Lime is a general term referring to products derived from heated or "calcined" limestone (calcium carbonate, CaCO3), principally quicklime and hydrated lime. Limestone is a naturally occurring sedimentary rock formed by the compression of the remnants of coral animals and plants on ocean floors. Once heated between 2000-2400º F in rotary or vertical kilns, it forms quicklime, which is then hydrated under controlled conditions to form slaked, or hydrated lime.
Producing high calcium quicklime (calcium oxide, CaO) requires high temperatures for dissociation of the calcium carbonate with the resultant formation of calcium oxide and the generation of carbon dioxide:
CaCO3 + Heat --------> CaO + CO2
Hydrated Lime Production
High calcium quicklime readily reacts with water resulting in the formation of hydrated lime. The reaction is highly exothermic (generates heat), and the overall process is known as "slaking". The reaction is accomplished using a "slaker", a specially designed mixer which completely incorporates the added water with the quicklime, resulting in a fine, dry powder. Further hydration may produce a putty slurry or "milk of lime".
In general, the chemical composition of the hydrated lime mirrors that of the quicklime from which it was derived. Hydration of a high calcium quicklime will produce a high calcium hydrated lime containing 72 to 74 calcium oxide(CaO), and 23 to 24 percent water chemically bonded to the oxide. Hydration results in conversion of the parent oxides to hydroxides as follows:
CaO + H2O --------> Ca(OH)2+ Heat
Hydrated lime is valuable in many industrial applications, including waste and water treatment, the drying and stabilization of soil, Ph and odor control, agricultural applications, asphalt modification and building product components such as plasters and whitewash. It is also frequently used in the scrubbing of sulfur dioxide from stack gases emitted from industrial boilers and power plants.