WO2006041271A1

WO2006041271A1 - Method of production of pure silicon

Info

Publication number: WO2006041271A1
Application number: PCT/KZ2005/000007
Authority: WO
Inventors: Marat Fatykhovich Tamendarov; Bolat Nygmetuly Mukashev; Khabibulla Abdullaevich Abdullin; Zhaksybek Abdrakhmetovich Kulekeev; Nuraly Sultanovich Bekturganov; Boris Anatolievich Beketov
Original assignee: The Ministry Of Education And Sciences Of Republic Kazakhstan Republican State Enterprise 'center Of Chemical-Technological Researches'; 'silicon Technologies', Limited Liability Partnersship
Priority date: 2004-10-12
Filing date: 2005-10-12
Publication date: 2006-04-20
Also published as: EA200700341A1; EP1805106A1; EA009888B1

Abstract

The method of production of pure silicon by aluminothermic reduction of silicon dioxide in silicon-containing phosphorous slags for PV industry and solar batteries manufacture. Phosphorous slag is loaded into open graphite crucible and then heated in the induction furnace at the eutectic melting temperature, whereupon it is added with aluminium. Obtained silicon separated from the slag emerges on the slag's surface. Then it is loaded with the new portion of slag and aluminium. The process should be repeated for several times, until full sedimentation of the reacted slag and its separation from silicon, which appeared in the top part of reactor. Obtained silicon is discharged into the graphite mould and cooled up to formation of the grain patterns with average sizes of crystals less than 1 mm. Than the silicon is crushed and sifted for the removal of small particles and exposed to hydrochemical treatment with aqueous solutions of mineral acids in two stages. Thus, this process allows receiving silicon powder with total purity of 99,99 %. Favorable results may also be obtained when up to 30 mole % of alkaline earth metal fluorides or other substances that increase the solubility in the slag or the aluminum oxide that is formed are added to the slag.

Description

Method of Production of Pure Silicon

The present invention relates to nonferrous metallurgy, especially to the aluminothermic method for the manufacture of pure silicon for PV industry, including solar batteries manufacture.

Solar energy is one of the main and actual problems of international science and engineering. Current Solar Cells (SC) manufacturing technologies use expensive silicon of a semiconduction- purity, which is produced through chlorosilane technological process. In the conventional process for the manufacture of high purity silicon for electronic components, metallurgic silicon, obtained by reduction of quartz with carbon, is converted into trichlorosilane by hydrogen chloride. The trichlorosilane can be purified by distillation and, finally, can be decomposed in the presence of hydrogen to give high-purity polycrystalline silicon. The silicon obtained in this manner then meets even the strict purity requirements for electronic components. There has therefore been no shortage of attempts to replace the classical purification process by a more cost-effective process.

Ordinary metallurgical silicon contains amounts of metallic and nonmetallic impurities, which do not allow its use for the solar cells manufacture. Nonmetallic impurities, such as boron and phosphorus can be reduced mainly due to the choice of suitable raw materials used for the pure silicon production, but it is worth mentioning that Fe, Al, Mn, Cu, Ni, and other metal-containing impurities can be reduced up to the definite grades. High-purified initial raw materials are expensive; therefore, it is desirable ensuring simple and inexpensive production method, which allows removing metallic impurities or reducing its concentrations up to admissible lowest degree, thus obtaining purified silicon, suitable for the use in the solar cells manufacture.

It is known, that the process of silicon crystallization recovers metallic impurities, which crystallize along silicon crystal boundary as intermetallic compounds or silicides. Therefore, silicon purification can be effective at the control during crystallization ensuring gathering and removing of those additives from silicon either by crucible-pulling method, or by crucible-free zone melting methods, or by dissolution of additives in mineral acids, which do not influence silicon. Crucible-pulling and crucible-free zone melting methods are very expensive. Moreover, they require continuous process for the production of solar silicon.

According to the US Patent Na 4457903, quartz may be reduced by aluminum in the presence of an aluminum sulphide slag to give elemental silicon. In this process, the aluminum acts simultaneously as a reducing agent for the quartz and as a solvent for the silicon that has formed, which can subsequently be crystallized out of the solution in an already very pure form, by cooling to a minimum temperature of approximately 600⁰C. This process, however, requires a lot of aluminum and, because of the odor and toxicity of the aluminum sulphide, requires additional protective measures. The most closely related method, which has been considered as prototype - is the method for silicon production by interaction of aluminium with siliceous slag of phosphate manufacture in a molar ratio of the slag and aluminium of 1: (0,3-0,5) at 1200 ... 1300⁰C in the presence of magnesium carbonate impurity in quantity of -20 % and cullet in quantity of 5-16% with regard to aluminium mass (see the Republic of Kazakhstan preliminary patent N° 4627, Bulletin Nώ, afrom Junelό, 97, C01B33/02). Main disadvantages of this process concern to a certain difficulties at leaching reaction control at acid purification due to thermal emission and formation silane and gaseous hydrogen, which can lead to spontaneous ignition or explosion and to insufficient purity of recovered silicon. Moreover, huge amounts of calcium in silicon result in huge losses of silicon in the form of fine particles, which can be lost within washing process after the leaching.

The object of this innovation was therefore to provide a process which may be used on a commercial production scale, and which, starting from quartz, permits the production of pure silicon for solar cells while avoiding the expensive gas-phase deposition, and without having the disadvantages of the previously known processes.

According to the present invention, the specified purpose can be achieved using the process, which is based on the following principles: the slag is load into a reactor and heated up to eutectic melting temperature. Then the melting liquid is added with aluminium in specified quantities, essential for the recovery of silica in phosphorous slag. Favorable results may also be obtained when up to 30 mole % of alkaline earth metal fluorides or other substances that increase the solubility in the slag or the aluminum oxide that is formed are added to the slag. The silicon, obtained as a result of oxidation-reduction reactions in the melting liquid, can be easily separated from slag and floats to the surface, due to its lower density, comparatively to slag. Unlike the prototype method, which uses cullet for the silicon fining, the present technique provides loading of new portion of slam and aluminium into the silicon melting liquid surface. As a result of repeated passage of the liquid slag through the layer of silicon into the lower part of the reactor, efficiency of the slag purification of silicon is increasing. Besides, interaction of liquid silicon with slag allows decreasing the content of calcium and aluminium, which quantity at conventional aluminothermic process can reach several percents. In this case, calcium and aluminium impurities can act as reducing agents for the silica in the new portion of slag.

Thus, the reactor is added with separate portions of the charge. The process proceeds until complete sedimentation of the reacted slag and its separation with silicon, which is deposited in the top part of the reactor. After that the silicon merges into graphite mould, separately from the slug, and slowly cooled until it is converted into frozen ingot with grain size less than lmm. Then silicon is preliminary crushed, diffused, and released from the small particles. Hydrochemical purification consists of the following two stages: First stage is characterized with application of HCl and FeC13 aqueous solution, second stage - HF and HN03 aqueous solutions. At the final stage the silicon powder is washed with deionized water and then dried. Comparative analysis of the present innovation project with prototype indicates that declared method differs from well-known technique with quantitative structure of the charge, methods for the injection of the charge into the reactor in separate portions, thus enabling to control temperatures of the liquid melt and eliminate necessity in its additional intermixing, providing effective process for the silicon's slag purification. The process also eliminates formation of silanes and possibility of its spontaneous ignition at acid treatment. The specified differences allow producing silicon with total purity of more than 99.99 %. One of the main advantages of the suggested method is that the siliceous slag simultaneously acts as a media for the silicon reducing reaction and media for extraction of impurities. Repeated passage of slag through the silicon melting liquid at its deposition in the reactor significantly strengthens silicon-fining efficiency. Insertion of charge in separate portions provides reaction completeness without mechanical mixing and allows supervising melting liquid temperature with the speed of the component addition.

Thermodynamic analysis of the aluminothermic reduction of SiO₂ in slag melting liquids has shown that in the melting liquids consisting of aluminium and slag with the content of CaO, SiO₂, MgO, Al₂O₃ and other oxides, the aluminium will mainly interacts with SiO₂ with reduction of silica to silicon. Comparison of SiO₂ equilibrium activity with silica activity in CaO-SiO₂-Al₂O₃ and CaO-SiO₂- Al₂O₃-MgO compounds see R.H. Rein, J.Chipman. Nrans. Metallurg. Soc. ASME, 227, N° 5, 1963] has shown that equilibrium in the compound "slag liquid melt— aluminium" can be achieved at the SiO₂ approximate content - 0,5 mass%. It testifies to a high degree of oxidation-reduction reactions completeness.

The aluminum serving as reducing agent is advantageously used in as pure as possible a form, in order to avoid entrainment of additional impurities. The use of electrolytically purified aluminum having a purity of at least 99.9% has proved particularly successful. If the impurities are substances that accumulate in particular in the slag, then lower degrees of purity of the aluminum may be tolerated. On the other hand, as regards impurities that dissolve only slightly in the slag, such as iron or phosphorus for example, care must be taken from the start that the aluminum is as pure as possible. The process of the present invention can be fully described in the following example:

Open graphite crucible was filled with 4000 g of slag. The slag was heated in the induction furnace at the melting temperature of (1300-1350⁰C). The amount of 000 g of granulated aluminium with total purity of at least 99,6 % was introduced into the siliceous slag liquid melt in the reaction chamber. Because of the exothermic nature of the reduction reaction of the phosphorous slag with aluminum, the molten temperature rises up to 1420-1600 C. In this temperature interval, the silicon in the molten state can be easily separated from the slag.

Only when the reaction subsides is it necessary to apply heat again, in order to prevent the temperature dropping below the melting point of the reagents of the reaction mixture. When the amount of reaction material, which has been added gradually, has completely reacted, may be observed by a fall in the reaction temperature, the system is given time to separate out. Secondary slag collects on the crucible bottom, while silicon formed collects on the surface of the slag, due to its lower density. When all oxidation-reduction reactions have finished, the silicon liquid melt should be consistently added with three portion of charge, containing 1000 g of phosphoric slag and 250 g of granulated aluminium. External heating of the graphite crucible increases temperature of the liquid melt up to 1500-1550⁰C. This temperature interval should be maintained within 30 minutes, thus allowing easier separation of the silicon from the slag. Total weight of the metal - 1260 g.

Obtained silicon should be discharged into the mould. This allows keeping rather slow speed of cooling for the grain pattern formation, where silicon crystallites are separated from each other with boundaries, which in turn provide effective segregation of metallic impurities. Phases of silicon crushing and dissemination are necessary for the removal of small particles, less than 0,060 mm. As silicon is characterized with higher degree of hardness, its crushing firstly provides destruction of the material on the crystal boundaries, thus removing significant quantities of impurity atoms at the mechanical effect stage.

The process of acid treatment can be realized in two stages: at the first stage the preliminary crushed material is leached in the acid solution containing 100 g/1 of FeCl₃ (10%) and HCl (2,5 %). Process conditions: ratio = 1:5, temperature - 80⁰C. Presence of FeCl₃ prevents formation of gaseous silane, thus eliminating possibility of its spontaneous ignition. After washing the silicon powder proceeds to the second stage. The second state of the leaching process should be carried out within 30 minutes at the room temperature and includes treatment of powder with hydrofluoric acid and nitric acid aqueous solutions, diluted with, for example 1-2 % HF, 4-5 % HNO₃. Application of solutions with higher concentrations will result in huge losses of silicon at its dissolution with impurities. The second stage can be characterized with dissolution of oxide film on the silicon crystals surfaces and with partial pickling of the silicon coating surface. Silicon oxide film and coating surfaces are contaminated with impurities, contained in silicon in insignificant quantities, which however, concentrate on the Si/SiO₂ phase boundary, and absorbed with crystal cavities and oxide film.

The silicon, purified at the second stage of the leaching process should be washed with water and dried. The product is sifted on 1 mm crushing unit, and then washed with distilled water or with water, purified in ion-exchange filter. Decantation allows removing small particles with sizes lesser than 0,050-mm during all washing stages. '

Thus, using the method, declared in the present invention we can receive silicon of the following purity:

Such silicon represents initial material, used for the "solar silicon' ingots growth by various methods of crucible pulling, or by crucible-free zone melting.

Claims

1. The method for the production of silicon by the reduction of silicon dioxide with aluminium in phosphorous slag at 1420-1600⁰C at the molar ratio of the amount of aluminium in the charge to the amount of silicon dioxide in the slag equal to 4:3, differs as the charge is loaded portion-by-portion into the silicon liquid melt, obtained silicon is discharged into the graphite mould and cooled up to formation of the grain patterns with average sizes of crystals less than 1 mm; and then the silicon is leached in aqueous solution of mineral acids.

2. The present method differs in the iteml, as it requires additional insertion of 30 molar percents of alkaline earth metal fluorides, necessary to increase dissolubility of alumina in the slag.

3. The present method differs in the iteml, because silicon is exposed to crushing and diffusion for the removal of small particles less than 0,060 mm in their size.

4. The present method differs in the iteml, as the silicon is leached in two stages: in aqueous solution containing FeCl₃ and HCl and in aqueous solution containing HF and HNO₃, correspondingly.

5. The present method differs in the item4, because after leaching the silicon should be washed out with distilled water, for the removal of fine particles.