US2836551A

US2836551A - Mercury cell electrolysis of brine

Info

Publication number: US2836551A
Application number: US344932A
Authority: US
Inventors: Harold H Heller; George H Saunders
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1953-03-27
Filing date: 1953-03-27
Publication date: 1958-05-27
Anticipated expiration: 1975-05-27

Description

May 1953 H. H. HELLER EI'AL 2,836,551

MERCURY CELL ELECTROLYSIS OF BRINE 4 Sheets-Sheet 1 Filed March 27, 1953 gm: wa w k Mi m; F flw J y 1953 H. H. HELLER ETAL 2,836,551

MERCURY CELL ELECTROLYSIS OF BRINE 4 Sheets-Sheet 2 Filed March 27, 1953 Zkverziars: Harald [i 5 6L661: 660 96. H Saunders,

May 27, 1958 Filed March 27. 1953 CELL v04 77 765 CELL VOLT/76E H. H. HELLER ETAL 2,836,551

MERCURY CELL ELECTROLYSIS 0F BRINE 4 Shegts-Sheet 3 CELL VOL 706E vs- PEYNOL D3 NUMBER REYNOLDS NUMBE/E Elk 0 I44 288 452 5'76 72 864 360 504 648 '792 936 I080 I224 I368 152 I008 52 1296 I440 CELL fl/WPEPES PER 5904025 3007 0F can/0'05 fizz/mivrsz May 27, 1958 H. H. HELLER ETAL 2,836,551

MERCURY CELL ELECTROLYSIS OF BRINE Filed March 27. 1953 4 Sheets-Sheet 4 Anode- Ca/hoafe \S aacfqgf2 Sa/ura/ed Joofum c/i/orfoe 5r/ne 0/ 80C.

0 2 4 6 8/0 /2 /4 l6 /8 Z0 Z2 24 26 Reyna/0Q Number fr; Thousands IN V EN TORS. Hare/00% He//er BY Geo/: 6 H Saunons ATTORNEY-.5

Unite 2,836,551 Patented May 27, 1958 Wis, assignors to she Bow Qhemical Company, Midland, Mich, a corporation of Delaware Application P/iarch 27, 1953, Serial N '0. 344,932

6 Claims. (Cl. 204-'99) This invention relates to an improved process for the electrolysis of alkali metal chloride brine in a mercurycathode cell. It also concerns a novel cell apparatus.

This application is a continuation-in-part of our copending application Serial No. 758,442, filed July 1, 1947, now abandoned.

In the usual mercury cell, a pool of mercury forms the cathode, and blocks of graphite are used as the anode. On electrolyzinga strong alkali metal chloride brine, the cation, e. g. sodium, migrates to the cathode and forms amalgam with the mercury. The chlorine ions move to the anode, where they are converted into chlorine gas, which is drawn oil from the cell. The mercury amalgam is conveyed to a denuding chamber where it is reacted with water to produce sodium hydroxide solution and hydrogen.

The mercury-cathode cell, while extensively developed, has been s to certain disadvantages which have limited its LlSCLlJlEESS. The amount of mercury required for the cathode pool has been extremely large. Further, the cell can be operated at economical Voltages, e. g. below 4 to volts, only at low current densities, ordinarily less than 145 amperes per square foot, because of excessive electrode polarization at higher currents. As a result, lar e electrode areas, and correspondingly large cell sizes, have been required. These factors have raised the capital investment for a mercury-cathode cell installation to a value so high as to be practical only under specialized circumstances. Further, the cells have been so sensitive to dissolved impurities that expensive brine treatment facilities have had to be provided. In addition, the inflexibility of continuous operation has been imposed by the high cost of shutdown and restarting.

It is therefore a principal object of this invention to provide an improved process for the operation of mercurycathode brine cells which eifectively avoids all of the limitations mentioned. A related object is to provide a cell design taking full advantage of the process improvernent.

The invention depends on the discovery that the operation of mercury-cathode electrolytic brine cell is vastly improved by propelling the brine between the anode and cathode at a how rate and degree of turbulence above a previously unknown critical value which is itself far higher than any heretofore used in practice. At this ilow condition significant electrode polarization is not encountered even at current densities many times the highest previously thought practical. Further, under these cond cos, the cell exhibits unprecedented tolerance and operate on salt brines from most th little or no'purifiication.

i cell of the invention is cons ucted with a trough-like steel cathode over which a film of mercury flows and a graphite anode parallel to it spaced therefrom only a short distance, e. g. inch or less. The space between the cathode and anode forms a passageway through the cell, and ispreferably the only such passageway. The brine, at an elevated temperature, is propelled from an inlet nozzle through the passageway at a considerable velocity, i. e. at a flow rate relative to the anode of over two feet per second. The passageway is so proportioned, and the weight rate of how of the brine is so controlled, as to maintain a high degree of turbulence, i. e. so that the Reynolds number of the brine is at least 400'.) and is preferably much higher. At the same time, sufiicient voltage is applied between the anode and cathode to provide a current density of at least 500 amperesper square foot, such voltage being ordinarily no more than 3.5 to 5 volts.

The stationary cathode surface is supplied with mercury at its upstream end and the motion of the rapidly flowing brine sweeps the mercury as a continuous .film along the surface of the cathode in the direction of the brine flow. The mercury film is maintained on the cathode in a layer sufiiciently thin that it does not ripple and short-circuit the cell.

Because of the fact that only a thin moving film of mercury is used, the total inventory of mercury required is quite small in comparison to that of prior cells. The ability to operate at high current densities allows design of units of small size and correspondingly low cost, and the absence of polarization aifords excellent energy efficiency. The tolerance for impurities almost eliminates the necessity of brine treatment. Wide flexibility in the rate of production of chlorine and alkali is realized, and shutdowns do not pose serious problems.

The accompanying drawings illustrate the structural characteristics and performance data of one form of cell utilizing the principles of the invention. Since the invention has particular advantages when used for the electrolysis of sodium chloride brine, the following description and drawings are of a cell and procedure particularly adapted to that material.

In said drawings,

Fig. 1 is a diagrammatic View of a system adapted to effect the electrolysis of sodium chloride brine in accordance with the principles of the present invention;

Fig. 2 is a sectional, side elevational view of the electrolytic cell indicated in Fig. 1;

Fig. 3 is a sectional, elevational View taken on line 33 in Fig. 2;

Fig. 4 is a curve showing the relation of the voltage across the cell illustrated in Fig. 2 to the Reynolds number of the electrolyte;

Fig. 5 is a curve showing the relation of the voltage across the cell to the current density in the cell illustrated in Fig. 2 where the electrolyte is being circulated a rate such that it has a Reynolds number of about 20,030; and

Fig. 6 is a family of curves similar to that of Fig. 4-, but for different operating conditions, showing the relation of the voltage across the cell to the Reynolds number of the electrolyte for a series of different electrode current densities.

As shown in Figs. 1 to 3, the entire cell may be supported upon the steel cathode block ll, as illustrated in the drawings. The cathode block 11 has a plane upper surface 13 which is horizontally disposed. A suitable electrical connecting terminal 15 is provided for-connecting the cathode 31 to the negative terminal of a direct current generator 17. At the upstream end of the cathode block 11, the lefthand side in Fig. 2, a mercury inlet 19 is provided which communicates with a transversely disposed trough 21 which extends the entire width of the cathode block 11. The trough 21 is provided with a curved edge 23 on the downstream side, to facilitate the movement of mercury out of the trough 21.

A mercury trap '25 is placed in the downstream end of the cathode '15 and includes a sloping surface 27 which brine is substantially eliminated, by carefully shaping the I system, a brine tank 75 is provided which has a brine brine outlet 31, as will be described in a subsequent para- 7 I graph.

a 11 and is of suitable height toprovide the constant head The head of electrolyte necessary for efficient operation. box 33 is provided with a brine inlet37, and this is connected by means. of a suitable'pipeline 39 to a source of brine. The flowspreader 35 comprises a narrow, 'elongated, nozzle which has a thickness about equal to the spacing between the cathode surface 13 and the anode 7 block 41 and a width about equal to the width of the cell.

The lower surface of the flow spreader extends over the mercury feed trough 21, and is provided with an undercut lip 43 which is adapted to form the mercury in the trough'21 into a bead, as illustrated at 45 in Fig. 2.

Vertically extending sidemembers 4-7 which define the ends of the flow spreader 35, the ends of the head box 33, and the sides of the brine passageway, are attached to the cathode block 11. The side members 47' are'fabricated from electrical insulating material, or they may be constructed fromconducting material if proper insulating spacers are provided. 7

' The anode chamber is rectangular in cross-section and extends upward far enough to accommodate the graphite anode block 41. The upstream end of the anode chamher is defined by a vertically extending member 49which is supported from the sidemembers 47 and is attached, at its lower end, to the upper surface of the flow spreader 35. The sides of the anode chamber are defined by the side walls 47, and thedownstream end is defined by a vertically-extending plate member 51 alsosupported from the walls 47, and which is connected at. its lower end to the brine outlet 31. I V

The graphite anode block 41 is of a size that is adapted to fill the anode chamber, and is supported from a 1 bridge frame 53 in a manner allowing vertical adjustment.

A suitable electrical connection55 is provided for a connection to the positive pole of the direct current generator 17; In the cell illustrated, the anode 41 is supported by means of a threaded shaft 57 which extends upward through the bridge frame 53 in which is disposed an adjusting nut 59. The anode connection 55 is made to the upper portion of the shaft 57 by means of nuts 61 and a lug 63. A suitable cover member 65 and packing gland 67 are also providedto'prevent the loss of brine out of the cell. V

The brine outlet 31 extends the width of the passageway between the anode 41 and cathode 11, and is shaped so that thetrap 25 for the mercury amalgam is formed at the juncture between the cathode surface 13 and the outlet 31. The upper surface 69 of the outlet 31 is shaped so that there isbut little resistance to the flow ofbrine out, through the outlet 31. The lower surface 7130f the outlet 31 is shaped as illustrated to aid in trapping the mercury amalgam that may become entrainedwiththe brine.

block, and the anode is a graphite block, The brine,

As before stated, the cathode fabricatedfromasteel outlet 77 connected to a suitable pump 79 for supplying brine to the electrolytic cell under the required pressure. The brine tank 75 contains a supply of solid salt 81 for refortifying the electrolyte after each pass through the cell. adapted to be connected to a brine purification system where unwanted salts of magnesiunniron and the like, may be removed when'necessary. A steam line 87, is provided with an outlet 89 under the level of the brine, so that the brine may be maintained at the'desired temperature. However, under certain conditions the heating efiect of the electric current in the cell will automatically maintain the brine at the proper temperature. A chlorine outlet pipe 91 whose opening is disposed above the level of the brine, extends upward through the top of the brine tank 75. An opening, with a conduit 93, adapted to be connected to the brineoutlet of the electrolytic cell, is also provided in the upper portion of the brine tank 75; I

The electrolytic cell illustrated is constructed in accordance with the invention and is connected, by means of

suitable conductors

95 and 97,to the conventional generator 17: which is supplied with suitable voltage and ampere-indicating

instruments

99 and 101, respectively. In the system illustrated only one cell isshown but, of

course, in an actual installation a plurality of cells would be used and the cathode of one cell would be connected to theanode of the next cell in the conventional manner, so that a voltage of from to 500 volts could be 7 used for the series of cells. The brine pump 79 is connectedto thebrine inlet on the head box by means of the pipeline 39, which is provided with an indicating manomete'r103 and a control valve 105.

The mercury'inlet 19 of the cell is connected by a suitableconduit 107 equipped with a control valve 109, to a constant-level mercury reservoir. 111. The amalgam outlet 29 of the cell is connected by a conduit 113 to the top of the stripping or denuding tower 115.

The mercury denuding tower 115 may be of any conventional type, but we have found that good results are obtained with a vertically extending tower which is packedwith carbon Raschig rings 117. A supply of pure I water, is conducted into the bottom of the tower 115 by means offapipeline 119, and its flow is controlled ,by

' amalgam and the water produces a caustic solution, by.-

drogen gas, and relatively. pure mercury which falls into the bottom of the tower 115 whereuponit is pumped, by meansof agear pump 123, back to the mercury reservoir 111 through the conduit 125. By the proper design of the tower, caustic concentrations as high as 50 may be obtained from the mercury.

A mercury overflowpipeline 127 for the constant-level reservoir ;111 is connected to the top of the denuding tower 115,:thu sallowing a slight excess of mercury tube a cooler, in whichentrained head box33,flow spreader 35, andoutlet 31, are all desirably fabricated from a suitable in'sulatingmaterial. The side members of the cell and the top of theanode compartment may be made of a conducting materiaL if in:

sulatingspacers, such asare illustrated .at 73'inif ig. 3,

carried in the system; A conduit 1290f suitable material conducts the sodium hydroxide from the top of the. de-

nudingtower 115 to a container 131. Another pipeline 7 133 conducts..the;hydrogen formed in the tower through mercuryis condensed, to a safe disposal point. 4 '7 V 1 filhen'beginning' operation of the system, the surface 13 of the cathode 11 is first flooded withimercury and the brine p ump 79 is energized to circulate the brine through the cell which movesthe mercury slowly across the:cathode surface 13. At this time the anode-cathode spacing is about /4'-inch," The electrical generator 17 is started, anda direct current is passed through the cell in 1 an amount which will give a current density of about amperes per'square foot. After this current has passed through the cellfor about 5 minutes, gradual and simul An inlet 83 and outlet are provided, which are percent 7 taneous changes are made in the rate of brine flow, the rate of mercury flow, the anode-cathode spacing, and the current density. The anode-cathode spacing is then reduced to about A; to 7 of an inch, and the rate of flow of brine is increased until the Reynolds number is over about 4,000, and the flow of mercury is adjusted by the valve 109 so that the film of mercury just wets the surface of the cathode. As soon as the Reynolds number in the cell reaches the critical figure of about 4,000, the resistance in the cell has greatly decreased and the current density can be greatly increased Without observing the usual voltage increase caused by polarization.

During periods when the cell is not operated the anodecathode distance is materially increased so that the cathode surface can be flooded with a heavy layer of mercury. Because of this thick layer of mercury on the cathode at the beginning of the starting period, care should be taken that the mercury layer does not come in contact with the anode and short-circuit the cell.

In some cases, for example, a cell designed for a limited range of operating conditions, the cathode-anode spacing may be fixed, with resulting simplification of the cell construction.

The chlorine formed in the cell is carried out with the circulating electrolyte and is flashed off in the brine reservoir 75 from which it is conducted to either a liquefying system or an apparatus adapted to utilize the chlorine gas.

The process of the invention is applicable generally to the electrolysis of strong alkali metal chloride brines, e. g. these which are at least 50 percent saturated. In the case of sodium chloride, a salt concentration of at least 270 grams per liter is desirable, with values approaching saturation, up to 320 grams per liter or more, being preferred because of the lower electrical resistivity of the stronger brine. This resistivity may also be minimized by electrolyzing the brine at an elevated temperature, usually at least 70 C. However, temperatures above 95 C. are ordinarily avoided because of the high partial pressure of water vapor in the evolving chlorine. Temperatures of 80 to 87 C. are perhaps the optimum. At this temperature, when the Reynolds number is 20,000, the proportion of chlorine gas as bubbles at the cell discharge is less than about 7 percent.

In operation, according tothe invention, the mercury flowing on the cathode does not form a pool, as in prior processes. Rather, the mercury appears to be smoothed out to a thin ripple-free film which is dragged along by the rapidly moving brine. The film thickness is in any case a small fraction, usually no more than 10 percent, of the anode-cathode gap, and need be only a few thousandths of an inch. The rate of mercury flow should in any case be high enough that the concentration of sodium in the amalgam remains less than about 0.6 percent by weight. On the other hand, excessive mercury flow rates ofier no advantage, it being desirable that the concentration of sodium in the amalgam rise by at least 0.05 percent during passage through the cell. Perhaps 0.1 to 0.2 percent sodium is the preferred range.

As previously stated, the invention requires for its success that the brine be propelled through the cell passageway at a good velocity and a high degree of turbulence. The velocity itself is extraordinary in comparison to prior practice, being at least 2 feet per second and much more advantageously over 3 feet per second, up to 5 or 6 feet per second or even more. However, the turbulence appears to be of even greater significance, and should be that corresponding to a Reynolds number of at least 40-00. Higher values above 8000 are of very great advantage, with a minimum of 10,000, 15,000, or even 20,000 being desirable and in some cases essential for minimum voltage at extremely high current densities.

The Reynolds number referred to herein is deemed to mean the dimensionless quantity which results from the equation:

Reynolds number= Where The dimensions of the cell passageway, of course, determine L the wetted perimeter. The anode-cathode spacing should ordinarily not exceed /s-inch, and is preferably less than At-inch, to minimize the electrical resistance of the cell. This small gap represents a very small part of the wetted perimeter. Accordingly, it will be seen that small changes in the electrode spacing have but small effect on the Reynolds number as long as there is no change in the weight rate of how of brine. The viscosity of the brine electrolyte will have some effect upon the Reynolds number, and as the temperature of the electrolyte rises, the viscosity is lowered which will raise the Reynolds number. The primary variable in the cell, however, under operating conditions is the weightrate of flow which is dependent upon the cross-sectional area of the passageway as well as upon the velocity of the circulating electrolyte.

In order to obtain a Reynolds number of about 20,000 in a cell which has an anode-cathode spacing of A; of an inch when the brine is maintained at 87 C. and has a sodium chloride concentration of 324 grams per liter it is necessary to move the brine through the cell at about 6.7 feet per second. To obtain this velocity of brine flow requires that about 30 gallons of brine per foot of electrode width be pumped through the cell every minute.

The invention includes the use of far higher electrode current densities than any heretofore employed. Operation is above 500 amperes per square foot, with values of at least 1000 amperes per square foot being preferred. Optimum conditions, at a Reynolds number over 10,000, call for a current density of 8 to 12 amperes per square inch. 7 The curve in Fig. 4 reproduces experimental data obtained on a cell constructed in accordance with Fi s. 1 to 3, using an inter-electrode gap of ,a-inch. The feed brine was saturated with sodium chloride and supplied at to 87 C. and the current density was maintained at 1000 amperes per square foot. Considerations other than voltage required that in order to operate the cell at all practically, the Reynolds number must approach 4000. As Fig. 4 shows, further rise in Reynolds number over 4,000 causes additional decrease in the cell voltage until a value of perhaps 10,000, above which the cell voltage is virtually independent of Reynolds number. it is in this region, where the voltage curve is flat, that electrode polarization has practically been eliminated. The observed voltage very" closely approaches the theoretical value, viz. the arithmetic sum of the theoretical decomposition Voltage of sodium chloride, the deposi tion potential of sodium ion in mercury, and potential due to the electrolyte resistance. Cathode concentration polarization and chlorine anode over-voltage are almost non-existent. Overall cell performance reaches an optimum at a Reynolds number of about 2 3,000, above which little or no beneficial effect is noted.

In Fig. 5 there is shown the relation between voltage and current density in a test cell according to the invention. Operation was with an anode-cathode spacing of about i' -inch, with substantially saturated sodium chloride brine at 80 to 90 C. and a Reynolds number of rent density of about 100 amperes per square foot 'in the conventional cell, illustrates the improved, efficiencies possible with the improved apparatus. 'In the tests, the concentration of sodium chloride in the brine electrolyte is maintained at about, 324, grams, per liter. About 0.3 percent of the dissolved salt was converted into chlorine and sodium at each pass between the electrodes.

Fig. 6 shows the relationships of voltage, Reynolds number, and current density obtained on a test cell con-v structed in accordance with Figs. lie 3; The'interelectrode spacing was y -inch. Saturated sodium chloride brine was supplied at 80 C. Separate voltage- Reynolds number curves are given for several current densities. 'As shown, there is, at' each current density, a minimum Reynolds number, marked by'the start of'the horizontal part of the curve, above which polarization is effectively eliminated. Even at a current density of 4 amperes per square inch (576 amperes per square'foot) the Reynolds number on this cell should be at least 10,000 for best operation. At higher current densities this optimum Reynolds number increases somewhat, being about 20,000 at 12 amperes per squ'are inch. The values shown in the curves of Fig. .6 depend in small part on the precise design of the cell. However, a similar family of curves is obtained for each cell in accordance with the invention. The data of Fig.6 may be used for design purposes Without serious error.

The improved cell of the invention is extremely pracabout l3l parts per million of iron (far exceeding the solubilitylimit of hydrous ferric oxide); Operation over a 373' hour period took place with only 0.6 percent hydrogen, in the chlorine. Similarly, up to at least 100 parts per million of-magnesiumionproduce only 0.4 percent hydrogen, and up to 335 parts per million have been tolerated in the simultaneous presence of 11 grams per liter of calcium ion without'adverse effect. fate may be tolerated up to its solubility limit '(about 6 grams per liter) and test concentrations of calcium ion as high at 12grams per liter have produced no difliculty. Essentially the same'tolerances may be obtained with the feed brine slightly alkaline, neutral, or with intentional acidification to a pH of 2 to 4.5. I a

As a'further demonstration of the ability of the cell to tolerate impurities, a prolonged test run was made on crude brinepurnped direct from' a salt well and fed to:

' 53 parts 'per million of magnesium, and abouti'l part per tical from the point of view of voltagereduction, and it also shows good currentefliciencies and excellentfgas purity. With the cell the manufacturing capacity of a plant can be easily varied with the demand for chlorine, and the plant can easily be shutdown with no lossin efliciency or damage to the cells. In a captive chlorine plant, a plant which runs in connection with another V procms such as in the paper industry, no chlorine liquefying equipment would be needed because the capacity would be varied from day to day. The extremely high electrical capacity, 1500 amperes per square foot current density, allows a comparatively few of the improved cell units to take the place of a large number of conventional cells, thus saving floor space and installation costs. Fur ther advantages of the cell are the low mercury volume in each cell and the high quality, salt-free caustic produced from the sodium amalgam. a y

In operation of conventional mercury-cathode brine cells, difiiculty is sometimes encountered because of the presence of an excessive proportion of hydrogeni'in the 'chlorine evolved. Since the explosive limit. is about 5 percent, good practice has called for not more than 1 to 2 percent hydrogen. It has long been realized that excessive hydrogen liberation at the cathode isassociated with the presence of non-alkali metal salt impurities in the shrine. 'In generahit has beenregarded as essential that the brine be given preliminary chemicaltreatment if necessary to insure that it contains (in salt form) no more than 1 part per million of iron, and 1 .part per million of magnesium, and that it also contains no'more-than 4 grams per liter of dissolved calcium sulfate. Quite in contrast, when operating according to the present invention, the tolerance for impurities is spectacularly increased. One, or even all, 'of the impurities mentioned may markedly exceed the values given withoutcausing excessive hydrogen deposition. For instance, in a test according to the invention at a Reynolds number-of 20,000, the feed brine, at 'a pH value of 3 to 4, contained million of iron. Operation was at 72 'C., a. Reynolds number of 18,000, an anode-cathode spacing of A-inch, and a current density of 9.76 amperes per square inch. The required cell voltage was only 4.92 volts. Operation was entirely satisfactory with only 1.0 percent hydrogen in the evolved chlorine. V I

Preliminary investigations have shown that apparatus and methods which are similar to:those described above for the production of chlorine and caustic from sodium chloridebrine can be used in the manufacture of other electrolytic products, It is believed that thehalide salts of the other alkali metals may be commercially electrolyzed in a cell similar to the cell disclosed. For example, a I

the higher oxidation salts. ofsodium and chlorine, e. g.

sodium chlorite and sodium chlorate, and possibly also chlorine dioxide, as well as chlorineandbromine may be prepared from alkali metal halides in such a cell.

We claim: i

, 1. In the electrolysis of a; strong alkali metal halide brine in a cell having a flowing mercury cathode andan anode parallel thereto and spaced therefrom, the space between the anode. and cathode forming a brine passageway through the'cell, the improvement which comprises propelling a stream of the brine through the passageway ata weight rate of flow relative to the anode suchthat theReynolds number of the brine in the passageway is at least 4000*and supplying sufficient electric current to the density of 'at least a rate of flow relativetothe anode over 2 feet per second such'that the Reynolds number of the brine in the passageway is, at least 8000 while supplying sufiicient electric current to the cell to provide an electrode current density of at least 1000 amperes per square foot 3. In the-electrolysis of sodium chloride brine containing at least 270 grams per liter NaCl in a cell having a substantially horizontal cathode. supporting a flowing,

mercury film and an: anode parallel thereto and spaced therefrom not over 1 inch; the space between the anode and cathode forming the sole brine passageway through the cell, theimprovement which comprises propelling a stream 'of the brine at a temperature of 70"to 0., through the passageway concurrent withthemercury at a rate of flow relative to the anode over '3 feet per second such that the Reynolds number of the brine in the passagewayis at'least 10,000 while supplying sufficient Calcium sul-' metal chloride 7 electric current to the cell to provide an electrode current density of 8 to 12 amperes per square inch, and controlling the rate of mercury flow such that the increase in sodium content thereof during passage through the cell is at least 0.05 percent by weight but the total concentration of sodium remains less than 0.6 percent.

4. In the electrolysis of a strong sodium chloride brine at an elevated temperature in a cell having a flowing mercury cathode and an anode parallel thereto and spaced therefrom less than %-inch, the space between the cathode and anode forming a brine passageway through the cell, wherein suflicient electric current is supplied to the cell to maintain an electrode current density of at least 1000 amperes per square foot, the method of efiecting electrolysis without causing appreciable electrode polarization which comprises propelling the brine as a stream through the passageway at a rate relative to the anode over 3 feet per second such that the Reynolds number of the brine in the passageway is at least 15,000.

5. In the electrolysis at an elevated temperature of a strong alkali metal halide brine in a cell having a flowing mercury cathode and an anode parallel thereto and spaced therefrom less than %-inch, the space between the cathode and anode forming a brine passageway through the cell, wherein the brine has dissolved therein as impurities at least one of the following: iron over 1 part per million, magnesium over 1 part per million,

and calcium sulfate over 4 grams per liter, the method of effecting electrolysis without causing appreciable electrode polarization and without encountering excessive production of hydrogen in the evolved chlorine due to the impurity which comprises propelling, the brine as a stream through the passageway at a weight rate of flow relative to the anode such that the Reynolds number of the brine in the passageway is at least 4000 while supplying sufficient current to the cell to provide an electrode current density of at least 500 amperes per square foot.

6. A process according to claim 5 wherein the brine is sodium chloride brine and wherein the velocity of the brine stream is above 3 feet per second and such that the Reynolds number of the brine in the passageway is at least 10,000 and wherein the electrode current density is at least 1000 amperes per square foot.

References Cited in the file of this patent UNITED STATES PATENTS 2,104,677 Sorensen Jan. 4, 1938 2,104,678 Sorensen Jan. 4, 1938 2,104,679 Sorensen Jan. 4, 1938 2,316,685 Gardiner Apr. 13, 1943 2,542,523 Hirsh Feb. 20, 1951 2,762,765 Kircher Sept. 11, 1956

Claims

1. IN THE ELECTROYSIS OF A STRONG ALKALI METAL HALIDE BRINE IN A CELL HAVING A FLOWING MERCURY CATHODE AND AN ANODE PARALLEL THERETO AND SPACED THEREFROM, THE SPACE BETWEEN THE ANODE AND CATHODE FORMING A BRINE PASSAGEWAY THROUGH THE CELL, THE IMPROVEMENT WHICH COMPRISES PROPELLING A STREAM OF THE BRINE THROUGH THE PASSAGEWAY AT A WEIGHT RATE OF FLOW RELATIVE TO THE ANODE SUCH THAT THE REYNOLDS NUMBER OF THE BRINE IN THE PASSAGEWAY IS AT LEAST 4000 AND SUPPLYING SUFFICIENT ELECTRIC CURRENT TO THE CELL TO PROVIDE AN ELECTRODE CURRENT DENSITY OF AT LEAST 500 AMPERES PER SQUARE FOOT.