RU2707053C1 - Method of cleaning metallurgical silicon from carbon - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy and can be used for direct purification of metallurgical silicon from carbon without using environmentally hazardous process operations to degree of purity of solar silicon used in photoelectric converters of solar energy into electrical energy. At the beginning of the cleaning process, temperature of 1,500 °C, which is increased to 1,600 °C to the end of the process, initial pressure is 10 Torr, which is reduced during purification to a final value of 0.5 Torr, initial content of water in a plasma jet, determined by the ratio of the number of moles of water n(H₂O) to number of moles of hydrogen n (H₂) as 1:40, reduced by 2–4 times to ratio n(H₂O):n(H₂), making 1:100 at the finish of technological process.

EFFECT: reduced time of purification of metallurgical silicon from carbon at low rates of evaporation of elementary silicon and removal of silicon from melt in SiO compound.

1 cl, 2 dwg

Description

The annual global production of metallurgical silicon is about 1 million tons. The basis of the process for producing metallurgical silicon is the following reaction carried out in arc melting furnaces:

SiO ₂ + 2C → Si + CO.

One of the applications of metallurgical silicon is the production of silicon for solar energy. Silicon for solar energy should have a purity of at least 6N.

To ensure the purity of solar silicon at the level of 99.9999% (6N), it is necessary to reduce the content of impurities in metallurgical silicon such as Fe, Al, Ca, Ti, Cr, B, P, O, and C.

One of the most harmful background impurities in silicon is carbon. In solar silicon, the carbon content in the volume should be reduced several times in comparison with metallurgical silicon. Its content should not exceed 5⋅10 ¹⁶ - 5⋅10 ¹⁷ cm ^-3 .

A known method of purification of "metallurgical" silicon in industry, the first stage of which is its chlorination or hydrochlorination. In this case, compounds are formed - chlorides (chlorosilanes) - such as SiCl4, SiH2Cl2 or SiHCl3, which are then purified from impurities in various ways (usually by distillation). After purification, chlorides (chlorosilanes) are reduced to elemental (most often polycrystalline) silicon. The most widely practiced method of reducing silicon tetrachloride or trichlorosilane with hydrogen. Such a process for producing silicon, called the "Siemens process" by the name of the company that first implemented it, is carried out in a special reactor [1].

The disadvantages of this method and device are their complexity, danger to staff and the environment, as well as the fact that this process still remains quite expensive.

A known method and device [2] for the purification of silicon, which are suitable for economical and mass production of high purity silicon for solar cells from metallurgical silicon containing boron and carbon in large quantities. The method consists in the fact that a plasma jet of an inert gas is directed to the surface of molten silicon held in a container lined with silica or refractory material based on silicon dioxide. To improve cleaning, inert gas as a plasma-forming gas is mixed with 0.1-10% of the vapor volume and / or less than 1 g of silicon dioxide powder per liter of inert gas in the normal state.

The disadvantage of this method is insufficient for practice to increase the rate of evaporation of impurities from silicon in comparison with the Siemens process.

From [3] there is known a method for the vacuum production of silicon suitable for the manufacture of solar cells from metallurgical silicon. Metallurgical silicon in the form of a melt is poured into a mold and gradually cooled to a solid state. When cooling silicon, the surface of the liquid is heated or insulated to slow down the solidification, while preliminary purification of the metallurgical grade silicon occurs. The resulting, more pure than the original silicon, is again melted and refined. Phosphorus is removed by melting at a pressure below atmospheric, boron and carbon - due to treatment with a mixture of water and inert gases. Refined silicon is cast into a bar and subjected to purification by zone melting of Fe, Al, Ti, and Ca.

The disadvantages of this method and device for its implementation are the complexity in the manufacture and complexity for industrial use.

Known [4] is a method of vacuum cleaning silicon, which consists in melting the charge in a crucible using electron beam heating and holding the melt to remove impurities. The process is carried out in three stages. At the first stage, oxidizing agents, such as water vapor, are introduced into the vacuum chamber to remove impurities whose vapor pressure is lower than the silicon vapor pressure. As a result of reactions, these impurities form readily removable compounds with high vapor pressure. Then, in a high vacuum, impurities having a vapor pressure higher than the silicon vapor pressure are removed, and in the third stage, directed melt crystallization is carried out to push the impurities, in particular metals, into the surface region of the crystallized volume, which is removed at the finish.

The disadvantage of this method is that the cleaning process is carried out by scanning the beam along the surface of the melt, which leads to the heating of silicon only slightly above its melting temperature. Therefore, on the one hand, the energy consumption for the process of purification from impurities with high vapor pressure increases, and on the other hand, the melt is not heated to a temperature corresponding to the necessary rate of evaporation of the mentioned impurities and compounds with vapor elasticity higher than that of silicon.

Known [5] is a method of vacuum cleaning silicon (prototype), which includes loading the purified silicon into a crucible, melting it using electron beam heating under vacuum, holding the melt in a crucible to evaporate impurities and crystallizing it to obtain purified silicon. In this case, the melt is exposed to during intensive heating of the central part of the melt surface and heat removal from the upper part of the crucible wall at the level of the melt surface and from the central part of the crucible bottom. Heat removal from the upper part of the crucible wall is carried out with greater intensity compared to heat removal from the central part of the crucible bottom. Crystallization of the melt is carried out with heat removal only from the bottom of the crucible with a uniform decrease in the intensity of heating of the surface of the melt. The proposed device contains a vacuum chamber, a crucible with silicon, an electron beam gun, a refrigerator mounted on the outer surface of the crucible wall in its upper part. They also contain a cooled tank in which the crucible is coaxially placed, a heat insulator located between the crucible and the cooled tank, and a heat-conducting element located between the cooled tank and the bottom of the crucible along their longitudinal axis.

The disadvantages of the method and devices for its implementation is insufficient for practice, the evaporation rate of the main impurities, in particular carbon, associated with the low temperature of the heating of the silicon melt by an electron beam, the difficulty of manufacturing and operating electron sources and their unreliability.

The present invention solves the problem of purification of silicon containing a large amount of carbon; reduction of time, energy and material costs.

The technical result is that it reduces the time for cleaning metallurgical silicon from carbon at low rates of evaporation of elemental silicon and the entrainment of silicon from the melt in the SiO compound.

The technical result is achieved due to the fact that the method of vacuum purification of silicon involves loading the purified silicon into a crucible, melting in a crucible using heaters and mixing silicon at a temperature slightly above the melting point of about 1500 ° C and a pressure of about 10 Torr while blowing the surface of the melt with an inert gas plasma jet (IG) with the addition of humidified hydrogen in the approximate molar ratio n (H _2): n (H ₂ O) ≈40: 1, followed by reduction of the pressure of the atmosphere in the process chamber to a value of order and 0.5 Torr, while increasing the temperature to about 1,600 ° C and reduction of approximately 3 times the water content in the plasma jet.

The essence of the method is shown in FIG. 1 and 2. In FIG. Figure 1 shows the temperature dependences of the amounts of equilibrium carbon compounds (carbides) calculated in accordance with the laws of thermodynamics at a pressure of 10 Torr, standard for silicon purification technologies, in a system consisting of n = 100 kmol of chemically inactive Ar, 1 kmol Si, 1 kmol H ₂ , 10000ppmH ₂ O and 100ppmC. Hereinafter, the vertical dashed line shows the melting point of silicon. From the presented data, it can be concluded that carbon from the melt, when it is blown with water vapor, is carried away mainly in the form of carbon monoxide CO. Hydrogen, in this case, is added to the mixture only to prevent the formation of a solid crust on the surface of the melt and does not affect the formation of carbides. Its maximum quantity is determined by safety and efficiency reasons.

» на кривой 1 Фиг. 2 отмечена интенсивность испарения СО, соответствующий температуре 1500°С выдержки расплава на данной стадии процесса. At the start of the cleaning process, the carbon content is high by the condition of the problem being solved, therefore, at the initial stage of cleaning, the bulk of the CO is removed at low energy costs by heating to a temperature just above the melting point of about 1500 ° C while blowing the surface of the melt with an inert gas plasma jet with the addition of moistened hydrogen in an approximate ratio of n (H ₂ ): n (H ₂ O) = 40: 1. Curve 1 in FIG. 2 represents the dependence of the amount vaporized CO depending on the temperature in the system consisting of 100 kmole chemically inactive Ar, 1 kmol Si, 1 kmol H _2, 10000ppmH ₂ O and 500 ppmC at a pressure P = 10 torr. The sign "

”On curve 1 of FIG. Figure 2 shows the rate of CO evaporation corresponding to a temperature of 1500 ° C of the melt holding at this stage of the process.

A decrease in water content lower than 10000 ppm in the simulated thermodynamic system is unacceptable due to a decrease, in this case, of the CO removal rate corresponding to the horizontal portion of dependence 1 in the temperature range from 800 ° C to 1600 ° C. An increase in water content is also undesirable due to an increase in the rate of ablation of silicon from the melt in the SiO compound.

So, at the initial stage of the technological process, CO gas carries carbon away from the melt, therefore, the carbon content decreases and, as a result, the probability and rate of CO formation decreases.

On curve 2 of FIG. 2 demonstrates the noted fact. Curve 2 of FIG. 2 is a graph of the equilibrium amounts of CO in the temperature thermodynamic system under consideration, but with lower carbon content: 100 kmole chemically inactive Ar, 1 kmol Si, 1 kmol H _2, 10000ppmH ₂ O and 100 ppmC. From the analysis of the dependence, we can conclude that the intensity of the formation of gaseous CO on curve 2, corresponding to the temperature T = 1500 ° C, is significantly reduced compared to the initial system containing 500 ppmC, and is indicated by a triangle on curve 2.

The restoration of the rate of evaporation of CO to its initial level is possible by reducing the pressure in the chamber. It is practically acceptable to reduce the pressure in the process chamber to a value of P = 0.5 Torr. Curve 3 of FIG. 2 corresponds to such pressure. The intensity of CO removal, equal to the intensity of CO evaporation in the initial system, is indicated by a circle on curve 3 and corresponds to a temperature of 1600 ° C. Thus, the final stage of the process of purifying silicon from carbon is carried out with a decrease in pressure to P = 0.5 Torr and an increase in the melt temperature to 1600 ° C. In this case, the temperature limit of 1700 ° C of the active evaporation of pure silicon is not exceeded here. Calculations show that under the indicated cleaning conditions it is permissible to reduce the molar water content by a factor of about three in the plasma stream of an inert gas compared to the initial ratio n (H ₂ ): n (H ₂ O) = 40: 1 without a noticeable decrease in the rate of CO evaporation in temperature range 1600 ° С. A decrease in water content makes it possible to compensate for the increase in the rate of undesirable ablation of silicon in the SiO compound with increasing temperature.

Thus, the solution of the technical problem is achieved by the fact that the method of purifying silicon from carbon in a closed vacuum chamber with a metallurgical silicon charge placed in it consists in melting the silicon charge, mixing the melt and blowing the melt surface with a plasma jet of an inert gas mixture with moistened hydrogen, while maintaining at the beginning of the cleaning process, a temperature of about 1500 ° C increases to about 1600 ° C by the end of the process, the initial pressure of about 10 Torr decreases during the cleaning process and to a final value of about 0.5 Torr, the starting water content of the plasma jet, defined by approximate molar ratio of water n (H ₂ O) to the number of moles of hydrogen n (H ₂₎ as a 1:40, decreases about 2 - 4 times to about ratios n (Н ₂ О): n (Н ₂ ) ≈1: 100 at the finish of the technological process.

LITERATURE

1. Reuschel Konrad. Method and apparatus for producing hyper-pure semiconductor material, particularly silicon. US2999735A. Priority date 1959-06-11 / Schweickert Hans, Reuschel Konrad, Gutsche Heinrich. Production of high-purity semiconductor materials for electrical purposes. US3011877A. Priority date 1956-06-25 / David L. Parsels. Method and apparatus for making elongated Si and SiC structures. US3961003A. Priority date 1972-05-17.

2. Yuge Noriyoshi etc. Method and apparatus for purifying silicon. EP0459421 B1, Priority date1990-05-30.

3. Baba Hiroyukiidr. Process for production of silicon for use in solar cells. RU2154606 C2. Priority date1997-03-24.

4. Norichika Yamauchi, Takehiko Shimada, Minoru Mori. Method and apparatus for refining silicon using an electron beam. US20070077191A1. Priority date2005-08-16.

5. Kravtsov A. A. The method of vacuum cleaning silicon and a device for its implementation (options). RF patent for invention №: 2403299 Date of publication November 10, 2010.

Claims (1)

A method of purifying silicon from carbon in a closed vacuum chamber with a metallurgical silicon charge placed therein, including melting a silicon charge, mixing the melt and blowing the melt surface with a plasma jet of an inert gas mixture with moistened hydrogen, characterized in that the temperature maintained at the beginning of the cleaning process is 1500 ° C increase to 1600 ° C by the end of the process, the initial pressure of 10 Torr, decrease during cleaning to a final value of 0.5 Torr, the initial plasma water content jet determines the amount ratio of moles of water n (H ₂ O) to the number of moles of hydrogen n (H ₂₎ as a 1:40, reduced in relation to 2-4 times n (H ₂ O): n (H ₂₎ is 1 : 100 at the finish of the process.

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