Glass forming is performed in the tin bath area through a strong thermal kinetic process to achieve the required thickness and glass surface quality. The extraction of the glass ribbon from the tin surface involves a complex interaction between glass, tin bath and atmosphere where thermo-chemical reactions are adding to the physical disturbance created in the lifting operation. The lift out rolls which are designed for the initial transfer of the glass from the forming zone down to the annealing furnace, are involved in a difficult compromise where roll surface behavior is influenced by the plasticity as well as the chemistry of the glass. Fluctuations in the tin bath operating conditions create strong variations in the lift out roll environment.
The Importance of the Lift Out Roll (LOR) for High Quality Glass Production
A typical lift out roll ensures the conveyance of the glass during the more plastic stage of the ribbon when any structural contact could have a damaging influence over the glass surface quality (chemical physical, optical). A very smooth and inert contact between glass and LOR is important to the glassmaker.
Tin Bath Extraction and Buildup Nodules
Under particular operating conditions, buildup nodules that may interfere with the glass ribbon have been observed in many occasions. Even in the presence of carbon scrappers, nodule build up may occur. Additional glass chemical interaction within the tin bath environment is also observed with sodium and alkali reactions coming as a secondary form of contamination. Due to the glassy structure of the developed buildup, an increase in reactivity may create an intermix composition with increasing reactive behavior and adhesion properties. The gas or metal turbulences present in the bath, are an additional factor for the formation of buildup compositions and precipitation.
Composite reactions are the consequence of the modified working environment, and a direct function of the process changes for the float line:
- Iron is present under iron sulfur and reacts with SnS + FeS with liquidus at 785°c inside the float.
- The equilibrium between the sulfur and oxide form of Fe is showing a liquidus around 920°C FeS+FeO
- The stability of SnS at temperature is also a function of the oxygen partial pressure as described in the diagram Sn-S-O, and conduction to the creation of the SnO phase as a temporary system for low oxygen partial pressure, before evolving toward full SnO2 when high O2 pressure is applicable.
- Under the tin sulfur conversion we can also find a similar movement of the FeS that is destabilized from FeS to FeO before to arise as Fe2O3 at the high oxygen level
SnS+ FeS --> SnO + FeO+ SO2 --> SnO2+Fe2O3
Chemistry and the Buildup of Nodules
On the roll surface the tin metal Sn is present as a metallic deposit from the glass handling. No direct (chemical) reaction between Sn or SnO or SnO2 is expected with the fused silica roll. The interaction of oxide forms of tin with fused silica is not prevalent below 890°C that is a liquid threshold for such system. However, under the pressure of the glass weight we can have an anchorage of the tin metal onto the fused silica, damaging the roll surface (consistency, spalling, and roughness).
At float exit temperature, under high oxygen concentration, the oxide forms of the different metallic phases are phases presents (FeO, Fe2O3, SnO, and SnO2). No reaction between the oxide forms and silica can be noticed. The presence of sodium sulfate helps the stability of iron sulfate in the melt as a reaction of activation Na2S+FeS with liquid phase around 650°C.
SnS+FeS+Na2S --> FeO/Fe2O3 + SO2 + Na2O
The decomposition of Fe-S system when in the presence of oxygen shows interdependence from temperature and type of iron oxide formation: below 560°C iron will form potential Fe3O4- Fe2O2 oxides, but when oxidation temperature moves above 560° (below 670°C), the possible formation of the FeO type oxide is prevalent against Fe3O4. The environment of oxide will stay with creation of SO2 in the atmosphere.
Iron or metallic elements are present in the melt as a consequence of multiple origins:
- glass material chemistry
- atmosphere presence under reducing condition
- tin contamination
- material from the bottom blocks (dissolution of alumino-silicates)
The quality of the glass is linked to multiple environmental and operational conditions that are affecting the glass surface when lifted from the tin bath. The bottom glass surface is influenced by liquid metal and solid roll contact, when the top glass surface is mainly affected by the atmosphere and process parameters.
From the various interfaces, we see specific service conditions and direct relation to the glass quality issues:
- Liquid to glass: will see tin influence like oxide concentration, soluble or precipitate phases, coming from the metal to refractory reaction (solubles, iron, and sulfur compounds, sodium to alumino-silicate reactants, tin oxides and sub-oxides). The solids formed by the different reactions are present as nodules or flakes, sticking to the bottom glass surface. The particles are typically embedded inside the glass at high enough temperature to create localized stresses or voids.
- Solid to glass: originated at the roll surface are acting at lower temperature than the molten tin, and are typically more aggressive for the glass surface. The effect of the contact could be seen as scratches on the glass surface when involving metal and metal alloys compounds. This wear effect could also be linked to the formation of scratches or haze on the bottom surface of the glass when the hardness of the parasite nodules are either of lower value than the glass or reacted with the LOR due to a sticking reaction or indentation.
- Atmosphere to glass: most prevalent when involving vapor phases of tin metal or sulfur, linked to tin bath internal pressure and heat flow stability. The relation with hydrogen and SO2 will be seen in another analysis of the tin bath environment (tin bath investigation and chemical stability).
We have introduced with Vesuvius Zyarock® Fused Silica Lift-Out Rolls a material with very high surface finish and extended capability to support the glass with minimum thermal and chemical interaction. The quality of the fused silica roll takes full advantage of the very low reactivity with tin and tin oxide at low temperature typically found in the dross box. Prevention of roll surface defects is an absolute requirement to ensure a perfect glass handling during the plastic stage and consolidation phase.
Metallic rollers used in the LOR position, will see buildup formation accelerated compared to ceramic rolls. The chemical reactions involving steel wet ability by liquid metals as well as the welding of metal / semi-oxides phases are inherent to the metal-to-metal compatibility. The creation of tin based metallic alloys is facilitated by the low temperature composites and surface degradation observed with metallic phase sticking to the steel.
In addition to the lower reactivity of Zyarock® Fused Silica Lift Out Rolls and resistance to build up, cleaning the fused silica ceramic rolls is simple and effective.
This information has been sourced, reviewed and adapted from materials provided by Vesuvius.
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