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Phosphoric acid

Uses of phosphoric acid• used in fertilizers and phosphate salts• in several acidic hard-surface (tile, porcelain, metal) cleaning and

sanitizing formulations, 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 proteinacidulant, buffering agent in jam and jelly preparation, nutrient and buffer in antibiotic manufacture and purification reagent in sugar refining.

• As bonding agent in refractory products, improves strength, load-bearingproperties, high temperature stability, 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/molP4O10 + 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 ) Venturiscrubbers; g) Receiver for wash acid; h) Off-gas fan; i) Separator

> 2000 °C

phosphorus combustion 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) Venturiscrubbers; f ) Dilute acid tank; g) Separator

phosphorus combustion 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

phosphorus combustion and P4O10 absorption are carried out separately

utilizes the heat of phosphorus combustion for steam generation

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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.– Fluorapatite Ca10(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 sulphateforms 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 + 4H3PO43Ca(H2PO4)2

Ca(H2PO4)2 + 3H2SO43CaSO4 + 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

fluoride but in the presence of silica this readily reacts to form fluosilicic acid, 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– Operating temperatures 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 concentration stage– 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

– Filtration rate• 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. – Phosphate losses

• 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 :• Acidulate under hemihydrate conditions; recrystallise to dihydrate

without intermediate hemihydrate separation; separate product– Hemi-dihydrate (HDH)process

• Acidulate under hemihydrate conditions; separate product; recrystallise hemihydrate to dihydrate; filter and return liquors to process

– Dihydrate-Hemihydrate (DH/HH) process • Acidulate under 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