Evolution of an alumina-magnesia/self-forming spinel castable. Part I: Microstructural features

Evolução de um refratário de espinélio auto-formado de alumina-magnésia. Parte I: Aspectos microestruturais

Evolution of an alumina-magnesia/self-forming spinel castable. Part I: Microstructural features.

(Evolução de um refratário de espinélio auto-formado de alumina-magnésia. Parte I: Aspectos microestruturais)

D. Gutiérrez-Campos, J. I. Diaz

Universidad Simón Bolívar, Dpto. de Ciencia de los Materiales,

Centro de Ingeniería de Superficies, Apdo. 89000, Caracas 1080-A, Venezuela

R. M. Rodriguez

Universidad Metropolitana, Dpto. de Química, Caracas, Venezuela

e-mail: [e-mail]

Abstract

Refractories containing magnesium aluminate spinel (MgAl₂O₄) are materials for emerging technology in several applications like cement and steelmaking processes. In order to deep the understanding of these castables, this work presents the microstructural characteristics of an alumina-magnesia/self-forming spinel castable. Several variables such as MgO content, firing temperature and spinel formation are analyzed through XRD and SEM analysis. The results showed that the processes of spinel formation and nucleation are not strongly affected by the MgO content, but that the crystal growth is enhanced for samples with 6.0 wt% MgO. Hibonite (CA₆) bonding in the castable matrix showed a needlelike structure that could increase hot properties of the material. MgO content in the castable seems to affect the hibonite development. The development of a self-forming spinel castable without any synthetic spinel grains appears to be promissory for optimum refractory linings.

Keywords: Refractories, magnesium aluminate, spinel, castable, alumina-magnesia, refractory lining, MgAl₂O₄, MgO-Al₂O₃.

Resumo

Refratários contendo espinélio de aluminato de magnésio (MgAl₂O₄) são materiais para tecnologia emergentes em várias aplicações tais como cimento e processos siderúrgicos. Com a finalidade de melhorar o entendimento destes refratários, este trabalho apresenta as características microestruturais de um refratário espinélio auto-formado de alumina-magnésia. Várias variáveis tais como teor de MgO, temperatura de queima e formação de espinélio são analisadas por meio de difração de raios X e microscopia eletrônica de varredura. Os resultados mostram que os processos de formação de espinélio e de nucleação não são fortemente influenciados pelo teor de MgO, mas que o crescimento de cristal é aumentado para amostras com 6.0% em peso de MgO. A ligação hibonita (CA₆) na matriz do refratário mostrou uma estrutura tipo agulha que poderia melhorar as propriedades a quente do material. O teor de MgO no refratário parece influenciar o desenvolvimento da hibonita. O desenvolvimento de um refratário tipo espinélio auto-formado sem quaisquer grãos de espinélio sintético parece ser promissor para uso em revestimentos refratários otimizados.

Palavras-chave: Refratários, aluminato de magnésio, espinélio, Alumina-magnesia, revestimento refratário, MgAl₂O₄, MgO-Al₂O₃.

INTRODUCTION

Magnesium aluminate spinel (MgAl₂O₄) is considered an excellent material for refractory products due to its outstanding high temperature mechanical, chemical and thermal properties. Spinel is artificially produced because there is little natural occurrence; thus, numerous studies have been presented for the synthesis of magnesium aluminate spinel (MgAl₂O₄) [1-9].

Refractories containing magnesium aluminate spinel (MgAl₂O₄) are becoming increasingly popular due to their exceptional behavior in several applications. Actually, spinel refractories have been successfully applied in aluminum, glass, lime and cement process vessels and also in steelmaking ladles [9]. Two demanding areas using spinel refractories are cement rotary kilns and metallurgical furnaces. In the first case, magnesia-spinel materials are generally used in the transition zones of the cement kilns and the development of this type of products is well documented elsewhere [2,5,10]. On the other hand, spinel products for steel making were originally developed for steel ladles. In this context, definite substitution of bricks by monolithic refractories was performed to reduce labor costs and in response to changing environmental conditions for bricklaying work. By 1988 the premier spinel castable for ladles was reported [11] and nowadays, they are widely applied for sidewall linings.

Several routes have been followed to develop spinel-containing castables. In the case of MgO-based materials, this has been achieved by the inclusion of synthetic spinel grains and/or the addition of alumina to form in-situ spinel in the matrix [12-14]. Similar ways have been observed for Al₂O₃-based products in which fine MgO had been used instead of alumina. Thus, the evolution of alumina-spinel castables has been achieved by two approaches:

(a) adding synthetic spinel as an aggregate in an alumina castable and/or

(b) including MgO to the binder phase of the castable.

"In-situ spinel bond" and "self-forming spinel product" are terms generally used to describe a material able to produce spinel in itself during firing and this could be accomplished with or without the inclusion of synthetic spinel aggregates. Many papers have been published in which synthetic spinel is used as an aggregate to develop an alumina-spinel castable [2,15, 16, 17]. Other studies reported the addition of synthetic spinel aggregate and fine MgO powder in order to reinforce alumina structure by in-situ spinel formation in the matrix [18-21]. However, just few published investigations analyze the inclusion of MgO in high-alumina low cement castables without any synthetic spinel grains. Recent publications [22,23] indicated that this last approach could produce more resistant concretes and it could be less costly than the synthetic spinel aggregate solution. Bier et al. [22] studied the importance of admixtures in the rheological properties of calcium aluminate cement based castables containing magnesia. Other work presented a comparison between alumina-magnesia systems and alumina-spinel castables from the corrosion, slag penetration and thermal shock resistance point of view and the results are remarkable for the use of alumina-magnesia/self-forming spinel castables in sidewall and impact-pad of steel ladles [23].

From the previous review, it seems necessary to increase the understanding of these refractories in order to develop optimum linings for severe applications. In this context, the first part of this work presents microstructural features of an alumina-magnesia/self-forming spinel castable at different firing temperatures in which MgO percentages have been changed. Due to its strong implications on the properties of castables, variables like MgO content, firing temperature and spinel formation are analyzed. In the second part, to be sequentially published, physical-chemical and mechanical properties, such as bulk density and cold crushing strength at several temperatures, are discussed and compared with properties of conventional alumina-spinel castables.

EXPERIMENTAL

The experiments conducted are based on a commercial grade high-alumina low cement castable in which different percentages of fine MgO were added to promote spinel formation. Variations in MgO content included 5.0, 5.5 and 6.0 wt %. Table 1 presents chemical compositions of castable mixes. All materials were weighed and dry mixed in a Hobart mixer for 5 minutes. Then, water was added to the dry mix and mixed for 3 more minutes. Since the MgO inclusion disturbs the workability of concretes, the content of water was varied in order to keep flowability and rheological characteristics in the mixes. Table 2 reports variations in water content with MgO percentages in the samples. Time and intensity of vibration was the same for all samples for testing. After casting, all the samples were kept inside the mold for 24 hours and then were released from the mold and maintained for additional 24 hours at ambient temperature. Samples were then dried at 110 ° C for 24 hours. Three different temperatures were evaluated: 1000, 1200 and 1400 ° C with 5 hours of soaking time. Structural changes and spinel formation, after each thermal treatment, were investigated by X-ray diffraction (Phillips XL-30) using Co Ka radiation (35 kV and 25 mA) with continuous scans of 2q between 12º and 100º.

Polished cross-sections were prepared for microstructural evaluation. Samples were cut with a diamond saw, vacuum embedded in epoxy resin, surface ground and polished with diamond paste up to 0.25 mm. A scanning electron microscope (Philips XL-30) equipped with an energy-dispersive X-ray spectrometer (EDAX-DX4) was used to analyze samples at 1400ºC.

RESULTS AND DISCUSSION

Fig. 1 shows the diffraction patterns of samples containing 5.0 wt% MgO at 1000, 1200 and 1400 °C. The principal crystalline phases were identified as Al₂O₃ and MgO. The MgAl₂O₄ phase was detected at all temperatures; however, peak intensities increase steadily as the synthesis temperature increases. This fact indicates that for materials in commercial grain-size range, spinel formation could be achieved even at intermediate temperatures around 1000-1200 °C and that the amount of the spinel phase would regularly increase at higher temperature levels (1400 °C). On the other hand, peak intensities for MgO decrease as temperature increases suggesting that spinel phase development is strongly controlled by MgO content. At 1400 °C, peaks related to the hibonite phase (CaO•6Al₂O₃ - CA₆) occur, indicating that the CaO introduced from de calcium aluminate cement has been completely converted. XRD patterns for specimens with 5.5 and 6.0 wt% MgO are displayed in Figs. 2 and 3. These XRD data confirm the trend that is observed in the 5.0 wt% MgO samples: a minor amount of spinel formation is detected at 1000 °C and strong diffracted intensities are observed at 1200 and 1400 °C. The CA₆ phase is detected in all the samples only at 1400 °C.

Certain other observations are also pertinent for XRD analysis. The peaks of MgAl₂O₄ detected for all mixes (5.0, 5.5 and 6.0 wt% MgO) at 1400 °C were sharp and strong enough to confirm cristallinity. The spinel content, as distinguished from the (3 1 1) peak intensity of the XRD patterns, related to MgO content at temperatures under study is shown in Fig. 4. It can be observed that at 1000 and 1200 °C, spinel formation is practically not affected by MgO content while at 1400 °C, MgAl₂O₄ formation increases with higher MgO content. By comparing XRD phase evolution in the three castables (5.0, 5.5 and 6.0 wt% MgO), it can be concluded that non appreciable differences were observed in the processes of spinel formation and nucleation; however, crystal growth is clearly defined for samples with 6.0 wt% MgO according to peak intensities. Similar analysis conducted on (1 1 4) peak intensity for CA₆ formation suggests that its development could be enhanced with higher MgO content (see Fig. 5). Since it had been reported [16] that the bonding strength between CA₆ and the alumina and spinel grain surfaces may have a strong influence on the fired castable properties, then it is suggested that the increased hibonite formation could benefit the hot properties of the castable.

Microstructural features of samples at 1400 °C analyzed by SEM-EDS are presented in Figs. 6 to 8. Fig. 6 (a) shows homogeneous distribution between fine grains in castable matrix and coarse aggregates for material containing 5 wt% MgO. EDS analysis revealed that castable matrix is mainly composed of Al₂O₃ (A), MgO (M) and CaO (C) while coarse aggregates are Al₂O₃. A detail of castable matrix from the same sample (5 wt% MgO) is displayed in Fig. 6 (b), where needlelike crystals were found. This interweaved crystalline structure (bright zone) revealed that bonding involving CA₆ links was present in this composition. The Al:Ca weight percent ratio of 13.2:1 (determine by EDS) was indicative of the presence of CA₆. The dark area contains Aland Mg in a proportion higher (3:1) to the stoichiometric spinel composition (2.55:1), suggesting some excess of reactive alumina after spinel formation in the castable matrix. A typical overall microstructure of the castable containing 5.5 wt% MgO is shown in Fig. 7. Microstructural characteristics are very similar to those observed in the castable with 5.0 wt% MgO. It can be noticed the Al₂O₃ aggregates and the fine grains are uniformly distributed in the castable matrix. According to EDS analysis, fine grain matrix is mainly composed of Al, Mg and Ca confirming spinel and hibonite formation in it. The Al:Mg ratio (2.79:1) in the matrix was closer to that of the stoichiometric spinel composition confirming more spinel formation than in the castable with 5.0 wt% MgO. This fact was also determined by the XRD data as mentioned in the previous analysis. Microstructure of castable containing 6.0 wt% MgO revealed similar features than other two castables (5.0 and 5.5 wt%) as shown in Fig. 8 (a). However, a microstructural detail of spinel formation was observed at higher magnification (see Fig. 8 (b)). The matrix micrograph shows "diffusion paths" between Al₂O₃ and MgO grains for spinel formation. The Al:Mg ratio (2.60:1) in the spinel spot was consistent with stoichiometric spinel composition. Al:Mg ratio through the "diffusion paths" was changing, depending on the starting grain (Al₂O₃ or MgO) showing a way for the counter-diffusion of ions. In Fig. 8 (c) is shown the morphology of a particle from the castable matrix in which hibonite and spinel formations are combined. At the lower corner (bright zone), Al₂O₃, MgO and CaO were detected. Since the three components co-exist, no relation between weight percentages ratio were taked into account. However, in the center of the particle (dark zone), Al₂O₃ and MgO detected by EDS were indicative of the spinel formation.

CONCLUSIONS

On the basis of the microstructural investigation of an alumina-magnesia/self-forming spinel castable it can be concluded that:

(1) Spinel formation in the matrix of the castable could be achieved at temperatures around 1000 – 1200 ° C and the amount of spinel phase would steadily increase with higher temperatures.

(2) MgO percentages did not show appreciable differences in the processes of spinel formation and nucleation; however, crystal growth was clearly defined for castable with 6.0 wt% MgO.

(3) Hibonite bonding seems to be enhanced with higher MgO content.

(4) Self-forming spinel castables can be developed without the addition of synthetic spinel aggregates and they could be technically promissory as lining for severe environments.

ACKNOWLEDGEMENTS

This work was executed thanks to technical support from Laboratorio "E" and Centro de Ingeniería de Superficies of USB. The authors are grateful to U. Marquez for performing test analysis. We also acknowledge the contributions of J. Lira-Olivares, E. Greaves, A. Rivas, M. Velez and J. Smith for their discussions and comments on the manuscript.

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(Rec. 10/98 por J. A. Varela, Ac. 01/99)

Publication Dates

• Publication in this collection
  11 May 2000
• Date of issue
  June 1999

History

• Received
  Oct 1998
• Accepted
  Jan 1999