NEET Notes: General Principles of Extraction of Metals

Introduction

The extraction of metals from their natural sources (ores and minerals) is a crucial part of metallurgy. This process involves various steps, including concentration of ores, reduction to obtain the metal, and refining to achieve the desired purity. Understanding the general principles of extraction is important for NEET aspirants, as it helps explain the industrial processes used to obtain metals for different applications.

Occurrence of Metals

1. Types of Ores

Metals occur in nature in the form of ores, which are minerals containing metal compounds in sufficient quantities to be economically extracted. Ores are classified based on the metal content:

• Oxide Ores: Bauxite (Al₂O₃·xH₂O), Hematite (Fe₂O₃)

• Sulphide Ores: Galena (PbS), Zinc Blende (ZnS)

• Carbonate Ores: Calamine (ZnCO₃), Malachite (CuCO₃·Cu(OH)₂)

• Halide Ores: Rock Salt (NaCl), Cryolite (Na₃AlF₆)

2. Minerals and Gangue

• Minerals: Naturally occurring compounds of metals found in the Earth's crust.

• Gangue (Matrix): Unwanted impurities like sand, clay, and rocky materials present in the ore.

Steps in Metallurgical Processes

1. Concentration of Ores

Concentration (or beneficiation) is the process of removing gangue from the ore to increase the metal content. Common methods include:

• Gravity Separation: Based on the difference in density between ore and gangue (e.g., washing of tin ore).

• Froth Flotation: Used for sulphide ores, where finely crushed ore is mixed with water, pine oil, and air to form froth, which carries the ore particles.

• Magnetic Separation: Used for magnetic ores like magnetite (Fe₃O₄), separating them from non-magnetic impurities.

• Leaching: A chemical method where an ore is dissolved in a suitable reagent, and the metal is selectively precipitated (e.g., cyanide process for gold and silver).

2. Extraction of Crude Metal from Concentrated Ore

The concentrated ore is converted into a free metal using various reduction methods:

(a) Conversion of Ores into Metal Oxides

Metals are generally extracted from their oxides. If an ore is in the form of a carbonate or sulphide, it must be converted into an oxide first:

• Calcination: Heating carbonate ores in limited oxygen to remove CO₂ and obtain metal oxide.

• Roasting: Heating sulphide ores in excess oxygen to convert them into oxides while removing SO₂ gas.

(b) Reduction of Metal Oxides

Metal oxides are reduced to free metals using different methods:

• Smelting: Extraction using a reducing agent such as carbon (e.g., iron from haematite using coke in a blast furnace).

• Self-reduction: Certain sulphide ores (e.g., Cu₂S) undergo reduction without an external reducing agent.

• Electrolytic Reduction: Used for highly reactive metals like sodium, aluminum, and potassium (e.g., electrolysis of molten alumina in the Hall-Héroult process).

3. Refining of Crude Metals

After extraction, metals often contain impurities, which need to be removed through refining methods:

(a) Distillation

Used for metals with low boiling points (e.g., zinc and mercury). Impure metal is vaporized and then condensed into a pure form.

(b) Liquation

Metals with low melting points (e.g., tin) are melted, and the pure metal flows away from impurities.

(c) Electrolytic Refining

The most commonly used method for refining metals like copper, aluminum, and silver. The impure metal is used as the anode, pure metal is deposited at the cathode, and impurities settle as anode mud.

(d) Zone Refining

Used for semiconductor-grade silicon and germanium. The impure metal rod is heated with a moving induction coil, and impurities move to one end, leaving pure metal behind.

(e) Van Arkel Method

Used for refining metals like titanium and zirconium, which are converted into volatile compounds and then decomposed to obtain pure metal.

Extraction of Some Important Metals

1. Extraction of Iron (Blast Furnace Process)

Iron is extracted from hematite (Fe₂O₃) using a blast furnace. The process involves:

• Calcination of Ore: Removal of moisture and CO₂.

• Reduction with Coke: Fe₂O₃ is reduced to Fe using CO gas.

• Formation of Slag: Limestone (CaCO₃) removes silica impurities as slag (CaSiO₃).

2. Extraction of Aluminium (Hall-Héroult Process)

• Ore Used: Bauxite (Al₂O₃·xH₂O)

• Electrolysis of Alumina: Dissolved in molten cryolite (Na₃AlF₆) to lower melting point and increase conductivity.

• Cathode Reaction: Al³⁺ ions gain electrons and deposit as pure aluminum metal.

3. Extraction of Copper (Self-Reduction Process)

• Ore Used: Copper pyrites (CuFeS₂)

• Roasting and Smelting: Sulphide ore is partially oxidized, and Cu₂S is reduced to pure copper.

• Electrolytic Refining: Increases purity up to 99.99%.

4. Extraction of Zinc (Roasting and Reduction)

• Ore Used: Zinc blende (ZnS)

• Roasting: ZnS is converted to ZnO by heating in oxygen.

• Reduction: ZnO is reduced using coke at high temperatures.

Environmental Impacts of Metallurgical Processes

1. Air Pollution

• Smelting releases harmful gases like SO₂, leading to acid rain.

• Mining operations generate dust, affecting air quality.

2. Water Pollution

• Toxic chemicals used in extraction (cyanide, mercury) can contaminate water bodies.

• Acid mine drainage affects aquatic ecosystems.

3. Land Degradation

• Large-scale mining destroys natural habitats and landscapes.

• Heavy metal contamination of soil reduces agricultural productivity.

Applications of Extracted Metals

1. Iron and Steel

• Used in construction, machinery, and transportation.

• Stainless steel is used in kitchenware and medical instruments.

2. Aluminum

• Used in packaging, aircraft, electrical wiring, and utensils.

• Light-weight, corrosion-resistant metal ideal for structural applications.

3. Copper

• Used in electrical wiring, plumbing, and coinage.

• Excellent conductor of electricity and heat.

4. Zinc

• Used for galvanization to prevent rusting of iron.

• Used in making alloys like brass and bronze.

Conclusion

The extraction of metals involves multiple steps, from ore concentration to refining. Various methods, including chemical and electrolytic processes, are used depending on the reactivity of the metal. Understanding these principles is essential for NEET aspirants as metallurgy plays a crucial role in both industrial and environmental chemistry.