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The kiln coating formation mechanism of MgO–FeAl2O4 brick
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CERAMICS INTERNATIONAL
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The kiln coating formation mechanism of MgO–FeAl2O4 brick Junhong Chenn, Mingwei Yan, Jindong Su, Bin Li, Jialin Sun School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Received 29 June 2015; received in revised form 19 August 2015; accepted 27 August 2015
Abstract MgO–FeAl2O4 brick used in the burning zone of a cement rotary kiln for 13 months was investigated. The bricks after usage consisted of three part, i.e. the original brick zone, the reaction part and the kiln coating zone. The phase and microstructure of the different part of the brick were analyzed using XRD and SEM with EDS. Based on this, the kiln coating mechanism of MgO–FeAl2O4 brick is proposed. The liquid phase of the cement clinker diffuses into the periclase crystal boundaries of MgO–FeAl2O4 brick, leading to the gradient of Fe and Al ions between the periclase boundary and crystal. Thus, calcium aluminate-ferrite phase (C4AF) is formed and further builds the network and “tree root” structure. This firmly combines C2S grains with the magnesia aggregates and thus the brick possesses good kiln coating formation performance. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: MgO–FeAl2O4 brick; Cement clinker; Kiln coating formation; Morphology
1. Introduction The quality of refractory used at the burning zone of cement rotary kilns is important for the safe operation of the kilns [1]. Magnesia chrome bricks are widely used at the burning zone of cement rotary kilns in the 1960s and 1970s. However the carcinogenic nature of Cr6 þ generated from Cr3 þ during service has led to its replacement by magnesia–spinel, magnesia–zirconia and doloma–zirconia brick [2–10]. Recently researchers have tried to synthesize hercynite (FeAl2O4) and develop MgO–FeAl2O4 bricks using hercynite and magnesia as raw material to replace magnesia chrome bricks used at the burning zone of cement rotary kilns [11–13]. Up to now, MgO–FeAl2O4 bricks have been successfully used in production lines of production scale exceeding 5000 tons a day [14]. Compared with magnesia chrome bricks, MgO–FeAl2O4 bricks have the following advantages. First, it has a good high temperature structural flexibility [11]. Second, the kiln coating protection layer is tightly and firmly combined with the working end of MgO–FeAl2O4 bricks even if the temperature in the rotary kiln is decreased to indoor n
Corresponding author. Tel.: þ86 6233 2666. E-mail address: [email protected] (J. Chen).
temperature. By comparison, magnesia-chrome bricks usually incur breakage between the kiln coating and the working end, as well as between the reaction part and the original brick when the temperature decreases to the indoor temperature. Although several studies on preparation and application of MgO–FeAl2O4 bricks have been reported [12,15–17], there are few research on the mechanism of the kiln coating formation. In this work, MgO–FeAl2O4 brick which had been used in a 7600 t/d (7600 tons a day) production line for 13 months was sampled. The phase and microstructure of the brick at different part were analyzed to explore the kiln coating formation mechanism.
2. Experiment The MgO–FeAl2O4 brick investigated was taken from a 7600 tons per day cement rotary kiln. The specifications of the cement rotary kiln were Ø5.6 m 87 m and the industrial fuel was pulverized coal. The chemical composition and physical properties of the original brick are listed in Table 1. The microstructure and composition of periclase crystals in magnesia aggregate of sintered MgO–FeAl2O4 brick are also
http://dx.doi.org/10.1016/j.ceramint.2015.08.148 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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shown in Fig. 1. It can be seen that Fe and Al ions exist in magnesia aggregates. The length of the brick changed from original 220 mm to 145 mm after 13 months service. According to the structural change, MgO–FeAl2O4 brick can be divided into three parts as shown in Fig. 2, i.e. the original brick zone (I), the reaction part (II) and the kiln coating zone (III). The phases were identified by X-ray diffraction (XRD; M21XVHF22, MAC Science, Yokohama, Japan) using Cu Kα radiation in the angular 10–901. The morphology and composition of the samples were investigated using scanning electron microscopy (FEI Nova 230 Nano). Table 1 The chemical composition and physical properties of the original brick. Chemical composition, wt%
Physical properties
MgO
Al2O3
Fe2O3
others
Apparent porosity (%)
Bulk density (g/cm3)
85.36
6.72
5.28
2.64
17 20
Z2.88
Fig. 1. SEM with EDS of periclase crystals in magnesia aggregate of sintered MgO–FeAl2O4 brick.
3. Results and discussion 3.1. Phase and structure analysis of the three part of MgO– FeAl2O4 brick 3.1.1. Phase and structure analysis of the original brick zone XRD pattern of the original brick zone is shown in Fig. 3. The characteristic peaks at the range of 15–401 are not obvious because the relative intensity of periclase (MgO) phase is high. At higher magnification, it can be seen that the original brick zone mainly consists of MgO, compound spinel ((MgxFe1 x)(FeyAl2 y)O4) and diopside (CaO MgO 2SiO2). SEM with EDS of the magnesia aggregate in the original brick zone is shown in Fig. 4. It can be seen that hercynite crystal exists. With EDS analysis (Area 1), it can be seen that it composes of Mg, Fe, Al and O elements. As for the reacted aggregate part, some bright spots in the periclase crystals exist at higher magnification. By EDS analysis, these bright spots are composed of Mg, Fe, Al and O elements (Area 2). This indicates Mg element in the periclase crystals diffuses into the hercynite grain. In addition, part of Fe and Al atoms in the hercynite grain have diffused into the periclase crystal to form the compound spinel, which has been confirmed in Fig. 1. 3.1.2. Structure and phase analysis of the reaction part Fig. 5 shows the XRD pattern of the reaction part (II) of the brick, which mainly consists of MgO, calcium aluminateferrite phase (Ca4Fe2Al2O10, C4AF) and dicalcium silicate (Ca2SiO4, C2S). While the spinel phase cannot be found, suggesting that spinel phase in MgO–FeAl2O4 brick incurs decomposition due to the seeping of the cement clinker liquid phase into the brick. The magnesia aggregate in this part is analyzed using SEM as shown in Fig. 6(a). It can be seen that the interface of the periclase crystal with the kiln coating is bright. In addition, the combination between the kiln coating and the magnesia aggregate is very tight. Fig. 6(b) shows the SEM image of the magnesia aggregate on the surface the MgO–FeAl2O4 brick at high magnification. Based on EDS analysis (Area 3 and 4), periclase crystals only contain Mg and O element, indicating the purity of periclase crystal on the surface of the MgO–FeAl2O4 brick is improved after usage. While Fe and
Fig. 2. The three part of MgO–FeAl2O4 brick after used at the burning zone for 13 months.
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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Fig. 3. XRD pattern of the original brick zone of (part I).
Fig. 4. SEM with EDS analysis of the magnesia aggregate in the original brick zone (part I).
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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Fig. 5. XRD pattern of the reaction part (part II).
Al atoms inside the periclase crystals have migrated out. The in situ amplified SEM image of the dense kiln coating (Fig. 6 (c)) shows that the kiln coating adhered to the magnesia aggregate consists of two parts, i.e. the gray granular phase and the bright white inter-granular phase. By the EDS analysis, the gray granular phase is composed of Ca, Si and O element (Area 5), while the bright white inter-granular phase consists of Ca, Fe, Al, Mn, Mg and O element (Area 6). In addition, EDS analysis (Area 4 and 6) further shows that the element composition between the periclase crystal boundaries is the same as that of the inter-granular phase of the kiln coating. Combining the XRD pattern (Fig. 5), the gray granular phase of the kiln coating should be C2S. The periclase inter-granular phase and the C2S inter-granular phase should be the calcium aluminate-ferrite phase (C4AF) 18. 3.1.3. Structure analysis of the kiln coating zone Fig. 7 shows SEM with EDS of the kiln coating zone. It can be seen that the kiln coating zone consists of the loose zone and the dense zone. The loose zone is mixed with some dark gray grains compared with the dense kiln coating zone. EDS analysis (Area 7) shows that these dark gray grains are f-CaO, while the dense kiln coating zone do not contain f-CaO. In view of the dense zone, the “tree root” structure and the network structure (blue part in Fig. 8) is firmly fix the kiln coating to the surface of the MgO–FeAl2O4 bricks. Based on the analysis of Figs. 6–8, due to the diffusion and migration of Fe and Al ions, the calcium aluminate-ferrite phase generated on the surface of the MgO–FeAl2O4 bricks is at last distributed among C2S grains of the dense kiln coating zone as the network combining phase. Therefore the “tree root” structure is rooted between the periclase crystals. 3.2. The kiln coating formation mechanism From above result, the kiln coating formation mechanism is proposed as following. Fig. 9 shows the schematic diagram of
the diffusion and migration of Fe and Al ions inside the periclase crystals. The sintering temperature of the cement clinker is in the range of 1400–1450 1C; the liquid phase in the cement clinker is up to 20–30%. In addition, the apparent porosity of MgO–FeAl2O4 bricks used at the burning zone of cement rotary kilns is usually 17–20 vol%. Therefore the liquid phase in the cement clinker will permeate into the MgO–FeAl2O4 bricks at elevated temperature. At high temperature, f-CaO in the liquid phase of the cement clinker exists as ionic state by the following reaction: CaO-Ca2 þ þ O2 The phases of SiO2, Fe2O3 and Al2O3 react with O2 from the dissociation of f-CaO to form the compound anions such as SiO4− 4 , AlO−2 , FeO−2 , etc. Ca2 þ in the liquid phase further reacts with these anions to form the complex compounds such as C2S, C3S, C3A and C4AF. As a result, Fe2O3 and Al2O3 in the liquid phase with high content of f-CaO basically exist in the form of C3A and C4AF. The content of Fe and Al ions is very small. When the cement clinker liquid phase permeates into MgO–FeAl2O4 brick, Fe and Al ions in the liquid phase of the periclase crystal boundaries mainly exist in the form of the compound anions such as AlO−2 and FeO−2 . Therefore a content gradient of Fe and Al ions exists between the periclase crystal boundaries and crystals. This leads Fe and Al ions inside the periclase crystals spontaneously diffuse and migrate into the periclase crystal boundaries. Since the periclase crystal boundaries accepts Fe and Al ions from the periclase crystals, the concentration of Fe and Al ions in the liquid phase increases. This concentration has a gradient relative to the concentration of Fe and Al ions in the liquid phase of the cement clinker. In addition, the concentration of f-CaO in the liquid phase of the cement clinker is relatively high. Therefore, Fe and Al ions in the liquid phase of the periclase crystal boundaries constantly migrate towards the liquid phase of the cement clinker on the surface of MgO–FeAl2O4
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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Fig. 6. SEM and EDS between the MgO–FeAl2O4 brick and the dense kiln coating (part II).
brick and form the calcium aluminate-ferrite phase (C4AF) on the surface. Finally, Fe and Al ions inside the periclase crystals in the form of the calcium aluminate-ferrite phase diffuse and migrate onto the surface of MgO–FeAl2O4 brick until Fe and Al ions inside the periclase crystals are completely migrated out. During the diffusion and migration process of Fe and Al ions, the liquid phase at the periclase crystal boundaries serves as the “transportation channel”. After diffusing and migrating to the surface of the MgO–FeAl2O4 bricks, Fe and Al ions inside the periclase crystals react with f-CaO from the cement clinker to form the calcium aluminate-ferrite phase, which results in the formation of the dense kiln coating on the surface of the MgO–FeAl2O4
bricks. With time continuing, the thickness of the dense kiln coating generated on the surface of the MgO–FeAl2O4 bricks increases. When the diffusion and migration of Fe and Al ions end, the increase of the thickness of the dense kiln coating accordingly stops. However because the cement clinker adheres to the dense kiln coating, the kiln coating finally consists of two zones, which has been confirmed by Fig. 7. 4. Conclusions MgO–FeAl2O4 brick has been used in the burning zone of a cement rotary kiln for 13 months. The good kiln coating formation mechanism is discussed based on the phase and
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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microstructure of the different part of the brick after usage. It is found that the liquid phase of the cement clinker diffuses into the periclase crystal boundaries of MgO–FeAl2O4 brick and
causes the content gradient of Fe and Al ions between the periclase boundary and crystal. This leads to the formation of calcium aluminate-ferrite phase (C4AF). C4AF firmly bonds
Fig. 7. SEM and EDS of the kiln coating zone (part III).
Fig. 8. SEM of the network structure and the “tree root” structure on the surface of the MgO–FeAl2O4 brick. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 9. The schematic diagram of the diffusion and migration of the Fe and Al ions inside the periclase crystals.
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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C2S grains with the magnesia aggregates to act as the “tree root” structure on the surface of MgO–FeAl2O4 brick. This is the main reason that MgO–FeAl2O4 bricks possess good kiln coating performance. Acknowledgments The authors express their appreciation to National Nature Science Foundation of China of No. 51172021, and the National ScienceTechnology Support Plan Projects (Grant 2013BAF09B01). References [1] D.H. Kim, C.J. Uym, S.J. Lee, The high-temperature properties of basic bricks for cement kilns with Fe2O3 bearing magnesia clinker, In: Proceedings of the Unified International Technical Conference on Refractories (UNITECR'2003), 2003, pp. 47–50. [2] W.E. Lee, R.E. Moore, Evolution of in-situ refractories in the 20th century, J. Am. Ceram. Soc. 81 (1998) 1385–1410. [3] D.J. Bray, Toxicity of chromium compounds formed in refractories, Am. Ceram. Soc. Bull. 64 (1985) 1012–1016. [4] S. Ghanbarnezhad, A. Nemati, M. Bavand-Vandchali, R. Naghizadeh, New development of spinel bonded chrome-free basic brick, J. Chem. Eng. Mater. Sci. 4 (2013) 7–12. [5] A. Ghosh, R. Sarkar, B. Mukherjee, S.K. Das, Effect of spinel content on the properties of magnesia–spinel composite refractory, J. Eur. Ceram. Soc. 24 (2004) 2079–2085. [6] Z.H. Zhang, N. Li, Effect of polymorphism of Al2O3 on the synthesis of magnesium aluminate spinel, Ceram. Int. 31 (2005) 583–589. [7] Z.Q. Guo, S. Palco, M. Rigaud, Bonding of cement clinker onto dolomabased refractories, J. Am. Ceram. Soc. 88 (2005) 1481–1487.
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[8] M. Chen, C. Lu, J. Yu, Improvement in performance of MgO–CaO refractories by addition of nano-sized ZrO2, J. Eur. Ceram. Soc. 27 (2007) 4633–4638. [9] S. Serena, M.A. Sainz, A. Caballero, The system Clinker–MgO–CaZrO3 and its application to the corrosion behavior of CaZrO3/MgO refractory matrix by clinker, J. Eur. Ceram. Soc. 29 (2009) 2199–2209. [10] E.A. Rodríguez, G.A. Castillo, T.K. Das, R. Puente-Ornelas, Y. González, A.M. Arato, J.A. Aguilar-Martínez, MgAl2O4 spinel as an effective ceramic bonding in a MgO–CaZrO3 refractory, J. Eur. Ceram. Soc. 33 (2013) 2767–2774. [11] Z.Q. Guo, Control of microstructure of basic refractories for cement rotary kilns, Bull. Chin. Ceram. Soc. 2 (2006) 51–56. [12] J.H. Chen, L.Y. Yu, J.L. Sun, Y. Li, W.D. Xue, Synthesis of hercynite by reaction sintering, J. Eur. Ceram. Soc. 31 (2011) 259–263. [13] B. Gerald, M.T. Homas, Magnesia-hercynite bricks—an innovative burnt basic refractory, in: Proceedings of UNITECR'99, Osaka, Japan, 1999, pp. 201–203. [14] J.H. Chen, S.J. Feng, J.L. Sun, J.S. Feng, Synthesis of hercynite and properties and applications of magnesia hercynite spinel brick, Refractories 45 (6) (2011) 457–461 Chinese version. [15] B. Lavina, F. Princivalle, A.D. Giusta, Controlled time–temperature oxidation reaction in a synthetic Mg-hercynite, Phys. Chem. Miner. 32 (2) (2005) 83–88. [16] G.P. Liu, N. Li, W. Yan, G.H. Tao, Y.Y. Li, Composition and structure of a composite spinel made from magnesia and hercynite, J. Ceram. Process. Res. 13 (2012) 480–485. [17] G.P. Liu, N. Li, W. Yan, C.H. Gao, W. Zhou, Y.Y. Li, Composition and microstructure of a periclase–composite spinel brick used in the burning zone of a cement rotary kiln, Ceram. Int. 40 (2014) 8149–8155. [18] H.F.W. Taylor, Cement Chemistry, 2nd edition, Thomas Telford Publishing, Thomas Telford Services Ltd., 1 Heron Quay, London E144JD, 1997.
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
The kiln coating formation mechanism of MgO–FeAl2O4 brick Junhong Chenn, Mingwei Yan, Jindong Su, Bin Li, Jialin Sun School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Received 29 June 2015; received in revised form 19 August 2015; accepted 27 August 2015
Abstract MgO–FeAl2O4 brick used in the burning zone of a cement rotary kiln for 13 months was investigated. The bricks after usage consisted of three part, i.e. the original brick zone, the reaction part and the kiln coating zone. The phase and microstructure of the different part of the brick were analyzed using XRD and SEM with EDS. Based on this, the kiln coating mechanism of MgO–FeAl2O4 brick is proposed. The liquid phase of the cement clinker diffuses into the periclase crystal boundaries of MgO–FeAl2O4 brick, leading to the gradient of Fe and Al ions between the periclase boundary and crystal. Thus, calcium aluminate-ferrite phase (C4AF) is formed and further builds the network and “tree root” structure. This firmly combines C2S grains with the magnesia aggregates and thus the brick possesses good kiln coating formation performance. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: MgO–FeAl2O4 brick; Cement clinker; Kiln coating formation; Morphology
1. Introduction The quality of refractory used at the burning zone of cement rotary kilns is important for the safe operation of the kilns [1]. Magnesia chrome bricks are widely used at the burning zone of cement rotary kilns in the 1960s and 1970s. However the carcinogenic nature of Cr6 þ generated from Cr3 þ during service has led to its replacement by magnesia–spinel, magnesia–zirconia and doloma–zirconia brick [2–10]. Recently researchers have tried to synthesize hercynite (FeAl2O4) and develop MgO–FeAl2O4 bricks using hercynite and magnesia as raw material to replace magnesia chrome bricks used at the burning zone of cement rotary kilns [11–13]. Up to now, MgO–FeAl2O4 bricks have been successfully used in production lines of production scale exceeding 5000 tons a day [14]. Compared with magnesia chrome bricks, MgO–FeAl2O4 bricks have the following advantages. First, it has a good high temperature structural flexibility [11]. Second, the kiln coating protection layer is tightly and firmly combined with the working end of MgO–FeAl2O4 bricks even if the temperature in the rotary kiln is decreased to indoor n
Corresponding author. Tel.: þ86 6233 2666. E-mail address: [email protected] (J. Chen).
temperature. By comparison, magnesia-chrome bricks usually incur breakage between the kiln coating and the working end, as well as between the reaction part and the original brick when the temperature decreases to the indoor temperature. Although several studies on preparation and application of MgO–FeAl2O4 bricks have been reported [12,15–17], there are few research on the mechanism of the kiln coating formation. In this work, MgO–FeAl2O4 brick which had been used in a 7600 t/d (7600 tons a day) production line for 13 months was sampled. The phase and microstructure of the brick at different part were analyzed to explore the kiln coating formation mechanism.
2. Experiment The MgO–FeAl2O4 brick investigated was taken from a 7600 tons per day cement rotary kiln. The specifications of the cement rotary kiln were Ø5.6 m 87 m and the industrial fuel was pulverized coal. The chemical composition and physical properties of the original brick are listed in Table 1. The microstructure and composition of periclase crystals in magnesia aggregate of sintered MgO–FeAl2O4 brick are also
http://dx.doi.org/10.1016/j.ceramint.2015.08.148 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
J. Chen et al. / Ceramics International ] (]]]]) ]]]–]]]
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shown in Fig. 1. It can be seen that Fe and Al ions exist in magnesia aggregates. The length of the brick changed from original 220 mm to 145 mm after 13 months service. According to the structural change, MgO–FeAl2O4 brick can be divided into three parts as shown in Fig. 2, i.e. the original brick zone (I), the reaction part (II) and the kiln coating zone (III). The phases were identified by X-ray diffraction (XRD; M21XVHF22, MAC Science, Yokohama, Japan) using Cu Kα radiation in the angular 10–901. The morphology and composition of the samples were investigated using scanning electron microscopy (FEI Nova 230 Nano). Table 1 The chemical composition and physical properties of the original brick. Chemical composition, wt%
Physical properties
MgO
Al2O3
Fe2O3
others
Apparent porosity (%)
Bulk density (g/cm3)
85.36
6.72
5.28
2.64
17 20
Z2.88
Fig. 1. SEM with EDS of periclase crystals in magnesia aggregate of sintered MgO–FeAl2O4 brick.
3. Results and discussion 3.1. Phase and structure analysis of the three part of MgO– FeAl2O4 brick 3.1.1. Phase and structure analysis of the original brick zone XRD pattern of the original brick zone is shown in Fig. 3. The characteristic peaks at the range of 15–401 are not obvious because the relative intensity of periclase (MgO) phase is high. At higher magnification, it can be seen that the original brick zone mainly consists of MgO, compound spinel ((MgxFe1 x)(FeyAl2 y)O4) and diopside (CaO MgO 2SiO2). SEM with EDS of the magnesia aggregate in the original brick zone is shown in Fig. 4. It can be seen that hercynite crystal exists. With EDS analysis (Area 1), it can be seen that it composes of Mg, Fe, Al and O elements. As for the reacted aggregate part, some bright spots in the periclase crystals exist at higher magnification. By EDS analysis, these bright spots are composed of Mg, Fe, Al and O elements (Area 2). This indicates Mg element in the periclase crystals diffuses into the hercynite grain. In addition, part of Fe and Al atoms in the hercynite grain have diffused into the periclase crystal to form the compound spinel, which has been confirmed in Fig. 1. 3.1.2. Structure and phase analysis of the reaction part Fig. 5 shows the XRD pattern of the reaction part (II) of the brick, which mainly consists of MgO, calcium aluminateferrite phase (Ca4Fe2Al2O10, C4AF) and dicalcium silicate (Ca2SiO4, C2S). While the spinel phase cannot be found, suggesting that spinel phase in MgO–FeAl2O4 brick incurs decomposition due to the seeping of the cement clinker liquid phase into the brick. The magnesia aggregate in this part is analyzed using SEM as shown in Fig. 6(a). It can be seen that the interface of the periclase crystal with the kiln coating is bright. In addition, the combination between the kiln coating and the magnesia aggregate is very tight. Fig. 6(b) shows the SEM image of the magnesia aggregate on the surface the MgO–FeAl2O4 brick at high magnification. Based on EDS analysis (Area 3 and 4), periclase crystals only contain Mg and O element, indicating the purity of periclase crystal on the surface of the MgO–FeAl2O4 brick is improved after usage. While Fe and
Fig. 2. The three part of MgO–FeAl2O4 brick after used at the burning zone for 13 months.
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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Fig. 3. XRD pattern of the original brick zone of (part I).
Fig. 4. SEM with EDS analysis of the magnesia aggregate in the original brick zone (part I).
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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Fig. 5. XRD pattern of the reaction part (part II).
Al atoms inside the periclase crystals have migrated out. The in situ amplified SEM image of the dense kiln coating (Fig. 6 (c)) shows that the kiln coating adhered to the magnesia aggregate consists of two parts, i.e. the gray granular phase and the bright white inter-granular phase. By the EDS analysis, the gray granular phase is composed of Ca, Si and O element (Area 5), while the bright white inter-granular phase consists of Ca, Fe, Al, Mn, Mg and O element (Area 6). In addition, EDS analysis (Area 4 and 6) further shows that the element composition between the periclase crystal boundaries is the same as that of the inter-granular phase of the kiln coating. Combining the XRD pattern (Fig. 5), the gray granular phase of the kiln coating should be C2S. The periclase inter-granular phase and the C2S inter-granular phase should be the calcium aluminate-ferrite phase (C4AF) 18. 3.1.3. Structure analysis of the kiln coating zone Fig. 7 shows SEM with EDS of the kiln coating zone. It can be seen that the kiln coating zone consists of the loose zone and the dense zone. The loose zone is mixed with some dark gray grains compared with the dense kiln coating zone. EDS analysis (Area 7) shows that these dark gray grains are f-CaO, while the dense kiln coating zone do not contain f-CaO. In view of the dense zone, the “tree root” structure and the network structure (blue part in Fig. 8) is firmly fix the kiln coating to the surface of the MgO–FeAl2O4 bricks. Based on the analysis of Figs. 6–8, due to the diffusion and migration of Fe and Al ions, the calcium aluminate-ferrite phase generated on the surface of the MgO–FeAl2O4 bricks is at last distributed among C2S grains of the dense kiln coating zone as the network combining phase. Therefore the “tree root” structure is rooted between the periclase crystals. 3.2. The kiln coating formation mechanism From above result, the kiln coating formation mechanism is proposed as following. Fig. 9 shows the schematic diagram of
the diffusion and migration of Fe and Al ions inside the periclase crystals. The sintering temperature of the cement clinker is in the range of 1400–1450 1C; the liquid phase in the cement clinker is up to 20–30%. In addition, the apparent porosity of MgO–FeAl2O4 bricks used at the burning zone of cement rotary kilns is usually 17–20 vol%. Therefore the liquid phase in the cement clinker will permeate into the MgO–FeAl2O4 bricks at elevated temperature. At high temperature, f-CaO in the liquid phase of the cement clinker exists as ionic state by the following reaction: CaO-Ca2 þ þ O2 The phases of SiO2, Fe2O3 and Al2O3 react with O2 from the dissociation of f-CaO to form the compound anions such as SiO4− 4 , AlO−2 , FeO−2 , etc. Ca2 þ in the liquid phase further reacts with these anions to form the complex compounds such as C2S, C3S, C3A and C4AF. As a result, Fe2O3 and Al2O3 in the liquid phase with high content of f-CaO basically exist in the form of C3A and C4AF. The content of Fe and Al ions is very small. When the cement clinker liquid phase permeates into MgO–FeAl2O4 brick, Fe and Al ions in the liquid phase of the periclase crystal boundaries mainly exist in the form of the compound anions such as AlO−2 and FeO−2 . Therefore a content gradient of Fe and Al ions exists between the periclase crystal boundaries and crystals. This leads Fe and Al ions inside the periclase crystals spontaneously diffuse and migrate into the periclase crystal boundaries. Since the periclase crystal boundaries accepts Fe and Al ions from the periclase crystals, the concentration of Fe and Al ions in the liquid phase increases. This concentration has a gradient relative to the concentration of Fe and Al ions in the liquid phase of the cement clinker. In addition, the concentration of f-CaO in the liquid phase of the cement clinker is relatively high. Therefore, Fe and Al ions in the liquid phase of the periclase crystal boundaries constantly migrate towards the liquid phase of the cement clinker on the surface of MgO–FeAl2O4
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Fig. 6. SEM and EDS between the MgO–FeAl2O4 brick and the dense kiln coating (part II).
brick and form the calcium aluminate-ferrite phase (C4AF) on the surface. Finally, Fe and Al ions inside the periclase crystals in the form of the calcium aluminate-ferrite phase diffuse and migrate onto the surface of MgO–FeAl2O4 brick until Fe and Al ions inside the periclase crystals are completely migrated out. During the diffusion and migration process of Fe and Al ions, the liquid phase at the periclase crystal boundaries serves as the “transportation channel”. After diffusing and migrating to the surface of the MgO–FeAl2O4 bricks, Fe and Al ions inside the periclase crystals react with f-CaO from the cement clinker to form the calcium aluminate-ferrite phase, which results in the formation of the dense kiln coating on the surface of the MgO–FeAl2O4
bricks. With time continuing, the thickness of the dense kiln coating generated on the surface of the MgO–FeAl2O4 bricks increases. When the diffusion and migration of Fe and Al ions end, the increase of the thickness of the dense kiln coating accordingly stops. However because the cement clinker adheres to the dense kiln coating, the kiln coating finally consists of two zones, which has been confirmed by Fig. 7. 4. Conclusions MgO–FeAl2O4 brick has been used in the burning zone of a cement rotary kiln for 13 months. The good kiln coating formation mechanism is discussed based on the phase and
Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148
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microstructure of the different part of the brick after usage. It is found that the liquid phase of the cement clinker diffuses into the periclase crystal boundaries of MgO–FeAl2O4 brick and
causes the content gradient of Fe and Al ions between the periclase boundary and crystal. This leads to the formation of calcium aluminate-ferrite phase (C4AF). C4AF firmly bonds
Fig. 7. SEM and EDS of the kiln coating zone (part III).
Fig. 8. SEM of the network structure and the “tree root” structure on the surface of the MgO–FeAl2O4 brick. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 9. The schematic diagram of the diffusion and migration of the Fe and Al ions inside the periclase crystals.
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C2S grains with the magnesia aggregates to act as the “tree root” structure on the surface of MgO–FeAl2O4 brick. This is the main reason that MgO–FeAl2O4 bricks possess good kiln coating performance. Acknowledgments The authors express their appreciation to National Nature Science Foundation of China of No. 51172021, and the National ScienceTechnology Support Plan Projects (Grant 2013BAF09B01). References [1] D.H. Kim, C.J. Uym, S.J. Lee, The high-temperature properties of basic bricks for cement kilns with Fe2O3 bearing magnesia clinker, In: Proceedings of the Unified International Technical Conference on Refractories (UNITECR'2003), 2003, pp. 47–50. [2] W.E. Lee, R.E. Moore, Evolution of in-situ refractories in the 20th century, J. Am. Ceram. Soc. 81 (1998) 1385–1410. [3] D.J. Bray, Toxicity of chromium compounds formed in refractories, Am. Ceram. Soc. Bull. 64 (1985) 1012–1016. [4] S. Ghanbarnezhad, A. Nemati, M. Bavand-Vandchali, R. Naghizadeh, New development of spinel bonded chrome-free basic brick, J. Chem. Eng. Mater. Sci. 4 (2013) 7–12. [5] A. Ghosh, R. Sarkar, B. Mukherjee, S.K. Das, Effect of spinel content on the properties of magnesia–spinel composite refractory, J. Eur. Ceram. Soc. 24 (2004) 2079–2085. [6] Z.H. Zhang, N. Li, Effect of polymorphism of Al2O3 on the synthesis of magnesium aluminate spinel, Ceram. Int. 31 (2005) 583–589. [7] Z.Q. Guo, S. Palco, M. Rigaud, Bonding of cement clinker onto dolomabased refractories, J. Am. Ceram. Soc. 88 (2005) 1481–1487.
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Please cite this article as: J. Chen, et al., The kiln coating formation mechanism of MgO–FeAl2O4 brick, Ceramics International (2015), http://dx.doi.org/ 10.1016/j.ceramint.2015.08.148