1.5.4 Extraction of Metals from Their Ores
Application of The Reactivity Series of Metals in The Extraction of Metals
- The method that is used in the extraction of metal from its ore depends on the position of the metal in the reactivity series of metals.
- metals that are located lower than carbon in the reactivity series of metals can be extracted using the reduction of metal oxide by carbon in a blast furnace.
- metals that are located higher than carbon in the reactivity series of metals are extracted by using electrolytic melting of metal compounds.
- The chart below give the method of extraction of certain metals by referring to the reactivity series.
Extraction of Metals via Electrolysis
- Diagram below shows the methods of extraction for different metals.
- Metals which are less reactive than carbon in reactivity series are extracted from their ore by displacement reaction using carbon.
- Copper and mercury can be extracted from their ore by burning directly in air.
- Silver (Ag) and gold (Au) need no extraction because they exist as element in nature.
- Those metals which are more reactive than carbon are extracted by electrolysis.
Extraction of Sodium
- Figure above shows the illustration of the designed used to extract sodium in industry by using electrolysis.
- In SPM, you need to know
- the electrolyte used
- the material used as anode and cathode
- the chemical reaction at anode and cathode
Electrolyte
Molten sodium chloride
NaCl —> Na+ + Cl–
Electrode:
Anode: Graphite
Cathode: Iron
Chemical Reaction
Anode:
2Cl– —> Cl2 + 2e
The negative chloride ions are attracted to the anode and then discharged to form chlorine gas.
Cathode
Na+ + e —> Na
Note:
- The sodium ions are discharged to form sodium atom.
- Due to high temperature, the sodium metal formed is in molten form.
- Metal sodium have lower density. Therefore it moves upward and been collected.
Extraction of Aluminium
- Figure above shows the illustration of the designed used to extract aluminium in industry by using electrolysis.
- In SPM, you need to know
- the electrolyte used
- the material used as anode and cathode
- the chemical reaction at anode and cathode
- why cryolite is added into molten bauxite in the process?
Electrolyte:
Molten bauxite (Aluminium Oxide).
Al2O3→ 2Al3+ + 3O2-
Electrode:
Anode: Graphite
Cathode: Graphite
Chemical Reaction
Anode:
2O2- → O2 + 4e
At the anode, oxygen gas which also has commercial value is collected.
Cathode
Al3++ + 3e → Al
Note:
- The aluminium ions are attracted towards the graphite cathode.
- The ions is discharged and become molten aluminium metal.
Q & A
Q: Explain why cryolite is added into molten bauxite in the process?
A: To reduce the melting point of bauxite.
Q & A
Q: Explain why the carbon electrodes need to be replaced periodically.
A:
- At the temperature of 980 °C, the oxygen burns the carbon anode.
- Also, this cell uses large quantities of electricity, and therefore needs cheap sources of power.
Extraction of Metals via Reduction by Carbon
Extraction of Iron
- Iron is extracted from its ore, that is hematite (Fe2O3) and magnetite (Fe3O4 ) through reduction by carbon in the form of carbon in a blast furnace.
- The mixture of iron ore, carbon, and limestone is entered into a blast furnace through the top of the furnace.
- Hot air is then put in through the lower part of the furnace
- Limestone (calcium carbonate) is disintegrated by hot air into calcium oxide and carbon dioxide gas.CaCO3 (s) → CaO (s) + CO2(g)
- Carbon also burns in hot air to produce carbon dioxide gasC (s) + O2 (g) → CO2 (g)
- Carbon dioxide that is produced reacts with excess carbon to produce carbon monoxide gas which is a type of reducing agent.CO2 + C → 2CO
- Carbon and carbon monoxide then reduces the iron ore to melted iron which flows to the lower part of the furnace.
2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g)
Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)
Fe3O4 (s) + C(s) → 3Fe(s) + 2CO2(g)
Fe3O4 (s) + 4CO(g) → 3Fe(s) + 4CO2(g) - In the blast furnace, calcium oxide that is produced from the disintegration of calcium carbonate, reacts with foreign matter such as sand (silicon dioxide) in the iron ore to form slag.CaO (s) + SiO2 (s) → CaSiO3 (s) (slag)
- The melted dross flows down to the bottom part of the furnace and floats on the layer of melted iron.
- The melted iron and dross then are taken out from the furnace separately.
- The melted iron is cooled in a mould to form cast iron, while the dross is used to make the foundation for roads and houses.
Extraction of Tin
- Tin exists as stanum(IV) oxide, SnO2 in the mineral casiterite, that is tin. Tin contains a lot of foreign matter such as sand, soil, sulphur, carbon and oil.
- Firstly the tin ore is made concentrated by the method of floatation. In this process, the ore is crushed and shaken in oily water. The foreign matter such as sand and soil drown while the tin ore sticks to the oil and floats on the surface of the water.
- The tin ore is then collected and roasted to take away foreign matter such as carbon, sulphur and oil.
- Lastly, the tin ore is mixed with carbon in the form of charcoal and is heated in a blast furnace at a high temperature.
- Stanum(IV) oxide in the ore is reduced to tin by the reducing agent carbon and carbon monoxide.
SnO2 (s) + 2C(s) → Sn (s) + 2CO (g)
SnO2 (s) + C(s) → Sn (s) + CO2 (g)
SnO2 (s) + 2CO(s) → Sn (s) + 2CO2 (g) - The melted tin that is formed collects at the base of the furnace and then is channeled out into a mould to form tin ingot.