Singapore Caustic Plant, Chlorine Plant | Kalf

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Caustic Soda Plant

KALF’s Chlor-alkali
The chlor-alkali is an industrial process for the electrolysis of NaCl.
It is the technology used to produce chlorine and sodium hydroxide
(caustic soda), which are commodity chemicals required by industry.

KALF design the process is conducted on a brine (an aqueous solution of NaCl),
in which case NaOH, hydrogen, and chlorine result. When using calcium
chloride or potassium chloride, the products contain calcium or potassium
instead of sodium. Related processes are known that use molten NaCl
to give chlorine and sodium metalor condensed hydrogen chloride to give
hydrogen and chlorine.

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KALF’s Membrane Technology
Basic membrane cell (Membrane Electrolyser) used in the electrolysis of brine. At the anode (A),
chloride (Cl−) is oxidized to chlorine. The ion-selective membrane (B) allows the countering Na+ to freely flow across,
but prevents anions such as hydroxide (OH−) and chloride from diffusing across. At the cathode (C),
water is reduced to hydroxide and hydrogen gas. The net process is the electrolysis of an aqueous solution
of NaCl into industrially useful products sodium hydroxide (NaOH) and chlorine gas.

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KALF Chlor-alkali Technology
The KALF’s chlor-alkali process involves the electrolysis of aqueous sodium chloride (a brine) in a membrane cell.
Saturated brine is passed into the first chamber of the cell where the chloride ions are oxidised at the anode,
losing electrons to become chlorine gas (A in figure):

2Cl− → Cl2 + 2e−

At the cathode, positive hydrogen ions pulled from water molecules are reduced by the electrons provided by
the electrolytic current, to hydrogen gas, releasing hydroxide ions into the solution (C in figure):

2H2O + 2e− → H2 + 2OH−

The ion-permeable ion exchange membrane at the center of the cell allows the sodium ions (Na+) to pass to
the second chamber where they react with the hydroxide ions to produce caustic soda (NaOH) (B in figure).
The overall reaction for the electrolysis of brine is thus:

2NaCl + 2H2O → Cl2 + H2 + 2NaOH

A membrane cell is used to prevent the reaction between the chlorine and hydroxide ions.
If this reaction were to occur the chlorine would be disproportionated to form chloride and hypochlorite ions:

Cl2 + 2OH− → Cl− + ClO− + H2O
Above about 60 °C, chlorate can be formed:

3Cl2 + 6OH− → 5Cl− + ClO3− + 3H2O

Because of the corrosive nature of chlorine production,
the anode (where the chlorine is formed) must be made
from a non-reactive metal such as titanium, whereas
the cathode (where hydroxide forms) can be made from
a more easily oxidized metal such as nickel.
In the membrane cell, the anode and cathode are separated
by an ion-permeable membrane.
Saturated brine is fed to the compartment with the
anode (the anolyte). A DC current is passed through the cell
and the NaCl splits into its constituent components.

The membrane passes Na+ ions to the cathode
compartment (catholyte), where it forms sodium hydroxide in solution.
The membrane allows only positiveions to pass through to prevent
the chlorine from mixing with the sodium hydroxide.

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The chloride ions are oxidised to chlorine gas at the anode,
which is collected, purified and stored. Hydrogen gas and hydroxide ions are formed at the cathode