Solutions for soda lime furnace bottoms

The factors that command glass furnace operations are well known to glassmakers worldwide, and especially true at times of economic pressure on the industry: An increase in specific cumulated pull to maximise fixed costs absorption, a decrease in energy consumption to optimise variable costs and comply with environmental policies, and an increasingly demanding glass quality requirement.

This contributes to a continuous shift in terms of glassmaking practices:

• Increase in electric direct heating (boosting) use;
• More widespread and intensive use of bubblers;
• Higher cullet percentage in the container and flat glass industry;
• Lower iron content in high end flat glass especially for solar market; and
• Deeper tank designs that tend to increase thermal convection.

These drivers result in an increase in both temperature and glass velocity which means higher stress on the tank bottom refractory assembly. The physical conditions of temperature and velocity lead some glass furnaces to fail and perform poorly, especially when furnace bottom design is inadequate.

A glass leak can be catastrophic leading to full loss of a furnace including structures and peripheral equipment (fans, chambers, piping, wires and combustion equipment) and above all puts human lives at risk
          Glass leaks also trigger scrutiny from a glassmaker’s business partners such as local environmental authorities, risk management consultants, shareholders, local staff and media. This increased scrutiny leads to technical overreaction, with countless countermeasures which can make operations more costly.
         Even when plant staff stop a glass leak quickly and damage is limited, its occurrence usually means glass quality problems are not far off and the furnace needs an early cold repair.
         Preventing glass leaks with proper tank bottom design costs far less than possible production loss, not to mention damage to plant equipment that can
result from these leaks.
         Since glass quality is almost always affected by a glass leak, preventing it leads to better glassmaking conditions and reduces the occurrence of glass defects that can come from non-glass contact refractory.

Glassmakers tend to increase the stress applied to a tank bottom and it is still not uncommon today to visit a tank with a shortened lifespan due to entire sections of the bottom paving disappearing.
           Figs 1 & 2 shows examples of bottom paving failure where the sub-paving layer was infiltrated with glass, causing an early cold repair.
           Although the quality of fused cast AZS exotic materials may be directly incriminated in some cases, SEFPRO’s services often conclude that the subpaving layer and the whole bottom assembly design are the root cause of furnace bottom failure.
           A typical furnace failure can be described as: A new furnace bottom has AZS paving on top, with a sub paving layer below and a safety layer made of fire clay insulation at the bottom. After heat-up, some joints may remain open and during fill-up of the glass tank, glass finds its way through the joints.
          After several weeks or months in operation, glass corrodes and penetrates the porosity in the sub-layer, which reacts with glass to form bubbles, which in turn allows glass renewal in the cavity, thus continuing the corrosion.
         After months or years, the slow infiltration of glass in the sub layer increases, generating an upward drilling phenomenon in the electrofused AZS which ends up disappearing completely, putting the sub paving layer in direct contact with the glass bath.
        Once this AZS paving has disappeared, a cold repair is mandatory to recover an acceptable glass quality; when a cold repair is not possible in the short term, it is common to increase melting temperature to improve glass quality.
        This worsens matters by increasing the corrosion rate of sub layers not designed for high temperature glass contact. In many cases, especially in the areas where the sub layers are under increased stress, a glass leak quickly appears causing the furnace to shut for repair.

The above scenario is triggered by seven factors:

1. Traditional AZS paving sometimes shows open joints after heat-up.
2. Insulated tank bottoms tend to prevent glass from freezing in the paving joints but rather in the paving sub-layer.
3. Open joints or cracks allow glass to reach the sub-layer.
4. Some sublayer materials react with glass to form bubbles.
5. Bubbles invariably corrode the refractory that is on their path to the surface (upward drilling mechanism).
6. High temperature accelerates renewal of the glass interface in joints and corrosion of sub-layer.
7. Porosity and/or corrosion in the sub-layer allows a slow glass circulation.

      Some factors can be reduced to the extend where their individual contributions become so negligible that the phenomenon does not unflod during the tank's lifetime.

      A common reaction to bottom corrosion by upward drilling from glass infiltration is to work on joint closing to prevent
glass infiltration.
      It is necessary to do so and to use a paving that has a good corrosion resistance and tight dimension tolerances to fight the open joints (Factor 1). Sefpro’s recommends using TJ paving with fully machined joint faces, with only 0.5mm maximum joint thickness at preassembly (Table 1).
      Closing the joints is not always enough to fight sub-layer corrosion:
- Depending on the heat-up conditions, some joints perfectly closed at assembly may open, and some expansion joints may not completely close.

- Factor 2 and time will lead to glass infiltration until glass devitrification temperature which hardly ever occurs in the paving layer but frequently in the
sub-layer or further down.
      Compared to zircon or zircon-mullite ramming mixes, and thanks to its unique electrofused AZS grains composition, SEFPRO’s ERSOL 50 offers more resistance to glass corrosion by infiltration and lower blistering when in contact with glass
      With ERSOL 50 as a sub-layer, it is possible to block factors four, five and seven. (Table 2)

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Table 1. Comparison of Tight Joint (TJ) quality

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Table 2. Comparison of ERSOL 50.

      To further decrease the sensibility of open joints and cracks to glass infiltration, a double layer concept was introduced. The upper layer acts as a monolithic, cohesive, corrosion resistant and glass quality friendly sub layer that will follow the expansion of the paving layer.
      The bottom layer, which adheres to the safety sub-layer that does not expand as much as the paving, can then release the differential expansion without stressing the upper ERSOL layer.
      To see the benefits of the double layer, a lab test assembly with ER1681TJ/double ERSOL layer/ERMOLD 300 was heated and cooled.
      After being fired, the upper ERSOL layer could be removed and handled crackfree while the lower layer showed some cracks due to the differential expansion (Fig 3). That way, factor three was ticked: Cracks created by differential expansion between safety layers and paving sub layer were removed with two layers of ERSOL.

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Fig 3 Upper layer of ERSOL – crack free.

      Some applications, in extra-clear or high quality glass, with high specific pull, intensive use of bubblers or boosting, apply extra stress on tank bottom assembly.
      In these situations the standard solution needs to be upgraded for more corrosion resistance.
      For particularly demanding furnaces, SEFPRO recommends the use of ERSOL SL, a new castable product that features improved corrosion resistance as a further assurance against glass leaks and to withstand longer campaigns.
      In the list of factors, its features lowers the contribution of factor’s six and seven (Fig 4).
      To go a step further for low iron glass furnaces where the bottom temperature is close to the crown, SEFPRO has developed the ER 2010 RIC AZS material, which has enhanced corrosion resistance compared to standard AZS.
      It shows an overall expansion of ER 2010 RIC compared to standard AZS (Fig 5).
      This difference was evidenced with a small scale test that
reproduces the behaviour of a closed joint after heat-up.
      Thanks to a video installation tracking the joints’ closure, it was possible to demonstrate the positive effect of ER 2010 RIC. At the working temperature, the joint is totally closed while it remains open with standard AZS; with this solution, the furnace bottom paving behaves as a monolithic, corrosion-resistant refractory less prone to blistering.

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Fig 4 The ERSOL SL’s features lessens the
contribution of factor’s six and seven.

 

Fig 5 Heating up simulation Showing 2010
RIC joint closure at high temperature

      ERSOL SL has been used in many glass tanks worldwide with promising results.
      Together with the Ermold safety layer, the two monolithic layers concept, and the ER 2010 TJ paving, ERSOL SL is a further step in Sefpro’s solution to offer durable, economical and safe furnace bottom assembly.