LABORATORY EVALUATION OF AGGLOMERATED SUPERFINE ALUMINA FOR SMELTING

Four samples of agglomerated alumina superfines, to be used as smelting feedstock, were produced using spray drying. The agglomerates were formed using either as-received or washed superfines (to reduce soda content) with the addition of either 5 or 10 wt% aluminium hydroxychloride binder. The feedstock formed from unwashed superfines and 10 wt% binder had an attrition resistance comparable with that of Smelter Grade Alumina (SGA). A segregation test on a mixture of 95 wt% SGA:5 wt% agglomerated alumina superfines showed that only a minimal degree of segregation of agglomerated superfines from SGA can be expected. The calcined agglomerated superfines feed contained residual chloride and this resulted in elevated levels of chloride in the cell fume compared with the 100% SGA feed.

A laboratory-scale aluminium smelting cell was used to establish whether the presence of the binder and other inorganic impurities in the agglomerated superfines had any detrimental effect on metal quality and current efficiency (CE). Smelting was conducted with agglomerated alumina superfines levels at 5 and 100 wt% of the alumina feed. Aluminium was deposited at a current density of ~0.75 A cm^-2 for 1-6 h at 965°C from an electrolyte similar in composition to that used in industrial cells. CEs of >95% were obtained in high purity electrolyte using 100% SGA as feed, and there appeared to be no significant reduction in CE with an agglomerated superfines feed. Trace metal analyses revealed no significant difference in the purity of aluminium deposits produced from the various feedstock compositions. Overall, the inclusion of superfine agglomerates at the 5 wt% level in SGA did not appear to adversely affect the performance of a laboratory-scale aluminium smelting cell.