Mercury

Extraction of mercury from cinnabar

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 the 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. Roasting and distillation in shaft furnace

The shaft furnace is a long cylindrical body having a cup and cone arrangement. This arrangement allows the charge (ore + coke +lime) to enter through the furnace but prevents mercury vapour from escaping. The shaft furnace contains a Y-shaped side tube that is passed through the cooler. The concentrated ore, coke and lime are mixed together and fed through cup and cone arrangement. The furnace is heated by the supply of air till it becomes red hot. In doing so, mercuric sulphide gets oxidized to mercuric oxide which decomposes into mercury.

\begin{align*} 2HgS + 3O_{2} &\xrightarrow[]{Roasting} 2HgO + 2SO_{2}\\ 2HgO &\xrightarrow[]{\Delta }\ \underset{Vapor}{2Hg} + O_{2} \end{align*}

Mercury vapour is passed to water-cooled Y-shaped tubes. These vapour gets condensed to give liquid mercury. The spent ore is taken out from the bottom of the furnace. The waste gas escapes out through the outlet of the condenser.

4. Purification

i. Filtration: Mercury is filtered through chamois leather where suspended oxide impurities are removed.

ii. Treatment with 5% nitric acid: Filtered mercury is passed through a long tube containing 5% nitric acid.

\begin{align*} 6Hg + 8HNO_{3} &\rightarrow 3Hg_{2}(NO_{3})_{2} + 2NO + 4H_{2}O\\ Hg_{2}(NO_{3})_{2} + &\underset{\substack{iron\ existing\\ in\ impure\\ mercury}}{Fe} \rightarrow Fe(NO_{3})_{2} + 2Hg \end{align*}

iii. Vaccum distillation: Mercury obtained from the above process still contains impurities like Ag, Au, etc. which is subjected to distillation under reduced pressure. Mercury gets distilled leaving behind the impurities.

Physical properties

1. It is silvery-white liquid metal. It is also called quicksilver.
2. It is the heaviest liquid known.
3. It melts at -39°C and boils at 357°C.
4. It is a good conductor of heat and electricity.
5. It’s vapour is highly poisonous.
6. It dissolves in many metals (except Fe and Pt) to form an amalgam. So, it is stored in iron vessels.

Chemical properties

1. Action with air: Mercury does not react with dry or moist air in ordinary conditions. When it is heated up to 350°C in the air, it slowly gets oxidized to mercuric oxide which decomposes on further heating.

\begin{align*} 2Hg + O_{2} &\xrightarrow[]{350\degree C} 2HgO\\ 2HgO &\xrightarrow[]{above\ 350\degree C}2Hg + O_{2} \end{align*}

2. Action with ozone: When mercury comes in contact with ozone, it is oxidized to mercurous oxide (Hg₂O), so that mercury loses its meniscus shape and leaves a black mark (like a tail) on the glass. This special property of mercury is called the tailing of mercury.

2Hg + O_{3} \rightarrow Hg_{2}O + O_{2}

3. Action with halogens: Hot mercury combines with halogens to give respective halides.

Hg + Cl_{2} \rightarrow \underset{mercuric\ chloride}{HgCl_{2}}

4. Action with sulphur: When mercury is warmed with sulphur, a black mass of mercuric sulphide is formed.

Hg + S \rightarrow HgS

5. Action with acids

• It doesn’t react with dilute and conc. HCl.
• It does not react with dil.H₂SO₄ but reacts with hot and conc. H₂SO₄.

Hg + \underset{conc.}{H_{2}SO_{4}} \rightarrow HgSO_{4} + 2H_{2}O + SO_{2}

• Action with HNO₃

\begin{align*} 6Hg + \underset{dil.}{8HNO_{3}} &\rightarrow 3Hg_{2}(NO_{3})_{2} + 2NO + 4H_{2}O\\ Hg + \underset{conc.}{4HNO_{3}} &\rightarrow Hg(NO_{3})_{2} + 2NO_{2} + 2H_{2}O \end{align*}

6. Action with aqua-regia: Mercury reacts with aqua-regia ( mixture of conc. HNO3 and conc. HCl in the ratio of 1:3) to form mercuric chloride.

\begin{align*} \underset{conc.}{HNO_{3}} + \underset{conc.}{3HCl} &\rightarrow \underset{\substack{(Nitrosyl\\ chloride)}}{NOCl} + 2H_{2}O +2[Cl]\\ Hg + 2[Cl] &\rightarrow \underset{Mercuric\ chloride}{HgCl_{2}} \end{align*}

*Aqua regia is also called kingly water or royal water.

7. Action with metals: Almost all metals except Fe and Pt dissolve in mercury and form amalgams.

\begin{align*}Na + Hg &\rightarrow \underset{sodium\ amalgum}{NaHg}\\ Zn + Hg &\rightarrow \underset{zinc\ amalgum}{ZnHg} \end{align*}

These amalgams are used as reducing agents.

Uses of mercury

• In the manufacture of caustic soda
• In the metallurgy of gold and silver.
• In mercury vapour lamps.
• In making dental plastic.
• It is used in silvering mirror.

Compounds of mercury

A. Calomel or mercurous chloride: Hg₂Cl₂

Preparation

i. From mercuric chloride

\begin{align*} HgCl_{2} + Hg &\rightarrow Hg_{2}Cl_{2}\\ HgCl_{2} + SnCl_{2} &\rightarrow Hg_{2}Cl_{2} + SnCl_{4} \end{align*}

ii. From mercurous nitrate

Hg_{2}(NO_{2})_{2} + 2HCl \rightarrow Hg_{2}Cl_{2} + 2HNO_{3}

iii. From mercuric sulphate

HgSO_{4} +2NaCl + Hg \rightarrow Hg_{2}Cl_{2} + Na_{2}SO_{4}

Physical properties

• It is a white powder.
• It is insoluble in water.
• It sublimes above 350°C.

Chemical properties

i. Action of heat

Hg_{2}Cl_{2} \xrightarrow[]{\Delta} HgCl_{2} + Hg

ii. Action with aqua regia

\begin{align*} \underbrace{\underset{conc}{3HCl} + \underset{conc.}{HNO_{3}}}_{aqua\ regia} &\rightarrow \underset{\substack{Nitrosyl\\ chloride}}{NOCl} + 2H_{2}O + 2[Cl]\\ Hg_{2}Cl_{2} + 2[Cl] &\rightarrow 2HgCl_{2} \end{align*}

iii. Action with stannous chloride

Hg_{2}Cl_{2} + SnCl_{2} \rightarrow 2Hg + SnCl_{4}

iv. Action with ammonia

Hg_{2}Cl_{2} + 2NH_{3} \rightarrow \underbrace{Hg + Hg(NH_{2})Cl}_{Black} + NH_{4}Cl

v. Action with NaOH

\begin{align*} Hg_{2}Cl_{2} + NaOH &\rightarrow Hg_{2}O + NaCl + H_{2}O\\ Hg_{2}O &\rightarrow HgO + Hg \end{align*}

Uses

• As a purgative drug.
• To make electrodes.
• As fungicides.

B. Corrosive sublimate or mercuric chloride: HgCl₂

Preparation

i. By the action of calomel and aqua regia

\begin{align*} \underbrace{\underset{conc}{3HCl} + \underset{conc.}{HNO_{3}}}_{aqua\ regia} &\rightarrow \underset{\substack{Nitrosyl\\ chloride}}{NOCl} + 2H_{2}O + 2[Cl]\\ Hg_{2}Cl_{2} + 2[Cl] &\rightarrow 2HgCl_{2} \end{align*}

ii. From mercury

Hg + Cl_{2} \rightarrow HgCl_{2}

iii. From mercuric sulphate

HgSO_{4} + 2NaCl \rightarrow HgCl_{2} + Na_{2}SO_{4}

iv. From mercuric oxide

HgO + 2HCl \rightarrow HgCl_{2} + H_{2}O

Physical properties

• It is a white crystalline solid.
• it is soluble in hot water.
• It is poisonous.

Chemical properties

i. Action of NaOH

HgCl_{2} + NaOH \rightarrow HgO + 2NaCl + H_{2}O

ii. Action with SnCl₂

\begin{align*} 2HgCl_{2} + SnCl_{2} \rightarrow SnCl_{4} + Hg_{2}Cl_{2}\\ Hg_{2}Cl_{2} + SnCl_{2} \rightarrow SnCl_{4} + 2Hg \end{align*}

iii. Action of heat

HgCl_{2} \rightarrow Hg + Cl_{2}

iv. Action with KI

\begin{align*} HgCl_{2} + 2KI &\rightarrow \underset{\substack{Scarlet\\ ppt.}}{HgI_{2}}+ 2KCl\\ HgI_{2} + 2KI &\rightarrow \underset{Nessler's\ reagent}{K_{2}[HgI_{4}]} \end{align*}

v. Action with ammonia

HgCl_{2} + 2NH_{3} \rightarrow Hg(NH_{2})Cl + NH_{4}Cl

Uses

• As fungicides.
• In preparation for Nessler’s reagent and calomel.
• As an antidote (medicine against poison).
• As skin and wood preservative.