US3481856A

US3481856A - Vertical mercury cathode electrolytic cells

Info

Publication number: US3481856A
Application number: US579945A
Authority: US
Inventors: Gotthard Csizi
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1965-09-25
Filing date: 1966-09-16
Publication date: 1969-12-02
Anticipated expiration: 1986-12-02
Also published as: BE687284A; DE1567907A1; SE300614B

Description

Dec. 2, 1969 G. (25121 3,481,856 4 VERTICAL MERCURY CATHODE ELECTROLYTIC CELLS Filed Sept. 16, 1966 Jfl 5 INVENTOR. GOTTHA/FD CS/Z/ United States Patent Int. Cl. C22d 1/04 U.S. Cl. 204-219 8 Claims ABSTRACT OF THE DISCLOSURE An electrolytic cell without diaphragms for the decomposition of alkali metal chlorides by the amalgam method which is divided into two chambers by a vertical cathode. Electrolysis of the alkali metal chloride solutions is carried out in one chamber which contains the anode whereas the other chamber, provided with a decomposing electrode, serves as a decomposition chamber for the amalgam formed. The cathode which vertically divides the cell consists of a supported member of metal wire cloth down the surface of which mercury flows in a thin film in contact with the decomposing electrode.

The present invention relates to an electrolytic cell without diaphragms for the decomposition of alkali meta chlorides by the amalgam method.

Two types of cell have become known for electrolytic decomposition of alkali metal chlorides by the amalgam method, one in which the mercury cathode is horizontal and the other in which it is vertical. Attention has hitherto been mainly directed to cells having horizontal mercury cathodes. This is because a flat mercury surface having large dimensions is easier to make, readjustment of graphite anodes is easier and the mercury can be recycled with less elaborate means in this type of cell. Vertical cells have the advantage of requiring less space, but they also have a number of disadvantages which have hitherto prevented their use in practice. The mercury runs down along the cathode too quickly so that it does not have enough time to react to form alkali amalgam of the desired concentration. Moreover the mercury tends to detach itself from the cathode walls in the form of drops instead of flowing down on the cathode walls in a thin film. This mercury, which is present in the drops in depolarized form, is attacked by chlorine however so that loss of mercury may be extremely high in this type of cell. Moreover the rate at which the mercury falls increases greatly with increasing height of the cell so that cells having vertical electrodes can only be built to a certain height.

Vertical mercury cathodes have already become known by which this disadvantage is partly avoided. Thus for example a cathode is known which consists of a lamellar member serving as cathode support whose surface down which the mercury flows is covered with a screen of electrically insulating material which separates the lumpy anode material inserted into the space between the cathode support and the wall of the cell casing from the cathode support. The rate at which the mercury falls is considerably retarded by such a cathode arrangement, but the anode material, consisting of lumps of graphite, has to be replenished from time to time.

Both types of cell have the disadvantage that a very large amount of mercury has to be used to operate the cells, the mercury being loaded with amalgam in the electrolytic cell and the amalgam decomposed into hydrogen and caustic soda solution by means of water in a special decomposer.

3,481,856 Patented Dec. 2, 1969 The object of the present invention is to provide an electrolytic cell without diaphragms and with vertical mercury cathode for the decomposition of alkali metal chlorides by the amalgam method, the cathode consisting of a vertical support member down the surface of which mercury flows in a thin film and the anode consisting of a metal which is resistant to chlorine and to corrosion, in which the abovementioned disadvantages do not occur. This cell is subdivided by a support member consisting of wire cloth into two separate chambers of which one chamber provided with the anode and into which the brine is introduced, serves as electrolysis chamber, and of which the other chamber filled with water is provided with a decomposing electrode in contact with the mercury cathode and serves as decomposition chamber for the amalgam formed.

In accordance with the invention, the cathode support member consists of wire cloth, preferably of iron. This wire cloth has about 10 to 60 meshes, preferably 35 to 45 meshes per square centimeter. The wire diameter is about 0.2 to 0.5 millimeter. Mercury is supplied by means of a suitable appliance to the upper end of the cloth and closes the meshes thereof as it flows down, thus forming the cathode surface. At the same time the cell is subdivided into two chambers separated by a liquid seal. One chamber, into which the alkali metal chloride solution is introduced, serves as the electrolysis chamber and is provided with an anode of corrosion-resistant metal, for example platinum, tantalum, niobium, zirconium or particularly titanium or an alloy thereof. This electrode may also be in the form of wire cloth.

The mercury, as it flows down from the upper portion of the support member, is greatly retarded in its rate of flow and made turbulent by continual impingement on the individual wires of the cloth. In the other chamber formed by the vertical cathode a decomposing electrode is provided which is in direct contact with the mercury flowing down. Water is passed through this chamber so that the amalgam formed on the surface of the cathode which faces the electrolysis chamber and which by reason of the turbulence of the mercury passes to the other surface of the cathode facing the decomposition chamber, is decomposed with the formation of hydrogen and caustic soda solution. Since the decomposing electrode, which may consist for example of graphite and which can be very accurately machined, is in contact with the support member for the mercury cathode at several places and thus serves as a support for the support member, the latter acquires a very flat surface. Consequently the metcury layer flowing down over the cathode support also acquires a very flat surface. It is therefore possible for the anode to be only a short distance away from the cathode without the risk of shortcircuit between anode and cathode because of any unevenness. Therefore it is possible (in contrast to cells having electrodes of comparable size but not having such level surfaces and which accordingly must have a larger electrode spacing) either to work at higher current densities at the same potential or to work at a lower potential at the same current density. Thus it is possible for example to use current densities of up to 15K amps/sq. m. or to use potentials of less than 4 volts at the conventional current densities of 8K amps/sq. m.

A preferred embodiment of a cell according to this invention is shown diagrammatically by way of example in the accompanying FIGURES 1 to 3. FIGURE 1 is a longitudinal section through such a cell, FIGURE 2 is a transverse section on the line A-B in FIGURE 1, and FIGURE 3 shows a preferred embodment of the anode in longitudinal section on an enlarged scale.

1 denotes metal wire cloth which serves as support member for the mercury flowing down. The cloth hangs vertically in a frame. This frame consists of a number of parts, namely parts 2 prepared from insulating material, for example steel covered with hard rubber, and

side walls

18 and 19 made from metallic conducting material. The wall 18 is made from a material which is resistant to chlorine, for example titanium, and the wall 19 is made from a material resistant to caustic alkalies, for example iron or nickel. Mercury is supplied uniformly to the whole width of the wire cloth by means of a distributing member 5, which may be constructed as an overflow, and is collected at the lower end of the. wire cloth by means of a channel 8. Thence it is withdrawn and conveyed by means of a pump along a slot 4 in the wall of the decomposition chamber to the top where it again enters the overflow 5. Mercury is supplied to the wire cloth at such a rate that all meshes are closed and thus a. closed surface is formed which separates the electrolysis and decomposition chambers (de scribed below) from each other. A decomposing electrode 6 is provided in the decomposition chamber; the electrode 6 consists of graphite and is provided with vertical slots 9 (see FIGURE 2). The lands 10 and the slots 9 have about the same width and are preferably about 4 to 6 mm. wide. The lands of this electrode contact the support member of the mercury cathode. In this way the support member is supported and moreover current is supplied to it via the decomposing electrode. In the other chamber formed by the mercury cathode, an anode 7 is provided with is iliustrated in greater detail in FIGURE 3. 11 denotes electrical connections to the anode. Alkali metal chloride solution is passed through line 12 into the electrolysis cham ber; chlorine is developed at the anode and sodium is deposited at the cathode with the formation of sodium amalgam. The exhausted salt solution is collected with the chlorine in the space 13 and removed thence. The sodium amalgam formed at the cathode, owing to the constant movement of the mercury, passes to the other side of the cathode and is separated at the decomposing electrode 2 into caustic soda solution and hydrogen. The decomposition chamber is supplied with water through ilne 14 and the caustic soda solution and hydrogen formed in the decomposition chamber are withdrawn through line 15. The water flows up through the vertical slots in the. decomposing electrode and passes together with the hydrogen formed through the upwardly inclined openings 16 to the outside and thence to the collecting space 15. 17 are rods for supplying current to the decomposing electrode.

The anode indicated by 7 in FIGURES 1 and 2 is shown in greater detail in FIGURE 3; it consists of a fine-mesh wire cloth, for example of titanium, whose surface may be activated by application of a thin layer of noble metal, for example platinum. The cloth is held between two

frames

102 and 103. Baflies 104 which are parallel and equidistant are arranged one above another in these frames; the baffles are inclined upwardly as viewed from the cathode. The baffles contact the cloth at their ends facing the cloth, and each baflie of one frame presses against a batfle of the other frame so as to form one surface interrupted by the cloth. The cloth is mechanically stiffened in this way. The distance between the baffles in a frame is about to mm. The alkali metal chloride solution flows upward in the space between the mercury cathode and the titanium cloth 101. Chlorine is liberated at the titanium cloth and is forced through the cloth with the exhausted salt solution by the incoming salt solution, flows upwardly in the space behind the cloth and collects in the collecting space 13 shown in FIGURE 1.

The anode arrangement may be held in position for example by pins mounted on the connections 11.

This anode arrangement offers the advantages that the chlorine developed is immediately forced through the cloth and thus no decrease in the free cross-section of the electrolyte between the cathode and the anode can take place by the formation of gas bubbles by which the passage of current between the two electrodes is interrupted.

The advantage of the cell according to this inven ion is that the immediate reaction of the sodium amalgam makes a decomposer separate from the cell unnecessary. It 15 possible to save considerable mounts of mercury which hitherto have been necessary in the case of decomposers separate from the actual electrolysis cells. Since the surface of the mercury cathode is very flat, as already stated, it is possible to locate the anode at only a short distance from the cathode and thus to achieve very high current densities at a low cell potential.

By connecting cells according to the invention in series it is possible to make units rather like filter presses which occupy only a very small space. The cells are connected in series so that mercury pumped upward along the decomposer wall serves as an electrical contact between the decomposing electrode of one cell and the anode of the next.

I claim:

1. In an electrolytic cell for the decomposition of an alkali metal chloride by the amalgam method without diaphragms, said cell having a vertical mercury cathode comprising a metal wire cloth adapted to support flowing mercury in a thin film over the surface thereof, the improvement which comprises: a cell divided into two separate chambers by said vertically positioned metal wire cloth which supports the flowing mercury, the first of said chambers on one side of said wire cloth containing an anode composed of a metal resistant to chlorine and arranged in close proximity to said vertical mercury cathode and the second of said chambers on the other side of said wire cloth containing a decomposing electrode mounted therein so as to be in direct contact with said vertical mercury cathode; means to supply the alkali metal chloride as an aqueous salt solution to said first chamber which serves as an electrolysis chamber; and means to convey water through said second chamber which serves as a decomposition chamber for the amalgam being formed.

2. An electrolytic cell as claimed in claim 1 wherein said decomposing electrode in said second chamber is slotted on its face which contacts said wire cloth to provide a passageway for water being conveyed through said second chamber.

3. An electrolytic cell as claimed in claim 2 wherein said decomposing electrode has alternating vertical slots and lands along said face of said wire cloth with the lands providing a flat supporting structure for said wire cloth.

4. An electrolytic cell as claimed in claim 2 wherein said decomposing electrode is constructed of graphite.

5. An electrolytic cell as claimed in claim 1 wherein distributor means are provided at the top of said metal wire cloth to supply mercury into said cell as a downwardly flowing film spread over the entire wire cloth within the cell, a collecting channel is arranged at the bottom of said cell to receive mercury discharged at the lower end of said wire cloth, and means are provided to conduct the mercury from said channel along the boundary wall of the cell back to said distributor means.

6. An electrolytic cell as claimed in claim 5 wherein a conduit is provided in the boundary wall of the cell for conducting the mercury from said channel back to said distributor means.

7. An electrolytic cell as claimed in claim 1 wherein the anode in said first chamber consists of a fine-mesh wire cloth arranged inwardly from the boundary wall of the cell and supported by a frame so as to be parallel to the vertical mercury cathode.

8. An electrolytic cell as claimed in claim 7 wherein said anode as a fine-mesh wire cloth is arranged between two frames, each of which carries a plurality of planar baffles on both sides of the cloth extending over its entire width, the baffles being parallel to each other and having their horizontal longitudinal axes arranged one above another so that the baffles contact the anode cloth with their longitudinal edges and each two opposite batlles form 5 6 a planar surface interrupted only by the cloth between 2,829,096 4/1958 Clement. their contacting longitudinal edges, said planar surface 3,002,914 12/1961 Clement 204219 being inclined upwardly at an angle in the direction eX- 3,046,215 7/1962 Sullivan et al. 204219 tending away from said vertical mercury cathode. 3,065,163 11/ 1962 Honsberg 204220 References Cited 5 JOHN H. MACK, Primary Examiner UNITED STATES PATENTS D. R. VALENTINE, Assistant Examiner 513,661 1/1894 Vaut1n 204250 U'SCLXR.

678,816 7/1901 Shinn. 692,531 2/1902 Le Sueur 204219