Extraction of metals

 

Define oxidation in terms of gain of oxygen and reduction in terms of loss of oxygen (C4.01)
 

 

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The reaction of a substance with oxygen is obviously oxidation:

2Mg(s) + O2(g) 2MgO(s)

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(l) 
 

 

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Reduction can be seen as the inverse process where oxygen is removed:

CuO(s) + H2(g) Cu(s) + H2O(g) on heating

Fe2O3(s) + 3CO(g) 2Fe(l) + 3CO2(g) in blast furnace
 

 

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In the latter reaction the carbon monoxide has been oxidised according to the first definition.
 

 

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Oxidation and reduction always occur together.

 



Define oxidation in terms of loss of electrons and reduction in terms of gain of electrons (C4.02)
 

 

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The reaction of magnesium with oxygen

2Mg(s) + O2(g) 2MgO(s)

gives a compound containing Mg2+ ions.
 

 

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The reaction of magnesium with chlorine gives ionic magnesium chloride

Mg(s) + Cl2(g) MgCl2(s)

which also contains Mg2+ ions. From the point of view of the magnesium the same process has occurred:

Mg Mg2+  +  2e
 

 

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Therefore oxidation is loss of electrons.
 

 

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The reaction of copper(II) oxide with hydrogen is reduction according to the first definition; it also results in copper(II) ions changing to copper metal, i.e.

Cu2+  +  2 e --    Cu

so that reduction is gain of electrons.
 

 

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OILRIG: Oxidation Is Loss Reduction is Gain (of electrons).
 

 

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Oxidation and reduction occur during electrolysis:

  • At the anode electrons are removed from the negative ion; at the Anode you get oxidAtion.
  • At the cathode electrons are added to the positive ion; at the Cathode you get reduCtion.

     


Understand that the extraction of metals involves reduction of their ores (C4.03)
 

 

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Apart from gold and silver which occur native (i.e. as the metal) metals have to be extracted from their ores, in which the metal exists as positive ions; Fe2+ in Fe2O3, Al3+ in Al2O3, Na+ in NaCl.
 

 

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The extraction involves adding electrons to the metal ion to make the metal, i.e. it is reduction.

 



Recall how the way in which a particular metal is extracted from its ores is related to its position in the reactivity series (C4.04)
 

 

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If a metal is reactive it forms ions easily; therefore conversion of its ions to the free metal is difficult.
 

 

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Reactive metals (alkali (group 1) and alkaline earth (group 2) metals, and aluminium) have to be extracted using electrolysis to reduce the ions.
 

 

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Less reactive metals (Zn, and other metals less reactive than Zn) can be extracted using carbon or carbon monoxide to reduce the ores, which are usually oxides or have been converted to the oxide.

 



Understand that processes involving the use of large amounts of electricity are relatively expensive (C4.05)
 

 

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Electrolysis uses very large amounts of electricity, which is expensive in terms of fuel costs.
 

 

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The process is much more expensive than reductions using C or CO; thus aluminium, the commonest metal in the Earths crust, is expensive, whereas iron is rather cheap.
 



Describe the extraction of aluminium from purified bauxite including simple cell diagram, nature of electrolyte and electrodes, and reactions (C4.06)
 

 

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Bauxite is a mixture of aluminium oxide, iron oxide, silicon dioxide and variable amounts of water.
 

 

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Pure aluminium oxide is extracted from bauxite and must be electrolysed.
 

 

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The aluminium oxide cannot be melted on an industrial scale since its melting temperature is over 2000oC. It is dissolved in molten cryolite, Na3AlF6, at about 950oC.
 

 

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The anode and the cathode of the cell are both graphite.

 

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At the anode, oxide ions are oxidised to oxygen which immediately reacts with the graphite anode, this having to be replaced from time to time:

2O2   O2 + 4e

C + O2 CO2
 

 

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Aluminium ions are reduced to molten aluminium at the cathode:

Al3+ +  3 e   Al

The molten metal is siphoned out of the cell from time to time.

 



Understand why aluminium is less reactive than expected and appreciate how anodising is achieved and the change that takes place during the process (C8.01)
 

 

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Aluminium is much less reactive than might be expected from its position in the Periodic Table and from the difficulty of its extraction from aluminium oxide.
 

 

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The lack of reactivity is due to a layer of aluminium oxide which sticks strongly to the surface of the metal and protects it from attack.
 

 

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The thickness of the oxide layer is increased by anodising; the aluminium object is made the anode of an electrolytic cell using a dilute sulphuric acid electrolyte:

 

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At the anode oxygen is liberated from hydroxide ions in the electrolyte:

4OH  (aq) O2(g) + 2H2O(l)

The oxygen is liberated as very fine bubbles and reacts with the aluminium to form aluminium oxide the layer can be made thicker by electrolysing for a longer time.
 

 

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The lead cathode is inert; hydrogen is liberated there from H+ ions in the electrolyte.
 

 

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The aluminium oxide layer can be coloured in the electrolytic process by incorporating dyestuffs into the electrolyte.
 

 

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Anodised objects can be made highly abrasion resistant by giving them thick oxide coatings.
 



Relate the uses of aluminium to its properties (C4.07)
Appreciate the need for alloying aluminium to increase its strength (C8.02)
Understand the important uses of aluminium and its alloys (C8.03)
 

 

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Aluminium is

  • soft and of low density it needs to be alloyed to be useful for engineering applications
  • malleable and ductile
  • a very good conductor of electricity
  • resistant to corrosion (see below)
     

 

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It is used as an alloy (Duralumin) with magnesium and some copper for making aircraft (because of low density; not light) and with magnesium for car bodies (Birmabrite).
 

 

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It is used with a steel core (for strength) for overhead power lines, and without the steel for underground cables.
 

 

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It is used to make a wide variety of items for construction, e.g. window frames, because of its ease of extrusion into complex shapes and its corrosion resistance.



Recall that carbon and carbon monoxide can reduce the oxides of less reactive metals (C4.08)

 

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A wide variety of oxides of less reactive metals can be reduced by carbon or carbon monoxide:

Fe2O3 + 3C 2Fe + 3CO
Fe2O3 + 3CO
2Fe + 3CO2
CuO + C
Cu + CO
CuO + CO
Cu + CO2

Heating the oxides in a blast furnace with coke will give both reactions at the same time, depending on the temperature.

 

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Carbon or carbon monoxide reduction of oxides can be used to obtain zinc and metals less reactive than zinc.

 

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The extraction of iron is the largest use of the process.



Describe the extraction of iron in the blast furnace, including outline diagram, raw materials, reactions and the formation and uses of slag (C4.09)

Explain the chemical reactions occurring in different parts of the blast furnace and the energy changes associated with them (C8.04)

 

 

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The structure of the blast furnace is essentially:

 

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The top of the furnace is charged with a mixture of:

  • coke: this produces heat and the reducing agent, CO;
  • iron ore, either Fe2O3 or Fe3O4
  • limestone, used to remove silica impurity from the ore.

 

 

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Coke is added to produce heat and the reducing agent, CO; these reactions occur near the air inlet.

C + O2 CO2     + heat; an exothermic reaction.

CO2 + C 2CO   an endothermic reaction
 

 

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CO reduces the iron ore; there is also some reduction with unburnt carbon higher up the furnace. The reaction with CO is endothermic and uses about half the energy in the furnace:

Fe2O3 +  3CO   2Fe  +  3CO2

Fe2O3 +  3C   2Fe  +  3CO
 

 

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The main impurity in the ore is silica, SiO2. This is acidic and reacts with CaO (a base) to give molten slag CaSiO3. CaO is produced from limestone by heating in an endothermic reaction:

CaCO3   CaO  + CO2

CaO + SiO2 CaSiO3
 

 

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The molten slag floats on top of the molten iron.
 

 

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It matters how impure the iron ore is removal of SiO2 as slag is an endothermic process and therefore costs fuel.
 

 

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Slag is used to make building blocks and for road-fill.

 



Understand that impure iron from the blast furnace and pure iron have very limited uses (C8.05)
 

 

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Pig-iron from the blast furnace contains about 4% carbon.

  • It is strong but very brittle so will not withstand sharp blows.
  • It is used for street furniture bollards, drain and manhole covers, for example.

 



Describe the production of mild steel by lowering the carbon content in the impure iron using high pressure oxygen (C8.06)
 

 

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The carbon content of pig-iron is lowered by blowing oxygen through the molten iron this oxidises the carbon to gaseous CO2.
 

 

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Mild steel contains about 0.15% carbon and is the most widely-used steel.
 



Understand the uses of mild steel (C8.07)
 

 

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Mild steel is used for almost all non-specialist steel products cars, domestic goods, constructional steel.
 

 

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Mild steel rusts easily in presence of air and water, so alloy steels are used where this is such a serious problem that the much greater coast of alloys is acceptable.
 



Understand the uses of alloy steels such as stainless steel, titanium steel and manganese steel (C8.08)
 

 

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Stainless steel consists of an alloy of iron, chromium and nickel:

  • It is expensive so is not used for large-scale construction.
  • Used for cutlery, kitchen fittings, surgical instruments, exhaust systems for cars (at a price!).
  • Stainless steel is attacked by high concentrations of acids.
  • High-quality stainless steel is non-magnetic. It will contain 18% Cr and 8% Ni and is called 18:8 stainless.
     

 

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Alloy steels are used for various purposes:

  • Titanium steels are used for cutting tools;
  • Vanadium steels are used for spanners and springs;
  • Manganese steel is especially hard and is used for safes;
  • Alnico contains Al, Ni and Co in addition to iron and is used for permanent magnets.
     


Describe the purification of copper by electrolysis, including a simple diagram of the cell (C4.10)
 

 

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Copper from the blast furnace contain numerous impurities; it is purified by electrolysis using the cell shown:

 

 

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The copper anode is not an inert electrode it does not remove electrons from an ion in solution, but loses electrons itself.
 

 

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At the anode copper is oxidised to copper(II) ions:

Cu   Cu2+ + 2 e

These dissolve into solution.
 

 

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The impurities:

         Iron, this forms Fe2+ ions in solution and means that the electrolyte has to be replaced periodically;

         Precious metals such as platinum, ruthenium, rhodium and palladium which are in the insoluble anode mud. This is processed to extract these elements.

 

Thanks to Kent White of TM Technologies for helpful comment on this page.

 

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