US1839756A

US1839756A - Method of electrolysis of fused bath and apparatus therefor

Info

Publication number: US1839756A
Application number: US336134A
Authority: US
Inventors: John J Grebe; Ray H Boundy
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1929-01-30
Filing date: 1929-01-30
Publication date: 1932-01-05
Anticipated expiration: 1949-01-05

Description

J. J. GREBE ET AL Filed Jan. 30, 1929 I N VEN TORS J s. @3 1H w, z

Jan. 5, 1932.

METHOD OF ELECTROLYSIS OF FUSED BATH AND APPARATUS THEREFOR U @(Jlib: 7 3 1 Q MTEWEW Patented Jan. 5, 1932 @UNITED ,STATES PATENT OFFICE JOHN J'QGREBEAND RAY H. BOUND Y, OI MIDLAND, MICHIGAN, ABSIGNOBB TO m DOWOHEMICAL COMPANY, OF MIDLAND, MICHIGAN, A CORPORATION OF MICEIGAN METHOD OF ELEOTROLYSIS F FUSED BATH AND AI'PABATUS THEREFOR Application filed January 80,

. chloride.

The. current efliciencies heretofore realized in the production of the alkali metals by the electrolysis of fused compounds thereof have not been satisfactory, being usually under reactions. .The metal usually obtained in such manner contained alkali metal compound, sometimes to the extent of 50 per cent of its weight, which circumstanceconstituted a serious obstacle to the success of the method. Still another obstacle was encountered in the rapid attack of the materials of which the cell. was constructed by the alkali metal and salt at the operating temperature.

We have now discovered a method of operation for electrolyzing a fused alkali metal halide bath at a temperature sufficient for the volatilization of the metal as formed, with the production of a substantially pure alkali I metal at current efficiencies well in excess of 50 per cent and have devised'a form of construction adapt d to withstand the severe conditions of ser ice without serious deteriora-- tion for long periods of continuous operation.

To the accomplishment of the foregoing and related ends, the invention, then, consists of the steps and means hereinafter fully described and particularly pointed out in the claims, the annexed drawing and the following description setting forth in detail certain 1829. Serial No. 338,184.

meansrand one mode of carrying out the invention, suchdisclosed means and mode'illustrating, however, but one of various ways in Whiich the principle of the invention may be use The single figure of the drawing accompanying this application illustrates in verti-,

cal cross-section an apparatus embodyin our invention with which we have obtain satisfactoryresults.

In said drawing,'l is a cylindrical steel caslng having a cover 2 which may be separable and removable, as indicated, or not. The casing 1 has a lining 3 and the cover 2 a lining 4 of suitable refractory material substantially 'unreact'ive with the'molten bath, such as fire-brick, forming and'enclosing a central cavity for the reception of the bath. Four anodes of graphitized carbon, three of which are shown at 5, 6 and 7, are inserted through the casing '1 and lining 3, extending into and across the bath cavity at or near the periphery thereof so as to provide there- .in four anode surfaces submerged in the bath and disposed in the form of a quadrangle. In modifications of the herein described structure, a different number of anodes may be used, if desired, in which case they are arranged to form the sides of the corresponding multilateral figure or poly on. The anodes are inserted through suitable collars packed against leakage of the bath or within which the bath itself will congeal and are supported at either end by embedding in the lining. A resistor heater 8 is preferably included in the structure and may have the form of a coil lying upon the bed of the cell, the leads for this heater being taken in and out at 9 and 10 respectively through suitable packable insulated outlets. This resistor heater furnishes a convenient means for fusing the initial charge or maintaining temperature during periods of shut-down for repairs or other times of interruption of operation.

A flanged steel collar 11, to which is applied a refractory insulating coating 12, preferably reinforced and attached firmly thereto by means of clinch members 13, the latter being conveniently in the form of wire loops or headed nails welded to said collar,

' the flange of collar 11. but is separated therefrom by a layer of insulating material 22. The level of the bath in the cell is maintained approximately as indicated, collar 11 and shell 17 dipping slightly below the surface thereof. Such collar with its insulating covering and such shell together form a doublewalled partition which divides the space above the bath level into an annular gas chamber 15 and a central well 16 for the col lection of the cathode products, such well opening into and being continuous with vapor chamber 21.

The collar 11 is preferably made of steel of substantial thickness, say inch thick, whereas the shell 17 is made of a very thin sheet steel, preferably inch, or less, thick. The flange 18 may he of thicker material for the purpose of securing greater rigidity of the assembled structure, but the shell 19 is made of thin metal, the object being to cut down the heat capacity of the cylindrical shells 17 and 19 and their ability to conduct or radiate heat away from the cell.

A metal curb 23 is placed around the chamber 21 and rests upon the cover 2 of the cell, forming a space 24 which may be filled with heat insulating material, as, for example, common salt. Shell 19 is carried up sutiiciently above the cover 20 to provide a shelf 25 which may be filled with salt or other heat insulating material. An insulating member or bushing 26 of soapstone or like material is fitted within the central opening of cover 20, through which depends the cathode 27. of iron or other suitable metal, carrying at its lower end a grid :28. Between the cathode stem and the bushing 26 is a sleeve 29 carrying an inner conical baffle 30, from which in turn is suspended a cylindrical shield 31. parts 29, 30 and 31 being preferably made of thin sheet steel. The cathode 27 1S movably mounted as shown and is supported by a vertically adjustable carrier, as, for example, a chain hoist. The upper portion of shell 17 rises somewhat above the flange 18 and forms therewith and the shell 19 an annular trough or channel 32 connected to an outlet pipe 33 for draining off the liquid sodium metal which condenses in chamber 21. i

An outlet connection 34 for the chlorine gas liberated in the anode chamber may be inserted through the cover 2 as shown. The

chlorine outlet also furnishes an opening through which raw material is fed to the cell. A feeding device consisting of a hopper 37 and feed pipe 38 is connected to outlet 34. The feed pipe is provided with a screw eon- "eyer 39 or other suitable mechanical means for feeding the salt to the cell against the stream of chlorine escaping through outlet 34 and pipe 10.

Spaces 35 are preferably left between the anodes and the lining of the celhl'or the purpose of atl'ording a short closed path for circulation of the lluid bath around the anodes. It will be observed that the inner surfaces of the anodes are the active faces from which the bulk of the chlorine is liberated. chlorine rises through the bath from the inner faces causing an upward current of the fluid bath which may return by circulating around the anodes as indicated by the arrows, thus largely localizing the circulation of anolyte to the vicinity of the anodes and preventing the carrying ovcr to any substantial extent of anode products into the cathode space and vapor chamber. Such circulation of electrolyte around the anodes also tends to maintain a uniform composition thereof, as well as to assist the disengagement of chlorine from the anode surface.

If desired, the conical bafiie 30 may be duplicated, two or more being used, a second bafiie being indicated by dotted lines at 36. Such conical baifie or battles assist in directing the vapor circulation in the chamber 21 and also form a radiant heat screen between the bath and the cover 20. The oflice of the shield 31 is to direct the circulation of convection currents of the vapors as indicated by the arrows. The cell operates more or less satisfactorily without the shield 81, but we find it advantageous for positively directing circulation of vapor in the desired manner.

The space between the shell 17 and the collar 11 is sealed, as shown, by the bath and together with the shell and collar forms a heat insulating jacket for the cathode chamber. \Ve have found that, if shell 17 is omitted, sodium metal condenses upon the collar 11, runs down into the bath in which it dissolves, forming locally a bath composition of highly alkaline reaction. The effect caused thereby is to attack the refractory covering 12 as well as to corrode and eat away that portion ofthe collar 11 dipping into the bath. If the thin sheet metal shell 17 be inserted in the manner indicated this trouble is almost entirely avoided. In some cases a plurality of shells 17 may be employed, if desired, thereby forming two or more insulating gas spaces in the partition surrounding the cathode chamber.

We have found that the operation of the cell may be controlled by varying the depth of immersion of the cathode and consequent- Such tit)

Leanna .ly the area effective for electrolysis. B restricting the effective cathode area and t iereby malntalmng a relatively high current density at the cathode surface. a correspondingly high resistance is created locally at that about 880 C.. preferably between 900 and 950 C., that the sodium is vaporized immediately as it is formed. The sodium vapors accompanied by some salt vapors rise above the bath in the chamber 21. We have observed that the salt vapors apparently lose heat rapidly by radiation and condense to form droplets throughout the vapor space which fall back into the bath as a rain. As the sodium vapors continue to rise and are deflected toward the sides of the chamber 21 by the bailies, they become sufliciently red in temperature to condense upon the wall thereof. run down and accumulate in the channel 32 whence the liquid metal is drawn off through the pipe 33. Any small amount of sodium metal accompanying the condensed halide and returned to the bath in the highly heated zone adjacent the cathode is promptly revolatilized therefrom and thus largely prevented from going into solution.

IVe have found that the presence of an inert gas in the vapor chamber aids materially in setting up and maintaining a suflicient circulation of convection currents therein. In practice nitrogen is most conveniently supplied as the inert gas, inasmuch as the vapor chamber is filled with air at the beginning of operation, from which the oxygen is quickly removed by combination with sodium. leaving an atmosphere of nitrogen in the chamber.

For observation and control of the cell temperature, which should preferably be maintained closely at a point just sufficient to liberate the metal as vapor, we employ thermo-couples located at strategic points, as at a, b and c.

As disclosed in our copending application. Scr. No. 290,691, filed July 5, 1928, now Patent No. 1.826.773, it is important that oxygen be prevented from entering the cell if a high current elliciency is to be realized. For the purpose of restricting the entrance of oxygen with the salt fed to the cell, we prefer to feed such salt in counter-current to the chlorine gas evolved from the anode chambel, suitable mechanical means therefor being indicated in the drawing. Feeding the salt to the cell through the chlorine outlet, in counter current to the chlorine, washes out substantially allof the air accompanying the salt as charged into the feed hopper. In this way the salt is introduced into the bath practically free from foreign entrained gases, thereby limiting very materially the entrance of oxygen into the'cell. An alternative method consists in meltin the salt outside of the cell and tapping t e molten salt through a pipe connection directly into the cell so as to prevent the entrance of air with the feed. In addition to the foregoing. when operating the cell, it is advisable, however, to employ the method of deoxidizing the bath disclosed in the above mentioned copending application.

In the operation of the cell herein de-' scribedwe are able to accomplish in a practical way the production of sodium by electrolyzing a fused bath of sodium chloride and separating the metal from the bathin vapor form by distillation as fast as it is formed. Detrimental side reactions are consequently almost entirely avoided, and under properly regulated conditions current efiiciencies in excess of per cent may be realed, the sodium produced having a purity as high as 99.9 per cent.

We obtain the metal in substantially pure form by means of a fractional condensation in the vapor chamber positioned directly abovh the active cathode surface. The mode of operation described permits condensation and return of vaporized salt immediately to the bath, while the sodium vapors are condensed on the walls of the vapor chamber and the liquefied metal collected in a trough or channel provided therefor. The arrangement of an internal shield and bafiles within the vapor chamber facilitates the separation of salt and metal vapors by positively directing the convection currents carrying the more volatile metal vapors from the hotter central zone to the cooler outer zone of the vapor chamber so that condensation occurs chiefly at or near the walls of such chamber Where the metal can be separately collected. The maintenance of convection currents in circulation within the vapor chamber is greatly aided by the presence therein of a volume of inert gas, such as nitrogen. Condensation of sodium vapors in the lower zone of the vapor chamber is largely prevented by providing a double-, or plural-walled, heat insulating jacket at such zone, having a gas space between the sections thereof, such double-walled construction also serving to protect the partition wall from chemical attack at the point where it dips into the molten bath. By providing for the circulation of the bath around the horizontally disposed anodes a uniformbath composition thereof, within practical limits, is secured, the rapid disengagement of chlorine from the anode surface is promoted and carrying over of anode products into the cathode region is substantially prevented. Finally the exclusion of air from the cell during operation, by washing the feed of raw material by means of the chlorine evolved from the chlorine chamber, or supplying the electrolyte in fused condition, maintains the bath largely or substantiall free from oxides, a condition essential or roper control of the electrolysis and for t e prevention of undered secondary reactions:

While we have described in detail a particular form of construction, various modifications thereof will be apparent to those skilled in the art, and we do not wish to limit ourselves to the'specific structure herein set forth. The circular form of construction has numerous advantages and, in general, is preferred, at least for a cell having a current capacity up to about 3000 amperes. Obviously a square or rectangular construction, however, can readily be devised, and in units of larger capacity than the above a rectangu lar arrangement may be more advantageous, as enabling an increase in the active electrode area of the cell without lengthening the distance between the active electrode surfaces. Furthermore, the horizontal disposition of anodes submerged in the bath, while preferred, is not essential to the invention in its broad aspects. It is possible, and under certain circumstances may be desirable, to provide a plurality of vertically or obliquely disposed anodes depending into the bath within the anode chamber or projecting upwardly through the bottom of the cell, suchrvertical or oblique anodes being in lieuof or auxiliary to the horizontal anodes as previously described.

It is further to be pointed out that, although the present electrolytic cell has been described more particularly with reference to the production of alkali metals, certain novel structural features have been disclosed which are of broader applicability, such that they may be readily adaptable to apparatus for the electrolysis of fused baths generally. The present application, therefore, is intended to cover in the broader sense such apparatus features as are hereinafter specifically referred to in the claims.

Other modes of applying the principle of our invention may be employed instead of the one explained, change being made as'regards the means and the steps herein disclosed, provided those stated by any of the following claims or their equivalent be employed.

WVe therefore particularly point out and distinctly claim as our invention 2-- 1. In an electrolytic apparatus of the character described, the combination of a closed vessel with a refractory lining forming a cavity to contain the fused bath, a plurality of anodes disposed about the periphery of said cavity and a cathode suspended centrally with respect to said anodes and dipping superficially into said bath.

2. In an electrolytic apparatus of the character described, the combination of a closed vessel with a refractory lining forming a cavity to contain the fused bath, a plurality of anodeshorizontally disposed about the periphery of said cavity, said anodes being spaced from the wall thereof for at least part of their length to permit free flow of electrolyte therebetween, and a cathode suspended centrally with respect to said anodes and dipping superficially into said bath.

3. In an electrolytic apparatus of the character described, the combination of a closed vessel with a refractory lining forming a cavity to contain the fused bath, a plurality of anodes disposed about the periphery of said cavity and a vertically adjustable cathode suspended centrally with respect to said anodes and dipping superficially into said ath. Y

4. In an electrolytic apparatus of the character described, the combination of a closed vessel with a refractory lining forming a cavity to contain the fused'bath, a plurality of anodes horizontally disposed about the periphery of said cavity, said anodes being spaced from the wall thereof for at least part of their length to permit free flow of electrolyte therebetween, and a vertically adjustable cathode suspended centrally with respect to said anodes and dipping superficially into said bath.

5. In an electrolytic apparatus of the character described, the combination of a closed vessel lined with refractory material forming a cavity to contain the fused bath, a plurality of anodes horizontally disposed about the periphery of said cavity and spaced therefrom for at least part of their length, a vertically adjustable cathode suspended centrally with respect to said anodes and dipping superficially into said bath and a partition also dipping superficially into said bath and dividing the space thereaboveinto an outer anode gas chamber and a central well surrounding said cathode.

6. Apparatus as claimed in claim 5, said partition having a double wall forming an enclosed gas space between the two sections thereof.

7 In an electrolytic apparatus of the character described, the combination of a closed vessel lined with refractory material forming a cavity to contain the fused bath, a plurality of anodes horizontally disposed about the periphery of said cavity and spaced therefrom for at least part of their length, a vertically adjustable cathode suspended centrally with respect to said anodes and dipping superficially into said bath, a double-walled partition also dipping superficially into said bath and dividingthe space thereabove into an outer anode gas chamber and a central well 'surroundin g said cathode, and a vapor chamber superposed above and continuous with said well and means associated with said chamber for collecting and condensing vapors evolved at said cathode.

v8. Apparatus claimed in claim 7, said vapor chamber being provided with internal baflies for directing the flow of convection currents therein, means for collecting condensed metallic vapors and means for Withdrawing such metal product.

9. In an electrolytic apparatus of the character described, the combination of a closed vessel lined with refractory material forming a cavity to contain the fused bath, a plurality of anodes horizontally disposed about the periphery of said cavity and spaced therefrom for at least part of their length, a vertically adjustable cathode suspended centrally with respect to said anodes and dipping superficially into said bath, a double-walled partition also dipping superficially into said bath and dividing the space thereabove into an outer anode gas chamber and a central well surrounding said cathode, means for withdrawing gaseous products from said anode chamber and means for feeding fresh halide into said anode chamber in counter current to such exit gases.

10. The process of producing an alkali metal which comprises electrolyzing a fused bath of an alkali metal halide, maintaining locally at the cathode a temperature above the volatilization point of said metal and separating the metal vapors evolved from said cathode.

11. The process of producing an alkali metal which comprises electrolyzing a fused bath of an alkali metal halide, maintaining locally at the cathode a temperature above the volatilization point of said metal, withdrawing vapors of metal and halide evolved at said cathode, fractionally condensing the same, returning condensed halide to the bath and removing the metal product.

12. The process of producing an alkali metal which comprises electmlyzin a fused bath of an alkali metal halide, maintaining at the cathode a temperature above the volatilization point of said metal, separately distilling ofi' anode and cathode products and fractlonally condensing said cathode product to separate metallic vapors from accompanying salt vapors.

13. The process of roducing sodium which comprises electrolyzin a fused bath of sodium chloride, maintaining locally at the oathode a temperature above the volatilization point of sodium, separately distilling 01f anode and cathode products and fractionally condensing in an atmosphere of nitrogen said cathode product to separate sodium vapors from accompanyin sa t vapors.

14. The process 0 producing sodium which comprises electrolyzing a fused bath of sodium chloride, maintaining locally at the cathode a temperature above the volatilization point of sodium, separately distilling 0E anode and cathode products, fractionally condensing in an atmosphere of nitrogen said cathode product to separate sodium JOHN J. GREBE. RAY H. BOUNDY.