ELECTROLYTIC METHODS AND SYSTEMS FOR THE PRODUCTION OF METALLURGICAL GRADE SILICON

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Patent Application No. 63/734,815 filed on Dec. 17, 2024, the contents of which are incorporated herein by reference in their entirety.

CONTRACTUAL ORIGIN

This invention was made with government support under Contract No. DE-AC36-08GO28308, awarded by the Department of Energy. The government has certain rights in this invention.

BACKGROUND

Metallurgical grade silicon (MGS) is traditionally produced via high-temperature reactions that use carbon-based reductants including coal, coke, and natural gas to displace oxygen from ores, thereby producing large amounts of carbon dioxide (CO₂) in the process along with requiring high electricity usage. These processes also rely on high temperatures, often exceeding about 2000° C., meaning they are very energy intensive. Thus, there remains a need for a process to create MGS with reduced CO₂ emissions.

SUMMARY

An aspect of the present disclosure is a system for producing metallurgical grade silicon (MGS), where the system includes an anode, a cathode in contact with a source of silicon, and an electrolyte solution, in which the anode comprises an inert material, and the system is configured to be operated at less than approximately 900° C. In some embodiments, the system also includes a heating element and the heating element is configured to heat the electrolyte solution to approximately 850° C. In some embodiments, the system also includes a voltage source and the voltage source is configured to provide a voltage to the anode and cathode of less than approximately 5 V. In some embodiments, the voltage is in the range of about 1 V to about 3 V. In some embodiments, the cathode comprises stainless steel. In some embodiments, the source of silicon comprises a plurality of silicon pellets or silica sand. In some embodiments, the electrolyte solution includes calcium chloride (CaCl₂). In some embodiments, the electrolyte solution is doped with at least one of calcium oxide (CaO), or silicon oxide (SiO₂). In some embodiments, the anode includes tin oxide (SnO₂)

An aspect of the present disclosure is a method of producing metallurgical grade silicon (MGS), where the method includes heating an electrolyte solution positioned within a container, applying a voltage between an anode and a cathode positioned within the container, collecting a product of MGS on the cathode, in which the cathode is in contact with a source of silicon, the anode comprises an inert material, and the electrolyte solution is heated to a temperature less than approximately 900° C. In some embodiments, the heating is performed using a heating element, and the heating element is configured to heat the electrolyte solution to approximately 850° C. In some embodiments, the applying is performed using a voltage source, and the voltage source is configured to provide a voltage to the anode and cathode of less than approximately 5 V. In some embodiments, the voltage is in the range of about 1 V to about 3 V. In some embodiments, the cathode comprises stainless steel. In some embodiments, the source of silicon comprises a plurality of silicon pellets or silica sand. In some embodiments, the electrolyte solution includes calcium chloride (CaCl₂)). In some embodiments, the electrolyte solution is doped with at least one of calcium oxide (CaO), or silicon oxide (SiO₂). In some embodiments, the anode comprises an inert material. In some embodiments, the inert material includes tin oxide (SnO₂). In some embodiments, the collecting comprises removing the MGS from the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present disclosure are illustrated in the referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting.

FIG. 1 illustrates an overview of the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure.

FIG. 2 illustrates a benchtop setup for performing the electrolytic method of producing MGS, according to some aspects of the present disclosure.

FIG. 3 illustrates an exemplary setup for performing the electrolytic method of producing MGS, according to some aspects of the present disclosure.

FIG. 4 illustrates current (A) in relation to potential (V vs O₂) for the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure.

FIG. 5 illustrates current (A) in relation to time) for the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure.

FIG. 6 illustrates energy dispersive spectroscopy (EDS) at 25 μm scale of the silicon product produced in tests of the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure.

FIG. 7 illustrates energy dispersive spectroscopy (EDS) at 10 μm scale of the silicon product produced in tests of the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure.

FIG. 8 illustrates characteristic X-ray images of silicon (Si), calcium (Ca), oxygen (O), and chloride (Cl), according to some aspects of the present disclosure.

DETAILED DESCRIPTION

The embodiments described herein should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein. References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, “some embodiments”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

As used herein the term “substantially” is used to indicate that exact values are not necessarily attainable. By way of example, one of ordinary skill in the art will understand that in some chemical reactions 100% conversion of a reactant is possible, yet unlikely. Most of a reactant may be converted to a product and conversion of the reactant may asymptotically approach 100% conversion. So, although from a practical perspective 100% of the reactant is converted, from a technical perspective, a small and sometimes difficult to define amount remains. For this example of a chemical reactant, that amount may be relatively easily defined by the detection limits of the instrument used to test for it. However, in many cases, this amount may not be easily defined, hence the use of the term “substantially”. In some embodiments of the present invention, the term “substantially” is defined as approaching a specific numeric value or target to within 20%, 15%, 10%, 5%, or within 1% of the value or target. In further embodiments of the present invention, the term “substantially” is defined as approaching a specific numeric value or target to within 1%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, or 0.1% of the value or target.

As used herein, the term “about” is used to indicate that exact values are not necessarily attainable. Therefore, the term “about” is used to indicate this uncertainty limit. In some embodiments of the present invention, the term “about” is used to indicate an uncertainty limit of less than or equal to ±20%, ±15%, ±10%, ±5%, or ±1% of a specific numeric value or target. In some embodiments of the present invention, the term “about” is used to indicate an uncertainty limit of less than or equal to ±1%, ±0.9%, ±0.8%, ±0.7%, ±0.6%, ±0.5%, ±0.4%, ±0.3%, ±0.2%, or ±0.1% of a specific numeric value or target.

Among other things, the present disclosure relates to a substantially carbon-free, molten electrolytic method and system for production of silicon with a purity of greater than approximately 95%. Unlike other molten electrolytic methods, which may somewhat decrease carbon emissions compared to incumbent processes but do not substantially reduce or eliminate them, the process of the present disclosure is substantially carbon free (meaning little to no carbon dioxide (CO₂) is released as a result of the process). The process of the present disclosure utilizes an anode which evolves oxygen, not CO₂. Silica may then be generated at the cathode, generating the desired product, silicon, as well as oxide (O²⁻) ions. In the process of the present disclosure these O²⁻ ions may react with a substantially inert anode to evolve oxygen (O₂).

In contrast to the methods of the present disclosure, in nearly all other electrolytic methods, generated O²⁻ ions react with a graphite anode to form CO₂. This can generate approximately 3.1 kg of CO₂ per kg of Si generated. This is only a marginal improvement over silicon production in contemporary submerged arc furnaces (SAFs), which also use graphite anodes and produce approximately 4-5 kg of CO₂ per kg of 2N Si generated.

A schematic of the process of the present disclosure is shown in FIG. 1. Silica (SiO₂) is reduced at the cathode, generating the desired product, silicon (Si) and oxide (O²⁻) ions. In the present disclosure, these O²⁻ ions react at an inert anode to evolve oxygen (O₂). The absence of process carbon and graphite reduces the carbon emissions to approximately zero, barring impurities present in the materials (such as the carbon present in stainless-steel cathode and in the containment vessel).

In some embodiments, the methods described herein may be performed at temperatures less than approximately 1000° C. For example, some embodiments, may be performed in the range of approximately 750° C. to approximately 900° C. Some embodiments may be performed at approximately 850° C. The heating may be provided by heating elements, such as resistive heating elements, a furnace, or other heating devices.

The methods of the present disclosure reduce SiO₂ to Si in molten CaCl₂) via Reaction 1 (below), which takes place at the cathode. The O²⁻ ions released by Reaction 1 are oxidized at the anode (shown in Reaction 2, below). This process may be performed in the range of approximately 750° C. to approximately 850° C., a significant temperature reduction compared to traditional SAF, which requires temperatures exceeding about 2000° C. with a voltage in the range of approximately 1 V to approximately 3 V applied between the working and counter electrodes.

Cathode ⁢ ( SiO 2 ⁢ pellets ) : SiO 2 + 4 ⁢ e - = Si + 2 ⁢ O 2 - ( Reaction ⁢ 1 ) Anode ⁢ ( Noncarbon - based ⁢ SnO 2 ) : 2 ⁢ O 2 - = O 2 + 4 ⁢ e - ( Reaction ⁢ 2 ) O 2 - + C = CO + 2 ⁢ e - ( Reaction ⁢ 3 ) Anode ⁢ ( Carbon - based ) : 2 ⁢ O 2 - + C = CO 2 + 4 ⁢ e - ( Reaction ⁢ 4 )

In contrast, traditional electrolytic methods include an anode made of a carbon material, where the oxide ions react with and consume the carbon electrode to produce carbon monoxide gas (Reaction 3) with some small fraction possibly going directly to CO₂ via Reaction 4. In graphite-based electrolytic technologies, production of CO and subsequent oxidation to CO₂ yields 3.1 t_CO2/t_Si (2 CO₂ per Si) (Reaction 3). Even if the anodic reaction is tailored to directly produce purely CO₂, the net emissions are 1.6 t_CO2/t_Si (Reaction 4). Studies which propose an alternative have not been reproduced and/or require highly expensive materials such as ruthenium and titanium. However, the anode of the present disclosure are substantially carbon-free inert materials that evolve O₂ instead of CO₂ (Reaction 2). Further, the substantially inert material will not be consumed as in the case of traditional carbon-based electrodes, increasing their lifespan.

In some embodiments, the electrode replacement (i.e., the anode) for this present disclosure is tin oxide (SnO₂). It is relatively stable under related O₂ evolving high-temperature molten salt electrochemical reactions. Some embodiments may include other conductive oxides as the anode or a coating on the anode, including those containing W, Au, and/or Pt.

Some embodiments may include improving the kinetics of the electrolysis process. This may be done by enhancing the electrolyte by doping with calcium oxide (CaO), magnesium oxide (MgO), and/or other oxide species. In some embodiments the electrolyte may be calcium chloride (CaCl₂)) with approximately 1.5 mol % CaO and approximately 1.5 mol % SiO₂ dispersed within. These modifications aim to lower the resistance of the electrolyte, increase the concentration of oxide ions at the surface, and thereby improve O₂ evolution kinetics.

In the exemplary benchtop set up shown in FIGS. 2 and 3, the cathode is a conductive metal such as stainless steel. In some embodiments, the cathode may be a silicon wafer wrapped with wire and/or graphite paste. The conditions at the cathode, where oxidizing reactions take place, are not as challenging as the anode from a materials perspective, since reduction reactions degrade materials. In some embodiments, the source of the silicon in the cell may be silicon oxide pellets (i.e., silica sand). The choice of cathode may be optimized based on impurities. In some embodiments, the cathode may be a stainless steel basket or other container holding solid silicon oxide pellets. The present disclosure may be capable of using silica sand or other substantially impure feedstocks containing silicon and still produce substantially high quality silicon. Energy consumption may be reduced with a reduced operating voltage or operating temperature compared to traditional methods. In some embodiments, pulsed voltammetry and/or reducing the overpotential using kinetic improvements may enable these energy reductions. The set up as shown in FIGS. 1-3 may include a substantially inert atmosphere, to avoid contaminating the reaction. In some embodiments, the temperature may be less than approximately 1000° C.

In some embodiments, the products may be characterized using scanning electron microscopy (SEM) and/or X-ray diffraction (XRD) and phase identification, and ICP-MS for ppm-level impurity detection in keeping with 5/5/3 standards. These tests may give information on both the yield (any remaining silicon oxide in the product) and the purity.

In some embodiments, dopants may be used to enhance the ionic properties of the salt mixture. Cyclic voltammetry (CV) and other electrochemical characterization techniques may be used to identify which reactions are limiting kinetics such as silicon dioxide reduction, oxygen evolution, and/or electrolyte transport. Doping the electrolyte with oxides may increase the concentration of O²⁻ in solution which not only improves conductivity, which subsequently reduces the cell voltage, but it also increases the concentration of O²⁻ at the counter electrode which increases the rate of O₂ evolution. In some embodiments, the voltage applied may be less than approximately 5 V.

In some embodiments, the produced silicon may be in a substantially solid state throughout the process of the present disclosure and thus may be lifted out of the molten salt when contained in a mesh basket or other container. In some embodiments, other methods of separating the silicon product from the water-soluble calcium chloride salt may be used. In some embodiments, the impurities in the silicon product may be analyzed to determine improved sources of electrodes/electrolytes and develop maximum tolerance levels.

In some embodiments, proper storage and handling techniques such as gloveboxes and desiccators may be used to minimize exposure to air. This may prevent the molten salt from corroding. In some embodiments, scavengers such as magnesium metal can be used to remove harmful species, forming inert or even potentially helpful products such as magnesium oxide.

FIG. 4 illustrates current (A) in relation to potential (V vs O₂) for the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure. FIG. 5 illustrates current (A) in relation to time) for the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure. As shown in FIGS. 4-5, cyclic voltammetry shows a silicon reduction peak. It appears that silicon was deposited at approximately −1.2 V for approximately 2 hours.

FIG. 6 illustrates energy dispersive spectroscopy (EDS) at 25 μm scale of the silicon product produced in tests of the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure. FIG. 7 illustrates energy dispersive spectroscopy (EDS) at 10 μm scale of the silicon product produced in tests of the electrolytic method of producing metallurgical grade silicon (MGS), according to some aspects of the present disclosure. FIG. 8 illustrates characteristic X-ray images of silicon (Si), calcium (Ca), oxygen (O), and chloride (Cl), according to some aspects of the present disclosure. As shown in FIGS. 6-8, in testing, there is evidence of aggregated spherical silicon formation near the cathode.

The foregoing discussion and examples have been presented for purposes of illustration and description. The foregoing is not intended to limit the aspects, embodiments, or configurations to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the aspects, embodiments, or configurations are grouped together in one or more embodiments, configurations, or aspects for the purpose of streamlining the disclosure. The features of the aspects, embodiments, or configurations may be combined in alternate aspects, embodiments, or configurations other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the aspects, embodiments, or configurations require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment, configuration, or aspect. While certain aspects of conventional technology have been discussed to facilitate disclosure of some embodiments of the present invention, the Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate aspect, embodiment, or configuration.