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

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.

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

• 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.

• 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

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 can’t 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.

Diagram

diaphragm_cell.swf

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.

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-filtration, 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.

membrane_cell.swf

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.

mercury_cell.swf

• 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.

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.

Important uses of Chlorine

• Chlorine's principal applications are in the production of a wide range of industrial and consumer products.[33][34] 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

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

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

• 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.