Reduction of oxide to metal
Some basic concepts of thermodynamics help us in understanding the theory of metallurgical transformations. . Where is the change in Gibbs energy, is the enthalpy change and is the entropy change.
K is the equilibrium constant of the 'reactant – product' system at the temperature,T. A negative implies a positive K in equation
When the value of is negative, only then the reaction will proceed. If is positive, on increasing the temperature (T), the value of T would increase (< T) and then will become negative.
If reactants and products of two reactions are put together in a system and the net of the two possible reactions is negative, the overall reaction will occur. So the process of interpretation involves coupling of the two reactions, getting the sum of their and looking for its magnitude and sign. Such coupling is easily understood through Gibbs energy () vs T plots for formation of the oxides.
Extraction of iron from its oxides
Extraction of copper from cuprous oxide [copper(I) oxide]
Extraction of zinc from zinc oxide
Electro chemical principles of metallurgy . Here n is the number of electrons and E is the electrode potential of the redox couple formed in the system.
This process of electrolysis is widely known as Hall-Heroult process.
Purified Al2O3 is mixed with Na3AlF6 or CaF2 which lowers the melting point of the mix and brings
conductivity. The fused matrix is electrolysed. Steel cathode and graphite anode are used. The graphite anode is useful here for reduction to the metal.
Copper from Low Grade Ores and Scraps
Copper is extracted by hydrometallurgy from low grade ores.
The G0 for this reaction is + 422 kJ. When it is converted to E0, we get E0 = – 2.2 V. Naturally, it will require an external e.m.f. that is greater than 2.2 V. But the electrolysis requires an excess potential to overcome some other hindering reactions. Thus, Cl2 is obtained by electrolysis giving out H2 and aqueous NaOH as byproducts. Electrolysis of molten NaCl is also carried out. But in that case, Na metal is produced and not NaOH.
Extraction of gold and silver involves leaching the metal with CN–.
Vapour phase refining
These are described in detail here.
This is very useful for low boiling metals like zinc and mercury. The impure metal is evaporated to obtain the pure metal as distillate.
In this method a low melting metal like tin can be made to flow on a sloping surface. In this way it is separated from higher melting impurities.
(c) Electrolytic refining
In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal. The more basic metal remains in the solution and the less basic ones go to the anode mud. This process is also explained using the concept of electrode potential, over potential, and Gibbs energy which you have seen in previous sections. The reactions are:
Copper is refined using an electrolytic method. Anodes are of impure copper and pure copper strips are taken as cathode. The electrolyte is acidified solution of copper sulphate and the net result of electrolysis is the transfer of copper in pure form from the anode to the cathode:
Impurities from the blister copper deposit as anode mud which contains antimony, selenium, tellurium, silver, gold and platinum; recovery of these elements may meet the cost of refining. Zinc may also be refined this way.
Vapour phase refining
In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal. So, the two requirements are:
The metal should form a volatile compound with an available reagent,
The volatile compound should be easily decomposable, so that the recovery is easy.
Following examples will illustrate this technique.
Mond Process for Refining Nickel: In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl:
The carbonyl is subjected to higher temperature so that it is decomposed giving the pure metal:
vanArkel Method for Refining Zirconium or Titanium: This method is very useful for removing all the oxygen and nitrogen present in the form of impurity in certain metals like Zr and Ti. The crude metal is heated in an evacuated vessel with iodine. The metal iodide being more covalent,volatilizes:
The metal iodide is decomposed on a tungsten filament, electrically heated to about 1800K. The pure metal is thus deposited on the filament.