General Principles & Processes of Isolation of Elements

Metallurgy — the extraction and refining of metals from their ores — is governed by thermodynamic principles illustrated in the Ellingham diagram. A mineral is any naturally occurring inorganic compound of a metal; an ore is a mineral from which the metal can be profitably extracted.

Ores are classified by type: oxide ores (bauxite Al₂O₃.2H₂O, haematite Fe₂O₃), sulphide ores (copper pyrite CuFeS₂, zinc blende ZnS, galena PbS), carbonate ores (calamine ZnCO₃, siderite FeCO₃), and halide ores (cryolite Na₃AlF₆, rock salt NaCl).

Concentration Methods: Hydraulic washing separates heavier ore from lighter gangue using flowing water (for oxide ores). Magnetic separation uses a magnetic roller for ores like chromite. Froth flotation is used for sulphide ores: ore is mixed with water and pine oil (collector), air is blown to create froth — hydrophobic sulphide particles attach to froth while hydrophilic gangue sinks. NaCN acts as a depressant, selectively suppressing ZnS in a ZnS-PbS mixture by forming soluble Na₂[Zn(CN)₄] on the ZnS surface. Leaching uses chemical solutions: gold extraction uses NaCN (4Au + 8NaCN + 2H₂O + O₂ → 4Na[Au(CN)₂] + 4NaOH; Au is recovered by Zn displacement). Bayer's process leaches bauxite with NaOH to form soluble NaAlO₂, separating impurities, then precipitates Al(OH)₃ and calcines it to pure Al₂O₃.

Extraction: Calcination heats ore in limited/absent air for carbonate and hydrated ores (ZnCO₃ → ZnO + CO₂). Roasting heats ore in excess air for sulphide ores (2ZnS + 3O₂ → 2ZnO + 2SO₂). Smelting reduces the oxide with carbon or CO at high temperature. Flux is added to combine with gangue, forming slag (acidic gangue + basic flux, or vice versa).

Ellingham Diagram: Plots of standard Gibbs free energy of oxide formation ($\Delta$-G-degree) versus temperature. A metal can reduce the oxide of another metal whose line lies above it (more positive $\Delta$-G-degree). The C + O₂ → CO₂ line is nearly horizontal, while the 2C + O₂ → 2CO line slopes downward (becomes more negative) because $\Delta$-S is positive (solid → gases). This means carbon becomes an increasingly better reducing agent at higher temperatures — explaining why coke reduces many metal oxides in blast furnaces.

Specific Extractions: Aluminium uses the Hall-Heroult process: purified Al₂O₃ dissolved in molten cryolite (Na₃AlF₆, lowers melting point from 2072 degrees C to ~950 degrees C) with CaF₂ for conductivity. Carbon anodes are consumed (C + $O^{2}$- → CO₂), and $Al^{3}$+ deposits at the carbon-lined steel cathode. Copper extraction involves roasting CuFeS₂ to a matte (Cu₂S + FeS), then self-reduction in a Bessemer converter: 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂, followed by Cu₂S + 2Cu₂O → 6Cu + SO₂. Blister copper (~98% pure, blistered by escaping SO₂) undergoes electrolytic refining. Iron extraction in a blast furnace uses haematite + coke + limestone: at the reduction zone (~500-800 K), Fe₂O₃ + 3CO → 2Fe + 3CO₂. Limestone decomposes to CaO, which combines with silica gangue: CaO + SiO₂ → CaSiO₃ (slag).

Refining Methods: Distillation works for volatile metals (Zn, Hg). Liquation melts low-melting metals like Sn. Electrolytic refining uses impure metal as anode, pure metal as cathode, and metal salt as electrolyte (Cu, Zn, Al); anode mud contains less electropositive metals like Au and Ag. Zone refining produces ultra-pure semiconductors (Si, Ge) by sweeping impurities into a molten zone. The Mond process purifies Ni: Ni + 4CO →(330-350 K) Ni(CO)₄ →(450-470 K) Ni + 4CO. The Van Arkel method purifies reactive metals (Ti, Zr): Ti + 2I₂ →(~870 K) TiI₄ →(~1700 K on W filament) Ti + 2I₂.

The key testable concept is the Ellingham diagram interpretation — specifically why the C → CO line slopes downward (positive $\Delta$-S), making carbon a better reducing agent at higher temperatures.