Homogenization Flashcards
What is striae?
~ 2D layers of compositional heterogeneity
What are cords?
~ 1D veins of compositional heterogeneity
What limits homogeneity?
~ poor batch mixing or segregation of the batch (fines falling to the bottom)
What causes compositional changes at the glass surface?
~ comp changes at glass surface can occur from higher vapor pressures of constituents such as alkali, lead, and boron oxides
How can volatization losses be minimized?
~ by reducing melt surface temp and reducing melt-free surface area, which occurs with electric melting/boosting
How do volatized melt components act in cold top melters?
~ volatilized melt components condense in the cold upper part of the batch blanket, so volitization losses are practically eliminated
What helps remove volatiles and particulates from exhaust gases?
~ scrubbers, electrostatic precipitators, and cloth filters
What are the effects of melt stirring?
~ enhances homogenization
~ requires establishing free convection flow patterns driven by density differences, in turn driven by temp gradients in the tank
What is the spring zone?
~ the near center of the tank that has the highest temp
~ molten glass flows toward the surface, replacing laterally outward moving melt on the surface towards colder regions
Where are the low temperature regions?
~ near batch entry (doghouse)
~ beyond the bridge wall where the melt undergoes final conditioning
~ near refractory regions have lower temp via heat losses through refractory walls
What are bubblers?
~ purposely induced gas bubbles to sweep out fine seed
~ combined with powered electrodes, create local upward currents
How are melters designed?
~ for convection flow, which limits forward mixing of material undergoing batch melting, fining, and homogenizing steps
What do mechanical barriers do?
~ provide more positive control of flow
~ the melting chamber is separated from the conditioning chamber by an internally-cooled bridge wall
