Patent Application
20060222817
This invention relates to metal electrodes and more particularly to edge strips formed on cathodes used in metal electrolysis and the method of making those edge strips.
In electrolysis of metal, electrical current is passed from anodes through a metal-containing electrolyte (e.g., sulfuric acid) to sheet-like blanks that serve as cathodes. The metal is deposited on the blanks by electrolysis. If the metal were allowed to grow along the entire blank, encircling it, it would be very difficult to harvest the metal growth, which is typically a 7-day growth that has accumulated on the blank. The blanks are pulled from the bath and flexed to cause the metal growth to be released. Along two or three edges of the blanks that form the cathodes, edge strips are positioned to prevent the metal growth from extending around the edges of the blanks.
Damage to the outer surfaces of the edge strips is a problem. Exterior surface cracks, cuts, gouges, and nicks on the outer surface of the edge strips are caused by everyday operating procedures such as transportation of the cathodes to receiving racks when harvesting the metal, stripping and transporting back to electrolytic cells.
Damaged areas can allow electrolyte to penetrate through or under the edge strips resulting in irreparable damage to the strips. Essentially a bad situation quickly becomes a hopeless one. As the damaged areas permit acid to penetrate through the edge strips, deposited metal grows. This means that the protected edge around the blank is now exposed. Once deposited metal contained in the electrolyte bath attaches itself to the exposed portions of the blank, it grows to form a nodule. This nodule continues to grow larger over the seven day growth period. The longer the blank is submerged in the electrolyte bath, the larger the nodule(s) will grow. As the nodule grows, it pushes against the edge strip, forcing the cracks, gouges and the like to enlarge and forcing the edge strip away from the blank to further expose the previously covered portions of the blank to the electrolyte bath. Once this happens, the edge strip must be replaced as repairs to it are not feasible.
Corners present an additional area of concern for three-sided edge strips. When installing a bottom edge strip on the blank, corner pieces are glued to the side edge strip. All three separate pieces are glued together. Those corners cannot be adequately sealed so eventually acid finds a void or deteriorates the glue exposing the previously covered portions of the blank, which in turn causes nodule growth that damages the edge strip as described.
A two-sided edge strip, on the other hand, has an additional bottom plug problem. These plugs are attached to the bottom of the edge strip, some are glued on and some are molded to the edge strip. Again, failures are caused by either acid attacking the glue or by mechanical abuse during transportation and stripping of deposited metal. Once again, nodules grow on the exposed portions of the blank that were intended to be covered by the edge strip. This exposure leads to the previously described irreparable damage.
In accordance with the present invention, edge strips are molded or sprayed in-situ onto cathode blanks. By this means, a polyurethane edge strip is mechanically attached to the external edges of the blanks. While this edge strip covers the same outer edge as the current plastic edge strips, it is narrower and allows more metal to deposit on each blank. This is a production increase with bottom line impact.
Three-sided edge strips formed in accordance with the invention are monolithic. Where previously corner pieces commonly admitted acid to the underlying portions of the blank, this inhibits the growth of deposited metal. Unlike prior edge strip technologies, spot repairs are easy and quick. Reapplication of the mold or respraying permits the formation in-situ of a new section of edge strip where a damaged area has been removed.
Polyurethane provides a significantly improved edge strip by virtue of its resistance to the electrolyte and its greater endurance to physical abuse. The edge strips last longer. The manufacturing process to apply the polyurethane edge strip provides substantial cost savings compared to current systems. Linear thermal elongation of the polyurethane edge strip offers superior performance when the blanks are flexed to remove metal deposited thereon.
In one preferred embodiment, the edge strip may comprise a polyurethane formulation, such as that used in tank liners.
In addition, because the improved edge strips need to be removed and replaced less often, substantial labor savings result. Because the polyurethane is substantially 100% solid and inert and even after usage is solvent, CFC, HCFC and VOC-free, disposal is a non-issue. There is no need to haul the polyurethane edge strips to a hazardous waste facility. The regulatory paper trail required for disposal is eliminated and labor is saved in the reduction of transportation of the polyurethane. There is 100% elimination of rolloffs and the maintenance associated with rolloffs.
According to an embodiment of the present invention, polyurethane edge strips are formed in-situ, directly on the stainless steel cathodes. In one exemplary embodiment the edge strips are formed on sheet-like metal blanks, which serve as cathodes, by molding. In another the strips are formed on the blanks by spraying.
Edge areas of the blanks are first sand blasted where the edge strips are to be formed so that the applied polyurethane will be well anchored. A commercially available primer for polyurethane is then applied to the areas to be covered by the polyurethane.
In an embodiment in which the strips are to be molded, mold parts (treated with a release agent) are fitted to the blanks along the edges where the strips are to be formed. Liquid polyurethane is then introduced into the molds in contact with the sandblasted and primed edge areas on both sides of the cathodes and around the edges. The polyurethane can be directed into the mold by gun or a movable tube connected to a pressurized source of the polyurethane. When the polyurethane has set, the mold-forming parts are removed.
When the technique for forming the edge strip is spraying, after sandblasting, the edge areas to be coated are defined by masking. Primer is applied. Liquid polyurethane is then sprayed onto the sandblasted and primed edge areas where the edge strips are to be formed.
Thus created in-situ, the edge strips are continuously adherent to the blanks at the steel-polyurethane interface and little opportunity is afforded for electrolyte to find its way beneath the edge strip to the covered metal portions of the cathode.
If damaged, the edge strips are readily repaired by removal of the polyurethane in the area of the damage. The underlying blank is sandblasted and primed. The mold is reapplied or masking is applied and fresh polyurethane is applied by molding or spraying to repair the damaged area.
In yet a further embodiment, voids in preexisting edge strips are filled with the polyurethane formulation, which advantageously strengthens the edge strips.
Turning to
Where a three-sided edge strip is employed, a further strip 22 similar to the strip 18 in
In
In an exemplary embodiment of the invention, each edge of the stainless steel blank 10 that is to have an edge strip formed along it has a strip at least ⅝ inches in width sandblasted to a 2 mil angled profile as shown in
Subsequent to sandblasting or otherwise abrading the edges in this manner, a suitable, commercially available polyurethane primer is applied to all edge areas that are to be covered by the edge strips to be formed.
In the exemplary embodiment discussed above, the sandblasted areas are masked with ¾ inch masking tape along the interior borders of the ⅝ inch wide strips and primer is brushed, rollered or sprayed onto the exposed portions of these ⅝ inch wide strips on both sides of the blanks 10. If sprayed on as a solvent diluted solution, the primer is allowed to dry completely. A suitable solvent is acetone sufficient to effect no more than a 5% dilution. If primer is rolled on undiluted, it is allowed to become tacky before any polyurethane is applied. The manufacturer's recommended time window for applying the polyurethane should be strictly observed.
When the edge strips are to be molded in place, polypropylene mold pieces 30 and 32 are first wiped clean using acetone and a clean rag. Mold piece surfaces that will contact the polyurethane have a release agent, such as a silicone spray or a wax applied thereto. The blank 10 is positioned on a mold piece 30. A mold positioning guide (not shown) is run between the mold and blank edge to assure proper positioning. The mold pieces 32 are then clamped along the edges on which edge strips are to be formed as illustrated in
In an embodiment of the present invention, the polyurethane is a polyurethane formulation, such as that used in tank liners. Examples of a suitable polyurethane formulations, for the purposes described herein, are Durabond™ Polyurethane or Hi-Chem™ Polyurethane, both of which are commercially available from Rhino Linings USA, Incorporated, of San Diego Calif. as described at the website http://www.rhinoliningsindustrial.com, incorporated herein by reference.
Desirably, the polyurethane formulation has good abrasion resistance, a strong adhesion to metal substrate and is flexible. In an embodiment, the polyurethane formulation has the following properties: a hardness (shore D) in the range of about 45-70, more particularly about 45-50 for processes using a lower temperature electrolyte solution and about 70 for higher temperature solutions; a tensile strength (psi) in the range of about 2100-2460, more particularly about 2100-2200 for lower temperature solutions and about 2460 for higher temperature solutions; an elongation factor (%) in the range of about 25-80%, more particularly about 50-80% for lower temperature solutions and 25-30% for higher temperature solutions; a Taber abrasion resistance (mg of loss/1000 cycles) of a CS17 wheel and 1000 grams of weight is in the range of about 25-53, more particularly 25-30 for lower temperature solution and 53 for higher temperature solution. It is also desired that the polyurethane formulation has characteristics that will hold up to exposure to 25% sulfuric acid.
In an alternative application of edge strips in accordance with the invention, as seen in
Again, because of the intimate association of the polyurethane with the underlying portions of the blank 10, little opportunity exists for the electrolyte to invade. If physical damage is done to edge strips so formed, the polyurethane in that area is removed, as previously described. The edge to which polyurethane is to be reapplied is then sandblasted and primed and the blank 10 is then masked and new polyurethane is sprayed in place.
With reference to
In an alternative application of edge strips in accordance with embodiments of the invention, the edge strips are injection molded around the blank 10. Polypropylene mold pieces 60 and 62 are first wiped clean using acetone and a clean rag. Mold piece surfaces that will contact the polyurethane have a release agent, such as a silicone spray or a wax applied thereto. The blank 10 is positioned to allow the solution to leave the exit holes in the mold at the point of least resistance (e.g., the bottom of the cathode to allow a gravity feed). The mold pieces 60 and 62 are then clamped along the edges on which edge strips are to be formed. Suitable clamps 36 are used for this purpose, such as vice grips or C clamps or other commercially available clamping devices as desired. Hot liquid polyurethane is introduced into a slot (not shown) formed along one edge of the mold pieces 60 and 62. In an exemplary embodiment, both sides of the recess formed by the mold parts 60 and 62 and blank edge is filled in a first pass. The polyurethane is permitted to cool and harden and then the molds 60 and 62 are removed to reveal the finished polyurethane edge strip 40.
While specific preferred embodiments have been described above, it will be readily understood that modifications can be made in the edge strips according to this invention and their method of manufacture without departure from the spirit and scope of this invention. For example, the blank 10 may comprise various types of metals, including stainless steel or aluminum. The metal deposited onto the blanks 10 by electrolysis may include copper, zinc, nickel and cobalt. The polyurethane formulation may include a variety of compositions. Moreover, various methods of electrolysis may be used in connection with the practice of the present invention, including known methods of electrorefining and equivalents thereof. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.
