RU2600055C1 - Method and device for refining technical silicon - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to production of technical silicium in ore-thermal furnaces and its further refinement. Method of refining technical silicon is implemented by directed crystallization, herewith the silicon melt is cooled down to 1,420 °C, immersed for 3-30 sec with metal crystallizing molds with the initial temperature of approximately 150-200 °C, impurities of metals in form of intermetallic compounds and solid solutions with silicon are extracted on the molds surfaces, after which the molds together with the impurities are removed from the melt and moved into a superheated flux for draining of silicon enriched with the impurities. Molds are made in the form of solids of revolution - balls, cones or cylinders. Device for implementation of this method comprises a mass exchanger with molten silicon and a settler with overheated flux for accumulation of silicon with increased content of impurities. Solids of revolution can be equipped with pins to increase the surfaces of crystallization.

EFFECT: invention ensures production of high-quality and cheap raw material with low content of impurities for further mass production of ultrapure silicon (SoG-Si).

3 cl, 2 dwg

Description

Technical field

The invention relates to a technology for the production of industrial silicon in ore-thermal furnaces and its further refining.

State of the art

Technical silicon contains from 1 to 4% impurities of iron, aluminum, calcium, other metals and non-metallic inclusions (GOST 2169-69).

Such a product can be used for the production of cast aluminum alloys, for example, silumin, and ferroalloys. However, it is completely unsuitable for use in the technology of obtaining ultra-pure “semiconductor” and “solar” silicon (SoG-Si), in which the maximum allowable content of metal impurities is limited to values of the order of 10 ^-7 at. % and less.

Known methods for the refinement of silicon by directional crystallization or crucibleless zone melting to produce single or polycrystals, for example, according to Czochralski (Semiconductor silicon technology. Edited by Falkevich ES, Metallurgy, 1992, pp. 254-255). They are the primary analogues of our invention, but using other physical principles and technological methods. Well-known analogues are precision methods that have existed for almost 100 years, but differ in low speed and high cost of redistribution and the final ultrapure product.

Crystallization in these methods occurs with the transfer of pure silicon into the solid phase, while impurities remain in the melt.

In applied electrochemistry and electrometallurgy, Rathenau’s proposal to use the so-called “touch cathode” for electrochemical separation of calcium metal from a melt is known (DRP, 155433-1903, Zf. Electrochem, 1904, 10, 508. Quoted from: Baymakov Yu.V., Vetyukov M.M .. Electrolysis of molten salts.M .: Metallurgy, 1966, p. 257-258). Such a cathode has been successfully used for the electrochemical production of metallic calcium from salt melt.

In electrochemistry, an unambiguous characteristic of the process speed is the current density, which already reached 12-60 a / cm ² on a calcium cathode in a calcium cathode and 16-30 a / cm ² under industrial conditions. It is worth noting that for the electrolytic separation of aluminum from a melt, the cathodic current density is only ~ 0.5 a / cm ² , i.e. 2 orders of magnitude smaller values. The calcium contact cathode with crystallization of this metal at a very high rate of metal phase evolution is the closest prototype of our invention.

Using a steel rod as a touch mold, we successfully isolated iron and silicon impurities from non-high-grade technical aluminum on it.

It is worth noting that the same group of physical effects can be applied to different metals. For example, the Czochralski method is suitable and applied not only to obtain semiconductor silicon, but also germanium. Silicon, aluminum and iron are among the most common elements. So their content in the Earth's lithosphere is 27.6, respectively; 8.8 and 5.1 mass. % (Handbook of a chemist. Edited by B. Nikolsky. T.I., L.-M., GHI, 1963, p. 22). The Si-Al-Fe system can be used to test inventions at any content of each of these elements. In particular, aluminum may be the basis, while silicon and iron may be impurities. Conversely, silicon can act as a solvent, and aluminum and iron can be impurities.

SUMMARY OF THE INVENTION

The invention is due to the fact that both silicon and the metal impurities present in it (except aluminum) have high melting points of the individual components and a tendency to form numerous intermetallic compounds:

Silicon - 1420 ° C;

Calcium - 850 ° C (Ca ₂ Si and CaSi ₂ );

Titanium - 1668 ° С (TiSi ₂ , TiSi and Ti ₅ Si ₃ );

Vanadium - 1900 ° C (V ₅ Si ₃ and V ₃ Si);

Chromium - 1882 ° С (CrSi, CrSi ₂ ; Cr ₃ Si; Cr ₅ Si ₃ );

Manganese - 1252 ° C (7 compounds);

Iron - 1539 ° С (FeSi; FeSi ₂ );

Nickel - 1453 ° С (Ni ₃ Si; Ni ₅ Si ₂ );

Cobalt -1492 ° С (CoSi; Co ₃ Si) [Hansen M., Anderko K. Structures of binary alloys, Eng., Vol. 1, M., 1962, 608 s; T. 2., M., 1962, 1488 s. Eliot R.P. The structure of double alloys, with the English., T. 1, M., 1970, 455 s; T. 2, M., 1970, 472 p. Shank F. Structures of double alloys. With the English., M., 1973, 759 pp.].

As analogues of the claimed solution, the Czochralski and crucible-free zone melting processes of directional melt crystallization can be considered [Semiconductor Silicon Technology. Ed. Falkevich E.S. M., Metallurgy, 1992, p. 254-256]. In these methods, a single crystal or polycrystalline material of a high degree of purity is grown, but with a very slow process. Low productivity provides a high level of costs, does not significantly reduce the cost of silicon SoG-Si for solar energy. While maintaining the Czochralski method for finishing refining operations, it is proposed in our invention to use mass crystallization of intermetallic compounds of metal IC and impurities from a melt of industrial silicon located in a boundary layer in a metastable supercooled state. The essence of the method is that the melt is cooled to the melting point of pure silicon at 1420 ° C.

Moreover, in any of the systems under consideration, there are extensive temperature regions located between the liquidus and solidus lines. In these areas, crystals of IC and solid solutions TP are in a liquid silicon melt in a state of deep supercooling and when impurities are introduced into such a metastable crystallizer system with a much lower initial temperature (~ 150-200 ° C), impurities are able almost instantly, in 3-30 s , fall out of the melt on its surface in the form of condensate. After that, the crystallizers together with impurities are removed from the melt.

The touch mold in the form of a steel rod will not allow to obtain that high speed and depth of purification of silicon from impurities that are necessary, because the crystallization of impurities is carried out near a three-phase crystallizer – melt – atmosphere interface. For an almost unlimited increase in the speed and depth of purification from impurities, the crystallizer must be placed in the deep layers of liquid silicon with the largest possible crystallization surface.

The sequence of refining operations is as follows.

The technical silicon melt is cooled to 1420 ° C, then a metal crystallizer with an initial temperature of ~ 150-200 ° C is immersed in it for 3-30 s. On the surfaces of the crystallizer in contact with the technical silicon melt, metal impurities in the form of intermetallic compounds and solid solutions with silicon are isolated. Next, the crystallizer, together with the crystals precipitated on it, is removed from the melt and transferred to the superheated flux, from where the draining impurities together with the residual mother silicon are periodically removed.

A device for implementing the inventive method comprises a mass transfer apparatus with molten technical silicon, in which a mold is arranged, made in the form of bodies of revolution: balls, cones or cylinders. Another part of the device is a superheated flux sump for the accumulation of silicon with a high content of impurities.

Rotation bodies for mass transfer apparatus in order to increase the surface of contact with the melt and the crystallization rate are performed with spikes.

Technical result

Such is the receipt of high-quality and cheap raw materials with a low content of impurities for further mass production of ultrapure silicon (SoG-Si). Our invention is a bridge for the transition from conventional technical silicon to highly pure material, which will ensure the competitiveness of “solar energy”. Another advantage of our method is an increase in the productivity of the process of removing the main amount of impurities.

List of drawings and description of the interaction

In FIG. 1 shows a device for implementing the method, consisting of a mass transfer apparatus 1 (A) and a sump for collecting impurities 2 (B).

The use of a massive ball 3 made by casting, for example, made of cast iron and placed in the central regions of the molten technical silicon 4, is shown as a crystallizer. The ball is suspended by means of a cylindrical rod 5 on a hoisting-and-transport device, with which it can be immersed in the melt, removed from it and move to the adjacent settling tank 2 with molten flux 6 to accumulate impurity-rich technical silicon 7.

The molds and, in particular, balls can be provided with spikes, as shown in FIG. 2, in order to increase the working surface and productivity of the silicon cleaning process. For a silicon level in a mass transfer apparatus of ~ 1200 mm, 250-300 spikes 8 can be made on a ball with a diameter of ~ 600 mm, with a radius at the base of 0.05 and a height of ~ 0.1 m, increasing the crystallization surface from 1.13 m ² to ~ 5.1 m ² .

Crystals of intermetallic compounds (IMS) and solid solutions (TP) almost instantly drop out of the molten technical silicon melt in the boundary layer near the massive and cold metal crystallizer, which, together with the crystallizer, are transferred to a settling tank with an overheated flux. Further, in this flux, the "dirty" technical silicon containing impurity crystals flows to the bottom of the sump, from where it is periodically removed to extract and use such valuable components as titanium, vanadium, nickel, etc.

Information confirming the possibility of carrying out the invention.

The directional crystallization method used in the preparation of calcium and the Czochralski method used in the production of semiconductor metals have been known for about 100 years. At the same time, the touch cathode in the calcium industry allows the process to be conducted at high speeds under conditions of isolating the desired product. The Czochralski method is used in the crystallization of a very pure product and in the growth of single crystals under conditions close to equilibrium, and at very low speeds.

In our invention, it was proposed to isolate not a pure product, but IC and TP crystals from metastable supercooled liquid on developed crystallization surfaces at very high process speeds. This technique will increase the quality of technical silicon. The possibility of manufacturing massive bodies of revolution, for example, cast iron balls by casting, is beyond doubt.

Verification of the invention was performed on an Al-Si-Fe system, which is qualitatively close to technical silicon, but based on aluminum. In laboratory experiments, a steel bar with a diameter of 13 mm to a depth of 0.3 cm was immersed in a crucible with molten technical aluminum at an initial melt temperature of 730 ° C and a melting point of aluminum of 660 ° C. The iron content in aluminum decreased from 1.5% to 0.3% and silicon from 0.25 to 0.07%. Moreover, the Al-Fe system contains a saturated solid solution containing 0.04% iron at a eutectic temperature, and the chemical compound FeAl ₃ (36.5% Fe) with a melting point of ~ 1147 ° С (!). The Al-Si system contains a saturated solid solution containing 1.65% Si at a eutectic temperature (Moskvitin V.I., Nikolaev I.V., Fomin B.A. Metallurgy of light metals. M., 2005, p. 258, 263).

Thus, the crystallization of intermetallic compounds and solid solutions from supercooled metastable states is also confirmed experimentally as a way of refining the base metal of the system. A touch mold placed deep in the molten technical silicon will increase the speed and depth of refining silicon by several orders of magnitude, as well as reduce the total cost of an ultrapure product at the finish stages.

Claims (3)

1. The method of refining technical silicon by the method of directional crystallization, characterized in that the silicon melt is cooled to 1420 ° C, immersed in it for 3-30 s with metal crystallizers with an initial temperature of about 150-200 ° C, metal impurities are isolated on their surfaces in the form intermetallic compounds and solid solutions with silicon, after which the crystallizers, together with impurities, are removed from the melt and transferred to the superheated flux, from where the draining impurities are periodically removed. 2. A device for implementing the method according to claim 1, characterized in that the crystallizers are made in the form of bodies of revolution — balls, cones or cylinders, the device comprising a mass transfer apparatus with a molten silicon and a settling tank with an overheated flux for accumulating silicon with a high content in it impurities. 3. The device according to p. 2, characterized in that the rotation bodies are made with spikes located on their working surface.

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