We continue to review some of the most important materials in heat treatment and metallurgy.
Calcium (chemical symbol: Ca)
Pure calcium is a bright, silvery-white metal that is typically found covered with an oxide layer due to its reactivity with oxygen in the atmosphere. Calcium is not a reliable structural material because it is very soft and ductile in addition to having low tensile strength and chemical reactivity. Therefore, calcium’s main use in metallurgy is as a reducing agent for aluminum, beryllium, copper, lead and magnesium alloys, as well as chromium, thorium, uranium, zirconium and other metals. Calcium compares favorably with sodium as a reducing agent, but it is preferred because of its greater chemical stability.
In nature, calcium makes up 4.1% of Earth’s crust by weight and 8% of the Moon’s crust in the form of calcite (Fig. 1). It also exists in cave stalagmites, stalactites, chalk, eggshells, marble, limestone, dolomite, pearls, coral and the shells of many sea creatures.
Calcium, in the form of calcium oxide (CaO), or lime, has been used for centuries in plaster and mortar. It was first isolated in 1808 in England by Cornish chemist and inventor Sir Humphry Davy. Sir Davy, famous for founding the field of electrochemistry, interestingly worked with Michael Faraday, the famed English scientist known for his important contributions to electromagnetism and electrochemisty.
Davy’s attempt to separate calcium by reducing moist lime through electrolysis was unsuccessful, although a similar technique worked with separating sodium and potassium. He then electrolyzed a mixture of lime and mercuric oxide and then distilled off the mercury, leaving pure calcium. He derived the name of calcium after the Latin word “calcis,” meaning lime.
Although pure calcium is not found unbound in nature, calcium compounds are common and widely used. Limestone, or calcium carbonate, is used directly as a building material in the form of bricks and indirectly as an ingredient in cement. To make cement, limestone is first heated to liberate carbon dioxide gas, leaving behind calcium oxide (quicklime). This is mixed with and reacts with water to produce calcium hydroxide, or slaked lime. Slaked lime is then used to make cement. Slaked lime is also used in water treatment to decrease acidity; in the chemicals industry as a soil conditioner; in the chemicals industry; and to remove impurities from molten iron ore.
Here are a few important facts about calcium.
- Atomic number: 20
- Atomic mass: 40.078 atomic mass units
- Number of protons: 20
- Number of neutrons: 20
- Number of electrons: 20
- Melting point: 839°C
- Boiling point: 1484°C
- Density: 1.55 grams per cubic centimeter
- Normal phase: Solid
- Family: Alkaline earth metals
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- Royal Society of Chemistry (www.rsc.org)