Novel silicon smelting process

Abstract

The invention relates to a process for silicon smelting of low-quality silicon feedstock material. More particularly, the invention relates to a process for the smelting of low-quality feedstock material to achieve silicon yields at acceptable energy efficiency levels, whilst demonstrating a significantly lower carbon footprint. According to a first aspect of the present invention, there is provided a process for silicon smelting of a feedstock material, the process including the steps of: (i) feeding a feedstock material containing a source of SiO2 having an average particle size of 36 µm < d50 < 25 mm, reductant and fluxes into an electrothermal furnace; (ii) heating the feedstock material containing a source of SiO2, reductant and fluxes, in the electrothermal furnace at a temperature of between 1400°C to 2500°C, whilst ensuring the continuous feeding of the feedstock material containing a source of SiO2, reductant and fluxes, by means of adjustable feeding chutes into the electrothermal furnace through a Loss-in Weight (LIW) system to control the power-to-feed balance to sufficiently melt the feedstock material containing a source of SiO2, reductant and fluxes, to a form a liquid silicon metal product, a liquid slag product and an off-gas containing SiO, CO and H2; wherein the electrothermal furnace is an Open Bath Furnace (OBF); (iii) recovering Si from the electrothermal furnace off-gas in a gas reactor by reducing SiO with H2 gas to form silicon metal; and (iv) recovering heat from the electrothermal furnace off-gas in a Waste Heat Recovery Unit (WHRU) by combustion of the process gas.

Description

[0001] NOVEL SILICON SMELTING PROCESS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a process for silicon smelting of low-quality silicon feedstock material. More particularly, the invention relates to a process for the smelting of low-quality feedstock material to achieve silicon yields at acceptable energy efficiency levels, whilst demonstrating a significantly lower carbon footprint.

[0004] BACKGROUND TO THE INVENTION

[0005] Silicon metal is used in the manufacture of silanes and silicones, as a “hardener” or alloying element to produce aluminium alloys, and in the manufacture of microprocessors and solar cells.

[0006] Silicon (Si) metal is traditionally produced by using a semi-closed or open Submerged-Arc Furnace (SAF) with coarse quartzite as the main ore feedstock. The process entails mixing of quartzite with a suitable reductant (coal, char and / or coke), woodchips and a small portion of slag flux (typically limestone) in a proportioning plant that is located upstream of the SAF.

[0007] The mixed feed material is transported to the furnace feed system which typically consists of multiple steel bins. The mixed material is then charged to the SAF intermittently by utilising several material feeders, while the electrical power is supplied to the SAF on a continuous basis.

[0008] The SAF operates with Alternating Current (AC) electricity, that is supplied to the furnace through carbon electrodes of which the tips are physically located below the raw material burden. Electrical arcs are generated on the electrode tips and provide the required heat and temperature to drive the required reduction reactions. Si metal is produced by reducing the silicon dioxide (SiC>2), contained in the quartzite, to silicon (Si) metal. The furnace vessel is typically circular in shape and lined with refractory material. The furnace is classified as an open or semi-closed furnace i.e., the furnace roof is situated above the furnace raw material burden and air is allowed to enter the area under the furnace roof to combust the process gas directly above the raw material burden.

[0009] Silicon carbide (SiC) is formed as an intermediate by-product that accumulates in the furnace. SiC has a very high melting temperature. This results in solid accretion of SiC in various locations in the furnace. To overcome this accretion formation, the furnace shell is rotated at slow speeds (typically 0.8 to 1 .67h) in order to allow the electrodes to burn these accretions away. It is well known that excess SiC accretion formation leads to poor Si recoveries.

[0010] Silicon monoxide (SiO) is also formed as an intermediate product in the SiC>2 reduction process. SiO exists as a gas inside the SAF and travels in an upward direction through the furnace raw material burden. During this upward travelling process, the SiO gas reacts with the solid-state reductant and forms SiC and Si. The main reason for including woodchips in the furnace feed material is to maintain the correct level of permeability / porosity inside the furnace raw material burden for the SiO gas to travel upwards.

[0011] A shortcoming of this process resides in that if the furnace raw material burden is not stoked sufficiently, cavities form where excess amounts of SiO gas will escape from the furnace raw material burden. This SiO combusts above the furnace burden to form SiO2 and this Si is lost to the gas cleaning system.

[0012] Management of the raw material burden through sufficient raw material charging and stoking practices are therefore critical in these known processes in order to maintain a sufficient level of Si recovery from the SAF. This is however associated with a tendency to lose material in the form of SiC>2-dust in the off-gases. This may often amount to at least a percentage of the amount of quartz supplied in the charge.

[0013] The Si metal is tapped as a molten material product intermittently by utilising a taphole. A taphole refers to a hole that is made in a controlled manner in the sidewall of the furnace to allow the molten Si metal to drain from the furnace. The Si metal is tapped into a refractory lined ladle that is used for transporting the Si metal to downstream processing and casting units. The SAF is typically fitted with multiple tapholes.

[0014] A known shortcoming of the use of an AC SAF for Si metal production is that these processes require a distinct quality of raw materials. The quartzite specifically needs to have sufficient strength under high temperature conditions to maintain its form. This is measured in laboratory tests and is defined by a Thermal Stability Index (TSI) number. If quartzite with a low TSI is charged to the furnace, the furnace burden permeability decreases causing localised gas eruptions and cave formation due to fines sintering, which, in turn, leads to localised gas eruptions. Similarly, reductant with specific reactivity properties must be used in the process to ensure that the SiO can react efficiently with the reductant.

[0015] Attempts to improve the performance of these processes have been demonstrated in recent studies. MINTEK, South Africa, has performed pilot scale test work using a closed Direct Current Furnace (DCF); however, the results obtained from the pilot scale demonstrated poor Si recoveries in yields of around 45% Si. (Abdellatif, M. “DC arc smelting of silicon: Is it technically feasible?’’, Southern African Pyrometallurgy 2011, March 2011).

[0016] Coupled with the drive to optimise Si metal production, the urgency to address and reduce carbon emissions is pressing in the energy sector. Thus, the global focus on decarbonization, including the continued substitution of lower-emission power sources, continued improvements in plant efficiencies, and the use of carbon capture, utilization and storage (CCUS) remains pinnacle to remaining competitive in the future. n addition hereto, in a recent report by the BBC, it has been reported that whilst more than one terawatt of solar capacity has been installed globally (averaging around 2.5 billion solar panels), the infrastructure to scrap and recycle decommissioned solar panels and modules is clearly lacking. In another recent report by the National Renewable Energy Lab (NREL), it is predicted that by 2030, the US alone could potentially accrue 1 million ton of waste from decommissioned solar panels and modules. This waste will grow exceedingly by 2050.

[0017] PCT / N02007 / 000092 discloses a method for the manufacture of pure silicon metal and amorphous silica by the reduction of quartz for use in solar panels by reducing quartz in a single step using hydrocarbon gas, preferably methane gas, as the reducing agent. However, PCT / N02007 / 000092 has significant drawbacks in that it not only requires a high-quality feed material having silicon metal of purity higher than 98% before the invention may be performed but also requires high furnace operating temperatures. Thus, PCT / N02007 / 000092 does not disclose the use of low-quality feed material for the production of high Si yields.

[0018] In the study by Abdellatif et aL, titled "DC Arc Smelting of Silicon: Is it Technically Feasible?" (Southern African Pyrometallurgy 201 1 ), the technical feasibility of producing silicon metal using a Direct Current (DC) Arc furnace was investigated. The researchers operated a graphite-lined furnace at power levels between 150 and 160 kW, processing a feed mixture of 28% petroleum coke and 72% lumpy silica. Over the course of the experiment, approximately 1.6 tonnes of premixed feed were processed in 16 feeding-tapping operations. The furnace encountered operational challenges such as feed system blockages and difficulty in maintaining continuous operation, necessitating manual feeding methods. Furthermore, the silicon metal produced demonstrated purity levels ranging from 18% to 78%, including impurities such as iron, calcium, and aluminium; partly attributed to contamination from oxygen lancing and sampling procedures. Silicon recovery to the metal phase averaged around 40%, peaking at just over 45% in later batches, which is substantially lower than the 70% to 85% recovery rates typical in commercial operations. The study was silent on methods for recovering silicon from the off-gas nor did it consider the recovery of heat from the process gas through waste heat recovery systems, which exposes these process’ shortfalls in terms of economic viability and environmental suitability.

[0019] WO 2018 / 141805 A1 discloses a process for producing elemental silicon from silicon oxides, particularly silicon dioxide. The process involves a two-step gas-phase reduction using gaseous reducing agents. In the first step, silicon dioxide is reduced to silicon monoxide at temperatures ranging from 1000°C to 2500°C, resulting in a gas phase containing silicon monoxide. In the second step, this silicon monoxide gas is further reduced by a gaseous reducing agent at temperatures of 1500°C or higher to form elemental silicon, which is then separated from the residual gas phase. While this process aims to be environmentally friendly by avoiding the use of chlorinated compounds, it relies on high-temperature gas-phase reactions that can present challenges in terms of energy efficiency and process control. The necessity of handling gases at such elevated temperatures may lead to increased operational complexities and equipment demands. Failing to incorporate recycle streams could result in less efficient energy utilisation and potential difficulties in scaling the process for industrial applications. The absence of these features may also impact the overall silicon yield and purity achievable through this method.

[0020] WO 2016 / 093704 A1 discloses a process in which silicon monoxide is reduced using hydrogen in a gas reactor. Whilst WO 2016 / 093704 A1 introduces an energy-efficient process for the production of 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 feed materials. Accordingly, current processes have not demonstrated high Si yields concurrent with the use of low-quality feed material whilst exhibiting reduced carbon emissions in processes operating at acceptable energy efficiency levels.

[0021] For purposes of the present specification, it will be appreciated that the following acronyms are used synonymously with the below referenced phrases.

[0022] AC Alternating Current

[0023] EAF Electric Arc Furnace

[0024] BF Blast Furnace

[0025] DCF Direct Current Furnace

[0026] HES Hollow Electrode System

[0027] OBF Open Bath Furnace

[0028] OSBF Open Slag Bath Furnace

[0029] SAF Submerged-Arc Furnace

[0030] SEC Specific Energy Consumption

[0031] SER Specific Energy Requirement

[0032] TSI Thermal Stability Index

[0033] WHRU Waste Heat Recovery Unit

[0034] OBJECT OF THE INVENTION

[0035] It is accordingly an object of the present invention to provide a novel process for silicon smelting of low-quality feedstock material which overcomes, at least partially, the abovementioned disadvantages and limitations and / or which will provide a useful alternative to existing technology whereby low-quality feed material is used whilst achieving silicon yields at acceptable energy efficiency levels with improved process control and demonstrating a significantly lower carbon footprint. SUMMARY OF THE INVENTION

[0036] According to a first aspect of the present invention, there is provided a process for silicon smelting of a feedstock material, the process including the steps of:

[0037] (i) feeding a feedstock material containing a source of SiC>2 having an average particle size of 36 pm < d5o < 25 mm, reductant and fluxes into an electrothermal furnace;

[0038] (ii) heating the feedstock material containing a source of SiC>2, reductant and fluxes, in the electrothermal furnace at a temperature of between 1400°C to 2500°C, whilst ensuring the continuous feeding of the feedstock material containing a source of SiC>2, reductant and fluxes, by means of adjustable feeding chutes into the electrothermal furnace through a Loss-in Weight (LIW) system to control the power-to-feed balance to sufficiently melt the feedstock material containing a source of SiC>2, reductant and fluxes, to a form a liquid silicon metal product, a liquid slag product and an off-gas containing SiO, CO and H2; wherein the electrothermal furnace is an Open Bath Furnace (OBF);

[0039] (iii) recovering Si from the electrothermal furnace off-gas in a gas reactor by reducing SiO with H2gas to form silicon metal; and

[0040] (iv) recovering heat from the electrothermal furnace off-gas in a Waste Heat Recovery Unit (WHRU) by combustion of the process gas.

[0041] Smelting in the present context is to be understood as the process of extracting silicon from a feedstock material containing a source of SiO2.

[0042] It is to be understood that an electrothermal furnace is a furnace with a heat source derived from electricity. The OBF electrothermal furnace may be either a Direct Current (DC) Electric Arc Furnace (EAF) or an Alternating Current (AC) EAF. The DC and AC EAF OBF of the present invention may be used in a closed furnace arrangement, meaning that no air enters the furnace and that a reducing environment exists below the furnace roof (closed open-bath furnace (Closed OBF)).

[0043] In terms of the present invention, the OBF electrothermal furnace may be operated on different arc modes, namely open-arc mode (multiple or single electrode), short open-arc mode, brush arc mode and immersed electrode (no open-arc) mode.

[0044] In terms of the invention, the DC EAF has a power capacity of up to 10OMW. The AC EAF has a power capacity of up to 10OMW.

[0045] The invention provides for the feedstock material containing a source of SiC>2, reductant and fluxes to be heated in the electrothermal furnace at a temperature of between 1400°C to 2500°C, with a preferred temperature range of between 1500°C and 1800°C.

[0046] It will be appreciated that the feedstock material containing a source of SiC>2 may be, but is not limited to, quartzite, quartzite fines, pre-processed quartzite sand (preferably pre-processed quartzite sand that has undergone a beneficiation process to remove contaminating elements), or pre-processed solar panels where unwanted components containing contaminating elements such as Iron (Fe) and Copper (Cu), have been removed in an up-stream process. The quartzite may contain between 40% and 100% SiC>2.

[0047] It will be appreciated that the Thermal Stability Index (TSI) of the quartzite which is used as a feedstock material is no longer an applicable factor for evaluating the suitability of quartzite for smelting in terms of the present process.

[0048] The feedstock material containing a source of SiC>2 may include quartzite fines having an average particle size of 36 pm < d5o < 25 mm, recycled quartzite scrap, and a combination thereof.

[0049] In terms of the invention, either cold or pre-heated feedstock material; alternatively, a combination of cold- and hot pre-heated feedstock material may be fed into the furnace.

[0050] The invention provides for a feed system consisting of a cold feeding and a hot feeding system that will charge the feedstock material according to desired process recipes. For instance, in an embodiment of the invention, a pre-heating unit, or multiple pre-heating units, may be fitted into the hot feeding system. In a further embodiment of the invention, the furnace may also utilize a Hollow Electrode System (HES) for feeding.

[0051] The invention provides for the reductant to be anthracite, coke, finer fraction coke, char or coal. In an embodiment of the invention, the reductant may be a source of bio-carbon. The reductant may be added to the electrothermal furnace as particulates having a particle size equal to or less than 0 to 50 mm.

[0052] The flux may be selected from the group consisting of burnt- or unburnt dolomite, burnt- or unburnt limestone, quartzite, bauxite and a combination of one or more thereof.

[0053] The invention provides for a plurality of adjustable feeding chutes which allow for the introduction of the feedstock material containing a source of SiC>2, reductant and fluxes, into the electrothermal furnace through a Loss-in Weight (LIW) system to control the power-to-feed balance within the furnace.

[0054] The invention provides for continuous replenishment of the feedstock material containing a source of SiC>2, reductant and fluxes utilizing the plurality of adjustable feeding chutes into the electrothermal furnace to ensure that the loss-in-weight and power-to-feed balance is controlled. The specific energy consumption (SEC) of a smelting process can be simply expressed as MWh per metric ton of total feed or power (MW) / feed rate (ton / h). SEC is the energy required to transform the feed materials at 25°C into the product streams at the desired temperatures at which they leave the furnace. SEC is thus inherently the power-to-feed ratio. It will be appreciated that the theoretical SEC changes quite significantly if the chemical composition or temperature of the raw materials deviates from the theoretical baseline.

[0055] From a product SEC point of view, conventional SAFs produce Si-metal with a SEC between 10.5 and 12.5MWh / t Si metal. The process of the present invention may produce Si metal with a similar SEC figure; pending the efficiency of the upstream pre-heating unit and downstream WHRU.

[0056] The invention provides for feedstock material containing a source of SiCh to be produced with 100% hydrogen to aid in reducing the CO2 footprint. It will be appreciated that the feedstock material containing a source of SiC>2 produced with 100% hydrogen will have a very low to 0% carbon footprint.

[0057] In terms of this embodiment, the invention provides for the reduction of SiO to Si using H2 gas as a reductant. In this embodiment, H2gas may be added to the process in the following manners; namely by -

[0058] (i) Feeding volatile rich reductants in the feeding system or the HES to produce additional H2and CO gas;

[0059] (ii) Injecting H2gas in the HES;

[0060] (iii) Injecting H2gas in the Waste Heat Recovery Unit (WHRU) or gas reactor;

[0061] (iv) Installing an upstream pre-heating unit to enhance the energy efficiency and thus the SER of the OBF electrothermal furnace; or (v) Installing a downstream WHRU to enhance the energy efficiency and thus the SEC of the process.

[0062] In an alternative embodiment, the invention provides for the injection or the addition of an alternative solid state H2source, in particular, high volatile coal.

[0063] The invention further provides for a furnace off-gas energy recovery system. The furnace off-gas energy recovery system may be fitted with a WHRU that recovers energy by combusting carbonmonoxide (CO), hydrogen (H2) and silicon monoxide gas (SiO). In this embodiment, the combustion heat may be transferred to steam in a steam boiler located in the WHRU.

[0064] In terms of the invention, the liquid silicon metal product material may be formed through the heating and melting or at least partial melting of the feedstock material containing a source of SiO2, reductant and fluxes.

[0065] The residence time of the feedstock material containing a source of SiO2in the electrothermal furnace may be controlled to control the degree of reduction of the feedstock material containing a source of SiO2in the OBF electrothermal furnace.

[0066] In terms of the invention, the liquid slag product may be used downstream in inter alia industrial silicon slag applications, solar silicon slag applications; semi-conductor silicon slag applications or use as a deoxidizer.

[0067] The silicon metal product produced by the present process may include:

[0068] (i) Si: 90 to 99.9999%

[0069] (ii) Al:0 to 10%

[0070] (iii) Ca:0 to 10%

[0071] (iv) Ti: O to 10% (v) C: O to 10%

[0072] The invention provides for the net carbon footprint to be between 40% and 95% of the traditional silicon smelting route.

[0073] In an embodiment, the present invention provides for a post silicon-monoxide-hydrogen gas reactor.

[0074] It will be appreciated that post tap hole operations can be integrated seamlessly into the process of the present invention.

[0075] It will be appreciated that the process in accordance with the present invention offers a number of significant advantages over technology that is currently employed.

[0076] In particular, the silicon smelting process of the present invention affords the following:

[0077] Metallurgical flexibility

[0078] 1. Fine quartzite can now be processed which is not possible in AC SAF’s currently employed;

[0079] 2. The TSI of the quartzite is no longer a determining factor for evaluating the suitability of quartzite for smelting;

[0080] 3. The inclusion of the upstream pre-heating unit and of the downstream WHRU aligns and potentially improves the SEC of an OBF relative to that of an AC SAF currently employed;

[0081] 4. Woodchips are no longer required in an OBF process of the present invention;

[0082] 5. Reductant quality requirements are more lenient - in particular, a substantially wider range of reductants can be utilised in the OBF process of the present invention compared to the AC SAF process; 6. The high quantities of SiO that are released to the off-gas in an OBF allows the Si-metal production process’ carbon footprint to be decreased by allowing the SiO to react with H2 in an external gas reactor;

[0083] 7. The SiO2 that is produced in the WHRU may be recycled back to the OBF as a high quality SiO2 feed stock to be re-used in the process;

[0084] Virtual independent power supply and precise temperature control

[0085] 8. Good temperature distribution within the OBF of the present invention;

[0086] 9. The process of the present invention affords significantly cleaner slag product;

[0087] 10. The process of the present invention is able to achieve and tolerate high temperature operation;

[0088] Stable operation

[0089] 11. In terms of the process of the present invention, it is no longer necessary or required to use stoking cars and operators;

[0090] Electrode consumption and maintenance

[0091] 12. The process of the present invention includes a unique electrode arm installation;

[0092] 13. The process of the present invention allows for the use of either pre-baked or paste electrodes:

[0093] 14. The process of the present invention affords decreased electrode consumption due to open-arc operation;

[0094] Structural benefits

[0095] 15. In terms of the DC OBF of the present invention, the structure thereof is simpler with one electrode, thereby affording unique and improved gas sealing;

[0096] 16. The process of the present invention achieves geometric and elegant simplicity, reducing uneven wear on sidewalls; Electrical power supply

[0097] 17. In terms of the DC OBF of the present invention, less harmonics and flicker is experienced;

[0098] Good at handling fines (open-arc mode)

[0099] 18. As mentioned herein above, the process of the present invention makes it possible for high quality fine quartzite sand to be utilised as a feedstock material for the smelting of silicon. Here, it is important to note that these benefits are linked to open-arc and openbath mode;

[0100] 19. The process of the present invention achieves benefits for the use of ultra-fines, when using the DC OBF, for instance pulling in dust.

[0101] As will become apparent to the person skilled in the art, the present invention affords a novel, improved process for the smelting of silicon which demonstrates improved overall Si recovery yields and SEC; and further provides a process route to produce Si metal with a decreased carbon footprint.

[0102] Importantly, high quality fine quartzite sand is available globally however the use thereof in silicon smelting processes is severally limited since the same is not able to be processed in currently employed AC SAF operations for the smelting of silicon.

[0103] In this regard, it will be appreciated that the process of the present invention affords a solution to this shortcoming. In particular, the AC or DC OBF electrothermal furnace process of the present invention therefore has the potential of processing low-quality quartzite fines to produce high quality Si-metal product directly from the electrothermal furnace. In this regard, the present process allows for the possibility of using used or decommissioned solar panels as a feedstock material containing a source of SiC>2 for the smelting of silicon; providing a solution for the beneficiation of solar panel waste.

[0104] It is to be understood that the steps of the process according to the invention need not necessarily be executed sequentially, as the process may be operated in a batch, semi-batch or continuous manner. Furthermore, it is envisaged that the steps of the process provided for need not necessarily be executed in the order listed herein.

[0105] The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrates, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

[0106] BRIEF DESCRIPTION OF THE DIAGRAMS

[0107] The invention will now further be described, by way of example only, with reference to the accompanying figures wherein:

[0108] Figure 1 is a cross-sectional front perspective view of an OBF electrothermal furnace in accordance with the process for the silicon smelting of feedstock material according to the present invention;

[0109] Figure 2 is a schematic diagram of the OBF electrothermal furnace of the process of Figure 1 ;

[0110] Figure 3 is a schematic diagram of a furnace feed system layout in accordance with the process of Figure 1 ;

[0111] Figure 4 is a schematic diagram of a furnace off-gas system employed in accordance with the process of Figure 1 ; and

[0112] Figure 5 is a schematic diagram of a furnace off-gas system with H2reduction employed in accordance with the process of Figure 1 .

[0113] DETAILED DESCRIPTION OF THE INVENTION

[0114] The invention described herein is not to be limited in scope by any specific embodiment or example herein disclosed, as the embodiments and examples are intended as illustrative of several aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention, as they will become apparent to those skilled in the art from the present description.

[0115] A process for silicon smelting of a feedstock material according to the invention is generally designated by reference numeral 10 in the accompanying figures.

[0116] As shown in the Figures, a proportioning plant (not shown) is used to pre-mix feedstock material according to a desired feed recipe. The feedstock material contains a source of SiC>2, such as fine quartzite, reductant, such as coke, and fluxes, typically limestone.

[0117] The pre-mixed feedstock material is delivered to the OBF electrothermal furnace 20 by a conveying system (not shown).

[0118] A pre-heater 30 is used to pre-heat the total feedstock material or the fine quartzite alone.

[0119] The OBF electrothermal furnace 20 is equipped with multiple feed chutes 40. Each feed chute 40 is equipped with a feed bin 50, feedstock material feeder and / or a pre-heating unit 30. A single feed chute 40, feeder 50 and pre-heater assembly 30 is shown in Figure 3.

[0120] The feedstock material will be charged on a continuous basis into the OBF electrothermal furnace 20 through a Loss-in Weight (LIW) system to accurately control a power-to-feed ratio of the furnace 20. The furnace 20 design consists of a steel vessel 60 that is lined with refractory materials (as shown in Figure 1).

[0121] The vessel 60 may be a circular- or rectangular-shaped unit and will be classified as a closed furnace (that is, no air will be forced into the furnace 20, resulting in a reducing gas environment above the furnace bath).

[0122] The furnace 20 will utilize a single or multiple electrode(s), depending on the power source and selected operating mode, and may be any of the following:

[0123] (i) DC option 1 : Single electrode (cathode) with a bottom located anode. The anode may be a pin-type, billet type or conductive hearth type;

[0124] (ii) DC Option 2: Twin electrodes (dual cathode) with a bottom located anode. The anode may be a pin-type, billet type or conductive hearth type;

[0125] (iii) DC option 3: Dual electrodes where one electrode acts as a cathode and the other as an anode;

[0126] (iv) DC option 4: Multiple dual electrodes positioned as pairs inside the furnace layout;

[0127] (v) AC Option 1 : Three electrode types with three electrodes arranged in a delta knapsack connection;

[0128] (vi) AC Option 2: A 6-in-line positioned electrode furnace; and

[0129] (vii) AC option 3: Multiple delta knapsack connected electrode arrangements.

[0130] The furnace 20 is operated with an open bath. Feedstock material is charged into the active energy zone where the arc 70 transfers energy to the furnace bath and in the peripheral regions against the furnace sidewalls 81 , as shown in Figure 2.

[0131] Some feedstock material may be charged to create a thin solid layer on top of the open bath to decrease radiation heat loss to the furnace 20 roof and freeboard. The furnace power supply can consist of the following arrangements:

[0132] (i) DC option 1 : Rectifying unit or multiple rectifying units that receives AC input to produce a DC output;

[0133] (ii) AC option 1 : Single 3 phase transformer to a single 3 electrode delta knapsack connection;

[0134] (iii) AC option 2: Three single phase transformers to a single 3 electrode delta knapsack connection;

[0135] (iv) AC option 3: Three single phase transformers to 3 electrode pairs; and

[0136] (v) AC option 4: Multiple 3 phase transformers to multiple electrode delta knapsack connections.

[0137] The furnace electrode(s) 80 can be Soderberg-, pre-baked carbon or pre-baked graphite type.

[0138] The feedstock material that is charged to the furnace 20 is smelted in the zone below the electrical arc 70, referred to as the Arc Attachment Zone (AAZ).

[0139] In the AAZ, the SiC>2 in the fine quartzite is turned into Si metal and SiO gas by the carbonaceous reduction of the SiC>2.

[0140] Some elements that are present in fine quartzite such as AI2O3 and CaO will not be reduced by the carbon source and will form a gangue layer, referred to as slag. Partial reduction is however expected that will be treated in a downstream, proven and well-known, refining process.

[0141] Due to the difference in densities, the two layers will separate, and one will lay on top of the other inside the furnace 20. The gangue minerals in the feed material determine the slag composition and density. Figure 2 illustrates a scenario where the slag is less dense than the metal, but can potentially be inverted, that is, the metal can potentially lie above the slag.

[0142] The Si-metal will accumulate in a bulk bath and will be tapped intermittently from the furnace 20 through a single or multi-taphole 90 system.

[0143] Similarly, the slag will also accumulate in a bulk bath and will also be tapped intermittently from the furnace 20 through a single or multi-taphole 90 system, situated at a different elevation level.

[0144] The furnace 20 may be fitted with a gas-tight roof 100. A SiO-CO-H2rich atmosphere will exist below the furnace roof 100. This region is known as the furnace freeboard.

[0145] The furnace roof 100 may be water-cooled steel or copper and may be lined with refractory material on the hot face. A suspended refractory roof can also be utilized.

[0146] The process 10 further provides for a furnace off-gas system as shown in Figures 4 and 5. The furnace off-gas system is fitted with a WHRU 120 that recovers energy by combusting carbonmonoxide (CO), hydrogen (H2) and silicon monoxide gas (SiO). The combustion heat is transferred to steam in a steam boiler located in the WHRU 120.

[0147] Here, the gas will exit the furnace 20 through an Off-Gas Duct (OGD) 110 to be delivered to a downstream gas facility (not shown) that may consist of:

[0148] (i) The WHRU 120 as shown in Figure 4, where all the furnace gas is combusted to generate heat that can be recovered to steam energy;

[0149] (ii) A gas reactor 130, in combination with a WHRU 120 as shown in Figure 5, where H2is injected into a gas reactor 130 to produce Si-metal by reducing the SiO to Si. The remaining gas is delivered to the WHRU 120. A solid state H2source can also be injected through a HES system (not shown). SiC formation is not foreseen, however, if the same is formed, the furnace 20 is be able to rotate and utilize multiple electrodes 80.

[0150] The furnace 20 further caters for reducing the SiO gas with H2gas. The H2gas may be added to the process 10 in the following manners:

[0151] (i) Feeding volatile rich reductants in the feeding system or the HES to produce additional H2and CO gas;

[0152] (ii) Injection of H2gas in the HES; and

[0153] (iii) Injection of H2gas in the WHRU 120 or gas reactor.

[0154] The net carbon foot print of the present process 10 is expected to be between 40% and 95% of the traditional silicon smelting processes.

[0155] Simultaneously, the process 10 provides an improved process for the smelting of silicon which demonstrates improved overall Si recovery yields and SEC; and further provides a process whereby fine quartzite sand (or any low-quality silicon feedstock material) may be used as a feedstock material for silicon smelting.

[0156] In this regard, it will be appreciated that the process of the present invention affords a number of significant and surprising solutions to address the shortcomings of the prior art, as detailed herein above. In particular, it is clear that the process of the present invention presents substantial advancements over Abdellatif et al. ("DC Arc Smelting of Silicon: Is it Technically Feasible?", Southern African Pyrometallurgy 201 1), WO 2018 / 141805 A1 , and WO 2016 / 093704 A1 . It will be appreciated that Abdellatif et al. is directed to the production of silicon using a DC arc furnace but does not make any suggestion on how to address silicon recovery from the offgas phase, a limitation that the process of the present invention surprisingly overcomes through the innovative use of hydrogen reduction in a gas reactor to recover SiO gas as silicon metal. WO 2018 / 141805 A1 describes a high-temperature process for reducing silicon oxides using gaseous agents, but it lacks energy-efficient steps, such as heat recovery. It will be appreciated that the teachings of WO 2018 / 141805 A1 provide no guidance as to how to optimise silicon yield and is completely silent on addressing this shortcoming.

[0157] WO 2016 / 093704 A1 , while proposing hydrogen reduction in a gas reactor, fails to address critical challenges like the formation of undesirable by-products (SiO and SiC) and contamination from substandard feedstock, both of which have been unexpectedly addressed by the present advanced process.

[0158] It is clear that the features of the present invention could not be “cherry picked” from the prior art and from the features of currently used technology. Moreso, the features of the present process contribute to a more environmentally sustainable and commercially advantageous process which is lacking in the art.

[0159] The description is presented by way of example only in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention and / or the equipment utilized therein in more detail than is necessary for a fundamental understanding of the invention.

Claims

CLAIMS1 . A process for silicon smelting of a feedstock material, the process including the steps of:(i) feeding a feedstock material containing a source of SiC>2 having an average particle size of 36 pm < d5o < 25 mm, reductant and fluxes into an electrothermal furnace;(ii) heating the feedstock material containing a source of SiC>2, reductant and fluxes, in the electrothermal furnace at a temperature of between 1400°C to 2500°C, whilst ensuring the continuous feeding of the feedstock material containing a source of SiC>2, reductant and fluxes, by means of adjustable feeding chutes into the electrothermal furnace through a Loss-in Weight (LIW) system to control the power- to-feed balance to sufficiently melt the feedstock material containing a source of SiC>2, reductant and fluxes, to a form a liquid silicon metal product, a liquid slag product and an off-gas containing SiO, CO and H2; wherein the electrothermal furnace is an Open Bath Furnace (OBF);(iii) recovering Si from the electrothermal furnace off-gas in a gas reactor by reducing SiO with H2gas to form silicon metal; and(iv) recovering heat from the electrothermal furnace off-gas in a Waste Heat Recovery Unit (WHRU) by combustion of the process gas.

2. The process according to claim 1 , wherein the electrothermal furnace is a Direct Current (DC) Electric Arc Furnace or an Alternating Current (AC) Electric Arc Furnace.

3. The process according to claim 1 or 2, wherein the electrothermal furnace is used in a closed furnace arrangement to prevent air from entering the electrothermal furnace and to create a reducing environment below a furnace roof.

4. The process according to any one of claims 2 and 3, wherein the electrothermal furnace is operated on arc modes selected from the group consisting of open-arc mode (multipleor single electrode), short open-arc mode, brush arc mode and immersed electrode (no open-arc) mode.

5. The process according to claim 2, wherein the electrothermal furnace has a power capacity of up to 10OMW.

6. The process according to claim 1 , wherein the feedstock material containing a source of SiC>2, reductant and fluxes are heated in the electrothermal furnace at a temperature of between 1500°C and 1800°C.

7. The process according to any one of claims 1 and 6, wherein the feedstock material containing a source of SiO2 is, but is not limited to, low-quality quartzite, quartzite fines, and pre-processed quartzite sand.

8. The process according to claim 7, wherein the feedstock material containing a source of SiC>2 is pre-processed quartzite sand that has undergone a beneficiation process to remove contaminating elements or pre-processed solar panels where unwanted components containing contaminating elements such as Iron (Fe) and Copper (Cu), have been removed in an up-stream process.

9. The process according to claim 7 or claim 8, wherein the quartzite contains between 40% and 100% SiC>2.

10. The process according to any one of claims 7, 8, or 9, wherein the feedstock material containing a source of SiC>2 includes quartzite fines having an average particle size of 36 pm < d5o < 25 mm, recycled quartzite scrap, and a combination thereof.11 . The process according to claim 1 , wherein the feedstock material containing a source ofSiC>2 is fed into the electrothermal furnace as either cold, hot pre-heated, or a combination of cold- and hot pre-heated feedstock material containing a source of SiC>2.

12. The process according to claim 1 , wherein the process provides a feed system consisting of a cold feeding system and a hot feeding system to charge the feedstock material containing a source of SiC>2 according to the desired process recipe.

13. The process according to claim 12, wherein the hot feeding system comprises at least one pre-heating unit.

14. The process according to claim 12, wherein the feed system is defined by utilizing a Hollow Electrode System (HES) for feeding the feedstock material containing a source of SiC>2 into the electrothermal furnace.

15. The process according to claim 1 , wherein the reductant is anthracite, coke, finer fraction coke, char, or coal, and wherein the reductant is added to the electrothermal furnace as particulates having a particle size of no more than 50 mm.

16. The process according to claim 15, wherein the reductant is a source of bio-carbon.

17. The process according to claim 1 , wherein the flux is selected from the group consisting of burnt- or unburnt dolomite, burnt- or unburnt limestone, quartzite, bauxite, and a combination thereof.

18. The process according to claim 1 , wherein the process provides a plurality of adjustable feeding chutes to allow for the introduction of the feedstock material containing a source of SiC>2, reductant and fluxes, into the electrothermal furnace through a Loss-in Weight (LIW) system to control the power-to-feed balance within the furnace.

19. The process according to claim 1 , wherein the process provides continuous replenishment of the feedstock material containing a source of SiC>2, reductant and fluxes utilizing the plurality of adjustable feeding chutes into the electrothermal furnace to ensure that the loss-in-weight and power-to-feed balance is controlled.

20. The process according to any one of the preceding claims, wherein the feedstock material containing a source of SiC>2 is produced with 100% hydrogen to aid in reducing the CO2 footprint.21 . The process according to any one of the preceding claims, wherein the process provides for the reduction of SiO to Si using H2gas as a reductant, and wherein the H2gas is added to the process in any one of the following manners -(i) Feeding volatile rich reductants in the feeding system or the HES to produce additional H2and CO gas;(ii) Injecting the H2gas in the HES;(iii) Injecting the H2gas in the Waste Heat Recovery Unit (WHRU) or gas reactor;(iv) Installing an upstream pre-heating unit to enhance the energy efficiency and thus the SER of the OBF electrothermal furnace; or(v) Installing a downstream WHRU to enhance the energy efficiency and thus the SEC of the process.

22. The process according to any one of the preceding claims, wherein the process provides for the injection or the addition of an alternative solid state H2source.

23. The process according to claim 22, wherein the alternative solid state H2is high volatile coal.

24. The process according to claim 1 , wherein the process provides a furnace off-gas energy recovery system which is fitted with a WHRU that recovers energy by combusting CO, H2 and SiO, and wherein the combustion heat is recovered to steam in a steam boiler located in the WHRU.

25. The process according to claim 1 , wherein the liquid silicon metal product material is formed through the heating and at least partial melting of the feedstock material containing a source of SiO2, reductant and fluxes.

26. The process according to claim 1 , wherein the residence time of the feedstock material is controlled to control the degree of reduction of the feedstock material containing a source of SiC>2 in the electrothermal furnace.

27. The process 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, use as a deoxidizer, and a combination thereof.

28. The process according to claim 1 , wherein the silicon metal product includes:(i) Si: 90 to 99.9999%(ii) Al:0 to 10%(iii) Ca:0 to 10%(iv) Ti: O to 10%(v) C: O to 10%29. The process according to any one of the preceding claims, wherein the process has a net carbon footprint of between 40% and 95% of a traditional silicon smelting route.

30. The process according to any one of the preceding claims, wherein the process providesa silicon-monoxide-hydrogen gas reactor.31 . The process according to any one of the preceding claims, wherein the process provides post tap hole operations.

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