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The Chemical Production of Phosphoric Acid

Phosphoric acid is needed in large quantities to be able to fill its many roles in industry, meaning that its production has to equal its large demand. This acid is most commonly created by two ways, with one being more energy intensive and more pure, while the other is faster and cheaper. These two chemically reacting process are know as the:

Wet Proccess 

The wet process is generally the more common route in the creation of H3PO4, as the form this technique produces is more readily used in fertilizers, and requires relatively low amounts of energy (Guichon, 2016). The wet process constitutes the addition of concentrated sulfuric acid (93%) to fluorapatite (phosphate rock), and the consecutive vigorous mixing of the two substances (University of York, 2013). Important to know, fluorapatite (Ca3(PO4)2) is the basis of phosphoric acid as this is the reactant that contains phosphate; a key component in phosphoric acids industrial uses.

 

 

 

 

The mixing of fluorapatite and sulfuric acid creates a double replacement reaction, forming both phosphoric acid and calcium sulfate. Better known as gypsum, the calcium sulfate produced is needed to be separated to allow for the collection and use of phosphoric acid. To achieve this, water is added and the gypsum is filtered out of the solution. The reaction below displays the formation of phosphoric acid:

 

Ca3(PO4)2 + 3H2SO4 → 2H3PO4 + 3CaSO4

It is important to note however, that insoluble impurities are produced by this reaction, and that these products include fluoride and other trace substances. The removal of fluoride is done through evaporation, while the rest of the impurities remain. These residual substances are why this method of phosphoric acid production results in a less pure and agriculturally applicable form of phosphoric acid (Waterose, 1996).

Fluorapatite 

Thermal Proccess 

The thermal process is used in the production of phosphoric acid, primarily, to reduce overall residual impurities and create a more pure phosphoric acid (Fang Chemicals, 2013). Such purified acids are used more actively in consumer based products such as food additives and more technical applications due to its high quality. The thermal process is initiated with the burning of elemental phosphorous (P4) in air at roughly 1800-3000°K. This reaction, classified that as a formation reaction, creates phosphoric anhydride, which is known for its affinity to water (US National Library of Medicine, 2013). This chemical combination can be described as follows:

 

P4(l) + 5O2(g) → 2P2O5(g)

 

This formation reaction takes place inside of a burning tower, one that enables the temperature to reach excessive highs. Once completed however, the product of the reaction is relocated into a hydrating tower, where the gaseous phosphoric anhydride is combined with water to obtain its H+ ions and form phosphoric acid (University of York, 2013). This final reaction is represented as follows:

 

2P2O5(g) + 3H2O → 2H3PO4(l)

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