US1937621A - Electrolytic apparatus - Google Patents

Description

3 Sheets-Sheet l INVENTOR azz-m .'A TTORNEY aange Dec. 5, 1933. G, BAUM ELECTROLYTIC APPARATUS Original Filed Jan. 3. 1927 1 wwwmw.

i Dec. 5, 1933. G BAUM 1,937,621

ELECTROLYTIC APPARATUS original Fi1ed Jan. 5, 1927. 3 sheets-sheet '2 ATTORNEY Dec. 5, 1933. G. BAU

ELEGTROLYTIC APPARATUS Original Filed Jan. 3, 1927 3 Sheets-Sheet 3 A TTORN EY Patented Dec. 5, 1933 UNITED STATES PATENT OFFICE ELECTROLYTIC APPARATUS Gustav Baum, Weissenstein above the Drau, Austria., assignor, by mesne assignments, to E. I. du Pont de Nemours and Company, a corporation of Delaware Original application January 3, 1927, Serial No. 158,457, now Patent No. 1,837,177, and in Austria January 28, 1926.

Divided and this application May 8, 1931. Serial No. 535,819

12 Claims. (Cl. 204-9) Persulphuric acid or the persulphates can be decomposed or hydrolyzed to give sulphuric acid, or sulphates, as the case may be, and hydrogen peroxide.

It has heretofore been proposed to electrolyze an aqueous solution of sulphuric acid in a cell having a platinum anode in the anolyte, and a lead cathode in the catholyte, the anolyte and the catholyte being separated by a porous diaphragm. Such arrangements have not heretofore to my knowledge been commercially successful for a number of reasons. Among these are excessive heat generated in the cell by internal resistance, decomposition of product and side reactions due to the heat generated, low current density, contamination of anolyte due to prolongation of the time of. electrolysis in the effort to increase the yield, polarization of the anode, etc. It has been attempted to overcome the objection of heating by cooling the anode and the electrolyte, but these expedients have only been of slight benefit to the yield or to the efficiency and have not reduced the expense of installation and of maintenance to such extent as to be of practical benefit.

I have discovered that a minimum internal resistance of a diaphragm cell of this type is not only necessary to reduce heating but is also of decided advantage in permitting higher current concentrations to be used, and that the higher the current concentration used, the greater the unit conversion to persulphuric acid. I have furyther found that in order to avoid side reactions or decomposition in a cell having high current concentration and minimum internal resistance, the amount of anolyte between the anode and the diaphragm must be as thin as possible in the form of a thin sheet or film, and that such a lm must be continually circulated both for the purpose of carrying away from the immediate anode sur- `face the high concentration of persulphuric acid as rapidly as formed, and also the heat, and further to reduce any tendency to polarization. In such a cell I ind it advantageous to cool both the anolyte and catholyte, but do not nd it necessary to use refrigeration for this purpose in order to maintain the desired working temperature at the desired high current concentration employed. Previous attempts at this electrolysis required electrolyte temperatures of below 10 C. 60

to 15 C.

The above advantages are all realized in an electrolytic cell wherein the anolyte, diaphragm, catholyte and cathode are concentric, with the anode at the center because thereby its entire surface is effectively utilized and its volume is at a minimum. The anolyte is preferably circulated lengthwise of the anode as a thin' sheet in a narrow annular space between the anode and diaphragm. Inasmuch as platinum is today the preferable anode material, the total cost .of platinum in a large plant becomes very material,

but by having a platinum anode at the center within the anolyte, or in a centrally located concentric anode chamber, the total investment for platinum is materially reduced. The lead cathode can conveniently be in the form'of a coiled tube encircling the diaphragm, through which cooling water can be passed to maintain the desired catholyte temperature. sistance of a cell constructed as above described is substantially less than that of any prior cell with which I am familiar, and a current concentration can be employed from two to upwards of The internal reve times that heretofore employed in any cell g5 with which I am familiar. I have been able by using current concentrations of between 300 and 550 amperes per liter, and preferably about 400 to 450 amperes per liter to obtain exceptionally valuable results.

Previous attempts to produce per-sulphuric acid or persulphatesv in high concentrations furthermore have not been successful since in all cases the electrolysis was carried on -in a large volume of liquid in a single vessel. This arrangement did not allow of rapid diffusion of the persulphuric acid from the anodes and decomposition and overheating resulted. I have overcome this by utilizing a small volume of liquid per unit of anode surface and by electrolyzing in this space with a high current concentration. In order now to increase the total overall concentration of persulphuric acid, a further feature of my invention comprises the operation in cascade I of a number of such cells. Operating in this way I have obtained solutions containing up to 30% or more of per-sulphuric acid with high yields at a temperature of about 20 C. These remarkable results have hitherto never been attained.

Inasmuch as a number of anodes and dia- 'or oval.

phragms as above described can be connected in parallel in one catholyte, it is Within the broad scope of this invention to connect single cells in cascadeor a plurality of units having a single catholyte and multiple vanode units in cascade. Besides the advantage realized in increase of overall concentration of per-sulphuric acid by cascading'of cells, I have found that while the anolyte may be circulated from cell to cell, and

the catholyte similarly circulated from cell to l,- a decided gain in yield is obtained if the anolyte from the last unit, after being treated to separate hydrogen peroxide, is then passed through the catholyte circulating system before being returned to the anolyte circulating system. Apparently the cathodic action on this regenerated slphuric acid removes substances which a cascade arrangement of cells for series operation in a continuous process; Fig. IV shows in detail the method of anode insertion inthe anode chamber; Fig.. V shows a slotted porous disk and Fig. VI shows one form of anode strip. The same numbers are used throughout to .designate the same or similar portions of each cell.

Referring to the drawings, 1 is the cell container, constructed of material resistant to the action of the electrolyte or lined with such material, such as lead or a resinuous compound. The container is provided with an overflow 2, at or near the top. The container is shown as having a round cross section but may be square In the center of the container is a cylindrical diaphragm 3 of thin porous material 'and closed at the bottom; this may be of unglazed porcelain. The space between the diaphragm and the container wall forms the cathode chamber. The cathode chamber is made large enough to carry a lead coil 14 which serves as the cathode and also for carrying cooling water. An overflow 4 is provided near the top of the diaphragm. cylinder above the container wall. Inside the diaphragm cylinder is a glass'tube l5 open at the top but sealed at the bottom having a glass tube 6 leading through the seal into the anode chamber at 7. An overflow 13 leads from near the top of the tube 5 over the top of the diaphragm. The tube 5 is of such diameter that a narrow annular space 8 of less than about 3 millimeters is formed between the tube and the diaphragm.v This narrow space is the anode chamber. The glass tube 5 rests on a slotted porous disk 21 (Fig. V).

The anode may be any suitable arrangement of non-attackable metal having the proper electrolytic characteristics, such as platinum, inserted in the anode chamber 8. I prefer to construct my anode as follows:

A lead ring 9 having a connector 10` is fitted over the tube 5 and rests on a shoulderll formed on the tube. On the circumference of the ring 9 I fasten several strips 12 o1' platinum of such length that they reach well into the anode chamber. The length of the strips and the number of the strips may be varied tu adjust the anode surface to any-amount desired, The amid? Surcarried up and over the edge of the container ipa/,cai

face is adjusted so as to give an anode current density of less than 2 amperes and preferably about 0.6.to 0.8 amperes per square centimeter. I have found that very thin platinum strips may be used; these are preferably reenforced by being rivetted or clamped to a strip of other metal not attacked during the electrolysis. Thus I have found that an anode strip of tantalum and platinum such as described in my U.S.P. 1,477,- 099 one form of which is shown in Fig. VI, gives satisfactory results even though the tantalum is in contact with the electrolyte. The strips are fastened to the lead anode ring by rivets, screws, or by brazing or soldering.

The cathode is formed by the coil of lead'tubing 14 arranged in the cathode chamber. A lead connector 15 is fastened to the coils to act as the cathode lead. VThe lower end of the cathode coil is as at 16 and connects with the cooling water supply 17 by a rubber or other non-conducting tube 18. 'I'he upper end 19 of the cathode coil is carried up and bent over so as to feed int`o a tube 20 inserted into the glass tube 5. Cooling of the anolyte can thus be effected. TheY cooling water enters from 17 passes into the coil 14 and out at 19 into the tube 20 to the bottom of 5 thence rising and overflowing through 13 to the sewer. The anolyte is cooled by contact with the outside of the glass tube 5. In some cases because of 155 structural features it is preferable to cool the anolyte by a separate cooling water supply. In this case the cathode cooling water is run directly to the sewer and a separate cold water supply led to the tube20 as shown in Fig. III.

In operation the anolyte is fed into the central tube 6 and flows to the bottom of the diaphragm cylinder and then rises in the anode chamber in contact with the anode and overows through 4. The catholyte is fed into the cathode chamber and overflows through 2. The passage of the current between the electrodes oxidizes the sulphuric acid or the sulphates to per-sulphuric acid or to the corresponding persulphate as the case may be.

As noted above, the current concentration is a most important factor in this electrolysis. It is now seen that I can easily apply high current densities per unit of volume. For example, if the anode chamber has an average diameter of about 5.0 centimeters, a depth about 50 cm. and a thickness of 0.2-0.3 centimeter its vlume will be about 0.18 to .23 liters. If 80 to 100 amperes are passed through there will be an anolyte current concentration lof between 300-550 amperes per liter.

'I'his type of cell construction enables meto obtain very low internal diaphragm resstances. 'I'he diaphragm can be made very thin as it does not support weight nor is it exposed to unbalanced pressures. I can thus obtain voltage` drops of 135 less than 0.5 volts across a porous ceramic diaphragm; the value of this is seen when itis realized that in previous work on this problem diaphragm resistances have been such that voltage drops of 0.8 to 1 volt have occurred'.

The rate of flow of the anolyte can of course vary within wide limits. I have found that a suitable rate in a cell of the above dimensions is about 3.25 cc/ampere/minute. Thus if the 4cell is carrying a current of 100 amperes the flow will 145. be about 0.325.1iters per minute. At'the rates and current densities noted above and at about 20 C. I have obtained a solution containing over 1% persulphuric acid from one passage through the een. v'rms solution can be recirmuae in uner 15G r anode chamber of this cell to increase the persulphuric acid content; successive passages through such a cell will add to the per-sulphuric acid concentration until a concentration of well over 25% is reached.

In order to rapidly secure high concentrations of per-sulphuric acid in substantial amounts the anolyte is preferably passed through the anode chambers of several cells at a rapid rate. Any suitable number of these cells, for example 20, may be arranged in a cascade system and interconnected as shown in Fig. III. The anolyte is fed into the tube 6 of the topmost cell, passes through the anode chamber and overiiows through 4 into the anode feed tube 6 of the next cell; the catholyte is likewise fed into the topmost cell and overows through 2 into the cathode chamber of the next cell. The voltage drop across each cell is from 4 to 8 volts. This allows a series electri-` cal connection of the cascade, giving for 20 cells a total voltage drop of about to 160 volts.

In order to best utilize the current supply sev-- eral of these series cascades may be arranged in parallel electrically. In this last case it is convenient to have a number of anode units including the diaphragm in one large cathode chamber containing one large lead cathode coil. These several anode units are connected together and then connected to the cathode of the preceding cell thus giving a series-parallel electrical connection. The anolytes from each anode unit in such an arrangement ow from one anode unit into a corresponding anode unit in the next set while the catholyte ows from the one connnon cathode chamber to the next.

When the anolyte leaves the lowest cell of such a cascade it contains a high concentration of persulphuric acid; the persulphuric acid is now decomposed to give hydrogen peroxide and sulphuric acid. The recovered acid is then raised to the upper cell and preferably made the catholyte supply. After passing the last cathode chamber the acid is returned to the upper cell and then added to the anolyte supply. This anolyte supply is adjusted by fresh acid and pure water so that its specic gravity will be preferably about 1.285 although other concentrations may be used.

The anode chambers of the cells ofthe dimensions given above in a bank of 20 would have a total volume of 3.6 to 4.6 liters. The electrolyte would then be subjected to anode action for a total period of about l0 to 15 minutes.

I have thus electrolyzed in a 17-cell bank of cells, as described above, a solution of sulphuric acid containing about 500 grams sulphuric acid per liter at a temperature of 20 C. to 21 C. and an anolyte current concentration of 400 amperes per liter. The anode current density was about 0.8 amperes/cm2. A 30.8% solution of persulphuric acid was obtained with a current efficiency of 71.5%.

Solutions of sulphates can also be employed. Thus, I have electrolyzed a solution containing 20% ammonium sulphate, 2% sulphuric acid and 'lt/2% K2SO4 at a temperature of 35 C. in the above apparatus. The per saltsv were obtained by cooling outside of the apparatus and showed a current efficiency of 74%. I have also obtained satisfactory results but lower current efliciencies at as low as 300 amperes per liter.

The cell described above is most suitable for4 my process but I do not wish to be limited to its structure since the high current concentration, thin flowing sheets of electrolyte and other features may be attained in other structures equiva- Ysaid glass cylinder lent to that described. `Nor do I wish to be limited to the exact rates, temperature and compositions given. My invention, as will be seen from the above, is applicable to a great variety of. solutions, therefore, in the appended claims wherein I refer to sulphuric acid solutions I mean not only an aqueous solution of sulphuric acid alone, but I also intend to include such solutions containing various addition agents, stabilizers, etc., from which per-sulphuric acidor persalts can be obtained by anodic action.

1. An anode for an electrolytic cell comprising an annular conducting ring having `suspended from its circumference an anode member comprising platinum.

2. An anode for an electrolytic cell comprising an annular lead ring having suspended from its circumference anode members consisting of an upper tantalum portion and a lower platinum portion mechanically united.

3. An anode for an electrolytic cell comprising an annular lead ring having suspended from its circumference anode members consisting of a tantalum portion and a platinum portion mechanically united.

4. An anode member for an electrolytic cell comprising a supporting tantalum member having a platinum member mechanically fastened to and suspended from it.

5. An anode chamber unit for electrolytic cells comprising a cylindrical porous diaphragm having glass cylinder inserted therein'so as to form a narrow annular anode chamber between said glass wall and said diaphragm. A

6. An anode chamber unit for electrolytic cells comprising a cylindrical porous f diaphragm having glass cylinder inserted therein so`as to form a narrow annular anode glass wall and said diaphragm of less than 0.3L

chamber betweenl said` vided-with an overflow and closed at the bottom, i

a glass cylinder sealed at theA bottom and lprovided with an overflow inserted in said porous cylinder so as to form a narrow annular anode chamber between said glass cylinder land said diaphragm and a glass tubey inside said glass cylinder and leading through said sealed bottom and communicating with said anode chamber.

8. In an apparatusv for the electrolytic productionof persulphuric acid a centrally located annular anode chamber having a depth between anode and diaphragm of less than 0.3 centimeters.

9. A cell for the electrolytic production of persulfuric acid comprising a centrally located anode assembly consisting of an anode member in a narrow anode chamber, said chamber being formed by an impervious cylinder and a porous diaphragm concentric therewith, and a concentric cathode chamber about said diaphragm.

l0. An electrolyticcell comprising a container, a coiled lead tube cathode, a cylindrical closedbottom porous diaphragm provided with an overflow inside the coils of said cathode, a glass cylinder sealed at the bottom, and provided with an overflow, inserted in said porouscylinder so as toform a narrow annular anode chamberbetween and said diaphragm, a glass tube inside said glass cylinder leading through said sealed bottom so as to communicate with said anode chamber and an anode suspended in said anode chamber.

11. An electrolysis system comprising a plurality of cells according to claim 9 in cascade flow arrangement and in series electrical connection, means connecting the anolyte overow of each cell but the lowest with the anolyte supply tube of the next lower cell, means connecting the catholyte overllow of each cell but the lowest with the cathode chamber of the next lower cell,

means for separately supplying anolyte and.

catholyte to the upmost cell and means for separately collecting said anolyte and catholyte owing from the lowest cell. 12. An electrolysis system for the production of per salts comprising a plurality of cells, having anode and cathode chambers,in cascade ow aring from the lowest cell.

GUSTAV BAUM.

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