Part 3. Phase Transformations within the Layer of a Dry Barrier Mixture for Aluminum Electrolysis Cells

Abstract

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...

Orthorhombic aluminium oxyfluoride, AlOF

“…This is believed to be due to the formation of AlX 3 and AlOX, where X is a halogen (Swihart & Catoire, 2000). Another occurrence of aluminium oxyhalides is the cathode surface in the aluminium electrolysis cell (Sharapova et al, 2005). Petrographic analysis of cell materials has revealed oxyfluoride glasses with variable compositions of AlOF 1Àx and with index of refraction n = 1.33-1.362.…”

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

“…Considering this and the results of studying samples (28-1v) SBS [4] and also the situation that for the comparatively thin (up to 0.7 cm) working zone of refractory ShPD M -45 it should be least changed, containing elemental fluorine and sodium in the form of traces, and there is no silicon entrained in it, and long-prismatic titanium-containing mullite without changes, it may be concluded that it is a titanium-containing glass phase [15] alongside a glass phase of albite composition that fufils a protective function and prevents the refractory cathode lining from further breakdown. Sodium vapor and gaseous fluorine compounds AlF, NaAlF 4 , CaF, MgF, i.e.…”

“…In the nepheline-glass matrix there is clearly recorded a-corundum, tetragonal titanium oxyfluoride TiOF, titanium oxyfluoride of nonstoichiometric composition (TONC), titanium fluoride of nonstoichiometric composition (TFNC). Results of x-ray structural studies of the working glassy zone of refractory ShPD M -45 after service are provided below 4 In comparing the characteristics of x-ray patterns for the test titanium fluorides and oxyfluorides with x-ray patterns of standard TiF 3 and TiOF, disturbance of the crystal lattice spacing is observed for titanium fluoride TiF 3 and titanium oxyfluoride TiOF of stoichiometric composition. This makes it possible to refer the test titanium fluorides titanium subfluoride, and titanium oxyfluoride to oxyfluoride of nonstoichiometric composition.…”

Structures and thermodynamic properties of aluminum oxyhalides: a computational study

“…During the combustion, one of the molecules formed is AlOCl [1]. Another occurrence of aluminum oxyhalides is on the cathode surface in the aluminum electrolysis cell [2]. Recent studies have shown AlOCl to be useful as a coagulating agent for cleaning wastewater from organic impurities [3].…”