Mass- and Heat Transfer during Dissolution of Alumina

Abstract

Dissolution of alumina in industrial aluminium cells is a complicated process, not least because it involves formation of agglomerates, and it involves mass- and heat transfer phenomena taking place simultaneously. In the present study, the diffusion coefficient of alumina in cryolitic melts was measured using a rotating alumina disc. It was found that the temperature dependence of the diffusion coefficient is relatively large. The enthalpies for heating of alumina, conversion, and dissolution are summarised. The addition of 1 wt% alumina causes adiabatic cooling of typically 10–12 °C in a normal industrial bath. The dissolution can be regarded as being purely mass transfer controlled, since the heat required for dissolution only brings about 1 °C temperature drop from the bath bulk to the alumina surface. The bath at the alumina surface is saturated in alumina and has a lower liquidus temperature than the bulk. Alumina dissolution can, therefore, take place in a supercooled bath.