Method and apparatus for producing direct reduced carburized metals

Abstract

The method and system for producing directly reduced metallic materials includes the following steps: a) charging the metallic material to be reduced into a furnace space (120); b) evacuating the existing atmosphere in the furnace to reduce the gas pressure in the furnace to less than 1 bar; c) supplying heat and hydrogen gas to the furnace space so that the heated hydrogen gas heats the charged metallic material to a sufficiently high temperature to reduce metal oxides present in the metallic material and result in the formation of water vapor, the supply of hydrogen gas being performed so as to build up a pressure of more than 1 bar in the furnace space; and d) condensing and collecting the water vapor formed in step c) in a condenser (160) below the charged metallic material before venting the built-up overpressure. The present invention further includes the following step: e) supplying a carbon-containing gas into the furnace space to carburize the heated and reduced metallic material before venting the built-up overpressure.

Description

[Technical field]

[0001] This invention relates to a method and apparatus for producing directly reduced, carburized metals, particularly carburized direct reduced iron (also called sponge iron). In particular, the invention relates to the direct reduction of metal ores under a controlled hydrogen atmosphere to produce such directly reduced metals, and to the supply of a carbon-containing gas as part of the process to carburize the reduced metal material. [Background technology]

[0002] The production of such directly reduced metals using hydrogen as a reducing agent is known, for example in SE 7406174-8 and SE 7406175-5 which describe a process in which a hydrogen atmosphere is passed through a charge of metal ore which is then reduced to form the directly reduced metal.

[0003] Furthermore, Swedish application SE1950403-4, unpublished at the priority date of the present application, discloses a process for the direct reduction of metallic materials in a closed hydrogen atmosphere. DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0004] The invention applies in particular to the batch charging and treatment of the material to be reduced and carburized.

[0005] The prior art has several problems, including efficiency in terms of heat loss and hydrogen gas usage, as well as control issues due to the need to measure when the reduction is complete.

[0006] Additionally, known methods for carburizing metallic materials include using carbon monoxide as a source of carburizing carbon, which results in the production and release of carbon dioxide and typically also results in the production of carbon monoxide.

[0007] It is therefore desirable to develop a thermally and energy efficient method for the direct reduction and carburization of metallic materials that does not release carbon monoxide or carbon dioxide into the atmosphere.

[0008] The present invention is intended to solve the above-mentioned problems. [Means for solving the problem]

[0009] The present invention therefore relates to a method for producing directly reduced metallic materials, comprising the steps of: a) charging the metallic material to be reduced into a furnace space; b) evacuating the existing atmosphere from the furnace space in order to reduce the gas pressure in the furnace space to less than 1 bar; c) supplying heat and hydrogen gas to the furnace space in such a way that the heated hydrogen gas heats the charged metallic material to a sufficiently high temperature to reduce the metal oxides present in the metallic material, as a result of which water vapor is formed, said hydrogen gas being supplied so as to build up a pressure of more than 1 bar inside the furnace space; and d) condensing and recovering the water vapor generated in step c) in a condenser below the charged metallic material before discharging the gas from the furnace space back to atmospheric pressure. The method is further characterized in that it comprises the steps of: e) supplying a carbon-containing gas into the furnace space and carburizing the heated and reduced metallic material with said carbon-containing gas before discharging the gas from the furnace space back to atmospheric pressure.

[0010] The present invention also provides a system for producing directly reduced metallic materials, comprising: an enclosed furnace space configured to contain a metallic material to be reduced; atmosphere exhaust means configured to evacuate an existing atmosphere from the furnace space such that the gas pressure in the furnace space is less than 1 bar; heat and hydrogen supply means configured to supply heat and hydrogen gas to the furnace space; a control unit configured to control the heat and hydrogen supply means such that the heated hydrogen gas heats the charge of metallic material to a sufficiently high temperature to reduce metal oxides present in the metallic material, resulting in the formation of water vapor; the supply of hydrogen gas is performed to build up a pressure of more than 1 bar inside the furnace space, and cooling and recovery means arranged below the charge of metallic material, configured to condense and recover the water vapor before exhausting the gas from the furnace space back to atmospheric pressure. The system further comprises a carbon-containing gas supply means configured to supply a carbon-containing gas to the furnace space, and the control unit is configured to control the carbon-containing gas supply means to supply a carbon-containing gas before exhausting gas from the furnace space back to atmospheric pressure, and to carburize the metal material heated and reduced by the carbon-containing gas.

[0011] The present invention will now be described in detail with reference to exemplary embodiments thereof and the accompanying drawings. [Brief description of the drawings]

[0012] [Figure 1a] FIG. 1a is a cross-sectional view of a simplified furnace for use in a system according to the invention, during a first operating state. [Figure 1b] FIG. 1b is a cross-sectional view of the simplified furnace of FIG. 1a in a second operating state. [Diagram 2] FIG. 2 is a schematic diagram of a system according to the present invention. [Diagram 3] FIG. 3 is a flow chart of a method according to the present invention. [Figure 4a]FIG. 4a is a schematic diagram showing possible relationships between H2 partial pressure, carburizing gas partial pressure, and temperature in the furnace space according to a first embodiment of the present invention. [Figure 4b] FIG. 4b is a schematic diagram showing a possible relationship between H2 partial pressure, carburizing gas partial pressure, and temperature in the furnace space according to a second embodiment of the present invention. [Figure 4c] FIG. 4c is a schematic diagram showing a possible relationship between H2 partial pressure, carburizing gas partial pressure, and temperature in the furnace space according to a third embodiment of the present invention. [Diagram 5] FIG. 5 is a graph showing the reducibility of H2 towards the metallic material being reduced as a function of temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In Figures 1a and 1b, the same parts are provided with the same reference numbers. Thus, Figures 1a and 1b illustrate a furnace 100 for producing directly reduced and carburized metallic materials. In Figure 2, two such furnaces 210, 220 are illustrated. The furnaces 210, 220 may be identical to the furnace 100 or may differ in details. However, it will be understood that everything said herein with respect to the furnace 100 is equally applicable to the furnaces 210 and / or 220 and vice versa.

[0014] Furthermore, it will be understood that everything said herein with respect to the present method is equally applicable to the present system 200 and / or furnace 100; 210, 220, and vice versa.

[0015] Such a furnace 100 has many similarities to the furnaces described in SE7406174-8 and SE7406175-5, and reference is made to these documents for possible design details. However, an important difference between these furnaces and the furnace 100 of the present invention is that the furnace 100 of the present invention is not configured to operate in a manner in which hydrogen gas is recirculated through the furnace 100 and back to a collection container located outside the furnace 100, and in particular does not operate in a manner in which hydrogen gas is recirculated out of the furnace 100 (or furnace space 120) and back to the furnace 100 (or furnace space 120) during the processing of one and the same batch of charge material to be reduced.

[0016] Instead, and as will become apparent from the description below, furnace 100 is arranged for batch reduction-carburizing operations of one charge of material at a time, and is arranged to operate as a closed system during each such batch process in the sense that hydrogen gas is supplied to furnace 100 but not removed therefrom during the batch reduction-carburizing process, and in the sense that carbon-containing gas is supplied to furnace 100 but not removed therefrom during the batch reduction-carburizing process.

[0017] That is, during the reduction process, the amount of hydrogen gas present inside the furnace 100 is constantly increasing. After the reduction is completed, the hydrogen gas is of course vented from inside the furnace 100, but there is no recirculation of the hydrogen gas during the reduction step. In some embodiments, this is also true for carbon-containing gases, as will become apparent below.

[0018] The furnace 100 is therefore part of a closed system consisting of a furnace space 120, which is configured to be pressurized to a pressure higher than 1 bar, for example at least 1.5 bar, or at least 2 bar, or at least 3 bar, or at least 4 bar, or at least 5 bar, or at least 6 bar. In any case, the furnace space 120 is constructed to withstand the operating pressures described herein. The upper part 110 of the furnace 100 has a bell shape. The upper part 110 of the furnace 100 can be opened to charge the material to be treated and can be closed gas-tightly using fastening means 111. The furnace space 120 is sealed with a refractory material such as a brick material 130.

[0019] Unless otherwise specified, the term "pressure" herein is intended to refer to the total pressure of gas, particularly within furnace volume 120, as opposed to "partial pressure," which refers to the partial gas pressure of a particular gas.

[0020] Furthermore, since atmospheric pressure is approximately 1 bar, the expressions "pressure greater than 1 bar" and "pressure greater than atmospheric pressure" are intended to have the same meaning, and the expressions "pressure less than 1 bar" and "pressure less than atmospheric pressure" are intended to have the same meaning.

[0021] The furnace space 120 is adapted to be heated by means of one or more heating elements 121. Preferably, the heating elements 121 are electric heating elements. However, it is also possible to use radiator combustion tubes or similar fuel heating elements. However, the heating elements 121 do not generate combustion gases that directly chemically interact with the furnace space 120, which must be kept chemically controlled for this purpose. The only gaseous substances supplied to the furnace space in the main heating step described below are preferably hydrogen gas and any carbon-containing gas used as a carbon source for carburizing the metallic material.

[0022] The heating element 121 may preferably be constructed of a heat-resistant metallic material such as a molybdenum alloy.

[0023] Also, additional heating elements may be arranged in the heated furnace space 120. For example, heating elements similar to heating element 121 may be provided on the side walls of the furnace space 120, for example at a height corresponding to the charge material, at least at a height corresponding to the container 140. Such heating elements can not only heat the gas, but also assist in heating the charge material by thermal radiation.

[0024] Furnace 100 also includes a lower portion 150 which together with upper portion 110 forms a sealed container when the furnace is closed using fastening means 111 .

[0025] In the lower part 150 of the furnace 100 there is a container 140 for the material to be treated (reduced, carburized). The container 140 may be supported on the refractory floor of the furnace space 120, for example along open and closed channels 172 formed in the floor, so that gases can pass under the container 140, where the channels 172 pass from an inlet 171 for hydrogen gas and carbon-containing gas, for example at the furnace floor, from the center of the furnace space 120 to the radial outside of the furnace space 120 and then upwards to the top of the furnace space 120. These flows are referred to by the flow arrows shown in FIG. 1a in the initial and main reduction and carburization steps described below.

[0026] The container 140 is preferably of open construction, meaning that gas can pass freely through at least the bottom / floor of the container 140. This can be achieved, for example, by forming a hole through the bottom of the container 140.

[0027] The material to be treated consists of metal oxides, preferably iron oxides such as Fe2O3 and / or Fe3O4. The material may be in granular form, such as in the form of pellets or balls. One suitable material to be charged for batch reduction is rolled balls of iron ore rolled in water to a ball diameter of about 1-1.5 cm. If such iron ore further contains oxides that vaporize at temperatures below the final temperature of the charge in the process, such oxides will condense in the condenser 160 and can be easily recovered in powder form. Such oxides may consist of metal oxides such as Zn oxide, Pb oxide, etc.

[0028] Advantageously, the furnace space 120 is not charged with very large amounts of material to be reduced. Each furnace 100 is preferably charged with at most 50 tonnes, for example at most 25 tonnes, for example between 5 tonnes and 10 tonnes, in each batch. This charge material may be stored in one single container 150 in the furnace space 120. Depending on the throughput requirements, several furnaces 100 can be used in parallel, and then the residual heat from one batch in one furnace 220 can be used to preheat another furnace 210 (see FIG. 2 and below).

[0029] This provides a system 200 suitable for installation and use directly at a mining site, without the need for expensive transportation of pre-reduced ore. Instead, directly reduced and carburized metal material can be produced on-site, packaged under a protective atmosphere, and shipped to another location for further processing.

[0030] Therefore, in the case of water-rolled iron ore balls, it is envisaged that the furnace 100 can be installed in association with an iron ore ball production system such that the charging of metal material into the container 140 of the furnace 100 is fully automatic. The container 140 is automatically circulated back from the iron ore ball production system to the system 100, where it is filled with iron ore balls to be reduced and carburized, inserted into the furnace space 120, subjected to the hydrogen / heat / carbon-containing gas treatment for reduction and carburization described herein, emptied out of the furnace space 120, returned to the iron ore ball production system, recharged, etc. More containers 140 than the furnace 100 may be used so that at each batch switch, the reduced and carburized charge material in a particular container is immediately exchanged in the furnace 100 with another container carrying material that has not yet been reduced and carburized. Such large systems, such as mines, can be implemented to be fully automated and very flexible in terms of throughput, using several small furnaces 100 rather than one very large furnace.

[0031] Below the vessel 140, the furnace 100 comprises a gas-gas heat exchanger 160, which is advantageously a tubular heat exchanger known per se. The heat exchanger 160 is preferably a countercurrent heat exchanger. Below the heat exchanger 160, a closed trough 161 is connected to the heat exchanger 160 for collecting and receiving condensate from the heat exchanger 160, and the trough 161 is also designed to withstand the operating pressure of the furnace space 120 in a gas-tight manner.

[0032] The heat exchanger 160 is connected to the furnace space 120, and preferably, the cooled / cooled gas that reaches the furnace space 120 passes through the heat exchanger 160 along the heat exchanger tubes provided on the outside / periphery, and then through the channel 172 to the heating element 121. Then, the heated gas that flows out of the furnace space 120 passes through and heats the charge material (described below), and then passes through the heat exchanger 160 via the heat exchanger tubes provided on the inside / center, and heats the cooled / cooled gas. Thus, the outgoing gas heats the incoming gas both by heat conduction due to the temperature difference between the outgoing gas and the incoming gas, and by the heat of condensation of the water vapor contained in the outgoing gas, the outgoing gas effectively heats the incoming gas.

[0033] Condensate formed from the outgoing gas is collected in the trough 161 .

[0034] The furnace 100 may include a set of temperature and / or pressure sensors in the trough 161 (122), at the bottom of the furnace volume 120, such as below the container 140 (123), and / or at the top of the furnace volume 120 (124). These sensors may be used by the control unit 201 to control the reduction and carburization processes, as described below.

[0035] Reference numeral 171 denotes an inlet conduit for heating / cooling gases. Reference numeral 173 denotes an outlet conduit for the used cooling gases.

[0036] Between the trough 161 and the inlet conduit 171 there may be an overpressure balancing channel 162 with a valve 163. If a predefined pressure difference, for example of at least 1 bar, builds up in the trough 161 due to the inflow of a large amount of water into the trough 161, such pressure difference may then be partially or fully balanced by gas release into the inlet conduit 171. The valve 163 may be a simple overpressure valve configured to open if the pressure in the trough 161 is higher than said predefined pressure difference relative to the pressure in the conduit 171. Alternatively, the valve may be operated by a control unit 201 (described below) based on measurements from the pressure sensor 122.

[0037] Condensed water may be directed downwardly from the condenser / heat exchanger 160, such as through a spout 164, into the trough and exits at the bottom of the trough 161, such as at a local low point 165 of the trough, preferably with the orifice of the spout 164 located completely below the main bottom 166 of the trough 161 as shown in Figure 1a. This reduces liquid water turbulence within the trough 161 and provides more controllable operating conditions.

[0038] The trough 161 is advantageously dimensioned so as to receive and contain all the water formed during the reduction of the charge material. The size of the trough 161 can therefore be adapted to the type and amount of one batch of reduced material. For example, the complete reduction of 1000 kg of Fe3O4 results in the formation of 310 liters of water, and the complete reduction of 1000 kg of Fe2O3 results in the formation of 338 liters of water.

[0039] In Figure 2 a system 200 is illustrated in which furnaces of the type illustrated in Figures 1a and 1b may be used. In particular, one or both of the furnaces 210 and 220 may be of the type illustrated in Figures 1a and 1b and at least according to claim 1 herein.

[0040] Reference number 230 denotes a heat exchanger of gas-gas type. Reference number 240 denotes a heat exchanger of gas-water type. Reference number 250 denotes a fan. Reference number 260 denotes a vacuum pump. Reference number 270 denotes a condenser. Reference number 280 denotes a container for used hydrogen gas. Reference number 290 denotes a container for fresh / virgin hydrogen gas. Reference number 310 denotes a container for fresh / virgin carbon-containing gas. Reference number 320 denotes a container for used carbon-containing gas, such as a mixture of the type of gas stored in container 310 and hydrogen gas. Reference numbers V1 to V19 denote valves.

[0041] Reference numeral 201 denotes a control unit connected to sensors 122, 123, 124 and valves V1-V19 and generally arranged to control the process as described herein. The control unit 201 may also be connected to a user control unit, such as a graphical user interface presented by a computer (not shown) to a user of the system 200 for monitoring and further control.

[0042] Figure 3 shows a method according to the invention, which uses a system 100 of the type shown diagrammatically in Figure 3, and in particular a furnace 100 of the type shown diagrammatically in Figures 1a and 1b. In particular, the method is a method for producing a directly reduced carburized metallic material using hydrogen gas as the reducing agent and a carbon-containing gas as the carburizing carbon source.

[0043] After such direct reduction and carburization, the metallic material may form a carburized sponge metal. In particular, the metallic material may be an iron oxide material and the product obtained after direct reduction may then be carburized sponge iron. The resulting reduced-carburized metallic material may be used in a subsequent method step to produce steel or the like.

[0044] In the first step, execution of the method is started.

[0045] In a subsequent step, the metallic material to be reduced is charged into the furnace space 120. This may be done by placing the loaded container 140 into the furnace space 120 in the orientation shown in Figures 1a and 1b, which may then be hermetically closed and sealed using fastening means 111.

[0046] In a subsequent step, the existing atmosphere is evacuated from the furnace space 120 so that a gas pressure of less than 1 bar is achieved inside the furnace space 120. It should be noted that this low gas pressure is lower than atmospheric pressure. This may be done by closing valves 1-8, 11 and 13-19, opening valves 9-10 and 12, and allowing the vacuum pump to suck out and thus evacuate the contained atmosphere inside the furnace space 120 via a conduit through the heat exchanger 240 and the fan 250. Valve 9 may then be opened to allow this evacuated gas to flow out to the surrounding atmosphere if the furnace space 120 is filled with air. If the furnace space 120 is filled with spent hydrogen and / or carbon containing gas, this is instead evacuated to a container 280 or 320, as the case may be.

[0047] In this example, the furnace atmosphere is vented via conduit 173, although it is realized that any other suitable outlet conduit disposed in furnace 100 may be used.

[0048] During this evacuation step, as well as other steps as described below, the control unit 201 may be used to control the pressure within the furnace volume 120, such as based on readings from pressure sensors 122, 123 and / or 124.

[0049] The evacuation may proceed until a pressure of at most 0.5 bar, preferably at most 0.3 bar, is reached in the furnace space 120 .

[0050] In the subsequent initial heating step, heat and hydrogen gas are supplied to the furnace space 120. The hydrogen gas may be supplied from the containers 280 and / or 290. Since the furnace 100 is closed as described above, the supplied hydrogen gas does not leak substantially at all during the process. In other words, the loss of hydrogen gas (apart from the hydrogen consumed in the reduction reaction) is very small or even non-existent. Instead, only the hydrogen that is chemically consumed in the reduction reaction is used during the reduction process. Furthermore, the reduction process requires only the amount of hydrogen gas required to maintain the pressure required for the reduction process and the chemical equilibrium between the hydrogen gas and the water vapor.

[0051] As mentioned above, container 290 holds fresh (unused) hydrogen gas, while container 280 holds hydrogen gas that has already been used in one or several reduction steps and then returned to system 200. The first time the reduction process is run, only fresh hydrogen gas provided from container 290 is used. During subsequent reduction processes, recycled hydrogen gas from container 280 (or 320, see below) is used, which is replenished with fresh hydrogen gas from container 290 as needed.

[0052] During any initial phase of the initial heating step (which is one of hydrogen gas introduction without heat supply up to a pressure in the furnace space 120 of about 2 bar), valves 2, 4-9, 11 and 13-19 are closed and valves 10 and 12 are open. Depending on whether fresh or recycled hydrogen gas is used, valves V1 and / or V3 are opened.

[0053] When the pressure in the furnace space 120 reaches or approaches atmospheric pressure (about 1 bar), the heating element 121 is switched on. It is the heating element 121 that supplies heat to the furnace space 120, preferably by heating the supplied hydrogen gas, which heats the material in the container 140. The heating element 121 is preferably arranged in a position through which the hydrogen / carbon-containing gas supplied to the furnace space 120 passes, so that during the reduction and carburization process the heating element 121 is substantially immersed (completely or substantially completely surrounded) by the freshly supplied hydrogen / carbon-containing gas. In other words, heat can advantageously be supplied directly to the hydrogen gas and / or the carbon-containing gas, whichever is simultaneously supplied to the furnace space 120 (in an earlier or later step described above). In Fig. 1a and Fig. 1b the preferred case is shown in which the heating element 121 is arranged at the top of the furnace space 120.

[0054] However, the inventors envision that heat may be provided to the furnace space 120 in other ways, such as by providing heat directly to the gas mixture within the furnace space 120 at a location away from where the supplied hydrogen / carbon-containing gas enters the furnace space 120. In other examples, heat may be provided to the supplied hydrogen / carbon-containing gas at a location outside the furnace space 120 before the thus heated hydrogen / carbon-containing gas can enter the furnace space 120.

[0055] During the remainder of the initial heating step, valves 5 and 7-19 are closed and valves 1-4 and 6, along with condenser 270, are controlled by a control unit to achieve a controlled supply of recycled and / or fresh hydrogen gas, as described below.

[0056] During this initial heating step, the control unit 201 is therefore configured to control the heat and hydrogen supply means 121, 280, 290 to supply heat and hydrogen gas to the furnace space 120 such that the heated hydrogen gas heats the charged metallic material to a temperature above the boiling point of the water contained in the metallic material, so that the contained water evaporates.

[0057] Hydrogen gas is slowly fed under the control of the control unit 201 throughout the initial heating step and the main reduction and carburization step (described below). The result is a relatively slow but steady flow of hydrogen gas that is continuously present vertically downward through the charge. In general, the control unit is configured to continuously add hydrogen gas to maintain the desired increasing (as in a monotonically increasing) hydrogen partial pressure curve (and also the total pressure curve) in the furnace space 120, in particular to counter the pressure decrease at the bottom of the furnace space 120 (and at the bottom of the heat exchanger 160) resulting from the constant condensation of water vapor in the heat exchanger 160 (see below). The total energy consumption depends on the efficiency of the heat exchanger 160, in particular its ability to transfer thermal energy from both the hot gas flowing through the heat exchanger 160 and the heat of condensation of the condensing water vapor to the incoming hydrogen gas. In the case of the example Fe2O3, the theoretical energy required to heat the oxide, thermally compensate the endothermic reactions and reduce the oxide is about 250 kWh per 1000 kg of Fe2O3. For Fe3O4, the corresponding figure is about 260 kWh per 1000 kg of Fe3O4.

[0058] An important aspect of the invention is that there is no recirculation of hydrogen gas during the reduction process. This has been discussed above at a schematic level, but in the example shown in FIG. 1a this means that hydrogen gas is fed through inlet conduit 171, for example via condenser 270, to the top of furnace space 121, where it is heated by heating element 121, and then passes slowly downwards, through the metal material to be reduced in container 140, through heat exchanger 130 and further downwards to trough 161. However, there is no available outlet hole from furnace space 120, in particular no outlet hole from trough 161. Conduit 173 is closed, for example by closing valves V10, V12, V13, V14. The fed hydrogen gas is therefore partly consumed in the reduction process and partly leads to an increase in gas pressure in furnace space 120. This process is then continued until complete or desired reduction of the metal material has occurred, as will be described in more detail below.

[0059] Thus, the heated hydrogen gas present in the furnace space 120 above the charge material in the container 140 is lowered onto the charge material via a slow feed of hydrogen gas forming a slowly downward moving gas stream, where it forms a gas mixture with the water vapor from the charge material and the carbon-containing gas previously added (see below).

[0060] The hot gas mixture then descends into the heat exchanger 160, forming a gas flow passing through the heat exchanger 160. In the heat exchanger 160, heat is then exchanged from the hot gas coming from the furnace space 120 to the cold fresh supply of hydrogen / carbon-containing gas coming from the conduit 171, whereby the latter is preheated by the former. That is to say, the hydrogen gas supplied in the initial step, as well as the hydrogen and / or carbon-containing gas supplied in the main reduction and carburization step (and / or the carbon supply step described below), are preheated in the heat exchanger 160.

[0061] Cooling of the hot gas stream causes water vapor contained in the cooled gas to condense into liquid water that is collected in the trough 161, but also produces heat of condensation. The heat exchanger 160 is preferably further configured to transfer such condensation heat energy from the condensed water to the cooled hydrogen / carbon-containing gas provided in the furnace volume 120.

[0062] Also, condensation of the contained water vapor reduces the pressure of the hot gases flowing downward from the furnace volume 120, allowing more space for the hot gases to pass downward through the heat exchanger 160.

[0063] Because the heated hydrogen gas is added slowly and because the thermal conductivity of the hydrogen gas is relatively high, the charge material relatively quickly, for example within 10 minutes, reaches the boiling point of the liquid water contained in the charge material, which is slightly above 100° C. As a result, the contained liquid water evaporates and mixes with the hot hydrogen gas to form water vapor.

[0064] Condensation of the water vapor in the heat exchanger 160 reduces the partial gas pressure of the water vapor at the bottom end of the structure, forcing the water vapor generated in the charge material to flow downwards on average. In addition to this effect, water vapor is substantially less dense than the hydrogen gas with which it is mixed.

[0065] In this manner, the water in the charge material within container 140 gradually evaporates and flows downward through heat exchanger 160 where it is cooled and condenses to a liquid state within trough 161 .

[0066] The cooled hydrogen gas supplied to the heat exchanger 160, and also the carbon-containing gas supplied, is preferably at or slightly below room temperature.

[0067] It will be appreciated that this initial heating step, during which the charge material is therefore dried from any liquid water it contains, is a preferred step in the method, in particular because it allows the charge material to be easily manufactured and provided as granular material, such as in the form of rolled balls of material, without the need to introduce expensive and complex drying steps prior to charging the material into the furnace space 120.

[0068] However, it is understood that it would be possible to charge already dry or dried material to the furnace volume 120. In this case, the initial heating step as described herein would not be performed, but the method would skip immediately to the main reduction and carburization steps (below).

[0069] Additionally, several mechanisms for this initial heating step are described above with reference to both added hydrogen gas and carbon-containing gas. These mechanisms are also present in the subsequent main reduction and carburization steps (described below). However, it is preferred that no carbon-containing gas is added in the initial heating step. In particular, it is preferred that hydrogen gas is the only gas added in the initial heating step.

[0070] In one embodiment of the present invention, the supply of hydrogen gas to the furnace space 120 during said initial heating step is controlled to be very slow so that a pressure equilibrium is substantially maintained during the initial heating step, preferably so that substantially equal pressure prevails throughout the furnace space 120 and the non-liquid filled parts of the trough 161 at all times. In particular, the supply of hydrogen gas may be controlled so that said equilibrium gas pressure does not increase or only increases substantially during the initial heating step. In this case, the supply of hydrogen gas is then controlled to increase the pressure in the furnace space 120 over time only after all or substantially all of the liquid water has evaporated from the charge material in the container 140. The point in time when this occurs may be determined, for example, as a point of upward change in the slope of the temperature-time curve measured by the temperature sensors 123 and / or 124, where the change in slope indicates the point in time when substantially all of the liquid water has evaporated but reduction has not yet started. Alternatively, the hydrogen gas supply may be controlled to increase the pressure when the measured temperature in the furnace space 120 as measured by temperature sensors 123 and / or 124 exceeds a predetermined limit, which may be between 100°C and 150°C, for example between 120°C and 130°C.

[0071] In the subsequent main reduction and carburization step, further heat and hydrogen gas are supplied to the furnace space 120 in a manner corresponding to that supplied during the initial heating step described above, so that the heated hydrogen gas heats the charge metal material to a temperature high enough to reduce metal oxides present in the metal material, resulting in the formation of water vapor.

[0072] During this main reduction and carburization step, therefore, the charged metal material is heated, by supplying additional hydrogen gas and heating while gradually increasing the pressure inside the furnace space 120, to a temperature which will now initiate and sustain the reduction chemical reaction.

[0073] In the example shown in Figures 1a and 1b, the top charge would therefore be heated first. For ferrous oxide materials, hydrogen gas begins to reduce the charge at approximately 350-400°C to form metallic iron, pyrophoric iron and water vapor according to the following equation: Fe2O3+ 3H2= 2Fe + 3H2O Fe3O4+ 4H2= 3Fe + 4H2O

[0074] This reaction is endothermic and is driven by thermal energy supplied via hot hydrogen gas flowing from top to bottom in the furnace space 120 .

[0075] Thus, water vapor is generated in the charge material during both the initial heating step and the main reduction and carburization step. This generated water vapor is continuously condensed and collected in a condenser located below the charged metal material. In the example shown in Figure 1a, the condenser is in the form of a heat exchanger 160.

[0076] According to the invention, the main reduction and carburization steps, including said condensation, are carried out in such a way that a pressure higher than 1 bar with respect to atmospheric pressure is established in the furnace space 120. In particular, hydrogen gas is supplied so that a pressure higher than 1 bar is achieved and maintained. It should be noted that such a pressure higher than 1 bar is a pressure higher than atmospheric pressure.

[0077] Further according to the invention, the method further comprises a carbon supplying step, i.e. supplying a carbon-containing gas to the furnace space 120 to carburize the metallic material heated by the supplied heat and reduced by reaction with the hydrogen gas, with the carbon-containing gas. This supply of carbon-containing gas is carried out as part of the main reduction and carburization step, in the furnace space 120, before the evacuation of the gas from the furnace space 120 back to atmospheric pressure. Such evacuation may be carried out as a step of the method, for example as part of a material cooling substep, as explained below.

[0078] The carbon-containing gas may be any carbon-containing gas capable of chemically reacting with the reduced metallic material to carburize it. Examples of suitable carbon-containing gases consist of various gaseous (at the temperature and pressure prevailing in the furnace space 120 during the execution of the method) hydrocarbons, such as methane, ethane, propane, propene and the like. Preferably, the carbon-containing gas does not contain more than trace amounts of carbon monoxide, since this effectively prevents both carbon monoxide and carbon dioxide from forming residual products after the completion of the carburization step of the present invention. In particular, it is preferred that no carbon monoxide is fed to the furnace space 120 during the carbon feeding step.

[0079] As described and exemplified below, the carbon supply step may be performed at least in part simultaneously with the supply of hydrogen gas and heat described above. In particular, the carbon supply step may be performed as part of the main reduction and carburization step described above.

[0080] As described above, when iron is reduced, free iron (Fe) is produced, and this free iron accepts carbon (C) to form Fe3C.

[0081] Figure 5 shows the ability of H2 to reduce Fe2O3 as a function of increasing temperature. As can be seen from Figure 5, reduction with hydrogen gas is particularly active in the temperature range of about 400° to 700°.

[0082] Carburization using the same Fe2O3 gaseous carbon source is most active in the range of approximately 650° to 900°.

[0083] For example, Fe3O4 exhibits similar behavior with reduction / carburization and temperature.

[0084] In other words, an efficient process is one in which most of the reduction of the metallic material occurs first at a relatively low temperature, and then most of the carburization of the metallic material occurs after further heating.

[0085] The carburization process may also be accelerated by the presence of water vapor generated during the reduction of the same metallic materials.

[0086] When methane is used as the carbon-containing gas and hematite / magnetite is used as the metallic material, the following carburizing chemical reaction occurs in the furnace space: Fe3O4+ 4H2= 3Fe + 4H2O 3Fe + CH4 = Fe3C + 2H2

[0087] The reaction of CH4 with Fe consists of a side reaction in which water vapor produced by the reduced hydrogen gas reacts with methane. CH4 + H2O = 2CO + 3H2

[0088] The carburization itself then takes place primarily via the well-known hydrogen-water reaction, where carbon monoxide and hydrogen react with the formed iron surface to form water vapor, which can then be liberated to take the place of the oxygen atoms previously liberated.

[0089] Because the surface of the reduced iron is rendered porous by the reduction, the total surface area of ​​the iron is usually very large, resulting in an efficient carburizing process, especially when the metallic material is provided as a granular material.

[0090] As can be seen from the above formula, a certain amount of hydrogen gas is generated by the carburizing process, which is why less hydrogen gas is required than would otherwise be necessary.

[0091] After the carbon supplying step is completed, the final carburized metal material preferably has a carbon content of 1% to 4% by weight.

[0092] The supply of hydrogen gas in the main reduction and carburization steps may preferably be maintained until a predetermined hydrogen partial pressure, or a predetermined total pressure higher than 1 bar, is reached inside the furnace space 120. Correspondingly, the supply of carbon-containing gas in the carbon supply step may be performed until a predetermined partial pressure, or a predetermined total pressure higher than 1 bar, is reached inside the furnace space 120.

[0093] The pressure in the furnace space 120 can be measured, for example, by pressure sensors 123 and / or 124. As mentioned above, according to the invention, it is possible not to vent hydrogen gas from the furnace space 120 until a pressure of more than 1 bar is reached, and preferably not to vent hydrogen gas from the furnace space 120 until the main reduction and carburization steps are fully completed. Correspondingly, it is preferred not to vent carbon-containing gas from the furnace space 120 until a pressure of more than 1 bar is reached, and more preferably not to vent carbon-containing gas from the furnace space 120 until the main reduction and carburization steps are fully completed.

[0094] In some embodiments, hydrogen gas is supplied at least until a hydrogen partial pressure of greater than 1 bar is reached within the furnace space 120, while hydrogen gas is not evacuated from the furnace space 120 until a hydrogen gas partial pressure of greater than 1 bar is reached.

[0095] In particular, the supply of hydrogen gas and the condensation of water vapor in the main reduction and carburization steps may be performed until a predetermined pressure higher than 1 bar is reached in the furnace space 120, said predetermined pressure being at least 2.3 bar, more preferably at least 2.5 bar or even about 3 bar or more. The same applies to the pressure-adjustable supply of carbon-containing gas in the carbon supply step.

[0096] The system may be designed so that no hydrogen or carbon-containing gas is discharged until the predetermined pressure is reached.

[0097] Alternatively, the supply of hydrogen gas and the condensation of water vapor in the main reduction and carburization steps may be carried out until a steady state is reached, in view of the fact that it is no longer necessary to supply more hydrogen gas in order to maintain the reached steady gas pressure in the furnace space 120. This pressure may be measured in a corresponding manner, as described above. Preferably, the steady state gas pressure may be at least 2.3 bar, more preferably at least 2.5 bar, or even about 3 bar or more. In this way, a simple way of knowing when the reduction process is completed is achieved.

[0098] Further alternatively, the supply of hydrogen gas and heat and the condensation of water vapor in the main reduction and carburization steps may be performed until the charge metal material to be reduced reaches a predetermined temperature, which may be at least 600° C., for example 640-680° C., preferably about 660° C. The temperature of the charge material may be measured directly, for example by measuring the thermal radiation from the charge material using a suitable sensor, or indirectly by a temperature sensor 123.

[0099] In some embodiments, the main reduction and carburization step, including said condensation of formed water vapor, is carried out for a continuous period of at least 0.25 hours, such as at least 0.5 hours, such as at least 1 hour, during which both the pressure and temperature of the furnace space 120 may increase monotonically.

[0100] In some embodiments, the main reduction and carburization steps may further be performed iteratively, with the control unit 201 in each iteration allowing a steady state pressure to be reached in the furnace space 120 before feeding additional amounts of hydrogen gas into the furnace space. Also, the heat supply may be repetitive (pulsed) and switched on for the entire duration of the main reduction and carburization steps.

[0101] It should be noted that during both the initial heating step and the main reduction and carburization steps, and particularly for at least the entire duration of these steps, there is a net downward flow of water vapor through the charge of metal material in container 140.

[0102] During the initial and main reduction and carburization steps, with the exception of a period associated with the start of the carbon feed step during which the total pressure in the furnace space 120 may drop, the condenser 270 may be controlled by the control unit 201 to maintain or increase the pressure by feeding additional hydrogen gas and / or carbon-containing gas at all times. The fed hydrogen gas is used to compensate for the hydrogen consumed in the reduction process and also to gradually increase the pressure to the desired final pressure. The carbon-containing gas may be fed using any one of a number of different strategies (as described below) and may be controlled, for example, to achieve a set target total pressure in the furnace space 120 during such feeding.

[0103] The formation of water vapor in the charge material causes a local gas pressure increase, which in effect creates a pressure fluctuation between the furnace space 120 and the trough 161. As a result, the formed water vapor sinks down through the charge material and condenses in the heat exchanger 160, which in turn reduces the pressure on the far side of the heat exchanger 160 (with respect to the furnace space 120). In this way, these processes result in a net downward movement of gas through the charge, and the newly added hydrogen gas compensates for the pressure loss in the furnace space 120.

[0104] The heat in the gas exiting the furnace space 120, particularly the heat of condensation of the water vapor, is transferred in the heat exchanger 160 to the incoming hydrogen / carbon-containing gas.

[0105] Thus, the reduction process is maintained as long as there is metal material to be reduced and therefore water vapor is produced, resulting in the downward gas movement. When the production of water vapor stops (because substantially all the metal material has been reduced), the pressure is homogenized throughout the interior of the furnace 100 and the measured temperature is similar throughout the furnace space 120 if no additional carbon-containing gas is supplied. For example, the measured pressure difference between a point of the gas-filled portion of the trough 161 and a point above the charge material is less than a predetermined amount, which may be at most 0.1 bar. Additionally or alternatively, the measured temperature difference between a point above the charge material and a point below the charge material on the furnace space 120 side of the heat exchanger is less than a predetermined amount, which may be at most 20° C. Thus, when such pressure and / or temperature homogeneity is reached and measured, the supply of hydrogen gas can be stopped by shutting off the supply of hydrogen gas.

[0106] Typically, the heating element 121 is not switched off until carburization is complete, which typically occurs at a later point in time.

[0107] Thus, the combined supply of hydrogen gas and heat in the main reduction and carburization steps may be performed until a predetermined minimum temperature and / or (over)pressure is reached and / or until a predetermined maximum temperature difference and / or maximum pressure difference is reached in the heat budget in the furnace 100. Which criterion(s) is / are used depends on prerequisites such as the design of the furnace 100 and the type of metallic material being reduced. For example, the supply of heat may be performed until a predetermined minimum temperature is reached and the supply of hydrogen gas may be performed until temperature uniformity is reached. In another example, the supply of heat and hydrogen gas may be performed until a steady pressure condition is reached, without further need for the supply of hydrogen gas.

[0108] Other criteria may also be used, such as a predetermined main heating time or the end of a predetermined heating / hydrogen supply program, and these can be empirically determined.

[0109] The carbon-containing gas can be supplied using one of several different strategies.

[0110] Example 1 In a first such strategy, reduction with hydrogen gas is directly followed by carburization of the metallic material. First, hydrogen gas and heat are supplied as described above, and the temperature and pressure in the furnace space 120 are slowly increased as the metallic material is reduced. The final pressure may be as described above, for example at least 1.1 bar, preferably at least between 2.3 and 2.5 bar.

[0111] In this and other examples, when complete reduction of the metallic material charge is completed, the furnace space 120 has reached a temperature of about 700° C. and the temperature of the hydrogen gas entering the furnace space is the same as the gas entering the heat exchanger 160.

[0112] Generally, in this first strategy, heat may be supplied in the main reduction and carburization step until the metallic material reaches a temperature of at least 500°C, such as at least 600°C, before starting the supply of carbon-containing gas in the carbon supply step.

[0113] In this state, when the reduction is completed, no carbon-containing gas is yet supplied. Before or in connection therewith, a part of the hydrogen gas may be discharged to lower the hydrogen gas partial pressure. That is, the valve V4 may be closed to terminate the supply of hydrogen gas. Then, using the condenser 270, the valve V6 may be closed and the valves V7 and V5 may be opened to exhaust a part of the hydrogen gas to the spent hydrogen storage container 280. When the pressure has dropped to a low pressure between 1.1 and 1.8 bar, for example between 1.3 and 1.6 bar, for example about 1.5 bar, the valves V7 and V5 are closed and the carbon supply step is started.

[0114] As shown in FIG. 4a, after this partial hydrogen gas venting, the total pressure in the furnace space 120 is about 1.5 in this example.

[0115] Generally, the carbon feeding step may be carried out at least partially, and preferably completely, at a pressure of the furnace space 120 which is lower than the pressure of the furnace space 120 which prevails at the time of completing the reduction process.

[0116] In the storage container 310 fresh hydrocarbon gas, for example methane, is stored, and in the container 320 previously used hydrocarbon gas (for example a mixture of methane and hydrogen) is stored. During the first use for carburization, if the pressure in the container 320 is higher than the pressure prevailing in the furnace space 120, valve V15 is opened, unless valve V17 is opened. Otherwise, valves V18 and V6 are opened to allow the condenser 270 to pump the amount of hydrocarbon required to maintain the pressure in the furnace space 120 to perform the carburization.

[0117] At this time, the newly reduced metal material can receive the supplied carbon. Carburization is performed in a state where the temperature of the furnace space 120 is increased by heating using the heating element 121. Depending on the composition of the metal material, carburization is terminated when the temperature reaches about 700°C to 1100°C. As mentioned above, a certain amount of hydrogen is generated as a result during carburization.

[0118] The following cooling and draining steps can then begin.

[0119] 4a shows a schematic chart of the process according to this first strategy, where the carbon-containing gas is added after the reduction is completed, showing the hydrogen gas partial pressure (solid line) as a function of the furnace space (120) temperature and the carbon-containing gas partial pressure (dashed line) as a function of the furnace space (120) temperature during the process.

[0120] It should be noted that FIG. 4a, as well as FIGS. 4b and 4c, is simplified in the sense that it ignores any residual gas present in the furnace volume 120 after the initial evacuation.

[0121] Example 2 The second strategy is to supply a carbon-containing gas before the reduction is complete.

[0122] During heating and initiated reduction, hydrogen gas is supplied to achieve an increasing total pressure in the furnace space 120 of at least 1.1 bar, preferably at least 2.3 bar. In this case, the carbon-containing gas is supplied immediately after the reduction is initiated, in other words after the temperature in the furnace space 120 has reached at least 350° C., for example 350-450° C., for example about 400° C. In this second strategy, generally, the carbon supply step is only started after the metallic material has reached a temperature between 350-450° C.

[0123] The supply of carbon-containing gas is then carried out by closing valve V1 or V3 and opening valve V15 (if this is the first reduction), otherwise valve V17 is opened. As a result, the furnace space 120 starts to fill with carbon-containing gas. That is, in the main reduction and carburization step, reduction and carburization are carried out in parallel, and the pressure is maintained by the supplied carbon-containing gas. If the pressure in the container 320 is not sufficient to supply the carbon-containing gas, valves V18 and V6 are opened instead and valves V12, V13 and V14 are closed, so that the condenser 270 slowly starts to bring in more carbon-containing gas, thereby allowing the pressure in the furnace space 120 to be maintained at the desired final pressure of at least 2.3 to 3.5 bar.

[0124] During the entire reduction process, both heat and more carbon-containing gas are applied at about 700° C., at which temperature the gas exiting the charge has the same temperature as the gas entering the charge, until the reduction nears completion. At this point, the temperature is increased to a final temperature above 700° C., preferably up to 1100° C., while pressure is maintained by a continuous supply of mixed gas from container 320 containing a mixture of hydrogen gas and carbon-containing gas.

[0125] The following cooling and draining steps can then begin.

[0126] FIG. 4b is a diagram corresponding to that shown in FIG. 4a, but for explaining this second strategy.

[0127] Example 3 The third strategy is to start the supply of a carbon-containing gas when maximum reduction is reached, which for hematite and magnetite occurs at about 550-570 °C.

[0128] In this strategy, depending on the available hydrogen gas pressure in container 290, the pressure is raised to at least 1.1 bar, preferably at least 2.3-2.5 bar, by supplying hydrogen gas from container 290 as described above via valve V1 or by opening valves V2 / V6 and using condenser 270. At the same time, heat is supplied to furnace space 120 as described above.

[0129] When the temperature of the gas leaving the charge approaches 500°C, the supply of hydrogen gas is cut off. At this point, the majority of the charge is already fully reduced and consists of pyrophoric iron ready to receive carbon supplied via the carbon-containing gas. This is achieved by controlling valves V1-V4 for hydrogen gas and opening valve V15 for fresh carbon-containing gas from container 310.

[0130] If the pressure in the container 310 is not sufficient, valves V15 and V1 are closed while valve V6 is opened and the condenser 270 is used to maintain the desired pressure. Carburization is performed after or partly in parallel with the reduction and the pressure is maintained by a supply of carbon-containing gas. As mentioned above, the carburization results in the production of a certain amount of hydrogen gas and the resulting unwanted pressure increase can be addressed by opening valves V7 and V19 to vent part of the atmosphere in the furnace space 120 to the container 320 and the condenser 270 pumps the hydrogen / carbon-containing gas mixture from the furnace space 120 to the container 320.

[0131] When the temperature of the outlet side of the charge is the same as that of the inlet side, preferably 650-750° C., for example 690-700° C., the temperature is raised under constant pressure, more precisely to a pressure of at least 1.1, preferably at least 2.3-2.5 bar, to an elevated temperature of at least 800° C., for example 800-1100° C. The constant pressure is maintained by feeding a carbon-containing gas, preferably fresh carbon-containing gas, from container 310 via valve V15 or, if necessary, via valves V16 and V6 using condenser 270.

[0132] The following cooling and draining steps can then begin.

[0133] Generally, in this third strategy, the carbon supply step is initiated only after the metallic material has reached a temperature between 450-550° C., after which the supply of hydrogen gas may be terminated, while the carbon supply step may then include continuing to supply heat to the furnace space 120.

[0134] Further, in this third strategy, heat is generally provided in the main reduction and carburization steps, particularly during the carbon supply step, until the metallic material reaches a temperature between 700-1100°C, such as between 800-1100°C.

[0135] As described above, the carbon supply step in this third strategy may include providing heat to the furnace space 120 at a constant pressure, which is controlled by a controlled supply of a carbon-containing gas, which may or may not be mixed with hydrogen gas.

[0136] Figure 4c is a diagram corresponding to that shown in Figure 4a, but for illustrating this third strategy. Note in particular the drop in hydrogen gas partial pressure above 600°C, which is due to hydrogen produced by the carburization reaction.

[0137] After complete reduction and carburization has occurred, the method according to the present invention includes a cooling and discharge step, which is described below.

[0138] Therefore, in a subsequent cooling step, the hydrogen gas / carbon-containing gas atmosphere in the furnace space 120 can be cooled to a temperature of up to 100° C., preferably about 50° C., and then evacuated and recovered from the furnace space 120.

[0139] In the case of a single furnace 100 / 220 that is not connected to one or more furnaces, the charge material may be cooled using a fan 250 arranged downstream of the air-water cooler 240, which in turn is arranged to cool the hydrogen / carbon-containing gas (which circulates in a closed loop by the fan 250 in a loop through the valve V12, the heat exchanger 240, the fan 250 and the valve V10, exits the furnace space 120 via the outlet conduit 173 and re-enters the furnace space 120 via the inlet conduit 171). This cooling circulation is indicated by the arrows in FIG. 1b.

[0140] The heat exchanger 240 therefore transfers the thermal energy from the circulated hydrogen / carbon-containing gas to water (or another liquid), from which the thermal energy can be used in a suitable manner, for example in a district heating system. A closed loop is achieved by closing all valves V1 to V19 except valves V10 and V12.

[0141] The hydrogen / carbon-containing gas in this case is circulated through the charge material in the container 140, absorbing thermal energy from the charge material and efficiently cooling the charge material while circulating the hydrogen / carbon-containing gas in a closed loop.

[0142] In another embodiment, the thermal energy available from the cooling of the furnace 100 / 220 is used to preheat another furnace 210. This is then achieved by the control unit 201 closing the valve V12 and opening the valves V13, V14 instead, in comparison with the cooling closed loop described above. In this way, the hot hydrogen / carbon-containing gas arriving from the furnace 220 is conveyed to a gas-gas type heat exchanger 230, preferably a countercurrent heat exchanger, in which the hydrogen gas being fed in the initial or main reduction and carburization step carried out in connection with the other furnace 210 is preheated. The somewhat cooled hydrogen / carbon-containing gas from the furnace 220 can then be circulated through a heat exchanger 240 for further cooling before being reintroduced into the furnace 220. Again, the hydrogen / carbon-containing gas from the furnace 220 is circulated in a closed loop using a fan 250.

[0143] Therefore, cooling of the hydrogen / carbon-containing gas in the cooling step may be performed via heat exchange with hydrogen gas supplied to the furnace space 120 of the other furnace 210 for the initial heating step and main heating step and condensation, as described above, in relation to the furnace space 120 of the other furnace 210.

[0144] Once the temperature of the hydrogen / carbon-containing gas becomes insufficient to heat the hydrogen gas supplied to furnace 210, control unit 201 again closes valves V13 and V14 and reopens valve V12, allowing the hydrogen / carbon-containing gas from furnace 220 to be taken directly into heat exchanger 240.

[0145] Regardless of how that thermal energy is processed, the hydrogen / carbon-containing gas from furnace 220 is cooled until it (and more importantly, the charge material) reaches a temperature below 100° C. to avoid reoxidation of the charge material when subsequently exposed to air. The temperature of the charge material can be measured directly in a suitable manner, as described above, or can be measured indirectly by measuring in a suitable manner the temperature of the hydrogen / carbon-containing gas exiting via outlet conduit 173.

[0146] Cooling of the hydrogen / carbon-containing gas may occur while maintaining the pressure of the hydrogen / carbon-containing gas, or the pressure of the hydrogen / carbon-containing gas may decrease as a result of the hot hydrogen / carbon-containing gas occupying a larger volume (of the closed loop conduits and heat exchangers) once valves V10 and V12 are opened.

[0147] In a subsequent step, the hydrogen / carbon-containing gas is evacuated from the furnace 220 space 120 and collected in a suitable container for spent gas. The furnace space 120 typically now contains a mixture of hydrogen and carbon-containing gas, which is then evacuated to a container 320 for spent carbon-containing gas using a vacuum pump 260, possibly in combination with a condenser 270. The control unit then opens valves V13, V14, V8 and V19 and closes valves V1-V7 and V15-V18. The vacuum pump 260 and the condenser 270 are then activated to pump the spent gas mixture into the container 320. The evacuation of the furnace space 120 is preferably performed until a pressure of at most 0.5 bar, or at most 0.3 bar, is detected inside the furnace space 120.

[0148] Since the furnace volume 120 is closed, only the hydrogen / carbon-containing gas consumed in the chemical reduction reaction is vented out of the system, and the remaining hydrogen gas is that required to maintain the hydrogen gas / water vapor balance in the furnace volume 120 during the main reduction and carburization steps. This vented hydrogen gas is sufficient to serve the subsequent batch operation of a new charge of metallic material to be reduced.

[0149] Thereafter, valves V7, V8 and V19 are closed, valve V9 is opened to admit air into the system for charge exchange, and valve V11 is opened to drain the condensate.

[0150] In a subsequent step, the furnace space 120 is opened, for example by releasing the fastening means 111 and opening the top 110. The container 140 is removed and replaced with a container containing a new batch of charge metal material to be reduced.

[0151] In a subsequent step, the removed reduced material may be placed under an inert atmosphere, such as a nitrogen atmosphere, to avoid reoxidation during shipping and storage.

[0152] For example, the reduced metal material may be placed in a flexible or rigid shipping container filled with an inert gas. A number of such flexible or rigid containers may be placed within the shipping container, with the space around the flexible or rigid containers filled with inert gas, allowing the reduced metal material to be safely transported without risk of re-oxidation.

[0153] The following table shows approximate equilibrium conditions for hydrogen gas H2 and water vapor H2O at different furnace space 120 temperatures. Temperature(°C): 400 450 500 550 600 H2(vol-%): 95 87 82 78 76 H2O(vol-%): 5 13 18 22 24

[0154] Approximately 417 Nm is needed to reduce 1000 kg of Fe2O3. 3 of hydrogen gas H2 is required to reduce 1000 kg of Fe3O4, and about 383 m 3 of hydrogen gas, H2, is required.

[0155] The table below shows the amount of hydrogen gas required to reduce 1000 kg of Fe2O3 and Fe3O4, respectively, at atmospheric pressure and in an open system (according to the prior art), but at different temperatures. Temperature (°C): 400 450 500 550 600 Nm 3 H2 / tonne Fe2O3: 8340 3208 2317 1895 1738 Nm 3 H2 / tonne Fe3O4: 7660 2946 2128 1741 1596

[0156] The table below shows the amount of hydrogen gas required to reduce 1000 kg of Fe2O3 and Fe3O4, respectively, at different pressures and temperatures. Temperature (°C): 400 450 500 550 600 Nm 3 H2 / tonne Fe2O3: 1 bar 8340 3208 2317 1895 1738 2 bar 4170 1604 1158 948 869 3bar 2780 1069 772 632 579 Nm 3 H2 / tonne Fe3O4: 1 bar 7660 2946 2128 1741 1596 2 bar 3830 1473 1064 870 798 3 bar 2553 982 709 580 532

[0157] As mentioned above, the main reduction and carburization step according to the invention is preferably carried out to a pressure of more than 1 bar and a high temperature. It has been found to be advantageous to use a combination of a heated hydrogen gas temperature of at least 500° C. and a pressure in the furnace space 120 of at least 2.3 bar during most of the main reduction and carburization step during which some reduction is ongoing.

[0158] Although preferred embodiments have been described above, it will be apparent to those skilled in the art that many modifications can be made to the disclosed embodiments without departing from the fundamental concepts of the invention.

[0159] For example, the shape of the furnace 100 may vary depending on detailed prerequisites.

[0160] Heat exchanger 160 is described as a tubular heat exchanger. Although this has proven to be particularly advantageous, it will be appreciated that other types of gas-gas heat exchangers / condensers are possible. Heat exchanger 240 may be of any suitable configuration.

[0161] The excess heat from the cooled hydrogen / carbon-containing gas can also be used in other processes requiring thermal energy.

[0162] The metallic material being reduced and carburized has been described as an iron oxide, however, the method and system of the present invention may also be used to reduce and carburize metallic materials that vaporize at temperatures below about 600-700°C, such as the Zn and Pb-containing metal oxides mentioned above.

[0163] Furthermore, the principle of combining direct reduction and carburization according to the present invention can also be used with metallic materials that have a higher reduction temperature than iron ore, such as used building materials, by appropriately adjusting the structure of the furnace 100.

[0164] Therefore, the invention is not limited to the described embodiments, but can be modified within the scope of the appended claims.

Claims

1. 1. A process for batchwise production of directly reduced metallic materials, comprising: a) charging the metal material to be reduced into the furnace space (120); b) evacuating the existing atmosphere from said furnace space (120) so as to bring the gas pressure in said furnace space (120) to less than 1 bar; c) supplying heat and hydrogen gas to the furnace space (120) such that the heated hydrogen gas heats the charged metal material to a sufficiently high temperature to reduce metal oxides present in the metal material, resulting in the formation of water vapor, said hydrogen gas being supplied in such a way as to build up a pressure of more than 1 bar inside the furnace space (120) and not to recirculate said hydrogen gas during the reduction of the metal material, whereby said hydrogen gas is supplied to the furnace space but not removed therefrom during the reduction, whereby said hydrogen gas is supplied to the top of the furnace space (120); and d) condensing and recovering the water vapor produced in step c) in a condenser (160) below the metal charge before exhausting the gases from the furnace space (120) back to atmospheric pressure; and e) supplying a carbon-containing gas to the top of the furnace space (120) prior to exhausting the gas back to atmospheric pressure from the furnace space (120) and carburizing the heated and reduced metal material with the carbon-containing gas; The method according to claim 1, further comprising:

2. The supply of the carbon-containing gas is not performed until the metallic material is at least partially reduced.

2. The method of claim 1 .

3. The hydrogen gas is supplied through an inlet conduit, and the carbon-containing gas is supplied through the inlet conduit.

3. The method according to claim 1 or 2.

4. The step a) of charging the metal material into the furnace space (120) comprises charging a container (140) containing the metal material into the furnace space (120); the container (140) is supported above a refractory floor of the furnace space (120) in such a manner that gas can be passed under the container (140) along open or closed channels (172) formed in the refractory floor; The channels (172) extend radially outward from the hydrogen gas and carbon-containing gas inlet (171) toward the outer periphery of the furnace space (120) and then pass upward toward the top of the furnace space (120); The hydrogen gas is supplied to the top of the furnace space (120) through the channel (172), and the carbon-containing gas is supplied through the channel (172).

4. The method according to claim 1, wherein the first and second electrodes are arranged in a first direction.

5. said steps c) and d) are carried out at least until a hydrogen partial pressure of more than 1 bar is reached in said furnace space (120), and no hydrogen gas is evacuated from said furnace space (120) until said partial pressure of more than 1 bar is reached; 5. The method according to claim 1, wherein the first and second electrodes are arranged in a first direction.

6. The step c) further includes, in an initial heating step, supplying heat and hydrogen gas to the furnace space (120), and the heated hydrogen gas heats the charged metal material to a temperature higher than the boiling point of water contained in the metal material, thereby evaporating the contained water.

6. The method according to claim 1, wherein the first and second electrodes are connected to a first electrode.

7. the hydrogen gas to be provided in step c) is preheated in a heat exchanger (160), the heat exchanger (160) being arranged to transfer thermal energy from the evaporated water to the hydrogen gas to be provided in step c); 7. The method according to any one of claims 1 to 6.

8. the carbon-containing gas to be provided in step e) is preheated in a heat exchanger (160), the heat exchanger (160) being arranged to transfer thermal energy from the evaporated water to the carbon-containing gas to be provided in step e); 8. The method according to any one of claims 1 to 7.

9. The supply of hydrogen gas in step c) and / or the supply of carbon-containing gas in step e) is performed until a predetermined pressure is reached.

9. The method according to any one of claims 1 to 8.

10. The predetermined pressure is a pressure of at least 2.3 bar.

10. The method of claim 9.

11. During step c), there is a net downward flow of steam through the charge metal material.

11. The method according to any one of claims 1 to 10.

12. the carbon-containing gas is a gaseous hydrocarbon and in step e) no carbon monoxide is supplied to the furnace space (120); 12. The method according to any one of claims 1 to 11.

13. said step e) being carried out at least in part simultaneously with said steps c) and d); 13. The method according to any one of claims 1 to 12.

14. said step d) comprises supplying heat to said furnace space (120) at a constant pressure, said pressure being controlled by a controlled supply of a carbon-containing gas, which may be mixed with hydrogen gas; 14. The method of claim 13.

15. A system (100; 200) for batchwise production of directly reduced metallic materials, comprising: an enclosed furnace volume (120) configured to contain the metallic material to be reduced; Atmosphere exhaust means (260) configured to exhaust the existing atmosphere from the furnace space (120) such that the gas pressure in the furnace space (120) is less than 1 bar; and heat and hydrogen supply means (171; 280, 290) configured to supply heat and hydrogen gas to the furnace space (120), The system (100; 200) further comprises: a control unit (201) configured to control said heat and hydrogen supply means (171; 280, 290) so that the heated hydrogen gas heats the charge metal material to a sufficiently high temperature to reduce metal oxides present in said metal material, resulting in the formation of water vapor; said supply of hydrogen gas is performed in such a way as to build up a pressure of more than 1 bar inside said furnace space (120) and not to recirculate said hydrogen gas during the reduction of said metal material, whereby said hydrogen gas is supplied to said furnace space but is not removed therefrom during said reduction, wherein said hydrogen gas is supplied to the top of said furnace space (120), cooling and recovery means (160, 161) arranged below the metal charge and configured to condense and recover the water vapor before exhausting the gas from the furnace space (120) back to atmospheric pressure; and said furnace space (120) is provided with carbon-containing gas supply means (171; 310, 320) configured to supply a carbon-containing gas to said top of said furnace space (120), the control unit (201) is configured to control the carbon-containing gas supply means (171; 310, 320) to supply a carbon-containing gas before exhausting gas back to atmospheric pressure from the furnace space (120) and to carburize the metal material heated and reduced by the carbon-containing gas, A system (100; 200) characterized in that

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