Extraction of Zinc — Revision Notes | Vyyuha Knowledge Platform

⚡ 30-Second Revision

Ores: — Zinc blende (ZnS), Calamine (

ext{ZnCO}_3

).

Concentration: — Froth flotation for ZnS.

Conversion to Oxide:

- Roasting (for ZnS): $ext{2ZnS(s) + 3O}_2\text{(g)} xrightarrow{\text{heat}} \text{2ZnO(s) + 2SO}_2\text{(g)}$ - Calcination (for $ext{ZnCO}_3$ ): $ext{ZnCO}_3\text{(s)} xrightarrow{\text{heat}} \text{ZnO(s) + CO}_2\text{(g)}$

Reduction: — Carbon reduction (pyrometallurgy):

ext{ZnO(s) + C(s)} xrightarrow{\text{1100-1200}^circ\text{C}} \text{Zn(g) + CO(g)}

Key Feature: — Zinc obtained as vapor due to low boiling point (

907^circ\text{C}

) at reduction temperature.

Refining:

- Fractional Distillation (separates based on b.p. differences). - Electrolytic Refining (hydrometallurgy): - Leaching: $ext{ZnO + H}_2\text{SO}_4 \to \text{ZnSO}_4\text{ + H}_2\text{O}$ - Cathode: $ext{Zn}^{2+}\text{ + 2e}^- \to \text{Zn}$ - Anode: $ext{2H}_2\text{O} \to \text{O}_2\text{ + 4H}^+\text{ + 4e}^-$

2-Minute Revision

Zinc is primarily extracted from its sulfide ore, zinc blende (ZnS). The process begins with concentrating the ore using froth flotation. The concentrated ZnS is then subjected to roasting, where it's heated in air to convert it into zinc oxide (ZnO) and release sulfur dioxide.

If the ore is calamine ( $ext{ZnCO}_3$ ), it undergoes calcination to yield ZnO. The zinc oxide is subsequently reduced to metallic zinc using carbon (coke) as a reducing agent at high temperatures ( $1100-1200^circ\text{C}$ ).

A critical point here is that zinc is obtained as a vapor because the reduction temperature is above its boiling point ( $907^circ\text{C}$ ), necessitating rapid condensation. The crude zinc, known as spelter, is then refined.

Common refining methods include fractional distillation, which separates impurities based on boiling point differences, and electrolytic refining. Electrolytic refining involves leaching ZnO with sulfuric acid to form zinc sulfate solution, followed by electrolysis where pure zinc deposits at the cathode and oxygen evolves at the anode, regenerating sulfuric acid.

This method yields very high-purity zinc.

5-Minute Revision

The extraction of zinc is a multi-step metallurgical process, predominantly from zinc blende (ZnS). The initial step is ore concentration, typically achieved by froth flotation for ZnS, leveraging its hydrophobic nature. The finely crushed ore is mixed with water, frothing agents, and collectors, and air is blown through to float the ZnS particles as froth.

Next, the concentrated ZnS undergoes roasting, a pyrometallurgical process where it's heated strongly in excess air at $900-1000^circ\text{C}$ . The reaction is: $ext{2ZnS(s) + 3O}_2\text{(g)} xrightarrow{\text{heat}} \text{2ZnO(s) + 2SO}_2\text{(g)}$ .

The byproduct $ext{SO}_2$ is usually converted to $ext{H}_2\text{SO}_4$ . If the ore is calamine ( $ext{ZnCO}_3$ ), it undergoes calcination (heating in absence of air): $ext{ZnCO}_3\text{(s)} xrightarrow{\text{heat}} \text{ZnO(s) + CO}_2\text{(g)}$ .

Both processes yield zinc oxide (ZnO), which is the form suitable for reduction.

The reduction of ZnO is primarily done by carbon (coke) in a pyrometallurgical process at $1100-1200^circ\text{C}$ : $ext{ZnO(s) + C(s)} xrightarrow{\text{1100-1200}^circ\text{C}} \text{Zn(g) + CO(g)}$ .

A key aspect is that zinc is produced as a vapor because the reduction temperature is well above its boiling point ( $907^circ\text{C}$ ). This zinc vapor must be rapidly condensed to prevent re-oxidation.

The thermodynamic feasibility of this reduction is explained by the Ellingham diagram, where the $Delta G^circ$ line for $ext{C} \to \text{CO}$ falls below that of $ext{Zn} \to \text{ZnO}$ above approximately $1000^circ\text{C}$ .

The crude zinc, known as spelter, contains impurities and requires refining. Fractional distillation can separate zinc from higher boiling point impurities like lead and lower boiling point impurities like cadmium.

However, electrolytic refining is the preferred method for high-purity zinc. In this hydrometallurgical route, roasted ZnO is leached with dilute sulfuric acid to form $ext{ZnSO}_4$ solution: $ext{ZnO(s) + H}_2\text{SO}_4\text{(aq)} \to \text{ZnSO}_4\text{(aq) + H}_2\text{O(l)}$ .

After purification of the solution, it's electrolyzed. At the cathode, $ext{Zn}^{2+}\text{(aq) + 2e}^- \to \text{Zn(s)}$ occurs, depositing pure zinc. At the anode, $ext{2H}_2\text{O(l)} \to \text{O}_2\text{(g) + 4H}^+\text{(aq) + 4e}^-$ occurs, regenerating sulfuric acid for reuse.

This method yields zinc of very high purity.

Prelims Revision Notes

Zinc Extraction: Key Facts for NEET UG

1. Ores of Zinc:

* Primary Ore: Zinc blende (Sphalerite), ZnS (sulfide ore). * Other Ores: Calamine ( $ext{ZnCO}_3$ , carbonate ore), Zincite ( $ext{ZnO}$ , oxide ore).

2. Concentration of Ore:

* For ZnS (sulfide ore): Froth Flotation method is used. * Principle: Differential wetting properties (sulfide particles are hydrophobic, gangue is hydrophilic). * Reagents: Collector (e.g., potassium ethyl xanthate), Frothing agent (e.g., pine oil), Depressant (e.g., NaCN for selective separation).

3. Conversion to Oxide:

* Roasting (for ZnS): Heating in excess air. * Reaction: $ext{2ZnS(s) + 3O}_2\text{(g)} xrightarrow{\text{900-1000}^circ\text{C}} \text{2ZnO(s) + 2SO}_2\text{(g)}$ * Byproduct $ext{SO}_2$ is used for $ext{H}_2\text{SO}_4$ production. * **Calcination (for $ext{ZnCO}_3$ ):** Heating in absence of air. * Reaction: $ext{ZnCO}_3\text{(s)} xrightarrow{\text{heat}} \text{ZnO(s) + CO}_2\text{(g)}$

4. Reduction of Zinc Oxide:

* Reducing Agent: Carbon (coke). * Process: Pyrometallurgical reduction at high temperatures. * Reaction: $ext{ZnO(s) + C(s)} xrightarrow{\text{1100-1200}^circ\text{C}} \text{Zn(g) + CO(g)}$ * Key Point: Zinc is obtained as vapor because the reduction temperature ( $1100-1200^circ\text{C}$ ) is significantly above zinc's boiling point ( $907^circ\text{C}$ ).

The vapor is rapidly condensed. * Thermodynamics (Ellingham Diagram): Carbon becomes a more effective reducing agent for ZnO above approximately $1000^circ\text{C}$ (crossover point where $Delta G^circ$ for $ext{C} \to \text{CO}$ becomes more negative than for $ext{Zn} \to \text{ZnO}$ ).

The reaction is endothermic.

5. Refining of Crude Zinc (Spelter):

* Spelter: Impure zinc obtained from reduction. * Methods: * Fractional Distillation: Separates based on boiling point differences. Removes higher boiling impurities (e.g., Pb, Fe) and lower boiling impurities (e.

g., Cd). * Electrolytic Refining (Hydrometallurgical): Produces high-purity zinc (99.99%). * Leaching: Roasted ZnO is dissolved in dilute $ext{H}_2\text{SO}_4$ : $ext{ZnO(s) + H}_2\text{SO}_4\text{(aq)} \to \text{ZnSO}_4\text{(aq) + H}_2\text{O(l)}$ * Purification: Removal of impurities (Fe, Cu, Cd) from $ext{ZnSO}_4$ solution.

6. Common Misconceptions:

* Roasting vs. Calcination: Roasting is for sulfides in air; Calcination is for carbonates/hydroxides in absence of air. * Direct reduction of ZnS: Not feasible thermodynamically. * Zinc's state after reduction: Vapor, not liquid.

Vyyuha Quick Recall

To remember the main steps of Zinc Extraction: Can Really Reduce Raw Zinc?

Concentration (Froth Flotation)

Roasting (Converts ZnS to ZnO)

Reduction (ZnO to Zn vapor with Carbon)

Refining (Electrolytic or Fractional Distillation)

Zinc (The final product!)