The crusting behavior of smelter aluminas
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has become more widely appreciated in recent y e a r s that the physical nature of the alumina used in aluminum reduction plants can greatly influence the operation of the cells. The use of a type of alumina for which the cell is not designed can lead to sludging, excessive open bath, poor current efficiency and g e n e r a l operational problems. The econ o m i c consequences may be serious. The satisfactory operation of an aluminum r e d u c tion cell requires a c r u s t t o be maintained on the molten bath for as l a r g e a proportion of the time as possible. Without such cover heat losses, and evaporation of relatively volatile fluorides, will be unacceptably high. A c r u s t can form either by a simple freezing of the melt (which is normally maintained at a temperature close to the liquidus), or by the addition of alumina to the surface. Freezing is important in p r a c tice and occurs fairly readily; it is, however, impeded by the circulation of the melt and the agitation caused by gas evolution from the anodes. Under suitable conditions a c r u s t will be f o r m e d by the addition of alum i n a , although this crust may b r e a k up through agitation or through its inherent weakness. The stimulus for this work was the observation (at a s m e l t e r with prebake anodes and center channel alumina feed), that a change from sandy t o floury alumina led to a considerable i n c r e a s e of open bath following routine two-hourly breaks and feeds. As open bath is a g e n e r a l symptom of a poorly operating cell, it was not certain that the crusting behavior of the alumina was the cause. It was therefore decided t o observe, under controlled conditions in the laborat o r y , the addition of different types of alumina to molten cryolite, and to deduce the factors which determine the nature and persistence of crusts so formed. Closely associated with crusting behavior is the problem of dispersion and dissolution of alumina in the melt. If, as a r e s u l t of methods of feeding or of
L. N. LESS is Research Scientist, British Aluminum Company Limited, Chalfont Technological Centre, Gerrards Cross, Buckinghamshire SL9 0QB, England. Manuscript submitted June 2 2 , 1976. METALLURGICAL
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the inherent properties of the alumina, all the added alumina enters the bath over a short time period, a slurry will tend to accumulate on or u n d e r the m e t a l pad and may lead to a persistent sludge. If, however, part of the added alumina f o r m s a c r u s t and is held on the surface, self-feeding over a long period can o c c u r and the tendency to sludging will be reduced. The solubilities of aluminas in cryolite m e l t s have been discussed by s e v e r a l authors ( e . g . Thonstad et al 1 and Kachanovskaya et a l e ) . T h e r e s e e m s t o be agreement that v-alumina dissolves more rapidly than o~-alumina, but that for both types dissolution is very rapid (a few seconds) if the alumina is well dispersed, and if the melt is not close to saturation. In practice, however, the alumina is unlike
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