CHEMISTRY OF LIME
|Hydrated Lime||Calcium Hydroxide||74.10||.||.||1076oF|
In everyday usage the terms "limestone" and "lime" are used by the general public interchangeably to mean the same material, however there are some significant differences between the two materials. Limestone is a sedimentary rock whereas lime is a manmade chemical which is produced from a sufficiently pure sedimentary rock by heating it to high temperature in a kiln. This process is referred to as "calcining" the limestone. (A chemical decomposition of limestone into the oxide and carbon dioxide.)
LIMESTONE: This term refers to a naturally occurring sedimentary rock which is relatively inert, except in the presence of a strong acid. With the proper purity the rock deposit can be used to produce "lime", a manmade chemical. Most often, limestone is found in nature in a mixed form known as "dolomite", which is a blend of calcium carbonate and magnesium carbonate in varying proportions. (In the Shelby County, AL area there are large deposits of limestone, primarily composed of calcium carbonate, which are used as the "raw material" for producing high calcium lime products.)
LIME: This term refers to either "quicklime", the product that is produced by heating the limestone to its decomposition temperature, or "hydrated lime", the product that is produced by the reaction of quicklime with water. (Lime in the form of high calcium quicklime, CaO readily reacts with water to form hydrated lime, which provides a pH of up to 12.454 when in an aqueous solution. Because of elemental differences between magnesium (Mg) and calcium (Ca) the compound magnesium oxide, MgO does not readily react with water at normal temperatures and pressures.
The production of high calcium quicklime (calcium oxide) requires a large amount of heat, which is generated in the kiln environment. The quarried and sized high calcium limestone travels through a rotary kiln and is subjected to these high temperatures where the calcium carbonate begins to decompose into calcium oxide and carbon dioxide. The minimum temperature for the decomposition of calcium carbonate into CaO and CO2 is 1648oF (898oC). For practical production purposes, however, the kiln temperature range is from an initial temperature of about 1750oF (954oC) to a final temperature of about 1950oF (1066oC). These temperatures can vary dependent upon the nature of the limestone being calcined.
"High Calcium" Limestone Calcination:
Hydrated Lime Production:
High calcium quicklime readily reacts with water to form hydrated lime. The reaction is highly exothermic and the process is known as "slaking". The reaction is usually carried out in a "slaker" (a specially designed mixer) which, through a process of rigorous mixing, makes certain that all of the quicklime has come into intimate contact with water and no unreacted quicklime remains. From a general viewpoint the hydrated lime produced can be in the form of dry hydrate, putty slurry, or "milk of lime". (At Cheney Lime we produce the dry hydrated lime that is sold in bulk or bags.) The exothermic reactions are shown below: (There are various types of slakers available on the market.)
"High Calcium" Quicklime Hydration:
Note: CaO will readily react with water under normal temperatures and pressures, whereas MgO will not.