Low-melting-point polymeric fiber performance as drying additives for refractory castables

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.     

The effect of permeability enhancement on dry-out behavior of CA- and microsilica gel-bonded castables as determined by NMR

This investigation deals with refractory monolithic materials that are broadly used in thermal treatment facilities as they are necessary e.g. for iron and steel, glass and cement production, thereby withstanding temperatures between 600 and 2000 °C. In the special case of hydraulic bond refractory castables, the components must be mixed with water for two reasons: firstly, to obtain a mouldable suspension; and secondly, to achieve a green strength via the hydraulic reaction of calcium aluminate cement that is high enough to enable a secure refractoriness of the concrete formwork. Prior to their first use in production, castables must have their pore water and hydraulic bond water carefully removed in order to avoid explosive spalling that can cause severe damages inside the furnaces.In this study, we investigate the one-dimensional drying behavior of two specific refractory castable compositions, a microsilica-containing low- and a no-cement castable (LCC/NCC) during first heat-up in the temperature regime between 20 and 300 °C. First results were already presented in a prior publication that demonstrate a specialized high-temperature Nuclear Magnetic Resonance (NMR) setup capable of continuously measuring moisture and temperature profiles on 74 mm-long cylindrical samples, without touching or moving the sample [1].In this paper we explore how the use of permeability-enhancing agents (fibers and MIPORE 20) beneficially affects the drying behavior and consequently allows higher heating rates. We also demonstrate that the NMR technique as applied here is sensitive enough to resolve differences in the dry-out behavior if said additives are used in the castable formulations.Our results demonstrate that incorporation of fiber and MIPORE 20 significantly alters the dry-out behavior. In particular, it can be resolved that as the fibers begin to melt, there is a noticeable increase in permeability that results in faster drying, as well as a decrease of the drying front temperature and therefore the generated maximum pressure.     

Gas permeability of alumina–spinel refractory castables bonded with hydratable magnesium carboxylate

The high level of gas permeability can effectively reduce the explosive spalling risk of refractory castables. The hydratable magnesium carboxylate (HMC) is expected to improve the permeability of castables owing to the thermal decomposition of the HMC hydrates. This study compared the gas permeability and explosive spalling resistance of HMC bonded refractory castables (HMCC) with calcium aluminate cement bonded refractory castables (CACC). Thermal decomposition of (Mg₃(C₆H₅O₇)₂∙11H₂O) (hydrates of HMC), drying behavior, and the pores size distribution of castables were investigated. The level of gas permeability of HMCC is higher than that of CACC, which was confirmed by the higher values of Darcian k₁ and non-Darcian k₂. The degas temperatures of HMC hydrates (156°C) and HMCC (432°C) are lower than those of CAC hydrates (289°C) and CACC (536°C) at a heating rate of 20°C/min, respectively. The large-size and more permeable pores in HMCC were obtained according to the mercury intrusion porosimeter (MIP) results, which formed the connected paths for gases (H₂O, CO₂, C₂H₄, CO, CH₄) released from the castables.     

Binding additives with sintering action for high-alumina based castables

Great efforts have been made recently to totally or partially replace calcium aluminate cement (CAC) by alternative materials in refractory castables, in order to attain an enhanced thermomechanical performance of these ceramic linings at intermediate temperatures (600–1200 °C). Besides that, using additives that induce earlier sintering/densification of the refractory microstructure may also reduce the energy costs derived from the production of pre-formed pieces. Based on these aspects, this work investigated the viability of replacing CAC by calcium carbonate (CaCO₃) or calcium hydroxide [Ca(OH)₂] to ensure a suitable binding action and effective sintering/densification of the designed compositions at intermediate temperatures. Six high-alumina castables containing these alternative additives or their blend were prepared and their green mechanical strength, apparent porosity and Young's modulus evolution with temperature were evaluated within the 30–1400 °C range. After that, the most promising compositions were characterized via X ray diffraction and thermomechanical tests, such as cold and hot modulus of rupture, thermal shock resistance, etc. Although the selected binders did not result in specimens with green mechanical strength values as high as the ones for the cement-bonded materials (2–8 MPa versus ∼18 MPa, respectively), they could be demolded and handled without any problems. CaCO₃ and/or Ca(OH)₂-bonded compositions presented a sintering effect at intermediate temperatures (600–1000 °C) due to the so-called “sintering-coarsening-coalescence” phenomenon. These transformations favored the faster sintering/densification of the tested castables, resulting in samples with improved cold and hot mechanical strength at 900 °C, reaching values within the range of 28–30 MPa instead of 10–13 MPa for the CAC-bonded one. After firing the evaluated compositions at higher temperatures (up to 1500 °C), all compositions presented similar results regarding their modulus of rupture or thermal shock resistance.     

Polypropylene Fibers and their Effects on Processing Refractory Castables

Polymeric fibers are efficient drying additives for refractory castables as they can reduce the risks of explosion during the first heat-up. When fibers are melted, they increase permeability, enhancing the drying rate and reducing vapor pressure. Despite these benefits, adding fibers can induce mixing and pumping difficulties due to particle entanglement. In the present work, an analysis involving rheology, dynamic permeability, drying and explosion likelihood of polypropylene fiber containing castables is presented. An optimized condition (fiber content and geometry) to maximize the performance of fibers as drying additives and to prevent mixing drawbacks is also highlighted.     

Nano-bonded refractory castables

Calcium aluminate cement (CAC) contents higher than 3 wt% in refractory castables can have some drawbacks in the various processing steps (mainly drying) and also in their refractoriness when in contact with SiO₂. The use of colloidal silica as an alternative binder has been studied by many researchers in recent years and recently reports have also explored the use of colloidal alumina for the same purpose. This article reviews the recent developments in nano-bonded refractory castables focusing on the use of colloidal silica or alumina. In the first part of the paper, a comparison of different binding systems for refractory castables is shown. The benefits of replacing CAC or hydratable alumina by colloidal binders are discussed. In the second part, the advantages of colloidal silica/alumina as a refractory binder are highlighted. Meanwhile, the characterization techniques and functional mechanisms of these binders are presented in order to understand the behavior of these systems. Finally, in the last section, the challenges for suitable use of colloidal binders are discussed and the future direction of nano-structured refractory castables is outlined.     

Dynamic Permeability Behavior during Drying of Refractory Castables Based on Calcium-Free Alumina Binders

Considerable efforts have been made to understand the dewatering behavior of refractory castables. Modeling of this process must be based on realistic heat and mass transfer data; hence, consistent values of castable properties are required, particularly permeability values, since these are the most important parameters that govern the process. Permeability values, however, are usually obtained at room temperature and therefore may not reflect the remarkable microstructural changes that take place during water removal. The problem has become more critical for castable compositions based on calcium-free binders, in which the dehydration process is still unclear and explosive spalling is more likely to occur. This study has investigated the behavior of the dynamic permeability of high-alumina calcium-free refractory castables subjected to a drying process up to 700°C. Samples were previously treated at temperatures ranging from 110° to 1650°C to provide information on the castables' reversible and irreversible microstructural changes. The results revealed fluctuations and a dramatic decrease in the permeability level near 200°C, which may help to explain the occurrence of explosive spalling in this class of castables.     

Direct observation of the moisture distribution in calcium aluminate cement and hydratable alumina-bonded castables during first-drying: An NMR study

The drying behavior for various calcium aluminate cement and hydratable alumina-bonded refractory castables was investigated in the first-drying temperature range (100°C-300°C). Using a specialized high-temperature Nuclear Magnetic Resonance setup, we were able to directly and nondestructively measure the spatially and temporally resolved moisture distribution, while simultaneously measuring the temperature distribution as well. These measurements show that the drying front position is a linear function of time, which can be explained on the basis of a simplified model where only vapor transport is considered. Based on the measurements and the model, one can directly determine the permeability at high temperatures. Moreover, the results demonstrate that the drying front speed and temperature strongly correlates with the control of key material parameters (eg, water demand, binder content, etc). In particular, microsilica fume-containing low-cement castables displayed the highest vapor pressures, while regular castables generated the lowest vapor pressures reflecting the permeability of these materials.     

The particle size distribution effect on the drying efficiency of polymeric fibers containing castables

Refractory castables present several placing methods, defined mainly by the application requirements and material characteristics. Considering the same chemical composition, the particle size distribution (PSD) is the key property related to the large differences in their rheology, creep and corrosion resistance. It also plays an important role on their fluids permeation and drying behaviors. Therefore, it is reasonable to consider that the benefits promoted by polymeric fibers, added as drying agents, would be affected by PSD changes. In this work, the permeability and drying behaviors of fiber containing refractory castables were correlated to their PSD. Typical pumpable, self-flowing and vibrated formulations were tested in combination with polypropylene fibers. Permeability measurements and explosion tests were associated to the maximum paste thickness (MPT) and interparticle separation (IPS) parameters and to the fine/coarse particles ratio. The different classes of castables presented distinct needs of drying additives and the fibers’ efficiency was strongly dependent on castables PSD.     

Drying behavior of dense refractory castables. Part 2 – Drying agents and design of heating schedules

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.     

Self-flowing high-alumina phosphate-bonded refractory castables

Phosphate refractories have a great potential to be applied in petrochemical industries as they present suitable properties at the temperature range used in fluid catalytic cracking units. This study addresses the development of high-alumina self-flowing castables bonded with H₃PO₄ solution (48 wt% concentration) or a mixture of phosphoric acid and monoaluminum phosphate (MAP) solutions, using MgO as a setting agent. Two polyphosphates (Budit 3H and 6H) and citric acid were evaluated as dispersant additives for these castables. The compositions were characterized by measuring their free-flow and temperature evolution over time, working and setting times, cold and hot mechanical strengths, drying behavior and explosion resistance, eroded volume and thermal shock resistance. The results indicated that high flowability (free flow >100%) could be attained when adding the selected polyphosphates to the mixtures, whereas citric acid acted mainly as a retarder agent for the castables’ setting. Moreover, free-flowing compositions with a suitable working time were obtained when combining H₃PO₄+MAP solutions as main binders. The thermo-mechanical tests pointed out that the most promising designed refractory (containing mixture of H₃PO₄+MAP and 0.5 wt% of Budit 3H) presented similar or even a better performance than a benchmark commercial vibratable product used in petrochemical units.     

Hydration of CAC cement in a castable refractory matrix containing processing additives

A growing demand for refractory castables with a particular behavior has been inducing a continuous technological evolution, where one of the most important aspects, is an in-depth knowledge of hydraulic binders. These materials greatly influence the rheological properties and mechanical strength evolution of castables, defining their workability range and demolding time, respectively. The hydration process of hydraulic binders is influenced by the presence of matrix and additives (dispersants and accelerators), which affect the setting and demolding time of shaped bodies. In this work, the influence of these variables on the hydration process of calcium aluminate cement was studied by means of temperature measurements, oscillatory rheometry and normal force measurement. These techniques were able to evaluate the setting behavior of different binders, either in plain water or in matrix-representative suspensions. In both cases, the dispersants presented a retarding effect on the hydration process, which was more significant for citric acid and diammonium citrate. The combination of these additives with an accelerator (Li₂CO₃) was shown to be an efficient tool to control the setting time of castables.     

Nanotechnology in castable refractory

In recent times nanotechnology has drawn significant attention in the field of refractory research. Different nano-powders and colloidal suspensions have been utilized to improve the properties of refractory castables. Various studies have been carried out worldwide with nano scaled binders; such as, hydratable alumina (HA), colloidal alumina (CA), colloidal silica (CS), micro silica, etc.; to improve the thermo mechanical properties of refractory materials. Nano scaled additives are also being applied to reduce the energy consumption and to improve the densification process at lower temperatures. In this paper, the contributions of nanotechnology in selection of raw materials, the binders and choice of additives to improve the quality of refractory materials, and the future of nanotechnology in refractory research are reviewed.     

Selection of binders for in situ spinel refractory castables

The expansive behavior of alumina–magnesia refractory castables is usually associated with in situ spinel formation. Nevertheless, when bonded with calcium aluminate cement (CAC), this class of materials can present additional expansion reactions due to CA₂ and CA₆ formation. Considering that these reactions impart a further contribution to the material's overall volumetric change, the objective of this work has been to analyze the effect of partial or complete replacement of CAC by hydratable alumina (HA). Taking into account that this substitution would affect various castable processing steps, properties such as the mechanical strength (during curing, intermediate or high temperatures), linear change behavior during heating, creep and thermal shock resistance were evaluated. In general, CAC-containing castables led to better mechanical strength and thermal shock resistance, whereas HA-containing castables presented higher creep resistance, lower apparent porosity and better volumetric stability. Due to the substantial reduction of the overall expansion of alumina–magnesia castables, the addition of hydratable alumina was pointed out as an interesting alternative to attain designed expansion levels.     

Heating rate effect on the moisture clog while drying refractory castables: A neutron tomography perspective

Due to their great performance and ease of installation, refractory castables are common ground materials to enable high-temperature processes. However, their fully operational condition is slowed down by the gradual drying stage required. Therefore, better understanding of the moisture transport is essential to improve their efficiency and reduce the likelihood of explosive spalling events due to vapor pressurization. Neutron tomography provides a relevant inner view of the moisture distribution across a sample and its evolution over time. In this work, the effect of the heating rate on moisture clog was investigated and compared with available laboratory and industrial observations. It was found out that higher heating rates resulted in a faster and longer lasting water accumulation ahead of the drying front, in agreement with other macroscopic studies and explaining the common reasoning behind using slower heating rates and safer industrial operations. This study highlights the potential of neutron imaging for the ongoing effort to maximize the efficiency of the refractory castables drying process by controlling the moisture accumulation without exclusively relying on slower heating rates.     

Mullite-based refractory castable engineering for the petrochemical industry

Refractory castables used in fluid catalytic converter (FCC) risers should present suitable particle erosion and thermal shock resistances at temperatures below 900 °C. Considering that calcium aluminate cement (CAC)-bonded refractories usually start their densification above 1200 °C, the use of sintering additives to induce faster densification is a promising technological alternative. Therefore, this work addresses the evaluation of mullite-based castables containing a boron-based sintering additive and CAC and/or hydratable alumina as the binder sources. Hot elastic modulus, cyclical thermal shock, hot modulus of rupture and cold erosion resistance measurements were carried out to evaluate the compositions. According to the attained results, adding 1.5 wt% of the evaluated sintering additive to the designed castables led to a remarkable increase of the hot modulus of rupture (maximum of 40.4 MPa at 800 °C for the CAC-containing refractory) and high erosion resistance (1.5–2.9 cm³) after pre-firing at 800 °C for 5 h. Moreover, the combination of CAC and hydratable alumina gave rise to an improved refractory (M–2CAC–2HA–S) showing a transient liquid formation at an increased temperature, high thermal shock resistance (no E decay after 8 thermal cycles, ΔT=800 °C) and high mechanical strength at 800 °C and 1000 °C.     

Al2O3-MgO refractory castables with enhanced explosion resistance due to in situ formation of phases with lamellar structure

This work evaluated an alternative route (formic acid addition and in situ generation of hydrotalcite phases) to reduce the explosion trend of dense MgO-bonded refractory castables during their drying step. Aqueous suspensions containing different magnesia sources (caustic or dead-burnt) and hydratable alumina were firstly analyzed in order to identify the likelihood of generating these in situ compounds with a lamellar structure in mixtures prepared with and without formic acid (hydrating agent). After that, high-alumina vibratable castables containing MgO and this carboxylic acid were evaluated and the following experimental tests were carried out: thermogravimetry, mechanical strength evaluation, apparent porosity and hot elastic modulus measurements. According to the obtained results, the thermal decomposition of the formed hydrotalcite-like phases led to samples’ mass loss over a broader temperature range, preventing their explosion even when a critical heating rate (20?°C?min^?1) was applied during the tests. Besides that, no deleterious effect to the refractories’ mechanical properties were observed during firing (i.e. softening at high temperatures). Thus, the addition of formic acid and the in situ formation of hydrotalcite-like phases is suggested as an alternative route to the conventional incorporation of amorphous silica or polymeric fibers into the castable dry-mixes to prevent the explosion of MgO-bonded refractories.     

The role of hydraulic binders on magnesia containing refractory castables: Calcium aluminate cement and hydratable alumina

Previous work by the authors has shown that the effects of calcium aluminate cement (CAC) and hydratable alumina (HA) can modify the magnesia hydration behavior in aqueous suspensions. As a consequence of these studies, the present paper highlights how varying the content of these binders can affect magnesia hydration in refractory castables using pH, apparent volumetric expansion, mechanical strength and porosity measurements and hydration–dehydration tests. Furthermore, as mechanical strength, porosity and refractoriness also play an important role in these materials, binder-free, magnesia-free and magnesia-and-binder-free samples were also tested as references. It was found that the deleterious effects of magnesia hydration can be greatly minimized by the binder and its selection content.     

Enhanced formation of magnesium silica hydrates (M-S-H) using sodium metasilicate and caustic magnesia in magnesia castables

Magnesium-silica-hydrates (M-S-H) is a promising binder for magnesia castables due to its bonding strength and progressive dehydration behavior over a wide temperature range during the heating-up stage. Sodium metasilicate and caustic magnesia were used to form M-S-H in magnesia castables. The results showed that M-S-H was remarkably produced in the caustic magnesia-microsilica slurries containing sodium metasilicate with increasing pH value, which activated the hydrolysis of microsilica into silicic ions and enhanced the M-S-H formation. When 0.3 wt% caustic magnesia and 0.05 wt% sodium metasilicate as additives were incorporated into magnesia castables, the cold crushing strength and cold modulus of rupture of castables after drying at 110 °C reached the maximum value of 68.3 MPa, which corresponded to ~ 40% improvement in comparison with those of caustic magnesia and sodium metasilicate-free magnesia castables. Besides, the enhanced formation of M-S-H bonding system contributed to a better explosion resistance of magnesia castables.     

In Situ Coagulation of High-Alumina Zero-Cement Refractory Castables

Coagulation methods originally developed for colloidal processing were investigated in this paper as alternative approaches to consolidate high-alumina refractory castables free of hydraulic binders (zero-cement). Three in situ reactions based on the direct coagulation casting (DCC) technique were evaluated to promote castable coagulation: (1) the autocatalytic hydrolysis of gluconic acid lactone, (2) the gradual dissolution of hydroxyaluminum diacetate particles in water, and (3) the enzyme-catalyzed hydrolysis of urea. The coagulating behavior of castables and matrix-representative suspensions was investigated with the help of zeta potential analysis, pH measurements, castable free-flow evaluation, and oscillatory rheological tests. The enzyme-catalyzed hydrolysis of urea seemed to be the most appropriate mechanism to promote the coagulation of initially self-flow zero-cement refractory compositions.