Electrolytic Cells

Abstract

An electrolytic cell with spaced-apart electrodes in a row so constructed that the inter-electrode spaces are in series so that the electrolyte flows through them in succession. Preferably the electrodes are at least 1 centimeter thick and the inter-electrode spaces communicate with one another only by way of holes through the electrodes. The electrodes may be wedge-shaped.

Description

This invention relates to electrolytic cells.

It is already known to make an electrolytic cell comprising a row of electrodes with spaces between them through which the electrolyte is caused to flow, in each case across the faces of the two electrodes on opposite sides of the space, each electrode being made throughout of electrically conductive material. A potential difference is applied between the end electrodes of the row, one of which acts as an anode and the other as a cathode, whilst each of the other electrodes, or the other electrode if the row comprises only three electrodes, has one face which acts as an anode and an opposite face which acts as a cathode and is therefore known in the art as a "bipolar" electrode, the cell being known as a "bipolar" cell. Such cells may, for example, be used in the electrolysis of sodium chloride solution or sea water to form sodium hypochlorite and hydrogen. In one known cell of this kind the electrodes consist of sheet titanium, 1/16th of an inch thick, with a thinner cladding of platinum on one face and because they are so thin the spacing between them must not be much less than 1/10th of an inch, for fear that they will bow and touch one another.

In the use of such cells for the electrolysis of sea water, by-products are formed and these create difficulties, a major one of which is the deposition of magnesium hydroxide within the cell which causes constriction or blockage of the paths through which the electrolyte must flow. Sometimes such an electrical leakage path is created by the deposit between two adjacent electrodes that the local heating caused by the electrical leakage results in damage to the electrode material.

In most hitherto known cells of the kind discussed above, the stream of electrolyte entering the cell splits up into several parallel paths, some of it flowing between one anode and the adjacent cathode, some flowing between another anode and the adjacent cathode, and so on. The individual streams join to form again a single stream, downstream of the gaps between the electrodes. If the electrolysis is to take place adequately in a single pass of the electrolyte through the cell, the rate of flow of electrolyte in each gap will be small, which makes the cell particularly prone to having magnesium hydroxide deposited on its electrodes. Instead, the electrolyte could be recirculated, i.e. passed through the cell repeatedly, at high velocity, which would mitigate the problem of deposition of magnesium hydroxide but increase the cost of the necessary ancillary equipment.

It is also known to have a bipolar electrolytic cell comprising a housing the interior of which is closed off from the atmosphere, a vertical row of spaced-apart electrodes in the housing which comprise upper and lower electrodes and between them some bipolar intermediate electrodes each of which has horizontal upper and lower faces, an inlet into the housing at the bottom of the housing for the supply of electrolyte liquid, an outlet from the housing at the bottom of the housing for the discharge of treated liquid and means for causing liquid to flow in a path from the inlet to the outlet in which it passes in succession through all the spaces between the electrodes in the row, in each case across the faces of the two electrodes on opposite sides of the space, the bipolar intermediate electrodes essentially comprising an aluminum anode plate, an iron cathode plate, an insulating sheet between the two plates, means for uniting the plates and openings in the plates and the sheet.

Such a cell and also a cell according to the present invention can be so constructed that when it is operated, passing the electrolyte once only through the cell, there will be less deposit magnesium hydroxide than when operating in a similar manner, and with the same throughput of electrolyte, a cell in which the electrolyte flows in parallel paths, with the same size, number and construction of electrodes and the same electrode spacing. This is largely on account of the greater velocity of the electrolyte across the electrode faces. However this known cell and a cell in accordance with the invention can also be operated with recirculation of the electrolyte.

In this known cell and a cell according to the invention, because the electrolyte flows through the interelectrode spaces in succession, it is always warmer and hence more conductive when passing through one space than it was when passing through the preceding space, so the downstream end of the cell operates more efficiently than the upstream end. The temperature rise is cumulative, a feature which is absent from the known cells with parallel flow paths for the electrolyte. Also it appears that the evolved hydrogen, which passes through the inter-electrode spaces in succession in contrast to what happens in the known cells, assists in keeping the electrodes free from magnesium hydroxide deposit, on account of its scouring action.

In the known cell, however, the composite construction of the bipolar intermediate electrodes adds to the cost. Also the fact that the spaces between the electrodes are bounded above and below by horizontal surfaces and the inlet and outlet are both at the bottom of the housing means that there are very large parts of the path of the electrolyte where it flows horizontally and another very large part where it flows downwardly. Thus liberated hydrogen can remain stagnant in the housing. This reduces the electrical efficiency of the apparatus and the hydraulic pressure drop across the cell is increased so that the power required to pass the requisite volume of electrolyte through the cell is increased.

According to the present invention there is provided a bipolar electrolytic cell comprising a housing the interior of which is closed off from the atmosphere, a vertical row of spaced-apart electrodes in the housing which comprise upper and lower electrodes and between them at least one bipolar intermediate electrode which has an anode and a cathode integral with one another and a lower face inclined to the horizontal, the lower face of the upper electrode also being inclined to the horizontal, an inlet into the housing at the bottom of the housing for the supply of electrolyte liquid, an outlet from the housing at the top of the housing for the discharge of treated liquid and means for causing liquid to flow in a never descending path from the inlet to the outlet in which it passes in succession through all the spaces between the electrodes in the row, in each case across the faces of the two electrodes on opposite sides of the space.

The or each bipolar intermediate electrode of a cell according to the invention has the anode integral with the cathode, that is to say there is not a body of electrically insulating material with electrically conductive material applied as a layer on the outside of the body, such as is known per se. The electrically conductive material used for the bipolar intermediate electrodes in a cell according to the invention could, however, be made up by mixing an electrically conductive substance with an electrically insulating substance. For example it could be plastics material reinforced throughout with carbon fibers. The electrodes could instead be made wholly of graphite or they could be made of graphite or metal with a cladding which protects the graphite or metal from the electrolyte and is more resistant than it to chemical action such as occurs in use of the cell.

Another disadvantage of all the known cells described above is that when they are used for the production of sodium hypochlorite the influent electrolyte is simultaneously in contact with one side of each electrode and the effluent sodium hypochlorite is simultaneously in contact with the other side of each electrode, so that in the case of the or each electrode other than the end electrodes the electrolyte and sodium hypochlorite afford electrical leakage paths between the opposite faces of the electrode. Also, the electrolyte and hypochlorite afford electrical leakage paths across the spaces between adjacent electrodes. The resultant electrical leakage results in an increase in the amount of electrical energy that must be used to produce a given amount of sodium hypochlorite. Of course, such electrical leakage paths cannot be avoided but substantial improvements can be made in this respect over the known cells with very thin electrodes and consequently short leakage paths and over the known cells with horizontal composite electrodes of iron and aluminum if, in accordance with a further development of the invention, the electrodes are made at least 1 centimeter thick, the spaces between them are interconnected only by holes through the electrodes and seals are provided around the edges of the electrodes, between them and the inside surface of a housing containing them. It is desirable also to line the holes with tightly fitting tubes of electrically insulating material, for example plastics material. With such thick electrodes, the electrode spacing can be made 0.08 inch or less.

Preferably, the end electrodes in the row each have one flat face which is perpendicular to the axis of the row and another flat face which is inclined by an angle other than 90.degree. to that axis. It is also preferable for each intermediate electrode to have two opposite flat faces inclined in opposite senses to said axis. According to a further development of the invention a cell according to the invention as defined above may have said upper electrode constructed as a bipolar electrode which has an anode and a cathode integral with one another and is also a lower electrode in a second vertical row of spaced-apart electrodes, which row comprises an upper electrode having its lower face inclined to the horizontal and at least one bipolar intermediate electrode which has an anode and a cathode integral with one another and a lower face inclined to the horizontal, the cell having means for causing liquid to flow in a second never descending path on its way from the inlet to the outlet in which it passes in succession through all the spaces between the electrodes in the second row, in each case across the faces of the two electrodes on opposite sides of the space, and means for dividing the stream of liquid to be treated into two streams of which one follows one of said paths and the other follows the other of said paths.

Preferably, the two rows of electrodes lie in a straight line and are disposed within a tubular housing which is in turn surrounded by another tubular housing, there being a space between the two housings divided by two partitions, extending along the housing, into two chambers. One of these chambers is an electrolyte inlet chamber and the other a treated liquid outlet chamber. There are two openings from the inlet chamber into the first inter-electrode spaces in the rows of electrodes and two openings into the outlet chamber from the last inter-electrode spaces in the rows of electrodes. This is a space-saving way of constructing a cell having two rows of electrodes operating in parallel.

In an embodiment of the invention which is particularly advantageous as regards ease of positioning of the electrodes, the electrodes are disposed in a housing which is composed of sections equal in number to the electrodes. Each electrode is assembled with its own housing section encircling it, a comparatively simple operation, after which the housing sections are assembled together with seals to prevent leakage between adjacent sections.

Examples in accordance with the invention are described below with reference to the accompanying drawings, in which:

FIG. 1 shows a side view of the interior of an electrolytic cell,

FIG. 2 shows a plan view of the cell,

FIG. 3 shows a side view of the interior of a second electrolytic cell,

FIG. 4 shows a plan view of the second cell,

FIG. 5 shows a detail of the second cell, and

FIG. 6 shows a side view, exploded, of part of the interior of a third electrolytic cell.

The cell shown in FIGS. 1 and 2 has a vertical row of five wedge-shaped electrodes, each at least 1 centimeter thick at the thinnest part. These electrodes are an anode 4 at the lower end of the row, a cathode 13 at the upper end of the row and three bipolar electrodes 3 each of which acts as a cathode at its lower face and as an anode at its upper face. These bipolar electrodes and every bipolar electrode mentioned below have an anode and a cathode integral with one another. All the electrodes are made of graphite and are circular as seen in plan, although they could have other shapes as seen in plan, the electrodes 4 and 13 being similar to one another and each having one flat face which is perpendicular to the longitudinal axis of the row of electrodes and an opposite flat face which is inclined to that axis by an angle other than 90.degree.. The electrodes 3 are similar to one another and each has two opposite flat faces which are inclined in opposite senses to the aforementioned axis. Each electrode 3 has its thickest part lying between the thinnest parts of the two adjacent electrodes. Each electrode 3 has a hole 15 through it from one flat face to the opposite one and near the thinnest part of the electrode. Each of the electrodes 4 and 13 has a hole 16 through it from one flat face to the opposite one and near the thinnest part of the electrode. In this example and the other examples described below, the holes could be lined with tightly-fitting tubes of electrically insulating material, but this is not shown.

The electrodes are spaced apart and prevented from moving sideways with respect to one another by pegs 16a of polytetrafluoroethylene. There are four pegs 16a between each two electrodes and parts of them project into holes (not shown) in the electrodes. The electrodes are enclosed within a tubular housing 5 made of polyvinyl chloride and are encircled by O-rings 2 which are squashed between the electrodes and the inner surface of the housing 5. Thus there are four enclosed spaces between adjacent ones of the electrodes and each space communicates with the or each adjacent one only by way of one of the holes 15.

Instead of the pegs 16a, there could be a ring of polyvinyl chloride between each two adjacent electrodes, the ring touching the inside of the housing 5 all round the periphery of the ring.

At each end of the housing 5 there is a gasket 6 and a rigid end plate 7 of electrically insulating material and the end plates are interconnected by bolts 14 provided with nuts 8, so that they clamp and render fluid-tight the assembly which is between them. An inlet tube 17 is fastened to the lower plate 7 and holes pass through this plate and the lower gasket 6 so that electrolyte can continuously enter the tube 17 and flow in a straight line through these holes and the hole 16 in the anode 4 into the space between the anode 4 and the adjacent electrode 3. The electrolyte then flows through all the spaces, between adjacent ones of the electrodes, in succession, in each case across the flat faces of the two electrodes on opposite sides of the space. An outlet tube 11 is fastened to the upper plate 7 and holes pass through this plate and the upper gasket 6 so that electrolyte can continuously flow in a straight line out of the space between the cathode 13 and the upper electrode 3, through these holes and the hole 16 in the cathode 13. Connecting conductors 10 and 12 pass through holes 18 in the end plates 7 and are screwed into the anode 4 and cathode 13, respectively.

Each electrode could be modified by providing the graphite with a cladding which protects the graphite from the electrolyte and is more resistant than the graphite to chemical action such as occurs in use of the cell. Instead of using graphite, the electrodes could be made of a metal, for example copper or aluminum, which is provided with a cladding which protects it from the electrolyte and is more resistant than the metal to chemical action such as occurs in use of the cell. Suitable materials for use as the cladding in these cases are titanium for the cathode 13 and the lower sides (below the O-rings) of the electrodes 3 and titanium covered with a layer of platinum for the anode 4 and the upper sides (above the O-rings) of the electrodes 3. Security of the cladding may be provided by screwing it on to the electrodes and/or sticking it on with conductive adhesive.

Another possibility is to make each electrode of a plastics material, for example an epoxy or polyester resin, reinforced with carbon fibers.

The above observations concerning possible materials for the construction of the electrodes apply also to the examples described below.

There could be any odd number greater than three of the electrodes 3, or only one. The housing 5 could be of polypropylene and it and the pegs 16a could be of some other material which is inert to the electrolyte.

Electrolytic cells constructed as described above may be used for the production of sodium hypochlorite by electrolysis of brine solution or sea water and of course they have other uses too.

An electrolytic cell according to the invention could have a row of only three or only four electrodes. There is described below and illustrated in FIGS. 3 to 5 of the accompanying drawings an electrolytic cell in accordance with the invention comprising two rows of electrodes, one row of 4 and one row of 3. One electrode is both the last in one row and the first in the other so that there are six electrodes in all. Within each row the electrolyte flows through both or all the inter-electrode spaces in succession, that is to say the spaces are connected in series. The two series arrangements of inter-electrode spaces are in parallel.

The cell has a row of four electrodes 20 to 23 and a row of three electrodes 23 to 25, each electrode being at least 1 centimeter thick at the thinnest part. The electrodes 20 and 25 are the same in shape as the electrodes 4 and 13 of FIG. 1 except that they do not have the holes 16. The electrodes 21 to 24 are the same in shape as the electrodes 3 in FIG. 1, except that only the electrodes 21, 22 and 24 have the holes 15 through them. The reference numerals 2 and 16A in FIGS. 3 and 4 have the same significance as they do in FIGS. 1 and 2.

The electrodes of FIGS. 3 and 4 are enclosed within a tubular housing 26 made of polyvinyl chloride which is formed with four slits 27, 28, 29 and 30, each of which is horizontal when the axis of the housing 26 is vertical and is as shown in FIG. 5 for the case of the slit 30. These slits are adjacent the first and last inter-electrode spaces in the rows of electrodes. The slits 27 and 29 form inlets for brine solution or sea water to flow into the inter-electrode spaces from a chamber 31. The slits 28 and 30 form outlets for treated liquid to flow out of the inter-electrode spaces into a chamber 32. A further tubular housing 33 of polyvinyl chloride surrounds the housing 26 and the chambers 31 and 32 are formed between the inner surface of the housing 33 and the outer surface of the housing 26. The chambers are separated by walls 34 extending along the housings.

End plates 35 of electrically insulating material are provided at the ends of the housings 26 and 33 and they are formed with recesses which receive O-rings 36 which seal the housings with respect to the end plates. The end plates are interconnected by bolts 37 provided with nuts 38. An inlet tube 39 projects into a hole in the lower plate 35 and communicates with the chamber 31 and an outlet tube 40 projects into a hole in the upper plate 35 and communicates with the chamber 32. Connecting conductors 41 and 42 pass through holes in the end plates 35 and are screwed into the electrodes 20 and 25, respectively. O-rings 43 surround and seal the inner ends of the holes through the plates 35.

There could be more than two paths connected in parallel, each path comprising a number of inter-electrode spaces connected in series, and whether or not this is so there could be more than the illustrated numbers of electrodes in the rows.

Finally, FIG. 6 shows a cell which is like the cell shown in FIGS. 1 and 2 but differs from the latter in the respects indicated below. Some details have been omitted from FIG. 6. The reference numerals in FIG. 6 which appear also in FIGS. 1 and 2 have the same significance.

There are only two electrodes 3 and the row of electrodes lies within a housing made up of individual sections 44 and 45, one section 44 for the anode 4, another for the cathode 13 and one section 45 for each of the electrodes 3. If there were more or less than two electrodes 3 there would be a corresponding number of sections 45 so that standard parts can be used to build the housings of cells of differing sizes. Each section 44 has a groove 46 on one side and each section 45 has a groove 46 on each side. These grooves receive O-rings 47 which are squashed between the sections to provide good seals between them, by tightening screws 48 which are screwed into the lower plate 7 and press on the lower section 44. There are actually three screws 48, spaced equally around the axis of the cell, but only one is shown. The electrodes are first individually assembled with the O-rings 2 and the sections 44 and 45, which is easier than positioning the electrodes in a long tubular housing as is necessary in the construction shown in FIGS. 1 and 2, and then these assemblies are put together, with the O-rings 47 and spacer rings 49 of polytetrafluoroethylene between adjacent electrodes.

Of course, the idea of building a housing of individual sections, as shown in FIG. 6, could also be applied to the construction according to FIGS. 3 to 5. In all the examples shown in the drawings there is a never descending path for fluid from the inlet to the housing at the bottom of the housing to the outlet of the housing at the top of the housing.

Claims

I claim:

1. A bipolar electrolytic cell comprising a housing the interior of which is closed off from the atmosphere, a vertical row of spaced-apart electrodes in the housing which comprise upper and lower electrodes and between them at least one bipolar intermediate electrode which has an anode and a cathode integral with one another and a lower face inclined to the horizontal, the lower face of the upper electrode also being inclined to the horizontal, an inlet into the housing at the bottom of the housing for the supply of electrolyte liquid, an outlet from the housing at the top of the housing for the discharge of treated liquid and means including sealing devices engaging the electrodes for causing liquid to flow in an always ascending path from the inlet to the outlet in which it passes in succession through all the spaces between the electrodes in the row, in each case across the faces of the two electrodes on opposite sides of the space.

2. A cell according to claim 1 wherein the electrodes are at least 1 centimeter thick and said means comprises portions of some of the electrodes defining holes through those electrodes, and seals around the edges of the electrodes between them and the inside surface of the housing.

3. A cell according to claim 1 in which the housing comprises sections equal in number to the electrodes, each electrode being encircled by its own housing section, and seals for preventing leakage between adjacent sections.

4. A cell according to claim 1 in which each intermediate electrode has its upper and lower faces inclined in opposite senses to the horizontal.

5. A cell according to claim 1 in which said upper electrode is a bipolar electrode which has an anode and a cathode integral with one another (and is also a lower electrode in a second vertical row of spaced-apart electrodes, which row comprises an upper electrode having its lower face inclined to the horizontal and at least one bipolar intermediate electrode which has an anode and a cathode integral with one another and a lower face inclined to the horizontal, means for causing liquid to flow in a second always ascending path on its way from the inlet to the outlet in which it passes in succession through all the spaces between the electrodes in the second row, in each case across the faces of the two electrodes on opposite sides of the space, and means for dividing the stream of liquid to be treated into two streams of which one follows one of said paths and the other follows the other of said paths.

6. A cell according to claim 5 in which the two rows of electrodes lie in a straight line and the housing is tubular, the cell further comprising a second tubular housing surrounding and spaced from the first tubular housing, partitions extending along the housings and dividing the space between them into two chambers and portions of the inner housing defining two openings from one chamber into the first inter-electrode spaces in the rows of electrodes and two openings from the last inter-electrode spaces in the rows of electrodes into the other chamber.

7. A cell according to claim 5 in which each bipolar electrode has its upper and lower faces inclined in opposite senses to the horizontal.