Title : New fused cast cruciform product ER55XX, for glass furnaces checkers, highly resistant under reducing atmosphere

Transcription

1 Title : New fused cast cruciform product ER55XX, for glass furnaces checkers, highly resistant under reducing atmosphere Authors Isabelle Cabodi, Michel Gaubil, Johan Combes (Saint-Gobain C.R.E.E) Bruno Escaravage, Jeremy Poiret (Saint-Gobain SEFPRO) Abstract Due to more and more drastic environmental rules in glass industry, glassmakers tends to reduce nitrogen or sulfur oxides emissions by using some techniques that may induce a more or less reducing atmosphere in the furnaces, especially at the top of regenerative chambers. Our experience allowed us to better understand where and what types of species may condense within the checkers as the waste gases cool down, depending on the atmosphere chemistry, and then to comprehend the atmosphere s aggressivity in regenerators. Considering that the corrosion resistance of the current checkers products located in the condensation area in such conditions could be limited, we developed on one hand some specific laboratory tests well describing this application, and on the other hand a new product highly resistant for this one. This new fused cast product (ER55XX) is exclusively made of spinel MgAl 2 O 4. It is not only resistant against NaOH(l), but also against NaOH(g) or Na2SO4(l), so also working in oxidizing atmosphere. Moreover, in such conditions, it doesn t form Nepheline. Finally, it exhibits a good resistance under thermal cycling. Context Considering the more and more drastic environmental rules in glass industry, glassmakers tends to reduce NOx & SOx emissions by using some techniques (such as primary settings) that may induce a more or less reducing atmosphere (meaning CO in excess) in glass furnaces, especially at the top of regenerative chambers. Impact of reducing conditions on alkali condensation within the checkers Our industrial and lab experience (measurements made on furnaces, used products analysis, laboratory tests, thermodynamic simulation) allowed us to better understand where and what types of species may condense within the checkers as the waste gases cool down, depending on the atmosphere chemistry. Then, under oxidizing atmosphere, NaOH(g) will condense mainly under Na 2 SO 4 (l) form, which is not very aggressive against fused cast products. This will not be the case under reducing atmosphere (primary settings, reburning process ). Indeed, when CO is in excess, SO 2 can be reduced to form H 2 S, then NaOH(g) condenses under Na 2 CO 3 (l) and NaOH (l) forms (cf figure 1). This NaOH(l) containing salt is strongly aggressive. Besides, this chemical attack will take place at a lower temperature, and in a more restricted area, compared to sodium sulfate condensation. In fact, the aggressivity of the atmosphere can be described by the index CO/SO2*NaOH(g), as illustrated by figure 2. Figure 1 : Condensation mechanisms in regenerators in oxidizing or reducing atmosphere

2 % NaOH in the condensate as a function of the ratio CO in excess / SO2 x NaOH(g) 3 3R process Primary settings 1200 %mol NaOH(l) 2,5 2 1,5 1 0, Min/Max Temperature of condensation( C) CO in excess / SO2 x NaOH(g) Figure 2 : nature of condensation salt & condensation area in temperature vs atmosphere s aggressivity This type of reducing atmosphere can decrease the checker s lifetime, whether made with standard grade fused cast or sintered products. When a checker made of fused cast cruciform products can resist above 15 years in oxidizing atmosphere, a strong degradation can be observed after few years in highly reducing and alkaline atmosphere (see picture 1). Picture 1 : AZS fused cast product and MgO-Zircon chimney block, 4 years old with 3 years under 3R process The corrosion of the AZS fused cast product, under these conditions, is due to the attack by NaOH under its liquid form, which penetrates the product on its skin and is able to go through this one by its vitreous phase and porosity (see picture 2 and table 1). Picture 2 : microprobe analysis on AZS fused cast product, 4 years old with 3 years under 3R process Sample 1 Sample 2 Fused cast AZS Al 2 O ZrO SiO Na 2 O others Table 1 : chemical analysis on AZS fused cast product, 4 years old with 3 years under 3R process The product, heterogeneously enriched in soda and chemically transformed (nepheline NaAlSiO 4 and sodium aluminates as NaAlO 2 formation), is moreover submitted to thermal cycling (of high amplitude at the bottom part of checkers) and probably also thermal shock (post-combustion of CO). The combination of this heterogeneous chemical & micro structural transformation plus thermal stresses can explain the degradation of the product in these conditions.

3 As the corrosion resistance of the current checkers products, located in the condensation area in such application, could be extremely limited, we developed on one hand some specific laboratory tests well describing this application, and on the other hand a new product highly resistant for this one. Lab tests description The first test consists in soaking some refractory samples (dimensions ~15x15x80mm) in crucibles containing various liquid alkaline species, as 100% NaOH, at a high temperature (950 C), until degradation. It is named soak test. The second one, named Channel furnace test, aims to reproduce the atmosphere encountered in a channel of checker (see picture 3). The furnace, with a channel shape, is equipped with a burner working under oxidizing or reducing atmosphere. We inject continuously an alkaline solution (diluted Na 2 SO 4, NaOH or Na 2 CO 3 ) in the burner flame. On the other side, a chimney extracts the waste gases. The used thermal gradient is between 1300 C (burner side) to 700 C (chimney side), so well covers the condensation area. The duration of the test depends on the aggressivity of the atmosphere and the shape of samples (small samples or whole pieces). We tested from 24h to 1 month duration. Picture 3 : channel furnace test The last test is a thermal cycling made on pieces pre-treated in channel furnace, consisting of 52 cycles between 900 C and 1350 C (see picture 4). This test aims at cumulating the effects of the chemical attack (in the channel furnace) and the thermal stresses Cyclage Thermique GN Températures en C Cyclage thermique Moyenne Maxi : 1326 C Moyenne Mini : 850 C Picture 4 : standard thermal cycling test Temps cumulé en minutes Description of the new fused cast spinel product (ER55XX) This new fused cast and cruciform shaped product, made of 100% spinel MgAl 2 O 4 (see chemistry in table 2), doesn t contain any inter crystalline phase (no silica, soda, lime ) what is in

4 general the weak phase for corrosion resistance. Its porosity is well distributed in the wings, as shown on picture 5. % in weight Al 2 O 3 ZrO 2 SiO 2 MgO Na2O AZS β alumina Al-Mg Spinel Table 2 : chemical compositions of fused cast products Masse % Picture 5 : phase diagram & cruciform product (entire / sawed by the middle / piece of wing) The product presents a columnar solidification, with a high size of crystals strongly imbricated (see picture 6), what is another advantage in case of chemical attack. Picture 6 : microstructure & microprobe analysis of the ER55XX spinel product Corrosion resistance in reducing conditions This product is very resistant against NaOH(l) containing salts (see results of soak test on picture 7 & channel furnace test results on picture 8), showing that the product can run under strong reducing and alkaline atmosphere. Its very high resistance against chemical attack is due to the high resistance of the spinel crystal itself, emphasized by the particular columnar microstructure which confers a strong cohesion to the product. Besides, in alkaline atmosphere, this product will not form the nepheline phase, like in AZS products, as it doesn t contain any silica phase. Picture 7 : fused cast AZS after 20, β alumina after 20 and spinel after 8h in 100% NaOH(l) at 950 C

5 Picture 8 : channel furnace tests on AZS & spinel samples under alkaline & reducing conditions Compared to the available sintered materials (chimney blocks, bricks ), the fused cast spinel is much more resistant in NaOH(l)(see table 3). Indeed, even if some of these materials have their coarse grains highly resistant (spinel grains, MgO grains), they always have a certain amount of bindering phase strongly weaker, which always remain the preferential path for NaOH to go through the products, inducing their damage. Table 3 : soak test results (in 100% NaOH(l) at 950 C) These comparative results are also confirmed by a channel furnace test (see picture 9). Picture 9 : channel furnace test results on fused cast spinel vs various sintered products Finally, we performed a thermal cycling test on the fused cast spinel product after treatment in channel furnace. We observed a very good resistance when combining a chemical attack and thermal strresses (see picture 10).

6 Picture10: wings spinel cruciform pieces pre-treated in the channel furnace (15 days under 500ppm NaOH and 1.5%CO) after 52 cycles 1350 C-900 C Corrosion resistance in oxydizing conditions This product is not only resistant towards NaOH(l) but also in Na 2 SO 4 (l), as illustrated on picture 11. That means that it can run equally well under reducing or oxidizing atmosphere, in condensation area. Picture 11 : ER55XX spinel product after 10h at 950 C in sodium sulfate. Corrosion resistance towards vapour phases Finally, the product is also well resistant against NaOH(g), as illustrated in picture 12. That means that this new product can also be applied above condensation area in checkers. Conclusion Picture 12 : AZS, β alumina and spinel from vapour phase corrosion test The more and more severe environmental regulation in glass industry will encourage glassmakers to use more or less reducing conditions to decrease NOx and SOx emissions. The new fused cast spinel cruciform shaped product (ER55XX) has been developed to strongly resist under reducing conditions, as much as under oxidizing conditions.

7 This product is manufactured at the plant of SEPR Le Pontet, France, and has been firstly installed on a float furnace in March One endoscopic observation, made in October 2009, showed the good behaviour of the product at that time (see picture 13). Picture 13 : installation of the new ER55XX on a float furnace & endoscopic observation from the top & the bottom of the checkers