Silver

Extraction of silver by cyanide process

1. Crushing and pulverization

Big lumps of ores are crushed using jaw crushers to get crushed ore which is pulverized using a pulverizer or stamp mill to get powdered ore.

2. Concentration by froth floatation process

The powdered ore is taken in a tank containing water and a small amount of pine oil. The mixture is heated by a blast of air. Impurities are wetted by water and get collected at the bottom of the tank. Ore particles are wetted by oil and come to the surface as froth. The froth is skimmed off to collect concentrated ore.

3. Cyanide treatment

The concentrated ore is mixed with sodium cyanide solution using a current of air. Then the solution of sodium argentocyanide is formed.

Ag_{2}S + 4NaCN \rightarrow \underset{\substack{Sodium\\ argentocyanide}}{2Na[Ag(CN)_{2}]} + Na_{2}S

The reaction is reversible. The air blown oxidizes sodium sulphide to sodium sulphate and drives this reaction to forward direction.

Na_{2}S + 2O_{2} \rightarrow Na_{2}SO_{4}

If the ore is horn silver, the following reaction occurs:

AgCl + 2NaCN \rightarrow Na[Ag(CN)_{2}] + NaCl

4. Precipitation of silver

The solution obtained above is filtered and the filtrate is treated with zinc where Ag is precipitated in the dark amorphous powder.

2Na[Ag(CN)_{2}] + Zn \rightarrow 2Ag + Na_{2}[Zn(CN)_{4}]

This silver is separated by filtration and fused with borax or KNO₃ to get a compact mass of silver known as spongy silver.

5. Purification

Silver thus obtained is purified by electrolytic refining. Impure silver is made anode and pure silver is made cathode. Acidified AgNO₃ solution is used as an electrolyte. On electrolysis, silver deposits from the anode and gets collected in the cathode.

At\ anode:\ Ag \rightarrow Ag^{+} + e^{-}\\ At\ cathode:\ Ag^{+} + e^{-} \rightarrow \underset{Pure}{Ag}

Uses of silver

• To prepare coins and ornaments.
• In photography and electrical purpose.
• To prepare electrode and alloy.

Compounds of silver

A. Lunar caustic or silver nitrate: AgNO₃

Preparation

i. By the action of nitric acid on silver:

\begin{align*}Ag + \underset{conc.}{HNO_{3}} &\rightarrow AgNO_{3} + H_{2}O + NO_{2}\\ Ag + \underset{dil.}{HNO_{3}} &\rightarrow AgNO_{3} + H_{2}O + NO \end{align*}

Physical properties

• It is a white crystalline solid.
• It is soluble in water and alcohol.

Chemical properties

i. Action of heat:

\begin{align*} 2AgNO_{3} &\xrightarrow{450\degree C}2AgNO_{2} + O_{2}\\ 2AgNO_{3} &\xrightarrow{above\ 450\degree C}2Ag + 2NO_{2} + O_{2} \end{align*}

ii. Decomposition by organic matter: If silver nitrate comes in contact with the skin in presence of sunlight, a permanent black stain is formed on the skin. This property has been used in the preparation of indelible (that cannot be removed) ink which is used during elections for marking the voters who have cast the vote.

AgNO_{3} \xrightarrow[nail\ or\ skin]{light}2Ag + 2NO_{2} + O_{2}

iii. Action with NaOH:

2AgNO_{3} + 2NaOH \rightarrow Ag_{2}O + 2NaNO_{3} + H_{2}O

iv. Action with NH₄OH:

2AgNO_{3} + NH_{4}OH \rightarrow \underset{Diamine\ silver\ nitrate}{2[Ag(NH_{3})_{2}]NO_{3}} + 3H_{2}O

v. Action with KCN:

\begin{align*} AgNO_{3} + KCN &\rightarrow AgCN + KNO_{3}\\ AgCN + \underset{excess}{KCN} &\rightarrow \underset{potassium\ argentocyanide}{K[Ag(CN)_{2}]} \end{align*}

vi. Displacement reaction:

Cu + AgNO_{3} \rightarrow Cu(NO_{3})_{2} + 2Ag

vii. Action with halides, sulphate, sulphide:

\begin{align*} AgNO_{3} + Cl^{-} &\rightarrow AgCl(white\ ppt.) + NO_{3}^{-}\\ AgNO_{3} + Br^{-} &\rightarrow AgBr(pale\ yellow\ ppt.) + NO_{3}^{-}\\ AgNO_{3} + I^{-} &\rightarrow AgI(pale\ yellow\ ppt.) + NO_{3}^{-}\\ 2AgNO_{3} + SO4^{--} &\rightarrow Ag_{2}SO_{4}(white\ ppt.) + 2NO_{3}^{-}\\ 2AgNO_{3} + S^{--} &\rightarrow Ag_{2}S(black\ ppt.) + 2NO_{3}^{-} \end{align*}

Uses

• In silvering of mirror.
• To prepare indelible ink.
• In photography.
• To prepare Tollen’s reagent: [Ag(NH₃)₂]OH

B. Horn silver or silver chloride: AgCl

Preparation

By the action of sodium chloride and silver nitrate.

NaCl + AgNO_{3} \rightarrow AgCl + NaNO_{2}

Physical properties of horn silver

• It is white solid.
• It is insoluble in water.

Chemical properties of horn silver

i. Action with ammonia:

AgCl + 2NH_{4}OH \rightarrow [Ag(NH_{3})2Cl] + 2H_{2}O

ii. Action with KCN:

AgCl + 2KCN \rightarrow K[Ag(CN)_{2}] + KCl

iii. Action with sodium thiosulphate:

AgCl + 2Na_{2}S_{2}O_{3} \rightarrow Na_{3}[Ag(S_{2}O_{3})_{2}] + NaCl

Uses

• In photography.

Frosted silver

The ornaments and articles of silver usually contain copper from 6-7%. To give a pure white appearance to the article, these are heated in the air so that the copper near the surface is oxidized while silver remains as such. The oxide formed on the surface is dissolved in dilute H₂SO₄ and a layer of pure silver is left on the surface. This process is called frosting of silver and silver obtained is called frosted silver.

Purity of gold

Pure gold is very soft and cannot be used for making jewellery. So, it is generally alloyed with silver or copper to make it harder and modify its colour. The purity of gold in jewellery is expressed in carat. It implies the number of parts by weight of gold in 24 parts by weight of the alloy. Pure gold is 24 carat. 18 carat gold means it contains 18 parts by weight of pure gold in 24 parts by weight of the alloy. Most of the jewellery is made up of 22 carat gold.