US3310482A

US3310482A - Electrolytic cell and anode assembly therefor

Info

Publication number: US3310482A
Application number: US222409A
Authority: US
Inventors: Charles K Bon; Richard E Carr; Marshall P Neipert
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1962-09-10
Filing date: 1962-09-10
Publication date: 1967-03-21
Anticipated expiration: 1984-03-21
Also published as: BE692581A

Description

Mwah 2L 11967 ELECTROLYTC CELL AND ANODE ASSEMBLY THEREFOR Filed Sept. l0, 1962 United States Patent 3,310,482 ELECTROLYTIC CELL AND ANODE ASSEMBLY THEREFOR Charles K. Bon, Richard E. Carr, and Marshall P. Neipert,

Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Sept. 10, 1962, Ser. No. 222,409 5 Claims. (Cl. 204-219) This invention relates to electrolytic cells, and particularly to slottype mercury-cathode cells.

A typi-cal mercury-cathode electrolytic cell for making chlorine and alkali comprises a long, narrow trough with a mercury pool cathode at the bottom and graphite anodes suspended from or supported by a rubber-lined cover. The feed brine is ilowed through the cell with very low turbulence. In operation of the cell, the chlorine ions in the brine are attracted to the anode and thus discharged to form chlorine gas, which is usually Withdrawn through an outlet line which leads from the rubber-lined cover. The cation, usually sodium, forms an -amalgam with the mercury. The amalgam is removed from the cell and treated with water in a separate denuder device to form alkali, the mercury being thus regenerated for re-use.

Caustic soda made with mercury-cathode cells is of higher concentration and purity than caustic soda made with diaphragm type cells, but the cost of producing this caustic has heretofore been higher at most installations as compared to the cost of caustic made with diaphragm cells.

Several factors contribute to the high cost of producing caustic soda by means of mercury-cathode cells. One important factor is the high initial cost of mercurycathode cells as compared with the cost of diaphragm type electrolytic cells. Another factor is that conventional mercury-cathode cells of the above-described type operate at relatively low current densities in order to avoid excessive polarization of electrodes and thus occupy considerable building space per unit of chlorine or caustic producing capacity.

Mercury-cathode cells of conventional design have also proven to be quite sensitive to impurities in the brine, thus necessitating that expensive brine treatment facilities be provided in order to remove bothersome impurities. Also, the amount of mercury required for the cathode has been large, and since some of the mercury is lost during the operation of the cell, the mercury has added to both the initial investment and the cost of operation of such cells.

An attempt to overcome some of these diiculties has resulted in what is known as a slot-type mercury cathode cell. Canadian Patent No. 476,519 to Heller and Saunders illustrates and claims a slot-type cell. In slot-type mercury cathode electrolytic cells the mercury cathode usually comprises a thin layer or lm of mercury which is swept through the cell at a high velocity as compared to the rate of ow of the cathode material in a conventional cell as previously described. Also, in many slottype cells, the chlorine is swept 4along the at lower surface of the anode and is removed periodically from vents or is fed into an end box. In general, slot-type mercury cathode cells are capable of operation at considerably higher current densities than are conventional mercury cathode cells.

However, it has been found that when the chlorine is swept along the lower surface of the anode for appreciable distances before being removed that the chlorine bubbles in the brine stream and clinging to the active surface of the anode from a substantial part of the resistance of the cell. The problem has been alleviated to some extent by placing rubber lined vent boxes between anode segments every two to four feet along the cell. Such means are expensive and result in anodes which still are longer than is desirable if the amount of chlorine bubbles in the cell electrolyte lying between the anode and cathode is to be held to small amounts which permit eflicient operation of the cell at high current densities.

Accordingly, a principal object of this invention is to provide an improved anode assembly for use in electrolytic cells.

Another object of this invention is to provide an improved anode assembly which is capable of efficient operation at high current densities in liquid cathode'type electrolytic cells.

A further object of this invention is to provide an improved self-venting anode assembly for use in a liquid cathode type electrolytic cell.

An additional object of this invention is to provide a liquid cathode type ele-ctrolytic cell which is capable of high production of halogen gas per unit area of anode surface, operates at high current densities at relatively low voltages and is relatively simple to construct and maintain.

In accordance with this invention there is provided an electrolytic cell comprising a cathode having a at surface and an anode having a flat surface disposed by means of a separator-gasket in predetermined approximately parallel position with respect to the iiat surface of the cathode. The anode has transversely extending slots in its surface which faces the cathode. The interior of the anode has hollow compartments which communicate with the slots by means of interconnecting bores. The

slots are disposed in pairs of closely adjacent slots sepa' rated from other pairs by a distance usually two or more times the spacing between the individual slots of a pair of slots.

The above and additional objects and advantages of this invention will best be understood when the following detailed description is read in connection with the Iaccompanying drawings, in which:

FIG. l is a simplified side elevational and diagrammatical view of an improved electrolytic cell assembly made in accordance with this invention;

FIG. 2 is a fragmentary longitudinal sectional view of the anode shown in FIG. 1;

FIG. 3 is a fragmentary plan view of the anode shown in FIG. 2; and

FIG. 4 is a sectional view taken along the line 4-4 of FIG. 2.

Refer-ring to the drawings, and particularly to FIG. 1, there is shown a flowing cathode electrolytic cell, indicated generally by the numeral 10, end box 12, amalgam level controller 14, amalgam decomposer 16, chlorine output header 18, brine input line 20, and brine output line 22.

The cell 10 comprises a cathode base plate 24, which extends beyond the cell to also serve as the base of the end box 12, a composite anode assembly, indicate-d generally by the numeral 26, composed of `

anode segments

28, 30, 32, 34 joined together to form a unitary structure, and a separator-gasket structure 36 which maintains the anode spaced and insulated from the cathode base plate and also maintains a liquid and gas tight seal between the anode, cathode and the surrounding atmosphere.

In operation current is applied across the cell with positive electrode terminals 38 on the anode 26 being connected to a positive lead of a direct current potential source (not shown) and a negative terminal 40 coupled to the cathode base plate, the terminal 40 being electrically coupled -to the negative lead of the previously mentioned potential source.

Mercury is pumped by means of the pump 41 and line 44 to the input end 45 of ythe cell 10 and then flows along and covers the top surface of the base plate 24, as is well known, forming the flowing cathode of the cell. Brine entering the cell through the input line 20 is fed into the cell, filling the space 'between the owing cathode and the anode 26. The brine and flowing cathode flow into the end box 12 where the brine (and any chlorine or other gas entrapped the-rin) is withdrawn. The amalgam flows out of the end box through the trap 42, through `the amalgam level controller 14 which maintains the level of the amalgam to provide the desired thickness of the owing cathode in the cell, and into the amalgam decomposer 16. In the decomposer 16 the sodium is released from the amalgam and the mercury then is pumped again into the cell.

The anode 26 is a composite structure which is composed of a plurality of block-like segments which lare joined together to substantially cover the space above the tlowing cathode surface from near the input end 45 of the cell to near the output or end box end 46 of the cell 10. The anode 26 is supported by the separator-gasket 36, as previously mentioned, with the lower or active surface 48 (see FIGS. 2 and 4) Iof the anode lying within the separator-gasket frame 36.

The anode 36 comprises the block-

like graphite segments

28, 30, 32, 34 which are the so-called active or electrolyzing segments plus graphite

edge framing segments

50, 52, 54 and 56 which extend around the perimeter of the anode at its upper part. Graphite cross member framing elements 58, 60, 62, for example, extend across and are bonded to the upper surface of the active segments of the anode, contacting the

graphite edge segments

50, 52. The anode top 64, a plate-like member separated from the rest of the anode structure by the sheet gasket 66, or by rubber coating, for example, is secured to the segments Si), 52, 54, 56, 58, 60 and 62, resulting in a series of enclosed

compartments

68, 70, 72, 74, 76, for example, being formed above the

active segments

28, 30, 32, 34 of the anode.

A series of

pairs

78, 80, 82, S4 and 86, for example, of slots are disposed along the bottom of the

active segments

28, 30, 32 and 34 of the anode. The slots are disposed generally perpendicularly with respect to the longitudinal axis of the cell. At least one pair of slots appears in or in conjunction with each segment or segments of active anode which lies under each of the

compartments

68, 70, 72, 74, 76 mentioned above.

The spacing between adjacent pairs of slots is between inches and 12 inches, with 7 inches being average. The slots of each pair of slots are separated from one another by a distance of between l and 3 inches, with 2 inch spacing being preferred. Communication between each of the slots 88, 90 of the pair of slots 86, for example, and the compartment above it is accomplished by rows of bores 92, 94, respectively, shown in FIGS. 2 and 3. As shown, each row 92, 94 contains 5 bores which are more or less spaced equally distant from one another across the width of an active anode segment. One bore is disposed midway along the length of its associated slots, a bore is at or near the ends of each slot, and a bore is disposed between the center bore and each bore which is near `the end of a slot. The bores 92, 94 are larger in diameter `than the width of the slots with which they communicate. Similar bores are provided in connection with the other slots. See bores 92a which communicate with slot 79 (part of pai-r of slots 78) shown in FIG. 4.

The width .of the slots of fthe various pairs of slots varies between 1A; inch and V16 inch.

Although the slots 88, 90 heretofore mentioned extend upwardly only part way through the anode segment in which it occurs, some slots may be formed by spacing apart two adjacent anode segments by the required amount, as at 100, 102. In such cases, since the slot formed by the spaced segments extends all the way through the active segments of the anode, no communications bores 92, 94 are needed to permit fluid to pass to or from (as the case may be) the compartment above the space between two active anode segments (segments 70 or 74, for example).

The space between the bottom surface 48 of the active anode and the upper surface of the cathode 24 is about 1/s inch. When the active surface of the anode is eroded so that the anode-cathode spacing exceeds 3/16 inch, the spacing is adjusted by lowering the anode.

The anode top 64, made of steel, is, as stated previously, protected from attack by the corrosive atmosphere of brine and chlorine, for example, as by the rubber gasket 66.

Chlorine output vents 104, 106, 10S, 110, for example, which extend upwardly through the anode top and communicate with the chlorine header 18, are either lined with rubber or are made of a material not subject to attack by brine or chlorine.

Electric current is applied to each of the active anode segments 28, 35), 32 or 34 by means of one or

more connector electrodes

38, 112, 114, for example. The connector electrodes, illustrated by the electrode 38 in FIGS. 2 and 3, comprises a more or less cylindrical plug 39 which extends into and contacts the active anode segment 34. A rod extends upwardly from the plug through a bore 116 in the top plate 64 and provides the means to which a buss bar may be attached. The top of the plug is the same height as the cross members 58, 60, 62, and therefore the gasket sheet 66, being compressed between the plug and the top plate, provides `a seal to prevent' the escape of chlorine through the bore 116.

Electric current is coupled to the cathode through the connector 40, previously mentioned.

During operation of the cell brine is pumped through the cell at a rate of between 1/2 to 11/2 gallons per minute per inch of width of the brine ow path across the cell. A preferred brine ow rate is between 3%. and one gallon per minute per inch of width of the brine ow path across the cell.

Current is applied across the cell at the r'ate of 3 amperes to 10 amperes per square inch of active anode surface, e.g. the anode surface facing the cathode surface, the .actual rate being a function of economics.

Bubbles of chlorine which are formed at the lower surface 48 of the anode are swept along with the brine until they come to the rst slot of a pair of slots in the bottom of the anode. They then rise up the rst slot of the pair due to the operating pressure within the cell and also because the g'as entrained brine is lighter than gas-free brine. The operating pressure within the cell is such that substantially no brine flows through the chlorine output headers. Upon entering the compartment above the active segment of the anode, the chlorine separates from the brine and is withdrawn through the

headers

104, 106, 108, 11), for example. The now normally heavy brine enters the anode-cathode space again by passing down the second or downstream slot of the pair. This action is repeated as the brine passes each successive pair of slots as it flows through the cell.

The spacing between the slots of a pair of slots is critical because spacing which is too little or too great may result in the failure of brine to circulate properly in the

compartments

68, 70, 72, 74 and/ or because uneven rates of wearing olf of different parts of the bottom of the active anode segments occurs if the spacing is too small or too great. For example, if the spacing is too small erosion or point discharge phenomena causes the part of the anode segment which lies between the slots of a pair of slots to wear at a more rapid rate than does that part of the lower surface of the anode which lies between pairs of slots. However, if the spacing between the slots of a pair of slots is too great, chlorine bubbles tend to cling to its surface. Since the bubbles are not as readily swept away because of the flow conditions between the slots and because graphite wear is a function of current density which in turn is a function of bubble retention,l

the wear on the lower surface tends to be less than in the spaces between pairs of slots.

If the lower surface of the anode is worn away more in the spaces between pairs of slots than between the slots of a pair of slots, then the lower surface of the anode will have ridges which will prevent the lowering of the anode to maintain the desired anode-cathode spacing for the entire lower surface of the anode.

It has been found that spacing of from 1 to 3 inches between the slots of a pair of slots results in a rate of wearing off of the lower surface of the anode segment between the slots which is about the same as the rate of wearing of the anode surface lying between pairs of slots.

If the erosion of the anode surface lying between the slots of 'a pair of slots is at a rate faster than the wear rate for the rest of the anode surface, the two slots of the pair will be less effective to function as desired, e.g. the downstream slot will be less effective as a means to return the more or less de-gassed brine to the main flow path for the brine as it passes through the cell because of increasingly greater amount of gasied brine will by-pass the upstream slot (as the surface wears) and will thus tend to become an upward flow path rather than a return path for de-gassed brine. Perhaps even more important, though, is the loss in cell operating efliciency as the anode material wears off and a substantial part of the lower surface of the anode is no longer in reasonable spacing with respect to the cathode.

High velocity type owing cathode `electrolytic cells made in accordance with this invention are characterized by high operating eiiiciency, long periods of operation without being shut down for overhauling, and their moderate cost per unit of chlorine producing capacity.

It has ben found that when an anode structure of the type described is used in an electrolytic cell about 95 percent of the chlorine' produced is withdrawn through the anode compartments, with only about 5 percent of the chlorine being recovered from the depleted brine which enters the end box 12.

' What is claimed is:

1. A high velocity type owing cathode electrolytic cell having an input end and an output end and including a composite anode structure, aicathode, means for maintaining said anode structure and cathode in predetermined spaced apart relationship, means for feeding electrolyte into the input end of said cell, and means for removing electrolyte from the output end of said cell, said cathode having a generally planar surface facing said anode structure, said anode structure having a graphite block which has a surface facing said cathode, a top member, said top member being spaced from said graphite block, side and end members, said side and end members joining said graphite and top generally along the periphery of said graphite block to thereby define a compartment between said top member and said graphite block, said graphite block having an array of transversely disposed pairs of slots which extend completely across its surface which faces said cathode communications means extending between each pair of slots and said compartment, said pairs of slots of said array being spaced apart between 5 and 12 inches, the spacing between slots of a pair of slots being between 1 and 3 inches, the width of said slots being between lz inch and D716 inch, and means for withdrawing gas from said compartment.

2. An electrolytic cell in accordance with claim 1, wherein said communications means comprises at least one bore extending between a slot and said compartment.

3. An electrolytic cell in accordance with claim 1, wherein said spacing between pairs of slots is 7 inches.

4. An electrolytic cell in accordance with claim 1, wherein said spacing between slots of a pair of slots is about 2 inches.

5. An electrolytic cell in accordance with claim 1, wherein said communications means comprises an array of spaced apart bores extending between each slot and the compartment above it.

References Cited by the Examiner UNITED STATES PATENTS 3,062,733 11/1962 Lynn et al 204-220 JOHN H. MACK, Primary Examiner.

B1 CURTIS, D. R. JORDAN, Assistant Examiners,

Claims

1. A HIGH VELOCITY TYPE FLOWING CATHODE ELECTROLYTIC CELL HAVING AN INPUT END AND AN OUTPUT END AND INCLUDING A COMPOSITE ANODE STRUCTURE, A CATHODE, MEANS FOR MAINTAINING SAID ANODE STRUCTURE AND CATHODE IN PREDETERMINED SPACED APART RELATIONSHIP, MEANS FOR FEEDING ELECTROLYTE INTO THE INPUT END OF SAID CELL, AND MEANS FOR REMOVING ELECTROLYTE FROM THE OUTPUT END OF SAID CELL, SAID CATHODE HAVING A GENERALLY PLANAR SURFACE FACING SAID ANODE STRUCTURE, SAID ANODE STRUCTURE HAVING A GRAPHITE BLOCK WHICH HAS A SURFACE FACING SAID CATHODE, A TOP MEMBER, SAID TOP MEMBER BEING SPACED FROM SAID GRAPHITE BLOCK, SIDE AND END MEMBERS, SAID SIDE AND END MEMBERS JOINING SAID GRAPHITE AND TOP GENERALLY ALONG THE PERIPHERY OF SAID GRAPHITE BLOCK TO THEREBY DEFINE A COMPARTMENT BETWEEN SAID TOP MEMBER AND SAID GRAPHITE BLOCK, SAID GRAPHITE BLOCK HAVING AN ARRAY OF TRANSVERSELY DISPOSED PAIRS OF SLOTS WHICH EXTEND COMPLETELY ACROSS ITS SURFACE WHICH FACES SAID CATHODE COMMUNICATIONS MEANS EXTENDING BETWEEN EACH PAIR OF SLOTS AND SAID COMPARTMENT, SAID PAIRS OF SLOTS OF SAID ARRAY BEING SPACED APART BETWEEN 5 AND 12 INCHES, THE SPACING BETWEEN SLOTS OF A PAIR OF SLOTS BEING BETWEEN 1 AND 3 INCHES, THE WIDTH OF SAID SLOTS BEING BETWEEN 1/8 INCH AND 3/16 INCH, AND MEANS FOR WITHDRAWING GAS FROM SAID COMPARTMENT.