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Redox chemistry of iron-manganese and iron-chromium interactions in soda-lime-silica glass melts
Glass melts are mostly not in a chemical equilibrium with a coexisting gas phase, it does not adjust before maintaining the melt at a constant temperature and a constant gas phase for hours. But the one-phase equilibrium between several polyvalent Clements in a melt above the glass transition temperature always exists, it adjusts spontaneously. Below the transition temperature the redox State is invariably frozen-in. The calculation of redox states must be based on the matter and Charge balance (stoichiometric equations) and equilibrium relations (mass-action law). Such a calculation of Schreiber's experimental results was consistently possible. Accordingly cooled glasses are free from Cr⁶⁺, if the glass contains Fe²⁺. To achieve this one has to melt oxidized chromium-containing glasses with sufficient iron content.
Α characteristic number of refining
It was already reported about the prediction of the bubble population behaviour with numerical models based on the population balance model. On the basis of this model, the similarity of analytical function and numerical computation is now presented for some cases of refining. Under these simplified conditions the bubble size distribution of analytical and numerical computation is identical. It is possible to predict the refining time in dependence on the melting depth. In the first step the prediction is possible for laboratory crucibles and pots. The results were compared with experimental values. Some differences between experimental and theoretical refining time were interpreted assuming that the melt volume is homogeneous. This assumption is necessary for these investigations. Α homogeneous melt is found rather in small crucibles than in larger pots. Starting from the population balance model a characteristic (dimensionless) number was found for the refining of glass melts. This characteristic number concept gives some new points of view for a better understanding of the refining process. It shows the temperature dependence of refining time as temperature dependence of glass viscosity and bubble growth rate. The refining capability is described with a simple term. The value of refining capability can be determined by refining experiments. This value can also be calculated if the temperature dependence of glass viscosity and bubble growth rate is known.
Redox conditions during tlie melting of batch
During the mehing down of glass batches a variety of gases are set free. The redox conditions of the melt can be influenced by these gases, because both the gases and the melt are at the same time in a temperature range of 200 K. For the quantification of this influence batches with different charges of coal were heated in comparison with a batch without additions of coal. The final temperature was 1000°C and the heating rate lOK/min. With the continuous measurement of the oxygen partial pressure and the content of CO in the gas the effect of coal on the reactions of gases during the heating of batches can be described.
Equilibrium constant - determination and influence on redox reactions in soda-lime-silica glass melts
The iron, arsenic and chromium redox equilibrium in an oxide glass-forming melt has been studied quantitatively The equilibrium constant K(T) and the thermodynamic Standard values were determined for the redox reaction in the melt at temperatures between 1000 and 1450 °C. These values were compared with values from the literature. The equilibrium constant is dependent on the temperature and on the concentration of the polyvalent elements. This dependence applies to the polyvalent elements investigated. For each polyvalent element it is possible to find a step in the funcdon lg([R^y]/[R^x) versus R_aO_b- This seems to be explained by the concentradon dependence of the equilibrium constant of the redox reaction. Possibilities to determine the equilibrium constants of the various polyvalent elements are described.
Modeling of the bubble population in glass melts
Previous studies of refining models have mostly dealt with the mathematical description of the behavior of individual bubbles in glass melts. A further step was the modeling of the ascent and growth of groups of bubbles, including restrictions with respect to the spatial distribution of the individual bubbles. Even though no algorithm was derived in this stage which described the total bubble balance during refining, these investigations have produced important results in the field of bubble growth and ascent. In this study, a general and comprehensive mathematical description of the bubble population during refining shall be given, on the basis of the population balance equation used in chemical engineering. The general balance equation for bubbles during refining is presented together with the corresponding computer model. Several experimental investigations into bubble size distribution are described, together with an analysis of the reduction of the bubble concentration in the pot. The obtained values were used as initial parameters and estimates in computer simulations. Some special results of modeling are shown and discussed.