Diaphragm cells/process

The diaphragm cell principles have been in use commercially since 1888, when the Greisheim cell was introduced in Germany. The principle is to reduce reverse flow and reaction of hydroxyl ions with chlorine by use of a porous diaphragm, across which a hydraulic gradient is set up, and the efficiency of the cell depends on the design of this separator. 

CP — e = Cr chloride ion loses an electron to form a chlorine atom 

Liquor containing ionic mixture flows through the diaphragm Cathode reaction (steel) 

CI2 + OH-= CP+HOC1 HOCl = H -hOCP 2HOC1 -h OCP = CIO3 + 2CP + 2H, 40H =02 + 2H20 + 4e With graphite electrodes, there is in addition 40H- -hC = CO2 + 2H2O + 4e 

It can be seen that in a commercial cell, the major byproducts are sodium hypochlorite, sodium chlorate, oxygen and carbon dioxide, which lead to overall current efficiencies in the range 90-96%. The electrolysis current only converts about half the 25% w/w sodium chloride feed to caustic soda, which leads to a maximum of about 12% w/w caustic in the product sodium chloride/sodium hydroxide liquor. 

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The brine feed to the electroly2ers of all the processes is usually acidified with hydrochloric acid to reduce oxygen and chlorate formation in the anolyte. Table 14 gives the specifications of the feed brines requited for the membrane and diaphragm cell process to reali2e optimal performance. 

Three forms of caustic soda are produced to meet customer needs purified diaphragm caustic (50% Rayon grade), 73% caustic, and anhydrous caustic. Regular 50% caustic from the diaphragm cell process is suitable for most appHcations and accounts for about 85% of the NaOH consumed in the United States. However, it caimot be used in operations such as the manufacture of rayon, the synthesis of alkyl aryl sulfonates, or the production of anhydrous caustic because of the presence of salt, sodium chlorate, and heavy metals. Membrane and mercury cell caustic, on the other hand, is of superior quaUty and... 

This yields a solution of highly pure alkali (free of chloride ions), which can be used in the manufacture of synthetic fibers. The mercury, which has been stripped of sodium, is returned to the electrolyzer. The cost of chlorine is higher in the mercurycell than in the diaphragm-cell process. In addition, the mercury-cell process is ecologically dangerous, owing to the possible escape of mercury into the environment hence, it has increasingly been discontinued in all countries. 

In the diaphragm-cell process, a solid cathode (iron) is used where hydrogen is evolved [reaction (15.4)]. Porous asbestos diaphragms are used to prevent mixing of the catholyte and anolyte, but owing to the finite permeability of these diaphragms, the alkaline solution that is produced near the cathode stiU contains important levels of chloride ions as an impurity. 

Summary of Raw Waste Loadings Found in Verification Sampling of Unit Product of Chlor-Alkali (Mercury Cell and Diaphragm Cell Processes)... 

Membrane cell A refinement of the Diaphragm cell process in which the diaphragm is made from a cation-exchange membrane. See also Castner-Kellner. 

Of the chlorine production capacity installed in Germany, which totalled 4.4 million tonnes in 2003, 50% were from the membrane cell process, 27% from the mercury cell process and 23% from the diaphragm cell process. The mercury cell process has been the subject of environmental policy criticism for years because of its use of mercury cathodes and resulting pollutant emissions. Hence, no new mercury plants will be... 

In production of chlorine by the diaphragm cell process (Fig. 1), salt is dissolved in water and stored as a saturated solution. Chemicals are added to adjust the pH and to precipitate impurities from both the water and the... 

Electrolytic Cell Operating Characteristics. Currently the greatest volume of clilorine production is by the diaphragm cell process, followed by that of the mercury cell and then the membrane cell. However, because of the ecological and economic advantages of the membrane process over the other systems, membrane cells are currently favored for new production facilities. The basic characteristics of the three cell processes are shown in Figure 5. 

Membrane cell process3 Diaphragm cell process... 

These include the cost of labour, the cost of replacing components and the rent and rates on the area of land. A comparison of the latter is very unfavourable to diaphragm cells where almost twice as much land is necessary because of the low current density and the need for evaporation plant. Membrane and mercury cells have similar land requirements because the former are packed together closely in a filter press and to some extent the electrodes scale vertically. All the processes are largely automated and hence the difference in labour costs is likely to arise because of a variable incidence of component and pipework failure. Certainly the need to replace diaphragms on a routine cycle increases the labour costs for the diaphragm cell process. 

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