Drying behavior optimization of dense refractory castables by adding a permeability enhancing active compound

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The drying step of dense refractory castables containing hydraulic binders is a critical process, which usually requires using slow heating rates due to the high explosion trend of such materials during their first thermal treatment. Thus, this work investigated the performance of alternative additives to induce faster and safer drying of self-flowing high-alumina refractory castables bonded with calcium aluminate cement (CAC) or hydratable alumina (HA). The following materials were analyzed for this purpose: polymeric fibers, a permeability enhancing compound (RefPac MIPORE 20) and an organic additive (aluminum salt of 2hydroxypropanoic acid). The drying behavior and explosion resistance of the cured samples were evaluated when subjecting the prepared castables to heating rates of 2, 5 or 20°C/min and the obtained data were then correlated to the potential of the drying agents to improve the permeability and mechanical strength level of the refractories at different temperatures. The collected results attested that the selected additives were more efficient in optimizing the drying behavior of the CAC-bonded compositions, whereas the HA-containing castables performed better when the aluminum-based salt was blended with a small amount of CAC (0.5 wt.%), which changed the binders hydration reaction sequence and optimized the permeability level of the resulting microstructure. Consequently, some of the designed compositions evaluated in this work showed improved drying behavior and no explosion was observed even during the tests carried out under a high heating rate (20°C/min).

Section: Refractory Castables' Processing and Characterizationsupporting

confidence: 79%

Novel drying additives and their evaluation for self-flowing refractory castables

Bezerra

Luz

Pandolfelli

2020

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“…However, as highlighted in Fig. 8, the obtained permeability results were higher than the ones commonly observed for calcium aluminate cement (CAC) or hydratable alumina (HA)-bonded refractory systems [27,37]. Hence, AL addition to the designed compositions induced the generation of a greater number of permeable paths, which is a key aspect for safe and fast drying, as the castable's mechanical strength itself may not be enough to withstand the high thermomechanical stress.…”

Section: -Characterization Of Mgo-bonded Refractory Castables With Anmentioning

confidence: 89%

Aluminum lactate role in improving hydration and drying behavior of MgO-bonded refractory castables

Fini

Miguel

Pinto

et al. 2020

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Developing MgO-bonded castables is still an important subject for refractory producers and end-users based on the expansive character of the in-situ Mg(OH)2 formation. Considering that magnesia undergoes hydration when exposed to water and the generated hydrated phase needs to be properly accommodated in the resulting microstructure to inhibit the generation of cracks, it is very important to find out alternatives to control/change the MgO hydration reaction rate, which may help to optimize the permeability and green mechanical strength of the castables. Therefore, fast and safer drying of such refractories can be carried out when adjusting Mg(OH)2 generation with proper additives. This research investigated the use of aluminum lactate (AL) as a likely additive to change the hydration and drying behavior of vibratable castables bonded with different MgO sources (dead burnt, caustic or fumed one). Firstly, XRD, TG and DSC measurements of magnesia-based aqueous suspensions were evaluated to identify the AL effect on changing the hydration reaction products during the curing and drying steps. After that, Al2O3-MgO refractories were prepared and their flowability, curing behavior, cold flexural strength, apparent porosity, permeability and explosion resistance were evaluated. The results indicated that, instead of Mg(OH)2, Mg6Al2(OH)16(OH)2.4.5H2O/ Mg6Al2(OH)16(CO3).4H2O was the main hydrated phase identified in the AL-containing compositions. Due to this change in the hydration behavior of the refractories, the mixtures prepared with dead-burnt or magnesia fumes plus organic salt presented a longer setting time. Besides that, crack-free samples with improved permeability and green mechanical strength could be obtained when adding 0.5 wt.% or 1.0 wt.% of aluminum lactate to the tested castable compositions. Consequently, 1.0 wt.% of the selected additive favored the design of refractories with enhanced properties and greater spalling resistance, as no explosion could be observed even when subjecting the prepared samples to severe heating conditions (20°C/min).

“…New tests were then carried out with similar samples, but without thermocouples to obtain the castables' mass loss under the same heating conditions. The As the incorporation of the drying agents into the compositions tends to increase the samples' permeability and pore volume content contained in the microstructure [33,35] (Table 3) and the set (refractory + slag powder) was…”

Section: -Refractory Castables Containing Different Drying Agentsmentioning

confidence: 99%

“…On the other hand, MgO-H2O, MgO-AcF and MgO-SM compositions dried at 110°C for 24h and then subjected to the TG measurements ( Fig. 1b) did not show the presence of free-water in their structure, whereas the mixtures comprised by MgO plus the organic aluminum salt (OAS) or the permeability enhancing active compound (MP) still presented a small mass change around 80-200°C, which might be related to the decomposition of gel-like phases [33,35] or the release of inter-lamellar water contained in hydrotalcite-like compounds [12], both of them derived from the action of these additives (as will be pointed out when discussing the XRD results). In fact, organic additives, such as OAS, may undergo hydrolysis in the first contact with water [43] and also favor the complexation of magnesium ions [20].…”

Section: -Physical-chemical Transformations Derived From the Interactmentioning

confidence: 99%

“…Considering that Refpac MIPORE® 20 contains CaAl2O4 and CaAl4O7 in its composition (as detected inFig. 3) and other organic components (including dispersants)[35], it was expected that some of these components might also react with water and partially consume the available liquid.Consequently, RefM-MP presented a water demand increase above 20% when compared to the reference castable (RefM). For RefM-PF and RefM-OAS compositions, just small changes in the liquid content (4.1-4.2 wt.%) were observed and both compositions reached the desired vibra-flow level.The addition of drying agents to the prepared castables led to significant differences in their curing behavior.…”

mentioning

confidence: 99%

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Fast drying of high-alumina MgO-bonded refractory castables

Pinto

Fini

Miguel

et al. 2020

Self Cite

Refractory producers face many challenges in terms of producing MgO-containing castables due to the high likelihood of magnesia to hydrate in contact with water, resulting in Mg(OH)2 generation. The expansive feature of this transformation affects the performance of such refractories, as (i) if this hydrated phase is not accommodated in the formed microstructure, ceramic linings with cracks and low green mechanical strength will be obtained; and (ii) if crack-free pieces are prepared, they should present low porosity and reduced permeability, which require special attention when heating these materials. In both cases, spalling/explosion may be favored during the drying step of MgO-containing compositions. Hence, this work investigated the ability of various additives in the optimization of the drying behavior of Al2O3-MgO castables. Vibratable compositions were tested after incorporating polymeric fibers (PF), an organic salt (OAS), SiO2-based additive (SM) or permeability enhancing active compound (MP) into the dry-mixtures. Various experimental measurements were performed to infer the role of the drying agents to prevent the samples' explosion and whether they would also influence other properties of the castables. As observed, OAS and MP helped to inhibit the MgO-bonded samples' explosion even under severe heating conditions (2-20°C/min) and increased their green mechanical strength and slag infiltration resistance when compared to the additive-free composition. On the other hand, the addition of polymeric fibers (PF) or silica-based compound (SM) to the formulations was not able to prevent the castables' explosion when using a high heating rate and other side effects (samples' cracking during drying at 110°C, high linear expansion and increased slag penetration during corrosion tests) could also be observed when testing these materials. Thus, the selection of suitable drying 2 agents is a key issue, as they may allow the development of MgO-bonded castables with enhanced properties and lower spalling risk during their first thermal treatment.