WO2008136681A1

WO2008136681A1 - Method and equipment for direct chlorination of metallurgical grade silicon

Info

Publication number: WO2008136681A1
Application number: PCT/NO2008/000154
Authority: WO
Inventors: Oddmund Wallevik
Original assignee: Norsk Hydro Asa
Priority date: 2007-05-07
Filing date: 2008-05-02
Publication date: 2008-11-13
Also published as: TW200906722A; NO20072350L

Abstract

Method and equipment for the direct chlorination of metallurgical grade silicon to produce silicon tetra chloride, including a reactor with means for the supply of silicon and injection of chlorine and means for the removal of the reaction product, silicon tetra chloride and any remnant material. The chlorination takes place in a disappearing bed reactor where the reactor is of the vertical tube shape type and where the silicon is fed continuously from the top of the reactor and chlorine is injected co-currently or counter-currently with the silicon in a controlled manner and whereby simultaneously heat generated under the reaction is rejected to maintain the reaction temperature preferably between 300 - 700°C. The invention provides a cheap, effective and easily controllable method and equipment for production of the subject product.

Description

"Method and equipment for direct chlorination of metallurgical grade silicon"

The present invention relates to a method and equipment for the direct chlorination of metallurgical grade silicon, Si, to produce silicon tetra chloride, SiCI₄, including a reactor with means for the supply of silicon and injection of chlorine and means for the removal of the reaction product, silicon tetra chloride and any remnant materials.

Silicon tetra chloride is useful as starting material for various organic silicon compounds and also as starting material for finely divided silica, high purity quarts, silicon nitride, silicon carbide and other silicon containing inorganic materials. Silicon tetra chloride is as well used for the production of high purity solar grade and semiconductor grade silicon, based on the reaction of SiCI₄ with liquid or gaseous.

Known typical processes for production of silicon tetra chloride are those wherein silicon carbide, ferrosilicon or similar silicon containing substances react with chlorine, and those wherein a mixture of silicon dioxide containing substances and carbon, for example, a mixture of silica stone and activated carbon is reacted with chlorine. The known processes, however, the disadvantages that the starting materials, silicon carbide and ferrosilicon are very expensive to produce due to high energy costs, and that an extremely high temperature, e.g. 1200 ⁰C or more, is required for smoothly producing the desired silicon tetra chloride at an acceptable yield.

From EP-A2- 0 077 138 is further known a process where silicon tetra chloride is produced at a relatively low temperature by feeding a mixture of silicon dioxide containing substance and carbon to the top of a reactor in which a reaction mixture forms a downwardly flowing moving bed and where gaseous boron trichloride is fed to a portion of the reactor and where gaseous chlorine in addition to an inert gas is fed to a portion below the boron trichloride feed portion. The silicon tetra chloride formation reaction is carried out in the moving bed and is recovered from a gas mixture discharged from the top of the reactor. Even though with this known process the temperature is more easily controllable, the process is complex, expensive and with low efficiency.

Beyond the above-mentioned known processes, it is further known to produce silicon tetra chloride by direct chlorination of metallurgical grade silicon in a fixed bed reactor. For a high reaction rate, which is needed to get a commercial scale output rate, the temperature can rise to 1200⁰C and above. Under these conditions it is possible that the silicon metal begins to melt leading to an uncontrolled reaction. It is a significant challenge to get rid of this generated energy (heat). Due to the restricted surface of a reactor and thereby small convection areas, an outside cooling wall-system may not be sufficient. Small size reactors (i.e. greater surface) may be used, but in such case the output will be small and hence this is not a commercially interesting option. Another problem is related to the requirements of the construction-materials. Due to the corrosive medium chlorine represents and the high temperature of the process there are hardly materials available which would meet the strong requirements of the reaction of silicon and chlorine.

To overcome the problems with the known direct chlorination process, the most commonly used SiCU-production processes are based mainly on the reaction of HCI with silicon metal. In this process, the reaction enthalpy is only less than one third (ca. 27%) of the direct chlorine process, 185 kJ/mol, and therefore less demanding in terms of temperature control and material selection. This reaction can take place in a fluidized bed reactor or a solid bed reactor. However, this process has the disadvantages that the chlorine must be converted to HCI in a separate process and the primary product SiCU hydrogen -chlorosilanes are formed, in fractions of 10 - 50% depending on the reaction temperature. These products are quite dangerous due to the risk of self-ignition. With the present invention is provided a process and equipment for continuous direct chlorination of metallurgical grade silicon where the above disadvantages with the known processes are overcome, where the silicon tetra chloride product is produced without additional reaction materials and which provides full control of the reaction temperature between 300 and 700 ⁰C and full recovery of heat from the process. The equipment and process according to the invention is further compact and cheap to manufacture and operate and is very efficient.

The process according to the invention is characterized by the features as defined in the attached independent claim 1.

The equipment is further characterized by the features as defined in the attached independent claim 7.

Dependent claims 2 - 6 and 7 - 12 define preferred embodiments of the invention.

The invention will be further described in the following by way of example and with reference to the drawings where:

Fig. 1 shows a principal sketch of the process equipment (process scheme) according to the invention on which the method according to the invention is based,

Fig. 2 shows in enlarged scale a reactor according to the invention as shown in Fig. 1.

When silicon is allowed to react with chlorine gas according to the equation below, about 5,9 kWh of heat is released per kilo silicon reacted, and the equilibrium is shifted completely towards SiCI₄ at all relevant temperatures.

Si + 2Cl₂ => SiCI₄ Not much written material is currently available regarding the kinetics of the chlorination reaction, but some data is found in a Brazilian work published in 2003, Seo E.S.M., Brocchi E. A., Carvalho R. J., Soares E. P., Andreoli M.: "A mathematical model for silicon chlorination", J. of Mat. Proc. Tech. 141 (2003) 370-378. The data published in the subject publication were obtained in a small fix bed reactor (15 cm²), with very shallow beds (5&10 mm) compared to the size of the particles. (The grain size was -4 +14 Tyler mesh, i.e. between 1.19 and 4,76 mm) The gas velocity was also quite low, with superficial velocity in the range 0.07 m/s. All by all, the design of the experiments, the scatter in the data and the simplifications in the analysis render the kinetic parameters extracted in the paper of uncertain value. When, however, the data obtained in non- compacted beds are lumped together, initial reaction rates in the range 100 -200 kmol Si/m³, h can be inferred for temperatures in the range 500 to 700 ⁰C. If first order kinetics with respect to chlorine gas is assumed, this would correspond to an average reaction rate in the order of 50-100 kmol Si/m³, h or 1.4 -2.8 XIm³, h. For chlorination of 5000 1 per annum of silicon with comparable grain size, this calls for a reactor volume in the range 0,5 -1 m³.

Based on further investigation and calculations the inventor found that chlorination of silicon would be possible to perform at temperatures well below 500 ⁰C, provided that the an optimal reactor design and cooling would be found. For the present purpose, a reaction temperature in the range 500 ⁰C and a reactor volume of about 1m³ was assumed. At a silicon consumption rate of 1000 kg/h, the heat production amounts to 5900 kWh /h. The conceivable idea of the inventor was that the reaction could be performed in a shell and tube reactor 1 as shown in Figs. 1 and 2 with the reaction taking place in each of the tubes in the reactor as further explained below. As an example, a reactor, 3 m high, with 1000 tubes, øJ- ø₀ 20/25 mm, has tube cross section area of 0.312 m², tube volume of 0.942 m³, and tube area of 188/235 m². With a heat transfer coefficient on the tube side of 200 W/m², ⁰C and 800 W/m² on the shell side, this would call for a tube temperature of 343 ⁰C and a cooling medium temperature of 311 ⁰C. The superficial inlet gas velocity would amount to 4 m/s, falling to 2 m/s at the outlet, giving an superficial gas residence time of 1s in the reactor. (For comparison, the gas residence time in the reactor reported in the Brazilian report above, can be inferred to be in the range 0.1 m/s)

The inventors choice of a shell and tube reactor emerged from such a simple exercise. The gas velocity appeared initially to be on the high side, but the gas velocity can to a certain degree be adjusted by the geometry of the reactor. The tube dimensions have to be considered in light of temperature gradients inside the tubes.

Thus, the equipment according to the invention include, as is shown in Fig. 1 , a continuous reactor A for the direct chlorination of metallurgical grade particulate silicon material, a heating/cooling circuit B with a heating/cooling medium for the heating/cooling of the reactor and a recovery unit C for the condensation and storing of the reaction product, silicon tetra chloride.

With further reference to Fig. 2, the reactor A is, as stated above, a shell and tube reactor where a number of vertical reactor tubes 3 are provided within a closed, outer shell or housing 1. The outer housing is provided at its upper end with an inlet 8 for the supply of metallurgical grade silicon particles, e.g. metallurgical grade, from a storage such as a silo or the like (not shown) via preferably a sluice feeding device 4. The silicon particles supplied to the reactor are distributed to the upwardly open inlet ends of the reactor pipes by means of a rotating distributor 6 (drive means not shown) which is designed to fill, but not pack each of the tubes by pushing the particles over their inlet ends. At the upper end of the housing 1 is provided an inlet 11 for the supply of chlorine gas, Cl₂ to the reactor, whereby the chlorine gas with this solution as shown in Fig. 1 may flow co-currently with the silicon material being fed to and moving downwardly in the reactor pipes 3. The vertical reactor pipes 3 are securely connected, in a suitable pattern and with a distance to one another, to endplates 2, 21 at the upper, respectively the lower end of the reactor housing 1 , thereby closing off from a middle space 19 within the reactor housing 1 respectively an upper space 34 within the housing where the silicon material and chlorine is fed to the reactor and a lower space 33 within the reactor housing where the reaction product, silicon tetra chloride and eventual contaminated rest material is collected and evacuated. A cooling medium is circulated, by means of the cooling/heating circuit B, through the middle space 19 within the housing via an inlet opening 15 at the upper end and an outlet opening 14 at the lower end of the middle space 19. A downwardly protruding plate or ring wall element 7 provides an annular space 35 between the outer wall 36 of the lower space 33 and the plate or wall element enabling escape of product gas to the product outlet 10.

The cooling/heating circuit B is used as a heating circuit to heat the vertical pipes to the required reaction temperature when starting the process and cooling the reactor pipes when operating the reactor under the process once the reaction has started. The cooling/heating circuit includes, beyond interconnecting piping 14, 25, 15, a reservoir 21 for storing and heating the heating/cooling medium and a heat exchanger 22 for supplying or removing heat from the heating/cooling medium. In the embodiment shown in Fig. 1 the reservoir is provided with a heater (not shown) to heat the cooling/heating medium to a temperature above the reaction temperature of the reactor. A pump (neither not shown) pumps the cooling/heating medium partly via the pipeline 25 through the heat exchanger 22 to cool part of the cooling/heating medium, and partly through the bypass loop 24 with a control valve 23 which controls the amount of cooling/heating medium bypassing the heater 22, thereby controlling the temperature of the cooling/heating medium entering the reactor A. The heat exchanger 22 may be used solely as a cooler, or may, according to an alternative embodiment, in addition be used as a heater to heat the cooling/heating medium from the reservoir thereby avoiding the use of a separate heater in the reservoir as mentioned above. The heating/cooling medium used to heat or cool the heat exchanger 22 may preferably be water/steam exchanged through supply respectively discharge pipelines 26, 27. Once the reaction is started in the reactor the heat generated is rejected by the heat exchanger 22. This rejected heat is thereby recovered and may be used in other processes or for heating purposes etc. The reactor A may preferably, in addition to the heating/cooling section 19, be provided with separate, direct cooling sections 18 and 20, respectively at the upper and lower end of the reactor as shown in Fig. 2. The direct cooling sections 18, 20 are separated from the middle cooling/heating section 19 by means of partition plates 24, 25 respectively, and water at ambient temperature is circulated through the sections via inlets 13, 17 and outlets 12, 16, respectively. The purpose of the upper cooling section 18 is to cool the upper feeding section of the pipes 3 with the solid phase distributor to ensure even pressure drop into all pipes and to prevent the reaction zone from spreading upwards by keeping the temperature below the reaction temperature. Further, the purpose of the lower cooling section 20 is to collect high boiling chlorides in the lower end of the disappearing bed reactor.

The reaction product, silicon tetra chloride, is evacuated from the lower space of the reactor through an outlet opening 10 and is collected in the recovery unit, C. Further, polluted remnants and un-reacted silicon from the process are collected in the bottom of the reactor and are evacuated in a continuous or semi-continuous manner through a sluice 5 or the like to a separator where the un-reacted silicon particles are separated from the polluted remnants and returned via a feed line to the inlet 8 of the reactor, whereas the polluted remnants are feed via a discharge line 29 to suitable storage or the like. It should be noted that the reactor according to the invention is completely sealed off from the surroundings and that the sluices 4 for the feeding of the silicon to the reactor and the sluice 5 to evacuate remnants from the reactor not necessarily must be of the rotating type as shown in the figures, but may be of any kind providing a tight seal and preventing leakage of chlorine or silicon tetra chloride to the environments.

The reactor and process according to the invention works, with reference to Figs. 1 and 2, in the following manner:

The reactor is initially heated by the cooling (heating) medium from the cooling circuit A to obtain the desired reaction temperature. Silicon particulate material is fed to the upper space 34 of the reactor through the sluice 4 and is distributed by the distributor 6 to the pipes 3 inlet, whereby the pipes are continuously filled up with silicon particles. At the same time chlorine gas is fed to the upper space 11 through the inlet opening 11 flowing downwardly with the silicon particles through the pipes, whereby the chlorine gas reacts with the silicon metal forming silicon tetra chloride, which passes downwards in the pipes towards the lower space 33 of the reactor. By the reaction between the silicon metal and the chlorine, the silicon is gradually consumed towards the lower end of the pipes 3, whereby the reactor may be called a "disappearing bed" reactor (the silicon metal is disappearing towards the bottom of the reactor).

The silicon tetra chloride arriving in the lower space 33 of the reactor is evacuated through the outlet 10 and flows to the recovery unit C where it is preferably condensed in a condenser/heat exchanger 30 with inlet/ outlet for a cooling medium 3, and stored in a suitable container or storage 32.

The surface temperature of the tubes may preferably be held in the range 340 to 400 ⁰C, depending on the reactor load. The tube temperature has to decrease with increasing load, and temperatures in the higher range might only be encountered during start up, in order to have the reaction started. The gas atmosphere in the reactor is a mixture of C^ and SiCI₄ with traces of other volatile inorganic chlorides including HCI, but, of cause, dry.

The corrosion of metals by dry Cl₂ and HCI is very much governed by the same mechanism, related to the vapour pressure if the formed chlorides. According to known design criteria, the upper design limits for nickel and high nickel alloys in dry chlorine gas (e.g. the 200 and 600 series) are about 500 ⁰C for vessels/pipes, and about 420 ⁰C for tubes/internals. Alloy 600 might thus seem to be a reasonable choice of reactor material. The corrosion rate for Alloy 600 in dry chlorine gas is, given by

ln(dr) (mm/yr) = -17.636 + 0.022*T(°K) The corrosion rate for the tubes at the estimated temperature of operation can thus be in the order of 0.017 mm/yr.

The inlet and outlet sections, which is cooled to ambient temperature, or slightly above, could be made from stainless steel (CrNi 18-9), which, even as tubes /internals, may be recommended up to about 250 ⁰C.

Even though the flow of reactants as shown in the figures is co-current, the flow may also be counter-current. Thus the chlorine may be fed from the lower part of the reactor and against the top, i.e. against the flow of the silicon metal.

The disappearing bed reactor according to the invention must be fed with silicon grains from the top. In any case, the gas inlet end must be cooled to prevent the reaction to spread outside the reactor tubes and the reactor must be large enough to ensure full conversion of the gas inside the reactor rubes.

The great advantage of co-current feeding is that both the reactants are cold when let into the feeding section so that uncontrolled spreading of the reaction is easy to prevent. The task of the cooled inlet section will only be to prevent the reactants from being heated up, and the cooled gas entering the reaction tubes will help to confine the reaction inside the tubes. The main disadvantage is that the small particles/dust that is formed in increasing amount downwards in the reactor will be driven further down with the gas flow, and when below the zone of full gas conversion, they will remain as dust and might tend to clog the tubes. This effect is counteracted with a continuous drainage of solid material, remnants, from the cooled bottom section, and the drained material may, except for a bleed of oxide dust, be re-circulated to the top of the reactor (not further shown). The pressure drop in the reactor is controlled by the rate of recirculation. Downwards in the reactor the increase in exposed surface will be counteracted of reduced concentration of chlorine gas in the gas phase, tending to balance the volumetric reaction rate. The cooling liquid on the shell side could in this case be circulated both ways, most probably counter current (i.e from bottom to top). With counter-current feeding, the fine particles that are formed as the grains go downwards are transported upwards with the gas flow, and finally blown out of the reactor with the product gas together with the dust fraction in the feed. The loss of feed as dust is small, of course depending of the quality of the feed, and further treatment of the dust might not be necessary. However, the permeability of the bed will vary with the position and the grain surface per volume unit exposed to gas will increase downward in the reactor. The rate of reaction per volume unit will thus increase downwards in the reactor due to both the increased surface and the increased concentration of chlorine in the gas phase. In this case the cooling liquid has to be circulated from bottom to top in order to provide the highest cooling capacity in the lower part.

For reactor control, it should be possible to keep the shell side temperature in the range 250 to 400 ⁰C. This limits the choice of cooling liquids, but commercially used nitrate- nitrite melt such as Hitec® which has a window of operation from 143 to 538 ⁰C may be used. The heat capacity of this liquid is 0.373 cal/g, ⁰C. For a cooling load of 5,9 MW with a temperature increase across the reactor of e. g. 30 ⁰C₁ the rate of circulation would be in the range 170 m³/h. The heat can then be used for high pressure steam production and, if economically justified, a substantial part of the energy recovered as electricity.

Claims

1. Method for the direct chlorination of silicon metal such as metallurgical silicon grade to produce silicon tetra chloride, including a reactor (A) with means for the supply of silicon and injection of chlorine and means for the removal of the reaction product, silicon tetra chloride and any remnant material, characterised in that the chlorination takes place in a disappearing bed reactor where the reactor is of the vertical tube shape type (3) encapsulated in housing or shell (1) and where the silicon in the form of particulate material is fed continuously from the top of the reactor and chlorine is injected co-currently or counter-currently with the silicon in a controlled manner, and whereby simultaneously heat generated under the reaction is rejected to maintain a controllable reaction temperature.

2. A method in accordance with claim 1 , characterised in that the reaction temperature is between 300 - 700⁰C

3. A method in accordance with claim 1 , characterised in that the heat is rejected by means of a cooling medium circulating through the housing (1) on the outside of the pipes (3) from a separate cooling circuit.

4. A method in accordance with claim 1 and 3, characterised in that the cooling medium is a nitrate-nitrite melt.

5. A method in accordance with claim 1 , characterised in that the silicon material is fed to the reactor (A) via a sluice (4) at the top of the reactor housing.

6. A method in accordance with claim 1 , characterized in that remnant material is removed from the lower end of the reactor through a sluice (5) in the reactor housing (1 ).

7. A method in accordance with claim 1 , characterized in that the reaction product, silicon tetra chloride is evacuated from the reactor (1) and is preferably condensed and collected in a separate storing unit (C).

8. Equipment for the direct chlorination of metallurgical grade silicon to produce silicon tetra chloride, including a reactor (A) with means for the supply of silicon and injection of chlorine and means for the removal of the reaction product, silicon tetra chloride and any remnant material, characterized in that the reactor is a continuous reactor (A) for the direct chlorination of metallurgical grade particulate silicon material, a cooling/heating circuit (B) with a cooling medium for the cooling of the reactor and recovery of heat from the process, and a product recovery unit (C) for the condensation and storing of the reaction product.

9. Equipment according to claim 8, characterized in that the reactor (A) is of a vertical tube shape type (3) encapsulated in housing or shell (1 ) and where the silicon in the form particulate material is fed continuously from the top of the reactor and chlorine is injected co-currently or counter- currently with the silicon in a controlled manner, and whereby simultaneously heat generated under the reaction is rejected by means of the cooling medium from the cooling circuit (B) to maintain the reaction temperature between 300 - 700⁰C.

10. Equipment according to claims 8 and 9, characterized in that, a sluice (4) is provided at the top of the reactor housing for the feeding of the silicon material to the reactor (A).

11. Equipment according to claims 8 and 9, characterized in that remnant material is removed from the lower end of the reactor through a sluice (5) in the reactor housing (1).

12. Equipment according to claims 8-11, characterized in that the cooling/heating circuit (B) includes a reservoir (21 ) and a heat exchanger (22) interconnected with the reactor (a) through external piping (14, 25, 15).

13. Equipment according to claim 12, characterized in that a bypass loop (24) with a control valve (23) is provided in the external piping (14, 25, 15) to bypass the heat exchanger (21).

14. Equipment according to claims 12 and 13, characterized in that the heat exchanger (22) is cooled/heated by means of water/steam.

15. Equipment according to claims 8, 9 and 11 , characterized in that the reactor (A) at its outlet end is provided with a separator (28) to separate the remnant materials.

16. Equipment according to claim 15, characterized in that a separate feeding arrangement is provided to return separated, un-reacted silicon metal from the separator (28) to the feed inlet (8) of the reactor.