HK1221268B - Electrolytic cell equipped with concentric electrode pairs - Google Patents

Description

Electrolytic cell equipped with coaxial electrode pair

Technical Field

The present invention relates to bipolar electrochemical cells and to a method of performing an electrolytic process therein.

Background

The present invention relates to a bipolar cell suitable for electrochemical processes with periodic polarity reversal. Periodic polarity reversal of the electrochemical cell, whereby each electrode alternately operates as an anode and as a cathode for a preset time interval, is a measure known in the art, in particular for preventing the formation of various types of scale on the surface of one of the electrodes, usually the cathode. The above is for example the typical case of cells for electrolyzing dilute alkaline brine to produce active chlorine at the anode (i.e. a mixture of hypochlorite and hypochlorous acid with possible traces of dissolved free chlorine and other species in equilibrium): especially in case brine is obtained from tap water containing carbonate ions and other anions of similar behavior, the cathode becomes a preferential deposition site for carbonates and other insoluble salts, which is supported by process-induced alkalisation nearby. Such deposits adversely affect the current transport through the electrode, whose electrical efficiency can eventually deteriorate irreversibly. The periodic reversal of the direction of the current and therefore of the polarity of the electrodes makes the surface work cathodically for half a cycle to start acting as an anode at the time of reversal, undergoing local acidification in favour of the dissolution of the previously formed precipitate. Other electrolytic processes that are sometimes subjected to periodic current reversal are, for example, the treatment of wastewater containing organic matter that is degraded at the anode and prone to form various types of deposits at the cathode; or the cathodic deposition of metal from an electrolytic bath and the simultaneous anodic degradation of organic matter, for the treatment of water in which both types of substances are present as impurities. In such cases, the anode is also often subjected to the deposition of a contaminating film, which in this case consists of organic residues that tend to oligomerize on the electrode surface and can sometimes be removed by the mechanical and chemical action of nascent hydrogen in the subsequent cathodic cycle. In order to preserve the regularity of operation and to maintain constant the operating parameters of the process required, the electrodes installed in the cell, designated to operate alternately as anode and as cathode, must preferably have the same dimensions, in addition to being spaced apart by a constant gap, so that both the supplied current and the operating voltage can be kept constant (except for the variation in sign). This means that the cell design for this type of process is mainly limited to flat type geometries, in other words considering the use of facing flat electrode pairs. However, in many cases this may constitute an unwanted limitation, including some negative consequences. In many cases, this type of process is in fact carried out in small-sized plants, for example in the case of water disinfection or the recovery of precious metals in jewellery waste, to be used in hospitals, hotels or in the household sector. For such types of applications, limiting the volume as much as possible, it may be important to choose a coaxial type of cell design (e.g., a cylindrical cell with an outer cathode wall and a central anode). In addition to better utilization of the available volume, this may have the advantage of improving current transport minimization edge effects, which are known to be heavy in flat geometries and very relevant in the case of small overall volume electrode areas. However, coaxial-type slots (both cylindrical and prismatic) are characterized by external electrodes having larger dimensions than internal electrodes, making operation with periodic current reversal more difficult. In fact keeping the current density constant from one cycle to the next and thus producing the desired substance, a corresponding change in the electrode area causes a corresponding change in the current density and therefore in the process voltage; on the other hand, it should be decided to operate at a constant voltage, the current density and therefore the productivity will oscillate between two values corresponding to two different electrode areas, hardly in line with the normal requirements of industrial processes.

The need has therefore been identified for an electrolytic cell providing a coaxial electrode geometry with a constant inter-electrode gap and with the same cathodic area as the anodic area.

Disclosure of Invention

In one aspect, the present invention relates to a bipolar cell defined by an outer body shell having in its interior:

an external pair of electrodes divided into two electrodes, separated at the edges by a separator element, destined to operate alternately one electrode as cathode and the other as anode, and vice versa;

-at least one intermediate electrode pair coaxial therewith, defining a first gap of substantially constant width therebetween, also divided into two electrodes, separated at the edges by a separation element, not directly supplied with electric current and intended to operate as a bipolar element;

-an inner electrode pair coaxial with the first two, defining a second gap of substantially constant width with the intermediate electrode pair; the internal pair of electrodes is also divided into two electrodes, separated at the edges by a separation element, designated to operate alternately one electrode as cathode and the other as anode, and vice versa, each of the two electrodes of the pair facing one of the two electrodes of the intermediate pair;

-means for electrically connecting one of the electrodes of the outer pair and the corresponding electrode of the inner pair not facing the electrode of the intermediate pair and therefore facing it, with one of the slot poles;

means for electrically connecting the remaining electrodes of the two inner and outer pairs with the other of the cells.

In one embodiment, the outer channel has an elongate shape and the electrode pair has a prismatic or cylindrical shape.

In another embodiment, the outer channel and the pair of electrodes have a spheroidal shape.

In one embodiment, there are more intermediate electrode pairs adapted to operate as bipolar elements in order to increase the yield of the cell.

In the cell constructed in this way, the anode area and the cathode area correspond to the sum of the areas of half of the outer electrode pair and half of the inner electrode pair: by reversing the polarity of the electrodes, the values of the anode and cathode areas are not changed.

In one embodiment, the channel and the electrode pair each have a prismatic or cylindrical shape. For example, a cylindrical cell is combined with an electrode pair that is also cylindrical to minimize the volume of the cell that does not participate in the electrolytic reaction. In one embodiment, two coaxial electrode pairs are coaxial with the tank. This may also have the advantage of minimising the volume of the cell that does not participate in the electrolysis reaction. In one embodiment, all the electrodes of the cell are made of titanium or other valve metal coated with a catalytic composition containing one or more components selected from the platinum group, for example platinum metal or oxides of platinum, ruthenium or iridium. In one embodiment, the above catalytic composition further comprises an oxide capable of facilitating the growth of a compact and protective film, such as an oxide of titanium, tantalum, niobium or tin. In the context of the present document, the term electrode made of titanium or valve metal is used to indicate an electrode obtained starting from a substrate of titanium or other valve metal (for example niobium, tantalum or zirconium), either pure or of a different alloy.

In an alternative embodiment, all of the electrodes of the cell are made of conductive diamond (e.g., boron doped diamond) either in bulk form or supported on a suitable conductive substrate (e.g., niobium or other valve metal).

For most known anode applications, the specified materials have the advantage of working in an optimized manner, including the evolution of anode products such as chlorine, oxygen, ozone or peroxides, while at the same time ensuring proper functioning also as cathode.

In one embodiment, the first gap and the second gap have a substantially constant width independently between 1 and 20mm, depending on the needs of each process, as will be clear to a person skilled in the art.

In another aspect, the invention relates to a method of performing an electrolytic process comprising feeding a process electrolyte in a gap of an electrolytic cell as described above and supplying a direct current to the cell poles, changing the direction of the applied current at preset time intervals, for example every 1-120 minutes. In one embodiment, the electrolytic process according to the invention consists of electrolysis of a salt solution with the production of active chlorine. In an alternative embodiment, the electrolytic process according to the invention consists of a wastewater treatment that degrades organic matter. In yet another embodiment, the electrolytic process according to the invention consists of recovering the metal by cathodic electrodeposition and optionally simultaneously degrading the organic substances.

Some embodiments exemplifying the invention will now be described with reference to the accompanying drawings, which have the sole purpose of illustrating the mutual arrangement of the different elements with respect to said specific embodiments of the invention; in particular, the drawings are not necessarily drawn to scale.

Brief description of the drawings

Fig. 1 shows a top view of a section of a cell comprising a cylindrical and prismatic electrode pair according to one embodiment of the present invention.

Figure 2 shows a top view of a cross section of a cell comprising a cylinder and a cylindrical electrode pair according to one embodiment of the present invention.

Detailed description of the drawings

Fig. 1 shows a top view of a section of one embodiment of the invention constituted by a tank defined by a cylinder 100 inside which three parallelepiped-shaped electrode pairs are placed, namely: an inner pair of electrodes 301 and 401 separated at the edges by a spacer element 101, an intermediate pair of electrodes 501 and 502, and an outer pair of electrodes 302 and 402 coaxial with the inner pair; the electrodes of the middle pair and the electrodes of the outer pair are also separated at the edges by an equivalent spacer element 101. The spacer element 101 keeps the electrodes in a fixed position, preventing their short-circuiting: in addition to performing these functions, the element 101 also avoids current concentration at the facing edges of each electrode pair. For this reason, the element 101 must be suitably dimensioned: the inventors found that for most of the applications tested, the element 101 was dimensioned such that the distance between the facing edges of each electrode pair was at least equal to the width of the respective gaps 102 and 112. The electrodes 402 and 501 face each other, as do the electrodes 302 and 502, to define a first gap 102, the first gap 102 having a substantially constant width except for corner regions.

Similarly, electrodes 301 and 501 face each other, as do electrodes 401 and 502, to define a first gap 112, the second gap 112 having a substantially constant width except for corner regions.

The electrodes 301 of the inner pair and the electrodes 302 of the outer pair, which do not face the bipolar electrode 501 and thus the same electrode 301, are connected to one pole 300 of a direct current power supply 200, which direct current power supply 200 is provided with means for reversing the direction of the current at preset time intervals; similarly, the other electrode 401 of the inner pair and the other electrode 402 of the outer pair are connected to the other pole 400 of the dc power supply 200. The regions 103 and 104 of the tank body outside the two adjacent gaps 102 and 112 are filled with an isolating material, so as to confine the process electrolyte within said gaps constituting the reaction zone. The cell may be fed from a terminal portion of the cell body 100 having the outlet in the opposite position, and optionally operated in a continuous mode with a single pass of electrolyte or in a batch mode.

Figure 2 shows a top view of a section of a similar embodiment of the invention, which differs from the previous one by the cylindrical shape of the electrode pair. In addition to maximizing the ratio of active electrode surface to total slot volume, this has the advantage of keeping the width of the gaps 102 and 112 constant, eliminating corner regions.

Some of the most significant results obtained by the inventors are illustrated in the following examples, which are not intended to limit the scope of the invention.

Examples

A cell corresponding to the embodiment of FIG. 2 (except for the two intermediate pairs equipped with bipolar electrodes) was fed from the opposite gap with a brine solution prepared from tap water containing 19g/l NaCl. The cell was equipped with a 60mm diameter outer electrode pair, a 30mm diameter inner electrode pair and a 50mm and 40mm diameter middle bipolar electrode pair (defining a gap of approximately 4mm width), respectively. All electrode pairs have a height of 50 mm. All the electrodes of each pair consist of titanium sheets activated on the side facing the gap with a mixture of ruthenium, palladium and titanium oxides (according to the prior art). The total reaction volume, corresponding to the volume of the two gaps, was 32.5 ml. At a total current of 5A applied (corresponding to about 1kA/m on the internal electrode assembly)²And 0.5kA/m on the outer electrode assembly²Current density of) and reversing the direction of current flow every 180 seconds, 2700ppm of active chlorine can be produced with a constant efficiency of 66% during a series of batch cycles (each lasting 60 minutes).

The test was repeated, applying a total current of 10A, always operating in a 60 minute batch cycle with current reversal every 180 seconds, resulting in preparation of 5530ppm of active chlorine with a constant efficiency of 68%. During this second test, an increase in pH from initial neutrality up to a value of 9.6 was observed.

The above description should not be taken as limiting the invention, which may be used according to different embodiments without departing from the scope thereof, and whose extent is solely defined by the appended claims.

Throughout the description and claims of this application, the terms "comprise" and variations thereof, such as "comprising" and "comprises," are not intended to exclude the presence of other elements, components, or additional process steps.

The discussion of documents, acts, materials, devices, articles and the like in this specification is included solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.

Claims (10)

1. A bipolar cell defined by an outer body of an elongate or spheroidal shaped casing having in its interior:

-an outer electrode pair;

-an inner electrode pair;

-at least one intermediate electrode pair,

the outer electrode pair is divided into a first outer electrode and a second outer electrode of the same size separated at an edge by a first separating member, the inner electrode pair is divided into a first inner electrode and a second inner electrode of the same size separated at an edge by a second separating member, the at least one middle electrode pair is divided into a first middle electrode and a second middle electrode of the same size separated at an edge by a third separating member, the inner electrode pair, the outer electrode pair and the middle electrode pair are coaxially disposed, and surfaces of the first outer electrode and the first middle electrode and surfaces of the second outer electrode and the second middle electrode face each other to define a first gap, surfaces of the first middle electrode and the first inner electrode and surfaces of the second middle electrode and the second inner electrode face each other to define at least one second gap, the first external electrode and the second internal electrode are connected to one pole of the cell, and the second external electrode and the first internal electrode are connected to the opposite pole of the cell.

2. The cell according to claim 1, wherein the outer electrode pair, the middle electrode pair and the inner electrode pair are a cylindrical or prismatic shaped electrode pair disposed in an elongated shaped body interior or a spheroid shaped electrode pair disposed in a spheroid shaped interior.

3. The cell according to claim 2, wherein the outer electrode pair, the at least one intermediate electrode pair and the inner electrode pair are coaxial with the cell body.

4. The cell according to any one of the preceding claims, wherein said first and second outer electrodes, said first and second intermediate electrodes and said first and second inner electrodes are made of conductive diamond in bulk or supported form or of titanium coated with a catalytic composition containing one or more elements of the platinum group.

5. The cell according to claim 4, wherein said catalytic composition comprises at least one component selected from the group consisting of metallic platinum, platinum oxide, ruthenium oxide and iridium oxide, and at least one oxide of an element selected from the group consisting of titanium, tantalum, niobium and tin.

6. The cell according to any one of claims 1-3 and 5, wherein said first gap and said second gap have constant widths independently from 1 to 20 mm.

7. The cell according to claim 4 wherein said first gap and said second gap have constant widths independently from 1 to 20 mm.

8. Method for performing an electrolytic process in a cell according to any one of claims 1 to 7, comprising feeding a process electrolyte in said first gap and said at least one second gap and supplying a direct current to the cell poles, changing the direction of said direct current at preset time intervals.

9. The method of claim 8, wherein the electrolytic process is selected from the group consisting of: the salt solution is electrolyzed with the production of active chlorine, the degradation of organic substances by electrolysis of the waste water, and the recovery of metals by cathodic electrodeposition and optionally the simultaneous degradation of organic substances.

10. A method according to claim 8 or 9, wherein said preset time interval has a duration of 1 to 120 minutes.