EP3565782A1 - Procédé de raffinage pour la production de silicium solaire, de carbure de silicium, de graphite de grande pureté et de microsphères creuses de silice - Google Patents

Procédé de raffinage pour la production de silicium solaire, de carbure de silicium, de graphite de grande pureté et de microsphères creuses de silice

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Publication number
EP3565782A1
EP3565782A1 EP17801231.6A EP17801231A EP3565782A1 EP 3565782 A1 EP3565782 A1 EP 3565782A1 EP 17801231 A EP17801231 A EP 17801231A EP 3565782 A1 EP3565782 A1 EP 3565782A1
Authority
EP
European Patent Office
Prior art keywords
plasma
silicon
getter
jhq
silicon carbide
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP17801231.6A
Other languages
German (de)
English (en)
Inventor
David Charles Lynch
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Plassein Technologies Ltd LLC
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Plassein Technologies Ltd LLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US15/399,592 external-priority patent/US11242252B2/en
Application filed by Plassein Technologies Ltd LLC filed Critical Plassein Technologies Ltd LLC
Publication of EP3565782A1 publication Critical patent/EP3565782A1/fr
Pending legal-status Critical Current

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Classifications

    • 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
    • C01B33/02—Silicon
    • C01B33/021—Preparation
    • C01B33/023—Preparation by reduction of silica or free silica-containing material
    • C01B33/025—Preparation by reduction of silica or free silica-containing material with carbon or a solid carbonaceous material, i.e. carbo-thermal process
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/205—Preparation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/90—Carbides
    • C01B32/914—Carbides of single elements
    • C01B32/956—Silicon carbide
    • C01B32/963—Preparation from compounds containing silicon
    • C01B32/97—Preparation from SiO or SiO2
    • 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
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/30—Particle morphology extending in three dimensions
    • C01P2004/32—Spheres
    • C01P2004/34—Spheres hollow

Definitions

  • the present invention relates to an integrated process for the production and refining of silicon, silica, and carbon products from unrefined ores.
  • Silicon dioxide (S1O2) is the most abundant mineral in the earth's crust. The manufacture of silicon for photovoltaics occurs in two stages. First, is the reduction of silica (removal of oxygen) to produce
  • any process for refining silicon is to remove impurities with the least cost.
  • the known processes for refining silicon ores typically starts with chunks of quartz and produces a metallurgical-grade silicon (m-Si) with 98.5% purity by employing a submerged arc furnace 10 (see Fig. 2) with very high temperatures in a coke (or coal) and woodchip reduction processing step 10.
  • TCS trichlorosilane
  • HCV(g) hydrogen chloride gas
  • SiCU(g) silicon tetrachloride
  • step 15 the TCS is decomposed, in a batch process, at 1050 to 1150°C in what is known as a hairpin reactor.
  • the silicon produced in that reactor has a purity of 9N.
  • An alternative to the Siemens process is converting purified TCS (produced in step 14) to silane (S1H4) in catalytic redistribution columns, step 16. That conversion involves multiple steps that include distillation.
  • a final distillation, step 17, is used to separate the S1H4 from SiCU.
  • the silane is decomposed in either a hairpin reactor (18) or in a fluidized bed reactor, step 19. Silane decomposes at a lower temperature than TCS, and thus there is a significant energy savings with the alternate process to that of the Siemens process (item 20).
  • the fluidized bed reactor, step 19, has an additional advantage in that it can be operated continuously.
  • the decomposition of S1H4 yields a 6 to 7 nines pure silicon.
  • step 13 Upgrading of m-Si (greater than 5N), step 13, can be accomplished by slagging and/or by blowing gases through the silicon melt whereby the boron and phosphorus impurities are removed followed by
  • hydrometallurgical treatment 20, and unidirectional solidification (UDS) 21 to achieve solar grade purity.
  • This approach requires that high purity quartz and coke be processed in the silicon submerged arc furnace (10) to produce a higher purity silicon than the typical metallurgical grade silicon used in the production of TCS in steps 11 and 12.
  • the Czochralski process may also be used to produce monocrystalline silicon that is useful in making conventional Mono-Si solar cells.
  • Today's cost to produce electronic grade silicon (Siemens process) is approximately $16 to $20 per kilogram.
  • Today's cost to produce medium grade silicon produced by the fluidized bed reactor is approximately $ 11 per
  • the goal of any process to produce Solar Silicon, s-Si, is to produce polysilicon at >5N purity on a metal basis, that has specific SEMI target impurities as indicated by Table I, and which substantially reduces the cost of production
  • a problem with the use of the submerged arc furnace 10 is that it cannot use low cost powdered silica widely available throughout the world without some other process to convert the powdered ore into briquettes or the like.
  • the disclosed process employs low cost silica powder (sand) as its input ore, thereby reducing costs.
  • the disclosed process also eliminates the use of explosive silane in the revised Siemens process and does not produce silicon tetrachloride that must be disposed of as in the
  • the disclosed process employs unconventional reducing agents (methane, propane or any other hydrocarbon that is easily vaporized). Elimination of solid reductant (coal, coke, and woodchips), as compared to conventional reduction of silica in the submerged arc furnace, eliminates major impurity sources. That advantage has significant consequences:
  • the process describe below uses a new plasma furnace design described in, e.g., US Pat. No. 8,253,057, hereby incorporated by reference and set forth here in its entirety.
  • the furnace variously described as JHQ herein after its inventor Jack Hunt, generates a rotating, donut-shaped disk, "dirty-air," stable plasma whose shape and rotational velocity can be controlled.
  • the size of the donut hole in the plasma is controlled by the size of the inner electrode, while the diameter of the donut-shaped plasma is dictated by the inner diameter of the outer electrode. That electrode forms a concave ring around the inner electrode with a surface area many times greater than that of the inner electrode, thereby ensuring it a long operational life.
  • the inner electrode is a consumable that can be fed continuously for steady state operation of the plasma furnace.
  • the donut-shaped disk plasma is created by superimposing an AC electric field over a DC field.
  • the combined fields significantly increase the volume of the plasma.
  • Both the temperature and thickness of the donut-shaped plasma are dictated by the magnitude of the AC and DC fields.
  • the expanded plasma volume allows for greater mass throughput without extinguishing the plasma, thus the use of the term "dirt-gas" plasma.
  • the size of the plasma furnace can be scaled with the power input.
  • the temperature of the plasma can exceed 4500 degrees C, and thus the electrodes are cooled and protected by an electromagnetic field to extend their operational life.
  • the residence time of particulate in the plasma can be controlled.
  • the particulate can either pass directly through the plasma or it can be swirled within the donut-shaped plasma. Control of the residence time is dictated by the arrangement of the exit port with respect to the inlet port, and by applying a slight reduction in pressure at the exit port.
  • the degree of heating particulate in the plasma is dictated by residence time, particulate size, the rate of mass through-put, and power to the plasma.
  • the plasma in the JHQ furnace is capable of rapid heating of particulate, even to the point of entirely vaporizing small particles of carbon coated silica if so desired. Rapid transfer of heat is essential to the process described below, as many of the reactions are highly endothermic. It is the rapid transfer of heat, the high mass throughput, and the physico-chemical longevity of the outer electrode that makes the JHQ plasma furnace ideal for production of solar grade silicon, electronic grade silicon carbide, high purity graphite and hollow silica microspheres.
  • the process described below produces polysilicon of the required purity at an operational cost of approximately $6 per kilogram. That cost may be reduced even further by subtracting the approximately $3 per kilogram energy produced by the process. The overall cost is significantly less than current alternatives.
  • Fig. 1 is a chart showing the prior art processes for producing
  • Fig. 2 is an illustration of a prior art coke reduction silica in the silicon submerged arc furnace, and chemical reactions that occur in the furnace for producing metallurgical grade polysilicon.
  • Fig. 3 illustrates a preferred process for producing solar grade silicon that also produces electrical energy.
  • Fig. 4A illustrates a typical silica ore having impurities in form of minerals.
  • Fig. 4B illustrates pre-treatment option number one.
  • Fig. 4C illustrates an alternative post-silicon production treatment operation that can be conducted separately (as shown in Fig. 3) or incorporated for example at the JHQ-2 step in the preferred process.
  • Fig. 5 illustrates the chemistry involved in cracking methane to produce carbon for reducing silicon dioxide.
  • Fig. 6 illustrates the chemistry involved in the formation of silicon carbide in the JHQ- 1 plasma furnace.
  • Fig. 7 illustrates impurity elimination in the JHQ-1 plasma furnace.
  • Fig. 8 illustrates the milling and posttreatment processes of the silicon carbide output from the JHQ- 1 plasma furnace before input into the second JHQ-2 plasma furnace in the preferred process.
  • Fig. 9 illustrates the milling step and then the silicon production in the JHQ-2 plasma furnace step.
  • Fig. 10 illustrates modification of the solar silicon process to produce pure silicon carbide, high purity graphite or hollow silica microspheres.
  • Fig. 11A illustrates the process of producing hollow silicon spheres from silica coated silicon carbides (or silicon).
  • Fig. 11B illustrates the relationship of the viscosity of silica to the pressure created by the SiC-Si02 reaction necessary to form the hollow sphere.
  • Fig. llC illustrates the relationship of the viscosity of silica to the pressure created by the Si-Si02 reaction necessary to form the hollow sphere.
  • FIG. 3 is a diagram of an example process for producing solar grade silicon(s-Si). It begins with the input of silica ore and ends with the production of solar grade silicon. It also discloses the production of electrical energy as a byproduct of the process.
  • the example process specifies particular weights or temperatures or other characteristics of the inputs and outputs of the various stages of the process. These are the amounts necessary to produce the given outputs at 100% efficiency unless otherwise indicated. The amounts actually necessary will vary according to the efficiency at any particular step.
  • Phosphorus content in ores suitable for producing s-Si ranges from 1 to 5 ppmw with the preferred process.
  • Ores with less than 0.05 ppmw B, and P from 0.5 to 5 ppmw are available, but Ca and Al contents are high; Al 700 to 1500 ppmw, and Ca 30 to 70 ppmw.
  • These ores should cost between US$0.02 to US$0.10 per kg.
  • silica waste product from a mining operation in North Carolina has a B content of ⁇ 0.05, P at 1.0, Ca at 75, Al at 14, and Fe at 2.1 with all numbers in ppmw.
  • the waste product is in powder form and can be purchased at US$0.50 per kg.
  • the first step 300 in the preferred process removes heavy
  • these impurities exist in the powdered silica in the form of separate mineral granules.
  • a shaker table, magnetic filter or electrostatic separators or any combination of the three methods may be used to remove the separate impurity minerals.
  • This initial treatment is the first purification operation in the preferred process, and opens up opportunity to use of low cost ore, ore that otherwise would be
  • the output from our pretreatment step 300 is then charged to a heat exchanger 310.
  • Methane gas and recycled silicon from UDS (350) and crusher (314) are also input to the heat exchanger.
  • the methane is cracked at temperatures below 900°C, specifically at temperatures between 800 to 900 °C.
  • the cracking process deposits carbon on the grains of silica. The process is illustrated and described in Fig. 5.
  • the methane decomposes into carbon and hydrogen, with the hydrogen and excess methane being expelled and carbon being deposited on the silicon and silicon dioxide particles.
  • the carbon produced in this process is significantly purer than the coke and woodchips used in the silicon submerged arc furnace.
  • This operation represents the second refining operation, namely the use of a high purity reductant.
  • the carbon is porous with a high surface area and is highly reactive.
  • the carbon deposited on the silica and recycled silicon serves two purposes:
  • the hot output gas from 312 is mixed with gases entering the combustion chamber 372, the details of which are described in connection with Fig. 3.
  • the primary output of the heat exchanger in this process step is both silicon dioxide and a small mass of silicon coated with carbon together with any impurities contained in them. These are charged while still at temperature to the first quantum furnace, JHQ-1 320. Iron or iron oxide (or copper or cuprous oxide), are also charged to the furnace 320 as getters. Details as to the physico-chemical processes occurring in JHQ-1 are presented in Figs. 6 and 7. Impurities in the silica and recycled silicon are volatilized as the solid reactants are gasified (SiO, CO and CO2) and porous carbon is converted to silicon carbide. The impurity elements preferentially dissolve in the molten getter where bonding and physical stress issues are minimal compared to the conditions the impurity elements experience in silicon carbide. After a period of time in the plasma the silicon carbide and getter fall through into a cyclone separator that is charged with a nitrogen gas to prevent unwanted oxygen or other contaminations from the
  • the output from the cyclone separator constitutes carbon monoxide, nitrogen gas at substantial temperatures (1300°C plus) which are then charged to a combustion chamber 372 which is used to operate a gas turbine generator 370 to produce electrical power.
  • JHQ-3 340
  • argon is used to both create the plasma and purge the cyclone separator.
  • Nitrogen is used where practical to reduce cost, but can be substituted with argon if necessary.
  • the particles must be heated to 2300°C or higher for complete conversion of reactants to silicon carbide. Higher temperatures can be tolerated in producing the desired product, silicon carbide, which is formed as particulate as the gas cools leaving the plasma.
  • silicon carbide and gas are separated using the cyclone separator and the nitrogen purge gas to prevent any back reactions.
  • the furnace is adjustable in terms of plasma shape, size and rotational velocity so that particle temperature can be controlled.
  • the carbon reacts directly with silicon and silicon dioxide where they contact. These reactions tend to be limited by the extent of contact, they tend to trap impurities, initially in the silica and then later in the carbide.
  • the carbon/silicon dioxide reaction cannot be stopped at temperatures above 1521°C, at ambient pressure.
  • silicon dioxide reacts with carbon to produce silicon carbide (SiC) and carbon monoxide (CO(g)). Also, any silicon present reacts with carbon to produce SiC.
  • SiC silicon carbide
  • CO(g) carbon monoxide
  • any silicon present reacts with carbon to produce SiC.
  • SiO(g) silicon monoxide
  • the reaction is limited by the extent of contact between the reactants.
  • concentration of the impurities in the silicon increases with the reaction's consumption of both the silica and the molten silicon in production of SiO(g). As the concentration of the impurities in the silicon increases, more of the impurities are volatilized. With all the molten silicon consumed by the reaction, all the impurities are
  • CO(g) also reacts with silicon dioxide to produce gaseous SiO(g) and carbon dioxide, C02(g). Carbon dioxide reacts with carbon producing more CO(g) for further reaction with silicon dioxide.
  • the SiO(g) gas also reacts with carbon producing SiC and more CO(g), although this reaction likely occurs at cooler portions of the plasma around 2100°C.
  • the optimum temperature range for the production of silicon monoxide gas through reaction of CO(g) with S1O2 is 2300°C to 2500°C.
  • Aluminum and calcium are typically found in silica ore. They are typically present as oxides and silicates in the silica. With the consumption of the S1O2 by the reactions identified above, the aluminum and calcium remain behind as their oxides; AI2O3 and CaO. These oxides are very stable, but not at the elevated temperatures in the plasma and in the presence of carbon and silicon carbide. The oxides decompose, aluminum and calcium are volatilized and their vapors dissolve in the getter.
  • Impurity elements (X) originally locked in the silica are chiefly released to the vapor phase as silica is converted to silicon carbide.
  • the impurity element in its vapor phase, dissolves in the molten getter, which is iron or copper (other metals that satisfy the requirements presented below can serve as getter), whichever is the input to the furnace or present in the ore.
  • the resulting reduction of the concentration of impurity elements in the vapor phase leads to the volatilization of the residual impurity elements in the carbide phase.
  • Those impurity elements are preferentially dissolved in the getter where bonding and stress issues are minimal compared to the physical conditions of the impurity elements in the silicon carbide.
  • the carbide Since silicon carbide was formed from porous carbon, the carbide has a high surface area, improving the rate of volatilization of the impurity element. As illustrated, the getter either partially wets the silicon carbide surface or exists as a separate compound exiting from the furnace.
  • the impurities found in silica ores are volatilized, or enter the metallic alloy (the getter) in JHQ- 1, or are retained in the SiC upon carbide formation in JHQ- 1.
  • the volatilized impurity elements preferentially dissolve in a getter. Molten metals at elevated temperatures take impurity elements into solution, provided the metal reduces the activity of the impurity element.
  • the activity of an element in solution is equal to its concentration in the solution times an activity coefficient. At elevated temperatures the value of the activity
  • the SiC plus getter from JHQ-1 are separated from the gas phase that consists of CO(g) and some volatile impurities in the cyclone separator, 322. Nitrogen gas is slowly passed through the collected solids to provide a protective atmosphere to prevent any back reactions that either oxidize the SiC or promotes retention of impurities leaving the furnace in the gas phase. If iron is used as the getter, cooling of the product must be slowed to allow austenitic iron to transform into ferritic iron. That transformation, later, significantly improves getter removal with a magnetic filter.
  • the separation of getter from SiC, as presented in Fig. 8, is accomplished by rod and/or ball milling.
  • the difference in malleable characteristics of the getter and SiC make the separation possible; unlike SiC the metallic getter deforms during milling creating stress at the interface between the getter and the carbide that leads to fracture and separation of getter and carbide.
  • Iron and copper have been identified as possible getters as they meet the requirements presented in Fig. 7. Iron is known to partially wet SiC at the processing temperature, where as copper does not. In milling SiC with iron getter, some of the carbide will be retained with the getter when it is separated due to physical interlocking crystals of getter and carbide. Very little SiC will remain with a copper getter.
  • the getter and the impurities dissolved in it are removed from further processing as presented in Fig. 8.
  • the removal is accomplished by physical means using a magnetic filter, electrostatic separator, or with a shaker table (step 326).
  • the difference in density between SiC and copper is such that a shaker table can be employed, as copper will not respond to a magnetic field.
  • a magnetic filter must also be used to remove the abraded iron fines produce in the milling
  • SiC is a highly abrasive material that will wear away the rods and balls used in milling. It is essential that the balls and rods be made of ferritic iron that readily responds to a magnetic field.
  • An electrostatic separator can be used to separate metallic getters plus the abraded iron particles from SiC.
  • Multiple treatments in step 326 can be used to remove 99% of the getter, and a similar percentage of impurities dissolved in the getter. This is the third refining operation.
  • the impurities, in the small percentage of the getter remaining with the SiC, will ultimately be removed from the silicon produced in step 330 through unidirectional solidification in step 350.
  • the next step is to convert SiC to Si.
  • pure silicon dioxide from a subsequent step (the JHQ-2 step 330) is charged to a ball mill with the silicon carbide from the posttreatment step 326.
  • the ball milling produces a mixture where SiC and S1O2 are in contact. That contact is essential to initiate reactions in step 330.
  • Carbon is optionally added at this point as well to initiate reactions in 330.
  • the carbon is only necessary as an initiator, or aide, to maintain the plasma.
  • the carbon must be of high purity to avoid contaminating silicon produced in 330.
  • This mixture of silicon carbide, and silicon dioxide (plus carbon if needed) is charged to a second JHQ-2 plasma furnace at step 330.
  • the solids are dropped directly into the plasma where they dwell for a period of time while they are heated. During that heating desired reactions take place, not only producing Si but ionized gases that respond to the electromagnetic field in JHQ-2 and thereby
  • the plasma acts as the heat source for the endothermic reactions.
  • the reaction products fall from the plasma into a cyclone separator 332, which has as a separate input nitrogen (or argon) so that the chemical reactions that take place at the elevated temperatures are not interfered with by reverse reactions with the gas leaving the plasma.
  • Argon replaces nitrogen if the option to combine steps 330 and 340 is implemented. More information on that option is provided later.
  • the S1O2 and SiC react to produce gas rich in SiO, as presented in Fig. 9.
  • the SiO(g) reacts with SiC to produce molten Si and CO(g).
  • the SiO(g) produced immediately reacting with the carbide producing more molten silicon. That silicon is now in contact with both the carbide and S1O2.
  • the molten silicon reacts with S1O2 producing more SiO(g) that reacts with SiC to produce more silicon and CO(g). Again the reactants provide the gaseous environment that sustains the plasma. The vast majority of impurities entering JHQ-2 will leave the furnace dissolved in the silicon.
  • the gas produced in 330 is separated from silicon in the cyclone separator (332).
  • the gas is rich in SiO and CO. Air is mixed with the gas in the combustion chamber (360) where SiO(g) is converted to S1O2, and CO(g) is oxidized to C02(g). Both reaction release heat that is transferred to the steps where electricity is produced (372 and 370). Before that heat is transferred the S1O2 produced in 360 is separated from the gas in 362. That silica is of high purity, and is recycled to step 328.
  • the silicon leaving the cyclone separator 332 may contain some particles of SiC, S1O2, and S13N5, plus some impurities as noted
  • the silicon particulate is smaller than 100 microns it must be stored with an inert atmosphere, as the silicon is pyrophoric. Production of larger particles of silicon is encouraged. Some exposure of the silicon particles to air, or air diluted with nitrogen or argon will form a thin S1O2 skin around the particles. This can be accomplished in the cyclone separator (332) as the silicon is cooled.
  • the silica skin plays an important role in eliminating any SiC remaining with the silicon in the final plasma furnace, 340.
  • SiC, S1O2, and S13N5 particles must either be eliminated or their size reduced to less than 5 microns in the silicon product as the particles interfere with wire sawing of ingots in the productions of wafers.
  • Silicon carbide particles due to their abrasive properties, are the most objectionable.
  • step 340 is the elimination of S1O2 and S13N4, and either partial or total elimination of SiC.
  • JHQ-3 silicon (with some dissolved impurities) in lump or granular form.
  • the lumps undergo further purification by unidirectional solidification, UDS (or continuous unidirectional solidification, CUDS) whereby impurities are largely pushed to the ends (top and tail) of the raw ingot, process 350.
  • the primary impurity removed in this operation is the residual portion of the getter remaining in the silicon produced in JHQ-2.
  • the silicon tops, tails and side wall crust from the ingot produced in UDS, together with the powdered form from JHQ-2, is recycled via crusher 314 back to the rotary heat
  • the process also produces energy that can be used to produce electrical energy via a gas turbine 370.
  • Gases from the various stages such as the heat exchanger stage 310 to the JHQ- 1 plasma furnace, the cyclone separator 362 which processes gases from JHQ-2, are input to the combustion chamber 372 where they produce substantial heat that is turned into steam to power turbine 370.
  • cyclone separators with JHQ-1 (320), and JHQ-2 (330) in the preferred process presented in Fig. 3 can be replaced with counter current feed streams with solids entering the plasma furnaces through the gas exiting the furnaces. This is possible as the gas in both furnaces are generated in the plasma, and a getter is used to capture impurities in JHQ-1.
  • the primary difficulty associated with this approach is loss of solid feed through elutriation. Elutriation can be eliminated or minimized through control of the mass feed rate that produces the exit gas, and through multiple feed tubes thereby
  • step 4 Eliminating steps 332, 360, 362, and 328 by allowing the SiO gas to condense as S1O2 and Si on SiC entering JHQ-2. This approach also reduces the energy consumption in JHQ-2, as the reaction is highly exothermic.
  • the process can also be used to produce silicon carbide in excess of 5N purity, pure graphite in excess of 6N purity, and hollow silica microspheres.
  • the materials are processed from step 300 (optional or processing) through step 326 (post treatment of silicon carbide and alloys) as described above.
  • getter is expanded since removal of the residual getter in the carbide, that enters the silicon, no longer needs to be removed by unidirectional solidification, step 350 in Fig. 3.
  • getters are tin, nickel, and cobalt. Tin, like copper, does not wet SiC, and has
  • elements can be separated from SiC using an electrostatic separator.
  • silicon carbide is present, with dissolved impurities although there remains some silicon nitride and silicon dioxide.
  • excess carbon be charged to the plasma furnace.
  • Silicon nitride as noted previously decomposes at temperatures above 1830°C and ambient pressure. Furthermore, it is difficult to break the triple bond in N2, and thus little, if any, S13N4 is expected to form. If the nitride formation becomes a problem, argon can be used to shield the solid product leaving JHQ- 1 instead of nitrogen.
  • the 3N to 4N SiC leaving JHQ-1 can be used to produce 5N to 6N (or higher) pure SiC, high purity graphite, or hollow silica microspheres, HSMS Increasing the purity of the SiC leaving the post- treatment (326) is accomplished by passing the carbide through an additional plasma furnace 1000.
  • the degree to which impurities are removed is dependent on the temperature to which the silicon carbide is heated to, the residence time in the plasma, the concentration of impurities in the carbide, and the mass ratio of purified argon to mass of SiC charged to JHQ-4 (1000).
  • the mass of impurity in the SiC is partitioned between carbide and the gas phase.
  • Increasing the ratio of mass of argon to mass of SiC further reduces the final impurity content in the carbide.
  • the carbide and gas should be separated at an elevated temperature in the cyclone separator (1002), or additional argon can be added to the gas leaving 1000 to achieve the same result.
  • Kinetic issues such as SiC particle size and surface area plus residence time in the plasma impact the rate of volatilization of impurities.
  • High purity graphite is produced by charging the particle output from the post treatment step 326 as input to JHQ-5 furnace 1020 with an argon plasma and an argon cyclone separator.
  • the furnace is operated at temperatures of 4200°C to volatilize the silicon and
  • Hollow silica micro-spheres are optionally formed by process steps 1030-1036, and in further reference to Fig. 11A-11B and Table II.
  • Silicon carbide particles from 326 are input to the sizing step 1030 where particles are separated by size having diameters of between 0.04 to 200 microns.
  • the particles of a specific size are then charged to a rotary kiln 1032 to produce an oxide layer on the exterior surface of the carbide particles. Sizing ensures a uniform final product. An oxidizing
  • atmosphere air, O2, CO2, or a mixture of any of the gases with an inert
  • SiC and oxidizing gas is heated to a temperature of 1400°C or lower.
  • An alternative approach to coating the carbide particle with a silica layer is to coat the SiC particle with fine silica powder (possibly that from operation 362 in Fig. 3) with either a silicone or hydrocarbon adhesive.
  • This approach while technically easier than using the rotary kiln, has less control as to the mass of silica coating the carbide particle.
  • the mass of silica coating a SiC particle must, on a molar basis, be more than twice that of the carbide.
  • the coated particles are input to JHQ-6 plasma furnace 1034 using air (or an inert gas). Upon heating the particulate to
  • the silica layer forms an impervious layer isolating the SiC particle from further contact with the surrounding gas.
  • the silicon carbide is reacted with the silicon dioxide producing SiO(g) and CO(g).
  • the pressure of the gas in equilibrium with the solid reactants, rises to 1 atmosphere at a temperature of approximately 1816°C.
  • the viscosity of the fused silica is about an order of magnitude lower than the softening point viscosity as shown in Fig. 11B.
  • the reaction between S1O2 and SiC tries to produce a gas with a pressure greater than 1 atmosphere. Initially at temperatures only slightly above
  • the fused silica will resist the increase in pressure, but will begin to expand slowly to return the internal gas pressure to 1 atmosphere. If the silica shell ruptures the oxidizing atmosphere will form another S1O2 skin on the surface of SiC. That reaction is highly exothermic, raising the temperature of the new silica skin and reducing its viscosity. The newly formed shell expands healing the original rupture.
  • An alternative approach to avoid ruptures is to operate the plasma furnace at pressures greater than 1 atmosphere. (Example: Operating the furnace at a total pressure of 3.5 atmospheres the internal pressure created by the reaction between S1O2 and SiC equals the external pressure at 1957°C. At that temperature the viscosity of the silica is two orders of magnitude lower than that at the softening point.)
  • the softened silica shell expands until all the SiC is consumed by the reaction as shown in Fig. 11 A.
  • Silicon particulate formed in JHQ-2 (330) can be substituted for SiC entering the sizing operation (1030) in Fig. 10.
  • the silicon particles are coated by either means used in coating SiC.
  • the coating on a molar basis of silica to silicon must be greater than 1.
  • the resulting particle of Si coated in S1O2 is charges to the plasma furnace 1034.
  • the reaction between Si and S1O2 produces SiO(g).
  • the physical processes for the growth of the hollow silica sphere are the same as those described for the SiC-Si02 combination.
  • the temperatures involved are slightly different as presented in Fig. 11C.
  • the hollow silicon dioxide spheres must be rapidly quenched to retain their size.
  • the decrease in temperature decreases the pressure of the gas inside the hollow sphere, but that decrease in temperature also increases the viscosity of the silica shell. Some reduction in size is expected.
  • the wall thickness of the sphere will increase. The overall shape will remain spherical, as the surface energy is minimized with the spherical shape.
  • the gas inside the hollow spheres on cooling will undergo the reverse of reactions El and E2, decreasing the total pressure inside the hollow sphere.
  • the reduction in pressure (below 1 atmosphere) will occur at temperatures where the viscosity of the silica is high enough to prevent any further reduction in the size of the sphere.
  • the gas pressure inside the spheres formed with the Si-Si02 system will go toward zero upon cooling, as the reverse of reaction E2 is known to readily occur at temperatures below 1427°C. That loss of the gas phase increases the resistance of the hollow sphere to thermal heat transfer, a property valued by consumers.
  • the degree that the gas pressure declines in spheres formed with the SiC-Si02 system is uncertain.
  • Table II sets forth the correspondence of the size of the silicon carbide particles input to the process with the size of the resulting hollow spheres.
  • the values in the table represent just a few examples of the size of hollow spheres that can be formed with SiC coated with S1O2. See, Fig. 11A that illustrates the overall process and what the Rl, R2, R3 and R3 in the table measure.
  • Rl is the radius of the silicon carbide core of the input particle
  • R2 is the radius of the silicon dioxide coating of the input particle
  • R3 is the radius of the inner wall of the silicon dioxide wall of the hollow sphere
  • R4 is the radius of the outer surface of the silicon dioxide wall of the hollow sphere produced by the present process.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geology (AREA)
  • Silicon Compounds (AREA)

Abstract

Un procédé de production de silicium de qualité solaire à partir d'une impureté contenant de la silice utilise une pluralité de fours à plasma pour effectuer une succession de réactions chimiques conjointement avec d'autres étapes de traitement pour produire du silicium de qualité solaire. Le four à plasma génère un air sale stable, un plasma en forme d'anneau dans lequel une matière particulaire peut être introduite. Le plasma dans les deux premiers étages est formé par des gaz provenant des réactions chimiques et dans le troisième étage par de gaz inertes. Des séparateurs à cyclone sont utilisés pour extraire des particules du plasma dans un gaz inerte qui empêche des réactions inverses comme le refroidissement particulaire.
EP17801231.6A 2017-01-05 2017-11-07 Procédé de raffinage pour la production de silicium solaire, de carbure de silicium, de graphite de grande pureté et de microsphères creuses de silice Pending EP3565782A1 (fr)

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US15/399,592 US11242252B2 (en) 2016-01-08 2017-01-05 Refining process for producing solar silicon, silicon carbide, high-purity graphite and hollow silica microspheres
PCT/US2017/060380 WO2018128708A1 (fr) 2017-01-05 2017-11-07 Procédé de raffinage pour la production de silicium solaire, de carbure de silicium, de graphite de grande pureté et de microsphères creuses de silice

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