Magnesia Refractories
Magnesia Refractories
A magnesia refractory is defined by the American Society for Testing and Materials (ASTM) as ‘a dead-burned refractory material consisting predominantly of crystalline magnesium oxide’. In addition, ASTM defines ‘dead-burned’ as “the state of a basic refractory material resulting from a heat treatment that yields a product resistant to atmospheric hydration or recombination with carbon dioxide’.
The chemical formula for magnesium oxide is MgO. However, no dead-burned magnesium oxide contains 100 % MgO. Chemical analysis of samples of such refractory raw material show some level (normally less than 30 % total) of silica, lime, iron oxide, alumina, and boron oxide, which, mineralogically, occur in triple-point pockets and films between the MgO crystallites in the dead burned magnesium oxide material for example, (i) as various calcium silicates, calcium magnesium silicates, calcium boron silicates, and calcium aluminates, (ii) as lime and iron oxide solid-solutions in the magnesia crystallites, and (iii) sometimes, as magnesio-ferrite exsolution intergrowths, within the magnesium oxide crystallites themselves.
Also, ASTM defines basic refractories as ‘refractories whose major constituent is lime, magnesia, or both, and which may react chemically with acid refractories, acid slags, or acidic fluxes at high temperatures’. In a postscript, the definition mentions that ‘commercial use of this term (basic refractories) also includes refractories made of chrome ore or combinations of chrome ore and dead-burned magnesite’. On the other hand, basic refractories show good chemical resistance to other basic refractories, basic slags, or basic fluxes at high temperatures.
Lime and magnesia also hydrolyze in water to form hydroxides, so the designation of these so-called basic refractories is truly meant to characterize their chemical behaviour.
Terminology
Magnesium oxide raw materials and products can be referred to interchangeably as one of the four names or terms and, thereby, lead to some confusion for people unfamiliar with commercial industry vernacular. The four terms are (i) MgO, (ii) magnesia, (iii) periclase, and (iv) magnesite.
Technically, MgO is the chemical formula for pure magnesium oxide. Magnesia is the chemical name applied to the oxide of magnesium. Periclase is the mineral name for magnesium oxide (this mineral is rarely found in nature, but is presently applied to high-grade with normally, less than 10 %. impurities, dead-burned magnesium oxide products produced synthetically from, for example, seawater or underground brines). Magnesite is the mineral name for magnesium carbonate (MgCO3), and was one of the original sources for magnesium oxide used in refractory products. Magnesite has to be dead-burned to remove the carbon dioxide (CO2), but the name has carried over to the dead-burned product of the magnesium carbonate. Present terminology is to use magnesite for dead-burned magnesium oxide produced from naturally occurring magnesite, especially those raw materials with impurities higher than 5 %, but its use secularly is not so differentiated.
Magnesia refractory raw materials
Normally, every refractory is composed of four major textural elements which are (i) primary building blocks, (ii) matrix, (iii) binder, and (iv) unfilled space. The primary building blocks of refractory bodies are given the name grains or aggregates. These components comprise raw materials larger than around 200 mm and constitute, by weight, around 70 % of the refractory product. Several carefully graded sizes of aggregates are used to construct a close-packed product texture. Matrix or filler materials of size smaller than 150 mm are then used to pack the spaces among the gapped aggregates. The terms binder, bond, or cement are used to describe the structural unit which eventually adheres the aggregates and / or matrix ingredients together to form the strength of the refractory product. Further, there is always unfilled space remaining in the refractory body, and these open volumes are called pores. Fig 1 shows the four textural elements of a refractory and their respective relationships.
Fig 1 Four textural elements of a refractory and their respective relationships
Raw materials of refractory products (i) can used directly, (ii) can be partially altered from naturally occurring mineral deposits, or (iii) are produced synthetically by different combinations of chemical processing and heat treatment. When the heat treatment is mild (around 900 deg C to 1,300 deg C), the raw material is described as being calcined. When the heat treatment is more robust (around 1,500 deg C to 2,200 deg C), the raw material is then described as being sintered (dead-burned materials are in this group). When the heat treatment proceeds to a molten state (e.g., for MgO, higher than 2,800 deg C), then the raw material is said to be fused.
Dead burned magnesium oxide – The principal magnesia refractory raw material is obviously magnesium oxide. Magnesium oxide has a very high melting point of around 2,800 deg C. This characteristic, together with its resistance to basic slags, ubiquitousness, and moderate cost, makes magnesium oxide products the choice for heat-intensive, metallurgical processes such as for the production of metals, cements, and glasses.
Since magnesium oxide does not occur extensively in nature, this material has to be obtained from other sources which are available in commercial quantities. The first source is from sintering naturally occurring magnesite, a mineral in which theoretical weight percent of MgO is 47.6, so around half the weight of magnesite is lost because of the carbon di-oxide evolution during sintering or dead burning.
Magnesite occurs in nature in two distinct textures namely (i) macro-crystalline, and (ii) crypto-crystalline. When high-purity, macro-crystalline magnesite is simply subjected to heat treatment, a low density, sintered product is produced which is not favourable for premium-quality refractory usage. The use of additional, more costly processes, such as fine grinding, briquetting, and modern shaft kilns for sintering, is needed to produce a product suitable for the refractories industry. However, macro-crystalline magnesite deposits occurring with minor levels of iron oxide (this magnesite variety is called breunnerite), do sinter to high density, and are quite suitable for certain refractory applications. On the other hand, the high reactivity of high-purity, fine-grained (around 1 micrometre) crypto-crystalline magnesite leads to this type of magnesite rather easily sintering to a high-density grain which is necessary for producing various refractory products.
The other very large, almost limitless, commercial source of high-purity magnesium oxide is obtained by processing seawater, inland brines, or salt deposits, all containing the soluble compound magnesium chloride (MgCl2). In these cases, the final products are referred to as ‘synthetic magnesia’. The introduction of new processing technologies (increasing the lime / silica ratio, reducing the boron content, using high-vacuum techniques in dewatering the filter cake, and using higher pressure briquetting) in the mid-1970s resulted in magnesium oxide qualities which exceeded previously available commercial, synthetic grains and led the refractories producers to employ these newer grades, and the naturally occurring and standard-quality, synthetic, magnesium oxide products largely fell out of favour.
A secondary source of magnesium oxide is from the mining and sintering of brucite deposits. This mineral is composed of magnesium hydroxide [Mg(OH)2], and has a theoretical MgO of around 70 %.
Magnesite bricks
Bricks made with dead-burned magnesite are an important category of basic refractories. Magnesite bricks are characterized by good resistance to basic slags as well as low vulnerability to attack by iron oxide and alkalis. They are widely employed in applications such as glass tank checkers, sub-hearth brick for electric arc furnaces, and sometimes as back-up linings in basic oxygen furnaces. These bricks are frequently 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.
Different 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 % maximum, to high purity synthetic magnesites containing 96 % MgO minimum. A large amount of work has been done to produce highly refractory magnesites. Since magnesia itself has an extremely high melting point (2,800 deg C), the ultimate refractoriness of a magnesite brick is frequently determined by the quantity and type of impurity within the grain. In practice, the refractoriness of a dead burned magnesite is improved by lowering the quantity of impurities, adjusting the chemistry of the impurities or both.
There are several 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, normally less than 0.02 % boron oxide, which have lime / silica ratios of two or higher. Frequently, the lime / silica ratio of these bricks is intentionally adjusted to a molar ratio of two to create a di-calcium silicate bond which gives the bricks high hot strength. Bricks with lime / silica ratios higher than two are frequently of higher purity than the di-calcium silicate bonded brick. This higher chemical purity makes them more desirable for certain applications.
The second category of magnesite brick normally has lime / silica ratios between zero and one, on a molar basis. These bricks can contain relatively high boron oxide contents (higher than 0.1 % B2O3) in order to impart good hydration resistance. Sometimes, for economic reasons, these bricks are made with lower purity natural dead burned magnesites with magnesia contents of 95 % or less. At other times, the bricks are made with very pure magnesites with MgO contents higher than 98 % for better refractoriness.
Mineral composition of magnesia refractories
The raw materials used for the production of magnesia refractories consist essentially of the basic oxide magnesium (MgO). These raw materials are (i) chemically precipitated magnesium hydroxide, ii) the mineral magnesite, which is naturally occurring magnesium carbonate, and to a minor extent (iii) the mineral brucite, naturally occurring magnesium hydroxide. Chemically precipitated magnesium hydroxide, prepared by causing slaked lime or slaked calcined dolomite to react with magnesium-bearing brine, contains small quantities of accessory minerals derived from the limestone or dolomite, and from the brine. The natural mineral magnesite contains small quantities of accessory minerals, such as dolomite, serpentine, talc, chalcedony, and quartz. Iron carbonate, in solid solution with the magnesium carbonate, is present in several magnesites.
Raw magnesia containing materials are ‘dead-burned’ in rotary kiln or shaft kiln to prepare them for use. The dead-burning consists of a high temperature heat treatment which drives off chemically combined water and / or carbon dioxide, and converts the remaining product into dense grains or lumps resistant to atmospheric moisture and carbon dioxide. In dead-burning, additions such as iron oxide, alumina, silica, or lime can be made to achieve desired compositions.
The temperature of dead-burning varies from around 1,540 deg C to 1,850 deg C, depending upon the type and purity of the product. In the process of dead-burning, magnesia hydroxide dissociates to form magnesia and water vapour, magnesium carbonate dissociates to form magnesia and carbon dioxide gas. The water vapour and the carbon dioxide gas escape with the kiln gases. The dead-burned product, which is known commercially as ‘dead burned magnesia’ consists mainly of aggregates of periclase crystals, with a fine-grained crystalline ground mass normally composed of silicates of magnesium and to a minor extent of calcium.
The colour of dead-burned magnesia low in iron oxide varies from white to buff or tan. Dead-burned magnesia which contains several percent of iron oxide is normally chocolate-brown in colour, and its microscopic examination shows that the periclase crystals normally contain dark moss-like inclusions. These have been identified as magnesio-ferrite (MgO.Fe2O3).
The most desirable quantities of the accessory constituents, and their relative proportions, depend upon the purposes for which the product is to be used. Dead-burned magnesia used in furnace hearths in general has a higher content of accessory constituents than that used for making bricks. In recent years, ultra high purity dead-burned magnesia containing 99 % have gained wide spread use.
The exact mineral composition of magnesia grains cannot be calculated accurately from the chemical composition, since adequate data regarding the six component system MgO-CaO-Al2O3-FeO-Fe2O3-SiO2 are not available, and since equilibrium conditions are not entirely reached in the brief period of exposure to high temperatures during dead-burning. However, the major components can be identified by painstaking study with X-ray diffraction.
The magnesia-lime-silica system – Consideration of the three component system MgO-CaO-SiO2 serves as a good starting point in studying the mineral composition of magnesia refractories. The composition triangles in this system are shown in Fig 2, which is explained here. Any point within the large equilateral triangle of Fig 2 represents a definite composition, consisting solely of MgO, CaO, and SiO2. For example, the point ‘A’ represents a composition of 25.6 % MgO, 35.9 % CaO, and 38.5 % SiO2, which corresponds to the composition of the mineral monticellite.
Fig 2 Magnesia-lime-silica system
The large equilateral triangle is divided by heavy lines into triangles of unequal size. These are known as ‘composition triangles’. Each of the three apexes of any composition triangle represents the chemical composition of a specific mineral. A line connecting any two apexes represents all compositions in which the two minerals indicated can coexist in equilibrium. The area within a triangle represents all compositions in which the three minerals indicated by the apexes can coexist in equilibrium.
The particular minerals to be found in any stable blend of magnesia, lime, and silica can be predicted from the triangle within which the composition lies. Under equilibrium conditions, the body consists of the three minerals represented by the apexes of the triangle, in relative amounts fixed by the distances from the point representing the composition, to the individual apexes.
The part of the MgO-CaO-SiO2 system, in which the magnesia is present in great excess and in which hence periclase (MgO) is invariably a component, is of particular interest in connection with the magnesia refractories. In Fig 2 the composition triangles of this part of the system appear as full lines. The magnesium and calcium silicates which can coexist in equilibrium with periclase are indicated. For any given composition, the particular silicates which can be present, and their relative proportions, are fixed by the lime / silica ratio. The part of the MgO-CaO-SiO2 system in which magnesia (MgO) is present in great excess, and in which the lime / silica ratio is less that 1.86 by weight (less than 2 on a molecular basis), either one or two magnesium-bearing silicate minerals are invariable be present at room temperatures as shown in Tab 1. If the lime / silica ratio of the high magnesia compositions is 1.86 or higher, no magnesium-bearing silicates are present.
| Tab 1 Minerals in MgO.CaO.SiO2 system which co-exist in equilibrium with periclase | ||||
| Case | Molecules of CaO to 1 molecule of SiO2 | Parts by weight of CaO to 1 part of SiO2 | Compatible components | |
| Minerals | Compositions | |||
| 1 | Under 1, (Less CaO than SiO2) | Less than 0.93 parts | Forsterite, Monticellite | 2MgO.SiO2, CaO.MgO.SiO2 |
| 2 | 1 | 0.93 parts | Monticellite | CaO.MgO.SiO2 |
| 3 | 1 – 1.5, (More CaO than SiO2) | 0.93 to 1.4 parts | Monticellite, Merwinite | CaO.MgO.SiO2, 3CaO.MgO.2SiO2 |
| 4 | 1.5 | 1.4 parts | Merwinite | 3CaO.MgO.2SiO2 |
| 5 | 1.5 to 2 | 1.4 to 1.86 parts | Merwinite, Di-calcium silicate | 3CaO.MgO.2SiO2, (2CaO.SiO2) |
| 6 | 2 | 1.86 | Di-calcium silicate | 2CaO.SiO2 |
| 7 | 2-3 | 1.86-2.8 parts | Di-calcium silicate, Tri-calcium silicate | 2CaO.SiO2, 3CaO.SiO2 |
| 8 | 3 | 2.8 parts | Tri-calcium silicate | 3CaO.SiO2 |
| 9 | Over 3 | Over 2.8 parts | Tri-calcium silicate, Lime | 3CaO.SiO2, CaO |
The lime / silica ratio has an important bearing upon the melting behaviour of magnesia-lime-silica compositions. Any combination of magnesia, lime, and silica alone, in which periclase is a component, and in which the quantity of lime by weight is less than 1.86 times the quantity of silica, develops a liquid phase at 1,575 deg C, and can develop liquid at a temperature as low as 1,490 deg C. If completely homogenous mixtures can be achieved, no combination of these oxides, in which the quantity of lime is equal to or more than 1.86 times the quantity of silica, forms liquid below 1,790 deg C. However, lower lime / silica ratios, in areas of even microscopic size, permits small quantities of liquid to form at temperatures lower than 1,790 deg C, and causes some coalescing together of particles, even though diffusion at higher temperatures causes the liquid to disappear.
Accessory minerals in magnesia refractories – Incomplete data are available concerning the complex six component system CaO-MgO-SiO2-Al2O3-FeO-Fe2O3, the system applicable to refractories which consist essentially of magnesia. However, the more important relationships are fairly well understood. In such mineral compositions, consisting mainly of periclase, if the quantity of lime is less than 0.93 times the quantity of silica, the minerals forsterite and monticellite form at high temperatures in the groundmass. If the quantity of lime is between 0.93 and 1.40 times the quantity of silica, monticellite and merwinite form, and if it is between 1.40 and 1.86 times the quantity of silica, merwinite and di-calcium silicate form.
In all these compositions, any ferrous oxide present dissolves in the magnesia, and alumina and ferric oxide combine with the magnesia, forming the spinel minerals MgO.Al2O3 and MgO.Fe2O3. If the quantity of lime equals or exceeds 1.86 times the quantity of silica, in the six-component system, the mineral relationships are much more complex, and the lime / silica ratio does not suffice to determine what minerals are present. The lime reacts with silica to form di-calcium silicate or tri-calcium silicate, and with alumina and ferric oxide to form calcium aluminate and calcium ferrite.
Several attempts have been made to formulate rules whereby the mineral compositions at room temperatures can be calculated from the chemical analyses, but frequently the calculated compositions are not in satisfactory agreement with the results of microscopic and X-ray determinations. Forsterite is a common bonding constituent of magnesia refractories of low lime content. It is highly refractory and has no undesirable mineral inversions. It is little affected by the presence of moderate quantities of ferric oxide, but reacts with lime to form the minerals monticellite and merwinite.
Monticellite and merwinite melt at relatively low temperatures, and in minor quantities can have some value in the development of the ceramic bond of magnesia bricks. Dead-burned magnesia compositions having a lime / silica ratio favourable to the development of an undesirable quantity of monticellite or merwinite can be corrected by the addition of sufficient lime so that there is 1.86 times as much lime as silica, by weight. When this is done, magnesia is displaced from the silicates, and the lime and silica combine to form the compound di-calcium silicate.
Di-calcium silicate is highly refractory. It does not react with magnesia (MgO), and it is stable in the presence of moderate quantities of ferric oxide. At one time, it has been regarded as an undesirable component of basic refractories, because of its tendency to turn completely into dust on cooling. This affect results from a mineral inversion at around 725 deg C which is accompanied by an abrupt volume increase of 10 %. However, means have been found to prevent the dusting of di-calcium silicate by adding small quantities of stabilizing minerals to the raw magnesia or magnesium hydroxide, before dead burning. These added materials inhibit the inversion of the silicate on cooling.
The lime compounds tri-calcium silicate and di-calcium ferrite (2CaO.Fe2O3), which can occur in magnesia refractories having high lime / silica ratio, are compatible with di-calcium silicate but do not coexist in equilibrium with monticellite, merwinite, or forsterite. Tri-calcium silicate is unstable below around 1,249 deg C and tends to dissociate into di-calcium silicate and free lime. Di-calcium ferrite is unstable above 1,436 deg C, and decomposes into free lime and a liquid high in iron oxide. It has been suggested that when this occurs the iron oxide can leave the liquid and combine with magnesia to form the mineral magnesio-ferrite (MgO.Fe2O3); and that, if the material is cooled rapidly, the di-calcium ferrite cannot re-form. Under these conditions, free lime is present in the dead-burned magnesite after cooling.
In basic refractories, the state of oxidation of the iron changes with the temperature, and with changes in the oxygen pressure of the furnace atmosphere. When heated in air, free ferric oxide loses oxygen at 1368 deg C and changes to the spinel mineral magnetite (FeO.Fe2O3). If the furnace atmosphere is strongly reducing, iron oxide is in the form of FeO and if the furnace atmosphere is strongly oxidizing, it is in the form of ferric oxide (Fe2O3).
An important property of magnesium oxide is its ability to absorb large quantities of iron oxide (either ferrous or ferric) without undue decrease in refractoriness. This property of magnesia, together with its very high melting temperature, is responsible, in large measure, for its value as a refractory. It accounts for the fact that refractories high in magnesia can be used advantageously in the presence of iron oxides, in either an oxidizing or a reducing atmosphere, and also, for some of the particular advantages of steel-encased basic brick.
The equilibrium diagram for the MgO.FeO system is shown in Fig 3, if the incongruent melting of FeO is neglected. Magnesium oxide and ferrous oxide form a continuous series of solid solutions which decrease in refractoriness with increasing proportions of ferrous oxide. However, even with more than 50 % FeO, the refractoriness is still high.
Fig 3 Equilibrium diagram of FeO.MgO system
Magnesium oxide reacts with ferric oxide to form magnesio-ferrite, which begins to melt only at 1,713 deg C, in spite of the fact that it contains 80 % Fe2O3. With less than 70 % Fe2O3, the temperature of incipient melting is even higher than 1,713 deg C. At ordinary temperatures, iron oxide in dead burned magnesia or in magnesia brick can be present within the periclase grains as MgO.FeO solid solution, or as inclusions of magnesio-ferrite particles. If the lime / silica ratio exceeds 1.86, iron oxide can be present also in the groundmass in combination with lime and alumina as brownmillerite (4CaO.Al2O3.Fe2O3), in combination with lime as di-calcium ferrite, or in solid solution with silicates.
While magnesio-ferrite is completely soluble in periclase at high temperatures, it separates out upon cooling, unless cooled very rapidly. This accounts for the presence of the dark coloured inclusions in periclase crystals frequently observed in microscopic examination.
Very dense grains of magnesia frequently are greenish in their interiors, which have been cooled out of contact with air, and chocolate-brown at their surfaces, which have been exposed to air during cooling. In the greenish centres the periclase crystals probably contain ferrous oxide in solid solution. In the brown surface portions the periclase crystals contain inclusions of magnesio-ferrite particles. Porous grains are less likely to have green centres than dense grains. Dead-burned magnesia sometimes contains somewhat more free lime than what is anticipated from their lime / silica ratio. The free lime has no disadvantage from the standpoint of melting, but it is very active chemically and slakes upon exposure to the air.
Production of dead burned magnesia
Exploration, drilling, assaying, and selective mining, either open-pit or underground, are the first four important steps for naturally occurring magnesite or brucite deposits. To produce high-grade grains, moderate grinding followed by beneficiation techniques using froth flotation or heavy media is then needed to remove gangue minerals before dead-burning in either rotary or shaft kilns. If the gangue minerals cannot be removed sufficiently to produce high-grade magnesia, then fine grinding is to be carried out before beneficiation. As a result, high pressure briquetting is needed for this fine-grained output in order to form a suitable pellet for dead-burning. Low-grade products can be produced by just coarse crushing the mined ore followed by dead-burning in crude shaft or rotary kilns. Products briquetted prior to dead-burning are of peach-pit–sized materials which when dead burned are normally less than 35 mm in size after firing.
When seawater is used as the source, the water is initially pretreated to remove the carbonic acid. The inland brines do not need this step. Then, these waters are mixed with calcined limestone or calcined dolomite in large reaction tanks. The use of calcined dolomite almost doubles the overall percentage of magnesium recovered per unit of processed water. Magnesium hydroxide precipitates out, and the resultant slurry is washed, thickened, and dewatered using very high-vacuum drum filters. The resulting filter cake can be fed directly to a rotary kiln and dead-burned to produce a standard-quality, dead-burned magnesia, but, more likely, the filter cake is calcined in multiple-hearth furnaces to produce highly active magnesia. Following high-pressure briquetting, the pellets are dead-burned in rotary kiln or shaft kiln, with the latter used for the premium magnesia products.
Synthetic dead-burned magnesia is always briquetted and hence is available as peach-pit–sized particles or portions thereof. The ultimate refractory-grade magnesia is produced by electro-melting previously produced refractory grades of magnesia, calcined magnesia, or even raw, naturally occurring magnesite. The resulting furnace charge or ingot is allowed to cool and crystallize slowly, resulting in the large periclase crystal sizes which are highly sought after for maximum slag resistance. The inner core produces the best product, while the crust is normally recycled due to its small crystallite size. The well-crystallized material is then crushed and sized into a variety of fractions for subsequent refractory-grade use.
Key characteristics of dead burned magnesium oxide
The key characteristics of dead burned magnesium oxide are described below.
MgO content– The MgO content of dead-burned magnesium oxide is normally included in the grade and / or brand of the particular product. Obviously, its overall purity plays an important role in determining the quantity of MgO content which is suitable for a particular end use. The MgO content of an aggregate is, by and large, but not entirely, directly proportional to its slag resistance. Several impurities, as mentioned previously, are located in triple-point and thin-film accessory mineral deposits between MgO crystallites composing the aggregates or grains.
The accessory minerals have lower melting temperatures than MgO. Hence, the quantity of impurities plays a major role in keeping the MgO crystallites apart in the aggregate and not available for high-strength, crystallite-to-crystallite direct bonding during the production sintering process. In metallurgical applications at high temperatures, grain-boundary softening leads to loss of MgO aggregate strength and affords critical pathways for slag attack, whereby corrosive agents breach around the excellent resistance of the MgO crystallites, As a consequence, the rationale for using an MgO refractory body can become meaningless when moderate to large quantities of such impurity phases are present. Since the mid-1980s, the trend has been to achieve the highest MgO content possible in the MgO grains.
MgO Impurities – A chemical analysis of a sample of the MgO aggregate yields the principal impurities namely silica, lime, alumina, iron oxide, and boric oxide. These impurities do not exist in the MgO aggregates as independent oxides as such. Rather, they combine together and / or with MgO from the MgO crystallites to form minerals. These minerals, under equilibrium conditions, can be predicted from phase equilibrium relationships in the MgO-CaO-SiO2-Al2O3-FeO-Fe2O3 system and normally confirmed by X-ray diffraction analyses. These minerals can be distributed in one or more of several locations namely (i) in triple points or as films along crystallite boundaries in the MgO aggregates, (ii) as solid solutions in the magnesia crystallites, and (iii) as spinellitic exsolutions in the magnesia crystallites.
However, complicating these general points are the phenomena of the solid solutions or solubilities of lime and / or iron oxide, respectively, in the MgO crystallites themselves. Although the lime solubility in the MgO phase is relatively small, the effect is particularly important in very high MgO-content materials. This fact is due to the impact that lime solubility has on altering the lime / silica ratio determined by chemical analysis and subsequent phase equilibrium assumptions. This ratio, in turn, controls the nature of the calcium silicate minerals which occur in the grains.
The importance of the lime / silica ratio can be appreciated from studying the MgO-CaO-SiO2 phase diagram (Fig 4). As mentioned earlier, the lime / silica ratio controls which minerals exist in the magnesia aggregates. If the lime / silica ratio is higher than 2.8, tri-calcium silicate, and free lime can exist, since the impurity or accessory phases with initial liquid formation in the magnesia aggregate occurs at around 1,850 deg C. An exact 2.8 ratio can yield only tri-calcium silicate. If the lime / silica ratio is between 2.8 and 1.87, tri-calcium silicate and di-calcium silicate can be present, while the temperature of initial liquid formation is around 1,790 deg C. At just lime / silica ratio of 1.87, only di-calcium silicate can exist. From lime / silica ratio of 1.87 to 1.4, di-calcium silicate and merwinite can be represented and an initial liquid can appear in the magnesia aggregate at 1,575 deg C. At just lime / silica ratio of 1.4, only merwinite can be present. A lime / silica ratio of 1.4 to 0.93 can produce a combination of merwinite and monticellite, with this accessory-phase mineral assemblage with MgO, liquid can begin to be generated if the temperature reaches around 1,490 deg C. Only monticellite is present at lime / silica ratio value of 0.93. Finally, a lime / silica ratio of less than 0.93 can yield the accessory-phase minerals monticellite and forsterite, and a temperature of around 1,500 deg C can begin to initiate melting in the MgO grain.
Fig 4 MgO-CaO-SiO2 phase diagram
Liquid formation normally signals initial signs of destruction and, consequently, deterioration of the refractoriness of the MgO grains, so the value of the lime /silica ratio plays a considerable role in this feature. Obviously, the quantity of available lime is a critical factor. Since lime can be dissolved in solid solution in the MgO crystallites, this circumstance complicates the overall phase assemblage. Moreover, the quantity of lime in solid solution in the MgO crystallites, in the presence of silicates, increases with the increasing lime content of the silicate; i.e., as the lime / silica ratio increases from 1.4 to over 3, the CaO in solid solution in MgO increases from 0.2 % CaO to around 2 % CaO. So, the solid solution of CaO in MgO causes the lime /silica ratio in the calcium silicates in the interstices of the MgO grains to decrease, and the refractoriness of the MgO aggregates is reduced. In addition, as the overall silica content decreases, this effect increases the impact on the lime / silica ratio, so lime solubility in MgO can cause a very considerable lowering of the lime / silica ratio and, in very pure MgO aggregates, a dramatic lowering of the temperature of initial melting. This lowering is much higher than anticipated if the lime solubility in MgO is ignored.
Iron oxide is also capable of dissolving in solid solution in the MgO crystallites. Its phase equilibrium relationship depends on the oxygen partial pressure of the system, but formation of magnesio-wustite and magnesio-ferrite solid solutions normally occur in MgO crystallites even with low levels of iron oxide being present. At higher iron oxide levels and with the lime /silica ratio less than around 2, the iron spinel reacts with the calcium-magnesium silicate accessory phases to further contribute to the quantity of liquid which can be formed at a particular temperature. Under these conditions, alumina also reacts with MgO to form spinel, which then also adds to the liquid formation.
Even when the lime / silica ratio is higher than 2, these impurities cause a loss of refractoriness of the MgO grains. Lime reacts with alumina and / or iron oxide to form calcium aluminates, calcium ferrites, or calcium-iron aluminates. For these reasons, these impurities need to be kept low.
Boric oxide is also a very undesirable impurity. Different studies in the 1960s and 1970s found that one of the major reasons for the good hot strength of dead-burned magnesite, which had such a dramatic effect on the increased life of pitch-impregnated, burned MgO brick produced from this material and used in the impact pads of basic oxygen furnaces in the mid-1960s and early 1970s, was its inherent low boron content (less than 0.005 % as B2O3). Subsequently, control of the boron content of synthetic magnesia aggregate to less than 0.02 % as B2O3 (ideally, less than 0.01 %) was critical to producing a high lime / silica ratio product which produced a burned MgO brick with hot properties similar to brick made with dead burned magnesite.
Further studies found that, apparently, boron combines with other impurities, such as CaO, to form very low temperatures of liquid formation, and the liquid formed possesses a very low wetting angle with MgO, resulting in a very thin, interstitial film around most of the MgO crystallites in commercial MgO grains, which prevents direct-bonded, MgO crystallite to MgO crystallite microstructure. However, there is one aspect where the boron-compound film around the magnesia crystallites comes in handy. Magnesia’s hydration resistance increases with the presence of that boron-compound film around the magnesia crystallites, so, where moisture is present in an application, bricks composed with these particular raw materials fit a special need.
MgO crystallite size – Since chemical reactions, such as in refractory corrosion, are a function of available surface area, one has to assume that the MgO crystallite size is important. Indeed, studies and results from refractory service over the last 15 years to 20 years corroborate this characteristic. Prior average crystallite size of dead-burned magnesium oxide ranged from 25 mm to 100 mm. Now synthetic, dead-burned magnesium oxide products are available with crystallite size ranging from 100 mm to 200 mm. Moreover, when the best corrosion resistance is needed, the industry trend is to use refractories containing at least some fused MgO whose crystallite size can be measured in the several mm range and individual MgO grains can be composed of fragments of single MgO crystals.
Bulk density of MgO grains – The true specific gravity of periclase is 3.58. Hence, the closer the bulk density of a sample of an MgO aggregate is to that value, the lower is its total porosity (open and closed pores). In refractory technology, normally, the densest body offers the best resistance to corrosion by slags and is the strongest to resist abrasion. Desired commercial MgO grains need to have bulk densities higher than 3,400 kg/cum.
Thermal expansion of MgO – The coefficient of thermal expansion of an essentially pure MgO refractory is very high. As an example, at 1,425 deg C, the linear expansion of a fused MgO or isostatically pressed and fired MgO of 99 % minimum purity with a minimum bulk density of 3,180 kg/cum is around 2 %. Of course, incorporating other refractory raw materials with magnesia in a refractory body alter this value. Care is needed in using these materials to account for this property in an engineering sense.
Production of magnesia refractories
The overall production process of magnesia refractory is just like that of baking a cake. The refractory technologists consider a particular refractory application for which the basic requirement is to have a long, lasting refractory life at an economical cost. The technologists contemplate the overall environmental and wearing mechanisms to which the refractory is going to be exposed including temperatures, gas pressures, chemically corrosive agents and / or mechanical stresses. Based on their experience and theory, they prepare the recipe for this refractory end use from the list of raw materials. In addition, throughout the entire production process, each step is to be carried out in accordance with total quality-control principles.
Several of the bulk raw materials, such as dead-burned magnesia, dead-burned dolomite are sized for the particular recipe to be produced. For a densely packed refractory, several graded, aggregate sizes are needed to achieve the density. The concept is based on the theory that each distinct particle-size class is capable of packing to 60 % of theoretical density, and that, if the particle-size ratios are high enough between each particle-size class, each distinct particle class fill 60 % of the remaining porosity, and so forth. One aggregate size combination, for example, can be -9.5 mm + 6.7 mm, -6.7 mm +3.4 mm, -3.4 mm +1.2 mm, -1.2 mm + 300 micrometers, -300 micrometers and finer, and -75 micrometers and finer.
Majority of modern, fully integrated refractory producers have a portion of their plant designed for such crushing, grinding, and screening. The coarser sizings can be achieved by crushing the as-received raw-material briquettes or fragments in coarse crushers, such as jaw crushers, gyratory and cone crushers. Examples of intermediate pulverizers are cage disintegrators and hammer mills. The fine grinding mills are rod mills and ball mills.
Screening or sieving these fractions is accomplished by passing each fraction over single or multi-decked, stationary screens or dynamic screens, such as inclined or horizontal vibrating screens, oscillating sieves, or reciprocating units. All product fractions are to be sieve-tested for conformity with internal specifications such as the specific sizing and its screening efficiency. Final products for each raw-material-size fraction are conveyed to individual holding bins or hoppers in the batching area.
The batching area is normally characterized by a batch weigh-hopper on wheels or tracks capable of traversing under rows of multiple hoppers or bins, each holding a specific size fraction of a specific raw material. Some batching systems are automated, while others are manual. In either case, the batch-card weights for each ingredient of a mix recipe are formulated based on the sieve analyses for the specific raw-material fractions being held in the hoppers or bins. If a pitch binder or similar is to be used in the mixing, the coarser fractions or the total batch have to be heated prior to discharging into the mixer. The preheating of the total batch needs a salt-bath-type heater in order to prevent the fines from being elutriated during heating.
The batch is then discharged into an appropriate mixer. A number of mixers are used. The most common type is a Muller mixer, a mixing bowl fitted with one or two heavy wheels which rotate while the mixer bowl revolves or the rotating wheels revolve while the mixer bowl is stationary. This equipment produces a kneading action to the batch materials in which that portion of the batch proceeding directly under the wheels is squeezed and pressed producing a ‘de-airing’ action together with intimate mixing of those ingredients.
In general, initially the binder is added to coat the coarse grains or aggregates of the mix, and then the recipe fines are added to be picked up in the sticky binder around these coarse grains. Studies have to be conducted for each recipe to optimize the mixer type-time requirements-mix temperature in order to yield a properly mixed batch with well-distributed components so that each unit of the mix has the same, randomly arranged percentage of each ingredient as the next.
At this point, the mix batch is conveyed to the press feed-hopper if a brick or a formed piece is to be made. If a monolithic is being produced, the mix batch goes to a holding hopper or bin over a bagging machine. After filling, the bags are palletized and shrink-wrapped before dispatch to the customers. Also, samples of the final monolithic product are taken for monitoring various quality characteristics such as chemical properties as well as grain sizing and other pertinent physical characteristics.
A formed or pressed refractory piece can be produced using a number of different processes. Special, large, complex pieces can be formed by hand moulding using an air hammer. Simple geometric shapes, such as straights, keys, wedges, arches, and combinations of these (i.e., key wedges, key arches, or key-arch wedges), are formed by using toggle presses, friction presses, or hydraulic presses. Typically, the highest possible forming pressures are used in order to achieve brick with the highest bulk densities to have maximum particle packing. Typical refractory recipes need forming pressures of 50 MPa to 100 MPa. Also, multiple de-airing cycles are needed to produce a lamination free product.
Process quality parameters monitored here are brick dimensions and weight and, consequently, brick bulk density. Statistical process control techniques are particularly easy to use at this point of the process.
The type of binder used in the mix recipe determines how a brick is handled following pressing. Majority of the bricks are placed or slid onto flat, individual trays which can be then placed onto steel racks, while other bricks can be stacked several bricks high on drying cars.
Following off-bearing from the press, inorganic-bonded bricks designed to be sold as chemically bonded or designated for subsequent firing to produce a burned brick are dried at low temperatures (around 150 deg C) in order to develop the green strength of the temporary binder-magnesia component reaction. Resin-bonded brick are processed to temperatures in the range of around 135 deg C to cure the resin bonds. Pitch-bonded brick have to be heated to around 290 deg C to develop appropriate green strength, primarily in the brick’s outer areas, in order to be handled during dispatch and installation without breaking. Heating in batch or tunnel ovens can be used for any of these bricks. After cooling, some bricks have completed their manufacturing cycle. Bricks for burning are appropriately stacked on kiln cars and sent to the kilns.
Burning or firing can be carried out in batch-type or tunnel kilns which are gas fired or oil fired. In some cases, oxygen partial pressures in the heat-up stages are controlled. Temperatures achieved during this process can range from 1,500 deg C to 1,750 deg C depending on the type of brick being produced. Overall heat up, at-temperature, and cool-down kiln schedules take two to three days. Following cool-down, the bricks are unloaded and wait final processing.
Some examples of post-treatments are (i) burned magnesia brick can be subsequently pitch-impregnated prior to use, or (ii) brick for cement kilns can have cardboard spacers glued on one or two brick faces. The last processing step for all shaped refractories involves careful inspection for product defects and compliance with proper workmanship standards and randomly selecting finished bricks for further testing, such as dimensions, weight, and other physical and chemical properties, according to a predetermined quality sampling plan, palletizing, and shrink-wrapping before dispatch. All pallets carry a date-made code. The palletized brick can be stored in cool, dry, clean storages before such dispatch.
Classifications of magnesia refractories
Refractory forms – Two forms of refractories products are universally recognized. The first is formed or pre-shaped refractories, which includes bricks of numerous shapes such as keys, wedges, and arches. The second is monolithics, which includes mortars and cements, plastics, ramming mixes, castables, and gunning mixes. Different classifications exist for these products.
Standard industrial classifications – There are several national standards and industrial standards which classify magnesia refractories. These standards describe classes distinguished by obvious differences in magnesium oxide (MgO) content. The International Organization for Standardization (ISO) standard, ‘ISO 10081-1 : 1991, Basic Refractory Products—Classification, Part 1: Products Containing Less Than 7% Residual Carbon’, not only classifies by refractory type, but also by MgO content or principal oxides content.
Application of magnesia refractories
Magnesia refractories are used in a multitude of heat processing industries. These refractories are widely used in ferrous, non-ferrous, and cement industries. They have been extensively used in the steel converter, electric arc furnace and ladle lining in steel-making processes. Within the steel industry, the types and quantities of magnesia refractories have dramatically changed over the years and continue to evolve as the steel-making process technology, which needs magnesia refractories, matures. In some cases, 99 % MgO, premium-quality, sintered magnesia is used, but, where extreme conditions exist, various quantities and qualities of fused-grain magnesia replace some or all of the premium-quality, sintered magnesia. Magnesia gunning materials are used extensively to repair and maintain magnesia brick linings.
A very important and highly specialized use of burned high-magnesia shapes is for small to medium-sized slide-gate plates for steel ladles with bore size less than 50 mm. The thermal expansion of magnesia is too high for large plates or bores, causing the latter to crack in service. In addition, burned high magnesia, tundish slide-gate plates and porous upper nozzles are fabricated to control the flow of manganese treated or calcium treated liquid steel into continuous casting machine moulds. Practically all consumable tundish linings are made from sprayable magnesia mixes. Where these magnesia spray materials are not used, dry, vibratable magnesia is employed. Forsterite can be part of these mix recipes.
The use of burned magnesia-spinel brick in the sintering or burning zone and adjacent transitional areas of cement kilns represents the biggest application of magnesia refractories in the non steel-making or so-called industrial industries. Around 30 years ago, burned magnesia-spinel (initially, in situ spinel and, later, sintered or fused spinel) brick appeared, and this trend was accelerated after 1986 when material safety data sheets highlighted the environmental problems of the various magnesia-chrome brick.
Another major area for magnesia refractories usage in industrial processes is glass tank regenerators. For soda-lime glasses, chemical-bonded high magnesia bricks (95 % to 98 % MgO) with a low lime / silica ratio have been used successfully in the upper checkers for several years. Today, in addition% MgO) with a high lime / silica ratio can be used in these areas, or, if high temperatures and batch carryover exist, burned creep-resistant magnesia-zircon bricks have been introduced recently and have yielded substantially better results than the former magnesia types.
In the intermediate checkers where operating temperatures exist above the temperature of alkali condensation, chemical-bonded or burned high-magnesia bricks (90 % to 94 % MgO) have been used in the past, whereas today conventional burned high magnesia bricks (95 % to 97 %) with a low lime / silica ratio are recommended. In the lower checkers, where alkali condensation does take place, burned and unburned forsterite brick types have been used in the past. The present concept is to use several burned high-magnesia brick types (95 % to 96 % MgO) with either low or high lime / silica ratios in order to have chrome-free regenerator packages, but, if there is a serious condensate-zone problem, then a burned condensate-resistant magnesia-zircon brick is a better choice for that area as well. For the regenerator top walls, target walls, and crown, today the emphasis is on chrome-free brick, so burned high-magnesia (96 %) with a high lime / silica ratio is being recommended with burned creep-resistant magnesia-zircon brick for severe carryover with high temperatures.
As seen from the many applications described above, a wide range of magnesia refractory products are available with a spectrum of chemical and physical properties.
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