Gel-Bonded Alumina Silicon Carbide Carbon-Based Blast-Furnace Trough Castable

Transcription

1 Gel-Bonded Alumina Silicon Carbide Carbon-Based Blast-Furnace Trough Castable Incorporation of an organic fiber improved the densification, strength, corrosion and penetration properties, under dried and fired conditions, of gel-bonded SiC C-based castables that included a -sol binder for the working lining of a blast furnace trough. Ritwik Sarkar, Somnath Mukherjee and Arup Ghosh Central Glass and Ceramic Research Institute, Kolkata, India T he iron and steel industry has undergone revolutionary changes because of the adaptation of various sophisticated processes to improve the quality and production of steel. The changes have been phenomenal. As a consequence, refractory materials used in various areas of the iron and steel industry need to face severe challenges to meet critical operational parameters at high temperatures. Refractories also must perform better with less down time, as has been imposed by steel manufacturers. Hence, the attention of refractory researchers, manufacturers and users has shifted toward unshaped refractories to substitute for conventional, shaped refractories. Among the various unshaped refractories, castables lead in the areas of research, development, manufacturing and application. The trend in castables has been to decrease cement content of the castable and finally to use no cement in the castable for high- or self-flow-ability. A conventional cementbonded castable has limits in high-temperature applications because of the presence of higher lime content. Lime forms low-melting phases on reaction with castable constituents, such as, and basic components. 1 Research has been confined for decades to decrease the cement/lime content in the castable and to improve its hotstrength properties. This has resulted in the continuing development of low-cement and ultra-low-cement castables. Use of -sol as a binder for castables has brought about the most significant changes in castable refractories since its first commercial application in the 1980s. 1 - sol, which is the colloidal form of,follows the principle of sol gel technology. Colloidal particles gel around the refractory particles of the castable. Hence, a gelbonded castable is basically a cementless castable that develops initial strength through gelation of the sol. A gel-bonded castable forms the final fired matrix through reaction between gel particles and reactive/finer components of the castable during firing. Absence of cement/lime nullifies the chances of forming lime-based, low-melting phases, which tremendously improves the hot properties of the castable. The American Ceramic Society American Ceramic Society Bulletin May

2 Table 1 Physical and Chemical Properties of Raw Materials Calcined Reactive Microfine Sillimanite Flake - Constituent WTA SiC sand graphite sol > Fe TiO CaO MgO Na 2 O K 2 O LOI SiC 98 C 98.5 Average 6 8 >99% >99% >98% >98% D 50 = ~14 size <45 µm <45 µm <90 µm <250 µm 43.2 nm Bulk density (g/cm 3 ) 3.61 Apparent porosity (%) 3.92 Characteristics of organic fiber: length, 5 mm; density, 0.9 g/cm 3 ; average diameter, 50 µm; tensile strength, MPa. Technological changes in the iron and steel industries have changed significantly the operating practice of the blast furnace. Increase in furnace capacity, operating temperature, blast pressure, hot-metal temperature and throughput are common to all units. The cast house of the blast furnace, which is used for transfer of hot metal from blast furnace to transport vessel, also has undergone radical changes to respond to the needs of improved practices. In the cast house, the blast furnace trough is of prime importance, because the hot metal and molten slag flows through it immediately after tapping. The trough refractory lining is subject to slag corrosion, slag oxidation, FeO corrosion from metal, abrasion and thermal/mechanical spalling caused by molten metal and slag. Hence, the essential requirements of the lining are slag and FeO corrosion resistance, oxidation resistance, high thermal-shock resistance, high strength, nonwetting by metal and slag, and high abrasion resistance. In the past, hydrous clay-based material was used in the trough, which provided short life. 2 The trend has shifted slowly to incorporate SiC in the trough-lining composition for better spalling and nonwetting characteristics. SiC-containing anhydrous ramming mass became the next application. This was replaced by SiC-based low-cement castables. Carbon then was incorporated in the refractory for better nonwetting and thermal-spalling properties. 3,4 Ultra-low-cement compositions followed. However, the presence of lime/cement in combination with (as the major constituent) and (as flow improver) formed low-melting calcium aluminosilicate phases, which resulted in poor high-temperature properties. Hence, it was decided that complete replacement of cement was essential to obtain much improved properties for handling molten iron and slag at temperatures >1500 C. In the present study, complete removal of cement is planned, using -sol as binder. Thus, the absence of a RO group (such as CaO, MgO or FeO) allows a better ceramic bond in the castable without formation of low-melting phases. At high-temperatures, the only bond formation is mullite. Mullite provides a high-temperature ceramic bond and hot strength in the The American Ceramic Society American Ceramic Society Bulletin May

3 products, and it imparts better thermal-shock resistance because of differential thermal expansion from. In the composition, is used as the main constituent with SiC and carbon to provide better slag/metal corrosion/erosion resistance and thermal-shock resistance. Fine metal powders are added to decrease the susceptibility of the carbon in the composition to oxidation. 5 Microfine and fine also are added as flow improver/modifier. The Experiment The starting raw materials were white tabular (WTA,Alcoa Chemicals), fine calcined (Indian Aluminium Co. Ltd.), reactive (Alcoa Chemicals), microfine (Elkem, India), SiC (Grindwel Norton Ltd., India), flake graphite (Graphite India Ltd.) and -sol (Dr. Khan s Laboratory, India). The raw materials were characterized for various physical and chemical properties (Table 1). Table 2 Batch Composition Component Batch 1 (%) Batch 2 (%) Batch 3 (%) Batch 4 (%) Batch 5 (%) WTA WTA WTA WTA WTA WTA < Reactive 2 Calcined Sillimanite sand SiC Flake graphite Microfine Metal powder Organic fiber (mm) sol (vol to wt%) All the dry raw materials were mixed according to specific batch compositions (Table 2). The batches were initially dry mixed in a planetary mixer. -sol then was added to the batch at an amount of 8 9 volume to weight percent (volume of sol in cubic centimeters added to 100 g of dry solid batch) and thoroughly mixed until castable consistency was achieved. Mixed batches then were cast into lubricated 50 mm cube-shaped and mm bar-shaped iron molds on a table that vibrated at ~4000 vibrations/min with an amplitude of 0.5 mm. Cast products then were aged inside the mold for 24 h, demolded, air dried for 24 h and oven dried at 110 C for 24 h. Dried products were fired at 1450 C with 2 h soaking time. Dried and fired cubes of various compositions were tested for bulk density (BD) and cold crushing strength (CCS). Bar-shaped fired products of all the compositions were tested for modulus of rupture (MOR) at ambient temperature and at 1400 C. Some of the compositions also were tested for slag corrosion using the static cup method with blast furnace (Bhilai, India) slag at 1450 C for 2 h. Chemical analyses of the starting raw materials were conducted using the acid-dissolution method. Densification studies were conducted using the conventional liquid-displacement method according to the Archimedes principle. Particle size of the fine materials was measured using laser particle-size analysis (Model Zetasizer 1000 HSA, Malvern Instruments, Malvern, U.K.). Phase analysis was accomplished using X-ray diffractometry (Model PW 1730, Philips, Eindhoven, Netherlands) in the range θ at a speed of 2 /min. Firings were conducted in a programmable electric furnace (Bysakh & Co., India). CCS and cold-mor were evaluated using a universal testing machine (Model 5500 R, Instron Corp., Danvers, Mass.). Hot- MOR was measured using three-point loading (Netzsch, Bayern, Germany). Castable Material Characteristics Five compositions of gel-bonded SiC C-based castables were studied using sol binder for the working lining of blast furnace trough areas. Incorporation of organic fiber improved densification and strength properties, under dried and fired conditions. The fibers The American Ceramic Society American Ceramic Society Bulletin May

4 also improved the hot-strength properties of the developed castable. Little corrosion and penetration of the slag component into the castable was observed under the static corrosion test using blast furnace slag of basicity The physical and chemical properties of the starting raw materials showed that all the raw materials were highly pure and that only small amounts of alkalis and iron oxides were present as impurities. The main constituent, WTA aggregate, was characterized by high purity and low porosity, which were highly suitable for castable applications. The calcined was micron sized and contained corundum as the only phase. The microfine also was highly pure with little alkali impurity and was micron sized. The SiC also was highly pure and of fine size. The sillimanite sand contained few impurities. The flake graphite was pure-grade with acceptable ash content and had a fine particle size. The -sol had ~40% solid content with particles in the nanometer range. B A C Batch composition (Table 2) showed that all the compositions were based on 75 wt%, 15 wt% SiC, 2 wt% flake graphite, 4 wt% sillimanite sand, 3 wt% microfine and 1 wt% metal powder. Main variation among the batches was particle-size of the WTA grains, presence of reactive,corresponding variation in calcined and presence of organic fiber. The sol content variation was volume to weight percent. Castable Physical Characteristics Figure 4 Photographs of castables: (A) corroded sample in top view; (B) corroded and cut sample of batch 2; (C) corroded and cut sample of batch 3; (D) corroded and cut sample of batch 4; and (E) corroded and cut sample of batch 5. The BD value of the dried and 1450 C fired products has been plotted (Fig. 1).There is little variation of density values among the various batches, under dried and fired conditions. Incorporation of organic fiber seems to have little effect on lowering density values. Also, a higher concentration of the coarser fraction shows little improvement in density. The dried and fired densities of all the compositions vary between 2.81 and 2.88 g/cm 3.A common trend of slightly lower density value is observed for all the compositions under fired condition than under dried condition, which may have been caused by the removal of liquid phase associated with sol during firing. The CCS study (Fig. 2) of the compositions shows some effects of additives and grain compositions on strength properties. A general trend of increase in strength (under dried and fired conditions) occurs with the addition of organic fiber. This may have been caused by better D E The American Ceramic Society American Ceramic Society Bulletin May

5 removal of moisture during drying and firing (without cracking) of the castables through the channel pores generated by the organic fibers. The incorporation of a greater amount of coarser fraction (batch 5) shows a slight decrease in strength values because of a lesser amount of matrix-forming component. The highest strength, under dried and fired conditions, is obtained for the batch that contains reactive (batch 4). Reactive provides better flow and compaction of the castable and forms a bond in the matrix phase under dried condition. Also, under fired conditions, reactivity and fineness of reactive might have enhanced the sintering and strength characteristics. Table 3 Chemical Composition of Slag Oxide Composition (%) Fe TiO Ca6.56 MgO 2.99 MnO 2.09 There is little variation in cold-mor (Fig. 3) of the fired castables. A marginal improvement is obtained with the addition of organic fiber, which may be associated with better moisture removal. Hot- MOR from three-point-bending strength tests at 1400 C show (Fig. 3) high-strength properties for almost all the compositions. The values reflect excellent strength retention of the compositions, even at 1400 C. The variation in the hot strength of various compositions is of little significant. The maximum strength is obtained for batch 2. Addition of organic fiber in the composition slightly improves the hot strength. Na 2 O 0.21 The slag corrosion test of some of the compositions shows (Fig. 4) K 2 O 1.29 no corrosion of the developed castable and almost no penetration LOI 0.22 of slag component into the castable, even at the most corrosive air slag refractory triple point. The chemical composition of the blast furnace slag has been determined (Table 3). The slag has a basicity (lime/silica ratio) of The static corrosion test shows that the developed castable is suitable in contact with molten blast-furnace slag. References 1 S. Banerjee, Recent Developments in Monolithic Refractories, Am. Ceram. Soc. Bull., 77 [10] (1998). 2 A.K. Bhattacharya, P. Chintaiah, D.P. Chakraborti and M.S. Mukhopadhyay, Ultra-low Cement Castables A New Generation of Trough Bodies for Increased Cast House Life, Interceram, 47 [4] (1998). 3 S. Zhang and W.E. Lee, Carbon-Containing Castable: Current Status and Future Prospects, Br. Ceram. Trans., 101 [1] 1 8 (2002). 4 I.R. Oliveira, R. Solomaro, V.C. Pandofell and A.R. Studort, High Carbon Content Refractory Castables, Am. Ceram. Soc. Bull., 82 [10] (internet edition) (2003). 5 S. Banerjee, Versatility of Gel Bond Castable/Pumpable Refractory, Refract. Appl., [2] 1 3 (2001). The American Ceramic Society American Ceramic Society Bulletin May

6 Bulk density (g m 3 ) Batch number Figure 1 Bulk density of ( ) green and ( ) fired castables as a function of batch composition. Cold crushing strength (MPa) Batch number Figure 2 Cold crushing strength of ( ) and ( ) fired castables as a function of batch composition.

7 MOR (MPa) Batch number Figure 3 MOR of fired castables as a function of composition (( ) cold and ( ) hot).