Zinc

Ores of Zinc

1. Zinc blende: ZnS
2. Calamine: ZnCO₃
3. Zincite: ZnO

Extraction of zinc from zinc blende

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. Roasting

The concentrated ore is roasted at 900°C in presence of air. Following changes takes place during roasting.

• Zinc sulphide is oxidized into its oxide.
  2ZnS + 3O₂ → 2ZnO + 2SO₂
• Volatile impurities are removed.

4. Reduction

Zinc oxide formed after roasting can be reduced by the vertical retort process. In this process, roasted ore is mixed with coke in the ratio of 2:1. The retort is heated by burning the producer gas (CO + N₂) at around 1400°C. On doing so, ZnO gets reduced to Zn by coke (C) as:

ZnO + C \rightarrow Zn + CO

The zinc vapours and carbon monoxide gas are carried in the condenser by the mild current of producer gas from the bottom of the furnace. Zinc obtained in the condenser is known as spelter zinc.

5. Purification or refining of Zinc

i. Distillation: The zinc spelter is subjected to distillation at 1000°C where impurities like iron (b.pt.= 3000°C) and lead (b.pt.= 1620°C) are left behind. Zinc (b.pt.=800°C) and cadmium (b.pt.=765°C) are collected as distillate. The distillate is distilled again at 800°C where cadmium is distilled off and zinc is left behind. This zinc is 99% pure.

ii. Electrolysis: The impure zinc is taken as anode and a thin sheet of pure zinc is taken as a cathode in a large tank. Acidified ZnSO₄ solution is taken as an electrolyte. On electrolysis, the anode dissolves and is deposited as pure zinc in the cathode.

ZnSO_{4}(aq) \rightleftharpoons Zn^{++} + SO_{4}\\ At\ anode: \underset{Impure}{Zn(s)} \rightarrow Zn^{++} + 2e^{-}\\ At\ cathode: Zn^{++} + 2e\rightarrow \underset{Pure}{Zn}

This obtained zinc is 99.5% pure.

Physical properties

1. It is blueish white solid when pure but tarnishes to grey when exposed to moist air.
2. It is good conductor of heat and electricity.
3. It melts at 420°C and boils at 907°C.
4. It is malleable and ductile at 100-150°C and brittle at 200°C.

Chemical properties

1. Action of air: Zinc is not affected by dry air but gets tarnished when exposed to moist air.

2Zn + H_{2}O + O_{2} + CO_{2}\rightarrow \underset{Basic\ zinc\ carbonate}{ZnCO_{3}.Zn(OH)_{2}}

When zinc is heated with air at 500°C, it burns with blueish white flame giving white woolly (dense white smoke) zinc oxide called philosopher’s wool or pompholyx.

2Zn + O_{2} \xrightarrow[]{500\degree C} \underset{Philosopher's\ wool}{2ZnO}

2. Action with acids: Zinc lies above hydrogen in the electrochemical series. Therefore, it produces H₂ gas from dilute, mineral and non-oxidizing acid. It shows amphoteric nature. Hence, it reacts with acid as well as the base.

1. Action with H₂SO₄:

\begin{align*}Zn + \underset{conc.}{2H_{2}SO_{4}} &\rightarrow ZnSO_{4} + SO_{2} + 2H_{2}O\\ Zn + \underset{dil.}{2H_{2}SO_{4}} &\rightarrow ZnSO_{4} +H_{2}\\ \end{align*}

2. Action with HCl

Zn + \underset{dil.\ or\ conc.}{HCl} \rightarrow ZnCl_{2} + H_{2}

3. Action with HNO₃

3. Action with alkali

Zn + 2NaOH \rightarrow \underset{Sodium\ zincate}{Na_{2}ZnO_{2}} + H_{2}

4. Displacement reactions: Zinc can displace less reactive metal from its salts.

\begin{align*} Zn + CuSO_{4} &\rightarrow ZnSO_{4} + Cu\\ Zn + 2AgNO_{3} &\rightarrow Zn(NO_{3})_{2} + 2Ag \end{align*}

Uses of zinc

1. It is used for making alloys.
2. It is used as a reducing and lab reagent.
3. It is used in the galvanization of iron.
4. It is used to prepare hydrogen gas.

*The process of depositing a thin layer of zinc over the surface of iron is called galvanization.

Compound of zinc

A. White vitriol: ZnSO₄.7H₂O

Preparation

i. From Zn, ZnO, ZnCO₃, Zn(OH)₂ with dil.H₂SO₄

\begin{align*} Zn + H2SO4 &\rightarrow ZnSO_{4} + H_{2}\\ ZnO + H_{2}SO_{4}&\rightarrow ZnSO_{4} + H_{2}O\\ ZnCO_{3} + H_{2}SO_{4} &\rightarrow ZnSO_{4} + H_{2}O + CO_{2}\\ Zn(OH)_{2} + H_{2}SO_{4} &\rightarrow ZnSO_{4} + 2H_{2}O\\ ZnSO_{4}(aq.) &\xrightarrow[]{crystallization} ZnSO_{4}.7H_{2}O \end{align*}

ii. From ZnS

ZnS + 2O_{2} \xrightarrow[]{below\ 800\degree C} ZnSO_{4}

Physical properties

• It is a white crystalline solid.
• It is efflorescent and loses water when exposed to air.
• It is highly soluble in water.

Chemical properties

i. Action of heat

\mathrm{ZnSO_4 \cdot 7H_2O}
\xrightarrow{70^\circ\mathrm{C}}
\mathrm{ZnSO_4 \cdot 6H_2O}
\xrightarrow{100^\circ\mathrm{C}}
\mathrm{ZnSO_4 \cdot H_2O}
\xrightarrow{300^\circ\mathrm{C}}
\mathrm{ZnSO_4}
\xrightarrow{800^\circ\mathrm{C}}
\mathrm{ZnO}

ii. Action of NaOH

\begin{align*} ZnSO_{4} + 2NaOH &\rightarrow Zn(OH)_{2} + Na_{2}SO_{4}\\ Zn(OH)_{2} + 2NaOH &\rightarrow Na_{2}ZnO_{2} + 2H_{2}O \end{align*}

iii. Action with barium sulphide

ZnSO_{4} + BaS \rightarrow \underset{\substack{Lithopone\\ (white\ pigment)}}{ZnS.BaSO_{4}}

iv. Formation of double salt

K_{2}SO_{4} + ZnSO_{4} + 6H_{2}O \rightarrow \underset{Double\ salt}{K_{2}SO_{4}.ZnSO_{4}.6H_{2}O}

v. Action with potassium ferrocyanide

2ZnSO_{4} + K_{4}[Fe(CN)_{6}] \rightarrow \underset{Zinc\ ferrocyanide\ (white\ ppt.)}{Zn_{2}[Fe(CN)_{6}] + K_{2}SO_{4}}

Uses

• Used as electrolyte.
• Used as an eye lotion.
• Used to check bleeding.
• Used to prepare lithopone, a white pigment.

*Rinman’s green: ZnO.CoO