US20070209945A1

US20070209945A1 - Method for producing silicon, method for separating silicon from molten salt and method for producing tetrafluoride

Info

Publication number: US20070209945A1
Application number: US11/673,788
Authority: US
Inventors: Alexsander Karelin; Vladimir Karelin; Valery Kazimirov
Original assignee: Gelios OOO
Current assignee: ZE POLY PTD Ltd; Gelios OOO
Priority date: 2004-08-12
Filing date: 2007-02-12
Publication date: 2007-09-13
Also published as: WO2006019334A1; RU2272785C1; CN101090862B; DE112005001969T5; ES2319072B1; ES2319072A1; CN101090862A; UA80662C2

Abstract

The invention relates to semiconductor silicon technology. The inventive method consists in electolytically decomposing a silicon tetrafluoride-saturated eutectic melt of a ternary system of alkali metal fluoride salts. For saturating the melt, a silicon tetrafluoride obtainable by fluorinating a silicon dioxide is used, wherein said fluorination is carried out in two stages, i.e. at a first stage, an elemental fluorine is supplied with excess and a silicon dioxide is supplied at the second stage. The silicon powder separation from the fluoride salt eutectic melt is carried out by dissolving the silicon particles-containing melt with the aid of anhydrous hydrogen fluoride and by subsequently filtering for isolating a solid phase in the form of a silicon powder.

Description

RELATED APPLICATIONS

This application is a Continuation of PCT application number PCT/RU2005/000400 filed on Aug. 1, 2005 (which was published in Russian under PCT Article 21(2) as International Publication No. WO 2006/019334 A1), which claims priority to Russian Application No. RU2004124626 on Aug. 12, 2004, both of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to methods of producing rare metals and non-metals. More specifically, the invention relates to the production of electrolytically pure silicon powder, which can be used in solar energy and semiconductor industries. More specifically, the invention relates to the methods of reducing silicon from gaseous SiF₄, as well as to the technology of producing SiF₄.
Technical silicon is produced in industrial scale by carbothermical reduction of quartz sand with carbon. Such metallurgical silicon is contaminated with admixtures and is not suitable for semiconductor industry and solar cell manufacture.
Semiconductor silicon is produced from technical (metallurgical) material by means of chlorination of thin grinded silicon powder with waterless hydrogen chlorine with the following purification of chlorine silane formed in the process of chlorination by means of rectification to necessary purity (book of E. S. Falkevich, et al. Technology of semiconductor silicon, Moscow: Metallurgy, 1992). In some countries silicon is produced by melting together technical silicon and magnesium, by decomposition of magnesium silicide, by the following low-temperature rectificative purification of monosilane and its thermal decomposition (book of I. P. Belov, et al. Monosilane in technology of semiconductor materials. Moscow: NIITEChim., 1989). The mentioned ways are many-stage, have low direct commodity output, i.e. prime cost of semiconductor silicon produced by these technologies is very high, and emissions of chemically dangerous substances into environment take place. All these factors serve as preconditions of search for new technologies of semiconductor silicon production. One of these technologies is technology of semiconductor silicon production on the basis of silicon tetrafluoride.
There is the way of production of fine-dyspersated silicon powder from gaseous silicon tetrafluoride (RU 2066296, MIIK C 01 B 33/03, issued Oct. 9, 1996), isolated from waste of uranium hexafluoride production, that is Na2SiF6. In the basis of the mentioned way is decomposition of gaseous silicon tetrafluoride by means of laser emission. Powerful continuous emission of CO2-laser (2-10 KW), directed into reaction chamber (confined space), where circulation of silicon tetrafluoride and hydrogen, binding fluorine in the process of decomposition of SiF4, is carried out simultaneously.
The way of production of silicon from silicon tetrafluoride according to laser technology with the use of waste of uranium hexafluoride production makes it possible to produce pure silicon of comparatively low cost. Though the purity of such silicon is about 99%, that is not enough for its utilization in solar energy or in semiconductor technique.
There is the technology of semiconductor silicon production, namely silicon for solar cells (RU 2035397, IPC C 01 B 33/02, issued Mat 20, 1995), including series of gas-transport reactions with the use of silicon tetrafluoride, from which, following its interaction with deposited water, silicon fluoride hydrogen acid is produced. Silicon is produced by its reduction from silicon fluoride hydrogen acid by atomic hydrogen at room temperature.
The main disadvantage of this technology is low rate of silicon output. As atomic hydrogen quickly looses atomation, the process of reduction of silicon at room temperature is impossible. The mentioned reason prevents using of this method for industrial purposes.
As a prototype of the claimed way of production of high-purity silicon powder with simultaneous production of elemental fluorine, the way of production of metallurgical silicon, described in the patent RU 2156220 (IPC C 01 B 33/00, issued Sep. 20, 2000) was adopted.
The prototype is characterized by isolation of silicon from solution of metallurgical silicon.
To produce such solution, which exists in silica or in quartz sand, silicon dioxide SiO2 is combined with three fluorine chlorine (ClF3) and, as a result of the reaction, silicon containing compounds of fluorine and chlorine (silicon tetrafluoride SiF4 and tetrachloride SiCl4) are produced. These compounds are electrolytically decomposed in electrolyzer with liquid low-melting cathode, such as melt of metal, namely zinc. Result of electrolysis: compound of fluorine and chlorine on anode; cathode, i.e. melt of zinc, absorbs isolated from the above mentioned silicon containing compounds of metal silicon, and as a solution of metal silicon it is processed to produce silicon as commodity output.
The way according to the patent RU 2156220 provides production of high-purity polycrystalline silicon. Though this way has no wide commercial application. One of the reasons is that by using of zinc cathode, the low solubility of silicon in zinc melt takes place. Thus, the process, characterized by vacuum-thermal distillation of silicon from zinc melt, with low concentration of silicon in melt demands multiple saturation of melt. It significantly increases cost of the finished product.
Another disadvantage of the prototype is periodicity of the process. In order to isolate silicon from liquid cathode melt, it is necessary on the definite stage, characterized by sufficient saturation of cathode melt with silicon particles, to stop electrolysis, to direct melt with silicon to the stage of silicon removing, and to charge the electrolytic device with new zinc melt, and after all these procedures to continue the process. Besides, the disadvantage of the method is that except fluorine, gaseous chlorine is also isolated on anode. Although, as noted in the claim and in the description of the invention according to the patent RU 2156220, both elements, isolated on anode, are transported to isolation of silicon from silicon containing raw material, i.e. elements are circulating, the necessity of isolation of one element from another complicates the technology, increases its hardware-controlled support and results in rise in price of the finished product.
In most of the known solutions by polycrystalline silicon production from salt melt in electrolytic way, reduced silicon is deposited on cathode and then is extracted by purification of cathode surface from silicon residue (RU 1416060, IPC C 25 C 3/00, issued Jul. 08, 1988; inventor's certificate No 460326, etc.) There are technologies to carry out extraction of silicon from electrolyte by its emergence on the melt surface or by its deposition in the form of residue on the bottom of electrolytic bath (example, the patent RU 215646, IPC C 25 II 1/00, issued Feb. 20, 2000). The main disadvantage of these methods is high labor intensiveness of silicon extraction. It significantly increases cost of the finished product.
As a prototype of the claimed way of separation silicon from salt melt, was made the decision according to the patent RU 2156220 (IPC C 01 B 33/00, issued Sep. 20, 2000). Metal silicon is produced by recrystallization of silicon at low pressure by evaporation from silicon solution in zinc melt. The mentioned way makes it possible to produce high-purity silicon but it can be used in isolating of silicon from metal melt, and is not realizable in extracting of silicon from salt melt. It is also more expensive than the claimed method.
There are ways of production of silicon tetrafluoride by its synthesis from silicon dioxide and fluorine containing reagents, e.g. natrium fluoride, anhydrous hydrogen fluoride, fluoride of lead, etc. All these ways require the application of chemically produced reagents, that results in additional material and energy resources.
There is the way of production of silicon tetrafluoride from hydrofluosilicic acid (the patent RU 2046095, IPC C 01 B 33/10, issued Oct. 20, 1995), including interaction of the acid solution with solution of organic basis by formation of hydrofluosilicic acid salt. The produced salt is washed, dried and is decomposed by processing of concentrated mineral acid, and after the stage of decomposition the separation of the produced silicon tetrafluoride from anhydrous hydrogen fluoride is carried out.
As a prototype of the claimed way of production of silicon tetrafluoride, was made a decision according to the patent GB 1080662, IPC C 01 B 33/08, issued Aug. 23, 1967. According to it, silicon tetrafluoride is produced from silicon dioxide by anhydrous hydrogen fluoride.
The main disadvantage of the prototype is ecological insecurity of the technology.
The produced silicon tetrafluoride is also characterized by large amount of impurities, that makes it unusable for technology of semiconductor silicon production.

SUMMARY OF THE INVENTION

The objective is the development of effective and more environmentally-friendly technology of production of electrolytically pure and relatively inexpensive semiconductor silicon with simultaneous production of high-purity silicon tetrafluoride and elemental fluorine, that can be used as circulating chemical compound and chemical element, and as well as marketable product with high quality characteristics.
The objective is reached as in the way of production of silicon from silicon tetrafluoride with simultaneous production of elemental fluorine by electrolysis and with isolation of elemental fluorine on anode according to the invention, electrolytic decomposition of eutectic melt of triple system of salt of alkaline metal, saturated with silicon tetrafluoride is carried out. The isolated silicon in the form of suspension of silicon powder and electrolyte, which is the above mentioned eutectic melt of triple systems of fluoride salt of alkaline metals, is isolated from electrolyzer. After removing of the mentioned suspension, i.e. within electrolyzer, the isolation of silicon powder from eutectic melt of triple systems of fluoride salt of alkaline metal is carried out.
The technical result of the method is the possibility to realize silicon production in continuous operation with high output of the finished product and its high quality. This result is caused by the following distinctive features:
Firstly, in the claimed technology, which can be characterized as fluoride technology of semiconductor silicon production, the distinctive feature of the claimed way is that in the process of production of high-purity silicon the eutectic melt of triple systems of fluoride salt of definite alkaline metals, i.e. chemical compounds, containing fluorine, is used. The saturation of this melt is carried out by silicon tetrafluoride, i.e. by chemical compound, which also contains fluorine. It is possible to produce electrolytically pure semiconductor silicon of high quality and high-purity elemental fluorine at one stage. Their cost will be significantly lower than that of these substances produced according to the known technologies.
Another distinctive feature of the claimed way of silicon production is that for isolating of product it is unnecessary to stop the process of electrolysis; the removing of silicon powder in compound with electrolyte melt can be continuous. To realize this operation, it is necessary to organize constant replenishment of electrolyzer with salt melt (i.e. electrolyte). It can be done in any way. Though the most preferable way is, when eutectic melt of triple systems of fluoride salt of alkaline metals is directed to the repeated use in electrolysis after separating of silicon powder, closing the process and providing non-waste production. It is also preferred to pick the suspension in electrolyzer in its interpolar gap while removing this suspension from electrolyzer of powder suspension and electrolyte. With these variants of the claimed way removing of electrolytically produced silicon goes with discharging of electrolyte and its constant replenishment, i.e. electrolyte is running. But because of the fact that removing of the suspension is carried out in interpolar gap of electrolyzer, electrolyte is made to “flow” not in full capacity but in “selected” part, i.e. it is constantly removed from the process in the capacity (part) that is the suspension with more concentration of isolated powder. It provides continuous and effective electrolysis during the whole process, as timely and continuous removing from the zone of the process of that part of electrolyte, which contains powder of reduced elements, reduces the risk of reverse decomposition to ions of isolated product. Besides, the reduced silicon is not deposited on cathode but is timely and continuously removed from the process of electrolysis. The powder of reduced silicon is not deposited on cathode surface, that provides stability of its work and as a result increases the efficiency of decomposition process of new and old ions on cathode.
Moreover, density of electrolyte with isolated silicon powder is lower than density of electrolyte without the powder, i.e. extrusion of suspension with column of electrolyte takes place.
The claimed way is more manufacturable, if the eutectic melt of the composition LiF—KF—NaF is used as eutectic melt of triple systems of fluoride salt of alkaline metals, and if electrolysis is carried out at 450-600° C. This salt melt is more preferable because of the fact that the melting temperature of the initial components of the melt is lower than the melting temperature of silicon. But for all that, temperature condition is more optimal for electrolysis and the process of silicon powder isolation.
In specific cases of realization of the invention the eutectic melt of fluoride salt KF—NaF is saturated with silicon tetrafluoride in range of 2-35% mass of SiF4. The parameter which is lower than 2% requires very high energy supply, that is not economically feasible. If the parameter is higher than 35%, it will lead to increasing of melting temperature of eutectic of fluoride salt, which is also not preferable.
The effectiveness of electrolysis increases if saturation of eutectic melt of triple systems of fluoride salt of alkaline metals is carried out by bubbling of silicon tetrafluoride into melt. It tales place because bubbling makes it possible to saturate melted electrolyte with supreme fluoride of the initial element in gas phase.
The removing of suspension of silicon powder in compound with electrolyte in the ratio: 2 parts of powder and 8 parts of electrolyte is more preferable because of the ways of separation of silicon from electrolyte. One of these methods is the way of separation of silicon from salt melt, claimed as self-dependent invention, as it can be used in other technologies. But, as the applicant says, it is copyrightable solution.
The claimed way reaches the same objective as the above mentioned way of production of high-purity semiconductor silicon and elemental fluorine, i.e. the objective of production of cost effective high-purity semiconductor silicon.
The objective was reached because in the way of separation of silicon from salt melt, according to the announced invention, silicon is separated from eutectic melt of fluoride salt KF—NaF. Besides, the mentioned melt with silicon particles is dissolved with waterless anhydrous hydrogen fluoride. The produced composition from HF+(LiF—KF—NaF) in the form of liquid phase and silicon particles, that are solid phase, is filtered with separation of solid phase in the form of silicon. The liquid phase is directed to distillation of anhydrous hydrogen fluoride, which is used at the phase of dissolving. In order to improve the process of dissolving, the consolidated melt with silicon particles is disintegrated before dissolving, and the dissolving process itself is carried out at −5° C. to+12° C. The preferable result of the process is the produced composition with the ratio of solid phase to liquid phase as 1:23, i.e. in 23 parts of HF+(LiF—KF—NaF) there is one part of solid silicon particles. Such ratio, predetermined by supplying of the corresponding amount of waterless anhydrous hydrogen fluoride to definite amount of melt with silicon particles is more optimal for further filtration of compound and separation of silicon powder.
In special case of realization of the method, the filtration is carried out by centrifugation with the use of centrifugal machine, manufactured by industry, or by the same centrifugal machine, physically updated, based on the concrete conditions of production.
For the claimed way the purification of silicon powder from metal impurities with washing of silicon powder with solution of compound of inorganic acid, specifically of the following composition: 2-3 M H₂SO₄+0,1-0,2 M HF at 5-75° C., with the following drying of silicon powder in inert atmosphere and at 80° C.-120° C. is applicable.
The above mentioned lower and upper limits of parameters of the claimed way were received experimentally on the basis of experimental research and analysis of results of the experiments with reaching the objective and technical result as production of high-purity semiconductor silicon powder.
The anhydrous hydrogen fluoride used as dissolvent after termination of cycle and after distillation of fluoride salt LiF—KF—NaF from eutectic melt, which is carried out by thermal way, is returned to the new cycle, and is used as dissolvent again. The eutectic melt of fluoride salt LiF—KF—NaF is used in the way of production of high-purity silicon powder from silicon tetrafluoride with simultaneous production of elemental fluorine, i.e. in the first invention of the claimed group, stated in item 1 of the claim.
The result of the above-stated solutions is production of electrolytically pure silicon powder, which is characterized by the content of the following components: silicon with weight content C₁, impurities of metals with weight content C₂, and admixtures of non-metals with weight content C₃. According to the invention, electrolytically pure silicon is produced by fluoride technology according to the way of item 1, i.e. by electrolytic decomposition of eutectic melt of triple systems of fluoride salt of alkaline metals not saturated with silicon tetrafluoride; is removed from electrolyzer in the form of suspension with electrolyte, isolated from electrolytic melt according to the way of item 8, and is characterized by the above mentioned composition under the condition:
0,01 ppba<=(C₁ 1+C₂+C₃)/C₁<=0,01 ppma,
where
ppba—atom content of impurities for milliard of silicon atoms.
ppma—atom content of impurities for million of silicon atoms.
Simultaneously, the claimed solutions provide production of high-purity elemental fluorine, including fluorine with weight content C₄and admixtures with their weight content C₅, produced by fluoride technology according to the invention of item 1 of the claim, i.e. it is isolated on anode by electrolytic decomposition of eutectic melt of triple system of fluoride salt of alkaline metals; is saturated with silicon tetrafluoride, and is characterized by the above mentioned composition under the condition;
0,95<=(C₄+C₅)/C₄<=1,01.
The objective of high-purity silicon powder production is reached by the way of production of silicon tetrafluoride used in described above technology, including the use of silicon dioxide as the initial compound, which is differ from the known solutions of silicon tetrafluoride production by the fact that fluorination of silicon dioxide is carried out by effect of elemental fluorine.
The fluorination is carried out in two stages: at the 1^ststage silicon dioxide is processed with elemental fluorine at 1100-1200° C.; supply of elemental fluorine is carried out with 20-30% of mass excess with regard to stoichiometrically necessary quantity. Gas phase is directed to the 2^ndstage of the process where the fluorination of silicon dioxide with its supply with 70-80% mass excess is carried out. The excess of elemental fluorine is used at the 1^ststage with its full absorption.
By realization of the claimed invention, fluorination is carried out in torch of plasma reactor. Fluorine, produced by realizing of production of high-purity silicon powder by electrolysis of eutectic melt of triple system of fluoride salt of alkaline metals, saturated with silicon tetrafluoride, can be used as elemental fluorine (i.e. fluorine produced according to the way, stated in item 1 of the claim).
The above mentioned intervals of temperature and correlation of reagents is optimal for realizing of the method. They are selected experimentally based on the objective and technical result.
The group of inventions stated in application meet the requirements of “unity of invention”. This requirement is observed as the invention (item 1 of the claim), which is the way of production of silicon and elemental fluorine on one stage. The inventions stated in item 8 is the way intended for use in the way according to item 1, as a part of its way, and the invention according to item 17 is solution for silicon tetrafluoride production, i.e. the substance, used in the way according to item 1. The offered ways have one and the same significance, and provide the possibility of recirculation of chemical elements and compounds formed during the process, setting conditions for completeness of technological cycle of high-purity silicon production, used for reduction in cost price of the final product—high-purity semiconductor silicon. Besides, the possibility of reutilization of waste material in technological process excludes emissions of chemically dangerous substances into environment. Their sterilization and refinement become unnecessary. Thus, the claimed group of inventions is a common, general conception of invention, and there is a technical interconnection between the inventions. The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:
The claimed inventions are illustrated by the enclosed drawings.
FIG. 1 shows block diagram, depicting the process (part 1) of production of high-purity silicon powder and elemental fluorine, including combination of operations of the way (part II) of separation of silicon powder from molten salt;
FIG. 2—block diagram, explaining the way of silicon tetrafluoride production.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The claimed ways are realized in the following way.
The equipment and hardware-controlled complexes, applied at plants of chemical industry and in metallurgy, are used. They are electrolyzers and similar reactors, torch (flaming) reactors; bubbling devices, equipment providing flotation, washing, etc., drying plants, transport systems for supply of gaseous, liquid and solid reagents, the known check-out equipment, etc.
The preparation of electrolyte is carried out before electrolysis in the way of production of high-purity silicon powder and elemental fluorine. Eutectic melt of fluoride salt LiF—KF—NaF is saturated with silicon tetrafluoride SiF₄in the range 2-35% mass of SiF₄. For this purpose SiF₄is bubbled into the mentioned melt by means of bubbling device 1 (FIG. 1, part 1), saturating it till every value in the given range.
Continuous supply of electrolyte saturated with silicon tetrafluoride is carried out into electrolyzer 2 with either liquid cathode, or solid cathode (stainless steel, silicon) and with inert anodes (carbide, nitride, silicon, graphite). Construction of the electrolyzer should be made with continuous removing of suspension of the isolated silicon powder and electrolyte from interpolar gap of electrolyzer.
Electrolytic decomposition of eutectic melt of LiF—KF—NaF, saturated with SiF₄, takes place during energy supplying. Electrolysis is carried out at 450-600° C. Li₂SiF₆and Na₂SiF₆are formed during electrolysis of eutectic melt of the above mentioned salt saturated with silicon tetrafluoride. Li₂SiF₆and Na₂SiF₆are changeable and decompose in the melt to SiF₄, LiF and NaF, and K₂SiF₆, which dissociates to positive ions K+ and negative ions SiF₆ ²⁺. The process is carried out according to the following reaction:
K₂SiF₆⇄2K¹⁺+SiF₆ ²⁻
Then SiF₆ ²⁻ is dissociated to ions: positive ions Si⁴⁺ and negative ions of fluorine (6F⁻), which are reduced: positive ions of silicon are reduced to metal silicon powder (Si) on cathode, negative ions of fluorine—to elemental fluorine (3F₂) on anode.
Silicon is produced in the form of suspension of Si powder in electrolyte melt in the ratio 2:8, i.e. 2 parts of silicon powder and 8 parts of electrolyte. Silicon powder in compound with electrolyte melt (i.e. suspension, including silicon powder and eutectic melt LiF—KF—NaF) is removed from electrolyzer.
Elemental fluorine, produced in the above mentioned way, is characterized by the composition, including fluorine with weight content C4, and impurities with their weight content C5 under the condition 0,95<=(C4+C5)/C4<=1,01, that was confirmed by pilot study.
Then silicon powder is isolated from electrolyte melt. This can be realized in every known way or by the claimed way of separation of silicon from molten salt (FIG. 1, part II).
To illustrate the claimed inventions, the process is carried out according to the claimed way of separation of silicon from salt melt, namely from eutectic melt of fluoride salt LiF—KF—NaF.
The consolidated electrolyte melt with silicon powder is disintegrated in the known way with the help of crusher 3. The consolidated composition in reactor 4 is dissolved by means of anhydrous hydrogen fluoride HF. Dissolving is carried out by intermixing and at −5° C. to +12° C. Suspension is produced from electrolyte, dissolved in anhydrous hydrogen fluoride, and silicon powder. This suspension is filtered with isolating of Si powder with the help of centrifugal machine 5. The separated silicon powder is directed to floatation machine 6. The silicon powder is washed out by means of device 7 in solution of inorganic acid of the composition 2-3 M H₂SO₄+0,1-0,2 M HF, and by means of wash device 8—with condensate (desalted water). Silicon powder, washed by desalted water with the help of aggregate 9 is filtered from water and is dried in dryer 10 in inert atmosphere at 80-120° C. The finished high-purity silicon powder, ready for use in solar energy and in semiconductor technique silicon powder, is packaged.
Electrolytically pure silicon powder, produced in the above mentioned way is characterized by composition, including silicon with weight content C1, impurity of metals with weight content C2 and impurity of non-metals with weight content C3, under the conditions:
0,01 ppba<=(C1+C2+C3)/C1<=0,01 ppma, where
ppba—content of atoms of impurities for milliard of silicon atoms;
ppma—content of atoms of impurities for million of silicon atoms.
Solution of electrolyte in HF, produced after filtration of silicon powder with the help of centrifugal machine 5, is directed to apparatus 11 where distillation of anhydrous hydrogen fluoride takes place at 500° C. The produced electrolyte with the composition LiF—KF—NaF is directed to the stage of electrolysis for realization of the way according to item 1.
Anhydrous hydrogen fluoride (gas) is condensed, and is directed in the form of liquid anhydrous hydrogen fluoride from capacitor 12 to reactor 4, using it as a dissolvent in dissolving of disintegrated consolidated electrolyte melt with silicon powder at the initial stage of the process.
Silicon tetrafluoride is used by producing of high-purity silicon powder with simultaneous production of elemental fluorine. Silicon tetrafluoride is produced with complex of equipment 13 and in the claimed way of silicon tetrafluoride production. The example of its realization is given below (FIG. 2).
The initial material of silicon tetrafluoride production is natural quartzite, quartz sand or another raw material, containing silicon dioxide in large quantity. As a rule, this raw material is characterized by the following composition: SiO2-97%, macro-impurities: Fe₂O₃, CaO, Al₂O₃.
The process is realized in two flame (torch) reactors 14 and 15, installed in series.
At the 1st stage silicon dioxide SiO₂is processed with elemental fluorine F2 (in the way according to item 1) at 1100-1200° C. Processing is carried out in torch of the flame reactor 14. Supply of elemental fluorine into reactor 14 is carried out with excess (20-30%) regarding to its stoichiometrically necessary quantity. Gaseous phase is withdrawn from reactor 14 and is directed to the 2nd stage of the process, i.e. to the 2nd flame reactor 15. Gaseous phase includes gaseous silicon tetrafluoride SiF₄, oxygen, produced during the reaction, and fluorine excess (O₂+F₂), which is not used in the reaction. Slurry, containing fluorides of macro-admixtures: aluminium tetrafluoride (AlF₃), calcium difluoride (CaF₂), FeF₃, is removed from reactor 14 with the help of auger device 16. Gaseous phase from the 1st stage is supplied to the 2nd flame reactor 15 simultaneously with silicon dioxide, which is supplied with 70-80% of mass excess. The full absorption of excess of elemental fluorine from the 1st stage takes place during the reaction in the 2nd flame reactor 15. The produced silicon tetrafluoride is used as reagent for saturating of electrolyte in the way of production of high-purity silicon powder and elemental fluorine, or it is removed from the process as the finished product. The excess of silicon dioxide is directed to the 1st reactor 14, closing the process.
By realizing the way of silicon tetrafluoride production the following reactions take place:
SiO₂+F₂(with excess) SiF₄+O₂+excess of F₂
SiF₄(from the 1st stage)+O₂+excess of F₂(from the 1st stage)+SiO₂(with excess)→SiF₄+O₂+SiO₂(excess)
Thus, the claimed way of silicon tetrafluoride production provides the full use of elemental fluorine in technological process. It can be fluorine, produced by electrolytic production of silicon powder.
The claimed inventions, forming the fluoride technology of high-purity semiconductor silicon, are energy- and resource-saving. The technology is characterized by ecological purity as the process is carried out over one cycle with the use of fluorine produced during electrolysis for production of silicon tetrafluoride; and also because the processed electrolyte is returned to the process. The produced products (silicon, fluorine, silicon tetrafluoride) are characterized by small quantity of impurities, and the cost of silicon as the finished product is significantly lower than that according to other technologies.

INDUSTRIAL APPLICABILITY

Based on the above-stated description of the group of invention and taking into account nature of the inventions, it is evident that all the claimed ways are intended for industrial usage.
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

1. An electrolytic method of producing silicon from silicon tetrafluoride with simultaneous formation of elemental fluorine on an anode comprising:

obtaining a eutectic melt of triple systems of fluoride salts of alkaline metals in an electrolyzer, the melt being saturated with silicon tetrafluoride;

electrolyzing the eutectic melt to obtain a suspension of silicon powder in the eutectic melt;

removing the suspension of the silicon powder from the electrolyzer;

separating the silicon powder from the eutectic melt of the triple systems of fluoride salts of alkaline metals.

2. The method according to claim 1, wherein removing the suspension of silicon powder comprises removing it from a gap between electrodes of the electrolyzer.

3. The method according to claim 1, further comprising recirculating the eutectic melt into the electrolyzer after separating the silicon powder for repeated use in electrolysis.

4. The method according to claim 1, wherein the composition of the eutectic melt is LiF—KF—NaF and wherein the electrolysis is carried out at 450-600° C.

5. The method according to claim 4, further comprising saturating the eutectic melt of fluoride salt LiF—KF—NaF with silicon tetradluoride in the range from 2 mass % to 35 mass % of SiF₄.

6. The method according to claim 4, further comprising bubbling of SiF₄into melt during saturating.

7. The method according to claim 1, wherein removing the suspension of the silicon powder from the electrolyzer occurs at a ratio of 2 parts of the silicon powder and 8 parts of the electrolyte.

8. The method of separating silicon from molten salts comprising:

dissolving a eutectic melt of fluoride salts LiF—KF—NaF with silicon particles in anhydrous hydrogen fluoride to obtain a liquid phase composition HF+(LiF—KP—NaF) with the silicon particles, the silicon particles being a solid phase in the form of a silicon powder;

filtering the liquid phase to separate the silicon powder; and

directing the liquid phase to distillation of the hydrogen fluoride used at the dissolving step.

9. The method of claim 8, wherein solidified melt with silicon particles is grinded before dissolving.

10. The method of claim 8, wherein dissolving is carried out at a temperature from a range from −5° C. to +12° C.

11. The method of claim 8, during the dissolving step a ratio between the solid phase and the liquid phase is 1:23.

12. The method of claim 8, wherein the filtrating step is carried out by means of centrifugation.

13. The method of claim 8, further comprising purifying the silicon powder by washing it with a mixture of inorganic acids.

14. The method of claim 13, wherein the washing step is performed with the mixture of inorganic acids comprising 2-3 M H₂SO₄+0,1-0,2 M HF at a temperature from a range from 5° C. to 75° C.

15. The method of claim 14, further comprising drying the silicon powder in an inert atmosphere at a temperature range from 80° C. to 120° C.

16. The method of claim 8, wherein distillation of the anhydrous hydrogen fluoride is carried out thermally.

17. A method of producing silicon tetrafluoride comprising:

fluorinating silicon dioxide by elemental fluorine in two steps by:

(1) acting on the silicon dioxide by elemental fluorine at a temperature between 1100° C.-1200° C. while supplying the elemental fluorine in an amount exceeding a stoichiometric amount by 20% mass to 30% mass, to form a gaseous phase; and

(2) supplying the silicon dioxide in an amount exceeding a stoichiometric amount by 70% mass to 80% mass and fluorinating it with the gaseous phase until all excess fluorine from step (1) is consumed.

18. The method of claim 17, wherein fluorinating is carried out in torch of a reactor.