HK1226784A1 - Electrolytic cell equipped with concentric electrode pairs - Google Patents

Description

Electrolytic cell equipped with concentric electrode pairs

The present application is a divisional application of the chinese invention patent application having an application date of 2013, 16.5 and 201380028818.8, and having the title of "electrolytic cell equipped with concentric electrode pair".

Technical Field

The present invention relates to a monopolar electrochemical cell and a method of performing an electrolysis process therein.

Background

The present invention relates to a unipolar electrochemical cell suitable for electrochemical processes carried out periodically with polarity reversal. The electrochemical cell is periodically reversed in polarity whereby the electrodes alternately function as an anode and a cathode at predetermined time intervals, which is a means known in the art, particularly to prevent the formation of various foulants on the surface of one electrode (typically the cathode). This is for example the typical case of a cell for the electrolysis of dilute alkaline brines to produce active chlorine (i.e. a mixture of hypochlorite and hypochlorous acid, possibly with traces of dissolved free chlorine and other species in equilibrium) at the anode; especially in the case of brine obtained from tap water, which contains carbonates and other anions of similar behavior, the cathode becomes the optimal deposition site for carbonates and other insoluble salts, which is enhanced by the alkalisation induced by the nearby process. Such deposits have a negative effect on the current transport of the electrode, the electrical efficiency of which deteriorates irreversibly over time. The periodic reversal of the direction of the current and therefore of the polarity of the electrodes makes the surface, which operates in cathodic mode for half a cycle, start to assume the anodic action after the reversal, being subjected to a local acidification which favours the dissolution of the previously formed deposit. Other electrolytic processes which are sometimes subject to periodic current reversal are for example the treatment of wastewater containing organic matter which degrades at the anode while tending to form various deposits at the cathode, or the cathodic deposition of metals from the electrolytic bath while the organic matter anodically degrades for the treatment of water in which both species are present as impurities. In these cases, the anode is also often subject to fouling film deposition, in this case containing organic residues, which tend to oligomerize on the electrode surface, and which can sometimes be removed by mechanical and chemical action of nascent hydrogen in the subsequent cathodic cycle. In order to maintain the regularity of operation and to maintain constant the operating parameters of the desired process, the electrodes installed in the cell, intended to operate alternately as anode and cathode, must preferably be of the same size, apart from being placed at intervals, which ensure constant supply current and operating voltage (apart from sign changes). This means that cell design for this type of process is limited primarily to planar geometries, in other words, the use of flat-surfaced electrode pairs is considered. However, in many cases this may constitute an undesirable limitation, involving a number of negative consequences. In many cases, in fact, such processes are carried out in small-sized units, for example in the case of the production of active chlorine for the disinfection of water used in hospitals, hotels or households, or the recovery of precious metals from jewelry waste. For such applications, it is important to limit the capacity as much as possible, choosing a coaxial concentric cell design, such as a cylindrical cell with an outer wall cathode and a central anode. This may have the advantage that, in addition to a better utilization of the available capacity, the current transport may be improved, minimizing edge effects, which are known to be more severe in planar geometries and are very relevant in cases where the total electrode area size is small. Coaxial concentric cells, whether cylindrical or prismatic, are characterized by outer electrodes of larger dimensions than the inner electrodes, making periodic current reversal operations more difficult. Maintaining the amperage substantially constant between one cycle and the next, thereby producing the desired species, a change in the respective electrode area causing a change in the amperage and thus the process voltage; on the other hand, if it is decided to operate at a constant voltage, the current intensity and therefore the productivity will oscillate between two values corresponding to the two different electrode areas, making it difficult to meet the normal requirements of an industrial process.

Accordingly, it is desirable to provide an electrolytic cell in a concentric electrode geometry having a constant inter-electrode gap and the same cathode area as the anode area.

Disclosure of Invention

Various aspects of the invention are set out in the appended claims.

In one aspect, the invention relates to a monopolar electrolytic cell having an interior bounded by an outer casing:

an outer pair of electrodes subdivided into two electrodes, separated at the edges by insulating elements, for alternately operating with one as cathode and the other as anode and vice versa;

-an inner electrode pair concentric with the outer electrode pair, defining a gap of generally constant width with the outer electrode pair, the inner electrode pair likewise being subdivided into two electrodes, separated at the edges by insulating elements, for alternately operating with one as cathode and the other as anode, and vice versa, each of the two electrodes of the electrode pair facing one of the two electrodes of the outer electrode pair;

-means for electrically connecting one electrode of the outer electrode pair and the corresponding electrode of the non-facing inner electrode pair with one cell pole;

means for electrically connecting the remaining electrodes of the two electrode pairs with the other cell electrode.

In one embodiment, the battery outer body has an elongated shape, and the electrode pair has a prismatic shape or a cylindrical shape.

In another embodiment, the cell outer body and the electrode pair have an elliptical shape.

In a cell constructed in this way, the anode area and the cathode area correspond to the sum of half the area of the outer electrode pair and half the area of the inner electrode pair: by reversing the electrode polarity, the anode and cathode area values were unchanged.

In one embodiment, the cell body and the electrode pair are each prismatic or cylindrical. For example, it may be advantageous to couple together a cylindrical cell body with a pair of electrodes that are also cylindrical, in order to minimize the volume of the cell that does not participate in the electrolytic reaction. In one embodiment, the two concentric electrode pairs are coaxial with the cell body. This may also have the advantage of minimising the volume of the cell which does not participate in the electrolysis reaction. In one embodiment, all 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, such as platinum metal or oxides of platinum, ruthenium, or iridium. In one embodiment, the above catalyst composition further comprises an oxide capable of promoting the growth of a dense protective film, such as an oxide of titanium, tantalum, niobium, or tin. In the context of the present description, the term electrode made of titanium or other valve metal is used to indicate an electrode starting from a substrate of titanium or other valve metal (for example niobium/tantalum/or zirconium), whether pure or of a different alloy.

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

For most known anode applications, certain materials have the advantage of operating in an optimal mode, involving the release of anode products, such as chlorine, oxygen, ozone, or peroxides, while ensuring proper functioning as a cathode.

In one embodiment, the gap between the two electrode pairs is generally of constant width, ranging between 1 and 20mm, depending on the individual process requirements, as will be clear to those skilled in the art.

In another aspect, the invention relates to a method of performing an electrolytic process comprising: the process electrolyte is fed into the gap of the aforementioned electrolytic cell and a direct current is supplied to the cell poles, the direction of the applied current being changed at predetermined time intervals, for example every 1 to 120 minutes. In one embodiment, the electrolytic process according to the present invention comprises electrolysis of a salt solution to produce active chlorine. In an alternative embodiment, the electrolytic process according to the invention comprises waste water treatment, degrading organic matter. In a further embodiment, the electrolytic process according to the invention comprises recovering the metal by cathodic electrodeposition, optionally with simultaneous degradation of organic species.

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

Drawings

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

Fig. 2 shows a top view of a cross-section of a cell according to an embodiment of the invention comprising a cylindrical body and a pair of cylindrical electrodes.

Detailed Description

Fig. 1 shows a cross-sectional top view of a cell defined by a cylindrical body, internally housing two parallelepiped-shaped electrode pairs, i.e. an inner electrode pair comprising electrodes 301 and 401, separated at the edges by an insulating element 101, according to an embodiment of the invention; and an outer electrode pair, coaxial with the inner electrode pair, containing electrodes 302 and 402, again separated at the edges by insulating element 101. The insulating element 101 keeps the electrodes in a fixed position, preventing their short-circuiting: in addition to performing these functions, the element 101 prevents current from concentrating on the opposing edges of each electrode pair. Therefore, the element 101 must have the appropriate dimensions: the inventors have found that for most test applications it would be advantageous to dimension the element 101 such that the distance between the opposing edges of each electrode pair is at least equal to the width of the gap 102. Electrodes 301 and 402 oppose each other, as do electrodes 302 and 401, to define gap 102 as a generally constant width, except for corner areas. The electrodes 301 of the inner electrode pair and the electrodes 302 of the outer electrode pair not facing the electrodes 301 are connected to one pole 300 of a direct current power supply 200, the direct current power supply 200 having means for reversing the direction of the current at predetermined time intervals; similarly, the other electrode 401 of the inner electrode pair and the electrode 402 of the outer electrode pair not facing the electrode 401 are connected to the other pole 400 of the direct current power supply 200. The regions 103 and 104 of the cell body outside the gap 102 are filled with an insulating material so as to confine the process electrolyte inside the gap 102, which constitutes a reaction region. The cell can be fed from the terminal part of the cylindrical body 100 with the outlet in the opposite position and optionally operated in a continuous manner, with a single pass of the electrolyte, or in batch mode.

Fig. 2 shows a cross-sectional top view of a similar embodiment of the invention, differing from the previous embodiment in that the electrode pair is cylindrical. In addition to maximizing the ratio of active electrode surface to total cell volume, this has the advantage of keeping the width of the gap 102 constant, eliminating corner regions.

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

Examples of the invention

A brine containing 9g/L NaCl was prepared from tap water and added to the gap 102 of a cell corresponding to the embodiment of FIG. 1, equipped with a total surface area of 15cm²And a total surface area of 7cm²The inner electrode pair of (2). The total height of the two electrode pairs was 5 cm. The electrodes of the two electrode pairs are made of titanium sheets, activated on the side facing the gap using a mixture of ruthenium, palladium and titanium oxides, as is known in the art. The total reaction volume corresponds to the interstitial volume and is 55 ml. By applying a total current of 2A, corresponding to a current density of 1.5kA/m over the inner electrode pair²And 0.7kA/m on the outer electrode pair²And by reversing the current direction every 180 seconds 3300ppm of active chlorine can be produced with a constant yield of 48% during a series of batch cycles of 15 minutes each, with the pH observed to rise from initial neutrality to a value of 11.3.

The foregoing description is not intended to limit the invention, which may be used according to different embodiments without departing from the scope of the invention, and its scope is only limited by the appended claims.

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

Discussion of documents, acts, materials, devices, articles and the like is included in this specification 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 monopolar type electrolytic cell defined by an elongated or elliptical outer body having an outer electrode pair and an inner electrode pair disposed inside the outer electrode pair, the outer electrode pair being subdivided into first and second outer electrodes having the same size, the first and second outer electrodes being separated at an edge by a first insulating member, the inner electrode pair being subdivided into first and second inner electrodes having the same size, the first and second inner electrodes being separated at an edge by a second insulating member, the inner and outer electrode pairs being concentrically disposed, wherein surfaces of the first and second outer electrodes and the first inner electrode and surfaces of the second and second inner electrodes are opposed to each other to define a gap, the first and second outer electrodes being connected to one electrode of the cell, the second outer electrode and the first inner electrode are connected to opposite poles of the battery.

2. The electrolysis cell of claim 1, wherein the pair of inner and outer electrodes is a pair of cylindrical or prismatic electrodes housed inside an elongated body, or a pair of elliptical electrodes housed inside an elliptical body.

3. The electrolysis cell of claim 2, wherein the outer electrode pair and the inner electrode pair are coaxial with the cell outer body.

4. An electrolysis cell according to any preceding claim, wherein the first and second outer electrodes and the first and second inner electrodes are prepared from conductive diamond in bulk or supported form, or from titanium coated with a catalytic composition containing one or more platinum group elements.

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

6. An electrolysis cell according to any preceding claim, wherein the gap has a constant width in the range 1-20 mm.

7. An electrolysis cell according to any preceding claim, wherein the first and second insulating members are dimensioned such that the distance between opposing edges of the first and second outer electrodes and the distance between opposing edges of the first and second inner electrodes is at least equal to the width of the gap.

8. A method of performing an electrolytic process within the electrolytic cell of claims 1-7, comprising: a process electrolyte is fed into the gap and a direct current is supplied to the battery poles, the direction of which is changed at predetermined time intervals.

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

10. The method of claim 8 or 9, wherein the predetermined time interval is 1-120 minutes in length.