Kinetics of Chromium Oxide Transformations in Glass Melting

Abstract

The kinetics of chromium oxide transformations in glass batch under heating and in glass melt has been studied. The different degrees of transformation of CrO 3 and Cr 2 O 3 is demonstrated. The conditions of equilibrium between chromium oxides in the case of their simultaneous presence in glass melt is determined. The conditions of the interaction between chromium and iron oxides in the glass melt are investigated, including the surface layer in which an equilibrium between the colorant oxides can be achieved in accordance with the reaction conditions and the composition of the gaseous atmosphere. The low reaction velocities in the glass melt are due to its high viscosity and low values of diffusion coefficients.The nature of the transformation of colorant compounds in glass melting is important for the technology of producing tinted glass, especially for automated glass container production. The processes of transformation of pigments occurring in the initial period of melting and later in the glass melt are insufficiently studied.The study in [1] considered the conditions for the transformation and equilibrium of iron oxides. It is demonstrated that during melting of glasses containing iron oxides the transformations of the latter mainly occur in the solid phase and the ratio of two forms of iron oxides in glass melt is determined by the melt temperature. The valence state of iron oxides in industrial glass-melting furnaces does not satisfy the condition of equilibrium with the ambient medium due to a high melt viscosity and low values of the diffusion coefficients.Chromium oxides are common colorants for container glasses; therefore, the present study is focused on the physicochemical parameters of transformations of chromium pigments.We investigated the conditions of transformations of chromium oxides in melting industrial container glass. Chromium oxides were introduced in the amount of 0.10 -0.25% (above 100%) of the main glass composition in the form of chromium anhydride CrO 3 , potassium bichromate K 2 Cr 2 O 7 , and chromium oxide Cr 2 O 3 . The content of chromium oxide in batches and glasses was found through chemical analysis using the diphenylcarbacide method. Laboratory electric furnaces adapted for the development and control of a corresponding atmosphere were used for experimental melting. The gaseous medium was characterized through partial oxygen pressure P O 2 . This parameter is universal and can be used to estimate both oxidizing and reducing conditions. Compounds of hexavalent chromium introduced in the batch decompose under heating and produce Cr 2 O 3 . The nature of this transformation is determined by the thermal resistance of oxides which, in turn, is related to dissociation pressure. Figure 1a shows the transformation kinetics of the initial compounds in air (log P O 2 = -1.68) upon introducing CrO 3 and K 2 Cr 2 O 7 into the batch. The parameter accepted as the main characteristic is the degree of transformation a, i.e., share of the decomposed colorant. It can be seen that the rate...

Alternative raw material research for decarbonization of UK glass manufacture

“…This indicates that more of the Cr 6+ is reduced to Cr 3+ in these glasses, due to the presence of higher levels of carbon in the batch, from the biomass ash. The equilibrium between chromium species in glass melt can be described by the following reaction 78 : $$$$\begin{equation*}4{\rm{Cr}}{{\rm{O}}_3} \rightleftharpoons 2{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_3} + 3{{\rm{O}}_2}\end{equation*}$$$$…”

“…This indicates that more of the Cr 6+ is reduced to Cr 3+ in these glasses, due to the presence of higher levels of carbon in the batch, from the biomass ash. The equilibrium between chromium species in glass melt can be described by the following reaction 78 :…”