RU2388691C2 - Method of purifying powdered silicon (versions) - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used to obtain highly pure silicon. The method involves chemical etching and high-temperature treatment of the surface of silicon while stirring particles of the powdered silicon at 800 - 1410° C and feeding silicon particles into a gas stream which is chemically active towards impurities and silicon. In another version a layer of porous silicon is formed on the surface of powdered silicon particles first.

EFFECT: invention shortens the purification process and consumption of chemically active gas.

8 cl, 3 ex

Description

The invention relates to metallurgy, in particular to the production of silicon of high purity.

Metallurgical silicon, containing more than 98.8% Si, is used to produce organosilicon compounds and trichlorosilane, the feedstock for the production of semiconductor silicon. The production of metallurgical silicon of high purity by the reduction of quartz in electric arc furnaces is difficult due to the insufficient purity of the furnace walls, electrodes, carbon reducing agents and quartz (I.M. G. (J. Ros. Chemical Society of D.I. Mendeleev), 2001, v. XLV, No. 5-6). Chemical preliminary cleaning of these materials requires significant capital expenditures; therefore, it is economically feasible to melt metallurgical silicon from conventional raw materials using conventional technology, and then refine it.

Subsequent purification of liquid metallurgical silicon from metal impurities by purging with gaseous halogen-containing reagents gives positive results, but is accompanied by harmful emissions into the atmosphere and worsens the environmental situation.

A known method is the acid purification of crushed metallurgical silicon, based on a large difference in the solubility of impurities in liquid and solid states (US patent No. 4241037, CL 01/33/02, 1979). In cooled silicon, impurities are located at the grain boundaries and are opened during crushing, which makes it possible to remove up to 90% of metallic impurities in aqueous solutions of inorganic acids and their mixtures at low temperatures. Silicon powder is treated with an aqueous solution of mineral acids containing 10% HCl and 10% HF, taken in an amount of 1.5-5 times the mass of silicon. After acid treatment for a long time (20-50 h) at a temperature of up to 353 K, the product is purified from impurities by washing, then filtered.

The disadvantage of this method is that it requires a significant consumption of inorganic acids. In accordance with this, the costs associated with the neutralization of spent acid solutions and their disposal increase. In addition, the use of acidic solutions containing a significant concentration of highly toxic hydrofluoric acid creates an environmentally hazardous environment, impairs the operation of equipment and complicates the process of neutralizing spent solutions.

The closest analogue is US Pat. US application No. 20070202029 "Method of removing impurities from metallurgical grade silicon to produce solar grade silicon", where metallurgical silicon is pulverized to a particle size of less than 5 mm and kept in vacuum at a temperature of from 1000 to 1410 ° C.

The disadvantage of this method is the long duration of the cleaning process (over 30 hours) and a limited set of removable impurities with high vapor pressure at a temperature of 1000-1410 ° C.

The objective of the invention is to develop a method for producing silicon powder of high purity, providing effective extraction from silicon of virtually any impurities.

The technical result of increasing the purity of metallurgical silicon is achieved by the fact that in the method of cleaning powdered silicon, including chemical etching of the silicon surface and high-temperature heat treatment, chemical etching is carried out at a temperature of from 800 to 1410 ° C when the silicon particles are immersed in a stream chemically active with respect to impurities and silicon gas, while chemical etching and heat treatment are carried out simultaneously with mixing particles of powdered silicon.

In a variant of the method for cleaning powdered silicon, including chemical etching of the silicon surface and high-temperature heat treatment, a layer of porous silicon is created on the surface of the particles of powdered silicon and chemical etching is carried out at a temperature of 800 to 1410 ° C when the silicon particles are immersed in a stream chemically active with respect to impurities and silicon gas, while chemical etching and heat treatment are carried out simultaneously with mixing particles of powdered silicon.

To reduce environmental pollution in a method for purifying silicon powder, a chemically active gas is created under reduced pressure by a plasma-chemical discharge by high-frequency currents.

To reduce the duration of the process in the cleaning method, chemical etching and heat treatment are carried out simultaneously with mixing particles of powdered silicon.

To further reduce the consumption of reactive gas in the purification method, chemical etching is carried out in a mixture of gases.

Purification of silicon is as follows.

Metallurgical silicon castings are crushed to a particle size of less than 1 mm (preferably about 0.1 mm), the resulting silicon powder is placed in a quartz chamber of the furnace, silicon is heated to the temperature of rapid diffusion of impurities in silicon (above 800 ° C, but below the melting point of silicon 1410 ° C) and gas containing halogens, such as chlorine or hydrogen chloride, is reactive with impurities and silicon through a furnace chamber. As a result of the interaction of impurities with a gas stream containing halogens, volatile compounds are formed that are transported by a gas stream from the furnace working zone and are trapped either in the external filter or condense in the low-temperature zone of the furnace. Thus, conditions for the outflow of impurities are created on the surface of particles of silicon powder and an impurity distribution gradient arises under the influence of which the diffusion of impurities from the bulk onto the silicon surface occurs.

The larger the specific surface area of silicon particles, the more intensive is the outflow of impurities to the surface; therefore, it is preferable to pre-treat the particles to form a layer of porous silicon that can increase the surface area by 10-100 times and as a result reduce the duration of the heat treatment process, which increases productivity equipment and reduces energy costs for the cleaning process. Simultaneously with the formation of a porous silicon layer, the first stage of removal of impurities from the surface of powdered silicon occurs.

The second stage of removal of the remaining impurities from silicon occurs in the heated chamber of the furnace during dry gas etching of the surface of silicon particles. The minimum value of the flow rate of chemically active gases is achieved by using reduced gas pressure and selecting the composition of their mixture, which provides the minimum interaction with silicon and the maximum with impurity elements.

The condition for immersing silicon particles in a stream of reactive gas is achieved by using continuous mixing of the particles using, for example, ultrasonic vibrations or rotation of the working chamber of the furnace.

The proposed method for purifying silicon is illustrated by the following examples.

Example 1. Powdered silicon with stirring is loaded into a chemical bath with a mixture of 1-2 parts of nitric acid per 100 parts of hydrofluoric acid for about 1 minute to form a layer of porous silicon and dissolve impurities located on the surface of silicon particles. The powder is washed with water and dried in an infrared oven. A further cleaning process continues in the gaseous medium. Dry etching of silicon powder is carried out in a quartz tube rotating along the longitudinal axis, placed horizontally in a tubular furnace with a temperature in the working zone of about 1150 ° C. A mixture of gases from iodine and chlorine is passed through the pipe. The process is conducted within 3 hours.

Example 2. In contrast to example 1, the process of dry etching of silicon powder is carried out in a plasma high-frequency discharge when sulfur hexafluoride + hydrogen chloride flows into the working chamber during the pumping of gases by a vacuum pump.

Example 3. In contrast to example 1, the process of dry etching of silicon powder is carried out, alternating every 15 minutes, a mixture of gases - nitrogen sparging through POCl ₃ to sulfur hexafluoride. At the same time, a phosphorosilicate glass film, which has good gettering properties with respect to impurities, appears and is subsequently etched on the silicon surface.

Claims (8)

1. The method of cleaning powdered silicon, including chemical etching of the silicon surface and high-temperature heat treatment, characterized in that the chemical etching is carried out at a temperature of from 800 to 1410 ° C when the silicon particles are immersed in a gas stream chemically active with respect to impurities and silicon, while etching and heat treatment are carried out simultaneously with mixing particles of powdered silicon. 2. The method according to claim 1, characterized in that the reactive gas is created by plasma-chemical discharge by high frequency currents under reduced pressure. 3. The method according to claim 1 or 2, characterized in that the chemical etching is carried out in a mixture of gases. 4. The method according to claim 1 or 2, characterized in that the chemical etching is carried out in a mixture of gases forming a glass-forming film on silicon. 5. A method of cleaning powdered silicon, including chemical etching of the silicon surface and high-temperature heat treatment, characterized in that a porous silicon layer is created on the surface of the silicon powder particles and chemical etching is carried out at a temperature of from 800 to 1410 ° C. when the silicon particles are immersed in a chemically active stream with respect to impurities and silicon gas, while chemical etching and heat treatment is carried out simultaneously with mixing particles of powdered silicon. 6. The method according to claim 5, characterized in that the reactive gas is created by plasma-chemical discharge by high frequency currents under reduced pressure. 7. The method according to claim 5 or 6, characterized in that the chemical etching is carried out in a mixture of gases. 8. The method according to claim 5 or 6, characterized in that the chemical etching is carried out in a mixture of gases forming a glass-forming film on silicon.