MXPA96001912A

MXPA96001912A - Method to reduce metal oxide in a rotating placement oven, heated by a dexided flame

Info

Publication number: MXPA96001912A
Application number: MXPA/A/1996/001912A
Authority: MX
Inventors: M Kundrat David
Original assignee: Armco Inc
Priority date: 1995-06-06
Filing date: 1996-05-21
Publication date: 1998-07-03

Abstract

The present invention relates to a metal oxide reduction method, characterized in that it comprises: providing a furnace having an annular platform and at least one fuel burner, placing a first layer of a mixture containing oxygen-bound metal and a reducer on a top surface of the platform, rotate the platform to pass to the burner to heat the first layer with an oxidation flame, load a second layer of the reducer covering the first layer, continue heating the layers at sufficient temperature and a sufficient time to form at least a partially reduced mixture, whereby the second layer prevents the re-oxidation of the partially reduced mixture within the hor

Description

METHOD TO REDUCE METAL OXIDE IN A ROTATING PLACE OVEN, HEATED BY AN OXIDATION FLAME BACKGROUND OF THE INVENTION The invention relates to a method for the reduction of a metal bound to oxygen in a furnace heated with an oxidation flame. More particularly, a first layer, including a mixture of a metal oxide and a reducer, is placed on an annular platform inside the furnace. The first layer is covered by a second reducing layer to prevent reoxidation of the reduced metal oxide. The reduced metal oxide can be used as a feedstock to supply metal units in a refining furnace for the alloy with the cast iron. It is known to reduce metal oxides and metal ores with the reduced product which is subsequently used as a feedstock in a refining vessel for making iron or iron alloys. The type of ovens commonly used to reduce metallic minerals are vertical draft, rotary kiln or rotary square oven. U.S. Patent 4,701,214, incorporated herein by reference, discloses the use of a rotary square oven, in which the iron ore, coal and lime are mixed together, pulverized and compacted into granules. The iron ore granules are fed two to three granules deep on a rotating platform inside the furnace. The iron ore granules are heated to 1000 ° C by the burners placed annularly above the turntable. A fuel reducing hydrogen and carbon monoxide are used in the burners. After they are reduced, the iron ore granules are fed to the melting furnace to be dissolved in molten iron. U.S. Patent 4,622,905 discloses the use of a rotary square oven to metallize iron oxide. The iron oxide granules and the charcoal powder are placed on a rotating platform inside the furnace. The granules are heated by a luminous flame of the burners, placed annularly above the turntable. The heating flame is formed from the combustion of charcoal powder and oxygen. U.S. Patent 5,186,741 describes the use of a rotary square oven for metallizing powders of iron oxide steel plants containing heavy metals. The iron oxide granules, carbonaceous material, a binder and optionally calcium oxide are formed into granules. The granules are placed on a rotating table in the rotary square oven. The granules are dried at a temperature not higher than 900 ° C for up to 15 minutes and then reduced to 30 minutes at a temperature of 1150 ° C. U.S. Patent 3,443,931 discloses the use of a rotary square oven to metallize iron oxide. The iron oxide granules and carbon are laid one or two depth granules on a turntable inside the furnace. The granules are heated in an atmosphere devoid of free oxygen, which has a temperature of up to about 1425 ° C by burners placed above the turntable. U.S. Patent 4,772,316 describes the use of a rotary-type furnace to metallize a chromium-containing iron ore to produce ferrochrome used as a master alloy in the manufacture of chromium steel. A mixture of the chromite mineral, carbon and flux, is heated in an atmosphere that contains carbon monoxide. The mixture is loaded at one end of the furnace, flows through the furnace and is discharged continuously from the other end of the furnace. The burner gases are passed through the discharge end of the furnace and flow in a direction opposite to the flow of the chromium ore. Coal is used, not only to reduce the gases of the combustion product of C02 and H20, to form carbon monoxide and hydrogen, but also to prevent the reoxidation of chromium metallizing. U.S. Patent 4,971,622 relates to the reduction and desulfurization of the chromite mineral. A chromite mineral is mixed with a carbonaceous material and heated to 1500 ° C in a rotary kiln. At least 90% by weight of the chromium oxide is reduced to a metallic state and almost 100% by weight of the iron oxide is reduced to a metallic state. This reduction product containing calcium oxide and excess carbon is fed to an electric arc furnace for desulfurization. It is also known to produce stainless steel by charging chromite ore and / or nickel ore directly into a refining nozzle having a lance that blows oxygen at the top and lower nozzles to blow the stirring gas. U.S. Patent 5,047,082 describes the production of stainless steel in an oxygen converter, using a nickel mineral with low sulfur content instead of ferronickel. The nickel mineral is reduced by the carbon dissolved in the molten iron and the residual trace present in the slag. U.S. Patent 5,039,480 describes the production of stainless steel in a converter, by sequentially melting and reducing the nickel mineral with low sulfur content and then the chromite mineral in place of ferronickel and ferrochrome melts. The minerals are reduced by the dissolved carbon in the molten iron and the residual trace present in the slag. There are disadvantages associated with the processing of minerals in a shooting furnace or a rotary kiln. In a draft furnace, the ore load moves downwards through the shaft to the hot reducing gases that are produced countercurrent through the mineral bed or column. The ore load does not mix with itself to any large extent during its descent through the draft. In an oven, as the ore moves from one end to the other by gravity, there is considerable mixing due to the rotation of the furnace. Both ovens involve the movement of the ore in relation to the walls of the kiln, making it more difficult to know and control the temperature of the ore. A shooting furnace and a rotary kiln tend to have problems of stickiness or agglomeration. In the kiln the ore tends to stick itself, while in the furnace decalcination, the ore tends to stick to the inner wall of the drum as well as to itself. And, those problems increase, at higher operating temperatures. Occasionally, during the operation, a furnace will develop local hot spots, where mineral melting can occur, resulting in severe mineral agglomeration, which adversely affects production. Both of the firing and calcination furnaces require chunks or hardened agglomerated ore due to the weight of the load * in the shot and impact in the furnace. A major disadvantage associated with the complete processing of minerals in a refining vessel during the fabrication of stainless steel is that the ore usually contains the metal in small amounts and is difficult to melt. Also, generally considerable energy is required to reduce the metal oxide to a suitable metal to form the alloy. The reclassification is done to a high degree of post combustion, but this necessitates the addition of solid carbonaceous material to the bath. This carbonaceous material is necessary to avoid the formation of the slag from the bath and to avoid the reoxidation of the alloys to the slag. The presence of the carbonaceous material in the slag in significant amounts results in a concentration of carbon enriched from the bath. This carbon has been removed during refinement. Melting of the ore may be undesirable, especially in the case of low grade minerals such as nickel ore, because as much as 80 percent of the weight of the ore is converted to slag. Nickel ores contain only about 1-3% by weight of Ni. Nevertheless, there remains a long-felt need to provide cheap metal units for making iron or alloy steel, such as steel forming alloy with chrome or stainless steel. There also remains a need to provide metal units of cheap metal oxides. Another long-felt need includes the development of a reliable and consistent process to provide cheap Cr and / or Ni units to form the alloy.

BRIEF DESCRIPTION OF THE INVENTION A principal object of the invention is to produce cheap metals to form an alloy, from a metal oxide for feeding into a refining vessel containing molten iron. Another object of the invention is to reduce the metal oxide in a furnace heated with an oxidation flame. Another object of the invention is to reduce the metal oxide by contact with a carbonaceous reducer in the heating with an oxidation flame to produce valuable alloy metals to produce stainless steel. Another object of the invention includes the prevention of the reoxidation of the reduced metal from a metal oxide in a furnace heated with an oxidation flame.

A further object of the invention includes providing cheap Cr and Ni units, "reduced in metal oxides and sulphides which are to be used as the feed charge to an electric furnace and / or a refining vessel during steel fabrication. alloy. The invention relates to a method for reducing metal bound to oxygen in a furnace heated with an oxidation flame and includes providing the furnace with an annular platform and at least one fuel burner, placing a first layer of a mixture of a metal attached to oxygen and a reducer on a top surface of the platform, rotate the platform that goes to the burner to heat the first layer by an oxidation flame, load a second layer of a reducer that covers the first layer, heat continuously the metal oxide at sufficient temperature and for sufficient time to at least partially reduce the metal oxide, whereby the second layer prevents the re-oxidation of the partially reduced metal by oxidation gases in the furnace. Another feature of the invention is for the first mentioned layer, which is heated to at least 1000 ° C, before it is covered with the second layer. Another feature of the invention is for the aforementioned mixture to include at least 10% by weight of fixed carbon.

Another feature of the invention is for the aforementioned blend to include pulverized chromite ore and pulverized charcoal. Another feature of the invention is for the chromium bound to the partially reduced oxygen, mentioned, in the mixture to be at least 40% metallized to chromium or chromium carbide. Another feature of the invention is for the first mentioned layer, which is not more than 40 mm deep. Another feature of the invention is for the second mentioned layer, which is not more than 10 mm deep. Another feature of the invention is for the aforementioned mixture to further include powdered sulfur-bearing nickel concentrate. Another characteristic of the invention is for the above-mentioned partially reduced mixture to contain at least 0.1% by weight of nickel as the metal or as the nickel sulfide. Another feature of the invention is for the first layer, mentioned, which is maintained at a temperature of at least 1300 ° C for at least 30 minutes. The advantages of the invention include an economical process to produce valuable metals, which can be used to form an alloy with the molten iron, partially reducing a metal oxide "in less than 30 minutes, achieving at least 40% chrome metallization. and 70% iron metallization by heating in a furnace with an oxidation flame, partially reducing an agglomerate of metal oxide without requiring the agglomerate to have high strength and preventing the reoxidation of the reduced metal oxide while in the furnace. rotating square. The above objects and other objects, features and advantages of the invention will become apparent from the consideration of the detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a diagram illustrating one embodiment of the process of the invention, FIGURE 2 is a diagram of a rotary square oven, used in the process of FIGURE 1 of the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY The invention relates to a method of reducing at least partially a metal bound to oxygen to a metal in a furnace heated with an oxidation flame. An essential feature of this invention includes protecting the metal with a cover or blanket of a carbonaceous material to avoid reoxidation, while being exposed to the oxidation flame. Another important feature of the invention includes heating the furnace having a platform adapted for rotation, while supporting the metal bonded to oxygen. The invention includes providing an oven, heated by an oxidation flame, having an annular platform and at least one fuel burner, adapted to be mounted in a position above the platform. A first layer of a mixture of a metal bonded to oxygen, i.e., metal oxide, and a reducer, is placed on a top surface of the platform. The metal oxide and the reductant can be in the form of powder, agglomerate, sintered, compacts, granules or a mixture thereof. After the mixture passes through a heating zone and is heated to at least about 1000 ° C, preferably to at least 1100 ° C, more preferably to at least 1200 ° C, a second layer of carbonaceous reducer is placed on the first layer. Although the second layer could be placed on the first layer before the first layer is heated, it is advantageous to retard the loading of the second layer until after the first layer reaches the metallization temperature. If the second layer is loaded before insufficient heating of the first layer, the second layer can be isolated, causing inefficient heating by the rotary kiln. By slowing the loading of the second layer, the heating times required to bring the metal oxide to the metallization temperature and to reduce the oxide are minimized. The platform is continuously rotated at a controlled rate that passes to the burner, in which the mixture remains stationary on the upper surface of the platform. The mixture is heated by the radiation of an oxidation flame emanating from the burner at sufficient temperature for a sufficient time to reduce at least partially the metal oxide to a metal and / or a carbide in the first layer, with excess remaining carbon. The second layer prevents reoxidation of the partially reduced metal oxide by oxidation gases leaving the flame until the reduced metal oxide is removed from the furnace. The partially reduced mixture can be used as the feedstock to form the alloy with the molten iron in a melting furnace or a refining vessel. The reason for heating the furnace with an oxidation flame instead of a reduction flame4 is because a higher temperature can be more efficiently achieved from an oxidation flame. The metal bonded to the oxygen of the invention is defined to include any one of several oxide minerals or concentrates formed of nickel ores, such as laterite or garnierite ores of low sulfur content, chromite or concentrates, iron ores, minerals of pulverized metal, particles of the fumes of stainless steel and their mixtures. Chromium bound to oxygen is especially desirable and occurs in chromite metal or concentrated in the form of spinel with other oxides, or in particles of stainless steel fumes such as CrOz, z = 1.0, 1.5 and / or 3. The spinel Chromite can be represented stoichiometrically as (Mg, Fe) (Cr, Al, Fe) 2O4 »where the ratio of Mg ++ to Fe ++ and the relative proportions of Cr + 3, Al + 3 and Fe + 3 to each other, are variable, depending on the source of the mineral. Generally, the ratio of total Cr to Fe (Fe +++ Fe +++) total is in the range of approximately 1 to 3, depending on the mineral source. During the reduction of the chromite spinel with carbon, small amounts of the metals Fe and Cr are precipitated first, forming the Fe3C and (Fe, Cr) 3C carbides in contact with the carbon. The initial reduction of ferrous ions is generally faster than that of ferric ions, which results in a low initial Cr / Fe ratio in the metal that precipitates. As the ratio of Cr / Fe in the reduced metal increases with time, the metal reacts with excess carbon to form the carbide (Cr, Fe) 7C3, according to the dictations of the Fe-Cr- phase diagram. C. During the reaction, the oxide MgAl204 initially present in the spinel remains in the same oxidation state. In the case of chromite reduction,% chromium metallization and% iron metallization can be understood in terms of the ratio of oxygen removed to total oxygen in the chromium bound to oxygen and the ratio of oxygen removed to total oxygen in iron bound to oxygen, respectively. The portion of the chromite spinel that participates in the metallization process can be represented by the stoichiometric formula FeO (CrFew) 03, where a «r * is the ratio of Fe +" + to Cr +++ .The metallization process can be represented by the reaction: FeO (CrFew) 03 + (x + y) C = F? O? .x (CrF? W)? 3.y + (x + y) CO, where x and y represent the proportions of oxygen removed to reducible ferrous ions (Fe ++) and to the reducible iron ions (Cr +++ and Fe + "+), respectively, before the reduction to 0% metallization begins, x = y = 0; Metallization% x = 1 ey = 3. Chromium metallization percentage is defined as chromium without oxygen, total in relation to total chromium, and% metallization of iron is defined as iron without total oxygen in relation to The total iron in the chromite, all the species were determined by chemical analysis.Oxygen-free or oxygen-free means the metals Cr and Fe as well as Cr and Fe combined as a carbide by the following reactions between the metal that precipitates and Coal in excess of that required for the metallization process: 3 (Fe, Cr) + C = (F?, Cr) 3C 7 (Cr, Fe) + 3C = (Cr, F?) TC3 The high sulfur content nickel oxide concentrate, which can be added to the chromite-carbon mixture in the invention, includes those having an S / Ni ratio of less than 1.0, Cu / Ni and lower Co / Ni. of 0.1 and the rest is iron oxide and residual amounts of MgO, Si203, A1203 and CaO. If the metal oxide is laterite nickel mineral or concentrate of this mineral, the nickel oxide will generally be hydrated in the form of (Fe, Ni) OOH. Due to the contact with the coal at high temperature, the "OH" would be released as hydrogen and carbon monoxide, the remaining nickel oxide (NiO) and iron oxide (FeO) would be partially reduced to metallic Ni and Fe. In case nickel laterite undergoes reduction, the% nickel metallization is defined as nickel without oxygen, total, in relation to total nickel according to the following reaction: (F? Nip) OOH + 2C = 1 / (1 + p) F? + p / (1 + p) Ni + 2C0 + 1 / 2H2, where p is the ratio of Ni to Fe in nickel laterite. If the origin of the metal oxide is particles of the fumes, the oxide can be in the form of MeO (where y = 1, 1.5 or 3 for Me = Cr; y = L for Me = Ni and y = 1.05, 1.33 or 1.5 for Me = Fe), all of which are partially reduced directly to metal and possibly to a metallic carbide. For example, in the case of stainless steel fume particles, the chromium oxide may be in the form of CrO, Cr203 or Cr03. The oval particles could be reduced by carbon directly to metallic Cr and Fe (Fe, Cr) 3C and (Cr, Fe) 7C3. The extent of the carbide formation depends mainly on the Cr / Fe ratio and the amount of carbon present in the particles. Stainless steel fume particles are understood to include dust, fine particles or sediment from stainless steel manufacturing furnaces, such as electric arc furnaces and refining vessels. If the metal oxide is in the form of lump ore, the ore could preferably be pulverized and then agglomerated into granules, sintered, compact forms and the like. The powder preferably would be agglomerated in a similar manner. Preferably, the reducer could also be pulverized and mixed with the pulverized ore or powder prior to agglomeration, to increase the rate of reduction by the carbon during heating in the rotary kiln. Suitable carbonaceous reductants include coke, pulverized coke, petroleum coke, charcoal, graphite and bituminous carbons, with low content and average content of volatile materials. The metal oxide is at least partially reduced in a rotary square oven and can be further reduced in a vessel for manufacturing ferrous base material such as Fe-Cr, Fe-Ni, Fe-Cr-Ni, alloy steel, stainless steel and similar. A furnace suitable for use in the invention is a continuous reheating furnace, having a circular inner wall circumscribed by a separate outer wall having a space therebetween. The space between the walls includes an annular platform or square of carbon adapted for rotation in relation to the burner. This type of oven is commonly mentioned as a rotary square oven, that is, RHF. The metal oxide to be reduced is usually loaded, dropped or extended to a controlled depth through the width of the coal square by a conveyor or a slide. After complete rotation of the platform, the reduced metal oxide is normally continuously removed by a suitable means such as a discharge screw. In a preferred embodiment, when the metal oxide is to be reduced is a chromite mineral, it has been determined that at least 40% of chromium metallization bound to oxygen and 70% of metallization of iron bound to oxygen in chromite can be achieved by pre-reducing the metal oxide with a carbonaceous reductant before final refining in a vessel containing molten iron. The amount of carbon fixed in the carbonaceous reducer mixed with the mineral or chromite concentrate, must at least be the same as the stoichiometrically required to theoretically reduce all chromium and iron bound to 100% metallization oxygen. The preferred amount of the fixed coal is 10-25% of the total mixture, preferably 15-20% and more preferably 20-25%, since a higher percentage of fixed coal can ensure a high probability of contact of the chromite and carbon grains, which facilitates the reduction of kinetics. Together with the fixed% carbon, the metallization rate depends on the grain mesh size, time and temperature. The degree of metallization of the chromium of 40% can be achieved by spraying the chromite mineral to a grain size of at least -200 mesh and the reducer to a grain size of at least -200 mesh, heating the mixture preferably to 1350 ° C. Preferably, the carbon mesh size should be -325. In the metallization of chromium and iron, the carbon present in excess of that required for the reduction, can carburize the metals as they are reduced to (Cr, Fe) 3C and (Cr, Fe) 7C3 insitu. Due to the complexity of the syn thetic processes that have different reaction rates for chromium bound to oxygen, than for iron bound to oxygen, the theoretical maximum metallization will not be reached for each at the same time. Almost complete metallization of the iron, but only an average metallization of the chromium, can be achieved by the processes to be economical in a reasonable amount of time, in the order of one hour. The extent of the metallization of difficult to reduce metal oxides, such as the chromite mineral, depends mainly on the reduction temperature of the metal oxide, the time at this temperature, the amount of the carbonaceous reducer available to reduce the oxide, metal and the grain sizes of the metal oxide and the carbon material. For the chromite or CrO (y = 1, 1.5 or 3), the chromium bound to the oxygen must be reduced to at least 40% metallization, which forms an amount of at least 5.0% by weight of the final reduced mixture. To achieve this metallization, the temperature of the chromite must be at least 1350 ° C for at least 30 minutes; the% of fixed carbon in the mixture should be 20-25% by weight and the mesh sizes of chromite and charcoal should be at least -200. The metals bound to oxygen of the mixture of the invention will be at least partially reduced in the RHF. Being partially reduced, it will be understood that it means a portion of the oxygen in the oxides bound to chromium and bound to iron in chromite or in the steel-making particles, has been removed by carbon as CO, leaving the metallic chromium, the iron and the carbides of them. The other portion of the oxides of the non-reduced mixture will remain as chromite, particles, chromium oxide, iron oxide and oxides attached to CaO, SiO2, MgO and Al20. For example, chromium bound to oxygen in chromite that is metallized to the extent of at least 40% means that 40% or more of the chromium bound to oxygen is reduced to chromium or chromium carbide. 'FIGURE 1 is a diagram illustrating one embodiment of the process of the invention. The number 10 generally refers to a sprayer, which can be used to grind a metal oxide such as a mineral to a powder. If sprayed, the metal oxide could preferably be mixed and generally agglomerated, sintered or compacted such as by a powder ore agglomeration machine 12. Of course, the metal oxide may not be agglomerated but mixed with the pulverized reducer. If they agglomerate as granules or do not agglomerate, the mixture would be fed in a RHF 14 during at least the partial metallization. The metal can be used as a feedstock in a refining vessel. Depending on the residual oxide, the metal can be fed into the container containing a molten bath, such as an electric arc furnace (EAF) 16 having an electrode 20 and / or a converter 18, such as an oxygen and argon decarburizer. (AOD) or a top and bottom blowing refining reactor (TBRR). Each of AOD and TBRR can be provided with an upper oxygen blowing lance 22, lower gas agitation nozzles 23, an iron bath 28 and a slag layer 30. When the metal is to be used as a feedstock for an AOD or TBRR, it may be advantageous to include desicating agents in the pulverized mixture, when the metal oxide is mineral chromite to preheat the deslagging agents in the RHF for the container of refining. Acceptable desicating agents include CaCo3, CaO, MgO, A1203, Si02 and CaF2. FIGURE 2 is an enlarged top view of the RHF 14 of FIGURE 1 of the invention. The furnace 14 includes a refractory, interior, annular wall 32, a refractory, exterior, annular wall 34, an annular refractory platform 33, between the walls 32 and 34, a plurality of fuel burners 36 placed on the periphery of the platform such form that is mounted on the outer wall 34 with a nozzle 38 in each burner extending through the openings in the wall in the positions just above the upper surface of the platforms 33, the fuel burners 40 on a annular structure 42 in interior positions of the platform 33, feeders 44 and 45, and a means 46, such as a propeller 48, for removing the reduced metal oxide from the platform 33. The platform 33 is rotated to pass the burners at controlled speed in a direction indicated by arrow 50. Exhaust gases from fuel combustion and vaporization of any dissociated sulfur during oxide reduction of metal, accompanied with the sulfur-bearing concentrate are evacuated through a vent hole 52. The residual energy of these exhaust gases can be reused in a heat recovery system (not shown) to preheat the fuel and air . The burners 36, 40 are located at a distance above the upper surface of the platform 33. The RHF preferably includes several burners mounted to one of the annular walls of the furnace, with the burners that are aligned in a direction towards the upper surface of the platform, in such a way that the radiation of the flames from the burners, can be transferred directly to the surface of the metal oxide mixture and the reducer deposited on the upper surface of the platform. After the metal oxide mixture is placed on it, the platform is rotated to pass to the burners. In this way, the platform works as a heat exchanger against flow, but with the transfer of heat mainly by radiation to the surface of the mixture and by conduction within the layer of the mixture. Unlike a furnace furnace in which the heating gases move vertically upwards through a column of ore, the gas flow in the RHF differs in that a thin layer of the metal oxide / reducing mixture that It will be reduced and rotated so that a flame of oxidation passes, where the heat is transferred to the metal oxide mainly by radiation from the flame and from the inside wall of the furnace and by conduction inside the mixing layer. That is, the metal oxide to be reduced rotates countercurrent with respect to the gas flame at a controlled rate. High temperatures are quickly reached, due to the high surface area and shallow depth, promoting the rapid reaction kinetics with the material that only needs contact with the reducer and to be exposed to the flame for less than an hour to reduce the oxide. metal The inherent disadvantages associated with a vertical shaft furnace, such as tackiness problems and material hardness are avoided with an RHF. Since only a thin layer of the metal oxide extends over the turntable and does not have to support a large load mass as in the case of a vertical shaft furnace or withstand the impact as in a rotary kiln, the agglomerate does not need have structural integrity. This is especially important when the metal oxide is pulverized, mixed with other materials, for example, charcoal and sulfur-bearing nickel concentrates and formed into granules. The nickel and iron sulphides are completely melted at 1200 ° C. The presence of these sulfides with a granule containing the mineral chromite will not harm the operation of the RHF, since the strength of the granule would not be a problem. The feeder 44 can be used to deposit a mixture of metallic miner L and a carbonaceous reducer in a continuous first layer, having a controlled, light depth across the width of the platform 33 with the layer having a uniform thickness. The thickness of the first layer should not be greater than 40 mm, preferably not greater than 35 mm and more preferably not greater than 30 mm. If the metal oxide and the reducer are pulverized and compacted into granules having a diameter of approximately 10 mm, the compact materials can be placed on the platform two to three granules deep, ie 25 mm. The thickness of the layer should not be greater than 40 mm, because the time required to reach the uniform temperature within the mixing layer increases exponentially with the thickness, requiring much longer times for an equivalent degree of metallization. Meanwhile, the heat must be supplied continuously to compensate for heat losses and to meet the heat requirements for the endothermic metalization reaction with the carbon. After the first layer has been heated preferably to a temperature of at least 1000 ° C, more preferably 1200 ° C, a second continuous layer having a uniform thickness of carbonaceous reductant is deposited on the first layer at a depth of not more than about 10 mm by the feeder 45 across the width of the platform 33. Preferably the second layer should cover the first layer and have a minimum depth of at least 2 mm, preferably at least 3 mm. The second layer should have a depth of 2-3 mm to provide enough carbon to reduce any of the oxidizing gases, for example C02, H20, of the flame that reaches the vicinity of the upper surface of the mixture, but not as deep as to be thermally insulating. The carbonaceous reducing layer should not exceed about 10 mm to allow adequate heat transfer by conduction to the underlying mixture layer that supports metallization. Preferably, the ratio of the depth of the second layer to the depth of the first layer should be controlled within the range of 0.05 to 0.3. It may be advantageous to include minerals of different type in the mixture, separated or in combination, during at least the partial reduction in the RHF. In the case when the reduced metal is used as a feedstock for the manufacture of nickel-containing stainless steel, the mixture may advantageously include chromium oxide and sulfur-bearing nickel concentrate. If the sulfur-containing nickel concentrate is used, the nickel and iron sulfides will begin to melt and the liquid sulfur will partially dissociate as gaseous sulfur in the furnace atmosphere long before the reduction of the chromium oxide begins. Sulfides can start to melt, when the temperature of the layers is as low as 640 ° C. When the nickel laterite is included in the mixture, the chromite is preferably not added to the mixture, because the metal product of the RHF must first be transferred to an EAF to melt and desscorify and the slag would contain a high level of chromium oxide, resulting in low Cr yields. The partially reduced metal product of the RHF would contain at least 1.0% by weight of Ni. The reaction products of nickel laterite ore or nickel laterite concentrate in an RHF are Ni, Fe, Fe C and remaining oxides MeO? where y = 1 for Me = Ni and y = 1.05, 1.33 or 1.5 for Me = Fe.

Example 1 Now the process to reduce a metal oxide will be described. A mineral containing chromium and charcoal can be sprayed separately, mixed together in a powder and compacted into low-strength granules. The dry mix would contain about 74% by weight of mineral or chromite concentrate, 25% by weight of fixed carbon and 1% by weight of bentonite binder. Fixed carbon means the remaining carbon in the charcoal after the coal contained in the volatile matter has been eliminated. The particle sizes of chromite and carbon must be between 200-325 mesh (44-74 μm). The mixture is then formed into granules having a diameter of approximately 10 mm. The granules should be placed completely across the width of the turntable as a first layer of two to three granules deep to a depth of approximately 25 mm. A second layer of granular carbon having a particle size within the range of 10-100 mesh (0.15-2.0 mm) is then deposited on the first layer at a depth of 3 mm after the first layer is heated to a temperature of 1200 ° C. The total depth of the two layers would be approximately 30 mm. The ratio of the depth of the second layer to the depth of the first layer would be approximately 0.1. To reduce the chromite mineral, the first layer preferably must be heated to a temperature of at least 1300 ° C, more preferably at least 1350 ° C for at least 30 minutes to achieve at least 40% metallization of chromium bound to oxygen in chromium to chromium metal and a chromium carbide and 70% of iron metallization bonded to oxygen in chromite to Fe as iron and iron carbide. The first layer should not be heated to a temperature higher than 1450 ° C, however, due to the significant melting of the product, metallized, for example, the carbide causes the metallized layer to be difficult to be discharged from the furnace platform of rotating square. In this example, the layer of the mixture could be preheated by an oxidation flame at a temperature of 1200 ° C in 10-15 minutes. Then, the carbon layer is added to the upper surface of the hot metal oxide layer. Then, the layers on the platform would continue to be heated by radiation from the oxidation flame for an additional 30 minutes during which time the metallization of the first metal oxide layer would occur. After metallization for 30 minutes, the mixture could be allowed to cool to about 200 ° C or be discharged hot, eg, 1000-1300 ° C, from furnace 14 by helix 48, both under a protective atmosphere of inert gas . If hot discharge from the RHF, the partially metallized mixture would be charged into a refining vessel such as an AOD or TBRR. The second carbonaceous layer protects metallized chromium and iron in any zone of the mixture that is depleted of coal, from which they become reoxidized by the products of combustion of the oxidation flame inside the furnace. However, any of the oxidation gases that try to penetrate the second layer, the gases are regenerated in reducing agents, for example, CO and H2, by the carbon in the second layer. Partially reduced chromite consisting of Cr and Fe metal, Cr and Fe, Cr and Fe carbides bonded to oxygen, unreduced and the stable oxides Si02, MgO, Al203 and CaO can be charged in the iron bath 24 covered by a slag 26 in the furnace 16 or an iron bath 28 covered by a slag 30 in the refining vessel 18, whereby inexpensive Cr units are supplied to make stainless steel.

Example 2 Now the process to reduce another metal oxide will be described. The nickel laterite mineral with low sulfur content contains approximately 1-3% by weight of NiO, 15-20% by weight of Fe203, 30-40 by weight of SiO2, 15-30% by weight of MgO, up to. 40% by weight of water immobilized as [(Fe, Ni) OOH] and small amounts of Al203 and Cr203. The laterite and coal can be pulverized into powders, mixed together and compacted into low strength granules as described in Example 1. The dry mix can contain 74 parts of laterite mineral and 25 parts of fixed carbon and 1 part of bentonite. The granules can be placed completely across the width of the turntable as a first layer at a depth of 25 mm.

After the pre-heating of the first granule layer at about 1200 ° C by an oxidation flame, the granulated carbon having a particle size within the range of 10 to 100 mesh could be deposited on the first layer at a depth of 2-3 mm with the ratio of the depth of the second layer to the depth of the first layer which is approximately 0.1. The granules could continue to be heated to maintain the temperature of 1200 ° C for an additional 30 minutes. Then, reduced nickel oxide granules could be allowed to cool to 200 ° C prior to discharge from the furnace or charged at a temperature directly to an iron bath in an EAF for the deslagging of the undesirable oxide constituents. By doing so, the reduced nickel mineral granules could be used to provide Ni and Fe units to make steel by forming a nickel alloy, such as AISI 304 stainless steel.

Example 3 In another example, a powder with chromium oxide-containing particles of an EAF or an AOD used for the manufacture of stainless steel could be partially reduced in an RHF. Such powder would normally have a particle size of -325 mesh and could contain 12-22% by weight of Cr203, 30-60% by weight of Fe2 ° 3 and the remainder essentially CaO, Si02, Al203 and MgO. The powder may also contain small amounts of heavy metals such as ZnO and PbO. The powder can be mixed with at least 15% by weight of pulverized carbon fixed coal and some pulverized chromite, compacted into low strength granules and deposited as a first layer at a depth of 25 mm, similar to that described in the Example 1. After pre-heating the layer to about 1200 ° C by an oxidation flame in about 10-15 minutes, the granulated carbon having a particle size within the range of 10 to 100 mesh could be deposited on the First layer at a depth of 2-3 mm with the ratio of the depth of the second layer to the depth of the first layer which is approximately 0.1. The granules could continue heating at a temperature of 1350 ° C and kept at this temperature for an additional 30 minutes. Then, the partially reduced mixture consisting of Cr and Fe metal, Cr and Fe carbides, any Cr and Fe bound to unreduced oxygen, remaining and the stable oxides of SiO2, MgO, Al203 and CaO could be allowed to cool to 200 ° C or be hot discharged from the furnace at a temperature of 1000 -1300 ° C both under a protective atmosphere of inert gas to an iron bath contained in an AOD or TBRR for melting and refining, so it captures the valuable Cr units . Any excess dissolved carbon in the bath would be removed by blowing oxygen either into the AOD or TBRR refining vessels. In this way, a steel fabrication powder containing chromium oxide could be used to supply cheap Cr and Fe units for the manufacture of stainless steel.

Example 4 Another example of how a mixture of chromite mineral and sulfur-containing nickel concentrate can be processed in an RHF will be described. The chromite mineral, the sulfur-containing nickel concentrate and coal could be pulverized into powders, mixed together and then compacted into low strength granules and deposited as a first layer at a depth of 25 mm, similar to that described in Example 1. After preheating the granule layer to approximately 1200 ° C by an oxidation flame in approximately 10-15 minutes, the granulated carbon having a particle size within the range of 10 to 100 mesh would be deposited on the first layer at a depth of 2-3 mm with the ratio of the depth of the second layer to the depth of the first layer which is approximately 0.1. The granules would continue to be heated to a temperature of 1350 ° C and maintained at this temperature for an additional 30 minutes. Partial desulfurization can occur by. dissociation in gaseous sulfur to the atmosphere of the RHF. The gaseous sulfur will be oxidized to sulfur dioxide as soon as it passes through the second layer in the atmosphere of the oxidation furnace. The reaction products of the heating of the sulfur-bearing nickel concentrate and the chromite mineral in the RHF would be Ni metallic, unreacted nickel sulphide, Cr metal and Fe metal, Cr and Fe and Cr and Fe carbons bound to unreduced oxygen when the fixed charcoal carbon mixed with the nickel concentrate mixture, which carries sulfur and chromite mineral at less equal to 10% by weight of the mixture. In this example, the sulfur-bearing nickel concentrate and the chromite mineral could be desulfurized and partially reduced, respectively in an RHF and then charged in an iron bath contained in an AOD for the fusion and refining of the metallized chromite and the Ni annex to provide Cr, Ni and Fe units to manufacture a steel forming alloy with nickel and chromium such as stainless steels AISI 304, 12 SR and 18 SR. The final fusion of chromium bound to oxygen, could occur mainly by carbon, for example (Fe, Cr) 3C and (Cr, Fe) 7C3, and excess carbon, transported to, and dissolved in the bath and, in the form secondary for any other of the reductores dissolved in the bath, such as silicon or aluminum. Any residual carbon that remains after this reduction period, would be removed from the iron bath by oxygen scavenging through the lance. The remaining sulfur in the iron bath that accompanies the metallized granules would be eliminated by the refining slag that covers the bath during the reduction period, when an inert gas such as argon of high purity, for example, 99.998% by volume, It is injected through the nozzles. At the time of filing this patent application, the market cost situation per kg for chromium in the form of ferrochrome and nickel in the form of ferronickel or coarse nickel shot was approximately $ 1.50 and $ 8 respectively. The cost per kg of chromium and nickel in the form of the feed charge produced in Examples 1-5, will be as little as approximately $ 1.20 and $ 6, respectively. An advantage of this invention is the potential realization of cost savings of approximately $ 0.30 or more per kilogram for the Cr feed material when fabricating ferritic stainless steel and cost savings of approximately $ 2 or more per kilogram for the material of Cr and Ni feed, when austenitic stainless steel is manufactured. It will be understood that various modifications can be made to the invention, without departing from the spirit and scope thereof. Therefore, the limits of the invention should be determined from the appended claims. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims

CLAIMS * 1. A metal oxide reduction method, characterized in that it comprises: providing a furnace having an annular platform and at least one fuel burner, placing a first layer of a mixture containing oxygen-bonded metal and a reducing agent on an upper surface of the platform, rotate the platform so that it passes to the burner to heat the first layer with an oxidation flame, load a second layer of the reducer covering the first layer, continue heating The layers at sufficient temperature and for a sufficient time to form at least a partially reduced mixture, whereby the second layer prevents re-oxidation of the partially reduced mixture within the furnace.
2. The method in accordance with the claim 1, characterized in that the mixture is heated to at least 1000 ° C before it is covered with the second layer.
3. The method according to claim 1, characterized in that the metal bonded to the oxygen is from the group of mineral chromite, laterite mineral, mineral garnierite, a concentrate produced from chromite mineral and particles of the stainless steel fumes.
4. The method according to claim 1, characterized in that the mixture is sprayed with the metal bonded to the oxygen and the reductant, each having a grain size of at least -200 mesh, the powder mixture is compacted into granules.
5. The method according to claim 1, characterized in that the mixture contains at least 10% by weight of fixed carbon.
6. The method of comfort with the claim 1, characterized in that the partially reduced mixture contains at least 5.0% by weight of chromium as metal or as chromium carbide.
7. The method in accordance with the claim 1, characterized in that the metal bound to oxygen is chromium, the partially reduced mixture having at least 40% of the chromium bound to oxygen, reduced to chromium or chromium carbide and at least 70% of iron bound to reduced oxygen to iron or iron carbide.
8. The method according to claim 1, characterized in that the mixture contains the mineral chromite, a reductant and a deslagging agent from the group consisting of CaCO3, CaO, MgO, Al203, Si02 and CaF2.
9. The method according to claim 1, characterized in that the mixture contains chromite mineral and sulfur-bearing nickel concentrate.
10. The method in accordance with the claim 9, characterized in that the partially reduced mixture contains at least 5% by weight of chromium as metal or as chromium carbide and at least 0.1% by weight of nickel as metal or as nickel sulphide.
11. The method according to claim 1, characterized in that the partially reduced mixture contains at least 1% by weight of nickel as metal.
12. The method in accordance with the claim 1, characterized in that the mixture contains at least a stoichiometric amount of the reductant required to theoretically reduce all the metal bound to the oxygen in the mixture.
13. The method according to claim 1, characterized in that the first layer is no more than 40 mm deep.
14. The method in accordance with the claim 1, characterized in that the second layer is no more than 10 mm deep.
15. The method according to claim 1, characterized in that the ratio of the depth of the second layer to the depth of the first layer is at least 0.05.
16. The method according to claim 1, characterized in that the ratio of the depth of the second layer to the depth of the first layer is 0.05-0.3.
17. The method according to claim 2, characterized in that the mixture is heated to a temperature of at least 1300 ° C and tained at this temperature for at least 30 additional minutes.
18. The method according to claim 1, characterized in that the metal bonded to the oxygen includes chromite mineral and the additional steps of feeding the partially reduced mixture into an iron bath contained in a refining vessel, reducing the chromium bound to the oxygen to chromium or chromium carbide and blowing oxygen into the iron bath to remove the excess carbon to form a stainless steel.
19. A method for reducing metal oxide, characterized in that it comprises: providing a furnace having an annular platform and at least one fuel burner, placing a first layer of a mixture containing oxygen-bonded metal and a reducer on an upper surface of the platform, rotate the platform so that it passes to the burner to heat the first layer with an oxidation flame at a temperature of at least 1000 ° C, load a second layer of the reducer covering the first layer, continue heating the layers for 30 seconds. additional minutes to at least partially reduce the metal bound to oxygen, the partially reduced mixture contains at least 1% metal as metal or as metal carbide, whereby the second layer prevents re-oxidation of the metal within the furnace.
20. A method of reducing metal oxide, characterized in that it comprises: providing a furnace having an annular platform and at least one fuel burner, placing a first layer of a mixture containing a reductant and chromium and iron attached to oxygen on a upper surface of the platform, rotate the platform to pass to the burner to heat the first layer with a flame of oxidation at a temperature of 1200 ° C, load a second layer of the reducer covering the first layer, continue heating the layers to at least 1300 ° C to partially reduce the chromium bound to oxygen, the partially reduced mixture containing at least 40% chromium or chromium carbide and at least 70% iron or iron carbide, so that the second layer avoid re-oxidation of chromium or iron inside the furnace, provide a refining vessel containing an iron bath, feed the partially reduced mixture into the ba ño, refining the bath until the chromium attached to oxygen and iron together with oxygen are reduced to chromium and iron so that a stainless steel is formed. SUMMARY OF THE INVENTION A method of reducing metal oxide in a rotary seat furnace (14), to manufacture a feed charge for a refining vessel when fabricating iron in alloy, alloy steel or stainless steel. The rotary square oven includes an interior, annular refractory wall (32), an exterior, annular refractory wall (34), a refractory platform (33), annular between the two walls. Stationary fuel burners (36, 40) are mounted to the walls (32, 34) at a position just above a top surface of the platform. A mixture of a metal oxide and a carbonaceous reducer is placed on the upper surface of the platform and rotated to pass the burners. The oxide is heated by an oxidation flame. Then, a second layer of a reducer is charged onto the first hot layer and both layers are heated for an additional period of time at a temperature of at least 1300 ° C to reduce the metal oxide. The metal oxide can be chromium ore, chromium ore concentrate, nickel ore, nickel ore concentrate and stainless steel fume particles. The reducer can be charcoal or coke. The reduced metals provide cheap metal units to form the alloy with the cast iron. .