Production of direct reduced iron in rotary hearth furnace with spiral shaped combustion chamber

Abstract

An apparatus and a method for producing direct reduced iron from iron oxide and carbonaceous material is described.

Description

PRODUCTION OF DIRECT REDUCED IRON IN ROTARY HEARTH FURNACE WITH SPIRAL SHAPED COMBUSTION CHAMBERCROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 568,396, filed March 21, 2024, which is incorporated herein by reference in its entirety.FIELD OF INVENTION

[0002] This invention generally relates to rotary hearth furnaces having a partition of the annular gas chamber.BACKGROUND

[0003] The steel industry is converting iron oxides into metallic iron in different processes. A common process is the blast furnace. Alternative processes are direct reduction processes producing DRI (direct reduced iron) or HBI (hot briquetted iron). DRI and HBI are important ingrediencies in the steel making processes based upon electric arc furnaces. Direct reduction processes may be gas based or solid carbonous material based. In gas-based processes, natural gas, coke oven gas, or hydrogen may be used as reducing agent. For processes with carbonous material, the reduction agent may be coal, petroleum coke, charcoal, or other materials having high level of carbon, such as some steel mill by-products, for example dust or sludge from the gas cleaning plant of the blast furnace. One option that is especially interesting is to produce green iron based on torrefied wood and / or biomass. For the processes with carbonous material, rotary hearth furnace (RHF) may be used. The rotary hearth furnace generally includes a rotating annular furnace bottom lined with refractory material on the top side and turns in an annular casing that may also be refractory lined, sometimes referred to as the gas chamber, and a plurality of burners being disposed on the side walls and on the top roof of the furnace.

[0004] Many conventional metallurgical processes, such as iron and steel making, may produce natural by-products and waste materials in the form of dust and sludge that may contain a high percentage of iron oxide. Due to its high monetary value, it may be desirable to reclaim the iron and or other metals in these materials for reuse; however, the presence of zinc and lead oxides or, in some instances, oil and / or grease, and other impurities, may make recycling the iron difficult and impractical. These materials may not be recovered by the conventional process of iron and steel making. It may be treated separately to recover iron in the form of DRI or HBI, on the one side and zinc, lead, and other metals on the other side, in a direct reduction process. This type of recycling may also be processed in a rotary hearth furnace.

[0005] The production of DRI based on iron ore and / or by-products may be accomplished through a reaction with carbon monoxide, hydrogen and / or solid carbon. Typically, iron oxideand a carbonaceous material, e.g., coal, may be charged into a furnace and heated. At higher temperatures (e.g., above 900°C) coal may react to produce carbon monoxide (C + CO? => 2 CO, this is known as the Boudouard reaction) which may react with the iron oxide (FejOa + 3 CO => 2 Fe + 3 CO2). In a process with carbonous material, the Boudouard reaction may generate more carbon monoxide than required to reduce the iron oxide to metallic iron. That excess carbon monoxide may be burned in the gas chambre above the hearth to generate the required heat for the process. The excess carbon monoxide may be higher towards the end of the reduction process. On the other hand, the required energy to generate the required heat may be more important at the beginning of the process. The carbon monoxide, carbon dioxide, and / or hydrogen gases may be referred to as process gas. A process of this kind may be carried out, for example, in a rotary hearth furnace.

[0006] The reduction process carried out in rotary hearth furnaces ("RHF") may be described in the following US patents Nos. 3,452,972, 3,770,417, 4,597,564, 4,701,214, 5,899,688, and 5,186,741 and in various publications. In most cases, the input material for the RHF may be ground to a very fine granulometry and subsequently may be formed to either pellets or briquettes. The furnace may be heated to temperatures of 1200-1400°C by burners installed in the roof or the side walls of the furnace. Pellets or briquettes may be charged onto the rotating hearth at one spot across the hearth and discharged after a little less than one rotation. The process gas may flow in counter-courant to the material and the excess carbon monoxide generated towards the end of the process may be burned in other areas of the furnace to generate the required heat. A problem with that kind of process may be the heat transfer because the input material may have a low thermal conductivity. Only the top of the material layer may be exposed to the heat generated by the combustion of the gas in the gas chamber, and because the heat transfer across the material layer may be very limited, the lower part of the layer may not reach the required temperature necessary for the chemical reactions described above. In addition, the heat transfer from the outside surface of the pellets or briquettes towards the center of those pellets or briquettes may be also very slow, so that the center part of those pellets or briquettes only reaches a low level of metallization. Low levels of metallization generally mean poor quality and low value of the DR1 or HB1.

[0007] In enhanced rotary hearth furnace operation, it has been tried to feed fine iron oxides mixed with carbonous material onto the hearth without agglomeration. In such cases, the material mix may be charged at one point on the inner or outer diameter of the furnace hearth. Fixed installed mixing devices, commonly called ploughs, may move the material either from the inner to the outer edge of the hearth or from the outer to the inner edge. The material movement basically describes the form of a spiral.

[0008] For the design described above, the beginning of the process may be either at the inner or the outer diameter of the hearth depending which side is the charging side. The metallizationof the iron oxide to metallic iron progresses across the hearth and reaches its maximum on the opposite side. The process gas still flows around the donut shaped gas chamber.

[0009] To operate such a process economically, it may be necessary to burn the process gas, which is mainly carbon monoxide, to generate the required heat. To achieve a good efficiency, it may be necessary that the process gas flows in opposite direction to the material. In the method described above, it may not be possible because the material moves across the hearth while the process gas moves around the circumference of the furnace. A possible solution may be described in international application number PCT / CA2011 / 000235.

[0010] Accordingly, more efficient and / or cost-effective apparatuses, systems, and methods for the production of direct reduced iron and recycling of iron and steel making by-products may be desirable.SUMMARY

[0011] According to various aspects, more efficient and / or cost-effective apparatuses, systems, and methods for the production of direct reduced iron are described.

[0012] The iron oxide may include iron ore and residues from iron and steel production. Fine iron oxide and carbonaceous material may be continuously fed onto the hearth of a rotary hearth furnace either together at one point on one side of the hearth or separately on multiple points. The material may be moved by sets of ploughs across the hearth from one side to the other at a temperature sufficient to cause the reducing agent to produce process gas containing carbon monoxide which may react with the iron oxide to form direct reduced iron that may be continuously discharged. Part of the process gas may be burned to generate the required temperature for the reactions. A subdividing wall in the gas chamber may guide the process gas containing the carbon monoxide in the opposite direction to the material on the hearth across the hearth of the furnace. The counter-flow of the gas may optimize the usage of the gas for reduction of iron oxide and for better efficiency of the heat usage generated by combustion of the process gas.

[0013] A method and apparatus for producing direct reduced iron from iron oxide and carbonaceous material is described. The iron oxide may comprise iron ore and / or residues from iron and steel production, such as broken pellets, mill scale, blast furnace dust and sludge, BOF dust and sludge, EAF dust a.s.o. A feed material comprising iron oxide and a carbonaceous material may be continuously fed onto one or several points mainly on the inner or the outer edge of the hearth of a rotary hearth furnace without previous agglomeration or with minimal agglomeration producing micro pellets. One or several sets of ploughs installed across the furnace hearth move the material outwards or inwards on the hearth basically in form of a spiral and will be discharged from the hearth with a last plough. To ensure that the temperatureis sufficient to cause reaction of the reducing agent and the iron oxide to form direct reduced iron, the process gas may be combusted in the chamber above the hearth. The chamber above the hearth may be subdivided in such a way that the flow of the process gas generated in the material layer may be controlled in such a way that the combustion of the said gas may take place where the heating energy is mostly required and / or desirable. To ensure an efficient usage of the process gas, the gas may flow in the opposite direction to the material. Vertical walls subdividing the chamber may be arranged in the shape that resembles the shape of a spiral. Burners and air injection nozzles installed in the roof of the furnace may be configured to allow specific heating, required and / or desirable for the process reactions, in the different zones of the furnace. Burners and air nozzles may also be used to inject combustion air to bum the process gas. Additional combustibles, such as natural gas or fine carbonous material, for example, may be added to the burners in some areas where more heat is required and / or desirable.DESCRIPTION OF THE DRAWINGS

[0014] The various embodiments described herein may be better understood by considering the following description in conjunction with the accompanying drawings.

[0015] FIG. 1 includes a schematic illustrating a rotary hearth furnace according to the present invention. A section of the furnace is cut out to better illustrate the layout with the ploughs and the subdivision of the gas chamber.

[0016] FIG. 2 includes a top view of the rotary hearth furnace shown in FIG. 1 with the roof of the furnace cut off.

[0017] FIG. 3 includes a cross-sectional view of one side of the rotary hearth furnace shown in FIG. 1.

[0018] FIG. 4 shows one option of the shape of the dividing wall of the gas chamber and the location of the charging funnel(s) and of the exhaust pipe(s) for a rotary hearth furnace according to the present invention.

[0019] FIG. 5 shows a horizontal section through 3 segments that form the vertical dividing wall in the gas chamber for a rotary hearth furnace according to the present invention.

[0020] FIG. 6 shows a vertical section through one element of the dividing wall for a rotary hearth furnace according to the present invention.DESCRIPTION

[0021] In the following description, certain details are set forth to provide a better understanding of various aspects of apparatuses, systems, and methods for the production of direct reduced iron. However, one skilled in the art will understand that these aspects may be practiced without these details and / or in the absence of any details not described herein. In other instances, well-known structures, methods, and / or techniques associated with methods of practicing the various aspects may not be shown or described in detail to avoid unnecessarily obscuring descriptions of other details of the various aspects.

[0022] The present invention may provide more cost-effective and / or efficient apparatuses, systems, and methods for the production of direct reduced iron, and / or recycling of iron and steel making by-products, and / or treating metal oxide fines to recover elemental iron from iron-bearing materials including iron-bearing ores, steel mill waste, and other metallurgical process waste.

[0023] The present invention may provide improved energy efficiency and a more efficient combustion of the process gas.

[0024] The present invention may provide an improved method to achieve more efficient production of direct reduced iron at moderate furnace temperatures relative to conventional methods. The present invention may provide more uniform heating of the various zones of the furnace.

[0025] The present invention may provide counter-courant travel of the feed material and process gases in the combustion chamber.

[0026] The present invention may provide a furnace having improved energy efficiency by using sensible heat and chemical energy of the process gas and other volatile matter to provide the energy / temperature to cause the direct reduction of iron oxide.

[0027] As described in more detail below, a rotary hearth furnace (RHF) according to the present invention generally comprising an annular combustion chamber subdivided by vertical walls to produce direct reduced iron is illustrated in FIGS. 1 and 2. The furnace may comprise burners, inlet(s) for feed materials, and outlet(s) for products and gases. Fine iron oxide materials, such as virgin iron ore, iron ore recycled from pellet plant or other iron ore processing plant and residues from the iron and steel making facilities, such as mills scale from rolling mills and dust or sludge from off-gas cleaning from blast furnace or from steel making, i.e., BOF shop or electric arc furnace and reducing agents, such as coal, charcoal, bituminous coal, coke fines or petroleum coke, may be fed to the furnace without previous agglomeration (because agglomeration may be equipment and labor intensive, and therefor very costly).Above listed materials may be fed through the inlet(s) and interact with one another in a reduction reaction in the combustion chamber before exiting the chamber through outlet(s) as direct reduced iron and gas by-products. The iron oxide materials, reducing agents, and other additives, such as lime or limestone may be provided onto the hearth by the same or different inlet(s). The inside of the combustion chamber may be lined with a refractory material, and / or the chamber walls may include an insulation material and / or cooling elements. The steel shell may be protected inside by a layer of insulation board, a layer of insulating bricks or castable and a layer of high temperature resistant bricks or castable. There may be cooling elements (cooled by air or water) added between the steel shell and the insulation bord. Also, there may be a fourth and fifth layer of refractory material to achieve a better insulation and or a better resistance to the deterioration by the aggressive components of the gas (i.e., FeO, S'). The feed materials placed onto the hearth may include iron oxide materials, reducing agents, and other additives. The feed materials, especially the carbonous material may be provided at various stages of the process through different charging funnels, located downstream of the main charging point. Coal with medium (14-22%) volatile matter content can be fed with the iron oxide. High volatile coal (28-36%, . bituminous coal or charcoal, that may have higher volatile matter of up to 50% may be fed at a later stage of the process in order to allow a more efficient combustion and usage of the calorific value of the volatile matter. The volatile matter escapes as soon as the coal reaches about 450°C. The off-gas exit may be located close to the charging funnel. If most of the volatile matter escapes close to the charging point, there may not be time enough to use the heating energy of the combusted volatiles to heat the cold material.

[0028] Some or all the process gas may flow from the side of the furnace where the material is discharged towards the opposite side. The rotary hearth furnace (RHF) according to the present invention may create counterflow of the process gas to the material to be able to use the calorific energy of the process gas to heat and keep the material at the required temperature. The process gas comprises a mix of combusted volatile matter from the coal and CO and CO2 generated by the process. As discussed above, the Boudouard reaction may generate carbon monoxide, which is mainly burned to carbon dioxide. The exit of the process gas may be close to the first charging funnel of material. The exit of the process gas may also be located in such a way that the process gas generated at the beginning of the process flows for a limited distance in the same direction as the feed material and may allow a more efficient combustion of that part of the process gas.

[0029] The reduction of iron oxide may be achieved through a reduction reaction of the iron oxide materials and the reducing agents. The iron oxide material may comprise iron ore and impurities or residue from iron and steel making, and other metallurgical processes. The impurities may comprise zinc, lead, or cadmium oxides. The iron oxide material may comprise small pellets, or fines. The fines may preferably have a particle size from 0.1-5 mm. The reducing agents may comprise solid carbon, such as coal, coke fines, charcoal, biomass, or petroleum coke, for example. Both the reducing agents and the additives shall have agranulometry of 0.2-30 mm. To produce one ton of DRI, about 1.5 ton of iron oxide is required and 350 kg to 600 kg of reducing agent. The additives are used to capture the sulfur that may be in the reducing agent. Because the difference of the sulfur content in the coal, the charcoal and the petroleum coke may vary from 0.1-3% such that the additives may need to be adjusted accordingly. Test in a laboratory furnace may define optimal compromises in granulometry, particle size distribution, purity, and other characteristics of the different materials available. The ranges of all those characteristics may be very wide and depend upon the availability in the region where the plant is located.

[0030] Referring to FIG. 1, FIG. 2. FIG. 3, FIG. 4 and FIG. 5, the present invention may generally comprise a rotary hearth furnace 1 to produce direct reduced iron comprising: a rotating hearth 2 within an annular combustion chamber 3, at least one inlet 15 disposed over the hearth to provide a feed material comprising iron oxide and a reducing agent. The material feed may be either on the outer or inner diameter of the hearth. At least one set of ploughs may move the material towards the opposite side of the hearth to be discharged after a mul ti tude of rotation. FIGS.1-5 show the option in which the feed material is charged onto the outer diameter of the hearth and discharged from the inner diameter. The feed material may also be charged at the inner diameter and discharged at the outer diameter. At least one set of ploughs mix, turn over and move the feed material towards the discharge side of the furnace. In case of multiple set of ploughs, one or more sets may turn over and move the material towards the opposite direction to increase the mixing of the material and to increase the residence time in the furnace, that may vary between 0.3-1.5 h. In general, the higher the temperature in the furnace the shorter the residence time. Depending on the available input material the temperature may be set to realize an optimum between energy consumption and productivity. The different sets of ploughs may mix the feed material, turn it over and, at the same time, move it from one side of the hearth to the opposite side. The annular combustion chamber 3 above the rotating hearth may be subdivided by a continuous vertical wall 7 attached to the roof and reaching down just above the material layer. The dividing wall may be configured to allow the process gas to flow in the opposite direction to the material, i.e., from the outside diameter to the inside diameter or in opposite direction.

[0031] The hearth 2 may comprise an annular hearth frame 8, a hearth heat insulating material and a plurality of refractory 9 arranged on the hearth frame 8.

[0032] The rotary hearth furnace 1 may comprise one or more sets of wheels 12 on tracks 11 to support the hearth 2 and enable rotation of the hearth 2. The tracks 11 may be attached to a bottom side of the hearth 2. The tracks 11 may travel on the wheels 12, and the hearth 2 may be rotated by a driving device (not shown) comprising an electric, a hydraulic, and / or a pneumatic motor. Alternatively, the wheels may be attached to the hearth frame 8 and travel on stationary tracks.

[0033] The rotary hearth furnace 1 may comprise a sealing system 10 to reduce and / or prevent gas from inside the combustion chamber 3 from escaping and / or air from outside the combustion chamber 3 from entering. The sealing system 10 may comprise an annular stationary chamber 30 attached to the furnace frame. That chamber 30 may be filled with an impermeable material 31, such as water or sand. An annular rotating portion 32 may be attached to the rotating hearth frame, wherein the rotating ring 32 is immerged into the impermeable material.

[0034] The rotary hearth furnace 1 may comprise one or more feeding conveyor 13 transporting the iron oxide and or the carbonous material to the inlet funnel(s) 15 of the furnace. A sluice system 14 may prevent process gas from escaping from the gas chamber or air penetrating into the furnace. The sluice system 14 may comprise a casing having two superposed gates in which a first gate is closed before the second gate opens. There may be room above both gates to hold material while that respective gate is closed. Alternatively, the sluice system 14 may comprise a rotary valve with a rotating part similar to a rotating door. For example, iron ore may be charged together with all or a portion of the carbonous material onto the hearth through funnel 15. A portion of the carbonous material may be charged through a second and third funnels 16. The rotating direction 44 of the hearth 2 may be clockwise or, as shown in FIG. 4, counterclockwise. The inlet funnel 15 may be located at any place on the charging side. The inlet funnel 15 may be closer or further from the process gas exhaust pipe 20, depending on the amount of volatile matter contained in the carbonous material. For higher content of volatile matter, the distance of the inlet funnel 15 to the process gas exhaust 20 may be increased.

[0035] The combustion chamber 3 may comprise an outer circumference wall 5, an inner circumference wall 4, and an annular roof 6. The combustion chamber 3 may comprise an annular shape having a U-shaped cross-section. The combustion chamber 3 may be lined with an insulation material and a refractory lining comprising one or more layers.

[0036] The rotary hearth furnace according to the present invention may improve the flow of the process gas relative to the material movement, by being counter-courant. The improved flow of process gas may be configured to allow a more controlled combustion, a more efficient usage of the energy of the process gas, and a better control of the temperature in the different zones of the furnace. The flow of the process gas is well controlled within the spiral shape of the combustion chamber. The rotary hearth furnace may comprise a plurality of independently controlled injection nozzles for combustion air located along the spiral. With the control of the amount of injected combustion air in the different zones, the temperature of each zone may be independently controlled.

[0037] The annular gas chamber may be subdivided by a vertical wall 7 that comprises a shape of a spiral. The spiral shape may be simplified by concentric rings that are interconnected asshown in FIG. 4. The vertical dividing wall 7 may comprise a plurality of segments 30. A section 3 of such segments 30 are shown on FIG 5. The ends of the segments may be configured so that parts of each end overlap. Each segment 30 may comprise high temperature refractory material. The segments 30 may also comprise at least one cooling pipe 31 covered with insulation material 32 and refractory material 33. The cooling pipe may have a U-shape or the shape of a multitude of interconnected Us or Cs. The cooling pipe may be attached to an upper flange 35 that supports the segment. The segments 30 may be supported by a flanged slot 34 attached to the shell plate of the furnace roof 6. The height of the segments may be adjustable, such as via shims 36 between the flanges on the roof 34 and the flange 35 supporting the segment 30.

[0038] The combustion chamber may comprise at least one process gas exhaust pipe 20. The process gas flows along the spiral-shaped chamber in counter-courant to the material. The exhaust pipe 20 can be installed at the end of the spiral-shaped chamber and the flow of gas and material are 100% counter-courant. As an alternative, the pipe 20 can be installed anywhere in the outer ring of the chamber. In that case there can be more time for the combustion of the volatile matter of the coal that is charged at the end of the chamber. In that design the process gas will flow in the same direction for part of the first ring of the spiral-shaped chamber, which, may allow a more efficient energy usage of the process gas. When the carbonous material is charged into the hot furnace, the volatile matter may escape as soon as the temperature of the coal reaches more than 400 or 450°C. To be able to efficiently burn the gas containing the volatile matter, the inlet funnel 15 and the exhaust pipe 20 may be configured such that the process gas has time to be combusted before it exits the furnace gas chamber through the exhaust pipe. To be able to better use the energy of the volatile matter, the carbonous material may be charged at one or more locations into the furnace. FIG. 4 shows possible positions 16 for additional inlet funnel(s) 15. The number and position of the inlet funnels and the exhaust pipe may depend upon the composition of the feed material. The choice may mainly be influenced by more or less volatile matter, faster or slower reacting carbon, and / or other factors that influence the combustion of the process gas. That way gas flowing in the same direction as the material for a quarter or halve a revolution may improve the usage of the energy of the process gas.

[0039] The rotary hearth furnace 1 may comprise a plurality of burners 18 disposed in the roof 6 to heat the combustion chamber 3. For the outer part and inner part of the gas chamber, some burners may be positioned at the inner and / or outer wall of the annular gas chamber. Because the energy requirements vary along the spiral-shaped combustion chamber because different stages of the process require more or less energy. To keep a more or less regular temperature chamber, the burners may provide adjusted energy in the different zones in the combustion chamber 3. The burners may be fired with combustible gases, liquids, and / or solids, such as, for example natural gas, synthetic gas, any fuel from petroleum or fine carbon. Additional combustion air or other gas comprising oxygen may be injected through the burners and / orthrough ports 19 adjacent to the burners to burn the process gas generated by the volatile matter of the carbonous material and the excess carbon monoxide generated by the Boudouard reaction in the material layer. The temperature of the combustion chamber 3 may be 900- 1400°C, typically between 1050-1250°C. The temperature at and close to the charging funnel 15 may be lower because of the energy required to heat the feed material. The burners and the combustion of the process gas may generate sufficient heat to cause the reduction reaction of the iron oxide and the reducing agent. The combustion air may or may not be preheated. Preheated combustion air already provides some of the required energy amount and sus reduces the consumption of fuel. A heat exchanger in the exhaust gas line may be used to preheat the combustion air.

[0040] The process gas may flow in counter-courant 43 to the rotation of the rotary hearth 44. The counter-flowing process gas may be used throughout the furnace as necessary or useful as explained above. By relocating the exhaust pipe to a location like the position 21, some of the process gas may flow in the direction of the rotating hearth and may improve the usage of the energy of the backflowing process gas as described above. There may be 2-12 burners per ring of the spiral-shaped chamber. In addition, there may be up to 20 injection nozzles per ring, such as 1-10, 11-20, or 1-20. The area around each burner with the adjacent nozzles may be defined as a different zone. The temperature in each zone may be independently measured and controlled by regulating the amount of injected oxygen through the burner and / or through the injection nozzles of that zone. The temperature in each zone may be set to be more or less equal to all of or an adjacent zone. It may also be adjusted differently to achieve the desired process reactions in a specific zone. The dividing wall 7 in combination with the location of the feed tunnel(s) 15 and in combination with the location of the exhaust pipe(s) may be configured to allow the combustion of the process gas to be controlled to achieve the optimum process efficiency. The counterflow of gas and material may be configured to allow combustion of the carbon monoxide gas generated by the process in areas where energy is needed. This control of the combustion of the process carbon monoxide gas may be configured to have a lower requirement of additional fuel, and thus, make the process more energy efficient. This control may provide a more efficient reaction process relative to conventional methods. As comparison, traditional RHF operated direct reduction rotary hearth furnace may, for example, require about 200 Nm3natural gas per ton of DRI as additional fuel. With efficient usage of the process gas the fuel consumption may be reduced to about zero. The off gas may leave the furnace through off gas pipes 20. The process gas funnel and pipe 20 may be cooled and / or refractory lined.

[0041] The rotary hearth furnace 1 may comprise at least one arm 23 disposed over the hearth 2. Each arm 23 may extend across the interior of the combustion chamber 3 and across a width of the hearth's 2 surface. The outer circumference wall 4 and the inner circumference wall 5 may support one or more of arms 23. FIG. 2 shows a rotary hearth furnace with eight of such arms23. The arms 23 may be radially spaced around the hearth 2. The plurality of arms 23 may or may not be evenly spaced around the hearth 2. A distance between the successive arms may increase, decrease, or be substantially the same.

[0042] The rotary hearth furnace 1 may comprise at least one set of ploughs 24 attached to the arm 23. The number of sets of ploughs 24 is equal to the number of arms 23. The ploughs 24 may extend towards the hearth's 2 surface. The ploughs 24 may be adjustable in height relative to the hearth's 2 surface by means of adjusting the height of the supporting arms 23, to adjust the thickness of material resting on the hearth's 2 surface, and / or ensure that the ploughs 24 do not interfere with the rotation of the hearth 2, due to thermal expansion.

[0043] The ploughs 24 may turn over and move the material 25 on the hearth 2 from one side of the hearth to the opposite side. After several rotations of the hearth 2, the material may be discharged from the hearth 2 by an extended plough 26. FIG. 4 shows an example of the material movement in a furnace with 4 sets of arms.

[0044] The ploughs 24 may be at an angle greater than 0° and less than 90°, to the arm 23 that is radial to the hearth surface to turn over and move the feed material in a sideways direction along the hearth. Typically, the angle of the plow to the arm shall be between 15° and 45°. The spacing between adjacent ploughs 24 may be the same or different. When a higher number of arms and sets of ploughs are used, one or more sets of ploughs may be angled in the opposite direction to move the material in the opposite direction. This may improve mixing and provide a more frequent turning over of the material on the hearth and at the same time extend the residence time of the material on the hearth. The ploughs may turn over, move, and mix the feeding material when the hearth rotates.

[0045] The arms and ploughs may be cooled by a coolant circulating therethrough. The coolant may comprise a gas or a liquid, such as water, or air and nitrogen. A mechanical pump may circulate the coolant through a pipe to flow through the arm.

[0046] The rotary hearth furnace 1 may comprise at least one stationary scraper (not shown) disposed on an inner edge of the hearth 2 and at least one stationary scraper disposed on an outer edge of the hearth 2. The scrapers may maintain the material on the hearth's surface and / or prevent or reduce the amount of material from falling off the hearth's surface.

[0047] The present invention may comprise a method for production of direct reduced iron in a rotary hearth furnace. A feed material comprising iron oxide may react with a carbonous material at a temperature of 900-1300°C to generate metallic iron. The carbonous material may comprise coal, charcoal, coke, petroleum coke, oil, grease, plastic waste, or wood, for example. At the same time, volatile components in the feed material, such as zinc, lead, chlorine, and alkalis, may be vaporized and evacuated with the off gas. Turning over, moving, and / or mixingthe feed material according to the present invention may improve the heat transfer and the generation of CO (the Boudouard reaction) in the material layer 25. As a result, the CO / CO2 ratio in the material layer may be sufficiently elevated to cause the reduction of the iron oxide by the CO. A CO concentration above 60% may be achieved in the layer of material. Any CO that escapes from the material layer may be combusted in the gas room above the feed material. The combustion of the CO generated by the process may produce sufficient heat to cause the reaction process. The burners and air injection nozzles may be inclined in such a way that the combustion of CO takes place in the upper part of the combustion chamber, thus far enough from the material layer, in order to prevent a reduction of the CO concentration in the material layer

[0048] A method to produce direct reduced iron may generally comprise providing an iron oxide material and a reducing agent charged into a rotary hearth furnace, moving the material by at least one set of ploughs and at the same time turn it over multiple times to evenly distribute heat from an outer surface of each piece of iron oxide material to its inner core and produce direct reduced iron. The process gas may flow in the combustion chamber in a direction opposed to a direction of material movement guided by the vertical dividing wall in the gas chamber. The method may comprise discharging the direct reduced iron from the rotating hearth after completing several revolutions of the rotating hearth.

[0049] The reducing agent may be mixed with the iron oxide material and may be fed together with the iron oxide material onto the hearth. The reducing agent may not be mixed with the iron oxide material and may be fed through a different inlet onto the rotating hearth. The feed material may travel in an opposed direction from the flow of the reducing gas.

[0050] The iron oxide material may be preheated separately from the reducing agent before being charged into the furnace. The sensible heat of the process gas exiting the furnace may be used to preheat the iron oxide material just before dropping onto the hearth.

[0051] The ploughs may extend outwardly from arms towards the hearth surface. A feed material comprising the iron oxide material may be provided onto the hearth surface and mixed with the reducing agent by the ploughs as the hearth rotates. Each set of ploughs may move the material in the same direction. Some sets of ploughs may turn over and move the feed material in the opposite direction as long as all the ploughs together keep moving the material in the desired direction. Moving the material in the opposite direction may extend the residence time of the material on the hearth. The present invention may be more efficient relative to conventional methods by repeatedly turning over and moving the feed material as the hearth rotates and thus improving heat transfer and reaction efficiency. The present invention may also be more efficient relative to conventional methods by and ensuring that the process gas flows in the opposite direction to the feed material. The metallization degree may be at least75%, 80%, 85%, 90%, 92%, 95%, 98%, or greater that 90%, such 95-98%. Degree of metallization of DRI may be the extent of conversion of iron oxide into metallic iron during reduction. It may be defined in percentage of the mass of metallic iron divided by the mass of total iron.

[0052] The method may comprise continuously introducing iron oxide containing material into the furnace, continuously rotating the hearth and having ploughs installed across the hearth to mix and turn over the material on the hearth, adding at least one reducing agent in form of a solid or liquid carbonous material to the iron oxide together with the iron oxide or through different feed funnels, mixing the reducing agent with the iron oxide by turning it over together with the iron oxide, forming a material mix mainly containing iron oxide with impurities and reducing agent, turning over the material mix several times in order to improve heat transfer and reaction efficiency; causing the process gas and feed material to travel counter currently across the hearth of the furnace, reacting the carbon dioxide, from by the reduction reaction of the iron oxide, with carbonous material to form carbon monoxide as reducing gas, reacting the reducing gas with the metal oxides to form directly reduced iron, feeding a gas containing oxygen through the roof or the inner or outer side wall of the furnace, burning the process gas with the oxygen to produce the required heat for the process, evaporating volatile metals, like zinc and lead, alkalis, chlorine and other impurities, withdrawing the volatile maters with the off gas from the top of the furnace, and recovering the directly reduced iron together with residues of the reducing agents after several rotations of the hearth.

[0053] The present invention may comprise a process in a rotary hearth furnace that includes mixing devices across the hearth. The feed material may comprise iron oxide and carbonous material. Unlike in conventional furnaces, the feed material may not need to be pelletized or briquetted. In conventional rotary hearth furnace direct reduction processes, the feed material may be charged all across the hearth at one point and discharged after a little less than one rotation. During the whole rotation, the material layer may not be moved. The feed material may be agglomerated to achieve a higher heat radiation. Agglomerating the feed material may be a cost intensive operation. According to the method according to the present invention, the material may be turned around multiple times such that the heat radiation contacts all or substantially all of the layer. The feed material may be charged onto the hearth at one or more points onto that hearth. A plurality of fixed arms across the hearth may each support a plurality of ploughs to mix the feed material while simultaneously pushing it slightly sideways. After several rotations, the direct reduced iron may be produced from the feed material and discharged from the hearth.

[0054] The feed material on the hearth may be continuously mixed and turned over, so that the heat transfer in the feed material is improved relative to conventional methods that do not mix and turn over material. The temperature in the furnace may be moderate (1000-1200°C) relative to conventional processes (1250-1350°C), or only slightly higher than the temperature requiredfor the process reactions, which is about 900°C. The continuous mixing and alternating turning provide the improved heat exchange to favor the process reactions, i.e., on one side the Boudouard reaction to generate carbon monoxide from carbon dioxide and carbon and on the other side the reaction of carbon monoxide with the metal oxide to generate metallic iron. Intensive mixing favors heat transfer and at the same time the intensity of the reactions through improved contact of the material with the produced gases. The result of the improved reactions may be the generation of a greater amount of carbon monoxide. The extra carbon monoxide may react with the injected oxygen containing gas and generate an effective amount of heat in the furnace. The dividing wall in the gas chamber may be configured such that the extra combustible gas may be burned in the zones of the furnace where the heat is required. The more efficient usage of process gas may reduce the external energy consumption of the process.

[0055] The apparatuses, systems, and methods of the present invention may achieve more efficient reduction of iron oxide relative to conventional techniques. The present invention may provide one or more of the following advantages: no preparation of feed material; high heat transfer through continuous mixing and turning of the feed material in the furnace; high efficiency of energy through controlled combustion of the process gas in different zones of the furnace; highly metallized iron may be produced at lower furnace temperature; and improved energy efficiency by controlling the flow of the process gas and controlling the combustion of the process gas in the different zones of the furnace and by using sensible heat to preheat combustion air and or iron oxide material.

[0056] Each of the characteristics and examples described above, and combinations thereof, may be said to be encompassed by the present invention. The present invention is thus drawn to the following non-limiting aspects:

[0057] Aspect (1) A rotary hearth furnace to produce direct reduced iron comprising: a rotary hearth within an annular subdivided combustion chamber; one or more feed funnels to provide a feed material comprising iron oxide and a reducing agent; and a plurality of sets of ploughs to turn over and move the feed material across the hearth.

[0058] Aspect (2) The rotary hearth furnace of aspect 1, wherein the gas chamber is subdivided by a vertical wall that ensures that the flow of the process gas is controlled and in counter- courant to the material flow.

[0059] Aspect (3) The rotary hearth furnace of aspects 1 and 2, wherein the subdividing wall in the gas chamber has the shape close to a spiral.

[0060] Aspect (4) The rotary hearth furnace of aspects 1-3, wherein the cross-section of the different zones of the gas chamber is increased in relationship to the volume of the process gas.

[0061] Aspect (5) The rotary hearth furnace of aspects 1-4, wherein the feed material is fed onto the hearth at one or several points and is turned over multiple times and moves across the hearth from one side to the other by a plurality, e.g., several sets, of fixed ploughs when the rotary hearth is rotating.

[0062] Aspect (6) The rotary hearth furnace of aspects 1-5, wherein the combustion chamber has a temperature from about 900°C to about 1300°C, such as about 900-1200°C, about 900- 1100°C, or about 900-1000°C.

[0063] Aspect (7) The rotary hearth furnace of aspects 1-6 characterized by a metallization degree greater than about 90%, such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%.

[0064] Aspect (8) A method to produce direct reduced iron using the rotary hearth furnace of aspects 1-7, wherein the rotary hearth is rotating within the subdivided annular combustion chamber, the method comprising: providing the feed material comprising iron oxide and a reducing agent onto the rotating hearth at a single point or at several points; turning over and moving the feed material gradually across; and controlling the gas flow in counter-courant to the material.

[0065] Aspect (9) The method of aspect 8 comprising displacing the feed material from one side of the hearth to the opposite side of the hearth with at least one, e.g., several sets of, plough(s).

[0066] Aspect (10) The method of aspects 8 and 9 comprising discharging the direct reduced iron from the rotating hearth after several revolutions of the rotating hearth by a last extended plough.

[0067] Aspect (11) The method of aspects 8-10 comprising flowing a process gas through the subdivided annular gas chamber from one side of the hearth to the opposite side in a direction opposed to the movement of the material on the hearth.

[0068] Aspect (12) The method of aspects 8-11, wherein the rotary hearth furnace consists of one circular rotating hearth.

[0069] Aspect (13) A rotary hearth furnace to produce direct reduced iron comprising: a rotary hearth within an annular combustion chamber; one or more inlet funnels to provide a feed material comprising iron oxide and a reducing agent; and a plurality of sets of ploughs to turn over and move the feed material from one side of the hearth to the opposite side; wherein the flow of the process gas is controlled by a subdividing wall in the annular gas chamber, and wherein the process gas flows in the opposite direction to the feed material across the hearth.

[0070] Aspect (14) The rotary hearth furnace of aspect 13, wherein the subdividing wall in the gas chamber has a shape of a spiral or a coil.

[0071] Aspect (15) The rotary hearth furnace of aspects 13 or 14, wherein the spiral shaped subdividing wall comprises a plurality of interconnected, concentric rings.

[0072] Aspect (16) The rotary hearth furnace of aspects 13-15, wherein the subdividing wall comprises a gap to the rotating hearth sufficient to allow material to pass.

[0073] Aspect (17) The rotary hearth furnace of aspects 13-16, wherein the subdividing wall comprises a series of individual segments.

[0074] Aspect (18) The rotary hearth furnace of aspects 13-17, wherein the segment of the dividing wall comprises solid refractory material.

[0075] Aspect (19) The rotary hearth furnace of aspects 13-18, wherein the segments of the dividing wall comprise a pipe that allows cooling with a circulating liquid or gaseous fluid.

[0076] Aspect (20) The rotary hearth furnace of aspects 13-19, wherein the iron oxide mixed with the carbonous reducing agent is fed onto the hearth at one point on either the inner or outer diameter of the rotating hearth.

[0077] Aspect (21) The rotary hearth furnace of aspects 13-20, wherein the iron oxide is fed alone or with a reducing agent and part or all of the reducing agent is fed separately through one or multiple secondary funnels onto the rotating hearth.

[0078] Aspect (22) The rotary hearth furnace of aspects 13-21, wherein the process gas generated by the reducing agent that is charged through at least one secondary funnel(s) flows one or multiple extra windings between the subdividing wall.

[0079] Aspect (23) The rotary hearth furnace of aspects 13-22, wherein the process gas generated by the reducing agent that is charged through the secondary funnel(s) is combusted in the extra windings and used to heat the iron oxide before the process gas exits the gas chamber from the last winding.

[0080] Aspect (24) The rotary hearth furnace of aspects 13-23, wherein burners and air nozzles are positioned all around the different windings of the combustion chamber.

[0081] Aspect (25) The rotary hearth furnace of aspects 13-24, wherein the process gas is extracted through an exhaust pipe in the last winding of the spiral shaped gas chamber.

[0082] Aspect (26) The rotary hearth furnace of aspects 13-25, wherein the combustion chamber has a temperature from about 800°C to about 1300°C, such as about 800-1200°C, about 800- 1100°C, about 900-1000°C, or about 800-900°C.

[0083] Aspect (27) A method to produce direct reduced iron using the rotary hearth furnace of any of the foregoing aspects, wherein the rotary hearth is rotating within the subdivided annular combustion chamber, the method comprising: providing the feed material comprising iron oxide and a reducing agent through one or more funnels onto the rotating hearth, separate or together; turning over and moving the feed material with one or more sets of ploughs from one side of the hearth to the opposite side; burning the process gas generated by the reducing agent in the subdivided combustion to control the temperature in each zone.

[0084] Aspect (28) The method of aspect 27 comprising a process gas exhaust in the last winding of the spiral shaped gas chamber.

[0085] Aspect (29) The method of aspect 27 and 28 comprising alternative gas exhaust pipe positions that allow a better efficiency of the process gas combustion.

[0086] Aspect (30) The method of aspects 27-29 comprising discharging the direct reduced iron from the rotating hearth after several revolutions of the rotating hearth.

[0087] Aspect (31) The method of aspects 27-30 comprising a process gas flow across the rotating hearth in a direction opposed to a direction of the material.

[0088] All documents cited herein are incorporated herein by reference, but only to the extent that the incorporated material does not conflict with existing definitions, statements, or other documents set forth herein. To the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern. The citation of any document is not to be construed as an admission that it is prior art with respect to this application.

[0089] While particular embodiments of apparatuses, systems, and methods for the production of direct reduced iron have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific devices, systems, and methods described herein, including alternatives, variants, additions, deletions, modifications and substitutions. This application including the appended claims is therefore intended to cover all such changes and modifications that are within the scope of this application.

Claims

What is claimed is:

1. A rotary hearth furnace to produce direct reduced iron comprising: a rotary hearth within an annular combustion chamber; one or more inlet funnels to provide a feed material comprising iron oxide and a reducing agent; a plurality of sets of ploughs to turn over and move the feed material from one side of the hearth to the opposite side; wherein the flow of the process gas is controlled by a subdividing wall in the annular gas chamber, and wherein the process gas flows in the opposite direction to the feed material across the hearth.

2. The rotary hearth furnace of claim 1, wherein the subdividing wall in the gas chamber has a shape resembling to a spiral or a coil.

3. The rotary hearth furnace of any of the foregoing claims, wherein the spiral shaped subdividing wall is composed of several concentric rings that are interconnected.

4. The rotary hearth furnace of any of the foregoing claims, wherein the subdividing wall has a gap to the rotating hearth big enough to let the material pas.

5. The rotary hearth furnace of any of the foregoing claims, wherein the subdividing wall is composed of a series of individual segments.

6. The rotary hearth furnace of any of the foregoing claims, wherein the segments of the dividing wall are made of solid refractory material.

7. The rotary hearth furnace of any of the foregoing claims, wherein the segments of the dividing wall comprise a pipe that allows cooling with a circulating liquid or gaseous fluid.

8. The rotary hearth furnace of any of the foregoing claims, wherein the iron oxide mixed with the carbonous reducing agent is fed onto the hearth at one point on either the inner or outer diameter of the rotating hearth9. The rotary hearth furnace of any of the foregoing claims, wherein the iron oxide is fed alone or with a part of the reducing agent and part or all of the reducing agent is fed separately through one or multiple secondary funnels onto the rotating hearth.

10. The rotary hearth furnace of any of the foregoing claims, wherein the process gas generated by the reducing agent that is charged through the secondary funnel(s) flows one or multiple extra windings between the subdividing wall.

11. The rotary hearth furnace of any of the foregoing claims, wherein the process gas generated by the reducing agent that is charged through the secondary funnel(s) can be combusted in the extra windings and be better used to heat the iron oxide before the process gas exits the gas chamber from the last winding.

12. The rotary hearth furnace of any of the foregoing claims, wherein burners and air nozzles are installed all around the different windings of the combustion chamber.

13. The rotary hearth furnace of any of the foregoing claims, wherein the process gas is extracted through an exhaust pipe in the last winding of the spiral shaped gas chamber.

14. The rotary hearth furnace of any of the foregoing claims, wherein the combustion chamber has a temperature from about 800°C to about 1300°C15. A method to produce direct reduced iron using the rotary hearth furnace of any of the foregoing claims, wherein the rotary hearth is rotating within the subdivided annular combustion chamber, the method comprising: providing the feed material comprising iron oxide and a reducing agent through one or several funnels onto the rotating hearth, separate or together; turning over and moving the feed material with one or several sets of ploughs from one side of the hearth to the opposite side; and burning the process gas generated by the reducing agent in the subdivided combustion to control the temperature in each zone;16. The method of claim 15 comprising a process gas exhaust in the last winding of the spiral shaped gas chamber17. The method of claim 15 and 16 comprising alternative gas exhaust pipe positions that allow a better efficiency of the process gas combustion.

18. The method of claim 15 to 17 comprising discharging the direct reduced iron from the rotating hearth after several revolutions of the rotating hearth.

19. The method of claim 15 to 18 comprising a process gas flow across the rotating hearth in a direction opposed to a direction of the material.

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