Calcination (also referred to as calcining) is a thermal treatment process in presence of air or oxygen
applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or
removal of a volatile fraction.[1] The calcination process normally takes place at temperatures below
the melting point of the product materials. Calcination is not the same process asroasting. In roasting,
more complex gas–solid reactions take place between the furnace atmosphere and the solids.
Contents
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1 Industrial processes
2 Calcination reactions
3 Oxidation
4 Alchemy
5 References
Industrial processes[edit]
An oven for calcination of limestone
The process of calcination derives its name from the Latin calcinare (to burn lime)[2] due to its most
common application, the decomposition of calcium carbonate (limestone) to calcium oxide (lime)
and carbon dioxide, in order to produce cement. The product of calcination is usually referred to in
general as "calcine," regardless of the actual minerals undergoing thermal treatment. Calcination is
carried out in furnaces or reactors (sometimes referred to askilns or calciners) of various designs
including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized bed reactors.
Examples of calcination processes include the following:
decomposition of carbonate minerals, as in the calcination of limestone to drive off carbon dioxide;
decomposition of hydrated minerals, as in the calcination of bauxite and gypsum, to remove
crystalline water as water vapor;
decomposition of volatile matter contained in raw petroleum coke;
heat treatment to effect phase transformations, as in conversion
of anatase to rutile or devitrification of glass materials
removal of ammonium ions in the synthesis of zeolites.
Calcination reactions[edit]
Calcination reactions usually take place at or above the thermal decomposition temperature (for
decomposition and volatilization reactions) or the transition temperature (for phase transitions). This
temperature is usually defined as the temperature at which the standard Gibbs free energy for a particular
calcination reaction is equal to zero. For example, in limestone calcination, a decomposition process, the
chemical reaction is
CaCO3 → CaO + CO2(g)
The standard Gibbs free energy of reaction is approximated as ΔG°r = 177,100 − 158 T (J/mol).[3] The
standard free energy of reaction is zero in this case when the temperature, T, is equal to 1121 K, or
848 °C.
Examples of chemical decomposition reactions common in calcination processes, and their
respective thermal decomposition temperatures include:
CaCO3 → CaO + CO2; 848 °C
See also calcination equilibrium of calcium carbonate
Oxidation[edit]
In some cases, calcination of a metal results in oxidation of the metal. Jean Rey noted
that lead and tin when calcinated gained weight, presumably as they were being oxidized.
Alchemy[edit]
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 sulfur; and mercury with aqua fortis.[4]
There was also philosophical calcination, which was said to occur when horns, hooves, etc.,
were hung over boiling water, or other liquor, until they had lost their mucilage, and were easily
reducible into powder.[4]
