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How iron is extracted from the ore?


Extraction of iron   :  Oxide ores of iron ,after concentration through calcination /roasting are mixed with limestone and coke and fed into a blast furnace from its top .
The cast iron is usually extracted from its oxide ore (haematite ) .This process involves the following steps :
 
 1.Concentration   : The ore is crushed in jaw crushers and is broken into small pieces about 1 inch in size .The crushed ore is concentrated by gravity separation process in which it is washed with water to remove clay ,sand etc .

2.Calcination :  The concentrated ore is calcined in a reverberatory furnace .During this process the following changes takes place .
(i)Moisture is removed 
(ii)The impurities such as sulphur ,phosphorus and arsenic are converted to their gaseous oxides which are volatile and escape .
                S  +     O2      _________>   SO2
               4As +   3O2    __________> 2As2O3
                P4   +  5O2   ___________> P4O10

If ferrous carbonate is present ,it changes to oxide .
                FeCO3       ______________>  FeO   +CO2
4FeO     +    O2         ______________> 2Fe2O3  (ferric oxide )

This prevents the loss of iron due to the formation of ferrous silicate (slag) during smelting 

(iii) Smelting  :The calcined ore is reduced with carbon that is smelted in the blast furnace.The calcined ore is mixed with coke and lime stone and is introduced from the top of the furnace .The added coke serve act as a fuel as a reducing agent while added lime serve as a flux .The burning of coke to CO supplies the heat required for the working temperature of the furnace .As the gas moves up ,they meet the descending charge and the temperature falls gradually.
  
The following reactions takes place in the furnace :
(i) Combustion zone : At the base the coke burns to produce CO2 which starts rising upward during the reaction .The reaction is exothermic and heat produced raises the temperature to about 2200 K.This region is called combustion zone .
    C  +  O2   _______> CO2 , ?H = -393.4 kJ
(ii)Fusion zone      :  As carbondioxide rises upward ,it comes in contact with layers of coke and gets reduced to carbonmonoxide .
    CO2  +  C________ > 2CO2 ,?H =163.2 kJ

This is an endothermic reaction and therefore the temperature is lowered to about 1570 K. The iron produced in the upper region melts here .Any Fe2O3 if present undergoes reduction by hot coke to iron .This region is called fusion zone .
 
Fe2O3   +  3C  ________ > 2Fe   +3CO  +heat

(iii) Slag formation zone    : In the middle portion of the furnace ,the temperature is about  1270 K .In this region the limestone decomposes to produce lime ((CaO) and carbondioxide (CO2).The lime stone produced act as a flux and combines with silica to produce slag .

   CaCO3   _______________> CaO   +  CO2CaO    +SiO2  _______> CaSiO3

                                            (limestone )   (lime)                        (calcium silicate )

The molten slag forms a separate layer above the molten iron .This region is called slag formation zone .

(iv) Reduction zone    :  The temperature near the top of the furnace is of the order of 875 K. The oxides of iron are reduced by carbon monoxide to iron .

 FeO       +        CO   __________> Fe            +CO2

3Fe2O3    +        CO  ___________> 2Fe3O4  + CO2
(iron ore)

Fe3O4       +     4CO  ____________> 3Fe     +4CO2
 
Fe2O3        +      CO  _____________> 2FeO   +CO2
The region of the furnace is called reduction zone .The spongy iron produced in the reduction zone moves down slowly  and melts in the fusion zone .At the lower hotter part ,the main reaction is :
 
FeO      +   C_________>  Fe     +CO

It dissolves some carbon ,silicon ,phosphorus and manganese and forms the lower layer at the base of furnace .It is removed from tapping hole from time to time .The iron thus obtained is called pig iron and it can be cast into variety of shapes .


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