PRODUCTION OF SILICON CARBIDE, MANGANESE CARBIDE AND FERROUS ALLOYS

Abstract: Manganese or silicon carbide is formed in a solid state reaction by mixing finely divided manganese or silicon oxide particles with finely divided carbonaceous material (e.g. coke) particles. The materials may be formed into agglomerates including excess carbonaceous material and heated in a suitable reactor vessel. An iron source, such as pig iron and/or iron scrap, can also be added. In that case, the manganese or silicon carbide dissolves in the molten iron to produce ferrosilicon or ferromanganese alloys.

Description: Production of SiC, MnC and Ferrous Alloys TECHNICAL FIELD The invention relates to the field of foundry and ceramic practices, and particularly to methods of manufacturing silicon carbide (SiC) , manganese carbide (MnC), ferrosilicon alloys and ferromanganese alloys. BACKGROUND ART The use of additives such as ferrosilicon and ferromanganese alloys is well known in the metallurgical arts. These materials are often sold in the form of master alloys, that is, alloys of these elements with iron in a relatively high alloy percentage (higher than that desired in a final product) for mixing with more iron or another iron alloy to provide the ultimately desired content of, manganese or .silicon. These iron-silicon or -manganese alloys are made by various techniques including the use of blast furnaces and electric arc furnaces. In a conventional process for forming an iron master alloy, a metal oxide such as a manganese oxide or a silicon oxide in coarse lump form is charged directly in combination with lump coke or coal into a furnace and heated to reduce the oxide and form molten silicon or manganese. This metal (including silicon, which is technically a metalloid) is then mixed with an iron melt to form an iron alloy. Silicon carbide is generally made by the Acheson process, which involves the use of a silicon source such as sand or quartz and a carbon source such as coke breeze in an electric furnace. The heating times are very long and the resulting silicon carbide is relatively expensive, coarse and of mixed quality. In these processes, the reduction reactions are in part between gas phases and the coarse lump material and in part between liquid metal and liquid slag. Chemical reaction rates are slow, requiring very large furnace size. Heat transfer to the reacting liquids is very difficult, requiring expensive electrical energy, and refractory demands are severe. Unlike iron, which is fully reducible by carbon from the iron oxide to elemental metal without melting, silicon and manganese are reducible by carbon only at temperatures above their melting points. Thus, while agglomerates of finely divided iron oxide and carbon can be completely reduced in the solid state, similar agglomerates of silicon or manganese oxides will undergo melting and disintegration before reduction is complete, and therefore require containment within a refractory vessel. The process in general reverts to a conventional liquid state reduction reaction process with slow reaction rate and expensive heating throughout the process. In addition, such processing normally dictates batch mode, which further increases energy costs, material handling costs and decreases productivity. The technical problem addressed by this invention is providing a process for the low cost manufacture of iron-based alloys containing manganese or silicon, and a process for forming silicon carbide or manganese carbide in a solid state reaction. DISCLOSURE OF THE INVENTION According to one aspect of the present invention, there is provided a process for forming a metal carbide, namely silicon carbide or manganese carbide, which includes the steps of forming a mixture of a finely divided carbonaceous material and a inely divided oxide of silicon or manganese; heating the mixture to a temperature below the melting point of the metal oxide to effect a solid state reaction forming the metal carbide without melting any of the components or product; and the product of that process. The resultant product then may be added to iron or an iron alloy to form a ferrous alloy. Another aspect of the present invention provides an alternative process for directly producing alloys of iron and a metal selected from the group consisting of silicon, manganese and mixtures thereof by: forming a mixture of a finely divided carbonaceous material arid a finely divided oxide of the alloying metal; heating the mixture and an iron metal source, whereby the oxide of. the alloying metal and the finely divided carbon source react in the solid state to form a carbide of the alloying metal, the iron metal source melts to form molten iron, the carbide thereafter dissolves in the molten iron and the alloying metal forms an alloy with the iron. The essence of the present invention is the utilization of conditions such that a solid state reaction between silicon and/or manganese oxide can be used to form the corresponding silicon and/or manganese carbide. In regard to silicon and manganese, excess carbon, that is an excess in the amount necessary for reduction to metal but at least enough to form a carbide, is necessary in order to prevent the materials from melting in the reactor before reduction of the metal oxides. In addition, the use of excess carbon prevents fuming (which increases the yield and reduces environmental control costs) , increases the reaction rate, provides low cost melt oxidation control and provides carburization and supplementary furnace fuel. The agglomerates of the silicon carbide or manganese carbide formed by this process have a greater porosity than that of commercially available silicon carbide pellets derived, for example, from Acheson process material. In addition, the individual silicon or manganese carbide particles within the resulting agglomerates may contain a controlled amount of unreacted carbon at the center of some (or all) of the particles. v The agglomerates of the present! process completely self reduce in a few minutes when heated in a furnace. Furnace size is only about one fifth for equivalent production by this method compared with traditional processes and no liquid state reduction reactions are required. The utilization of the two stage reduction from a carbon bearing-metal oxide agglomerate to a metallic alloy by the utilization of an intermediate carbide ^ formation in the overall reduction process has a number of advantages, and is necessary for silicon and manganese since the intermediate carbide formation is essential to the solid state reaction. In particular, the use of this process for silicon and.manganese gives fast solid state reactions with a minimum environmental effect. The intermediate product (the silicon carbide or manganese carbide product) - is readily shippable. In addition, the product can be made using low cost ores and carbon and waste product material such as molding sand, furnace dust, coke breeze, carbon fines, etc. The ultimate iron " alloy as produced is useful as an inoculant for cast iron and may be used in the simultaneous refinement and carburization of cast iron or steel. In addition, the process of the present invention may be conducted in conventional reactor vessels such as shaft furnaces, cupolas or the like. The processes of the present invention utilize lower process temperatures than conventionally available processes and have less loss and therefore higher yield of the materials. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of one aspect of the present invention. Figure 2 is a schematic representation of a further aspect of the present invention. MODES FOR CARRYING OUT THE INVENTION Figure 1 is a schematic representation of the process which may be utilized to form silicon or manganese carbide. Finely divided particles of silicon oxide or manganese oxide 1 are mixed with finely divided carbon particles 2 to form agglomerates 3 which are then passed into the reactor vessel 10, e.g., a vertical shaft furnace, along with coke particles 4 and, if desired, flux material 5. The silicon oxide or manganese oxide 1 can be any of the commonly available oxides. In one of the aspects of the present invention, the silicon oxide is sand, preferably used core or mold sand which is a waste by- product of most foundries. This material is particularly advantageous in the process of the present invention because of its low cost and on-site availability to the iron foundry. Other sources of the silicon or manganese oxides may also be used. The carbon particles 2 can be coke (including char) , calcined coal or other finely divided forms of carbon, including coke breeze, which is also often readily available as a by-product in the iron foundry. The carbon particles are mixed with the metal oxide in an amount which is stoichiometrically in excess of that required to form the metal carbide by direct reduction of the metal oxide. A particularly ■ advantageous aspect of the invention is that the excess carbon mixed with the metal oxide is in effect used twice, that is, to accomplish two functions in the two- stage, iron alloy production. One is to maximize efficiency of oxide to carbide conversion (this occurs first) , and the other (which occurs last) is improved melt oxidation control by maximally shifting oxidation control from silicon to carbon to reduce cost and metallurgical variability of the cast iron. The amount of excess carbon in the SiC + C aggregate that can be used this way can vary from about 1 to about 3 mole C per mole SiC and depends on the temperature and dilution ratio of FeO in the melt slag, this ratio being in turn a function of the quality of iron metal (scrap and pig) and operating practice. When the SiC is formed in situ by this process, i.e., effectively by the reaction Sι0 2 + 3C SiC + 2CO the extra 1 to 3 moles of carbon is seen to equal 33% to 100% of stoichiometric requirements. Thus, in the case of silicon, generally at least 30%, preferably at least 50%, and in some cases 100% excess carbon source is desired. In the case of manganese, at least 2 1/2%, preferably at least 5% and in some cases 10% excess carbon, is desired. The carbon particles 2-should be finely divided so as to provide sufficient carbon surface for the reaction of gaseous metal oxide, that is about 150 microns or less, preferably about 75 microns or less, most preferably about 50 microns or less. Preferably, the carbon particles are also porous so as to provide even further carbon surface. Optimally, the metal oxide vapors which are generated at a rate determined jointly by metal oxide particle size and temperature are consumed by carbide conversion at a rate as fast as that generation rate. The metal oxide particles 1 preferably should be of a size to provide sufficient reactivity " at the reaction temperature without undue (i.e., more than 5%) loss by fuming. These particles are somewhat finer than the carbon particles and can be in the range of about 100 microns or less, preferably 75 microns or less, most preferably about 50 microns or less. As noted, the metal oxide particles 1 and carbon particles 2 are preferably formed into agglomerates 3 prior to being passed into the reactor . 10. The agglomerate 3 can contain a bonding agent such as the oxides, hydroxides and carbonates of calcium and magnesium and mixtures thereof with lime and/or hydrated lime being the most preferred as set forth in U.S. Patent No. 3,770,416 and related patents such as U.S. Patents 4,580,029; 4,412,840; and 4,239,530, all to Goksel. Sodium silicate or portland cement are also possible binding agents. The agglomerates 3 should be of suitable size such that their movement in the reactor vessel 10 matches that of the coke particles 4, that is, interstitial flow of the agglomerates 3 through the coke particles 4 is precluded. Additionally, the agglomerates 3 should have a sufficient size distribution and eccentricity as to provide for minimum resistance to gas flow. Generally the agglomerates 3 should be egg-shaped with a ratio of diameters of major axis to minor axis of from about 1.05 to about 1.2 although spherical particles (major to minor diameter ratio : 1 to 1) may also be utilized. The use of slightly egg-shaped pellets increases the void -space and relieves the back pressure of gas in the reactor vessel 10. The agglomerates 3 may also be briquetted cubes or any other shape or cut extrusions of any convenient cross section. The agglomerates 3 generally have a size of from 10 to about 100 mm, preferably from about 25 to about 40 mm. The coke particles 4 may advantageously be blast furnace (or metallurgical) coke which are generally non- spherical, e.g., rectangular- or cubic-shaped, particles having major dimensions of from 25 to 50 mm. The coke particles 4 are advantageously slightly larger than the agglomerates 3, but the particular size of the agglomerates 3 may be chosen in accordance with the above characteristics. The agglomerates 3 are mixed with coke particles 4 and the optional and conventional flux material 5 (such as, for example, limestone) in a preheating zone generally indicated as 11 in reactor 10 in Figure 1. The particles travel downwardly through the reactor 10 into reaction zone 12. Just below the reaction zone 12, a high temperature heat source such as a plasma heated gas is injected at inlet 13 into the reactor along with hot air to provide the reaction zone 12 with the proper temperature. Heat from the reaction zone 12 travels upwardly into the preheating zone 11. The amount of heat and hot air added as well as the residence time in the reaction zone 12 is dependent on the materials. That is, silicon oxides in the case of alloying iron will generally.be sized to react in the range of from about 1530 to 1800°C, preferably from about 1550 to 1750°C, most preferably from about 1580 to 1700°C (temperatures higher than those indicated will require more excess carbon to prevent excessive fume formation) . The manganese oxides react at a temperature of from about 1250 to 1500"C, preferably from about 1300 to 1450°C, most preferably from 1325 to 1375"C. The reaction temperature is dependent on the particle size, that is, in general, the finer the particle size of these oxides, the lower the reaction temperature can be ' within the range to effect the reaction of the silicon and manganese oxides. The reactants are maintained in the reaction zone 12 for a time sufficient to convert the silicon or manganese oxide to the corresponding carbide without exceeding.the melting point of any of the components or the product. Thus, there is a solid state reaction between these materials, which offers significant benefits to the overall process as described below. In the case of silicon carbide production for non- metallurgical use (ceramic or " abrasive industries), the reactor is controlled to provide a post-formation heating of the reacted product to about 2200°C in order to convert the silicon carbide from beta to alpha form (the more desired form) . Below the plasma and hot air input 13, cooling air 14 is admitted into the reactor 10 to cool the reactive particles, which then exit the reactor 10 through an exit generally indicated 15 into a quenching unit 16 which can be a water quench of conventional type from which the manganese or silicon carbide particles (generally indicated as 17) can be recovered. These particles have utility in the formation of iron alloys and may be utilized in any other situation in which such material is desired. For example, silicon carbide particles are of significant interest in the ceramics industry. Generally, the temperature of the solids in the preheating zone is from about 200 to 500°C and the coke which is added is used as fuel in the process. The plasma heated gas is at 1000°C or higher with the hot air being approximately 500°C. The plasma can provide all of the necessary heat in reactor vessel 10 (in which case the amount of coke particles 4 can be reduced or even eliminated) . The cooling air 14 which enters the bottom of the reactor also burns excess carbon off the surface or interior of the particles. It has been found that the agglomerates of the silicon carbide or manganese carbide formed by this process have a greater porosity than that of commercially available silicon carbide pellets derived, for example, from Acheson process material. In addition, the individual silicon or manganese carbide particles within the resulting agglomerates may contain a controlled amount of unreacted carbon at the center of some (or all) of the particles. Whether such carbon centers remain is primarily a function of the residence time within the reactor. For utilization in, for example, a solid state reaction forming an iron alloy such as carburizing a powder metal compact or for the liquid state refinement of iron alloy such as inoculation of cast iron, this carbon presence will be generally beneficial in amounts of up to about 50% by weight of the particle. In those cases in which it is not desired, more complete reaction, e.g., longer residence time, will remove substantially all the carbon from the particles of the agglomerate. In Figure 2, there is shown a schematic representation of a reactor 20 which may be used for the direct formation of an iron-manganese or iron-silicon alloy including the intermediate oxide to carbide solid state reaction. In Figure 2, the metal oxide particles 21 and carbon particles 22 represent the same materials as identified as 1 and 2, respectively, in Figure l . Again, these particles are formed into agglomerates 23 of the same type, size and in the same manner as agglomerates 3 in Figure 1. Coke particles 24 are added to the reactor vessel 20 along with the optional flux 25. However, in this embodiment, an iron source 26 which is a source of iron metal, such as iron scrap, pig iron or iron alloys, also is added. Generally, in the iron-silicon case the charge to the vessel 20 will include a load portion having approximately 85 to 96%, preferably about 92 to 95%, (by weight) iron source and about 4 to 15%, preferably about 5 to 8% (by weight) of the agglomerates, plus working material of about 7 to 14%, preferably about 8 to 12% (by weight of the load portion) of coke and up to 2%, generally about 1% (by weight) of flux. It is possible in a particular embodiment of the invention to incorporate sufficient lime or other fluxing material into the metal oxide-carbon agglomerates to meet all fluxing requirements, so that no flux will be required in the furnace (reactor) charge. Since the coke is substantially lower in weight than that of the other materials, it generally constitutes about 15 to 35% by volume of the overall (load portion plus working material) charge. In the iron-manganese case, the load portion may be up to 99%, but is preferably (for ferromanganese alloy production) from about 0 to about 10% (by weight) iron source and concomitantly at least about 1%, and, in the ferromanganese alloy production, preferably from about 90 to about 100% (by weight) of the agglomerates. The working material is the same as in the iron-silicon case. Air is added through an inlet 27 into the reactor 20 to burn the coke particles 24 to fuel the process. If desired, a plasma heating gas may also be used to provide heat. The gas temperature in the reaction zone will generally be in the neighborhood of 1700 to 2200°C. In this aspect of the invention, while the formation of the metal carbide is in the solid state (as described above with reference to Figure 1) , the iron metal source 26, e.g., iron scrap or pig iron, melts to form molten iron in the reaction zone at a temperature of about 1300 to 1500°C. " The silicon carbide and/or manganese carbide formed by the solid state reaction dissolves in the molten iron and the silicon or manganese -alloys with the iron. Thus, the product 29 taken from the reactor 20 is an iron alloy containing silicon in an amount of about 1 to 5 weight percent or manganese in an amount of up to about 80 weight percent. While the air entering through inlet 27 in the reaction can be at room temperature, preheating of the air by conventional preheating means saves on the amount of coke needed to reach the reaction temperature. Although the above has been described with respect to the utilization of manganese oxides or silicon oxides, various mixtures of these oxides may also be used and, with certain limitations, mixtures with other oxides may be used. For example, an iron oxide-containing silicon oxide should not normally be used in the process of the present invention because too much iron oxide (e.g., above about 2 percent by weight) causes the pellet to melt and disintegrate. Therefore, such iron oxide- containing silicon oxides are not normally desired (however, when available silicon oxide materials are contaminated with iron oxide in amounts above 2% by weight of the pellet, up to about 5% iron oxide may be accommodated by adding lime in an amount equal to the excess iron oxide over 2%) . When utilizing manganese oxide, iron ore sources which include iron oxides may be used. While the presence of high amounts of iron with silicon oxides are as described above not acceptable, iron and manganese form combined carbides of greater stability than iron carbide alone (the opposite of silicon carbide) . Also, manganese carbide solubility in liquid iron is very high, so iron master alloys containing as high as 70 to 80 percent manganese can easily be made. INDUSTRIAL APPLICABILITY The invention provides an inexpensive method of manufacturing and a new structure of silicon carbide and manganese carbide, which are useful in the ceramic and foundry industries. The invention also provides an inexpensive method of manufacturing ferrosilicon and ferromanganese alloys directly from silicon or manganese oxide, carbon and ferrous oxide.

Claims: CLAIMS 1. A process for forming a metal carbide, namely, silicon carbide or manganese carbide, by heating a mixture of carbon and a metal oxide, namely, silicon oxide or manganese oxide, characterized in that: the mixture is formed of a finely divided carbonaceous material and a finely divided oxide of silicon or manganese; and the mixture is heated to a temperature below the melting point of the metal oxide to effect a solid state reaction forming the metal carbide without melting any of the components or product. 2. The process of claim 1, further characterized in that the mixture is formed into an agglomerate prior to heating. 3. The process of claim 1, further characterized in that the finely divided carbonaceous material has a particle size of about 150 microns or less. 4. The process of claim 3, further characterized in that the finely divided carbonaceous material has a particle size of about 75 microns or less. 5. The process of claim 4, further characterized in that the finely divided carbonaceous material has a particle size of about 50 microns or less. 6. The process of any of claims 1, 3, 4 or 5, further characterized in that the carbonaceous material comprises calcined coal or coke. 7. The process of claim 1, further characterized in that the finely divided metal oxide has a particle size of about 100 microns or less. 8. The process of claim 7, further characterized in that the finely divided metal oxide has a particle size of about 75 microns or less. 9. The process of claim 8, further characterized in that the finely divided metal oxide has a particle size of about 50 microns or less. 10. The process of claim 1, further characterized in that the mixture contains the carbonaceous material in excess of that stoichiometrically required to form the metal carbide. 11. The process of claim 10, further characterized in that the carbonaceous material is present in an amount of up to about 100% of that stoichiometrically required to form the metal carbide. 12. The process of claim 11, further characterized in that the carbonaceous material is present in an amount of up to about 50% of that stoichiometrically required to form the metal carbide. 13. The process of claim 12, further characterized-in that the carbonaceous material is present in an amount of up to about 30% of that stoichiometrically required to form the metal carbide. 14. The process of any of claims 1-13, further characterized in that the metal oxide comprises silicon oxide. 15. The process of claim 14, further characterized in that the finely divided silicon oxide is comprised at least in part of foundry molding sand. 16. The process of claim 14, further characterized in that ' the mixture is heated to a temperature of from about 1530 to 1800°C. 17. The process of claim 10, further characterized in that the carbonaceous.material is present in an amount of up to about 10% in excess of that stoichiometrically required to form the metal carbide. 18. The process of claim 17, further characterized in that the carbonaceous material is present in an amount of up to about 5% in excess of that stoichiometrically required to form the metal carbide. 19. The process of claim 18, further characterized in that the carbonaceous, material is present in an amount of up to about 2 1/2% in excess of that stoichiometrically required to from the metal carbide. 20. The process of any of claims 1-10, 17, 18 or 19, wherein the metal oxide comprises manganese oxide. 21. The process of claim 20, further characterized in that the mixture is heated to a temperature of from about 1250 to 1500°C. 22. The process of claim 1, further characterized in that the heating is provided at least in part by the oxidation of carbonaceous material. 23. The process of claim 22, further characterized in that at least a part of the carbonaceous material used for heating is different from the carbonaceous material of the mixture. 24. The process of claim 23, further characterized in that the carbonaceous material used for heating is coke. 25. The process of claim 24, further characterized in that the carbonaceous material used for heating is metallurgical coke. 26. The process of claim 1, wherein the heating is performed in a vertical shaft furnace with the metal carbide being removed from the bottom of the furnace. 27. A process for forming manganese carbide by heating a mixture of manganese oxide and carbon, characterized in that: the mixture is formed of manganese oxide particles having an average diameter of 100 microns or less, and particles of a carbonaceous material having an average diameter of 150 microns or less, said carbonaceous material being present in the mixture in an amount in excess of that stoichiometrically necessary for direct reduction of the manganese from the oxide to the metal; forming the mixture into an agglomerate; and passing the agglomerate and further carbonaceous material into a reactor and heating the same at a temperature of from 1250 to 1500°C for a time sufficient to convert substantially all of the manganese oxide to manganese carbide in a solid state reaction without melting any of the components or product. 28. The process of claim 27, further characterized in that the manganese carbide particles are recovered from the reactor. 29. The process of. claim 27, further characterized in that an iron metal source is added to the vessel, the iron metal source being melted in the reactor, the manganese-carbide dissolving in the iron source to form ferromanganese alloy which is recovered from the reactor. 30. A process for producing alloys of iron and an alloying metal selected from the group consisting of silicon, manganese and mixtures thereof, characterized by: forming a mixture of a source of finely divided carbonaceous material and a source of finely divided oxide of the alloying metal; and heating the mixture and an iron metal source such that the oxide of the alloying metal and the carbonaceous react in the solid state to form a . carbide of the alloying metal, the iron metal source melts to form molten iron, the carbide thereafter dissolves in the molten iron and the alloying metal forms an alloy with the iron. 31. The process of claim 30, further characterized in that the source of finely divided carbonaceous material is calcined coal or coke and has a particle size of about 150 microns or less. 32. The process of claim 30, further characterized in that the source of finely divided oxide of the alloying metal has a particle size of 100 microns or less. 33. The process of any of claims 30-32, further characterized in that the alloy metal is manganese and the manganese oxide source includes an iron metal source. 34. The process " of any of claims 30-32, further characterized in that the iron metal source comprises at least partly iron or steel scrap. 35. The process of claim 34, further characterized in that the iron metal source comprises at least partly pig iron. 36. The process of any of claims 30-32, further characterized in that the oxide is silicon oxide and the carbonaceous material is present in an amount of up to about 100% in excess of that stoichiometrically required for direct reduction of the silicon oxide to silicon carbide. 37. The process of any of claims 30-32, further characterized in that the oxide is- manganese oxide and the carbonaceous .material is present in an amount of up to about 10% in excess of that stoichiometrically required for direct reduction of the manganese oxide to manganese carbide. 38. The process of any of claims 30-32, further characterized in that the mixture of finely divided carbonaceous material and finely divided oxide of the alloying metal is formed into agglomerates prior to heating. 39. The process of claim 38, further characterized in that the alloying metal is manganese and the agglomerates and iron metal source are heated to a temperature of from about 1250 to about 1500°C. 40. The process of claim.38, further characterized in that the alloying metal is silicon and the agglomerates and iron metal source are heated to a temperature of from about 1530 to about 1800°C. 41. The process of any of claims 30-32, further characterized in that oxidation of a carbonaceous material provides at least part of the heating. 42. The product of' the process of claim 1. 43. The product ofJ the process of claim 28. 44. The product of the process of claim 30.

Inventors: LILLIBECK NORMAN PAUL, LILLIBECK, NORMAN, PAUL
Applicants: DEERE & CO, DEERE & COMPANY
Application year: 1989
Main classification: C01B31
IPC codes: C01B31, C22C33