Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
1998-12-10
2001-03-20
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S525000, C205S526000, C205S531000
Reexamination Certificate
active
06203687
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for shutting down an electrolysis cell with a membrane and an oxygen-reducing cathode (or an oxygen diffusion cathode).
More precisely, the invention relates to a method for shutting down an electrolysis cell with a membrane and an oxygen-reducing cathode which produces an aqueous solution of sodium hydroxide and chlorine by electrolysis of an aqueous NaCl solution, the said cell having been turned off intentionally or following an operational incident, then turned on again.
BACKGROUND OF THE INVENTION
The electrolysis cells with a membrane and an oxygen-reducing cathode have resulted, on the one hand, from the remarkable improvements obtained recently in terms of fluorinated ion-exchange membranes, which have made it possible to develop methods for electrolysing sodium chloride solutions by means of ion-exchange membranes. This technique makes it possible to produce hydrogen and sodium hydroxide in the cathode compartment, and chlorine in the anode compartment, of a brine electrolysis cell.
Furthermore, in order to reduce energy consumption, it has been proposed to use an oxygen-reducing electrode as the cathode, and to introduce a gas containing oxygen into the cathode compartment in order to prevent hydrogen evolution and to significantly reduce the electrolysis cell voltage.
In theory, it is possible to reduce the electrolysis voltage by 1.23 V by using the cathode reaction with supply of oxygen represented by (1) instead of the cathode reaction without supply of oxygen represented by (2):
2H
2
O+O
2+
4e
−
→4OH
−
(1)
E=+0.40 V (relative to a standard hydrogen electrode).
4H
2
O+4e
−
→2H
2
+4OH
−
(2)
E=0.83 V (relative to a standard hydrogen electrode).
A conventional membrane electrolysis cell using the gas technology comprises a gas diffusion electrode (cathode) which is placed in the cathode compartment of the electrolysis cell and divides the said compartment into a solution compartment, on the ion-exchange membrane side, and a gas compartment on the opposite side.
An electrochemical cell of this type therefore generally consists of 3 separate compartments:
an anode compartment,
a sodium hydroxide compartment, placed between a cation-exchange membrane (Nafion N966, Flémion F892) and the cathode,
and a gas compartment.
The cathode is generally made of a silvered nickel grid covered on either side with platinized carbon.
One of the faces is coated with a fluorocarbon micropore layer in order to make it more hydrophobic.
Platinum represents 5% to 20% by weight of the carbon/platinum combination, and its average mass per unit surface area may range from 0.2 to 4 mg/cm
2
.
Conventional electrolysis cells with a membrane and a cathode evolving hydrogen, that is to say those employing reaction (2) mentioned above, are sometimes turned off to perform a variety of maintenance operations, or else following an incident. In such cases as these, the electrodes are de-energized, that is to say they are no longer supplied with electrical power.
Industrially, these outage phases can be managed in the following way: turning off the power and continuing the flow and addition of fluids (water and brine). The following procedure may also be adopted: turning off the power, emptying the sodium hydroxide and brine compartments, then filling with 20% strength sodium hydroxide solution (i.e. about 4 M) in the case of the cathode compartment, and with 220 g/l of brine in the case of the anode compartment (eliminating the active chlorine).
This operation is intended to preserve the performance of the membrane.
When conditions of this type are applied during outage phases to electrolysis cells with a membrane and an oxygen-reducing cathode, a significant increase in the cathode potential is observed when the electrolysis is resumed. This cathode alteration affects the voltage of the cell and leads to a significant increase in the energy consumption, which may be up to 100 kWh/tonne of sodium hydroxide produced.
Without tying applicant to an explanation, it is reasonable to assume that, in view of the simultaneous presence of oxygen and sodium hydroxide, the carbon of the de-energized cathode reacts with the oxygen and sodium hydroxide to form sodium carbonate, which deposits on the cathode. It reduces its porosity and electrical conductivity.
In order to overcome these drawbacks, Patent Application EP 0064874 has proposed a procedure which consists in completely replacing the gas (containing oxygen) in the gas compartment with nitrogen, and in keeping the nitrogen in the said gas compartment throughout the outage period.
Under these conditions, it is observed that after very short outages (a few hours), the cathode potential is little altered on restarting.
SUMMARY OF THE INVENTION
A method has now been found for shutting down an electrolysis cell with a membrance and oxygen-reducing cathode, characterized in that, after the electrical power and oxygen supplies to the said cell have been disconnected, the gas compartment is emptied and filled with demineralized water having a pH equal to or less than 7, the cathode is rinsed with demineralized water from the gas compartment until a pH equal to or less than 7 is obtained, for example a pH equal to that of the demineralized water which was introduced, and the said gas compartment is kept filled with the said demineralized water throughout the shutdown period.
The use of demineralized water is superior than the use of nitrogen because it permits the elimination of carbonated ions.
According to the present invention, the demineralized water may be acidified by means of inorganic acids such as HCl, or H
2
SO
4
so as to obtain a pH of between 0 and 7. Preferably, use will be made of demineralized aqueous solutions of the said inorganic acids, having concentrations in mol-g/l of between 0.1 and 1, thereby providing pH values well below 7, e.g. a pH between 0.1 and 1.
In the shutdown method according to the present invention, the anolyte and water supplies may be maintained, or alternatively the anode compartment may be emptied then filled with a clean anolyte of the same type and same concentration (this operation making it possible to eliminate the active chlorine) and the sodium hydroxide compartment may be emptied then filled with a sodium hydroxide solution of low molar concentration (molarity), generally between 0.5 and 5 mol-g/l, and preferably close to 1 mol-g/l.
The temperature of the liquids which are introduced into the various compartment of the electrolysis cell which has been shut down is between 20° C. and 80° C., and preferably between 30° C. and 60° C.
These temperatures are maintained throughout the period during which the cell is shut down.
This shutdown method applies more particularly to shutting down cells with a membrane and an oxygen-reducing cathode which have 3 compartments.
REFERENCES:
patent: 4185142 (1980-01-01), Solomon et al.
patent: 6017279 (1992-06-01), None
JP 6017279—English Abstracts Aug. 1994.
Elf Atochem S.A.
Millen White Zelano & Branigan
Phasge Arun S,.
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