Extracting Metals

• Extracting Metals with Carbon:

  • The Earth’s crust contains metals and metal compounds such as gold, copper, iron oxide and aluminium oxide
  • Useful metals are often chemically combined with other substances forming ores
  • A metal ore is a rock that contains enough of the metal to make it worthwhile extracting
  • Common examples of oxide ores are iron and aluminium ores which are called haematite and bauxite respectively
• Extracting Metals with Carbon:

  • Unreactive metals do not have to be extracted chemically as they are often found as the un-combined element
  • Examples include gold and platinum which can both be mined directly from the Earth’s crust
  • They are known as native metals
  • The position of the metal on the reactivity series determines the method of extraction
  • Metals placed above carbon are extracted using electrolysis
  • Lower placed metals can be extracted by heating with carbon which reduces them, two common examples being iron and carbon 
• The extraction method depends on the position of a metal in the reactivity series:

  
• Extraction of Copper:

  • Most copper ores exist as copper (II) sulfide 
  • The copper can be extracted in two stages
  • Stage 1: The copper sulfide is heated in the air to produce the oxide 
  • 2CuS (s) + 3O2 (g)    →   2CuO (s) + 2SO2 (g)
  • Stage 2: The copper oxide is reduced by carbon
  • 2CuO (s) + C (s)  →   2Cu (s) + CO2 (g) 
  • This is an example of a redox reaction, whereby both reduction and oxidation have taken place 
• Extraction of Iron:

  • Iron is extracted in a large container called a blast furnace from its ore, haematite 
  • Modern blast furnaces produce approximately 10000 tonnes of iron per day 
  • The process is demonstrated and explained below 
• The demonstration:

  
• The demonstration:

  
• • The raw materials: iron ore (haematite), coke (an impure form of carbon), and limestone are added into the top of the blast furnace
  • Hot air is blown in the bottom
  • Zone 1:
  • Coke burns i the hot air forming carbon dioxide 

  C (s)  +  O2 (g)  →  CO2 (g)
• • Zone 2:
  • At the high temperatures in the furnace, more coke reacts with carbon dioxide forming carbon monoxide

  CO2 (g)  +  C (s)  →  2CO (g)

  • Zone 3:
  • Carbon monoxide reduces the iron (III) oxide in the iron ore to form iron 
  • This will melt and collect at the bottom of the furnace, where it is tapped off:

  Fe2O3 (s)  +  3CO (g)  →  2Fe (I)  +  3CO2 (g)
• • Limestone (calcium carbonate) is added to the furnace to remove impurities in the ore.
  • The calcium carbonate in the limestone decomposes to form calcium oxide:

  CaCO3 (s)  →  CaO (s)  +  CO2 (g)

  • The calcium oxide formed reacts with the silicon dioxide, which is an impurity in the iron ore, to form calcium silicate
  • This melts and collects as a molten slag floating on top of the molten Iron, which is tapped off separately:

  CaO (s)  +  SiO2 (s)  →  CaSiO3 (l)
• Extracting Metals using Electrolysis:

  • Some metals are too reactive to be reduced by carbon 
  • For these metals they are extracted using electrolysis, e.g. aluminium from aluminium oxide (bauxite) 
• Extracting Metals using Electrolysis: To extract aluminium: 

  • Bauxite is first purified to produce aluminium oxide Al2O3
  • Aluminium oxide is then dissolved in molten cryolite. This is because aluminium oxide has a melting point of over 2000°C which would use a lot of energy and be very expensive. The resulting mixture has a lower melting point without interfering with the reaction
  • The mixture is placed in an electrolysis cell, made from steel, lined with graphite. 
  • The graphite lining acts as the negative electrode, with several large graphite blocks as the positive electrodes.
• • Aluminium is produced at the cathode: 

  Al3+ +  3e-   →  Al 

  • Oxygen is produced at the anode:

  2O2- →   O2 + 4e-

  • The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell.
  • A lot of electricity is required for this process of extraction, this is a major expense
  • The carbon in the graphite anodes reacts with the oxygen produced at the anode to produce CO2 , so the anodes have to be replaced regularly
• Diagram showing the extraction of aluminium by electrolysis: