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جواب:

Corrosion of refractory silica brick used to line the roof or “crown” of many glass-melting furnaces is a serious problem in furnaces using oxygen-fuel rather than air-fuel mixtures. In this work, we report equilibrium calculations that support a corrosion mechanism in which alkali hydroxide gas (NaOH or KOH), produced by reaction of water vapor in the combustion gas with the molten glass, reacts with the silica brick in the furnace crown to produce an alkali silicate liquid with a composition that depends on the temperature of the crown. Our reported calculations predict the variable-composition liquid-solution corrosion product phase as a function of key furnace variables. Critical thermodynamic data needed for the liquid corrosion product were generated using a modified associate species solution model and critical analysis of thermochemical information found in the literature for the and systems. Excellent agreement with reported and phase diagrams and with experimentally measured activities for and is achieved. The results of our current calculations are for temperatures between 1273 and 1973 K (1000-1700°C) under either air-fired or oxy-fired conditions, and are used to define a “critical temperature,” above which corrosion is not expected to occur for a given NaOH(g) or KOH(g) partial pressure. © 2001 The Electrochemical Society. All rights reserved

The files below may be used to model the corrosion of various refractories used in glass melting furnaces. Those listed under "Silica corrosion" are designed to simulate corrosion of low-density silica bricks used in furnace crowns. The corrosive mechanism is assumed to be

 

M₂O(in glass melt) + H₂O(g, combustion gas) 2MOH(g), M = Na or K (1)

2MOH(gas) M₂O(dissolved in liquid SiO₂) + H₂O(g, combustion gas) (2)

Files listed under "Alumina corrosion" are designed to simulate corrosion of high-purity alumina (either or ) according to the reactions below. Similar reactions apply to alumina corrosion by KOH (although the temperature ranges differ). Details can be found in Ref. 2.

NaOH reaction with alumina:

T < 2158 K:    2 NaOH(g) + 9 Al₂O₃ 2 NaAl₉O₁₄ + H₂O(g, combustion gas)

T > 2158 K:    2 NaOH(g) Na₂O(in Al₂O₃-rich liquid) + H₂O(g, combustion gas)

NaOH reaction with alumina:

4 NaAl₉O₁₄ + 2 NaOH(g) 3 Na₂Al₁₂O₁₉ + H₂O(g)

NaAl₉O₁₄ + 8 NaOH(g) 9 NaAlO₂ + 4 H₂O(g)

Silica Corrosion (See Ref. 1 for more details.)

Low-density silica corrosion by NaOH (Na-Ca-Si-O-C-H-N system) (File format: ChemSage)

Low-density silica corrosion by KOH (K-Si-O-C-H-N system) (File format: ChemSage)

Alumina Corrosion (See Ref. 2 for more details.)

Alumina refractory corrosion by NaOH (Na-Al-O-C-H-N system) (File format: ChemSage)

Alumina refractory corrosion by KOH (K-Al-O-C-H-N system) (File format: ChemSage)

References:

1. M. D. Allendorf, K. E. Spear "Thermodynamic Analysis of Refractory Corrosion in Glass Melting Furnaces," J. Electrochem. Soc., 148, B59 (2001).
2. K. E. Spear, M. D. Allendorf "Thermodynamic Analysis of Alumina Refractory Corrosion by Sodium or Potassium Hydroxide in Glass Melting Furnaces," J. Electrochem. Soc., 149, B551-B559, 2002.

Operating Guide

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Very little information exists in published literature on defect chemistry and frequency as a function of furnace age and AZS reuse, so researchers at Vesuvius Monofrax, Inc. attempted to correlate glass defect chemistry with AZS refractories of varying ages. They analyzed post-campaign AZS refractory blocks from three types of glass melting furnaces: a soda-lime container furnace, a soda-lime tubing furnace and a lead silicate TV funnel furnace. The results of this study are providing a better understanding of the link between AZS refractory corrosion and glass defects.

Figure 1. A 34% ZrO2 AZS superstructure (left and inset) and glass contact block (far right) after a 10-year campaign in a soda-lime container furnace.

Soda-Lime Container Furnace, 10-Year Campaign

The holes in the blocks are from drilling core samples for characterization. Polished sections for microscopy were prepared from the core samples. The chemistry and microstructure were analyzed as a function of depth using SEM/EDS techniques. In addition, physical properties such as the bulk density and the apparent porosity were also measured as a function of depth.

Figure 2. The matrix phase chemistry as a function of depth for both the glass contact and superstructure AZS refractory samples from the soda-lime container furnace. The photomicrographs show a corrosion depth of ~150 mm in the superstructure block compared to ~50 mm at the glass contact.

The chart on the upper portion of Figure 2 shows the matrix phase chemistry as a function of depth for both the glass contact and superstructure AZS refractory samples. Both samples exhibit similar changes in chemistry-i.e., an increase in the concentration of the alkali/alkaline earth species, an increase in the alumina concentration and a decrease in the concentration of silica. However, the superstructure refractory sample shows a greater depth of chemical change than the glass contact sample. An example of this is the depth of the nephelitic zone, mentioned above, which was found to be greater in the superstructure sample than in the glass contact sample.

The results shown in Figure 2 suggest that the glass contact refractory corrosion lessens with time, while the superstructure corrosion continues over the life of the furnace campaign.

Figure 3. A glass contact block (AZS sidewall and superstructure) after a five-year campaign in a soda-lime tubing furnace. Inset: A viscous knot from the soda-lime tubing furnace.

Soda-Lime Tubing Furnace, Five-Year Campaign

Figure 4. The matrix phase as a function of depth in the AZS block. The open symbols represent the cord chemistry. (The absence of ZrO2 in the cord is indicative of a non-AZS source.) The photomicrograph shows the microstructure of the AZS block at the glass/refractory interface.

The chart in the upper portion shows the chemical analysis of the matrix phase (glassy in the as-is AZS refractory) as a function of depth in the AZS block. The chemistry of this tubing glass is similar to that of the soda-lime container glass described previously, and the chemistry profile of the AZS block was found to be similar to that seen in the container glass tank. The alkali/alkaline earth and alumina contents increased, while the silica content decreased in the matrix toward the glass/refractory interface. This resulted in the formation of a nephelitic matrix phase.

Figure 5. A comparison of the depth of corrosion in the glass contact AZS blocks taken from the container and tubing furnaces.

Figure 5 compares the depth of corrosion in the glass contact AZS blocks taken from the container and tubing furnaces. Though the corrosion depth is greater in the container furnace, this could be due to the differences in the campaign durations (10 years for the container vs. 5 years for the tubing), location in the furnace and temperature.

Figure 6. An evaluation of an AZS superstructure core sample from the charge end wall of a TV funnel furnace after a four-year campaign.

TV Funnel Furnace, Four-Year Campaign

Figure 7. The concentration of alkali and alkaline earth species in the B1 core sample as a function of depth.

Figure 8. Reflected light images of AZS samples obtained from the charge end of the TV funnel furnace.

Summary of Results

In addition to studying the corrosion behavior, researchers also measured changes in the physical properties of the post-campaign AZS samples. The level of apparent porosity was, in general, higher than that seen in the as-is AZS. This increase is most likely due to the formation of new crystalline phases (that can be higher in density and thus lower in volume) at the expense of the glassy phase, and also from liquid phase rundown from the refractory surface into the glass bath.

Figure 9. AZS refractory corrosion and knot/cord-type glass defects.

AZS Corrosion and Glass Defect Formation

The defect represented by a triangle symbol in Area 1 is believed to have definitely originated from the melt contact corrosion of AZS refractories. The defects represented by square symbols in Area 3 are believed to have definitely originated from the superstructure corrosion of AZS refractories. Defects noted in Areas 2 and 4 represent some uncertainty about the source; however, the defects in Area 2 are most likely from an AZS melt contact source, and the defects in Area 4 are most likely from an AZS superstructure source.

Since the defects shown in this chart came from furnaces melting many types of glass chemistries, and the majority of the defects appear to have originated from superstructure AZS corrosion, it seems logical to conclude that glass defect formation continues as a given glass melting furnace ages.

Minimizing Defects

Post-campaign refractory evaluation has also shown an increase in apparent porosity, which may be due to the formation of new phases and/or a matrix (liquid) phase rundown. The chemistry of both knots and cords appears similar to the AZS hot face chemistry following corrosion. Though both glass contact and superstructure corrosion products can lead to knot/cord defects, superstructure corrosion is a more potent and long-term source of defects. Additionally, used AZS can contain new crystalline phases such as nepheline, kalsilite, leucite, beta-alumina and zircon. And a mismatch in the coefficient of thermal expansion (CTE) with unaltered AZS may lead to spalling, which can also cause defects.

As mentioned earlier, the glassy matrix phase in AZS (which is effectively 1/3 of the total volume) provides large pathways for the corrosive alkaline and alkaline earth species to diffuse into the body of the refractory. This in-diffusion promotes the dissolution of crystalline alumina, resulting in an expansion of the glassy phase volume. The data from post-campaign AZS superstructure blocks show significant chemical alteration of the glassy phase in up to several inches of the block thickness.

Given that the majority of the knot and cord defects analyzed at Monofrax are similar in chemistry to that of the AZS glassy phase following superstructure corrosion, and that AZS superstructure corrosion can progress over the life of the furnace campaign, it is reasonable to conclude that superstructure AZS corrosion is an ongoing source of glass defects. However, the rate of defect generation is a more complex issue and depends on many other factors besides refractory degradation. These factors include furnace temperature profile, throughput and furnace exhaust control, which were not analyzed in this study.

Obtaining a correlation between glass defect frequency and furnace variables would require meticulous recordkeeping of the furnace process conditions and defect levels throughout the furnace campaign. Glass manufacturers can either engage in this type of long-term study, or find alternatives to reduce defects based on the current understanding of refractory degradation.

There is no doubt that all AZS refractories experience an expansion of the glassy phase volume due to superstructure corrosion and can therefore serve as a source of liquid phase rundown, promoting glass defects. Therefore, the best solution would be to avoid using AZS refractories in the superstructure lining of airfuel furnaces altogether.

One suitable alternative is a fusion-cast alpha-beta alumina refractory, which contains a very small amount of crystalline boundary phase (~2% by volume) bearing nepheline-type chemistry. Comparative studies of AZS and alpha-beta alumina refractories superstructure corrosion in airfuel and oxyfuel furnaces have shown significantly lower chemical alteration of the alpha-beta alumina than that seen in AZS. Furthermore, over the last 10 years, alpha-beta alumina refractories have been successfully used in the crown and superstructure of oxyfuel-fired glass melting furnaces, showing excellent physical and chemical stability over multiple campaigns.

By understanding how glass defects occur, manufacturers can take the appropriate steps to minimize these defects and improve glass quality.

For more information about glass furnace refractories, contact Vesuvius Monofrax, Inc., 1870 New York Ave., Falconer, NY 14733-1797; (716) 483-7200; fax (716) 661-9296; e-mail amul_gupta@us.vesuvius.com; or visit http://www.monofrax.com.

Reference

The main requirements for the operation are : glass quality, long furnace life, operational flexibility

Saint-Gobain SEFPRO response is based on ER 1195 RT and Scimos Z,  high zirconia fused-cast refractories.

With this kind of glass, ER 1195 RT and Scimos Z , present a four times better corrosion resistance than a regular 40% Zirconia Fused-Cast AZS. It is therefore recommended in all heavy wear zones.

The main requirements are the following :

• Glass Quality
• Long furnace life
• Operational flexibility

   

  Saint-Gobain SEFPRO response is based on High Zirconia fused-cast ER1195 RT and Scimos Z.

   

   

  

   

   

  With this type of glass, ER 1195 RT and Scimos Z present a corrosion resistance that is four times better than for a 40% zirconia fused-cast AZS. It is therefore recommended in all zones of heavy wear.

Throughout the entire campaign, the Float furnace at the head of the production line must ensure :

• Quality glass production at low operating cost
• Consistent and safe operation within the local environmental constraints
• Ability to adapt quickly to the market shift.

In order to achieve these targets which are a must for a good return on investment, Saint-Gobain SEFPRO provides the suitable refractories solutions like:

• The "Dalles-Ersol" concept for melter bottom pavings with ER 1681 Dalle TJ tight-joints.
• In a furnace producing Flint Extra White glass, the ER 2010 RIC, AZS material for better closure of the joints, low blistering & improved corrosion resistance.
• Super low exudation fused-cast AZS ER 2001 SLX and fused-cast Alumina Jargal H for superstructure, to improve glass quality and furnace output.
• Cruciform Regenerator Packing solutions, allowing the highest levels of thermal efficiency, high resistance to corrosion and reduced risk of plugging
• A full range of unshaped materials 

زاک با برش متفاوت

NORMAL Casting: Cavity is located under the casting scar.