TW202225486A - Electrolyser for electrochlorination processes and a self-cleaning electrochlorination system - Google Patents

Abstract

The present invention concerns a chlorination electrolyser comprising, a housing provided with an inlet and an outlet suitable for the circulation of brine; at least one pair of bipolar electrodes facing each other and positioned within said housing. The electrolyser is characterised in that each bipolar electrode of said at least one pair comprises: a valve metal substrate; an active coating comprising at least one layer of a catalytic composition comprising ruthenium and titanium disposed over said substrate; a top coating comprising at least one layer of a composition comprising oxides of tantalum, niobium, tin, or combinations thereof disposed over said active coating. The invention also concerns a self-cleaning electrochlorination system comprising such an electrolyser, a method for its production, its use in normal and low salinity pools for hypochlorite mediated water disinfection and a method for hypochlorite-mediated water disinfection.

Description

Electrolyzers and self-cleaning electrochlorination systems for electrochlorination

The present invention relates to a chlorination electrolytic cell operation under the condition of polarity reversal, a manufacturing method of the chlorination electrolytic cell, and a self-cleaning electrochlorination system. .

Electrochlorination methods consist in the production of hypochlorite in brine via an electrolytic reaction. The resulting sodium hypochlorite can be used in various applications related to water disinfection and oxidation, such as water treatment for drinking water, microbial control in swimming pools or cooling towers.

Sodium hypochlorite is effective against bacteria, viruses and fungi and has the advantage that microorganisms cannot develop resistance to its effects.

In contrast to chlorine gas or chlorine tablets, which can be added to water to achieve similar results, in electrochlorination methods, the active chemicals are produced on-site, thereby avoiding transportation, environmental and/or storage problems. The method is carried out by applying a suitable current to an electrolytic cell comprising at least two electrodes and an electrolyte containing brine, ie a mixture of salt and water, of varying concentrations depending on the application. As a result of the electrochemical reaction, sodium hypochlorite and hydrogen gas are produced.

Titanium electrodes with active coating compositions containing mixtures of valve and noble metals, especially rare transition metals from the platinum group, have been used successfully in the past as anodes in these types of batteries. However, over time, the electrodes form scale on their active surfaces, which negatively affects the hypochlorite production efficiency of the electrolytic cell.

To prevent/reduce scale formation, the electrodes can be periodically reversed in polarity to facilitate their self-cleaning. Reversing the polarity also reduces ionic bridging between electrodes and prevents uneven electrode wear.

Under polarity reversal conditions, where each electrode operates alternately as cathode and anode, some elements occasionally used in the active coating composition become unstable and dissolve in the electrolyte after a few reversal cycles, resulting in Electrode life is insufficient.

In general, polarity reversal for active coatings of electrodes is a detrimental operation that can quickly It is rapidly deactivated due to delamination.

To reduce these problems, it is necessary to provide higher coating loadings for bipolar electrodes used under polarity reversal conditions than when each electrode operates only as an anode or cathode. In general, electrode durability depends on polarity reversal frequency and coating loading.

Increasing the coating loading has a negative impact on the cost of the electrode, both in terms of material quantity and longer production process. Furthermore, since many active coating compositions rely on rare transition metals that are in short supply, the increased loading also exacerbates any associated sourcing issues.

It would be desirable to have self-cleaning electrodes for use in electrochlorination systems, exhibiting improved lifetime and efficiency over a broad range of possible applications and operating conditions, and possibly maintaining reduced production costs. In addition, it is also desirable to use this type of electrochlorination system in normal salinity and low salinity pools, i.e. in pools with salinity at or below 6 g/L (usually, in low salinity applications sodium chloride ( NaCl) is 0.5 to 2.5 g/L, and sodium chloride (NaCl) is 2.5 to 4 g/L in normal salinity applications).

World Patent Application No. WO2019/215944A1 discloses an electrolytic cell for ozone generation, which is equipped with electrodes with a thick dielectric surface layer, in order to increase the oxygen overvoltage for generating oxygen at local noble metal sites in the intermediate layer. These electrodes are neither suitable for chlorine production nor for operation under polarity reversal conditions.

The present invention relates to a chlorination electrolytic cell comprising a housing with an inlet and an outlet suitable for brine circulation, and at least one pair of bipolar electrodes facing each other and positioned within the housing. Each bipolar electrode includes: (i) a valve metal substrate; (ii) an active coating comprising at least one layer of a catalytic composition disposed on the substrate, the catalytic composition including ruthenium and titanium; and (iii) ) topcoat comprising at least one layer of a composition comprising oxides of tantalum, niobium, tin or combinations thereof and positioned over the active coating.

In another aspect, the present invention relates to a self-cleaning electrochlorination system, comprising: (i) the above-mentioned chlorination electrolytic cell; (ii) an electrolyte comprising 1 to 30 g/L of sodium chloride (NaCl) saline solution in the Circulation within the cell; and (iii) an electronic system to periodically reverse the polarity of the bipolar electrode pair to which it is electrically connected and positioned outside the cell housing.

In another aspect, the present invention relates to the production of a chlorination electrolytic cell according to the present invention method of making.

In another aspect, the present invention relates to the use of the above-described chlorinated electrolytic cells in normal salinity and low salinity pools for hypochlorite mediated water disinfection.

In another aspect, the present invention relates to a method of hypochlorite-mediated water disinfection using the above-described chlorinated electrolytic cell under polarity reversal conditions.

In one aspect, the invention relates to a chlorination electrolytic cell comprising a housing having an inlet and an outlet suitable for brine circulation; and at least one pair of bipolar electrodes facing each other and positioned within the housing, wherein the pair of bipolar electrodes Each of the bipolar electrodes includes: (i) a valve metal substrate; (ii) an active coating comprising at least one layer of catalytic composition disposed on the substrate, the catalytic composition including ruthenium and titanium; and ( iii) a top coat comprising at least one layer of a composition comprising oxides of tantalum, niobium, tin or a combination thereof disposed over the active coating.

The at least one layer of the catalytic composition comprising ruthenium and titanium is a substantially homogeneous layer in terms of its electrical properties. The at least one layer of the catalytic composition is also homogeneous in terms of its morphological properties, and essentially constitutes a solid solution comprising ruthenium and titanium, preferably a homogeneous solid solution, wherein the metals are mainly oxides, namely ruthenium oxide and Titanium oxide.

The chlorination cell according to the present invention can be used for hypochlorite-mediated water disinfection in various applications, such as pools, wastewater disinfection (eg municipal water treatment, black and grey water treatment, seawater chlorination, ... ).

Chlorinating cells can advantageously be operated under polarity reversal conditions, thus ensuring self-cleaning of the electrodes and avoiding scale formation.

Each electrode of the pair of electrodes can be coated on one or both sides. Conventionally, two opposing electrodes should be positioned with the coated sides facing each other.

The chlorination cell may include a plurality of bipolar electrode pairs forming a stack with coated electrodes arranged generally parallel to each other.

The enclosure should be designed to allow the bipolar electrodes to be electrically connected to an external generator. The generator may advantageously be equipped with a system to reverse electrode polarity at a preset frequency, typically in the range of 30 minutes to 10 hours, as is well known in the art, depending on application and operating conditions such as water contamination and water hardness.

The valve metal substrate can be any geometric shape commonly used in the art such as, but not limited to, flat plates, perforated plates, screens, louvers. The substrate is preferably made of titanium due to its durability, cost, and ease of surface preparation.

The substrate should preferably be cleaned, sandblasted and etched to ensure proper adhesion prior to applying the reactive coating.

The active coating can be applied directly on the valve metal substrate using roll coater, brush and spray techniques. Alternatively, the present invention allows an intermediate coating to be inserted between the substrate and the active coating, eg, to improve the adhesion of the active coating. In this case, the latter should still be considered to be disposed on the substrate, albeit indirectly.

In one embodiment, the catalytic composition of the chlorinated electrolyzer according to the present invention, expressed in weight percent of elements, includes 25% to 45% of ruthenium and 55% to 75% of titanium.

In another embodiment, the catalytic composition may optionally include 2% to 5% of a doping element selected from scandium, strontium, hafnium, bismuth, zirconium, aluminum, copper, rhodium, iridium, platinum, palladium, and the like. A group formed by combining with each other. These dopants can advantageously help increase the life of the chlorination cell and the efficiency of free available chlorine.

Applying an insulating top coat of tantalum, niobium or tin oxide (combined or alone) over the active coating according to any of the above embodiments allows the loading of ruthenium (Ru) to be reduced to 38% for electrode supply Set life goals without compromising efficiency.

Due to the scarcity of ruthenium and the attendant procurement and cost issues, the reduction in ruthenium (Ru) loading provides significant advantages, especially compared to the metal oxides used in the topcoat compositions of the present invention.

The inventors have found that a top coat of tin oxide is particularly effective in the practice of the present invention because tin (Sn) appears to form an oxide that allows chloride ions, Cl, compared to tantalum (Ta) or niobium (Nb) ^- preferably diffusion to the active layer. The tin (Sn) topcoat also forms a less cracked surface, as it has a lower tendency to form dislocations, such as those typical of cracks that can be observed on tantalum oxide surfaces. A less cracked surface prevents the electrolyte from dissolving unstable parts of the active layer.

In another embodiment, the topcoat is preferably thin enough (between 0.5 to 7 microns) as it can help maintain the free available chlorine (FAC) efficiency of the active layer.

In any of the above embodiments, the active coating may have a ruthenium loading of 1 to 30 g/m2, which is useful for salinities higher than 6 g/L (but preferably lower than 30 g/L) Applications such as seawater chlorinator applications, as well as applications with salinity below 6 g/L such as 0.5 to 4 g/L found in pools, may work.

In pool applications, the total loading of the topcoat is preferably 2 to 6 grams per square meter.

Without limiting the invention to a particular theory, the topcoat according to the invention forms a network rather than a barrier: it reduces mechanical wear of the active coating surface due to bubble friction and maintains partial dissolution when polarity reversal occurs material, thereby preventing delamination of the coating and dissolution of ruthenium and other optional dopants in the electrolyte. At the same time, the porosity and thinness of the top coat allows the electrolyte to reach the catalytic center of the active coating.

In another aspect, the present invention relates to a self-cleaning electrochlorination system, comprising: (i) the above-mentioned chlorination electrolytic cell; (ii) an electrolyte comprising 1 to 30 g/L of sodium chloride (NaCl) saline solution in the Circulation within the cell; and (iii) an electronic system for periodically reversing the polarity of the bipolar electrodes of the cell, preferably positioned outside the housing of the cell and electrically connected to the bipolar electrodes.

The electronic system used to periodically reverse the polarity of the bipolar electrodes is equipped with an internal clock that allows the polarity of the bipolar electrodes to be reversed at preset time intervals (in the range of 30 minutes to 10 hours).

In pool applications, the inventors observed that the self-cleaning electrochlorination system according to the present invention performs particularly well when the electronic system reverses the polarity of the bipolar electrode pair at preset intervals of 1 to 4 hours.

A stack comprising 5 to 15 bipolar electrode pairs in parallel has been found to be beneficial for the practice of the present invention.

Electronic systems according to the present invention can advantageously operate at current densities of about 200 to 600 amperes/square meter (preferably 200 to 400 amperes/square meter).

In another aspect, the present invention relates to a method for manufacturing the above-described chlorination electrolytic cell, comprising the steps of manufacturing each electrode of the at least one pair of bipolar electrodes according to the following sequential steps:

a) applying an active coating solution comprising precursors of both ruthenium and titanium to the valve metal substrate, thereby obtaining a coated substrate;

b) baking the coated substrate at a temperature of 450 to 550° C. for 2 to 10 minutes;

c) repeating steps a) and b) until the desired ruthenium loading is achieved;

d) applying a topcoat solution comprising a precursor species of tantalum, niobium, tin or a combination thereof to the coated substrate;

e) baking the coated substrate at a temperature of 450 to 550° C. for 2 to 10 minutes;

f) repeating steps d) and e) until the desired loading of tantalum, niobium, tin or a combination thereof is achieved; and

g) Final heat treatment in a temperature range of 450 to 550°C.

The precursors of ruthenium and titanium and the precursors of tantalum, niobium or tin are compounds selected from methoxide, ethoxide, propoxide, butoxide, chloride, nitrate, iodide, bromide of these metals , sulfate or acetate, and mixtures thereof.

Optionally, after step a) and/or after step d), the coated substrate is air-dried at a temperature of 20 to 80° C. for 2 to 10 minutes.

In general, the chlorination cell according to the present invention, especially with regard to bipolar electrode architectures, can be successfully used in all applications for hypochlorite production subject to polarity reversal to reduce noble metal loading of active coatings Or exhibit extended lifetime if the same load is applied without affecting free available chlorine (FAC) efficiency.

The inventors have found that chlorinated electrolyzers work particularly well in pool applications (operating at salinities of 0.5 to 4 grams per liter).

In yet another aspect, the object of the present invention is the use of a chlorinated electrolytic cell according to the present invention for hypochlorite-mediated water disinfection in normal and low salinity pools, i.e. for use at salinity equal to or In pools operating below 6 g/L (typically 0.5 to 2.5 g/L of sodium chloride (NaCl) in low salinity applications and 2.5 to 4 g/L in normal salinity applications grams/liter).

The following examples are included to illustrate specific ways in which the invention may be put into practice, the utility of which has been largely demonstrated within the range of claimed values.

The present invention also relates to a method for hypochlorite-mediated water disinfection, comprising the steps of:

a) circulating an electrolyte comprising 1 to 30 g/l of sodium chloride (NaCl) saline solution in at least one chlorination cell as defined above, the chlorination cell comprising one or more pairs of bipolar electrodes;

b) applying an electric current to the bipolar electrode pair to generate hypochlorite in the sodium chloride (NaCl) saline solution; and

c) periodically reversing the polarity of at least one pair of bipolar electrodes during application of the current.

According to an embodiment of the present invention, the polarities of the at least one pair of bipolar electrodes are reversed at a time interval selected from a range of 1 minute to 20 hours, preferably 30 minutes to 10 hours , more preferably in the range of 1 to 4 hours.

In a preferred embodiment of the present invention, current is applied to the at least one pair of bipolar electrodes.

It should be understood by those skilled in the art that the apparatus, compositions and techniques disclosed below represent apparatus, compositions and techniques that the inventors have found to function well in the practice of the invention; It is to be understood that many changes can be made in the specific embodiments disclosed and still obtain a like or similar result without departing from the scope of the invention.

Experimental preparation

In all electrode samples used in the following examples and comparative examples, valve metal substrates for a pair of bipolar electrodes were fabricated starting from grade 1 titanium plates with dimensions of 100 mm x 100 mm x 1 mm, used in an ultrasonic bath. Acetone degreased, then sandblasted and etched with 22% full boiling hydrogen chloride (HCl).

By dissolving the chloride salts of ruthenium and titanium in a 10% aqueous hydrogen chloride (HCl) solution to obtain the catalytic solutions used for the preparation of electrode samples E1, E2a, E2b and samples C1 to C3, expressed by weight percent of elements, ruthenium: The ratio of titanium (Ru:Ti) was equal to 28:72 and the final concentration of ruthenium in each catalytic solution was equal to 45 grams per liter.

The solution thus prepared was stirred for 30 minutes.

In all electrode samples E1, E2a, E2b, C1 to C3, the titanium substrate was coated with the above catalytic solution using a brush coating method with a gain of 0.8 g/m2 of ruthenium.

After each brush coat, the samples were baked at a temperature of 500 to 550°C for 10 minutes.

The above coating procedure was repeated for each sample E1, E2a, E2b, C1 to C3 until the total ruthenium loading according to Table 1 below was reached:

Example 1

Sample E1 formed from the test preparation was further coated with a topcoat solution derived from a tin (Sn) acetate solution diluted with acetic acid until a final concentration of 40 grams/liter was reached. The topcoat solution was applied by brush in 4 coats with a total tin loading of 4.5 g/m². After each coat was applied, the samples were then baked at a temperature of 500 to 550°C for 10 minutes.

After the final coat was applied, the samples were post-baked at a temperature of 500 to 550°C for 3 hours.

Testing of sample electrode E1 was performed according to the following accelerated testing procedure: A pair of identical electrode samples were placed in a housing with inlet and outlet at 25°C, with a gap of 3 mm between the electrodes, and containing 4 g/L NaCl (NaCl) and 70 g/L sodium sulfate (Na ₂ SO ₄ ) in 1 liter of water.

The electrode pair was run at a current density of 1000 amps/square meter with polarity reversal every 1 minute during the test. The electrode pair was maintained under test conditions until the cell voltage exceeded 8.5 volts ("accelerated life", measured in hours for ruthenium per gram/square meter in the catalytic composition).

The results are recorded in Table II.

The E1 lifetime performance (in hours, corresponding to 145 hours on line (HOL)) was chosen as the target performance for the bipolar electrode, as listed in Table II.

The sample was measured for free available chlorine (FAC) at a temperature of 25°C at 300 amperes/square meter and sodium chloride (NaCl) at 3 g/liter in water.

Example 2

The experimentally prepared samples E2 (ie E2a and E2b) were further coated with a topcoat solution prepared by dissolving 80 grams of tantalum chloride (TaCl5) in ₁ liter of 20. % strength hydrogen chloride (HCl) and the solution was stirred at room temperature for 30 minutes. For each E2 sample, the topcoat solution was applied by brush in 1 layer with a total tantalum (Ta) loading of 1 gram/square meter. The samples were first baked at a temperature of 300 to 350°C for 10 minutes, and then at a temperature of 500 to 550°C for 10 minutes.

Sample E2 was tested according to the same test procedure described in Example 1 .

Analyzing the results of sample E2, the only sample that meets the target performance of E1 is E2b; its performance characteristics are shown in Table 2.

Comparative Example 1

Samples C (ie C1 to C3 ) prepared from the experiments were post-baked at temperatures of 500 to 550° C. for 3 hours and tested according to the test procedure described in Example 1 .

Analyzing the results of sample C, the only sample meeting the target performance of E1 was C3; the characteristics of its performance are listed in Table II.

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

Throughout the specification and scope of this application, the word "comprising" and variations such as "comprising" and "comprising" are not intended to exclude the presence of other elements, components or additional procedural steps.

Discussions of patent documents, acts, materials, devices, articles, etc. are included in this specification for the sole purpose of providing the context of the present invention. It is not suggested or indicated that any or all of these matters formed part of the prior art base or common general knowledge in the field relevant to the present invention prior to the priority date of each claimant of this application.

Claims (14)

A chlorination electrolytic cell, comprising: - a housing with inlet and outlet suitable for salt water; - at least one pair of bipolar electrodes, facing each other and positioned within the housing; It is characterized in that, each bipolar electrode in the at least one pair of bipolar electrodes comprises: - valve metal substrate; - an active coating, comprising at least one layer of catalytic composition disposed on the substrate, the catalytic composition including ruthenium and titanium; - a top coating comprising at least one layer of a composition comprising oxides of tantalum, niobium, tin or a combination thereof disposed on the active coating. The chlorination electrolytic cell of claim 1, wherein the catalytic composition is expressed in weight percentages of elements, including ruthenium accounting for 25% to 45% and titanium accounting for 55% to 75%. The chlorinated electrolytic cell according to the second item of the patent application scope, wherein the catalytic composition still includes 2% to 5% of doping elements selected from the group consisting of scandium, strontium, hafnium, bismuth, zirconium, aluminum, copper, rhodium, iridium , platinum, palladium and their combination with each other. The chlorination electrolytic cell of any one of claims 1 to 3, wherein the active coating has a ruthenium loading of 1 to 30 g/m2. The chlorination electrolytic cell according to any one of claims 1 to 4 of the claimed scope, wherein the top coating is composed of tin oxide. The chlorination electrolytic cell according to any one of items 1 to 5 of the claimed scope, wherein the top coating has a thickness of 0.5 to 7 microns. The chlorination electrolytic cell of any one of claims 1 to 6 of the claimed scope, wherein the top coat has a total load of 2 to 6 grams per square meter. The chlorination electrolytic cell according to any one of claims 1 to 7 of the claimed scope, wherein the valve metal base material is titanium. A self-cleaning electrochlorination system, comprising: - Such as the chlorination electrolytic cell of any one of items 1 to 8 of the scope of the application; - an electrolyte comprising 1 to 30 g/L sodium chloride (NaCl) saline solution circulated in the chlorination cell; and - an electronic system for periodically reversing the polarity of at least one pair of bipolar electrodes and for being electrically connected to the electrodes. A method for making a chlorinated electrolytic cell as claimed in any one of claims 1 to 8, comprising the steps of making each electrode of at least one pair of bipolar electrodes according to the following sequential steps: a) Coating an active coating solution including ruthenium and titanium precursors on the valve metal substrate to obtain a coated substrate; b) Bake the coated substrate at a temperature of 450 to 550°C for 2 to 10 minutes; c) repeating steps a) and b) until the desired ruthenium loading is achieved; d) coating the coated substrate with a topcoat solution comprising precursors of tantalum, niobium, tin, or a combination thereof; e) Bake the coated substrate at a temperature of 450 to 550°C for 2 to 10 minutes; f) Repeat steps d) and e) until the desired loading of tantalum, niobium, tin or a combination thereof is achieved; and g) final heat treatment in a temperature range of 450 to 550°C; wherein the ruthenium and titanium precursors and the tantalum, niobium or tin precursors are compounds selected from metal methoxides, ethoxides and propoxides , butoxide, chloride, nitrate, iodide, bromide, sulfate or acetate and mixtures thereof. A use of a chlorinated electrolytic cell as claimed in Claims 1 to 8 in normal salinity and low salinity pools for hypochlorite mediated water disinfection. A method for hypochlorite-mediated water disinfection, comprising the steps of: a) Circulating an electrolyte comprising 1 to 30 g/L of sodium chloride (NaCl) saline solution in at least one chlorination cell as in any one of claims 1 to 8, the chlorination cell including one or more pairs of bipolar electrodes; b) applying an electrical current to the pair of bipolar electrodes to generate hypochlorite in the saline solution; and c) periodically reversing the polarity of at least one pair of bipolar electrodes during application of the current. The method of claim 12, wherein the polarity of the at least one pair of bipolar electrodes is reversed at time intervals selected from the range of one minute to 20 hours. The method of claim 12 or 13, wherein a current is applied to the at least one pair of bipolar electrodes at a current density selected from the range of 200 to 600 amperes/square meter.