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Magnesia-Carbon
Brick: a Retrospective
Ruth Engel
Introduction
The steel industry is the biggest consumer of refractories and the main
user of magnesia based ones. Consequently, improvements and new
developments of magnesia based refractory types reflect the demands
of this industry.
The continual need to increase process vessel availability, improve
life and decrease refractory lining cost has been the driver of many
technological changes. The development of what today is understood as
a magnesia-carbon brick, a resin bonded magnesia brick with graphite
and often antioxidant additions, resulted from the quest to develop
better refractories for the high wear areas of converters. This overview
will cover the early days of magnesia-carbon brick and some of the
developments that have taken place with this technology.
History
Since, at least, the 1870s pressed basic raw materials, in particular
burned dolomite, mixed with pitch have been available to line vessels
for use in different processes.
The advent of the BOF in the late 1950s and early 60s was accompanied
with developments in tar bonded dolomite, mag- dolomite and magnesia
brick [1]. High strength burned MgO brick was introduced in the late
1960s and by the 1970s BOFs were being lined with pitch bonded
magnesia brick and also burned and pitch impregnated MgO brick. The
latter became the standard for the charge pad and other high wear areas
which was the beginning of zoned linings [2]. Pitch bearing brick required
some manufacturing fine tuning in order to facilitate their installation at
the user’s site as excess pitch covered the brick bonding them together
while in transit to the installation site, made them slippery to handle
and modified their dimensions. During this time period the expected
converter life was in the 200 to 500 heat range.
By the late 1970s the need for better refractories became acute as a
result of moving from ingot to continuous casting which increased the
maximum temperature of the steel in the converter from the 1500o
C -
1600o
C range to 1650o
C or 1700o
C or even higher. In addition, changes
in steelmaking processes added stresses to the vessels further increasing
refractory wear.
Between 1975 and 1980 magnesia-carbon refractories were developed
and started to be used in Japan (Figure 1) first for electric arc furnace
hot spots and shortly thereafter for BOFs. This entailed the replacement
of pitch with resin and allowed for a dramatic increase in the carbon level
as a result of graphite additions. Subsequently, antioxidants were added
to the magnesia-carbon brick to protect the carbon from oxidation.
Figure 1: Annual production of MgO-C refractories [3]
The use of magnesia carbon refractories in the high wear areas of
the BOF led to a significant reduction in refractory consumption and
those brick were soon being incorporated into other areas as well until
the whole vessel was lined with them. Although refractories are an
important component of the improvement in BOF life, other changes
such as better temperature control, development and improved control of
slag chemistry, implementation of slag splashing technology, improved
refractory repair mixes, etc. have all contributed to the ever increasing
BOF life. By 2008 a North American BOF was expected to be relined after
20,000 to 35,000 heats while a European one after 2,500 to 3,000 heats
[4] and these numbers have almost doubled by today.
Magnesia
Much confusion can arise from the term magnesia refractory. The American
Society for Testing and Materials (ASTM) defines it as “a dead-burned
refractory material consisting predominantly of crystalline magnesium
oxide”, but that is not the terminology found in the literature. The
terms magnesite, magnesia and MgO are used interchangeably to signify
magnesium oxide. In addition, older publications use the term periclase
for MgO, even though this is the name of the naturally occurring mineral
of that composition. Over time different raw materials have been the
source for the MgO used in refractories. These changes have been
driven by technology, economics, availability and the expectation of
continuously improving refractory properties. Today, most magnesia
refractories contain sintered and/or fused grain, both being a synthetic
product.
Sintered magnesia, also called dead burned magnesite, is produced
by heating, sintering, a Mg bearing mineral or a synthetic compound
to drive off volatile gasses. When the resultant product is further
beneficiated by melting in an electric arc furnace, it becomes fused
magnesia. The amount and chemistry of the impurities present affect
the high temperature grain properties. Many papers contain sections
discussing the properties of magnesia grain, the changes that have taken
place in its production over time and the effect of impurities at high
temperature [1,5,6, etc.]. It should be noted that there has been a
continuous improvement in magnesia grain quality. This has resulted in
the availability of bigger grains with higher density and the continuous
lowering of the SiO2 and other impurities which lead to low melting point
phases at high temperature [6]. The decrease in SiO2 can be observed in
the changes in lime to silica ratios from 2.6, in the early days, to about
4 with improvements in technology and the current level of 10 or higher.
Use of these improved grains has contributed to the production of brick
with ever better physical properties and slag resistance.
Carbon
Carbon is added to refractories because it is not wetted by slags.
Traditionally, refractory chemical corrosion resistance was improved
by decreasing the size or number of pores, but this could make them
more sensitive to thermal shock damage. Consequently, methods were
developed to incorporate carbon to fill the spaces between the magnesia,
or other refractory oxide grains. The carbon then forms a film blocking
the slag penetration thereby minimizing the damage it causes the brick.
The early carbon sources were tars which have been replaced by pitches
or resins. The carbon level so achieved can be further increased with the
addition of carbon black and graphite.
Pitch could be used to hold the refractory components together as in
pitch bonded brick or forced into the brick’s pores as in burned pitch
impregnated brick. The residual carbon contained after tempering and
coking, was approximately 5 % for pitch bonded brick and 2.5 % or slightly
higher, for burned impregnated brick. Contrast this with magnesia-carbon
brick which can have 8 to 30% carbon while the most common range
is between 10 and 20%. The carbon in burned impregnated brick also
affected their porosity by decreasing it from approximately 18% to about
12%, but did not detrimentally affect the brick’s physical properties.
The graphite used in refractories is a naturally occurring flake material
selected for its high temperature stability and chemical inertness. The
important properties are flake size, carbon content and ash or, impurity
level. A consequence of the flake shape is that during manufacture they
preferentially align themselves leading to anisotropic brick properties.
In particular, its thermal conductivity is higher in the long vs. short
direction.
Magnesia-Carbon
The ground work for magnesia-carbon refractories was set, among others,
by Herron et.al. [8], who showed that the carbon in pitch impregnated
burned magnesia brick prevented slag from penetrating the pores.
Dense zone formation, thought to be a characteristic of magnesia-carbon
refractories, was known before these brick were developed [9, 10]. Work
using laboratory generated and field samples of pitch bonded and pitch
impregnated magnesia and dolomite brick showed that an MgO dense
zone formed at high temperature protecting the carbon and thereby
preventing slag penetration. Figure 3 shows a part of a “relatively
continuous dense magnesia zone between the carbon-bearing zone and
the carbon-free zone” observed in a pitch impregnated brick fired in
the laboratory using conditions that simulated the BOF environment.
The dense zone was thought to be the result of the reduction of the
MgO in the presence of carbon and at steelmaking temperatures, which
vaporized the Mg as a gas. There was a point at which the oxygen
potential and temperature relationship was no longer stable leading to
the precipitation of Mg vapor to form a magnesia dense zone [10].
Figure 3: Laboratory fired pitch containing 95% MgO brick, showing “bridging” of MgO
grains (x130) [10]
The concept of adding a metallic component to a refractory was already
patented in 1935 [11], a long time before the technology was available
to produce magnesia-carbon brick. In 1983 a patent for the addition of
metallic Mg to a chemically bonded brick was issued. The idea was to
increase the amount of Mg gas generated which should lead to a thicker
dense zone [12]. This concept was reduced to practice, and a commercial
product was successfully manufactured. To carry out production several
hurdles had to be overcome, not the least of which was how to handle
metallic Mg, a spontaneously flammable material in the presence of
nitrogen, carbon dioxide, oxygen and/or water.
With the increased use of magnesia-carbon refractories came the
realization that graphite, in the 10% and higher level, behaved in a
completely different manner than the residual carbons from pitch or
resins. Figure 4 shows that the higher C levels decreased slag penetration,
increased the brick’s thermal conductivity leading to more effective
cooling of the hot face, and reduced its modulus of elasticity (Young’s
modulus) with the latter two improving its thermal shock resistance.
Figure 2: Magnesia carbon brick micrographs showing the graphite flake alignment: A)
perpendicular and B) parallel to brick’s long dimension [7]
50000
100000
5 10 15
Year
Production
(T/ano)

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Figure 4: Effect of graphite on slag resistance, thermal conductivity and Young’s modulus
on the MgO-C brick [13]
To ensure the brick’s carbon would be available for a long time its rate of
oxidation had to be reduced. The formation of the dense zone to protect
it was already known and ways to increase its speed of formation and/
or its stability were being investigated. To this end Al and/or Si metals
were added to commercial magnesia-carbon brick, but a major problem
remained, the laboratory determined dense zone could not be detected
in post mortem brick [14]. It was speculated that the technique used
for brick removal from actual field installations somehow destroyed it
or that slag penetration during cooling was to blame. By 1997, this
problem was no longer an issue and dense zones from used brick were
being analyzed [15].
Rymon-Lipinski [16] with his seminal work, provided the theoretical
basis for determining the steps in which Mg, Al, and Si metals react to
provide the carbon protection in a magnesia-carbon brick. Table 1 shows
the expected reactions as a function of temperature and position within
the BOF lining.
Zone I (CO2
, CO, N2
)
MgO + MA MgO + M2
S
MA = MgO Al2
O3
MgO M2
S = 2MgO SiO2
(Spinel)
Zone II (CO, N2
)
> 2,080 K: > 1,800 K:
MgO + AlN + C MgO + M2
S + C
< 2,080 K: MgO + C < 1,800 K:
MgO + MA + C MgO + SiC + C
Zone III (N2
)
MgO + AlN + C > 1,600 K: > 1,700 K:
MgO + Mg + C MgO + SiC + C
< 1,600 K: < 1,700 K:
MgO + Mg3
N2
+ C MgO + Si3
N4
+ C
Zone IV
(Neutral Atmosphere)
>1,380 K: > 1,810 K:
MgO + Mg + C MgO + M2
S + Mg +C
<1,380 K: MgO + Mg + C < 1,810 K:
MgO + Al4
C3
+ C MgO + SiC + C
Zone V
(Unchanged Material)
MgO + Al + C MgO + Mg + C MgO + Si + C
Table 1: Reaction products for Al, Mg and Si with the refractory at different temperatures
in a BOF lining with Zone I representing the hot face and Zone V the as received brick
[16].
Figure 5 shows the expected dense zone morphology of a metal containing
magnesia-carbon brick. Compare this to the one shown in Figure 3 which
is more of a bridging between grains. In the latter case the amount of
contained carbon is considerably lower than that found in a modern
magnesia-carbon brick which results in the MgO grains being very close
to each thereby providing a scaffold for the newly precipitated material.
Figure 5: Dense zone formation in a magnesia-carbon refractory [17]
If the metal added to the brick was Al then the dense zone will consists
of an MgAl spinel not pure MgO. Figure 6 shows the reaction products
from the hot face to the cold face of a used magnesia-carbon brick
with Al addition. Note, that in addition to the MgAl spinel, nitride and
carbide phases also formed while the brick was in service.
Figure 6: Distribution of reaction products in used MgO-C brick [18]
Although much has been written about the addition of Al, Si and Mg
to magnesia-carbon brick other additives have also been investigated.
At some point, interest was shown in additions of glass, Mg-B, TiO2,
borides and carbides [19, 20, 21]. Of these, B4C has been found to be a
useful addition to magnesia-carbon brick. The microstructures resulting
from the addition of different antioxidants to magnesia-carbon brick was
published by Zhang et. al. [22] and a review of more current additives
was assembled by Luz and Pandolfelli in 2007 [23].
Antioxidants also improve the strength of the brick as compared to
similar ones without it. Figure 7 shows the effect of Al on the brick’s
strength after firing to different temperatures [24].
Figure 7: Relative crushing strength of 15% magnesia-graphite samples with and without
Al additions. B.O.= before oxidation [24]
At the same time that the early research on antioxidants was being
carried out, magnesia and carbon as raw materials and in contact with
each other were studied to establish the important parameters for
achieving good refractory properties which would lead to increased life
in actual applications [25].
Dolomite
Baker, et. al. [10] showed evidence of a dense zone formation in pitch
bonded dolomite, similar to that observed in magnesia brick. Following
up on this work, studies with antioxidant additions showed that boron
bearing compounds were more effective than metals in forming a dense
zone, but their presence had a detrimental effect on slag corrosion [26]
so it was not pursued.
Summary
Although magnesia-carbon refractories showed low wear in actual
applications the move from the laboratory to the steel plant was slow
until the rate of carbon oxidation could be controlled. Much research
was carried out to determine its role and how to reduce its oxidation.
This work traced the development of the early magnesia carbon brick and
the role of antioxidants in the dense zone formation. Their role can be
summarized as follows:
At low temperatures, less than around 1200o
C, there is a decrease in the
brick’s porosity
Above about 1400o
C graphite will oxidize in the presence of MgO and
both Mg and C will be lost from the refractory.
If the refractory is above about 1500o
C for long periods of time and, in
the presence of carbon, an Mg vapor forms which reacts to form the MgO
or an MgAl -spinel dense zone at the brick’s hot face. This prevents the
ingress of air or slag into the brick thereby “resisting” oxidation.
The commonly added antioxidants of Al and Si are used because they are
low cost and afford effective protection.
Borides, in particular B4C, are added because they fill pores at
intermediate temperatures thereby preventing air to react with the
refractory components.
It is interesting to note that brick based on this technology are used
with great success, not only in the BOFs, but also in most EAFs and
ladles and the metal addition concepts were expanded to encompass
Al2O3-SiC-C (ASC) brick, SENs, slide gate plates, etc.
Post script: the biggest challenge to writing this overview was acquiring
copies of the cited papers and many others that I used for background.
I would like to thank all the libraries which have kept journals and
conference proceedings from years gone by.
References
I would like to thank N. Sullivan for his critical reading of this manuscript.
[1] A.M. Garbers-Craig: Presidential address: How Cool are Refractory Materials?, The Journal
of The Southern African Institute of Mining and Metallurgy, V 108, p. 1, 2008
[2] E. Ruh: International Ceramic Monographs, V 1, no. 2, pp. 772-93, 1994
[3] T. Hayashi: Recent development of Refractories Technology in Japan, First International
Conf. on Refractories, Japan, 1983
[4] H.J. Junger, C. Jandl, J. Cappel: Relationships Between Basic Oxygen Furnace Maintenance
Strategies and Steelmaking Productivity, Iron & Steel Tech.,V5, #11, 2008
[5] C. Alvarez, E. Criado, C. Baudin: Refractarios de magnesia-grafito, Bol. Soc. Esp. Ceram.
Vidri., V 31, #5, 1992
[6], R. A. Landy: Magnesia Refractories, Chapter 5 in Refractories Handbook, ed. C. Schacht,
2004
[7] W E Lee @ http://core.materials.ac.uk
[8] R.H. Herron, C.R. Beechan, R.C. Padfield: Slag attack on carbon-bearing basic refractories,
Amer. Ceram. Soc. Bull, V 46, p 1163–1168, 1967
[9] B. Brezny, R. Landy: Microstructural and Chemical Changes of Pitch Impregnated
Magnesite Brick under Reducing Conditions, Trans. and Journal of the British Ceramic Soc.,
V 71, p. 163, 1972
[10] B. H. Baker, B. Brezny, R.L. Shultz: Role of Carbon in Steel Plant Refractories, Am. Ceram.
Soc. Bull., V 55, # 7, p 649, 1976
[11] US Patent #2,013,625, 1935
[12] US Patent #4,407,972, 1983
[13] T. Ayashi : Recent Trends in Japanese Refractory Technology, Transactions ISIJ, V21,
1981
[14] J.D. Smith, R.E. Moore: Thermochemical evaluation of Dense MgO Layer Formation,
Proceedings UNITECR, 1993
[15] M. Karakus, J.D. Smith, R.E. Moore: Mineralogy of the Carbon Containing Steelmaking
Refractories, Proceedings UNITECR, 1997
[16] T. Rymon-Lipinski: Reaktionen von Metallzusatzen in Magnesia-Kohlenstoffsteinen in
einem Sauerstoffknoverter, Teil 1, p 1049 (47), Teil 2, p 1055 (53), Stahl u. Eisen, V. 108,
# 22, 1988
[17] I. Strawbridge, D.G. Apostolopoulos: High Purity Magnesias and Graphites in Magnesia-
Carbon Refractories, Stahl und Eisen, Special, 1994
[18] K. Ichikawa, H. Nishio, O. Nomura, Y. Hoshiyama “Suppression Effects of Aluminum on
Oxidation of MgO-C Bricks, Taikabutso Overseas, V 15, #2, 1995
[19] K. Kurata, T. Matsui, K. Kono: Oxidizing Prevention of MgO-C Bricks for Converter,
Taikabutso Overseas, V 11, #2, 1991
[20] T. Suruga: Effect of Mg-B Material Addition to MgO-C Bricks, Taikabutso Overseas, V 15,
#2, 1995
[21] C.G. Aneziris, J. Hubalkova, R. Barabas: Microstructure evaluation of MgO–C refractories
with TiO2- and Al-additions, J. of the European Ceramic Society, V 27, 2007
[22] S. Zhang, N.J. Marriott, W.E. Lee: Thermochemistry and Microstructures of MgO-C
Refractories Containing Various Antioxidants, J. of the European Ceram. Soc.,V 21, p 1037,
2001
[23] A. P. Luz, V. C. Pandolfelli: Review Article: Performance of the Antioxidants in Carbon
Containing Refractories”, Cerâmica, V 53, 2007
[24] M. Rigaud, P. Bombard, X. Li, B. Gueroult: Phase Evolution in Various Carbon-Bonded
Basic Refratories, Proceedings UNITECR, 1993
[25] H. Ishii, M. Nagafune, I. Tsuchiya: Reaction between Magnesia and Carbon at High
Temperature, Taikabutso Overseas, V 11, #2, June, 1991
[26] F. Ye, M. Rigaud, X. Zhong: Effects of Boron Bearing Additives on Oxidation and Corrosion
Resistance of Doloma-Based Carbon Bonded Refractories, Proceedings UNITECR, 1997
MgO-A2O3
ALN
A14C3
A1
Without additive
With 4 wt% Al
6000
5000
7000
4000 0
2
4
6
8
10
12
5
0 10 15 20
Thermal
conductivity
Graphite content (%)
Youngs
modulus
(kg/mm2)
Depth
of
slag
penetration
(mm)
Thermal
conductivity
(kcal/m.hr.oC)
at
1000oC
(mm)
25
Young’s modulus
Depth of slag
penetration
0
10
20
30
40
50
60
MgO-A2O3
AlN
Al4C3
Al
Without additive
With 4 wt% Al
10
50
0
0 100 200 300
Back
Hot face
Relative
intensity
of
X.R.D
peaks
(mm)
Graphite Content: 15 wt%
Firing Time: 5 hours
Without additive
With 4 wt% Al
60
50
40
30
20
10
0
B.O. 1000 1200
Relative
Cold
Crushing
Strength
(kgk/cm2)
Hot face -
slag
Dense
zone
Unreacted
brick
Temperature o
C