Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-02-15
2003-05-27
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S288200, C204S297010, C204S280000, C204S281000, C205S292000, C205S575000, C205S576000
Reexamination Certificate
active
06569300
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to electrolytic processes and apparatus for refining metals and, in particular, to an improved stainless-steel clad electrode.
2. Description of the Prior Art
The principle of electrolysis has been utilized for decades to extract metals and other cations from an electrolytic solution. The extraction process is carried out by passing an electric current through an electrolyte solution of the metal of interest, such as copper, gold, silver, or lead. The metal is extracted by electrical deposition as a result of current flow between a large number of anode and cathode plates immersed in cells of a dedicated extraction tank house. In electrowinning processes, a solution of metal-rich electrolyte and various other chemicals, as required to maintain an optimal rate of deposition, is circulated through the extraction cells. The cathode is generally constructed of a metal alloy, such as titanium or copper alloys, and various grades of stainless steel which are resistant to corrosive acid solutions. In the most efficient processes, each cathode consists of a thin sheet of metal of uniform thickness, e.g., 2-4 mm, disposed vertically between parallel sheets of anodic material, so that a uniform current density is present throughout the surface of the cathode. On passing of an electric current through the anodes, electrolyte and cathodes, a pure layer of metal is electrodeposited on the cathode surface which becomes plated during the process.
Similarly, in a metal purification process in a refinery using electrodeposition, an anode of impure metal is placed in an electrolytic solution of the same metal and subjected to an electric current passing through the anode, electrolyte and cathode of each cell. The anode goes into solution and the impurities drop to the bottom of the tank. The dissolved metal then follows the current flow and is deposited in pure form on the cathode which typically consists of a mother plate of stainless steel. When a certain amount of pure metal has been plated onto the mother plate, the cathode is pulled out of the tank and stripped of the pure metal.
In both processes, the pure metal deposit is grown to a specific thickness on the cathode during a predetermined length of time and then the cathode is removed from the cell. It is important that the layer of metal deposited be recovered in uniform shapes and thicknesses and that its grade be of the highest quality so that it will adhere to the cathode blank during deposition and be easily removed by automated stripping equipment afterwards. The overall economy of the production process depends in part on the ability to mechanically strip the cathodes of the metal deposits at high throughputs and speeds without utilizing manual or physical intervention. To that end, the cathode blanks must have a surface finish that is resistant to the corrosive solution of the tank house and must be strong enough to withstand continuous handling by automated machines without pitting or marking. Any degradation of the finish of the blank causes the electrodeposited metal to bond with the cathode resulting in difficulty of removal and/or contamination of the deposited metal.
Also immensely important in the production and refining of metals by electrolytic extraction is the relationship of electrical power consumption with rates of metal production. A cell efficiency of ninety-five percent or better is the typical goal for the best operations. In order to achieve this level of efficiency, the voltage profile across the cathodic deposition surface must be held constant and variations avoided. Shorts due to areas of high current density caused by nodulization or by curved cathode surfaces which touch the anode must be prevented. Therefore, the details of construction of cathode blanks are very important to minimize operational problems and ensure high yields.
U.S. Pat. No. 4,186,074 to Perry describes a cathode for the electrolytic refining of copper that consists of a stainless steel hanger bar which is point-welded to a stainless steel mother plate hanging from the bar in a vertical position, as illustrated in FIG.
1
. The hanger bar has a flat bottom face for maximum surface contact and corresponding maximum electrical conductance with support bus-bars through which the system is energized. In order to reduce the electrical resistance resulting from the welds between the hanger bar and the mother plate, the hanger bar and the upper edge of the mother plate are uniformly clad with copper, thereby creating a low-resistance boundary between the bar and the sheet. Additionally, in order to prevent deposit build-up along the lateral edges of the mother plate which would impede the automated separation of the product at the end of each cycle, these lateral edges are masked with an insulating strip riveted to the electrode.
U.S. Pat. No. 5,492,609 to Assenmacher disclosed an improved cathode that includes a copper hanger-bar with a rounded underside that ensures the automatic vertical position of the mother plate with respect to horizontal supporting bus-bars irrespective of warpage or construction defects. The mechanical connection between the hanger bar and the mother plate is achieved by inserting the latter's upper edge into a receiving groove in the underside of the hanger bar and by welding the entire length of connection, thereby establishing a large boundary surface for good electrical conductance.
Another development in the art is a hanger bar that consists of hollow stainless-steel tubing spot-welded to the mother plate in conventional manner. The hanger bar and the spot welds are then covered with plated copper to improve electrical conductance. The problem with this hanger-bar configuration is the fact that the copper plating tends to come off the spot welds with use, thereby creating current paths with different conductivity and nonuniform current densities.
Another type of electrode, developed primarily for use in chloride-rich environments, consists of a copper hanger-bar clad in titanium and welded to a titanium mother plate. A copper tube is first covered with a layer of titanium to produce the hanger bar. The mother plate is then either fingered around or welded to the hanger bar, or both, thereby requiring a more durable and efficient titanium-to-titanium connection between the two components. Because of the materials involved, this type of electrode is relatively expensive and its use is reserved to specialized applications.
Bartsch et al. (U.S. Pat. No. 4,647,358) have disclosed a cathode with a hanger bar consisting of a hollow copper tube clad in a tubular sheath of high grade steel. The steel sheath is preferably a tube with slightly larger diameter than the copper tube and is slidably fitted over it. The electrode's mother plate, which is also made of steel, is then welded to the steel sheath cladding the hanger bar. In order to improve the contact between the copper and steel tube sections of the hanger bar, they may be deformed to an oval shape by the application of pressure. Additional contact pressure points may be provided, such as by center-punch blows, to improve current flow between the copper and steel tubes of the hanger bar. A tight seal between the copper and steel tubes also prevents diffusion of liquid and gas and ensures a satisfactory flow of current through the cross-sections of the welds.
While these inventions have provided substantial improvements over the prior art, the maintenance of a uniform current density remains a problem due to wear, pits or other faults developed in the electrode, especially at the points of connection between the hanger bar and the mother plate. The center-punch blows of the Bartsch et al. cathode tend to create stresses in the steel wrap that may cause corrosion attacks. Although its steel-clad copper hanger bar is very desirable to improve electrical conductivity with the mother plate, its open-ended tubular configuration allows contamination and acidic corrosion inside
Bell Bruce F.
Durando Antonio R.
Durando Birdwell & Janke PLC
T. A. Caid Industries Inc.
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