Magnesium: Important Ores, Metallurgy, Properties, Uses

Magnesium

The magnesium comes from the name of the mineral magnesite, which in turn is believed to stem from the name Magnesia. The British chemist Humphry Davy discovered the pure element magnesium in 1808.

Due to its low density, it is considered to be a structural unit.

Important Ores

Magnesium does not occur in the native state. In the combined state it occurs very abundantly in the earth crust.

Magnesite, MgCO₃       Dolomite, MgCO₃, CaCO₃

Epsomsalt, MgSO₄, 7H₂O  Carnallite MgCl₂ KCl.6H₂O

However magnesium ion Mg²⁺, is the third most abundant dissolved ion in the oceans, after Cl^- and Na⁺. The oceans are the best sources for magnesium. It is widely distributed in the vegetable kingdom being present in chlorophyll, the green colouring matter of the leaves.

Metallurgy

Magnesium is prepared on a large scale by the electrolysis of either fused magnesium chloride or magnesia.

1. Electrolysis of fused magnesium chloride

The purified carnallite ore is the principal source for this process. A mixture of equal quantities of carnallite and NaCl is fused to a clear liquid at 973K. The alkali chloride prevents hydrolysis of magnesium chloride and increases the conductivity of the fused mass.

The electrolysis of the fused mass is carried out in an atmosphere of coal gas in air tight iron cell which can hold 6-7 tonnes of the electrolyte. The temperature of the elctrolyte bath is maintained at 970K. The iron cell itself acts as a cathode unlike the anode consists of a carbon or graphite rod surrounded by a porcelain tube through which the liberated chlorine escapes. Molten magnesium being lighter than the electrolyte, rises to the surface and is periodically removed with perforated ladle. The electrolysis is carried out in an atmosphere of coal gas so as to avoid the oxidation of molten magnesium. The metal thus obtained is 99.9% pure. It may be further purified by remelting with a flux of anhydrous magnesium chloride and sodium chloride.

Physical

Pure magnesium metal is a relatively active silvery white metal. At slightly below its melting point, it is malleable and ductile and can be drawn into wire or rolled into ribbon in which form it is generally sold. It is a very light metal.

Chemical Properties

1.Action of Air : It does not tarnish in dry air but a layer of white oxide is formed on its surface in moist air.

2.With air on burning : It burns in air or oxygen with a dazzling light rich in ultraviolet rays, forming magnesium oxide and magnesium nitride.

2Mg + O2  -- >  2Mg O

3Mg + N2  -- >  Mg3N2

3.With CO2

It continues to burn in CO₂,

2 Mg + CO₂    -- >   2 MgO + C

4.Action of Water

When heated with steam it burns brilliantly producing magnesium oxide and hydrogen.

Mg  H ₂O -- >  MgO  H ₂

steam

5. Action of Acids

Dilute HCl or H2SO4 gives hydrogen with magnesium. With dilute HNO3, part of the hydrogen liberated is oxidised by nitric acid, which itself is reduced to a variety of products depending upon the concentration. With concentrated HNO3, it gives ammonium nitrate.

4Mg + 10HNO3 -- >  4 Mg(NO3)2 + NH4NO3 + 3H2O

6. Displacement of Metals

It is a strongly electropositive metal and hence Mg displaces nearly all the metals from the solutions of their salts eg.

Mg + 2AgNO3 -- > Mg(NO3)2+ 2Ag

7. Reducing Action

Mg has great affinity for oxygen and it liberates sodium, potassium, boron and silicon from their oxides at high temperatures.

K2O + Mg    -- > MgO + 2K

B2O3 + 3Mg  -- > 3MgO + 2B

Uses of Magnesium

1.     In flashlight photography, pyrotechnics and in fireworks.

2.     As a reducing agent in the preparation of boron and silicon and deoxidiser in metallurgy.