Fast drying of high-alumina MgO-bonded refractory castables

doi:10.1016/j.ceramint.2020.01.134

Ceramics International

Volume 46, Issue 8, Part A, 1 June 2020, Pages 11137-11148

https://doi.org/10.1016/j.ceramint.2020.01.134 Get rights and content

Abstract

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 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.

Introduction

The mixing stage of refractory castables requires the addition of water or some other liquid to help mold it into the desired shape. After that, drying becomes the most important processing step, as the added water must be withdrawn from the microstructure before firing, which implicates in heat and mass transfers of H₂O in liquid or vapor form [1,2]. According to Oummadi et al. [2], two main issues can be related to drying: (i) it requires significant energy consumption, and (ii) it may lead to mechanical stresses (due to pressure build-up associated with steam trapped inside the dense structure) and eventually the spalling and/or explosion of the ceramic. Thus, this subject has motivated many studies focused on better understanding the water distribution and advance of the drying front in green ceramic bodies, by using alternative experimental techniques (i.e., magnetic resonance imaging and neutron tomography) [[2], [3], [4], [5]]. Moreover, other research groups have also been developing numerical models of heat and mass transfers in moist ceramics in order to predict safe and optimized heating schedules for refractory compositions [[6], [7], [8], [9]].

Usually, the main castable systems that present higher likelihood to undergo spalling or explosion during their drying step are the ones containing hydraulic binders (calcium aluminate cement, hydratable alumina or magnesia), as such components react with water, giving rise to hydratable phases that decompose and release water vapor at temperatures above 100 °C [[10], [11], [12], [13]]. Considering that the precipitated hydrates fill in the pores of the refractory structure, resulting in materials with reduced permeability, and the vapor pressure inside the castables during heating increases exponentially as a function of the temperature (according to Antoine's equation [1]), the higher the decomposition temperature of the hydrates is, the greater the damage risk will be for such materials.

Although magnesia has been pointed out as a likely binder for alumina-based castables [[12], [13], [14], [15], [16]], there are many challenges and concerns for producing high quality and crack-free compositions in this system. In this case, the bonding effect is derived from Mg(OH)₂ (brucite) formation (Eq. (1)) in the interstices and voids among the coarse and fine particles of the refractory. However, considering the density mismatch between MgO (ρ_MgO = 3.5 g/cm³) and brucite (ρ_Mg(OH)2 = 2.3 g/cm³), this reaction can result in an expressive volumetric expansion, leading to the generation of cracks in the molded samples during curing and drying steps [12,15]. ${M g O + H_{2} O \to M g (O H)}_{2}$

Aiming to take advantage of the MgO hydration and optimize the bonding potential of this material, some approaches are suggested in the literature, such as: (i) inducing faster Mg(OH)₂ formation before the composition setting time, when the molded castable still has enough room and some freedom to accommodate stresses [14,17], (ii) changing the morphology of the hydrated phase to better fit the crystal growth in the designed microstructure [[18], [19], [20]], and (iii) using hydrating agents (i.e., carboxylic acids, and others) to activate a higher number of sites and increase the Mg(OH)₂ nucleation rate on MgO surface, which might limit the crystal growth [12,13].

As MgO can readily react with water in its liquid or vapor form, the brucite formation can still take place during the initial stages of the drying process, resulting in an additional risk to the integrity of the refractories containing this binder. Additionally, Mg(OH)₂ decomposition takes place at relatively high temperatures (380–420 °C) [21,22], which coupled to the reduced permeability of the resulting microstructure, may result in high steam pressure levels and, consequently, the explosion of the castable even under usual heating schedule profiles.

The drying behavior of MgO-bonded castables can be adjusted by using polymeric fibers [23], silica-based compounds [[24], [25], [26], [27]], organic additives (such as lactates [28]) or favoring the formation of hydrotalcite-like phases [Mg_xAl_y(OH)_2x+2y](CO₃)_y/2.nH₂O] in the compositions [29,30]. The addition of fibers is a simple and versatile solution, but their presence decreases the flowability of the mixtures, whereas the MgO and SiO₂ reactions at high temperatures usually give rise to low melting point phases in alumina systems. On the other hand, the in situ formation of hydrotalcite-like phases seems to be a promising alternative, as such compounds commonly present their thermal decomposition in various steps in the 200–400 °C range, which might decrease the spalling risk associated with the steam pressure generation [30].

Considering the aspects discussed above and the availability of novel drying agents (SioxX-Mag and Refpac MIPORE 20) designed for monolithic refractory systems, this work investigated and compared the performance of various additives in the optimization of the drying behavior of high-alumina MgO-bonded castables. Vibratable compositions were formulated and tested after the incorporation of polymeric fibers, an organic aluminum salt (aluminum salt of 2-hydroxypropanoic acid), SioxX-Mag (silica-based additive) and Refpac MIPORE 20 (permeability enhancing active compound originally designed for cement-bonded compositions), into the dry-mixtures.

Section snippets

MgO-based suspensions

Firstly, mixtures of dead-burnt magnesia (M-30, MgO = 98.12 wt%, CaO = 0.80 wt%, SiO₂ = 0.34 wt% and others = 0.74 wt%, d < 212 μm, RHI-Magnesita, Contagem, Brazil), distilled water and additives (Table 1) were prepared to investigate the likely influence of the latter compounds in the brucite formation during curing (30 °C) and drying (110 °C) conditions. A reference composition (MgO + distilled water) was analyzed and compared to the other prepared systems. Formic acid (AcF, molecular mass:

Physical-chemical transformations derived from the interaction of the selected additives with MgO

Firstly, mixtures of MgO + additives were prepared and analyzed in order to identify the most likely reactions that could take place when keeping the prepared samples at 30 °C and/or 110 °C for 24 h. Although 30 wt% of distilled water was added to MgO for the preparation of the MgO–H₂O mixture (Table 1), this composition presented an aqueous layer (excess of liquid that did not react with magnesia) at the top of the molded samples after their curing step at 30 °C for 24 h. Hence, this unreacted

Conclusions

Designing and processing crack-free high-alumina MgO-bonded castables are huge challenges for refractory producers, as not only cracks and flaws can be generated during the curing step of the prepared samples (due to brucite formation and its non-accommodation in the microstructure), but also the first heating treatment of such materials may lead to their spalling or explosion, as a consequence of a combination of low permeability and high steam pore pressure build-up in the ceramic lining.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (grant number: 303324/2019-8). The authors are also grateful to Almatis, RHI-Magnesita, Imerys Aluminates S.A. and Elkem for supplying the raw materials used in this study.

References (50)

S. Oummadi et al.
Distribution of water in ceramic green bodies during drying
J. Eur. Ceram. Soc.
(2019)
D. Dauti et al.
Modelling of 3D moisture distribution in heated concrete: from continuum towards mesoscopic approach
Int. J. Heat Mass Transf.
(2019)
B. Nait-Ali et al.
Thermal conductivity of ceramic green bodies during drying
J. Eur. Ceram. Soc.
(2017)
K. Fey et al.
Pressure driven heat-up curves - a numerical and experimental investigation
Int. J. Therm. Sci.
(2017)
K. Fey et al.
Experimental and numerical investigation of the first heat-up of refractory concrete
Int. J. Therm. Sci.
(2016)
F.A. Cardoso et al.
Drying behavior of hydratable alumina-bonded refractory castables
J. Eur. Ceram. Soc.
(2004)
T.M. Souza et al.
Systemic analysis of MgO hydration effects on alumina-magnesia refractory castables
Ceram. Int.
(2012)
A.P. Luz et al.
MgO fumes as a potential binder for in situ spinel containing refractory castables
Ceram. Int.
(2018)
J. Wang et al.
Characterizations of the thermal decomposition of brucite prepared by sol–gel technique for synthesis of nanocrystalline MgO
Mater. Lett.
(1998)
T.M. Souza et al.
Phosphate chemical binder as an anti-hydration additive for Al₂O₃–MgO refractory castables
Ceram. Int.
(2014)

G. Ye et al.

Hydration of hydratable alumina in the presence of various forms of MgO

Ceram. Int.

(2006)

A.P. Luz et al.

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

Ceram. Int.

(2019)

A.P. Luz et al.

Thermal shock damage evaluation of refractory castables via hot elastic modulus measurements

Ceram. Int.

(2013)

M.A.L. Braulio et al.

Basic slag attack of spinel-containing refractory castables

Ceram. Int.

(2011)

T. Shido et al.

Reactant-promoted reaction mechanism for water gas shift reaction on MgO

J. Catal.

(1993)

L. Zhang et al.

Aluminum lactate – an attractive precursor for sol – gel synthesis of alumina-based glasses

J. Non-Cryst. Solids

(2007)

K. Ghanbari Ahari et al.

Hydration of refractory oxides in castable bond systems - I: alumina, magnesia, and alumina-magnesia mixtures

J. Eur. Ceram. Soc.

(2002)

H.L. Zhang et al.

A numerical investigation of the influence of pore pressures and thermally induced stresses for spalling of concrete exposed to elevated temperatures

Fire Saf. J.

(2013)

M.A.L. Braulio et al.

Spinel-containing alumina-based refractory castables

Ceram. Int.

(2011)

M.A.L. Braulio et al.

Selection of binders for in situ spinel refractory castables

J. Eur. Ceram. Soc.

(2009)

A.P. Luz et al.

Refractory Castable Engineering

(2015)

E. Keita et al.

MRI evidence for a receding-front effect in drying porous media

Phys. Rev.

(2013)

Z.-X. Gong et al.

Development of drying schedules for one-side-heating drying of refractory concrete slabs based on a finite element model

J. Am. Ceram. Soc.

(1996)

Z.-X. Gong et al.

Review of R&D in drying refractories

Dry. Technol.

(2007)

M.D.M. Innocentini et al.

Permeability and drying behavior of PSD-designed refractory castables

Am. Ceram. Soc. Bull.

(2003)

Cited by (20)

Mechanical properties and gas permeability of silica-gel modified hydratable magnesium carboxylate bonded refractory castables in drying process
2024, Ceramics International
The hydratable magnesium carboxylate (HMC) bonded castables have a high-level gas permeability while the mechanical strength decreased sharply during drying process. Silica-gel with high activity is a porous structure after drying and can potentially increase the intermediate-temperature strength without reducing the gas permeability of the castables. In this study, the mechanism of silica-gel modified the HMC, and the effects on the properties of the castables were investigated. The surface of the hydration products of HMC is covered with a thin protective layer of siloxane network. The proportion of fine permeable pores (0.1 μm ∼1 μm) increased, optimizing the connected network channels in the castables. Compared to the pure HMC bonded castables, with the HMC/silica-gel mass ratio of 2, the cold modulus of rupture of the castables increased by 2 times after heat treatment in the range of 400 °C ∼600 °C. Especially, the permeability constants Darcian k₁ and non-Darcian k₂ increased by 37 times and 38 times after heat treatment at 600 °C, respectively.
Low-carbon and low-cost preparation of non-sintering bauxite-based solid thermal energy storage materials
2024, Solar Energy
In recent years, the rapid growth of clean energy has driven the rapid development of solid thermal energy storage (STES) technology in China. However, the preparation process of commonly used sintered magnesia bricks have the problems of high carbon emissions (7 t CO₂/t brick) and high cost, which limits the large-scale application of STES technology. This paper proposes a non-sintering strategy for preparing bauxite-based STES materials. The results demonstrate that the prepared material exhibits good mechanical strength at both room temperature (>15 MPa) and medium-to-high temperatures (>10 MPa), resistance to high-temperature creep (>1250 °C), acceptable thermal conductivity (2.23 W·m⁻¹·K⁻¹), and stable thermal cycling performance with a flexural strength of at least 10 MPa after 20 thermal cycles (400–1000 °C), which satisfies the demand for high-temperature energy storage. Furthermore, the carbon emissions of the non-sintering material is only 30 % of that of sintered magnesia brick, and the production costs are significantly reduced, making them a promising alternative to current sintered STES bricks.
Combined transient 3D simulation and experimental methods to assess a slow heat-up curve used to dry a refractory concrete
2023, International Journal of Thermal Sciences
Citation Excerpt :
Therefore, the thermal stresses must be controlled while providing time for the water vapour to escape without raising the internal pressures above the tensile strength of refractory concrete [2]. In heated high-performance concrete (HPC), the addition of polypropylene fibres has shown to be efficient to mitigate spalling [3,4]; nonetheless, in the case of some refractory concretes, fibres have shown to be detrimental for important properties like corrosion resistance, mechanical strength, flowability and slag penetration [5–7]. Currently, the heat-up curves are defined by refractory manufacturers from empirical experience [2,8,9].
Explosive spalling is a major concern when drying refractory concretes. For safety, empirically defined overestimated slow heat-up curves are used in industry. Polypropylene fibres reduce spalling; however, they are inconvenient for certain castables due to properties deterioration, such as slag penetration. There is neither a generally accepted methodology to define monolithic refractories dry-out schedules nor agreement about a mathematical model to predict spalling phenomena. This paper presents original numerical and experimental results of castable drying investigation using slow heating. Modifications in experimental methods were made to reproduce refractory lining boundary conditions. A conservative fully implicit finite volume method in three-dimensions was developed to predict pressure and moisture migration. The influence of the methods used to characterize the properties needed for the mathematical model was discussed. The combined analyses suggest that the first holding time used was not productive for drying; however, its effect on curing must be reviewed.
Drying behavior of dense refractory castables. Part 2 – Drying agents and design of heating schedules
2022, Ceramics International
Citation Excerpt :
Among various organic additives suggested to improve the properties of cured magnesia containing compositions, aluminum lactate (aluminum salt of 2-hydroxypropanoic acid) is an interesting option as it can act as a chelating agent of Mg2+ ions and induce the generation of hydrotalcite-like compounds [MgxAly(OH)2x+2y](CO3)y/2. nH2O] in such systems, which might lead to crack-free refractory pieces [37,100–104]. Previous works [105,106] also identified an additional benefit by incorporating this organic compound into magnesia-containing monolithic products.
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.
Low-melting-point polymeric fiber performance as drying additives for refractory castables
2022, Ceramics International
Citation Excerpt :
During monolithic material processing, water is added to mix and homogenize the raw materials, increase packaging efficiency and ensure the formation of hydrated phases. The latter are generated due to the reaction of binders (i.e., calcium aluminate cement, hydratable alumina, etc.) with the liquid, which induces the precipitation of the hydrates and, consequently, provides the green mechanical resistance of these ceramic products at early ages [3–7]. After the consolidation of the ceramic pieces, the next step consists of removing free water (liquid retained in the open porosity and adsorbed on the surface of the particles) and the structural one (chemically bonded water from the hydrates’ structure).
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.
Drying behavior of dense refractory ceramic castables. Part 1 – General aspects and experimental techniques used to assess water removal
2021, Ceramics International
Citation Excerpt :
Therefore, water withdrawal in magnesia-bonded castables must be carried out as fast as possible because due to the low porosity and permeability of the designed microstructures, the steam release associated with brucite decomposition might be hindered [26,55]. Fig. 10 illustrates the drying rate profile of high-alumina MgO-bonded castables when the green samples were submitted to heating rates of 5 and 20 °C/min [20]. As observed, the reference composition containing 94 wt% of alumina and 6 wt% of MgO did not withstand the stresses derived from the ebullition stage and exploded in both tested conditions at temperatures close to 150 °C (before brucite decomposition).
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.

View all citing articles on Scopus

View full text

Fast drying of high-alumina MgO-bonded refractory castables

Abstract

Introduction

Section snippets

MgO-based suspensions

Physical-chemical transformations derived from the interaction of the selected additives with MgO

Conclusions

Declaration of competing interest

Acknowledgements

J. Eur. Ceram. Soc.

Int. J. Heat Mass Transf.

J. Eur. Ceram. Soc.

Int. J. Therm. Sci.

Int. J. Therm. Sci.

J. Eur. Ceram. Soc.

Ceram. Int.

Ceram. Int.

Mater. Lett.

Ceram. Int.

Ceram. Int.

Ceram. Int.

Ceram. Int.

Ceram. Int.

J. Catal.

J. Non-Cryst. Solids

J. Eur. Ceram. Soc.

Fire Saf. J.

Ceram. Int.

J. Eur. Ceram. Soc.

Refractory Castable Engineering

MRI evidence for a receding-front effect in drying porous media

Phys. Rev.

Development of drying schedules for one-side-heating drying of refractory concrete slabs based on a finite element model

J. Am. Ceram. Soc.

Review of R&D in drying refractories

Dry. Technol.

Permeability and drying behavior of PSD-designed refractory castables

Am. Ceram. Soc. Bull.