Write short notes on: [2.5+2.5] i) Carbon reduction process ii) Froth floatation process.

i) Carbon Reduction Process

The carbon reduction process is a metallurgical method used to extract metals from their oxides by using carbon (usually in the form of coke) as a reducing agent. This process is primarily applied to extract metals that are less reactive than carbon, such as iron, zinc, tin, and lead.

Principle

The process relies on the thermodynamic stability of metal oxides and carbon monoxide (CO). Carbon reacts with oxygen at high temperatures to form carbon monoxide, which then reduces the metal oxide to its pure metal. The general reaction can be represented as:

Fe₂O₃+3 COHigh temperature (≈1200°C)2 Fe+3 CO₂

$$\mathrm{Metal}\mathrm{Oxide}+\mathrm{Carbon}\to \mathrm{Metal}+\mathrm{Carbon}\mathrm{Monoxide}$$

Steps Involved

1. Roasting of Ore (if sulfide ore):

   • If the ore is a sulfide (e.g., zinc blende, ZnS), it is first roasted in the presence of oxygen to convert it into an oxide: $$2\mathrm{ZnS}+3\mathrm{O}{}_2\to 2\mathrm{ZnO}+2\mathrm{SO}{}_2$$

2. Reduction with Carbon (Coke):

   • The metal oxide is then heated with coke (impure carbon) in a blast furnace (for iron) or a reverberatory furnace (for other metals). Carbon monoxide (CO), produced from the reaction of carbon with oxygen, acts as the reducing agent: $$\mathrm{C}+\mathrm{O}{}_2\to \mathrm{CO}{}_2$$ $$\mathrm{CO}{}_2+\mathrm{C}\to 2\mathrm{CO}$$ The metal oxide is then reduced by CO: $$\mathrm{ZnO}+\mathrm{CO}\to \mathrm{Zn}+\mathrm{CO}{}_2$$

3. Separation of Impurities:

   • The molten metal sinks to the bottom of the furnace, while impurities (slag) float on top and are removed.

Example: Extraction of Iron (Blast Furnace Process)

• Raw Materials: Hematite (Fe₂O₃), coke (C), limestone (CaCO₃), and hot air.
• Key Reactions:
  • Formation of CO (reducing agent): $$\mathrm{C}+\mathrm{O}{}_2\to \mathrm{CO}{}_2$$ $$\mathrm{CO}{}_2+\mathrm{C}\to 2\mathrm{CO}$$
  • Reduction of iron oxide: $$\mathrm{Fe}{}_2\mathrm{O}{}_3+3\mathrm{CO}\to 2\mathrm{Fe}+3\mathrm{CO}{}_2$$
• Product: Molten iron (pig iron) is collected at the bottom.

Limitations

• This process is not suitable for highly reactive metals (e.g., aluminum, sodium, calcium) because carbon cannot reduce their oxides.
• It produces carbon dioxide, contributing to environmental pollution.

CO₂CO₂

---

ii) Froth Flotation Process

The froth flotation process is a physical separation method used to concentrate sulfide ores (e.g., galena PbS, sphalerite ZnS, chalcopyrite CuFeS₂) from gangue (unwanted impurities). It relies on the difference in wettability between the ore and gangue particles.

Principle

• Hydrophobic particles (ore) attach to air bubbles and float to the surface, forming a froth.
• Hydrophilic particles (gangue) remain wetted by water and sink to the bottom.

Steps Involved

1. Crushing and Grinding:

   • The ore is crushed to a fine powder (~0.2 mm) to liberate the mineral particles from gangue.

2. Pulp Preparation:

   • The ground ore is mixed with water to form a slurry (pulp).

3. Addition of Reagents:

   • Collectors (e.g., xanthates, thiols): Make ore particles hydrophobic.
   • Frothers (e.g., pine oil, cresols): Stabilize the froth.
   • Depressants (e.g., NaCN, Na₂CO₃): Prevent certain minerals from floating (e.g., ZnS in the presence of PbS).
   • Activators (e.g., CuSO₄): Help certain minerals (e.g., sphalerite) to float.

4. Flotation:

   • Air is blown through the pulp, creating bubbles.
   • Hydrophobic ore particles attach to bubbles and rise to the surface, forming a froth layer.
   • The froth is skimmed off, dried, and roasted to remove sulfur.

5. Separation of Concentrate:

   • The collected froth contains concentrated ore, while the remaining pulp (tailings) is discarded.

Example: Concentration of Galena (PbS)

• Collector: Potassium ethyl xanthate (C₂H₅OCSSK).
• Frother: Pine oil.
• Process:
  • PbS particles become hydrophobic and attach to air bubbles.
  • Froth is skimmed, dried, and roasted to obtain lead oxide (PbO) or lead sulfide (PbS) concentrate.

Advantages

• Highly selective for sulfide ores.
• Economical for low-grade ores.
• Environmentally friendly compared to smelting (less energy-intensive).

Limitations

• Not suitable for oxide ores (e.g., bauxite, hematite).
• Requires careful reagent control to avoid contamination.
• Produces tailings, which may contain toxic chemicals.

---

2 ZnS+3 O₂Roasting (air, 900°C)2 ZnO+2 SO₂