Calcination: Encyclopedia - Calcination

Calcination

Calcination is the process of heating a substance to a high temperature, but below its melting or fusing point, to bring about thermal decomposition or a phase transition in its physical or chemical constitution. The process, which usually takes place in long cylindrical kilns, often has the effect of making a substance friable.

The objects of calcination are usually:

• to drive off water, present as absorbed moisture, as "water of crystallization", or as "water of constitution" (as in the conversion of ferric hydroxide to ferric oxide);
• to drive off carbon dioxide (as in the calcination of limestone into lime in a limekiln), sulphur dioxide, or other volatile constituent;
• to oxidize (oxidative calcination) a part or the whole of the substance (commonly used to convert metal sulfide ores to oxides in the first step of recovering such metals as zinc, lead, and copper);
• the reduction (reductive calcination) of metals from their ores (smelting).

There are a few other purposes for which calcination is employed in special cases (e.g. bone char).

Calcination reactions may include thermal dissociation, including the destructive distillation of organic compounds (i.e., heating a highly carbonaceous material in the absence of air or oxygen, to produce solids, liquids, and gases). Examples of other calcination reactions include the concentration of alumina by heating bauxite; polymorphic phase transitions such as the conversion of anatase to the rutile; and thermal recrystallizations such as the devitrification of glass. Materials that are commonly calcined include phosphate, aluminum oxide, manganese carbonate, petrol coke, and sea water magnesite.

In Alchemy, calcination is believed to be one of the 12 vital processes required for the transformation of a substance.

Categories: Chemical processes | Alchemical processes

Adapted from the Wikipedia article "Calcination", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki