What is Caustic Soda and How is it Produced?
Caustic soda is mainly produced in the industry using one of the chlor-alkali methods. The chlor-alkali process is a process that, according to the following reaction, by applying direct current and electrolysis of sodium chloride, the chlorine ion in the salt is converted into elemental chlorine at the anode. Sodium also reacts with the reduced hydrogen of water at the cathode and produces caustic soda.
Other products that can be obtained during the chlor-alkali process are hydrogen, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate. The resulting product is often mixed in water in liquid form, which is then converted into the forms of soda flakes, tablets, and granules.
Uses of Caustic Soda
Among the uses and uses of caustic soda, which is the main product of the chlor-alkali process, are its use in the production of chemicals, including aluminum oxide or alumina, use in drilling and extracting oil and gas, neutralizing chemical reactions, production of detergents, paper pulp, and so on.
Production Methods:
Today, the chlor-alkali industry has three methods:
- Mercury cell method
- Diaphragm cell
- Membrane cell
Each of these methods has its own advantages and disadvantages in terms of cost, pollution, and environmental standards, which we will discuss below.
Mercury cell
The liquor produced by each of these three methods contains some salt-containing impurities. The mercury cell method contains a small amount of this type of impurity, and the liquor produced by this method has a high purity in terms of the presence of salts such as sodium chloride and sodium chlorate, but the presence of mercury in the liquor cannot be ignored.
Removing mercury impurities by heating and passing the produced soda through activated carbon filters requires high costs and energy consumption, which will not be economical. Also, the separated mercury can enter nature and groundwater, which will cause a lot of pollution.
Usually, to purify the caustic soda produced by any of the above methods, there is a need to evaporate and cool the soda. This requires a high amount of energy, which is different for each cell depending on the temperature and pressure and the type of cell designed.
Mercury cell with the highest electricity and energy consumption, about 3600 kWh, and membrane cell with energy consumption of 2800 kWh, are considered the most and least energy-consuming methods, respectively. Diaphragm cell is also somewhere between these two methods in terms of energy consumption.
The mercury cell method is an old method that is less used today due to high energy consumption and the proven dangers of mercury to health and the environment.
Diaphragm cell
About 84% of the world’s sodium and chlorine consumption is provided by the membrane method and the diaphragm method.
The advantages of using the membrane and diaphragm process include the separation and non-mixing of the two parts of the catholyte (sodium with water in the cathode) and the anolyte (salt water in the anode). In the diaphragm process, a permeable asbestos separator separates the two parts of the anolyte and catholyte, and in the membrane process, an ion exchange membrane is used as a separator.
In the process of producing caustic soda and chlorine using a mercury cell, the cathode itself acts as a separator by forming an alloy of sodium and mercury (sodium amalgam) and then reacts with water to produce caustic soda and hydrogen in a separate reactor. This leads to the production of mercury when using mercury cells and the production of asbestos in diaphragm cells, which is a harmful and toxic substance and poses irreparable risks to the environment.
Disadvantages
Among the environmental damages caused by chlor-alkali methods are the production of solid waste compounds, including salt; waste of used membranes, cathodes and anodes, etc. To prevent environmental degradation, diaphragm cells have devised measures, including the reuse of RuO2/TiO2 from nickel cathodes and the re-coating of used anodes with inert metals. This not only reduces the production of environmental waste, but also significantly reduces costs.
But a serious problem with diaphragm tubes is the release of asbestos in these tubes. The use of asbestos has been banned in many countries around the world due to the many risks it poses to human health.
Membrane cell
The disadvantages and drawbacks of the two previous methods led scientists to think of an alternative method with lower costs and environmental risks. In 1970, with the discovery of the destructive effects of mercury and asbestos, the membrane cell method was rapidly developed.
In the membrane cell, the anode and cathode are separated by a selective membrane made of perfluoropolymer with cation exchange groups. This selective membrane transfers sodium ions but prevents the permeability of hydroxide ions from the catholyte to the anolyte. Sodium ions migrate along with water to the cathode by passing through the membrane and react with ionized hydrogen at the cathode, which produces high-purity caustic soda.
In this method, it should be noted that due to the corrosiveness of chlorine, the anode must be made of metals such as titanium that do not react. Also, using a metal anode instead of graphite prevents the production of elements such as lead and chlorine in municipal wastewater and also improves the life of the cell.
