Metallurgy and Industrial Extraction of Zinc (Zn)

Natural Occurrence & Major Ores

Zinc is a moderately reactive metal and, therefore, is not found in its native (free) state in nature. It primarily occurs in combined forms as various ores.

Principal Ores of Zinc

1. Zinc Blende (Sphalerite): ZnS (Sulphide ore) – This is the most significant and abundant ore for industrial zinc extraction. It often occurs mixed with galena (PbS) and iron pyrites (FeS₂).
2. Calamine (Smithsonite): ZnCO₃ (Carbonate ore) – An important secondary ore.
3. Zincite: ZnO (Oxide ore) – Less common but also a source of zinc.

Concentration of the Ore

The initial step in metallurgy is the concentration of the ore, which involves removing gangue (undesired rocky material) from the ore. The method chosen depends on the nature of the ore and the gangue.

For Zinc Blende (ZnS) – Froth Flotation Process

The sulphide ore, Zinc Blende, is concentrated using the froth flotation method due to its hydrophobic nature and the ability to selectively separate it from other sulphides or gangue.

1. Principle: This process separates finely ground sulphide ore particles from gangue by preferentially wetting the ore with oil (e.g., pine oil, xanthates) and the gangue with water. Air is then blown through the mixture to create froth.
2. Procedure:
   • The finely crushed ore is mixed with water to form a slurry.
   • Collectors: Substances like pine oil, fatty acids, or xanthates are added. They selectively attach to the sulphide ore particles, making them water-repellent (hydrophobic).
   • Frothers: Chemicals like cresols or aniline are added to stabilize the froth.
   • Depressants: Sodium cyanide (NaCN) or sodium hydroxide (NaOH) can be added when ZnS is associated with PbS. NaCN selectively prevents ZnS from coming with the froth, allowing PbS to be floated first. Later, ZnS can be floated.
   • Air is vigorously blown into the mixture, creating froth that carries the ore particles to the surface, while the gangue settles down. The froth is then skimmed off.

Reduction to Crude Metal

After concentration, the ore is converted into a form suitable for reduction, typically an oxide, and then reduced to crude metal.

1. Conversion of Ore to Zinc Oxide (ZnO)

Both sulphide and carbonate ores are first converted to zinc oxide.

a. Roasting (ZnS ore)

• Process: Zinc Blende (ZnS) is heated strongly in the presence of excess air at high temperatures (typically 900-1000°C) in a fluidized bed roaster. This converts the sulphide to oxide and sulfur dioxide gas.
• Reaction: 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g)
• Note: The SO₂ produced is generally captured and used for the manufacture of sulfuric acid (H₂SO₄).

b. Calcination (ZnCO₃ ore)

• Process: Calamine (ZnCO₃) ore is heated in the absence of air at 800-900°C in rotary kilns. This decomposes the carbonate into zinc oxide and carbon dioxide.
• Reaction: ZnCO₃(s) → ZnO(s) + CO₂(g)

2. Reduction of Zinc Oxide (ZnO) to Crude Zinc

There are two primary methods for reducing zinc oxide to metallic zinc:

a. Smelting (Carbon Reduction Process)

• Principle: Zinc has a relatively low boiling point (907°C), which is lower than the reduction temperature required (1100-1400°C). Therefore, zinc is obtained as a vapor and must be rapidly condensed.
• Process:
  • Concentrated ZnO (from roasting or calcination) is mixed with powdered coke (carbon) and heated in vertical retorts or electrothermic furnaces.
  • Reaction: ZnO(s) + C(s) → Zn(g) + CO(g) (at 1100-1400°C)
  • The zinc vapor produced is quickly condensed in specialized condensers to form liquid zinc (known as “spelter”). Rapid cooling is essential to prevent re-oxidation of zinc or the formation of zinc dust.
• Crude Metal: The spelter obtained typically contains impurities like lead, cadmium, and iron.

b. Electrolytic Process (Hydrometallurgy)

This method is increasingly popular, especially for lower-grade ores or when high-purity zinc is required.

1. Leaching: The roasted or calcined ZnO is leached with dilute sulfuric acid to form zinc sulfate solution.
   • Reaction: ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l)
2. Purification of ZnSO₄ Solution: The solution must be rigorously purified to remove impurities (e.g., Fe, Cu, Cd, Ni, Co) which can interfere with the electrolysis or co-deposit with zinc. This is typically done by adding zinc dust, which displaces more electropositive metals from the solution.
   • Example: CuSO₄(aq) + Zn(s) → ZnSO₄(aq) + Cu(s)
3. Electrolysis: The purified ZnSO₄ solution is electrolyzed using inert lead anodes and pure aluminium cathodes.
   • At Cathode: Zn²⁺(aq) + 2e⁻ → Zn(s) (Pure zinc deposits on the aluminium cathode).
   • At Anode: H₂O(l) → 2H⁺(aq) + ½O₂(g) + 2e⁻ (Oxygen gas is evolved, and sulfuric acid is regenerated).

• Product: This process yields very high-purity zinc (99.9% to 99.99%).

Refining and Purification

The crude zinc obtained from the reduction processes often contains impurities and requires further purification to meet industrial specifications.

1. Fractional Distillation (for Smelted Zinc)

• Principle: This method utilizes the significant differences in boiling points between zinc and its common impurities (e.g., Pb, Fe, Cd). Zinc has a boiling point of 907°C.
• Process:
  • Crude molten zinc (spelter) is fed into a distillation column.
  • Zinc vaporizes, along with any more volatile impurities (like cadmium, boiling point ~767°C). Less volatile impurities (like lead, boiling point ~1749°C; iron, boiling point ~2861°C) remain in the liquid phase.
  • The zinc vapor is condensed, and further separation can be achieved by a second distillation if cadmium is present.
• Purity: This process can yield zinc with purity up to 99.95%.

2. Electrolytic Refining (for Smelted or High-Purity Requirements)

While the electrolytic process already yields high-purity zinc, further electrolytic refining can be done if ultra-high purity is required.

• Process:
  • Impure zinc acts as the anode.
  • A thin sheet of pure zinc or aluminium acts as the cathode.
  • An electrolyte containing zinc sulfate solution is used.
  • During electrolysis, pure zinc is deposited on the cathode, while more electropositive impurities (e.g., Fe, Ni) remain in the anode sludge, and less electropositive impurities (e.g., Cd) dissolve in the electrolyte.
• Purity: Can achieve purities exceeding 99.99%.