US3403083A

US3403083A - Operation of chlor-alkali cells

Info

Publication number: US3403083A
Application number: US510225A
Authority: US
Inventors: John E Currey; Rutherford John; Dudley P Fernandes; Robert J Leonard
Original assignee: Hooker Chemical Corp
Current assignee: Occidental Chemical Corp
Priority date: 1965-11-29
Filing date: 1965-11-29
Publication date: 1968-09-24
Anticipated expiration: 1985-09-24

Description

'.J. E. CURREY ETAL OPERATION OF' CHLOE- ALKALI CELLS Sept. 24, 1968 `5 Sheds-Sheet l Filed NOV. 29, 1965 sept. 24, 196s J E, CURREY- TAL 3,403,083

OPERATION OF CHLORALKALI CELLS Filed Nov. 29, 1965 5 Shees-Shee(l 2 ODG-J .Emu Z :Coz mmh: mmm mwm@ 0mm :@WN ONN Oom ow.

5 Sheets-Sheet 5 J. E. cuvRRr-:Y ETAL OPERATON OF CHLOR-ALKALI CELLS l sept. 24, 1968 Filed NOV. 29 1965 @Nm oo. n

United States Patent O 3,403,083 OPERATION F CHLOR-ALKALI `CELLS John E. Currey, Niagara Falls, N.Y., John Rutherford, Luliug, La., Dudley P. Fernandes, Montague, Mich., and Robert J. Leonard, Niagara Falls, N.Y., assignors to Hooker lChemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Nov. 29, 1965, Ser. No. 510,225 20 Claims. (Cl. 204-98) This invention relates to chlor-alkali diaphragm cells and more particularly to a method of operating chloralkali diaphragm cells alone or in groups under controlled conditions of pH, anolytesalt concentration and temperature, to thereby increase the efficiency of such cells, to control the proportions of chlorine and caustic soda produced, to reduce anode consumption, to produce a cell liquor of increased caustic soda content and to effect numerous other improvements in cell operations, as hereinafter disclosed.

Chlor-alkali diaphragm cells have for years been run in large groups of 50 to 100 or more cells, each cell running independently of the other cells. The products from these cells were combined into three major eflluent streams so that the total groups of cells produce an etiiuent stream of chlorine, an eiiluent stream of hydrogen and an effluent stream of cell liquor. The cell liquor was a mixture of about 9 to 12 weight percent caustic and about 10 to 18 weight percent salt.

With each cell running individually, only very limited control could be maintained on the operation of the cell such as could be achieved through changes in the decomposition voltage Iand regulation of the brine concentration and feed rates which in turn, depended largely on the porosity and flow rate through the diaphragm. This resulted in each cell operating at different eiciencies and under different conditions of temperature, anolyte concentration, pH, and so forth. As an example, cells having new anodes maintain an operating temperature which is lower than the most etlicient operating temperature. In turn, cells having old, worn anodes tend to operate at ternperatures above the most efficient operating temperature and often result in the expenditure of excessive amounts of current in heating the electrolyte to its boiling point.

The major items of 'expense in operating diaphragm cells are (l) power, (2) cell renewal and (3) caustic evaporation. All of these items are directly related to the cell operation in (1) 'current efliciency, (2) cell voltage, (3) anode life, (4) diaphragm life and (5) cell liquor caustic and salt concentration. If the current efficiency and cell voltage could be advantageously controlled to reduce power costs and extend the anode life, and if the diaphragm life could also be extended, two of the major cost items in operating diaphragm cells would be reduced. Further, if in advantageously controlling the first four factors enumerated above, the cell liquor caustic and salt concentrations could be changed to a more advantageous ratio, then all three of the major cost items in operating diaphragm cells could be reduced, thereby greatly improving chlor-alkali diaphragm cell operations.

It is an object `of this invention to provide a method whereby the current efficiency of Chlor-alkali diaphragm cells is improved. It is another object of this invention to provide a method whereby chlor-alkali cell voltages are advantageously improved to -operate at the most efficient level. A further object of this invention is to provide a method whereby the anode life of Chlor-alkali diaphragm cells is extended. Yet, another object of this invention is to provide a method of operation whereby the porosity of the diaphragm becomes less critical, so that the diaphragm life can be extended to equal the anode life. Another -object of this invention is to improve the caustic-salt con- 3,403,083 Patented Sept. 24, 1968 centration ratio in the cell liquor by producing a higher concentration of caustic in the cell liquor. These and other `objects will become apparent to those skilled in the art from the description of the invention which follows.

In accordance with the invention, a process is provided for operating a group of Chlor-alkali diaphragm `cells comprising imposing a decomposition volt-age across the electrodes of a group of Chlor-alkali diaphragm cells, feeding a solution of brine to the anolyte compartments of each of said cells at a rate in excess of the amount which ows through the diaphragm of said cells, withdrawing the excess brine feed solution from the anolyte compartments of said cells, combining and replenishing the withdrawn solutions with additional amounts of salt and returning the replenished anolyte brine solution to the anolyte compartments -of said cells. Preferably, the feed brine is a saturated or nearly saturated solution. In this anolyte recirculation method, the feed rate of brine to the anolyte compartment is greater than the amount which flows through the diaphragm and preferably is in an amount greater than one time and up to about ten times the amount which ows through the diaphragm into the catholyte compartment. In addition to operating a series of cells with the brine feed in parallel, the process can also be readily effected with each cell on an individual basis. Even further, improvements are obtained wherein temperature and pH adjustments, as by the addition of acid, are made with the anolyte solution in the cell.

The anolyte recirculation method of the present invention provides greatly improved control of the anolyte liquor to thereby provide the highest operating etiiciencies in Chlor-alkali diaphragm cells. The process can be operated with a group 4of cells, thereby establishing controlled conditions of temperature, anolyte pH, anolyte and cell liquor concentration, and the like, which are substantially the same in all of the cells, thereby effectively operating all or nearly all of the cells under the most desirable conditions.

The process of the present invention can -be used in the electrolysis of any alkali metal chloride. However, because sodium chloride is preferred and is normally the alkali metal chloride used, the description hereinafter is directed more particularly to sodium chloride. It is to be understood, however, that other alkali metal chlorides can be used, particularly potassium and lithium chlorides.

The invention will be further described by reference to the drawings in which:

FIG. 1 is a partially schematic flow sheet illustrating the process of the present invention, particularly as it relates to operation of a group of cells;

FIG. 2 is a vertical, partial section of a typical chloralkali cell modified in accordance with the present invention;

FIG. 3 is a side elevation of FIG. 2 further showing a typical modification of the Chlor-alkali cell; and

FIG. 4 is a graph showing the average 4relationship between cell liquor strength, current efliciency and rate lof HC1 addition for chlor-alkali diaphragm cells operated in accordance with the present invention.

It has been found that the anolyte salt concentration has an effect on the current efficiency. Normally, increasing the chloride concentration in the anolyte compartment results in benefits of higher current efficiency, purer chlorine, lower voltage, lower graphite consumption, higher caustic concentration and less chlorate in the cell liquor so that it is normally preferred to operate at the highest salt concentration. However, the solubility of sodium chloride in the feed brine limits the amount of sodium chloride which can be practicably fed to a normally operated cell.

Since both chlorine and sodium ions are being removed from the cell at the electrodes, the bulk anolyte solution becomes depleted of salt to such an extent that a normal cell has an anolyte salt concentration considerably below the saturation point, even though the ybrine was fed to the cell as a nearly saturated solution. Typically, a normal diaphragm cell was fed with a brine solution containing about 320 to 330 grams per liter of sodium chloride (a saturated solution at about 90 degrees centigrade contains about 333 grams per liter) but has an anolyte sodium chloride concentration of only about 270 grams per liter. Better cell operation is obtainable at higher anolyte salt concentrations.

It has now been found that by the present anolyte recirculation method, the advantages of higher sodium chloride concentration in the anolyte compartment can be realized by feeding a stream of Ibrine to the anolyte compartment at a rate faster than the brine can pass through the diaphragm into the catholyte compartment. Thus, it is possible to increase the sodium chloride concentration in the anolyte compartment by continuously adding concentrated brine and removing depleted anolyte solution. The excess brine is withdrawn from the anolyte compartment and is resaturated with sodium chloride. Also, it is preferred to adjust the temperature to the desired operating range and to add H-Cl in an amount to obtain the desired anolyte pH prior to returning the brine to the anolyte compartment. The practical over-all result is that a leveling effect is obtained in all of the cells and the sodium chloride concentration in the anolyte compartment is increased to a level higher than that previously obtainable. By the present method, the sodium chloride concentration in the anolyte compartment can be maintained at any level up to the saturation concentration and particularly may be maintained within the preferred range of 260 to 330 grams per liter of sodium chloride. Because the recirculation of the anolyte liquor without an enrichment of sodium chloride results in substantial improvements in the operation of Chlor-alkali cells, the present invention can also be operated with lower NaCl concentrations, such as about l30 to 260 grams per liter of NaCl.

The process of the present invention is effected, as illustrated by FIG. l, by feeding a concentrated stream of feed liquor to a group of chlor-alkali diaphragm cells 16 `by means of

lateral feed lines

12, 13, 14 and 15. The group or series of cells may be 2 to 100 or more cells from which anolyte liquors are Withdrawn and combined for recirculation. The feed rate of brine to the cells 16 by means of the

lateral lines

12, 13, 14 and 15, is at a rategreater than the amount of liquor which flows from the anolyte compartment through the diaphragm in the chlor-alkali cell into the catholyte compartment and more preferably, the brine feed rate is 1.5 times up to about ten times the flow through the diaphragm. The most preferred flow rate averages, for a group of cells, is about two to five times that flowing through the diaphragm. The excess feed liquor is withdrawn from the cells 16 via

lines

18, 20, 22 and 24. These lines are combined and returned to salt saturator 28 via line 26. Cell liquor 40 is withdrawn from the catholyte compartments of the cells via

lines

35, 36, 37 and 38 using suitable withdrawal means.

In salt saturator 28, additional sodium chloride 30 and Water 31, or mixtures thereof, are mixed with the brine to resaturate it with sodium chloride prior to returning the brine to the anolyte compartments of the cells 16 via line 10. Normal salt saturation techniques are used in the salt saturation step. In addition to resaturating the withdrawn anolyte liquor, sufficient additional brine is prepared or mixed with the anolyte liquor to replace brine which passes through the diaphragm in the electrolytic cell.

Another variable `which affects the anode current efficiency is the anolyte temperature. The provisions for heat exchange means 32 associated with brine saturator 28, as shown, or other heating means, 'provide for maintaining the cells at the most eficient operating temperatures. Heat exchange means 32 maintains the vsaturator 28 at the proper temperature for saturating the recirculating anolyte solution so that the maximum practical salt concentration is supplied to the cells. Normally, saturated brine fed to `the cells contains about 26 to.27 percent NaCl by .weight or about 327 grams per liter'of NaCl, which is the saturation concentration at about 65 degrees centigrade. Additional heat is provided after the saturator to superheat the brine to a temperature of approximately to 80 degrees centigrade to prevent the deposition of salt crystals in the feed lines to the cells. 'This latter temperature (superheat) is regulated so that the temperature of the anolyte in the cell is maintained between about and 100 degrees centigrade by the additional heat provided by the electrochemical reaction taking place in the cell.

Alternatively, the saturator can be operated at a higher temperature, such as 75 to 8O degrees centigrade, and a small stream of unsaturated brine or water may be added after the saturator to reduce the salt concentration in the brine to about 327 grams per liter to prevent salt drop out in the lines to cell. Again it is preferred that the temperature of the saturated brine to cell Ibe regulated so that the operating temperature in the anolyte compartment of the cell is maintained at the most preferred temperature of about 93 to 100' degrees centigrade.

In an operating group of electrolytic cells, the amount of heat required by heat exchanger 32 varies primarily with the requirements to heat the additional water or brine added in the salt saturator 28. The heating of brine prior to feeding it to the cell is not in itself, new. However, the effect of rapid anolyte turnover and the mixing of the anolyte effluents from a group of cells produces a cumulative heat exchange effect which results in all of the cells operating at more efficient temperatures independent of the cell age, electrode decomposition, particular cell characteristics and the like factors which previously dictated the individual cell operating temperature.

As a result of the changes effected in the anolyte compartment by means of the present anolyte recirculation method, further beneficial changes result in the entire cell operation. It was previously known that the addition of hydrochloric acid to a chlor-alkali diaphragm cell having a porous asbestos diaphragm resulted in a tightening of the diaphragm and a restriction of liquid flow through the diaphragm when the anolyte pH dropped to too low a level. When this occurred, the liquid level in the anolyte chamber increased and often the cell would have to be removed from service due to the high level. Because of the variations in porosity of deposited diaphragms and the changes effected by acid additions, the flow rates through the diaphragms varied with each cell such that previously the brine feed had to be individually controlled in each cell to maintain the desired cell liquor strength to compensate for the added acid. To regulate the anolyte pH Within the most desirable pH range of 2 to about 4 while maintaining a proper cell level was indeed, a difficult task. To further complicate the matter, changes in the flow through the diaphragm affect the back migration of hydroxyl ions which changes the acid requirement for each cell. Thus, as a practical matter, large acid additions have not previously been feasible in large scale operations.

The present method of anolyte recirculation substantially reduces the need for individual cell attention due to changes in diaphragm porosity, hydroxyl ion back migration, and the like. The rapid anolyte turnover or flow rate produces a leveling effect in all the cells, whereby the desired pH range is maintained independent of the particular porosity of the diaphragm and back migration. In addition, the effects of a restricted diaphragm are of lesser importance because the anolyte recirculation method maintains the same anolyte liquid level independent of the flow through the diaphragm.

In a preferred embodiment of the present invention, hydrogen chloride 34 is added to the saturated or nearly saturated brine withdrawn from salt saturator 28 via line 10. When HC1 is added, it is added in an amount suflicient to maintain an anolyte pH within the range of about 0.2 to about 4.5 and more preferably about 1.5 to 4. The most preferred pH range is about 2.0 to 3.0. The lowest pH- values are best used with a diaphragm material other than asbestos, such as chlorinated polyvinyl chloride, polypropylene, and the like. The amount of HC1 required for this adjustment varies with the particular operating conditions and can be in amount up to about 20 percent HCl based on the amount of chlorine liberated at the anode; that is, 20 percent of the chlorine produced is from the HC1 addition. With greater amounts of HC1 being added, the pH of the brine fed to the cell can be as low as about 0.2. When no HCl is added, the pH of the brine fed to the cell is as high as about 7, because the recirculated anolyte lowers the brine pH from the normally alkaline pH o-f about 9 to a neutral or slightly acidic pH. The HC1 added can be added either as a gas or as an aqueous solution.

The pH of the anolyte has been found to be important in establishing high current efliciencies in the cell, and especially in attempting to improve the efliciency of already highly eiiicient cells. In normal cell operations, the back migration of the hydroxyl ions into the anolyte results in an increase in the anolyte pH while the chlorine evolved therein lowers the pH. Cells running individually will vary widely in anolyte pH. Normally, a low anolyte pH is obtained in cells with new diaphragms and a high anolyte pH is found in cells with older diaphragms. As the mechanism of back migration of hydroxyl ions is presently understood, the migration increases for any particular diaphragm as the concentration of caustic in the catholyte cell liquor increases. In turn, the concentration of caustic in the catholyte cell liquor increases because of a decrease in the ow of brine into the catholyte chamber as a result of a decrease in the porosity of the diaphragm. The decrease in the `diaphragm porosity results from the deposition of calcium and magnesium compounds and other substances in the diaphragm pores during use. Thus, over-all brine quality and the nature of the diaphragm are factors which bear significantly on the changes in anolyte pH and its attendant lower cell elliciency.

In normal cell operation, when the anolyte pH ncreases for any reason, there is no built-in compensating effect to keep it at its proper value. The present invention provides the means for maintaining anolyte pH within the desired range by (1) recirculating the anolyte from a group of cells to obtain the cumulative effect of the anolyte pH of all of the cells so as to result in the cells operating at a pH which is the average thereof and/or by (2) the addition of HC1 to the brine feed, with anolyte recirculation. Thus, the cell can always be kept operating at the most effective pH for peak elliciency substantially independently of the porosity of the diaphragm and the concentration of the caustic in the catholyte chamber.

Being able to use a tight diaphragm or a diaphragm of lower porosity has the added benefit of enabling cell operation at higher caustic concentrations in the catholyte compartment. Whereas previously the catholyte solution (cell liquor) contained about 9 to 12 percent sodium hydroxide, caustic concentrations can now be in the range of 12 to about 22 percent in the catholyte cell liquor or about 145 to 270 grams per liter of NaOH, while the desired anolyte pH is maintained. As Will be readily realized, the process can also be operated to obtain normal cell liquor strengths of about 110 to 150 grams per liter of NaOH. By controlling the anolyte pH at the desired elicient operating level, such as by increasing the recirculation rate and/or using brinewith enough HC1 dissolved in it to compensate for increased back migration of the hydroxyl ion, the former limiting factor of the hydroxyl back migration is mitigated. By operating the cells to increase the caustic concentration in the catholyte compartment a higher ratio of caustic to sodium chloride in the cell liquor is obtained. Thus, over twice the normal caustic concentration can be obtained in the cell liquor, thereby greatly reducing the evaporation and concentration costs normally otherwise incurred.

FIG. 2 and FIG. 3 illustrate a Chlor-alkali diaphragm cell modified so as to utilize the present anolyte recirculation method. A typical cell 44 having an anolyte compartment 46 separated from a catholyte compartment 48 by means of a porous diaphragm 50 is used. Catholyte compartment 48 has an overflow means 49 by which cell liquor is withdrawn from the cell. Within the anolyte compartment 46 are brine feed means` 52, chlorine gas removal means 54, anodes 56 and anolyte liquid Withdrawal means 64. Attached to the anolyte compartment is sight glass 60 which shows the level of anolyte liquor within the anolyte compartment.

Anolyte liquor withdrawal means 64 preferably has means for regulating the anolyte liquid level 62. Anolyte liquor withdrawal means 64 is thus preferably a pas'- sageway for liquids which is capable of being rotated about an axis passing through hole 66 through which anolyte withdrawal means 58 is attached. Thus, anolyte liquid level 62 can be changed to increase or decrease the hydrostatic head within anolyte compartment 46, as may be preferable when increasing or decreasing the caustic concentration in the catholyte compartment. Handle 68 is provided to aid in rotating anolyte liquor withdrawal means 64 when adjusting the anolyte level 62.

It is obvious that numerous modifications can be used to provide anolyte withdrawal means from the anolyte compartment. Such modifications can be either with or without anolyte liquid level adjustment means. An example of other adjustment means is an externally mounted inverted U tube which could be rotated to change the liquid level within the cell. Also, means similar to those used in regulating the catholyte liquid level could be used. Such other modifications will be readily apparent to those skilled in the art from the description herein.

Anolyte liquor withdrawn from anolyte compartment 46 by means of withdrawal means 58 is passed into stack 70 for return to the resaturator. Stack 70 has transparent sight glass 72 therein whereby the anolyte eluent liquor can be observed.

The following examples illustrate certain preferred embodiments of the present invention. Unless otherwise indicated, all parts and percentages used herein are by weight and all temperatures are in degrees centigrade.

EXAMPLE 1 A group of 23 Hooker type S-l cells was operated in the normal method by feeding brine to the anolyte compartments of each cell at a feed concentration of 310 grams per liter of NaCl. The brine feed was at a pH of 9, which is the normal brine pH of feed liquor. A decomposition voltage of about 4 volts at about 12,000 amperes per cell was passed through the cells in the normal manner thereby producing gaseous chlorine at the anode and hy drogen and caustic soda (cell liquor) at the cathode. The caustic soda was withdrawn from the catholyte compartment of each cell as cell liquor. The group of cells was continuously operated for several weeks, during which time the operating conditions of the cells were noted. The brine feed rate during the period of operation averaged 2.7 liters per minute per cell which corresponds to the flow through the diaphragm of each cell. It was found that the average current eiciency of the cells for this period was 95.5 percent and that the anolyte temperature within the cells varied from cell to cell within the range of 92 degrees centigrade to 104 degrees centigrade, the average being about 95A degrees centigrade. The brine strength Within the anolyte compartments averaged 260 grams per liter of sodium chloride.

A similar group of 23 Hooker type S-1 cells was operated in accordance with the present invention wherein 24 hour period under the noted conditions of operation. Table II illustrates the advantages to be gained in a commercial size plant producing 200 tons per day of chlorine based on six months of operation.

the cells were fed a brine solution in parallel, as shown in In Tables I and II, Example 2 illustrates the results FIG. 1, at a rate in excess of the ow through the diaobtained under conventional cell operations. Examples 3 phragm and Where the excess anolyte liquor was Withthrough 8 show the results obtained utilizing the anolyte drawii', as shown in FIG. 1, using the anolyte liquor withrecirculation method of the present invention. Example 3 drawal means illustrated in FIGS. 2 and 3. Again, these illustrates the cell operation without an HCl addition 'of cells were fed with a brine solution containing 310 grams 10 Examples 4 through 8 illustrate the cell operation with per liter of sodium chloride but the feed rate was in- HCl additions of varying percentages. The percentages creased from the normal rate of about 2.7 liters per minof HCl addition noted are based by weight on the amount ute to about 7.0 liters per minute, or about an average of chlorine produced by the cell. of 2.5 to 2.75 times the ow through the diaphragms of The cell operating temperatures averaged about 95 said cells. The excess anolyte liquor, which varied slightly degrees centigrade, varying from 94 to 96 degrees centifrom cell to cell within the range of about 4 to 5 liters grade, for the anolyte recirculation examples. The operaper minute, the average being about 4.3 liters per minute, ting temperatures of the standard Hooker type S-3 cells was withdrawn from the cells, mixed together and realso averaged about 95 degrees centigrade, but the insaturated to 310 grams per liter of NaCl prior to being dividual cells varied widely from about 92 degrees centireturned to the anolyte compartments of the cells. Under grade to about 100 degrees centigrade. these conditions, the sodium chloride concentration in the All of the cells were operated at 30,000 amperes. The anolyte compartment averaged 295 to 300 grams per liter brine feed rate to the standard S-3 cells averaged about of sodium chloride. The pH of the anolyte liquor with- 6.44 liters per minute. The brine feed rate to the anolyte drawn from the anolyte compartments averaged 3.5 to recirculation Examples 3 through 8, averaged about 20 4.0. The resaturated and replenished brine returned to liters per minute, with the amount of withdrawn anolyte the cell had a pH of 6.5. The temperature within the liquor ranging from about 13.5 to 17 liters per minute. anolyte compartments of the cells varied from 94 to 96 The lesser Withdrawal rate, i.e., about 13.5 liters per degrees centigrade, the average being 95 degrees centiminute, was used in Examples 3 through 5 to correspond grade. After extended operation, it was found that the to a flow through the diaphragm about equal to that of group of cells using the anolyte recirculation method of EXamPlC 2, and the greatest Withdrawal rate was used in the present invention had improved current eiciencies Examples 7 and S. The lesser flow rates through the diacompared to the cells operated in a conventional manner. phragm resulted in producing more concentrated caustic The average current eiciency of the cells operating by in the catholyte compartment. The particular flow rate the anolyte recirculation method was 96.4 percent. or tllle cells uedhin the present method was regulated y c anging t e ydrostatic rhead in the anolyte com- EXAMPLES 2 8 partment by means of the apparatus of FIGS. 2 and 3. The method of the present invention was operated in Slight variations in the withdrawal rate were noted due accordance with FIG. l and the method described in the to differences in diaphragm porosity. specification wherein cells modified as shown in FIGS. 2 All of the cells were fed with a nearly saturated brine and 3 were used, this method was compared with normal 40 containing about 315 grams per liter of sodium chloride cell operations as in a production size operation. The com- `at a feed temperature of about degrees centigrade. The parison was made between a group of 10 standard 30,000 anolyte sodium chloride concentration for the cells of arnperes Hooker type S-3 cells which produce about one Example 2 was about 270 grams per liter whereas the ton of chlorine per cell per day and an equal number of cells of Examples 3 through 8 had sodium chloride con- S-3 cells modified as in FIGS. 2 and 3, which were op- 45 centrations between 280 and 310 grams per liter, `the erated in accordance with the present invention. Table I average being about 300 grams per liter. tabulates the average results obtained during an average Table I shows the results obtained as follows:

TABLE I Example Number Normal Anolyte Recireulation Cells Percent HC1 Added 0 0 3 7 13 19 Cell Liquor, grams per liter of NaOH. 140 140 140 170 210 260 Anolyte pH at 25 degrees centigrade.. 3.8 3.8 3.4 2.7 2.7 2.7 2.7 Anode Current Eniciency,percent 95.2 96.0 90.5 97.2 97.2 97.2 97.2 Cen Life, days 250 259 300 375 375 375 375 Typical Cell Voltage... 4.00 3.99 3. 99 3. 99 4.00 4.01 4. 02

TABLE II.-DAILY PRODUCTION RATES FOR 200 TON PER DAY PLANT Method of Example Number 2 3 4 5 6 7 8 Tons of salt used 331 331 328 321 310 290 262 Tons of water to be evaporated to make 50 percent NaOH- 1, 369 1, 369 1, 355 1, 331 918 641 488 Tons of fresh brine to be heated 2, 621 2, 654 2, 588 2, 529 2, 020 1, 610 1,340

CHANGES DUE TO ANOLYTE RECIRCULATION Tons o1 NaOH not produced- 0 0 2 6 15 28 42 Tons ofsalt not used 0 0 3 10 21 41 69 Tons of water not evapolat rl 0 14 38 351 728 881 Tons of brine not heated. I) -33 33 92 601 1, 011 1, 281

Percent current reduct 1on.. 0 84 1. 37 2. 1 2. 1 2. 1 2. 1

Percent voltage reduetlon U 25 25 25 0 25 50 Percent reductions in cells renewed 0 3. 5 16. 7 33. 3 33. 3 33. 3 33. 3

A comparative analysis of Example 2, which is the average of conventional cell operation, with the examples of the anolyte recirculation method las illustrated in Examples 3 through 8, shows that even without the addition eiciencies and the over-al1 effects of the anolyte recirculation method of the present invention. All of the examples utilized a brine feed having a sodium chloride concentration of 315 grams per liter. The cells were opof HC1 to control the anolyte pH, improved results are 5 erated at 12,000 amperes. Example 10 shows the operaobtained in current reduction, voltage reduction and a tion of a normal cell without anolyte recirculation. Exreduction in the percentage of cells renewed. With the ample 11 illustrates the anolyte recirculation process of addition of HC1, increased anode efficiency is noted in the present invention without acid addition, wherein the addition to markedly improved cell life. The current rebrine recirculation is at a rate of about 2.3 times that duction and reduction in percentage of cells renewed is lo flowing through the diaphragm which corresponds to a also markedly improved. By regulating the flow of brine feed rate of about 6.2 liters per minute, 3.5 liters of through the diaphragm by raising or lowering the hydrowhich were recycled. The withdrawn anolyte solution is static head -in the anolyte compartment, the amount of resaturated and the temperature adjusted prior to returnsodium hydroxide contained in the cell liquor can be also ing the brine to the cell to provide'an anolyte temperature changed so as to greatly improve the caustic salt ratio in 15 of about 98 degrees centigrade. the catholyte cell liquor. A decrease in the ow rate Examp1es 12 through 14 employed the same recirculathrough the diaphragm, as by lowering the hydrostatic tion method as in Example 11 but with a reduced hydrohead, increases the caustic concentration in the catholyte Statie head within the anolyte compartment to increase compartment thereby increasing the demand for HC1 to the caustic concentration in the catholyte compartment maintain the desired pH due to an increase in the back by reducing the ow through the diaphragm. At a feed migration of hydroxyl orlS through tho diaphragm Higher rate of 3.6 liters per minute about 1.6 liters per minute of caustic concentration greatly reduces the amount of water anolyte liquor were Withdrawn from lhe anolyte compart. to be evaporated to make 50 Percent Sodium hydroxide ment for recirculation. The reduced hydrostatic head reand reduces :the amount of fresh feed brine to be heated. duced the ow through the diaphragm to about 2 liters The economic advantages of the process are clearly illusper minute, The feed brine was aeidied with HC1 to a trated in Table II. pH of 0.2, the amount of HC1 added was chosen to provide As is seen from an examination of Tables I and II, the the noted anolyte pI-L Present method Carl be Used .to regulate the Proportion Examples 15 through 17 were run in the same general of chlorine'to sodium hydroxlde produced.l As .has often manner as Examples 12 through 14, but with the anolyte been the Sltuatlon the demand for Chlolfnfe 1S greater 30 recirculation rate being about 1.5 times that flowing than that of sodium hydrox1de..Therefore, 1t1 s often prefthrough the diaphragm into the catholyte compartment erabie to produce .more chlorme than .caustl Thls can This corresponded to a feed rate of about 3.3 liters per readily be accomphshed by the present Invention' minute of brine of which about 1.1 liters per minute of EXAMPLE 9 anolyte liquor were withdrawn for recirculation, the re- This example illustrates the preferred operating condima1r1mg 22 me Per minute Passed through the dla' tions of the present invention under varied conditions of Phragm mto the catholyte Compartment- Current. Calculated cell Anolyte pH Anolyte, Cell liquor, Exemple Efficiency, life-days based at 25 degrees .grams per grams per Number Percent on six months centigrade liter of NaCl liter of NaOH ot operation 95. 2 250 3. 8 240 140 96. o 259 3. s 30o 140 9e. o 375 2. 7 279 252 93. 9 (l) 4. 05 233 259 se. 3 375 2. 6 280 245 96.2 375 2.7 248 21o 92.4 (1) 4.3 248 223 97l 2 (l) 2. 1 251 213 l Not determined.

HC1 additions and cell liquor caustic concentration. FIG. 4 shows the average relationship between cell liquor strength, current eiciency and r-ate of HC1 addition for ch-lor-alkali diaphragm cells operated at an average anolyte temperature of 94 degrees centigrade and an anolyte recirculation rate sufficient to provide an anolyte NaCl concentration of 300 grams -per liter at a feed concentration of 310 to 315 grams NaCl per liter. The data given in FIG. 4 are the averages obtained from numerous runs using Hooker type S-l production cells operated in accordance with the invention at 12,000 amperes. Curve A illustrates the relationship between HC1 additions and the amount of caustic which is retained in the cell liquor to obtain an anolyte pH at a level which will provide a 98 percent current eiciency. Changes in this relationship bring labout corresponding changes in current eciencies as is illustrated by curves B, C and D.

The illustrated curves are for constant conditions of anolyte temperature and anolyte sodium chloride concentrations. Changes in these conditions will displace or alter the slopes of the curves.

EXAMPLES 10-17 Individual Hooker type S-l cells were operated in accordance with the present invention to determine the effects of high catholyte caustic concentrations on cell ab out 4.

While there have been described various embodiments of the present invention, the methods described are not intended to be understood as limiting the scope of the.

invention. It is realized that changes therein are possible and it is further intended that each ele-ment recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner. It is intended to cover the invention broadly in Whatever for-m its principles may be utilized.

We clai-m:

1. A process -for operating a group of chlor-alkali diaphragm cells comprising imposing a decomposition voltage across the electrodes of said chlor-alkali diaphragm cells, feeding a solution of brine to the anolyte compartment of each of said cells at a rate in excess of the amount which flows through the diaphragm of said cells, Withdrawing the excess brine feed solution from the anolyte compartments of said cells, combining and replenishing the withdrawn solutions with additional :amounts of a chloride selected from the group consisting of an alkali metal chloride, hydrogen chloride and mixtures thereof,

and returning the replenished brine solution to the anolyte compartments of said cells.

2. The process of claim 1 wherein the temperature of the brine feed solution is adjusted to obtain an anolyte temperature of about 85 to 100 degrees centigrade.

3. The process of claim 1 wherein the brine feed solution fed to the cell contains 130l to 330 grams per liter of sodium chloride.

4. The process of claim 1 wherein the brine fed to the anolyte compartment is nearly saturated sodium chloride solution.

5. T he process of claim 4 wherein the brine feed solution is fed to the anolyte compartment at a rate of 1.5 to about times the rate at -which the brine passes through the diaphragm into the catholyte compartment of the cell.

6. The process of claim 1 wherein acid is Iadded to the brine feed solution in an amount to obtain an anolyte pH within the range of about 1.5 to 4.0i.

7. The process of claim 6 wherein HCl is added to the brine feed solution in an amount to obtain an anolyte pH of about 2 to 3.

8. The process of claim 6 wherein the brine feed solution is aciditied by the addition of up to about i weight percent HCl based on the amount of chlorine produced by the cell.

9. A process for operating a Chlor-alkali diaphragm cell comprising feeding a solution of brine to the anolyte compartment of the Chlor-alkali diaphragm cell while imposing a decomposition voltage across the electrodes of said cell, feeding said brine solution at a rate of about 1.5 to 10 times the amount which flows through the diaphragm of the cell into the catholyte compartment of the cell, withdrawing the excess brine feed solution from the anolyte compartment of the cell, replenishing the withdrawn solution with salt, adjusting the temperature of the solution to the desired feed temperature `and returning the resaturated and adjusted brine solution to the anolyte compartment of the cell.

10. The process of claim 9 wherein the temperature of the brine feed solution is adjusted so as to obtain an anolyte temperature of about 93 to 100 degrees centi grade.

11. The process of claim 9 wherein the brine fed to the anolyte compartment contains about 280 to 330 grams per liter of NaCl and the rate of feed is sufficient to maintain an anolyte sodium chloride concentration in the anolyte compartment of about 280 to 330` grams per liter. t

12. The process of claim 9 wherein the pH of the anolyte solution -in the cell is maintained between about 1.5 and 4 by the addition of up to :about 20 weight percent hydrogen chloride based on the amount of chlorine produced in the cell.

13. The process of claim 9 wherein the amount of anolyte feed solution passing through the diaphragm into the catholyte compartment is regulated by |changing the level to which the anolyte liquor is withdrawn by adjustment of the anolyte liquor withdrawal means thereby changing the hydrostatic head within the anolyte compartrnent.

14. The process of claim 9 wherein the brine feed solution fed to the cell contains about 130 to 330 grams per liter of sodium chloride.

15. The process of claim 9 wherein the brine feed solution is a nearly saturated sodium chloride solution.

16. The process of claim 15 wherein said brine solution is fed to the cell at a rate of about 2 to about 5 times the amount which llows through the diaphragm of the cell into the catholyte compartment, the excess brine feed solution is withdrawn from the anolyte compartment and resaturated with salt, the temperature of the resaturated solution is adjusted to obtain an anolyte temperature of about '93 to 100 degrees centigrade, the acidity of the brine feed solution is adjusted by the addition of hydrogen chloride to obtain an anolyte pH of about 2 to about 3 and the resaturated and adjusted brine solution is returned to the anolyte compartment of the cell.

17. The process of claim 16 wherein the amount of HCl added to the brine feed solution is in an amount of up to about 2() weight percent HCl based on the weight of chlorine -produced by the cell, said HCl addition being in an amount commensurable with the back migration of the hydroxyl ion from the catholyte compartment to provide a pH in the anolyte compartment of about 2 to 3.

18. The process of claim 17 `wherein the caustic concentration in the catholyte compartment of the cell is increased by lowering the hydrostatic head of anolyte liquor within the anolyte compartment and increasing the `amount of HCl added to the brine to provide an anolyte pH of about 2 to 3.

19. A process for Iincreasing the caustic concentration of cell liquor in a Chlor-alkali diaphragm cell comprising `feeding t-o the anolyte compartment of said chlor-alkali cell a nearly saturated solution of brine while imposing a decomposition voltage across the electrodes of said cell, said brine feed solution being acidied with HC1, feeding said solution at a rate in excess of the amount which flows through the diaphragm into the catholyte compartment of the cell, adjusting the hydrostatic head of the anolyte liquor to lessen the flow rate through the diaphragm of said cell, regulating the HCl acidification of the brine to produce an anolyte pH below about 4, withdrawing the excess brine feed solution from the anolyte compartment, resaturating said withdrawn solution with salt and returning the resaturated brine solution to the anolyte compartment of the cell.

20. The process of claim 19 wherein the hydrostatic ,head within the anolyte compartment of the cell is redu-ced to lessen the ilo-w through the diaphragm and thereby increase the concentration of caustic within the catholyte compartment.

References Cited UNITED STATES PATENTS 2,628,935 2/1953 Earnest et al. 204--95 2,925,371 2/196() Winckel et al. 204--239 XR 2,954,333 9/ 1960 Heiskell et al 204--98 3,043,757 7/1962 Holmes 204-95 3,052,612 9/1962 Henegar et al. 204--128 3,055,821 9/ 1962 Holmes et al. 2011-270 HOWARD S. WILLIAMS, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

Claims

1. A PROCESS FOR OPERATING A GROUP OF CHLOR-ALKALI DIAPHRAGM CELLS COMPRISING IMPOSING A DECOMPOSITION VOLTAGE ACROSS THE ELECTRODES OF SAID CHLOR-ALKALI DIAPHRAGM CELLS, FEEDING A SOLUTION OF BRINE TO THE ANOLYTE COMPARTMENT OF EACH OF SAID CELLS AT A RATE IN EXCESS OF THE AMOUNT WHICH FLOWS THROUGH THE DIAPHRAGM OF SAID CELLS, WITHDRAWING THE EXCESS BRINE FEED SOLUTION FROM THE ANOLYTE COMPARTMENTS OF SAID CELLS, COMBINING AND REPLENISHING