Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Water – sewage – or other waste water
Patent
1997-07-28
1999-11-02
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Water, sewage, or other waste water
205770, 205771, 204284, 204294, C01D 140, C25C 702
Patent
active
059763493
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the purification of solutions the pH of which is higher than 14, by an electrochemical route, with the aim of reducing the metal impurities to trace form.
In particular, the process according to the invention is suitable for the purification of alkali metal hydroxide solutions the pH of which is higher than 14.
Furthermore, the present invention relates to a particular cathode capable of being applied in the process according to the invention.
Electrochemical methods for obtaining solutions the pH of which is higher than 14, and more particularly of alkali metal hydroxide solutions, are now well known and developed industrially.
Electrolysis in diaphragm cells may be mentioned among the electrochemical methods for obtaining these solutions. Similarly, electrolysis in mercury cells is known. This process consists, in a first step, in producing an amalgam of sodium and mercury from a solution saturated with sodium chloride and then, in a second step, in reacting the said amalgam with water to give sodium hydroxide. Finally, electrolysis in a membrane cell may be mentioned, in which the anode and cathode compartments do not communicate.
These two last methods make it possible to produce alkali metal hydroxide solutions containing relatively few metal impurities. In fact, in contrast to the diaphragm cells, there is practically no corrosion due to the reaction medium in the electrolyser itself. In the case of mercury cells the problem of anode and cathode corrosion does not arise because the anode happens to be mercury and the cathode graphite or vice versa, according to the cell compartment. In the case of membrane cells it must be noted that the materials of construction of membrane cells are much more resistant than those usually employed for diaphragm cells: the membrane is very brittle and must therefore be contaminated as little as possible with metals produced by the corrosion of the cell materials, failing which the cell performance is considerably reduced. Finally, the subsequent stages of concentration of the soda are lesser in the case of the mercury and membrane cells than in that of the diaphragm cells, because the soda obtained is more concentrated. This decreases proportionately the risks of corrosion of the equipment employed for the said concentration.
Thus, the alkali metal hydroxide solution produced in diaphragm cells is found to be contaminated by the presence of metallic elements originating from the electrodes and also from the equipment used during the concentration stage. These metals are encountered in a quantity of the order of a few parts per million, a content that is higher than that of alkali metal hydroxide solutions originating from the other two types of cells which have been mentioned.
It is therefore necessary to have available solutions the pH of which is higher than 14, and more particularly alkali metal hydroxide solutions, which have a degree of purity with regard to these metals that is comparable with the hydroxide solutions obtained by electrolysis in mercury and membrane cells.
Furthermore, it is necessary to perform a purification of such solutions by using methods which are simple and economical.
Methods for electrochemical purification of alkali metal hydroxide solutions do exist. They usually employ vessels including, as cathode, hollow graphite cylinders the porosity of which is controlled. The disadvantage of a process of this type lies in the manufacture of these cylinders and in the fact that the equipment is large in bulk. In fact, the cross-section of the electrode is 5 m.sup.2 /m.sup.3 of electrode working volume.
The subject-matter of the present invention is therefore a process for the purification of solutions the pH of which is higher than 14 which does not exhibit the abovementioned disadvantages.
Thus, the process according to the invention consists in treating solutions the pH of which is higher than 14 in an electrolysis cell in which the cathode includes a fibrous sheet based on a mixture of fibres at
REFERENCES:
patent: 5092974 (1992-03-01), Kondo et al.
Kuntzburger Frederic
Magne Jean-Claude
Phasge Arun S,.
Rhodia Chimie
Seugnet Jean-Louis
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