Explosion bonding of bipolar electrode backplates

#The present invention relates generally to an electrolytic cell of the filter press type wherein a series of bipolar electrodes with diaphragms or membranes sandwiched in between can be used for electrochemical production of alkali metal hydroxides and halogens. More particularly, the present disclosure relates to an improved method for connecting the backplates of the bipolar electrodes by an explosion bonding process which provides the essential electrical and mechanical connection therebetween while leaving sufficient air space to allow hydrogen gas to escape from within the cell thereby preventing hydrogen embrittlement of the titanium anode backplate.

#Chlorine and caustic (sodium hydroxide) are essential and large volume commodities which are basic chemicals required in all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkali metal chlorides, with a major proportion of current production coming from the diaphragm type electrolytic cells. These cells generally have a plurality of electrodes disposed within the cell structure to present a plurality of rows of alternatively spaced anodes and cathodes. These electrodes are generally foraminous in nature and made of a screen or mesh material so that a hydraulically permeable diaphragm may be formed over the cathode. This compartmental cell will allow fluid flow through the cell structure. Brine (sodium chloride) starting material is continuously fed into the cell through the anode compartment and flows through the diaphragm backed by the cathode. To minimize back-diffusion and migration through the hydraulically permeable diaphragm, the flow rate is always maintained in excess of the conversion rate so that resulting catholyte solution has unreacted alkali metal chloride present. This catholyte solution, containing sodium hydroxide, unreacted sodium chloride, and certain other impurities, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and a sodium chloride solution to be reused in the diaphragm electrolytic cell. This is a serious drawback since the costs of this concentration and purification process are rising rapidly.

#With the advent of technological advances such as the dimensionally stable anode which permits ever narrowing gaps between the electrodes, and the hydraulically impermeable membrane, other electrolytic cell structures are being considered. The geometry of the diaphragm cell structure makes it inconvenient to place a planar membrane between the electrodes, hence the filter press electrolytic cell structure with planar electrodes has been proposed as an alternate electrolytic cell structure.

#FIG. 1 is a perspective view of the anode and cathode backplates of a bipolar electrode with the mechanical and electrical connection effected therebetween by explosion bonded solid metallic strips therebetween according to the concepts of the present invention.

#FIG. 2 is a perspective view of an alternate embodiment of a bipolar electrode according to the concepts of the present invention.

#FIG. 3 is a side section view of the second embodiment of the bipolar electrode taken substantially along line 3--3 of FIG. 2.

#Referring to the drawings, numeral 10 generally refers to a bipolar electrode assembled by a method according to the concepts of the present invention. The bipolar electrode 10 has an anode 12 which is generally foraminous in nature and can be made of a screen or mesh material of an appropriate metallic substance. Such foraminous anodes 12 may be made of any conventional electrically conductive electrolytically active material resistance to electrolyte and preferably, a valve metal such as titanium or tantalum or alloys thereof bearing on the surface a noble metal, and noble oxide (either along or in combination with the valve metal oxide), or other electrocatalytically active corrosive resistant material. Anodes of this preferred class are called dimensionally stable anodes and are well know and widely used in the industry. Foraminous anodes such as anode 12 shown in FIG. 1 are generally preferred because of their greater electrolytically active surface areas which facilitate the electrochemical reaction and flow within the compartments of an electrolytic cell. Each bipolar electrode 10 also has a cathode 14 on the reverse side thereof. The cathode 14 similarily may be made of any conventional electrically conductive material resistant to the catholyte, examples including iron, mild steel, stainless steel, and nickel. The cathodes are preferably foraminous, similar to the anodes. The bipolar electrode 10 has an anode backplate 16 and a cathode backplate 18 each of which acts as a supporting base for the anode 12 and the cathode 14 respectively. Generally the anode backplate 16 will be made of the same material as the anode 12 such that conventional resistance weldments may be accomplished between the anode 12 and the anode backplate 16. Similarly, the cathode backplate 18 and cathode 14 are generally made of the same material for ease of connection therebetween. The bipolar electrode 10 as shown in FIG. 1 when fitted into a filter press electrolytic cell will have either a frame surrounding the peripheral edge of the bipolar electrode or the backplates 16 and 18 may be pan shaped so as to present clamping flanges so that a liquid tight engagement between a series of these bipolar electrodes 10 can be accomplished. For ease of illustration, these supporting structures or frames have not been shown. The anode 12 and cathode 14 are connected respectively to the anode backplate 16 and cathode backplate 18 by riser posts or current distributors 20 which are also made of a material corresponding to the materials of the anode 12 and anode backplate 16 and the cathode 14 and cathode backplate 18 respectively. This facilitates use of conventional welding techniques for attaching the anode 12 and cathode 14 to their respective backplates.

#It is desirable to use a valve metal for the anode 12 and the anode backplate 16 since this compartment contains an anolyte which normally has highly corrosive concentrations of free halide which can cause corrosion of the anode 12 and anode backplate 16. The anode backplate 16 will generally have a thickness of 0.040 to 0.080 inch (1.016 to 2.032 mm) when titanium is used. The cathode 14 and cathode backplate 18 need not be of such an expensive valve metal since the catholyte is not nearly so corrosive and generally steel will be used for the cathode 14 and cathode backplate 18. The cathode backplate 18 will generally have a thickness of 0.080 to 0.50 inch (2.032 to 12.7 mm) with a preferred thickness of 0.25 inch (6.35 mm) when steel is used. Since it is believed that hydrogen ions generated at the cathode can migrate to the anode backplate and anode of prior art constructions causing hydrogen embrittlement it is necessary to leave some kind of barrier to these ions between the anode backplate 16 and cathode backplate 18. Any insulative material can be used which will resist the flow of atomic hydrogen therethrough and it has been found that air provides such an insulative property very inexpensively since the atomic hydrogen generally combines to form molecular hydrogen which is vented off before the atomic hydrogen reaches the anode backplate 16. Copper also provides a good barrier to atomic hydrogen flow but would be rather expensive if solid copper sheet was used between the backplates. Copper does provide excellent electrical properties though, so to provide this kind of insulative barrier at a lower cost, a spaced series of metallic electrical conductor 22 such as copper strips is placed between the anode backplate 16 and cathode backplate 18 so as to conduct an electrical current and yet provide an insulative zone between the anode backplate 16 and cathode backplate 18 to prevent hydrogen embrittlement of the anode backplate 16. This metallic electrical conductor 22 can be of any substance capable of carrying the necessary amount of electrical current while providing an insulator against hydrogen ion movement with copper being the preferable form because of the cost and electrical conductivity thereof.

#The method of the present invention employs an explosion bonding technique to bond the metallic electrical conductor 22 to the anode backplate 16 and cathode backplate 18 in either a one step or two step process. This process can be generally achieved by supporting a layer of one material parallel to the surface of the other material, the inside surfaces being spaced apart slightly and placing on the outer surface of one layer a detonating explosive having a velocity of detonation less than 120 percent of the velocity of sound in that metal in the system having the highest sonic velocity and thereafter initiating the explosive layer. Usually it is desirable to use an explosive having a detonation velocity not greater than the velocity of sound in that metal with the higher sonic velocity. The metal layers must be separated from each other a distance at least sufficient for the explosive propelled layer to achieve an adequate velocity before impact with the stationary layer, a spacing of 0.001 inch (0.0254 mm) between the facing surfaces of the two layers represents the minumum spacing to produce consistently adequate results. The maximum separation will almost depend almost entirely upon the reduction of the velocity of the propelled layer caused by the air layer between the two metals. By increasing the explosive loading or evacuating the space between the layers, spacings much greater than 0.001 inch (0.0254 mm) are feasible. In general, however, separation of more than 0.5 inch (12.7 mm) is not convenient or necessary.