General Principlesand Processes of Isolation of Elements Class 12 Chapter 6 | Chemistry | Class 12 | CBSE|
General Principlesand Processes of Isolation of Elements
Elements are the basic units of all types of matter in the universe.
Modes of Occurrence of Elements
Elements are found to occur in nature either in free state or in the combined state i.e in the form of compounds This is mainly due to the reason that different elements have different chemical reactivities .
i) Native State
Elements which are not attacked by moisture , oxygen and carbon dioxide of the air occur in native state . Example : Carbon , Sulphur
ii) Combined State
Elements which are readily attacked by moisture , oxygen and carbon dioxide of the air occur in combined state in the form of their compounds called minerals.
Elements vary in abundance . Among metals , aluminum is the most abundant . It is third most abundant element in earth ‘s crust.
Aluminum is a major component of many igneous minerals including mica and clays . Many gemstones are impure forms of alumina (Al2O3) .
Iron is the second most abundant metal in earth’s crust .
The mineral from which the metal is conveniently and economically extracted is called an ore.
Note : All ores are minerals but all minerals are not ores.
Gangue or Matrix
Ores as they are obtained from earth ‘s crust are never pure. They are usually associated with earthy and silicous impurities ( in addition to impurities of other minerals ) called gangue or matrix .
The scientific and technological process of extracting metal from their ores is called metallurgy.
The process employed for extraction of a metal depends upon its physical and chemical properties and the impurities associated with it. Since different metals have different properties . Therefore , a single universal method cannot be applied for extraction of all the metals . Still there are some procedures which are common to metallurgies of various metals and are termed as metallurgical operations .
Some important metallurgical operations are :
Crushing and Grinding of the ore
Concentration or benefaction of the ore
Extraction of crude metal from the concentrated ore
Refining of the crude metal
Crushing and Grinding of the Ore
The ores usually occur in nature in the form of big lumps . These lumps are broken into small pieces with the help of crushers or grinders . This process is called crushing .These small pieces are then reduced to a fine powder with the help of stamp mill or ball mill . This process is called pulverisation.
Concentration or Benefaction of Ore
The removal of unwanted earthy and siliceous impurities (i.e. gangue or matrix ) from the ore is called ore -dressing or concentration of ores and the process used to concentrate an ore is called the benefaction process . The finely grounded ore is concentrated by the following methods:
1) Hydraulic Washing
The process by which lighter earthy particles are freed from the heavier ore particles by washing with water is called Levigation or Hydraulic washing or Gravity separation . For this , the powdered ore is either agitated with water or washed with running stream of water .
This method of concentration is employed when either the ore or the impurities associated with it are magnetic in nature .
Example : Chromite (FeCr2 O4) – an ore of chromium
Magnetite (Fe3O4) – an ore of iron
In this method ,the powdered ore is dropped over a conveyer belt moving around two rollers – one of which has an electromagnet in it . As the ore particles roll over the belt , magnetic particles are attracted by the magnetic roller . As a result , two heaps are formed separately . The heap collected below the magnetic roller contain the magnetic particles while the heap formed away from the magnetic roller contain the non magnetic impurities .
3)Froth Floatation Method
This method is widely used for concentration of sulphide ores such as zinc blende (ZnS) , copper pyrites (CuFeS2) , galena (PbS). This method is based upon the fact that the surface of sulphide ores is preferentially wetted by oils while that of gangue is preferentially wetted by water .
The ore is crushed into fine powder and mixed with water to form suspension in a tank. To it, collectors and froth stabilizers are added . Collectors ( e.g. pine oil, xanthate) enhance the non – wettability of ore particles while froth – stabilizers (e.g. cresols , aniline ) stabilize the froth .
The suspension is agitated (shaked) by rotating paddle which draws in air causing frothing .
During this , ore particles which are wetted by oil become lighter and thus rise to surface along with froth while gangue particles which are preferentially wetted by water become heavier and thus settled at bottom of tank . The froth is skimmed off . It is allowed to collapse and finally dried to get concentrated ore.
This process consists in treating the powdered ore with a suitable reagent (acid , base or other chemicals)which can selectively dissolve the ore but not the impurities.
Extraction of Crude Metal from Concentrated Ore
The process used to obtain metal in free state from concentrated ore.
Since many metal can be obtained from their respective oxides by reduction (electronation) . Thus, extraction of metal involves two chemical process:
Conversion of the ore into metal oxide , i.e., de-electronation of ore
Reduction or electronation of metal oxide to the free metal
i) Conversion of the ore into metal oxide
Calcination is the process of converting an ore into its oxide by heating it strongly below its melting point either in absence or limited supply of air.
Note : Calcination is commonly used to convert metal carbonates and hydroxides to their respective oxides.
During this process of calcination , following chemical changes occur :
moisture is driven out.
Volatile impurities of S, AS and P are removed as volatile oxides .
Water is removed from hydrated oxides and hydroxide ores.
Al2O3.2H2O (Bauxite) → Al2O3 (Alumina)+ 2H2O
Carbonate ores are converted into their oxide by loss of carbon dioxide.
CaCO3 (Limestone) → CaO (Calcium Oxide) + CO2 ↑
Roasting is the process of converting an ore into its metal oxide by heating strongly below its melting point in excess of air.
Note : This process is commonly used for sulphide ores.
The following changes occur during roasting.
Moisture is removed
Organic matter is destroyed
Non-metallic impurities like that of sulphur , phosphorous and arsenic are oxidised and removed as volatile gases.
S8 + 8 O2 → 8SO 2 ↑ (sulphur dioxide)
P4 + 5O2 → P4O10 ↑ (Phosphorous pentoxide)
Ores are generallyconverted into metal oxide
2ZnS (Zinc sulphide) + 3O2 → 2ZnO + 2SO2↑
Important Points to Remember
If the calcinated or roasted ore still contain non-fusible impurities of earthy matter , an additional substance called flux is added during reduction process.
Flux is a substance that chemically combines with gangue which may still be present in roasted or calcinated ore to form an easily fusible material called the slag.
Flux + Gangue → Slag
ii) Reduction or Electronation of metal oxide to the free metal
The process of extracting the metal by heating the metal oxide with suitable reducing agent is called pyrometallurgy.
The oxides of less reactive metal such as Zinc , iron , copper can be reduced by number of reducing agent such as carbon(coke). When carbon is used as reducing agent it combines with oxygen of metal oxide to form carbon monoxide.
MxOy (Metal oxide) +y C (carbon) → x M (Metal) + y CO (Carbon monoxide)
Note : Reduction of metal oxide is easier if metal formed is in liquid state.
The process of extracting a metal by reduction of its oxide with carbon (in form of coke , charcoal or carbon monoxide ) is called smelting.
Thermodynamic Principles of Metallurgy
We know , ΔG = ΔH – TΔS where ΔG is gibb’s free energy , ΔH is the enthalpy change , ΔS is entropy change and T is the absolute temperature.
The free energy change is also related to equilibrium constant K of the reactant product system at temperature T ΔGº = –RT ln K . Thus , if ΔG is negative than K will be positive .
The criteria of feasibility of a reaction at any temperature is that ΔG must be negative Thus if ΔS is +ve then on increasing temperature TΔS will increase and when TΔS > ΔH , ΔG will be negative and reaction will proceed in forward direction .
A reaction with ΔGpositive can still made to occur by coupling it with another reaction having large negative ΔG so that the net ΔG of two reaction is negative .
Applications of Pyrometallurgy
a) Extraction of Iron from its Oxide
The chief ores of iron are :
Haematite , Fe2O 3 (red oxide of iron)
Limonite , Fe2O 3.3H 2O (hydrated oxide of iron )
Magnetite , Fe3O4 (magnetic oxide of iron )
Siderite or Spathic ore , FeCO3
Iron Pyrites (Fool ‘s gold) , FeS2
Extraction of iron involves the following steps:-
Concentration: The ore is crushed in jaw crushers and crushed ore is concentrated by gravity separation process.
Calcination : The concentrated ore is then calcinated i.e heating strongly in presence of limited amount of air
FeCO3 → FeO + CO2 ; 4FeO + O2 → 2Fe2O3
Ferrous oxide is reduced to Ferric oxide thereby preventing the loss of iron as slag.
Smelting : The calcinated ore is reduced with carbon , i.e, smelted in a blast furnace . It is a tall cylindrical furnace made of steel and lined inside with fire bricks . It is slightly narrow at top and gain at the bottom . The furnace is provided with double cup and cone arrangement which helps to feed the charge from top without letting any gas from inside to escape . Near the top , furnace is also provided with an outlet for waste gas . At base , furnace is provided with
small pipes called tuyeres through which blast of hot air is admitted.
a taping hole for withdrawing molten iron
a slag hole for withdrawing slag.
Reduction of Iron Oxide takes place at different temperature ranges:
a) At temperature range 500-883 K (Reduction zone )
3Fe2O3 + CO → 2Fe3O4 + CO2
Fe3O4 + 4CO → 3Fe + 4CO2
Fe2O3 + CO → 2FeO + CO2
b) At temperature range 900 -1500 K (Combustion zone)
CO2 + C → 2CO
FeO + CO → Fe + CO2 (Spongy iron )
Slag Formation zone
CaCO3 → CaO + CO2
CaO + SiO2 → CaSiO3 (Slag)
c) At temperature range 1500-2100 K (Melting zone )
C + O2 → CO2
FeO + C → Fe + CO
The molten iron is called pig iron which has 4% carbon and small amount of S , P , Si and Mn .
Note : Cast Iron is made by melting pig iron with scrap iron and ore using hot air blast . It contains 3% of carbon. Wrought Iron is the purest form of commercial iron.
b) Extraction of Copper from Cuprous Oxide
It is very easy to reduce cuprous oxide to metallic copper by heating with coke . But most of the ores of copper are sulphides . Thus sulphide ores are first roasted in a reverberatory furnace to convert them into oxides .
2Cu2S + 3O2 → 2Cu2O + 2SO2
The oxide can then be reduced to metallic copper using coke as reducing agent
Cu2O + C → 2Cu + CO
In actual process , the ore is heated in reverberatory furnace after mixing with silica . In the furnace iron silicate and copper matte are formed . Copper matte is then charged into silica lined convertor .
Following reaction take place
2FeS + 3O2 → 2FeO + 2SO2
FeO + SiO2→ FeSiO3
2Cu2S + 3O2 → 2Cu2O + 2SO2
2Cu2O + Cu2S → 6Cu + SO2
C)Extraction of Zinc from Zinc Oxide
The reduction of zinc oxide is carried out using coke .
ZnO + C → Zn + CO
The metal is distilled off and collected by rapid chilling .
Electro-chemical Principles of Metallurgy
The process of Electrolysis has been used to carry out the reduction of molten metal salts.
ΔGº = –nFEº where n is number of electrons involved in reduction process , Eº is standard electrode potential of redox couple present in system .
More reactive metals have large negative value of standard electrode potential and are difficult to reduce.
If difference in value of Eº is positive and consequently , ΔGº is negative then more reactive metal will displace less reactive metal from the solution .
Application of Electrolysis to Metallurgy
The process of extraction of metals by electrolysis of their fused salts is called electro-metallurgy. In this process , electrons serve as reducing agent.
Extraction of Aluminium from Alumina
Fused Alumina (Al2O3) is a bad conductor of electricity . Therefore, cryolite and fluorspar are added to purified alumina which not only make alumina a good conductor of electricity but also reduce the melting point of mixture.
The process of obtaining aluminium by electrolysis of a mixture of purified alumina and cryolite is called Halland Heroult process.
The reaction taking place during electrolysis are as follows :
At Cathode : Al3+ (melt) + 3e- → Al (l)
At Anode : C + O2- (melt) → CO + 2e– ; C + 2 O2- (melt) → CO2 + 4e–
The electrolysis of molten mass is carried out in electrolytic cell made of iron using carbon electrodes.
The molten electrolyte is covered with layer of powdered coke to prevent oxidation and loss of heat due to reduction.
The metal obtained by any of the above method are still impure and are called crude metals . The process of purifying the crude metal is called Refining .
Common methods used for Refining of metals are discussed below :
This method is employed for purification of volatile metals like zinc , mercury , cadmium . The impure metal is heated in an iron retort and vapours are condensed in separate receivers . The pure metal distills over leaving behind non volatile (having higher boiling point ) impurities in the retort.
This method is used for purification of such metals as contain impurities which are less fusible than the metal themselves, i.e., the melting point of metals are lower than those of impurities. The crude metal is heated in an inert atmosphere of carbon monoxide on the sloping hearth of reverberatory furnace ; the metal melts and flows down into the receiver leaving the infusible impurities on the hearth.
c) Electrolytic Refining
A large number of metals such as copper , silver , gold , lead , nickel , chromium etc are refined by this method ; cathode is made of pure metal and a salt of metal (which has to be purified) soluble in water acts as an electrolyte . On passing electric current , pure metal is deposited at cathode from anode . The reactions are
Anode : M → Mn+ + ne–
Cathode : Mn+ + ne– → M
d) Zone Refining
This method is based upon principle that impurities are more soluble in molten state than in solid state of the metal . In other words , when the melt of an impure metal is allowed to cool , the pure metal crystallises out while the impurities remain in the melt . The process is usually carried out in an inert atmosphere to prevent the oxidation of the metal. The element thus obtained are of high purity .
d)Vapour Phase Refining
In this method , crude metal is freed from impurities by first converting it into a suitable volatile compound by heating it with a specific reagent at lower temperature and then decomposing the volatile compound at some higher temperature to give the pure metal .
This method is illustrated by the following two processes:
1)Mond Process: It is used for refining of Nickel . When impure Nickel is heated in a current of CO , it forms volatile nickel tetracarbonyl complex leaving behind impurities . The complex thus heated at high temperature gives pure nickel.
Ni (impure nickel) + 4CO → Ni(CO)4 (Nickel tetracarbonyl) → Ni (pure nickel) + 4CO
2)van Arkel method : This method is used for preparing ultra pure metals by removing all oxygen and nitrogen present in form of impurity in metals such as zirconium and titanium . In this, crude zirconium is heated in vessel with iodine . The covalent volatile zirconium tetraiodide thus formed is separated . It is then decomposed by heating over tungsten to give pure zirconium.