US2232128A - Electrolysis of alkaline metal chlorides and apparatus therefor - Google Patents

Description

E. M LLER Feb. 18, 1941.

Filed June 24, 1937 mw 4 uw *bwl l \Wl YN. Y J 1 n 1 A. w

rich Hllen Stro- HCM Patented Feb. 18, 1941v PATENT OFFICE ELECTROLYSIS 0F ALKALINE METAL CHLO- RIDES AND APPARATUS THEREFOB Erich Mller. Dresden, Germany, assignor, by

mesne assignments. to Th. Goldschmidt Corporation, New York, N. Y., a corporation of Delav v./liplilieation Jane 24, 1931, serial No. 150,021

In Germany April 13, 1937 2 Claims.

'lhis invention relates to electrolysis of alkaline metal chlorides and apparatus therefor; and it includes a process which comprises passing an alkaline metal chloride solution through an electrolytic cell while electrolyzing the same, passing a stream of mercury through said electrolytic cell; the mercury serving as cathode and being converted to alkaline metal amalgam during the process; passing the resulting amalgam through an amalgam-decomposing cell having an inclined bottom. of catalytic material, formed in spaced parallel tracks, in contact with an amaigam-decomposing solution; the said mercury be-` ing passed into said electrolytic cell intermittently in pulsations of suificient magnitude to produce a pulsating flow'through said cell and into the amalgam decomposing cell where it is broken up into droplets which roll in said tracks along the inclined bottom, thereby producing a' maximum surface exposure; collecting the mercury from the decomposing cell and returning it to the electrolytic cell. My invention further includes an apparatus for conducting said process, said apparatus comprising an electrolytic cell and an amalgam-decomposing cell, means for introducing mercury intermittently in a pulsating iiow into said electrolytic cell, means'for passing an alkaline metal chloride solution through said cell, means for passing the resulting amalgamated mercury from said electrolytic cell into said decomposing cell without substantial disturbance of, said pulsating flow, means including asloping cell bottom formed in spaced parallel tracks ior passing the partially amalgamated mercury through said decomposing cell in the form of droplets and in contactwith a decomposing liquor, vand means for returning the resulting mercury to the electrolytic cell; allas morefully hereinafter setforth and as claimed.

Many different methods have been proposed in the prior art for electrolyzing alkali metal chloridel solutions. In several of these methods use ismade of a owing mercurycathode. And a number of diierent electrolytic cell designs have been proposed for this process. 'Ihe mercury which acts as cathode usually ows over the floor of the electrolytic cell while a. sodium chloride solution is passed over this mercury. 'I'his solution is electrolyzed as it passes through the cell between the mercury cathode and the anodes, usually of carbon. Chlorine is evolved at the anodes and is passed out of the cell and recovered. The mercury in passing through the cell is partially converted to sodium amalgam and this is then decomposed by means of water or an aqueous solution into mercury and causticlsoda in a separate decomposing cell. The hydrogen formed during the decomposition is collected separately. A catalyst, such as graphite, is usually employed in the decomposing cell to assist in4 the decomposition of the amalgam. 'I'he mercury which is recovered from the latter cell is then recycled to the electrolytic cell to be used in a repetition of the process. In the commercial operation of this lu process various difficulties have arisen.

When the conventional carbon anodes are employed in the electrolytic cell it has been found that these anodes tend to disintegrate slowly. This causes the production` of fine carbonparticles which, of course, tend to float on the sur- `face of the mercury or of the amalgam. These carbon impurities, as Well as the iron amalgam impurities which are usually present, due to a slight reaction between the mercury and the iron of the containing vessels, tend to act catalytically causing. the decomposition of the amalgam in the electrolytic cell. This leads to the formation of hydrogen gas which, of course, is liable to form explosive mixtures with the chlorine evolved in the electrolytic cell.

The difilculties caused by impurities in the mercury can be mitigated to some extent by the provision of a plurality of transverse weirs across the oor of the electrolytic cell over which the mercury is forced to pass seriatim, the iloor of the cell being stepped downwardly from the entrance to the exit. The impurities are thus forced to pass over the weirs and a return is rendered impossible.

The same impurities cause diillculties in the amalgam-decomposing cell. In the conventional decomposing cell the alkali metal amalgam is slowly passed across the iloor of the cell in countercurrent to a. flow of water, the floor of the cell being usually constructed of catalytic material such as graphite. Grids of catalytic material are also provided which project above the surface of the mercury. It has been found that the passages between the grids become obstructed by the impurities in the amalgam. Furthermore in this type of decomposing cell the floor of the cell is completely covered with amalgam which therefore also covers the surface of any catalytic ma- 50 terial ofwhich the floor is constructed. This surface is thereby rendered inactive. Even when a grid of catalytic material is employed, the decomposition of the amalgam in the cell is slow. One cause for this is that the more dilute amal- 55 gam is more dense than the more concentrated amalgam. 'I'he latter therefore tends to float and to become'highly viscous on its surface, especially if it contains any substantial quantities 'of impurities. The iiuidity of the surface of the amalgam is considerably less than that of the heavier and more dilute amalgam on the oorA of the cell. The amalgam therefore tends to ow along the floor of the cell to the exit without coming in contact with the decomposing liquor and therefore without decomposing. The surface of the amalgam tends to become stagnant and a Idelayed decomposition as well as a lowered efficiency is an inevitable result.V

I have found that the enumerated diiiculties which arise in the decomposing cell can be eliminated in a surprisingly easy and commercially feasible manner. 'Ihis is accomplished by slightly tilting the floor of the cell downwardly towards the exit and providing a'pulsating ilow of the amalgam to the cell. The amalgam then tends to form in droplets which are flattened more or less and which roll across the oor of the cell thus continuously supplying fresh amalgam surfaces to the action of the decomposing liquor and also exposing the floor of the cell, thus rendering active its catalytic surface. 'I'he three-phase line- ',contact of catalyst, amalgam and decomposing The rate of decomposition is increased still further by the provision of a catalytic grid comprising spaced parallel bars running longitudinally along the floor of the cell, these bars forming spaced tracks for the flow of amalgam droplets. This prevents coalescence of the droplets and provides a uniform flow` of droplets across the floor of the cell. Furthermore the total quantity of mercury-amalgam mixture in the decomposing cell is greatly reduced and this reduces the quantity of mercury required in the process yas a whole. A pulsating ilow of mercury through the electrolytic cell is employed in combination with a similar flow through the decomposing cell. It is usually possible to produce a positive pulsating flow at one point in the cycle, the pulsations carrying over from one cell to the other. If a posi tive pulsating flow is provided for the electrolytic cell, by means of a pump for example, these pulsations will cause the mercury to flow in pulsations throughout the length of this cell and, if the amalgam leaving this cell is caused to flow into the decomposing cell in a manner preventing disturbance of these pulsations, the flow into the latter cell will also be in the form of pulsations, these pulsations being usually suicient to produce a flow of droplets across the floor of the decomposing cell in the manner described previously.

Itis possible, of course,v to produce independent and positive pulsations in the flow of the mercury through the decomposing cell. 'Ihis can be accomplished by vibrating or rocking the decomposing cell, for example. If thedecomposing cell is vibrated these vibrations tend to break up the droplets of amalgam and to increase further the surface exposed to the action of the catalyst and decomposing liquor.

The pulsating flow in the electrolytic cell has the added advantage of producing a positive propulsion of the impurities towards the cell exit.

These floating impurities are lifted over any weirs provided in this cell by the pulsations and may be collected in a well at the lower end and removed without disturbing the operation of the process.

It is also advantageous to provide a similar well at the lower end of the decomposing cell from which the impurities may be removed prior to repassing the mercury into the electrolytic cell.

It is advantageous but not essential to provide a counter current flow of water, as decomposing liquid, and of amalgam in the decomposing cell. The fresh water then comes into Ycontact with the most dilute amalgam while the most concentrated caustic alkali solution in the. cell is contacted with the amalgam as it enters the cell. The stirring action which is produced by the evolution of hydrogen in this cell can be minimized by the use of a low head of decomposing liquor in the cell.

My invention can be explained in greater detail by reference to the accompanying drawing which shows an assembly of apparatus elements within the purview of my invention and useful in @the conduct of my process. In this showing,

gam-decomposing cell with related apparatus, l

while Fig. 2 is a vertical transverse section through the amalgam-decomposing cell, taken along the line 2-2 of Fig. l.

In the drawing like parts are represented by like reference numerals. The electrolytic cell is designated generally by the reference-numeral I, the amalgam-decomposing cell being similarly designated at 2. The bottom of the electrolytic c ell is stepped, as shown, forming a series of trays 3B, 3b, 3c and 3d which are separated by the transverse wiers 4I, 4b, 4, 4d and 4'3 over which the mercury 5 passes on its way through the cell.V

hollow cylinder 1 which is provided with a slot 8.v

The resulting pulsating iiow of mercury passes under the dam I2, over the Weir IIa upon the tray 3a, then over the weir 4b and so on from tray to tray finally passing over Weir 4e through the opening I4 formed below the dam I5 and into the well I3 at the vopposite end of the cell. The well I3 is provided with 'a cover 20. The solution of alkali metal chloride enters the cell through the pipe I I and leaves at I'I. The chlorine evolved is removed at I 8. The anodes, usually of carbon or graphite, are shown at I9. course, forms the cathode. Electrical connections to the anodes and to the mercury cathode are made as shown in the drawing. 'Ihe connection tothe mercury cathode may be made through the oor of the cell if this is made of metal.

The mercury is transformed into alkali metal amalgam in passing through the electrolytic cell and this is collected in the well I3. Various impurities collect, on top of this product and may be drawn 01T through the opening below the cover 20. The amalgam, substantially free from im-` purities, is drawn olf by the tube 2| which connects with the bottom of the well I3. The amalv 2,232,128 gam is then passed into the well 22 of the decom-A posingvessel or cell;

The decornposing cell 2 is provided with a sloping bottom 35 which is advantageously made of catalytic material. such as graphite. A grid 23, usually o f'the same material. is supported on the bottom oi the cell. This grid is made of a series of spaced parallel strips, -as shown best in Fig. 2, which serve to divide the mercury flowing along .the bottom of the cell into a plurality o1 parallel streams.

The pulsating ieed'to the electrolytic cell produces corresponding pulsations through the tube 2| to the decomposing cell. This pulsating feed causes the amalgam to flow over the bottom of the decomposing cell in the form of a series of droplets as indicated in Fig. 1, each impulse propromoting a very rapid decomposition ofthe alkali metal amalgam.

The amalgam is decomposed in passing through the cell forming mercury which passes under the dam and falls into the Well 24 at the left hand end of the decomposing cell. Any impurities which collect on the top of the mercury in well-24 canbe removed by skimming, for example, through the opening 26 at the top koi! the well 24, without disturbing the operation of the process. The mercury is drawn oil. from the bottom of the well 24 and falls into a container 21. The pump 23 draws the mercury from the container 2l, and delivers it to the vessel 6 from which point it is again fed to the electrolytic cell,

to complete the cycle.

The decomposing cell is supplied with water or other decomposing liquor at 29 and the caustic solution which is formed during decomposition of the amalgam ilows out of the cell at 30. The hy drogen generated in the cell is removed at 3 I.

While, as stated previously, the pulsations produced by the cylinder 1, as the mercury is fed into the electrolytic cell, are generally sufficient to produce a corresponding pulsating ilow into the decomposing cell, it isv frequently advantageous,`especially in ,the case of large-scale operations, to provide for more positive pulsations in the ow of the mercury along the floor of the decams a wave of the partially amalgamated mercury flows over from the well 22 upon the iioor of the decomposing cell. 'I'his amalgamated mercury is broken up into droplets by means of the grid 23 which droplets then roll over the oor Voi the cell as indicated in the drawing. Substantially the same result can be accomplished by the use of one ofthe conventional electrical vibrators which may be attached to the floor of the decomposing cell. The amplitude and the frequency of the vibrations produced in this manner can be readily controlled in order to produce droplets of posing cell in such manner that the catalytic surface thereof is exposed to the decomposing liquor. Subdivision of the mercury is advantageously produced both longitudinally as well as transversely of the cell, by means of a grid, for example.

While I have described what I consider to be the best embodiments of my invention, it is obvious that various modiiications can be made withoutdeparting 4from the purview thereof. For example, either or both the electrolytic cell and the decomposing cell can be vibrated or rocked to produce the desired pulsating ilow of mercury and amalgam. 'I'he pulsating iiow of amalgam to the decomposing cell can be supplied by means oi" a slotted piston similar to that shown at I in the drawing if desired. My process can be applied to the decomposition of any of the alkaline metal chlorides, including chlorides 'of the alkali metals and of the alkaline earth metals. The mercury or amalgam can be passed in counter current to or in parallel with the ilow of the aqueous liquor in either or both of the cells. As a decomposition liquor, water, a caustic alkali solution or any other aqueous solution can be employed. It is not essential that'the 4ilooi of the decomposing cell be made oi' catalytic material since the use of a pulsating ow of mercury through this cell, in accordance with my invention, has advantages even when this iioor is of inert material. The impurities which collect in wells I3 and 24 may be removed mechanically if desired either continuously or` intermittently. Other modicatlons which fall within the scope of the following claims will be immediately evident to those skilled in this art.

vWhat I claim is:

1. In the process of electrolyzing alkaline metal chloride solutions wherein such a solution is electrolyzed in a cell employing a flowing mercury cathode and the resulting amalgam is transferred by means of a tube to an amalgam decomposing cell having a floor of catalytic material formed into continuous spaced parallel tracks sloping downwardly from the entrance to the exit, the steps which comprisel introducing mercury into the electrolytic cell intermittently in pulsations of suilcient magnitude to produce a pulsating ilow through said cell, through said tube and into said amalgam decomposing cell at the upper edge of said sloping floor, the ilow of amalgam alongthe upper edge oi' said sloping iioor dividing into a plurality oi streams, the pulsations in said iiow being sufcient in magnitude and the slope of said iioor being suiTlcient to cause said streams of amalgam to break into droplets at each pulsation, whereby said droplets roll in said tracks to the'exit of the amalgam decomposing cell while being converted into mercury, collecting said mercury at the lower end of said sloping floor and returning it to the electrolytic cell. y

2. In an apparatus for the electric decomposition of alkaline metal chloride solutions, an electrolytic cell provided with a ilowing mercury cathode fordecomposing such solutions in combination with an amalgam decomposing cell having a floor of catalytic material formed-in continuous spaced parallel tracks which'slope downwardly from the catrame thereofA to the exit,

ymeansfor introducing mercury into said electroof 'said amalgam decomposing cell; the pulsations' in said iiow being sumcient in magnitude and the slope of said iloor being sufficient to subdivide the amalgam into droplets at each pulsation whereby said droplets flow in said tracks across said s1oping iloor to the exit o! said amalgam decomposing cell while being converted into mercury, and means ior transferring the resulting mercury to said electrolytic' cell. A

ERICH Mmm.

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