The Fertiliser Industry Revision Notes for Grade 12 NSC Matric Physical Sciences

The Fertiliser Industry

The fertiliser industry represents one of the most important sectors in industrial chemistry, producing essential nutrients that support global food production. Understanding the interconnected processes involved in fertiliser manufacturing is crucial for NSC Physical Sciences students.

infoNote

The industrial production of fertilisers involves multiple interconnected chemical processes. Each process builds upon others to create a complex network of chemical manufacturing that ultimately produces the nitrogen, phosphorus, and potassium compounds needed for plant growth.

Raw material production

Producing hydrogen through steam reforming

Steam reforming is the primary industrial method for producing hydrogen gas, which serves as a crucial raw material in fertiliser production.

The process involves reacting methane (from natural gas) with steam at high temperatures (700-1100°C) to produce syngas - a mixture of carbon monoxide and hydrogen:

$\text{CH}_4\text{(g)} + \text{H}_2\text{O(g)} \rightarrow \text{CO(g)} + 3\text{H}_2\text{(g)}$

This reaction is endothermic, meaning it requires continuous heat input to maintain the process.

lightbulbExample

Step-by-Step: Water Gas Shift Reaction

Step 1: The water gas shift reaction occurs at lower temperatures (around 130°C)

Step 2: Steam reacts with carbon monoxide to produce additional hydrogen: $\text{CO(g)} + \text{H}_2\text{O(g)} \rightarrow \text{CO}_2\text{(g)} + \text{H}_2\text{(g)}$

Step 3: This exothermic reaction strips oxygen atoms from steam to oxidise carbon monoxide to carbon dioxide, while simultaneously producing more hydrogen gas and releasing energy to help maintain the overall process.

Obtaining nitrogen through fractional distillation

Fractional distillation is a separation method that exploits the different boiling points of air components to isolate nitrogen gas.

lightbulbExample

Process Steps: Air Separation

Air is compressed and heated, then cooled using liquid nitrogen
The compressed air expands and liquefies at extremely low temperatures (-200°C)
The liquefied air mixture is separated in a fractional distillation column
Nitrogen gas is collected at -190°C while oxygen remains as a liquid at -185°C

chatImportant

This process is essential because nitrogen makes up 78% of air but exists as unreactive N₂ molecules that plants cannot use directly.

Key industrial processes

The Haber process for ammonia production

The Haber process converts nitrogen and hydrogen gases into ammonia, which forms the foundation for most nitrogen-containing fertilisers.

The balanced equation is: $\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}$

chatImportant

Essential conditions:

Catalyst: Iron (Fe)
Pressure: 200 atmospheres
Temperature: 450-500°C

The reaction is exothermic and reversible. High pressure favours ammonia formation because there are fewer gas molecules on the product side (4 reactant molecules → 2 product molecules). However, high temperature actually decreases the yield due to Le Chatelier's principle, but is necessary to achieve reasonable reaction rates.

infoNote

The ammonia produced is immediately removed from the reaction vessel to prevent the reverse reaction from occurring, maximising the yield of product.

The Ostwald process for nitric acid production

The Ostwald process converts ammonia into nitric acid through a three-step oxidation sequence.

lightbulbExample

Step-by-Step: Ostwald Process

Step 1: Ammonia oxidation (strongly exothermic) $4\text{NH}_3\text{(g)} + 5\text{O}_2\text{(g)} \rightarrow 4\text{NO(g)} + 6\text{H}_2\text{O(g)}$

This step requires a platinum catalyst and produces nitrogen monoxide and water vapour.

Step 2: Nitrogen monoxide oxidation
$2\text{NO(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{NO}_2\text{(g)}$

Step 3: Nitrogen dioxide absorption $3\text{NO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightarrow 2\text{HNO}_3\text{(aq)} + \text{NO(g)}$

infoNote

The nitrogen monoxide produced in step 3 is recycled back into the process, making it highly efficient. The nitric acid is then concentrated to the required strength for further chemical processes.

The Contact process for sulphuric acid production

The Contact process produces sulphuric acid, which is needed for manufacturing certain fertilisers like ammonium sulphate.

lightbulbExample

Step-by-Step: Contact Process

Step 1: Sulphur combustion $\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}$

Step 2: Sulphur dioxide oxidation (using vanadium(V) oxide catalyst) $2\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{SO}_3\text{(g)}$

Step 3: Sulphur trioxide absorption $\text{SO}_3\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{SO}_4\text{(l)}$

This process produces sulphuric acid with 98-99% purity, making it suitable for industrial applications.

Fertiliser production

Ammonium nitrate production

Ammonium nitrate is produced by combining ammonia and nitric acid in an acid-base neutralisation reaction:

$\text{NH}_3\text{(l)} + \text{HNO}_3\text{(l)} \rightarrow \text{NH}_4\text{NO}_3\text{(s)}$

Ammonium nitrate appears as white crystalline crystals and is highly soluble in water. This makes it an excellent nitrogen source for plants, but also requires careful handling since it can be explosive under certain conditions.

infoNote

The compound contains both ammonium cations (NH₄⁺) and nitrate anions (NO₃⁻), providing plants with readily available nitrogen in two different chemical forms.

Urea production

Urea [(NH₂)₂CO] has the highest nitrogen content (46.4%) of all commonly used solid nitrogen fertilisers.

The production involves two reactions: $2\text{NH}_3\text{(g)} + \text{CO}_2\text{(g)} \rightleftharpoons \text{H}_2\text{NCOONH}_4\text{(s)}$

$\text{H}_2\text{NCOONH}_4\text{(s)} \rightleftharpoons \text{(NH}_2\text{)}_2\text{CO(aq)} + \text{H}_2\text{O(l)}$

lightbulbExample

Urea Production Process

Step 1: Ammonia reacts with carbon dioxide to form ammonium carbamate (H₂NCOONH₄) as an intermediate product

Step 2: The ammonium carbamate decomposes to form urea and water

Result: The final urea product is highly water-soluble and provides an efficient way to deliver nitrogen to plants.

Ammonium sulphate production

Ammonium sulphate is produced by reacting ammonia with sulphuric acid:

$2\text{NH}_3\text{(g)} + \text{H}_2\text{SO}_4\text{(l)} \rightleftharpoons \text{(NH}_4\text{)}_2\text{SO}_4\text{(s)}$

The solution is concentrated by evaporating water until ammonium sulphate crystals form. This fertiliser provides both nitrogen and sulphur, making it valuable for crops that require both nutrients.

Phosphoric acid and super phosphates

Phosphate fertilisers begin with producing phosphoric acid from phosphate rock (fluorapatite):

$\text{Ca}_5\text{(PO}_4\text{)}_3\text{F(s)} + 5\text{H}_2\text{SO}_4\text{(l)} \rightleftharpoons 5\text{CaSO}_4\text{(s)} + \text{HF(l)} + 3\text{H}_3\text{PO}_4\text{(l)}$

lightbulbExample

Super Phosphate Production

Single super phosphate (SSP) is produced by: $2\text{Ca}_5\text{(PO}_4\text{)}_3\text{F(s)} + 7\text{H}_2\text{SO}_4\text{(l)} \rightleftharpoons 3\text{Ca(H}_2\text{PO}_4\text{)}_2\text{(s)} + 7\text{CaSO}_4\text{(s)} + 2\text{HF(l)}$

Triple super phosphate (TSP) is produced by: $\text{Ca}_5\text{(PO}_4\text{)}_3\text{F(s)} + 7\text{H}_3\text{PO}_4\text{(l)} \rightleftharpoons 5\text{Ca(H}_2\text{PO}_4\text{)}_2\text{(s)} + \text{HF(l)}$

chatImportant

These processes convert insoluble phosphate rock into water-soluble forms that plants can absorb and utilise.

Compound fertilisers and NPK ratios

The nitrophosphate process produces complex fertilisers containing multiple nutrients. It involves acidifying calcium phosphate with nitric acid to create a mixture of phosphoric acid and calcium nitrate:

$\text{Ca}_5\text{(PO}_4\text{)}_3\text{(s)} + 6\text{HNO}_3\text{(l)} + 12\text{H}_2\text{O(l)} \rightarrow 2\text{H}_3\text{PO}_4\text{(aq)} + 3\text{Ca(NO}_3\text{)}_2\text{(aq)} + 12\text{H}_2\text{O(l)}$

When ammonia is added, this produces compound fertilisers containing nitrogen, phosphorus, and calcium. Adding potassium chloride or potassium sulphate creates NPK fertilisers that provide all three primary plant nutrients.

infoNote

NPK ratios (such as 4:2:3) indicate the relative proportions of nitrogen, phosphorus, and potassium in the fertiliser, allowing farmers to select products that match their soil's specific nutrient requirements.

Advantages and disadvantages of inorganic fertilisers

Advantages of inorganic fertilisers

Inorganic fertilisers offer several important benefits for modern agriculture:

Precise nutrient control: Manufacturers can produce fertilisers with exact nutrient concentrations to match specific crop requirements
Water solubility: Inorganic fertilisers dissolve readily in soil water, making nutrients immediately available to plant roots
High efficiency: Much smaller quantities are needed compared to organic fertilisers to achieve the same nutritional effect
Large-scale production: Industrial processes can produce vast quantities to meet global agricultural demands
Consistent quality: Manufacturing processes ensure uniform nutrient content across all batches

Disadvantages of inorganic fertilisers

However, inorganic fertilisers also present significant challenges:

Manufacturing costs: Industrial production requires substantial investment in chemicals and energy, making fertilisers expensive
Environmental pollution: Manufacturing processes release pollutants into the atmosphere, contributing to air quality problems
Soil contamination: Nutrients not absorbed by plants can accumulate in soil, potentially altering soil chemistry
Water pollution: Excess fertilisers wash away from fields and contaminate groundwater, rivers, and dams through leaching and runoff
Dependence on industrial processes: Agricultural productivity becomes reliant on continuous industrial chemical production

bookmarkSummary

Key Points to Remember:

The fertiliser industry connects multiple industrial processes: steam reforming → Haber process → Ostwald process → fertiliser production
Key equations to memorise: Haber process ( $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ ), Ostwald process (3 steps), and Contact process (3 steps)
Fractional distillation separates air components based on different boiling points to obtain pure nitrogen gas
NPK fertilisers provide the three primary plant nutrients: Nitrogen, Phosphorus, and Potassium in specific ratios
Inorganic fertilisers offer precise nutrient control and high efficiency but cause environmental concerns through pollution and water contamination

The Fertiliser Industry (Grade 12 NSC Matric Physical Sciences): Revision Notes