20.2 Chemical Industries in Pakistan & Environmental Chemistry

Chemical Industries in Pakistan & Environmental Chemistry

Intermediate chemistry ends with applied topics: how important chemicals are manufactured at scale, and how human activity changes the air and water. Pakistan's large fertilizer, cement, sugar and paper industries make these topics practical as well as examinable.

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1. Fertilizers

Plants need the macronutrients nitrogen (N), phosphorus (P) and potassium (K), together with smaller amounts of other elements. A good fertilizer is soluble enough to be taken up, stable in storage, and supplies the nutrient in a form plants can use.

Manufacture of Urea

Urea, $\text{NH}_2\text{CONH}_2$, is the most widely used fertilizer in Pakistan. Ammonia from the Haber process (using natural gas as the source of hydrogen) reacts with carbon dioxide (a by-product of making that hydrogen) under high pressure:

$2\text{NH}_3 + \text{CO}_2 \rightarrow \text{NH}_2\text{COONH}_4 \quad (\text{ammonium carbamate})$

$\text{NH}_2\text{COONH}_4 \rightarrow \text{NH}_2\text{CONH}_2 + \text{H}_2\text{O}$

The molten urea is sprayed down a tall prilling tower against a rising current of air, forming small solid pellets (prills).

Nitrogen content: $\frac{2 \times 14}{60} \times 100 \approx 46.7\%$. This is higher than ammonium nitrate (35%) or ammonium sulphate (about 21%), which is one reason urea is popular. In the soil, urea is hydrolysed to ammonia and then oxidized by bacteria to nitrates that plants absorb.

Phosphatic Fertilizers

Rock phosphate, $\text{Ca}_3(\text{PO}_4)_2$, is insoluble. Treating it with sulphuric acid gives single superphosphate, a mixture of soluble calcium dihydrogen phosphate and calcium sulphate; treating it with phosphoric acid gives the more concentrated triple superphosphate.

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2. Cement

Portland cement is the binding material of concrete.

Raw materials: limestone (calcium carbonate) and clay (silica, alumina and iron oxide), with a small amount of gypsum added at the end.

Manufacture:

1. Raw materials are crushed, mixed in the correct proportion and ground (dry or wet process).
2. The mixture is heated in a long, slowly rotating, inclined rotary kiln, fired by coal, oil or gas.
3. In the drying zone water evaporates; in the decomposition zone (around $900^\circ\text{C}$) limestone breaks down, $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$; in the hottest burning zone (about $1400$–$1500^\circ\text{C}$) the lime combines with silica, alumina and iron oxide to form clinker.
4. The cooled clinker is ground with 2–3% gypsum ($\text{CaSO}_4\cdot 2\text{H}_2\text{O}$), which slows the setting so the cement can be worked.

Main compounds in clinker: tricalcium silicate ($3\text{CaO}\cdot\text{SiO}_2$), dicalcium silicate ($2\text{CaO}\cdot\text{SiO}_2$), tricalcium aluminate ($3\text{CaO}\cdot\text{Al}_2\text{O}_3$) and tetracalcium aluminoferrite ($4\text{CaO}\cdot\text{Al}_2\text{O}_3\cdot\text{Fe}_2\text{O}_3$).

Setting and hardening: when mixed with water these compounds hydrate. The aluminate reacts fastest and is responsible for initial setting; the silicates hydrate more slowly and give long-term strength. Cement plants also emit a lot of carbon dioxide and dust, so they need pollution control.

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3. The Paper Industry

Paper is made of cellulose fibres. In Pakistan, raw materials include wheat straw, rice straw and bagasse (sugar-cane waste) as well as wood and recycled paper.

1. Pulping: the raw material is chopped and cooked to separate cellulose fibres from lignin (the material that binds them). In the neutral sulphite semi-chemical (NSSC) process, the material is digested with a solution of sodium sulphite buffered with sodium carbonate, which dissolves much of the lignin.
2. Washing and screening remove dissolved material and uncooked pieces.
3. Bleaching (for example with chlorine dioxide, hypochlorite or hydrogen peroxide) whitens the pulp.
4. Beating and sheet-forming: the pulp is beaten, mixed with fillers and sizing agents, spread on a moving wire mesh to drain, then pressed, dried on heated rollers and wound into reels.

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4. The Atmosphere and Air Pollution

Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and about 0.04% carbon dioxide by volume.

Acid Rain

Rain is naturally slightly acidic (pH about 5.6) because dissolved $\text{CO}_2$ forms carbonic acid. Acid rain has a pH below 5.6:

$2\text{SO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4 \qquad 4\text{NO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HNO}_3$

It damages buildings made of limestone and marble ($\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$), corrodes metals, harms forests and makes lakes too acidic for fish. Reducing sulphur in fuels, flue-gas desulphurization with lime, and catalytic converters reduce it.

Smog

• Classical (industrial) smog: smoke and fog together with $\text{SO}_2$, typical of cold, damp conditions with heavy coal burning.
• Photochemical smog: in strong sunlight, nitrogen oxides and hydrocarbons from vehicle exhaust react to form ozone, aldehydes and peroxyacetyl nitrate (PAN). It causes eye irritation, breathing problems and damage to plants. It is a secondary pollutant, formed in the air rather than emitted directly.

Ozone Depletion

The stratospheric ozone layer absorbs harmful ultraviolet radiation. Chlorofluorocarbons (CFCs), once used in refrigerators, aerosol sprays and foams, are stable enough to reach the stratosphere, where UV light breaks them and releases chlorine atoms:

$\text{CF}_2\text{Cl}_2 \xrightarrow{\text{UV}} \text{CF}_2\text{Cl} + \text{Cl}$

$\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \qquad \text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2$

The chlorine atom is regenerated, so one atom can destroy many thousands of ozone molecules. A thinner ozone layer increases skin cancer, cataracts and crop damage. The Montreal Protocol (1987) phased out CFCs, and the ozone layer is slowly recovering.

The Greenhouse Effect

Carbon dioxide, methane, water vapour, nitrous oxide and CFCs absorb infrared radiation emitted by the Earth's surface and re-emit part of it back down, warming the lower atmosphere. A natural greenhouse effect keeps the Earth habitable, but increased concentrations from burning fossil fuels and deforestation intensify it, causing global warming, melting glaciers and changing weather patterns.

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5. Water Pollution and Treatment

Sources: untreated sewage, industrial effluents (heavy metals such as lead, mercury and chromium from tanneries and factories), pesticides and fertilizer run-off, and oil spills.

• Eutrophication: excess nitrates and phosphates from fertilizers and detergents cause algae to grow rapidly; when they die, bacteria decompose them and use up dissolved oxygen, killing fish.
• Biochemical oxygen demand (BOD): the amount of dissolved oxygen used by microorganisms to decompose the organic matter in a water sample, usually over 5 days at $20^\circ\text{C}$. A high BOD means heavy organic pollution; clean water has a low BOD.
• Chemical oxygen demand (COD): the oxygen needed to oxidize the pollutants chemically (for example with acidified dichromate); it is usually higher than BOD because it includes material that microorganisms cannot break down.

Water treatment for drinking: screening, coagulation with alum (aluminium hydroxide flocs trap fine particles), sedimentation, sand filtration, and disinfection with chlorine (or ozone or UV light). Excess chlorine must be avoided because it can form harmful by-products.

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6. Solid Waste

Solid waste includes household rubbish, plastics, industrial and hospital waste. It is managed by:

• Reduce, reuse, recycle: paper, glass, metals and some plastics can be recycled.
• Composting of biodegradable waste into manure.
• Sanitary landfill: waste is compacted in lined pits and covered with soil to limit leakage and smell.
• Incineration: burning reduces volume and can generate energy, but it must be controlled to limit air pollution (burning plastics such as PVC releases toxic gases).
• Hospital and other hazardous wastes need separate collection and high-temperature incineration.