US3344053A

US3344053A - Chlorine cell

Info

Publication number: US3344053A
Application number: US364565A
Authority: US
Inventors: Marshall P Neipert; James J Leddy
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1964-05-04
Filing date: 1964-05-04
Publication date: 1967-09-26
Anticipated expiration: 1984-09-26

Description

2 Sheets-Sheet 1 INVENTORS.

Arrow/EY Sept. 26, 1967' M. P. NEIPERT ETAL CHLORINE CELL Filed May 4, 1964 lm e r m, @J W w d P o n e c o wf /fm c o @e M m .z a e o C f m a. D D C 5 C o Screen Marsha/ l? Neie/ Jam es J. L eddy Sept. 26, 1967 M. P. NEIPERT ETAL 3,344,053

cHLo'RINE CELL Filed May 4, 1964 2 sheets-sheet 2 United States Patent() 3,344,053 CHLORINE CELL o Marshall P. Neipert and .lames .1. Leddy, Midland, M1ch., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed May 4, 1964, Ser. No. 364,565 4 Claims. (Cl. 204-266) This invention relates generally to a diaphragm-type electrolytic cell having a cathode screen and used for the production of chlorine and, more particularly, is concerned with such a cell having a second screen, that is, an insulated grating or screen positioned adjacent to and substantially parallel to the diaphragm, on the cathode screen side thereof.

The second screen, mentioned above, will be hereinafter referred to as the secondary screen.

Chlorine is ordinarily produced by the electrolysis of the chloride of an alkali metal, e.g., sodium chloride, from an aqueous solution in an electrolytic cell. The chlorine is collected at the anode and hydrogen and a solution containing the corresponding alkali metal hydroxide, e.g., sodium hydroxide, at the cathode.

In conventional electrolytic cells, it is known to place a diaphragm between the anode and cathode in an effort to prevent products formed on the surfaces of the cathode from migrating toward the anode. The diaphragms are made ordinarily from asbestos in the form of paper, woven fabric or pulp made to adhere to a cathode. Ordinarily, the cathode is formed of perforated metal or wire mesh and the asbestos, from a thick aqueous slurry, is drawn against the perforated cathode by -means of suction and thereafter made to adhere thereto by drawing out most of the remaining water and partially or completely drying the asbestos insitu. The use of such diaphragms, however, has not been very successful in preventing the back migration to the anode of products formed on the cathode. In fact, there is a build-up of hydroxyl ions around the cathode and within the diaphragm when the diaphragm is imposed directly on the cathode surface. The hydroxyl ion produced on the cathode surface by the decomposition of water is, due to its negative charge, attracted back to the anode. The result is an increase of the pH value in the anolyte chamber of the cell and the subsequent attack of the hydroxyl ions by chemical and/ or electrochemical processes on the anode (which ordinarily is graphite). The carbon anode is thus consumed to carbon dioxide. The cell operating life is markedly reduced, the current efiiciency of the cell is reduced and its acid consumption is increased.

We have unexpectedly discovered an improvement in such electrolytic chlorine cells which substantially reduces the back migration, to the anode, of products formed at the cathode thereby increasing the cell current efficiency due to redu-ction of cathode products entering and lost in the anolyte chamber. In addition, our improved electrolytic cell which is operated at a controlled pH, has decreased acid consumption, increased life in the graphite anode and decreased maintenance and replacement costs of the cell in general.

Thus, it is an object of the instant invention to provide an improved diaphragm-type chlorine cell which utilizes two screens, one a cathode screen and the second an electrically neutral screen on which the diaphragm is placed.

An additional object is to provide an improved diaphragm-type chlorine cell which has decreased acid con- Patented Sept. 26, 1967 ICC sumption, increased anode life and generally increased operating life over conventional cells used prior hereto.

Other uses, objects and advantages of the instant invention will become apparent from reading the detailed description thereof disclosed hereinafter in conjunction with the drawings.

In the annexed drawings, FIGURE l is a schematic side elevational view of a conventional chlorine cell containing a diaphragm.

FIGURE 2 is a schematic side elevational view of one embodiment of the instant invention.

FIGURE 3 is an enlarged horizontal section of the cell of FIGURE 4 on line 4-4 of FIGURE 3.

FIGURE 4 of the drawing is a side elevational view of one form of the cell of the invention taken on line 3--3 of FIGURE 3, and

FIGURE 5 is a fragmentary sectional view showing a folded cathode-diaphragm assembly in accordance with this invention.

FIGURES 1 and 2 in the drawings, in accordance with the instant invention, depict an improved electrolytic cell for the production of chlorine in accordance with the present invention having a diaphragm therein of a known material, eg., asbestos, vertically positioned in the cell between the anode and the cathode so as to divide the cell into an anolyte and a catholyte compartment. The improvement of the present invention as shown in the cells shown in FIGURES 1 and 2 consists of a protectiveperforated sheet or meshed grating, i.e., secondary screen, having the exposed surfaces coated with a nonconducting, electrically insulating, resinous material inert to the reactants of the cell, said sheet or screen being positioned substantially parallel and adjacent to or abutting said diaphragm on the cathode side thereof, between the diaphragm and cathode screen. Thus, the diaphragm is held away from the cathode screen itself. In a cell constructed in this manner, the cell products formed on the cathode screen are substantially prevented from owing through the openings in the cathode screen and migrating to the anolyte compartment.

Referring to the drawings in more detail, there is shown in FIGURES 3 and 4 a cell of the invention having concrete body structure 10', bottom 12, top 14, front wall 16, -rear wall 18 and opposing

parallel side walls

20 and 22. Openings are provided in concrete walls 16- and 18.

Vertically positioned in structure 10 just inwardly ofthe opening provided therefor in wall 18 is metal cathode 24 which has flattened border 26 thereabout. Adjacent to cathode 24 on the anode side thereof is a novel protective coated grating or screen 28 coated with an elec-trically insulated coating 30, e.g., polytetrauoroethylene. Adjacent to and substantially following the contour of said novel screen is asbestos diaphragm 32 which separates the cell into anolyte compartment 34 and catholyte compartment 36. The novel screen 28 is thereby positioned between cathode 24 and asbestos rscreen 3-2.

Covering the opening in wall 18, as defined by wall edge 38, is plate 40. Plate 40' has ridge 42 thereon which follows the general outline of wall edge 38 and which is raised inwardly toward border 26 of cathode 24. Studs 46 secure plate 40 to wall 18 and also hold ridge 42 of plate 40 in firm contact with border 26 of cathode 24. To plate 40 is attached terminal 48 which in turn makes contact with negative lead-in line 50` attached to a source of direct current. Plate 40, therefore, serves as both the outer enclosing wall of catholyte compartment 36 and a bus bar for carrying negative electricity to cathode 24.

Graphite block S2 is inserted in the opening provided therefor in wall 16. Graphite block 52 forms both outer wall of anolyte compar-tment 34 and a portion of the anode of the cell. Perpendicular to block 52 are horizontal extensions 54 which, together with block 52, constitute the anode. Terminal 56 is secured to anode block 52 by means of contact bar 58 and has positive lead-in line 60 attached thereto.

Inlet pipe 62 in bottom 12 provides for the ingress of an aqueous solution of an alkali chloride, referred to hereinafter as brine, into anolyte compartment 34. Outlet 64 in top 14 provides for an escape of chlorine gas evolving at the anode. In plate 40 is pipe 66 for egress of the alkali hydroxide and outlet 68 for escape of hydrogen gas and other gases, if any, from catholyte compartment 36. Gasket '70 provides a snug tit between outlet 64 and the opening provided therefor in top 14. Gasket 72 provides a snug fit between the plate 40 and wall 18 about the opening defined by wall 38.

In operating the cell of the invention, a brine consisting essentially of a concentrated salt solution, for example, about 31S grams of sodium chloride per liter of water, is pumped into anolyte compartment 34 through inlet 62. The electrolysis of the brine is started by setting up an electrical potential between anode body d and cathode 24 by attaching lead-in line 60 to the positive take-oit of a D.C. source of electricity (not shown) and lead-in line 50 to the corresponding negatice take-olf of the same source of electricity. The brine is permitted to rise in anolyte compartment 34 until it is at about level 76 which is below that which would result in an overflow out through outlet 64.

The brine comes into contact with the anode comprising block 52 and extension 54 forming chlorine, and cathode 24 forming ultimately sodium hydroxide and hydrogen gas. The chlorine thus formed rises and passes out through outlet 64. The alkali hydroxide, referred to as the caustic, eg., sodium hydroxide, thus formed, together with water from the brine and some undecomposed salt, flows to the bottom of the catholyte compartment 36l and passes out through outlet 66. The hydrogen rises and passes out through outlet 68.

The novel coated secondary screen 28 reduces back migration of hydroxyl ions from the cathode to the anode. In addition, the instant coated secondary screen prevents both the combining of chlorine and hydrogen, which in the absence of separatory means, often takes place with explosive violence and prevents the reaction between alkali hydroxide and chlorine, which would result in the formation of the hypochlorite and later the chlorate. The chlorate, if formed, would attack the carbon of the anode to form CO2, cause excessive graphite consumption, contaminate the gaseous products with CO2 and thereby lessen the efficiency of the cell. Such loss of eciency is hereby prevented by the structure of the cell of the invention.

The cathode, as is commonly employed in conventional chlorine-producing cells, is formed of a metal screen. It may comprise a substantially flat screen or it may comprise a series of sinuous repeating folds or ngers, each fold or nger being in a vertical plane and about 2 to 3 inches from center to center of each fold. If the cathode is comprised of a series of folds, the novel protective screen of the invention is shaped so that it follows the contour of the cathode.

The protective or secondary screen used herein is not unlike that protective screen defined in U.S. Patent No. 2,944,956 entitled Chlorine Cell Having Protected Diaphragms by Blue and Neipert, and `is substantially resistant to the action of both -the alkali hydroxide and the wet chlorine. The screen has openings in it which preferably comprise at least half of its area and consists preferably of (l) an inner structural material which is cornposed of either strands, bers, filaments or cords which may be derived fnom either natural .or synthetic sources,

and (2) an outer adhesive coating consisting of a polymer which renders the screen non-conductive of electric current and highly resistant to chemical attack and to the excessive absorption and softening effect of caustic, acidic or salt solutions.

In making the secondary screen, structural materials suitable for use include, but are not limited to, steel, iron and titanium. This material is fabricated into an interwoven or meshed configuration and thereafter treated with an aqueous dispersion of the polymer as by dipping, spraying or brushing application and thereafter cured in accordance with the recommended procedure for such polymer. As an alternative practice, the individual untreated bers may be treated with the polymer and cured prior t-o fabrication and thereafter fabricated into the screen of the invention.

As an alternative construction, the protective or secondary screen may consist of a pressed or molded sheet of non-conducting and chemically resistant polymer. Coatings suitable for use herein include halolluorinated polymers and synthetic rubbers. The preferred material is a polymerized fluoroor uoro-chloro-substituted ethylene resin. Examples of the polymers that can be used herein include polytetratluoroethylene and polymonoclorotriuoroethylene. Other satisfactory polymers are electrically non-conducting synthetic rubbers.

The cell is assembled in the conventional manner. The asbestos diaphragm is iirmly adhered to the anode side of the secondary screen.

The instant chlorine cell may employ conventional anod-es, e.g., carbon anod, in the form of solid blocks or in the form of shapes rmly attached together.

The following example is illustrative of the instant invention and in no way is meant to limit it thereto.

EXAMPLE An improved chlorine cell assembled in accordance with the instant invention and consisting of a l2 square inch graphite anode, an anolyte Compartment, a polytetraiiuoroethylene-coated steel screen (i.e., the secondary screen) on which a slurry of asbestos (i.e., the diaphragm) was drawn, a catholyte compartment and a cathode screen utilizing sodium chloride as the electrolyte was run for several weeks at C. A similar conventional chlorine cell (i.e., a standard cell) which did not have a secondary screen was run along with Ithe improved cell. Samples of anolyte liquor were taken periodically from both cells and the pI-I value was determined. At a caustic strength 4of 102 grams of NaOH/ liter, the anolyte pH in the cell containing the secondary screen was 1.8 wherein it was 2.8 in the standard cell. Table l below shows the caustic concentration in the new cell and in the standard cells at various pH values in the anolyte compartment of each cell.

Talble 1 Grams NaOHfLlter Anolyte PolytetrapH uoro- Standard ethylene Screen (Improved Cell) Table 2 b-elow shows the pH in the anolyte compartment of the improved cell having the secondary screen and the standard cell operating at various caustic concentrat-ions.

Thus, it is seen that the improved chlorine cell utilizing the secondary screen has a markedly decreased pH value in the anolyte compartment than does the standard cell at various caustic concentrations nreach of the two cells. In addition, the improved cell can operate at markedly greater caustic concentrations than; can the standard cell and still has about the same pH Value in the anolyte compartment as does the standard. These results are explained by the fact that the improved cell utilizing the secondary screen substantially curtails back migration to the anode of hydroxyl ions formed at the cathode screen.

In a similar manner to that described in the foregoing example, improved cells utilizing monochlorotriuoroethylene, rubber, a magnetite-coated screen or screens made entirely from polytetrauoroethylene or monochlorotriuoroethylene can be used to curtail back migration of hydroxyl ions and thus increase cell life.

Various modifications can be made inthe instant invention without departing from the spirit or scope thereof, for it is to be understood that we limit ourselves only as defined in the appended claims.

What is claimed is:

1. In an improved diaphragm-type chlorine cell for the electrolytic decomposition of a chloride brine in the production of chlorine gas, said cell containing a cathode, anode and non-conducting diaphragm which divides the cell into an anolyte compartment and a catholyte compartment, the improvement comprising a substantially rigid screen composed of meshed filaments positioned parallel to and adjacent to said diaphragm on the cathode side thereof between the diaphragm and the cathode, the filaments of said screen being coated with a non-conducting, resinous, protective coating which is inert to the reactants of the cell.

2. The improved diaphragm-type chlorine cell as dened in claim 1 wherein said protective coating is a halouorinated polymer.

3. The improved diaphragm-type chlorine cell in accordance with claim 2 wherein said halouorinated polymer is a member selected from the group consisting of polymerized monochlorotriuoroethylene and tetratluoroethylene.

4. An improved cell for electrolysis of brine comprismg:

(a) a cell chamber,

(b) an anode within said chamber,

(c) a metal cathode Within said chamber having openings therein for the passage of fluids therethrough;

(d) a duid-permeable electrically non-conductive diaphragm dividing said chamber into a catholyte compartment and an anolyte compartment; and

(e) a structurally self-supporting substantially rigid perforate sheet of polymer selected from the group consisting of polytetraiuoroethylene and polymonochlorotrifluoroethylene positioned substantially parallel and adjacent to said diaphragm in the catholyte compartment between the diaphragm and the cathode.

References Cited UNITED STATES PATENTS 548,162' 10/ 1895 Hargreaves et al 2011-283 2,944,956 7/ 1960 Blue et al. 204-266 FOREIGN PATENTS 15,759 11/ 1915 Great Britain.

I OHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.