Notes › Inorganic Chemistry

Metallurgy (Isolation of Elements)

1. Minerals and Ores

A mineral is a naturally occurring compound of a metal; an ore is a mineral from which the metal can be extracted economically. All ores are minerals, but not all minerals are ores.

  • Aluminium: bauxite $AlO_x(OH)_{3-2x}$, often written $Al_2O_3 \cdot xH_2O$.
  • Iron: haematite $Fe_2O_3$, magnetite $Fe_3O_4$, siderite $FeCO_3$.
  • Copper: copper pyrites $CuFeS_2$, malachite, cuprite $Cu_2O$.
  • Zinc: zinc blende (sphalerite) ZnS, calamine $ZnCO_3$, zincite ZnO.

2. Concentration of Ores

  • Hydraulic washing (gravity separation): uses the difference in density between ore and gangue.
  • Magnetic separation: when either ore or gangue is magnetic (e.g. magnetite, chromite).
  • Froth flotation: for sulphide ores. Pine oil acts as a frother, xanthates as collectors and cresols or aniline as froth stabilisers. NaCN is used as a depressant to separate ZnS from PbS.
  • Leaching: chemical dissolution. Bauxite is leached with NaOH (Baeyer process); gold and silver are leached with dilute NaCN in the presence of air.

3. Converting to Oxide

  • Calcination: heating in limited or no air to remove water, $CO_2$ or volatile matter (carbonate and hydrated ores).
  • Roasting: heating in excess air below the melting point; converts sulphides to oxides, e.g. $2ZnS + 3O_2 \to 2ZnO + 2SO_2$.

4. Reduction: The Ellingham Diagram

An Ellingham diagram plots $\Delta_f G^\circ$ of oxides against temperature. A metal can reduce the oxide of any metal whose line lies above its own at that temperature. The $C \to CO$ line slopes downwards, so carbon becomes a better reducing agent at high temperature; this is why coke reduces iron oxide in the blast furnace.

Extraction of Iron (Blast Furnace)

  • Zone of reduction (500–800 K): $3Fe_2O_3 + CO \to 2Fe_3O_4 + CO_2$, then $Fe_3O_4 + 4CO \to 3Fe + 4CO_2$.
  • Higher zone (900–1500 K): $C + CO_2 \to 2CO$ and $FeO + CO \to Fe + CO_2$.
  • Limestone decomposes to CaO, which combines with silica to form slag ($CaSiO_3$).
  • Pig iron has about 4% carbon; cast iron about 3%; wrought iron is the purest commercial form.

Extraction of Copper and Zinc

Copper matte ($Cu_2S$ + FeS) is converted in a Bessemer converter: $2Cu_2O + Cu_2S \to 6Cu + SO_2$. Escaping $SO_2$ gives the solidified copper a blistered appearance (blister copper). Zinc oxide is reduced with coke at 673 K.

Extraction of Aluminium (Hall–Héroult Process)

Purified $Al_2O_3$ is mixed with cryolite ($Na_3AlF_6$) and $CaF_2$, which lower the melting point and increase conductivity, and electrolysed with carbon electrodes. The carbon anode is burnt away by the oxygen released and must be replaced periodically.

5. Refining

  • Distillation: low-boiling metals such as Zn and Hg.
  • Liquation: low-melting metals such as Sn.
  • Electrolytic refining: impure metal as anode, pure metal as cathode (Cu, Zn). Anode mud contains Ag, Au and Pt.
  • Zone refining: impurities are more soluble in the melt; gives ultra-pure Si, Ge, Ga for semiconductors.
  • Vapour-phase refining: Mond process for Ni (via $Ni(CO)_4$) and van Arkel method for Zr and Ti (via $ZrI_4$/$TiI_4$).
  • Chromatography: for elements available in small amounts.

Practice questions

  1. Froth flotation is mainly used to concentrate:

    1. Oxide ores
    2. Sulphide ores
    3. Carbonate ores
    4. Halide ores
    Answer

    B. Sulphide particles are preferentially wetted by oil and carried up in the froth.

  2. In the extraction of aluminium, cryolite is added to:

    1. Reduce Al₂O₃
    2. Lower the melting point and increase conductivity
    3. Remove impurities as slag
    4. Act as the anode
    Answer

    B. Pure Al₂O₃ melts above 2300 K; cryolite lowers the working temperature and improves conductivity.

  3. Zone refining is used for:

    1. Copper
    2. Silicon
    3. Iron
    4. Aluminium
    Answer

    B. Very high-purity semiconductors (Si, Ge, Ga) are produced by zone refining.

  4. The Mond process is used to refine:

    1. Zirconium
    2. Nickel
    3. Titanium
    4. Zinc
    Answer

    B. Ni forms volatile Ni(CO)₄ at about 330–350 K, which decomposes at 450–470 K to give pure Ni.

  5. Roasting differs from calcination in that roasting:

    1. Is done in the absence of air
    2. Is done in excess air
    3. Is done above the melting point
    4. Uses carbon
    Answer

    B. Roasting heats the ore in excess air (e.g. converting sulphides to oxides); calcination uses little or no air.

  6. In the blast furnace, the slag formed is:

    1. CaCO₃
    2. CaSiO₃
    3. FeSiO₃
    4. CaO
    Answer

    B. CaO + SiO₂ → CaSiO₃ removes silica gangue.

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