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Preparation and characterization of MgO powders obtained from different magnesium salts and the mineral dolomite
www.elsevier.nl/locate/poly Polyhedron 19 (2000) 2345– 2351
Preparation and characterization of MgO powders obtained from different magnesium salts and the mineral dolomite E. Alvarado a, L.M. Torres-Martinez a, A.F. Fuentes a, P. Quintana b,* b
a Facultad de Ciencias Quimicas, DES, UANL, Monterrey, N.L., Mexico Cin6esta6 del IPN, Unidad Merida, km.6 Antigua Carretera a Progreso, C.P. 97310, Merida, Yucatan, Mexico
Received 13 October 1999; accepted 11 May 2000
Abstract The characterisation of the physical properties of MgO powders, obtained from three commercial magnesium compounds, MgSO4·7H2O, MgNO3·6H2O and Mg(CH3CO2)2·4H2O and the mineral dolomite (natural source from Mexico), synthesised by chemical precipitation, is presented. The decomposition of the precipitated Mg(OH)2 was analysed by DTA/TGA and the crystallisation process was observed by XRD. The variation of the properties with the nature of the precursors at 960°C was studied: as the crystallite size, density, specific surface area, degree of agglomeration, and the total porosity. The microstructural differences between the MgO agglomerates were examined by SEM, at different temperatures. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Magnesia; Dolomite; Crystallite size; Degree of agglomeration; X-ray diffraction; SEM
1. Introduction Magnesia (MgO) is the most important product of the magnesium compound industry. It is usually produced for commerce by the thermal decomposition of magnesite (MgCO3), from magnesium hydroxide (Mg(OH)2) obtained from a seawater process or from magnesium rich-brines as the mineral dolomite [1]. Mexico is one of the major producers of MgO; the states of Nuevo Leon and Coahuila contain large deposits of high-purity dolomite; major capacity expansions took place in 1992 when Industria Pen˜oles started with the production of high-purity electrofused MgO in Ramos Arizpe, Coahuila [1]. The most important process for the production of MgO from seawater or brine is based on the fact that Mg(OH)2 can be precipitated from solutions of magnesium salts by the addition of a strong base. The Mg(OH)2 precipitate is washed, filtered and then calcined to produce MgO. Decomposition of solutions of magnesium salts is also important for the synthesis of * Corresponding author. Tel.: + 52-99-81-2960; fax: + 52-99-812917. E-mail address: [email protected] (P. Quintana).
ceramic powders. Many alternative techniques have been developed offering advantages of easy preparation of oxide powders, accurate compositional control, homogeneity and high purity [2–6]. Caustic-calcined MgO is produced at temperatures B 900°C and characterised by its moderate to high chemical reactivity. This type of magnesia has numerous applications as mineral supplement for animal feeds, in fertilisers, as a raw material for various MgO chemicals, to manufacture ceramics, cements, paper, petroleum additives, etc. Meanwhile, calcined dolomite as a source of alkali is used in agriculture, to neutralise soil acidity and adds calcium and magnesium to it; it can also can be used in the manufacture of refractories, as mineral filler, as a constituent in a glass batch, etc. [1]. The physical properties of MgO powders are affected by the type of precursor, the temperature of calcination and the impurities. Actually, one of the main problems detected in the production of caustic magnesia powders is to achieve a real density near to the ideal value, in order to obtain ceramic bodies with suitable compacting conditions. The properties of refractories in industrial processes with low densities, present undesired characteristics such as the formation of low eutectic
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points due to the presence of impurities, vulnerability to chemical attack and a deficient mechanical resistance [7 – 9]. Studies on the synthesis of materials with appropriate refractory properties had been aimed to obtain powders with certain characteristics, which may help to improve the process of sintering [10]. Among these, the following can be mentioned: high surface area, uniform particle size and chemical homogeneity. The formation of large agglomerates must be avoided, since these generate fractures in their surroundings and if present in excess, they can accelerate or delay the sinterability phenomena. The morphology of powders of MgO has been observed using scanning electron microscopy (SEM), to study the origin of the degree of agglomeration in caustic MgO and to analyse the effects during the process of sintering [7,8,11,12]. The aim of this work was to study and characterise the physical properties of MgO powders obtained by chemical precipitation from three commercial chemical compounds to be compared with dolomite (natural source from Coahuila, Mexico). Different measurements were carried out at 960°C (the surface area, the primary particle size, the crystallite size and the apparent density) to determine the degree of agglomeration between the different types of precursor. The crystallization of magnesia was studied by X-ray diffraction, and the development of the morphology was observed by SEM.
2. Experimental Powders of MgO were synthesised by chemical precipitation from four different types of precursors, three of them commercial reagent-grade hydrated salts: MgSO4·7H2O, MgNO3·6H2O and Mg(CH3CO2)2·4H2O and from the mineral dolomite (MgCO3·CaCO3) obtained from Coahuila, Mexico. The chemical analysis of dolomite used in this experiment, measured by X-ray fluorescence with Cr radiation (Diano 8000), indicates a purity of 99.8%. The principal impurities (wt%) were SiO2 (0.01), Al2O3 (0.01), Fe2O3 (0.04), Na2O (0.02) and P2O5 (0.03). The purity, for the three reagent-grade salts, taken from the catalogues of the manufacturers (Productos Quimicos Monterrey, SA), was 99.8%. Each hydrated compound was dissolved in deionized water at room temperature to yield 0.8 M, an excess of ammonium hydroxide (13 M) was added, and vigorously stirred at 50°C, pH 10. A white precipitate of Mg(OH)2 was obtained, which was thoroughly washed with distilled water and dried at 100°C for 2 h. Magnesium hydroxide powders were calcined in Pt crucibles, at different temperatures to produce MgO powders. The heating treatment was carried out in electric furnaces under air atmosphere, from 500 to 1000°C, raising the temperature 100°C every 40 min.
Natural dolomite was initially milled to 200-mesh. The decarbonation procedure was done with a progressive heating from 500 to 800°C, with a stepwise of 100°C every 0.5 h. The sample was held for 2 h and finally the temperature was increased to 900°C (0.5 h) to obtain dolima (MgO·CaO). The hydration of dolima (0.9 M) was made with a vigorously agitation and then it was mixed with a MgCl2 (0.5 M) solution at 50°C, pH 11. Once Mg(OH)2 appeared as a precipitate, it was washed, dried and calcined under the same conditions described above, finally a powder of MgO was obtained. The decomposition characteristics of the salts and dolomite were determined by simultaneous differential/ thermogravimetric analysis (DTA/TGA, TA-Instruments model SDT-2960). The heating rate of the sample was 10°C min − 1, with a flowing N2 atmosphere, using alumina as a reference material. The powders from each calcined treatment were characterised by X-ray diffraction and the crystallite size was calculated using the Scherrer’s formula, using KCl as an internal standard. A Cu Ka radiation (u= 1.5405 A, ) and Ni filter was used. The difractograms were registered in a range of 10–115 2q (30 kV, 20 mA), in a step-scan mode of 0.01° 2q per step and counting for 5 s/step (model Siemens D-5000). MgO apparent density was obtained by the liquid pycnometer method according to ASTM C699, using toluene; several measurements were done until data reproducibility was obtained. The specific surface area and primary particle size of MgO powders was measured by the BET technique (Stro¨hlein), N2 as an absorption gas was used. The morphology of Mg(OH)2 and MgO agglomerates at different temperatures were examined by SEM (Leica S440). Powders were pelletized and coated with carbon to avoid charging during exposure to the electron beam and covered with an Au thin film by sputtering in a vacuum system.
3. Results and discussion The decomposition path of the precipitated, Mg(OH)2, was studied by DTA/TGA analysis. Results obtained from the salts were similar. A very small and a strong endothermic peak was observed; the first peak is related to the dehydration processes at temperatures B 200°C and the second to the decomposition of magnesium hydroxide, the temperature was 420, 310 and 390°C, for sulfate, acetate and nitrate, respectively. Furthermore, sulfate salt shows a continuous weight loss of 7%, between 600 and 920°C. The total weight loss was around 40% (Fig. 1). These results are in good agreement with the ones presented elsewhere [4]. In the case of dolomite, the first endothermic peak was very strong with a weight loss of 13.44 wt% and the decom-
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position reaction was held at 370°C with a weight loss of 25.9%; finally at 800°C evaporation of CO2 occurs (Fig. 1). Therefore, the complete decomposition of the acetate and nitrate can be achieved at reasonably low temperatures (B 450°C); however for the dolomite and sulfate salt this was near 800 and 950°C, respectively. The X-ray diffraction studies on the crystallisation process for the magnesium salts showed the formation
Fig. 3. Diffractograms of MgO powders obtained at 960°C from different precursors: (a) dolomite, (b) nitrate, (c) sulfate, (d) acetate. Traces of MgSO4 ( ) and NaF () are indicated.
Fig. 1. DTA/TGA of Mg(OH)2 obtained from two different precursors.
Fig. 4. Development of crystallite size with temperature on the crystallisation of MgO.
Fig. 2. Comparison of the X-ray diffraction patterns of Mg(OH)2 at 100°C, between sulfate and dolomite. The latest shows the presence of CaCO3 (*).
of brucite Mg(OH)2 at 100°C with a very poor crystallinity, although dolomite presented stronger diffraction peaks as can be seen in Fig. 2. At higher temperature, crystals of periclase (MgO) were initially detected at 500°C after 40 min, with broad diffraction peaks, which became nice and sharp at 960°C, mainly on dolomite, since it had a higher crystallinity (Fig. 3). Additional diffraction lines were detected, as traces of NaF and magnesium sulfate phases. Crystallite size was calculated, by means of Scherrer’s equation [13] on the (200) diffraction maximum, obtained from the different powders heated 40 min at each temperature (500 –1000°C). The behaviour of the crystallite growth with temperature is shown in Fig. 4. The results are very similar for the chemical com-
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Fig. 5. Microstructure of Mg(OH)2 powders obtained from the different precursors, heated at 100°C for 40 min.
pounds. Near to the decomposition temperature of brucite, the magnesium oxide formed has a very small crystallite size, showing a plateau with a slight increment, and with a crystallite growth of 10 – 40 nm. On raising the calcination temperature, over 850°C, a rapid increase in the crystallite size occurs; where dolomite had the highest value of 110 nm at 1000°C, which is double the value obtained for the other precursors. These data are in good agreement with the values
reported in the literature for oxides prepared from different chemical precursor salts [14,15]. The apparent density measurements determined by pycnometry, zapp, at 960°C, are shown in Table 1. The density values of sulfate approach the theoretical data of periclase (3.59 g cm − 3), meanwhile, dolomite shows the lowest value probably due to the higher crystallite size. It has been reported that it is difficult to obtain high-density magnesia from purified raw materials [11].
Table 1 Physical properties of MgO powders obtained at 960°C (40 min) from different precursors Precursor
zapp (g cm−3)
zDRX (g cm−3)
Surface area a (m2 g−1)
Surface area (m2 g−1)
GBET (nm)
GDRX (nm)
GBET/GDRX
Ptot (%)
Sulfate Nitrate Acetate Dolomite
3.53 3.46 3.44 3.42
3.576 3.584 3.581 3.585
137.6 124.2 105.0 47.3
64.4 30.8 28.9 16.5
25.9 54.2 57.7 101.1
41.6 54.9 57.9 53.1
0.62 0.99 0.99 1.90
1.3 3.46 3.94 4.60
a
Values obtained for Mg(OH)2 at 100°C, 40 min.
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Fig. 6. SEM micrographs of MgO powders at 960°C, showing the formation of agglomerates.
Density from X-ray diffraction data was also calculated, zXRD (same as ztrue), using KCl as an internal standard and a least-square refinement program [16] to determine precisely the lattice parameter. The values obtained show good agreement with the theoretical value of periclase. The percentage of total porosity in the different magnesia samples was calculated from
%PT = 1−
zapp × 100 ztrue
according to [17], where the highest value was observed on powders derived from dolomite, 4.6%, and the lowest for sulfate, 1.3%. Acetate and nitrate had 3.5% and 3.9%, respectively. The variation of the specific surface area for brucite and MgO powders can be observed in Table 1. The surface area values were higher for the powders obtained from the reagent-grade salts than those obtained from the mineral dolomite. In all cases MgO had a
much lower value than the corresponding Mg(OH)2. The samples obtained from sulfate show the highest values, which agrees with the smallest particle size. Meanwhile, nitrate and acetate show intermediate values. In contrast, the powders prepared from dolomite had values almost four and two orders of magnitude lower than the data obtained for sulfate and from nitrate and acetate, respectively. This effect can be related to the zapp value, which depends on the utilised precursor and the presence of impurities. The surface area obtained for periclase was in a good agreement with other reports [7,15], B100 m2 g − 1, but much higher than [14,18] due to the different route of preparation. The degree of agglomeration GBET/GXRD was obtained from the primary particle size, GBET, determined from the specific surface area, and from the crystallite size, GXRD [8] (Table 1). The lowest agglomeration degree, 0.62, was for the sulfate sample. MgO powders obtained from the nitrate and acetate precursors were
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Fig. 7. Microstructure of MgO powders showing the initial stage of sintering at 1200°C (40 min).
nearly at unity, the ideal value, since the crystallite sizes were almost in accord with the primary particle sizes [8]. Meanwhile, the mineral dolomite almost doubles this value to 1.90; similar to the powder derived from the seawater magnesia process [7]. The analysis by SEM, showed the morphology of Mg(OH)2 and MgO powders originating from different calcination temperatures (Figs. 5 – 7). At 100°C, the brucite phase is observed, which shows microstructural differences for the four precursors (Fig. 5). Samples derived from dolomite and nitrate are formed of small particles consisting of dispersed agglomerates between 5 and 10 mm, and acetate contains flakes forming large agglomerates (30 mm). At 960°C, the periclase powders for these three precursors show the presence of fine particles, less than 1 mm, forming agglomerates with a more homogeneous distribution (Fig. 6). Meanwhile, the microstructure of MgO powders calcined 1 h at 1200°C (Fig. 7), differed significantly from the precipitated hydroxide. The magnesium salt compounds
showed the formation of grains with a homogeneous distribution (less than 500 nm), it was also possible to observe the initial formation of necks between particles. However, dolomite powders showed a more inhomogeneous distribution of grains, between 0.5 and 1 mm, with a higher porosity (dark zones between the grains) due to a rapid grain growth, which brings the entrapment of pores. Magnesium hydroxide derived from sulfate show a plate-like shape, the particles were joined to each other forming spherical particles around 3 mm, which tend to form large agglomerates (30 mm). At higher temperature (960°C), a well-defined plate-like morphology was observed (with a high aspect ratio of length to thickness), which can be related to the periclase phase (Fig. 6); a similar morphology has been reported elsewhere [19]. Also, the particle size increases to 5 mm. A significant change in particle shape was observed at 1200°C, pseudomorph agglomerates were found in the calcined product (Fig. 7).
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The powder activity provides the driving force for sintering, and increases with the surface area, thus with a decrease in the particle size. Moreover, a smaller value of GBET/GXRD indicates that the primary particle is composed of softer agglomerations of crystallites [20]. Therefore, magnesium oxide powders derived from sulfate, which showed the highest surface activity, had the softest agglomeration of crystallites among the four powders, as can be seen on the SEM image at 1200°C (Fig. 7). It has been reported elsewhere [21], that a good densification of aggregates occurs when the growth of crystallites and pores are retarded. Therefore, from the GBET/GDRX data (Table 1), and from the images of SEM at 1200°C, we can conclude that powders from nitrate and acetate show the best physical properties. The mineral dolomite should be easily sintered to highdensity bodies, and can be used to produce magnesia (which should be dead-burnt, i.e. chemically inactive) for the manufacture of heavy-duty basic refractories [17].
4. Conclusions We present a comparative study on the characterisation of different physical properties of MgO powders prepared from three precursor magnesium salts and from the mineral dolomite. The samples were characterised by X-ray diffraction, thermal analysis, N2 adsorption and SEM. The samples at low temperatures are composed of brucite, and after 500°C pure periclase appears, where the degree of crystallinity increases with the temperature of calcination. The apparent density and the specific surface area for brucite and periclase show the following decreasing sequence: sulfate\nitrate \ acetate \ dolomite. Meanwhile, the particle size, the degree of agglomeration and the percentage of the total porosity, present the inverse tendency. Magnesium powders from sulfate precursors, showed the best approach to the ideal density value, due to their high surface area, and consequently, to their smallest primary particle size. Also, the specific area value can be related to the microstructure of lamellars disposed around a central nucleus. Microstructure studies showed that powders derived from acetate and nitrate promotes sufficient diffusion of matter into neck regions between individual particles, favouring the initial stage of sintering and a uniform grain growth. This agrees with the values obtained
.
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for the degree of agglomeration, which are near to one. Dolomite had the biggest primary particle size due to it low surface area, therefore the degree of agglomeration is much higher in comparison with the magnesium salts. The rapid growth of crystallite size promotes the entrapment of pores between the grains, which is reflected in the higher total porosity value.
Acknowledgements We thank CONACYT (Mexico) for supporting this project (3824P-A9607). P.Q. also acknowledges the program Catedra Patrimonial (970030-R98) and M.Sc. D.H. Aguilar for helpful comments and assistance in preparing the manuscript.
References [1] A.N. Copp, Am. Ceram. Soc. Bull. 75 (1996) 135. [2] L. Hench, D. Ulrich ((Eds.), Science of Ceramic Chemical Processing, Wiley, New York, 1986. [3] J.W. Evans, L.C. De Jonghe, The Production of Inorganic Materials, Macmillan, New York, 1991. [4] T.J. Gardner, G.L. Messing, Thermochim. Acta 78 (1984) 17. [5] T.J. Gardner, G.L. Messing, Am. Ceram. Soc. Bull. 63 (1984) 1498. [6] Bokhimi, A. Morales, T. Lopez, R. Gomez, J. Solid State Chem. 115 (1995) 411. [7] K. Itatani, M. Nomura, A. Kishioka, M. Kinoshita, J. Mater. Sci. 21 (1986) 1429. [8] K. Itatani, A. Itoh, F.S. Howell, A. Kishioka, M. Kinoshita, J. Mater. Sci. 28 (1993) 719. [9] U. Chong, D.H. Jeong, Proceedings of UNITCR, Refractories – A, World Wide Technology, 1997, p. 1037. [10] R.W. Rice, AIChE J. 36 (1990) 481. [11] K. Yamamoto, K. Umeya, Am. Ceram. Soc. Bull. 60 (1981) 636. [12] M.H. Bocanegra, J. Mater. Sci. 28 (1993) 3467. [13] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, USA, 1978. [14] S. Ardizzone, C.L. Bianchi, B. Vercelli, J. Mater. Res. 13 (1998) 2218. [15] S.E. Wanke, R.M.J. Fiedorow, in: K.K. Unger, et al. (Eds.), Characterization of Porous Solids, Elsevier, Amsterdam, 1998, p. 601. [16] J. Rodriguez-Carbajal, AFFMA, Mesh Refinement Program, 1985 Filhol 1972. [17] S.F. Estefan, M.B. Morsi, G.A. El-Shobaky, I.F. Hewaidy, Powder Tech. 49 (1987) 143. [18] V.R. Choundhary, V.H. Rane, R.V. Gadre, J. Catal. 145 (1994) 300. [19] J. Staron, S. Palco, Am. Ceram. Soc. Bull. 72 (1993) 83. [20] A.M.M. Gadalla, H.W. Hennicke, Powder Met. Int. 5 (1973) 196. [21] A.W. Hey, D.T. Livey, Trans. Brit. Ceram. Soc. 65 (1966) 627.
Preparation and characterization of MgO powders obtained from different magnesium salts and the mineral dolomite E. Alvarado a, L.M. Torres-Martinez a, A.F. Fuentes a, P. Quintana b,* b
a Facultad de Ciencias Quimicas, DES, UANL, Monterrey, N.L., Mexico Cin6esta6 del IPN, Unidad Merida, km.6 Antigua Carretera a Progreso, C.P. 97310, Merida, Yucatan, Mexico
Received 13 October 1999; accepted 11 May 2000
Abstract The characterisation of the physical properties of MgO powders, obtained from three commercial magnesium compounds, MgSO4·7H2O, MgNO3·6H2O and Mg(CH3CO2)2·4H2O and the mineral dolomite (natural source from Mexico), synthesised by chemical precipitation, is presented. The decomposition of the precipitated Mg(OH)2 was analysed by DTA/TGA and the crystallisation process was observed by XRD. The variation of the properties with the nature of the precursors at 960°C was studied: as the crystallite size, density, specific surface area, degree of agglomeration, and the total porosity. The microstructural differences between the MgO agglomerates were examined by SEM, at different temperatures. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Magnesia; Dolomite; Crystallite size; Degree of agglomeration; X-ray diffraction; SEM
1. Introduction Magnesia (MgO) is the most important product of the magnesium compound industry. It is usually produced for commerce by the thermal decomposition of magnesite (MgCO3), from magnesium hydroxide (Mg(OH)2) obtained from a seawater process or from magnesium rich-brines as the mineral dolomite [1]. Mexico is one of the major producers of MgO; the states of Nuevo Leon and Coahuila contain large deposits of high-purity dolomite; major capacity expansions took place in 1992 when Industria Pen˜oles started with the production of high-purity electrofused MgO in Ramos Arizpe, Coahuila [1]. The most important process for the production of MgO from seawater or brine is based on the fact that Mg(OH)2 can be precipitated from solutions of magnesium salts by the addition of a strong base. The Mg(OH)2 precipitate is washed, filtered and then calcined to produce MgO. Decomposition of solutions of magnesium salts is also important for the synthesis of * Corresponding author. Tel.: + 52-99-81-2960; fax: + 52-99-812917. E-mail address: [email protected] (P. Quintana).
ceramic powders. Many alternative techniques have been developed offering advantages of easy preparation of oxide powders, accurate compositional control, homogeneity and high purity [2–6]. Caustic-calcined MgO is produced at temperatures B 900°C and characterised by its moderate to high chemical reactivity. This type of magnesia has numerous applications as mineral supplement for animal feeds, in fertilisers, as a raw material for various MgO chemicals, to manufacture ceramics, cements, paper, petroleum additives, etc. Meanwhile, calcined dolomite as a source of alkali is used in agriculture, to neutralise soil acidity and adds calcium and magnesium to it; it can also can be used in the manufacture of refractories, as mineral filler, as a constituent in a glass batch, etc. [1]. The physical properties of MgO powders are affected by the type of precursor, the temperature of calcination and the impurities. Actually, one of the main problems detected in the production of caustic magnesia powders is to achieve a real density near to the ideal value, in order to obtain ceramic bodies with suitable compacting conditions. The properties of refractories in industrial processes with low densities, present undesired characteristics such as the formation of low eutectic
0277-5387/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 0 ) 0 0 5 7 0 - 2
2346
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points due to the presence of impurities, vulnerability to chemical attack and a deficient mechanical resistance [7 – 9]. Studies on the synthesis of materials with appropriate refractory properties had been aimed to obtain powders with certain characteristics, which may help to improve the process of sintering [10]. Among these, the following can be mentioned: high surface area, uniform particle size and chemical homogeneity. The formation of large agglomerates must be avoided, since these generate fractures in their surroundings and if present in excess, they can accelerate or delay the sinterability phenomena. The morphology of powders of MgO has been observed using scanning electron microscopy (SEM), to study the origin of the degree of agglomeration in caustic MgO and to analyse the effects during the process of sintering [7,8,11,12]. The aim of this work was to study and characterise the physical properties of MgO powders obtained by chemical precipitation from three commercial chemical compounds to be compared with dolomite (natural source from Coahuila, Mexico). Different measurements were carried out at 960°C (the surface area, the primary particle size, the crystallite size and the apparent density) to determine the degree of agglomeration between the different types of precursor. The crystallization of magnesia was studied by X-ray diffraction, and the development of the morphology was observed by SEM.
2. Experimental Powders of MgO were synthesised by chemical precipitation from four different types of precursors, three of them commercial reagent-grade hydrated salts: MgSO4·7H2O, MgNO3·6H2O and Mg(CH3CO2)2·4H2O and from the mineral dolomite (MgCO3·CaCO3) obtained from Coahuila, Mexico. The chemical analysis of dolomite used in this experiment, measured by X-ray fluorescence with Cr radiation (Diano 8000), indicates a purity of 99.8%. The principal impurities (wt%) were SiO2 (0.01), Al2O3 (0.01), Fe2O3 (0.04), Na2O (0.02) and P2O5 (0.03). The purity, for the three reagent-grade salts, taken from the catalogues of the manufacturers (Productos Quimicos Monterrey, SA), was 99.8%. Each hydrated compound was dissolved in deionized water at room temperature to yield 0.8 M, an excess of ammonium hydroxide (13 M) was added, and vigorously stirred at 50°C, pH 10. A white precipitate of Mg(OH)2 was obtained, which was thoroughly washed with distilled water and dried at 100°C for 2 h. Magnesium hydroxide powders were calcined in Pt crucibles, at different temperatures to produce MgO powders. The heating treatment was carried out in electric furnaces under air atmosphere, from 500 to 1000°C, raising the temperature 100°C every 40 min.
Natural dolomite was initially milled to 200-mesh. The decarbonation procedure was done with a progressive heating from 500 to 800°C, with a stepwise of 100°C every 0.5 h. The sample was held for 2 h and finally the temperature was increased to 900°C (0.5 h) to obtain dolima (MgO·CaO). The hydration of dolima (0.9 M) was made with a vigorously agitation and then it was mixed with a MgCl2 (0.5 M) solution at 50°C, pH 11. Once Mg(OH)2 appeared as a precipitate, it was washed, dried and calcined under the same conditions described above, finally a powder of MgO was obtained. The decomposition characteristics of the salts and dolomite were determined by simultaneous differential/ thermogravimetric analysis (DTA/TGA, TA-Instruments model SDT-2960). The heating rate of the sample was 10°C min − 1, with a flowing N2 atmosphere, using alumina as a reference material. The powders from each calcined treatment were characterised by X-ray diffraction and the crystallite size was calculated using the Scherrer’s formula, using KCl as an internal standard. A Cu Ka radiation (u= 1.5405 A, ) and Ni filter was used. The difractograms were registered in a range of 10–115 2q (30 kV, 20 mA), in a step-scan mode of 0.01° 2q per step and counting for 5 s/step (model Siemens D-5000). MgO apparent density was obtained by the liquid pycnometer method according to ASTM C699, using toluene; several measurements were done until data reproducibility was obtained. The specific surface area and primary particle size of MgO powders was measured by the BET technique (Stro¨hlein), N2 as an absorption gas was used. The morphology of Mg(OH)2 and MgO agglomerates at different temperatures were examined by SEM (Leica S440). Powders were pelletized and coated with carbon to avoid charging during exposure to the electron beam and covered with an Au thin film by sputtering in a vacuum system.
3. Results and discussion The decomposition path of the precipitated, Mg(OH)2, was studied by DTA/TGA analysis. Results obtained from the salts were similar. A very small and a strong endothermic peak was observed; the first peak is related to the dehydration processes at temperatures B 200°C and the second to the decomposition of magnesium hydroxide, the temperature was 420, 310 and 390°C, for sulfate, acetate and nitrate, respectively. Furthermore, sulfate salt shows a continuous weight loss of 7%, between 600 and 920°C. The total weight loss was around 40% (Fig. 1). These results are in good agreement with the ones presented elsewhere [4]. In the case of dolomite, the first endothermic peak was very strong with a weight loss of 13.44 wt% and the decom-
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position reaction was held at 370°C with a weight loss of 25.9%; finally at 800°C evaporation of CO2 occurs (Fig. 1). Therefore, the complete decomposition of the acetate and nitrate can be achieved at reasonably low temperatures (B 450°C); however for the dolomite and sulfate salt this was near 800 and 950°C, respectively. The X-ray diffraction studies on the crystallisation process for the magnesium salts showed the formation
Fig. 3. Diffractograms of MgO powders obtained at 960°C from different precursors: (a) dolomite, (b) nitrate, (c) sulfate, (d) acetate. Traces of MgSO4 ( ) and NaF () are indicated.
Fig. 1. DTA/TGA of Mg(OH)2 obtained from two different precursors.
Fig. 4. Development of crystallite size with temperature on the crystallisation of MgO.
Fig. 2. Comparison of the X-ray diffraction patterns of Mg(OH)2 at 100°C, between sulfate and dolomite. The latest shows the presence of CaCO3 (*).
of brucite Mg(OH)2 at 100°C with a very poor crystallinity, although dolomite presented stronger diffraction peaks as can be seen in Fig. 2. At higher temperature, crystals of periclase (MgO) were initially detected at 500°C after 40 min, with broad diffraction peaks, which became nice and sharp at 960°C, mainly on dolomite, since it had a higher crystallinity (Fig. 3). Additional diffraction lines were detected, as traces of NaF and magnesium sulfate phases. Crystallite size was calculated, by means of Scherrer’s equation [13] on the (200) diffraction maximum, obtained from the different powders heated 40 min at each temperature (500 –1000°C). The behaviour of the crystallite growth with temperature is shown in Fig. 4. The results are very similar for the chemical com-
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Fig. 5. Microstructure of Mg(OH)2 powders obtained from the different precursors, heated at 100°C for 40 min.
pounds. Near to the decomposition temperature of brucite, the magnesium oxide formed has a very small crystallite size, showing a plateau with a slight increment, and with a crystallite growth of 10 – 40 nm. On raising the calcination temperature, over 850°C, a rapid increase in the crystallite size occurs; where dolomite had the highest value of 110 nm at 1000°C, which is double the value obtained for the other precursors. These data are in good agreement with the values
reported in the literature for oxides prepared from different chemical precursor salts [14,15]. The apparent density measurements determined by pycnometry, zapp, at 960°C, are shown in Table 1. The density values of sulfate approach the theoretical data of periclase (3.59 g cm − 3), meanwhile, dolomite shows the lowest value probably due to the higher crystallite size. It has been reported that it is difficult to obtain high-density magnesia from purified raw materials [11].
Table 1 Physical properties of MgO powders obtained at 960°C (40 min) from different precursors Precursor
zapp (g cm−3)
zDRX (g cm−3)
Surface area a (m2 g−1)
Surface area (m2 g−1)
GBET (nm)
GDRX (nm)
GBET/GDRX
Ptot (%)
Sulfate Nitrate Acetate Dolomite
3.53 3.46 3.44 3.42
3.576 3.584 3.581 3.585
137.6 124.2 105.0 47.3
64.4 30.8 28.9 16.5
25.9 54.2 57.7 101.1
41.6 54.9 57.9 53.1
0.62 0.99 0.99 1.90
1.3 3.46 3.94 4.60
a
Values obtained for Mg(OH)2 at 100°C, 40 min.
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Fig. 6. SEM micrographs of MgO powders at 960°C, showing the formation of agglomerates.
Density from X-ray diffraction data was also calculated, zXRD (same as ztrue), using KCl as an internal standard and a least-square refinement program [16] to determine precisely the lattice parameter. The values obtained show good agreement with the theoretical value of periclase. The percentage of total porosity in the different magnesia samples was calculated from
%PT = 1−
zapp × 100 ztrue
according to [17], where the highest value was observed on powders derived from dolomite, 4.6%, and the lowest for sulfate, 1.3%. Acetate and nitrate had 3.5% and 3.9%, respectively. The variation of the specific surface area for brucite and MgO powders can be observed in Table 1. The surface area values were higher for the powders obtained from the reagent-grade salts than those obtained from the mineral dolomite. In all cases MgO had a
much lower value than the corresponding Mg(OH)2. The samples obtained from sulfate show the highest values, which agrees with the smallest particle size. Meanwhile, nitrate and acetate show intermediate values. In contrast, the powders prepared from dolomite had values almost four and two orders of magnitude lower than the data obtained for sulfate and from nitrate and acetate, respectively. This effect can be related to the zapp value, which depends on the utilised precursor and the presence of impurities. The surface area obtained for periclase was in a good agreement with other reports [7,15], B100 m2 g − 1, but much higher than [14,18] due to the different route of preparation. The degree of agglomeration GBET/GXRD was obtained from the primary particle size, GBET, determined from the specific surface area, and from the crystallite size, GXRD [8] (Table 1). The lowest agglomeration degree, 0.62, was for the sulfate sample. MgO powders obtained from the nitrate and acetate precursors were
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Fig. 7. Microstructure of MgO powders showing the initial stage of sintering at 1200°C (40 min).
nearly at unity, the ideal value, since the crystallite sizes were almost in accord with the primary particle sizes [8]. Meanwhile, the mineral dolomite almost doubles this value to 1.90; similar to the powder derived from the seawater magnesia process [7]. The analysis by SEM, showed the morphology of Mg(OH)2 and MgO powders originating from different calcination temperatures (Figs. 5 – 7). At 100°C, the brucite phase is observed, which shows microstructural differences for the four precursors (Fig. 5). Samples derived from dolomite and nitrate are formed of small particles consisting of dispersed agglomerates between 5 and 10 mm, and acetate contains flakes forming large agglomerates (30 mm). At 960°C, the periclase powders for these three precursors show the presence of fine particles, less than 1 mm, forming agglomerates with a more homogeneous distribution (Fig. 6). Meanwhile, the microstructure of MgO powders calcined 1 h at 1200°C (Fig. 7), differed significantly from the precipitated hydroxide. The magnesium salt compounds
showed the formation of grains with a homogeneous distribution (less than 500 nm), it was also possible to observe the initial formation of necks between particles. However, dolomite powders showed a more inhomogeneous distribution of grains, between 0.5 and 1 mm, with a higher porosity (dark zones between the grains) due to a rapid grain growth, which brings the entrapment of pores. Magnesium hydroxide derived from sulfate show a plate-like shape, the particles were joined to each other forming spherical particles around 3 mm, which tend to form large agglomerates (30 mm). At higher temperature (960°C), a well-defined plate-like morphology was observed (with a high aspect ratio of length to thickness), which can be related to the periclase phase (Fig. 6); a similar morphology has been reported elsewhere [19]. Also, the particle size increases to 5 mm. A significant change in particle shape was observed at 1200°C, pseudomorph agglomerates were found in the calcined product (Fig. 7).
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The powder activity provides the driving force for sintering, and increases with the surface area, thus with a decrease in the particle size. Moreover, a smaller value of GBET/GXRD indicates that the primary particle is composed of softer agglomerations of crystallites [20]. Therefore, magnesium oxide powders derived from sulfate, which showed the highest surface activity, had the softest agglomeration of crystallites among the four powders, as can be seen on the SEM image at 1200°C (Fig. 7). It has been reported elsewhere [21], that a good densification of aggregates occurs when the growth of crystallites and pores are retarded. Therefore, from the GBET/GDRX data (Table 1), and from the images of SEM at 1200°C, we can conclude that powders from nitrate and acetate show the best physical properties. The mineral dolomite should be easily sintered to highdensity bodies, and can be used to produce magnesia (which should be dead-burnt, i.e. chemically inactive) for the manufacture of heavy-duty basic refractories [17].
4. Conclusions We present a comparative study on the characterisation of different physical properties of MgO powders prepared from three precursor magnesium salts and from the mineral dolomite. The samples were characterised by X-ray diffraction, thermal analysis, N2 adsorption and SEM. The samples at low temperatures are composed of brucite, and after 500°C pure periclase appears, where the degree of crystallinity increases with the temperature of calcination. The apparent density and the specific surface area for brucite and periclase show the following decreasing sequence: sulfate\nitrate \ acetate \ dolomite. Meanwhile, the particle size, the degree of agglomeration and the percentage of the total porosity, present the inverse tendency. Magnesium powders from sulfate precursors, showed the best approach to the ideal density value, due to their high surface area, and consequently, to their smallest primary particle size. Also, the specific area value can be related to the microstructure of lamellars disposed around a central nucleus. Microstructure studies showed that powders derived from acetate and nitrate promotes sufficient diffusion of matter into neck regions between individual particles, favouring the initial stage of sintering and a uniform grain growth. This agrees with the values obtained
.
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for the degree of agglomeration, which are near to one. Dolomite had the biggest primary particle size due to it low surface area, therefore the degree of agglomeration is much higher in comparison with the magnesium salts. The rapid growth of crystallite size promotes the entrapment of pores between the grains, which is reflected in the higher total porosity value.
Acknowledgements We thank CONACYT (Mexico) for supporting this project (3824P-A9607). P.Q. also acknowledges the program Catedra Patrimonial (970030-R98) and M.Sc. D.H. Aguilar for helpful comments and assistance in preparing the manuscript.
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