The chlor-alkali industry

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The chlor-alkali industry

• C.12.1 Discuss the production of chlorine and
  sodium hydroxide by the electrolysis of sodium
  chloride
• C.12.2 Outline some important uses of the
  products of this process
• C.12.3 Discuss the environmental impact of the
  processes used for the electrolysis of sodium
  chloride

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Introduction of chloride

• Chloride is a powerful oxidizing agent with a
  standard electrode potential of 1.36V.
• Since it has a very high value of standard
  electrode potential, very few chemical oxidizing
  agents can oxidize chloride ions to chlorine
  (Apart from fluorine).
• ? the manufacture of chlorine gas depends on
  using electrons themselves.

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How is chlorine made ?

• Chlorine gas is formed during the electrolysis of
  molten sodium chloride in the industrial
  production of sodium metal.
• Chlorine is produced by passing an electric
  current through a solution of brine (common salt
  dissolved in water). The chemical term for salt
  is sodium chloride (NaCl).
• Essential co-products are caustic soda (sodium
  hydroxide (NaOH)) and hydrogen (H2). All three
  are highly reactive, and technologies have been
  developed to separate them and keep them apart

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• Caustic soda is an alkali and widely-used in many
  industries, including the food industry, textile
  production, soap and other cleaning agents, water
  treatment and effluent control.
• Hydrogen is a combustible gas used in various
  processes including the production of hydrogen
  peroxide and ammonia as well as the removal of
  sulphur fro petroleum derivatives.

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• Chlorine has been manufactured industrially for
  more than 100 years. During this time, the
  industry's firm commitment to the best safety,
  health and environmental practices has ensured
  continuous improvement.
• There are three methods to produce Chlorine
• The membrane cell process
• The diaphragm cell process
• The mercury cell process

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The diaphragm cell process

• In the diaphragm cell process
• The positive electrode( made of titanium) and
  negative( made of steel) electrodes are separated
  by a permeable diaphragm.
• Hydrogen is formed at negative electrode
• 2H2O(l) 2e- ? H2(g) 2OH-(ag)
• Chlorine is formed at positive electrode
• 2Cl-(ag) ? Cl2(g) 2e-
• The diaphragm is made of asbestos
• Sodium chloride solution can flow between the
  electrodes
• Chlorine and hydrogen gas cant flow through(
  preventing the OH- ions flowing towards the
  positive electrode)
• The Sodium hydroxide solution formed accumulates
  in the cathode compartment and is piped off.
• The Resulting solution contains about
• 10 sodium hydroxide
• 15 unused sodium chloride by mass
• The solution is concentrated by evaporation and
  the sodium chloride crystallizes out leaving a
  50 solution of sodium hydroxide.

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Diagram
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Modern version

• Another version of the diaphragm cell is known as
  an ions exchange membrane cell.
• It uses a partially permeable ion exchange
  membrane rather than asbestos. The membrane is
  made of a flourinated polymer and is permeable to
  positive ions but not negative ions.

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The membrane cell process

• The anode and the cathode are separated by an
  ion-exchange membrane. Only sodium ions and a
  little water pass through the membrane.
• The brine is de-chlorinated and re-circulated.
  Solid salt is usually needed to re-saturate the
  brine. After purification by precipitation-filtrat
  ion, the brine is further purified with an ion
  exchanger.
• The caustic solution(NaOH) leaves the cell with
  about 30 concentration and, at a later stage in
  the process, is usually concentrated to 50. The
  chlorine gas contains some oxygen and must often
  be purified by liquefaction and evaporation.

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The mercury cell process

• In the mercury cell process, negative electrode
  is made of flowing mercury.
• Sodium is above hydrogen in the electrochemical
  series, sodium is preferentially discharged as it
  forms an alloy (known as an amalgam) with the
  mercury
• Na(ag) e- Hg(l) ? Na/Hg(l)
• The mercury flows out of the electrolysis cell
  into a separate chamber ? Reacts with water to
  produce hydrogen and sodium hydroxide solution
• The mercury is recycled back into the
  electrolytic cell.
• Na/Hg(l) H2O(l) ? Na(ag) OH-(ag) 1/2 H2(g)
  Hg(l)
• The cell is made of PVC-lined steel and the
  positive electorde where is chlorine is formed is
  made of graphite.
• 2Cl-(ag) ? Cl2(g) 2e-
• As the brine is usually re-circulated, solid salt
  is required to maintain the saturation of the
  salt water. The brine is first de-chlorinated and
  then purified by a precipitation-filtration
  process.

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• The products are extremely pure. The chlorine,
  along with a little oxygen, generally can be used
  without further purification.
• Of the three processes, the mercury process uses
  the most electricity, but no steam is required to
  concentrate the caustic solution. The use of
  mercury demands measures to prevent environmental
  contamination. Also, mercury must be removed from
  the hydrogen gas and caustic soda solution.
• Mercury losses have been considerably reduced
  over the years. Increasingly, chlorine producers
  are moving towards membrane technology, which has
  much less impact on the environment.
• In 2007, emissions for all mercury cells across
  Western Europe reached an all-time low of 0,97
  grammes per tonne of chlorine capacity.
• Forty-two mercury-based chlorine plants remain to
  be voluntarily phased out or converted to
  non-mercury technology by 2020 at a cost of more
  than EUR 3,000 million. These plants account for
  an ever decreasing part (37.7 in 2007) of
  European chlorine capacity.

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Important uses of Hydrogen

• A perfect fuel.
• The byproducts of hydrogen combustion are
  electricity, water and heat.
• Energy conversion devices using hydrogen are
  highly efficient and produce very little or no
  harmful emissions
• It is used as a shielding gas in welding methods
  such as atomic hydrogen welding.
• H2 is used as the rotor coolant in electrical
  generators at power stations, because it has the
  highest thermal conductivity of any gas
• In more recent applications, hydrogen is used
  pure or mixed with nitrogen (sometimes called
  forming gas) as a tracer gas for minute leak
  detection. Applications can be found in the
  automotive, chemical, power generation,
  aerospace, and telecommunications industries
• Disadvantage
• High cost
• Low density ? storage problem.

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Important uses of Chlorine

• Chlorine's principal applications are in the
  production of a wide range of industrial and
  consumer products.3334 For example, it is
  used in making plastics, solvents for dry
  cleaning and metal degreasing, textiles,
  agrochemicals and pharmaceuticals, insecticides,
  dyestuffs, etc.
• Chlorine is an important chemical for water
  purification (such as water treatment plants), in
  disinfectants, and in bleach
• Chlorine is usually used (in the form of
  hypochlorous acid) to kill bacteria and other
  microbes in drinking water supplies and public
  swimming pools.
• Elemental chlorine is an oxidizer(Chemistry) ,
  Weapon (World War I,Irag War),... etc

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Important uses of sodium hydroxide

• General applications
• Sodium hydroxide is the principal strong base
  used in the chemical industry. In bulk it is most
  often handled as an aqueous solution, since
  solutions are cheaper and easier to handle. It is
  used to drive chemical reactions and also for the
  neutralization of acidic materials. It can be
  used also as a neutralizing agent in petroleum
  refining. It is sometimes used as a cleaner.
• Soap production
• Sodium hydroxide was traditionally used in soap
  making (cold process soap, saponification).
  Persians and Arabs began producing soap in this
  way in the 7th century, and the same basic
  process is used today.
• Biodiesel
• For the manufacture of biodiesel, sodium
  hydroxide is used as a catalyst for the
  transesterification of methanol and
  triglycerides. This only works with anhydrous
  sodium hydroxide, because combined with water the
  fat would turn into soap, which would be tainted
  with methanol. It is used more often than
  potassium hydroxide because it is cheaper and a
  smaller quantity is needed

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Environmental impact of the chlor-alkali industry

• The mercury cell has been replaced with either
  diaphragm or membrane cells in many countries.
• The replacement is because of environmental
  problem since in practice some of the mercury
  leaks into the environment and can build up in
  the food chain to toxic levels. In theory, all
  of the mercury is recycled
• The membrane cell is preferably chosen among
  three methods because it is
• cheaper to run due to the development of modern
  polymers.
• The consumption of electric energy is the lowest
  of the three processes and the amount of steam
  needed for concentration of the caustic is
  relatively small (less than one tonne per tonne
  of caustic soda)
• Chlorine producers across Europe are
  progressively moving towards this method of
  making their product as the membrane cell process
  is the most environmentally sound way of
  manufacturing chlorine. In 2007, membrane cell
  capacity accounted for 45.6 of total installed
  chlorine production capacity in Europe

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• Besides all of the good uses of Chlorine. People
  are also concerned about overusing of chlorinated
  organic compounds.
• Several of them have been shown to be
  carcinogenic(causing cancer)
• The C-Cl bond can break homolytically(each atom
  getting one of the two electrons) in the presence
  of ultraviolet light at higher altitudes to from
  chlorine radicals which can contribute to ozone
  depletion.