Sedimentation behavior of high-temperature concen-trated colloidal suspension based on potassium cryolite

Abstract

The paper describes the sedimentation behavior of concentrated high-temperature oxide-fluoride slurries with alumina particles volume fraction range 0.24 ≤ φ ≤ 0.32 at ∼700 °C. The behavior is of interest due to perspectives of the non-Newtonian fluids usage in the future aluminium reduction technology. To characterize sedimentation behavior several techniques were used: density analysis, X-Ray diffraction, microphotography and electron scanning microscopy. Sample with −63 µm dispersed phase as well as smelter-grade alumina which contains particles in a range of 10–150 µm has been examined. It has been found that particle settling occurs with the initial velocity 0.97·10−5 m·s−1 at φ = 0.24 and gradually reduces reaching zero at φ close to 0.32 which was considered as a maximum packing fraction. MS−1 sedimentation mode has been identified, the type of sedimentation was found to be changed from type 1 to type 3 in the sample with φ = 0.24. The alumina partial density distribution in suspension as a function of time was determined. Alumina average motion velocity during sedimentation was calculated to be 154·10−3 kg·m−2·s−1 at φ = 0.24 in the top layer of the sample within the first 150 s of sedimentation. It had been reducing drastically before it reached zero after 300 s. An alumina flow was higher in the top layers. Some of the rheological properties (such as relaxation time, which increased from 7.2·10−3 to 17.7·10−3 s after the increase of φ from 0.24 to 0.28) has been determined.