Basic Refractories:
Basic refractories were so named because they exhibit resistance to corrosive
reactions with chemically basic slags, dusts and fumes at elevated
temperatures. While this is still a useful definition, some classes of basic
refractories have been developed that exhibit excellent resistance to rather
acidic slags. Some types of direct bonded chrome-magnesite brick, such as
those used in primary copper applications, fall into this latter category.
Broadly speaking, basic refractories generally fall into one of five
compositional areas:
1. Products based on deadburned magnesite or magnesia.
2. Products based on deadburned magnesite or magnesia in combination with
chrome-containing materials such as chrome ore.
3. Deadburned magnesite or magnesia in combination with spinel.
4. Deadburned magnesite or magnesia in combination with carbon.
5. Dolomitic products. One of the more important types of magnesite brick
are those that have low boron oxide contents and dicalcium silicate bonds.
These chemical features give the brick excellent refractoriness, hot strength
and resistance to load at elevated temperatures. Another category of
magnesite brick contains a higher boron oxide content to improve hydration
resistance. Basic refractories containing chrome continue to be an important
group of materials due to their excellent slag resistance, superior spalling
resistance, good hot strengths and other features. Historically, silicates in the
groundmass or matrix formed the bond between the chrome ore and periclase
in the brick. However, the advent of high purity raw materials in combination
with high firing temperatures made it possible to produce “direct bonded”
brick, where a ceramic bond between the chrome ore and periclase particles
exists. These direct bonded brick exhibit superior slag resistance and strengths
at elevated temperatures. Magnesite-spinel brick have increased in importance
due to a desire to replace chrome-containing refractories because of
environmental concerns. Brick made with spinel and magnesite have better
spalling resistance and lower coefficients of thermal expansion than brick
made solely with deadburned magnesite. These features minimize the chance
of the brick cracking during service. Basic brick containing carbon include
pitch impregnated burned magnesite brick with carbon contents up to 2.5%,
pitch bonded magnesite brick containing about 5% carbon and magnesitecarbon brick which contain up to 30% carbon. Development of the more
corrosion resistant magnesite-carbon brick has resulted in decreased
consumption of pitch impregnated and pitch bonded magnesite brick. In
addition, in many instances the magnesite-carbon brick have replaced
magnesite-chrome brick in applications such as electric arc furnaces. It is
anticipated that magnesite-carbon brick will continue to grow in importance
as new products are developed and additional uses for these products are
found. Dolomitic products are an important class of refractories that are used
for example in rotary cement kilns and AOD’s. Dolomite brick offer a good
balance between low cost and good refractoriness for certain uses. They also
offer good metallurgical characteristics for certain “clean steel” applications.
1. Magnesite Brick
Brick made with dead-burned magnesite are an important category of basic
refractories. Magnesite brick are characterized by good resistance to basic
slags as well as low vulnerability to attack by iron oxide and alkalies. They
are widely employed in applications such as glass tank checkers, subhearth
brick for electric arc furnaces, and sometimes as backup linings in basic
oxygen furnaces. They are often impregnated with pitch in the latter
application. Magnesite compositions are also widely used to control the flow
of liquid steel in continuous casting systems, either as the slide gate refractory
or as a nozzle.
Various grades of dead-burned magnesite are available for the production of
magnesite brick. They range from natural dead-burned materials, with MgO
contents of 90% or less, to high purity synthetic magnesites containing 96%
MgO or greater. A large amount of work has been done to produce highly
refractory magnesites. Since magnesia itself has an extremely high melting
point, i.e., 5070°F (2800°C), the ultimate refractoriness of a magnesite brick
is often determined by the amount and type of impurity within the grain. In
practice, the refractoriness of a dead- burned magnesite is improved by
lowering the amount of impurities, adjusting the chemistry of the impurities
or both. There are many types of magnesite refractories, both burned and
chemically-bonded. For simplification, they can be divided into two
categories on the basis of chemistry. The first category consists of brick made
with low boron magnesites, generally less than 0.02% boron oxide, that have
lime- to-silica ratios of two to one or greater. Often, the lime-to-silica ratio of
these brick is intentionally adjusted to a molar ratio of two to one to create a
dicalcium silicate bond that gives the brick high hot strength. Brick with
lime-to-silica ratios greater than two to one are often of higher purity than the
dicalcium silicate-bonded brick. This greater chemical purity makes them
more desirable for certain applications. The second category of magnesite
brick generally has lime-to-silica ratios between zero and one, on a molar
basis. These brick may contain relatively high boron oxide contents (greater
than 0.1% B2O3) in order to impart good hydration resistance. Sometimes, for
economic reasons, these brick are made with lower purity natural dead burned
magnesites with magnesia contents of 95% or less. At other times, the brick
are made with very pure magnesites with MgO contents greater than 98% for
better refractoriness.
2. Magnesite-Chrome and Chrome - Magnesite Brick
A major advance in the technology of basic refractories occurred during the
early 1930’s, when important discoveries were made regarding combinations
of chrome ore and dead- burned magnesite. Chrome ores are often represented
by the generic formula RO•R2O3, where the RO constituent consists of MgO
and FeO, and the R2O3 constituent consists of Al2O3, Fe2O3 and Cr2O3. It
should be recognized that most of the iron content of raw chrome ores is
present as part of the RO constituent. Chrome ores also contain siliceous
impurities as interstitial gangue minerals. These are generally olivine,
orthopyroxene, calcic plagioclase, chlorites, serpentine and talc. If raw
chrome ore were fired in the absence of dead-burned magnesite, the FeO that
is present would oxidize readily to Fe2O3. This would result in an imbalance
between the RO and R2O3, as the RO decreases and the R2O3 increases. Two
solid phases would appear:
(1) a spinel consisting mainly of MgO•R2O3
(2) a solid solution of the excess R2O3 constituents (Fe2O3,Cr2O3 and Al2O3).
Frequently, the solid solution is easily visible under the microscope as needlelike inclus- ions.
When a chrome ore is heated with added magnesia, as in a chrome- magnesite
or magnesite-chrome brick, MgO enters the chrome spinel to replace the FeO
as it oxidizes to Fe2O3. The MgO also combines with the newly formed Fe2O3
to maintain the spinel structure. The new spinel will have essentially the
formula MgO•R2O3. The reaction of chrome ore with dead burned magnesite
increases the refractoriness of the spinel minerals, since spinels formed by
MgO with Cr2O3, Al2O3 and Fe2O3 have higher melting points than the
corresponding spinels formed by FeO. In addition, the added magnesia also
reacts with the accessory silicate minerals of low melting points present in the
groundmass of the ore, and converts them to the highly refractory mineral
forsterite, 2MgO•SiO3. These reactions explain why magnesite-chrome and
chrome-magnesite refractories have better hot strength and high temperature
load resistance than refractories made from 100% chrome ore.
a) Direct-Bonded Magnesite-Chrome Brick
While the reactions between chrome ore and magnesite outline the
fundamental chemistry of magnesite-chrome brick, a significant advance in
the quality of these products occurred in the late 1950’s and early 1960’s with
the introduction of “direct-bonded” brick. Prior to that time, most magnesitechrome brick were silicate-bonded. Silicate-bonded brick have a thin film of
silicate minerals that surrounds and bonds together the magnesite and chrome
ore particles. The term direct- bonded describes the direct attachment of the
magnesia to the chrome ore without intervening films of silicate. Directbonding was made possible by combining high purity chrome ores and
magnesites, and firing them at extremely high temperatures. High strength at
elevated temperatures is one of the single most important properties of direct-
bonded brick. They also have better slag resistance and better resistance to
“peel spalling” than silicate-bonded brick. Direct-bonded magnesite-chrome
brick are available with various ratios of magnesite-to-chrome ore. The
balance of properties of the brick is a function of the magnesite-to-chrome ore
ratio. For example, a direct bonded brick containing 60% magnesia would
generally be regarded as having better spalling resistance than one containing
80% magnesia, although the latter might be considered a better choice in a
high- alkali environment. This changing balance of properties as a function of
the ratio of magnesite-to-chrome ore makes it possible to choose products
best suited for an individual application.
b) Chrome-Magnesite Brick
Burned chrome-magnesite brick may be of either the direct-bonded or
silicate- bonded variety. The direct-bonded brands are used under more severe
service conditions.
c) Chemically-Bonded Magnesite- Chrome and Chrome-Magnesite Brick
Some magnesite-chrome brick are chemically-bonded rather than burned.
These chemically-bonded brick do not have the high temperature strength,
load resistance or slag resistance of burned compositions. They are widely
used, usually as lower cost compositions to balance out wear profiles in
various applications. Chemically bonded magnesite-chrome brick are
sometimes used with steel casing. In service, the steel oxidizes and forms a
tight bond between the brick. The technique of steel casing has accounted for
improved service life in many applications.
d) Fused Magnesite- Chrome Grain Brick
Products have been developed that contain fused magnesite-chrome grain to
offer improved slag resistance. Fused grain is made by melting dead burned
magnesite and chrome ore in an electric arc furnace. The melted material is
then poured from the furnace into ingots and allowed to cool. The resulting
ingots are crushed and graded into grain for brick making. Brick made from
this grain, are called “rebonded fused magnesite-chrome brick”. Fused
magnesite-chrome grain has extremely low porosity and is chemically inert.
In addition, brick made from this grain have a tendency to shrink on burning
rather than expand, as is characteristic of many direct-bonded magnesitechrome brick. As a result of these features, the rebonded fused magnesitechrome brick have lower porosity and superior slag resistance as compared to
direct-bonded magnesite- chrome brick.
This type of brick is used in AOD’s, degassers and sometimes in the more
severe areas of nonferrous applications. The fused grain brick used in North
America typically contain 60% magnesia. Some compositions contain a
combination of fused and unfused materials for better spalling resistance, to
lower cost, or to achieve a balance of properties that is appropriate to the
particular application in which they will be used.
e) Coburned Magnesite- Chrome Grain Brick
Some magnesite-chrome brick are made from coburned magnesite-chrome
grain, often referred to merely as coburned grain. Coburned grain is made by
combining fine magnesia and chrome ore and dead-burning in, for example, a
rotary cement kiln. The resulting grain is dense and exhibits a direct-bonded
character. Like brick made with fused magnesite-chrome grain, brick made
with coburned grain shrink in burning and thus can have lower porosity than
certain classes of direct-bonded magnesite-chrome brick. Brick made with
coburned grain find wide variety of uses, such as in vacuum degassers in
steelmaking and in certain nonferrous industries, such as primary copper and
nickel production.
3. Magnesite-Spinel Brick
Magnesite-spinel brick have been more broadly used in recent years. The
term “spinel” as used in describing this type of brick refers to the mineral
MgO•AI2O3. In discussing magnesite- chrome brick and chrome ores, the
term “spinel” is often used to refer to the family of minerals that crystallize in
the cubic system and have the general formula RO•R2O3, where RO may be
MgO, and FeO and R2O3 may be Fe2O3, Al2O3 and Cr2O3 While usage of the
term “spinel” in this broader sense is accepted practice, the mineral spinel has
the chemical formula MgO•Al2O3. It has become accepted usage to use the
term magnesite-spinel brick to refer to the products containing MgO•Al2O3.
A family of magnesite-spinel brick has been developed by combining the
constituent raw materials in various ways. Some magnesite-spinel brick are
made by adding fine alumina to compositions composed mainly of magnesia.
On firing, the fine alumina reacts with the fine magnesia in the matrix of the
brick to form an in situ spinel bond. An alternative is to add spinel grain to a
composition containing magnesia. One of the principal benefits of combining
spinel and magnesia is that the resulting compositions have better spalling
resistance than brick made solely with dead burned magnesite. This feature
results in the avoidance or inhibition of peel spalling caused by temperature
cycling and infiltration of constituents from the service environment. Spinel
additions also lower the thermal expansion coefficients of magnesite
compositions. This can reduce thermal stresses that could contribute to
cracking in certain environments. A desire to use chrome-free basic brick for
environmental reasons has increased the importance of magnesite- spinel
brick. Trivalent chromium (Cr+3 ) present in magnesite-chrome brick can be
converted to the hexavalent state (Cr+6 ) by reaction with alkalies, alkaline
earth constituents and other compounds that are present in some service
environments. These factors have led to broad use of magnesite-spinel brick
in rotary cement kilns. They have excellent spalling resistance, good thermal
expansion characteristics and have been shown to provide excellent service
results in many rotary kilns.
4. Carbon-Containing Basic Brick
The idea of adding carbon to a magnesite refractory originally stemmed from
the observation that carbon is not easily wetted by slag. Thus, one of the
principal functions of carbon is to prevent liquid slag from entering the brick
and causing disruption. Until the mid 1970’s brick based on carbon in
combination with magnesite were mainly used in basic oxygen steelmaking
furnaces; but since that time they have been more broadly utilized in electric
arc furnaces and steel ladle applications. Carbon-containing basic brick can be
categorized as follows:
1. Pitch-impregnated, burned magnesite brick containing about 2.5% carbon;
2. Pitch-bonded magnesite brick containing about 5% carbon;
3. Magnesite-carbon brick containing 8% to 30% carbon (in this class, carbon
contents ranging from 10% to 20% are most common).
While all brick in these categories contain both magnesite and carbon, the
term “magnesite-carbon brick” as typically used in the United States refers to
brick with carbon contents greater than 8%. Pitch-impregnated and pitchbonded magnesite brick can be thought of as products containing just enough
carbon to fill their pore structures. In magnesite-carbon brick, however, the
carbon addition is too large to be considered merely a pore filler. These brick
are considered composite refractories in which the carbon phase has a major
influence on brick properties.
a) Burned Pitch-Impregnated Magnesite Brick
One category of burned pitch- impregnated magnesite brick is made with a
dicalcium silicate bond. Dicalcium silicate has an extremely high melting
point of about 3870°F (2130°C). Use of this bond in combination with tight
chemical control of other oxides gives these brick excellent hot strength and
an absence of fluxes at temperatures commonly found in metallurgical
processes. The carbon derived from the impregnating pitch when the brick is
heated in service prevents slag constituents from chemically altering the
dicalcium silicate bond, preserving the hot strength and high refractoriness.
The carbon also prevents the phenomenon of peel spalling, where the hot face
of a brick cracks and falls away due to slag penetration in combination with
temperature cycling. Dicalcium silicate bonded burned magnesite brick that
have been impregnated with pitch are used in a number of applications. In
basic oxygen furnaces, this type of brick is sometimes used in charge pads,
where its high strength enables it to resist cracking and disruption caused by
the impact of steel scrap and liquid metal being added to the furnace. These
brick are also widely used as a tank lining material, i.e. as a backup lining
behind the main working lining of a basic oxygen furnace. They are also used
in subhearths of electric arc furnaces. Not all pitch impregnated burned
magnesite brick are dicalcium silicate bonded, however. One important class
of brick that deserves mention has a low lime to silica ratio and a high boron
oxide content. These chemical features cause the brick to have relatively low
hot strength, but at the same time, result in very good hydration resistance.
Thus, brick such as this are the products of choice where it is judged that there
is potential for hydration to occur.
b) Pitch-Bonded Magnesite Brick
Pitch-bonded magnesite brick are used in various applications, but mainly in
basic oxygen furnaces and steel ladles. These products have excellent thermal
shock resistance and high temperature strength, and good slag resistance.
Pitch-bonded magnesite brick were the principal working lining materials for
basic oxygen furnaces for many years. Although in severe service
environments they have been replaced to a large extent by more erosion
resistant graphite-containing magnesite- carbon brick, they continue to play
an important role in, for example, lower wear areas of basic oxygen furnaces.
c) Magnesite-Carbon Brick
The high carbon contents of magnesite- carbon brick are generally achieved
by adding flake graphite. The high oxidation resistance of flake graphite
contributes to the reduced erosion rates of these brick. In addition, the flake
graphite results in very high thermal conductivities compared to most
refractories. These high thermal conductivities are a factor in the excellent
spalling resistance of magnesite-carbon brick. By reducing the temperature
gradient through a brick, the high thermal conductivities reduces the thermal
stresses within the brick. High thermal conductivity also results in faster
cooling of a magnesite- carbon brick between heats and thus reduces potential
for oxidation. In recent years, product development efforts have been directed
towards producing magnesite-carbon brick with good slag resistance and high
temperature stability. High temperature stability refers to the ability of the
brick to resist internal oxidation-reduction reactions that can reduce hot
strength and adversely affect the physical integrity of the brick at elevated
temperatures (i.e. the oxides in the brick are reduced by the carbon). A high
degree of slag resistance and good high temperature stability have been found
to be advantageous in the hotter and more corrosive service environments.
The high temperature stability of magnesite-carbon brick has been achieved
by utilization of high purity graphites and magnesites. Since flake graphite is
a natural, mined material, there are impurities associated with it. These may
be minerals such as quartz, muscovites, pyrite, iron oxides and feldspars.
Although much purification is accomplished through flotation processes, most
flake graphites contain a limited amount of these impurities. These mineral
impurities are often referred to as graphite “ash”. Some of the ash
constituents, especially the silica and iron oxide, are easily reduced by carbon
and thus will result in a loss of carbon from the brick and a reduction in hot
strength at elevated temperatures.
Magnesia can also be reduced by carbon at high temperatures. For best high
temperature stability, high purity magnesites are used. Magnesites with very
low boron oxide contents are especially desirable. The service environments
in which these carbon-containing basic brick are used are very diverse due to
process changes in the steelmaking industry and due to broader use of the
products. A great deal of work has been done to develop special additives to
improve the performance of carbon-containing brick in these applications.
These additives include powdered metals such as aluminum, magnesium and
silicon. One reason for adding these metals is to improve oxidation resistance.
The metals consume oxygen that would otherwise oxidize carbon. The
aluminum and silicon also cause the pore structure of a magnesite-carbon
brick to become finer after the brick is heated. It is believed that the pores
become finer due to formation of aluminum carbide (Al4C3) and silicon
carbide (SiC) by reaction between the metals and the carbon in the brick. The
finer pores result in decreased permeability of the brick and inhibit oxidation
by making it more difficult for oxygen to enter the brick structure. Another
reason for adding metals is to improve the hot strength of magnesite-carbon
brick. It has been suggested that the improvement in hot strength is due to the
formation of carbide “bridges” within the matrix of the magnesite-carbon
brick. Another way that metals may improve hot strength is simply by
protecting the carbon bond in these brick from oxidation. Silicon has been
employed as an additive to inhibit the hydration of aluminum carbide that is
formed in aluminum-containing magnesite-carbon brick. Aluminum carbide
can react with atmospheric humidity or any other source of water to form an
expansive reaction product that can disrupt the brick. This is illustrated by the
following equation: Al4C3+12H2O•CH4+4Al(OH)3.
This reaction represents a potential problem for applications with intermittent
operations such as some steel ladles or electric arc furnaces. Adding silicon to
an aluminum- containing brick greatly extends the time before which
hydration will occur. Boron-containing compounds such as boron carbide
(B4C) are used to improve oxidation resistance in certain critical applications
such as tuyere elements in bottom blown basic oxygen furnaces. In addition,
magnesite-carbon brick are sometimes impregnated with pitch in order to
improve oxidation resistance as well as to promote brick to brick bonding in
service.
5. Dolomite Brick
Dolomite brick are available in burned and carbon-bonded compositions. The
carbon-bonded varieties include both pitch and resin-bonded versions. Some
of the carbon-bonded products contain flake graphite and are somewhat
analogous to magnesite-carbon brick. Dolomite brick are widely applied in
applications as diverse as argon-oxygen decarburization vessels (AOD’s),
rotary cement kilns and steel ladles.