Novel silicon melting process

Abstract

The present invention relates to a silicon smelting process of low quality silicon feedstock. More specifically, the present invention relates to a smelting process of low quality feedstock to achieve silicon yield at an acceptable level of energy efficiency, while having a significantly lower carbon footprint. According to a first aspect of the present invention, there is provided a method of silicon smelting of a feedstock, the method comprising the steps of: (i) feeding a feedstock comprising a Si02source having an average particle size of 36 µm < d 50 <25 mm, a reducing agent and a fluxing agent to an electric furnace; (ii) heating the feedstock comprising the Si02source, the reducing agent and the fluxing agent in the electric furnace at a temperature of 1400 °C to 2500 °C, while ensuring a continuous feeding of the feedstock comprising the Si02source, the reducing agent and the fluxing agent into the electric furnace by means of an adjustable feeding slot, controlling the power- feed balance by means of a loss in weight (LIW) system, and smelting the feedstock comprising the Si02source, the reducing agent and the fluxing agent sufficiently to form a liquid metallic silicon product, a liquid slag product and a waste gas comprising SiO, CO and H2, wherein the electric furnace is an open bath furnace (OBF); (iii) recovering Si from the electric furnace waste gas by forming metallic silicon from SiO by reduction with H2 gas in a gas reactor; and (iv) recovering heat from the electric furnace waste gas by combustion of process gas in a waste heat recovery unit (WHRU).

Description

Technical Field

[0001] This invention relates to a method for smelting silicon from low-quality silicon feedstocks. More specifically, this invention relates to a method for smelting low-quality feedstocks to achieve silicon yields at acceptable energy efficiency levels while having a significantly lower carbon footprint. Background Technology

[0002] Metallic silicon is used to produce silanes and organosilicones, as a "hardener" or alloying element in the production of aluminum alloys, and also in the production of microprocessors and solar cells.

[0003] Traditionally, metallic silicon (Si) is produced using coarse quartzite as the primary ore feedstock through a semi-enclosed or open submerged arc furnace (SAF). This method requires mixing the quartzite with a suitable reducing agent (coal, char, and / or coke), sawdust, and a small portion of slag-forming flux (typically limestone) in a batching unit located upstream of the SAF.

[0004] The mixed feed material is conveyed to a furnace feeding system, typically consisting of multiple steel bins. The mixed material is then intermittently added to the SAF using several material feeders, while the SAF is continuously powered.

[0005] SAF operates using alternating current (AC), which is supplied to the furnace through carbon electrodes, the tips of which are physically positioned below the raw material load. An electric arc is generated at the electrode tips, providing the necessary heat and temperature to drive the desired reduction reaction. Metallic Si is produced by reducing silica (SiO2) contained in quartzite to metallic silicon (Si).

[0006] The furnace vessel is typically circular and lined with refractory material. These furnaces are classified as either open or semi-closed, meaning the furnace top is located above the raw material load and allows air to enter the area below the top to directly combust process gases above the raw material load.

[0007] Silicon carbide (SiC) forms and accumulates in the furnace as an intermediate byproduct. SiC has a very high melting temperature. This leads to the formation of solid SiC accretions at various locations within the furnace. To overcome this accretion formation, the furnace shell is rotated at a low speed (typically 0.8° / h to 1.6° / h), thereby burning off these accretions at the electrodes. It is well known that excessive SiC accretion leads to poor Si recovery.

[0008] During the reduction of SiO2, silicon monoxide (SiO) is also formed as an intermediate product. SiO exists in gaseous form within the SAF (Sinking Furnace Air Facility) and moves upwards through the furnace feedstock. During this upward movement, the SiO gas reacts with the solid reducing agent to form SiC and Si. The main reason for including sawdust in the furnace feedstock is to maintain a standard level of permeability / porosity within the furnace feedstock, allowing the SiO gas to move upwards.

[0009] The drawback of this method is that if the furnace feedstock is not fully stoked, cavities will form, causing excessive SiO gas to escape from the feedstock. This SiO burns above the feedstock to form SiO2, resulting in the loss of this portion of Si into the gas purification system.

[0010] Therefore, in these known methods, managing the feed load through adequate feed and ignition operations is crucial for maintaining sufficient levels of Si recovered from the SAF. However, this is related to the tendency for material to be lost in the exhaust gas as SiO2 dust. This loss can typically account for at least a percentage of the quartz supply in the feed.

[0011] Metallic Si, produced as a molten material, is intermittently discharged through tapholes. A taphole is a controlled opening formed in the side wall of the furnace to allow molten Si to exit. The Si is then discharged into a refractory-lined ladle, which is used to transport the Si to downstream processing and casting units. SAFs are typically equipped with multiple tapholes.

[0012] A known drawback of using AC SAF to produce metallic Si is that these methods require raw materials of a specific quality. Quartzite, in particular, needs to possess sufficient strength at high temperatures to maintain its shape. This is measured in laboratory tests and defined by the Thermal Stability Index (TSI) value. If quartzite with a low TSI value is supplied to the furnace, the furnace load permeability decreases, leading to localized gas ejections, and the formation of voids due to the sintering of fine powder further contributes to localized gas ejections. Similarly, this method must use a reducing agent with specific reactivity to ensure that SiO reacts effectively with the reducing agent.

[0013] Recent studies have demonstrated attempts to improve the performance of these methods. MINTEK in South Africa conducted pilot-scale tests using a closed-circuit direct-current furnace (DCF); however, the results from the pilot-scale tests showed poor Si recovery, approximately 45% Si. (Abdellatif, M. "DC arc smelting of silicon: Is it technically feasible?”, Southern African Pyrometallurgy 2011, March 2011).

[0014] Furthermore, the drive to optimize silicon metal production has highlighted the growing urgency of addressing and reducing carbon emissions in the energy sector. Therefore, global focus on decarbonization, including the continued replacement of energy sources with low-emission energy, the continuous improvement of plant efficiency, and the use of carbon capture, utilization, and storage (CCUS), remains crucial for maintaining competitiveness in the future.

[0015] Furthermore, a recent BBC report points out that while more than 1 terawatt of solar capacity (an average of approximately 2.5 billion solar panels) has been installed globally, there is a clear lack of infrastructure for dismantling and recycling retired solar panels and modules. Another recent report from the National Renewable Energy Lab (NREL) predicts that by 2030, the United States alone could generate 1 million tons of waste from retired solar panels and modules. By 2050, this amount of waste will increase dramatically.

[0016] PCT / NO2007 / 000092 discloses a method for producing pure metallic silicon and amorphous silicon dioxide by reducing quartz used in solar panels. This method uses a hydrocarbon gas (preferably methane) as a reducing agent to reduce quartz in a single step. However, PCT / NO2007 / 000092 has significant drawbacks. It requires high-quality feedstock with metallic silicon purity exceeding 98% before implementation and also necessitates high furnace operating temperatures. Therefore, PCT / NO2007 / 000092 does not disclose the use of low-quality feedstock for high Si yield production.

[0017] The study by Abdellatif et al., entitled “DC Arc Smelting of Silicon: Is it Technically Feasible?” (Southern African Pyrometallurgy 2011), explored the technical feasibility of producing metallic silicon using a direct current (DC) arc furnace. The researchers operated a graphite-lined furnace at power levels ranging from 150 kW to 160 kW, processing a feed mixture of 28% petroleum coke and 72% bulk silica. During the experiment, approximately 1.6 tons of premixed feed were processed in 16 feed-discharge operations. The furnace encountered operational challenges, such as feed system blockage and difficulty in maintaining continuous operation, necessitating a manual feeding method. Furthermore, the produced metallic silicon exhibited purity levels ranging from 18% to 78%, containing impurities such as iron, calcium, and aluminum; this was partly attributed to contamination from oxygen lancing and sampling processes. The average recovery rate of silicon to the metallic phase was approximately 40%, peaking at just over 45% in later batches, which is far below the typical 70% to 85% recovery rate in commercial operations. The study did not mention methods for recovering silicon from waste gas, nor did it consider recovering heat from process gases via waste heat recovery systems, exposing the shortcomings of these methods in terms of economic feasibility and environmental suitability.

[0018] WO2018 / 141805A1 discloses a method for producing elemental silicon from silicon oxides, particularly silicon dioxide. The method comprises a two-step gas-phase reduction using a gaseous reducing agent. In the first step, silicon dioxide is reduced to silicon monoxide at a temperature of 1000°C to 2500°C, producing a gaseous phase containing silicon monoxide. In the second step, this silicon monoxide gas is further reduced by the gaseous reducing agent at a temperature of 1500°C or higher to form elemental silicon, which is then separated from the residual gaseous phase. While this method aims to be environmentally friendly by avoiding the use of chlorinating compounds, it relies on a high-temperature gas-phase reaction, which can pose challenges for energy conservation and process control. The necessity of handling the gas at such high temperatures can lead to increased operational complexity and equipment requirements. Failure to introduce a circulating liquid flow can result in lower energy efficiency and potential difficulties in scaling up the process in industrial applications. The absence of these features may also affect the total silicon yield and purity achievable through this method.

[0019] WO2016 / 093704A1 discloses a method for reducing silicon monoxide using hydrogen in a gas reactor. While WO2016 / 093704A1 introduces an energy-efficient method for producing metals using carbothermic reduction, it does not address the specific challenges of silicon smelting, particularly the formation of SiO and SiC, or the need for high-quality feedstock.

[0020] Therefore, current methods have failed to achieve high Si yields while using low-quality feed materials, and have not shown reduced carbon emissions when operating at acceptable energy efficiency levels.

[0021] For the purposes of this specification, it should be understood that the following acronyms are used synonymously with the phrases referenced below.

[0022]

[0023] Purpose of the invention Therefore, the object of the present invention is to provide a novel method for silicon smelting using low-quality feedstocks, which at least partially overcomes the aforementioned disadvantages and limitations, and / or provides a useful alternative to the prior art, thereby achieving silicon yield at an acceptable level of energy efficiency while using low-quality feedstocks, with improved process control and exhibiting a significantly lower carbon footprint. Summary of the Invention

[0024] According to a first aspect of the present invention, a method for smelting silicon as a raw material is provided, the method comprising the following steps: (i) Containing particles with an average particle size of 36µm <d 50 The raw materials, reducing agent and flux of the SiO2 source with a diameter of <25mm are fed into the electric furnace; (ii) At a temperature of 1400°C to 2500°C, raw materials, reducing agents, and flux containing SiO2 sources are heated in an electric furnace, while an adjustable feed trough ensures that the raw materials, reducing agents, and flux containing SiO2 sources are continuously fed into the electric furnace. A weight loss (LIW) system is used to control the power-feed balance, so that the raw materials, reducing agents, and flux containing SiO2 sources are fully melted, thereby forming liquid silicon metal products, liquid slag products, and waste gas containing SiO, CO, and H2; wherein the electric furnace is an open furnace (OBF). (iii) In a gas reactor, Si is recovered from the exhaust gas of an electric furnace by reducing SiO with H2 gas to form metallic silicon; and (iv) In the waste heat recovery unit (WHRU), heat is recovered from the exhaust gas of the electric furnace by burning process gases.

[0025] In the context of this article, smelting should be understood as the process of extracting silicon from a raw material containing a SiO2 source.

[0026] It should be understood that an electric furnace is a furnace whose heat source is electricity. An OBF electric furnace can be a direct current (DC) electric arc furnace (EAF) or an alternating current (AC) EAF. The DC EAF OBF and AC EAF OBF of the present invention can be used in a closed furnace configuration, which means that no air enters the furnace and a reducing environment exists below the furnace top (closed open furnace (closed OBF)).

[0027] In this invention, the OBF electric furnace can operate in different arc modes, namely, open arc mode (multiple electrodes or single electrode), short open arc mode, brush arc mode and immersion electrode (no open arc) mode.

[0028] For the purposes of this invention, the power capacity of the DC EAF is at most 100MW. The power capacity of the AC EAF is at most 100MW.

[0029] The present invention provides a method for heating a raw material containing a SiO2 source, a reducing agent and a flux in an electric furnace at a temperature of 1400°C to 2500°C (preferably 1500°C to 1800°C).

[0030] It should be understood that the raw materials containing SiO2 sources can be, but are not limited to, quartzite, fine quartzite, pretreated quartz sand (preferably pretreated quartz sand that has undergone beneficiation to remove contaminating elements), or pretreated solar panels containing unwanted components such as contaminating elements like iron (Fe) and copper (Cu) that have been removed in the upstream process. Quartzite can contain 40% to 100% SiO2.

[0031] It should be understood that the thermal stability index (TSI) of the quartzite used as raw material is no longer an applicable factor for evaluating the suitability of quartzite for smelting in this method.

[0032] Raw materials containing SiO2 sources may include those with an average particle size of 36µm. <d 50 Fine quartzite <25mm, recycled quartzite waste, and combinations thereof.

[0033] In this invention, cold raw materials or preheated raw materials may be fed into the furnace; alternatively, a combination of cold raw materials and preheated raw materials may be fed into the furnace.

[0034] This invention provides a feeding system comprising a cold feeding system and a hot feeding system, which delivers raw materials according to the desired process formulation. For example, in embodiments of the invention, one or more preheating units can be assembled into the hot feeding system. In other embodiments of the invention, the furnace can also utilize a hollow electrode system (HES) for feeding.

[0035] The reducing agent provided by this invention is anthracite, coke, fine-powder coke, charcoal, or coal. In embodiments of this invention, the reducing agent can be a biomass carbon source. The reducing agent can be added to the electric furnace in particulate form, wherein the particle size is equal to or less than 0 mm to 50 mm.

[0036] Fluxes can be selected from calcined or uncalcined dolomite, calcined or uncalcined limestone, quartzite, bauxite, and combinations thereof.

[0037] The present invention provides multiple adjustable feed troughs that introduce raw materials containing SiO2 sources, reducing agents and fluxes into an electric furnace, and control the power-feed balance in the furnace through a weight loss (LIW) system.

[0038] The present invention provides a method for continuously replenishing raw materials containing SiO2 source, reducing agent and flux into an electric furnace using multiple adjustable feed troughs to ensure that weight loss and power-feed balance are controlled.

[0039] The specific energy consumption (SEC) of a smelting process can be simply expressed as the MWh consumed per metric ton of total feed, or power (MW) / feed rate (ton / h). SEC refers to the energy required to transform a 25°C feed material into the desired temperature of the product stream as it leaves the furnace. Therefore, SEC is essentially a power-feed ratio. It should be understood that the theoretical SEC will change considerably if the chemical composition or temperature of the feed deviates from the theoretical baseline.

[0040] From the perspective of product SEC, the SEC of metallic Si produced by conventional SAF is 10.5 MWh / t metallic Si to 12.5 MWh / t metallic Si. Depending on the efficiency of the upstream preheating unit and the downstream WHRU, the method of the present invention can produce metallic Si with similar SEC values.

[0041] This invention provides a method for producing carbon from feedstock containing SiO2 using 100% hydrogen, thereby helping to reduce the CO2 footprint. It should be understood that the carbon footprint of producing carbon from 100% hydrogen using feedstock containing SiO2 is extremely low, potentially as low as 0%.

[0042] In this embodiment, the present invention provides a method for reducing SiO to Si using H2 gas as a reducing agent. In this embodiment, H2 gas can be added to the method in the following manner: (i) Feeding a reducing agent rich in volatiles into the feed system or HES to generate additional H2 and CO gases; (ii) Inject H2 gas into the HES; (iii) Inject H2 gas into a waste heat recovery unit (WHRU) or a gas reactor; (iv) Install a preheating unit upstream to improve energy efficiency, thereby increasing the SER of the OBF electric furnace; or (v) Install a WHRU downstream to improve energy efficiency, thereby improving the SEC of the method.

[0043] In an alternative embodiment, the present invention provides the injection or addition of an alternative solid H2 source, particularly high volatile coal.

[0044] The present invention also provides a furnace exhaust gas energy recovery system. This furnace exhaust gas energy recovery system can be equipped with a waste heat recovery unit (WHRU) that recovers energy by burning carbon monoxide (CO), hydrogen (H2), and silicon monoxide gas (SiO). In this embodiment, the combustion heat can be transferred to steam in a steam boiler located within the WHRU.

[0045] In the present invention, liquid metal silicon product materials can be formed by heating and melting, or at least partially melting, raw materials containing a SiO2 source, a reducing agent and a flux.

[0046] The residence time of raw materials containing SiO2 sources in the electric furnace can be controlled to control the degree of reduction of raw materials containing SiO2 sources in the OBF electric furnace.

[0047] In the present invention, the liquid slag product can be used downstream, especially in industrial silicon slag applications, solar silicon slag applications, semiconductor silicon slag applications, or as a deoxidizer.

[0048] Silicon metal products produced by this method may include: (i) Si: 90% to 99.9999%; (ii) A1: 0% to 10%; (iii) Ca: 0% to 10%; (iv) Ti: 0% to 10%; (v) C: 0% to 10%.

[0049] The net carbon footprint provided by this invention is 40% to 95% of that of the conventional silicon smelting route.

[0050] In an embodiment, the present invention provides a post-positioned silicon monoxide-hydrogen gas reactor.

[0051] It should be understood that the operations after the slag outlet can be seamlessly integrated into the method of the present invention.

[0052] It should be understood that the method according to the present invention has many significant advantages over currently used techniques.

[0053] In particular, the silicon melting method of the present invention provides the following benefits: Metallurgical flexibility 1. Fine quartzite can now be processed, which is impossible with the currently used AC SAF; 2. The TSI of quartzite is no longer a decisive factor in assessing the suitability of quartzite for smelting; 3. Including an upstream preheating unit and a downstream WHRU, the SEC of the OBF is comparable to that of the currently used AC SAF, and may be improved; 4. In the OBF method of the present invention, wood chips are no longer required; 5. More lenient requirements on the quality of reducing agents—in particular, compared with the AC SAF method, the OBF method of the present invention can use a significantly wider range of reducing agents; 6. The large amount of SiO released into the exhaust gas in the OBF can be reduced by reacting SiO with H2 in an external gas reactor, thereby reducing the carbon footprint of the Si metal production process; 7. The SiO2 generated in the WHRU can be recycled back into the OBF and reused in this method as a high-quality SiO2 feedstock; Essentially independent power supply and precise temperature control 8. The OBF of the present invention has a good internal temperature distribution; 9. The method of the present invention provides a significantly cleaner slag product; 10. The method of the present invention can achieve and withstand high-temperature operation; Stable operation 11. With respect to the method of the present invention, the use of a stoking car and operators is no longer required or necessary; Electrode Consumption and Maintenance 12. The method of the present invention includes a unique electrode arm mounting; 13. The method of the present invention allows the use of pre-baked electrodes or paste electrodes; 14. Due to the arc-opening operation, the method of the present invention reduces electrode consumption; Structural benefits 15. The DC OBF of the present invention has a simpler structure, with only one electrode, providing a unique and improved gas seal; 16. The method of the present invention achieves a simple and elegant geometric structure and reduces uneven wear on the sidewalls; power supply 17. With respect to the DC OBF of the present invention, less harmonics and flicker are experienced; Skilled at handling fine powder (open arc mode) 18. As described above, the method of the present invention can use high-quality fine silica sand as a raw material for silicon smelting. It is important to note here that these benefits relate to both the arc-opening mode and the open-pool mode; 19. When using DC OBF, the method of the present invention realizes the benefits of using ultrafine powder, such as inhaled dust.

[0054] It will be apparent to those skilled in the art that the present invention provides a novel and improved silicon smelting method with improved total Si recovery and SEC; the present invention also provides a process route for producing metallic Si with a reduced carbon footprint.

[0055] Importantly, high-quality fine silica sand is available globally; however, its use in silicon smelting methods is severely limited because it cannot be processed in the currently used AC SAF operation for silicon smelting.

[0056] In this regard, it should be understood that the method of the present invention provides a solution to this drawback. In particular, the AC OBF electric furnace process or DC OBF electric furnace process of the present invention therefore has the potential to process low-quality fine quartzite and directly produce high-quality metallic Si products in an electric furnace.

[0057] In this respect, this method makes it possible to use used or decommissioned solar panels as raw materials containing SiO2 sources for silicon smelting; thus providing a solution for the beneficiation of solar panel waste.

[0058] It should be understood that the steps of the method of the present invention do not necessarily need to be performed sequentially, as the method can be operated in batches, semi-batch, or continuously. Furthermore, it is contemplated that the steps of the provided method do not necessarily need to be performed in the order listed herein.

[0059] The above-described features, characteristics, and advantages of the present invention, as well as other features, characteristics, and advantages, will become apparent from the following detailed description taken in conjunction with the accompanying drawings, which illustrate the principles of the invention by way of example. This description is given for illustrative purposes only and does not limit the scope of the invention. Reference numerals cited below refer to the accompanying drawings. Attached Figure Description

[0060] The invention will now be further described by way of example and in conjunction with the accompanying drawings, wherein: Figure 1 A front perspective cross-section view of an OBF electric furnace in a silicon smelting method for raw materials according to the present invention; Figure 2 for Figure 1 A schematic diagram of the OBF electric furnace in the method; Figure 3According to Figure 1 A schematic diagram of the furnace feeding system layout in the method; Figure 4 According to Figure 1 A schematic diagram of the furnace exhaust gas system used in the method; and Figure 5 According to Figure 1 A schematic diagram of the furnace exhaust gas system using H2 reduction in the method. Detailed Implementation

[0061] The scope of the invention described herein is not limited to any specific embodiments or examples disclosed herein, as these embodiments or examples are intended to illustrate some aspects of the invention. Any equivalent embodiments should be considered within the scope of protection of the invention, as such equivalent embodiments will be readily apparent to those skilled in the art based on this specification.

[0062] The silicon smelting method of the raw materials according to the present invention is generally indicated by reference numeral 10 in the accompanying drawings.

[0063] As shown in the figure, a batching device (not shown) is used to premix raw materials according to the required feed formulation. The raw materials include a SiO2 source (such as fine quartzite), a reducing agent (such as coke), and a flux (typically limestone).

[0064] The premixed raw materials are conveyed to the OBF electric furnace 20 via a conveying system (not shown).

[0065] The preheater 30 is used to preheat all raw materials or only fine quartzite.

[0066] The OBF electric furnace 20 is equipped with multiple feed troughs 40. Each feed trough 40 is equipped with a feed bin 50, a raw material feeder and / or a preheating unit 30. Figure 3 A single feed trough 40, a feeder 50, and a preheater assembly 30 are shown.

[0067] Raw materials are continuously fed into the OBF electric furnace 20, and the power-feed ratio of the furnace 20 is precisely controlled by a weight loss (LIW) system.

[0068] The furnace 20 is designed to consist of a steel container 60 lined with refractory material (e.g., Figure 1 (As shown).

[0069] The container 60 can be a circular or rectangular unit and is classified as a closed furnace (i.e., no air is forced into the furnace 20, resulting in a reducing gas environment above the molten pool).

[0070] Depending on the power supply and the selected operating mode, furnace 20 will utilize one or more electrodes, which can be any of the following: (i) DC Option 1: Single electrode (cathode) with an anode at the bottom. The anode can be needle-type, billet-type, or conductive furnace bottom type; (ii) DC Option 2: Dual electrode (dual cathode) with an anode at the bottom. The anode can be needle type, billet type, or conductive furnace bottom type; (iii) DC option 3: dual electrode, with one electrode as the cathode and the other as the anode; (iv) DC option 4: Multiple dual electrodes, located in pairs within the furnace arrangement; (v) AC Option 1: Three-electrode type, with the three electrodes connected in a triangular backpack configuration; (vi) AC Option 2: A furnace with six electrodes arranged in a straight line; and (vii) AC option 3: Multiple triangular backpack-style electrode configuration.

[0071] Furnace 20 operates in an open molten pool manner. Raw materials are fed into the active energy zone, where the electric arc 70 transfers energy to the molten pool and feeds the materials into the surrounding area near the furnace sidewall 81, such as... Figure 2 As shown.

[0072] Some raw materials can be fed in to form a thin solid layer on top of the open molten pool, thereby reducing radiative heat loss to the furnace top and the freeboard of the furnace cavity.

[0073] The furnace power supply can consist of the following configurations: (i) DC Selection 1: One or more rectifier units that receive AC input to generate DC output; (ii) AC Option 1: A three-phase transformer connected to a single three-electrode delta backpack connection; (iii) AC Option 2: Three single-phase transformers connected to a single three-electrode delta backpack connection; (iv) AC Option 3: Three single-phase transformers connected to three pairs of electrodes; and (v) AC Option 4: Multiple three-phase transformers connected to multiple electrode triangle backpack wiring.

[0074] The furnace electrode 80 can be of the Soderberg type, prebaked carbon type, or prebaked graphite type.

[0075] In the area below the electric arc 70 (referred to as the arc attachment zone (AAZ)), the raw materials fed into the furnace 20 are smelted.

[0076] In AAZ, SiO2 in fine quartzite is converted into metallic Si and SiO gas through the carbon reduction reaction of SiO2.

[0077] Some elements present in fine-grained quartzite (such as Al2O3 and CaO) are not reduced by carbon sources and form a gangue layer, also known as slag. However, partial reduction is expected, which can be addressed in downstream, proven, and well-known refining processes.

[0078] Due to the difference in density, the two layers will separate, with one layer stacked on top of the other within the furnace 20. The coal gangue minerals in the feed material determine the composition and density of the slag.

[0079] Figure 2 This illustrates a scenario where the slag density is lower than that of the metal, which could lead to an inversion, meaning the metal might be on top of the slag.

[0080] Metallic Si will accumulate in the main molten pool and be intermittently discharged from the furnace 20 through one or more slag outlets 90.

[0081] Similarly, slag will also accumulate in the main molten pool and will be intermittently discharged from the furnace 20 through one or more slag outlets 90 located at different height levels.

[0082] The furnace 20 may be equipped with an airtight furnace roof 100. Below the furnace roof 100, there will be an atmosphere rich in SiO-CO-H2. This area is called the furnace cavity free zone.

[0083] The furnace roof 100 can be water-cooled steel or water-cooled copper, and can be lined with refractory material on the hot surface. A suspended refractory material furnace roof can also be used.

[0084] Method 10 also provides, for example Figure 4 and Figure 5 The furnace exhaust gas system shown is equipped with a WHRU120, which recovers energy by burning carbon monoxide (CO), hydrogen (H2), and silicon monoxide gas (SiO). Combustion heat is transferred to steam in a steam boiler located within the WHRU120.

[0085] In this document, the gas will exit the furnace 20 through the exhaust gas duct (OGD) 110 and will be transported to a downstream gas equipment (not shown), which may consist of the following: (i) If Figure 4 The WHRU120 shown in the diagram is in which all furnace gases are burned to generate heat that can be recovered as steam energy; (ii) The combination of gas reactor 130 and WHRU120 as follows Figure 5As shown, H2 is injected into gas reactor 130 to produce metallic Si by reducing SiO to Si. The remaining gas is delivered to WHRU 120. Alternatively, a solid H2 source can be injected via an HES system (not shown).

[0086] SiC formation was not anticipated, but even if SiC were formed, the furnace 20 could rotate and utilize multiple electrodes 80.

[0087] Furnace 20 is also suitable for reducing SiO gas using H2 gas. H2 gas can be added to method 10 in the following manner: (i) Feeding a reducing agent rich in volatiles into the feed system or HES to generate additional H2 and CO gases; (ii) Injecting H2 gas into the HES; and (iii) Inject H2 gas into WHRU120 or a gas reactor.

[0088] The net carbon footprint of this method is expected to be 40% to 95% of that of conventional silicon smelting methods.

[0089] Meanwhile, method 10 provides an improved silicon smelting method with improved total Si recovery and SEC; and also provides a method for using fine silica sand (or any low-quality silicon raw material) as a silicon smelting raw material.

[0090] In this regard, it should be understood that the method of the present invention provides many important and surprising solutions to the shortcomings of the prior art as detailed above. In particular, it is evident that the method of the present invention represents a substantial advancement over Abdellatif et al. ("DC Arc Smelting of Silicon: Is it Technically Feasible?", Southern African Pyrometallurgy 2011), WO2018 / 141805A1, and WO2016 / 093704A1. It should be understood that Abdellatif et al. addressed the production of silicon using a DC arc furnace, but did not offer any suggestions on how to recover silicon from the waste gas phase, while the method of the present invention surprisingly overcomes this limitation by innovatively using hydrogen reduction in a gas reactor to recover SiO gas into metallic silicon.

[0091] WO2018 / 141805A1 describes a high-temperature method for reducing silicon oxide using gaseous reagents, but it lacks energy-saving steps, such as heat recovery. It should be understood that the teachings of WO2018 / 141805A1 do not provide guidance on how to optimize silicon yield and make no mention of addressing this shortcoming.

[0092] While WO2016 / 093704A1 proposed hydrogen reduction in a gas reactor, it failed to address key challenges such as the formation of undesirable byproducts (SiO and SiC) and contamination from substandard feedstock, both of which are unexpectedly solved by this advanced method.

[0093] Clearly, the features of this invention cannot be “selected” from existing and currently used technologies. More importantly, the features of this method contribute to a more environmentally sustainable and commercially advantageous approach, which is lacking in the prior art.

[0094] This specification is presented by way of example only and is intended to provide the most useful and readily understandable explanation of the principles and concepts of the invention. In this regard, no attempt is made to provide a more detailed description of the structural details of the invention and / or the devices utilized therein, beyond what is necessary for a basic understanding of the invention.

Claims

1. A method for smelting silicon from a raw material, the method comprising the following steps: (i) Containing particles with an average particle size of 36µm <d 50 The raw materials, reducing agent and flux of the SiO2 source with a diameter of <25mm are fed into the electric furnace; (ii) The raw materials, reducing agent, and flux containing the SiO2 source are heated in the electric furnace at a temperature of 1400°C to 2500°C, while an adjustable feed trough ensures that the raw materials, reducing agent, and flux containing the SiO2 source are continuously fed into the electric furnace. The power-feed balance is controlled by a weight loss (LIW) system to ensure that the raw materials, reducing agent, and flux containing the SiO2 source are fully melted, thereby forming liquid silicon metal product, liquid slag product, and waste gas containing SiO, CO, and H2; wherein the electric furnace is an open furnace (OBF). (iii) In a gas reactor, Si is recovered from the exhaust gas of the electric furnace by reducing SiO with H2 gas to form metallic silicon; and (iv) In a waste heat recovery unit (WHRU), heat is recovered from the exhaust gas of the electric furnace by burning process gases.

2. The method according to claim 1, wherein, The electric furnace is a direct current (DC) electric arc furnace or an alternating current (AC) electric arc furnace.

3. The method according to claim 1 or 2, wherein, The electric furnace is used in a closed furnace configuration to prevent air from entering the furnace and to create a reducing environment under the furnace top.

4. The method according to any one of claims 2 and 3, wherein, The electric furnace operates in an electric arc mode, which is selected from a group consisting of free arc opening mode (multi-electrode or single-electrode), short arc opening mode, brush arc mode, and immersion electrode (no arc opening) mode.

5. The method according to claim 2, wherein, The power capacity of the electric furnace is at most 100MW.

6. The method according to claim 1, wherein, The raw materials containing the SiO2 source, reducing agent, and flux are heated in the electric furnace at a temperature of 1500°C to 1800°C.

7. The method according to any one of claims 1 and 6, wherein, The raw materials containing SiO2 sources are, but are not limited to, low-quality quartzite, fine quartzite, and pretreated quartz sand.

8. The method according to claim 7, wherein, The raw material containing the SiO2 source is pretreated quartz sand that has undergone mineral processing to remove polluting elements, or pretreated solar panels that have had unwanted components containing polluting elements, such as iron (Fe) and copper (Cu), removed in upstream processes.

9. The method according to claim 7 or claim 8, wherein, The quartzite contains 40% to 100% SiO2.

10. The method according to any one of claims 7, 8 or 9, wherein, The raw material containing the SiO2 source includes materials with an average particle size of 36µm. <d 50 Fine quartzite <25mm, recycled quartzite waste, and combinations thereof.

11. The method according to claim 1, wherein, The raw material containing the SiO2 source is fed into the electric furnace in the form of cold raw material containing the SiO2 source, hot preheated raw material containing the SiO2 source, or a combination of cold and hot preheated raw material containing the SiO2 source.

12. The method according to claim 1, wherein, The method provides a feeding system consisting of a cold feeding system and a hot feeding system to supply raw materials containing a SiO2 source according to the required process formulation.

13. The method according to claim 12, wherein, The hot feed system includes at least one preheating unit.

14. The method according to claim 12, wherein, The feeding system is configured to feed raw materials containing a SiO2 source into the electric furnace using a hollow electrode system (HES).

15. The method according to claim 1, wherein, The reducing agent is anthracite, coke, fine coke, charcoal, or coal, and is added to the electric furnace in the form of particles with a particle size of no more than 50 mm.

16. The method according to claim 15, wherein, The reducing agent is a biological carbon source.

17. The method according to claim 1, wherein, The flux is selected from the group consisting of calcined or uncalcined dolomite, calcined or uncalcined limestone, quartzite, bauxite, and combinations thereof.

18. The method according to claim 1, wherein, The method provides multiple adjustable feed troughs that introduce the raw materials containing the SiO2 source, reducing agent, and flux into the electric furnace, and controls the power-feed balance within the furnace through a weight loss (LIW) system.

19. The method according to claim 1, wherein, The method provides a way to continuously replenish the raw materials containing the SiO2 source, reducing agent, and flux into the electric furnace using the plurality of adjustable feed troughs, thereby ensuring that weight loss and power-feed balance are controlled.

20. The method according to any one of the preceding claims, wherein, The raw materials containing the SiO2 source are produced using 100% hydrogen, which helps reduce the CO2 footprint.

21. The method according to any one of the preceding claims, wherein, The method provides a way to reduce SiO to Si using H2 gas as a reducing agent, wherein the H2 gas is added to the method in any of the following ways: (i) Feeding a reducing agent rich in volatiles into the feed system or the HES to generate additional H2 and CO gases; (ii) Injecting the H2 gas into the HES; (iii) Injecting the H2 gas into the waste heat recovery unit (WHRU) or gas reactor; (iv) Install a preheating unit upstream to improve energy efficiency, thereby increasing the SER of the OBF electric furnace; or (v) Install a WHRU downstream to improve energy efficiency, thereby improving the SEC of the method.

22. The method according to any one of the preceding claims, wherein, The method provides for injecting or adding an alternative solid-state H2 source.

23. The method according to claim 22, wherein, The substitute solid H2 is high-volatile coal.

24. The method according to claim 1, wherein, The method provides a furnace exhaust gas energy recovery system equipped with a WHRU that recovers energy by burning CO, H2 and SiO, wherein the heat of combustion is recovered as steam in a steam boiler located in the WHRU.

25. The method according to claim 1, wherein, The liquid metal silicon product material is formed by heating and at least partially melting the raw materials, reducing agent, and flux containing the SiO2 source.

26. The method according to claim 1, wherein, The residence time of the raw material is controlled to control the degree of reduction of the raw material containing the SiO2 source in the electric furnace.

27. The method according to claim 1, wherein, The liquid slag product is used in downstream applications, including silicon slag applications, solar silicon slag applications, semiconductor silicon slag applications, as a deoxidizer, and combinations thereof.

28. The method according to claim 1, wherein, The metallic silicon product comprises: (i) Si: 90% to 99.9999%; (ii) A1: 0% to 10%; (iii) Ca: 0% to 10%; (iv) Ti: 0% to 10%; (v) C: 0% to 10%.

29. The method according to any one of the preceding claims, wherein, The net carbon footprint of the method is 40% to 95% of that of the conventional silicon smelting route.

30. The method according to any one of the preceding claims, wherein, The method provides a silicon monoxide-hydrogen gas reactor.

31. The method according to any one of the preceding claims, wherein, The method provides instructions for operations after the slag outlet.

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