CHEMISTRY · Stage 9 · Chapter 12: Chemical Reactions 3 and Rates
Chemistry 12.4 Extracting metals — Ores, Carbon Reduction and Electrolysis
5 min read 7 parts Lesson 4 of 13
Lesson summary
12.4 Extracting metals — Ores, Carbon Reduction and Electrolysis — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define ore and link the position of a metal in the reactivity series to how it is extracted.Compounds in rocks — in the Earth's crust most metals are …
- Native metals
- Carbon in the series
- Below carbon
- Above carbon
- Blast furnace
- Oxidation and reduction
Part 1 of 7Ores
- Compounds in rocks — in the Earth's crust most metals are joined to other elements in compounds mixed with other substances.
- Ore = a rock that a metal can be extracted from, containing enough metal compound to be worth extracting.
- Iron is found as haematite (iron oxide), aluminium as bauxite (aluminium oxide), copper in malachite (copper carbonate) and lead as lead sulfide.
- Native metals — unreactive metals such as gold can be found as the metal itself.
- Reduction = removing oxygen (or the non-metal) from a compound to get the metal.
Part 2 of 7Extraction and the reactivity series
- Carbon in the seriescarbon is a non-metal but can be placed in the series between aluminium and zinc.
- Below carbonzinc, lead, iron, copper and silver are extracted from their oxides by heating with carbon, which is cheap and easy to obtain.
- Above carbonsodium, calcium, magnesium and aluminium are extracted by electrolysis.
| Position | Method | Examples |
|---|---|---|
| Above carbon | Electrolysis | Potassium, sodium, calcium, magnesium, aluminium |
| Below carbon | Heating with carbon | Zinc, iron, lead |
| Very low | Found native or by simple separation | Gold, silver |
Part 3 of 7Extracting iron with carbon
- Separate iron oxide from the compounds it is mixed with.
- Heat the iron oxide with carbon (charcoal or coke): iron oxide + carbon → iron + carbon dioxide.
- Blast furnace — iron ore, coke and limestone are fed in at the top and hot air is blown in near the bottom.
- Coke burns to carbon dioxide, which reacts with more coke to form carbon monoxide.
- Carbon monoxide reduces iron oxide to molten iron: iron oxide + carbon monoxide → iron + carbon dioxide.
- Limestone removes impurities such as sand by forming slag, which floats on the iron and is tapped off.
- Oxidation and reduction — iron oxide is reduced (loses oxygen) and carbon is oxidised (gains oxygen).
- Use — iron is made into steel by adding other elements, and the 55 km Hong Kong-Zhuhai-Macau Bridge, the longest sea-crossing bridge, is made of steel.
- Cost and environment — the process is relatively cheap, but it releases carbon dioxide, a greenhouse gas.
Part 4 of 7Extracting lead
- Source — lead exists as lead sulfide in the crust, and people have used lead for up to 8000 years.
- Heat lead sulfide in air: lead sulfide + oxygen → lead oxide + sulfur dioxide.
- Heat the lead oxide with carbon: lead oxide + carbon → lead + carbon dioxide.
- Uses — lead is now used for roofing and to protect from X-rays.
Part 5 of 7Extracting reactive metals by electrolysis
- Aluminium — carbon cannot remove oxygen from aluminium oxide, so electricity is used instead.
- Dissolve pure aluminium oxide in a special solvent (molten cryolite), which lowers the melting temperature and saves energy.
- Pass a very large electric current (about 100 000 amps) through the solution to split up the aluminium oxide.
- Liquid aluminium forms at the negative carbon electrode and oxygen at the positive electrode.
- Carbon anodes — the oxygen burns away the carbon anodes, so they must be replaced regularly.
- Electrolysis in general — positive metal ions move to the negative electrode (cathode) and gain electrons to form the metal.
- Other reactive metals are extracted this way, for example sodium from molten sodium chloride.
- Cost — electrolysis uses a great deal of electricity, which is why aluminium is made near sources of cheap power.
Part 6 of 7Extracting gold
- Native gold — gold is at the bottom of the series, is unreactive and is found as the element.
- Panning — gold in stream beds mixed with sand and gravel is separated by placing the mixture in a pan with water, because gold has a higher density and sinks to the bottom.
Part 7 of 7Sustainability
- Miningit takes land, uses energy and may pollute.
- Recyclingit saves ores and energy; recycling aluminium needs only about 5% of the energy of making it from ore.
- Balancegovernments and industry balance economic needs with environmental effects.
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