The chloralkali process is an industrial process for the electrolysis of NaCl .
It is the technology used to produce chlorine and sodium hydroxide, which are
commodity chemicals required by industry. 35 million tons of chlorine
were prepared by this process in 1987. Industrial scale production began in
1892. Usually the process is conducted on a
brine, 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 metal or condensed hydrogen chloride to give hydrogen and
chlorine. The process has a high energy
consumption, for example over 4 billion kWh per year in West Germany in 1985.
Because the process gives equal amounts of chlorine and sodium hydroxide, it is
necessary to find a use for these products in equal proportions.
Procedures Three production methods are in use.
While the mercury cell method produces chlorine-free sodium hydroxide, the use
of several tonnes of mercury leads to serious environmental problems. In a
normal production cycle a few hundred pounds of mercury per year are emitted,
which accumulate in the environment. Additionally, the chlorine and sodium
hydroxide produced via the mercury-cell chloralkali process are themselves
contaminated with trace amounts of mercury. The membrane and diaphragm
method use no mercury, but the sodium hydroxide contains chlorine, which must
be removed. = Membrane cell =
The most common chloralkali process involves the electrolysis of aqueous
sodium chloride 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: 2Cl− → Cl
2 + 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: 2H
2O + 2e− → H2 + 2OH− The ion-permeable ion exchange membrane
at the center of the cell allows the sodium ions to pass to the second
chamber where they react with the hydroxide ions to produce caustic soda.
The overall reaction for the electrolysis of brine is thus:
2NaCl + 2H 2O → Cl
2 + H 2 + 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:
Cl 2 + 2OH− → Cl− + ClO− + H
2O Above about 60 °C, chlorate can be
formed: 3Cl
2 + 6OH− → 5Cl− + ClO 3− + 3H
2O Because of the corrosive nature of
chlorine production, the anode must be made from a non-reactive metal such as
titanium, whereas the cathode 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 cathode. A DC current is passed through
the cell and the NaCl splits into its constituent components. The membrane
passes Na+ ions to the anode compartment, where it forms sodium
hydroxide in solution. The membrane allows only positive ions to pass
through to prevent the chlorine from mixing with the sodium hydroxide. 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. = Diaphragm cell =
In the diaphragm cell process, there are two compartments separated by a
permeable diaphragm, often made of asbestos fibers. Brine is introduced
into the anode compartment and flows into the cathode compartment. 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 and hydrogen.
The diaphragm prevents the reaction of the caustic soda with the chlorine. 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 with about three tonnes of steam per tonne of caustic
soda. The salt separated from the caustic brine can be used to saturate
diluted brine. The chlorine contains oxygen and must often be purified by
liquefaction and evaporation. = Mercury cell =
In the mercury-cell process, also known as the Castner-Kellner process, a
saturated brine solution floats on top of the cathode which is a thin layer of
mercury. Chlorine is produced at the anode, and sodium is produced at the
cathode where it forms a sodium-mercury amalgam with the mercury. The amalgam is
continuously drawn out of the cell and reacted with water which decomposes the
amalgam into sodium hydroxide and mercury. The mercury is recycled into
the electrolytic cell. Mercury cells are being phased out due to concerns about
mercury poisoning from mercury cell pollution such as occurred in Canada and
Japan. Manufacturer Associations
The interests of chloralkali product manufacturers are represented at
regional, national and international levels by associations such as Euro
Chlor and The World Chlorine Council. Laboratory procedure
Electrolysis can be done with beakers, one containing a brine solution and one
containing pure water connected by a salt bridge. Anodes are made ideally
from platinum metals, which resist corrosion. Since corrosion is less
severe at the cathode, it can be stainless steel or silver.
See also Gas diffusion electrode
References ^ Greenwood, Norman N.; Earnshaw, Alan.
Chemistry of the Elements. Butterworth-Heinemann. ISBN 0080379419.
^ Holleman, A. F.; Wiberg, E., Inorganic Chemistry, San Diego: Academic Press,
ISBN 0-12-352651-5 Further reading
Bommaraju, Tilak V.; Orosz, Paul J.; Sokol, Elizabeth A.(2007). "Brine
Electrolysis." Electrochemistry Encyclopedia. Cleveland: Case Western
Rsserve University.