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Phosphoric Acid — General Information
Phosphoric acid, also called orthophosphoric acid, is used in the production of phosphorus fertilizers, feed additives, and in other industries.
Worldwide, the annual use of raw phosphate products exceeds 150 million tons. Of this, about 85% is used for fertilizer production.
General Information
It has the formula H3PO4, and at 15 degrees it looks like an oily substance. It mixes very well with water. It melts at 42.35 degrees (in its anhydrous state). At high concentration it crystallizes into hemihydrates, for example, H3PO4 x 0.5H2O.
Under normal conditions it is not very active, reacting with some metals (used, for example, for rust treatment before painting), as well as with hydroxides and carbonates. However, at temperatures above 80 degrees it reacts actively even with silicate-containing materials, among others.
If zinc or Mn is added to H3PO4, corresponding protective films will form when it acts on a metal surface — this process is called phosphating.
When orthophosphoric acid is heated, a water-loss process occurs, which can be described by the formula:
2H3PO4 = H2O + H4P2O7 + 71.17 kJ, and further H4P2O7 = H2O + 2HPO3 + 100.46 kJ
This produces so-called superphosphoric acid, in which the P2O5 content can reach up to 88.6% once the system reaches equilibrium. If water is added to this system (diluting it), orthophosphoric acid is formed again, releasing heat.
Production of H3PO4
There are two industrial production methods:
- Extraction — produces the purest substance.
- Thermal.
Let’s look at each of them.
Extraction Method
This method is based on the reaction of natural phosphorus compounds with sulfuric acid. The process can be written with the formula:
Ca3(PO4)2 + 3H2SO4 = 2H3PO4 + 3CaSO4, after which the resulting substance is filtered and brought to the required concentration by evaporating excess water.
Thermal Method
For laboratory methods there is a fairly simple approach — it is based on the action of Nitric acid on phosphorus, which can be described by the reaction:
3P + 5HNO3 + 2H2O = 3H3PO4 + 5NO, the nitric acid is taken as a 32% solution.
Phosphorus is obtained by reducing natural phosphates and then burning the phosphorus.
On an industrial scale, oxidation (combustion) of phosphorus in an air atmosphere is used, followed by hydration (dissolving in water) and separation of P4O10, with subsequent condensation of the substance itself.
P4O10 + 6H2O = 4H3PO4
P4O10 is obtained in two ways:
- Oxidation of phosphorus vapor — this method is used quite rarely on an industrial scale.
- Oxidation of liquid phosphorus.
Processes are also distinguished by the method of cooling the gases (vapors):
- Evaporative — cooling of evaporating dilute orthophosphoric acid or water. The particular feature is that the amount of off-gas is quite significant, so this method is used only for small production volumes.
- Circulation-evaporative — the downside of this method is the need to cool a large amount of substance.
- Heat-exchange-evaporative — this method in some ways combines both methods described above — removing excess heat through walls and through water evaporation.
Other Varieties
Phosphorous acid
Has the formula H3PO3. The anhydrous form looks like crystals that melt at 74 degrees. It forms salts called phosphites. When heated, it converts into phosphoric acid according to the formula:
4H3PO3 = PH3 + 3H3PO4
Hypophosphoric acid
Has the formula H4P2O6. Decomposes over time. When heated, it also decomposes into phosphoric and phosphorous acids.
Hypophosphorous acid
Has the formula H3PO2. Its salts are called hypophosphites. The acid and its salts are very active reducing agents; salts of metals such as Nickel, Copper, and others can be reduced to free metal ions.
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