Process and system for reducing metal oxides
A two-reactor process and system for reducing metal oxides addresses the challenges of using green hydrogen by minimizing equipment size and costs through strategic feeding and recycling, achieving efficient and cost-effective metal oxide reduction.
Patent Information
- Authority / Receiving Office
- AE · AE
- Patent Type
- Applications
- Filing Date
- 2023-12-22
AI Technical Summary
The mining industry faces challenges in using green hydrogen due to its high cost and the need for large equipment to handle by-product gases, leading to expensive production facilities and utility demands, while conventional methods using fossil fuels are environmentally unfriendly.
A two-reactor process and system for reducing metal oxides, where pre-reduced metal oxides are fed to the upper half of a second reactor or a solid gas separator before entering, with a specific ratio of reducing agents and recycling gases to minimize equipment size and reduce losses.
This approach reduces capital expenditure, minimizes hydrogen loss, and optimizes the reduction process by decreasing reactor volumes and operational costs, while utilizing green hydrogen efficiently.
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Abstract
Description
Full specificationPROCESS AND SYSTEM FOR REDUCING METAL OXIDES FIELD OF THE INVENTIONThe present invention relates to a process for reducing metal oxides and a system for reducing metal oxides. BACKGROUND OF THE INVENTIONThe mining industry consumes a lot of energy. Therefore, it is important to optimise the processes related to metals and minerals in order to achieve both economic and environmental advantages. The step of reducing metal oxides in the mining industry has a number of issues, for example the need to use green reducing agents, such as “green” hydrogen. Conventionally, hydrogen production was predominantly based on fossil fuels with respective CO2 emissions yielding to so called "grey" hydrogen. This “grey” hydrogen is not significantly expensive but not environmentally friendly. Recently, "green" hydrogen, which is in contrast based on renewable energy and does not have any CO2 emissions, has been utilized in the mining industry. However, the "green" hydrogen is expensive and even small losses of the "green" hydrogen during production can drastically affect the profitability of the production process. Additionally, metal reduction processes can generate large volumes of gas as a by-product. These large volumes require large process equipment, which results in expensive production facilities and a large need for utilities. As such, there is a need for processes and systems for reducing metal oxides that overcome the above challenges. SUMMARYAccording to a first aspect, a process for reducing metal oxides is provided, the process comprising:a. feeding metal oxides and a reducing agent to a first reactor to obtain pre-reduced metal oxides and a first gaseous by-product; andb. feeding the pre-reduced metal oxides and a reducing agent to a second reactor to obtain further reduced metal oxides and a second gaseous by-product;wherein the pre-reduced metal oxides are fed to the upper half of the second reactor and / or wherein the pre-reduced metal oxides are fed to a solid gas separator prior to being fed to the second reactor. According to a second aspect, a system for reducing metal oxides is provided, the system comprising:a. a first reactor comprising means for feeding metal oxides and means for feeding a reducing agent; andb. a second reactor comprising means for feeding pre-reduced metal oxides and means for feeding a reducing agent;wherein the means for feeding pre-reduced metal oxides is arranged in the upper half of the second reactor and / or wherein the system comprises a solid gas separator between the first and the second reactor. BRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:Figure 1 is a schematic illustration of a process for reducing a metal.Figure 2 is a schematic illustration of a process for reducing a metal comprising separating gas and solids between two reactors and a condenser. DETAILED DESCRIPTIONIt is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above, instead they may vary within the scope of the claims. Throughout this specification, the term “essentially” may refer to close to all, such as 97 wt.%, or 98 wt.%, or 99 wt.%, or 99.9 wt.%, or 100 wt.%. Throughout this specification, “at least x%” may refer to x to 100%. A process for reducing metal oxides According to a first aspect, a process for reducing metal oxides is provided, the process comprising:a. feeding metal oxides (100, 200) and a reducing agent (101, 201) to a first reactor (106, 206) to obtain pre-reduced metal oxides (116b, 216b) and a first gaseous by-product (116a, 216a); andb. feeding the pre-reduced metal oxides (116b, 216b) and a reducing agent (110, 210) to a second reactor (114, 214) to obtain further reduced metal oxides (115, 215) and a second gaseous by-product (117a, 217a);wherein the pre-reduced metal oxides (116b, 216b) are fed to the upper half of the second reactor (114, 214) and / or wherein the pre-reduced metal oxides (216b) are fed to a solid gas separator (208) prior to being fed to the second reactor (214). Pre-reduced metal oxides (116b, 216b) may refer to metal oxides that contain less oxygen compared to the metal oxides prior to a reduction step. Further reduced metal oxides (115, 215) may refer to metal oxides that contain even less oxygen than the pre-reduced metal oxides (116b, 216b). The pre-reduced metal oxides (116b, 216b) may be fed to the upper half of the second reactor (114, 214), such as to the upper 1 / 3 of the second reactor (114, 214), or to the upper 1 / 4 of the second reactor (114, 214). For the avoidance of doubt, the pre-reduced metal oxides (116b, 216b) may be fed to somewhere between the top of the second reactor (114, 214) and the upper half of the second reactor (114, 214), such as to somewhere between the top of the second reactor (114, 214) and the upper 1 / 3 of the second reactor (114, 214), or to somewhere between the top of the second reactor (114, 214) and the upper 1 / 4 of the second reactor (114, 214). In one embodiment, the pre-reduced metal oxides (216b) are fed to a solid gas separator (208) prior to being fed to the second reactor (214), which after the pre-reduced metal oxides (216b) may be fed to anywhere between the bottom and top of the second reactor (214). Alternatively, the pre-reduced metal oxides (216b) may be fed to the solid gas separator (208) and then to somewhere between the top of the second reactor (214) and the upper half of the second reactor (214), such as to somewhere between the top of the second reactor (214) and the upper 1 / 3 of the second reactor (214), or to somewhere between the top of the second reactor (214) and the upper 1 / 4 of the second reactor (214). The reducing agent (101, 201, 110, 210) may be fed to the bottom of the first and second reactors (106, 206, 114, 214) through a nozzle grid, a tuyere or an open nozzle, e.g. a venturi nozzle. For the avoidance of doubt, the metal oxides (100, 200) may be converted to pre-reduced metal oxides (116b, 216b) in the first reactor (106, 206) by contacting the metal oxides (100, 200) with the reducing agent (101, 201). The mixture in the first reactor (216) including the pre-reduced metal oxides (116b, 216b) may then be transported from the first reactor (106, 206), optionally via the solid gas separator (208), to the second reactor (114, 214). In the second reactor (114, 214), the pre-reduced metal oxides (116b, 216b) may be further reduced to obtain further reduced metal oxides (115, 215) by contacting the pre-reduced metal oxides (116b, 216b) with the reducing agent (110, 210). The first and second gaseous by-products (116a, 216a, 117a 217a) may be formed in the reduction processes. The inventors have found that it is useful to feed the pre-reduced metal oxides (116b, 216b) to the upper half of the second reactor (114, 214). The reason being that the pre-reduced metal oxides (116b, 216b) often are conveyed to the second reactor (114, 214) together with a gas, such as a by-product or a diluent, and when fed to the upper part of the second reactor (114, 214), the gas can be discharged from the second reactor (114, 214) through the roof of the reactor. The process according to the first aspect may comprise at least partially separating the first gaseous by-product (216a) from the pre-reduced metal oxides (216b) through the solid gas separator (208). The process may comprise at least partially separating the first gaseous by-product (216a) from the pre-reduced metal oxides (216b) through the solid gas separator (208) between the first and the second reactor (214), i.e. prior to feeding the pre-reduced metal oxides (216b) to the second reactor (214), such that the amount of by-product fed to the second reactor (214) is decreased. The process may comprise separating essentially all of the first gaseous by-product (216a) from the pre-reduced metal oxides (216b) through the solid gas separator (208). When separating the pre-reduced metal oxides (216b) from the first gaseous by-product (216a) between the first and second reactor (214), it is not necessarily required to feed the pre-reduced metal oxides (216b) to the upper half of the second reactor (214). However, this could be beneficial since some by-product could still be present with the pre-reduced metal oxides (216b). Additionally, some unreacted reducing agent (201) may be present in the feed (116, 216) exiting the first reactor (206), which may also be separated from the pre-reduced metal oxides (216b) in the solid gas separator (208). For the avoidance of doubt, the pre-reduced metal oxides (116b, 216b) may be solid and the first gaseous by-product (116a, 216a) and reducing agent (101, 201) gaseous in the temperature and pressure in which the process is operated. The inventors have found that a number of advantages are achieved when at least partially separating the first gaseous by-product (216a) from the pre-reduced metal oxides (216b) through the solid gas separator (208). For example, the volume of the second reactor (214) as well as all the equipment used in connection with the second reactor (214) may be reduced in size. This significantly reduces the capital expenditure of the system needed for the process. The process according to the first aspect may comprise feeding the reducing agent (101, 201, 110, 210) to the first reactor (106, 206) and the second reactor (114, 214) in a ratio of 1:17 to 1:7, or 1:14 to 1:9, or 1:12 to 1:10. For the avoidance of doubt the above ratio may refer to the split of reducing agent (101, 201) fed to the first reactor (106, 206) compared to the split of the reducing agent (110, 210) fed to the second reactor (114, 214). It has been found that the above split provides the added utility of achieving the desired oxygen removal in the two reactors. About 5 - 45 wt.-%, or 18 – 40 wt.-%, or 20 – 35 wt.-%, or 25 – 33 wt.-% of the oxygen of the metal oxides (100, 200) according to the first aspect may be removed in the first reactor (106, 206). About 50 - 90 wt.-%, or 60 – 80 wt.-%, or 65 – 75 wt.-%, of the oxygen of the metal oxides (116b, 216b) may be removed in the second reactor (114, 214). For the avoidance of doubt, the above wt.-% may refer to the wt.-% of oxygen that has been removed when compared to the total amount of oxygen present in the metal oxide prior to the reduction process. When removing the above amount of oxygen in the first reactor (106, 206), the metal oxides (100, 200) and pre-reduced metal oxides (116b, 216b) remain in the Wustite stage in the first reactor (106, 206). When the metal oxides (100, 200) and pre-reduced metal oxides (116b, 216b) remain in the Wustite stage, high temperatures can be used in the first reactor (106, 206) without the issue of the particles becoming sticky and agglomerating. Higher temperatures promote the reduction process. Therefore, the above oxygen removal amount allows the residence time to be lower in the first reactor (106, 206) and thus the time required for the whole reduction process in both the first and second reactor (114, 214) together is lower compared to a process where the reduction is taking place in only one reactor or in several reactors where the temperature is not significantly altered between the reactors. The process may further comprise at least partially recycling a second reactor gas (117, 217) in the second reactor (114, 214). The second reactor gas (117, 217) may comprise the second gaseous by-product (117a, 217a) and the reducing agent (110, 210). The second reactor gas (117, 217) may essentially consist of the second gaseous by-product (117a, 217a) and the reducing agent (110, 210). These gases or at least one of these gases may be used as a fluidisation gas in the second reactor (114, 214). As such, the process may comprise recycling at least a part the second reactor gas (117, 217) in the second reactor (114, 214) in an amount sufficient to fluidise the second reactor (114, 214). For the avoidance of doubt, some of the metal oxide particles may be carried over to the second reactor gas (117, 217) to be recycled. Therefore, the process may further comprise separating solids from the second reactor gas (117, 217) when recycling the second reactor gas (117, 217). The solids may be returned to the second reactor (114, 214). By recycling the second reactor gas (117, 217), the unreacted reducing agent (110, 210) may be utilised as a fluidisation gas in the second reactor (114, 214). By separating and returning solids from the second reactor gas (117, 217) to the second reactor (114, 214), the operation of the recycling of the second reactor gas (117, 217) is not disturbed. Further, the loss of reducing agent (110, 210) is minimised as the reducing agent (110, 210) is recycled. The process according to the first aspect may further comprise at least partially removing the second gaseous by-product (217a) from the second reactor gas (217) through condensing to create a separated reducing agent (217b) and feeding the separated reducing agent (217b) to the second reactor (214). By condensing the second gaseous by-product (217a), it is not necessary to bleed any of the second reactor gas (217) from the process. As such, the loss of unreacted reducing agent (110, 210) is prevented. The process may comprise removing essentially all of the second gaseous by-product (217a) from the second reactor gas (217). The first and second gaseous by-products (116a, 216a, 117a, 217a) according to the first aspect may comprise water vapour. In one embodiment, the first and second gaseous by-products (116a, 216a, 117a, 217a) comprise at least 50 wt.% water vapour, such as at least 70 wt.% water vapour, such as at least 95 wt.% water vapour. The first and second gaseous by-products (116a, 216a, 117a, 217a) may essentially consist of water vapour. It is beneficial to obtain water as the first and second by-products (116a, 216a, 117a, 217a) as this can be removed from the process through condensing. The reducing agent (101, 201, 110, 210) according to the first aspect may comprise hydrogen. The reducing agent (101, 201, 110, 210) may essentially consist of hydrogen. It is beneficial to use hydrogen as the reducing agent (101, 201, 110, 210) as water vapour is then obtained as the first and second gaseous by-products (116a, 216a, 117a, 217a). The metal oxides (100, 200) according to the first aspect may comprise iron oxides, the pre-reduced metal oxides (116b, 216b) may comprise pre-reduced iron oxides, and the further reduced metal oxides (115, 215) may comprise further reduced iron oxides. The metal oxides (100, 200) may essentially consist of iron oxides, the pre-reduced metal oxides (116b, 216b) may essentially consist of pre-reduced iron oxides, and the further reduced metal oxides (115, 215) may essentially consist of further reduced iron oxides. The first reactor (106, 206) and the second reactor (114, 214) according to the first aspect may be fluidised bed reactors, preferably wherein the first reactor (106, 206) may be operated in the turbulent or fast fluidisation regime and the second reactor (114, 214) may be a circulated fluidised bed reactor. The temperature in the first reactor (106, 206) may be at least 80 °C higher than the temperature in the second reactor (114, 214), such as at least 150 °C higher, or at least 250 °C higher, or at least 300 °C higher, or at least 400 °C higher. The temperature in the first reactor (106, 206) may be 80 to 400 °C higher than the temperature in the second reactor (114, 214), such as 150 to 400 °C higher, or 250 to 400°C higher, or 300 to 400 °C higher. The temperature in the first reactor (106, 206) may be 750 - 1050 °C, or 850 - 950 °C and the temperature in the second reactor (114, 214) may be 520 - 800 °C, or 620 - 700 °C. The pressure in the first reactor (106, 206) may be 3,5 - 4,5 bar, or 4 bar and the pressure in the second reactor (114, 214) may be 3,5 - 4,5 bar, or 4 bar. It has been found that these conditions provide the optimal process for reducing metal oxides with regards to time, cost, and quality of the end product. The process may be a continuous process such that all equipment is connected, and the material can freely flow through the process equipment. A process control system may be applied to the process to optimise the process parameters. A system for reducing metal oxides The description related to the process above apply to the system below and the description related to the system below apply to the of the process above. According to a second aspect, a system for reducing metal oxides is provided, the system comprising:a. a first reactor (106, 206) comprising means for feeding metal oxides (103, 203) and means for feeding a reducing agent (104, 204); andb. a second reactor (114, 214) comprising means for feeding pre-reduced metal oxides (121, 221) and means for feeding a reducing agent (112, 212);wherein the means for feeding pre-reduced metal oxides (121, 221) is arranged in the upper half of the second reactor (114, 214) and / or wherein the system comprises a solid gas separator (208) between the first (206) and the second reactor (214). The means for feeding pre-reduced metal oxides (121, 221) may be arranged in the upper half of the second reactor (114, 214), such as in the upper 1 / 3 of the second reactor (114, 214), or in the upper 1 / 4 of the second reactor (114, 214). For the avoidance of doubt, the means for feeding pre-reduced metal oxides (121, 221) may be arranged somewhere between the top of the second reactor (114, 214) and the upper half of the second reactor (114, 214), such as somewhere between the top of the second reactor (114, 214) and the upper 1 / 3 of the second reactor (114, 214), or somewhere between the top of the second reactor (114, 214) and the upper 1 / 4 of the second reactor (114, 214). In one embodiment, the solid gas separator (208) is arranged between the first (206) and the second reactor (214) such that feed (216) exiting the first reactor (206), such as pre-reduced metal oxides (216b), can flow through the solid gas separator (208) before entering the second reactor (214). Then, the means for feeding pre-reduced metal oxides (221) may be arranged anywhere in the second reactor (214) such as somewhere between the top of the second reactor (214) and the upper half of the second reactor (214), such as somewhere between the top of the second reactor (214) and the upper 1 / 3 of the second reactor (214), or somewhere between the top of the second reactor (214) and the upper 1 / 4 of the second reactor (214). The solid gas separator (208) being arranged between the first and the second reactors (206, 214) may refer to the solid gas separator (208) being arranged such that material, such as the pre-reduced metal oxides (216b), may flow through the solid gas separator (208) prior to being fed to the second reactor (214). For the avoidance of doubt, the solid gas separator (208) is not necessarily physically located between the two reactors, but it can be located between the reactors. The means for feeding metal oxides (103, 203) and means for feeding pre-reduced metal oxides (121, 221) may comprise pumps and a pipes. The means for feeding reducing agent may comprise pumps and pipes. The means for feeding reducing agent may be arranged in the bottom of the first and second reactors (106, 206, 114, 214. The means for feeding metal oxides (103, 203) may be arranged in the bottom third of the first reactor (106, 206). The solid gas separator (208) may comprise a gas cyclone. The metal oxides (100, 200) according to the second aspect may comprise iron oxides and the pre-reduced metal oxides (116b, 216b) may comprise pre-reduced iron oxides and / or the reducing agent (101, 201, 110, 210) may comprise hydrogen. The first reactor (106, 206) according to the second aspect may be configured to produce pre-reduced metal oxides (116b, 216b) and a first gaseous by-product (116a, 216a) and the solid gas separator (208) may be configured to at least partially separate the first gaseous by-product (216a) from the pre-reduced metal oxides (216b). The first gaseous by-product (116a, 216a) may comprise water vapour. The system according to the second aspect may further comprise means for recycling gas (122, 222) in the second reactor (114, 214). The means form recycling gas may be arranged such that gas, i.e. a second reactor gas (117, 217), exits the second reactor (114, 214) from the top of the second reactor (114, 214) and is conveyed to the bottom of the second reactor (114, 214). The means for recycling gas (122, 222) may comprise a pipe and a compressor. The system according to the second aspect may further comprises a condenser (220) in connection with the means for recycling gas (222) in the second reactor (214). The second reactor (114, 214) according to the second aspect may be configured to produce further reduced metal oxides (115, 215) and a second gaseous by-product (117a, 217a) and the condenser (220) may be configured to at least partially remove the second gaseous by-product (217a) from the second reactor gas (217). The condenser (220) may be configured to condense at least a part of the second gaseous by-product (217a) and to separate at least a part of the second gaseous by-product (217a) from the second reactor gas (217). The condenser (220) may be configured to condense essentially all of the second gaseous by-product (217a) and to separate essentially all of the second gaseous by-product (217a) from the second reactor gas (217). The condenser (220) may be a scrubber or a packing tower sprayed with liquid water or a combination of both. The second reactor gas (117, 217) may further carry some pre-reduced metal oxides (116b, 216b) or further reduced metal oxides (115, 215) in solid form. Therefore, the means for recycling gas (122, 222) may further comprise a second solid gas separator. The second solid gas separator may be configured to separate the pre-reduced metal oxides (116b, 216b) or further reduced metal oxides (115, 215) from the second reactor gas (117, 217) and to return the pre-reduced metal oxides (116b, 216b) or further reduced metal oxides (115, 215) to the second reactor (114, 214). The second solid gas separator may be a gas cyclone. The second gaseous by product (117a, 217a) may comprise water vapour. The first reactor (106, 206) and the second reactor (114, 214) according to the second aspect may be fluidised bed reactors, preferably the first reactor (106, 206) is operated in the turbulent or fast fluidisation regime and the second reactor (114, 214) is a circulated fluidised bed reactor. The system may further comprise means for heating the reactors. The means for heating the reactors may comprise gas-gas heat exchangers and / or gas- or electrical fired gas heaters. EXAMPLES This example illustrates feeding 200 t / h of preheated ore with an Fe content of 64.1% at 884°C through a conveyor and mixed with 600 Nm3 / h of H2O vapor into the first reactor. A process gas (a potion of 7% of the total process gas) of 31380 Nm3 / h at 560°C containing 91% H2, 8% N2, and 1% H2O, was fed into the first reactor as reducing media. The prereduction took place in the first reactor at a temperature of 774°C and the reduction degree of the solids was 22.5% with an iron total 68.3%. The solids were separated from the reacted gas in a cyclone separator and fed into the second reactor via a transfer seal pot. The effect of the prereduction (or oxygen removal before being fed into the second reactor) on the overall process can be observed by keeping a H2 / H2O ratio of 6.45 in the second reactor off gas. As a result, the recycle gas flow in the reduction section was decreased by 23% representing significant savings in both Capex and Opex, given the impact of recycle gas volume flow on the size of the equipment in the reduction circuit, the lower heat required to increase the process gas temperature for the reduction process as well as the electric power consumption required by the recycle gas compressor is decreased.
Claims
1. A process for reducing metal oxides, the process comprising:a. feeding metal oxides (100, 200) and a reducing agent (101, 201) to a first reactor (106, 206) to obtain pre-reduced metal oxides (116b, 216b) and a first gaseous by-product (116a, 216a); andb. feeding the pre-reduced metal oxides (116b, 216b) and a reducing agent (110, 210) to a second reactor (114, 214) to obtain further reduced metal oxides (115, 215) and a second gaseous by-product (117a, 217a);wherein the pre-reduced metal oxides (116b, 216b) are fed to the upper half of the second reactor (114, 214) and / or wherein the pre-reduced metal oxides (116b, 216b) are fed to a solid gas separator (208) prior to being fed to the second reactor (114, 214). 2. The process according to claim 1, wherein the process comprises at least partially separating the first gaseous by-product (216a) from the pre-reduced metal oxides (216b) through the solid gas separator (208). 3. The process according to claims 1 or 2, wherein the process comprises feeding the reducing agent (101, 201, 110, 210) to the first reactor (106, 206) and the second reactor (114, 214) in a ratio of 1:17 to 1:7, or 1:14 to 1:9, or 1:12 to 1:
10. 4. The process according to any preceding claims, wherein about 5 - 45 wt.-%, or 18 – 40 wt.-%, or 20 – 35 wt.-%, or 25 – 33 wt.-% of the oxygen of the metal oxides (100, 200) is removed in the first reactor (106, 206). 5. The process according to any preceding claims, wherein the process further comprises at least partially recycling a second reactor gas (117, 217) in the second reactor (114, 214). 6. The process according to claim 5, wherein the process further comprises at least partially removing the second gaseous by-product (217a) from the second reactor gas (217) through condensing to create a separated reducing agent (217b) and feeding the separated reducing agent (217b) to the second reactor (214). 7. The process according to any preceding claims, wherein the first and second gaseous by-products (116a, 216a, 117a, 217a) comprise water vapour. 8. The process according to any preceding claims, wherein the reducing agent (101, 201, 110, 210) comprises hydrogen. 9. The process according to any preceding claims, wherein the metal oxides (100, 200) comprise iron oxides, the pre-reduced metal oxides (116b, 216b) comprise pre-reduced iron oxides, and the further reduced metal oxides (115, 215) comprise further reduced iron oxides. 10. The process according to any preceding claims, wherein the first reactor (106, 206) and the second reactor (114, 214) are fluidised bed reactors, preferably wherein the first reactor (106, 206) is operated in the turbulent or fast fluidisation regime and the second reactor (114, 214) is a circulated fluidised bed reactor. 11. The process according to any preceding claims, wherein the temperature in the first reactor (106, 206) is at least 80 °C higher than the temperature in the second reactor (114, 214), such as at least 150 °C higher, or at least 250 °C higher, or at least 300 °C higher, or at least 400 °C higher. 12. A system for reducing metal oxides, the system comprising: a. a first reactor (106, 206) comprising means for feeding metal oxides (103, 203) and means for feeding a reducing agent (104, 204); andb. a second reactor (114, 214) comprising means for feeding pre-reduced metal oxides (121, 221) and means for feeding a reducing agent (112, 212);wherein the means for feeding pre-reduced metal oxides (121, 221) is arranged in the upper half of the second reactor (114, 214) and / or wherein the system comprises a solid gas separator (208) between the first (206) and the second reactor (214). 13. The system according to claim 12, wherein the metal oxides (100, 200) comprise iron oxides and the pre-reduced metal oxides (116b, 216b) comprises pre-reduced iron oxides and / or wherein the reducing agent (101, 201, 110, 210) comprises hydrogen. 14. The system according to claims 12 or 13, wherein the first reactor (106, 206) is configured to produce pre-reduced metal oxides (116b, 216b) and a first gaseous by-product (116a, 216a) and wherein the solid gas separator (208) is configured to at least partially separate the first gaseous by-product (216a) from the pre-reduced metal oxides (216b). 15. The system according to claim 14, wherein the first gaseous by-product (216a) comprises water vapour. 16. The system according to any of claims 12 to 15, wherein the system further comprises means for recycling gas (122, 222) in the second reactor (114, 214). 17. The system according to claim 16, wherein the system further comprises a condenser (220) in connection with the means for recycling gas (222) in the second reactor (214). 18. The system according to claim 17, wherein the second reactor is configured to produce further reduced metal oxides (115, 215) and a second gaseous by-product (117a, 217a) and wherein the condenser (220) is configured to at least partially remove the second gaseous by-product (217a) from the second reactor gas (217). 19. The system according to claim 18, wherein the second gaseous by product (117a, 217a) comprises water vapour. 20. The system according to any of claims 12 to 19, wherein the first reactor (106, 206) and the second reactor (114, 214) are fluidised bed reactors, preferably wherein the first reactor (106, 206) is operated in the turbulent or fast fluidisation regime and the second reactor (114, 214) is a circulated fluidised bed reactor.