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blast furnace

2. The Extraction of Iron and Steelmaking

Doc Brown's GCSE/IGCSE O Level KS4 science–CHEMISTRY Revision Notes

 Mining of Minerals and Methods of Extracting of Metals 

How do we extract iron from its mineral ores like the iron oxide ores haematite and magnetite? How do we convert it into steel? The raw materials needed i.e. limestone, coke, air and iron ore and the chemistry of the blast furnace are fully described. Why convert iron into steel? How do you make steel? The manufacture of steel alloys is further described. Scroll down for revision notes on extraction procedures and theory which should prove useful for school/college assignments/projects on ways of extracting metals from their ores.

Equation note: The equations are often written three times: (i) word equation, (ii) balanced symbol equation without state symbols, and, (iii) with the state symbols (g), (l), (s) or (aq) to give the complete balanced symbol equation.

Metal extraction index

extract1extract2

1. Introduction to Metal Extraction

2. Extraction of Iron and Steel Making (this page)

3. Extraction of Aluminium and Sodium

4. Extraction and Purification of Copper

5. Extraction of Lead, Zinc, Titanium and Chromium

6. Economic & environmental Issues and recycling

 

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2a. The Extraction of Iron

 

blast furnace

 

Raw Materials:

  • Iron Ore e.g. haematite ore (iron(III) oxide) the source of iron.

    • Fe2O3

  • or magnetite ore

    • Fe3O4

  • coke (carbon, C), both fuel and reducing agent.

  • hot air (for the oxygen in it) to burn the coke

    • O2

  • limestone (calcium carbonate) to remove certain impurities like silica.

    • CaCO3

 

(c) doc b

 

 

memory help - element quiz

 

  • Iron ore is used to make iron and steel eg haematite and magnetite. Iron is produced (extracted) in a blast furnace by reducing iron oxides with carbon and it is the carbon that removes the oxygen from the iron oxides – the carbon is known as the reducing agent. Coke is a cheap and readily made reducing agent.
  • Iron is not a very reactive metal, so, because its position in the reactivity series of metals, iron can be extracted using carbon in a blast furnace because iron is below carbon (iron is less reactive than carbon). Therefore, iron can be displaced from its eg its oxides, by heating with the theoretically 'more reactive' carbon in a sort of displacement reaction.
  • Iron oxide ore is mined in many parts of the world.
    • Examples of rich or high quality ores are
    • haematite formula Fe2O3
    • and magnetite formula Fe3O4.
  • A solid mixture of magnetite/haematite ore, coke and limestone is continuously fed into the top of the blast furnace.
  • The double role and function of coke (carbon)
  • 1st Coke function (i) As a fuel
    • The coke is ignited at the base and hot air blown in to burn the coke (carbon) to form carbon dioxide in an oxidation reaction (C gains O).
    • The heat energy is needed from this very exothermic reaction to raise the temperature of the blast furnace to over 1000oC to effect the ore reduction. The furnace contents must be he
    • carbon + oxygen ==> carbon dioxide
      • C + O2 ==> CO2
        • C(s) + O2(g) ==> CO2(g)
  • 2nd Coke function (ii) As a reducing agent
    • At high temperatures the carbon dioxide formed, reacts with more coke (carbon) to form carbon monoxide
    • carbon dioxide + carbon ==> carbon monoxide
      • CO2 + C ==> 2CO
        • CO2(g) + C(s) ==> 2CO(g)
    • Note: that carbon dioxide, CO2, is reduced by oxygen loss to the carbon, and the carbon is oxidised by oxygen, O gain to carbon dioxide.
  • The carbon monoxide is the molecule that actually removes the oxygen from the iron oxide ore.
    • REDOX definition reminders – reduction is a process of oxygen loss (or electron gain) and oxidation is a process of oxygen gain (or electron loss).
    • This a reduction reaction, which can be described in two ways.
    • The Fe2O3 loses its oxygen O (reduction)
      • or for more advanced students
        • the ion Fe3+ gains three electrons to form Fe,
          • noting that electron gain is reduction.
    • The carbon monoxide, CO, is known as the reducing agent because it is the oxygen (O) remover and gets oxidised to carbon dioxide in the process (CO gains oxygen).
  • This frees the iron, which is molten at the high blast furnace temperature, and trickles down to the base of the blast furnace and run off.
    • An example of the main reduction reaction is ...
    • iron(III) oxide + carbon monoxide ==> iron + carbon dioxide
      • Fe2O3 + 3CO ==> 2Fe + 3CO2
        • Fe2O3(s) + 3CO(g) ==> 2Fe(l/s) + 3CO2(g)
        • Note on state symbols (l/s), iron is initially formed as a liquid and then obviously solidifies on cooling.
    • note, as in the two reactions above, oxidation and reduction always go together!
      • Other possible iron ore reduction reactions are direct reduction of the iron oxide by carbon itself ...
      • iron(III) oxide + carbon ==> iron + carbon monoxide
      • Fe2O3 + 3C ==> 2Fe + 3CO
        • Fe2O3(s) + 3C(g) ==> 2Fe(l/s) + 3CO(g)
      • or
      • iron(III) oxide + carbon ==> iron + carbon dioxide
      • 2Fe2O3 + 3C ==> 4Fe + 3CO2
        • 2Fe2O3(s) + 3C(g) ==> 4Fe(l/s) + 3CO2(g)
    • The iron is initially formed in its liquid state because of the high temperatures of the blast furnace (well over 1000oC) but when cooled it is cast into solid ingots, or the liquid iron can be transported directly in special insulated 'torpedo' wagons to a steel making plant on the same industrial site complex..
    • At the highest temperatures in a blast furnace the reactions can be written as a direct reduction of the oxide with carbon, and, carbon monoxide (CO) can be formed as well as carbon dioxide (CO2) e.g.
      • for haematite
        • iron(III) oxide + carbon ==> iron + carbon monoxide
        • Fe2O3 + 3C ==> 2Fe + 3CO
          • Fe2O3(s) + 3C(s) ==> 2Fe(l/s) + 3CO(g)
      • or 
        • iron(III) oxide + carbon ==> iron + carbon dioxide
        • 2Fe2O3 + 3C ==> 4Fe + 3CO2
          • 2Fe2O3(s) + 3C(s) ==> 4Fe(l/s) + 3CO2(g)
      • for magnetite
        • 'iron tetroxide' + carbon ==> iron + carbon monoxide
        • Fe3O4 + 4C ==> 3Fe + 4CO
          • Fe3O4(s) + 4C(s) ==> 3Fe(l/s) + 4CO(g)
      • or 
        • 'iron tetroxide' + carbon ==> iron + carbon dioxide
        • Fe3O4 + 2C ==> 3Fe + 2CO2
          • Fe3O4(s) + 2C(s) ==> 3Fe(l/s) + 2CO2(g)
      • Name note: The correct name for iron tetroxide is diiron(III)iron(II) oxide, but I wouldn't worry about it!
    • I'm afraid the chemistry of the blast furnace can get very complicated indeed!
  • The role of limestone in the extraction of iron
    • The original ore contains acidic mineral impurities such as silica (SiO2, silicon dioxide).
    • These react with the calcium carbonate (limestone) to form a molten slag, the main ingredient being calcium silicate.
    • There are two ways to show the formation of the waste slag, which is mainly calcium silicate.
    • (i) calcium carbonate + silica ==> calcium silicate + carbon dioxide
      • (i) CaCO3 + SiO2 ==> CaSiO3 + CO2
      • Reaction (i) is a sort of displacement reaction i.e. the less volatile high melting/boiling silicon dioxide (silica) displaces the more volatile gaseous carbon dioxide.
    • However, this is sometimes shown in two stages, i.e. reactions (ii) and (iii):
      • (ii) CaCO3 ==> CaO + CO2
      • (iii) CaO + SiO2 ==> CaSiO3
      • (ii) is the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide, and the reaction needs a high temperature of over 900oC, but that's no problem in the blast furnace!
      • (iii) is the combination of the basic calcium oxide and the acidic silicon dioxide to form calcium silicate.
    • However, whichever way you represent the reaction, its all the same in the end.
      • If you 'add up' chemical reactions (ii) and (iii) you get (i), check for yourself.
  • The molten slag forms a layer above the more dense molten iron and they can be both separately, and regularly, drained away. The iron is cooled and cast into pig iron ingots OR transferred directly to a steel producing furnace.
  • The waste gases and dust from the blast furnace must be appropriately treated to avoid polluting the environment.
    • The highly toxic carbon monoxide can actually be burnt to provide a source of heat energy, and in the exothermic reaction it is converted into relatively harmless carbon dioxide.
      • carbon monoxide + oxygen ==> carbon dioxide
      • 2CO + O2 ==> 2CO2
        • 2CO(g) + O2(g) ==> 2CO2(g)
    • Acidic gases like sulphur dioxide from sulphide ores, can be removed by bubbling through an alkali solution such as calcium hydroxide solution ('limewater') where it is neutralised and oxidised to harmless calcium sulphate. Cleaning a gas in this way is called 'gas scrubbing'.
    • Any contaminated water must be purged of harmful chemicals before being released into a river or recycled via water treatment plant.
    • The waste slag is used for road construction or filling in quarries which can then be landscaped.
  • Iron from a blast furnace is ok for very hard cast iron objects BUT is too brittle for many applications due to too high a carbon content from the coke.

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2b. STEEL MAKING

  • Why do we need to convert most iron into steel alloys?

    • Most metals in everyday use are alloys.

    • Iron is a good conductor of heat and can be bent or hammered into shape (malleable), quite strong physically – made stronger when alloyed with other materials.

    • This makes iron useful as structural material and for making things that must allow heat to pass through easily and useful construction materials.

    • Steel is an alloy because it is a mixture of a metal (iron) with other elements (carbon and perhaps other metals too).

    • Iron from the blast furnace contains about ~96% iron with ~4% of impurities including carbon, silica and phosphorus.

    • In this state the cast iron is too hard and too brittle for most purposes.

    • Cast iron is hard and can be used directly for some purposes eg manhole covers, ornamental railings because of its strength in compression and is very hard wearing.

    • However, if all the impurities are removed, the resulting very pure iron is too soft for any useful purpose.

    • Therefore, strong useful steel is made by controlling the amount of carbon and selected metals to produce an alloy mixture with the right physical properties fit for a particular application e.g. steel for car bodies, chrome stainless steel, extremely hard and tough tungsten–iron steel alloys etc.

    • The real importance of alloys is that they can be designed to have properties for specific uses in terms of eg compression/tensile strength or corrosion reduction ie less susceptible to rusting.

    • Low–carbon steels (0.1% carbon) are easily shaped for car bodies

    • High–carbon steels (1.5% carbon, often with other metals too) are hard wearing and inflexible and can be used for cutting tools, bridge construction.

    • Stainless steels have chromium (and maybe nickel) added and are much resistant to corrosion (from oxygen/water) than iron or plain steel which readily rust. Objects made of iron or plain steel, particularly those exposed to the weather, regularly have to be painted or coated with some other protective layer from the effects of water and oxygen.

  • The properties of iron can be altered by adding small quantities of other metals or carbon to make steel, one of the useful metal alloys in widespread use today.

  • Steels are alloys since they are mixtures of iron with other metals or with non–metals like carbon or silicon.

  • Making Steel:

    • (1) Molten iron from the blast furnace is mixed with recycled scrap iron

    • (2) Then pure oxygen is passed into the mixture and the non–metal impurities such as silicon or phosphorus are then converted into acidic oxides (the BOS oxidation process) ..

      • e.g. Si + O2 ==> SiO2, or 4P + 5O2 ==> P4O10

    • (3) Calcium carbonate (a base) is then added to remove the acidic oxide impurities (in an acid–base reaction). The salts produced by this reaction form a slag which can be tapped off separately.

      • e.g. CaCO3 + SiO2 ==> CaSiO3 + CO2 (calcium silicate slag)

    • Reactions (1)–(3) produce pure iron.

    • Calculated quantities of carbon and/or other metallic elements such as titanium, manganese or chromium are then added to make a wide range of steels with particular properties.

    • Because of the high temperatures the mixture is stirred by bubbling in unreactive argon gas!

    • Economics of recycling scrap steel or ion: Most steel consists of >25% recycled iron/steel and you do have the 'scrap' collection costs and problems with varying steel composition* BUT you save enormously because there is no mining cost or overseas transport costs AND less junk lying around! (NOTE: * some companies send their own scrap to be mixed with the next batch of 'specialised' steel they order, this saves both companies money!). More on this below.


  • NOTE on RECYCLING Iron and Steel

    • About 42% of iron/steel in goods/components manufactured in iron/steel is recycled iron/steel, whether it be steel pans, car bodies, bridge girders, stainless steel cutlery etc.

    • This makes good economics because recycling saves on several costs AND allows a mineral resource like iron's haematite or magnetite ore resources to last a lot longer – slower depletion of the Earth's mineral ore resources will make it last longer.

      • Transport costs may be less (ie within UK now), but much more importantly

        • mining costs are omitted – energy/machinery involved in digging out the ore, crushing it, transporting the ore,

        • and the cost of actually extracting the metal from its finite ore resource – chemicals needed (coke, limestone), constructing and running a blast furnace

    • So, scrap metal merchants are doing a roaring trade at the moment.

    • The savings are partly reduced by the cost off collecting waste/scrap metal.

    • There are particular problems due to the varying composition of alloys, but if the composition is known, or obtained from chemical analysis, the different compositions can be blended together to a desired alloy composition.

  • The social, economic and environmental impacts of exploiting metal ores including RECYCLING are discussed on a separate page.


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WHERE NEXT? Other associated KS4 Science GCSE/IGCSE chemistry web pages on this site


Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences GCSE/IGCSE/O level Chemistry Revision–Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science WJEC/CBAC GCSE science–chemistry CCEA/CEA GCSE science–chemistry (and courses equal to US grades 8, 9, 10) also useful revising and introduction to metal extraction for A level AS/A2/IB chemistry students


keywords formula equations: blast furnace equations C + O2 ==> CO2 CO2 + C ==> 2CO Fe2O3 + 3CO ==> 2Fe + 3CO2 Fe2O3 + 3C ==> 2Fe + 3CO 2Fe2O3 + 3C ==> 4Fe + 3CO2 2Fe2O3 + 3C ==> 4Fe + 3CO2 Fe3O4 + 4C ==> 3Fe + 4CO Fe3O4 + 2C ==> 3Fe + 2CO2 CaCO3 + SiO2 ==> CaSiO3 + CO2 2CO + O2 ==> 2CO2 steel making Si + O2 ==> SiO2, or 4P + 5O2 ==> P4O10


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Extracting iron from its ores with carbon and limestone in a blast furnace, how to change iron into steel, steelmaking

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