Question

he process of converting hydrated alumina into anhydrous alumina is called
A. Roasting
B. Smelting
C. Dressing
D. Calcination

Answer 1

: Hydrated alumina has aluminium oxide and water molecules. The chemical formula of hydrated alumina is ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}\,$. For the conversion of hydrated alumina into anhydrous alumina it is heated in absence of oxygen. The chemical formula of anhydrous alumina ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$.

Complete step-by-step answer:
Roasting is also used to extract metal from concentrated ore. Roasting is used for the metals which are present in the form of sulphide. Roasting is the heating of metal in an oxygen rich environment. So, volatile matter removes leaving behind the oxide of metal only.

Smelting is the heating of ore with the presence of reducing agents. So, the ore decomposes. The impurities are removed in form of slag or gases left behind a base metal.

Dressing is the removal of unwanted material from the ore. This process is used before the concentration of ore.

Calcination is used to extract metal from concentrated ore. Calcination is used for the metals which are present in the form of carbonate or oxide. Calcination is the heating of metal in an environment that lacks oxygen. So, volatile matter removes leaving behind the oxide of metal only.

When hydrated alumina is heated in absence of oxygen the impurities are removed as carbon dioxide and sulphur dioxide and we get the aluminium oxide.

The reaction is shown as follows:
${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}\,\mathop \to \limits^\delta {\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\,{\text{ + }}\,\,{\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}\,$

As hydrated alumina is heated in absence of oxygen and form oxide so, it is a calcination.So, the process of converting hydrated alumina into anhydrous alumina is called calcination.

Therefore, option (D) calcination is the correct answer.

Note: Calcination and roasting both processes both have the same object but different in two ways. Calcination is used for metal carbonate oxide whereas roasting is used for metal sulphide. Calcination takes place in absence of oxygen whereas roasting takes place in presence of oxygen. As In dressing, unwanted sand material and other things are removed by the help of simple things like a cutter or knife.