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” (1). Furthermore, 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” (2). The chemical formula for magnesium oxide is MgO. However, no dead-burned magnesium oxide contains 100 wt. % MgO. Chemical assays of any such refractory raw material show some level (generally less than 30 wt. % total) of silica, lime, iron oxide, alumina, and boron oxide, that, mineralogically, occur (1) in triple-point pockets and films between the MgO crystallites in the dead-burned magnesium oxide material as, for example, various calcium silicates, calcium magnesium silicates, calcium boron silicates, and calcium aluminates;
(2) as lime and iron oxide solid-solutions in the magnesia crystallites; and (3) sometimes, as magnesioferrite exsolution intergrowths, within the magnesium oxide crystallites themselves.
Also, ASTM defines basic refractories as “refractories whose major con-stituent is lime, magnesia, or both, and which may react chemically with acid refractories, acid slags, or acidic fluxes at high temperatures” (3). (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.”) Conversely, basic refractories exhibit excellent chemi-cal resistance to other basic refractories, basic slags, or basic fluxes at high tem-peratures. Lime and magnesia also hydrolyze in water to form hydroxides, so the 109
designation of these so-called basic refractories is truly meant to characterize their chemical behavior.
B. Terminology
Magnesium oxide raw materials and products can be referred to interchangeably as one of four names or terms (4) and, thereby, lead to some confusion for people unfamiliar with commercial industry vernacular: (1) MgO; (2) magnesia; (3) periclase; and (4) magnesite. Technically, (1) MgO is the chemical formula for pure magnesium oxide; (2) magnesia is the chemical name applied to the oxide of magnesium; (3) periclase is the mineral name for magnesium oxide (this mineral is rarely found in nature, but is presently applied to high-grade [gen-erally, less than 10 wt. %. impurities], dead-burned magnesium oxide products produced synthetically from, for example, seawater or underground brines);
and (4) 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, but the name has carried over to the dead-burned product of the magnesium carbonate—current U.S. terminology is to use magnesite for dead-burned magnesium oxide produced from naturally occurring magnesite, especially those raw materials with impuri-ties greater than 5 wt. %, but its use secularly is not so differentiated).
II. MAGNESIA REFRACTORY RAW MATERIALS
A. General Concepts and Terminology
Generally, every refractory is composed of four major structural elements that are depicted in Figure 1 (5):
1. The primary building blocks of refractory bodies are given the name grainsor aggregates; these components comprise raw materials larger than about 200 mm and constitute, by weight, about 70% of a refractory product. Several carefully graded sizes of aggregates are used to con-struct a close-packed product texture.
2. Matrixor filler materials smaller than 150 mm are then used to pack the spaces among the gapped aggregates.
3. The terms binder, bond, or cement are used to describe the structural unit that eventually adheres the aggregates and/or matrix ingredients together to form the refractory product’s strength.
4. There is always unfilled space remaining in the refractory body, and these open volumes are called pores.
Some raw materials of refractory products can used directly, can be partially altered from naturally occurring mineral deposits, or are produced
synthetically by various combinations of chemical processing and heat treatment.
If the heat treatment is mild (900– 13008C), the raw material is described as being calcined; if the heat treatment is more robust (1500 –22008C), the raw material is then described as being sintered (dead-burned materials are in this group); and if the heat treatment proceeds to a molten state (e.g., for MgO, in excess of 28008C), then the raw material is said to be fused.
B. Dead-Burned Magnesium Oxide (6, 7) 1. Sources of Magnesium Oxide
The principal magnesia refractory raw material is obviously magnesium oxide.
Magnesium oxide has a very high melting point of about 28008C. This characteri-stic, 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 that are available in commercial quantities.
The first source is from sintering naturally occurring magnesite, a mineral whose world reserves exceed 1010mt; the theoretical wt. % of MgO is 47.6, so about half the weight of magnesite is lost due to CO2evolution during sintering or dead-burning.
Magnesite occurs in nature in two distinct textures: macrocrystalline and cryptocrystalline. When high-purity, macrocrystalline magnesite, as found in Figure 1 Schema of the four textural elements of a refractory and their respective relationships. [From Ref. 5. Reprinted with permission of the Canadian Ceramics Society (M. Rigaud).]
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China, North Korea, and Russia, is simply subjected to heat treatment, a low-density, sintered product is produced and is not favorable for premium-quality refractory usage; the use of additional, more costly processes, such as fine grind-ing, briquettgrind-ing, and modern shaft kilns for sintergrind-ing, is required to produce a pro-duct suitable for the refractories industry. However, macrocrystalline magnesite deposits occurring with minor levels of iron oxide (this magnesite variety is called breunnerite) exist in Austria and Slovakia, 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 (1 mm), cryptocrystalline magnesite leads to this type of magnesite rather easily sintering to a high-density grain that is necessary for producing various refractory pro-ducts. Greece and Turkey were major sources for high-performance, refrac-tory-grade, dead-burned magnesia in the 1960s and 1970s; in the late 1980s, a cryptocrystalline source, claimed to be the largest single deposit of this type in the world, was found in Australia.
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); these final products are referred to as “synthetic magnesia.” The introduction of new proces-sing technologies (increaproces-sing the CaO/SiO2wt. 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 that 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 favor. The largest seawater facilities of this type are found in Japan, while other important plants are located in Great Britain, the United States, and Ireland.
Inland brine operations are found in the United States, Mexico, and Israel, while underground magnesium chloride salt deposits over 1000 m below ground level are recovered in The Netherlands.
Finally, 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 about 70 wt. %. A major source of brucite was located in Nevada.
2. Production of Dead-Burned Magnesia
This section only outlines the generalities of the manufacturing processes since there are quite diverse procedures employed for each of the manifold occurrences described in the previous section.
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 required to remove gangue minerals before dead-burning in either rotary or shaft kilns. If the gangue minerals can’t be removed sufficiently to produce high-grade magnesia, then fine grinding must be performed before beneficiation; consequently, high-pressure briquetting is required for this fine-grained output in order to form a suit-able pellet for dead-burning. Low-grade products can be manufactured by just coarse crushing the mined ore followed by dead-burning in crude shaft or rotary kilns. Products briquetted prior to dead-burning are delivered to major refractory maufacturers as peach-pit – sized raw materials; materials that are just dead-burned are generally 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; inland brines don’t require this step (M. Wajer, per-sonal communication, 2002). Then, these waters are mixed with calcined lime-stone 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 a highly active magnesia.
Following high-pressure briquetting, the pellets are dead-burned in rotary or shaft kilns, with the latter used for the premium magnesia products.
In The Netherlands, solution mining of underground magnesium salt deposits produces magnesium chloride brine that then goes through the process just described to produce dead-burned magnesia (8). In Israel, Dead Sea water, after solar evaporation in shallow ponds to increase the brine concentration, is initially processed through a special ion exchanger to remove boron, and then the brine is thermally decomposed in the Aman spray roaster to MgO and hydrochloric acid, the latter a byproduct recovered for fertilizer. The MgO is then hydrated to mag-nesium hydroxide, washed, vacuum drum-filtered, calcined in a multihearth fur-nace, and dead-burned in a shaft kiln (Dead Sea Periclase web site, 2002).
Synthetic dead-burned magnesias will always be briquetted and thus avail-able to the refractory industry as peach-pit – sized particles or portions thereof.
The ultimate refractory-grade magnesia is manufactured by electromelting 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 that are highly sought after for maximum slag resistance. The inner core produces the best product, while the crust is usually recycled due to its small crys-tallite size. The well-crystallized material is then crushed and sized into a variety of fractions for subsequent refractory-grade use.
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3. Key Dead-Burned Magnesium Oxide Characteristics
MgO Content. The MgO content of dead-burned magnesium oxide is gen-erally included in the grade and/or brand of the particular commercial product.
Obviously, its overall purity plays an important role in determining what MgO con-tent is suitable for a particular end use. The MgO concon-tent of an aggregate is, by and large, but not entirely, directly proportional to its slag resistance. Many impurities, as mentioned previously, are located in triple-point and thin-film accessory mineral deposits between MgO crystallites composing the aggregates or grains; these acces-sory minerals have lower melting temperatures than MgO. Therefore, the amount 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 criti-cal 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 may become meaningless when moderate to large quantities of such impurity phases are present.
Since the mid-1980s, the trend has been to obtain the highest MgO content possible in commercially produced MgO grains.
MgO Impurities. As mentioned in the introduction, a chemical analysis of a sample of commercial MgO aggregate will yield the following principal impu-rities: SiO2(silica); CaO (lime); Al2O3(alumina); Fe2O3(iron oxide); and B2O3 (boric oxide). These impurities do not exist in the MgO aggregates as indepen-dent oxides per se. Rather, they combine together and/or with MgO from the MgO crystallites to form minerals that, under equilibrium conditions, can be pre-dicted from phase equilibrium relationships in the MgO – CaO – SiO2– Al2O3– FeO–Fe2O3system and generally confirmed by X-ray diffraction analyses. These minerals can be distributed in one or more of the following locations: (1) in triple points or as films along crystallite boundaries in the MgO aggregates; (2) as solid solutions in the magnesia crystallites; and (3) as spinellitic exsolutions in the magnesia crystallites.
However, complicating these general points are the phenomena of the solid solutions or solubilities of CaO and/or iron oxide, respectively, in the MgO crys-tallites themselves. Although the lime solubility in the MgO phase is relatively small, the effect is particularly important in very high MgO-content materials (9 – 12). This fact is due to the impact that lime solubility has on altering the CaO/SiO2wt. ratio determined by chemical assay and subsequent phase equili-brium assumptions; this ratio, in turn, controls the nature of the calcium silicate minerals that occur in the grains.
The importance of the CaO/SiO2wt. ratio can be appreciated from study-ing the MgO – CaO – SiO2phase diagram; its representation is seen in Figure 2
(13). As mentioned earlier, the CaO/SiO2wt. ratio controls which minerals exist in the MgO aggregates. If the wt. ratio of CaO/SiO2is greater than 2.8, tricalcium silicate, Ca3SiO5, and free CaO can exist as the impurity or accessory phases with initial liquid formation in the MgO aggregate occurring at 18508C. An exact 2.8 ratio can yield only tricalcium silicate. If the wt. ratio of CaO/SiO2 is between 2.8 and 1.87, tricalcium silicate and dicalcium silicate, Ca2SiO4, can be present, while the temperature of initial liquid formation is 17908C; at just 1.87, only dicalcium silicate can exist. From 1.87 to 1.4 wt. ratio of CaO/
SiO2, dicalcium silicate and merwinite, Ca3MgSi2O8, can be represented and an initial liquid can appear in the magnesia aggregate at 15758C. At just 1.4 CaO/SiO2 wt. ratio, only merwinite can be present. A CaO/SiO2wt. ratio of 1.4 to 0.93 can produce a combination of merwinite and monticellite, CaMgSiO4; with this accessory-phase mineral assemblage with MgO, liquid can begin to be generated if the temperature reaches 14908C. Only monticellite is present at 0.93. Finally, a CaO/SiO2 wt. ratio of less than 0.93 can yield the Figure 2 MgO – CaO – SiO2phase diagram. (From Ref. 13.)
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accessory-phase minerals monticellite and forsterite (Mg2SiO4), and a tempera-ture of15008C can begin to initiate melting in the MgO grain.
Liquid formation usually signals initial signs of destruction and, conse-quently, deterioration of the refractoriness of the MgO grains, so, as discussed in the previous paragraph, the value of the CaO/SiO2wt. ratio plays a significant role in this feature. Obviously, the amount of available lime is a critical factor.
Since lime can be dissolved in solid solution in the MgO crystallites, this circum-stance complicates the overall phase assemblage; moreover, the amount 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 CaO/SiO2wt. ratio increases from 1.4 to over 3, the CaO in solid solution in MgO increases from 0.2 wt. % CaO to2 wt. % CaO. So, the solid solution of CaO in MgO causes the CaO/SiO2wt. 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 CaO/SiO2wt. ratio, so lime solubility in MgO can cause a very significant lowering of the CaO/SiO2wt. ratio and, in very pure MgO aggregates, a dramatic lowering of the temperature of initial melting; this lowering is much greater than anticipated if the lime solubility in MgO is ignored.
Iron oxide is also capable of dissolving in solid solution in the MgO crystal-lites; its phase equilibrium relationship depends on the oxygen partial pressure of the system, but formation of magnesiowustite, (Mg,Fe)O, and magnesioferrite, (Mg,Fe)Fe2O4, solid solutions commonly occur in MgO crystallites even with low levels of iron oxide being present. At higher iron oxide levels and with the CaO/SiO2wt. ratio less than2, the iron spinel reacts with the calcium-mag-nesium silicate accessory phases to further contribute to the amount of liquid that 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 CaO/SiO2wt. ratio is greater 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 (14).
For these reasons, these impurities need to be kept low.
One further point about the need to keep the iron oxide content of MgO grains low. In the presence of carbon in magnesia-carbon refractories, iron oxide reacts with carbon; therefore, this carbon oxidation – iron oxide reduction reaction contributes to the loss of carbon from the refractory, an undesirable effect that is explored in a later section.
Boric oxide is also a very undesirable impurity. Researchers in the 1960s and 1970s found that one of the major reasons for the excellent hot strength of dead-burned Grecian magnesite, which had such a dramatic effect on the increased life of pitch-impregnated, burned MgO brick manufactured 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 wt. % as B2O3). Subsequently, control of the boron content of synthetic magnesia aggregate to less than 0.02 wt % as B2O3 (ideally, less than 0.01 wt .%) was critical to producing a high CaO/SiO2product that could pro-duce a burned MgO brick with hot properties similar to brick made with dead-burned Grecian magnesite. Subsequent research 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 crys-tallites in commercial MgO grains, which prevents direct-bonded, MgO
mid-1960s and early 1970s, was its inherent low boron content (less than 0.005 wt. % as B2O3). Subsequently, control of the boron content of synthetic magnesia aggregate to less than 0.02 wt % as B2O3 (ideally, less than 0.01 wt .%) was critical to producing a high CaO/SiO2product that could pro-duce a burned MgO brick with hot properties similar to brick made with dead-burned Grecian magnesite. Subsequent research 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 crys-tallites in commercial MgO grains, which prevents direct-bonded, MgO