Membrane Cell ProcessIn the membrane cell, the anode and cathode are separated by an ion-permeable membrane. Saturated brine is fed to the compartment with the anode (the anolyte). A DC current is passed through the cell and the NaCl splits into its constituent components. The membrane passes Na + ions to the cathode compartment (catholyte), where it forms sodium hydroxide in solution. The membrane allows only positive ions to pass through to prevent the chlorine from mixing with the sodium hydroxide. The chloride ions are oxidised to chlorine gas at the anode, which is collected, purified and stored. Hydrogen gas and hydroxide ions are formed at the cathode. Fig 1.1 Process flow of Membrane cell method This means that ions can pass. an anode assembly is suspended. In the bottom of this trough a shallow film of mercury (Hg) flows along the brine cell co-currently with the brine. The cathode and anode compartments are separated by a water-impermeable membrane. liberating chlorine gas (Cl2) at the anode and metallic sodium (Na) at the cathode. In the primary electrolyser (or brine cell). The chlorine gas is accumulated above the anode assembly and discharged to the purification process. The liquid amalgam flows from the electrolytic cell to a separate reactor. Mercury Cell Process The mercury cell process involves two "cells".The membrane process is summarised in the diagram below. the sodium immediately forms an amalgam. called the decomposer or denuder. but water cannot. purified and saturated brine containing approximately 25% sodium chloride flows through an elongated trough that is slightly inclined from the horizontal. . The sodium-free mercury is fed back into the electrolyser and reused. Concentrated sodium chloride solution (NaCl(aq)) flows though the anode compartment and water flows though the cathode compartment. Electric current flowing through the cell decomposes the brine passing through the narrow space between the electrodes. where it reacts with water in the presence of a graphite catalyst to form sodium hydroxide and hydrogen gas. Fig 1. As it is liberated at the surface of the mercury cathode.2 Membrane Cell Compartment. The sodium ions and hydroxide ions combine in the cathode compartment to produce sodium hydroxide. Closely spaced above the cathode. Fig 1.4 Mercury Cell System .3 Mercury Cell Flow Process Fig 1. 5 Diaphragm Cell System .Diaphragm Cell Process In the diaphragm cell process. The salt separated from the caustic brine can be used to saturate diluted brine. a permeable diaphragm. Evaporative process is achieved with about three tonnes of steam per tonne of caustic soda. Similarly to the Membrane Cell. The chlorine contains oxygen and must often be purified by liquefaction and evaporation. Figure 1. separates the anode and cathode compartments. When using asbestos diaphragms. The caustic soda must usually be concentrated to 50% and the salt removed. and water is split into caustic soda and hydrogen at the cathode. often made of asbestos fibers. The diaphragm prevents the reaction of the caustic soda with the chlorine. the diaphragm process inherently gives rise to environmental releases of asbestos. Diluted caustic brine leaves the cell. Brine is introduced into the anode compartment and flows into the cathode compartment. chloride ions are oxidized at the anode to produce chlorine. 6 Process Flow of Diaphragm Cell Diaphragm cells can operate with less pure brine than required by membrane cells and at a lower voltage than mercury cells.Figure 1. Among the three manufacturing processes. . diaphragm cell process is the most suitable and generally accepted manufacturing processes of caustic soda.