Evolution of an alumina-magnesia/self-forming spinel castable. Part II: physico-chemical and mechanical properties

Evolução de um refratário de espinélio auto-formado de alumina-magnésia Parte II: Propriedades físico-químicas e mecânicas

Evolution of an alumina-magnesia/self-forming spinel castable. Part II: physico-chemical and mechanical properties

Parte II: Propriedades físico-químicas e mecânicas)

D. Gutiérrez-Campos¹, J. I. Diaz¹, R. M. Rodriguez²

¹Universidad Simón Bolívar, Dpto. de Ciencia de los Materiales,

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

²Universidad Metropolitana, Dpto. de Química, Caracas, Venezuela

e-mail: [e-mail]

Abstract

This study was carried out in conjunction with the investigation, reported in Part I, on the microstructural characteristics of an alumina-spinel castable with several percentages of MgO content. Bulk density and cold crushing strength of samples were evaluated dried and at three fired states (1000, 1200, 1400 °C). Results indicate little influence of MgO additions on physico-chemical properties of the alumina-magnesia/self-forming spinel castable. Characteristics compared with those reported for conventional alumina-spinel castables did not show large difference in values. Therefore, the alumina-magnesia/self-forming spinel castable could be a possible material for substitution of the conventional alumina-spinel castable.

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

Resumo

Este estudo foi desenvolvido, juntamente com a pesquisa descrita na parte I (Cerâmica 45, 292-293 (1999) 93-98), sobre as característic as microestruturais de um refratário espinélio de alumina com várias porcentagens de teor de MgO. A densidade aparente e a resistência à moagem a frio das amostras foram avaliadas a seco e em três temperaturas (1000 ^oC, 1200 ^oC e 1400 ^oC). Os resultados mostram pouca influência das adições de MgO nas propriedades físico-químicas do refratário de espinélio auto-formado de alumina-magnésia. A comparação das características com as reportadas para refratários convencionais de espinélio de alumina não mostrou valores muito diferentes. Consequentemente, o refratário de espinélio auto-formado de alumina-magnésia pode ser um material para substituição do refratário convencional de espinélio de alumina.

Palavras-chave: Refratários, aluminato de magnésio, espinélio, alumina-magnésia, revestimento refratário, MgAl₂O₄, MgO-Al₂O₃

INTRODUCTION

Alumina-spinel castables have been successfully applied in steel ladles due to its outstanding behavior to severe process conditions [1-7] . This material has shown excellent corrosion resistance and structural spalling resistance. In general, refractories containing magnesium aluminate spinel (MgAl₂O₄) have undergone several evolutionary steps that include various qualities of synthetic spinel aggregates and significant bond improvements. Nevertheless, the techniques employed to produce this type of products can be summarized as follows:

1. including synthetic spinel aggregate to castable formulation

2. adding synthetic spinel grains and fine MgO powder to form in-situ spinel bond in the binder phase of the castable

3. adding fine MgO powder to promote a self-forming spinel product.

Thus, the development of alumina-spinel castables can be achieved with or without the addition of synthetic spinel grains. Considerable activity has occurred recently in the area of spinel containing castables and numerous papers have been published that show the potential of castables with synthetic spinel grains [2, 3, 8-11] . On the other hand, little information exists about the development of alumina-magnesia castables without any synthetic spinel grains [12, 13] . This latter instance represents a more economical solution and could produce castables with better high-temperature properties.

In this context, the microstructural features of an alumina-magnesia/self-forming spinel castable at different firing temperatures were presented in the first part [14] of this series. It was shown that for materials in commercial grain-size range, spinel formation could be achieved even at intermediate temperatures (1000 – 1200 °C) and that the amount of MgAl₂O₄ phase would regularly increase at higher temperature levels. It was also indicated that hibonite bonding, detected at 1400 °C, seemed to be enhanced with higher MgO content.

In this study, a series of castable samples with several MgO contents were tested and the results of their physico-chemical and mechanical properties are compared with those found in conventional alumina-spinel castables. Five ranges of MgO additions were performed to evaluate bulk density and cold crushing strength at different firing temperatures. Results are compared with properties of conventional alumina-spinel castables.

EXPERIMENTAL

Five different mixes with 5.0, 5.25, 5.50, 5.75 and 6.0 wt% of fine MgO were prepared. Casting procedure includes the following: dry mixing in a Hobart mixer for 5 minutes, water additions to the dry mix and wet mixing for 3 more minutes. Water content was adjusted to keep flowability and rheological characteristics in the samples. Table I displays chemical compositions and water requirements of castable mixes. The tempered castables were formed in oil-coated m etal molds. The specimens were allowed to air-set in the molds for 24 hours, removed from the molds and maintained for additional 24 hours at room temperature. After curing, the cubes were dried for 24 hours at 110 °C. When drying process was accomplished, the samples were heat-treated. Heat-treatments were performed in an electrically heated furnace for 5 hours at 1000, 1200 and 1400 °C. Five 51 by 51-mm cube specimens of castables were prepared for each condition tested.

Cold crushing strength was determined on a hydraulic testing machine (Forney – Mod. ST0021) using a loading rate of 0.05 in./min as stated in the A.S.T.M. C 133 standard [15] . Bulk density of the test cubes was calculated by dividing the cube weight by its volume as determined from its linear dimensions according to the A.S.T.M. C 134 standard [16] . Five determinations were averaged for each condition investigated.

RESULTS AND DISCUSSION

Bulk density values of castable compositions with different MgO percentages for the dried and three fired states are shown in Fig.1. The range of each set of data is lower than 0.1 indicating homogeneity of castable structure and consistency of testing procedure. Castable bulk densities in the dried state ranged from about 2.85 to 2.88 g/cm³. Values for samples fired in the intermediate temperature ranges (1000 – 1200 °C) tend to be slightly lower due to cement dehydration. Also, it had been reported that some expansion occurs during the synthesis of spinel from 1100 °C to 1500 °C due to spinel formation [17] . This fact could have also contributed to decrease bulk densities at those temperatures. Lower bulk densities observed at 1400 °C indicate that sintering mechanism was not enough to overcome expansion phenomenon during spinel formation. It can be observed that MgO content does practically not affect bulk density values at any temperature. However, highest values of bulk density were obtained at 6.0 wt% of MgO content and a slight tendency to increase bulk densities could also be observed at that MgO percentage for any studied temperature.

Strength data for various castable compositions at several temperatures are shown graphically in Fig. 2. Cold crushing strength values for each studied temperature have similar tendency at every MgO content. This fact suggests that strength is little affected by MgO content. It is noteworthy that cold crushing strength values for 1200 °C show an increase of about 16% compared to strength values for 1000 °C while data reported for 1400 °C displayed an increase of 67% compared to that found at 1200 °C. From this context, it could be assumed that increased values at 1400 °C had two contributions: sintering and spinel formation. On the other hand, spinel and hibonite formations have being confirmed at 1400 °C in a previous work [14] ; and they could possibly benefit the hot properties of the castable.

Figure 2:
Relation between Cold Crushing Strength and MgO content at several temperatures.

In order to compare physico-chemical properties of studied castable with those found in the literature, two conventional alumina-spinel materials containing synthetic spinel aggregates are reported in Tables II and ^III. Characteristics of a conventional alumina-spinel castable with 5.0 wt% of MgO are shown in Table II. It can be seen that at any tested temperature the differences between bulk densities for castable under investigation with similar MgO percentage (5.0 wt%) and those presented in Table II are less than 6.5%. Same comparison for cold crushing strength at high temperatures (1400 and 1500 °C) indicates a difference of about 16.5%. Under these considerations, it seems that the alumina-magnesia/self-forming spinel material could have a similar structural performance than castables with synthetic spinel aggregates. ^{Table III} presents the characteristics of a conventional alumina-spinel castable with 6.0 wt% of MgO. The difference in bulk density values among both materials (conventional versus alumina/self-forming) is in the order of 7% and about 6% for cold crushing strength data. Property comparisons indicate that the alumina-magnesia/self-forming spinel castable could be a possible material for substitution of the conventional alumina-spinel castable.

CONCLUSIONS

Analysis of physico-chemical properties for the alumina-magnesia/self-forming spinel castable indicate that MgO content seems to have little effect in bulk densities and cold crushing strength values. On the other hand, to confirm the influence of MgO additions on castable performance, hot properties investigations should be conducted. The characteristics reported for the studied material are not so far from those found in a conventional alumina-spinel castable.

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, for his comments on the manuscript.

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

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Publication Dates

• Publication in this collection
  31 Mar 2000
• Date of issue
  May 1999

History

• Accepted
  Jan 1999
• Received
  Oct 1998