Lime is a versatile material that has been used by humans for thousands of years, playing a significant role in various aspects of daily life. It is not a single substance but rather a term encompassing several products derived from limestone, primarily composed of calcium carbonate. This material finds widespread application across numerous industries, including construction, agriculture, and water treatment.
From Limestone to Quicklime
The journey from limestone to quicklime begins with the extraction of limestone, a sedimentary rock primarily composed of calcium carbonate (CaCO₃). This raw material is quarried or mined, then crushed, screened, and washed to prepare it for the next stage. The size of the crushed limestone can vary, from several inches down to dust-sized particles, depending on the specific requirements of the production process.
The core process for making lime is called calcination, also known as lime-burning. This involves heating the prepared limestone to very high temperatures, typically ranging from 900°C to 1000°C (1650°F to 1830°F). This thermal decomposition takes place in industrial kilns, such as rotary kilns or vertical kilns. Rotary kilns are rotating cylinders that are slightly inclined, allowing the limestone to move through as it heats.
During calcination, the intense heat causes a chemical transformation where calcium carbonate (CaCO₃) decomposes into calcium oxide (CaO) and carbon dioxide (CO₂). This reaction can be represented as: CaCO₃ → CaO + CO₂. Carbon dioxide is released, leaving behind calcium oxide, commonly known as quicklime or burnt lime. This process is endothermic, meaning it absorbs heat to drive the reaction.
The quicklime produced is then cooled after leaving the calcining zone. This product is a white, lumpy, and caustic powder.
Different Forms of Lime and Their Production
Quicklime (calcium oxide, CaO) is a highly reactive substance produced from limestone calcination. It appears as a dense, dull-white or grayish-white solid. Quicklime’s strong reactivity, especially with water, makes it suitable for applications requiring rapid chemical interactions, such as in steelmaking to remove impurities.
Another common form of lime is hydrated lime, also known as slaked lime (calcium hydroxide, Ca(OH)₂). It is produced by carefully adding water to quicklime, a process called hydration or slaking. The reaction is represented by the chemical equation: CaO + H₂O → Ca(OH)₂. This reaction is notably exothermic, meaning it releases a significant amount of heat.
The slaking process can be controlled to produce different forms of hydrated lime. When just enough water is added to hydrate the quicklime while keeping it as a powder, it is known as dry hydration, resulting in a fine, dry powder. If excess water is used, the process is called slaking, yielding a lime slurry. Hydrated lime is a fine, stable, bright white powder, which is less reactive than quicklime and generally safer to handle and store.