Effect of carbon recycle materials on properties of bench scale prebaked anodes for aluminum smelting
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butt* additions are an economic necessity
*In a l u m i n u m smelting in Hall cells using prebaked anodes, the c o n s u m m a b l e carbon anodes cannot be used to extinction or the submerged portion of the metal current collectors to which they are attached would dissolve in the molten salt bath. The unreacted portion of an anode withdrawn from the cell is commonly called an anode " b u t t . "
in manufacture of prebaked anodes for aluminum smelting. Although average butts content is fixed by rate of generation and is usually in the 15 to 25 wt pct range, at times butt-free anodes may be produced and at other times butts content may be 40 pct or more. Only limited work on the effect of anode butts content has been reported.~,2 The work discussed in this report was undertaken to more completely determine the effect of butts content on prebaked anode properties and on binder requirement. In addition to butts, green scrap generated in the mixing and forming operations is often added to prebaked anodes. Effects of mixer scrap and green anode scrap were determined by comparing anodes made with 100 pct of each type of scrap with anodes made conventionally with the same coke and pitch. Environmentally acceptable disposal of used potlining is a problem of considerable concern. Although impurity levels would seem to preclude significant use in anodes of cells producing commercially pure aluminum, use in selected cells to produce alloyed aluminum may be a disposal route for some of this material. Use of 1.5 wt pct in Soderberg anodes has been reported 3 to have little effect on cell operation. The work described below was aimed at determining effects of used potlining additions on prebaked anode performance.
DAVID BELITSKUS is Section Head, Physical Chemistry Division, Alcoa Laboratories, Alcoa Technical Center, Alcoa Center, P A 15069. Manuscript submitted June 6, 1980.
GENERAL EXPERIMENTAL PARAMETERS Experimental details unique to each aspect of this work will be reported as appropriate below. General experimental parameters are as follows: Low-to-medium bulk density calcined petroleum cokes (such as cokes " I " to " L " in work reported previously 4 were used in all cases. A single lot of coal tar pitch having a softening point (cube-in-air) of 110 ~ and a quinoline insoluble fraction of 11 wt pct was used. Base aggregate consisted of petroleum coke (no butts) having the particle size distribution: 10 wt pct +8 mesh, 19 pct - 8 + 1 4 mesh, 14 pct -14+28 mesh, 12 pct -28+48 mesh, 10 pct -48+100 mesh, 14 pct -100+200 mesh, and 21 pct - 2 0 0 mesh. Aggregate and pitch were blended for 30 min at 140 ~ in a 3.8-1itre (l-gallon) sigma-blade mixer. Fifty-millimeter diameter, approximately 150 m m long specimens were pressed at 27.6 M P a (4000 psi) in a mold preheated at 140 ~ Green apparent density was calculated using volumes determined by water displacement. Green anodes were packed in calcined coke and baked at an upheat rate of 25 ~ per h to 1135 ~ and held at that temperature for 10 h. Volume change during baking, p
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