RU2348581C2 - Method of silicon tetrafluoride extraction from gas mix and aggregate for implementation of method - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: silicon tetrafluoride is extracted from oxygen-containing gas mix obtained by quartz sand burning in elementary fluorine. The mix is cooled in two sequential heat exchangers to the temperature above triple point of silicon tetrafluoride, with preliminary compression to the pressure correlating to fluid state of silicon tetrafluoride at selected cooling temperature. To ensure process safety, all equipment is placed in sealed cases with automatic operation system.

EFFECT: obtaining liquid high-grade silicon tetrafluoride, gradual and complete filling of storage and usage containers by silicon tetrafluoride.

12 cl, 3 dwg, 2 ex

Description

FIELD OF THE INVENTION

The invention relates to the field of inorganic chemistry, and more particularly to means for producing high-purity silicon tetrafluoride by isolating it from a gas mixture.

State of the art

The most common method in the modern industry for the separation of silicon tetrafluoride (SiF ₄ ) from fluorine-containing gas mixtures is the absorption method. For example, there is a known method of separating SiF ₄ from a gas mixture (see patent No.2019504, IPC: СВВ 33/10, publ. 1994.09.15), in which gaseous SiF4 is captured at a temperature of 300 ° С on granules of sodium fluoride. A well-known technological scheme for the purification of fluorine-containing gases in the production of superphosphates (see Kuznetsov I.E. et al. Equipment for sanitary cleaning of gases. Kiev: Technique, 1989, pp. 94-95), in which the process of absorption of silicon tetrafluoride by aqueous solutions is carried out at a temperature of 30-40 ° C. Both methods make it possible to isolate SiF ₄ from gas mixtures; however, it turns out to be bound by another substance.

In connection with the development of science and production, there has recently been an increasing need to obtain high-purity silicon tetrafluoride in order to use it as a raw material for producing “solar” quality silicon.

Obtaining a pure liquid silicon tetrafluoride as a commercial product is possible by the method described in the materials of the patent for invention No. 2046095, IPC СВВ 33/10, publ. 1995.10.20, namely, by separation of the previously obtained anhydrous mixture of hydrogen fluoride (HF) gases and silicon tetrafluoride. According to this method, first, HF is isolated from the gas mixture by low-temperature condensation at t = -78 ° C. Then silicon tetrafluoride is condensed in a special container cooled with liquid nitrogen. The mentioned method is the closest in its purpose to the claimed and therefore selected for the prototype.

However, the method carries out the separation of a simple gas mixture comprising only two components, the remaining impurities were removed at the stages preceding the receipt of the said mixture. In addition, liquid nitrogen used for cooling has a temperature of about -196 ° C. At this temperature and atmospheric pressure, SiF ₄ passes from the gas phase directly to the solid, i.e. crystallizes (see materials on the dependence of silicon tetrafluoride on temperature and pressure: O. Ruff et al., Inorganic Chemistry journal (Z.anorg.Ch.) No. 196, 1931, pp. 413-414). When the cooling of the tank is removed, the temperature of the latter rises and SiF ₄ passes into a liquid, the volume of which is much smaller than the volume occupied by crystalline silicon tetrafluoride. This implies the incomplete use of the useful volume of container containers for liquid silicon tetrafluoride. At the same time, work in the field of low temperatures (with liquid nitrogen) has its own difficulties.

A known installation that allows you to select silicon tetrafluoride from the composition of gas mixtures obtained in the production of superphosphates (see Kuznetsov I.E. and other equipment for sanitary cleaning of gases. Kiev: Technique, 1989, pp. 94-95), including horizontal and vertical absorbers. However, it is impossible to obtain pure silicon tetrafluoride in this facility, since in any case, it is bound by another substance.

Closest to the claimed invention is a device designed to separate a mixture of anhydrous gaseous hydrogen fluoride and silicon tetrafluoride (see the description of the patent for the invention No. 2046095, IPC С01В 33/10, publ. 1995.10.20), including a capacitor and a supply pipe connected to it SiF ₄ gas is a special container cooled by liquid nitrogen. The device allows to obtain pure SiF ₄ , however, it has the following significant drawback - the need to use containers with special cooling with liquid nitrogen. Storage and transportation of SiF ₄ in these containers is difficult, and when cooling is removed, SiF ₄ transfers from a crystalline state to a liquid whose volume is much smaller than the volume of solid SiF ₄ , as a result of which the container will be filled only by one third of its volume. In addition, the need to work with liquid nitrogen, the use of cryogenic technology has its own difficulties.

Disclosure of invention

The objective of the invention is to expand the application of the method, more specifically for oxygen-containing mixtures, increase its efficiency and economy, in particular, by eliminating the need to work in the field of low temperatures and the possibility of efficient use of the useful volume of containers for the isolated silicon tetrafluoride.

The problem is solved due to the fact that in the method of separation of silicon tetrafluoride from a gas mixture by condensation, according to the claimed invention, an oxygen-containing mixture obtained by burning quartz sand in elemental fluorine is used as a source gas mixture, which is compressed and then cooled to a temperature not lower than the temperature of the triple point of silicon tetrafluoride, and compress the mixture to a pressure corresponding to the liquid state of silicon tetrafluoride at a selected cooling temperature, while m equipment for the implementation of compression, cooling and collection processes are placed in airtight boxes.

The problem is also solved due to the fact that the installation for the separation of silicon tetrafluoride from the gas mixture, containing a heat exchanger and a related container for silicon tetrafluoride, according to the invention, is equipped with a membrane compressor with a profiled lubricating fluid placed at the inlet of the heat exchanger and the associated gas pipeline high pressure, and a second heat exchanger with a capacity for silicon tetrafluoride, the inlet of which is connected by a high pressure gas pipeline to the output of the first heat exchanger, and in The outlet is communicated through a gas reducer with means for collecting and discharging non-condensed gaseous products, each container for silicon tetrafluoride is made in the form of at least one cylinder for liquefied gas, connected to the corresponding heat exchanger via a valve connection and a pipeline for liquid silicon tetrafluoride, while equipment is placed in airtight boxes.

The above set of distinctive features allows us to obtain a number of positive technical results: to obtain silicon tetrafluoride in liquid form, due to which to ensure full use of transport capacities, eliminate the need for liquid nitrogen and the need to work in the low temperature region, and finally, combine the process of separation of silicon tetrafluoride with its purification from related impurities, and the degree of purification of silicon tetrafluoride as a result of such selection is very high, which It turns by the following examples.

One of the significant distinguishing features of the proposed method, which allows achieving the above positive results, is the use of natural quartz sand combustion products in elemental fluorine as the initial gas mixture containing silicon tetrafluoride. The gaseous mixture of the products of the mentioned combustion includes SiF ₄ (content ≈40%) and oxygen (О ₂ ≈40%), which makes up the bulk of the mixture, as well as carbon dioxide, fluorine, nitrogen, boron trifluoride, phosphorus pentafluoride, and other volatile fluorides formed as a result of burning of impurities contained in quartz sand. Silicon tetrafluoride (SiF ₄ ) and oxygen have a significant difference in condensation temperatures, which allows the condensation method to be used for their separation. The remaining volatile impurity fluorides, although they have condensation temperatures close to the condensation temperature of silicon tetrafluoride, however, their content is low, and due to the known property of vapor pressure, they will not have time to condense simultaneously with it.

As an oxygen-containing gas mixture, a mixture obtained by burning quartzite in a mixture of elemental fluorine with oxygen can also be used. The process of its separation is similar to the above.

Thus, in contrast to the known solutions, for example, such as patent No. 2035397, where the purity of SiF _{4 is} achieved due to the multi-stage washing and temperature treatment of the raw material for producing silicon tetrafluoride - silica sand, the use of catalysts and other reagents and additional purification of SiF ₄ from the remaining impurities by means of sedimentation tanks or activated carbon, as well as from the prototype, where silicon tetrafluoride is isolated from a simple mixture, the preparation of which was preceded by a multi-stage purification of raw materials aschey only two components, the claimed method avoids multi-step pretreatment for the starting raw material of silicon tetrafluoride, SiF _4, and cleaning of the gaseous impurities combined with the process of its isolation.

As is known, the triple point of silicon tetrafluoride corresponds to: t = -90.2 ° C and P = 1318 mm Hg. (reference Gmelins HandBuch der anorganischen Chemie 8 Auflase Silicium Teil 8 1959 Veriag Chemie, GMBH, Weinheim / Bergstrasse) [1].

The transition of silicon tetrafluoride from a gaseous state directly to a liquid can be carried out only at elevated pressure. Under normal atmospheric pressure, silicon tetrafluoride does not have a liquid state and, upon cooling more than -140 degrees, passes from a gaseous state directly to a solid one. The need to carry out the process of separation of SiF ₄ from a gas mixture at elevated pressure leads to an increased production hazard. Placing the equipment and performing operations of compression, cooling and collection of liquid SiF ₄ in sealed boxes allows solving the aforementioned problem and not only ensuring production safety, but also providing an additional technical result, namely, the safety of such a valuable product as high-purity silicon tetrafluoride.

The gas mixture is supplied for cooling under high pressure, and the speed of its movement through the heat exchangers is quite high. In order to ensure the most complete (i.e., high-quality, effective) separation of silicon tetrafluoride, it is advisable to carry out the cooling process of the compressed gas mixture in at least two stages. Liquefied silicon tetrafluoride is removed at each cooling stage, and non-condensed gas impurities are discharged for sanitary cleaning only with the last.

Sealed boxes are preferably made of metal. The volume of each pressurized box is selected (calculated) based on the condition that in case of depressurization of the equipment placed in it and leakage of silicon tetrafluoride or its gas mixture into the box volume, the overpressure of the above gases in the box will not exceed 76 mm Hg (0.1 atm.). The design parameters of the boxes are calculated by any known methods.

In case of depressurization of the equipment, it is possible to absorb silicon tetrafluoride from the box, for example, in a sorption column with granules of sodium fluoride.

A positive aspect of the proposed solution is the ability to use for the collection, storage and transportation of SiF ₄ conventional cylinders for liquefied gas without special cooling. The ability to simultaneously connect several cylinders to the heat exchanger ensures the continuity of the production process.

The inventive installation provides the possibility of implementing the inventive method and the possibility of obtaining the above-mentioned positive results.

The use of a membrane compressor with a fluorinated lubricating fluid in the installation, in which the pumping part with moving elements is separated from the gas flow by the membrane, is a necessary condition to eliminate contamination of the compressed gas mixture and to ensure high purity of the separation process, and hence high purity of the obtained silicon tetrafluoride.

To cool the gas mixture, it is preferable to use recuperative heat exchangers (recuperators), in which heat is transferred through the dividing wall. In the specific case of the installation, tubular condensers with a single water cooling system were used as the latter.

To cool the gas mixture, such well-known liquid refrigerants as saline solutions (brines) can be used, however, it is preferable to use industrial water with a temperature of +4 to + 24 ° C, the regeneration process of which is simple and has long been debugged. The lower temperature limit mentioned is due to the proximity of the freezing point of water, and the upper one is due to the simplicity of technical implementation.

To calculate the compression pressure of the gas mixture using the well-known logarithmic dependence p (t):

where P is the pressure in mmHg and T is the temperature of the gas mixture in Kelvin.

Ideally, the temperature of the chilled gas mixture is equal to the temperature of the selected refrigerant, but in real conditions the temperature is affected by: the volume of the mixture and the cooling time associated with the speed of flow through the heat exchanger, which is taken into account in the calculations.

Possibility of using water having a temperature above 0 ° С for cooling a gas mixture, i.e. a temperature higher than the critical temperature of SiF ₄ known from the literature, equal to -14.15 ° C at p _k = 36.66 Atm = 27862 mm Hg, was confirmed by experimental studies by the applicant, as a result of which it was found that when SiF was compressed ₄ above critical parameters, it does not transform into a gaseous state (as expected), but into a liquid state. This anomalous phenomenon, inherent only to fluorides, probably occurs due to the high polarization of bonds in the SiF ₄ molecule and due to the formation of associated molecules, the so-called silicon bonds, identical to the formation of hydrogen bonds between hydrogen fluoride molecules.

Figure 1 shows graphs of critical states of silicon tetrafluoride, delimiting, respectively: the upper is the gaseous and liquid state of silicon tetrafluoride, and the lower is the liquid and crystalline state. The graphs were constructed on the basis of extrapolation over a wider temperature range of the aforementioned logarithmic dependence (1), known from the literature (see Gmelins HandBuch der anorganischen Chemie 8 Auflase Silicium Teil 8 1959 Veriag Chemie, GMBH, Weinheim / Bergstrasse), as well as based on experimental studies.

Non-condensed gas impurities are decompressed through a gas reducer and sent for sanitary cleaning.

Sanitary cleaning of non-condensed gas impurities can be carried out in a column with a solid sorbent absorbing acid gases or in a scrubber with a liquid absorption solution.

In order to increase production safety and eliminate the human factor, the installation is fully automated.

Cylinders for liquefied silicon tetrafluoride are universal removable containers for its storage and transportation. Moreover, the inventive installation provides for the possibility of connecting the above cylinders through a gas reducer to the silicon tetrafluoride removal line with the aim of direct supply to the stage of further use.

Brief Description of the Drawings

The method and operation of the installation are illustrated by drawings, where Fig. 1 shows graphs of critical states of silicon tetrafluoride, shows the gaseous, liquid and crystalline states of SiF ₄ ;

figure 2 shows a diagram of an installation for the separation of silicon tetrafluoride;

figure 3 is a table showing the content of components in the source gas mixture and the content of impurities in the silicon tetrafluoride extracted from the gas mixture.

The implementation of the invention

The proposed method was tested in laboratory conditions on the inventive installation. Installation for the separation of silicon tetrafluoride from the gas mixture contains (see figure 2) a membrane compressor 1 with a profiled lubricating fluid, heat exchangers in the form of tubular condensers 2 and 3, tanks 4 and 5 for collecting liquid SiF ₄ , a gas reducer 6 and a tank 7 for collection of non-condensed gaseous products. Tanks 4 and 5 are made in the form of cylinders for liquefied gas. In a specific implementation case, the container 4 is made in the form of two cylinders 4 'and 4 "connected to the condenser 2 by pipelines 8' and 8" for liquid silicon tetrafluoride by means of valve connections 9 'and 9 ", respectively.

Capacity 5 is implemented in the form of a single cylinder connected to the condenser 3 by a pipe 10 for liquid SiF ₄ by means of a valve connection 11. In specific cases, the installation is the number of cylinders. connected to capacitors, may vary depending on production needs.

The compressor 1 is placed in a sealed metal box 12 and is connected by a high pressure gas line 13 to a condenser 2. The first cooling stage equipment, including a condenser 2 with 4 'and 4 "cylinders, is placed in a sealed metal box 14, while the condenser 2 is connected by a gas pipe 15 to the condenser 3. The equipment of the second cooling stage, including a condenser 3 and a container 5, is placed in a sealed metal box 16, while the condenser 3 is connected by a gas pipe 17 to a gas reducer 6 connected to a tank 7 for collecting ondensirovannyh gaseous products.

All boxes 12, 14, 16 are made of metal. The volume of each box is calculated by a known method, based on the thickness of the steel used and the conditions for maintaining tightness in case of excessive pressure inside the box = 76 mm Hg

Tubular condensers 2 and 3 are connected by a single water cooling system, including a collection tank 18 of cooling water, connected by a pipe to pump H, a pipe 19 for supplying water from pump N to capacitors 2 and 3, regulating valves of the supply pipes, pipe 20 for draining water from the condensers to the system cooling for regeneration. The technology of water regeneration is simple and developed and can be implemented in any known manner.

The pressure and flow rate of the circulating refrigerant and the shared gas mixture are controlled by pressure sensors (P _in ) and water flow (G _in ) located on the pipeline 19, and pressure sensors (P _cm ) and gas flow rate (G _cm ) installed on the pipeline 9. The installation is controlled remotely.

Example 1

Carried out the separation of silicon tetrafluoride from a gas mixture obtained by burning quartz sand in elemental fluorine, having the following composition:

SiF ₄ - the basis;

O ₂ - 40.0 wt.%;

N ₂ - 2.0 wt.%;

CO ₂ - 3.0 wt.%;

F ₂ - 0.05 wt.%;

BF ₃ - 0.1 ppm;

PF ₅ - 0.1 ppm vol., Where ppm is the number of atoms per million silicon atoms.

Industrial water with a temperature of + 4 + 5 ° C was used as a refrigerant.

Compression pressure was calculated by the formula (I):

If t = + 4 ° C, then T = t + 273 = 4 + 273 = 277.

Log ₇ P = 10.469-13 52.8 / 277 = 5.585, P = 7 ^5.585 = 52465.4 mm Hg

As can be seen in figure 1 (point S1), the calculated pressure corresponds to the region of the liquid state of SiF ₄ . Due to the fact that they carry out the separation of a multicomponent mixture, the pressure to which the mixture is compressed must exceed the extreme (critical) value of pressure at the indicated cooling temperature, corresponding to the upper curve of the graph. This is necessary for the complete separation of silicon tetrafluoride from the mixture.

The original gas mixture of the specified composition is compressed in the compressor 1 to a pressure. 52465 mm Hg, which is detected by P _cm pressure sensors of the gas mixture.

Compressed mixture through the gas pipeline 9 is fed into the condenser 2 of the first cooling stage. When cooling, silicon tetrafluoride passes from a gaseous state to a liquid state - it condenses on the surface of the pipes and flows through them to the lower part of the condenser 2, from where it is discharged through a pipe 8 '(8 ") to a collection tank 4' (4").

Due to the fact that the gas mixture is passed through the heat exchanger under high pressure, all silicon tetrafluoride contained in the mixture does not have time to condense. Silicon tetrafluoride and gas impurities that are not condensed at the first stage are fed through a gas pipeline 15 to the condenser 3 of the second stage, where the silicon tetrafluoride remaining in the gas mixture, being in contact with the surface of the heat exchanger tubes, is cooled, condensed, and similarly to the first stage, it is discharged into the collecting tank 5. Oxygen has a lower the condensation temperature remains in the gaseous state, and the remaining gases: nitrogen (N ₂ ), carbon dioxide (CO ₂ ), fluorine (F ₂ ), boron trifluoride (BF ₅ ) and phosphorus pentafluoride (PF ₅ ), which have a low percentage of keeping in the initial mixture (see table, figure 3), due to vapor pressure, they do not have time to condense with SiF ₄ , although they have condensation temperatures close to it. The aforementioned gaseous products that are not condensed at the second stage are passed through a pipeline 17 through a gas reducer 6, which reduces the pressure of the gas mixture, and are collected in a tank 7, from where they are discharged for sanitary cleaning or for other use.

SiF ₄ isolated from the initial gas mixture was analyzed for impurities. It was found (see figure 3) that the silicon tetrafluoride obtained by the claimed method on the inventive installation contains the following impurities:

O - 6.6 · 10 ³ ppm;

N - 1.0 · 10 ³ ppm;

F - 1.58 ppm;

B - 0.002 ppm;

P is 0.003 ppm, where ppm is the number of atoms per million silicon atoms.

The above analysis data confirm that silicon tetrafluoride obtained by the proposed method and device is a highly pure substance and can be used as a starting material for the production of solar-grade polycrystalline silicon.

The isolated liquid silicon tetrafluoride can be stored and transported in cylinders 4 ', 4 ", 5, disconnected by valves 9', 9" and 11 from the pipelines 8 '8 "and 10 for supplying liquid silicon tetrafluoride, respectively. However, the inventive installation provides the possibility of stationary use of cylinders and the possibility of including it in the production line, for example, to obtain polycrystalline silicon. In this case, the cylinders 4 ', 4 "and 5 are equipped with reducers P, serving as exhaust elements for the evaporation of silicon tetrafluoride, and connected es said supplying reducers to the backbone silicon tetrafluoride in operation further use.

Example 2

Silicon tetrafluoride was extracted from a gas mixture obtained by burning quartz sand in a mixture of oxygen and elemental fluorine having the following composition: SiF4 —48%, oxygen —47%, nitrogen, argon, carbon dioxide, boron trifluoride, and phosphorus pentafluoride — the rest .

Industrial water with a temperature of t = + 24 ° C was used for cooling, the compression pressure was calculated as follows:

T = t + 273 = 24 + 273 = 297.

Log ₇ P = 10.469-1352.8 / 297 = 5.914, then P = 7 ^5.914 = 99519.9 mm Hg

In figure 1, there is no point S2 corresponding to the state calculated above, however, if the graphs are extended, it can be seen that the calculated value P = 99519.9 mm Hg. also correspond to the region in which silicon tetrafluoride is in the form of a liquid.

The selection of silicon tetrafluoride is carried out on the installation of figure 2, while the selection process occurs analogously to example 1.

As can be seen from example 2, the higher the cooling temperature of the gas mixture, the higher the compression pressure of the gas mixture, and the pressure increases much more than the temperature.

It should be noted that formula (1) is valid for a wider temperature range than the interval (+ 4 + 24 ° C), namely from t = -20 ° and above + 24 °, which allows it to be used for calculations when cooling the gas mixture using saline solutions (brines) having a freezing point lower than water.

Example 3. As the refrigerant used saline solution, while the mixture was cooled to t = -15 ° C. Compression pressure was calculated by the formula (1):

T = -15 + 273 = 258, Log ₇ P = 10.469-1352.8 / 258 = 5.2434

P = 7 ^5.2434 ≈27000 mmHg

The process of separation of silicon tetrafluoride, carried out in the installation of figure 2, proceeds analogously to example 1.

Claims (12)

1. The method of separation of silicon tetrafluoride from a gas mixture by condensation, characterized in that as the initial gas mixture using an oxygen-containing mixture obtained by burning quartz sand in elemental fluorine, which is compressed, and then cooled to a temperature not lower than the temperature of the triple point of tetrafluoride silicon, and compress the mixture to a pressure corresponding to the liquid state of silicon tetrafluoride at a selected cooling temperature, while the equipment for the implementation of compression processes, cooling Denia and collecting placed in sealed boxes. 2. The method according to claim 1, characterized in that the volume of each pressurized box is selected based on the condition that in case of depressurization of the equipment located in it, the excess pressure of silicon tetrafluoride or its gas mixture in the box will not exceed 76 mm Hg 3. The method according to claim 1, characterized in that the cooling is carried out at least in two stages with the conclusion of the liquefied silicon tetrafluoride on each of them and the removal of non-condensed gas impurities only with the latter. 4. The method according to claim 1, characterized in that in the case of depressurization of the equipment, silicon tetrafluoride from the box is sent for absorption to the sorption column with granules of sodium fluoride NaF. 5. The method according to claim 1, characterized in that the non-condensed gas impurities are passed through a gas reducer and sent for sanitary cleaning. 6. The method according to claim 5, characterized in that the sanitary cleaning is carried out in a column with a solid sorbent absorbing acid gases sorbent. 7. The method according to claim 5, characterized in that the sanitary cleaning is carried out in a scrubber with a liquid absorbing solution. 8. Installation for the separation of silicon tetrafluoride from a gas mixture, containing a heat exchanger and an associated container for silicon tetrafluoride, characterized in that it is equipped with a membrane compressor with a profiled lubricating fluid located at the inlet of the heat exchanger and the associated high pressure gas pipeline, and a second heat exchanger with a container for silicon tetrafluoride, the input of which is connected by a high pressure gas pipeline to the output of the first heat exchanger, and the output is communicated through a gas reducer with collection means and the output of non-condensed gaseous products, each container for silicon tetrafluoride is made in the form of at least one cylinder for liquefied gas, connected to the corresponding heat exchanger by means of a valve connection and a pipeline for liquid silicon tetrafluoride, while the above-mentioned equipment is placed in sealed boxes. 9. The installation according to claim 8, characterized in that the sealed boxes are made of metal and ensure the preservation of tightness at an overpressure in the box up to 76 mm Hg 10. Installation according to claim 8, characterized in that the heat exchangers are made in the form of tubular condensers with a single cooling system. 11. Installation according to claim 8, characterized in that each box is equipped with an automated service system. 12. The apparatus of claim 8, wherein the cylinders for liquefied silicon tetrafluoride are configured to be connected through a gas reducer to a silicon tetrafluoride discharge line.

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