The development of innovative chrome-free basic bricks based on
electrofused MgO-CaZrO3 technology using as ceramic bonding
two different types of spinel (hercynite and magnesium-aluminium
spinel) to improve their properties is presented. Microstructural,
physical, mechanical, and thermal properties of electrofused
MgO-CaZrO3 matrix were determined. Static and dynamic resistance
tests by chemical attack of cement raw constituents were carried
out. The results showed that thermo-mechanical properties
of new bricks significantly improved with increasing spinel content.
Microstructural analysis revealed that the spinel phase aided
to develop a strong bond between the magnesia and calcium zirconate
refractory phases. Finally, this refractory matrix exhibits a
good thermal stability and excellent chemical resistance against
cement raw meal

GUADALUPE ALAN CASTILLO RODRIGUEZ

English

Repositorio Institucional de CIMAV (Centro de Investigación en Materiales Avanzados)

Industrial Application120654th International Colloquium on Refractories 2011 – Refractories for IndustrialsIMPROVEMENT OF AN ELECTROFUSED MGO-CAZRO3 REFRACTORY MATRIX BY THE ADDITION OF HERCYNITE SPINEL AND MAGNESIUM-ALUMINIUM SPINEL FOR THE CEMENT INDUSTRYG-Alan Castillo, José Contreras, Fabiola Dávila, Laura García, Cristian Gómez, Yadira González, Ana-María Guzmán,  Edén Rodríguez*Programa Doctoral en Ingeniería de Materiales, FIME – UANL; Nuevo León, MéxicoJ.A. Aguilar-MartínezCentro de Investigación en Materiales Avanzados, S.C. (CIMAV); Alianza Norte No. 202,  Parque de Investigación e Innovación Tecnológica (PIIT), Nueva carr. Aeropuerto Km. 10 Apodaca N.L. México, 66600ABSTRACTThe development of innovative chrome-free basic bricks based on electrofused MgO-CaZrO3 technology using as ceramic bonding two different types of spinel (hercynite and magnesium-alumin-ium spinel) to improve their properties is presented. Microstruc-tural, physical, mechanical, and thermal properties of electrofused MgO-CaZrO3 matrix were determined. Static and dynamic resist-ance tests by chemical attack of cement raw constituents were car-ried out. The results showed that thermo-mechanical properties of new bricks significantly improved with increasing spinel con-tent. Microstructural analysis revealed that the spinel phase aided to develop a strong bond between the magnesia and calcium zir-conate refractory phases. Finally, this refractory matrix exhibits a good thermal stability and excellent chemical resistance against cement raw meal.Keywords: MgO-CaZrO3 refractories, Hercynite, Magnesium al-uminium spinel, rotary cement kiln.INTRODUCTIONRecently, the substitution of fossil fuels by the use of second-ary fuels and industrial waste (eg. used tires, coal ash and sewage sludges), associated with the modernization and improvements of the cement rotary kiln themselves have drastically affected the performance life of the refractory bricks. Those changes have driven the refractory industry to create and develop improved re-fractory lining products[1].Newest refractory trends, for improving the properties of lining are related to the optimization of the matrix by careful design of the phase combinations and the microstructure characteristics. There is no doubt that, one of the main microstructural character-istic that has to taking into account for contributing to the devel-opment of a reliable refractory matrix is the bond. By increasing the bonding strength, the resistance against many kinds of stresses during performance and structural spalling would be improved. In addition, a high strength brick frequently has been linked to a rig-id matrix; however cement rotary kilns require refractory bricks with sufficient structural flexibility and stress absorbent to prevent cracking and peeling off from the hot face.By other hand, MgO-CaZrO3 materials are well known to be com-patible phases, do not form a liquid phase up to temperatures higher than 2060ºC and they are highly resistant in aggressive basic environments and atmospheres with high alkali contents at temperature up to 1400ºC. In this kind of refractory matrix, the formation of a so-called elastic direct bonding between MgO and CaZrO3 and the high refractoriness of CaZrO3 (2340ºC), allow to reach a high hot mechanical strength and excellent corrosion re-sistance against alkali, earth alkali oxides and basic slags[2-14]. Kozuka et al. in Japan have studied the behavior of magnesia-calcium zirconate refractory bricks in rotary cement kilns. The conclusion of this study proved that the bricks showed superior corrosion resistance and good coating adherence but peeled off easily in high stressed areas[15,16].According to the above mentioned, the objectives of this research are the study, development and the evaluation of physical, me-chanical, thermo-mechanical and chemical properties, as well as microstructural characteristics of a new free-chrome refrac-tory brick composes of a matrix made of electrofused magnesia-calcium zirconate (MgO-CaZrO3) with little addition of magne-sium aluminate spinel (MgAl2O4) and hercinite (FeAl2O4) acting as bonding linkage. Due to the importance of the development of a strong bonding structure in the refractory matrixes, the influ-ences of varying the amount of both spinel phases (MgAl2O4 and  FeAl2O4) on the refractory matrix were established. Additionally, static and dynamic resistance tests to chemical attack by cement raw meal were carried out.EXPERIMENTAL PROCEDURERaw materialSince, raw material characteristics play an important role in get-ting suitable final refractory properties; the use of high purity raw material was a necessary issue (even in industrial scale). The chemical composition of the raw material used for the develop-ment of new refractory bricks is given in table 1. For the sintered magnesia a wide particle size distribution (coarse, intermediate and fine grains) was used; for electrofused magnesia-calcium zir-conate mixture only coarse and intermediate grains were used. Fine particle grains (‹45µm) were utilized for ZrO2, MgAl2O4 and FeAl2O4. The addition of a fine particle size for spinel and zirconia is due to a well known superior reaction force with the surround-ing matrix. The crystalline phases identified by XRD analysis for the starting raw meal were: MgO (periclase-MgO), MgO- CaZrO3 (periclase-MgO, calcium zirconate-CaZrO3 and non-ste-quiometric calcium zirconate-CaO0.15ZrO0.85O1.85), MgAl2O4 (per-iclase-MgO and spinel-MgAl2O4), FeAl2O4 (hercinite-FeAl2O4) and ZrO2 (baddeleyite-ZrO2).Refractory matrixIn order to evaluate the effect of magnesium aluminate and her-cynite spinels on an electrofused MgO-CaZrO3 matrix, nine re-fractory formulations were studied; the first one without addition of spinel (formulation 1) and the rest with variations in the spi-nel contents. The refractory formulations are shown in table 2. The addition of a small quantity of fine zirconia particles in all formulations responds mainly to avoid free lime in the refracto-ry matrix and promotes the formation of an in. situ. calcium zir-conate phase.Preparation and characterization of refractory bricksMixtures with compositions according to the proportions given in table 2 were prepared. Dextrine was added to bind the mixtures and for providing maximum stability of the green samples dur-Industrial Application 1216October 19th and 20th, 2011 · EUROGRESS, Aachen, Germanying subsequent handling prior to firing. The powders were uni-axial pressed into a metallic mould obtaining refractory bricks of 228x114x76 mm. After pressing, green compacts were sintering at 1650ºC for 7 h in an industrial tunnel kiln. Physical properties such as apparent porosity, bulk density, cold crushing strength, cold modulus of rupture and hot modulus of rupture at 1260°C were measured based on ASTM standards. Phase analyses of the sintered samples were carried out by X-ray diffraction study (Philips X’pert) using Cu-K radiation in the diffraction range of 5–90°. Sintered samples morphology was examined using scan-ning electron microscope (JSM-6490, Joel) with an energy dis-persive scanning attachment for qualitative and quantitative mi-croanalysis.Static and dynamic chemical attack test by clinker raw meal pow-ders (see table 3) were executed for evaluating corrosion resist-ence and coatability of free-spinel MgO-CaZrO3 matrix and MgO-CaZrO3 matrixes with spinel addition. The static method intends to make raw meal contact with the refractory and evalu-ates clinker adherences by heating. Using a crucible (114x114x76 mm) made of refractory brick, a hole was bored in the sample brick with the dimensions 50x50 mm and filled it with clink-er raw meal. The crucible sample was heated in an gas furnace for 4 h at 1450 ºC. The adherence of the clinker on the refracto-ry was determined according to the following criteria: nill adher-ence (clinker pellet is removed by inverting the sample up side down), moderate adherence (clinker pellet is removed by slight-ly hitting) and strong adherence (clinker pellet is practically fused with the refractory).For the dynamic method a laboratory rotary kiln was lined with the refractory to be tested. The temperature was increased to 1450 ºC with an air/O2 burner. The raw meal (the amount of 3–5 kg) was fed continually during 4 h rotating the kiln to 1 r.p.m. After the test, the degree of penetration and the microstructure damage of the refractory was evaluated by electronic microscopy on polished sections.Tab. 3: Chemical composition of clinker raw meal.Compositions (% by weight)CaO SiO2 Al2O3 Fe2O3 MgO K2O Na2O44.79 11.86 3.03 1.60 0.88 0.47 0.24RESULTS AND DISCUSSIONPhysical propertiesPhysical properties from each refractory formulation are provid-ed in table 4. According to the results an average magnitude of apparent porosity for the refractory formulations was established around 17-18%. Whereas the porosity of refractory bricks mani-fests itself in different coating adhesion levels, the average mag-nitude reached in this study was suitable because it permits a de-sirable good coating adherence avoiding thermal overloads on the brick hot face.The results obtained for cold crushing strength shown an increase tendency in magnitude with spinel addition and a maximum cold crushing strength magnitude corresponding to 3MA formulation for magnesium aluminate spinel (MgAl2O4) and 9H formulation for hercynite spinel. This behaviour can be explained in porosi-ty terms, since according to those values 3MA and 9H refractory formulations registered the minimum porosity magnitude.For cold and hot modules of rupture the addition of both magne-sium aluminate and hercynite spinels resulted in higher magni-tudes compared to MZ formulation. In addition, it was found that the most favourable spinel content to reaches the maximum val-ue in both cold and hot modules of rupture was 2MA formulation for MgAl2O4 addition and 8H formulation for FeAl2O4 addition.X-ray diffraction analysisThe X-ray diffraction (XRD) study obtained from each refractory formulation revealed the presence of the same phases prior to sin-tering. This indicates that there were no new phases formed. How-ever, the diffraction patterns of formulation 2MA and 6H did not detect spinel phase peak intensity due to the small amount used (see table 5).Microstructural analysisFigure 1 shows a SEM image of the sintered microstructure of the formulation without spinel content (MZ). It can be observed a homogeneous microstructure matrix forms by two well-distribut-ed phases. Using energy dispersive spectroscopy analysis (EDS) were identified as magnesia (grey grains) and calcium zirconate (white grains). At higher magnification an excellent bond linkage between MgO and CaZrO3 grains was observed.Tab. 4: Physical properties of the refractory bricks.BrickTypePropertiesApparent porosity (%)Bulk density (g/cm3)Cold crushing strength (MPa)Modulus of rupture (MPa)Hot modu-lus of rup-ture @ 1260ºC (Kg/cm2)MZ 18.4 2.94 39.6 9.6 592MA 17.7 3.01 54.2 12.7 1443MA 17.1 3.02 56.5 11.6 1264MA 18.1 2.98 47.2 8.7 1165MA 18.6 2.97 49.3 9.5 1216H 18.4 3.00 44.2 9.7 1117H 18.2 3.01 49.4 10.5 1188H 18.1 3.00 47.5 10.4 1329H 17.9 3.00 55.5 11.2 80Tab. 1: Chemical composition of raw materials.Compositions (% by weight)Raw Material MgO ZrO2 CaO Al2O3 Fe2O3 SiO2Sintered MgO 98.91 – 0.85 0.08 0.05 0.11ElecrofusedMgO-CaZrO350.00 36.43 13.57 – – –MgAl2O4 34.00 – 0.20 64.0 1.0 0.40FeAl2O4 4.6 – 0.19 49.06 45.78 0.13ZrO2 – 97.72 – – – 2.10Tab. 2: Refractory formulations.Compositions (% by weight)RefractoryformulationMgO CaZrO3 Mg Al2O4FeAl2O4 ZrO2MZ 85.5 14.0 – – 0.52MA 83.0 14.0 2.5 – 0.53MA 81.9 14.0 3.6 – 0.54MA 80.7 14.0 4.8 – 0.55MA 79.5 14.0 6.0 – 0.56H 83.0 14.0 – 2.5 0.57H 81.9 14.0 – 3.6 0.58H 80.7 14.0 – 4.8 0.59H 79.5 14.0 – 6.0 0.5Industrial Application122654th International Colloquium on Refractories 2011 – Refractories for IndustrialsTab. 5: Physical properties of the refractory bricks.BrickCrystalline phasesMgO CaZrO3N.SCaZrO3ZrO2Mg Al2O4Fe Al2O4MZ xxxxx xxx x – – –2MA xxxxx xxx x – – –3MA xxxxx xxx x – x –4MA xxxx xxx x – xx –5MA xxxx xxx x – xx –6H xxx x – – –7H xxx x – – x8H xxx x – – xx9H xxx x – – xx*N.S meaning non stechiometric CaZrO3 (CaO0.15ZrO0.85O1.85).In the figure 2, it was shown a representative microstructure corre-sponding to the refractory formulations with addition of MgAl2O4 spinel (2MA, 3MA, 4MA and 5MA). This microstructural analy-sis revealed as in MZ formulation, the presence of well-distribut-ed magnesia (grey grains) and calcium zirconate grains (light gray grains) in the microstructure as mainly phases. Magnesium alumi-nate spinel phase was not identified at low magnifications; howev-er, at higher magnifications it was revealed that spinel was located in the boundaries between MgO and CaZrO3 particles acting as a bond linkage among these phases.It is proposed that spinel phase was localized by diffusion in these specific zones due to its melting temperature is lower than both magnesia and calcium zirconate phases. Besides, the fine particle size of spinel phase contributed to the arrangement in those spe-cific zones. A high magnification SEM image for 5MA formula-tion is shown in figure 3, in which spinel phase was identified in the boundaries between magnesia and calcium zirconate particles. This was corroborated by EDS analysis.The representative microstructure corresponding to the refracto-ry formulations with addition of FeAl2O4 spinel (6H, 7H, 8H and 9H) is shown in figure 4.By this analysis, it can be seen in the microstructure the presence of well-distributed magnesia (gray grains) and calcium zirconate grains (white grains) as mainly phases. Hercynite spinel was also located in the boundaries between MgO and CaZrO3 particles act-ing as a bond linkage between these phases.Coatability and corrosion res istanceIn table 6 is shown the results obtained by static and dynamic methods for determining the coating adhesion in the refractory formulations. In general, the hot face in every refractory formu-lation was no corroded by cement liquid phase; however a brown molten phase entered up to a range of 6 to 9 mm depth. In addi-tion, the increase of spinel content leads to a diminished in coating adhesion. This behaviour can be attributed to spinel phases due to their lack of coating adhesion. According to the microstructural analysis corresponding to static and dynamic tests a slightly den-sification was observed without corrosion evidence. It is impor-tant to mention that bonding strength still remained in all formu-Fig. 1: Sintered microstructure of MZ formulation.Fig. 3: SEM micrograph of the 5MA formulation. The rectangle indicates the spinel location in the microstructure.Fig. 2: General microstructure with MgAl2O4 spinel addition. Fig. 4: General microstructure with FeAl2O4 spinel addition.Industrial Application 1236October 19th and 20th, 2011 · EUROGRESS, Aachen, Germanylations. Figure 5 and 6 revealed a slightly dense microstructure presented in all formulations due to clinker penetration. Besides, a magnesia-calcium zirconate unaltered bonding was presented.Tab. 6: Adherence test with clinker raw meal.FormulationStatic test Dynamic testDistance from hot face (mm)Adherence AdherenceMZ 9.0 strong moderate2MA 6.4 strong moderate3MA 5.6 strong moderate4MA 5.7 moderate moderate5MA 5.7 moderate moderate6H 3.6 moderate moderate7H 3.4 moderate moderate8H 4.3 nill moderate9H 5.5 nill moderateCONCLUSIONSThis research demonstrated that the addition of magnesium-alu-minate and hercynite spinels to the electrofused MgO-CaZrO3 matrix improved their thermo-mechanical properties. The micro-structural analysis revealed that both spinel (MgAl2O4 and FeA-l2O4) were located in the boundaries between MgO and CaZrO3 particles acting as a bond linkage among these phases. Coating adherence was moderate and a general slightly dense microstruc-ture with a magnesia-calcium zirconate unaltered bonding was presented after raw meal clinker attack.REFERENCES[1] Bartha, P. and Klischat, H. J; “Classification of magnesia bricks in rotary cement kilns according to specification and service-ability”; ZKG International. 47:277–280, 1994.[2] J.L. Rodríguez-Galicia, A.H. de Aza, J.C. Rendón-Angeles, P. Pena; “The Mechanism of corrosion of MgO CaZrO3–cal-cium silicate materials by cement clinker”; Journal of Euro-pean Ceramic Society. 27:79-89, 2007.[3] Serena S, Sainz MA, Caballero A; “Corrosion behavior of MgO/CaZrO3 refractory matrix by clinker”; Journal Europe-an Ceramic Society. 24:2399–2406, 2004.[4] Serena S, Sainz MA, Caballero A; “The system clinker–MgO–CaZrO3 and its application to the corrosion behavior of CaZrO3/MgO refractory matrix by clinker”; Journal Euro-pean Ceramic Society. 29:2199–2209, 2009.[5] Edén Rodríguez, G-Alan Castillo, José Contreras, J.A. Agu-ilar Martínez, Yadira González, Ana-María Guzmán, Laura I. García Ortiz; “Desarrollo de un refractario MgO-CaZrO3 dopado con MgAl2O4 para la industria cementera”; Ciencias UANL. 15:23–30, 2011.[6] Edén Rodríguez, G-Alan Castillo, Ana María Guzmán; “Ef-fect of spinel MgAl2O4 on microstructural and mechano-chemical properties evolution in a sintered MgO-CaZrO3 re-fractory matrix”; Industrial Ceramics. 31:1–5, 2011.[7] Rodríguez. E.A., Castillo. G.A., and Guzmán A.M; “Devel-opment of a chrome-free refractory based on electrofused magnesia-calcium zirconate technology using spinel addition for use in cement rotary kilns”; UNITECR´09 Congress. 072, 2009.[8] Rodríguez JL, Baudín C, Pena P; “Relationships between phase constitution and mechanical behaviour in MgO– CaZrO3–calcium silicate materials”; Journal of European Ceramic Society. 24:669–679, 2004.[9] Rodríguez JL, Pena P; “Obtención de materiales de magne-sia-circonato dicálcico por sinterización reactiva de mezc-las dolomita – circón”; Estudio del procesamiento. 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Guzmán; “Microstructure and properties of hercynite-magnesia-calcium zirconate refractory mixtures”; Materials Characterization. 54:354–359, 2005.[15] Kozuka, H., Kaita, Y., Tuchiya, Y., Honda, T. and Ohta, S; “New kind of chrome-free (MgO-CaO-ZrO2) bricks for burning zone of rotary cement kiln”; UNITECR. 93, Sao Paulo, Brasil. 1027–1037, 1993.[16] H. Kozuka, Y. Kajita, Y. Tuchiya, T. Honda and S. Ohta; “Fur-ther improvements of MgO-CaO-ZrO2 refractory bricks”; UNITECR´05. 1995.Fig. 5: General microstructure with MgAl2O4 spinel addition test-ed with clinker raw meal at 1450 ºC for 4 h.Fig. 6: General microstructure with FeAl2O4 spinel addition test-ed with clinker raw meal at 1450 ºC for 4 h.

IMPROVEMENT OF AN ELECTROFUSED MGO-CAZRO3 REFRACTORY MATRIX BY THE ADDITION OF HERCYNITE SPINEL AND MAGNESIUM-ALUMINIUM SPINEL FOR THE CEMENT INDUSTRY

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IMPROVEMENT OF AN ELECTROFUSED MGO-CAZRO3 REFRACTORY MATRIX BY THE ADDITION OF HERCYNITE SPINEL AND MAGNESIUM-ALUMINIUM SPINEL FOR THE CEMENT INDUSTRY

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