Polymeric Fiber Geometry Affects Refractory Castable Permeability

Transcription

1 Polymeric Fiber Geometry Affects Refractory Castable Permeability Selected values of polymeric fiber length and diameter maximize the permeability level of castable refractories, which decreases destructive spalling. R. Salomão, A.M. Zambon and V.C. Pandolfelli Federal University of São Carlos, São Carlos, S.P., Brazil everal publications have reported the benefits of adding polymeric fiber Sto aid the drying step of refractory castables. 1 3 Fibers have been described as permeability magnifier additives that allow an easier release of the pressurized water vapor entrapped when castables undergo a heating process. This mechanism can greatly decrease the risk of explosive spalling and the resulting idle time of equipment lined with refractory castables. 3 The permeability increase is associated with the channels generated by the fibers after their melting and decomposition. 3 7 The chemical composition of the fibers (and, consequently, their thermal behavior) is an important factor when setting the permeability increase temperature. High-temperature-resistant fibers (such as polyester and aramidic fibers) generate permeability increases at temperatures >200 C (235 and 400 C, respectively), whereas more susceptible fibers (such as polypropylene) can generate permeability increases at temperatures <170 C. 4,5 Because the channels are open, the benefits attained are related to the efficiency that fibers present to connect regions with low (packed matrix) and high (open pores, matrix-aggregated interfaces and channels left by fiber decomposition) permeability in the structure of the castable. 4 6 In this case, the geometric aspects of the fibers (volumetric amount and shape of the fibers) are important. The permeability of the fired polymeric-fiber-containing castables has been described as presenting an exponential dependence on the volumetric amount of fibers added. 3,4 This can be correlated to the permeable paths generated when the number of fibers (or volumetric amount) added to the castable is increased. Some aspects related to the shape of the fibers and the corresponding effect in the castable permeability have been recently evaluated by the authors. 6,7 A minimum fiber length has been suggested as necessary to promote an increase in permeability. The deleterious effect of aggressive mixing conditions on the fiber length also has been highlighted. The American Ceramic Society American Ceramic Society Bulletin April

2 Table 1 Polymeric Fiber Characteristics Diameter Chemical Nominal Initial Length after (µm) composition length (mm) length (mm) mixing (mm) 8 Poly(ethylene terefalate) (PET) ± ± Polypropylene (PP) ± ± Polyaramid (PAr) ± ± Polypropylene (PP) ± ± Polyethylene (HDPE) ± ± Polypropylene (PP) ± ± 0.98 Figure 1 Schematic and comparative representation of the cross section of tested fibers. In the present work, these aspects are reviewed and compared with new results that describe the effect of changes in the diameter of fibers on permeability. To establish comparative relationships in the present work and those previously reported in the literature, 4 7 an equivalent refractory castable formulation is used as well as mixing, curing, drying and testing conditions. When these two sets of results are compared, the best conditions to optimize the increase in permeability for a constant amount of fibers is attained. Castable Preparation and Mixing The self-flow refractory castable formulation was designed according to the Andreasen particle-packing model (q coefficient of 0.21 and free-flow index of 120%) using the software PSDIS- TRIBUTION DESIGNER, which was developed by the authors research group. The raw materials comprised a mix of fine matrix powders (24 wt%, d p < 100 µm) and coarse aggregate grains (76 wt%, d p,max = 4.5 mm) that contained 98 wt% of alumina and 2 wt% of calcium aluminate cement (CA 14, Almatis Inc., Leetsdale, Pa.). Water was added (4.12 wt% (15 vol%)) for the mixing step and cement hydration. Six different types of 6 mm long polymeric fibers (0.36 vol%) were added to the formulation (Table 1, Fig. 1). The fiber incorporation was conducted after water was added (4.5 wt%) and the castable was homogenized. Lower shear levels and, consequently, less decrease in fiber length were generated using this technique. 7 Fiber Length Measurements The diameter and length of the samples of the as-received fibers and those extracted from the castables by flotation were measured using a digital camera (Model FD, Mavica) and image analyzer software (IMAGEPRO EXPRESS ). For each mixing condition, at least 800 length measurements were conducted. Permeability Evaluation After the mixing process was completed, the formulations were cast into cylindrical molds (70 mm diameter 22 mm thickness) for the permeability measurements. Curing was conducted at 50 C and at a relative humidity of ~100% for 72 h, after which the samples were left in an acclimatized chamber (Model 2002, Vötsch) for 96 h at the same curing temperature. The permeability to airflow of green and fired (at 900 C for 6 h) samples was evaluated at room temperature using an apparatus described elsewhere. Permeability constants (Darcian The American Ceramic Society American Ceramic Society Bulletin April

3 (k 1 ) and non-darcian, (k 2 )) were obtained according to the Forchheimer equation 8 expressed for flow compressible fluids. Figure 2 Permeability measurements (k 2 ) for green and fired samples that contain fibers with different diameters (6 mm in length) and lengths 7 (15 µm in diameter), for a constant fiber amount (0.36 vol%). The permeability measurements (k 2 ) conducted in green and fired (at 900 C for 6 h) samples that contained fibers with different diameters and equal nominal lengths were plotted (Fig. 2). Data already reported in the literature 6,7 concerning the permeability behavior of samples containing fibers with different lengths, but with the same diameter, also were plotted for comparison. Both cases show that the permeability of the green samples was not affected by fiber addition. Because the fibers remained as a solid body at room temperature, there was no opening of the channels with the curing time. 3,9 On the other hand, after thermal treatment, different levels of permeability (k 2 ) were observed for each value of fiber diameter and fiber length previously added to the compositions. Publications that described the effect of length variations reported that, for a constant amount and diameter of fibers, a minimum length value was required to promote permeability increase after thermal treatment. 6,7 This effect was explained considering that, when the same type of yarn was chopped as staples of different lengths, the number of fibers generated was distinct for the same volume content: the smaller the fiber, the higher the number of individual fibers and vice-versa (Fig. 3). In the castables, the addition of short fibers (<3 mm) generated many channels; however, because of their small lengths, they did not promote permeability increase. On the other hand, longer fibers (12 and 24 mm) were susceptible to the mixing conditions, which resulted in a large fraction of shorter fibers (<1 mm) after mixing. Because of this length decrease, the number of fibers able to connect different regions was not as high as those observed for castable-containing fibers with intermediate lengths (3 6 mm). 7 Fiber lengths in this range maximized the permeability increase. This result showed that the designed castable that contained polymeric fibers was strongly dependent on the initial fiber geometry and on the processing conditions if higher drying rates were expected. A similar effect was observed for the results of k 2 after thermal treatment of castables containing fibers with different diameters. Samples containing fibers with 500 and 1000 µm diameter presented a permeability level similar to those achieved by the fiber-free castable. This requirement of a maximum diameter to promote significant permeability increase could have been associated to the major decrease in the number of channels generated when the diameter of the fibers increased (and the length was kept constant at 6 mm). Although the channels formed were long enough, their decreased number did not allow an efficient network and, consequently, permeability increase (Figs. 3 and 4(a)). The American Ceramic Society American Ceramic Society Bulletin April

4 For the samples that contained thinner fibers (100, 20, 15 and 8 µm diameter), the permeability levels achieved were significantly higher (up to 2 orders of magnitude). Moreover, a clear relationship was observed between the diameter of the fibers and the k 2 values: the thinner the fibers, the higher the permeability level generated. This also could have been associated with the number of the channels formed (Figs. 3 and 4(b)). The length measurements conducted after mixing indicated (based on Table 1) a small length decrease, except for the 8 µm diameter fibers. Therefore, a constant fiber length could be assumed, and a superior number of channels were generated as the diameter was decreased. The samples that contained 8 µm diameter fibers showed smaller permeability levels when compared with those that contained 15 and 20 µm ones. In this case, the lower permeability increase could be associated with the length decrease that resulted from mixing. Despite being less aggressive, the mixing process used preserved the length of the thicker fibers. 7 Nevertheless, the 8 µm diameter fibers seemed to be sensitive to mixing because of their thinner cross section. Concluding Remarks The Forchheimer Equation The Forchheimer equation expressed for a flow compressible fluid is (P 2 i P 2 0 )/2P 0 L = (µ/k 1 )v s + (ρ/k 2 )v 2 s where P i and P 0 are, respectively, the absolute air pressure at the entrance and exit of a sample, v s the fluid velocity, L the sample thickness, µ and ρ the viscosity and the density of the fluid (air), respectively, and k 1 and k 2 the Darcian and non-darcian constants, respectively. The linear term of the Forchheimer equation, (µ/k 1 )v s,represents energy losses due to viscous friction and prevails at low fluid velocities. The quadratic term, (ρ/k 2 )v s 2, denotes the contribution of inertia and turbulence on the pressure decrease at higher fluid velocities. 8 The values of the permeability constants are obtained by fitting the experimental data of (P i 2 P 0 2 )/2P 0 L versus v s in the Forchheimer equation, using the least-squares method. The correct design of the geometrical parameters of the polymeric fibers (length and diameter) is important in the optimization of the castable permeability after firing. Suitable selected values of fiber length and diameter maximize the permeability level. This geometrical dependence is associated with the ability that the used fibers have to generate an efficient connection among the different regions of the castable structure. For the short fibers (length <3 mm and diameter of 15 µm), despite their large number, the length of the permeable paths is not sufficient to generate connections. For the thicker fibers (diameter >50 to 100 mm), the same effect is observed and is associated to the fewer fibers per volume of castable. Acknowledgments The authors are grateful to FAPESP, Alcoa S.A. and Magnesita S.A. for supporting this work. References 1 P.H. Havranek, Am. Ceram. Soc. Bull., 62 [2] (1983). 2 T.R. Kleeb and J.A. Caprio, pp in Advances in Ceramics, Vol. 13, New Developments in Monolithic Refractories.Edited by R. Fisher. American Ceramic Society, Columbus, Ohio, R. Salomão and V.C. Pandolfelli, J. Tech. Assoc. Refract. Jpn., 24 [2] (2004). 4 M.D.M. Innocentini, R. Salomão, C. Ribeiro, F.A. Cardoso, L.R.M. Bittencourt and V.C. Pandolfelli, Am. Ceram. Soc. Bull., 81 [6] (2002). The American Ceramic Society American Ceramic Society Bulletin April

5 5 M.D.M. Innocentini, R. Salomão, C. Ribeiro, F.A. Cardoso, L.R.M. Bittencourt and V.C. Pandolfelli, Am. Ceram. Soc. Bull., 81 [7] (2002). 6 R. Salomão, F.A. Cardoso, M.D.M. Innocentini, L.R.M. Bittencourt and V.C. Pandolfelli, Am. Ceram. Soc. Bull., 82 [4] (2003). 7 R. Salomão, V.G. Domiciano, C.S. Isaac, R.G. Pileggi and V.C. Pandolfelli, Am. Ceram. Soc. Bull., 83 [1] (2004). 8 M.D.M. Innocentini, A.R.F. Pardo and V.C. Pandolfelli, J. Am. Ceram. Soc., 85 [6] (2002). 9 M.D.M. Innocentini, C. Ribeiro, R. Salomão, F.A. Cardoso, L.R.M. Bittencourt and V.C. Pandolfelli, J. Am. Ceram. Soc., 85 [8] (2002). The American Ceramic Society American Ceramic Society Bulletin April

6 Figure 3 Number of paths formed in the castable structure after fiber burnout as a function of the fiber diameter (6 mm in length) and length 7 (15 µm in diameter), for a constant fiber amount (0.36 vol%).

7 Figure 4 Schematic representation of the thick and thin fibers disposition in the castable structure (for the same length and volume amount).