PSD, Polymeric Fibers and the Permeability of Refractory Castables

Transcription

1 The permeability of castables designed using various PSDs was affected in distinct ways by the addition of polymeric fibers having the same diameter but variable lengths. PSD, Polymeric Fibers and the Permeability of Refractory Castables R. Salomão, C.S. Isaac, F.A. Cardoso, M.D.M. Innocentini and V.C. Pandolfelli Federal University of São Carlos, São Carlos, Brazil Polymeric fibers are usually added to refractory castables to decrease the risk of explosion during the drying step. 1 The fiber actuation mechanism is temperature activated 2 and is associated with physicochemical changes in the polymeric material (such as shrinkage, melting and burnout) that leave permeable channels in the castable structure. 3 6 The connections generated among the regions of low (pores in the matrix) and high (open pores, interfaces between matrix and aggregates, and channels left by fibers after thermal treatment) permeability facilitate the release of pressurized steam during heatup. 7 Table I. Volumetric Amounts of Matrix and Aggregates for the Tested Formulations Amount (vol%) Formulation Matrix Aggregates q = q = d p < 100 µm. 100 µm < d µm < 5.6 µm. The balance between the number and length of permeable channels created after fiber burnout is an important parameter to maximize increases in permeability for the same volumetric amount of polymer added. Considering a constant fiber diameter, changes in fiber length modify the number of channels generated. 7 Fibers ranging in length from 3 to 24 mm have proved to successfully increase the permeability in a self-flow castable (Andreasen s coefficient q = 0.21, 61 vol% of fine matrix, 39 vol% of coarse aggregates and 15 vol% of water) (Table I). On the other hand, for the same formulation, the performance reported for short fibers (<3 mm) is not efficient. 7 Despite the number of fibers added, their length is apparently insufficient to provide connections for the permeable path of the castable structure. The existence of a minimum fiber length required to generate an increase in castable permeability indicates that, should changes occur in the structural parameters responsible for variations in the permeable paths, 8 as in the case of particle-size distribution (PSD), their interaction with the fibers also is modified. In the present work, the measurements of permeability of two castable formulations, designed with distinct Andreasen s PSD coefficient (q) and containing polymeric The American Ceramic Society American Ceramic Society Bulletin October

2 fibers with various lengths, are correlated to the distance between the permeable paths to identify the minimum fiber length required to promote a designed permeability increase (Fig. 1). CASTABLE PREPARATION Ultra-low-cement (ULC) high-alumina refractory castables with two PSDs were prepared based on Andreasen s packing model, with distribution coefficient values (q) of 0.21 and The raw material consisted of a mixture of fine matrix powders (39 45 vol%, d p < 100 µm) and coarse aggregates (55 61 vol%, 100 µm < d p < 5.6 mm). All the fused alumina used as aggregates, the calcined alumina and the high-alumina cement (CA14) were supplied by Alcoa-Brazil and Alcoa-U.S. An amount of 15 vol% (4.12 wt%) of water was continuously added to the composition during mixing at a constant rate. Fig. 1. PSD for castable formulations. Table II. Typical Characteristics of the Tested Fibers Characteristic Value Chemical composition Polypropylene Density (g/cm 3 ) Denier (g/9000m)/tex (g/1000m) 1.26/0.15 Diameter (µm) 15 Nominal length (mm) 0.1, 0.5,1,3,6,12,24 Melting point ( C) 165 Thermal degradation ( C) TRM Compostos, Brazil. Wrigley Fibers, U.K. Fitesa SA, Brazil. Oxidizing atmosphere, 5 C/min, C; Netzsch DSC 204 and Netzsch TG 209. Polypropylene fibers (Table II) having seven different lengths ( mm long 15 mm diameter) were added to the compositions in an amount of 0.36 vol% (0.09 wt%). The compositions were processed in a paddle mixer and cast in cylindrical molds (70 mm diameter 22 mm thick) for the permeability measurements. The samples were cured at 50ºC and at a relative humidity of ~100% for 24 h, after which they were left in an acclimatizing chamber (Model 2002, Vötsch) at the same temperature for 48 h to remove residual moisture. PERMEABILITY MEASUREMENTS An evaluation was made of the permeability to airflow of green and fired (900ºC for 6 h) samples using a technique described elsewhere, 9 which allowed the permeability constants, k 1, to be obtained following the Forchheimer equation, expressed for flow of compressible fluids as, 9 (P i 2 P 0 2 )/2P 0 L = (µ/k 1 )v S + (ρ/k 2 )v S 2 (1) where P i and P 0 are the absolute air pressure at the entrance and exit of the sample, respectively, v S the fluid velocity, L the sample thickness, µ the air viscosity and ρ the fluid density. The constants k 1 are the Darcian permeability and non-darcian permeability, respectively. ADDITION OF FIBER AND RESULTING PERMEABILITY The permeability of the fiber-free green samples (cured and dried at 50ºC) was significantly higher for the q = 0.26 than for the q = 0.21 formulation (100% increase for k 1 The American Ceramic Society American Ceramic Society Bulletin October

3 Fig. 2. Permeability versus fiber length for the PSD studied. and 400% for k 2 ). This finding has been reported in the literature and related to differences in the PSD of the formulations. 8 The higher the q coefficient of Andreasen s particle-packing model, the greater the volumetric amount of coarse aggregates and the number of permeable aggregate/matrix interfaces. For each PSD, the permeability values of the fiber-containing green samples were at the same level as fiber-free samples, displaying no significant variations with various fiber lengths and, hence, demonstrating that the presence of fibers in the green stage failed to generate useful permeable paths in the castable structure (Fig. 2). After thermal treatment at 900ºC, the fiber-free samples exhibited a slight increase in permeability, which may have been associated with the decomposition of hydrates and changes in the permeable voids in the castable structure. 9 The differences in the permeability constants for the formulations were maintained (50% increase for k 1 and 180% for k 2 in the q = 0.26 composition). For the q = 0.21 castable, the values of permeability constants in the samples containing short fibers (0.1, 0.5 and 1 mm) remained similar to those of the fiber-free samples. For the q = 0.26 composition, the samples containing the same short fibers displayed significant increases in k 1 (up to 100%) (up to 900%), compared with the fiberfree samples. The samples containing long fibers (3, 6, 12 and 24 mm) showed considerably higher permeability values than the fiber-free samples, for both compositions (up to 900% for k 1 and 30000% for k 2 for the q = 0.26 formulation and up to 1400% for k 1 and 2500% for k 2 for the q = 0.21 sample). In the samples containing long fibers, the k 1 values reached by the q = 0.26 formulation were higher than those reached by the q = 0.21 formulation (up to 33% for k 1 and 500% for k 2 ). The American Ceramic Society American Ceramic Society Bulletin October

4 The differences in the permeability values of samples containing long fibers after thermal treatment were reported as a consequence of decreases in length during the mixing process. 7 The longer the fiber, the greater the damage and the larger the amount of short fibers generated. The fraction of short-length fibers (short fibers) did not contribute to permeability increases, as shown above. PERMEABILITY INCREASE VS PSD In this study, it was observed that fibers having the same length and diameter produced various effects on the permeability of castables designed with distinct PSD (Fig. 3). This can be attributed to the differences in the number of permeable paths in the castables and the resulting differences in the distances among them. In the q = 0.21 formulation, the smaller volumetric fraction of aggregates decreases the number of interfaces, thereby increasing the distances among them. Therefore, fibers with lengths above a minimum value (>1 mm) are necessary to generate effective connections among the permeable paths. Channels with shorter lengths than this minimum value are less efficient for fluid permeation; they perform as closed pores. On the other hand, in the q = 0.26 formulation, the higher volumetric fraction of aggregates increases the number of interfaces and decreases the distances among them, enabling even short fibers (0.1, 0.5 and 1 mm) to connect the permeable regions. Also, because of the decreased matrix content in the q = 0.26 composition, the volumetric fiber content/matrix ratio is higher. Hence, for the fibers to effectively increase the permeability during the drying process, the type and length of the fibers and the castable PSD must be considered. Fig. 3. Influence of PSD on the mechanism of permeability increase by the addition of fiber (0.36 vol% of polypropylene fibers). The American Ceramic Society American Ceramic Society Bulletin October

5 CONCLUDING REMARKS After thermal treatment, the permeability of the castables designed using various PSDs was affected in distinct ways by the addition of polymeric fibers having the same diameter but variable lengths. The short fibers (0.1, 0.5 and 1 mm) promoted permeability increases similar to the one observed in the fiber-free sample in the q = 0.21 formulation, while producing a more pronounced permeability increase in the q = 0.26 formulation. This difference was associated with the number of and the distance among the permeable interfaces in the castables. The shorter the distance between the interfaces, or the higher the Andreasen coefficient, the shorter the fiber length required to promote connections and increased permeability. The long fibers (3, 6, 12 and 24 mm) caused significant permeability increases in both formulations. The increase in permeability in the q = 0.26 formulation was up to fivefold higher than in the q = 0.21 formulation. Acknowledgments The authors are grateful to FAPESP, ALCOA S.A. and MAGNESITA S.A. for supporting this work and to the fiber suppliers FITESA (Brazil), TRM Compostos (Brazil) and Wrigley Fibers (U.K.). References 1 P.H. Havranek, Recent Developments in Abrasion- and Explosion-Resistant Castables, Am. Ceram. Soc. Bull., 62 [2] (1983). 2 M.D.M. Innocentini, C. Ribeiro, R. Salomão, L. Bittencourt and V.C. Pandolfelli, Assessment of Mass Loss and Fluid Dynamic Phenomena during the Dewatering Process of Refractory Castables Containing Polypropylene Fibers, J. Am. Ceram. Soc., 85 [8] (2002). 3 M.D.M. Innocentini, R. Salomão, C. Ribeiro, F.A. Cardoso, L.R.M. Bittencourt, R.P. Rettore and V.C. Pandolfelli, Permeability Behavior of Fiber-Containing Refractory Castables Part I, Am. Ceram. Soc. Bull., 81 [7] (2002). 4 M.D.M. Innocentini, R. Salomão, C. Ribeiro, F.A. Cardoso, L.R.M. Bittencourt, R.P. Rettore and V.C. Pandolfelli, Permeability Behavior of Fiber-Containing Refractory Castables Part II, Am. Ceram. Soc. Bull., 81 [8] (2002). 5 J.M. Canon, T.P. Sandler, J.D. Smith and R.E. Moore, Effect of Organic Fiber Additions on Permeability of Refractory Concrete ; pp in UNITECR 97 (New Orleans, La., 1997). Edited by M. Stett. American Ceramic Society, Westerville, Ohio, T.R. Klebb and J.A. Caprio, Properties and Service Experience of Organic, Fiber-Containing Monoliths ; pp in Advances in Ceramics, Vol. 13, New Developments in Monolithic Refractories. Edited by R.E. Fisher. American Ceramic Society, Columbus, Ohio, R. Salomão, F.A. Cardoso, M.D.M. Innocentini, L.R.M. Bittencourt and V.C. Pandolfelli, Effort of Polymeric Fibers on Refractory Castable Permeability, Am. Ceram. Soc. Bull., 82 [4] (2003). 8 M.D.M. Innocentini, A.R. Studart, R.G. Pileggi and V.C. Pandolfelli, How PSD Affects Permeability of Castables, Am. Ceram. Soc. Bull., 80 [5] (2001). 9 M.D.M. Innocentini, A.R.F. Pardo and V.C. Pandolfelli, Permeability of High-Alumina Refractory Castables Based on Various Hydraulic Binders, J. Am. Ceram Soc., 85 [6] (2002). The American Ceramic Society American Ceramic Society Bulletin October