Phosphorus is an essential plant nutrient and is taken up by plant roots, usually as the dihydrogenphosphate ion, H2PO4-, derived from phosphoric acid, H3PO4. The manufacture of fertilizers that are used to rectify phosphorus deficiencies in soils depends on the availability of supplies of phosphoric acid.
Uses of phosphoric acid
By far the largest use of phosphoric acid is the manufacture of phosphates for use as fertilizers.
Figure 1 Uses of phosphoric acid.
While Figure 1 shows the worldwide uses of phosphoric acid, the proportion varies country by country. For example, the proportion used in the US to make fertilizers is even larger, 90%.
Although their use as builders in detergents has been phased out in many countries, it is still an important use for the acid. It is also used widely in beverages and in animal feeds.
Annual production of phosphoric acid
Phosphoric acid is made by two processes:
a) the 'wet' process
b) thermal process
The thermal method normally produces a more concentrated and purer product, but is energy intensive (because of the manufacture of phosphorus itself. Acid produced by the 'wet' route is less pure but is used in the manufacture of fertilizers.
(a) Wet process
Phosphoric acid is produced from fluorapatite, known as phosphate rock, 3Ca3(PO4)2.CaF2, by the addition of concentrated (93%) sulfuric acid in a series of well-stirred reactors. This results in phosphoric acid and calcium sulfate (gypsum) plus other insoluble impurities. Water is added and the gypsum is removed by filtration along with other insoluble materials (e.g. silica). Fluoride, as H2SiF6, is removed at a further stage by evaporation.
Although the reaction takes place in stages involving calcium dihydrogenphosphate, the overall reaction can be represented as:
However, there are side reactions; for example with calcium fluoride and calcium carbonate present in the rock:
Fluorosilicilic acid is an important by-product from this and from the manufacture of hydrogen fluoride. It can be neutralised with sodium hydroxide to form sodium hexafluorosilicate. The acid is also used to make aluminium fluoride, used in turn in the manufacture of aluminium.
The crystal structure of the calcium sulfate formed depends on the conditions of the reaction. At 340-350 K, the principal product is dihydrate, CaSO4.2H2O. At 360-380 K, the hemihydrate is produced, CaSO4.1/2H2O.
Calcium sulfate is filtered off and the acid is then concentrated to ca 56% P2O5 using vacuum distillation.
The product from the 'wet process' acid is impure but can be used, without further purification, for fertilizer manufacture (see below). Alternatively it can be evaporated further to ca 70% P2O5, a solution called superphosphoric acid which is used directly as a liquid fertrilizer.
To make industrial phosphates, the acid is purified by solvent extraction using, for example, methyl isobutyl ketone (MIBK) in which the acid is slightly soluble and concentrated to give 60% P2O5 content. This acid can be further purified using solvents to extract it from heavy metals and defluorinated (by evaporation) to produce a product of food grade quality.
The raw materials for this process are phosphorus and air:
Initially, phosphorus is sprayed into the furnace and is burnt in air at about 1800-3000 K.
Most processes use undried air and many involve the addition of steam to the phosphorus burner to produce and maintain a film of condensed polyphosphoric acids which protect the stainless steel burner tower (externally water cooled). The products from the burner tower pass directly into a hydration tower where the gaseous phosphorus oxide is absorbed in recycled phosphoric acid:
Alternatively, the phosphorus may be burnt in dried air. The phosphorus pentoxide is condensed as a white powder and separately hydrated to phosphoric acid.
This method allows heat to be recovered and reused.
Burning and direct hydration, as previously described, creates highly corrosive conditions. The equipment is constructed from stainless steel or is carbon brick-lined. To reduce corrosion, the walls of the burner and hydrator towers are cooled, but the reactor products emerge at a temperature too low for useful heat recovery.
The product acid has a concentration of ca 85%.
The salts of phosphoric acid, the phosphates, are the compounds that are widely used in agriculture, industry and in the home.
(a) Ammonium phosphates
Monoammonium dihydrogenphosphate and diammonium hydrogenphosphate are much used as fertilizers and are made by mixing the correct proportions of phosphoric acid with anhydrous ammonia in a rotating drum.
The choice of which ammonium phosphate to use depends on the proportion of nitrogen and phosphorus needed for the crop.
(b) Calcium phosphates
The calcium phosphates are used extensively as fertilizers. Calcium dihydrogenphosphate, Ca(H2PO4)2, is produced by the reaction of phosphate rock with sulfuric acid:
This is known as superphosphate. It contains ca 20% P2O5.
This is known as triple superphosphate. The higher level of phosphate is achieved because the product is no longer diluted with calcium sulfate.
(c) Sodium phosphates
Sodium phosphates are produced by reaction of phosphoric acid and a concentrated solution of sodium hydroxide in the appropriate (stoichiometric) proportions. The product crystallizes out.
Date last amended: 18th March 2013