Future application of aluminosilicate refractories for aluminum electrolyzers. Part 4. Wettability of aluminosilicate refractories with molten industrial electrolyte and sodium fluoride

Abstract

Optical microscopy is used to determine wetting angles for aluminosilicate refractories with molten industrial electrolyte and NaF. Kinetic wetting parameters and interaction of electrolyte with refractory are determined. It is shown that removal of silicon from refractory also occurs in the form of silicon monoxide. It is established that aluminosilicate refractories, prepared with addition of mullite-corundum chamotte, are wetted worse by fluoride salts, but the corrosion resistance of them is better than for refractories prepared by normal technology.In an aluminum electrolyzer surface forces at the boundary of the liquid -gas -solid substance determine the intensity of penetration of materials by molten electrolyte and their corrosion rate. The overall index characterizing equilibrium of surface tension at a three-phase interface is the contact wetting angle q: q = arccos(s s -g -s s -l )/s l -g , where s s -g and s l -g are interphase energies at the boundaries of the solid and liquid with vapor respectively; s s -l is the interphase energy at the solid -liquid boundary.The value of q is affected by the nature of all phases participating in equilibrium, their physicochemical properties, preparation methods, temperature conditions, and polarization. The contact wetting angle, connected with the work of adhesion W a by the relationship W a = s l -g (1 + cosq), may serve as a criterion for selecting materials used in electrolyzers. Knowing the contact wetting angle it is possible to improve production schemes for preparing the cathode assembly of an aluminum electrolyzer.Wetting of refractory materials by cryolite has been described in [1]. According to data in [2] the contact wetting angle for aluminosilicate materials by molten electrolyte is close to zero. The corrosion resistance of refractory material is mainly affected by the diameter of its permeable pores (and consequently gas permeability), the optimum distribution of pres with to size, porosity, density [2,3]. According to data in [3] the microstructure of a refractory is subject more intense chemical reaction than its macrostructure. A high mullite content, particularly in the microstructure, provides a high resistance to the chemical action of gases released from the heated anode in the furnace (residues of cryolite Na 3 AlF 6 in ash) [3], since mullite is chemically more stable than cristobalite or glass.Previous studies by the authors of this article [4] established that aluminosilicate refractory ShPD M -45, produced with addition of mullite-corundum chamotte, is more resistant to the action of molten industrial electrolyte than refractory ShPD-43 prepared by normal technology. The aim of the present work is to study wetting of different aluminosilicate refractories by molten industrial electrolyte with a.o. = 2.4 and chemically pure sodium fluoride in the absence of polarization. The results obtained serve as a characteristic for the test refractories from the point of view of their future application in electrolyzers with hearth units containin...