Question

Explain the role of each of the following.
A) ${\text{NaCN}}$ in the extraction of silver.
B) ${\text{Si}}{{\text{O}}_{\text{2}}}$ in the extraction of copper.

Answer 1

Hint: Sodium cyanide is used for leaching the roasted ore of silver. Silicon dioxide is used to remove the impurity formed during the oxidation of copper ore.

Complete step by step answer:
A) The roasted ore of silver is leached using a solution of sodium cyanide. The process in which the valuable metals are converted to their soluble salts and the impurities remain insoluble is known as leaching. The leaching process of silver metal by sodium cyanide is carried out in the presence of oxygen.
The role of sodium cyanide in this process of leaching is to dissolve the silver metal to form a silver cyanide complex. The reaction is as follows:
${\text{4Ag + 8NaCN + 2}}{{\text{H}}_2}{\text{O + }}{{\text{O}}_2}{\text{ }} \to {\text{ 4Na}}\left[ {{\text{Ag}}{{\left( {{\text{CN}}} \right)}_2}} \right]{\text{ + 4NaOH}}$
The pure silver metal can be obtained from the silver cyanide complex by a displacement reaction with zinc metal. The reaction is as follows:
${\text{2Na}}\left[ {{\text{Ag}}{{\left( {{\text{CN}}} \right)}_2}} \right]{\text{ + Zn }} \to {\text{ N}}{{\text{a}}_2}\left[ {{\text{Zn}}{{\left( {{\text{CN}}} \right)}_4}} \right]{\text{ + 2Ag}}$
Thus, the role of ${\text{NaCN}}$ in the extraction of silver is leaching the roasted ore of silver.
B) The most common ore of copper is copper matte. Copper matte contains copper sulphide along with small amounts of iron sulphide. The amount of copper sulphide is 80 - 95 % and the remaining amount is iron sulphide.
The extraction of copper from copper matte is done in a blast furnace. During the extraction, copper matte is added to the blast furnace along with powdered coke and silicon dioxide.
The oxidation of copper matte occurs in the blast furnace and as a result cuprous oxide and iron oxide are produced.
The role of silicon dioxide is to remove the ferrous oxide by combining with it. Iron oxide combines with silicon dioxide and forms iron silicate. The reaction of iron oxide with silicon dioxide is as follows:
${\text{FeO + Si}}{{\text{O}}_2}{\text{ }}\xrightarrow{\Delta }{\text{ FeSi}}{{\text{O}}_2}$
The iron silicate $\left( {{\text{FeSi}}{{\text{O}}_2}} \right)$ formed in the reaction is known as slag.

Note: The most common ore of silver is argentite or silver glance and that of copper is copper matte. Potassium cyanide can be used instead of sodium cyanide.