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Breanne Leakey

on 18 March 2013

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Transcript of Chlorine

I H L C Objectives 6.1 Describe the chemical process involved in the electrolysis of brine using the diaphragm cell 6.2 Discuss the economic advantages of chlorine production by the diaphragm cell method, include sodium hydroxide 6.3 Discuss the industrial importance of the halogens and their compounds 6.4 Asses the impact of the chlor-alkali industry on the environment Chlorine is manufactured by electrolyzing brine (i.e. sodium chloride solution). The Manufacturing of Chlorine The electrolysis of sodium chloride solution actually produces three useful substances, chlorine, sodium hydroxide and hydrogen At the Anode The negative ions, chloride and hydroxide, get attracted towards the positively charged anode. It is actually easier to liberate hydroxide ions (to give oxygen) than chloride ions (to give chlorine), but there are far more chlorine ions arriving at the anode than hydroxide ions The major reaction at the anode is therefore 2Cl- Cl2 + 2e- The two chloride ions each give up an electron to the anode, and the atoms produced combine to give chlorine gas The chlorine is, however, contaminated with small amounts of oxygen because of a reaction involving hydroxide ions giving up electrons as well The chlorine has to be purified to remove this oxygen 4OH-(aq) 2H2O(l) + O2(g) + 4e- The sodium hydroxide solution leaving the cell is concentrated by evaporation. During this process, most of the sodium chloride crystallises out as solid salt. The salt can be separated, dissolved in water, and passed through the cell again.

Even after concentration, the sodium hydroxide will still contain a small percentage of sodium chloride.The sodium ions and hydrogen ions (from the water) are attached to the negative cathode. It is much easier for a hydrogen ion to pick up an electron than for a sodium ion. So this reaction happens: As the hydrogen ions are converted into hydrogen gas, the water equilibrium tips to the right to replace them. The net effect of this is that there is a build up of sodium ions and these newly produced hydroxide ions around the cathode. In other words, sodium hydroxide solution is being formed around the cathode. Why do we need to keep all these products separate? If chlorine comes into contact with hydrogen, it produces a mixture which will explode violently on exposure to sunlight or heat. In other words hydrogen chloride would be produced. Clearly the two gases need to be kept apart It is also possible for chlorine to react with sodium hydroxide to produce a mixture of sodium chloride and sodium chlorate (I) (aka sodium hypochlorite). This mixture is commonly sold as bleach Therefore, if you are trying to manufacture chlorine and sodium hydroxide rather than bleach, you have to keep the chlorine and sodium hydroxide apart as well. The diaphragm cell, which will be shown next, is designed to keep all the products separate. At the Cathode: 2H+(aq) + 2e- H2 (g) H2O (l) + OH-(aq) H+(aq) These are removed during electrolysis Equilibrium shifts to replace them, therefore producing more hydroxide ions The Diaphragm Cell The diaphragm is made of a porous mixture of asbestos and polymers. The solution can seep through it from the anode compartment into the cathode side. Note the fact that there is a higher level of liquid on the positive side. That makes sure that the flow of liquid is always from left to right, thus preventing any of the sodium hydroxide solution formed finding its way back to where the chlorine is being produced. R N E Products of the Process OH- Chlorine Chlorine is produced at the titanium anode according to the equation: 2Cl- Cl2 + 2e- It is contaminated with some oxygen because of the reaction: 4OH-(aq) 2H2O(l) + O2(g) + 4e- The chlorine is purified by liquifying it under pressure. The oxygen stays as a gas when it is compressed at ordinary temperatures. Hydrogen The hydrogen is produced at the steel cathode: 2H+(aq) + 2e- H2 (g) Sodium Hydroxide Na + H + Cl - H + It is highly contaminated with unchanged sodium chloride solution. A dilute solution of sodium hydroxide solution is also produced at the cathode The sodium hydroxide solution leaving the cell is concentrated by evaporation. During this process, most of the sodium chloride crystallises out as solid salt. The salt can be separated, dissolved in water, and passed through the cell again. Even after concentration, the sodium hydroxide will still contain a small percentage of sodium chloride. Economic Advantages of Chlorine Production Producing chlorine using the diaphragm method, as previously stated, not only produces chlorine, but it also produces other useful substances, such as: Hydrogen Sodium Hydroxide Uses of Sodium Hydroxide:
Used in unclogging drains; Drano
Used in food processing; for example peeling fruits and vegetables, processing cocoa and chocolate, thickening of ice cream, poultry scalding and soda processing
Used in processes to make products including plastics, soaps rayon and textiles
Revitalizing acids in petroleum refining Uses of Hydrogen
Used to process (‘upgrade’) fossil fuels
Used to produce ammonia- used in common household cleaning products and fertilizer.
Hydrogen is used as a hydrogenating agent to produce methanol and convert unhealthy unsaturated fats and oils to saturated fats and oils.
Used in the production of hydrochloric acid- used widely in chemical industries.
Hydrogen is used for rocket fuel.
Tritium, a radioactive isotope of hydrogen, is produced in nuclear reactions. It can be used to make hydrogen bombs and acts as a radiation source in luminous paints
Can be used to make water. Industrial Importance of the Halogens and their Compounds F2 (Flourine) non-stick coatings for saucepans
aerosal propellants
refrigerator coolants and artificial blood Cl2 (Chlorine) Largely used in the manufacture of organochlorine compoounds
Used as solvents for dry-cleaning and degreasing,
Plastics (for example PVC)
Insecticides ( such as DDT)
Anesthetics (Chloroform)
Used for treatment of water for drinking and swimming baths and in household bleach (sodium hypochlorite) Used in the manufacture of fluorocarbons which have many uses such as: Industrial Importance of the Halogens and their Compounds Cont'd I2 (Iodine) Br2 (Bromine) Used to make:
Flame Retardants
Photographic chemicals
Inhaled anaesthetics such as halothane, CF3CHBrCl Used to cure Goiters
Used in the manufacture of photographic films
Used as Skin disinfectant (antiseptic) Impact of the Chlor -Alkali Industry on the Environment There are five major environmental concerns that have made a significant impact on the growth of the chlor-alkali industry over the past twenty years and will dictate the future growth as well. 1. Dioxin Emission into the environment The use of chlorine as a bleaching agent in the pulp and paper industry led to increased amounts of dioxin and chlorinated organics, not only in pulp mill effluents, but also in paper and paper-based products.

In the U.S., chlorine consumption in the pulp and paper industry, decreased from 15% in 1987 to 7% in 1998. The U.S. Environmental protection agency promulgated "Cluster Rules" in late 1998, mandating the use of elemental chlorine-free bleaching.

These rules, which went into effect in April 2001, lowered the chlorine utilization in the North American pulp and paper bleaching operations in favor of sodium chlorate, hydrogen peroxide and oxygen. 2. Ozone Layer Depletion Chlorofluorocarbons (CFCs), carbon tetrachloride (CCl4) and 1,1,1, -trichloroethane were all major contributers to the depletion of the ozone layer and some countries have band the use of such compounds.

Chlorinated methanes and ethanes are under great scrutiny due to the environmental and occupational concerns associated with them.
Nevertheless, their production will continue because of their use in the manufacture of HCFC-22.

HCFC-22 is less harmful than the CFC's towards ozone depletion and is an intermediate in the production of tetrafluoroethylene for use in the production of Teflon and other fluoro polymers.

HCFC's are currently substituted for the CFC's, until they are phased out. HFC's containing no chlorine are not subject to this restriction. 3. Problematic Disposal of Polyvinylchloride Plastics Their lack of biodegradability,
Generation of dioxins when they are incinerated for energy recovery and for controlled waste recycling
The formation of hydrochloric acid during the thermal decomposition of PVCs In 1987, approximately 38% of all U.S. chlorine production was consumed in vinyl chloride monomer (VCM) production to satisfy the growing polyvinyl chloride (PVC) demand. Through 2010, VCM demand is expected to grow annually because of the demand for PVC in the construction, packaging, and other industries The major environmental issues with PVC include: 4. Mercury Emissions The manufacturing process of using a mercury cell resulted in mercury waste being dumped into the environment and causing methlymercury poisoning through consumption of contaminated fish (Minamata Bay Incident).

The victims suffered from severe neurological damage, which later became known as Minamata Disease. All told, thousands were afflicted and more than 900 died Since then, there was a significant move away from mercury-cell technology to diaphragm and ion-exchange-membrane-cell operations and currently only 35% of the world capacity (mostly in western and center Europe and about 10% of U.S. production) of chlorine is produced using the mercury-cell process. There will be no new construction of mercury-cell plants. 5. Asbestos Even with all these constraints, the chlor-alkali industry is projected to grow at a rate of 1 to 3% depending on pessimistic or optimistic reasoning. Much of this will be dictated on how effectively the industry responds to the concerns of the environmentalists and the government agencies Asbestos is used as a separator material in diaphragm cells. However, asbestos is a toxic material, causing lung cancer, asbestosis, and mesothelioma. As a result, in 2007, a bill was adopted to ban most uses of asbestos in the United States. Chlor-Alkali plants were exempt because few cost effective alternatives exist for this technology. However, the EPA could revoke this exemption if unreasonable risks to health or the environment are found. Some ways that the industry has avoided these risks have included surveillance and monitoring programs for asbestos related diseases and use of proper safety equipment and filtration systems during times of unavoidable exposure. In other countries, the use of asbestos in diaphragm cells had already been banned. & THE END
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