Electrolysis of Magnesium Chloride

Magnesium Chloride after the reduction reaction in titanium sponge production

process can be electrolyzed to Mg and Cl2 in a electrolytic cell. Electrolysis of MgCl2

with anode as graphite and cathode as Carbon steel will produce Magnesium in the

cathode and Cl2 in the anode. Electrolysis should be carried out at a temperature of

700o C well above the melting point of Mg which is around 6500 C. Electrolysis of

MgCl2 is difficult compared to electrolysis of NaCl (commercially done to produce

sodium hydroxide and chlorine) because MgCl2 cannot be used as such as an

electrolyte due its inherent properties like: -

a) High melting point.

b) Low electrical conductivity.

In order to electrolyze MgCl2 additives are added to improve the property of the

electrolyte. The composition (by weight) of the electrolyte (presently experimented by

DMRL) is: -

a) NaCl- 55% (Increase the electrical conductivity)

b) KCl - 25% (Lowers the melting point and viscosity of MgCl2)

c) CaF2- 2% (Easy coalescence of the metallic mixture)

d) MgCl2- 18%

Reactions involved

a) In the Electrolyte

MgCl2 Mg2+ + 2Cl2- (1)

b) In the Cathode

Mg2+ + 2e- Mg (2)

c) In the Anode

2Cl2- Cl2 + 2e- (3)

Overall Reaction

MgCl2 Mg + Cl2 (4)

Material Balance

Basis – 2000 T of MgCl2 (21052.6 kg moles)

From equation (4)

Weight of Magnesium produced = 21052.6 x 24

= 505.26 T

Weight of Chlorine produced = 21052.6 x 71

= 1494.7 T

The oxidation potential of Mg2+ is 2.37 V and oxidation potential of Cl2- is -1.36V

therefore the cell potential should be at least 2.37-(-1.36) = 3.73 V. The weight of

Magnesium deposited on the cathode will depend on the current given into the cell,

the time of electrolysis, efficiency of the cell. The chlorine produced will be 2.95

times (1494.7/505.26) the amount of Mg deposited on the cathode.

Q (quantity of Mg deposited) in Kg = (I (current in A) x t (time in Seconds) x 12) /

96500

Magnesium

The quantity of Magnesium produced will depend on the current given into the

electrolytic cell and the cell efficiency.

Magnesium produced in the cell being lighter than the electrolyte will float over the

electrolyte. This magnesium has to be tapped out of the cell and can be used in the

reactor for reducing Titanium tetra chloride. The purity of Magnesium produced by

the electrolytic cell depends on the separation technique used for separating

magnesium and electrolyte while removing magnesium from the cell.

Chlorine

Chlorine produced in the anode will be driven out of the cell using the suction

produced by the liquid seal compressor. The chlorine produced in the cell is cooled to

400 C by the chlorine cooler using cooling water as the cooling medium. The cooled

gas is then scrubbed using water at 30 0C and then by chilled water at 18 o C. This is

done to remove the impurities entrained in the gas while exiting the cell. The washed

gas is then dried in a sulfuric acid drier. The dried chlorine gas is then compressed in

the liquid seal compressor to 3.5 Kg/cm2 and fed to the chlorinators in the

chlorination section of TP.

Engineering Challenges

1) Efficiency of the cell: The efficiency of the cell can be reduced if the conductivity

of the electrode is reduced. The concentration of the electrolyte has to be

maintained through out the process by adding additives.

2) Feeding of molten magnesium chloride into the cell.

3) Removal of magnesium after electrolysis: After electrolysis magnesium floats

over the electrolyte, it should be tapped out of the cell leaving the electrolyte

undisturbed.