Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-05-15
2001-09-11
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C148S269000, C148S281000, C419S002000, C419S027000, C419S038000, C419S046000, C205S299000, C204S281000, C204S290010
Reexamination Certificate
active
06287433
ABSTRACT:
BACKGROUND OF THE INVENTION—PRIOR ART
Anodes for Electrowinning of Metals
Electrowinning involves the recovery of a metal, usually from its ore, by dissolving a metallic compound in a suitable electrolyte and reducing it electrochemically through passage of a direct electric current. Increasingly more metals are being produced by electrowinning because of the stringent air pollution restraints on the more conventional concentrating, smelting and electrorefining processes.
Anode material suitable for electrowinning of metals has been a source of difficulty. The requirements are insolubility in the sulfate electrolytes, resistance to the mechanical and chemical effects of oxygen liberated on the anode surface, low oxygen overvoltage and resistance to breakage in handling.
At present, lead alloy anodes have been used in most plants for the electrowinning of metals such as copper, nickel, zinc, etc. A particular problem with lead anodes is prevention of lead transfer from the anode to the electrowon metal deposited on the cathode.
Copper electrowinning consists of using an aqueous solution of copper sulfate containing free sulfuric acid, electrolyzing it with an insoluble anode, and depositing its copper content as pure copper on the cathode. Oxygen is released at the anode. Copper electrowinning cells are open concrete tanks lined with plastic or rubber, approximately 1 meter (m) across, 1 m deep and 5-15 m long. Electrodes measuring about 1 m×1 m hang vertically at intervals of about 50 millimeters (mm). The electrodes are arranged so that they are alternately anodic and cathodic, and all anodes and cathodes in a single tank are usually connected in parallel. Copper is deposited on cathode starting sheets of pure copper, stainless steel, or titanium. The electrolyte contains from 25 to 40 grams per liter (g/L) of copper and from 100 to 160 g/L of sulfuric acid. Electrowinning of copper is carried out at current densities from 160 to 270 amperes per square meter (A/m
2
) and electrolyte temperature 30-50 centigrade (° C.) (see Encyclopedia of materials science and engineering/edited by M. B. Bever.—Oxford: Cambridge, Mass.: Pergamon; MIT Press, 1986, pp. 1444-1445).
Conventional lead anodes for copper electrowinning are stabilized with antimony, calcium and/or tin, and by adding cobalt to the electrolyte, both of which inhibit electrocorrosion of lead. However, copper obtained by electrowinning using lead alloy anodes is not pure enough for wire drawing due to high lead content. The continued improvement of the anode material is critical to increase the life of the anode and the purity of the product, and to make electrowon copper suitable for most commercial uses.
Anodes for Electrolytic Manganese Dioxide Production
Electrolytic manganese dioxide (EMD) is manufactured at a large scale due to the remarkable electrochemical characteristic of EMD—its ability to function superbly as a solid-state oxygen electrode in dry-cell batteries.
EMD is produced by electrolyzing acidified manganese sulfate solution and depositing the product on the insoluble anode. Hydrogen is released at the anode.
The cells for EMD production are usually rectangular open steel troughs, lined with a corrosion-resistant nonconductive material. The electrodes measuring from about 1 m×1 m to 2 m×2 m are flat plates or series of cylindrical rods or tubes. The spacing between anode and cathode ranges from 25 to 50 mm. Cathodes are made from graphite, soft or hard lead, or stainless steel. The composition of electrolyte is maintained at about 80-150 g/L MnSO
4
and 50-100 g/L H
2
SO
4
. The electrolysis is conducted at an anode current density of about 70-120 A/m
2
and a temperature of 90-95° C. The process is terminated when the EMD layer deposited on the anode reaches a specific desired thickness. The product is stripped from the anode manually, or by an automated system. A number of continuous processes have been devised to generate the EMD as a precipitate at the bottom of the cell and to remove it without interruption of the electrolysis (see Encyclopedia of chemical technology/edited by H. F. Mark. New York: Wiley, 1984, pp.867-868).
The anodes are made mostly of graphite which tolerates high current densities without passivation. However, the gradual corrosive attack at operating conditions causes the excessive wear and the lowering of mechanical strength of graphite anodes. Anodes work for about 300 days before they break in the EMD-removal operation.
Lead, especially hard lead (with 3-8 weight percent (wt %) Sb), is also used as anode material for EMD production. At higher current densities lead contamination of the product takes place. More than 0.2 wt % Pb in EMD is undesirable because it shortens the lifetime of the batteries.
Considerable efforts to find practical titanium-based anodes and improve methods of their manufacturing to meet EMD industry's requirements are being continued.
Titanium-based Anodes
The excellent corrosion resistance of titanium in a variety of solutions and its self-oxidizing, valve-metal characteristic are recognized to be of value for electrochemical processes. With respect to mechanical stability, titanium also is the ideal anode material. However, as an anode in acid solutions, it does not pass current satisfactory because of the buildup of noncorrodible oxide coatings on the surface and passivation. By using titanium as a base metal, a series of composite anodes have been developed. To prevent formation of titanium oxide, the inert coatings on the titanium anode surface have been used. These anodes have been described as precious metal anodes (PMA), noble-metal-coated titanium (NMT), dimensionally stable anodes (DSA) and platinized titanium anodes (PTA). Noble metals and their oxides are used in the coating of titanium, in particular, ruthenium oxide, platinum oxide, platinum, platinum-iridium, which are deposited thermally or electrolytically on the titanium substrate. Several methods of applying coatings to titanium surface, using cobalt oxide, lead dioxide, manganese dioxide, mixed oxides and titanium carbide, have also been developed to improve performance of titanium anodes. All these anodes appear to have limited commercial use due to high cost and coating failure in the operating conditions of metal electrowinning and EMD production (See Encyclopedia of chemical technology/edited by H. F. Mark. New York: Wiley, 1984, pp.172-173).
Titanium-lead Anodes
Several U.S. patents relate to the anodes based on titanium-lead composite material.
U.S. Pat. No. 4,121,024, issued to Turillon et al. on Oct. 17, 1978, relates to a titanium-lead electrode for a lead-acid storage battery. The electrode is fabricated from a porous sintered metal which is lighter than lead, such as sintered titanium. This porous metal is then infiltrated with lead, a lead alloy, or by a metal wetted by pure lead. The electrode further includes a base with a protective layer of pure lead and a negative active battery mass adhered to the layer of pure lead.
The present invention does not relate to the use of the electrode for a lead-acid storage battery and does not include a protective layer of pure lead and a negative active battery mass adhered to the layer of pure lead.
U.S. Pat. No. 4,260,470, issued to Fisher on Apr. 7, 1981, relates to anodes for electrowinning of metals. The anodes are fabricated from a plurality of infiltrated sintered metal strips, such as sintered titanium infiltrated with lead. According to the patent, the strips are then connected together by longitudinally extending current carrying ribs of lead metallurgically bonded to and sheathed by the lead infiltrated sintered titanium.
Overlapping member made of said infiltrated sintered metal is a part of the next adjacent strip.
In the present invention lead ribs are not used for strips joining.
U.S. Pat. No. 4,297,421, issued to Turillon et al. on Oct. 27, 1981 discloses a composite electrode constructed from a continuous matrix of titanium infiltrated by lead. The process involves oxidizing the lead in at l
Bell Bruce F.
Niro, Scavone, Holler & Niro
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