Calcium compounds

Where Found

Calcium compounds are widely distributed. Naturally occurring calcareous materials include calcite, aragonite, chalk, marble, shell, and coral, all of which are predominantly calcium carbonate (CaCO₃); other common calcium-bearing minerals include fluorite (CaF₂) and apatite (Ca₅(PO₄)₃(F,Cl,OH)). Principal calcium ores are limestone (predominantly calcite), dolostone (chiefly dolomite, MgCa(CO₃)₂), and gypsum rock (mostly the mineral gypsum, or calcium sulfate, CaSO₄ · 2H₂O), all widespread sedimentary rocks. Calcium is present in all soil, most water, and all plant and animal life.

Primary Uses

Calcium compounds are used for a variety of applications, including chemical manufacture, construction, and agriculture. Uses for elemental calcium include metal refining, alloy manufacture, and uranium and plutonium processing.

Technical Definition

Calcium (abbreviated Ca), atomic number 20, is a metallic element belonging to Group 2 of the periodic table of the elements (alkaline-earth metals). It is chemically similar to strontium and barium. Its average molecular weight is 40.08. Its specific gravity is 1.55 at 20° Celsius (68° Fahrenheit), its melting point is approximately 850° Celsius (1,562° Fahrenheit), and its boiling point is 1,484° Celsius (2,696° Fahrenheit).

Description, Distribution, and Forms

Calcium has a cubic crystalline structure and is silver-white in color. It is relatively soft, ductile, and malleable. A chemically active element, it occurs in nature only in combination with other elements. It tarnishes readily in air, reacts with water to form a hydroxide, and combines with oxygen, carbon, hydrogen, chlorine, fluorine, phosphorus, sulfur, and other elements to form many compounds.

Calcium is the fifth most abundant element in the Earth’s crust, of which it makes up roughly 3.6 percent. Calcium is not found uncombined in nature, but it is widely distributed in its many naturally occurring compounds. Calcium is found in all soil and most water, as well as in all plant and animal life. Bones and teeth are composed mostly of calcium and phosphorus, with calcium predominant.

Calcium is widely distributed in its naturally occurring compounds, which are found in many deposits throughout the world in rocks of varying geologic age. The chief ores of calcium—limestone, dolostone, and gypsum—are sedimentary in origin and can be found in thick, extensive beds. Limestone, largely or entirely composed of calcium carbonate, is generally formed by the deposition and consolidation of the skeletons of invertebrate marine organisms, although some limestones are the result of chemical precipitation from solution. Dolostone, composed chiefly of the mineral dolomite, originates from the partial replacement of the calcium in limestone with magnesium. The mineral gypsum, or calcium sulfate, precipitates to form deposits as seawater evaporates. Calcium in carbonate form is found not only in limestone and dolostone but also in chalk, marble, shell, and coral. Fluorite is a calcium fluoride mineral found in igneous deposits. Phosphate rock, which contains calcium in the form of the mineral apatite, occurs in both igneous and sedimentary deposits. Calcium is also naturally present in soil and most water.

Calcium combines with other elements to form a wide variety of natural compounds, including calcium carbonate, calcium sulfate, fluorite, and apatite. Some of these compounds may take several forms: Calcium carbonate, for instance, may exist as calcite (the predominant mineral in limestone, chalk, and most marble), Iceland spar, aragonite, shell, or coral, while calcium sulfate occurs in nature as gypsum rock, gypsite, alabaster, satin spar, and selenite.

History

In general, the earliest known uses of calcium compounds were as construction materials. From calcareous substances such as limestone, dolostone, marble, gypsum, and lime (calcium oxide, CaO, made by heating limestone or other calcium-rich materials), ancient peoples and the civilizations to follow made mortar, plaster, cement, stucco, building stone, and ornamental carvings. Limestone, lime, gypsum, and other calcareous materials have also been used for centuries as soil conditioners. Beginning in the late eighteenth century, calcium compounds were increasingly employed in industrial processes and chemical manufacture: Limestone, for example, found use as a key ingredient in the Leblanc and Solvay processes, two early commercial processes for manufacturing soda ash from salt. Elemental calcium was not isolated until 1808, when English chemist Sir Humphry Davy produced it as a mercury amalgam by electrolysis of calcium chloride in the presence of a mercury cathode. By 1904, calcium was obtained commercially by electrolysis of molten calcium chloride in the presence of an iron cathode. The advent of World War II necessitated greater quantities of calcium. To meet the increased demand, an aluminothermic reduction process was developed for commercial use.

Obtaining Calcium

Calcium is obtained from lime through the aluminothermic reduction process. Limestone or a similar calcareous material is heated to produce lime and carbon dioxide. After lime is ground finely, it is heated with finely divided aluminum in a retort under a high vacuum to produce calcium metal.

Uses of Calcium

Natural and manufactured calcium compounds are used for a variety of applications. Limestone serves as a flux in iron smelting, a key component in portland cement, a building stone, and a raw material for lime production. Lime, an essential industrial compound, is used in chemical manufacture, construction, water softening, industrial waste treatment, and soil deacidification. Gypsum is widely used in agriculture as a soil conditioner and in the construction industry in plaster, wallboard, cement, and tiles. Calcium chloride is employed as a filler in rubber, plastics, and ceramics. Calcium hypochlorite is a disinfectant used in swimming pools and in municipal and industrial bleaching and sanitation processes. Calcium nitrate is used as a concrete additive to reduce setting time and minimize the corrosion of steel reinforcement bars. Calcium carbide is used in the production of acetylene gas and calcium cyanamide, a fertilizer. Arsenate and cyanide compounds of calcium were used as insecticides in the twentieth century, but were phased out due to toxicity. Elemental calcium is a component of alloys used in maintenance-free batteries. It is used in lead refining to remove bismuth and in steel refining to remove sulfur and oxygen. It plays a critical role in uranium and plutonium processing, making calcium a strategic resource. Elemental calcium is also used in the preparation of vitamin B and chelated calcium supplements.

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