General Principles and Processes of Isolation of Metals (Metallurgy)

Mineral: Naturally occurring inorganic compound found in earth's crust. Ore: Mineral from which metal can be extracted economically.

Common Ores:

Types of Ores:

• Oxides: $Al_2O_3$, $Fe_2O_3$, $SnO_2$
• Sulfides: $ZnS$, $PbS$, $CuFeS_2$
• Carbonates: $ZnCO_3$, $CaCO_3$
• Halides: $NaCl$, $KCl$, $CaF_2$
• Native (free metal): Au, Ag, Cu, Pt

Gangue/Matrix: Impurities mixed with the ore (sand, clay, rock).

Steps in Metallurgy:

1. Crushing and grinding (size reduction)
2. Concentration of ore (removing gangue)
3. Conversion to oxide (roasting/calcination)
4. Reduction to free metal
5. Refining/purification

Concentration Methods:

1. Hydraulic (Gravity) Separation:

• For heavy ores; water washes lighter gangue away
• Used for hematite, tin ores

2. Magnetic Separation:

• For magnetic ores (e.g., magnetite $Fe_3O_4$)
• Ore passed over magnetic belt; magnetic particles separated

3. Froth Flotation:

• For sulfide ores
• Powdered ore + water + pine oil (collector) + frothing agent (water glass, sodium ethyl xanthate)
• Sulfide ore particles become hydrophobic; rise with froth
• Gangue particles hydrophilic; sink

Selectivity: Adding depressants — e.g., NaCN suppresses ZnS while allowing PbS or CuFeS₂ to float; useful when both are present.

4. Leaching (Chemical):

• Treating ore with reagent that selectively dissolves the metal
• Examples:
• Bauxite: $Al_2O_3 + 2NaOH + 3H_2O \to 2Na[Al(OH)_4]$ (Baeyer's process); ferric oxide impurity insoluble
• Argentite: $Ag_2S + 4NaCN \to 2Na[Ag(CN)_2] + Na_2S$ (cyanide process)
• Gold: $4Au + 8NaCN + O_2 + 2H_2O \to 4Na[Au(CN)_2] + 4NaOH$
• Recovery: $Zn + 2[Au(CN)_2]^- \to [Zn(CN)_4]^{2-} + 2Au$

Conversion to Oxide: Sulfide ores, carbonate ores converted to oxide before reduction (oxides more easily reduced).

Roasting: Heating ore in excess air (below MP).

• Converts sulfides to oxides
• Removes volatile impurities (S, As, P)
• Examples:
• $2ZnS + 3O_2 \to 2ZnO + 2SO_2$
• $2PbS + 3O_2 \to 2PbO + 2SO_2$
• $4FeS_2 + 11O_2 \to 2Fe_2O_3 + 8SO_2$
• $2Cu_2S + 3O_2 \to 2Cu_2O + 2SO_2$

Calcination: Heating ore in absence/limited air.

• Decomposes carbonates and hydrates; expels CO₂ and water
• Examples:
• $CaCO_3 \to CaO + CO_2$
• $ZnCO_3 \to ZnO + CO_2$
• $Al_2O_3 \cdot 2H_2O \to Al_2O_3 + 2H_2O$ (from bauxite)

Differences between Roasting and Calcination:

Reduction of Metal Oxide:

1. Carbon Reduction (Smelting): Carbon (coke) reduces oxides at high T.

• $ZnO + C \to Zn + CO$
• $PbO + C \to Pb + CO$
• $SnO_2 + 2C \to Sn + 2CO$
• $Fe_2O_3 + 3C \to 2Fe + 3CO$ (blast furnace, with CO too)

2. CO Reduction:

• $Fe_2O_3 + 3CO \to 2Fe + 3CO_2$ (blast furnace, upper region)

3. Aluminothermy (Thermite Process): $Al$ as reducing agent.

• $Cr_2O_3 + 2Al \to Al_2O_3 + 2Cr$
• $Fe_2O_3 + 2Al \to Al_2O_3 + 2Fe + heat$ (welding rails)

4. Electrolytic Reduction: For active metals (above Mn in series).

• Al from molten cryolite-alumina (Hall-Héroult)
• Na from molten NaCl (Down's process)
• Mg from MgCl₂

5. Hydrogen Reduction:

• $MoO_3 + 3H_2 \to Mo + 3H_2O$
• $WO_3 + 3H_2 \to W + 3H_2O$

6. Self-reduction (Auto-reduction):

• For Cu, Hg, Pb sulfides:
• $Cu_2S + 2Cu_2O \to 6Cu + SO_2$
• $HgS + Hg(O_2) \to Hg + SO_2$
• $2PbS + 3O_2 \to 2PbO + 2SO_2$, then $PbS + 2PbO \to 3Pb + SO_2$

Flux: Substance added to combine with gangue, forming slag (lower MP, easily removable).

• Acidic flux ($SiO_2$) for basic gangue (CaO)
• Basic flux (CaO) for acidic gangue ($SiO_2$)

$CaO + SiO_2 \to CaSiO_3$ (slag in iron extraction)

Smelting: Reduction with carbon in presence of flux at high T in a furnace.

Refining: Purification of crude metal.

Methods of Refining:

1. Distillation: For low-BP metals (Hg, Zn).

• Crude metal vaporized; pure metal condensed; impurities left behind.

2. Liquation: For metals with low MP (Sn, Bi).

• Crude metal melted on inclined surface; molten metal flows down; impurities remain.

3. Poling: For ores containing oxide impurities (Cu, Sn).

• Molten metal stirred with green wood; hydrocarbons released reduce metal oxide impurities.

4. Electrolytic Refining: Most common.

• Crude metal as anode; pure metal as cathode; metal salt as electrolyte
• At anode: $M \to M^{n+} + ne^-$ (more reactive impurities also dissolve)
• At cathode: $M^{n+} + ne^- \to M$ (only the desired metal deposits at controlled voltage)
• Anode mud: noble metal impurities (Ag, Au, Pt) collect under anode — economically valuable
• Used for Cu, Zn, Ag, Au, Al

5. Zone Refining: Based on different solubilities in melt and solid.

• Bar of impure metal; circular heater moves along it
• Zone of molten metal forms; impurities preferentially in liquid (lower MP)
• Heater moves slowly; impurities pushed to one end
• Used for ultra-pure Si, Ge, Ga (semiconductors)

6. Vapour Phase Refining: Volatile compound formation/decomposition.

• Mond process (Ni): $Ni + 4CO \xrightarrow{350K} [Ni(CO)_4] \xrightarrow{450K} Ni + 4CO$
• Van Arkel method (Ti, Zr): $Zr + 2I_2 \xrightarrow{870K} ZrI_4 \xrightarrow{2070K} Zr + 2I_2$
• Pure metal deposited on hot tungsten filament

7. Chromatographic Refining: For very pure substances; based on adsorption.

Ellingham Diagram:

Plot of $\Delta G^\ominus$ of formation of metal oxides vs temperature. Useful to predict feasibility of reduction.

Features:

• Y-axis: $\Delta G^\ominus$ (formation of oxide, per mole of O₂ used)
• X-axis: Temperature (K)
• Slopes:
• For $2M(s) + O_2 \to 2MO(s)$: $\Delta S^\ominus < 0$ (gas → solid), so $\Delta G$ increases with T → positive slope
• For $2C(s) + O_2 \to 2CO(g)$: $\Delta S^\ominus > 0$, $\Delta G$ decreases with T → negative slope (only line going down)
• For phase changes (MP, BP), kinks in line

Key Result: At any T where C line is below metal-oxide line, carbon can reduce that oxide.

Examples:

• CuO + C → Cu + CO at any T above ~$500$ K
• $Fe_2O_3$ + C → Fe + CO above ~$1000$ K
• $Al_2O_3$ + C: C line above Al₂O₃ for normal T; reduction not feasible by C even at $1700$ K (need electrolysis)

Limitations of Ellingham Diagram:

• Doesn't tell about rate
• Doesn't tell about thermodynamic stability over wide ranges
• Doesn't account for kinetic factors

Extraction of Aluminium (Hall-Héroult):

1. Concentration: Bauxite ($Al_2O_3 \cdot 2H_2O$) purified by Baeyer's process (NaOH leaching).

2. Electrolysis:

• Pure $Al_2O_3$ + cryolite ($Na_3AlF_6$) + fluorspar ($CaF_2$) molten at ~$950°$C (cryolite lowers MP from $2000°$C; CaF₂ improves conductivity)
• Cathode: carbon-lined steel; Anode: carbon rods (consumed)
• At cathode: $Al^{3+} + 3e^- \to Al$ (molten Al, denser, collects at bottom)
• At anode: $2O^{2-} \to O_2 + 4e^-$; $O_2$ reacts with C anode: $C + O_2 \to CO_2$; $2C + O_2 \to 2CO$
• Anodes are continuously replaced as they erode

Extraction of Iron (Blast Furnace):

1. Concentration: Hematite ($Fe_2O_3$) crushed and washed; magnetic separation. 2. Roasting: Removes moisture, volatile matter. 3. Smelting in Blast Furnace:

• Charge: roasted ore + coke + limestone ($CaCO_3$)
• Hot air blasted in
• Reactions at different zones:

Lower (hot, $\sim 1800$ K): $$C + O_2 \to CO_2$$ $$CO_2 + C \to 2CO$$ (reduction of CO₂)

Middle (~$1000-1500$ K): $$Fe_2O_3 + 3CO \to 2Fe + 3CO_2$$ (main reduction) $$Fe_3O_4 + 4CO \to 3Fe + 4CO_2$$ $$CaCO_3 \to CaO + CO_2$$ $$CaO + SiO_2 \to CaSiO_3$$ (slag formation; CaO acts as flux)

Upper (~$500-1000$ K): $$Fe_2O_3 + 3CO \to 2Fe + 3CO_2$$ (continued reduction by CO) $$3Fe_2O_3 + CO \to 2Fe_3O_4 + CO_2$$

Molten iron (pig iron) at bottom; slag ($CaSiO_3$, lighter) floats on top.

Pig iron contains $4\%$ C and other impurities. Converted to:

• Cast iron ($3\%$ C): melting pig iron with scrap iron and coke
• Wrought iron ($0.2-0.5\%$ C): purest form; puddling process
• Steel ($0.2-1.5\%$ C): Bessemer process or open hearth

Extraction of Copper:

Copper extracted from sulfide ores (mainly copper pyrites $CuFeS_2$).

1. Crushing and Concentration: Froth flotation. 2. Roasting in air: $2CuFeS_2 + O_2 \to Cu_2S + FeS + SO_2$ $2Cu_2S + 3O_2 \to 2Cu_2O + 2SO_2$ $2FeS + 3O_2 \to 2FeO + 2SO_2$ 3. Smelting (with silica flux): $FeO + SiO_2 \to FeSiO_3$ (slag) Cu₂S + Cu₂O → matte 4. Bessemerization (in air through molten matte): $2FeS + 3O_2 \to 2FeO + 2SO_2$ → slag $2Cu_2S + 3O_2 \to 2Cu_2O + 2SO_2$ $2Cu_2O + Cu_2S \to 6Cu + SO_2$ (self-reduction) 5. Refining: Electrolytic refining gives $99.9\%$ pure Cu; anode mud contains Ag, Au.

Extraction of Zinc:

1. Concentration: Zinc blende (ZnS) by froth flotation. 2. Roasting: $2ZnS + 3O_2 \to 2ZnO + 2SO_2$ 3. Reduction: $ZnO + C \to Zn + CO$ at $1100°$C in a retort.

• Zn distills off (BP $907°$C); collected as molten metal.

4. Refining: Electrolytic refining gives high-purity Zn; or distillation.