Metals are rarely found pure. Learn how ores are concentrated, how aluminium is won by electrolysis, and why alloys beat pure metals.
A mineral is a naturally occurring compound of a metal, and an ore is a mineral from which the metal can be extracted profitably. The earthy impurities in an ore are called gangue. Extraction involves concentrating the ore, converting it to an oxide by roasting or calcination, reducing the oxide to the metal, and refining it.
What is the difference between roasting and calcination?
Roasting heats a sulphide ore in excess air, for example 2ZnS + 3O₂ → 2ZnO + 2SO₂. Calcination heats a carbonate or hydroxide ore in little or no air, for example ZnCO₃ → ZnO + CO₂
Which method is used to concentrate a sulphide ore?
Froth flotation: the powdered ore is mixed with water and oil, and air blown through carries the ore particles to the surface in the froth
How is zinc oxide reduced to zinc?
By heating it with carbon (coke): ZnO + C → Zn + CO
The chief ore of aluminium is bauxite, Al₂O₃·2H₂O. The purified oxide, alumina, is dissolved in molten cryolite (Na₃AlF₆) with a little fluorspar (CaF₂) and electrolysed in a graphite-lined tank, which is the cathode, using graphite anodes at about 950 °C. Molten aluminium collects at the bottom. Oxygen is released at the anode and burns it away to carbon dioxide. Overall: 2Al₂O₃ → 4Al + 3O₂.
Why is alumina dissolved in molten cryolite before electrolysis?
Pure alumina melts above 2000 °C and conducts poorly when molten; cryolite lowers the working temperature to about 950 °C and makes the melt conduct better
Why do the graphite anodes have to be replaced from time to time?
Oxygen released at the anode reacts with the hot carbon to form carbon dioxide, so the anodes burn away
What is the percentage of aluminium in Al₂O₃? (Al = 27, O = 16)
54 ÷ 102 × 100 = 52.94%
An alloy is a homogeneous mixture of a metal with one or more other elements. Alloys are often harder and stronger than the pure metals, resist corrosion better, and may have lower melting points. Brass is copper and zinc, bronze is copper and tin, duralumin is aluminium with copper, magnesium and manganese, and stainless steel is iron with chromium, nickel and carbon.
Name the metals present in brass and in bronze.
Brass contains copper and zinc; bronze contains copper and tin
Why is duralumin used to make aircraft parts?
It is light like aluminium but much stronger, because of the copper, magnesium and manganese in it
1. What is gangue?
2. Name the chief ore of aluminium.
3. Why is cryolite added to alumina before electrolysis?
4. What is formed at the cathode when alumina is electrolysed?
5. Why must the graphite anodes be replaced?
6. What name is given to heating a carbonate ore in limited air?
7. Which method is used to concentrate sulphide ores?
8. Write the equation for the roasting of zinc sulphide.
9. Name the alloy of copper and zinc.
10. What mass of aluminium can be obtained from 102 kg of pure alumina? (Al = 27, O = 16)