US2824054A

US2824054A - Process and apparatus for the manufacture of solutions of alkali metal hydroxides

Info

Publication number: US2824054A
Application number: US479704A
Authority: US
Inventors: Rene A Crabbe
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1954-01-06
Filing date: 1955-01-04
Publication date: 1958-02-18
Anticipated expiration: 1975-02-18
Also published as: NL193762A; FR1117036A; DE946620C; GB764403A; CH327267A; BE525535A; NL85436C

Description

Feb. 18, 1958 R. A. CRABBE 2,824,054

PROCESS AND APPARATUS FOR THE MANUFACTURE-OF SOLUTIONS OF ALKALI METAL HYDROXIDES Filed Jan. 4, 1955 United States Patent ice PROCESS AND APPARATUS FOR THE MANUFAC- TURE 0F SGLUTIONS 0F ALKALI METAL HY- DROXIDES Ren Al Crabbe, Boitsfort-Brussels, Belgium, assignor to Solvay & Cie., Brusseis, Belgium, a Belgian company Application January 4, 1955, Serial No. 479,704 Claims priority, application Belgium January 6, 1954 11 Claims. (Cl. 204--99) This invention relates to a process for the manufacture of solutions of alkali metal hydroxides by the decomposition of am-algams obtained by electrolysis of the corresponding salts such as for example chlorides or sulphates of sodium or potassium.

The decomposition of the 'amalgams is carried out in apparatus whose functioning can be compared to that of a so called denuder cell in which the amalgam forming the anode is decomposed in contact with the cathode constituted, for example, by a set of successive graphite grids. The apparatus for the decomposition of amalgams is often called a denuder.

It is known that the activity of denuders depends on that of the cathodes and that this activity progressively decreases with use. Immediately after having replaced a set of cathode elements it is possible to work in the presence of solutions having elevated concentration of alkali metal hydroxide, but gradually it becomes necessary to work with more dilute solutions in order to decompose the amalgam at a still satisfactory rate. Finally it becomes necessary to replace the cathode elements.

As in practice, the tendency prevails to produce more and more highly concentrated solutions, it is necessary to replace the cathodes more and more frequently. These replacements provoke disturbances in the normal working of the denuders. Independently of the fluctuationsin the alkali metal hydroxide concentration of the lyes, suspension of solid impurities in the lyes is produced by the handling of the grids therein. Another disadvantage derives from the fact that in the case of total replacement of the set of cathode elements of a denuder it is necessary to stop the cell which supplies the denuder with amalgam.

Attempts have been made to obviate these disadvantages by improving the construction of the elements constituting the cathode so as to impart to them a prolonged activity. Up to now, it has, however, not been possible to avoid the said disturbances. f

It is an object of the invention to provide a process which permits to produce very pure solutions of alkali metal hydroxides at elevated concentration in a continuous and indefinitely prolonged manner by keeping constant the total activity of the whole set of cathodes of a denuder.

In denuders of the horizontal type the cathode layer is. formed by elements which have the form of a grid whose graphite bars form the cathode elements proper. These bars can be in direct contact with the amalgam to be decomposed; they can likewise be kept at some distance from the amalgam layer, but in this case the shortcircuit of the denuder is safeguarded by contact pieces, preferably metallic ones, which are immersed in the mercury. These graphite bars lose finally their activity of decomposition even if they have not become white or have not acquired a metallic look. This loss of activity can be explained by the fact that, in the decomposition of amalgam, not only the ions of the alkali metal are liberated and react with water, but also, by the fact 2,824,054 Patented Feb. 18, 1958 that under the action of the generated electric current, metallic contaminations of the amalgam decompose on the cathode bars and modify the electric characteristics of same.

The process of the invention comprises the moving of the set of grids from one end to the other end of the denuder at a rate which will be in accordance with the rate of the activity loss of the graphite, removing one grid at said last end and placing of a fresh or regenerated grid into the space which has become free in the denuder. In practice, this displacement can be carried out in a discontinuous manner, but it is possible to provoke a slow but continuous displacement without transgressing the scope of the invention.

The grids removed from the end of the denuder are then regenerated in known manner, such as for example, by steaming, or by applying mechanical means suitable for the cleaning of surfaces, etc.

It has been found that by applying the process according to the invention there are obtained alkali metal hydroxides which are superior in purity to those obtained by known processes.

The new process is applicable to amalgam of alkali metals however obtained. When the latter are obtained from an electrolytic cell which is coupled with the denuder (an arrangement usually applied in the electrolysis of aqueous solutions) the working. of the denuder has to be interrupted when it is necessary to replace the set of worn anodes, even if the said cell comprises a device for compensating for the wear of the anodes during working. On the contrary, it is conceivable that the continuous working of the denuder could be unlimited when the same denuder receives the amalgam from two or more cells. This uninterrupted working may also be ensured by coupling the denuder according to the invention with an electrolytic cell or" the type described in Belgian specification 499,224 of Nov. 7, 1950, according to which the anodes are successively replaced in proportion as they wear down.

The invention also comprises a device for carrying out the process in a denuder of the horiontal type substantially consisting in providing the cell with a compartment for introducing fresh cathodes and a compartment for removing the least active cathodes, the compartment placed on the side of the inlet of the concentrated alkali metal amalgam, being advantageously provided with a guide which prevents the contact between the cathode and the concentrated amalgam.

The accompanying drawing illustrates diagrammatically and by way of example a longitudinal section through a denuder for carrying out the process according to the present invention.

As shown in the drawing the denuder comprises a tank 1, an inlet 2 for the amalgam, an outlet 3 for the mercury, an outlet 4 for the lye, and a hydraulic mercury guard 5. The tank is closed by a lid 6 provided with gas outlets 7. The compartment of decom osition of amalgam is delimited by two weirs 3 and d which are gas-tight and immersed in the solution of alkali metal hydroxide It The compartment for the decomposition is situated between two other compartments: the first of which 11 serves for introducing fresh cathodes and the second one 12 which serves for removing the cathodes which have become inactive.

The introduction of fresh cathodesis carried out while the denuder is working, and it is therefore necessary to avoid any contact between these cathodes and the amalgam at the moment of their introduction. For this reason, the compartment for introducing the cathodes is pro vided with a slide 13 which keeps the fresh cathodes above the amalgam and conducts them directly into the decomposition compartment. The compartment for removing cathodes may similarly be provided with a slide 20. When a fresh cathode 14 is introduced the whole series or train of cathodes 15 moves forward by the range of one cathode, after the oldest one 16 has previously been removed in order to allow the translation of the other cathodes along the amalgam 17. The compartment for introducing new cathodes and the compartment for withdrawing used cathodes are provided with hinged

lids

18 and 19. The apparatus comprises also an inlet (not shown) for introducing fresh water into the denuder.

The drawing is not limitative, as any design of denuders of the horizontal type may be used if it can be fitted with an entrance and exit compartment for the cathodes, provided that the compartment at the side of the concentrated amalgam is furnished with a device avoiding the contact between the fresh cathode and the concentrated amalgam as long as the said cathode remains outside the decomposition compartment. The said entrance compartment may be situated also at the other end of the apparatus.

I claim:

i. In a process for continuously producing an alkali metal hydroxide solution of uniform high concentration and purity by the decomposition of alkali metal amalgams in an elongated decomposition zone in the presence of a plurality of graphite cathode elements, said elements decreasing in activity from a first end of said elongated decomposition zone to a second end thereof, the steps which comprise removing from said second end of said zone the cathode element of least activity, introducing a fresh cathode element into said first end of said zone, and moving all of the cathode elements toward said second end of said decomposition zone a distance corresponding to that occupied by the cathode element removed.

2. In a process for continuously producing an alkali metal hydroxide solution of uniform high concentration and purity by the decomposition of alkali metal amalgams in an elongated decomposition zone in the presence of a plurality of graphite cathode elements, said elements decreasing in activity from a first end of said elongated decomposition zone to a second end thereof, the steps which comprise removing from said second end of said zone the cathode element of least activity, introducing a fresh cathode element into said first end of said zone, and moving all of the cathode elements toward said second end of said decomposition zone a distance corresponding to that occupied by the cathode element removed, and repeating said steps a plurality of times.

3. In a process for continuously producing an alkali metal hydroxide solution of uniform high concentration and purity by the decomposition of alkali metal amalgams in an elongated decomposition Zone in the presence of a plurality of graphite cathode elements, said elements decreasing in activity from a first end of said elongated decomposition zone to a second end thereof, the steps which comprise removing from said second end of said zone the cathode element of least activity, introducing a fresh cathode element into said first end of said zone, and moving all of the cathode elements toward said second end of said decomposition zone a distance corresponding to that occopied by the cathode element removed, and repeating said steps a plurality of times at a rate related to a predetermined concentration and purity of said alkali metal hydroxide solution.

4. A denuding cell of the horizontal type for continuously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated substantially horizontal trough having a sole for supporting the amalgam as it flows through the cell, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment in a position for electrically-conductive contact with said amalgam, said inlet and said outlet compartments each being of a size sulficient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said cathode elements to and from said compartments and said main reaction chamber, at least said inlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of all contact with said amalgam until it is introduced into the main reaction chamber, and said guide means in said inlet compartment being constructed to guide said cathode element automatically from a position substantially above the amalgam to the position occupied by said cathode elements in the main reaction chamber.

5. A denuding cell of the horizontal type for continuously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated substantially horizontal trough having a sole for supporting the amalgam as it flows through the cell, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment, said cathode elements being positioned to be in direct contact with said amalgam, said inlet and said outlet compartments each being of a size sufficient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said cathode elements to and from said compartments and said main reaction chamber, at least said inlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of all contact with said amalgam until it is introby said cathode elements in the main reaction chamber.

-6. A denuding cell of the horizontal type for continuously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated substantially horizontal trough having a sole for supporting the amalgam as it flows through the cell, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment, and electrically-conducting contact pieces for separating said cathode elements from the amalgam but maintaining them in electrically-conducting contact therewith, said inlet and said outlet compartments each being of a size sufiicient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said cathode elements to and from said compartments and said main reaction chamber, at least said inlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of all contact with said amalgam until it is introduced into the main reaction chamber, and said guide means in said inlet compartment being constructed to guide said cathode element automatically from a position substantially above the amalgam to the position occupied by said cathode elements in the main reaction chamber.

ously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated substantially horizontal trough having a sole for supporting the amalgam asfit flows through the cell, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment in a position for electrically-conductive contact with said amalgam, said inlet and said outlet compartments each being of a size sufficient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said cathode elements to and from said compartments and said main reaction chamber, said main reaction chamber having a cover and said compartments having independent covers whereby the end compartments may be opened without opening the main reaction chamber, at least said inlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of all contact with said amalgam until it is introduced into the main reaction chamber, and said guide means in said inlet compartment being constructed to guide said cathode element automatically from a position substantially above the amalgam to the position occupied by said cathode elements in the main reaction chamber.

8. A denuding cell of the horizontal type for continuously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated substantially horizontal trough having a sole for supporting the amalgam as it flows through the cell, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment in a position for electricallyconductive contact with said amalgam, said inlet and said outlet compartments each being of a size sufiicient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said cathode elements to and from said compartments and said main reaction chamber, said inlet compartment and said outlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of contact with said amalgam, said guide means in said inlet compartment being constructed to guide Said cathode element automatically from a position substantially above the amalgam to the position occupied by said cathode elements in the main reaction chamber.

9. A denuding cell of the horizontal type for continuously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated substantially horizontal trough adapted to contain the solution at a predetermined level and having a sole for supporting the amalgam as it flows through the cell and a top, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment in a position for electrically-conductive contact with said amalgam, said inlet and said outlet compartments each being of a size sutficient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said. cathode elements to and from said compartments and said main reaction cham ber, said means defining the inlet compartment being a weir extending from the top of the trough to a point below said level whereby to be immersed in the solution, at least said inlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of all contact with said amalgam until it is introduced into the main reaction chamber, and said guide means in said inlet compartment being constructed to guide said cathode element automatically from a position substantially above the amalgam to the position occupied by said cathode elements in the main reaction chamber.

10. A denuding cell of the horizontal type for continuously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated sub stantially horizontal trough having a sole for supporting the amalgam as it flows through the cell, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment in a position for electrically-conductive contact with said amalgam, said inlet and said outlet compartments each being of a size sufiicient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said cathode elements to and from said compartments and said main reaction chamber, said inlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of all contact with said amalgam until it is introduced into the main reaction chamber, said guide means being constructed to guide said cathode element from a position substantially above the amalgam to the position occupied by said cathode elements in the main reaction chamber, and said outlet compartment containing guide means to facilitate withdrawal of the cathode of least activity.

11. A denuding cell of the horizontal type for continuously producing an alkali metal hydroxide solution of high and substantially constant strength by the decomposition of the corresponding alkali metal amalgam, said cell comprising, in combination, an elongated substantially horizontal trough adapted to contain the solution at a predetermined level and having a sole for supporting the amalgam as it flows through the cell and a top, means defining an inlet compartment at a first end of said trough, means defining an outlet compartment at a second end of said trough, and a main reaction chamber between said compartments, a plurality of graphite cathode elements disposed in laterally adjacent relationship extending from said inlet compartment to said outlet compartment in a position for electrically-conductive contact with said amalgam, said inlet and said outlet compartments each being of a size sufficient to accommodate a cathode element outside of said main reaction chamber, and said means defining said compartments permitting movement of said cathode elements to and from said compartments and said main reaction chamber, said means defining the outlet compartment being a weir extending from the top of the trough to a point below said level whereby to be immersed in the solution, at least said inlet compartment being provided with guide means for supporting a cathode element outside of said main reaction chamber and holding it out of all contact with 7 said amalgam until it is introduced into the main reaction chamber, and said guide means in said inlet compartment being constructed to guide said cathode element automatically from a position substantially above the amalgam to the position occupied by said cathode elements in the 5 main reaction chamber.

8 References Cited in the file of this patent UNITED STATES PATENTS 2,551,248 Deprez et a1. May 1, 1951 FOREIGN PATENTS 235,756 Switzerland Apr. 16, 1945

Claims

1. IN A PROCESS FOR CONTINUOUSLY PRODUCING AN ALKALI METAL HYDROXIDE SOLUTION OF UNIFORM HIGH CONCENTRATION AND PURITY BY THE DECOMPOSITION OF ALKALI METAL AMALGANS IN AN ELONGATED DECOMPOSITION ZONE IN THE PRESENCE OF A PLURALITY OF GRAPHITE CATHODE ELEMENTS, SAID ELEMENTS DECREASING IN ACTIBITY FROM A FIRST END OF SAID ELONGATED DECOMPOSITION ZONE TO A SECOND END THEREOF, THE STEPS WHICH COMPRISE REMOVING FROM SAID SECOND END OF SAID ZONE THE CATHODE ELEMENT OF LEAST ACTIVITY INTRODUCING A FRESH CATHODE ELEMENT INTO SAID FIRST END OF SAID ZONE, AND MOVING ALL OF THE CATHODE ELEMENTS TOWARD SAID SECOND END OF SAID DECOMPOSITION ZONE A DISTANCE CORRESPONDING TO THAT OCCUPIED BY THE CATHODE ELEMENT REMOVED.