explain role:1.CO in the extraction of nickel.

2.zinc in the extraction of silver.

3.iodine in refining of zinconium.

4.depressant in froth-floation method.

5.graphite rods in metallurgy of aluminium.

  1. CO forms a volatile compound with Ni, Nickel carbonyl, under mild conditions(330-350K).  Impurities like iron and cobalt do not react under these conditions.  Nickel carbonyl being a gas is easily separated from solid impurities.  Ni is recovered as nickel carbonyl is easily decomposed by heating at 450-470K.
         Ni (s) + 4 CO(g)     ------>     Ni(CO)4 (g) 
         Ni(CO)4 (g) -------->   Ni (s) + 4 CO (g)
  1. Zn acts as a reducing agent and displaces silver from it compound, sodium argento cyanide, Na[Ag(CN)2].
        Zn(s) +  2Na[Ag(CN)2](aq)    -------->   Na2[Zn(CN)4](aq) + 2Ag(s)
  1. Zirconium is reacted with iodine to form a volatile zirconium iodide.  This is decomposed by heating on a tungsten filament electrically heated to 1800K.
         Zr + 2I2 ------->    ZrI4
         ZrI4  -------->    Zr + 2I2
   4.  A depressant is used in froth floatation method to separate two different sulphide ores in the froth and water             fractions.  Depressant NaCN selectively prevents ZnS to move into the froth and not PbS.  PbS remains in
       the water phase.   

   5.  Graphite rods act as anode in electrolysis of molten alumina. Graphite is used as anode as it is a good
        conductor of electricity and has a high melting point.      
       Electrolysis occurs at high temperatures as alumina has a melting point over 2000°C.

 ​ ​ ​ ​    C (s)  + O2-   ----->    CO(g)  +  2e-
 ​ ​      ​C (s)  + 2O2-   ------>    CO2(g)  +  4e-

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