Extraction of Metals

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  • Created by: chunks-42
  • Created on: 14-05-15 09:35

Sulphide Ores --> Oxides

1)An ore is a naturals substance that a metal can be economically extracted from. In other words, a rock you can get quite a bit of metal out of.

2)Metals are often found in ores as sulphides (such as lead sulphide and zinc sulphide), or oxides (like titanium dioxide and iron (III) oxide). The metal element needs to be removed from these compounds - that's where the chemistry comes in.

3)The first step to extract a metal from a sulphide ore is to turn into an oxide. This is done by roasting the sulphide in air.

E.g. zinc sulphide + oxygen --> zinc oxide + sulphur dioxide

         2ZnS          +    3O2   -->   2ZnO      +   2SO2

4)Here's the bad news: sulphur dioxide gas causes acid rain. Acid rain can cause harm to plants and aquatic life, and damage limestone buildings, so sulphur dioxide can't be released into the atmosphere.

5)But here's the good news: by converting the sulphur dioxide to sulphuric acid a pollutant is avoided, and a valuable product is made - sulphuric acid's in demand because it's used in many chemical and manufacturing processes.

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Oxides --> The Metal

1) The methods for reducing the oxide depends on the metal you're trying to extract.

2) Carbon (as coke - a solid fuel made from coal) and carbon monoxide are used as reducing agents for quite a few metals - usually the ones that are less reactive than carbon.

You need to know three examples of the extraction of metals with carbon monoxide and carbon.

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Reduction of Iron (III) Oxide

Iron (III) oxide is reduced by carbon or carbon monoxide to iron and carbon dioxide

2Fe2O3 + 3C ---> 4Fe + 3CO2

Fe2O3 + 3CO ---> 2Fe + 3CO2

This happens in a blast furnace at temperatures greater than 700 degrees celsius.

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Reduction of Manganese (IV) Oxide

Manganese (IV) oxide is reduced with carbon (as coke) or carbon monoxide in a blast furnace.

MnO2 + C ---> Mn + CO2

MnO2 + 2CO ---> Mn + 2CO2

This needs higher temperatures than iron (III) oxide - about 1200 degrees celsius.

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Reduction of Copper Carbonate

Copper can be extracted using carbon.

One ore of copper is malachite, containing CuCO3. This can be heated directly with carbon.

2CuCO3 + C ---> 2Cu + 3CO2

Another method involves heating the carbonate until it decomposes, then reducing the oxide with carbon.

CuCO3 ---> CuO + CO2

2CuO + C ---> 2Cu + CO2

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Tungsten

1) Tungsten can be extracted from its oxide with carbon, but that can leave impurities which make the metal more brittle. If pure tungsten is needed, the ore is reduced using hydrogen instead.

WO + 3H2 ---> W + 3H2O

This happens in a furnace at temperatures above 700 degrees celsius.

2) Tungsten is the only metal reduced on a large scale using hydrogen. Hydrogen is more expensive but it's worth the extra cost to get pure tungsten, which is easier to work with.

3) Hydrogen is highly explosive when mixed with air though, which is a bit of a hazard.

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Aluminium

1) Aluminium is too reactive using reduction by carbon. A very high temperature is needed, so extractng aluminium by reduction is too expensive to make it worthwhile.

2) Aluminium's ore is called bauxite - it's aluminium oxide, Al2O3, with various impurities. First of all, these impurities are removed. Next, its dissolved in molten cryolite (sodium aluminium fluoride, Na2AlF6), which lowers its melting point from a scorching 2050 degrees celsius, to a cool 970 degrees celsius.

Electrolysis of aluminium

1) Aluminium is produced at the cathode and collects as the molten liquid at the bottom of the cell.

Al3+ + 3e- ---> Al

2) Oxygen is produced at the anode.

2O2- ---> O2 + 4e-

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Titanium

1) Titanium is a pretty abundant metal in the Earth's crust. In its pure form, titanium is a strong, light metal that is highly resistant to corrosion. Pretty much perfect really, so how come it's not used any more... Well basically, it's just too difficult and explosive to produce.

2) The main ore is rutile (titanium (IV) oxide, TiO2). You can't extract titanium from it by carbon reduction because you get titanium carbide which ruins it ... TiO2 + 3C ---> TiC + 2CO

The extraction of titanium- is a batch process with several stages

1) The ore is converted to titanium (IV) chloride by heating it to about 900 degrees celsius with carbon in a steam of chlorine gas. TiO2 + 2Cl2 + 2C ---> TiCl4 + 2CO

2) The titanium chloride is purified by fractional distillation under an inert atmosphere of argon or nitrogen.

3) Then the chloride gas gets reduced in a furnace at almost 1000 degrees celsius. It's heated with a more reactive metal, such as sodium or agnesium. An inert atmosphere is used to prevent side reactions. TiCl4 + 4Na --> Ti + 4NaCl / TiCl4 + 2Mg --> Ti + 2MgCl

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Recycling Metals

Once you've got the metal out of the ore, you can keep recycling it again and again. As usual, there are pros and cons:

Advantages

  • Saves raw materials - ores are a finite resource.
  • Saves energy - recycling metals takes less energy than extracting metal. This saves money too.
  • Reduces waste sent to landfill.
  • Mining damages the landscape and spoil heaps are ugly. Recycling metals reduces this.

Disadvantages

  • Collecting and sorting metals from other waste can be difficult and expensive.
  • The purity of recycled metal varies - there's usually other metals and other impurities mixed in.
  • Recycling metals may not produce a consistent supply to meet demand.
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Scrap Iron and Copper Extraction

1) Some scrap metal can be putto other uses. For example, scrap iron can be used to extracr copper from solution. This method is mainly used with low grade ore - ore that only contains a small percentage of copper.

2) Acidified water dissolves the copper compounds in the ore. The solution is collected and scrap iron is then added. The iron dissolves and reduces the copper (II) ions. The copper precipitates out of the solution. Cu2+ + Fe ---> Cu + Fe2+

2) This process produces copper more slowly than carbon reduction and has a lower yield, which is why it's not used with ores that have a high copper content. It's cheaper than carbon reduction though, because you don' need high temperatures, and better for the environment because there's no CO2 produced.

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