Lime slaking is a fundamental chemical process that transforms a raw material into a highly versatile compound used across numerous industries. It involves the controlled addition of water to quicklime, a process known as hydration. This reaction is one of the most ancient chemical transformations mastered by humans, serving as a basis for industrial chemistry and civil engineering for millennia. The final product is known as slaked or hydrated lime.
The Chemical Transformation
The process begins with quicklime, the common term for calcium oxide (CaO). This material is produced by heating limestone to high temperatures, driving off carbon dioxide in a process called calcination. The resulting quicklime is highly reactive and chemically unstable in the presence of moisture.
When water (H2O) is introduced, the slaking reaction begins immediately, producing calcium hydroxide (Ca(OH)2), or slaked lime. The chemical transformation is represented by the formula: CaO + H2O → Ca(OH)2 + Heat. This reaction is strongly exothermic, meaning it releases a significant amount of thermal energy.
The intense heat generated is a defining characteristic of the reaction. A large quantity of heat is released, which can raise the mixture’s temperature close to the boiling point of water. This energy release ensures the reaction is rapid and complete, converting the raw calcium oxide into the more stable calcium hydroxide. Slaked lime is less caustic and more easily handled than its precursor, quicklime.
Methods and Forms of Hydrated Lime
The slaking method employed determines the final physical form of the calcium hydroxide product. Industrial manufacturers often use dry slaking, where only the minimum amount of water necessary to complete the chemical reaction is used. This results in a fine, dry powder known commercially as hydrated lime. This powdered form is easy to package, transport, and store, making it the most common product form.
Conversely, wet slaking involves using an excess amount of water, which yields a paste or a liquid suspension. Using a slight excess of water results in a thick, highly plastic material called lime putty. Adding a large amount of water creates a dilute suspension called a lime slurry or milk of lime. The choice between dry or wet slaking is dictated by the intended application, with the powdered form preferred for dry mixing and the slurry for liquid-phase treatments.
Major Industrial Applications
The highly alkaline nature of slaked lime makes it a powerful material used across several major sectors. One significant use is in municipal water and sewage treatment, where it adjusts the water’s pH level. Increasing the alkalinity helps soften the water and effectively removes impurities like heavy metals and phosphates by causing them to precipitate out of the solution.
In the construction industry, slaked lime is a fundamental component of traditional building materials. It is mixed with sand and water to create mortars, plasters, and stucco, where it provides excellent workability and binding strength. Historically, its use in whitewash also provided a mild disinfecting and protective coating for surfaces.
Slaked lime also serves as an industrial neutralizing agent for acidic waste streams. Its strong alkalinity allows it to safely manage and treat acidic effluents from chemical plants and industrial processes. This use supports environmental compliance by helping to neutralize dangerous acids and aiding the safe disposal of industrial byproducts.
Handling the Exothermic Reaction
The highly exothermic nature of the slaking reaction requires careful safety protocols to manage the rapid heat release. When quicklime contacts water, the temperature can rise quickly, creating a risk of boiling water and violent steam eruptions that can cause severe thermal burns. Safe practice involves slowly adding the quicklime powder to the water, rather than pouring water onto the lime, to control the reaction rate and heat distribution.
Both quicklime and the resulting slaked lime are strongly alkaline and corrosive materials, posing a risk of chemical burns upon skin or eye contact. Workers must wear personal protective equipment, including chemical splash goggles, face shields, and waterproof gloves, to prevent direct exposure. Adequate ventilation is also necessary to prevent the inhalation of fine dust particles, which can irritate the respiratory system.