Premature shutdown of electrolytic baths after a short service life leads to an increase in the cost of aluminum and serious economic losses. The most important element governing the service life of an electrolytic bath is the hearth that is lined with refractory hearth blocks. Presence of one concealed crack, that is exposed in the start-up period, may lead to failure of the whole electrolyzer in the first year of service. There are many producers and users of hearth blocks, and there are many specifications for cathode hearth blocks, although the requirements for them are selected in a very empirical way without considering actual service conditions. Scientifically based requirements for the material of hearth blocks is still in the formation stage. In 2004 in four aluminum enterprises under major repair hearth blocks were used from nine different producers that made it possible during evaluation of the statistics for shutting down young baths to determine the effect of the structure and properties of hearth blocks on the service life of electrolyzers and to formulate requirements for hearth block material. The contribution of permeable porous cathode carbon blocks on premature shutdown of electrolyzers by a mechanism of hearth uplift due to formation of a lens of electrolyte and refractory reaction products is demonstrated. Critical gas permeability and the maximum size of permeable pores in refractory hearth units are determined. Applicability of the first and second Hasselman criteria for the heat resistance of hearth block materials is demonstrated for predicting shutdown of young baths by a crack development mechanism in blocks due to thermomechanical stresses.

Carbon is far from a perfect material for the bottom blocks of electrolysis cells operating on the Hall-Heroult principle. A perfect material should meet the following requirements [1]: high electrical conductivity; high thermal stability; reliable electrical contact with current leads; minimum open porosity; good wettability by molten aluminum; chemical inertness; high wear resistance; high resistance to intercalation and migration of sodium through the block material towards the refractory layer; and minimum capillary flow of the electrolyte through pores towards the refractory.None of the carbon materials available actually meet these properties. Since the 1980s, studies have been conducted to design a non-carbon electrode for the Hall -Heroult electrolysis cell. It was thought that the non-carbon cathode is capable of providing a longer service life and a significant power savings by narrowing the electrode gap and decreasing the cathode voltage [2 -5]. The suitable candidate materials seemed to be carbides, nitrides, and borides of high-melting metals and their composites. Viewed from the chemical standpoint, three requirements are placed on the material of coatings: (i) no interaction between material and molten aluminum; (ii) no interaction between material and electrolyte, and (iii) non-solubility of material in molten aluminum.Thermodynamically, few materials meet the requirement of being non-reactive towards molten aluminum. However, it should be kept in mind that a film of molten electrolyte is always formed on the cathode surface (in the Hall -Heroult process). Therefore the possible interaction of a high-melting material with molten electrolyte should never be ruled out (in the absence of gaseous oxygen).No complete theoretical analysis of high-melting materials and related systems with regard for the above three requirements has ever been conducted. According to thermodynamic calculations [6], the isobaric-isothermal potential of interaction between borides and the components of molten alumina-based electrolytes at 1100°C is arranged in order of increasing reactivity as TiB 2 < ZrB 2 < HfB 2 < VB 2 < NbB 2 < TaB 2 < MoB 2 < CrB 2 < WB 2 . The reactivity of transition metal carbides to aluminum increases in the order TiC < ZrC < NbC < HfC < VC < CrC < TaC < MoC < WC.Nitrides of high-melting transition metals interact with aluminum to yield aluminum nitride [7]. No experimental evidence supportive of the existence of borides and carbides for high-melting metals found on the left side of the two above series have been reported [8].A thermodynamic analysis of the interaction between high-melting compounds and the components of an electrolyte-alumina melt (with an alumina concentration of 10 wt.% at 1230 K (957°C) gives the following series of increase in stability: TiN < TiC < TiB 2 and ZrN < ZrC < ZrB 2 . Thus, titanium boride and titanium carbide are the first to meet more fully the three requirements stated above. Attempts were made to use composites TiB 2 -TiC [9] and TiB 2 -C [10] as the cathode-we...