JEE Chemistry General Principles and Processes Of Isolation Of Elements Notes Set 03

Terms Used in Metallurgy

Aluminium is the most abundant metal in the earth's crust. Metallurgy is the science of extracting metals from ores. Pyrometallurgy uses heat (roasting, smelting), hydrometallurgy uses aqueous solutions (leaching, cyanide process) and electrometallurgy uses electrolysis.

Steps of Extraction

1. Crushing and grinding of the ore.
2. Concentration (dressing) to remove gangue.
3. Conversion to the oxide: calcination or roasting.
4. Reduction of the oxide to the metal.
5. Refining of the crude metal.

Concentration of the Ore

In flotation of an ore containing ZnS and PbS, NaCN is used as a depressant. It forms a complex with ZnS and stops it from floating, but PbS comes up in the froth: \( \ce{NaCN + ZnS -> Na2[Zn(CN)4]} \).

Leaching

1. Red bauxite (Al₂O₃·2H₂O with Fe₂O₃): Baeyer's process uses hot concentrated NaOH, which dissolves Al₂O₃ as sodium aluminate while Fe₂O₃ stays insoluble. Hall's process uses fusion with Na₂CO₃, then water extraction and CO₂ to precipitate Al(OH)₃.
2. White bauxite (with SiO₂): Serpeck's process heats the ore with coke in nitrogen at about 1800 °C to give AlN, which hydrolyses to Al(OH)₃ and NH₃. Silica is lost as volatile silicon.
3. Silver and gold (cyanide process): the ore dissolves in aerated cyanide as a complex. \( \ce{Ag2S + 4NaCN + 2O2 -> 2Na[Ag(CN)2] + Na2SO4} \). Zinc then displaces the metal: \( \ce{2Na[Ag(CN)2] + Zn -> Na2[Zn(CN)4] + 2Ag} \). Oxygen makes the first reaction irreversible by oxidising the sulphide.

Calcination and Roasting

For PbS, CuS and HgS only partial roasting is done because the oxide formed goes on to react with the unchanged sulphide (autoreduction). In iron extraction roasting converts FeO to Fe₂O₃, so that iron is not lost as FeSiO₃ slag.

Reduction of the Oxide

1. Carbon reduction: oxides of less reactive metals (Pb, Zn, Fe, Sn) are heated with coke. Oxides of Al, Cr, Mn are not reduced by carbon. CO is a better reductant than C except for Zn.
2. Self reduction: \( \ce{2Cu2O + Cu2S -> 6Cu + SO2} \) for Cu, Pb, Hg after partial roasting.
3. Metal displacement: lean copper ore is dissolved in acid and scrap iron displaces copper. Silver and gold are displaced by zinc.
4. Electrolytic reduction of fused salts for Na, K, Mg, Ca, Al. In Down's process molten NaCl with some CaCl₂ is electrolysed between a graphite anode and an iron cathode. In the Hall-Heroult process Al₂O₃ is dissolved in cryolite and CaF₂, which lowers the melting point from 2050 °C to about 950 °C and improves conductivity.
5. Aluminothermite (Goldschmidt) process: Al reduces Cr₂O₃, Fe₂O₃ and Mn oxides, because Al₂O₃ is very stable and the reaction gives a lot of heat. Thermite is Fe₂O₃ and Al in about 3 : 1.

Refining of Metals

Extraction of Copper from Copper Pyrites

1. Froth flotation of the finely ground ore, followed by roasting at a moderate temperature: some CuFeS₂ is oxidised and FeS goes to FeO.
2. Smelting with coke and silica in a blast furnace: FeO + SiO₂ → FeSiO₃ (slag). The molten mixture of Cu₂S and FeS is copper matte.
3. Bessemer converter: silica is added and air is blown. FeS is oxidised to FeO and removed as slag, and Cu₂S is partly oxidised to Cu₂O, which reacts with Cu₂S (self reduction) to give blister copper (about 98% Cu).
4. Poling and electrolytic refining give 99.99% pure copper. The electrolyte is CuSO₄ with dilute H₂SO₄.

Extraction of Aluminium

1. Bauxite is concentrated by Baeyer, Hall or Serpeck process to pure Al₂O₃ (after calcination of Al(OH)₃ at about 1500 °C).
2. Pure alumina in molten cryolite and CaF₂ is electrolysed at about 950 °C with a graphite anode and a carbon-lined iron cathode, giving 99.8% Al.
3. Hoope's electrolytic process raises the purity to 99.98%.

Extraction of Iron

1. Ore is concentrated by gravity separation and then calcined and roasted: Fe₃O₄ → FeO + Fe₂O₃ and FeCO₃ → FeO + CO₂. FeO is then converted to Fe₂O₃ so that it does not form slag with silica.
2. Reduction in the blast furnace with coke and limestone (about 8 : 4 : 1 by weight of ore, coke, CaCO₃). The slag is CaSiO₃ and the product is pig iron.
3. Forms of iron: pig iron and cast iron (2.5 to 5% carbon), steel (0.25 to 2%) and wrought iron (0.12 to 0.25%, the purest). Melting point order: wrought iron > steel > cast iron.
4. Heat treatment of steel: quenching (sudden cooling) makes it hard and brittle, annealing (slow cooling) makes it soft and tempering (reheating to 500-575 K then slow cooling) removes brittleness.