10.2 Metal Extraction

Overview:

• Metal extraction refers to the process of obtaining metals from their ores. Different metals require different methods of extraction based on their properties.

• Some metals, like gold and platinum, are found in their pure state due to being non-reactive. However, more reactive metals are typically found as compounds, such as oxides, sulfides, and carbonates. Extraction methods differ for these metals, with processes like reduction and electrolysis being common.

Steps of Metal Extraction:

1. Crushing the Ores:

• Ores are first crushed to smaller pieces using a jaw crusher. Then, they are powdered further using a ball crusher.

2. Condensation of Ores:

• Ores contain impurities that need to be separated. The process of removing these impurities is called condensation.

• Methods of condensation:

• Hydrolytic Method: Used for oxide ores. The heavier ore particles settle, while impurities are washed away with water.

• Froth Floatation: Used for sulfide ores. The ore is mixed with oil and air, causing the ore to float as froth while impurities settle at the bottom.

• Magnetic Separation: Used if one of the materials (ore or gangue) is magnetic. Magnetic ores are attracted and separated from non-magnetic gangue.

• Chemical Method: Used when a reaction occurs between the ore and a chemical, like sodium hydroxide with bauxite to extract aluminium oxide.

3. Conversion of Condensed Ore to Oxides:

• Calcination: Ore is heated in the absence of air to remove moisture and other volatile impurities.

• Example: ZnCO₃ → ZnO + CO₂.

• Roasting: Ore is heated in the presence of air, especially for sulfide ores, to convert them into oxides.

• Example: 2PbS + 3O₂ → 2PbO + 2SO₂.

4. Conversion of Metallic Oxides to Free Metals:

• The metallic oxides from calcination and roasting are reduced to pure metals using different methods, depending on the metal’s reactivity.

• Electrolysis: Used for highly reactive metals like potassium, sodium, calcium, magnesium, and aluminum.

• Carbon Reduction: Used for medium reactive metals like zinc, iron, lead, and copper.

• Auto-reduction: Used for less reactive metals like copper, mercury, and silver.

Reactivity Series of Metals:

• The method of reduction is based on the metal’s position in the reactivity series:

• Highly reactive metals (top of the series) are reduced by electrolysis.

• Medium reactive metals are reduced by carbon reduction.

• Less reactive metals are reduced by self-reduction (heating alone).

Reduction Methods:

• Electrolysis:

• Used for metals like aluminum. The process involves passing an electric current through a molten mixture of aluminum oxide and cryolite (Na₃AlF₆) to extract aluminum.

• At the cathode: Al³⁺ + 3e⁻ → Al (metal).

• At the anode: 2O²⁻ → O₂ + 4e⁻ (gas).

• Carbon Reduction:

• Example: Zinc (Zn) is extracted from zinc oxide (ZnO) by heating with carbon.

• ZnO + C → Zn + CO.

• Self-reduction:

• Used for less reactive metals. Example: Mercury is extracted from cinnabar (HgS) by heating it, leading to the formation of mercury and sulfur dioxide (SO₂).

Purification of Metals:

• After reduction, metals often contain impurities. These can be removed by:

1. Adding a Smelting Agent: A substance added to remove impurities. If the impurities are alkaline, an acidic smelting agent is used (like SiO₂), and vice versa.

2. Electrolysis: Impure metal is made the anode, and pure metal is deposited at the cathode. This method is commonly used for purifying copper.

• Example: Cu (impure) → Cu²⁺ + 2e⁻ at the anode, Cu²⁺ + 2e⁻ → Cu (pure) at the cathode.

Example - Copper Purification:

• Copper extracted from reduction methods is 98% pure. Electrolysis purifies it to 99.9%. Copper sulfate (CuSO₄) is used as the electrolyte, with an impure copper anode and pure copper cathode.

Extraction of Iron:

• Hematite (Fe₂O₃) is the main ore of iron, extracted in a blast furnace.

• Steps:

1. Concentration: Remove impurities.

2. Calcination/Thermal Oxidation: Heat the ore to remove moisture.

3. Smelting: Iron is extracted by reacting with carbon (coke) in the furnace.

• Fe₂O₃ + 3C → 2Fe + 3CO.

Environmental Concerns and Solutions:

• Pollution: During extraction, gases like sulfur dioxide (SO₂) may be released, contributing to acid rain. Using scrubbers or capturing gases can prevent environmental damage.

Other Metal Extraction Methods:

• Mercury: Extracted from cinnabar (HgS) by heating.

• Zinc: Extracted from zinc blend (ZnS) by roasting and reduction with carbon.

• Lead: Extracted from galena (PbS) by roasting and reduction with carbon.

Group Assignment:

1. Sodium extraction using sodium chloride and calcium chloride mixture.

2. Aluminum extraction using cryolite and aluminum oxide mixture.

3. Environmental pollution from copper extraction byproduct gases.

4. Reactions in a blast furnace for extracting iron and dealing with gangue.