Self-Repairing Function in the Carbon-ContainingRefractory

Akira Yamaguchi*

Okayama Ceramics Research Foundation, Bizenn, Okayama 705-0021, Japan

Self-repairing function is observed in carbon-containing refractory such as MgO–C, Al2O3–C, and so forth. Non-oxidessuch as pure metallic, alloys, carbides, and nitrides are intentionally added to the refractory composition to bring about thisfunction. As a basis of the development of a self-repairing refractory, the self-repairing mechanism in the carbon-containingrefractory is described.

Introduction

In a refractory composed only of oxides, one way toincrease corrosion resistance is to decrease porosity (in-creasing density). When the porosity becomes too low,however, the thermal shock resistance of the refractorybecomes poor. The relationship between the corrosionresistance and the thermal shock resistance of the re-fractory is well known, and is a delicate balance in areaswhere thermal shock and corrosion are issues. Mixingcarbon in oxides solved the thermal shock problem be-cause of carbon’s low thermal expansion and high ther-mal conductance, which increases the refractory’sthermal shock resistance. The corrosion resistance isalso increased because the penetrated slag into the re-fractory was suppressed by the characteristics of carbon,which is hardly wetted by slag. In the results, the cor-rosion resistance and the thermal shock resistance of therefractory are both simultaneously improved.

Carbon, however, has a large defect—it is easilyoxidized in air at a high temperature. One way to de-crease this oxidation is by adding non-oxides such asmetal, alloy, and carbide, which react with the air andexpand, sealing pores.

Behavior and Effect of Nonoxides Added to aCarbon-Containing Refractory

Initially, non-oxides were added to prevent theoxidation of carbon. It was learned, however, that thesenon-oxides not only acted as antioxidants, but they alsohad other effects on the refractory including:

(1) Decrease of carbon-loss by the reduction ofCO(g) to C(s).

(2) Decrease of porosity.(3) Increase of the modulus of rupture (MOR).(4) Crystallization of amorphous carbon precipi-

tated from binder.(5) Improvement of the oxidation resistance and

corrosion resistance by forming a surface-protective layer.

Int. J. Appl. Ceram. Technol., 4 [6] 490–495 (2007)

Ceramic Product Development and Commercialization

*akira-yamaguchi@occ.ecweb

r 2007 The American Ceramic Society

Each of these areas that additives benefit carbon-containing refractories is described in great depth interms of the mechanisms. Their use can be engineeredin a refractory to increase the service life.

Decrease of Carbon-Loss by the Reduction of CO(g)to C(s)

Graphite is rapidly oxidized under an oxidation at-mosphere over about 7001C, and it generates primarilyCO2(g) and CO(g). CO(g) is mainly produced in theopen pores of the carbon-containing refractory at a hightemperature, because the equilibrium partial pressure ofCO(g) is remarkably higher than that of CO2(g) underthe existence of condensed carbon above about10001C.1 At the atmospheric pressure, CO(g) existsat a partial pressure of 0.1 Mpa in the pores in therefractory.

Carbon antioxidants of pure metals, alloys, nitrides,and carbides become an oxide that is stabilized under a0.1 Mpa CO(g) atmosphere with solid carbon. For ex-ample, Al and AlN become effective antioxidants be-cause Al2O3 is stable under 0.1 Mpa CO(g). Thefollowing reactions occur in metal (M) and carbide(MC) in contact with CO(g):

xMþ yCOðgÞ !MxOy þ yC ð1Þ

xMCþ yCOðgÞ !MxOy þ ðx þ yÞC ð2Þ

The effect of antioxidants is to suppress oxidationloss of carbon by reducing CO(g) to C(s). In addition,the reactions bring about a volume expansion, whichcan seal pores. Volume changes of 149% and 289% areobtained from the oxidation of Al and SiC, respectively,

2Alðs; lÞ þ 3COðgÞ ¼ Al2O3ðsÞ þ 3CðsÞ ð3Þ

SiCðsÞ þ 2COðgÞ ¼ SiO2ðsÞ þ 3CðsÞ ð4ÞThe change was calculated by taking the true den-

sity of Al, Al2O3, SiC, SiO2, and amorphous carbon as2.70, 3.98, 3.21, 2.32, and 1.60 g/cm3, respectively.

Decrease of Porosity

When the non-oxide added changes to an oxide, avolume expansion is generated as described in Eqs (3)and (4). If this expansion is absorbed by the pores inthe refractory, the porosity of the refractory decreases.Figure 1 shows an example,2 namely open porosity de-creasing in a (77.5 mass% MgO–17.5% C) refractory

with additives at above about 8001C, but remainingconstant in a refractory without additives.

Phenol resin was mixed in powdered raw materialsas a binder, and the mixture was pressed to form ashaped refractory using a vacuum friction press, whichcreated a very low porosity (several percent or less).Pores are formed by decomposing binder gases throughabout 8001C. The porosity, however, decreases by thereaction products of the additive above about 8001C.The rate of porosity decrease varies depending on thecomposition, amount of additive, and particle size.Therefore, it is important to choose an additive thatperforms the best in the service environment of the re-fractory.

Increase of the MOR

Figure 23 shows the MOR of MgO–C refractorieswith and without metal additives during heating. TheMOR of the refractory without an additive decreasesduring heating. On the other hand, the MOR of therefractory with non-oxide additives begins to increasefrom about 600 to 10001C. This is brought about bycarbides, nitrides, and/or oxides that are formed by re-actions between the additives and carbon, CO(g) and/orN2(g) from the air. These reaction products deposit asplate crystals and/or whiskers as shown in Fig. 3, and in

Fig. 1. Changes in the open porosity of the carbon-containingrefractory with/without metal additives.

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addition, react with the aggregate oxide to form a newsolid phase. These changes are considered to bring aboutthe increase of MOR of the refractory.

Crystallization of Amorphous Carbon Precipitatedfrom Binder

Free carbon deposits are created from the decom-position of phenol resin added as a binder. Free carbonis also formed by the reaction between the addednon-oxide and CO(g) as shown by expressions (1)–(4). These carbons2 are amorphous and oxidized at low-er temperatures more easily than graphite. Their truedensity is between 1.2 and 1.6 g/cm3. However, onlywith the use of B4C or MgO does crystallization of thecarbons occur.

Figure 44 shows X-ray diffraction patterns of freecarbon from phenol resin and the free carbon with5 mass% B4C, which were heated in Ar gas at 800–16501C for 3 h. When B4C is added, crystallization ofthe carbon is promoted. MgO also promotes crystalli-zation of the free carbon, as shown in Fig. 5. This figureshows the X-ray diffraction pattern of free carbon heatedat 17001C and of a mixture of MgO and the carbonfrom phenol resin, which is heated at 1100–16001C inAr gas.

Improvement of the Oxidation Resistance andCorrosion Resistance by Forming a Surface-Protective Layer

It is well accepted that a protective layer is formedtoward the surface of MgO/carbon refractories duringservice by the addition of metals, metal alloys, carbides,or nitrides. The following three examples are explained.

Fig. 2. Changes in the modulus of rupture of the carbon-containing refractory with/without additives.

Fig. 3. Whisker formed in a MgO–C refractory with Al metaladditives during heating.

Fig. 4. X-ray diffraction patterns in free-carbon from phenol resinand carbon in samples containing 5 mass% B4C, when sampleswere heated in Ar gas from 8001C to 16501C.

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Example 1: Formation of a dense protective layerthrough Al–Mg alloy additions to a MgO–C refractory.

When Al–Mg alloy additions are made to a MgO–C refractory used in steelmaking, a dense magnesia layeris formed when the refractory contacts slag containingiron oxide. The formation of this protective layer isshown in Fig. 6.

When this refractory is heated during use, the Mgvolatilizes at a low temperature and Mg(g) is oxidizednear the refractory surface, condensing Mg in a layer asMgO. This MgO condensation becomes a driving forcefor the continual formation of a dense MgO protectivelayer. It is important for the MgO layer to grow underuse, because it is corroded during service. The growthmechanism for MgO can be explained thermodynam-ically by the following methods:

MgOðsÞ þ CðsÞ ¼MgðgÞ þ COðgÞ ð5Þ

The partial pressure of Mg(g) and CO(g) in thepores of the refractory becomes significant above about14001C, acting as a driving force for the reaction. WhenAl metal coexists with Mg, it reacts with CO(g) as fol-lows, reducing the partial pressure of CO(g) and in-creasing the partial pressure of Mg(g):

2Alðl ; gÞ þ 3COðgÞ ¼ Al2O3ðsÞ þ 3CðsÞ ð6Þ

In practice, oxygen diffuses through the MgOdense layer from outside the refractory to inside the re-fractory microstructure. At the refractory surface, incontact with the slag, the dense MgO that is formedreacts with Fe2O3 to form MgFe2O3. Iron diffusion to

the inside of this layer leads to the formation of(Mg,Fe)O (wustite). The next reaction occurs in thedense (Mg,Fe)O layer, causing Fe metal deposition, andthe growth of a magnesia layer:

ðMg;FeÞOþMgðgÞ ¼MgOþ Fe ð7Þ

Fe metal from reaction (7) deposits on the inside ofthe dense layer in the refractory

Example 2: Formation of a surface-protective layerin the B4C addition MgO–C refractory.

B4C added as an antioxidant to the MgO–C re-fractory reacts with CO(g) as shown in the followingequation:

B4CðsÞ þ 6COðgÞ ¼ 2B2O3ðlÞ þ 7CðsÞ ð8Þ

The reaction is brought about through various gasphases of the B–C–O system, especially B2O2(g) andB2O3(g), which have high equilibrium partial pressures.Initially, B4C(s) reacts with CO(g) to form B2O2(g), asshown in the following equation:

B4CðsÞ þ 4COðgÞ ¼ 2B2O2ðgÞ þ 5CðsÞ ð9Þ

When B2O2(g) diffuses near the surface of the re-fractory where CO(g) partial pressure is high, it iscondensed as B2O3 according to the following reaction:

B2O2ðgÞ þ COðgÞ ¼ B2O3ðlÞ þ CðsÞ ð10Þ

From Eqs (9) and (10), Eq. (8) is obtained. OnceB2O3 has condensed on the surface, it reacts with MgOto form Mg3B2O6, which melts above 13401C. Figure 7shows the dense layer formed near the surface of therefractory, which is composed of MgO1Mg3B2O6.

Fig. 5. X-ray diffraction patterns in free-carbon refractory samplescontaining phenol resin heated at 17001C and a mixture of thefree-carbon and MgO.

Fig. 6. Dense magnesia layer formed near the surface of theMgO–C refractory with Al–Mg alloy.

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This liquid phase suppresses the diffusion of the oxygento the refractory.

Example 3: Formation of a surface-protective layerin the SiC addition Al2O3–C refractory.

A dense protective layer is formed in Al2O3–Crefractory by SiC additions as shown in Fig. 8. Themechanisms by which the protective layer form is start-ed by SiC, which exists near the refractory surface; itreacts with CO(g) to form SiO(g) and C(s) as shown inthe following equation:

SiCðsÞ þ COðgÞ ¼ SiOðgÞ þ 2CðsÞ ð11Þ

The deposited C(s) condenses near the surface ofSiC grain. The SiO(g), on the other hand, diffuses nearthe surface of the refractory and reacts with CO(g) toform SiO2 and C(s) as indicated in the following equa-tion:

SiOðgÞ þ COðgÞ ¼ SiO2ðs; lÞ þ CðsÞ ð12Þ

SiO2, which is deposited near and builds up therefractory surface, reacts with the slag to form a high-viscosity slag, which contains some free carbon. Thehigh-viscosity slag contributes to the corrosion and ox-idation resistance of the refractory by forming a protec-tive layer.

In general, the formation of a layer near the surfaceof a refractory increases the service life of the underlingrefractory by a number of different mechanisms de-pending on the type of additive in the refractory. Anadditive of appropriate type, particle size, and dosageshould be chosen depending on the refractory type andthe service requirements.

In case of the nitride additives, the viscosity of theslag melt near the surface of the refractory is increasedby dissolution of N2(g), which occurs by the decompo-sition of the nitride when in contact with slag. A smallamount of nitride addition seems to be very effective atdecreasing refractory/slag interactions.

The self-repairing function in carbon-containingrefractories, which has been described above, is dia-grammatically shown in Fig. 9.

Fig. 7. Dense liquid phase formed near the surface of the MgO–Crefractory with B4C.

Fig. 8. Dense protective layer formed in an Al2O3–C refractorywith SiC.

Fig. 9. Phenomenon of self-repairing function in a carbon-containing refractory through metal, alloy, carbide, or nitrideadditions.

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The Present and Future State of the Additives toRefractories Contributed to the Self-RepairingFunction

Carbon-containing refractories with metal or metalalloys or with carbides or nitrides are representative ex-amples of a refractory with a self-repairing mechanismthat prolongs the refractory service life. Table I showsthe additives that are most commonly used or underdevelopment. Al and SiC are the additives most fre-quently used for a MgO–C and a Al2O3–C refractory,respectively.

Carbides are excellent additives because they createdeposits as a free carbon in a refractory. Al4O4C,5,9

Al2OC,5 Al4SiC4,6 Al8B4C7,7,8 and so forth, are con-sidered promising future additives because of theirpotential to form free carbon.

Summary

It has been shown that added non-oxides, additionsto carbon-containing refractory, such as metal, alloys,

carbides, nitrides, and so on, bring about a self-repairingfunction. The self-repairing function includes

(1) reduction of CO(g) to C(s) by non-oxides andaiding in reducing carbon loss,

(2) decrease of the porosity of the refractory,(3) strengthening the modulus of rupture of the

refractory,(4) promotion of the crystallization of free carbon

from the binder, and(5) formation of the protective layer, the refractory

surface, and increase of oxidation resistance and corro-sion resistance.

References

1. A. Yamaguchi, ‘‘Control of Oxidation–Reduction Reactions in MgO–C Re-fractories,’’ Taikabutsu (Japanese), 37 [7] 365–370 (1983).

2. A. Yamaguchi and J. Yu, ‘‘Behavior of Carbon from Pitch and Resin Added toCarbon-Containing Refractories,’’ J. Ceram. Soc. Japan, 102 [1] 73–74(1994).

3. A. Watanabe and H. Takahashi, et al., ‘‘Behavior of Different MetalsAdded to MgO–C Brick,’’ Taikabutsu (Japanese), 38 [11] 740–746 (1986).

4. S. Zhang and A. Yamaguchi, ‘‘Effect of B4C on the Crystallization and Ox-idation Resistance of Carbon from Resin,’’ J. Ceram. Soc. Japan, 102 [9] 830–839 (1994).

5. S. Zhang and A. Yamaguchi, ‘‘Hydration Resistances and Reactions withCO(g) of Al4O4C and Al2OC Formed in Carbon-Containing Refractorieswith Al,’’ J. Ceram. Soc. Japan, 104 [5] 393–398 (1996).

6. A. Yamaguchi, S. Zhang, J. Yu, and S. Hashimoto, ‘‘Behavior of AntioxidantsAdded to the Carbon-Containing Refractories,’’ Unified international Tech-nical Conference on Refractories, 3 140–147 (1995).

7. A. Yamaguchi, Y. Nakano, and T. Wang, ‘‘Effect and Behavior of Al–B–CSystem Antioxidants Added to MgO–C Refractories,’’ Can. Metall. Q., 39 [4]59–68 (2000).

8. T. Wang and A. Yamaguchi, ‘‘Antioxidation Behavior and Effect of Al8B4C7

Added to Carbon Containing Refractories,’’ J. Ceram. Soc. Japan, 108 [9]818–822 (2000).

9. J. Zhao, W. Lin, A. Yamaguchi, J. Ommyouji, and J. Sun, ‘‘Synthesis ofAl4O4C from Starting Raw Materials of Alumina and Graphite,’’ Taikabutsu(Japanese), 59 [6] 288–295 (2007).

Table I. Additive for the Self-Repairing Refractory

Sort

Metal Al, Si, MgAlloy Al–Si, Al–Mg, Al–Ca–Mg–SiCarbide SiC, B4C, Al4SiC4, Al8B4C7, Al4O4C, Al2OCBoride B4C, ZrB2, CaB2

Nitride Si3N4, AlN

Bold, practical; italic, trial.

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