💯 Sodium Hydroxide Production Explained
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Sodium hydroxide is a white translucent solid. It is deliquescent, that is, it absorbs water from the atmosphere and dissolves in this water to form a solution. It is very soluble in water. Sodium hydroxide is among the most widely used industrial chemicals and ranks in the top ten manufactured chemicals.
The industrial production of sodium hydroxide from sodium chloride solution is carried out by electrolysis. Three different methods are used in the electrolytic conversion of concentrated sodium chloride solution into sodium hydroxide, chlorine, and hydrogen. These are the mercury process, diaphragm process and membrane process. Each uses different methods to physically separate the cathode and anode compartments, ensuring the electrolysis products are kept apart. This is necessary because chlorine gas will react with sodium hydroxide and hydrogen. One possible reaction is the formation of sodium hypochlorite (bleach).
The mercury process
The mercury process uses an electrolytic process to convert sodium chloride into sodium hydroxide, hydrogen gas, and chlorine gas. This process for producing sodium hydroxide also requires large amounts of energy for the desired chemical reaction. In this instance, the chemical reactions are:
Chloride is oxidised at the titanium anode:
When the sodium metal is formed at the mercury cathode, it forms an amalgam (a metal alloy), and the sodium is then transported to the secondary cell and reacts with water to form sodium hydroxide and hydrogen gas, which is a spontaneous reaction:
In this process, hydrogen gas and chlorine gas are not produced simultaneously because of the amalgam produced, and thus a very explosive mixture is avoided. This process has significant environmental issues: mercury is a heavy metal that bioaccumulates (builds up in living things), and its compounds are poisonous to most plants and animals. Although this process produces high-purity sodium hydroxide, it is very energy-intensive, much more so than the alternative processes. For these reasons, the mercury process is being phased out and replaced by electrolytic cells.
Diaphragm cell
The name comes from the diaphragm used to separate the sodium hydroxide solution from the chlorine. In this process, the stock solution (brine) is treated with sodium carbonate to remove any calcium (Ca2+) and magnesium (Mg2+) impurities that can clog the diaphragm, and then the purified brine is acidified with hydrochloric acid and placed into an electrolytic cell.
The diaphragm is made from asbestos that has been deposited on a steel or iron cathode. The anode is an inert metal or carbon rod. The process involves the production of chlorine gas at the anode and sodium hydroxide and hydrogen gas at the cathode. The overall reactions are the same as in the mercury cell but there is no need for the formation or transportation of sodium metal. All sodium metal is immediately converted to sodium hydroxide in the presence of water.
In order to sell the sodium hydroxide, it must be concentrated by evaporation of the water—a costly process. However, the diaphragm process requires much less energy than the mercury process and does not produce harmful mercury wastes. Unfortunately, this process is not as effective as the mercury process and does not produce as pure a product as the mercury process. The products typically contain about 1% unreacted sodium chloride, which is insufficiently pure for processes such as the production of rayon. Another environmental issue is the use of asbestos, a known respiratory irritant, and the possible health risks associated with its use and disposal.
Membrane cell
This is the most recently developed process (dating from the 1970s). A membrane cell is a diaphragm cell with an improved diaphragm called a membrane. The membrane is made from polytetrafluoroethylene, which has been modified to incorporate anionic groups so that it acts as an ion-exchange membrane. Sodium ions are able to pass through it but not chloride or hydroxide ions, which prevent chloride ions from contaminating the cathode part.
It has three main advantages over the other two processes:
• lower energy consumption—much less than the mercury cell and almost identical to the diaphragm cell;
• negligible environmental impact—no toxic waste products;
• produces a pure product—similar to the mercury cell and significantly purer than the diaphragm cell without evaporation of excess water.
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Sodium hydroxide is a white translucent solid. It is deliquescent, that is, it absorbs water from the atmosphere and dissolves in this water to form a solution. It is very soluble in water. Sodium hydroxide is among the most widely used industrial chemicals and ranks in the top ten manufactured chemicals.
The industrial production of sodium hydroxide from sodium chloride solution is carried out by electrolysis. Three different methods are used in the electrolytic conversion of concentrated sodium chloride solution into sodium hydroxide, chlorine, and hydrogen. These are the mercury process, diaphragm process and membrane process. Each uses different methods to physically separate the cathode and anode compartments, ensuring the electrolysis products are kept apart. This is necessary because chlorine gas will react with sodium hydroxide and hydrogen. One possible reaction is the formation of sodium hypochlorite (bleach).
The mercury process
The mercury process uses an electrolytic process to convert sodium chloride into sodium hydroxide, hydrogen gas, and chlorine gas. This process for producing sodium hydroxide also requires large amounts of energy for the desired chemical reaction. In this instance, the chemical reactions are:
Chloride is oxidised at the titanium anode:
When the sodium metal is formed at the mercury cathode, it forms an amalgam (a metal alloy), and the sodium is then transported to the secondary cell and reacts with water to form sodium hydroxide and hydrogen gas, which is a spontaneous reaction:
In this process, hydrogen gas and chlorine gas are not produced simultaneously because of the amalgam produced, and thus a very explosive mixture is avoided. This process has significant environmental issues: mercury is a heavy metal that bioaccumulates (builds up in living things), and its compounds are poisonous to most plants and animals. Although this process produces high-purity sodium hydroxide, it is very energy-intensive, much more so than the alternative processes. For these reasons, the mercury process is being phased out and replaced by electrolytic cells.
Diaphragm cell
The name comes from the diaphragm used to separate the sodium hydroxide solution from the chlorine. In this process, the stock solution (brine) is treated with sodium carbonate to remove any calcium (Ca2+) and magnesium (Mg2+) impurities that can clog the diaphragm, and then the purified brine is acidified with hydrochloric acid and placed into an electrolytic cell.
The diaphragm is made from asbestos that has been deposited on a steel or iron cathode. The anode is an inert metal or carbon rod. The process involves the production of chlorine gas at the anode and sodium hydroxide and hydrogen gas at the cathode. The overall reactions are the same as in the mercury cell but there is no need for the formation or transportation of sodium metal. All sodium metal is immediately converted to sodium hydroxide in the presence of water.
In order to sell the sodium hydroxide, it must be concentrated by evaporation of the water—a costly process. However, the diaphragm process requires much less energy than the mercury process and does not produce harmful mercury wastes. Unfortunately, this process is not as effective as the mercury process and does not produce as pure a product as the mercury process. The products typically contain about 1% unreacted sodium chloride, which is insufficiently pure for processes such as the production of rayon. Another environmental issue is the use of asbestos, a known respiratory irritant, and the possible health risks associated with its use and disposal.
Membrane cell
This is the most recently developed process (dating from the 1970s). A membrane cell is a diaphragm cell with an improved diaphragm called a membrane. The membrane is made from polytetrafluoroethylene, which has been modified to incorporate anionic groups so that it acts as an ion-exchange membrane. Sodium ions are able to pass through it but not chloride or hydroxide ions, which prevent chloride ions from contaminating the cathode part.
It has three main advantages over the other two processes:
• lower energy consumption—much less than the mercury cell and almost identical to the diaphragm cell;
• negligible environmental impact—no toxic waste products;
• produces a pure product—similar to the mercury cell and significantly purer than the diaphragm cell without evaporation of excess water.
5 سال پیش
در تاریخ 1398/07/17 منتشر شده
است.
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