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

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).     

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.     

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.     

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 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.     

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.     

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.     

Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures

Colloidal silica as nanostructured binder for refractory castables has attracted many attentions in recent years. In the present study, phase composition, microstructure and reactivity evolution of silica gel at different heating conditions were investigated to find suitable system for colloidal silica application. The results showed that atmosphere and carbon slightly affected phase composition of the silica gel at elevated temperatures, and the crystalline phases were composed of major α-cristobalite and minor α-tridymite. The morphology and particle size of the silica gel were greatly affected by atmosphere and carbon during heating. The spherical nano-silica particles with sizes of 40–50 nm rapidly grew into macroscale rod-like particles with temperature increasing from 800-1000 °C to above 1200 °C in air, and sintering of silica particles was observed. However, the size and morphology of the spherical nano-silica particles retained at high temperature in a reducing atmosphere, and many well developed columnar mullite crystals and some SiC whiskers formed on heating silica gel, alumina fines and carbon at 1500 °C, which was due to carbon inclusions retarding the growth of nano-silica particles and the nano silica remained high reactivity at high temperature. Thus, colloidal silica was suitable for application in carbon-containing refractory castables.     

ZnO and MgO as inducers of spinel-like phase formation on alumina-based castables

Environmental issues regarding Cr⁶⁺ formation lead to replacing chrome-containing refractories with greener alternatives. MgO-containing compositions have been extensively investigated for this purpose, however, few studies evaluated the likelihood of using other chemical elements as inducers of spinel-like phase formation in refractory castables. In this study, the addition of zincite in alumina-based castables was evaluated and compared with its MgO-counterpart. In-situ elastic modulus, assisted sinterability and differential scanning calorimetry pointed out that the gahnite (ZnAl₂O₄) formation took place at lower temperatures (～ 1100 °C) than MgAl₂O₄ (～ 1300 °C). On one hand, this feature induces anticipated strengthening of the Zn-containing compositions, giving rise to the possibility of firing these compositions at lower temperatures. On the other, the faster kinetics of gahnite formation led to a significant Kirkendall effect, changing the morphology of the pores created during sintering, which became preferentially located at the interface of alumina aggregates, negatively affecting some mechanical properties of the castable.     

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.     

Acetic Acid Role on Magnesia Hydration for Cement‐Free Refractory Castables

Different approaches to master magnesia hydration in refractory castables have been recently proposed. Among them, the use of hydrating agents can change the Mg(OH)₂ crystals morphology and its distribution in the resultant microstructure, minimizing the drawbacks related to the reaction expansion. In this work, the hydrating effect of acetic acid in Al₂O₃–MgO cement‐free castable properties during curing, drying, and firing steps was evaluated by elastic modulus, thermogravimetric, apparent porosity, and SEM analyses. Based on the attained results, adding acetic acid resulted in hydroxide crystals with distinct morphology and flexibility leading to a better accommodation of Mg(OH)₂ in the designed microstructure, which inhibited the samples' cracking during curing. In addition, the drying behavior of the evaluated compositions was further optimized by incorporating polypropylene fibers. Thus, this study highlights a novel perspective for fine MgO powders application, indicating that brucite morphology engineering may be a key aspect for the development of advanced Al₂O₃–MgO cement‐free refractory castables.     

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.     

The physical and mechanical properties of alumina-based ultralow cement castable refractories

In this study, the effects of the type of alumina on the physical, chemical and mechanical properties of the ultralow cement castable (ULCC) refractories were investigated. Brown fused alumina, tabular alumina and rotary bauxite-based ULCC refractories were prepared by mixing each type of alumina with silicon carbide, carbon, cement, metallic silicon and microsilica. The density, porosity and cold crushing strength (CCS) of the refractory castables were measured after drying at 110 °C for 24 h and firing at 1450 °C for 5 h. The slag penetration resistance of the refractory castables was determined using slag corrosion tests. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD) were used to characterize the castables. It was found that all three refractory castables had strong slag penetration resistance and that the tabular alumina-based refractory castable had the largest specific cold crushing strength with an acceptable percent of porosity among the refractory castables.     

High-alumina refractory castables bonded with novel alumina-silica-based powdered binders

In recent years, nano-binders (mainly colloidal suspensions) have been proposed as alternative materials for applications that require CaO-free refractory lining or improved mechanical behavior at intermediate temperatures (700?°C<?T?<?1200?°C). Despite the benefits of these suspensions, nano-bonded castables usually present limited green mechanical strength and different on site logistics to handle the liquid. Considering the availability of novel alumina-silica-based powdered binders, this work investigated the role of submicron alumina and SioxX®-Zero (both supplied by Elkem company) on rheological and mechanical properties of vibratable high-alumina castables, aiming to identify whether they can be suitable options to replace colloidal silica suspensions. Cold and hot mechanical strength and apparent porosity in the range of 110–1400?°C, cyclic thermal shock resistance, creep tests and hot elastic modulus of the designed formulations were evaluated. According to the results, SioxX®-Zero-bonded compositions presented good flowability levels and their sintering process started around 800?°C. Adding boron carbide to the same formulations resulted in transient liquid sintering of the silica-containing refractories, which allowed the development of compositions with improved thermo-mechanical performance in the 600–1400?°C temperature range. Furthermore, the submicron alumina-bonded samples presented fast sintering, resulting in E values close to 200?GPa (the highest value so far registered in our lab for a coarse grain size formulation) after one heating-cooling thermal-cycle up to 1400?°C.     

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.     

Main trends on the simulation of the drying of refractory castables - Review

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.     

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.     

Analysis on mechanism of explosive spalling resistance of aluminum powder on ladle corundum-based refractory castable

The effect of the particle size (50–325 mesh) and content (0–0.1 wt%) of metallic aluminum powder on the explosive spalling resistance of corundum-based refractory castables were investigated in this study based on air permeability, pore size distribution, heat release, fracture energy and microstructural analyses. The experimental results show that the addition of Al powder significantly improves their explosive spalling resistance. As observed, the explosive spalling resistance of the castables was mainly influenced by the permeability mechanism and the fracture energy. Permeable paths were generated in the microstructure of the sample during the overlapping period between the curing process and the H₂ gas forming one, derived from the Al–H₂O reaction. The highest permeability level was obtained when 0.075 wt % of Al powder with size of 100 mesh was incorporated into the corundum-based refractory castables. At the same time, the fracture energy of the castable samples was increased accordingly.     

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.     

Drying behavior of dense refractory ceramic castables. Part 1 – General aspects and experimental techniques used to assess water removal

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.