7.2 Zinc
🎯 Learning Outcomes
By the end of this topic, students should be able to:
- Describe the occurrence of zinc
- Describe the extraction of zinc
- Describe properties (with air, acid, alkali, displacement reaction)
- State the uses of zinc
- Explain the chemistry (preparation, properties, and uses) of white vitriol
1 Zinc — Occurrence & Ores
Occurrence
Zinc doesn’t exist in a free state, but exists as the following ores.
| Ore | Formula |
|---|---|
| Zinc Blende (most important ore) | ZnS |
| Calamine | ZnCO₃ |
| Zincite | ZnO |
| Franklinite | ZnO·Fe₂O₃ |
| Willimite | 2ZnO·SiO₂ |
2 Extraction — Crushing to Roasting
Zinc is extracted from zinc blende by the Carbon Reduction Process, which involves the following steps.
The ore is crushed in big jaw crushers and finely ground in ball mills.
- The powdered ore is mixed with water containing a little pine oil in a small tank.
- Air is blown through the mixture.
- ZnS particles stick to the froth and float to the top.
- Impurities (gangue) settle at the bottom and are removed.
- The concentrated ore is heated to about 900°C in air.
- Main reaction: the sulphide ore gets converted into oxide.
- During this process, some of the zinc sulphide is also converted into zinc sulphate.
- This later decomposes to give ZnO.
- Moisture is removed.
- Impurities such as S, P, As, etc. are removed as their respective volatile oxides.
3 Extraction — Reduction & Refining
- Carried out in a vertical retort furnace.
- Roasted ore (ZnO) + coke (2:1 ratio) is made into briquettes.
- The briquettes are fed into the retort through the vertical furnace’s charging door.
- The furnace is externally heated by producer gas (CO + N₂) at about 1400°C.
- The oxide is reduced to metallic zinc and is vaporized.
- The vapour (Zn + CO) is carried to the water-cooled condenser by the mild current of producer gas passed through the bottom of the furnace.
- The condensed zinc is called zinc spelter (purity ~97–98%).
- The zinc thus obtained contains Fe, Pb, Cd, As, Sb, etc. as impurities.
- Impure zinc is made an anode, and pure zinc a cathode, placed in an acidified ZnSO₄ solution.
- When electricity is passed, the metal from the anode dissolves into the solution.
- The pure metal is deposited at the cathode.
- The impurities remain at the bottom of the anode as anode mud.
- By this process, 99.5% pure zinc is obtained.
4 Properties of Zinc
- Bluish-white shining metal.
- Brittle at room temperature (becomes malleable at 100–150°C).
- Good conductor of heat and electricity.
4. Action of Air
Dry air — no reaction. Moist air — forms a protective basic carbonate layer:
On heating to 500°C, zinc burns with a greenish-blue flame, forming zinc oxide fumes. The white fluffy zinc oxide formed during burning is called Philosopher’s wool.
5. Action of Acid
With dilute hydrochloric acid and sulphuric acid, zinc gives hydrogen gas.
Concentrated sulphuric acid reacts with zinc to give sulphur dioxide.
With nitric acid, zinc gives a variety of products depending upon the concentration of the acid.
(a) Very dilute HNO₃ → ammonium nitrate
(b) Dilute HNO₃ → nitrous oxide (laughing gas)
(c) Moderately concentrated HNO₃ (1:1) → nitric oxide
(d) Concentrated HNO₃ → nitrogen peroxide
6. Action with Alkali
When zinc is boiled with concentrated caustic alkali like NaOH, hydrogen is liberated.
In aqueous solution, sodium zincate exists mainly as sodium tetrahydroxozincate(II), Na₂[Zn(OH)₄].
7. Displacement Reaction
Zinc displaces less electropositive metals like Cu, Ag, Au, etc. from their aqueous solutions.
5 Uses of Zinc
- Galvanizing iron (coating iron with zinc to prevent rusting) — used in roofing sheets and construction materials.
- Making alloys: brass, German silver, die casting alloys, etc.
- As an electrode in electrochemical cells/batteries.
- Production of zinc compounds like ZnO, ZnSO₄, etc.
- In dyes, drugs, and paints.
6 White Vitriol (ZnSO₄·7H₂O)
Preparation
From zinc metal (with dilute H₂SO₄)
From zinc compounds
When oxides, carbonates, or hydroxides of zinc are treated with dilute H₂SO₄, ZnSO₄ is formed.
From zinc blende
When zinc blende (ZnS) is roasted below 800°C with excess air, ZnSO₄ is formed.
The aqueous solution obtained by any of the above methods is evaporated till the crystallization point to obtain white vitriol.
Properties
- White crystalline solid, highly soluble in water.
- Efflorescent and loses water when exposed to air.
Action of Heat
When exposed to air above 30°C, it loses one water molecule. It loses six molecules of water at 100°C, and at 300°C anhydrous salt is formed. This anhydrous salt decomposes at 800°C to give SO₂ gas.
Action with Alkali
When aqueous ZnSO₄ is treated with NaOH solution, a white precipitate of Zn(OH)₂ is formed.
If excess NaOH is added, the precipitate dissolves due to the formation of sodium zincate.
Formation of Double Salts
When an equimolar solution of potassium sulphate and zinc sulphate is subjected to crystallization, a double salt is formed (also written as ZnK₂(SO₄)₂·6H₂O).
Action with Potassium Ferrocyanide
When white vitriol is treated with potassium ferrocyanide, a white precipitate of zinc ferrocyanide is formed.
Action with Barium Sulphide
Zinc sulphate reacts with barium sulphide to give lithopone (a white paint). It is not blackened by atmospheric hydrogen sulphide.
Uses of White Vitriol
- In medicine (eye drops and lotion).
- In preparation of white pigment, lithopone.
- As an electrolyte in the electrorefining of zinc.
- In the preparation of double salts and other zinc compounds.
📘 Chapter Summary
- Zinc does not occur free in nature — zinc blende (ZnS) is its most important ore.
- Extracted by the carbon reduction process: roasting converts ZnS to ZnO, which is reduced by coke in a vertical retort furnace.
- Electrolytic refining gives 99.5% pure zinc; impurities settle as anode mud.
- Reacts with air, acids, and alkali — it is amphoteric, dissolving in both acids and strong bases.
- Forms the important compound white vitriol, ZnSO₄·7H₂O.