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Overview of General Metallurgical Processes

Which of the ...

Question

Which of the following is not correctly matched with respect to the processes involved in the extractive metallurgy of the respective metal?

A

A l_{2} O_{3} .2 H_{2} O \to A l

: Leaching, precipitation, calcination and electrolytic reduction (molten state)

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B

A g_{2} S \to A g

: Leaching and displacement method

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C

P b S \to P b

: Froth flotation process, roasting and self reduction

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D

K C l . M g C l_{2} .6 H_{2} O \to M g

: Dehydration by simple heating and electrolytic reduction in aqueous phase

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Solution

The correct option is D $K C l . M g C l_{2} .6 H_{2} O \to M g$ : Dehydration by simple heating and electrolytic reduction in aqueous phase
A) Extraction of $A l$ from its ore follows these steps:
Leaching, precipitation, calcination and electrolytic reduction (molten state)
$A l_{2} O_{3} (s) + 3 H_{2} O (l) + 2 N a O H (a q) L e a c h i n g - ---- \to 2 N a [A l (O H)_{4}] (a q)$

$2 N a [A l (O H)_{4}] (a q) + C O_{2} (g) \to A l_{2} O_{3} . x H_{2} O (s) + 2 N a H C O_{3} (a q)$

$A l_{2} O_{3} . x H_{2} O (s) 1470 K - ------ \to C a l c i n a t i o n A l_{2} O_{3} (s) + x H_{2} O (g)$

Electrolytic reduction of molten pure $A l_{2} O_{3}$ is mixed with $N a_{2} A l F_{6}$ or $C a F_{2}$

This method is known as Hall-Heroult process
$C a t h o d e : A l^{3 +} (m e l t) + 3 e^{-} \to A l (l)$

$A n o d e : C (s) + O^{2 -} (m e l t) \to C O (g) + 2 e^{-}$
$C (s) + 2 O^{2 -} (m e l t) \to C O_{2} (g) + 4 e^{-}$
B) In case of $A g$ , extraction follows leaching and displacement method as:
Leaching:
$A g_{2} S + 4 N a C N ⇌ 2 N a [A g (C N)_{2}] + N a_{2} S$
$N a_{2} S + 2 O_{2} \to N a_{2} S O_{4}$

Displacement by zinc in aqueous solution:
$2 N a [A g (C N)_{2}] + Z n \to N a_{2} [Z n (C N)_{4}] + 2 A g ↓$
C) Extraction of $C u$ follows these as:
Roasting:
$2 P b S + 3 O_{2} Δ - \to 2 P b O + 2 S O_{2}$

Self reduction:
$2 P b O + P b S \to 2 P b + S O_{2}$
D) Extraction of $M g$ follows:
Calcination:
$M g C l_{2} .6 H_{2} O Δ (c a l c i n a t i o n) - -------- \to D r y H C l (g) M g C l_{2} + 5 H_{2} O$

It is not made anhydrous by simple heating because it gets hydrolysed
$M g C l_{2} .6 H_{2} O Δ - \to M g O .5 H_{2} O + 2 H C l$

Electrolytic reduction:
Electrolytic reduction of molten anhydrous carnalite.
$M g C l_{2} ⇌ M g^{2 +} + 2 C l^{-}$

At cathode: $M g^{2 +} + 2 e^{-} \to M g$ (99% pure);

At anode: $2 C l^{-} \to C l_{2} + 2 e^{-}$
Option D represents the incorrect statement.
Ans-option D

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