Metallurgy and Industrial Extraction of Copper (Cu)

Natural Occurrence & Major Ores

Copper (Cu) is found in both native (free) and combined states. Its primary commercial extraction occurs from sulfide ores.

Sulfide Ores: These are the most common and economically significant.
- Chalcopyrite (Copper Pyrites): CuFeS₂ (Most important ore)
- Chalcocite (Copper Glance): Cu₂S
- Bornite (Peacock Ore): Cu₅FeS₄
Oxide Ores:
- Cuprite (Red Copper Ore): Cu₂O
Carbonate Ores:
- Malachite: CuCO₃·Cu(OH)₂
- Azurite: 2CuCO₃·Cu(OH)₂

The most crucial step for sulfide ores is concentration, which increases the percentage of the desired metal.

This method is predominantly used for the concentration of sulfide ores like chalcopyrite.

Principle: Based on the differential wetting properties of ore and gangue particles. Sulfide ore particles are preferentially wetted by oil (hydrophobic) and float with the froth, while gangue particles are wetted by water (hydrophilic) and sink.
Procedure:
1. The finely powdered ore is mixed with water to form a slurry.
2. Collectors (e.g., pine oil, eucalyptus oil, xanthates) are added to enhance the non-wettability of the sulfide ore particles.
3. Frothing agents (e.g., cresols, anilines) are added to stabilize the froth.
4. Depressants (e.g., NaCN or Na₂S for ZnS) may be used to selectively prevent other sulfide ores (like zinc sulfide) from forming froth if they are present as impurities.
5. Air is blown vigorously through the mixture.
6. The oil-coated ore particles adhere to air bubbles, rise to the surface, and form a froth, which is skimmed off.
7. The gangue particles, being heavier and wetted by water, settle at the bottom.

The concentrated copper ore (primarily chalcopyrite, CuFeS₂) undergoes a series of pyrometallurgical steps to yield crude copper.

Process: The concentrated ore is heated strongly in a controlled supply of air in a reverberatory furnace.
Purpose:
- Removes moisture and volatile impurities like Arsenic (As), Antimony (Sb), and Sulfur (S) as their volatile oxides (e.g., SO₂, As₂O₃).
- Partially converts sulfide ores (CuFeS₂) into oxides and sulfates.
- Converts Iron sulfide (FeS) to Iron oxide (FeO), which is easier to remove.
Key Reactions:
- 2CuFeS₂(s) + O₂(g) → Cu₂S(s) + 2FeS(s) + SO₂(g) (Partial oxidation)
- 2FeS(s) + 3O₂(g) → 2FeO(s) + 2SO₂(g)
- S(s) + O₂(g) → SO₂(g)
- 4As(s) + 3O₂(g) → 2As₂O₃(g) (Volatile)

Process: The roasted ore is mixed with silica (sand, SiO₂) and heated strongly in a reverberatory furnace at temperatures around 1400-1500°C.
Purpose:
- To remove iron impurities as a fusible slag.
- To form copper matte.
Reactions:
- The iron oxide (FeO) formed during roasting, along with any remaining FeS, reacts with the added silica to form molten iron silicate (slag), which is lighter and floats on the surface.
- FeO(s) + SiO₂(s) → FeSiO₃(l) (Slag)
- The remaining Cu₂S and any unoxidized FeS melt to form a heavy, dark liquid known as copper matte.
Product: Copper matte (primarily Cu₂S with a small amount of FeS) and a separate layer of slag (FeSiO₃).

Process: The molten copper matte is transferred to a large pear-shaped Bessemer converter, lined with refractories (silica and magnesia). Hot air (sometimes enriched with oxygen) and silica are blown through the molten matte.
Purpose:
- To oxidize the remaining FeS to FeO and remove it as slag.
- To oxidize Cu₂S to Cu₂O, which then undergoes self-reduction with unreacted Cu₂S to produce crude copper.
Reactions:
- First, residual FeS is oxidized and removed as slag:
  - 2FeS(s) + 3O₂(g) → 2FeO(s) + 2SO₂(g)
  - FeO(s) + SiO₂(s) → FeSiO₃(l) (Slag)
- Once all FeS is removed, Cu₂S is oxidized, and then self-reduction occurs:
  - 2Cu₂S(l) + 3O₂(g) → 2Cu₂O(s) + 2SO₂(g)
  - 2Cu₂O(s) + Cu₂S(l) → 6Cu(l) + SO₂(g) (Self-reduction)
Product: Blister copper (approximately 98% pure). It is named for the rough, blistered appearance of its surface, caused by the escape of SO₂ gas during solidification.

Process: Blister copper is melted in a reverberatory furnace and stirred with green wood or bamboo poles.
Purpose: To reduce any remaining copper oxide (Cu₂O) present in blister copper back to metallic copper. The hydrocarbons from the green wood act as reducing agents.
Reaction:
- 2Cu₂O(s) + C(s) → 4Cu(l) + CO₂(g)
Product: Yields copper that is about 99.5% pure, which is still insufficient for electrical applications.

For high-purity applications, especially electrical wiring, copper requires further refining.

This is the most important and widely used method for obtaining high-purity copper (99.9% to 99.99%).

Setup:
- Anode: Thick blocks of impure (blister) copper.
- Cathode: Thin sheets of pure copper.
- Electrolyte: An aqueous solution of copper sulfate (CuSO₄) acidified with dilute sulfuric acid (H₂SO₄).
Process:
1. When an electric current is passed through the electrolytic cell, the impure copper anode undergoes oxidation.
2. Copper from the anode dissolves into the electrolyte as Cu²⁺ ions.
  - Anode Reaction (Oxidation):
    - Cu (impure anode) → Cu²⁺(aq) + 2e⁻
3. More electropositive impurities (e.g., Fe, Zn, Ni) present in the impure anode also dissolve into the electrolyte as their respective ions (Fe²⁺, Zn²⁺, Ni²⁺).
4. Less electropositive impurities (e.g., Ag, Au, Pt) do not dissolve and fall to the bottom of the cell as anode mud or anode sludge, which is a valuable source for these precious metals.
5. At the cathode, Cu²⁺ ions from the electrolyte migrate and are reduced to pure copper metal, which deposits onto the pure copper sheets.
  - Cathode Reaction (Reduction):
    - Cu²⁺(aq) + 2e⁻ → Cu(s) (Pure copper deposition)
6. The concentration of Cu²⁺ ions in the electrolyte remains relatively constant as copper dissolves from the anode at the same rate it deposits on the cathode.
Result: High-purity copper (often 99.99% pure), essential for electrical conductors due to its excellent conductivity.