Calcination

Calcination (also referred to as calcining) is a thermal treatment process applied to ores and other solid materials in order to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials. Calcination is to be distinguished from roasting, in which more complex gas-solid reactions take place between the furnace atmosphere and the solids, however calcination takes place in the absence of air.

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Industrial processes

The process of calcination derives its name from its most common application, the decomposition of calcium carbonate (limestone) to calcium oxide (lime). 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 as kilns) of various designs including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized bed reactors.

Examples of calcination processes include the following:

• decomposition of hydrated minerals, as in calcination of bauxite, to remove crystalline water as water vapor;
• decomposition of carbonate minerals, as in the calcination of limestone to drive off carbon dioxide;
• 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

Calcination reactions

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 Gibb's free energy of reaction 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 Gibb's free energy of reaction is approximated as ΔG°_r = 177,100 - 158 T (J/mol).^[1] The standard free energy of reaction is zero in this case when the temperature, T, is equal to 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

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

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. ^[2]

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.^[2]

Geological

Calcination can also occur under layers of hot volcanic ash. For example, calcination occurred under the ash of the Tungurahua volcano in Ecuador that erupted in 2006.^{[citation needed]}

References

1. ^ Gilchrist, J.D. Extraction Metallurgy, 3rd ed. Pergamon Press, Oxford, 1989, p. 145.
2. ^ ^{a b} This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.