Refractories

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 magnesite-carbon 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 needle-like 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 magnesite- chrome 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. Direct-bonding 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 magnesite-chrome brick. As a result of these features, the rebonded fused magnesite-chrome 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 pitch-bonded 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.