Principles & Processes of Extraction of Elements (Metallurgy)

Metallurgy: Key Definitions

• Metallurgy: methods of extraction of metals from ores and alloy formation.
• Minerals: compounds of metals found in earth’s crust (obtained by mining).
• Ore: a mineral from which a metal can be extracted profitably and easily.
• Gangue / Matrix: unwanted impurities (clay, sand, pebbles, etc.) present in ore.
• Flux: added during smelting to convert infusible impurities into fusible slag.
• Slag: product formed by reaction of flux with gangue.
• Earth’s crust: Oxygen is the most abundant non-metal; Aluminium is the most abundant metal.

Modes of Occurrence of Elements

1) Native (Free) State

• Elements not attacked by moisture, O₂ and CO₂ occur in native state (e.g., Au, Pt, noble gases).

2) Combined State

• Elements attacked by moisture, O₂ and CO₂ occur as compounds (minerals).
• Metals are generally found in oxidised form (oxides, carbonates, sulphides, silicates, etc.).

Common Ore Types & Examples

Flux & Slag

General: Flux + infusible gangue → slag

Acidic Flux

• Used to remove basic impurities.
• Common acidic flux: SiO₂ (silica).
• Acidic flux + basic gangue → slag

Basic Flux

• Used to remove acidic impurities.
• Examples: CaO, MgO
• Basic flux + acidic gangue → slag

Properties of Slag

• Molten slag is not miscible with molten metal.
• Melting point of slag is lower than that of the metal.
• Slag has lower density than molten metal, so it floats on top.

Refractory Materials

• Withstand very high temperature without decomposing/softening.
• Acidic: silica, quartz, sandstone
• Basic: lime, dolomite, magnesite
• Neutral: graphite, chromite, bone ash

Extraction of Metals: Main Steps

• A) Concentration (beneficiation / ore dressing)
• B) Calcination and Roasting
• C) Reduction to metal
• D) Refining (purification)

A) Concentration of Ore

Removal of impurities from ore.

1) Gravity Separation (Levigation)

• Based on difference in specific gravities of ore and gangue.
• Lighter gangue washed away; heavier ore settles.

2) Froth Flotation (Sulphide Ores)

• Based on different wetting: gangue wetted by water; sulphide ore wetted by oil.
• Frothers: create stable froth (e.g., pine oil).
• Collectors: make ore particles water-repellent (e.g., ethyl xanthate, potassium ethyl xanthate).
• Activators / Depressants: help separation of mixed sulphide ores (depressant: NaCN/alkali; activator: CuSO₄).

3) Magnetic Separation

• Used when one component is magnetic and the other is non-magnetic.

4) Chemical Separation (Leaching)

• Ore is treated with a reagent that dissolves the ore; impurities remain insoluble.
• Used for Au and Ag (cyanide complexes).
• Used in Bayer process: Al₂O₃ leached with NaOH as sodium meta-aluminate (when Fe₂O₃ is impurity).

B) Calcination & Roasting

Calcination

• Heating oxide/hydroxide/carbonate ores below melting point in limited air (no external substance added).

Examples:

• ZnCO₃ → ZnO + CO₂
• CuCO₃·Cu(OH)₂ → 2CuO + CO₂ + H₂O
• PbCO₃ → PbO + CO₂

Advantages of Calcination

• Moisture removed
• Organic matter destroyed
• Carbonates/hydroxides converted to oxides
• Mass becomes porous and workable

Roasting

• Heating sulphide ores in excess air to remove sulphur and convert sulphides to oxides.

Advantages of Roasting (Key Reactions)

• S + O₂ → SO₂↑
• 4As + 3O₂ → 2As₂O₃↑
• 4Sb + 3O₂ → 2Sb₂O₃↑

Roasting Examples

• 2PbS + 3O₂ → 2PbO + 2SO₂↑
• 2ZnS + 3O₂ → 2ZnO + 2SO₂↑
• 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂↑
• 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂↑
• 2FeS + 3O₂ → 2FeO + 2SO₂↑

C) Reduction to the Metal

After calcination/roasting, metal compounds are reduced to metal using suitable method.

1) Cathodic / Electrolytic Reduction

• Used for highly electropositive metals (alkali, alkaline earth, lanthanides, Al).
• Generally done by electrolysis of fused salts (especially chlorides) for most such metals.

2) Chemical Reduction (Smelting)

Smelting: extraction involving melting of ore with a reducing agent (and flux when needed).

(a) Active Metals as Reducing Agents

• Na, Mg, Al, Ca used to reduce oxides/chlorides of less active metals.
• Ti, Zr (via chloride route): TiO₂ + 2C + 2Cl₂ → TiCl₄ + 2CO↑
• Kroll process: TiCl₄ + 4Na → Ti + 4NaCl

Goldschmidt (Thermite) Process

• Al reduces metal oxides (Cr, Mn) producing intense heat.
• Cr₂O₃ + 2Al → Al₂O₃ + 3Cr + heat
• 3MnO₂ + 4Al → 2Al₂O₃ + 3Mn + heat

(b) Carbon as Reducing Agent

Metal oxide + C + flux → Metal + CO↑ + slag

Carbon Reduction (Examples)

• Tin: SnO₂ + 2C + CaO → Sn + 2CO↑ + slag
• Slag formation (example): CaO + SiO₂ → CaSiO₃
• Zinc: ZnO + C → Zn + CO↑ (after roasting ZnS to ZnO)

(c) Carbon Monoxide as Reducing Agent

• 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂↑ (≈500°C)
• Fe₃O₄ + CO → 3FeO + CO₂↑ (≈600°C)
• FeO + CO → Fe + CO₂↑ (≈700°C)

D) Refining (Purification) of Metals

1) Liquation

• For low melting metals with high melting impurities (Pb, Sn, Sb, Bi).
• Impure metal heated on sloping hearth; pure molten metal flows down.
• Dross: infusible matter left behind during liquation.

2) Zone Refining

• For very high purity metals (Ge, Si, Ga).
• Based on impurities remaining dissolved in molten metal.

3) Oxidation Processes

• Used when impurities have higher affinity for oxygen than the metal.
• Important processes: cupellation, bessemerisation.
• Cupellation purifies Ag containing Pb impurity.

4) Vapour Phase Refining

• Metal converted to volatile compound, then decomposed to pure metal.

Mond’s Process (Ni)

• Ni + 4CO → Ni(CO)₄ (volatile) at 25–50°C
• Ni(CO)₄ → Ni + 4CO at ~180°C

Van-Arkel Process (Ti, Zr, Hf, Si, U)

• Impure Zr + 2I₂ → ZrI₄ (volatile)
• ZrI₄ → Zr + 2I₂ (on hot W filament ~1800 K)

5) Electro-refining

• Used for Cu, Ag, Zn, Sn, Pb, Al, Ni, Cr (etc.).
• Impure metal = anode; pure metal sheet = cathode; electrolyte = soluble salt of metal.
• Pure metal deposits at cathode; soluble impurities stay in solution; insoluble impurities form anode mud/sludge.

Electro-refining of Copper (Example Setup)

• Anode: blister copper (~98%)
• Cathode: pure copper sheet
• Electrolyte: CuSO₄ (aq, ~15%) + dil. H₂SO₄ (~5%)

6) Cupellation (Pb in Ag)

• Crude metal heated in a cupel (bone ash/cement); hot air blown.
• Pb impurity oxidised to litharge (PbO) and removed, leaving pure Ag.