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AGGLOMERATION OF ALUMINA DURING DISSOLUTION IN ALUMINIUM CELLS

Thonstad, J., Kobbeltvedt, O., and Rolseth, S.

A laboratory study was made of the dissolution characteristics of alumina in cryolite melts. The most important feature is the tendency of alumina to agglomerate, which is associated with the conversion from gamma alumina (g), into alpha alumina (α). The dissolution process is mass transfer controlled, and dispersed alumina dissolves very rapidly (seconds), while agglomerates dissolve slowly (minutes). Dissolution tests were designed to simulate the conditions in industrial cells with point feeding. High superheat and preheating of the alumina promoted rapid dissolution. To study the agglomeration phenomenon loose alumina of commercial low-a quality was placed in a steel frame lined with aluminium foil, and immersed into alumina-saturated cryolite bath. It was found that sintering of the alumina particles took place while heating between 700 °C and 950 °C. When the alumina was permeated by molten bath, the alumina body still retained its shape. With alpha alumina this was not the case. This shows that the g ® a phase transformation plays an important role in the dissolution process.