Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1983-10-19
1985-08-20
Valentine, Donald R.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
423327, 501127, C25C 308, C01B 3326, C04B 3510, C04B 3516
Patent
active
045362734
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a diffusion barrier for the bottom lining of electrolysis furnaces for the preparation of aluminium by electrolysis of alumina according to the Hall-Heroult process. If desired, said diffusion barrier may represent the only insulating lining in the furnace. The diffusion barrier is intended to form a barrier against liquid metal and particularly against liquid and gaseous bath components which normally penetrate into the lining through the pore system, joints and cracks in the materials involved. As a consequence of the penetration the heat conductivity of the lining will increase, and the heat loss from the furnace will increase. Metal and bath components may also react with the insulating materials in the lining, and the reaction products may be of low viscosity and penetrate further downwards into the lining.
According to Chapman, J. C. and Wilder H. J., Light Metals, 1978, vol. 1, page 303 the methods which have previously been used to prevent--or limit--the penetration into the insulating lining of aluminium electrolysis furnaces, may be divided into three main groups: between the cathode (carbon lining) and the insulating bricks. included, whereby a dense layer ("crust") is formed through the reaction between the alumina and the bath components present. graphite material, "Grafofil" from Union Carbide Corporation, supported by a thin steel sheet which also serves as a barrier against sodium gas.
From U.S. Pat. Nos. 3,773,643 and 3,779,699 it is known to use sheet glass as a diffusion barrier in electrolysis furnaces for the preparation of aluminium by electrolysis of aluminium chloride. However, such sheets may suitably also be used in the electrolysis of alumina dissolved in a fluoride melt.
Although the use of sheet glass represents an essential improvement it will not always provide a complete safeguarding against the leakage of liquid bath components, particularly sodium fluoride. This is particularly the case if cracks in the glass or gaps between the glass sheets should occur so that the glass does not bond sufficiently together. Thus, there exists a need for a further safeguarding against the leakage of liquid material and penetration into the insulating lining underneath.
According to the present invention there may be established a diffusion barrier, possibly in combination with sheet glass, of materials (bricks, insulating bricks or granular materials) having such a composition that upon reaction with penetrating sodium fluoride-containing melt they form solid compounds at the operation temperature of the furnace. Thereby the amount of molten phase is reduced so that melt infiltration of the insulating lining underneath is inhibited or prevented. As a further safeguarding against penetration a metal sheet may also be placed on the underside, possibly on the underside of the sheet glass if such is used, or the metal sheet may be interposed between two glass sheets.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a phase diagram.
FIG. 2 illustrates the construction of a diffusion barrier of the present invention.
FIG. 3 illustrates an X-ray diffractogram of a sample obtained in a experiment described below.
Examination--see for instance Dell, M. B., J. Met. 23, 18 (1971)--of materials from used bottom linings of aluminium electrolysis furnaces indicates that the molten phase which has come in contact with the insulating lining consists of cryolite, Na.sub.3 AlF.sub.6, with a certain excess of sodium fluoride, and minor amounts of dissolved calcium fluoride, CaF.sub.2, and alumina, Al.sub.2 O.sub.3. The eutectic temperature in the partial system NaF--CaF.sub.2 --Na.sub.3 AlF.sub.6 has been determined (Fedotieff, P. P., Iljinsky, W. P., Z. anorg. Allgem. Chem. 129, 93 (1923)) to be about 780.degree. C., and the eutectic composition may be read to be about 70.2 percent NaF, 6.4 percent AlF.sub.3 and 23.4 percent CaF.sub.2 on a molar basis (cf. FIG. 1, point E.sub.1). This corresponds to 55.5 percent by weight of NaF, 10.1 percent by weight of AlF.sub.3 and 34.4 percent by
REFERENCES:
patent: 3723286 (1973-03-01), Hunt et al.
patent: 3764509 (1973-10-01), Etzel et al.
patent: 4175022 (1979-11-01), Vadla et al.
patent: 4411758 (1983-10-01), Hess et al.
Sintef
Valentine Donald R.
LandOfFree
Diffusion barrier for aluminium electrolysis furnaces does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Diffusion barrier for aluminium electrolysis furnaces, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Diffusion barrier for aluminium electrolysis furnaces will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-559407