Calcination

Calcinationisthermal treatmentof a solid chemical compound (e.g. mixedcarbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambientoxygen(i.e. gaseous O₂fraction of air), generally for the purpose of removing impurities orvolatilesubstances 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) throughcombustionto 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 chemicalfluxinsmelting.Industrial calcination generally emitscarbon dioxide(CO₂).

Acalcineris 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 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.

Industrial processes

Calcination is carried out infurnacesor reactors (sometimes referred to askilnsor calciners) of various designs including shaft furnaces,rotary kilns,multiple hearth furnaces,andfluidized bed reactors.

Examples of calcination processes include the following:

decomposition of carbonate ores, as in the calcination oflimestoneto drive offcarbon dioxide;
decomposition of hydrated minerals, as in the calcination ofbauxiteandgypsum,carbonate oreto removewater of crystallizationas water vapor;
decomposition of volatile matter contained in rawpetroleum coke;
heat treatment to effect phase transformations, as in conversion ofanatasetorutileordevitrificationofglassmaterials;
removal ofammonium ionsin thesynthesisofzeolites;
defluorination of uranyl fluoride to createuranium dioxideandhydrofluoric acidgas;
heat treatment ofanthracitethrough 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 (forphase transitions). This temperature is usually defined as the temperature at which the standardGibbs free energyfor 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 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 1121K, or 848 °C.

Oxidation

In some cases, calcination of a metal results inoxidationof 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 Reynotes 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 Lavoisierexplored this experiment with similar results time later.^[7]

Alchemy

Inalchemy,calcination was believed to be one of the12 vital processesrequired for the transformation of a substance.

Alchemists distinguished two kinds of calcination,actualandpotential.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 bypotentialfire, such as corrosive chemicals; for example, gold was calcined in areverberatory furnacewithmercuryandsalammoniac;silver with common salt andalkali salt;copperwith salt andsulfur;ironwith 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]

References

^"Calcination".The IUPAC Compendium of Chemical Terminology.2014.doi:10.1351/goldbook.C00773.
^"High-Temperature Processing with Calciners".
^Mosby's Medical, Nursing and Allied Health Dictionary, Fourth Edition, Mosby-Year Book Inc., 1994, p. 243
^Gilchrist, J.D. (1989).Extraction Metallurgy(3rd ed.). Oxford: Pergamon Press. p. 145.ISBN 978-0-08-036612-8.
^Rey, Jean (1953).Essays of Jean Rey, doctor of medicine, on an enquiry into the cause wherefore tin and lead increase in weight on calcination (1630).E. & S. Livingstone for the Alembic Club.OCLC 154124030.
^"Tin: its oxidation states and reactions with it".
^"Lavoisier tin calcination".
^^a ^bThis 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.
^Daintith, John, ed. (2008)."Phlogiston theory".A Dictionary of Chemistry(6th ed.).Oxford University Press.doi:10.1093/acref/9780199204632.001.0001.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.

[1] "Calcination".The IUPAC Compendium of Chemical Terminology.2014.doi:10.1351/goldbook.C00773.

[2] "High-Temperature Processing with Calciners".

[3] Mosby's Medical, Nursing and Allied Health Dictionary, Fourth Edition, Mosby-Year Book Inc., 1994, p. 243

[4] Gilchrist, J.D. (1989).Extraction Metallurgy(3rd ed.). Oxford: Pergamon Press. p. 145.ISBN 978-0-08-036612-8.

[5] Rey, Jean (1953).Essays of Jean Rey, doctor of medicine, on an enquiry into the cause wherefore tin and lead increase in weight on calcination (1630).E. & S. Livingstone for the Alembic Club.OCLC 154124030.

[6] "Tin: its oxidation states and reactions with it".

[7] "Lavoisier tin calcination".

[cyclo-8] 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.

[9] Daintith, John, ed. (2008)."Phlogiston theory".A Dictionary of Chemistry(6th ed.).Oxford University Press.doi:10.1093/acref/9780199204632.001.0001.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.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]