Fertilisers

Lessons

Senior 4 · Chemistry

Fertilisers

4 lessons

Lesson 3 of 4

Manufacture of Fertilisers

25 min

Bread from air

Almost every bag of nitrogen fertiliser starts as nitrogen from the air and hydrogen from natural gas, joined in the Haber process. In the 2024–25 financial year, MINAGRI reports, 92 520 tonnes of fertiliser were applied to Rwanda's food crops: 22 633 t of urea, 23 777 t of DAP, 24 510 t of NPK and 21 600 t of potassium chloride and blends, plus 16 762 t of lime to correct soil acidity. Blending is the step closest to the farm: in April 2019 MINAGRI and Morocco's OCP group launched a mobile soil-testing laboratory with the Rwanda Agriculture Board (RAB), and announced a planned 38 million US dollar fertiliser blending facility, so that NPK grades could be matched to Rwandan soils. This lesson follows the chemistry from raw materials to the bag.

Definition

Feedstock: a raw material fed into an industrial process. Example: natural gas (methane) is the feedstock for the hydrogen used in ammonia plants.

Phosphate rock: a mineral rich in calcium phosphate, mainly fluorapatite, $\mathrm{Ca_5(PO_4)_3F}$ , often simplified to $\mathrm{Ca_3(PO_4)_2}$ . It is almost insoluble in water, so plants cannot use it directly.

Blending and granulation: mixing straight fertilisers in set proportions to make a compound grade (blending), or forming the mixture into uniform granules (granulation), so every granule carries the same nutrients.

Figure 15.3: From raw materials to fertilisers. Ammonia is the starting point for every nitrogen fertiliser; sulfuric acid is needed to make phosphate rock soluble.

1. Ammonia: the Haber process

Nitrogen (from air) and hydrogen (made from natural gas and steam) combine over an iron catalyst at about 450 °C and 200 atm: $\mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)} \qquad \Delta H = -92\ \mathrm{kJ\,mol^{-1}}$ You studied the choice of these conditions in Senior 3 and in Unit 10. The carbon dioxide formed when hydrogen is made from methane is a useful by-product: it is the second raw material for urea.

2. Nitric acid and ammonium nitrate

In the Ostwald process ammonia is oxidised over a platinum–rhodium gauze at about 900 °C, and the nitrogen monoxide is oxidised further and absorbed in water: $\mathrm{4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O} \qquad \mathrm{2NO + O_2 \rightarrow 2NO_2} \qquad \mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}$ Neutralising the acid with more ammonia gives ammonium nitrate; the hot, concentrated solution is sprayed down a tower to form solid pellets (prills): $\mathrm{NH_3(g) + HNO_3(aq) \rightarrow NH_4NO_3(aq)}$

3. Urea

Liquid ammonia and carbon dioxide react at high pressure in two equilibrium steps: $\mathrm{2NH_3 + CO_2 \rightleftharpoons NH_2COONH_4} \qquad \Delta H = -117\ \mathrm{kJ\,mol^{-1}}\ \text{(ammonium carbamate; fast)}$ $\mathrm{NH_2COONH_4 \rightleftharpoons CO(NH_2)_2 + H_2O} \qquad \Delta H = +15.5\ \mathrm{kJ\,mol^{-1}}\ \text{(slow)}$ The second, endothermic step needs a high temperature (about 190 °C), which works against the first, exothermic step; a high pressure (well over 100 atm) pushes the first equilibrium to the right, because three molecules of gas become one molecule of carbamate. Unreacted ammonia and carbon dioxide are separated and recycled. The urea solution must be cooled quickly: above about 160 °C two urea molecules join to form biuret, which is toxic to crops: $\mathrm{2CO(NH_2)_2 \rightarrow NH_2CONHCONH_2 + NH_3}$

4. Ammonium sulfate

Ammonia is absorbed in sulfuric acid (from the Contact process): $\mathrm{2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4}$ . Ammonium sulfate is also formed as a by-product of some other chemical industries, for example in making methyl methacrylate, the raw material of acrylic plastic.

5. Phosphate fertilisers

Phosphate rock is made soluble by treating it with acid. With sulfuric acid it gives single superphosphate, a mixture of calcium dihydrogenphosphate and calcium sulfate: $\mathrm{Ca_3(PO_4)_2 + 2H_2SO_4 \rightarrow Ca(H_2PO_4)_2 + 2CaSO_4}$ With more sulfuric acid the rock gives phosphoric acid (the "wet process"); the calcium sulfate (gypsum) is filtered off: $\mathrm{Ca_3(PO_4)_2 + 3H_2SO_4 \rightarrow 2H_3PO_4 + 3CaSO_4}$ Phosphoric acid then makes triple superphosphate (no gypsum, so richer in phosphorus) and diammonium phosphate: $\mathrm{Ca_3(PO_4)_2 + 4H_3PO_4 \rightarrow 3Ca(H_2PO_4)_2} \qquad \mathrm{H_3PO_4 + 2NH_3 \rightarrow (NH_4)_2HPO_4}$

6. Potash and NPK

Potassium chloride is mined as the mineral sylvite (often mixed with rock salt) and purified by dissolving and recrystallising or by flotation. Finally, measured amounts of nitrogen, phosphate and potash fertilisers are blended, or granulated together, to make NPK grades such as 17-17-17.

A crystal of sylvite, natural potassium chloride, from the Carlsbad potash district of New Mexico, USA (about 5.5 cm across). Purified sylvite is the potash in most NPK fertilisers. Photo: Robert M. Lavinsky, CC BY-SA 3.0, via Wikimedia Commons

Worked example 1: urea from a tonne of ammonia

Calculate the maximum mass of urea that can be made from 1.00 tonne of ammonia, and the mass of carbon dioxide needed. ( $M$ : $\mathrm{NH_3}$ = 17.03, urea = 60.06, $\mathrm{CO_2}$ = 44.01)

Solution. Overall: $\mathrm{2NH_3 + CO_2 \rightarrow CO(NH_2)_2 + H_2O}$ . $n(\mathrm{NH_3}) = \frac{1.00 \times 10^{6}\ \mathrm{g}}{17.03\ \mathrm{g\,mol^{-1}}} = 5.87 \times 10^{4}\ \mathrm{mol} \qquad n(\text{urea}) = n(\mathrm{CO_2}) = 2.94 \times 10^{4}\ \mathrm{mol}$ $m(\text{urea}) = 2.94 \times 10^{4} \times 60.06 = 1.76 \times 10^{6}\ \mathrm{g} = 1.76\ \text{t} \qquad m(\mathrm{CO_2}) = 2.94 \times 10^{4} \times 44.01 = 1.29\ \text{t}$ More than a tonne of carbon dioxide is locked into urea for every tonne of ammonia used (it is released again when urea breaks down in the soil).

Worked example 2: percentage yield of DAP

A plant reacts 1.00 tonne of pure phosphoric acid with excess ammonia and obtains 1.30 tonnes of diammonium phosphate. Calculate the percentage yield. ( $M$ : $\mathrm{H_3PO_4}$ = 97.99, DAP = 132.06)

Solution. $\mathrm{H_3PO_4 + 2NH_3 \rightarrow (NH_4)_2HPO_4}$ : 1 mol of acid gives 1 mol of DAP. $n(\mathrm{H_3PO_4}) = \frac{1.00 \times 10^{6}}{97.99} = 1.021 \times 10^{4}\ \mathrm{mol} \qquad \text{theoretical DAP} = 1.021 \times 10^{4} \times 132.06 = 1.35 \times 10^{6}\ \mathrm{g}$ $\%\ \text{yield} = \frac{1.30}{1.348} \times 100 = 96.5\%$

Warning

Common mistakes. (1) Writing urea as made directly in one step: it forms through ammonium carbamate, in two equilibria. (2) Forgetting that phosphate rock must be treated with acid: $\mathrm{Ca_3(PO_4)_2}$ is insoluble and nearly useless to plants as it is. (3) Writing $\mathrm{NH_4H_2PO_4}$ for DAP: that is MAP (mono-ammonium phosphate); DAP is $\mathrm{(NH_4)_2HPO_4}$ . (4) Applying Le Chatelier's principle to temperature alone: in urea manufacture the two steps pull in opposite directions, so the conditions are a compromise.

Safety. Fertiliser plants handle ammonia (toxic and corrosive), concentrated acids, hot pressurised liquids and, in ammonium nitrate plants, a material that can explode in a fire. In a school laboratory, small amounts of ammonium salts can be made by neutralisation with dilute acid and dilute ammonia, wearing goggles; concentrated ammonia is opened only in a fume cupboard.

Tip

Every nitrogen fertiliser starts as ammonia; every soluble phosphate starts with an acid. Ammonia + acid = ammonium salt: + $\mathrm{HNO_3}$ → ammonium nitrate; + $\mathrm{H_2SO_4}$ → ammonium sulfate; + $\mathrm{H_3PO_4}$ → DAP. Ammonia + $\mathrm{CO_2}$ → urea.

Stamicarbon: How Urea Is Produced (4 min). Engineers from a company that designs urea plants explain how ammonia and carbon dioxide are turned into urea fertiliser.

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Practice

Name the raw materials needed to make (a) ammonium nitrate and (b) single superphosphate.
Write the three equations of the Ostwald process.
Write equations for the manufacture of (a) ammonium sulfate and (b) DAP.
Explain why phosphate rock is treated with acid before it is used as a fertiliser.
Explain why urea manufacture uses high pressure, and why the temperature is a compromise.
Calculate the mass of ammonium nitrate that can be made from 1.00 tonne of ammonia, if half of the ammonia is first converted to nitric acid with a 95% yield and all the nitric acid is then neutralised by the other half ( $M$ : $\mathrm{NH_3}$ = 17.03, $\mathrm{NH_4NO_3}$ = 80.05).
Calculate the mass of sulfuric acid needed to convert 1.00 tonne of $\mathrm{Ca_3(PO_4)_2}$ into single superphosphate ( $M$ : $\mathrm{Ca_3(PO_4)_2}$ = 310.18, $\mathrm{H_2SO_4}$ = 98.08).
Explain why biuret must be kept out of urea fertiliser, and how its formation is limited in the plant.

Show answersHide answers

(a) Air (nitrogen and oxygen), natural gas and water (hydrogen); (b) phosphate rock and sulfur (for sulfuric acid), with air and water.
$\mathrm{4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O}$ ; $\mathrm{2NO + O_2 \rightarrow 2NO_2}$ ; $\mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}$ .
(a) $\mathrm{2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4}$ ; (b) $\mathrm{H_3PO_4 + 2NH_3 \rightarrow (NH_4)_2HPO_4}$ .
Calcium phosphate is almost insoluble, so roots cannot absorb its phosphate; acid converts it to soluble $\mathrm{Ca(H_2PO_4)_2}$ or $\mathrm{H_3PO_4}$ salts.
The first step ( $\mathrm{2NH_3 + CO_2} \rightleftharpoons$ carbamate) reduces the number of gas molecules, so high pressure favours it. The second step is endothermic and slow, so it needs a high temperature, but that shifts the exothermic first step back; about 190 °C is the compromise.
$n(\mathrm{NH_3}) = 5.87 \times 10^{4}\ \mathrm{mol}$ ; half = $2.94 \times 10^{4}\ \mathrm{mol}$ → $\mathrm{HNO_3} = 0.95 \times 2.94 \times 10^{4} = 2.79 \times 10^{4}\ \mathrm{mol}$ ; $\mathrm{NH_4NO_3} = 2.79 \times 10^{4} \times 80.05 = 2.23 \times 10^{6}\ \mathrm{g} = 2.23\ \text{t}$ .
$n = \frac{1.00 \times 10^{6}}{310.18} = 3224\ \mathrm{mol}$ ; $\mathrm{H_2SO_4} = 2 \times 3224 = 6448\ \mathrm{mol}$ ; mass $= 6448 \times 98.08 = 6.32 \times 10^{5}\ \mathrm{g} = 0.632\ \text{t}$ .
Biuret is toxic to crops. It forms when urea is held above about 160 °C, so the urea solution is cooled quickly after synthesis and excess ammonia is kept in the reactor.

Quick check

Put the steps in making ammonium nitrate in order.

Tap the cards in the right order.

Key points

Ammonia (Haber) is the source of all nitrogen fertilisers: with $\mathrm{HNO_3}$ (Ostwald) it gives ammonium nitrate, with $\mathrm{H_2SO_4}$ ammonium sulfate, with $\mathrm{H_3PO_4}$ DAP, and with $\mathrm{CO_2}$ urea.
Urea forms in two equilibria via ammonium carbamate; high pressure and about 190 °C are a compromise; biuret must be avoided.
Insoluble phosphate rock is made soluble with acid: single superphosphate ( $\mathrm{H_2SO_4}$ ), phosphoric acid (wet process), triple superphosphate ( $\mathrm{H_3PO_4}$ ).
Potash (KCl) is mined; NPK grades are made by blending or granulation. MINAGRI reports 92 520 t of fertiliser applied to food crops in 2024–25.

Sources and credits

Image: Urea process plant UFFL 01.jpg by Mar11, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Image: Sylvite-mrz218b.jpg by Robert M. Lavinsky, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Rwandan data: Annual Report 2024–2025, MINAGRI (section 1.1.6.1); Rwanda, Morocco inaugurate mobile laboratory to improve soil fertility management, MINAGRI news, 30 April 2019.
Process chemistry adapted from the Wikipedia articles Urea, Superphosphate, Diammonium phosphate and Ammonium nitrate, Wikipedia contributors, licensed under CC BY-SA 4.0, adapted by Ganzaa.
Video: How urea is produced, Stamicarbon, embedded from YouTube.
The text, flow chart, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Bread from air

21. Ammonia: the Haber process

32. Nitric acid and ammonium nitrate

43. Urea

54. Ammonium sulfate

65. Phosphate fertilisers

76. Potash and NPK