US2834728A - Method and apparatus for protecting the cathodes of electrolytic cells - Google Patents

Description

M y 13, 1958 P. GALLONE 2,834,728

METHOD AND APPARATUS FOR PROTECTING THE CATHODES OF ELECTROLYTIC CELLS Filed March 1, 1954 3 Sheets-Sheet l INVENTOR- May 13, 1958 P GALLQNE 2,834,728

METHOD AND APPARATUS FOR PROTECTING THE CATHODES OF ELECTROLYTIC CELLS 5 Sheets-Sheet 2 Filed March 1, 1954 y 13, 1958 P. GALLONE 2,834,728

METHOD AND APPARATUS FOR PROTECTING THE CATHODES OF ELECTROLYTIC CELLS Filed March 1, 1954 s Sheets-Sheet s United States (Patent @fitice 2,834,728 Patented May 13, 1958 METHOD AND APPARATUS FOR PROTECTING THE CATHODES OF ELECTROLYTIC CELLS Patrizio Gallone, Milan, Italy, assignor to Oronzio De Nora Impianti Elettrochimici Application March 1, 1954, Serial No. 413,332

Claims priority, application Italy March 2, 1953 Claims. (Cl. 204-147) This invention relates to a method and apparatus for protecting the cathode of an electrolytic cell from attack by the electrolyte and electrolysis product during periods when the cell is cut out of the main electrolysis circuit.

The method and apparatus of. my invention may be used, in general, in all electrolysis apparatus in which the cathode has a reactivity with respect to the electrolyte and the electrolysis products when the flow of the electrolysis current between the anode and the cathode of the cell is stopped.

In the following description, which is for illustrative purposes, reference will be made particularly to amalgam cells for the electrolysis of alkali chlorides, wherein the cathode consists of a flowing layer of mercury or of an amalgamated metallic surface. It will be obvious, however, that the principles of my invention may be applied 1 to other types of electrolytic cells than those hereinafter specifically described.

In order to shut down a cell in a bank of electrolytic chlorine cells, for example, and keep the other cells in operation, the normal practice, up to the present time, has been to short-circuit the cell to be shut down, by cutting off the current to this cell. This procedure is objectionable because, during such period of inoperation, the cathode of the cell is exposed 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 dechlorinated brine.

One means for overcoming this objection is illustrated and described in United States Patent No. 2,508,523, which shows the installation of a number of auxiliary protective anodes in the cell which form, with the cathode,

a circuit independent of the main anodes when the cell is cut out of the main electrolysis circuit. These auxiliary anodes are used to maintain a small polarizing current over the cathode by means of an auxiliary source of electromotive force, during the period in which the main anodes are short-circuited with the cathode.

The method which is suggested in Patent No. 2,508,523, has several disadvantages. The installation of the additional auxiliary anodes involves a complicaiton in the design and construction of the cell. In addition, as there is very limited space left free around the main anodes, in which the auxiliary anodes may be located, the polarizing current cannot be uniformly distributed over the cathode. Furthermore, in order to maintain the polarizing current, an additional amount of current must be dissipated through the main anodes which are short-circuited with the cathode, and finally, due to the small dimensions of the auxiliary anodes, even though the polarizing current is kept to the lowest value which can still afford protection to the cell, its amount, plus thatwhich is dis vsipated through the main anodes, requires a rather high 2 current density over the auxiliary anodes so that there potential can go as high as the chlorine discharge point, which involves potential danger to the inactive cell. All these disadvantages are overcome by the use of our invention.

One of the objects of my invention is to provide a method of protecting the cathodes of electrolytic cells during shut-down of the cell, by maintaining a small polarizing current between the main anodes and the cathode of the cell.

Another object of my invention is to provide a method of protecting the cathodes of electrolytic cells, whereby it is possible to cut a cell out of the electrolysis circuit without short-circuiting' the cell.

Another object of my invention is to provide means by which an electrolytic cell may be cut out of the main circuit and the main anodes used as protective anodes during the period of inactivity of the cell.

Another object of my invention is to provide improved switch means for cutting an electrolytic cell out of the electrolysis circuit and simultaneously impressing a protective circuit between the main anodes and the cathode of the cell.

Another object of my invention is to provide a method and apparatus which permits a cell to be cut out of the electrolysis circuit without short-circuiting the cell and.to provide automatically for the maintenance of a protective current between the anodes and the cathode ofthe cell during periods in which this cell is out of operation.

Various other objects and advantages of my invention will appear as this description proceeds.

Referring, now, to the drawings which illustrate a preferred form of embodiment of our invention:

Fig. 1 is a diagrammatic illustration of a pair of electrolytic mercury cells to which my invention has been applied;

Fig. 2 is a part sectional end view of a switching means suitable for use in carrying out the principles of my invention;

Fig. 3 is a side view of the switch illustrated in Fig. 2, taken from the right of Fig. 2;

Fig. 4 is a sectional view of the switch illustrated in Fig. 3, taken along the line 4--4 of Fig. 3;

Fig. 5 is a part sectional view taken along the line 55 of Fig. 4; and

Fig. 6 is a detailed view showing the switch in one of its positions.

In the embodiment of the invention illustrated in Fig. 1, two amalgam cells, 1 and 2, are shown in which the mercury cathode flows along the sloped bottom of the cells and the electrolysis current, during operation of the cell, flows between the anodes A and the cathode B. Brine is circulated between the anodes and the cathode and for the production of chlorine and caustic soda is decomposed into chlorine and sodium, which later is amalgamated with the mercury flowing along the bottom of cells 1 and 2.

In the embodiment illustrated, the cell 1 is represented as in the process of being cut out of the main electrolysis circuit, While the cell 2 is shown in the main electrolysis line. The cell 1 or any other cell in the electrolysis circuit may be cut out of the circuit by disconnecting the negative bus bar 3 coming from a cell in the row from the positive 'bus bar 4 of the cell 1 to be cut out of 'operation, while the following cell 2 remains connected through the negative bus bar 7 and the positive bus bar 4, so that the electrolysis current is carried through the bus bar 3, switch member 5, bus bar 7 and bus bar 4 to the cell 2, while the cell 1 is cut out of the electrolysis circuit. The switch members 5, which may be of a detailed construction such as that illustrated in Figs. 2 to 6 inclusive, are operated by means of a switch handle 6 and switch bar 8 to movethe switch element to the position to disconnect cell 1 from the electrolysis circuit or with reference to cell 2, to either. include the cell in the electrolysis current, or to disconnect the cell from the electrolysis current.

In Fig. 1, switch member 5 is illustrated in an inter.- mediate position with reference to cell 1 in which the switch member is passing from a position Where itconnects bus bar 3 with bar 4 to a position where this connection is broken and connection established between bar 3 and bar 7. The switch element 5 is constructed so that when a cell is being cut into or out of the circuit, the electrical connection between the bus bar 3 and the bar 7 will be established before the connection between the bar 3 and the bar 4 is broken, and vice versa.

In addition to the switching means for switching a cell into or out of the electrolysis circuit, means are provided for simultaneously applying a protective polarizing voltage between the anodes A and the mercury layer constituting the cathode B. For this purpose, the switch bar 8 is provided with an auxiliary contact member 9 which is constructed to make contact with the contacts 9a and 9b as soon as the cell is cut out of the main circuit to connect the anodes A and cathode B with the positive and negative pole respectively of a source of polarizin-g voltage 10.

A ballast resistor 11 is preferably inserted in the polar- 1 izing circuit in order to protect the voltage source from damaging current surges, which might arise if the voltage source 10 were connected without ballast to a cell which had not yet acquired or had lost its polarizing conditions.

The polarizing voltage source 10 which is provided for each cell preferably consists of a battery of very small capacity and a trickle charge system 12fed by alternating current means so that the voltage source 10 is maintained at the desired potential. However, any suitable source of polarizing voltage 10 may be used.

,In order to insure immediate protection of the mercury cathodes in case of a temporary failure of the main electrolysis power under operating conditions, an auxiliary contact 13 may also be provided on the electrolysis power circuit breaker which will close automatically when the electrolysis circuit breaker opens. Through the closing of the contact 13 an exciting current will be sent to the relay 14, which is provided in each polarizing circuit, so that the relay 14 will close the polarizing circuit, upon any failure of the electrolysis power, even when the cell disconnect switch is closed and the auxiliary contact 9 is in open position, as illustrated in connection with cell 2. In the operation of our invention, as illustrated in Fig. 1, if it is desired to cut the cell 1 out of the electrolysis circuit, the switch handle 6 is moved in a clockwise direction to move the contact switch member 5 from the position in which it makes contact between the bus bar 3 and the positive bar 4 to the position in which the member 5 is out of contact with the bar 4 but is in contact with the negative bar 7, leading to the next cell. The contact between the bar 3 and the bar 4 is not broken until the contact between the bar 3 and the bar 7 has been made. At the same time the switch bar 8 moves the contact member 9 into contact with contacts 9a and 9b to complete the polarizing circuit from the positive side of the source 10 to the anodes A and from the negative side of the source 10 to the cathode B, so that the polarizing voltage is immediately impressed on the cell which is cut out of the circuit.

It will be understood that the illustrations in Fig. 1 are purely diagrammatic and that insulation, part of the wiring and other parts, not essential to the understanding of our invention, have been omitted so as not to unduly complicate the drawings.

When a current failure occurs while a cell is in operation, the circuit between the source 10 and the anodes A and cathode B is completed by the closing of the contacts of the relay 14 which is operated automatically by the contact 13 on any failure or break in the electrolysis power line.

7, In addition to the obvious advantages afforded by the above described arrangement, another very important advantage is that the protection of the cell is elfected through the same anodes of large dimension which serve for the electrolysis process, so that the polarizing current is uniformly distributed over the entire cathode and, therefore, a small current is sufficient to insure protection of the cathode. We have found that by the above arrangement, a cathodic current density as low as 0.15 amps/sq. ft. is sufficient to afford the desired protection.

Inasmuch as the anode surface area is nearly the same as the cathode area, the anodic current density will also be nearly the same as the cathodic current density. Using the above stated value of 0.15 amps/sq. ft.,-the voltage difference at the cell terminals amounts to about 1 volt, so that the anode potential is much lower than the chlorine discharge potential. It is therefore unnecessary to provide means for neutralizing any possible evolution of chlorine gas, as is required when using auxiliary anodes of small dimensions.

Another advantage which arises from keeping the polarizing voltage at a very low value resides in the fact that under the above stated conditions, polarization is suflicient to protect the mercury from the attack by chlorine, but not the iron suspension that sometimes accumulates in mercury and is often a cause of serious troubles. The small polarizing current will protect the mercury, but will allow its iron content to be set free and go into solution as chloride.

While any suitable switch element 5 adapted to make contact between the two bus bars to be connected before the contact between the two bars to be disconnected is broken may be used, we have found the switch illustrated in greater detail in Figs. 2 to 6 particularly well adapted for use according to our invention. This switch consists of the contact members 5 mounted on switch bar 8 and adapted to make or break contact between bars 3, 4 and '7 corresponding to the bars illustrated diagrammatically in Fig. l.

Contact members 33, 44 and 77 are connected, respectively, to the bars 3, 4-and 7 and are supported by a U-shaped bracket 21, to which the contact members 33, 44 and 77 are connected, respectively, at 22, 23 and-24, suitable insulation being provided at these connections as indicated more particularly in Fig. 4, so that electrical contact between the contact bars 33, 44 and 77 will be made only through the switch members 5. A spring member 25, which passes through the contact bars 44 and 77, presses against blocks 30 to maintain the contact bars 44 and 77 in their desired position and in contact with the switch members 5 when contact is to be made with the switch members 5. The brackets 21 are also provided with lower bearing members 34 and removable cap bearing members 35, so as to provide a journal through which the shaft 8 extends and in which bearings the shaft 8 may be rotated to control the position of the switch members 5.

The position of the switch members 5 is changed by the rotation of the shaft 8, which is connected through keys 26 to the separate elements of the switch members 5 in such a way as to allow some radial play for the switch contacts 5.

The shaft 8 is provided with two collars 27, which are rigidly fixed to the shaft, and which support two rods 28,

which are parallel to the shaft 8 and push against springs 29 through pressing plates 32. Springs 29' bear on the ends of the switchmembers 5 through pressing plates 31 formed as inverted U-shaped members which also guide the'springs 29. The springs 29 are, therefore, compressed between the rods 28 and the switch members 5, so that the switch members will be permanently pushed against the ends of the contact bars 33, 44 or 77, dependingupon the position to which the switch members 5 are rotated. Separate springs 29 are provided at each end of each switch member 5 so that the individual switch members may automatically adjust their position with reference to the contact bars 33, 44 and 77.

Movement of the switch members by turning the shaft 8 causes the switch members 5 to slide over the ends of the contacts 33, 44 and 77 to make or break the contact. The switch members 5 are preferably made of a silver alloy or of any other metal particularly suitable for electrical contacts, and the switch members 5 are preferably divided into three sections, as indicated particularly in Figs. 3 and 5, so as to provide a plurality of contact points and insure good contact between the switch members and the contact bars 33, 44 or 77 at all times. The contact bars 33, 44 and 77, which are usually made of copper, may have their ends silver-plated or may have a suitable contact alloy brand to the ends of the contact bars.

The operation of the switch members 5 will be readily understood from the above description. When the control shaft 8 is turned to switch-in position, for a particular cell, the switch members 5 will be pressed against the bars 44 and 33, so that the electrolysis current will be led by the contact 44 and anode bar 4 to the anode of this cell and after passing through the cell, will be conducted through the cathode bar 7 connected across the base of the cell to the next adjacent cell. When it is desired to cut a cell out of the electrolysis circuit, the shaft 8 will be rotated so that the switch members 5 will be turned to connect contact 33 with contact 77 and to disconnect contact 44 from contact 33. During both the switching in and switching out operations, the switch members 5 will temporarily pass through the intermediate position as indicated in Figs. 1, 2 and 4, in which position the switch members 5 will contact all of the three contact bars 33, 44 and 77, so that there will be no interruption of the continuity of the circuit to the remaining cells in the circuit as the switch members 5 move from one position to another.

This intermediate position of the switch members 5 will correspond to a momentary short-circuiting condition for the cell being cut out of the circuit, but this shortcircuiting condition will last for only a short period of time as the contact member 9 will be brought into contact with the contacts 9a and 9b as soon as the switch member 5 has reached the position indicated in Fig. 6 in which it has broken contact with the contact bar 44 and remains in contact with the contact bars 33 and 77. As soon as the contact member 9 has completed the circuit between the auxiliary source of power and the cell which is cut out of the main circuit, a polarizing current will be impressed between the main anodes A and the cathode B, so that the short-circuiting effect will be of only short duration.

Each cell is preferably provided with two or more switch elements 5 as described above; preferably, the cells will be provided with as many switches as there are anodes in the cell, and as many cathode bus bars as are necessary to feed the anodes in parallel to each cell. One operating shaft 8 will be used to simultaneously move all of the switch members 5 for one cell, so that all of the contacts will be made or broken by the same operation and also by the same operation all of the anodes will receive polarizing current from the source 10 as soon as the cell is cut out of operation.

While I have described, for illustrative purposes, a preferred application of my invention to mercury cells and a preferred embodiment of our switching device, it will \be understood that this is for illustrative purposes only and that the principles of my invention may be applied to other types of electrolysis cells and that other types of switching devices may be used, without depart- 6 ing from the spirit of our invention or the scope of the following claims.

I claim:

1. The method of protecting the cathodes of electrolytic cells, having anodes and a mercury cathode said anodes and said cathode being of substantially the same surface area, a main electrolysis circuit, and a source of polarizing current, when the cell is cut out of the main electrolysis current, which comprises disconnecting said source of polarizing current from the anodes and cathode of said cell when the cell is in the main electrolysis circuit, automatically connecting said source of polarizing current, having a potential lower than the decomposition potential of the electrolysis solution to the electrolysis anodes and the mercury cathode of the cell when the cell is cut out of the main electrolysis circuit.

2. The method of protecting the cathode of an electrolytic cell having a set of anodes and a mercury cathode in the main electrolysis circuit, said anodes and said cathode being of substantially the same surface area, during periods, when the cell is disconnected from the main electrolysis current, which comprises disconnecting a cell from the main electrolysis current while maintaining the current flowing through the other cells of the circuit, and substantially simultaneously connecting a small polarizing current having a potential lower than the decomposition potential of the electrolysis solution to said anodes and said cathode of said cell when said cell is cut out of the circuit.

3. The method of protecting the cathode of an electrolytic cell having a set of anodes and a mercury cathode in the main electrolysis circuit, said anodes and said cathode being of substantially the same surface area, during periods when the cell is disconnected from the main electrolysis current which comprises disconnecting a cell from the main electrolysis current, simultaneously conmeeting the other cells of the circuit with the main electrolysis current, and substantially simultaneously connecting a source of polarizing current of a magnitude to provide a cathodic current density of about 0.15 amps/sq. ft. to said anodes and said cathode of said cell when said cell is cut out of the circuit.

4. In a switching device for electrolytic cells comprising fixed and movable contacts, a rotary shaft, a movable contact member formed of a plurality of segments of a cylinder spaced closely adjacent to one another longitudinally of the axis of said shaft, springs spaced on each side of the axis of said rotary shaft pressing outwardly against each end of each of said segments, means to limit the outward movement of said segments by said springs, connections between said springs and said segments and the rotary shaft whereby rotary movement of said shaft causes rotary movement of said segments past said fixed contact members, said connections also permitting radial tipping movement of said segments relative to said shaft.

5. A switching device for electrolytic cells comprising a plurality of fixed contact members attached to bus bars of an electrolytic cell and forming an arc of a circle at their contact points, said fixed contact members being composed substantially of short sections of bus bar material, spring means pressing some of said contact members toward said arc of said circle, and movable contact members moving in the arc of said circle comprising a rotary shaft placed perpendicular to the arc of said circle formed by the fixed contact points, contact members formed of a plurality of segments of a cylinder spaced closely adjacent to one another longitudinally of the axis of said shaft, said segments of a cylinder having a length of arc sufficient to make contact with all of the three fixed contact point simultaneously, springs spaced on each side of the axis of said rotary shaft pressing out wardly against each end of each of said segments, means to limit the outward movement of said segments by said springs, connections between said springs and said seg- 7 ments and the rotary shaft whereby rotary movement of said shaft cause rotary movement of all of said segments as a single unit past said fixed contact members, said connections also permitting radial tipping movement of said segments relative to said shaft.

References Cited in the file of this patent UNITED STATES PATENTS 8 MacLean May 27, 1930 Seaman Dec. 31, 1940 Krebs May 23, 1950 W FOREIGN PATENTS Switzerland Aug. 1, 1922 Great Britain Sept. 4, 1924 France Sept. 27, 1950

Claims (2)

1. THE METHOD OF PROTECTING THE CATHODES OF ELECTROLYTIC CELLS, HAVING ANODES AND MERCURY CATHODE SAID ANODES AND SAID CATHODE BEING OF SUBSTANTIALLY THE SAME SURFACE AREA, A MAIN ELECTROLYSIS CIRCUIT, AND A SOURCE OF POLARIZING CURRENT, WHEN THE CELL IS CUT OUT OF THE MAIN ELECTROLYSIS CURRENT, WHICH COMPRISES DISCONNECTING SAID SOURCE OF POLARIZING CURRENT FROM THE ANODES AND CATHODE OF SAID CELL WHEN THE CELL IS IN THE MAIN ELECTROLYSIS CIRCUIT, AUTOMATICALLY CONNECTING SAID SOURCE OF POLARIZING CURRENT, HAVING A POTENTIAL LOWER THAN THE DECOMPOSIANODES AND THE MERCURY CATHODE OF THE CELL WHEN THE CELL IS CUT OUT OF THE MAIN ELECTROLYSIS CIRCUIT.

4. IN A SWITCHING DEVICE FOR ELECTROLYTIC CELLS COMPRISING FIXED AND MOVABLE CONTACTS, A ROTARY SHAFT, A MOVABLE CONTACT MEMBER FORMED OF A PLURALITY OF SEGMENTS OF A CYLINDER SPACED CLOSELY ADJACENT TO ONE ANOTHER LONGITUDINALLY OF THE AXIS OF SAID HSFT, SPRINGS SPACED ON EACH SIDE OF THE AXIS OF SAID ROTARY SHAFT PRESSING OUTWARDLY AGAINST EACH END OF EACH OF SAID SEGMENTS, MEANS TO LIMIT THE OUTWARD MOVEMENT OF SAID SEGMENTS BY SAID SPRINTS, CONNECTIONS BETWEEN SAID SPRINGS AND SAID SEGMENTS AND THE ROTARY SHAFT WHEREBY ROTARY MOVEMENT OF SAID SHAFT CAUSES ROTARY MOVEMENT OF SAID SEGMENTS PAST SAID FIXED CONTACT MEMBERS, SAID CONNECTIONS ALSO PERMITTING RADICAL TIPPING MOVEMENT OF SAID SEGMENTS RELATIVE TO SAID SHAFT.

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