Method for obtaining purified silicon metal

Abstract

This invention relates to a method for obtaining purified silicon metal, the method comprising the steps of: providing liquid silicon in a crystallization mold, wherein the crystallization mold includes a wall lined with a crucible; solidifying the liquid silicon by unidirectional solidification from the crucible-lined wall of the mold while stirring the liquid silicon by means of electromagnetic stirring (EMS); stopping the unidirectional solidification when a certain percentage of the liquid silicon has solidified; and pouring out excess liquid silicon; wherein the step of pouring out excess unsolidified liquid silicon comprises the steps of: inverting and tilting the crystallization mold under electromagnetic stirring (EMS); and emptying the excess unsolidified liquid silicon from the crystallization mold by turning off the electromagnetic stirring (EMS). The invention also relates to an apparatus comprising: a crystallization mold; an electromagnetic stirrer; and a pouring mechanism. Finally, the invention describes the use of the apparatus in a method for obtaining purified silicon metal.

Description

Technical Field

[0001] This invention relates to the production of solid silicon metal. Specifically, this invention relates to a method for obtaining purified solid silicon metal starting from low-quality silicon metal. Background Technology

[0002] New applications for silicon metal have typically demanded increasingly higher purity grades of raw materials. This trend began in the 1990s when there was a need to improve the uniformity and iron content of silicon metal to obtain chemical-grade silicon for the production of silicone resins. About a decade later, silicon photovoltaic (PV) technology required even higher purity silicon metal, known as polycrystalline silicon grade. Between 2005 and 2010, the scarcity of high-purity silicon attracted the most important metallurgical silicon producers to develop metallurgical processes capable of achieving the 99.9999% purity (6N) specification for solar-grade silicon. In recent years, new applications for silicon metal (anodes for cells, thermoelectric generators or coolers, advanced ceramics, etc.) have required even higher purity grades and / or custom compositions. Some of the purity grades currently available for a wide range of applications are listed below.

[0003] Metallurgical grade Si: %Fe<0.5; %Ca<0.3; %Al<0.1.

[0004] • Chemical grade Si: %Fe < 0.4%; %Ca < 0.1%; minimum < %Al < maximum.

[0005] • Polycrystalline silicon grade Si: %Fe<0.3; %Ca<0.03; minimum<%Al<maximum; ppmw P<limit; ppmw B<limit; trace elements<limit.

[0006] • Solar grade Si: %Fe<0.0001; %Ca<0.0001; %Al<0.0001; ppmw P<1; ppmw B<1; ppmw others<10.

[0007] Furthermore, society is moving towards a more circular economy. This stance necessitates new processes that allow for the recycling of non-compliant silicon, waste from other industries (i.e., solar kerf losses), and / or discarded materials. All these waste materials tend to have high levels of impurities. Therefore, it is often necessary to purify these waste materials so that they can be sold for standard or new applications. In these respects, several methods for purifying metallurgical or low-grade silicon are known in the art.

[0008] In US 4,094,731, liquid silicon is poured into a mold equipped with mechanical stirring (i.e., carbon rods) and / or gas injection to wash the exposed surfaces of the growing solid silicon crystals and prevent the top surface of the mother liquor from freezing. This allows excess liquid silicon to be removed after sufficient solid separation has been achieved. While the authors claim this method is suitable for reducing the iron (Fe) content to 1 / 20 of the initial content, their best example showed a reduction in surface Fe content from 0.48% to 0.12%. However, the average for the examples ranged from 0.53% to 0.24%, meaning the initial Fe content was reduced by less than 60%. Additionally, the formation of silicon spheres was reported, which significantly reduced process yield. Finally, the graphite rods can contaminate the liquid silicon and are expensive consumables. Furthermore, using air / oxygen can oxidize the silicon, and inert gases are also expensive.

[0009] Also known in the art are documents BR102013005759 A2 and BRPI0806050 B1, which describe a method in which a mold containing mother liquor and refined solid silicon is centrifuged to prevent dendrites in the grown solid silicon phase from solidifying and potentially trapping impurities. BR102013005759 A2 provides only a laboratory-scale example. The difficulty in implementing these inventions in an industrial environment lies in the step of rotating a heavy industrial mold filled with liquid silicon at very high speeds to prevent dendrite solidification. BRPI0806050 A2 uses graphite electrodes to ensure that the top surface remains liquid. However, said graphite electrodes are expensive consumables and may contaminate the liquid silicon.

[0010] Document US 4,747,906 discloses a process in which silicon is melted in a mold and solidification is performed on a cooling graphite rod rotating at very high speed. The purification rate achieved using this method is remarkable (a 99.9% reduction in impurities). However, the process appears to be both complex and expensive. On the one hand, the machine design is quite complex, and the operations required to solidify and remove the solid portion do not seem straightforward. On the other hand, the high consumption of inert gases, the need to maintain silicon in a liquid state for a longer period compared to other solidification solutions, and the requirement to use a new crucible for each batch of the process inevitably lead to higher costs.

[0011] Document WO 2009 / 033255A1 describes an alternative non-contact method for stirring liquid silicon in a solidification mold. According to this alternative method, liquid silicon is poured into the mold, which is then electromagnetically stirred to control thermal extraction and dendrite formation. The authors were able to achieve low contaminant carbon concentrations by avoiding the graphite consumables used for stirring. Examples show a 98% reduction in iron content. However, the method of removing excess liquid silicon from the center of the ingot is clearly cumbersome and potentially contaminated. Therefore, it seems difficult to replicate this process in an industrial environment.

[0012] In view of the above, there is a need in the art for an efficient method for purifying solid silicon that avoids the use of potentially contaminating carbon tools (i.e., for stirring) and is simple and economical to operate, making it easily implementable in an industrial environment. Summary of the Invention

[0013] The inventors have developed a process that overcomes the aforementioned difficulties in the methods described in the prior art. In this new method, liquid silicon is solidified in a crystallization mold lined with a crucible by means of non-contact stirring with electromagnetic stirring (EMS) until a specified percentage of the liquid silicon has solidified, thereby obtaining a hollow solid silicon ingot. This hollow solid silicon ingot has a higher purity grade compared to the raw silicon used as raw material. On the other hand, the excess liquid silicon is rich in impurities that were originally present in the silicon raw material.

[0014] Therefore, in a first aspect, the present invention relates to a method for obtaining purified silicon metal, the method comprising:

[0015] a) Liquid silicon is supplied in a crystallization mold having walls lined with a crucible.

[0016] b) Solidifying the liquid silicon by unidirectional solidification from the wall of a crucible-lined vessel while stirring it with electromagnetic stirring (EMS) until a certain percentage of the liquid silicon has solidified; and

[0017] c) Pour out any excess unsolidified liquid silicon.

[0018] Step (c) of pouring out excess unsolidified liquid silicon includes the following steps:

[0019] c1) Invert and tilt the crystallization mold under electromagnetic stirring (EMS);

[0020] c2) Excess unsolidified liquid silicon is drained from the crystallization mold by turning off the electromagnetic stirring (EMS).

[0021] In another aspect, the present invention also relates to an apparatus comprising:

[0022] a) Crystallization mold;

[0023] b) An electromagnetic stirrer; and

[0024] c) Discharge mechanism.

[0025] Additionally, the present invention relates to the use of the crystallization mold according to the invention in a method for obtaining purified silicon metal. Attached Figure Description

[0026] Figure 1 : A graph showing the effect of stirring on solidification distribution. C S : Concentration of solute in solid; C O : Initial concentration of solute in the liquid; k: Segregation coefficient; z: Distance in the direction of unidirectional solidification; D / V / k is a coefficient representing the initial transient boundary layer, where D (or D0) L ) is the diffusivity of impurities in the liquid, V (sometimes referred to as R in the literature) is the growth rate of the solid, and k is the segregation coefficient of the impurities.

[0027] Figure 2 Solute redistribution in the absence of solid diffusion: (a) Type 1; (b) Type 2; (c) Type 3; (d) Final segregation curves.

[0028] Figure 3 The schematic diagram of the method of the present invention depicts the following steps: (a) providing solid silicon in the furnace / crystallization mold of the present invention; (b) melting the solid silicon by means of a high-frequency generator; (c) solidifying the liquid silicon by unidirectional solidification from the wall of the mold while stirring the liquid silicon with a low-frequency generator by means of electromagnetic stirring (EMS); (d) stopping unidirectional solidification and pouring out the excess liquid silicon when a certain percentage of the liquid silicon has solidified; and (e) removing the solid silicon metal ingot from the crystallization mold.

[0029] Figure 4 The schematic diagram of the method of the present invention depicts the following steps: (a) pouring liquid silicon into the crystallization mold of the present invention; (b) while stirring the liquid silicon with a low-frequency generator by means of electromagnetic stirring (EMS), solidifying the liquid silicon by unidirectional solidification from the wall of the mold; (c) stopping unidirectional solidification and pouring out excess liquid silicon when a certain percentage of the liquid silicon has solidified; and (d) removing the solid silicon metal ingot from the crystallization mold.

[0030] Figure 5The schematic diagram of the method of the present invention depicts the following steps: (a) providing solid silicon in the furnace / crystallization mold of the present invention; (b) melting the solid silicon by means of a high-frequency generator; (c) solidifying the liquid silicon by unidirectional solidification from the wall of the mold while stirring the liquid silicon with a low-frequency generator by means of electromagnetic stirring (EMS); (d) stopping unidirectional solidification and pouring out the excess liquid silicon when a certain percentage of the liquid silicon has solidified; (e) providing additional solid silicon in the furnace / crystallization mold of the present invention; and (b') melting the additional solid silicon together with a purified solid silicon metal ingot by means of a high-frequency generator.

[0031] Figure 6: Schematic diagram of the apparatus of the present invention. (A) The figure shows the melting unit and the solidification unit, wherein the solidification unit depicts an electromagnetic stirrer, a support structure and a tilting unit, a dragging mechanism and a crucible. (B) The step of pouring liquid silicon into a crystallization mold is shown. (C) The step of solidifying liquid silicon by unidirectional solidification from the wall of the mold while stirring the liquid silicon by means of electromagnetic stirring (EMS) is shown. (D) The step of tilting the crystallization mold while the electromagnetic stirrer is still operating (power on) is shown. (E) The step of pouring out excess liquid silicon by turning off the electromagnetic stirrer (power off) is shown. (F) The step of removing a solid silicon metal ingot from the crystallization mold is shown.

[0032] Figure 7 The graph shows the relationship between the coefficient of the initial and final iron concentrations in Example 10 and the ingot production. Detailed Implementation

[0033] As explained above, the inventors have developed a process for obtaining purified metallic silicon and / or for purifying silicon metal, wherein a lower purity grade of silicon is used as the starting material. This lower purity grade of silicon is melted into liquid silicon, and then the liquid silicon is solidified in a crucible-lined crystallizing mold by non-contact stirring using electromagnetic stirring (EMS) until a specified percentage of the liquid silicon solidifies on the walls of the crucible-lined crystallizing mold. As a result, the excess unsolidified liquid silicon remaining in the center of the mold is enriched with impurities originally present in the silicon raw material, thus obtaining a higher purity grade of solid silicon hollow ingot.

[0034] Methods for obtaining purified silicon metal

[0035] In a first aspect, the present invention relates to a method for obtaining purified silicon metal, the method comprising:

[0036] a) Liquid silicon is supplied in a crystallization mold having walls lined with a crucible.

[0037] b) Solidifying the liquid silicon by unidirectional solidification from the wall of a crucible-lined vessel while stirring it with electromagnetic stirring (EMS) until a certain percentage of the liquid silicon has solidified; and

[0038] c) Pour out any excess unsolidified liquid silicon.

[0039] Step (c) of pouring out excess unsolidified liquid silicon includes the following steps:

[0040] c1) Invert and tilt the crystallization mold under electromagnetic stirring (EMS);

[0041] c2) Excess unsolidified liquid silicon is drained from the crystallization mold by turning off the electromagnetic stirring (EMS).

[0042] In the method of this invention, the step of pouring out excess unsolidified liquid silicon is performed under controlled stirring conditions. At the start of the pouring step, the excess liquid silicon is still subjected to electromagnetic stirring. This allows the excess liquid silicon to remain inside the crystallization mold, even when the crystallization mold is inverted and rotated, provided the electromagnetic stirrer remains powered on. This prevents any undesirable spillage of excess liquid silicon during the operation of inverting and rotating the crystallization mold. In the context of this invention, the inverted position of the crystallization mold corresponds to a position where the crystallization mold is rotated between 90 and 270 degrees. Therefore, in a preferred embodiment of the invention, the expression "inverting and tilting the crystallization mold" or "inverting and rotating the crystallization mold" corresponds to the action of bringing the crystallization mold to a position where the crystallization mold is rotated between 90 and 270 degrees, 135 and 225 degrees, 150 and 210 degrees, 160 and 200 degrees, or 170 and 190 degrees. In a particular embodiment, the crystallization mold is rotated by 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, or 270 degrees. Preferably, the inverted position corresponds to the position where the crystallization mold is rotated 180 degrees. This action is performed while the excess liquid silicon is still being electromagnetically stirred, so that the liquid metal is largely retained in the container in the inverted position due to the electromagnetic field. The electromagnetic field prevents any undesirable spillage of excess liquid silicon during the inverted rotation of the crystallization mold and allows for controlled release of excess liquid silicon at the precise moment when the electromagnetic stirring is turned off (i.e., disconnected). That is, when the power to the electromagnetic stirrer is turned off, the liquid silicon suddenly falls into the auxiliary mold, thus emptying the interior of the crystallization mold.

[0043] As a result of this method, the excess unsolidified liquid silicon is enriched with the impurities found in the initial liquid silicon, and at the same time, the solid silicon metal ingots exhibit a higher purity level compared to the initial liquid silicon.

[0044] As will be apparent to those skilled in the art, the method of the present invention for obtaining purified silicon metal is equally suitable for purifying solid silicon. That is, the method is suitable for increasing the purity grade of solid silicon used as a raw material (i.e., from a lower purity grade to a higher purity grade). Therefore, the method of the present invention can have a specific use in recycling non-compliant silicon or waste materials from other applications that would otherwise be discarded as waste. For example, the method of the present invention can be used to recover solar-grade silicon from slurry waste from cut loss. In an alternative example, lower-grade metallurgical solid silicon can be used as a starting material. Some of the purity grades currently available for a wide range of applications are listed below in order from lower to higher purity grades:

[0045] Metallurgical grade silicon: %Fe < 0.5%; %Ca < 0.3%; %Al < 0.5%;

[0046] • Chemical grade silicon: %Fe < 0.4%; %Ca < 0.1%; minimum < %Al < maximum;

[0047] • Polycrystalline silicon grade silicon: %Fe<0.3; %Ca<0.03; minimum<%Al<maximum; ppmwP<limit; ppmwB<limit; trace elements<limit.

[0048] • Solar-grade silicon: %Fe<0.0001; %Ca<0.0001; %Al<0.0001; ppmwP<1; ppmwB<1; ppmw others<10.

[0049] Silicon purity grades are often defined using the "N-value" notation, where:

[0050] • The purity of metallurgical grade silicon (MG-Si) is 98%;

[0051] • Solar-grade silicon (SG-Si) has a purity of 99.9999 or 6N (“six nines”) (i.e., one part per million of impurities); and

[0052] • Electronic grade silicon (EG-Si) has a purity of 99.9999999 or 9N (“nine nines”) (i.e., less than one part per ten million of impurities).

[0053] In certain embodiments, solid silicon is provided in the form of chunks, powder, pellets, and / or bricks.

[0054] In a particular embodiment, the method of the present invention may include additional steps before step (a) or after step (c), but not any additional intermediate steps between step (a) and step (c). That is, in a particular embodiment, the method consists of the steps described above. In another particular embodiment, the method of the present invention further includes step (d) of removing a solid silicon metal ingot from a crystallization mold.

[0055] The step of providing liquid silicon in the crystallization mold can be achieved by directly melting solid silicon in the crystallization mold. Therefore, in a particular embodiment, the method includes a prior step of melting silicon directly from solid silicon in the crystallization mold. Alternatively, the step of providing liquid silicon in the crystallization mold can be achieved by melting solid silicon elsewhere prior to step (a) and then transferring the liquid silicon to the crystallization mold after melting. Therefore, in a particular embodiment, the method includes a prior step of obtaining liquid silicon from an electrometallurgical reduction furnace in which quartz is reduced or from a furnace in which silicon is melted from solid silicon and transferring the liquid silicon melt to the crystallization mold. Methods for transferring liquid silicon melt are known to those skilled in the art. In a particular embodiment, the melt is transferred to the crystallization mold by pouring it into the mold or by means of a transfer bag.

[0056] A crystallization mold includes walls lined with a crucible (i.e., a crucible). In a particular embodiment, the crystallization mold is a crucible. In another particular embodiment, the crystallization mold has walls (i.e., a bottom wall and side walls) lined with crucible material. That is, the walls are lined with a refractory material. As will be readily understood by those skilled in the art, a crucible is a container in which a metal or other substance can be melted or subjected to very high temperatures. That is, the crucible is made of a material capable of withstanding very high temperatures. In the context of this invention, the melting point of silicon must be taken into account, which is 1414°C. Therefore, it is desirable that the crucible-lined walls of the mold be capable of withstanding temperatures above 1000°C, above 1100°C, above 1200°C, above 1300°C, above 1400°C, above 1500°C, above 1600°C, above 1700°C, or above 1800°C. The crucible is typically ceramic or metal. In a particular embodiment, the crucible is made of a material selected from the group consisting of: high-alumina cement (HAC), mullite, silicon carbide (SiC), graphite, quartz, refractory SiC composites, refractory metals (preferably ferroalloys), and combinations thereof. In a particular embodiment of the invention, the mold may be made entirely of a refractory crucible material capable of withstanding very high temperatures. In an alternative embodiment, the mold is not necessarily made of a refractory material, but rather a crucible material is lined within a space configured to receive liquid silicon, such that the crucible-lined mold can withstand very high temperatures. Within the context of the invention, the mold can have any shape or form. For example, the mold can be rectangular, cylindrical, conical, pyramidal, or bell-shaped, or any other shape known to those skilled in the art. In a particular embodiment, the wall of the crucible-lined mold includes a bottom wall and side walls. In another particular embodiment, the wall of the crucible-lined mold has a conical or pyramidal shape.

[0057] In certain circumstances, it is recommended to prevent the uncontrolled formation of a silicon layer on the crucible-lined wall when liquid silicon is poured into a crystallization mold or when solid silicon is melted directly in the crystallization mold. Therefore, in an embodiment, the crucible-lined wall is heated before starting the method of the invention, i.e., before step (a), in order to minimize the risk of forming a silicon layer on the crucible-lined wall. This minimizes the formation of an initial silicon layer with the same impurity concentration as present in the mother liquor silicon melt.

[0058] This invention provides a process for purifying silicon metal that meets the aforementioned requirements. This process is capable of removing impurities with segregation coefficients less than 1. Some of these impurities are: Al, As, Ba, Bi, Ca, Cd, Fe, Co, Cr, Cu, Fe, K, La, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Se, Sn, Sr, Ti, V, Zn, Zr, C, and B. Table 1 shows the segregation coefficients of the major impurities in silicon. If the segregation coefficient is greater than 1 (e.g., oxygen has a segregation coefficient of 1.25), this invention will not reduce its concentration through segregation, but it is still a good system for removing slag inclusions (oxygen-rich inclusions).

[0059] Table 1. Segregation coefficients of selected impurities in silicon

[0060]

[0061]

[0062] Adapted from GLColetti, D. MacDonald and D. Yang, 2012, Role of impurities in solar silicon, Advanced Silicon Materials for Photovoltaic Applications, S. Pizzini (ed.), Wiley, pp. 79–125.

[0063] One-way solidification is a well-known process commonly used in the purification of metallic silicon, where low-grade solid silicon is melted and solidified under specific conditions. When recycling waste or using metallurgical silicon, it is important to consider that impurities present in the liquid metal can contribute to supercooling during solidification, which may lead to dendritic solidification. Dendritic solidification is an inefficient type of one-way solidification because it results in the entrapment of a specific amount of impurity-rich liquid. To prevent or minimize the risks associated with dendritic solidification, impurity segregation into the liquid portion should be enhanced. Therefore, careful monitoring of the agitation / stirring rate of the liquid in the solid-liquid mesophase is necessary to control the boundary layer thickness (d) and improve the results of impurity segregation into the liquid portion. In particular, increasing the agitation / stirring rate of the liquid in the mesophase will decrease the boundary layer thickness (d), and therefore improve the results of impurity segregation into the liquid portion. Typical values ​​for the boundary layer thickness under different conditions are: 10 -3 cm (vigorous stirring) <d<10 -1cm (natural convection). When the solidification rate is very low or when mixing efficiency is high, the effective segregation coefficient (k) e The segregation coefficient (k) tends to be closer to that under complete stirring. o However, if the solidification rate increases or if effective mixing is absent, the diffusion component in the boundary layer becomes more important, and the effective segregation coefficient (k) becomes more crucial. e It tends to be closer to 1 (i.e., C) S / C O =0). Therefore, the effective segregation coefficient (k) e The value of ) is k o ≤k e ≤1, such as Figure 1 As shown. Additionally, Figure 2 Three different types of solute redistribution modes that may occur when diffusion in the solid is negligible during solidification are shown. In type 1, mixing caused by liquid diffusion or convection in the liquid is complete. Therefore, this mode achieves the most efficient segregation. On the other hand, in type 3, liquid diffusion is limited, and there is no mixing caused by convection in the liquid. Therefore, the final solute segregation is minimal. Type 2 is an intermediate case, thus some degree of solute segregation can be obtained, however, it is not as efficient as in type 1. These three cases can be summarized and characterized according to the parameters described in Table 2 below.

[0064] Table 2. Solute redistribution patterns

[0065]

[0066] Δ (boundary layer); k (segregation coefficient); BPS (BPS theory: Burton, Prim, and Slichter); C S (Final concentration of solute in solid); C O (The original concentration of the solute in the melt).

[0067] Therefore, the agitation / stirring speed of the liquid is extremely important in order to improve the redistribution of impurities between the solid and liquid phases. It is recommended that the liquid velocity in the solid-liquid interphase be 1 m / s, but more preferably in the range of 3 m / s.

[0068] Controlling the unidirectional solidification rate. Specifically, the unidirectional solidification rate depends on several factors, such as the initial temperature of the mold, the initial temperature of the liquid, and the power and frequency of the electromagnetic generator. These factors will affect the unidirectional solidification rate (i.e., the solidification speed). Preferably, the unidirectional solidification rate is within 10... -7 m / s to 10 -4 Between m / s. In a specific embodiment, the solidification rate is 10 m / s. -6 m / s to 10 -4m / s. The unidirectional solidification rate (or solidification speed) can be strictly controlled by providing a device that adjusts the temperature of the crucible-lined wall in the crystallization mold as needed. That is, the crucible-lined wall is maintained at a temperature sufficient to induce silicon crystal growth. Therefore, in embodiments, the crystallization mold includes an insulator for the crucible-lined wall and / or a temperature control device for the crucible-lined wall. In embodiments, insulation of the crucible-lined wall is achieved by providing a suitable refractory lining. In embodiments, the temperature control device includes a heating element capable of heating the crucible-lined wall.

[0069] Another factor regulated in the context of this invention is the speed at which liquid silicon is stirred by means of electromagnetic stirring (EMS). In the context of this invention, the term "electromagnetic stirring" or "EMS" refers to a process in which high levels of stirring speed can be achieved through the interaction between a magnetic field from an electromagnetic stirrer and a conductive metal bath, thus avoiding the need for physical contact between the stirrer and the liquid metal bath. One type of electromagnetic technology is electromagnetic stirring, in which fluid flow is generated by a Lorentz force provided by a linear induction motor. Electromagnetic stirring can be provided by any suitable electromagnetic stirrer (i.e., any electromagnetic stirrer capable of inducing an azimuthal electromagnetic field in liquid silicon), such as an electrostatic induction coil. In specific embodiments, electromagnetic stirring (EMS) is performed at frequencies of 1 Hz to 200 Hz, 2 Hz to 150 Hz, 3 Hz to 100 Hz, 4 Hz to 90 Hz, 5 Hz to 80 Hz, 6 Hz to 70 Hz, 7 Hz to 60 Hz, 8 Hz to 50 Hz, 9 Hz to 40 Hz, 10 Hz to 30 Hz, 11 Hz to 29 Hz, 12 Hz to 28 Hz, 13 Hz to 27 Hz, 14 Hz to 26 Hz, or 15 Hz to 25 Hz. In specific embodiments, electromagnetic stirring (EMS) is performed at frequencies of 1 Hz to 200 Hz, 1 Hz to 100 Hz, 1 Hz to 50 Hz, 1 Hz to 40 Hz, 1 Hz to 30 Hz, 1 Hz to 20 Hz, or 2 Hz to 15 Hz. In embodiments, electromagnetic stirring (EMS) is performed at frequencies of 1 Hz to 100 Hz, preferably 1 Hz to 30 Hz, more preferably 1 Hz to 20 Hz, and most preferably 2 Hz to 15 Hz. In a particular embodiment, electromagnetic stirring (EMS) is performed at frequencies of 1 Hz, 2 Hz, 3 Hz, 4 Hz, 5 Hz, 6 Hz, 7 Hz, 8 Hz, 9 Hz, 10 Hz, 11 Hz, 12 Hz, 13 Hz, 14 Hz, 15 Hz, 16 Hz, 17 Hz, 18 Hz, 19 Hz, 20 Hz, 21 Hz, 22 Hz, 23 Hz, 24 Hz, 25 Hz, 26 Hz, 27 Hz, 28 Hz, 29 Hz, or 30 Hz.

[0070] If desired, the method of the present invention can be carried out under an inert atmosphere or an oxidizing atmosphere. Therefore, the method can be carried out under an atmosphere selected from the group consisting of argon, nitrogen, air, oxygen-enriched air, mixtures of nitrogen and oxygen, and pure oxygen. In certain embodiments, if desired and as required, the method can be carried out by injecting a suitable gas via a spray gun to regulate the temperature of the liquid silicon melt (i.e., for cooling) or to prevent oxidation.

[0071] Unidirectional solidification is maintained until a certain percentage of the liquid silicon has solidified. That is, unidirectional solidification is maintained until a certain percentage of the liquid silicon has solidified, thereby obtaining solid silicon of a higher purity grade. The specific degree of solidification of the liquid silicon (i.e., the solidification percentage) is determined by the desired purity in the solid silicon ingot, as the presence of impurities in the solid ingot increases with the solidification percentage. In particular, the concentration of impurities (Ci) in the initial silicon feedstock is higher than the concentration of said impurities (Cf) in the solid silicon ingot, expressed as a ratio Ci / Cf as a function of the ingot yield (i.e., solidification percentage), except when the ingot yield is 100 (i.e., when 100% of the liquid silicon feedstock has solidified). For an ingot yield of 100, the ratio Ci / Cf is 1. Therefore, the specific degree of solidification (i.e., solidification percentage) of the liquid silicon can be selected based on the desired reduction in the concentration of a particular impurity in the final solid ingot. Unidirectional solidification is preferably stopped when 10% to 90% of the liquid silicon has solidified. In other words, unidirectional solidification preferably stops when 10% to 90% of the excess unsolidified liquid silicon remains. Therefore, in specific embodiments, the percentage of solidified liquid silicon is at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, or at least 90%. In specific embodiments, the percentage of solidified liquid silicon is 20% to 90%, preferably 40% to 85%, more preferably 50% to 80%, and most preferably 60% to 80%. In the context of this invention, stopping unidirectional solidification means not allowing the liquid silicon melt to completely solidify and proceed to the next step. As those skilled in the art will recognize, when the desired percentage of liquid silicon has solidified, the crystallization mold comprises (a) a solid hollow ingot comprising high-purity solid silicon, and (b) an internal liquid core comprising excess liquid silicon rich in impurities.

[0072] Therefore, unidirectional solidification is maintained until a certain percentage of the liquid silicon solidifies, wherein the percentage of solidified liquid silicon corresponds to the desired purity grade. The method of the present invention includes solidifying the liquid silicon by unidirectional solidification from the wall of a crucible-lined vessel while stirring the liquid silicon using electromagnetic stirring (EMS) until a certain percentage of the liquid silicon solidifies, that is, until the desired purity grade is achieved. As used herein in the context of the method of the present invention, "purity grade" refers to a purity value used as a reference for analytical data obtained from laboratory examination of a solid silicon sample. A purity grade can be an absolute value; a relative value; a value with an upper and / or lower limit; a range of values; an average; a median; a mean; or a value compared to a specific control or baseline value. Purity grade values ​​can be relative and based on individual sample values, such as values ​​obtained from a sample of solid silicon to be purified but at an earlier time point (i.e., before purification of the solid silicon). In this case, the purity grade of the solid silicon to be purified (i.e., the raw material) can be used as a reference value. Once this reference value is established, the purity grade achieved during solidification using the method of the present invention can be compared to this reference value and thus assigned a level of "increased" or "decreased" value. For example, a purity grade that is at least 1.1 times, 1.5 times, 5 times, 10 times, 20 times, 30 times, 40 times, 50 times, 60 times, 70 times, 80 times, 90 times, 100 times, 500 times, 1000 times, or even more higher than the reference value is considered an "increased" purity grade value. Similarly, those skilled in the art can deduce that the purity grade is necessarily related to the concentration of impurities in the solid silicon to be purified (i.e., the raw material) and to the concentration of impurities in the purified solid silicon, such that the concentration of impurities in the purified solid silicon is reduced. A concentration value of at least 0.9 times, 0.75 times, 0.2 times, 0.1 times, 0.05 times, 0.025 times, 0.02 times, 0.01 times, 0.005 times, 0.001 times, or even less of the same impurity / contaminant in the purified solid silicon (i.e., the raw material) is considered a “reduced” concentration of the impurity / contaminant.

[0073] In yet another embodiment, the method further includes melting a purified solid silicon metal ingot and repeating steps (a) to (e) of the method of the present invention, thereby obtaining a purified solid silicon metal ingot of a higher purity grade. The ingot may be remelted on its own, i.e., remelted without the addition of any other materials, or alternatively, may be remelted together with additionally loaded solid silicon raw materials. Suitable solid silicon raw materials have been described elsewhere herein. In this way, a solid silicon ingot of a higher purity grade can be obtained.

[0074] In another embodiment, a solid hollow ingot can be cleaned mechanically to remove a thin layer (1 mm to 10 mm) containing excess impurities resulting from rapid solidification of the liquid silicon upon contact with the inner wall of the crucible and from contact with the crucible itself. These impurities may contaminate the ingot through impurity diffusion or trapping of small portions separated from the crucible. This cleaning process can be accomplished by appropriate methods, such as cutting or milling using various means (sandblasting, high-pressure water jetting, dry ice blasting, or diamond tools), and can be automated using robots.

[0075] All terms and embodiments described elsewhere in this document are equally applicable to these aspects of the invention.

[0076] Crystallization molds and their uses

[0077] In one aspect, the present invention also relates to an apparatus comprising:

[0078] a) Crystallization mold;

[0079] b) An electromagnetic stirrer; and

[0080] c) Discharge mechanism.

[0081] The crystallization mold includes a wall lined with a crucible. As will be readily understood by those skilled in the art, a crucible is a container in which a metal or other substance can be melted or subjected to very high temperatures. That is, the crucible is made of a material capable of withstanding very high temperatures. In the context of this invention, the melting point of silicon must be taken into account, which is 1414°C. Therefore, it is desirable that the wall of the mold lined with the crucible be able to withstand temperatures above 1000°C, above 1100°C, above 1200°C, above 1300°C, above 1400°C, above 1500°C, above 1600°C, above 1700°C, or above 1800°C. The crucible is typically ceramic or metal. In the context of this invention, the mold can have any shape or form. For example, the mold can be rectangular, cylindrical, conical, pyramidal, or bell-shaped, or any other shape known to those skilled in the art. In a particular embodiment, the wall lined with the crucible includes a bottom wall and side walls. In another particular embodiment, the wall lined with the crucible has a conical or pyramidal shape. In a particular embodiment, the crucible is made of a material selected from the group consisting of: high-alumina cement (HAC), mullite, silicon carbide (SiC), graphite, quartz, refractory SiC composites, refractory metals (preferably ferroalloys), and combinations thereof. Optionally, the apparatus of the present invention further includes an insulator for the wall of the crucible lining and / or a temperature control device for the wall of the crucible lining.

[0082] In certain embodiments, the crystallization mold of the present invention can be used as a furnace to directly melt solid silicon prior to unidirectional solidification, in which case the mold is equipped with any suitable type of heating device. Heating devices associated with furnaces (e.g., induction furnaces, resistance furnaces, and gas furnaces) are well known in the art. In a preferred embodiment, heating is provided by an induction device.

[0083] In embodiments, the crystallization mold of the present invention is configured to be fully or partially open, or partially closed with the orifice serving as a liquid inlet and a gas outlet. In specific embodiments, the crystallization mold of the present invention is configured to have a top that is open by 5% to 95%, 5% to 75%, 5% to 50%, 5% to 25%, or 5% to 10%. In specific embodiments, the crystallization mold of the present invention is configured to have a top that is open by 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, or 5%. In specific embodiments, the crystallization mold of the present invention is configured to have a top that is open by 6.25%. In a specific operation of the crystallization mold of the present invention, an open or partially open top can be used to ensure that the top surface remains liquid while the liquid silicon is being solidified by unidirectional solidification from the crucible-lined wall of the crystallization mold, simultaneously by means of electromagnetic stirring (EMS). In an alternative embodiment, the crystallization mold of the present invention is configured to have a temperature-controlled cover that performs the same function to ensure that the surface in contact with the temperature-controlled cover remains liquid while the liquid silicon is being solidified by means of electromagnetic stirring (EMS) and simultaneously by unidirectional solidification from the crucible-lined wall of the crystallization mold.

[0084] The apparatus of the present invention is equipped with an electromagnetic stirrer. The electromagnetic stirrer can be positioned at any location on the crystallization mold of the present invention. In a particular embodiment, the electromagnetic stirrer is attached to or embedded in the wall of the crystallization mold. In another embodiment, the electromagnetic stirrer is attached to the bottom of the crystallization mold.

[0085] The apparatus of the present invention includes a pouring mechanism. The purpose of the pouring mechanism is to help drain remaining liquid from the mold. In a particular embodiment, the pouring mechanism is a tilting mechanism. In a preferred embodiment, the tilting mechanism is configured such that the crystallization mold can be tilted from 90 degrees to 180 degrees.

[0086] In certain embodiments, the mold also includes a dragging mechanism. The dragging mechanism is particularly useful when the furnace is a separate unit from the crystallization mold. During operation, the furnace needs to be tilted to pour liquid silicon into the crystallization mold. After pouring, the crystallization mold of the present invention is moved away from the furnace by means of the dragging mechanism for easier operation.

[0087] In related aspects, the present invention also relates to the use of the crystallization mold according to the invention in a method for obtaining purified silicon metal. That is, the present invention relates to a method for obtaining purified silicon metal by means of a crystallization mold according to the invention. As will be apparent to those skilled in the art, the use of the crystallization mold according to the invention for obtaining purified silicon metal is also suitable for purifying solid silicon from a lower purity grade to a higher purity grade.

[0088] In the use of this invention, when using a furnace independent of the crystallization mold of this invention, liquid silicon is loaded into the crystallization mold. Alternatively, solid silicon can be melted directly in the crystallization mold. The crucible-lined walls of the crystallization mold are typically heated to a temperature above 100°C to prevent uncontrolled formation of a silicon layer on the crucible-lined walls when the liquid silicon melt is poured into the crystallization mold or when solid silicon is melted directly in the crystallization mold. However, the temperature of the crucible-lined walls of the crystallization mold should be sufficient to induce silicon crystal growth. Although solidification is then allowed to occur, an electromagnetic stirrer generates alternating current (AC) that induces a rotational electromagnetic force in the liquid silicon. Centripetal solidification occurs in the radial direction, causing the purified silicon ingot to grow inward from the crucible wall and impurities to concentrate in the liquid phase. In the context of the use of the crystallization mold of this invention, unidirectional solidification is preferably stopped when 40% to 85% of the liquid silicon has solidified. Therefore, in a specific embodiment, the percentage of solidified liquid silicon is 40% to 85%, preferably 50% to 80%, and more preferably 60% to 80%. In the context of this invention, stopping unidirectional solidification means not allowing the liquid silicon melt to completely solidify and proceed to the next step. Once a set time corresponding to a specific degree of solidification of the liquid silicon is reached, the rotator is inverted while keeping the power to the electromagnetic stirrer on. In a specific embodiment, the inverted position corresponds to a position where the crystallization mold is rotated 90 to 270 degrees, 135 to 225 degrees, 150 to 210 degrees, 160 to 200 degrees, or 170 to 190 degrees. Preferably, the inverted position corresponds to a position where the crystallization mold is rotated 180 degrees. In the inverted position, the liquid metal is largely retained within the container due to the electromagnetic field. When the power to the electromagnetic stirrer is turned off, the liquid silicon suddenly falls into the auxiliary mold, thus emptying the interior of the crystallization mold. In the final step, the purified hollow ingot is removed. In a particular embodiment, the purified hollow ingot is removed from the bottom of the crucible with the aid of a mechanical actuator.

[0089] All the terms and embodiments described above also apply to all aspects of the invention. It should be noted that the singular forms “a,” “an,” and “described” used in this specification and the appended claims include plural indicators unless the context clearly indicates otherwise. Similarly, in accordance with generally acceptable patent practice, the term “comprises / comprising,” as used herein, also describes “consists of / consisting of.”

[0090] Example

[0091] The invention is described herein by way of examples, which should be interpreted as illustrative only and not as limiting the scope of the invention.

[0092] Example 1

[0093] 1000 kg of commercial silicon with an Fe content of 0.23% w was poured directly from the outlet of the metallurgical furnace into a transfer ladle. The temperature after pouring was 1545 °C, and oxygen injection (3 to 9 Nm³) was carried out during pouring and transfer to the solidification furnace. 3 Conventional gas purification was performed using O2 / h (standard cubic meters of oxygen per hour). The bales were then tilted into a solidification furnace, which was set to operate at 25 Hz for 40 minutes, achieving a yield of 80% solids. Then, in... Figure 4 Following the steps shown, excess liquid is drained into an auxiliary mold. The resulting hollow ingot has an iron content of 288.5 ppm (by weight).

[0094] Table 3. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP OES) Analysis of Example 1

[0095] Al Ca Cr Cu Fe P Ti raw materials 369.0 359.0 7.9 11.4 2329.0 19.2 192.0 Excess liquid 1034 949 22 29 6553 39 537 solid ingot 47.2 73.4 0.9 2.9 288.5 9.8 25.0

[0096] The unit is ppm by weight.

[0097] Example 2

[0098] 1000 kg of low-Fe silicon with an Fe content of 1115.6 parts per million (ppm) was heated to 1600 °C at 756 Hz using an induction heater. The melt was then rotated for 40 minutes using a low-frequency electromagnetic stirrer at 15 Hz, achieving a yield of 80% solids. Then, in… Figure 3 Following the steps shown, the liquid is released into an auxiliary mold. The resulting hollow ingot has an iron content of 120.6 ppm (by weight).

[0099] Table 4. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP OES) Analysis of Example 2

[0100] Al Ca Cr Cu Fe P Ti raw materials 51.1 140.5 23.4 3.9 1115.6 29.8 14.7 Excess liquid 210.5 359.7 63.3 6.1 2839.1 58.5 28.9 solid ingot 6.0 27.9 2.6 0.6 120.6 14.7 1.3

[0101] The unit is ppm by weight.

[0102] Example 3

[0103] 1000 kg of commercial 3N silicon with an Fe content of 309.6 parts per million (ppm) by weight was heated to 1600 °C using an induction heating device at 756 Hz. Subsequently, the melt was rotated for 40 minutes using a low-frequency electromagnetic stirrer at 20 Hz, achieving a yield of 80% solids. Then, in… Figure 3 After the described steps, excess liquid is drained into an auxiliary mold. The resulting hollow ingot has an iron content of 9.2 ppm (by weight).

[0104] Table 5. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP OES) Analysis of Example 3

[0105] Al Ca Cr Cu Fe P Ti raw materials 38.7 16.2 4.4 2.3 309.6 25.1 1.2 Excess liquid 216.5 79.8 22.1 11.4 1647.6 124.9 5.9 solid ingot 0.1 0.1 0.1 0.1 9.2 10.6 0.3

[0106] The unit is ppm by weight.

[0107] Example 4

[0108] 1000 kg of photovoltaic silicon nicking loss waste with an Fe content of 14629.2 parts per million (ppm) by weight was heated to 1600 °C using an induction device at 756 Hz. Subsequently, the melt was rotated for 40 minutes using a low-frequency electromagnetic stirrer at 20 Hz, achieving a yield of 80% solids. Then, in… Figure 3 Following the described steps, the liquid is released into an auxiliary mold. The resulting hollow ingot has an iron content of 1375 ppm (by weight).

[0109] Table 6. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP OES) Analysis of Example 4

[0110] Al Ca Cr Cu Fe P Ti raw materials 887.0 79.1 20.3 11.2 14629.2 74.9 26.0 Excess liquid 4356.2 451.7 61.5 34.5 67542.2 161.3 70.8 solid ingot 17.2 9.6 10.5 5.4 1375.0 53.8 14.8

[0111] The unit is ppm by weight.

[0112] Example 5

[0113] 1000 kg of photovoltaic silicon nicking loss waste with an Fe content of 169.2 parts per million (ppm) by weight was heated to 1600 °C using an induction device at 756 Hz. Subsequently, the melt was rotated for 30 minutes using a low-frequency electromagnetic stirrer at 20 Hz, achieving a 60% solids yield. Then, in… Figure 3After the described steps, excess liquid is drained into an auxiliary mold. The resulting hollow ingot has an iron content of 23.8 ppm (by weight).

[0114] Table 7. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES) Analysis of Example 5

[0115] Al Ca Cr Cu Fe P Ti raw materials 2414.0 441.3 2.1 3.4 169.2 5.9 4.2 Excess liquid 9883.7 2089.3 8.4 14.6 762.3 15.6 18.6 solid ingot 46.7 4.3 0.6 0.5 23.8 3.2 0.5

[0116] The unit is ppm by weight.

[0117] Example 6

[0118] 1000 kg of photovoltaic silicon nicking loss waste with an Fe content of 15034.4 parts per million (ppm) by weight was heated to 1600°C using an induction device at 756 Hz. Subsequently, the melt was rotated for 40 minutes using a low-frequency electromagnetic stirrer at 30 Hz, achieving a yield of 80% solids. Once the liquid was released into an auxiliary mold, it was then... Figure 5 Following the previously described steps, 200 kg of 99% silicon bulk material was added as refill, which was then remelted and purified. The resulting hollow ingot had an iron content of 148.7 ppm (by weight).

[0119] Table 8. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES) Analysis of Example 6

[0120]

[0121] The unit is ppm by weight.

[0122] Example 7

[0123] 160 kg of commercial silicon with an Fe content of 5010.0 parts per million (ppm) was heated to 1560 °C at 756 Hz using an induction heater. The furnace was then tilted, and the molten material was discharged into a solidification furnace set to operate at 21 Hz for 30 minutes, achieving a yield of 80% solids. Then, in… Figure 4 Following the described steps, the liquid is released into an auxiliary mold. The resulting hollow ingot has an iron content of 200.4 ppm (by weight).

[0124] Table 9. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP OES) Analysis of Example 7

[0125]

[0126] The unit is ppm by weight.

[0127] Example 8

[0128] 160 kg of 3N silicon with an Fe content of 338.1 parts per million (ppm) was heated to 1560 °C at 756 Hz using an induction heater. The furnace was then tilted, and the molten material was discharged into a solidification furnace set to operate at 21 Hz for 25 minutes, achieving a 75% solids yield. Then, in… Figure 4 Following the described steps, the liquid is released into an auxiliary mold. The resulting hollow ingot has an iron content of 12 ppm (by weight).

[0129] Table 10. - Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP OES) Analysis of Example 8

[0130] Al Ca Cr Cu Fe P Ti initial liquid 345.0 9.0 4.6 4.3 331.8 12.8 3.3 Excess liquid 1707.8 16.4 21.7 17.6 1597.1 48.6 15.4 solid ingot 4.3 7.4 0.6 1.2 12.0 4.3 0.5

[0131] The unit is ppm by weight.

[0132] Example 9

[0133] Exemplary operating mode when using a furnace with a crystallization mold independent of the present invention: Once the silicon has melted in the furnace, the mold must be placed at the end of the furnace channel, where the crucible temperature is above 100°C. Figure 6A This temperature is easily reached 15 to 20 minutes after the electromagnetic stirrer is started. Then tilt the furnace and pour out the liquid silicon at a rate of 20 to 50 kg / min. Figure 6B After pouring out the contents, the furnace should be returned to its normal upright position, and the crystallization mold of the present invention should be moved a few meters away from the furnace for easier operation. Figure 6C Then solidification is allowed to occur, which typically lasts 20 to 40 minutes. An electromagnetic stirrer generates alternating current (AC), which induces a rotational electromagnetic force in the liquid silicon. Centripetal solidification occurs in the radial direction, causing the purified silicon ingot to grow inward from the crucible wall and impurities to concentrate in the liquid phase. The heat released during material solidification is extracted through the crucible walls. These walls can be insulated, equipped with temperature control devices, or directly exposed to air. If the crucible material is already refractory, the walls can be directly exposed to air.

[0134] Once the set time corresponding to the specific solidification degree of the liquid silicon is reached, invert the stirrer 180° while keeping the power on. In this position, most of the liquid metal remains within the container due to the electromagnetic field. Figure 6D When the power to the electromagnetic stirrer is turned off, the liquid silicon suddenly falls into the auxiliary mold, thus emptying the interior of the crystallization mold. Figure 6E In the final step, the purified hollow ingot is removed. Figure 6F ).

[0135] Example 10

[0136] The specific degree of solidification (i.e., the percentage of solidification) of liquid silicon is determined by the desired purity in the solid silicon ingot, because the presence of impurities in the solid ingot increases with the percentage of solidification. Specifically, when the entire solid silicon has solidified, the concentration of impurities in the solid silicon ingot is equal to the concentration of impurities in the silicon feedstock at the start of the process. In the case of iron, Figure 7 The diagram shows that the iron concentration (Ci) in the initial silicon feedstock is higher than the iron concentration (Cf) in the solid ingot, expressed as the ratio Ci / Cf as a function of ingot yield (i.e., solidification percentage), except when the ingot yield is 100 (that is, when 100% of the liquid silicon feedstock has solidified). For an ingot yield of 100, the ratio Ci / Cf is 1. Therefore, a specific degree of solidification (i.e., solidification percentage) of liquid silicon can be selected based on the desired reduction in the concentration of a particular impurity in the final solid ingot.

Claims

1. A method for obtaining purified silicon metal, the method comprising: a) Liquid silicon is supplied in a crystallization mold having walls lined with a crucible. b) The liquid silicon is solidified by unidirectional solidification from the wall of the crucible while being stirred by means of electromagnetic stirring, until a certain percentage of the liquid silicon is solidified, wherein the solidification occurs centripetally in the radial direction, such that the purified silicon ingot grows inward from the wall of the crucible and the impurities are concentrated in the liquid phase. as well as c) Pour out any excess unsolidified liquid silicon. Step (c) of pouring out excess unsolidified liquid silicon includes the following steps: c1) The crystallization mold is inverted and tilted under electromagnetic stirring, wherein the inverted position corresponds to the position where the crystallization mold is rotated from 90 degrees to 270 degrees. c2) The excess unsolidified liquid silicon is drained from the crystallization mold by turning off the electromagnetic stirring.

2. The method of claim 1, wherein the method comprises the following prior steps: a) Melting silicon directly from solid silicon in the crystallization mold.

3. The method of claim 1, wherein the method comprises the following prior steps: a) Obtaining liquid silicon from an electrometallurgical reduction furnace in which quartz is reduced, or from a furnace in which silicon is melted from solid silicon, and b) Transfer the liquid silicon into the crystallization mold.

4. The method according to claim 2 or 3, wherein the solid silicon is in the form of a block, powder, and / or granules.

5. The method according to claim 2 or 3, wherein the solid silicon is in the form of bricks.

6. The method of claim 1, wherein the percentage of the solidified liquid silicon is 20% to 90%.

7. The method of claim 6, wherein the percentage of the solidified liquid silicon is 40% to 85%.

8. The method of claim 7, wherein the percentage of the solidified liquid silicon is 50% to 80%.

9. The method of claim 8, wherein the percentage of the solidified liquid silicon is 60% to 80%.

10. The method of claim 1, wherein the wall lined with the crucible comprises a bottom wall and side walls, or wherein the wall lined with the crucible has a conical or pyramidal shape.

11. The method of claim 1, wherein the crucible in the wall lined with the crucible is made of a material selected from the group consisting of: high alumina cement (HAC), mullite, silicon carbide, graphite, quartz, refractory SiC composite material, refractory metals and combinations thereof.

12. The method according to claim 11, wherein the refractory metal is an iron alloy.

13. The method of claim 1, wherein the wall of the crucible is heated to minimize the formation of a silicon layer on the wall of the crucible.

14. The method of claim 1, wherein electromagnetic stirring is performed at a frequency of 1 Hz to 100 Hz.

15. The method of claim 14, wherein electromagnetic stirring is performed at a frequency of 1 Hz to 30 Hz.

16. The method of claim 15, wherein electromagnetic stirring is performed at a frequency of 1 Hz to 20 Hz.

17. The method of claim 16, wherein electromagnetic stirring is performed at a frequency of 2 Hz to 15 Hz.

18. The method of claim 1, wherein the solidification rate is 10. -7 m / s to 10 -4 m / s.

19. The method of claim 18, wherein the solidification rate is 10. -6 m / s to 10 -4 m / s.

20. The method of claim 1, wherein the method further comprises melting a purified solid silicon metal ingot and repeating steps (a) to (c) of claim 1.

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