Formation and oxidation of silicon in container glass melts

Abstract

The input of recycled container glass is increasing continuously in container glass production, unfortunately also causing the rise of rejects because of inclusions. In the group of glass inclusions caused by recycled cullet small silicon spheres are the main problem due to their different thermal expansion coefficient in relation to soda-lime-silica glass. Aluminium is introduced together with the recycled cullet into the melting process. Silica is partly reduced by aluminium to metallic silicon which forms small spheres in the produced containers. The kinetics of formation and dissolution of these silicon inclusions was investigated over a long period with respect to the melting parameters in several plants. Single reaction steps were repeated in the laboratory. Aluminium metal introduced as foils or pieces reduces nearly the same amount of silicia in flint, green and amber glass melts. Between 800 and 1000 °C silicon formation starts as a brown powder with a great specific surface area. This silicon modification melts at temperatures over 1400 °C and crystallizes to metallic silicon spheres. The highly varying number of silicon inclusions in glasses of different colour is not directly proportional to the aluminium input. It depends on different dissolution velocities. Because of their small density the silicon phases rise up to the glass bath surface and are oxidized. This reaction is a function of temperature and residence time at the surface of the glass bath. In amber glass melts the oxidation velocity of the silicon inclusions is greater than in flint glass because of much higher surface temperatures compared with flint glass melts.