Online Notes on "General Principles & Processes Of Isolation of Elements" (Chapter=>6 ) Part 3

These are Online Notes on "General Principles & Processes

Of Isolation of Elements" (Chapter=>6 ) Part 3 for practice of CBSE BOARD, CBSE NEET, CSIR NET Chemical Sciences etc.

21.    Types of iron:

a.    Pig iron: The iron obtained from blast furnace is called pig iron. It is impure from of iron contains 4% carbon and small amount of S,.P, Si and Mn. It can be casted into variety of shapes.

b.   Cast iron: It is made by melting pig iron with scrap iron and coke using hot air blast. It contains about 3% of carbon content. It is extremely hard and brittle.

c.   Wrought iron: It is the purest form of commercial iron. It is also called malleable iron.

It is prepared by oxidative refining of pig iron in reverberatory furnace lined with haematite which oxidises carbon to carbon monoxide.

Fe2O3 + 3C ® 2Fe + 3CO

The substance which reacts with impurity to form slag is called flux

e.g. limestone is flux. 

S    +   O2     ®   SO2

4P   +   5O2      ®    2P2O5

Si    +   O2    ®   SiO2

CaO   +  SiO2 ®   CaSiO3 (slag) 

3CaO + P2O5  ® Ca3(PO4)2 (slag)

The metal is removed and freed from slag by passing through rollers.

22.   Electrolytic Reduction (Hall – Heroult Process): Purified bauxite ore is mixed with cryolite (Na3AlF6) or CaF2 which lowers its melting point  and increases electrical conductivity.

Molten mixture is electrolysed using a number of graphite rods as anode and carbon lining as cathode.

The graphite anode is useful for reduction of metal oxide to metal.

2Al2O3 + 3C = electrolysis=> 4Al + 3CO2

Al2O3   ==electrolysis==>      2 Al³  + 3 O-²

At cathode:  Al³+  (melt) + 3e-  ® Al (l)

At anode:    C(s) + O²-  (melt) ® CO (g) + 2e- 

                    C(s) + 2O²-  (melt) ® CO2 (g) + 4e-

Graphite rods get burnt forming CO and CO2. The aluminium thus obtained is refined electrolytically using impure Al as anode, pure Al as cathode and molten cryolite as electrolyte.

At anode:        Al   ®     Al³+ + 3e-             (Impure)

At cathode:  Al³+  + 3e-  ® Al (pure)

23.           Electrolysis of molten NaCl:

NaCl ® Na++ Cl- (Molten)

At cathode:  Na++ e-  ® Na At anode:          2Cl-  ® Cl2  + 2e-

Thus sodium metal is obtained at cathode and Cl2 (g) is liberated at anode.

24.       Refining: It is the process of converting an impure metal into pure metal depending upon the nature of metal.

25.           Distillation: It is the process used to purify those metals which have low boiling points, e.g., zinc, mercury, sodium, potassium. Impure metal is heated so as to convert it into vapours which changes into pure metal on condensation and is obtained as distillate.

26.           Liquation: Those metals which have impurities whose melting points are higher than metal can be purified by this method. In this method, Sn metal can be purified. Tin containing iron as impurities heated on the top of sloping furnace. Tin melts and flows down the sloping surface where iron is left behind and pure tin is obtained.

27.     Electrolytic refining: In this method, impure metal is taken as anode, pure metal is taken as cathode, and a soluble salt of metal is used as electrolyte. When electric current is passed, impure metal forms metal ions which are discharged at cathode forming pure metal.

At anode:    M (Impure)  ®     Mⁿ+  + n

At cathode:  Mⁿ+ + ne-   ®   M(pure)

28.   Zone refining: It is based on the principle that impurities are more soluble in the melt than in the solid state of the metal.

The impure metal is heated with the help of circular heaters at one end of the rod of impure metal. The molten zone moves forward along with

the heater with impurities and reaches the other end and is discarded. Pure metal crystallizes out of the melt.

The process is repeated several times and heater is moved in the same direction. It is used for purifying semiconductors like B, Ge, Si, Ga and In.

29.      Vapour phase refining: Nickel is purified by Mond’s process. Nickel, when heated in stream of carbon monoxide forms volatile Ni(CO)4 which on further subjecting to higher temperature decomposes to give pure metal.

Ni (impure)  + 4 CO ¾¾33¾0 -¾35¾0K¾®Ni(CO) = 450 – 470K => Ni(pure)  + (CO)

                                                                        

30.        van- Arkel method: It is used to get ultra pure metals. Zr and Ti are purified by this process. Zr or Ti are heated in iodine vapours at about 870 K to form volatile ZrI4 or TiI4 which are heated over tungsten filament at 1800K to give pure Zr or Ti

Ti + 2I2   ¾¾¾® TiI4 ¾¾¾® Ti + 2I2

Impure                                   Pure

Zr + 2I2   ¾¾¾®ZrI4 ¾¾¾®Zr + 2I2

Impure                                  Pure

31.        Chromatographic method: It is based on the principle of separation or purification by chromatography which is based on differential adsorption on an adsorbent. In column chromatography, Al2O3 is used as adsorbent. 

The mixture to be separated is taken in suitable solvent and applied on the column. They are then eluted out with suitable solvent (eluant). The weakly adsorbed component is eluted first. 

This method is suitable for such elements which are available only  in minute quantities and the impurities are not very much different in their chemical behaviour from the element to be purified.

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