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Improvement and Application of Refractory Materials in the Chute Area of CDQ Furnace Fang Chang Rongdaijie Li Shiming Sheng Junbo Zhang Yiping (MCC Wuhan Yejian Technology Research Company Wuhan Iron and Steel Group Coking Company Wuhan Iron and Steel Group International Trade Company) Coke quenching is one of the important links in the coke production process. There are two methods of coke quenching: wet coke quenching and dry coke quenching. Dry coke quenching uses inert gas as a carrier to achieve the purpose of coke quenching. The dry quenching furnace is the main equipment in the dry coke quenching device. It consists of the upper cone (furnace top), pre-storage section (annular air channel), chute and cooling chamber. The hot coke is in reverse contact with cold inert gas (N2) in the dry quenching furnace for heat exchange, and the coke is cooled. Compared with wet coke quenching, dry coke quenching has the advantages of saving energy, reducing water waste, improving coke quality and protecting the environment. With the progress of society and the sustainable development of environmental protection, energy and resources, dry coke quenching has become the development trend of my country's coking industry. At present, there are many problems with refractory materials that seriously affect the service life of CDQ furnaces, mainly concentrated in the support beams (commonly known as corbels) and lintels in the chute area of CDQ furnaces. To this end, we conducted a damage investigation on the refractory materials in the chute area of the CDQ furnace, combined with the process characteristics of the CDQ furnace, proposed improvement measures, and successfully applied them to Wuhan Iron and Steel No. 7 CDQ furnace, achieving satisfactory results. 1 Damage analysis of refractory materials in the chute area 1.1 Damage status of the CDQ furnace support beams and lintel refractory materials Wuhan Iron and Steel Co., Ltd. No. 7 CDQ furnace was put into operation in December 2003. When the furnace was shut down for maintenance in November 2004, it was found that the support beams and lintel refractory materials were severely damaged. There were 10-25mm cracks in the straight seams on the front of the support beams, and 3-15mm cracks on the sides of the ramp support beams. The refractory bricks are generally loose, and the 36 support beams have cracks, fractures and peeling off the surface of the refractory bricks to varying degrees. In some places, one-third of the refractory bricks have even peeled off. Due to the damage of the supporting beams, the lintel bricks in the pre-existing section also sank, broke and collapsed in many places. 1.2 Damage analysis of the refractory masonry of the chute support beam 1. 2. 1 Factors affecting the service life of the refractory material of the support beam The structure of the chute area of the dry quenching furnace is complex, and the working conditions in the furnace change greatly. There are many factors listed below that affect the service life of the refractory material of the chute support beam. (1) In terms of overall structural design, the ramp support beam must bear the weight of all the refractory materials in the upper straight section of the pre-stored section. High temperature loads will inevitably lead to a gradual reduction in structural strength. ； (2) During the top-down movement of coke and the bottom-up movement of inert gas, the refractory material of the chute support beam withstands the impact, wear, airflow and dust erosion of the coke. ； (3) The temperature at the lower part of the ramp support beam is about 300°C, and the temperature at the upper part is about 1000°C. There is a temperature gradient of nearly 700°C in the refractory material of the support beam from bottom to top, causing stress accumulation inside the refractory material; (4) Chemical etching. Harmful media entrained by coking, reducing gases generated by cooling, coal ash dust, changes in the waste heat recovery process system (spray, jet, penetration, etc.) are also factors that lead to melting, erosion and damage of refractory materials. Among these causes of damage, thermal stress caused by temperature fluctuations is the main factor causing damage to the refractory materials of support beams and lintels. 1.2.2 Unreasonable support beam structure and refractory brick type design The support beams and lintels of the CDQ furnace ramp play a supporting role in the upper part of the CDQ furnace pre-storage section, and the support beams and lintels must bear all the weight of the upper part. The weight of nearly 300 tons on the upper part of the ramp is exerted on the 36 support beams of the ramp. If the support beams are designed to be too thin, they will be unable to bear the weight of the upper masonry and will be easily damaged. At present, the support beam designs of large CDQ furnace ramps are too thin and need to be improved. In addition, the staggered joints at the ends of the ramp support beam refractory brick combination are too small, and the hooks and tongues are designed too much and too large, resulting in a smaller contact area between the upper and lower refractory brick planes and a significant reduction in the stress area, which to some extent will aggravate the damage of the refractory bricks. 1.2.3 The quality of mullite-bonded silicon carbide bricks is unstable. At present, most of the ramp support beams of large-scale dry quenching furnaces in China use mullite-bonded silicon carbide bricks. Mullite bonded silicon carbide bricks are multi-phase materials formed by adding alumina and silicon oxide to form a continuous mullite phase in the matrix and wrapping the silicon carbide aggregate. It has the characteristics of silicon carbide such as high mechanical strength, high thermal conductivity, low expansion coefficient, good thermal shock stability, and good chemical corrosion resistance. It is a high-quality refractory material. Experience shows that the microstructure of mullite-bonded silicon carbide bricks is related to factors such as silicon carbide particle distribution, mullite phase development, firing temperature, and firing time. These factors directly affect the performance of mullite-bonded silicon carbide bricks. Therefore, raw materials and processes must be strictly controlled. Otherwise, the product will become brittle, the strength will become worse, and the thermal vibration stability will decrease due to secondary mulliteization, vitrification and the formation of cristobalite. Table 1 shows the technical indicators of mullite-bonded silicon carbide bricks, among which the measured values of the products are bricks for Wuhan Iron and Steel No. 7 dry quenching furnace. As can be seen from Table 1, the actual measured values of the product are quite different from the technical indicators. Due to inherent shortcomings such as low high-temperature strength and poor thermal shock stability, it is bound to be unable to meet the complex working conditions in CDQ furnaces. Table 1 Some indicators of refractory bricks for CDQ furnace ramp support beams Mullite-silicon carbide bricks Refractory degree °C Volume density g/cm3 Apparent porosity% Normal temperature compressive strength MPa Load softening temperature (0.2MPa) ℃ High temperature flexural strength (1100℃×0.5h) MPa Thermal vibration stability (1100℃↔ Cold water) times required index ≥ 1770 ≥ 2.5 ≤ 21 ≥ 85 ≥ 1600 ≥ 20 ≥ 50 actual measured value > 18002.614.992 > 165012.8713 Table 2 Technical indicators of phosphate bonded refractory mud used in the chute area of dry quenching furnace Phosphate mud required indicators Actual measured values Phosphate mud required indicators Actual measured values SiC ,% ≥3030.82 Flexural bonding strength MPa 800℃ ≥65.17Al2O3 ,% ≥4044.54 Flexural bonding strength MPa Normal temperature 1 day - 01000℃ ≥88.82 2 days at normal temperature - 0 refractory degree, ℃ ≥17701770 3 days at normal temperature - 0 bonding time, s 60～12096110℃ ≥47.4 load softening temperature, ℃ ≥15001520400℃ ≥64.71 particle size% +0.5mm ≤10600℃ ≥63.95-0.074mm ≥5072.8 1.2.4 Improper selection of refractory mud. The support beams and lintels in the chute area must bear the full weight of the upper masonry, so the support beams are required to have sufficient overall structural strength. It can be seen from the damaged support beams that the bricks are not bonded to each other, the mud strength is low, and the support beams do not form a whole, so they cannot bear the weight of the upper masonry. Table 2 lists the technical indicators of phosphate combined mullite-silicon carbide refractory mud used in the chute area of Wuhan Iron and Steel No. 7 dry quenching furnace. It can be seen from Table 2 that the refractory mud used in masonry support beams has no technical requirements for normal temperature flexural strength, and the actual measured value is 0. Therefore, during the construction process, it was not enough to withstand the gradually increasing weight of the upper refractory material. Therefore, under the action of gravity, slight deviations occurred between the bricks to varying degrees. The refractory mud caused damage to the bonding surface due to the extrusion stress, resulting in the initial damage of the support beams and lintels. 1.2.5 The strength of refractory mud has an obvious downward trend. The support beams of large-scale dry quenching furnaces in China generally use phosphate-bonded refractory mud. It was observed from the furnace demolition site that the refractory mortar was loose where the bricks were peeling off. Follow-up tests found that the flexural bonding strengths of the mud samples after insulation at 1000°C for 3 hours and 6 hours were 8. 82MPa and 7. 25MPa respectively. The strength decreased with the extension of the firing time, so in-depth research was conducted on this. The mud was made into a 70 × 70 × 70 mm sample, and the P2O5 content of the inner and outer layers of the sample was measured in its original form and in a reducing atmosphere at 1000°C for 3 hours and 6 hours respectively. The test results are shown in Table 3. Table 3 P2O5 content of phosphate bonded refractory mud in reducing atmosphere P2O5 volume fraction in the original sample, % P2O5 volume fraction under 1000℃, 3h reducing atmosphere, % P2O5 volume fraction under 1000℃, 6h reducing atmosphere, % inner layer outer layer inner layer outer layer 22.8221.6823.3420.7823.51 According to the data analysis in Table 3, the P2O5 content in the inner and outer layers of the sample changes significantly from that of the original sample, and the change increases with the lengthening of the holding time. The P2O5 content in the inner layer is less than the original, and the P2O5 content in the outer layer is more than the original, indicating that P2O5 migrates from the inner layer to the outer layer. This is consistent with the fact that P2O5 begins to sublimate and decompose when the phosphate binder is above 300°C. After long-term use at high temperatures, P2O5 is completely sublimated and eliminated. If it cannot be transitioned to ceramic bonding at this time, its strength will inevitably be significantly reduced. 2 Improvement measures for the refractory materials of the chute area support beam 2.1 Improve the brick structure of the chute support beam refractory bricks. While the CDQ furnace ramp and support beam structures remained unchanged, the brick shape of the large CDQ furnace ramp support beam refractory bricks was redesigned. The refractory bricks adopt "hidden buckle" technology (patent has been applied for). The brick type using this patented technology can not only increase the plane contact area between bricks, but also reduce the concentrated stress caused by the sudden change in the shape of the refractory bricks. It can also further strengthen the containment effect between the upper and lower layers and the same layer of bricks. 2.2 Use high-quality Sialon bonded silicon carbide bricks. In view of the working conditions of the CDQ furnace ramp and the load borne by the support beam refractory masonry, the materials and physical and chemical indicators of the refractory bricks for the support beams of the large CDQ furnace ramps were improved and Sialon bonded silicon carbide bricks were used. This brick is a compound composed of silicon, aluminum, oxygen, and nitrogen. It has the characteristics of silicon nitride and alumina. It has extremely high thermal vibration stability and good high-temperature mechanical properties. The compressive strength can be greater than 200MPa, the 1250°C water-cooled thermal vibration stability is greater than 250 times, and the thermal flexural strength is greater than 40MPa. Its performance is far better than that of mullite-bonded silicon carbide bricks. 2.3 Using sol-bonded refractory slurry According to the structure and masonry characteristics of the refractory bricks of the chute support beams, we have developed a new type of refractory slurry specially used for the construction of refractory bricks of the chute support beams of large dry quenching furnaces. The refractory mud in the chute area was changed from the original phosphate mud to sol-bonded silicon nitride silicon carbide mud. During the development process, the normal temperature flexural bonding strength was increased. Table 4 lists its technical specifications. Table 4 Technical indicators of silicon nitride combined with silicon carbide refractory mud Project dry quenching furnace chute area refractory mud technical indicators Actual measured value SiC, % ≥5050.8Si3N4, % Appropriate flexural bonding strength MPa Normal temperature 1 day 1.01.68 Normal temperature 2 days 1.52.29 Normal temperature 3 days 3.54.05110℃ 6.08.67400℃ 6.010.03600℃ 6.09.63800℃ 6.010.711000℃ 8.09.82 Linear change rate (1000℃), % ±0.5-0.26 Bonding time, s 60～12096 Particle size, % +0.5mm ≤10-0.074mm ≥5072.8 2.4 Strictly controlling the construction quality of refractory materials. On the basis of improving the materials and physical and chemical indicators of refractory bricks and refractory mud, strictly controlling the construction quality of refractory materials is also one of the important measures. In order to strictly control the construction progress and quality of the masonry process, strict quality assurance measures have been formulated for the refractory brick masonry of the ramp support beams. It is stipulated that only three layers of bricks can be laid on the ramp support beams a day (the upper layer of bricks can be laid only after the strength is achieved) and the mortar joints are strictly controlled within the range of 3±1mm. It is not allowed to use mortar joints to adjust the elevation of each layer of refractory bricks. 3 Conclusion The dry quenching device of Wuhan Iron and Steel Coking was put into operation from December 2003 to March 2006, and it was shut down three times for maintenance in less than three years. The reason for the maintenance is that the refractory bricks of the chute support beams are loose and cracked. The 36 chute support beams have refractory bricks cracking and falling off to varying degrees. The brick falling phenomenon is serious. The lintel bricks in the pre-stored section have sunk, broken, and damaged in many places. Such frequent maintenance not only increases production costs, but also seriously affects the normal production of subsequent processes. When it was overhauled again in March 2006, the above-mentioned comprehensive improvement technologies were adopted, including improving the brick structure, selecting high-quality Sialon-bonded silicon carbide bricks, improving the bonding strength of refractory mud at room temperature, etc. The 36 support beams and lintels of the ramp were carefully constructed and put into use. When the furnace was shut down for normal annual maintenance in September 2007, it was observed that the support beams and lintels were smooth and complete, the brick joints were dense, and the refractory bricks had no cracks, peeling, or fractures. The dry quenching furnace has been in use so far, and no abnormal phenomena such as falling bricks have been found, and the effect is extremely obvious. This post was last edited by ryn on 2009-3-19 09:00 ]