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Q.Explain the role of the following:

(a) NaCN in the separation of ZnS and PbS.
(b) SiO2SiO_2 in the metallurgy of Cu containing Fe as impurity.
(c) Iodine in the refining of Ti.
CBSECBSE Class XII Board 2020Subjective· 3mImportance★★★★★est
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Three classic separation and purification techniques in metallurgy: (a) NaCN selectively dissolves ZnS as a complex while PbS remains insoluble; (b) SiO₂ acts as an acidic flux to remove FeO as fusible slag; (c) Iodine enables van Arkel refining of Ti through reversible iodide formation.

(a) NaCN in the separation of ZnS and PbS

The challenge here is to separate two sulfide ores that often occur together. Both zinc and lead form sulfides, but their chemical behavior toward complexing agents differs sharply.

Sodium cyanide exploits the fact that zinc forms stable soluble complexes with cyanide ions, while lead does not. When NaCN solution is added to a mixture of ZnS and PbS:

The chemistry:

ZnS+4NaCN⟶Na2[Zn(CN)4]+Na2S\text{ZnS} + 4\text{NaCN} \longrightarrow \text{Na}_2[\text{Zn(CN)}_4] + \text{Na}_2\text{S}

Zinc sulfide dissolves to form the soluble sodium zincate complex [Zn(CN)4]2−[\text{Zn(CN)}_4]^{2-}. Lead sulfide, on the other hand, remains completely insoluble because lead does not form stable cyanide complexes under these conditions.

The separation:

  1. The mixture is treated with aqueous NaCN solution.
  2. ZnS dissolves as the complex ion, passing into the solution phase.
  3. PbS remains as an insoluble residue and can be filtered off.
  4. From the filtrate, zinc can be recovered by further processing (acidification releases Zn²⁺, or electrolysis can deposit metallic zinc).

This is a classic example of differential complexation — using ligands that bind selectively to one metal ion over another.

(b) SiO₂ in the metallurgy of Cu containing Fe as impurity

Copper ores frequently contain iron as an impurity, and during smelting both metals can be present as their oxides or sulfides. The problem is that iron oxide (FeO) has a high melting point and would contaminate the copper if not removed.

Silicon dioxide acts as an acidic flux. A flux is a substance added to combine with unwanted impurities (the gangue) to form a low-melting, easily separable slag.

The reaction:

FeO+SiO2⟶FeSiO3\text{FeO} + \text{SiO}_2 \longrightarrow \text{FeSiO}_3

Iron(II) oxide reacts with silica to form iron(II) silicate (fayalite), which is fusible and immiscible with molten copper. This slag floats on top of the denser molten copper and can be skimmed off.

Why SiO₂?

Iron oxide is a basic oxide. Silica, being acidic, neutralizes it in a classic acid-base reaction at high temperature. The resulting silicate has a much lower melting point than either FeO or SiO₂ alone, and it does not dissolve in the copper matte.

Note

In the reverberatory furnace or during roasting and smelting of copper pyrites (CuFeS₂), silica is deliberately added to ensure clean separation of iron as slag, leaving behind a purer copper matte for further refining.

(c) Iodine in the refining of Ti …

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