Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used
chemical compound. It is a white,
caustic,
alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
ne,
crystalline solid at room temperature. The broadly used term "''
lime''" connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of
calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, ''quicklime'' specifically applies to the single chemical compound calcium oxide. Calcium oxide that survives processing without reacting in building products such as cement is called free lime.
Quicklime is relatively inexpensive. Both it and a chemical derivative (
calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
, of which quicklime is the
base anhydride
A base anhydride is an oxide of a chemical element from group 1 or 2 (the alkali metals and alkaline earth metals, respectively). They are obtained by removing water from the corresponding hydroxide base. If water is added to a base anhydride, ...
) are important commodity chemicals.
Preparation
Calcium oxide is usually made by the
thermal decomposition of materials, such as
limestone or
seashell
A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer usually created by an animal or organism that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washe ...
s, that contain
calcium carbonate (CaCO
3; mineral
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
) in a
lime kiln. This is accomplished by heating the material to above ,
[Merck Index of Chemicals and Drugs, 9th edition monograph 1650] a process called
calcination or ''lime-burning'', to liberate a molecule of
carbon dioxide (CO
2), leaving quicklime. This is also one of the few chemical reactions known in
prehistoric
Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use of ...
times.
: CaCO
3(s) → CaO(s) + CO
2(g)
The quicklime is not stable and, when cooled, will
spontaneously react with CO
2 from the air until, after enough time, it will be completely converted back to calcium carbonate unless
slaked with water to set as
lime plaster or
lime mortar.
Annual worldwide production of quicklime is around 283 million tonnes. China is by far the world's largest producer, with a total of around 170 million tonnes per year. The United States is the next largest, with around 20 million tonnes per year.
Approximately 1.8t of limestone is required per 1.0t of quicklime. Quicklime has a high affinity for water and is a more efficient
desiccant
A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids that absorb water. Desiccant ...
than
silica gel. The reaction of quicklime with water is associated with an increase in volume by a factor of at least 2.5.
Uses
* The major use of quicklime is in the
basic oxygen steelmaking (BOS) process. Its usage varies from about per ton of steel. The quicklime neutralizes the acidic oxides,
SiO2,
Al2O3, and
Fe2O3, to produce a basic molten slag.
* Ground quicklime is used in the production of
aerated concrete
Autoclaved aerated concrete (AAC) is a lightweight, precast, foam concrete building material suitable for producing concrete masonry unit like blocks. Composed of quartz sand (SiO2 with impurities), calcined calcium sulfate (CaSO4, a.k.a ...
blocks, with densities of ca. .
* Quicklime and
hydrated lime can considerably increase the load carrying capacity of clay-containing soils. They do this by reacting with finely divided silica and alumina to produce calcium silicates and aluminates, which possess cementing properties.
* Small quantities of quicklime are used in other processes; e.g., the production of glass, calcium aluminate cement, and organic chemicals.
* Heat: Quicklime releases
thermal energy by the formation of the hydrate,
calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
, by the following equation:
[Collie, Robert L. "Solar heating system" issued May 11, 1976]
::CaO (s) + H
2O (l) Ca(OH)
2 (aq) (ΔH
r = −63.7kJ/mol of CaO)
: As it hydrates, an exothermic reaction results and the solid puffs up. The hydrate can be reconverted to quicklime by removing the water by heating it to redness to reverse the hydration reaction. One litre of water combines with approximately of quicklime to give calcium hydroxide plus 3.54
MJ of energy. This process can be used to provide a convenient portable source of heat, as for on-the-spot food warming in a
self-heating can
Self-heating food packaging is active packaging with the ability to heat food contents without external heat sources or power. Packets typically use an exothermic chemical reaction. Packets can also be self-cooling. These packages are useful fo ...
, cooking, and heating water without open flames. Several companies sell cooking kits using this heating method.
* It is known as a
food additive
Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives have been used for centuries as part of an effort to preserve food, for example vinegar (pickling), salt (salt ...
to the
FAO as an acidity regulator, a flour treatment agent and as a leavener. It has
E number
E numbers ("E" stands for "Europe") are codes for substances used as food additives, including those found naturally in many foods such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...
E529.
* Light: When quicklime is heated to , it emits an intense glow. This form of illumination is known as a
limelight
Limelight (also known as Drummond light or calcium light)James R. Smith (2004). ''San Francisco's Lost Landmarks'', Quill Driver Books. is a type of stage lighting once used in theatres and music halls. An intense illumination is created when ...
, and was used broadly in theatrical productions before the invention of electric lighting.
* Cement: Calcium oxide is a key ingredient for the process of making
cement.
* As a cheap and widely available alkali. About 50% of the total quicklime production is converted to
calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
before use. Both quick- and
hydrated lime are used in the treatment of drinking water.
* Petroleum industry: Water detection pastes contain a mix of calcium oxide and
phenolphthalein. Should this paste come into contact with water in a fuel storage tank, the CaO reacts with the water to form calcium hydroxide. Calcium hydroxide has a high enough pH to turn the phenolphthalein a vivid purplish-pink color, thus indicating the presence of water.
* Paper: Calcium oxide is used to regenerate sodium hydroxide from sodium carbonate in the chemical recovery at Kraft pulp mills.
* Plaster: There is archeological evidence that
Pre-Pottery Neolithic B humans used limestone-based
plaster for flooring and other uses. Such
Lime-ash floor Lime-ash floors were an economic form of floor construction from the 15th century to the 19th century, for upper floors in parts of England where limestone or chalk were easily available. They were strong, flexible, and offered heat insulation, good ...
remained in use until the late nineteenth century.
* Chemical or power production: Solid sprays or slurries of calcium oxide can be used to remove
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
from exhaust streams in a process called
flue-gas desulfurization
Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide () from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes such as waste incineration.
Methods
...
.
* Mining: ''Compressed lime cartridges'' exploit the exothermic properties of quicklime to break rock. A
shot hole
Shot hole disease (also called Coryneum blight) is a serious fungal disease that creates BB-sized holes in leaves, rough areas on fruit, and concentric lesions on branches. The pathogen that causes shot hole disease is '' Wilsonomyces carpophil ...
is drilled into the rock in the usual way and a sealed cartridge of quicklime is placed within and
tamped. A quantity of water is then injected into the cartridge and the resulting release of steam, together with the greater volume of the residual hydrated solid, breaks the rock apart. The method does not work if the rock is particularly hard.
* Disposal of corpses: Historically, it was mistakenly believed that quicklime was efficacious in accelerating the decomposition of corpses. The application of quicklime can, in fact, promote preservation. Quicklime can aid in eradicating the stench of decomposition, which may have led people to the erroneous conclusion.
Weapon
In 80 BC, the Roman general
Sertorius deployed choking clouds of caustic lime powder to defeat the Characitani of
Hispania, who had taken refuge in inaccessible caves. A similar dust was used in China to quell an armed peasant revolt in 178 AD, when ''lime chariots'' equipped with bellows blew limestone powder into the crowds.
Quicklime is also thought to have been a component of
Greek fire
Greek fire was an incendiary weapon used by the Eastern Roman Empire beginning . Used to set fire to enemy ships, it consisted of a combustible compound emitted by a flame-throwing weapon. Some historians believe it could be ignited on contact w ...
. Upon contact with water, quicklime would increase its temperature above and ignite the fuel.
David Hume, in his ''
History of England
England became inhabited more than 800,000 years ago, as the discovery of stone tools and footprints at Happisburgh in Norfolk have indicated.; "Earliest footprints outside Africa discovered in Norfolk" (2014). BBC News. Retrieved 7 February ...
'', recounts that early in the reign of
Henry III, the English Navy destroyed an invading French fleet by blinding the enemy fleet with quicklime. Quicklime may have been used in medieval naval warfare – up to the use of "lime-mortars" to throw it at the enemy ships.
Substitutes
Limestone is a substitute for lime in many applications, which include agriculture, fluxing, and sulfur removal. Limestone, which contains less reactive material, is slower to react and may have other disadvantages compared with lime, depending on the application; however, limestone is considerably less expensive than lime. Calcined gypsum is an alternative material in industrial plasters and mortars. Cement, cement kiln dust, fly ash, and lime kiln dust are potential substitutes for some construction uses of lime. Magnesium hydroxide is a substitute for lime in pH control, and magnesium oxide is a substitute for dolomitic lime as a flux in steelmaking.
Safety
Because of vigorous reaction of quicklime with water, quicklime causes severe irritation when inhaled or placed in contact with moist skin or eyes. Inhalation may cause coughing, sneezing, and labored breathing. It may then evolve into burns with perforation of the nasal septum, abdominal pain, nausea and vomiting. Although quicklime is not considered a fire hazard, its reaction with water can release enough heat to ignite combustible materials.
Natural occurrence
It is noteworthy that CaO is also a separate mineral species, named lime. A pyrometamorphic mineral, it is rare, as it is unstable in moist air quickly turning into
portlandite, Ca(OH)
2.
References
External links
Lime Statistics & Informationfrom the
United States Geological Survey
Factors Affecting the Quality of Quicklime''American Scientist''(discussion of
14C dating of mortar)
{{Authority control
Alchemical substances
Bases (chemistry)
Calcium compounds
Cement
Dehydrating agents
Desiccants
Disinfectants
E-number additives
Limestone
Rock salt crystal structure
Oxides