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Thermal behavior of SrSO4–SrCO3 and SrSO4–SrCO3–Al2O3 mixtures
Materials Characterization 58 (2007) 859 – 863
Thermal behavior of SrSO4–SrCO3 and SrSO4–SrCO3–Al2O3 mixtures J. Torres T. ⁎, J. Almanza R., A. Flores V., M. de J. Castro R., M. Herrera T. CINVESTAV—Unidad Saltillo Carretera Saltillo—Monterrey Km 13, A. P. 663. 25000, Saltillo, Coahuila, México Received 16 August 2005; received in revised form 15 August 2006; accepted 15 August 2006
Abstract The high temperature behavior of SrSO4, SrCO3 and Al2O3 mixtures was studied. A mixture of 1:1 mole of SrSO4 and mechanically activated SrCO3 was mixed and characterized using thermal gravimetric analysis. Some samples were uniaxially pressed and sintered at 1100, 1200 and 1300 °C for 8 h and then analyzed using X-ray diffraction and scanning electron microscopy. Additionally, a mixture of SrSO4:SrCO3:Al2O3 was uniaxially pressed and sintered at 1500 °C. The decomposition temperature of SrCO3 was decreased 18° by milling for 180 min. Samples sintered at 1300 °C showed a microstructure free of porosity. X-ray diffraction analysis showed the presence of SrO and SrSO4 after sintering at 1100, 1200 and 1300 °C. The mixture containing alumina showed the formation of a strontium aluminum oxide sulfate compound in addition to strontium aluminate. © 2006 Elsevier Inc. All rights reserved. Keywords: Strontium sulfate; Strontium carbonate; Alumina
1. Introduction Celestite (SrSO4) ore is converted to strontianite (SrCO3) by hydrometallurgical and pyrometallurgical processes [1,2]. Strontium carbonate is the main component in the production of TV and computer screens [3] and in the production of nitrates, chromates, chlorides, etc, with applications in industries such as pyrotechnic [4,5], pharmaceutical [6], and foundry [7] among others. Strontium carbonate decomposed into SrO above 900 °C. It has been reported that SrO is a cement forming compound as used in glass-ionometer cement [8]. The addition of SrO retards the setting time, which is convenient in some applications, and increases the compressive strength ⁎ Corresponding author. E-mail address: [email protected] (J. Torres T.). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.08.005
when added in amounts less than 20 wt.%. The use of SrO in cements has some disadvantages because of the rapid hydration at room temperature, however a high temperature treatment could be useful in preventing slaking, as it has been reported to have occurred with MgO [9]. On the other hand, alkaline earth sulfates have been used as nonwetting additives in concrete refractories in contact with aluminum melts [10–12]. The addition of these sulfates reduces the corrosion of silico–aluminate refractories by liquid aluminum. Alkaline earth oxides, particularly BaO, have high thermal stability and reacts with alumina to form compounds that increase thermal shock resistance and reduce thermal expansion in refractories applications [13]. One possible application for a mixture of SrO and SrSO4 is as cement additives in refractory concretes, however there is scarce information about the effect of the heat treatment of SrCO3–SrSO4 mixtures on reduction
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of slaking as well as their stability. On the other hand, several refractory compositions contain alumina, and the compounds formed at a high temperature while SrO and SrSO4 in contact with alumina have not been reported. In this work heat treated mixtures of SrCO3–SrSO4 have been characterized by XRD and SEM. The effect of mechanical activation of SrCO3 is also reported. A preliminary result of formation of compounds at high temperature of mixtures of SrCO3–SrSO4–Al2O3 was studied as well. 2. Experimental A mixture of SrCO3 and SrSO4 (reactive grade) in a 1:1 molar ratio was mixed and analyzed by TGA. Reactive grade SrCO3 was mechanically activated by milling for different time intervals, 15, 30, 45, 60, 75, 90, 105 and 120 min. Particle size distribution of the samples for each activation time was measured using a particle size analyzer (COULTER). Samples of SrCO3 that were mechanically activated at different times and SrSO4 were analyzed by TGA with a heating rate of 10 °C/min to 1300 °C using a flow (10 L/h) of argon gas. It was later decided to use powders of SrCO3 mechanically activated for 30 min increments and include powder mechanically activated for longer periods of time, 150 and 180 min. The results of TGA of mixtures containing mechanically activated and nonactivated strontium carbonate were compared. Powders of mixtures containing SrCO3 mechanically activated for 90 and 180 min, were uniaxially pressed at 100 MPa and sintered at 1100, 1200 and 1300 °C for 8 h under argon atmosphere with a heating rate of 10 °C/min. The surfaces of the sample discs were analyzed by XRD and
Fig. 1. Particle size distribution analysis for mechanically activated SrCO3 at several times.
Fig. 2. Effect of mechanical activation time on weight reduction of SrCO3 with temperature for a 1:1 mol mixture of SrCO3:SrSO4.
the sintered disks were fractured and prepared for SEM analysis. Additionally, a mixture containing alumina was prepared. The first step was to mix 1 mol of mechanically activated SrCO3 (180 min), and 1 mol of SrSO4. Then a mixture of 50/50 mol% of SrCO3– SrSO4/Al2O3 (reactive grade) was prepared, the powder was uniaxially pressed at 100 MPa and sintered at 1500 °C, for 8 h in an argon gas flow (10 L/min) in a tube furnace. The samples were analyzed using XRD and SEM. 3. Results and discussion Fig. 1 shows the volume accumulative percent versus particle size of mechanically activated SrCO3. Above
Fig. 3. XRD patterns for the samples of 180 min mechanically activated SrCO3 and SrSO4 after heat treatment at a) 1100 °C, b) 1200 °C and c) 1300 °C; 1).
J. Torres T. et al. / Materials Characterization 58 (2007) 859–863
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Fig. 4. SEM micrographs for the samples of a 1:1 mol mixture of SrCO3:SrSO4 after heat treatment at 1200 °C with additions of activated carbonate at a) 90 min and b) 180 min.
45 min of milling, there was no apparent change in particle size distribution. From Fig. 1 it was determined that mechanical activation was effective above 45 min of milling. The effect of mechanical activation time on the decomposition of SrCO3 with temperature for mixtures of 1:1 mol SrCO3: SrSO4 is shown in Fig. 2. The decomposition started around 800 °C for all the samples and all the curves showed the same reduction in weight, around 14%. There is not a significant effect of mechanical activation of SrCO3 on the decomposition temperature. For a 90% of decomposition, for SrCO3 with 0 and 180 min of mechanical activation, there was a difference of only 18 °C on decomposition temperature. The sintering experiments were performed using SrCO3 activated for 90 and 180 min in order to better observe the effect of milling time on the sintering temperature. XRD diffractograms of the samples containing SrCO3 mechanically activated for 180 min and sintered at 1100, 1200 and 1300 °C are shown in Fig. 3. It was observed that above 1100 °C the decomposition of SrCO3 was complete. Peaks corresponding to strontium
oxide and strontium sulfate were detected. There was no evidence of any other compound formed during these heat treatments, neither of any hydroxides. In spite that there was no apparent change in behavior in the samples for milling times above 90 min there were some microstructure differences in the samples. Fig. 4 shows the comparison of the fracture surface for samples containing SrCO3 mechanically activated for 90 and 180 min and sintered at 1200 °C for 8 h. The sample with 90 min of activation (Fig. 4a) shows isolated pores and small grains; neck growth typical of an intermediate stage of sintering can be observed. The sample with 180 min of activation showed grain growth and a more dense structure, some cracks are apparent along the grain boundaries and no porosity was observed. Samples sintered at 1300 °C for 8 h showed a dense microstructure, Fig. 5a shows the sample containing SrCO3 activated for 90 min, there was no porosity remaining and it had a larger grain size in comparison to the sample sintered at 1200 °C. The sample containing mechanically activated SrCO3 for 180 min, Fig. 5b, showed no porosity, but some grain growth. Particle size
Fig. 5. SEM micrographs for the samples of a 1:1 mol mixture of SrCO3:SrSO4 after heat treatment at 1300 °C with additions of activated carbonate at a) 90 min and b) 180 min.
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distribution did not change significantly after 1 h of milling. However there is a marked difference in samples sintered as milling time increased. More dense samples were obtained for higher milling times, the mechanical energy applied to the materials increased the density of cracks and defects and also induce the deformation of crystal lattice. In order for sintering to occur, it is necessary to lowering free energy of the system. One of the driving forces for sintering is the curvature of the particle surfaces. Surface diffusion and lattice diffusion does not cause densification, however, reduces the curvature of the neck surface. This reduction in the neck surface, reduce the driving force necessary for densification mechanisms to take place such as grain boundary diffusion. All the defects created during the mechanical activation contribute to the mass transfer necessary for sintering. As the time of mechanical activation increased the number of defects increased, leading to a more dense material. The fact that no Sr(OH)2 was detected by XRD diffraction demonstrated that heat treatments reduce slaking. However, small amounts of hydroxide could be present since XRD technique has a lower limit of detection of about 3% of any compound. This lead us to infer that the density/particle size requirements has not been determined, appropriate experiments has to be set in order to determine the particle size and effective activation time required to obtain a dense material. These conditions have to be related to the rate of hydration of the SrO. The heat treated mixture could be useful for application as cement additives. There has been stated that Sr is a cement forming ion [8]. The addition of Sr in calcium aluminate cement containing refractories represent a study area of future interest since Ca2+ and Sr2+ have similar ionic radii [14]. Of particular interest is the presence of SrSO4 which is a stable compound at high temperatures, up to 1693 °C [15]. It has been reported also that the use of alkaline earth sulfates decreased the corrosion of silica containing concretes by molten aluminum [10–12]. The use of these compounds SrO: SrSO4 in cements for refractory concrete in contact with molten aluminum represent an potential area of research for these materials. Mixtures of SrCO3–SrSO4 sintered could provide an additive for the source of a non-wetting material for refractory concrete in contact with molten aluminum. Most of the refractories in contact with aluminum are alumina based materials. The addition of SrO and SrSO4 could reduced the corrosion and increase strength. On the other hand, the use of silica based refractories in the aluminum industry is of great interest since it will reduce the cost of the lining. The addition of
Fig. 6. XRD pattern for a sample of SrCO3:SrSO4/Al2O3 after sintering at 1500 °C for 8 h with addition of mechanically activated SrCO3 (180 min) 1) Sr4Al6O12SO4, 2) Sr3Al2O6.
additives like sintered mixtures of SrO–SrSO4 would increase the refractoriness of SiO2 castables and the corrosion resistant. Fig. 6 shows the results of XRD analysis of the mixtures of 50/50 mol% of a mixture of SrSO4–SrCO3/ Al2O3 sintered at 1500 °C for 8 h. Two main phases were identified, Sr4Al6O12SO4 and Sr3Al2O6. This second phase is a compound that is stable up to 1660 °C, according to the corresponding SrO–Al2O3 binary phase diagram [16]. The addition of alumina to the mixture promotes the formation of a strontium aluminum oxide sulfate. There is a lack of information about the stability of the Sr4Al6O12SO4 compound, but appears that this compound is also stable at high temperature. This compound has, however, not been properly characterized. 4. Conclusions Mechanical activation reduces the decomposition temperature of SrCO3 about 18 °C. Heat treatments of mixtures of SrCO3:SrSO4 delay the formation of Sr (OH)2, however proper experiments have to be preformed to elucidate the effect of sintering on hydration. Samples of mixtures of SrCO3:SrSO4 sintered at 1300 °C showed a dense microstructure free of porosity. It was found that the addition of Al2O3 to the mixtures promotes the formation of Sr4Al6O12SO4 which appear to be a stable compound at high temperature, however this materials has not been properly characterized. The use of these materials as cement additives in refractory concretes is a research area of interest.
J. Torres T. et al. / Materials Characterization 58 (2007) 859–863
References [1] Iwai M, Toguri M. The leaching of Celestite in Sodium Carbonate Solution. Hidrometallurgy 1989;22:87–100. [2] Plewa J, Steindor J. Kinetic of SrSO4 reduction with CO. Termochim Acta 1998;138:55–6. [3] Barsotti AE, Morse DE. Industrial minerals 1999. Industrial Minerals in the United States. Strontium Orber J A Min Eng 2000;52:62–3. [4] Kirk RE, Othmer DF Estroncio, Compuestos. Enciclopedia de la Tecnología Química, tomo VII, Ed. UTHEA, 1962, p. 472–6. [5] Joshi MN, Grover AK, Balachandra J. Preparation of the Metals Strontium from Celestite. Indian J Technol 1973;11:74–7. [6] Safdar M, Asan R. Preparation of strontium salts from Celestite. Pakistan Sci Res 1996;8(2):161–6. [7] Langlais J, Harris R. Strontium extraction by aluminothermic reduction. Can Metall Q 1992;31(2):127–31. [8] Deb S, Nicholson JW. The effect of strontium oxide in glass– ionomer cements. J Mater Sci Mater Med 1999;10(8):471–4. [9] Carniglia SC, Barna GL. Handbook of Industrial Refractories Technology; Principles, Types, Properties and Applications. Westwood, NJ. USA: Noyes Publication; 1992. p. 46. [10] Teiken J, Krietz L. Additive Effects on the Physical Properties of Castable Systems for Aluminium Contact Applications.
[11]
[12] [13]
[14]
[15]
[16]
863
Refractories for the Next Millenium. Proccedings of the 9th Symposium, Refractories for the Aluminum Industry, 51st Pacific Coast Regional Meeting of The American Ceramic Society; Bellevue, Washington, USA. October 27–29 1999. p. 5–22. Allahervrdi M, Afshar S, Allaire C. Additives and the corrosion resistance of aluminosilicate refractories in molten Al–5Mg. J Met 1998;2:30–4. Afshar S, Allaire C. Furnaces: improving low cement castables by non-wetting additives. J Met 2001;8:24–7. El-Hemaly SAS, Khalil NM, Girgis LG. Refractory castables based on barium aluminate cements. Br Ceram Trans 2003;102 (4):169–74. Prodjosantoso AK, Kennedy BJ. Solubility of SrAl2O4 in CaAl2O4—a high resolution powder diffraction study. Mater Res Bull 2003;38(1):79–87. Torres J. Analisis de la Reversivilidad del Sistema SrS–SrSO4 en Atmosferas de CO–CO2. Ph.D Thesis, CINVESTAV-IPN, Unidad Saltillo, México:1999. Douy A, Capron M. Crystallization of spray-dried amorphous precursors in the SrO–Al2O3 system: a DSC study. J Eur Ceram Soc 2003;23(1):2075–81.
Thermal behavior of SrSO4–SrCO3 and SrSO4–SrCO3–Al2O3 mixtures J. Torres T. ⁎, J. Almanza R., A. Flores V., M. de J. Castro R., M. Herrera T. CINVESTAV—Unidad Saltillo Carretera Saltillo—Monterrey Km 13, A. P. 663. 25000, Saltillo, Coahuila, México Received 16 August 2005; received in revised form 15 August 2006; accepted 15 August 2006
Abstract The high temperature behavior of SrSO4, SrCO3 and Al2O3 mixtures was studied. A mixture of 1:1 mole of SrSO4 and mechanically activated SrCO3 was mixed and characterized using thermal gravimetric analysis. Some samples were uniaxially pressed and sintered at 1100, 1200 and 1300 °C for 8 h and then analyzed using X-ray diffraction and scanning electron microscopy. Additionally, a mixture of SrSO4:SrCO3:Al2O3 was uniaxially pressed and sintered at 1500 °C. The decomposition temperature of SrCO3 was decreased 18° by milling for 180 min. Samples sintered at 1300 °C showed a microstructure free of porosity. X-ray diffraction analysis showed the presence of SrO and SrSO4 after sintering at 1100, 1200 and 1300 °C. The mixture containing alumina showed the formation of a strontium aluminum oxide sulfate compound in addition to strontium aluminate. © 2006 Elsevier Inc. All rights reserved. Keywords: Strontium sulfate; Strontium carbonate; Alumina
1. Introduction Celestite (SrSO4) ore is converted to strontianite (SrCO3) by hydrometallurgical and pyrometallurgical processes [1,2]. Strontium carbonate is the main component in the production of TV and computer screens [3] and in the production of nitrates, chromates, chlorides, etc, with applications in industries such as pyrotechnic [4,5], pharmaceutical [6], and foundry [7] among others. Strontium carbonate decomposed into SrO above 900 °C. It has been reported that SrO is a cement forming compound as used in glass-ionometer cement [8]. The addition of SrO retards the setting time, which is convenient in some applications, and increases the compressive strength ⁎ Corresponding author. E-mail address: [email protected] (J. Torres T.). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.08.005
when added in amounts less than 20 wt.%. The use of SrO in cements has some disadvantages because of the rapid hydration at room temperature, however a high temperature treatment could be useful in preventing slaking, as it has been reported to have occurred with MgO [9]. On the other hand, alkaline earth sulfates have been used as nonwetting additives in concrete refractories in contact with aluminum melts [10–12]. The addition of these sulfates reduces the corrosion of silico–aluminate refractories by liquid aluminum. Alkaline earth oxides, particularly BaO, have high thermal stability and reacts with alumina to form compounds that increase thermal shock resistance and reduce thermal expansion in refractories applications [13]. One possible application for a mixture of SrO and SrSO4 is as cement additives in refractory concretes, however there is scarce information about the effect of the heat treatment of SrCO3–SrSO4 mixtures on reduction
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of slaking as well as their stability. On the other hand, several refractory compositions contain alumina, and the compounds formed at a high temperature while SrO and SrSO4 in contact with alumina have not been reported. In this work heat treated mixtures of SrCO3–SrSO4 have been characterized by XRD and SEM. The effect of mechanical activation of SrCO3 is also reported. A preliminary result of formation of compounds at high temperature of mixtures of SrCO3–SrSO4–Al2O3 was studied as well. 2. Experimental A mixture of SrCO3 and SrSO4 (reactive grade) in a 1:1 molar ratio was mixed and analyzed by TGA. Reactive grade SrCO3 was mechanically activated by milling for different time intervals, 15, 30, 45, 60, 75, 90, 105 and 120 min. Particle size distribution of the samples for each activation time was measured using a particle size analyzer (COULTER). Samples of SrCO3 that were mechanically activated at different times and SrSO4 were analyzed by TGA with a heating rate of 10 °C/min to 1300 °C using a flow (10 L/h) of argon gas. It was later decided to use powders of SrCO3 mechanically activated for 30 min increments and include powder mechanically activated for longer periods of time, 150 and 180 min. The results of TGA of mixtures containing mechanically activated and nonactivated strontium carbonate were compared. Powders of mixtures containing SrCO3 mechanically activated for 90 and 180 min, were uniaxially pressed at 100 MPa and sintered at 1100, 1200 and 1300 °C for 8 h under argon atmosphere with a heating rate of 10 °C/min. The surfaces of the sample discs were analyzed by XRD and
Fig. 1. Particle size distribution analysis for mechanically activated SrCO3 at several times.
Fig. 2. Effect of mechanical activation time on weight reduction of SrCO3 with temperature for a 1:1 mol mixture of SrCO3:SrSO4.
the sintered disks were fractured and prepared for SEM analysis. Additionally, a mixture containing alumina was prepared. The first step was to mix 1 mol of mechanically activated SrCO3 (180 min), and 1 mol of SrSO4. Then a mixture of 50/50 mol% of SrCO3– SrSO4/Al2O3 (reactive grade) was prepared, the powder was uniaxially pressed at 100 MPa and sintered at 1500 °C, for 8 h in an argon gas flow (10 L/min) in a tube furnace. The samples were analyzed using XRD and SEM. 3. Results and discussion Fig. 1 shows the volume accumulative percent versus particle size of mechanically activated SrCO3. Above
Fig. 3. XRD patterns for the samples of 180 min mechanically activated SrCO3 and SrSO4 after heat treatment at a) 1100 °C, b) 1200 °C and c) 1300 °C; 1).
J. Torres T. et al. / Materials Characterization 58 (2007) 859–863
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Fig. 4. SEM micrographs for the samples of a 1:1 mol mixture of SrCO3:SrSO4 after heat treatment at 1200 °C with additions of activated carbonate at a) 90 min and b) 180 min.
45 min of milling, there was no apparent change in particle size distribution. From Fig. 1 it was determined that mechanical activation was effective above 45 min of milling. The effect of mechanical activation time on the decomposition of SrCO3 with temperature for mixtures of 1:1 mol SrCO3: SrSO4 is shown in Fig. 2. The decomposition started around 800 °C for all the samples and all the curves showed the same reduction in weight, around 14%. There is not a significant effect of mechanical activation of SrCO3 on the decomposition temperature. For a 90% of decomposition, for SrCO3 with 0 and 180 min of mechanical activation, there was a difference of only 18 °C on decomposition temperature. The sintering experiments were performed using SrCO3 activated for 90 and 180 min in order to better observe the effect of milling time on the sintering temperature. XRD diffractograms of the samples containing SrCO3 mechanically activated for 180 min and sintered at 1100, 1200 and 1300 °C are shown in Fig. 3. It was observed that above 1100 °C the decomposition of SrCO3 was complete. Peaks corresponding to strontium
oxide and strontium sulfate were detected. There was no evidence of any other compound formed during these heat treatments, neither of any hydroxides. In spite that there was no apparent change in behavior in the samples for milling times above 90 min there were some microstructure differences in the samples. Fig. 4 shows the comparison of the fracture surface for samples containing SrCO3 mechanically activated for 90 and 180 min and sintered at 1200 °C for 8 h. The sample with 90 min of activation (Fig. 4a) shows isolated pores and small grains; neck growth typical of an intermediate stage of sintering can be observed. The sample with 180 min of activation showed grain growth and a more dense structure, some cracks are apparent along the grain boundaries and no porosity was observed. Samples sintered at 1300 °C for 8 h showed a dense microstructure, Fig. 5a shows the sample containing SrCO3 activated for 90 min, there was no porosity remaining and it had a larger grain size in comparison to the sample sintered at 1200 °C. The sample containing mechanically activated SrCO3 for 180 min, Fig. 5b, showed no porosity, but some grain growth. Particle size
Fig. 5. SEM micrographs for the samples of a 1:1 mol mixture of SrCO3:SrSO4 after heat treatment at 1300 °C with additions of activated carbonate at a) 90 min and b) 180 min.
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J. Torres T. et al. / Materials Characterization 58 (2007) 859–863
distribution did not change significantly after 1 h of milling. However there is a marked difference in samples sintered as milling time increased. More dense samples were obtained for higher milling times, the mechanical energy applied to the materials increased the density of cracks and defects and also induce the deformation of crystal lattice. In order for sintering to occur, it is necessary to lowering free energy of the system. One of the driving forces for sintering is the curvature of the particle surfaces. Surface diffusion and lattice diffusion does not cause densification, however, reduces the curvature of the neck surface. This reduction in the neck surface, reduce the driving force necessary for densification mechanisms to take place such as grain boundary diffusion. All the defects created during the mechanical activation contribute to the mass transfer necessary for sintering. As the time of mechanical activation increased the number of defects increased, leading to a more dense material. The fact that no Sr(OH)2 was detected by XRD diffraction demonstrated that heat treatments reduce slaking. However, small amounts of hydroxide could be present since XRD technique has a lower limit of detection of about 3% of any compound. This lead us to infer that the density/particle size requirements has not been determined, appropriate experiments has to be set in order to determine the particle size and effective activation time required to obtain a dense material. These conditions have to be related to the rate of hydration of the SrO. The heat treated mixture could be useful for application as cement additives. There has been stated that Sr is a cement forming ion [8]. The addition of Sr in calcium aluminate cement containing refractories represent a study area of future interest since Ca2+ and Sr2+ have similar ionic radii [14]. Of particular interest is the presence of SrSO4 which is a stable compound at high temperatures, up to 1693 °C [15]. It has been reported also that the use of alkaline earth sulfates decreased the corrosion of silica containing concretes by molten aluminum [10–12]. The use of these compounds SrO: SrSO4 in cements for refractory concrete in contact with molten aluminum represent an potential area of research for these materials. Mixtures of SrCO3–SrSO4 sintered could provide an additive for the source of a non-wetting material for refractory concrete in contact with molten aluminum. Most of the refractories in contact with aluminum are alumina based materials. The addition of SrO and SrSO4 could reduced the corrosion and increase strength. On the other hand, the use of silica based refractories in the aluminum industry is of great interest since it will reduce the cost of the lining. The addition of
Fig. 6. XRD pattern for a sample of SrCO3:SrSO4/Al2O3 after sintering at 1500 °C for 8 h with addition of mechanically activated SrCO3 (180 min) 1) Sr4Al6O12SO4, 2) Sr3Al2O6.
additives like sintered mixtures of SrO–SrSO4 would increase the refractoriness of SiO2 castables and the corrosion resistant. Fig. 6 shows the results of XRD analysis of the mixtures of 50/50 mol% of a mixture of SrSO4–SrCO3/ Al2O3 sintered at 1500 °C for 8 h. Two main phases were identified, Sr4Al6O12SO4 and Sr3Al2O6. This second phase is a compound that is stable up to 1660 °C, according to the corresponding SrO–Al2O3 binary phase diagram [16]. The addition of alumina to the mixture promotes the formation of a strontium aluminum oxide sulfate. There is a lack of information about the stability of the Sr4Al6O12SO4 compound, but appears that this compound is also stable at high temperature. This compound has, however, not been properly characterized. 4. Conclusions Mechanical activation reduces the decomposition temperature of SrCO3 about 18 °C. Heat treatments of mixtures of SrCO3:SrSO4 delay the formation of Sr (OH)2, however proper experiments have to be preformed to elucidate the effect of sintering on hydration. Samples of mixtures of SrCO3:SrSO4 sintered at 1300 °C showed a dense microstructure free of porosity. It was found that the addition of Al2O3 to the mixtures promotes the formation of Sr4Al6O12SO4 which appear to be a stable compound at high temperature, however this materials has not been properly characterized. The use of these materials as cement additives in refractory concretes is a research area of interest.
J. Torres T. et al. / Materials Characterization 58 (2007) 859–863
References [1] Iwai M, Toguri M. The leaching of Celestite in Sodium Carbonate Solution. Hidrometallurgy 1989;22:87–100. [2] Plewa J, Steindor J. Kinetic of SrSO4 reduction with CO. Termochim Acta 1998;138:55–6. [3] Barsotti AE, Morse DE. Industrial minerals 1999. Industrial Minerals in the United States. Strontium Orber J A Min Eng 2000;52:62–3. [4] Kirk RE, Othmer DF Estroncio, Compuestos. Enciclopedia de la Tecnología Química, tomo VII, Ed. UTHEA, 1962, p. 472–6. [5] Joshi MN, Grover AK, Balachandra J. Preparation of the Metals Strontium from Celestite. Indian J Technol 1973;11:74–7. [6] Safdar M, Asan R. Preparation of strontium salts from Celestite. Pakistan Sci Res 1996;8(2):161–6. [7] Langlais J, Harris R. Strontium extraction by aluminothermic reduction. Can Metall Q 1992;31(2):127–31. [8] Deb S, Nicholson JW. The effect of strontium oxide in glass– ionomer cements. J Mater Sci Mater Med 1999;10(8):471–4. [9] Carniglia SC, Barna GL. Handbook of Industrial Refractories Technology; Principles, Types, Properties and Applications. Westwood, NJ. USA: Noyes Publication; 1992. p. 46. [10] Teiken J, Krietz L. Additive Effects on the Physical Properties of Castable Systems for Aluminium Contact Applications.
[11]
[12] [13]
[14]
[15]
[16]
863
Refractories for the Next Millenium. Proccedings of the 9th Symposium, Refractories for the Aluminum Industry, 51st Pacific Coast Regional Meeting of The American Ceramic Society; Bellevue, Washington, USA. October 27–29 1999. p. 5–22. Allahervrdi M, Afshar S, Allaire C. Additives and the corrosion resistance of aluminosilicate refractories in molten Al–5Mg. J Met 1998;2:30–4. Afshar S, Allaire C. Furnaces: improving low cement castables by non-wetting additives. J Met 2001;8:24–7. El-Hemaly SAS, Khalil NM, Girgis LG. Refractory castables based on barium aluminate cements. Br Ceram Trans 2003;102 (4):169–74. Prodjosantoso AK, Kennedy BJ. Solubility of SrAl2O4 in CaAl2O4—a high resolution powder diffraction study. Mater Res Bull 2003;38(1):79–87. Torres J. Analisis de la Reversivilidad del Sistema SrS–SrSO4 en Atmosferas de CO–CO2. Ph.D Thesis, CINVESTAV-IPN, Unidad Saltillo, México:1999. Douy A, Capron M. Crystallization of spray-dried amorphous precursors in the SrO–Al2O3 system: a DSC study. J Eur Ceram Soc 2003;23(1):2075–81.