NPK fertilisers

• NPK fertilisers contain nitrogen, phosphorus and potassium
• NPK fertilisers are formulations of different salts
• How are compounds containing nitrogen made?

  Main compound is ammonium nitrate, produced in the haber process. This can then be used to produce nitric acid. We react the nitric acid with more ammonia to make ammonium nitrate
• Where does the potassium come from in NPK?

  Potassium comes from potassium chloride or potassium sulfate, which are mined directly from the ground and do not need to be processed.
• Where does the phosphorus come from in NPK?

  Phosphate rocks. This must be treated with nitric acid to produce phosphoric acid and calcium nitrate. This cannot be directly added to plants so phosphoric acid is neutralised with ammonia. This produces ammonium phosphate
• What if we treat phosphoric rocks with sulfuric acid?

  A mixture of calcium phosphate and calcium sulfate is made. This is called single superphosphate and this can be used in NPK
• What if we treat phosphoric rocks with phosphoric acid?

  Triple superphosphate and this can be found in NPK
• Ammonium nitrate in school labs:

  • use dilute solutions of ammonia and nitric acid
  • since it is safer
  • can only produce small amount (batch process)
• Ammonium nitrate in industry:

  • used as a gas and the nitric acid is concentrated
  • dangerous because it is exothermic
  • so heat must be safely removed
  • continuous process, so lots is produced constantly