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(11) | EP 0 610 946 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Activated cathode for chlor-alkali cells and method for preparing the same |
| (57) An activated cathode assembly for use in diaphragm or membrane electrolysis cells
comprising pairs of interleaved cathodes and anodes (B) having openings, said cathodes
being provided with a corrosion resistant ion exchange membrane or porous diaphragm,
said cells (C) being provided with inlets for feeding brine and outlets for the withdrawal
of chlorine (H), hydrogen (I) and caustic (L), the cathode assembly comprising a rigid
cathode structure covered with a thin and flexible corrosion resistant, perforated
or expanded sheet or mesh (F) provided with an electrocatalytic coating for hydrogen
evolution in an alkaline environment, the cathode structure and sheet or mesh (F)
being in electrical and mechanical contact with each other; method of preparing said
assembly and an electrolysis cell using said assembly. |
STATE OF THE ART
[0001] Chlor-alkali electrolysis is certainly the electrolytic process of greatest industrial interest. In general terms, said electrolysis process may be illustrated as the splitting of a starting reactant, which is an aqueous solution of sodium chloride (hereinafter defined as brine), to form gaseous chlorine, sodium hydroxide in an aqueous solution and hydrogen. This splitting is made possible by the application of electrical energy which may be seen as a further reactant. Chlor-alkali electrolysis is carried out resorting to three technologies: with mercury cathodes cells, with porous diaphragms cells or with ion exchange membranes cells. This latter represents the most modern development and is characterized by low energy consumptions and by the absence of environmental or health drawbacks. Of the others, the mercury cathodes cells are probably destined for a sharp decline in use because of the severe restrictions adopted by most countries as regards the release of mercury to the atmosphere and soil. In fact, the most modern cell designs allow one to meet the severe requirements of the present regulations, but the public opinion rejects "a priori" any process which could lead to the possible release of heavy metals in the environment.
[0002] The diaphragm process has also problems as the main component of the diaphragm is asbestos fibers, which is recognized to be a mutagenic agent. The most advanced technology foresees a diaphragm made by depositing a layer of asbestos fibers mixed with certain polymeric binders onto cathodes made of iron meshes. The structure thus obtained is then heated whereby the fusion of the polymeric particles permits the mechanical stabilization of the agglomerate of asbestos fibers. As a consequence, the release of fibers during operation (particularly in the drain liquids of the plant) is minimized, as well as the release to the atmosphere due to various expedients adopted during manipulation of the asbestos in the deposition step.
[0003] However, this appears to be only sufficient to prolong the life of the diaphragm technology, in view of the ever increasing difficulty in the supply of asbestos fibers due to the progressive closing of the mines. For this reason, porous diaphragms have been developed where the asbestos fibers are substituted by fibers of inorganic materials considered to be completely safe, such as zirconium oxide, mechanically stabilized by polymeric binders. The deposition and the stabilization by heating in oven are carried out following the same procedure adopted for asbestos diaphragms.
[0004] In the last few years, graphite anodes have been nearly completely substituted by dimensionally stable anodes made of a titanium substrate coated by an electrocatalytic film based on noble metal oxides. In the plants using the most advanced technologies, the dimensionally stable anodes are of the expandable or non-expandable type. Expandable anodes as described for example in U.S. patent 3,674,676, which permit one to minimize the gap between the anode and the cathode, with the consequent reduction of the cell voltage, have the shape of a box with a rectangular cross-section, rather flat, the electrode surfaces of which are kept in a contracted position by means of suitable retainers while the anode is inserted between the cathodes during assembling of the cell. Before start-up, the anode electrode surfaces are released and are moved towards the surfaces of the diaphragms by suitable spreading means or extenders. Spacers may be introduced between said electrode surfaces and the diaphragms. These technological improvements brought the cost of production of chlorine and caustic obtained by the diaphragm technology quite close, even if somewhat higher, to those obtained by the membrane technology.
[0005] It is therefore the current opinion of industry that diaphragm cells plants may still remain in operation for a long time and the future of these plants could be even more promising if the following inconveniences still penalizing the technology are overcome:
- cell voltages higher than that theoretically obtained by the expansion of the anodes.
It is well known that the cell voltage linearly decreases with the decrease of the
anode-cathode gap. Said result is connected to the lower ohmic drop in the brine layer
between the diaphragm and the anode. However, for anode-cathode distances below a
certain limit, usually 3.5-4 mm, the cell voltage remain more or less constant or
even increases (see Winings et al. in Modern Chlor-Alkali Technology, 1980, pages
30-32).
This negative behaviour, quite unsatisfactory, is commonly attributed to the chlorine bubbles which are entrapped in the thin brine layer between the anode and the diaphragm. The problem is partially solved by resorting to the use of internal hydrodynamic means as described in US patent 5,066,378. Said means are directed to promote a strong circulation of brine capable of removing the chlorine bubbles; - increase of the cell voltage in the electrolysis which increase is commonly ascribed to gas entrapping inside the pores, favoured by insufficient hydrophilic properties of the material forming the diaphragm, in particular in the case of diaphragms containing polymeric binders, as suggested by Hine in Electrochemical Acta Vol. 22, page 429 (1979). The increase of cell voltage may also be due to precipitation of impurities contained in the brine inside the diaphragms;
- deposition of metallic iron or electrically conductive compounds of iron, such as magnetite, formed by reduction at the cathode, with growth of dendrites in the diaphragm and evolution of hydrogen in the anode compartment (hydrogen in the chlorine which is explosive). This problem is most likely to occur with diaphragms characterized by a scarcely tortuous porosity, as discussed by Florkiewicz et al. at the 35th Seminar of the Chlorine Institute, New Orleans, Louisiana, USA, March 18, 1992;
- decrease of the faradic efficiency in the electrolysis run;
- reduced life of the diaphragm.
OBJECTS OF THE INVENTION
[0006] It is an object of the invention to provide an improved cathode assembly for use in a diaphragm chlor-alkali electrolysis cell which permits the substantial elimination of the inconveniences of the prior art and a method for preparing the same.
[0007] It is another object of the invention to provide an improved diaphragm electrolysis cell and the method for operating the same.
[0008] These and other objects and advantages of the invention will become obvious from the following description.
SUMMARY OF THE INVENTION
[0009] The novel cathode assembly of the invention for use in monopolar or bipolar diaphragm or membrane electrolysis cells comprises pairs of interleaved anodes and cathodes having openings, said cathodes being provided with a corrosion resistant ion exchange membrane or a porous diaphragm, said cells being provided with inlets for feeding brine and outlets for the withdrawal of chlorine, hydrogen and caustic. Said cathode assembly comprises a thin and flexible corrosion resistant, perforated or expanded sheet or mesh provided with an electrocatalytic coating for hydrogen evolution in an alkaline environment, applied between each of said cathodes and said diaphragm, the cathode and the sheet or mesh being in electrical and mechanical contact, preferably by a plurality of contact points.
[0010] Preferred embodiments of the present invention will be now described making reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Fig. 1 is a cross sectional longitudinal view of a conventional diaphragm cell for chlor-alkali electrolysis comprising anodes of the expandable type and the cathode assemblies of the present invention.
[0012] Fig. 2 a cross sectional view of a detail of fig. 1 illustrating the cathode assemblies of the present invention.
DESCRIPTION OF THE INVENTION
[0013] In fig. 1, the diaphragm electrolysis cell comprises a base (A) on which expandable anodes (B) are secured by means of conductor bars (D). The cathode (C) is made of a perforated or expanded sheet or mesh of interwoven iron wire so shaped as to form a multiplicity of rather flat parallelepipeds (so called fingers). The thickness of the sheet or mesh is such as to ensure a sufficient rigidity to the cathode structure. Further, the dimensions of the openings in the sheet or mesh have suitable values so as to permit easy deposition of the diaphragm starting from a suspension of fibers and possibly of a polymeric binder as well as good adhesion of the diaphragm after deposition. The anodes, usually of the dimensionally stable type, are interleaved with the fingers.
Said fingers (C) are provided with a diaphragm (not shown in the figure). Spacers (not shown in the figure) may be optionally inserted between the surfaces of said anodes and the diaphragms. The cover (G) is made of corrosion resistant material with outlets (H) for chlorine and brine inlets (not shown). Hydrogen and caustics are released through (I) and (L) respectively.
[0014] Referring to fig. 2, the fingers (C) are provided with the thin, foraminous sheet or mesh (F) of the present invention, coated with the diaphragm (E) constituted by fibers and possibly by a polymeric binder.
[0015] The cathode assembly of the invention is provided with catalytic properties to ensure a decrease of the cell voltage to 3.10-3.15 V and allows for utilizing the cathode structures of existing cells made of interwoven iron wires or of a perforated sheet, thus minimizing the financial investment cost. The application to the fingers (C) of the cathode structure of conventional chlor-alkali cells of a fine and flexible, foraminous screen (F) made of a mesh or expanded or perforated sheet provided with an electrocatalytic coating for hydrogen evolution in an alkaline environment gives unexpected advantages.
The improved cathode of the invention is characterized by a composite layer structure, wherein the more internal layer is formed by the fingers (C) of the conventional cells, while the more external layer is formed by the mesh or sheet (F), provided with the electrocatalytic coating, which is mechanically and electrically connected to the fingers. The fingers therefore act both as supports and current distributors.
[0016] The mesh or sheet (F) of the present invention, provided with the electrocatalytic coating, may be made of iron, chromium, nickel, copper and alloys thereof. The materials most commonly used, due to their availability on the market, are iron, stainless steel and nickel. When the first two are selected, preferably before the catalytic activation a thin layer of nickel, some microns thick, is applied by galvanic deposition. The dimensions of the openings are not critical but must be suitably selected in order not to interfere with the deposition of the diaphragm or not to spoil the adhesion of the diaphragm to the cathode. The mesh or sheet of the present invention must be thin to permit the greatest flexibility, which is necessary during fixing to the cathode structures (fingers) of conventional cells.
The electrocatalytic coating must be advantageously capable of resisting current reversals and to minimize the deposition of metallic iron or electrocondutive compounds of iron, such as magnetite when the fresh brine is polluted by some parts per million of iron. The current reversals, as is well known, occur whenever a monopolar cell of a production line must be excluded from operation. The exclusion is carried out by short-circuiting the cell with a suitable jumper switch means and the cell is then removed from the production line and sent to the service area, while copper rods are inserted in its place. In this way the production is not interrupted. During short-circuiting, the cell is crossed by high reverse current which may easily damage the cathode coating. In order to avoid this problem, suitable short-circuiting devices have been developed which minimize the intensity of the reverse current but are extremely expensive. An alternative method consists in providing the cathodes with coatings capable of undergoing even strong currents reversals.
Among the types having these characteristics, most suitable coatings are made of a substrate of nickel metal containing a dispersion of electrocatalytic particles as described in BE 848458, obtained by galvanic deposition from a bath containing suitable nickel salts and particles of electrocatalytic material held in suspension by mechanical stirring with the possible addition of suitable suspending agents. A similar coating obtained by galvanic deposition comprises a nickel matrix having suspended therein particles of electrocatalytic material and particles of a material capable of absorbing high quantities of hydrogen in the form of hydrides, as described in US 5,035,790.
[0017] The said coatings are also capable of resisting the aggressive attack by the active chlorine dissolved in the brine which flows and diffuses through the diaphragm during the first minutes of the shut-down of the cells.
[0018] As regards the problem connected to the presence of remarkable quantities of iron in the fresh feed brine, this practically brings about the formation of dendrites of metallic iron or electroconductive iron oxide such as magnetite, capable of crossing the diaphragm and causing hydrogen discharge directly in the anodic compartment with the formation of dangerous hydrogen/chlorine mixtures. A first obvious alternative solution is represented by subjecting the fresh brine to a suitable pre-treatment and excluding from the circuit any steel component which with time and with the formation of defects may become a continuous source of poisoning of the brine. It is evident that these countermeasures involve high costs. A second alternative is to resort to very active electrocatalytic coatings, which operate at such a potential that the formation of metal iron or magnetite, if not impossible, is at least strongly slowed down. This alternative, particularly if combined with a geometrical shape of the substrate capable of degrading the adhesion of the dendrites favouring the detachment by the hydrogen bubbles, is extremely efficient and permits one to eliminate the need for additional investment costs for the installation of equipments and filtering systems for brine purification. Coatings of this type are described in US patent No. 4,724,052.
[0019] The method of preparation of the cathode assembly of the present invention comprises preparation of the perforated or expanded sheet or fine mesh screen provided with the electrocatalytic coating, and the pre-treatment of the cathode structure of conventional cells. In the case of used cathodes, the original diaphragms must be carefully removed to eliminate even the minimum residue of fibers and polymeric binder. This removal may be efficiently carried out in compliance with the current health regulations by using strong water jets under pressure and collecting the liquid which is to be sent to a treatment section. The polished structure thus obtained must be free of any rust or deposits of any nature. This may be obtained by hydrosand-blasting or chemical pickling using acid solutions added with suitable filming corrosion inhibitors. In the case of new cathode structures, the pre-treatment is limited only to hydrosand-blasting or chemical pickling.
[0020] After careful final washing and drying under forced air circulation, the fine electrocatalytic expanded sheet or mesh must be readily applied. The mesh or sheet is cut into strips of suitable dimensions and the strips are then pressed carefully onto the surface of the fingers of the cathode structure. The strips are then mechanically fixed so as to make the electrical continuity between the electrocatalytic sheet or mesh and the cathode structure as extended as possible. To attain this goal, the activated sheet or mesh must be particularly flexible and capable of conforming to the profile of the cathode structure, which may certainly present distorsions of various kinds. Further, the number of contact points must be very high. As a consequence, the most advantageous fixing method is electrical spot welding. It must be noted that the weldings spots are only to ensure electrical continuity and a particular mechanical resistance is not required. In fact, the activated sheet or screen applied to the external of the cathode structure, are subjected to a hydraulic head during operation which tends to keep them pressed against the cathode structure themselves. The composite cathode assembly thus obtained is then subjected to deposition of the diaphragm, which is carried out according to the conventional techniques, without particular variations, if the dimensions of the openings in the sheet or mesh are suitably selected.
[0021] In the following Examples, there are described several preferred embodiments to illustrate the present invention. However, it should be understood that the invention is not intended to be limited to the specific embodiments. For example, it is evident to one skilled in the art that the cathode assembly of the present invention may apply also for membrane cells of the so called bag cell type, which are obtained from existing chlor-alkali diaphragm cells using ion exchange membranes in the form of a bag capable of enveloping the cathode fingers.
EXAMPLE 1
[0022] Two MDC55 electrolysis cells from a chlorine production line were shut down and disassembled. The diaphragms were removed from the fingers by washing with water jets under pressure and a subsequent pickling in inhibited 6% hydrochloric acid at 70°C, for about one hour. The structures were then carefully washed with industrial water until a pH 5 was obtained, then with water alkalinized by 1% by weight of sodium carbonate and then with demineralized water, followed by drying under forced hot air circulation.
[0023] The fingers of the two cells were readily provided respectively with an activated nickel mesh and an activated expanded sheet prepared according to the teachings of Example 1 of US patent 4,724,052.
The mesh was made of nickel wire having a diameter of 0.3 mm and forming square openings of 2 x 2 mm and the expanded, flattened sheet had square openings having dimensions of 5 x 5 mm. After flattening, the thickness of the sheet was 0.5 mm. The application was carried out by maintaining the activated mesh and the sheet pressed against the surfaces of the fingers and then spot welding with a portable welding machine. The welding points formed a square reticulate with a distance among the points of 30 mm.
[0024] The two composite cathode structures were then coated with a diaphragm comprising asbestos fibers and a suitable fluorinated polymeric binder of the type MS2, forming up to a thickness of 3 mm.
The coated cathode structures were then treated in oven according to the conventional technique to obtain mechanical stabilization of the fibers by the polymeric binder.
[0025] The two cells were re-inserted in the production line, with the following average parameters:
- cell voltage: 3.35 Volts
- current density: 2200 Ampere/square meter
- fresh feed brine: 315 grams/liter, flow rate: 1.6 cubic meters/hour
- outlet liquid: 125 grams/liter of caustic, 190 grams/liter of sodium chloride at 95°C
- oxygen content in chlorine: 3.2%
- current efficiency: 93%
- dimensionally stable expandable anodes provided with 3 mm spacers.
[0026] The voltage of the two cells equipped with the composite cathode assembly of the present invention, detected once stationary operating conditions were reached, was about 3.07 Volts, that is 0.28 Volts lower than the average value typical of the production line. The voltage then slowly increased to 3.10 V in 15 days and remained thereafter constant. No noticeable variations in current efficiency or oxygen content in chlorine were detected.
[0027] After 47 days of operation, the cell equipped with the activated mesh made of nickel wire was subjected to 12 daily short-circuitings. Upon reaching stationary operating condition, the voltage had only negligibly increased to 3.12 Volts. Similar results were obtained using activated meshes made of a wire having a diameter of 0.5 mm forming openings of 5 x 10 mm. The above data demonstrate that the selection of the mesh geometry may be made over a wide range and it is possible to operate chlor-alkali diaphragm cells equipped with activated cathodes even in the event of severe anomalies, as it is the case during short-circuiting, without experiencing appreciable voltage increases.
EXAMPLE 2
[0028] The cell of Example 1, provided with the same activated mesh made of a nickel wire, was fed with fresh brine containing 0.01 grams/liter of iron. For comparison purposes, also a reference cell from the production line having an operating lifetime of 50 days, was also fed with the same polluted brine. The reference cell was shut down after 28 days of operation when the hydrogen content in the chlorine reached 0.8%. The cell equipped with the cathode assembly of the invention showed a hydrogen content in chlorine of 0.2% substantially unvaried during the whole operation.
1. A cathode assembly for use in diaphragm or membrane electrolysis cells equipped with
pairs of interleaved anodes (B) and cathodes (C) having openings, said cathodes (C)
being provided with a corrosion resistant ion exchange membrane or a porous diaphragm
(E), said cell being further provided with inlets for feeding brine and outlets for
the withdrawal of chlorine (H), hydrogen (I) and caustic (L) characterized in that
said cathode assembly comprises a a thin and flexible corrosion resistant, foraminous
sheet or mesh (F) provided with an electrocatalytic coating for hydrogen evolution
in an alkaline environment, applied between each of said cathodes (C) and each diaphragm
(E), each cathode (C) and each sheet or mesh (F) being in electrical and mechanical
contact.
2. The cathode assembly of claim 1 characterized in that said cathode (C) and said sheet
or mesh (F) are in electical contact at a plurality of contact points.
3. The cathode assembly of claim 2 characterized in that the contact points are spaced
not more than 50 mm apart.
4. The cathode assembly of claim 2 characterized in that the contact points are electrical
spot weldings.
5. The cathode assembly of claim 1 characterized in that the mesh or sheet (F) has a
thickness of 0.5 mm.
6. The cathode assembly of claim 1 characterized in that the mesh (F) is made of wires
having a diameter not more than 0.5 mm and has openings with a length less than 10
mm.
7. The cathode assembly of claim 1 characterized in that the sheet (F) is an expanded
sheet having openings with a width less than 10 mm.
8. The cathode assembly of claim 1 characterized in that the mesh or sheet (F) is made
of a metal selected in the group of iron, nickel, chromium, copper or alloys thereof.
9. The cathode assembly of claim 1 characterized in that the electrocatalytic coating
is obtained by galvanic deposition and comprises a nickel matrix containing a dispersion
of electrocatalytic particles.
10. The cathode assembly of claim 1 characterized in that the electrocatalytic coating
is comprised of mixed oxides of the platinum group metals and at least one metal capable
of absorbing hydrogen.
11. A method of producing an activated cathode assembly of claim 1 comprising derusting
a cathode (C), pressing a thin foraminous sheet or a mesh (F) provided with an electrocatalytic
coating for hydrogen evolution in an alkaline environment against the cathode (C)
and providing for electrical and mechanical contact between the cathode (C) and the
sheet or mesh (F).
12. The method of claim 11 characterized in that the cathode (C) and the sheet or mesh
(F) are in electric contact at a plurality of contact points.
13. The method of claim 11 characterized in that the derusting is effected by hydrosand-blasting
or by pickling in an inhibited acid solution.
14. A diaphragm or membrane electrolysis cell for electrolysis comprising at least one
pair of interleaved anodes (B) and cathodes (C) provided with openings and separated
by a porous diaphragm or a corrosion resistant ion exchange membrane (E), the cell
being provided with at least one inlet for feed brine and outlets for the withdrawal
of chlorine (H), of caustic (I) and hydrogen (L), characterized in that it further
comprises a thin and flexible foraminous sheet or mesh (F) provided with an electrocatalytic
coating for hydrogen evolution applied between each cathode (C) and each sheet or
mesh (F), said cathode (C) and said sheet or mesh (F) being in electric and mechanical
contact with each other.