The wettability of carbon/TiB 2 composite materials by aluminum in cryolite melts
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INTRODUCTION
THE Hall-Heroult
process for the production of aluminum is energy inefficient, tq This is due, in part, to the use of carbon cathode linings in the reduction cell construction. The properties of carbon, in particular, the nonwettability by aluminum, necessitate that the interpolar gap in an aluminum reduction cell be maintained at 40 to 50 mm.[2] This results in an ohmic voltage drop in the electrolyte of from 1.5 to 2.5 V, which represents between 30 to 40 pct of the total electrical energy consumption of the process. 0.41 The shortcomings of carbon have stimulated an extensive investigation into the possibility of replacing carbon cell linings with other materials, tSl A candidate replacement material is carbon/TiB2 composite, t6'71 The major advantage of this material over carbon is that it can be wetted by aluminum. An aluminum reduction cell of a drained cathode design operating at a reduced anodecathode distance, with attendant energy savings, may then be feasible. Previous studies, which were largely qualitative, have claimed TiB2 is wetted by aluminum under aluminum conditions, t2'5-s] Rhee t91 investigated the wettability of hot-pressed TiB2 (98.5 pct of theoretical density) by liquid aluminum at 2 • 10 -7 torr or less using the sessile drop technique. It was reported that the contact angle decreased with temperature from approximately 90 deg at 980 K to about 55 deg at 1110 K. Samsonov e t a l . t~~ studied the contact angle of A1 on hot-pressed TiB2 (porosity < 6 pct) under vacuum by the sessile drop method and found a nonwetting contact angle of about 140 deg at 900 ~ At 1150 ~ a reduction in contact angle was observed over time, from an initial nonwetting angle of about 110 deg to a wetting angle of about 75 deg after 20 minutes. At 1250 ~ the contact angle decreased over time from an initial angle of about 70 deg to an angle of about 30 deg after 20 minutes. Both of these studies were conducted in the absence of cryolite melts. K.D. WATSON, Graduate Student, and J.M. TOGURI, Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, ON M5S 1A4, Canada. Manuscript submitted October 30, 1990. METALLURGICAL TRANSACTIONS B
Liao and Liu tm measured the contact angle of aluminum on hot-pressed TiB2 and cathode carbon coated with a TiB2 paste in cryolite-alumina melts. An X-ray radiographic sessile drop technique was used. The TiB2 coating paste contained TiB2 powder resin, pitch, and additives. Two pastes were used, (1) one containing 40 mass pct TiB2 and (2) the other 60 mass pct TiB2. They reported contact angle values for paste 1 of between 63 and 65 deg and for paste 2 of between 51 and 57 deg. The hot-pressed TiB2 was found to be completely wetted by aluminum. In the present study, the contact angles formed by aluminum on pure hot-pressed TiB2, graphite, carbonaceous cement, and carbon/TiB2 composites in the presence of cryolite melts were determined. II.
EXPERIMENTAL
The sessile drop technique incorporating X-ray imaging of the drop profile was use
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