Part 4. Phase transformations within the heat-insulating layer of aluminum electrolysis cells

Abstract

The heat insulating layer of an aluminum electrolyzer is shown to become gradually impregnated with sodium and fluoride compounds. Vermiculite, while subjected to deep dehydration, enters into reaction with electrolysis products, which resulted in damage to the refractory chamotte layer. The saturation with gaseous compounds proceeds along the cleavage plane of vermiculite. Sodium tetrafluoraluminate is shown to form under the electrolyzer's service conditions. Alongside the pure fluorides, complex fluorides of variable composition have been identified. Fluorides and metallic aluminum have been shown to form by condensation and disproportionation of lower fluorides. Electric corrosion is suggested as a factor affecting the structural integrity of the electrolyzer's casing.A major route towards increasing the service life of aluminum electrolyzers consists in improving the heat insulation of the electrolyzer's casing to provide a reliable protection of the bath from heat losses and to maintain a scull coating built-up on the refractory lining. At present, vermiculite plates have found use as the heat-insulating material in some aluminum electrolyzers. In this paper, we present the results of a study of phase transformations that occurred in the vermiculite heat-insulating coating after 45.6 months of service of the Al electrolyzer. The sampling scheme and chemical analysis methodology can be found in [1]. Sampled specimens labeled 1-1s, 3-1s, 5-2s, 7-3v, 11-2s, 14-3s, 15-1v, and 22-1n were examined [1].Vermiculite of natural occurrence is a gold-colored mineral, of complex and variable composition (Mg, Fe 2+ , Fe 3+ ) 3 [(Al, Si) 4 O 10 ](OH) 2 × 4H 2 O. Based on chemical analysis data, the precursor vermiculite plates had an elemental composition (wt.%) of Mg 9.78, Al 4.23, and Si 19.61. The initial density of vermiculite plate material was 0.4 g/cm 3 . The apparent density of specimens sampled from the post-service heat-insulating layer was 1.623 -1.187 g/cm 3 (specimens 3-1s and 15-1v).Macroscopic specimens sampled from the post-service heat-insulating layer looked like: 1-1s, gray brittle chips with white inclusions or a powder of gray color; 3-1s, brittle chips or a powder of yellow-gray color with vermiculite flakes; 5-2s, a powder of gray color with vermiculite flakes; 7-3v, rather large lumps or a powder of pink color with vermiculite platelets; 11-2s, a powder of red-brown color with little balls and flakes of residual vermiculite; 14-3s, a powder of red color with vermiculite flakes; 15-1v, chips or a powder of white-gray color with flakes of residual vermiculite; and 22-1n, a powder of dark-gray color with vermiculite flakes.The phase composition and quantitative estimates of phase transformation within the heat-insulating layer as determined by petrographic analysis are given in Table 1. With time, the vermiculite plates of the heat-insulating layer became impregnated with sodium and fluoride salts. The plates, originally made up of pressed and partially dewatered vermiculite, gradually transformed in...

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.…”

“…In mass spectrogram of condensed sublimate from a slag ingot ANF-6 (ANF is the grade of flux) after electroslag remelting steel ShKh015 CaF was also observed [6]. The presence of the compound 2MgO·MgF 2 ·3CaF 2 in the thermally insulated layer of an aluminum electrolyzer after service [7] and also CaO and CaF 2 , previously detected by the authors in [8], provides a basis for assuming that all of them are products of calcium sub-fluoride CaF disproportionation. This is also indicated in studies by the authors of this article.…”

Prospects of the use of aluminosilicate refractories for aluminum electrolyzers. Part 3. The role of the glass phase formed in the operation of aluminum electrolyzers

“…Samples with a low total content of elemental silicon (due to its removal) are characterized by the presence of oxyfluoride glass with N = 1.362 and the following composition (in mass percent): 17 Al, 18.91 Na, 10.62 F, and traces of Si. Thus, the data of chemical and crystallographic analyses show that the oxyfluoride glasses consist of both a silica skeleton and a corundum skeleton.The authors of [5,7] have observed immiscibility of the oxyfluoride and aluminosilicate glasses. They detected pure aluminosilicate glass with N = 1.521 (nepheline glass with N = 1.51) containing 17.15% Al and 31.54% Si in a refractory layer.…”

“…It was interesting to determine the critical content of aluminum at which the glasses separate into oxyfluoride and aluminosilicate components. An analysis of the glass phase of a heat-insulating layer [5] has shown the presence of aluminosilicate glasses with N = 1.464 in it (Le Chatelier glass with N = 1.458) The content of elemental aluminum in such glasses is 8.41%, which corresponds to the lowest mass fraction of Al at which the glass separates into aluminosilicate and oxyfluoride components. In addition, we should mention a sodium-fluoride kind of glass.…”

“…The authors of [5,7] have observed immiscibility of the oxyfluoride and aluminosilicate glasses. They detected pure aluminosilicate glass with N = 1.521 (nepheline glass with N = 1.51) containing 17.15% Al and 31.54% Si in a refractory layer.…”