Search

EP-4735818-A1 - NOVEL SILICON SMELTING PROCESS

EP4735818A1EP 4735818 A1EP4735818 A1EP 4735818A1EP-4735818-A1

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 SiO 2 having an average particle size of 36 µm < d 50 < 25 mm, reductant and fluxes into an electrothermal furnace; (ii) heating the feedstock material containing a source of SiO 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 SiO 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 SiO 2 , reductant and fluxes, to a form a liquid silicon metal product, a liquid slag product and an off-gas containing SiO, CO and H 2 ; 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 H 2 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.

Inventors

  • MOOLMAN, Wynand David
  • LOUW, Stephan Christiaan
  • GELDENHUYS, ISABELLA JOHANNA

Assignees

  • Metix (Proprietary) Limited

Dates

Publication Date
20260506
Application Date
20240918

Claims (20)

  1. 1 . 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 < d 5 o < 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 H 2 ; 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 H 2 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.
  2. 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. 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. 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 (multiple or single electrode), short open-arc mode, brush arc mode and immersed electrode (no open-arc) mode.
  5. 5. The process according to claim 2, wherein the electrothermal furnace has a power capacity of up to 10OMW.
  6. 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. 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. 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. 9. The process according to claim 7 or claim 8, wherein the quartzite contains between 40% and 100% SiC>2.
  10. 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 < d 5 o < 25 mm, recycled quartzite scrap, and a combination thereof.
  11. 11 . The process according to claim 1 , wherein the feedstock material containing a source of SiC>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. 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. 13. The process according to claim 12, wherein the hot feeding system comprises at least one pre-heating unit.
  14. 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. 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. 16. The process according to claim 15, wherein the reductant is a source of bio-carbon.
  17. 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. 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. 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. 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.

Description

NOVEL SILICON SMELTING PROCESS FIELD OF THE INVENTION 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. BACKGROUND TO THE INVENTION 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Attempts to improve the performance of these processes have been demonstrate