Question

When Bauxite is boiled with aqueous $NaOH$ solution, sodium metal aluminate solution is formed. This part in metallurgy of aluminium is called as:
A. Liquation
B. Amalgamation
C. Leaching
D. Calcination

Answer 1

Firstly one should be aware of the fact that bauxite is an ore of aluminum. Secondly, the metallurgy is a process of extracting metals (in pure form) from the ores( that are naturally occurring on the earth’s surface.) using a suitable method. These methods may be froth floatation processes, roasting, calcination and many more processes exist.

Complete answer:

Liquation is one of the methods used in metallurgical processes to separate metal from an alloy or an ore. The alloy (in its crude form) is exposed to high temperature till one of the metals present in the alloy starts to melt and is removed away from rest of the material and can be collected.
Amalgamate, in literary terms, means merging into a single entity. So, as the word itself suggests, amalgamation is the process in which an ore of gold or silver is mixed with mercury to get the pure metal. This method is used to extract gold and silver from ores, in which the ore is crushed, metal is then dissolved into mercury. The amalgam is further heated, resulting in mercury to evaporate leaving behind the pure metal.
Leaching is one of the metallurgical processes often used where the impurities remain insoluble and the metals get converted into soluble salts when a certain chemical is added into the ore.
Thus, the ore (bauxite) is concentrated by chemical reaction with a suitable reagent (hot concentrated $NaOH$ solution.) that dissolves the metal (aluminium) and the impurities like $F{e_2}{O_3},Si{O_2}$ are removed.

The reaction follows as mentioned below:
$A{l_2}{O_3}.2{H_2}O + 2NaOH \to 2NaAl{O_2} + 3{H_2}O$
(Bauxite) (Sodium Hydroxide) (Sodium meta aluminate)
This process is called leaching.

In calcination, the ore is exposed to the heat in the absence of air. In case of bauxite, when it is exposed to high temperature, the alumina is formed. The reaction occurs in the following manner:
$A{l_2}{O_3}.2{H_2}O\xrightarrow{{\Delta (heat)}}A{l_2}{O_3} + 2{H_2}O$
(Bauxite) (Alumina)

Thus, option C. Leaching is the correct answer to the given question.

Note: Amalgamation is an exothermic process in which heat is released as the final product is formed.In leaching, we study one more example; noble metals are leached with a dilute aqueous solution of sodium cyanide or potassium cyanide in the presence of air.
The reaction is:
$A{g_2}S + 4NaCN \to 2Na\left[ {Ag{{(CN)}_2}} \right] + N{a_2}S$
(Argentite) (Sodium argento cyanide)