Electrolyzer and self-cleaning electrolytic chlorination system for electrolytic chlorination process

Abstract

This invention relates to a chlorination electrolyzer 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 disposed within the housing. The electrolyzer is characterized in that each of the at least one pair of bipolar electrodes comprises: a valve metal substrate; an active coating disposed on the substrate comprising at least one layer of a catalytic composition containing ruthenium and titanium; and a top coating disposed above the active coating, comprising at least one layer of a composition containing oxides of tantalum, niobium, tin, or combinations thereof. The invention also relates to a self-cleaning electrolytic chlorination system comprising such an electrolyzer, a method for producing it, its use in normal and low-salinity tanks for hypochlorite-mediated water disinfection, and a method for hypochlorite-mediated water disinfection.

Description

Technical Field

[0001] This invention relates to a chlorination electrolyzer operating under polarity reversal conditions, its production method, and a self-cleaning electrolytic chlorination system. Background Technology

[0002] Electrolytic chlorination involves producing hypochlorous acid in brine through an electrolytic reaction. The resulting sodium hypochlorite can be used in various applications involving water disinfection and oxidation, such as drinking water, swimming pool water treatment, or microbial control in cooling towers.

[0003] Sodium hypochlorite is effective against bacteria, viruses, and fungi, and has the advantage that microorganisms cannot develop resistance to its effects.

[0004] Unlike chlorine gas or tablets, which can be mixed with water to achieve similar results, the active material is produced in situ in the electrolytic chlorination method, thus avoiding transportation, environmental, and / or storage problems. Depending on the application, the method is carried out by applying a suitable current to an electrolytic cell containing at least two electrodes and an electrolyte containing a brine, i.e., a mixture of salt and water at different concentrations. The electrochemical reaction results in the production of sodium hypochlorite and hydrogen gas.

[0005] In the past, titanium electrodes with active coating compositions containing a mixture of valve metals and precious metals (especially rare transition metals from the platinum group) have been successfully used as anodes in these types of tanks. However, over time, rust scale has formed on the active surface of the electrodes, which adversely affects the hypochlorous acid production efficiency of the tank.

[0006] To prevent / reduce rust formation, periodic polarity reversal can be applied to the electrodes to promote their self-cleaning. Reversing polarity also reduces ion bridging between electrodes and prevents uneven electrode wear.

[0007] Under polarity reversal conditions, where each electrode alternately operates as a cathode and as an anode, after several reversal cycles, some elements accidentally used in the active coating composition become unstable and dissolve in the electrolyte, resulting in insufficient electrode lifetime.

[0008] Typically, polarity reversal is a detrimental operation to the active coating of the electrode, rapidly causing it to deactivate due to delamination.

[0009] To mitigate these issues, bipolar electrodes are needed that can be used under polarity reversal conditions with significantly higher coating loads than when each electrode operates solely as an anode or cathode. Typically, electrode durability depends on the polarity reversal frequency and the coating load.

[0010] Increasing the coating loading negatively impacts electrode costs in terms of material quantity and lengthy production processes. Furthermore, because many active coating compositions rely on rare transition metals that are scarce and unavailable, increased loading also exacerbates any procurement-related issues.

[0011] Self-cleaning electrodes are desired for electrolytic chlorination systems that exhibit improved lifespan and efficiency under a wide range of possible applications and operating conditions, while potentially maintaining reduced production costs. Furthermore, such electrolytic chlorination systems are expected to be used in normal and low-salinity ponds, i.e., ponds with salt levels equal to or less than 6 g / L (typically 0.5–2.5 g / L NaCl in low-salinity applications and 2.5–4 g / L NaCl in normal-salinity applications).

[0012] International patent application WO 2019 / 215944 A1 describes an electrolyzer for generating ozone, equipped with electrodes having a thick dielectric surface layer to increase the overpotential for oxygen generation at concentrated noble metal sites in the intermediate layer. These electrodes are unsuitable for either chlorine production or operation under polarity reversal conditions. Summary of the Invention

[0013] This invention relates to a chlorination electrolyzer comprising a housing and at least one pair of bipolar electrodes facing each other and disposed within the housing, the housing being provided with an inlet and an outlet suitable for brine circulation. Each bipolar electrode comprises: (i) a valve metal substrate; (ii) an active coating disposed on the substrate, comprising at least one layer of a catalytic composition containing ruthenium and titanium; and (iii) a top coating comprising at least one layer of a composition containing an oxide of tantalum, niobium, tin, or a combination thereof and disposed above the active coating.

[0014] In another aspect, the present invention relates to a self-cleaning electrolytic chlorination system comprising: (i) the chlorination electrolyzer described above; (ii) an electrolyte circulating within the electrolyzer, comprising a 1-30 g / L NaCl aqueous solution; and (iii) an electronic system for periodically reversing the polarity of a pair of bipolar electrodes electrically connected thereto and located outside the housing of the electrolyzer.

[0015] In another respect, the present invention relates to a method for manufacturing a chlorination electrolyzer according to the present invention.

[0016] In another aspect, the present invention relates to the use of the chlorination electrolyzer described above in normal and low-salinity pools for hypochlorite-mediated water disinfection.

[0017] In another aspect, the present invention relates to a method for hypochlorite-mediated water disinfection using the chlorination electrolyzer described above under polarity reversal conditions. Detailed Implementation

[0018] In one aspect, the present invention relates to a chlorination electrolyzer comprising:

[0019] A housing having an inlet and an outlet suitable for brine circulation and at least one pair of bipolar electrodes facing each other and disposed within the housing, wherein each of the pair of bipolar electrodes comprises: (i) a valve metal substrate; (ii) an active coating disposed above the substrate, comprising at least one layer of a catalytic composition containing ruthenium and titanium; and (iii) a top coating disposed above the active coating, comprising at least one layer of a composition containing an oxide of tantalum, niobium, tin, or a combination thereof.

[0020] At least one layer of the catalytic composition comprising ruthenium and titanium is substantially homogeneous in terms of its electrical properties. At least one layer of the catalytic composition is also homogeneous in terms of its morphological properties and substantially constitutes a solid solution comprising ruthenium and titanium, preferably a homogeneous solid solution, wherein the metals are primarily oxides, namely ruthenium oxide and titanium oxide.

[0021] The chlorination electrolyzer according to the present invention can be used for hypochlorite-mediated water disinfection in various applications, such as ponds, wastewater disinfection (e.g., municipal water treatment, blackwater and greywater treatment, seawater chlorination, etc.).

[0022] It can advantageously operate under polarity reversal conditions, thereby ensuring self-cleaning of the electrodes and preventing the formation of rust.

[0023] Each electrode in the pair may be coated on one or both sides. Conventionally, two opposing electrodes should be arranged so that they have coated sides facing each other.

[0024] Chlorination electrolyzers may contain multiple bipolar electrode pairs, resulting in a stack of coated electrodes arranged substantially parallel to each other.

[0025] The housing should be designed to allow electrical connection of one or more bipolar electrode pairs to an external generator. As is known in the art, the generator may advantageously be equipped with a system that reverses the electrode polarity at a preset frequency (typically in the range of 30 min to 10 h, depending on the application and operating conditions such as water contaminants and water hardness).

[0026] The valve metal substrate can have any geometry commonly used in the art, such as, but not limited to, slabs, perforated plates, meshes, and louvers. Preferably, the substrate is made of titanium for its durability, cost, and ease of surface preparation.

[0027] Before applying an active coating, it is preferable to clean, sandblast, and etch the substrate to ensure proper adhesion.

[0028] The active coating can be applied directly over the valve metal substrate using roller coaters, brushes, and spraying techniques. Alternatively, the claimed invention allows an intermediate coating to be inserted between the substrate and the active coating, for example, to improve the adhesion of the active coating. In this case, the latter should still be considered to be applied over the substrate, albeit indirectly.

[0029] In one embodiment, the catalytic composition of the chlorination electrolyzer according to the invention comprises 25%-45% ruthenium and 55%-75% titanium, with respect to the elements expressed as a weight percentage.

[0030] In another embodiment, the catalytic composition may optionally contain 2%-5% of a dopant element selected from scandium, strontium, hafnium, bismuth, zirconium, aluminum, copper, rhodium, iridium, platinum, palladium, and combinations thereof. These dopants can advantageously contribute to improved lifetime and free chlorine efficiency of the chlorination electrolyzer.

[0031] Applying an insulating top coating of tantalum, niobium, or tin oxide onto the active coating according to any of the above embodiments (in combination or individually) allows for a reduction in Ru load of up to 38% for a given lifetime target of the electrode without affecting efficiency.

[0032] The reduction in Ru loading offers significant advantages due to its scarcity and the resulting procurement and cost issues, especially compared to the metal oxides used in the topcoat compositions of this invention.

[0033] The inventors discovered that the tin oxide top coating works particularly well in carrying out the invention because, compared to Ta or Nb, Sn appears to form a coating that allows Cl to form. - Ions diffuse more readily into the oxide layer of the active layer. The Sn top coating also forms a less cracked surface due to its lower tendency to form dislocations, which cause the typical cracks observed, for example, on tantalum oxide surfaces. The less cracked surface prevents the electrolyte from dissolving unstable portions of the active layer.

[0034] In another embodiment, the top coating is preferably thin enough, between 0.5 and 7 micrometers, because it helps maintain the free available chlorine (FAC) efficiency of the active layer.

[0035] In any of the above embodiments, the active coating may have a concentration of 1-30 g / m³. 2 The ruthenium loading can be effective for applications with salinity greater than 6 g / l (but preferably less than 30 g / l), such as seawater chlorinators, and for applications with salinity less than 6 g / l, such as 0.5-4 g / l found in ponds.

[0036] In pond applications, the top coating has a concentration of 2-6 g / m³. 2 The preferred total load.

[0037] Without being limited to a specific theory, the top coating according to the invention forms a mesh rather than a barrier layer: it reduces mechanical wear on the surface of the active coating due to friction from air bubbles and maintains partial dissolution of the material when polarity reversal occurs, thereby preventing coating delamination and the dissolution of ruthenium and other optional dopants in the electrolyte. Simultaneously, the porosity and thinness of the top coating allow the electrolyte to reach the catalytic centers of the active coating.

[0038] In another aspect, the present invention relates to a self-cleaning electrolytic chlorination system comprising: (i) a chlorinator electrolyzer as described above; (ii) an electrolyte containing a 1-30 g / L NaCl aqueous solution circulating within the electrolyzer; and (iii) an electronic system for periodically reversing the polarity of the bipolar electrodes of the electrolyzer, the electronic system preferably being located outside the housing of the electrolyzer and electrically connected to the bipolar electrodes.

[0039] The electronic system that periodically reverses the polarity of 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).

[0040] In pond applications, the inventors observed that the self-cleaning electrolytic chlorination system according to the invention performed particularly well when the electronic system reversed the polarity of the bipolar electrode pair at preset intervals of 1-4 hours.

[0041] It has been found that stacking 5-15 bipolar electrode pairs connected in parallel is advantageous for implementing the present invention.

[0042] The electronic system according to the invention can advantageously operate at about 200-600 A / m 2 Preferred 200-400A / m 2 It operates at current densities.

[0043] In another aspect, the present invention relates to a method for producing the chlorination electrolyzer described above, comprising the steps of manufacturing each of at least one pair of bipolar electrodes in the following sequential stages:

[0044] a) Apply an active coating solution containing ruthenium and titanium precursors to a valve metal substrate to obtain a coated substrate;

[0045] b) Bake the coated substrate at 450-550℃ for 2-10 minutes;

[0046] c) Repeat steps a) and b) until the desired load of ruthenium is achieved;

[0047] d) Apply a topcoat solution containing a precursor of tantalum, niobium, tin or a combination thereof to the coated substrate;

[0048] e) Bake the coated substrate at 450-550℃ for 2-10 minutes;

[0049] f) Repeat steps d) and e) until the desired load of tantalum, niobium, tin or a combination thereof is achieved;

[0050] g) Perform final heat treatment at a temperature in the range of 450-550℃.

[0051] Precursors for ruthenium and titanium, as well as for tantalum, niobium, or tin, are compounds selected from the following: metal methoxides, ethanolates, propoxides, butoxides, chlorides, nitrates, iodides, bromides, sulfates, or acetates, and mixtures thereof.

[0052] Optionally, after step a) and / or step d), the coated substrate may be air-dried at a temperature of 20-80°C for 2-10 minutes.

[0053] Generally, the chlorination electrolyzer according to the invention, particularly with regard to the bipolar electrode structure, can be successfully used in all applications of hypophosphite production, which undergoes polarity reversal to reduce the noble metal load on the active coating, or exhibits extended lifespan if the same load is applied, without compromising FAC efficiency.

[0054] The inventors discovered that the chlorination electrolyzer works particularly well in pond applications operating at salinity levels of 0.5-4 g / L.

[0055] In another aspect, the present invention relates to the use of the chlorination electrolyzer according to the invention in normal and low-salinity tanks for hypochlorite-mediated water disinfection, i.e., for use in tanks operating at a salt level equal to or less than 6 g / L (typically 0.5–2.5 g / L NaCl in low-salinity applications and 2.5–4 g / L NaCl in normal-salinity applications).

[0056] The following examples illustrate specific ways of putting the invention into practice, and its practicality has been extensively verified within the claimed value range.

[0057] This invention also relates to a hypochlorite-mediated water disinfection method, comprising the following steps:

[0058] a) In at least one chlorination electrolyzer as defined above, an electrolyte containing an aqueous solution of 1-30 g / L NaCl salt is circulated, said chlorination electrolyzer comprising one or more bipolar electrode pairs;

[0059] b) Apply current to the bipolar electrode pair to generate hypochlorite in the NaCl aqueous solution;

[0060] c) The polarity of the at least one pair of bipolar electrodes is periodically reversed during the application of the current.

[0061] According to one embodiment of the invention, the polarity of the at least one pair of bipolar electrodes is reversed at time intervals selected from the range of 1 minute to 20 hours, preferably 30 minutes to 10 hours, and particularly preferably 1 to 4 hours.

[0062] In a preferred embodiment of the invention, the current is selected from 200 to 600 A / m 2 The range is preferably selected from 200 to 400 A / m 2 A current density within a certain range is applied to the at least one pair of bipolar electrodes.

[0063] Those skilled in the art will understand that the devices, components, and techniques disclosed below represent those that the inventors have discovered and that work well in the implementation of the invention; however, those skilled in the art will understand, in view of the content of this disclosure, that many changes can be made to the specific embodiments disclosed and still obtain the same or similar results without departing from the scope of the invention.

[0064] Experimental preparation

[0065] In all the electrode samples used in the following examples and counterexamples, the valve metal substrates for the bipolar electrode pairs were manufactured as follows: starting with a Grade 1 titanium plate of size 100mm × 100mm × 1mm, degreased with acetone in an ultrasonic bath, and subsequently subjected to sandblasting and etching with boiling HCl at a concentration of 22%.

[0066] Catalytic solutions for preparing electrode samples E1, E2a, E2b and samples C1-C3 were obtained by dissolving ruthenium and titanium chloride salts in 10% aqueous HCl at a Ru:Ti ratio of 28:72 (by elemental weight), wherein the final concentration of ruthenium in each catalytic solution was equal to 45 g / L.

[0067] Stir the solution prepared in this way for 30 minutes.

[0068] In all electrode samples E1, E2a, E2b, and C1-C3, brush coating was used at a concentration of 0.8 g / m³. 2 The ruthenium yield was achieved by coating a titanium substrate with the catalytic solution described above.

[0069] After each coating is applied, bake the sample at 500-550°C for 10 minutes.

[0070] Repeat the above coating process for each sample E1, E2a, E2b, C1-C3 until the total ruthenium load is achieved according to Table 1 below.

[0071]

[0072] Table 1

[0073] Example 1

[0074] The sample E1 prepared for the experiment was further coated with a topcoat solution obtained by diluting a Sn acetate solution with acetic acid to a final concentration of 40 g / L. Four layers of the topcoat solution were applied by brush, resulting in a total Sn loading of 4.5 g / m³. 2 After each layer, the sample is then baked at 500-550℃ for 10 minutes.

[0075] After the final layer, the sample is baked at 500-550°C for 3 hours.

[0076] Test sample electrode E1 according to the following accelerated testing procedure:

[0077] At 25°C, a pair of identical electrode samples were placed in a housing with an inlet and an outlet, characterized by a 3 mm inter-electrode gap and containing 1 L of an aqueous solution of 4 g / L NaCl and 70 g / L Na2SO4.

[0078] Electrode pair at 1000 A / m 2 It operates at a current density and undergoes polarity reversal every minute during the test duration. The electrode pair is maintained under test conditions until the cell voltage exceeds 8.5 volts (“accelerated lifetime”, for each g / m³ of catalytic composition). 2 Ruthenium is measured in hours.

[0079] The results are recorded in Table 2.

[0080] E1 lifetime performance in hours (corresponding to 145 hours on-line (HOL)) was selected as the target performance for the bipolar electrode, as reported in Table 2.

[0081] At a temperature of 25℃, at 300A / m 2 The FAC of the sample was measured in water with 3 g / L NaCl.

[0082] Example 2

[0083] The topcoat solution, obtained by dissolving 80 g of TaCl5 in 1 L of 20% HCl and stirring the solution at room temperature for 30 minutes, was further coated onto samples E2, namely E2a and E2b, prepared for the experiment. For each E2 sample, one layer of the topcoat solution was applied by brush, with a total Ta loading of 1 g / m³. 2 First, bake the sample at 300-350℃ for 10 minutes, then bake it at 500-550℃ for 10 minutes.

[0084] Sample E2 was tested according to the same test procedure described in Example 1.

[0085] The results of analyzing sample E2 show that the only sample that meets the target performance of E1 is E2b; its performance is characterized in Table 2.

[0086] Counterexample 1

[0087] The samples C, namely C1-C3, obtained from the experiment were baked at 500-550°C for 3 hours and then tested according to the test procedure described in Example 1.

[0088] The results of analyzing sample C show that the only sample that meets the target performance of E1 is C3; its performance is characterized in Table 2.

[0089]

[0090] Table 2

[0091] The preceding description should not be intended to limit the invention, which can be used in various embodiments without departing from its scope, and the scope of the invention is defined only by the appended claims.

[0092] Throughout the specification and claims of this application, the term "comprising" and its variations, such as "containing" and "comprising," are not intended to exclude the presence of other elements, components, or additional method steps.

[0093] Discussions of documents, laws, materials, devices, articles, etc., included in this specification are for the purpose of providing background information on the invention only. It is not recommended or implied that any or all of these matters constitute part of the prior art or are common general knowledge in the relevant field prior to the priority date of each claim of this application.

Claims

1. A self-cleaning electrolytic chlorination system comprising a chlorination electrolyzer and an electrolyte circulating within the chlorination electrolyzer, the electrolyte comprising a 1-30 g / L NaCl aqueous solution, wherein the chlorination electrolyzer comprises: - Provides a housing with inlet and outlet suitable for brine circulation; - At least one pair of bipolar electrodes facing each other and placed within the housing; The characteristic is that each of the at least one pair of bipolar electrodes comprises: - Valve metal substrate; - An active coating disposed above the substrate, comprising a layer of at least one catalytic composition containing a solid solution of ruthenium and titanium; - A top coating layer disposed above the active coating, comprising at least one layer of an oxide containing tantalum, niobium, tin, or a combination thereof. The active coating has a concentration of 1-30 g / m 2 The ruthenium loading, and The top coating has a thickness of 0.5-7 micrometers. The self-cleaning electrolytic chlorination system further includes an electronic system that periodically reverses the polarity of at least one pair of bipolar electrodes and electrically connects them thereto.

2. A self-cleaning electrolytic chlorination system comprising a chlorination electrolyzer and an electrolyte circulating within the chlorination electrolyzer, the electrolyte comprising a 1-30 g / L NaCl aqueous solution, wherein the chlorination electrolyzer comprises: - Provides a housing with inlet and outlet suitable for brine circulation; - At least one pair of bipolar electrodes facing each other and placed within the housing; The characteristic is that each of the at least one pair of bipolar electrodes comprises: - Valve metal substrate; - An active coating disposed above the substrate, comprising at least one layer of a catalytic composition containing ruthenium and titanium; A top coating layer disposed above the active coating, comprising at least one layer containing tin oxide, wherein the top coating layer does not contain niobium or tantalum oxide. The active coating has a concentration of 1-30 g / m 2 The ruthenium loading, and The top coating has a thickness of 0.5-7 micrometers. The self-cleaning electrolytic chlorination system further includes an electronic system that periodically reverses the polarity of at least one pair of bipolar electrodes and electrically connects them thereto.

3. The self-cleaning electrolytic chlorination system according to any one of claims 1 or 2, wherein the catalytic composition comprises 25%-45% ruthenium and 55%-75% titanium, the elements being expressed as weight percentages.

4. The self-cleaning electrolytic chlorination system according to claim 3, wherein the catalytic composition further comprises 2%-5% of a dopant element selected from the group consisting of scandium, strontium, hafnium, bismuth, zirconium, aluminum, copper, rhodium, iridium, platinum, palladium, and combinations thereof.

5. The self-cleaning electrolytic chlorination system according to any one of claims 1 or 2, wherein the top coating is composed of tin oxide.

6. The self-cleaning electrolytic chlorination system according to claim 1 or 2, wherein the top coating has a content of 2-6 g / m 2 Total load.

7. The self-cleaning electrolytic chlorination system according to claim 1 or 2, wherein the valve metal substrate is titanium.

Images