Maintaining stability in a toughening furnace environment

Vesuvius Researching The Development Of An SO2 Injection Practice

The quality of the glass product is linked to the cleanliness of the furnace environment. This article examines how to maintain stable working conditions during thermal toughening, in an effort to reduce dust-originated defects.

The glass toughening furnace is a demanding operation, in which final glass properties are linked to the control of the furnace environment. Fused silica rollers are widely used to convey glass in high temperature environments. Vesuvius, a provider of refractory products for glass tanks and rollers for the glass forming part of the process, is researching an SO2 injection practice aimed at creating singular working conditions that influence the roll operation and understanding chemical reactions in the furnace. Premature fatigue of the furnace environment is investigated through chemical and mechanical microanalysis. Thermodynamic evaluation of the operation is given to relate glass surface quality with atmosphere and thermal furnace conditions. Corrective actions are therefore proposed to limit unstable working conditions and reduce particle and dust-originated defects in the furnace environment.
Thermal toughening
In the process of thermal toughening, glass is heated to a temperature just below its softening point and then quenched rapidly in air. The outside of the glass cools quickly, while the inside contracts and cools slowly. When the glass eventually reaches a uniform temperature, a compressive stress will have been created in the surface and a compensating internal tensile stress will have developed. The actual value of the surface compressive stress depends on such factors as glass thickness and heat transfer coefficient, as well as the heat treatment schedule. Limiting external factors that could modify the quality of the glass surface has always been a trend in the industry. Injecting SO2 The reaction between Na ions and injected SO2 creates a sulphate surface on the glass that is stable under normal working conditions. During handling of the glass sheet, the sodium sulphate powder is transferred to active zones of the furnace environment. A deposit of sulphate is observed at the interface of the glass and the conveyor rolls, which is considered of potential benefit for the contact surface longevity. In effect, the sodium sulphate is slowly evolving in a continuous interface of neutral reactivity towards the glass chemistry. The continuous gaseous interaction contributes to the creation of the solid lubricant Na2SO4, stable at a temperature that is standard practice for glass heat treatments: Toughening or annealing zones. The yield of formation of Na2SO4 is directly linked to the moisture level, concentration and temperature (if the action of sodium sulphate is well-known, some products of a reaction could present a negative effect on the quality of the operating conditions, mainly in the lowtemperature transient domain). Metastable forms of sodium sulphate could be generated as a function of the environmental conditions and lead to complex reactions that may induce unexpected results in temperature. Unstable sodium sulphate species could form and interact with the furnace environment and the glass to capture particles and dust that may create glass defects, if corrective actions are not taken to avoid undesirable operating conditions. Glass quality and Zyarock roll efficiency is enhanced by the control of the furnace adjustment. These factors influencing the working environment and furnace atmosphere are related to the thermodynamic stability of some of the sulphate species in the operation. Air flow interaction Air flow inside the furnace induces multiple side-effects that are linked to glass defect observation. The following are noticed from all thermal disturbances: – Induced forced convection introduced as a key for fast glass thermal stability (top and bottom sticking particles). – Geometrical deformation of the glass sheet under high thermal stresses (thermal gradient leading to increased load and wear behaviour). -Dust transfers inside the furnace environment (refractory and external dust particles generated through the working environment), but not in direct relation to the glass contact (glass dust, lining material and particles in suspension). -Chemically reactive by-products
issued from the toughening atmosphere and the glass chemistry (usually observed for SO2 injection environment). Furnace equilibrium is necessary to obtain thermal stability, to create reduced dynamic conditions for dust particles and to avoid the high-density movement inside the chamber. The introduction of SO2 injection is to be considered when all other working
conditions have been achieved to limit the undesirable wear behaviour and localised contact stresses usually found with quick thermal cycles, extreme glass thickness and chemistry requirement: Coated glass, thin glass and low absorption coefficient.
Thermal inertia Roll designs have been challenged for an increased response time that could be reduced further: The differential thermal transfer from the handling pieces to the glass sheet during the heating part of the cycle. Hollow rolls, as well as textured rolls, have been designed for such a purpose, but their use should be tightly linked to good control of operating parameters and stable working conditions, as they are not to be defined to overcome inconsistent working changes.
-The use of hollow rolls allows for a quick response time when the design of the roll ensures equal thermal stability and best use of radiative effect observed within the internal cavity of the roll. Material consistency is then used to achieve multiple mode heat (conductive and radiative) through the ceramic roll.
-Hollow rolls with uneven wall thickness do not provide the desired thermal benefits and add a mechanical imbalance.
-Hollow rolls will require shorter time to ramp up to stable operating conditions.
-Differential thermal capacity of 20°C is observed between solid and hollow rolls in transient mode.
-Homogeneous thermal profile can be achieved with hollow rolls assuming there is a consistent ceramic wall thickness.
-Radiative and conductive thermal modes should be investigated to understand thermal influence of hollow rolls on endcap stability. Mechanical stability The development of an appropriate driving system, capable of handling thermal leaks from the furnace and variable thermal conditions to the fused silica roller, can be achieved by adopting a mechanical system instead of the standard glued rolls endcap typically used. A new generation of mechanical drives has been completed to allow for a wider thermal gradient on older furnaces and also allow for the most demanding process changes, as recently seen with convection furnaces.Whichever roll is selected, the driving system should be adapted to maintain best furnace operation consistency:
-Provide a reliable driving system through mechanical attachment
-Increase operating temperature
-Reduce hot TIR overtime dispersion
-Reduce vibration
-Secure torque transmission
-Reduce chemical usage 99 Increase operational safety. Glass quality is linked to the cleanliness of the furnace environment. The thermal consistency of the toughening environment links glass properties to the furnace. When operating conditions are optimum, the introduction of SO2 to the furnace achieves the final touch of contact characteristic, with the formation of a sodium sulphate inter-layer.Strict control of atmosphere confinement and gas flow in the furnace should be conducted to avoid the formation of high oxidation levels for the sulphate, which could lead to viscous liquids and create service problems in the transition stage of the operation. The furnace environment is to be considered with secondary reactions involving dust and particles from the lining, oxido-reduction and corrosion that can compromise the glass quality in normal operating conditions.

About the company
Vesuvius is a global leader in metal flow engineering, providing a full range of engineering services and solutions to its customers worldwide, principally serving the steel and foundry industries.