lime addition Lime addition is controlled to maintain the pH above while maintaining a water content in the hemihydrate under the specified value In order to have a good lime made of Ca(OH)2 in solution, lime milk is produced on site from CaO Crusher Two type are used The first one a rotary crusher crushes the cured gypsum The second one crushes the over size from sieve following the primary crusher: it can also be a rotary crusher or can be a hammer crusher Vibrating screen It is used to separate the biggest gypsum particles which are sent back to the second crusher SUMMARY Thanks to the application of the two-stage process principle, the Central-Prayon process allows: • to use coarsely ground phosphate or screened phosphate; • to use undilute sulphuric acid; • a P2O5 recovery over 98 % • a to % saving on the consumption of sulphuric acid; • the production of 32 – 37 % P2O5 phosphoric acid with a free H2SO4 content of about 1%; • the production of calcium sulphate containing about 0.4 % of P2O5 which can be rendered insoluble by neutralisation with a base, so that the product may replace natural calcium sulphate for most of its applications This possibility of utilisation of calcium sulphate is appreciated not only in countries where the utilisation of this product brings an important economic improvement of the process, but also in countries where it solves of the pollution problems resulting of the dumping or the disposal of residually calcium sulphate 18/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP It should be noted that the figures mentioned in this chapter constitute ranges of values obtained with most commercial phosphates on the market According to the particular conditions of a given project, some of these values can be improved, possibly by modifying certain elements of the plant or by accepting the reduction of other process performances This faculty results directly from the fact that a two stage process is utilised, in which the second stage precipitates the calcium sulphate in the form of hemihydrate; in fact, though this technique involves some problems in connection with filtration and handling of hemihydrate, it offers important advantages These are among other : finishing the operation with very aggressive conditions, allows reducing the P2O5 content of the hemihydrate to very low values; rendering this P2O5 totally insoluble by neutralisation, during rehydration; the obtaining high filterability hemihydrate; this not only increases the capacity of the filters but also supplies a calcium sulphate which is less moist than gypsum This lesser humidity is an important source of saving when utilising the calcium sulphate, which generally has to be dried This drying can even be avoided in case of rehydration in the solid state as this operation takes place by natural absorption of the major part of the impregnation water and the gypsum obtained is sufficiently dry for most of the applications it is intended for (After complete rehydration, the product contains to 10 % moisture versus 20 to 25 % for dihydrate phosphogypsum filter cake) ACID CONCENTRATION The acid concentration section is designed to concentrate 510 mtpd of P2O5 from +/-32 % P2O5 to +/-52 % P2O5 by evaporation Evaporator The evaporation system consists of a vapour Flash chamber P2O5 entrainment separator, heat exchanger, circulation pump, and associated vacuum and acid product handling equipment Vacuum is maintained by a barometric condenser and a vacuum pump The evaporator body acts as flash chamber in which the hot acid boils as it is exposed to the reduced pressure Acid is continuously recirculated at a high rate by an elbow-mounted, axialflow pump, upward through the single pass shell and tube heat exchanger and back into the vapour body 19/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Low pressure steam is used as the heating medium in the evaporator heat exchanger The steam condensate collected in the condensate receiver is returned to the utility facility Vapour leaving the vapour body passes through the evaporator P2O5 separator where P2O5 droplets are removed from the vapours and are returned to the evaporator acid circulation system The vapours then pass to the fluorine recovery system which is described later Vapour, mainly water, leaving the fluorine recovery system condensates in the direct contact barometric condenser, which uses cooling water The water from the condenser overflows from the barometric condenser seal tank It is collected in the water return sump and is returned by pump to the cooling tower system Non condensable gases leaving the condenser are withdrawn by a vacuum pump and discharged to atmosphere Product acid overflows from the evaporator vapour body to a transfer pump and is pumped to the 52 % acid storage tank Wash Tank A wash tank is provided to allow the evaporator to be drained and washed on a regular basis The drained effluents are slowly returned to the phosphoric acid plant reactor to recover the P2O5 content Fluorine Recovery The fluorine recovery system is designed to recover 18 to 20 % wt fluosilicic acid which is low in P2O5 content from the evaporator vapours Process water is fed as make up to the fluorine scrubber seal tank Vapours from the evaporator P2O5 separator pass to the fluorine scrubber Recirculated fluosilicic acid solution is sprayed into the fluorine scrubber to absorb the HF and SiF4 in the vapours to from 18 -20 % H2SiF6 solution The solution is fed to the fluosilicic acid storage tank under density control Acid Storage and clarification The phosphoric acid clarification and storage system is designed to provide storage of filter product acid (weak acid with +/-31% strength) and storage for concentrated product acid (+/-52 % phosphoric acid) 20/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP Weak Acid Storage The weak acid storage is to store weak acid delivered from the filter acid pump The suspended solids are kept in suspension by center-mounted agitator Weak acid from the tank overflow is pumped to the evaporator where it is concentrated to +/-52 % P2O5 UTILITIES Water circuit There are basically two types of water supplied to the Phosphoric Acid Plant: Process water - Fresh water Cooling water - Water from recirculated acidic cooling tower Process water Fresh water enters the plant through the following equipment items : Concentration vacuum pump Flash Cooler Vacuum Pump Fume Gas Scrubber Filters Vacuum Pump Fluorine Scrubber Seal Tank Various seal water duties on pumps Water used in vacuum pumps are then heated and used for the internal wash of the hemihydrate filter That pure hot water is used to solubilize hemihydrate that could stay in the filter cell Part of it is then used for the prewash of the filter Prewash acts as a mechanical wash to remove solids that could stick to the cloth before slurry feeding Waters coming from internal wash, prewash, cloth washes, pump seal are then transferred in Rehydratation tank The aim of this tank is to convert hemihydrate that cloth wash water contains into dihydrate to avoid scaling of pipe and also to rehydrate hemihydrate produced during filter start-up/shut-down or during off-set periods Indeed, during start-up, as the vacuum is not set on the filter, water soluble losses are high In order to recycle these losses, the hemihydrate is reslurried and then rehydrated before expedition to the stack The water of rehydratation, is then filtered and used back in the process with P2O5 recycling 21/22 PRAYON TECHNOLOGIES Aff.2898: FOSKOR – revamping into CPP The main utilisation of recycled water are the filter cloth wash (dihydrate, hemihydrate and rehydratation filter), internal wash of the hemihydrate filter and hemihydrate cake wash As all the water can not be recycled, some water goes to the neutralisation Excess water of Fume Gas Scrubber will be used for cooling tower make up Cooling water The feed of cooling water is at 32°C and is fed to the following equipment items: Evaporator Condenser Dihydrate filter condenser Hemihydrate filter condenser Rehydratation filter condenser Concentration condenser The cooling water is returned in the cooling water return trench Steam and condensate Steam The low pressure steam is received at the Battery Limits dry saturated at bars g and is immediately reduced to less than 3.5 bars g by a reducing valve The steam flow control valve lets down the steam to the pressure required by the heat exchanger 22/22