REFRACTORIES FOR MOLTEN ALUMINUM CONFINEMENT ABSTRACT

Transcription

1 REFRACTORIES FOR MOLTEN ALUMINUM CONFINEMENT Claude Allaire CIREP-CRNF Department of Engineering Physics & Materials Engineering Ecole Polytechnique of Montreal Montreal, Quebec, H2M 2N9 ABSTRACT Refractories for molten aluminum confinement are subjected in service to thermomechanical abuse and to corrosion, which lead to metal contamination and lining repair or replacement. The mechanisms of deterioration of refractories, as well as the refractory technology evolution with regard to molten aluminum confinement are presented.

2 INTRODUCTION Today, twenty million tons of aluminum is produced world wide, as compared to thirteen million ten years ago. The Canadian aluminum production has more than doubled during that same period to reach 11 % of the actual world wide production. The minimization of the aluminum production costs and the maximization of the quality of the metal produced are at the origin of such an increase. To pursue with such improvements, the primary aluminum industry is now looking for better refractories permitting them to get more metal units through their castings centers as well as to avoid having refractory particles (or inclusions) in their ingots. Among the processes which need more considerations are the melting and holding furnaces, the troughs and the filtering units whose refractory lining is recognized, at the present time, to impart most of the ingot contamination. In such processes, refractories are subjected to thermomechanical abuse and to corrosion. Both, but mainly corrosion, promote the contamination of the molten metal in service. The mechanisms of deterioration of refractories, as well as the refractory technology evolution with regard to molten aluminum confinement are presented in the following sections. 1. Refractory degradation and metal contamination associated with molten aluminum confinement 1.1. Thermomechanical abuse Thermomechanical abuse in molten aluminum confinement occurs by thermal shocks, mechanical impacts and erosion. Increasing the strength (σ R ) of refractories is generally required to impart them with high erosion resistance 1,3. Moreover, it was shown 4,5 that both the mechanical impacts and thermal shock resistance of aluminosilicate castables fired at 1200 o C and less are also increased when their strength is higher. More precisely, the optimization of the resistance to thermal shocks and mechanical impacts of such materials, in the above firing conditions, requires the maximization of both their strength and their resistance to long crack propagation (R st ) 4,5 (see Fig. 1 and 2). The parameter R st is given by the following equation: R st = (γ wof /Eα 2 ) 1/2, γ wof = Work of fracture (or fracture energy) E = Young's modulus α = Coefficient of thermal expansion However, it is the short crack propagation resistance (R ) of these same materials that should be maximized to promote them optimal thermal shock resistance, when their firing temperature exceeds 1200 o C 5 (see Fig. 3). The parameter R'''' is given by the following equation: R'''' = Eγ wof /(σ R 2 (1-ν)) ν = Poisson coefficient In these higher firing conditions, castables with higher thermal shock resistance should theoretically, unlike the previous case, have a lower mechanical impact resistance. The latter is in fact proportional to the material's strength irrespectively of the firing temperature while the former, according to the R parameter, varies inversely with that property.

3 1.2. Corrosion and metal contamination Metal contamination in molten aluminum confinement is mostly related to the following characteristics of the refractories: Their chemical and mineralogical compositions The type of binder (hydraulic, chemical or ceramic) Their permeability These characteristics have a great influence on the susceptibility of the refractories to be corroded in service. Such corrosion promotes the formation of inclusions in the molten metal, which can also originate from its direct oxidation at the metal line. An increase of the refractory's permeability favors its corrosion by the molten metal and permits gas migration such as oxygen and water vapor. The latter contributes to the contamination of the molten metal by hydrogen. Finally, alkalis that are often present in the metal charge may favor the corrosion of the refractories. Inversely, the corrosion of these materials may release alkalis in the molten metal Origin of inclusions The products of oxidation of molten aluminum alloys are a function of the alloy composition. For example, at operating conditions of aluminum melting furnaces, the presence of 0,3 to 18 wt. % magnesium favors the formation of spinel (MgAl 2 O 4 ). At less than about 0,3 wt. % Mg, alumina (Al 2 O 3 ) is the most stable oxide in these conditions, while at more than 18 wt. %, it is magnesia (MgO). These oxides may result from the action of oxygen gas on the molten metal, which leads to the formation of a skim at the metal line. This direct oxidation mechanism may also intervene at the refractory/metal interface due to the oxygen gas migration through the refractory porosity. These oxides may also result from the reduction of the silica contained in the refractory (or any other oxide less stable than alumina) by the action of the molten metal. Fig. 4 shows three different zones of the refractory lining in aluminum treatment furnaces whose operating conditions are the following 6 : Zones Firing temperature Operating Oxygen partial of the refractory temperature pressure 1 > 1000 O C > 1000 O C High 2 > 1000 O C < 1000 O C Moderate 3 < 1000 O C < 1000 O C Low In each of these zones the corrosion mechanism of the refractory is the following 6 :

4 ZONES MECHANISMS 1 Direct oxidation 2 Direct oxidation and Reduction of silica 3 Reduction of silica The reactions behind each above mechanisms are the following: Direct oxidation 2 Al + 3/2 O 2 Al 2 O 3 (1) Reduction of silica 2 Al + 3/2 SiO 2 3/2 Si + Al 2 O 3 (2) Direct oxidation The direct oxidation mechanism, known as DIMOX TM 7,8, explains the formation of alumina from alloys such as Al- Mg-Si, by the migration of the molten metal through a discontinuous and growing layer of alumina toward the oxygen gas source. The principal steps associated to this mechanism are the following (see Fig. 5): 1) Formation at the surface of the alloy of a primary porous spinel layer and an overlying magnesia layer; 2) Migration of the alloy through the primary spinel layer and reduction of its magnesium content; 3) Decomposition of the magnesia layer in contact with the molten metal impoverished in magnesium; 4) Diffusion of the oxygen toward the primary spinel layer leading first to the nucleation and then to the growth of discontinuous alumina crystals. This discontinuity of the crystals is favored by the presence of the magnesium that lowers the molten metal surface tension and favors their wetting. The release oxygen also contributes to the formation of a dense secondary spinel layer underneath the magnesia layer; 5) Slow solid diffusion of the magnesium through the magnesia layer favoring the accumulation of magnesium gas underneath this layer; 6) Cracking of the magnesia layer when the magnesium gas reaches a sufficiently high partial pressure. As shown in Fig. 5, the oriented alumina crystals that form during direct metal oxidation create channels though which penetrate the molten metal by capillarity. When such a composite grows into contact with refractories, such as in zone 1 of aluminum treatment furnaces (see Fig. 4), the molten aluminum contained in these channels can promote the corrosion of the refractory. When this happens, the composite sticks on the refractory lining of the furnace and cannot be removed without damaging it.

5 Reduction of oxides The reduction of oxides involves the refractories in a direct role. Aluminum tends to be oxidized by less stable oxides such as silica. Magnesium does as well which leads to the formation of spinel. Such a reduction mechanism is favored in the more reactive regions of the refractory, in particular those containing vitreous phases surrounding more stable crystalline phases. In such a case, the latter may be released to form inclusions in the metal Effect of alkalis The alkalis may originate in the refractories, which limits their corrosion resistance and thus may contribute to the molten metal contamination. The alkalis may also originate in the metal charge which favors the corrosion of the refractories Alkalis contained in the refractory In general, refractories contain less than 2 wt. % alkalis (expressed as Na 2 O). They may be present as amorphous phases, such as sodium silicate, or as crystalline compounds, such as β-alumina (NaAl 11 O 17 ). Alkalinosilicious amorphous phases are highly susceptible to reduction by aluminum, which favors the liberation of alkalis into the molten metal 9. β-alumina is also reactive with aluminum, as shown by the following reaction 10 : 6 NaAl 11 O Al 6 Na + 34 Al 2 O 3 (3) The presence of AlF 3 in the refractory, as a non-wetting agent, may also contribute to the release of alkalis into the molten metal, according to the following reaction 10 : 6 NaAl 11 O AlF 3 6 NaF + 34 Al 2 O 3 (4) NaF + Al 3 Na + AlF 3 (5) Alkalis contained in the metal charge The charge of molten aluminum may contain a certain amount of alkalis which originate either from the electrolytic cells, during metal transfer, or from the added flux used for metal refining or cleaning, or for other purposes. An example of conversion of such salt by the action of the molten metal is given by reaction (5). As shown by the following reactions, sodium in the molten metal can be oxidized during operation of aluminum confinement devices, either from the oxidizing atmosphere or from the refractories that contain reducible oxides such as silica 10 : 4 Na + ½ O 2 Na 2 O (6) 4 Na + SiO 2 2 Na 2 O + Si (7) In the presence of Na 2 O, the protective alumina layer which may be formed at the metal/refractory interface in molten aluminum confinement devices, such as zone 3 in melting furnaces (see Fig. 4), tends to be converted into sodium aluminate according to the following reaction 10 :

6 Al 2 O 3 (protective layer) + Na 2 O 2 NaAlO 2 (8) Because of the difference in density between alumina (d = 3,96 g/cm 3 ) and sodium aluminate (d = 2,69 g/cm 3 ), such conversion promotes the corrosion of the refractory due to the resultant cracking of the protective alumina layer which allows penetration of the molten metal. Another effect of alkalis on the corundum growth process has been shown previously in laboratory tests using the CIREP Bellyband Test set-up, which simulates the operating conditions in aluminum treatment furnaces 6, 11 (see Fig. 6). It was shown that the kinetics of corundum growth above the metal line in such furnaces is about 25 times less when cryolite is added on top of the molten metal. As shown in Fig. 7, such a difference in growth kinetics leads to different corundum morphologies (corundum ''mushrooms'' or ''large balls''). With cryolite, the corundum that forms has a coarser microstructure that involves a higher metal content with thicker metal channels. These results are opposite those obtained by Xiao and Bedy 12 who showed an increase in corundum growth from aluminum alloys in the presence of NaOH. The authors suggested that NaOH act as a dopant which favors the wetting of the alumina layer by the molten aluminum. More work is required to understand the different effects of alkalis on corundum growth, which is a major preoccupation for the aluminum producers with regard to their melting and holding furnaces 13, Source of hydrogen Hydrogen in molten aluminum alloys originates from the following reaction 15 : 2 Al + 3 H 2 O (g) Al 2 O H Al (9) The water vapor in this reaction may come from the combustion gas, the surrounding air, the metal charge, the flux or the refractories. The latter are often hygroscopic and some, such as refractory castables, include a hydraulic binder. In this case, the mixing water content includes 16 : The amount of water required to hydrate the cement phases; The amount of water required to fill the interstices between the aggregates; The amount of water required to create a thin water film around each particle. With the exception of the hydration water, the mixing water of a castable is eliminated during drying. The hydration water is eliminated during firing. A minimum temperature of 550 o C is required to complete the dehydration of aluminosilicate castables 17. During the first firing of a castable in an aluminum treatment furnace, the water vapor produced migrates toward the furnace shell and condenses under the 100 o C isotherm (see Fig. 8). During subsequent furnace shutdowns, the migration of water toward the hot face is less favored then toward the floor, due the higher capillary action of the insulating ceramic materials (as compared to castables) to which should be added the action of gravity. However, during operation, the water under the 100 o C isotherm can migrate, in the vapor state, toward the hot face where there is a negative pressure promoted by reaction (9). Each part of a castable whose maximum temperature has reached less than 550 o C during its first firing should not be exposed to a higher temperature during the subsequent operating periods of the furnace. One of the factors that may contribute to the displacement of the isotherm in a furnace during operation is the infiltration of molten metal into the hot face region of the castable. In such a case, the 550 o C isotherm tends to be closer to the cold face of the lining due to the increase in thermal conductivity in the penetrated zone. To minimize the contamination of molten aluminum by hydrogen, the use of refractories of low permeability is required.

7 2. Refractory technology evolution associated with molten aluminum confinement Because of their various advantages as compared to bricks, non-shaped refractories have, in the last two decays, increasingly been used in the metallurgical industry, particularly in the aluminum cast-houses where aluminosilicate castables are today widely used as the major substitute for bricks. Such materials are currently made of a mixture of aluminosilicate aggregate and binder, and include the presence of fillers and additives. The aggregates in such castables are most often made of mixtures of grog, mullite, bauxite and tabular alumina, depending on the required alumina content of the material. Fume silica and calcined alumina are generally used as fillers. The binder used in these castables is currently made of calcium aluminate hydraulic cements whose alumina content is typically between 60 to 80 wt. %. Finally, the additives used are mostly deflocculants and dispersants. The above formulation of castables resulted from various improvements made by the refractory manufacturers during the last two decades, mainly to increase the strength of such materials, by reducing both their hydraulic cement and mixing water content. These improvements lay in the use of fillers (submicronic particles) and deflocculants, as well as of a selective granulometry which permit the solids content to be maximized. This confers thixotropic behavior on the material during mixing with water 18. Such rheological behavior is in fact required to minimize the mixing water content and thus to maximize the material s strength even at low temperature. In molten aluminum confinement, the addition of fume silica in the above materials has an additional purpose. It serves to impart strength to the castable by promoting mechanical bonding at temperatures above 950 o C due to silica crystallization. Without this additive, strength development of such refractories, due to the mullitization of their minerals, is effective at temperature exceeding the operating temperature of most molten aluminum confinement devices. However the use of fume silica or any other type of amorphous silica constitutes a weakness in the corrosion resistance of the refractory, which becomes highly reducible by the action of molten aluminum alloys. Another improvement in refractory castables with regard to molten aluminum confinement concerns their resistance to corrosion. As previous work showed 6, a lowering of the permeability increases the resistance of such materials to corrosion by molten aluminum in conditions where either oxide reduction or direct metal oxidation are favored. Moreover, the use of non-wetting additives such as AlF 3, CaF 2 and BaSO 4 have also been shown to be advantageous in these conditions 9, 19, 20,21,22. In section , it has been mentioned that AlF 3 may not be a good candidate for such an additive if the refractory contains alkalis that may lead to the formation of β-al 2 O 3. In contrast, the use of barium sulfate in such conditions has been shown 19 to be effective for converting weak phases in the refractory into more stable and less reducible phases, such as celsian (BaAlSiO 4 ). However, the efficiency of barium sulfate was shown 21 to be limited to temperatures not exceeding 1050 o C (with AlF 3 and CaF 2, this temperature limit was shown to be less than 950 o C). Another limitation with BaSO 4, as well as all other conventional non-wetting additives, is that it may protect only the matrix of the refractory (i.e. the particles less that 50 Tyler mesh) 23 against corrosion. The corrosion resistance of the aggregates (i.e. particles higher than 50 Tyler mesh) is in fact independent of the presence of such additives and is only a function of their composition, as shown in Fig. 9. Moreover, as shown in this figure, the corrosion of the aggregates can even promote the corrosion of the refractory's matrix despite the presence of a non-wetting agent. Mouldables are a special class of non-shaped refractories which also received increasing interest during the last few years with regard to molten aluminum confinement, particularly for the lining of troughs. These materials use fine aggregates, super plastifiers and a high content of amorphous silica, typically from 25 to 40 wt. %. The latter is mostly used to confer high thermal shock resistance to the material. However, this high content of amorphous silica makes the non-wetting additives much less efficient. Previous work 24 showed that some additives, which can be designated as ''anti-oxidant'', might also contribute to increased corrosion resistance of refractories in conditions where direct metal oxidation is favored. Fig. 10 shows

8 the effect of adding such an additive to aluminoslicate low cement castables, on their resistance to corrosion in these conditions (The results shown in this figure have been obtained from laboratory testing using the CIREP Bellyband Test set-up). Such additives seem to promote the formation of a protective oxide layer between the refractory and the growing corundum above the metal line. These results suggest that inhibitors preventing direct metal oxidation inside the refractories through their open porosity may improve their resistance to corrosion at high temperature in aluminum confinement devices. CONCLUSION In molten aluminum confinement, refractories are deteriorated by the action of thermomechanical abuse and corrosion, which promote the contamination of the molten metal in service. Thermomechanical abuse intervenes by thermal shocks, mechanical impacts and erosion. The strength and the resistance to both short and long crack propagation of refractories should be maximized to reduce their deterioration by such abuse. Non-wetting agents have been incorporated into alumonosilicate refractory castables to minimize the reduction of their oxides by the action of molten aluminum alloys, up to 1050 o C. For higher temperature, a new type of additives, designated as ''anti-oxidant'', have been introduced to impart corrosion resistance to such materials under direct metal oxidation conditions. ACKNOWLEDGMENT The author is very grateful to the industrial partners of the CIREP-CRNF group at Ecole Polytechnique of Montreal for their financial support during the realization of the research work on refractories for the aluminum industry. The CIREP-CRNF staff members are also acknowledged for their contribution to the experimental work and for their helpful discussion with the author. REFERENCES 1. MACKENZIE, J., The abrasion Resistance of Refractory Bricks, The United Steel Companies Limited, Research and Development Department, J. Trans. Brit. Ceram. Soc., Vol. 50, pp , BAAB, K.A. and KRANER, H. M., J. Amer. Ceram. Soc., 31, 293, SHAPLAND, J. T., Abrasion-Resistance Steel plant Castables, Blast Furnace Steel Plant, Vol. 52, pp , RATLE, Alain, PANDOLFELLI, V., ALLAIRE, C. et RIGAUD, M., Correlation Between Thermal Shock and Mechanical Impact Resistance of Refractories, British Ceram. Transactions, Vol. 96, No. 6, pp , NTAKABURIMVO, N., SEBBANI, M. J. E., and C. ALLAIRE, Influence of the Firing Temperature on the Correlation Between Thermal Shock and Mechanical Impact Resistance of Refractories, British Ceramic Transactions, manuscrit No. BCT 394, accepted for publication the 25 septembre QUESNEL, S., ALLAIRE, C. et AFSHAR, S., Criteria for Choosing Refractories in Aluminum Holding and Melting Furnaces, Light Metals, 1998, Ed. by Barry Welch, TMS Proceedings, pp , NEWKIRK, M. S. and al., Preparation of Lanxide TM Ceramic Matrix Composites : Matrix Formation by the Directed Oxidation of Molten Metals, Ceram. Eng. Sci. Proc., Vol. 8, No. 7-8, pp , 1987.

9 7. SALAS, O. and al., Nucleation and Growth of Al 2 O 3 /Metal Composites by Oxidation of Aluminum Alloys, J. of Mat. Research, Vol. 6, No. 9, pp , ALLAHVERDI, M., ALLAIRE, C. et AFSHAR, S., Effect of BaSO 4, CaF 2 and AlF 3 as well as Na 2 O on the Aluminosilicates Having a Mullite-like Composition, Canadian Ceram. Soc., Vol. 66, No. 3, pp , August ALLAIRE, C. and DESCLAUX, P., Effect of Alkalies and of a Reducing Atmosphere on the Corrosion of Refractories by Molten Aluminum, J. Am. Ceram. Soc., Vol. 74, No. 11, pp , AFSHAR, S., ALLAIRE, C., QUESNEL, S., et ALLAHVERDI, M., Corrosion des réfractaires aluminosiliceux au contact de l aluminium liquide, Al 13 - Le magazine de l aluminium, publié par le Centre Québécois de Recherche et de Développement de l Aluminium, Vol. 2, No. 2, pp , oct XIAO, P. and BERBY, B., Alumina/Aluminum Composites Formed by the Directed Oxidation of Aluminum Using Sodium Hydroxide as a Surface Dopant, J. Am. Ceram. Soc., Vol. 77, No. 7, pp , FURNESS, A.G. and PYGALL, C. F., Hot Face Refractory Failure in Aluminum Melting and Holding Furnaces, Trans, J, Br. Ceram. Soc., Vol. 82, pp , CLAVAUD, B. and VENISSIEUX, V. J., Refractories Used in Melting Furnaces for Aluminum Alloys Interceram Proceeding XXIII Intl. Colloquium on Refractories, Sept. 1998, pp , GRUZLESKI, J. E. and CLOSSET, B. M., The Treatment of Liquid Aluminum-Silicon Alloys, American Foundrymeris Society, Inc. USA, CARNIGLIA, C. and BARNA, L. G., Handbook of Industrial Refractories Technology : Principle, Types, Properties and Applications, Published by Noyes Publication, U.S.A., LEE, F. M., The Chemistry of Cement and Concrete, Chemical Publishing Co. Inc. N. Y., NANDI, P. and al., Mironized α-al 2 O 3 in Zero-Cement Castables, Am. Ceam. Soc. Bull., Vol. 75, No. 11, pp , ALLAHVERDI, M., AFSHAR, S. and ALLAIRE, C., Additives and the Corrosion Resistance of Aluminosilicate Refractories in Molten Al-5 Mg, JOM, pp , February DULBERG, J. L. and al., Molten Aluminum Resistant Refractory Composition Containing Ceramic Fibers, United States Patent No. 5,039,634, August 13, DROUZY, M. L. and al., Nonfibrous Castable Refractory Concrete Having High Deflection Temperature and High Compressive Strength and Process, United States Patent No. 4,174,972, Nov. 20, PORTFIELD, A. D., Aluminum Resistant Refractory Composition, United States Patent No. 4,806,509, Feb. 21, RICHTER, T., VEZZA, T., ALLAIRE, C., Castable with Improved Corrosion Resistance Against Aluminum, Eurogress Aachen (Germany), pp , ALLAIRE, C. and GUERMAZI, M., Protective Refractories Against Corundum Growth in Aluminum Treatment Furnaces, TMS 2000, Light Metals Proceeding, accepted for publication the 30 septembre 1999.

10 s R xr st s R xr st ,50 5,00 5,50 6,00 6,50 7,00 7,50 Figure 1 - Effect of the parameter σ R xr st on the thermal shock resistance of refractory castables pre-fired at 1200 o C Residual modulus of rupture (MPa) after thermal shock 1200 o C 1450 o C 80 Residual modulus of rupture (MPa) After thermal shocks Number of impacts required to damage the material Figure 2 - Effect of the parameter σ R xr st on the mechanical impact resistance of refractory castables pre-fired either at 1200 or 1450 o C 5.

11 8,0 7,5 7,0 R'''' 6,5 6,0 5,5 5,00 5,50 6,00 6,50 7,00 7,50 8,00 Residual modulus of rupture (MPa) after thermal shock Figure 3 - Effect of the parameter R'''' on the thermal shock resistance of refractory castables pre-fired at 1450 o C 5. O 2 BELLYBAND ZONE O Maximum level of metal Usual level of metal Minimum level of metal MOLTEN METAL (T < 900 C) Figure 4 Zoning in aluminum treatment furnaces 6.

12 O 2 ~ 1 µm Grain boundary Crack MgO MgAlO 2 4 (Dense) Al (0.3 % Mg) ~ 3 mm MgAlO 2 4 (Porous) Al ( > 0.3 % Mg, x % Si) Figure 5 Direct Metal Oxidation mechanism (DIMOX TM ) 7. Furnace wall Heating elements Hole in the roof Crucible Metal line Insulating fiber board support A-16 Alumina powder Stainless steel pan Figure 6 - The CIREP Bellyband test set-up 6.

13 Mushroom Large balls Figure 7 - Effect of alkalis on the morphology of the corundum that forms during direct metal oxidation; (a) with cryolite and (b) without cryolite addition on top of the molten metal. Figure 8 Hydrogen pick-up during operation of aluminum treatment furnaces

14 Fig. 9 - Effect of the aggregates' composition on their resistance to corrosion by molten alumimun (Note that in zone 3, the corrosion of the aggregates induces the corrosion of the surrounding matrix)

15 Without additive With additive Corroded zone Metal line Metal line Protective deposit Figure 10 - Effect of adding a ''anti-oxidant'' additive in an aluminoslicate low cement castable on its resistance to corrosion in conditions where direct metal oxidation is favored 24.