ELECTROCHEMICAL IRON PRODUCTION

RELATED APPLICATIONS

[0001]This patent application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent App. No. 63/632,588, filed Apr. 11, 2024, entitled “LOW CARBON EMISSION IRON PRODUCTION FROM IRON OXIDE,” incorporated herein by reference in entirety.

STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002]This patent application was developed, either in whole or in part, with U.S. Government support under Contract Nos. 2222928, awarded by the National Science Foundation (NSF). The Government has certain rights in the Invention.

BACKGROUND

[0003]Iron production is essential to an industrialized society for buildings, transportation and other infrastructure needs. Iron is a significant component in steel and other alloys, and may often be employed in conjunction with concrete to form structural members (beams) capable of spanning significant distances, such as for bridges and large buildings. Iron production is often associated with large furnaces and heat intensive processes that generate substantial carbon emissions. Nonetheless, the demand for iron is unavoidable in any society having substantial industry.

SUMMARY

[0004]A low heat, electrochemical cascade process generates iron metal (Fe₂) from iron ore and a sequence of alkaline electrolytic solutions. An intermediate phase favors iron oxide in a layered double hydroxide (LDH) form, resulting from conditioning silicates in the alkaline solution, over chemically inert Fe₃O₄ formation. The alkaline electrolytic solution mitigates production of hydrogen over acidic approaches by inhibiting a hydrogen evolution reaction (HER) that forms parasitic hydrogen gas. An electrolyte containment generates an electrolyte flow for the cascading electrochemical reaction as the raw iron oxide transforms to iron metal while avoiding conventional shortcomings of low value products of Fe₃O₄ (magnetite) and hydrogen gas, and instead favors generation of iron metal. Additional electrolyte salts can further form iron alloys.

[0005]Configurations herein are based, in part, on the observation that iron and iron production are unavoidable necessities in an industrialized society. Unfortunately, conventional approaches to iron production and related processes are heavily carbon and heat intensive, and generate substantial carbon emissions such as CO and CO₂. Conventional iron approaches employ carbon intensive oxygen furnaces or electric arc furnaces, or chemical processes that also employ high temperatures, as well as volatile and/or expensive reactants.

[0006]Accordingly, configurations herein substantially overcome the shortcomings of conventional iron production by employing a cascading electrochemical reaction in an alkaline environment that avoids unwanted hydrogen and Fe₃O₄ by flowing different electrolyte streams through a containment as the cascading process proceeds by favoring Fe—CO₃ LDH production over Fe₃O₄ based on pH and conditioning silicates in the electrolyte stream.

[0007]In further detail, configurations herein define a method of forming green iron with reduced carbon emissions in an electrochemical cascade process by generating a strong alkaline solution of ferrate ions, and electrochemically reducing the ferrate ions in the alkaline solution by adding sodium silicate and carbonate ions for forming an Fe—CO₃ layered double hydroxide (LDH) intermediate phase that reduces formation of magnetite. Iron metal may be formed by adding sodium silicate to Fe(OH)₂ resulting from the Fe—CO₃ LDH intermediate phase.

[0008]This results in iron production may be implemented by forming iron in a reduced carbon emission electrochemical cascade from combining hematite with a highly alkaline solution to form ferrate(VI), and reducing the pH of the highly alkaline solution to form a mildly alkaline solution including goethite and magnetite. Fe(OH)₂ is formed from the mildly alkaline solution via an intermediate phase of Fe—CO₃ LDH (layered double hydroxide) formed from the magnetite, thereby limiting magnetite accumulation. Addition of a mildly alkaline electrolyte to the mildly alkaline solution forms an iron interphase to inhibit hydrogen gas formation and yield iron metal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[0010]FIG. 1 is a context diagram of an iron production environment suitable for use with configurations herein;

[0011]FIG. 2 is a schematic diagram of the electrochemical cascade process as disclosed herein;

[0012]FIG. 3 is a Pourbaix diagram of Fe—H₂O depicting compounds employed herein;

[0013]FIG. 4 is an Fe—H₂O—CO₂ Pourbaix diagram depicting the Fe—CO₃ LDH stable phase connecting Fe(OH)₂ and Fe₃O₄ in alkaline solutions; and

[0014]FIGS. 5A and 5B show an example apparatus for implementing the cascading electrochemical iron production as in FIGS. 1 and 2.

DETAILED DESCRIPTION

[0015]The cascading electrochemical process for iron production is described below in an example configuration. Other configurations may exhibit scaling of the disclosed approach for increased iron demand. Conventional approaches cannot achieve the absence of carbon emissions by shifting the iron generating reactions towards the Fe—CO₃ LDH formation over Fe₃O₄.

[0016]Scalable, energy-efficient, zero-emission ironmaking is beneficial to net zero carbon goals. Configurations herein exhibit two phenomena: (i) electrolyte additives, including molecular crowding agent (MCA) sodium silicate (Na₂SiO₃), which alter materials and electrolyte interactions to promote a solid state FeOOH→Fe₃O₄→Fe(OH)₂→Fe reduction by inhibiting hydrogen evolution reaction (HER) and minimizing chemically inert Fe₃O₄ accumulation, and (ii) mildly alkaline and conditioned electrolytes which promote a highly reversible FeOOH↔Fe(OH)₂ redox, while minimizing Fe₃O₄ accumulation, via a SO₄²⁻ anion intercalated layered double hydroxide (LDH) intermediate phase, denoted as Fe—SO₄ LDH.

[0017]FIG. 1 is a context diagram of an iron production environment 100 suitable for use with configurations herein. Referring to FIG. 1, a containment 110 provides an electrolyte flow 122 of alkaline solutions, denoted as I . . . IV to facilitate the electrochemical reactions 120 facilitated by an anode 112 and cathode 114 opposed or separated in the containment. The containment 110 allows a flow of a series of electrolyte solutions through the containment 110, and may be complemented by applying a voltage source to electrodes (anode 112 and cathode 114) submerged on opposed sides of the containment 110 for ion attraction and reduction.

[0018]The resulting cascade 130 of iron compounds transitions with the electrolyte flow. The containment 110 is invoked to flow the strong alkaline solution defined by a first electrolyte into a containment for combining with Fe₂O₃ to form Fe^IVO₄²⁻. Iron production commences by treating Fe₂O₃ 102 to form soluble ferrate ions Fe^VIO₄²⁻ 104 in the alkaline solution (Solution I) in the containment 110 at cascade step 132. Fe₂O₃ is also known as hematite, iron(III) oxide or ferric oxide.

[0021]A third electrolyte flow includes sodium sulfide to form the iron metal. Electrolyte III flows at cascade step 136, where electrolyte III favors electrochemical production of green Fe in alkaline solution conditioned with sodium silicate and interphase-formation Na₂S additives, following the Fe(OH)₂→Fe pathway. Additional electrolytes such as electrolyte IV at cascade step 138 facilitate alloys, such as electrochemical production of green Fe-M alloys (M: Ni, Mn) in alkaline solutions containing NaOH, sodium silicate, Na₂S and M ions (e.g., Ni(OH)₃⁻, MnO₄⁻), following the Fe+Mⁿ⁺→Fe-M pathway. Note that the iron metal yield at step 136 may conclude the electrochemical cascade 120, alloy production may be considered optional.

[0022]FIG. 2 is a schematic diagram of the electrochemical cascade process as in FIG. 1. By forming an interplay of iron-based materials, electrolytes (e.g., CO₃²⁻ anion, sodium silicate, Na₂S additives, water, ion hydration), and electrochemical kinetics (e.g., charge transfer and anion transport) the disclosed approach achieves an efficient and sustainable manufacturing of green Fe from hematite (Fe₂O₃, the dominant species in iron ore). The proposed approach enables selective green Fe production by avoiding inert Fe₃O₄ accumulation and parasitic H₂ formation, confronting current green ion production technologies with higher energy efficiency and lower cost.

[0023]FIG. 2 depicts the electrolytes II-IV (following the initial use of solution I) to generate a strong alkaline solution of ferrate ions from Fe₂O₃, usually from raw iron ore, typically using a highly alkaline solution such as 6-8M sodium hydroxide. Step 134 depicts electrochemically reducing the ferrate ions in the alkaline solution by adding sodium silicate and carbonate ions for forming an Fe—CO₃ layered double hydroxide (LDH) intermediate phase that reduces formation of magnetite. Step 136 depicts forming iron metal by adding sodium silicate to Fe(OH)₂ resulting from the Fe—CO₃ LDH intermediate phase, where the potential 140 drops below the HER 142 for minimizing parasitic hydrogen.

[0024]Conventional approaches to low-emission steelmaking technologies encounter several challenges. One obstacle is accumulation of electrochemically inert Fe₃O₄ from reduction of Fe₂O₃ to Fe in alkaline solutions. Alkaline electrowinning starts with soluble iron species, either in the form of Fe³⁺ ions from acidic leaching or ferrate ions (Fe^VIO₄²⁻), by treating iron ore (hematite, Fe₂O₃) with a highly alkaline solution at elevated temperatures.

[0025]FIG. 3 is a Pourbaix diagram of Fe—H₂O depicting compounds employed herein. The Fe—H₂O Pourbaix diagram of FIG. 3 shows that Fe³⁺ or Fe^VIO₄²⁻ ions quickly form FeOOH (goethite) in an alkaline solution before finally being reduced to Fe. Among various reaction intermediates formed during FeOOH reduction, Fe₃O₄ has a close-packed atomic structure, is electrochemically inert, and doesn't fully participate in the sequential electrochemical process once formed, leading to inefficiency and reduced product.

[0026]A Rietveld refinement revealed the time-resolved phase ratio analysis: (i) FeOOH→Fe₃O₄→Fe(OH)₂→Fe reduction pathway occurs in alkaline solution, where Fe forms from the reduction of Fe(OH)₂ instead of Fe₃O₄; (ii) There are still 44.5% of total Fe atoms remain as Fe₃O₄ even when Fe forms, contributing significantly to the high overpotential of the reduction process and energy loss. The issue of Fe₃O₄ accumulation is beneficial to address to achieve a high yield of Fe. The Fe—H₂O Pourbaix diagram suggests that Fe₃O₄ can be reduced into soluble HFe^IIO₂⁻ ions and further to metallic Fe. However, this conversion requires extremely high alkalinity. For example, forming 10⁻⁵ M HFe^IIO₂⁻ ions requires at least 10 M NaOH (pH=15), while 10⁻⁴ M HFe^IIO₂⁻ requires 100 M NaOH (pH=16). Even 10⁻⁴ M is still too low a concentration for a viable electrodeposition (HFe^IIO₂⁻→Fe).

[0027]H₂ is generated during the electrodeposition of metal ions, especially when the reduction potential for HER is close to or more favorable than that for the metal ions (e.g., Mn²⁺, Zn²⁺, Fe²⁺, Ni²⁺, Co²⁺, or Pb²⁺). The HER overpotential in alkaline systems is generally higher than that in acidic systems. Still, parasitic H₂ formation is one of the main reasons (along with Fe₃O₄ accumulation) for the low energy efficiency of ironmaking. FIG. 3 also shows that the reduction potential of HER (H₂O→H₂) is about 50 mV more positive than Fe(OH)₂ reduction (Fe(OH)₂→Fe) in alkaline solutions. Once Fe(OH)₂→Fe occurs, Fe catalyzes H₂ formation simultaneously because Fe is a more active HER catalyst than Fe(OH)₂ with nearly two orders of magnitude higher HER exchange currents. Parasitic HER causes the poor Coulombic and energy efficiency of the iron electrolysis process, where the electrical power is used to reduce H₂O instead of Fe(OH)₂. Similarly, several metal ions alloying with Fe (e.g., Zn, Ni, Co) have more negative reduction potential than HER, making HER an undesired competing reaction during the electrochemical process. Therefore, it would be further beneficial to address the issue of H₂ formation to produce Fe or Fe alloys with good energy efficiency.

[0028]Configurations disclosed herein present an improved Fe reduction pathway by forming CO₃2- intercalated LDH to mitigate Fe₃O₄ accumulation. An FeOOH/Fe(OH)₂ redox in a weak alkaline with Na₂SO₄ additive which showed a formation of a sulfate anion-intercalated LDH intermediate phase, so-called “green rust (GR)” [Fe²⁺_1-xFe³⁺_x(HO⁻)₂]^x+[(A²⁻)_x/2]^x-, (A: SO₄²⁻, CO₃²⁻) facilitates FeOOH→Fe₃O₄→LDH→Fe(OH)₂ reduction pathways by effectively decreasing Fe₃O₄ accumulation. Some approaches to alkaline Fe redox are conducted in a highly alkaline electrolyte with a high pH solution to improve water and ion transport. However, configurations herein show that the “high-pH paradigm” may have led the material community to overlook the more reversible anion intercalation (the LDH phase) that would take place in the mildly alkaline solution (pH from 9 to 13), where water transport and Fe₃O₄ accumulation are alleviated. Configurations herein approach mitigation of Fe₃O₄ accumulation by forming the anion-intercalated LDH intermediate phase.

[0029]FIG. 4 is an Fe—H₂O—CO₂ Pourbaix diagram depicting the Fe—CO₃ LDH stable phase connecting Fe(OH)₂ and Fe₃O₄ in alkaline solutions. Referring to FIG. 5, configurations herein form an anion-intercalated Fe—CO₃ LDH phase could facilitate FeOOH→→Fe reduction and mitigate Fe₃O₄ accumulation. Such an approach is supported by the following: the Fe—H₂O—CO₂ Pourbaix diagram of FIG. 4 shows that Fe—CO₃ LDH is a stable phase connecting Fe(OH)₂ and Fe₃O₄ in mildly alkaline solutions (pH 8-13). The Fe—CO₃ LDH region 400 effectively replaces a portion of the Fe₃O₄ accumulation 402. The complete FeOOH/Fe₃O₄↔Fe(OH)₂ conversion can be facilitated by Fe—CO₃ LDH (green rust) intermediate phase that helps mitigate Fe₃O₄ accumulation by facilitating FeOOH→Fe₃O₄→CO₃—Fe LDH→Fe(OH)₂→Fe pathway. Referring to FIGS. 1-4, configurations herein flow a mild alkaline electrolyte solution through the containment 110 for replacing the strong alkaline solution, where the mild alkaline solution has a pH lower than the strong alkaline solution, in a range between pH 8-13 or pH 9-13.

[0030]A further enhancement provides that the disordered local structure of iron reduction intermediates, such as Fe(OH)₂, induced by interaction with silicates, may tune the reduction pathway toward more complete reactions. Notably, the shrinking core mass-transfer model has been used to describe the reduction of iron oxide, considering that the diffusion of the oxygenous anions through iron atoms was the rate-determining step in the electrochemical reduction process. Thus, disordered materials have a modified local environment for the Fe—O polyhedra and provide a lower percolation threshold of ion transport in the solid state so that ions (oxygen and iron) can diffuse quickly to facilitate the Fe₃O₄→Fe(OH)₂ reduction.

[0031]Returning to the containment and cascading electrochemical process of FIGS. 1 and 2, the cascading process performs a series of redox reactions involving iron compounds as a series of solutions and electrolytes favor the evolution of iron ore to iron metal. The disclosed method of iron production therefore includes combining, in the containment 110, Fe₂O₃ with a first solution defined by a 6-8 M concentration of sodium hydroxide to form Fe^IVO₄²⁻, shown as step 132, followed by flowing, into the containment 110, a second electrolyte defined by a 0.01-0.1 M concentration of sodium hydroxide, Na₂SiO₃, and CO₃²⁻ anions. FeOOH forms in the containment from the Fe^IVO₄²⁻, to which sodium carbonate is added to the containment 110 for forming Fe₃O₄ and an intermediate phase of Fe—CO₃ layered double hydroxide (LDH) to result in formation of Fe(OH)₂, such that the intermediate phase favors the Fe(OH)₂ formation over Fe₃O₄ formation, depicted in step 134.

[0032]A third electrolyte including sodium sulfide is added to the containment to inhibit hydrogen gas production via an Fe²⁺:S²⁻ interphase, resulting in the iron metal (Fe₂), corresponding to step 136. Alternate configurations may add sodium silicate for suppressing a hydrogen evolution reaction (HER) from forming hydrogen gas.

[0033]Finally, an optional step of alloy formation includes adding magnesium ions to form a magnesium-iron alloy, or adding nickel ions to form a nickel-iron alloy, as disclosed at step 138.

[0034]The equations in Table I below show the full process denoting iron compounds and the corresponding electrolytes and/or solutions flown through the containment 110. The ferrate ion is formed by oxidizing Fe₂O₃ in a highly alkaline solution (Eq. 1). Generally, the principal iron ores contain hematite (Fe₂O₃), magnetite (Fe₃O₄), and goethite (FeOOH).

[0035]Electrolyte II facilitates electrochemical reduction Fe^VIO₄²⁻→FeOOH→Fe₃O₄→Fe—CO₃LDH→Fe(OH)₂. Configurations herein propose that disordered reduction intermediates form during the electrodeposition of ferrate ions and consecutive into FeOOH and Fe₃O₄ on the carbon cathode, assisted by electrolyte additives (e.g., Na₂SiO₃ or other additives). Disordered reduction intermediates and diffusive CO₃²⁻ anion will promote FeOOH→Fe₃O₄→Fe—CO₃ LDH→Fe(OH)₂ reduction pathway to avoid Fe₃O₄ accumulation (Eqs. 2-5). Disordered close-packed metal oxides, accompanied by the cation/anion vacancies and partial reduction of transition metal, have demonstrated interesting redox behavior, especially for battery materials.^26,27 Therefore, disordered reduction intermediates facilitate CO₃²⁻ transport with reduced electrostatic interaction with the [FeO₆] octahedra framework. In addition, divalent CO₃²⁻ anion has a planar geometry and will be an effective intercalant. Electrochemical measurements and X-ray scattering analysis will validate the beneficial role of disordered materials and diffusive CO₃²⁻ anion.

[0036]Electrolyte III induces green iron formation Fe(OH)₂→Fe. The strong interaction between Fe and S²⁻ (from low concentration Na₂S additive) will form an insoluble Fe:S interphase on the surface of Fe(OH)₂. This interphase will facilitate complete Fe(OH)₂→Fe conversion (Eq. 6) by applying lower reduction potential without H₂ gas generation to improve Coulombic efficiency.

[0037]Alloys may be formed using electrolyte IV according to Fe+M²⁺→Fe-M in Electrolyte IV Fe-M (M: Ni, Mn) alloy formation relies on how an M:S interphase forms to inhibit HER (Eq. 7). The composition of the solutions and electrolytes is detailed in Table II:

[0038]FIGS. 5A and 5B show an example apparatus for implementing the cascading electrochemical iron production as in FIGS. 1 and 2. The containment 110 facilitates maintaining an electric field between a cathode plate and an anode plate in the containment 110. The pH in the containment remains generally alkaline. The containment allows flow-cell operation of the cascade process using consecutive electrolyte streams corresponding to the solutions/electrolytes I-IV. The electrochemical flow cells of FIG. 5A and the full-cell of FIG. 5B allows for the cascade electrochemical reduction of Fe₂O₃→→Fe and Fe-M alloys. As shown in FIGS. 5A and 5B, peristaltic pumps 150-1 . . . 150-2 are employed to flow electrolyte streams through the working electrode (carbon paper), during which (a) Fe^VIO₄²⁻→FeOOH→Fe₃O₄→Fe—CO₃ LDH→Fe(OH)₂ will occur in Electrolyte II; (b) Fe(OH)₂→Fe will occur in electrolyte III. The pumps 150 circulate the solutions and electrolyte I-IV in sequence as shown in Table 2 for facilitating the equations in Table 1. A cyclic or return tank 123 collects the flowed solutions/electrolytes I-IV. The cascade cell (half-cell) is also suitable to validate the Fe and Fe-M formation. Iron formed on the paper 115 or other collection medium is harvested, optionally as an alloy.

[0039]While the system and methods defined herein have been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.