Concentration, Oxidation and Reduction — Revision Notes

⚡ 30-Second Revision

Concentration: — Removal of gangue.

- Hydraulic Washing: Density difference. - Magnetic Separation: Magnetic properties difference. - Froth Flotation: Wetting properties difference (sulfide ores, collectors, frothers, depressants). - Leaching: Chemical solubility (Al, Au, Ag).

Oxidation: — Conversion to oxide form.

- Roasting: Heating sulfide ores in excess air ($2ZnS + 3O_2 \to 2ZnO + 2SO_2$). - Calcination: Heating carbonate/hydrated ores in limited air ($CaCO_3 \to CaO + CO_2$).

Reduction: — Conversion of oxide to metal.

- Smelting: With carbon/CO (e.g., $ZnO + C \to Zn + CO$). Flux forms slag. - Auto-reduction: Sulfide + Oxide $o$ Metal (e.g., $2Cu_2O + Cu_2S \to 6Cu + SO_2$). - Electrolytic Reduction: For reactive metals (e.g., Hall-Héroult for Al, using cryolite). - Reduction by other metals: Thermite process ($Cr_2O_3 + 2Al \to 2Cr + Al_2O_3$).

2-Minute Revision

Metal extraction starts with Concentration, separating valuable ore from unwanted gangue. This can be done physically via hydraulic washing (density), magnetic separation (magnetic properties), or froth flotation (wetting properties, for sulfide ores using collectors and frothers).

Chemically, leaching dissolves the ore selectively (e.g., bauxite with $NaOH$, gold with $NaCN$). Next, Oxidation prepares the ore for reduction, typically by converting it to an oxide. Roasting heats sulfide ores in air ($SO_2$ byproduct), while Calcination heats carbonate/hydrated ores in limited air ($CO_2/H_2O$ byproduct).

Finally, Reduction converts the metal oxide to pure metal. Methods include smelting (using carbon/CO, often with a flux to remove impurities as slag), auto-reduction (sulfide reacting with oxide), reduction by more reactive metals (like aluminium), or electrolytic reduction for highly reactive metals (e.

g., aluminium from molten alumina in cryolite).

5-Minute Revision

The journey from raw ore to pure metal involves three crucial stages: concentration, oxidation, and reduction. Concentration aims to remove gangue (impurities). Hydraulic washing is for heavy ores, leveraging density differences.

Magnetic separation is for magnetic ores or gangue. Froth flotation is vital for sulfide ores; collectors make ore hydrophobic, frothers stabilize froth, and depressants allow selective separation (e.

g., $NaCN$ for $ZnS/PbS$). Leaching is a chemical method where the ore dissolves selectively, like bauxite in $NaOH$ (Bayer's process) or gold/silver in $NaCN$ (cyanide process, with $O_2$ as oxidant, followed by $Zn$ reduction).

Oxidation prepares the concentrated ore for reduction. Roasting heats sulfide ores in excess air to form oxides and release $SO_2$ (e.g., $2ZnS + 3O_2 \to 2ZnO + 2SO_2$). Calcination heats carbonate or hydrated ores in limited air to remove $CO_2$ or $H_2O$, forming oxides (e.g., $CaCO_3 \to CaO + CO_2$).

Reduction extracts the pure metal. Smelting uses carbon or $CO$ as a reducing agent at high temperatures, often with a flux (e.g., $SiO_2$ for basic gangue, $CaO$ for acidic gangue) to form slag.

Auto-reduction is a self-reduction process for less reactive metals like copper, where partially roasted sulfide reacts with its oxide ($2Cu_2O + Cu_2S \to 6Cu + SO_2$). Reduction by other metals (e.

g., aluminium in the thermite process for $Cr, Mn$) is used for certain oxides. For highly reactive metals like aluminium, electrolytic reduction (Hall-Héroult process) is employed, where alumina is dissolved in molten cryolite to lower its melting point and increase conductivity, then electrolyzed.

Prelims Revision Notes

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Concentration (Ore Dressing):

* Purpose: Removal of gangue (impurities) from ore. * Hydraulic Washing (Gravity Separation): Based on density difference. Used for heavy oxide ores ($Fe_2O_3, SnO_2$) and native gold. * Magnetic Separation: Based on magnetic properties.

Used for magnetic ores ($Fe_3O_4, CrO_2$) or separating magnetic impurities (e.g., wolframite from cassiterite). * Froth Flotation: For sulfide ores ($PbS, ZnS, CuFeS_2$). Based on differential wetting (ore by oil, gangue by water).

* Collectors: Pine oil, fatty acids, xanthates (enhance non-wettability of ore). * Frothers: Pine oil, cresols, aniline (stabilize froth). * Depressants: $NaCN$ (for $ZnS/PbS$ separation, depresses $ZnS$ by forming $Na_2[Zn(CN)_4]$).

* Leaching (Chemical Method): Selective dissolution of ore. * Bayer's Process (Aluminium): Bauxite ($Al_2O_3 cdot 2H_2O$) + $NaOH \to Na[Al(OH)_4]$ (soluble). Impurities insoluble. $Al(OH)_3$ precipitated, then calcined to $Al_2O_3$.

* Cyanide Process (Au, Ag): $4M + 8CN^- + 2H_2O + O_2 \to 4[M(CN)_2]^- + 4OH^-$. Metal recovered by displacement with $Zn$ ($2[M(CN)_2]^- + Zn \to [Zn(CN)_4]^{2-} + 2M$).

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Oxidation: — Conversion of concentrated ore to metal oxide.

* Roasting: Heating sulfide ores in excess air, below melting point. * Purpose: Convert sulfide to oxide, remove volatile impurities ($S \to SO_2$, $As \to As_2O_3$). * Example: $2ZnS(s) + 3O_2(g) \to 2ZnO(s) + 2SO_2(g)$.

* Calcination: Heating carbonate/hydrated ores in limited/absence of air, below melting point. * Purpose: Remove volatile matter ($CO_2$ from carbonates, $H_2O$ from hydrated oxides). * Example: $CaCO_3(s) \to CaO(s) + CO_2(g)$; $Al_2O_3 cdot 2H_2O(s) \to Al_2O_3(s) + 2H_2O(g)$.

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Reduction: — Conversion of metal oxide to elemental metal.

* Smelting (Carbon Reduction): Using $C$ (coke) or $CO$ as reducing agent at high temperatures. * Ellingham Diagram: Predicts feasibility. Carbon reduces oxides where its $Delta G^circ$ line is below the metal oxide's line.

* Flux: Added to remove non-fusible gangue as fusible slag. * Acidic gangue ($SiO_2$) $o$ Basic flux ($CaO$). Reaction: $SiO_2 + CaO \to CaSiO_3$ (slag). * Basic gangue ($FeO$) $o$ Acidic flux ($SiO_2$).

Reaction: $FeO + SiO_2 \to FeSiO_3$ (slag). * Example: $ZnO(s) + C(s) \to Zn(s) + CO(g)$. * Auto-reduction (Self-reduction): For less reactive metals (Cu, Pb, Hg). Partially roasted sulfide reacts with its oxide.

* Example: $2Cu_2O(s) + Cu_2S(s) \to 6Cu(s) + SO_2(g)$. * Reduction by Other Metals: More reactive metal reduces less reactive metal's oxide. * Thermite Process: $Cr_2O_3 + 2Al \to 2Cr + Al_2O_3$.

* Electrolytic Reduction (Electrometallurgy): For highly reactive metals (Al, Na, Mg). * Hall-Héroult Process (Aluminium): $Al_2O_3$ dissolved in molten cryolite ($Na_3AlF_6$) and fluorspar ($CaF_2$).

Cryolite lowers melting point and increases conductivity. Cathode: $Al^{3+} + 3e^- \to Al$. Anode: $C + O^{2-} \to CO/CO_2$.

Vyyuha Quick Recall

To remember the main concentration methods: Hydrogen Makes Frogs Leap.

Hydraulic washing
Magnetic separation
Froth flotation
Leaching