Grade 10 Chapter 8: Metallurgy

Introduction

Metallurgy is the branch of science and technology concerned with the properties, physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures (alloys). It is a domain that combines elements of physics, chemistry, and materials science. This chapter explores the fundamental principles of extracting metals from their ores and refining them for various applications.

Occurrence of Metals in Nature

Metals occur in nature in different forms depending on their reactivity:

1. Native State (Free State):
2. Less reactive metals like gold, silver, platinum, and copper sometimes occur in nature in their elemental form
3. These metals do not readily combine with other elements

4. Example: Gold nuggets, native copper

5. Combined State:

6. Most metals occur in combined form as:

   • Oxides (e.g., Fe₂O₃, Al₂O₃)
   • Sulfides (e.g., ZnS, PbS)
   • Carbonates (e.g., CaCO₃, FeCO₃)
   • Halides (e.g., NaCl, KCl)
   • Sulfates (e.g., CaSO₄, BaSO₄)
   • Silicates (e.g., feldspar, mica)

7. Minerals and Ores:

8. Minerals: Naturally occurring chemical substances with definite chemical composition and physical properties
9. Ores: Minerals from which metals can be extracted profitably
10. All ores are minerals, but not all minerals are ores

Metallurgical Processes

The extraction of metals from their ores involves several steps:

1. Concentration of Ores (Ore Dressing)

The process of removing impurities (gangue) from the ore is called concentration or ore dressing. Methods include:

a) Physical Methods:

Hydraulic Washing (Gravity Separation): - Based on the difference in specific gravity between the ore and gangue - Lighter gangue particles are washed away by water, leaving heavier ore particles - Used for heavy ores like iron ore, tin ore

Magnetic Separation: - Based on magnetic properties of the ore - Magnetic ores (like magnetite Fe₃O₄) are separated from non-magnetic gangue using electromagnets - Used for iron ores and some manganese ores

Froth Flotation: - Based on the difference in wetting properties of ore and gangue - Ore is mixed with water, pine oil (froth stabilizer), and a collector (like xanthates) - When air is blown, ore particles attach to air bubbles and float as froth, while gangue settles - Used for sulfide ores of copper, zinc, and lead

Leaching: - Selective dissolution of the ore in a suitable solvent - Example: Bauxite (Al₂O₃·2H₂O) is leached with NaOH to form sodium aluminate - Used for aluminum extraction from bauxite

b) Chemical Methods:

Calcination: - Heating the ore strongly below its melting point in the absence of air - Drives off volatile impurities and converts carbonates to oxides - Example: ZnCO₃ → ZnO + CO₂

Roasting: - Heating the ore strongly below its melting point in the presence of excess air - Converts sulfides to oxides - Example: 2ZnS + 3O₂ → 2ZnO + 2SO₂

2. Extraction of Crude Metal

The concentrated ore is converted to the metal through reduction processes. The method depends on the reactivity of the metal:

a) Pyrometallurgy (Extraction by Heat):

Smelting: - Reduction of metal oxides with carbon (coke) at high temperatures - Example: Fe₂O₃ + 3C → 2Fe + 3CO - Used for iron, copper, zinc, and lead extraction

Self-reduction: - Some sulfide ores are partially roasted to convert part of the sulfide to oxide - The oxide then reacts with the remaining sulfide - Example: 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ Cu₂S + 2Cu₂O → 6Cu + SO₂ - Used in copper extraction

Thermite Process: - Reduction of metal oxides using aluminum powder - Highly exothermic reaction - Example: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + Heat - Used for welding and extraction of some metals

b) Hydrometallurgy (Extraction in Aqueous Solutions):

• Leaching the ore with suitable reagents
• Recovering the metal by displacement or electrolysis
• Example: Extraction of gold using cyanide process Au + 2CN⁻ + ½O₂ + H₂O → [Au(CN)₂]⁻ + 2OH⁻
• Used for gold, silver, and copper extraction

c) Electrometallurgy (Extraction by Electrolysis):

• Electrolytic reduction of molten metal compounds
• Example: Extraction of aluminum from molten alumina (Al₂O₃) dissolved in cryolite (Na₃AlF₆)
• Used for highly reactive metals like aluminum, sodium, potassium, and magnesium

3. Refining of Metals

The crude metal obtained from extraction processes contains impurities and needs to be refined. Methods include:

Distillation: - For volatile metals with low boiling points - Example: Zinc (boiling point 907°C)

Liquation: - Based on the difference in melting points - The impure metal is heated to a temperature where the metal melts but impurities remain solid - Example: Refining of tin

Electrolytic Refining: - The impure metal is made the anode - Pure metal is the cathode - Suitable electrolyte containing metal ions - During electrolysis, pure metal deposits on the cathode - Impurities either dissolve in the electrolyte or fall as anode mud - Example: Refining of copper, silver, gold

Zone Refining: - Based on the principle that impurities are more soluble in the molten state than in the solid state - A heating coil is moved slowly along a rod of impure metal - Impurities concentrate in the molten zone and are carried to one end - Used for semiconductor materials like silicon, germanium

Extraction of Iron from its Ores

Iron is one of the most important metals in modern civilization. Its extraction from iron ore (mainly hematite, Fe₂O₃) illustrates many principles of metallurgy.

Blast Furnace Process

Raw Materials: - Iron ore (hematite, Fe₂O₃) - Coke (carbon) - Limestone (CaCO₃) as flux - Hot air

Structure of Blast Furnace: - Tall cylindrical furnace (25-35 meters high) - Lined with heat-resistant bricks - Widest in the middle (bosh) - Narrow at the top and bottom

Chemical Reactions: 1. Combustion Zone (Bottom): - C + O₂ → CO₂ + heat - CO₂ + C → 2CO (carbon monoxide is the main reducing agent)

1. Reduction Zone (Middle):
2. Fe₂O₃ + 3CO → 2Fe + 3CO₂

3. Fe₂O₃ + 3C → 2Fe + 3CO

4. Slag Formation:

5. CaCO₃ → CaO + CO₂
6. CaO + SiO₂ → CaSiO₃ (slag)

Products: - Pig Iron: Contains 2-4.5% carbon and impurities like Si, Mn, P, and S - Slag: Mainly calcium silicate, used in cement manufacturing and road construction

Types of Iron and Steel

Cast Iron: - Contains 2-4.5% carbon - Brittle but resistant to corrosion - Used for making pipes, stoves, radiators

Wrought Iron: - Contains less than 0.25% carbon - Tough, malleable, and ductile - Used for making chains, nails, and ornamental ironwork

Steel: - Contains 0.25-2% carbon - Combines strength with ductility - Various types depending on composition: * Mild Steel (0.25-0.45% C): For machinery, structural steel * Medium Carbon Steel (0.45-0.8% C): For rails, gears * High Carbon Steel (0.8-1.5% C): For cutting tools, springs * Stainless Steel: Contains Cr, Ni; resistant to corrosion * Alloy Steels: Contain elements like Mn, Ni, Cr, V for specific properties

Extraction of Aluminum from Bauxite

Aluminum is the most abundant metal in Earth's crust but was once more precious than gold due to difficulties in extraction.

Hall-Héroult Process

Raw Material: Purified alumina (Al₂O₃) from bauxite ore

Process: 1. Bauxite is purified by Bayer's process to obtain pure alumina 2. Alumina is dissolved in molten cryolite (Na₃AlF₆) at about 950°C 3. Electrolysis is carried out with: - Carbon-lined steel tank as cathode - Carbon rods as anode - Molten mixture as electrolyte

Reactions: - At cathode: Al³⁺ + 3e⁻ → Al - At anode: C + O²⁻ → CO + 2e⁻ C + 2O²⁻ → CO₂ + 4e⁻

Challenges: - High energy consumption (13-15 kWh per kg of aluminum) - Carbon anodes are consumed and need replacement - Environmental concerns from fluoride emissions

Properties and Uses of Aluminum: - Low density (2.7 g/cm³) - Good conductor of heat and electricity - Corrosion resistant due to oxide layer - Used in aircraft, automobiles, packaging, construction, and electrical transmission

Extraction of Copper from its Ores

Copper is one of the oldest metals used by humans and is essential for electrical applications.

Process for Sulfide Ores (Chalcopyrite, CuFeS₂)

1. Concentration: Froth flotation

2. Roasting: 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂

3. Smelting: FeS + O₂ + SiO₂ → FeSiO₃ (slag) + SO₂

4. Converting: Cu₂S + O₂ → 2Cu + SO₂

5. Refining: Electrolytic refining

6. Impure copper as anode
7. Pure copper as cathode
8. CuSO₄ solution as electrolyte
9. Impurities like Au, Ag settle as anode mud

Properties and Uses of Copper: - Excellent conductor of heat and electricity - Ductile and malleable - Resistant to corrosion - Used in electrical wiring, electronics, plumbing, and alloys like brass and bronze

Corrosion and its Prevention

Corrosion is the deterioration of metals due to chemical reactions with their environment, particularly oxygen and moisture.

Types of Corrosion

Dry Corrosion: - Direct reaction of metal with gases like O₂, Cl₂, SO₂ - Forms a layer of metal compound on the surface - Example: Tarnishing of silver with H₂S

Wet Corrosion: - Electrochemical process in presence of moisture - Involves formation of electrochemical cells - Example: Rusting of iron

Rusting of Iron

Chemical Process: 4Fe + 3O₂ + 2H₂O → 2Fe₂O₃·H₂O (hydrated iron(III) oxide or rust)

Electrochemical Mechanism: 1. Anodic reaction: Fe → Fe²⁺ + 2e⁻ 2. Cathodic reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻ 3. Formation of rust: Fe²⁺ + 2OH⁻ → Fe(OH)₂ 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃ 2Fe(OH)₃ → Fe₂O₃·3H₂O (rust)

Prevention of Corrosion

Barrier Protection: - Painting - Greasing and oiling - Plastic coating - Electroplating with less reactive metals

Sacrificial Protection: - Connecting the metal to a more reactive metal (sacrificial anode) - The more reactive metal corrodes instead of the protected metal - Example: Zinc coating on iron (galvanization)

Alloying: - Adding elements to form corrosion-resistant alloys - Example: Stainless steel (Fe + Cr + Ni)

Cathodic Protection: - Making the metal to be protected the cathode in an electrochemical cell - Example: Underground pipelines connected to magnesium anodes

Alloys

Alloys are homogeneous mixtures of two or more metals, or metals with certain non-metals.

Purpose of Alloying

• Increase hardness
• Increase tensile strength
• Improve corrosion resistance
• Lower melting point
• Modify color
• Improve castability

Types of Alloys

Ferrous Alloys (containing iron): - Steel (Fe + C) - Stainless Steel (Fe + Cr + Ni) - Tool Steel (Fe + C + W/Mo/V)

Non-ferrous Alloys: - Brass (Cu + Zn) - Bronze (Cu + Sn) - Duralumin (Al + Cu + Mg + Mn) - Solder (Pb + Sn) - Amalgams (Hg + other metals)

Important Alloys and Their Uses

Metallurgy of Gold and Silver

Gold Extraction (Cyanide Process)

1. Concentration: Gravity separation followed by amalgamation

2. Cyanide Leaching: 4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻

3. Recovery: 2[Au(CN)₂]⁻ + Zn → 2Au + [Zn(CN)₄]²⁻

4. Refining: Electrolytic refining with gold anode, gold cathode, and AuCl₃ electrolyte

Silver Extraction

1. Concentration: Froth flotation

2. Extraction from Argentite (Ag₂S):

3. Roasting: 2Ag₂S + 3O₂ → 2Ag₂O + 2SO₂

4. Reduction: 2Ag₂O → 4Ag + O₂

5. Extraction by Cyanide Process (similar to gold)

6. Refining: Electrolytic refining with silver anode, silver cathode, and AgNO₃ electrolyte

Environmental Aspects of Metallurgy

Metallurgical processes can have significant environmental impacts:

Environmental Concerns

• Air Pollution: SO₂ emissions from smelting sulfide ores
• Water Pollution: Acid mine drainage, heavy metal contamination
• Land Degradation: Mining activities, slag disposal
• Energy Consumption: High energy requirements for extraction and refining
• Greenhouse Gas Emissions: CO₂ from reduction processes

Sustainable Practices

• Recycling of Metals: Reduces need for primary extraction
• Improved Extraction Technologies: More efficient, less polluting processes
• Waste Management: Proper disposal and utilization of slag and other wastes
• Rehabilitation of Mining Sites: Restoration of mined areas
• Energy Efficiency: Reducing energy consumption in metallurgical processes

Summary

Metallurgy is a crucial field that enables the extraction and processing of metals for countless applications in modern society. The extraction process typically involves concentration of ores, conversion to metal, and refining to achieve the desired purity. Different metals require different extraction methods based on their chemical properties and reactivity.

The environmental impact of metallurgical processes is significant, and sustainable practices are increasingly important in modern metallurgy. Recycling of metals, improved extraction technologies, and proper waste management are key aspects of sustainable metallurgy.

Understanding the principles of metallurgy helps us appreciate the complexity involved in obtaining the metals that form the backbone of our technological civilization.

Key Terms

• Metallurgy
• Ore
• Gangue
• Concentration
• Froth flotation
• Calcination
• Roasting
• Smelting
• Flux
• Slag
• Pyrometallurgy
• Hydrometallurgy
• Electrometallurgy
• Refining
• Alloy
• Corrosion
• Galvanization