The Chlorine Generation Process:
Most of the packaged chlorine available today is manufactured by the salt process. There are three types of electrolytic cells: diaphragm, mercury and membrane. There are other methods of production, which are designed to fit the raw material containing the chlorine ion. These methods include the electrolysis of hydrochloric acid, thesalt process, and the HCl oxidation process. Electrochemical Cell The electrochemical cell is composed of an anode, a cathode, and a separator which isolates the liquids contained in the anode chamber and the cathode chamber. The function of the separator is to isolate the two chambers while allowing the migration of selected ions from the anode chamber to the cathode chamber. Brine composed ofsodium chloride and water is introduced into the anode chamber where oxidation of the sodium chloride takes place. Chlorine gas is released at the anode. The sodium ions are attracted to the negatively charged cathode and transported through the separator. Ideally, all of the chloride would be contained on the anode side of the cell. Water is reduced at the cathode and hydrogen gas is evolved. Theremaining hydroxide ion combines with the sodium ion to form sodium hydroxide solution (caustic), which exits the cathode chamber. Ideally, all of the hydroxide ions would be contained on the cathode side of the cell. Membrane Cell In the membrane process, the anolyte (the solution in the anode chamber) and the catholyte (the solution in the cathode chamber) are separated by a cation exchangemembrane that selectively transmits sodium ions but suppresses the migration of hydroxyl ions from the catholyte to the anolyte. This produces a catholyte effluent with a strong caustic soda solution with a very low sodium chloride content. The advantage of the membrane process are its energy efficiency and its ability to produce, without any harmful effect on the environment, a strong, high-qualitysolution of caustic soda (NaOH). The overall chemical reaction is: NaCl + H2O + electric current --> NaOH + 1/2 Cl2 + 1/2 H2 The two reactions that make up this overall reaction are: The principal anode reaction: 2Cl- --> Cl2 + 2eChlorine formed at the anode saturates the anolyte and an equilibrium is established as follows: Cl2 + (OH)- --> Cl- + HOCl HOCl H+ + OCl-
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How Salt Generators Work Page 2
This shows that the chlorine is formed and then makes the chloride ion (Cl-), and the hypochlorous acid (HOCl), which further dissociates in equilibrium to hypochlorite ion (OCl-) and hydrogen ion (H+). The principal cathode reaction is: 2H+ + 2OH- + 2e- --> H2 + 2OHThe hydrogenion (H+) present in the H2O in the catholyte evolves at the cathode as hydrogen gas (H2), leaving behind the hydroxyl or hydroxide ion (OH-) in the catholyte. Because chlorine has evolved at the anode, the sodium ion (Na+) is free to join the hydroxide ion (OH-) as it migrates from the anolyte chamber to the catholyte chamber. The porous diaphragm is used to inhibit the migration of the OH- ionsfrom the cathode to the anode. Ideally, we produce pure hydrogen gas, pure chlorine gas and pure sodium hydroxide in this process. Mercury Cell The mercury cell has two essential parts: (1) the elctrolyzer and (2) the amalgam decomposer. In the elctrolyzer, a salt solution is electrolyzed, making use of a special anode and a flowing mercury cathode. Chlorine gas is liberated at the anode, and sodiumis deposited at the surface of the flowing mercury cathode, in which it dissolves to form a liquid amalgam. The amalgam flows to the decomposer where it is decomposed with water to form sodium hydroxide and hydrogen gas. The mercury is then recovered and reused. In the 1970s, mercury cell operations in the U.S. were found to be discharging effluents containing mercury in excess of the safe...