Calcination isthermal treatment of a solid chemical compound (e.g. mixedcarbonateores) whereby the compound is raised to high temperature without melting under restricted supply of ambientoxygen (i.e. gaseous O2 fraction of air), generally for the purpose of removing impurities orvolatile substances and/or to incurthermal decomposition.[1]
The root of the word calcination refers to its most prominent use, which is to remove carbon fromlimestone (calcium carbonate) throughcombustion to yieldcalcium oxide (quicklime). This calcination reaction is CaCO3(s) → CaO(s) + CO2(g). Calcium oxide is a crucial ingredient in moderncement, and is also used as a chemicalflux insmelting. Industrial calcination generally emitscarbon dioxide (CO2).
Acalciner 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]
The process of calcination derives its name from the Latincalcinare 'to burn lime'[3] due to its most common application, the decomposition of calcium carbonate (limestone) tocalcium oxide (lime) andcarbon dioxide, in order to createcement. The product of calcination is usually referred to in general as "calcine", regardless of the actual minerals undergoing thermal treatment.
Calcination is carried out infurnaces or reactors (sometimes referred to askilns or calciners) of various designs including shaft furnaces,rotary kilns,multiple hearth furnaces, andfluidized bed reactors.
Examples of calcination processes include the following:
Calcination reactions usually take place at or above the thermal decomposition temperature (for decomposition and volatilization reactions) or the transition temperature (forphase transitions). This temperature is usually defined as the temperature at which the standardGibbs free energy for a particular calcination reaction is equal to zero.
In limestone calcination, a decomposition process that occurs at 900 to 1050 °C, the chemical reaction is
Today,[when?] this reaction largely occurs in acement 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.
In some cases, calcination of a metal results inoxidation of the metal to produce ametal oxide. In his essay "Formal response to the question, why Tin and Lead increase in weight when they are calcined" (1630),Jean Rey notes that "having placed two pounds six ounces of fine English tin in an iron vessel and heated it strongly on an open furnace for the space of six hours with continual agitation and without adding anything to it, he recovered two pounds thirteen ounces of a white calx". He claimed "That this increase in weight comes from the air, which in the vessel has been rendered denser, heavier, and in some measure adhesive, by the vehement and long-continued heat of the furnace: which air mixes with the calx (frequent agitation aiding) and becomes attached to its most minute particles: not otherwise than water makes heavier sand which you throw into it and agitate, by moistening it and adhering to the smallest of its grains", presumably the metal gained weight as it was being oxidized.[5]
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]
Inalchemy, calcination was believed to be one of the12 vital processes required for the transformation of a substance.
Alchemists distinguished two kinds of calcination,actual andpotential. 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 bypotential fire, such as corrosive chemicals; for example, gold was calcined in areverberatory furnace withmercury andsalammoniac; silver with common salt andalkali salt;copper with salt andsulfur;iron with salammoniac andvinegar; tin withantimony; lead with sulfur; and mercury withnitric acid.[8]
There was alsophilosophical calcination, which was said to occur when horns, hooves, etc., were hung over boiling water, or other liquor, until they had lost theirmucilage, and were easily reducible into powder.[8]
According to the obsoletephlogiston theory, the 'calx' was the true elemental substance that was left after phlogiston was driven out of it in the process ofcombustion.[9]
This article incorporates text from a publication now in thepublic domain: Chambers, Ephraim, ed. (1728)."Calcination".Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al.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.