Calcination
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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), sulfur 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.
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[edit] Alchemy
In alchemy, calcination was believed to be one of the 12 vital processes required for the transformation of a substance.
Alchemists distinguished two kinds of calcination, actual and potential. Actual calcination is that brought about by actual fire, from wood, coals, or other fuel, raised to a certain temperature. Potential calcination is that brought about by potential fire, such as corrosive chemicals; for example, gold was calcined in a reverberatory furnace with mercury and sal ammoniac; silver with common salt and alkali salt; copper with salt and sulfur; iron with sal ammoniac and vinegar; tin with antimony; lead with sulfure; and mercury with aqua fortis. [1]
There was also philosophical calcination, which was said to occur when horns, hooves, etc, were hung over boiling water, or other liquor, till they had lost their mucilage, and were easily reducible into powder. [1]
[edit] Geological
Calcination can also occur under layers of hot volcanic ash. eg. Tungurahua volcano in Ecuador eruption in 2006.
[edit] See also
- philosophical calcination
[edit] References
- ^ a b This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.
