corrosion resistance of alumina-magnesia castable to slag

Corrosion resistance of alumina-magnesia castableto slag in steel ladle lining

Yoshitaka SADATOMI,³ Naoya ENOMOTO* and Junichi HOJO*

Technical Research Laboratories, Krosaki Harima Corporation, 1–1 Higashihama-machi, Yahatanishi-ku, Kitakyushu 806–8586*Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819–0395

The corrosion resistance of alumina-magnesia castable to a basic slag was evaluated to find the highly-resistive castablecomposition for steel processing. Stable solid phases were CA6, alumina and spinel after firing of the castable of Al2O3MgOCaOSiO2 system. The slag corrosion was simulated thermodynamically for CA6, alumina, spinel and castables. The solubility ofeach oxide component is expected in the order of MgAl2O4 > CA6 > Al2O3 at 1600°C because of the formation of CA2 on CA6and the formation of CA2 and CA6 on alumina, suggesting the protection of castable from slag. However, CA2 and CA6disappear at 1800°C and the solubility changes in the order of CA6 > Al2O3 >MgAl2O4. Although the protection of castable byCA6 is also expected, the CA6 disappears at a large amount of slag and a high temperature. The solubility of castable decreaseswith increasing MgO/CaO ratio owing to spinel formation. The practical immersion test demonstrated the high corrosionresistance of high MgO/CaO castable against slag, exhibiting the importance of spinel formation.©2011 The Ceramic Society of Japan. All rights reserved.

Key-words : Refractory, Corrosion resistance, Castable, Slag, Steel ladle

[Received June 10, 2011; Accepted August 30, 2011]

1. Introduction

Alumina-magnesia castables have been used as refractory forlining steel ladles.1),2) The castables are composed of aluminaaggregates and fine-grain matrix consisting of alumina, magnesia,fumed silica and alumina cement. Corundum (¡-Al2O3), spinel(MgAl2O4), CA6 (CaAl12O19) and glass phase are generated byheat in contact with molten steel.3) Slag is a molten liquid ofmetal oxides floating on molten steel, and corrodes the castbles.The mechanism of slag corrosion is classified as follows: wettingof molten slag to refractory, dissolution of refractory componentinto slag, penetration of molten slag into refractory, andformation of reaction products. The deterioration by slagpenetration and chemical reaction leads to spalling of the surfacelayer of refractory. The authors have studied on the reactionbehaviors of alumina-magnesia and alumina-spinel castables withmolten slag.4) They reported that alumina-rich spinel forms inthe alumina-magnesia castable and that the excessive alumina isreleased to molten slag and increases the viscosity of slag,leading to low wettability and retarded slag-penetration. Next,the authors focused on the reaction and dissolution behaviors ofrefractories in molten slag. The slag corrosion depends on thechemical compositions of refractory and slag. However, the slagcomposition is changed depending on the demanded steel. Theimportance is to find an appropriate composition of refractoryavailable to various slag compositions. Thermodynamic simu-lation is useful to anticipate the stable solid and liquid phases inthe final stage of slag-refractory reaction.5)8) F. Melcher et al.7)

thermodynamically analyzed the stability of alumina-magnesiaand alumina-spinel castables in steel slags, reporting thatcastables with a large amount of spinel are supposed to have abetter slag resistance, whereas the formation of CA6 and CA2

(CaAl4O7) layers at a low temperature has a positive effect onthe corrosion resistance. Generally speaking, however, when thecontent of spinel is large, the castable has an excellent corrosionresistance.9)

In the present study, the corrosion resistance of alumina-magnesia castable was thermodynamically simulated with em-phasis on MgO/CaO ratio. The assumed slag was a highly basicand corrosive one for acidproof steel refining. The simulatedresults were compared to the practical data by an immersion testof the castables with the slag.

2. Experiments

2.1 Preparation of alumina-magnesia castablesFour kinds of alumina-magnesia castables (Cast 14) were

prepared at the formulation in Table 1. The raw materials wereof industrial grade in Krosaki Harima Co. The castables werecomposed of coarse alumina aggregates less than 8mm,intermediate alumina aggregates less than 1mm, and matrixconsisting of fine grains of 75¯m or less of alumina, magnesia,alumina cement and fumed silica. The fractions of coarseaggregates, intermediate aggregates and matrix were 50, 19 and

Table 1. Formulation of alumina-magnesia castables

Chemical composition/mass%

Cast 1 Cast 2 Cast 3 Cast 4

Coarse aggregate(18mm) Al2O3 50 50 50 50

Intermediate aggregate(1mm½) Al2O3 18.9 18.9 19.0 18.9

Matrix(75¯m½) Al2O3 24.2 23.4 22.5 21.7

MgO 4.8 6.0 7.1 8.3CaO 1.4 1.0 0.7 0.4SiO2 0.7 0.7 0.7 0.7

³ Corresponding author: Y. Sadatomi; E-mail: [email protected]

Journal of the Ceramic Society of Japan 119 [12] 916-921 2011 Paper

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31mass%, respectively. The MgO/CaO ratio of the matrix waschanged in the range of 3.420.8 in mass. The alumina cementwas composed of calcium aluminates (CA, CA2, etc). Thecontent of calcia in the matrix was changed with the content ofalumina cement. The alumina content in the matrix was adjustedat the stoichiometry to form spinel and CA6. An appropriateamount of conditioning agent (dispersing agent, retardant, etc.)was added to adjust the fluidity and hardening time of castable.The powders were mixed with 6.1mass% water for 3min, and

casted into a polystyrene mold of 40mm © 40mm © 160mm.The casted sample was removed from the mold after curing for2 days at 20°C in a temperature-controlled room and dried forover 24 h at 110°C in a drying chamber. The firing was conductedfor 3 h at 1500°C in an ambient atmosphere with an electricfurnace. The apparent porosities of the fired castables weremeasured by water immersion method.The fired castables were cut with a diamond wheel and

polished with a diamond slurry. The microstructure was observedwith an electron probe microanalyzer (EPMA, JEOL Ltd.,JXA-8200), and the elementary distribution was locally analyzedby energy dispersive X-ray spectroscopy (EDS) to determinethe crystalline species of grains. The crystalline phases wereidentified by X-ray diffractometry (XRD, Rigaku Co. Ltd.,RU-200B) using CuK¡ radiation.

2.2 Corrosion testThe model slag of Al2O3CaOMgOSiO2FexOyMnO sys-

tem was supplied by Krosaki Harima Co. After melting andcooling, the solidified slag was pulverized. The chemicalcomposition was analyzed by X-ray fluorescence analysis (XRF,Rigaku Co. Ltd., Simultix 14). Al2O3 32.0%, CaO 55.7%, SiO2

8.9%, MgO 2.0%, FeO 0.8%, Fe2O3 0.1%, MnO 0.5% in mass.The basicity defined as C/S (=CaO/SiO2) was 6.3.The Corrosion test was carried out by a crucible method. The

castables were prepared in a size of 50mm in diameter and50mm in height. After firing at 1500°C for 3 h, the castables weremachined to make a hole in a size of 20mm in innerdiameter and30mm in depth. The 12.5 g of pulverized slag was charged intothe test crucible and heated at 1675°C for 12 h. After testing, thecooled slag and crucible were cross-sectioned and the appearanceof slag penetration and damage of crucible wall was recordedwith a digital camera. The corrosion index was determined byimage analysis of the cross-section, in which the final cross-sectioned area of crucible hole was evaluated and normalized bythat of Case 1. The microstructure was observed by EPMA, andthe Al2O3/MgO molar ratio of spinel was analyzed by EDS.

2.3 Thermodynamic simulationThe stable phases after firing of alumina-magnesia castables

and after reaction of slag with castables and their stable oxidecomponents (CA6, alumina, spinel) were thermodynamicallysimulated under the condition of chemical equilibrium by usingthe FactSage program. In the calculation, ChemSheet was used assoftware and FACT database as data source.10) The data of puresolids were selected for the compound species, and FACT-MULL, FACT-Mel, FACT-SLAG, FACT-WOLL, FACT-MeOand FACT-CORU were selected for the solution species. Thesimulation was compared with the results of immersion tests toevaluate the corrosion resistance of alumina-magnesia castableagainst molten slag.

3. Results and discussion

3.1 Stable phases of alumina-magnesia castableafter firing

The stable phases after firing of alumina-magnesia castablewere thermodynamically simulated in the temperature range of10001800°C. The thermodynamic simulation of change instable phase of Cast 1 and Cast 4 with temperature is shown inFig. 1. The formation of gehlenite (Ca2Al2SiO7), anorthite(CaAl2Si2O8) and CA6 is expected as reaction product amongalumina, calcia and silica, and the formation of spinel as reactionproduct between alumina and magnesia. The stability of theCa-compound is in the order of Ca2Al2SiO7 < CaAl2Si2O8 <CA6. The Ca-compounds decompose with a rise of temperature,leading to the formation of liquid. Spinel is stable and remains ata relatively high temperature. Although alumina is consumed byreaction with calcia, silica and magnesia, corundum is morestable phase than the Ca-compounds at a high temperature. Atthe firing temperature of 1500°C, CA6, alumina and spinel areexpected as stable solid phase. MgO/CaO ratio is high in theorder of Cast 1 < Cast 2 < Cast 3 < Cast 4. With increasingMgO/CaO ratio, the contents of CA6, alumina and liquiddecrease, whereas the spinel content increases.The backscattered electron image of the microstructure of

Cast 1 after firing at 1500°C is shown in Fig. 2. The imagedemonstrates the microstructure of fine-grain matrix. The distri-bution of Ca, Al, Mg and Si of each grain was measured by EDSspot analysis to distinguish CA6 and spinel grains. Acicular grainswere assigned to CA6 and tiny grains to spinel. With an increasein MgO/CaO ratio from Cast 1 to Cast 4, the number of CA6grains decreased and the number of spinel grains increased. TheXRD patterns of fired castables are shown in Fig. 3. Since thecastables included a large amount of alumina, strong XRD peaks

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Fig. 1. (Color online) Thermodynamic simulation of change in stable phase of castable with temperature.

Journal of the Ceramic Society of Japan 119 [12] 916-921 2011 JCS-Japan

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of ¡-Al2O3 existed. The formation of CA6 and spinel wasconfirmed by XRD. The XRD peaks of CA6 decreased and theXRD peaks of spinel increased with increasing MgO/CaO ratio.

3.2 Reactivity of CA6, alumina and spinel withslag

CA6, alumina and spinel are stable solid phases after firingcastables. To clarify the reactivity of castables with slag, thereactivity of these oxide components with slag was simulated at1600 and 1800°C. In the practical use of castables, the lowestoperation temperature is about 1600°C and the highest operationtemperature is about 1800°C. The weight of oxide component isfixed at 50 g, and the slag is added by 1 g interval up to 50 g.Figure 4 shows the thermodynamic simulation of change in

stable phase on CA6, alumina and spinel in slag. Spinel has noreaction product, whereas large amounts of CA2 and CA6 form

in CA6 and alumina at 1600°C. The reason is that large amountsof calcia and alumina are included in slag and react with CA6 andalumina. As a result, the amount of slag, which is required tocompletely dissolve the oxide component, is large in CA6 andalumina. This means that CA6 and alumina are more stable thanspinel. The formation of CA2 and CA6 is not observed at1800°C. The change in the amount of liquid formed from theoxide component by addition of slag is shown in Fig. 5. At1600°C, the amount of liquid slightly decrease by addition ofslag because of the formation of Ca-compounds, and increases ata large amount of added slag because of the dissolution ofCa-compounds. Because of the formation of large amounts ofCa-compounds, the sequence of solubility is expected inMgAl2O4 > CA6 > Al2O3 at 1600°C. The amount of liquidsignificantly increases at 1800°C, and CA2 and CA6 disappearon CA6 and alumina. Therefore, the sequence of solubility isexpected in the order of CA6 > Al2O3 >MgAl2O4. As seen here,there is a possibility that CA2 and CA6 work as protective layeron CA6 and alumina at a low temperature, but spinel isessentially stable at a high temperature.

3.3 Reactivity of alumina-magnesia castable withslag

The stable phases of Cast 14 in slag were thermodynamicallysimulated by the similar method as in Section 3.2. Intermediatealumina aggregates and fine-grain matrix were considered forcalculation, because coarse alumina aggregates seemed not toparticipate in the reaction. Figure 6 shows the thermodynamicsimulation of change in stable phase on Cast 1 and Cast 4 in slag.Stable solid phases are CA6, alumina and spinel at 1600°C. CA6is unstable in castables at this temperature according to Fig. 1.The existence of CA6 means that CA6 is stabilized by largeamounts of calcia and alumina in slag. CA2 does not form incastables differently from CA6 and alumina in Fig. 4. This isbecause the amounts of CA6 and alumina are reduced by mixingwith magnesia in castables and alumina is consumed to formspinel. With an increase in MgO/CaO ratio from Cast 1 toCast 4, the amounts of CA6 and alumina decrease, whereas theamount of spinel increases. CA6 disappears at 1800°C, and theamount of alumina decreases and the amount of spinel increaseswith an increase in MgO/CaO ratio. The required amount ofadded slag for complete dissolution is raised owing to theformation of spinel at 1600 and 1800°C. The change in theamount of liquid formed from the castables by addition of slag isshown in Fig. 7. Even without added slag, the liquid existsbecause a liquid phase exists in castables as shown in Fig. 1. Thechange in the amount of liquid is divided into 4 stages at 1600°C.The direct dissolution of alumina takes place at Stage I. Thedissolution is retarded by CA6 at Stage II. The CA6 dissolves atStage III, and then spinel dissolves at Stage IV. There are Stage Iand Stage IV at 1800°C because there is no CA6. The first slopeis assigned to the dissolution of alumina and the second slopeto the dissolution of spinel. The protective effect by CA6 isexpected at a small amount of added slag and a low temperature.Under an ordinary operation condition, the solubility of castableis essentially in the order of Cast 1 > Cast 2 > Cast 3 > Cast 4,depending on the formation of spinel.

3.4 Corrosion resistance of alumina-magnesiacastable in slag

The results of corrosion tests of Cast 1 to Cast 4 in slag at1675°C are demonstrated in Fig. 8. Judging from the externalview and the corrosion index, the erosion of crucible wall was

Fig. 2. Backscattered electron image of castable after firing at 1500°C.

25 30 35 40 45

2θ(Cu Kα)/degree

Inte

nsity

/a.u

.

( Cast 1 )

( Cast 2 )

( Cast 4 )

( Cast 3 )

Fig. 3. XRD patterns of castables after firing at 1500°C. ¡-Al2O3,Spinel, CA6.

Sadatomi et al.: Corrosion resistance of alumina-magnesia castable to slag in steel ladle liningJCS-Japan

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serious in the order of Cast 1 > Cast 2 > Cast 3 > Cast 4. Thisresult agrees with the thermodynamic simulation, supporting theusefulness of spinel formation in castables. It was, further, foundthat the slag penetration was retarded at high MgO/CaO ratio. Theapparent porosities of Cast 1, Cast 2, Cast 3 and Cast 4 afterfiring at 1500°C were 23.6, 22.4, 22.2 and 22.3 vol%, respec-tively. The porosity of Cast 1 was a little high, but those of othercastables were identical. Hence, it was considered that the otherfactors rather than porosity affected the degree of slag penetration.The backscattered electron images of microstructures of Cast 2after corrosion test are shown in Fig. 9. The images demonstratethe microstructures of slag penetration layer immersed withmolten slag and non-penetration layer inside. The Al/Mg atomicratio of each grain was measured by EDS spot analysis to findspinel grains and calculate the Al2O3/MgO molar ratio. Theresults of spot analyses are described below Fig. 9. The Al2O3/MgO molar ratio of spinel was 1.01.1 in the slag penetrationlayer, whereas it was 1.51.6 in the non-penetration layer. Thus,the alumina-rich spinel formed in the castables and changed to

stoichiometric one in the slag penetration layer. The authorsreported in the previous paper4) that the alumina-rich spinelreleased the excessive alumina to molten slag, which led to theincrease in viscosity of slag and the retardation of slag penetration.It should be also noted that, according to the thermodynamicsimulation, CA6 in castables tends to disappear, meaning thedissolution of calcia into slag. The viscosity of molten slag isknown to decrease with an increase in calcia content.11),12) Theamount of dissolved calcia is low in high MgO/CaO castable. Thedissolution of alumina from alumina-rich spinel and the decreasein the amount of dissolved calcia may cause the increase inviscosity of slag, leading to the retardation of slag penetration,in the front of penetrating molten-slag even though the slagcontained large amounts of calcia and alumina.As described above, there is a possibility that the corrosion of

castable is suppressed by the formation of Ca-compounds at asmall amount of added slag and a low temperature, which isalso expected in the thermodynamic simulation of reactivity ofoxide components. At a large amount of added slag and a high

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Fig. 4. (Color online) Thermodynamic simulation of change in stable phase on CA6, alumina and spinel in slag at 1600 and1800°C.


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temperature, the Ca-compounds are useless and the corrosionresistance depends on spinel formation. The C/S of slag used inthe present work is high, meaning that it is highly corrosive.

Under such a corrosive condition, the magnesia-rich castable hada superior corrosion resistance against the molten slag because ofthe excellent stability of spinel generated in the castables.

Cast 1 Cast 4

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Fig. 7. Thermodynamic simulation of change in the conversion of solid to liquid by addition of slag at 1600 and 1800°C.

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095997

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( Cast 4 )

Fig. 8. (Color online) Cross-sections of castable crucibles after corrosion test at 1675°C.

Sadatomi et al.: Corrosion resistance of alumina-magnesia castable to slag in steel ladle liningJCS-Japan

920

4. Conclusion

The corrosion resistance of alumina-magnesia castable againststeel slag was evaluated thermodynamically and experimentallywith emphasis on the MgO/CaO ratio of castables. The resultsare concluded as follows.1) Stable oxide phases are CA6, alumina and spinel after firing

of castables at 1500°C. The amount of CA6 decreases and theamount of spinel increases with an increase in MgO/CaO ratio.2) The thermodynamic simulation of solubility of oxide

component in slag exhibits the sequence of MgAl2O4 > CA6 >Al2O3 at 1600°C. This is attributed to the formation of CA2 andCA6 in CA6 and alumina owing to the large contents of calciaand alumina in slag. At 1800°C, however, the sequence ofsolubility changes to CA6 > Al2O3 >MgAl2O4 because theCa-compounds are unstable at a high temperature.3) The existence of CA6 is expected on castables in slag at

1600°C, similarly to the simulation of solubility of oxidecomponent, suggesting the protection of castables by CA6.However, CA6 disappears at a large amount of added slag and ahigh temperature. Eventually, the corrosion resistance depends onthe content of spinel. The high corrosion resistance of the highMgO/CaO castable was proved in the immersion test.

References1) S. Kataoka, Proceeding of the UNITCER ’95, 1, Nov. 1922,

Kyoto, Japan (1995) pp. 127.2) T. Nishitani, S. Matsuo and T. Genba, Proceeding of the

UNITECR ’89, 1, Nov. 14, Anaheim, USA (1989) pp. 529540.

3) M. Fuhrer, A. Hey and W. E. Lee, J. Eur. Ceram. Soc., 18,813820 (1998).

4) Y. Sadatomi, N. Enomoto and J. Hojo, J. Ceram. Soc. Japan,119, 595600 (2011).

5) W. E. Lee, B. B. Argent and S. Zhang, J. Am. Ceram. Soc., 85,29112918 (2002).

6) H. Sarpoolaky, S. Zhang, B. B. Argent and W. E. Lee, J. Am.Ceram. Soc., 84, 426434 (2001).

7) F. Melcher, V. Reiter and H. Harmuth, Interceram, 57, 257260 (2008).

8) J. Berjonneau, P. Prigent and J. Poirier, Ceram. Int., 35, 623635 (2009).

9) S. Zhang and W. E. Lee, “Refractories Handbook”, MechanicalEngineering, Vol. 178, Marcel Dekker (2004) pp. 215257.

10) GTT-technologies, http://gtt-technologies.de/ (2011.05.22).11) J. S. Machin, T. B. Yee and D. C. Hanna, J. Am. Ceram. Soc.,

35, 322325 (1952).12) J.-Y. Choi and H.-G. Lee, ISIJ Int., 43, 13481355 (2003).

Al2O3/MgO molar ratioa: 1.0, b: 1.1, c: 1.1

50 μm

ba

c

Penetration layer

Al2O3/MgO molar ratiod: 1.6, e: 1.5, f: 1.6

50 μm

e

f

d

Non-penetration layer

Fig. 9. Backscattered electron images of microstructures of Cast 2 after corrosion test. The EDS spot analysis shows theAl2O3/MgO molar ratio at (a)(f) positions.


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corrosion resistance of alumina-magnesia castable to slag

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