Phosphoric acid production technologies and process options

Phosphorus is a non-metal element calculated in the form of P2O5 in nature. Its main ore is fluorapatite (contains three combination elements, calcium, phosphorus and fluorine and some naturally occurring radioactive impurities). The rock phosphate treated to obtain yellow phosphorus that used to produce Orthophosphoric acid. It is a weak acid but highly consumed in fertilizer making.  Its acidic strength lies between the sulphuric acid and acetic acid. We can store it in solid form when it is 100 percent concentrated and the temperature maintained below 40oC. P2O5 content in pure phosphoric acid would be 72.43 on weight percentage basis.

Phosphoric acid production technologies:

  • Wet process:
    a) Sulphuric acid leaching (90% of total world production). Dihydrate process: Digestion step operation conducted at 165-195oC producing 25-35% P2O5 with Phosphogypsum) chemical reaction:Ca10(PO4)6F2 + 10H2SO4 + 20H2O → 10CaSO4.2H2O + 6H3PO4+2HF
  • Hydrochloric acid leaching
  •  Purified wet process
  • Thermal process: Classification based on combustion chamber
    a) Wetted wall
    b) Water-cooled
    c) Air-cooled
    d) Hydrator-absorber
  • Electric furnace process: Produce elemental phosphorus
    a) Direct conversion
    b) Oxidation and hydration in a remote location plant
  • Blast furnace

Dihydrate process for phosphoric acid production:

Phosphate rocks (calcium fluorapatite) contain radionuclide like radium, uranium and thorium in most ores. Most industrial complexes are designed by the best suitable method for this type ores is wet process using dihydrate method. In treatment, the naturally occurring radioactive materials were removed by solvent extraction as by-products. Fractionation is also preferred by it not efficient and safe as compared to solvent extraction. Suitable solvents to remove toxic cadmium and uranium are polyalkyl phoshasene and trioctyl amine. Including the solvent extraction leads to the new process option named as a purified wet process.

Phosphogypsum (i.e dihydrated calcium sulfate)obtain from phosphoric acid production unit contains traces of radioactive elements of uranium, radium and metals like Copper, Zinc, Strontium, Yttrium, Chromium, Zirconium, Cadmium, Barium, Tin and Lead. . The impurities of iron, magnesium and aluminum affect the granulometery of the fertilizer like di-ammonium phosphate derived from H3PO4 and NH3 reaction.

In the phosphoric acid that is produced from sulfuric acid treatment methods the by-product Uranium extracted as yellow paste and it is the source for 5% of the world uranium production. Uranium, radium and thorium can be detected from samples using alpha spectrometry or neutron activation analysis. All metal can be analyzed by the x-ray fluorescence.

When phosphate is decomposed in an agitated reactor. Most of the reactors are designed to use turbine or paddle blades agitators and for better life wedge shape agitators are utilized for thorough mixing. Experimental and plant operation date is required to do analysis and simulation of a chemical process of actual operation plants.

Units present in the process:

  • Grinding unit
  • Reaction section
  • Filtration section
  • Hydration unit
  • Recovery unit (rare earth metal and radioactive elements)

Main effects caused by the process are:

  • Recycling acid stream from washing filters
  • Slurry recycle ratio it is used determine the solubility of calcium and sulfate.

Hemihydrate and Hemidihydrate (two stages) process in phosphoric acid production plant complex

A hemihydrate technology is known for high energy and P2O5 recovery efficient process to produce strong H3PO4. It is alternative to dihydrate process. Similar to wet process after H2SO4 addition to the phosphate rock mixed in low concentrated phosphoric acid. The mixture produces calcium fluoride and carbonate along with the calcium sulfate as a primary precipitate. Filtration operation is introduced at each stage improve the recovery and formation that is hydration, anhydrite or hemihydrate.

In hemihydrate process sulfuric acid added directly and calcium sulfate is removed completely as by-product without recycling but hemidihydrate process is a two-stage process:

  • The first stage is hemihydrate produced in high-grade stainless steel reaction tanks
  • Second stage transformation of hemihydrate to dihydrate for P2O5 recovery and reduce the acid wash requirement for purification of by-products.

Fertilizers derived from H3PO4

  • Triple superphosphate
  • Ammonium phosphate
  • Potassium phosphates
  • Magnesium ammonium phosphate

Household consumables products containing phosphoric acid:

  • Coke cola drink