Metallurgy is the branch of chemistry dealing with the extraction and refining of metals from their ores. A mineral is any naturally occurring inorganic compound of a metal found in the earth's crust, while an ore is a mineral from which the metal can be profitably and economically extracted. Not every mineral is an ore — the distinction lies in economic viability of extraction.

Classification of Ores

Ores are grouped by their anion type. Oxide ores include bauxite ($Al_{2}O_{3}$·$2H_{2}O$) and haematite ($Fe_{2}O_{3}$). Sulphide ores include copper pyrite ($CuFeS_{2}$), zinc blende (ZnS), and galena (PbS). Carbonate ores include calamine ($ZnCO_{3}$) and siderite ($FeCO_{3}$). Halide ores include cryolite ($Na_{3}AlF_{6}$). Knowing which ore belongs to which metal is directly tested in NEET.

Step 1 — Concentration (Ore Dressing)

Raw ore contains gangue (unwanted rocky impurities). Concentration removes gangue before extraction.

• Hydraulic washing exploits density difference: a stream of water carries lighter gangue away, leaving heavier ore particles behind. Used for oxide ores (haematite, cassiterite).
• Magnetic separation uses a rotating magnetic drum. Magnetic ores (chromite, wolframite) are attracted and separated from non-magnetic gangue, or vice versa.
• Froth flotation is the most-tested method. Ore is crushed, mixed with water, and pine oil (collector) is added. Compressed air creates froth. Sulphide ore particles are hydrophobic — they attach to air bubbles and float, while hydrophilic gangue sinks. NaCN acts as a depressant for ZnS in ZnS–PbS mixtures by forming soluble $Na_{2}$[Zn(CN)_{4}] on the ZnS surface, making it hydrophilic so only PbS floats.
• Leaching dissolves the metal with a chemical reagent. Gold leaching uses NaCN: 4Au + 8NaCN + $2H_{2}O$ + $O_{2}$ → 4Na[Au(CN){2}] + 4NaOH; gold is recovered by zinc displacement. Bayer's process leaches bauxite with hot NaOH, dissolving Al as NaAl$O_{2}$, then precipitating Al(OH){3} on dilution and calcining to pure $Al_{2}O_{3}$.

Step 2 — Extraction from Concentrated Ore

• Calcination: ore heated in limited or absent air. Used for carbonate and hydrated ores. Example: $ZnCO_{3}$ → ZnO + C$O_{2}$. Products are metal oxide + C$O_{2}$ or $H_{2}O$.
• Roasting: ore heated in excess air. Used for sulphide ores. Example: 2ZnS + 3$O_{2}$ → 2ZnO + 2S$O_{2}$. Products are metal oxide + S$O_{2}$.
• Smelting: reduction of metal oxide with carbon (coke) or CO in a blast furnace at high temperature, with a flux to form slag.
• Flux: acidic gangue needs basic flux (limestone/CaO); basic gangue needs acidic flux (Si$O_{2}$). Flux + gangue → slag (discarded).

Ellingham Diagram

The Ellingham diagram plots standard Gibbs free energy of oxide formation ($\Delta G$°) vs. temperature for metals and carbon. Key rules:

• The metal oxide whose line is lower (more negative $\Delta G$°) is more thermodynamically stable.
• A metal whose line lies lower can reduce the oxide of a metal whose line lies higher.
• The C + $O_{2}$ → C$O_{2}$ line is nearly horizontal ($\Delta S$ ≈ 0).
• The 2C + $O_{2}$ → 2CO line slopes downward ($\Delta S$ > 0, since 1 mol solid + 1 mol gas → 2 mol gas). At high temperatures this line crosses the oxide lines of many metals, enabling carbon to act as a reducing agent — the thermodynamic basis of blast furnace smelting.

Specific Metal Extractions

Aluminium (Hall-Heroult process): Purified $Al_{2}O_{3}$ from Bayer's process is dissolved in molten cryolite ($Na_{3}AlF_{6}$), which lowers the melting point from 2072 °C to ~950 °C. $CaF_{2}$ is added to increase conductivity. At the carbon anode: C + $O^{2-}$ → C$O_{2}$ (anodes are consumed and periodically replaced). At the carbon-lined steel cathode: $Al^{3+}$ + 3$e^{-}$ → Al. Molten aluminium settles at the bottom.

Copper (self-reduction): $CuFeS_{2}$ is concentrated by froth flotation, then roasted to matte ($Cu_{2}S$ + FeS). In the Bessemer converter: 2$Cu_{2}S$ + 3$O_{2}$ → $2Cu_{2}O$ + 2S$O_{2}$; then $Cu_{2}S$ + $2Cu_{2}O$ → 6Cu + S$O_{2}$. Blister copper (~98% pure) is named for S$O_{2}$ blisters. Refined by electrolysis — anode mud contains Au and Ag.

Iron (blast furnace): Haematite + coke + limestone are charged at the top. At the reduction zone (500–800 K): $Fe_{2}O_{3}$ + 3CO → 2Fe + 3C$O_{2}$. CaO (from $CaCO_{3}$ decomposition) combines with silica: CaO + Si$O_{2}$ → $CaSiO_{3}$ (slag). Molten iron tapped from the bottom is pig iron.

Refining Methods

• Distillation: for volatile metals (Zn, Hg).
• Liquation: for low-melting metals like Sn (melt on a sloped hearth).
• Electrolytic refining: impure metal anode, pure metal cathode, metal salt electrolyte. Anode mud retains less electropositive metals (Au, Ag).
• Zone refining: impurities concentrate in the molten zone; heater sweeps them to one end. Produces ultra-pure Si, Ge for semiconductors.
• Mond process (Ni): Ni + 4CO → Ni(CO){4} at 330–350 K; Ni(CO){4} → Ni + 4CO at 450–470 K.
• Van Arkel method (Ti, Zr): Ti + $2I_{2}$ → $TiI_{4}$ at ~870 K; $TiI_{4}$ → Ti + $2I_{2}$ at ~1700 K on a hot tungsten filament.