General Metallurgy

Metal

Metal are the class of chemical elements with certain characteristic properties. In everything usage, metals are elements and alloys such as iron, aluminium, copper, etc. which are lustrous, malleable solid usually good conductors of heat and electricity.

Till today more than 110 elements are known. To facilitate their study, these must be divided into certain classes. These have been mainly divided into metals and non - metals. One important factor, which distinguishes metals from non-metal in their electronic configuration. Metal have fewer electrons in their outermost energy level are lost rapidly and positive ions are obtained.

Na ( sodium )→ Na⁺(sodium ion ) + e^-

Mg→ Mg²⁺ + 2e^-

Al→ Al³⁺ + 3e^-

sodium, potassium, calcium , magnesium , aluminium, copper, iron, silver, gold , zinc , platinum etc are some familiar examples of metals.

Hydrogen, carbon, nitrogen, oxygen, fluorine, phosphorous, sulphur, chlorine, bromine, iodine, etc are well known non-metals.

Metals are dividedinto two main groups.

1. Reactive Metal Elements : This group of metal includes metals like alkali metals and alkaline earth metal. They are high in the electrochemical series and tend to form compounds by losing electrons to give positive ions.
2. Transition Metal Elements : This group of metal is less reactive, have metallic bonds, formed by positive ions with free electrons.

Distinction Between Metals and Non-metals

The basic differences between metals and non-metals are listed below.

Metalloids (Semi-metals)

Elements which have intermediate properties between metals and nonmetals are called metalloids. Metalloids act as an electron donor with non-metals and as electron acceptor with metals. The elements like germanium, arsenic and antimony possess the character of both the metals and non-metals. Like non-metals, they show following properties .

1. They exhibit allotropy.
2. They are poor conductors of heat and electricity.
3. They are neither malleable nor ductile but sometimes volatile.
4. They are oxidized by nitric acid when arsenic and antimonic acids are obtained .
5. They form hydrides with hydrogens like AsH₃ , SbH₃ and these are gases.

Like metals , Arsenic possess metallic lustre, forms alloys and conducts electricity. Like copper, it also forms arsenious sulphide, As₂S₃ when H₂ is passed through its salt solution. Antimony resembles arsenic in almost all respects.

Alloy & Amalgams

I. Alloys:

An alloy is a homogenous mixture of two or more metals or metals with non-metals. As alloy possess metallic properties and it is prepared to get a product possessing desirable properties which the constituent elements do not posses. some examples of common alloys are :

1. Brass - Cu: 60 - 80 % and Zn: 40-20%
2. Bronze - Cu : 75 - 90% and Sn : 25 - 10%
3. Stainless steel - Cr : 12% Ni : 2% and Fe :86%
4. Manganese steel - Mn : 12 - 15% and Fe : 88 - 85 %
5. Invar - Ni : 36% and Fe : 64 %
6. Duralumin - Al : 95 % , Cu : 4% , Mg : 0.5% and Mn : 0.5%
7. Type metal - Pb : 82 % , Sb : 15% and Sn: 3%
8. Pewter - Sn : 80% and Pb : 20 %
9. Bell metal - Cu : 80% and Sn : 20%
10. Rose metal - Bi : 50 % , Pb : 28 % and Sn: 22 %

Depending on the presence or absence of Fe as one component of alloys, they have been classified into two categories.

1. Ferrous alloys :-An alloy in which one of the constituent elements is iron, is called ferrous alloy. eg Steel (Fe + C ), Manganese steel ( Fe + Mn) etc .
2. Non-Ferrous alloys :-An alloy which does not contain iron as one of the constituents is called non-ferrous alloys. eg. Brass (Cu + Zn ) , Bronze (Cu+ Sn) etc .

II. Amalgams :

A homogeneous mixture of metals with mercury is called an amalgam. It is also defined as an alloy containing mercury as one of the constituents. eg. Sodium amalgam, Zinc amalgam etc. Most of the metals dissolve in liquid mercury to form the amalgam. But Fe, Co and Ni do not form the amalgam.

Occurrence of metals

The metals occur in the earth's crust along with a number of rocky material and impurities. These are known minerals. Thus, the naturally occurring chemical substances in the earth's crust which are obtained by mining are known as minerals. Those minerals from which metal can be extracted economically are called ores, Thus, all ores are minerals, but all the minerals are not ores. Ores are usually associated with earthy materials called gangue or matrix.

Some important ores are listed below.

1. Oxides ores -Haematite ( Fe₂O₃), Bauxite (Al₂O₃.2H₂O), Pyrolusite ( MnO₂), Corrundum (Al₂O₃). Cuprite ( Cu₂O) etc.
2. Sulphide ores -Copper pyries ( CuFeS₂), Copper glance ( Cu₂S), Iron pyrites ( FeS₂), Galena ( PbS), Zinc blende ( ZnS) , Cinnabar ( HgS ) etc .
3. Halide ores - Rock Salt ( NaCl ), Horn silver ( AgCl ), Carnallite ( KCl.MgCl₂.6H₂O) etc
4. Carbonate ores -Magnesite ( MgCO₃), Dolomite ( CaCO₃.MgCO₃), Limestone (CaCO₃), Malachite (CuCO₃.Cu(OH)₂), Calamine (ZnCO₃) etc
5. Silicate ores -Felspar ( K₂O.Al₂O₃.6SiO₂), Mica ( MgO.3Al₂O₃.6SiO₂) etc

Occurrence of Metals in Nepal

The country has yet to make the best use of these minerals for economic development. A table below records the occurrence of some common minerals in different parts of the country.

Table : Some Common Minerals in Nepal

Reference :

Ghosh, A.K. Chemical Calculations. 15th Edition. India: Scientific Book Company, 1991.

M.L Sharma & P.N. Chaudhary. Advanced Level Chemistry. 2nd Edition . Vol. I. Kathmandu, Nepal: Ekta Books, 2011.

Palak, K.R. Fundamentals of Chemistry. Kathmandu, Nepal: Ratna Pustak Bhandar, 2000.

Pathak, Prof. Dr. Tulsi Prasad. Rectified Chemistry. Kalikasthan, Kathmandu : Airawati Prakashan (P.) Ltd., 2014.