The Chloralkali Industry

The chloralkali process is responsible for the production of chlorine gas (Cl2), sodium hydroxide (NaOH) hence the name chloralkali. The process is also responsible for the production of hydrogen gas (H2). These three substances are used to make products that are essential to people’s lives. The chloralkali process does have environmental impacts; however, there are three methods used in the cholralkali process and work is being done to favour processes that are more environmentally friendly.

The benefits of the chloralkali process to people’s lives far outweigh the disadvantages which makes it a process fundamental to human life. The products produced by the chloralkali process are very important and have a wide range of uses. The element chlorine produced in the process is responsible for: the production of disinfectant, PVC production and for the extraction of metals. Sodium hydroxide the other main product of the chloralkali industry is used to: purify bauxite, produce soap and to produce dyes.

Hydrogen another by-product is used to produce rocket fuel and for hydrogenation. These products are essential for human life which makes it clear that the chloralakali process is beneficial to people. The chloralkali industry revolves around three main methods of production. Each method uses an electrolytic cell in which a current is run through brine (saturated NaCl), resulting in chemical reactions at the electrodes and separation of sodium and chlorine. The three cells that are used are the mercury cell, the diaphragm cell and the membrane cell.

Each of these processes differ in the methods of production and also differ in the way the half-reactions are isolated. All three of the cells have different environmental impacts; however, the risks associated with the mercury cell and diaphragm cell have led to the membrane cell being the most widely used. The membrane cell is the most widely used cell and accounts for 52% of chlorine production across Europe because it consumes a low amount of energy, is relatively inexpensive and produces very pure sodium hydroxide.

The cell operates by separating the anode and the cathode with an ion-exchange membrane. Only sodium ions and a little water pass through the membrane. In the process the concentrated salt solution (Na+ Cl-) is pumped into the section with the anode. Chlorine ions are oxidised to form Cl2. The positive sodium ions bond with the OH- that formed at the cathode to form NaOH which is siphoned off regularly. Hydrogen gas is also produced at the cathode. The NaOH has to be concentrated slightly but the amount of steam to do so is small.

The consumption of electric energy is the lowest of the three processes and the amount of steam needed for concentration of NaOH is relatively small. The mercury cell is an older cell and its main advantage is producing very pure hydrogen and chlorine as well as producing a high concentration of NaOH. In the mercury cell there is a liquid mercury cathode and sodium forms an amalgam with it. The amalgam reacts with the water in a separate reactor known as a decomposer where hydrogen gas and sodium hydroxide are produced.

The mercury cell uses a huge amount of electricity and the use of mercury can cause huge environmental problems because it is very poisonous and it is a bio-accumulative which means once it enters the food chain poisoning from mercury can get worse as it moves up the various trophic levels. Accidents like the one at Minamata at Japan affected people greatly and caused poisoning of people. The diaphragm cell is the least widely used cell, with the diaphragm process accounting for only 14% of total European chlorine production, which can be attributed to the huge environmental and health risks that the diaphragm cell poses.

In the diaphragm cell process the anode compartment is separated from the cathode area by a permeable asbestos diaphragm. The asbestos fibres produced from the diaphragm process can cause cancer. The only advantage of the diaphragm process is that it has low electrical energy consumption but not as low as the membrane cell. The chloralkali industry is clearly extremely important and has many benefits to human life. It would be unfeesable to discontinue such a large industry. However, there are big environmental problems associated with the mercury and diaphragm cells.

In order to make the industry more environmentally friendly the membrane cell should be universally used because of its low energy consumption and small environmental affects. Therefore the chloralkali industry is essential to human life and has to be continued. There are environmental impacts associated with the mercury and diaphragm processes but these could be overcome by using the membrane cell. Therefore the advantages of the chloralkali industry far outweigh the negative impacts associated with the production processes.

Word Count: 800 Bibliography • Wikipedia, the free encyclopedia. (2011) Chloralkali Process [online] Available from http://en. wikipedia. org/wiki/Chloralkali_process (07/04/2011) • Chlorine Online. (2010) How is chlorine made? [online] Available from http://www. eurochlor. org/makingchlorine (07/04/2011) • Dr Andrew A Lindley (1997) MERCURY CELLS AND ALTERNATIVE TECHNOLOGIES [online] Available from http://www. oxford. co. za/download_files/business/oxford_press_vol1_issue1. pdf (07/04/2011) •