METALLURGY
1. Metallurgy relates to the science and technology of metals
2. Metals are shiny and malleable substances, but they don't occur that way. They are found in native state and
combined state.
The least chemically reactive metals (Copper, Gold, Silver) occur in their native state. Reactive metals (Alkali metals)
occur in a combined state
3. A naturally occurring substance obtained by mining that contains the metal in a free
state or in the form of compounds like oxides, sulphides etc... is called a Mineral.
4. Minerals that contain a high percentage of metal, from which it can be extracted conveniently and economically are
called Ores.
Iron is found in 800 minerals. Only a few of them are ores (Hematite magnetite)
5. All ores are minerals. All minerals are not ores
List of Ores
1. Aluminium
Bauxite
Al2 O3 .nH2 O
China Clay
Al2 O3 .2SiO2 .2H2 O
Diaspore
Al2 O3 .H2 O
Kaolinite
Al2 Si2 O5 (OH)4
2. Iron
Haematite
F e2 O3
Magnetite
F e3 O4
Siderite
F eCO3
Iron Pyrite
F eS2
Limonite
F e2 O3 .3H2 O
3. Copper
Copper Pyrite
CuF eS2
Copper Glance
Cu2 S
Cuprite
Cu2 O
Malachite
CuCO3 .Cu(OH)2
Azurite
2CuCO3 .Cu(OH)2
4. Zinc
Zinc blende/Sphalerite
ZnS
Calamine
ZnCO3
Zincite
ZnO
5. Lead
METALLURGY
Galena
P bS
Angelsite
P bSO4
1
Cerrusite
P bCO3
6. Tin
Cassiterite (Tin Stone)
SnO2
7. Silver
Silver Glance (Argentite)
Ag2 S
Pyrargyrite (Ruby Silver)
Ag3 SbS3
Chlorargyrite (Horn Silver)
AgCl
Stefinite
Ag5 SbS4
Proustite
Ag3 AsS3
6. Three metallurgical processes:
a. Concentration of the ore
b. Extraction of crude metal
c. Refining of crude metal
7. Ores are associated with nonmetallic impurities, rocky materials and siliceous matter known as Gangue. The first
step is to remove it. This is Concentration of Ore. Methods:
a. Gravity separation
i. Ore is crushed into a powdered form and treated with rapid running water.
High specific gravity ore is separated from low specific gravity gangue.
Applied to oxide ores. Gold, Hematite (F e2 O3 ) , Tin stone (SnO2 )
b. Froth floatation
i. Ore is suspended in water, mixed with frothing agent and collector. Froth is generated and the ore particles
rise with the froth. The froth is skimmed and dried.
Eucalyptus and pine oil is used as frothing agent. Sodium Ethyl Xanthate acts as a collector. Froth is
generated by blowing air into the mix. The collector molecules make the ore water repellant so that it may go
up when the froth is generated. Depressing agents prevent impurities from entering the froth.
Applies to sulphide ores. Galena (P bS ) contains ZnS as impurity. When NaCN is added, it depresses the
floating property of ZnS by reacting and forming a complex Na2 [Zn(CN)4 ] on the surface of ZnS .
Diagram
c. Leaching
i. Crushed ore is allowed to dissolve in a suitable solvent, the metal present in the ore is converted to its
soluble salt or complex while the gangue remains insoluble.
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1. Cyanide Leaching
a. The crushed ore of gold is leached with an aerated dilute solution of sodium cyanide. Gold is
converted into a soluble cyanide complex. The gangue, aluminosilicate remains insoluble.
−
−
4Au(s) + 8CN(aq)
+ O2(g) + 2H2 O(l) ⟶ 4[Au(CN)2 ]−
(aq) + 4OH(aq)
b. Gold can be recovered by reacting the deoxygenated leached solution with zinc. In this process, the
gold is reduced to its elemental state (zero oxidation state) and the process is called Cementation.
Zn(s) + 2[Au(CN)2 ]−
⟶ [Zn(CN)4 ]2−
+ 2Au(s)
(aq)
(aq)
2. Ammonia Leaching
a. When nickel, copper and cobalt ore is treated with aqueous Ammonia under suitable pressure,
ammonia selectively leaches these metals by forming their soluble complexes respectively from the
ore leaving behind the gangue, iron(III) oxides/hydroxides and aluminosilicate. The complexes
formed are:
[Ni(NH3 )6 ]+2 , [Cu(NH3 )4 ]2+ , [Co(NH3 )5 H2 O]+3
3. Alkali Leaching
a. The ore is treated with aqueous alkali to form a soluble complex.
Ex. Bauxite is heated with a solution of NaOH or NaCO3 at 470 - 520 K at 35 atm to form soluble
sodium meta-aluminate leaving behind the impurities.
Al2 O3(s) + 2NaOH(aq) + 3H2 O ⟶ 2Na[Al(OH)4 ](aq)
b. The hot solution is decanted, cooled, and diluted. This solution is neutralized by passing CO2 gas,
to the form of hydrated Al2 O3 precipitate.
2Na[Al(OH)4 ](aq) + 2CO2(g) ⟶ Al2 O3 .H2 O(s) + 2NaHCO3(aq)
The precipitate is filtered off and heated around 1670 K to get pure alumina.
4. Acid Leaching
a. Leaching of sulphide ores can be done by treating them with hot aqueous sulphuric acid. Insoluble
sulphide is converted into soluble sulphate.
2ZnS(s) + 2H2 SO4(aq) + O2 ⟶ 2ZnSO4(aq) + 2S(s) + 2H2 O
d. Magnetic Separation
i. Ore is poured onto an electromagnetic separator consisting of a belt moving over two rollers of which one is
magnetic. The magnetic part of the ore is attracted towards the magnet and falls as a heap close to the
magnetic region while the nonmagnetic part falls away from it.
It is only for ferromagnetic ores. It's based on the difference in the magnetic properties between ores and
impurities. Ex. Wolframite, Chromite, Pyrolusite.
Diagram
8. To Extract crude Metal, the ore is converted to a suitable oxide, then that oxide is reduced.
a. Oxidation
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i. Roasting
1. The concentrated ore is oxidized by heating it with excess oxygen in a suitable furnace below the melting
point of the metal.
It is applied to sulphide ores. Example,
Δ
2P bS + 3O2
Δ
2ZnS + 3O2
2PCu2 S + 3O2
2P bO + 2SO2 ↑
2ZnO + 2SO2 ↑
Δ
2Cu2 O + 2SO2 ↑
It removes arsenic, sulphur and phosphorous impurities
4As + 3O2 ⟶ 2As2 O3 ↑
S8 + 8O2 ⟶ 8SO2 ↑
P4 + 5O2 ⟶ P4 O10 ↑
ii. Calcination
1. During Calcination, the concentrated ore is strongly heated in the absence of air.
It is carried out in limited air supply. It also removes organic matter.
It is meant for Carbonate Ores.
P bCO3
Δ
P bO + CO2 ↑
CaCO3 ⟶ CaO + CO2 ↑
ZnCO3
Δ
ZnO + CO2 ↑
The water of crystallization present in the hydrated oxide escapes as moisture
F e2 O3 .3H2 O
Δ
F e2 O3 (s) + 3H2 O(g) ↑
Al2 O3 .2H2 O
Δ
Al2 O3 (s) + 2H2 O(g) ↑
b. Reduction
i. Smelting
1. A flux is mixed with the reducing agent and the concentrated ore and the mixture is heated at elevated
temperature in a furnace.
Flux is a chemically fusible substance that forms a slag when mixed with gangue. C, CO, Al are
reducing agents. Appropriate reducing agents are found by using The Ellingham diagram.
Example (Iron Oxide)
F e2 O2(s) + 3CO(g) ⟶ 2F e(s) + 3CO2(g) ↑
Flux - Limestone. Gangue - Silica. Slag - Calcium silicate.
CaO(s) + SiO2(s) ⟶ CaSiO3(s)
Example (Copper Pyrite)
2CuF eS2(s) + O2(g) ⟶ 2F eS(l) + Cu2 S(l) + SO2(g)
2F eS(s) + 3O2(g) ⟶ 2F eO(l) + 2SO2(g)
Flux - Silica. Gangue - Ferrous Oxide. Slag - Ferrous Silicate.
F eO(s) + SiO2(s) ⟶ F eSiO3(s)
Copper and Ferrous Sulphides combine and form a copper matte. It is again fed into the furnace with
Silica as flux. Ferrous Sulphide is oxidized to F eO (slag). Copper Sulphide is oxidised to its oxide
and gets converted to metallic copper.
2Cu2 S(l,s) + 3O2(g) ⟶ 2Cu2 O(l,s) + 2SO2(g)
2Cu2 O(l) + Cu2 S(l) ⟶ 6Cu(l) + SO2(g)
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This copper is called blistered copper
ii. Reduction by Carbon
1. Metal oxides are mixed with coke and heated in a blast furnace.
Applied to Metals that forms carbide with carbon. Examples.
ZnO(s) + C(s) ⟶ Zn(s) + CO(g) ↑
Mn3 O4(s) + 4C(s) ⟶ 3Mn(s) + 4CO(g) ↑
Cr2 O3(s) + 3C(s) ⟶ 2Cr(s) + 3CO(g) ↑
iii. Reduction by Hydrogen
1. Metal Oxides are mixed with Hydrogen to reduce into metal
Applied to Metals with less electro-positive character than Hydrogen.
Ag2 O(s) + H2(g) ⟶ 2Ag(s) + H2 O(l)
F e3 O4(s) + 4H2(g) ⟶ 3F e(s) + 4H2 O(l)
Nickel Oxide is reduced using Water Gas (CO + H2 )
2NiO(s) + CO(g) + H2(g) ⟶ 2Ni(s) + CO2(g) + H2 O(l)
iv. Reduction by Metal
1. The Metal oxide is mixed with aluminum powder and placed in a fire clay crucible. An ignition mixture is
used to initiate the reaction. A Large amount of heat is evolved.
Metal oxides (Cr2 O3 ) is reduced by the aluminothermic process. At 2400o C , it reduces to aluminum
powder. The ignition mixture contains Mg and BaO2
BaO2 + Mg ⟶ BaO + MgO
Cr2 O3 + 2Al
Δ
2Cr + Al2 O3
Sodium, Potassium and Calcium can also be used for reduction instead of Aluminum.
v. Auto Reduction
1. Roasting the crude metal without a reducing agent is Auto reduction.
Example
Mercury is obtained by roasting Cinnabar
HgS(s) + O2(g) ⟶ Hg(l) + SO2 ↑
9. Thermodynamic principle of Metallurgy
a. Consider the reduction of metal oxide. y2 Mx Oy(s)
⟶
2x
y M(s)
+ O2(g)
b. Reduction will occur only if the free energy change for the reaction is negative (spontaneous). Hence, the
reducing agent selected must offer the lowest ΔG for the reaction.
c.
ΔG = ΔH − T ΔS . ΔH - Enthalpy. T - Temperature. ΔS - Entropy change. For the equilibrium process,
ΔGo = −RT lnKp
d. Harold Ellingham calculated ΔGo values at various temperatures. He drew a plot with temperature on the xaxis and standard free energy change on the y-axis. The result was a straight line with ΔS slope and ΔH yintercept.
e. The graphical representation of standard Gibbs free energy of reaction for the formation of various metal oxides
with temperature is called Ellingham Diagram.
Ellingham Diagram
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f. Observations from Ellingham Diagram:
i. Slope is positive
Consumption of oxygen results in a decrease in randomness. This implies that ΔS is negative and T ΔS is
positive in the straight-line equation.
ii. Slope in negative for the formation of Carbon Monoxide
Since 2 moles of CO is consumed, ΔS is positive. We can infer that CO is stable at higher temperatures.
iii. As temperature increases, ΔG becomes less negative and becomes 0. Below this temperature, ΔG is
negative, above this, ΔG is positive.
Metal oxides are less stable at high temperatures and they decompose easily.
iv. There is a sudden change in slope for some metals. It is because of phase transition in that particular
temperature. Ex. MgO, HgO (Melting or evaporation)
g. Applications of Ellingham Diagram:
It is used to select suitable reducing agents and appropriate temperatures and to infer the relative stability of
metal oxides at different temperatures.
1. A metal can reduce the oxide of the other metal if it is located above it in the diagram.
2. The formation of Ag2 O and HgO is at the upper part of the diagram.
Decomposition occurs at 600 K and 700 K. These oxides are unstable at moderate temperatures, they will
decompose on heating even without reducing agent.
3. Graph of Carbon cuts across many lines, meaning, it can reduce all those metal oxides at high temperatures.
Example, Reduction of iron with Carbon
2F eO(s) + 2C ⟶ 2F e(l,s) + 2CO(g) , ΔG = −130 kJ mol −1 , at 1000K
standard free energy for reducing 1 mole F eO is ΔG3/2
= −65 kJ mol −1
h. Limitations of Ellingham diagram:
i. It explains the thermodynamic feasibility of a reaction and doesn't explain the rate of reaction. It doesn't
consider the other reactions that might take place.
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ii. Interpretation of ΔG is based on the assumption that the reactant is in equilibrium, which is not always true.
10. Electrochemical properties of metallurgy
a. Here, metal salts are taken in a fused form and the metal ions are reduced by electrolysis.
ΔGo = −nF E o . n - Number of electrons. F - Faraday. E o - Electrode potential.
E 0 is positive and ΔG is negative, the reduction is spontaneous. Then the reaction proceeds such that the e.m.f
of the net redox reaction is positive.
When more reactive metal is added to a solution of less reactive metal, the more reactive metal will go into the
solution.
b. Hall-Héroult Process: Electrochemical extraction of aluminum
i. Electrolysis is carried out in an iron tank with carbon (Cathode) immersed in an electrolyte with Graphite
(Anode). 20% solution of alumina mixed with cryolite is added. 10% calcium chloride solution is used to
reduce the melting point. The fused mixture is obtained at 1270 K.
Al2 O3 ⟶ 2Al 3+ + 3O 2−
3+
Cathode : 2Al(melt)
2−
Anode : 6O(melt)
+ 6e− ⟶ 2Al(l)
⟶ 3O2 + 12e−
Reaction with Carbon : C(s)
+ O 2− ⟶ CO + 2e− ,C(s) + 2O 2− ⟶ CO2 + 4e−
The anode is consumed during the reaction. Pure Aluminum is formed at the cathode.
3+
2−
4Al(melt)
+ 6O(melt)
+ 3C(s) ⟶ 4Al(l) + 3CO2(g)
11. Refining Process
a. Distillation
i. The impure metal is heated to evaporate and the vapors are condensed to get pure metal.
Used for volatile metals with low boiling point. Ex. Zinc (1180 K), Mercury (630 K)
b. Liquation
i. The impure metal is placed in a reverberatory furnace and is heated in the absence of air. Pure metal flows
down a slope and the impurities are left behind.
It is applicable if the impurities have a higher melting point than metals. Ex. Tin (904 K), Lead (600 K),
Mercury (234 K), Bismuth (545 K)
c. Electrolytic refining
i. Rods of impure metal (Anode) and thin strips of pure metal (Cathode) is immersed in an aqueous solution of
a salt of the metal. The metal dissolves from the anode into the solution and the metal ions from the solution
are deposited at the cathode.
Less electropositive impurities settle at the bottom.
Example: Electrolytic refining of Silver
Pure silver is taken as cathode and impure silver as anode with Silver Nitrate is the electrolyte.
At anode : Ag(s)
+
+
⟶ Ag(aq)
+ 1e−
At cathode : Ag(aq)
+ 1e− ⟶ Ag(s)
Anode silver atoms lose electrons and enter the solution. Silver cations migrate towards the cathode and
are discharged by gaining electrons. Copper and Zinc can also be refined similarly
d. Zone Refining
When molten metal is allowed to solidify, the impurities tend to stay in the molten region
1. Impure metal rods are taken and heated on one end using a heater till it melts. When the heater is moved to
the other end, the impurities follow it. The process is repeated by moving it back and forth for several hours
to achieve the desired purity.
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It is carried out in an inert atmosphere to prevent oxidation. Ex. Ge, Si, Ga (semiconductors)
e. Vapor phase method
Metal is treated with a reagent to form a volatile compound, which decomposes to give pure metal.
1. Mond's process of refining Nickel:
a. Impure nickel is treated with the steam of CO to form a highly volatile compound. Impurities are left
behind. On heating at 460 K, pure metal is obtained.
Ni(s) + 4CO(g) ⟶ [Ni(CO)4 ](g)
[Ni(CO)4 ](g) ⟶ Ni(s) + 4CO(g)
2. Van Arkel method for Zirconium and Titanium:
a. When metal undergoes thermal decomposition, it forms pure metal.
b. When titanium is heated in an evacuated vessel with I at 550 K, it forms volatile Titanium tetra-iodide.
Impurities are left behind. Titanium tetraiodide is decomposed and pure titanium is obtained. Iodine is
reused.
T i(s) + 2I2(s)
T iI4(vapour)
550K
Δ
1800K
Δ
T iI4(vapour)
T i(s) + 2I2(s)
12. Applications of metals
a. Aluminum
i. It is the most abundant. Good conductor of heat and electricity. Resists corrosion
ii. It is used as a heat sink. Used to make cooking vessels
iii. Used to make chemical reactors, medical equipment, refrigerator unit, gas pipes
iv. It is also used to make electrical cables. It is alloyed with copper, manganese, magnesium and silicon to
create Duralumin, which is used in aircraft.
b. Copper
i. It is the first metal used by humans
ii. It is used to make coins and ornaments along with gold
iii. it is also used to make wires, pipes and electrical parts
c. Iron
i. It is the most useful metal
ii. It is used in construction, machinery, guns and tools
iii. Its alloy is used to make magnets.
iv. Cast iron is used to make pipes, valves and pump stoves
v. Stainless steel is used in architecture, surgical equipment and jewelry.
Nickel Steel is used in making cables, automobiles and aircrafts parts.
Chrome Steel is used to make cutting tools and crushing equipment.
d. Zinc
i. Used in Galvanizing Iron and steel to prevent corrosion
ii. Used to produce die-castings. Used in making paints, rubber, cosmetics, medicines, plastic, ink, battery,
textile and electrical equipment.
iii. Zinc sulphide is used in making luminous paints, fluorescent lights and x-ray screens.
iv. Brass (Zinc alloy) is used in water valves and communication equipment as it has high corrosion resistance.
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e. Gold
i. It is expensive and precious. It was used for coinage and as a standard for monetary systems
ii. It is used extensively in Jewelry making in alloyed form. It is used for electroplating watches, artificial limbs,
dental fillings and electrical connectors.
iii. Gold nanoparticles are used in solar cells.
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