Electrolytic Refining | Chemistry Class 12 - NEET PDF Download

(D) Electrolytic refining :

Anode : Impure Zn

Cathode : Aluminium Electrode

Electrolyte : ZnSO₄(aq) H₂SO₄

Extraction of Cr From FeCr₂O₄(FeO.Cr₂O ₃) Chromite ORE :

Conc. Step : Chromite ore is concentrated by gravity sep. Method to remove light impurites followed by magnetic sep. method to remove non-magnetic impurities.

Chemical Method to Separate Cr₂O₃ :

4FeO.Cr₂O₃  7O₂ (air) 8 Na₂CO₃/16NaOH  2Fe₂O₃  8Na₂CrO₄  8CO₂/8H₂O

2Na  (aq) Cr₂O₇^2-(aq) SO₄^2- (aq)  2Na (aq) CrO₄^2- (aq)

(Orange Solution) yellow solution

Na₂Cr₂O₇.2H₂ O  NaCrO₂  Cr(OH)₃¯

(orange crystal) (green ppt)

Cr₂O₃ (green solid)

Al-reduction method : (Goldsmith alumino thermite process)

Impure Cr is purified by electrolytic refining method.

Anode : Impure chromium

Cathode : Hg-electrode

Electrolyte : CrCl₃  HCl

Extraction of Mn from MnO₂ (Pyrolusite) :

Pyrolusite is concentrated by gravity separation method followed by magnetic separation method to remove non-magnetic impurity.

Calcination :

MnO₂ gives explosive rxn. with Al therefore in this step it is converted into mixed oxide Mn₃O₄.

3MnO₂  Mn₃O₄(MnO Mn₂O₃) + O₂­

Al-reduction method : (Goldsmith alumino thermite process)

Impure Mn is purified by electrolytic refining method.

Anode : Impure Mn

Cathode : Hg-electrode

Electrolyte : MnSO₄  H₂SO₄

8. Aluminium :

Ores :

(i) Bauxite Al₂O₃.2H₂O (main ORE) ; Diaspore Al₂O₃.H₂O ; Corundum Al₂O₃

(ii) Mica K₂O.3Al₂O₃.6SiO ₂.2H₂O ; Kaolin Al₂O₃.2SiO₂.2H ₂O

(iii) Cryolite Na₃AIF₆

Extraction of Al from Bauxite :

Purification of Bauxite : 

 Electrolysis of pure fused Al₂O₃ (Hall - Heroult method)

Cathode - iron tank lined with graphite

Anode - Carbon rods dipped in molten electrolyte

Electrolyte - molten (20% Al₂O₃  60% Cryolite 20% CaF₂) Temp ≈ 1100°C

Cryolite lowers the melting point of mixture and makes the molten mix conducting.

Na₃AlF₆  3NaF+ AlF₃

AlF₃ → Al ³  + 3F^-

At cathode Al ³ + 3e^- → Al

At anode 3F^- → 3F+ 3e^-

Al₂O₃ + 6F → 2AIF₃  +  O₂­

C +  O₂ → CO

CO+   O₂ → CO₂

Anodes are periodically changed as they are consumed by oxygen liberated at anode.

Electrolytic refining (Hoppe's Method)

Iron box lined with carbon, contains the three molten layers.

Bottom layer : Impure Al as anode

Middle layer : (Na₃AlF₆  BaF₂) Molten electrolyte

Tope layer : molten pure Al as cathode.

On passing the current aluminium is deposited at cathode from the middle layer and an equivalent amount is taken from andoe (bottom layer) levaing behind the impurites. In this way aluminium obtained is 99.98% pure.

9. Magnesium : (Dow's process)

Ores :

Carnallite MgCl₂.KCl 6H₂O (main ore) ; Epsom MgSo₄.7H₂O magnesite MgCO₃ ; Kiesserite MgSO₄. H₂O ; Dolomite MgCO₃. CaCO₃ ; Kainite K₂SO₄. MgSO₄. MgCl₂.6H₂O (Mg is also found in Talc, sabestos and chlorophyll)

Sea water contains 0.13% magnesium as chloride and sulphate. It involves following steps.

(a) Precipitation of magnesium as magnesium hydroxide by slaked lime :

MgCl₂ + Ca(OH)₂ → Mg(OH)₂ + CaCl₂ (ppt. reaction)

(b) Preparation of hexahydrated magnesium chloride :

Mg(OH)₂  + 2HCl(aq) → MgCl₂  + 2H₂O (Neutralisation reaction)

The solution on concentration and crystallisation gives the crystals of MgCl₂.6H₂O

(c) Preparation of anhydrous magnesium chloride :

MgCl₂.6H₂O  MgCl₂ + 6H₂O

* It is not made anhydrous by simple heating because it gets hydrolysed.

MgCl₂.6H₂O  MgO+ 5H₂O +2HCl

(d) Electrolysis of fused anhydrouns MgCl₂ :

Magnesium chloride obtained by any of the above methods is fused and mixed with sodium chloride and calcium chloride in the temperature range of 972 - 1023K. The molten mixture is electrolysed. Magnesium is liberated at the carhode (iron pot) and chlorine is evolved at graphite anode.

MgCl₂  Mg²   2Cl^-

At cathode : Mg²  + 2e^- → Mg(99% pure) ;

At anode : 2Cl^- → Cl₂ + 2e^-