Metallurgy – Lesson Summary

Class 10

Physical Science

Lesson Summary: Principles of Metallurgy

INTRODUCTION

  • Process of extraction and purification of metals from its ore
  • Earth’s crust is made of minerals. Metals can be extracted from minerals.
  • A mineral from which a metal can be extracted in good yield and in a cost-effective manner is called as Ore.
Ore Name Chemical Formula Ore Type Metal
Bauxite Al2O3.2H2O Oxide ore Al-Aluminium
Pyrolusite MnO2 Oxide ore Mn-Manganese
Hamatite Fe2O3 Oxide ore Fe-Iron
Magnetite Fe3O4 Oxide ore Fe-Iron
Zincite ZnO Oxide ore Zn-Zinc
Zinc Blende ZnS Sulphide ore Zn-Zinc
Cinnabar HgS Sulphide ore Hg-Mercury
Galena PbS Sulphide ore Pb-Lead ore
Epsom MgSO4.7H2O Sulphate ore Mg-Magnesium
Gypsum CaSO4.2H2O Sulphate ore Ca-Calcium
Horn Silver AgCl Chloride ore Ag-Silver
Rock Salt NaCl Chloride ore Na-Sodium
  • Metallurgical Process involves 3 steps
    • Concentration (ore is taken from crust)
    • Extraction (metal is extracted from ore)
    • Refining (pure metal is taken from impure metal)

CONCENTRATION

  • Unwanted rocky materials, dust, soil, sand etc are removed from ore.
  • 4 ways to concentrate an ore
    • Hand-Picking
    • Washing
    • Froth Floatation
    • Magnetic Separation
  • Hand-Picking
    • This method is opted when the impurities have a different texture/size/colour than the ore. Ore particles are hand picked.
  • Washing
    • This method is opted when impurities are smaller/less denser than ore.
    • Ore is kept on an inclined plane surface and washed with water.
    • Impurities are washed with water, leaving the ore concentrated.
  • Froth Floatation
    • This method is used for sulphide ores.
    • Powdered ore is added in a floatation cell containing water and few drops of pine oil.
    • Compressed air is passed into the floatation cell.
    • Ore particles along with froth raise to the top of the cell, while impurities (gangue) settle at the bottom of the cell.
  • Magnetic Separation
    • This method is used for ores that have magnetic property (Eg: iron ore)
    • Powdered ore is passed on a moving belt running across 2 wheels. One of the wheel is magnetic and the other is non magnetic.
    • Magnetic ore particles are attracted towards the magnetic wheel.

EXTRACTION

  • Extraction methods are chosen depending on the reactivity of the metal.
Reactivity Example Method of extraction
Highly Reactive Na, K, Mg, Ca, Al Electrolysis
Moderately Reactive-Sulphate ore and Sulphide ore Fe, Cu, Zn, Pb Roasting + Reduction
Moderately Reactive-Carbonate ore Fe, Cu, Zn, Pb Calcination + Reduction
Less Reactive Ag, Au, Hg Roasting + Reduction

 

  • Extraction of Highly Reactive Metals
  • Electrolysis:-
    • For highly reactive metals, reduction method of extraction is not economically viable.
    • Electrolysis is done with molten (fused) ore. Electrolysis of aqueous solution is not preferable because ions from water will be discharged at the cathode.
    • Process
      • Extraction of Sodium from Rock Salt
      • Cathode: Steel
      • Anode: Graphite
      • Electrolyte: NaCl fused ore
      • On passing electricity, electrolysis occurs.
        • At the Cathode: Sodium ion gets an electron and deposits as Sodium metal
        • At the Anode: Chlorine ions combine to form Chlorine gas. Each Chlorine ion gives off 1 electron.
      • Suitable impurities, if needed, are added to the ore to decrease the melting point.

 

  • Extraction of Moderately Reactive Metals
  • Roasting:-
    • Pyrochemical process
    • Heating sulphide ores in presence of oxygen
    • Metal oxide is obtained

 

 

 

  • Calcination:-
    • Pyrochemical process
    • Heating carbonate ores in the absence of oxygen
    • Metal oxide is obtained

 

 

  • Reduction:-
    • The metal oxides obtained from roasting or calcinations are reduced to their corresponding metals
    • Reduction with carbon
      • Usually done for metals like – Zn, Fe, Ni, Sn, Cu, Pb

 

 

 

 

  • Reduction with other metals
    • More reactive element can reduce a metal oxide by displacing the metal from its oxide.
    • Aluminum – more reactive metal when compared to Zn, Mn, Cr, Fe. Aluminum powder is used as reducing agent.
    • Using Aluminum to reduce metal oxides is a highly exothermic reaction. This reaction is called Thermite reaction, because both Heat and Al2O3 (bauxite) are formed.
    • Thermite reaction
      • A mixture of iron oxide, Fe2O3 and Al powder is heated in presence of Mg ribbon.
      • Al reduces iron oxide to give Fe, Heat and Al2O3. Heat produced is high enough to melt the iron
      • Molten iron, so produced, is used to join railings of tracks or cracked machine parts – thermite welding
    • Auto Reduction
      • Certain sulphide ores (Cu2S) are roasted partially
      • On further heating, Cu2S and Cu2O react together to from metal and SO2

 

 

  • Extraction of Less Reactive Metals
  • Roasting and Reduction by heating
    • Mercury ore (HgS) is heated to from HgO, which on heating is reduced to Hg

 

 

  • Treating with KCN
    • Silver sulphide is treated with KCN to form potassium silver cyanide. When Zinc dust is added to potassium silver cyanide, it gives Ag

 

PURIFICATION

  • Metals obtained from extraction process contain some impurities. Refining of impure metals is called Purification
  • Distillation
    • Purification of low boiling metals – Zn, Hg
    • Impurities – High boiling
    • Extracted metal is distilled to obtain pure metal
  • Liquation
    • Purification of metals with low melting point– Sn
    • Impurities – High boiling
    • Extracted are metals are heated and pure metal is collected from a sloppy surface
  • Poling
    • Molten metal is stirred with poles of wood
    • On heating, impurities get converted to gases or get oxidized and form as removable slag over the molten metal.
  • Electrolytic Refining
    • Usually done for metals like Cu, Zn, Sn, Pb, Cr, Ni, Ag, Au
    • Thick block of impure metal = Anode
    • Thin strip of pure metal = Cathode
    • Aqueous solution of the corresponding metal salt = Electrolyte
    • On passing electric current, metal from anode goes into solution, metal from electrolyte deposits at cathode
    • Soluble impurities from anode goes into solution, insoluble impurities from anode, deposit at the bottom of the anode. This is called anode mud.
    • Anode mud contains valuable metals like Sb, Te, Se, Ag, Au
    • Electrolytic Refining of Copper
      • Acidified Copper sulphate solution, i.e., CuSO4 + H2SO4 + H2O = electrolyte
      • Impure copper metal = Anode, i,e., connected to positive terminal of battery
      • Pure copper metal = Cathode , i,e., connected to negative terminal of battery
      • On passing electric current, impure Cu from anode goes into CuSO4 solution, pure Cu2+ from CuSO4 gets deposited at cathode as Cu
      • As the process goes on, anode thickness decreases and anode thickness increases. Pure copper is obtained at the cathode.
      • Cathode reaction

 

  • Anode reaction

 

CORROSION

  • Reactive metals like copper, iron, silver gets oxidized in the presence of moisture and O2
  • Reddish brown dust –Hydrated Iron oxide; Green coating on copper – Copper carbonate; Blackening of silver – Silver sulphide
  • Corrosion of iron is rusting
    • Rusting occurs in the presence of both moisture and O2
      • When iron nail is kept in a test tube with water and oil = no rusting
      • When iron nail is kept in a test tube with dry air and dry CaCl2 = no rusting
      • When iron nail is kept in a test tube with water and air = rusting
    • Corrosion is an electrochemical process
      • Anode = where oxidation of Fe occurs

 

  • Cathode = reduction of oxygen occurs in the presence of H+ ion. This is done by the electrons released from anode

 

 

  • Overall Reaction

 

  • Fe2+ is further oxidized to Fe3+ which forms hydrated iron oxide

 

  • Prevention of Corrosion
    • By painting
    • By applying grease or oil
    • By galvanization – electroplating zinc on iron objects.
    • By tin or chromium plating
    • By alloying
      • Alloy is a homogenous mixture of two or more metals.
      • Iron is alloyed with Cr and Ni to obtain stainless steel.
      • Alloys are generally stronger than the metals from which they are made; more resistant to corrosion; have lower electrical conductivity than pure metals and improved properties than pure metals.
      • Other common alloys
        • Gold + Silver or copper = 22 carat gold
        • Copper + Zinc = Brass
        • Mercury + other metals = Amalgam

OTHER PROCESSES

  • Smelting
    • Ore is mixed with flux and fuel and heated strongly
    • Flux is a substance added to remove impurities (gangue) from ore.
    • If gangue is acidic, flux is basic and vice versa
    • During smelting, gangue and flux react to form a removable substance called slag

 

 

 

 

 

 

 

 

 

  • Furnace
    • Pyrochemical process is carried out in furnace.
    • Furnace has 3 parts – Hearth, Chimney, Fire Box
    • Ore is kept at hearth of the furnace
    • Fuel is kept in fire box
    • Waste or gases go out of the chimney
    • Types of furnace – Reverberatory furnace, Blast furnace, Retort furnace
    • Blast furnace
      • Fire box an hearth are combined in a big chamber
      • Haematite ore is smelted in blast furnace
    • Reverberatory furnace
      • Fire box an hearth are kept in separate chambers
      • Vapours from the burning fuel touch the ore and heat it.
    • Retort furnace
      • Fire box an hearth are kept in separate chambers
      • Flames of the fuel do not touch the ore.