Compositions: ceramic – Ceramic compositions – Refractory
Patent
1997-03-11
1998-04-28
Marcantoni, Paul
Compositions: ceramic
Ceramic compositions
Refractory
501143, 501153, 501154, 106692, C04B 35185
Patent
active
057444137
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Conventional virgin aluminum production typically involves the reduction of alumina which has been dissolved in a cryolite-containing electrolyte. The reduction is carried out in a Hall-Heroult cell containing a carbon anode and a carbon cathode which also serves as a container for the electrolyte. When current is run through the electrolyte, liquid aluminum is deposited at the cathode while gaseous oxygen is produced at the anode. Because the heat flux of the cell must be strictly maintained, insulating materials, such as dense firebrick, are used under the carbon cathode container.
Although cryolite is quite useful in dissolving alumina, it nonetheless compromises the long term life of the cell. Upon electrolysis, the cryolite ionizes into highly reactive species, in particular, sodium and fluorine ions and aluminum radicals. Over time, these species pass through the carbon cathode containment and contact the firebrick. Upon contact, the species attack the firebrick and gradually degrade its physical integrity to the point where the firebrick must be replaced.
Previous attempts to solve this problem have met with mixed success. Vapor barriers such as steel sheets have been placed between the cathode and the firebrick. However, these sheets are eventually consumed by the cryolite and produce contamination. Glass sheets have been similarly placed in an attempt to produce a viscous liquid barrier between the cathode and the firebrick, with similar results. Other efforts have used castables and dry vibrated cements ("DVC") such as mullite as the protective barrier. Although these materials react with the cryolite to produce a relatively stable assemblage, the silica and alumina contained therein are highly susceptible to attack from the ionized cryolite species.
One effort to improve upon high alumina castable barriers, U.S. Pat. No. 4,126,474, added barium to the castable. Although this effort improved cryolite resistance, barium is a heavy metal and so carries with it environmental concerns.
DE 4128963 discloses a fluorine-resistant refractory material comprising lime particularly suited for use as a lining in aluminum production vessels.
U.S. Pat. No. 5,389,583 and corresponding EP 0 464 757 A1 disclose a novel ceramic composition suitable for use as an electrical refractory wherein the improvement concerns at least partial replacement of a feldspar component. Each of the examples disclosed in U.S. Pat. No. 5,389,583 requires at least 10% clay.
JP-A-57129880 discloses refractories having graphite, clay and 5-15% potash feldspar, which are used as linings for materials which contact high temperature molten metals.
Accordingly, it is the object of the present invention to provide a dry vibrated cement ("DVC") or castable refractory which offers superior resistance to cryolite penetration without posing environmental concerns.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a Hall-Heroult cell for aluminum production comprising a cryolite-resistant liner comprising: consisting of feldspar and nepheline.
Also in accordance with the present invention, there is provided a cryolite-resistant refractory composition consisting of: consisting of feldspar and nepheline, and
DETAILED DESCRIPTION OF THE PRESENT INVENTION
It has been unexpectedly found that small amounts of feldspar and/or nepheline impart the necessary bonding, sealing, penetration resistance and erosion resistance to alumina-silica cement mixtures (e.g., a mullite-based cement) to provide a cryolite-resistant refractory product more capable of withstanding the rigorous conditions of the aluminum reduction cell.
Without wishing to be tied to a theory, it is believed that the sealant increases resistance to cryolite penetration in two ways. First, at typical operating temperatures, the sealant forms a viscous glass which seals the pores of the alumina-silica refractory from cryolite penetration. It is believed the large ionic radii of the cations in the alkali- and alkaline earth-b
REFERENCES:
patent: 4615953 (1986-10-01), Ichikawa et al.
patent: 5556817 (1996-09-01), Cortellini
DiMauro Thomas M.
Marcantoni Paul
Saint-Gobain Norton Industrial Ceramics Corporation
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