Electrochemical mechanism of the oxygen bubble formation at the interface between oxidic melts and zirconium silicate refractories

Abstract

The formation of oxygen and oxygen bubbles at the interface of new, untreated, zirconium silicate (ZS) refractories and oxidic melts, which is of fundamental and economical interest, was investigated. The experiments led to a reaction mechanism which is based on an internal reduction of polyvalent ionic impurities of ZS and an oxidation of oxide ions of the melt at the ZS surface. The redox reaction proceeds via a reaction layer below the ZS surface, which forms an inherently short-circuited, internal, galvanic cell with a growing thickness. The reaction rate is controlled by solid state diffusion of charge-balancing alkali ions. The oxygen formation can be displaced to other electrodes, and the internal reduction can be accelerated and halted by an outer voltage. It was shown that the reaction mechanism is generally valid for alkali ion-containing melts whh oxidizable anions, e.g. for sodium halide melts. Oxygen fugacity measurements and bubble analyses were evaluated critically because of secondary factors, e.g. other gas bubbles often also generated at ZS, which influence the gas economy of the melts. A formal similarity with the electrochromic reaction is pointed out.