Background

Davy did not differentiate calcium metal from lime until 1808. It is a metallic element, and is the fifth most abundant in the Earth’s crust, where it constitutes more than 3% of the total. It is never found in its elemental form in nature. It occurs abundantly as limestone (CaCO3), gypsum (CaSO4.2H2O) and fluorite (CaF2); where apatite is the fluorophosphate or chlorophosphate of calcium.

The metal is obtained 98.6% pure by electrolysis of the fused anhydrous chloride to which calcium fluoride is added to lower the melting point. By further subliming, it can be obtained 99.5% pure.

Calcium metal is yellowish to silvery white and is considered hard. Chemically is belongs to the alkaline earth elements. Upon heating in air calcium burns with a brilliant white light with a yellow-red flame, where it forms mainly the nitride. It is oxidised easily, and in air forms a white coating of nitride.
Applications

Uses for calcium include:

· Due to its strong affinity for oxygen and sulphur is utilised as a cleanser for nonferrous alloys.

· As a deoxidiser, desulphuriser and decarburiser for various ferrous and nonferrous alloys, where it is implemented in the form of a stick or lump of metal.

· It is used as a alloying agent for aluminium, beryllium, copper, lead and magnesium alloys.

· Serves as a ‘getter’ for residual gases in vacuum tubes.

· Crystalline calcium is employed in the form of a very reactive free-flowing powder of 94 to 97% purity and contains 2.5% calcium oxide (CaO) and smaller amounts of magnesium and these impurities.

· It can be used to harden lead.

Many calcium compounds are used in the foodstuffs, pharmaceuticals and medical industries such as:

· Oyster shells (where the shell is CaCO3) are a commercially important source of calcium for animal feeds.

· Edible calcium (calcium lactate, Ca(C3H5O3)2 .5H2O) for food products derived from milk.

· Calcium lactobionate, a white powder, used as a suspending agent in pharmaceuticals – containing 4.94% available calcium.

· Calcium diphosphate (commonly know as phosphate of lime) (CaHPO4.H2O) is soluble in dilute citric acid solutions and is used to add calcium and phosphorous to foods. It is also used as a polishing agent in toothpaste.

· Calcium monophosphate (CaH4P2O8.H2O) a stable, water-soluble white powder is used as a leaving agent in baking. The anhydrous form monocalcium phosphate (CaH4(PO4)2) is used in pre-mixed flour mixes. Where each particle has a coating of a phosphate that has a time dependent solubility to delay solution when liquids are added.

· Tricalcium phosphate (Ca3(PO4)2) water-insoluble white powder, used to supply calcium and phosphorous to foods. Acts as a polishing agent in dentifrices, and as an antacid.

Other useful calcium compounds include:

· Calcium sulphate (CaSO3.2H2O), used to bleach paper pulp and textiles, it can also be used a disinfectant.

· Calcium silicate (CaO.SiO2), used as a reinforcing agent in rubber. As an absorbent to control the viscosity of liquids. It can be employed as a filler in paints and coatings, and when used in coatings it can reduce sheen.

· Calcium metasilicate (CaO.SiO3) or the mineral know as wollastonite, is a brilliant white powder and is therefore used in flat paints for paper coatings, as a filler plastics, as a coating for welding rods and for electrical insulators (tile and other ceramics).

· Calcium acetate (Ca(C2H3O2)2.H2O) (also know as lime acetate, acetate of lime and vinegar salts) used in liming rosin and for the manufacture of metallic soaps and synthetic resins.

· Dolomite (a natural compound) is used directly as a flux in melting iron.

· Calcium silicide is implemented in the making of special steels to inhibit carbide formation.

· Calcium from limestone is an essential element in Portland cements.