Metallurgy (Extraction)

Extraction Of Metals (from Class 10)

The process of extracting a metal from its ore is called metallurgy. The method used depends on the reactivity of the metal.

Extracting Metals Low In The Activity Series:

Metals like Copper (Cu), Silver (Ag), Gold (Au), Platinum (Pt) are very unreactive. Their ores are usually found in the elemental form or as sulphides or oxides.

Extraction from Sulphides:

These metals can be easily obtained by heating their sulphides in the presence of air (roasting).
Example: Extraction of Mercury from cinnabar (HgS).
Roasting: $2HgS(s) + 3O_2(g) \xrightarrow{\Delta} 2HgO(s) + 2SO_2(g)$
Heating the oxide: $2HgO(s) \xrightarrow{\Delta} 2Hg(l) + O_2(g)$
Example: Extraction of Copper from copper glance ($Cu_2S$).
Roasting: $2Cu_2S(s) + 3O_2(g) \xrightarrow{\Delta} 2Cu_2O(s) + 2SO_2(g)$
Further heating of $Cu_2O$ with remaining $Cu_2S$: $Cu_2S(s) + 2Cu_2O(s) \xrightarrow{\Delta} 6Cu(s) + SO_2(g)$

Extraction from Oxides:

These metal oxides can be reduced to metals by heating. Since these metals are low in the reactivity series, simple heating is enough to cause reduction.

Extracting Metals In The Middle Of The Activity Series:

Metals like Zinc (Zn), Iron (Fe), Tin (Sn), Lead (Pb) are moderately reactive. They are usually found as oxides, carbonates, or sulphides.

Conversion to Oxide:

Sulphide ores are roasted in excess air to convert them into oxides.
Example: Zinc blende (ZnS) roasting: $2ZnS(s) + 3O_2(g) \xrightarrow{\Delta} 2ZnO(s) + 2SO_2(g)$
Carbonate ores are calcined (heated in the absence of air) to convert them into oxides.
Example: Calamine ($ZnCO_3$) calcination: $ZnCO_3(s) \xrightarrow{\Delta} ZnO(s) + CO_2(g)$

Reduction of Metal Oxide:

The metal oxides are then reduced to metals using a reducing agent like carbon (coke).
Example: Reduction of Zinc Oxide: $ZnO(s) + C(s) \xrightarrow{\Delta} Zn(l) + CO(g)$
Example: Reduction of Iron Oxide: $Fe_2O_3(s) + 3C(s) \xrightarrow{\Delta} 2Fe(l) + 3CO(g)$
In some cases, more reactive metals are used as reducing agents (e.g., reduction of $ZnO$ by heating with carbon). For metals like Iron, the reduction is carried out in a blast furnace.

Extracting Metals Towards The Top Of The Activity Series:

Metals like Potassium (K), Sodium (Na), Calcium (Ca), Magnesium (Mg), Aluminium (Al) are highly reactive. They are found as oxides, carbonates, and chlorides in the Earth's crust. They cannot be reduced by heating with carbon because the metals displace carbon from their oxides.

Extraction by Electrolysis:

These metals are extracted by the electrolysis of their molten chlorides or oxides.
Example: Extraction of Sodium (Na) from molten Sodium Chloride (NaCl).
At Cathode (-): $Na^+ + e^- \rightarrow Na(l)$
At Anode (+): $2Cl^- \rightarrow Cl_2(g) + 2e^-$
Example: Extraction of Aluminium (Al) from molten cryolite ($Na_3AlF_6$) mixed with Aluminium Oxide ($Al_2O_3$).
Aluminium is produced by the electrolysis of its molten oxide ($Al_2O_3$) dissolved in cryolite. Cryolite is added to lower the melting point of $Al_2O_3$ and increase its conductivity.
At Cathode (-): $Al^{3+} + 3e^- \rightarrow Al(l)$
At Anode (+): $2O^{2-} \rightarrow O_2(g) + 4e^-$ (Oxygen reacts with carbon anode to form $CO$ and $CO_2$).

Extraction Of Crude Metal From Concentrated Ore (from Class 12)

The process of extracting a metal from its concentrated ore involves several steps, the first of which is converting the concentrated ore into a form that can be easily reduced.

Conversion To Oxide:

Most metal ores are not oxides. Therefore, it is often necessary to convert the concentrated ore into a metal oxide before reduction. This conversion depends on the nature of the ore:

1. Roasting:

Definition: Heating of the concentrated ore in a limited supply of air below its melting point.

When Used: Primarily used for sulphide ores.

Purpose: To convert sulphide ores into oxides and to remove volatile impurities.

Reactions:

$2ZnS(s) + 3O_2(g) \xrightarrow{\Delta} 2ZnO(s) + 2SO_2(g)$
$2PbS(s) + 3O_2(g) \xrightarrow{\Delta} 2PbO(s) + 2SO_2(g)$
$2Cu_2S(s) + 3O_2(g) \xrightarrow{\Delta} 2Cu_2O(s) + 2SO_2(g)$

2. Calcination:

Definition: Heating of the concentrated ore in the absence or limited supply of air, above its melting point.

When Used: Primarily for carbonate ores and sometimes for hydrated oxides or sulphides.

Purpose: To decompose carbonates into oxides and to remove volatile impurities and water of crystallization.

Reactions:

$ZnCO_3(s) \xrightarrow{\Delta} ZnO(s) + CO_2(g)$
$CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)$
$MgCO_3 \cdot CaCO_3(s) \xrightarrow{\Delta} MgO(s) + CaO(s) + 2CO_2(g)$
$Al_2O_3 \cdot 2H_2O(s) \xrightarrow{\Delta} Al_2O_3(s) + 2H_2O(g)$

Reduction Of Oxide To Metal:

Once the ore is converted to its oxide, the metal can be extracted by reducing the metal oxide. The choice of reducing agent and method depends on the position of the metal in the activity series and the stability of the metal oxide.

1. Reduction by Heating (Thermal Decomposition):

For metals low in the activity series, their oxides can be decomposed by simple heating.

$2HgO(s) \xrightarrow{\Delta} 2Hg(l) + O_2(g)$
$Ag_2O(s) \xrightarrow{\Delta} 4Ag(s) + O_2(g)$

2. Reduction with Carbon:

This is a common method for metals in the middle of the activity series. Carbon is a cheap and effective reducing agent. The metal oxide is heated with carbon (coke) in a furnace.

$ZnO(s) + C(s) \xrightarrow{\Delta} Zn(l) + CO(g)$
$Fe_2O_3(s) + 3C(s) \xrightarrow{\Delta} 2Fe(l) + 3CO(g)$
$PbO(s) + C(s) \xrightarrow{\Delta} Pb(l) + CO(g)$

This process is often carried out in a blast furnace, especially for iron.

3. Reduction using More Electropositive Metals (Thermite Process/Aluminothermy):

For metals that are highly reactive and have stable oxides (like Chromium and Manganese), carbon reduction is not effective. In such cases, a more electropositive metal, typically Aluminium, is used as the reducing agent. This reaction is highly exothermic and produces molten metal.

The Thermite Reaction:

$Fe_2O_3(s) + 2Al(s) \xrightarrow{\Delta} 2Fe(l) + Al_2O_3(s) + \text{Heat}$
$Cr_2O_3(s) + 2Al(s) \xrightarrow{\Delta} 2Cr(l) + Al_2O_3(s) + \text{Heat}$
$MnO_2(s) + 2Al(s) \xrightarrow{\Delta} Mn(l) + Al_2O_3(s) + \text{Heat}$

This process is used for welding cracked iron parts, railway tracks, etc.

4. Reduction by Displacement with Less Reactive Metals:

For some metal oxides and sulphides, displacement by a more reactive metal is used.

Example: Extraction of Silver from its cyanide solution (leaching) using Zinc.
$Zn(s) + 2Na[Ag(CN)_2](aq) \rightarrow Na_2[Zn(CN)_4](aq) + 2Ag(s)$

5. Electrolytic Reduction:

Highly reactive metals like Sodium, Potassium, Calcium, Magnesium, and Aluminium are extracted by the electrolysis of their molten salts (chlorides or oxides).

Extraction of Aluminium: Electrolysis of molten alumina ($Al_2O_3$) dissolved in cryolite ($Na_3AlF_6$).

Cathode (-): $Al^{3+} + 3e^- \rightarrow Al(l)$
Anode (+): $2O^{2-} \rightarrow O_2(g) + 4e^-$ (Oxygen reacts with carbon anode)

Extraction of Sodium: Electrolysis of molten $NaCl$.

Cathode (-): $Na^+ + e^- \rightarrow Na(l)$
Anode (+): $2Cl^- \rightarrow Cl_2(g) + 2e^-$