The nitrophosphate process (also known as the Odda process) was a method for the industrial production of nitrogen fertilizers invented by Erling Johnson in the municipality of Odda, Norway around 1927.
- Ca3(PO4)2 + 6 HNO3 + 12 H2O → 2 H3PO4 + 3 Ca(NO3)2 + 12 H2O
The mixture is cooled to below 0 °C, where the calcium nitrate crystallizes and can be separated from the phosphoric acid.
- 2 H3PO4 + 3 Ca(NO3)2 + 12 H2O → 2 H3PO4 + 3 Ca(NO3)2·4H2O
The resulting calcium nitrate produces nitrogen fertilizer. The filtrate is composed mainly of phosphoric acid with some nitric acid and traces of calcium nitrate, and this is neutralized with ammonia to produce a compound fertilizer.
- Ca(NO3)2 + 4 H3PO4 + 8 NH3 → CaHPO4 + 2 NH4NO3 + 3(NH4)2HPO4
- Ca(NO3)2 + 2 NH3 + CO2 + H2O → 2 NH4NO3 + CaCO3
Both products can be worked up together as straight nitrogen fertilizer.
Although Johnson created the process while working for the Odda Smelteverk, his company never employed it. Instead, it licensed the process to Norsk Hydro, BASF, Hoechst, and DSM. Each of these companies used the process, introduced variations, and licensed it to other companies. Today, only Yara (Norsk Hydro), BASF, Borealis Agrolinz Melamine GmbH, and GNFC still use the Odda process. Due to the alterations of the process by the various companies who employed it, the process is now generally referred to as the nitrophosphate process.
Due to the byproduct ammonium nitrate which has lower value, the production of ammonium nitrophosphate fertilizer is not economical compared to diammonium phosphate which is produced from cheaper sulfuric acid or gypsum.