Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2001-01-18
2002-09-10
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S385000, C205S386000, C205S388000, C205S390000, C205S399000, C204S243100, C204S247500, C204S280000, C204S291000, C204S292000, C204S290010, C204S290150
Reexamination Certificate
active
06447667
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to production of a metal by electrolysis of a metal compound in a cell having a cermet anode. More specifically, the invention relates to protection of the cermet anode and its support structure assembly from thermal shock during cell start-up.
BACKGROUND OF THE INVENTION
A number of metals including aluminum, lead, magnesium, zinc, zirconium, titanium, and silicon can be produced by electrolytic processes. Each of these electrolytic processes employs an electrode in a highly corrosive environment.
One example of an electrolytic process for metal production is the well-known Hall-Heroult process producing aluminum in which alumina dissolved in a molten fluoride bath is electrolyzed at temperatures of about 960-1000° C. As generally practiced today, the process relies upon carbon as an anode to reduce alumina to molten aluminum. The carbon electrode is oxidized to form primarily CO
2
, which is given off as a gas. Despite the common usage of carbon as an electrode material in practicing the process, there are a number of serious disadvantages to its use.
Because carbon is consumed in relatively large quantities in the process, approximately 420 to 550 kg carbon per ton of aluminum produced, the electrode must be constantly repositioned or replenished to maintain proper anode-cathode spacing to produce aluminum efficiently. If prebaked electrodes are used a relatively large facility is needed to produce sufficient electrodes to operate a smelter. In order to produce aluminum of sufficient purity to satisfy customer standards, the electrodes must be made of having relatively low metal content carbon, and availability and cost of raw materials to make the carbon are of increasing concern to aluminum producers.
Because of disadvantages inherent in use of carbon for electrodes, some cermet materials have been developed that can operate as electrodes with a reasonable degree of electrochemical efficiency and withstand the high temperatures and corrosive environment of the smelting cell. Cermet electrodes are inert non-consumable and dimensionally stable under cell operating conditions. Replacement of carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing costs. Significant environmental benefits are achievable because inert electrodes produce essentially no CO
2
or fluorocarbon or hydrocarbon emissions. Some examples of inert anode compositions are found in U.S. Pat. Nos. 4,374,050; 4,374,761; 4,339,088; 4,455,211; 4,582,585; 4,584,172; 4,460,905; 5,279,715; 5,794,112; 5,865,980; and 6,126,799, all assigned to Alcoa Inc. These patents are incorporated by reference.
Although cermet electrodes are capable of producing aluminum having an acceptably low impurity content, they are susceptible to cracking during cell start-up when subjected to temperature differentials on the order of about 900-1000° C. In addition, ceramic components of the anode support structure assembly are also subject to damage from thermal shock during cell start-up and from corrosion during cell operation. Accordingly, there still remains a need for a means of protecting cermet electrodes and the anode support structure from thermal shock and corrosion.
A principal objective of the present invention is to provide a coating for protecting a cermet anode and its support structure assembly from thermal shock during cell start-up.
A related objective of the invention is to provide a coating for protecting the support structure assembly from corrosion during cell operation.
Additional objectives and advantages of our invention will be apparent to persons skilled in the art from the following detailed description of some preferred embodiments.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided an electrolytic cell for production of a metal by electrolytic reduction of a metal compound contained in a molten salt bath. Metals that may be produced electrolytically in accordance with the invention include aluminum, lead, magnesium, zinc, zirconium, and titanium.
A preferred embodiment of the invention relates to production of aluminum by electrolytic reduction of alumina dissolved in a molten bath containing aluminum fluoride and sodium fluoride. An electrolytic current is passed between a cermet anode and a cathode through the salt bath, producing aluminum at the cathode and oxygen at the anode.
The molten salt bath comprises aluminum fluoride and sodium fluoride, and may also contain calcium fluoride, magnesium fluoride, and/or lithium fluoride. The weight ratio of sodium fluoride to aluminum fluoride is preferably about 0.7 to 1.1. The bath ratio is preferably about 0.8 to 1.0 and more preferably about 0.96.
As used herein, the term “inert anode” refers to a substantially non-consumable anode having satisfactory resistance to corrosion and dimensional stability during the metal production process. At least part of the inert anode comprises a cermet material. As used herein, the term “cermet” refers to a material having a ceramic phase and a metal phase. Inert anodes of the present invention may be made entirely of a cermet material over a central metal core. When the cermet is provided as an outer coating its thickness is preferably about 0.1 to 50 mm, more preferably about 1 to 10 or 20 mm. The ceramic phase preferably makes up about 50-95 wt % of the cermet material, the metal phase about 5-50 wt %. More preferably, the ceramic phase comprises about 80-90 wt % of the cermet and the metal phase about 10-20 wt %.
The ceramic phase of the cermet can be composed of any suitable oxide material including one or more metal oxides selected from the group consisting of Ni, Fe, Zn, Co, Al, Cu, Ti, V, Cr, Zr, Nb, Ta, W, Mb, Hf, and any of the rare earth metal oxides and at least one additional oxide from the above list. A particularly preferred ceramic phase embodiment comprises iron, nickel, and zinc oxides.
The metal phase of the cermet material comprises a base metal, such as Cu and/or Ag replaced in whole or in part by, or mixed or alloyed with one or more metals selected from the group consisting of Co, Ni, Fe, Al, Sn, Nb, Ta, Cr, Mo, W, and the like. The metal phase also comprises a noble metal such as one or more metals selected from Ag, Pd, Pt, Au, Rh, Ru, Ir, and Os. A preferred metal phase comprises copper as the base metal with the addition of at least one noble metal selected from Ag, Pd, Pt, Au, and Rh.
The metal phase may be continuous or discontinuous. When the metal phase is continuous it forms an interconnected network or skeleton that increases electrical conductivity. When the metal phase is discontinuous, discrete particles of the metal are at least partially surrounded by the ceramic phase, which may increase corrosion resistance.
The types and amounts of base metal and noble metal contained in the metal phase are selected in order to reduce unwanted corrosion, dissolution, or reaction of the inert anodes, and to withstand the high temperatures to which the inert anodes are subjected during the electrolytic production process. For example, in the electrolytic production of aluminum, the production cell typically operates at a sustained smelting temperature above 800° C., usually about 900-980° C. Accordingly, the metal phase of inert anodes in such cells should have a melting point above 800° C., more preferably above 900° C., and optimally above about 1000° C.
The metal phase typically comprises about 50-99.99 wt % base metal and about 0.01 to 50 wt % noble metal. Preferably, the metal phase comprises about 70-99.95 wt % of the base metal and about 0.05-30 wt % of the noble metal. More preferably, the metal phase comprises about 90-99.9 wt % base metal and about 0.1-10 wt % noble metal. For every numerical range or limit set forth herein, all numbers within the range or limit including every fraction or decimal between its stated minimum and maximum are considered to be designated and disclosed by this description.
Inert anodes useful in practicing the present invention are
Bates Calvin
Stewart Patricia A.
Wieserman Larry F.
Alcoa Inc.
Bell Bruce F.
Klepac Glenn E.
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