US3551309A

US3551309A - Process for electrolysis of alkali metal chloride

Info

Publication number: US3551309A
Application number: US687790A
Authority: US
Inventors: Luther L Dunn Jr; Norval Magnusson
Original assignee: Georgia Pacific LLC
Current assignee: Georgia Pacific LLC
Priority date: 1967-12-04
Filing date: 1967-12-04
Publication date: 1970-12-29
Anticipated expiration: 1987-12-29

Description

Dec. 29, 1970 N, JR ETAL 3,551,309

PROCESS FOR ELECTROLYSIS 0F ALKALI METAL CHLORIDE Filed Dec. 4, 1967 Alla/l V kombk uom FI/ter Deco/"poser 4 ll k&-2 \0\-\uhQn F C L! 3. c a U. r al 0/ f/ 6 6 r 5 in. 00 m. E 1

INVEN TOR.

United States Patent Office 3,551,309 Patented Dec. 29, 1970 ABSTRACT OF THE DISCLOSURE A process for the electrolysis of an alkali metal chloride brine in a mercury cathode electrolytic cell using a portion of the dechlorinated brine for the acidification of the resaturated, purified brine being returned to the cell.

This invention pertains to a process for the electrolysis of an alkali metal chloride in a mercury cathode cell. More particularly it pertains to an electrolysis process employing particular conditions and steps to minimize the acid requirement for the process.

In the operation of mercury cathode cells, substantially saturated brine is circulated through the cell where it is passed between a fixed anode and a flowing mercury cathode to subject it to electrolysis. Only a small fraction of the alkali metal chloride in the brine is electrolyzed in passing through the cell. The depleted brine discharged from the cell, while still highly concentrated, is resaturated by being contacted with solid chloride salt and returned to the cell. In addition to the electrolysis of the alkali metal chloride, other reactions take place in the electrolytic cell. Hydrochloric acid is formed in the cell by some of these reactions but is generally lost through further side reactions.

Since it is essential in the electrolysis to use a brine substantially free of impurities, such as iron, magnesium and others, the brine, after resaturation, is treated to remove the impurities which it may have picked up in being contacted with the solid salt. A purification method commonly used to remove the impurities is to precipitate them under alkaline conditions. Addition of an alkali such as sodium or potassium hydroxide or carbonate has to be made to the brine, since the depleted brine is usually at a pH of 2-3 upon being discharged from the cell. After the removal of the precipitated impurities, the purified brine is acidified prior to the introduction into the cell. Thus, considerable quantities of alkali and acid are used in the process.

It is, therefore, an object of this invention to provide an improved process for the electrolysis of an alkali metal chloride. Another object is to provide a process wherein the amount of chemicals used in the treatment of the brine is substantially reduced. A further object is to more elfectively utilize the acid generated in the cell during electrolysis.

The above and other objects are attained, according to the invention, by saturating and purifying only a portion of the dechlorinated brine and recycling a portion of the dechloroinated brine to the cell feed to acidity the resaturated brine. The brine flow rate through the cell is increased proportionately to the amount of recycle to maintain substantially the same chlorine production capacity. The brine is acidified in passing through the cell by the dissolution of chlorine. A portion of the hydrochloric acid thus formed is utilized by the recycle to acidify the resaturated portion of the brine. The recycle may be increased to the extent that the combined portions are at the proper cell feed acidity.

Further, additional advantages are attained, according to the invention, by operating the cell under controlled conditions of impurity levels and flow rates of the brine in the cell. In operating the cell under the controlled conditions, the undesirable side reactions are minimized. As the result, the brine feed does not have to be acidified to the extent otherwise necessary, and the acid formation in the cell is utilized to a greater extent to lower the pH of the brine to its final level.

The invenion will be explained in more detail in conjunction with the attached drawing which is a schematic flow diagram illustrating an embodiment of the invention. In the process shown in the drawing, a substantially saturated brine, such as a sodium or potassium chloride brine, is fed to the electrolytic cell at an inlet temperature of 60 to C. and passed through the cell electrolyzing some of the alkali metal chloride in the brine. The electrolysis is effected at a rate such that the temperature rise obtained of the brine in the cell is in the range of from 0.12 to 040 C., per foot of cell length. The depleted brine discharged from the electrolytic cell is passed to Dechlorinator 4. While the dechlorinator is shown as one block, it may consist of a number of units. For example, the depleted brine after coming from the cell may be subjected to a preliminary dechlorination by being subjected to a partial vacuum and then further dechlorinated, as by air stripping, to reduce the chlorine content to less than ten parts of chlorine per million parts of brine. In the dechlorination operation, hydrochloric acid may be added, if the concentration of hypochlorous acid in the depleted bine is excessive or the brine contains chlorate. However, generally, in the operation of the cell under the controlled conditions of impurity levels and brine flow rates, this addition is not necessary. The depleted brine is substantially free of chlorate and the pH of the brine is usually in the range of 2.0 to 2.5 which is sufficient for the removal of the limited about of hypochlorous acid found in the brine.

From the dechlorinator, the dechlorinated brine is divided into two portions. One portion is recycled to storage tank 5, while the other remaining portion is passed to saturator 6 where it is contacted with solid alkali metal chloride and is resaturated. The portion recycled is generally about 20 to percent of the amount of brine being resaturated.

Prior to resaturation, the portion of the brine to be resaturated is usually neutralized by the addition of an alkali, such as the hydroxide or carbonate of the same alkali metal as in the brine. The pH of the brine does not change appreciably on dechlorination, especially if the brine does not contain large amounts of hypochlorous acid. Thus, the brine leaves the dechlorinator at a pH in the range of 2 to 2.5 or close to that of the depleted brine. A sufficient amount of the alkali may be added ahead of the saturator just to neutralize the brine to a pH of 7 prior to contact with the salt. On the other hand, sufficient alkali may be added to raise the pH to the level desired in the later purification step. When the brine to the saturator is only neutralized to a pH of 7 or so, further addition of alkali is made to the saturated brine in the purification step. The impurities precipitated out under alkaline conditions are removed by settler 7 and filter 8. The purified, saturated brine is then intermixed in storage tank 5 with the recycled dechlorinated brine which was not subjected to the saturation and purification steps. Before returning the combined portions of the brine to the cell as feed, the brine from storage tank 5 is passed through heat exchanger 9 to bring the brine to the cell inlet temperature. Relatively small amounts of hydrochloric acid may be added to the brine feed to make the final adjustment to bring the pH to the acidity usually employed. In the operation of the cells under the controlled conditions, normally a higher pH, in the range of 4.5 to 6.5, is used. The chlorine formed in the electrolysis is withdrawn from the cell and processed further while is not shown on the drawing. The amalgam formed is contacted with water in Decomposer where the alkali metal amalgam reacts with the water to form a hydroxide solution and hydrogen. The mercury from the decomposer is returned to the cell.

Since, generally, the electrolytic cells are operated at at relative constant current density, the concentration of the alkali metal in the mercury is primarily dependent on the rate of flow of mercury through the cell. The mercury being returned from the decomposer usually contains less than 0.010 weight percent of the alkali metal. Thus, the rate of flow of mercury through the cell is preferably maintained at rates normally used such that the concentration of the alkali metal in the mercury at the exit is in the range of from 0.20 to 0.25 weight percent. However, at times the maximum concentration of the alkali metal in the mercury may be increased to about 0.30 or even 0.35 weight percent for a short period of time.

The flow rate of the brine through the cell is generally increased by an amount proportional to the recycle used to obtain about the same chlorine production as without the recycle. Under the controlled conditions, a flow rate of about from 8 to 14 gallons per minute per foot of width of the cell, preferably in the range from 9 to 11 gallons per minute per foot, is employed. Electrolytic cells are relatively flexible in this respect and can be operated under various brine flow rates. Since the brine in the cell flows through passages in the cell other than just the passage or gap between the anode and the cathode, the velocity of the brine may vary in difierent parts of the cell. Thus, the flow rates are expressed in volume per unit time per linear foot of cell width. The flow rates, as expressed, are relatively indicative of the velocities obtained in the cell. However, an increase in the flow rate increases the brine level in the cell so that the passage of the additional brine through the cell may not be due entirely to increased velocity.

By increasing the rate of flow of the brine, other beneficial results are obtained in addition to the advantages of the physical effects of the higher velocity. The higher brine level in the cell is advantageous. Also, a lower temperature gradient of the brine in the cell is obtained permitting a more uniform operation of the cell which improves efiiciency. While the discharged temperature of the brine from the cell may be widely varied, there is a practical limit above which it is undesirable to operate the cell due to the high vapor pressure of water. This results in a large amount of water being drawn off with the chlorine which has to be processed later to be removed. Thus, in the operation of the cell, a higher inlet temperature may be used with increased flow rates of the cell without exceeding the practical discharge temperature.

A sufiicient amount of the decholrinated brine is subjected to resaturation and purification steps to dissolve in the saturator the amount of solid chloride salt which was depleted from the total brine in passage through the cell. The actual amount of the dechlorinated brine required may vary somewhat from the amount generally used without the recycle, since the concentration desired of the alkali metal chloride in the brine is at a higher level to compensate for the dilution efiFect obtained by the combination of this stream with the less concentrated recycle stream. Generally, the temperature increase obtained during the electrolysis of the brine sufiiciently increases the solubility of the alkali metal chloride so that the required amount of salt can be dissolved at the higher concentration level with about the same or somewhat less amount of brine used Without recycle. The solubility of the alkali metal chloride may be further increased by heating the portion of brine to be resaturated by means of an external heater. However, this additional heating is usually unnecessary. Generally, the brine may be resaturated to a sufiiciently high concentration without external heating to maintain a substantially saturated brine, having a concentration in the range of 295 to 310 grams of the alkali metal chloride per liter of brine, with a recycle ratio of 40 to percent preferably used. This amount or ratio of recycle is sufiicient to bring the pH of the saturated, purified portion down to a pH close to the pH used for the brine feed. Since the pH of the dechlorinated brine is in the range from 2 to 2.5, the portion of the dechlorinated brine being recycled is sufiicient to acidify or lower the pH of the treated brine to about the degree necessary. However, if the amount of dechlorinated brine recycled is not sutficient to lower the pH to the level desired, an acid may be added to supplement the dechlorinated brine in the acidification. Even though a sufiicient amount of dechlorinated brine is available for recycle to effect the desired acidification, it is often desirable, for operational purposes, to use a small amount of hydrochloric acid for the final control of the pH.

It is apparent that the amount of dechlorinated brine recycled or the acid required to acidify the saturated, purified brine will depend upon the conditions used in the purification of the brine. Using the usual methods for the purification of the brine, the brine may be subjected to difierent degrees of alkalinity depending upon the impurity or impurities to be removed. Generally, a pH in the range of 9 to 12 may be used; however, in the presence of certain impurities, for example aluminum, part of the treatment may be effected at a pH of 8.5 or lower. In the treatment employed, the impurity level in the resaturated, purified portion of the brine is reduced to a point such as to maintain the desired low impurity level in the feed brine after the addition of the recycle portion of the dechlorinated brine. Under the controlled conditions, the impurity levels in the cell feed are maintained such that the listed metals are present in less than the following parts per million parts of brine:

Chromium .01 Molybdenum .01 Vanadium .01 Titanium .10 Iron 10 Aluminum 1.0 Magnesium 3 .0 Calcium 15.0

However, it may be desirable to further lower the concentration of some of these metals in particular combinations. Some metals, such as cobalt, nickel, iron, copper, aluminum and magnesium may function as promoters and have synergistic effect with other metals. The presence of these promoter metals may require the lowering of the concentration of the affected metal by at least the concentration of the promoter.

In the operation of a plant, DeNora type electrolytic cells were operated according to the invention for a period of 24 hours. A recycle of dechlorinated brine equal to about 78 percent of the amount of brine being resaturated and treated was maintained. A brine flow rate through the cells of about 10.3 gallons per minute per foot of cell width was employed. The brine feed containing about 297 grams of sodium chloride per liter was charged to the cells at a temperature of about 66 C. and at a pH of about 6.1. The depleted brine containing about 270 grams of sodium chloride per liter was discharged from the cell at a temperature of about 79 C. and at a pH of 2.2. Upon leaving the cell, the depleted brine was dechlorinated by evacuation and also by air stripping to decrease the chlorine content of the brine from about 80 parts of free chlorine per million parts of brine to about 1 part per million. The pH of the dechlorinated brine was about 2.4.

The portion of the dechlorinated brine from the air strippers which was to be resaturated was neutralized with caustic to a pH of about 10.5 and resaturated without further heating to about 318 grams of sodium chlochlorine produced by the plant was about 4 pounds per ton.

In operation of the plant without the recycle, the brine was passed through the cell at a flow of 6.8 gallons per minute per foot of cell width. The depleted brine was dechlorinated as above and passed through the resaturator and purified to the level above prior to being returned through the cell. The hydrochloric acid requirements were about 105 pounds per ton of chlorine produced.

The hydrogen content of the chlorine gas stream was about 0.1 volume percent during both of the operations.

What is claimed is:

1. In the electrolysis of a substantially saturated alkali metal chloride brine in a mercury cathode electrolytic cell, wherein the brine is passed through the cell to electrolyze the alkali metal chloride in the brine at a rate to obtain a particular chloride production at a predetermined current density; the depleted brine, discharged from the cell after passage through the cell, is contacted with solid salt to resaturate the brine; the resaturated brine purified under alkaline conditions; and the resaturated, purified brine acidified and returned to the cell; the improvement which comprises dechlorinating the depleted brine; recycling a portion of the dechlorinated brine to the cell feed; resaturating the remaining portion of the dechlorinated brine at a temperature such as to dissolve the amount of alkali metal chloride depleted from the total brine in passing through the cell; purifying the resaturated brine under alkaline conditions; combining the resaturated, purified portion of the dechlorinated brine with the recycled portion of dechlorinated brine to acidify the resaturated, purified portion of the brine; and passing the combined portions through the cell at a flow rate increased proportionately to the amount of recycle to obtain substantially the same chlorine production from the cell at the predetermined current density.

2. In the electrolysis of a substantially saturated a1- kali metal chloride brine in a mercury cathode electrolytic cell, wherein a feed brine is passed 'to a cell at a particular inlet temperature; the depleted brine, discharged from the cell at an increased temperature after passage through the cell, is contacted with solid salt to resaturate the brine; the resaturated brine purified under alkaline conditions; and the resaturated, purified brine acidified and recycled through the cell; the improvement which comprises dechlorinating the depleted brine, recycling a portion of the dechlorinated brine to the cell feed, neutralizing the remaining portion of the dechlorinated brine;

resaturating the neutralized portion of the dechlorinated ibrine at a temperature such as to dissolve the amount of alkali metal chloride depleted from the total brine in passing through the cell; purifying the resaturated portion of the brine under alkaline conditions; combining the resaturated, purified portion of the dechlorinated brine to acidify the resaturated, purified portion of the brine; and passing the combined portions of the brine as feed brine to the cell at the inlet temperature, said portion of dechlorinated brine recycled being in the range of to 100 percent of the remaining portion of the dechlorinated brine neutralized, resaturated, and purified.

3. A process according to claim 2 wherein the alkali metal chloride brine is a sodium chloride brine.

4. A process according to claim 3 wherein the recycled portion of the dechlorinated brine is from 40 to 80 6 percent of the portion of the dechlorinated brine neutralized, resaturated, and purified.

5. In the electrolysis of a substantially saturated alkali metal chloride brine in a mercury cathode electrolytic cell, wherein a feed brine is electrolyzed in the cell by passage through the cell at a rate to obtain a particular chlorine production; the depleted brine, discharged from the cell after passage through the cell, is contacted with solid salt to resaturate the brine; the brine purified under alkaline conditions; the resaturated, purified brine acidified; and returned to the cell; the improvement which comprises passing the feed brine through the cell at a rate of 8 to 14 gallons per minute per foot of cell width to electrolyze the alkali metal chloride at a rate such that a temperature rise obtained in the cell of the brine' is in the range of 012 C. to 0.40 C. per foot of cell length; dechlorinating the depleted brine until the chlorine content is reduced to at least 10 parts of chlorine per million parts of brine; recycling a portion of the dechlorinated brine to the cell feed; neutralizing the remaining portion of the dechlorinated brine; contacting the neutralized dechlorinated portion of the brine with a solid alkali metal chloride salt at a temperature to dissolve the amount of salt depleted from the total brine in passage through the cell; purifying the resaturated brine under alkaline conditions; combining the resaturated, purified portion of the brine with the recycled portion of the dechlorinated brine to acidify the purified portion of the brine; and returning the combined portions as feed brine to the electrolytic cell at a pH in a range of 4.5 to 6.5; said resaturated portion of the brine being purified to the extent that the feed brine returned to the cell contains less than the following parts of the listed metals per parts per million of brine;

Chromium 0.01 Molybdenum 0:01 Vanadium 0.01 Titanium 0.1 Iron 0.10 Aluminum 1.0 Magnesium 3.0 Calcium 15.0

6. A process for the electrolysis of an alkali metal chloride brine in a mercury cathode electrolytic cell, which comprises charging a substantially saturated feed brine to the electrolytic cell at an inlet temperature in the range of 60 to C.; passing the brine through the cell at a flow rate of 8 to 14 gallons per foot of cell width to thereby electrolyze the alkali metal chloride in the brine to obtain a depleted brine; said electrolysis being at a rate such that a temperature rise obtained in the cell of the brine is in the range of O.l2 to 0.40 C., per foot of cell length; dechlorinating the depleted brine until the chlorine content is reduced to at least 10 parts of free chlorine per million parts of brine, recycling 2. portion of the dechlorinated brine to the cell feed; neutralizing the remaining portion of the dechlorinated brine; said portion of dechlorinated brine recycled being in an amount in the range of 20 to percent of the remaining portion of the dechlorinated brine neutralized; contacting the neutralized dechlorinated portion of the brine with a solid alkali metal chloride salt at a temperature to dissolve the amount of salt depleted from the total brine in passage through the cell; purifying the resaturated brine by pre cipitation of the impurities under alkaline conditions; combining the resaturated purified portion of the brine with the recycled portion of the dechlorinated brine to acidify the purified portion of the brine; and passing the combined portions as feed brine to the electrolytic cell at a pH of from 4.5 to 6.5; said resaturated portion of the brine being purified to the extent that the feed brine returned to the cell contains less than the following parts of the listed metals per million of brine;

Chromium 0.01 Molybdenum 0.01 Vanadium 0.01 Titanium 0.1

Iron 0.1

Aluminum 1.0 Magnesium 3.0 Calcium 15.0

7. A process according to claim 6 wherein the brine is a sodium chloride brine.

8. A process according to claim 7 wherein the brine is passed through the cell at a .rate in the range of 9.5 to 12 feet per minute per foot of cell width.

9. A process according to claim 8 wherein the portion of the dechlorinated brine recycled is in the range of 40 to 80 percent of the amount of the remaining portion of the dechlorinated brine neutralized and resaturated.

References Cited UNITED STATES PATENTS Goerg 20499 OTHER REFERENCES 10 Angel et al., J. of the Electrochemical S0c., November 1952, pp. 435-441.

T, TUNG, Primary Examiner US. Cl. X.R. 204125 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 551, 309 Dated December 29, 1970 'Invent fl Irrther L. Dunn. Jr. and Norval l larznusson It is certified that error appears in the above-identified patent and that said Letters Patent; are hereby corrected as shown below:

Column 5, line 6 4, after "brine' inser*t--with the recycled portion of the dechlor'inated brine--.

Sigued and sealed this 30th day of March 1971.

(SEAL) Attest:

EDWARD M.FIE TCHERJR. Attesting Officer WILLIAM E. SCHUYLER, JR. Commissioner of Patents