SE446104B - WHEN OPERATING AN ELECTRIC LIGHT CELL WITH ANODO AND CATHODE REDUCE THE DISTANCE BETWEEN CELL ELECTROPRODES - Google Patents

Description

For porous membranes, it is desirable to minimize the distance between the electrodes (ie, the intermediate electrode gap), which reduction has a remarkable effect on the operating voltage and thus on the energy efficiency of the electrolytic process.

Commercial membranes are sensitive to current density, which must be maintained within certain optimal limits for efficient membrane operation. The current density must be almost constant over the entire surface to avoid the occurrence of mechanical and electrical stresses, which would irreversibly destroy the diaphragm.

In the known membrane cells the optimization of these parameters largely depends on design tolerance limits and due to the size of the electrode surfaces in the commercial cells relative to the smaller electrode gaps (of the order of a few millimeters) the inevitable deviations from the most accurate parallelism between anodes cause and the cathode surfaces more or less marked variations of the current density across the surface of the membrane. As a result, previous efforts to ensure a correct current tightness locally on different surfaces of the membrane are not successful.

According to the invention, the problem is solved by pressing the membrane applied to the anode against the porous anode surface of a cathodically polarized, open, static bed of electrically conductive, catholyte-resistant, porous filling material, which bed substantially fills the space between the membrane and the walls of the cathode chamber, which are electrically conductive, L intends to maintain a substantially constant and minimal gap between the electrodes to maintain the current density in the cell substantially constant, so that mechanical and electrical stresses on the membrane are minimized.

The solution according to the invention is both original and effective by actually pressing the membrane between the working perforated surfaces of the anode and the cathode, and thus ensures both a uniform and substantially minimal spacing of the electrodes and for uniform distribution. of the current density across the membrane and for minimal ohmic losses in the electrolyte. Furthermore, these optimal conditions are achieved with almost total relief from design tolerances.

The preferred embodiment of the cell according to the invention comprises a cathode container of steel or other conductive material, which is resistant to corrosion in the catholyte environment and which is closed at the upper end with a plate or lid of titanium or other anodic oxide film-forming metal, which is passable under anodic polarization conditions and having at least one but preferably a series of tubular anodes welded into holes in the titanium cover plate, which extend over almost the entire depth of the container with the walls of the tubular anodes (except the top of the anode walls near the welds at the titanium plate) perforated to be permeable to liquids and gases.

The anodes are dimensionally stable and typically those of titanium or other anodic oxide film-forming metal are coated on at least a portion of the active surface with an electrically conductive, electrocatalytic deposit of materials resistant to the anodic conditions and non-passivable, preferably a deposit of precious metals such as platinum. , palladium, rhodium, ruthenium and iridium or oxides or mixed oxides thereof. The lower ends of the tubular anodes are closed by plugs of inert material, preferably plastic material provided with coaxial, threaded holes. The permeable walls of the tubular anodes are completely covered on the outside of the membranes to define the anode space inside the tubular anodes.

The lower end of the container is closed by a plate preferably of inert plastic material and is provided with means for feeding saline or other anolyte to the interior of the various tubular anodes, typically by means of inlets. of plastic material, the flanges of which form a seal against the bottom plate of the container. The anolyte is fed through tubular connectors screwed into the threaded holes of the closure plugs of the tubular anodes.

The container is provided with an outlet in the upper part for the outlet of cathode gas, with a discharge opening in the lower part for discharging the catholyte and with an inlet pipe for recirculation of diluted catholyte or water to the cathode chamber.

The anodes, which are welded to the container lid, communicate through the holes in the lid with a chamber above the container, where the anode gas is separated from the electrolyte, escapes from an outlet and flows to a gas recovery system and the electrolyte is recycled to a saturation system.

The cathode in the cell consists of a porous static bed of loosely conductive cathode material in the form of chips, beads, spheres, cylinders, Raschig rings, metal wool or other particles, with which the container is completely filled to a height corresponding to at least the height of the permeable walls of the tubular anodes, which are covered by the membranes. The filling of cathode material is in contact with the inner walls of the container and with the outer surfaces of the membranes of the various tubular anodes and presses against the membranes. The conductive cathode filler material may be graphite, lead, iron, nickel, cobalt, vanadium, molybdenum, zinc or alloys thereof, compounds between metals, hydrating compounds, carbiding compounds and nitridating compounds of metals or other materials which have good conductivity and good resistance force against the cathode conditions.

Materials having low hydrogen overpotential, such as iron, nickel and alloys thereof, are particularly suitable for saline electrolysis; For example, for the reduction of FeIII to FeII in acid sulfate catholyte solution with an anionic membrane and oxygen evolution on the anode, particulate matter with high hydrogen overpotential, such as lead and lead alloys, are preferred. The cathode filling material may also comprise plastic, ceramics or other inert non-conductive material coated with a layer of the mentioned electrically conductive and cathodically resistant materials.

The titanium plate or lid, at which the tubular anodes are welded, is insulated from the cathode chamber by an insulating gasket. It is connected to the positive terminal of the power distribution network and the cathode chamber is connected to the negative terminal of the distribution network.

The mass of the cathode charge is cathodically polarized and acts as a cathode and the porosity of the static bed of the cathode material allows rapid development of the cathode gas and helps to cathodically protect the inner walls of the cathode container.

The electrode gap is reduced to slightly more than the thickness of the membranes by the local deflection of the electrolytic current flow loops on the geometrically undefined surfaces of the cathode material, represented by the particles in the bed directly adjacent the surfaces of the membranes and on the geometrically undefined surfaces of the the walls of the tubular anodes on which the membranes are applied.

The gap between the cathode filler material and the anodes remains substantially constant throughout the electrolysis process.

This configuration of the cell provides excellent uniformity of the current density throughout the electrode area without sudden, localized differences, which would seek to destroy the membranes by the occurrence of mechanical and electrical stresses. Another advantage of the preferred embodiment of the cell according to the invention, which comprises a plurality of tubular anodes, is its compactness, since the ratio between the extent of the cathode surfaces and the volume occupied by the cell is very larger than in previous commercial membrane cells.

In the drawing figures, the anodes are shown as circular tubes in a rectangular container, which is the preferred construction due to greater uniformity of the current density and lower cost. It will be appreciated that anode tubes of other shapes, such as oval, rectangular, hexagonal and other polygonal shapes, may be used and are within the scope of the word "tube", as used herein, and that the cell container may be rectangular, cylindrical or have other shapes. A less preferred embodiment of the invention is a cylindrical container enclosing a single concentric, cylindrical anode. According to this embodiment, however, a number of cells are necessary to achieve the desired capacity. It will also be appreciated that although the cell has been described in connection with the production of chlorine, it may be used for electrolytic processes which yield other products.

In the accompanying drawing, which shows the preferred embodiment of the invention, Fig. 1 is a sectional view of a typical embodiment and Fig. 2 is a sectional plan view along the line 1-1 in Fig. 1 with parts above the section line shown in broken lines.

As shown in Fig. 1, the cell comprises a rectangular cathodic container 1 of steel or nickel or alloys thereof or of other conductive and catalytically resistant material. A lid 2 of titanium or other anodically passivable valve metal is bolted to the container 1 and closes the container at the top. An insulating gasket 3 is arranged between the cathode container 1 and the titanium lid 2. Tubular anodes 4 of titanium are welded in holes in the lid 2 and extend above the lid as shown. The walls of the tubular anodes 4 are provided with holes or other perforations, which start at a small distance under the lid 2 and extend to the bottom of the anodes 4. The perforated portions 6 of the anodes may be formed of mesh-like or expanded titanium sheet welded to the impermeable cross-section 5 or formed connected thereto. The surface of the perforated portions 6 of the tubular anodes 4 is suitably coated with an electrocatalytic deposit which is non-passivable and resistant to the anodic conditions, typically containing noble metals or oxides of noble metals. The tubular anodes are closed at the upper end by a plug or closure 7 of titanium welded at the lower end of each anode 4 or preferably as indicated in Fig. 1 of chemically resistant plastic material, for example polyvinyl chloride or the like, provided with a coaxially threaded hole 7a.

The cationic membrane 8, preferably tubular, is drawn over the anode 4 and attached to the impermeable upper part of the anodes and to the outer cylindrical surface of the plug 7 by means of strips of plastic material 9. This attachment is particularly easy and provides a perfect hydraulic seal between the membranes and the perforated sections of the anodes 4, which is difficult to obtain in conventional filter press cells.

The cationic membrane 8 is preferably permeable to cations and impermeable to the hydrodynamic flow of liquid and gas. Suitable materials for the membranes other than fluoridated polymers or copolymers containing sulfone groups. Such materials are sufficiently flexible and are produced in tubular form by extrusion or hot gluing of flat sheets. The thickness of such membranes is of the order of one tenth of a millimeter.

The container 1 is turned 180 ° to facilitate filling and is filled with the cathode material 10. The container is then closed with a rectangular plate 11 perforated at the bottom of each anode 4 and preferably of inert plastic material. A rectangular saline distribution box 12 also of inert plastic material is welded to the plate 11 and is closed by a closure plate 13 equipped with a saline inlet opening 14. A gasket may be arranged between the plate 11 and the flange bottom of the rectangular container 1.

The flanges of the plate 11 may be bolted to the bottom flange of the container 1 and the closure plate 13 may be bolted to the bottom of the distribution box 12. The saline distribution box is connected to the interior of the anodes 4 by means of tubular connectors 15 which are flanged at one end and screwed into the threaded holes 7a of the closure plugs 7. Seals or gaskets 15 are arranged between the flanges of the connecting members 15 and the saline distribution box 12.

The cathode chamber is filled with particulate material to approximately the top of the permeable sections 6 of the tubular anodes 4.

The cathode container is near the upper part at a level above that of the particulate bed 10 provided with one or more outlets 17 for hydrogen and in its lower part with at least one adjustable gooseneck outlet 18 for dispensing the catholyte.

A manifold or spray tube 24 above the level of the particulate material 10 extends horizontally over substantially the entire length of the container 1 and is provided with a series of holes to allow the supply of water or catholyte to the cathode chamber for dilution and control of the concentration of alkali metal hydroxide, which generated in the cathode chamber.

Preferably, water is continuously supplied to the cathode space through the manifold 24 to dilute the hydroxide formed on the cathode and maintain the hydroxide concentration of the catholyte drain medium from the cell between 25 and 43% by weight.

Each of the tubular anodes 4 is connected at the top to a rectangular tank 19, which extends over the entire top of the cell container 1. The electrolyte level of the tank 19 is constantly maintained by a gooseneck discharge tube 20 for the electrolyte. The electrolyte discharged from the tube 20 is fed to a resaturation system before being returned to the cell through the electrolyte inlet 14.

The halogen generated on the anodes is separated from the electrolyte in the tank 19 and escapes through the outlet 21.

The plate or lid 2, at which the tubular anodes 4 are welded, is directly connected to the positive terminal of the electric power supply system by means of the connection 22 and the cathode container 1 is connected to the negative terminal by means of the connection 23.

Fig. 2 is a sectional view taken along line 1-1 of Fig. 1 with the elements of the cell described in connection with Fig. 1 indicated by the same reference numerals. The location of the manifold 24 is indicated by dashed lines above the level of the particles of the cathode material 10 in the cathode container 1.

The cell shown comprises six tubular anodes in a rectangular housing but it will be appreciated that the number of anodes may vary in the transverse direction, that several rows of anodes may be used, that the shape of the cell and anodes may be different from that shown and that other modifications and changes can be done within the scope of the invention.

The extent of the cylindrical surfaces of the tubular anodes 4 is very large relative to the volume of the container 1, allowing high production rates in a compact cell with substantially uniform current density through the cell compared to cells commonly used. commercially. In operation, concentrated brine (120-310 g / l), for example, of NaCl is fed through the inlet 14 to the distribution box 12 and rises through each of the tubular anodes 4, on which electrocatalytically coated surfaces chlorine is formed. The sodium ions pass through the cationic membrane and combine with the hydroxyl ions, which are released on the cathode by electrolysis of the water to form sodium hydroxide. The chlorine rises through the electrolyte contained within the tubular anodes 4, and to the tank 19, where it separates from the liquid and escapes through the outlet 21. The rising chlorine bubbles give a rapid upward flow of the electrolyte in the tubes 4.

The leached brine flows through the constant level outlet 20 and is returned to the resaturation system, before being returned to the cell through the inlet 14.

The hydrogen released on the surfaces of the porous cathode bed adjacent the membrane 8 rises through the particle bed 10 and is collected in the upper space of the cathode container, from which it escapes through the outlet 17. The sodium hydroxide solution is released through the adjustable gooseneck 18. The adjustable gooseneck 18 maintains level of the catholyte at substantially the same level as the top of the cathode bed 10.

The catholyte can be passed through a sodium hydroxide recovery system located outside the cell and the dilute sodium hydroxide solution effluent is reintroduced into the cathode space I through the manifold 24.

The operating temperature can vary between 30 and 100 ° C and is preferably maintained at about 85 ° C. The pH of the anolyte can vary between 1 and 6 and the current density can vary between 1000 and 2000 A / m 2.

Although the cell of the invention has been described in connection with

Claims (3)

In the drawings, it will be appreciated that numerous modifications and alternatives may be employed in constructing the cell within the scope of the invention, that other electrolysis processes may be performed in the apparatus described and that instead of titanium other anodic oxide film-forming metals such as tantalum, zirconium, molybdenum, niobium, tungsten and yttrium can be used in the construction of the cell. P a t e n t k r a v A method of reducing the distance between the cell electrodes when operating an electrolytic cell with anode and cathode space, the cell comprising at least one electrocatalytically coated porous anode (4) of anodic oxide film-forming metal which is permeable to both gas and electrolyte in the anode space. , a porous cathode permeable to both gas and electrolyte in the cathode space, an ion-selective cationic ion exchange membrane (8) substantially impermeable to liquid and gas flow and separating anode space and cathode space, means for passing electrolytic current through the cell and means for collecting due to electro lysed anode gas and liquid separated from cathode gas and liquid, characterized in that the membrane (8) applied to the anode is pressed against the porous anode surface of a cathodically polarized, open, static bed (10) of electrically conductive , catholyte-resistant, porous filling material, which bed substantially fills the space between the membrane and the walls (1) of the cathode chamber, which is electrically conductive, in order to maintain a substantially constant and minimal gap between the electrodes in order to maintain the current density in the cell substantially constant, so that mechanical and electrical stresses on the membrane are minimized. 2. A method according to claim 1, characterized in that as the cathode filling material (10) any of the materials graphite, lead, iron, nickel, cobalt, vanadium, molybdenum, zinc or alloys thereof, compounds between metals, or compounds formed by hydridization, carbidization and nitridation of metals. 3. A method according to claim 1 or 2, characterized in that materials in the form of chips, spheres, beads, cylinders, Raschig rings, compressed metal wool or wire or particles of other shapes are used as cathode filling material (10).