6 methods of preparation of caustic soda

6 METHODS OF
PREPARATION OF CAUSTIC
SODA
www.worldofchemicals.com

Introduction
 Sodium hydroxide, also known as caustic soda or
lye, is an inorganic compound with the chemical
formula NaOH. It is a white solid, and is a highly
caustic metallic base and alkali salt. It is available
in pellets, flakes, granules, and as prepared
solutions at a number of different concentrations.
 Sodium hydroxide is industrially produced as a 50
per cent solution by variations of the electrolytic
chloralkali process. Chlorine gas is also produced
in this process. Solid sodium hydroxide is
obtained from this solution by the evaporation of
water.

Castner - Kellener
Process
Principle - In castner-kellner method NaOH is prepared by the electrolysis of
aqueous solution of NaCl (Brine).

 Castner-kellner cell - It is a rectangular tank of steel.
Inside of tank is lined with ‘ebonite.’ Anode is made of
titanium. Flowing layer of mercury (Hg) at the bottom
of tank serves as cathode.
 Ionization of NaCl - 2NaCl e- 2Na+ + 2Cl-
 When electric current is passed through brine, +ve
and -ve ions migrate towards their respective
electrodes. Na+ ions are discharged at mercury
cathode. The sodium deposited at mercury forms
Sodium Amalgam. Chlorine produced at the anode is
removed from the top of the cell.
 Reaction at cathode
2Na+ +2 e- à 2Na
(Na forms amalgam)
Na + Hg à Na/Hg
Na+ ions are discharged in preference to H+ ions
due to high over voltage.

 Reaction at anode
2Cl- à Cl2 + 2e-
 Formation of NaOH
Amalgam moves to another chamber called
‘denuder,’ where it is treated with water to
produce NaOH which is in liquid state. Solid
NaOH is obtained by the evaporation of this
solution.
2Na/Hg + 2H2O à 2NaOH + H2 + 2Hg
NaOH obtained is highly pure and the process
is very effeicient.

Nelson Diaphragm Cell
 Principle: Electrolyte used in this process is
aqueous NaCl (Brine).
 Procedure: Porous diaphragm of asbestos or
metal oxide with polymer separates anode and
cathode compartments. Diaphragm prevents
hydroxide ions entering anode compartment
and prevents chloride ions entering cathode
compartment. Saturated brine enters anode
compartment where chlorine gas is produced.

Nelson Diaphragm Cell
Anode (positive electrode): carbon (graphite) or titanium coated with
Ru-Ti oxide.
Cathode (negative electrode): steel mesh

 Reaction at anode(oxidation):
o 2Cl- (aq) àCl2(g) + 2e
o Cathode reaction (reduction):
o 2H2O (l) + 2e à H2(g) + 2OH-(aq)
o Na+ migrates across diaphragm to cathode compartment
combining with OH- to form NaOH.
o Overall cell reaction (showing Na+ spectator ions):
o 2H2O (l) + 2Cl- (aq) + 2Na+ (aq) -----> 2Na+(aq) + 2OH-(aq)
+ H2(g) + Cl2(g)
o Product contains sodium chloride and sodium hydroxide.
NaOH(s) can be crystallised out.

Loewig’s process
 Loewig’s process for caustic soda preparation depends on
the formation of sodium ferrate (Na2Fe2O4), which is then
decomposed with water. The soda liquors are mixed with
ferric oxide, and the mass evaporated to dryness and
calcined at a bright red heat, usually in a revolving furnace.
By the calcination, a reaction between the sodium carbonate
and the iron oxide is brought about, carbon dioxide escaping
and sodium ferrate remaining in the furnace. The mass is
washed with cold water until all soluble matter is removed;
then water at 900 C is run over the sodium ferrate, by which it
is decomposed, caustic soda formed, and iron oxide
regenerated; the last is returned to the calcining process. The
ferric oxide used is a natural iron ore, very clean and free
from silica or other impurities; that made by calcining a
precipitated ferric hydroxide is not well adapted to the
process, as it gives a product difficult to lixiviate.

White liquor oxidation
process
 In the white liquor oxidation process, the
nitrogen sulfide (Na2S) in the white liquor is
oxidized by air to polysulfides, which are used
in the digestion process. The digestion
process is where cellulose and semi-cellulose
(polysaccharide) are separated from the chips
which serve as the wood material for making
pulp. White liquor being a chemical such as
Na2S, NaOH etc.

 This air-oxidation process is situated in between the
caustification process and the digestion process. The white
liquor that is to be air-oxidized is taken from the line that
connects the caustification to the digestion process, and the
polysulfides generated are channeled back into process. This
means that no modification to the existing system is
necessary. The air-oxidation process is comprised mainly of a
white-liquor filtration device and an oxidation reactor. The
upper-current type is used in order for the filter to efficiently
remove the SS, the major components being CaCO3 and
other substances generated during the caustification process.
The oxidation reactor is filled with oxidation catalysts, then air
supplied via a blower keeps the oxidation reaction going.
 The NaOH generated along with the polysulfides in the
reaction is used effectively in the digestion process, which
also serves to reduce the load placed on the caustification
process.

Carmichael method
 In Carmichael’s apparatus, an asbestos diaphragm, impregnated
with Portland cement, is used. The diaphragm rests horizontally on
the cathode at the bottom of the cell; above it is a bell to collect the
hydrogen given off. On anode is a grating of copper rods, covered
with hard rubber, through which platinum points project into the
brine. This anode is suspended in the top of the cell, and the
chlorine set free is thus only momentarily in contact with the liquid.
The salt solution is fed into the cell at the top, in a rapid stream of
drops while the mixture of caustic soda and salt flows continuously
from the bottom. The supply of brine is so regulated that the caustic
formed at the cathode is drawn off before it has time to diffuse
through the liquid. The solution drawn from the cell contains about
20 per cent of caustic soda, and about 75 per cent of the salt is
decomposed. The reaction is carried on at a temperature of about
80° C in the top of the cell near the anode, while the region around
the cathode is kept as cool as possible.
 Being removed from the immediate action of the chlorine, the
diaphragms are very durable.

LeSueur’s process
 The process uses Lunge’s apparatus. The cathode, of iron
wire gauze is placed in a slanting position. On it rests the
diaphragm, consisting of two parts, a sheet of parchment
paper and it double sheet of asbestos cemented together by
blood albumin, coagulated and hardened by treatment with
potassium bichromate. An earthenware bell enclosed the
anode, which was made of lead, carrying carbon rods dipping
into the salt solution. Caustic soda is formed in the solution
outside the bell, and owing to the inclined position of the
cathode, the hydrogen was expected to escape readily, thus
preventing polarization. But it proved in practice that the
earthenware bells were disintegrated by the caustic soda
solution, while the hydrogen set free. The diaphragms are
rapidly destroyed, lasting only from 24 to 48 hours. The
anodes are consumed more slowly, lasting about six weeks.
The process yields a solution of caustic containing 10 per
cent NaOH.

Thank You
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6 methods of preparation of caustic soda

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