Behaviour of aluminium oxide in KF-AlF3-Al2O3 melts and suspensions

Abstract

The article addresses the properties of melts and alumina suspensions based on the molten KF-AlF 3 -Al 2 O 3 system, the kinetics of alumina dissolution and the rheological properties that determine the sedimentation stability. The study of such suspensions has become topical due to the prospects of their use as electrolytes in the production of aluminium using carbon-free anodes. The effects of the temperature, the particle size and the phase composition of the dispersed material and its volume fraction in the suspension on the dissolution kinetics and the sedimentation velocity are studied. The experiments were carried out over the melts with cryolite ratios 1.3 and 1.5 in the range of 750-850 °C. Three different types of aluminium oxide were used. The Reynolds numbers for sedimentation have indicated the Stokesian regime. Typical alumina dissolution rates were in the range 0.028-0.167 g•kg -1 •s -1 , which is close to the values reported previously. Sedimentation velocities were in the range (0.05-3.61)•10 -2 m/s, which is several times higher than those obtained previously for 700 °C and φ = 0.24-0.32. It is shown that mechanical activation of alumina increases the performance of the suspension in terms of the sedimentation stability and the dissolution rate increasing.

Electrochemical Sensor for Monitoring the Alumina Dissolution and Concentration in a Cryolite-Alumina Melt

“…-measuring directly in the electrolytic cell a property or parameter of the investigated system, reliably and reasonably correlating with the concentration of alumina dissolved in the melt; -analysis of melt samples taken from the electrolytic cell by physicochemical methods. -The following methods are used for studying the solubility and kinetics of alumina dissolution in cryolite: -potentiometry (according to the emf of the concentration element) [6][7][8][9] ; -the method of a rotating disk electrode 10,11 ; -linear and square-wave voltammetry (according to the peak of the anode current) [12][13][14][15] ; -chronopotentiometry (in transition time) [16][17][18] ; -thermal analysis (by liquidus temperature) [19][20][21][22] ; -impedance spectroscopy (by resistance or electrical conductivity of the melt) [23][24][25] ; -stationary polarization (by anode overvoltage) 26,27 ; -optical method (by changing the structure of the melt) 28 ;…”

Recovery of Noble Metals from Spent Catalysts: A Review

“…Moreover, the temperature of the process can be lowered to 700-850 °C by replacing sodium cryolite with low-temperature fluoride or chloride melts with considerable solubility of Al2O3. The promising option, the molten 1.3KF-AlF3 system has been studied elsewhere [89][90][91]. This process is patented [92] and now is under the laboratory scale studies [93] While Pt is successfully concentrated in the alloy, Re requires additional effort to be extracted as it is oxidized in a wide range of temperatures.…”

Anode Process on Platinum in Low Temperature KF-AlF3-Al2O3 Based Melts: An Update

“…This firstly concerns the need to separate the anolyte and catholyte using Al 2 O 3 suspension or ceramic diaphragms. [21][22][23] Otherwise, dissolved oxides from the anode will be constantly reduced on the surface of the aluminum cathode (by metallic aluminum) and in the volume of the melt (by dissolved aluminum).…”