Metallurgy

Chapter 8 deals with the physical and chemical properties of metals and nonmetals, ionic compounds and the extraction and purification of metals.

Physical Properties of Metals

• Metals mainly exist in solid state.
• Mercury and gallium exist in liquid state at room temperature.
• Metals possess metallic lustre.
• Metals show ductility and malleability.
• Metals are generally good conductors of heat and electricity.
• Metals are generally hard, but lithium, sodium and potassium are soft.
• Metals generally have high melting and boiling points.
• Tungsten has a very high melting point of about 3422°C.
• Some metals produce sound when struck. Such metals are called sonorous metals.

Physical Properties of Nonmetals

• Some nonmetals are solids and some are gases.
• Bromine is a liquid nonmetal at room temperature.
• Nonmetals generally do not possess lustre.
• Iodine is an exception because its crystals are shiny.
• Nonmetals are generally not hard.
• Diamond is an exception and is the hardest natural substance.
• Nonmetals generally have low melting and boiling points.
• Nonmetals are generally poor conductors of heat and electricity.
• Graphite is an exception because it conducts electricity.

Metals + Oxygen

Metals combine with oxygen on heating in air and form metal oxides.

Figure 8.1 – Combustion of metal

Sodium

Sodium combines with oxygen even at room temperature and forms sodium oxide.

4Na + O₂ → 2Na₂O

Sodium readily catches fire in air. Therefore, it is kept in kerosene.

Na₂O + H₂O → 2NaOH

Magnesium

2Mg + O₂ → 2MgO

MgO + H₂O → Mg(OH)₂

Metals + Water

Figure 8.2 – Reaction of a metal with water

• Sodium and potassium react rapidly and vigorously with water and liberate hydrogen.

2Na + 2H₂O → 2NaOH + H₂ + heat

2K + 2H₂O → 2KOH + H₂ + heat

Calcium reacts slowly and less vigorously with water. Hydrogen bubbles collect on its surface, so calcium floats.

2Ca + 2H₂O → 2Ca(OH)₂ + H₂

Aluminium, iron and zinc react with steam and form oxides.

2Al + 3H₂O(g) → Al₂O₃ + 3H₂

3Fe + 4H₂O(g) → Fe₃O₄ + 4H₂

Zn + H₂O(g) → ZnO + H₂

Metals + Acids and Nitric Acid

Figure 8.3 – Reaction of metals with dilute acid

Aluminium, magnesium, iron and zinc react with dilute sulphuric or hydrochloric acid to form salts and liberate hydrogen gas.

Mg + 2HCl → MgCl₂ + H₂

2Al + 6HCl → 2AlCl₃ + 3H₂

Fe + 2HCl → FeCl₂ + H₂

Zn + 2HCl → ZnCl₂ + H₂

Nitric acid

Reaction of metals with nitric acid forms nitrate salts and nitrogen oxides according to the concentration of nitric acid.

Cu + 4HNO₃(conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

3Cu + 8HNO₃(dil.) → 3Cu(NO₃)₂ + 2NO + 4H₂O

Metals + Salt Solutions

Figure 8.4 – Reaction of metal with solution of salts of other metals

If a metal A displaces metal B from the solution of its salt, metal A is more reactive than metal B.

Metal A + Salt solution of metal B → Salt solution of metal A + Metal B

In the textbook activity, iron displaces copper from copper sulphate. Therefore, iron is more reactive than copper.

Reactivity series

Figure 8.5 – Reactivity series of metals

K > Na > Li > Ca > Mg > Al > Zn > Fe > Sn > Pb > Cu > Hg > Ag > Au

Metals + Nonmetals

Metals lose electrons and form positive ions called cations.

Nonmetals gain electrons and form negative ions called anions.

The driving force behind many reactions is the tendency to attain the electronic configuration of the nearest noble gas.

Example: Sodium chloride

Sodium gives one electron and chlorine takes one electron.

2Na + Cl₂ → 2NaCl

Similarly, magnesium forms MgCl₂ and potassium forms KCl.

Chemical properties of nonmetals

Generally, nonmetals combine with oxygen to form acidic oxides. In some cases neutral oxides are formed.

C + O₂ → CO₂   (acidic)

2C + O₂ → 2CO   (neutral)

S + O₂ → SO₂   (acidic)

Generally, nonmetals do not react with water or dilute acids; halogens are exceptions.

Ionic Compounds

The electrostatic force of attraction between oppositely charged ions is called an ionic bond.

The positive and negative charges balance each other, so an ionic compound is electrically neutral.

General properties

• Ionic compounds are crystalline in nature.
• They are generally hard and brittle.
• They have strong electrostatic attraction between ions.
• They generally have high melting and boiling points.
• The size and charge of ions affect crystal structure.

Figure 8.6 – To verify the properties of ionic compounds

In the solid state ions cannot move freely. In the molten state and in aqueous solution, ions become mobile and conduct electricity.

More Properties of Ionic Compounds

1. Solid and hard: strong attraction between oppositely charged ions.
2. Brittle: ionic compounds can break into pieces when pressure is applied.
3. High melting and boiling points: large energy is needed to overcome strong attraction.
4. Water soluble: water molecules interact with ions and help form aqueous solutions.
5. Electrical conductivity: solid ionic compounds do not conduct electricity because ions cannot move. Molten and aqueous ionic compounds conduct electricity because ions are mobile.

Metallurgy, Minerals, Ores and Gangue

Occurrence of metals

• Most reactive metals occur in combined state as oxides, carbonates, sulphides or nitrates.
• Very unreactive metals such as silver, gold and platinum generally occur in free state.

Concentration of Ores

The concentration method depends on the physical properties of the ore and gangue, reactivity of the metal, available purification facilities and environmental factors.

Separation based on gravitation

Heavy ore particles can be separated from lighter gangue particles by gravitational methods.

Wilfley table method

Powdered ore is placed on a continuously vibrating inclined table. Water is released over it. Lighter gangue is carried away by water while heavier ore particles are retained by the wooden riffles.

Figure 8.8 – Wilfley table method

Hydraulic and Magnetic Separation

Hydraulic separation

Finely ground ore is released into a tank. A forceful jet of water is introduced from the lower side. Lighter gangue flows away with water while heavier ore particles are collected at the bottom.

Figure 8.9 – Hydraulic separation

Magnetic separation

An electromagnetic machine is used. Magnetic particles are attracted to the magnetic roller, while nonmagnetic particles continue along the conveyor belt and are collected separately.

Figure 8.10 – Magnetic separation

Example: Cassiterite contains nonmagnetic SnO₂ and magnetic FeWO₄; these can be separated by electromagnetic separation.

Froth Floatation and Leaching

Froth floatation method

This method is based on hydrophilic and hydrophobic properties.

• Sulphide ore particles are wetted mainly by oil.
• Gangue particles are wetted by water.
• Pine oil or eucalyptus oil is added.
• Pressurised air is passed through the mixture.
• Froth is formed and sulphide ore particles float with it.

Figure 8.11 – Froth floatation method

Examples: zinc blende (ZnS) and copper pyrite (CuFeS₂).

Leaching

In leaching, the ore is soaked in a suitable solution. The desired component dissolves while gangue does not dissolve and can be separated.

Leaching is used in the extraction of aluminium, gold and silver. Bauxite is soaked in aqueous NaOH or aqueous Na₂CO₃.

Extraction of Highly Reactive Metals

Highly reactive metals occur at the top of the reactivity series. Examples are potassium, sodium, aluminium, calcium and magnesium.

These metals have a large tendency to form cations by losing electrons. Their extraction is carried out by electrolytic reduction.

Example: Sodium from molten sodium chloride

Metal is deposited at the cathode and chlorine gas is liberated at the anode.

Extraction of Aluminium – Concentration

Aluminium is a reactive metal and occurs mainly as bauxite.

Bayer's process

Bauxite is ground and leached with concentrated NaOH at about 140–150°C under high pressure. Aluminium oxide reacts with NaOH and forms soluble sodium aluminate.

Al₂O₃ + 2NaOH → 2NaAlO₂ + 3H₂O

Iron oxide does not dissolve and is separated by filtration. Silica dissolves to form sodium silicate.

Hall's process

Powdered bauxite is heated with aqueous sodium carbonate to form soluble sodium aluminate. Insoluble impurities are filtered out and carbon dioxide is passed through the filtrate to precipitate aluminium hydroxide.

Al₂O₃·2H₂O + Na₂CO₃ → 2NaAlO₂ + CO₂ + 2H₂O

2NaAlO₂ + 3H₂O + CO₂ → 2Al(OH)₃ + Na₂CO₃

Alumina and Electrolytic Reduction

The precipitated aluminium hydroxide is filtered, washed, dried and calcined at about 1000°C to obtain alumina.

2Al(OH)₃ → Al₂O₃ + 3H₂O

Figure 8.12 – Extraction of aluminium

• Electrolysis of molten alumina is carried out in a steel tank.
• The inner graphite lining acts as cathode.
• Graphite rods act as anodes.
• Cryolite (Na₃AlF₆) and fluorspar (CaF₂) lower the melting point to about 1000°C.
• Aluminium is deposited at the cathode.
• Oxygen is liberated at the anode.

Extraction of Moderately Reactive Metals

Iron, zinc, lead and copper are moderately reactive metals. They generally occur as sulphide or carbonate ores.

Roasting

2ZnS + 3O₂ → 2ZnO + 2SO₂

Calcination

ZnCO₃ → ZnO + CO₂

Reduction and Thermit Reaction

Reduction of zinc oxide

Zinc oxide is reduced using carbon.

ZnO + C → Zn + CO

Apart from carbon, reactive metals such as sodium, calcium and aluminium can act as reducing agents.

Thermit reaction

Ferric oxide reacts with aluminium to form iron and aluminium oxide with a large amount of heat.

Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat

Figure 8.13 – Thermit welding

Another example:

3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + heat

Less Reactive Metals and Refining

Less reactive metals

Metals at the bottom of the reactivity series are less reactive. Gold, silver and platinum generally occur in free state.

Extraction of copper

Copper is mainly found as Cu₂S and is obtained by heating in air.

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂

2Cu₂O + Cu₂S → 6Cu + SO₂

Refining of metals

Corrosion of Metals

Corrosion is the gradual deterioration of a metal due to reaction with substances in the environment.

Figure 8.14 – Effects of corrosion

Examples

1. Iron reacts with moist air and forms reddish rust, Fe₂O₃·H₂O.
2. Carbon dioxide in moist air reacts with copper and forms a greenish layer of copper carbonate, CuCO₃. This is called patination of copper.
3. Silver reacts with hydrogen sulphide and forms black silver sulphide, Ag₂S.
4. Oxidation of aluminium forms a thin layer of aluminium oxide.

Prevention of Corrosion – Galvanizing

Corrosion can be reduced by preventing direct contact of metal with air and moisture.

Painting, oiling, greasing and varnishing

A protective layer prevents contact of the metal with moisture and oxygen. However, painting is suitable only for a limited time because a scratch can expose the metal.

Galvanizing

A thin layer of zinc is applied to iron or steel to prevent corrosion.

Figure 8.15 – Galvanizing process

Zinc is more electropositive than iron, so zinc corrodes first and protects the iron underneath.

Tinning and Anodization

Tinning

A layer of molten tin is deposited on a metal surface. This is called tinning or kalhaee. It is used to prevent damage due to corrosion and the poisonous greenish layer that can form on copper or brass vessels.

Anodization

Metals such as copper and aluminium are coated with a thin and strong layer of their oxides by electrolysis.

Figure 8.16 – Anodization

In anodization, the copper or aluminium article is used as the anode. In aluminium, the oxide layer prevents contact with oxygen and water and prevents further oxidation.

Electroplating and Alloying

Electroplating

Figure 8.17 – Electroplating

Examples: silver-plated spoons and gold-plated ornaments.

Alloying

Bronze contains 90% copper and 10% tin. Stainless steel contains 74% iron, 18% chromium and 8% carbon.

Figure 8.18 – Coins made from various alloys

When one of the metals in an alloy is mercury, the alloy is called an amalgam. Examples include sodium amalgam and zinc amalgam.

Board Exam Quick Revision

Important definitions

• Metallurgy: science and technology of extraction and purification of metals.
• Mineral: naturally occurring metal compound with impurities.
• Ore: mineral from which metal can be separated economically.
• Gangue: soil, sand and rocky impurities present in ore.
• Concentration: separation of gangue from ore.
• Roasting: strong heating of sulphide ore in air.
• Calcination: strong heating of carbonate ore in limited air.
• Alloy: homogeneous mixture of metals or a metal with a nonmetal in a certain proportion.
• Electroplating: coating a less reactive metal on a more reactive metal by electrolysis.