5. Extraction & purification of lead, zinc, titanium and chromium

 Doc Brown's Chemistry GCSE/IGCSE/O Level Revision Notes - Mining of Minerals, Methods of Extracting of Metals from Ores These revision notes on the extraction of copper and the electrolytic refining of copper, useful for the new AQA, Edexcel and OCR GCSE (9–1) chemistry science courses.

Summary for this page

How do you extract zinc from zinc oxide, zinc sulfide or zinc carbonate ores? How to extract titanium from titanium dioxide? How do we extract chromium metal from chromium containing minerals? 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 sometimes 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.

5a. The Extraction of Lead

• Because its position in the reactivity series of metals, lead can be extracted using carbon (coke) in a smelting furnace. Because lead is below carbon, that is less reactive than carbon, lead can be displaced by carbon from lead oxides or lead sulfides in a sort of displacement reaction.

• The principal ore of lead is galena, chemical formula PbS lead sulphide (lead(II) sulfide)

  • There is also the less common lead ore cerussite, chemical formula PbCO₃, lead carbonate (lead(II) carbonate)

• The crushed ores are concentrated by a technique known as froth flotation.

• The ores are roasted to drive off unwanted water and convert them to a more suitable chemical form e.g. lead oxide, for reduction to lead metal. The poisonous acidic gas sulfur dioxide (sulphur dioxide) is formed.
  • Roasting galena in air converts the lead sulphide into lead(II) oxide ...
    • lead(II) sulfide + oxygen ==> lead oxide + sulfur dioxide
    • 2PbS + 3O₂ ==> 2PbO + 2SO₂
    • 2PbS(s) + 3O₂(g) ==> 2PbO(s) + 2SO₂(g)
    • ... but harmful and polluting sulfur dioxide is made in the process, which must be dealt with!
  • Roasting cerussite coverts the lead carbonate into lead oxide ...
    • lead(II) carbonate ==> lead(II) oxide + carbon dioxide
    • PbCO₃ ==> PbO + CO₂
    • PbCO₃(s) ==> PbO(s) + CO₂(g)
    • ... an example of a thermal decomposition
• The more concentrated lead oxide can then be reduced in a coke fired blast furnace ...
  • lead(II) oxide ==> lead + carbon dioxide
  • 2PbO + C ==> 2Pb + CO₂
  • 2PbO(s) + C(s) ==> 2Pb(l) + CO₂(g)
  • In this reaction lead oxide is reduced to lead, oxygen loss and the carbon is oxidised to carbon dioxide by oxygen gain.
  • ... the lead oxide is reduced (oxygen loss) and the carbon/coke reducing agent is oxidised (oxygen gain) in the smelting process.
  • ... as in the blast furnace from iron, the liquid lead can be drained off from the lead smelter.
  • The above balanced equations, are a simplification of what can be quite complicated chemistry, BUT they do adequately describe and illustrate the chemical processes for obtaining lead from its ores.
• The overall process from lead ore to very pure lead is quite complicated and lead ores contain other valuable metals like silver, so there more stages in the process than are described here.

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

5b. The Extraction and Purification of Zinc

• Because its position in the reactivity series of metals, zinc can be 'just' extracted using carbon (coke) in a high temperature smelting furnace. – it is a sort of displacement reaction.

• Zinc is extracted from either zinc blende/sphalerite ore (zinc sulphide) or sometimes calamine/Smithsonite ore (zinc carbonate).
• (1) The zinc sulphide ore is roasted in air to give impure zinc oxide and forming the nasty poisonous gas sulfur dioxide (sulphur dioxide).
  • zinc sulfide + oxygen ==> zinc oxide + sulfur dioxide
  • 2ZnS + 3O₂ ==> 2ZnO + 2SO₂
    • 2ZnS(s) + 3O₂(g) ==> 2ZnO(s) + 2SO₂(g)
  • Note: calamine ore can be used directly in a zinc smelter because on heating it also forms zinc oxide.
    • ZnCO₃(s)  ==> ZnO(s) + CO₂(g) (endothermic thermal decomposition)
• (2) The impure zinc oxide can be treated in two ways to extract the zinc:
  • (a) It is roasted in a smelting furnace with carbon (coke, reducing agent) and limestone (to remove the acidic impurities). The chemistry is similar to iron from a blast furnace.
    • carbon + oxygen ==> carbon dioxide
    • C + O₂ ==> CO₂ (very exothermic oxidation, raises temperature considerably)
      • C(s) + O₂(g) ==> CO₂(g)
    • carbon + carbon dioxide ==> carbon monoxide
    • C + CO₂ ==> 2CO (C oxidised, CO₂ reduced)
      • C(s) + CO₂(g) ==> 2CO(g)
    • zinc oxide + carbon monoxide ==> zinc + carbon dioxide
    • ZnO + CO ==> Zn + CO₂ (zinc oxide reduced by CO, Zn undergoes O loss)
      • ZnO(s) + CO(g) ==> Zn(l) + CO₂(g)
      • In this reaction zinc oxide is reduced to lead, oxygen loss and the carbon monoxide is oxidised to carbon dioxide by oxygen gain.
    • or direct reduction by carbon:
      • ZnO + C ==> Zn + CO (ZnO reduced, C oxidised)
        • ZnO(s) + C(s) ==> Zn(l) + CO(g)
        • not sure if 2ZnO + C ==> 2Zn + CO₂ happens? but probably does too.
        • In this reaction zinc oxide is reduced to lead, oxygen loss and the carbon is oxidised to carbon dioxide by oxygen gain.
    • REDOX definition reminders – reduction is a process of oxygen loss (or electron gain) and oxidation is a process of oxygen gain (or electron loss).
    • The carbon monoxide acts as the reducing agent i.e. it removes the oxygen from the oxide.
    • The impure zinc is  then fractionally distilled from the mixture of slag and other metals like lead and cadmium out of the top of the furnace in an atmosphere rich in carbon monoxide which stops any zinc from being oxidised back to zinc oxide.
    • The slag and lead (with other metals like cadmium) form two layers which can be tapped off at the base of the furnace.
    • The zinc can be further purified by a 2nd fractional distillation or more likely by dissolving it in dilute sulphuric acid and purified electrolytically as described below.
  • (b) Two stages
    • (i) It is dissolved and neutralised with dilute sulphuric acid to form impure zinc sulphate solution.
    • ZnO + H₂SO₄ ==> ZnSO₄ + H₂O
      • ZnO(s) + H₂SO₄(aq) ==> ZnSO₄(aq) + H₂O(l)
    • or using calamine ore (zinc carbonate) directly:
      • ZnCO₃ + H₂SO₄ ==> ZnSO₄ + H₂O + CO₂
        • ZnCO₃(s) + H₂SO₄(aq) ==> ZnSO₄(aq) + H₂O(l)+ CO₂(g)
    • (ii) Quite pure zinc is produced from the solution by electrolysis. It can be deposited on a pure zinc negative electrode (cathode) in the same way copper can be purified. The other electrode, must be inert e.g. for laboratory experiments, carbon (graphite) can be used and oxygen is formed.
      • Zn²⁺_(aq) + 2e^– ==> Zn_(s)
        • A reduction process, electron gain, as zinc metal is deposited on the (–) electrode.
      • You can't use solid zinc oxide directly because its insoluble and the ions must free to carry the current and migrate to the electrodes in some sort of solution.
      • For more details of the type of electrolysis system used, see purification of copper (just swap Zn for Cu in the method/diagram).
      • PLEASE note: In the industrial production of zinc by electrolysis (called electro–winning) the negative (–) cathode is made of aluminium (Al, where zinc deposits) and the positive (+) electrode is made of a lead–silver alloy (Pb–Ag, where oxygen gas is formed).
      • Why these particular electrode metals are used in this 'electrowinning' process I'm not quite sure, but aluminium is so unreactive that it is effectively inert, and lead and silver are also of low chemical reactivity.

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

5c. Extraction of Chromium and Titanium by Displacement

The production–extraction of chromium and titanium is very expensive because the processes use lots of energy, involves many stages and uses costly reactive metals in the process (eg Na, Mg and Al from electrolysis)

• Titanium is another very useful metal but expensive to produce.

• Titanium is too reactive (too strongly bound to oxygen atoms) to be obtained by simple reduction of its oxide and has to be extracted by a costly multiple stage process involving chlorine and a more reactive metal like sodium or magnesium and all these three are themselves made from the costly process of electrolysis!

  • So the process ain't cheap! BUT titanium is a very valuable metal!

• Titanium ore is mainly the oxide TiO₂, which is converted into titanium tetrachloride TiCl₄ by heating with carbon and chlorine.

  • titanium dioxide + carbon + chlorine ==> titanium(IV) chloride + carbon dioxide

  • TiO₂ + C + 2Cl₂ ==> TiCl₄ + CO₂

• The chloride is then reacted with sodium or magnesium to form titanium metal and sodium chloride or magnesium chloride.

• This reaction is 'simple' displacement reaction, ie, the less reactive titanium is displaced by a more reactive metal such as sodium or magnesium.

• This reaction is carried out in an atmosphere of inert argon gas so non of the metals involved becomes oxidised by atmospheric oxygen.

  • titanium(IV) chloride + magnesium ==> titanium + magnesium chloride

  • TiCl₄ + 2Mg ==> Ti + 2MgCl₂

  • or

  • titanium(IV) chloride + sodium ==> titanium + sodium chloride

  • TiCl₄ + 4Na ==> Ti + 4NaCl

  • These are displacement reactions in which a more reactive metal (Mg or Na) displaces a less reactive metal (Ti).

• Overall the titanium oxide ore is reduced to titanium metal (overall O loss, oxide => metal) and the magnesium or sodium acts as a reducing agent.

• Titanium is a strong metal that has a low density and a high resistance to corrosion which makes a good structural material.

  • Titanium alloys are amongst the strongest lightest of metal alloys and used in aircraft production.

  • As well as its use in aeroplanes it is an important component in nuclear reactor alloys and for replacement hip joints because of its light and strong nature.

  • It is one of the main components of Nitinol 'smart' alloys. Nitinol belongs to a group of shape memory alloys (SMA) which can 'remember their original shape'. For example they can regain there original shape on heating (e.g. used in thermostats in cookers , coffer makers etc.) or after release of a physical stress (e.g. used in 'bendable' eyeglass frames, very handy if you tread on them!). The other main metal used in these very useful intermetallic compounds is nickel.

    • Nitinol is an acronym for 'Nickel Titanium Naval Ordinance Laboratory' betraying, like so many technological developments, its military origins, but now acquiring many 'peaceful' uses.

• Chromium ore is processed and purified into chromium(III) oxide. This is reacted, very exothermically, in a Thermit style reaction, with aluminium (see reactions of aluminium) to free the chromium metal. It is not possible to extract chromium with carbon (coke), like titanium, it is to strongly bound to oxygen atoms, so a more reactive metal than chromium, must be used to displace it.

  • chromium(III) oxide + aluminium ==> aluminium oxide + chromium

  • Cr₂O₃ + 2Al ==> Al₂O₃ + 2Cr

    • Cr₂O₃(s) + 2Al(s) ==> Al₂O₃(s) + 2Cr(s)  (equation with state symbols)

  • REDOX definition reminders – reduction is a process of oxygen loss (or electron gain) and oxidation is a process of oxygen gain (or electron loss).

  • The chromium(III) oxide is reduced to chromium by O loss, the aluminium is oxidised to aluminium oxide by O gain, and the aluminium is the reducing agent i.e. the O remover.

• These are examples of metal displacement reactions e.g. the less reactive chromium or titanium are displaced by the more reactive sodium, magnesium or aluminium.

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