MXPA02004576A - Improved design of diaphragm electrolyser - Google Patents

Abstract

The invention relates to an improved design of electrolyser, in particular to the design of a diaphragm electrolyser for the production of chlorine and alkali from aqueous solutions of alkali chlorides, comprising at least one anode fixed to an anodic base (11) and in electrical contact therewith by means of a current collecting stem separated from the electrolyte circulating in the anodic compartment by means of a hydraulic seal system (16), wherein a conductive and deformable contact element (18) is interposed between said current collecting stem and said anodic base, and wherein the hydraulic seal system comprises at least one O ring.

Description

ELECTROLYZER DESIGN FOR IMPROVED DIAPHRAGM FIELD OF THE INVENTION Chlorine production is one of the most widespread processes on the world stage of industrial chemistry.

BACKGROUND OF THE INVENTION The current annual production, which can be estimated at around 50 million tons, comes almost entirely from the electrolysis of alkali chlorides in aqueous solutions; in these processes, chlorine is developed through the anodic discharge of chloride ions, typically with the concomitant production of alkali to the cathodic compartment; in the most typical case, the hydrogen evolution reaction also occurs at the cathode. The last of the three types of electrolytic cell most commonly used for this purpose - mercury cathode, diaphragm and cathode - the latter is still the largest global amount of chlorine produced on the world market. Figure 1 shows a modern diaphragm cell, comprising an anode base (1), made of a copper body covered with a thin sheet of titanium, in which the anodes (2) are held by means of current collector shafts. copper (4), also protected with a titanium coating. The reason for these bimetallic constructions comes from the fact that copper, used for its excellent electrical properties, would be easily corroded by the anolyte (chlorinated brine), against which, on the contrary, titanium shows good resistance characteristics. The cathode (3), on one side of which, and precisely on the side facing the anode, is a diaphragm, is made of perforated sheets or iron meshes. The lid (5), made of a chlorine-resistant plastic material, is provided with an outlet conduit for the produced chlorine gas (6) and an inlet conduit for the brine feed (not shown). The hydrogen and the alkaline solution (for example, caustic soda solution) produced at the cathode leave respectively conduits (7) and (8). The diaphragm, which has the purpose of separating the anodic and cathodic compartments, was traditionally made of asbestos fibers and a plastic binder; the need to abandon the use of asbestos, harmful to health, together with the search for higher yields and longer life of the elements, has led to a radical study of traditional diaphragms as regards materials. The diaphragms of today are typically made of zirconium oxide fibers, or plastic materials.

SUMMARY OF THE INVENTION While diaphragms based on asbestos were the components that determined the lifetime of the entire cell (an average of 10-14 months), the availability of new generation diaphragms, known as "NAD" ("non-asbestos") diaphragm ", diaphragm without asbestos) would allow to extend the operating time of a diaphragm cell from a minimum of 36 to a maximum of 60 months, before its degradation. Current experience, however, indicates that another factor limits the total life time of the diaphragm electrolytic cells for the production of chlorine, and is substantially associated with the corrosion phenomena in the anodic compartment. BRIEF DESCRIPTION OF THE DRAWINGS In particular, the sealing between the bimetallic current collector shaft (4) with which the anodes (2) are fastened, and the copper anodic base (1) is made by means of a lining (9) , as shown in figure 2. The experience on the best garrisons currently available allows to foresee a life time of 12-24 months in the typical operating conditions. The multiplicity of joints in a cell, where several dozen anodes are present (typically 40 to 90), further increases the likelihood for a garrison to suffer a rupture, or in any case to result in a spill, well before that the NAD diaphragm's life time is over. When a spill occurs in correspondence of anodic shafts (4), it is necessary to stop the cell because the following phenomena, each of which is critical, are verified: - deterioration of the bimetal of the anodic shaft (4), due to the corrosive action of the electrolyte. - deterioration of the copper base, due to the same phenomenon - risk of grounding the cell. On the other hand, the stoppage of the cell and its opening to replace the fittings also implies the need to replace the diaphragms, which during the exercise suffer from a permanent deformation that prevents their use in a successive assembly. Ensuring leak-tightness of the anolyte with respect to the anodic current collector shafts during the maximum life time of the NAD diaphragm (60 months) is a problem of fundamental importance in the economy of the chlor-alkali diaphragm electrolysers, since it would not be It is acceptable to even damage in part the improvements that the NAD technology has introduced in the duration of the diaphragms. Figure 2 represents the state of the art in the field of sealing of the anodic current collector shaft. In particular, the embodiment shown in FIG. 2 comprises a current collector shaft (4), for example a shaft of 1% "(31.75 mm) with a UNC female thread of%", adapts to the housing of a fastening bolt ( 10) with the corresponding male thread. The electrical contact between the anodic base (1) and the current collector shaft (4) is maximally assured by the tightening of the exposed copper part of said shaft (4) to the copper current collector bottom (11). the anodic base (1). The simultaneous flow of current from the copper bottom (11) to the clamping bolt (10) through the thread of the clamping nut (12) is considered negligible, due to the number of conduction interfaces and the tiny section involved. The separation between the copper base (11) of the anodic base (1) and the anolyte is carried out, as described above, by means of an anodic coating (13) made of a titanium sheet, for example a sheet of thickness 1 mm, perforated and activated in correspondence of the shafts (4) which is also a fundamental integral part of the anode seal. The lining (9) is typically a toroid made of a hydrocarbon-based elastomer eg EPM or EPDM), pressed against the anodic coating (13) by means of a collar (14). The collar (14) is preferably made of a palladium titanium alloy, to have a sufficient resistance to interstitial corrosion, and may for example have a diameter of 50.0-50.8 mm and be welded at a distance of 4.7 mm from the end of the stem (4). The lining (9) therefore works under predetermined deformation, which in the case of the exemplary dimensions mentioned above, will be 3.7 mm in the coated area. The typical initial thickness can be, for example, 6 mm, thus achieving the typical compression ratio of 40%; even considering the entire region of contact between the toroidal rubber lining (9) and the anodic lining (13) as effective support area of support, it is evident as its amplitude is restricted; for example, for a collar (14) with a diameter of 50 mm in correspondence of a hole in the lining (13) having a diameter of 35 mm, the resulting amplitude of the clamping support zone is only 7.5 mm. The clamping pressure transmitted by the bolt (10), normally made of brass or copper-nickel alloy, is limited by the mechanical resilience of the threaded portion of the current collector shaft (4); An indicative value, typical of pieces with% "UNC" thread, is around 8 kg.m. The prior art described above has the following limitations: - the packing material (EPM, EPDM) has an inadequate resistance to chlorine, together to a high surface area exposed to the aggressive environment - The use of composite gaskets with a PTFE protective coating is impossible due to the high ratio between the thickness at rest and under compression (about 2: 1) and the high compression ratio (40 %) - On the other hand, the use of PTFE-derived materials, such as Gylon (marketed by Garlock, USA) or Permanite ™ Sigma (marketed by TBA, Great Britain), is impeded by low compressibility and by due to the need to use very high mechanical loads to ensure tightness - The compression load is not well defined, since the lining works at predetermined deformation, as described above. factors strongly limits the life time of the anodic seal fittings (9), affecting, as described above, the entire economy of the exercise of the diaphragm cells. An attempt to solve the problems mentioned above is described in Swedish Patent Application 97 020 79, and in the corresponding technology marketed by Akzo Nobel under the trademark Tibac ™. This invention consists in the direct welding of the collar (14) to the anodic coating (13), executed by means of a laser. In this way, no polymeric material is used for sealing, with obvious advantages in terms of reliability, since all polymeric material for fittings is subject to corrosion to some extent. By means of this technique, however, some undeniable disadvantages are introduced: the anodes (2) can no longer be detached from the anodic coating (13) and consequently from the base (1), with negative consequences in terms of manageability during the assembly and maintenance procedures, and the possibility of reactivating the anodes conveniently once their catalytic coating is exhausted. In addition, the weld seam is of considerable extension, and the risks of spillage due to local defects are therefore high. A second partial solution to the problem consists of the use of a lining (9), provided with a step (15) and with the shape shown in figure 3. The constructive principle contemplates the exposure to chlorine of a reduced surface of elastomer. In this way, the possibility of removing the anodes (2) from the anodic base (1) is preserved, while guaranteeing a prolonged life time of the lining (9) in view of its decreased exposure to corrosive agents. This invention has been proven in any case not yet sufficient to ensure adequate reliability, the linings (9) still being subject to spills induced by corrosion in a longer, but still unpredictable, average time. In addition, the constructive tolerances, on which the state of compression of the step (15), which is very thin, depend, become more critical; of the state of compression of the step (15) depends, in turn, the chemical resistance of this. Finally, in this type of fitting the sealing is entrusted to the inner profiled ring which, in case of infiltration, is intended for a rapid collapse being thinner than a traditional lining.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Under a first aspect, it is an object of the present invention to provide a diaphragm electrolytic cell design for the production of chlorine and alkali of improved reliability with respect to the prior art, ensuring that the exercise time without maintenance or substitution of components is limited only for the life time of the NAD diaphragms. Under a different aspect, it is an object of the present invention to provide a sealing system for anodes of diaphragm electrolytic cells for the production of chlorine and alkali to prevent the corrosion phenomena of the fittings for a minimum period of five years, at the same time. time preserving the possibility of removing each individual anode from the anodic coating.

Under a different aspect, it is an object of the present invention to provide a sealing system for anodes of diaphragm electrolytic cells applicable not only to newly constructed cells, but also to cells designed and constructed according to the prior art, possibly already in exercise, that allows to prevent or counteract the appearance of corrosion problems due to the faults of its sealing systems. Under a different aspect, it is a further object of this invention to provide a sealing system for anodes of diaphragm electrolytic cells for the production of chlorine and alkali applicable to cells designed and constructed according to the prior art that have previously suffered from marked corrosion phenomena. , including the degradation of the anodic current collector bottom (11). A new configuration of hydraulic sealing and electrical contact between the anode base (1) and the anodes (2) of a diaphragm electrolytic cell for the production of chlorine and alkali, which allows to completely overcome the limitations of the prior art, is described below. The constructive principle of the invention comprises a sealing system based on the interposition of a dimensionally conductive intermediate stratum adaptable between the anodic base (1) and the bottom of the current collector shaft (4). According to this new cell design, contrary to what happens in the prior art, the deformable component when clamping the cell is an integral part of the electrical contact, and not of the hydraulic seal. The innovative features of the cell design of the invention are summarized in Figure 4, and described below. The hydraulic seal is based on a ring gasket (16) instead of the flat gasket (9), optionally provided with a step (15), of the prior art. The ring gasket (16) must have the following characteristics: - to be produced from chemically inert, and possibly elastic, construction materials - To have sufficient dimensions to compensate for local irregularities - to rely exclusively on the anodic coating (13) - To have a low deformation load (for example, substantially lthan that of a spiral-metallic seal).

The anodic current collector shaft (4) is also provided with an additional ferrule (17) or with an equivalent element to delimit a receiving slit for the ring gasket (16); In a preferred embodiment of the invention, the ferrule (17) is obtained by turning a titanium-palladium ring, as shown in figure 5, supporting it to the collar (14) and optionally welding it; in the latter case, this embodiment is particularly suitable for being applied to cells constructed according to the prior art, wherein the collar (14) is already present, and the ferrule (17) is succvely welded, preferably according to the geometry shown in the figure 4, where it is evident how the external position of the weld with respect to the bimetal of the shaft (4) prevents damage to the structural integrity of the latter by subjecting it to a high temperature. In the case of new constructions, the collar (14) and the collar (17) can be constructed as a single piece, provided with a suitable slit to receive the ring gasket. In the selection of construction material for ring packing, the chemical inertnof the latter is particularly important; in particular, purely elastomeric ring packings are not an acceptable solution. Suited to the object are ring packings made from an elastomeric core coated with an inert film, for example a fluorided film. Composites ring gaskets of this type can be selected, for example, from the following categories: - Ring packings coated with FEP, a polymer characterized by very low chlorine diffusion. An example of commonly available FEP-coated ring gasket is the FEP-O-SEAL ™, marketed by the Swiss company Angst-Pfister, which has a fluorinated film thickness of 0.25 mm. A material preferably used for the elastomeric core is Viton, which is very resistant to attack by dry chlorine, that is to say under conditions that can be verified due to the diffusion of chlorine through the fluorided film of the ring gasket. - Ring gaskets coated with PTFE; in this case, the thickness of the protective film must be higher (preferably 0.75 - 0.8 mm), and the core preferably should have accentuated elastic characteristics. Preferably, a silicone rubber material, on which the protective film is applied by welding, is selected as an elastomeric core. The ring packings selected according to the criteria described above, due to the protection relative to the thickness and to the reduced exposure to the liquid, can remain in exercise for many years; the elastomeric bores described above are suitable for a continuous operation up to temperatures between 150 and 180 ° C, against the typical 90-95 ° C of the diaphragm process; In addition, the possible irregularities or imperfections of the anodic coating are compensated by the pressure exerted by the collar. The electrical contact must be made by means of a deformable element (18), and at the same time must be effective to sustain a high intensity of current; this can in fact reach 2000 A. As shown in figure 4, the amplitude of the intermediate space between the copper current collector bottom (11) and the end of the anodic shaft (4) is determined by the thickness of the additional ferrule. of titanium - palladium (17) in the case that a preexisting cell is modified. As specified above, in the case that a new cell is constructed, the ferrule (17) or equivalent element is integral to the collar (14), and the position of this integral part will determine the amplitude of the intermediate space between the current collector bottom of copper (11) and the end of the anodic shaft (4). The tolerance of this amplitude depends in all ways on constructive factors, among which the most decisive is the orthogonality between the ferrule (17) and the bimetallic shaft (4), as shown in figure 6. The deformability of the electrical contact element (18) serves precisely to compensate for such deviations, and the optimal selection of said component is decisive for the electrical efficiency of the entire process. A suitable solution for producing the deformable contact element (18) is the use of massive silver, a metal that has the following characteristics: - low contact potential even at very low clamping pressures - high deformability, which renders it adaptable to eventual irregularities in the thickness at limited pressures, in addition to a tendency to seal the two copper surfaces by coupling as if it were a true metal lining.

However, in the description below reference will be made to contact elements of pure silver, for example "Fine Silver" of purity 99.9%, it will be understood that other silver materials having equivalent characteristics in terms of electrical conductivity and mechanical deformability may be advantageously employed. For example, the silver alloy known as "Sterling Silver" or "Silver / Copper Alloy", which contains about 7.5% copper, is widely used for all types of electrical contact, and may be suitable for this purpose. Other silver alloys that can be used are the silver / zinc / antimony alloy known as "Silanca", as well as the alloys called "Coin Silver", which contain 2.5% aluminum or copper. In a diaphragm cell of the invention, which corresponds to the illustration in figure 4, it is possible, by means of a silver deformable contact element (18), to carry out a direct current flow up to 3500 A while maintaining a potential drop of contact less than 1 mV, at the usual clamping pressures required for sealing. A particularly preferred geometry for decreasing the use of silver in the intermetallic contact element is the washer type, shown in Figure 7. In this case, the importance of ensuring the compression of the entire washer under exercise conditions is evident. For this reason, the washer (19), made of a continuous central base typically thickens a few millimeters, is provided on both sides with regular asperities, for example concentric grooves (20), which act as points or preferential support regions of Contact. In a typical embodiment, the total height of the piece is 3.7 mm and the grooves, initially 1.5 mm, are subject to a compression of 0.85 mm on each side, corresponding to an absorption of 1700-2000 kg of contact. With these parameters, slots (20) having surface ridges equivalent to 40% projected area of the washer, were determined potential drops between 2 and 3 mV under a current flow of 2000 A, which is still a fully value acceptable. Another preferred embodiment, which allows a simpler construction with respect to the washer contact, is given by the "ring" contact element, as shown in Figure 8. The ring (21) is obtained by simply cutting a silver cannula; This type of contact has the advantage of a rapid initial deformability, and therefore of a quick adaptability, however it is not suitable for too high clamping pressures. Another preferred embodiment of the invention provides for the use of a closed shaped contact element, for example as shown in Figure 9. The peculiar feature of this embodiment is the location of the contact on small surfaces, subjected to a high pressure. The shape of the petal shown in figure 9 is of assistance to the assembly, because it imparts self-centering characteristics to the piece; as it is evident, many different contact elements of closed conformation can achieve the same function, being technically equivalent. Although all these types of contact elements require replacement when the anodes are removed (for example for mechanical repairs, or for reapplication of the electrocatalytic coating), since they are subject to mechanical deformation in the plastic regime, the pure silver used for its construction can be easily and entirely recovered at the end of the life cycle of the piece. The fixing of the anodic structures on the bottom of the cell is effected by acting on the clamping nut (12); The typical torsional moment is about 8 kg.m. The sealing system of the invention, based on the use of ring packings, does not require any elastic apparatus, such as Belleville springs inserted between copper bottom (11) and nut (12), since the general adjustment is defined by the Rigid contact of the ferrule surface (17) with the lining (13); the same goes for the silver contact element, confined in the slot delimited by the collar-ferrule system. The above-described cell design, therefore, completely overcomes the problems arising from the use of corrosive linings, is apt to exercise also at high current density, and presents an outstanding flexibility in terms of possible ways of implementation. The constructive solutions just described have the sole purpose of exemplifying some possible ways of implementing the invention without limiting its extension, which is only defined by the following claims.

Claims (12)

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 diaphragm cell for electrolysis of alkali chlorides, comprising at least one anode secured to an anodic base and in electrical contact with it by means of a current collector shaft separated from the electrolyte circulating in the anodic compartment through a system of hydraulic sealing, characterized in that a conductive and deformable contact element is interposed between said current collector shaft and said anode base, and the hydraulic sealing system comprises at least one ring gasket, positioned in correspondence of said current collector shaft and housed in a slit delimited by said anode base and by a containment element secured to said current collector shaft.

2. The cell of claim 1, characterized in that said ring gasket is made of an elastomeric core, coated with a film of a material with low diffusivity of chlorine and chemically inert.

3. - The cell of claims 1 or 2, characterized in that said contact element is made of silver.

4. The cell of claim 3, characterized in that said contact element is a massive silver contact element.

5. The cell of claim 3, characterized in that said silver contact element has the shape of a washer provided with grooves.

6. - The cell of claim 3, characterized in that said contact element is ring-shaped.

7. - The cell of claim 3, characterized in that said contact element has a closed form.

8. The cell of claim 7, characterized in that said closed form imparts self-centering characteristic to said contact element.

9. The cell of claim 1, characterized in that said containment element is a ferrule welded on a collar.

10. The cell of claim 1, characterized by being obtained by modifying a pre-existing cell, wherein said containment element is a collar and an additional containment element is welded thereto, and wherein said hydraulic sealing system comprising the less a ring gasket is provided to replace a pre-existing hydraulic sealing system comprising at least one flat gasket, optionally provided with at least one step and interposed between said collar and said anode base, wherein said pre-existing sealing system is removed before of the assembly of said hydraulic sealing system comprising at least one ring gasket.

11. The cell of claim 1, characterized in that said additional containment element is a ferrule. 12.- A diaphragm electrolytic cell characterized in that it contains the distinctive elements illustrated in the designs and in the description. P) hl? OO ¡oo 7é SUMMARY OF THE INVENTION The invention relates to an improved electrolyser design, in particular to the design of a diaphragm electrolyzer, for the production of chlorine and alkali from aqueous solutions of alkali chlorides, comprising at least one anode secured to an anodic base (11). ) and in electrical contact with it by means of a current collector shaft separated from the electrolyte circulating in the anodic compartment by means of a hydraulic sealing system (16), wherein a conductive and deformable contact element (18) is interposed between said current collector shaft and said anodic base, and wherein the hydraulic sealing system comprises at least one ring gasket.