Electrochemical cell operation

Abstract: A method is disclosed for maintaining or increasing electrolyte flow rate through a microporous diaphragm in an electrochemical cell for the production of hydrogen peroxide by maintaining in the electrolyte a stabilizing agent. Flow rate is maintained or increased by complexing metal ions or compounds with the stabilizing agent.

Description: The present invention relates to the electrochemical production of alkaline hydrogen peroxide solutions. The production of alkaline hydrogen peroxide by the electroreduction of oxygen in an alkaline solution is well known from US-A- 3 607 687 (Grangaard) and US-A-3 969 201 (Oloman et al). Improved processes for the production of an alkaline hydrogen peroxide solution by electroreduction of oxygen are disclosed in US-A- 4 431 494 (McIntyre et al.) and in CA-A- 1 214 747 (Oloman). These Patent Specifications describe methods for the electrochemical generation of an alkaline hydrogen peroxide solution designed to decrease the hydrogen peroxide decomposition rate in an aqueous alkaline solution (US-A- 4 431 494) and to increase the current efficiency (CA-A- 1 214 747). In US-A- 4 431 494 there is disclosed utilizing a stabilizing agent in an aqueous electrolyte solution in order to minimize the amount of peroxide decomposed during electrolysis, thus, maximizing the electrical efficiency of the cell, i.e., more peroxide is recovered per unit of energy expended. In CA-A- 1 214 747 there is disclosed overcoming the continually decreasing current efficiency of electrochemical cells for the generation of alkaline peroxide by the electroreduction of oxygen in an alkaline solution by the inclusion of a complexing agent in the aqueous alkaline electrolyte which is utilized at a pH of 13 or more. Both US-A-4 431 494 and CA-A- 1 214 747 disclose using chelating agents as the stabilizing agent or complexing agents, respectively. Both of these specifications disclose the use of alkali metal salts of ethylene-diaminetetraacetic acid (EDTA) as useful stabilizing agents. Electrochemical cells for the electroreduction of oxygen in an alkaline solution are disclosed in US-A-4 872 957 (Dong et al) and US-A- 4 921 587, (also Dong et al.). In those Specifications electrochemical cells are disclosed having a porous, self-draining, gas diffusion electrode and a microporous diaphragm. A dual purpose electrode assembly is disclosed in US-A- 4 921 587. The diaphragm can have a plurality of layers and may be a microporous polyolefin film or a composite thereof. The present invention provides a method for the electroreduction of oxygen in an alkaline solution in an electrochemical cell having a cell diaphragm or cell separator which is characterized as comprising a microporous film. Plugging of the pores of the film diaphragm during operation of the cell is avoided by the use of a stabilizing agent which can be a chelating agent. The present invention is a method for the electroreduction of oxygen in an alkaline solution in order to prepare an alkaline hydrogen peroxide solution. In the method of the present invention, the electrolyte flow rate through the cell separator is maintained constant or increased during electroreduction by the incorporation of a stabilizing agent in the electrolyte used in the cell. It is believed that this prevents the deposition of insoluble compounds present as impurities in the electrolyte, on or in the pores of the cell separator or diaphragm. It has been found, as disclosed in US-A- 4 431 494, that the efficiency of a process for the electrolytic production of hydrogen peroxide solutions utilizing an alkaline electrolyte can be improved by the incorporation of a stabilizing agent in the electrolyte solution. The amount of peroxide decomposed during electrolysis is thus minimized in accordance with the teaching of US-A- 4 431 494. In the process of that Specification, an electrolytic cell separator is disclosed as a permeable sheet of asbestos fibres or an ion exchange membrane sheet. Similarly, in CA-A-1 214 747, it is disclosed that the gradual reduction of current efficiency of an electrochemical cell for the electroreduction of oxygen in an alkaline solution has been found to gradually decrease over time so as to make the process uneconomic. The incorporation of a complexing agent which is preferably of the type which is effective to complex chromium, nickel, or particularly iron ions at a pH of at least 10 is utilized even though the pH of the alkaline electrolyte is at least about pH 13. The use of electrolytic cell separators or diaphragms consisting of a polypropylene felt is disclosed. Neither of US-A- 4 431 494 and CA-A 1 214 747 would suggest the use of stabilizing agents or complexing agents in an aqueous alkaline electrolyte solution for the electroreduction of oxygen in an alkaline solution to complex with or solubilize metal compounds or ions present in the electrolyte solution. Where a microporous polymer film is utilized as the cell separator or diaphragm, the fine pores of the diaphragm are subject to plugging during operation of the cell. This is because the asbestos diaphragm or polypropylene felt diaphragm disclosed, respectively, in US-A- 4 431 494 and CA-A- 1 214 747 are not subject to plugging of the pores of the diaphragm in view of the fact that the porosity of these asbestos or polypropylene felt diaphragms is much greater than that of the microporous polymer film which is disclosed as useful in US-A- 4 872 957 and US-A- 4 921 587. In WO/88/3965 a method is disclosed for operating a cell for manufacturing hydrogen peroxide in which air free from carbon dioxide is directed over a cathode surface. A microporous polypropylene film separator is employed as a separating means having 38% porosity with an effective pore size of 0.02 meter. Slits are punctured through the film approximately 0.7 mm in length in a 1 cm x 1 cm matrix. The electrolyte in the reservoir was 4% NaOH containing 0.05% EDTA. It has now been discovered that the presence of a stabilizing agent in an aqueous alkaline solution which is utilized as an electrolyte in an electrochemical cell for the electroreduction of oxygen allows the maintenance of a constant or increased flow rate of electrolyte through the cell separator where the diaphragm is composed of a microporous polymer film. The microporous polymer film cell separator can be utilized in multiple layers in order to control the flow of electrolyte through the diaphragm. The use of multiple film layers allows substantially the same amount of electrolyte to pass to the cathode at various electrolyte head levels irrespective of the electrolyte head level to which the cell separator is exposed. Uniformity of flow of electrolyte into a porous and self-draining electrode is important to achieve high cell efficiency. In accordance with the present invention, there is provided a method of producing alkaline hydrogen peroxide solution which comprises electro-reduction of oxygen in an alkaline solution as electrolyte using an electrochemical cell containing a microporous polymer film cell separator, maintaining a concentration of stabilizing agent sufficient to complex with or solubilize a substantial proportion of transition metal compounds and other metal compounds or ions present as impurities in the electrolyte and thus to maintain a constant or increased electrolyte flow rate through the microporous film cell separator during cell operation. Preferred features of the invention are the subject matter dependent claims 2 to 15. To be suitable for use as a stabilizing agent, a compound must be chemically, thermally, and electrically stable to the conditions of the cell. Compounds that form chelates or complexes with the metallic impurities present in the electrolyte have been found to be particularly suitable. Such compounds include the reaction product of a metal and an acid selected from an amino carboxylic acid, an amino polycarboxylic acid, and a polyamino polycarboxylic acid. Representative chelating compounds include alkali metal salts of ethylene-diaminetetraacetic acid (EDTA), alkali metal salts of diethylene triamine pentaacetic acid (DTPA), alkali metal stannates, alkali metal phosphates, alkali metal, triethanolamine (TEA) and 8-hydroxyquinoline. Most particularly preferred are salts of EDTA because of their availability, low cost and ease of handling. The stabilizing agent should be present in an amount which is, generally, sufficient to complex with or solubilize at least a substantial proportion of the impurities present in the electrolyte and, preferably, in an amount which is sufficient to inactivate substantially all of the impurities. The amount of stabilizing agent needed will differ with the amount of impurities present in a particular electrolyte solution. An insufficient amount of stabilizer will result in the deposition of substantial amounts of compounds or ions on or in the pores of the microporous film cell separator during operation of the cell. Conversely, excessive amounts of stabilizing agents are unnecessary and wasteful. The actual amount needed for a particular solution may be, generally, determined by monitoring the electrolyte flow rate as indicated by cell voltage during electrolysis, or, preferably, by chemically analyzing the impurity concentration in the electrolyte. Stabilizing agent concentrations of from about 0.05 to about 5 grams per litre of electrolyte solution have, generally, been found to be adequate for most applications. Alkali metal compounds suitable for electrolysis in the improved electrolyte solution are those that are readily soluble in water and will not precipitate substantial amounts of HO 2 -. Suitable compounds, generally, include alkali metal hydroxides and alkali metal carbonates such as, for example, sodium carbonate. Alkali metal hydroxides such as, for example, sodium hydroxide and potassium hydroxide are preferred because they are readily available and are easily dissolved in water. The alkali metal compound, generally, should have a concentration in the solution of from about 0.1 to about 2.0 moles of alkali metal compound per litre of electrolyte solution (moles/litre). If the concentration is substantially below 0.1 mole/litre, the resistance of the electrolyte solution becomes too high and excessive electrical energy is consumed. Conversely, if the concentration is substantially above 2.0 moles/litre, the alkali metal compound peroxide ratio becomes too high and the product solution contains too much alkali metal compound and too little peroxide. When alkali metal hydroxides are used, concentrations from about 0.5 to about 2.0 moles/litre of alkali metal hydroxide are preferred. Impurities which are catalytically active for the decomposition of peroxides are also present in the electrolyte solution. These substances are not normally added intentionally but are present only as impurities. They are usually dissolved in the electrolyte solution, however, some may be only suspended therein. They include compounds or ions of transition metals. These impurities commonly comprise iron, copper, and chromium. In addition, compounds or ions of lead can be present. As a general rule, the rate of flow of electrolyte decreases as the concentration of the catalytically active substances increases. However, when more than one of the above-listed ions are present the effect of the mixture is frequently synergistic, i.e., the electrolyte flow rate when more than one type of ion is present is reduced more than occurs when the sum of the individual electrolyte flow rate decreasing ions present as compared to that flow rate which results when only one type of ion is present. The actual concentration of these impurities depends upon the purity of the components used to prepare the electrolyte solution and the types of materials the solution contacts during handling and storage. Generally, impurity concentrations of greater than 0.1 part per million will have a detrimental effect on the electrolyte flow rate. The solution is prepared by blending an alkali metal compound and a stabilizing agent with an aqueous liquid. The alkali metal compound dissolves in the water, while the stabilizing agent either dissolves in the solution or is suspended therein. Optionally, the solution may be prepared by dissolving or suspending a stabilizing agent in a previously prepared aqueous alkali metal compound solution, or by dissolving an alkali metal compound in a previously prepared aqueous stabilizing agent solution. Optionally, the solutions may be prepared separately and blended together. The prepared aqueous solution, generally, has a concentration of from about 0.01 to about 2.0 moles alkali metal compound per litre of solution and about 0.05 to about 5.0 grams of stabilizing agent per litre of solution. Other components may be present in the solution so long as they do not substantially interfere with the desired electrochemical reactions. A preferred solution is prepared by dissolving about 40 grams of NaOH (1 mole NaOH) in about 1 litre of water. Next, 1.5 ml. of an aqueous 1.0 molar solution of the sodium salt of EDTA (an amino carboxylic acid chelating agent) is added to provide an EDTA concentration of 0.5 gram per litre of solution. The preferred solution is ready for use as an electrolyte in an electrochemical cell. In addition to use of the preferred EDTA stabilizing agents above, it has been found that alkali metal phosphates, 8-hydroxyquinoline, triethanolamine (TEA), and alkali metal heptonates are useful stabilizing agents. The phosphates that are useful are exemplified by the alkali metal pyrophosphates. Representative preferred chelating agents are those which react with a polyvalent metal to form chelates such as, for example, the amino carboxylic acid, amino polycarboxylic acid, polyamino carboxylic acid, or polyamino polycarboxylic acid chelating agents. Preferred chelating agents are the amino carboxylic acids which form co-ordination complexes in which the polyvalent metal forms a chelate with an acid having the formula: (A) 3-n ―N―B n where n is two or three; A is a lower alkyl or hydroxyalkyl group; and B is a lower alkyl carboxylic acid group. A second class for use in the process of preferred acids utilized in the preparation of chelating agents of the invention are the amino polycarboxylic acids represented by the formula: wherein two to four of the X groups are lower alkyl carboxylic groups, zero to two of the X groups are selected from lower alkyl groups, hydroxyalkyl groups, and and wherein R is a divalent organic group. Representative divalent organic groups are ethylene, propylene, isopropylene or alternatively cyclohexane or benzene groups where the two hydrogen atoms replaced by nitrogen are in the one or two positions, and mixtures thereof. Exemplary of the preferred amino carboxylic acids are the following: (1) amino acetic acids derived from ammonia or 2-hydroxyalkyl amines, such as, for example, glycine, diglycine (imino diacetic acid), NTA (nitrilo triacetic acid), 2-hydroxy alkyl glycine; di-hydroxyalkyl glycine, and hydroxyethyl or hydroxypropyl diglycine; (2) amino acetic acids derived from ethylene diamine, diethylene triamine, 1,2-propylene diamine, and 1,3-propylene diamine, such as, for example, EDTA (ethylene diamine tetraacetic acid), HEDTA (2-hydroxyethyl ethylenediamine tetraacetic acid), DETPA (diethylene triamine pentaacetic acid); and (3) amino acetic acids derived from cyclic 1,2-diamines, such as, for example, 1,2-diamino cyclohexane N,N-tetraacetic acid, and 1,2-phenylenediamine. Suitable electrolytic cells are described in US-A-4 921 587 and US-A- 4 872 957. Generally, such electrolytic cells for the production of an alkaline hydrogen peroxide solution have at least one electrode characterized as a gas diffusing, porous and self-draining electrode and a cell separator which is, generally, characterized as a microporous polymer film. The cell diaphragm, generally, comprises a microporous polymer film cell separator and preferably, comprises an assembly having a plurality of layers of a microporous polyolefin film cell separator material or a composite comprising a support fabric resistant to degradation upon exposure to electrolyte and said microporous polyolefin film. Generally, the polymer film cell separator can be formed of any polymer resistant to the cell electrolyte and reaction products formed therein. Accordingly, the cell separator can be formed of a polyamide or polyester as well as a polyolefin. Multiple layers of said porous film or composite are utilized to provide even flow across the cell separator irrespective of the electrolyte head level to which the cell separator is exposed. No necessity exists for holding together the multiple layers of the cell separator. At the peripheral portions thereof, as is conventional, or otherwise, the cell separator is positioned within the electrolytic cell. Multiple cell separator layers of from two to four layers have been found useful in reducing the variation in flow of electrolyte through the cell separator over the usual and practical range of electrolyte head. Portions of the cell separator which are exposed to the full head of electrolyte as compared with portions of the cell separator which are exposed to little or no electrolyte head pass substantially the same amount of electrolyte to the porous, self-draining, gas diffusing cathode. As an alternative means of producing a useful multiple layer vertical cell separator, a cell separator can be used having variable layers of the defined porous composite cell separator material. Thus, it is suitable to utilize one or two layers of the defined porous composite material in areas of the cell separator which are exposed to relatively low pressure (low electrolyte head pressure). This is the result of being positioned close to the surface of the body of electrolyte. Alternatively, it is suitable to use two to six layers of the defined composite porous material in areas of the cell separator exposed to moderate or high pressure (high electrolyte head pressure). A preferred construction is two layers of the defined composite porous material at the top or upper end of the cell separator and three layers of the composite at the bottom of the cell separator. For use in the preparation of hydrogen peroxide, a polypropylene woven or non-woven fabric support layer has been found acceptable for use in the formation of the composite cell separators. Alternatively, there can be used as a support layer any polyolefin, polyamide, or polyester fabric or mixtures thereof, and each of these materials can be used in combination with asbestos in the preparation of the supporting fabric. Representative support fabrics include fabrics composed of polyethylene, polypropylene, polytetrafluoroethylene, fluorinated ethylenepropylene, polychlorotrifluorethylene, polyvinyl fluoride, asbestos, and polyvinylidene fluoride. A polypropylene support fabric is preferred. This fabric resists attack by strong acids and bases. The composite cell separator is characterized as hydrophilic, having been treated with a wetting agent in the preparation thereof. In a 0.025 mm (1 ml) thickness, the film portion of the composite has a porosity of about 38% to about 45%, and an effective pore size of 0.02 to 0.04 µm. A typical composite diaphragm consists of a 0.025 mm (1 mil) thick microporous polyolefin film laminated to a non-woven polypropylene fabric with a total thickness of 0.127 mm (5 mils). Such porous material composites are available under the trade designation CELGARD from Celanese Corporation. Utilizing multiple layers of the above described porous material as an electrolytic cell separator, it is possible to obtain a flow rate within an electrolytic cell of about 0.01 to about 0.5 millilitres per minute per 2.54 cm (square inch) of diaphragm, generally over a range of electrolyte head of about 15.24 cm to about 1.83 m (about 0.5 foot to about 6 feet), preferably about 0.3015 to about 1.219 m (about 1 to about 4 feet). Preferably, the flow rate over said range of electrolyte head is about 0.03 to about 0.3 and most preferably is about 0.05 to about 0.1 millilitres per minute per 2.54 cm (square inch) of cell separator. Cells operating at above atmospheric pressure on the cathode side of the cell separator would have reduced flow rates at the same anolyte head levels since it is the differential pressure that is responsible for electrolyte flow across the cell separator. Self-draining, packed bed, gas diffusing cathodes are disclosed in the prior art such as, for example, in US-A- 4 118 305; US-A- 3 969 201; US-A 4 445 986; and US-A- 4 457 953. The self-draining, packed bed cathode is typically composed of graphite particles. However, other forms of carbon can be used as well as certain metals. The packed bed cathode has a plurality of interconnecting passageways having average diameters sufficiently large so as to make the cathodes self-draining, that is, the effects of gravity are greater than the effects of capillary pressure on an electrolyte present within the passageways. The diameter actually required depends upon the surface tension, the viscosity, and other physical characteristics of the electrolyte present within the packed bed electrode. Generally, the passageways have a minimum diameter of about 30 to about 50 µm (microns). The maximum diameter is not critical. The self-draining, packed bed cathode should not be so thick as to unduly increase the resistance losses of the cell. A suitable thickness for the packed bed cathode has been found to be about 0.762 to about 6.35 mm (about 0.03 inch to about 0.25 inch), preferably about 1.524 to about 0.508 mm (about 0.06 inch to about 0.2 inch). Generally the self-draining, packed bed cathode is electrically conductive and prepared from such materials as, for example, graphite, steel, iron and nickel. Glass, various plastics, and various ceramics can be used in admixture with conductive materials. The individual particles can be supported by a screen or other suitable support or the particles can be sintered or otherwise bonded together but none of these alternatives is necessary for the satisfactory operation of the packed bed cathode. An improved material useful in the formation of the self-draining, packed bed cathode is disclosed in US-A-4 457 953. The cathode comprises a particulate substrate which is at least partially coated with an admixture of a binder and an electrochemically active, electrically conductive catalyst. Typically, the substrate is formed of an electrically conductive or nonconductive material having a particular size smaller than about 0.3 millimetre to about 2.5 centimetres or more. The substrate need not be inert to the electrolyte or to the products of the electrolysis of the process in which the particle is used but is preferably chemically inert since the coating which is applied to the particle substrate need not totally cover the substrate particles for the purposes of rendering the particle useful as a component of a packed bed cathode. Typically, the coating on the particle substrate is a mixture of a binder and an electrochemically active, electrically conductive catalyst. Various examples of binder and catalyst are disclosed in US-A- 4 457 953. In operation, the electrolyte solution described above is fed into the anode chamber of the electrolytic cell. At least a portion of it flows through the separator, into the self-draining, packed bed cathode, specifically, into passageways of the cathode. An oxygen-containing gas is fed through the gas chamber and into the cathode passageways where it meets the electrolyte. Electrical energy, supplied by the power supply, is passed between the electrodes at a level sufficient to cause the oxygen to be reduced to form hydrogen peroxide. In most applications, electrical energy is supplied at about 1.0 to about 2.0 volts at about 7.750x10 -3 to 77.5x10 -3 amp/cm 2 (about 0.05 to about 0.5 amp per square inch). The peroxide solution is then removed from the cathode compartment through the outlet port. The concentration of impurities which would ordinarily plug the pores of the microporous cell separator during electrolysis is minimized during operation of the cell in accordance with the process of the present invention. The impurities have been substantially chelated or complexed with the stabilizing agent and are rendered inactive. Thus, the cell operates in a more efficient manner. The following Examples illustrate the various aspects of the process of the invention but are not intended to limit its scope. Where not otherwise specified throughout this specification and appended claims, parts, percentages and proportions are by weight. EXAMPLE 1 (control, forming no part of the present invention) An electrolytic cell was constructed effectively as taught in US-A- 4 872 957 and US-A- 4 891 107. The cathode bed was double-sided, measuring 68.88 cm x 30.48 cm (27 inches x 12 inches) and two stainless steel anodes of similar dimensions were used. The cell separator was Celgard 5511 arranged so that three layers were utilized for the bottom 66.04 cm (26 inches) of active area, and one layer was used for the top 2.54 cm (1 inch) of active area. The cell operated with an anolyte concentration of about one molar sodium hydroxide, containing about 1.5 weight % 41° Baume sodium silicate, at a temperature of about 20°C. The anolyte had a pH of 14. Oxygen gas was fed to the cathode chip bed at a rate of about 3.5 litres per minute. A current density of between about 0.053 and 0.081 amp/cm 2 amperes per square inch) was maintained over a period of 67 days. All anolyte hydrostatic head values are given in inches of water column above the top of the cathode active area. Performance over this period is summarized in Table 1 below, and shows a steady deterioration of current efficiency with time. Cell Performance Characteristics Before Chelate Addition Day of Oper. Curr. Dens. Asi (Amp/cm 2 ) Cell Volt. (Volts) Prod. Flow Rate (ml/min) Anolyte Head Inches (cm) of water Product Weight Ratio (NaOH/H 2 O 2 ) Current Efficy. (%) 1 0.48 (0.074) 2.08 68 42 (106.68) 1.64 89 5 0.45 (0.070) 2.15 57 24 (60.96) 1.57 85 20 0.40 (0.062) 2.24 60 38 (96.52) 1.72 86 40 0.40 (0.062) 2.31 58 44 (111.76) 1.77 77 55 0.34 (0.053 2.40 39 28 (71.12) 1.77 74 64 0.41 (0.064) 2.33 56 46 (116.84) 1.92 73 67 0.41 (0.064) 2.32 55 46 (116.84) 1.94 71 EXAMPLE 2 On day 67, 0.02% by weight of EDTA was added to the anolyte of the cell of Example 1. The first analysis was performed seven hours later. On succeeding days, further EDTA was added to maintain approximately 0.02% by weight in the anolyte feed. The cell performance characteristics over a subsequent 5-day period are shown in Table 2. Cell Performance Characteristics After Chelate Addition Day of Oper. Curr. Dens. Asi (Amp/cm 2 ) Cell Volt. (Volts) Prod. Flow Rate (ml/min) Anolyte Head Inches (cm) of water Product Weight Ratio (NaOH/H 2 O 2 ) Current Efficy. (%) 67 0.50 (0.078) 2.14 76 50(127.00) 2.12 71 68 0.49 (0.076) 2.14 61 36 (91.44) 2.05 68 70 0.49 (0.076) 2.15 63 40 (101.60) 1.94 69 71 0.48 (0.074) 2.15 61 42 (106.68) 1.99 67 The addition of EDTA caused a sudden unexpected improvement in cell performance, notably in the reduced cell voltages and increased product flow rates at the same or lower anolyte heads. If the results are normalized to a similar current density, the improvement can be seen in the reduction in power required to produce one pound of hydrogen peroxide at the same ratio as follows: Day of Oper. Cell Voltage (Volts) Cell (normalised to 0.4 Asi (0.062 amps/cm 2 ) (volts) Current Efficiency % Power Consumpt. KWH/lb KWH/kg 67 2.32 2.29 71 2.29 5.05 70 2.15 1.93 69 2.01 4.43 The results show a substantial lowering of cell voltage at a higher current after addition of 0.02 weight % EDTA to the anolyte. The product flow rate also increased initially and this was reduced by lowering of the anolyte hydraulic head. Most important, the power consumption has been reduced from 5.05 to 4.43 kilowatt-hours per kg (2.29 to 2.01 kilowatt-hours per pound) of hydrogen peroxide. Without desiring to be bound by theory, it is thought that these observations were due to the chelate complexing of transition metal compounds or ions (impurities) that were deposited in the pores of the membrane and/or deposited directly on the composite cathode chips themselves. If insoluble impurities were deposited in the membrane pores, then some current paths would be blocked and the cell voltage would rise. On depositing transition metals on composite chips, it is expected that the hydrophobicity of the chips will decrease, allowing a thicker film of liquid to build up. This in turn would impede oxygen diffusion to the active reduction sites, again resulting in an increase in cell voltage. EXAMPLE 3 On completion of the test described in Example 2, the cell was shut down and the anolyte diluted with soft water and the pH adjusted with sulphuric acid to give a pH of 7. At this point EDTA was added to give a 0.02 weight % solution, and the anolyte was allowed to recirculate through the cell overnight. The anolyte was made up to about one molar NaOH, and contained 1.5% added sodium silicate. On the following day, the cell was restarted. The cell was operated for a six day period, during which the performance characteristics were as shown in Table 4. Cell Performance Characteristics After Chelate Addition at pH 7 Day of Oper. Curr Densty. Asi (Amps/cm 2 ) Cell Volt (volts) Prod. Flow Rate (ml/min) Anolyte Head Inches (cms) of water Prod. Wght. Ratio (NaOH/H 2 O 2 ) Current Efficy. (%) 76 0.36 (0.056) 1.62 56 43 (109.22) 1.90 78 77 0.52 (0.081) 2.02 61 40 (101.60) 1.87 68 78 0.49 (0.076 2.04 59 42 (106.68) 1.82 69 81 0.49 (0.076) 2.10 58 41 (104.14) 1.92 66 In Table 4, the further improvement in cell operation over the previous operation as shown in Example 2, Table 2, is seen in the further lowering of the cell voltage and the further reduction in the cell product ratio to an average of 1.88. Again, the improvement is seen more clearly if the cell voltage is normalized to 0.062 amps/cm 2 (0.4 Asi) and the power to produce one pound of hydrogen peroxide at the same or lower product ratio is compared to operation prior to EDTA treatment. Day of Oper. Cell volt (volts) Cell Voltage (Normalized to 0.4 Asi (0.062 amps/ cm 2 (volts) Current Efficy. % Power Consumpt. KWH/lb (KWH/kg) 67 2.32 2.29 71 2.29 (5.05) (Example 2) 70 2.15 1.93 69 2.01 4.43 78 2.04 1.81 69 1.88 4.14 (Example 3) In Table 5, it can be seen that consecutive treatment of the alkaline peroxide cell with the chelate has improved the power consumption to 4.14 Kilowatt-hours per kg (1.88 kilowatt-hours per pound) of hydrogen peroxide. The action of EDTA may be more effective at the lower, neutral pH than at the higher pH (13.5 to 14.2) at which the cell is normally operated. This is because metal ions, particularly iron ions, can undergo hydrolysis at higher pH values, precipitating metal hydroxide which would impede flow (of fluid and current) through the membrane. EXAMPLE 4 In a commercially operating plant for the production of hydrogen peroxide (the plant electrochemical cells having microporous cell membranes), the failure of the water softening apparatus resulted in the supply water becoming approximately 120 parts per million in hardness (expressed as calcium carbonate) for several hours. The normal process water contains less than 2 parts per million of hardness on the same basis. Subsequent to this hardness excursion, the cell voltages were observed to rise by approximately 100 millivolts. Cell voltages during this period of hardness excursion are shown in Table 6 below. During subsequent operation of the plant, a solution of ethylene diamine tetracetic acid (EDTA) was added to the cell anolyte at a rate so as to maintain a concentration of 0.02% by weight over a period of 3.5 hours. Over this period, the cell voltages fell, as indicated by comparison of the values shown in Table 7 below with those shown in Table 6. It postulated that increased liquid flow through the membrane which occurs subsequent to treatment with EDTA results in reduced voltages at comparable currents. CELL PERFORMANCE AFTER HARDNESS EXCURSION CELL No. VOLT CELL No. VOLT CELL No. VOLT CELL No. VOLT 1 1.869 13 1.709 25 1.977 37 1.806 2 1.827 14 1.698 26 2.036 38 1.736 3 1.739 15 1.670 27 1.836 39 1.664 4 1.908 16 1.741 28 1.670 40 1.752 5 1.700 17 1.641 29 1.698 41 1.670 6 1.920 18 1.792 30 1.789 42 1.756 7 1.778 19 1.778 31 1.850 43 1.753 8 1.747 20 1.786 32 1.717 44 1.787 9 1.677 21 1.700 33 1.895 45 1.870 10 1.773 22 1.844 34 1.733 46 1.731 11 1.833 23 1.938 35 1.748 47 1.839 12 1.778 24 1.625 36 1.775 48 1.752 CELL PERFORMANCE AFTER EDTA TREATMENT CELL No. VOLT CELL No. VOLT CELL No. VOLT CELL No. VOLT 1 1.817 13 1.645 25 1.931 37 1.742 2 1.772 14 1.650 26 2.003 38 1.675 3 1.669 15 1.606 27 1.797 39 1.610 4 1.844 16 1.681 28 1.616 40 1.694 5 1.641 17 1.572 29 1.661 41 1.614 6 1.856 18 1.727 30 1.731 42 1.692 7 1.712 19 1.722 31 1.811 43 1.692 8 1.734 20 1.725 32 1.659 44 1.725 9 1.614 21 1.637 33 1.848 45 1.803 10 1.722 22 1.800 34 1.722 46 1.661 11 1.783 23 1.883 35 1.681 47 1.781 12 1.727 24 1.548 36 1.720 48 1.684

Claims: A method of producing alkaline hydrogen peroxide solution which comprises electro-reduction of oxygen in an alkaline solution as electrolyte using an electrochemical cell containing a microporous polymer film cell separator, maintaining a concentration of stabilizing agent sufficient to complex with or solubilize a substantial proportion of transition metal compounds and other metal compounds or ions present as impurities in the electrolyte and thus to maintain a constant or increased electrolyte flow rate through the microporous film cell separator durinq cell operation. A method according of claim 1, wherein the electrochemical cell comprises a porous, substantially uniform electrolyte flow rate producing, microporous polymer film cell separator and said stabilizing agent is selected from the group consisting of triethanolamine (TEA) and alkali metal heptonates. A method according to claim 2, wherein the microporous polymer film contacts at least one electrode characterized as porous, gas diffusing, and self-draining. A method according to claim 1, 2 or 3, wherein said electrochemical cell comprises a dual purpose electrode assembly comprising: (A) an electrode frame defining an opening, the opening filled with (B) a porous, self-draining, gas diffusion electrode containing an internally positioned current distributor and at least one external face and (C) a liquid permeable, microporous polymer film cell separator contacting each external face of the electrode. A method according to claim 3, wherein the gas diffusing electrode is a cathode and the cell separator comprises plural layers of a microporous polyolefin film. A method according to claim 5, wherein the cathode comprises a bed of particles having pores formed between the particles of sufficient size and number to allow both gas and liquid to flow therethrough and the cell separator comprises a microporous polypropylene film. A method according to claim 6, wherein the cell separator comprises 2 to 4 layers of the microporous polypropylene film or a plurality of layers of a composite comprising the microporous polypropylene film and a support fabric resistant to deterioration on exposure to an aqueous solution of an ionizable compound and electrolysis products thereof, the support fabric being laminated to the microporous polypropylene film. A method according to claim 7, wherein the aqueous solution of an ionizable compound comprises an alkali metal hydroxide and said cell separator has a flow rate of about 0.03 to 0.3 ml per minute per 6.45 x 10 -4 m 2 (square inch) of cell separator over an electrolyte head of 0.1524 m to 1.83 m ( 0.5 foot to 6 feet). A method according to claim 8, wherein the microporous polypropylene film cell separator is characterized as having a porosity of about 38 percent to about 45 percent, an effective pore size of 0.02 to 0.04 µm, and a thickness of about 0.025 mm (1 mil). The method of claim 1 wherein said cell has an electrolyte flow rate of 0.03 to 0.3 millilitres per minute per 6.45 x 10 -4 m 2 (square inch) through said separator at an electrolyte head of 0.1524 to 1.83m (0.5 foot to 6 feet) wherein the concentration of said stabilizing agent is 0.05 to 5grams per litre of electrolyte, and wherein said impurities are present in concentration of greater than 0.1 part per million in said electrolyte. A method according to claim 1 wherein said cell is periodically shut down, the pH of said electrolyte is lowered to about 7, and said electrolyte containing a concentration of a stabilizing agent sufficient to complex with or solubilize a substantial portion of the transition metal compounds or ions, or other metal compounds or ions, present as impurities in said electrolyte is recirculated. The method of claim 11 wherein said stabilizing agent is a chelating agent which is the reaction product of a metal and an acid selected from the group consisting of an amino carboxylic acid, a polyamino carboxylic acid, an amino polycarboxylic acid, and a polyamino polycarboxylic acid. The method of claim 11, wherein said stabilizing agent is selected from the group consisting of an alkali metal salt of ethylene diamine tetraacetic acid (EDTA), an alkali metal salt of diethylene triamine pentacetic acid (DTPA), alkali metal stannates, alkali metal phosphates, 8-hydroxyquinoline, triethanolamine (TEA) and alkali metal heptonates. The method of claim 13 wherein said electrochemical cell comprises a porous, substantially uniform, electrolyte flow rate producing, microporous polypropylene film cell separator. The method of claim 11 additionally comprising restarting the operation of said cell.

Inventors: CLIFFORD ARTHUR L, DONG DENNIS, ROGERS DEREK J, CLIFFORD, ARTHUR L., DONG, DENNIS, ROGERS, DEREK J.
Applicants: H D TECH INC, H-D TECH INCORPORATED
Application year: 1992
Main classification: C25B1
IPC codes: C25B1