Calcination
Calcination is 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.[1]
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 (CO2), 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.[2]
Etymology
The process of calcination derives its name from the Latin calcinare 'to burn lime'[3] 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.
Industrial processes
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.
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 zeolites;
- defluorination of uranyl fluoride to create uranium dioxide and hydrofluoric acid gas;
- heat treatment of anthracite through electrically fired calcining furnace or gas calcination which results in development of graphitic structure.
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 [J/mol] is approximated as ΔG°r ≈ 177,100 J/mol − 158 J/(mol*K) * T.[4] The standard free energy of reaction is 0 in this case when the temperature, T, is equal to 1121 K, or 848 °C.
Oxidation
In some cases, calcination of a metal results in
At room temperature, tin is quite resistant to the impact of air or water, as a thin oxide film forms on the surface of the metal. In air, tin starts to oxidize at a temperature of over 150 °C: Sn + O2 → SnO2.[6]
Antoine Lavoisier explored this experiment with similar results time later.[7]
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
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.[8]
According to the obsolete phlogiston theory, the 'calx' was the true elemental substance that was left after phlogiston was driven out of it in the process of combustion.[9]
References
- .
- ^ "High-Temperature Processing with Calciners".
- ^ Mosby's Medical, Nursing and Allied Health Dictionary, Fourth Edition, Mosby-Year Book Inc., 1994, p. 243
- ISBN 978-0-08-036612-8.
- OCLC 154124030.
- ^ "Tin: its oxidation states and reactions with it".
- ^ "Lavoisier tin calcination".
- ^ a b This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "Calcination". Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al.
- ISBN 9780191726569– via Oxford Reference.
In the early 18th century Georg Stahl renamed the substance phlogiston (from the Greek for 'burned') and extended the theory to include the calcination (and corrosion) of metals. Thus, metals were thought to be composed of calx (a powdery residue) and phlogiston; when a metal was heated, phlogiston was set free and the calx remained. The process could be reversed by heating the metal over charcoal (a substance believed to be rich in phlogiston, because combustion almost totally consumed it). The calx would absorb the phlogiston released by the burning charcoal and become metallic again.