Question

: The chemical treatment of the ore for concentration is done in the case of:
(This question has multiple correct options)
A.Aluminium
B.Silver
C.Copper
D.Gold

Answer 1

Concentration of ores is the removal of unwanted impurities (i.e., gangue or matrix) from the ore. After the ore is finely ground, it can be concentrated by many methods.
When the concentration is done by treating the powdered ore with suitable chemical reagents which can selectively dissolve the ore but not the impurities, the process is called leaching. It is generally used for ores of precious metals and also for bauxite ore.

Complete step by step answer:
The bauxite ore, which is an aluminium ore, usually contains impurities of iron oxide, silica and titanium oxide. The powdered ore is heated with a concentrated solution of sodium hydroxide at 473 to 523 K and 35 to 36 bar pressure. Under these conditions, the alumina dissolves to form sodium meta-aluminate and silica as sodium silicate and leaves behind the impurities.
$\mathop {{\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\left( {\text{s}} \right)}\limits_{{\text{Alumina}}} + 2{\text{NaOH}}\left( {{\text{aq}}} \right){\text{ + 3}}{{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{l}} \right)\xrightarrow{{{\text{473 - 523K}}}}\mathop {{\text{Na}}\left[ {{\text{Al}}{{\left( {{\text{OH}}} \right)}_{\text{4}}}} \right]\left( {{\text{aq}}} \right)}\limits_{{\text{Sod}}{\text{.meta - aluminate}}}$
$\mathop {{\text{Si}}{{\text{O}}_{\text{2}}}\left( {\text{s}} \right)}\limits_{{\text{Silica}}} + {\text{2NaOH}}\left( {{\text{aq}}} \right)\xrightarrow{{{\text{473 - 523K}}}}\mathop {{\text{N}}{{\text{a}}_{\text{2}}}{\text{Si}}{{\text{O}}_{\text{3}}}\left( {{\text{aq}}} \right)}\limits_{{\text{Sod}}{\text{.silicate}}} + {{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{l}} \right)$
The resulting solution is then filtered and cooled and its pH adjusted downward either by dilution or by neutralization with carbon dioxide when aluminium hydroxide gets precipitated. After this, the solution is seeded with a fresh sample of hydrated alumina when hydrated alumina gets precipitated leaving sodium silicate in the solution.
${\text{2Na}}\left[ {{\text{Al}}{{\left( {{\text{OH}}} \right)}_{\text{4}}}} \right]\left( {{\text{aq}}} \right){\text{ + 2C}}{{\text{O}}_{\text{2}}}\left( {\text{g}} \right) \to {\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}.{\text{x}}{{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{s}} \right) + 2{\text{NaHC}}{{\text{O}}_{\text{3}}}\left( {{\text{aq}}} \right)$
The hydrated alumina is then filtered and washed to get pure alumina.
${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}.{\text{x}}{{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{s}} \right)\xrightarrow{{{\text{1473K}}}}{\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\left( {\text{s}} \right) + {\text{x}}{{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{g}} \right)$
Similarly, silver and gold is also extracted by leaching by converting their ores into soluble complexes. The finely powdered argentite or the native gold is treated with a dilute solution of sodium or potassium cyanide while an air current is passed continuously. The silver and gold, thus get oxidized which then combine with cyanide ions forming their respective soluble cyanides while the impurities are filtered off.
${\text{4M + 8C}}{{\text{N}}^{\text{ - }}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O + }}{{\text{O}}_{\text{2}}} \to 4{\left[ {{\text{M}}{{\left( {{\text{CN}}} \right)}_{\text{2}}}} \right]^ - } + 4{\text{O}}{{\text{H}}^ - }$

Hence, the correct options for this question are A, B and D.

Note:
Another widely used method for concentration is the froth floatation process which is used for sulphide ores like zinc sulphide, copper pyrites and galena. It is based on the fact that the surfaces of sulphide ores are more favourably wetted by oils while the surfaces of gangue are more favourably wetted by water. Thus, during this process, the ore particles become lighter and rise to the surface along with the froth while the gangue particles become heavier and settle at the bottom of the tank. The froth is skimmed off and then allowed to collapse and dried to get the concentrated ore.