5.2 The fertilizer industry  (Page 2/3)

However, all these natural processes of maintaining soil nutrients take a long time. As populations grow and the demand for food increases, there is more and more strain on the land to produce food. Often, cultivation practices don't give the soil enough time to recover and to replace the nutrients that have been lost. Today, fertilisers play a very important role in restoring soil nutrients so that crop yields stay high. Some of these fertilisers are organic (e.g. compost, manure and fishmeal), which means that they started off as part of something living. Compost for example is often made up of things like vegetable peels and other organic remains that have been thrown away. Others are inorganic and can be made industrially. The advantage of these commercial fertilisers is that the nutrients are in a form that can be absorbed immediately by the plant.

Fertiliser

A fertiliser is a compound that is given to a plant to promote growth. Fertilisers usually provide the three major plant nutrients and most are applied via the soil so that the nutrients are absorbed by plants through their roots.

When you buy fertilisers from the shop, you will see three numbers on the back of the packet e.g. 18-24-6. These numbers are called the NPK ratio , and they give the percentage of nitrogen, phosphorus and potassium in that fertiliser. Depending on the types of plants you are growing, and the way in which you would like them to grow, you may need to use a fertiliser with a slightly different ratio. If you want to encourage root growth in your plant for example, you might choose a fertiliser with a greater amount of phosphorus. Look at the table below, which gives an idea of the amounts of nitrogen, phosphorus and potassium there are in different types of fertilisers. Fertilisers also provide other nutrients such as calcium, sulfur and magnesium.

The industrial production of fertilisers

The industrial production of fertilisers may involve several processes.

1. Nitrogen fertilisers Making nitrogen fertilisers involves producing ammonia , which is then reacted with oxygen to produce nitric acid . Nitric acid is used to acidify phosphate rock to produce nitrogen fertilisers. The flow diagram below illustrates the processes that are involved. Each of these steps will be examined in more detail.

   

   1. The Haber Process The Haber process involves the reaction of nitrogen and hydrogen to produce ammonia. Nitrogen is produced through the fractional distillation of air. Fractional distillation is the separation of a mixture (remember that air is a mixture of different gases) into its component parts through various methods. Hydrogen can be produced through steam reforming . In this process, a hydrocarbon such as methane reacts with water to form carbon monoxide and hydrogen according to the following equation: ${\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CO}+3{\mathrm{H}}_2$ Nitrogen and hydrogen are then used in the Haber process. The equation for the Haber process is: ${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\to 2{\mathrm{NH}}_3\left(\mathrm{g}\right)$ (The reaction takes place in the presence of an iron (Fe) catalyst under conditions of 200 atmospheres (atm) and 450-500 degrees Celsius)

      Interesting fact

      The Haber process developed in the early 20th century, before the start of World War 1. Before this, other sources of nitrogen for fertilisers had included saltpeter (NaNO ${}_3$ ) from Chile and guano. Guano is the droppings of seabirds, bats and seals. By the 20th century, a number of methods had been developed to 'fix' atmospheric nitrogen. One of these was the Haber process, and it advanced through the work of two German men, Fritz Haber and Karl Bosch (The process is sometimes also referred to as the 'Haber-Bosch process'). They worked out what the best conditions were in order to get a high yield of ammonia, and found these to be high temperature and high pressure. They also experimented with different catalysts to see which worked best in that reaction. During World War 1, the ammonia that was produced through the Haber process was used to make explosives. One of the advantages for Germany was that, having perfected the Haber process, they did not need to rely on other countries for the chemicals that they needed to make them.
   2. The Ostwald Process The Ostwald process is used to produce nitric acid from ammonia. Nitric acid can then be used in reactions that produce fertilisers. Ammonia is converted to nitric acid in two stages. First, it is oxidised by heating with oxygen in the presence of a platinum catalyst to form nitric oxide and water. This step is strongly exothermic, making it a useful heat source. $4{\mathrm{NH}}_3\left(\mathrm{g}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right)\to 4\mathrm{NO}\left(\mathrm{g}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$ Stage two, which combines two reaction steps, is carried out in the presence of water. Initially nitric oxide is oxidised again to yield nitrogen dioxide: $2\mathrm{NO}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{NO}}_2\left(\mathrm{g}\right)$ This gas is then absorbed by the water to produce nitric acid. Nitric oxide is also a product of this reaction. The nitric oxide (NO) is recycled, and the acid is concentrated to the required strength. $3{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{g}\right)$
   3. The Nitrophosphate Process The nitrophosphate process involves acidifying phosphate rock with nitric acid to produce a mixture of phosphoric acid and calcium nitrate: ${\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2+6{\mathrm{HNO}}_3+12{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}_3{\mathrm{PO}}_4+3\mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2+12{\mathrm{H}}_2\mathrm{O}$ When calcium nitrate and phosphoric acid react with ammonia, a compound fertiliser is produced. $\mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2+4{\mathrm{H}}_3{\mathrm{PO}}_4+8{\mathrm{NH}}_3\to {\mathrm{CaHPO}}_4+2{\mathrm{NH}}_4{\mathrm{NO}}_3+8{\left({\mathrm{NH}}_4\right)}_2{\mathrm{HPO}}_4$ If potassium chloride or potassium sulphate is added, the result will be NPK fertiliser.
   4. Other nitrogen fertilisers
      • Urea ((NH ${}_2$ ) ${}_2$ CO) is a nitrogen-containing chemical product which is produced on a large scale worldwide. Urea has the highest nitrogen content of all solid nitrogeneous fertilisers in common use (46.4%) and is produced by reacting ammonia with carbon dioxide. Two reactions are involved in producing urea:
        1. $2{\mathrm{NH}}_3+{\mathrm{CO}}_2\to {\mathrm{H}}_2\mathrm{N}-{\mathrm{COONH}}_4$
        2. ${\mathrm{H}}_2\mathrm{N}-{\mathrm{COONH}}_4\to {\left({\mathrm{NH}}_2\right)}_2\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}$
      • Other common fertilisers are ammonium nitrate and ammonium sulphate. Ammonium nitrate is formed by reacting ammonia with nitric acid. ${\mathrm{NH}}_3+{\mathrm{HNO}}_3\to {\mathrm{NH}}_4{\mathrm{NO}}_3$ Ammonium sulphate is formed by reacting ammonia with sulphuric acid. $2{\mathrm{NH}}_3+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$
2. Phosphate fertilisers The production of phosphate fertilisers also involves a number of processes. The first is the production of sulfuric acid through the contact process . Sulfuric acid is then used in a reaction that produces phosphoric acid. Phosphoric acid can then be reacted with phosphate rock to produce triple superphosphates.
   1. The production of sulfuric acid Sulfuric acid is produced from sulfur, oxygen and water through the contact process. In the first step, sulfur is burned to produce sulfur dioxide. $\mathrm{S}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{SO}}_2\left(\mathrm{g}\right)$ This is then oxidised to sulfur trioxide using oxygen in the presence of a vanadium(V) oxide catalyst. $2{\mathrm{SO}}_2+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{SO}}_3\left(\mathrm{g}\right)$ Finally the sulfur trioxide is treated with water to produce 98-99% sulfuric acid. ${\mathrm{SO}}_3\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)$
   2. The production of phosphoric acid The next step in the production of phosphate fertiliser is the reaction of sulfuric acid with phosphate rock to produce phosphoric acid (H ${}_3$ PO ${}_4$ ). In this example, the phosphate rock is fluoropatite (Ca ${}_5$ F(PO ${}_4$ ) ${}_3$ ). ${\mathrm{Ca}}_5\mathrm{F}{\left({\mathrm{PO}}_4\right)}_3+5{\mathrm{H}}_2{\mathrm{SO}}_4+8{\mathrm{H}}_2\mathrm{O}\to 5{\mathrm{CaSO}}_4+\mathrm{HF}+3{\mathrm{H}}_3{\mathrm{PO}}_4$
   3. The production of phosphates and superphosphates When concentrated phosphoric acid reacts with ground phosphate rock, triple superphosphate is produced. $3{\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2·{\mathrm{CaF}}_2+12{\mathrm{H}}_3{\mathrm{PO}}_4\to 9\mathrm{Ca}{\left({\mathrm{H}}_2{\mathrm{PO}}_4\right)}_2+3{\mathrm{CaF}}_2$
3. Potassium Potassium is obtained from potash , an impure form of potassium carbonate (K ${}_2$ CO ${}_3$ ). Other potassium salts (e.g. KCl AND K ${}_2$ O) are also sometimes included in fertilisers.