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188 IMMC 2016 | 18th International Metallurgy & Materials Congress

Production of Magnesia-Rich Magnesium Aluminate Spinel

Nuray Canikoğlu, H. Özkan Toplan

Sakarya University - Türkiye

Abstract Synthesis of magnesia-rich magnesium aluminate (MgAl2O4) spinel (66%wt. Al2O3 - 34%wt. MgO) after mechanical activation of a powder mixture containing Al2O3 and magnesite waste was investigated. The mechanical activated powder mixture was compacted under a uniaxial pressure of 100 MPa and sintered in the range of 1400°C-1700°C in open atmosphere. The sintered samples were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM). In addition to, the bulk density, the apparent porosity (%) and the microhardness (HV) were measured. The optimum condition for magnesia-rich magnesium aluminate (MgAl2O4) spinel sintering was determined as 1600°C-1h from the activated magnesite waste and Al2O3 powder mixture. In this sintering condition, the microhardness and the bulk density of sample were obtained as 1786 HV and 2,91 g.cm-3, respectively. 1. Introduction Aluminates form in binary systems with alkali, alkaline earth or rare-earth oxides and share the high melting point, high hardness, high strength and resistance to chemical attack of the pure Al2O3 end-member (Table 1) [1]. Magnesium aluminate (MgAl2O4) spinel is an important raw material for refractories and other advanced engineering ceramics, because of its many excellent properties such as high melting point (2135°C), high thermal shock resistance, and excellent slag corrosion

resistance, good mechanical strength [2,3]. Magnesium aluminate powders are commercially synthesized using different techniques. These methods include sol-gel [4,5], co-precipitation [6], hydrothermal [7], microwave- assisted combustion processing [8], microemulsion [9], metal-organic processing [10], spray drying [11], mechanochemical synthesis [12] and solid state synthesis [13] techniques. If the amount of magnesia is higher than the magnesia content of pure spinel composition, then it is known as magnesia rich spinel (MR). If the amount of alumina is higher than the alumina content of pure spinel composition, then it is known as alumina rich spinel (AR). Commercially there are many grades of spinal such as (MR 66, AR 78, AR 90) and by varying the composition [14]. Preparation of spinel can be made by reaction of magnesium and aluminum compounds. When relative cheap raw materials, such as burned magnesite, sea water magnesia, calcined alumina or gibbsite were used, the calcination temperature necessary to obtain complete conversion was about 1450-1600°C [15]. In this work, we presented Mg-rich MgAl2O4 spinel (66%wt. Al2O3 - 34%wt. MgO) production using magnesite waste and alumina by solid state synthesis. In produced samples, the effects of sintering temperature were investigated. The sintered samples were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM). In addition to, the bulk density, the apparent porosity (%) and the microhardness (HV) were measured

Table 1. The physical properties of selected binary aluminate ceramics

Melting

Temperature (K)

Density (g.cm-3)

Hardness (Knoop/100g)

(kg.mm-2)

Compressive Strength (MPa)

Tensile Strength (MPa)

Young’s Modulus

(GPa) -Al2O3 2327 3,98 2000-2050 2549 255 393

CaAl2O4 2143 2,98 MgAl2O4 2408 3,65 1175-1380 1611 129 271 LiAlO2 1883 2,55 350

Y3Al5O12 2243 4,55 1315-1385 280 282

2. Experimental Procedure In this work, the magnesia-rich magnesium aluminate (MgAl2O4) spinel was prepared using magnesite waste and alumina (Al2O3) powders as starting raw materials. Magnesite waste was supplied from KUMA Company,

Turkey and Alumina was supplied from Dura-Bagno Company, Turkey. The chemical compositions of the raw materials as XRF analysis are given in Table 2. The mixture containing 66wt.% Al2O3 - 34wt.% MgO was prepared. The powders were mixed by ball milling for 10 h with alumina balls. The mixture was activated

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mechanically a high-energy planetary ball mill (Fristch) with a rotation speed 600 rpm for 1h. The samples were uniaxially pressed (10 mm diameter x 7 mm height) at 100 MPa and sintered at different temperatures ranging from 1400-1700°C for 1 h in an electric furnace. X-ray diffraction analysis was performed using a Rigaku Ultima X-ray diffractometer and CuK radiation. A Joel 6060 LV scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS) was used for microstructural analysis of sintered samples after the thermal etching (the sintering temperature 50°C under). Bulk density, % apparent porosity, % water absorption values of the sintered samples were measured using Archimedes Principle. Also, hardness values of the sintered samples were measured by Vickers microhardness method. Table 2. Chemical compositions of magnesite waste and alumina powder

Compound (wt.%)

Magnesite waste

Alumina

Al2O3 0,28 99,85 MgO 45 - SiO2 15,56 - CaO 1,79 -

Fe2O3 3,36 - Na2O - 0,06 L.O.I* 33,96 0,29

* Loss on ignition 3. Results and Discussion In Figure 1 was given XRD analysis of sintered samples from 1400°C to 1700°C. According to XRD analysis, MgAl2O4 spinel phase was formed at 1400°C, also enstatite (MgSiO3) phase was determined. The intensities of this peak were decreased with the increased sintering temperature. In this study, MgAl2O4 spinel was synthesized from magnesite waste at a similar temperature and time as literature [16]. So that, with using waste material was synthesized spinel phase more economically.

Figure 1. XRD analysis of the sintered samples at different temperatures for 1h

The bulk density graphic of sintered samples at temperatures between 1400°C and 1700°C was given in Figure 2. Here, the densities were changed from 3,02 g/cm3 to 2,77 g/cm3 from 1400°C to 1700°C with increased sintering temperature. As the literature, the bulk density values were obtained around 3 g/cm3 for MgAl2O4 spinel [17].

Figure 2. Bulk density changes of sintered samples In Figure 3 was shown the apparent porosity (%) values of sintered samples at the different temperatures. The apparent porosities of samples were decreased with increasing sintering temperature. These results have shown that the samples are sintered better with increasing temperature. Sinhamahapatra et all. [18] have producted the magnesia rich magnesium aluminate spinel with the natural magnesite and the synthetic caustic magnesia. But, spinel and periclase and forsterite are found in the samples due to presence of silica as impurities. In our study, only magnesium aluminate spinel was formed at 1600°C. The magnesite contains substantial amount of CaO and SiO2, these may form low melting phases, which promotes densification of samples at lower sintering temperatures. The decrease in apparent porosity of samples; from 1400°C to 1700 °C also indicates that the densification occurred through liquid phase sintering (Figure 7).

Figure 3. Apparent porosity (%) values of sintered samples

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

190 IMMC 2016 | 18th International Metallurgy & Materials Congress

The Vickers hardness of the sintered samples at different temperatures was given in Figure 4. The hardness of samples was increased from 1400°C to 1600°C, later it was decreased up to 1700°C. This situation can be explained by density reduction given in Figure 2 and with increased the glassy phase seen in XRD analysis (Figure 7). The maximum hardness was found in sintered sample at 1600°C as 1786 HV. In this study, the optimum sintering temperature was determined as 1600°C for Mg-rich MgAl2O4 spinel production with activated magnesite waste and Al2O3 powders.

Figure 4. Microhardness values of sintered samples SEM analyses of sintered samples at different temperatures were presented in Figure 5. As seen from micrographs, the structure was more densificated, the porosity was decreased and the grain size was increased with increasing sintering temperature. The morphological features were almost similar for all sintering temperatures. The average grain size was observed range from 5 to 20 m in the sintered sample at 1600°C for 1h. EDS analysis of this sample was given in Figure 6. The analysis was applied for different regions and Mg, O, Al elements were detected in the grains (2 and 3 points), these elements belong to MgAl2O4 spinel. Mg, O, Al, Si and Ca elements were detected between the grains (1 point). It is thought that the oxides in the magnesite waste (Table 1) occur the glassy phase in the regions. The presence the glassy phase was proved in the XRD analysis given in Figure 7.

(a) (b)

(c) (d)

(e) (f)

(g) Figure 5. SEM analyses of the sintered samples at (a) 1400°C, (b) 1450°C, (c) 1500°C, (d) 1550°C, (e) 1600°C, (f) 1650°C and (g) 1700°C for 1h

ELEMENT 1 2 3 O 33.041 32.981 30.852 Mg 17.282 18.224 19.044 Al 29.855 48.795 50.104 Si 17.664 Ca 0.323

Figure 6. EDS analysis of the sintered sample at 1600°C

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Figure 7. XRD analysis of the sintered sample at 1600°C for 1h. 4. Conclusion In this study, the magnesium-rich magnesium aluminate (MgAl2O4) spinel was synthesized from the activated magnesite waste and alumina powder. Firstly, mixed powders were pressed and sintered at 1400, 1450, 1500, 1550, 1600, 1650 and 1700°C for 1 h in the open atmosphere. The sintered samples were examined with same characterization methods. The optimum sintering temperature for samples was determined as 1600°C. In the produced MgAl2O4 sample at this temperature, the bulk density, the micro hardness and the apparent porosity was measured as 2,91 g/cm3, 1786 HV, 0.61%, respectively. References [1] J. F. Shackelford, R. H. Doremus, Ceramic and Glass Materials, Springer, 2008. [2] S. Zhang, D. D. Jayaseelan, G. Bhattacharya, W. E. Lee, J. Am. Ceram. Soc., 89 (2006) 1724-1726. [3] A. Wajler, H. Tomaszewski, E. D. Ciesla, H. Weglarz, Z. Kaszkur, Journal of the European Ceramic Society 28 (2008) 2495-2500. [4] A. Saberi, F. G. Fard, M. W. Porada, Z. Negahdari, C. Liebscher, B. Gossler, Ceramics International, 35 (2009) 933-937. [5] S. Sanjabi, A. Obeydavi, Journal of Alloys and Compounds, 645 (2015) 535-540. [6] M. M. Rashad, Z. I. Zaki, H. El-Shall, Journal of Mater Sci, 44 (2009) 2992-2998. [7] X. Zhang, Materials Chemistry and Physics, 116 (2009) 415-420. [8] I. Ganesh, R. Johnson, G.V.N. Rao, Y.R. Mahajan, S.S. Madavendra and B.M. Reddy, 31(2005) 67-74. [9] F. Meyer, A. Dierstein, C. Beck, W. Härtl, R. Hempelmann, S. Mathur und M. Veith, Nanostruct. Mater. 12 (1999) 71-74.

[10] C. Pacurariu, I. Lazau, Z. Ecsedi, R. Lazau, P. Barvinschi, G. Marginean, Journal of the European Ceramic Society, 27 (2007) 707-710. [11] U. Kanerva, T. Suhonen, J. Lagerbom, E. Levänen,. Ceramics International. 41 (2015) 8494-8500. [12] M.S. Abdi, T. Ebadzadeh, A. Ghaffari, M. Feli, Advanced Powder Technology, 261 (2015) 75-179. [13] I. Ganesh, S. Bhattacharjee, B.P. Saha, R. Johnson, Y.R. Mahajan, Ceramics International, 27 (2001) 773-779. [14] A. Kumar, Development of Magnesia Spinel brick using Pre-synthesized and in-situ spinel, Thesis, Department of Ceramic Engineering National Institute of Technology, 2014, Rourkela, India. [15] A.D. Mazzoni, M.A. Sainz, A. Caballero, E.F. Aglietti, Materials Chemistry and Physics, 78 (2002) 30-37. [16] Z. Zhang, N. Li, Ceramics International, 31 (2005) 583-589. [17] S. Sinhamahapatra, K. Dana, A. Ghosh, V. P. Reddy, H. S. Tripathi, Ceramics International, 41 (2015) 1073-1078. [18] S. Sinhamahapatra, H. S. Tripathi, A. Ghosh,Ceramics International, 42 (2016) 5148-5152.