US2786810A

US2786810A - Anodes and cover of electrolytic cells

Info

Publication number: US2786810A
Application number: US308682A
Authority: US
Inventors: Brown Collingwood Bruce
Original assignee: Dominion Tar and Chemical Co Ltd
Current assignee: Domtar Inc
Priority date: 1952-09-09
Filing date: 1952-09-09
Publication date: 1957-03-26
Anticipated expiration: 1974-03-26
Also published as: GB747775A

Description

March 26, 1957 c. B. BROWN 2,786,810

' ANODES AND COVER OF ELECTROLYTIC CELLS File c l Sept. 9, 1952 3 Sheets-Sheet 1 IHVEHTOL- March 26, 1957 c B. BROWN 2,786,810

ANODES AND COVER OF ELECTROLYTIC CELLS Filed Sept. 9, 1952 3 Sheets-Sheet 2 *TLTJELHSE March 1957 Filed Sept. 9; 19.52

PIC-l5.

c. 5. BROWN 2,786,810

ANODES AND COVER OF ELECTROLYTIC CELLS 3 Sheets-Sheet 5 diliiiiiiiiiiiilin A"/ I IIIIIIIII/Iillll/ United States Patent ANODES AND COVER OF ELECTROLYTIC CELLS Collingwood Bruce Brown, Montreal, Quebec, Canada, assignor to Dominion Tar and Chemical Company, Montreal, Quebec, Canada, a corporation of Canada Application September 9, 1952, Serial No. 308,682

3 Claims. (Cl. 204219) This invention relates to an electrolytic cell for the manufacture of chlorine and in particular to an electrolytic cell having a flowing mercury cathode.

, Two types of electrolytic cells having mercury cathodes are in commercial operation at the present time. These are primarily devoted .to the electrolysis of brine to produce chlorine, caustic soda and/or other sodium compounds. The particular type of electrolytic cell with which my invention is concerned is one which has a flowing mercury cathode of which there are several commercial variations known as the Krebs cell, the De Nora cell, the Mathieson cell, etc. These cells consist of two parts: a primary cell in which the brine is electrolysed to produce chlorine and sodium amalgam, and a secondary cell in which the amlgam is decomposed with water to produce aqueous caustic soda solution, hydrogen and mercury. My invention is concerned with the structure and operation of the primary cell.

The primary cell in which the brine is electrolysed is a trough having a U-shapcd cross section and inclined in a longitudinal direction to provide for the necessary flow of mercury and brine. Such troughs are made of some suitably resistant material. For example, some are made of concrete, others of steel lined with rubber or ceramic tile. Provision is made, usually, in the bottom of the trough, for an electrical connection to the flowing mercury cathode.

The anode assembly of such cells consists of graphite blocks suspended from the cover or top of the cell by means of stems, also made of graphite. These anodes arearranged in rows across the primary cell with the longitudinal axis of the block parallel to the longitudinal axis of the primary cell. The electrical connections to the anodes are made by means of clamps around the part of the stem which protrudes above the cover. The escape of the chlorine through the pores of the graphitestems is prevented by impregnating them with some suitable material.

The cover to the cell, which is made of some suitable corrosion resistant material, in addition to serving as a holder for the anodes also serves as the chlorine collector. Usually this cover is mounted on the trough, i. e., bottom part of the primary cell, in such a manner that it can be raised orlowered and so permit adjustment of the gap between the anode and cathode (called the electrolysis gap) to any desired dimension.

These electrolytic cells are operated in the following manner. Brine usually enters the primary cell at the higher end and flows beneath, around and over the anode blocks toward the lower end, co-currently with the mercury, leaving a space beneath the cover for the collection of chlorine. When a voltage drop is applied across the cell between the graphite anode and the mercury cathode, chlorine and sodium amalgam are produced. The chlorine rises to the top of the brine and is collected beneath the cover and eventually led away. The amalgam flows along the bottom of the primary cell to the secondary cell or decomposer where it is converted by means of water to sodium hydroxide solution, hydrogen, and mercury containing some residual amalgam which is returned to the primary cell. The electrolysed brine leaves the lower end of the cell, is brought back to strength by the addition of salt, treated for removal of impurities, and recycled.

While cells of this type operate reasonably well they, nevertheless, have several disadvantages. For instance, one disadvantage is that chlorine is evolved on the lower surface of the anode. This chlorine tends to form a layer of bubbles because the velocity of the brine in the electrolysis gap is so low that the bubbles are not swept away. This layer of chlorine bubbles obstructs the passage of the current and requires a higher voltage in order to get the required quantity of current to flow. That the presence of bubbles hinders the flow of current is demonstrated by the fact that the anode block wears most at the outer edges probably due to greater flow of current (but possibly to a condition conductive to more rapid oxidation) and it is the outer edges from which the chlorine can escape most readily. Many attempts to facilitate and provide for the removal of these bubbles. such as holes or slots in the anode, have been tried with partial though not complete success. The chief disadvantages of the use of slots or holes are that (l) the effective area of the anode is reduced and (2) the construction of the anode is more costly.

Another disadvantage with anodes of the type used up to now is that of obtaining a satisfactory mechanical and electrical connection between the stem and the anode block. In order to keep the consumption of electric power as' low as possible, it is necessary to keep the voltage drop across this connection as low as possible. The best methods used to date to achieve this involve eX- pensive machining operations, as for instance in the De Nora'cell where the stem and block are threaded and screwed together.

A voltage drop is also present in the stern itself and in the clamp connection at the top. At present such voltage drops are kept to a minimum by increasing the cross section of the stem and using high pressures and corrosion resistant materials for the clamp, etc. But electrical connections of this type are expensive and require considerable labour to install and maintain.

. Still another disadvantage of the type of anode assembly which is used today is that about half of the graphite is wasted. The anode block ultimately wears in a non-uniform manner to such a thickness that it can no longer be used safely or economically and must be discarded. The stem is usually used twice and then it is discarded.

Another disadvantage of the cells which are used today is that there is insuflicient mixing of the brine, i. e., the brine in the electrolysis gap does not become mixed with that flowing above and around the blocks. As a consequence the brine concentration in the electrolysis gap may often reach undesirably low levels, particularly at the exit end of the cell.

Many of these disadvatages are overcome by my invention. This is achieved by placing graphite blocks parallel to each other with their longitudinal axes at right angles to the longitudinal axis of the primary cell. These blocks extend from one side of the cell to the other and are supported on two or more spacer strips extending along the bottom of the primary cell. The thickness of these strips provides the required gap between the anode and the cathode. Between adjacent anode blocks is a narrow space to permit the escape of chlorine. Over each space gap is a hood to collect the chlorine. The anodes, which are coated on their upper surface with some suitable sealing material to prevent the escape of chlorine through the pores of the graphite, together with the hoods form the cover of the cell. The

anodes are so disposed in the primary cell that the brine is prevented from flowing above and around the anodes, but is caused to flow only in the electrolysis gap, i. e., the gap between the anode and the cathode- As a conse quence the brine fiows through the electrolysis gap at a much greater velocity.

As the anode wears away .on its under surface, the electrolysis gap is adjusted by periodically removing the anode and machining the under side so that it is once again a level smooth surface.

My invention can be more fully appreciated and understood by a study of a particular embodiment. Such an embodiment is shown in the drawings; Fig. l is a plan of my new electrolytic cell; Fig. 2 is a section along the longitudinal axis; Fig. 3 is a section across the cell.

The primary cell consists of an elongated trough 1 having a flat U-shaped cross section made of steel, the walls of which are lined with rubber. This trough is gently inclined in a longitudinal direction. At the higher end of the primary cell is a mercury inlet 2 and a brine inlet 3; at the lower end is a brine outlet 4 and an amalgam outlet 5 leading to the secondary cell 6. The graphite anodes 7 which some as part of the cover of the primary cell 1 are placed in the cell parallel to each other with their longitudinal axes at right angles to the longitudinal axis of :the primary cell. These anodes extend to each side of the primary cell. Between adjacent anodes is a space 8 to permit the escape of chlorine from beneath the anode. Along each anode in a shallow groove is a copper distributor cable 9 enclosed in lead It). This cable is attached by means of cable connectors 11 to the cell distribution busbar 12. The spacers 8 between the anodes are covered throughout their length by hoods 13 which'are supported by the anodes 7. These hoods 13 are connected individually by take-oils 14 to the chlorine collector header 15. The anodes 7 are sup ported on strips of hard rubber is running the full length of the cell. The strips 16 serve to support the anodes at the requisite distance from the mercury cathode 17. The space 18 between the anode 7 and the cathode 17 is tilled wtih brine, the level of which in the cell is indicated by 19. The space between the ends of the anodes 7, the hoods 13, and the wall of the cell are sealed with putty as shown at 20. Similarly, the joints between the hoods l3 and the anodes '7 are sealed with putty as shown at 21. Each graphite anode 7 is grooved longitudinally along each side as shown at 22 to receive the hood 13. The upper surface of the anodes is coated with a sealing material 23 to prevent the escape of chlorine through thepores of the anodes.

The advantages of my invention are many.

1. The usual cover is eliminated thereby reducing capital and maintenance costs.

2. The elimination of the stem and connections to the anode block results in a reduction in voltage loss and graphite consumption. The carbon consumption is reduced by having a greater proportion of the graphite available for use in electrolysis. The voltage loss is reduced by eliminating one connection and shortening the path of the current through the graphite.

3. With little or no increase in the quantity of brine from the cell and the concentration of sodium in the mercury stream is more uniform.

6. The assembling and dismantling of this new primary cell can be more easily accomplished than is possible with present cells.

It is obvious that while the above is a preferred form of my invention, there are many variations which it could take without departing from the spirit of my invention.

For instance, the hoodsfor collecting the chlorine may have a variety of shapes. In cross section they might have the shape of an inverted V or of a semi-circle. In addition if desired a single hood may be made to cover several of the spaces between the anodes. The chlorine hoods could also be made to cover one anode block only; be grouted, sealed or gasketed to that block; and sealed or gasketed to each other to contain the chlorine. Furthermore these hoods may be constructed of any one of a vareity of corrosion resistant materials, such as, concrete, rubber lined steel, glass lined steel, etc. The chlorine collection hoods could also be eliminated by using a specially extruded and/ or machined graphite section as combined anode and hood.

The electrical connections need not take the precise form shown in the particular description, but may be made in a variety of ways without departing from the spirit of my invention. For example, the electrical connections might be made by means of prongs from a copper busb-ar extending into holes in the graphite blocks or by clamping the connections around a raised extruded portion.

It is also possible without departing from the spirit of my invention to have the strips which support the anodes made of concrete or porcelain as an integral part of the primary cell.

The sealing material used on the upper surface of the anodes to prevent the escape of chlorine may be one of a variety of materials, such as, pitch, asphalt, rubber, wax, etc.

Having thus described my invention, I claim:

1. An electrolytic cell comprising an elongated trough of U-shaped cross section, inclined in a longitudinal direction to provide for the flow of mercury cathode and electrolyte; electrical connections to said flowing mercury cathode; an anode assembly serving as cell cover, said anode assembly comprising graphite blocks disposed parallel to each other with their longitudinal axes at right angles to the longitudinal axis of the cell, narrow gaps between adjacent blocks for the removal of chlorine therethrough and concrete hoods in gas-tight relation to adjoining blocks forming an enclosed space above each of said gaps for receiving the chlorine, said graphite blocks being supported above the cathode by means of hard rubber strips resting on the bottom of the trough and so arranged that substantially the whole of the under surface of said graphite blocks is in contact with the electrolyte, the exterior surface 'of said graphite blocks opposite to said under surface being coated with a chlorine fed to the cell, the velocity of the brine in the electro* lysis gap is increased several times. in any case, it is increased sufficiently to sweep away the chlorine bubbles from theunder-surface of the anode. The sweeping away of the bubbles effects a considerable reduction in the voltage drop across the electrolysis gap. This permits operation of the electrolytic cell at a lower voltage with the same current, or at a higher current with the same voltage.

4. With the entire quantity of brine in the electrolysis gap, the concentration of the brine throughout the cell is more uniform permitting operation at a lower voltage.

5. With the brine velocity equal to or greater than that of the mercury, the debris is more readily removed impervious composition, said exterior surface of each of said graphite blocks having a shallow groove along its length for receiving electrical conducting means; metallic conductors for supplying electric current enclosed in said grooves, sealed and protected against corrosion.

2. An electrolytic cell comprising an elognated trough of U-shaped cross section, inclined in a longitudinal direction to provide for the flow of mercury cathode and electrolyte; electrical connections to said flowing mercury cathode; an anode assembly serving as cell cover, said anode assembly comprising graphite blocks disposed parallel to each other across the cell, narrow gaps between adjacent blocks for the removal of chlorine therethrough and chlorine receiving hoods along each of said gaps forming an enclosed space thereabove, said hoods being maintained in gas-tight relationship with adjoining blocks by means of a chlorine impervious composition applied along the edges of contact with the blocks, said blocks being supported above the cathode by means of spacers resting on the bottom of said trough and so arranged that substantially the Whole of the under surface of said blocks is in contact with the electrolyte, the exterior surface of said blocks opposite to said under surface being coated With a chlorine impervious composition; and means for conducting electricity to individual anode blocks.

3. An electrolytic cell comprising an elongated trough of U-shaped cross section, inclined in a longitudinal direction to provide for the flow of mercury cathode and electrolyte; electrical connections to said flowing mercury cathode; an anode assembly serving as cell cover, said anode assembly comprising graphite blocks disposed parallel to each other with their longitudinal axes at right angles to the longitudinal axis of the cell, narrow gaps between adjacent blocks for the removal of chlorine theretlu'ough and hoods extending the full length of each gap forming an enclosed space thereabove, said hoods being maintained in gas-tight relationship with adjoining blocks by means of a chlorine impervious composition applied along the edges of contact with the blocks, said blocks being supported above the cathode by means of spacers resting on the bottom of said trough and so arranged that substantially the whole of the under surface of said blocks is in contact with the electrolyte, the exterior surface of said graphite blocks opposite to said under surface being coated with a chlorine impervious composition; and means for conducting electricity to individual anode blocks.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES FIAT Report No. 816, P. B. 33,221, May 15, 1946. Horizontal Mercury Chlorine Cell, pages 21 and 22. Publication of the. Office of Technical Services.

Claims

1. AN ELECTROLYTIC CELL COMPRISING AN ELONGATED TROUGH OF U-SHAPED CROSS SECTION, INCLINED IN A LONGITUDINAL DIRECTION TO PROVIDE FOR THE FLOW OF MERCURY CATHODE AND ELECTROLYTE; ELECTRICAL CONNECTIONS TO SAID FLOWING MERCURY CATHODE; AN ANODE ASSEMBLY SERVING AS CELL COVER, SAID ANODE ASSEMBLY COMPRISING GRAPHITE BLOCKS DISPOSED PARALLEL TO EACH OTHER WITH THEIR LONGITUDINAL AXES AT RIGHT ANGLES TO THE LONGITUDINAL AXIS OF THE CELL NARROW GAPS BETWEEN ADJACENT BLOCKS FOR THE REMOVAL OF CHLORINE THERETHROUGH AND CONCRETE HOODS IN GAS-TIGHT RELATION TO ADJOINING BLOCKS FORMING AN ENCLOSED SPACE ABOVE EACH OF SAID GAPS FOR RECEIVING THE CHLORINE, SAID GRAPHITE BLOCKS BEING SUPPORTED ABOVE THE CATHODE BY MEANS OF HARD RUBBER STRIPS RESTING ON THE BOTTOM OF THE TROUGH AND SO ARRANGED THAT SUBSTANTIALLY THE WHOLE OF THE UNDER SURFACE OF SAID GRAPHITE BLOCKS IS IN CONTACT WITH THE ELECTROLYTE, THE EXTERIOR SURFACE OF SAID GRAPHITE BLOCKS OPPOSITE TO SAID UNDER SURFACE BEING COATED WITH A CHLORINE IMPERVIOUS COMPOSITION, SAID EXTERIOR SURFACE OF EACH OF SAID GRAPHITE BLOCKS HAVING A SHALLOW GROOVE ALONG ITS LENGTH FOR RECEIVING ELECTRICAL CONDUCTING MEANS; METALLIC CONDUCTORS FOR SUPPLYING ELECTRIC CURRENT ENCLOSED IN SAID GROOVES, SEALED AND PROTECTED AGAINST CORROSION.