Detailed Notes on Metallurgy and Alloys

Important Terms

• Minerals: Naturally occurring substances containing minerals.
• Ore: A mineral from which metal can be extracted economically and profitably. All ores are minerals, but not all minerals are ores.
• Gangue (Matrix): Rocky and earthy materials associated with ores, considered impurities (e.g., $SiO_2$ - Silica impurities).

Occurrence of Metals

Metals occur in two states:

1. Combined State: Active metals found as sulphides, oxides, carbonates, sulphates, etc.
   • Sulphide ores:
     • Copper pyrites - $CuFeS_2$
     • Argentite - $Ag_2S$
     • Zinc blend - $ZnS$
     • Cinnabar - $HgS$
     • Galena - $PbS$
   • Oxide ores:
     • Cuprite - $Cu_2O$
     • Zincite - $ZnO$
     • Bauxite - $Al<em>2O</em>3 . 2H_2O$
     • Haematite - $Fe<em>2O</em>3$
   • Carbonate ores:
     • Malachite - $CuCO<em>3 . Cu(OH)</em>2$
     • Lime stone - $CaCO_3$
     • Dolomite - $MgCO<em>3 . CaCO</em>3$
     • Calamine - $ZnCO_3$
   • Sulphate ores:
     • Barytes - $BaSO_4$
     • Anglesite - $PbSO_4$
2. Free State: Metals with very low reactivity found in elementary or free form (native metals).
   • e.g., silver, gold, platinum.

Iron Ores in India

• Found in: Bihar, Orrisa, Karnataka, Tamilnadu, Madhya Pradesh, Maharashtra, Goa.
• Indian iron ores contain approximately 62% to 68% iron.
• India's iron ore reserves total around 21,000 million tonnes.

Metallurgy

• The process of extraction of metals from their ores economically and profitably.

Main Operations of Metallurgy

1. Processing of Ores
2. Concentration
3. Reduction

Processing of Ores

• Ores from mines are in big stones, which are cut into small pieces.
• Treated in jaw crushers to get smaller-sized ore.
• Passed through ball mills to obtain fine powdered ore.

Concentration of Ores

• The process of removal of gangue present in the ores.

Types of Processes

1. Physical Process
2. Chemical Process

Physical Processes

1. Gravity Separation Process
   • Crushed ore is treated with running water.
   • Light gangue particles are washed away.
   • Heavy metallic particles settle at the bottom due to gravity.
   • Used for concentrating heavy oxide ores like tinstone, haematite, and ores of noble metals.
2. Electro-magnetic Separation Process
   • Used when ores contain magnetic impurities.
   • Powdered ore falls on a non-magnetic belt passing over a magnetic roller.
   • Magnetic impurities are attracted to the roller, separating them from the ore.
   • Example: Removing wolframite ($FeWO<em>4$) from tin-stone ($SnO</em>2$) or separating magnetite ($Fe<em>3O</em>4$) from roasted pyrite or separating $SnO_2$ from iron and manganese tungstates.
3. Froth Floatation Process
   • Used for sulphide ores.
   • Sulphide particles attach to oil bubbles in a mixture of oil and water.
   • The mixture is agitated by blowing compressed air.
   • Sulphide particles rise to the surface with oil bubbles, forming froth.
   • Impurities are wetted by water and settle at the bottom.
   • Used for ores like galena ($PbS$), zinc blend ($ZnS$), copper pyrites ($CuFeS_2$).
   • Oils used: pine oil, cresylic oil; sometimes alcohol is also used.

Chemical Processes

1. Roasting
   • Heating ores in the presence of air to a temperature below their melting point.
   • Carried out in a reverberatory furnace with open doors and windows.
   • Purposes:
     • To convert sulphide ores into easily reducible form (oxide form).
       • $2ZnS + 3O<em>2 2ZnO + 2SO</em>2$
     • To remove moisture present in the ore.
     • To remove volatile matter present in the ore.
2. Calcination
   • Heating the ores in the absence of air to a temperature below their melting point.
   • Carried out in a reverberatory furnace with closed doors and windows.
   • Chemical changes:
     • Moisture removal.
     • Conversion of carbonate ores into oxides.
       • $CaCO<em>3 CaO + CO</em>2$
       • $CuCO<em>3 . Cu(OH)</em>2 2CuO + H<em>2O + CO</em>2$
       • $ZnCO<em>3 ZnO + CO</em>2$
     • Removal of volatile matter (organic and sulphur) as oxides.
     • Organic substances convert into $CO_2$.
     • The ore becomes porous.

Roasting vs. Calcination

Roasting

1. Heating ores in the presence of air below its melting point.
2. Used to convert sulphide ores into their oxides.
3. Removes moisture present in ores.
4. Sulphide ores are chemically changed into a suitable form.

Calcination

1. Heating ores in the absence of air below its melting point.
2. Used to convert carbonate ores into their oxides.
3. Removes volatile matter present in ores.
4. Ore becomes porous.

Reduction

• Reducing the metal oxide (obtained by roasting or calcination) to the metallic state.
• Methods:
  • Smelting process
  • Aluminothermic process
  • Electrolytic reduction process

Smelting Process

• Calcined ore (metal oxides) mixed with coke and flux ($CaO$).
• Heated to a high temperature in a furnace.
• Metal oxides are reduced into metal, with coke acting as a reducing agent.
  • $Fe<em>2O</em>3 + 3C 2Fe + 3CO$
• Flux removes remaining gangue (impurities) by melting it.
  • $SiO<em>2 + CaO CaSiO</em>3$
• Flux: A substance used to remove the gangue still present in the ores while melting it.
• Slag: The product of flux combined with the gangue, forming a fusible compound.
• Acidic gangue requires basic flux, and basic gangue requires acidic flux.
  • $CaO + SiO<em>2 CaSiO</em>3$
  • $FeO + SiO<em>2 FeSiO</em>3$

Aluminothermic Process (Thermite Process)

• Aluminum is used as a reducing agent for stable metal oxides.
• Aluminum powder and metallic oxide (1:3 parts) are heated at high temperature (thermite mixture).
• Large amount of heat evolved (exothermic process).
  • $Cr<em>2O</em>3 + 2Al Al<em>2O</em>3 + 2Cr + heat$
  • $Fe<em>2O</em>3 + 2Al Al<em>2O</em>3 + 2Fe + heat$

Electrolytic Reduction Process

• Very active metals (sodium, potassium, calcium, aluminum) are extracted by electrolysis of their fused (molten) anhydrous salts.
• Example: Sodium metal extracted by electrolysis of fused sodium chloride salt using platinum electrodes.
  • $NaCl Na^+ + Cl^-$
  • At cathode: $Na^+ + e^- Na$
  • At anode: $Cl^- Cl + e^-; Cl + Cl Cl_2$

Iron Ores

• Haematite ($Fe<em>2O</em>3$) - Ferric Oxide: Contains about 70% iron; it is a red-colored ore.

Flue Gases

• Waste gases from blast furnace.
• Average composition:
  • $CO = 25-28\%$
  • $CO_2 = 12-15\%$
  • $N_2 = 60\%$
  • $H_2 = 3-4\%$
• Calorific value: about 760 Kcal/m³ at normal conditions.
• Used as a fuel in the same industry.

Refining of Crude Metal

• Metal obtained from reduction contains impurities, and so it is refined.
• Methods:
  1. Liquation
  2. Polling
  3. Distillation
  4. Electrorefining

Liquation

• Purifies metals that are easily fusible compared to impurities.
• Crude metal is allowed to fall on a sloping heated hearth of a reverberatory furnace.
• The metal melts and flows down, leaving infusible impurities behind.
• Used to refine lead and tin.

Polling

• Hot molten crude metal is stirred with green logs of wood.
• Wood gases reduce any metal oxide impurities.
• Absorbed air oxidizes easily oxidizable impurities, which escape as scum.
• Example: Blister copper is refined by polling method.

Distillation

• Volatile metals (zinc and mercury) are refined by distillation.

Electrorefining

• Most metals are refined by this method.
• Example: Electrorefining of crude copper.
• Crude copper rod serves as anode, and pure copper rod serves as cathode.
• Both electrodes are dipped into a solution of $CuSO_4$.
  • Reactions:
    • $Cu Cu^{2+} + 2e^-$ (Crude copper dissolves)
    • $Cu^{2+} + 2e^- Cu$ (Pure copper is deposited)

Physical Properties of Metals

• Hardness: Resistance to abrasion by other materials and application of heat and pressure. Diamond is the hardest material.
• Toughness: Withstanding bending or torsion without fracture.
• Ductility: The property allows to be drawn into wires. Gold is the most ductile metal.
• Malleability: The property allows to be beaten into thin sheets. Gold is the most malleable metal.
• Weldability: The property allows to be welded together.
• Machinability: The property allows to be easily cut by cutting tools to produce a desired shape and surface finish.
• Tensile Strength: The property allows to withstand a force acting upon it with a tendency to break it by tearing and expressed in Kg/cm of cross section.
• Thermal Conductivity: The ability allows to conduct electricity. Silver is the best electrical conductor, followed by copper.
• Magnetic Property: Materials attracted towards a magnet (iron, nickel, steel, cobalt, chromium, manganese).

Chemical Properties of Metal

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Purposes of Heat Treatment

• To change the structure of steel.
• To alter the magnetic properties of steel.
• To increase the hardness and toughness of steel.
• To increase corrosion resistance.
• To make the steel easily workable.
• To remove gases.

Types of Heat Treatment

1. Hardening or Quenching
2. Tempering
3. Annealing
4. Normalising

Hardening or Quenching

• Steel is heated to a high temperature (800-900°C) and cooled suddenly (quenching) in some medium (water, 6-20% $NaCl$ solution, 4-6% $Na<em>2CO</em>3$ solution, oil, etc.).
• Purposes:
  • To develop higher hardness.
  • To increase cutting ability.
  • To improve strength, elasticity, ductility, and toughness.
• The rate of cooling and the medium used for cooling dictates the hardness in steel.

Tempering

• Hardened (or quenched) steel is re-heated to a definite temperature (200-600°C) and then cooled at a suitable rate.
• Purposes:
  • To stabilize the strength of metal.
  • To reduce the hardness, brittleness, and tensile strength of metal.
  • To reduce the ductility of metal.

Annealing

• Steel is heated to a certain temperature and cooled at a controlled and slow rate.
• Annealing temperature depends on the percentage of carbon in steel:
  • Less than 0.12%: 875 - 925 °C
  • 0.12 to 0.5%: 840 - 970°C
  • 0.5 to 0.8%: 780 - 870°C
  • 0.8 to 1.5%: 760 - 768°C
• Purposes:
  • To soften the steel.
  • To improve the machinability of steel.
  • To increase the ductility and toughness.
  • To improve the internal stresses.

Normalising

• Similar to annealing, but steel is heated to a slightly higher temperature and cooled in air.
• Purposes:
  • To improve the internal stresses.
  • To increase the ductility and toughness.
  • To improve machinability of steel.
  • To soften steel.

Annealing vs. Normalising

Annealing

1. Steel is heated to a certain temperature and cooled slowly at a controlled rate.
2. Steel becomes more soft, malleable, and ductile.
3. Time required is more.
4. Consumption of fuel or electric power for heating is more.

Normalising

1. Steel is heated to a slightly high temperature and cooled in air.
2. due to this, steel becomes homogeneous and soft.
3. Time required is less.
4. Consumption of fuel or electric power for heating is less.

Advantages of Normalizing over Annealing

• Cooling in air increases the rate of cooling.
• Mechanical properties are increased.
• Time required for normalizing is less than annealing.
• Fuel or electric power consumption is greater in annealing than normalizing.

Introduction to Alloys

• Metals as such are not useful for many engineering purposes; properties are modified when mixed with other suitable metals or non-metals.
• Example: Pure aluminum is ductile but weak/soft; when alloyed with copper or magnesium, it becomes as hard as steel.

Definition of Alloy

• A homogeneous mixture of two or more elements, one of which must be a metal.
• Alloy must exhibit metallic characteristics.
• The metal present in the greatest proportion is the base metal; others are alloying elements.
• Classified into two groups: Ferrous (Iron-based) and Nonferrous (containing elements other than iron as the principle constituent).

Preparation of Alloys

1. Fusion:
   • Components are fused together in a crucible.
   • The component with a higher melting point is melted first, then the component with a lower melting point is added.
   • The molten mass is stirred with a graphite rod.
   • The surface is covered with a carbon layer to avoid oxidation.
   • Example: copper-tin alloy.
2. Electro-deposition:
   • Prepared by simultaneous deposition of different component metals from their electrolyte solution by the electrolysis process.
   • Example: Brass (copper and zinc) is obtained by the electrolysis of a mixed solution of copper and zinc cyanides dissolved in potassium cyanide.
3. Compression:
   • Prepared by compressing mechanically finely powdered compounds together under a high pressure.
   • Example: Solder alloy is obtained by rolling together the sheets of tin and lead under high pressure. Wood’s metal also obtained by this method.
4. Reduction:
   • Alloys are also obtained by the reduction of a suitable of one component metal in the presence of other component metal
   • Example: Aluminium bronze is prepared by reducing aluminium oxide, in the presence of copper in an electric furnace.

Purpose of Making Alloys

1. To increase the hardness of metal (e.g., gold and silver hardened by adding copper).
2. To lower the melting point (e.g., Wood's metal has a low m.p. of 61°C).
3. To increase the tensile strength (e.g., pure iron's tensile strength increases with 1% carbon).
4. To increase corrosion resistance (e.g., stainless steel is rust-proof and acid-proof).
5. To get good casting. So they can be fusible easily.
6. To modify colour (e.g. alunminium, bronze).

Classification of Alloys

1. Ferrous Alloys
2. Nonferrous Alloys

Alloy Steel

• Contains iron as one of the main components.
• Also contains carbon, silicon, phosphorous, manganese, and nickel.
• The added elements are called alloying elements.

Effect of Alloying Elements

• Carbon: Increases hardness and tensile strength but reduces ductility.
• Silicon: Used as a scavenger to make the metal sound and free from blow holes; improves tensile strength and hardness (maximum 0.25%).
• Manganese: Used as a deoxidizer.
• Phosphorous: Increases the tensile strength of steel.
• Sulphur: Improves the machinability of steel (should not exceed 0.6% to prevent brittleness).
• Nickel: Makes steel more elastic, tough, and corrosion-resistant; improves ductility (present at about 0.4 to 0.6%).

Aluminium Alloys

Duralumin

• Composition:
  • $Al = 95\%$
  • $Cu = 4\%$
  • $Mg = 0.5\%$
  • $Mn = 0.5\%$
• Properties:
  • Soft, tough, ductile, and corrosion-resistant.
  • Good conductor of heat and electricity.
  • Non-magnetic.
  • Good machinability.
• Uses:
  • Aeroplane parts, auto parts, railway parts, tubes, cables, surgical instruments, housing cases, etc.

“Y” Alloys

• Not provided in the text.

Bearing Alloys

• Properties:
  • Soft or hard, depending on the percentage of tin and lead.
  • Solders have melting range between 182° to 250°c.
  • Adhere to metallic surfaces better.

Babbit Metals

• An important example for bearing alloys that is used as Tin metal; Composition:
  • $Sn = 88\%$
  • $Sb = 8\%$
  • $Cu = 4\%$
• Properties:
  • Silvery white metal.
  • Soft.
  • Low coefficient of friction.
• Uses:
  • Engine bearings (distributes the load uniformly).

Aluminium Bearing Alloys

• Chemical composition: Al = balance; Si = 0.025 % Sn = 6.5 ; Mg = 0.08% Cu = 0.5 to 1.5%

Platinoid

• Chemical composition:
  • $Cu = 60\%$
  • $Ni = 14\%$
  • $Zn = 24\%$
  • $W = 2\%$
• Possesses high electrical resistance.
• Used for the manufacture of Wheatstone bridges, rheostats, potentiometers, etc.

Some Important Alloys

Monel Metal

• An alloy of copper and nickel; Composition:
  • $Ni = 66 - 67\%$
  • $Cu = 28 - 30\%$
  • $Fe = 1 - 2\%$
  • $Mn = 0.9 - 1\%$
• Properties:
  • Bright, strong & tough.
  • Easy to clean.
  • Attractive appearance.
  • Resistant to chemical action.
• Uses:
  • Turbine blades & automobile engine parts.
  • Machine parts for processing foods, dyes, papers, chemicals, bolts, screws, nails etc.

Wood's Metal

• Fusible alloy of lead.
• Composition:
  • $Pb = 25\%$
  • $Bi = 50\%$
  • $Sn = 12.5\%$
  • $Cd = 12.5\%$
• Uses:
  • A soldering metal and is used For making fire-alarms and automatic sprinkles.
  • Used for making safety plugs for cookers, milk pot etc.
  • Used for castings for dental works, and for boilers and electric fuses etc.

Alnico

• Also known as aluminum-nickel-cobalt steel. Composition:
  • $Al = 12\%$
  • $Ni = 20\%$
  • $CO = 6\%$
  • $Steel = 62\%$
• Properties:
  • High magnetic permeability, Strong.
  • Lifts 4450 times of its own weight.
• Uses:
  • Making small powerful permanent magnets for speakers in radio and television.

Rose Metal

• Chemical composition:
  • $Bi = 50\%$
  • $Pb = 28\%$
  • $Sn = 22\%$
• Properties & Uses:
  • Readily fusible, used for making fire alarms, fuse wires, castings for dental works, and in automatic sprinkler systems.