Oxygen behaviour in the process of float glass manufacturing

Abstract

By means of solid electrolyte probes the oxygen content was determined in inert gas atmosphere and in the tin melt. The probes can be used within a temperature range of 500 °C for tin melt and of 600 °C for inert gas atmosphere up to 1100 °C. A higher emf value causes an oxygen decrease, a lower emf value leads to an oxygen increase in the system. From the measured emf values the oxygen partial pressures in the inert gas atmosphere and the oxygen contents in the tin melt can be calculated by means of the respective mathematical relations. The investigations confirmed that the leveling of an oxygen-unsaturated tin melt is very time-consuming if forming gas of 10 vol.% H2 is used. However, at temperatures above 700 °C (900 °C may be best) the oxygen unsaturated tin melt is reached relatively quickly. At 500 and 600 °C the melt will stay in the oxygen-saturated state for days. Setting of an oxygen-unsaturated Sn melt is most efficient under pure hydrogen. If the Sn melt is covered by a glass melt, the oxygen content above the glass melt and in the tin melt is always higher than without the glass. Small amounts of oxygen are steadily transferred from the glass melt to the atmosphere and to the tin. Furthermore, the effects of air, H2O vapour, CO and CO2 on the system were investigated in detail. A rather complicated behaviour in inert gas atmosphere and in the melt was observed. It depends on doping, temperature and type of inert gas atmosphere. Carrying SO2 in the carrier gas stream (forming gas and H2, respectively) leads to changes of oxygen content both in the tin melt and in the gas phase. The equilibrium state in the Sn melt is reached more slowly than in the gas phase. The emf curve for the gas phase falls below that of the tin melt, i.e. the main part of SO2 is decomposed in the gas atmosphere and only then is available as oxygen for the Sn melt. The restoration of the starting state of oxygen in both phases takes much time (about 40 h). However, at the Sn surface and at the wall of the vessel there are depositions consisting of SnS2, SnS and SnO2. As a further impurity in the floating chamber H2S is considered. The measurements show that there are no essential changes in oxygen behaviour in the system. The Sn melt, however, is strongly interspersed with SnS, such that it is no longer suitable for the float glass process. For the float glass process a tin melt unsaturated with oxygen is required. If saturation is achieved, SnO2 is formed, which is deposited as slag onto the tin surface, and thus is detrimental to the float glass process. Oxygen dissolved in tin can be removed by the addition of an dement, the oxide of which is more stable than that of the oxide of tin. Na, Mg, C and Zr are used as deoxidation agents. Using Zr, which is dissolved in Sn, a nearly oxygen-free melt is obtained, too. The advantage of the use of this getter consists in the fact that no slag is formed on the Sn surface. The introduction of a getter into the production process of float glass manufacture could lead to an essential improvement of the process technology.