CHEMICAL NAME = calcium oxide CAS NUMBER = 1305–78–8 MOLECULAR FORMULA = CaO MOLAR MASS = 56.0 g/mol
COMPOSITION = Ca(71.4%) O(28.6%) MELTING POINT = 2,572°C
BOILING POINT = 2,850°C DENSITY = 3.3 g/cm3
Calcium oxide is a white caustic crystalline alkali substance that goes by the common name lime. Th e term lime is used both generically for several calcium compounds and with adjec-tives to qualify diff erent forms of lime. Th is entry equates lime, also called quicklime or burnt lime, with the compound calcium oxide. Hydrated lime, made by combining lime with water, is calcium hydroxide and is often referred to as slaked lime (Ca(OH)2). Dolomite limes contain magnesium as well as calcium. Limestone is the compound calcium carbonate. Th e term lime comes from the Old English word lïm for a sticky substance and denotes lime’s traditional use to produce mortar. Calx was the Latin word for lime and was used to name the element calcium.
Calcium oxide dates from prehistoric times. It is produced by heating limestone to drive off carbon dioxide in a process called calcination: CaCO3(s) CaO(s) + CO2(g). At tem-peratures of several hundred degrees Celsius, the reaction is reversible and calcium oxide will react with atmospheric carbon dioxide to produce calcium carbonate. Effi cient calcium oxide production is favored at temperatures in excess of 1,000°C. In prehistoric times limestone was heated in open fi res to produce lime. Over time, lined pits and kilns were used to produce lime. Brick lime kilns were extensively built starting in the 17th century and the technology to produce lime has remained relatively constant since then.
Modern lime kilns operate at approximately 1,200°C to 1,300°C. Limestone, which has been crushed and screened into pieces with diameters of several inches, is fed into the top of the kiln. Air fed into the kiln’s bottom fl uidizes the limestone, allowing for greater reaction
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effi ciency. Rotating horizontal kilns several meters in diameter and as long as 100 meters are also used to produce lime. In these kilns, limestone is fed into one end that is elevated and moves down the rotating kiln by gravity as it is heated and converted into lime. Much of the lime produced is hydrated with water in a process called slaking to produce slaked lime or calcium hydroxide (Ca(OH)2): CaO(s) + H2O(l) → Ca(OH)2(s). Th e hydration of calcium oxide is highly exothermic and one reason for converting lime to slaked lime is for safety.
Th e oldest use of lime is as a mortar. Th ere is evidence that Egyptians used lime mortars as early as 4000 b.c.e. Lime mortars consist of a mixture of lime, water, and sand. Mortars are based on the reaction of slaked lime reacting with atmospheric carbon dioxide to produce cal-cium carbonate and water: Ca(OH)2 + CO2(g) → CaCO3(s) + H2O(l). Th e conversion of slaked lime to limestone is a slow process and lime mortars can take several years to cure. Modern mortars contain a mixture of substances such that curing times can be controlled. Th e most common mortar is cement, which uses lime as a major ingredient in cement. Portland cement is made by heating limestone, clay, and sand in rotating kilns at a temperature of 2,700°C. Th e limestone is converted to lime and the clay and sand are sources of silicates, iron, and alumi-num. Portland cement is 85% lime and silicates by mass. Th e “Portland” name comes from an English bricklayer named Joseph Aspdin (1799–1855) who patented the cement in 1824 and noted that it resembled limestone quarried from the Isle of Portland in the English Channel.
Lime has also been used since ancient times in the production of glass. Early glasses were made by producing a melt of silica (SiO2), sodium carbonate (soda ash, Na2CO3), and lime and by letting the melt cool into an amorphous solid. Other additives were included in the melt to impart color or various other properties to the glass; for example, copper produced a blue glass and calcium strengthened it. Most modern glass produced is called soda-lime glass and consists of approximately 70% silica, 15% soda (Na2O), and 5% lime.
Lime is traditionally one of the top ten chemicals produced annually. Approximately 15 million tons of calcium oxide and 22 million tons of all compounds called limes (burnt, slaked, dolomite) are produced annually in the United States. Global production is approxi-mately 135 million tons. Th e major uses of lime are metallurgy, fl ue gas desulfurization, construction, mining, papermaking, and water treatment. About one third of calcium oxide production in the United States is used for metallurgical processes, principally in the iron and steel industry. Calcium oxide is used to remove impurities during the refi ning of iron ore.
Calcium oxide combines with compounds such as silicates, phosphates, and sulfates contained in iron ores to form slag. Slag is immiscible with molten iron, allowing the slag and iron to be separated. Lime is also used for purifi cation in other metal refi ning and to control pH in min-ing processes such as leachmin-ing and precipitation. Th e calcium oxide is also used in remediation of mine wastes to recover cyanides and to neutralize acid mine drainage.
Both lime and slaked limes are use to reduce sulfur emissions, which contribute to acid precipitation, from power plants, particularly coal-fi red plants. By using lime, more than 95%
of the sulfur can be eliminated from the emissions. Calcium oxide reacts with sulfur dioxide to produce calcium sulfi te: CaO(s) + SO2(g) → CaSO3(s). Sulfur dioxide is also removed by spraying limewater in the fl ue gas. Limewater, also called milk of lime, is a fi ne suspension of calcium hydroxide in water. Other pollutants removed with lime include sulfur trioxide, hydrofl uoric acid, and hydrochloric acid.
Lime is used in drinking water treatment to control pH, soften water, and control turbid-ity. Lime, in combination with sodium carbonate, is used to precipitate the major bivalent
64 | Th e 100 Most Important Chemical Compounds
cations Ca2+ and Mg2+ that cause hardness. Th e bivalent ions Fe2+, Sr2+, Mn2+, and Zn2+ are removed as well. Calcium oxide is also used to treat wastewater. It is used to keep pH between 6.0 and 9.0 and aids in the precipitation of solids, especially nitrogen and phosphorus com-pounds. Phosphates from wastewater discharges can lead to algae blooms and eutrophication in receiving waters. Calcium oxide precipitates out the phosphate from the discharge as cal-cium phosphate: 3CaO(s) + 3H2O(l) + 2PO43-(aq) → Ca3(PO4)2(s) + 6OH−(aq). Lime is used to treat sludge and biosolids to decrease odor and control pH.
Lime is used in the paper industry to produce the bleaching agent calcium hypochlorite (Ca(OCl)2). Slaked lime is used to recover sodium hydroxide from soda ash after the pulping process: Ca(OH)2(aq) + Na2CO3(aq) → 2NaOH(aq) + CaCO3(s). Th e recovered calcium carbonate can be used to regenerate lime. Lime is also used to treat process water in the paper industry.
Lime is used in the chemical industry as a feedstock. It is heated with carbon in the form of coke to make calcium carbide (CaC2): 2CaO(s)+ 5C(s) 2CaC2(s) + CO2(g). Calcium carbide is used to produce acetylene. Some of the other chemicals made with lime include calcium hypochlorite, citric acid, and sodium alkalis. Lime is used to produce precipitated calcium carbonate (PCC), which is a fi ne-grained form of calcium carbonate.
To produce PCC, lime is hydrated to produce slaked lime and the slaked lime is combined with water to produce limewater. Carbon dioxide is added to the limewater, causing calcium carbonate to precipitate as PCC. PCC is used widely in plastics production, papermaking, pharmaceuticals, and the petrochemical industry.
Calcium oxide is sometimes used in the building industry to treat soils at construction sites. Treating the soil serves several purposes. Calcium oxide serves as a drying agent by com-bining with water in the soil to form calcium hydroxide. Th e heat generated in this process aids in the drying. Th e addition of CaO changes the soil chemistry and combines with clays to stabilize the soil. Th is provides a stronger building foundation and reduces soil plasticity.
Lime has limited use as a liming agent. Liming refers to the process of adding substances that neutralize acidity by increasing the pH of the soil. Th is is referred to as sweetening the soil. Lime, as both CaO and Ca(OH)2, can be used as liming agents, but these are not the pre-ferred forms for agricultural purposes. Lime and slaked lime have high neutralizing capacity, but lime is diffi cult to store and is caustic. Clumping on the soil surface of these forms is also a problem. Th e lime used in agriculture and gardening is typically crushed limestone (calcitic lime) or dolomitic limestone (a calcium-magnesium carbonate). Calcitic and dolomitic limes are what is meant by agricultural lime.