Production of Phosphatic Fertilisers

Rock phosphate contains phosphorus mainly in poorly water-soluble apatite minerals, so plants cannot readily use it directly. Treatment with sulphuric acid converts this phosphate either into soluble monocalcium phosphate or into phosphoric acid, which is subsequently used to manufacture concentrated phosphatic fertilisers.

Direct acidulation: Single Superphosphate

Finely ground rock phosphate is mixed with sulphuric acid and water. The acid converts apatite into monocalcium phosphate, the water-soluble fertilising component, while calcium sulphate remains in the product as gypsum.

• The reaction mixture is allowed to cure so that acidulation proceeds sufficiently before the material is granulated or dispatched.
• Single Superphosphate supplies both phosphorus and sulphur because the calcium sulphate is retained in the fertiliser.

Phosphoric acid route

For concentrated fertilisers, rock phosphate is digested with sulphuric acid by the wet-process phosphoric acid route. This produces phosphoric acid and gypsum; the gypsum is separated by filtration, and the acid is concentrated.

• Phosphoric acid is reacted with ammonia to produce Diammonium Phosphate or Monoammonium Phosphate.
• It can also react with additional rock phosphate to manufacture Triple Superphosphate, whose phosphorus is largely present as soluble monocalcium phosphate.

Why acidulation creates plant-available phosphorus

Sulphuric acid supplies hydrogen ions that break down the stable calcium phosphate structure of apatite. The resulting phosphoric acid, monocalcium phosphate and ammonium phosphates dissolve much more readily in soil water, making phosphate available for plant uptake.

• In the phosphoric acid route, much of the sulphate leaves with gypsum rather than remaining in the final ammonium phosphate.
• Grinding, acid concentration, reaction conditions and curing influence the completeness of conversion and the proportion of water-soluble phosphate.