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The main ores, Occurrence and extraction of zinc

Answer Text:
3. Zinc
Main ores;
(i). Zinc blende; ZnS
(ii). Calamine; `ZnCO_3`.
Extraction:
- Is done by electrolysis or reduction of its oxide using carbon.
Preliminary steps:
- The ore is first concentrated by froth floatation.
- The ore is roasted in air to convert it to the oxide.
Equations:
-From Zinc blende:
`2ZnS(s) + 3O_2(g) to 2ZnO(s) + 2SO_2(g)`
-From Calamine:
`ZnCO_3(s) to ZnO(s) + CO_2(g)`
- After obtaining the oxide the metal is extracted by either reduction or reduction:
(a). The reduction method.
- The oxide is mixed with coke and limestone and heated in a furnace.
Diagram: Furnace for zinc extraction by reduction:

- The limestone (`CaCO_3`) decomposes to liberate `CO_2` which is then reduced by coke to form carbon (II) oxide.
Equations:
`CaCO_3(s) to CaO(s) + CO_2(g)`
Then:
`CO_2 (g) + C(s) to 2CO (g)`
- The resultant carbon (II) oxide and coke are the reducing agents in the furnace, at about `1400^oC`.
- They reduce the oxide to the metal; which is liberated in vapour form.
Equations:
`ZnO(s) + C(s) to Zn (g) + CO (g)`
`ZnO(s) + CO(s) to Zn (g) + CO2 (g)`
- At the furnace
temperatures zinc exists in vapour form, and leaves at the top of the furnace.
- Liquid zinc being lighter settles above molten lead and is run off;
- The vapour is condensed in a spray of molten lead to prevent re-oxidation of zinc.
- The resultant zinc is 98- 99% pure and can be further purified by distillation.
- `SO_2` is a by-product and is the main source of pollution in the extraction of zinc.
- Usually it is channeled to a contact process plant for the manufacture of sulphuric acid.
- Alternatively it can be scrubbed off to prevent pollution of the environment.
- Less volatile impurities remain in the furnace.
- The silica impurities combine with the quicklime/ calcium oxide (CaO) from limestone to form calcium silicate.
Equation:
`CaO(s) + SiO(s) to CaSiO_3(s)`
- The silicates together with other less volatile impurities form slag, at the bottom of the furnace from where it is run off.
Summary: Flow chart and the extraction of zinc

(b). Electrolytic extraction of zinc.
- Zinc metal is obtained from the oxide via a series of
steps:
Step I: Preparation of electrolyte:
- The ZnO obtained from roasting the ore is converted to zinc sulphate by reacting it with dilute sulphuric (VI) acid.
Equation:
`ZnO(s) + H_2SO_4(aq) to ZnSO_4(aq) + H_2O(l)`
- Any lead (II) oxide impurity present in the zinc oxide reacts with the acid to form lead (II) sulphate.
Equation:
`PbO(s) + H_2SO_4(aq) to PbSO_4(s) + H_2O(l)`
- The insoluble lead (II) sulphate is then precipitated and separated by filtration;
- The zinc sulphate is then dissolved in water and the solution electrolysed.
Step II: The electrolytic process:
Electrolyte:
- Zinc (II) sulphate solution;
Ions present:
- `Zn^(2+)` and `H^(+)` as cations; and `SO4^(2-)` and `OH^(-)` as anions;
Cathode:
Lead containing 1% silver.
Anode:
- Aluminium sheets;
Chemical reactions:
Cathode:
Observations:
- Deposits of a grey solid.
Explanations:
- `Zn^(2+)` and `H^(+)` migrate to the
cathode.
- The `Zn^(2+)` are discharged in preference to `H^(+)`;
Reason:
- The cathode is relatively reactive. Thus since zinc is more reactive then hydrogen, its ions undergo reduction faster;
Equation:
`Zn^(2+) (aq) + 2^(e-) to Zn(s)`;
Note:
If graphite electrodes were used, hydrogen gas would have been evolved instead;
Anode:
Observations:
- Evolution of a colourless gas that relights a glowing splint;
Explanations:
- OH- and `SO4^(2-)` migrate to the cathode.
- The `OH^(-)` are discharged in preference to `SO4^(2-)`; giving off oxygen gas
Reason:
The OH- ions have a higher oxidation potential than `SO4^(2-)` and therefore easily giving electrons for reduction at the cathode
Equation:
`4OH^(-) to (aq) 2H_2O(l) + O_2(g) + 4^(e-)`
Note:
- Over 80% of zinc is extracted by the electrolytic method.
- Zinc extracted by the electrolytic method is much more pure.
Note: - Industrial plants that can be set up near the zinc extraction plant.
- Contact process plant, to make use of the `SO_2` byproduct.
- Lead accumulators factories, to utilize the zinc produced.
- Paper factory using, `SO_3` and hence `SO_2` in bleaching.
- Brass factory for alloying zinc and copper.
- Steel factory to use zinc in galvanization.