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

Explanation

(i) zone refining . This method is used for produacation of semiconductors and other metals of very high puity e.g. G.e Si. B, Ca and In .
it is based on the imurites ara emore soluble in the mothen state (Melt) thean in the soild state of the metal .
the imopure metal in the from of bara is heated at one end with a moving circular heater is slowly moved along the length of the pure metal crystallises out of the melt whereas the imurits pass into the adject molten Zone this process is repeated several times till the imuriites are comeletely driven to one end of the rod is then cut discareded .
(ii) Electrolysis refings :many metals ,such as Cu,Au Al ,Pb etc. purified by this metod , the immpure metals is made the anode while a thin sheet metal acts as a a cathode the elecrolytic solution consists of a salt or a complex salt solution of hte metal on passing the currect the pure metal is deposited on the caathode while the impuriies fall down as anode mud .
(iii) Vapour-phase refining : the crude metal is freed from impurities by first converting it into a suitable volatile compound by heating it with a specfic regent at a lower and then decompossing the volatile comooiund ar some hiogher temparature to give pure metal.
(a) Mond's processs : when impure nikel is heated is a current of CO 330-350 K it forms volatile nikel tertracarbony complex leaving behind the impurites ,the complex then heated to a higher temeperature (450 -4710 K) when it undergoes thermal decompostion giving pure nikel.
`underset( "imapure") Ni+4COoverset(330-350K) to Ni (Co)_(4)`
`Ni(Co)_(4) overset(450-470K) underset("pure") Ni + 4CO`
(b) Van arkel method : this method is used for preparing ultra - pure metals by removing all the oxygen and nitrogen present as imurites in matals like zironium and titanum (which areused in space technolgy ) crude Zr is Heated in a evanatured vessel with iodine at 870 K Zirconium tertraiodide thus formed is separated . it is then decomposed by heating over a tugsten flament at 1800 -2075 K give pure Zr :
` underset("impure") Zr(s) +2l_(2)(g) to Zr l_(4)`
`Zrl_(4) underset("Tungsten filament ") overset( 1800-2075 K) to underset("Pure") Zr(s) +2l_(2)(g)`