Short Answer TypeGive a brief account of the following processes as used in extraction of metals:
(a) Smelting
(b) Magnetic separation
What do you understand by the following terms: (i) Roasting, (ii) Calcination, (iii) Smelting?
Long Answer TypeDescribe mainly with balanced chemical equations.
(a) Copper from copper pyrites by self reduction.
(b) Zinc from zinc blende by carbon reduction.
(c) Silver from silver glance by cyanide process.
(a) How is aluminium extracted from alumina?
(b) Give balanced equations for the extraction of aluminium from bauxite by electrolysis.
(c) What is the function of cryolite in the extraction of Al from Al2O3.
a) it is extracted by electrolysis. The ore is first converted into pure aluminium oxide by the Bayer Process, and this is then electrolysed in solution in molten cryolite - another aluminium compound.
The alumina is dissolved in a mixture of molten cryolite and fluorspar which lowers the melting point. It is then electrolysed in a rectangular steel tank with carbon lining, which serves as cathode. Anode is set of thick carbon rods suspended from the top into the fused Al2O3. The temperature is maintained between 1200 and 1310k. Oxygen is evolved at the anode.
Al2O3 -------> 2Al3+ + 3O2-
At cathode: 2Al3+ + 6 e- -------> 2Al
At cathode: 3O2- --------> 1/5O2 + 6e-
Aluminium formed at the cathode gets collected at the bottom of the electrolytic cell from where it is removed periodically.
b) In the Baeyer's process, the bauxite ore is heated with concentrated NaOH solution under pressure alumina dissolves as sodium meta aluminate. The other materials present in the ore are left as insoluble part. This solution is filtered in ore are left as insoluble part. This solution is filtered off. From the solution Al(OH)3 is precipitated by adding freshly prepared Al(OH)3 to the cold dilute solution and agitating.
Al2O3 + 2NaOH -------> 2NaAlO2 + H2O
NaAlO2 + 2H2O -------> NaOH + Al(OH)3
The precipitated Al(OH)3 is dried and ignited to get pure alumina.
2Al(OH)3 --------> Al2O3 + 3H2O
c) Cryolite is added as an impurity to the electrolyte. The cryolite, lowers the melting point of alluminium oxide to 980. The rule is as it is for all impurities they lower the melting point and raise the boiling point, so the reduction of the melting point allows the alluminium to melt at lower temperatures, making electrolysis work at lower temperatures, hence requires lesser energy, making the process cheaper, and more cost effective.
Short Answer Type
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