US3409519A - Method of protecting electrolytic cells - Google Patents

Description

P. GALLONE ET AL METHOD OF PROTECTING ELECTROLYTIC CELLS Nov. 5, 1968 2 Sheets-Sheet 1 Filed Oct. 4, 1965 INVENTOR% o PATRIZIO GALL NE GIOVANNI TRISOGLIO Z w/W/M ATTORNEYS J Hut.

Nov. 5, 1968 P. GALLONE ET AL 3,409,519

ECTROLYT IC CELLS 2 Sheets-Sheet 3 Filed Oct. 4, 1965 INV RIZ O V MN W W ATTORNEYS United States Patent 3,409,519 METHOD OF PROTECTING ELECTROLYTIC CELLS Patrizio Gallone and Giovanni Trisoglio, Milan, Italy, as-

signors to Oronzio de Nora Impianti Electtrochimici, S.A.S., Milan, Italy, a corporation of Italy Filed Oct. 4, 1965, Ser. No. 492,526 Claims priority, application Italy, Oct. 10, 1964, 52,182/ 64 6 Claims. (Cl. 204-99) The invention relates to a novel apparatus and a novel method of protecting the cathode of an electrolytic cell from the corrosive action of the electrolyte or the electrolysis products dissolved in the electrolyte as soon as the electrolysis current is interrupted.

In the following description, for illustrative purposes, reference will be made particularly to amalgam cells for the electrolysis of alkali metal chlorides wherein the cathode consists of a flowing layer of mercury or of an amalgamated metallic surface. It is obvious though that the principles of the invention are applicable to other types of cells than those specifically described herein.

An electrolytic cell such as a chloro-alkali cell is shut down by cutting oif the electrical current to the cell, but during a period of inoperation, the mercury cathode of the cell is subjected to chemical attack by the free chlorine dissolved in the brine remaining in the cell and is thus turned into the anode of a short-circuited galvanic battery and as a consequence, the attack on the mercury in the cell is increased by the concomitant electrochemical process which takes place within the short-circuited cell in the presence of the chlorinated brine.

The only practical method of the prior art used to overcome this problem has consisted of maintaining a cathodic current on the cell to be protected of suflicient intensity to preserve its immunity. The said cathodic current is impressed upon the cell by suitable auxiliary electrical means according to known methods designated as cathode protection methods.

With particular regard to some industrial processes of foremost importance, the modern trend is to use electrolytic cells of ever increasing size and current capacity, so that eflicient and economical cathode protection becomes even more diflicult and problematic. As an example, in a typical mercury cathode cell for alkali chloride electrolysis, the results obtained from practical experience indicate that, in the absence of any protection, the mercury is quickly attacked by free chlorine dissolved in brine. In order to avoid any appreciable build-up of mercurous chloride and mercuric chloride when the contacting brine is saturated with free chlorine at atmospheric pressure, it is necessary to apply a cathodic current density in the order of 100 A./m. Moreover, cathodic protection must be brought to bear almost at once, and therefore automatically at any interruption of the electrolysis current in order to prevent any appreciable mercury loss and the danger that the free chlorine may attack the metal surface of the underlying cell structure as well. This requires an auxiliary source of cathodic current entirely independent of the power supply for the electrolysis process. Therefore, this requirement can only be met by a battery of accumulations, particularly since the substitution of the cathodic current for the electrolysis current must be almost instantaneous.

Consider as an example a cell with a mercury cathode surface of 20 m2, which at the rated current density of 6000 A./m. corresponds to a current capacity of 120,000 A. Such cell size and current capacity would still be considered economical in a modern chloro-alkali plant not exceeding a daily production of 100 tons/day, but for any larger plant capacity a larger cell size and current rating would be more profitable. Nonetheless, the auxiliary battery required for cathodic protection would have to be rated for a current output of no less than 2000 A., at least during the residence time of the chlorinated brine inside the cell, which may extend to over 15 minutes before the brine hold up that was present in the cell during the electrolysis is evacuated after any shutdown and replaced with fresh brine. If the latter is thoroughly dechlorinated, the cathodic current density required for cathodic protection of mercury and steel drops to a very low value of the order of a fraction of an ampere per square meter. There are, however, some instances in which the solid salt, used for brine resaturation, is of such a purity grade that only a fraction of the refortified brine stream may be submitted to a chemical treatment and then made to join again the untreated fraction, so as not to exceed the tolerable level of impurities content in the feed stream flowing back tothe electrolysis process. In other cases, the available salt is of such a high purity grade that no chemical treatment is required on the refortified brine stream and, consequently, no dechlorination is needed before resaturation. In any such case the cathodic protection battery is to be rated at a permanent current output that for the cell size considered before must be no less than 2000 A.

Besides the high cost involved by a storage battery meeting the requirements as outlined above, it must also be remembered that such a device would in general find application only for applying cathodic protection to the entire cell bank in case of any planned or accidental shutdown. It would require an even more expensive arrangement if cathodic protection were independently provided, which is desirable, to each single cell during the period when the cell is to be shorted out of the electrolysis circuit while the other cells are being kept in operation. This arrangement requires a considerably more elaborate set up as, for instance, described in US. Patent Nos. 2,834,728 and 3,057,984, both granted to Gallone.

Another drawback of cathodic protection resides in that the metal surfaces receiving the cathodic current tend to become the seat of hydrogen evolution with the ensuing danger that hydrogen may build up an explosive mixture not only with the chlorine gas formerly produced by electrolysis and still present in the system, but also with the chlorine gas and/or oxygen that will develop at the counterelectrode performing as the anode in the cathodic protection circuit as well as with the atmospheric oxygen that might inevitably enter the system.

It is an object of the invention to provide a novel economical method of protecting electrolytic cells from corrosive agents during periods of inoperation.

It is an additional object of the invention to provide a novel method of protecting electrolytic cells without cathodic protection.

It is another object of the invention to provide a novel electrolytic cell in which the cell is not subjected to corrosive action during periods of inoperation.

These and other objects and advantages of the invention will become obvious from the following detailed description.

The novel method of the invention for protecting electrolytic cells from corrosive agents during periods of inoperation comprises introducing a non-corrosive fluid onto the surface of the cathode adjacent to the electrolyte when the electric current to the cell is halted. It is preferable for the fluid to be introduced to contain a reducing agent to reduce the corrosive agent.

The corrosion rate at which mercury or any other metal, such as iron, are attacked by an aqueous solution of a free halogen, such as chlorine, is strictly related to the rate of diffusion of the free halogen through the diffusion layer separating the metallic phase from the bulk ice of the solution. It was possible to prove, by experimental investigation, that the diffusion rate is practically a linear function of the partial pressure of the halogen gas. It is, therefore, possible to obtain a drastic fall in the rate of attack and even to reach conditions of practical immunity, if the partial pressure of the aggressive agent within the bulk of the solution is conveniently diminished, at least in the immediate proximity of the diffusion layer. This can be obtained most conveniently not only by introducing a reactant into the aqueous solution, so as to chemically reduce the aggressive oxidant, but also and much more simply, by bubbling through the solution an inert gas, such as nitrogen or air, onto the immediate proximity of the surface to be protected.

Whenever the fugacity of the aggressive oxidant such as chlorine, is depressed by the use of a reducing agent, this may be conveniently dissolved or dispersed in a liquid phase prior to its injection. Typical examples of such liquids are aqueous solutions of bisulfite, or sulfurous acid, or thiosulfate. However, if the reducing agent is in the gaseous state, such as sulfur dioxide, it will be even more convenient to have it bubble throughout the bulk of the solution either as such or in admixture with an inert gas by injecting it at points as near as possible to the surface to be protected.

In order to avoid any contamination of the cell gas delivery line with the protective gas being bubbled into the cell immediately after shutdown, it is advisable to intercept the delivery line and establish at the same time a communication between the cell and the vent line as soon as electrolysis is discontinued. This can most conveniently be achieved by means of an automatic control acting on a valve system interlocked with the electrolysis circuit breaker or with any other device capable of a response to the ceasing of the electrolytic process.

Referring now to the drawings:

FIG. 1 is a partial sectional view in elevation of a typical mercury cathode cell equipped with graphite anodes.

FIG. 2 is a plan view partially cut out, taken along line II-II of FIG. 1.

FIG. 3 is a partial cross-sectional view of the interior of a typical mercury cell embodying a perforated pipe system for the injection of a protective fluid onto the mercury cathode when the anodes are dimensionally stable mesh anodes.

In FIGS. 1 and 2 the graphite anode 1 overhangs the mercury cathode 2 flowing on cell bottom 3. According to a well known and widely used arrangement the lower graphite plate surface is provided with slots 4, and the innermost surface of each slot communicates with the top surface of the plate through a series of vertical ducts 5, which are drilled throughout the graphite structure and in combination with the slots facilitate the gas release from the anode-to-cathode gap. A suspending block 6 of graphite is fixed into a central cavity 20 in the top of the anode plate 1 and is in turn connected with a threaded stem metal 7 which passes through the cell cover 8 which, in the present example, is made of a flexible and plastic material, such as rubber, resistant to the oxidizing attack of the cell gas, according to the arrangement described in US. Patent No. 2,958,635 granted to De Nora. The opening through which the stem 7 is sticking out of the cover is made gas-tight by fastening locknut 9 onto the stem 7, so as to press the cover between outer washer 10 and the inner graphite block 6. Another set of two or more nuts 13 screwed onto the stem 7 serve to establish the mechanical and electrical connections of the anode with the bus bars and the supporting structure.

The above-described details, although being already known by themselves, have been illustrated in order to make a clearer distinction with the other novel parts and characteristics of the assembly that are suitable to apply the present invention and will be described hereunder.

The holding stem 7 is bored along its axis and the bore contains a tube 12 of chemically resistant material. The stem 7 is screwed into the threaded cavity 23 worked out in the top of block 6 by means of a wrench applied to the hexagonal upper end 15. In this way, the lower end of the stem is pressed against the annular gasket 14 at the bottom of the recess to form a gas-tight assembly and thus prevent any chemical attack of the stem 7 by any anodic gas that might leak through the porosity of the graphite block.

The tube 12 ends into a second cavity 18 provided in the bottom of block 6 and is held tight by a terminal flange 17 against the innermost cavity surface, when the tube 12 is pulled from above by screwing nut 15 onto its upper threaded end until the nut is pressing against the upper end of stem 7.

The lower cavity 18 in the block communicates through radial slots 19 with the annular empty space 20 left between the lower recess in the block 6 and the central top cavity of plate 1, into which the block is fitted. A number of horizontal and intercommunicating ducts 21 are bored throughout the graphite plate and some of them end in the empty space 20. Said horizontal ducts 21 perform the function of a manifold for a series of vertical d-ucts 22 bored through the bottom of the plate, between the slots 4 and ending in said manifold.

According to the above-described device when a liquid ora gaseous fluid is introduced through the top of tube 12 under a pressure sufficient to oppose the hydrostatic head of the electrolytic solution, it will penetrate into cavity 18 and through the annular space 20 and ducts 21, 22, it will be expelled from the lower anode surface so as to displace the electrolyte from within the anode-tocathode gap.

In the example of FIG. 3 a mercury cathode cell is schematically represented as a structure consisting of bottom 30, side walls 31 and flexible cover 32. In this example, the anodes are made of perforated metal structures 33 suspended from metallic stems 34. Such anodes have a relatively small thickness and have a large ratio of empty to solid space so that any fluid introduced into the electrolyte just above the anodes will easily pass through the perforated structure and reach the interelectrodic gap. Accordingly, the method of the invention can be effected by means of a relatively simple arrangement to take advantage of the dimensionally stable anodes which are extremely stable due to the very limited amount of chemical and mechanical wear, so that they can be given a permanent adjustment without the structural arrangements that would be needed to adjust for gradual wear in the course of operating life. It is, therefore, simple to arrange immediately above these anodes a set of horizontal perforated pipes 35 through which the protective fluid. will be injected into the cell. The perforations through each pipe will preferably be limited to that part of its surface that is facing the cathode.

Whenever the active surface of the perforated anodes 33 consists of a thin coat of a noble metal such as platinum, iridium, rhodium or of noble metal alloy, the protective method of the invention acquires significance also as regards the anodes if applied in a cell with a mercury cathode. Indeed one of the advantages afforded by the dimensionally stable structure of the latter resides in the possibility to give the anodes a permanent adjustment in such a way as to leave only a very narrow distance from the mercury cathode surface with an intermediate gat of no more than 3 mm. The voltage losses caused by the electrolyte resistance are thus substantially reduced in comparison with the performance that it is possible to achieve with graphit anodees. With graphite anodes, it is well known that any decrease of the interelectrodic distance to less than 5 mm. would bring about hardly any improvement and on the contrary, because of the difficulty for the gas to escape away from the gap, the results would thereby become worse in most cases, whatever the slot and hole configuration of the graphite plate.

However, the drawback of such a narrow gap generally resides in the danger of the anodes becoming contaminated with solid calomel in case of a cell shutdown if the aggressive action of the dissolved free chlorine or mercury is not promptly suppressed. In fact, such contamination brings about a scaling on the thin coat of noble metal and by bridging the gap between the anode and cathode surface may cause a local short circuit. Consequently, on restarting the electrolysis current, the thin coat of noble metal may become seriously deteriorated. However, by thoroughly suppressing any calomel formation by the present invention, not only will the mercury inventory remain unimpaired, but it will also be possible to make use of all the advantages afforded by these anodes since the narrower distance at which they can be adjusted from the cathode will allow operation at a lower voltage without any danger for their integrity.

Various modifications of the method and apparatus of the invention may be made without departing from the spirit or scope thereof and it is to be understood that the invention is to be limited only as defined in the appended claims.

We claim:

1. A method of protecting electrolytic cells from corrosive agents during periods of inoperation which comprises introducing a non-corrosive fluid onto the cathode surface adjacent to the electrolyte when the electric current to the cell is halted.

2. The method of claim 1 wherein the fluid is a chemically inert liquid.

3. The method of claim 1 wherein the fluid is a liquid containing an agent for reducing the corrosive agent.

4. The method of claim 1 wherein the fluid is a chemically inert gas.

5. The method of claim 1 wherein the fluid is in the gaseous state and contains an agent for reducing the corrosive agent.

6. A method of protecting an alkali metal electrolysis cell having a flowing mercury cathode from the corrosive effects of chlorine during periods of inoperation which comprises introducing a non-corrosive fluid onto the mercury cathode surface adjacent to the electrolyte when the electric current is halted.

References Cited UNITED STATES PATENTS 1,565,943 12/1925 Klopstock 204-99 2,834,728 5/1958 Gallone 204147 3,310,482 3/1967 Bon et a1. 2042l9 FOREIGN PATENTS 316,694 8/ 1929 Great Britain. 988,610 4/1965 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner. D. R. JORDAN, Assistant Examiner.

Claims (1)

1. 6. A METHOD OF PROTECTING AN ALKALI METAL ELECTROLYSIS CELL HAVING A FLOWING MERCURY CATHODE FROM THE CORROSIVE EFFECTS OF CHLORINE DURING PERIODS OF INOPERATION WHICH COMPRISES INTRODUCING A NON-CORROSIVE FLUID ONTO THE MERCURY CATHODE SURFACE ADJACENT TO THE ELECTROLYTE WHEN THE ELECTRIC CURRENT IS HALTED.

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