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
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            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.
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              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.
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          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 900C:                       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 500C                    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.
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