Phosphoric acid has many industrial uses including in fertilizers, cleaning products, food processing, and more. It is produced commercially via either the thermal or wet process. The thermal process involves combusting white phosphorus to form P4O10, then hydrating it to form H3PO4. The wet process reacts phosphate rock with sulfuric acid to form H3PO4 and calcium sulfate. There are various wet process techniques including dihydrate, hemihydrate, and recrystallization methods that aim to control calcium sulfate precipitation and recover phosphoric acid.

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Phosphoric acid
Uses of phosphoric acid
• used in fertilizers and phosphatesalts
• in several acidic hard-surface (tile, porcelain, metal) cleaning and
sanitizingformulations, as well as an acid cleaner for food processing
equipment
• good pickling agent, leaves the steel coated with a thin film of iron
phosphates
• in cola beverages as a tart flavoring agent, used as a general protein
acidulant,buffering agent in jam and jelly preparation, nutrient and buffer
in antibiotic manufactureand purification reagent in sugar refining.
• As bondingagent in refractory products, improves strength,load-bearing
properties, high temperaturestability,and good abrasion resistance.
• as catalysts in the petroleum and chemical industries for alkylation,
dehydrogenation,polymerization and isomerization reactions

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Production
• H3PO4 is the second largest volume mineral
acid produced after sulfuric acid.
• H3PO4 is produced commercially by either
– the thermal (furnace) process
– the wet process
• Thermal acid, manufactured from elemental
phosphorus, is more expensive and
considerably purer than wet-process acid.
Thermal process
• Two steps
– combustion of white phosphorus with excess air
to give P4O10 and
– hydration of P4O10 to form H3PO4
• The acid obtained by hydration of the oxide is
generally termed thermal phosphoric acid

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Thermal process
P4+5O2  P4O10 ΔH= -3053 kJ/mol
P4O10 + 6 H2O  H3PO4 ΔH= -377 kJ/mol
• The IG and TVA processes for the production of
phosphoric acid are distinguished by the method
of absorption and hydration of P4O10.
• The Hoechst process differs from these in that it
utilizes the heat of phosphorus combustion for
steam generation.
Thermal phosphoric acid (IG process)
a) Nozzle; b) Combustion tower; c) Overflow cup; d) Heat exchanger; e) and f ) Venturi
scrubbers; g) Receiver for wash acid; h) Off-gas fan; i) Separator
> 2000 °C
phosphoruscombustion and P4O10 absorption are carried out in
a single stage in a common tower

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Thermal phosphoric acid (TVA process)
a) Storage tank; b) Combustion chamber; c) Absorption tower; d) Cooled receiver; e) Venturi
scrubbers; f ) Dilute acid tank; g) Separator
phosphoruscombustion and P4O10 absorption are carried out separately
Thermal phosphoric acid (Hoechst process)
a) Storage tank; b) Air dryer; c) Burner; d) Combustion chamber; e) Separator; f ) Superheater
phosphoruscombustion and P4O10 absorption are carried out separately
utilizesthe heat of
phosphorus
combustionfor
steamgeneration

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Wet process
• Wet-process acid, is produced directly from phosphatic ores,
and is characterized by relatively high production volume,
low cost, and low purity.
– FluorapatiteCa10(PO4)6(F,OH)2
– Francolite Ca10(PO4)6–x(CO3)x(F,OH)2+x
• The tricalcium phosphate in the phosphate rock is converted
by reaction with concentrated sulphuric acid into phosphoric
acid and the insoluble salt calcium sulphate.
Ca3(PO4)2 + 3H2SO4  3CaSO4 + 2H3PO4
• The insoluble calcium sulphate is then separated from the
phosphoric acid, most usually by filtration.
Wet process
• The reaction between phosphate rock and sulphuric acid is
self-limiting because an insoluble layer of calcium sulphate
forms on the surface of the particles of the rock.
• This problem is kept to a minimum by initially keeping the
rock in contact with recirculated phosphoric acid to convert it
as far as possible to the soluble monocalcium phosphate and
then precipitating calcium sulphate with sulphuric acid.
Ca3(PO4)2 + 4H3PO4 3Ca(H2PO4)2
Ca(H2PO4)2 + 3H2SO4 3CaSO4 + 6H3PO4
• Calcium sulphate exists in a number of different crystal forms
depending particularly on the prevailing conditions of
temperature and P2O5 concentration

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Wet process
• Operating conditions so that the calcium sulphate will be
precipitated in either the dihydrate or the hemihydrate form,
– 26-32% P2O5 at 70-80°C for dihydrate precipitation
– 40-52% P2O5 at 90-110°C for hemihydrate precipitation
• There are many impurities in phosphate rock, interfere in the
reaction or filtration
– Fluorine is present in most phosphate rocks to the extent of 2-4% by
weight.
– This element is liberated during acidulation,initially as hydrogen
fluoridebut in the presence of silica this readily reacts to form
fluosilicicacid, H2SiF6.
– Other components such as magnesium and aluminium can also react
with HF to form compounds (MgSiF6 and H3AlF6).

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Wet process
• Emphasis must also be placed on another group of
impurities such as arsenic, cadmium, copper, lead, nickel,
zinc and mercury, which may pass into the acid during
acidulation.
• Phosphate rocks contain naturally-occurring uranium and
the radioactive components of the uranium decay series
are associated with the phosphate material.
• The uranium goes into the product acid solution and any
radium is co-precipitated with the phosphogypsum.
• Impurities such as iron, aluminium, sodium, potassium,
chlorine, etc have some influence during the production of
phosphoric acid and on the quality of the acid produced.
Dihydrate process
• This is the most diffused process
• Advantages of dihydrate systems are
– There is no phosphate rock quality limitation
– On-line time is high
– Operatingtemperatures are low
– Start-up and shut-down are easy
– Wet rock can be used (saving drying costs)
• The disadvantages are
– Relatively weak product acid (26-32% P2O5)
– High energy consumption in the acid concentrationstage
– 4-6% P2O5 losses, most of them co-crystallised with the calcium
sulphate

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Dihydrate process
• Grinding
– ball or rod mills in dry or wet conditions : particle size
distribution 60-70% less then 150μm
• Reaction
– Stirred tank reactors or a single stirred tank in some
processes.
– The operating conditions for dihydrate precipitation are
26-32% P2O5 and 70-80°C.
– Temperature is controlled by passing the slurry through a
flash cooler or by using an air circulating cooler.
Dihydrate process
• Filtration
– Pressure or vacuum assisted : initial separation is followed
by washing, to ensure a satisfactory recovery of soluble
P2O5. The remaining liquid is removed from the filter cake.
– Five tonnes of gypsum are generated for every tonne
(P2O5) of product acid produced.
– The most common filtration equipment is of three basic
types: tilting pan, rotary table or travelling belt.
• Concentration
– Evaporation by hot combustion gas from a burner
– Evaporators : forced circulation design

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Dihydrate process

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Hemihydrate (HH) process
• Operating conditions are selected in this
process so that the calcium sulphate is
precipitated in the hemihydrate form.
• It is possible to produce 40-52% P2O5 acid
directly, with consequent valuable savings in
energy requirements.
• The stages are similar to those of the
dihydrate process but grinding may be
unnecessary.
Hemihydrate (HH) process

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Hemihydrate (HH) process
• Advantages of this process
– the reduction or elimination of evaporation heat
requirement
– Capital savings
– Purer acid
• Acid from the HH process tends to contain substantially less
free sulphate and suspended solids and lower levels of
aluminium and fluorine than evaporated dihydrate process
acid of the same strength.
– Lower rock grinding requirements
• A satisfactory rate of reaction can be achieved from much
coarser rock than in the dihydrate process, because of the
more severe reaction conditions in the HH process.
Hemihydrate (HH) process
• Disadvantages of HH systems are
– Filtrationrate
• Hemihydrate crystals tend to be small than dihydrate crystals thus hemihydrate slurries
are more difficult to filter unless modifiers are used to suppress excessive nucleation.
– Phosphatelosses
• Due to the higher P2O5 concentration of the slurry being filtered, the amounts of both
soluble and insoluble P2O5 remaining in the filter cake are greater
– Scaling
• Hemihydrate is not a stable form of calcium sulphate and revert to gypsum even before
the acid filtration. anti-scale agent is required in a HHplant filter to avoid scaling
– Filter cake impurity
• The cake is more acidic than gypsum filter cake because of the extra P2O5 losses and it
also contains more fluorine and cadmium
– Corrosion
• particularly agitators and slurry pumps, because of the higher temperature (100°C) and
acid concentration (40-50% P2O5 ) compared to a dihydrate plant.

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Hemihydrate (HH) process
Recrystallisation processes
• Losses of P2O5 can be lowered if the calcium sulphate is
made to recrystallise to its other hydrate.
• There are only three basic routes
– Hemihydrate recrysallisation(HRC) process :
• Acidulateunder hemihydrateconditions; recrystallise to dihydrate
without intermediatehemihydrateseparation; separate product
– Hemi-dihydrate (HDH)process
• Acidulateunder hemihydrateconditions; separate product;
recrystallise hemihydrateto dihydrate; filter and return liquors to
process
– Dihydrate-Hemihydrate (DH/HH) process
• Acidulateunder dihydrate conditions; separate product;
recrystallise hemihydrate;filter and return liquors to process

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HRC
HRC

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HDH
HDH

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DD/HH
DD/HH