PROTECTING REFRACTORIES AGAINST CORUNDUM GROWTH IN ALUMINUM TREATMENT FURNACES C. Allaire and M. Guermazi CIREP-CRNF, Dept. of Eng. Physics & Materials Engng., Ecole Polytechnique (CRIQ campus), 8475 Christophe Colomb Street Montreal, Quebec, H2M 2N9 Abstract In aluminum treatment furnaces, corundum growth may take place at the metal line which promotes the deterioration of their refractory sidewall. While conventional non-wetting additives may increase the resistance to corrosion of refractories below the metal line in such furnaces, their efficiency is significantly reduced at the high operating temperature conditions prevailing at their metal line. The purpose of this paper is to present a new additive which permit to protect refractories against the action of corundum growth in the above application.
Introduction It is well known that refractories used in aluminum holding and melting furnaces are subjected under severe corrosion conditions at and below the metal line. These refractories are degraded in service by the reaction with molten aluminum below the metal and by reaction with the corundum that grows along the walls above the metal line. Since the furnace conditions differ from one area to another, it is difficult to have a refractory material that resists to corrosion at the metal line as well as below it. The different areas of such furnaces are shown schematically in Fig. 1 [1]. The challenge facing the research on such refractory materials is to improve the rating of a refractory material in different areas of the aluminum holding and melting furnaces. The corrosion process therefore results in a decrease in mechanical properties of the refractory, contamination of the aluminum alloy, and the formation of dross or corundum, which in turns results in a loss of metal. All of these result in considerable loss of refractory material that has to be replaced more frequently, which increases the cost of operation. up to 1300 o C and more [5]. Few work has been made about sidewall build-up at the belleyband area of such furnaces. Davis [6] suggested that the molten metal penetrates the porous refractory, migrates upward above the metal line to the refractory surface, then is exposed to an oxidizing atmosphere where corundum develops. However, the inverse situation has also been suggested to be more favored. It was in fact observed, from laboratory simulation tests, that the corrosion of the refractory sidewall above the metal line results principally from the action of the corundum that grows from the metal line [7]. The primary objective of this study is to investigate the effects of wollastonite (CaSiO 3 ) as a potential new additive to the refractories to improve their corrosion resistance at the bellyband of aluminum treatment furnaces. As a first step, only one refractory material that has well-established corrosion properties has been tested. Bellyband Area 1 2 Maximum metal line level Usual metal line level Minimum metal line level 3 Figure 1: Zoning in an aluminum treatment furnace [1]. The focus of many researches on refractories is on improving the resistance to aluminum attack. Adding elements or compounds to the refractory has generally resulted in minimizing the reaction with aluminum. More specifically, some additives, called nonwetting agents, have been patented and their primary roles are to lower the wettability and/or the reactivity of refractories by and/or with molten aluminum and consequently to improve their corrosion resistance [2,3,4]. Among these additives are: aluminum borate, barium sulfate, calcium fluoride and aluminum fluoride. They usually improve the corrosion resistance of refractory castables below the metal line in aluminum treatment furnaces. At the bellyband area in the above furnaces (see Fig. 1), two particular conditions exist: (1) - the presence of a triple interface (molten metal/ solid refractory/ gaseous atmosphere) and (2) - the presence of a thermal gradient. It has been reported that the temperature below the metal line may be as low as 800 o C while on the sidewall above the metal line it may reach a temperature of Materials Experimental The composition of the as-received material is given in Table I. This refractory is a low cement castable denoted as CS after mixing with 5,5 to 6 wt. % water, for casting under vibration. The wollastonite used has a size less than 325 mesh. In the early stages of the investigation on the role of the additive, 7 wt. % wollastonite were added to the base refractory castable prior to casting. About 5,5 to 6 wt. % water was then added and the castable was vibated for 5 minutes (material CW). Later, the wollastonite was added in the matrix only, and in this case, the refractory castable was sieved and particles less than 170 mesh were removed. These fine particles were then replaced by wollastonite to make an addition of 7 wt. % of the total refractory castable weight (material CWF). In this manner, the total amount of water required for mixing the refractory powder had to be
increased to 6,6 wt. % to account for the increased water absorption by the wollastonite. After shaping, each above refractory material was cured at room temperature for 24 hours and then dried at 110 0 C for 48 hours prior to firing. After firing, the materials were submitted to X-ray analysis, as well as to density, porosity, modulus of rupture and air permeability measurements, using conventional techniques. Full immersion tests were used to evaluate the metal penetration resistance of the refractories. The tests were performed, and the materials were rated according to the Alcan standard immersion test procedure and rating criteria [8]. For each test, two 25 mm x 25 mm x 51 mm samples were immersed in 2 kg of a 5 % (w/w) Mg aluminum alloy maintained at 850 o C for four days in a silicon carbide-based crucible. The magnesium level was maintained at 5 % (w/w) by daily additions of this metal to the alloy. Each sample had three original and three machined surfaces. For the immersion tests, the refractory samples were tested in two conditions: 1) prefired at 1200 o C, and 2) pre-fired at 815 o C. Hole in the roof Heating elements Furnace wall Table I - Composition of the as-received refractory material. Crucible Metal line Compounds Content (wt. %) Alumina Al 2 O 3 59,6 Silica SiO 2 34,0 Iron Oxide Fe 2 O 3 0,8 Lime CaO 3,8 Magnesia - MgO 0,2 Titania TiO 2 1,3 Alkalis Na 2 O + K 2 O 0,3 Non-Wetting Agents Yes Corrosion Tests Three types of tests were used to evaluate the corrosion resistance of the refractory: the CIREP Bellyband Corrosion Test, the Alcan Standard Immersion Test, and a partial immersion finger test. The CIREP Bellyband Corrosion Test was previously designed to evaluate the resistance of refractories to corundum attack at the metal line of aluminum treatment furnaces. The test procedure and the sample preparation method are explained elsewhere [7]. The crucible samples are cylinders with the following dimensions: diameter 7,62 cm, height 7,62 cm. The cavity consists of a hole (3,81 cm diameter x 4,8 cm deep) machined with a diamond tool, thus leaving a 1.9 cm wall minimum thickness. About 90 g of a 5 wt. % Mg alloy is used for each test. This quantity fills two thirds of the cavity. The test is carried out for 4 days. The set-up configuration is shown in Fig. 2. It uses a bottom loading type furnace permitting to achieve a temperature drop from about 1100 to 900 o C across the air (atmosphere) interface and the molten aluminum alloy. The latter is contained in the refractory crucible sample which is pre-fired at 1200 o C. The refractory sample is then cut longitudinally and two ratings are given according to the criteria described in reference [1]. The first rating is given based on the observation of the portion of the sample that is above the metal line. The second one corresponds to the observation of the sample below the metal line. Insulating fiber board support A-16 Alumina powder Stainless steel pan Figure 2: Experimental set-up using a bottom-loading type of furnace [7]. Complementary, corrosion tests were pursued under partial immersion conditions. For these tests, the dimensions of the finger type samples used were 2,54 cm x 2,54 cm x 22,86 cm. The tests were performed using E406 Salamander clay working graphite crucibles from Morganite Thermal Ceramics Ltd., coated with a micawash coating. In these crucibles, 700 g of a 5 % (w/w) Mg aluminum alloy were required to immerse a sample 10 cm deep. At the bottom of the crucibles, a graphite support was placed to hold the sample in a centered vertical position. For these tests, the samples were pre-fired at 1200 o C and tested at 1150 o C for 4 days, in air. Selection Criteria Based on their resistance to corrosion, four criteria have been proposed to help select the refractory materials best suited for use in each zone of aluminum holding or melting furnaces. [1]. These zones are shown in Fig. 1. The refractories of zone 1 are the ones usually subjected to corrosion by the corundum attack mechanism which is also operative above the metal line in the bellyband corrosion test. Since refractories of zone 2 are most of the time in contact with molten metal, a good rating in the immersion test is required. In addition, since the top part of these refractories is exposed to the atmosphere, the effect of oxygen cannot be neglected. The bellyband test rating below the metal line takes this effect into account since the oxygen is brought from the outside of the sample by air diffusion through its thickness. For evaluating unshaped refractories to be used in zone 2 and above, a pre-firing temperature of 1200 o C represents more adequately the firing temperature, in practice. For the refractories of zone 3, a good rating in the immersion test with, in the case of unshaped products, a pre-firing temperature of 815 o C, is sufficient because these refractories are much better protected from the high temperature and the oxidizing atmosphere by the molten metal. Results and Discussion Table II shows the comparative characteristics of the tested materials.
Table II - Characteristics of the different tested materials. MATERIAL CS CW CWF RESULTS FROM THE BELLYBAND TEST Rating above metal line 5 2 2 Rating below metal line 2 2 2 RESULTS FROM THE FULL IMMERSION TEST Rating @ 1200 o C 4 4 4 Penetration depth (mm) 4 3 2 Rating @ 815 o C 4 4 4 Penetration depth (mm) 4 3 2 Apparent Porosity (%) @ 1200 o C 17,0 ± 0,1 18,2 ± 1,1 16,7 ± 0,6 Cold modulus of rupture (MPa) @ 1200 o C 10,44 ± 1,17 11,77 ± 0,50 12,56 ± 0,48 Permeability (centidarcy) @ 1200 o C 0,56 ± 0,17 0,46± 0,08 0,26 ± 0,07 Corrosion Tests In the full immersion test, material CS showed an average metal penetration of about 4 mm and has a moderate-to-poor (rating = 4) resistance to corrosion when pre-fired at 815 o C and 1200 o C, as shown in Fig. 3 (a) and (b), respectively. In the bellyband test, it showed poor (rating = 5) resistance above the metal line and good-to-moderate (rating = 2) resistance below the metal line, as shown in Fig. 4. Apparently for this material, the non-wetting agent was not so effective. Material CW, in the full immersion test, showed an average metal penetration of about 3 mm and has a moderate-to-poor (rating = 4) resistance to corrosion when pre-fired at 815 o C and 1200 o C, as shown in Fig. 5 (a) and (b), respectively. However, comparing to material CS, it showed an improved resistance to corrosion at both pre-firing temperatures. In the bellyband test, it showed good-to-moderate (rating = 2) resistance both above and below the metal line where minor metal penetration (less than 1 mm) was detected, as shown in Fig. 6. (a) (b) Figure 3: Cross-sections of sample after carrying out the Alcan Immersion Tests; (a) - CS pre-fired at 815 o C, (b) - CS pre-fired at 1200 o C.
In the full immersion test, material CWF showed moderate-topoor (rating = 4) resistance to corrosion when pre-fired at 815 o C and 1200 o C, as shown in Fig. 7 (a) and (b), respectively. However this material showed the lowest extent of metal penetration after both pre-firing temperature (about 2 mm in average). In the bellyband test, it showed good-to-moderate (rating = 2) resistance both above and below the metal line with almost no metal penetration observed, as shown in Fig. 8. For both materials CW and CWF, it appears that the incorporation of the wollastonite in the base refractory material (CS) resulted in an improved corrosion resistance, both in the bellyband test and in the full immersion test. This improvement of the corrosion resistance due to the addition of wollastonite was also observed during the finger test. Fig. 9 (a) shows a cross-section of the CS sample after completion of such test. Below the metal line, little penetration of the metal was observed at the base of the sample, Metal line Figure 4 - Crucible of CS sample showing severe signs of corundum attack above the metal line and no sign of metal penetration below the metal line. (a) (b) Figure 5: Cross-sections of sample after carrying out the Alcan Immersion Tests; (a) - CW pre-fired at 815 o C, (b) - CW pre-fired at 1200 o C.
Metal line Figure 6: Crucible of CW sample showing no sign of corundum attack above the metal line and no sign of metal penetration below the metal line (The arrow indicates the presence of a thin interfacial layer between the metal and the refractory sample). (a) (b) Figure 7: Cross-sections of sample after carrying out the Alcan Immersion Tests; (a) - CWF pre-fired at 815 o C, and (b) - CWF pre-fired at 1200 o C.
Metal line Figure 8: Crucible of CWF sample showing no sign of corundum attack above the metal line and no sign of metal penetration below the metal line. which decreases moving upwards (towards the metal line). However, the sample was not resistant to corundum attack above the metal line, and has largely been penetrated in that zone. On the other hand, when wollastonite was added (material CW), it made the refractory more resistant both above the metal line (no reaction with the oxides) and below it (no metal penetration), as shown in Figure 9 (b). Wollastonite was used as an additive coming from the idea that calcium silicate-based refractory powder is though to act as a barrier against the oxidation of aluminum melts [9]. During high temperature oxidation of aluminum, calcium is believed to be liberated from the wollastonite into the aluminum liquid, and then to be deposited on the metal bath/crucible interface. This deposit would act as a surface protective (i.e., protects the surface of aluminum from being exposed to oxygen and therefore preventing the formation of alumina). Thus, calcium would act in an opposite manner as magnesium in aluminum alloys. While magnesium is required for the formation of alumina during the directed oxidation of molten aluminum alloys to break the protective oxide layer [10], calcium would maintain this layer and prevents the aluminum surface from being oxidized. After carrying out the bellyband tests, there was a white continuous thin layer that was observed between the aluminum alloy and the crucible inner surface, which supports the above idea (see Fig. 6). Materials characterization: Table III shows the results of X-ray analysis made on the three different materials: CS, CW and CWF. The first important observation is the decrease in the amount of amorphous phase present in the refractory when wollastonite was added. It was previously established that the presence of the amorphous phase in the refractories degrades the corrosion resistance of the latter since the glassy phase is more vulnerable to aluminum attack [11]. The second important point is the increase in the amount of anorthite (CaAl 2 Si 2 O 8 ) present in the refractories when wollastonite was added to the material. Afshar and Allaire [12] have investigated the effect of BaSO 4 on the corrosion of aluminosilicate refractories. They concluded that barium compounds react with SiO 2 and Al 2 O 3 to form BaAl 2 Si 2 O 8 and that this phase, rather than BaSO 4, is the one that provides the protection against corrosion. If the same analogy is used in the case of wollastonite, it is the anorthite (CaAl 2 Si 2 O 8 ) that would provide the enhanced protection against corrosion. However, material CWF pre-fired at 815 o C was found to provide a better corrosion resistance in the immersion test than the one pre-fired at 1200 o C. X-ray analysis on these two materials revealed that wollastonite is present after a pre-firing at 815 o C (4,5 wt.%) and was not detected after a prefiring at 1200 o C. Furthermore, the amount of anorthite phase is lower (4,01 wt.%) in the material pre-fired at 815 o C than the one pre-fired at 1200 o C (14,42 wt.%), as shown in Table IV.
corundum attack metal line (a) (b) Figure 9 - Cross-sections of (a) - CS showing signs of corundum attack above the metal line and (b) - CW showing resistance to corrosion above and below the metal line, after carrying out the finger test. Table III - Results of X-ray analysis of the present materials prior to corrosion. Minerals CS @ 1200 o C CW @ 1200 o C CWF @ 1200 o C % Crystallinity 64 68 74 Al 2 O 3 (Corundum) 13,77 11., 3 11,33 Al 2 (SiO 4 )O 43,09 43,73 37,58 Al 6 Si 2 O 13 19,08 14,05 18,99 CaAl 2 Si 2 O 8 9,38 15,12 14,42 SiO 2 (Cristobalite) 10,25 10,81 13,23 TiO 2 2,87 4,03 3,26 SiO 2 (Quartz) 1,25 0,73 1,04 NaAl 11 O 17 0,33 0,50 0,14 CaSiO 3 (Wollastonite) - - - Others - - -
Table IV - Phase analysis results showing the effect of pre-firing temperature on the CWF material. Minerals CWF @ 1200 o C CWF @ 815 o C % Crystallinity 74 63 Al 2 O 3 (Corundum) 11,33 10,24 Al 2 (SiO 4 )O 37,58 33,91 Al 6 Si 2 O 13 18,99 21,80 CaAl 2 Si 2 O 8 (Anorthite) 14,42 4,01 SiO 2 (Cristobalite) 13,23 0,35 TiO 2 3,26 3,18 SiO 2 (Quartz) 1,04 1,.85 NaAl 11 O 17 0,14 - CaSiO 3 (Wollastonite) - 4,50 Others - 4,15 It can be concluded that the presence of wollastonite mineral in these refractory materials is more responsible for their improved corrosion resistance. As shown in table II, additions of wollastonite to the refractory material did not result in significant changes in either apparent porosity or density. However, there was an increase in the mechanical properties as reflected by an increase of the room temperature modulus of rupture values. This should confer to the material a higher resistance to mechanical abuse when used in aluminum treatment furnaces. There is also a noticeable decrease in the gas permeability values upon additions of wollastonite (see table II). This means that the material becomes less permeable to molten metal and oxygen penetration, and thus should become more resistant to corrosion in aluminum treatment furnaces, both at above and below the metal line. Conclusions The present work revealed a new additive material, wollastonite, which can be incorporated into refractory castables to impart them a higher resistance to corundum attack at the bellyband of aluminum treatment furnaces. These improved refractories may also be useful below that area in such furnaces since their resistance to molten metal penetration is also increased. Among the factors responsible for such improvements is the reduced material permeability in the presence of wollastonite. Another beneficial effect of that additive is that it promotes an increase of the mechanical properties of the material, which thus becomes more resistant to mechanical abuse in the above mentioned application. Acknowledgements The authors gratefully acknowledge Harbison-Walker Refractory Company for its financial support during the realization of this work. References 1. S. Quesnel, S. Afshar and C. Allaire, Criteria for Choosing Refractories in Aluminum Holding and Melting Furnaces, Light Metals 1998, Edited by Barry Welch, The Minerals, Metals & Materials Society, (1998), 1391-1402. 2. F. T. Felice, Aluminum Resistance Refractory Compositions, US Patent No. 4,522,926, June 11, 1985. 3. R. W. Tallaey, R.A. Henrichen and W. T. Bakker,, Refractory for Aluminum Melting Furnaces, US Patent No. 4,126,474, November 21, 1978. 4. A. D. Porterfield, Aluminum Resistance Refractory Compositions, US Patent No. 4,806,509, February 21, 1989. 5. A. M. Wynn, Testing of Castable Refractories for Resistance to Molten Aluminum Alloys, Br. Ceram. Trans. J., 91 (5), (1992), 153-158. 6. E. G. Davis, Evaluation of Refractories for Aluminum Recycling Furnaces, Bureau of Mines (Paper Presented at ACS rev. Div. Meeting, October 10, 1987). 7. S. Quesnel, S. Afshar and C. Allaire, Corrosion of Refractories at the Bellyband of Aluminum Melting and Holding Furnaces, Light Metals 1996, Edited by Wayne Hale, The Minerals, Metals & Materials Society, (1996), 661-667. 8. C. Allaire, Refractories for the Lining of Holding and Melting Furnaces, Proceeding of the International Symposium on Advances in Production and Fabrication of Light Metals and Metals Matrix Composites, (1992), 1-12. 9. S. Lee and D. K. Kim, The Effect of Oxide Additives in Filler Materials During Directed Melt Oxidation Process, Ceramc Eng. Sci. Proc., 11 (7-8), (1990), 795-805.
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