Metals and Non-metals • Topic 3 of 3

Extraction of Metals & Corrosion

Most metals are too reactive to occur free in nature; only the least reactive ones like gold, silver and platinum are found native. The science of extracting metals from their ores is called metallurgy.

Minerals and ores

The elements or compounds of a metal found naturally in the earth's crust are minerals. A mineral from which a metal can be extracted profitably is called an ore. Common ores are oxides (e.g. haematite Fe2O3, bauxite Al2O3), sulphides (e.g. zinc blende ZnS) and carbonates (e.g. calamine ZnCO3).

Enrichment (concentration) of the ore

Ores contain unwanted earthy impurities called gangue. Removing the gangue to concentrate the ore is called enrichment and is done by methods such as froth flotation, magnetic separation or washing.

Extraction depends on reactivity

  • Highly reactive metals (K, Na, Ca, Mg, Al): extracted by electrolysis of their molten compounds, because no cheaper reducing agent is strong enough. Example: aluminium is obtained by electrolysis of molten Al2O3.
  • Moderately reactive metals (Zn, Fe, Pb, Cu): the ore is first converted to its oxide, then reduced. Sulphide ores are roasted (heated strongly in air) and carbonate ores are calcined (heated in limited air) to give the oxide, e.g. 2ZnS + 3O2 → 2ZnO + 2SO2 and ZnCO3 → ZnO + CO2. The oxide is then reduced with carbon: ZnO + C → Zn + CO.
  • Less reactive metals (Hg, Cu in part): their oxides can be reduced by heat alone, e.g. 2HgO → 2Hg + O2.

Refining

The metal obtained is usually impure and is purified (refined) most often by electrolytic refining, in which the impure metal is the anode, a thin strip of pure metal is the cathode and a salt of the metal is the electrolyte.

Corrosion

The slow eating away of a metal by the action of air and moisture is called corrosion. The rusting of iron forms hydrated iron(III) oxide, Fe2O3·xH2O; silver tarnishes black (Ag2S) and copper turns green. Rusting needs both oxygen and water.

Prevention of corrosion

Corrosion is prevented by painting, oiling, greasing, galvanisation (coating iron with zinc), tin-plating, chromium plating and by making alloys. Alloys are homogeneous mixtures of a metal with other metals or non-metals, made to improve properties — for example steel (iron + carbon) is harder than iron, stainless steel (iron + chromium + nickel) resists rusting, and brass (copper + zinc) and bronze (copper + tin) are stronger than pure copper.

Metallurgy flow: enrichment, then roasting or calcination, then reduction, then refiningOre + gangueConcentrated ore(enrichment)Metal oxide(roasting / calcination)Crude metal(C reduction / electrolysis)Pure metal (refining)
Solved Examples
1
Worked Example
Distinguish between a mineral and an ore.
Solution
  1. A mineral is any naturally occurring element or compound of a metal found in the earth's crust.
  2. An ore is a mineral from which the metal can be extracted conveniently and profitably.
  3. So every ore is a mineral, but not every mineral is an ore.

Answer: A mineral is a natural source of a metal; an ore is a mineral rich enough to extract the metal profitably.

2
Worked Example
Differentiate between roasting and calcination with one equation each.
Solution
  1. Roasting is heating a sulphide ore strongly in the presence of excess air to form the oxide: 2ZnS + 3O2 → 2ZnO + 2SO2.
  2. Calcination is heating a carbonate ore in limited air to form the oxide: ZnCO3 → ZnO + CO2.
  3. Both convert the ore to its oxide, ready for reduction.

Answer: Roasting (sulphides, in air) and calcination (carbonates, limited air) both give the metal oxide.

3
Worked Example
Why is aluminium extracted by electrolysis and not by reduction with carbon?
Solution
  1. Aluminium is a highly reactive metal, near the top of the reactivity series.
  2. It holds its oxygen so strongly that carbon cannot reduce Al2O3.
  3. Electrolysis of molten Al2O3 supplies enough energy to free the metal.

Answer: Aluminium is too reactive for carbon reduction, so it is obtained by electrolysis of molten Al2O3.

4
Worked Example
Write the conditions necessary for rusting of iron and the formula of rust.
Solution
  1. Rusting requires the presence of both oxygen (air) and water (moisture).
  2. If either is absent, iron does not rust.
  3. The rust formed is hydrated iron(III) oxide, Fe2O3·xH2O.

Answer: Both air (oxygen) and water are needed; rust is hydrated iron(III) oxide, Fe2O3·xH2O.

5
Worked Example
What is galvanisation and how does it protect iron?
Solution
  1. Galvanisation is the process of coating iron or steel with a thin layer of zinc.
  2. The zinc layer keeps air and moisture away from the iron surface.
  3. Even if the coating is scratched, the more reactive zinc corrodes first (sacrificial protection), protecting the iron.

Answer: Galvanisation coats iron with zinc; the zinc blocks air and water and corrodes preferentially, protecting the iron.

6
Worked Example
What are alloys? Give the composition of steel, stainless steel and brass.
Solution
  1. An alloy is a homogeneous mixture of a metal with one or more metals or a non-metal.
  2. Steel = iron + a small amount of carbon; stainless steel = iron + chromium + nickel.
  3. Brass = copper + zinc.

Answer: Alloys are homogeneous metal mixtures: steel (Fe + C), stainless steel (Fe + Cr + Ni), brass (Cu + Zn).

Key Points

  • Minerals are natural metal compounds; an ore is a mineral from which a metal can be extracted profitably; gangue is the earthy impurity.
  • Highly reactive metals (K, Na, Ca, Mg, Al) are extracted by electrolysis of molten compounds.
  • Sulphide ores are roasted and carbonate ores are calcined to give the oxide, which is then reduced by carbon (ZnO + C -> Zn + CO).
  • Crude metal is purified by electrolytic refining (impure metal anode, pure metal cathode); rust is hydrated Fe2O3.xH2O, needing both air and water.
  • Corrosion is prevented by painting, oiling, galvanisation, tin/chromium plating and alloying; alloys (steel, stainless steel, brass, bronze) improve metal properties.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.A mineral from which a metal can be extracted profitably is called:
Explanation: An ore is a mineral rich enough in metal to be extracted conveniently and profitably.
Q2.Heating a carbonate ore in limited air to form the oxide is called:
Explanation: Calcination is heating a carbonate ore (in limited air) to give the metal oxide, e.g. ZnCO3 -> ZnO + CO2.
Q3.Aluminium is extracted from its oxide by:
Explanation: Aluminium is too reactive for carbon reduction, so molten Al2O3 is electrolysed.
Q4.Galvanisation is the coating of iron with:
Explanation: Galvanisation coats iron with a protective layer of zinc.
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