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.     

Modelling the pressure dependence and the influence of added polymeric fibers on the permeability of refractory concretes

In order to facilitate the water evacuation during drying, polymeric fibers are added to refractory concretes. A permeability model using Bruggeman's approach is developed to predict the permeability increase due to the fibers addition. This model (without adjustable parameters) knowing the fibers geometry (given by the manufacturer) and added amount (mass fraction varying between 0 and 0.20%) is validated by a thorough comparison with experimental results. The refractory permeability is measured at ambient temperature varying the quantity of added organic fibers after the samples were fired at different temperatures (from 80 °C to 500 °C). The analysis of results is especially careful to take into account the influence on the permeability, on both pressure (Klinkenberg effect) and firing temperature. The agreement between theoretical and experimental results is shown to be very satisfactory.     

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.     

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.     

Chelants to inhibit magnesia (MgO) hydration

Magnesium oxide (MgO) presents excellent refractoriness and high resistance to basic slag. However, in the presence of water, MgO undergoes an expansive hydration reaction generating Mg(OH)₂, which can lead to swelling and cracking. In this work, additives called chelants were added to dead burnt magnesia suspensions in order to check their effectiveness as inhibitors of the magnesium oxide hydration. Zeta potential, ionic conductivity, pH and temperature measurements were used to provide information related to the magnesia surface and the chelant adsorption. Assessment of the hydration degree and volumetric expansion provided indications of the amount of Mg(OH)₂ formed, as well as its likelihood to damage the ceramic bodies. The results showed that citric acid can inhibit hydration to some extent, whereas ethylenediamine tetraacetic acid (EDTA) was more effective in preventing volume expansion. An addition of 0.3 wt% of these chelants was sufficient to prevent hydration and avoid expansion.     

Sintering effect of calcium carbonate in high-alumina refractory castables

Calcium hexaluminate (CA₆) presents interesting properties and morphology, which can be readily changed depending on specific additives and refractory processing conditions. Aiming to investigate the role of calcium carbonate in inducing the formation of elongated CA₆ grains and also identify its sintering effect during the thermal treatments of high-alumina castables, this work focused on evaluating compositions containing calcium aluminate cement, CaCO₃ or their blend with the help of in situ techniques (hot elastic modulus, assisted sintering) and other traditional methods (mechanical strength, thermal shock resistance, etc.). A sintering effect derived from CaCO₃ addition to alumina castables could be identified during the hot E measurements, pointing out the ability of this compound to undergo a sintering-coarsening-coalescence transformation at 500–900?°C. This process also enhanced the mechanical strength and thermal shock resistance of the designed refractories at intermediate temperatures. Acicular CA₆ grains were formed in all analyzed compositions after firing at 1400?°C.     

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.     

六铝酸钙材料及其在铝工业炉中的应用

摘要 较系统地介绍了六铝酸钙材料的一些独特优良性能以及六铝酸钙的合成及其制品;同时还阐述了六铝酸钙材料在铝工业中的应用。关键词 六铝酸钙(CA6);铝工业炉;耐火浇注料;博耐特（Bonite）     

Citric acid as anti-hydration additive for magnesia containing refractory castables

Magnesia hydration is a key concern in refractory castable processing. The volumetric expansion that follows this reaction can result in cracks or even explosion during the first heating-up. Citric acid (CA) and other chelants can significantly reduce MgO hydration rate in aqueous suspensions by forming an insoluble magnesium citrate protective coating on the magnesia particles’ surface. In the present work, the performance of CA as an anti-hydration additive in refractory castables was evaluated by hydration tests, mechanical strength and apparent volumetric expansion (AVE) measurements and thermogravimetry. The results attained have shown that CA effectiveness depends strongly on the amount added and by the interaction with other raw materials in the composition, in particular calcium aluminate cement.     

One-step synthesis of in-situ carbon-containing calcium aluminate cement as binders for refractory castables

In-situ carbon-containing calcium aluminate cement (CCAC) was synthesized through carbon-bed sintering with calcium citrate tetrahydrate and Al₂O₃ as raw materials. The synthesized product was characterized by X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and infrared carbon–sulfur analysis. The results show that after sintering at 1500?°C for 4?h, the phase compositions of the product approached that of the commercial cement Secar71. The in-situ carbons in the product had partially graphitized domains and porous structures, were uniformly embedded in calcium aluminate, and the carbon content of the product was 1.45%. The floating ratios and oxidation ratios of the CCAC were lower than those of carbon back/Secar71 (S71CB) composite powders, implying that the water dispersion and oxidation resistance of CCAC were improved. Furthermore, the cold crushing strength (CCS), and cold modulus of rupture (CMOR) of the corundum-based castables bonded with CCAC, and S71CB, respectively, were compared. The CCS and CMOR values of the castables bonded with CCAC after being fired at 1100?°C for 3?h are higher by 20% and 21%, respectively, than those of the castables bonded with S71CB, suggesting that CCAC can be applied as a promising binder for the refractory castables.     

Microstructure and refractory properties of spinel containing castables

The bauxite-based and kaolin-based refractory castables investigated were carefully prepared. They are composed of 90 wt.% well-graded (coarse, medium, and fine) bauxite or kaolin aggregates, 10 wt.% binding matrix and adequate amount of distilled water. The binder mixture was calcium aluminate cement (CAC) containing 80% alumina and magnesium-aluminate spinel (MA-spinel) either preformed or in situ. The castable batches were cast into cubes (25 mm × 25 mm × 25 mm), left at 100% relative humidity for 24 h cured for 7 days under water, and dried at 110 °C for 24 h. The samples were then subjected to firing at 1550 °C for a soaking time of 1 h.     

Review of R&D in Drying of Refractories

This article reviews the published research and development work on refractory drying. Current practice and potential problems are analyzed. Some selected important research results are presented. It is shown that significantly improved drying procedures can be achieved through computer simulation of the drying process.     

MgO fumes as a potential binder for in situ spinel containing refractory castables

MgO is pointed out as an alternative binder for refractory materials, mainly for systems where the presence of CaO might not be desired. Selecting the most suitable magnesia source is an important step as its purity and reactivity should influence the hydration reaction, leading to binding effect or cracks. This work investigated the design of vibratable high-alumina compositions bonded with MgO fumes [which is a very fine powdered oxide (d < 3?µm) resulting from the production process of electrofused magnesia] and/or dead-burnt magnesia (d < 212?µm). Acetic and formic acids were added to the castables during their processing steps in order to adjust the density of active sites for Mg(OH)₂ formation and control the crystal growth of this phase. The green mechanical strength and thermomechanical performance (cold and hot mechanical strength, thermal shock, refractoriness under load, corrosion, etc.) of designed MgO-bonded compositions were analyzed. Improved green mechanical strength and crack-free samples were obtained when adding up to 6?wt% of MgO fumes to the refractories and processing them with aqueous solutions with 3?wt% of formic acid. The compositions with 6?wt% of magnesia fumes resulted in samples with flexural strength in the range of 12.0?MPa after curing at 50?°C/24?h and similar green mechanical strength (12.9?MPa) as the ones bonded with 4.0?wt% of calcium aluminate cement after drying at 110?°C for 24?h, which highlights the great potential of this MgO source. Despite the enhanced green mechanical strength, alumina-based castables containing 6?wt% of MgO (fumes, dead-burnt or their blend) showed low mechanical strength at intermediate temperatures and high linear expansion, as a consequence of the in situ spinel phase formation above 1200?°C. Thus, better densification, improved HMOR, thermal shock resistance and corrosion behavior were obtained for the castables prepared with less MgO fume contents.