Chemistry THERMODYNAMIC AND ELECTROCHEMICAL PRINCIPLES INVOLVED IN THE EXTRATION OF METALS
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THERMODYNAMIC AND ELECTROCHEMICAL PRINCIPLES INVOLVED IN THE EXTRATION OF METALS

Ellingham Diagram :

The changes in Gibbs energy that occur when one mole of oxygen is used may be plotted against temperature for a number of reaction of metals to form their oxides. Such a graph is shown in Fig.1 and is called an Ellingham diagram for oxides. Similarly, we can plot Ellingham diagrams for halides.


The Ellingham diagram for oxides show the following important features :

(i) Ellingham diagram normally consist of plots of `Delta G^o` vs `T` for the formation of oxides of elements, i.e. for the reaction.

(ii) The graphs for metal oxide all slope upwards because the change in Gibbs energy becomes less negative with increase in temperature.

(iii) Each plot follows a straight line unless there is some change in phase.

(iv) When the temperature is increased, a point will be reached when the line crosses `Delta G = 0` line. Below this temperature the `Delta_f G^o` of oxide is negative and hence the oxide is stable. Above this temperature `Delta_f G^o` of the oxide is positive and hence the oxide becomes unstable and decomposes on its own into metal and oxygen.

`ast` `text(Conversion of the oxide to metallic form)`
The roasted or calcined ore is converted into metallic form through reduction by using different reducing techniques which will depend upon the nature of the ore, some of the methods are mentioned below :

Thermodynamic Principles :

Consider a reaction such as formation of an oxide.

`2M(s) + O_2 (g) -> 2MO(s)`

In this reaction, the randomness of the system decreases because gases have more randomness than solids.

Hence, `Delta S` for this reaction is negative. Thus, if temperature is increased then `TDeltaS` becomes more negative. Since `T Delta S` is subtracted in equation, `Delta G` becomes less negative. On the other hand, if `Delta S` is positive, on increasing the temperature the value of `Delta G` decreases and becomes more negative. For example, in the reaction, `2C(s) + O_2 (g) -> 2CO(g)`, `Delta S` is positive and `Delta G` decrease and becomes more negative as the `T` increase.

(a) `text(Reduction by Carbon)` : The oxides of less electropositive metals like `Pb`, `Zn`, `Fe`, `Sn`, `Cu` etc. are reduced by strongly heating with coal or coke.

(b) `text(Reduction by aluminium (Alumino-thermic reduction))` : See fig.

(i) Aluminium acts as reducing agent due to its high electropositive nature.

(ii) Oxides such as `Cr_2O_3`, `MnO_4` are reduced by this method.

(iii) The process is also known as `text(Gold Schmidt thermite process)`.

(c) `text(Reduction by heating in air (Auto-reduction))``//``text(Self reduction)` :

(i) Employed for metals of less active such as `Hg`, `Cu` and `Pb`

(ii) Due to unstable nature in the oxide form at high temperature, no reducing agent is required for their reduction.

`text(Example)` : `HgS + O_2 overset(Delta)-> Hg + SO_2 uparrow`

`undersettext(Roasting)[2Cu_2S + 3O_2] overset(Delta) -> 2Cu_2O + 2SO_2`

`2Cu_2O + Cu_2S -> undersettext[(Auto-reduction)](6Cu + SO_2)`

(d) `text(Electrolytic Reduction (Electro-metallurgy))` :

(i) Employed for highly electropositive metals such as `Na`, `K`, `Ca`, `Mg` etc.

(ii) These metals are extracted by the electrolysis of their oxides, hydroxides or chlorides in fused state.

`text(Example)` : On fusion : `NaCl ⇋ undersettext[(ions become mobile)][Na^(+) + Cl^(-)]`

On electrolysis :

At cathode : `Na^(+) + e^(-) -> Na`

At anode : `Cl^(-) -> Cl + e^(-)`

`Cl + Cl -> Cl_2`

(iii) Aluminium is obtained by the electrolysis of electrolyte which consists of mixture of alumina, cryolite and calcium fluoride (Hall-Herault process)

(e) `text(Other method are following)` :

(i) Reduction by carbon monoxide (employed for iron (III) oxide)
(ii) Reduction by water gas (employed for nickel oxide)
(iii) Amalgamation method (employed for noble metals)

Hydrometallurgy (Reduction by Precipitation) :

(i) Process in which more electropositive metal displace less electropositive metals from salt solution.

(ii) First the concentrated ore is dissolved in strong reagent and remove insoluble precipitates.

(iii) Now the metal is precipitated by addition of more electropositive metal.

Example: Silver sulphide dissolved in sodium cyanide which forms a soluble complex, then silver is precipitated by the addition of zinc powder.

`Ag_2S + 4 NaCN -> undersettext[sodium dicyanoargentate (I)](2 Na [Ag (CN)_2] + Na_2S)`

`2 Na [Ag (CN)_2] + Zn -> Na_2 [Zn(CN)_4] + 2 Ag) downarrow`

`text(Note)` : This type of precipitation process is called `text(cementation)`.

`text(Refining or purification)` :

(i) The metals after reduction process consists of number of impurities like `Si`, `P`, slag, oxides, other metals etc.

(ii) Removal of all these impurities to get pure metal is called as refining.

(iii) Methods of refining are as under :

`text(Liquation)` :

(i) This is based on the principle of difference in melting points of metal and impurity.

(ii) Employed for purification of low melting point metals like `Pb`, `Sn` etc.

`text(Distillation process)` :

(i) This is based on difference in boiling points of metals and impurities.

(ii) Employed for low boiling point metals like `Zn`, `Hg` etc.

`text(Oxidation process)` :

(i) This is a selective oxidation method.

(ii) Used for refining those metals in which the impurities have greater tendency to get oxidised than the metals itself.

(iii) The impurities converted into oxide & skimmed off from the metal.

(iv) Various oxidation processes used for different metals bear different names, e.g., poling, pudding, bessemerisation(See fig.) and cupellation (for Ag).

Cupellation :

The process in which a impure sample of metal (say `Pb` in `Ag`) is fused in a bone ash crucible (Cupel) on the hearth of furnace in the blast of air. The impurity (`Pb`) present is oxidized and blown away with air. Some `PbO` is absorbed by Cupel.

`text(Electrorefining)` :

(i) Employed for refining of highly electropositive melals like `Al`, `Cu`, `Ag`, `Zn`, `Sn`, `Pb`, `Cr` and `Ni`.

(ii) Impure metal is made as anode, thin pure metal sheet is kept as cathode and the electrolysis comprising with soluble salt solution of the melal.

(iii) On passing the electric current, pure metal from the anode dissolved and is deposited on the cathode.

(iv) The soluble impurities goes into the solution (remains in the solution after the completion of refining) while the insoluble impurities settle down below the anode as `text(anode mud)`.

See fig.1.


`text(Van-Arkel process)` :

(i) Employed to get metal in very pure form of small quantities.

(ii) In this method, the metal is converted into a volatile unstable compound (e.g. iodide), and impurities are not affected during compound formation.

(iii) The compound thus obtained is decomposed to get the pure metal.

(iv) Employed for purification of metals like titanium and zirconium

`undersettext(Impure)[Ti(s)] + 2I_2(g) overset(523 K)-> Ti I_4(g)`

`TiI_4(g) overset(1700 K)-> Ti (s) + 2I_2(g)`

`text(Zone refining)` :

(i) Employed for metals which requires in very high purity like semi conductors.

(ii) The method is based on the principle that an impure metal on solidification will deposit crystals of pure metal and the impurities will remain behind in the molten part of the metal.

(iii) Used to purify the elements such as silicon, germanium.

See fig.2.

`text(Amalgamation process)` : Nobel metals ores like `Ag`, `Au`, `Pt` in finely powdered state are mixed with water to form slurry. The slurry is then flown over `Cu` or brass plates coated with `Hg` arranged in slanting position. The metal particles forming amalgam with `Hg` are retained on these plates. The amalgam is scrapped off and then distilled in iron retorts where `Hg` distillates over leaving free metals `Hg` vapours.

See fig.3.

`text(Mond's process)` :

(i) Nickel is purified by using `CO` gas. This involves the formation of nickel tetracarbonyl.

`undersettext(lmpure nickel)(Ni) + 4CO overset(330-350 K)-> Ni(CO)_4 oversettext(450 -470 K)-> Ni + 4CO`
 
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