General Principles and Processes of isolation of elements class 12 chemistry

General Principles and Processes of Isolation of Elements
 
Minerals: 
 

They are the naturally occurring chemical substances present in the earth’s crust that may or may not be extracted profitably through mining.

Ores: 

They are the rocky materials that contain sufficient amount of mineral so as to extract the metal profitably or economically are known as ores.

Gangue: 

They are the earthy or undesirable material present in the ore.

Flux:
The substance that is added to the ore so as to convert the non-fusible gangue to a fusible compound is known as flux. There is both acidic and basic flux.

Slag:
The fusible compound formed by the combination of flux and gangue is called slag.

 
 
 Metallurgy:

It is the entire scientific process used for isolation of the metal from its ores.

Steps of metallurgy include

  •      Concentration of the ore
  •      conversion of the concentrated ore  to oxide
  •      Reduction of the oxide to metal
  •      Refining of the metal

Concentration of the ore:

The process of removal of unwanted material like sand, clay,  rock, etc. from the ore is known as concentration ore-dressing or benefaction, which includes several steps that depend upon the physical properties of the metal compound and the impurity  (gangue). It also depends on the type of metal, environmental factors and available facilities.

Hydraulic washing: 

Ore is first washed with a stream of water under pressure to wash away the lighter impurities thereby leaving the heavy ores behind. It is based on the difference in the densities of the ore and gangue particles.

 Magnetic separation:

It is based on the difference in the magnetic and non-magnetic properties of the two components of the ore  (pure and impure). Magnetite (Fe3O4), chromite(FeCr2O4) and pyrolusite (MnO2) can be concentrated from the unwanted gangue and to remove tungsten ore particles from cassiterite (SnO2).

Froat flotation process:

It is based on the principle that sulphide ore is preferentially wetted by the pine oil or fatty acids or xanthates, etc.  while the gangue particles are wetted by the water. To enhance the non-wettability of the mineral particles, collectors are added. Froat stabilizers such as cresols, aniline, etc. are added to stabilize the froat.

When two sulphide ores are present then by adjusting the proportion of oil to water or by adding depressants, we can separate the two sulphide ores.

For example, NaCN is used as a depressant for ore containing zinc sulphide and lead sulphide. Selectively, it allows PbS to come with the froth and prevents ZnS from coming to the froat.

Leaching:

  •      Purification of bauxite by leaching  (baeyer’s process)

Step 1 :-  Al2O3(s) + 2NaOH + 3H2O —-> 2Na[Al(OH)4] (aq)

Step 2 :- 2Na[Al(OH)4] (aq) + CO2 (g) —–> Al2O3.xH2O (s) + 2NaHCO3 (aq)

Step 3 :-  Al2O3.xH2O (s) —-> Al2O3(s) + xH2O (g)

  •     Concentration of gold  and silver  ores by leaching:

4M(s) + 8CN- (aq) +2H2O (aq) +O2 (g) —-> 4[M(CN)2]- (aq) +4OH- (aq)

2[M(CN)2]- (aq) +Zn (s) —-> [Zn(CN)4]- (aq) + 2M (s)

Where M = Ag or Au

Conversion of ore   into oxide:

It is easier to reduce oxide than sulphide or carbonate ore.  Therefore,  the ore can be converted into oxide either by:- Roasting and Calcination

Roasting:

The ore is heated in a continuous supply of air at a temperature below the metal’s m.p. to convert the ore into  its oxide. Sulphide ores are thus converted into oxide by roasting. It also helps us to remove impurities and volatile oxides.

Example :-  2 ZnS + 3 O2 —-> 2 ZnO + 2 SO2

Calcination:

It is a process of heating of carbonate ores in a limited supply of air to convert it into oxides. It helps us to remove moisture and volatile impurities.

Example :- CaCO3heat—-> CaO + CO2

Pyrometallurgy: The process of extraction of a metal by heating the metal oxide with a suitable reducing agent is known as pyrometallury.

Hydrometallurgy: The extraction of metal by dissolving the ore in a suitable reagent followed by displacement of the metal by a more electropositive metal is known as Hydrometallurgy.

Reduction of oxide to metal :

Suitable reducing agent are needed depending upon the reducing power of metal. The common reducing agents used are C or CO or any other metal like Al, Mg, etc.

Thermodynamic principles of metallurgy: 

Gibbs free energy change at any temperature is given by ΔG= ΔH-TΔS  where ΔG is free energy change, ΔH is enthalpy change and ΔS  is entropy change. The relationship between ΔGо and K is ΔGо= -2.303RTlogK  where K is equilibrium constant R=8.314JK-1.  T is the temperature in Kelvin. The negative ΔG means a positive value of K i.e., products are formed more than the reactants.  the reaction will proceed in the forward direction. If ΔS is positive on increasing temperature the value of TΔS increases so that TΔS > ΔH  and ΔG will become negative.

Coupled reaction: 

If reactant and product of 2 reactions are put together in a system and the net ΔG of two possible reactions is negative the overall reaction will take place. This reaction is called coupled reactions.

Ellingham diagram: 

The plots between ΔfGо of formation of oxides of elements versus temperature are called Ellingham diagrams.  Such diagrams help in predicting the feasibility of a   thermal reduction of an ore. ΔG   must be negative at a given temperature for a reaction to be feasible.

 

Limitations of Ellingham diagram

Kinetics of reduction is not taken into consideration.

Reduction of iron oxide in a blast furnace:

Reduction of oxides takes place in different zones.

At 500 -800 Kelvin 

3 Fe2O3 + CO —> 2 Fe3O4 + CO2

Fe3O4 + 4CO —> 3 Fe +  4 CO2

Fe2O3 + CO —> 2 FeO + CO2

At 900-1500 K 

C + CO2 —> 2 CO

FeO + CO —> Fe + CO2

CaCO3heat—> CaO + CO2

Silica (impurity)  reacts with calcium oxide to form calcium silicate, i.e., slag.  It floats over molten iron and prevents oxidation of iron.

CaO + SiO2 —> CaSiO3 (calcium silicate, i.e., slag)

 
 

Types of iron

pig iron:  It is the iron which is obtained from the blast furnace. It is the impure form of iron. It contains 4% C and a small amount of S, P, Si, and Mn. 

Cast iron: By melting pig iron with scrap iron and coke using hot air blast, cast iron is obtained. It contains 3% of C content and is hard and brittle.

Wrought iron: It is the purest form of commercial iron. By the oxidative refining of iron in a reverberatory furnace lined with haematite, wrought iron is prepared which oxidizes C to CO. It is also known as malleable iron

Fe2O3 + 3 C —> 2 Fe + 3 CO

S + O —> SO

4 P4 + 5 O2 —> 2 P2O5

Si + O2 —> SiO2

CaO + SiO2 —> CaSiO3 (slag)

3 CaO + P2O5 —> Ca3(PO4)2 (slag)

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

Electrolytic reduction  (hall Herault process):

The pure bauxite ore is mixed with cryolite( Na3AlF6) or  CaF2. It lowers its melting point and increases the electrical conductivity.  The molten mixture is then electrolyzed using graphite rods as anode and carbon lining as a cathode. Graphite anode is useful for reduction of metal oxide to metal.

2 Al2O3 + 3C —> 4 Al + 3 CO2

Al2O3 –electrolysis —> 2 Al3+ + 3 O2-

At cathode:- Al3+(melt) +3e —> Al(l)

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

C(s) + 2 O2-(melt) —> CO2 (g) + 4e

Graphite rods get Burnt forming carbon monoxide and carbon dioxide.  The aluminium obtained is then refined electrolytically by using an impure aluminium as an anode, a pure aluminium as a cathode and molten cryolite as the electrolyte.

At anode (impure) :-  Al —> Al3+ + 3e

At cathode:- Al3+ +3e —> Al (pure)

Refining:

An impure metal is converted into pure metal depending upon the nature of metal, this process is called refining.

Distillation:

It is the process which is used to purify those metals which have low b.p. Example: zinc, Mercury, Sodium, Potassium.  The impure metal is heated to convert it into vapours which in turn changes into pure metal on condensation and is obtained as distillate.

Liquation:

The impurities having higher m.p. than metal can be purified by this method.  In this method, tin metal can be purified. Tin containing iron as an impurity is heated on the top of the sloping furnace. Tin melts and flows down the sloping surface where iron is left Behind and pure tin is obtained.

Electrolytic refining:

An impure metal is taken as an anode,  pure metal is taken as a cathode and a soluble salt of metal is used as an electrolyte in this method.  When an electric current is passed through it, then the impure metal forms metal ions which are discharged at the Cathode forming a pure metal.

At anode:- M(impure) —>Mn+ +ne

At cathode:- Mn+ +ne —> M(pure)

Zone refining:

The main principle of this method is impurities are more soluble in the melt than in the solid state of the metal.  The impure metal is heated at one end of the rod with the help of circular heaters. The impurities in the molten zone move forward and reach the other end and are discarded.  Pure metal crystallizes out of the melt.  The process is repeated several times and the heater is moved in the same direction.It is used for purifying semiconductors like  Boron, Germanium, silicon,  gallium, and Indium.

Vapour phase refining:

Nickel is purified by Mond ’s process.  

Ni(impure) + 4 CO –330-350k—> Ni(CO)4450-470k—>Ni(pure) + 4CO

Van Arkel method: 

It is used to get Ultra Pure metal.  zirconium and Titanium are purified by this process. Zr or Ti are heated in iodine vapours at about 870 Kelvin to form a volatile ZrI4  or TiI4  which are again heated over tungsten filament at 1800  Kelvin to give pure Zr and Ti.

Ti(Impure) + 2I2—> TiI4 —> Ti(pure) + 2I2

Zr(Impure) +2I2 —> ZrI4 —> Zr(pure) + 2I2

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