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. A preferred embodiment of the process of synthesis of a high purity corrosive gas generated by decomposition of a precursor solid at high temperature deploys a dry vacuum pump and a compressor in series so that the corrosive gas is pressurized as it fills storage containers. Accordingly, the reaction of water with silicon tetra-fluoride to produce corrosive hydrogen fluoride gas is prevented.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of priority to the International (PCT) Patent application PCT/US11/45351 filed on 26 Jul. 2011 for a “Calcining Chamber and Process”, which is incorporated herein by reference which in turn claims the benefit of priority from the US Non-Provisional Patent Application for the “Contamination Free Compression of a Corrosive Gas” that was filed on Jul. 21, 2011, having application Ser. No. 13/188,353, and is incorporated herein by reference, and also claims the benefit of priority from the US Provisional Patent Application for the “Contamination Free Compression of a Corrosive Gas” that was filed on Jul. 26, 2010, having application Ser. No. 61/367,627, and is also incorporated herein by reference. 
         [0002]    The present application also claims the benefit of priority to the International (PCT) application PCT/US11/43723 filed on 12 Jul. 2011 for a “Calcining Chamber and Process”, which is incorporated herein by reference, which in turn claims the benefit of priority form the US Provisional Patent Application for the “Calcining Chamber and Process” that was filed on Jul. 23, 2010, having application Ser. No. 61/367,320, which is also incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0003]    The present field of invention is apparatus and method related to the production, compression, and storage of corrosive gases, and in particular to the production of silicon tetrafluoride (SiF 4 ) by calcining sodium fluorosilicate (SFS). 
       BACKGROUND OF INVENTION 
       [0004]    Numerous chemical processes to produce high purity materials, and in particular contaminant free electronic grade materials such as semiconductors, utilize highly reactive gas. One method of producing such high purity gases is by the calcining of a solid precursor in which the contaminants are rejected by either remaining as solids in the precursor or by phase segregation in the synthesis of the precursor. 
         [0005]    Gases used to synthesize such materials are generally highly reactive, and hence can attack or corrode congenital hardware and equipment used in there production unless special precautions are taken in sealing the materials of contraction of the equipment used to contain the synthetic process. 
         [0006]    A particularly challenging problem can involve rotary seals, in particular stirring shafts. This is particularly an issue in a calcining process in which heat transfer from the walls of the vessel to the interior of the solid would be slow without stirring, which also enable the rapid release of the gas produced by the thermal decomposition process. 
         [0007]    One non limiting example of such a process is thermal decomposition of sodium fluorosilicate (SFS) to produce silicon tetrafluoride (SiF 4 ) which among other uses is, can be reacted with liquid sodium metal to produce Silicon metal. As sodium must be highly pure for use as a semiconductor in electronic and photovoltaic applications, it is of paramount importance that the SiF 4  is not only pure, but does not become contaminated by reaction with the process equipment. SIF 4  itself is toxic and highly corrosive. Further, it readily reacts with water to process hydrofluoric acid, which is more corrosive. 
         [0008]    Calcining SFS is particularly problematic because it must first be dried at under about 400° C. to remove up to about 0.5% absorbed water. The water must also be removed from, but preferably prevented from entering any part of the apparatus that then is potentially exposed to even small quantities of SIF4 gas to prevent the formation of hydrofluoric acid (HF). 
         [0009]    Accordingly, it is an object of the invention to provide a method and apparatus for calcining solid materials at high temperatures with stirring that neither contaminates the gas produced nor allows it to leak from the chamber. 
       SUMMARY OF INVENTION 
       [0010]    In the present invention, the first object is achieved by providing an apparatus comprising a sealable chamber, rotatable shaft descending downward from the upper portion of said chamber, a stirring blade disposed at the end of said shaft distal from the upper portion of said chamber that substantially conforms to the curvature of at least the bottom of said chamber, an upper ferro-fluidic seal connecting the upper end of said rotatable shaft to a drive shaft external to said chamber, a lower dual lip seal disposed between the upper fluidic seal and the interior of said chamber that surrounds said rotatable shaft, a first portal in fluid communication with a first region surrounding said rotatable shaft disposed between the upper ferro-fluidic seal and lower lip seal for the selective evacuation and blanketing of said first region, a second portal in fluid communication with a second region surrounding said rotatable shaft disposed between dual lip seals for the selective evacuation and blanketing of said second region. 
         [0011]    A second aspect of the invention is characterized by a process for synthesizing silicon tetra fluoride comprising the steps of providing a heatable chamber having a sealable stirring rod, charging the chamber with solid sodium fluorosilicate (SFS), stirring the solid sodium fluorosilicate, heating the SFS to at least 400° C., removing water from the chamber, heating the SFS to at least 700° C., removing the SiF 4  from the chamber, wherein the sealable stirring rod is isolated from the outside of the chamber by a ferro-fluidic seal and the interior of the chamber is isolated from the ferro-fluidic seal by a lip seal. 
         [0012]    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. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a cross sectional elevation of the calcining apparatus and chamber. 
           [0014]      FIG. 2  is a cross sectional elevation of the stirring rod seal region of the calcining chamber of  FIG. 1   
           [0015]      FIG. 3  is a top plan view of the calcining chamber of  FIGS. 1 and 2 . 
           [0016]      FIG. 4  is a schematic diagram of another aspect of the invention. 
           [0017]      FIG. 5  is a schematic diagram of an alternative embodiment of the invention to that illustrated in  FIG. 4   
           [0018]      FIG. 6  is a schematic diagram of another alternative embodiment of the invention to that illustrated in  FIGS. 4 and 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Referring to  FIGS. 1 through 6  wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved calcining chamber and process, generally denominated  100  herein. 
         [0020]    In accordance with the present invention, calcining apparatus  100  includes a heatable calcining chamber  110  having an internal region  101  that is capable of having the contents therein mixed with rotatable stirring blade  120  situated in close proximity to the bottom  111  of heatable calcining chamber  110 . The rotatable stirring blade  120  is disposed at the distal end of the stirring shaft  130  that descend down from the top  112  of the heatable calcining chamber  110 , entering at portal  115 . Between portal  115  and the opening into the wider heatable calcining chamber  110  is a generally cylindrical channel housing  116 . Within cylindrical channel housing  116  a lower shaft lip seal  140  that surrounds the shaft  130 . Above this lower lip seal  140  is a ferro-fluidic seal  150 , so that the shaft can extend though portal  115  for rotation by motor  170 . 
         [0021]    Thus, there is an annular cavity  143  around both the lip seal  140  and another annular cavity  153  around the ferro-fluidic seal  150 , each having the inner surface of the generally cylindrical housing  116 . The drive shaft of the ferro-fluidic seal is connected to a motor  170  that the drives the shaft and stirrer. The annulus  143  about lip seal  140  is preferably flushed with an inert gas or evacuated via the external portal  245  formed in the housing. Likewise the annulus  153  about ferro-fluidic seal  150  is preferably flushed with an inert gas or evacuated via the external portal  246  formed in the housing. 
         [0022]    More preferably, the lip seal  140  has two round sealing gaskets ( 141   a  and  141   b ) disposed one above the other to form an inner annular region  243 , which optionally has it&#39;s own portal  245  for evacuation or flushing with an inert gas. The round sealing gaskets  141   a  and  141   b  are preferably made of an inert fluorocarbon resin filled with carbon or graphite fiber to add strength and stiffness. Other mechanical seal devices such as face seals could also be used in place of the lip seals for various applications. 
         [0023]    The cylindrical housing  116  is preferably surrounded by a sealable annulus through which cooling water flows when the chamber  110  is heated to prevent over heating of the valves and seal means. This, and other cooling means discussed below, allow the operation of the chamber at high temperatures without damaging the mechanical and moving components on the exterior and their related feedthroughs. 
         [0024]      FIG. 3  illustrates the position on numerous entry ports  104  on the upper half or top  112  of the chamber  110 . Support of the motor  170  and the rotary coupled shaft  130  is preferably totally external, with no internal contact of the stirring blade and shaft in the interior of chamber  110  to prevent contamination. Further, the stirring blade  120  and shaft  130  are preferably Inconel 625 metal plated or clad with pure nickel  200 . Chamber  110  is preferably itself explosion clad nickel  200  on Inconel 625 alloy. These materials are specifically chosen for their high-temperature compatibility with SiF 4  gas, however other materials could also be chosen in other applications. 
         [0025]    In a preferred embodiment of the invention, the stirring blade  120  is preferably helically spiraled with a tilted leading edge. Anther important aspect of the invention is the provision of a cooling channel  131 , in stirring shaft  130 , which receives cooling fluid at inlet  132 , which is then drained from channel  131 . 
         [0026]    Most, preferably chamber  110  includes a sealable cylindrical extension or discharge chamber  180  that extends downward from the center thereof, which terminates discharge port  106  having a gas and vacuum tight valve  185 . The discharge chamber may terminate with multiple gas tight valves to provide a load lock chamber for removing the residual solids from the calcining phase without admitting outside air into chamber  110 . 
         [0027]    In addition, it is also preferred to deploy heaters  105  surrounding the discharge chamber  180 . The heaters  105  are preferably infrared heaters that do not touch the outside of the chamber  110 . A cooling jacket  190  surrounds the infrared heaters, which receives cooling fluid at inlet  192 , which is then drained from jacket  190  at outlet  193 . Another cooling jacket is the annulus  181  that surrounds the discharge chamber  180 . There is also an annular cooling jacket  186  disposed about discharge valve  185 . 
         [0028]    Another aspect of the invention is a process for the synthesis of SiF 4  from SFS using the above apparatus. In the first phase the chamber  110  is charged with SFS and sealed prior to heating the contents to at least above about 100° C., but more preferably up to about 400° C. to remove the absorbed water. Prior to initiating this dehydration phase the annular region  153  surrounding the ferro-fluidic seal  150  is flushed with a dry inert carrier gas, preferably dry Argon gas, to preventing moisture ingress. The lower annular region  243  is evacuated to remove the water vapor produced by dehydration of SFS or alternatively also flushed with dry inert gas at a pressure below that of region  153 , but above that of the chamber  101 . The interior  101  of chamber  110  is preferably also flushed with a dry inert gas (Argon) during the dehydration process, or alternatively can be evacuated during dehydration of SFS. Thus, the inert gas in the region of lip seal  140  will be at positive press with respect this region preventing moisture ingress. The dehydration preferably occurs with continues rotation of the shaft  130  and stirring bar  120  to accelerate the heating of the SFS charge to uniform temperature and insure complete dehydration. Chamber interior  101  is flushed with dry argon during dehydration, while a vacuum pump removes the carrier gas and moisture. 
         [0029]    In the subsequent process step of heating the SFS to the decomposition temperature of at least 500° C., but more preferably circa 700 to 800° C., the primary route for evacuation of SiF 4  is a chamber portal  104 . However, both the lower  243  and upper annular region  153  are also differentially pumped to remove any SiF 4  that leaks through the lip seals. The chamber  110 , as shown in  FIG. 3 , may have multiple top portal  104  for charging reactant SFS, and pumping off moisture during dehydration, as well as removing SiF 4  during calcining. 
         [0030]    Alternatively, during the above calcining process, the upper annular region  153  can be flushed with an inert gas and the lower annular region  243  can be evacuated so that any SiF 4  that leaks past the lip seal is rapidly diluted by this carrier gas and removed before it can interact with the ferro-fluid materials. The evacuation also prevents any inert carrier gas from leaking past the lower lip seal into the chamber interior  101  where it would dilute the product SiF 4  being generated therein. Thus, after dehydration of the SFS charge is complete, the source of the inert flushing gas is closed and the pump or line removing this inert gas and moisture is shut off or closed. Then the heaters  105  are energized while blade  120  is rotated by attached rod  130  so that the dry SFS charge is mixed as it reaches the decomposition temperature. The product SiF 4  is removed by a separate vacuum pumping system that provides an internal pressure in chamber  110  of preferably between about 20-50 torr. 
         [0031]    In the preferred mode of dehydration of SFS, the upper chamber is flushed with dry argon, but pumped at a sufficient speed to provide a local pressure of about 850 torr, the lower region is also flushed with dry argon to provide a local pressure of above 800 torr, and the chamber interior  101  is also flushed with dry argon to provide a pressure of about 750 torr. The flushing with dry argon in this stage also prevents any accumulate of fine particulate at the lip seal  140 . 
         [0032]    On calcining however, the upper annular chamber  153  and lower annular chamber  243  could be sealed off or evacuated. If they are evacuated it is preferred that the lower annular chamber  243  be pumped at a speed so the local pressure is about 5 torr, while the upper annular chamber  153  reaches a higher local pressure of about 20 torr, and the interior  101  of the chamber  110  having a local pressure of about 20 to 200 torr, but more preferably 20 to 50 torr. Under the latter conditions of lower pressure in the chamber  110  it was discovered that the clumping of SFS powder during calcining was generally minimized if not avoided, provided the mixing from stirring blade  120  was at a high enough speed. It was further discovered that avoiding such clumping apparently provided more efficient mixing during calcining as it lead to a notable increases throughput and completeness of the decomposition reaction, improving the process yield. 
         [0033]    It should be noted that absent the stirring of reactant SFS, the charge in the chamber  110  would turn to solid block on heating, and the remaining sodium fluoride sinter together 
         [0034]    Accordingly, it should now be appreciated that the use or deployment of the above non-leaking calcining chamber with stirring results in several mutual benefits, which include a high throughput and efficiency of a decomposition reaction, as well as the avoidance of contamination from the stirring blade along with greater safety from the high reliability of rotation shaft seal mechanism. 
         [0035]    Another particularly challenging problem in producing SiF4 and other corrosive gases is an efficient means to remove them from a reaction chamber and compress them for storage. Conventional vacuum pumps can be used, but must deploy a cryo-trap to condense the gas in front of the vacuum pump to prevent contamination of the product gas, as well as damage to the pump. This then requires a second process to warm up the condensed solid, to form a gas that can be compressed for storage in inert high pressure contains. The process is time consuming and inefficient and not well suited for continuous product processes. 
         [0036]    One non limiting example of such a process is thermal decomposition of sodium fluorosilicate (SFS) to produce silicon tetrafluoride (SiF4) which among other uses is, can be reacted with liquid sodium metal to produce silicon metal. As silicon must be highly pure for use as a semiconductor in electronic and photovoltaic applications, it is of paramount importance that the SiF4 is not only pure, but does not become contaminated by reaction with the process equipment. SiF4 itself is toxic and highly corrosive. Further, it readily reacts with water to process hydrofluoric acid, which is more corrosive. At has recently been discovered that this process is most efficient and has a higher yield when the SFS powder is agitated and stirrer at pressure of about 50 to 200 torr. Hence, there is a need to collect the SIF4 gas at such pressures. 
         [0037]    As illustrated in  FIG. 4-6 , another embodiment of the invention is the pumping apparatus  400  which is deployed to collect and compress SiF4 gas that is formed by the thermal decomposition of dry SFS at or above 700° C. It has been discovered that optimum pressure for such decomposition is generally from about 20 to 200 torr. 
         [0038]    A decomposable solid, such as SFS, is introduced into a heatable chamber  110 . The chamber  110  is evacuated, and then heated to the heat the solid to the decomposition temperature so that a pure gas is released. The gas is removed at an exhaust portal  111  by the action of a first dry vacuum pump  4120  in communication therewith. This first vacuum pump  4120  delivers the exhausted gas to a compressor  4130 , with compresses the gas into one or more storage tanks  4140 . To prevent contamination of the gas from seal region  4125  of the vacuum pump  4120 , a small portion of the compressed gas is continuously bled off of the compressor  4130  (as shown in  FIG. 4 ) from the feed line to the tanks  4140 , and fed back to flush the seal regions  4125  of the first vacuum pump  4120 . Alternatively, as shown in  FIG. 5 , the compressed gas in the storage tank  4140  can be fed back to flush the seal region  4145  of the first vacuum pump  4120 . 
         [0039]    U.S. Pat. No. 4,734,018, which is incorporated herein by reference, discloses one such dry vacuum pump that is generally suitable for use in the inventive apparatus and method. The pump deploys multiple a labyrinth seals between the bearings that support a rotary member that turns the pumps compressor shaft. The labyrinth seals thereof may be flushed with the bled of gas from the compressor as described above. 
         [0040]    U.S. Pat. No. 6,189,176, which is incorporated herein by reference, discloses a high pressure glass cleaning purge of silicon oxide dust from a dry vacuum pump while installed on a crystal grower. 
         [0041]    The dry vacuum pump and the compressor must not have any leaks that allow gas to leak in from the environments, as well as prevent the leakage of the pure gas formed from thermal decomposition out. 
         [0042]    The portions of each pump apparatus that are exposed to the pure gas are constructed of materials that are substantially non-reactive therewith, thus avoiding contamination of the by-producers of such a reaction. Such materials include pure nickel for forming, cladding or coating metal components, and flouropolymers for resilient and flexible components. 
         [0043]    In the start up phase when the compressor has not yet produced a sufficient quantity of pure gas to flush or purge the seal regions of the first vacuum pump, such pure gas can be provided from a storage tank. 
         [0044]    While the dry vacuum pump can evacuate to low pressure, the gas thus removed can only be compressed at the output port to few psi. Hence there was also a need for then deploying a compressor that receive the output of the dry vacuum pump at about 2 psig, and compressing it in a first stage to 60 psig, and in the second stage from about 60 psig to preferably at least about 300 psig for storage in tanks. Further, it is also desirable that at least one particulate filter is deployed between the first dry vacuum pump and the compressor.  FIG. 6  illustrates such an to apparatus  400  having a first compressor  4131  connected to receive the output of the dry vacuum pump and a second compressor  4132  connected thereto for another stage of compression beyond about 60 psig to preferably about 300 psig. 
         [0045]    It is also preferred to deploy a control system that simultaneously maintains each pump at a speed to provide the optimum pressure for the other pump. In start up, the compressor starts first, then the dry vacuum pump after the optimum operating pressure is reached, and the vacuum pump seal region is fed with the compressed SiF4 gas. As shown in  FIG. 4 , control system  4200  is also operative to modulate a valve  4135  that controls the bleed of compressed gas from compressor  130  to the seal region  125  of pump  120 . In contrast, in  FIG. 2 , controller  4200  is operative to modulate a valve  4145  that controls the flow of gas from tank  4140  to the seal region  4125  of pump  4120 . 
         [0046]    The gas mixture that flushes the seal region is preferably either trapped with a cryo-pump or captured by reaction with a solid leaving a safely disposable residue or a material that can be returned to chamber for re-processing. 
         [0047]    While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form 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.