HIGH-EFFICIENCY ELECTROLYSIS OF IRON ORE FOR IRON PRODUCTION

CROSS REFERENCE TO RELATED APPLICATIONS

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BACKGROUND

Iron (Fe) occurs naturally as iron ores such as hematite, magnetite, goethite, limonite, wustite which are made up iron oxides such as Fe₂O₃, Fe₃O₄, FeO etc. In iron metal and steel production, these iron oxides are typically reduced to form iron metal. A majority (about 71%) of global crude steel production currently utilizes a reduction process known as blast furnace-basic oxygen furnace (BF-BOF). This process usually uses carbon (such as coke) as a reductant. The carbon binds oxygen in the iron ore and generates carbon dioxide (CO₂). Thus, the most common process for producing iron and steel produces a significant amount of carbon dioxide. In fact, iron and steel production as a whole currently accounts for about 7% of total industry carbon dioxide production.

Some other processes have been used to directly reduce iron ore to iron metal sponge via a solid-state process using a reducing gas. However, the reducing gas used in such processes typically originates from reformed natural gas, syngas, or coal, which also have a heavy carbon footprint. Therefore, the most common processes currently used to produce iron and steel produce a significant amount of carbon dioxide, which can be harmful to the environment.

SUMMARY

Methods of producing iron metal by electrolysis of iron ore, and iron ore electrolysis reactors that can be used in such methods are presented. In one example, a method of producing iron metal by electrolysis of iron ore can include introducing iron ore into an anode chamber. The anode chamber can include a first electrolyte and an anode. The anode can oxidize water to form O₂gas and H⁺ ions in the anode chamber. The iron ore can be dissolved to form Fe³⁺ and/or Fe²⁺ ions in the anode chamber. The Fe³⁺ and/or Fe²⁺ ions can be transferred from the anode chamber to a cathode chamber through a cation exchange membrane that separates the anode chamber from the cathode chamber. The cathode chamber can include a second electrolyte and a cathode. The Fe³⁺ and/or Fe²⁺ ions can be reduced to form iron metal at the cathode.

In another example, an iron ore electrolysis reactor can include an anode chamber to receive a feed of iron ore via an iron ore feed inlet. The anode chamber can include a first aqueous electrolyte and an anode. The reactor can also include a cathode chamber that includes a cathode and a second aqueous electrolyte. The second aqueous electrolyte can include a dissolved iron salt and a dissolved co-salt. The co-salt can include magnesium, calcium, or a combination thereof. The reactor can also include a cation exchange membrane separating the anode chamber from the cathode chamber. The cation exchange membrane can be configured to transfer Fe³⁺ and/or Fe²⁺ ions from the anode chamber to the cathode chamber.

In yet another example, an iron ore electrolysis flow reactor can include an anode chamber that includes a first electrolyte, an anode, and an iron ore inlet. A cation exchange membrane can be positioned at least partially above the anode chamber and in contact with the first electrolyte. A cathode chamber can be positioned at least partially above the cation exchange membrane. The cathode chamber can include a second electrolyte and a cathode. The cation exchange membrane can be configured to transfer Fe³⁺ and/or Fe²⁺ ions from the anode chamber to the cathode chamber. A recycle flow line can be connected to the cathode chamber to cycle the second electrolyte through the cathode chamber. An iron metal collector can be connected to the recycle flow line to collect iron metal particles from the second electrolyte.

There has thus been outlined, rather broadly, features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken with the accompanying drawings and claims, or may be learned by the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustration an example method of producing iron metal by electrolysis of iron ore, in accordance with the present disclosure.

FIG. 2 is a side cross-sectional view of an example iron ore electrolysis reactor, in accordance with the present disclosure.

FIG. 3 is a side cross-sectional view of another example iron ore electrolysis reactor, in accordance with the present disclosure.

FIG. 4 is a side cross-sectional view of an example iron ore electrolysis flow reactor, in accordance with the present disclosure.

FIG. 5 is a SEM micrograph of iron powder produced using a method in accordance with the present disclosure.

FIG. 6 shows in XRD of titanium electrodes after 1 hour of electrolysis in accordance with an example of the present disclosure.

These drawings are provided to illustrate various aspects of the invention and are not intended to be limiting of the scope in terms of dimensions, materials, configurations, arrangements or proportions unless otherwise limited by the claims.

DETAILED DESCRIPTION

While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. Thus, the following more detailed description of the embodiments of the present invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the present invention, to set forth the best mode of operation of the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the present invention is to be defined solely by the appended claims.

Definitions

In describing and claiming the present invention, the following terminology will be used.

The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a salt” includes reference to one or more of such materials and reference to “the membrane” refers to one or more of such membranes.

As used herein, “faradaic efficiency” can also be called “Faraday efficiency” or “faradic efficiency.” This refers to the fraction of electric current that flows through an electrochemical cell that effects a desired electrochemical reaction. The remaining fraction of the electric current, which does not contribute to the desired electrochemical reaction, can be consumed by faradaic losses such as unwanted side reactions and heat generation.

As used herein with respect to an identified property or circumstance, “substantially” refers to a degree of deviation that is sufficiently small so as to not measurably detract from the identified property or circumstance. The exact degree of deviation allowable may in some cases depend on the specific context.

As used herein, “adjacent” refers to the proximity of two structures or elements. Particularly, elements that are identified as being “adjacent” may be either abutting or connected. Such elements may also be near or close to each other without necessarily contacting each other. The exact degree of proximity may in some cases depend on the specific context.

As used herein, the term “about” is used to provide flexibility and imprecision associated with a given term, metric or value. The degree of flexibility for a particular variable can be readily determined by one skilled in the art. However, unless otherwise enunciated, the term “about” generally connotes flexibility of less than 2%, and most often less than 1%, and in some cases less than 0.01%.

As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

As used herein, the term “at least one of” is intended to be synonymous with “one or more of.” For example, “at least one of A, B and C” explicitly includes only A, only B, only C, or combinations of each.

Numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also to include individual numerals such as 2, 3, 4, and sub-ranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as “less than about 4.5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.

Any steps recited in any method or process claims may be executed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus-function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) “means for” or “step for” is expressly recited; and b) a corresponding function is expressly recited. The structure, material or acts that support the means-plus function are expressly recited in the description herein. Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents, rather than by the descriptions and examples given herein.

Methods of Producing Iron Metal by Electrolysis of Iron Ore

The present disclosure describes processes for producing iron metal from iron through electrolysis. The major products of the processes can be iron metal and oxygen. Carbon is not involved in any of the chemical reactions that occur in this process, and no carbon dioxide is produced. If the electricity used for electrolysis is supplied from renewable resources such as solar or wind power, then the entire iron production process can be carbon-emission-free.

Electrolysis of iron ore can break iron ore directly into iron metal and oxygen gas. Due to its simplicity and potential carbon-emission-free nature, electrolysis has received attention in recent years. Among different electrolysis processes, molten oxide electrolysis (MOE) is the most studied because it enables the direct production of iron metal in the liquid state from iron oxide feedstock. This produces liquid iron metal that can be easily separated from the molten oxide electrolyte, and the high solubility of iron oxide in the electrolyte enables high operation current, such as 1 A/cm². However, the high operation temperature (often greater than 1,538° C.) can result in significant energy inefficiency due to heat loss. The MOE reactor can also suffer from severe corrosion due to the oxygen-rich environment, high temperature, the presence of ceramic-solubilizing molten oxide, and the presence of metal-solubilizing molten metal. Moreover, MOE has a low faradaic efficiency (34-46%) due to the corrosion of cathode. Using molten salt electrolyte (such as halide or carbonate) can significantly reduce the operating temperature to 750-900° C., but the reaction rate is limited by the sluggish solid-state reduction of iron oxide and/or the diffusion of the dissolved iron oxide in the electrolyte. In addition, it is difficult to separate the produced solid iron from the molten salt.

Electrolysis in aqueous electrolyte can significantly lower the operating temperature to below 150° C. This can allow for simpler reactor design and reduce the energy consumption compared to the generation and maintaining of high temperature in the MOE process. Electrowinning, i.e., the electrolysis of suspended iron ore particles in alkaline solutions, has been demonstrated at 100-120° C. In the electrowinning process, a solid-state shrinking-core conversion of iron oxide to iron metal occurs when iron ore particles pass by the cathode in the stirred electrolyte. However, the reaction rate is restricted to about 0.1 A/cm², due to the limited reaction area (electrode-particle contact surface) and the slow electron/oxygen anion transport in the iron oxide particles. Oxygen bubbles formed on the cathode can be trapped due to the high viscosity of the suspension. The trapped bubbles can block the electrode surface and further compromise the reaction rate. Additionally, the formed product is usually a mixed-phase solid particle containing both iron metal and iron oxide, making it difficult to separate the iron metal.

Electrolysis in acidic electrolyte can potentially address the above issues. Electrolysis in acidic electrolyte can be performed at much lower temperature than MOE. Iron ore can dissolve into the acidic electrolyte and form iron ions (i.e. Fe³⁺ and/or Fe²⁺ ions). The reduction of Fe³⁺ and/or Fe²⁺ ions to Fe metal, i.e., electrochemical deposition of Fe, is a liquid-solid two-phase reaction that can be much more facile than the solid-state reaction in an alkaline electrolyzer. The high solubility of Fe³⁺ and Fe²⁺ in acidic electrolyte (>3.5M) allows high rate operation with good mass transfer. The acidic electrolyte can also have much lower viscosity than a suspension electrolyte used in an alkaline electrolyzer. Additionally, the obtained precipitate from the acidic electrolyte is pure Fe metal with no Fe oxide in the bulk.

Despite these positive features of electrolysis in acidic electrolyte, the electrolysis of Fe ore in acidic electrolyte has not been successfully demonstrated. One reason for this is that there is a large amount of H⁺ present in acidic electrolyte. The reduction of H⁺ is both thermodynamically and kinetically more favorable than the reduction of Fe³⁺ and Fe²⁺. Therefore, electrolysis in acidic electrolyte tends to favor the reduction of H⁺ to produce hydrogen gas over the reduction of Fe³⁺ and Fe²⁺. This side reaction leads to a very low faradic efficiency during Fe deposition as well as the corrosion of formed Fe.

The methods of producing iron metal described herein involve electrolyzing iron ore in acidic electrolyte. However, the methods described herein can have much higher faradaic efficiency than other electrolysis methods. This can be achieved by minimizing the hydrogen evolution reaction (HER) that occurs as a side reaction at the cathode, in which H⁺ is reduced instead of Fe³⁺ or Fe²⁺. The methods described herein can continuously convert Fe ore into Fe metal and oxygen at low temperatures, such as in the range of 25-90° C. In some examples, the methods can use an electrochemical reactor having two compartments separated by a cation exchange membrane. The compartments can be referred to as an “anode chamber” and a “cathode chamber.” Both chambers can be filled with electrolyte, which may be two different electrolytes or the same electrolyte in some examples. In the anode chamber, water can be oxidized to form O₂gas and H⁺ ions at an anode. Iron ore particles can be fed into the anode chamber. The iron ore can be dissolved by the H⁺ ions in the anode chamber, and the reaction of the iron ore with the H⁺ ions can produce Fe³⁺ and/or Fe²⁺ ions. The Fe³⁺ and/or Fe²⁺ ions can migrate across the cation exchange membrane into the cathode chamber, where they can be reduced to Fe metal at the cathode. The Fe metal can be a solid that is easily separated from the liquid electrolyte in the cathode chamber.

The chemical reactions involved in this process, using Fe₂O₃as an example feedstock, are shown below.

$\begin{matrix} H_{2} O \to 2 H^{+} + 2 e^{-} + 0.5 O_{2} (g) & (I) \end{matrix}$

$\begin{matrix} {Fe}_{2} O_{3} + 6 H^{+} \to 2 {Fe}^{3 +} + 3 H_{2} O & (II) \end{matrix}$

$\begin{matrix} {Fe}^{3 +} + 3 e^{-} \to Fe (s) & (III) \end{matrix}$

Equations (I) through (III) show the individual chemical reactions that can take place in the methods described herein. Reaction (I) can take place at the anode, where water is oxidized to form H⁺ ions and oxygen gas. Reaction (II) can take place in the aqueous electrolyte within the anode chamber. Iron ore can be loaded into the anode chamber and the iron ore can react with the H⁺ ions to form Fe³⁺ ions. Reaction (III) can occur at the cathode, where the Fe³⁺ ions are reduced to form Fe solid metal. The overall reaction that occurs in the electrochemical reactor is shown as Equation (IV) below:

$\begin{matrix} {Fe}_{2} O_{3} \to 2 Fe (s) + 1.5 O_{2} (g) & (IV) \end{matrix}$

Thus, the iron oxide in the ore is the sole reactant and the only products are iron metal and oxygen gas. The materials in the aqueous electrolytes and the anode and cathode are not consumed by the reaction.

As mentioned above, electrolysis of iron ore in acidic electrolyte can be challenging because of the hydrogen evolution reaction (HER) that competes with the reduction of Fe³⁺ and/or Fe²⁺ at the cathode. This challenge is addressed by the methods and systems described herein. First, the design of the electrochemical reactor creates a pH gradient from the anode to the cathode. The local pH is very low near the anode because the of oxygen evolution reaction that occurs at the anode. As the H⁺ ions migrate from the anode toward the cathode, the H⁺ ions react with iron ore to form Fe³⁺ ions. Although some H⁺ ions may migrate through the cation exchange membrane into the cathode chamber, the concentration of H⁺ in the cathode chamber can be much lower because most of the H⁺ ions have already been consumed by the reaction with iron ore. The pH at the cathode can be higher, such as in the range of 4 to 5. This pH range can suppress HER and favor the reduction of Fe³⁺ ions. Additionally, in some examples, the cathode chamber can contain a specific type of electrolyte that can further suppress HER. This electrolyte can include an iron salt and a co-salt that includes magnesium or calcium. The characteristics of this electrolyte are described in more detail below. With the pH gradient and the HER-suppressing electrolyte, the methods described herein can be used to make iron metal with high faradaic efficiency, such as greater than 90%, greater than 95%, or greater than 99%.

With this description in mind, FIG. 1 is a flowchart illustrating one example method 100 of producing iron metal by electrolysis of iron ore. The method includes: introducing iron ore into an anode chamber, wherein the anode chamber includes a first electrolyte and an anode 110; oxidizing water at the anode to form O₂gas and H⁺ ions in the anode chamber 120; dissolving the iron ore to form Fe³⁺ and/or Fe²⁺ ions in the anode chamber 130; transferring the Fe³⁺ and/or Fe²⁺ ions from the anode chamber to a cathode chamber through a cation exchange membrane that separates the anode chamber from the cathode chamber, wherein the cathode chamber includes a second electrolyte and a cathode 140; and reducing the Fe³⁺ and/or Fe²⁺ ions to form iron metal at the cathode 150.

The methods described herein can be performed using an iron ore electrolysis reactor that includes the components described in the method. In particular, an iron ore electrolysis reactor can include: an anode chamber that contains a first electrolyte and an anode; a cathode chamber that contains a cathode and a second electrolyte, and a cation exchange membrane separating the anode chamber from the cathode chamber.

One example iron ore electrolysis reactor is shown in FIG. 2. This figure shows a side cross-sectional view of an iron ore electrolysis reactor 200. The reactor includes an enclosure 210 that defines an anode chamber 212 and a cathode chamber 214, with a cation exchange membrane 220 separating the anode chamber from the cathode chamber. A first electrolyte 230 is contained in the anode chamber, and a second electrolyte 232 is contained in the cathode chamber. The first and second electrolytes are both in contact with the cation exchange membrane so that Fe³⁺ and/or Fe²⁺ ions 257 can migrate from the anode chamber to the cathode chamber through the membrane. The membrane prevents other materials, such as solid particles and anions, from crossing between the chambers. An anode 240 is at least partially submerged in the first electrolyte in the anode chamber. A cathode 242 is at least partially submerged in the second electrolyte in the cathode chamber. When the reactor is operating, iron ore particles 250 are fed into the anode chamber. Water is oxidized at the anode to form O₂gas 252 and H⁺ ions 254. Electrons 256 taken from the water travel out through the anode. The H⁺ ions react with the iron ore to form Fe³⁺ and/or Fe²⁺ ions. The Fe³⁺ and/or Fe²⁺ ions migrate across the cation exchange membrane to the cathode. Electrons flow into the cathode and reduce the Fe³⁺ and/or Fe²⁺ ions to form iron metal particles 258. The iron metal particles can accumulate in the cathode chamber and then the particles can be separated from the electrolyte and collected. It is noted that this figure is not necessarily drawn to scale, and that in practice the iron ore and iron metal particles can be much smaller compared to the other components of the reactor.

The example shown in FIG. 2 was a relatively simple electrolysis reactor design. Other example electrolysis reactors used in the methods described herein can have a variety of other designs. FIG. 3 is a cross-sectional view of a slightly more complex design for an electrolysis reactor 200. This example includes an enclosure 210 that defines an anode chamber 212 and a cathode chamber 214. A cation exchange membrane 220 separates the anode chamber from the cathode chamber as in the previous example. However, in this example, the anode chamber is divided into a water oxidation compartment 216 and an iron ore feed compartment 218. A second cation exchange membrane 222 divides the anode chamber into these two sub-chambers. Water can be oxidized in the water oxidation compartment at the anode 240 to form O₂gas 252 and H⁺ ions 254 while electrons 256 flow out through the anode. In this example, the enclosure also defines an oxygen gas outlet 260 in the water oxidation compartment. The oxygen gas can form bubbles in the first aqueous electrolyte 230. The bubbles can then rise to the surface and the oxygen gas can escape through the oxygen outlet. Iron ore particles 250 are fed into the iron ore feed compartment through an iron ore inlet 262 defined by the enclosure in the iron ore feed compartment. The iron ore feed compartment is between the water oxidation compartment and the cathode chamber. The iron ore feed compartment can contain a third aqueous electrolyte 234, which may have the same composition as the first aqueous electrolyte or may have a different composition in some examples. The second cation exchange membrane 222 prevents the iron ore particles from entering the water oxidation compartment. However, H⁺ ions formed at the anode can migrate through the second cation exchange membrane into the iron ore feed compartment. These H⁺ ions react with the iron ore to generate Fe³⁺ and/or Fe²⁺ ions 257. The Fe³⁺ and/or Fe²⁺ ions can then migrate through the first cation exchange membrane into the cathode chamber. There, the Fe³⁺ and/or Fe²⁺ ions can be reduced by electrons from the cathode 242 to from iron metal particles 258. This example reactor also includes an iron metal collector 264 positioned below the cathode. The iron metal collector is a volume holding a non-aqueous liquid 266 that is substantially immiscible with the second aqueous electrolyte 232. The non-aqueous liquid can be denser than the second electrolyte so that the non-aqueous liquid is held in the iron metal collector volume below the cathode, and the non-aqueous liquid does not mix with the second aqueous electrolyte. When iron metal particles form at the cathode, the particles can fall down into the non-aqueous liquid. The non-aqueous liquid can protect the iron particles from chemical reactions with water that could occur in the aqueous electrolyte.

In some examples, it can be useful to agitate the electrolytes to increase the mass transfer of Fe³⁺ and/or Fe²⁺ ions from the anode chamber to the cathode. The agitation can include stirring, shaking, laminar flow, turbulent flow, or other suitable methods of moving the liquid electrolytes. A flow line can be used to generate a continuous flow of electrolyte in some examples. The flow line can be a recycle line that circulates the electrolyte through the anode chamber or cathode chamber. The first electrolyte, second electrolyte, or both can be agitated. In examples that include a third electrolyte, the third electrolyte may also be agitated. In certain examples, the first electrolyte in the anode chamber can be stirred and the second electrolyte in the cathode chamber can be circulated with a recycle line.

FIG. 4 shows a side cross-sectional view of an example iron ore electrolysis flow reactor 300 that incorporates circulation flow and stirring of the electrolytes. This example includes an enclosure 310 that defines an anode chamber 312 and a cathode chamber 314. In this design, the cathode chamber is positioned above the anode chamber. A cation exchange membrane 320 separates the anode chamber from the cathode chamber. An anode 340 is positioned horizontally at the bottom of the anode chamber. A cathode 342 is positioned horizontally at the top of the cathode chamber, so that the cathode is above the anode. The anode chamber is filled with a first aqueous electrolyte 330. A small airspace is present at an upper portion at the top of the anode chamber near an O₂gas outlet 360. This can prevent the electrolyte from escaping through the O₂gas outlet while also directing rising O₂gas bubbles toward the O₂gas outlet. The O₂gas outlet is also used as an iron ore inlet in this example. Of course, a separate O₂gas outlet and iron ore feed inlet can be used instead as an alternative. Iron ore particles 350 are fed into the anode chamber through the iron ore inlet/O₂gas outlet. This example also includes a stir bar 370 in the anode chamber, which can stir the first electrolyte to increase mass transfer of Fe³⁺ and/or Fe²⁺ ions 357 to the cation exchange membrane. The stirring can also increase the reaction rate of H⁺ ions 354 with iron ore to form the Fe³⁺ and/or Fe²⁺ ions. The cathode chamber is filled with a second electrolyte 332. This example includes a recycle flow line 372 connected to the cathode chamber to circulate the second aqueous electrolyte through the cathode chamber. The second electrolyte can flow in a clockwise direction as shown in the figure so that the second electrolyte can be constantly in motion. This can help to carry Fe³⁺ and/or Fe²⁺ ions to the cathode surface. The Fe³⁺ and/or Fe²⁺ ions can be reduced by electrons 356 to form iron metal particles 358 at the cathode surface. The iron metal particles are denser than the second electrolyte and the iron metal particles can sink downward from the cathode. The flowing motion of the second aqueous electrolyte can carry the iron metal particles to an iron metal collector 364. The iron metal collector contains a non-aqueous liquid 366 that is substantially immiscible with the second electrolyte (i.e. whether aqueous or non-aqueous electrolyte is chosen). As in the previous example, the non-aqueous liquid is denser than the second electrolyte so that the non-aqueous liquid is held by gravity in the iron metal collector as the second electrolyte flows over the top of the non-aqueous liquid. The iron metal particles can accumulate in the non-aqueous liquid for later collection. In this example, the cation exchange membrane is inclined downward in the direction of flow of the second electrolyte. This allows gravity to help direct the iron metal particles toward the iron metal collector. Both gravity and the flow direction of the second electrolyte work together to carry the falling iron metal particles toward the iron metal collector. Additionally, when oxygen bubbles form in the anode chamber, the bubbles can rise up to the cation exchange membrane, which forms a ceiling for the anode chamber. The buoyancy of the oxygen bubbles can cause the bubbles to move along the bottom surface of the inclined cation exchange membrane toward the O₂gas outlet.

It is noted that the example shown in FIG. 4 includes a combination of features including an inclined cation exchange membrane, positioning of the cathode chamber above the anode chamber, a recycle flow line, an iron metal collector connected to the cathode chamber, and other features. In various examples, these features can be included individually or in a variety of combinations in electrolysis reactors. For example, an electrolysis reactor can include a recycle flow line connected to the cathode chamber, but the cathode chamber may be positioned to the side of the anode chamber instead of above the anode chamber. In another example, an electrolysis reactor can include an inclined cation exchanged membrane but may not include a recycle flow line. Thus, the methods and reactors described herein can incorporate these features in any combination.

When the electrolysis reactor includes an inclined cation exchange membrane, in some examples the membrane can have an angle inclination from about 1° to about 45°, or from about 3° to about 35°, or from about 5° to about 20° with respect to horizontal. Although cation exchange membranes often have a flat planar shape, in some examples the membrane can have another shape such as curved or bent. In certain examples, a portion of the cation exchange membrane can be inclined and another portion can be horizontal or vertical. The cation exchange membrane can be positioned in contact with both the first electrolyte and the second electrolyte so that ions can migrate from the first electrolyte to the second electrolyte. The cation exchange membrane can also block transfer of some other materials, such as iron ore particles and iron metal particles. In some examples, the cation exchange membrane can be a NAFION™ cation exchange membrane (available from The Chemours Company, USA) or a DARAMIC® membrane (available from DARAMIC, USA).

In further detail regarding the anode, the anode can be made from a conductive material that can resist corrosion at low pH levels. In some examples, the anode can be made of a metal such as platinum, gold, silver, copper, zinc, bismuth, tin, copper, indium, lead, stainless steel, nickel, titanium, aluminum, tungsten, or alloys thereof. Carbon-based materials can also be used, such as glassy carbon, graphite, carbon felt, carbon cloth, carbon paper, porous carbon, activated carbon, or others. The anode can also include a catalyst for the oxygen evolution reaction (i.e., oxidation of water to form oxygen gas). In certain examples, the catalyst can include nickel, platinum, cobalt, ruthenium, palladium, or alloys thereof, or manganese oxide, lead oxide, ruthenium oxide, cobalt oxide, or a combination. The combination of anode material and catalyst (if used) can be selected to provide a lower overpotential for the oxygen evolution reaction and good corrosion resistance in the acidic electrolyte.

The cathode can also be made of a conductive material. The PH level at the cathode is usually higher than the pH at the anode. However, the pH at the cathode can be slightly acidic, such as in the range of 4 to 5. In some examples, it can be useful to use a cathode material that does not bond to iron metal or form alloys with iron. This can allow iron metal particles formed on the cathode surface to easily detach from the cathode. When the iron metal forms and alloy with the material of the cathode or otherwise bonds to the surface of the cathode, then the iron can build up and form a coating on the cathode that can be difficult to remove. However, in some examples this type of cathode can be used and the iron metal can be removed from the surface of the cathode later by various methods. An iron cathode can be used in some cases, and the electrolysis process can deposit additional iron metal onto the iron cathode. In certain examples, the cathode can be made of a glassy carbon material that does not bond to iron metal. In further examples, the cathode can be made of graphite, carbon felt, carbon cloth, carbon paper, porous carbon, activated carbon, or other carbon-based materials. The cathode can also be made of metal, such as platinum, gold, silver, copper, zinc, bismuth, tin, copper, indium, lead stainless steel, nickel, titanium, aluminum, tungsten, or alloys thereof.

The cathode surface area can affect the rate at which Fe³⁺ and/or Fe²⁺ ions are reduced to form iron metal. Therefore, the size and surface are of the cathode can be selected depending on the desired rate of iron metal production. In some examples, the cathode can have a reaction area from about 0.01 m²to about 10 m², or from about 0.1 m²to about 5 m², or from about 0.2 m²to about 2 m², or from about 0.5 m²to about 1 m². The reaction area can be the area of the cathode that is in contact with the second electrolyte, where Fe³⁺ and/or Fe²⁺ ions can be reduced to iron metal.

The production rate of the electrolysis reactor can range from about 100 grams of iron metal per day to about 100 kg per day, or from about 1 kg per day to about 50 kg per day, or from about 5 kg per day to about 20 kg per day, in some examples. The desired rate of production of iron metal can also affect the amount of electric current that will be used. In some examples, the methods of producing iron metal can consume a current from about 10 A to about 2,000 A, or from about 100 A to about 1,500 A, or from about 200 A to about 1,000 A, or from about 500 A to about 700 A.

The methods described herein can also include controlling the pH in the anode chamber and in the cathode chamber. As mentioned above, the pH can be very low near the anode, where the oxygen evolution reaction occurs. For example, the pH at the anode can be less than 1 or even a local pH less than 0. The average pH of the first electrolyte can be less than 1 or from 0 to 1 in some examples. The second electrolyte in the cathode chamber can have a higher pH by comparison. In some examples, the aqueous electrolyte can have a pH from about 4 to about 5. If there is a third electrolyte in between the first electrolyte and the second electrolyte (as when the anode chamber is divided into a water oxidation compartment and an iron ore feed compartment) then the third electrolyte can have a pH somewhere between the pH of the first and second electrolytes. The design and operation of the electrolysis reactor can ensure that the pH in the cathode chamber stays in the appropriate range. If the pH is too high, then precipitation of Ca(OH)₂and Fe(OH)₃may occur. A low pH can reduce the faradaic efficiency of the iron deposition reaction. To maintain a stable pH in the cathode chamber, any H⁺ that is consumed in the cathode chamber can be replenished by H⁺ migrating from the anode chamber. This can be achieved by balancing H⁺ consumption rate in the cathode chamber, which can be controlled by adjusting the operation current, with H⁺ flux through the membrane, which can be controlled by adjusting pH in the anode chamber and the thickness and/or selectivity of the membrane.

In further detail regarding the electrolytes, in some examples the first and second electrolytes can be aqueous electrolytes. For example, the first aqueous electrolyte can include a perchlorate. The perchlorate is an anion that can be paired with a cation to form a salt. In various examples, the first aqueous electrolyte can include a salt such as sodium perchlorate, lithium perchlorate, calcium perchlorate, magnesium perchlorate, manganese perchlorate, or other salts. Other anions include sulfate, nitrate, phosphate, tetraboron fluoride, bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate. The concentration of salt in the first aqueous electrolyte can be from about 0.5 mol/L to about 10 mol/L, or from about 1 mol/L to about 6 mol/L, or from about 3 mol/L to about 5 mol/L. In further examples, the first aqueous electrolyte can also include a buffer agent such as sodium citrate. This can help regulate the pH in the anode chamber.

The second electrolyte in the cathode chamber can help to suppress the hydrogen evolution reaction at the cathode. In some examples, the second electrolyte can include a dissolved iron salt. In some examples, the iron salt can be soluble in water in an amount from about 0.1 mol/L to about 3.0 mol/L. In further examples, the iron salt can be soluble in amount from about 0.3 mol/L to about 3.0 mol/L, or from about 0.5 mol/L to about 3.0 mol/L, or from about 1.0 mol/L to about 3.0 mol/L, or from about 1.0 mol/L to about 2.0 mol/L, or from about 0.5 mol/L to about 2.0 mol/L.

The second electrolyte can include the dissolved iron salt in any desired amount up to the solubility limit of the iron salt. In some examples, the concentration of iron salt in the electrolyte can be from about 0.05 mol/L to about 3.0 mol/L. In further examples, the concentration can be from about 0.1 mol/L to about 2.0 mol/L, or from about 0.1 mol/L to about 1.0 mol/L, or from about 0.1 mol/L to about 0.5 mol/L, or from about 0.3 mol/L to about 0.7 mol/L, or from about 0.5 mol/L to about 1.0 mol/L, or from about 0.5 mol/L to about 2.0 mol/L, or from about 0.5 mol/L to about 3.0 mol/L. These are concentrations of the iron ions. If the iron salt includes multiple anions per iron atom or multiple iron atoms per anion, then the concentration of the anion in the electrolyte may be different.

The iron salt can be a salt of iron(III) or a salt of iron(II), or a combination thereof. Non-limiting examples of iron salts can include iron(III) chloride (FeCl₃), iron(II) chloride (FeCl₂), iron(II) bromide (FeBr₂), iron(II) iodide (FeI₂), iron(II) sulfate (FeSO₄), ammonium iron sulfate, iron(II) perchlorate, iron(II) nitrate, iron(II) acetate, Fe(BF₄)₂, iron(II) bis(trifluoromethanesulfonyl)imide (FeTFSI₂), iron(II) trifluoromethanesulfonate (Fe(CF₃SO₃)₂), and combinations thereof. These salts can dissolve in the electrolyte to provide Fe²⁺ ions and dissolved anions that originate from the iron salt. In a particular example, the second aqueous electrolyte can include FeCl₃at a concentration of about 0.1 M.

In addition to the iron salt, the second electrolyte can also include a dissolved co-salt when an aqueous electrolyte is used. The co-salt can include magnesium, calcium, or a combination thereof in some examples. In other examples, the co-salt can include lithium, sodium, potassium, ammonium, magnesium, calcium, zinc, manganese, cobalt, nickel, tin, indium, or a combination thereof. In some examples, the dissolved co-salt can be present at a concentration such that the cation of the co-salt (e.g., Mg²⁺, Ca²⁺, etc.) is at a concentration from about 4 mol/L to about 5 mol/L. In further examples, the concentration of the co-salt cation can be from about 4 mol/L to about 4.5 mol/L, or from about 4.5 mol/L to about 5 mol/L, or from about 4.3 mol/L to about 4.7 mol/L.

The anion of the co-salt can be a halogen such as chloride, bromide, or iodide in some examples. In other examples, the anion of the co-salt can be a multiatomic anion, such as sulfate, perchlorate, nitrate, phosphate, bis(trifluoromethanesulfonimide), trifluoromethanesulfonate, acetate, tetraboron fluoride, or a combination thereof. In further examples, the multiatomic anion can include a multi-dentate anion. For example, multi-dentate anions can be used such as, but not limited to, oxalate (C₂O₄²⁻), sulfate (SO₄²⁻), mesylate (CH₃SO₃⁻), and the like.

In further examples, the second electrolyte can also include a buffer agent such as sodium citrate. This can help to regulate the pH in the cathode chamber. In another example, one or both of the first and second aqueous electrolytes can consist essentially of water, the calcium salt and the magnesium salt.

One or more of the first, second and optional third electrolytes can comprise a deep eutectic solvents (DES). In some examples, the DES can be used without an aqueous electrolyte as described previously, i.e. as a non-aqueous electrolyte. In other examples, the DES can be used in combination with an aqueous electrolyte as described herein to form a hybrid electrolyte. DES as electrolytes and can include an iron salt mixed with an organic molecule. Without being bound to any theory, it appears that the iron salt can bring a small amount of crystallization water into the electrolyte. The organic molecule can donate protons to form hydrogen bonds with the anion of the iron salt. For example, the iron salt can be chloride, bromide, iodide, sulfate, phosphate, perchloride, tetraboron fluoride, nitrate, bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate. Non-limiting examples of suitable organic molecules can include amides such as acetamide, urea, propionamide, methyl carbamate, and thiourea. Further examples for the organic molecule can also include alcohols such as ethylene glycol, methanol, ethanol, and sucrose. In another example, the organic molecule can be carboxylic acid such as citric acid, glacial acetic acid (AcOH). propanoic acid, and the like. In a further example, the organic molecule can be an amino acid such as glycine. In still further examples, the organic molecule can be amine such as diethylamine HCl, triethylamine HCl (TEHCl). In some cases, the hybrid of aqueous electrolyte and DES electrolyte can be used. Such a hybrid contains the above-mentioned iron salt, organic molecules, and water.

The aqueous electrolyte, DES electrolyte and the hybrid electrolytes can be used in the cathode chamber, anode chamber, or the middle chamber in the reactor. In other words, any one of more of the first electrolyte, second electrolyte and third electrolyte can be aqueous electrolyte, DES electrolyte, or a hybrid electrolyte.

The methods described herein can also include controlling the concentration of Fe³⁺ and/or Fe²⁺ ions present in the second electrolyte. In some experimental results, it was found that Fe³⁺ and/or Fe²⁺ ions in the second electrolyte was depleted by converting the Fe³⁺ and/or Fe²⁺ ions into iron metal at the cathode. The depletion of Fe³⁺ and/or Fe²⁺ ions in the second electrolyte can be avoided by matching the rate at which Fe³⁺ and/or Fe²⁺ ions migrate across the cation exchange membrane with the rate at which the Fe³⁺ and/or Fe²⁺ ions are reduced to form iron metal. This rate can also be matched with rate at which iron ore particles dissolve in the anode chamber. The dissolution rate of iron ore particles can depend on particle size, stirring rate, pH of the first electrolyte in the anode chamber, and the temperature of the first electrolyte. Therefore, adjusting these parameters can help match the rate at which Fe³⁺ and/or Fe²⁺ ions is transferred and the rate at which Fe³⁺ and/or Fe²⁺ ions are reduced to form iron metal.

In some examples, the first and second electrolytes can be at a relatively low temperature while the iron metal is being produced (much lower than the high temperatures used to melt iron and iron ore in some other production methods). The temperature of the first and second electrolytes can be from about 0° C. to about 150° C. in some examples, from 25° C. to 150° C. in some examples, or from about 25° C. to about 90° C., or from about 25° C. to about 50° C., or from about 50° C. to about 90° C. in other examples. In some cases, using higher temperatures such as around 90° C. can increase the dissolution rate of iron ore and the rate of diffusion of ions in the electrolysis reactor. On the other hand, heating the materials to a higher temperature can also increase the energy consumption of the process. However, the relatively low temperatures used in the methods described herein can save a significant amount of energy compared to other process. By comparison, the blast furnace process and the molten oxide electrolysis process both utilize temperatures over 1500° C. Direction reduction of iron process often use temperatures from about 800° C. to about 1200° C. Thus, the methods described herein use much lower temperatures and this can save significantly on energy costs.

The faradaic efficiency of the process described herein can be from about 80% to about 99.9%, or from about 90% to about 99.9%, or from about 95% to about 99.9%, or from about 95% to about 99%, in some examples. These levels of faradaic efficiency can be higher than other processes that utilize electricity to reduce iron. Electrowinning processes have had a faradaic efficiency in the range of about 86% to about 97%. Molten oxide electrolysis processes have had even lower faradaic efficiency in the range of about 34% to about 46%. Therefore, the methods described herein also save costs of electricity and reduce undesired side reactions that would lower the faradaic efficiency.

The experimental results described below utilized an operating current of around 50 mA/cm², referring to the total electric current divided by the surface are of the cathode on which the iron reduction reaction occurred. However, the operating current can be increased in scaled-up processes. In some examples, the operating current used in the methods described herein can be from about 100 mA/cm²to about 1,000 mA/cm²or from about 500 mA/cm²to about 1,000 mA/cm². The operating current can be proportionate to the rate at which iron metal is produced. In some examples, the rate of mass transfer of Fe³⁺ and/or Fe²⁺ ions to the cathode can also affect the rate of iron production. The methods can include controlling the rate of Fe³⁺ and/or Fe²⁺ ions mass transfer to the cathode so that this rate matches the rate at which the Fe³⁺ and/or Fe²⁺ ions is reduced (which is controlled by the operating current). As explained above, this can involve adjusting the dissolution rate of iron ore in the anode chamber, agitating the electrolytes, selecting the cation exchange membrane thickness, and other parameters.

The total energy cost of producing iron metal using the methods described herein can be lower than many previous processes. The theoretical minimum energy needed to produce iron metal from Fe₂O₃(a common iron ore) is about 1.84 kWh/kg. This is based on the free energy different between Fe₂O₃and Fe metal. In practice, the methods described herein can have an energy cost from about 2 kWh/kg to about 10 kWh/kg, or from about 2.5 kWh/kg to about 5 kWh/kg, or from about 2.5 kWh/kg to about 3 kWh/kg, or less than 2.7 kWh/kg in some examples. By comparison, the blast furnace process uses about 4.98 kWh/kg of iron metal; the direct reduction of iron process uses about 3.5 to 5.5 kWh/kg of iron metal; the molten oxide electrolysis process uses about 2.78 to about 4.63 kWh/kg of iron metal; and electrowinning process uses about 3.6 kWh/kg.

The methods described herein, the energy cost can be affected by the distance between the anode and cathode, as well as the overpotential of the anode for the oxygen evolution reaction. Resistance between the anode and cathode can be reduced by moving the anode and cathode closer together, increasing the conductivity of the electrolytes, and increasing operation temperature. The overpotential of the anode can be reduced by increasing anode surface area, selecting effective anode catalysts, and increasing the operating temperature. As an example, the wherein the anode and cathode can be separated by a distance from about 0.1 cm to about 10 cm, 2 cm to about 10 cm, or from 0.1 cm to 2 cm, or from 10 cm to 50 cm.

The types of iron ore used as feedstock for the methods described herein can include any iron-oxide-containing ore. Several examples include hematite, magnetite, goethite, limonite, siderite, wustite, or a combination thereof. Iron oxide compounds in the ore can include Fe₂O₃, Fe(OH)O, Fe₃O₄, FeO, and others. Many iron ores include a small fraction (3.9-6.0%) of non-Fe impurities. Typical Fe ore contains the oxide of P, Al, Mn, Si and Ca as impurities. Among them, the oxide of Al and Si (i.e., alumina and silica) are resistant to the corrosion of acid. Therefore, they can be removed from the reactor by periodically separate the insoluble solids, which can be done by including a flowing channel to the anode chamber to allow any solid particles to be flushed out and collected. The oxide of Ca can dissolve into the electrolyte and produce Ca²⁺, but the Ca²⁺ ions are not likely to affect the process, especially when the electrolyte includes a calcium salt already. The oxide of Mn can dissolve and produce Mn ions. Since the reduction potential of Mn²⁺ (−1.18 V vs SHE) is much lower than that of Fe²⁺ (−0.44V vs SHE), it is unlikely that Mn will be reduced together with Fe. However, upon long-duration operation, an increasing amount of Mn²⁺ can build up and may increase electrolyte viscosity and decrease conductivity. Therefore, these ions can be removed from the electrolyte. This can be accomplished by periodically precipitating Mn²⁺ by adding base (such as NaOH), organic ligands (such as bipyridine) or natural zeolite. The oxide of phosphorous can dissolve into the electrolyte and form phosphoric acid, which will can gradually increase the pH of the electrolyte and reduce the faradic efficiency. The phosphoric acid can be removed from the electrolyte by adding Ca²⁺, which can precipitate the phosphate anion as calcium phosphate.

The iron ore can be introduced into the anode chamber as particles. In various examples, the iron ore can have an average particle size from about 1 micrometer to about 5 mm, or from about 1 micrometer to about 1 mm, or from about 10 micrometers to about 1 mm, or from about 100 micrometers to about 1 mm, or from about 10 micrometers to about 100 micrometers. As a guideline, the raw iron ore can be mechanically ground to the desired particle size. The particle size can be tuned to match the dissolution rate of iron ore with the iron deposition rate. The rate at which the iron ore particles are fed can depend on the size of the reactor and the desired rate of iron production. In some examples, the iron ore can be fed into the electrolysis reactor at a rate from about 5 kg/day to about 100 kg/day, or from about 10 kg/day to about 100 kg/day. If the process is a continuous process, then iron ore particles can be introduced continuously or semi-continuously into the reactor. The methods described herein can also be performed as a batch process. A batch of a desired amount of iron ore can be loaded in the anode chamber and then the entire batch can be converted to iron metal over an operating time period. As a general guideline, the iron ore can have a mole fraction of iron oxide of 90% to 95%, or 80% to 95%, or 60-95%.

The iron metal forming at the cathode can form solid particles. In some examples, the average particle size of the iron metal can be from about 1 micrometer to about 5 mm, or from about 1 micrometer to about 1 mm, or from about 10 micrometers to about 1 mm, or from about 100 micrometers to about 1 mm, or from about 10 micrometers to about 100 micrometers.

Example: Electrochemical Reactor

A test reactor was constructed with two glass chambers. The first chamber was filled with a first aqueous electrolyte and an anode was partially submerged in the first aqueous electrolyte. The first aqueous electrolyte included calcium perchlorate (Ca(ClO₄)₂) at a concentration of 4.5 mol/L. The anode was a platinum wire. The second glass chamber was filled with a second aqueous electrolyte and a cathode was partially submerged in the second aqueous electrolyte. The second electrolyte included FeCl₃salt at a concentration of 0.1 mol/L and CaCl₂salt at a concentration of 4.5 mol/L. The cathode was titanium foil. The distance between the anode and the cathode was about 6 cm. The glass chambers were separated by a NAFION™ cation exchange membrane (available from The Chemours Company, USA), with the first aqueous electrolyte in contact with the cation exchange membrane on one side and the second aqueous electrolyte in contact with the cation exchange membrane on the opposite side.

Iron ore powder was added to the anode chamber. Initially, the electrolyte in the anode chamber was clear and the electrolyte in the cathode chamber was yellow due to the presence of Fe³⁺ ions in the second aqueous electrolyte. An electrolysis reaction was then started using a potentiostat connected to the anode and cathode. The electric current used for electrolysis was about 50 mA per square centimeter of cathode area. The temperature in the reactor during electrolysis was about 25° C. After 4 hours of electrolysis, oxygen bubbles formed on the anode and the electrolyte in the anode chamber had become slightly yellow due to the formation of Fe³⁺ ions from dissolving the iron ore. The second electrolyte in the cathode chamber had changed color to slightly greenish, due to the formation of iron metal and Fe²⁺ ions by reducing Fe³⁺ ions. After 24 hours of electrolysis, the first electrolyte in the anode chamber had become very yellow. The second electrolyte in the cathode chamber had become mostly clear, except for a slight yellow color near the cation exchange membrane. Black iron metal powder covered the cathode and additional iron powder settled to the bottom of the cathode chamber. This indicates that Fe³⁺ ions formed by dissolving the iron ore were migrating through the cation exchange membrane, and then being converted to iron metal at the cathode. In this experiment, the iron ore was converted to iron metal with a faradaic efficiency of 80-90%. It is noted that this is much higher than the faradaic efficiency of electrolysis of iron in an acidic electrolyte (such as a common Fe₂(SO₄)₃electrolyte, which usually has a faradaic efficiency of 2-8% with hydrogen evolution reaction dominating the cathodic reaction.

The black powder formed on the titanium foil cathode was confirmed to be iron by X-ray diffraction. FIG. 5 is a scanning electron microscopy (SEM) image of the iron powder. The iron powder had a particle size of a few micrometers. The iron tended to attach to the titanium cathode, making separation difficult. But, different cathode materials such as glassy carbon are expected to allow the iron particles to detach more easily. FIG. 6 shows in XRD of titanium electrodes after 1 hour of electrolysis. The strong peaks of iron confirm the formation of iron on the Ti substrate after electrolysis.

While the flowcharts presented for this technology may imply a specific order of execution, the order of execution may differ from what is illustrated. For example, the order of two more blocks may be rearranged relative to the order shown. Further, two or more blocks shown in succession may be executed in parallel or with partial parallelization. In some configurations, one or more blocks shown in the flow chart may be omitted or skipped.

Reference was made to the examples illustrated in the drawings and specific language was used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the technology is thereby intended. Alterations and further modifications of the features illustrated herein and additional applications of the examples as illustrated herein are to be considered within the scope of the description.

Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more examples. In the preceding description, numerous specific details were provided, such as examples of various configurations to provide a thorough understanding of examples of the described technology. It will be recognized, however, that the technology may be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the technology.

Although the subject matter has been described in language specific to structural features and/or operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features and operations described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. Numerous modifications and alternative arrangements may be devised without departing from the spirit and scope of the described technology.

HIGH-EFFICIENCY ELECTROLYSIS OF IRON ORE FOR IRON PRODUCTION

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