EXTRA CLEAR GLASS REFRACTORY SELECTION: A FOLLOW UP

Transcription

1 EXTRA CLEAR GLASS REFRACTORY SELECTION: A FOLLOW UP L. Massard (1), M. Gaubil (1), J. Poiret (2) (1) Saint-Gobain CREE, Cavaillon, France (1) Saint-Gobain SEFPro, Le Pontet, France ABSTRACT Extra clear glasses are widely used for solar applications. The extra clear glass properties (higher transmission coefficient, lower glass redox) would induce an evolution of glass furnace running conditions. As a consequence, the refractory interface temperature and glass flow rate will increase and the glass capability of dissolving oxygen bubble will be lower. Theses new conditions would affect the lifetime of glass furnace and glass quality. New fused cast AZS has been developed for these applications to improve and secure the lifetime of glass furnace and the glass quality regarding oxygen bubbles and AZS stones defects. In this paper, SEFPRO will present the properties of this new product. 1. INTRODUCTION Extra white glasses are widely used in industry as substrate or support for many solar applications such as photovoltaic panel, thermal solar or solar concentration. The extra clear glass is characterized by lower iron content (<200 ppm) and redox values. In terms of property, theses characteristics lead to higher transmission coefficient due to higher glass thermal conductivity, as seen on figure y Thermal conductivity (W /m K) at 1300 C 1 0,001 0,01 0,1 1 % FeO Figure 1: Thermal conductivity evolution versus FeO content at 1300 C The higher transmission coefficient of extra white glass will modify glass furnace running conditions. At first, due to the rise of heat transfer into the glass, the refractory interface temperature should increase especially for the bottom soldier block and the pavement compared to usual conditions. Then, the thermal change should lead to an increase of

2 glass flow rate (i.e. glass thermal convection) in EWG. Finally, due to low iron content, theses new properties could induce some changes on blistering due to the evolution on the glass/refractory interface and the low capability of this glass to dissolve oxygen bubbles. In a point of view of glass furnace, it could lead to an increase of localized corrosion by upward drilling and decrease of glass quality. 2. CONSEQUENCES ON REFRACTORY At the refractory point of view, these conditions may imply corrosion highly enhanced. Figure 2 presents pictures of used soldier block in extra white glass. We notice that the corrosion is characterized by high corrosion level of bottom block and an accelerated corrosion at block joints. Figure 2: Examples of corroded soldier blocks 2.1 Corrosion of bottom block The accelerated corrosion of bottom block could be explained by the increase of the corrosion speed linked to the rise of the refractory temperature interface and glass convection. From dynamic corrosion lab-test results, we can see, on figures 3 & 4, that temperature remains the main factor of corrosion with a exponential dependence of corrosion speed to the temperature and corrosion speed is proportional to square root of glass flow. The increase of corrosion could impact the glass quality by increasing stones and cords defects. Figure 3: Corrosion speed vs. Temperature for AZS refractory (MGR results 6 round/min)

3 Figure 4: Corrosion speed vs. Glass flow rate for AZS refractory (MGR results) Theses results show the high interest of increasing bottom block resistance by using void free soldier block. In fact, as seen on figures below, the void free soldier block is characterized by good homogeneity in chemistry and density and induce, as a consequence, better corrosion resistance homogeneity. This fact has been validated by corrosion test (PFT). 300 mm 600 mm 900 mm 200 mm 500 mm 800 mm 900 mm ZrO2 (%) , ,0 850 mm , , , , , , , , Hauteur (mm) ZrO2 SiO2 Height Figure 5: Macroscopic and chemistry homogeneity of void free soldier block SiO2 (%) 100 mm AZS RT (250 x 410 x 1200) mm 2.1 Corrosion of block joint Theses conditions may imply high localized corrosion that could be a source of unexpected glass leakage. The corrosion is characterized by an accelerated corrosion at block joint by upward drilling process.

4 The upward drilling phenomenon inside the joint is mainly due to the presence of bubbles (raw materials, oxygen blistering sensitivity with EWG) and a opened joint with AZS material. Concerning the oxygen blistering sensitivity, tests (crucible tests during 30 hours) in atmospheric conditions show the higher capability on AZS material in EWG (in blue) compared to standard glass (in red) especially at low temperature. Also, theses tests shows the negative effect on blistering of zirconia content in the refractory and low iron content of the glass. This phenomenon is emphasized for product with high zirconia content. Figure 6: Blistering measurements for AZS ER1681 (32% ZRO2) and ER1711 (41% ZrO2) in EWG (in blue) and standard glass (in green) The influence of atmosphere has been evaluated too. A blistering test with a Argon atmosphere has been realized and compared to air atmosphere. The test reveals that, by suppression of external oxygen, we can stop the blistering at low temperature. So, the various results indicate that the low temperature oxygen blistering seems to be linked to an electrochemical process controlled by oxidation/reduction reactions, electronic conductivity of zirconia and external air. GLASS REFRACTORY AIR Alkali Phv M + Fe 3+ O Fe 2+ O 2 e - Zr e - OXIDATION REDUCTION e - M + Fe 2+ Fe 3+ + e - M Y+ + x e- M y-x e- OXIDATION M y-x M Y+ + x e- Figure 7: Low temperature oxygen blistering mechanism e - REDUCTION + 4 e - -

5 This kind of phenomenon can occur in «zirconia based refractory» at block s joint. Due to thermal gradient inside the block and volume contraction imposed by zirconia transformation, we can observe some opened joint at high temperature. This phenomenon should be emphasized in extra white glass at the block s bottom due to the temperature rising. As a consequence, the glass can penetrate the joint and reaches some low-temperature area of refractory where zirconia is monoclinic so electronic conductor. So, the electrochemical blistering proceeds. The consequences for glass furnace are the possibility to have more bubbles defects in the glass and to enhance corrosion by upward drilling. 3. NEW AZS REFRACTORY SOLUTION FOR EXTRA WHITE GLASS To answer to these new conditions and complementary to the new self leveling refractory mortar, presented last year, new fused cast AZS, called ER2010 RIC, has been developed for these applications to improve and secure the lifetime of glass furnace and the glass quality regarding oxygen bubbles and AZS stones defects. The table 1 gives the typical chemical composition of this product. ZrO2 Al2O3 SiO2 Na2O Others 36 Cplt. 14 <1.1 <4 Table 1: Typical chemical composition of ER2010 RIC The product is dedicated to the closure of the block s joint, the decrease of blistering capability at low temperature, and the improvement of corrosion resistance. These objectives have been reached by the addition of yttria. Yttria contributes to dope zirconia grains and as a consequence to modify the thermal expansion curve. The new curve is characterized by a lower zirconia transformation temperature which allows to obtain the joint closure at running temperature and to reduce the domain where zirconia is an electronic conductor. 0,9 0,8 ER2010 RIC AZS 0,7 % Thermal expansion 0,6 0,5 0,4 0,3 0,2 0, T ( C) Figure 8: Thermal expansion curves Regarding to the blistering (crucible tests during 30 hours), the figure 9 & 10 show that the capability of these new product is highly reduced, especially at low temperature.

6 Bubbles AZS AZS ER2010 RIC T ( C) Figure 9: Blistering results in EWG (crucible tests during 30 hours) AZS 36 ZrO2 ER2010 RIC Figure 10: Comparison on blistering at low temperature Finally, with the new chemical composition, the corrosion resistance has been improved compared to standard 32% AZS for bottom paving application. AZS 32% ZrO2 ER2010 RIC Total corrosion V (cm3) Index Flux-line corrosion V Index Table 2: MGR results at 1500 C during 48 hours in EWG

7 We can notice that this product has a good industrial feasibility and is actually in test as soldier block in extra white patterned glass furnace. 4. CONCLUSION Extra clear glasses are widely used for solar applications. The extra clear glass properties and especially the higher transmission coefficient would induce an evolution of glass furnace running conditions. As a consequence, bottom furnace refractory corrosion (bottom soldier block and pavement) may be enhanced. We have shown that the corrosion increase is mainly due to combined factors such as temperature and glass flow rate, low temperature blistering sensitivity and opened joints. To solve this problem, new fused cast product, called ER2010 RIC, has been developed for soldier block and bottom paving application with a better close join, low blistering capability, an improved corrosion resistance. This new product will contribute to improve and secure the lifetime of glass furnace and the glass quality regarding oxygen bubbles and AZS stones defects. Finally, SEFPRO can propose optimized solutions for extra white glass application that consist of using: for soldier block, the new AZS solution associated with void free filling (cf. figure 11), for paving application, new fused cast AZS tiles with a optimized joint machining (TJ) in glass contact, and a new AZS mortar layer that allow to close the thermal expansion join after heating up and present a better corrosion resistance (cf. figure 12 & 13). Figure 11: Pictures of ER2010 RIC void free soldier block

8 ER 1681 ( mm) ER 2010 RIC (120 mm) ERSOL 50 (50 mm) ERSOL 50 (50 mm) Bonded AZS New SL MORTAR (50 mm) New SL MORTAR (50 mm) Bonded AZS Insulation Insulation Standard solution Optimized solution Figure 12: New optimized pavement solution for extra white glass application Figure 13: Pictures of the SEFPRO optimized solution for paving application