WO2017072655A1 - Procédé pour l'enrichissement et la séparation de cristaux de silicium contenus dans un métal en fusion destiné à la purification du silicium - Google Patents

Procédé pour l'enrichissement et la séparation de cristaux de silicium contenus dans un métal en fusion destiné à la purification du silicium Download PDF

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WO2017072655A1
WO2017072655A1 PCT/IB2016/056404 IB2016056404W WO2017072655A1 WO 2017072655 A1 WO2017072655 A1 WO 2017072655A1 IB 2016056404 W IB2016056404 W IB 2016056404W WO 2017072655 A1 WO2017072655 A1 WO 2017072655A1
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crystals
molten metal
temperature
molten
salt
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PCT/IB2016/056404
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English (en)
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György 55 % DR. KAPTAY
József 30 % SZABÓ
Gergely Bálint 15 % TÓTH
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Bay Zoltán Alkalmazott Kutatási Közhasznú Nonprofit Kft. Mérnöki Divízió
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Priority to EP16816738.5A priority Critical patent/EP3368477A1/fr
Publication of WO2017072655A1 publication Critical patent/WO2017072655A1/fr

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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
    • 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 invention relates to a method for the enrichment and separation of silicon (Si) crystals from a molten metal for the purification of silicon, for which contaminated (metallurgical-grade) Si is dissolved in a suitable molten metal, preferably molten aluminium at a relatively high temperature, then the silicon crystals precipitated, recrystallized in a purer form upon cooling are in part or in full separated from the molten metal in a suitable manner. Finally, the surface of the purified Si crystals is cleaned mechanically and with a suitable solvent, while the remaining Al-Si alloy can be reused. From the purified and enriched silicon, by known post-treatment steps, solar grade silicon can be obtained at a lower cost.
  • the production of polycrystalline Si with the Siemens method includes the following steps: the conversion of MG-Si into a volatile Si compound, e.g. S1HCI 3 , the purification thereof by fractional distillation, from the decomposition of the Si-compound SeG-Si is obtained, and the by-products are recycled.
  • the most common method of the electronics industry for the production of monocrystalline Si is the Czochralski method (B. Ceccaroli, O. Lohne: Solar grade silicon feedstock; Handbook of Photovoltaic Science and Engineering, A. Luque, S.
  • K 2 SiF 6 are used as an Si-source, or a mixture of chlorides and oxides is applied during the electrolysis
  • the working temperature of the slag phase can be influenced by adding salts, for example the addition of CaF 2 to a CaO or CaO-Si0 2 system, or Na 2 C0 3 to a slag forming Si0 2 has been previously studied.
  • a promising group of methods is solvent refining, in which MG-Si is dissolved in a suitable solvent from which Si can be recrystallized to a purer form (M.D. Johnston, L.T. Khajavi, M. Li, S. Sokhanvaran, M. Barati: High-temperature refining of metallurgical-grade silicon: a review; JOM, 2012, vol. 64, 935-945). If the solvent is selected properly, then during this process most of the elements contaminating the original MG-Si remain in the solvent, resulting in the purification of Si.
  • the solvents are typically molten metals, from which aluminium (Al), iron (Fe), copper (Cu), tin (Sn), etc. are suitable for the process (J.
  • the degree of purification of Si depends on the properties of the solvent-molten metal.
  • one of the most promising solvents is molten Al.
  • MG-Si is dissolved in molten Al at a higher temperature, then the solubility of Si is reduced by decreasing the temperature in a controlled manner.
  • Al-Si based melt supersaturated with Si is formed, from which, upon cooling, Si crystals precipitate that are significantly purer than the starting MG-Si.
  • Yoshikawa, . Morita An evolving method for solar-grade silicon production: solvent refining; JOM, 2012, vol. 64, 946-951; M.D. Johnston, L.T. Khajavi, M. Li, S. Sokhanvaran, M. Barati: High-temperature refining of metallurgical-grade silicon: a review; JOM, 2012, vol. 64, 935-945) the purified Si can be separated from the metal matrix by acid leaching, however, this process generates significant amounts of acid waste solutions containing light and heavy metals.
  • the aim of the invention is to eliminate the disadvantages of the known solution using a molten metal solvent, and to develop a new, improved method capable of efficiently separating the purified Si crystals precipitated from the molten metal upon cooling from the molten metal in such a way that the purified Si crystals become readily accessible at the end of the process, and thus solar grade silicon can be produced at a low cost.
  • the key element of this method is that a salt of suitably selected composition (or in a certain ratio slag) is added to a molten metal or molten metal alloy suitable for the purification of Si, which melts at the temperature of the process and functions as a melt in such a way that it pulls the Si crystals precipitated from the molten metal upon cooling through the molten metal/molten salt interface into the inside of the molten salt at least in part, and separates them from the inside of the molten metal.
  • the invention is furthermore based on the recognition that if relatively large Si crystals are formed and the density of the molten salt is lower than the density of the Si crystals and the molten metal, then in a static case the separation of the Si crystals from the molten metal is not perfect, as although the larger portion of each Si crystal reaching the surface passes to the molten salt, however, for reasons of gravity their smaller portion adheres to the molten metal. Then preferably the system is cooled, the salt is removed mechanically and with a suitable solvent, and the Si crystals protruding from the surface of the solidified metal are recovered by dissolving the surface layer of the solidified metal with a suitable solvent.
  • the invention is furthermore based on the recognition that the previous method can be made more efficient if the molten metal/Si/molten salt system is mixed/dispersed, and by this the passage of the Si crystals from the molten metal to the molten salt is accelerated, which is an advantage especially when a larger volume of molten metal is used.
  • the invention is furthermore based on the recognition that in the case of mixing and dispersion the efficiency of the separation of the Si crystals and the molten metal can be increased if the density of the molten salt is lower than the density of the molten metal and the Si crystals, as with the reduction of the mixing speed the larger molten metal droplets sediment more quickly than the smaller Si crystals, and the molten salt temporarily rich in Si crystals can be poured from the surface of the molten metal before the sedimentation of the Si crystals, from which after solidification the Si crystals can be readily removed mechanically and with a suitable solvent, and the material and heat energy of the remaining molten metal can be reused in full.
  • the invention is furthermore based on the recognition that the Si crystals precipitated from the molten metal can be floated from the system by introducing small bubbles of an inert gas into the lower part of the molten metal, on the surface of which the Si crystals gather and together with the bubbles rise to the top of the molten metal, then to the top of the molten salt, from where the purified Si crystals can be skimmed off in the form of a foam or scum.
  • the Si crystals can be readily removed from the solidified salt in part mechanically, and in part with a suitable solvent, and the material and heat energy of the remaining molten metal can be reused.
  • the invention is furthermore based on the recognition that the method can be made even more efficient by selecting the density of the molten metal and the molten salt in such a way that the densities of the phases are characterized by the following relations: the density of the Si is the lowest, the density of the molten salt is medium, and the density of the molten metal is the highest.
  • the buoyant force will also help the separation of the Si and the molten metal (molten metal at the bottom, molten salt at the top), the Si crystals precipitated upon cooling are first driven to the top of the molten metal by the buoyant force, then they are pulled through the molten metal/molten salt interface by the molten salt, then they are driven to the top of that as well, and finally the purified Si crystals can be skimmed off from the top of the molten salt.
  • the Si crystals can be readily removed from the solidified salt in part mechanically, and in part with a suitable solvent, and the material and heat energy of the remaining molten metal can be reused.
  • the invention is furthermore based on the recognition that the efficiency of the separation of the Si crystals precipitated from the molten metal upon cooling can be increased and the specific metal consumption can be reduced by mechanically filtering out the Si crystals with a suitable ceramic filter.
  • the invention relates to a method for the enrichment and separation of silicon crystals from a molten metal, in the first step of which the contaminated Si is dissolved in a molten metal of suitable composition at a relatively high temperature, preferably in a furnace, in the second step upon controlled cooling a part of the dissolved Si crystallizes from the molten metal, in the third step by means of a suitable molten salt (that may be present in the first and second steps as well, but then its presence is not necessary) the Si crystals are transferred into the molten salt, in the fourth step the Si crystals are possibly concentrated in as small a volume of the molten salt as possible (this is not a process step, it is required only for economic reasons), in the fifth step the molten salt rich in Si crystals is removed (skimmed off or poured) from the molten metal and the furnace, and finally the molten salt rich in Si crystals is cooled, and after the solidification of the salt the adherent salt is cleaned from the surface of the Si crystals
  • the contaminated Si is dissolved in such an amount and at such a relatively high temperature in such a metal alloy/molten metal, firstly, which is stable in the molten state under the conditions of the process, secondly, which allows the complete dissolution of the added Si, thirdly, from which primary Si crystals precipitate upon cooling, fourthly, which is preferred by the elements contaminating the original Si over the Si crystals and thus allows the purification of the Si from these impurities, fifthly, which helps the floatation of the Si crystals to the molten metal/molten salt interface and from there the passage of the Si crystals into the molten salt phase, sixthly, which after the removal of the Si crystals, in the molten state, can be used for the production of other products, therefore both the material and the thermal energy can be reused.
  • such a molten salt is selected for the implementation of the method, firstly, which is stable in the molten state under the conditions of the process, secondly, which does not react with either the molten metal selected above or the Si crystals, thirdly, which deoxidizes the molten metal/molten salt interface, fourthly, which allows the spontaneous passage of the Si crystals from the molten metal to the molten salt through the molten metal/molten salt interface, fifthly, which helps the best possible separation of the Si crystals from the molten metal, sixthly, which can be separated/washed from the purified Si crystals with minimal cost and environmental impact, seventhly, the remaining part of which can be recycled into the process the most times possible, eightly, the replacement of which has the least possible environmental impact.
  • the crucible or the inner wall of the furnace (and all other equipment required during the process, such as the mixer or the equipment suitable for measuring the salt rich in Si) is made of (or covered with) such a material, firstly, which is stable in the solid state under the conditions of the process, secondly, which does not react chemically with either the molten metal or the molten salt and/or slag, thirdly, which does not contaminate the Si crystals, fourthly, which is sufficiently durable to allow the economically viable implementation of the Si purification, fifthly, the replacement of which has the least possible environmental impact.
  • the process is performed under such a gas atmosphere, firstly, which does not react chemically with either the crucible, the molten metal, or the molten salt, secondly, which does not contaminate the Si crystals, thirdly, the use of which has the least possible financial and environmental impact.
  • such a solvent is used for the secondary cleaning (following mechanical cleaning) of the Si crystals from the solidified salt and/or metal adhering to them, firstly, which well and quickly dissolves the salt and/or metal used in the process, secondly, which does not contaminate the Si crystals, thirdly, the use of which has the least possible financial and environmental impact. It is advisable to use sonication to accelerate the dissolution.
  • a ceramic filter made of such a material and with such a pore volume is used for the concentration of the Si crystals precipitated from the Al-Si melt, which does not contaminate either the Al-Si melt or the Si crystals, which is suitable for filtering out most of the precipitated Si crystals, which can be readily separated from the suspension rich in Si crystals and therefore can be reused, and the replacement of which has a minimal environmental impact.
  • the contaminated (from primary metallurgical processes, and/or waste) silicon is dissolved at a relatively high temperature (Tl) in a relatively high concentration (CI) in a molten metal.
  • Tl relatively high temperature
  • CI relatively high concentration
  • the system is cooled at a suitable cooling rate, and the process is completed at a lower temperature (T2) and a lower Si concentration (C2) of the molten metal.
  • T2 relatively high temperature
  • C2 lower Si concentration
  • a greater (C1-C2) difference typically requires a greater (T1-T2) difference, although the relationship between the two is not linear. According to the above, therefore the (T1-T2) difference also has an optimum value.
  • Tl is limited in part by the melting point of Si, in part by economic considerations, and in part by the stability of the molten metal, the molten salt, the ceramic filter and the crucible. Increasing Tl beyond the limit decreases the degree of Si purification.
  • T2 Reducing T2 increases the degree of Si purification, but it is limited by the fact that each particular composition of the primary Si-free alloy has an eutectic temperature T2* at which, in addition to the Si crystals, other crystals also precipitate from the molten metal. As it is advisable to avoid that in order to keep the Si crystals pure, it is advisable to keep the value of T2 above T2* during the process.
  • T2 shall not be lower than the liquidus temperature of the molten salt (if a molten salt is also used) - therefore optimally the melting point of the molten salt shall be adjusted (through its composition) to a temperature below T2*, or at least below temperature T2.
  • temperature Tl shall be between the temperature of the metal-Si eutectic closest to pure Si and the melting point of Si, while temperature T2 shall be lower than temperature Tl, but higher than the eutectic temperature T2* closest to pure Si.
  • the amount of purified Si crystals that can be extracted per unit of a primary Si-free molten metal of a particular composition at particular parameters Tl, T2, C 1 and C2 has a theoretical maximum.
  • the Si extraction efficiency shall be measured by comparison to this theoretical maximum.
  • molten salts of different compositions can provide different extraction efficiencies.
  • the extraction efficiency can be increased, but this function reaches saturation: excessively increasing the specific amount of molten salt does not increase significantly the extraction efficiency, but increases the incidental costs, therefore the specific amount of molten salt used per unit of molten metal has an optimum value.
  • the higher the cooling rate used during the process the more productive the process.
  • increasing the cooling rate can lead to increasing molten metal inclusions in the growing Si crystals, contaminating the formed Si crystals. Therefore, the cooling rate used during the process also has an optimum value.
  • the purer the used molten metal and molten salt that is the lower the initial concentration of elements critical from the point of view of Si purification in them
  • the higher the degree of Si purification the higher the degree of Si purification.
  • this relationship is not linear, therefore the use of a purer and purer molten metal and molten salt increases less and less the Si purification efficiency.
  • the cost of a purer molten metal and molten salt is higher, therefore the purity of the used molten metal and molten salt also has an optimum value.
  • the metal and salt (or their melts) remaining after the process can be reused in another Si purification cycle.
  • the degree of Si purification gradually decreases, but the cost of the process also decreases, therefore the number of reuse cycles also has an optimum value.
  • the contamination level of the molten metal and molten salt no longer reusable for the purification of Si is so low that they have a good chance of being usable in other processes.
  • the remaining, slightly contaminated Al-Si melt is suitable, for example, for the production of Al-Si castings.
  • the process embodiment of the invention depends on the density ratio of the phases.
  • three types of processes are used:
  • the common characteristic of type 1 processes is that the density of the molten salt is lower than the density of the molten metal and the Si crystals (the density ratio of Si and the molten metal is insignificant). Then for reasons of gravity the molten metal is at the bottom and the molten salt is at the top, and the Si crystals transferred into the molten salt are, at least in part, pulled back by gravity into the molten metal over time.
  • the type 1 processes have three versions.
  • the type 1A process only a molten salt is added on top of the molten metal, typically no other action (either mixing or bubbling) is used. Then the Si crystals precipitated from the molten metal are enriched at the molten metal/molten salt interface. After cooling the system, the Si crystals enriched at this interface are recovered using a small amount of solvent, with a relatively low loss of solvent and metal. Then the remaining metal (with the Si remaining in it) can be reused. The yield of the process can be improved by slow mixing.
  • the molten metal and molten salt (after the precipitation of the Si, at temperature T2) are mixed/dispersed with a suitable mixer at a suitably high speed, by this the passage of the Si crystals to the molten salt is accelerated and implemented, then by suitably reducing the mixing speed (and raising the mixer) the molten metal is left to sediment, but before the sedimentation of the Si crystals the molten salt rich in Si crystals is poured from the surface of the molten metal. After solidification the salt is removed from the Si crystals mechanically and with a suitable solvent. Then all the molten metal remains and can be reused.
  • an inert gas is introduced into the molten metal from below, with the smallest possible bubble size, and the Si crystals are floated.
  • the Si crystals will be enriched on the surface of the bubbles in such a way that the average density of the bubbles and the Si crystals adhering to them is lower than the density of the molten salt.
  • the solidified Si crystals can be separated from the salt mechanically and with a suitable solvent. The remaining molten metal and molten salt can be reused.
  • the common characteristic of type 2 processes is that from the phases the density of the Si crystals is the lowest, the density of the molten salt is medium, while the density of the molten metal is the highest. Then the molten metal is at the bottom and the molten salt is at the top.
  • the Si crystals precipitated in the molten metal upon cooling are driven to the molten metal/molten salt interface by the buoyant force, where the suitably selected molten salt pulls them from the molten metal, then the Si crystals are driven to the top of the molten salt by the buoyant force, and from there they can be removed together with the upper part of the molten salt.
  • the solidified Si crystals can be separated from the salt mechanically and with a suitable solvent. The remaining molten metal and molten salt can be reused.
  • a ceramic filter is used instead of the molten salt, by filtering the Al-Si (liquid)/Si (solid) suspension, the precipitated Si crystals are significantly enriched in the part blocked by the ceramic filter, while the melt passing through the filter contains practically no Si crystals.
  • the latter part can be reused, while the purified Si crystals can be leached from the part blocked by the filter in an economically viable manner.
  • the type 1A, IB, 1C, 2 and 3 processes described above can be combined with one another.
  • the material of the Si-free molten metal can be Al, Ca-, Cu-, Fe-, In-, Mg-, Ni-, Sb-, Sn- Zn, preferably, aluminium is used.
  • Al with a purity of e.g. 99.7 wt%, produced by primary metallurgical processes can be used, but a higher degree of Si purification can be achieved by using purer Al.
  • the advantage of dissolving Si in aluminium is that no intermetallic phase is formed, and it can be managed at a relatively low temperature.
  • the molten salt is preferably a mixture of sodium chloride (NaCl), potassium chloride (KC1) and sodium fluoride (NaF), but alkali metal halides of other composition and their mixture can also be used, such as for example a mixture of potassium chloride (KC1), potassium fluoride (KF) and potassium hexasilicofluoride (K ⁇ SiFg).
  • KC1 potassium chloride
  • KF potassium fluoride
  • K ⁇ SiFg potassium hexasilicofluoride
  • the material of the crucible is preferably corundum, although crucibles made of other materials can also be used.
  • the material of the gas (and if needed, the bubbles) in the simplest case is preferably air, but for higher purity it is advisable to use inert gases (e.g. argon).
  • inert gases e.g. argon
  • water, or an aqueous solution preferably with dissolved aluminium chloride: A1C1 3
  • a concentrated acid preferably hydrochloric acid (10 to 37 wt% HC1), sulphuric acid (40 to 98 wt% H 2 S0 4 ), nitric acid (20 to 63 wt HNO 3 ), hydrofluoric acid (10 to 48 wt% HF), or a mixture of these is used for partially dissolving the Al.
  • hydrochloric acid 10 to 37 wt% HC1
  • sulphuric acid 40 to 98 wt% H 2 S0 4
  • nitric acid 20 to 63 wt HNO 3
  • hydrofluoric acid 10 to 48 wt% HF
  • the system is cooled gradually, the purified Si crystals can be removed from the interface of the solidified Al and salt mechanically, and by dissolving a small amount of salt and Al. In this case most of the Al-Si remains and can be reused either in this or in other processes.
  • the type IB process mixing is applied at temperature T2.
  • This can be external (electromagnetic) mixing, but a mechanical mixer can also be used.
  • the volume of the molten salt shall be larger than the volume of the molten metal, as only then can the dispersion of the molten metal droplets in the molten salt be achieved.
  • Mixing shall be performed by lowering the mixer below the molten metal/molten salt level, at a high mixing speed. Then molten metal droplets are dispersed in the molten salt, as a result of which the specific surface area of the molten metal increases, through which the Si crystals quickly and fully pass from the molten metal into the molten salt.
  • the approximately spherical molten metal droplets sediment relatively quickly, while the smaller and non-spherical Si crystals sediment only more slowly, thus after the sedimentation of the molten Al the molten salt containing the Si crystals can be poured from the system.
  • the solidified salt can be removed from the Si crystals mechanically and by leaching with an aqueous solution, the efficiency of the latter can be increased by sonication. Then the material and heat of the remaining Al-Si melt can be reused in full either in this or in other processes.
  • small gas bubbles are introduced at the bottom of the molten Al at temperature T2.
  • the bubbles Due to the buoyant force the bubbles start to rise, together with the Si crystals adhering to their surface. Finally the bubbles raise (float) the Si crystals to the top of the molten metal and the molten salt, from where they can be skimmed off.
  • the solidified salt can be removed from the Si crystals mechanically and by leaching with an aqueous solution, the efficiency of the latter can be increased by sonication. Then the material and heat of the remaining Al-Si melt can be reused in full either in this or in other processes.
  • the density of the Al-based melt shall be increased, secondly a molten salt of higher density shall be selected.
  • a copper (Cu) additive is used to increase the density of the Al-based melt, and thus the Si-free molten metal becomes an Al-Cu alloy.
  • Sodium iodide (Nal) is used as the main component of the molten salt, in which a small amount of sodium fluoride (NaF), and cryolite (Na3AlFe) is dissolved.
  • the purified Si crystals are driven to the top of the molten metal and the molten salt by the buoyant force, and from there they can be removed together with a small amount molten salt, then the salt can be cleaned from the Si crystals mechanically and with a suitable aqueous solution. Then the material and heat of the remaining Al-Cu-Si melt can be reused in full either in this or in other processes.
  • the Si crystals precipitated at temperature T2 are in part or in full filtered from the melt rich in Al by means of a ceramic filter.
  • the ceramic filter can be the same as the one used nowadays by aluminium foundries for the filtration/purification of the melt. Its material is preferably aluminium oxide (corundum), or an appropriately surface treated version thereof, with a pore size between 10 micrometers and 1 mm. Filtration is assisted by gravity, as well as by any pressure difference created on the two sides of the filter or the melt.
  • the process can be implemented in a batch or a continuous manner.
  • Figure 1 is a binary phase diagram of Al-Si, showing temperatures Tl and T2 (and T2*), and concentrations C 1 and C2,
  • Figure 2 is a schematic representation of the two steps of the type 1A process
  • Figure 3 is a schematic representation of the five steps of the type IB process
  • Figure 4 is a schematic representation of the three steps of the type 1C process
  • Figure 5 is a schematic representation of the three steps the of the type 2 process
  • Figure 6 is a schematic representation of the type 3 process
  • Figure 7 is an image of the cut Al-28Si experimental product according to Example 1
  • Figure 8 is an image of the end product according to the type 1A process
  • Figure 9 is an image of the end product produced by slow mixing during cooling according to the type 1A process
  • Figures 10a, 10b and 10c show the result of the type IB process, where 10a shows: Al-rich spheres bordered by Si crystals obtained in the salt matrix during the emulsification process, 10b shows: Si crystals obtained after the dissolution of the salt, and 10c shows: a piece of the Al-Si alloy stuck in the crucible, with many Si crystals protruding from its surface.
  • Figures 11a and l ib show the end product of the result of the type 1C process, where 11a shows: a specimen prepared by floatation, with Si crystals protruding from its lateral surface, and 1 lb shows: a part separated from it, rich in Si crystals.
  • Figure 12 shows a polished cross-section of the specimen prepared by means of the buoyant force according to the type 2 process
  • Figures 13a and 13b show the result of the type 3 process, where 13a shows: an image of the specimen stuck to the ceramic filter and 13b shows: a polished cross-section of the specimen passing through the filter.
  • the starting material for the experiments is a hypereutectic Al-Si alloy, which is either produced in a preliminary step, or the required amount of MG-Si is dissolved in the molten Al above the liquidus temperature in the first step of the experiment, and afterwards the produced starting material is treated according to the particular additional process.
  • the material of the crucible is corundum.
  • the starting alloy used for the experiments was Al-28Si, with a temperature Tl of 800 °C, and a temperature T2 of about 600 °C, making it a low temperature process among the Si purification processes.
  • FIG 1 shows the Al-Si equilibrium phase diagram [ASM-93]: a schematic representation of the principle of the purification of Si by means of molten Al.
  • Tl high temperature
  • T2 lower temperature
  • T2* eutectic temperature
  • T2* 577 °C
  • the Al-Cu-Si phase diagram applies, based on a similar principle, but with different details.
  • the system is cooled to room temperature TO (not indicated in the Figure), and although this results in the precipitation of almost the total Si content, only a part thereof can be dissolved from the metal/salt interface, because the other part crystallizes eutectically in the total volume.
  • This process is the purification process of the contaminated Si, as most of the impurities of Si remain in the Al-rich melt.
  • maximum 17.6 wt% primary Si crystals can crystallize from the Al-28Si melt.
  • temperature T2 600 °C was selected as the end of the cooling phase; then between 800 and 600 °C, in principle, 16.3 wt% Si crystallizes from the Al-28Si base melt.
  • the cooling rate was about 1 °C/minute.
  • Negative example 1 distribution of the recrystallized Si in the Al-28Si alloy
  • the Si crystals are more preferably enriched on the surface of the molten Al, or in the molten salt or the filter.
  • Example 2 enrichment of the recrystallized silicon by means of a molten salt
  • Figure 2 shows a schematic representation of the type 1A process.
  • the Al-28Si alloy and a NaCl-KCl mixture of equimolar composition containing 10 wt% of NaF was melted in an aluminium oxide crucible (the density of the molten salt was lower than the density of the molten aluminium and the solid silicon).
  • the system was maintained at a temperature of 850 °C for about 30 minutes, then it was crystallized by cooling it below the liquidus point of the molten metal.
  • the molten metal containing the Si crystals was not mixed, furthermore the system was solidified in the crucible, without pouring it out.
  • the first step of the type IB process in a corundum crucible (a), at the bottom an Al-Si melt (d), above that a NaCl-KCl-NaF melt (c), and above that argon gas (b) is kept at a (high) temperature Tl.
  • the temperature is gradually reduced from temperature Tl to temperature T2, then purified Si crystals precipitate from the Al-Si melt (e).
  • this system is mixed with a mixer (f) in such a way that the mixer is lowered and a high mixing speed is used, then the Al-Si melt is dispersed in the molten salt, while the Si crystals pass into the molten salt.
  • the mixer In the fourth step at temperature T2 the mixer is raised and the mixing speed is reduced, as a result of this the Al-Si melt sediments at the bottom, the Si crystals dispersed in the molten salt sediment more slowly.
  • the mixer (f) is removed and most of the molten salt containing the Si crystals is poured from the molten metal.
  • a mixture of salt/Si crystals cooled to room temperature TO is obtained in a separate crucible (g), from which the Si crystals can be recovered mechanically and by aqueous leaching.
  • the remaining Al-Si melt of temperature T2 can be reused.
  • the Al-28Si melt + molten salt system was dispersed with a paddle mixer having a graphite mixing head, at a mixing speed of 800 rpm. Then both the molten salt and the molten metal were poured out, the molten metal rolled out from the crucible in the form of smaller or larger spherical droplets, the droplets were embedded in the salt matrix, as shown by the photo in Figure 10a. A part of the Si crystals recovered after dissolving the salt is shown in Figure 10b. Some Al-rich alloy remained in the crucible as well, with many Si crystals enriched on its surface, as shown in Figure 10c. Therefore the type IB process can separate the Si crystals from the remaining Al-Si melt more efficiently compared to the type 1A process.
  • Example 4 enrichment of the recrystallized silicon by floatation
  • the essence of the process is that an inert gas is introduced into a hypereutectic Al-Si melt from below, whereby the gas bubbles carry with them the Si crystals precipitated during cooling, helping their passage from the molten metal alloy to the molten salt above it.
  • the argon gas was bubbled through the system through a stainless steel pipe with an internal diameter of 2 mm, ending in a graphite capillary tube, over the temperature range from 710 to 610 °C for a total of 120 minutes, at a flow rate of about 25 - 35 cm3/min.
  • the gas bubbles rising in the melt carried most of the Si crystals to the molten metal/molten salt interface, and a smaller part of them into the molten salt.
  • Example 5 enrichment of the recrystallized silicon by means of the buoyant force
  • the essence of separation by means of the buoyant force is that the density relations of the solid Si/molten metal and molten salt system are adjusted by preferably changing the compositions in such a way that the density of the solid Si crystals is the lowest, it is exceeded by the density of the molten salt, and it is exceeded by the density of the molten metal. Then the Si crystals precipitated in the molten metal are driven upwards in the molten metal, and in part pass into the molten salt.
  • the density of the molten metal was adjusted by alloying with copper, in fact an alloy of a composition of Al (60 wt%) + Si (20 wt ) + Cu (20 wt%) was used.
  • the calculated density of the so produced alloy at 800°C is around 2.82 g/cm .
  • the composition of the salt mixture Nal (89.6 wt ) + NaF (7.9 wt%), Na 3 AlF 6 (2.5 wt ), its calculated density at 800 °C is about 2.64 g/cm 3 .
  • the density of the solid Si crystals at 800 °C is about 2.33 g/cm 3 .
  • the result is shown in Figure 12. A large number of Si crystals cover the surface of the alloy having an increased density (not only its top, but its sides as well). Many of the surfacing Si crystals extend from the alloy into the molten salt.
  • Example 6 enrichment of the recrystallized silicon by filtration
  • FIG. 6 shows the experimental arrangement for filtration.
  • the crucible used in the process is divided into two parts by a filter.
  • the material of the filter is preferably aluminium oxide (corundum), or an appropriately surface treated version of it, with a pore size between 10 ⁇ and 1 mm.
  • the cooled starting material, containing the purified Si crystals and the remaining Al-Si melt is placed in the upper part. Due to gravity, or any pressure difference created below and above the melt (below it reduced by a vacuum pump and/or above it increased by an inert gas), the Al-Si melt poor in Si passes through the pores of the filter, and can be poured/led out and reused.
  • the purified Si crystals are highly enriched in the filter, or on the top thereof, from where the Si crystals purified from impurities can be leached in an economically viable manner. This process can be implemented in a continuous manner by using a vertical temperature gradient.
  • the Si crystals dispersed in an Al-Si melt (without a molten salt) were filtered from the melt with a corundum filter, using only gravity as the driving force.
  • the filter and the alloy stuck in it are shown in Figure 13a; the Si content of this alloy was 52 ⁇ 4 wt%.
  • a polished cross-section of the Al-Si alloy passing through the filter is shown in Figure 13b; the Si content of this alloy was 17 + 2 wt%.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
  • Silicon Compounds (AREA)

Abstract

L'invention concerne un procédé pour l'enrichissement et la séparation de cristaux de silicium contenus dans un métal en fusion destiné à la purification du silicium, pour lequel le Si contaminé (de qualité métallurgique) est dissous dans un métal en fusion approprié, de préférence de l'aluminium en fusion à une température relativement élevée, puis les cristaux de silicium précipités, recristallisés sous une forme plus pure après refroidissement, sont en partie ou complètement séparés du métal en fusion de manière appropriée. Enfin, la surface des cristaux de silicium purifiés est nettoyée mécaniquement et avec un solvant approprié, tandis que l'alliage d'Al-Si restant peut être réutilisé. À partir du silicium purifié et enrichi, au moyen d'étapes de post-traitement connues, du silicium de qualité solaire peut être obtenu à un coût inférieur.
PCT/IB2016/056404 2015-10-29 2016-10-25 Procédé pour l'enrichissement et la séparation de cristaux de silicium contenus dans un métal en fusion destiné à la purification du silicium WO2017072655A1 (fr)

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CN108796606A (zh) * 2018-07-07 2018-11-13 孟静 太阳能级多晶硅制备装置
CN109850904A (zh) * 2018-12-28 2019-06-07 宁夏大学 利用半固态法提高合金法提纯多晶硅收率的方法
CN110228810A (zh) * 2019-07-24 2019-09-13 信阳师范学院 一种高效去除硅中硼和磷的方法
CN110304634A (zh) * 2019-07-05 2019-10-08 昆明理工大学 一种高效节能提纯工业硅的方法
CN110467185A (zh) * 2019-09-10 2019-11-19 中国科学院合肥物质科学研究院 一种硅材料除磷提纯添加剂以及提纯方法
CN110482556A (zh) * 2019-09-10 2019-11-22 中国科学院合肥物质科学研究院 一种用于硅材料低温精炼除硼的造渣剂及其使用方法
EP3643680A1 (fr) * 2018-10-23 2020-04-29 SiQAl UG (haftungsbeschränkt) Production couplée de silicium et d'alumine de grande pureté
CN113508090A (zh) * 2019-03-27 2021-10-15 瓦克化学股份公司 生产工业硅的方法
CN114778266A (zh) * 2022-04-28 2022-07-22 中国第一重型机械股份公司 一种化学分析试样高温熔融消解方法及装置
CN115650239A (zh) * 2022-09-13 2023-01-31 昆明理工大学 一种高效去除冶金级硅中杂质的方法

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CN108796606A (zh) * 2018-07-07 2018-11-13 孟静 太阳能级多晶硅制备装置
EP3643680A1 (fr) * 2018-10-23 2020-04-29 SiQAl UG (haftungsbeschränkt) Production couplée de silicium et d'alumine de grande pureté
WO2020084015A1 (fr) * 2018-10-23 2020-04-30 Siqal Ug (Haftungsbeschränkt) Production couplée de silicium et d'alumine de haute pureté
CN109850904A (zh) * 2018-12-28 2019-06-07 宁夏大学 利用半固态法提高合金法提纯多晶硅收率的方法
CN109850904B (zh) * 2018-12-28 2022-05-17 宁夏大学 利用半固态法提高合金法提纯多晶硅收率的方法
CN113508090A (zh) * 2019-03-27 2021-10-15 瓦克化学股份公司 生产工业硅的方法
CN113508090B (zh) * 2019-03-27 2024-01-12 瓦克化学股份公司 生产工业硅的方法
CN110304634A (zh) * 2019-07-05 2019-10-08 昆明理工大学 一种高效节能提纯工业硅的方法
CN110228810A (zh) * 2019-07-24 2019-09-13 信阳师范学院 一种高效去除硅中硼和磷的方法
CN110467185B (zh) * 2019-09-10 2020-11-10 中国科学院合肥物质科学研究院 一种硅材料除磷提纯添加剂以及提纯方法
CN110482556B (zh) * 2019-09-10 2020-12-08 中国科学院合肥物质科学研究院 一种用于硅材料低温精炼除硼的造渣剂及其使用方法
CN110482556A (zh) * 2019-09-10 2019-11-22 中国科学院合肥物质科学研究院 一种用于硅材料低温精炼除硼的造渣剂及其使用方法
CN110467185A (zh) * 2019-09-10 2019-11-19 中国科学院合肥物质科学研究院 一种硅材料除磷提纯添加剂以及提纯方法
CN114778266A (zh) * 2022-04-28 2022-07-22 中国第一重型机械股份公司 一种化学分析试样高温熔融消解方法及装置
CN114778266B (zh) * 2022-04-28 2024-05-07 中国第一重型机械股份公司 一种化学分析试样高温熔融消解方法及装置
CN115650239A (zh) * 2022-09-13 2023-01-31 昆明理工大学 一种高效去除冶金级硅中杂质的方法
CN115650239B (zh) * 2022-09-13 2024-04-26 昆明理工大学 一种高效去除冶金级硅中杂质的方法

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