Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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neutral post in the energy balance, cullet vigorously react with soda ash; very fine cullet compete with sand for soda ash, delaying sand dissolution.

      Owing to their extremely low viscosities, the salt‐like primary melts play an important role in bringing about high turnover rates during batch melting. When they are lacking, as in alkali and boron‐free continuous fiber glasses, the products in contrast remain in a granular state until they reach their lowest eutectic temperature (compare with Figure 6b in Chapter 6.1).

Photos depict the early stages of batch melting, manually sketched after the scanning electron microscopy micrograph. (a) Open-pore stage with granular solids and gas, the gas composition being dominated by the equilibrium between CO2 and O2 from trapped air and the furnace atmosphere. (b) Closed-pore stage with the development of a widespread primary liquid, a large ratio s of effective liquid interface and liquid volume, and a gas composition dominated by CO2, redox active materials, and polyvalent ions in the primary melt. (c) Reaction-foam stage characterized by large volumes of granular solids, bubbles, and melt, and by progressive melting of solids and decreasing s ratios. (d) Rough-melt stage, the melt being the predominant phase coexisting with considerable amounts of bubbles and undissolved grains and showing on top a seam of the primary foam formed.

      4.3 Sand Dissolution

Schematic illustration of a dissolving sand grain. The grain is surrounded by a solid reaction layer followed by a liquid high-viscosity diffusion seam with decreasing SiO2 concentration, hence decreasing acidity, from inside to outside. Gas bubbles – mostly O2 – precipitate at the interface solid/liquid, upon complete dissolution of the sand grain, a bubble cluster remains in the melt.

      In addition, temperature‐induced reduction of ferric iron takes place as described by the reaction

      (2)equation

      describing how firm [Fe3+O4] oxygen complexes give rise to the weak [Fe2+O6] complexes formed by ferrous iron. The equilibrium constant of the reaction is given by

      (3)equation

      so that, at constant redox state, tiny oxygen bubbles emerge at the boundary of the dissolving grain. Any dissolving sand grain leaves behind it a cluster of small bubbles, removal of these bubbles makes sense only if their generation is over. This is one of the reasons why sand dissolution and the fining process need to take place in separate parts of the furnace.

      In summary, successful sand dissolution is a prerequisite for successful fining. Even apparently small differences in the grain‐size distributions of sands have a big impact in this respect. This statement will be demonstrated for two different sands. Let us assume that a spherical sand grain with radius r dissolves according to Jander's kinetics:

      (4)equation