WO2022073579A1 - Process of producing technical silicon from silicon metal-containing material - Google Patents

Process of producing technical silicon from silicon metal-containing material Download PDF

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Publication number
WO2022073579A1
WO2022073579A1 PCT/EP2020/077825 EP2020077825W WO2022073579A1 WO 2022073579 A1 WO2022073579 A1 WO 2022073579A1 EP 2020077825 W EP2020077825 W EP 2020077825W WO 2022073579 A1 WO2022073579 A1 WO 2022073579A1
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silicon
production
slag
technical
anyone
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PCT/EP2020/077825
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French (fr)
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Javad MOHSSENI
Danilo Coladetti CURTOLO
Karl Bernd FRIEDRICH
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Wacker Chemie Ag
Rheinisch-Westfälische Technische Hochschule Aachen (RWTH)
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Priority to PCT/EP2020/077825 priority Critical patent/WO2022073579A1/en
Publication of WO2022073579A1 publication Critical patent/WO2022073579A1/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/02—Silicon
    • C01B33/037—Purification

Definitions

  • the present invention relates to a process of production of technical silicon from pellets of silicon metal-containing material and slag .
  • Silicon of technical quality ( Si content ⁇ 99 . 9 wt-% , also known as technical silicon) is mainly used in silicon thermal processes , in metal extraction, as a deoxidi zer in steel production and serves as an alloying component in casting alloys of aluminium, copper, titanium and iron and as a starting material for chemical compounds .
  • Kerf is the residual powder material obtained from the sawing slurry during the production of silicon wafers .
  • it consists mostly of fine metallic silicon particles and a few impurities form the sawing process .
  • a maj or portion of the silicon is wasted in the PV industry in the form of kerf slurry .
  • kerf slurry Upon slicing the silicon wafers , a considerable portion of this highly valuable material is lost . Therefore , the recovery and recycling of high-purity silicon from kerf slurry waste will be of steadily increasing importance in the coming years .
  • the kerf powder was successfully purified via directional solidification.
  • the dissolved impurities are segregated from the solidifying melt via fractional crystallization. While this process is technically viable, it generates a low yield, as a significant section of the produced ingot will have a much higher concentration of impurities [Wang TY, Lin YC, Tai CY, Sivakumar R, Rai DK, Lan CW.
  • impurities [Wang TY, Lin YC, Tai CY, Sivakumar R, Rai DK, Lan CW.
  • thermal plasma process that has been developed by DeSousa et al. to recover silicon from the slurry waste. They suggested that the energetic characteristics of thermal plasma are unique for materials processing.
  • plasma was generated with direct current arc-plasma torch by injecting argon and hydrogen mixture into an electrical arc. Slurry waste in form of powdered particles was injected into the plasma jet. The high temperature of the plasma jet then melts the input material, having the molten particles to settle in a graphite crucible. The end product upon cooling down is a solid silicon ingot [De Sousa M, Vardelle A, Mariaux G, Vardelle M, Michon U, Beudin V. Use of a thermal plasma process to recycle silicon kerf loss to solar-grade silicon feedstock.
  • the slag treatment research found throughout literature were solely focused as a method to upgrade the metallurgical grade silicon (MG-Si) into solar grade silicon (SG-Si) , employing different slag compositions and process parameters.
  • the main goal of such investigations was to remove phosphor and boron from the metallic silicon by oxidizing and entrap such elements in the slag phase [Meteleva-Fischer Y V., Yang Y, Boom R, Kraaijveld B, Kuntzel H. Slag Treatment Followed by Acid Leaching as a Route to Solar-Grade Silicon. JOM 2012;64:957-67. https : //doi .org/ 10.1007/sll 837-012-0383-4 ] .
  • the present invention provides a process of production of technical silicon, wherein silicon metal-containing material comprising at least 50 wt-% of metallic silicon and slag comprising at least together 80 wt-% of AI2O3, CaO and SiO2 are mixed with water to give a mixture, the mixture is agglomerated into the form of wet pellets, the wet pellets are dried to give pellets, and the pellets are heated up to 1450 to 1800°C wherein the liquid technical silicon phase is separated from the slag phase . At the heating temperature, a phase separation occurs and the metallic silicon is separated from the slag phase. The result is the entrapment of all the non-metallic impurities in the slag phase, while the technical silicon phase remains cleaned and can be recovered.
  • each pellet behaves like an individual reactor, providing an efficient and localized entrapment of the non-metallic impurities present in the silicon metal-containing material.
  • the slag originated from the silicon production, particularly by reduction of silicon dioxide with carbon in a smelting reduction furnace.
  • the use of slag as a purification media has been previously studied with the focus of upgrading the primary source of metallurgical grade silicon into solar grade silicon, as well as removing non-metallic impurities from silicon metal-containing material. In all these studies, an artificial slag was employed. No literature could be found on the use of the slag originated from the silicon production for the purpose of kerf recycling. This factor poses a great innovation from the environment and economic perspective, as it employs a wide-available and -recyclable source of slag.
  • the ratio between slag and silicon metal-containing material is between 1:20 and 2:1, preferably between 1:10 and 1.5:1, in particular between 1:5 and 1.1:1 the pelleting of both materials also assures that the homogeneity of the mixture can be assured throughout the whole process .
  • the silicon metal-containing material preferably comprises silicon residues which are preferably selected from byproducts or wastes of the silicon-producing or silicon-processing industries , examples being
  • classi fying processes are , for example , sieving and/or si fting;
  • the material in question here may be neutrali zed catalyst material from chlorosilane reactors before and/or after recovery of Cu; more particularly from the processes of Muller-Rochow direct synthesis , hydrochlorination or low-temperature conversion for the production of silanes .
  • the silicon metal-containing material preferably comprises at least 80 wt-% , more preferably at least 90 wt-% , in particular at least 95 wt-% of metallic silicon .
  • the silicon metalcontaining material is kerf as defined above, with a metallic silicon concentration of at least 95 wt.%, preferably 98 wt . % .
  • the metallic silicon preferably has an Si content of at least 98% by mass, preferably at least 99% by mass, in particular at least 99.9% by mass.
  • a XRF (X-Ray fluorescence) analysis shows that, preferably, the slag comprises at least together 90 wt-%, more preferably at least together 95 wt-%, in particular at least together 99 wt-% of A1 2 O 3 , CaO and SiC>2.
  • the slag can comprise elements selected from Mg, Fe and Ba, each at least most 10 wt- % , preferably at most 5 wt-%, in particular at most 1 wt-%.
  • the slag preferably comprises at most 1 wt-%, more preferably at most 0.1 wt-%, of halogen.
  • the slag preferably comprises at most 0.05 wt-%, more preferably at most 0.01 wt-%, of F.
  • the slag preferably is grinded before mixing with the silicon metal-containing material.
  • the slag is grinded to a grain size of at most 0.5 mm, more preferably at most 0.3 mm, in particular at most 0.1 mm.
  • the silicon metal-containing material is also grinded.
  • the silicon metal-containing material is grinded to a grain size of at most 0.5 mm, more preferably at most 0.3 mm, in particular at most 0.1 mm.
  • the silicon metal-containing material and slag preferably are mixed and afterwards the dry mixture is mixed with water.
  • the ratio between dry mixture and water is between 15:1 and 2:1, preferably between 10:1 and 5:1.
  • water is used as main binder medium.
  • the resulting mixture is agglomerated to give wet pellets, preferably by using pelletizing plates or pressed into briquettes .
  • surface activating substances such as Na2O/NaOH or K2O/KOH can be also employed to increase the mechanical resistance and formability of the agglomerates to give wet pellets. Later, the wet pellets are dried at air or at 100 to 300°C, preferably at 120 to 250°C.
  • the average size of the pellets is 0.1 to 3 cm, more preferably 0.5 to 2 cm.
  • the dried pellets are then preferably charged in a crucible and preferably heated in a furnace, in particular inductively heated, more preferably in a middle-frequency induction heating furnace.
  • the crucible used can be made of graphite or any other material which withstands the high temperatures involved in the process, while having no significant reaction with the melt.
  • the pellets are preferably heated to a temperature from 1600°C to 1780°C, more preferably 1700°C to 1760°C.
  • the temperature can be measured near the crucible wall by a thermocouple type B (Platinum Rhodium - 30% / Platinum Rhodium - 6%) or any other thermocouple suitable for the temperature range employed.
  • the heating preferably is conducted in a closed system with low amount of oxygen. This can be obtained by the pre-consumption of the available oxygen by the oxidation of the initially charged pellets, or by replacing the furnace atmosphere with inert gas, preferably Argon. Alternatively, the process can be run in an open atmosphere when a molten slag layer is present at the surface of the melt . This slag acts then as a protective medium against the oxidation of the silicon . For that , an amount of slag should be first pre-melted with the subsequent addition of the pellets .
  • pellets Due to the heating the pellets are trans formed into a mixture of liquid technical silicon phase and slag phase .
  • the molten mixture of silicon and slag is then preferably kept at the target process temperature for a period of at least 15 minutes , more preferably of at least 20 minutes , to ensure a proper phase separation between the slag and the liquid technical silicon phases .
  • This separation can be improved by applying a forced convection in the melt .
  • This convection can be a product of the inductively heating system, mechanical agitation, or gas formation and/or purging .
  • the technical silicon phase can be removed from the melt by means of mechanically removal or by separately pouring the melted slag and silicon phase in di f ferent recipients .
  • the mechanical removal preferably takes place in such a way that the Si is frozen on a cooling element or skimmed of f the liquid .
  • the slag and technical silicon phase layers order can be influenced by changing the slag density . This can be controlled by adj usting the content of SiC>2 in the slag .
  • SiC>2 When a higher amount of SiC>2 is present in the slag ( or arti ficially added) , its density is decreased and will tend to float over the silicon melt .
  • This slag configuration will act as a protective layer and is suitable for the application in an open system .
  • the preferably way of separating the both phases is by means of mechanical removal or by pouring the top silicon layer. Since this configuration exposes the molten technical silicon to the furnace atmosphere, it is recommended that there is a very low amount of oxygen available in the atmosphere to prevent the losses of silicon via oxidation.
  • the produced technical silicon preferably has a Si content of at least 90 wt-%, more preferably at least 95 wt-%, in particular at least 98 wt-%.
  • All steps of the process preferably are carried out at atmospheric pressure. If not otherwise described, all temperatures are 20°C.
  • the mixture was pelletized using 39,8 wt . % of water as binder medium.
  • the pelletizing took place in a pelletizing disc of 500 mm diameter rotating at 25RPM with a 50° inclination angle. The results were pellets with 1 cm in size.
  • the pellets were then charged in a graphite crucible place in a closed middle frequency (8 KHz) induction furnace with an atmosphere consisting of 800 mbar Argon and no oxygen present. The charge was then heated up to 1750°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min. Upon cooling, the crucible was sectioned and the phase separation was revealed. The silicon fraction was analyzed via XRF (X-ray fluorescence) and a purity of 98.3 wt-% was obtained.
  • XRF X-ray fluorescence
  • Example 2 Using the same slag powder from Example 1, a mixture with kerf powder using a proportion 1:1 was pelletized using 30,0 wt . % of water as binder, performed similarly as Example 1. The pellets were charged and heated in a middle frequency (8 KHz) induction furnace at 1650°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min. The result was a phase separation, where the silicon fraction obtained had a purity of 98.9%.
  • a middle frequency (8 KHz) induction furnace at 1650°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min.
  • the result was a phase separation, where the silicon fraction obtained had a purity of 98.9%.
  • Example 2 Using the same slag powder from Example 1, a mixture with kerf powder using a proportion 1:1 was pelletized using 30,0 wt . % of water as binder, performed similarly as Example 1. The pellets were charged and heated in a middle frequency (8 KHz) induction furnace at 1750°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min. The result was a phase separation, where the silicon fraction obtained had a purity of 97.8%.
  • a middle frequency (8 KHz) induction furnace at 1750°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min.
  • the result was a phase separation, where the silicon fraction obtained had a purity of 97.8%.

Abstract

The present invention provides a process of production of technical silicon, wherein silicon metal-containing material comprising at least 50 wt-% of metallic silicon and slag comprising at least together 80 wt-% of Al2O3, CaO and SiO2 are mixed with water to give a mixture, the mixture is agglomerated into the form of wet pellets, the wet pellets are dried to give pellets, and the pellets are heated up from 1450 to 1800°C wherein the liquid technical silicon phase is separated from the slag phase.

Description

Process of producing technical silicon from silicon metalcontaining material
The present invention relates to a process of production of technical silicon from pellets of silicon metal-containing material and slag .
Silicon of technical quality ( Si content <99 . 9 wt-% , also known as technical silicon) is mainly used in silicon thermal processes , in metal extraction, as a deoxidi zer in steel production and serves as an alloying component in casting alloys of aluminium, copper, titanium and iron and as a starting material for chemical compounds .
Kerf is the residual powder material obtained from the sawing slurry during the production of silicon wafers . Preferably it consists mostly of fine metallic silicon particles and a few impurities form the sawing process . A maj or portion of the silicon is wasted in the PV industry in the form of kerf slurry . Upon slicing the silicon wafers , a considerable portion of this highly valuable material is lost . Therefore , the recovery and recycling of high-purity silicon from kerf slurry waste will be of steadily increasing importance in the coming years .
Several researches have been done about kerf recycling . Despite some of them being technically success ful , most of them lacks the economic feasibility needed the large-scale industrial process of such valuable residue .
The exact route to handle raw silicon kerf slurry varies a lot . Often it starts with removal of polyethylene glycol using acetone , removal of Fe particles via acid dissolution, physical removal of most non-metallic impurities i . e . SiC by density separation using a heavy liquid with density value between Si and the non-metallic particle (e.g. sodium polytungstate in water) . Finally, the residual product is a powder (kerf) consisting mostly of Si and a few residual impurities. Throughout the literature, kerf recycling was investigated for the fields of directional solidification [Wang TY, Lin YC, Tai CY, Sivakumar R, Rai DK, Lan CW. A novel approach for recycling of kerf loss silicon from cutting slurry waste for solar cell applications. J Cryst Growth 2008;310:3403-6. https://doi.Org/10.1016/j .j crysgro .2008.04.031] , hydrometallurgy [Huang K, Deng H, Li J, Zhu H. Separation of Si/SiC Wiresaw Cutting Powder Through Sedimentation by Adjusting the Solution pHs . EPD Congr . 2012, Hoboken, NJ, USA: John Wiley & Sons, Inc.; 2012, p. 297-304. https://doi.org/10.1002/9781118359341.ch34] , high temperature plasma melting [Wang TY, Lin YC, Tai CY, Sivakumar R, Rai DK, Lan CW. A novel approach for recycling of kerf loss silicon from cutting slurry waste for solar cell applications. J Cryst Growth 2008;310:3403-6. https://doi.Org/10.1016/j .jcrysgro.2008.04.031] and [De Sousa M, Vardelle A, Mariaux G, Vardelle M, Michon U, Beudin V. Use of a thermal plasma process to recycle silicon kerf loss to solar-grade silicon feedstock. Sep Purif Technol 2016; 161 : 187- 92. https://doi.Org/10.1016/j . seppur .2016.02.005] , centrifugation separation [Li JW, Guo ZC, Tang HQ, Wang Z, Sun ST. Si purification by solidification of Al-Si melt with super gravity. Trans Nonferrous Met Soc China (English Ed
2012;22:958-63. https://doi.org/10.1016/S1003-6326 dl) 61270-3] , as well as slag treatment.
The kerf powder was successfully purified via directional solidification. In this process, the dissolved impurities are segregated from the solidifying melt via fractional crystallization. While this process is technically viable, it generates a low yield, as a significant section of the produced ingot will have a much higher concentration of impurities [Wang TY, Lin YC, Tai CY, Sivakumar R, Rai DK, Lan CW. A novel approach for recycling of kerf loss silicon from cutting slurry waste for solar cell applications. J Cryst Growth 2008;310:3403-6. https://doi.Org/10.1016/j .j crysgro .2008.04.031] .
When recycling kerf slurry originated from the diamond sawing process, the most important metallic contaminants are the nickel and iron deriving from the abrasion with the outer layer of the diamond-embedded wire. After chemical treatment, usually with HNO3 and HC1, the total metal impurities are drastically reduced. Higher purity levels can be achieved by performing a deeper chemical etching which removes a thicker layer of silicon surface, thus directly impacting the silicon recycling yield [Lombardi I, Fragiacomo G, Zehetmeier C, Rohr C, Gaumann B, Ktinzli A. High yield recycling process of silicon kerf from diamond wire watering. 24th Eur Photovolt Sol Energy Conf Hamburg, Ger 2009:1256-8] .
A thermal plasma process that has been developed by DeSousa et al. to recover silicon from the slurry waste. They suggested that the energetic characteristics of thermal plasma are unique for materials processing. In their study, plasma was generated with direct current arc-plasma torch by injecting argon and hydrogen mixture into an electrical arc. Slurry waste in form of powdered particles was injected into the plasma jet. The high temperature of the plasma jet then melts the input material, having the molten particles to settle in a graphite crucible. The end product upon cooling down is a solid silicon ingot [De Sousa M, Vardelle A, Mariaux G, Vardelle M, Michon U, Beudin V. Use of a thermal plasma process to recycle silicon kerf loss to solar-grade silicon feedstock. Sep Purif Technol 2016;161:187-92. https://doi.Org/10.1016/j . seppur .2016.02.005 ] . In the centrifugation separation method, the kerf is initially dissolved in aluminum, which acts as solvent trapping most of the metallic impurities. Upon reaching supersaturation, the formed primary Si crystals can be recovered by super gravity. The separated silicon obtained goes then to an acid leaching process for the removal of residual aluminum contamination [Li JW, Guo ZC, Tang HQ, Wang Z, Sun ST. Si purification by solidification of Al-Si melt with super gravity. Trans Nonferrous Met Soc China (English Ed 2012;22:958-63. https : //doi . org/ 10.1016/S1003- 6326 ( 11 ) 61270-3 ] . Similar process has been used in industrial scale for the upgrade of MG-Si into SoG-Si .
The slag treatment research found throughout literature were solely focused as a method to upgrade the metallurgical grade silicon (MG-Si) into solar grade silicon (SG-Si) , employing different slag compositions and process parameters. The main goal of such investigations was to remove phosphor and boron from the metallic silicon by oxidizing and entrap such elements in the slag phase [Meteleva-Fischer Y V., Yang Y, Boom R, Kraaijveld B, Kuntzel H. Slag Treatment Followed by Acid Leaching as a Route to Solar-Grade Silicon. JOM 2012;64:957-67. https : //doi .org/ 10.1007/sll 837-012-0383-4 ] .
The present invention provides a process of production of technical silicon, wherein silicon metal-containing material comprising at least 50 wt-% of metallic silicon and slag comprising at least together 80 wt-% of AI2O3, CaO and SiO2 are mixed with water to give a mixture, the mixture is agglomerated into the form of wet pellets, the wet pellets are dried to give pellets, and the pellets are heated up to 1450 to 1800°C wherein the liquid technical silicon phase is separated from the slag phase . At the heating temperature, a phase separation occurs and the metallic silicon is separated from the slag phase. The result is the entrapment of all the non-metallic impurities in the slag phase, while the technical silicon phase remains cleaned and can be recovered.
Since both silicon metal-containing material and slag are pelletized together, the agglomeration of the powder materials decreases the losses due to their high surface area. It also improves the storage and transport of these materials in the plant. Moreover, during the melting phase, each pellet behaves like an individual reactor, providing an efficient and localized entrapment of the non-metallic impurities present in the silicon metal-containing material.
Preferably used is the slag originated from the silicon production, particularly by reduction of silicon dioxide with carbon in a smelting reduction furnace. The use of slag as a purification media has been previously studied with the focus of upgrading the primary source of metallurgical grade silicon into solar grade silicon, as well as removing non-metallic impurities from silicon metal-containing material. In all these studies, an artificial slag was employed. No literature could be found on the use of the slag originated from the silicon production for the purpose of kerf recycling. This factor poses a great innovation from the environment and economic perspective, as it employs a wide-available and -recyclable source of slag.
If the ratio between slag and silicon metal-containing material is between 1:20 and 2:1, preferably between 1:10 and 1.5:1, in particular between 1:5 and 1.1:1 the pelleting of both materials also assures that the homogeneity of the mixture can be assured throughout the whole process .
The silicon metal-containing material preferably comprises silicon residues which are preferably selected from byproducts or wastes of the silicon-producing or silicon-processing industries , examples being
- those arising in the production or the mechanical processing of silicon, such as poly- , multi- or monocrystalline silicon;
- those arising in the production of granulated silicon metal in, for example , fluidi zed bed, centri fugal , gas atomi zation or water granulation processes ;
- those arising in the production of technical-grade silicon by means of carbothermic reduction of SiCb ;
- those arising in the course of mechanical processing and optionally of one or more classi fying processes of technical- grade silicon . The mechanical processing may more particularly be crushing and/or grinding . Typical classi fying processes are , for example , sieving and/or si fting;
- those arising in the production of silanes . For example , the material in question here may be neutrali zed catalyst material from chlorosilane reactors before and/or after recovery of Cu; more particularly from the processes of Muller-Rochow direct synthesis , hydrochlorination or low-temperature conversion for the production of silanes .
There is normally no need for these silicon residues to be puri fied before being used in the method of the invention; that is , the silicon-containing materials can be used without further puri fication steps .
The silicon metal-containing material preferably comprises at least 80 wt-% , more preferably at least 90 wt-% , in particular at least 95 wt-% of metallic silicon . According to a preferred embodiment, the silicon metalcontaining material is kerf as defined above, with a metallic silicon concentration of at least 95 wt.%, preferably 98 wt . % .
The metallic silicon preferably has an Si content of at least 98% by mass, preferably at least 99% by mass, in particular at least 99.9% by mass.
A XRF (X-Ray fluorescence) analysis shows that, preferably, the slag comprises at least together 90 wt-%, more preferably at least together 95 wt-%, in particular at least together 99 wt-% of A12O3, CaO and SiC>2.
Besides the elements Ca, Si, Al and 0 the slag can comprise elements selected from Mg, Fe and Ba, each at least most 10 wt- % , preferably at most 5 wt-%, in particular at most 1 wt-%.
The slag preferably comprises at most 1 wt-%, more preferably at most 0.1 wt-%, of halogen.
The slag preferably comprises at most 0.05 wt-%, more preferably at most 0.01 wt-%, of F.
The slag preferably is grinded before mixing with the silicon metal-containing material. Preferably the slag is grinded to a grain size of at most 0.5 mm, more preferably at most 0.3 mm, in particular at most 0.1 mm.
If necessary, the silicon metal-containing material is also grinded. Preferably the silicon metal-containing material is grinded to a grain size of at most 0.5 mm, more preferably at most 0.3 mm, in particular at most 0.1 mm.
Kerf has not to be grinded.
The silicon metal-containing material and slag preferably are mixed and afterwards the dry mixture is mixed with water. Preferably the ratio between dry mixture and water is between 15:1 and 2:1, preferably between 10:1 and 5:1.
At this stage, water is used as main binder medium. The resulting mixture is agglomerated to give wet pellets, preferably by using pelletizing plates or pressed into briquettes .
The usage of surface activating substances such as Na2O/NaOH or K2O/KOH can be also employed to increase the mechanical resistance and formability of the agglomerates to give wet pellets. Later, the wet pellets are dried at air or at 100 to 300°C, preferably at 120 to 250°C.
Preferably, the average size of the pellets is 0.1 to 3 cm, more preferably 0.5 to 2 cm.
The dried pellets are then preferably charged in a crucible and preferably heated in a furnace, in particular inductively heated, more preferably in a middle-frequency induction heating furnace. The crucible used can be made of graphite or any other material which withstands the high temperatures involved in the process, while having no significant reaction with the melt. The pellets are preferably heated to a temperature from 1600°C to 1780°C, more preferably 1700°C to 1760°C. The temperature can be measured near the crucible wall by a thermocouple type B (Platinum Rhodium - 30% / Platinum Rhodium - 6%) or any other thermocouple suitable for the temperature range employed.
The heating preferably is conducted in a closed system with low amount of oxygen. This can be obtained by the pre-consumption of the available oxygen by the oxidation of the initially charged pellets, or by replacing the furnace atmosphere with inert gas, preferably Argon. Alternatively, the process can be run in an open atmosphere when a molten slag layer is present at the surface of the melt . This slag acts then as a protective medium against the oxidation of the silicon . For that , an amount of slag should be first pre-melted with the subsequent addition of the pellets .
Due to the heating the pellets are trans formed into a mixture of liquid technical silicon phase and slag phase .
The molten mixture of silicon and slag is then preferably kept at the target process temperature for a period of at least 15 minutes , more preferably of at least 20 minutes , to ensure a proper phase separation between the slag and the liquid technical silicon phases . This separation can be improved by applying a forced convection in the melt . This convection can be a product of the inductively heating system, mechanical agitation, or gas formation and/or purging .
The technical silicon phase can be removed from the melt by means of mechanically removal or by separately pouring the melted slag and silicon phase in di f ferent recipients . The mechanical removal preferably takes place in such a way that the Si is frozen on a cooling element or skimmed of f the liquid .
The slag and technical silicon phase layers order can be influenced by changing the slag density . This can be controlled by adj usting the content of SiC>2 in the slag . When a higher amount of SiC>2 is present in the slag ( or arti ficially added) , its density is decreased and will tend to float over the silicon melt . This slag configuration will act as a protective layer and is suitable for the application in an open system . In the case of the slag having higher density than the technical silicon melt i.e. lower SiC>2 content, the preferably way of separating the both phases is by means of mechanical removal or by pouring the top silicon layer. Since this configuration exposes the molten technical silicon to the furnace atmosphere, it is recommended that there is a very low amount of oxygen available in the atmosphere to prevent the losses of silicon via oxidation.
The produced technical silicon preferably has a Si content of at least 90 wt-%, more preferably at least 95 wt-%, in particular at least 98 wt-%.
All steps of the process preferably are carried out at atmospheric pressure. If not otherwise described, all temperatures are 20°C.
Examples
Example 1
A slag originated from the primary silicon production by reduction of silicon dioxide with carbon in a smelting reduction furnace with the composition of 17 wt-% AI2O3, 55 wt-% SiC>2, and 28 wt-% CaO was grounded into powder using a vibrating-disc mill for ca. 60 seconds, obtaining at the end a powder with a maximum 0.3 mm, which is then mixed with Kerf powder having a silicon metal content of 95 wt-% at a weight ratio of 1:3 (25 wt-% of slag and 75 wt-% of kerf) . The mixture was pelletized using 39,8 wt . % of water as binder medium. The pelletizing took place in a pelletizing disc of 500 mm diameter rotating at 25RPM with a 50° inclination angle. The results were pellets with 1 cm in size.
The pellets were then charged in a graphite crucible place in a closed middle frequency (8 KHz) induction furnace with an atmosphere consisting of 800 mbar Argon and no oxygen present. The charge was then heated up to 1750°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min. Upon cooling, the crucible was sectioned and the phase separation was revealed. The silicon fraction was analyzed via XRF (X-ray fluorescence) and a purity of 98.3 wt-% was obtained.
Example 2
Using the same slag powder from Example 1, a mixture with kerf powder using a proportion 1:1 was pelletized using 30,0 wt . % of water as binder, performed similarly as Example 1. The pellets were charged and heated in a middle frequency (8 KHz) induction furnace at 1650°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min. The result was a phase separation, where the silicon fraction obtained had a purity of 98.9%.
Example 3
Using the same slag powder from Example 1, a mixture with kerf powder using a proportion 1:1 was pelletized using 30,0 wt . % of water as binder, performed similarly as Example 1. The pellets were charged and heated in a middle frequency (8 KHz) induction furnace at 1750°C (measured at crucible wall with a Type B thermocouple) and kept for 30 min. The result was a phase separation, where the silicon fraction obtained had a purity of 97.8%.

Claims

Claims
1 . A process of production of technical silicon, wherein silicon metal-containing material comprising at least 50 wt-% of metallic silicon and slag comprising at least together 80 wt-% of AI2O3, CaO and SiO2 are mixed with water to give a mixture , the mixture is agglomerated into the form of wet pellets , the wet pellets are dried to give pellets , and the pellets are heated up from 1450 to 1800 ° C wherein the liquid technical silicon phase is separated from the slag phase .
2 . The process of production of technical silicon according to Claim 1 , wherein the slag is originated from the silicon production by reduction of silicon dioxide with carbon in a smelting reduction furnace .
3 . The process of production of technical silicon according to anyone of the preceding claims , wherein the ratio between slag and silicon metal-containing material is between 1 : 20 and 2 : 1 .
4 . The process of production of technical silicon according to anyone of the preceding claims , wherein the silicon metalcontaining material preferably comprises at least 80 wt-% of metallic silicon .
5 . The process of production of technical silicon according to anyone of the preceding claims , wherein the silicon metalcontaining material is kerf , which is the residual powder material obtained from the sawing slurry during the production of silicon wafers .
6. The process of production of technical silicon according to anyone of the preceding claims, wherein the slag is grinded to a grain size of at most 0.5 mm before it is mixed with silicon metal-containing material and water.
7. The process of production of technical silicon according to anyone of the preceding claims, wherein the average size of the pellets is 0.1 to 3 cm.
8. The process of production of technical silicon according to anyone of the preceding claims, wherein the heating is carried out under an inert gas atmosphere.
9. The process of production of technical silicon according to anyone of the preceding claims, wherein the molten mixture of silicon and slag is kept at the target process temperature for a period of at least 15 minutes.
10. The process of production of technical silicon according to anyone of the preceding claims, wherein the produced technical silicon has a Si content of at least 95 wt-%.
PCT/EP2020/077825 2020-10-05 2020-10-05 Process of producing technical silicon from silicon metal-containing material WO2022073579A1 (en)

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