Melting of Glass Bottles and Jars (Part C)

The physical, chemical, physicochemical phenomena, and reactions that occur during the melting of the glass batch materials are roughly: (1) Evaporation of the attached moisture in the raw materials; (2) Escape of gases such as CO2, SO2, and SO3 produced by the thermal decomposition of the raw materials; (3) Melting of low-melting components in the batch materials to form a liquid phase, and the formation of a multi-component liquid phase by eutectic components, with some local, special low-melting eutectic components also forming a liquid phase at low temperatures; (4) Mutual melting of the above various liquid phases, and the dissolution of unmelted solid components in the molten liquid; (5) Solid-state reactions between the raw materials, and between the generated silicate compounds and other oxides, to form new compounds; (6) Volatilization of certain components in the molten glass at high temperatures (such as K2O, Na2O, B2O3, PbO, SiF4, As2O3, etc.); (7) Dissolution of various gases covering the molten glass surface (combustion gases, air, and gases escaping from the thermal decomposition of raw materials) into the molten glass, and these dissolved gases turn back into small bubbles when saturation is reached. These phenomena and reactions do not necessarily occur in the above order, and some reactions occur simultaneously.

In the tank furnace, the batch materials on the molten glass surface flow longitudinally. Observing the position where the batch materials disappear on the molten glass surface can indicate the melting condition of the batch materials.

To prevent the production of unmelted batch materials and to obtain homogeneous molten glass, the melting of the batch materials must be completed as quickly as possible. The main factors affecting the melting time of the batch materials are: (1) Chemical composition of the glass; (2) Melting temperature and temperature distribution in the furnace; (3) Particle size and distribution of the raw materials; (4) Amount and particle size of cullet added; (5) Proportion of raw materials; (6) Uniformity of the batch materials; (7) Bubbling and electric boosting. The molten glass that has completed melting contains a large number of residual bubbles, and glass in this state cannot be used for forming. The process of removing these residual bubbles from the molten glass is called fining (or refining).

According to Stokes’ Law, the rising velocity of a bubble in molten glass can be expressed by the following formula:

Where:

V — Rising velocity of the bubble (cm/s)

g — Acceleration due to gravity (cm/s^2)

r — Radius of the bubble (cm)

d — Density of the molten glass (g/cm^3)

d’ — Density of the gas in the bubble (g/cm^3)

η — Viscosity of the molten glass (g/cm·s)

When the temperature rises to the fining temperature, the viscosity of the glass decreases, the rising velocity of the bubbles increases, and the bubbles are more easily removed. However, the smaller the bubble, the more difficult it is to float up. Therefore, to accelerate fining, a fining agent should be added to the batch materials. In the tank furnace, the use of bubbling and electric boosting can also accelerate glass melting and fining. Small bubbles that cannot be removed during the fining process can be absorbed by the glass, become smaller, or disappear during the forming process as the temperature of the molten glass decreases and the gas pressure in the glass decreases.

The main factors affecting the fining quality of molten glass are: particle size of raw materials, batch material form, fining agent, refractory materials, impurities, temperature, and atmosphere. The specific method for controlling the fining quality of molten glass is to take samples at specific locations in the tank furnace, measure the number of bubbles in a certain amount of glass, and record and compare this with other conditions.

During the glass melting process, the molten glass is chemically non-uniform due to factors such as the volatilization of glass components and the erosion of refractory materials. The process of bringing the molten glass to a chemically uniform state is called homogenization of the molten glass.

The homogenization of molten glass is based on diffusion. The natural convection of the molten glass in the furnace plays a significant role in promoting the diffusion of the molten glass. This promoting effect can be enhanced by electric boosting, bubbling, or forced melting. Because the viscosity of molten glass is relatively high, melting, fining, and homogenization are not distinctly separate but occur simultaneously.