Extraction of aluminium, copper and iron

Aluminium from bauxite

Baeyer's process makes pure alumina. Hall's process turns alumina into aluminium by electrolysis.

1. 1
   Al₂O₃·2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O

   Bauxite is ground and heated with sodium hydroxide under pressure. It forms soluble sodium meta-aluminate; impurities are filtered off.
2. 2
   NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH

   Diluting with water gives a white precipitate of aluminium hydroxide.
3. 3
   2Al(OH)₃ → Al₂O₃ + 3H₂O

   The precipitate is heated strongly to give pure alumina.
4. 4★
   Electrolyte: alumina dissolved in molten cryolite (Na₃AlF₆) and fluorspar (CaF₂)

   These lower the melting point and make the melt conduct electricity better.
5. 5
   Cathode: iron tank lined with graphite. Anode: graphite rods. About 900 to 950 °C, 5 to 6 V
6. 6
   2Al₂O₃ → 4Al³⁺ + 6O²⁻; cathode: 4Al³⁺ + 12e⁻ → 4Al; anode: 6O²⁻ → 3O₂ + 12e⁻

   Molten aluminium collects at the bottom. Oxygen burns the anode rods to CO₂, so they are replaced from time to time.

★ key step

Copper from copper pyrite (CuFeS₂)

1. 1
   Concentration by froth flotation

   Copper pyrite is a sulphide ore.
2. 2
   Roasting: 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂

   Heated strongly in excess air; moisture and volatile impurities go off.
3. 3
   Smelting: the roasted ore is heated with coke and sand in a blast furnace

   This gives copper matte, a molten mixture of Cu₂S and FeS, and a slag that is thrown away.
4. 4★
   Bessemerisation: 2FeS + 3O₂ → 2FeO + 2SO₂; FeO + SiO₂ → FeSiO₃ (slag); 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂; 2Cu₂O + Cu₂S → 6Cu + SO₂

   Hot air is blown through the molten matte in a Bessemer converter. FeS is removed as iron silicate slag and Cu₂S becomes copper. Escaping SO₂ leaves a blistered surface: blister copper, about 98% pure.
5. 5
   Electrolytic refining: anode Cu → Cu²⁺ + 2e⁻; cathode Cu²⁺ + 2e⁻ → Cu

   Anode: impure copper. Cathode: thin strip of pure copper. Electrolyte: acidified copper sulphate solution. Impurities settle as anode mud.

★ key step

Iron from haematite (Fe₂O₃)

1. 1
   Concentration by gravity separation

   Haematite is heavier than its gangue.
2. 2
   Roasting and calcination in a reverberatory furnace

   Removes moisture and impurities such as sulphur and arsenic as their oxides.
3. 3
   Charge: roasted ore, coke and limestone in the ratio 8 : 4 : 1, with a blast of hot air from below
4. 4
   Combustion zone (bottom, about 1500 °C): C + O₂ → CO₂ (heat given out)
5. 5
   Fusion zone (middle, about 1000 °C): CO₂ + C → 2CO; CaCO₃ → CaO + CO₂; CaO + SiO₂ → CaSiO₃ (slag)

   The first two reactions take in heat.
6. 6★
   Reduction zone (top, about 400 °C): Fe₂O₃ + 3CO → 2Fe + 3CO₂
7. 7
   Molten iron sinks to the bottom and slag floats on it. Both are tapped off separately.

   This iron is called pig iron.

★ key step

How to draw the blast furnace

1. 1
   Draw a tall tower, wider in the middle and narrower at the top and bottom. Label it blast furnace lined with fire bricks.
2. 2
   At the top draw a cup and cone. Label it charge: roasted ore, coke, limestone.
3. 3
   Near the top draw a side pipe. Label it waste gases out.
4. 4
   Mark three zones from top to bottom: reduction zone (400 °C), fusion zone (1000 °C), combustion zone (1500 °C), each with its reaction.
5. 5
   Near the bottom draw pipes on both sides. Label them hot air blast in.
6. 6
   At the bottom draw two outlets, the upper one slag out and the lower one molten iron out. Show slag as a layer above the iron.

How to draw the electrolytic cell for aluminium

1. 1
   Draw a rectangular tank. Label it iron tank lined with graphite: cathode (−).
2. 2
   Fill it with liquid. Label it alumina in molten cryolite and fluorspar.
3. 3
   Draw thick rods dipping in from the top. Label them graphite rods: anode (+).
4. 4
   Shade a layer at the bottom with a tap. Label it molten aluminium.
5. 5
   Draw bubbles at the rods. Label them oxygen. Write 900 to 950 °C, 5 to 6 V beside the cell.