Method of producing green steel

Abstract

The present invention relates to a method of producing green steel by reduction of iron oxides using hydrogen. The inventive method makes use of mining waste as starting material for H2 generation by SDE process or a sulfur-iodine-process. Side products can be utilized in the steelmaking process. This is achieved by a method according to the present invention comprising the following steps: a) a part or all of the iron oxide used as raw material for steelmaking is reduced by hydrogen, b) a part or all hydrogen required for the reduction of iron oxide is generated via a SO2-depolarized electrolyzer (SDE) process or a sulfur-iodine-process, and c) a part or all of the diluted sulfuric acid obtained from step b) is used for at least one of i. steel pickling, ii. reaction with steel mill dust for generation of iron sulfate (FeSO4 or Fe2(SO4)3), and iii. increasing the concentration of said diluted sulfuric acid by vacuum evaporation of water using off-heat from steelmaking or pyrite roasting.

Description

[0001] Method of producing green steel

[0002] TECHNICAL FIELD

[0003] The invention relates to a method of producing green steel utilizing green hydrogen (H2) for direct iron (Fe) reduction.

[0004] TECHNICAL BACKGROUND

[0005] Green hydrogen (i.e., hydrogen produced by carbon free energy sources) is the most promising energy vector seen for the future to reach decarbonization in steelmaking and to meet climate goals.

[0006] Pyrite is today mainly seen in mining as an economically worthless material (i.e. waste) , producing environmental hazards such as acid mine drainage (AMD) . It is actively supressed in the flotation process to prevent that it contaminates the target metal concentrate, and therefore ends up in the tailings, i.e., in the mine waste. Only if pyrite contains economically valuable concentrations of trace elements, such as Au, Ag, or other critical elements like, for example, Co, Cd, Te, Bi, or Se, some specific pyrite processing might be of economic interest.

[0007] Industrial decarbonization requires large amounts of green hydrogen. Water-splitting by electrolysis is the standard path to produce hydrogen. Alkaline Electrolysis (AEL) or Proton Exchange Membrane Electrolysis (PEM) are the standard techniques, whereby AEL has been used at industrial scale for more than 100 years, while PEM is the latest technology with several years of industrial application. The large amounts of energy required for the H2gas production usually are covered by natural gas (grey hydrogen) or nuclear energy (pink hydrogen) and have now to be replaced by renewable energy sources (i.e., wind, solar, hydro) for the production of green hydrogen . Whereas classical water splitting requires enormous amounts of energy (1.23 V) , SO2 depolarized electrolysis has the advantage that it theoretically requires only 14% of the energy to produce hydrogen (0.17 V) . The Hybrid-Sulfur (HyS) process, where SO2 is thermochemically produced from sulfuric acid (H2SO4) using solar energy (at around 900°C) and then is used via a SCp-depolarized electrolyzer (SDE) , or the sulfur- iodine-cycle (S-I cycle) to produce hydrogen is seen as an energetically more convenient technology (Sattler et al. 2017) .

[0008] Thermochemical water-splitting cycles were the focus of research over the past 50 years. In these operations, SO2 is required as base reagent, which together with water is used to produce H2gas and diluted H2SO4. This process has the advantage that its theoretical energy requirement is only 14% of the energy demand for, e.g., water-splitting by electrolysis (0.17 vs 1.23 V) . Currently, the sulfur-iodine- cycle (S-I-cycle; Fig. 1) and the hybrid sulfur cycle (HyS- cycle, Fig. 2) are the most promising candidates for industrial application .

[0009] The hybrid sulfur cycle (HyS-cycle) makes use of a SO2- depolarized electrolyzer (SDE) and is described in US 4,412,895. A schematic description of an SDE cycle is given in Fig. 2. The S-I-cycle is described in US 4,089, 940 and makes use of the Bunsen reaction instead and is therefore also called three step cycle (Fig. 1) .

[0010] Both cycles require the thermal splitting of sulfuric acid (H2SO4) at around 850°C to produce sulfur dioxide (SO2) , leading to high energy requirement and taking away a significant part of the energy savings for H2production.

[0011] The sulfur-iodine-cycle (S-I-cycle) and the hybrid sulfur cycle (HyS-cycle involve the generation of diluted sulfuric acid (10-90%, typically 30-70%) , which needs to be concentrated and cracked into H2O, SO2 and O2 with the SO2 being recycled into the H2generation step. References

[0012] Safari F, Dincer I (2020) A review and comparative evaluation of thermochemical water splitting cycles for hydrogen production. Energy Conversion and Management 205:112182. doi: https: / / doi.Org / 10.1016 / i.enconman.2019.112182.

[0013] Sattler C, Roeb M, Agrafiotis C, Thomey D (2017) Solar hydrogen production via sulphur based thermochemical water-splitting. Solar Energy 156:30-47. doi: https: / / doi.Org / 10.1016 / j.solener.2017.05.060.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] [Fig. 1] Fig. 1 shows H2gas production via a sulfur-iodine- cycle (S-I-cycle) by means of a schematic diagram for a three- step S-I thermochemical cycle with the Bunsen reaction in the central part after (Safari and Dincer 2020) .

[0016] [Fig. 2] Fig. 2 shows H2gas production via a hybrid sulfur cycle (HyS-cycle) by means of a schematic diagram of a Hybrid Sulfur thermochemical cycle with a S02-depolarized

[0017] Electrolyzer (SDE) to separate H2.

[0018] [Fig. 3] Fig. 3 shows a schematic presentation of the inventive method of DRI production from pyrite roasting with H2production via a S02-depolarized Electrolyzer (SDE) . Squares are educts and circles are products.

[0019] SUMMARY OF THE INVENTION

[0020] Technical Problem

[0021] Traditional steelmaking processes rely heavily on carbon-based fuels, such as coke, for the reduction of iron oxide, which results in significant carbon dioxide (C02) emissions, contributing to environmental pollution and climate change. Additionally, the production and procurement of raw materials for steelmaking can impose significant energy demands, and the management of by-products and waste streams poses environmental and economic challenges.

[0022] Therefore, there is a need for a more sustainable, cost- effective, and environmentally friendly method of producing steel by an optimized and efficient process and with avoiding waste problems.

[0023] Main challenges are:

[0024] • Reducing CO2Emissions

[0025] • Optimizing the steelmaking process to be more energyefficient and cost-effective

[0026] • Utilization of by-products, thereby minimizing waste and enhancing resource efficiency

[0027] Solution to Problem

[0028] The present invention overcomes the above problems by providing a method of producing steel wherein a) a part or all of the iron oxide used as raw material for steelmaking is reduced by hydrogen, b) a part or all of the hydrogen required for the reduction of iron oxide is generated via a SO2- depolarized electrolyzer (SDE) process or a sulfuriodine-process, and c) a part or all of the diluted sulfuric acid obtained from step b) is used for at least one of i. steel pickling, ii. reaction with steel mill dust for generation of iron sulfate (FeSO4or Fe2(SO4)3) , and iii. increasing the concentration of said diluted sulfuric acid by vacuum evaporation of water using off-heat from steelmaking or pyrite roasting . A key feature of the inventive method is the use of a S02- depolarized electrolyzer (SDE) process or a sulfur-iodine- process to produce H2from S02.

[0029] These features allow generation of green H2(without CO2emission) from SO2with a much better energy balance than traditional water electrolysis and also much better energy balance than the Sulfur-Iodine Cycle (Fig. 1) and the Hybrid- Sulfur Cycle (Fig. 2) as prior art. Both cycles (Fig. 1 and Fig. 2) require the thermal splitting of sulfuric acid (H2SO4) at around 850°C to produce sulfur dioxide (SO2) , thereby leading to a high energy requirement and taking away a significant part of the energy savings for H2production. In contrast thereto, fresh SO2is used in the inventive process and diluted sulfuric acid is produced.

[0030] Hydrogen is considered to be „green H2" if it is produced using electricity from renewable sources.

[0031] The presented technology preferably takes advantage of the internal energy content of pyrite which can be harnessed by relying on its exothermal oxidation reaction. Thus, the here presented invention avoids producing S02by high temperature cracking of H2SO4. S02is produced by oxidation of pyrite, other sulfides, elementary sulfur or other S containing materials.

[0032] The inventive method aims to address the problems of prior art by introducing a hydrogen-based reduction process, leveraging the S02-depolarized electrolyzer (SDE) or sulfur-iodine- process for hydrogen production, and effectively integrating by-products into the steelmaking workflow.

[0033] The present inventors have found that green steel can be produced by using green hydrogen, wherein the energy required for the production of green hydrogen can be reduced significantly by utilizing an S02-depolarized electrolyzer (SDE) process or a sulfur-iodine-process. The SDE process currently requires 16-31 MWh of electricity to produce 1 t of hydrogen, while PEM requires 50-60 MWh / 1 H2. This amounts to a tremendous saving of about 50% (or even >50%) electricity by using SDE process.

[0034] A prerequisite for using the S02-depolarized electrolyzer (SDE) process or the sulfur-iodine-process is that SO2can be economically supplied, e.g. by oxidizing S containing materials .

[0035] The SO2entering the inventive process ends up as diluted H2SC>4 with a concentration of 10-90% by mass, typically 30-70% by mass .

[0036] If pyrite is used as sulfur source, iron oxide (hematite Fe203 and / or magnetite FeaCy) is obtained.

[0037] As is explained in greater detail below, it is a preferred feature of the inventive method that both by-products (diluted sulfuric acid and iron oxide) obtained from the low-energy H2generation can be utilized in the steelmaking process or in steel mill.

[0038] Preferably, SO2used in step b) is generated from waste materials comprising sulfur, preferably the SO2is obtained by roasting of pyrite.

[0039] The overall concept is supplying H2for the steel industry for enabling production of green steel by a technology that requires significant less energy as conventional H2generation. This technology requiring significant less energy for H2generation is SDE or S-I technology. SO2required for SDE or S-I-technology preferably is supplied by roasting of pyrite. All by-products generated are utilized: Iron oxide from pyrite roasting is taken as a raw material for steel production, and diluted H2SC>4 is used for steel pickling, generation of iron sulfate from steel mill dust, for MgSCy or (NH4)2SO4 fertilizer, or for conversion into higher concentrated H2SO4. The burning of elementary sul fur, the burning of H2S , and / or S02enrichment and separation from industrial processes represent further options for obtaining S02gas for use in the inventive method .

[0040] When S02for use in step b ) is obtained by roasting pyrite , the iron oxide obtained from the roasting of pyrite preferably is used for steelmaking . In another embodiment the iron oxide obtained from the roasting of pyrite roasting is used as an additive in cement manufacturing . This is done preferably, i f the impurities in the iron oxide are too high for steelmaking .

[0041] Previously, little practical use has been found for diluted H2SO4 with a concentration of 10-90 % by mass , typically 30-70% by mass , which is obtained when generating H2via a SO2- depolari zed electrolyzer ( SDE ) process or a sul fur-iodine- process . However, the inventors have found an application for such diluted H2SO4 that even avoids transportation : Preferably, a part or all of the H2SO4 obtained in step (b ) is used for steel pickling . Alternatively, the diluted H2SO4 may be used for the production of FeSCg heptahydrate from iron oxide or metallic Fe . H2SO4 generated in step (b ) can be concentrated by water evaporation using o f f-heat from steelmaking, preferably to a concentration of >70% by mass , more preferably >75% by mass , most preferably to 75 - 82 % by mass .

[0042] Preferably, excess heat liberated in the roaster ( in a case of obtaining SO2by roasting pyrite ) or in the steelwork is utili zed for vacuum concentrating the diluted H2SO4 generated in step b ) .

[0043] Preferably, a pyrite concentrate that has been obtained from mine waste by separation and / or concentration treatment is used for the generation of SO2for use in step b ) .

[0044] The advantage of using pyrite is that all constituents ( Fe and S ) are usable and of high value for the inventive method . Fe ends up as metallic Fe (via Fe-oxides after pyrite roasting) , and S ends up as H2SO4.

[0045] When employing conventional Fe ore as raw material only about 70% per weight of the material are utili zed whereas 30% per weight of the material ( oxygen) are transported over large distances without any merit .

[0046] Preferably, membranes comprising perfluorated compounds or membranes comprising polybenzimidazole ( PBI ) are used in the hydrogen generation process of step (b ) .

[0047] Preferably, an electrode comprising at least one selected from platinum, palladium, gold and Fe-N-C as catalyst is used in the hydrogen generation process of step (b ) .

[0048] In another embodiment step b ) makes use of a SDE process utili zing green electricity .

[0049] In another embodiment a part of the SO2obtained by roasting pyrite is oxidi zed to SO2and the SO2is used for concentrating diluted H2SC>4 generated in step b ) or the H2SC>4 obtained after water evaporation .

[0050] In another embodiment the diluted sul furic acid obtained from step b ) is used for production of MgSO4or (NH4)2SO4.

[0051] Preferably, green electricity is used in a flexible way :

[0052] In times of oversupply of electricity => H2generation via conventional electrolysis of H2O .

[0053] In times of shortage of electricity => H2generation via SDE process or sul fur-iodine-process , preferably by using green electricity that has been stored in storage media .

[0054] The inventive method combines several advantages in an amazing manner : Environmental benefits and economic advantages combine with the high sustainability of the inventive method . Using hydrogen instead of carbon-based fuels significantly reduces C02emissions in the steelmaking process, thereby contributing to lower greenhouse gas emissions.

[0055] The S02-depolarized electrolyzer (SDE) process and the sulfuriodine-process are efficient tools for enabling a significantly lower energy consumption for H2generation.

[0056] Only about 1 / 3 to 1 / 2 of the energy is required by SDE for the production of 1 t of H2(16-31 MWh / kg H2) compared to classical water electrolysis via DEM (50-60 MWh / kg H2) .

[0057] Lowering energy consumption for H2generation results in lower energy consumption for steel production.

[0058] Additionally, logistic costs and environmental impact are minimized by the inventive method: Pyrite not only can serve as source for sulfur or SO2but also as source for Fe : In contrast to normal iron oxide ore pyrite offers not only Fe, but Fe and S for utilization in the steelmaking process.

[0059] Finally, efficient utilization of by-products is a key advantage of the inventive method. Diluted sulfuric acid and iron oxide can be used directly in the steelmaking process. This avoids tedious and expensive treatment and / or transportation of the by-products.

[0060] Furthermore: SO2production offers excess heat energy for other use .

[0061] Moreover: The preferred use of pyrite as a sustainable source of sulfur and Fe helps to eliminate the environmental burden (acid mine drainage) associated with mining tailings.

[0062] The surprising overall efficiency of the inventive method is represented by the combination of the advantages mentioned above . Summarizing the benefits of the inventive method:

[0063] • reduction of C02emissions in the steelmaking process

[0064] • significant lower energy consumption

[0065] • minimized logistic cost and environmental impact

[0066] • efficient utilization of by-products

[0067] Lower emissions, lower energy consumption, synergies in logistics and by-products result in an amazing environmental and economic advantage and step change for steelmaking.

[0068] Further embodiments:

[0069] There is an increasing demand to find environmentally friendly uses for pyrite (and other sulfide minerals) to avoid the formation of acid mine drainage and other environmental hazards. Desulfurization of mine waste, i.e., separation of pyrite from the gangue mineralogy, provides pyrite and silicate minerals that can be used as construction material (e.g. construction sand) or industrial minerals, like quartz for fibre optic, photovoltaic or glass industries, i.e., a circular economy approach is achieved, and waste is minimized. The here-presented process makes it possible to transform pyrite from today' s highly problematic mine tailings from a waste product into a valuable resource for producing green hydrogen and associated green steel and therefore into an asset mineral for the future.

[0070] The trend towards a decarbonized industry will have the side effect of a lack of sulfuric acid, as most of today's sulfuric acid is produced by the desulfurization of fossil fuels, such as gas, oil, and coal.

[0071] This is especially relevant in light of today's global sulfuric acid production mainly being based on the desulfurization of fossil fuels, such as gas, oil or carbon. If the decarbonization of the global industry is successful, an alternative sulfur source for today's sulfuric acid production will be required. Pyrite, the most abundant sulfide in the earth crust will be the main future resource for obtaining sulfur and for generating CO2 free hydrogen and steel.

[0072] Preferably, the inventive process takes a waste (e.g. pyrite obtained from mine tailings) as starting material and surprisingly ends up with several valuable products and no new waste material: a) desulfurized silicate fraction (desulfurized tailings) suitable for construction or other purposes where industrial minerals are required, b) iron oxide concentrates suitable for steelmaking or as additive in cement manufacturing, c) hydrogen which can be used directly in the steelworks, and d) sulfuric acid that can be used for steel pickling

[0073] The process produces enough green hydrogen to fulfil the demand for reducing the produced Fe2Oa calcine into direct reduced iron (DRI ) .

[0074] The desulfurization of mine waste or active ore streams (separation of pyrite) , allows the tailings (gangue minerals like mainly silicates) to be used for other industrial processes (e.g., construction, glass industry, semi-conductors etc.) , for example, as sulfur concrete or geopolymers as a replacement for cement. Thus, the complete mining waste material can be transformed into different products in the sense of the circular economy with a minimum of waste production and carbon emissions.

[0075] The Fe203 calcine also can be acid or cyanide leached to recover potentially economically interesting trace elements (e.g., Au, Ag, Cd, Co, Te, Bi, Se) contained in the original pyrite concentrate . The SO2 gas used in step (b) of the claimed method can be obtained by the roasting of a pyrite concentrate or smelting of sulfide concentrates. This process involves a reaction according to the following Equation 1, where pyrite oxidizes to iron oxide and SO2.

[0076] 4 FeS2+ 11 O2-> 2 Fe2O3+ 8 SO2Equation 1

[0077] The produced SO2 will be introduced to the S03-depolarized Electrolyzer, where hydrogen and diluted sulphuric acid will be produced (Equation 2)

[0078] S02+ 2 H20 -> H2SO4 + H2Equation 2

[0079] The calcine from the pyrite roasting (mainly the iron oxides hematite and magnetite) will be reduced with the produced hydrogen to direct reduced iron (DRI) (Equation 3) .

[0080] Fe2O3+ 3 H2-> 2 Fe° + 3 H2O Equation 3

[0081] The overall inventive method is represented by Equations 1 to

[0082] 3 is summarized in Equation 4.

[0083] FeS2+ 11 / 4 O2+ 10 / 4 H2O -> Fe° + 2 H2SO4+ 1 / 2 H2Equation 4

[0084] Mass relations:

[0085] 1000 t Pyrite => 665, 5 t Fe3O3+ 33, 6 t H2+ 1635 t H2SO4 (100%)

[0086] 1000 t Pyrite => 465,5 t Fe + 8,40 t H2(according to eq. 3)

[0087] H2SO4 + Fe => FeSO4(complete conversion)

[0088] H2SO4 + Fe2O3=> FeSO4(complete conversion)

[0089] The inventive process employs a proton exchange membrane (PEM) for migration of protons in the case of employing a SDE process step. Preferably perfluorated membranes, e.g. copolymers made from tetrafluoroethylene and perfluoro-3, 6-dioxa-4-methyl-7- octenesulfonic acid can be used for membranes. Tradename of such compounds is Nafion®.

[0090] Membranes comprising perfluorated compounds may be used at 25- 80°C, or more preferably at 60-80°C. Also, membranes comprising or based on polybenzimidazole (PBI) at 100 - 200°C may be used, more preferably at 110-130°C, as the general reactivity, for example the oxidation rate and proton conductivity, are temperature dependent. Preferably, the membrane is acid doped with sulfuric acid (H2SO4) , and more preferably with phosphoric acid (H3PO4) , to increase the acid conductivity .

[0091] Composite membrane systems with graphene-oxide (GO) , Ti02 or other inorganic compounds as filler may also be employed. Such composite membrane systems exhibit greater chemical stability to the acidic environment and higher proton conductivity.

[0092] Pt-based electrocatalysts are still the most dominant catalysts used for both electrodes due to their high efficiency. Also, gold and carbon coatings are used.

[0093] Preferably, the anodic catalysts in the electrolysis cell are Pt and / or Pd, Pt-Cr, Au, Pt / C, Pt / SiC-TiC or combinations thereof, more preferable Pt-Cr, Au, Pt / C, Pt / SiC-TiC or combinations thereof, even more preferably Pt / C, Pt / SiC-TiC or combinations thereof.

[0094] Also, Platin group metal-free catalysts (e.g., Fe-N-C catalysts) can be used to prevent alloy formation and lower the sensitivity to trace element poisoning of the electrolyzer .

[0095] In practice, the specific choice of membranes and catalysts will depend on balancing performance, cost, and durability to meet the specific requirements of the hydrogen generation setup . The inventive method combines a couple of advantages in an amazing manner: Environmental benefits and economic advantages combine high sustainability of the inventive method.

[0096] The S02-depolarized electrolyzer (SDE) process or the sulfuriodine-process are efficient tools for enabling a significantly lower energy consumption for H2generation, resulting in lower energy consumption for steel production.

[0097] Additionally, logistic costs and environmental impact are minimized by the inventive method: Pyrite not only can serve as source for sulfur or S02, but also source for Fe .

[0098] Finally, efficient utilization of by-products is a key advantage of the inventive method. Diluted sulfuric acid and iron oxide can be utilized directly in the steelmaking process or at the steel mill. This avoids tedious and expensive treatment and / or transportation of the by-products.

[0099] And not to forget: Utilization of pyrite as a sustainable source of sulfur and Fe helps to eliminate the environmental burden (acid mine drainage) associated with mine waste.

[0100] The surprising overall efficiency of the inventive method is represented by the combination of the advantages mentioned above .

Claims

CLAIMS1. A method of producing steel wherein a) a part or all of the iron oxide used as raw material for steelmaking is reduced by hydrogen, b) a part or all of the hydrogen required for reduction of the iron oxide is generated via a SCy-depolarized electrolyzer (SDE) process or a sulfur-iodine- process, and c) a part or all of the diluted sulfuric acid obtained from step b) is used for at least one of i. steel pickling, ii. reaction with steel mill dust for generation of iron sulfate (FeSO4or Fe2(SO4)3) , and iii. increasing the concentration of said diluted sulfuric acid by vacuum evaporation of water using off-heat from steelmaking or pyrite roasting .

2. The method according to claim 1, wherein SO2 used in step b) is obtained by roasting of pyrite and the iron oxide obtained from pyrite roasting is utilized in the steelmaking process as iron source.

3. The method according to claim 1, wherein SO2 used in step b) is generated from waste materials comprising sulfur, preferably sulfur obtained by fossil fuel purification.

4. The method according to claim 1, wherein the diluted sulfuric acid obtained from step b) is used for production of MgSO4or (NH4)2SO4.

5. The method according to claim 2, wherein the iron oxide obtained by the roasting of pyrite is used as an additive in cement manufacturing.

6. The method according to claim 1, wherein a part or all of H2SO4generated in step (b) is used for steel pickling.. The method according to claim 1 , wherein H2SO4 generated in step (b ) is concentrated by water evaporation to a concentration of >70% by mass , preferably >75% by mass , most preferably to 75 - 82 % by mass .8 . The method according to any one of the preceding claims wherein heat from pyrite roasting or from steelmaking is utili zed for vacuum concentrating of diluted H2SO4 generated in step b ) .9 . The method according to any one of claims 2- 8 wherein a part of SO2 obtained by roasting pyrite is oxidi zed to SO3 and the SO3 is used for concentrating diluted H2SO4 generated in step b ) or the H2SO4 obtained according to claim 7 or claim 8 .10 . The method according to claim 2 , wherein a pyrite concentrate that has been obtained from mine waste by separation and / or concentration treatment is used for the generation of SO2 for use in step b ) .11 . The method according to any one of the preceding claims , wherein membranes comprising perfluorated compounds or membranes comprising polybenzimidazole are used in the hydrogen generation process of step (b ) .12 . The method according to any one of the preceding claims , wherein an electrode comprising at least one selected from platinum, palladium, gold and Fe-N-C as catalyst is used in the hydrogen generation process of step (b ) .13 . The method according to any one of the preceding claims , wherein step b ) makes use of a SDE process utili zing green electricity .

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