* GCSE-science-CHEMISTRY 5. Extraction & purification of zinc & of titanium & chromium at Doc Brown's Chemistry

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 Mining of Minerals and Methods of Extracting of Metals 

5. Extraction & purification of zinc & of titanium & chromium

Scroll down for revision notes on extraction procedures and theory

Useful for school/college assignments/projects on ways of extracting metals from their ores

1. Introduction to Metal Extraction * 2. Extraction of Iron and Steel Making

3. Extraction of Aluminium and Sodium * 4. Extraction and Purification of Copper

5. Extraction of Zinc, Titanium and Chromium * 6. Economic & environmental Issues - metal extraction

GCSE Multiple choice Quizzes on metal extraction: Foundation (easier) or Higher (harder) and word-fill

Revision Notes KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  (revise courses equal to US grades 9-10)

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

    

reactivity5. The Extraction and Purification of Zinc
  • 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.
    • 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.
      • C(s) + O2(g) ==> CO2(g) (very exothermic oxidation, raises temperature considerably)
      • C(s) + CO2(g) ==> 2CO(g) (C oxidised, CO2 reduced)
      • ZnO(s) + CO(g) ==> Zn(l) + CO2(g) (zinc oxide reduced by CO, Zn undergoes O loss)
      • or direct reduction by carbon: ZnO(s) + C(s) ==> Zn(l) + CO(g) (ZnO reduced, C oxidised)
      • 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(s) + H2SO4(aq) ==> ZnSO4(aq) + H2O(l)
      • or using calamine ore/zinc carbonate directly:
        • 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.

  top index

The Extraction of Chromium and Titanium by Displacement

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

    • TiO2 + C + Cl2 ==> TiCl4 + CO2

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

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

    • TiCl4 + 2Mg ==> Ti + 2MgCl2 or TiCl4 + 4Na ==> Ti + 4NaCl

  • 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.

  • 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.

    • Cr2O3(s) + 2Al(s) ==> Al2O3(s) + 2Cr(s) 

    • 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.

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(chinese) 督布朗的化学 开采矿物和金属的提取方法 5。提取与纯化锌铬及钛＆ 向下滚动修订的笔记和理论的提取程序 项目有用的学校/学院作业/对矿石中提取金属的方法从 提取锌 ， 钛，铬 *

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