Nitrophosphate process
The nitrophosphate process (also known as the Odda process) was a method for the industrial production of nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...

s invented by Erling Johnson in the city of Odda, Norway around 1927.

The process involves acidifying phosphate rock with nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

 to produce a mixture of phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

 and calcium nitrate
Calcium nitrate
Calcium nitrate, also called Norgessalpeter , is the inorganic compound with the formula Ca2. This colourless salt absorbs moisture from the air and is commonly found as a tetrahydrate. It is mainly used as a component in fertilizers but is found other applications...

Ca3(PO4)2 + 6 HNO3 + 12 H2O → 2 H3PO4 + 3 Ca(NO3)2 + 12 H2O

The mixture is cooled to below 0 degrees Celsius, 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
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 to produce a compound fertilizer.
Ca(NO3)2 + 4 H3PO4 + 8 NH3 → CaHPO4 + 2 NH4NO3 + 3(NH4)2HPO4

If potassium chloride
Potassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...

 or potassium sulfate
Potassium sulfate
Potassium sulfate is a non-flammable white crystalline salt which is soluble in water...

 is added, the result will be NPK fertilizer. The process was an innovation for requiring neither the expensive sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 nor producing gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...


The calcium nitrate mentioned before, can as said be worked up as calcium nitrate fertilizer but often it is converted into ammonium nitrate and calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 using carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 and ammonia.
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
Norsk Hydro
Norsk Hydro ASA is a Norwegian aluminium and renewable energy company, headquartered in Oslo. Hydro is the fourth largest integrated aluminium company worldwide. It has operations in some 40 countries around the world and is active on all continents. The Norwegian state holds a 43.8 percent...

BASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...

, Hoechst
Hoechst or Höchst may refer to:* Hoechst AG, a former German life-sciences company* Hoechst stain, one of a family of fluorescent DNA-binding compounds* Höchst , Germany* Höchst im Odenwald, Germany* Höchst, Austria...

, and DSM. Each of these companies used the process, introduced variations, and licensed it to other companies. Today, only Yara
Yara International
Yara International ASA is a Norwegian-based chemical company. Its largest business area is the production of nitrogen fertilizer, however it also encompasses the production of dry ice, nitrates, ammonia, urea and other nitrogen-based chemicals....

(Norsk Hydro), BASF, AgroLinz, 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.
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