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Exam Master Review Sheet - Metallurgy

Chemca Formula Sheet - Metallurgy

CHEMCA

EXAM MASTER FORMULA SHEET

Inorganic Chemistry: General Principles of Metallurgy

Isolation of Elements, Ellingham Diagram & Refining

1. Principal Ores of Common Metals

Minerals are naturally occurring chemical substances; Ores are minerals from which a metal can be extracted profitably and conveniently.

Metal Principal Ores (Name & Formula)
Aluminium (Al) Bauxite: $\ce{AlO_x(OH)_{3-2x}}$ (where $0 < x < 1$)
Cryolite: $\ce{Na3AlF6}$
Kaolinite (clay): $\ce{[Al2(OH)4Si2O5]}$
Iron (Fe) Haematite: $\ce{Fe2O3}$ | Magnetite: $\ce{Fe3O4}$
Siderite: $\ce{FeCO3}$ | Iron Pyrites: $\ce{FeS2}$
Copper (Cu) Copper Pyrites: $\ce{CuFeS2}$ | Malachite: $\ce{CuCO3 \cdot Cu(OH)2}$
Cuprite: $\ce{Cu2O}$ | Copper Glance: $\ce{Cu2S}$
Zinc (Zn) & Lead (Pb) Zinc Blende/Sphalerite: $\ce{ZnS}$ | Calamine: $\ce{ZnCO3}$ | Zincite: $\ce{ZnO}$
Galena: $\ce{PbS}$ | Anglesite: $\ce{PbSO4}$

2. Concentration of Ores (Benefaction)

Froth Floatation

Specifically used for removing gangue from Sulphide Ores. Based on differential wettability (ore wetted by oil, gangue by water).

  • • Collectors: Pine oil, fatty acids, xanthates (attach to ore particles and enhance non-wettability).
  • • Froth Stabilizers: Cresols, Aniline.
  • • Depressants: Used to separate two sulphide ores. E.g., $\ce{NaCN}$ is used to separate $\ce{ZnS}$ and $\ce{PbS}$. It selectively prevents $\ce{ZnS}$ from coming to the froth by forming the soluble complex $\ce{Na2[Zn(CN)4]}$.
Magnetic Separation

Used when either the ore or the impurities are magnetic.

High Yield Examples:

  • Separating Wolframite (magnetic, $\ce{FeWO4}$) from Cassiterite (non-magnetic, $\ce{SnO2}$).
  • Separating Chromite ($\ce{FeCr2O4}$) or Magnetite ($\ce{Fe3O4}$) from non-magnetic gangue.
Leaching (Chemical Concentration)

Ore is soluble in a suitable chemical reagent while impurities remain insoluble.

1. Baeyer's Process (Leaching of Alumina):

Impure Bauxite + hot conc. NaOH. Impurities ($Fe_2O_3, TiO_2$) are left behind.

$\ce{Al2O3(s) + 2NaOH(aq) + 3H2O(l) -> 2Na[Al(OH)4](aq)}$

The solution is then seeded and $CO_2$ is passed to precipitate pure $Al(OH)_3$, which is heated to get pure $Al_2O_3$.

2. MacArthur-Forrest Cyanide Process (Au/Ag):

Leaching with dilute $NaCN$ or $KCN$ in presence of air ($O_2$).

$\ce{4Au(s) + 8CN^-(aq) + 2H2O(aq) + O2(g) -> 4[Au(CN)2]^-(aq) + 4OH^-(aq)}$

Metal is recovered by displacement with Zinc: $\ce{2[Au(CN)2]^- + Zn -> [Zn(CN)4]^{2-} + 2Au v}$

3. Ellingham Diagram & Thermodynamic Principles

Graphical representation of Gibbs Free Energy of formation of oxides ($\Delta G^\ominus$) vs Temperature ($T$). Used to predict the feasibility of thermal reduction of ores.

\[ \Delta G^\ominus = \Delta H^\ominus - T\Delta S^\ominus \]

Reaction is feasible if $\Delta G^\ominus$ is NEGATIVE.

The Reduction Rule: A metal can reduce the oxide of any other metal which lies above it in the Ellingham diagram (because the net $\Delta G$ will be negative). E.g., $Al$ can reduce $Cr_2O_3$.
Crossover Logic for Iron: Below 710°C, the $C \to CO$ line is above the $Fe \to FeO$ line, but $CO \to CO_2$ is below it. Therefore, CO is a better reducing agent below ~710°C. Above 710°C, Carbon (C) is better.

4. Industrial Extraction Processes

Iron (Blast Furnace)

Ore: Haematite. Reducing Agent: $CO$ (at low T) and $C$ (at high T). Flux: Limestone ($CaCO_3$).

  • • Reduction Zone (500-800 K):
    $\ce{3Fe2O3 + CO -> 2Fe3O4 + CO2}$
    $\ce{Fe3O4 + 4CO -> 3Fe + 4CO2}$
  • • Slag Formation Zone (~1200 K):
    $\ce{CaCO3 -> CaO + CO2}$
    $\ce{CaO + SiO2 -> CaSiO3}$ (Slag)
  • • Combustion Zone (~2200 K):
    $\ce{C + O2 -> CO2 + Heat}$
Pig Iron: ~4% C | Cast Iron: ~3% C | Wrought Iron: Purest (<0.5% C)

Aluminium (Hall-Heroult Process)

Electrolytic reduction of fused Alumina ($\ce{Al2O3}$).

Cryolite ($\ce{Na3AlF6}$) & Fluorspar ($\ce{CaF2}$): Added to lower the melting point and increase electrical conductivity.

  • • Cathode (Steel vessel lining):
    $\ce{Al^3+ + 3e- -> Al(l)}$
  • • Anode (Graphite rods):
    $\ce{C(s) + O^2- -> CO(g) + 2e-}$
    $\ce{C(s) + 2O^2- -> CO2(g) + 4e-}$

*Anodes must be replaced periodically as they burn away.*

Self Reduction (Auto Reduction)

Used for less electropositive metals like $\ce{Cu, Pb, Hg}$. No external reducing agent is required.

$\ce{2Cu2O + Cu2S -> 6Cu + SO2 \uparrow}$ (Blister Copper)
Copper Matte

During smelting of copper pyrites, iron is removed as slag ($\ce{FeSiO3}$).

Copper Matte = $\ce{Cu2S}$ + $\ce{FeS}$ (small amount)

5. Refining of Crude Metals

Vapour Phase Refining

Metal is converted to a volatile compound, collected, and decomposed to give pure metal.

1. Mond Process (For Nickel):
$\ce{Ni + 4CO ->[330-350 K] Ni(CO)4(g)}$
$\ce{Ni(CO)4 ->[450-470 K] Ni(s) + 4CO}$
2. van Arkel Method (For Zr / Ti):

Used to remove all oxygen and nitrogen impurities.

$\ce{Zr + 2I2 ->[870 K] ZrI4(g) ->[2075 K, W filament] Zr(s) + 2I2}$
Physical Methods
  • Distillation: For low B.P. metals like $Zn, Hg, Cd$.
  • Liquation: For low M.P. metals like $Sn, Pb, Bi$. Impurities remain solid while metal flows down a sloping hearth.
  • Zone Refining: For extremely pure Semiconductors ($Si, Ge, Ga, In$). Based on the principle that impurities are more soluble in the melt than in the solid state of the metal.
  • Chromatography: Based on differential adsorption. Useful for purification of elements available in minute quantities (Lanthanoids).
Electrolytic Refining (e.g., Copper)
  • Anode: Thick block of Impure metal.
  • Cathode: Thin strip of Pure metal.
  • Electrolyte: Soluble salt of the metal (e.g., acidified $\ce{CuSO4}$).

Anode Mud contains unreacted noble metal impurities: Antimony, Selenium, Tellurium, Silver, Gold, Platinum.

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