US 3705090 A
Description (OCR text may contain errors)
United States Patent 3,705,090 NOVEL ELECTROLYTIC CELL AND METHOD FOR PRODUCING CHLORINE AND METAL HYDROXIDES Grafton L. Berger-on and James J. Leddy, Midland, Mich., g isglnors to The Dow Chemical Company, Midland, Continuation-impart of application Ser. No. 796,249, Sept. 16, 1968. This application Oct. 1, 1970, Ser.
Int. Cl. C23b 5/68 US. Cl. 204266 5 Claims ABSTRACT OF THE DISCLOSURE A novel electrolytic cell for producing both chlorine and alkaline earth hydroxides simultaneously, in which brine supply means are connected to each of the anode and cathode chambers.
BACKGROUND OF INVENTION This application is a continuation-in-part of application Ser. No. 796,249, filed Sept. 16, 1968, now Patent No. 3,598,707. The invention relates generally to a diaphragm type electrolytic cell employed in the production of chlorine.
A well-known method of producing chlorine is by the electrolysis of the chloride of an alkali metal, e.g. sodium chloride, in a diaphragm type electrolytic cell, and collecting chlorine at the anode and hydrogen and a solution containing the corresponding alkali hydroxide, e.g. sodium hydroxide, at the cathode.
A frequent problem with producing chlorine by this method is that anions, e.g. hydroxyl ions, produced in the cathode chamber migrate through the diaphragm and interact with the graphitic anode. Such reaction is detrimental to the efficiency of the cell for a number of reasons, among which are: the formation of oxygen which contaminates the chlorine and ultimately attacks the carbon of the anode to form carbon dioxide; the formation of hypochlorites and ultimately chlorates; and increased power consumption and, as a consequence thereof, increased costs per unit of chlorine and alkali hydroxide produced.
Another problem frequently encountered in conventional chlorine-producing cells is that acid must be added to the anolyte to reduce the rate at which the graphitic anode is consumed in the electrolysis reaction.
The detrimental effects of hydroxyl migration in chlorine cells are generally reduced by maintaining a fluid pressure differential between the anolyte and catholyte in favor of the anolyte. The differential is usually maintained by feeding brine predominantly into the anode chamber.
The problems confronted in the operation of chlorine cells are further compounded when the brine contains relatively large amounts of alkaline earth metal chlorides. Whereas sodium chloride brine produces water-soluble sodium hydroxide during electrolysis, alkaline earth brines produce relatively insoluble hydroxides such as magnesium hydroxide. The insoluble hydroxides quickly coat and/or impregnate the cathode and/or the diaphragm. This causes increased power consumption and necessitates frequent shutdowns of the cell so that the cathode and diaphragm can be cleaned or replaced.
Various mechanical means of eliminating coating or impregnation with insoluble hydroxides have been developed, such as fitting the cell with mechanical devices which periodically wipe or scrape the surfaces of the anode and diaphragm, as described in US. Patent 3,294,665.
The unique cell of our invention provides a superior means for producing chlorine and an alkaline earth hydroxide by electrolysis from a brine consisting essentially of alkaline earth chlorides in aqueous solution.
The crux of the present invention resides in carefully feeding an alkaline earth metal brine into both the anode and cathode chambers, but predominantly into the cath' ode chamber; maintaining a substantially similar fluid pressure in both the cathode and anode chambers; maintaining a certain predetermined range of hydrogen ion concentration (pH) in the cathode chamber, and using an electrode with a polished surface.
The cell and process of the present invention differ from electrolyte chlorine production systems previously employed in many respects such as, for example: in the present invention the alkaline earth chloride brine is fed into both anode and cathode chambers but predominantly into the cathode chamber whereas in conventional chlorine cells the brine is fed into the anode chamber, and migrates through the diaphragm to the cathode chamber. In the present invention, the fluid pressure differential between anolyte and catholyte is desirably zero, e.g. the fluid pressures of catholyte and anolyte are substantially equal, whereas in conventional chlorine cells, the anolyte fluid pressure is usually considerably greater than the catholyte fluid pressure. Additionally, in the present invention it is not necessary to add acid to the anolyte as is often done in conventional cells as the pH inherently is sufficiently low, often one or less, to assure proper operation of the system.
Generally, it is an object of the present invention to provide a unique cell and electrolytic process for producing chlorine and alkaline earth metal hydroxides from electrolysis of brines comprising alkaline earth metal ions.
It is another object of the present invention to substantially reduce and/or eliminate diaphragm and cathode impregnation or coating with insoluble hydroxides during electrolysis of an alkaline earth metal brine.
It is another object of the present invention to substantially eliminate the formation of oxygen at the graphitic anode in chlorine producing cells, to achieve a relatively low anolyte pH without addition of acid and, ultimately to greatly reduce the rate of consumption of the graphitic anode.
DESCRIPTION OF THE DRAWING The drawing is a sectional view of an electrolytic cell of the present invention.
DESCRIPTION OF THE INVENTION The process of the present invention comprises providing an electrolytic cell divided into cathode and anode chambers by a diaphragm, and passing therethrough a brine comprising an aqueous solution of the chlorides of alkaline earth metals, While maintaining the pH of the catholyte liquor at from about 8 to about 11, and maintaining the liquid pressures of the catholyte and anolyte liquors substantially equal to one another.
Gaseous chlorine is produced in the anode chamber and can be collected by suitable means as it exits from the chamber. The insoluble metal hydroxide is produced as a slurry in the cathode chamber and is pushed from the cell by the brine stream and collected by an appropriate solids removal system, such as a settling tank or filtering system.
In the process described above, the cathode chamber has disposed therein a ferrous cathode consisting essentially of iron or steel, and having a smooth, polished surface in contact with the catholyte.
During operation of the cell, the temperature of the brine can vary over a wide range, however for optimum efiiciency in operation, the brine temperature preferably should range from about 60 C. to about 90 C.
For purposes of the present application the terms anolyte, and anolyte liquor designate those portions of the brine stream passing through the electrolytic cell which are contained within the anode chamber of the cell. Correspondingly, the terms catholyte, and catholyte liquor designate those portions of the electrolyte stream contained within the cathode chamber.
In maintaining the pH of the catholyte liquor at from about 8 to about 11, it should be noted that a principal reaction in the cathode chamber is the electrolysis of water to produce hydrogen and hydroxyl ions. The hydroxyl ions combine with alkaline earth metal ions to form insoluble hydroxides. The pH of the catholyte is therefore directly related to the flow of brine through the cathode chambers. If a higher pH is desired, the flow can be reduced, whereas if a lower pH is desired the fiow can be increased thereby bringing more alkaline earth ions into the chamber to react with the hydroxyl ions.
The pH of both the anolyte and catholyte can be determined by inserting commonly employed pH measuring devices into the cell, or alternatively by inserting such devices into the effluent exiting from each chamber. Similarly the flow rate through each chamber can be determined by inserting appropriate instruments into the efiiuent stream, or alternatively inserting a metering pump into the conduit carrying the brine from its source of supply to the anode or cathode chamber. Since the liquid pressure of the catholyte or anolyte is directly related to the flow rate of brine through the chambers, the rate of flow can be varied until the catholyte and anolyte liquid pressures are substantially equal to each other.
The absolute rate of brine flow through the chambers is dependent upon the size and shape of the chamber, and concentration and nature of the components of the brine.
Brine solutions which are employed in the present invention comprise alkaline earth chlorides and especially the chlorides of magnesium and calcium.
While the anionic component of the brine is usually chloride ion, other anions can be present if during electrolysis they do not tend to form signficant amounts of gas in the anode chamber. For example, a brine containing calcium chloride could be employed if it contained relatively little carbonate ion.
In practice, with a brine comprising magnesium and calcium ions, the primary hydroxide produced in the cathode chamber will be magnesium hydroxide. However, the nature of the insoluble hydroxide can be controlled by choosing a brine of appropriate composition after consultation with a standard table showing the relative positions of the alkaline earth metals in the electromotive series.
Preferably, the brine employed will consist of an aqueous solution of magnesium-/and calcium chlorides. The brine can also contain other metal values such as iron, aluminum, copper, etc. With the calcium-magnesium chloride brine, it has been found that an operating electrolyte pH of about 9.5 in the cathode chamber produces good yields of chlorine gas. The pH of the anolyte will be relatively low, and by appropriate operation of the process can be reduced to about 1 or less without the need for adding additional acid as practiced conventionally.
One embodiment of a cell especially suitable for conducting the process of the present invention is depicted schematically in the figure.
With reference to the figure, the electrolytic cell comprises a housing 1 having inner walls which define a chamber 4, containing a diaphragm 5. The diaphragm is attached in sealing engagement to the inner walls of the housing 1, and divides said chamber 4 into a cathode chamber 6, and an anode chamber 7. The cathode chamber contains during operation, a catholyte 26, and the anode chamber contains an anolyte 27.
The anode chamber has disposed therein, an electrode. Preferably, the electrode is a graphitic anode 8, and is affixed to an inner wall of the housing 1.
The cathode chamber has disposed therein an electrode which is preferably a smooth steel cathode 9 which is af fixed to an inner wall of the housing 1, in contact with the cathode chamber.
The cathode chamber 6 and anode chamber 7 are fitted with brine supply means 2 and 3 respectively which are in operative communication with a source of supply (not shown). Preferably the supply means will be a conduit consisting of material substantially nonreactive with the brine electrolyte stream, such as glass, plastics, and certain types of steel.
The cathode chamber 6 is also fitted with a liquid discharge means 10 which is in operative communication with a solids removal system 11. Generally, the means of communication will be by a conduit consisting of material substantially nonreactive with the brine electrolyte stream. The solids removal system can be a settling tank, filtering device or any other device employed to separate the solid portions of a slurry from the liquid portion thereof. The residual liquor passes from the solids removal system and can be disposed of.
Optionally, the solids removal system 11 can be fitted with a solids-depleted aqueous brine conduit 13 which communicates with the cathode chamber, said conduit being in communication with a waste time conduit 12 prior to communicating with the cathode chamber.
The bottom of the anode chamber 7 is fitted with a liquid discharge means 15. Optionally, the means of liquid discharge can be a conduit for disposal of waste brine which recommunicates with the anode chamber, thereby converting the liquid discharge means 15 into a means of recirculating brine. Conduit 15 is fitted with a waste brine conduit 20 prior to communication with the anode chamber.
The anode 8 and cathode 9 are adapted to communicate with a source of electrical power 17. Such communication could be achieved by fitting both electrodes with electrical leads 18, which communicate with said power source 17.
The anode chamber 7 is fitted with gas discharge means 21, and the cathode chamber 6 is fitted with gas discharge means 22. The anode gas discharge means 21, is in operative communication with a chlorine storage system (not shown).
In operation, brine is supplied to chambers 6 and 7 through conduits 2 and 3 respectively. The supply of brine is regulated so that there is substantially no fluid pressure differential built up between the anolyte and catholyte.
As operation of the cell proceeds, a slurry of particulate water-insoluble metal hydroxide forms in the catholyte. The flow of brine through the cathode chamber will cause the hydroxide slurr'y to exit from the chamber through conduit 10. The slurry then passes through a suitable solids removal system 11. The residual hydroxidedepleted brine exits from the solids removal system and continues on through conduit 13 to reenter the cathode chamber. Portions of the hydroxide-depleted brine can be discarded through conduit 12 prior to reentry into the cathode chamber. Discarding of brine reduces the liquid pressure of the catholyte and increases the metal ion concentration therein.
Optionally, the recirculation of hydroxide-depleted brine can be abandoned by disconnecting conduit 13 where it communicates with the cathode chamber. In this case, the supply of fresh brine through conduit 2 should be increased so that the liquid pressure of the catholyte and anolyte will remain approximately equal.
At the anode, electrolysis produces chlorine gas as the brine enters the anode chamber through conduit 3, and exits through conduit 15.
The brine exiting from the anode chamber through conduit can either be discarded, or recirculated back through the anode chamber by having conduit 15 communicate with anode chamber through the top member of said chamber.
If the anolyte brine is recirculated, conduit 15 is equipped with a waste brine conduit which permits reduction of the liquid pressure of the anolyte by discarding part of the brine to be recirculated.
As chlorine gas is formed in the anode chamber, it passes from said chamber through conduit 21 to a suitable chlorine storage s'ystem. The hydrogen gas formed at the cathode passes from the cathode chamber through conduit 22 and can be discarded or recovered, if desired.
The electrolytic cell casing is made of any material substantially resistant to corrosion by the brines employed, such as for example glass.
The diaphragm is composed of a substantially inert porous material with suflicient mechanical integrity to remain intact during operation of the cell. The permeability of the diaphragm is such that from about 0.5 to about 2 cubic feet per minute of air can be made to pass through one cubic inch of the diaphragm when a pressure equivalent to /2 inch of water is applied to the air. The thickness of the diaphragm is from about 0.1 to about 0.4 inch, and preferably is about 0.2 inch. The diaphragm material must also be chemically resistant to brine corrosion. Some suitable diaphragm materials are for example, woven polyacrylonitrile, polypropylene, fiber-mass and reinforced asbestos. Felts of the above material can also be employed. Porous ceramics and glasses such as Alurrdum and porcelain are also useful.
The cathode is compoesd of a ferrous-based material such as iron or mild steel, and the surface of the cathode in contact with the brine solution is of such a uniform nature as to be substantially smooth, and level with relatively few surface irregularities. Such a smooth steel electrode surface readily can be obtained for example, by subjecting the surface to the action of a suitable finely divided abrasive.
The anode is generally composed of graphite of any type commonly employed in electrolytic cells. Other suitable anodes include cobalt oxide coated anodes described in US. Patent 3,399,966 incorporated therein by reference. Also titanium coated with metals of the platinum sub group are useful. The platinum-coated anodes are described in US. Patents, 3,236,756 and 3,428,544 incorported therein by reference.
The distance between the cathode and anode can vary over a wide range. Preferably, however the distance will range from about 0.2 to about 1.0 inch. In operation, the current density generally ranges from about 0.25 to about 1.0 ampere per square inch.
The electrical leads in contact with the electrodes may consist of any suitable electrical conducting material. The electrical leads may communicate directly with a commonly employed source of electrical power, or optionally another electrolytic cell such as a cell of the present invention can be inserted before the leads communicate with the source of electrical power.
EXAMPLE A substantially debrominated brine containing about 19 percent by weight of calcium chloride and about 3 percent by weight of magnesium chloride was electrolyzed in an electrolytic cell substantially similar to the cell depicted in the figure. (The distance between the cell electrodes was about 0.5 inch.) The diaphragm employed was a Dynel brand 5D-9 spun fabric diaphragm with a 60 by 40 thread count, and a weight of 12 /2 ounces/ yard As depicted in the figure, a recirculating system was employed. The solids removal system consisted of a settling tank.
The temperature of the brine was about 70 C.
The pH of the system was maintained at about 9.5. The feed rate of fresh brine into the cathode chamber was about 4 mL/minute. The feed rate of recirculated hydroxide-depleted brine into the catholyte was about 420 mL/min. The feed rate of brine into the anolyte was about 6-8 ml./min. The anolyte feed brine was hydroxide-depleted recirculated brine. The flow rate of waste anolyte was about 3.8 ml./min.
The anode operated at about 99.7 percent current efficiency and the resulting chlorine gas contained by weight about 0.10 percent CO about 0.02 percent 0 and about 0.02 percent H The current density employed was about 0.5 amps/infi.
The resulting slurry of magnesium hydroxide exiting as underflow from the settling tank contained about 25 percent by weight solids. The average particle size of the slurried magnesium hydroxide particles was about 1.45 microns. The magnesium hydroxide isolated from the slurry contained trace amounts of boron, calcium, silicon, aluminum, iron, and chloride.
What is claimed is:
1. An electrolytic cell to produce chlorine at an anode, hydrogen at a cathode and an alkaline earth metal hydroxide in a cathode chamber comprising:
anode and cathode chambers separated by a vertically positioned ion-permeable diaphragm,
a vertically positioned anode disposed in the anode chamber,
a vertically positioned cathode disposed in the cathode chamber,
a chlorine gas discharge means in combination with said anode chamber,
a hydrogen gas discharge means in combination with said cathode chamber,
a brine supply means adapted to regulatably supply the brine to said anode and cathode chambers to provide substantially equal brine pressures in combination with the anode and cathode chambers,
a liquid discharge means in combination with said anode chamber, and
a solids removal system positioned to receive the alkaline earth hydroxide emanating from the cathode chamber.
2. A cell as in claim 1 wherein the solids removal system is fitted with a conduit for solids-depleted liquors, said conduit communicating with the cathode chamber thereby to recirculate solids-depleted liquors through the chamber.
3. A cell as in claim 1 wherein the solids removal system is a settling tank.
4. A cell as in claim 1 wherein the anode is graphite and the cathode is carbonaceous steel having a substantially smooth and level face for electrolysis of brine, with the distance between the anode and the smooth cathode face being from about 0.2 to about 1.0 inch.
5. A cell as in claim 1 wherein the diaphragm is a planar sheet of material substantially resistant to brine corrosion with a generally uniform thickness of from about 0.1 to about 0.4 inch and with an air permeability of from about 0.5 to about 2 cubic feet per minute at a pressure equivalent to /2 inch of water.
References Cited UNITED STATES PATENTS 2,882,210 4/1959 Jenks 204128 3,378,479 4/ 1968 Colvin et al. 204272 514,318 2/1894 Greenwood 204-258 3,547,800 12/1970 Pan 204-149 JOHN H. MACK, Primary Examiner W. I. SOLOMON, Assistant Examiner U.S. Cl. X.R.