MXPA98006399A - Cell of improved electrolysis of type diaphragmapara chlorine-alc electrolysis - Google Patents

Abstract

The invention relates to a chlor-alkali diaphragm electrolysis cell, comprising a cover, a conductive base for supporting the anodes and a cathode in the form of a box provided with an internal wall, an external wall and tubular fingers made of mesh or perforated sheet covered with a porous diaphragm. One or more copper sheets for the distribution of the electric current are fixed to the external walls of the cathode. The connection between the copper sheets and the external walls of the cathode is effected by means of screws with the interposition of a deformable conductive element provided with residual elasticity on compression. The welds for the assembly or assembly of the cathode walls are free of stress or internal stress

Description

IMPROVED ELECTROLYSIS CELL OF DIAPHRAGM TYPE FOR CHLORINE-ALLCALI ELECTROLYSIS BACKGROUND OF THE INVENTION The production of chlorine and caustic soda by the electrolysis of aqueous solutions of sodium chloride (hereinafter defined as brines) is one of the most important industrial processes. Chlorine is, in fact, the raw material necessary to obtain a wide variety of solvents, intermediate chemical compounds and plastic materials, such as perchlorethylene, propylene oxide, polyvinyl chloride and polyurethane. Chlor-alkali electrolysis is normally carried out using three different technologies, that is, the diaphragm, the mercury cathode and the membrane technology. Membrane technology has been developed in recent years and is currently used in the construction of new plants. However, much of the global production of chlorine and caustic soda is still obtained through diaphragm and mercury technologies, which experienced a slow evolution over time in terms of energy savings, operation reliability and control. of pollution due to the possible emission of the fibers used to P1484 / 98MX produce the diaphragm or mercury leaks. From the economic point of view, this continuous improvement made, in fact, less interesting or attractive to replace existing diaphragm or mercury plants by modern membrane cells. In particular, as regards the diaphragm cells, which are the object of the present invention, their structure consists essentially of three parts: a cover, a base on which the anodes are fixed and a cathode provided with elements of hollow interior with a rather flat section, known as fingers, interspersed between the anodes. The base structure is clearly illustrated in U.S. Patent No. 3,591,483. It preferably comprises a conductive sheet, such as for example a copper plate, provided with holes to which the anodes are fixed. The side of the plate facing the anodes is protected by a rubber sheet or, preferably, by a thin sheet of titanium. The anodes may be in the form of a box, as described in U.S. Patent No. 3,591,483. However, in a more advanced solution, as described in U.S. Patent No. 3,674,676, the anodes comprise two opposed moving surfaces supported by flexible means that allow them P1484 / 98MX expand by minimizing the distance anode-cathode fingers and the consequent reduction of cell voltage, that is, energy consumption. The cathode structure is still as described in U.S. Patent No. 3,390,072. This comprises a hollow box (without cover and without base), whose outer wall is made of four carbon steel plates welded along their vertical edges. The box is additionally provided with an internal wall which is welded to the same fingers made of perforated sheet or metal mesh, covered by a porous diaphragm. The geometry of the connections between the external, internal walls and the fingers has been optimized as described in DE 4117521A1, which specifies the dimensions of the various parts, which allows the minimization of the corrosive action of the catholyte on the steel to the carbon. The porous diaphragm deposited on the fingers is made of a mixture containing asbestos fibers or other inert materials such as zirconium oxide and a polymeric material. The mixture, in the form of a suitable aqueous suspension, is deposited by vacuum filtration. The polymeric material provides the binder function that is obtained by subjecting the cathode, with the diaphragm deposited on its fingers, to a heat treatment at 250-350 ° C in a suitable stove. Temperature P1484 / 98MX appropriate and the necessary time are selected depending on the polymeric material used. Suitable materials are polymers with different degrees of fluorination, such as polyvinylidene fluoride, ethylene-chlorotrifluoroethylene copolymers and polytetrafluoroethylene. In order to improve the current distribution to the fingers, the thickness of the external wall must be selected appropriately. The aforementioned U.S. Patent No. 3,390,072 describes the use of one or more copper sheets applied to the outer wall to avoid the use of excessively thick carbon steel plates. These copper sheets can be applied by arc welding or by explosion adhesion. This second method, although much more expensive, is the most commonly preferred, since it ensures a homogeneous electrical contact throughout the interface between copper and carbon steel. Conversely, in the case of copper sheets applied by arc welding, the electrical contact is located essentially in the welded areas. Therefore, in the latter case, the copper sheets are less efficient to distribute homogeneously the electric current between the various fingers and to minimize the losses, that is, the dispersion of the electrical energy due to the resistance P1484 / 98MX electric structure. While the performance of both the cover and the conductive base provided with the anodes is satisfactory, the cathode, as briefly illustrated, is adversely affected by more serious disadvantages or drawbacks, which the present invention seeks to overcome, as explained in the following discussion. These drawbacks can be summarized as follows: a) fractures in the welding areas that connect the plates of the external wall, the inner wall and the cathode fingers. This problem, known in the art, is well represented in the Figure on page 176 of the "Corrosion Data Survey", NACE Editions, 1985. From the Figure, it is very soon clear that certain combinations between the concentration of the caustic soda and the temperature cause fractures in the carbon steel parts that have internal stresses or stresses, such as welded headers. The Figure also indicates that the fractures are removed if the carbon steel parts are subjected to a thermal treatment of release or stress relief. This treatment, which consists of heating at 600 ° C for about an hour, can not be applied to the cathodes of the prior art, due to the large difference between the coefficients of P1484 / 98MX thermal expansion of carbon steel and copper, which could cause sharp distortions. On the other hand, the heat treatment only to the structure of the carbon steel would be useless, since the subsequent welding of the copper sheets would again involve internal stresses. This situation imposes limitations both on the concentration of the caustic soda produced at the cathode and on the electrolysis temperature, which reduces but does not eliminate the risk of fractures. b) distortions in the cathode structure and fractures in the weld areas between the copper foil and the carbon steel walls due to thermal fatigue during the stabilization phase of the diaphragm at 250-350 ° C. These problems are also due to the different coefficients of thermal expansion of copper and carbon steel, as mentioned above. Even if the diaphragm stabilization temperatures are substantially lower than those that are normal for the release or stress relief treatment, the inconveniences are equally serious since most diaphragms commonly used today have an average life of 9- 15 months and, therefore, its preparation, including stabilization, is repeated more than once during the lifetime of the cathode. c) contamination of the copper salt of the P1484 / 98MX suspension used to deposit diaphragm. Since the cathode is completely submerged in the tank containing the suspension and since the suspension contains significant amounts of chlorides and is saturated with air, inevitably both the carbon steel parts and the copper parts will be subject to corrosion. The progressive accumulation of the copper concentration in the suspension can lead to the degradation of the quality of the diaphragm, in particular, that of the diaphragms of greater value which were foreseen for a longer useful life. It is an object of the present invention to provide a novel cathode structure made of separable parts, which overcomes the disadvantages of the aforementioned prior art.

SUMMARY OF THE INVENTION The present invention relates to a chlor-alkali diaphragm electrolysis cell equipped with an improved cathode, characterized in that the copper sheet or sheets for the distribution of the electric current are not integrated to the cathode but can be disconnect easily. Therefore, the carbon steel structure, after assembling the various parts P1484 / 98MX d by welding but, without the copper sheets, it can be subjected to a thermal stress-release treatment before the operation of the electrolysis cell. Additionally, the carbon steel structure can be sent alone to the furnace for stabilization of the porous diaphragm after each re-deposition. In order to improve the distribution of current between the carbon steel structure and the copper sheet or sheets, an element of high conductivity is interposed, which can be made either of a deformable layer interposed between the copper foil and the steel surface of the outer wall or of a layer thermally applied to the steel surface or of a combination thereof. By the present invention, all drawbacks which negatively affect the cathodes of the prior art are avoided, i.e., fractures during operation, distortions during the stabilization phase of the diaphragm and contamination of the aqueous suspensions used for the deposition of the diaphragm. Additionally, for the cathodes of the present invention, any limitation of the caustic soda concentration produced and the electrolysis temperature may be due solely to process reasons and not to the need to maintain the integrity of the cathode structure over time.

P1484 / 98 X BRIEF DESCRIPTION OF THE DRAWINGS The invention will be illustrated with reference to the Figures, in which: Figures 1, 2 and 3 are exploded views of the components of the connection system between the copper foil and the external wall of the Carbon steel of the cathode of the invention. Figure 4 illustrates the system of Figure 2 after assembly. Figure 5 shows a different design of the screwing arrangement of Figure 4. Figure 6 is a diagram showing the ohmic drop in the connection of Figure 2 as a function of both the different materials and the mechanical load applied by means of of the screws. Figure 7 is a diagram of a further cross section of an external wall of the cathode of the invention that includes the connection system of Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In Figure 1, the external wall 1 of the cathode of the invention is provided with threaded holes 2 to house the screws 3, which have the capacity of P1484 / 98MX press the copper foil 4 against the outer wall. The outer wall 1 is provided with a high conductivity element 12 consisting of a metallic layer applied thereto by thermal spraying or metallization methods, such as torch or plasma metallization. Contrary to the teachings of some prior art, the adjustment of the spray or metallization machine is such that the layer of the conductive element 12 is provided with pores, that is, it is porous. Experimental data have shown that the porosity, defined as the ratio of the volume of voids to the volume of solids, should be at least 10% and preferably 20 to 30%. The porosity is necessary, because with the assembly of the components shown in Figure 1, some deformability of the conductive element 12 is required to compensate for all the deviations of the flatness of the contact surfaces. Referring now to Figure 2, a further embodiment of the invention is illustrated, wherein the high conductivity element 5 separating the copper foil 4 and the outer wall 1 is a material showing deformation properties and residual elasticity to deformation. This material can be selected from the group consisting of single or superimposed meshes, sheets or sheets of expanded metal without P1484 / 98MX flatten, metallic foams, such as for example those of the type marketed by Sumitomo, Japan, under the trade name Cellmet®. Figure 3 represents a particularly preferred embodiment of the invention, wherein the external wall 1 of the cathode of the invention is provided with the conductive element 12 of Figure 1 and the deformable element 5 of Figure 2 is additionally placed or located between the wall external 1 and copper foil 4. In this case, both elements 5 and 12 cooperate to deform as much as required for optimum continuous contact between the surfaces of the wall 1 and the copper foil 4; in addition, the element 12 provides the lowest resistance interface both towards the outer wall 1 thanks to the metallurgical connection between the carbon steel of the wall 1 and the sprayed metal particles, and towards the element 5 thanks to the typical surface of conductive oxide of the metals of both elements 5 and 12. When the components of Figure 2 are assembled or assembled (Figure 4), each screw 3 can apply a load in the range of 5-10 tons with a pressure between the sheet of copper 4, the deformable conductive element 5 and the outer wall 1 in the range 0.5-2 kg / mm. As shown in Figure 5, in order to P1484 / 98MX improve the stability of the contact pressure, the threaded holes 2 can be in a receptacle 6 which by welding 7 is fixed on the side of the external wall 1 opposite that which is in contact with the copper foil 4. In addition , between the head of the screw 3 and the copper foil 4, a suitable spring, not shown in the figures can be inserted for reasons of simplicity, in order to keep the pressure exerted by the screw as constant as possible, independently of dimensional modifications caused by temperature variations. The connection between the copper foil 4 and the outer wall 1 of the invention can be provided with a peripheral package, not shown in the Figures, which ensures the sealing of the contact area and avoids the risk of corrosion in the area of the interface of contact due to aggressive agents that may be present in the surrounding environment. The packaging also has the function of preventing the possible washing liquids of the electrolysis cell from penetrating the contact area, causing the oxidation of the carbon steel surface. The carbon steel surface only needs to be free of rust, which is easily achieved by sand blasting. As explained above, there is no need to machine, since P1484 / 98 X possible deviations of the profile are easily compensated by the conductive elements 5 and / or 12 of the invention. Figure 6 shows the ohmic drops of the cathode connection of Figure 2 as a function of the clamping or closing pressure, the type of conductive element and the improvement achieved by the addition of a conductive grease, such as it can be Alcoa EJC, No. 2. The current density through the connection is 0.25 A / mm, that is, approximately twice the current density typical of normal industrial operation. As regards the type of metal used for conductor elements 5 and 12, the results obtained indicate that silver or nickel ensure better performance than copper, but copper is also acceptable. When a metal foam is used as in the connection in Figure 2, it can be characterized by 80 pores per inch (ppi), whose behavior is shown in Figure 6. However, acceptable results have also been obtained with 30 pores per inch. Only with thicker foams, on the order of approximately 7 ppi, the results have been less satisfactory. Figure 7 shows a cross section of the outer wall of an improved cathode, provided with the P1484 / 98MX connection system of the invention and with pins for current transmission. The various parts are identified by the same numbers used in the other Figures. The inner wall 8 has several anodic fingers fixed thereto and the pins 9 are fixed by the welds 10 and 11 to the outer wall 1 and the internal wall 8. The pins 9 allow the transfer of the electric current directly from the area of contact between the copper foil 4 and the outer wall 1 towards the inner wall 8 and then towards the fingers covered by the diaphragm. This arrangement makes it possible to shorten the path of the electric current from the copper foil to the fingers and, therefore, reduce ohmic drops, that is, the dispersion of electrical energy. The use of pins is known in the art but was limited to the upper and lower portions of the outer wall with respect to the copper sheet. In fact, until now, it was not possible to weld the spikes in correspondence with the central area of the copper foil to avoid damaging the carbon / copper steel interface. The present invention solves this problem since the copper sheets are only subsequently applied and, therefore, this limitation is eliminated. A further objective of the present invention is to provide a process for the preparation of the cathode for P1484 / 98 X the cell of the present invention. This process is directed towards the preparation of a cathode whose welding is free of internal stresses. This is obtained by subjecting the structure made of carbon steel, without the copper plates, to a thermal treatment of release or stress relief, as a guide, at 550-600 ° C for one hour. The carbon steel structure is subsequently subjected to the diaphragm deposition process. A further objective of the present invention is to provide a process for the preparation of the diaphragm of the cell. This process is characterized in that the carbon steel structure of the cathode, which has been thermally relaxed and is again without the copper plates, is subjected to the deposition of the diaphragm in accordance with the known procedures and to its stabilization by treatment in an oven or stove, as a guide, at 250-350 ° C, depending on the type of polymeric binder used. Only at the end of this treatment, the cathode structure is connected to the copper plates, as described above. Even though the invention has been described with reference to the specific modalities, it must be understood that modifications, substitutions, omissions and changes to it are possible without deviating from the spirit of the invention.

P1484 / 98MX and are intended to be included in the appended claims. P1484 / 98MX

Claims (14)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property: 1. A cell for chlor-alkali electrolysis with diaphragm comprising: a cover; a conductive base that supports the anodes; a cathode in the form of a box provided with an outer wall and an inner wall, assembled from carbon steel plates by means of welding; the cathode comprises one or more copper sheets for conducting and distributing electric current and tubular fingers made of mesh or perforated sheet covered by a porous diaphragm deposited from an aqueous suspension of fibers and polymeric material, the fingers are fixed to the inner wall, the cover and the cathode are provided with an inlet and outlets for feeding the brine and the discharge of the chlorine and hydrogen released and the produced caustic soda, characterized in that one or more copper sheets are fixed to the external wall by means of screws and a conductive element is interposed in the middle, the conductive element has the ability to deform and maintain the elasticity to deformation and, because the one or more copper sheets and the P1484 / 98MX cathode are easily disconnected. The cell according to claim 1, characterized in that the conductive element is made of nickel, silver or copper. 3. The cell according to claim 1, characterized in that the conductive element is made of one or more superimposed meshes or unrolled expanded metal sheets. The cell according to claim 1, characterized in that the conductive element is a metal foam. The cell according to claim 1, characterized in that the conductive element is a metal layer applied to the external wall by spraying or thermal metallization. The cell according to claim 1, characterized in that the conductive element comprises a metallic foam and a metal layer applied to the external wall by spraying or thermal metallization. The cell according to claim 1, characterized in that it additionally comprises a spring inserted between each of the heads of the screws and the copper foil. The cell according to claim 1, characterized in that it additionally comprises a packaging P1484 / 98MX inserted between the copper foil and the external wall of the cathode along the periphery of the conductive element. The cell according to claim 3 and 4, characterized in that the surfaces of the external wall that are in contact with the conductive element are covered with a conductive grease. The cell according to claim 1, characterized in that the welds are free of internal stresses or stresses. The cell according to claim 1, characterized in that it additionally comprises spikes applied to the external walls to connect the internal walls and fingers in the area corresponding to one or more of the copper sheets. 12. A process for the manufacture of the cathode of the cell according to claim 10, characterized in that the free welds of stresses or internal stresses are obtained by heat treatment of relieving or releasing cathode voltages without the one or more sheets of copper. 13. The process for producing the diaphragm of the cell according to claims 1-11, characterized in that the cathode is immersed in a suspension of fibers and polymeric binder for deposition by vacuum filtration when removing one or more sheets of P1484 / 98MX copper. The process according to claim 13, characterized in that after the deposition of the diaphragm, a stabilization phase is carried out by heating the cathode covered with the diaphragm at 250-300 ° C without the one or more sheets of copper. P1484 / 98MX