# Wet and themal process

Phosphoric acid is produced industrially by two general routes – the thermal process and the wet process, which includes two sub-methods. The wet process dominates in the commercial sector. The more expensive thermal process produces a purer product, which is used for applications in the food industry.WetWet-process phosphoric acid is prepared by addingsulfuric acidtotricalcium phosphaterock...

Phosphoric acid is produced industrially by two general routes – the thermal process and the wet process, which includes two sub-methods. The wet process dominates in the commercial sector. The more expensive thermal process produces a purer product, which is used for applications in the food industry.

### Wet

Wet-process phosphoric acid is prepared by adding sulfuric acid to tricalcium phosphate rock, typically found in nature as apatite. The reaction is:

• ${displaystyle {ce {{overset {apatite}{Ca5(PO4)3X}}+{overset {sulfuric~acid}{5H2SO4}}+10H2O->{3H3PO4}+{overset {calcium~sulfate}{5CaSO4.2H2O}}+HX}}}$

where X may include OH, F, Cl, and Br.

The initial phosphoric acid solution may contain 23–33% P2O5 (32–46% H3PO4), but can be concentrated by the evaporation of water to produce commercial- or merchant-grade phosphoric acid, which contains about 54–62% P2O5 (75–85% H3PO4). Further evaporation of water yields superphosphoric acid with a P2O5 concentration above 70% (corresponding to nearly 100% H3PO4; however, pyrophosphoric and polyphosphoric acids will start to form, making the liquid highly viscous).[13][14]

Digestion of the phosphate ore using sulfuric acid yields the insoluble calcium sulfate (gypsum), which is filtered and removed as phosphogypsum. Wet-process acid can be further purified by removing fluorine to produce animal-grade phosphoric acid, or by solvent extraction and arsenic removal to produce food-grade phosphoric acid.

The nitrophosphate process is similar to the wet process except that it uses nitric acid in place of sulfuric acid. The advantage to this route is that the coproduct, calcium nitrate is also a plant fertilizer. This method is rarely employed.

### Thermal

Very pure phosphoric acid is obtained by burning elemental phosphorus to produce phosphorus pentoxide, which is subsequently dissolved in dilute phosphoric acid. This route produces a very pure phosphoric acid, since most impurities present in the rock have been removed when extracting phosphorus from the rock in a furnace. The end result is food-grade, thermal phosphoric acid; however, for critical applications, additional processing to remove arsenic compounds may be needed.

Elemental phosphorus is produced by an electric furnace. At a high temperature, a mixture of phosphate ore, silica and carbonaceous material (coke, coal etc.) produces calcium silicate, phosphorus gas and carbon monoxide. The P and CO off-gases from this reaction are cooled under water to isolate solid phosphorus. Alternatively, the P and CO off-gases can be burned with air to produce phosphorus pentoxide and carbon dioxide.