HK1166773A - Process and equipment for synthetizing silicon tetrafluoride - Google Patents

Abstract

Solid materials capable of producing toxic and/or corrosive gases by thermal decomposition are heated in a stirred in a sealable crucible. The stirring rod is supported on a downward extending shaft using a combination of a lip seal or other mechanical seal and a ferro-fluidic seal or rotary feed through. The lip seal region is evacuated to reduce the chance that the small upward flow of corrosive gas will detrimentally react with components of the ferro-fluid. In a process for calcining sodium fluorosilicate to product silicon tetra-fluoride gas, the lip seal and ferro-fluidic seal regions are purged and/or blanked to prevent the absorption of water during an initial drying phase. Accordingly, the reaction of water with silicon tetra-fluoride to produce corrosive hydrogen fluoride gas is prevented.

Description

Process and equipment for synthesizing silicon tetrafluoride

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent application, filed on 7/23/2010 with the same title, having application number 61/367,320 and incorporated herein by reference.

Technical Field

The field of the invention is apparatuses and methods related to sealing stirring shafts, and in particular, apparatuses and methods related to sealing stirring shafts in chambers for calcining solids to produce gases that may be corrosive and highly reactive while avoiding contamination.

Background

Many chemical processes for producing high purity materials, and in particular, non-contaminating electronic grade materials such as semiconductors, utilize highly reactive gases. One method of producing such high purity gases is to calcine the solid precursor, with the contaminants being eliminated by leaving the solid as in the precursor or by phase segregation in the synthesis of the precursor.

The gases used to synthesize such materials are typically highly reactive and, therefore, unless special precautions are taken to seal the contracting materials of the equipment used to house the synthesis process, the gases may attack or corrode the original hardware and equipment used in production.

A particularly challenging problem may relate to rotary seals, in particular stirring shafts. This is particularly problematic in calcination processes where the heat transfer from the vessel wall to the interior of the solid would be slow without agitation, which also enables the gas produced by the thermal decomposition process to be released quickly.

One non-limiting example of such a process is to perform thermal decomposition of Sodium Fluorosilicate (SFS) to produce silicon tetrafluoride (SiF)₄)，SiF₄Among other uses, may be reacted with liquid sodium metal to produce silicon metal. Since sodium must be highly pure for use as a semiconductor in electronic and photovoltaic applications, it is of paramount importance that SiF is a crystalline form₄Not only pure, but also free from contamination by reaction with process equipment. SiF₄Inherently toxic and highly corrosive. In addition, SiF₄Readily react with water to form more corrosive hydrofluoric acid.

Calcination of SFS is particularly problematic because the SFS must first be dried at about 400 ℃ to remove up to about 0.5% of the absorbed water. Water must be removed from any portion of the apparatus that may be subsequently exposed to even a small amount of SiF4 gas (but preferably is prevented from entering that portion) to prevent the formation of hydrofluoric acid (HF).

Disclosure of Invention

It is an object of the present invention to provide a method and apparatus for calcining solid materials at high temperatures by stirring which neither contaminates the gases produced nor allows them to escape from the chamber.

In the present invention, the first object is achieved by providing an apparatus comprising: a sealable chamber; a rotatable shaft extending downwardly from an upper portion of the chamber; an agitator paddle disposed at an end of the shaft distal from the upper portion of the chamber, the agitator paddle generally conforming to at least a curvature of a bottom of the chamber; an upper ferrofluid seal connecting an upper end of the rotatable shaft to a drive shaft external to the chamber; a lower dual lip seal disposed between the upper fluid seal and an interior of the chamber surrounding the rotatable shaft; a first inlet in fluid communication with a first region surrounding the rotatable shaft disposed between the upper ferrofluid seal and a lower lip seal for selectively evacuating and covering the first region; a second inlet in fluid communication with a second region surrounding the rotatable shaft disposed between dual lip seals for selectively evacuating and covering the second region.

A second aspect of the invention is characterized by a process for the synthesis of silicon tetrafluoride, the process comprising the steps of: providing a heatable chamber having a sealable stir bar; filling the chamber with solid Sodium Fluorosilicate (SFS); stirring solid sodium fluosilicate; heating the SFS to at least 400 ℃; removing water from the chamber; heating the SFS to at least 700 ℃; mixing SiF₄Removed from the chamber, wherein the sealable stir bar is isolated from the exterior of the chamber by a ferrofluid seal, and the interior of the chamber is isolated from the ferrofluid seal by a lip seal.

A process for synthesizing silicon tetrafluoride as described above, wherein the process further comprises the step of blanketing the ferrofluid seals with a dry inert gas during the step of removing water from the chamber.

The process for synthesizing silicon tetrafluoride as described above, wherein the process further comprises the step of evacuating the ferrofluid seal areas during the step of removing the SiF4 from the chamber.

Compared with the prior art, the invention has the following beneficial effects:

the invention heats solid materials capable of generating toxic and/or corrosive gases by thermal decomposition in a stirred manner in a sealable crucible. A combination of a lip seal or other mechanical seal and a ferromagnetic seal or rotary feedthrough is used to support the stir bar on the downwardly extending shaft. The lip seal area is evacuated to reduce the chance that a small upward flow of corrosive gas will adversely react with the components of the ferrofluid. In a process for calcining sodium fluorosilicate to produce silicon tetrafluoride gas, the lip seal and ferrofluid seal zones are purged and/or emptied to prevent absorption of water during an initial drying stage. Thus, the reaction of water with silicon tetrafluoride to produce corrosive hydrogen fluoride gas is prevented.

The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.

Drawings

FIG. 1 is a cross-sectional elevation view of a calcination apparatus and a calcination chamber.

FIG. 2 is a cross-sectional elevation view of the stir bar seal area of the calciner of FIG. 1.

Fig. 3 is a top plan view of the calcination chamber of fig. 1 and 2.

Detailed Description

Referring to fig. 1 through 3, wherein like reference numerals refer to like components in the various figures, there is illustrated a novel and improved calcination chamber and process, generally referred to herein as 100.

In accordance with the present invention, the calcining apparatus 100 comprises a heatable calcining chamber 110, the heatable calcining chamber 110 having an interior region 101 capable of having the contents mixed therein with a rotatable stirring blade 120 in close proximity to the bottom 111 of the heatable calcining chamber 110. Rotatable stirring paddle 120 is disposed at the distal end of stirring shaft 130, stirring shaft 130 extending downward from the top 112 of heatable calcining chamber 110, entering at inlet 115. Between the inlet 115 and the opening into the wider heatable calcining chamber 110 is a generally cylindrical channel enclosure 116. Within the cylindrical passage housing 116 is a lower shaft lip seal 140 that surrounds the shaft 130. Above this lower lip seal 140 is a ferrofluid seal 150 so that the shaft may extend through the inlet 115 for rotation by a motor 170.

Thus, there is an annular cavity 143 around the lip seal 140 and another annular cavity 153 around the ferrofluid seal 150, each cavity having a generally cylindrical inner surface of the outer shell 116. The drive shaft of the ferrofluid seal is connected to a motor 170 which drives the shaft and agitator. The annular space 143 around the lip seal 140 is preferably flushed with an inert gas via an external inlet 245 formed in the housing, or the annular space 143 is evacuated. Also, it is preferred to flush the annular space 153 around ferrofluid seal 150 with an inert gas via an external inlet 246 formed in the housing, or to evacuate the annular space 153.

More preferably, the lip seal 140 has two circular sealing gaskets (141a and 141b) disposed one above the other to form an inner annular region 243, the inner annular region 243 optionally having its own inlet 245 for evacuation or flushing with an inert gas. The circular sealing washers 141a and 141b are preferably made of inert fluorocarbon resin filled with carbon or graphite fibers to increase strength and rigidity. Other mechanical sealing devices, such as face seals, may also be used in place of lip seals for various applications.

The cylindrical housing 116 is preferably surrounded by a sealable annular space through which cooling water flows when the chamber 110 is heated to prevent overheating of the valve and sealing components. This and other cooling means discussed below allow the chamber to be operated at high temperatures without damaging external mechanical and moving components and their associated feedthroughs.

Fig. 3 illustrates the location of a number of access ports 104 on the top half or top 112 of the chamber 110. The support for the motor 170 and the rotary coupling shaft 130 is preferably completely external, wherein the stirring blades have no internal contact with the shaft inside the chamber 110 to prevent contamination. In addition, the stirring vanes 120 and the shaft 130 are preferably Inconel 625 metal plated or coated with pure nickel 200.Chamber 110 is preferably itself explosion clad nickel (explosion clad) 200 on inconel 625 alloy. Due to the material pairs SiF₄The high temperature of the gas is compatible and the material is selected specifically, although other materials may be selected in other applications.

In the preferred embodiment of the present invention, the stirring blade 120 is preferably helically spiraled with an inclined leading edge. Another important aspect of the present invention is the provision of a cooling channel 131 in the stirring axle 130, said cooling channel 131 receiving a cooling fluid at an inlet 132 (which is subsequently discharged from the channel 131).

Most preferably, chamber 110 contains a sealable cylindrical extension or exhaust chamber 180 extending downward from its center, said exhaust chamber 180 terminating in an exhaust port 106 having an air-tight and vacuum-tight valve 185. The discharge chamber may be terminated with a plurality of airtight valves to provide a load lock chamber for removing residual solids from the calcination stage without allowing external air to enter the chamber 110.

In addition, it is also preferable that the heater 105 is disposed to surround the discharge chamber 180. The heater 105 is preferably an infrared heater that does not contact the outside of the chamber 110. Surrounding the infrared heater is a cooling jacket 190, which cooling jacket 190 receives a cooling fluid at an inlet 192, which cooling fluid is then discharged from jacket 190 at an outlet 193. Another cooling jacket is an annulus 181 surrounding the discharge chamber 180. An annular cooling collar 186 is also disposed about the exhaust valve 185.

Another aspect of the present invention is a method for synthesizing SiF from SFS using the above-described apparatus₄The process of (1). In the first stage, the chamber 110 is filled with SFS and the chamber 110 is sealed to remove absorbed water before heating the contents to at least above about 100 ℃ (but more preferably up to about 400 ℃). Before initiating this dehydration stage, the annular region 153 surrounding ferrofluid seal 150 is flushed with a dry inert carrier gas (preferably dry argon) to prevent ingress of moisture. The lower annular zone 243 is vented to remove water vapor generated by the dehydration of the SFS, or at a pressure lower than zone 153 but higher than chamber 110The lower annular zone 243 is also flushed with dry inert gas at pressure. The interior 101 of the chamber 110 is preferably also flushed with dry inert gas (argon) during the dehydration process, or the interior 101 may be evacuated during the dehydration of the SFS. Accordingly, the inert gas in the region of the lip seal 140 will be at a positive pressure relative to this region, thereby preventing moisture ingress. Dewatering preferably occurs with continued rotation of the shaft 130 and stir bar 120 to accelerate heating of the SFS charge to even out the temperature and ensure complete dewatering. The chamber interior 101 is flushed with dry argon during dehydration while the vacuum pump removes carrier gas and moisture.

In a subsequent process step of heating the SFS to a decomposition temperature of at least 500 ℃ (but more preferably about 700 ℃ to 800 ℃), the SiF is evacuated₄Is the chamber inlet 104. However, both the lower annular region 243 and the upper annular region 153 are also pumped differently to remove any SiF leaking past the lip seal₄. The chamber 110 (shown in FIG. 3) may have multiple top inlets 104 for loading the reactant SFS, and for extracting moisture during dehydration, and for removing SiF during calcination₄。

Alternatively, during the calcination process described above, the upper annular region 153 may be flushed with an inert gas and the lower annular region 243 may be evacuated so that this carrier gas quickly dilutes any SiF leaking past the lip seal₄And in SiF₄It may be removed prior to interaction with the ferrofluid material. The evacuation also prevents any inert carrier gas from leaking past the lower lip seal into the chamber interior 101, which would dilute the product SiF being produced therein in the chamber interior 101₄. Thus, after completion of the dehydration of the SFS charge, the source of inert purge gas is turned off, and the pump or line that removes this inert gas and moisture is shut off or shut down. Subsequently, the heater 105 is energized while the attached rod 130 rotates the paddles 120, causing the dry SFS charge to mix as it reaches the decomposition temperature. Removal of product SiF by a separate vacuum suction system₄The vacuum pumping system provides preference in chamber 110An internal pressure of between about 20 torr and 50 torr.

In a preferred dehydration mode of the SFS, the upper chamber is flushed with dry argon gas while pumping at a sufficient rate to provide a partial pressure of about 850 torr, the lower zone is also flushed with dry argon gas to provide a partial pressure above 800 torr, and the chamber interior 101 is also flushed with dry argon gas to provide a pressure of about 750 torr. Flushing with dry argon at this stage also prevents any fine particles from accumulating at the lip seal 140.

However, during calcination, the upper annular chamber 153 and the lower annular chamber 243 may be sealed or evacuated. If evacuated, it is preferred to pump the lower annular chamber 243 at a rate such that the partial pressure is about 5 torr, while the upper annular chamber 153 reaches a higher partial pressure of about 20 torr, and the partial pressure of the interior 101 of the chamber 110 is about 20 torr to 200 torr (but more preferably 20 torr to 50 torr). Under the latter lower pressure conditions in the chamber 110, we have found that if the mixing from the stirring blade 120 is at a sufficiently high speed, agglomeration of the SFS powder during calcination is substantially minimized, if not avoided. It has further been found that avoiding such agglomeration clearly provides more efficient mixing during calcination, as it results in a significant increase in yield and allows the decomposition reaction to proceed to completion, thereby improving process yield.

It should be noted that without stirring the reactant SFS, the charge in the chamber 110 will become a solid mass upon heating and the remaining sodium fluoride will sinter together.

Thus, it should now be understood that the use or deployment of the above-described non-leaking calciner with agitation results in several reciprocity, including higher throughput and efficiency of the decomposition reaction, as well as avoidance of contamination from the agitator blades, and higher safety due to the high reliability of the rotating shaft sealing mechanism.

Although the present invention has been described in connection with the preferred embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.

Claims (4)

1. A process for the synthesis of silicon tetrafluoride, the process comprising the steps of:

a) providing a heatable chamber having a sealable stir bar,

b) the chamber was filled with solid sodium fluorosilicate,

c) stirring the solid sodium fluosilicate to obtain a mixture,

d) heating the SFS to at least above about 100 c,

e) the water is removed from the chamber and,

f) heating the SFS to at least about 500 c,

g) mixing SiF₄Is removed from the chamber in a manner such that,

h) wherein the sealable stir bar is isolated from the exterior of the chamber by a ferrofluid seal and the interior of the chamber is isolated from the ferrofluid seal by a lip seal.

2. The process for the synthesis of silicon tetrafluoride according to claim 1, further comprising the step of blanketing the ferrofluid seals with a dry inert gas during the step of removing water from the chamber.

3. The process for the synthesis of silicon tetrafluoride according to claim 1 or 2, wherein the process further comprises subjecting the SiF to₄A step of evacuating the ferrofluid seal region during the step of removing from the chamber.

4. An apparatus, characterized in that the apparatus comprises:

a) the chamber may be sealed and the lid closed,

b) a rotatable shaft extending downwardly from an upper portion of the chamber,

c) an agitating paddle disposed at an end of the shaft distal from the upper portion of the chamber, the agitating paddle substantially conforming to at least a curvature of a bottom of the chamber,

d) an upper ferrofluid seal connecting an upper end of the rotatable shaft to a drive shaft external to the chamber,

e) a lower dual lip seal disposed between the upper fluid seal and an interior of the chamber surrounding the rotatable shaft,

f) a first inlet in fluid communication with a first region surrounding the rotatable shaft disposed between the upper ferrofluid seal and lower lip seal for selectively evacuating and covering the first region,

g) a second inlet in fluid communication with a second region surrounding the rotatable shaft disposed between dual lip seals for selectively evacuating and covering the second region.