CN101137576A - Preparation method of high-purity silicon - Google Patents

Abstract

An object of the invention is to provide a method for producing a large amount of inexpensive and high purity silicon useful in a solar battery. The method includes steps of preparing molten silicon, preparing a slag, bringing the molten silicon and the slag into contact with each other, and exposing at least the slag to vacuum pressure.

Description

Preparation method of high-purity silicon

This application claims the benefit of Japanese Patent Application No. 2005-062560 filed in Japan on March 7, 2005 and Japanese Patent Application No. 2006-034362 filed in Japan on February 10, 2006, which are hereby incorporated by reference Incorporate it in its entirety into this article.

technical field

The invention relates to a method for preparing high-purity silicon. The high-purity silicon is used in solar cells.

Background technique

For silicon used in solar cells, its purity must be 99.9999% by weight or more, requiring that each metal impurity in silicon does not exceed 0.1 mass ppm. In particular, it is required that the impurity boron (B) does not exceed 0.3 mass ppm. Although silicon prepared by Siemens Process used for semiconductors can meet the above requirements, the silicon is not suitable for solar cells. This is because silicon produced by the Simens Process is expensive, and solar cells need to be cheap.

In order to produce high-purity silicon at low cost, some methods have been proposed.

Methods of unidirectionally curing silicon metal have been widely known for a long time. In this method, the difference in solubility of impurities between the solid and liquid phases is used to unidirectionally solidify molten silicon metal to form purer solid silicon. This method can be effectively used to purify silicon from various metal impurities. However, this method cannot be used to purify silicon from boron because the difference in solubility of boron between solid and liquid phases is too small to purify silicon from boron.

A method of vacuum melting silicon is also widely known. This method removes low-boiling impurities from silicon by maintaining molten silicon in a vacuum state, and effectively removes carbon impurities from silicon. However, this method cannot be used to purify silicon from boron, since boron generally does not form low boiling species in molten silicon.

As mentioned above, boron has been recognized as a problematic component because boron is the most difficult impurity in silicon to remove but greatly affects the electrical properties of silicon. A method whose main purpose is to remove boron from silicon is described below.

JP56-32319A discloses a method of cleaning silicon by acid, a method of vacuum melting of silicon, and a method of unidirectional solidification of silicon. In addition, this document discloses a purification process for the removal of boron using slag, in which impurities are transferred from the silicon to the slag placed on molten silicon. In the patent document JP56-32319A, the boron distribution ratio (boron concentration in slag/boron concentration in silicon) is 1.357, obtained by using slag including (CaF ₂ +CaO+SiO ₂ ) in purified silicon The boron concentration was 8 mass ppm. However, the boron concentration in this purified silicon does not meet the requirements of silicon for solar cells. The disclosed slag purification cannot industrially improve the purification of silicon from boron because commercially available feedstocks for the slag used in this process usually contain boron on the order of a few mass ppm and the purified silicon cannot be obtained unless the partition ratio is sufficiently high. Inclusion of the same level of boron concentration as in slag is avoided. As a result, when the distribution ratio of boron is about 1.0, the concentration of boron in the purified silicon obtained by the slag purification method is preferably as low as about 1.0 mass ppm. Although it is theoretically possible to reduce the boron concentration by purifying the raw material for slag, this is not industrially feasible because it is not economically justifiable.

JP58-130114A discloses a slag purification method in which ground crude silicon is melted together with a mixture of slag containing alkaline earth metal oxides and/or alkali metal oxides. However, the minimum boron concentration in the resulting silicon is 1 mass ppm, which is not suitable for use in solar cells. In addition, new impurities are inevitably added when grinding silicon, which also makes the method unsuitable for solar cells.

Non-patent literature, "Shigen to Sozai" (Resource and Material) 2002, vol.118, p.497-505 discloses another example of slag purification, wherein slag contains (Na ₂ O+CaO+SiO ₂ ), and boron The maximum allocation ratio is 3.5. A distribution ratio of 3.5 is the maximum value disclosed in the past, however, considering the boron concentration of the slag raw material available in practice, this slag purification is still not suitable for solar cells.

As mentioned above, traditional slag purification methods cannot obtain a high distribution ratio of boron that is actually available, and are not suitable for obtaining silicon for use in solar cells. The reason why the distribution ratio of boron tends to be lower when purifying silicon from boron is that silicon is as easily oxidized as boron. In the slag purification method, boron in silicon tends to be unoxidized, and unoxidized boron is hardly absorbed in the slag. The slag purification method is widely used to remove boron from steel because boron is much more easily oxidized than steel. Due to the fundamental difference in properties between steel and silicon, slag purification methods in the steel industry cannot simply be used to remove boron from silicon.

A method of combining the conventional slag purification method with other methods will now be described.

JP2003-12317A discloses another purification method. In this method, fluxes CaO, CaCO ₃ and Na ₂ O are added to silicon, mixed and melted. Then, the molten silicon is purified by blowing an oxidizing gas. However, silicon purified by this method has a boron concentration of about 7.6 mass ppm and is not suitable for use in solar cells. Also, blowing a stable oxidizing gas into molten silicon at low cost is difficult from an engineering point of view. Therefore, the method disclosed in JP2003-12317A is not suitable for the purification of silicon.

USP 5,972,107 and USP 6,368,403 disclose methods for the purification of silicon from boron in which a special torch is used and in addition to feeding oxygen, hydrogen and CaO, BaO and/or _CaF into molten silicon, Water vapor and SiO ₂ are also provided.

The techniques in USP5,972,107 and USP6,368,403 not only require expensive equipment such as special blowpipes, but also require complex operations, which are difficult to implement from an industrial point of view.

The conventional techniques mentioned above can be classified into two categories. The first category includes a method of supplying only slag onto molten silicon (disclosed in JP56-32319A and JP58-130114A, hereinafter referred to as "simple slag purification method"). The second category includes contacting oxidizing gas with molten silicon and providing slag and/or slag raw materials such as SiO ₂ onto the molten silicon (disclosed in JP2003-12317A, USP5,972,107 and USP6,368,403, hereafter referred to as For "complicated slag purification method"). Another method for the purification of silicon from boron is provided by the present inventors in WO 2005/08513A1.

Contents of the invention

An object of the present invention is to provide a method for producing high-purity silicon simply and at low cost by purifying crude silicon from impurities, especially boron, to a level usable for solar cells.

The present inventors have proposed the following proposal after studying the preparation of silicon.

The first embodiment is a method of producing high-purity silicon, which includes: preparing molten silicon; preparing slag; contacting the molten silicon and the slag with each other; and exposing at least the slag to vacuum pressure.

The second embodiment is a method of producing high-purity silicon, which includes: producing molten silicon; producing slag; bringing the molten silicon and the slag into contact with each other; separating the slag from the molten silicon; exposing the slag to vacuum pressure; The molten silicon and the slag exposed to vacuum pressure are brought into contact with each other.

A third embodiment is the method according to the first embodiment or the second embodiment, further comprising: providing an oxidant to the molten silicon together with the slag.

A fourth embodiment is the method according to the third embodiment, wherein the oxidizing agent is provided so as to be in direct contact with the molten silicon.

A fifth embodiment is the method according to the first embodiment or the second embodiment, wherein the vacuum pressure is in a range of 10-10,000Pa.

A sixth embodiment is the method according to the third embodiment, wherein the oxidizing agent is a material comprising as a main component at least one selected from the group consisting of alkali metal carbonates, alkali metal carbonate hydrates, alkali metal Hydroxide, alkaline earth metal carbonate, alkaline earth metal carbonate hydrate, or alkaline earth metal hydroxide; and the method according to the fourth embodiment, wherein the oxidizing agent comprises at least one selected from the group consisting of Substance materials: alkali metal carbonates, alkali metal carbonate hydrates, alkali metal hydroxides, alkaline earth metal carbonates, alkaline earth metal carbonate hydrates or alkaline earth metal hydroxides.

A seventh embodiment is the method according to the third embodiment, wherein the oxidizing agent is a material comprising at least one selected from the group consisting of sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, magnesium carbonate as a main component , calcium carbonate, hydrates of the above-mentioned carbonates, magnesium hydroxide, or calcium hydroxide; and the method according to the fourth embodiment, wherein the oxidizing agent is a material comprising at least one selected from the following as a main component : Sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, magnesium carbonate, calcium carbonate, hydrate of each of the above carbonates, magnesium hydroxide or calcium hydroxide.

The method of the present invention can reduce the boron concentration in silicon to 0.3 mass ppm or less without using expensive equipment such as a plasma device or a gas blowing device, thereby being applicable to solar cells. In addition, the combination of the present invention with the conventional unidirectional solidification process or the conventional vacuum melting process can provide high-quality and low-cost raw material silicon for solar cells.

Description of drawings

FIG. 1 is a schematic diagram showing a first embodiment of the present invention.

Fig. 2 is a schematic diagram showing a second embodiment of the present invention.

Fig. 3 is a schematic diagram showing part of a third embodiment of the present invention.

Fig. 4 is a schematic diagram showing a third embodiment of the present invention.

Figure 5 is a schematic diagram showing the mechanical means of applying vacuum pressure used in the present invention.

Fig. 6 is a graph showing the relationship between the evaporation rate of boron and the vacuum pressure.

Figure 7a is an explanatory diagram providing an example of a mixture of slag and oxidizing agent on molten silicon.

Fig. 7b is an explanatory diagram providing another example of a mixture of slag and oxidizing agent on molten silicon.

Fig. 7c is an explanatory diagram providing an example of placing an oxidizing agent on slag on molten silicon.

Detailed ways

As mentioned above, conventional slag purification techniques can be divided into two categories, namely, the first type or simple slag purification method, in which only slag is fed onto molten silicon; the second type or complex slag purification method, in which oxidizing gas Used with slag. The process of the invention is characterized by the removal of boron by slag purification under vacuum conditions, which cannot be assigned to any of the conventional categories. Although the above vacuum melting process is known in which impurities such as phosphorus are removed from silicon by evaporation by keeping molten silicon in a vacuum state, the vacuum melting process does not use slag.

In conventional slag purification, no further chemical change has been assumed whether the boron in the slag is elemental boron or boron oxide. Based on the above assumptions, the following conclusions are made: comparing boron in silicon (elemental form, oxide form or other boron compound form), boron in slag (elemental form, oxide form or other boron compound form) and boron Thermodynamic stability between compound gases, boron can be removed from silicon by evaporation if the boron compound gas is more stable than boron in silicon. Conversely, if boron in slag is more stable than boron in silicon, boron transfers from silicon to slag. As a result, when boron in silicon is transferred to slag without evaporation, it can be concluded that boron in slag is more stable than boron compound gas so that it is more difficult to evaporate than boron in silicon. Since there are no examples reported of removing boron from silicon using a vacuum melting process, it has been concluded that boron in slag cannot be evaporated under vacuum. For this reason, vacuum treatment of slag was never performed.

The present inventors have found that when the boron in the slag is chemically altered, vaporizable boron compounds (low boiling point materials) can be formed in the slag. In the present invention, based on the above fact, the evaporation of the boron compound formed in the slag can be accelerated by keeping the slag in a vacuum state. As the boron content in the slag decreases, the boron compounds in the slag evaporate and the boron from the silicon is transferred to the slag according to the boron distribution ratio. As a result, the boron content in silicon can be reduced.

A more specific example is described below. The slag purification is carried out with respect to molten silicon covered with slag based on _SiO2 slag and sodium carbonate on it. After the boron in silicon is transferred into the slag in the form of boron element and/or boron oxide, the boron element and/or boron oxide chemically changes into boron-containing low-boiling point materials. Such low boiling point materials include boron and oxygen containing, and/or boron, oxygen and sodium containing compounds, which are characterized by their ease of evaporation and removal from the slag. That is, in slag at high temperature, this boron containing low boiling point compounds has a much higher vapor pressure than ordinary boron oxides. Thus, boron-containing low-boiling materials evaporate as they form on the slag surface. However, since slag is generally highly viscous, low-boiling point materials formed in the slag (not on the surface) form tiny bubbles and hardly separate from the slag. These microscopic bubbles are constantly in contact with molten silicon and dissolved in the silicon during the purification process by the agitation of the slag. Therefore, the evaporation rate of boron from slag is limited at atmospheric pressure. In the present invention, keeping the slag under vacuum increases the bubbles of the boron-containing low-boiling point material in the slag. Therefore, the air bubbles of the low-boiling point material easily reach the surface of the slag and are separated from the slag. As a result, the evaporation rate of boron from the slag increases, which can be predicted from the intrinsic vapor pressure of the boron-containing low-boiling material. As the pressure around the slag decreases, the probability of collisions between evaporated molecules and ambient gas molecules decreases, and thus, the rate of evaporation of low-boiling materials from the slag surface increases.

The present inventors have also found that boron distribution ratios as high as 7-11 can be obtained when slag purification is performed by placing an oxidizing agent such as sodium carbonate directly on molten silicon. High-purity silicon with a boron concentration of 0.1 mass ppm or the like can be obtained only by the effect of removal by evaporation, and at the same time, high-purity silicon can be obtained more easily by utilizing a high distribution ratio.

In conventional simple slag purification, a large amount of slag is required for the purification, since the removal of boron from silicon depends only on a property-dependent partition ratio. In particular, when the distribution ratio is as low as about 1, it is theoretically difficult to achieve a lower boron concentration in silicon than in slag. In the present invention, since boron in slag can be removed as a boron compound by evaporation, there is no lower limit to the concentration of boron in silicon determined by the above boron concentration in slag. In addition, the amount of slag required can be relatively small, which is an advantage of the simpler slag purification of the present invention.

In conventional complex slag purification, there is a problem of expensive manufacturing equipment in addition to complicated operations because special blowpipes are used. In addition, since a large amount of oxidizing gas has to be in contact with the molten silicon, another problem is the loss problem due to oxidized silicon, which reduces the yield. However, in the present invention, only slag is partially exposed to vacuum pressure, and no special equipment or other complicated operations are required. Due to the absence of oxidizing gases, the loss of oxidized silicon is very small. These are the advantages of the present invention over traditional complex slag purification.

Equipment structure

The configuration of the apparatus used in the first embodiment of the present invention will be described below based on FIG. 1 . The plant is designed to expedite the removal of boron from slag by evaporation by keeping the entire refining furnace including the slag under vacuum. Crucible 2 placed in purification furnace 1 is heated by heater 3 . The molten silicon 4 is housed in the crucible 2 and kept at a certain temperature. The oxidizing agent 5 is supplied through the oxidizing agent supply pipe 7 , and the slag 6 is supplied to the molten silicon 4 in the crucible 2 through the slag supply pipe 8 . Between the molten silicon 4 , the oxidizing agent 5 and the slag 6 the reaction and purification involving the removal of boron is initiated. After feeding the oxidant 5 and slag 6, the flow valve 17 of the gas supply pipe 10 is closed and the vacuum valve 16 of the gas discharge pipe 11 is opened. Then, turn on the vacuum pump 15 to evacuate the gas in the purification furnace 1 . In this state, purification is performed and the pressure inside the furnace is maintained at a preferred value by controlling the vacuum pump while monitoring the pressure gauge 14 . When the consumption of the oxidant 5 (either by reaction with the molten silicon 4 and the slag 6 or by evaporation) and the transfer of boron to the slag 6 is almost complete, the vacuum pump 15 is turned off, the vacuum valve 16 is closed, and the flow valve 17 is opened to restore the furnace internal pressure to atmospheric pressure. The slag and oxidant held on the molten silicon 4 are discharged from the crucible 2 into the waste slag receiver 9 by tilting the crucible 2 using the crucible tilting device 12 . Then, the crucible 2 is set to the initial position, and if necessary, the slag 6 and the oxidizing agent 5 are supplied to the molten silicon 4 again, and the purification process is repeated.

The configuration of an apparatus used in the second embodiment of the present invention is described below based on FIG. 2 . The apparatus is designed to expedite the removal of boron from the slag by evaporating boron compounds by maintaining a portion of the slag exposed to vacuum pressure. Its basic construction and operation are the same as those in FIG. 1 . In FIG. 2 , the same parts as those in FIG. 1 are omitted, and a structure/mechanism that exposes only a part including slag to vacuum pressure is mainly shown. Only the parts that are different from those in Fig. 1 are described. Referring to Fig. 2, the vacuum chuck 19 is located on the top of the crucible 2, in which molten silicon 4, oxidant 5 and slag 6 are layered sequentially from the bottom under atmospheric pressure. The vacuum chuck 19 is lowered by the up-down mechanism 18 to be placed in the slag. Then, close the flow valve 17, open the vacuum valve 16, and open the vacuum pump 15 to evacuate the gas in the vacuum chuck 19. Only a limited portion of the slag 6 is exposed to vacuum pressure, the remainder in the furnace being kept at atmospheric pressure. The pressure in the vacuum chuck 19 is monitored by a pressure gauge 14, and the vacuum pump 15 is controlled to maintain an appropriate pressure in the vacuum chuck. When the consumption of the oxidant and the transfer of boron to the slag 6 is almost complete, the vacuum pump 15 is turned off, the vacuum valve 16 is closed, and the flow valve 17 is opened to restore the internal pressure of the vacuum chuck 19 to atmospheric pressure. Then, if necessary, replace the slag in the vacuum chuck 19 with new slag around the vacuum chuck, and repeat the same vacuum process. The slag discharge process is the same as that described in Figure 1. The vacuum chuck 19 may be made of SiC-coated carbon fiber-reinforced carbon with pressure and corrosion resistance. In case the bottom of the vacuum chuck 19 does not adhere to the bottom of the crucible, the level of slag and molten silicon in the vacuum chuck 19 is raised during the vacuum and the liquid level outside the vacuum chuck 19 is lowered. If the horizontal cross-sectional area of the vacuum chuck 19 is larger than the horizontal cross-sectional area of the crucible, all material outside the vacuum chuck is sucked into the vacuum chuck, which can cause problems. In view of this, the horizontal cross-sectional area of the vacuum chuck is preferably one quarter or less of that of the crucible. In the case where the bottom of the vacuum chuck 19 is tightly adhered to the bottom of the crucible, material outside the vacuum chuck rarely flows into the vacuum chuck. Thus, in this case, the suction cup can have the same or smaller cross-sectional area as the crucible. Since the boron purification rate increases as the cross-sectional area of the vacuum chuck increases, the cross-sectional area of the vacuum chuck is preferably one-tenth or more of the cross-sectional area of the crucible.

For the third embodiment of the invention, a method of vacuuming only slag alone is described. The examples shown in FIGS. 1 and 2 consider a method in which the entire furnace is kept under vacuum pressure or a method in which a vacuum chuck fixed to an up-down mechanism is used in a purification furnace. However, the slag can be more easily vacuumed if it is separated from the silicon. This method is explained with reference to FIGS. 3 and 4 . First, silicon purification was performed using the heating furnace of FIG. 3 in which argon gas under atmospheric pressure was contained inside, and other conditions were the same as those of the embodiment of FIG. 1 . Second, the slag discharged into the waste slag receiver 9 is transferred out of the furnace through the door 20 in the refining furnace 1 . Third, the slag is placed in a vacuum heating furnace 21 together with the waste slag receiver 9 and exposed to vacuum pressure while heating. The vacuum heating furnace 21 can be much smaller than the refining furnace 1 because the furnace 21 is only used for a small amount of slag. Fourth, after the boron compounds in the slag have evaporated sufficiently, the slag is pulled out of the vacuum heating furnace 21 together with the waste slag receiver 9 . Then, the slag is fed together with the oxidizer onto the molten silicon, which has been purified once in the previous stage, through the slag feeding pipe for the purification furnace 1 in the previous step again. Then, the same process as described for the embodiment of FIG. 1 is performed. In this case, the vacuum device can be very compact since only a small vacuum furnace 21 is required.

For another method for exposing slag to vacuum pressure, more mechanical means can be used. For example, a piston-cylinder mechanism as shown in Figure 5 could be used. The molten slag 6 is filled at the bottom of the cylinder 23 and the piston 22 is inserted to contact the slag 6 completely. The piston 22 is then pulled up by an actuator (not shown) to provide vacuum pressure in the slag 6 . Because the slag is in a liquid state, the interior of the slag can be uniformly subjected to negative pressure (absolute pressure). If enough power is provided to the piston, it can result in a very effective vacuum pressure. The gas generated from the slag 6 is discharged to the outside by the vacuum pump through the discharge pipe 24 passing through the piston 22 so that the piston 22 can be kept in contact with the slag 6 .

Oxidizing agent: As the oxidizing agent, any oxidizing agent can be used as long as it satisfies the conditions of oxidizing ability, purity, ease of operation, and price. Preferably, however, the oxidizing agent is a material comprising as a main component at least one substance selected from the group consisting of alkali metal carbonates, hydrates of alkali metal carbonates, alkali metal hydroxides, alkaline earth metal carbonates, alkaline earth Hydrates of metal carbonates or alkaline earth metal hydroxides. These materials are preferred for the following reasons. First, they are highly oxidative. Second, they rarely cause silicon contamination by dissolving in silicon. Third, they have the property of forming stable slags of low melting point and low viscosity by reacting with slags, which are easy to handle in terms of discharge and waste disposal. Fourth, they have the ability to accelerate the formation of easily vaporized boron compounds in slag. More preferably, the oxidizing agent is a material comprising at least one material selected from the following as a main component: sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, magnesium carbonate, calcium carbonate, each of the above-mentioned carbonates Hydrate, Magnesium Hydroxide or Calcium Hydroxide. These materials are preferred for the following reasons. First, these materials can form a _SiO2 film on the surface of molten silicon, which can inhibit the contact between molten silicon and slag, and these materials form slag and are removed together with the slag. Second, these materials are mass-produced products that ensure high-purity products. The aforementioned alkaline earth metals include beryllium and magnesium.

Slag: For slag, SiO ₂ that is not contaminated with silicon, such as high-purity silica sand, or Al ₂ O ₃ , such as high-purity alumina, are preferred base materials. It is also preferable to add sodium carbonate or the like to the slag in advance in order to change boron into an easily evaporated boron compound, or to supply sodium carbonate or the like separately from the slag to molten silicon to chemically change boron in the slag. As described later, since it is preferable to purify at a temperature close to the melting point of silicon, it is also desirable to aim at lowering the melting point and viscosity of the slag. Since sodium carbonate reduces the viscosity of slag, it can be added independently to _SiO2 . Alternatively, additives may be added instead of oxidizing agents. The additive may include CaO for a milder reaction rate for purification. For the slag, commercially available high-purity soda glass after crushing and heating can be used. As for the temperature of the slag, it is preferably 2000° C. or lower in view of avoiding silicon contamination and/or excessive reaction rate.

Slag, Oxidizer Feeding Operation: There are two preferred methods of feeding slag. In the first method, slag feedstock is mixed and heated to form a molten or glassy material, which is then fed to molten silicon. In the second method, the slag feedstock is processed to form a granular solid, which is then added separately from the oxidizing agent. In consideration of anti-scattering and/or handleability, the particle diameter of the granular solid is preferably in the range of 1 mm to 200 mm.

As the oxidizing agent, commercially available granular materials such as soda ash can be used without problems. As for the particle diameter, it is preferably in the range of 1 to 50 mm from the standpoint of reactivity and feeding operability. If a strong reaction is allowed, the reaction rate can be increased by directly supplying the molten oxidizing agent onto the molten silicon after previously heating the oxidizing agent to a temperature slightly higher than the melting point. It should be noted, however, that the oxidizing agent is preferably fed at a temperature below its decomposition temperature, since large amounts of basic carbonates decompose/vaporize at temperatures above 1000°C.

Regarding the positional relationship between the slag fed on the molten silicon and the oxidizing agent fed, it is preferable to place the oxidizing agent directly on the molten silicon. Since boron in molten silicon can be oxidized mainly by direct contact with an oxidizing agent, the contact area between molten silicon and the oxidizing agent is preferably as large as possible. The boron oxidation rate can be increased by increasing the contact area by stirring the molten silicon. The present inventors have found that boron in molten silicon is oxidized primarily by direct contact with an oxidizing agent and is immediately absorbed by the slag as boron oxide. This provides a high boron distribution ratio. It is not necessary to place the oxidizer under the slag if the reaction rate needs to be reduced because the reaction rate is too fast for the operation. Conversely, the oxidizing agent can be fed into the slag (as shown in Figures 7a and 7b) or placed on the slag (as shown in Figure 7c).

The co-feeding of slag and oxidizer means that slag and oxidant are fed within a short time interval. Feeding in a short time interval means, for example, feeding the slag before most of the oxidant is consumed (due to reaction with molten silicon and/or decomposition/vaporization at high temperature). More specifically, for example, there is no problem in starting to feed the slag within 20 minutes after initially feeding several tens of kilograms of the oxidizing agent.

Atmosphere of operation: In the conventional technology, since the boron concentration in the purified slag reaches the equilibrium concentration in the molten silicon, it is difficult to reuse the spent slag for another silicon purification. In the present invention, the increased boron in the slag can be removed from the slag by evaporation by exposing the slag to vacuum pressure. This makes it possible to reuse the spent slag and leads to a reduction in the total amount of slag used and lower manufacturing costs. The conditions of the non-evacuated operating atmosphere are as follows: A reducing atmosphere such as hydrogen should be avoided so as not to inhibit the oxidation of boron in the molten silicon. In case graphite is used as the crucible and/or refractory lining, in order to avoid deterioration of the crucible and/or refractory lining due to oxidation, an oxidizing atmosphere such as air should be avoided. Therefore, an inert atmosphere such as an argon atmosphere is preferred.

The conditions of the operating atmosphere with evacuation are as follows: In general, argon is preferred as the atmosphere gas. If the operating pressure is 100Pa or less, air can be used because the effect of air is negligible. The pressure of the operating atmosphere is preferably 10-10,000 Pa. If the pressure exceeds 10,000 Pa, the evaporation rate of boron will decrease. However, there is still some effect at pressures in excess of 10,000 Pa, so pressures slightly in excess of 10,000 Pa may be used for devices for some reason. At 10 Pa, the increase in the evaporation rate of boron reaches saturation. Apparently, there is no problem in using a pressure lower than 10 Pa for the evaporation rate. However, a specific type of vacuum pump is required to maintain this low pressure, which leads to an increase in factory cost. Additionally, when molten silicon comes into contact with slag, this applied low pressure causes the reaction between Si and _SiO2 to accelerate to generate large amounts of SiO gas, which results in a yield of very low percentage silicon. Therefore, operation at 10 Pa is preferably avoided.

Other Operating Conditions: It is desirable for the crucible to be used to be stable to molten silicon and oxidizing agents. For example, graphite and/or alumina may be used. In order to utilize the elution of the crucible material as part of the raw material for the slag, a crucible whose main material is _SiO2 can be used.

As for the operating temperature, it is preferable to avoid high-temperature operation as much as possible from the viewpoint of durability and prevention of contamination to the refractory lining. The temperature of the molten silicon is preferably between the melting point of silicon and 2000°C. Obviously, the temperature of silicon must be at or above the melting point of silicon.

Example

Example 1

A furnace as shown in FIG. 1, which is a modification of a general vacuum heating furnace, was used as a purification furnace for purifying silicon. Into a graphite crucible with a diameter of 500 mm placed in the purification furnace was charged 50 kg of metallic silicon particles having a boron concentration of 12 mass ppm and an average particle diameter of 5 mm. The crucible was heated to 1500°C in an argon atmosphere and the resulting molten silicon was maintained. In the second heating furnace, a mixture of 20 kg of high-purity silica sand and 5 kg of powdered sodium carbonate ( _Na CO ₃ ) was added in a graphite crucible, wherein the boron concentration of the silica sand was 1.5 mass ppm and the average particle size was 10 mm, and the amount of sodium carbonate The boron concentration was 0.3 mass ppm, and the mixture was heated to 1600° C. and kept to form slag. Then, 15 kg of powdery sodium carbonate (Na ₂ CO ₃ ) having a boron concentration of 0.3 mass ppm was fed onto the molten silicon in the purification furnace through the oxidant feed pipe, and the slag prepared in the second heating furnace was put together with the crucible Transfer to the refining furnace and tilt the crucible to feed the slag through the slag feed tube onto the molten silicon. The time from feeding the oxidant to feeding the slag is about 5 minutes. After the feeding of slag is completed, the purification furnace is sealed and evacuated by a vane type vacuum pump until the pressure inside the furnace reaches 1000Pa. The temperature of the molten silicon was kept at 1500° C., and purification was performed for 30 minutes. During the purification process, the gas in the furnace was sampled and analyzed to find that a large amount of Na-containing gas in the furnace was a boron-containing low boiling point material, for example, including boron and oxygen, and/or a compound including boron, oxygen, and sodium. After the purification was completed, the vane type vacuum pump was turned off and the furnace atmosphere was restored to the original argon atmosphere pressure, the crucible was tilted to discharge the slag and remaining oxidant into the waste slag receiver, and the molten silicon was sampled. Sampling was performed by dipping one end of a high-purity alumina tube heated to a temperature above the melting point of silicon into molten silicon and drawing molten silicon through the tube. Solidified silicon was formed outside the furnace by quenching the unheated portion of the tube, and the solidified silicon was separated from the alumina tube for analysis as a sample. The weight of the sample is about 100 g. The component analysis method of the sample is inductively coupled plasma (ICP) analysis, which is widely used in industry. Then, the oxidizing agent and slag are fed again onto the molten silicon to repeat the purification under the same vacuum pressure. A total of three purifications were performed. The boron concentration of the finally obtained sample was 0.09 mass ppm, which satisfies the boron concentration requirement of silicon used for solar cells.

Example 2

A furnace as shown in FIG. 2, which is a modification of a general vacuum heating furnace, was used as a purification furnace for purifying silicon. A vacuum chuck made of SiC-coated carbon fiber-reinforced carbon with a diameter of 300 mm and a height of 1 m was coupled to an air cylinder located outside the furnace so that the vacuum chuck could move up and down by operating the air cylinder. The same crucible, the same silicon raw material, and the same slag were prepared in the same manner as in Example 1, and the oxidizing agent and slag were supplied onto molten silicon. After the vacuum chuck moved down and was tightly adhered and fixed to the bottom of the crucible, a vane-type vacuum pump connected to the vacuum chuck through a tube was turned on to evacuate the inside of the vacuum chuck to a pressure of 10,000 Pa. Under these conditions, the purification of the silicon was carried out by keeping the molten silicon at a temperature of 1500° C. for 30 minutes. After the purification is completed, the vane type vacuum pump is turned off and the atmosphere in the vacuum chuck is restored to the initial argon atmosphere pressure, and the vacuum chuck is moved upward to separate from the slag. The crucible was then tilted to drain the slag and remaining oxidant into a spent slag receiver and the molten silicon was sampled. Sampling was performed in the same manner as in Example 1. Then, the oxidizing agent and slag are fed again on the molten silicon to repeat the purification under the same vacuum pressure. A total of three purifications were performed. The boron concentration of the finally obtained sample was 0.10 mass ppm, which satisfies the boron concentration requirement of silicon used for solar cells.

Example 3

A furnace as shown in FIG. 3, which is a modification of a general vacuum heating furnace, was used as a purification furnace for purifying silicon. In the same manner as in Embodiment 1, the same crucible, the same silicon raw material, and the same slag were prepared, and the oxidizing agent and slag were supplied onto molten silicon. Purification of silicon was performed under the pressure of an argon atmosphere, and the molten silicon was kept at a temperature of 1500° C. for 20 minutes. Then, the crucible is tilted to discharge the slag into the waste slag receiver, and the slag in the waste slag receiver is placed outside the furnace in another small-sized vacuum heating furnace. This small-sized vacuum heating furnace with an internal volume of 1 m ³ has a heat-resistant general structure, and is connected to a vane type vacuum pump. After holding the slag at 1500° C. for 20 minutes in a small-sized vacuum heating furnace under a vacuum pressure of 100 Pa, the slag was fed again together with an oxidizing agent onto molten silicon previously purified in the furnace. The same purification operation was repeated three times in total. The boron concentration of the finally obtained sample was 0.12 mass ppm, which satisfies the boron concentration requirement of silicon used for solar cells.

Example 4

In this example, all parameters were the same as in Example 1 except that _MgCO3 was used as the oxidant. The boron concentration of the finally obtained sample was 0.2 mass ppm, which satisfies the boron concentration requirement of silicon used for solar cells.

All patents, publications, co-pending applications and provisional applications cited in this application are hereby incorporated by reference into this application.

It is obvious that the invention as described above may be varied in many ways. Such changes are not to be regarded as departing from the spirit and scope of the invention, and all changes obvious to those skilled in the art are included within the scope of the claims.

Claims (16)

1. the preparation method of HIGH-PURITY SILICON, it comprises:

The preparation molten silicon;

The preparation slag;

Described molten silicon and described slag are contacted with each other; With

Be exposed to vacuum pressure to the described slag of major general.

2. the method for claim 1, it also comprises:

Oxygenant is provided to described molten silicon with described slag.

3. method as claimed in claim 2 wherein, provides described oxygenant so that it directly contacts with described molten silicon.

4. the method for claim 1, wherein the scope of described vacuum pressure is 10-10,000Pa.

5. method as claimed in claim 2, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: alkaline carbonate, alkaline carbonate hydrate, alkali metal hydroxide, alkaline earth metal carbonate, alkaline earth metal carbonate hydrate and alkaline earth metal hydroxides.

6. method as claimed in claim 3, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: alkaline carbonate, alkaline carbonate hydrate, alkali metal hydroxide, alkaline earth metal carbonate, alkaline earth metal carbonate hydrate and alkaline earth metal hydroxides.

7. method as claimed in claim 2, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: the hydrate of yellow soda ash, salt of wormwood, sodium bicarbonate, saleratus, magnesiumcarbonate, lime carbonate, above-mentioned each carbonate, magnesium hydroxide and calcium hydroxide.

8. method as claimed in claim 3, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: the hydrate of yellow soda ash, salt of wormwood, sodium bicarbonate, saleratus, magnesiumcarbonate, lime carbonate, above-mentioned each carbonate, magnesium hydroxide and calcium hydroxide.

9. the preparation method of HIGH-PURITY SILICON, it comprises:

The preparation molten silicon;

The preparation slag;

Described molten silicon and described slag are contacted with each other;

Separate slag from described molten silicon;

Slag is exposed to vacuum pressure; With

Described molten silicon and the slag that is exposed to vacuum pressure are contacted with each other.

10. method as claimed in claim 9, it also comprises:

Oxygenant is provided to molten silicon with slag.

11. method as claimed in claim 10 wherein, provides described oxygenant so that it directly contacts with described molten silicon.

12. method as claimed in claim 9, wherein, the scope of described vacuum pressure is 10-10,000Pa.

13. method as claimed in claim 10, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: alkaline carbonate, alkaline carbonate hydrate, alkali metal hydroxide, alkaline earth metal carbonate, alkaline earth metal carbonate hydrate and alkaline earth metal hydroxides.

14. method as claimed in claim 11, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: alkaline carbonate, alkaline carbonate hydrate, alkali metal hydroxide, alkaline earth metal carbonate, alkaline earth metal carbonate hydrate and alkaline earth metal hydroxides.

15. method as claimed in claim 10, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: the hydrate of yellow soda ash, salt of wormwood, sodium bicarbonate, saleratus, magnesiumcarbonate, lime carbonate, above-mentioned each carbonate, magnesium hydroxide and calcium hydroxide.

16. method as claimed in claim 11, wherein, described oxygenant is to comprise at least a material that is selected from following material as main ingredient: the hydrate of yellow soda ash, salt of wormwood, sodium bicarbonate, saleratus, magnesiumcarbonate, lime carbonate, above-mentioned each carbonate, magnesium hydroxide and calcium hydroxide.

Images