Chemistry: electrical and wave energy – Processes and products
Patent
1993-06-08
1994-08-16
Valentine, Donald R.
Chemistry: electrical and wave energy
Processes and products
204129, 204267, 204269, 204279, C25B 102, C25B 126, C25B 900
Patent
active
053384148
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an electrolytic cell comprising an anodic end wall and a cathodic end wall facing each other and supporting alternately arranged plate-shaped anodes and cathodes extending substantially perpendicularly to said end walls. At least some of the anodes and/or cathodes cooperate with the opposite end wall via electrically insulating spacer members, thus enabling compressive forces to be transmitted between the cell end walls. The invention also relates to an electrolyser comprising two or more cells according to the invention. Further, the invention relates to a method of performing electrolyses.
Sodium chlorate is extensively used in the cellulose industry for producing the bleaching agent chlorine dioxide. Chlorate can also be used for producing rocket fuel and weedkillers, and for enriching uranium.
The production of sodium chlorate is described in detail in the available literature, see e.g. "Ullmann's Encyclopedia of Industrial Chemistry", 5th Ed., 1986, Vol. 6, pp 501-511, 521-525. Industrial production is performed by electrolysing sodium chloride in electrolytic cells, e.g. comprising alternately arranged plate-shaped anodes and cathodes. An electrolyser generally consists of a plurality of electrolytic cells electrically connected in series. In the cells, hypochlorite is formed which is conducted together with the electrolyte to reactors where it is converted into chlorate. A part of the product is withdrawn as a solution or as crystals, while the remaining electrolyte is recycled to the cells together with freshly-supplied sodium chloride. The electrolyte is highly corrosive, which places high demands on the construction materials. The cathodically protected parts may consist of iron or steel, while the other parts in general must be made of titanium or fluoroplastics, which is most expensive. The anodes usually are made of titanium and coated with a catalytically active layer based on platinum-group metals, while the cathodes most often consist of iron or steel. For economical operation, the electrical energy supplied must be used as efficiently as possible.
The cell voltage U in a chlorate cell with iron cathodes and activated metal anodes can be expressed in volts by the formula the cell resistance, i is the current density (kA/m.sup.2) while f.sub.korr is a temperature-dependent term which is zero at the temperature where the decomposition voltage is determined and which decreases by about 10.sup.-3 V/.degree. C.
For optimal production in a cell, the current density should be as high as possible. At given values for k and f.sub.korr, this can only be achieved by increasing the cell voltage, excessively high values resulting in secondary reactions causing current losses. To permit a high current density, the cell constant, i.e. the electric resistance in and between the cells, should be as low as possible. Known chlorate electrolysers generally operate with a k-value of 0.18 to 0.25. For optimal function, it is also necessary to have as uniform a current distribution as possible between the electrodes in each cell. Meeting the above-mentioned requirements normally involve substantial costs, especially with respect to the anodic parts made of titanium which is a considerably poorer electric conductor than iron.
Another problem is the formation of deposits on the cathodes, increasing the cell voltage. Chlorate plants are therefore regularly shut down with electrolyte remaining in the cells which results in a decrease of the pH and dissolving of the deposits, but also in severe corrosion of the cathodes which therefore normally have to be changed after a few years operation.
From e.g. FR, A, 2,283,245 it is known to provide contact between series-connected electrolytic cells by arranging them in a filter-press-like frame where they are pressed against each other, this making it easier to disassemble the electrolyser. In known electrolysers of the filter press type, the anodes and the cathodes consist of plates extended parallel no the end walls and arranged with insula
REFERENCES:
patent: 4060475 (1977-11-01), Fournier et al.
patent: 4075077 (1978-02-01), Hodges
patent: 4194953 (1980-03-01), Hatherly
patent: 4194961 (1980-03-01), Williams
Ullman's Encyclopedia of Industrial Chemistry, 5th Edition, 1986, vol. 6, pp. 501-511.
Dalenius Olov
Tenfalt Mikael
Ullman Anders
Wanngard Johan
Permascand AB
Valentine Donald R.
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