WO2013078220A1

WO2013078220A1 - Method for producing solar grade silicon from silicon dioxide

Info

Publication number: WO2013078220A1
Application number: PCT/US2012/066088
Authority: WO
Inventors: Reinaldo Rodrigues Bittar; Vishu Dutt Dosaj; Robert J. Harmer
Original assignee: Dow Corning Silicio do Brasil Industria e Comercio Ltda; Dow Corning Corp
Current assignee: Dow Corning Silicio do Brasil Industria e Comercio Ltda; Dow Silicones Corp
Priority date: 2011-11-22
Filing date: 2012-11-20
Publication date: 2013-05-30
Anticipated expiration: 2014-05-22

Abstract

A method for producing a solar grade silicon including preheating silica to a temperature of from about 1300 degrees Celsius (°C) to about 1400°C and combining aluminum with the preheated silica to melt the aluminum and produce an aluminum-silicon alloy. The aluminum-silicon alloy is fractionally crystallized to produce first silicon crystals, which are acid leached to produce high-purity second silicon crystals. The high-purity second silicon crystals are refined to produce refined silicon, which is further purified to produce solar grade silicon.

Description

METHOD FOR PRODUCING SOLAR GRADE SILICON

FROM SILICON DIOXIDE

[0001] The present invention relates to the production of solar grade silicon, and more specifically, for producing the solar grade silicon with a multi-step process including reducing silicon dioxide by molten aluminum.

[0002] Generally, solar grade silicon is a metallurgical grade silicon that is approximately ninety-seven percent, by weight, pure and that has been refined to over ninety-nine, by weight, percent pure. To attain solar grade silicon, contaminants are removed by various techniques. For example, one technique dissolves the contaminants atomically. However, one problem associated with this technique is that it is expensive. Other techniques are also plagued by problems. For example, gettering and hydrogen passivation are not feasible options if a low grade silicon (i.e., an inexpensive material) is used for producing solar grade silicon. As such, a need exists for an economic and efficient process of producing a solar grade silicon.

SUMMARY OF THE INVENTION

[0003] In one embodiment, this invention is a method for producing a solar grade silicon includes preheating silica to a temperature of from about 1300 degrees Celsius ("°C") to about 1400°C and combining aluminum with the preheated silica to melt the aluminum and produce an aluminum-silicon alloy. The aluminum-silicon alloy is fractionally crystallized to produce first silicon crystals, which are acid leached to produce second silicon crystals, which are generally high-purity crystals having a silicon purity that is greater than the silicon purity of the first silicon crystals. The second silicon crystals are refined to produce refined silicon, which is further purified to produce solar grade silicon.

[0004] In another embodiment, this invention is a method for producing a solar grade silicon from a silicon dioxide includes preheating silica in a rotary kiln to a predetermined temperature and adding aluminum to the preheated silica to melt the aluminum. An inert atmosphere is provided in the rotary kiln, and contents in the kiln are agitated to produce an aluminum-silicon alloy. The aluminum-silicon alloy is crystallized to produce first silicon crystals, which are contacted by an acid-water mixture to produce second silicon crystals. The second silicon crystals are removed from the acid-water mixture, the second silicon crystals being high-purity silicon crystals having a silicon purity that is greater than the first silicon crystals. The second silicon crystals are washed and, then, dried. The dried second silicon crystals are melted in a gaseous mixture of oxygen and nitrogen gases in the presence of a calcium oxide-based slag to refine the dried second silicon crystals and produce refined silicon. The refined silicon is directionally solidified to produce solar grade silicon. [0005] In yet another embodiment, this invention is a method for producing a solar grade silicon includes preheating silica to a predetermined temperature, the silica being in the form of particles having an average diameter of from about 1 millimeter (mm) to about 5 millimeters. The aluminum is combined with the preheated silica to melt the aluminum at a temperature from about 1000 °C to about 1 100°C and produce an aluminum-silicon alloy. The aluminum-silicon alloy is crystallized to produce first silicon crystals, which are acid leached to produce second silicon crystals (with a higher silicon purity than the first silicon crystals). The second silicon crystals are refined in a gaseous mixture of oxygen and nitrogen gases, in the presence of a calcium oxide-based slag, to produce refined silicon, which is directionally solidified to produce solar grade silicon.

[0006] These and additional features and advantages will be more fully understood from the following detailed description, accompanying drawings, and appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0007] The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the invention which is defined by the appended claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings in which:

[0008] FIG. 1 is a schematic flowchart of an exemplary method for producing a solar grade silicon, according to one or more of the embodiments presented herein.

[0009] FIG. 2 is a schematic flowchart of an exemplary method for producing an aluminum- silicon alloy according to one or more of the embodiments presented herein.

[0010] FIG. 3 is a schematic diagram of one embodiment of a system for producing an aluminum-silicon alloy.

DETAILED DESCRIPTION

[0011] Referring to FIG. 1 , embodiments of the present disclosure provide an embodiment of the method for producing a solar grade silicon (also referred to as SoG-Si) using a multi-step process. For example, the process begins with a first step in which an aluminum-silicon alloy is produced through aluminothermic reduction of silica. The product results in a high purity aluminum-silicon alloy containing about fifty weight percent ("wt%") silicon and aluminum. In a second step, the aluminum-silicon alloy is fractionally crystallized by slow cooling to result in first silicon crystals (or flakes) containing approximately ninety wt% silicon and ten wt% aluminum.

[0012] In a third step, the isolated first silicon crystals are placed in contact with an acid-water mixture for sufficient time to dissolve the aluminum in the crystals. Individual or combinations of different acid sources can be used in this step, including, for example, one or more of hydrochloric acid (HCI), nitric acid (HN03), hydrofluoric acid (HF), sulfuric acid (H2S04), and acetic acid. After contact with the acid-water mixture, the crystals are separated from the acid- water mixture (e.g., removed by filtration), washed with water, and dried. The dried product results in high-purity second silicon crystals containing approximately 2,500 parts per million by atom ("ppma") of aluminum.

[0013] In a fourth step, the second silicon crystals of the third step are further refined to obtain refined silicon containing up to one percent aluminum - 10,000 parts per million by weight ("ppmw") - and only trace amounts of other impurities, including calcium. The refined silicon can optionally be in the form of silicon crystals. By melting and refining the second silicon crystals in a gaseous mixture of oxygen and nitrogen gases in the presence of a calcium oxide-based slag, the aluminum in the silicon crystals can be refined to below 50 ppmw. Although the aluminum is refined by adding calcium and silicon-based compounds, the final product is not contaminated by calcium. The product results in a refined (high purity) silicon containing approximately less than 40 ppmw aluminum and calcium. For example, the refined silicon can be in crystalline form or in amorphous form.

[0014] In a fifth (and final) step, the refined silicon is further purified by crystallization to obtain high-yield crystallization. For example, the crystallization can be performed using any directional solidification technique, e.g., rotational casting. The product results in solar grade silicon in which B<1 and P<1. For example, the solar grade silicon can be crystalline or amorphous. In another example, if the solar grade silicon is in crystalline form, the crystalline solar grade silicon may be monocrystalline or polycrystalline. Each of the above steps will be discussed in more detail, below.

[0015] Step 1 - Producing An Aluminum-Silicon Alloy. Typically, an aluminum-silicon alloy comprises elements of silicon and aluminum and has only trace amounts of contaminating elements such as boron and phosphorous. Typically, the aluminum-silicon alloy produced in Step 1 comprises from about 20 to about 55 wt% silicon and from about 80 to about 45 wt% aluminum. For example, in one embodiment, the resulting aluminum-silicon alloy comprises about 50 wt% silicon and about 50 wt% aluminum.. Trace amounts is defined as less than about 100 ppmw. According to some examples, methods and systems for producing an aluminum-silicon alloy use a rotary kiln, preheated silica, and/or an inert gas atmosphere for the more efficient use of aluminum such that there are fewer losses of aluminum during the process and more aluminum is available to alloy with the silicon.

[0016] Furthermore, the by-product slag (generally aluminum oxide) which is produced by the aluminothermic reaction forms as a solid that can be readily removed from the reactor once the molten aluminum-silicon alloy has been tapped and drained off. These methods and systems, taken individually or in combination, improve the overall efficiency of producing an aluminum-silicon alloy and reduce the amount of aluminum lost during processing. [0017] Silica may be reduced by aluminum in an aluminothermic reaction at temperatures above about 900°C according to the general reaction:

4AI + 3Si0₂ = 3Si + 2AI₂0₃ (Reaction I)

As a more specific example of Reaction I, silica may be reduced according to the following reaction:

4 Al + 3 Si0₂ = 2.54Si + 0.62 Al + 1.23 Al₂0₃ + 0.46 AI₂Si0₅ (Reaction II) The silicon produced from the reduction of silica may then be combined with aluminum to produce an aluminum-silicon alloy comprising from about 20 to about 55 wt% silicon and from about 80 to about 45 wt% aluminum. In one embodiment, the aluminum-silicon alloy comprises about 50 wt% aluminum and about 50 wt% silicon.

[0018] Referring now to FIG. 2, an exemplary method 10 for producing an aluminum-silicon alloy is shown. The method 10 comprises preheating silica in step 100 to a predetermined temperature. Preferably, the silica is in the form of small particles, typically having an average diameter of from between about 1 .0 to about 5.0 millimeters ("mm") or from between about 2 to about 5 mm. Silica used in the process can be produced by methods readily known in the art or purchased commercially. The silica may further comprise a variety of physical states or phases of silica. For example, the silica may comprise sand, crushed quartz, quartz fines, fused silica or any other phase or physical state, or any combinations thereof, operable to be preheated and mixed with aluminum to form an aluminum-silicon alloy as described herein.

[0019] Aluminum is combined with the preheated silica in step 200 for a time sufficient to melt the aluminum. As described above, the aluminum may be coated with silica particles prior to combining it with the preheated silica. Aluminum may be supplied in any convenient form including ingots, shot, plates, or pellets. The aluminum may be substantially pure, or may be in the form of an aluminum-silicon alloy such as, for example, an alloy containing 80 wt% aluminum and 20 wt% silicon. Once the aluminum has melted, the mixture is preferably agitated in step 300, such as, for example, in a rotary kiln to produce the aluminum-silicon alloy. As described above, the temperature of the silica is such that added aluminum will melt and the aluminothermic reaction will take place. Alternatively, as described above, the aluminum may also be preheated to melt it prior to combination with the preheated silica. Both processes may occur substantially simultaneously such that molten aluminum is produced at about the same time that the silica is preheated. In the alternative, either process may occur before the other so long as the resultant product is maintained in a state applicable for combination. For example, where preheating the silica occurs first, the preheated silica may be stored in an insulated housing to reduce or minimize the amount of heat loss thereby preserving its preheated state. Likewise, where melting the aluminum occurs first, the resultant molten aluminum may be stored in an insulated housing to reduce or minimize the amount of heat loss thereby preserving its molten state. [0020] Preheating the silica in step 100 comprises preheating the silica to a predetermined temperature which will typically be in the range of from about 1000°C to about 1550°C, from about 1300°C to about 1400°C, or to about 1300°C. Heating temperatures may vary or fluctuate throughout the method 10. Thus, where it is said that silica is preheated to a temperature of about 1300°C, the actual temperature may fluctuate and may not be held constant at said temperature.

[0021] In one embodiment, silica is preheated in a rotary kiln 1 1 . As illustrated schematically in FIG. 3, the kiln 1 1 comprises a generally cylindrical shape oriented in a generally horizontal configuration and having an outer wall 12. Lining the interior of outer wall 12 is an insulating refractory material 14. The kiln 1 1 also includes a kiln opening 16, and a kiln cover 18. For example, in operation, silica 15 is loaded into the kiln 1 1 via the kiln opening 16. The kiln opening 16 is further adapted to receive a heat source such as, for example, a burner 20 which may comprise a gas burner 20 fed with a source of gas 22. The burner 20 is used to heat the silica within the kiln 1 1 to the desired predetermined temperature. Once the predetermined temperature is reached, the burner 20 is removed, and the kiln opening 16 is closed by the kiln cover 18. In one embodiment, solid aluminum at ambient temperature is added through kiln opening 16 after the predetermined temperature has been reached. The insulating refractory 14 may comprise any temperature resistant and insulating material operable to reduce heat loss from the kiln 1 1 and which does not contaminate the aluminum- silicon alloy product. For example, the refractory 14 may comprise a silica-containing alumina, graphite, silicon carbide or silicon nitride. As shown, rotary kiln 1 1 is driven by a gear 24 operatively communicating with a drive motor 26.

[0022] Referring both to FIGs. 2 and 3, the method 10 alternatively comprises melting the aluminum to produce molten aluminum prior to combining it with the preheated silica. The aluminum may be contained in a furnace 28 such as, for example, an induction furnace, although other types of furnaces may be used. As discussed above, the melting may occur before, after, or during the preheating of the silica. The aluminum is heated to above its nominal melting temperature of about 660°C, but is not superheated as in the prior art. In one embodiment, this comprises heating the aluminum to a temperature of from about 1000°C to about 1200°C, from about 1050°C to about 1 150°C, or to about 1 100°C. By heating the aluminum to a lower temperature than prior art processes, less aluminum is oxidized and/or vaporized and lost during processing. The lower temperatures which are used retard oxidation of the aluminum. In one embodiment, the aluminum is melted in an induction furnace 28. In another embodiment, any other heating device may be used such as an electric arc furnace or gas furnace.

[0023] In one embodiment, after preheating of the silica and melting of the aluminum to produce molten aluminum, the preheated silica and the molten aluminum are combined, for example, in a reactor such as a rotary kiln. In one embodiment, the reactor comprises the container in which the silica is preheated such that the molten aluminum is directly added to the preheated silica with no transfer of the preheated silica. For example, where the silica is preheated in kiln 1 1 , the molten aluminum is added to the kiln via conduit 30 for the aluminum and silica to mix. Where the kiln comprises a kiln opening 16 as discussed above, the molten aluminum may be added into the kiln via the kiln opening 16 and the kiln opening may subsequently be closed with the kiln cover 18 to reduce heat loss and/or material loss of the preheated silica and molten aluminum.

[0024] Furthermore, any suitable method of transport may be used to combine the preheated silica and molten aluminum. In one embodiment, insulated transport devices may be used to transfer the molten aluminum to where the silica was preheated. In another embodiment, channels may be predisposed between a furnace housing the molten aluminum such that the molten aluminum flows from one location to another as induced by gravity and/or the opening and closing of gates. Any other alternative form of repositioning may otherwise be used such that the silica remains preheated and the aluminum remains molten when the two are combined.

[0025] The method 10 may further comprise agitating the contents of the reactor such as kiln 1 1 , either internally or externally, such as by rotating the chamber in step 300 to encourage mixing of the preheated silica and aluminum. Specifically, agitating the contents in the chamber may comprise rotating the chamber while it houses the preheated silica and the aluminum such that the two thoroughly mix with one another. Where aluminum is added as a solid, this agitation increase heat transfer to more quickly melt the aluminum. In one embodiment, the rotation occurs about the horizontal axis of the chamber such that the bottom portion of the chamber, or the portion closest to the floor, rotates toward the top portion of the chamber, or the portion farthest from the floor. In another embodiment (not shown), the reactor may internally comprise a stirrer, such as a graphite stirrer, that moves within the chamber while the reactor rotates to further encourage mixing. In one embodiment, where the silica is preheated in a rotary kiln 1 1 , the reactor may comprise the rotary kiln such that molten aluminum from furnace 28 may be directly added to the rotary kiln to encourage mixing.

[0026] The agitation of the contents in the reactor in step 300 (such as the rotation of a rotary kiln), may began at any time during the method 10. For example, in one embodiment the contents of the reactor may be agitated after the preheating of the silica but before the silica and the aluminum are combined. In another embodiment, the contents of the reactor may be agitated after the preheated silica and the aluminum are combined. In yet another embodiment, the contents of the reactor may be continuously agitated throughout the entire method 10. In one embodiment, the kiln 1 1 may be rotated at various speeds effective for such agitation. The speeds depend on the overall weight of the preheated silica and/or the aluminum and the desired degree of agitation. In one embodiment, the kiln is rotated at speeds of from about 0.1 revolutions per minute ("rpm") to about 30 rpm, from about 3 rpm to about 30 rpm, or from about 3 rpm to about 15 rpm. In another embodiment, the kiln's rotation speed may fluctuate throughout the method 10. In yet another embodiment, the kiln may be rotated at any other speed or between any other steps such that the rotation encourages the mixing of the preheated silica and aluminum. The kiln may also be rotated for a time sufficient to mix the preheated silica with the aluminum to cause melting of the aluminum and to produce an aluminum-silicon alloy. In one embodiment, the preheated silica and aluminum are combined and rotated in kiln 1 1 for from about 10 minutes to about 200 minutes, for from about 20 minutes to about 150 minutes, or from about 30 minutes to about 120 minutes. The time in which the preheated silica and aluminum are mixed depends in part on the overall batch size, the silica preheating temperature, the temperature of the aluminum, the rate of heat loss from the reactor, and/or the degree of agitation thereof in the reactor.

[0027] In another embodiment, an inert atmosphere is provided to the reactor to purge In another embodiment, an inert atmosphere is provided to the reactor to purge the atmosphere in the reactor of molecular oxygen (herein after "oxygen" e.g., O2 gas). For example, the inert atmosphere may substantially comprise argon, helium or any combinations thereof, or any other inert atmosphere that contains little or no oxygen and which does not react with the silica and aluminum. The inert atmosphere may be provided through any available method that allows for the continued mixing of the preheated silica and aluminum within the chamber. In one embodiment illustrated in FIG. 3, the kiln 1 1 may comprise an inlet port 34 in which an inert gas may be pumped into the chamber. The chamber may further comprise an outlet port 36 that, in combination with the inlet port, allows for the flushing of the kiln atmosphere such that an inert atmosphere is continuously provided. In another embodiment, an inert gas may be pumped into the reactor as the reactor is being sealed such that it is sealed with a substantially inert atmosphere. In yet another embodiment, a porous plug may be disposed in a wall of the reactor and used to inject inert gas. In such an embodiment, the location of the porous plug may have the additional benefit of further encouraging mixing between the preheated silica and aluminum in the reactor. Any other method may alternatively be employed such that the atmosphere within the reactor while the preheated silica and aluminum are mixed contains little or no oxygen. By minimizing the amount of oxygen in the reactor, less aluminum and silicon are lost by oxidation, and the silicon content of the alloy is enhanced.

[0028] As a result of the mixing between preheated silica and aluminum, the preheated silica is reduced to silicon in accordance with Reaction I. The silicon is combined with aluminum to form the aluminum-silicon alloy. Along with the aluminum-silicon alloy, a by-product slag is also produced, wherein the by-product slag typically comprises Si0₂ and Al₂0₃. At the temperatures of operation contemplated by several embodiments of the present invention, the by-product slag remains a solid.

[0029] Method 10 also comprises separating the aluminum-silicon alloy from the by-product slag in step 400. Separating the aluminum-silicon alloy may be accomplished in any number of ways. For example, the byproduct slag remains a solid, while the aluminum-silicon alloy is a molten liquid. The reactor may be tilted such that the aluminum-silicon alloy is poured out of the reactor while the by-product slag remains behind. In another embodiment, a tap hole 38 may be provided in the reactor to drain off the molten aluminum-silicon alloy into a casting or the like where the alloys cools and solidifies. In another embodiment, a screen or porous ladle adapted to withstand the temperature of the molten aluminum-silicon alloy may be employed to remove the by-product slag and/or other contaminants from the reactor. Any other alternative process or method for separating out the aluminum-silicon alloy may otherwise be used where such method substantially isolates the aluminum-silicon alloy from the by-product slag and any other additives or particulates.

[0030] An aluminum-silicon alloy is produced by preheating silica such that the temperature at which the aluminum is heated is lower than the processes used by the prior art to provide molten aluminum. The lower temperature of the aluminum during melting and processing serves to decrease any oxidative and/or vapor losses of aluminum. Additionally, the contents of the reactor utilized in combining the preheated silica and the aluminum may be agitated, such as by rotation, to encourage mixing between the two. In one embodiment, the purging of oxygen gas by the addition of an inert gas atmosphere further aids in the improvement of the overall efficiency of the alloying process.

[0031] Step 2 - Fractional Crystallization. After producing the aluminum-silicon alloy, the next step is directed to the fractional crystallization of the aluminum-silicon alloy to obtain first silicon crystals that contain approximately ten percent aluminum. This step can be performed using known methods, including methods described in U.S. Patent No. 7,727,503 ("the '503 Patent") to Scott Nichol, issued June 1 , 2010, and titled "Method for Purifying Silicon," which is incorporated by reference in its entirety.

[0032] According to an exemplary method described in the '503 Patent, a step includes contacting the first molten liquid with a first gas, to provide dross and a second molten liquid. Specifically, in this step of the described method of the '503 Patent, the majority of aluminum/copper is removed by slowly cooling an induction furnace while mixing to about 700° C from 1000° C, over about 4 hours. Slow cooling with mixing causes the crystals of silicon to slowly form improving their purity and size. Once 700° C is reached, a large tamper is used to compact the crystals in the bottom of the furnace. The tamper may have many small holes in it, e.g., approximately 3/16 inches in diameter, to allow the liquid to escape through the tamper. The tamper is pushed straight down with enough force to tightly compact the crystals at the bottom of the furnace and not damage the furnace. While the tamper is down, the furnace is tilted to pour the liquid aluminum-silicon out of the furnace. The furnace is then reheated to remelt remaining silicon crystals, which are preferably approximately 90% silicon and approximately 10% aluminum. This step may be repeated to improve the purity of the silicon.

[0033] Step 3 - Acid Leaching of Silicon Crystals. In the third step, the isolated first silicon crystals are placed in contact with an acid-water mixture for sufficient time to dissolve the aluminum in the crystals. This step can be performed using known methods, including methods described in U.S. Patent Application Publication No. 2010/0329959 A1 ("the '959 Publication") to Scott Nichol ef a/., published December 30, 2010, and titled "Use of Acid Washing to Provide Purified Silicon Crystals" (U.S. Patent Application Serial No. 12/670,263), which is incorporated by reference in its entirety.

[0034] According to an exemplary method described in the "959 Publication, a step includes contacting the first silicon crystals with an acid-base alcohol or chemical capable of dissolving the aluminum to provide washed silicon crystals and used acid. The washed silicon crystals are separated from the used acid sufficiently (e.g., by filtration) to provide purified silicon crystals. In an example, first silicon crystals were placed in a solution of 8 wt % HCL+water and the aluminum was dissolved off of the first silicon crystals in 72 hours to form second silicon crystals. The second silicon crystals were, then, strained from the acid and dried.

[0035] Step 4 - Refining the Second Silicon Crystals. In the fourth step, various methods can be used to further refine the second silicon crystals to reduce aluminum and calcium to less than 40 ppmw. Generally, an exemplary method directed to refining the second silicon crystals of step 3 includes adding a calcium source selected from the group consisting of Ca, CaO, and CaC0₃, and optionally with Si0₂ to an aluminum-containing silicon. The second silicon crystals are heated to melt them, and the molten silicon is exposed to oxygen to produce refined silicon (e.g., refined silicon in the form of silicon crystals) and a by-product slag such that the refined silicon contains an amount of aluminum that is less than the amount of aluminum in the second silicon crystals of step 3. The source of calcium, and optionally silica, may be combined with the second silicon crystals either prior to, during, or after heating takes place. However, preferably the second silicon crystals are melted prior to the addition of the source of calcium and optionally silica.

[0036] The initial amount of aluminum in the second silicon crystals will influence the overall refining method, such as, for example, whether the refining method is repeated and performed multiple times to reduce the amount of aluminum in the refined silicon. For ease of explanation, the second silicon crystals will also be referred to below as the "aluminum- containing silicon." [0037] In one embodiment, the aluminum-containing silicon may be provided to the induction furnace directly. The aluminum-containing silicon is then heated to form molten silicon. During the heating, the silicon may be heated to any temperature sufficient to reduce the silicon to a molten state. For example, in one embodiment, the silicon may be heated to a temperature of 1400°C to 1700°C, or from 1500°C to 1600°C, or to approximately 1550°C. Heating temperatures may vary throughout the heating step. Thus, where it is said that aluminum- containing silicon is heated to a temperature of about 1550°C, the actual temperature may fluctuate and may not be held constant at said temperature. Furthermore, the temperature at which the aluminum-containing silicon is heated to may depend on the temperatures desired during later steps in the refining method. For example, where in subsequent steps of the refining method, it may be preferred to utilize a lower temperature (for example, a temperature less that about 1500°C) as opposed to a higher temperature, the temperature that the aluminum-containing silicon is heated to may take into account predicted heat losses for subsequent steps. Thus, the actual temperature used for melting the aluminum-containing silicon into molten silicon may depend, in part, on both anticipated heat loss as well as the desired temperature for subsequent steps of the overall refining method.

[0038] The heating and melting of the aluminum-containing silicon in may be accomplished in a variety of ways. For example, in one embodiment, the silicon may be melted using an induction furnace, a resistance furnace, or any other technique used in the art to heat and melt silicon. In some embodiments, it may be desirable to employ an induction furnace with a cycle frequency less than 3000 Hertz to allow for vigorous stirring so that substantially all of the silicon is dispersed as it is melted. Any other heating apparatus operable to melt the aluminum-containing silicon may alternatively be used such that the silicon is melted.

[0039] After the aluminum-containing silicon is heated to a molten state, the molten silicon is exposed to oxygen. For example, in one embodiment, the molten aluminum-containing silicon is transferred to a ladle. The ladle may comprise any metallurgical ladle operable to reduce heat loss from the molten silicon and permit the injection of oxygen into the molten material. In one embodiment, the ladle may comprise a porous plug or a lance from the top of the refining ladle operable to facilitate the injection of oxygen into the molten silicon while the molten silicon is disposed in the ladle. In another embodiment, the molten silicon may be transferred into any other container, receptacle or other storage device capable of allowing the injection of a gas or mixture of gases into the molten silicon.

[0040] Once the molten aluminum-containing silicon is transferred to the ladle, a source of calcium is added to the molten aluminum-containing silicon. Optionally, a source of silica (Si0₂) may also be added. Alternatively, a source of calcium, and optionally a source of silica, may be placed in the bottom of the refining ladle before transferring the molten into the ladle. [0041] Without intending to be bound by any one particular theory, it is believed that the addition of a source of calcium, or optionally a mixture of a source of calcium and silica, results in the removal of aluminum via the production of a by-product slag once the molten material is exposed to oxygen gas such as, for example, by injecting oxygen gas into the molten material. That is, by adding appropriate amounts of the calcium source to the molten silicon and heating the molten silicon to a desired temperature, all as described herein, the calcium is believed to distribute itself as a dissolved element in the molten silicon and as an oxide in the by-product slag. The source of calcium can be produced by means readily available to those skilled in the art or purchased commercially. The amount of calcium to be added to the molten aluminum-containing silicon depends on the initial amount of aluminum in the silicon and the total weight of silicon being refined.

[0042] By knowing 1 ) the initial amount of aluminum in the molten silicon, and 2) the targeted amount of aluminum in the byproduct slag, one may derive, using thermodynamic principles, the proper amount of calcium to be added to the molten silicon to effect a desired reduction in the amount of aluminum in the molten silicon. For example, to achieve an aluminum content in the refined silicon product of less than about 50 ppmw, the initial silicon-containing aluminum may be heated to about 1550 °C and, depending on the initial aluminum content, an appropriate amount of calcium is added.

[0043] If the source of calcium is added as calcium oxide or calcium carbonate, the weight added should be proportionately calculated to achieve the proper weight based on calcium. Optionally, silica (Si0₂) may be added to the aluminum-containing silicon to aid in aluminum reduction in the refined product. Generally, for aluminum concentrations of from about 0.05 to about 0.3 wt.% in the aluminum-containing silicon material, calcium may be added in amounts ranging from about 0.05 to about 3.0 wt. % calcium, based on the total weight of the aluminum-containing silicon material. The ratio of the weight % of calcium added to the weight % of aluminum present in the aluminum-containing silicon material is from about 1 to about 10, from about 2 to about 7, from about 3 to about 6.5, and from about 4.5 to about 5.5.

[0044] In one embodiment, the source of calcium may be added in the form of precipitated calcium carbonate (CaC0₃). In such an embodiment, calcium carbonate is added to the molten silicon as a weight percent of the molten silicon. The added calcium carbonate can range from greater than 0.1 wt % to 10 wt %, from 1 wt % to 5 wt %, or to about 2.5 wt % of the weight of the molten silicon. As discussed above, the amount of calcium carbonate added to the molten silicon may depend on the initial level of aluminum in the silicon, the targeted amount of aluminum in the by-product slag, as well as the desired amount of aluminum and calcium in the refined silicon. The calcium carbonate may be introduced into the molten silicon by injecting powdered calcium carbonate, or, optionally, a mixture of calcium carbonate and silica through a lance with oxygen, nitrogen, or a mixture of nitrogen and oxygen. The optional addition of silica (Si0₂) with the calcium carbonate operates to provide a desired final byproduct slag composition.

[0045] The source of calcium, in any of its forms (such as calcium, calcium oxide, or calcium carbonate), can be added to the molten silicon in a variety of ways. For example, in one embodiment, calcium carbonate may be added directly to the batch of molten silicon. In another embodiment, calcium oxide may be added serially such that a first charge of calcium oxide mixes with the molten aluminum-containing silicon before a second charge of calcium oxide is added. After the source of calcium and, optionally silica, are added, the molten silicon is exposed to oxygen.

[0046] The combined effect of adding a source of calcium to the molten aluminum-containing silicon and exposing it to oxygen results in the production of refined silicon having lower aluminum content and a by-product slag. The by-product slag comprises calcium oxide, aluminum oxide, and silica, while the refined silicon comprises silicon and an amount of aluminum which is less than the initial amount of aluminum in the original aluminum- containing silicon material. In addition, the refined silicon will also comprise an amount of boron that is less than an initial amount of boron in the original aluminum-containing silicon material. Specifically, aluminum and other impurities (such as boron) that were initially present in the aluminum-containing silicon are at least partially removed via the production of the byproduct slag wherein the amount of each component present in the by-product slag (e.g., calcium oxide, aluminum oxide, and silica) is a result of the aluminum and other impurities removed from the molten silicon batch. For good mixing to occur between the by-product slag and the refined silicon, that desirably the melting point of the by-product slag should be below that of the silicon. Furthermore, the density, viscosity, and melting point of the by-product slag permits one to predict the degree of refining (i.e. the amount of aluminum removed) of the molten silicon in the production process.

[0047] Exposing the molten aluminum-containing silicon to oxygen comprises adding oxygen to the molten silicon using any available method and in any amount which operates to oxidize the aluminum and calcium in the molten silicon into a by-product slag such that refined silicon is produced as discussed above. For example, where the ladle housing the molten silicon contains a porous plug, oxygen can be introduced via the porous plug. In one embodiment, oxygen can be added as a gaseous mixture that comprises oxygen and nitrogen. For example, in such an embodiment the gaseous mixture comprises a mixture of from about 50 to about 95% oxygen and from about 5 to about 50% nitrogen, or from about 60 to about 90% oxygen and from about 10 to about 40% nitrogen, or about 80% oxygen and about 20% nitrogen (all percentages by weight).

[0048] In addition, the gas flow rate of the oxygen may be varied depending on the overall amount of molten silicon as well as the specific composition of the molten silicon. For example, in one exemplary method, the gas flow rate for 320 kilograms ("kg") of molten aluminum-containing silicon is about 5-14 Normal-cubic meter per hour ("Nm³/h"). The molten silicon is exposed to oxygen for a time sufficient to produce by-product slag and refined silicon. Typically, such exposure will be, for example, from about 10 minutes to about 120 minutes, from about 30 minutes to about 60 minutes, or for about 45 minutes.

[0049] Once the molten silicon material containing the source of calcium is exposed to oxygen such that by-product slag and refined silicon are produced, the by-product slag is separated from the refined silicon. The by-product slag comprises a higher density phase than that of the refined silicon phase so that as the refined silicon is poured from the ladle, the higher density by-product slag remains behind. In another embodiment, the refined silicon may be passed through a screen or filter to separate it from the by-product slag.

[0050] Depending in part on the initial amount of aluminum present in the original aluminum- containing silicon, the refining method can be repeated to achieve further removal of aluminum from the refined silicon. For example, the initial amount of aluminum present in the original aluminum-containing silicon can be determined and compared with a predetermined threshold value. The threshold value can be chosen such that if the remaining amount of aluminum in the refined silicon batch is above that threshold value, the refining method is repeated. However, if the remaining amount of aluminum in the refined silicon batch is equal to or less than the chosen threshold value, no further purification is necessary, and the refined silicon can be used, for example, for solar applications. In one exemplary embodiment, the threshold value for the remaining aluminum content in the refined silicon may be chosen to be in the range of from about 10 ppmw to about 100 ppmw, from about 20 ppmw to about 75 ppmw, or about 50 ppmw.

[0051] By adding a source of calcium and, optionally silica, along with oxygen or an oxygen- containing gas to the aluminum-containing silicon, refined silicon can be produced having less than about 50 ppmw aluminum and only trace amounts of other impurities. The following examples demonstrate the measured effect of adding calcium carbonate and oxygen to an aluminum-containing silicon material for the removal of aluminum. Comparative Example 1 demonstrates that when aluminum-containing silicon is refined with a mixture of oxygen and nitrogen without adding a source of calcium, the aluminum content of the silicon is not appreciably lowered (i.e., little or no aluminum is removed from the silicon). Examples 2 and 3 demonstrate that when aluminum-containing silicon is refined with oxygen and a source of calcium (e.g., calcium carbonate), refined silicon is produced having an amount of aluminum that is less than the initial amount of aluminum in the original aluminum-containing silicon material.

[0052] Step 5 - High-Yield Crystallization. In the final step, the refined silicon of step 4 is even further refined using any directional solidification technique, such as, for example, a rotational casting process. In the rotational casting process, impurities of different densities are separated and concentrated using centrifugal force, controlled crystallization of the silicon, or a combination thereof.

[0053] According to various embodiments, a method of refining the refined silicon may include

(I) providing a mold comprising a longitudinal axis, a mold cavity defined by an inner mold surface and a hollow bore extending along the longitudinal axis, and an outer mold surface;

(II) pre-heating the mold; (III) introducing a predetermined amount of molten silicon into the heated mold cavity while continuously rotating the mold around the longitudinal axis at a speed sufficient to form a hollow body of molten silicon comprising an inner surface and an outer surface that is in contact with the inner mold surface, wherein the body extends along the longitudinal axis of the mold; and (IV) cooling the outer mold surface while continuously rotating the mold to effect directional solidification of the molten silicon from the outer surface of the body to the inner surface of the body. In some embodiments, the inner silicon surface may be heated to control the rate of directional solidification from the outer surface of the body to the inner surface of the body.

[0054] The method may include (I) providing a mold having a longitudinal axis, a mold cavity defined by an inner mold surface and a hollow bore extending along the longitudinal axis, and an outer mold surface. Mold cavity dimensions and the volume of molten silicon introduced can be configured to provide castings of varied size, weight, diameter, and wall thickness. In some embodiments, the mold may be of varied shapes or diameters, provided that the diameter of the mold cavity is uniform and concentric to the diameter of the outer mold surface. In some embodiments, the mold has a shape selected from cylindrical and tapered. In some embodiments, the mold may be of a material suitable for high temperature applications. Examples of suitable materials include steel, cast iron, steel alloys, molybdenum, titanium, ceramic and other materials suited to the operating temperature and stresses of the process. Materials may be solid or composite layered to form the mold body. In some embodiments, the mold may be maintained at an orientation that is substantially vertical or substantially horizontal. In some embodiments, one or more end-caps may be utilized with the mold to prevent leakage of the molten silicon. Good results have been obtained with a cylindrical steel mold maintained at a substantially horizontal orientation. In some embodiments, a suitable mold is one that is capable of obtaining and maintaining a rotational speed that will generate centrifugal acceleration of up to 400 G on its inner surface and the molten silicon within its cavity.

[0055] The inner mold surface has a high temperature, non-reactive refractory material suitable for providing a mold release and thermal interface for the silicon introduced into the mold. Examples of suitable materials include silica, silicon carbide, silicon nitride, boron nitride, alumina, magnesia, alumina-silicate, and combinations thereof. In some embodiments, the refractory material has at least 1 % (w/w) of silica. In some embodiments, the refractory material has from about 10 to about 100% (w/w) of silica. For example, the refractory material may comprise from about 10-15%, 15-20%, 20-25%, 25-30%, 30-35%, 35- 40%, 40-45%, 45-50%, 50-55%, 55-60%, 60-65%, 65-70%, 70-75%, 75-80%, 80-85%, 85- 90%, 90-95%, 95-100% (w/w) of silica. Good results have been obtained with a refractory material comprising from about 30 to about 98 % (w/w) of silica. The refractory material is uniformly applied to the inside surface of the mold and may be applied in any suitable manner including spray coating or hand loading into the spinning mold.

[0056] The method provided may further include (II) heating the mold prior to introducing a predetermined amount of molten silicon. In some embodiments, the outer mold surface is heated to a temperature of from about 25 to about 700 °C. For example, the temperature may be 25-50 °C, 50-100 °C, 100-150 °C, 150-200 °C, 200-250 °C, 250-300 °C, 300-350 °C, 350- 400 °C, 400-450 °C, 450-500 °C, 500-550 °C, 550-600 °C, 600-650 °C, 650-700 °C, or combinations thereof. In some embodiments, the inner mold surface is heated to a temperature of from about 25 to about 1600 °C. For example, the temperature may be 25-50 °C, 50-100 °C, 100-150 °C, 150-200 °C, 200-250 °C, 250-300 °C, 300-350 °C, 350-400 °C, 400-450 °C, 450-500 °C, 500-550 °C, 550-600 °C, 600-650 °C, 650-700 °C, 700-750 °C, 750- 800 °C, 800-850 °C, 850-900 °C, 900-950 °C, 950-1000 °C, 1000-1050 °C, 1050-1 100 °C, 1 100-1 150 °C, 1 150-1200 °C, 1200-1250 °C, 1250-1300 °C, 1300-1350 °C, 1350-1400 °C, 1400-1450 °C, 1450-1500 °C, 1500-1550 °C, 1550-1600 °C, or combinations thereof. In some embodiments, the inner mold surface is heated to a temperature that is above the melting temperature of the silicon to be introduced into the mold. In some embodiments, the outer mold surface and the inner mold surfaces are heated. The mold may be heated by any suitable heating device, and the devices used for heating the inner and outer mold surfaces may be the same or different. Examples of suitable heating devices include a hydrogen/oxygen torch, an oven, a fuel gas heater/burner, an electric heater, or combinations thereof. Good results have been obtained with heating the outer mold surface to a temperature of from about 25 °C to about 350 °C and the inner mold surface to a temperature of from about 1 100 °C to about 1550 °C.

[0057] The method may also include (III) introducing a predetermined amount of molten silicon into the heated mold while continuously rotating the mold around the longitudinal axis at a speed sufficient to form a hollow body of molten silicon comprising an inner surface and an outer surface that is in contact with the inner mold surface, wherein the body extends along the longitudinal axis of the mold. If the rotational speed of the mold and the fluidity/temperature of the molten silicon are adequate, the molten silicon is uniformly distributed along the inner mold surface throughout the length of the mold. According to some embodiments, rotation of the mold around the longitudinal axis at a speed sufficient to generate equivalent gravitational acceleration of from about 1 to about 400 g is sufficient to form the body of molten silicon. For example, rotational speed may be sufficient to generate 1- 15 g, 15-30 g, 30-45 g, 45-60 g, 60-75 g, 75-90 g, 90-105 g, 105-120 g, 120-135 g, 135-150 g, 150-165 g, 165-180 g, 180-195 g, 195-210 g, 210-225 g, 225-240 g, 240-255 g, 255-270 g, 270-285 g, 285-300 g, 300-315 g, 315-330 g, 330-345 g, 345-360 g, 360-375 g, 375-390 g, 390-400 g, and combinations thereof. Good results have been obtained with rotational speeds sufficient to generate from about 3 to about 120 g. In some embodiments, the rotational speed can be lower during introduction of the molten silicon followed by rapid acceleration. In alternative embodiments, the molten silicon can be introduced into a stationary mold, followed by rapid acceleration to cause uniform distribution. Suitable equivalent gravitational acceleration (g) varies with respect to mold size, mold cavity size, desired casting size, volume of silicon feedstock introduced, desired purity, and other application-specific factors. Thus, the present invention is not limited to the rotational speeds or equivalent gravitational acceleration described herein.

[0058] The molten silicon may be introduced into the mold in any suitable manner, but is typically introduced in a manner allowing its initial speed to be in the direction of the mold's rotation in order to provide a uniform distribution on the inner mold surface. Examples of suitable pouring devices include, but are not limited to, a ladle, an angled nozzle spout, a straight nozzle spout, or a pouring boot. In some embodiments, the molten silicon can be introduced at one end of the mold, from both ends of the mold, from the interior of the mold (via use of a lance or other distributor), or combinations thereof. In some embodiments, the molten silicon can be filtered for impurities prior to, or concurrent with, its introduction into the mold, and any suitable filter may be utilized. Examples of suitable filters include, but are not limited to, silicon carbide, aluminum oxide, and aluminum oxide/graphite ceramic filters. Good results have been obtained with pre-filtering the molten silicon by pouring through a silicon carbide ceramic foam filter. In some embodiments, the molten silicon can be introduced and maintained within the spinning mold while under a vacuum or inert ambient conditions.

[0059] According to various embodiments, after introduction of the molten silicon, the method may include continuing rotation of the heated mold at a sufficient temperature and duration to provide sufficient time for particle and slag migration through the melt into the outer surface of the silicon body. Higher density "sinking" slag and other impurities will be concentrated on the outer surface of the silicon body closest to the refractory layer and lighter density "floating" slag and other impurities will concentrate at the inner surface of the silicon body. The use of a synthetic slag can also be employed to assist in the migration and concentration of impurities within the silicon body, and/or to assist in the provision of a thermal barrier as a means to control heat loss from the inner surface of the liquid silicon body. Such slag can be introduced into the molten silicon during the pouring process into the mold. In some embodiments, the mold cavity/hollow silicon body may be heated during this process in order to maintain a temperature of from about 1 100 to 1600 °C. For example, temperature may be maintained at 1 100-1 150 °C, 1 150-1200 °C, 1200-1250 °C, 1250-1300 °C, 1300-1350 °C, 1350-1400 °C, 1400-1450 °C, 1450-1500 °C, 1500-1550 °C, 1550-1600 °C, and combinations thereof. In some embodiments, the outer mold surface may be heated during this process in order to maintain a temperature of from about 25 to 700°C. For example, temperature may be maintained at 25-50 °C, 50-100 °C, 100-150 °C, 150-200 °C, 200-250 °C, 250-300 °C, 300- 350 °C, 350-400 °C, 400-450 °C, 450-500 °C, 500-550 °C, 550-600 °C, 600-650 °C, 650-700 °C, and combinations thereof.

[0060] Mold and silicon body temperature may be controlled by any suitable device. Examples of suitable devices include a hydrogen/oxygen torch, an oven, a fuel gas heater/burner/torch, an electric heater, a water box, a water spray, a water jet, compressed air and other gases, and combinations thereof. Good results have been obtained by use of an external fuel gas burner to heat the outer mold surface, or a water spray jet to cool the outer mold surface, or a propane/oxygen torch to heat the inner mold surface/hollow silicon body.

[0061] In some embodiments, a hydrogen/oxygen torch may also be used to refine silicon. The torch is directly combusted within the mold cavity/hollow silicon body, wherein the resultant combustion gas introduces water vapor, and/or unreacted hydrogen or oxygen into the molten silicon to promote refining of the silicon through oxidation and vaporization of the entrained impurities. Targeted impurities for removal include sodium, calcium, potassium, boron, and phosphorus. The refining of molten silicon with a hydrogen/oxygen torch may also be, but is not required to be, practiced in combination with controlling the speed of the rotating mold to cause slippage or raining of the molten silicon in order to achieve mixing, which increases the surface area of the molten silicon exposed to the torch combustion gases, thereby allowing for removal of volatile impurities.

[0062] In some embodiments, the speed of the mold is decreased after the heated mold has been rotated at a sufficient temperature and duration to cause one or more higher density impurities in the molten silicon to concentrate near the outer surface of the body and one or more lower density impurities to concentrate near the inner surface of the body. For example, after the silicon body has been formed, the mold may be rotated at a temperature and duration sufficient to cause at least silicon carbide to concentrate near the outer surface of the body. In some embodiments, the speed may be decreased to speeds sufficient to generate equivalent gravitational acceleration (relative to free fall, i.e., g-force) of from about 1 to about 25 g. For example, reduced speed may be sufficient to generate 1 -5 g, 5-10 g, 10-15 g, 15-20 g, 20-25 g, and combinations thereof. Good results have been obtained by decreasing the speed of the mold to speeds sufficient to generate from about 3 to about 10 g. [0063] The method provided herein may further include (IV) cooling the outer mold surface while continuously rotating the mold to effect directional solidification of the molten silicon from the outer surface of the body to the inner surface of the body. By cooling the outer mold surface (and controlling the temperature of the inner surface of the silicon body), controlled silicon crystal growth (in a radial direction from the silicon/refractory interface towards the inner surface of the silicon body) can be achieved. In some embodiments, such directional solidification occurs at a rate of from about 0.1 to about 3 millimeters/minute (mm/min). In some embodiments, such directional solidification occurs at a rate of from about 0.5 to about 1.5 mm/min. However, other rates of solidification are possible and that the present invention is not limited to the rates of solidification described herein. Any suitable cooling device may be used to cool the outer surface of the mold, thereby controlling the rate of directional solidification. Examples of suitable cooling devices include a water box, a water spray, compressed air and other gases, liquefied gases, and a water jet.

[0064] In silicon directional solidification, maximization of the segregation velocity during directional solidification can be achieved through the mixing of the liquid silicon at the liquid/solid interface. According to embodiments of the instant method, this mixing effect can be achieved through slippage of the liquid silicon by the controlled rapid acceleration and deceleration of the spinning mold (via controlling drive motor speed control through variable frequency drive technology); by rotating the mold at or near raining speed; through recirculation currents generated within the rotating mold cavity; and combinations thereof.

[0065] According to various embodiments, the method may include varying the speed of the mold to that sufficient to cause slippage or raining of the molten silicon to achieve mixing of the liquid silicon at the liquid/solid interface. While, the step of raining is typically performed prior to directional solidification, it may also be done after the onset of directional solidification. According to various embodiments, the method may include rapidly varying the speed of the mold in order to cause slippage of the molten silicon, thereby achieving mixing of the liquid silicon at the liquid/solid interface. In some embodiments, the rotation of the mold is rapidly decreased to speeds sufficient to generate equivalent gravitational acceleration of from about 3 g to about 25 g. For example, rotational speed may be decreased to speeds sufficient to generate equivalent gravitational acceleration of from about 3 g-5 g, 5 g-10 g, 10 g-15 g, 15 g- 20 g, 20 g-25 g, or combinations thereof. In some embodiments, the rotational speed of the mold may be rapidly increased to speeds sufficient to generate equivalent gravitational acceleration of from about 140 g to about 300 g. For example, rotational speed may be increased to speeds sufficient to generate equivalent gravitational acceleration of from about 140 g - 160 g, 160 g - 180 g, 180 g - 200 g, 200 g - 220 g, 220 g - 240 g, 240 g - 260 g, 260 g - 280 g, 280 g -300 g, or combinations thereof. Good results have been obtained by rapidly decreasing the rotational speed of the mold to speeds sufficient to generate equivalent gravitational acceleration of from about 3 g to about 10 g, followed by rapidly increasing the rotational speed of the mold to speeds sufficient to generate equivalent gravitational acceleration of from about 150 g to about 200 g.

[0066] According to various embodiments, the method may include the use of recirculation flows within the spinning mold to achieve mixing of the liquid silicon at the liquid/solid interface. Recirculation flow is generated within the molten silicon, which disperses the saturated impurity boundary during the directional solidification process. In some embodiments, mold vibration is generated through imbalance of the spinning mass to promote this effect.

[0067] According to various embodiments, after a desired yield of solidified silicon is achieved, the rotational speed of the mold may be decreased, the mold elevated, and the remaining liquid silicon poured from the end of the mold, thereby leaving a hollow solidified silicon casting within the mold.

[0068] According to various embodiments, after a desired yield of solidified silicon is achieved, the mold rotation can be stopped, the mold end-cap(s) opened, and the remaining liquid silicon poured from the end of the mold, thereby leaving a hollow solidified silicon casting within the mold.

[0069] The hollow silicon casting may include an inner surface and an outer surface that is in contact with the inner mold surface. The molten silicon removed has a higher concentration of impurities as compared to the remaining solidified silicon in the casting and can be used as a secondary product or be recycled for other purposes. In some embodiments, the rotational speed of the mold may be decreased and the remaining molten silicon removed when from about 10 to about 90% (w/w) of the molten silicon has solidified. For example, the molten silicon can be removed when solidification is 10-20%, 20-30%, 30-40%, 40-50%, 50-60%, 60- 70%, 70-80%, 80-90%, and combinations thereof. Good results have been obtained by decreasing the speed of from about 0 to about 3 g and removing the remaining molten silicon when from about 50 to about 80 % (w/w) of the molten silicon has solidified.

[0070] According to various embodiments, after the molten silicon is removed, a heating device (such as a hydrogen/oxygen torch) can be used to melt a thin layer of silicon from the hollow casting to release concentrated impurities trapped within the dendritic structure of the crystallized silicon. The resulting molten silicon is also removed. Good results have been obtained by melting a 1-5 mm layer of silicon from the hollow casting. However, the desired depth of melting will depend upon the specific application and that the present invention is not limited to the depths described herein.

[0071] According to various embodiments, after the molten silicon is removed, a heating device (such as a hydrogen/oxygen torch) can be used to melt a thin layer of silicon from the hollow casting to release concentrated impurities trapped within the dendritic structure of the crystallized silicon. The resulting molten silicon is also removed. Good results have been obtained by melting a 1-5 mm layer of silicon from the hollow casting. However, one of skill in the will understand that the desired depth of melting will depend upon the specific application and that the present invention is not limited to the depths described herein.

[0072] According to various embodiments, after a desired percentage of the molten silicon has solidified and the remaining molten silicon has been removed, the method comprises cooling the mold and casting to a sufficient temperature (for example, 150-250°C), and separating the silicon casting from the mold. The casting can be extracted from the centrifugal mold via a machine mounted hydraulic extraction mechanism. In some embodiments, the mold cavity may be tapered (for example, 2-5 degrees) to facilitate easier removal of the casting from the refractory interface. Additionally, supplemental heat from only the external heating device can also be applied to the external surface of the mold to facilitate an expansion of the outer mold surface relative to the casting outer surface.

[0073] In some embodiments, residual higher density impurities from the outer surface of the silicon casting and residual lower density impurities from the inner surface of the silicon casting can be removed by surface treatment. Treatment of the inner and outer surfaces of the casting in order to remove additional impurities may be achieved by any suitable process. Examples include melting or chipping, sawing, vaporizing, particle blasting, or use of other ablative processes to remove a predetermined amount of the surface where undesired impurities are concentrated. Good results have been obtained by removing the impurities on the outer and inner surfaces of the casting by chipping and quartz grit blasting. After the casting has been cooled, removed from the mold, and further refined by surface treatment, it may be crushed and packaged per suitable material handling processes.

[0074] The method provided herein allows for efficient, cost-effective, high throughput methods for the bulk purification of silicon. For example, the provided method may be used to reduce the concentration of one or more of sodium, calcium, potassium, boron, phosphorus, and silicon carbide in silicon. The method can be used with any grade of silicon feedstock, including chemical grade, metallurgical grade, electronics grade, and solar grade silicon, as well as silicon-containing alloys. The purified silicon prepared according to the methods provided herein may be used in a variety of applications with or without further refinement. However, the degree of refinement achievable within one casting is dependent upon, among other things, the grade of silicon feedstock. Accordingly, the methods described herein may need to be repeated more than once in order to achieve the desired purity of refined silicon. A specific example is described below.

[0075] Approximately 121 kg of silicon metal was melted in a 1000 pound (lb; 450 kg) "Box" InductoTherm induction furnace lined with an Engineered Ceramics "Hycor" model CP-2457 crucible and sealed with Vesuvius "Cercast 3000" top cap refractory. During the melting process, a nitrogen gas purge was introduced into the induction furnace headspace to reduce the formation of SiO gas and silicon dioxide.

[0076] The silicon melt was heated to 1524 °C, prior to being poured into a Cercast 3000 refractory lined, transfer ladle. The transfer ladle was preheated to 800 °C, using a propane/air fuel torch assembly. After pouring, the temperature of the silicon melt in the transfer ladle was measured at 1520 °C prior to pouring into the centrifugal casting machine. The silicon was sampled from both the furnace and transfer ladle to establish a baseline material elemental analysis.

[0077] A model M-24-22-12-WC centrifugal casting machine manufactured by the "Centrifugal Casting Machine Company" was fitted with a refractory lined, nominal 420 mm diameter x 635 mm long steel casting mold. The silicon casting produced in this experiment measured 372 mm in diameter x 635 mm long x 74 mm wall thickness. Advantage W5010 mold wash was sprayed onto the inner surface of the rotating casting mold to provide a base coating of approximately 1 mm thick. The steel mold was rotated at 58 rpm and was preheated to 175 °C using an external burner assembly. The mold was then sped up to 735 rpm and hand-loaded with a sufficient volume of Cercast 3000 refractory to centrifugally create a 19 mm thick refractory layer within the mold. The mold was then transferred into a heat treatment oven whereby the mold was maintained at 175 °C for an additional 4 hours before being allowed to slowly cool to ambient temperature.

[0078] Vesuvius "Surebond SDM 35" was hand loaded into the mold cavity and the mold was spun at 735 rpm to uniformly generate a 6 mm thick inner shell of refractory. After 30 minutes of spinning, the mold assembly was stopped and allowed to air dry for 12 hours.

[0079] A propane/oxygen torch was used to preheat the mold inner refractory surface to 1315 °C. The torch nozzle was positioned flush to the 100 mm opening in the end-cap and was directed into the mold and allowed to vent out the rear 100 mm opening in the opposing end- cap.

[0080] A transfer ladle, supported on a "Challenger 2" model 3360 weigh scale device, was used to measure 120 kg of silicon into the casting mold. Silicon metal was poured from the transfer ladle at 1520°C into a refractory coated mold that was rotating at 735 rpm.

[0081] Mold speed was maintained at 735 rpm for 4 minutes to allow for impurity and slag separation. The mold speed was then slowly reduced to a point in which the material visually appeared as pooling in the bottom of the spinning mold and droplets appeared to be slumping at the top of the mold (near raining point). Mold speed was measured as 140 rpm and was maintained for 30 minutes with only ambient air cooling. The mold speed was then increased to 735 rpm and was maintained for 63 minutes of directional solidification. An alumina ceramic rod was inserted through the 100 mm opening in the mold cap to verify that the core of the casting was still liquid. The experiment was concluded when the casting was visually deemed solid and the dip rod was unable to penetrate the inner surface of the casting.

[0082] Experimental temperature data was recorded for the mold outside temperature using a Fluke 65 infrared thermometer measurement instrument. Internal mold, and ladle temperatures were measured using an Omega OS524 instrument. Mold rpm was measured using an Extech model TACH+IR instrument. Liquid silicon melt temperatures were measured using a HelectroNite model Heraeus instrument.

[0083] After 100% solidification, the casting was allowed to spin for an additional 45 minutes to provide air-cooling to the mold prior to removal from the centrifugal casting machine. The mold and casting were then removed and allowed to cool slowly overnight.

[0084] A hydraulic press was used to extract the casting from the steel mold body. The refractory shell was separated and the casting was blasted with silica grit to remove remaining traces of refractory.

[0085] The casting was sectioned, polished, and etched for visual inspection of crystal grain growth. The casting was core drilled and sliced into approximately 6 mm thick samples using a Buehler "Isomet 4000" sample slicer. Individual sample slice thicknesses were recorded along with the original total drilled core length. Saw kerf was calculated based on the comparison of total slice thickness relative to original drilled core length. Slice 01 was visually contaminated with a porous slag material and slice 12 contained visual refractory contamination from the casting to refractory interface.

[0086] Furnace and ladle melt samples were also submitted for analysis. Each sample slice was washed in a solution of 35% HCI mixed at a ratio of 1 :4 with de-ionized water. Each sample slice was allowed to soak for 20 minutes in the solution before being rinsed in a container of 100% de-ionized water. After the water rinsing, each slice was then dipped into acetone to speed air drying of the sample. Samples were ground in a Fritsch model "Pulverisette 0" mill and were analyzed using ICP-OEMs analysis. Specific boron and phosphorous data were tabulated into a spreadsheet, such that the slice closest to the refractory (casting O.D.), was indicated as the first data point. Volumetric % for each slice was calculated relative to the total casting volume through the summation of progressive slice and saw kerf thicknesses. Each slice was represented in a spreadsheet as a % of the total casting cylindrical volume.

[0087] Analytical data was compared against the theoretical expectations as a function of crystallization depth in Table 1. The data contained in Table 1 demonstrates functional directional solidification verified through Inductively coupled plasma-mass spectrometry ("ICP- MS") elemental analysis for boron and phosphorous for each sample slice. Results indicate at or near maximum theoretical value up to 82% yield (+/- 10% analytical error). Slices #12 and #1 were omitted due to contamination from concentrated impurities and refractory. Table 1

ppmw = parts per million by

weight

[0088] The above example illustrates some embodiments of the methods described herein. In particular, it illustrates the ability to perform the pouring and centrifugal casting of a silicon body within a centrifugal casting machine mold, as well as the ability to use torches to heat external and internal surfaces of the mold body. In addition, it demonstrates slippage and raining at a 3g mold speed, and the ability to rapidly accelerate the mold and silicon to full speed (100g) from at/near raining point (3g). Moreover, it demonstrates pouring of molten silicon from the end-cap openings of the mold for demonstration of yield control, and the ability to perform purification of the silicon metal through directional solidification (Table 1 ) at 0.78 mm/min. Finally, the example illustrates casting extraction and surface treatment.

[0089] Example 1 - Illustrative Example of Producing a Silicon Grade Silicon. According to an illustrative example, refined silicon is produced in accordance with the first four steps described below. The fifth step, which was not performed, would have provided a solar grade silicon.

[0090] Initially, in the first step, an aluminum-silicon alloy was produced by initially feeding a recommended amount of quartz particles inside a rotary kiln. The quartz particles and a refractory lining of the rotary kiln was heated from about 1200°C to about 1400°C using a burner. Molten aluminum or aluminum ingots were fed into the rotary kiln. The decision whether to feed liquid or solid aluminum was based on the amount of heat stored in the refractory lining of the rotary kiln. The rotary kiln was sealed and an inert gas, e.g., argon, was injected. The rotary kiln was kept in rotation between about 5 revolutions per minute to about 15 revolutions per minutes, for about 45 minutes to about 100 minutes. The produced aluminum-silicon alloy was poured into a cast mold.

[0091] In the second step, the aluminum-silicon alloy was fractionally crystallized by first feeding a necessary amount of the aluminum-silicon alloy into an induction furnace. A fractional crystallization unit was heated to a temperature between about 1 100°C to about 1200°C. The heating process curve was carried up to achieve a molten aluminum-silicon alloy. The molten aluminum-silicon alloy was heated up to between about 1000°C to about 1200°C. The molten aluminum-silicon alloy was poured into the fractional crystallization unit and kept for about 6 to about 24 hours. Additional time might be considered if necessary to achieve a desirable temperature from about 650°C to about 800°C. The remaining molten aluminum-silicon alloy was poured into a cast mold and the cover of the fractional crystallization unit was removed. The produced first silicon crystals were removed.

[0092] In the third step, acid leaching was used on the first silicon crystals. Initially, a recommended amount of the first silicon crystals was fed into a vessel and, then, the vessel was filled with water to keep all lumps of the first silicon crystals immersed. An acid solution was fed, from time to time, to achieve a desirable concentration. The solution was dispersed to assure that all the first silicon crystals reacted. The first silicon crystals were maintained immersed in the acid solution to reduce hydrogen generation and, then, the solution was removed to a specific tank. Produced second silicon crystals were washed with water to completely remove residues of the acid solution. The second silicon crystals were dried and packed.

[0093] In the fourth step, the second silicon crystals were further refined. Initially, a recommended amount of the second silicon crystals were fed and melted into the induction furnace. The molten silicon crystals were heated from about 1450°C to about 1550°C and, then, poured into a metallurgical ladle with a porous plug installed in an orifice on the bottom of the ladle. Materials were added to form a slag into the metallurgical ladle. Oxygen and nitrogen gases were injected via the orifice with a controlled flow rate and pressure. The refining process was kept for about 30 minutes to about 60 minutes. The refined molten silicon crystals were poured into a cast mold and left to cool, after which they were crushed.

[0094] Although not actually performed, a fifth step would further refine the refined silicon of the fourth step using any directional solidification technique, such as, for example, the rotational casting process described in more detail above. [0095] Example 2 - Step 4. In this comparative example, aluminum-containing silicon was refined without the addition of calcium. To start, 310 kg of aluminum-containing silicon was melted and poured into a metallurgical ladle. The initial amount of aluminum in the silicon (i.e., the amount of aluminum before refining) was measured to be 1054 ppmw. A gaseous mixture comprising 70 wt% oxygen and 30 wt% nitrogen was introduced into the metallurgical ladle via a plug installed proximate the bottom of the ladle. The gaseous mixture was injected for about 65 minutes at a flow rate of approximately 14 Nm³. After refining and separation from the by-product slag, the amount of aluminum remaining in the refined silicon was be measured to be 1040 ppmw. The temperature of the molten silicon before refining was 1559 °C, while the temperature of the molten silicon after exposure to the oxygen containing gas was 1409 °C.

[0096] Example 3 - Step 4. As in Example 2, aluminum-containing silicon was used as the starting material. Calcium carbonate was added to the molten silicon prior to the introduction of oxygen. To start, 288 kg of aluminum-containing silicon was melted and poured into a metallurgical ladle. The initial amount of aluminum in the aluminum-containing silicon was measured to be 1920 ppmw. Seven (7) kg of precipitated calcium carbonate was then added to the molten silicon material. A gaseous mixture comprising 80 wt% oxygen and 20 wt% nitrogen was introduced into the metallurgical ladle via a plug installed proximate the bottom of the ladle. The gaseous mixture was injected for about 45 minutes at a flow rate of approximately 14 Nm³. After refining and separation of the by-product slag, the amount of aluminum remaining in the refined silicon was measured to be 44 ppmw, for a reduction in aluminum of 1876 ppmw. The temperature of the molten silicon before refining was 1544 °C, while the temperature of the molten silicon after refining was 1410 °C.

[0097] Example 4 - Step 4. As in Example 2, aluminum-containing silicon was used as the starting material. Calcium carbonate was added to the molten aluminum-containing silicon prior to the introduction of oxygen. To start, 283 kg of aluminum-containing silicon was melted and poured into a metallurgical ladle. The initial amount of aluminum in the aluminum- containing silicon was measured to be 1275 ppmw. Seven (7) kg of precipitated calcium carbonate was then added to the molten silicon material. A gaseous mixture comprising 80 wt% oxygen and 20 wt% nitrogen was introduced into the metallurgical ladle via a plug installed proximate the bottom of the ladle. The gaseous mixture was injected for about 45 minutes at a flow rate of approximately 14Nm³. After refining, the amount of aluminum remaining in the refined silicon was measured to be 18 ppmw, for a reduction of 1257 ppmw. The temperature of the molten silicon before refining was 1557 °C, while the temperature of the molten silicon after refining was 1408 °C.

[0098] It is noted that terms like "specifically," "preferably," "commonly," and "typically" and the like, are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. It is also noted that terms like "substantially" and "about" are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. Unless indicated otherwise, wt%, ppma, and ppmw of Al, B, P, and other contaminating elements in silicon may be determined by ICP-MS elemental analysis, e.g., using an Agilent 7500cs ICP-MS with Octopole Reaction System (ORS) instrument.

[0099] Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.

[00100] We claim:

Claims

1. A method for producing a solar grade silicon, the method comprising:

preheating silica to a temperature of from about 1300 degree Celsius (°C) to about 1400°C;

combining aluminum with the preheated silica to melt the aluminum and produce an aluminum-silicon alloy;

fractionally crystallizing the aluminum-silicon alloy to produce first silicon crystals; acid leaching the first silicon crystals to produce second silicon crystals, the second silicon crystals being high-purity silicon crystals having a silicon purity that is greater than the first silicon crystals;

refining the second silicon crystals to produce refined silicon; and

purifying the refined silicon to produce a solar grade silicon.

2. The method of claim 1 , wherein the aluminum-silicon alloy contains about fifty weight percent (wt%) silicon.

3. The method of claims 1 or 2, wherein the first silicon crystals contain about ten weight percent (wt%) aluminum.

4. The method of any one of claims 1-3, wherein the second silicon crystals contain about 2,500 parts per million by atom (ppma) of aluminum.

5. The method of any one of claims 1-4, wherein the refined silicon contains less than about 40 parts per million by weight (ppmw) of aluminum and calcium.

6. The method of any one of claims 1-5, further comprising cooling the aluminum-silicon alloy during the fractionally crystallizing.

7. The method of any one of claims 1-6, further comprising selecting for the acid leaching an acid from a group consisting of hydrochloric acid (HCI), nitric acid (HN03), hydrofluoric acid (HF), sulfuric acid (H2S04), and acetic acid.

8. The method of any one of claims 1-7, further comprising, before the refining step, removing the second silicon crystals by filtration, the second silicon crystals being subsequently washed and dried.

9. A method for producing a solar grade silicon from a silicon dioxide, the method comprising:

preheating silica in a rotary kiln to a predetermined temperature;

adding aluminum to the preheated silica to melt the aluminum;

providing an inert atmosphere in the rotary kiln;

agitating the preheated silica and melted aluminum in the rotary kiln to produce an aluminum-silicon alloy;

crystallizing the aluminum-silicon alloy to produce first silicon crystals;

contacting the first silicon crystals with an acid-water mixture to give second silicon crystals;

separating the second silicon crystals from the used acid-water mixture, the separated second silicon crystals having a silicon purity that is greater than the first silicon crystals;

washing the separated second silicon crystals;

drying the washed second silicon crystals;

melting the dried second silicon crystals in a gaseous mixture of oxygen and nitrogen gases in the presence of a calcium oxide-based slag to refine the dried second silicon crystals into refined silicon; and

directionally solidifying the refined silicon to produce a solar grade silicon.

10. The method of claim 9, wherein the aluminum-silicon alloy contains about fifty weight percent (wt%) silicon.

1 1 . The method of claims 9 or 10, wherein the first silicon crystals contain about ten weight percent (wt%) aluminum.

12. The method of any one of claims 9-1 1 , wherein the second silicon crystals contain about 2,500 parts per million by atom (ppma) of aluminum.

13. A method for producing a solar grade silicon, the method comprising:

preheating silica to a predetermined temperature, the silica being in the form of particles having an average diameter of from about 1 millimeter (mm) to about 5 millimeters; combining aluminum with the preheated silica to melt the aluminum at a temperature from about 1000 degrees Celsius (°C) to about 1 100°C and produce an aluminum-silicon alloy;

crystallizing the aluminum-silicon alloy to produce first silicon crystals;

acid leaching the first silicon crystals to produce second silicon crystals, the second silicon crystals having a silicon purity greater than the first silicon crystals; refining the second silicon crystals in a gaseous mixture of oxygen and nitrogen gases, in the presence of a calcium oxide-based slag, to produce refined silicon; and

directionally solidifying the refined silicon to produce a solar grade silicon.

14. The method of claim 13, wherein the aluminum-silicon alloy contains about fifty weight percent (wt%) silicon.

15. The method of claims 13 or 14, wherein the first silicon crystals contain about ten weight percent (wt%) aluminum.