Question

Give reasons:
(i)Name the method of refining of nickel
(ii)What is the role of cryolite in the extraction of aluminium?
(iii)What is the role of limestone in the extraction of iron from its oxides?

Answer 1

Hint: Cryolite is a compound of sodium, aluminium and fluorine, which in molten state can help in extraction of aluminium. Limestone can react with iron ores and give iron by reacting with the impurities in it. While refining of nickel, nickel oxide is converted to nickel and the purity of the nickel obtained is very high.

Complete step by step solution:
-The method of refining nickel is called the Mond process. This process is also known as the carbonyl process and it is a technique to extract and purify nickel. In this process, nickel oxides are converted to nickel metal with a very high purity attained in a single process. In this process, carbon monoxide combines with nickel reversible and readily gives nickel carbonyl, which then is heated at 220-250 degree Celsius. The reaction is as follows

\[NiO(s)+{{H}_{2}}\to Ni(s)+{{H}_{2}}O(g)\]
\[Ni(s)+4CO(g)\to Ni{{(CO)}_{4}}(g)\]
\[Ni{{(CO)}_{4}}(g)\to Ni(g)+4CO(g)\]

-Aluminium is extracted from its ore by electrolysis process. For the extraction process, aluminium oxide is melted so that electricity can pass through it. It is expensive to melt aluminium oxide, instead it is dissolved in molten cryolite. Cryolite is used as a solvent which reduces some of the energy costs involved in extracting of aluminium. It allows the ions in aluminium oxide to move freely at a low temperature. The overall extraction reaction is

\[2A{{l}_{2}}{{O}_{3}}+3C\to 4Al+3C{{O}_{2}}\]

-Limestone is used during the extraction iron. Limestone can decompose in a hot furnace and form calcium oxide. This calcium oxide can react with the sandy impurities in the ore to form a slag. Limestone is a flux, during the extraction of iron from its haematite ore. The reaction is as follows
\[\underset{limestone}{\mathop{CaC{{O}_{_{3}}}}}\,\to CaO+C{{O}_{2}}\]
\[CaO+Si{{O}_{2}}\to CaSi{{O}_{3}}(slag)\]

Thus, answers are
(i)Mond process
(ii)Molten cryolite can lower the melting point of alumina, can act as a solvent and melted alumina and cryolite can conduct electricity.
(iii)Sandy bits of rocks in iron ore can be eliminated by using limestone. It forms slag with silicon present in the sand.

Note: It is to be noted that molten cryolite is used instead of solid cryolite for the extraction of aluminium. Limestone can only eliminate the slag during extraction of iron from its ore. While purification of nickel from its oxides, nickel carbonyl is formed, in which CO reversibly combines with nickel.