WO2011099870A1 - Method for recovering solar grade silicon - Google Patents

Method for recovering solar grade silicon Download PDF

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Publication number
WO2011099870A1
WO2011099870A1 PCT/NO2011/000053 NO2011000053W WO2011099870A1 WO 2011099870 A1 WO2011099870 A1 WO 2011099870A1 NO 2011000053 W NO2011000053 W NO 2011000053W WO 2011099870 A1 WO2011099870 A1 WO 2011099870A1
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Prior art keywords
silicon
slag
mixture
slag forming
weight
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PCT/NO2011/000053
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French (fr)
Inventor
Knut Henriksen
Eli Ringdalen
Ingeborg Solheim
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Metallkraft As
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Priority to EP11706954A priority Critical patent/EP2534100A1/en
Publication of WO2011099870A1 publication Critical patent/WO2011099870A1/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
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • H01L31/182—Special manufacturing methods for polycrystalline Si, e.g. Si ribbon, poly Si ingots, thin films of polycrystalline Si
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/546—Polycrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention concerns a method for recovering particulate silicon.
  • the world may face an energy shortage due the expected levelling and subsequent decline in so-called proven world oil reserves within the coming one or two decades. Further, according to IPCC of the UN, it is necessary to reduce the man- made emissions of long-lived greenhouse gases by at least 80 % of the present global emission-level within a few decades to avoid dangerous climate changes.
  • the major part of man-made emissions of long-lived greenhouse gases is C0 2 resulting from burning of fossil fuels.
  • Electric energy may be produced directly from the sun light by photovoltaic conversion of light energy into electric energy by use of photovoltaic or solar cells.
  • Solar cells have several advantageous aspects by being long lasting and almost maintenance free sources of electric energy which may be placed on the locations where the electric power are needed, and which produce no pollution, make no noise, need no fuel, need no cooling water, and need no moving parts. All you need is some space and access to sunlight - and preferably some storage facility for electric energy.
  • the silicon needs a higher purity than attainable by metallurgical refining.
  • the presently dominating process for refining silicon to solar grade silicon has been a chemical refining route where metallurgical grade silicon has been reacted to form gaseous or liquid species, such as i.e. silane or halogenated silanes, which have subsequently been purified in multiple distillations until required purity. Then the purified gaseous or liquid specie has been reduced, by i.e. thermal decomposition, to form a purified silicon metallic phase which is directionally solidified to form high purity mono- or polycrystalline silicon ingots.
  • the process of forming SoG-Si feedstock involves many process steps and consumes huge amounts of energy. It is thus both from an environmental and cost perspective important to employ the SoG-Si feedstock efficiently, that is reduce the loss of the feedstock in subsequent processing to solar cells.
  • the presently most common process route for producing solar panels is based on forming solar cells from thin disks (wafers) of crystalline silicon.
  • the main steps for forming silicon wafers are; dividing the crystalline silicon ingots into a number of silicon bricks by dividing the ingots by sawing, polishing and grinding the silicon bricks to into the right dimension, and then slicing each brick into several hundred wafers.
  • the last step is often denoted wafer cutting and is usually performed by use of a wire saw.
  • the wafer cutting process consists of starting with a brick of silicon, either multi-, or mono-crystalline Si. Typical dimensions of this brick are 0.25 m long by 125 x 125 mm or 156 ⁇ 156 mm. This brick is then glued and mounted onto a holder and placed into a wire saw where there is a spool of wire with a suspension of grit particles of SiC in a slurry. The wire is guided onto the brick by a threading unit that spaces the wires at intervals along the brick. The slurry is continuously fed and acts as both the cutting material and the coolant. At the end, the wire cuts through the brick and the process stops. Then the wafer set is demounted, the wafers are separated, singulated, cleaned, and then collected.
  • kerf-loss is the silicon that is cut and lost as particles with a size in the range from sub micron to a few microns in the sawing slurry.
  • the kerf-loss is about equal to the wafer thickness until 120 ⁇ . This means that about 50 % of the costly and energy- demanding SoG-Si is lost during cutting of wafers.
  • commercial recycle processes have been found due to problems with impurities introduced in the sawing process, such that the sawing dust is presently discharged as waste.
  • the impurities stems from the abrasive grains and metallic impurities from the sawing thread, from the sawing cooling fluid, and from oxides formed on the metallic grains of silicon due to contact with oxygen.
  • the level of impurities in the saw dust is comparable to the corresponding impurity levels in metallurgical grade silicon.
  • the sawing slurry from the wire saw usually contains a mixture of silicon particles, SiC-particles, glycol and particulate metallic impurities from the sawing wire.
  • the liquid fraction may be separated from the solids by i.e. settling, flotation, filtration, etc., and then the liquid may be purified and reused as sawing slurry.
  • WO 2004/098848 discloses a method for separating the SiC particles and the silicon particles in the slurry: Method for cleaning of silicon carbide particles from fine grain particles adhering to said silicon carbide particles, typically in the form of agglomerates of metal particles, subsequent to production (cutting) of silicon wafers and after removal of any present solute or dispersing agent from the particles.
  • the particles (1) of contaminated silicon carbide are firstly exposed to a mechanical treatment in a first step (2) of cleaning in a per se known classifying apparatus where a first coarse fraction (3) of particles, agglomerates, larger than the original silicon carbide particles, are separated out and treated in a process (4) where the agglomerates are broken down to individual grains, without crushing said individual grains, and thereafter recycled (5) to said first step (2) of cleaning.
  • a first fine fraction (6) is discharged from said first step (2) and transferred to a second step (7) of cleaning conducted in a per se known classifying apparatus from which the particles of silicon carbide are discharged in the form of a second coarse fraction (8), while the contaminants separated out in said second step of cleaning, are discharged in the form of a second fine fraction (9).
  • the silicon particles are contained in the discharge of the second fine fraction (9).
  • saw dust from production of silicon wafers at particle sizes 40 ⁇ may be recovered as silicon feedstock by heating the powder in a ladle by use of an arc discharge in an argon atmosphere. The heating is stopped before all powder is melted to form an outer non-melted layer that acts as an impurity shield towards the ladle.
  • the argon atmosphere may be admixed with hydrogen.
  • US 2008/0295294 discloses a process for producing silicon feedstock by thermal decomposition of silane to elementary silicon dust in a free space reactor.
  • the document reports of problems with melting the silicon powder due to an oxide layer on the particles.
  • the solution of the melting problem according to this document, is mixing the powder with silicon lumps and then melting the mixture in an argon atmosphere at 100 mbar.
  • Another document reporting similar problems with melting of small silicon particles due to an oxide layer is US 2007/0148034. This document teaches to dry pressing the silicon powder into pellets and then melt the pellets.
  • US 4 354 987 discloses a method where powder formed by thermal decomposition of silane gas is directly melted in a hydrogen atmosphere. The process is contained in a closed system, and thus the problem with oxide on the particle surface is avoided. The powder is reported to melt by heating to a temperature in the range 1460 - 1600 °C.
  • WO 2006/009456 shows another example of a reactor where silane gas is first thermally decomposed to elemental silicon powder and then melted in the same reactor without exposure to oxygen before melting.
  • the HBO-gas is then removed from the melt together with the purge gas.
  • WO 2008/031229 discloses a method for refining molten silicon in a rotatable drum furnace heated by an oxy-fuel burner providing an oxidising atmosphere above the liquid silicon comprising H 2 , 0 2 , CO and C0 2 .
  • the oxidizing atmosphere is obtained by employing a mixture of oxygen to natural gas in the range from 1 : 1 to 4:1, preferably in the range from 1.5:1 and 2.85:1.
  • the melt is covered by a slag comprising including one or more metal oxides which are able to extract Al, Ba, Ca, K, Mg, Na, Sr, Zn, C and B.
  • the document informs that numerous slag recipes known in the art may be applied, for example, a synthetic slag that includes Si0 2 , AI 2 0 3 , CaO, CaC0 3 , Na 2 0, Na 2 C0 3 , CaF, NaF, MgO, MgC0 3 , SrO, BaO, MgF 2 , or K 2 0, or any combination thereof may be added to the molten silicon to remove Al, Ba, Ca, K, Mg, Na, Sr, Zn, C, or B, or any combination thereof from the melt.
  • the document contains experimental data showing B removal of 23 - 26 %.
  • US 3 871 872 describe the treatment of silicon with a slag to remove calcium and aluminium impurities by adding a slag comprising Si0 2 , CaO, MgO and AI 2 C «3 to molten silicon metal.
  • US 4 534 791 describe the treatment of silicon with a slag to remove calcium and aluminium impurities by treating silicon with a molten slag comprising Si0 2 , CaO, MgO and A1 2 0 3 , Na 2 0, CaF 2 , NaF, SrO, BaO, MgF 2 , and K 2 0.
  • the experiments made by Suzuki and Sano were carried out by placing 10 g of silicon and 10 g of slag in a graphite crucible, melting the mixture and keeping the mixture molten for two hours.
  • the low distribution coefficient of boron between slag and molten silicon means that a high amount of slag has to be used and that the slag treatment has to be repeated a number of times in order to bring the boron content from 20-100 ppm, which is the normal boron content of metallurgical silicon, down to below 1 ppm, which is the required boron content for solar grade silicon.
  • a further object of the invention is to provide a method for recovering high-purity silicon lost as kerf from production of wafers to be used as feed stock in the photovoltaic industry.
  • the invention is based on the discovery that small droplets of molten silicon dispersed in a slag phase with a relatively low viscosity and with a relatively low wetting towards the silicon droplets, may be agglomerated to lumps of pure silicon by flushing the melt by a gas.
  • This combination of features makes it possible to produce lumps of silicon metal from small particles of silicon, such as from i.e. kerf loss where the silicon particles typically are about 1 ⁇ in diameter by melting the particulate Si together with a sufficient amount of slag forming compounds, passing a dispersed spray of gas bubbles through the melt and then cool the melt to the solid state followed by separating the agglomerated silicon lumps from the slag.
  • This discovery has solved a long standing problem in the photovoltaic industry - how to recover high purity silicon lost as kerf.
  • the present invention relates to a method of refining and converting particulate silicon to silicon lumps, where the method comprises the following steps:
  • particulate silicon with at least one slag forming compound in a weight ratio, particulate silicon: slag forming compound(s), from 1 :10 to 3:1 in a crucible, wherein the at least one slag forming compound will form a liquid slag at an operating temperature above the liquidus temperature of silicon, and which has a viscosity lower than 2 Ns/m 2 and a wetting angle towards silicon of at least 20 °,
  • slag forming compound as used herein, means any known or
  • the slag may be formed from a single slag forming compounds, or by mixtures of two or more slag forming compounds. Examples of suitable slag forming
  • any slag forming compound forming a slag satisfying the above-given condition may be employed. It is believed that the flotation effect will be enhanced by reducing the viscosity of the slag. Thus it may be advantageous to use small amount slag compounds that reduces the viscosity of the slag compounds in order to enhance the flotation effect of the gas on the molten silicon droplets.
  • CaF 2 and Na 2 0 are examples of slag compounds that will contribute to a lower slag viscosity.
  • wetting angle between compound A and B means the angle formed between the planar surface of a solid substrate of compound A and the tangent of the surface plane of a liquid droplet of compound B at the interface between the droplet and substrate at the end of the droplet, see illustration in Figure 1 where the wetting angle between substrate and droplet is marked as angle a.
  • Reference numeral 1 is the surface of the solid compound A and numeral 2 shows the liquid droplet of compound B.
  • the wetting angle may be measured or calculated from the surface tension of the compounds.
  • heating above the liquidus temperature of the slag means a temperature where the slag forming compound(s) is/are transformed to a completely liquid phase. Both the silicon and slag forming compound(s) must be molten to obtain the claimed effect of the invention.
  • the intended temperature of the heating the mixture in the method according to the first aspect of the invention depends on which is highest, the melting temperature of silicon or the liquidus temperature of the slag. In practice the temperature of the molten mixture will usually be raised to a temperature in the range from about 10 to about 150 °C above the liquidus temperature of the compound in the mixture with the highest liquidus temperature. Other temperatures outside this range however may also be applied.
  • injecting a stream of gas through at least one point of the bottom of the crucible means that a stream of gas bubbles are made to pass through the crucible bottom and enter the liquid mixture, raise up through the molten phase and escape to the ambient atmosphere above the molten phase.
  • the inventive method may function with use of any gas including both reactive and inert gases, size and flux of gas bubbles being flushed through the molten phase.
  • suitable gases include, but are not limited to, noble gases and hydrogen gas.
  • a mixture of gases This applies both to the gas forming the atmosphere in the hot zone employed to melt the mixture in the crucible and the gas being flushed through the bottom of the crucible and into the molten mixture. It is advantageous to employ a high number of small and dispersed gas bubbles to obtain a high bubble surface area to which the molten silicon droplets may adhere and an effective flushing of the entire bulk liquid phase.
  • the melt After melting of the mixture and flushing with gas to form the Si-agglomerates, the melt is solidified and cooled to a temperature allowing separating the solidified Si- agglomerates/lumps from the solidified slag.
  • the separation of the slag and Si- lumps may be achieved by any known or conceivable process for separating solid metallic spherules from a solidified slag phase.
  • the slag system Si0 2 - CaO is known to undergo several phase transformations during cooling which may cause the slag to self-disintegrate to a fine powder, and thus open up for the possibility of an easy separation of slag and solidified silicon particles/lumps.
  • This slag system has one phase transformation from ⁇ to ⁇ at about 490 °C which is accompanied by a 12 % volume expansion which usually results in disintegrating of the slag to a fine powder with particles sizes of less than 100 ⁇ in diameter.
  • This fine powder may easily, by use of sieves etc. be separated from the silicon agglomerates which can have particle sizes up to several times larger than 100 ⁇ .
  • the invention may employ the slag system Si0 2 - CaO in the following manner:
  • particulate silicon slag forming mixture in a weight ratio, particulate silicon: slag forming mixture, in the range from 1 :10 to 3:1
  • the slag system Si0 2 - CaO consists of only these two slag forming compounds, we have from the phase diagram shown in Figure 2 that the amounts of Si0 2 and CaO should be such that the weight% Si0 2 is within the range from 43 to 65 weight%.
  • additional slag compounds such as i.e. CaF 2 which lowers the liquidus temperature, optionally in combination with operation at higher temperatures, allows use of the slag system Si0 2 - CaO with Si0 2 contents outside this range from 43 to 65 weight% Si0 2 -
  • the slag system Si0 2 - CaO which makes the system suitable for use in the present invention.
  • the density of the slag system Si0 2 - CaO will range from 2526 kg/m 3 to 2758 kg/m 3 with temperatures ranging from 1550 to 1650 °C when the weight% Si0 2 ranges from 43 to 63. From [1] it is known that the density of molten silicon at this temperature range is from 2446 to 2481 kg/m .
  • the slag system according to the invention has the advantage that the small droplets which adhere to and are transported by the flotation gas bubbles will agglomerate to lumps of molten silicon which will remain floating at the surface layer of the molten slag due to the density difference.
  • the viscosity of the slag system Si0 2 - CaO is investigated by [5] and found to be from about 0.2 to 1.4 Ns/m 2 for mixtures with a Si0 2 -content ranging from 43 to 60 weight% at temperatures from 1500 to 1600 °C. See figure 3.
  • the viscosity of the slag system is less than 2 Ns/m and may be employed as such in the method according to the first and second aspect of the invention without use of viscosity reducing additives.
  • one or more viscosity reducing additives may advantageously be added to the Si0 2 - CaO slag system.
  • the wetting properties of the Si0 2 - CaO slag system towards molten silicon is calculated by [6] and found to form contact angles at from about 87 ° when the Si0 2 is present at 44 weight% and up to about 101 ° when the Si0 2 is present at 63 weight%. See figure 4. These calculations show that the Si0 2 - CaO slag system has very low wetting towards molten silicon, with contact angles just below or above 90 ° which is considered to be non-wetting, and thus will function well in the method according to the first and second aspect of the invention.
  • the wetting properties of the slag system make the slag selective towards Si and SiC such that it will retain SiC-particles stronger than it does towards Si- droplets. Contact between SiC and molten Si and hence transfer of contaminants from SiC to Si is then reduced. The gas bubbles will be more likely to capture and transport silicon droplets out of the bulk liquid slag phase when they do not contain solid SiC grains. This is a huge advantage in that it opens the possibility of employing the Si0 2 - CaO slag system to recover silicon from kerf resulting from sawing of silicon wafers in the photovoltaic industry, and thus provide a solution to a long standing problem in this industry - how to recover the approx. 50 % of the solar grade silicon feedstock being lost as kerf remains.
  • Figure 1 is a drawing showing a contact angle between a liquid droplet placed on a solid substrate.
  • Figure 2 is the phase diagram for the Si0 2 - CaO slag system.
  • Figure 3 shows the viscosity of the slag system Si0 2 - CaO with a Si0 2 -content ranging from 43 to 60 weight% at temperatures from 1500 to 1600 °C as determined by [5].
  • Figure 4 shows wetting angles calculated by [6] between the Si0 2 - CaO slag system and silicon for compositions with Si0 2 present from 43 weight% up to 63 weight%.
  • Figure 5 shows wetting angles calculated by [6] between the Si0 2 - CaO slag system and SiC for compositions with Si0 2 present from 43 weight% up to 63 weight%.
  • Figure 6 shows photographs of the crucible, melt and silicon-lumps resulting by applying the invention on kerf from diamond sawing of silicon wafers.
  • Figure 7 shows photographs of the silicon-lumps resulting by applying the invention on kerf from SiC-slurry band sawing of silicon wafers.
  • kerf Approximately 285 g of kerf remains, from cutting of photovoltaic silicon wafers by diamond based wire or band saws, was mixed with 445 g Si0 2 , 638 g CaO and 57 g CaF 2 (resulting in 1140 g slag with composition of 39 weight% Si0 2 , 56 weight% CaO and 5 weight% CaF 2 ) and packed in a semiclosed graphite crucible.
  • the graphite crucible was made of a commercially available graphite "ISEM 3" delivered by Toyo Tanso Co. Ltd. In the crucible bottom there was placed a plug of porous graphite "EG 92", also delivered by Toyo Tanso Co. Ltd.
  • the crucible with the mixture or slag forming compounds and kerf remains was placed in an induction furnace.
  • the crucible and content was heated up to a temperature of 1600 °C.
  • the temperature was maintained at this level and argon gas was flushed trough the porous plug in the crucible bottom for a period of 3 hours, then the crucible and melt was allowed to cool to a temperature of about 490 °C by turning off the heating of the furnace.
  • the heating system of the furnace was turned on to maintain the temperature at this level for about 15 minutes, and then the heating was turned off again to allow the crucible with content to cool to room temperature.
  • the slag system 39 weight% Si0 2 , 56 weight% CaO and 5 weight% CaF 2 has a density of 2600 - 2700 kg/m and a viscosity of about 0.2 Ns/m in the temperature range 1550 - 1600 °C.
  • the slag system is almost non- wetting towards silicon with a contact angle of 85 °, and shows a significantly higher wetting towards SiC with contact angles of 15 - 55 °C.
  • Figure 6 shows photographs of the crucible cut in half after a test run with the same conditions as above, except that the slag/crucible was cooled rapidly down to room temperature such that the slag did not disintegrate.
  • photograph a) it is seen the walls and bottom of crucible 10 as a dark shading. The porous plug 20 is placed in the bottom. Inside the crucible 10, the slag 30 is seen to have three almost spherical lumps of silicon metal 40 with sizes of 1 - 2 cm diameter just below the upper surface 50. This section of the melt is enlarged in photograph b).
  • Photograph c) is a micrograph of the metal surface of one silicon lump, and shows that it consists mainly of a pure silicon phase and an Si-Fe-phase at grain boundaries. An analysis of the Si-phase gave the composition in part per million weight (ppmw) shown in Table 1. The table shows that the problematic elements for photovoltaic
  • P and B are present in the silicon at levels acceptable for use as feedstock for the photovoltaic industry.
  • metallic impurities which are present at too high levels, i.e. Fe, Ca and Ti, but these may be removed by conventional metallurgical refining of silicon.
  • Figure 7a shows a photograph of the resulting pearls and lumps of Si after separation from the slag.
  • Photograph 7b is a micrograph showing a similar metallic structure as photograph 6c.
  • the total weight of the silicon pearls/lumps was 185 g, giving a silicon yield of 64 %.
  • the chemical composition of the silicon phase is given in Table 2.
  • Element B Fe Cu Zn Al Mg Ca Ti V Cr Mn Co

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Abstract

The invention concerns a method of refining and converting particulate silicon to silicon lumps, where the method comprises mixing the particulate silicon with at least one slag forming compound in a weight ratio, particulate silicon:slag forming compound(s), from 1:10 to 3:1 in a crucible, wherein the at least one slag forming compound will form a liquid the slag at the operating temperature of the process that has a viscosity lower than 2 Ns/m2 and a wetting angle towards silicon of at least 20°, heating the mixture of particulate silicon and slag forming component(s) to a temperature above the liquidus temperature of the slag, injecting a stream of gas through at least one point of the bottom of the crucible and maintain the gas flushing for a period of time, solidifying and cooling the molten mixture, and separating formed silicon agglomerates/lumps from the slag.

Description

Method for recovering solar grade silicon
The invention concerns a method for recovering particulate silicon. Background
The world may face an energy shortage due the expected levelling and subsequent decline in so-called proven world oil reserves within the coming one or two decades. Further, according to IPCC of the UN, it is necessary to reduce the man- made emissions of long-lived greenhouse gases by at least 80 % of the present global emission-level within a few decades to avoid dangerous climate changes. The major part of man-made emissions of long-lived greenhouse gases is C02 resulting from burning of fossil fuels.
Thus, both from the perspective of global warming and future declining availability of oil, it is necessary to find new energy sources both to substitute the present fossil fuel based energy supply and to encompass the expected rise in demand for energy in the coming decades.
The sun bathes the earth with vastly more energy than the humanity needs today or anytime in the foreseeable future. Thus, converting sun light to heat energy and/or electricity has the potential to solve the energy shortage. Electric energy may be produced directly from the sun light by photovoltaic conversion of light energy into electric energy by use of photovoltaic or solar cells.
Solar cells have several advantageous aspects by being long lasting and almost maintenance free sources of electric energy which may be placed on the locations where the electric power are needed, and which produce no pollution, make no noise, need no fuel, need no cooling water, and need no moving parts. All you need is some space and access to sunlight - and preferably some storage facility for electric energy.
The main obstacle which has prevented solar cells from becoming a major player in the energy markets of the world has been prohibitively high production costs of solar cells. However, that situation is in some energy markets of the world resolved by the lowering of the production costs to a present level of about 2 USD/W, and as a result, there has been a growth in the global production volumes of solar cells by 25 - 40 % annually in recent years. However, to make solar cells competitive world wide, it is assumed that it will be necessary to bring the production costs of solar cells to below 1 USD/W.
Presently, about 90 % of the world production of solar cells is based on high-purity crystalline silicon, and the metal is expected to maintain its position in the solar cell market. Silicon metal for use in photovoltaic cells is presently made by
carbothermal reduction of silicon dioxide: Si02 + C -» Si + C02. This method produces metallurgical grade silicon which may attain a purity of over 99.9 % by use of metallurgical refinement techniques.
However, in order to function optimally as photovoltaic material, the silicon needs a higher purity than attainable by metallurgical refining. The presently dominating process for refining silicon to solar grade silicon has been a chemical refining route where metallurgical grade silicon has been reacted to form gaseous or liquid species, such as i.e. silane or halogenated silanes, which have subsequently been purified in multiple distillations until required purity. Then the purified gaseous or liquid specie has been reduced, by i.e. thermal decomposition, to form a purified silicon metallic phase which is directionally solidified to form high purity mono- or polycrystalline silicon ingots. The process of forming SoG-Si feedstock involves many process steps and consumes huge amounts of energy. It is thus both from an environmental and cost perspective important to employ the SoG-Si feedstock efficiently, that is reduce the loss of the feedstock in subsequent processing to solar cells.
The presently most common process route for producing solar panels is based on forming solar cells from thin disks (wafers) of crystalline silicon. The main steps for forming silicon wafers are; dividing the crystalline silicon ingots into a number of silicon bricks by dividing the ingots by sawing, polishing and grinding the silicon bricks to into the right dimension, and then slicing each brick into several hundred wafers. The last step is often denoted wafer cutting and is usually performed by use of a wire saw.
The wafer cutting process consists of starting with a brick of silicon, either multi-, or mono-crystalline Si. Typical dimensions of this brick are 0.25 m long by 125 x 125 mm or 156 χ 156 mm. This brick is then glued and mounted onto a holder and placed into a wire saw where there is a spool of wire with a suspension of grit particles of SiC in a slurry. The wire is guided onto the brick by a threading unit that spaces the wires at intervals along the brick. The slurry is continuously fed and acts as both the cutting material and the coolant. At the end, the wire cuts through the brick and the process stops. Then the wafer set is demounted, the wafers are separated, singulated, cleaned, and then collected.
An inherent problem of the cutting process is kerf-loss, which is the silicon that is cut and lost as particles with a size in the range from sub micron to a few microns in the sawing slurry. For wafer cutting, the kerf-loss is about equal to the wafer thickness until 120 μιη. This means that about 50 % of the costly and energy- demanding SoG-Si is lost during cutting of wafers. However, up to date no, commercial recycle processes have been found due to problems with impurities introduced in the sawing process, such that the sawing dust is presently discharged as waste. The impurities stems from the abrasive grains and metallic impurities from the sawing thread, from the sawing cooling fluid, and from oxides formed on the metallic grains of silicon due to contact with oxygen. For some species, the level of impurities in the saw dust is comparable to the corresponding impurity levels in metallurgical grade silicon. Prior art
The sawing slurry from the wire saw usually contains a mixture of silicon particles, SiC-particles, glycol and particulate metallic impurities from the sawing wire. The liquid fraction may be separated from the solids by i.e. settling, flotation, filtration, etc., and then the liquid may be purified and reused as sawing slurry.
WO 2004/098848 discloses a method for separating the SiC particles and the silicon particles in the slurry: Method for cleaning of silicon carbide particles from fine grain particles adhering to said silicon carbide particles, typically in the form of agglomerates of metal particles, subsequent to production (cutting) of silicon wafers and after removal of any present solute or dispersing agent from the particles. The particles (1) of contaminated silicon carbide are firstly exposed to a mechanical treatment in a first step (2) of cleaning in a per se known classifying apparatus where a first coarse fraction (3) of particles, agglomerates, larger than the original silicon carbide particles, are separated out and treated in a process (4) where the agglomerates are broken down to individual grains, without crushing said individual grains, and thereafter recycled (5) to said first step (2) of cleaning. A first fine fraction (6) is discharged from said first step (2) and transferred to a second step (7) of cleaning conducted in a per se known classifying apparatus from which the particles of silicon carbide are discharged in the form of a second coarse fraction (8), while the contaminants separated out in said second step of cleaning, are discharged in the form of a second fine fraction (9). The silicon particles are contained in the discharge of the second fine fraction (9).
From EP 0 158 563 it is known that saw dust from production of silicon wafers at particle sizes 40 μπι may be recovered as silicon feedstock by heating the powder in a ladle by use of an arc discharge in an argon atmosphere. The heating is stopped before all powder is melted to form an outer non-melted layer that acts as an impurity shield towards the ladle. The argon atmosphere may be admixed with hydrogen.
US 2008/0295294 discloses a process for producing silicon feedstock by thermal decomposition of silane to elementary silicon dust in a free space reactor. The document reports of problems with melting the silicon powder due to an oxide layer on the particles. The solution of the melting problem, according to this document, is mixing the powder with silicon lumps and then melting the mixture in an argon atmosphere at 100 mbar. Another document reporting similar problems with melting of small silicon particles due to an oxide layer is US 2007/0148034. This document teaches to dry pressing the silicon powder into pellets and then melt the pellets.
US 4 354 987 discloses a method where powder formed by thermal decomposition of silane gas is directly melted in a hydrogen atmosphere. The process is contained in a closed system, and thus the problem with oxide on the particle surface is avoided. The powder is reported to melt by heating to a temperature in the range 1460 - 1600 °C.
WO 2006/009456 shows another example of a reactor where silane gas is first thermally decomposed to elemental silicon powder and then melted in the same reactor without exposure to oxygen before melting.
A report from the National Renewable Energy Laboratory, "Production of Solar Grade (SoG) Silicon by Refining Liquid Metallurgical Grade (MG) Silicon", NREL/SR-520-30716, August 2001 (http://www.nrel.gov/docs/fy01osti/30716.pdf), shows that it is possible to refine MG silicon in the molten stage to SoG silicon feedstock. The document teaches that all impurity elements present in MG silicon may be reduced to the ppma level by a combination of vacuum treatment, slagging, and blowing with a reactive gas (moist argon gas, optionally moist hydrogen gas) during the liquid stage, and then followed by a directional solidification. It is obtained reduction of most impurities to < 1 ppma and boron and phosphorus to < 10 ppma. The report states that the removal of boron (and phosphorous) are the most difficult elements, and that the removal of boron is probably obtained by the following reaction:
SiO (g) + 1/2 H2 (g) + B (1) -> Si (1) + HBO (g) (I)
The HBO-gas is then removed from the melt together with the purge gas.
WO 2008/031229 discloses a method for refining molten silicon in a rotatable drum furnace heated by an oxy-fuel burner providing an oxidising atmosphere above the liquid silicon comprising H2, 02, CO and C02. The oxidizing atmosphere is obtained by employing a mixture of oxygen to natural gas in the range from 1 : 1 to 4:1, preferably in the range from 1.5:1 and 2.85:1. The melt is covered by a slag comprising including one or more metal oxides which are able to extract Al, Ba, Ca, K, Mg, Na, Sr, Zn, C and B. The document informs that numerous slag recipes known in the art may be applied, for example, a synthetic slag that includes Si02, AI203, CaO, CaC03, Na20, Na2C03, CaF, NaF, MgO, MgC03, SrO, BaO, MgF2, or K20, or any combination thereof may be added to the molten silicon to remove Al, Ba, Ca, K, Mg, Na, Sr, Zn, C, or B, or any combination thereof from the melt. The document contains experimental data showing B removal of 23 - 26 %. US 3 871 872 describe the treatment of silicon with a slag to remove calcium and aluminium impurities by adding a slag comprising Si02, CaO, MgO and AI2C«3 to molten silicon metal. And US 4 534 791 describe the treatment of silicon with a slag to remove calcium and aluminium impurities by treating silicon with a molten slag comprising Si02, CaO, MgO and A1203, Na20, CaF2, NaF, SrO, BaO, MgF2, and K20.
In the article "Thermodynamics for removal of boron from metallurgical silicon by flux treatment of molten silicon" by Suzuki and Sano published in the proceedings of the 10th European photovoltaic solar energy conference in Lisbon, Portugal, 8-12 April 1991, removal of boron by flux or slag treatment is investigated. It was found that treatment of silicon with the slag systems CaO-Si02, CaO-MgO-Si02, CaO- BaO-Si02 and CaO-CaF2-Si02 gave a maximum distribution coefficient of boron, defined as the ratio between ppmw B in slag and ppmw B in silicon, of about 2.0 when the slag system CaO-BaO-Si02 was used. It was further found that the boron distribution coefficient increases with increasing alkalinity of the slag, reaches a maximum and then decreases. The experiments made by Suzuki and Sano were carried out by placing 10 g of silicon and 10 g of slag in a graphite crucible, melting the mixture and keeping the mixture molten for two hours. The low distribution coefficient of boron between slag and molten silicon means that a high amount of slag has to be used and that the slag treatment has to be repeated a number of times in order to bring the boron content from 20-100 ppm, which is the normal boron content of metallurgical silicon, down to below 1 ppm, which is the required boron content for solar grade silicon.
Objective of the invention
It is an object of the invention to provide a method to produce silicon material from silicon particles.
A further object of the invention is to provide a method for recovering high-purity silicon lost as kerf from production of wafers to be used as feed stock in the photovoltaic industry.
The object of the invention may be achieved with the invention as defined in the independent claim and in the following description.
Description of the invention
The invention is based on the discovery that small droplets of molten silicon dispersed in a slag phase with a relatively low viscosity and with a relatively low wetting towards the silicon droplets, may be agglomerated to lumps of pure silicon by flushing the melt by a gas. This combination of features makes it possible to produce lumps of silicon metal from small particles of silicon, such as from i.e. kerf loss where the silicon particles typically are about 1 μπι in diameter by melting the particulate Si together with a sufficient amount of slag forming compounds, passing a dispersed spray of gas bubbles through the melt and then cool the melt to the solid state followed by separating the agglomerated silicon lumps from the slag. This discovery has solved a long standing problem in the photovoltaic industry - how to recover high purity silicon lost as kerf.
Thus in a first aspect, the present invention relates to a method of refining and converting particulate silicon to silicon lumps, where the method comprises the following steps:
- mixing the particulate silicon with at least one slag forming compound in a weight ratio, particulate silicon: slag forming compound(s), from 1 :10 to 3:1 in a crucible, wherein the at least one slag forming compound will form a liquid slag at an operating temperature above the liquidus temperature of silicon, and which has a viscosity lower than 2 Ns/m2 and a wetting angle towards silicon of at least 20 °,
- heating the mixture of particulate silicon and slag forming component(s) to a temperature above the liquidus temperature of the slag,
- injecting a stream of gas through at least one point of the bottom of the crucible and maintain the gas flushing for a period of time,
- solidifying and cooling the molten mixture , and
- separating formed silicon agglomerates/lumps from the slag.
The term "slag forming compound" as used herein, means any known or
conceivable chemical compound or mixture of two or more compounds that will form a liquid slag, and which satisfies the condition that the resulting slag when heating the slag forming compound to at temperature above the liquidus temperature obtains a viscosity lower than 2 Ns/m2 (also denoted as Pa s in the literature) and a wetting angle towards liquid silicon of at least 20 °. Experiments performed by the inventor shows that use of slag within these limits, the silicon droplets will be adhered to the gas bubbles and transported up to or just below the surface of the melt. There the gas phase (gas bubbles) will leave the liquid melt, and the adhered molten silicon droplets will due to the wetting conditions adhere to each other and form large agglomerates. It has been obtained silicon lumps up to 40 - 50 mm in diameter when melting silicon powder with particle sizes of about 1-10 μπι in diameter.
The slag may be formed from a single slag forming compounds, or by mixtures of two or more slag forming compounds. Examples of suitable slag forming
compounds are one or more of the following: Si02, CaO, Li20, MgO, A1203, Na20, CaF2, NaF, SrO, BaO, MgF2, K20, CaC03, Na2C03, and MgC03. This list is not exhaustive, as mentioned above, any slag forming compound forming a slag satisfying the above-given condition may be employed. It is believed that the flotation effect will be enhanced by reducing the viscosity of the slag. Thus it may be advantageous to use small amount slag compounds that reduces the viscosity of the slag compounds in order to enhance the flotation effect of the gas on the molten silicon droplets. CaF2 and Na20 are examples of slag compounds that will contribute to a lower slag viscosity.
The term "wetting angle between compound A and B" as used herein means the angle formed between the planar surface of a solid substrate of compound A and the tangent of the surface plane of a liquid droplet of compound B at the interface between the droplet and substrate at the end of the droplet, see illustration in Figure 1 where the wetting angle between substrate and droplet is marked as angle a.
Reference numeral 1 is the surface of the solid compound A and numeral 2 shows the liquid droplet of compound B. The wetting angle may be measured or calculated from the surface tension of the compounds. These techniques are known to a person skilled in the art.
The term "heating above the liquidus temperature of the slag" as used herein means a temperature where the slag forming compound(s) is/are transformed to a completely liquid phase. Both the silicon and slag forming compound(s) must be molten to obtain the claimed effect of the invention. Thus the intended temperature of the heating the mixture in the method according to the first aspect of the invention depends on which is highest, the melting temperature of silicon or the liquidus temperature of the slag. In practice the temperature of the molten mixture will usually be raised to a temperature in the range from about 10 to about 150 °C above the liquidus temperature of the compound in the mixture with the highest liquidus temperature. Other temperatures outside this range however may also be applied.
The term "injecting a stream of gas through at least one point of the bottom of the crucible" as used herein means that a stream of gas bubbles are made to pass through the crucible bottom and enter the liquid mixture, raise up through the molten phase and escape to the ambient atmosphere above the molten phase. The inventive method may function with use of any gas including both reactive and inert gases, size and flux of gas bubbles being flushed through the molten phase.
Examples of suitable gases include, but are not limited to, noble gases and hydrogen gas. There may be applied a mixture of gases. This applies both to the gas forming the atmosphere in the hot zone employed to melt the mixture in the crucible and the gas being flushed through the bottom of the crucible and into the molten mixture. It is advantageous to employ a high number of small and dispersed gas bubbles to obtain a high bubble surface area to which the molten silicon droplets may adhere and an effective flushing of the entire bulk liquid phase. There may be applied one or more injection points for the gas in the bottom of the crucible. After melting of the mixture and flushing with gas to form the Si-agglomerates, the melt is solidified and cooled to a temperature allowing separating the solidified Si- agglomerates/lumps from the solidified slag. The separation of the slag and Si- lumps may be achieved by any known or conceivable process for separating solid metallic spherules from a solidified slag phase.
It is advantageous to employ a self-disintegrating slag to alleviate the post- solidification separation of the formed silicon aggregates and slag. The slag system Si02 - CaO is known to undergo several phase transformations during cooling which may cause the slag to self-disintegrate to a fine powder, and thus open up for the possibility of an easy separation of slag and solidified silicon particles/lumps.
This slag system has one phase transformation from β to γ at about 490 °C which is accompanied by a 12 % volume expansion which usually results in disintegrating of the slag to a fine powder with particles sizes of less than 100 μπι in diameter. This fine powder may easily, by use of sieves etc. be separated from the silicon agglomerates which can have particle sizes up to several times larger than 100 μπι.
From the phase diagram of the Si02 - CaO system shown in Figure 2, we find that when the amounts of Si02 and CaO are within the range around 43 to 65 weight% Si02 this slag system has a liquidus temperature below 1550 °C. Outside this range, the liquidus temperature increases rapidly. However, addition of additional slag compounds such as i.e. CaF2 which lowers the liquidus temperature allows use of the slag system Si02 - CaO with ranges outside 43 to 65 weight% Si02. It is also possible to go outside the range of 43 to 65 weight% Si02 by increasing the temperature above 1550 °C, or by employing a combination of increased
temperature and addition of additives that reduces the liquidus temperature.
Thus, as an example of a suited slag system, the invention may employ the slag system Si02 - CaO in the following manner:
- preparing a slag forming mixture comprising CaO and Si02,
- mixing particulate silicon with the slag forming mixture in a weight ratio, particulate silicon: slag forming mixture, in the range from 1 :10 to 3:1
- loading the mixture in a crucible having at least one gas inlet in the bottom,
- heating the mixture to a temperature in the range from 1450 to 1650 °C to form a melt,
- injecting a stream of gas through the at least one gas inlet in the bottom of the crucible and maintain the gas flushing for a period of time,
- solidifying and cooling the melt to a temperature of about 490 °C, optionally maintaining this temperature for a period of time ranging from 5 minutes to 5 hours, and then
- cooling the solidified melt to below about 200 °C and separate the formed silicon lumps from the disintegrated slag. If the slag system Si02 - CaO consists of only these two slag forming compounds, we have from the phase diagram shown in Figure 2 that the amounts of Si02 and CaO should be such that the weight% Si02 is within the range from 43 to 65 weight%. However, addition of additional slag compounds such as i.e. CaF2 which lowers the liquidus temperature, optionally in combination with operation at higher temperatures, allows use of the slag system Si02 - CaO with Si02 contents outside this range from 43 to 65 weight% Si02-
There are a number of advantageous properties of the slag system Si02 - CaO which makes the system suitable for use in the present invention. One advantageous property is that the liquid slag is denser than liquid silicon. From the literature [2, 3, 4] it may be found that the density (in kg/m3) of the slag system Si02 - CaO as function of temperature and mole fractions may be given as: pslag = (1786 + 0.224Γ)χ &θ2 + (3253 - 0.0657 CaO
This formula gives that the density of the slag system Si02 - CaO will range from 2526 kg/m3 to 2758 kg/m3 with temperatures ranging from 1550 to 1650 °C when the weight% Si02 ranges from 43 to 63. From [1] it is known that the density of molten silicon at this temperature range is from 2446 to 2481 kg/m . Thus the slag system according to the invention has the advantage that the small droplets which adhere to and are transported by the flotation gas bubbles will agglomerate to lumps of molten silicon which will remain floating at the surface layer of the molten slag due to the density difference.
The viscosity of the slag system Si02 - CaO is investigated by [5] and found to be from about 0.2 to 1.4 Ns/m2 for mixtures with a Si02-content ranging from 43 to 60 weight% at temperatures from 1500 to 1600 °C. See figure 3. Thus the viscosity of the slag system is less than 2 Ns/m and may be employed as such in the method according to the first and second aspect of the invention without use of viscosity reducing additives. However, one or more viscosity reducing additives may advantageously be added to the Si02 - CaO slag system.
The wetting properties of the Si02 - CaO slag system towards molten silicon is calculated by [6] and found to form contact angles at from about 87 ° when the Si02 is present at 44 weight% and up to about 101 ° when the Si02 is present at 63 weight%. See figure 4. These calculations show that the Si02 - CaO slag system has very low wetting towards molten silicon, with contact angles just below or above 90 ° which is considered to be non-wetting, and thus will function well in the method according to the first and second aspect of the invention.
Also, calculations made by [6] of the wetting of the Si02 - CaO slag system towards SiC show that the slag system has a significantly better wetting towards SiC than Si. The calculated contact angles are given in Figure 5 and are in the range from about 68 ° when the Si02 is present at 44 weight% and up to about 85 ° when the Si02 is present at 63 weight%. However, in experimental work by the present inventors, the measured wetting angles have been substantially lower than the calculated but with the same trends and differences with regard to Si02 content in the slag and the difference in wettability between Si and slag. Thus the invention according to the first and second aspect is found to function with wetting angles covering a broad range. The wetting properties of the slag system make the slag selective towards Si and SiC such that it will retain SiC-particles stronger than it does towards Si- droplets. Contact between SiC and molten Si and hence transfer of contaminants from SiC to Si is then reduced. The gas bubbles will be more likely to capture and transport silicon droplets out of the bulk liquid slag phase when they do not contain solid SiC grains. This is a huge advantage in that it opens the possibility of employing the Si02 - CaO slag system to recover silicon from kerf resulting from sawing of silicon wafers in the photovoltaic industry, and thus provide a solution to a long standing problem in this industry - how to recover the approx. 50 % of the solar grade silicon feedstock being lost as kerf remains.
List of figures
Figure 1 is a drawing showing a contact angle between a liquid droplet placed on a solid substrate.
Figure 2 is the phase diagram for the Si02 - CaO slag system.
Figure 3 shows the viscosity of the slag system Si02 - CaO with a Si02-content ranging from 43 to 60 weight% at temperatures from 1500 to 1600 °C as determined by [5].
Figure 4 shows wetting angles calculated by [6] between the Si02 - CaO slag system and silicon for compositions with Si02 present from 43 weight% up to 63 weight%.
Figure 5 shows wetting angles calculated by [6] between the Si02 - CaO slag system and SiC for compositions with Si02 present from 43 weight% up to 63 weight%.
Figure 6 shows photographs of the crucible, melt and silicon-lumps resulting by applying the invention on kerf from diamond sawing of silicon wafers.
Figure 7 shows photographs of the silicon-lumps resulting by applying the invention on kerf from SiC-slurry band sawing of silicon wafers.
Example embodiments of the invention
The invention will be described in further detail by way of example embodiments, these should not be interpreted as a limitation of the general idea of recovering fine particulate silicon metal by melting the silicon powder in a slag which wets the silicon poorly and then collect and agglomerate them into relatively large lumps of silicon by use of flotation with an inert gas.
First example embodiment
Approximately 285 g of kerf remains, from cutting of photovoltaic silicon wafers by diamond based wire or band saws, was mixed with 445 g Si02, 638 g CaO and 57 g CaF2 (resulting in 1140 g slag with composition of 39 weight% Si02, 56 weight% CaO and 5 weight% CaF2) and packed in a semiclosed graphite crucible. The graphite crucible was made of a commercially available graphite "ISEM 3" delivered by Toyo Tanso Co. Ltd. In the crucible bottom there was placed a plug of porous graphite "EG 92", also delivered by Toyo Tanso Co. Ltd.
The crucible with the mixture or slag forming compounds and kerf remains was placed in an induction furnace. The crucible and content was heated up to a temperature of 1600 °C. The temperature was maintained at this level and argon gas was flushed trough the porous plug in the crucible bottom for a period of 3 hours, then the crucible and melt was allowed to cool to a temperature of about 490 °C by turning off the heating of the furnace. When the temperature reached about 490 °C, the heating system of the furnace was turned on to maintain the temperature at this level for about 15 minutes, and then the heating was turned off again to allow the crucible with content to cool to room temperature.
The slag system 39 weight% Si02, 56 weight% CaO and 5 weight% CaF2 has a density of 2600 - 2700 kg/m and a viscosity of about 0.2 Ns/m in the temperature range 1550 - 1600 °C. The slag system is almost non- wetting towards silicon with a contact angle of 85 °, and shows a significantly higher wetting towards SiC with contact angles of 15 - 55 °C.
Figure 6 shows photographs of the crucible cut in half after a test run with the same conditions as above, except that the slag/crucible was cooled rapidly down to room temperature such that the slag did not disintegrate. In photograph a) it is seen the walls and bottom of crucible 10 as a dark shading. The porous plug 20 is placed in the bottom. Inside the crucible 10, the slag 30 is seen to have three almost spherical lumps of silicon metal 40 with sizes of 1 - 2 cm diameter just below the upper surface 50. This section of the melt is enlarged in photograph b). Photograph c) is a micrograph of the metal surface of one silicon lump, and shows that it consists mainly of a pure silicon phase and an Si-Fe-phase at grain boundaries. An analysis of the Si-phase gave the composition in part per million weight (ppmw) shown in Table 1. The table shows that the problematic elements for photovoltaic
applications, P and B, are present in the silicon at levels acceptable for use as feedstock for the photovoltaic industry. There are metallic impurities which are present at too high levels, i.e. Fe, Ca and Ti, but these may be removed by conventional metallurgical refining of silicon. Thus the example embodiment verifies that a method for recovering kerf losses from the photovoltaic industry and reuse the metal as feedstock is obtained.
Table 1 Chemical analysis of Si-lump from re-melting of kerf from diamond saw
Figure imgf000013_0001
Second example embodiment
Kerf from cutting of photovoltaic silicon wafers by use of band saws with SiC as the abrasive was collected and allowed to settle to separate glycol/water from solid residues (mainly Si- and SiC-grains) and dried to form a powder. Then the powder mixture was classified to take out the main portion of the SiC-grains and to form a powder mixture with approx. 65 weight% Si.
Approximately 285 g of this powder was mixed with 445 g Si02, 638 g CaO and 57 g CaF2 (resulting in 1140 g slag with composition of 39 weight% Si02, 56 weight% CaO and 5 weight% CaF2) and packed a closed graphite crucible. The graphite crucible was made of a commercially available graphite "ISEM 3" delivered by Toyo Tanso Co. Ltd.
The remaining test conditions were exactly as given in the first example
embodiment. Figure 7a shows a photograph of the resulting pearls and lumps of Si after separation from the slag. Photograph 7b is a micrograph showing a similar metallic structure as photograph 6c. The total weight of the silicon pearls/lumps was 185 g, giving a silicon yield of 64 %. The chemical composition of the silicon phase is given in Table 2.
Table 2 Chemical analysis of Si-lump from re-melting of kerf from SiC band saw
Element B P Fe Cu Zn Al Mg Ca Ti V Cr Mn Co
Amount 0.88 29 950 220 0.48 220 350 18000 170 50 12 350 25 [ppmw] References
S. Kimura and K. Terashima, A review of measurement of thermophysical properties of silicon melt, Journal of Crystal Growth, 1997. 180(3-4): p. 323-333 K. Ono, K. Gunji, and T. Araki, Nippon Kinzoku Gakhaishi, 1969. 33(3): p. 299 S. I. Popel and Y. O. Esin, Journal of Applied Chemistry of the USSR, 1956. 29: 707
J. W. Tomlinson, M. S. R. Heynes, and J. O. M. Bockris, The structure of liquid silicates. Part 2.— Molar volumes and expansivities, Transactions of the Faraday Society, 1958. 54: p. 1822 - 1833
K. Tang and M. Tangstad, Modeling Viscosities of Ferromanganese Slags, in INFACON 11. 2007: India.
Memo by Kai Tang

Claims

1. A method of refining and converting particulate silicon to silicon lumps, where the method comprises the following steps:
- mixing the particulate silicon with at least one slag forming compound in a weight ratio, particulate silicon:slag forming compound(s), from 1:10 to 3:1 in a crucible, wherein the at least one slag forming compound will form a liquid slag at an operating temperature above the liquidus temperature of silicon, and which has a viscosity lower than 2 Ns/m2 and a wetting angle towards silicon of at least 20 °,
- heating the mixture of particulate silicon and slag forming component(s) to a temperature above the liquidus temperature of the slag,
- injecting a stream of gas through at least one point of the bottom of the crucible and maintain the gas flushing for a period of time,
- solidifying and cooling the molten mixture , and
- separating formed silicon agglomerates/lumps from the slag.
2. A method according to claim 1, wherein the slag forming compounds are one or more of Si02, CaO, Li20, MgO, A1203, NazO, CaF2, NaF, SrO, BaO, MgF2, K20, CaC03, Na2C03, and MgC03.
3. A method according to claim 2, wherein
- the slag forming compounds are a mixture of CaO and Si02,
- the mixture of slag forming compounds and particulate silicon is heated to a temperature in the from 1450 to 1650 °C to form a melt,
- the melt after gas injection is solidified and cooled to a temperature of about
490 °C, optionally maintaining this temperature for a period of time ranging from 5 minutes to 5 hours, and then
- cooling the solidified melt to below about 200 °C and separating the formed silicon lumps from the disintegrated slag.
4. A method according to claim 3, wherein the slag forming compounds also includes addition of CaF2.
5. A method according to claim 3, wherein the slag forming mixture consists of 39 weight% Si02, 56 weight% CaO and 5 weight% CaF2.
6. A method according to claim 3, wherein the weight% Si02 of the slag forming mixture is within the range from 43 to 65 weight%.
7. A method according to any of the above claims, wherein the injection gas is chosen among the noble gases and hydrogen gas, or a mixture of two or more of these gases.
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