10.4 The Haber process and the use of NPK fertilisers

10.4.1 The Haber process

- The Haber process manufactures ammonia for nitrogen-based fertilisers by combining nitrogen from the air and hydrogen, typically derived from natural gas, then passing the purified gases over an iron catalyst at around 450°C and 200 atmospheres.
- Some nitrogen and hydrogen react to form ammonia, but the equilibrium is reversible, so some product reverts to the reactants.
- nitrogen + hydrogen ⇌ ammonia
- Cooling liquefies the ammonia for removal while unreacted gases are recycled.
- Understanding the dynamic equilibrium explains the need to balance rate and position: higher temperature increases rate but shifts equilibrium unfavorably, while higher pressure favors ammonia formation but increases costs.
- Commercial conditions reflect the availability and cost of raw materials, energy, and the need to maximise yield efficiently.
- For example, if the cost of energy is high, the process may operate at a slightly lower temperature to save on energy costs, even if it means a lower yield of ammonia.

10.4.2 Production and uses of NPK fertilisers

- Compounds of nitrogen, phosphorus and potassium are used as fertilisers to improve agricultural productivity.
- NPK fertilisers contain compounds of all three elements.
- Industrial production of NPK fertilisers can be achieved using a variety of raw materials in several integrated processes:

• Reaction of ammonia with nitric acid to form ammonium nitrate before blending.
• Potassium chloride, potassium sulfate and phosphate rock are obtained by mining, but phosphate rock cannot be used directly as a fertiliser.
• Phosphate rock is treated with nitric acid or sulfuric acid to produce soluble salts that can be used as fertilisers.
• This produces the salts calcium nitrate and phosphoric acid when treated with nitric acid, and when treated with sulfuric acid, it produces single superphosphate, which is a mixture of calcium sulfate and calcium phosphate.

• 25cm³ of ammonia in a conical flask
• Add dilute sulphuric acid, 1cm³ at a time, and swirl after each addition
• Dip a glass rod into the solution and then test a drop on blue litmus paper
• Keep adding until the litmus test turns pink and repeat until concordant results
• Pour the solution into an evaporating dish and heat it gently on water bath until half of the water has evaporated
• Leave the rest to crystallise

Laboratory vs Industrial methods

- In the laboratory, NPK fertilisers can be prepared by mixing appropriate amounts of ammonium nitrate, potassium chloride and phosphate rock, but this is not practical for large-scale production due to the need for precise measurements and the handling of large quantities of materials.
- Industrially, the production of NPK fertilisers involves more complex processes that allow for the efficient handling of raw materials and the production of large quantities of fertiliser. This includes the use of chemical reactors, separation techniques and blending processes to ensure the final product meets quality standards.