Method of producing green hydrogen from pyrite recovered from mine waste

Abstract

The present invention relates to a method of producing green hydrogen and associated products from pyrite separated from mine waste (e.g., disposed tailings or active tailings streams) in an energetically self-sustained process. This is achieved by a method according to the present invention comprising the following steps: (a) separation and enrichment of a mine waste material comprising pyrite to obtain a pyrite concentrate, (b) oxidation of the pyrite concentrate to obtain SO2 gas; (c) separation of the SO2 gas; (d) utilization of SO2 gas from step (c) to generate H2 gas and H2SO4 via a SO2-depolarized electrolyzer (SDE) process or a sulfur-iodine-cycle (S-I-cycle) process.

Description

[0001] Method of producing green hydrogen from pyrite recovered from mine waste

[0002] TECHNICAL FIELD

[0003] The invention relates to a method of producing green hydrogen from pyrite separated from mine waste in an energetically sel fsustained process .

[0004] TECHNICAL BACKGROUND

[0005] In 2022 , with the Ukrainian war and the increasing global ef fects of climate change and environmental pollution, the western world decided to increase the speed of decarboni zation of their industries and search for replacement technologies to be independent from fossil fuel s and their suppliers . Green hydrogen ( i . e . , hydrogen produced by carbon free energy sources ) is the most promising energy vector seen for the future to reach decarboni zation and the climate goals . Additionally, the metal mining industry encounters more and more di f ficulties to find new resources and to open new mines due to the associated environmental pollution of their operations , producing a potential bottleneck in future metal supply due to the predicted increase in demand of metals for the decarboni zation of the global industrial processes .

[0006] Pyrite is the most common sul fide mineral in the earth crust . Other important sul fide ore minerals include chalcopyrite , molybdenite , sphalerite , galena, and pentlandite . When these minerals are exposed to weathering at the Earth' s surface , be it through natural processes or as a result of mining activities , they react with oxygen in the air in the presence of water to form acidic drainage ( so-called acid mine drainage (AMD) formation) , causing severe environmental pollution and therefore the loss of the social license to operate the mining industry . Roasting of sulfide minerals, and pyrite in particular, was applied in the last century to produce sulfuric acid, but when desulfurization of fossil fuels became the main source for sulfuric acid production, the last pyrite mine stopped operation in the 1950s in the Iberian Pyritic Belt.

[0007] Thus, pyrite is today mainly seen in mining as an economically worthless material, producing environmental hazards such as AMD. It is therefore 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 interesting trace elements as Au or Ag, specific pyrite processing might be economically interesting.

[0008] One of the side effects of the current industrial and energetic transition to a decarbonized industry is the increased demand for metals to produce the necessary energy vector (i.e., green hydrogen) and their industrial application (e.g., increased copper demand due to electrification of the transport sector) . This will lead to increased mining activities around the globe with the associated increased mine waste production and environmental pollutions, such as AMD. Additionally, ore grades have continuously been dropping in the last years, thereby making recovery techniques for low-grade ores in the future more relevant (e.g. bioleaching or acid leaching with H2SO4) .

[0009] The 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 H2 gas 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 .

[0010] Whereas classical water splitting requires enormous amounts of energies (1.23 V) , SO2 depolarized electrolysis has the advantage that it needs only 14% of the energy to produce hydrogen (0.17 V) . The Hybrid-Sulfur (HyS ) process , where SO2 is thermochemically produced from sulfuric acid using solar energy (at around 900°C) and then is used via a SDE or the S- I cycle to produce hydrogen is seen as an energetically more convenient technology (Sattler et al. 2017) .

[0011] Outotec discloses a method for the production of hydrogen and concentrated (93-100%) sulfuric acid (see U.S. Pat. No. 7,794, 685 B2) . The method includes the use of SO2 gas to produce H2 gas via sulfur-iodine-cycle (S-I-cycle) or hybrid sulfur cycle (HyS-cycle) , which are known thermochemical water-splitting processes. The employed SO2 gas may originate from sulfur combustion or may be the by-product of a sulfide smelter or roaster process.

[0012] 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 H2 gas 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) . An overview of the different processes is given in U.S. Pat. No. 7,794, 685 B2. Currently, the sulfur-iodine-cycle (S-I-cycle) and the hybrid sulfur cycle (HyS-cycle) are the most promising candidates for industrial application. The hybrid sulfur cycle (HyS-cycle) makes use of a SC^-depolarized electrolyzer (SDE) and is described in the U.S. Pat. No. 4,412,895. A schematic description of a SDE is given in Fig. 1 of this document. The S-I-cycle, which is also called the general atomics process ( "GA-process" ) , is described, for example, in U.S. Pat. No. 4, 089, 940. These two processes involve the generation of diluted sulfuric acid (20-60%) , which is not a commercially useful product, because it is difficult to handle and to transport due to its highly corrosive character.

[0013] U.S. Pat. No. 7,794, 685 B2 tries to overcome the problem of formation of diluted sulfuric acid by concentrating the same to 93-100% sulfuric acid. For this purpose, part of the SO2 stream (about 50%) is used to oxidize SO2 to SO3, which is then used to reach commercial concentrations (93-100%) of the sulfuric acid after several evaporation steps. However, with this method only about 50% of the SC^-stream can be used for hydrogen production, thereby loosing important hydrogen potential .

[0014] Sulfuric acid and ammonia (based today on grey hydrogen production) are the most-produced chemicals worldwide in industrial processes, and are mainly employed for fertilizer production, such as (NH4)2SO4. The world production of ammonia and sulfuric acid in 2021 is estimated to be 150 million metric tons and 260 million metric tons, respectively. Their production is directly linked to the fossil fuel economy and usually involves a high energy demand. The emissions of CO2 from ammonia production alone are estimated at 2.07 t CO2 eq / t NH3, which represents 1.5% of the global anthropogenic CO2 emissions .

[0015] The production of NH3has a more than 100 years long history with the Haber-Bosch process, which is the standard approach for ammonia production. The process converts atmospheric nitrogen (N2) to ammonia (NH3) by reaction with hydrogen (H2) using a metal catalyst under high temperatures and pressures:

[0016] Though this reaction is exothermic (i.e., it releases energy, albeit not very much) , it results in a decrease in entropy, which is the central reason why it is very challenging to carry out .

[0017] The invention presented here makes it possible to unlock the full energy, metal and environmental potential of pyrite for a circular economy in mining to produce green hydrogen and associated products and to eliminate the mine waste problem.

[0018] References

[0019] Calvo G, Mudd G, Valero A, Valero A (2016) Decreasing Ore Grades in Global Metallic Mining: A Theoretical Issue or a Global Reality? Resources 5:36.

[0020] Fediuk R, Mugahed Amran YH, Mosaberpanah MA, Danish A, El- Zeadani M, Klyuev SV, Vatin N (2020) A Critical Review on the Properties and Applications of Sulfur-Based Concrete. Materials 13:4712.

[0021] Santander M, Valderrama L (2019) Recovery of pyrite from copper tailings by flotation. Journal of Materials Research and Technology 8:4312-4317. doi: https: / / doi.Org / 10.1016 / j . jmrt .2019.07.041.

[0022] 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.

[0023] BRIEF DESCRIPTION OF THE DRAWINGS

[0024] [Fig. 1] Fig. 1 shows H2 gas production via a S02-depolarized electrolyzer (SDE) process.

[0025] [Fig. 2] Fig. 2 shows a schematic flow sheet of an embodiment of the presently claimed method.

[0026] [Fig. 3] Fig. 3 shows a schematic flow sheet of another embodiment according to the presently claimed method. SUMMARY OF THE INVENTION

[0027] Technical Problem

[0028] In view of the foregoing, there still is a strong demand for finding a cost efficient and environmentally friendly way of using pyrite, which mainly represents a hazardous mineral in mine waste, to produce industrially valuable products via an optimized and efficient process and to eliminate the waste problem of the mining industry.

[0029] Solution to Problem

[0030] The present invention overcomes the above problems by providing a method of converting pyrite recovered from mine waste (disposed or active tailings) into green hydrogen and associated products, the method comprising the following steps :

[0031] (a) separation and enrichment of a mine waste material comprising pyrite to obtain a pyrite concentrate,

[0032] (b) oxidation of the pyrite concentrate to obtain SO2 gas;

[0033] (c) separation of the SO2 gas;

[0034] (d) utilization of SO2 gas from step (c) to generate H2 gas and H2SO4 via a S02-depolarized electrolyzer (SDE) process or a sulfur-iodine- cycle (S-I-cycle) process.

[0035] The present inventor has unexpectedly found that green hydrogen and associated products can be produced in an energetically self-sustained process from a pyrite concentrate (> 62% pyrite) obtained from mine waste (e.g., by re-mining of disposed mine tailings or from active tailings stream by flotation and / or gravity separation or ore sorting) . In this context, "green hydrogen" means hydrogen produced without significant CO2 emissions.

[0036] Pyrite is a reactive mineral that undergoes oxidation and transformation to Fe(III) oxides with a volume increase. Therefore, it cannot be present in construction material, as the expansion through oxidation would destabilize concrete. Thus, after separation of pyrite from the remainder of mine wastes, which are principally silicate minerals (quartz, micas and feldspars, i.e., sand) , the resulting desulfurized tailings can be used for construction material, or, if the quality of the mineral for other industrial processes is high enough, further separation processes can provide industrial minerals like quartz for fibre-glass or photovoltaic industries .

[0037] Additionally, the inventive process has the advantage that green hydrogen as an energy vector of strong global demand can be produced with a very low energy demand from a waste material. Moreover, the CO2 emissions of the inventive process are negative, as the combustion of the pyrite delivers 80 kW heat per ton of pyrite that can be transformed into electricity and used for the hydrogen production via SDE, and the flotation separation process or other associated industrial processes.

[0038] Furthermore, the inventive process takes a waste (pyrite waste) as starting material and surprisingly ends up with several valuable products and no new waste material.

[0039] The products are:

[0040] - desulfurized silicate fraction (desulfurized tailings) suitable for construction or other purposes where industrial minerals are required,

[0041] - iron oxide concentrates suitable for steelmaking or as additive in cement manufacturing,

[0042] - heat from pyrite combustion that can be used either for chemical processes or transformed into electricity, - hydrogen which can be used directly in the mine or further processed to derivates like ammonia, and

[0043] - sulfuric acid that can be used for leaching low-grade ores or for bioleaching directly in the mine, or further concentrated .

[0044] The process also makes possible the recovery of economically interesting elements associated to pyrite (e.g., Au, Ag, Te, Bi, Se) .

[0045] For example, the diesel demand of an average copper mine (1 L diesel / t rock) can be replaced with the on-site produced green hydrogen and a sulfuric acid demand for ore leaching in the mine of up to 1:2 of high-grade to low-grade ore can be satisfied for bioleaching or acid leaching.

[0046] Alternatively, the process produces enough green hydrogen to fulfil the demand for reducing the produced Fe2Os calcine into direct reduced iron (DRI) .

[0047] The Fe2C>3 calcine also can be acid or cyanide leached to recover potentially economically interesting trace elements (e.g., Au, Ag, Te, Bi, Se; Sb, Sn, Cd, Co) associated to the original pyrite concentrate.

[0048] When the process is combined with a Hybrid-Sulfur (HyS) solar reactor, which splits sulfuric acid via SO3 to SO2, the basis for a sulfur-based green hydrogen production is provided to produce ammonia and combine it with the produced sulfuric acid to obtain fertilizer.

[0049] As stated in the introduction, it can be said that today' s hydrogen and fertilizer production is extremely rich in CO2 emissions due to the required hydrogen and sulfuric acid production .

[0050] The everlasting trend towards a decarbonized industry will have the side effect that there will be a lack of sulfuric acid, as most of today's sulfuric acid is produced by desulfurization of fossil fuels like gas, oil, and coal.

[0051] Additionally, there is an increasing demand to find an environmentally friendlier use of pyrite (and other sulfide minerals) to avoid the formation of acid mine drainage and other environmental hazards. If desulfurization of mine waste, i.e., separation of pyrite from the gangue mineralogy, is economically and environmentally attractive, then the rest of the material, which usually represents the majority of the extracted materials (mainly silicate minerals) , 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. Therefore, the here- presented process makes it possible to transform today's highly problematic waste mineral pyrite into green hydrogen and associated products and therefore into an asset mineral for the future.

[0052] This is especially relevant in the light that today' s global sulfuric acid production is mainly based on the desulfurization of fossil fuels like gas, oil or carbon. If decarbonation of the global industry is successful, a replacement source for the sulfuric acid production is required, as it is the main educt, together with ammonia, for fertilizer production. The only relevant source together with native sulfur resources is pyrite, the most abundant sulfide in the earth crust.

[0053] Effects of the invention

[0054] The method of the present invention makes use of the exothermic oxidation of the pyrite concentrate in step (b) , which not only has the capacity to cover the energy needed for the H2 gas production in step (d) , but also for the concentration of sulfuric acid, if required, in step (d) . Therefore, this method allows for the production of green sulfuric acid, i.e., the production of H2SO4 without the emission of CO2 or association to fossil fuel production. In particular, step (b) is exothermic and thus liberates heat which can be used to power the further step (d) . In this context, the heat may be fully or partially used for the production of electricity. Thereby, the method of the present invention allows for the replacement of CO2 emission intense industrial processes by an emission free process.

[0055] The final products of the process are green hydrogen and green sulfuric acid, as well as a calcine containing mainly hematite, which can be used as Fe-oxide ore for Direct Reduced Iron (DRI) . The amount of hydrogen produced in the process also is enough to cover the demand of DRI if, for example, the hydrogen is not used in the mine for diesel replacement.

[0056] Hydrogen and sulfuric acid also are the base educts for fertilizers like (NH4)2SO4, which finds application in a number of commercial uses, such as fertilizer, insecticides, herbicides and / or fungicides. This is the basis for also feeding the growing world population in future with these materials .

[0057] By the desulfurization of the mine waste or the active ore stream (separation of pyrite) , the tailings (gangue minerals like mainly silicates) can be used for other industrial processes (e.g., construction, glass industry, semi-conductors etc.) , for example, a use as sulfur concrete (Fediuk et al. 2020) or geopolymers as a replacement for cement. Thus, the complete waste material can be transformed into different products in the sense of the circular economy with a minimum of waste production and carbon emissions. DETAILED DESCRIPTION

[0058] The present invention relates to a method of producing green hydrogen and associated products from pyrite recovered from mine waste or active ore streams, the method comprising the following steps:

[0059] (a) separation and enrichment of a mine waste material comprising pyrite to obtain a pyrite concentrate,

[0060] (b) oxidation of the pyrite concentrate to obtain S02gas;

[0061] (c) separation of the SO2 gas;

[0062] (d) utilization of SO2 gas from step (c) to generate H2 gas and H2SO4 via a SC^-depolarized electrolyzer (SDE) process or a sulfur-iodine- cycle (S-I-cycle) process.

[0063] The method includes the production of SO2 gas via exothermic oxidation of a pyrite-containing material. Usually, this step is performed in a sulfide roaster or smelter.

[0064] The exothermic oxidation of pyrite plays a crucial rule to produce green hydrogen without the need of an external energy source. When used in the traditional manner, thermochemical processes (S-I cycle or SDE) employ SO2 gas that is produced by the energy-demanding decomposition of sulfuric acid (Eq. 1) •

[0065] 2 H2SO4 -> 2 SO2+ 2 H2O + O2Equation 1

[0066] In contrast thereto, the SO2 gas used in step (d) of the claimed method is obtained by the roasting or smelting of a pyrite concentrate in step (b) of the claimed method (Eq. 2) .

[0067] 4 FeS2+ 11 02-> 2 Fe2O3+ 8 S02Equation 2 Preferably, the material comprising pyrite originates from the treatment of mine waste associated with sulfide metal mining (e.g., disposed or active tailings) , the desulfurization of ore streams in metal and / or the desulfurization in coal mining. This is particularly advantageous from the viewpoint of environmental friendliness, because it prevents and / or remediates environmental pollution.

[0068] The material comprising pyrite may comprises further sulfide- minerals, such as chalcopyrite (CuFeS2) , bornite (CusFeS , enargite (CU3ASS4) , chalcocite (CU2S) , molybdenite (M0S2) , sphalerite (ZnS) , galena (PbS) , pentlandite ( ( Fe, Ni )9S8) , acanthite (Ag2S) , pyrrhotite (Fei-xS) , millerite (NiS) , covellite (CuS) , realgar (AsS) , orpiment (AS2S3) , stibnite (Sb2S3) , and / or marcasite (FeS2) , together with trace amounts of gangue minerals.

[0069] From the viewpoint of overall efficiency, the concentrate produced in step (a) of the claimed method preferably comprises >62% by mass, more preferably >90% by mass of pyrite. Common methods, such as froth flotation as reported by Santander and Valderrama (2019) , and / or gravity separation or ore sorting can be used to concentrate the pyrite-containing material. A pre-concentration of the pyrite-containing material is particularly preferable from the standpoint of reaching an energetically self-sustained roasting process. The higher the concentration of pyrite, the more SO2 gas, and in turn hydrogen, may be generated via the present method.

[0070] The pyrite concentration of the material may be determined by assessing the total sulfur concentration or the sulfide sulfur concentrations (based on sequential extractions) , or by quantitative mineralogy (e.g., QEMSCAN, MLA, ZEISS- Mineralogic) .

[0071] The oxidation in step (b) is preferably performed at a temperature of 600 °C-1000 °C . Optionally, the temperature may be in the range of 700-900°C or 800-900°C, which are preferred in view of obtaining an energetically auto-sustained roasting process .

[0072] The oxidation in step (b) may be performed using air as oxygen source. The efficiency of the oxidation reaction in step (b) may be increased by using oxygen enriched air or pure oxygen.

[0073] Preferably, O2 obtained by air fractionation is used for the pyrite oxidation in the step (b) .

[0074] Aside from SO2, the oxidation in step (b) generates Fe2O3 / FesO4 (see Eq. 2 above) . In one embodiment, part of the sulfuric acid generated in step (d) is used together with these oxides to produce iron sulfate.

[0075] In a second embodiment, the generated Fe2O3 / Fe3O4 is reacted with the produced hydrogen to produce Direct Reduced Iron (DRI) (see Fig . 3 ) .

[0076] This makes possible the complete conversion of the pyrite in the pyrite-containing material into green products, in line with the circular economy principles. The higher the pyrite concentration in the pyrite-containing material, the higher the conversion rate of the starting materials into green products .

[0077] In step (c) the SO2 gas is separated from the off-gas generated in the roaster or smelter used for the oxidation in step (b) .

[0078] Preferably, the oxidation in step (b) takes place in the presence of air as oxygen source. The introduced air delivers the oxygen needed for the generation of SO2 gas (see Eq. 2 above) . Since air is mainly composed of N2 gas, the off-gas generated in the roaster or smelter used for the oxidation in step (b) will contain SO2 gas and N2 gas in such a case.

[0079] The off-gas containing both, SO2 gas and N2 gas, is subjected to gas cleaning and separation of the SO2 from the N2 in step (c) . Moreover, the N2 preferably also is separated from the off-gas generated in the roaster or smelter used for the oxidation in step (b) . Methods for the cleaning and separation of gases such as SO2 and N2 are well-known and established in the art and any of these known methods may be employed for the separation process. The purification and separation of both, the SO2 and the N2, is beneficial for improving the efficiency of the follow-up processing.

[0080] In step (d) , the separated SO2 is transferred to a SO2- depolarized electrolyzer (SDE) process as shown in Fig. 1 or to a S-I cycle process, whereby H2 gas and H2SO4 are generated.

[0081] Preferably >30% of the SO2, more preferably >50% of the SO2, even more preferably >70% of the SO2, even more preferably >90% of the SO2 used in step (d) originates from the oxidation of pyrite.

[0082] Optionally, the burning of elementary sulfur, burning of H2S, and / or solar thermochemical decomposition of H2SO4 for producing SO2 enrichment and separation from industrial processes can be employed as further sources of SO2 gas used in step (d) .

[0083] The SO2 flow used in step (d) has passed the gas cleaning step but can still contain some trace elements of impurities as pyrite can be the host mineral of a broad range of trace elements in ore deposits. Therefore, the hydrogen production unit (SDE) used for the proposed invention must be trace element resistant in order to ensure efficiency and durability .

[0084] In the SDE electrolyzer, at the anodic flow, SO2 is introduced and at the cathodic flow H2O. SO2 is oxidized by catalysts to HSO3- and SO42~ and the produced protons migrate through a proton exchange membrane (PEM) to the cathode. Nafion® membranes at 25-80°C, or more preferably at 60-80°C, are used to separate the protons. Preferably, polybenzimidazole (PBI) membranes at 100 - 200°C are used, more preferably at 110-130°C, as the general reactivity, for example the oxidation rate and proton conductivity, are temperature dependent.

[0085] Composite membrane systems with graphene-oxide (GO) or TiO2 as filler are preferably used as they exhibit greater chemical stability to the acid environment and increase the proton conductivity. Additionally, they make the membrane hydrophobic to prevent SO2 and H2S crossovers to the cathodic compartment, which can lead to sulfur generation and precipitation, and thus cause a decrease of efficiency and durability of the SDE.

[0086] The membrane is preferably acid doped with sulfuric acid (H2SO4) , and more preferably with phosphoric acid (H3PO4) , to increase the acid conductivity of the membrane.

[0087] To make the anode and cathode more chemically resistant to the corrosive environment, gold and carbon coatings are preferred. The anodic catalysts in the electrolysis cell are Pt and / or Pd, more preferable Pt-Cr, Au, even more preferably Pt / C, Pt / SiC-TiC or combinations. Most preferably, Platin group metal free catalysts (e.g., Fe-N-C catalysts) are used to prevent alloy formation and lower the sensibility to trace element poisoning of the electrolyzer.

[0088] Separation of the base metal (Cu, Zn, Pb, Ni) sulfides from pyrite prevents the presence of traces of these elements associated to the SO2 stream and thus prevents the formation of Pt, Pd and Au alloys with these metals in the electrolyzer, thereby increasing the efficiency and durability of the SDE.

[0089] The concentration of the H2SO4 generated in step (d) usually is in the range of 10-90%, but may also be in the range of 10- 80%, 10-70%, 20-70%, 30-60% or 30-50% by mass.

[0090] Optionally, a part of the H2SO4 generated in step (d) can be used to produce H2SO4 having a concentration of 93-100% by weight. Preferably, excess heat liberated in the roaster through the exothermic oxidation of the pyrite concentrate in step (b) is used to provide energy needed for the concentration of the H2SC>4 generated in step (d) to produce concentrated H2SO4.

[0091] The concentrated H2SO4 may be used together with the iron oxide obtained in step (b) to produce iron sulfate.

[0092] Alternatively, the concentrated H2SO4 can be used to produce phosphoric acid, e.g. by reaction with apatite. The phosphoric acid may then be used to produce phosphate fertilizers, such as ammonium phosphate ( (NH4)3PO4) or calcium dihydrogen phosphate (Ca (H2PC>4)2) . Optionally, a part of the H2SO4 generated in step (d) may be directly used to produce other sulfate fertilizers, such as MgSO4.

[0093] The hydrogen produced by the SDE or S-I-cycle process may be subjected to reaction with N2 gas to produce NH3via the Haber- Bosch process or through electrochemical NH3synthesis (Eq. 3) .

[0094] 3H2+ N2-> 2 NH3Equation 3

[0095] Preferably, N2obtained by air fractionation is used for the synthesis of NH3. This makes it possible to simultaneously produce O2for use in step (b) to thereby simplify the overall process structure and to make it more efficient.

[0096] The NH3and the H2SO4 produced in step (d) may be reacted with each other to produce ammonium sulfate ( (NH4)2SO4) (Eq. 4) .

[0097] 2 NH3+ H2SO4-> (NH4)2SO4Equation 4

[0098] If required, the (NH4)2SO4 produced can be dried to obtain granular (NH4)2SO4. The drying process can be performed by a water evaporation step. The NH3produced may take the form of gaseous NH3, which can be fed directly into the H2SO4. This is particularly preferable if the introduction of additional water is to be avoided.

[0099] Optionally, a part of the NH3produced may be used to generate ammonium phosphate (NJhHPCy, ammonium nitrate NH4NO3, and / or urea CO(NH2)2.

[0100] The excess heat liberated in the roaster through the exothermic oxidation of the pyrite concentrate in step (b) is used to provide energy needed for the H2gas production via a SDE or a S-I-cycle process in step (d) and / or energy needed for the synthesis of NH3if this is carried out. If required, renewable energy sources (i.e., wind, solar, hydro) may be used as supplementary energy sources in order to ensure that the overall process can be operated without net CO2 emissions.

[0101] Additional renewable energy sources may also be used to produce green H2gas by the energetic convenient H2SO4 splitting via SDE. The Fe2O3 / Fe3O4 oxides obtained in step (b) may be reduced by said green H2gas to elemental Fe and / or DRI .

[0102] In one embodiment, excess heat from step (b) is used for providing energy in the SDE process in step (d) . Generally, the SDE process requires energy in the form of electricity. Accordingly, the excess heat may be converted into electric energy by appropriate means in case the SDE cycle process is used .

[0103] In another embodiment, excess heat from step (b) is used for providing energy in the S-I-cycle process in step (d) . Since the S-I-cycle process mainly requires energy in the form of heat, the excess heat from step (b) may be directly used for the S-I-cycle process in step (d) .

[0104] The excess heat from step (b) may also be used as energy source for the NH3synthesis. In a case where the synthesis of NH3is carried out , it is preferable that 40- 80% of excess heat from step (b ) is used for providing energy in the SDE or S- I-cycle process in step ( d) and / or 20- 60% of excess heat from step (b ) is used for providing energy for the synthesis of NH3.

[0105] The method of the invention combines

[0106] - the separation and enrichment of pyrite from mine waste or active ore streams , e . g . by flotation or gravity techniques and

[0107] - the exothermic oxidation of pyrite-containing material with green H3gas and sul furic acid production in a single industrial process that employs a single mineral resource and can be carried out without net CO2 emissions . The produced products can be consumed on site by the very same mining operation, thereby preventing transport from outside and remediating and preventing environmental pollution onsite .

[0108] A schematic overview about the method described above is shown in Fig . 2 .

[0109] Exemplary embodiments

[0110] Exemplary embodiments of the claimed invention are described in the following .

[0111] <1> A method of producing green hydrogen from mine waste material comprising pyrite comprising the following steps :

[0112] ( a ) separation and enrichment of a mine waste material comprising pyrite to obtain a pyrite concentrate ,

[0113] (b ) oxidation of the pyrite concentrate to obtain SO2 gas ; (c) separation of the SO2 gas;

[0114] (d) utilization of SO2 gas from step (c) to generate H2 gas and H2SO4 via a SC^-depolarized electrolyzer (SDE) process or a sulfur-iodine- cycle (S-I-cycle) process.

[0115] <2> The method according to <1> , wherein the mine waste material comprising pyrite originates from the treatment of mine waste, the desulfurization of ore streams in metal mining and / or the desulfurization in coal mining.

[0116] <3> The method according to <1> or <2>, wherein the pyrite concentrate obtained in step (a) comprises >62% by mass, preferably >90% by mass of pyrite.

[0117] <4> The method according to any one of <1> to <3>, wherein the H2SO4 obtained in step (d) has a concentration of 10 - 80% preferably 30-60%, by mass. <5> The method according to any one of <1> to <4>, wherein air is used as oxidizing agent in step (b) .

[0118] <6> The method according to any one of <1> to <5>, wherein step (c) further includes separation of N2 from the exhaust gas of step (b) .

[0119] <7> The method according to <6>, wherein the separated N2 is used to produce NH3. <8> The method according to any one of <1> to <7>, wherein H2SO4 obtained in step (d) is used to prepare (NH4)2SO4.

[0120] <9> The method according to <8>, wherein the (NJh^SCy is produced by feeding gaseous NH3into the H2SO4 obtained in step (d) .

[0121] <10> The method according to any one of <1> to <9>, wherein H2 obtained in step (d) is used to produce NH3. <11> The method according to <10>, wherein at least a part of the produced NH3is used to generate ammonium phosphate (NH4)3PO4, ammonium nitrate NH4NO3, and / or urea CO(NH2)2-

[0122] <12> The method according to any one of <1> to <11>, wherein a part of the H2SO4produced in step (d) is used to generate ammonium sulfate, potassium sulfate, MgSO4, phosphoric acid, ammonium phosphate (NH4)3PO4, and / or calcium phosphate

[0123] Ca (H2PO4)2.

[0124] <13> The method according to any one of <1> to <12>, wherein excess heat generated in step (b) and optionally renewable energy sources are used to provide energy needed for the H2gas production via SDE or S-I-cycle process in step (d) and / or energy needed for the synthesis of NH3.

[0125] <14> The method according to any one of <1> to <13>, wherein >50% of SO2, preferably >70% of SO2, more preferably >90% of SO2used in step (d) originates from the oxidation of pyrite.

[0126] <15> The method according to <14>, wherein the burning of elementary sulfur, burning of H2S, and / or S02enrichment and separation from industrial processes represent further sources of S02gas used in step (d) .

[0127] <16> The method according to any one of <1> to <15>, wherein the oxidation in step (b) is performed at a temperature of 600°C - 1000°C.

[0128] <17> The method according to any one of <1> to <16>, wherein air is fractionated into O2and N2, and the O2is used for the pyrite oxidation in step (b) and the N2is used for the synthesis of NH3.

[0129] <18> The method according to any one of claims 1-17, wherein the H2SO4obtained in step d) is used for leaching of oxide and / or low-grade sulfide ores. <19> The method according to <1> to <18>, wherein the obtained H2SO4 is not further concentrated prior to being used for leaching .

[0130] <20> The method according to any one of <1> to <19>, wherein the desulfurized mine waste material remaining after separation of the pyrite concentrate in step a) comprises <2%, preferably <1%, more preferably <0.5%, most preferably <0.1%, of pyrite.

[0131] <21> The method according to any one of <1> to <20>, wherein iron oxide obtained in step b) is reduced to iron using H2 generated in step (d) .

[0132] <22> The method according to any one of <1> to <21>, wherein Nafion or FBI membranes are used for the SDE process of step (d) .

[0133] <23> The method according to any one of <1> to <22>, wherein an electrode comprising at least one selected from platinum, palladium, gold and Fe-N-C as catalyst is used in the SDE process of step (d) .

[0134] Specific embodiments

[0135] The method according to the invention is described in more detail by means of the following embodiment.

[0136] In the oxidation step (b) , lOOOt per day of pyrite-containing material having a pyrite concentration of 100% is provided in a pyrite roaster in the presence of air and roasted at a temperature of 800-900°C.

[0137] [Mass balance]

[0138] Step (b) produces 1067t SO2 gas and provides 2383t N2 gas in form of off-gas. Overall, the pyrite roasting releases 467t Fe in the form of 667t Fe2Os (hematite) and 533t S in the form of 1067t SO2.

[0139] The SO2 gas is separated from the off-gas of step (b) and is used to produce 33t hydrogen via a SDE process with 1633t H2SO4 being obtained as secondary by-product (step (d) ) .

[0140] The produced hydrogen (33t) can optionally be reacted with 156t N2 separated from the off-gas of step (b) to produce 189t NH3via Haber-Bosch process.

[0141] The produced NH3can be mixed with 544t of the H2SO4 obtained in step (d) to produce 733t of (NH4)3SO4 in one industrial process from one mineral resource without net CO2 emissions.

[0142] The invention thus has the capacity to produce 33t H2 per lOOOt of 100% pyrite roasting (33 kg H2 per It of pyrite) , which is sufficient to replace the diesel demand of an average copper mine with 1 1 diesel / t rock(Calvo et al. 2016) .

[0143] Alternatively, the produced 33t of green hydrogen can be used to reduce the 667t of Fe2O3, a process which requires 5% of Fe-oxide weight as H2, thus 33t.

[0144] The sulfuric acid can be used directly on the mine site as diluted H2SO4 from the SDE (20-70%) for bioleaching or acid leaching of up to 1:2 ratios between high-grade ore and low- grade ore (7-16 kg ^SCy / t leach ore) .

[0145] As an example, an open pit porphyry copper mine (2% pyrite) with 50.000 tpd processing capacity of sulf idle ore will produce, with an additional flotation circuit for pyrite, 1000 tpd of pyrite concentrate, thus 33 tpd of hydrogen. This corresponds to 100 tpd of diesel, as hydrogen has a 3 times higher energy density as diesel, which corresponds roughly to 120.000 L of diesel, which makes it possible to mobilize 120.000 tpd material in the mine, which corresponds to a typical ratio between 1:1 and 1:2 between high grade ore and low-grade / sterile mobilized in a mine.

[0146] The sulfuric acid may also be used to produce other sulfate fertilizers, such as MgSCy, or may be converted into sulfuric acid having a concentration of 93-100% by weight. The concentrated H2SO4 may be used together with the iron oxide obtained in step (b) to produce iron sulfate or may be used to produce phosphoric acid, which in turn may be used to produce further fertilizer, such as (NJhHPCy or Ca(H2PO4)2- To cover the hydrogen demand for all production lines, i.e. alternative fertilizer production, renewable energy sources are optionally used as supplementary energy sources.

[0147] [Energy balance]

[0148] Since pyrite predominantly contains FeS2, formation of Fe2Os (hematite) occurs during step (b) , so that the reaction enthalpy can be taken as -1666 kJ / mol of Fe2Os as a first estimate (Eq .

[0001] ) .

[0149] This means that if lOOOt / day of pyrite are introduced in step (b) , about 48 mol / s of Fe2Os will leave the roaster, which coincides with a heat release of about 80 MW. Since not all reactants are cooled to room temperature, a fraction of the total 80 MW will not be recovered.

[0150] Step (d) requires an energy amount of about 21 MW and concentration of sulfuric acid an amount of about 42 MW, depending on the exact reaction conditions.

[0151] Since step (b) provides a theoretical overall heat release of 80 MW, the invention has the capacity to produce 33t H2 per lOOOt of 100% pyrite roasting per day (33 kg H2 per It of pyrite) without the need for external energy sources, as most of the heat produced in step (b) can be used for providing energy in the SDE process in step (d) and / or concentration of H2SO4 (see also Fig. 3) .

[0152] All together the method is energy balanced.

Claims

CLAIMS1. A method of producing green hydrogen from mine waste material comprising pyrite comprising the following steps:(a) separation and enrichment of a mine waste material comprising pyrite to obtain a pyrite concentrate,(b) oxidation of the pyrite concentrate to obtain SO2 gas;(c) separation of the SO2 gas;(d) utilization of SO2 gas from step (c) to generateH2 gas and H2SO4 via a SC^-depolarized electrolyzer (SDE) process or a sulfur-iodine- cycle (S-I-cycle) process.

2. The method according to claim 1, wherein the mine waste material comprising pyrite originates from the treatment of mine waste, the desulfurization of ore streams in metal mining and / or the desulfurization in coal mining.

3. The method according to claim 1 or 2, wherein the pyrite concentrate obtained in step (a) comprises >62% by mass, preferably >90% by mass of pyrite.

4. The method according to any one of claims 1-3, wherein the H2SO4 obtained in step (d) has a concentration of 10 - 80% preferably 30-60%, by mass.

5. The method according to any one of claims 1-4, wherein air is used as oxidizing agent in step (b) .

6. The method according to any one of claims 1-5, wherein step (c) further includes separation of N2 from the exhaust gas of step (b) .

7. The method according to claim 6, wherein the separated N2 is used to produce NH3.

8. The method according to any one of claims 1-7, wherein H2SO4 obtained in step (d) is used to prepare (NH4)2SO4.

9. The method according to claim 8, wherein the (NJh^SCy is produced by feeding gaseous NH3into the H2SO4 obtained in step (d) .

10. The method according to any one of claims 1-9, wherein H2obtained in step (d) is used to produce NH3.

11. The method according to claim 10, wherein at least a part of the produced NH3is used to generate ammonium phosphate (NH4)3PO4, ammonium nitrate NH4NO3, and / or urea CO(NH2)2-12. The method according to any one of claims 1-11, wherein a part of the H2SO4 produced in step (d) is used to generate ammonium sulfate, potassium sulfate, MgSCy, phosphoric acid, ammonium phosphate (NH4)3PO4, and / or calcium phosphate Ca (H2PO4) 2.

13. The method according to any one of claims 1-12, wherein excess heat generated in step (b) and optionally renewable energy sources are used to provide energy needed for the H2gas production via SDE or S-I-cycle process in step (d) and / or energy needed for the synthesis of NH3.

14. The method according to any one of claims 1-13, wherein >50% of SO2, preferably >70% of SO2, more preferably >90% of SO2 used in step (d) originates from the oxidation of pyrite.

15. The method according to claim 14, wherein the burning of elementary sulfur, burning of H2S, and / or SO2enrichment and separation from industrial processes represent further sources of SO2 gas used in step (d) .

16. The method according to any one of claims 1-15, wherein the oxidation in step (b) is performed at a temperature of 600°C - 1000°C.

17. The method according to any one of claims 1-16, wherein air is fractionated into O2 and N2, and the O2 is used for the pyrite oxidation in step (b) and the N2 is used for the synthesis of NH3.

18. The method according to any one of claims 1-17, wherein the H2SO4 obtained in step d) is used for leaching of oxide and / or low-grade sulfide ores.

19. The method according to claim 18, wherein the obtained H2SO4 is not further concentrated prior to being used for leaching .

20. The method according to any one of claims 1-19, wherein the desulfurized mine waste material remaining after separation of the pyrite concentrate in step a) comprises <2%, preferably <1%, more preferably <0.5%, most preferably <0.1%, of pyrite.

21. The method according to any one of claims 1-20, wherein iron oxide obtained in step b) is reduced to iron using H2 generated in step (d) .

22. The method according to any one of claims 1-21, wherein Nafion or FBI membranes are used for the SDE process of step (d) .

23. The method according to any one of claims 1-22, wherein an electrode comprising at least one selected from platinum, palladium, gold and Fe-N-C as catalyst is used in the SDE process of step (d) .

Images