[1]Patented Sept. 21, 1937 2 9 0 9 3 9 7 7 0 UNITED STATES PATENT OFFICE 2,093,770 ELECTRICAL PURMCATION OF IJQUEDS Jean BiMter, Vienna, Austria APiplication January 13, 1933, S . erial No. 651,507 In Germany January 15. 1932 10 Claims. MY invention relates to the electrical purification of liquids such as water and more-perticularly to methods by which raw water and similar liquids in which inorganic salts are dissolved are freed from these conipounds eith6r paitly or completely by the action of an electrical current. It is an object of my invention to render such proeesses more efficient and less expensive than was heretofore possible. 10 Another object of my invention is the provision of an Improved apparatus for carrying out these methods. nrther objects of my invention will appear as the specification proceeds. 15 After all attempts had failed to purify water by electrical means without using diaphragms or with the use of a single diaphragm for each cell', suggestions were made to desalt water by electroosmotic means in so-called threecompart20 merit cells, each cell being provided with a positively charged diaphragm at the anode side and with a negatively charged diapiliagm at the cathode side. Apart from the fact that electroosmosis is not, capable of bringing about a de25 salting effect for reasons given further below, these suggestions were not commercially successful either because no suitable materials for the diaphragms could be found. At present the electrical desalting of liquids is generally carried 30 out by electro-dialysis, although the effect thus obtained is erroneously ascribed to electroosmotic phenomena. In these processes the liquid to be treated is kept separated from the electrgdes by two dia35 phragms both of which usually consist of substant4ally non-charged materials such as sail cloth or the like. If only a partial purifleation of the liquid is desired, it has been suggested to use ceramic diaphragms at both sides Gf the cell. 40 In order to arrive at the desired desalting effeet, all authors and inventors describe and consider it indispensable to wash or flush the electrode compartments with raw water, with distilled water or even witli water partially purified in the 45 cell itself. By sit , ch washiiig steps it iz intended to systematically keep the concentration of electrolytes in the electrode compartments as low as possible, and to systematically remove from the cells, together with the wash water, a-s much of 50 the electrolytes as possible. It is generally believed to-day that the recovery of completely desalted water by electrodialysis is only possible if the electrgde compartments or at least the last electrode com55 partments of a series of cells contain liqwds (CL 204- 25) in which only a small percentage of electrolytes is dissolved. It is further bereved that a complete desalting necessitates the use of such diaphragm materials wwch do not cause an electroos motic migration of water from the elec- 5 trode compartments into the middle compart-@ ment or middle chamber which contains the liquid to be purified. This latter consideration accounts for the fact that in the complete desalting of Hquids dia- 10 phrag ms consisting of Mter fabrics are to-day in use 6xclusively, although they are not durable and are very rapidly destroyed when used for the desalting of water which is rich in chlorides such as is found in many places., The advantage 15 offere d by these diaphragms, viz. that they are, charg ed only to a negligible extent and that conseque ntly they do not exert a distinct electroendos motic effect, is, ho*ever, considered to be decisi ve. 20 It will,be understood, ho*ever, that methods for desalting liquids by electrital means oanncit meet with a striking technical and commercial succe ss and that they cannot compete with the proce sses of recovering pure water @ by distilla- 25 tion, unless the diaphragms used are very durable and can be used for the desalting of water of any composition occurring in actual practice. Sub stantially all diaphragm materials. which have proven tG be durable in des@alting Of .30 liquids are comparatively strongly charged negatively , at least so when in contact with a dilute solutio n of potassium chloride, i. 6. the standard solution for testing the charge of diaphragm materi als. In contact with anode liquori@, the 35 charg e of, the diaphragm materials, as a rule, win not greatly differ therefrom. Starting from this experi ence and from the fact thtit'all these diaphrag ms exert strong electro-oamotic or electroendos motic effects, I have attempted to so. im- 40 prove the methgds for purifying Uquids by the action of the electrical current that they result in a substantially complete desalting effect even with diaphragms Gf this type. Acc ording to my invention I obtain tWs re- 45 sult by the adoption of step's, wh.ich are to a large extent contrary to those heretofore adopted. While hitherto it was usual to keep the cone@ntration of the electrolytes in the electrode liquors as low as possible by means of washing water in- 50 troduc ed Into the electrode comp artments, I intenti gnally provide for an Increased acid content of the anodic electrgde liquor, thereby also ingrea @ing its conductivity. This .enables me to operat e the cell at an increased current,density 55
[2]and at an elevated temperature. An increased concentration of the cathodic electrode liquor may be advaritageous for siniilar reasons. According to one modification of my invention I do not wash or flush the eiectrode compgxtments at all, or I wash them but slightly in order to secure constant levels of liquid in the respective compartments but not in order to keep low the electrolyte contents of the electrode liquors. 10 I may avoid substantially any addition of liquid to, as well as any removal of liquid from the anode compartments. In such a case the anodic electrode liquor becomes more and more rich In acid, because the passing of the electrical current 15 through the cell results iri an immigration of anions such aS S04-- into the anode compartment, and these ions when discharged at the anode, form equivalent amounts of acid with attendant evolution of oxygen. Acids of this type, 20 more particularly sulfuric acid, are contained almost in every raw water in the form of salts. When using diaphragms which are distinctly charged under the conditic , ns actually occurring in the cells and which therefore cause distinct 25 electro-endosmotic effects, it will be understood that the use of such diaphragms at least at the anode side of the cells, is particularly contemplated in the present invention. It is, however, not possible to altogether avgid any migration 30 of liquid tlirough the diaphragms. The intended purpose, i. e. the increasing of the acid contents of the anodic el6etrode liquor may, however, be automatically fulflued by compensating for the electro-endosmotic migration of hquid by the 35 addition or withdrawal of corresponding amounts of liquids. Most of the diaphragm materials which can be used at the anode side of the cell are chekrged negativel@, and consequently cause an emigration 40 of anodic electrode liquor from the anode compartment Into the middle compartment, and this is frequently true to ' such an extent that the anode compartments soon become empty unless additional Hquid is Introduced thereinto. Such 45 a supply of Hquid can, however, readily be provided for in an automatic manner. In the drawings annexed to this specification and forming part thereof several forms of apparatus embodying my invention are iuustmted 50 diagramm tically by way of example. In the drawings Flg. 1 is a sectional elevation of a cell for the electrical puriftcation of water' Flg. 2 is a simflar view of a eell provided with ;5 a mercury cathode, Flg. 3 is a sectional elevation of a modified form of a oell similar to that shown in Flg. 2, Mg. 4 is an elevation and, partly, a section of a number of cells connected in series, 60 Flg. 5 is an elevation and, partl3@, a section of an arrtlngement of superimposed cells connected with a heat-exchanger, Flg. 6 is a sectional elevation of a cell provided with heating means, 65 Mg. 7 is a plan view of a cell shown In Mg. 6. Referring to the drawings, the ceR shown in Mg. 1 comprlses a tank IS provided with an overflow pipe 22 situated.somewhat belgw the top rim of the tank. Within the ceH, which is preferably 70 cylindrical, a cylindrical cathode I 1, preferabiy made of perforated sheet iron, Is arranged, the inner face of which is tightly covered with a so-called futer diaphragm 10 made of asbestos paper or a gitnilar flexible readily permeable ma75 terial. A rubber disk 29 tightly closes the bottom of the cylindrical vessel consisting of the cathode II and the diaphragm IO, and an overflow 20 permits keeping the liquid within this vessel at a higher level than prevails in the tank 13. 5 To the bottom of the cathodic vessel extends a raw water supply 14. The anocuc diaphragm I ,which may be made for Instance of ceramic material and also forms a cylindrical vessel, is concentrically arranged In the cell. The anode 10 consists of a coil of wound-up platinum wire 25. No means are provided for the withdrawal of anodic electrode liquor, and the liquid supply 23 is regulated by a valve 24 operaied through suitable levers by a fl(jat i 2 also arranged in the 15 anode compartment. The parts 12, 23, and 24 are so adjusted that the level of liquid in the anodic compartment substantimly corresponds to the level in the middle compartments, which In turn depends on the height of the overflow 20. 20 , The anodic diaphragm represents a cup, which may readily be withdrawn from the interior I of the ceR, as it is not secured to any parts thereof. Substantially the same is t-,rue with regard to the structural unit consisting of the cathode 25 II, cathodic diaphragm IO, the overflow 20 and perliaps the disk 29. When operating this cell, the water to be purlfled is continuously supplied to the middle compartment through the supply pipe 14, and the 50 purifled water automatically passes off through the overflow 20. The voltage apphed to the anode and the cathode causes not only a purification of the water in the middle compartment, but also a gradually increasing acid concentration of the 35 anodic electrode liquor filling the central or anode compartment. By electro-endosmotic action some Hquid from the anode compbxtment is passed througli the pores of the diaphragm I into the middle com- 40 partment, and this liquid is replaced by raw water automatically supplied to the anode compartment through the pipe 23 controlled by the float 12. The ele@tro-endosmotic migration of water 45 through the cathodic diaphragm 10 may vary its sign. Initially some water will be transferred from the middle or anode compartment 28 Into the cathode compartment 29, although the amounts thereof wiR be comparatively sma'll in 50 view of the hydrostatic difference of pressure prevathng between the middle compartment and the cathode compartment, which difference tends to force part of the liquid from the middle compartment to the cathode compartment. In the 55 course of operation the charge of the diaphragm 10 niay be reduced by alkahne metal compounds originating from the water and deposited In and on the diiiphragm. I have frequently observed that even the sign of the charge of the cathodic 60 diaphragm Is altered so that the electro-endosmotic effect, if any, carries liquid from the cathc>de compartment into the niiddle compartment. It wUl be appreciated that this cell Is of a 65 lparticularly simple design, since there Is no necessity of tightening the three compartments relative to each other. There Is further no liquid supply to the cathode compartment nor a liquid discha@rge from the anode compartment, with at- 70 tendant simplifleation of the cell. The supply of liquid to the anode compartment Is restricted to the amount required to replace the liquid carried off electro-endosmoticaliy, In contradistinction to the usual passing of considerable quanti- 75
[3]2,003,770 3 ties of washing liquor through the electrode comPartments of the cells. If the liquid supphed through the pipe 23 is absolutely pure water, the coniposition and concentration of th6 anodic electrode liquor will be substantially the same as if any migration of liquid into or from the anode compartment were altogether prevented. Pbr reasons which will be stated. further below, the addition of foreign 10 cations to the anolyte is as a rule not advantageous, but the unfavorable effect is rather low .if the liquid emigrating from the anode compartment,is replac.ed by raw water. In such.a case the conductivity 6f the anolyte is increased still 15 more rapidly and this advantage compensates for the above mentioned disadvantage. In certain cases. for Instarice vjhen starting the operation of a cell, some acid may be added to the anolyte contained In the anode compartment or 20 to the liquid supplied through pipe 23. The cell shown in Flg. 2 consists of a steel vessel 44, the side walls of which are covered with an insulating lining 46. The bottom is covered with a layer 43 of mercury, which is 25 supplied through the funnel 31 and withdrawn through a discharge pipe 32 baving the shape of a syphon. The layer of mercury 43 forms the cathode and is in contact with the bottom of the metallic vessel 44, which is in turn connected 30 with the negative pole of the source of electricity. Within the vessel is further provided a cylindrical diaphragm 42 closed at the bottom and provided with the anode 45 which is connected with the positive pole. The tank is cbvered by 35 a lid 30 provided with a gas outlet 26 and with various pipe@ (not shown) which serve for the supply of water to be purjfled to the space between the mercury layer 43 and the anodic diaphragm 42, and for the supply of additional 40 liquid into the anode' compartment, the latter supply pipe being controlled by a float-regulated valve similar to that described with reference to Fig. 1. As mercury is capable of alloying with alkali 45 metals and alkaline earth metals, the mercury cathode need not be protected by a cathodic diaphragm. As far as the cathode is con@erned, this cell may be operated in the same manner as the cells which are In current use for the 50 electrolytic manufacture of caustic alkalis, the - mercury being passed through the cell at such a rate that the mercury alloy forming remains liquid. It is then freed from alkali end alkaline earth metals and the regenemted mercury Is re55 turned into the cell in cycle. The cell shown in ng. 3 differs from that shown in Fig. 2 mainly in that it is provided with a so-called vertical mercury cathode 48 and with two anodes. The tank 50 lined with Insulating r ,,o material. 52 is, divided into three compartments by 'two anodic diaphragms 47. The two side compartments contain anodes 51 consisting of a plurality of straight platinum wires, and the liquid in these compartments is maintained at a 65 constant level by an automatic leveling device (not shown) corresponding to that shown in Mg. 1. The middle compartment is provided with a vertical wire@gauze partition 49 on which trickles dow-n mercury supplied through the sup70 ply pipe 53, thus forming constantly renewed mercury surfaces which act as a cathode, the mercury supply 53 being connected with the negative pole. At the bottom of the middle 6ompartment an alloy of mercury and alkali form75 ing and alkaline earth metals collects, which is led off through the overflow pipe or syphon 54 to be re-generated and the mercurv returned to the mercury supply pipe 53. The water tG be purified Is supplied to the bottom of the niiddle compartment and with5 drawn from the top of this,compartment in a purifled form through two pipes which are not shown in the diawings as their arrangement is obvious. Each of the ceus shown @ in Flg. 4 comprises a 10 tank 59 closed by a lid 65 and having its bottom - covered with an electricany insulating layer 64, consisting for instance of rubber. Within this ceH a cylindrical cathode 58 is arranged, which consists of perforated sheet me.tal and is,covered 15 on its inner surface, with a cathodic diaphragm 57 consisting for instance of asbestos paper. The spacing device 8 holds th6 cathode 58 in the correct posi ion within the c chamber 5 and presses it down against the rubber layer 64- 20 Concentrically to the cathode an anodic diaphragm 62 Is arranged which is made of ceramic material and with a constricted portion 55 tightly passes through an opening in the bottom of the cell. Outside of the cell the anodic diaphragm 25 is connected with a T-plece 2, one branch of which is closed by a cock 3, while the other branch 4, which consists of a comparatively narrow pipe, leads to a leveling device S. which Is connected with a supply pipe 6 and with an over- 30 flow pipe 7. Within the anode compartment 63 a magnetite anode 61 Is provided, while the cathode compartment 62 contains a platinum cathode 57. The various cells of the series are arranged at gradu- 35 ally decreasing levels. The overflow pipe 60 of the middle compartment of each cell is connected with the middl6 compartment of the following cell, to the bottom of which the pipe 60 leads. The leveling devices of the cens are connected 4( with each other in such way that the overflow p@pe 7 of each of them is connected with the supply pipe 6 of the immediately succeeding cell. In operating these cells the water to be purified Is supplied through pipe 60 to the niiddle 45 compartment of the first of them. It then passes through the middle compartments of all the cells of the series and Is withdrawn from the last of these cells in a purifled state. A small quantity of liquid, which may also consist of r)o raw water, is supplied through pipe 6 to tht leverng device 5 of the first cell, leaves this device 5 through the overflow 7, enters the leveling device of the next cell, and so on. If in any of the anode compartments of the series a lack of 55 liquid should occur, the corresponding amount bf water will be supplied to this compartment through the corresponding connection 4. Similarly, If the level of liquid within one of the anode compart ments should tend to rise, the surplus 60 liquid will be discharged through the connection 4 to the corresponding leveiing device 5. It will be understood that the arrangement here shown affords a particularly simple leveling of the liquid within the various anode compartments at pre- 65 deterinin ed levels, the liquid in these cornpartments (the anolyte) becoming enriched in acid for the same reasons as were explained above with reference to Mg. 1. A somewhat similar arrangement of cells which 70 are, however, super-imposed to one knother, is shown In Fig. 5. Each cell comprises a tank 75, a cathode 12 covered on its interior face with an asbestos diaphragm 73, and a ceramic anodic diaphragm 67 surrounding an anode (not shown) 75
[4]4 2,003,770 and passing with 9, restricted portion through the bottom of the cell. All . these particulars altogether correspond to those described with reference tci Flg. 4. The leveling device 68 is, however, arranged inside of the anode compartment; it consists of a cylindrical vessel open at the top, closed at the bottom and communicating with the anolyte through an orifice 74, which renders ihe same services as the connection 4 10 shown in Mg. 4. The overffow pipe 70, which is open at the top, is arranged to maintain the desired height of level within the anode compartment and carries off any superfluous liquid to the leveling device 68 of the next succeeding cell. Raw water is supplied to the leveling device 68 of the topmost cell through. a supply pipe 60, and it will be understgod that the pipe 6 in co-' operation with the leveling 4evices 68 and the overflow pipes 70 acts in the same way and rend20 @rs the same services as the leveling arrangement 74, 68, 69, and 70 shown In Flg. 5, one advantage of this vertical 'arrangement consisting In that hose connections or packings are dispensed with. I 25 Overflow from the cell chamber 7i. flows off through overflow pipe 78. The cathode and anode compartments 79 and 80, resp6etively, naturally correspond to the siinilar , compartmenii of the previously described forms of apparatus. 30 Below the last cell of this serles a heat-exchanger 15 is arranged comprising a shell 15 and a plurality of tubes 16 arranged between two tube plates 35 and 36, the liquid to be preheated being supplied through the pipe 33 and passing .35 through the interstices- 17 between these. tubes to pipe 34. The hot purified liquid discharged from the last ceU is fed through the collecting funnel 76 to the top of the heat-exchangeri then passes through the tubes 16 and isultimately 40 withdrawn In a cooled state through the overflow pipe IS. The preheated liquid passes through pipe 34 ,to the top of the topmost cell, is fed through the collecting funnel 14 to the bottom of the middle 45 coziipartment of this cell, and dwharged through the overflow pipe 77 in a partially purified state. .Tt Is then collected by the collecting funnel 76 of the next succeeding cell, passed througli its niiddle compartment, and so 6n. It wUl be un50 derstood that the operation of this arrangement is fuuy automatic, as only raw water is supplied through pipe 33 and purified water withdrawn through the overflow 18 in a continuous manner. While 'in Fligs. I to 5 I have shown cells of difr)5 ferent design, it wiR be understood that none of theiii is superior to the others, election depending upon the r-oml@6sition of the liquid td be purified, ,tile desired extent of purifleation" the desired ouiput, energy efficiency and similar circum60 stances. A ceB yielding a particularly bigh output per unit of bottom area is that shown In Mgs. 6 and 7. It comprises a tank 86 in which two concentric cathodes P3 and 84 are arranged, both 65 consisting of p(*forated sheet metal. These cathodes are covered at their opposite faces with flexible diaphragms 90 consisting for Instance of asbestos paper or asbestos fabric. Two cylliidric anodic diaphragms Of made for in70 stance of ceraniie material are arranged within the annular space defined by the two cathodes 83 and 84, and these anodic diaphragms are sp*ed from each other by a tightly inserted r" 92 made of concrete or the hke. The dou76,,ble-walled anodic diaphragm thus formed rests on feet 40 which allow Ilquid to pass from one of the cathodes to another one. The w2ode compartment 91 de:ftned by the two anodic diaphragms I contains anodes 89 conslstfng of straight platinum wires and a float 86 lnflueneing a valve (not shown) of a supply pipe (not shown) for liquid, wldch acts In the $me way and renders the same services as the supply pipe 23 shown in Flg. 1. The top of the cell Is tightly covered with a lid 41 provided with a projeciing 10 pipe 93 for leading off evolved hydrogen and an annular opening through which pasi the upper rims of the cathode cylinders. One side of the lid is provided with an overflow 20 for the purifled liquid., In the central portion of the cell 15 is arranged a, cylindrical heating vessel 37 of sheet Iron or the like which is provided with a steam supply pipe 38 and a discharge pipe 31 for condensed water. 7bis cell is operated as follows: 20 The water to be purified is continuously supplied to the basin 87 represented by the r-entral portion of the lid 4 i of the cell and from this basin flows over the rim of the inner cathode 83 down Into the space defined by this cathode and 25 the Inner anodic diaphragm 81. At the bottom of this space the water passes Into the space deflnbd by the outer anodic diaphragm 81 and the outer cathode 84 and leaves this space by flowing over the rim of the cathode 84. it then 30 leaves the cell In a puri:ded state through the overflow 88. As under the conditions prevailing In the cell the diaphragm 81 is negatively charged, some liquid will emigrate from the anode compart- 35 me'nt into the middle compartment which Is form . ed of two annular spaces connected only at their lowest portions. The liquid thus withdrawn from the anode compartment is replaced bY additi.onal liquid, for instance by raw waiter, 40 supplied In an amount which is controued by the float 85, so that the levels of hquid in the anode compartment and In the middle compartment remain constant. and preferably substantially equal. 45 If water rich In chlorides and also containing considerable amounts oi other dissolved ln6rganic matter Is to be purified, or if even seawater containing up to 35,000 mgs. per liter of inorganic matter is to be desalted, ceus of the 50 type shown In Plgs. 2 and 3 provided with inercury cathodes should be used at least for the Initial desalting step. Experience has shown that this iind other mercury cells are partlcularly suitable for such a purpose, provided that 55 the mercury cathode Is used In combination with a negatively charged anodic diaphragm, for instance with a ceramic diaphragm, and that the anodic Ithuor or anolyte Is kept, in accordance with one of the basic ideas underlying the 60 present Invention, at a constant level and at a comparatively, high acid concentration. The desalting of liquids havi'ng a medium or low salt contents and the coniplete desalting of sea-water or other kinds of water originally hav- 65 ing a comparatively high salt contents after having been partly desalted In mercury, cers of the above mentioned type is more advantageously carried out in cells provided with diaphragms both at the anode and at the cathode 70 sides, such as are shown for Instance in Ogs. 1, 4, and 5. Cells of the type shown In Flg. 4 are particularly advantageous for loads or yields ranging from 2 to 40 liters water per hour, while the 75
[5]2,098,770 ceus according to Fig. 1 or 4 are designed for outputs of 100 to 5000 liters water per hour. In any case anolytes are used which contain not less electrolyte than the liquid to be @urifled, these anolytes, in the course of electrolysis becoming inore and more rich in acid. Such an increase in concentration, which was heretoforeconsidered to be highly detrimental, exerts, in accordance with the results of my investiga10 tions. a distinctly advantageous effect, which cannot be explained without abandoning the heretofore established hypothesis that a corr@7 mercial operation of desalting cells be impossible unless mgre ions are withdrawn from the 15 middle compartment than are supplied to it. The fact that equal quantities of electricity must pass through any,,,of the section planes which can be assumed b6tween the electrodes parallel to the diaphragms, and that each of these planes 20 muat.consequently be crossed by equal numbers of ions, indicates that the above mentioned supposition cannot be correct. In contradistinction thereto I believe the phenomena occurring in electrical desaiting processes, for instaxice in the 25 electrolytid removal of Na2SO4 from its solution, to be substantially as follows, If such a solution of Na2SO4 iS subjected to the action of direct current in the middle compartment of a cell of the type here in question, the 30 catholyte or cathodical electrode liquor becomes alkaline and the anolyte or anodical electrode liquor becomes acid, as Is well known to those skilled In the art. Under the influence of the current H+ ions Pass from the anolyte into the 35 solution to be purified and S04-- ions pass from this solution Into the anolyte, the ratio of these two ions corresponding to the migration coefficients of sulfuric acid, approximating 0.807 to 0.193 in the,case of 40, N TO solutions. S04 ions thus introduced into the anolyte or otherwise contained therein are dis45 charged @t the anode, but as the discharged radical S04 immediately reacts with water @under formation of sulfuric acid and oxygen, the anolyte becomes more and more rich in sulfuric acid. In an analogous manner OH- ions are pgssing 50 from the cathode compartment into the middle compartment, while Na+ ions migrate in the other direction, the ratio again corresponding tq the coeffleients of migration (i. e. to the so@colled transference numbers) of the corresponding@ cOMr,5 pound (NaOH), approximating 0.81 to 0.19. Again the Na+ ions are discharged at the cathode, but the metal @thus formed @immediately reacts with water with the formation of hydrogen and NAOH, with which the catholyte bec6mes more oo and more enriched. In the middle compartment all,6f the immigrated OH- ions react with the equivalent amount of H+ ions under @ the formation of undissociated walter, there remaining over 0.007 equivalents of o5 =gen ions. The emigration of 0.19 equivaof Na+ Ions and 0.183 eqwvalents of S04 ions results in the disappearance of 0.19 equivalents of sodium sulfate, the remaining surplus Of 0.007 equivalents S04@- ions fepresenting, to70 gether with the above mentioned remainder of 0.007 equivalents H+ ions, an amount of 0-007 equivalents H2SO4, by which the solution underlying the @urification becomes acidified. The explanation here given is not an altogether 74 complete one, since the anolyte emit@ into the middle compartment not only H+ Ions, but also all of the other cations which It contains, the proportion of the various emigrating cations corresponding to their respective migrating velocities. Similarly the -catholyte emits into the 5 riiiddle compartment not 6nly OH- ions, but also other anions which It contains, the relative proportion of the emigrating anions corresponding to their relative migrating velocity. However, as - the H+ and OH- lons,display a migrating velocity 10 which Is by far greater than that of all other ions, the influence of these other ions Is not important and can be taken into account by apportioning small corrections to the flgures given above, the character of the prgcess being not in- 15 fluenced thereby. On the other hand these phenomena indicate that the acidification of the solution contained in the middle compartment can be reduced to a certain extent by adding foreign cations to the anolyte used. 20 The theory given above (to which I do not wish to be bound) readily explains why the liquid to be purified becomes acid in the course of electrolytic purification, and why the current yield in electrolytical desalting processes cannot exceed a 25 proportion of about 19% of the yield co@lculated in conformity with Faraday's law if the desalting Is effected through electrodialysis exclusively. In the first steps of the process, the current effleiency may, however, be still higher because of 30 the action of electrolysis besides the electrodialysis. It further Indicates that the current yield will be the'better, the less foreign cations are contained in the anolyte beside H+ ions, and the less foreign anions are contained in the catho- 35 lyte beside OH- Ions. This means that the maximum current yield is obtained if the anolyte consists of a pure acid ond the cathoiyte of a pure base. My theory explains the desalting effect to be 40 due to the replacing of the contaminating ions contained in the middle compartment by the more speedily migrating H+ and OH- ions which then disappear practically completely by forming water. As the desalting effect is consequently 45 due to the high migrating velocity of the ions of water, no desalting effect may be obtained if the liquors contained in the electrode compartments are kept neutral, be It by a continuous mixing of the anolyte and the catholyte or by an intensive Go washing of the electrode compartment with pure water or neutral solutions. The character of these phenomena is not materially altered If the anodic diaphragm is charged positively and the cathodic diaphragm negativeIY. The charge of the diaphragms results in changes in the movabilities or rnigration veloci'- ties of the iom contained In their pores, some types of ions b6ing accelerated while others are 'retarded. By a suitable election of the charge of 60 the diaphragm the acidity of the solution in the middle compartment may be influenced to a certain extent, but a desalting effect cannot be obtained thereby. There is probably nothing that could be correctly styled "an electro-osmotic de- 65 salting effect", which wowd only be possible if the liquid electro-osmotically forced through a, diaphragm were more concentrated than the liquid to be desalted.. A presumption in this direction was obviously the basis of the well known 70 suggestion to use positively charged anodic diaphragms and negatively charged cathodic diaphragnis. According to my experiences and experiments these presumptions are, however, not correct, and I have even ascertained that an 75
[6]6 2,098,770 electro-osmotic or electro-endosmotic migration of liquid from the electrode corxipartments irito the middle compartinent is far from being detriinental. I have not yet been able to ascertain the exact composition of the liquid which eniigra-tes from the electrode compartments, but my experiences make me believe that they differ in composition from the electrode liquids being purer than said electrode liquids, although I have not been 10 able to show that- they consist of pure water. I therefore believe It to be a mere prejudice that completely desalted water coiild only be obtained by washing the'electrode compartments, or at least the last electrode compartment of a jr, serles of cells, In order to keep the concentration of the Ions therein as low as possible. The number of ions electrolyticauy emigrating at a given current density from the eiectrode compartments Into the middle compartment is 20 practicary independent from their respective concentrations in the electrode hquors. This does not mean, however, that conditions are most favorable If the concentration of tons In the electrode compartments is particularly low; In the 26 practice of my Invention I use electrode licluors ofcomparatively high roncentrations in order to Obtain higher curient densities and higher temperatures, it - being further advantageous for the reasons given above that the anolyte be dis30 tinctly acid and the catholyte distinctly alkaline. It haa tumed out to be advantageous if the acid concentration in the anode compartment, expre,ssed In cheniical equivalents, Is at least a hundred-fold or, with higher current densities, even 35 some hundredtthousand-fold higher than the concentration of the purified liquid also expressed In chemical equive@ents. At room temperature the anolyte may display an electrical conductivity, due only'to the acid contents of from 40 1 x 10-3 to I x 100 reciprocal ohms per cubic centimetre. @ I It Is true that with an Increased concentration of the electrode Hquors the recontamination of the Uquid In the middle compartment by com45 mon diffusion, i. e. by an altogether non-electri6alphenomenon, is also Increased. Practically this diMLsion Is, however, not important during the opemtion of the'cell, provided that diaphragms of low permeability to liquids are used. It wiH be 50 appreciated that with an increased concentration of the electrode liquors their electrical conductivity is also increased which in turn permits a higher current density and a more rapid flow of the liquid to be purified through the cell, the r)5 increase of the volume of liquid purifted within a given pertod of operation being at least proportional to the increased amount. of saltg reIntroduced Into the liquid compartment py diffusion. Only during the intervals of operation the 60 increased diffusion Is disadvantageous, but in such a c-ase I may empty out the anode compartments, for instance by means Gf the cock 3, and re-ful them when re-starting the operation. Or I may intensively wash the middle compartments o5 with raw water immediately before re-starting the operation. The above is trae only with diaphragms baving a relatively low permeability ,, such as for Instance with ceramic diaphragms which I have found to 7o work highly satisfactorfly at the anode side of my cells. With diaphragms made of sail cloth, asbestos fibres or the like conditions are different. I may use such diaphragms for instance at the cathode side of my cells, but then I do not allow 75 the catholyte to become so conrentrqtcd as is stated above with reference to the anolyte. Such an operation of a cell is readily possible In view of the fact that in the puriftcation of raw water the alkalinity of the catholyte is increased much more slowly than the acidity of the anolyte, the reason 5 being that part of the cations Immigrating into the catholyte consists of alkaline-earth metal cations, which are precipitated here, and that part of the caustic alkalies formed in the cathode compartment Is consumed In the precipitation of 10 alkaline-earth metitl compounds such as for tnstance.calcium bicarbonate or carbonate. From the explanation given obove It will be further un'derstood that a washing of the cathode compartments is not absolutely necessary either, 15 but I may nevertheless supply some liquid to these compartr@ents In order to prevent their becoming altogether dry, as might otherwise happen. It is true that empty cathode eompartments are not absolutely detrimental but they may cause con- 20 tact resistances to arise which render it difficult to al3p]y high current densities. Originally, cathodic diaphragms made of asbestos fabrics are slightly negatively charged and therefore cause a slight electro-osmotic emigration of liquid 25 frorii the nxiddle compartment Into the cathode compartments. In the course of -operation however, alkaline-earth metol hydroxides are deposited within the pores and on the Inner face of the diaphram, reducing the negative charge 30 of the latter and even rendering It fthtly poaltive. By such a positively charged cathodic diaphragm some liquid is, however, transferred electro-osmotically from the cathode compirtment Into the niiddle compartment, and this effect may 35 lower the levels of the liquid In the cathode compartments by 25 to 75 or even 100 cms. Plor this reason I have frequently found It advantageous to keep the level of liquid constant in the cathode compartments also, for instanre by coniinuously 40 adding thereto a quantity of raw water balancing the amount of liquid carried off by electroosmotical emigration. This means, that I may provide constant levels of liquid within all the compartments o'f a ceR, the level In the middle 45 compartment being for Instance equal to that in the anode compartment and somewhat higher than that In the cathode compartment. According to these modifirations of my invention the electrode compartments, instead of being 50 washed with highly diluted watery, solutions or 'even with pure water, as was heretofore customary, are only replenished with lfquid to partially or completely compensate for the emigration of liquid from one or both of these com- 55 Partments to the nilddle compartment. The liquid thus supplied to this middle compartment by electro-osmotic action Is also subjected to Purification, and In view thereof my Invention may involve a, feeding of the middle compart- 60 ment in -part through the anode compartment, and in the course of operation, also through the r-athode compartment. This may be true to such an extent that the mlddle compartments receive 25 or 35 P'er cent of their feed tbrough the elec- 65 trode compartments. This ratio varies, however, In the course of operation; as a rule the electroendosmotic immigration of liquid from the anode compartments slowly decreases while the immfgmtion from the cathode compartments slowly 70 increases. The graduauy increasing electrical conductivity of the anolyte also reduces the electroendosmotic effect of the anodical diaphragm, and If the concentmtion of the acid in the anode com- 75
[7]2,098,770 7 partment Is raised to a certain limit, this diaPhragm may become altogethe.r non-charged or even slightly charged in an inverse sense. With diaphragms made of the so -called Pukall mass (a, ceramic material manufactured by the Staatliche Porzellanmanufaktur at Berlin) the charge substantially disappears at an acid concentration of about 5 per cent, and these diaphragms even become slightly positively charged io in contact with a solution 6ontaining 10-20 per cent of acid. In view of this, changeabuity of the charge of the diaphragms the term "negatively charged anodical diaphragms" Is here used to mean such diaphragms which are negatively 15 charged under norrngl conditions, i. e. In contact with a dilute solution of 'potassium chloride, or with other neutral or slightly acid solutions. In contact with strongly acid solutions substantially all materials iri question which are resis20 tive to acid, become slightly positively charged and this phenomenon leads me to avoid an aeld concentration above a certain upper limit, which depends upon commercial consideration%. When using for Instance a cell of the type shown in Flg. 25 1 of the drawings, this cell being altogether closed at the bottom, an electroendosrnotic effect of the anodic diaphragm ca@using an immigration of liquid Into the anode compartment would gradually raise the level of liquid within this compart30 ment, until the anolyte flows over. In order to prevent this, I avoid acid concentrations in the anode compartments above about 5% by withdrawing some liquid from these compartments at intervals; the liquid withdrawn is then auto35 matically replaced by raw water. It will be noted that such an operation of the cells permits the use of cup-shaped anodic diaphragms open only on their top, and not requiring any tight, seals. An addition of foreign cations to the anolyte 40 or of foreign anions to the catholyte enables me to solve certain by-problems in the electrolytic desalting of liquids. As mentioned above, such additions allow to control the acidity or alkalinitv of the water In puriflcation. By similar means I 45 may avoid an undesired haziness of the purified liquid or remove certain colloidal substances tberefrom by precipitation. In order to obtain such effects I may add to the anolyte cations such as those of magnesium or aluminium which when rJ'O lmmigrated into the middle compartment precipitate in the form of the corresponding hydroxides which exert a strong absorbing and clarifying effect. Similar effects ar6 obtained for similar reasons, if aluminates, stannates, zincates 55 or similar metallates are added to the catholyte. Electrode liqugrs of an increased electrical conductivity not only reduce the voltage required and consequently the consumption of energy for a given output, but they further enable me to 60 highly increase the current density and to consequently obtain a higher capacity of a cell of a given size. With an increased current density higher temperatures can be obtained and maintained in the middle compartments of the cells, 65 and this results in a further increase in conductivity with attendant inerease in current density. in the practice of my invention I may use unusua.lly high current densities, and I have found it 70 advantageous to operate my cers at average current densities equal to or exceeding that defined by the formula z I ovc 75 wherein I is the current intensity in amps., a is the active area of the nilddle compartments of the cells in square decimetres, 5 is the avera e current density in the middle COM, 9 partm ents, V Is the average voltage applied to the electrodes of a cell, and c Is the conductivity of the liquid to be purified, expressed In reciprocal ohms per cubic centimetre. 10 I have further ascertained that at elevated tempe ratures a purer product can be obtained. Exper ience has taught me that there is a lower limit of electrical conductivity of the purificid water which can hardly be understepped, at least 15 not without extraordinaxily increasing the consumpti on of energy. Now, as the same electrical condu ctivity corresponds to comparatively higher electr olyte contents of the liquid at lower temperatu res, and to lov@er electrolyte contents at 20 higher temperatures, I prefer carrying out the desalti ng Process at comparatively high temperatures ranging from 40 or even 70 to 90 or 95, C. In order to provide for the desired temperature at a minimum consumption of h@at, I preheat the 25 water to be purified with the hot purified water in a heat exchanger of the type shown In Fig. 5 (f the drawings, or of any other usual type. By this comparatively simple provision I obtain an efficie ncy of these cells which is, at high current 30 densiti es, up to 30% higher than the normal one. With lower current densities the influence is not so Important, but I ajso obtain a somewhat more compl ete desalting effect and a somewhat lower ccnsu mption Gf energy. 35 Wit h the higlily conductive anode liquors and the particularly high current densities which I may use In the practice of my invention-the curre nt densities being materially higher than was heretofore possible in the electrical purifl- 40 cation of raw water-the use of platinum anodes beco mes eofnmercially feasible and advantageous, the required quantity of this very expensive metal being a comparatively small one. This is particular ly true if the platinum is used- in the form 45 of wires such as for instance in the form of wire coils as shown in Mg. 1, or of straight separate wires as shown in Mg. 3. The cost of installation of such platinum electrodes may even be lower than that of magnetite electrodes. On the other 50 hand platinum anodes submerged in an acid eleetrolyte cause a formation of considerable quantities of ozone which can be utilized for sterilizing the liquid to be purified. Wit h the new method here described in detail, 55 which is mainly characterized by the use of anolytes of a comparatively high acid concentration, and with cells of the type here described, I obtain current yields up to about 90% of the theoretical maximum value and this, when calcu- 60 lated according to Faraday's law is a very favorable yield, while on the other hand, in proper practi ce, the yield is never below 80% of said theore tical maximum. The energy consumption depen ds on the average voltage applied to the 65 cells, and the output, which is improved by an elevat ed operating temperature, is as a rule large enoug h to render the installation of such cells, even if provided with platinum anodes, less ex- @0 pensiv e than that of distillation plants of equal output and efficiency. This is true with regard to output s up to 5 cbms. per hour. Var ious changes may be made In the details disclosed in the foregoing specification without 7r,
[8]8 2,093 770 departing from the invention or sacrifleing th,e advantages thereof. In the claims affixed to this specification no selection of any particular modification of ihe invention is intended to the exclusion of other modifications thereof and the right to subsequently make claim to any moclifleation not covered by these claims is expressly reserved. 10 I
