Modern sidelinings in aluminium smelters : properties and fluoride attack of S3N4-bonded SiC materials

Abstract

Lifetime improvement of the materials used in sidelining construction is needed to increase productivity in moderen aluminium production. The aim of this study is to analyse the characteristics of Si3N4-bonded SiC and to describe its resistence to fluoride attack.

Si3N4-SiC ceramics are prepared by in-situ reaction bonding of Si3N4. Two ranges of SiC particles are chosen in order to study their impact in the properties of the material. The nitridation is carried out in steps from 1200 to 1400°C using the nitrogen demand cycle. A complete characterisation of these materials is done and shows principally a high level of porosity in the network of silicon nitride needles binding large blocks of silicon carbide.

The resistince of Si3N4-SiC to fluoride attack is investigated by means of a complete thermodynamic study. A complementary a finger test is carried out by exposing the materials to the combined effect of oxidation and corrosion by molten cryolite at 980°C. These studies show that Si3N4-SiC is inert to the cryolitic melt but the addition of an oxidising gas renders it vulnerable to attack. The attack is double: penetration of the cryolite into the porous silicon nitride network, and degradation by cryolite of the formed oxide layer.

It is concluded that penetration by cryolite into the pores of Si3N4-SiC is the main couse of degradation. The solution consists in reducing as much as possible the porosity by either adding a third compound to fill the pores and/or coating the surface.