Outline the principles of refining of metals by the following methods.
(a)Zone refining (b) Electrolytic refining (c) poling (d) Vapour phase refining.

Explanation

a) Zone refining :
`rarr` Zone refining is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.
`rarr` A circular mobile heater is fixed at one end of a rod of impure metal.
`rarr` The molten Zone moves along with the heater moves forward the pure metal crystallises out of the melt and the impurities pass into the adjacent moltan zone.
`rarr` The above process is repeated several times and the heater is moved in the same direction form one end to the other end. At one end impurities get concentrated. This end is cut off.
`rarr` This method is very useful for producing semicondutor grade metals of very high purity.
Eg : Ge,Si, B,Ga etc...

Electrolytic refining : This process is used for less reative metals like Cu,Ag,Al ,Au etc.
`rarr` In this process anode is made by impure metal and a thin strip of pure metal acts as cathode .
`rarr` On electrolysis metal dissolves from anode and pure metal gets deposited at cathode.
`{:(MrarrN^(n+)+"ne"^(-)),("Impure"),(M^(n+)+"ne"^(-)rarr"M (Cathode)"),(" Pure metal"):}`
Impurities settle down anode in the form odf anode mud.
c) Poling : When the metals are having the metal oxides as impurities this method is employed The impure metal and is then covered by carbor powder. Then it is stirred with green wood poles .The reducing gases formed the green wood and the carbon, reduce the oxides to the metal .Eg : Cu &Sn metals are refined by this method .
d) Vapour phase refining : In this method the metal is converted into its volatile compound and collected . It is then decomposed to give pure metal.
i) The metal should form a volatile compound with an available reagent.
ii) The volatile compound should be easily decomposable .So the the recovery is easy.
E.g : Mond's process :
`rarr` In Mond's process , nickel is heatded in a stream of carbon monoxide forming a volatile complex,nickel tetra carbonyl.
`Ni+4COoverset(330-350K)rarr[Ni(CO)_(4)]`
`rarr` Nickel tetra carborngly is strongly heated to decompose and gives the pure Nickel
`Ni(CO)_(4)overset(450-470K)rarrNi+4CO`