Request a call back

Join NOW to get access to exclusive study material for best results

Metallurgy

Metallurgy Synopsis

Synopsis


  • Comparison of Metals and Non-Metals
    Physical Properties of Metals and Non-Metals

Chemical Properties of Metals and Non-Metals:

 

     Occurrence of Metals

  • The Earth’s crust is the major source of metals.
  • Most of the metals are reactive and hence they do not occur as free elements in nature.
  • Less reactive elements such as copper, silver, gold and platinum occur in a free state as metals.
  • Copper and silver metals also occur in the combined state in the form of compounds.
  • Mineral & Ores
    Minerals: The naturally occurring compounds of metals, along with other impurities are known as minerals.
    Ores: The minerals from which metals are extracted profitably and conveniently are called ores.
    Gangue: Earthly impurities including silica, mud, etc. associated with the ore are called   gangue.
    Metallurgy: The process used for the extraction of metals in their pure form from their ores is referred to as metallurgy.
  •  Extraction of Metals
  • The reactivity of elements differs for different metals.
  • Metals at the top of the reactivity series (K, Na, Ca, Mg, Al, etc.) are reactive and they are never found in nature as free elements.
  • Metals in the middle of the reactivity series (Zn, Cu, Pb, etc.) are moderately reactive.
  • Metals at the bottom of the series (Au, Ag, Pt, etc.) are the least reactive and occur in a free state.

     
  • Three major steps involved in the extraction of metals from its ore are

  • Enrichment of Ores
  • The ores of metal are usually contaminated with a large amount of impurities such as sand, soil, etc. called gangue.
  • Before extracting the metal from an ore, it is necessary to remove these impurities.
  • The method used for removing gangue from the ore depends on the differences between the physical and chemical properties of the gangue and the ore.
  • Conversion of Concentrated ore into Metal
  • The extraction of a metal from its concentrated ore is essentially a process of reduction of the metal compound present in the ore.
  • The method of reduction to be used depends on the reactivity of the metal to be extracted.

Extraction of Less Reactive Metals
Metals at the bottom of the reactivity series are very unreactive and the oxides of these metals can be reduced by heating itself.

  1.  Extraction of Mercury
    Cinnabar, an ore of mercury is first heated in the air and is converted into mercuric oxide.
    begin mathsize 12px style 2 HgH subscript left parenthesis straight s right parenthesis end subscript plus 3 straight O subscript 2 left parenthesis straight g right parenthesis end subscript rightwards arrow with Heat on top 2 HgO subscript left parenthesis straight s right parenthesis end subscript plus 2 SO subscript 2 left parenthesis straight g right parenthesis end subscript end style
    Mercuric oxide is then reduced to mercury on further heating.
    begin mathsize 12px style 2 Hg straight H subscript left parenthesis straight s right parenthesis end subscript rightwards arrow with Heat on top 2 Hg subscript left parenthesis straight s right parenthesis end subscript plus straight O subscript 2 left parenthesis straight g right parenthesis end subscript end style
  2.  Extraction of Copper
    Concentrated copper (I) sulphide ore is roasted in the air when a part of it is oxidised to copper oxide.

    begin mathsize 12px style 2 straight C subscript straight u 2 end subscript straight S subscript left parenthesis straight s right parenthesis end subscript plus 3 straight O subscript 2 rightwards arrow with Roasting on top 2 straight C subscript straight u 2 end subscript straight O subscript left parenthesis straight s right parenthesis end subscript plus 2 SO subscript 2 left parenthesis straight g right parenthesis end subscript end style
    When a good amount of copper (I) sulphide is converted into copper oxide, the supply of oxygen is stopped. The copper (I) oxide formed above reacts with the remaining copper (I) sulphide to form copper metal and sulphur dioxide.

    begin mathsize 12px style 2 straight C subscript straight u 2 end subscript straight S subscript left parenthesis straight s right parenthesis end subscript plus straight C subscript straight u subscript 2 straight S subscript left parenthesis straight s right parenthesis end subscript rightwards arrow with Heat on top 6 straight C subscript straight u left parenthesis straight s right parenthesis end subscript plus SO subscript 2 left parenthesis straight g right parenthesis end subscript end style

Extraction of Moderately Reactive Metals

  • The moderately reactive metals in the middle of the reactivity series are extracted by the reduction of their oxides with carbon, aluminium, sodium or calcium.
  • It is easier to obtain metals from their oxides (by reduction) than from carbonates or sulphides. So, before reduction can be done, the ore is converted into a metal oxide.
  • The concentrated ores can be converted into metal oxides by the process of calcination or roasting.
    1. Calcination is the process in which a carbonate ore is heated strongly in the absence of air to convert it into a metal oxide.
      For example:
      When zinc carbonate is heated strongly in the absence of air, it decomposes to form zinc oxide and carbon dioxide.
      begin mathsize 12px style ZnCO subscript 3 left parenthesis straight s right parenthesis space space rightwards arrow with Calcination on top space space ZnO left parenthesis straight s right parenthesis space plus space CO subscript 2 left parenthesis straight g right parenthesis end style
    2. Roasting is the process in which a sulphide ore is strongly heated in the presence of air to convert it into a metal oxide.
      begin mathsize 12px style 2 ZnS left parenthesis straight s right parenthesis space plus space 3 straight O 2 left parenthesis straight g right parenthesis space space rightwards arrow with Roasting on top space space 2 ZnO left parenthesis straight s right parenthesis space plus space 2 SO 2 left parenthesis straight g right parenthesis end style 
  • The metal oxides are converted to free metal by using reducing agents such as carbon, aluminium, sodium or calcium.
    For example:
    1. Zinc metal is extracted by the reduction of zinc oxide with carbon. Thus, when zinc oxide is heated with carbon, zinc metal is produced.
      ZnO(s) + C(s) → Zn(s) + CO(g)
       Manganese metal is extracted by the reduction of MnO2 with aluminium powder as the reducing agent.
      3MnO2(s) + 4Al(s)  → 3Mn(l) + 2Al2O3(s) + Heat
    2. Aluminium reduces iron oxide to produce iron metal with the evolution of heat. Due to this heat, the iron metal is produced in the molten state.
    3. Fe2O3(s) + 2Al(s)  → 2Fe(l) + Al2O3(s) + Heat

The reaction of iron (III) oxide with aluminium is used to join the railway tracks or cracked machine parts. This reaction is known as the thermite reaction.

Extraction of Highly Reactive Metals

  • Metals high up in the reactivity series are very reactive.
  • These metals have a strong affinity for oxygen. So, oxides of sodium, magnesium, calcium and aluminium cannot be reduced by carbon.
  • These metals are obtained by electrolytic reduction.
  • Sodium, magnesium and calcium are obtained by the electrolysis of their molten chlorides.
  • For example:
    1. Sodium metal is extracted by the electrolytic reduction of molten sodium chloride.
      2NaCl(l)  2Na(s)  + Cl2(g)
      At Cathode :               Na+  +  e-   → Na
      At Anode :                   2Cl-           → Cl2    +  2e-   
    2. Aluminium is extracted by the electrolytic reduction of molten aluminium oxide.
      2Al2O3(l)  4Al(s)  + 3O2(g)
      At Cathode :               Al+3  +  3e-   → Al
      At Anode :                   2O2-            → O2    +  4e 
  • Refining of Metals
    • The metals produced by reduction processes are not very pure and still contain impurities which must be removed to obtain pure metals.
    • The most widely used method for refining impure metals is electrolytic refining.
    • Electrolytic refining means refining by electrolysis. Metals such as copper, zinc, tin, lead, chromium, nickel, silver and gold are refined electrolytically.
    • For refining of an impure metal by electrolysis
  • A thick block of impure metal is made anode.
  • A thin strip of pure metal is made cathode.
  • A water soluble salt is taken as an electrolyte.
  • On passing current through the electrolyte, the impure metal from the anode dissolves into the electrolyte.
  • An equivalent amount of pure metal from the electrolyte is deposited on the cathode.
  • The soluble impurities go into the solution, whereas the insoluble impurities settle down at the bottom of the anode and are known as the ‘anode mud’.