Foaming and fining behavior in glass melts

The foaming behavior of various glasses was studied using a direct observation method and an electrical probe method. The experimental results showed that two distinct foaming processes existed during glass melting: the primary foaming caused by the decomposition of carbonates, and the high temperature foaming caused by the redox reactions and gas oversaturation in the glass melts. Among many fundamental properties affecting foaming behavior, the volatilization and surface tension were studied. The volatilization at the glass melt surface greatly increases the viscosity and increases the stability of the foam. Water vapor highly enhances the weight loss of the glass melt. At very high temperatures, the rapid depletion of alkali on the surface will form a silica-rich layer which is a diffusion barrier for further alkali loss. Surface tension decreases with increasing water vapor content in the atmosphere.; Foaming in the glasses with Na2SO4, fining agent is caused by the decomposition of sulfate at high temperature. The solubility of SO42- groups decreases as temperature increases, and oxygen and sulfur dioxide gases release from the melt. Foam forms when the oversaturation is high enough and the gas release is aggressive. Al 2O3 and Li2O decreases the foaming temperature because they decrease the solubility of sulfate due to their unique role in the glass structure. However, the substitution of Na2O by K 2O increases the foaming temperature. The compositional effects are also explained by the relationship between the sulfate and gas solubility and the basicity of glass melt. When Sb2O3, As 2O3, and CeO2 are used as fining agents, the high temperature foaming is caused by the oxygen release from the redox reactions of the multivalent fining; agent ions in the glass melt. A combined fining agent of antimony, arsenic, or cerium oxide and sodium sulfate shows a reduced foam volume for both high temperature and primary foaming, providing a potential route to control the foaming process in glass melting. The mechanisms for this foaming reduction are believed to be the decrease in the dissolution rate of silica sand grains and the modification of the gas diffusion in the glass melt. It was shown that the fining efficiency of the glasses could be increased by combining the fining agents.; Water vapor affects the redox reaction of sulfur in glass melts and decreases the solubility of sulfate, thus decreasing the foaming temperature when only Na2SO4 is used as fining agent. The considerable increase in maximum foam volume is ascribed to the fact that water accelerates the volatilization of the glass melt and increases the viscosity of glass melt in the bubble walls. For the Sb2O3 fined soda-lime-alumina-silica glasses, oxygen can slightly increase the maximum volume of the high temperature foam and makes its decay slower. Water vapor also increases the maximum foam volume and decreases the decay rate. The mechanisms of water effects are discussed based on the volatilization of glass melt and redox reactions of antimony ions. For the sulfate fined soda-lithia-lime-alumina-silica glasses, the foam stability was considerably reduced by changing the atmosphere from CO 2 + 50%H2O to other dry atmospheres. The stability of the foam in the antimony-fined SLAS glasses also showed a decrease when the above atmosphere change was made. The mechanisms were proposed based on the gas diffusion sulfate redox reaction, and water release from glass melt due to the atmosphere change.