Mercury-chlor-alkali cell, development

H. Y. Castner (US/UK) and C. Kellner (Vienna) independently developed commercial mercury-cathode cell for chlor-alkali production... 

The development of the membrane cell cut the energy consumption in chlor-alkali production. A good cell will produce a ton of caustic for around 2400 kWh. Membrane caustic can only be produced up to around 35%. Several cell designers have tried to develop a cell and membrane combination that would allow 50% caustic to be made, but this has proved to be commercially elusive so far. Membrane cells have probably reached the theoretical limit on energy consumption for a commercial plant. In Japan, power consumption has been cut by 30% over the last 20 years as the conversion from mercury cell progressed. 

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

The ozone concentration in the atmosphere is only a few pphm. In certain chemical plants as in electrolytic mercury cell houses in the chloralkali industry, the ozone concentration is higher. Although the atmospheric ozone level is low, it reacts with rubber double bonds rapidly and causes cracking of rubber products. Especially when rubber is under stress (stretching and bending as in the case of flexible cell covers), the crack development is faster. Neoprene products resist thousands of parts per hundred million of ozone for hours without surface cracking. This nature of neoprene is quite suitable for cell house application in chlor-alkali industries. Natural rubber will crack within minutes when subjected to ozone concentration of only 50 pphm. 

Many of the world s major chlor-alkali companies have developed their own mercury cells and the designs will differ in the way they seek to obtain the maximum electrode area and in the arrangement of the auxiliary equipment. The development of the cells during almost a century of electrolytic chlorine and caustic soda production and the variation in the cells recently available are described in the texts at the end of the chapter. 

The chlor-alkali industry s development of ion-exchange membrane-based electrolytic cells in the early 1970s was driven by environmental concerns associated with mercury and asbestos and the desire to reduce energy costs associated with electrolysis and caustic evaporation [1] (Ihble 4.8.1). 

J.H. Collins and J.H. Entwisle, Development and Operation of High-Current Density Mercury Cells. In M.O. Coulter (ed.). Modem Chlor-Alkali Technology, Ellis Horwood, Chichester (1980). 

The major processes for manufacturing chlorine and caustic soda are based on electrolytic procedures and have been since the 1890s. In the UK the introduction of the Castner Kellner process in 1897 was a major development in chlor-alkali technology. At present over 90% of the world s requirement for chlorine is obtained electrolytically from aqueous sodium chloride and the rest from the molten salt. The proliferation of cells used in the 1930s and 1940s has now been reduced to three cell types, two diaphragm cells and the mercury cell. The latter is being phased out, however, because of the hazardous nature of mercury. 

Many of the world s major chlor-alkali companies have developed their own mercury cells and the designs will differ in the way they seek to obtain the... 

In 1800, Cruickshank was the first to prepare chlorine electrochemically [38] however, the process was of little significance until the development of a suitable generator by Siemens and of synthetic graphite for anodes by Acheson and Castner in 1892. These two developments made possible the electrolytic production of chlorine, the chlor-alkali process, on an industrial scale. About the same time, both the diaphragm cell process (1885) and the mercury cell process (1892) were introduced. The membrane cell process was developed much mpre recently (1970). Currently, more than 95 % of world chlorine production is obtained by the chlor-alkali process. Since 1970 graphite anodes have been superseded by activated titanium anodes in the diaphragm and mercury cell processes. The newer membrane cell process uses only activated titanium anodes. 

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