Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
1998-10-06
2001-02-13
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S372000, C205S377000, C205S378000, C205S384000, C205S391000, C204S243100, C204S247300, C204S290010, C204S291000, C204S292000, C204S294000
Reexamination Certificate
active
06187168
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrolysis of a metal oxide dissolved in a molten salt bath wherein a solid oxide ion conductor separates the anode from the bath. More specifically the invention relates to production of aluminum from alumina dissolved in a fluoride-chloride molten salt bath.
BACKGROUND OF THE INVENTION
Processes for making aluminum by electrolysis of alumina dissolved in a molten salt bath are known in the prior art. Electrolytic processes in commercial use today for making aluminum involve passage of an electric current between a cathode and an anode in molten salt baths containing cryolite or other fluoride salts. The aluminum and sodium fluorides predominate, and lesser amounts of potassium, lithium, calcium, and magnesium fluorides may also be included.
In order to reduce the consumption of anode material in conventional electrolysis cells containing alumina-cryolite mixtures, Marincek U.S. Pat. Nos. 3,562,135 and 3,692,645 proposed a layer of oxygen-ion-conducting material in direct electrical contact with the anode. The oxygen-ion-conducting material is preferably zirconium oxide stabilized in a fluorite lattice by addition of calcium oxide, magnesium oxide, or yttrium oxide. However, stabilized zirconium oxide dissolves in molten cryolite and cells of Marincek's design are not in commercial use today.
In order to reduce the operating temperatures of aluminum electrolysis cells, prior art inventors have proposed molten salt baths containing various mixtures of fluoride and chloride salts. Two issued patents disclosing molten salt baths containing both fluoride and chloride salts are Lewis U.S. Pat. No. 2,915,442 and Wallace et al U.S. Pat. No. 2,915,443.
One important advantage of cells operated with mixed fluoride and chloride electrolytes is a lower bath density of about 1.74 g/cm
3
compared with about 2.2 g/cm
3
in cryolite baths. This lower density improves the stability of the bath-metal interface and allows an opportunity for a reduced interpolar distance, thereby improving cell productivity. Another advantage is reduced operating temperature, thereby enabling the use of materials that eliminate any need for a frozen layer of bath surrounding the cell top and sides. Reducing the cell temperature also reduces metal solubility in the bath, thereby improving current efficiency. Additionally, the chloride-fluoride bath electrical conductivity is higher than in conventional fluoride baths, effectively reducing resistive losses and improving current efficiency.
Reduced operating temperatures also lower sodium solubility, thereby minimizing distortion of the cathode blocks and improving overall dimensional stability of the interior lining. Reduced temperatures also extend cell life by reducing formation of aluminum carbide and its erosive effect on the cell block.
In spite of the advantages of mixed fluoride and chloride electrolytes, they are not used commercially. One concern about such electrolytes is the potential for producing chlorofluorocarbon compounds at the cell anode.
A principal objective of the present invention is to provide a process for production of metals by electrolysis in a molten salt bath, wherein the production of chlorofluorocarbon compounds as a by-product of the process is avoided by interposing a solid oxide ion conductor between the molten salt bath and the anode.
A further objective of the invention is to provide a process for production of metals by electrolysis in a molten salt bath, wherein the process can be retrofitted to existing electrolysis cells.
A related objective of the invention is to provide a novel electrolysis cell for carrying out the process of the invention.
One important advantage of our invention is that a mixed fluoride-chloride molten salt bath enables cell operation at a lower temperature than with all fluoride molten salt baths. Lower temperature operation reduces corrosion on the solid conductor of oxide ions, and enables operation at higher current densities without forming a crust on the cathode. Lower temperature and increased current density in a conventional, all fluoride molten salt bath precipitates sodium at the cathode thereby increasing voltage drop so that the cell eventually loses its ability to pass current.
Additional objectives and advantages of the present invention will become readily apparent to persons skilled in the art, from the following detailed description.
SUMMARY OF THE INVENTION
The potential for producing chlorofluorocarbon compounds in an electrolysis cell can be reduced or even eliminated entirely by separating the fluoride-chloride molten salt bath from the anode by a solid conductor of oxide ions. The solid conductor is preferably zirconia stabilized in cubic form by a divalent or trivalent metal oxide.
In accordance with the present invention, there is provided a cell for electrolyzing a metal oxide to make a metal. The metal oxide may be selected from the group consisting of aluminum oxide, magnesium oxide, silicon dioxide, titanium dioxide, lithium oxide, lead oxide, and zinc oxide in order to produce aluminum, iron, magnesium, silicon, titanium, lithium, lead, and zinc, respectively. Other metals may also be recovered, as will be appreciated by those skilled in the art. The metal oxide is preferably aluminum oxide, for production of aluminum.
The electrolytic cell of the invention includes an anode, a cathode, a molten salt bath, and a solid conductor of oxide ions between the anode and the cathode.
The anode may be made from a cermet such as a sintered combination of iron and nickel oxides with copper and silver. The anode material is preferably an inert substance that produces oxygen. Alternatively, the anode material is a carbonaceous substance that produces a carbon oxide, namely, carbon dioxide and/or carbon monoxide. The carbonaceous material may be prebaked carbon produced by molding petroleum coke and coal tar pitch, and then baking at 1000-1200° C.
The cathode may be made from a refractory hard metal (RHM), carbonaceous material coated with an RHM such as titanium diboride, or a carbonaceous material. As used herein, the term “refractory hard metal” refers to the borides, carbides, and nitrides of titanium, zirconium, and hafnium. The cathode is preferably wetted by aluminum which can be achieved by using a wettable material or a coating wettable to aluminum. Less preferably, the cathode may be a carbonaceous material such as prebaked carbon produced by molding a mixture of petroleum coke and coal tar pitch, and then baking at 1000-1200° C.
The molten salt bath contains an oxide of the metal to be produced, together with at least two salts selected from cryolite (Na
3
AlF
6
), the chlorides of sodium and potassium, and the fluorides of sodium, aluminum, and potassium. The molten salt bath may also contain other chlorides and fluorides in lesser amounts, including LiCl, MgCl
2
, CaCl
2
, LiF, MgF
2
, and CaF
2
. The oxide may be an oxide of aluminum, magnesium, zinc, silicon, titanium, lithium, or lead, and is preferably alumina (Al
2
O
3
).
The molten salt bath may contain about 30-90 wt. % chlorides and about 10-70 wt. % fluorides. In a preferred embodiment, the molten salt bath comprises about 15-35 wt. % NaCl, 25-45 wt. % KCl, and 30-50 wt. % cryolite.
The solid conductor has a molecular framework structure permitting oxide ions to move from the salt bath to the anode, without exposing the anode to fluorides and chlorides in the molten salt bath. The solid conductor may be zirconia, ceria or hafnia, and is preferably zirconia stabilized in cubic form by a divalent or trivalent metal oxide. Some suitable divalent and trivalent oxides include MgO, NiO, SrO, CaO, Y
2
O
3
, Sc
2
O
3
, La
2
O
3
, Gd
2
O
3
and Ce
2
O
3
.
The solid conductor may be applied as a layer, coating or film on the anode or it may be a thin membrane laminated to the anode. A preferred solid conductor is connected electrically with the anode and has a thickness of less than about 1 mm.
REFERENCES:
patent: 2915442 (1959-12-01), Lewis
patent: 2915443 (
LaCamera Alfred F.
Ray Siba P.
Aluminum Company of America
Klepac Glenn E.
Valentine Donald R.
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