Chemistry · Class 12 Science

Ch 1Metallurgy — Class 12 Chemistry, concept-first.

Metallurgy is the branch of science and technology concerned with metals -- how they occur in nature, how pure metal is extracted from that natural source, and how the extracted metal is refined and put to use.

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Occurrence of Metals — Minerals and Ores

Metals differ enormously in how they occur in nature, and that difference is governed almost entirely by chemical REACTIVITY.

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Chapter contents

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Introduction

1.1

Occurrence of Metals

In general, pure metals are shiny and malleable, but most metals are found in nature as compounds rather than free elements, and how reactive a metal is decides which form it occurs in.

1.1.1

Mineral and Ore

Mineral. A mineral is any naturally occurring substance, obtained by mining, that contains a metal either in the free state or combined as a compound (an oxide, sulphide, carbonate, silicate, etc.).

1.2

Concentration of Ores

Ores as mined are never pure: they are always associated with unwanted nonmetallic impurities, rocky material and siliceous matter, which are collectively called GANGUE (or matrix).

1.2.1

Gravity Separation or Hydraulic Wash

Gravity separation (also called hydraulic washing) exploits the DIFFERENCE IN SPECIFIC GRAVITY (density) between the valuable ore and the lighter gangue that surrounds it.

1.2.2

Froth Flotation

Froth flotation is the standard concentration method for SULPHIDE ores -- for example galena (PbS) and zinc blende (ZnS) -- and works on a completely different principle from gravity separation: it ex…

1.2.3

Leaching

Leaching is a CHEMICAL (rather than purely physical) method of concentration, based on the solubility of the metal-bearing compound of the ore in a suitable solvent, combined with the chemistry of wha…

1.2.4

Magnetic Separation

Magnetic separation exploits a DIFFERENCE IN MAGNETIC PROPERTY between the ore and its impurities, and applies specifically to FERROMAGNETIC ores -- ores that are themselves strongly attracted by a ma…

1.3

Extraction of Crude Metal

Once the ore has been concentrated (Section 1.2), the next stage is extracting the actual CRUDE metal from it.

1.3.1

Conversion of Ores into Oxides

This section covers the two industrial methods used to convert a concentrated ore into the oxide of the metal of interest -- roasting and calcination -- which are chosen according to whether the start…

1.3.2

Reduction of Metal Oxides

Once the metal oxide has been obtained (Section 1.3.1), it must be REDUCED to the crude, elemental metal.

1.4

Thermodynamic Principle of Metallurgy

Having seen (Section 1.3.2) that several different reducing agents -- carbon, carbon monoxide, hydrogen, aluminium, or a reactive metal -- can in principle be used to reduce a given metal oxide, the n…

1.4.1

Ellingham Diagram

Building the diagram. The change in Gibbs free energy for a reaction is given by ΔG = ΔH - TΔS ...... (1), where ΔH is the enthalpy change, T the temperature in kelvin, and ΔS the entropy change.

1.4.2

Applications of the Ellingham Diagram

The Ellingham diagram's central practical use is to help select a suitable reducing agent and an appropriate temperature range for a given reduction.

1.5

Electrochemical Principle of Metallurgy

Just as thermodynamic principles guide the choice of a chemical reducing agent (Section 1.4), ELECTROCHEMICAL principles find their own applications in metallurgical processes -- and become essential…

1.5.1

Electrochemical Extraction of Aluminium -- Hall-Heroult Process

The Hall-Heroult process is the standard industrial ELECTROCHEMICAL extraction of aluminium metal from alumina (Al2O3), and is the worked example of electrochemical metallurgy given in this unit.

1.6

Refining Process

The metal obtained at the end of the extraction stage (Section 1.3 or 1.5, depending on the route used) is described as CRUDE metal, because it generally still carries residual impurities -- unreacted…

1.6.1

Distillation

Distillation is used to refine metals that are themselves LOW-BOILING and VOLATILE -- the two examples given are zinc (boiling point 1180 K) and mercury (boiling point 630 K).

1.6.2

Liquation

Liquation is used to remove HIGH-MELTING impurities from metals that themselves have a relatively LOW melting point -- the examples given are tin (Sn, mp 904 K), lead (Pb, mp 600 K), mercury (Hg, mp 2…

1.6.3

Electrolytic Refining

Electrolytic refining purifies a crude metal by ELECTROLYSIS, carried out in an electrolytic cell containing an aqueous solution of a salt of the metal of interest as electrolyte.

1.6.4

Zone Refining

Zone refining is based on the principle of FRACTIONAL CRYSTALLISATION -- specifically, on the fact that impurities are generally MORE SOLUBLE IN THE MOLTEN (liquid) form of a metal than in its solid,…

1.6.5

Vapour Phase Method

In the vapour phase method of refining, the impure metal is first treated with a suitable reagent chosen specifically because it reacts with the metal to form a VOLATILE compound; this volatile compou…

1.7

Applications of Metals

Having covered how metals are extracted and refined, this final section surveys some of the everyday and industrial APPLICATIONS of five metals that have run through the unit as worked examples -- alu…

1.7.1

Applications of Al

Aluminium is the most abundant metal in the earth's crust, and is valued industrially for being a good conductor of both electricity and heat while also resisting corrosion well.

1.7.2

Applications of Zn

Metallic zinc is widely used to GALVANISE iron and steel structures -- coating them in a thin protective layer of zinc -- specifically to protect the underlying iron/steel from rusting and corrosion,…

1.7.3

Applications of Fe

Iron is one of the most widely useful of all metals, and its alloys turn up almost everywhere in engineering and daily life -- bridges, electricity transmission pylons, bicycle chains, cutting tools a…

1.7.4

Applications of Cu

Copper holds the distinction of being the first metal used by human beings, and the extended use of its alloy BRONZE (copper with tin) went on to define an entire era of human technological history, t…

1.7.5

Applications of Au

Gold is one of the most expensive and precious of all metals. It has been used for coinage, and has served as a standard for the monetary systems of some countries.

SUMMARY

- Metallurgy relates to the science and technology of metals. - A naturally occurring substance obtained by mining, containing a metal free or combined (e.g.

CONCEPT MAP

The unit's own concept map is rooted at 'Metallurgy' and fans out into the major stages and principles covered.

EVALUATION

40 Q

This is the textbook's own end-of-chapter evaluation for the Metallurgy unit, testing material across the full span of the chapter through two question formats.

+Choose the Best Answer24 questions

+Write Brief Answer16 questions

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

+Show 21 questions21 questions