Iron- and manganese oxides: culprits of refractory erosion.Once you understand these havoc-wreaking oxides, you can control their sources to prolong your induction melter's refractory refractory Material that is not deformed or damaged by high temperatures, used to make crucibles, incinerators, insulation, and furnaces, particularly metallurgical furnaces. life. Many foundries have battled refractory wear or erosion in coreless induction furnaces used for melting the various grades of iron and steel. Channel induction furnaces melting iron often experience erosion of the uppercase or inductor inductor, electric device consisting of one or more turns of wire and typically having two terminals. An inductor is usually connected into a circuit in order to raise the inductance to a desired value. refractories. When erosion occurs, the foundry must shut down the furnace as the lining thickness has been eroded past an acceptable limit. Erosion of refractories occurs when there is a localized or uniform reduction of the refractory thickness due to chemical or mechanical means. Chemical erosion between the refractory and the ferrous ferrous (fĕr`əs), iron in the +2 valence state. Containing or having to do with iron. The difference between ferrous and ferric is the number of valence electrons they contain (ferrous contains two and ferric contains three), which melt is commonly referred to as "corrosion." This type of erosion is often the direct result of iron- and manganese manganese (măng`gənēs, măn`–) [Lat.,=magnet], metallic chemical element; symbol Mn; at. no. 25; at. wt. 54.938; m.p. about 1,244°C;; b.p. about 1,962°C;; sp. gr. 7.2 to 7. oxide attack on the refractories. Iron Oxide The material used to coat the surfaces of magnetic tapes and lower-capacity disks. To understand the chemical erosion of the refractory due to iron oxide attack, the various forms of iron oxide must be identified and classified according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. Gibbs Free Energy Gibbs free energy: see free energy. of Formation at a given iron melting temperature Melting temperature may refer to:
FeO (s) [approaches] FeO (l) [TABULAR DATA FOR TABLE 1 OMITTED] [TABULAR DATA FOR TABLE 2 OMITTED] Commonly known as "rust," [Fe.sub.2][O.sub.3] (hematite hematite (hĕm`ətīt), mineral, an oxide of iron, Fe2O3, containing about 70% metal, occurring in nature in red to reddish-brown earthy masses and in steel-gray to black crystalline forms. ) is very unstable and as its Gibbs Free Energy of Formation illustrates, it will dissociate dis·so·ci·ate v. dis·so·ci·at·ed, dis·so·ci·at·ing, dis·so·ci·ates v.tr. 1. To remove from association; separate: into FeO when exposed to ferrous melt temperatures. 2 [Fe.sub.2][O.sub.3] [approaches] 4FeO + [O.sub.2] [Fe.sub.3][O.sub.4] (magnetite magnetite (măg`nətīt), lustrous black, magnetic mineral, Fe3O4. It occurs in crystals of the cubic system, in masses, and as a loose sand. ), is moderately stable. However, it is possible (but not likely) that if the temperature is increased and held under reducing melt conditions for an extended period of time, it will dissociate to [Fe.sub.2][O.sub.3] and FeO. This is strictly dependent on the availability of oxygen in the melt. [Fe.sub.3][O.sub.4] [approaches] [Fe.sub.2][O.sub.3] + FeO 2[Fe.sub.2][O.sub.3] [approaches] 4FeO + [O.sub.2] The partial pressure of oxygen present determines which form of iron oxide is more stable. If this oxide reacts with the various refractory systems, refractoriness is lowered, dropping the service temperature of the refractory. This results in rapid general wear of the refractory. Manganese Oxide Similar to the different forms of iron oxide, degrees of stability exist for the different forms of manganese oxide. Again, using Gibbs Free Energy of Formation values at a given iron temperature, and the Ellingham Free Energy Diagram, the degree of stability has been verified in Table 2. Although it's difficult to find conclusive data on all four oxides of manganese, MnO is determined to be the most stable and Mn[O.sub.2] would be the least stable. If the dissociation dissociation, in chemistry, separation of a substance into atoms or ions. Thermal dissociation occurs at high temperatures. For example, hydrogen molecules (H2 reactions are compared to those of iron oxide, the [Mn.sub.2][O.sub.3] would behave similarly to the [Fe.sub.2][O.sub.3] and [Mn.sub.3][O.sub.4] would behave similarly to [Fe.sub.3][O.sub.4]. Accurate modeling of the manganese oxide stability must include both partial pressure of oxygen and temperature considerations. Silica Refractory System As different grades of iron are melted in silica refractory-lined coreless induction furnaces, oxides are present within the melt. Figure 1 shows the melting points of 2FeO*Si[O.sub.2] (fayalite fay·a·lite n. A yellowish to black mineral, Fe2SiO4, of the olivine group. [German Fayalit, from Fayal, Faial. ) and 2MnO-Si[O.sub.2] (tephroite) to be 2223F (1217C) and 2453F (1345C), respectively. At typical iron or steel melting temperatures, these compounds remain in the liquid state within the molten metal and will resolidify in the furnace where significant heat loss is experienced. Figure 2 shows iron oxide attack on a silica refractory. Iron oxide reacts with silica (Si[O.sub.2]) in this way: 2FeO + Si[O.sub.2] [approaches] 2FeO*Si[O.sub.2] fayalite [Delta][G.sub.form] = -3370 cal/mole at 2642F (1450C) Manganese oxide reacts with silica in this way: 2MnO + Si[O.sub.2] [approaches] 2MnO*Si[O.sub.2] tephroite [Delta][G.sub.form] = -4010 cal/mole at 2642F (1450C) In both cases, erosion of the silica refractory occurs. With increased stirring from lower frequency coreless induction furnaces, the chemical erosion is more uniform throughout the entire molten metal contact surface. In medium-frequency, high-powered coreless induction furnaces, silica erosion is more pronounced in the middle of the coil. Alumina alumina (əl  `mĭnə) or aluminum oxide, Al2O3, chemical compound with m.p. about 2,000°C; and sp. gr. about 4.0. Refractory System Alumina refractory linings are often used for channel furnaces melting or holding iron, and are frequently used in coreless furnaces for melting iron. In steel coreless melting, alumina is the refractory of choice. Figure 1 also illustrates the effect of FeO and MnO on the [Al.sub.2][O.sub.3] system. In the +90% alumina refractory system, iron oxide can react with the alumina to form a spinel spinel, magnesium aluminum oxide, MgAl2O4, a mineral crystallizing in the isometric system, usually as octahedrons. It occurs as an accessory mineral in basic igneous rocks, in aluminum-rich metamorphic rocks, and in contact-metamorphosed compound FeO*[Al.sub.2][O.sub.3] (hercynite). At iron melting temperatures, hercynite doesn't form as readily as it does at steel melting temperatures. It is often referred to as metal saturation. FeO + [Al.sub.2][O.sub.3] [approaches] FeO*[Al.sub.2][O.sub.3] hercynite [Delta][G.sub.form] = - 5404 cal/mole at 2642F (1450C) When manganese oxide is in the presence of a +90% alumina refractory, it behaves similarly to iron oxide, forming a spinel compound, MnO*[Al.sub.2][O.sub.3] (galaxite), according to the following reaction: MnO + [Al.sub.2][O.sub.3] [approaches] MnO*[Al.sub.2][O.sub.3] galaxite [Delta][G.sub.form] = -8485 cal/mole at 2642F (1450C) Although the initial formation of these compounds remains within the refractory hotface without any negative consequence, continuous exposure to increasing amounts of these oxides promotes uncontrolled expansion of saturated refractory, which leads to hotface spalling [ILLUSTRATION FOR FIGURE 3 OMITTED]. In the mullite-based (3[Al.sub.2][O.sub.3]*2Si[O.sub.2]) or mullite-forming refractories for ferrous induction applications, the working linings for coreless induction furnaces for iron alloys and steel are continuously exposed to iron- and manganese oxides [ILLUSTRATION FOR FIGURE 1 OMITTED]. In mullite-based products, a reaction between the iron- or the manganese oxides and the silica portion of the mullite can occur, but will require a higher melt temperature to drive the reduction reaction. When comparing the Gibbs Free Energy of Formation for mullite, fayalite and tephroite, the reactions between the iron- and manganese oxides and the silica will have the following values: [Delta][G.sub.form] 3[Al.sub.2][O.sub.3]*2Si[O.sub.2] (mullite) = -3012 cal/mole @ 2642F (1450C) [Delta][G.sub.form] 2 FeO*Si[O.sub.2] (fayalite) = -4010 cal/mole [Delta][G.sub.form] 2MnO*Si[O.sub.2] (tephroite) = -3370 cal/mole Iron- and manganese oxides will react with the silica component of the mullite to form fayalite and tephroite, respectively. These reactions readily occur with mullite-forming refractories because there is more free silica present within the matrix. In both cases, the overall result is a general wear of the hotface. Spinel-based (MgO*[Al.sub.2][O.sub.3]) or spinel-forming refractories used in induction furnaces for ferrous melts, will experience little reaction at typical iron melting temperatures. At steel melting temperatures, however, reactions between the iron- and manganese oxide, and the spinel compound will occur more readily, forming a modified spinel [(Fe, Mn, Mg)O*[Al.sub.2][O.sub.3]]. In spinel-forming refractories that are alumina-enriched, it is possible to form either hercynite or galaxite [ILLUSTRATION FOR FIGURE 3 OMITTED]. Chrome-alumina-forming refractories are also affected by iron- and manganese oxides. Typically, chrome alumina refractories form a solid solution of chrome oxide and alumina with the following formulation [(Al,Cr).sub.2][O.sub.3]. The iron- or the manganese oxides will replace one of the [Al.sub.2][O.sub.3] to form [(Fe,Mn)O*[(Al,Cr).sub.2][O.sub.3]]. In summary, both oxides react with the alumina to form complex spinels, except in the case of mullite. As previously discussed, the initial effect on the refractory matrix is minimal, however, prolonged exposure to increasing amounts of FeO and MnO will cause hotface spalling due to uncontrolled expansion of the saturated refractory. In mullite systems, the overall result is a reaction with the silica portion of the mullite, causing general erosion. Magnesia Magnesia, ancient cities, Lydia Magnesia (măgnē`zhə), two ancient cities of Lydia, W Asia Minor (now W Turkey). They were colonies of the Magnetes, a tribe of E Thessaly. Refractory System Magnesia-based refractories are typically used in inductors of channel furnaces for melting or holding iron. In steel melting, dry vibratable magnesia has been used for specific high temperature melting in coreless induction furnaces, especially if high manganese-containing steel grades are melted. The limiting factor A factor or condition that, either temporarily or permanently, impedes mission accomplishment. Illustrative examples are transportation network deficiencies, lack of in-place facilities, malpositioned forces or materiel, extreme climatic conditions, distance, transit or overflight rights, on its usage in larger coreless furnaces for steel is the unfavorable expansion that is attributed to the magnesia itself, leading to severe cracking due to thermal shock Thermal shock in mechanical models Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high . When iron- or manganese oxides are introduced into a magnesia system, both readily form a solid solution with the MgO, due to the similar valences of the [Fe+.sup.2] ion, the[ Mn+.sup.2] ion and the [Mg+.sup.2] ion. These can be easily substituted into a complex MgO solid solution compound. For example, in a MgO*Si[O.sub.2], forsterite forsterite See under olivine. compound, iron oxide is readily absorbed into the matrix to form a (Fe,Mg)O*Si[O.sub.2]. Similarly, in a spinel-forming MgO compound, the presence of FeO will cause the formation of (Fe,Mg)O*[Al.sub.2][O.sub.3]. This compound is moderately stable, but the overall refractoriness of the matrix is reduced by the presence of iron oxide, as shown in Fig. 4. Manganese oxide has a similar effect on magnesia refractories, forming solid solution compounds with the complex MgO compounds. Sintering/Bonding Agents The sintering sintering, process of forming objects from a metal powder by heating the powder at a temperature below its melting point. In the production of small metal objects it is often not practical to cast them. agent or bonding system plays an equally important role in the effects of iron- and manganese oxides on refractories. Sodium silicate sodium silicate, any one of several compounds containing sodium oxide, Na2O, and silica, Si2O, or a mixture of sodium silicates. Sodium orthosilicate is Na4SiO4 (or 2Na2O·SiO2); sodium is often a sinter sinter Mineral deposit with a porous or vesicular texture (having small cavities). Siliceous sinter is a deposit of opaline or amorphous silica that occurs as an incrustation around hot springs and geysers and sometimes forms conical mounds (geyser cones) or terraces. agent that is used to give refractories some low temperature or intermediate temperature strength. Unfortunately, the silicate portion is affected by iron- and manganese oxides in a way similar to that which a silica-based refractory is affected, forming low temperature fayalite or tephroite. As this sintering agent is dissolved, the matrix will erode. Boron boron (bōr`ŏn) [New Gr. from borax], chemical element; symbol B; at. no. 5; at. wt. 10.81; m.p. about 2,300°C;; sublimation point about 2,550°C;; sp. gr. 2.3 at 25°C;; valence +3. oxide/boric acid is used to allow for a stable, "low temperature glass" to form within the matrix. Iron-and manganese oxides will react with this "glassy" phase, lowering refractoriness and resulting in a wear condition. Calcium aluminate a·lu·mi·nate n. A chemical compound containing aluminum as part of a negative ion. Noun 1. aluminate - a compound of alumina and a metallic oxide cement is in castable refractories. Iron oxide will react with the complex CaO*[Al.sub.2][O.sub.3] compound, to form wollastonite wol·las·ton·ite n. A white to gray mineral, essentially CaSiO3, found in metamorphic rocks and used in ceramics, paints, plastics, and cements. [After William Hyde Wollaston. , a complex compound that is equally stable at iron melting temperatures. Manganese oxide will react with the CaO*[Al.sub.2][O.sub.3] similarly to the iron oxide. The overall effect is a lowering of the refractoriness of the matrix, but the bonding agent will continue to function. Clay is often used to add intermediate strength to the matrix or to add a "buffer agent" to help absorb refractory expansion. There is a silicate portion in the clay bond that is affected by both iron- and manganese oxides in a similar manner to the silica-based refractories. The overall result is a general wear of the refractory as the clay bond is washed out. Phosphate/Phosphoric acid is used to give low temperature strength to the matrix. Both iron-and manganese oxides will react with the phosphate ion to form iron or manganese phosphate, respectively. These are considered to be low melting point compounds that negatively impact refractoriness. Case Studies Oxide materials from spent magnesia-alumina spinel refractories in a steel coreless melt were examined. A sample was obtained from an operation melting low-carbon steel Noun 1. low-carbon steel - steel with less than 0.15% carbon mild steel, soft-cast steel steel - an alloy of iron with small amounts of carbon; widely used in construction; mechanical properties can be varied over a wide range in coreless furnaces. The material analyzed came from an area adjacent to a metal fin perpendicular to the hotface in the sintered sin·ter n. 1. Geology A chemical sediment or crust, as of porous silica, deposited by a mineral spring. 2. A mass formed by sintering. v. sin·tered, sin·ter·ing, sin·ters v. refractory. Penetration of FeO resulted in formation of hercynite (FeO*[Al.sub.2][O.sub.3]) as determined by x-ray diffraction (XRD XRD X-Ray Diffraction XRD Crossroad XRD X-Ray Diode ). The refractory used in this case was a 90% alumina, 10% magnesia refractory. Examination of the XRD pattern in Fig. 5 shows the presence of hercynite (h), unreacted alumina (Al) and magnesia (M), as well as spinel (s). Another sample came from an operation melting tool steel charge in coreless furnaces and was located at a fin site parallel to the hot face, roughly 0.5 in. from the metal contact surface. In both cases, FeO and MnO had penetrated these linings via metal finning that resulted from thermal shock. With the same material, the XRD pattern in Fig. 6 shows the alumina (A), spinel (S) and magnesia (M) phases expected from such a starting refractory composition. The secondary phases of galaxite (MnO*[Al.sub.2][O.sub.3]) (G) and magnesium (iron ([+.sup.3]) - aluminum) spinel (x) provide evidence of refractory attack by iron- and manganese oxide components from the melt. It is interesting that the iron is present in a +3 state, while in the previous example it is present in a +2 state. This can be verified by the type of spinel formation present in the XRD pattern. These examples show that attack of magnesia-alumina refractory linings by metal oxides from the melt is a pertinent factor in campaign life. Minimizing their penetration by control of grain and pore size, or by manipulation of the chemistry of the lining material are means of reducing such attack and increasing lining life. Extend Your Refractory Life When induction melting any ferrous alloy, iron oxide is formed within the melt due to residual tramp surface oxides, oxidation of the ferroalloy ferroalloy Alloy of iron (less than 50%) and one or more other metals, important as a source of various metallic elements in the production of alloy steels. The principal ferroalloys are ferromanganese, ferrochromium, ferromolybdenum, ferrotitanium, ferrovanadium, or entrapped gas as a result of low frequency induction stirring. This can lead to shortened lining life due to erosion, and will also increase the amount of inclusions within the ferrous metal. If the oxide level is controlled, the erosion can be retarded. Iron- and manganese oxides have played havoc on refractories in induction furnaces. Both oxides affect the overall refractoriness of most refractory systems and bonding agents. General refractory wear will be evident. By identifying the contributing sources of both oxides (as listed in the sidebar) and controlling their additions, you will prolong the refractory life and improve metal quality. RELATED ARTICLE: Tips for Oxide Control To control the amount of iron- and manganese oxide within the melt, it is important to know the compositions of the metal and of the typical slag. Historical data of these compositions will allow one to identify immediately any drastic changes. This is important for prolonging refractory life, but even more important, is the effect on metal quality (inclusions). To control iron oxide: * minimize use of heavily oxidized oxidized having been modified by the process of oxidation. oxidized cellulose see absorbable cellulose. iron or steel charge. This includes the use of pig iron pig iron: see iron. pig iron Crude iron obtained directly from the blast furnace and cast in molds (see cast iron). The crude ingots, called pigs, are then remelted along with scrap and alloying elements and recast into molds to produce and various ferroalloys. * use higher grades of ferroalloys. * keep the silicon level in the melt at the middle of the metal specification (exception: ductile-base chemistry). * keep charge materials dry. Moisture generates iron oxide, as well as an unsafe condition. * use a cover. Excessive exposure of the molten metal to the atmosphere adds oxygen to the metal's surface. * minimize excessive induction stirring. Dissolved oxygen may become trapped. * add silicon carbide silicon carbide, chemical compound, SiC, that forms extremely hard, dark, iridescent crystals that are insoluble in water and other common solvents. Widely used as an abrasive, it is marketed under such familiar trade names as Carborundum and Crystolon. to help reduce the iron oxide to iron, silicon and carbon monoxide carbon monoxide, chemical compound, CO, a colorless, odorless, tasteless, extremely poisonous gas that is less dense than air under ordinary conditions. It is very slightly soluble in water and burns in air with a characteristic blue flame, producing carbon dioxide; . To control manganese oxide: * minimize manganese steel Man`ga`nese´ steel 1. Cast steel containing a considerable percentage (10-14) of manganese, which makes it very hard and tough and highly resistant to wear. See Alloy steel, above. Noun 1. in carbon steel charge. * use higher grades of ferroalloys. * use a cover. * add silicon carbide to help reduce the manganese oxide to manganese, silicon and carbon monoxide. |
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