CA1105880A - Method and apparatus for electrochemical contaminant removal from liquid media - Google Patents
Method and apparatus for electrochemical contaminant removal from liquid mediaInfo
- Publication number
- CA1105880A CA1105880A CA356,588A CA356588A CA1105880A CA 1105880 A CA1105880 A CA 1105880A CA 356588 A CA356588 A CA 356588A CA 1105880 A CA1105880 A CA 1105880A
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- Canada
- Prior art keywords
- electrode elements
- outer electrode
- electrochemical cell
- electrodes
- cell according
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
- C02F2103/02—Non-contaminated water, e.g. for industrial water supply
- C02F2103/023—Water in cooling circuits
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Abstract
ABSTRACT OF THE DISCLOSURE
Contaminating ions are removed from an aqueous medium, e.g. hexavalent chromium ions from cooling tower waters, by an electrochemical method which includes formation of an insoluble iron compound or complex of the contaminant ion, utilizing an anode of iron, iron alloy or insoluble iron compound. The electrodes are treated which an acid solution at regular intervals to remove any substances formed on or lodged between the electrodes.
The electrolytic cell includes a plurality of closely spaced generally parallel plate electrodes with electric potential applied to the two end electrodes, and an electrode near each end electrodes is of a size protecting the end electrode from flux field effects.
Contaminating ions are removed from an aqueous medium, e.g. hexavalent chromium ions from cooling tower waters, by an electrochemical method which includes formation of an insoluble iron compound or complex of the contaminant ion, utilizing an anode of iron, iron alloy or insoluble iron compound. The electrodes are treated which an acid solution at regular intervals to remove any substances formed on or lodged between the electrodes.
The electrolytic cell includes a plurality of closely spaced generally parallel plate electrodes with electric potential applied to the two end electrodes, and an electrode near each end electrodes is of a size protecting the end electrode from flux field effects.
Description
~CK RO~NI)_O~ ~HE IN FNTION
This invention relates to the art of electroche~ical treatment of liquids to remove contaminants therefrom.
One area of use of the present invention is the electrochemical treatment by means of consumable electrodes of ionizing liquid media to remove contaminant ions, although the principles of the present invention can be variously applied. Insoluble reaction products of such treatment may tend to accumulate in a manner blocking the flow passages between closely spaced electrode surfaces in a manner inter-fering with proper operation of the treatment apparatus.
Where the apparatus for carrying out such treatment comprises a stacked arrangement of a plurality of closely spaced consumable electrodes with electrical connection being made to the two outer or end electrodes, such electrodes may encounter rapid wear and early failure in the electrode region adjacent the gas-liquid interface. In addition, uneven dissolution of electrodes in apparatus of this type can give rise to problems in cell operation and maintenance.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a new and improved apparatus for electrochemically removing by means of consumable electrodes contaminant ions from ionizing media.
In one particular aspect the present application, which is a division of co-pending Canadian Patent Application No.
244,174, filed January 26, 1976,provides in an electrochemical cell for removing a contaminant ion from an ioni~ing medium and having a cell body:
a) a pair of outer electrode elements positioned in said body in spaced apart relation;
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1,) mc~ s for al-plying ele(~trical poten~ials of opposite polarity directly to said outer eleetrode elements;
c~ addition~l electrode elements bet~een said ollter electrode elemellts in closel~ spaced relation to each other;
d) means in said cell body defining a flow path for an ionizing medium in operative relation to said electrode elements;
e) whereby the electric potentials applied to said outer electrode elements induce an electric charge on said additional electrode elements to cause electric current flow , thorugh said ionizing medium; and f) one of said additional electrode elements in neighboring relation to a corresponding one of said outer electrode elements being of sufficient size so as to extend beyond said neighboring outer electrode element so as to shield said neighboring outer electrode element from flux field effects causing excessive current density.
The foregoing and additional advantages and characteri~ing features of the present invention will become clearly apparent upon a reading of the ensuing detailed description wherein:
BRIEF DESCRIPTION OF THE DRAWING FIGURES
Fig. 1 is a schematic diagram of apparatus for removing contaminants from liquids by the method of this invention and illustrating one phase in the operation thereof;
Fig. 2 is a schematic diagram similar to Fig. 1 and , illustrating another operation phase;
Fig. 3 is a schematic diagram similar to Fig. 1 and , .
, 30 ; jl/ _3_ ~,' l illustrating another operating phase;
Fig. 4 is a schematic diagram similar to Fig. l and illustrating another operating phase;
Fig. 5 is a vertical, sectional view taken about on line 5-5 in Fig. l showing an electrolytic cell according to the present invention.
Fig. 6 is a vertical sectional view ta~,en about on line 6-6 in Fig. 5;
Fig. 7 is a cross-sectional view taken about on line 7-7 in Fig. 6; and Fig. 8 is an enlarged fragmentary sectionalview of a portion of the cell of the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED E~BODIMENTS
A system for electrochemically removing contaminants from ionizing media according to the present invention is shown in ` Fig. l. A detailed description of an electrochemical method , and apparatus to which the present invention is applicable is found in Canadian patent application Serial No. 188,657 filed December 20, 1973, now Canadian Patent No. l,046,448, entitled "Electrochemical Contaminant Removal from Aqueous ~edia", assigned to the assignee of the present invention. Briefly, the method is for electrochemically removing from an ionizing i~ .
medium a contaminant ion which is capable of forming an insoluble iron compound or complex, and the method includes passing an electric current between an anode electrode which has a surface or a portion of a surface thereof of iron, iron alloy or insoluble iron compound, and cathode el~ctrode, through the ionizing medium containing the ion to be removed, so as to produce anodically an insoluble iron , ~ -4-1 compo~nd, species or complex while cathodica]ly reacting with the conta~linant ion to form insoluble products, and removing such insoluble material from the ionizing medium. In preferred embodiments of the invention the contaminant, which is initially toxic, is converted to non-toxic or less toxic form which is insoluble and is removed. For example, hexavalent chromium in chromate or dichromate salt form is changed to the less toxic trivalent form which is capable of forming insoluble complexes and is removed as a complex of iron.
Referring now to Fig. 1, the apparatus includes a pair of electrolytic cells 10, 12 which are identical ln construction and operation, although for some applications it may be desirable to have the cells of different relative size or capacity. Each of the cells 10, 12 includes anodes of iron such as hot and cold rolled steel or stainless steel, iron alloy or insoluble iron compounds such as ferrous oxide or magnetite, and cathodes whi.ch preferably are also of iron but may be of other suitable electrode materials compatible with the anode and electrolyte.
Electrical potential is applied to the anodes and cathodes in a manner which will be described.
Liquid to be treated, for example a waste liquid effluent such as cooling tower blowdown water, is transmitted to the system by a conduit 14 connected to the inlet 16 of cell 10.
Outlet 18 of cell 10 is connected by a conduit 20 to the inlet 22 of cell 12. The outlet 24 of cell 12 is connected through conduit 26, valve 27 and conduit 2~ to a suitable solids handling arrangement. The latter can comprise, for example, a conventional clarifier, filter, inclined tube or .; .
1 plate clarifier, or lagoon for separati.ng clarified efEluent from the sludge, and the sludge can be subjected to inci.neration, drying in sludge beds, centri.fuging or vacuum filtration. A
d.c. power supply 30 is connected by lines 32, 33 to cell 10 and by lines 34, 35 to cell 12, the input power to supply 30 being delivered by lines 36,37.
In operation, as indicated by the broken lines in Fig. 1, liquld from conduit 1~ enters cell 10 in the uppper portion through inlet 16 and flows downwardly through an annular passage to the bottom region of the cell where the flow -then is directed upwardly through an arrangement of stacked, closely spaced and vertically disposed electrode plates and then the liquid leaves the top of the cell 10 through outlet 1~. The liquid flows : through conduit 20 i.nto inlet 22 of cell. 12 in the upper portion and flows first downwardly and then upwardly through an arrangement of vertica.lly stac~ed, closely spaced electrode plates in a manner similar to that of the flow through cell 10. The liauid leaves cell 12 through outlet 24 thereof. The electric current passed through each cell, in particular between the anodes and cathodes thereof and through the ionizing medium containing the ion to be removed, produces an insoluble iron compound or complex ; with the contaminant ion. In particular, hydrogen gas and hydroxide ion are produced at the cathode on the passage of current, simul.taneously at the anode iron is oxidized to ferrous ion, and the ferrous ion produced at the anode and the hydroxide ion produced at the cathode diffuse into solution and react forming ferrous hydroxide. The ferrous hydroxide reduces the . contaminant ion so as to produce a.n insoluble iron compound or complex with the contaminant ion. The hydrogen gas is vented :
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1 to the atmosphere through a suitable relief valve (not shown) at the top of the cell The contaminant to be insolubilized by the foregoing reaction may be in various liquid media in which ionic reactions may occur, the contaminant generally will be substantially completely or entirely dissolved in th~ medium, and non-interfering solvents, solutes and suspended ma-terials may also be present.
The anodes will have the active surfaces thereof at least partially of a material which releases iron in the ionizing medium by action of the electric current, usually at least 50~/O
and preferably over 90% and in most preferred cases the entire electrode being of such material. Mixtures of iron, iron alloys and insoluble iron compounds may be employed. The cathodes are preferably also of iron, although because the cathode is relatively inert with respect to removal of contaminant ions from the ionizing medium being treated, the nature of the cathode material is not critical but should be compatible with the anode and electrolyte and not interfere with the insolubiliza-tion of the soluble contaminant to be removed. Generally, the proportion of contaminant in the ionizing medium will be less than 1% although as much as 15% may sometimes be present.
Normally, the initial contaminant concentration will be no less than 0.03 parts per million, in most instances ~rom 1 to 5,000 ; p.p.m. and usually from 2 to 500 p.p.m. The current density in , amperes per square foot (ASF) is in the range from 0.05 to 500.
, The pH is maintained within the range of about ~ to 11, more preferably from 6 to 10. The dwell time in the apparatus, determined by the flow of electri.city, can be as little as 0.001 minute to as long as one hour but normally at least 5 ', :
if - 7 ' ,, . ~' :
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1 seconds or 0.1 minute is required. For a more detailed description of the foregoing reaction, reference should be made to the aforementioned application Serial No. 188,657.
In the use of cold and hot rolled steel anodes for the electrochemical generation of ferrous species according to the foregoing reactions, a problem encountered in commercial scale systems is the formation of adherent hydrous oxide gelatinous precipitates which can lead to blocking of flow channels in the apparatus, especially the passages bet~een the closely spaced stacked electrode plates. Another problem encountered is the formation of an insoluble insulating iron oxide layer on the electrode surface, often underneath the gelatinous percipitates, which can give rise to high over-voltage and the generation of oxygen at the anode.
In accordance with the present invention there is provided the further step of treating the electrodes with a solution containing acid to remove any substances on or lodged between the surfaces of the electrodes, the step being done at regular intervals. As shown in Fig. 1, the system of the present invention includes means in the form of a tanl; 40 for storing the acid solution. Tank 40 is connected through a conduit 42, valve 44 and conduit 46 to a second outlet 48 of cell 10. Outlet ~:8 is located near the bottom of cell 10.
A conduit 50 connected at one end to a bottom outlet 52 of cell 12 and at the other end to conduit 46 places tank 40 also in communication with cell 12. The apparatus further includes a pump 56, one side of which is connected through a conduit 58 and through valve 44 to conduits 42 and 46. The other side of 'pump 56 is connected by a conduit 60 and through a valve 62 ''' ` -8-., 1 to conduits 64 and 66. Conduit 66 leads into conduit 1~, and conduit 64 leads to a valve 68. Valve 68, in turn, is connected between a pair of conduits 70 and 72. Conduit 70 leads into the system outlet conduit 28, and conduit 72 is connected to tank 40. A conduit 74 is connected at one end to conduit 72 and at the other end to valve 27 joining conduits 26 and 28.
A typical and preferred mode of operation would include continuous operation of the cells treating incoming liquid from conduit 14 for a period of about one day. During such operation pump 56 is not operating and valves 44, 62 and 68 are '` set in a manner controlling the flow as indicated in Fig. 1, with valve 27 allowing flow from conduit 26 to conduit 28.
check valve 78 in conduit 50 prevents any bypass flow away from the plates of cell 10 through conduit 50. At the end of the day, ' the acid solution contained in tank 40 is flowed or circulated through cells 10 and 12 in the following manner. First the flow of incoming liquid in conduit 14 is stopped, su.ch as by a valve r (not shown) in conduit 14 upstream of conduit 66, and the liquid remaining in cells 10, 12 is removed. This is accomplished by operating pump 56 and by placing valve 27 ln a position blocki.ng , flow between conduits 26 and 28 to conduit 74, valve 44 in :. a position connecting only conduits 46 and 58, valve 62 in a position connecting only conduits 60 and 6~1, and valve 68 in a position connecting only conduits 64 and 70 as shown in Fig. 2.
The liquid in cells 10, 12 thus flows outwardly therefrom into conduits 50 and 46, being drawn through pump 56 and transmitted along conduit 64 and through valve 68 and conduit 70 into . outlet conduit 28 as indicated by the broken lines in Fig. 2.
~1hen all or substantially all of the liquid is removed from ,?s~
1cells 10, 12 the acid solution stored ln tank 40 is conveyed serially through cell 10 and cell 12 and returned to tanl~ 40 in the following manner. Pump 5~ is operated and valve 4~ is placed in a position connecting only conduits 42 and 58, valve 62 in a position connecting only conduits 60 and 66, valve 27 in a position connecting only conduits 26 and 7~ and valve ~8 blocking all flow as shown in Fig. 3. The acid solution is dra~7n from tank 40 by pump 56 and is flowed serially through cells 10, 12 and returned to tan~ 40 as indicated by the broken lines in Fig. 3.
10According to a preferred mode of the present invention the acid solution from tank ~0 ls recirculated through the cells 10, 12 for a time from about 5 minutes to about 10 minutes.
flow rate of about 25 gallons per minute was found to provide satisfactory results. The flow rate must be sufficient to dissolve the gelatinous precipitates and the oxide layers and to remove the material or sludge from the cell, i.e. sufficient to provide the required mechanical washing action. An aqueous solution at about three percent to about five percent concentration of technical grade hydrochloric acid was found to provide effective removal of t~le ferric oxide layer and gelatinous precipitates while at the same time not dissolving or otherwise damaging the electrode plates. Other acids which satisfy this requirement can of course be used. It is preferred to circulate the acid solution through cells 10, 12 with no electrical power being supplied to the cells, although it has been found that the washing operation can be performed in a relatively shorter period of time if the electric power is applied. ~7hile good results have been obtained with the foregoing washing procedure done once per day, there may be situations where a greater level of electric current is used during the treating opera-tion thereby 1, ~ ,..`5 ~g ~
1 requiring that the acid washing procedure of the present invention be performed at a rate more frequently than once per day. ~t the end of the washing cycle, the acid solution is removed from cells 10, 12 and returned to tan~ 40. This is done by operating pump 56 and by placing valve 44 in a position connecting only conduits 46 and 58, valve 62 in a position connecting only conduits 60 and 64, valve 68 in a position connecting only conduits 64 and 72 and valve 27 i.n a position blocking all flow as shown in Fig. 4. The acid solution in cells 10, 12 thus flows from cells 10, 12 through conduits 50 and 46, through conduits 64 and 72 into tank 40 as indicated by the broken lines in Fig. 4. Then the apparatus is ready for another operating cycle for treatment of liquid obtained from input conduit 1~!.
The solution stored in tank 40 can be periodically replenished with additional acid and at an appropriate time later on, the solution in tank 40 is removed and rep].aced with a new solution. The solution removed will have an acid strength of less than 1% and it can be simply bled into an effluent stream or it could be stored in ponds or tanks and periodically neutralized. In any event the nature of the solution in tank 40 is such taht it does not pose any disposal problem. According to a preferred mode of the present invention once per month and prior to the last day of treatment, the content of tar.k 40 is bled into the treated effluent and discharged into the solids handling arrangement for solids retnoval and final disposal.
. This is done by setting the valves appropriately and operating pump 56 to draw solution from tank 40 through pump 56, conduit 64, valve 68, conduit 70 and into outlet conduit 28. Tank 40 also is provided with a conventional valved drain outlet 78.
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1 By virtue of ~he method o~ the present invention, the cells lO and 12 can be operated at the original electrical current and volta~e quantities. Heretofore, ~oatings formed on the electrode surfaces would require higher operating voltage to obtain the same level of current. ~Tith tl~ ferric oxide coating being removed from the electrode plates by the method of the present invention, there is no problem of having to operate at a higher voltage to obtain the same level of current. Thus the method of the present invention prevents the occurrence of large increases of voltage from occurring across cells of this type which, without the foregoing procedure, can occur after about fourty hours of operation and cause cessation of high efficiency generation of ferrous species at the anode. Treat-ing the electrodes with acid solution according to the present invention also removes any foreign matter which might flow into the cell with the effluent and accumulate. Thus, the acid wash recirculation technique allows each electrochemical cell in a system of this type to be operated at about 9% of the time based upon a 24 hour day and yield at least 75% utilization of the ; 20 iron electrodes in the cell. While the method of the present invention has been illustrated by a system including the two electrolytic cells 10 and 12, the method is of course applicable to an arrangement having only one cell or a plurality of any number of cells.
Figs. 5-8 illustrate in further detail an electrolytic cell according to the present invention, for example cell 12 shown in Fig. 1. The cell includes a housing of electrically insulatin~ and inert material, for example a suitable plastic material such as polyvinyl chloride which can be reinforced with :"
", , 1 fiberglass. I'he housing comprises four main parts including a generally cup-shaped base or bottom portion 90, a generally cylindrical central portion or section 92 which fits at one end thereof into the base 90, a generally cup-shaped upper portion 94 which is set on the upper end of central section 92, and a cap or closure 96 secured to the top portion. The central section 92 is provided with an opening to receive a pipe fitting 98 defining inlet 22, and section 92 and base 90 are provided with aligned openings for receiving a pipe fitting 100 deflning out-let 52 adJacent the lower portion of the cell. Similarly, the upper portion 94 is provided with an opening for receiving a pipe fitting 102 defining outlet 2~. When assembly of the cell is completed, it is closed by cap element 96 which is connected to an armular flange 104 of top portion 9~, connection being made by a plurality of suitable fasteners 106 with a sealing gasket 108 positioned therebetween. Upper portion 94~ has a generally planar bottom surface portion 110 provided with a gen-erally square shaped central opening defined by depending opposed wall portions 112-115. In an assembled cell positioned for use, the bottom 110 of housing portion 94 is generally horizontal, ; being secured for example by bonding to the upper edge of section 92.
The cell of the present invention further comprises a frame or holder 118 for positioning a plurality of planar electrodes in closely spaced and substantially parallel relation in the cell body. The frame or holder 118 is hollow rectangular in shape, in the present illustration square ln crossection, and includes wall portions 119-122. The walls are of inert and electrically insulating material, for example a suitable plastic such as polyvinyl chloride which can be rei~forced with fiber-glass. Walls 119-122 are preferably honded together 1 at the edges to provide the hollow rectangular frame or housing, but frame 119 could be of an integral molded conskruction. One pair of opposed walls is provlded with longitudinal and closely spaced parallel grooves facing inwardly of frame 118, the opposed grooves being in alignment. The holder 118 containing - the electrodes fitted therein is received adjacent the upper end of the cell as viewed in Figs. 5 and 6 relatively snugly with-in the opening in the base of upper portlon 94 defined by the wall portions 112-115 as to be supported and firmly held thereby. The holder and electrode plates are supported adjacent the bottom of the cell by the following preferred arrangement.
A pair of spaced-apart, generally parallel supporting elements ; 126, 128 are secured on the bottom of the cell, being disposed generally perpendicular to the planes of the electrode elements.
Thus the electrodes and the holder rest upon the upper surfaces of the elements so that an open space for liauid flow is provided between the bottom edges of the electrode plates and the bottom of the cell. The supporting elements are held in place by a pair of right angle bracket members 130, 132 fastened thereto and secured in a suitable manner to the bottom of the base portion 90.
As previously described, a plurality of electrode plates are positioned within holder 118, being firGly held therein in closely spaced, generally parallel relationship. As shown in Fig. 5, the plates extend along the entire longitudinal dimension of the holder, being slightly greater in overall length. The plates are all of the same overall length~ith the exception of four of the plates for a purpose to be shown. In the present illustration a total o~ fourteen plates is show, al-though a smaller or larger number thereof can be employed ~, : -14-., l depending upon the operating conditions. The end or outer electrode elements 134 and 136 of the arrangement each are oE
slightly greater length thereby including extending portions 138 and 14-0, respectively, for the purpose of making electrical connection thereto. In particular, and referring in detail to Fig. 8, an electrically conductive threaded rod or bolt 142 extends through an opening provided in the extending portion 138 of end electrode 134. The end of bolt 142 is secured thereto by an arrangement of conductive lock washers 144 and nuts 146 threaded thereon. The other end of the bolt is fastened to the housing wall 94 by an arrangement of rubber grommets 1~3 and nuts 150 threaded thereon. The electrical line 34 is connected to the outer end of bolt 142 and held thereto by a sleeve 152 threaded on the bolt and against the nut 150 located external to housing wall 94. The connection is protected by an insulating cover piece 154 of generally semispherical shape with the edge thereof abutting the outer surface of housing portion 94. A fastener 155 of insulating material extends through an aperture in cover 154 and threads into the other end of sleeve 152 to hold cover 154 in place. An identical arrangement is provided for connect-ing electrical lead 35 through a bolt to the extending portion 140 of the outer electrode 136 at the opposite end of the stack or series. Each cell in the sys-tem would have similar arrange-ments.
In the electrolytic cell of the present invention, electric potential is applied directly from power supply 30 to only the outer or end electrodes 134 and 136. Depending upon the relative polarity of the applied potentials, one of the end electrodes serves as an anode and the other as a cathode.
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1 Electric potential then is applied to the intermediate or additional electrodes 158 by induction. In particular, and referring ~o the electrode arrangement of Fig 5, assuming that line 34 is at a relatively positive potential, and line 35 at a relatively negative potential, lines of flux emanating from the end electrode 134 will induce relatively negative charges on the surface of the very next electrode 158 facing the end elec-trode 134 and this, in turn, causes relatively positive charges to appear on the opposite surface of that electrode 158. The inductive effect proceeds through the remainder of the electrodes 158 in the arrangemcnt such that there are negative charges present on the left hand surface of each electrode 158 in the arrangement as viewed in Fig. 5 and relative positive charges on the right hand surfaces thereof. The direct application of negative potential to end electrode 136 induces relatively pos-itive charges on the right hand surfaces of t`he electrodes 158 and negative charges on the left hand surfaces in a similar manner. The foregoing inductive action is enhanced by the degree of close spacing of the electrode plates and by the degree of conductivity of the liquid in the cell which serves as an electrolyte.
During the foregoing mode of operation, where electrical potential applied to the end electrodes 134, 136 induces a charge in the electrodes 158 therebetween, the lines of flux travel not only through the region between the plates but also outwardly from the edges of the plates, particularly from the upper edges as viewed in Fig. 5 in the region above the electrodes - and below the cell cover 96. These firnge lines of flux em-anating ~rom the plate edges travel in generally curved paths i 1 between relatively positive and negative potential points. A
problem observed to arise in commercial scale apparatus involved partial dissolution of the end plate electrodes 134,136 at the gas-liquid interface. Such dissolution can proceed from the top of the electrodes to the point of the electrical lead-in connec-tion whereupon the cell becomes disabled. This is believed to occur as a result of complete or continuous flux loops from the electrodes encountering the end electrodes which induce current therein, the current density being the highest at the region of the gas-liquid interface. This causes the end electrodes to erode at a rate greater than that of the other electrode plates.
In accordance with the present invention, an electrode plate 158 near or adjacent the corresponding one of the end electrodes 134, 136 to which electric potential is applied extends beyond the end or outer electrode by an amount sufficient to protect the electrodes from flux field effects causing excessive current density. The electrode 158 extends upwardly as viewed in Fig.
5 into the region above the electrodes and below the cell cover 96. This serves tobreak the flux lines in that region in a manner preventing them from travelling to an end electrode thereby protection those electrodes. Thus, flux lines emanating from electrodes into this upper region and tending toward an end electrode would first ercounter -the extended electrode and be blocked or cut, in any event not reaching the end electrode.
In the present instance the second neighboring or adjacent elec-trode 158 relative to end electrode 13~ is made of greater length including the extending portion 16~~. Slmilarly, the sec-ond neighboring or adjacent electrode 158 from end electrode 136 is made of greater length including the extending portion 168.
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1 For example, a flux line leavir.g end electrode 138 in the space above the plates would be broken or cut by portion 164 and thus be unable to encounter end electrode 140. Similarly, flux leav-i.ng end electrode 140 in the space above the plates would be bro-ken or cut by portion 168 and thus not travel to end electrode 138. In an illustrative cell, the intermediate electrodes 158 are 48 inches in length, the end electrodes are 49 inches in length, and the two electrodes 158 having the extending portions 164 and 168, respectively, are 50 inches in length. All plates can be about one-eight inch thick and spaced apart about one-eighth inch. In the arrangement of the present illustra~ion, the second neighboring electrode is extended rather than the first or immediately adjacent electrode, in order to provide s-ufficient space for connecting bolts 150 to the end plates 134,136.
The shielding or protecting funccion provided by the extending portions 16~,168 of the two electrodes 158 is en-hanced by coating the extending portions on the surface facing the corresponding end electrode with an electrically insulating material. In the present illustration the surface facing the corresponding end electrode is coated as indicated by coating 172 shown in Fig. 8 although both surfaces could be coated.
Approximately the upper ten to twenty percent of the surface length of the electrode is coated, i.e. the upper six to eight , inches thereof. One form of material found to provide satis-factory results is an epoxy material available commercially as Pro Tek~ fromthe Baltimore Paint and Chemical Corporation, made by mixing equal quantities of Duratyle~ epoxy hibuild reactor component #2 and Duratyle~ epoxy regular high ' "~
~' : 1 gloss base A.
The extending portions 164 I 168 of the two electrodes 158 adjacent end electrodes 134 and 136, respectively, according to the present invention together with the insulating coating thereof eliminate difficulties associated with current density effects which otherwise lead to early failure of the end electrodes.
In conjunction with this, the distance between the top of the electrodes and the top of the cell is made as small as possible while allowing for adequate liquid outflow from the cell, inas-much as the top of the cell tends to cut any lines flux incidentthereon. The liquid level in the cell is controlled so as to submerge all of the electrodes but just slightly over the extending portions 164, 168 of the shielding electrodes. The electrodes should not be submerged to an excessive degree be-cause the liquid electrolyte can provide a relatively easy path for flux fields.
In addition to bipolar operation of the cell, the polarity of the electrical potential applied to the end plates 134,136 is reversed at regular intervals determined by the cell current density which, in turn, is determined by the operating conditions. Reversing or switching the polarity reverses the direction of current flow of current in the electroly-te between electrodes thereby assuring that dissolution of the end electrodes 134,136 occurs at an equal rate, and that the dissolution of the electrodes 158 between them occurs uniformly on both sides.
In addition, reversing the current flow in the foregoing manner eliminates the possibility of bucl~ling of the electrodes which could cause shorting between adjacent electrodes ancl also mechanical difficulties in removing a spent electrode from the l cell. Reversing polarity of the potential applied to electrodes 134,136 is accomplished by a suitable switching means assoeiated with power supply 20 which can be manually operated or :~ automatically controlled by a timer in a known manner. The rate at which the polarity is reversed depends upon the current density, and by way of illustration typical rates are in the range from about one reversal per hour to two reversals per week.
The method and apparatus of the present invention is 10 illustrated further by the foregoing examples.
Example I
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; Cooling tower blowdown water was supplied to the inlet of ¢~ apparatus similar that of Fig. 1 at a rate of 25 gallons per minute with a hexavalent chrome concentration of about 3.5 p.p.m.
The apparatus has a total effective electrode weight of 217 pounds, and was operated at about 4.0 amperes and betwen 11.5 volts and 14.0 volts. The method of the present invention was performed continuously with the acid washing step being done for a period of 15 to 30 minutes per day. Each day 1.05 pounds of 20 hexavalent chrome was removed and the effluent contained less than 0.05 p.p.m. of hexavalent chrome, often falling to undetect-. able limits.
¢ At the time the electrodes were changed, it was observed that only about 60% of the available iron had been utilized and that the electro~es still were in quite good condition and could have been used for a longer period of time. The sludge analysis indicated approximately 3.5 pounds of iron per pound of chrome compared to a theoretical useage of 3.2. The two cells contain-ed a total active electrode weight of 204 pounds. Assuming that .,.
l 75~/O of the electrodes could ~e used before replacement, there is 153 pounds of useable iron available for chrome reduction.
At thls rate, the cells could operate approximately 42 days prior to electrode replacement. At 75% electrode utilization, equivalent iron usage would be 4.3 pounds per pound oE chrome and at 90% utilization the value would be 3.6 pounds per pound of chrome. In the event that the electrodes are not run for the standard period, the apparent iron usage goes up. In this example, during the time the electrodes operated with 60% of the electrode life, their apparent iron usage was 5.7 pounds per pound of chrome whereas actually they were using 3.5 pounds.
In this example, the operation for about 25 days removed 27 pounds of hexavalent chrome for an iron to chrome ration of 5.7.
Example II
Cooling tower blowdown water was supplied to the inlet of apparatus similar to that of Fig. 1 but of greater capacity than that llsed in Example I at a rate of 88 gallons per minute with a hexavalent chrome concentration of about 8.0 p.p.m. The higher concentration and flow of this particular blowdown water required a current of about 14 amperes and a potential between 40-50 volts. The cells were operated 24 hours per day except for the acid wash procedure and treated 8.3 pounds of hexavalent chrome each day. The effluent contained less than 0.05 p. p. m. of hexavalent chrome. The apparatus had available 485 pounds of useable iron based on 75~/O electrode utilization, and the plant required the removal of 8.3 pounds of hexavalent chrome each da,. Under the foregoing conditions, operation for approximately 17 days prior to electrode replace-ment was predicted.
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1 Operating data from the electrochemical method and apparatus of the present invention for chromate removal in the treatment of cooling tower blowdown water has shown that chrome : levels consistently under 0.05 p.p.m. can be achieved with no pH adjustment of the incoming water provided that it is in the range of approximately 5 to 10. Zinc ion frequently accom-panies chromate in cooling tower blowdown water, and in most cases the effluent from the apparatus has a pH in a range ade-quate to enable removal of the bulk of the zinc which is present.
It is therefore apparent that the present invention accomplishes its intended objects. Treating the electrodes , with an acid solution removes any substances on or lodged between the electrode surfaces arising from the reaction or ' from the incoming liquid. Applying electrical potential direct~
ly to only the outer or end electrodes, with charge being induced in the additional closely spaced electrodes therebetween, advantageously permits cell operation to be at a relatively low current and relatively high voltage which, in turn, allows use of relatively more efficient power supply apparatus. The ~i 20 electrode arrangement, including the provision of the shielding electrodes protects the end electrodes, to which direct electri-cal connection is made, from flux field effects and excessive current density. Changing the polarity of applied potential at regular intervals insures relatively even dissolution of the electrodes. While the foregoing embodiments of the present in-vention have been described in detail, this is for the purpose of illustration, not limitation.
This invention relates to the art of electroche~ical treatment of liquids to remove contaminants therefrom.
One area of use of the present invention is the electrochemical treatment by means of consumable electrodes of ionizing liquid media to remove contaminant ions, although the principles of the present invention can be variously applied. Insoluble reaction products of such treatment may tend to accumulate in a manner blocking the flow passages between closely spaced electrode surfaces in a manner inter-fering with proper operation of the treatment apparatus.
Where the apparatus for carrying out such treatment comprises a stacked arrangement of a plurality of closely spaced consumable electrodes with electrical connection being made to the two outer or end electrodes, such electrodes may encounter rapid wear and early failure in the electrode region adjacent the gas-liquid interface. In addition, uneven dissolution of electrodes in apparatus of this type can give rise to problems in cell operation and maintenance.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a new and improved apparatus for electrochemically removing by means of consumable electrodes contaminant ions from ionizing media.
In one particular aspect the present application, which is a division of co-pending Canadian Patent Application No.
244,174, filed January 26, 1976,provides in an electrochemical cell for removing a contaminant ion from an ioni~ing medium and having a cell body:
a) a pair of outer electrode elements positioned in said body in spaced apart relation;
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~3~f3~
1,) mc~ s for al-plying ele(~trical poten~ials of opposite polarity directly to said outer eleetrode elements;
c~ addition~l electrode elements bet~een said ollter electrode elemellts in closel~ spaced relation to each other;
d) means in said cell body defining a flow path for an ionizing medium in operative relation to said electrode elements;
e) whereby the electric potentials applied to said outer electrode elements induce an electric charge on said additional electrode elements to cause electric current flow , thorugh said ionizing medium; and f) one of said additional electrode elements in neighboring relation to a corresponding one of said outer electrode elements being of sufficient size so as to extend beyond said neighboring outer electrode element so as to shield said neighboring outer electrode element from flux field effects causing excessive current density.
The foregoing and additional advantages and characteri~ing features of the present invention will become clearly apparent upon a reading of the ensuing detailed description wherein:
BRIEF DESCRIPTION OF THE DRAWING FIGURES
Fig. 1 is a schematic diagram of apparatus for removing contaminants from liquids by the method of this invention and illustrating one phase in the operation thereof;
Fig. 2 is a schematic diagram similar to Fig. 1 and , illustrating another operation phase;
Fig. 3 is a schematic diagram similar to Fig. 1 and , .
, 30 ; jl/ _3_ ~,' l illustrating another operating phase;
Fig. 4 is a schematic diagram similar to Fig. l and illustrating another operating phase;
Fig. 5 is a vertical, sectional view taken about on line 5-5 in Fig. l showing an electrolytic cell according to the present invention.
Fig. 6 is a vertical sectional view ta~,en about on line 6-6 in Fig. 5;
Fig. 7 is a cross-sectional view taken about on line 7-7 in Fig. 6; and Fig. 8 is an enlarged fragmentary sectionalview of a portion of the cell of the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED E~BODIMENTS
A system for electrochemically removing contaminants from ionizing media according to the present invention is shown in ` Fig. l. A detailed description of an electrochemical method , and apparatus to which the present invention is applicable is found in Canadian patent application Serial No. 188,657 filed December 20, 1973, now Canadian Patent No. l,046,448, entitled "Electrochemical Contaminant Removal from Aqueous ~edia", assigned to the assignee of the present invention. Briefly, the method is for electrochemically removing from an ionizing i~ .
medium a contaminant ion which is capable of forming an insoluble iron compound or complex, and the method includes passing an electric current between an anode electrode which has a surface or a portion of a surface thereof of iron, iron alloy or insoluble iron compound, and cathode el~ctrode, through the ionizing medium containing the ion to be removed, so as to produce anodically an insoluble iron , ~ -4-1 compo~nd, species or complex while cathodica]ly reacting with the conta~linant ion to form insoluble products, and removing such insoluble material from the ionizing medium. In preferred embodiments of the invention the contaminant, which is initially toxic, is converted to non-toxic or less toxic form which is insoluble and is removed. For example, hexavalent chromium in chromate or dichromate salt form is changed to the less toxic trivalent form which is capable of forming insoluble complexes and is removed as a complex of iron.
Referring now to Fig. 1, the apparatus includes a pair of electrolytic cells 10, 12 which are identical ln construction and operation, although for some applications it may be desirable to have the cells of different relative size or capacity. Each of the cells 10, 12 includes anodes of iron such as hot and cold rolled steel or stainless steel, iron alloy or insoluble iron compounds such as ferrous oxide or magnetite, and cathodes whi.ch preferably are also of iron but may be of other suitable electrode materials compatible with the anode and electrolyte.
Electrical potential is applied to the anodes and cathodes in a manner which will be described.
Liquid to be treated, for example a waste liquid effluent such as cooling tower blowdown water, is transmitted to the system by a conduit 14 connected to the inlet 16 of cell 10.
Outlet 18 of cell 10 is connected by a conduit 20 to the inlet 22 of cell 12. The outlet 24 of cell 12 is connected through conduit 26, valve 27 and conduit 2~ to a suitable solids handling arrangement. The latter can comprise, for example, a conventional clarifier, filter, inclined tube or .; .
1 plate clarifier, or lagoon for separati.ng clarified efEluent from the sludge, and the sludge can be subjected to inci.neration, drying in sludge beds, centri.fuging or vacuum filtration. A
d.c. power supply 30 is connected by lines 32, 33 to cell 10 and by lines 34, 35 to cell 12, the input power to supply 30 being delivered by lines 36,37.
In operation, as indicated by the broken lines in Fig. 1, liquld from conduit 1~ enters cell 10 in the uppper portion through inlet 16 and flows downwardly through an annular passage to the bottom region of the cell where the flow -then is directed upwardly through an arrangement of stacked, closely spaced and vertically disposed electrode plates and then the liquid leaves the top of the cell 10 through outlet 1~. The liquid flows : through conduit 20 i.nto inlet 22 of cell. 12 in the upper portion and flows first downwardly and then upwardly through an arrangement of vertica.lly stac~ed, closely spaced electrode plates in a manner similar to that of the flow through cell 10. The liauid leaves cell 12 through outlet 24 thereof. The electric current passed through each cell, in particular between the anodes and cathodes thereof and through the ionizing medium containing the ion to be removed, produces an insoluble iron compound or complex ; with the contaminant ion. In particular, hydrogen gas and hydroxide ion are produced at the cathode on the passage of current, simul.taneously at the anode iron is oxidized to ferrous ion, and the ferrous ion produced at the anode and the hydroxide ion produced at the cathode diffuse into solution and react forming ferrous hydroxide. The ferrous hydroxide reduces the . contaminant ion so as to produce a.n insoluble iron compound or complex with the contaminant ion. The hydrogen gas is vented :
`5~
1 to the atmosphere through a suitable relief valve (not shown) at the top of the cell The contaminant to be insolubilized by the foregoing reaction may be in various liquid media in which ionic reactions may occur, the contaminant generally will be substantially completely or entirely dissolved in th~ medium, and non-interfering solvents, solutes and suspended ma-terials may also be present.
The anodes will have the active surfaces thereof at least partially of a material which releases iron in the ionizing medium by action of the electric current, usually at least 50~/O
and preferably over 90% and in most preferred cases the entire electrode being of such material. Mixtures of iron, iron alloys and insoluble iron compounds may be employed. The cathodes are preferably also of iron, although because the cathode is relatively inert with respect to removal of contaminant ions from the ionizing medium being treated, the nature of the cathode material is not critical but should be compatible with the anode and electrolyte and not interfere with the insolubiliza-tion of the soluble contaminant to be removed. Generally, the proportion of contaminant in the ionizing medium will be less than 1% although as much as 15% may sometimes be present.
Normally, the initial contaminant concentration will be no less than 0.03 parts per million, in most instances ~rom 1 to 5,000 ; p.p.m. and usually from 2 to 500 p.p.m. The current density in , amperes per square foot (ASF) is in the range from 0.05 to 500.
, The pH is maintained within the range of about ~ to 11, more preferably from 6 to 10. The dwell time in the apparatus, determined by the flow of electri.city, can be as little as 0.001 minute to as long as one hour but normally at least 5 ', :
if - 7 ' ,, . ~' :
,, .
`5~
1 seconds or 0.1 minute is required. For a more detailed description of the foregoing reaction, reference should be made to the aforementioned application Serial No. 188,657.
In the use of cold and hot rolled steel anodes for the electrochemical generation of ferrous species according to the foregoing reactions, a problem encountered in commercial scale systems is the formation of adherent hydrous oxide gelatinous precipitates which can lead to blocking of flow channels in the apparatus, especially the passages bet~een the closely spaced stacked electrode plates. Another problem encountered is the formation of an insoluble insulating iron oxide layer on the electrode surface, often underneath the gelatinous percipitates, which can give rise to high over-voltage and the generation of oxygen at the anode.
In accordance with the present invention there is provided the further step of treating the electrodes with a solution containing acid to remove any substances on or lodged between the surfaces of the electrodes, the step being done at regular intervals. As shown in Fig. 1, the system of the present invention includes means in the form of a tanl; 40 for storing the acid solution. Tank 40 is connected through a conduit 42, valve 44 and conduit 46 to a second outlet 48 of cell 10. Outlet ~:8 is located near the bottom of cell 10.
A conduit 50 connected at one end to a bottom outlet 52 of cell 12 and at the other end to conduit 46 places tank 40 also in communication with cell 12. The apparatus further includes a pump 56, one side of which is connected through a conduit 58 and through valve 44 to conduits 42 and 46. The other side of 'pump 56 is connected by a conduit 60 and through a valve 62 ''' ` -8-., 1 to conduits 64 and 66. Conduit 66 leads into conduit 1~, and conduit 64 leads to a valve 68. Valve 68, in turn, is connected between a pair of conduits 70 and 72. Conduit 70 leads into the system outlet conduit 28, and conduit 72 is connected to tank 40. A conduit 74 is connected at one end to conduit 72 and at the other end to valve 27 joining conduits 26 and 28.
A typical and preferred mode of operation would include continuous operation of the cells treating incoming liquid from conduit 14 for a period of about one day. During such operation pump 56 is not operating and valves 44, 62 and 68 are '` set in a manner controlling the flow as indicated in Fig. 1, with valve 27 allowing flow from conduit 26 to conduit 28.
check valve 78 in conduit 50 prevents any bypass flow away from the plates of cell 10 through conduit 50. At the end of the day, ' the acid solution contained in tank 40 is flowed or circulated through cells 10 and 12 in the following manner. First the flow of incoming liquid in conduit 14 is stopped, su.ch as by a valve r (not shown) in conduit 14 upstream of conduit 66, and the liquid remaining in cells 10, 12 is removed. This is accomplished by operating pump 56 and by placing valve 27 ln a position blocki.ng , flow between conduits 26 and 28 to conduit 74, valve 44 in :. a position connecting only conduits 46 and 58, valve 62 in a position connecting only conduits 60 and 6~1, and valve 68 in a position connecting only conduits 64 and 70 as shown in Fig. 2.
The liquid in cells 10, 12 thus flows outwardly therefrom into conduits 50 and 46, being drawn through pump 56 and transmitted along conduit 64 and through valve 68 and conduit 70 into . outlet conduit 28 as indicated by the broken lines in Fig. 2.
~1hen all or substantially all of the liquid is removed from ,?s~
1cells 10, 12 the acid solution stored ln tank 40 is conveyed serially through cell 10 and cell 12 and returned to tanl~ 40 in the following manner. Pump 5~ is operated and valve 4~ is placed in a position connecting only conduits 42 and 58, valve 62 in a position connecting only conduits 60 and 66, valve 27 in a position connecting only conduits 26 and 7~ and valve ~8 blocking all flow as shown in Fig. 3. The acid solution is dra~7n from tank 40 by pump 56 and is flowed serially through cells 10, 12 and returned to tan~ 40 as indicated by the broken lines in Fig. 3.
10According to a preferred mode of the present invention the acid solution from tank ~0 ls recirculated through the cells 10, 12 for a time from about 5 minutes to about 10 minutes.
flow rate of about 25 gallons per minute was found to provide satisfactory results. The flow rate must be sufficient to dissolve the gelatinous precipitates and the oxide layers and to remove the material or sludge from the cell, i.e. sufficient to provide the required mechanical washing action. An aqueous solution at about three percent to about five percent concentration of technical grade hydrochloric acid was found to provide effective removal of t~le ferric oxide layer and gelatinous precipitates while at the same time not dissolving or otherwise damaging the electrode plates. Other acids which satisfy this requirement can of course be used. It is preferred to circulate the acid solution through cells 10, 12 with no electrical power being supplied to the cells, although it has been found that the washing operation can be performed in a relatively shorter period of time if the electric power is applied. ~7hile good results have been obtained with the foregoing washing procedure done once per day, there may be situations where a greater level of electric current is used during the treating opera-tion thereby 1, ~ ,..`5 ~g ~
1 requiring that the acid washing procedure of the present invention be performed at a rate more frequently than once per day. ~t the end of the washing cycle, the acid solution is removed from cells 10, 12 and returned to tan~ 40. This is done by operating pump 56 and by placing valve 44 in a position connecting only conduits 46 and 58, valve 62 in a position connecting only conduits 60 and 64, valve 68 in a position connecting only conduits 64 and 72 and valve 27 i.n a position blocking all flow as shown in Fig. 4. The acid solution in cells 10, 12 thus flows from cells 10, 12 through conduits 50 and 46, through conduits 64 and 72 into tank 40 as indicated by the broken lines in Fig. 4. Then the apparatus is ready for another operating cycle for treatment of liquid obtained from input conduit 1~!.
The solution stored in tank 40 can be periodically replenished with additional acid and at an appropriate time later on, the solution in tank 40 is removed and rep].aced with a new solution. The solution removed will have an acid strength of less than 1% and it can be simply bled into an effluent stream or it could be stored in ponds or tanks and periodically neutralized. In any event the nature of the solution in tank 40 is such taht it does not pose any disposal problem. According to a preferred mode of the present invention once per month and prior to the last day of treatment, the content of tar.k 40 is bled into the treated effluent and discharged into the solids handling arrangement for solids retnoval and final disposal.
. This is done by setting the valves appropriately and operating pump 56 to draw solution from tank 40 through pump 56, conduit 64, valve 68, conduit 70 and into outlet conduit 28. Tank 40 also is provided with a conventional valved drain outlet 78.
., .
' ~
1 By virtue of ~he method o~ the present invention, the cells lO and 12 can be operated at the original electrical current and volta~e quantities. Heretofore, ~oatings formed on the electrode surfaces would require higher operating voltage to obtain the same level of current. ~Tith tl~ ferric oxide coating being removed from the electrode plates by the method of the present invention, there is no problem of having to operate at a higher voltage to obtain the same level of current. Thus the method of the present invention prevents the occurrence of large increases of voltage from occurring across cells of this type which, without the foregoing procedure, can occur after about fourty hours of operation and cause cessation of high efficiency generation of ferrous species at the anode. Treat-ing the electrodes with acid solution according to the present invention also removes any foreign matter which might flow into the cell with the effluent and accumulate. Thus, the acid wash recirculation technique allows each electrochemical cell in a system of this type to be operated at about 9% of the time based upon a 24 hour day and yield at least 75% utilization of the ; 20 iron electrodes in the cell. While the method of the present invention has been illustrated by a system including the two electrolytic cells 10 and 12, the method is of course applicable to an arrangement having only one cell or a plurality of any number of cells.
Figs. 5-8 illustrate in further detail an electrolytic cell according to the present invention, for example cell 12 shown in Fig. 1. The cell includes a housing of electrically insulatin~ and inert material, for example a suitable plastic material such as polyvinyl chloride which can be reinforced with :"
", , 1 fiberglass. I'he housing comprises four main parts including a generally cup-shaped base or bottom portion 90, a generally cylindrical central portion or section 92 which fits at one end thereof into the base 90, a generally cup-shaped upper portion 94 which is set on the upper end of central section 92, and a cap or closure 96 secured to the top portion. The central section 92 is provided with an opening to receive a pipe fitting 98 defining inlet 22, and section 92 and base 90 are provided with aligned openings for receiving a pipe fitting 100 deflning out-let 52 adJacent the lower portion of the cell. Similarly, the upper portion 94 is provided with an opening for receiving a pipe fitting 102 defining outlet 2~. When assembly of the cell is completed, it is closed by cap element 96 which is connected to an armular flange 104 of top portion 9~, connection being made by a plurality of suitable fasteners 106 with a sealing gasket 108 positioned therebetween. Upper portion 94~ has a generally planar bottom surface portion 110 provided with a gen-erally square shaped central opening defined by depending opposed wall portions 112-115. In an assembled cell positioned for use, the bottom 110 of housing portion 94 is generally horizontal, ; being secured for example by bonding to the upper edge of section 92.
The cell of the present invention further comprises a frame or holder 118 for positioning a plurality of planar electrodes in closely spaced and substantially parallel relation in the cell body. The frame or holder 118 is hollow rectangular in shape, in the present illustration square ln crossection, and includes wall portions 119-122. The walls are of inert and electrically insulating material, for example a suitable plastic such as polyvinyl chloride which can be rei~forced with fiber-glass. Walls 119-122 are preferably honded together 1 at the edges to provide the hollow rectangular frame or housing, but frame 119 could be of an integral molded conskruction. One pair of opposed walls is provlded with longitudinal and closely spaced parallel grooves facing inwardly of frame 118, the opposed grooves being in alignment. The holder 118 containing - the electrodes fitted therein is received adjacent the upper end of the cell as viewed in Figs. 5 and 6 relatively snugly with-in the opening in the base of upper portlon 94 defined by the wall portions 112-115 as to be supported and firmly held thereby. The holder and electrode plates are supported adjacent the bottom of the cell by the following preferred arrangement.
A pair of spaced-apart, generally parallel supporting elements ; 126, 128 are secured on the bottom of the cell, being disposed generally perpendicular to the planes of the electrode elements.
Thus the electrodes and the holder rest upon the upper surfaces of the elements so that an open space for liauid flow is provided between the bottom edges of the electrode plates and the bottom of the cell. The supporting elements are held in place by a pair of right angle bracket members 130, 132 fastened thereto and secured in a suitable manner to the bottom of the base portion 90.
As previously described, a plurality of electrode plates are positioned within holder 118, being firGly held therein in closely spaced, generally parallel relationship. As shown in Fig. 5, the plates extend along the entire longitudinal dimension of the holder, being slightly greater in overall length. The plates are all of the same overall length~ith the exception of four of the plates for a purpose to be shown. In the present illustration a total o~ fourteen plates is show, al-though a smaller or larger number thereof can be employed ~, : -14-., l depending upon the operating conditions. The end or outer electrode elements 134 and 136 of the arrangement each are oE
slightly greater length thereby including extending portions 138 and 14-0, respectively, for the purpose of making electrical connection thereto. In particular, and referring in detail to Fig. 8, an electrically conductive threaded rod or bolt 142 extends through an opening provided in the extending portion 138 of end electrode 134. The end of bolt 142 is secured thereto by an arrangement of conductive lock washers 144 and nuts 146 threaded thereon. The other end of the bolt is fastened to the housing wall 94 by an arrangement of rubber grommets 1~3 and nuts 150 threaded thereon. The electrical line 34 is connected to the outer end of bolt 142 and held thereto by a sleeve 152 threaded on the bolt and against the nut 150 located external to housing wall 94. The connection is protected by an insulating cover piece 154 of generally semispherical shape with the edge thereof abutting the outer surface of housing portion 94. A fastener 155 of insulating material extends through an aperture in cover 154 and threads into the other end of sleeve 152 to hold cover 154 in place. An identical arrangement is provided for connect-ing electrical lead 35 through a bolt to the extending portion 140 of the outer electrode 136 at the opposite end of the stack or series. Each cell in the sys-tem would have similar arrange-ments.
In the electrolytic cell of the present invention, electric potential is applied directly from power supply 30 to only the outer or end electrodes 134 and 136. Depending upon the relative polarity of the applied potentials, one of the end electrodes serves as an anode and the other as a cathode.
'~-~.
1 Electric potential then is applied to the intermediate or additional electrodes 158 by induction. In particular, and referring ~o the electrode arrangement of Fig 5, assuming that line 34 is at a relatively positive potential, and line 35 at a relatively negative potential, lines of flux emanating from the end electrode 134 will induce relatively negative charges on the surface of the very next electrode 158 facing the end elec-trode 134 and this, in turn, causes relatively positive charges to appear on the opposite surface of that electrode 158. The inductive effect proceeds through the remainder of the electrodes 158 in the arrangemcnt such that there are negative charges present on the left hand surface of each electrode 158 in the arrangement as viewed in Fig. 5 and relative positive charges on the right hand surfaces thereof. The direct application of negative potential to end electrode 136 induces relatively pos-itive charges on the right hand surfaces of t`he electrodes 158 and negative charges on the left hand surfaces in a similar manner. The foregoing inductive action is enhanced by the degree of close spacing of the electrode plates and by the degree of conductivity of the liquid in the cell which serves as an electrolyte.
During the foregoing mode of operation, where electrical potential applied to the end electrodes 134, 136 induces a charge in the electrodes 158 therebetween, the lines of flux travel not only through the region between the plates but also outwardly from the edges of the plates, particularly from the upper edges as viewed in Fig. 5 in the region above the electrodes - and below the cell cover 96. These firnge lines of flux em-anating ~rom the plate edges travel in generally curved paths i 1 between relatively positive and negative potential points. A
problem observed to arise in commercial scale apparatus involved partial dissolution of the end plate electrodes 134,136 at the gas-liquid interface. Such dissolution can proceed from the top of the electrodes to the point of the electrical lead-in connec-tion whereupon the cell becomes disabled. This is believed to occur as a result of complete or continuous flux loops from the electrodes encountering the end electrodes which induce current therein, the current density being the highest at the region of the gas-liquid interface. This causes the end electrodes to erode at a rate greater than that of the other electrode plates.
In accordance with the present invention, an electrode plate 158 near or adjacent the corresponding one of the end electrodes 134, 136 to which electric potential is applied extends beyond the end or outer electrode by an amount sufficient to protect the electrodes from flux field effects causing excessive current density. The electrode 158 extends upwardly as viewed in Fig.
5 into the region above the electrodes and below the cell cover 96. This serves tobreak the flux lines in that region in a manner preventing them from travelling to an end electrode thereby protection those electrodes. Thus, flux lines emanating from electrodes into this upper region and tending toward an end electrode would first ercounter -the extended electrode and be blocked or cut, in any event not reaching the end electrode.
In the present instance the second neighboring or adjacent elec-trode 158 relative to end electrode 13~ is made of greater length including the extending portion 16~~. Slmilarly, the sec-ond neighboring or adjacent electrode 158 from end electrode 136 is made of greater length including the extending portion 168.
'~ ~q~5~ ~
1 For example, a flux line leavir.g end electrode 138 in the space above the plates would be broken or cut by portion 164 and thus be unable to encounter end electrode 140. Similarly, flux leav-i.ng end electrode 140 in the space above the plates would be bro-ken or cut by portion 168 and thus not travel to end electrode 138. In an illustrative cell, the intermediate electrodes 158 are 48 inches in length, the end electrodes are 49 inches in length, and the two electrodes 158 having the extending portions 164 and 168, respectively, are 50 inches in length. All plates can be about one-eight inch thick and spaced apart about one-eighth inch. In the arrangement of the present illustra~ion, the second neighboring electrode is extended rather than the first or immediately adjacent electrode, in order to provide s-ufficient space for connecting bolts 150 to the end plates 134,136.
The shielding or protecting funccion provided by the extending portions 16~,168 of the two electrodes 158 is en-hanced by coating the extending portions on the surface facing the corresponding end electrode with an electrically insulating material. In the present illustration the surface facing the corresponding end electrode is coated as indicated by coating 172 shown in Fig. 8 although both surfaces could be coated.
Approximately the upper ten to twenty percent of the surface length of the electrode is coated, i.e. the upper six to eight , inches thereof. One form of material found to provide satis-factory results is an epoxy material available commercially as Pro Tek~ fromthe Baltimore Paint and Chemical Corporation, made by mixing equal quantities of Duratyle~ epoxy hibuild reactor component #2 and Duratyle~ epoxy regular high ' "~
~' : 1 gloss base A.
The extending portions 164 I 168 of the two electrodes 158 adjacent end electrodes 134 and 136, respectively, according to the present invention together with the insulating coating thereof eliminate difficulties associated with current density effects which otherwise lead to early failure of the end electrodes.
In conjunction with this, the distance between the top of the electrodes and the top of the cell is made as small as possible while allowing for adequate liquid outflow from the cell, inas-much as the top of the cell tends to cut any lines flux incidentthereon. The liquid level in the cell is controlled so as to submerge all of the electrodes but just slightly over the extending portions 164, 168 of the shielding electrodes. The electrodes should not be submerged to an excessive degree be-cause the liquid electrolyte can provide a relatively easy path for flux fields.
In addition to bipolar operation of the cell, the polarity of the electrical potential applied to the end plates 134,136 is reversed at regular intervals determined by the cell current density which, in turn, is determined by the operating conditions. Reversing or switching the polarity reverses the direction of current flow of current in the electroly-te between electrodes thereby assuring that dissolution of the end electrodes 134,136 occurs at an equal rate, and that the dissolution of the electrodes 158 between them occurs uniformly on both sides.
In addition, reversing the current flow in the foregoing manner eliminates the possibility of bucl~ling of the electrodes which could cause shorting between adjacent electrodes ancl also mechanical difficulties in removing a spent electrode from the l cell. Reversing polarity of the potential applied to electrodes 134,136 is accomplished by a suitable switching means assoeiated with power supply 20 which can be manually operated or :~ automatically controlled by a timer in a known manner. The rate at which the polarity is reversed depends upon the current density, and by way of illustration typical rates are in the range from about one reversal per hour to two reversals per week.
The method and apparatus of the present invention is 10 illustrated further by the foregoing examples.
Example I
.
; Cooling tower blowdown water was supplied to the inlet of ¢~ apparatus similar that of Fig. 1 at a rate of 25 gallons per minute with a hexavalent chrome concentration of about 3.5 p.p.m.
The apparatus has a total effective electrode weight of 217 pounds, and was operated at about 4.0 amperes and betwen 11.5 volts and 14.0 volts. The method of the present invention was performed continuously with the acid washing step being done for a period of 15 to 30 minutes per day. Each day 1.05 pounds of 20 hexavalent chrome was removed and the effluent contained less than 0.05 p.p.m. of hexavalent chrome, often falling to undetect-. able limits.
¢ At the time the electrodes were changed, it was observed that only about 60% of the available iron had been utilized and that the electro~es still were in quite good condition and could have been used for a longer period of time. The sludge analysis indicated approximately 3.5 pounds of iron per pound of chrome compared to a theoretical useage of 3.2. The two cells contain-ed a total active electrode weight of 204 pounds. Assuming that .,.
l 75~/O of the electrodes could ~e used before replacement, there is 153 pounds of useable iron available for chrome reduction.
At thls rate, the cells could operate approximately 42 days prior to electrode replacement. At 75% electrode utilization, equivalent iron usage would be 4.3 pounds per pound oE chrome and at 90% utilization the value would be 3.6 pounds per pound of chrome. In the event that the electrodes are not run for the standard period, the apparent iron usage goes up. In this example, during the time the electrodes operated with 60% of the electrode life, their apparent iron usage was 5.7 pounds per pound of chrome whereas actually they were using 3.5 pounds.
In this example, the operation for about 25 days removed 27 pounds of hexavalent chrome for an iron to chrome ration of 5.7.
Example II
Cooling tower blowdown water was supplied to the inlet of apparatus similar to that of Fig. 1 but of greater capacity than that llsed in Example I at a rate of 88 gallons per minute with a hexavalent chrome concentration of about 8.0 p.p.m. The higher concentration and flow of this particular blowdown water required a current of about 14 amperes and a potential between 40-50 volts. The cells were operated 24 hours per day except for the acid wash procedure and treated 8.3 pounds of hexavalent chrome each day. The effluent contained less than 0.05 p. p. m. of hexavalent chrome. The apparatus had available 485 pounds of useable iron based on 75~/O electrode utilization, and the plant required the removal of 8.3 pounds of hexavalent chrome each da,. Under the foregoing conditions, operation for approximately 17 days prior to electrode replace-ment was predicted.
~' ~ ~ i5~
1 Operating data from the electrochemical method and apparatus of the present invention for chromate removal in the treatment of cooling tower blowdown water has shown that chrome : levels consistently under 0.05 p.p.m. can be achieved with no pH adjustment of the incoming water provided that it is in the range of approximately 5 to 10. Zinc ion frequently accom-panies chromate in cooling tower blowdown water, and in most cases the effluent from the apparatus has a pH in a range ade-quate to enable removal of the bulk of the zinc which is present.
It is therefore apparent that the present invention accomplishes its intended objects. Treating the electrodes , with an acid solution removes any substances on or lodged between the electrode surfaces arising from the reaction or ' from the incoming liquid. Applying electrical potential direct~
ly to only the outer or end electrodes, with charge being induced in the additional closely spaced electrodes therebetween, advantageously permits cell operation to be at a relatively low current and relatively high voltage which, in turn, allows use of relatively more efficient power supply apparatus. The ~i 20 electrode arrangement, including the provision of the shielding electrodes protects the end electrodes, to which direct electri-cal connection is made, from flux field effects and excessive current density. Changing the polarity of applied potential at regular intervals insures relatively even dissolution of the electrodes. While the foregoing embodiments of the present in-vention have been described in detail, this is for the purpose of illustration, not limitation.
Claims (13)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In an electrochemical cell for removing a contaminant ion from an ionizing medium and having a cell body:
a) a pair of outer electrode elements positioned in said body in spaced apart relation;
b) means for applying electrical potentials of opposite polarity directly to said outer electrode elements;
c) additional electrode elements between said outer electrode elements in closely spaced relation to each other;
d) means in said cell body defining a flow path for an ionizing medium in operative relation to said electrode elements;
e) whereby the electric potentials applied to said outer electrode elements induce an electric charge on said additional electrode elements to cause electric current flow through said ionizing medium; and f) one of said additional electrode elements in neighbor-ing relation to a corresponding one of said outer electrode elements being of sufficient size so as to extend beyond said neighboring outer electrode element so as to shield said neighboring outer electrode element from flux field effects causing excessive current density.
a) a pair of outer electrode elements positioned in said body in spaced apart relation;
b) means for applying electrical potentials of opposite polarity directly to said outer electrode elements;
c) additional electrode elements between said outer electrode elements in closely spaced relation to each other;
d) means in said cell body defining a flow path for an ionizing medium in operative relation to said electrode elements;
e) whereby the electric potentials applied to said outer electrode elements induce an electric charge on said additional electrode elements to cause electric current flow through said ionizing medium; and f) one of said additional electrode elements in neighbor-ing relation to a corresponding one of said outer electrode elements being of sufficient size so as to extend beyond said neighboring outer electrode element so as to shield said neighboring outer electrode element from flux field effects causing excessive current density.
2. An electrochemical cell according to Claim 1, wherein all of said electrode elements are of iron.
3. An electrochemical cell according to Claim 1, further including a coating of electrically insulating material on the surface of the extending portion of said one additional electrode element disposed toward said neighboring outer electrode element.
4. An electrochemical cell according to Claim 1, further including another additional electrode element neighboring the other of said outer electrode elements, said another additional electrode element being of sufficient size so as to extend beyond the said other outer electrode element so as to shield said other outer electrode element from flux field effects causing excessive current density.
5. An electrochemical cell according to Claim 4, wherein all of said electrode elements are of iron.
6. An electrochemical cell according to Claim 4 further including a coating of electrically insulating material on the surfaces of the extending portions of said additional electrode elements disposed toward the outer electrode elements.
7. An electrochemical cell according to Claim 1, wherein in use said cell body is disposed generally vertically and said electrode elements are planar and disposed in generally vertically parallel relation in said body, said outer electrode elements having potential applied thereto near the tops thereof and there being a space between the tops of said electrode elements and the top of said cell body, and wherein an additional electrode element adjacent each of said outer electrode elements extends beyond the top of the corresponding outer electrode element into said space by an amount sufficient to protect said outer electrode elements from flux field effects and excessive current density.
8 . An electrochemical cell according to Claim 7, wherein all of said electrode elements are of iron.
9 . An electrochemical cell according to Claim 7 further including a coating of electrically insulating material on the surfaces of said extending portions of said additional electrode elements disposed toward said outer electrode elements.
10. An electrochemical cell according to Claim 1 further including means for changing the relative polarity of the potentials applied to said outer electrode elements.
11. An electrochemical cell according to Claim 1 wherein said electrical potential applying means is connected to said outer electrode elements adjacent one end of each outer electrode element and wherein said one additional electrode element extends beyond said corresponding outer electrode elements adjacent said one end of said outer electrode elements.
12. An electrochemical cell according to Claim 1, wherein said electrode elements are planar and disposed in generally parallel relation in said cell body, said outer electrode elements being shorter in length than said one additional electrode element, the remainder of said additional electrode elements being of substantially uniform length and shorter than said outer electrode elements and said one additional electrode element.
13. An electrochemical cell according to Claim 1, wherein said one additional electrode element is adjacent said corresponding one of said outer electrode elements.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA356,588A CA1105880A (en) | 1975-02-07 | 1980-07-18 | Method and apparatus for electrochemical contaminant removal from liquid media |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/547,940 US4123339A (en) | 1975-02-07 | 1975-02-07 | Method and apparatus for electrochemical contaminant removal from liquid media |
| US547,940 | 1975-02-07 | ||
| CA244,174A CA1100090A (en) | 1975-02-07 | 1976-01-26 | Method and apparatus for electrochemical contaminant removal from liquid media |
| CA356,588A CA1105880A (en) | 1975-02-07 | 1980-07-18 | Method and apparatus for electrochemical contaminant removal from liquid media |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1105880A true CA1105880A (en) | 1981-07-28 |
Family
ID=27164297
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA356,588A Expired CA1105880A (en) | 1975-02-07 | 1980-07-18 | Method and apparatus for electrochemical contaminant removal from liquid media |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1105880A (en) |
-
1980
- 1980-07-18 CA CA356,588A patent/CA1105880A/en not_active Expired
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