Chlorine is the chemical element with atomic number 17 and
symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly
VIIa or VIIb). As the chloride ion, which is part of common salt and other
compounds, it is abundant in nature and necessary to most forms of life,
including humans. In its common elemental form (Cl2 or "dichlorine")
under standard conditions, it is a pale green gas about 2.5 times as dense as
air. It has a disagreeable, suffocating odor that is detectable in
concentrations as low as 3.5 ppm and is poisonous. Chlorine is a powerful
oxidant and is used in bleaching and disinfectants. As a common disinfectant,
chlorine compounds are used in swimming pools to keep them clean and sanitary.
In the upper atmosphere, chlorine based molecules have been implicated in the
destruction of the ozone layer.
Chlorine was discovered in 1774 by Swedish chemist Carl Wilhelm Scheele, who
called it dephlogisticated muriatic acid (see phlogiston theory) and
mistakenly thought it contained oxygen. Scheele isolated chlorine by reacting
MnO2 with HCl. Chlorine was given its current name in 1810 by Sir
Humphry Davy, who insisted that it was in fact an element.
Chlorine gas, also known as bertholite, was first used as a weapon in
World War I by Germany on April 22, 1915 in the Second Battle of Ypres. As
described by the soldiers it had a distinctive smell of a mixture between pepper
and pineapple. It also tasted metallic and stung the back of the throat and
chest. It was pioneered by a German scientist later to be a Nobel laureate,
Fritz Haber of the Kaiser Wilhelm Institute in Berlin, in collaboration with the
German chemical conglomerate IG Farben, who developed methods for discharging
chlorine gas against an entrenched enemy. It is alleged that Haber's role in the
use of chlorine as a deadly weapon drove his wife, Clara Immerwahr, to suicide.
After its first use, chlorine was utilized by both sides as a chemical weapon,
but it was soon replaced by the more deadly gases phosgene and mustard gas.
Chlorine gas has also been used by insurgents in the Iraq War as a chemical
weapon to terrorize the local population and coalition forces. On March 17,
2007, for example, three chlorine filled trucks were detonated in the Anbar
province killing 2 and sickening over 350. Other chlorine bomb attacks resulted
in higher death tolls, with more than 30 deaths on two separate occasions. Most
of the deaths were caused by the force of the explosions rather than the effects
of chlorine, since the toxic gas is readily dispersed and diluted in the
atmosphere by the blast. The principal objective of the insurgents is to create
widespread panic. The Iraqi authorities have tightened up security for chlorine,
which is essential for providing safe drinking water for the population.
Chlorine has isotopes with mass numbers ranging from 32 to 40. There are two
principal stable isotopes, 35Cl (75.77%) and 37Cl
(24.23%), giving chlorine atoms in bulk an apparent atomic weight of
Trace amounts of radioactive 36Cl exist in the environment, in a
ratio of about 7x10−13 to 1 with stable isotopes. 36Cl is
produced in the atmosphere by spallation of 36Ar by interactions with
cosmic ray protons. In the subsurface environment, 36Cl is generated
primarily as a result of neutron capture by 35Cl or muon capture by
40Ca. 36Cl decays to 36S and to
36Ar, with a combined half-life of 308,000 years. The half-life of
this hydrophilic nonreactive isotope makes it suitable for geologic dating in
the range of 60,000 to 1 million years. Additionally, large amounts of
36Cl were produced by irradiation of seawater during atmospheric
detonations of nuclear weapons between 1952 and 1958. The residence time of
36Cl in the atmosphere is about 1 week. Thus, as an event marker of
1950s water in soil and ground water, 36Cl is also useful for dating
waters less than 50 years before the present. 36Cl has seen use in
other areas of the geological sciences, including dating ice and sediments.
Chlorine gas is diatomic, with the formula
Cl2. It combines readily with nearly all other elements, although it
is not as extremely reactive as fluorine. At 10°C and atmospheric
pressure, one liter
of water dissolves
3.10 L of gaseous chlorine, and at 30°C, 1 L of water dissolves only 1.77 liters
Chlorine gas in a plastic container. It is
not advisable to store chlorine in this
This element is a member of the salt-forming halogen series and is extracted
from chlorides through oxidation often by electrolysis. As the
chloride ion, Cl−, it is also the most abundant dissolved ion in ocean water.
In nature, chlorine is found primarily as the chloride ion, a component of
the salt that is deposited in the earth or dissolved in the oceans — about 1.9%
of the mass of seawater is chloride ions. Even higher concentrations of chloride
are found in the Dead Sea and in underground brine deposits. Most chloride salts
are soluble in water, thus, chloride-containing minerals are usually only found
in abundance in dry climates or deep underground. Common chloride minerals
include halite (sodium chloride), sylvite (potassium chloride),
and carnallite (potassium magnesium chloride hexahydrate). Over 2000
naturally-occurring organic chlorine compounds are known.
Industrially, elemental chlorine is usually produced by the electrolysis of
sodium chloride dissolved in water. Along with chlorine, this chloralkali
process yields hydrogen gas and sodium hydroxide, according to the following
- 2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH
Chlorine gas extraction
Chlorine can be manufactured by electrolysis of a sodium chloride solution
(brine). The production of chlorine results in the co-products caustic soda
(sodium hydroxide, NaOH) and hydrogen gas (H2). These two products,
as well as chlorine are highly reactive. Chlorine can also be produced by the
electrolysis of a solution of potassium chloride, in which case the co-products
are hydrogen and caustic potash (potassium hydroxide). There are three
industrial methods for the extraction of chlorine by electrolysis of chloride
solutions, all proceeding according to the following equations:
- Cathode: 2 H+ (aq) + 2 e− → H2 (g)
- Anode: 2 Cl− (aq) → Cl2 (g) + 2 e−
Overall process: 2 NaCl (or KCl) + 2 H2O → Cl2 +
H2 + 2 NaOH (or KOH)
Mercury cell electrolysis
Mercury cell electrolysis, also known as the Castner-Kellner process, was the
first method used at the end of the nineteenth century to produce chlorine on an
industrial scale. The "rocking" cells used have been improved over the years.
Today, in the "primary cell", titanium anodes (formerly graphite ones) are
placed in a sodium (or potassium) chloride solution flowing over a liquid
mercury cathode. When a potential difference is applied and current flows,
chlorine is released at the titanium anode and sodium (or potassium) dissolves
in the mercury cathode forming an amalgam. This flows continuously into a
separate reactor ("denuder" or "secondary cell"), where it is usually converted
back to mercury by reaction with water, producing hydrogen and sodium (or
potassium) hydroxide at a commercially useful concentration (50% by weight). The
mercury is then recycled to the primary cell.
The mercury process is the least energy-efficient of the three main
technologies (mercury, diaphragm and membrane) and there are also concerns about
It is estimated that there are still around 100 mercury-cell plants operating
worldwide. In Japan, mercury-based chloralkali production was virtually phased
out by 1987 (except for the last two potassium chloride units shut down in
2003). In the United States, there will be only five mercury plants remaining in
operation by the end of 2008. In Europe, mercury cells accounted for 43% of
capacity in 2006 and Western European producers have committed to closing or
converting all remaining chloralkali mercury plants by 2020.
Diaphragm cell electrolysis
In diaphragm cell electrolysis, an asbestos (or polymer-fiber) diaphragm
separates cathode and anode, preventing the chlorine forming at the anode from
re-mixing with the sodium hydroxide and the hydrogen formed at the cathode. This
technology was also developed at the end of the nineteenth century. There are
several variants of this process: the Le Sueur cell (1893), the Hargreaves-Bird
cell (1901), the Gibbs cell (1908), and the Townsend cell (1904).The cells vary
in construction and placement of the diaphragm, with some having the diaphragm
in direct contact with the cathode.
The salt solution (brine) is continuously fed to the anode compartment and
flows through the diaphragm to the cathode compartment, where the caustic alkali
is produced and the brine partially depleted.
As a result, diaphragm methods produce alkali that is quite dilute (about
12%) and of lower purity than do mercury cell methods. But diaphragm cells are
not burdened with the problem of preventing mercury discharge into the
environment. They also operate at a lower voltage, resulting in an energy
savings over the mercury cell method, but large amounts of steam are required if the
caustic has to be evaporated to the commercial concentration of 50%.
Membrane cell electrolysis
Development of this technology began in the 1970s. The electrolysis cell is
divided into two "rooms" by a cation permeable membrane acting as an ion
exchanger. Saturated sodium (or potassium) chloride solution is passed through
the anode compartment, leaving at a lower concentration. Sodium (or potassium)
hydroxide solution is circulated through the cathode compartment, exiting at a
higher concentration. A portion of the concentrated sodium hydroxide solution
leaving the cell is diverted as product, while the remainder is diluted with
deionized water and passed through the electrolyzer again.
This method is more efficient than the diaphragm cell and produces very pure
sodium (or potassium) hydroxide at about 32% concentration, but requires very
Other electrolytic processes
Although a much lower production scale is involved, electrolytic diaphragm
and membrane technologies are also used industrially to recover chlorine from
hydrochloric acids solutions, producing hydrogen (but no caustic alkali) as a
Furthermore, electrolysis of fused chloride salts (Downs process) also
enables chlorine to be produced, in this case as a by-product of the manufacture
of metallic sodium or magnesium.
Before electrolytic methods were used for chlorine production, the direct
oxidation of hydrogen chloride with oxygen or air was exercised in the Deacon
- 4 HCl + O2 → 2 Cl2 + 2 H2O
This reaction is accomplished with the use of CuCl2 as a catalyst
and is performed at high temperarature (about 400°C). The amount of extracted
chlorine is approximately 80%. Due to the extremely corrosive reaction mixture,
industrial use of this method is difficult and several pilot trials failed in
the past. Nevertheless, recent developments are promising.
Another earlier process to produce chlorine was to heat brine with acid and
- 2 NaCl + 2H2SO4 + MnO2 →
Na2SO4 + MnSO4 + 2 H2O +
Using this process, chemist Carl Wilhelm Scheele was the first to isolate
chlorine in a laboratory. The manganese can be recovered by the Weldon
Small amounts of chlorine gas can be made in the laboratory by putting
concentrated hydrochloric acid in a flask with a side arm and rubber tubing
attached. Manganese dioxide is then added and the flask stoppered. The reaction
is not greatly exothermic. As chlorine is denser than air, it can be easily
collected by placing the tube inside a flask where it will displace the air.
Once full, the collecting flask can be stoppered.
In the laboratory, small amounts of chlorine gas can also be created by
adding concentrated hydrochloric acid (typically about 5M) to sodium
hypochlorite or sodium chlorate solution.
Large-scale production of chlorine involves several steps and many pieces of
equipment. The description below is typical of a membrane plant. The plant also
produces simultaneously sodium hydroxide (referred to in the industry as caustic
soda) and hydrogen gas. A typical plant consists of brine production/treatment,
cell operations, chlorine cooling & drying, chlorine compression &
liquefaction, liquid chlorine storage & loading, caustic handling,
evaporation, storage & loading and hydrogen handling.
Key to the production of chlorine is the operation of the brine
saturation/treatment system. Maintaining a properly saturated solution with the
correct purity is vital, especially for membrane cells. Many plants have a salt
pile which is sprayed with recycled brine. Others have slurry tanks that are fed
The raw brine is partially or totally treated with sodium hydroxide, sodium
carbonate and a flocculant to reduce calcium, magnesium and other impurities.
The brine proceeds to a large clarifier or a filter where the impurities are
removed. The total brine is additionally filtered before entering ion exchangers
to further remove impurities. At several points in this process, the brine is
tested for hardness and strength.
After the ion exchangers the brine is considered pure, and is transferred to
storage tanks to be pumped into the cell room. Brine fed to the cell line is
heated to the correct temperature to control exit brine temperatures according
to the electrical load. Brine exiting the cell room must be treated to remove
residual chlorine and control pH before being returned to the saturation stage.
This can be accomplished via dechlorination towers with acid and sodium
bisulfite addition. Failure to remove chlorine can result in damage to the
cells. Brine should be monitored for accumulation of chlorate and sulfate and
either have treatment systems in place or purging of the brine loop to maintain
safe levels, since chlorate can diffuse through the membranes and contaminate
the caustic, while sulfate can damage the anode surface coating.
The building that houses the many electrolytic cells is usually called a cell
room or cell house, although some plants are built outdoors. This building
contains support structures for the cells, connections for supplying electrical
power to the cells and piping for the fluids. Monitoring and control of the
temperatures of the feed caustic and brine is done to control exit temperatures.
Also monitored are the voltages of each cell which vary with the electrical load
on the cell room that is used to control the rate of production. Monitoring and
control of the pressures in the chlorine and hydrogen headers is also done via
pressure control valves.
Direct electrical current is supplied via rectifiers. Plant load is
controlled by varying the current to the cells. As the current is increased flow
rates for brine, caustic and deionized water are increased while lowering the
Cooling and drying
Chlorine gas exiting the cell line must be cooled and dried since the exit
gas can be over 80º C and contains moisture that allows chlorine gas to be
corrosive to iron piping. Cooling the gas allows for a large amount of moisture
from the brine to condense out of the gas stream. Cooling also improves the
efficiency of the compression and liquefaction stage that follows. Chlorine
exiting is ideally between 18º C and 25º C. After cooling the gas stream passes
through a series of towers with counter flowing sulfuric acid. These towers
progressively remove any remaining moisture from the chlorine gas. After exiting
the drying towers the chlorine is filtered to remove any sulfuric acid
Compression and liquefaction
Several methods of compression may be used: liquid ring, reciprocating, or
centrifugal. The chlorine gas is compressed at this stage and may be further
cooled by inter- and after-coolers. After compression it flows to the
liquefiers, where it is cooled enough to liquefy. Non condensible gases and
remaining chlorine gas are vented off as part of the pressure control of the
liquefaction systems. These gases are routed to a gas scrubber, producing sodium
hypochlorite, or used in the production of hydrochloric acid (by combustion with
hydrogen) or ethylene dichloride (by reaction with ethylene).
Storage and loading
Liquid chlorine is typically gravity-fed to storage tanks. It can be loaded
into rail or road tankers via pumps or padded with compressed dry gas.
Caustic handling, evaporation, storage and
Caustic fed to the cell room flows in a loop that is simultaneously bled off
to storage with a part diluted with deionized water and returned to the cell
line for strengthening within the cells. The caustic exiting the cell line must
be monitored for strength, to maintain safe concentrations. Too strong or too
weak a solution may damage the membranes. Membrane cells typically produce
caustic in the range of 30% to 33% by weight. The feed caustic flow is heated at
low electrical loads to control its exit temperature. Higher loads require the
caustic to be cooled, to maintain correct exit temperatures. The caustic exiting
to storage is pulled from a storage tank and may be diluted for sale to
customers who require weak caustic or for use on site. Another stream may be
pumped into a multiple effect evaporator set to produce commercial 50% caustic.
Rail cars and tanker trucks are loaded at loading stations via pumps.
Hydrogen produced may be vented unprocessed directly to the atmosphere or
cooled, compressed and dried for use in other processes on site or sold to a
customer via pipeline, cylinders or trucks. Some possible uses are hydrochloric
acid or hydrogen peroxide production, desulfurization of petroleum oils and use
as a fuel in boilers or fuel cells.
Production of chlorine is extremely energy intensive. Energy consumption per
unit weight of product is not far below that for iron and steel manufacture and
greater than for the production of glass or cement.
Since electricity is an indispensable raw material for the production of
chlorine, the energy consumption corresponding to the electrochemical reaction
cannot be reduced. Energy savings arise primarily through applying more
efficient technologies and reducing ancillary energy use.