Magnesia Chrome Refractories
Magnesia Chrome Refractories
Magnesia chrome refractories are basic refractories. These refractories are based on dead burned magnesite or magnesia in combination with chrome containing materials such as chrome ore (chromite). Major constituents of these refractories are magnesium oxide (MgO) and chromium oxide (Cr2O3). Basic refractories containing chrome continue to be an important group of materials because of their excellent slag resistance, superior spalling resistance, good hot strengths, and other features.
Based on the content of chrome, these refractories are called chrome magnesia refractories or magnesia chrome refractories. Chrome containing refractories are divided into three main groups according to the chrome oxide content. Magnesia chrome refractory has less than 30 % of Cr2O3, chrome-magnesia refractory has higher than 30 % Cr2O3, and Picro-chromite refractory has higher than 75 % Cr2O3.
Basic refractories are so named since they show resistance to corrosive reactions with chemically basic slags, dusts and fumes at high temperatures. While this is still a useful definition, some classes of basic refractories have been developed which show good 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.
Chrome-containing refractories have been around since 1879 and are critical for different metallurgical applications. Refractory grade chromite is important as a source of chrome in these chrome containing-refractories.
Modern refractories technology began in the late 18th century with the growth of the iron industry and later the steel industry during the Industrial Revolution. The greater demands placed on refractories needed materials other than alumino-silicates and silica. Magnesite and chrome brick were all introduced in the late 1880s. It was only in 1931 that the superior hot strength of blends of chromite and magnesite was recognized and chrome-magnesia bricks were introduced with tonnage usage in open hearth (OH) and electric steel-making furnaces. In the mid1960s low-silica, magnesia chrome and reconstituted fused grain magnesia chrome refractories were introduced worldwide.
The early chrome refractories consisted of moulded and fired chromite. These refractories had several issues because of their bursting and crumbling as a result of alternative exposure to oxidizing and reducing atmospheres. They also shrank and softened at high temperatures. The addition of magnesia ‘solved’ several of these issues, and this led to the development of the magnesia chrome, chrome magnesia series of refractories during the 1930s.
Historically, silicates in the ground mass or matrix formed the bond between the chrome and magnesia 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 and magnesia particles exists. These direct bonded brick show superior slag resistance and strengths at high temperatures.
The effect of a silicate melt on brick properties was established in the 1950s and 1960s, and in the steel industry in particular, the demand increased for lower SiO2 (silica) bricks for OH and electric arc furnaces. Magnesia chrome brick became the preferred brick because of its superior slag resistance and stability at high temperatures.
In the firing process, FeO in the chromite oxidizes to Fe2O3 and diffuses at high temperature into the MgO. Magnesio-ferrite (MgO·Fe2O3), a refractory spinel, is formed. The chromite is ‘stabilized’, thereby reducing the risk of undergoing subsequent redox (reduction oxidation) reactions. There is also development of direct bonding between MgO crystals and MgO and chromite grains.
A combination of low silica and good bonding gives bricks a high hot strength and good spalling resistance. High Cr2O3 contents give rise to low wettability by fayalitic slags and a high chromite spinel content gives rise to low slag solubilities.
By fusing (melting together) the chromite and magnesia the chromite spinel is completely stabilizes and completely disperses as small spinel crystals throughout the magnesia. Optimum performance is then obtained.
The usefulness of chrome as a refractory is based on four factors namely (i) it has a high melting point, (ii) it has a moderate thermal expansion with thermal expansion of magnesite chrome brick as 1.1 % and that of chrome magnesite brick as 1 %, (iii) neutral chemical behaviour since chrome containing materials can tolerate slags ranging from slightly acid to basic, and (iv) relatively high corrosion resistance. Chrome has exceptionally good resistance to pyro-metallurgical slags. Slags which are acidic and contain high levels of iron, i.e. are silica-rich fayalite (2FeO·SiO2), rapidly attack and deeply penetrate alumino-silicate refractories. The resistance of chrome against fayalitic slags, which are very common in several non-ferrous metallic smelting processes, is exceptional.
Chromium plays an important role in a wide range of industrial processes. In refractories, chromite is a cost effective material which has properties ideal for a number of metallurgical applications ranging from ferro-alloys and steel, to base metals, to platinum group of metals. Applications for chrome-bearing refractories include all pyro-metallurgical extraction processes for copper, nickel, lead, and platinum group of metals. These refractories are the preferred refractory in these applications because of their superior thermal shock resistance and chemical inertness in contact with liquid metal and slags, in comparison to other basic refractories, including magnesia-spinel and magnesia refractories. Chrome based refractories are typically used in cement kilns, secondary steel refining furnaces, foundry sands, glass melting furnaces, and incinerators. The steel industry, large quantities of fused grain bricks are still used in vacuum degassing furnaces. These refractories are also used for lining of CLU converters in the ferro-alloy industry. Another use of these refractories is for the lining of foundry electric-arc furnaces. In some cases, alternative materials such as magnesium aluminum spinels, spinel–bonded magnesite and high alumina refractories have replaced chrome containing refractories.
Chrome ore as a raw material for refractories
The use of chromium in refractories is important in metallurgical applications. ‘Chrome ore’ and ‘chromite’ are terms used synonymously in the refractory industry. There are three grades of chrome ores for industrial usage. These are (i) metallurgical, (ii) refractory, and (iii) chemical. Only the refractory grade ore is used for chrome refractories. Chromite is one of the principal ore of chromium in which the metal exists as a complex oxide (FeO.Cr2O3). The prepared chromite can be widely used as refractory in high temperature applications. Chromite contains certain quantity of FeO. The chromite containing less SiO2 and more Cr2O3 is ideal raw material for refractories. Gangue minerals are the main source of SiO2 and CaO.
The mineral chromite is the ore of chromium. A typical composition of a chromite suitable for refractory purpose is 30 % to 50 % Cr2O3, 13 % to 30 % Al2O3, 12 % to 24 % Fe2O3, 14 % to 20 % MgO, less than 10 % SiO2, and up to 1 % CaO. The usage of chromite as a refractory is based on its high melting point of 2,180 deg C, moderate thermal expansion, neutral chemical behaviour, and relatively high corrosion resistance. Chromite improves thermal shock and slag resistance, volume stability, and mechanical strength. A major advance in the technology of basic refractories occurred during the early 1930s, when important discoveries were made regarding combinations of chrome ore and dead burned magnesite.
Chrome ore has been a widely used refractory raw material for several years. In particular, the steel industry greatly expanded its use in the late 1950s after laboratory and post-mortem studies revealed that bricks with ‘direct bonding’ between the chrome ore and magnesia aggregates, in which the two grains are joined predominantly by a solid-state diffusion mechanism, produced a brick which out-performed earlier magnesia chrome bricks in the roofs of the OH furnaces. Since then, direct-bonded magnesia-chrome bricks have enjoyed good success in steel ladles, cement kilns, and copper converters.
Chrome ores have good resistance to slag attack. This raw material has been considered to be chemically neutral. In combination with magnesia, the magnesia chrome refractory products have good refractoriness and spalling resistance. However, under fluctuating temperature and oxygen partial pressure or both, the chromite grains expand or grow continuously, eventually causing the chrome containing refractories to become more porous and distort the furnace steel super-structures. Also, chrome ore can have poor resistance to iron oxide attack, and, in the presence of calcium-containing furnace burdens, soluble, hexavalent chromium compounds are formed, leading to an environmentally unfriendly product re disposal issues because of its potentially carcinogenic characteristics.
Chrome ores are frequently 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 is to 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 normally olivine, ortho-pyroxene, calcicplagioclase, chlorites, serpentine and talc.
If raw chrome ore is fired in the absence of dead-burned magnesite, the FeO which is present oxidizes readily to Fe2O3. This results in an imbalance between the RO and R2O3, as the RO decreases and the R2O3 increases. Two solid phases appear, which are (i) a spinel consisting mainly of MgO.R2O3, and (ii) 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 inclusions.
When a chrome ore is heated with added magnesia, as in a chrome magnesia bricks or magnesia chrome bricks, 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 has 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 ground mass of the ore, and converts them to the highly refractory mineral forsterite, 2MgO.SiO3. These reactions explain why magnesia chrome and chrome magnesite refractories have better hot strength and high temperature load resistance than refractories made from 100 % chrome ore.
The dominant phase in chrome ore is a mineral which has a spinel structure with a complicated chemical composition because of almost unlimited solid solution substitutions. Its overall chemical composition can be best understood by considering the general formula for spinel, AB2O4, where A represents a divalent cation, most frequently Fe2+ and / or Mg2+, and B represents a trivalent cation, normally Fe3+, Al3+, and / or Cr3+. Hence, the general formula can be written as (Fe,Mg)(Fe,Al,Cr)2O4. Another representation of the spinel composition is to consider a right triangular prism with the lower triangular area having end member apices magnetite (FeFe2O4), ferro-chromite (FeCr2O4), and hercynite (FeAl2O4), and the upper triangular area having the end member apices magnesio-ferrite (MgFe2O4), picro-chromite (MgCr2O4), and spinel (MgAl2O4). Then the composition of the chromite occurs at some point within this triangular volume and can be considered to be a mixture of these end members.
One or more of silicate gangue minerals accompany the chromite in the ore. These minerals can range from anhydrous magnesia lime olivines, pyroxenes, and amphiboles to hydrous serpentine varieties. Further, with respect to particle packing of refractories, chrome ores need to be considered as both aggregate and matrix components.
Chromite in fired magnesia chromite refractories has characteristic texture and give superb qualities to the brick. Chromite in the bricks is stable when they are blended with magnesia to manufacture magnesia chrome bricks.
Types of magnesia chrome refractory bricks
The magnesia chrome refractory bricks are the most common chromite bearing refractories. They are basically made of magnesia and chromite. The magnesia is mainly synthetic. The bricks are durable or corrosion resistant against slag in a wide compositional range. The magnesia chrome bricks can be classified into several types based on their manufacturing processes.
Silicate bonded refractories – In the silicate bonded refractories, the magnesia crystallites and the chromite grains are bonded together by silicates. Silicate bonded brick have a thin film of silicate minerals which surrounds and bonds together the magnesite and chrome ore particles. These bricks have limited refractoriness, but can have good thermal shock resistance, and high pressure flexibility.
Direct bonded magnesite chrome refractory brick – While the reactions between chrome ore and magnesite outline the fundamental chemistry of magnesite chrome brick, an important advance in the quality of these products occurred in the late 1950s and early 1960s with the introduction of ‘direct-bonded’ brick. Prior to that time, majority of the magnesia chrome bricks were silicate bonded. The term direct bonded describes the direct attachment of the magnesia to the chrome ore without intervening film of silicate.
In direct bonded refractories by lowering the impurity content and high temperature firing, one can produce a direct bonded brick in which the chrome reacts with the MgO to form a highly refractory spinel, MgO·(Al,Cr,Fe)2O3. Direct bonding is 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 the direct bonded brick. They also have better slag resistance and better resistance to ‘peel spalling’ than silicate bonded brick.
Sieved magnesia and chromite are mixed, pressed and fired at 1,750 deg C or higher temperature. These are simple, economical and most common bricks. The name ‘direct bond’ comes from direct contact between grains of magnesia and chromite in texture. The direct contact is achieved with the usage of chromite carrying less silicates as gangue minerals. On the contrary, less pure brick of this type is used to be called ‘silicate bonded bricks’ since silicates existed and bonded between magnesia and chromite grains. The direct bonded bricks have higher strength at high temperature and are durable or corrosion resistant since less low melting temperature phase exists in the grain boundaries.
Direct bonded magnesite chrome bricks 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 is normally regarded as having better spalling resistance than one containing 80 % magnesia, although the latter can 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 ratio makes it possible to choose products best suited for an individual application. Burned chrome magnesite brick can be of either the direct bonded or silicate bonded variety. The direct bonded bricks are used under more severe service conditions.
Rebond bricks – Magnesia and chromite are once melted in an electric arc furnace to make fused magnesia chromite. The magnesia and chromite are purified through this process. Obtained fused magnesia chromite has better sinterability. They are crushed, sieved, pressed and fired to manufacture rebond bricks. The bricks are highly corrosion resistant but expensive. The bricks can be an alternative to fused cast bricks.
Unfired (unburnt) bricks – Magnesia and chromite (and sometimes fused magnesia chromite) are mixed and pressed. The bricks are simple, economical, and save energy, but they are less resistant bricks.
Chemically bonded magnesite chrome and chrome magnesite refractory brick – In case of chemically bonded refractories, the refractories are normally produced with magnesium salts and are not burned. Some magnesia chrome bricks are chemically bonded rather than burned. These chemically bonded bricks do not have the high temperature strength, load resistance, or slag resistance of burned compositions. They are widely used, normally as lower cost compositions to balance out wear profiles in various applications.
Chemically bonded magnesia chrome bricks 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 several applications.
Fused cast bricks – In case of fusion cast refractories, magnesia clinker and chromite grains are fused before brick making. Magnesia and chromite are melted in an electric arc furnace and cast in moulds to shape bricks. They have the highest resistance to corrosion but have many internal cavities or voids which make the bricks less reliable, and are extremely expensive.
Fused magnesia chrome refractory brick – Fused magnesia chrome refractory brick contains fused magnesia chrome grain to offer improved slag resistance. Fused grain is made by melting dead burned magnesia 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. Bricks made from this grain, are called ‘rebonded used magnesia-chrome bricks’.
Fused magnesia chrome grain has extremely low porosity and is chemically inert. In addition, bricks made from this grain have a tendency to shrink on burning rather than expand, as is characteristic of many direct bonded magnesia chrome refractory bricks. As a result of these features, the rebonded fused magnesia chrome refractory bricks have lower porosity and superior slag resistance as compared to direct bonded magnesia chrome refractory bricks.
This type of brick is used in AOD (argon oxygen decarburization) furnace, degassing units, and sometimes in the more severe areas of non-ferrous applications. The fused grain brick used typically contains 60 % magnesia. Some compositions contain a combination of fused and unfused to lower cost, or to achieve a balance of properties which is appropriate to the particular application in which they are used.
Co-burned magnesia chrome refractory brick – In case of co-burned refractories, the magnesia clinker and chromite grains are sintered before brick making. Some magnesia chrome refractory bricks are made from co-burned magnesia chrome grain, frequently referred to merely as co-burned grain. Co-burned grain is made by combining fine magnesia and chrome ore and dead burning in, for example, a rotary kiln. The resulting grain is dense and shows a direct bonded character. Like brick made with fused magnesia chrome grain, bricks made with co-burned grain shrink in burning and hence can have lower porosity than certain classes of direct bonded magnesia chrome bricks.
Bricks made with co-burned grain find wide variety of uses, such as in vacuum degassing furnaces in steelmaking and in certain non-ferrous industries, such as primary copper and nickel production.
Magnesia chrome grains
Magnesia chrome materials occur as either a sintered (also called ‘pre-reacted’ or ‘co-clinkered’) or a fused grain. The former is produced by co-grinding calcined magnesia and milled, low silica chrome ore, granulating or briquetting this mixture, drying, and dead burning in either a rotary or a shaft kiln. The fused grain is also manufactured by blending the two aforementioned raw materials and then fusing them in a Higgins-type or tilt pour electric furnace followed by carefully slow cooling to allow the growth of large magnesia crystallites containing large exsolutions of spinel solid solutions outlined by partial spinel-solid solution crystals.
Different sizes of these products can be there. As with chrome ore, magnesia chrome grains, in the presence of calcium containing contaminants, can lead to the formation of soluble, hexavalent chromium compounds and carcinogenic concerns. Different combinations of magnesia and chrome ore can be produced. These refractory raw materials can be used as both aggregates and matrix materials.
Production of magnesia chrome refractory brick
These days the magnesia chrome refractory bricks are normally made from fused magnesia chrome grains. The fused grains with the desired particle size distribution (typically 0.1 micrometers to 7 mm) are mixed and then pressed to form a brick. The bricks are sintered in a tunnel kiln for around 48 hours to 72 hours, depending on its size. The sintering temperature is normally around 1,700 deg C to 1,800 deg C and the sintering atmosphere is partially reducing to help the densification of fused grains.
Fig 1 shows the schematic view of the microstructure of a magnesia chrome refractory brick. The large grains (greater than 1 mm) called aggregates are bonded together by smaller particles (less than 1 mm) called the matrix or binding system. During sintering the matrix densifies and form a bond with the aggregates, hence giving strength and integrity to the brick.
The matrix is the key component of the magnesia chrome bricks since it is the first component of the brick which reacts with the corrosive environment. Hence, the composition, the sintering behaviour, and the interaction between binding system (matrix) and aggregates (fused magnesia chrome grains) are important in designing a microstructure for the refractory applications.
Fig 1 Schematic view of a magnesia chrome refractory
The binding system for magnesia chrome grains is normally fine fused grains and a small quantity of MgO, Al2O3, and Fe2O3 to improve the densification. These oxides react to form spinel during the sintering and enhance the densification probably through synthesis and sintering process. No chromium oxide is normally to be added since Cr2O3 reduces the densification of the binding system. Hence, it is expected that the binding system is vulnerable to fast corrosion especially when it is in contact with the liquid slags when used in the non-ferrous industry.
The phase analysis of fused magnesia chrome grains indicates that they are composed of magnesia and a complex spinel solid solution. Hence, a fully dense binding system (matrix) with the same composition or with high Cr2O3 composition is ideal for the magnesia chrome brick. Moreover, a composition with a spinel crystal structure is favourable since it can bond better to aggregate with the same crystal structure. In addition, reducing the sintering temperature of magnesia chrome refractory from 1,700 deg C to less than 1,500 deg C through designing an appropriate binding system is significant for industry since high sintering temperature increases the production costs and contributes to the greenhouse effect by the CO2 (carbon di-oxide) emission associated with fossil combustible, normally used for the sintering.
Magnesia chrome refractories have been used for more than 50 years for refractory linings in non-ferrous metal smelting and refining furnaces, owing to their high corrosion resistance against high silica and iron containing slags and gaseous environments. However, the challenge of using these refractories is the rapid oxidation of chromium during sintering. To avoid this, the sintering of magnesia chrome refractories is to take place at a relatively high temperatures (around 1,750 deg C) and in a controlled atmosphere, which leads to high production costs.
While the sintering mechanisms of MgO and chromite are separately well understood, there is limited studies focusing on the sintering mechanism of magnesia chrome refractories. Chromite is defined as a solid solution between two or more spinels, wherein one of them contains chromium in its composition. The magnesia chrome refractories contain spinel solid solutions which can experience different degrees of disordering, because of the cations movement between tetra and octahedral sites. Hence, the correlation between sintering properties and the degree of disordering has to be identified.
Chromite is a mineralogical name for FeCr2O4. However, it normally refers to any chromium containing spinel or solid solution with spinel crystal structure. There are six important chromite end members which include FeFe2O4, MgFe2O4, FeAl2O4, MgAl2O4, FeCr2O4, and MgCr2O4. Among them, MgCr2O4, MgAl2O4 and MgFe2O4 are stable in harsh environments where high temperature (greater than 1,300 deg C), thermal shock resistance, oxidizing reducing atmosphere, and corrosion resistance against liquid slags are necessary. Due to these characteristics, MgCr2O4, MgAl2O4, MgFe2O4 and their solid solutions have been used as refractory materials for more than 50 years.
MgCr2O4 is a normal spinel (space group Fd3m) with a lattice parameter of 8.333 +/- 0.002 Angstrom and theoretical density of 4.412 g/cc (grams per cubic centimeter). According to the MgO-MgCr2O4 phase diagram (Fig 2), it is the only compound in this system, and its melting temperature is around 2,350 deg C. At high temperatures and in air atmosphere, the intrinsic defect in MgCr2O4 is the chromium vacancy. Since the formation of chromium vacancies can be due to the evaporation mechanisms, the formed vacancies can improve the self-diffusion rate or the diffusion rates of other cations, but they cannot help the densification rate.
Fig 2 MgO-MgCr2O4 phase diagram
The mixture is shaped using a pressing machine, dried, and then fired normally in a tunnel kiln. Typical firing condition is at 1,800 deg C for several hours. In the tunnel kiln, temperature of the bricks increases gradually. Components of the chromite such as Cr2O3, Fe2O3 and Al2O3 dissolve into magnesia in the process and make solid solution. Majority of the Fe2O3 and Cr2O3 can dissolve into magnesia (Fig 3a). Firing experiment at 1,530 deg C in a study has shown Mg2+ and Fe2+ diffused rapidly in this process. In the study it has been pointed that the effect of the initial oxidation state of the starting material, i.e. chromite, and oxidation of Fe2+ to Fe3+ strongly affect the reaction. The chromite as a raw material contains some FeO, but bricks fired in a near neutral atmosphere are oxidized and contain only Fe2O3.
Fig 3 Properties and microstructure of magnesia chrome bricks
After the firing at the maximum temperature, temperature of the bricks decreases gradually. In this process dissolved components exsolve from the magnesia and form ‘secondary spinel’ around or between magnesia grains. The secondary spinel i.e. chromite forms direct bond between grains and makes bricks dense, strong (i.e. well-sintered) and durable. Quantity and size of the secondary spinel are strongly affected by cooling rate and atmosphere.
It has been shown that majority of secondary spinel in larger size can be crystallized under slower cooling rate and higher oxygen partial pressure. Less secondary spinels are crystallized from liquid phase existing in grain boundaries. Solubility of Cr2O3 and Al2O3 is higher in magnesia and lower in liquid in case of higher Cr2O3 / (Al2O3+Cr2O3) ratio at 1,700 deg C. Liquid improves mass transportation in the body, helps reorganization of texture, and contributes to the sintering. However, over-abundant silicates hinder direct bond.
The liquid phase is calcium-magnesium-silicate melt derived mainly from gangue minerals of chromite as a raw material. Another raw material, magnesia, is synthetic and contains less impurities. The melt is crystallized as forsterite, monticellite, merwinite, di-calcium silicate, tri-calcium silicate or lime as per the CaO / SiO2 ratio in the melt.
There are three types of chromite in fired magnesia chrome bricks based on their forming mechanism. These are (i) type 1, (ii) type 2, and (iii) type 3. Type 1 are formed from blended chromite (primary chromite spinel). Type 2 are formed from precipitation from liquid phase while cooling and the type 3 is formed from exsolution precipitation from magnesia while cooling.
Another study has classified the chromites into four types namely (i) type 1, (ii) type 2a, (iii) type 2b, and (iv) type 3 as shown in Fig 4a. Type 1 is the blended primary chromite. Type 2a, chromite is present as a rim around the blended chromite. Type 2b is the chromite at magnesia grain boundaries, and thee type 3 precipitates within magnesia grains.
Fig 4 Types of chromites and slag viscosity in CaO-Cr2O3-SiO2 system
Properties of magnesia chrome bricks
Magnesia chrome refractory offers the advantages of high resistance to the extremely corrosive environment and low thermal expansion at operating temperatures. Magnesia chrome refractories mainly consists of magnesia phase with large complex spinel particles. Based on the grade of raw materials and production methods applied, different grades of magnesia chrome refractory are produced for use in different parts of the furnace. The magnesia phase mainly acts as a bonding phase to join the large spinel particles.
Normally refractories are durable if they are resistant against corrosion, spalling and slag penetration, and strong at high temperatures. Spalling, or peeling off of refractory surface, occurs when rapid temperature change happens beneath the surface. The chromite in a magnesia chrome brick strongly affects resistance against corrosion and slag penetration. Cr2O3 component in the chromite raises slag viscosity and make protective layer on the hot face and suppresses slag penetration. Fig 4b shows steep rise of SiO2-CaO slag viscosity with Cr2O3. On the other hand, the chromite is effective to stop crack propagation in the brick.
SiO2, Al2O3, Fe2O3 and CaO components in the brick also have influence on the properties of the refractory. They are determined by the mixing ratio of magnesia and chromite, and their chemical composition. SiO2, Al2O3, and Fe2O3 are mainly from chromite and CaO from both of magnesia and chromite. SiO2 and CaO in magnesia chrome bricks form calcium magnesium silicate as liquid phase at high temperature. Total quantity of these components has close relation with development of the direct bond in the brick, and affects softening point and hot strength of the brick. CaO/SiO2 ratio also has influence on hot strength. Hot bend strength has maximum value at the CaO/SiO2 ratio of 2.
SiO2 degrades corrosion resistance. Al2O3 is an important component to improve spalling resistance. However, Al2O3 rich chromite is less corrosion resistant. Fe2O3 intensifies hot strength of the brick, but diminishes spalling resistance. Fe2O3 in the bricks is easily reduced to FeO or Fe during the service under reducing environment. After the reduction the bricks lose strength and wear easily. Chromite in a strongly reduced brick is destroyed and decomposed into metallic iron and chromium.
Environment issue related to magnesia chrome bricks
Chromium exists in a number of different oxidation states which give it the ability to modify other chemical compounds, or to act as a catalyst in promoting chemical reactions. Hence, it is widely used in the chemical industry. The most important oxidation states are described below.
Trivalent chromium Cr (III) – Trivalent chromium is the most stable oxidation state of chromium. Trivalent chromium compounds are stable. They normally have low solubility in water, and do not present a considerable environmental hazard. The most common example of trivalent chromium is green chrome oxide (Cr2O3), which is widely used as a pigment in paints and as a component in alumino silicate refractories. Chromium in chromite is also in the trivalent form.
Cr (IV) – Cr (IV) oxide (CrO2) is a black, conducting, ferromagnetic compound used in the production of audio and video tapes.
Hexavalent chromium Cr (VI) – The most common examples of hexavalent chromium compounds are chromic acid, and the di-chromates of sodium and potassium, which are used in the chemical industry and to surface-treat steels to improve corrosion resistance. Hexavalent chromium compounds are soluble, toxic, and are known to increase the risk of respiratory cancer.
When chrome based refractory materials are exposed to high temperatures and pressures combined with certain chemical phases, a possibility exists that toxic by-products can form. In particular, the transition in the oxidation state of the chrome from Cr3+ to Cr6+ is of particular concern, as hexavalent chromium compounds are classified as carcinogenic and harmful to health.
As chromite comes into contact with alkali and alkaline earth oxides, the transition from Cr3+ to Cr6+ is accelerated. In particular it is clear that exposing chromium containing materials to alkali or calcium oxide rich environments most likely results in the accelerated formation of Cr6+. The reaction in the chromium containing refractories begins along the grain boundaries and can hence spread throughout the structure of the refractory at a fairly rapid rate where conditions and the environment favour it.
The Cr6+ content, following the CaO-Cr2O3 phase diagram, increases with the exposure to temperatures below 1,022 deg C and with an increase in CaO (from 0 % to 42 % CaO). In the case of magnesia chrome refractories, temperature, basicity (CaO/SiO2 ratio), and the chromite grain size, all play a role in Cr6+ formation. Hence the formation of Cr6+ can be minimized by carefully controlling the levels of CaO in the refractory and by avoiding the use of fine chromite during brick making. The use of fused magnesia-chrome or chrome magnesia grains also help minimizing the potential to form Cr6+ within the refractory structure.
Because of the likely formation of Cr6+ when exposed to alkali or CaO environments, there has been a move away from chromium containing refractories in those applications where these chemical and certain physical conditions exist. A good example of this being the cement and glass industries. This move has also taken place in other industries and applications where the formation of Cr6+ is not at all likely, but a view has been taken that chromium based refractories are environmentally damaging and can be harmful to health.
The move to replace chrome containing refractories has seen development of several other possibilities. These include magnesia alumina spinels, spinel bonded magnesia, very high alumina materials, zirconia-containing materials, and various fused-cast products.
Occupational exposure limits to hexavalent chromium range from 1 mg/cum (milligrams per cubic mere) to 0.01 mg/cum on an 8 hour TWA (time weighted average). Values vary from one country to another country. If the maximum nuisance dust level of 10 mg/cum is assumed of a material at a hexavalent chromium level of 450 ppm (parts per million) with unused chromium bearing refractories varying between 20 ppm and 200 ppm, exposure to hexavalent chromium is 0.005 mg/cum which is well below the TWA maximum. Limits for landfill disposal for Cr (VI) also varies from country to country.
One of the problems with comparing exposure limits is that environmental limits and the limits of occupational organizations are based on different extraction and analytical test methods, which all yield different results. In addition, the results from different laboratories do not always agree. The Ceramic Research Association in Britain has carried out extensive work on the development of a reliable and significant test method for the determination of hexavalent chrome.
Under certain operating conditions, however, toxic and hazardous hexavalent chromium is formed. The major industries in which this occurred have moved to alternative lining materials and the indications are that in other user industries hexavalent chromium do not present an occupational or environmental disposal hazard. In areas, where hexavalent chromium can be identified as a problem, a joint approach by the refractory producer and the refractory user, through product development or recycling of used lining materials, is the most cost effective solution.