Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-06-27
2001-05-01
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C420S563000, C420S566000, C148S706000
Reexamination Certificate
active
06224723
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved electrowinning anode particularly for zinc electrowinning. The anode consists of a rolled lead-silver alloy, preferably a lead-calcium-silver alloy, with controlled surface grain structure. Because the anode is used in zinc electrowinning, it should contain no tin. The surface grain structure is formed by a combination of anode chemistry, rolling and heating, preferably while rolling. When placed in a zinc electrowinning cell, the anode surface is rapidly covered with an adherent oxide coating.
BACKGROUND OF THE INVENTION
A zinc electrowinning tankhouse uses cast lead-silver alloy anodes. Silver is added to lead anodes for electrowinning to reduce the rate of corrosion of the anodes in use. Lead anodes used in zinc electrowinning generally contain 0.5-1.0% silver. Lead-silver anodes used for zinc electrowinning contain no tin because tin will plate on the negative anode and prevent zinc deposits.
To produce good quality zinc the cathode in an electrowinning cell must contain less than 10 ppm lead. In order to reduce lead contamination of the cathode, the lead anode must be coated with a protective layer of PbO
2
/MnO
2
. The silver present in the anode decreases the rate of initial oxidation of the anode surface leading to an extended time period before a stable oxide film is produced. Conditioning new anodes by developing a PbO
2
/MnO
2
layer on the surface normally takes many weeks. The complete formation of this layer may take as long as 60-90days. Until the anode is fully conditioned, the zinc cathodes in electrowinning cells experience high lead contents, high numbers of nodules and poor current efficiency. In addition, zinc production is substantially reduced as manganese ions are recirculated between anode and cathode as MnO
2
spalled off the anode is reduced at the cathode to produce MnSO
4
. The production of zinc from a cell containing new unconditioned anodes may produce as much as one-third less zinc than corresponding conditioned cells.
Once a stable layer of PbO
2
/MnO
2
is formed on the anode, the current efficiency of the zinc electrowinning process increases dramatically, and the lead contamination of the resultant cathodes also decreases dramatically. Production of a stable PbO
2
, or PbO
2
/MnO
2
layer via pretreatment of the anode is described by Ecgett et al. in U.S. Pat. No. 3,880,733, Gaunce et al. in U.S. Pat. No. 3,392,094, Fountain et al. in U.S. Pat. No. 3,755,112, as well as R. H. Farmer in “Electrometallurgy” ed. H. Baker 1969. As described therein, a stable PbO
2
layer/MnO
2
layer is typically created by the immersion of the anodes in a preconditioning solution in which the anodes are electrolyzed to produce corroded layers. In some cases the anodes are first immersed in water or water and air to produce a PbO, Pb(OH)
2
, or PbCO
3
film which is more readily oxidized to a protective PbO
2
layer than the normal cast or rolled surface. Rodrigues and Meyer, in “EPD Congress 1996” ed. G. Warren, describe the use of sandblasting to aid in preconditioning anodes.
Lead-silver alloy anodes are relatively weak. In use, they can become warped and bent leading to short circuits between the anode and cathode, low current efficiency, and lead contamination of the cathodes in the area of the short circuit. To improve the mechanical properties of the lead-silver anodes alloying elements such as calcium, strontium, barium and others have been added to the anodes to improve the mechanical properties. For example, UK patent application GB 2149424A by M. J. Thorn teaches an alloy containing 0.4-1.0% Ag, 0.05-0.15% Ca/Sr, less than 0.0002% antimony and optionally barium to reduce calcium losses during remelting.
Production of cast lead-silver or lead-silver-calcium anodes often results in the formation of numerous holes, voids or laps in the anode surface. In use, these can initiate internal corrosion in localized areas which can weaken the anode and cause warping. When the anodes are periodically cleaned of the adhering MnO
2
deposit, the internal corrosion may cause cracking which can lead to premature anode failure.
To reduce the presence of internal porosity or laps, lead-silver or lead-calcium-silver alloys have been rolled into sheets. These sheets have been joined to a copper busbar by various means but primarily by welding the rolled sheet to lead which has been cast around the copper busbar. The rolled sheet generally has a smooth surface on which it is more difficult for the PbO
2
/MnO
2
corrosion product to produce an adherent film. In addition, the grail structure is uniform and is oriented in the rolling direction producing a grain structure with few grain boundaries available for corrosion and attachment of the oxidized film.
The improvement taught by this invention is the rolling of a cast billet of lead-silver alloys and treatment of the alloy during or after rolling at a temperature sufficiently high to produce a surface on which the PbO
2
/MnO
2
layer more readily adheres due to a grain structure having many grain boundaries. The grain structure is nonuniform (i.e., not oriented in the rolling direction). These anodes have more satisfactory mechanical characteristics than prior art cast anodes and can be conditioned much more rapidly than prior art rolled anodes.
SUMMARY OF THE INVENTION
This invention relates to a lead-silver anode for zinc electrowinning having a randomly oriented grain structure with many grain boundaries. The anode is formed by rolling a cast lead-silver alloy and heat treating the alloy either during or after rolling at a temperature sufficiently high to cause recrystallization of the alloy and to prevent most or all of any calcium, barium and/or strontium present in the alloy from precipitating from solution. Because the anode is used for zinc electrowinning it contains 0% tin. In anodes formed via this procedure, finely divided silver particles form during solidification and prevent gross grain structure growth while the high temperatures result in a material with a recrystallized grain structure with many grain boundaries. The material is also without stresses induced by rolling. A temperature greater than about 100° C. and preferably above about 150° C. is typically required to produce the proper grain structure.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, a lead-silver anode containing 0% tin for use in zinc electrowinning, is formed preferably by rolling a cast lead-silver alloy at a temperature high enough to cause recrystallization of the alloy. The temperature is also high enough to prevent precipitation of any alloying elements, such as barium, calcium or strontium, during the rolling process. As a result, an alloy is formed having, a grain structure with many grain boundaries to which the PbO
2
/MnO
2
layer may adhere more readily than in prior art alloys rolled at lower temperatures.
A lead alloy suitable for use in the practice of the invention may contain as little as about 0.30-0.45% silver. A preferred alloy also contains no more than about 0.08% calcium and preferably at least 0.03% calcium. A more preferred alloy contains about 0.04-0.07% calcium and about 0.3 to 0.5% silver, most preferably about 0.065% calcium and about 0.35% silver. The alloy may contain other alloying elements, including barium, strontium and other materials which enhance the mechanical properties of an anode. The alloy may also contain small amounts of aluminum to reduce the oxidation of the reactive alloying elements. The alloy must not contain any tin as tin will prevent zinc from depositing.
If the silver content of the lead alloy used to make the anode of the invention is too low, there are insufficient silver particles to restrict the growth of the grains during the hot rolling process. If the silver content is too high, the cost of the alloy is excessive.
If the calcium content of the lead alloy is too low, the improved mechanical properties attributable to calcium will not be achieved. If the calcium content of the invention is hi
Morgan Clifford E.
Prengaman R. David
Bell Bruce F.
Gillis Theresa M.
RSR Technologies, Inc.
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