Mineral Composition and Phase Transformations in the Refractory Lining of the Bottom of Aluminum Electrolysis Cells. Part 2.

Abstract

Specimens of post-service ShA-5-grade chamotte refractory are analyzed for total elemental aluminum and silicon. Products of the electrolysis of cryolitic melt Na (gas), AlF, Al 2 O, and, possibly, Na 2 F are shown to diffuse through pores of the carbon refractory bottom lining blocks of an aluminum electrolysis cell. The lower compounds and Na(gas) as they reach the refractory layer enter into a reaction with the refractory material. The Gibbs free energy of hypothetic reactions in the Al -Si -O -F -Na system involving a gas phase has been calculated. It is inferred that the performance characteristics of the ShA-5 refractory fail to meet standard operational requirements.In a previous communication [1], the changes in mineral and phase composition of ShA-5-grade chamotte refractory specimens sampled from the bottom lining of a aluminum electrolysis cell in service for 45.6 months have been reported. In this paper, we focused attention on a thermodynamic analysis of the processes involved in the wear of the refractory lining of the electrolysis cell bottom.The results of a post-service analysis of the ShA-5 refractory have indicated an increase in total aluminum in some test specimens (24-1m and 33-1m) and a decrease in total silicon, especially in specimens 6-3m, 34-3m, and 40-1m. The increase in total aluminum defies explanation as being due to the simple penetration of metallic aluminum into the refractory layer. The mode of occurrence and dispersion of the penetrated metal suggest that the infiltration of aluminum is effected via gas phase; that is, the electrolysis products AlF and Al 2 O 3 penetrate through pores and joints in the carbon bottom lining blocks to undergo disproportionation during condensation from the gas phase by the reactions 3AlF (gas) = 2Al + AlF, 3Al 2 O (gas) = 4Al + g-Al 2 O 3 , 3AlO = Al + g-Al 2 O 3 .

Study of the mineral and phase composition of aluminosilicate refractory ShPDM-45 after service in an aluminum electrolyzer of OAO Zaporozh’e aluminum combine

“…The presence of metal beads based on Fe, Si-Al in a sample after service indicates that a reducing atmosphere is observed in the refractory layer [16], and this is confirmed by data of previous studies [3,6]. Silicon, aluminum, iron-containing phases are reduced preferentially and also partly calcium-and titanium-containing compounds.…”

“…In the skin there is glassy SiO with N = 1.54. In chips of glass under an optical microscope with 210-fold magnification there are microlytes of oxides of aluminum, magnesium, trivalent titanium, and aluminum oxyfluorides [3] with N = 1.33. Isotropic platelets were recorded by an immersion method with N = 1.448 and N = 1.582, that it was assumed were related to titanium oxyfluoride (Fig.…”

Prospects for using aluminosilicate refractories for aluminum electrolyzers. Part 2. Resistance of aluminosilicate refractories to the action of commercial electrolyte

“…Taking account of the considerable resistance of refractory ShPD M -45 to the action of commercial electrolyte, almost complete absence of a transition zone in this refractory and the detection of clear crystals of long-prismatic mullite in the transition zone and less changed zone, and also data of studies in [7,9,10,11], it is possible to conclude that long-prismatic titanium-containing mullite of increased density is resistant to the action of the fluoride-ion. Normal short-prismatic mullite in refractory ShPD M -45 is less resistant to aluminum electrolyzer operating conditions since initially it recrystallizes into pragite.…”

“…The low resistance of ShA-5 articles in the refractory layer of an aluminum electrolyzer hearth after service is shown in publications [9,10].…”