Patent ID: 12227820

FIGS.1aand1bshare the same reference numerals for same parts.

Hence,FIGS.1aand1billustrate a furnace100for producing direct reduced metal material. InFIG.2, two such furnaces210,220are illustrated. The furnaces210,220may be identical to furnace100, or differ in details. However, it is understood that everything which is said herein regarding the furnace100is equally applicable to furnaces210and/or220, and vice versa.

Furthermore, it is understood that everything which is said herein regarding the present method is equally applicable to the present system200and/or furnace100;210,220, and vice versa.

The furnace100as such has many similarities with the furnaces described in SE7406174-8 and SE7406175-5, and reference is made to these documents regarding possible design details. However, an important difference between these furnaces and the present furnace100is that the present furnace100is not arranged to be operated in a way where hydrogen gas is recirculated through the furnace100and back to a collecting container arranged outside of the furnace100, and in particular not in a way where hydrogen gas is recirculated out from the furnace100(or heated furnace space120) and then back into the furnace100(or heated furnace space120) during one and the same batch processing of charged material to be reduced.

Instead, and as will be apparent from the below description, the furnace100is arranged for batch-wise reducing operation of one charge of material at a time, and to operate during such an individual batch processing as a closed system, in the sense that hydrogen gas is supplied to the furnace100but not removed therefrom during the batch-wise reducing step.

In other words, the amount of hydrogen gas present inside the furnace100always increases during the reduction process. After reduction has been completed, the hydrogen gas is of course evacuated from within the furnace100, but there is no recirculation of hydrogen gas during the reduction step.

Hence, the furnace100is part of a closed system comprising a heated furnace space120which arranged to be pressurized, such as to at least 5 bars, or at least 6 bars, or at least 8 bars, or even at least 10 bars. An upper part110of the furnace100has a bell-shape. It can be opened for charging of material to be processed, and can be closed in a gas-tight manner using fastening means111. The furnace space120is encapsulated with refractory material, such as brick material130.

The furnace space120is arranged to be heated using one or several heating elements121. Preferably, the heating elements121are electric heating elements. However, radiator combustion tubes or similar fuel-heated elements can be used as well. The heating elements121do not, however, produce any combustion gases that interact directly chemically with the furnace space120, which must be kept chemically controlled for the present purposes. It is preferred that the only gaseous matter provided into the furnace space during the below-described main heating step is hydrogen gas.

The heating elements121may preferably be made of a heat-resistant metal material, such as a molybdenum alloy.

Additional heating elements may also be arranged in the heated furnace space120. For instance, heating elements similar to elements121may be provided at the side walls of the furnace space120, such as at a height corresponding to the charged material or at least to the container140. Such heating elements may aid heating not only the gas, but also the charged material via heat radiation.

The furnace100also comprises a lower part150, forming a sealed container together with the upper part110when the furnace is closed using fastening means111.

A container140for material to be processed (reduced) is present in the lower part150of the furnace100. The container140may be supported on a refractory floor of the furnace space120in a way allowing gas to pass beneath the container140, such as along open or closed channels172formed in said floor, said channels172passing from an inlet171for hydrogen gas, such as from a central part of the furnace space120at said furnace floor, radially outward to a radial periphery of the furnace space120and thereafter upwards to an upper part of the furnace space120. See flow arrows indicated inFIG.1afor these flows during the below-described initial and main heating steps.

The container140is preferably of an open constitution, meaning that gas can pass freely through at least a bottom/floor of the container140. This may be accomplished, for instance, by forming holes through the bottom of the container140.

The material to be processed comprises a metal oxide, preferably an iron oxide such as Fe2O3and/or Fe3O4. The material may be granular, such as in the form of pellets or balls. One suitable material to be charged for batch reduction is rolled iron ore balls, that have been rolled in water to a ball diameter of about 1-1.5 cm. If such iron ore additionally contains oxides that evaporate at temperatures below the final temperature of the charged material in the present method, such oxides may be condensed in the condenser160and easily collected in powder form. Such oxides may comprise metal oxides such as Zn and Pb oxides.

Advantageously, the furnace space120is not charged with very large amounts of material to be reduced. Each furnace100is preferably charged with at the most 50 tonnes, such as at the most 25 tonnes, such as between 5 and 10 tonnes, in each batch. This charge may be held in one single container150inside the furnace space120. Depending on throughput requirements, several furnaces100may be used in parallel, and the residual heat from a batch in one furnace220can then be used to preheat another furnace210(seeFIG.2and below).

This provides a system200which is suitable for installation and use directly at the mining site, requiring no expensive transport of the ore before reduction. Instead, direct reduced metal material can be produced on-site, packaged under a protecting atmosphere and transported to a different site for further processing.

Hence, in the case of water-rolled iron ore balls, it is foreseen that the furnace100may be installed in connection to the iron ore ball production system, so that charging of the metal material into the furnace100in the container140can take place in a fully automated manner, where containers140are automatically circulated from the iron ore ball production system to the system100and back, being filled with iron ore balls to be reduced; inserted into the furnace space120; subjected to the reducing hydrogen/heat processing described herein; removed from the furnace space120and emptied; taken back to the iron ore ball production system; refilled; and so forth. More containers140may be used than furnaces100, so that in each batch switch a reduced charge in a particular container is immediately replaced in the furnace100with a different container carrying material not yet reduced. Such a larger system, such as at a mining site, may be implemented to be completely automated, and also to be very flexible in terms of throughput, using several smaller furnaces100rather than one very large furnace.

Below the container140, the furnace100comprises a gas-gas type heat exchanger160, which may advantageously be a tube heat exchanger such as is known per se. The heat exchanger160is preferably a counter-flow type heat exchanger. To the heat exchanger160, below the heat exchanger160, is connected a closed trough161for collecting and accommodating condensed water from the heat exchanger160. The trough161is also constructed to withstand the operating pressures of the furnace space120in a gas-tight manner.

The heat exchanger160is connected to the furnace space120, preferably so that cool/cooled gases arriving to the furnace space120pass the heat exchanger160along externally/peripherally provided heat exchanger tubes and further through said channels172up to the heating element121. Then, heated gases passing out from the furnace space120, after passing and heating the charged material (see below), pass the heat exchanger160through internally/centrally provided heat exchanger tubes, thereby heating said cool/cooled gases. The outgoing gases hence heat the incoming gases both by thermal transfer due to the temperature difference between the two, as well as by the condensing heat of condensing water vapour contained in the outgoing gases effectively heating the incoming gases.

The formed condensed water from the outgoing gases is collected in the trough161.

The furnace100may comprise a set of temperature and/or pressure sensors in the trough161(122); at the bottom of the furnace space120, such as below the container140(123) and/or at the top of the furnace space120(124). These sensors may be used by control unit201to control the reduction process, as will be described below.

171denotes an entry conduit for heating/cooling hydrogen gas.173denotes an exit conduit for used cooling hydrogen gas.

Between the trough161and the entry conduit171there may be an overpressure equilibration channel162, with a valve163. In case an overpressure builds up in the trough161, due to large amounts of water flowing into the trough161, such an overpressure may then be released to the entry conduit171. The valve163may be a simple overpressure valve, arranged to be open when the pressure in trough161is higher than the pressure in the conduit171. Alternatively, the valve may be operated by control device201(below) based on a measurement from pressure sensor122.

Condensed water may be led from the condenser/heat exchanger160may be led down into the trough via a spout164or similar, debouching at a bottom of the trough161, such as at a local low point165of the trough, preferably so that an orifice of said spout164is arranged fully below a main bottom166of the trough161such as is illustrated inFIG.1a. This will decrease liquid water turbulence in the trough161, providing more controllable operation conditions.

The trough161is advantageously dimensioned to be able to receive and accommodate all water formed during the reduction of the charged material. The size of trough161can hence be adapted for the type and volume of one batch of reduced material. For instance, when fully reducing 1000 kg of Fe3O4, 310 liters of water is formed, and when fully reducing 1000 kg of Fe2O3, 338 liters of water is formed.

InFIG.2, a system200is illustrated in which a furnace of the type illustrated inFIGS.1aand1bmay be put to use. In particular, one or both of furnaces210and220may be of the type illustrated inFIGS.1aand1b, or at least according to the present claim1.

230denotes a gas-gas type heat exchanger.240denotes a gas-water type heat exchanger.250denotes a fan.260denotes a vacuum pump.270denotes a compressor.280denotes a container for used hydrogen gas.290denotes a container for fresh/unused hydrogen gas. V1-V14denote valves.

201denotes a control device, which is connected to sensors122,123,124and valves V1-V14, and which is generally arranged to control the processes described herein. The control device201may also be connected to a user control device, such a graphical user interface presented by a computer (not shown) to a user of the system200for supervision and further control.

FIG.3illustrates a method according to the present invention, which method uses a system100of the type generally illustrated inFIG.3and in particular a furnace100of the type generally illustrated inFIGS.1aand1b. In particular, the method is for producing direct reduced metal material using hydrogen gas as the reducing agent.

After such direct reduction, the metal material may form sponge metal. In particular, the metal material may be iron oxide material, and the resulting product after the direct reduction may then be sponge iron. Such sponge iron may then be used, in subsequent method steps, to produce steel and so forth.

In a first step, the method starts.

In a subsequent step, the metal material to be reduced is charged into the furnace space120. This charging may take place by a loaded container140being placed into the furnace space120in the orientation illustrated inFIGS.1aand1b, and the furnace space120may then be closed and sealed in a gas-tight manner using fastening means111.

In a subsequent step, an existing atmosphere is evacuated from the furnace space120, so that an underpressure is achieved inside the furnace space120as compared to atmospheric pressure. This may take place by valves1-8,11and13-14being closed and valves9-10and12being open, and the vacuum pump sucking out and hence evacuating the contained atmosphere inside the furnace space120via the conduit passing via240and250. Valve9may then be open to allow such evacuated gases to flow out into the surrounding atmosphere, in case the furnace space120is filled with air. If the furnace space120is filled with used hydrogen gas, this is instead evacuated to the container280.

In this example, the furnace atmosphere is evacuated via conduit173, even if it is realized that any other suitable exit conduit arranged in the furnace100may be used.

In this evacuation step, as well as in other steps as described below, the control device201may be used to control the pressure in the furnace space120, such as based upon readings from pressure sensors122,123and/or124.

The emptying may proceed until a pressure of at the most 0.5 bar, preferably at the most 0.3 bar, is achieved in the furnace space120.

In a subsequent initial heating step, heat and hydrogen gas is provided to the furnace space120. The hydrogen gas may be supplied from the containers280and/or290. Since the furnace100is closed, as mentioned above, substantially none of the provided hydrogen gas will escape during the process. In other words, the hydrogen gas losses (apart from hydrogen consumed in the reduction reaction) will be very low or even non-existent. Instead, only the hydrogen consumed chemically in the reduction reaction during the reduction process will be used. Further, the only hydrogen gas which is required during the reduction process is the necessary amount to uphold the necessary pressure and chemical equilibrium between hydrogen gas and water vapour during the reduction process.

As mentioned above, the container290holds fresh (unused) hydrogen gas, while container280holds hydrogen gas that has already been used in one or several reduction steps and has since been collected in the system200. The first time the reduction process is performed, only fresh hydrogen gas is used, provided from container290. During subsequent reduction processes, reused hydrogen gas, from container280, is used, which is topped up by fresh hydrogen gas from container290according to need.

During an optional initial phase of the initial heating step, which initial phase is one of hydrogen gas introduction, performed without any heat provision up to a furnace space120pressure of about 1 bar, valves2,4-9,11and13-14are closed, while valves10and12are open. Depending on if fresh or reused hydrogen gas is to be used, valve V1and/or V3is open.

As the pressure inside the furnace space120reaches, or comes close to, atmospheric pressure (about 1 bar), the heating element121is switched on. Preferably, it is the heating element121which provides the said heat to the furnace space120, by heating the supplied hydrogen gas, which in turn heats the material in the container140. Preferably, the heating element121is arranged at a location past which the hydrogen gas being provided to the furnace space120flows, so that the heating element121will be substantially submerged in (completely or substantially completely surrounded by) newly provided hydrogen gas during the reducing process. In other words, the heat may advantageously be provided directly to the hydrogen gas which is concurrently provided to the furnace space120. InFIGS.1aand1b, the preferred case in which the heating element121is arranged in a top part of the furnace space120is shown.

However, the present inventor foresee that the heat may be provided in other ways to the furnace space120, such as directly to the gas mixture inside the furnace space120at a location distant from where the provided hydrogen gas enters the furnace space120. In other examples, the heat may be provided to the provided hydrogen gas as a location externally to the furnace space120, before the thus heated hydrogen gas is allowed to enter the furnace space120.

During the rest of the said initial heating step, valves5and7-14are closed, while valves1-4and6are controlled by the control device, together with the compressor270, to achieve a controlled provision of reused and/or fresh hydrogen gas as described in the following.

Hence, during this initial heating step, the control device201is arranged to control the heat and hydrogen provision means121,280,290to provide heat and hydrogen gas to the furnace space120in a way so that heated hydrogen gas heats the charged metal material to a temperature above the boiling temperature of water contained in the metal material. As a result, said contained water evaporates.

Throughout the initial heating step and the main heating step (see below), hydrogen gas is supplied slowly under the control of the control device201. As a result, there will be a continuously present, relatively slow but steady, flow of hydrogen gas, vertically downwards, through the charged material. In general, the control device is arranged to continuously add hydrogen gas so as to maintain a desired increasing (such as monotonically increasing) pressure curve inside the furnace space120, and in particular to counteract the decreased pressure at the lower parts of the furnace space120(and in the lower parts of the heat exchanger160) resulting from the constant condensation of water vapour in the heat exchanger160(see below). The total energy consumption depends on the efficiency of the heat exchanger160, and in particular its ability to transfer thermal energy to the incoming hydrogen gas from both the hot gas flowing through the heat exchanger160and the condensation heat of the condensing water vapour. In the exemplifying case of Fe2O3, the theoretical energy needed to heat the oxide, thermally compensate for the endothermic reaction and reduce the oxide is about 250 kWh per 1000 kg of Fe2O3. For Fe3O4, the corresponding number is about 260 kWh per 1000 kg of Fe3O4.

An important aspect of the present invention is that there is no recirculation of hydrogen gas during the reduction process. This has been discussed on a general level above, but in the example shown inFIG.1athis means that the hydrogen gas is supplied, such as via compressor270, through entry conduit171into the top part of the furnace space121, where it is heated by the heating element121and then slowly passes downwards, past the metal material to be reduced in the container140, further down through the heat exchanger130and into the trough161. However, there are no available exit holes from the furnace space120, and in particular not from the trough161. The conduit173is closed, for instance by the valves V10, V12, V13, V14being closed. Hence, the supplied hydrogen gas will be partly consumed in the reduction process, and partly result in an increased gas pressure in the furnace space120. This process then goes on until a full or desired reduction has occurred of the metal material, as will be detailed below.

Hence, the heated hydrogen gas present in the furnace space120above the charged material in the container140will, via the slow supply of hydrogen gas forming a slowly moving downwards gas stream, be brought down to the charged material. There, it will form a gas mixture with water vapour from the charged material (see below).

The resulting hot gas mixture will form a gas stream down into and through the heat exchanger160. In the heat exchanger160, there will then be a heat exchange of heat from the hot gas arriving from the furnace space120to the cold newly provided hydrogen gas arriving from conduit171, whereby the latter will be preheated by the former. In other words, hydrogen gas to be provided in the initial and main heating steps is preheated in the heat exchanger160.

Due to the cooling of the hot gas flow, water vapour contained in the cooled gas will condense. This condensation results in liquid water, which is collected in the trough161, but also in condensation heat. It is preferred that the heat exchanger160is further arranged to transfer such condensation thermal energy from the condensed water to the cold hydrogen gas to be provided into the furnace space120.

The condensation of the contained water vapour will also decrease the pressure of the hot gas flowing downwards from the furnace space120, providing space for more hot gas to pass downwards through the heat exchanger160.

Due to the slow supply of additional heated hydrogen gas, and to the relatively high thermal conductivity of hydrogen gas, the charged material will relatively quickly, such as within 10 minutes or less, reach the boiling point of liquid water contained in the charged material, which should by then be slightly above 100° C. As a result, this contained liquid water will evaporate, forming water vapour mixing with the hot hydrogen gas.

The condensation of the water vapour in the heat exchanger160will decrease the partial gas pressure for the water vapour at the lower end of the structure, making the water vapour generated in the charged material on average flow downwards. Adding to this effect, water vapour also a substantially lower density than the hydrogen gas with which it mixes.

This way, the water contents of the charged material in the container140will gradually evaporate, flow downwards through the heat exchanger160, cool down and condense therein and to up in liquid state in the trough161.

It is preferred that the cold hydrogen gas supplied to the heat exchanger160is room tempered or has a temperature which is slightly less than room temperature.

It is realized that this initial heating step, in which the charged material is hence dried from any contained liquid water, is a preferred step in the present method. In particular, this makes it easy to produce and provide the charged material as a granular material, such as in the form of rolled balls of material, without having to introduce an expensive and complicating drying step prior to charging of the material into the furnace space120.

However, it is realized that it would be possible to charge already dry or dried material into the furnace space120. In this case, the initial heating step as described herein would not be performed, but the method would skip immediately to the main heating step (below).

In one embodiment of the present invention, the provision of hydrogen gas to the furnace space120during said initial heating step is controlled to be so slow so that a pressure equilibrium is substantially maintained throughout the performance of the initial heating step, preferably so that a substantially equal pressure prevails throughout the furnace space120and the not liquid-filled parts of the trough161at all times. In particular, the supply of hydrogen gas may be controlled so that the said equilibrium gas pressure does not increase, or only increases insignificantly, during the initial heating step. In this case, the hydrogen gas supply is then controlled to increase the furnace space120pressure over time only after all or substantially all liquid water has evaporated from the charged material in the container140. The point in time when this has occurred may, for instance, be determined as a change upwards in slope of a temperature-to-time curve as measured by temperature sensor123and/or124, where the change of slope marks a point at which substantially all liquid water has evaporated but the reduction has not yet started. Alternatively, hydrogen gas supply may be controlled so as to increase the pressure once a measured temperature in the furnace space120, as measured by temperature sensor123and/or124, has exceeded a predetermined limit, which limit may be between 100° C. and 150° C., such as between 120° C. and 130° C.

In a subsequent main heating step, heat and hydrogen gas is further provided to the furnace space120, in a manner corresponding to the supply during the initial heating step described above, so that heated hydrogen gas heats the charged metal material to a temperature high enough in order for metal oxides present in the metal material to be reduced, in turn causing water vapour to be formed.

During this main heating step, additional hydrogen gas is hence supplied and heated, under a gradual pressure increase inside the furnace space120, so that the charged metal material in turn is heated up to a temperature at which a reduction chemical reaction is initiated and maintained.

In the example illustrated inFIGS.1aand1b, the topmost charged material will hence be heated first. In the case of iron oxide material, the hydrogen gas will start reducing the charged material to form metallic iron at about 350-400° C., forming pyrophytic iron and water vapour according to the following formulae:
Fe2O3+3H2=2Fe+3H2O
Fe3O4+4H2=2Fe+4H2O

This reaction is endothermal, and is driven by the thermal energy supplied via the hot hydrogen gas flowing down from above in the furnace space120.

Hence, during both the initial heating step and the main heating step, water vapour is produced in the charged material. This formed water vapour is continuously condensed and collected in a condenser arranged below the charged metal material. In the example shown inFIG.1a, the condenser is in the form of the heat exchanger160.

According to the invention, the main heating step, including said condensing, is performed until an overpressure has been reached in the furnace space120in relation to atmospheric pressure. The pressure may, for instance, be measured by pressure sensor123and/or124. As mentioned above, according to the invention no hydrogen gas is evacuated from the furnace space120until said overpressure has been reached, and preferably no hydrogen gas is evacuated from the furnace space120until the main heating step has been completely finalized.

More preferably, the supply of hydrogen gas in the main heating step, and the condensing of water vapour, is performed until a predetermined overpressure has been reached in the furnace space120, which predetermined overpressure is at least 4 bars, more preferably at least 8 bars, or even about 10 bars in absolute terms.

Alternatively, the supply of hydrogen gas in the main heating step, and the condensing of water vapour, may be performed until a steady state has been reached, in terms of it no longer being necessary to provide more hydrogen gas in order to maintain a reached steady state gas pressure inside the furnace space120. This pressure may be measured in the corresponding way as described above. Preferably, the steady state gas pressure may be at least 4 bars, more preferably at least 8 bars, or even about 10 bars. This way, a simple way of knowing when the reduction process has been completed is achieved.

Alternatively, the supply of hydrogen gas and heat in the main heating step, and the condensing of water vapour, may be performed until the charged metal material to be reduced has reached a predetermined temperature, which may be at least 600° C., such as between 640-680° C., preferably about 660° C. The temperature of the charged material may be measured directly, for instance by measuring heat radiation from the charged material using as suitable sensor, or indirectly by temperature sensor123.

In some embodiments, the main heating step, including said condensation of the formed water vapour, is performed during a continuous time period of at least 0.25 hours, such as at least 0.5 hours, such as at least 1 hour. During this whole time, both the pressure and temperature of the furnace space120may increase monotonically.

In some embodiments, the main heating step may furthermore be performed iteratively, in each iteration the control device201allowing a steady state pressure to be reached inside the furnace space120before supplying an additional amount of hydrogen gas into the furnace space. The heat provision may also be iterative (pulsed), or be in a switched on state during the entire main heating step.

It is noted that, during the performing of both the initial heating step and the main heating steps, and in particular at least during substantially the entire length of these steps, there is a net flow downwards of water vapour through the charged metal material in the container140.

During the initial and main heating steps, the compressor270is controlled, by the control device201, to, at all times, maintain or increase the pressure by supplying additional hydrogen gas. This hydrogen gas is used to compensate for hydrogen consumed in the reduction process, and also to gradually increase the pressure to a desired final pressure.

The formation of water vapour in the charged material increases the gas pressure locally, in effect creating a pressure variation between the furnace space120and the trough161. As a result, formed water vapour will sink down through the charged material and condense in the heat exchanger160, in turn lowering the pressure on the distant (in relation to the furnace space120) side of the heat exchanger160. These processes thus create a downwards net movement of gas through the charge, where newly added hydrogen gas compensates for the pressure loss in the furnace space120.

The thermal content in the gas flowing out from the furnace space120, and in particular the condensing heat of the water vapour, is transferred to the incoming hydrogen gas in the heat exchanger160.

Hence, this process is maintained as long as there is metal material to reduce and water vapour hence is produced, resulting in said downwards gas movement. Once the production of water vapour stops (due to substantially all metal material having been reduced), the pressure equalizes throughout the interior of the furnace100, and the measured temperature will be similar throughout the furnace space120. For instance, a measured pressure difference between a point in the gas-filled part of the trough161and a point above the charged material will be less than a predetermined amount, which may be at the most 0.1 bars. Additionally or alternatively, a measured temperature difference between a point above the charged material and a point below the charged material but on the furnace space120side of the heat exchanger will be less than a predetermined amount, which may be at the most 20° C. Hence, when such pressure and/or temperature homogeneity is reached and measured, the main heating step may end by the hydrogen gas supply being shut off and the heating element121being switched off.

Hence, the main heating step may be performed until a predetermined minimum temperature and/or pressure has been reached, and/or until a predetermined maximum temperature difference and/or maximum pressure difference has been reached in the heated volume in the furnace100. Which criterion(s) is/are used depends on the prerequisites, such as the design of the furnace100and the type of metal material to be reduced. It is also possible to use other criteria, such as a predetermined main heating time or the finalization of a predetermined heating/hydrogen supply program, which in turn may be determined empirically.

In a subsequent cooling step, the hydrogen atmosphere in the furnace space120is then cooled to a temperature of at the most 100° C., preferably about 50° C., and is thereafter evacuated from the furnace space120and collected.

In the case of a single furnace100/220, which is not connected to one or several furnaces, the charged material may be cooled using the fan250, which is arranged downstream of the gas-water type cooler240, in turn being arranged to cool the hydrogen gas (circulated in a closed loop by the fan250in a loop past the valve V12, the heat exchanger240, the fan250and the valve V10, exiting the furnace space120via exit conduit173and again entering the furnace space120via entry conduit171). This cooling circulation is shown by the arrows inFIG.1b.

The heat exchanger240hence transfers the thermal energy from the circulated hydrogen gas to water (or a different liquid), from where the thermal energy can be put to use in a suitable manner, for instance in a district heating system. The closed loop is achieved by closing all valves V1-V14except valves V10and V12.

Since the hydrogen gas in this case is circulated past the charged material in the container140, it absorbs thermal energy from the charged material, providing efficient cooling of the charged material while the hydrogen gas is circulated in a closed loop.

In a different example, the thermal energy available from the cooling of the furnace100/220is used to preheat a different furnace210. This is then achieved by the control device201, as compared to the above described cooling closed loop, closing the valve V12and instead opening valves V13, V14. This way, the hot hydrogen gas arriving from the furnace220is taken to the gas-gas type heat exchanger230, which is preferably a counter-flow heat exchanger, in which hydrogen gas being supplied in an initial or main heating step performed in relation to the other furnace210is preheated in the heat exchanger230. Thereafter, the somewhat cooled hydrogen gas from furnace220may be circulated past the heat exchanger240for further cooling before being reintroduced into the furnace220. Again, the hydrogen gas from furnace220is circulated in a closed loop using the fan250.

Hence, the cooling of the hydrogen gas in the cooling step may take place via heat exchange with hydrogen gas to be supplied to a different furnace210space120for performing the initial and main heating steps and the condensation, as described above, in relation to said different furnace210space120.

Once the hydrogen gas is insufficiently hot to heat the hydrogen gas supplied to furnace210, the control device201again closes valves V13, V14and reopens valve V12, so that the hydrogen gas from furnace220is taken directly to heat exchanger240.

Irrespectively of how its thermal energy is taken care of, the hydrogen gas from furnace220is cooled until it (or, more importantly, the charged material) reaches a temperature of below 100° C., in order to avoid reoxidation of the charged material when later being exposed to air. The temperature of the charged material can be measured directly, in a suitable manner such as the one described above, or indirectly, by measuring in a suitable manner the temperature of the hydrogen gas leaving via exit conduit173.

The cooling of the hydrogen gas may take place while maintaining the overpressure of the hydrogen gas, or the pressure of the hydrogen gas may be lowered as a result of the hot hydrogen gas being allowed to occupy a larger volume (of the closed loop conduits and heat exchangers) once valves V10and V12are opened.

In a subsequent step, the hydrogen gas is evacuated from the furnace220space120, and collected in container280. This evacuation may be performed by the vacuum pump260, possibly in combination with the compressor270, whereby the control device opens valves V3, V5, V6, V8, V10and V12, and closes the other valves, and operates the vacuum pump260and compressor270to displace the cooled hydrogen gas to the container280for used hydrogen gas. The evacuation is preferably performed until a pressure of at the most 0.5 bars, or even at the most 0.3 bars, is detected inside the furnace space120.

Since the furnace space120is closed, only the hydrogen gas consumed in the chemical reduction reaction has been removed from the system, and the remaining hydrogen gas is the one which was necessary to maintain the hydrogen gas/water vapour balance in the furnace space120during the main heating step. This evacuated hydrogen gas is fully useful for a subsequent batch operation of a new charge of metal material to be reduced.

In a subsequent step, the furnace space120is opened, such as by releasing the fastening means111and opening the upper part110. The container140is removed and is replaced with a container with a new batch of charged metal material to be reduced.

In a subsequent step, the removed, reduced material may then be arranged under an inert atmosphere, such as a nitrogen atmosphere, in order to avoid reoxidation during transport and storage.

For instance, the reduced metal material may be arranged in a flexible or rigid transport container which is filled with inert gas. Several such flexible or rigid containers may be arranged in a transport container, which may then be filled with inert gas in the space surrounding the flexible or rigid containers. Thereafter, the reduced metal material can be transported safely without running the risk of reoxidation.

The following table shows the approximate equilibrium between hydrogen gas H2and water vapour H2O for different temperatures inside the furnace space120:

Temperature (° C.):400450500550600H2(vol-%):9587827876H2O (vol-%):513182224

At atmospheric pressure, about 417 m3hydrogen gas H2is required to reduce 1000 kg of Fe2O3, and about 383 m3hydrogen gas H2is required to reduce 1000 kg of Fe3O4.

The following table shows the amount of hydrogen gas required to reduce 1000 kg of Fe2O3and Fe3O4, respectively, at atmospheric pressure and in an open system (according to the prior art), but at different temperatures:

Temperature (° C.):400450500550600Nm3H2/tonne Fe2O3:83403208231718951738Nm3H2/tonne Fe3O4:76602946212817411596

The following table shows the amount of hydrogen gas required to reduce 1000 kg of Fe2O3and Fe3O4, respectively, at different pressures and for different temperatures:

Temperature (° C.):400450500550600Nm3H2/tonne Fe2O3:1 bar834032082317189517382 bars4170160411589488693 bars278010697726325794 bars20858025794744345 bars16686424633793486 bars1390535386316290Nm3H2/tonne Fe3O4:1 bar766029462128174115962 bars3830147310648707983 bars25539827095805324 bars19157375324353995 bars15325894263483196 bars1277491355290266

As described above, the main heating step according to the present invention is preferably performed up to a high pressure and a high temperature. During the majority of the main heating step, it has been found advantageous to use a combination of a heated hydrogen gas temperature of at least 500° C. and a furnace space120pressure of at least 5 bars.

Above, preferred embodiments have been described. However, it is apparent to the skilled person that many modifications can be made to the disclosed embodiments without departing from the basic idea of the invention.

For instance, the geometry of the furnace100may differ, depending on the detailed prerequisites.

The heat exchanger160is described as a tube heat exchanger. Even if this has been found to be particularly advantageous, it is realized that other types of gas-gas heat exchangers/condensers are possible. Heat exchanger240may be of any suitable configuration.

The surplus heat from the cooled hydrogen gas may also be used in other processes requiring thermal energy.

The metal material to be reduced has been described as iron oxides. However, the present method and system can also be used to reduce metal material such as the above mentioned metal oxides, such as of Zn and Pb, that evaporate at temperatures below about 600° C.

The present direct reduction principles can also be used with metal materials having higher reduction temperatures than iron ore, with suitable adjustments to the construction of the furnace100, such as with respect to used construction materials.

Hence, the invention is not limited to the described embodiments, but can be varied within the scope of the enclosed claims.