Calcination
Encyclopedia
Calcination is a thermal treatment process applied to ore
s and other solid materials 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.
) to calcium oxide
(lime
) and carbon dioxide
, in order to produce 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 furnace
s or reactors (sometimes referred to as kiln
s or calciners) of various designs including shaft furnaces, rotary kiln
s, multiple hearth furnaces, and fluidized bed reactor
s.
Examples of calcination processes include the following:
for a particular calcination reaction is equal to zero. For example, in limestone calcination, a decomposition process, the chemical reaction is
The standard Gibbs free energy of reaction is approximated as ΔG°r = 177,100 − 158 T (J/mol). The standard free energy of reaction is zero in this case when the temperature, T, is equal to 1121 K, or 848 °C.
Examples of chemical decomposition reactions common in calcination processes, and their respective thermal decomposition temperatures include:
See also calcination equilibrium of calcium carbonate
Jean Rey
noted that lead
and tin
when calcinated gained weight, presumably as they were being oxidized.
, 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
.
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.
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
s and other solid materials to bring about a thermal decomposition
Thermal decomposition
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes....
, phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
of the product materials. Calcination is to be distinguished from roasting
Roasting (metallurgy)
Roasting is a step in the processing of certain ores. More specifically, roasting is a metallurgical process involving gas–solid reactions at elevated temperatures with the goal of purifying the metal component. Often before roasting, the ore has already been partially purified, e.g. by froth...
, in which more complex gas–solid reactions take place between the furnace atmosphere and the solids.
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 (limestoneLimestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
) to calcium oxide
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
(lime
Lime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
) and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, in order to produce cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
. 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 furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
s or reactors (sometimes referred to as kiln
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
s or calciners) of various designs including shaft furnaces, rotary kiln
Rotary kiln
A Rotary kiln is a pyroprocessing device used to raise materials to a high temperature in a continuous process. Materials produced using rotary kilns include:* Cement* Lime* Refractories* Metakaolin* Titanium dioxide* Alumina* Vermiculite...
s, multiple hearth furnaces, and fluidized bed reactor
Fluidized bed reactor
A fluidized bed reactor is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid is passed through a granular solid material at high enough velocities to suspend the solid and cause it to behave as though it were a fluid...
s.
Examples of calcination processes include the following:
- decomposition of carbonate minerals, as in the calcination of limestoneLimestoneLimestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
to drive off carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
; - decomposition of hydrated minerals, as in calcination of bauxiteBauxiteBauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
, to remove crystalline waterWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
as water vapor; - decomposition of volatile matter contained in raw petroleum cokePetroleum cokePetroleum coke is a carbonaceous solid derived from oil refinery coker units or other cracking processes. Other coke has traditionally been derived from coal....
; - heat treatment to effect phase transformations, as in conversion of anataseAnataseAnatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but,...
to rutileRutileRutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
or devitrificationDevitrificationDevitrification is the opposite of vitrification, i.e., the process of crystallization in a formerly crystal-free glass. The term is derived from the Latin vitreus, meaning glassy and transparent.-Devitrification in glass art:...
of glassGlassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
materials - removal of ammonium ions in the synthesis of zeolites
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 Gibbs free energyGibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
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 Gibbs free energy of reaction is approximated as ΔG°r = 177,100 − 158 T (J/mol). The standard free energy of reaction is zero in this case when the temperature, T, is equal to 1121 K, or 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
Jean Rey (physician)
Jean Rey was a French physician and chemist.Born at Le Bugue, in the Périgord , he studied medicine at the University of Montpellier...
noted that lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
and tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
when calcinated gained weight, presumably as they were being oxidized.
Alchemy
In alchemyAlchemy
Alchemy is an influential philosophical tradition whose early practitioners’ claims to profound powers were known from antiquity. The defining objectives of alchemy are varied; these include the creation of the fabled philosopher's stone possessing powers including the capability of turning base...
, 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
Reverberatory furnace
A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases...
with mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
and sal ammoniac
Sal ammoniac
Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless to white to yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and a brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and has a low specific...
; silver with common salt and alkali salt
Alkali salts
Alkali salts or basic salts are salts which are the product of the neutralization of a strong base and a weak acid.Rather than being neutral, as some salts are, basic salts are, as their name suggests, bases. What makes these compounds basic is that the conjugate base from the weak acid hydrolyzes...
; copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
with salt and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
; iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
with sal ammoniac and vinegar
Vinegar
Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...
; tin with antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
; lead with sulfur; and mercury with aqua fortis
Aqua fortis
Aqua fortis, or "strong water," in alchemy, is a solution of nitric acid in water. Being highly corrosive, the solution was used in alchemy for dissolving silver and most other metals with the notable exception of gold, which can only be dissolved using aqua regia...
.
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
Mucilage
Mucilage is a thick, gluey substance produced by most plants and some microorganisms. It is a polar glycoprotein and an exopolysaccharide.It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as...
, and were easily reducible into powder.