JEE Chemistry General Principles and Processes Of Isolation Of Elements Notes Set 01

Occurrence of Metals

A naturally occurring compound of a metal is a mineral. If the metal can be extracted from it profitably, the mineral is an ore. The earthy and rocky impurities mixed with an ore are called gangue (matrix).
Abundance in the earth's crust: O > Si > Al > Fe > Ca. Aluminium is the most abundant metal and iron the second.

Concentration of Ores

In froth floatation a collector (e.g. sodium ethyl xanthate) makes the ore particles water repellent, a frother (pine oil) stabilises the froth, and a depressant (NaCN) prevents one sulphide from floating. For a PbS and ZnS mixture, NaCN forms a soluble complex with ZnS so that only PbS floats.
Leaching bauxite (Baeyer's process): Al₂O₃ dissolves in hot concentrated NaOH as sodium aluminate while Fe₂O₃ and TiO₂ remain; SiO₂ also dissolves as silicate. CO₂ precipitates Al(OH)₃, which is ignited to alumina.
Silver and gold: 4M + 8CN⁻ + 2H₂O + O₂ → 4[M(CN)₂]⁻ + 4OH⁻, then 2[M(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2M.

Conversion of the Concentrated Ore to Metal

Calcination: heating in limited or no air below the melting point to drive off water and CO₂, e.g. CaCO₃ → CaO + CO₂, Al₂O₃·2H₂O → Al₂O₃ + 2H₂O.
Roasting: heating a sulphide ore in excess air, e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂.
Smelting: reduction of the oxide with carbon at high temperature with a flux, giving molten metal and slag. A flux combines with gangue: basic flux (CaO) removes acidic SiO₂ as CaSiO₃; acidic flux (SiO₂) removes basic FeO as FeSiO₃.

Thermodynamic Principle and Ellingham Diagram

A reduction is feasible when the overall ΔG is negative. ΔG° = ΔH° − TΔS°. Forming a metal oxide uses up oxygen gas, so ΔS is negative and the Ellingham line (ΔG° versus T) slopes upward. For C → CO, ΔS is positive and the line slopes downward.
A reducing agent can reduce an oxide when its own line lies below that of the metal oxide at the temperature used. Hence carbon reduces ZnO and Fe₂O₃ but not Al₂O₃ or MgO under practical conditions, and Al reduces Cr₂O₃ and Mn₃O₄ (thermite).
Sulphides are roasted to oxides first, because ΔfG° of CS₂ and H₂S are not favourable for reduction of sulphides by C or H₂.

Extraction of Iron

Charge: roasted ore (Fe₂O₃), coke and limestone, fed at the top of the blast furnace with a hot air blast from the bottom.
Zone of combustion (bottom, about 2200 K): C + O₂ → CO₂. Higher up, CO₂ + C → 2CO. Zone of reduction: Fe₂O₃ + 3CO → 2Fe + 3CO₂ and FeO + CO → Fe + CO₂.
Limestone decomposes (endothermic) to CaO, which combines with SiO₂ to give CaSiO₃ slag. Molten slag floats over molten iron at the bottom.
Pig iron has about 4% carbon; cast iron about 3%; wrought iron (purest) 0.1 to 0.25%; steel 0.1 to 1.5% (made by the basic oxygen or open-hearth processes, with Mn as scavenger and phosphorus removed as Thomas slag, Ca₃(PO₄)₂). Quenching steel makes it hard and brittle, tempering reduces the brittleness, annealing softens it.

Extraction of Copper

Chalcopyrite is concentrated by froth floatation and partly roasted: 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂; 2FeS + 3O₂ → 2FeO + 2SO₂.
Smelting with silica removes iron as FeSiO₃ slag and leaves matte (Cu₂S + FeS). In the Bessemer converter: 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂, then self-reduction 2Cu₂O + Cu₂S → 6Cu + SO₂, giving blister copper (about 98% Cu). It is refined by poling or electrolysis.

Extraction of Aluminium (Hall-Heroult)

Pure Al₂O₃ from bauxite is dissolved in molten cryolite with CaF₂ (melting point lowered to about 1230 K, conductivity raised) and electrolysed using a carbon-lined steel cell (cathode) and graphite anodes.
Cathode: Al³⁺ + 3e⁻ → Al (molten Al collects at the bottom). Anode: O²⁻ is discharged and oxygen burns the carbon anodes to CO and CO₂, so they are replaced periodically.

Extraction of Other Metals

Refining of Metals

In electrolytic refining of copper, the less reactive metals Ag, Au and Pt do not dissolve and collect as anode mud; the more reactive Fe and Zn dissolve and stay in solution.