General Principles and Processes of Isolation of Elements — Notes

Mineral: naturally occurring compound of a metal. Ore: a mineral from which the metal is extracted profitably. Gangue: earthy impurities. Steps of metallurgy: concentration → conversion to oxide → reduction → refining.

Concentration (benefaction):

• Hydraulic washing (gravity separation): ore heavier than gangue (tin stone, lead ores).
• Magnetic separation: one component magnetic (magnetite, chromite; wolframite from tin stone).
• Froth flotation: sulphide ores; collectors (xanthates, fatty acids) make ore water-repellent, frothers (pine oil, cresols) stabilise froth, depressants (NaCN) keep one sulphide (ZnS) down while another (PbS) floats.
• Leaching: ore dissolved selectively. Bauxite: $\ce{Al2O3 + 2NaOH + 3H2O -> 2Na[Al(OH)4]}$; alumina precipitated by CO₂ and heated at 1470 K. Gold/silver: $\ce{4Au + 8CN- + 2H2O + O2 -> 4[Au(CN)2]- + 4OH-}$, then displaced by zinc.

• Calcination: heating in limited air or none; removes CO₂, H₂O, volatile matter. $\ce{ZnCO3 -> ZnO + CO2}$; $\ce{Al2O3.2H2O -> Al2O3 + 2H2O}$.
• Roasting: heating in excess air below the melting point; sulphides → oxides. $\ce{2ZnS + 3O2 -> 2ZnO + 2SO2}$; $\ce{2PbS + 3O2 -> 2PbO + 2SO2}$. SO₂ is used for sulfuric acid.
• Flux converts leftover gangue into fusible slag: acidic gangue + basic flux ($\ce{CaO + SiO2 -> CaSiO3}$); basic gangue + acidic flux ($\ce{FeO + SiO2 -> FeSiO3}$).

Reduction of an oxide by a reducing agent is feasible when the overall $\Delta G < 0$. Coupling: $\ce{2M + O2 -> 2MO}$ ($\Delta G_1$) and $\ce{2C + O2 -> 2CO}$ ($\Delta G_2$) give $\ce{2MO + 2C -> 2M + 2CO}$ with $\Delta G = \Delta G_2 - \Delta G_1$, negative when the carbon line lies below the metal line.

Ellingham diagram: $\Delta G^\circ$ (per mol O₂) for oxide formation vs $T$.

• Metal oxide lines slope upwards ($\Delta S < 0$, gas consumed); kinks at melting or boiling points.
• $\ce{2C + O2 -> 2CO}$ slopes downwards ($\Delta S > 0$); $\ce{C + O2 -> CO2}$ is nearly flat; $\ce{2CO + O2 -> 2CO2}$ slopes upwards.
• An element whose line lies below another oxide’s line can reduce that oxide at that temperature. Carbon reduces FeO above about 1000 K and ZnO at higher temperatures, but not Al₂O₃ or MgO in practice.
• Limitation: it says nothing about rate (kinetics).

• Iron (blast furnace): haematite + coke + limestone; hot air at the bottom. $\ce{C + O2 -> CO2}$, $\ce{CO2 + C -> 2CO}$; CO reduces iron oxides in the upper zone (500–800 K): $\ce{3Fe2O3 + CO -> 2Fe3O4 + CO2}$, $\ce{Fe3O4 + 4CO -> 3Fe + 4CO2}$; lower (900–1500 K): $\ce{FeO + C -> Fe + CO}$. Slag: $\ce{CaO + SiO2 -> CaSiO3}$. Pig iron about 4% C; cast iron about 3% C; wrought iron purest; steel.
• Copper: roast, then smelt with silica (FeO → FeSiO₃ slag), giving matte (Cu₂S + FeS). Converter: $\ce{2Cu2S + 3O2 -> 2Cu2O + 2SO2}$; self-reduction $\ce{2Cu2O + Cu2S -> 6Cu + SO2}$ → blister copper.
• Zinc: $\ce{ZnO + C -> Zn + CO}$ at about 1673 K; zinc distils off.
• Aluminium (Hall–Héroult): electrolysis of Al₂O₃ in molten cryolite (Na₃AlF₆) + CaF₂ at about 1250 K. Cathode (carbon-lined steel): $\ce{Al^{3+} + 3e- -> Al}$. Graphite anodes burn: $\ce{2Al2O3 + 3C -> 4Al + 3CO2}$.

Electrochemical principle: $\Delta G^\circ = -nFE^\circ$; a voltage greater than the required $E$ drives a non-spontaneous reduction. Oxidation: chlorine from brine, $\ce{2Cl- + 2H2O -> 2OH- + H2 + Cl2}$.

• Distillation: low-boiling metals (Zn, Hg).
• Liquation: low-melting metals (Sn) flow away from infusible impurities.
• Electrolytic refining: impure metal anode, pure metal cathode, salt solution electrolyte; Cu: $\ce{Cu -> Cu^{2+} + 2e-}$ (anode), $\ce{Cu^{2+} + 2e- -> Cu}$ (cathode). Ag, Au, Pt collect as anode mud.
• Zone refining: impurities more soluble in the melt; moving heater sweeps them to one end. Si, Ge, Ga, B, In of very high purity.
• Vapour phase refining: Mond process $\ce{Ni + 4CO ->[330 K] Ni(CO)4}$, decomposed at 450–470 K; van Arkel method $\ce{Zr + 2I2 -> ZrI4}$, decomposed on a hot tungsten filament (Zr, Ti).
• Chromatography: separates elements present in small amounts by differential adsorption.

Uses: Al — foil, cables, alloys; Cu — wires, pipes, brass, bronze, coins; Zn — galvanising, dry cells, brass; Fe — cast iron, steel and alloy steels.