WO2009061205A1 - Method for extending up-time and improving temperature control in the direct chlorination of silicon metal in a fluidised bed and fixed bed reactors - Google Patents

Method for extending up-time and improving temperature control in the direct chlorination of silicon metal in a fluidised bed and fixed bed reactors Download PDF

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Publication number
WO2009061205A1
WO2009061205A1 PCT/NO2008/000362 NO2008000362W WO2009061205A1 WO 2009061205 A1 WO2009061205 A1 WO 2009061205A1 NO 2008000362 W NO2008000362 W NO 2008000362W WO 2009061205 A1 WO2009061205 A1 WO 2009061205A1
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WIPO (PCT)
Prior art keywords
reactor
silicon
process according
reaction zone
absorbing material
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PCT/NO2008/000362
Other languages
French (fr)
Inventor
Per Bakke
Robert Gibala
Grete Viddal ØI
Oddmund Wallevik
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Hycore Ans
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Publication of WO2009061205A1 publication Critical patent/WO2009061205A1/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/08—Compounds containing halogen
    • C01B33/107—Halogenated silanes
    • C01B33/1071—Tetrachloride, trichlorosilane or silicochloroform, dichlorosilane, monochlorosilane or mixtures thereof
    • C01B33/10715—Tetrachloride, trichlorosilane or silicochloroform, dichlorosilane, monochlorosilane or mixtures thereof prepared by reacting chlorine with silicon or a silicon-containing material
    • C01B33/10721—Tetrachloride, trichlorosilane or silicochloroform, dichlorosilane, monochlorosilane or mixtures thereof prepared by reacting chlorine with silicon or a silicon-containing material with the preferential formation of tetrachloride
    • C01B33/10726—Tetrachloride, trichlorosilane or silicochloroform, dichlorosilane, monochlorosilane or mixtures thereof prepared by reacting chlorine with silicon or a silicon-containing material with the preferential formation of tetrachloride from silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
    • B01J8/18—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles
    • B01J8/20—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles with liquid as a fluidising medium
    • B01J8/22—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles with liquid as a fluidising medium gas being introduced into the liquid

Definitions

  • the present invention relates to a method for improving temperature distribution and up-time in the direct chlorination of silicon metal in fluidised bed and fixed bed reactors.
  • MCSi metallurgical grade silicon
  • the present invention solves the problem with accumulation of liquid salt in the reactor as a porous substance inert to the reactants and products of the reaction is introduced to the reactor to absorb the liquid salt. Further, the present invention improves temperature control as the reaction takes place over a larger volume in the reaction zone due to improved particle distribution and increased total mass in the reaction zone.
  • Fig. 1 is a CaCI 2 -MnCI 2 phase diagram showing liquidus temperature as a function of mole fraction MnCI 2 ,
  • Fig. 2 is a table showing results of analyses of metal and kieselgel from chlorination performed under two tests (tests I and II)
  • Liquid crude Si tapped from electric arc furnaces for carbothermic reduction of silica may typically contain 1-3% impurities, for example:
  • Refining of liquid Si by addition of oxygen and slag forming agents such as silica sand, lime/limestone, dolomite, olivine and calcium fluoride, in a ladle during tapping from the electric reduction furnace is common practice among major producers and can give significant reduction of Al, Ca (and Mg) content.
  • Typical concentrations after refining can be 0.3% Al and 0.03% Ca (so-called 3303 quality) or even lower.
  • These refined qualities are typically used for Rochow synthesis for production of methyl chlorosilanes.
  • Other impurities including Ti, B, P, As, Sb, Cu and Mn depend on raw material selection (quartz and carbon sources).
  • Chlorides of Cu, Mn, Fe, Mg, K, Ca, Cr, Sr, Ba, Ni, V and Zr may accumulate in the reactor and depending on the temperature in the reactor, these may contribute to a liquid salt obstructing the fluidization.
  • the melting temperature of many of the individual salts may be higher than the reaction temperature, mixtures of salts may form and these will generally have lower liquidus temperatures than the melting temperature of the individual salts.
  • CaCk and MnCI 2 are examples, shown in a phase diagram in Fig.1. In this system the eutectic temperature is 59O 0 C, while the pure CaCI 2 and MnCI 2 melts at 772 and 65O 0 C, respectively.
  • the presence of other chlorides like e.g. MgCI 2 and SrCI 3 will contribute to an even lower liquidus temperature.
  • the present invention represents a solution to the above-mentioned problem with liquid salt accumulating in the reactor.
  • Kieselgel is a granular, porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid. It is commonly used as a dessicant to control local humidity in order to avoid damage and possible spoilage of goods being sensitive to humidity. Kieselgel's high surface area (typically 500-800 m 2 /g) and pore volume (typically 0.6-0.8 cm 3 /g) allows it to adsorb water readily.
  • Kieselguhr diatomaceous earth
  • This powder is very light, due to its high porosity.
  • the typical chemical composition of diatomaceous earth is 86% silica, 5% sodium, 3% magnesium and 2% iron.
  • kieselgel In an attempt to absorb liquid salts that accumulate in a fluidised bed reactor for direct chlorination of silicon metal, kieselgel (Merck, 1.07733.100) with size fraction 0.2-0.5mm was added to the reaction zone of the reactor.
  • test I 5% kieselgel (15g) was mixed with the first 300 grams of MGSi of size 0.1-0.5mm. In total 2.1 kilogram SiCI 4 was produced from 0.40 kilogram Si.
  • test II In a second test (test II) 2.0 grams kieselgel was added to the reactor before the test started. During this test 3.7 kilograms SiCI 4 was produced from 0.66 kilograms Si.
  • the enrichment of Ca corresponds to a factor of three (1.4-4.3%) while for Al it is a factor of six (0.5-3.0%) in the kieselgel from the fluidised bed compared to the metal residue. Comparing the kieselgel from the fluidised bed with the kieselgel reference show enrichment of Ca and Al of 40 and 100, respectively.
  • kieselgel to the reaction zone in a reactor for direct chlorination of silicon metal may be done continuously or intermittently, based on calculated liquid chloride formation, directly to the zone via separate supply means, or indirectly with the supply of Si (not shown in figures).
  • the supply of kielselgel to the reaction zone will eventually lead to build-up such material, including absorbed chlorides, in the reaction zone.
  • the chloride enriched kieselgel may however be removed from the reaction zone on an intermittent basis by short stoppage of the reactor, or on a continuous basis by automatic discharge of the material from the zone through a sluice or the like (neither not shown).
  • kieselgel, kieselguhr or any other silica based or other material having high surface area and which is chemically inert to the reacting species at the operation temperature may be used to absorb liquid chlorides from the reaction zone according to the invention.
  • Such materials may be expanded perlite, exfoliated vermiculite or dried calcium bentonite.
  • An additional requirement is also that the absorbing material must have a sufficient mechanical strength so that it does not disintegrate into fine particles that are carried with the gas and eventually may contaminate the SiCI 4 product.

Abstract

Method for extending up-time and improving temperature control in the direct chlorination of silicon metal in a fluidised bed or fixed bed reactor. The reactor includes a reaction zone and means for the supply of silicon and chlorine as well as means for removal of reaction products. A liquid chloride absorbing material, kieselgel, kieselguhr, silica based, or other equivalent material having high surface area and which is chemically inert to the reacting species at the operation temperature, is added to the reaction zone of the chlorination reactor, whereby the chloride enriched material is removed from the reaction zone on an intermittent or continuous basis.

Description

"Method for extending up-time and improving temperature control in the direct chlorination of silicon metal in a fluidised bed and fixed bed reactors"
The present invention relates to a method for improving temperature distribution and up-time in the direct chlorination of silicon metal in fluidised bed and fixed bed reactors.
The reaction of silicon metal with chlorine gas is characterized by an extremely high positive reaction enthalpy; at room temperature 687kJ/mol SiCI4. This high release of energy causes major challenges and explains why other processes than direct chlorination historically have been more attractive for industrial use.
Another problem is related to the impurities in the metallurgical grade silicon (MGSi). From a chlorination point of view, there are basically two types of impurities; those who forms volatile chlorides which mainly escape from the reactor alongside the SiCI4 gas, and those who accumulate as solids or liquids in the fluidized bed, causing gradually evolving operational problems with improper fluidization, irregular temperature distribution, erroneous temperature control and risk of chlorine penetration. An inevitable consequence is decreased up-time of the reactor due to maintenance needs.
The present invention solves the problem with accumulation of liquid salt in the reactor as a porous substance inert to the reactants and products of the reaction is introduced to the reactor to absorb the liquid salt. Further, the present invention improves temperature control as the reaction takes place over a larger volume in the reaction zone due to improved particle distribution and increased total mass in the reaction zone.
The method according to the invention is characterized by the features as defined in the accompanying independent claim 1.
Preferred embodiments of the invention are defined in the accompanying dependent claims 2 - 8.
The invention will be further described in the following by way of example and with reference to the figures, where:
Fig. 1 is a CaCI2-MnCI2 phase diagram showing liquidus temperature as a function of mole fraction MnCI2,
Fig. 2 is a table showing results of analyses of metal and kieselgel from chlorination performed under two tests (tests I and II)
Liquid crude Si tapped from electric arc furnaces for carbothermic reduction of silica may typically contain 1-3% impurities, for example:
Fe: 0.2-1%
Al: 0.4-0.7%
Ca: 0.2-0.6%
Ti: 0.1-0.2%
C: 0.1-0.15%
Refining of liquid Si by addition of oxygen and slag forming agents such as silica sand, lime/limestone, dolomite, olivine and calcium fluoride, in a ladle during tapping from the electric reduction furnace is common practice among major producers and can give significant reduction of Al, Ca (and Mg) content. Typical concentrations after refining can be 0.3% Al and 0.03% Ca (so-called 3303 quality) or even lower. These refined qualities (Chemical grade) are typically used for Rochow synthesis for production of methyl chlorosilanes. Other impurities including Ti, B, P, As, Sb, Cu and Mn depend on raw material selection (quartz and carbon sources).
When exposed to chlorine, aluminium and iron forms AICI3 and FeCI3, respectively, and at typical reaction temperatures, 4000C or above, these follow the SiCU gas out of the reaction zone. If downstream of the reactor the temperature is allowed to drop to temperatures where these components precipitate (FeCI3 solidifies at 3080C while the sublimation temperature Of AICI3 is 1870C), build-up of deposits can occur, eventually causing clogging and maintenance needs.
Chlorides of Cu, Mn, Fe, Mg, K, Ca, Cr, Sr, Ba, Ni, V and Zr may accumulate in the reactor and depending on the temperature in the reactor, these may contribute to a liquid salt obstructing the fluidization. Although the melting temperature of many of the individual salts may be higher than the reaction temperature, mixtures of salts may form and these will generally have lower liquidus temperatures than the melting temperature of the individual salts. CaCk and MnCI2 are examples, shown in a phase diagram in Fig.1. In this system the eutectic temperature is 59O0C, while the pure CaCI2 and MnCI2 melts at 772 and 65O0C, respectively. The presence of other chlorides like e.g. MgCI2 and SrCI3 will contribute to an even lower liquidus temperature.
The present invention represents a solution to the above-mentioned problem with liquid salt accumulating in the reactor.
Kieselgel (silica gel) is a granular, porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid. It is commonly used as a dessicant to control local humidity in order to avoid damage and possible spoilage of goods being sensitive to humidity. Kieselgel's high surface area (typically 500-800 m2/g) and pore volume (typically 0.6-0.8 cm3/g) allows it to adsorb water readily.
Also used as an absorbent for liquids is Kieselguhr (diatomaceous earth), which is a naturally occurring, soft, chalk-like sedimentary rock that is easily crumbled into a fine white to off-white powder. This powder is very light, due to its high porosity. The typical chemical composition of diatomaceous earth is 86% silica, 5% sodium, 3% magnesium and 2% iron.
Examples
In an attempt to absorb liquid salts that accumulate in a fluidised bed reactor for direct chlorination of silicon metal, kieselgel (Merck, 1.07733.100) with size fraction 0.2-0.5mm was added to the reaction zone of the reactor.
In the first test (test I), 5% kieselgel (15g) was mixed with the first 300 grams of MGSi of size 0.1-0.5mm. In total 2.1 kilogram SiCI4 was produced from 0.40 kilogram Si.
In a second test (test II) 2.0 grams kieselgel was added to the reactor before the test started. During this test 3.7 kilograms SiCI4 was produced from 0.66 kilograms Si.
Upon termination of the tests and subsequent dismantling of the test equipment a fraction of material mainly consisting of kieselgel was taken out of the fluidised bed and analysed by XRF. In the table, Fig. 2, the analyses of kieselgel from the fluidised bed (named as Kieselgel from FB) are compared with the XRF analyses of kieselgel added to the reactor (named as Kieselgel (ref)). The analyses of the input MGSi (named as Metal input) and the metal phase mainly consisting of partly reacted Si taken out from the reactor (named Residues from FB) are also included.
In the kieselgel from fluidised bed there are no distinctions between Si bound as SiO2 and metallic Si. In the original kieselgel the SiO2 content is analysed.
In test I it is seen that there is an enrichment of Ca and Al corresponding to roughly a factor of four (0.28-1.2% for Ca and 0.18-0.81 % for Al) in the kieselgel from the fluidised bed compared to the metal residue. Comparing the kieselgel from the fluidised bed with the kieselgel reference show enrichment of Ca and Al of more than a factor of 10 for both Ca and Al.
In test II, the enrichment of Ca corresponds to a factor of three (1.4-4.3%) while for Al it is a factor of six (0.5-3.0%) in the kieselgel from the fluidised bed compared to the metal residue. Comparing the kieselgel from the fluidised bed with the kieselgel reference show enrichment of Ca and Al of 40 and 100, respectively.
In both tests it is noticed that the content of Fe is smaller in the kieselgel than in the metal residue. Other elements are present in significantly lower quantities than Fe, Al and Ca. However, in both tests it seems that there is an enrichment of Sr in the kieselgel, while in test Il enrichment of Mg in the kieselgel is detected. For the remaining elements, including Mn, the results are inconclusive.
Based on these tests, especially test II, it is evident that kieselgel has a significant ability to absorb chlorides of Al and Ca.
Another benefit observed during test Il (where 2 grams of kieselgel was added before start) was the positive influence on behavior of the fluidized bed, in the way that the mixing of kieselgel particles with the silicon led to an extension of the reaction zone, resulting in improved temperature control with direct cooling. The same effect was also noticed during the beginning of test I before the volume of kieselgel in the reactor grew too high.
The addition of kieselgel to the reaction zone in a reactor for direct chlorination of silicon metal may be done continuously or intermittently, based on calculated liquid chloride formation, directly to the zone via separate supply means, or indirectly with the supply of Si (not shown in figures).
On the other hand, the supply of kielselgel to the reaction zone will eventually lead to build-up such material, including absorbed chlorides, in the reaction zone. The chloride enriched kieselgel may however be removed from the reaction zone on an intermittent basis by short stoppage of the reactor, or on a continuous basis by automatic discharge of the material from the zone through a sluice or the like (neither not shown).
Within the scope of protection as defined in the claims kieselgel, kieselguhr or any other silica based or other material having high surface area and which is chemically inert to the reacting species at the operation temperature may be used to absorb liquid chlorides from the reaction zone according to the invention. Such materials may be expanded perlite, exfoliated vermiculite or dried calcium bentonite. An additional requirement is also that the absorbing material must have a sufficient mechanical strength so that it does not disintegrate into fine particles that are carried with the gas and eventually may contaminate the SiCI4 product.

Claims

Claims
1. Method for extending up-time and improving temperature control in the direct chlorination of silicon metal in a fluidised bed or fixed bed reactor, the reactor including a reaction zone and means for the supply of silicon and chlorine as well as means for removal of reaction products, characterized in that a liquid chloride absorbing material, kieselgel, kieselguhr, silica based, or other equivalent material having high surface area and which is chemically inert to the reacting species at the operation temperature, is added to the reaction zone of the chlorination reactor, whereby the chloride enriched material is removed from the reaction zone on an intermittent or continuous basis.
2. Process according to claim 1 , characterized in that the liquid chloride absorbing material is fed directly to the reaction zone of the reactor.
3. Process according to claim 1 , characterized in that the liquid chloride absorbing material is mixed with and introduced to the reactor together with the silicon feed.
4. Process according to claim 1 -3 characterized that the the liquid chloride absorbing material is added in an amount corresponding to 2-10 wt% of the silicon content of the silicon containing feed inside the reactor.
5. Process according to claims 1 -4 characterized in that the size of the liquid chloride absorbing is such that it stays inside the reactor and does not follow the gas phase out of the reactor.
6. Process according to claims 1 and 4-5 characterized in that the liquid chloride absorbing material is introduced to the reactor before the silicon is added.
7. Process according to claims 1 and 4-5 characterized in that the liquid chloride absorbing material is partly introduced to the reactor before the silicon is added, and partly mixed with and introduced to the reactor together with the silicon feed.
8. Process according to claims 1 -7 characterized in that the liquid chloride absorbing material is one of or a combination of the following material: expanded perlite, exfoliated vermiculite or dried calcium bentonite.
PCT/NO2008/000362 2007-11-05 2008-10-10 Method for extending up-time and improving temperature control in the direct chlorination of silicon metal in a fluidised bed and fixed bed reactors WO2009061205A1 (en)

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NO20075614A NO20075614L (en) 2007-11-05 2007-11-05 Method for extended uptime and improved temperature control by direct chlorination of silicon metal in a "fluid bed" or "fixed bed reactor"
NO20075614 2007-11-05

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2942950A (en) * 1956-12-28 1960-06-28 Monsanto Chemicals Process for the production of silicon tetrachloride
EP0256876A2 (en) * 1986-08-20 1988-02-24 Dow Corning Corporation A process for preparing halosilanes

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2942950A (en) * 1956-12-28 1960-06-28 Monsanto Chemicals Process for the production of silicon tetrachloride
EP0256876A2 (en) * 1986-08-20 1988-02-24 Dow Corning Corporation A process for preparing halosilanes

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TW200927650A (en) 2009-07-01

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