Question

Which of the following is not correctly matched with respect to the processes involved in the extractive metallurgy of the respective metal ?
A. `Al_2O_3. 2H_2OtoAl` : Leaching, precipitation , calcination and electrolytic reduction (molten state)
B. `Ag_2StoAg` Leaching and displacement method
C. `PbS to Pb` Froth flotation process , roasting and self reduction
D. `KCl. MgCl_2. 6H_2OtoMg` Dehydration by simple heating and electrolytic reduction in aqueous phase.

Answer 1

Correct Answer - A
`Al_2O_3(s)+3H_2O(l)+2NaOH(aq)overset("Leaching")to2Na[Al(OH)_4](aq)`
`2Na[Al(OH)_4](aq)+CO_2(g)toAl_2O_3. xH_2O(s)+2NaHCO_3(aq)`
`Al_2O_3 . xH_2O(s)underset("Calcination")overset(1470K)toAl_2O_3(s)+xH_2O(g)`
Electrolytic reduction of molten pure `Al_2O_3` mixed with `Na_3AIF_6` or `CaF_2` Method is known as Hall-Heroult process
Cathode: `Al^(3+)("melt")+3e^(-)toAl(l)`
Anode: `C(s)+O^(2-)("melt")toCO(g)+2e^(-)`
`C(s)+2O^(2-)("melt")toCO_2(g)+4e^(-)`
(B)Leaching:
`Ag_2S+4NaCN hArr 2Na[Ag(CN)_2]+Na_2S`
`Na_2S+2O_2toNa_2SO_4`
Displacement by zinc in aqueous solution:
`2Na[Ag(CN)_2]+ZntoNa_2[Zn(CN)_4]+2Agdarr`
(C )Roasting:
`2PbS+3O_2overset(Delta)to2PbO+2SO_2`
Self reduction: `2PbO+Pbsto3Pb+SO_2`
(D)Calcination:
`MgCl_2 . 6H_2O underset(Dry HCl(g))overset(Delta("calcination"))toMgCl_2+6H_2O`
It is not made anhydrous by simple heating because it gets hydrolysed
`MgCl_2 . 6H_2O oversetDeltatoMgO+5H_2O+2HCl`
Electrolytic reduction :
Electrolytic reduction of molten anhydrous camalite .
`MgCl_2 hArr Mg^(2+)+ 2Cl^(-)`
At cathode: `Mg^(2+) + 2e^(-) to Mg`("99% pure" )
At anode: `2Cl^(-)to Cl_2+2e^(-)`