Search

Drying is the most critical process of the first heating cycle of monolithic dense refractories, as the reduced permeability of the resulting microstructure may lead to explosive spalling and mechanical damage associated with dewatering. The first part of this review series pointed out the various drying stages, the role of the binder components and the techniques that can be used to follow the water release in as-cast refractory materials, when they are exposed to heat. Although defining a suitable heating schedule is a great challenge, some tools can be applied to minimize the spalling risks associated with steam pressurization. In this context, this second review article points out (i) the main drying agents and how they affect the resulting castables' microstructure (organic fibers, metallic powders, permeability enhancing active compounds, silica-based additives and chelating agents), and (ii) the effects related to the procedures commonly applied during the designing of heating routine (i.e., the role of the heating rate, ramp versus holding time), as well as the influence of the castable's dimension on the overall drying behavior. Considering the recent advances regarding the design of refractory formulations and their processing, one may expect that incorporating suitable drying additives into the prepared composition should lead to a suitable and safer water release in such dense consolidated structures. Besides that, novel engineering opportunities, such as the use of in-situ based experimental techniques (i.e., neutron and X-ray tomography) to obtain more accurate data and the development of numerical models, might help in simulating and predicting the steam pressure developed in refractory systems during their first heating. Consequently, instead of designing conservative drying schedules based on empirical knowledge, the novel optimized heating procedures should be based on technical and scientific information. [ABSTRACT FROM AUTHOR]

Polymeric fibers can be used as drying additives for refractory compositions as their softening and decomposition at the beginning of the heating procedures of those ceramics induces the generation of permeable pathways, providing safer steam release out of the dense microstructure. Thus, it is required to select fibers that favor the preparation of castables with suitable permeability levels at temperatures as low as possible to enhance their explosion resistance during drying. Therefore, this work addresses the production of low-melting-point polymeric fibers (T m ∼ 70–150 °C) and the evaluation of their performance when incorporated into dense high-alumina calcium aluminate cement- or hydratable alumina-bonded refractory castables. EVA (ethylene vinyl acetate copolymer), PCL (polycaprolactone) and PLA (polylactic acid) fibers were produced and added to the designed compositions. Based on the collected results, changes in fiber morphology during their softening and melting process had a great impact on the drying behavior of the castables under different heating rates. Hydratable alumina-bonded compositions were more susceptible to explosions due to such transformations, whereas the CAC-bonded ones exhibited explosion resistance at the hardest heating schedule (20 °C/min). Micrographic analysis of the fiber morphology during heating provided relevant information that helped to better understand the drying process and the permeability evolution of the evaluated dense refractory castables. [ABSTRACT FROM AUTHOR]

Despite the continuous evolution on the performance of refractory ceramic products, monolithic materials still require special attention during their processing steps as various phase transformations may take place during the curing, drying and firing stages. Drying is usually the longest and the most critical process observed during the first heating cycle of refractory linings, as the enhanced particle packing and reduced permeability of the resulting microstructure may lead to recurrent explosive spalling and mechanical damage associated with dewatering and the development of high steam pressure at the inner regions of such dense materials. In this context, this review article mainly addresses (i) the theoretical aspects related to the drying process of dense refractories, (ii) the influence of the phase transformations derived from the binder additives, and (iii) the usual and advanced experimental techniques to assess the water removal from consolidated castable pieces. Many studies have pointed out that due to the complex nature of this phenomenon (i.e., considering combined thermal stresses and pore pressure, heterogeneous microstructure, evolving pore structure with temperature, etc.), the mechanisms behind the water withdrawal and castables' explosive spalling are lacking further understanding and, consequently, it has been difficult to save time and energy during the first heating of industrial equipment lined with ceramic materials. On the other hand, different methods are used for refractory spalling assessment and many efforts have been carried out in applying in situ imaging techniques (such as NMR and neutron tomography) to follow the moisture evolution during such thermal treatments. These novel techniques, also addressed in this review, might be of particular importance to provide more accurate data for the validation of many state-of-the-art numerical models, which can be used to predict the steam pressure developed in refractory systems and help in the design of proper heating schedules for such products. [ABSTRACT FROM AUTHOR]

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 2-hydroxypropanoic 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). [ABSTRACT FROM AUTHOR]

Abstract New challenges have been identified over recent decades regarding the increasing complexity of the dry out step of refractory castables, mainly due to the advances on the particle size distribution design (and consequent reduction in the ceramic linings' permeability) and growing demand for shorter equipment maintenance time, which generally results in pressures to speed up the drying. Some efforts focused on the design of novel permeability enhancing active compounds (PEAC) have been receiving great attention, as such products may be an interesting alternative for producers and end-users. This work evaluates the addition of a permeability enhancing active compound (AC) to high-alumina based castables and also analyzes the effect of different parameters (i.e., particle packing models, hydraulic binders, drying temperature, etc.) which could affect the dry out behavior of such refractories. Initially AC-containing castables, additive-free and mixtures with a polymeric fiber with low melting point were analyzed. Further investigations of the selected AC action mechanism were carried out in a second moment, where novel castables containing distinct binders (Secar 51, Secar 71 or Alphabond 300) and with or without this drying agent were characterized regarding their flexural strength, apparent porosity, permeability and explosion resistance at different temperatures and heating rates. The obtained results indicated that adding this active compound into the alumina-based castables favored a change in the cement hydration path, giving rise to gel-like hydrates (instead of AH 3 and C 3 AH 6 phases) that underwent decomposition at low temperatures (100–140 °C). Consequently, such transformations resulted in the samples' permeability increase, which inhibited their explosion during the heating up step. However, other important properties such as flowability, setting time and green mechanical strength of the AC-containing compositions were also affected, due to the interaction of this additive with water and the fine refractories' raw materials and the generation of a CAH-gel phase in the castables' microstructure. [ABSTRACT FROM AUTHOR]

Using statistical methods (such as Design of Experiments – DOE) is an important issue due to the increasing demand to reduce costs and optimize properties of different processes and products. In order to assess the feasibility of these techniques in the refractory field, the Response Surface Method (RSM) and Tukey test were applied to optimize the modulus of rupture of high-alumina calcium aluminate cement (CAC)-bonded castables after curing and drying, considering three continuous factors (the amount of water, cement and dispersant) and a categorical one (the drying process). In addition to the cold mechanical strength, the flowability and apparent porosity of all the castable samples were also measured. The Tukey test pointed out the composition with the highest strength after curing and after drying. Based on the attained RSM models, the first one (considering all four factors) indicated the composition with the highest modulus of rupture after curing, but it could not properly predict the castables' properties after drying. The other two RSM models analysed different processing conditions (post curing and post drying, separately) highlighting that the complexity of the drying stage related to the hydration of the cement phases invalidated the models used for the post drying condition. The major difficulty in using statistical techniques comes from the transient and non-equilibrium conditions experienced by refractory ceramic castables during their processing and working conditions. [ABSTRACT FROM AUTHOR]

The current trend to replace shaped refractories for monolithics is related to various technical aspects, i.e., lack of joints, easier installation and free format. However, the main drawback of these non-shaped products is their longer first heat-up demand, otherwise mechanical failures or even explosive spalling may take place due to steam pressurization within the well packed consolidated structure. As a consequence, over-conservative drying schedules are commonly used in industrial equipment lined with such ceramics, which increase the energy consumption and the production down-time. Based on this scenario and considering the recent advances in numerical simulations, this article discusses the implications of applying different heating rates during the drying of a refractory castable lining of a steel ladle. Additionally, a novel methodology based on the correlation of the maximum vapor pressure and the mechanical strength of the ceramic material was also proposed. Using some of the castable properties measured as a function of temperature, simulation results pointed out that higher vapor pressures and a displacement of the maximum pressure peak are attained by applying high heating rates and thicker linings. This behavior is associated with the water migration to the inner regions of the refractory structure. It was observed that these conditions displace the maximum resistance ratio position towards the hot face, where most common spallings are observed. The work also emphasizes the importance of carrying out accurate measurements of physical properties as a function of the temperature, otherwise the outcome can be far from the reality of the working conditions. [ABSTRACT FROM AUTHOR]

Refractory castables develop microstructures after curing that behave as partially saturated porous media. Upon heating (during its drying stage), the steam generated by the physical and chemically bond water can result in pore pressurization and explosive spalling. Numerical modeling can provide guidelines for designing safer heat-up profiles and also a better understanding of the mechanisms that lead to catastrophic damage. This work aims to review the fundamentals and models available, providing insightful thoughts on the current trends of the drying phenomena of ceramic compositions. The review also highlights that there are models better oriented to result in reasonable predictions of pore pressure values and others focused on a more accurate representation of the main physical phenomena that take place during heating. According to the findings, there are still various challenges to attain accurate models with high applicability capable of yielding safer and more efficient drying of refractory castables. [ABSTRACT FROM AUTHOR]

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, Al 2 O 3 –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 , Mg 6 Al 2 (OH) 16 (OH) 2.4.5H 2 O/Mg 6 Al 2 (OH) 16 (CO 3).4H 2 O 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). [ABSTRACT FROM AUTHOR]

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 Al 2 O 3 –MgO castables. Vibratable compositions were tested after incorporating polymeric fibers (PF), an organic salt (OAS), SiO 2 -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 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. [ABSTRACT FROM AUTHOR]