Furnaces: Improvement of Low Cement Castables by Non-Wetting Additives Saied AFSHAR and Claude ALLAIRE CIREP-CRNF Department of Engineering Physics & Materials Engineering, Ecole Polytechnique of Montreal (CRIQ campus), 8475 Christophe Colomb Street, Montreal Quebec H2M 2N9 Canada Abstract: Aluminosilicate castables are widely used in the aluminum transformation furnaces. Commercial additives such as BaSO 4 , CaF 2 and AlF 3 are generally added in such castables to prevent the chemical reactions occurring between molten aluminum and the furnace refractory lining. This work presents and analyses the effect of various amounts of the above additives as well as the influence of pre-firing temperatures on the corrosion behavior of an aluminosilicate low cement castable matrix in contact with liquid Al – 5% Mg alloy. The results of the present study may help choose the type and the appropriate quantity of the non- wetting agents according to the operating temperature of the aluminum furnaces.
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Furnaces: Improvement of Low Cement Castables by Non-WettingAdditives
Saied AFSHAR and Claude ALLAIRE
CIREP-CRNFDepartment of Engineering Physics & Materials Engineering,
Ecole Polytechnique of Montreal (CRIQ campus),8475 Christophe Colomb Street,
Montreal Quebec H2M 2N9Canada
Abstract:
Aluminosilicate castables are widely used in the aluminum transformation furnaces.
Commercial additives such as BaSO4, CaF2 and AlF 3 are generally added in such castables to
prevent the chemical reactions occurring between molten aluminum and the furnace refractory
lining. This work presents and analyses the effect of various amounts of the above additives
as well as the influence of pre-firing temperatures on the corrosion behavior of an
aluminosilicate low cement castable matrix in contact with liquid Al – 5% Mg alloy. The
results of the present study may help choose the type and the appropriate quantity of the non-
wetting agents according to the operating temperature of the aluminum furnaces.
Introduction:
Some specific additives, so called “non-wetting” agents, are generally employed by refractory
producers in order to enhance the corrosion resistance of aluminosilicate castables. Among
these additives, BaSO 4, CaF 2 and AlF 3 are the most common ones. Although, more than a
decade, the non-wetting agents are used in the aluminosilicate refractories, their beneficial
effect is not yet totally approved.
Recently, a particular attempt has been made to characterize the proper role of barium sulfate
in the protection mechanism of refractories [1-3]. It has been pointed out that barite, by
reacting with alumina and silica among the fine particles of the refractories, forms hexa-
celsian and or celsian in the firing temperature ranging between 900 to 1200 oC [1,2,4]. Such
transformations reduce the amount of free silica within the matrix and should consequently
improve the corrosion resistance of the refractory. However, the protective effect of barium
sulfate appeared to be significantly reduced for the cases that firing temperatures exceeded
1050 oC [1,2].
Compared to barium sulfate, the effect of AlF 3 and CaF 2 has been less studied. The
experiences carried out on the aluminosilicate ceramics, having a mullite-like composition,
have not been conclusive to show the efficiency of the fluoride additives [3,5]. However,
based on the mineralogical analysis results reported, it has been postulated that the fluoride
additives act as mineralizers, favoring the mullite formation during high temperature firing of
aluminosilicate materials [6].
The purpose of this study was to analyze and to compare the efficiency of the three mentioned
commercial additives used to protect a low cement castable against aluminum attack. In order
to establish their appropriate quantity, various amounts of additives as well as some
combinations of them have been used. The effect of firing temperature, which limits the
performance of such additives, has been also studied.
Experimental procedures:
Materials: An aluminosilicate low cement castable (see Table I for chemical composition),
which did not contain originally any non-wetting additive, was used in this study. Since the
beneficial effect of non-wetting agents concerns mainly the fine particles of refractories
[1,2,7,8], the aggregates of the above castable, having a size greater than 500 µm, were
removed from the refractory using a sieve. The remaining fine powders, referred here to as
“refractory matrix”, were then used as the basic testing material for sample preparation. The
weight ratio of refractory matrix to the castable was about 50%. The chemical composition of
the refractory matrix is also given in Table I.
Table I: Chemical composition of materials in weight percent
The results of the study on the effects of the non-wetting agents indicated that:
- At 7 wt% content, the additives improved the corrosion resistance of the fine portion of
the aluminiosilicate castable.
- Firing temperature influenced differently the protective effect of the additives: with AlF 3
and BaSO 4, increasing firing temperature reduced the corrosion resistance, while with
CaF2, an improvement was observed.
- The beneficial effect of BaSO 4 was attributed to the formation of some barium
aluminosilcate compounds, whose quantity appeared to be proportional to barite content.
- It seems that the improvement induced by the addition of AlF 3 is due to its action in
converting silica to mullite via fluorotopaz formation.
- Adding CaF 2 favored the anorthite formation. Parallel to corrosion resistance
improvement, the amount of anorthite appeared to be increased with firing temperature.
- In terms of corrosion resistance improvement, no practical interest was found when the
mixes of the additives were used.
From the present work, it can be postulated that the efficiency of non-wetting additives, to
protect the castables, is in their potential role in converting silica to some aluminosilcates
based crystalline phases that are more resistant to aluminum attack.
References:
1 - Afshar, S. and Allaire, C., “The Corrosion of Refractories by Molten Aluminum”, JOM,pp. 23-27, May 1996.
2 – Afshar, S., Allaire, C., Quesnel, S. and Allahverdi, M., “Corrosion des RéfractairesAluminosiliceux au Contact de l’Aluminium Liquide”, Centre Québecois de Recherche et deDéveloppement de l’Aluminium, le Journal Al-13, Vol. 2, No. 2, pp. 21-36, 1996.
3 – Allahverdi, M., Afshar, S. and Allaire, C., “Corrosion Resistance of AluminosilicateCeramics to Molten Al-5% Mg Alloy”, in Proceedings of International Symposium onAdvances in Refractories for Metallurgical Industries II, 35 th Annual Meeting of Metallurgistsof CIM, Montreal (Canada), pp. 295-303, 1996.
4 – Schmutzler, H.J. and Sandhage, K.H., “Transformation of Ba-Al-Si Precursors to Celsianby High-Temperature Oxidation and Annealing”, Met. Mater. Trans. B, 26B, pp. 135-148,1995.
5 - Allahverdi, M., Afshar, S. and Allaire, C., “Additives and the Corrosion Resistance ofAluminosilicate Refractories in Molten Al-5% Mg”, JOM, pp. 30-34, February 1998.
6 - Allahverdi, M., Allaire, C., and Afshar, S., “Effect of BaSO 4, CaF 2 and AlF 3 as well asNa2O on the Aluminosilicates Having a Mullite-like Composition”, Canadian Ceram. Soc.,Vol. 66, No. 3, pp. 223-230, August 1997.
7 – Richter, T., Vezza, T., Allaire, C. and Afshar, S., “Castable with Improved CorrosionResistance Against Aluminum”, Eurogress Aachen (Germany), pp. 86-90, 1998.
8 - Afshar, S. and Allaire, C., “The Corrosion of Refractory Aggregates by MoltenAluminum”, JOM, pp. 43-46, May 2000.
9 – McDonald, H.A., Dore, J.E. and Peterson, W.S., “How Molten Aluminum Affects PlasticRefractories”, J. Metals, pp. 35-37, 1958.
10 – Isselbächer, E. et al., “ Comportement des Matériaux Réfractaires vis-à-vis des Bainsd’Aluminium du Point de Vue de la Formation d’Inclusions Dures”, Aluminium (6), pp. 375-379, June 1971.
11 – Allaire, C., “Refractories for the Lining of Holding and Melting Furnaces”, Advanced inProduction and Fabrication of Light Metals and Metal Matrix Composites, ed. M.M.Avedesian et al. (Montreal) Canada: CIM, pp. 163-174, 1992.
12 – Moyer, J.R. and Rudolf P.R., “ Stoichiometry of Fluorotopaz and Mullite Made fromFluorotopaz”, J. Am. Ceram. Soc., 77 (4), pp. 1087-1089, 1994.
13 – Moyer, J.R., “Phase Diagram for the SiF 4 Join in the System SiO 2-Al2O3-SiF4”, J. Am.Ceram. Soc., 79 (11), pp. 2965-2968, 1996.
14 - Moyer, J.R., “Phase Diagram for Mullite-SiF 4”, J. Am. Ceram. Soc., 78 (12), pp. 3253-3258, 1995.
15 – Bale, C.w., Pelton, A.D. and Thompson, W.T., “F*A*C*T* 2.1 – User Manual”, EcolePolytechnique of Montreal / Royal Military College, Canada, ( http://www.crct.polymtl.ca),July 1996.