Increased demand for glass for solar applications is driving the development of specific glass qualities that are designed for the various solar technologies. If the requirement is different function of the application we have seen appearing a variety of different dimensions, thicknesses and grades requiring specific conditions to prevent contamination by iron particles. When dedicating the glass plant to produce Solar grades for thin film or to PV module encapsulation, the melting and forming process are impacted by the float lines' past applications. Temperatures and chemistries are not only defined by the batch but also by the working environment found in the different furnace zones. Achieving high quality glasses required for the Solar market could be done not only by adjusting the working parameters in the fusion area, but also through the control of the tin batch and transition zone immediately, and in particular at the LOR position when entering the annealing area.
Tin Bath Environment for Producing Solar Grade Glasses
When producing solar grade glasses in a float line, we should consider the passed conditions of the float and in particular the operating conditions in the tin bath. As strong interactions are involving oxide systems (refractory, glass), metallic tin, sulfur atmosphere, reducing conditions and extreme temperature gradients exchanges are developing through the tin metal in a first step, before being transported outside of the tin bath during the reduction of the glass thickness required for high performance low thicknesses.
Metal Equilibrium and LOR Interaction
If tin is present as a predominant metallic deposit due to the glass hot handling, other elements are frequently found due to the float line environment and the selection of the support systems for the different rollers between the forming and annealing zones.
At float exit temperature, under high oxygen concentration, the oxide forms of the different metallic phases that are present (FeO, Fe2O3, SnO, and SnO2). The presence of sodium sulfide helps the stability of iron sulfate in the melt as a reaction of Na2S+FeS with liquids around 650°C.
SnS+FeS+Na2S ……… FeO/Fe2O3 + SO2 + Na2O
The decomposition of the Fe-S system when in presence of oxygen is shows an interdependence from temperature and type of iron oxide formation: below 560°C iron will form Fe3O4- Fe2O3 oxides, but when the oxidation temperature moves above 560° (below 670°c) the possible formation of FeO type oxide is prevalent against Fe3O4. The environment of oxide will stay with emission of SO2 in the atmosphere.
Producing Low Iron Glass
Low iron glass specification requires that the glass is produced at a low redox ratio, which implies that the glass is manufactured under oxidizing conditions. Oxygen partial pressure is considerably different at the glass melt and at the molten tin contact zone where high redox is observed. Operating the float furnace imposes conditions drastically different from pattern glass production lines that are also producing solar glass qualities. The presence of the tin bath area creates specific working conditions: oxygen atoms are stripped away from the sulfate remaining in the glass, reducing it to sulfide ions. When the sulfide ions combine with ferric iron, the resulting compound (FeS) affects the glass coloration.
Producing Photovoltaic Glass
When moving to photovoltaic glass, special compensating adjustments have to be made to the glass composition to reduce the influence of iron present in the raw materials; such composition changes could rely on sodium concentration at the same time as specific chemical additives capable to control the iron effect. SO2 treatment could be part of the fabrication of the soda-lime to fix the sodium and reduce the reactivity.
Higher SO2 in the annealing lehr and later in the tempering furnace will reduce the glass surface roughness; degree of staining may be conducted function of the selected process route and the float line capability.
The control of the metallic iron in the glass is only a part of the requirement for solar glass quality as the glass sheet surface should be free from chemical and structural defects, as they are generated from the float furnace and in particular from the tin bath. The specific conditions that will create a redox adapted to the solar glass formation will also imply the modification of the working zones and the creation of “metallic particles” for non dedicated solar float units. Metallic sulfites, ferric iron, sodium silicates, etc… generated in the tin bath will move toward the LOR and further down the line.
Similar operating parameters should be observed when looking for automotive or architectural glasses than when producing solar glass, but with consequences that could be different in term of end product quality:
- Conversion of the tin metal to a stable oxide form SnO, SnO2. Conversion which is conducted with crystallographic change allowing to surface modification and possible volumetric stress
- inability of the carbon scrapper to remove the tin metal, and creation of even more pressure to force the tin onto the LOR surface texture
- Iron sulfate precipitate in the liquid bath as Fe metal, if the equilibrium between sodium and tin is driving the reaction
- SnS vapor formation leading to possible Sn metal droplets or formation of SnS2 concentrations in the cold zone of the tin bath. The solubility between SnS and SnS2 inside the Sn is however conducive to a sulfur concentration in the melt (High S), and enhances further reaction with glass and gas environment Na, Fe, Cl, etc…
- Metals, metal sulfurs & metal oxides (precipitation or soluble) will be entrapped with the tin metal and transported to the colder part of the bath, where they will deposit on surface of the roll. The cleaning operation (graphite) should be enough to remove low sticking elements.
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 buildup mechanism of reaction between the different metallic impurities coming from the tin bath and the metallic roll on the lift out area, is conducive to understand that various forms of metallic compounds could be found on the metallic roll surface: direct metallic reaction, sulfates and oxide reacted compounds. Each of the forms can lead to permanent metallic roll damage, and improper working conditions in glass support.
Plasma Coated LOR
Alternative solutions developed to remove the direct metal contact by using plasma coated LOR’s. These are designed to reduce the reactivity of the metal roll against metallic, sulfate and oxide residues coming from the tin bath with the glass sheet, but such systems are highly dependent on the ageing and reactivity of the selected plasma chemistry when exposed to the environment.
The adequation of plasma with the metallic LOR is also subject to thermal damage, inherent to the glass sheet extraction and the LOR design:
- differential thermal expansion
- stressed plasma surface
- metal to coating interface
- chemistry of the coating to handle metal roll compatibility and glass contact behavior
Fused Silica LOR
The use of a complete and permanent ceramic LOR is the ultimate solution to avoid the mechanical compatibility of the roll surface, the compromise between the tin bath unstable residues and the environment. Fused silica stability in the tin bath environment is limited to the reactivity of the fused silica itself which is well above the typical temperatures present in the LOR area for most of the solar as electronic applications.
To better understand the substitution potential for the metallic or plasma LOR by fused silica LOR, we have to review the different mechanisms of tin metal buildup and deposition onto the surface of silica under glass weight pressure, as no direct (chemical) reaction between Sn, SnO or SnO2 is expected with the silica roll. The interaction of oxide forms of tin with fused silica is not prevalent below 890°C that is a liquidus for such system.
Tin Bath Transition Zone
Under large tin metal overflow, or limited cleaning operation, some of the metal will react with the metallic roll surface and will become difficult to remove, starting buildup formation. The LOR area is a zone of variable operating conditions (temperature, oxidation, pressure, maintenance) function of the operating parameters decided by the glass maker. Oxidation of the tin metal could take place, leading to tin oxide conversion which is impacts the roll surface quality and ultimately, the good service of the material for future glass contact.
Low pressure carbon cleaning tools and protective non oxide environment are recommended for the LOR operation. The non wetting behavior of the fused silica against metallic elements provides a good prevention to sticking glass defects. Particular attention to maintenance, and cleaning the LOR should be taken when oxidizing conditions are allowed, hot cleaning of the roll is recommended to minimize metal to oxide conversion.
The operating conditions in the float line determine the type of technical improvement to select to achieve optimal line performance. From severe oxidizing to highly pressurized conditions decided for line maintenance, with fused silica LOR & LHR Vesuvius is proposing specific rolls to help glass makers produce high quality glass under best operating practices.
In term of hardness, the precipitation or crystallization of stable oxides from metals and soluble systems should be considered as a nucleation of hard particles. The shape of the precipitate follows the general mechanism of the roll, and most of the reaction compounds are present under a “flat” or “flaky” aspect ratio which is reduces the negative effect on the glass surface. Further nucleation and consolidation of the flakes will produce a local “punching” defect that will require a physical removal or cleaning operation to avoid glass damage.
The temperature of the application is not a parameter which should affect the hardness of the deposit on a chemical base, but it may affect the “shape” of the precipitation and the sticking behavior by incorporating sodium (or glass) elements in addition to metal oxide compounds.
Introducing Annealing Zone
If metallic deposits are possible in the lift out zone, most of the problems in the transfer zone will come from the increased oxygen partial pressure that allows for the conversion of the metals to oxide forms. The combination of metals and oxides on the surface of the glass sheet, conduct to modify the buildup chemistry, developing stronger potential for sticking on surface of the rolls when exposed to equilibrium conditions. Tin metal is the driving force for glass defects that are not compatible with clean surface cosmetic and ready surface preparation for surface treatment.
The full potential of glass chemistry is enhanced by the capability to limit glass surface transfers from the tin bath, and maintain maximum interface quality over the LOR and annealing zones, to avoid surface defects and keep efficiency for glass coating application. Fast cooling in annealing to maintain thickness and surface flatness, requires reducing the stresses generated by thermal gradients. Rapid temperature changes demand a very high quality roll to glass interface as friction coefficient creates a necessary behavior during annealing.
The cleaning operation and maintenance of the tin bath and transfer rolls in reducing and low oxidizing working conditions will need to be correlated to the metallic deposit on the rolls.
Knowledge of the float history prior to conversion to solar glass production, as well as knowledge of the fusion, chemical stabilization, conditioning, temperature gradient changes in the float bath, atmosphere partial pressures and thermal flows may need to be investigated to understand the possible variation in sulfur, metals and oxides in the furnace forming zones (SnS, SnS2, Na2S, FeS, etc…)
LOR and transition LHR roll surface should stay “defect free”:
- No metallic inclusion
- Control of surface damage (machining, handling, operation)
- Techniques to “reduce” tin forced in the silica sub-layer should be implemented.
- Cleaning the roll during service should remain a good practice
The transportation of the glass sheet when coming out of the tin bath area is an important step to maintain the high quality of the glass and to ensure the proper cooling action required to meet the defined application. The capability and process controls implemented in the lehr are dependent of the glass specifics, like thickness, chemistry but also of the lehr furnace design and operating parameters. Formation of buildup is initiated by tin “adhesion” to the roll surface. Low adhesion of tin oxide and sodium sulphate on roll surface is required.
The main concern is the surface damage of the roll, and the penetration on a sub-layer of the tin metal, associated with thermal cycle conducting to oxide transformation as a base for metallic and sulfate buildup.
This information has been sourced, reviewed and adapted from materials provided by Vesuvius.
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