calcining

Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O₂ fraction of air), generally for the purpose of removing impurities or volatile substances and/or to incur thermal decomposition.

The root of the word calcination refers to its most prominent use, which is to remove carbon from limestone (calcium carbonate) through combustion to yield calcium oxide (quicklime). This calcination reaction is CaCO₃(s) → CaO(s) + CO₂(g). Calcium oxide is a crucial ingredient in modern cement, and is also used as a chemical flux in smelting. Industrial calcination generally emits carbon dioxide (), making it a major contributor to climate change.

A calciner is a steel cylinder that rotates inside a heated furnace and performs indirect high-temperature processing (550–1150 °C, or 1000–2100 °F) within a controlled atmosphere.

Industrial processes

The process of calcination derives its name from the Latin ''calcinare'' (to burn lime) due to its most common application, the decomposition of calcium carbonate ( limestone) to calcium oxide ( lime) and carbon dioxide, in order to create 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 as kilns or calciners) of various designs including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized bed reactors.

The process involves the burning of the metal ore in limited oxygen to extract the ores.

Examples of calcination processes include the following:
*decomposition of carbonate ores, as in the calcination of limestone to drive off carbon dioxide;
*decomposition of hydrated minerals, as in the calcination of bauxite and gypsum,carbonate ore to remove water of crystallization 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;
*defluorination of uranyl fluoride to create uranium dioxide and hydrofluoric acid gas.

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 Gibbs free energy for a particular calcination reaction is equal to zero.

Limestone calcination

In limestone calcination, a decomposition process that occurs at 900 to 1050°C, the chemical reaction is
: CaCO₃(s) → CaO(s) + CO₂(g)

Today, this reaction largely occurs in a cement kiln.

The standard Gibbs free energy of reaction in /molis approximated as Δ''G''°_''r'' ≈ 177,100 J/mol − 158 J/(mol*K) *''T''. The standard free energy of reaction is 0 in this case when the temperature, ''T'', is equal to 1121K, or 848 °C.

Oxidation

In some cases, calcination of a metal results in oxidation of the metal to produce a metal oxide. In the seventeenth century, Jean Rey noted that lead and tin when calcinated gained mass, 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 salammoniac; 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 nitric acid.

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.

References

{{Extractive metallurgy

Chemical processes
Metallurgical processes
Alchemical processes