The Electrolysis of Brine
Although the chemistry of brine electrolysis had been demonstrated by Davy and Faraday in the early 19th century, the electrolysis of brine on an industrial scale (to produce chlorine, caustic soda and hydrogen) had to await the 1870’s invention of a dynamo capable supplying high currents required by the process.
Development work on electrolysis cells began in the 1880’s. Two basic types were investigated; the “mercury cathode” and the “diaphragm” cell. In 1892, in England, Castner and Baker patented the Rocking Mercury Cell, while in Austria, Kellner patented a mercury cell of similar design. Following an 1895 patent agreement with Solvay in Belgium, (who then held the patent rights to the Kellner Cell) the Castner-Kellner Company was formed and a large installation rocking cells erected at Weston Point, Runcorn in 1897. The North American patent rights were licenced by Mathieson and a larger installation of rocking cells was installed at Niagara Falls in the same year.
In Canada an installation of Le Sueur diaphragm cells began operation at Rumford Falls in 1893 and yielded the world’s first marketable supplies of electrolytic caustic soda. In this same year a small installation of Hargreaves-Bird diaphragm cells was operating at Farnworth near Widnes. In 1901.the Electrolytic Alkali Company was formed and a large installation of Hargreaves-Bird cells was installed at Cledford, Middlewich.
The archives of this works were unfortunately destroyed in the 1930’s following the formation of ICI and the subsequent closure of the works. As mentioned above (see Vacuum Process) this works installed the first commercial multi-effect brine evaporator and it seems most likely that the pure vacuum salt was required to resaturate the weak brine from the electrolysis process which was then recycled to the cells. At Weston Point the weak brine from the mercury brine electrolysis cells was merely discharged into the tidal Mersey.
The development of the brine electrolysis process in the 1890’s brought further change to the pattern of the salt-based chemical industry. Both the Leblanc and Ammonia Soda processes produced the sodium-derived product and chlorine derived product (or waste product) in separate stages, and only produced caustic soda on demand by conversion from the soda ash by the lime-soda process. The electrolysis process on the other hand produced caustic soda, chlorine and hydrogen in equivalent yields. While the hydrogen if not required could be vented to atmosphere the caustic soda( a corrosive liquid) and chlorine ( a toxic gas) both had stockpiling problems and ideally required an equal, ie balanced, market demand for the two products.
In the early days of the process the chlorine was primarily converted into bleaching powder by absorption on lime, but by 1903, Castner Kellner were investigating the liquid chlorine market and it was then possible to import cylinders of the liquefied gas from Germany. The liquid chlorine market was greatly expanded with the subsequent use of chlorine as a war gas. Chlorine from the electrolysis process that is not liquefied may be converted into hydrochloric acid by burning with hydrogen, or into sodium hypochlorite bleach by absorption in caustic soda,.
In the early the 20th century Castner-Kellner and United Alkali,( latterly as ICI), held most of the market share of the chlorine–caustic soda production in the UK. At Castner Keller works the installation of rocking cells ceased in 1902 and all subsequent expansion was with the long mercury cell based on a Solvay design. Rocking cells remained in operation at Castner Kellner Works until 1928. However the rocking cells which had been installed at Niagara Falls by Mathiesen in 1896 were to remain in operation until 1963.
An unusual transaction of about 1908 was the lease or sale by Castner-Kellner to the Salt Union, of an existing bank of rocking cells. These were transferred to the nearby Salt Union works where they produced caustic soda to be used for purification of the brine by the Salt Union vacuum plants and probably also the brine which the Castner –Keller works obtained from the Salt Union’s pipeline. The chlorine from the Salt Union rocking cells was made into bleach. This cellroom remained operational until 1938.
In 1914, the UK brine electrolysis industry was represented by the Castner Kellner mercury cell works at Weston Point and the Electro-Bleach Company’s Hargreaves Bird diaphragm cell Works at Middlewich. The wartime demand for chlorine led to government pressure and the United Alkali Company imported Gibbs diaphragm cells from the USA in 1915. There were-not-well-documented government operated cellrooms of German type diaphragm cells, certainly at Middlewich, and possibly others elsewhere. The United Alkali Company’s early Gibbs Cellrooms were at Sullivan Works, Widnes, and Hardshaw Brook Works at St Helens. Later installations were at Allhusen’s Works, Gateshead, Hillhouse at Fleetwood and Wade Works at Northwich.
During the 1914-1918 war years the Staveley Iron and Chemical Company at Staveley near Chesterfield had greatly expanded its chemical activities beyond the production of the basic coal tar chemicals such as benzene and toluene, producing sulphuric acid and nitric acid and becoming a major war time supplier of picric acid, TNT and guncotton. The company planned to extend its range into chlorinated organics and purchased brine bearing land at Sandbach where the British Soda Company was formed to primarily produce salt for a planned installation of mercury cells at Staveley. Since such cells were not commercially available they decided to construct their own with the aid of technical staff head-hunted from Castner-Kellner. The first installation of Staveley cells operated with apparent success from about 1922 and in 1926 the company went into partnership with the Krebs Company of Paris and Berlin to fabricate a new cell, based on the experience gained. This was marketed worldwide as the Krebs-Staveley Cell. Photographs of the Krebs Cells and Castner-Kellner Cells of this vintage show an interesting similarity.
Up to the 1930’s the UK had been the centre of mercury cell technology while on the continent and in North America the bulk of the chlor-alkali production was in diaphragm cells. After 1933 with the merging of the German chemical giants into IG Farben there was a massive German move into mercury cell design and operation. This trend continued with the post-war reconstruction and by the 1950’s there were German designs of mercury cell available on the open market. German cells were installed at Sandbach (Murgatroyds) and Ellesmere Port (Associated Octel) in the mid 50’s and a few years later the 1926 Krebs cellroom at Staveley Chemicals was replaced with German cells.
Within ICI, in addition to Castner Kellner at Weston Point, there were cellrooms of ICI-Solvay mercury cells at nearby Rocksavage, at Hillhouse, Fleetwood and at Billingham and Wilton.
In the immediate post-war years, ICI’s installations of Gibbs diaphragm cells then still in service, continued to operate and this UK complement of diaphragm cells was added to in 1949 when Murgatroyd’s at Sandbach installed Type S Hooker Diaphragm cells. At that time Hooker Cells of this type accounted for a major proportion of the total chlor-alkali production of the USA.
In the 1960’s, the remaining ICI Gibbs Cells at Billingham/ on Tees Side, Wade Works at Northwich, and possibly also those at Hillhouse Works near Fleetwood, were replaced with an ICI diaphragm cell similar in design and construction to the Hooker Cell.
An important development of the 1960’s was the outcome of extensive research throughout the industry into replacements for carbon as a material of construction for the chlorine anode that was non-consumable. Titanium had become a readily available material and proved to be an ideal material of construction for plant handling wet chlorine. But this corrosion resistance at the same time made the base metal unsuitable as electrode material. The discovery that a platinum metal coating of titanium overcame this problem (Cotton, 1957) but platinised titanium proved to be electrolytically inferior to graphite as a chlorine cell anode. Platinised titanium electrodes were successfully used in the cathodic protection of sea water structure and opened a new avenue of research which led to the discovery that anode coatings based on ruthenium oxides had the characteristics required for a chlorine cell anode. (Henri Beer 1967 Patent).
Mercury cells with ruthenium oxide coated titanium anodes based on Beer’s patent were introduced by the Italian De Nora Company and were selected by BP Chemical for a large installation at Baglan Bay in South Wales to produce chlorine for the manufacture of PVC. Meanwhile ICI had been involved in further development of anode coatings based on precious metal oxide mixtures and the new design of permanent metal anode was used in all ICI cellrooms.
The introduction of the permanent metal anode has probably been the most important advance in chlor-alkali technology of the last century.
Recent years have seen other developments in both mercury and diaphragm cell technology. Cells increased in size and current carrying capacity. The latter being made possible by the invention of high-current semiconductor rectification.
However, there has been much more recent emphasis on the elimination of the use of mercury cathodes and asbestos diaphragms in cell operation. Since the 1970s, much work has gone into the development of synthetic diaphragms and ion-exchange membranes. Parallel to this effort has been the successful reduction of the environmental impact of the mercury and asbestos used in existing cells. These have been successfully reduced to such extremely low levels that cellrooms have been allowed to continue operation for an economic working life.
The development of diaphragm and ion exchange membranes has had predictable set-backs and, in addition to the need for materials capable of standing up to the corrosive conditions of a chlorine cell, an efficient and economic membrane life demands a feed brine of extreme purity and stringent quality control.