Extraction of metals

  • Created by: Lola
  • Created on: 05-04-12 06:55

Basic ideas

  • Only less reactive metals, such as gold, silver, copper and platinum are found uncombined in the Earth's crust. We call these metals native metals
  • Most metals are found as compunds with oxygen or sulphur in metal ores
  • A metal ore is a rock from which it is more economical to extract the metal
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Pros of mining:

  • Brings jobs
  • Boosts local economy through incoming miners and more money
  • Very profitable if a rich vein of ore is discovered
  • Open -cast mining is cheap and more predictable

Cons of mining:

  • Destroys habitats of wildlife
  • Causes pollution of groundwater
  • Makes unattractive heaps of waste rock
  • Incoming miners can bring disease
  • Very expensive - can be a risk, as veins of ore can be broken, so there is little in the area. Often mines contain water which needs to be pumped out
  • Risk of illness due to dust and damp conditions
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Phytomining and Bioleaching

  • Phytomining involves the use of plants which absorb metal ions from the soil and then concentrate the metal in their tissues.
  • After the plants are harvested, they are burned and the metal is extracted from their ash, which is known as bio-ore.
  • Metals such as gold, nickel and thalium can be extracted this way
  • Bioleaching involves the use of bacteria, which can extract metals from ores containing sulphur.
  • The bacteris carry out oxidation to turn metal sulphides into sulphate ions and pure metals.
  • Bioleaching produces less air pollution that conventional methods of extraction
  • About 20% of the extracted copper in the world comes from bioleaching
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Reduction of metal ores

Carbon, which is a non-metal, is often used to remove oxygen from metal oxides. The removal of oxygen is called reduction. Carbon can reuce metal oxides which contain elements less reactive than carbon, e.g. zinc, iron and copper

Pros of using carbon:

  • Carbon can be found in very cheap forms, such as charcoal  or coke
  • The waste product produced does not need to be dealt with - it escapes into the air

Cons of using carbon:

  • Carbon dioxide, which is produced, is a greenhouse gas
  • Carbon can form alloys with the metal, which will make them more brittle

Reactive metals can be used to displace less reactive metals

  • In the Thermit process, aluminium is used to displace iron from iron oxide
  • Iron can be used to displace copper from copper sulfate solution
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The Blast furnace

  • The main iron ore used is haematite which consists of iron(III) oxide and silicon dioxide (sand)
  • Iron ore, coke and limestone are added at the top of the blast furnace.
  • The coke reacts with the oxygen in the air to form carbon dioxide. This is a strongly exothermic reaction which supplies the heat for the furnace
  • In the hottest part of the furnace, carbon reacts with the carbon dioxide to form carbon monoxide
  • Carbon monoxide removes the oxygen from the iron (III) oxide; it is a reducing agent. The molton iron then collects at the bottom of the furnace
  • Limestone decomposes in the furnace to make calcium oxide and carbon dioxide
  • Calcium oxide is basic, so reacts with the acidic silicon dioxide to form calcium silicate (****). **** is less dense than iron to floats on the molten iron and is taped off and solidified. It is used for making cement
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Steel and other alloys

  • An alloy is a mixture containing at least one metal and at least one other element (often a metal). Alloys are usually harder and stronger than the pure metal. For instance, pure gold is too soft for jewellery, so it is mixed with copper. However, sometimes the alloy may be more brittle than the pure metal
  • Iron from the blast furnace is about 10% carbon. This cast iron is rather brittle, due to the carbon atoms making it hard for the iron atosm to slide over one another. Cast iron is usually melted and oxygen, and unreactive argon gas, is blown into it, so that carbon dioxide and steel are formed.
  • There are many types of steel, with the more carbon it contains the harder it is. Other metals are added to give different properties, for example nickel and chronium make the steel resistant to corrosion (stainless steel), and tungsten gives it a higher boiling point. Low carbon steels are easily shaped so are often used for the bodies of cars
  • Other alloys include brass, a mixture of copper and zinc, bronze, a mixture of copper and tin and solder, a mixture of lead and tin
  • Smart alloys are shape memory alloys. If they are bent into another shape, then reheated, they return to their original shape. They are often used to repair broken bones or in dental braces
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Recycling metals

There are several reasons that we should recycle metals:

  • We use less of the metal ore, which is a finite resource and will run out eventually
  • We use  less energy as energy is always needed to extract metals from their ores
  • We do less environmental damage in mining the ores
  • We cause less air pollution in extracting the metal
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