Effect of nano iron oxide as an additive on phase and microstructural evolution of Mag-Chrome refractory matrix

doi:10.1016/j.jeurceramsoc.2009.03.032

Journal of the European Ceramic Society

Volume 29, Issue 13, October 2009, Pages 2679-2684

https://doi.org/10.1016/j.jeurceramsoc.2009.03.032 Get rights and content

Abstract

Nano iron oxide, up to 8 wt.%, was added to Mag-Chrome refractory matrix through stirring in ultrasonic bath in an alcohol media. The phase and microstructure of samples heated up to 1650 °C were studied by XRD and SEM/EDS respectively.

It was found out that the formation of magnesioferrite spinel was encouraged at lower temperatures in the presence of nano iron oxide. The dissolution of iron oxide and ionic migration improved the sintering process in the matrix of the refractory. The presence of nano iron oxide also influenced the bonding structure in a way that direct bonding was enhanced while silicate bonding was hindered.

Introduction

Mag-Chrome bricks have been playing a crucial role as a refractory material in various industries such as secondary metallurgy (AOD, VOD, etc.), non-ferrous furnaces (copper converter) and cement making (rotary kiln) due to their high temperature stability, low thermal expansion and outstanding erosion–corrosion performance at high temperatures.¹ Mag-Chrome refractories are basically categorized into silicate, direct and rebonded grades in terms of bonding between grains. Silicate bonds are mainly comprised of low melting compositions such as monticillite (CaO·MgO·SiO₂), merwinite (3CaO·MgO·2SiO₂) and some amounts of dicalcium silicate and forsterite according to the molar ratio of CaO/SiO₂. These silicate phases facilitate liquid phase sintering process but deteriorate thermo chemical and thermo mechanical properties of refractory in high temperature applications.[2], [3] Hence, developing direct bonded Mag-Chrome brick containing low amounts of silicate phase is of high importance based on their excellent properties at high temperatures.

In order to enhance direct bond formation between grains in Mag-Chrome refractories, SiO₂ content of the refractory is to be less than 3 wt.%.¹ Such low content of silicate phases in the matrix, however, leads to high sintering temperature and high energy cost. One of the key factors in reducing the sintering temperature of direct bonded bricks as reported nearly four decades ago is lowering the dihedral angle between periclase and periclase–chromite grains. White claims that introducing some oxides such as ZrO₂ and TiO₂ into the refractory could decrease dihedral angle and lower the firing temperature by promoting direct bond between grains.² In this regard Haldar and Ghosh have investigated the effect of TiO₂ and ZrO₂ addition in Mag-Chrome refractories respectively.[4], [5] They concluded that addition of TiO₂ and ZrO₂ improved the physical properties of Mag-Chrome refractories and enhanced direct bond formation. These additives, however, did not lower the sintering temperature significantly. Recently, Huizhong studied addition of nano Fe₂O₃ into the matrix of Mag-Chrome refractory and reported that the sintering temperature was reduced about 150 °C but he did not report any phase and microstructural evolutions.⁶

The aim of this work is to introduce nano iron oxide as an additive to Mag-Chrome refractories matrix and study the phase and microstructural developments during the sintering process.

Section snippets

Experimental

Commercial Chinese Dead Burnt magnesia and Iranian Chrome ore were taken as raw materials (Table 1). In order to obtain a direct bonded magnesia–chromite system, a formulation containing 75 wt.% Chinese magnesia and 25 wt.% Iranian chrome ore was adopted. High grade nano iron oxide (α-Fe₂O₃) with average particle size of 20–50 nm was supplied from Nanoamor Co., USA (Table 2).

Two-hour simultaneous stirring and ultrasonication in methyl alcohol as dispersing media was applied to prepare a stable and

Results and discussion

In order to prepare a stable suspension carrying nano iron oxide powder, it is necessary to conduct a specific procedure to break the agglomerates. High surface tension of iron oxide particles leads to severe agglomeration and therefore we found that a liquid media such as methyl alcohol was required to disperse the particles in the suspension. However this media was found to be influential only if simultaneous ultrasonic stirring was adopted.[7], [8]

Fig. 1a and b shows the SEM images of nano

Conclusion

Nano iron oxide addition enhanced the formation of magnesioferrite spinel at lower temperatures. Also substantial amount of spinel was found to precipitate during cooling process. Solid state sintering was promoted by accelerated counter diffusion of Mg and Fe ions between chromite and magnesia particles as well.

Nano iron oxide additive encouraged the formation of direct bonding in refractory matrix. This phenomenon was observed to be accompanied by decreasing the viscosity of liquid silicate

Acknowledgement

The authors wish to express their great gratitude for appreciable assistance of Dr. A. Saberi in characterizing the samples.

References (13)

M.K. Haldar et al.
Effect of compositional variation on the synthesis of magnesite–chrome composite refractory
Ceramic International
(2004)
A. Ghosh et al.
Effect of MgO and ZrO₂ additions on the properties of magnesite–chrome composite refractory
Ceramic International
(2007)
C.-S. Lee et al.
Dispersion control of Fe₂O₃ nanoparticles using a mixed type of mechanical and ultrasonic milling
Materials Letters
(2003)
K. Goto et al.
The direct bond in magnesia chromite and magnesia spinel refractories
Journal of American Ceramic Society
(1995)
Alper.
High Temperature Oxides
(1970)
J.H. Chester
Refractories, Production and Properties
(1973)

There are more references available in the full text version of this article.

Cited by (27)

Functionalization of nano-MgCr<inf>2</inf>O<inf>4</inf> additives by silanol groups: a new approach to the development of magnesia-chrome refractories
2021, Ceramics International
This work describes a novel approach to improve the dispersion of a nanoparticle MgCr₂O₄ additive in the matrix of magnesia-chrome refractories by functionalizing the nanoparticle surfaces with silanol groups. The effect of the silanol groups on the nanoparticles within the refractory matrix was shown by FTIR spectroscopy, zeta potential measurements, UV–visible spectroscopy, Dynamic Light Scattering (DLS) and TEM to result in a decrease of the average particle size of the MgCr₂O₄ additive by about 50% due to de-agglomeration of the nano particles by the silanol groups. Silanol functionalization of the nano-additives in the magnesia-chrome refractories fired at 1600 °C resulted in a 65 Kgf/cm² increase in their compressive strength, while the hot modulus of rupture and corrosion resistance values of these refractories fired at 1400 °C were similar to those containing unfunctionalized nano-additives fired at 1600 °C. XRD and SEM results suggest the improvement in corrosion resistance is related to the formation of spinel phases in the refractories containing silanol-modified nano-additives.
Role of nano titania on the thermomechanical properties of silicon carbide refractories
2020, Ceramics International
Nano titania incorporated silicon carbide (SiC) refractory has been prepared through sol-gel route to analyze its effect on SiC physical and thermo-mechanical properties. The X-ray diffraction (XRD) results revealed that as-synthesized titania was of anatase phase with an average crystallite size of 9.15 nm. The scanning electron microscopy (SEM) analysis showed that the nano titania particles were spherical in shape. After firing the samples containing SiC with different amounts of nano titania at 1450 °C, bulk density, apparent porosity, cold crushing strength, modulus of elasticity, cold and hot modulus of rupture and thermal spalling index were examined. XRD and SEM analysis were also carried out to determine the ceramic phases and microstructure of the fired samples, respectively. Upon analysis of the obtained results, it showed that addition of nano titania improved the SiC properties and also led to the in-situ formation of TiC, which aided in enhancing the thermal shock resistance and hot strength of the fired SiC refractory samples.
A comparison of the effect of nanostructured MgCr<inf>2</inf>O<inf>4</inf> and FeCr<inf>2</inf>O<inf>4</inf> additions on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories
2020, Ceramics International
Citation Excerpt :
The observed results from BET, TEM and the Scherrer equation show that these spinel additives are nanosized. The main reason for adding these two nanosized spinels to the magnesia-chrome matrix was that these are formed as secondary spinels in magnesia-chrome refractories during thermal processing in the absence of additives, and are thought to increase direct bonding between the grains, assisting densification and improving the mechanical properties [1,3]. The XRD pattern and SEM/EDX micrographs taken at different magnifications of the magnesia-chrome matrix without the nano-spinel additives (sample MC0), Figs. 5 and 6 (a,b), confirm that these secondary spinels formed in the present magnesia-chrome matrix sample are MgCr2O4 and FeCr2O4.
The effect on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories of additions of nanostructured MgCr₂O₄ and FeCr₂O₄ is reported. The nanostructured additives, synthesized by the citrate-nitrate route and calcined at several different temperatures, were characterized by XRD, BET and TEM. Additions of 0.5 and 1 wt % of these nanostructured oxides were made to magnesia-chrome refractories and calcined at 1650 ^OC in a shuttle kiln. Their microstructures were analyzed by SEM/EDX and their physical and mechanical properties (permanent linear change (PLC), bulk density, apparent porosity, cold crushing strength (CCS) and hot modulus of rupture (HMOR) were determined according to the respective DIN standards. The addition of the nanostructured oxides to the magnesia-chrome refractories facilitated the formation of secondary spinels, influencing the physical and mechanical properties. FeCr₂O₄ additions increased the size of the secondary spinel due to liquid phase formation in the presence of magnetite impurities in the FeCr₂O₄ nano-powder. The addition of nano-sized MgCr₂O₄ and FeCr₂O₄ to the base formulation of the refractory increased the CCS from 67.4 MPa to 82.8 MPa and 81.0 MPa respectively, while nano-sized MgCr₂O₄ increased the HMOR value from 5.48 MPa to 5.91 MPa and nano-sized FeCr₂O₄ increased the HMOR from 5.48 MPa to 5.72 MPa. This smaller increase than that obtained with FeCr₂O₄ additions is attributed to liquid phase formation in the presence of magnetite, as observed by XRD.
Effects of nano-alumina content on the formation of interconnected pores in porous purging plug materials
2017, Ceramics International
Citation Excerpt :
Micro-powder has been widely used in the field of refractory materials since the 1970s, and the overall performance of these materials has been improved compared to the conventional materials. The particle size of nano-materials is smaller than that of micro powder, and nano-technology has been recognized as the most promising research field of the 21st century, and is currently used in many research applications [4–16]. Badiee et al. studied the effect of the addition of nano-titania on the properties of high-alumina, low-cement, self-flowing refractory castables [4].
The physical properties and microstructure of porous purging plug materials added with different nano-alumina contents and firing temperatures were investigated by means of X-ray diffraction, scanning electron microscopy, air permeability, pore size distribution, mean pore size, apparent porosity, bulk density, and cold crushing strength (CCS) tests. The results showed that the addition of nano-alumina had a great effect on the physical properties and microstructure of the porous purging plug materials. With increasing nano-alumina content in the composition, the main phase was α-Al₂O₃ in all compositions and the mean pore size, apparent porosity and air permeability all increased due to the increased number of pores and pore size of the specimens which facilitated the formation of interconnected pores. When the sintering temperature was changed from 1550 °C to 1650 °C, some of the smaller pores vanished due to solid phase sintering, which reduced the apparent porosity, and some open pores connected to form interconnected pores, which promoted increased air permeability. In addition, the strength and porosity were found to follow the relationship σ = σ₀ exp (-b P). When the apparent porosity increased, the CCS decreased, and vice versa.
Growth mechanism of in situ diamond-shaped mullite platelets and their effect on the properties of Al<inf>2</inf>O<inf>3</inf>–SiC–C refractories
2017, Ceramics International
The microstructure evolution and properties of Al₂O₃–SiC–C refractories with different Fe₂O₃ content (0, 0.5, 1.0 wt%) heat-treated at 1400 °C were investigated. Also, the in-situ growth mechanism of diamond-shaped mullite platelets was discussed. The experimental results indicate that there were only mullite and SiC whiskers formed in the absence of the additive. However, both whisker-shaped and diamond-shaped mullite phases could be generated aside from SiC whiskers, which were satisfactorily correlated with Fe₂O₃ addition. Interestingly, the introduction of Fe₂O₃ changed the morphology of mullite whiskers from the curved structure (without additive) to the straight and elongated shape (with additive). Furthermore, these two-dimensional mullite platelets were better developed resulting in the larger size and higher yield with 1.0 wt% Fe₂O₃. XRD, SEM, TEM and HRTEM analyses reveal that with the addition of Fe₂O₃, mullite phases in refractory samples grew preferentially toward the crystal facets along (130) and (4 $\bar{2}$ 0) directions of mullite nanocrystals, thus, a diamond-shaped structure was generated. As the well-developed mullite platelets were formed, the cold modulus of rupture (CMOR) and the cold crushing strength (CCS) increased from 6.5 MPa to 10.3 MPa and from 38.7 MPa to 55.7 MPa, respectively. Meanwhile, after three thermal shock cycles, the CCS of samples containing 1.0 wt% Fe₂O₃ only decreased by 2.6 MPa, and the residual strength ratio of CCS (CCSst) reached the maximum value of 95.3%. These results suggest that the in situ synthesized platelet-shaped mullite phase observably improved the thermal shock resistance of Al₂O₃–SiC–C refractories.
The impact of trivalent oxide nanoparticles on the microstructure and performance of magnesite-dolomite refractory bricks
2017, Materials Chemistry and Physics
In this research, the impact of trivalent oxide nanoparticles on the microstructure and performance of the magnesite-dolomite refractory bricks was examined. Up to 4 wt% Fe₂O₃, Al₂O₃, and Cr₂O₃ nanoparticles were added to the specimens as additives. Physical and mechanical properties such as bulk density (BD), apparent porosity (AP), hydration resistance (HR), and cold crushing strength (CCS) were examined. XRD and SEM analysis were used to detect the ceramic phase's formation and microstructure analysis, respectively. Results show that the use of Fe₂O₃, Al₂O₃, and Cr₂O₃ nanoparticles improve the physical and mechanical properties of the MgO-CaO materials. Also, it revealed that specimens contain Fe₂O₃ and Al₂O₃ nanoparticles improve hydration resistance through the liquid phase sintering mechanism by formation some low melting phases such as C₂F(2CaO·Fe₂O3), C₃A(3CaO·Al₂O₃), and C₁₂.A₇(12CaO·7Al₂O₃). However the specimens contain Cr₂O₃ nanoparticles improve the hydration resistance through the solid state sintering mechanism by formation phases such as CaCr₂O₄ and MgCr₂O₄. In general, hydration resistance improvement trend of the MgO-CaO specimens includes trivalent nanoparticles is Al₂O₃<Fe₂O₃< Cr₂O₃.

View all citing articles on Scopus

View full text

Effect of nano iron oxide as an additive on phase and microstructural evolution of Mag-Chrome refractory matrix

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgement

Ceramic International

Ceramic International

Materials Letters

The direct bond in magnesia chromite and magnesia spinel refractories

Journal of American Ceramic Society

High Temperature Oxides

Refractories, Production and Properties