Quick lime

Quicklime, chemically known as calcium oxide (CaO), is a white, caustic, alkaline substance obtained by heating limestone (calcium carbonate) in a kiln to a high temperature. It is one of the oldest and most widely used industrial chemicals, with applications ranging from construction and metallurgy to agriculture and environmental management. Quicklime reacts vigorously with water to form slaked lime (calcium hydroxide), releasing heat in the process—a reaction fundamental to its industrial and chemical significance.

Production and Chemical Properties

Quicklime is produced through the calcination of limestone, a thermal decomposition process carried out in lime kilns at temperatures between 900°C and 1100°C. The reaction can be represented as:
CaCO₃ (s) → CaO (s) + CO₂ (g)
This endothermic reaction drives off carbon dioxide, leaving behind calcium oxide. The quality and reactivity of quicklime depend on the purity of the limestone, the burning temperature, and the method of calcination.
Quicklime is:

• White or greyish-white in appearance.
• Highly hygroscopic, readily absorbing moisture and carbon dioxide from the air.
• Exothermic when hydrated, reacting vigorously with water to produce heat and calcium hydroxide (slaked lime):CaO + H₂O → Ca(OH)₂ + heat

Because of this reactivity, quicklime must be stored in airtight containers to prevent it from degrading into calcium carbonate by atmospheric carbonation.

Historical Background

The use of lime dates back to ancient civilisations. The Egyptians and Romans employed lime-based mortars in construction, and the Chinese used it for plaster and soil stabilisation. In medieval Europe, lime kilns became widespread, producing quicklime for use in masonry, agriculture, and sanitation.
The industrial production of quicklime expanded during the Industrial Revolution, when it became essential for steelmaking, chemical processing, and water treatment. It remains a vital industrial material in the modern era, integral to processes that underpin construction, environmental engineering, and manufacturing.

Industrial and Commercial Applications

Quicklime is a versatile compound with numerous industrial applications:

• Construction: Quicklime is used in the production of lime mortar, cement, and plaster. When mixed with sand and water, it forms slaked lime, which sets through carbonation to form calcium carbonate, giving strength and durability to structures.
• Steel Industry: It acts as a flux in the extraction and refining of metals, particularly in the basic oxygen steelmaking process, where it removes impurities such as silica, phosphorus, and sulphur from molten iron.
• Chemical Industry: It serves as a reagent and neutralising agent in chemical synthesis, and in the manufacture of calcium carbide, soda ash (sodium carbonate), and bleaching powder (calcium hypochlorite).
• Water Treatment: Quicklime is used to soften water by removing carbonate hardness and to neutralise acidic waste streams in wastewater treatment plants.
• Agriculture: Farmers apply quicklime to reduce soil acidity, improve soil structure, and provide calcium as a nutrient for crops. It is also used to disinfect animal pens and manage organic waste.
• Environmental Uses: It plays a role in flue-gas desulphurisation, reducing sulphur dioxide emissions from power plants, and in stabilising sewage sludge and contaminated soils.

Types and Grades of Quicklime

Commercial quicklime varies in purity and physical form depending on its intended use. Common classifications include:

• High-calcium quicklime: Derived from nearly pure limestone, containing over 95% calcium oxide.
• Dolomitic quicklime: Produced from dolomitic limestone, containing both calcium oxide and magnesium oxide.
• Hydraulic lime precursor: Quicklime partially combined with silica, alumina, or iron oxide, used in building materials where water resistance is required.
• Ground, lump, or pebble lime: Classified according to particle size after calcination.

The reactivity of quicklime—particularly its rate of hydration and heat release—varies according to its composition and calcination conditions.

Safety and Handling

Quicklime is highly caustic and reactive, requiring careful handling. Contact with moisture, including perspiration, causes irritation and burns. Appropriate personal protective equipment (PPE)—such as gloves, goggles, and respiratory protection—should be worn when working with the substance.
Key safety measures include:

• Avoiding direct contact with skin and eyes.
• Preventing exposure to moisture to avoid unwanted hydration or heat release.
• Storing in sealed, dry containers away from acids and combustibles.

In case of accidental contact, the affected area should be flushed with copious amounts of water.

Environmental Considerations

While quicklime plays an important role in pollution control, its manufacture is energy-intensive and contributes to carbon dioxide emissions. Each tonne of calcium oxide produced releases roughly 0.79 tonnes of CO₂. Modern industry seeks to mitigate these emissions through carbon capture and utilisation technologies, improved kiln efficiency, and alternative fuels.
Despite these challenges, quicklime remains crucial for sustainable waste management, soil remediation, and air pollution control, where it helps offset its environmental impact.