Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-07-24
2001-12-25
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S354000, C205S357000, C205S483000, C205S484000, C205S538000, C205S543000, C205S544000, C205S545000, C204S291000, C204S292000, C204S293000, C204S243100, C204S247300, C148S679000
Reexamination Certificate
active
06332969
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the electrolytic production of metals such as aluminum. More particularly, the invention relates to electrolysis in a cell having an inert electrode comprising at least two metal oxides, copper and a noble metal.
BACKGROUND OF THE INVENTION
The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes should allow a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce no CO
2
or CF
4
emissions. The use of a dimensionally stable inert anode together with a wettable cathode also allows efficient cell designs and a shorter anode-cathode distance, with consequent energy savings.
The most significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not react with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1000° C. It must be relatively inexpensive and should have good mechanical strength. It must have high electrical conductivity at the smelting cell operating temperature, about 950-970° C., so that the voltage drop at the anode is low. In addition, aluminum produced with the inert anodes should not be contaminated with constituents of the anode material to any appreciable extent.
A principal objective of our invention is to provide an efficient and economic process for making an inert electrode material, starting with a reaction mixture comprising compounds of iron and at least one other metal, copper and a noble metal.
A related objective of our invention is to provide a novel inert electrode comprising ceramic phase portions and alloy phase portions, wherein interior portions of the alloy phase portions contain more copper than noble metal and exterior portions of the alloy phase portions contain more noble metal than copper.
Some other objectives of our invention are to provide an electrolytic cell and an electrolytic process for producing metal, utilizing the novel inert electrode of the invention.
Additional objectives and advantages of our invention will occur to persons skilled in the art from the following detailed description thereof
SUMMARY OF THE INVENTION
The present invention relates to a process for making an inert electrode and to an electrolytic cell and an electrolytic process for producing metal utilizing the inert electrode. Inert electrodes containing the composite material of our invention are useful in producing metals such as aluminum, lead, magnesium, zinc, zirconium, titanium, lithium, calcium, silicon and the like, generally by electrolytic reduction of an oxide or other salt of the metal.
In accordance with our invention, a starting mixture is treated in a gaseous atmosphere at an elevated temperature. The mixture comprises particles containing compounds of at least two different metals and an alloy or mixture of copper and a noble metal. The compounds are preferably oxides and more preferably iron oxide and at least one other metal oxide which may be nickel, tin, zinc, yttrium, zirconium, chromium, or tantalum oxide. Nickel, zinc, and chromium oxides are preferred. Other suitable compounds of the metals include metal salts that are converted to oxides when exposed to oxygen at elevated temperatures. Such salts include the halides, carbonates, nitrates, sulfates and acetates.
The noble metal may be silver, gold, platinum, palladium, rhodium, iridium, or a mixture of such noble metals. Mixtures and alloys of copper and silver containing up to about 30 wt. % silver are preferred. The silver content is about 0.2-30 wt. %, preferably about 2-30 wt. %, more preferably about 4-20 wt. %, and optimally about 5-10 wt. %, remainder copper. The starting mixture preferably contains about 50-90 parts by weight of the metal oxides and about 10-50 parts by weight of the copper and noble metal.
The alloy or mixture of copper and silver preferably comprises particles having an interior portion containing more copper than silver, and an exterior portion containing more silver than copper. More preferably, the interior portion contains at least about 70 wt. % copper and less than about 30 wt. % silver, while the exterior portion contains at least about 50 wt. % silver and less than about 30 wt. % copper. Optimally, the interior portion contains at least about 90 wt. % copper and less than about 10 wt. % silver, while the exterior portion contains less than about 10 wt. % copper and at least about 50 wt. % silver. If desired, all or part of the silver may be replaced with one or more other noble metals.
The alloy or mixture may be provided in the form of copper particles coated with silver or other noble metal. The noble metal coating may be provided, for example, by electrolytic deposition or electroless deposition, chemical vapor deposition, or physical vapor deposition.
Particles having an average particle size of about 2-100 microns are suitable. The copper interior portion or core comprises about 75-99.8 wt. % and the noble metal exterior portion or coating comprises about 0.2-25 wt. % of the particles. When the particles are copper coated with silver, the copper interior portion preferably comprises about 85-99 wt. % and the silver exterior portion about 1-15 wt. % of the particles.
The starting mixture is treated or sintered at an elevated temperature in the range of about 750-1500° C., preferably about 1000-1400° C. and more preferably about 1300-1400° C. In a particularly preferred embodiment, the sintering temperature is about 1350° C.
The gaseous atmosphere contains about 5-3000 ppm oxygen, preferably about 5-700 ppm and more preferably about 10-350 ppm. Lesser concentrations of oxygen result in a product having a larger metal phase than desired, and excessive oxygen results in a product having too much of the phase containing metal oxides (ceramic phase). The remainder of the gaseous atmosphere preferably comprises a gas such as argon that is inert to the metal at the reaction temperature.
In a preferred embodiment, about 1-10 parts by weight of an organic polymeric binder are added to 100 parts by weight of the metal oxide and metal particles. Some suitable binders include polyvinyl alcohol, acrylic polymers, polyglycols, polyvinyl acetate, polyisobutylene, polycarbonates, polystyrene, polyacrylates, and mixtures and copolymers thereof. Preferably, about 3-6 parts by weight of the binder are added to 100 parts by weight of the metal oxides, copper and silver.
Inert anodes made by the process of our invention have ceramic phase portions and alloy phase portions or metal phase portions. The ceramic phase portions may contain both a ferrite such as nickel ferrite or zinc ferrite, and a metal oxide such as nickel oxide or zinc oxide. The alloy phase portions are interspersed among the ceramic phase portions. At least some of the alloy phase portions include an interior portion containing more copper than noble metal and an exterior portion containing more noble metal than copper. The noble metal is preferably silver.
At least part of the ceramic phase portion should have a spinel structure. Some preferred spinels have the formulas NiFe
2
O
4
, Ni
1+x
Fe
2-x
O
4
, and Ni
1-x
Fe
2+x
O
4
, wherein x is less than about 0.4.
Other suitable spinels have the following formulas:
Ni
x
Zn
y
Fe
2±z
O
4
, wherein x+y is about 0.8-1.2 and z is less than or equal to 0.3;
Ni
x
Zn
y
Fe
m
Cr
n
O
4
, wherein x+y is about 0.8-1.2 and m+n is about 1.5-3; and
Ni
x
Zn
y
Fe
m
Cr
n
Ta
p
O
4
, wherein x+y is about 0.8-1.2 and m+n+
Dawless Robert K.
Hosler Robert B.
Ray Siba P.
Woods Robert W.
Alcoa Inc.
Bell Bruce F.
Klepac Glenn E.
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