Principles and Methods of Extraction — Revision Notes

⚡ 30-Second Revision

Ores — Economic source of metal. Gangue: Impurities.
Concentration — Remove gangue.

- Hydraulic Washing: Density difference. - Magnetic Separation: Magnetic properties. - Froth Flotation: Sulfide ores, differential wettability (collectors, frothers, depressants). - Leaching: Chemical dissolution (e.g., Bauxite with NaOH, Au/Ag with NaCN/O2).

Conversion to Oxide

- Calcination: Heat in absence of air (carbonates, hydroxides) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$. - Roasting: Heat in presence of air (sulfides) $2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$.

Reduction — Metal oxide to crude metal.

- Smelting: With C/CO (e.g., Fe, Zn). Flux removes gangue as slag. - Ellingham Diagram: Predicts feasibility; slope $= -\Delta S^\circ$. Carbon is better at high T. - Electrolytic Reduction: For highly reactive metals (Al - Hall-Héroult, molten cryolite lowers MP, increases conductivity).

Refining — Purify crude metal.

- Distillation: Low BP metals (Zn, Cd, Hg). - Liquation: Low MP metals (Sn, Pb). - Electrolytic Refining: Cu, Zn, Ag, Au (impure anode, pure cathode, anode mud). - Zone Refining: Semiconductors (impurities more soluble in molten zone). - Vapor Phase Refining: - Mond Process: Ni with CO $\text{Ni(CO)}_4$. - Van Arkel Method: Zr, Ti with $\text{I}_2 \rightarrow \text{ZrI}_4$. - Chromatography: Differential adsorption.

2-Minute Revision

Metal extraction, or metallurgy, begins with crushing and grinding the ore. The first major step is concentration, removing unwanted gangue. Methods include hydraulic washing (density), magnetic separation (magnetic properties), froth flotation (for sulfide ores, using oil/water differential wettability with collectors and frothers), and leaching (chemical dissolution, like bauxite in NaOH or gold in cyanide).

Next, the concentrated ore is converted into a form suitable for reduction, typically an oxide. This involves calcination (heating carbonates/hydroxides in limited air to remove $\text{CO}_2$/$\text{H}_2\text{O}$) or roasting (heating sulfide ores in excess air to form oxides and release $\text{SO}_2$).

The metal oxide is then reduced to crude metal. This can be done by smelting with carbon or CO, using thermodynamic principles (Ellingham diagram) to select the right reducing agent and temperature.

For highly reactive metals like aluminium, electrolytic reduction (Hall-Héroult process, using molten cryolite) is employed. Finally, the crude metal is refined to achieve high purity. Key refining methods include distillation (for volatile metals), liquation (for low-melting metals), electrolytic refining (for copper, zinc, etc.

, forming anode mud), zone refining (for semiconductors, based on differential solubility in molten state), and vapor phase refining (Mond process for nickel, Van Arkel for zirconium/titanium). Each method exploits specific physical or chemical differences between the metal and its impurities.

5-Minute Revision

Metallurgy is the comprehensive process of extracting metals from their ores. It starts with crushing and grinding the ore to a fine powder. The first chemical step is concentration (beneficiation), which removes gangue.

Hydraulic washing uses gravity for heavy ores. Magnetic separation is for magnetic ores or impurities. Froth flotation is crucial for sulfide ores (e.g., $\text{ZnS}$, $\text{PbS}$); collectors (xanthates) make ore particles hydrophobic, frothers (pine oil) create froth, and depressants (NaCN) can selectively separate different sulfides.

Leaching is a chemical method where the ore dissolves selectively, like bauxite ($\text{Al}_2\text{O}_3$) in hot concentrated $\text{NaOH}$ to form soluble sodium meta-aluminate, or gold/silver in dilute $\text{NaCN}$ solution in presence of air.

After concentration, the ore is prepared for reduction. Calcination involves heating carbonate or hydroxide ores (e.g., $\text{CaCO}_3$, $\text{MgCO}_3$) in the absence of air to decompose them into oxides and release $\text{CO}_2$ or $\text{H}_2\text{O}$. Roasting involves heating sulfide ores (e.g., $\text{ZnS}$, $\text{PbS}$) in excess air to convert them into oxides and release $\text{SO}_2$. Both aim to convert the metal into an oxide form, which is generally easier to reduce.

The next stage is reduction of the metal oxide to crude metal. This often occurs via smelting, where the oxide is heated with a reducing agent (like carbon or carbon monoxide) and a flux. Flux (e.

g., $\text{SiO}_2$ for basic gangue, $\text{CaO}$ for acidic gangue) combines with gangue to form fusible slag. The Ellingham diagram is a thermodynamic tool that plots $\Delta G^\circ$ of oxide formation vs.

temperature, helping select the most effective reducing agent. A metal can reduce the oxide of another metal if its own oxide formation line lies below the other's. For highly reactive metals like aluminium, electrolytic reduction is used (Hall-Héroult process).

Here, pure alumina is dissolved in molten cryolite ($\text{Na}_3\text{AlF}_6$) and fluorspar ($\text{CaF}_2$) to lower the melting point and increase conductivity, then electrolyzed.

Finally, refining purifies the crude metal. Distillation is for low boiling point metals (Zn, Cd, Hg). Liquation is for low melting point metals (Sn, Pb). Electrolytic refining is common for copper, zinc, silver, and gold, where impure metal is the anode, pure metal is the cathode, and less electropositive impurities collect as anode mud.

Zone refining is for ultra-pure semiconductors (Si, Ge), based on impurities being more soluble in the molten zone. Vapor phase refining includes the Mond process for nickel (forming volatile $\text{Ni(CO)}_4$) and the Van Arkel method for zirconium/titanium (forming volatile iodides).

Chromatographic methods are used for highly similar elements. Understanding these principles and specific examples is key for NEET.

Prelims Revision Notes

Principles and Methods of Extraction (Metallurgy)

I. Basic Terminology:

Mineral: — Naturally occurring chemical substance in Earth's crust.
Ore: — Mineral from which metal can be profitably extracted (e.g., Bauxite for Al, Haematite for Fe).
Gangue (Matrix): — Unwanted impurities in ore.

II. Steps in Metallurgy:

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Crushing & Grinding: — Reduce ore size.
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Concentration (Beneficiation): — Removal of gangue.

* Hydraulic Washing (Gravity Separation): Based on density difference. For heavy oxide ores (e.g., Haematite, Tin stone). * Magnetic Separation: For magnetic ores/impurities (e.g., Chromite, Pyrolusite, Wolframite).

* Froth Flotation: For sulfide ores (e.g., $\text{ZnS}$, $\text{PbS}$, $\text{CuFeS}_2$). * Principle: Differential wettability (ore wetted by oil, gangue by water). * Reagents: Collectors (xanthates), Frothers (pine oil), Froth stabilisers (cresols), Depressants (NaCN for $\text{ZnS}$).

* Leaching (Chemical Separation): Selective dissolution of ore. * Bauxite (Baeyer's Process): $\text{Al}_2\text{O}_3\cdot x\text{H}_2\text{O} + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{Na}[\text{Al(OH)}_4]$.

Then $\text{Na}[\text{Al(OH)}_4] + \text{CO}_2 \rightarrow \text{Al}_2\text{O}_3\cdot x\text{H}_2\text{O} \downarrow$. Finally, calcination to $\text{Al}_2\text{O}_3$. * Gold/Silver (MacArthur-Forrest Cyanide Process): $4\text{M} + 8\text{CN}^- + 2\text{H}_2\text{O} + \text{O}_2 \rightarrow 4[\text{M(CN)}_2]^- + 4\text{OH}^-$.

Then $2[\text{M(CN)}_2]^- + \text{Zn} \rightarrow [\text{Zn(CN)}_4]^{2-} + 2\text{M}$.

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Extraction of Crude Metal from Concentrated Ore:

* Conversion to Oxide: * Calcination: Heating in absence/limited air. For carbonates/hydroxides. $\text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2$. * Roasting: Heating in excess air.

For sulfide ores. $2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$. * Reduction of Metal Oxide: * Smelting (Pyrometallurgy): Heating with reducing agent (C, CO) and flux. * Flux: Removes non-fusible gangue as fusible slag.

* Acidic flux ($\text{SiO}_2$) for basic gangue (e.g., $\text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3$). * Basic flux ($\text{CaO}$) for acidic gangue (e.g., $\text{SiO}_2 + \text{CaO} \rightarrow \text{CaSiO}_3$).

* Reducing Agents: C, CO, more reactive metals (e.g., $\text{Cr}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Cr}$). * Thermodynamic Principles (Ellingham Diagram): Plot of $\Delta G^\circ$ vs.

T for oxide formation. Slope $= -\Delta S^\circ$. Lower line can reduce upper line. Carbon is better reducing agent at high T. * Electrolytic Reduction: For highly reactive metals (Alkali, Alkaline earth, Al).

* Hall-Héroult Process (for Al): Molten $\text{Al}_2\text{O}_3$ dissolved in molten cryolite ($\text{Na}_3\text{AlF}_6$) and fluorspar ($\text{CaF}_2$). Cryolite lowers MP and increases conductivity.

Cathode: $\text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al}$. Anode: $\text{C} + \text{O}^{2-} \rightarrow \text{CO} + 2\text{e}^-$.

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Refining of Crude Metal: — Purification.

* Distillation: For low boiling point metals (Zn, Cd, Hg). * Liquation: For low melting point metals (Sn, Pb). * Electrolytic Refining: For Cu, Zn, Ag, Au. Impure metal = Anode, Pure metal = Cathode, Metal salt solution = Electrolyte.

Less electropositive impurities form anode mud. * Zone Refining: For semiconductors (Si, Ge, Ga). Principle: Impurities more soluble in molten state than solid state. * Vapor Phase Refining: Metal forms volatile compound, then decomposes.

* Mond Process (for Ni): $\text{Ni}(s) + 4\text{CO}(g) \xrightarrow{330-350,\text{K}} \text{Ni(CO)}_4(g) \xrightarrow{450-470,\text{K}} \text{Ni}(s) + 4\text{CO}(g)$. * Van Arkel Method (for Zr, Ti): $\text{Zr}(s) + 2\text{I}_2(g) \xrightarrow{870,\text{K}} \text{ZrI}_4(g) \xrightarrow{1800,\text{K}} \text{Zr}(s) + 2\text{I}_2(g)$.

* Chromatographic Methods: For elements in minute quantities or with similar impurities.

Vyyuha Quick Recall

Can Really Like Some Refined Metals

Concentration
Roasting/Calcination
Leaching (a type of concentration)
Smelting (reduction)
Reduction (general term, includes smelting, electrolytic)
Metals (referring to the final refining step for pure metals)