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Metal extraction Converting sulphides to oxides The occurrence of metals in the Earth's crust Sulphides ores are usually converted to oxides before •Rocks that contain a high enough percentage of a extraction by roasting with oxygen. metal to be extracted commercially are known as The most important ore of zinc is mostly zinc sulphide ores . 2 ZnS + 3O 2ZnO + 2SO •Aluminium and iron are the most abundant metals 2 2 in the Earth's crust. or copper can be extracted from copper(I) sulphide Cu2S + 2O 2CuO + SO •Most compounds are found as oxides or 2 2 sulphides, but sulphides are usually converted to The oxide is then reduced with a suitable reducing agent oxides before extraction. such as carbon. Sulphur dioxide is an acidic gas that can form acid rain if released into the atmosphere. The sulphur dioxide can be used, however, to form sulphuric acid by the contact process Which method is used depends on: Methods of extraction •the energy requirements The extraction of metals involves reduction, usually extraction uses large amounts of energy (electricity and / of metal oxides. or heat) This reduction of the metal compound is usually done in one of the following ways: •the cost of the reductant •by heating with carbon (in the form of coke) Carbon , which is cheap, is widely used, but sometimes •by heating with hydrogen more reactive metals are required which are very costly •by heating with a more reactive metal (active metal) •by electrolysis •the metal purity required the higher the required purity, the greater the cost in obtaining that purity N Goalby chemrevise.org 1 Method 1: Reduction of metal oxides with carbon Carbon and carbon monoxide are cheap, effective and readily In theory all metals can be extracted this way, but for available reducing agents some •the temperature required is excessively high Iron, manganese and copper are extracted using C and CO •metal carbides are formed rather than the metal itself (e.g. tungsten, titanium) o Fe O + 3 CO 2 Fe + 3 CO Occurs at about 1200 C 2 3(s) (g) (l) 2(g) excess C is easily removed in the Fe O + 3 C 2 Fe + 3 CO Occurs at higher temps 2 3(s) (s) (l) (g) steel making process MnO2 + C Mn + CO2 2 CuO + C 2 Cu + CO The CuO comes from the thermal decomposition of malachite 2 CuCO CuO + CO 3 2 Pollution problems arise from using carbon, giving CO (a greenhouse gas) and CO (toxic) 2 A mixture of limestone, cokeandhaematite is added at the top of the blast furnace. Waste gases from the blast furnace. Hot air is ‘blasted’ into the blast furnace. Impurities collect at the bottom in a layer, ‘slag’ The iron, pig iron, collects at the bottom of the blast furnace to be tapped off. N Goalby chemrevise.org 2 Method2:Extraction by electrolysis When a metal is more reactive than carbon (e.g. aluminium), it can The main cost in this process is not be extracted by reduction with carbon; electrolysis is usually used. the electricity – so a cheap supply Electrolysis is not used if the metal has to be very pure. is needed – plants are often built near hydroelectric power stations. The raw materials for the extraction of aluminium are purified aluminium Electrode Reactions oxide (Al O ) and cryolite (Na AlF ). 2 3 3 6 3+ - cathode Al + 3e Al The aluminium oxide must be molten or dissolved to conduct electricity. 2- - anode 2O O2 + 4e It is dissolved in molten cryolite (this lowers the melting point of the The oxygen gas formed at the anode then aluminium oxide, increases conductivity and therefore reduces cost). reacts with the graphite (carbon) anode and Without using the cryolite, the aluminium oxide melts at too high a forms carbon dioxide C + O CO temperature which makes the process uneconomic. 2 2 The anode therefore disintegrates and has to be replaced frequently. Oxygen gas formed at the anode, reacts with the carbon to form CO2.The anode disintegrates and has to The anode is be replaced frequently. made from + graphite - The cathode is made from graphite The electrolyte contains molten aluminium oxide dissolved in molten The aluminium ions are attracted to cryolite, at a temperature the cathode, gain three electrons of 950°C to form liquid aluminium. N Goalby chemrevise.org 3 Method 3: Reaction with a more reactive metal Titanium is a very useful metal because it is abundant, Titanium cannot be extracted with carbon has a low density and is corrosion resistant – it is used for because titanium carbide (TiC) it is formed making strong, light alloys for use in aircraft for example. rather than titanium (similar reactions take place for vanadium, tungsten and Titanium is extracted by reaction with a molybdenum). Titanium cannot be extracted by more reactive metal (e.g. Mg, Na). electrolysis because it has to be very pure. Steps in extracting Titanium TiO + 2 Cl + 2 C TiCl + 2 CO 1. TiO (solid) is converted to TiCl (liquid) at 900C: 2 2 4 2 4 2. The TiCl is purified by fractional distillation in an Ar 4 TiCl4 + 4 Na Ti + 4 NaCl atmosphere. TiCl + 2Mg Ti + 2 MgCl 3. TheTiis extracted by Mg or Na in an Ar atmosphereat 500C 4 2 Titanium is expensive because TiO is converted to TiCl as it can be 2 4 1. The expensive cost of the Na or Mg purified by fractional distillation, TiCl4 2. This is a batch process which makes it expensive because the being molecular (liquid at room temperature) rather than ionic like process is slower (having to fill up and empty reactors takes TiO (solid at room temperature). time) and requires more labour and the energy is lost when the 2 reactor is cooled down after stopping This all makes titanium expensive 3. The process is also expensive due to the Ar, and the need to even though it is a relatively abundant metal. It is only therefore remove moisture (as TiCl is susceptible to hydrolysis). used to a limited amount even 4 though it has useful properties 4. High temperatures required in both steps TiCl 4 Argon and air argon Steel reactor lined with Titanium sponge molybdenum is removed by hand after the TiCl 4 Titanium reactor has cooled sponge magnesium Magnesium Magnesium chloride chloride is tapped off. The magnesium is recovered using electrolysis. N Goalby chemrevise.org 4
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