8. Phosphorus Industries

8.1 Phosphorus

• Three forms of elemental phosphorus: white, red, black.

  • White is the least stable form, with time by light and heat turns to red. White phosphorus containing small amounts of red phosphorus appears as yellow; hence it is also called as yellow phosphorus.

  • Red and black phosphorus and polymeric in nature.

  • Stability of phosphorus (\(\ce{P}\)) allotropes: \[\text{Black $\ce{P}$} > \text{Red $\ce{P}$} > \text{White $\ce{P}$}\]

• Uses:

  • Early small-scale uses of white (or yellow) phosphorus were for the manufacture of matches and fireworks. However, this practice was discontinued after 1900s, because of safety issues.

  • Today, either red phosphorus or phosphorus sesquisulfide (\(\ce{P4S3}\)), both much safer, are used for matches, flares, and other incendiary devices.

8.2 Phosphate Rock

• Domestic phosphate rocks are essentially fluorapatite admixed with various proportions of other compounds of calcium, fluorine, iron, aluminum, and silicon.

• Fluorapatite [\(\ce{CaF2.3Ca3(PO4)2}\)]: This is major source to produce phosphorous.

• Phosphate rock, in pulverized form, has limited direct use as a fertilizer, chiefly because of the relatively slow availability of the \(\ce{P2O5}\). However, it is mainly used as a raw material for the manufacture of phosphoric acid, super phosphate, phosphorus, and phosphorus compounds.

• In the phosphate industry, the phosphate content of the rock is usually expressed as tricalcium phosphate (\(\ce{Ca3(PO4)2}\)) and traditionally referred to as Bone Phosphate of Lime (BPL). The phosphate rock contains about 70% BPL. \[\text{BPL} = \ce{P2O5} \times 2.1853\] This factor 2.1853 is the ratio of molecular weights of \(\ce{Ca3(PO4)2}\) to \(\ce{P2O5}\). The term BPL is reminiscent of the time when bones were the principal source of phosphate in the fertilizer industry.

• Manufacturers of phosphoric acid and phosphorus fertilizers normally stipulate a minimum content of 28% \(\ce{P2O5}\), and most marketed grades of Phosphate Rock contain more than 30% \(\ce{P2O5}\) (65% BPL).

8.3 Phosphoric Acid

• \(\ce{H3PO4}\). It is also called as ortho-phosphoric acid.

• Three main routes are employed for the commercial production of phosphoric acid:

  • Dry Process: It is also called as the combustion process yields a furnace acid. It is obtained via the combustion of yellow phosphorus in air followed by hydration of phosphorus pentoxide product with water. \[\begin{aligned} \ce{P4} + \ce{5O2} &\rightarrow \ce{2P2O5} \\ \ce{2P2O5} + \ce{6H2O} &\rightarrow \ce{4H3PO4}\end{aligned}\] Phosphoric acid produced by this method is of higher purity(75–85%) and is used in manufacture of detergents and pharmaceuticals.

  • Wet Process: The other two processes operate by acidulation of phosphate rock with strong acids, hence are referred to as wet processes.

    • The Dorr process uses sulfuric acid.

    • The Haifa process uses hydrochloric acid.

    Wet process phosphoric acid normally contains 26 to 30 percent \(\ce{P2O5}\). The production of wet process phosphoric acid generates considerable quantity of acidic cooling water which contains considerable amount of phosphorus and fluoride. Upon precipitation of these salts, this water can be used as cooling-water.

• Phosphoric Acid by Acidulation:

  • Acidulation with \(\ce{H2SO4}\): The oldest, and still the lowest cost route to phosphoric acid is via the addition of high concentrations of sulfuric acid to finely ground phosphate rock. This acidulation step releases phosphoric acid from the calcium phosphate salts and produces insoluble gypsum. 93% sulfuric acid is normally used to make the strongest phosphoric acid possible and to decrease evaporation costs involved in the process. \[\ce{CaF2.3Ca3(PO4)2} + \ce{10H2SO4}+ \ce{20H2O} \rightarrow 10\; \ce{CaSO4.2H2O} + \ce{2HF}\uparrow +\; \ce{6H3PO4}\]

  • Acidulation with \(\ce{HCl}\): Here, the coproduct calcium chloride (\(\ce{CaCl2}\)) is very water soluble, which requires a solvent extraction (using \(\ce{C4}\) to \(\ce{C5}\) alcohols) to obtain the product free of calcium chloride. \[\ce{CaF2.3Ca3(PO4)2} + 20\; \ce{HCl} \rightarrow \ce{10CaCl2} + \ce{2HF}\uparrow +\; \ce{6H3PO4}\]

  • Nitric acid has also been used to obtain phosphoric acid from phosphate rock. Process difficulties in obtaining a pure phosphoric acid using nitric acid have resulted in this chemistry only being utilized to prepare granular fertilizers, rather than acid preparation.

• Food-grade applications require removal of traces of arsenious oxide. Arsenic is present to the extent of 50–180 ppm (as \(\ce{As2O3}\) equivalent) in the feed phosphorus because of the similarity of its chemical properties to those of phosphorus.

• 100% \(\ce{H3PO4}\) is a colorless solid, with melting point of 42\(^\circ\)C. The usual laboratory concentration is 85% \(\ce{H3PO4}\).

8.4 Super Phosphates

Single Super Phosphate (SSP) (\(\ce{Ca(H2PO4)2.H2O}\))

• It is made by reacting phosphate rock with sulfuric acid. \[\ce{CaF2.3Ca3 (PO4)2} + \ce{7H2SO4} + \ce{14H2O} \rightarrow \ce{3Ca(H2PO4)2} + \ce{7CaSO4.2H2O} + \ce{2HF}\]

• SSP is a straight phosphatic multi-nutrient fertilizer which contains 16–20% \(\ce{P2O5}\), 12% sulphur, 21% calcium and some other essential micro nutrients in small proportions. SSP is a poor farmer’s fertilizer (price-wise).

• SSP has found wide-spread use as a fertilizer where both P and S are needed. As a source of P alone, SSP often costs more than other more concentrated fertilizers, therefore it has declined in popularity.

Triple Super Phosphate (TSP) (\(\ce{Ca(H2PO4)2.H2O}\))

• It is made by the action of phosphoric acid on phosphate rock; no diluent calcium sulfate is formed. \[\ce{CaF2. 3Ca3 (PO4)2} + \ce{14H3PO4} \rightarrow \ce{10Ca (H2PO4)2} + \ce{2HF}\uparrow\]

• This material is a much more concentrated fertilizer than ordinary super phosphate, containing from 45–46% of available \(\ce{P2O5}\), or nearly 3 times the amount in regular phosphate.

• With the advent of ammonium phosphate fertilizers, the popularity of TSP has declined. This is because of the total nutrient content (\(\ce{N + P2O5}\)) is lower in TSP than ammonium phosphates. For example, the mono-ammonium phosphate, which by comparison, contains 11% \(\ce{N}\) and 52% \(\ce{P2O5}\).

8.5 Ammonium Phosphates

• Ammonium phosphates usually are manufactured by neutralizing phosphoric acid with ammonia in which control of the pH (acidity/alkalinity) determines which of the ammonium phosphates will be produced.

  • Di-ammonium phosphate (\(\ce{(NH4)2HPO4}\), DAP)

  • Mono-ammonium phosphate (\(\ce{NH4H2PO4}\), MAP)

• In 2002, production percentages for phosphate fertilizers are: DAP – 67%; MAP – 26%; and TSP – 7%.

• All fertilizers should be managed to avoid losses in surface water runoff from fields. Phosphorus loss from agricultural land to adjacent surface water can contribute to undesired stimulation of algae growth—known us eutrophication.