Extraction of Silver, Lead And Magnesium

ChemistryGeneral Principles And Processes Of Isolation Of ElementsFor JEE aspirants

Extraction of Silver

Important Ores of Silver:

$∙$ Argentite or silver glance, $A g_{2} S$

$∙$ Horn Silver AgCl

$∙$ Pyragyrite (Ruby Silver), $3 A g_{2} S, S b_{2} S_{3}$

Metallurgy of Silver:

Silver is mainly extracted from argentite $(A g_{2} S)$ by Cyanide Process

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Extraction of Lead

Ores:

Galena PbS

Anglesite $P b S O_{4}$

Cerussite $P b C O_{3}$

Lanerkite PbO, $P b S O_{4}$

Croeoisite $P b C r O_{4}$

Extraction: Steps of extraction are shown in the following flow sheet:

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Reactions involved:

$P b O + C P b + C O$

$P b O + C O P b + C O_{2}$

$P b S + 2 P b O 3 P b + S O_{2} ↑$

$P b S + P b S O_{4} 2 P b + 2 S O_{2} ↑$

$C a O + S i O_{2} C a S i O_{3} (s l a g)$

$F e_{2} O_{3} + C O 2 F e O + C O_{2}$

$F e O + S i O_{2} F e S i O_{3} (s l a g)$

Lead thus obtained contains other metals like Cu, Sb, Sn, Ag, Au, etc. and these make lead hard. In reverberatory furnace, impurities are oxidized and removed and this makes lead soft.

Extraction of Magnesium

Magnesium from sea – water is obtained by the Dow's process. It consists of electrolysis of molten magnesium chloride using an iron cathode and a graphite anode. Following are the steps that involved during the process.

$∙$ $M g^{+ 2}$ is precipitated as magnesium hydroxide by the addition of slaked lime, $C a (O H)_{2}$ to the sea water.

$M g^{2 +} + C a (O H)_{2} M g (O H)_{2} + C a^{2 +}$

$∙$ Magnesium hydroxide on treatment with hydrochloric acid is converted to chloride which is crystallized as $M g C l_{2} .6 H_{2} O$

$M g (O H)_{2} + 2 H C l M g C l_{2} + 2 H_{2} O$

$∙$ Now for electrolysis, magnesium chloride is fused as follows :

On passing a current of dry HCl gas it get partially dehydrated and the chloride thus obtained is added to a molten mix. of sodium chloride and calcium chloride (temperature range 973 – 1023 K). Under this condition $M g C l_{2}$ melt with the loss of water.

If hydrated magnesium chloride is heated strongly, it hydrolyses to yield magnesia (magnesium oxide) which is a refractory.

$M g C l_{2} .6 H_{2} O M g O + 2 H C l + 5 H_{2} O$

$∙$ Molten mixture of $M g C l_{2} . N a C l an d C a C l_{2}$ is electrolysed. Magnesium is deposited at the cathode and chlorine is evolved at the anode. The chlorine is used to make HCl acid which in turn is required for making magnesium chloride.

Cathode: $M g^{2 +} + 2 e^{-} M g$

Anode: $2 C l^{-} C l_{2} + 2 e^{-}$

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