The diaphragm is usually made of asbestos and separates the feed brine (anolyte) from thecaustic-containing catholyte.
Chemical Process Industry (Production of Caustic Soda & Chlorine)
What is electrolysis??
Process of breaking down a compound into its
constituent elements by passing electricity
Basically chlorine and caustic soda is produced
by electrolysis of salt water (brine).
In this process there are three types of
* Mercury Cell * Diaphragm Cell
* Membrane Cell
• Salt is first dissolved in the dissolution tank.
• The obtained saturated brine(salt) is then sent
to a purification tank to remove impurities.
• Industrial water is also purified before entering the cell.
• This shows the need of electricity in electrolytic cell.
• Electric current flowing through the cell decomposes the brine
passing through the narrow space between the electrodes,
liberating chlorine gas (Cl2) at the anode and metallic sodium (Na)
at the cathode.
• The chlorine gas is accumulated above the anode assembly and
discharged to the purification process.
• As it is liberated at the surface of the mercury cathode and the
sodium immediately forms an amalgam (a 'mixture' of two metals)
• The liquid amalgam flows from the electrolytic cell to a
separate reactor, called the decomposer, where it reacts with
water in the presence of a graphite catalyst to form caustic
soda (sodium hydroxide) and hydrogen gas.
• The sodium-free mercury is fed back into the electrolyser
The reaction in the electrolyser is:
2 Na+ +2Cl-+ 2 Hg → 2 Na-Hg + Cl2(g)
The reaction in the decomposer is:
2 Na-Hg + 2 H2O → 2 Na++ 2 OH- + H2 (g) + 2 Hg
• In the diaphragm cell process, there are two compartments separated by
a permeable diaphragm.
• Brine is introduced into the anode compartment and flows into the
• Similarly to the Membrane Cell, chloride ions are oxidized at the anode
to produce chlorine, and at the cathode, water is split into caustic soda
• The diaphragm prevents the reaction of the caustic soda with the
• A diluted caustic brine leaves the cell.
• The caustic soda must usually be concentrated to 50% and the salt
removed. This is done using an evaporative process.
• This technology uses water-impermeable ion-conducting
• The membrane is made of a special resin which permits cations
(positive ions) to pass through.
• The anode chamber of a membrane electrolytic cell is filled with brine,
and the cathode chamber with water.
• The brine in the anode chamber contains sodium (Na+) and chloride
• These ions migrate when a current is applied: the positively charged
sodium ions pass through the membrane to the cathode chamber,
while the negatively charged chloride ions are discharged on the
anode surface to form chlorine gas (Cl2).
• Water in the cathode chamber partly dissociates into hydrogen (H+)
and hydroxide (OH-) ions.
• The hydrogen ions capture electrons on the cathode surface to form
hydrogen gas (H2).
• The hydroxide ions are attracted to the anode, but blocked by the
membrane, and react with the sodium ions from the anode chamber to
form caustic soda (sodium hydroxide, NaOH).
• The chlorine is washed and cooled to remove salt, and further
dehydrated before being delivered or liquefied.
• Chlorine is liquefied and stored low temperature. The liquid
chlorine from the bulk tank can be used as a feedstock.
Caustic soda is further concentrated in a vaporizer to a
concentration of about 50% for delivery.
• In the mercury cell process, 50% caustic soda is obtained
directly from the decomposers.
• The caustic soda is normally pumped through a cooler, then
through a mercury removal system and then to the intermediate
and final storage sections. In some cases the caustic is heated
before filtration. The most common method for removal of
mercury from caustic soda is a plate (or leaf) filter with carbon
• In the case of diaphragm and membrane technologies the
caustic soda is concentrated by evaporation before final
It never gets contaminated with any sodium chloride solution
due to presence of water-impermeable ion-conducting
The caustic soda is more quality.
Operates at low voltage.
Less environmental impact
Wear goggles all the time to avoid contact with gases.
Handle the gases with care in loading area and also
storage area as it is explosive.
Always follow the safety hazards.
Irritates respiratory system
Can be fatal