RU2645138C1 - Method for purification of metallurgical silicon - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method of purification of metallurgical silicon includes tapping silicon melt from the furnace, purifying silicon melt in a ladle with overheated steam at the temperature 120-400°C in a mixture with compressed air fed through porous plugs in the ladle bottom at steam-to-compressed air ratio 1:8-1:30 at silicon temperature 1500-1760°C. The number of points for supplying steam-air mixture to the bucket can be from one to four.

EFFECT: improved quality of silicon due to reduction of calcium, phosphorus, boron, iron in silicon.

5 cl, 1 tbl, 8 ex

Description

The invention relates to ferrous metallurgy, and in particular to methods of purification (refining) of industrial silicon from impurities.

Technical silicon is obtained by carbothermic reduction of mineral raw materials, quartz or quartzite in electric furnaces. In the process of reducing smelting, in addition to the leading silicon element, other elements are also restored, which are impurities in the mineral raw materials and are part of the reductant ash. All recovered elements, in the overwhelming majority, worsen the quality of technical silicon (Elkin KS, Zelberg B.I. et al. Silicon production. Metallurgist Handbook, St. Petersburg, MANEB, 2013, 364 pp.). Increased requirements for high-quality aluminum alloys, in which silicon is the main alloying element, for silicon, intended for further processing into polycrystalline, limit the content of phosphorus, boron, iron, calcium.

A known method of refining silicon melt (SU 835063, СВВ 33/02, publ. 07/27/1996), including processing the melt with a flux consisting of SiO ₂ , NaF, Al ₂ O ₃ , CaO, while blowing the melt through a graphite tube with oxygen . The disadvantage of this method of refining silicon is the low degree of removal of aluminum and calcium. The content of phosphorus and boron in silicon remains the same.

A known method of refining silicon and its alloys (patent RU 2146650, C01B 33/037, publ. 20.03.2000), including processing the melt in the ladle in the presence of flux, which includes pure silica sand, lime and / or fluorspar, melt processing is carried out in two stages: at the first stage, the melt is blown with a mixture of oxygen with air and / or an inert gas during the process of pouring the melt from the furnace into the ladle until it is filled with continuous and uniform supply of flux to the surface of the melt, in the second stage after filling the ladle, the melt is processed They are blown with air and / or inert gas until the melt temperature in the ladle reaches 1450-1550 ° C, and the melt is purged with gases through the porous part of the ladle bottom. The disadvantage of this method is that with this method there is a decrease in the content of only aluminum and calcium.

The known method and device (patent RU No. 2159213, C01B 33/037, publ. 25.02.1999). The method includes heating the crude silicon in a crucible to obtain a melt and treating the melt with a plasma torch containing an inert gas, a reducing gas, and water vapor. Heating and processing of silicon by a plasma torch is carried out simultaneously with the rotation of the crucible around its axis until a hollow cylinder is molten, while the plasma torch is directed along the axis of rotation, and the finished product is drained when the specified level of impurities is reached. A device for purifying silicon according to this method consists of a crucible and a plasma torch with gas supply channels. In this case, the crucible is a cylindrical shell with two flanges at the ends, lined and lined with quartz glass from the inside, a plasmatron is inserted into the hole of the flange, and on the opposite side in the second flange there is a hole for gas exit, removal of impurities and silicon discharge into the mold .

The disadvantages of this method and device due to the fact that the effectiveness of this method is extremely small. Low efficiency of jet plasmatron, low efficiency of quartz sand thermal insulation, high consumption of argon, as the process is conducted under atmospheric pressure.

A known method of refining metallurgical silicon with dry argon plasma with water injection on the melt surface, including heating the crude silicon in a crucible to obtain a melt and treating its surface with an inert gas plasma torch directed at an acute angle to the melt surface, the surface of the silicon melt being treated with a jet dry argon plasma, at the same time feeding to it portions of distilled water with a volume of from 0.01 to 0.05 cm ³ under a pressure of 1000-1500 kgf / cm ² through a nozzle nozzle, after which form an ingot of polycrystalline silicon by the method of controlled directional crystallization (patent RU 2465199, C30B 29/06, C01B 33/037, publ. 12/27/2012).

The disadvantage of this method is the low efficiency of this method due to the high energy costs.

A known method of purification of metallurgical silicon moistened by an alternating current plasma in vacuum (patent RU 2465202, C30B 29/06, publ. 27.12.2012). The method includes heating the silicon in a crucible to obtain a melt and treating the melt with a plasma torch directed at an acute angle to the surface containing inert gas and water vapor, while heating and melting the crude silicon is carried out in a cylindrical quartz crucible in a vacuum using a graphite heater. The technical result is aimed at obtaining from metallurgical silicon with a purity of 98-99.9% an ingot of polycrystalline silicon of a purity of 99.9999%, with a phosphorus content of not more than 0.1 ppm, boron from 0.1 to 1 ppm, suitable for the manufacture of photoconverters in an industrial way. The disadvantages of this method are the complexity of the hardware design of the process and the high cost of cleaning silicon.

The closest is the method of refining metallurgical silicon (patent RU 2465200, C01D 33/037, publ. 10/27/12), including heating the crude silicon in a crucible to obtain a melt and treating the melt with a plasma torch containing an inert gas, a reducing gas and water vapor, at silicon is treated with an argon plasma jet with an admixture of water vapor directed vertically from bottom to top through a hole in the bottom of a quartz crucible in a vacuum; silicon is preliminarily heated to a temperature of 1400 ° C by induction Foot heater and graphite cylinder. The disadvantages of this method are the significant consumption of energy for heating silicon, the high consumption of plasma-forming gas and the insufficient productivity of the installation.

By technical nature, by the presence of common features, this technical solution was adopted as the closest analogue.

The basis of the invention is a task aimed at improving the quality of silicon.

In this case, the technical result is a decrease in the content of calcium, phosphorus, boron, and iron in silicon.

This goal is achieved by the fact that liquid silicon in the ladle is cleaned of impurities of calcium, phosphorus, boron and iron by supplying compressed air and superheated water vapor through a porous plug in the bottom of the bucket, taken in the ratio of superheated water vapor-compressed air 1: 8-1 :thirty. Superheated steam is mixed in the main with air and the steam-air mixture is fed into the ladle at a temperature of 120-250 ° C. Silicon temperature during cleaning is maintained at 1500-1750 ° C.

The prior art involves in the process of refining to clean the silicon in the bucket from aluminum and calcium during the release of the molten silicon from the furnace into the bucket and after filling the bucket. Other impurities (phosphorus and boron) are not removed with such cleaning methods. To remove phosphorus and boron from a silicon melt, sophisticated and expensive equipment is used in the form of plasmatrons of various designs, which significantly increases the energy costs of silicon purification. The equipment claimed by these methods has low productivity.

The studies conducted by the authors made it possible to establish that the process of cleaning the silicon melt in the ladle can be carried out using a steam-air mixture of compressed air and superheated water vapor. The authors experimentally determined the limits of the ratio of water vapor and compressed air and the temperature conditions of this process of purification of silicon in the bucket.

Comparison of the proposed silicon purification technology not only with the technology of the prototype, but with technologies by analogs shows that:

- it is known the refining of the melt in the process of pouring it from the furnace into the ladle;

- the use of air and / or oxygen in the process of refining metals is known;

- the use of fluxes in the process of refining silicon is known;

- known processing of the melt through the porous part of the bottom of the bucket.

- it is known to use inert and reducing plasma gases and water vapor for silicon purification.

A comparative analysis of the known technical solutions did not reveal identical and equivalent features to the proposed solution, namely:

- supply through a porous plug in the bottom of the bucket of compressed air and superheated water vapor,

- application for the purification of silicon overheated water vapor and compressed air in a ratio of 1: 8-1: 30;

- purification of silicon without the use of fluxes, which may introduce additional impurities;

- use of a special device to obtain a vapor-air mixture.

Thus, the proposed solution meets the criteria of the invention - inventive step and industrial applicability.

An example implementation of the method.

When silicon melt was discharged from the furnace, silicon was cleaned simultaneously with the melt being fed into the ladle by feeding a mixture of superheated water vapor and compressed air through a porous plug in the bottom of the ladle. At the first stage, the amount of superheated steam in the steam-compressed air mixture was worked out, the calculation was carried out taking into account the amount of unsaturated water vapor per ton of silicon melt.

Prototype. In a pre-heated ladle, a melt with a temperature of 1720 ° C was poured from the furnace. Before purification of silicon, samples were taken to determine the amount of impurities in the silicon melt. They amounted to Ca - 0.76%, Fe - 0.31%, P - 0.0028%, B - 0.0032%. After purging the bucket, the amount of impurities in silicon was measured: Ca - 0.09%, Fe - 0.32%, P - 0.0027%, B - 0.0032%, Si - 99.43%.

Example 1. A molten silicon was poured into the ladle from a furnace with a temperature of 1760 ° C, with impurities: Ca — 0.70%, Fe — 0.32%, P — 0.0025%, B — 0.0035%. At the same time as the melt was fed into the ladle, a mixture of superheated steam and compressed air was continuously fed through porous plugs to the bottom of the bucket. After filling the bucket, silicon was further cleaned until the desired result was obtained. The amount of unsaturated water vapor was 1 kg / t of silicon melt. The ratio of steam to compressed air is 1: 1. After purging the bucket, the amount of impurities in silicon was: Ca - 0.009%, Fe - 0.31%, P - 0.0024%, B - 0.0033%, Si - 99.55%.

Example 2. In the bucket spent the release of silicon with a temperature of 1690 ° C. The amount of impurities before purification was Ca - 0.67%, Fe - 0.24%, P - 0.0027%, B - 0.0033%. A mixture of superheated steam and compressed air was fed into the bucket through the plugs in the bottom, the ratio was maintained 1: 1, with an amount of unsaturated water vapor of 3 kg / t of melt. After cleaning, the following result was achieved: Ca - 0.007%, Fe - 0.23%, P - 0.0025%, B - 0.0031%, Si - 99.61%.

Example 3. In a ladle with a molten silicon, temperature 1645 ° C, the silicon was cleaned with the composition of impurities: Ca - 0.87%, Fe - 0.30%, P - 0.0023%, B - 0.0038%, steam and compressed air, with a ratio of 1: 1 and a steam quantity of 5 kg / t. After purification, the content of impurities was achieved: Ca - 0.008%, Fe - 0.27%, P - 0.0021%, B - 0.0035%, Si - 99.61%.

Example 4. A silicon melt with a temperature of 1710 ° C and the amount of impurities: Ca - 0.73%, Fe - 0.31%, P - 0.0025%, B - 0.0036% was purified with an amount of water vapor of 8 kg / t melt. The cleaning result: Ca - 0.009%, Fe - 0.27%, P - 0.0022%, B - 0.0032%, Si - 99.63%.

Example 5. The temperature of silicon 1630 ° C. The amount of impurities: Ca - 0.67%, Fe - 0.28%, P - 0.0026%, B - 0.0034%. The amount of unsaturated water vapor in the mixture with air was 10 kg / t of melt. Result: Ca - 0.008%, Fe - 0.23%, P - 0.0021%, B - 0.0029%, Si - 99.64%.

Example 6. The temperature of silicon 1580 ° C. The amount of impurities: Ca - 0.81%, Fe - 0.30%, P - 0.0022%, B - 0.0036%. The amount of unsaturated water vapor in the vapor-air mixture is 12 kg / t of melt. Result: Ca - 0.0010%, Fe - 0.23%, P - 0.0018%, B - 0.0030%, Si - 99.67%.

Example 7. The temperature of silicon is 1685 ° C, the amount of impurities: Ca - 0.74%, Fe - 0.31%, P - 0.0024%, B - 0.0035%. The amount of steam in the vapor-air mixture is 13 kg / t of silicon. Result: Ca - 0.016%, Fe - 0.28%, P - 0.0021%, B - 0.0030%, Si - 99.58%.

Example 8. The temperature of silicon is 1705 ° C, the amount of impurities: Ca - 0.72%, Fe - 0.30%, P - 0.0026%, B - 0.0033%. The ratio of steam and compressed air is 1: 1, the amount of steam is 13 kg / t of silicon melt. Result: Ca - 0.018%, Fe - 0.29%, P - 0.0023%, B - 0.0031%, Si - 99.56%.

The optimal amount of steam in the mixture with compressed air was 1-12 kg / t of silicon melt. When the amount of steam in the mixture is more than 12 kg per ton of melt, the amount of silicon in the melt decreases due to its more intense oxidation.

In further experiments, the quantitative ratio of superheated steam-compressed air was changed, the number of supplied points of the gas-vapor mixture in the ladle was changed, the temperature of the superheated steam in the oxidizing mixture was changed, and the temperature of the silicon melt in the melt ladle was fixed. The test results are summarized in table 1.

The optimal conditions for the purification of silicon from impurities of iron, calcium, phosphorus and boron are:

- the ratio of superheated steam to compressed air was 1: 8-1: 30 (position 7-29), with a lower ratio, the oxidation of impurities is insufficient (positions 1-6), with a larger ratio, the efficiency of removing impurities decreases (positions 30-32); the probable reason is the increase in the proportion of impurities, in comparison with the optimal cleaning conditions, an increase in the proportion of impurities due to an increase in silicon losses from oxidation;

- the number of supply points of the oxidizing mixture is optimally 1-4 (positions 7-29), with a larger number of supply points, the cleaning efficiency decreases (positions 30-32);

- the optimum temperature of superheated steam was 120-400 ° C (items 7-9, 11-22, 24-25, 27-29), at a lower temperature, the oxidation of impurities is low (items 2, 23), at a higher temperature, the cleaning efficiency decreases (positions 10, 26);

- the optimum temperature of the silicon melt during cleaning is in the range of 1500-1760 ° C. At temperatures below 1500 ° C, the cleaning process is not very effective (positions 1, 12, 21, 31), at temperatures above 1760 ° C, the removal efficiency of impurities decreases (positions 11, 17, 32).

Claims (5)

1. The method of purification of metallurgical silicon, including purging the silicon melt through the porous bottom of the ladle, characterized in that the cleaning of the silicon melt poured from the furnace is carried out by supplying superheated steam with a temperature of 120-400 ° C in a mixture with compressed air at a ratio of superheated water vapor - compressed air 1: 8-1: 30 at a temperature of silicon 1500-1760 ° C. 2. The method according to p. 1, characterized in that the flow rate of unsaturated water vapor is maintained in the range of 1-12 kg per 1 ton of metallurgical silicon. 3. The method according to p. 1, characterized in that the flow rate of compressed air is maintained within 50-500 m ³ . 4. The method according to p. 1, characterized in that the number of supply points in the bucket of steam-air mixture is 1-4. 5. The method according to p. 1, characterized in that the maintenance of the specified temperature of the molten silicon in the bucket is carried out by supplying air and water vapor.