Description:
STATE OF THE ART
Diaphragm-type cells for the electrolysis of aqueous alkali metal halide brines generally employ a foraminous or perforated metallic cathode and a fluid-permeable diaphragm overlaying the cathode thereby permitting hydraulic flow of electrolyte from the anode chamber through the diaphragm and cathode into the cathode chamber. Such cells first made their appearance in the early part of the twentieth century. The fluid permeable diaphragm, by separating the anode and cathode chambers, avoids the disturbing effects of convection currents and gas evolution, and generally inhibits migration of hydroxyl ions towards the anode. Those diaphragm-type cells most widely used today are of the circulating electrolyte type, wherein the diaphragms and cathodes may be arranged horizontally or vertically, but in most instances, at least in the United States, the arrangement is vertical. Such cells are in wide-spread use in the industry for the production of chlorine and caustic soda from sodium chloride brines and, through the use of various sophisticated modifications, considerable efficiency has been obtained from the cells which have been operated at current densities approaching 1 ampere per square inch. However, despite their widespread acceptance, these cells nevertheless have certain drawbacks and disadvantages which limit the further modification and improvement thereof.
Most of these limitations may be attributed to the fact that the majority of cells in operation to-date employ graphite as the anode material. Generally these anodes take the form of flat, vertically-disposed graphite blades which have their lower ends embedded in the cell bottom or base. A typical means of implanting these anode blades in the cell base is described in U.S. Pat. No. 2,987,463 and consists in inserting the anode blades into the slots formed by a plurality of conductive metal grids, usually copper grids. In order to improve electrical contact, it is then the standard procedure to apply a bonding layer of an electrically conductive material such as molten lead, which layer serves both to increase electrical conductivity and rigidly set the anode blades in the conductive grid. Over this electricallyconductive bonding layer there is then applied an electrically insulating coating, for example asphalt, which prevents access of the corrosive anolyte to the metal. In turn, a layer of concrete is applied over this asphalt layer to complete the base construction. Obviously, there are a number of disadvantages to such a cumber-some technique, which are mainly due to the use of graphite anode.
In the last few years, dimensionally stable metal anodes with an electrically conductive, electro-catalytic coating thereon have been adapted for commercial use in diaphragm-type cells which have permitted changes in the design of the cell, particularly the design and mounting of the anodes. In a commercial cell which has become widely used in that of U.S. Pat No. 3,591,483, the cell base is made of a conducting material and anodes are positioned thereon by risers passing through the cell base and provided with a flange, preferably made of titanium, on the lower portion thereof and a non-conductive, preferably rubber, sheet on the cell base to act as a compressible seal between the anodes and the cell base and between the cell can and the cell base.
Experience with cells of this type, however, has shown that there is a tendency for crevice corrosion to appear on the riser flange where it rests on the insulating sheet of rubber. Crevice corrosion is due to a special type of galvanic cell arising from a difference in the electrolyte composition in the crevice and the electrolyte in contact with the main body of the metal part or flange. The crevice corrosion could cause leaks of electrolyte from the cell and other problems leading to premature shut down of the cell for repairs.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a novel method of preventing crevice corrosion in valve metal connections in diaphragm-type cells.
It is a further object of the invention to provide an improved diaphragm cell using as the non-conductive, insulating sheet a rubber sheet having in the area around the anode riser a mixture of nickel and nickelous oxide.
These and other objects and advantages of the invention will become obvious from the following detailed description.
THE INVENTION
The invention relates to an improvement in diaphragm cells wherein the anodes are supported by risers which are secured in the base and the base is insulated from the interior of the cell by a protective sheet, the improvement comprising providing in the area of the cell base where crevice corrosion normally occurs a mixture of metallic powder nickel and nickelous oxide in the protective blanket or gasket sealing means. Usually, it is effected by incorporating into the protective sheet at least in the area thereof through which the risers pass a mixture of metallic nickel powder and nickelous oxide.
The said mixture may be incorporated throughout the entire protective sheet or only in the area where the risers pass therethrough. The protective sheets are usually made of chlorinated rubber or neoprene type material and if the said mixture is to be incorporated throughout the sheet, the mixture of metallic nickel powder and nickelous oxide may be substituted for the zinc and magnesium oxide filling and curing agents. The nickel and nickelous oxide provide the needed curing effect during vulcanization of the neoprene blanket and the omission of the zinc and magnesium compounds from the protective blanket avoids their possible chlorination which would aggravate the crevice corrosion problem.
If the protective sheet is not a neoprene type material but a metal sheet such as a titanium sheet, the gaskets used to seal the cell and the area around the anode risers are made of neoprene type material and the nickel-nickelous oxide mixture is incorporated in this material to prevent crevice corrosion. It is also possible to merely insert into the neoprene type blanket, small gasket portions which can be inserted into the blanket in those areas wherein the anode risers pass through the protective blanket.
The neoprene type material used for the protective blanket or the gasketing material are well known but the most commonly used types are the G and W types. Neoprene is made by polymerizing 2-chlorobutadiene in the presence of suitable catalysts, emulsifying agents, modifiers and protective agents. The G-type neoprene differs from the W-types in that G contains a thiuram disulfide stabilizer and are interpolymerized with sulfur.
The additives in various neoprenes are well known and perform various functions. For example, plasticizer and softeners such as naphthenic oils and other petroleum derivatives are used to increase flexibility thereof. Fillers may be carbon black, clays, calcium carbonate, silicon dioxide and other mineral fillers. Antioxidants are added to provide maximum protection from heat, ozone, and/or discoloration. Processing acids may also be incorporated therein which class are included lubricate, tackifiers and agents to control viscosity and nerve.
After the neoprene has been vulcanized and has been formed into the desired shape, the area of the neoprene in the crevice corrosion problem area should be cut or scraped to remove the film inherently formed on the surface of the neoprene during the vulcanization process.
The amount of metallic nickel and nickelous oxide to be incorporated into the neoprene may be 25 to 45 preferrably 30 to 40 ,% by weight. The weight ratio of the nickel to nickelous oxide may vary from 3:1 to 1:1 , preferably 1.5:1 to 2:1 .
Referring now to the drawings:
FIG. 1 is a simplified end view of a typical diaphragm-type electrolytic cell of the invention with the cell can and cathodes removed for clarity.
FIG. 2 is a simplified side view of a portion of the diaphragm-type electrolytic cell of FIG. 1 with cell can and cathodes not shown.
In FIG. 1, the cell base 1 is constructed of a material such as aluminum or copper and hence serves as both the supporting means for the cell and the conductor. The power supply is attached directly to this base, for example, by means of a nut 9 and bolt 11. The non-conductive sheet 3 covers essentially all of the cell base 1 and is constructed of an elastic material such as neoprene containing metallic nickel powder and nickelous oxide. The protrusions 5 and 6 on this non-conductive sheet 3 perform separate functions. Protrusion 5 serves as a gasket on which the cell can rests. A small amount of putty 29 lines the inside of the protrusion to insure that no leakage occurs. Protrusion 6 serves as a deflector to prevent brine or water from getting between the non-conductive sheet 3 and the cell base 1. The valve metal anode 19 with the electrocatalytic coating thereon is connected, for example by welding, to the anode riser 13, which riser extends through the non-conductive sheet and cell base and is fastened on the bottom of the cell base by means of a nut 17. Examples of suitable electrocatalytic, electrically conductive coatings are described in U.S. Pat. Nos. 3,632,498 and 3,711,385. The preferred coating for chlorine production is TiO2 + RuO2. This is one of the problem areas in which crevice corrosion generally occurs. The neoprene blanket is scraped in this area to remove the film on the blanket formed during vulcanization. The riser is also provided with a flange 15 which upon tightening the nut 17, forms a hydraulic seal with the non-conductive sheet of material 3 thereby preventing leakage of anolyte through the cell base. While it is indicated in FIG. 1 that two anodes extend across the width of the cell, this number is not critical and may be changed as conditions warrant.
FIG. 2, is a partial side view along the length of a cell, again with the conventional cathodes and cell can removed. This figure shows essentially the same features as in FIG. 1, however, there is also indicated on the anode 19 the electrically conductive surface 21, greatly exaggerated for illustration, in fact being on the order of from 1 to 5 microns in thickness.
In the following example, there are described various embodiments to illustrate the invention. However, it should be understood that the invention is not intended to be limited to the specific embodiments.
EXAMPLE
160 g of neoprene W, 160 g of carbon black, 16 g of aromatic petroleum plasticizer, 15 g of naphthalenic petroleum plasticizer, 0.8 g of ethylene thiourea as accelerator, 139 g of powdered metallic nickel, 76 g of green nickelous oxide, 6.4 g of magnesium oxide in oil as a scorch resistant agent and 11.5 g of monobenzone type anti oxidant were throughly admixed and vulcanized in the usual fashion. The resulting neoprene had a durometer hardness rating of 65-70. A 40 mil sheet was cut into circular gaskets 2 inches in diameter, said gaskets being abraded to expose the nickel.
In order to determine the degree of protection from crevice corrosion, a sandwich of alternating discs of titanium and neoprene prepared above were clamped in a press of a titanium frame, nuts and bolts. A second test was run with alternating layers of titanium and conventional neoprene without the metallic nickel-nickelous oxide mixture. The sandwiches were then submerged for 13 days into a refluxing aqueous solution of 25 g/l of sodium chloride with a pH of 1.3 after which the sandwiches were checked for corrosion. The sandwich with conventional neoprene exhibited heavy titanium corrosion and build up of deposits while the sandwich of the invention was free of corrosion.
Various modifications of the invention may be made without departing from the spirit or scope thereof, and it is to be understood that the invention is to be limited only as defined in the appended claims.