Metallurgy-ICSE-Class 10|Biswajit Das

1. Introduction to Metallurgy

Metallurgy is the science and technology of extracting metals from their ores and refining them for use. An ore is a naturally occurring rock or mineral from which a metal can be extracted profitably. A mineral is a naturally occurring chemical substance with a specific crystal structure and chemical composition. Not all minerals are ores. For example, bauxite is an ore of aluminum, but clay is a mineral of aluminum that is not economically viable to extract from. Gangue or matrix refers to the unwanted rocky and earthy impurities present in an ore.

2. General Principles of Metallurgy

The metallurgical process for a particular metal depends on its position in the reactivity series. The process typically involves three main steps:

1. Concentration of the Ore (Ore Dressing): This is the process of removing the unwanted gangue from the ore. It is usually a physical process.
   • Hydraulic Washing (Gravity Separation): This method is used for heavy oxide ores (e.g., hematite). The ore is washed with water; the lighter gangue particles are washed away, leaving the heavier ore particles behind.
   • Magnetic Separation: This is used for magnetic ores (e.g., magnetite). The powdered ore is passed over a magnetic roller, which separates the magnetic ore from the non-magnetic gangue.
   • Froth Flotation: This is used for sulfide ores (e.g., zinc blende). The powdered ore is mixed with oil and water, and air is bubbled through. The ore particles stick to the oil and rise as a froth, while the gangue sinks.
   • Chemical Method (Leaching): This involves treating the ore with a suitable chemical reagent to dissolve the ore, leaving the insoluble gangue behind.
2. Extraction of the Metal: This involves converting the concentrated ore into the crude metal. The method used depends on the metal’s reactivity.
   • Reduction by Carbon (Smelting): Metals of medium reactivity (like iron, lead, and zinc) are extracted by heating their oxides with carbon or carbon monoxide.ZnO+CΔ​Zn+CO
   • Electrolytic Reduction: Highly reactive metals (like sodium, potassium, and aluminum) are extracted by the electrolysis of their molten salts.
   • Self-Reduction: Less reactive metals (like copper and mercury) are extracted by heating their sulfide ores in the presence of air. A part of the sulfide gets oxidized to oxide, which then reacts with the remaining sulfide to form the metal and sulfur dioxide.
   • Reduction by more reactive metals: A more reactive metal can displace a less reactive metal from its compound.
3. Refining of the Metal: This is the process of purifying the crude metal obtained from the extraction process.
   • Liquation: Used for low-melting metals like tin and lead. The impure metal is heated on a sloping hearth; the metal melts and flows away, leaving the impurities behind.
   • Distillation: Used for low-boiling metals like zinc and mercury. The impure metal is heated, vaporized, and then condensed to get a pure metal.
   • Electrolytic Refining: This is the most widely used and effective method. An impure block of the metal is made the anode, a pure thin strip of the same metal is the cathode, and a solution of the metal salt is the electrolyte. During electrolysis, the impure metal from the anode dissolves and deposits as a pure metal on the cathode.

3. Alloys

An alloy is a homogeneous mixture of two or more metals, or a metal with a non-metal. Alloys are made to improve the properties of the base metal, such as hardness, strength, and corrosion resistance.

• Examples of Common Alloys:
  • Brass: An alloy of copper and zinc. It is harder than pure copper and is used for making utensils and musical instruments.
  • Bronze: An alloy of copper and tin. It is also harder and more resistant to corrosion than pure copper.
  • Stainless Steel: An alloy of iron, chromium, and nickel. It is highly resistant to rusting.
  • Duralumin: An alloy of aluminum, copper, magnesium, and manganese. It is lightweight and has high tensile strength, making it ideal for aircraft parts.
  • Solder: An alloy of lead and tin. It has a low melting point and is used for joining metals.

4. Corrosion

Corrosion is the slow and continuous degradation of a metal due to a chemical reaction with its environment. The most common example is the rusting of iron.

• Rusting: It is the chemical reaction of iron with oxygen and water to form hydrated ferric oxide (Fe2​O3​⋅xH2​O). The process is an electrochemical phenomenon.
• Prevention of Corrosion:
  • Barrier Protection: By applying a coat of paint, grease, oil, or electroplating the metal surface with a non-corroding metal.
  • Galvanization: Coating the iron with a layer of zinc. Zinc is more reactive and gets oxidized first, protecting the iron.
  • Alloying: Making alloys that are resistant to corrosion, like stainless steel.

5. Extraction of Aluminium

The extraction of aluminum is a two-step process that involves the purification of its ore and the subsequent electrolytic reduction of the purified ore. Aluminum is a highly reactive metal, and its strong affinity for oxygen means it cannot be extracted by simple reduction with carbon. Therefore, electrolysis is used.

Step 1: Purification of Bauxite Ore (Baeyer’s Process)

Aluminum’s chief ore is bauxite (Al2​O3​⋅2H2​O). Bauxite contains impurities like silicon dioxide (SiO2​) and iron oxides (Fe2​O3​). The Baeyer’s process is used to remove these impurities to obtain pure alumina (Al2​O3​).

1. Digestion: Powdered bauxite is digested with a hot, concentrated sodium hydroxide (NaOH) solution at high pressure. Alumina is an amphoteric oxide, so it reacts with the base to form soluble sodium aluminate. The impurities (iron oxides and silicon dioxide) do not react and remain as an insoluble residue.Al2​O3​⋅2H2​O+2NaOH→2NaAlO2​+3H2​O
2. Filtration: The insoluble residue, known as “red mud” (mainly iron oxides), is filtered out.
3. Precipitation: The hot sodium aluminate solution is cooled, diluted with water, and seeded with freshly prepared aluminum hydroxide (Al(OH)3​) to start the precipitation. The sodium aluminate hydrolyzes to form a white precipitate of aluminum hydroxide.2NaAlO2​+4H2​O→2Al(OH)3​+2NaOH
4. Calcination: The precipitated aluminum hydroxide is filtered, washed, and then heated strongly to a temperature of around 1000∘C to yield pure, anhydrous alumina.2Al(OH)3​1000∘C​Al2​O3​+3H2​O

Step 2: Electrolytic Reduction of Alumina (Hall-Héroult Process)

Pure alumina has a very high melting point (over 2050∘C), making direct electrolysis impractical. Therefore, it is dissolved in a molten electrolyte to lower the melting point and increase conductivity.

1. Electrolyte: The electrolyte is a molten mixture of:
   • Alumina (Al2​O3​) (the substance to be electrolyzed).
   • Cryolite (Na3​AlF6​) which acts as a solvent and lowers the melting point of alumina to about 950∘C.
   • Fluorspar (CaF2​) which increases the conductivity of the electrolyte.
2. Electrolytic Cell: The electrolysis is carried out in a large steel tank lined with graphite, which acts as the cathode. A series of thick graphite rods dipped into the molten electrolyte act as the anode.
3. Electrode Reactions:
   • At the Cathode (Reduction): Aluminum ions (Al3+) from the molten alumina gain electrons and are reduced to molten aluminum. The molten aluminum, being denser than the electrolyte, sinks to the bottom of the cell and is periodically tapped out.Al3++3e−→Al
   • At the Anode (Oxidation): Oxide ions (O2−) from the molten alumina lose electrons and are oxidized to oxygen gas. The hot oxygen gas reacts with the graphite anodes, forming carbon dioxide, which causes the anodes to be consumed and requires them to be replaced regularly.2O2−→O2​+4e−C+O2​→CO2​