Results for specimens of the refractory lining sampled from the bottom of an aluminum electrolysis cell with a service life of 3.5 years examined by methods of petrography, chemical analysis, and electron probe x-ray microanalysis are reported. The main products of conversion are sodium aluminosilicates, sodium silicates, a glassy phase of variable composition, oxyfluoride glasses, and eutectics. Some of the specimens analyzed are high in b-alumina (20 -50%). Fluorides are represented by NaF (about 7%), cryolite Na 3 AlF 6 (2 -5%), malladrite Na 2 SiF 6 (2%), and NaF × MgF 2 (1 -2%), and the metallic phase, by aluminum (2 -7%) and ferrosilicon (3 -10%). The apparent density of the used refractory material is 2.5 -2.62 g/cm 3 .Data have been reported [1 -4] on the use of a-alumina as a component of the cathode lining for aluminum electrolysis cells, on fluoride salts as impregnators for the lining, and on the phase transformations in a-Al 2 O 3 involved. In [5,6], results of an inspection of a chamotte refractory specimen sampled from the bottom lining of the base plate of a dismantled aluminum electrolysis cell were given.Here we report results of a study of specimens sampled from different parts of the chamotte refractory lining of the bottom of an aluminum electrolysis cell that had been in service for 3.5 years. The lining of the bottom of the electrolysis cell was composed of a heat-insulating vermiculite layer 100 mm thick, a layer of dry barrier mixture (DBM) 110 mm thick, and a refractory layer of ShA-5-grade chamotte refractory 65 mm thick.Methods of petrography, chemical analysis, and electron probe x-ray microanalysis (EPXMA) were used in our study. The specimens were observed in reflected and transmitted light under MBI-6 and MIN-8 microscopes at magnifications of´90 -400, and in reflected light at magnifications of 950 -1800 using an immersion oil. The EPXMA was carried out on an MS-46 Cameca microprobe analyzer.A total of 50 specimens were sampled from the disused refractory lining. The sampling scheme is shown in Fig. 1. Chemical analysis results are summarized in Table 1.In this communication, results of a comparative analysis of specimens of intact (pre-service) ShA-5 chamotte refractory and six specimens of post-service chamotte 6-3m; 24-1m, 33-1t, 34-3m, 38-2m, and 40-1m are given.The commercial ShA-5 chamotte refractory contained (according to manufacturer's specifications) 37 -41% Al 2 O 3 , with an open porosity of 22.4 -24%. The apparent density of post-service ShA was 2.50 -2.62 g/cm 3 (specimens 6-3m and 38-2m).The inspected specimens of refractory layer were present in a range of colors -green, gray, yellow-brown, occasionally milky in appearance, hard and stony to touch, in places exhibiting a zonal pattern. Specimen 33-1t was powdery, of gray-brown color. The surface of solid, powder-like, and milky specimens, when tested with phenolphthalein, gave an alkaline reaction. The characteristic crimson color took different times to develop from specimen to specimen -either instantly or in a ...

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...

Data are provided for laboratory studies of the resistance of aluminosilicate refractories to the action of commercial electrolyte. It is established that refractory ShPD M -45, prepared using a mullite-corundum chamotte, is most resistant to electrolyte action. Results are provided for studies in the change of mineral composition and phase transformations in aluminosilicate refractories during reaction with commercial electrolyte. It is shown that long-prismatic titanium-containing mullite is more resistant to the action of the fluoride ion than short-prismatic material.The service life of an aluminum electrolyzer is mainly governed by the resistance of refractory linings of the cathode assembly to the action of an incoming corrosive medium. There is information about tests of refractory materials for cryolite resistance. Different test procedures, their characteristics and comparisons are provided in [1,2].Data from laboratory studies are provided in this article for the resistance of aluminosilicate refractories ShA-5, ShPD-43 and ShPD M -45 [3] to the action of a commercial electrolyte with a cryolite ratio (c.r.) of 2.4. Tests were performed by a petrographic method in reflected and transmitted light in MBI-6 and MIN-8 microscopes respectively. The resistance of aluminosilicate refractories in contact with molten commercial electrolyte was determined with a steady-state method by heating test articles in a Tamman furnace followed by a study of the change in mineral and phase composition of the refractories. Cylindrical holes of identical diameter and depth were made in articles, and they were filled with a uniform amount of commercial electrolyte with c.r. of 2.4 containing, apart from the main components, calcium and magnesium fluorides. The prepared samples were placed in a hermetically sealed alundum crucible and heated in a Tamman furnace to 1000°C with isothermal soaking at 1000°C for 1 h. Cooling took place in the furnace.After testing refractory specimens changed in color. At the surface of specimens from the side of the opening reaction zones were seen in the form of concentric circles of different color. The diameter of the original hole increased (Fig. 1), and its relative increase for specimens of ShA-5, ShPD-43, and ShPD M -45 was 47.5, 33.3 and 16.1% respectively.A specimen of ShA-5 after testing had a dingy lilac color. Over its surface (at the top of the hole and over the side) glassy new formations were seen in the form of circles with a diameter of 4 mm and leakages from the direction of the depression (hole), i.e. coarse pores with a diameter of 1.0 -1.5 mm. The electrolyte was converted into glass. With separation over the length of the depression two zones seen: a working zone 5 -10 mm thick of green color, and a glassy and transition zone yellow in color. The base of the specimen was the least changed zone (Fig. 2a ).A specimen of ShPD-43 after testing had a gray color. At its surface from the direction of the hole in the form of circles there was a working zone green in color and then t...

Comparative analysis is provided for the structure and properties of aluminosilicate refractories grade ShPD. It is established that refractory ShPD M -45, produced with the use of an addition of mullite-corundum chamotte surpasses in all characteristics refractories prepared by the normal technology. Refractory ShPD M -45 may be recommended for use in aluminum electrolyzers.The service life of the lining of the cathode assembly of an aluminum electrolyzer is an important engineering parameter. In order to avoid penetration of subsidiary products of an electrolyzer into the lining a refractory layer is laid down under hearth blocks of the electrolyzer. Sodium vapor and fluorine compounds, diffusing into the refractory layer, lead to a change in its chemical-mineral and phase compositions and destruction of the refractory itself, as for the cathode lining of the aluminum electrolyzer as a whole, and therefore the question of selecting the grade of refractory remains important.There is currently use of aluminosilicate refractories for the refractory layer. The chemical and phase composition of chamotte refractories is determined by the raw material composition and properties. In turn the properties of the aluminosilicate articles depend not only on the Al 2 O 3 content within them, but also on the overall total sum of fluxes and often on their chemical nature [1]. The degree of mullitization, the composition of mullite, and the glassy phase in chamotte refractories, depend on the properties of the original raw material, firing temperature and duration.Properties are provided in the present communication for the characteristics and phase composition of chamotte dense articles of ShPD produced by traditional technology (ShPD-43), and with the use of addition of mullite-corundum chamotte (ShPD M -45). Studies were carried out petrographically in reflected and transmitted light in microscopes MBI-6 and MN-8 respectively. Articles made of ShPD M -45 with addition of mullite-corundum chamotte in production properties and phase composition somewhat exceed those for articles made of ShPD-43 prepared by normal technology. The physicochemical indices of ShPD, prepared by both production methods, are provided in Table 1.The average apparent density of specimens of the corresponding type of refractory was determined by hydrostatic weighing. Open porosity of ShPD M -45 articles was lower by a factor of 1.5, and the average density was higher by a factor of 1.1 than for articles without mullite-corundum chamotte. Since open porosity is connected with the nature of pores,

It has been established that transformations within the layer of a dry barrier mixture (DBM) are selective in character and are mainly confined to the upper zones of the innermost part (core) of DBM. The DBM layer undergoes a solid-liquid sintering in these zones. The glass phase thus formed prevents the electrolysis products from penetration into the DBM layer. Modifier admixtures -titanium and iron oxides, products of electrolysis and degradation of the refractory layer AlF, SiF 4 , and sodium oxide -decrease the viscosity of aluminosilicate melt. The grains of a disthene-sillimanite concentrate undergo cracking along the cleavage plane. Imperfections of the crystal lattice promote transformations in the grains of normal electrofused corundum. Oxyfluoride AlOF and an oxyfluoride of variable composition AlOF 1 -x are formed in the cathode lining of the electrolysis cell.In this paper we present the results of a study of the dry barrier mixture (DBM) sampled for analysis from the bottom lining of an aluminum electrolysis cell after 45.6 months of service life. This work is a continuation of our previous studies reported in [1 -3].Originally, the DBM was a mixture of granular disthene-sillimanite concentrate (GDSC) and normal electrofused corundum (NEC) of fraction 20 mm. According to TU U 14-10-017-98 Specifications, GDSC contains, wt.%: Al 2 O 3 , not less than 57; iron oxides, not higher than 0.8%; CaO, not higher than 0.4; TiO 2 , not higher than 2.5. GDSC consists mainly of aluminum silicates -disthene Al 2 [SiO 4 ]O and sillimanite Al[AlSiO 5 ]. Disthene and sillimanite are the same in chemical composition, but they differ structurally. Both minerals are eutomous. The strength of disthene is emphasized by its name (di + Greek sthene force) [4]. So, its hardness in the cleavage plane lengthwise is 4.5; across, it is 6. The hardness of sillimanite (named after the American chemist and geologist Benjamin Silliman, 1779 -1864) is 6.5 -7. Occasionally, quartz grits and glass with N = 1.54 are found among other impurities in GDSC.The NEC micropowder consists mainly of a-Al 2 O 3 . The NEC chemical composition is, wt.%: Al 2 O 3 , not less than 94; Fe 2 O 3 , not higher than 0.6; TiO 2 , not higher than 2.8; SiO 2 , not higher than 1.3; CaO, not higher than 0.7. A specific feature of NEC is the occurrence of Ti 3+ in it in the form of a solid solution [5,6]. Oxides of titanium and iron were also found in the basal jointing of a crystalline electrocorundum. These impurities increase the degree of the a-Al 2 O 3 crystal lattice imperfection. In the powdered NEC, rutile TiO 2 occurs as an individual impurity phase. The mentioned features of constituents GDSC and NEC cannot leave unaffected the DBM performance characteristics.Specimens 9-2n, 12-3s, 18-3s, 20-1v, 21-1n, 26-2v, 28-1v, 29-1n, 39-2s, 41-1v, and 44-1n were examined petrographically. The sampling scheme and chemical analysis of the DBM post-service specimens were as described in [1]. The density of DBM post-service specimens determined by hydrostatic weigh...