Extraction of Crude Metal from Concentrated Ore — Revision Notes

⚡ 30-Second Revision

Calcination — Heat in absence of air, for $MCO_3, M(OH)_x$. Products: $MO, CO_2/H_2O$. Ex: $ZnCO_3 \rightarrow ZnO + CO_2$.
Roasting — Heat in excess air, for $MS$. Products: $MO, SO_2$. Ex: $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$.
Reduction — Convert $MO \rightarrow M$. Agents: C, CO, Al, Electrolysis.
Flux — Removes gangue. Acidic flux ($SiO_2$) for basic gangue ($FeO$). Basic flux ($CaCO_3$) for acidic gangue ($SiO_2$).
Slag — Fusible product of flux + gangue. Ex: $FeSiO_3, CaSiO_3$.
Self-reduction — For $Cu, Pb, Hg$. Ex: $2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$.
Electrolytic Reduction — For highly reactive metals (Al, Na, Mg).

2-Minute Revision

The extraction of crude metal from concentrated ore is a sequence of chemical transformations. It starts with converting the concentrated ore into a more reducible form, typically an oxide. This is achieved via calcination for carbonate/hydroxide ores (heating without air, releasing $CO_2/H_2O$) or roasting for sulfide ores (heating with excess air, releasing $SO_2$).

Once the metal is in its oxide form, it undergoes reduction to yield the crude metal. The choice of reducing agent is crucial: carbon or carbon monoxide for moderately reactive metals (e.g., iron, zinc), more reactive metals like aluminium for stable oxides (e.

g., chromium), or electrolytic reduction for highly reactive metals (e.g., aluminium, sodium). During these high-temperature processes, a flux is added to react with unwanted impurities (gangue) to form a molten, easily separable slag.

This entire sequence aims to isolate the metal in a relatively impure state, ready for further refining.

5-Minute Revision

The process of extracting crude metal from concentrated ore is a cornerstone of metallurgy, involving several key steps. First, the concentrated ore is prepared for reduction. If it's a carbonate or hydroxide ore, it undergoes calcination, where it's heated strongly in the absence of air.

This thermally decomposes the ore, driving off volatile components like carbon dioxide or water, leaving behind the metal oxide. For example, $ZnCO_3(s) \xrightarrow{\text{heat}} ZnO(s) + CO_2(g)$. If the ore is a sulfide, it undergoes roasting, heated in the presence of excess air.

This oxidizes the sulfide to a metal oxide, releasing sulfur dioxide. For instance, $2ZnS(s) + 3O_2(g) \xrightarrow{\text{heat}} 2ZnO(s) + 2SO_2(g)$.

Once the metal is in its oxide form, the next crucial step is reduction. This involves using a suitable reducing agent to remove oxygen and obtain the free metal. For moderately reactive metals like iron and zinc, carbon (coke) or carbon monoxide are effective reducing agents.

Example: $ZnO(s) + C(s) \xrightarrow{\text{heat}} Zn(g) + CO(g)$. For very stable oxides of highly reactive metals (e.g., chromium, manganese), more reactive metals like aluminium are used in the aluminothermic process: $Cr_2O_3(s) + 2Al(s) \xrightarrow{\text{heat}} 2Cr(l) + Al_2O_3(s)$.

For the most reactive metals (e.g., aluminium, sodium), electrolytic reduction of their fused salts is necessary, as chemical reducing agents are insufficient. Some less reactive metals like copper can undergo self-reduction, where partially oxidized sulfide reacts with unoxidized sulfide to yield the metal: $2Cu_2O(s) + Cu_2S(s) \xrightarrow{\text{heat}} 6Cu(s) + SO_2(g)$.

Throughout these high-temperature processes, flux is added to remove unwanted rocky impurities called gangue. The flux reacts with the gangue to form a fusible, molten substance called slag, which is lighter than the molten metal and can be easily separated.

Acidic fluxes ($SiO_2$) remove basic gangue ($FeO$), while basic fluxes ($CaCO_3$) remove acidic gangue ($SiO_2$). For example, $FeO + SiO_2 \rightarrow FeSiO_3$ (slag). The metal obtained at this stage is 'crude' and requires further refining.

Prelims Revision Notes

1
Ore Concentration — Precedes extraction. Removes major gangue.
2
Conversion to Oxide — Essential for easier reduction.

* Calcination: * Conditions: Heating below melting point, in absence/limited air. * Ores: Carbonates ($MCO_3$), Hydroxides ($M(OH)_x$). * Purpose: Remove volatile impurities ($CO_2, H_2O$), make ore porous.

* Reactions: $MCO_3 \rightarrow MO + CO_2$; $M(OH)_x \rightarrow MO_{x/2} + x/2H_2O$. * Example: $ZnCO_3 \rightarrow ZnO + CO_2$. * Roasting: * Conditions: Heating below melting point, in excess air.

* Ores: Sulfides ($MS$). * Purpose: Convert sulfide to oxide, remove sulfur as $SO_2$. * Reactions: $2MS + 3O_2 \rightarrow 2MO + 2SO_2$. * Example: $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$.

1
Reduction of Oxide — Obtaining crude metal from oxide.

* Carbon Reduction (Smelting): * Agents: Coke (C), Carbon monoxide (CO). * Metals: Fe, Zn, Pb (moderately reactive). * Reactions: $MO + C \rightarrow M + CO$; $MO + CO \rightarrow M + CO_2$.

* Example: $ZnO + C \rightarrow Zn + CO$. * Aluminothermic Process: * Agent: Aluminium (Al). * Metals: Cr, Mn (very stable oxides). * Reaction: $Cr_2O_3 + 2Al \rightarrow 2Cr + Al_2O_3$.

* Electrolytic Reduction: * Method: Electrolysis of fused salts/oxides. * Metals: Al, Na, Mg (highly reactive). * Example: $Al^{3+} + 3e^- \rightarrow Al$ (from molten $Al_2O_3$ in cryolite).

* Self-reduction (Auto-reduction): * Metals: Cu, Pb, Hg (less reactive). * Mechanism: Partially roasted sulfide reacts with unroasted sulfide. * Reaction: $2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$.

1
Flux and Slag

* Gangue: Unwanted impurities in ore. * Flux: Substance added to react with gangue. * Slag: Fusible product of flux + gangue, separates from molten metal. * Acidic Flux: $SiO_2$ (removes basic gangue like $FeO$). Reaction: $FeO + SiO_2 \rightarrow FeSiO_3$. * Basic Flux: $CaCO_3$ (decomposes to $CaO$, removes acidic gangue like $SiO_2$). Reaction: $CaO + SiO_2 \rightarrow CaSiO_3$.

1
Crude Metal — Impure metal obtained after reduction, requires refining.

Vyyuha Quick Recall

Calcinate Carbonates Coolly (no air). Roast Sulfides Rigorously (with air). Reduce Oxides Reactants Right. Flux Gangue Slag Separate.