Classifications

• • 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
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/0053—Details of the reactor
  • B01J19/0073—Sealings
• • 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
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/0053—Details of the reactor
  • B01J19/0066—Stirrers
• • 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
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/02—Apparatus characterised by being constructed of material selected for its chemically-resistant properties
• • 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
  • C01B33/027—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material
  • C01B33/033—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material by reduction of silicon halides or halosilanes with a metal or a metallic alloy as the only reducing agents
• • 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
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/02—Apparatus characterised by their chemically-resistant properties
  • B01J2219/0204—Apparatus characterised by their chemically-resistant properties comprising coatings on the surfaces in direct contact with the reactive components
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/141—Feedstock

Definitions

• Silicon is,, at present, the most important material in modern semiconductor technology and is finding increased use in solar cells for the photovoltaic generation of electricity.
• the process and apparatus is described primarily in the context of production of silicon for solar cell use. However, it is
• the silicon fluoride is prepared from an aqueous solution of fluosilicic acid, a low cost waste by-product of the phosphate fertilizer industry by treatment with a metal fluoride which precipitates the corresponding fluosilicate.
• This salt is filtered, washed, dried and thermally decomposed to produce the corresponding silicon tetrafluoride and metal fluoride which can be recycled to the precipitation step.
• the silicon tetrafluoride is then reduced by a suitable reducing metal and the products of reactions are treated to extract the silicon.
• Each of the steps is described in detail using sodium as typical reducing agent, and sodium fluoride as typical precipitating fluoride but the concept applies as well to other reducingmetals and metal fluorides that can reduce silicon fluoride and form fluosilicates.
• N-a 2 SiF 6 +4Na Si+6NaF or this can be expressed:
• reaction product After the reaction product has been cooled at least to 200 degrees C it is finely divided and is treated with water or heat treated with dilute 1:1 sulfuric acid. Hydrogen fluoride gas is liberated (which latter can then be made into hydrofluoric acid or a metallic fluoride) metallic sulphates are produced and the silicon separates out on the surface in amorphous form as shining metallic froth.
• the silicon obtained in this way is in the form of an impalpable redish or grey-brown powder which discolors strongly and which, even if the raw products were impure, contains a minimum of 96-97% silicon.
• the yield amounts to about 87% of the theoretically possible yield.
• sodium fluosilicate Na 2 SiF 6 is precipitated from fluosilicic acid followed by thermal decomposition of the fluosilicate to silicon tetrafluoride SiF 4 .
• the SiF 4 is then reduced by an alkali metal, preferably Na, to obtain silicon which is separated from the mix, preferably by leach separation.
• an alkali metal preferably Na
• Purity of reaction products are maintained through isolation from reaction container walls by non-contaminating salts, such as NaF, CaF 2 , BaF 2 , MgF 2 , et al, but preferably NaF, so that the reaction products do not adhere to the walls and are easily removed from the container without the use of contaminating tools.
• the invention has for its principal object the provision of a process for obtaining silicon of sufficient purity to
• a further object of this invention is to provide a process by means of which silicon can be obtained which is substan tially free of impurities starting with relatively inexpen sive and impure fluosilicic acid.
• a still further object of this invention is to provide a high purity silicon by isolation of silicon producing reaction products from contaminating reaction chamber walls in such a way that the reaction product does not adhere to the walls and is easily removed.
• Figure 1 is a flow diagram illustrating a preferred embodiment of the process for producing high purity silicon by the leach process
• Figure 2 is a graph illustrating the time, temperature and pressure characteristics of the silicon fluoride and sodium reaction showing time in minutes plotted along the axis of abscissae and temperature in degrees C and pressure (torr) plotted along the axis of ordinates;
• Figure 3 is a somewhat diagramatic central vertical section through a solid sodium dispenser and reactor unit showing reaction products and a reactor liner according to the present invention
• Figures 4 and 5 are central vertical sections through broken away portions of reactors showing other reactor and liner configurations according to the present invention.
• Figure 6 is a diagrammatic central vertical section through a drip feed liquid sodium dispenser and reactor showing a reactor liner suitable for such a unit.
• a preferred embodiment of the process for production of pure silicon starting with inexpensive commercial grade fluosilicic acid is illustrated in the flow diagram of Figure 1.
• the overall process consists of three major operations which encompass a series of steps.
• the first major operation includes the step of precipitation of sodium fluosilicate from fluosilicic acid followed by generation of silicon tetrafluoride gas.
• the second major operation comprises the reduction of silicon tetrafluoride to silicon, preferably by sodium
• the third operation (brackets 14) involves the separation of silicon from the mixture of silicon and sodium fluoride.
• the preferred starting source of silicon is an aqueous solution of fluosilicic acid (H 2 SiF 6 ), a waste product of the phosphate fertilizer industry, that is inexpensive and available in large quantities.
• Fluosilicic acid of commercial grade [23 weight percent (w%)] has also been used directly as received without purification or special treatment and is shown as the silicon source 16 in Figure 1.
• fluosilicic acid is obtained by treating silica, or silicates (natural or artificially made) with hydrogen fluoride. The SiF 6 ion is then precipitated in sodium fluosilicate Na 2 SiF 6 , by adding a sodium salt to the solution (step 18).
• Other salts such as NaF, NaOH, NaCl, or similar salts of the elements in groups IA and IIA of the periodic table are all candidates.
• the major selection criteria are, low solubility of the corresponding fluosilicate, high solubility of impurities in the supernatant solution, high solubility of the precipitating fluoride salt, and non-hydroscopic character of the fluosilicate.
• the preferred fluosilicates in order of preference are Ba 2 SiF 6 , K 2 SiF 6 and BaSiF 6 .
• the hydrogen of the fluosilicic acid is displaced by the sodium to form sodium fluosilicate, a highly stable, nonhydroscopic, white powder, and sodium fluoride which is recycled.
• equation form the reaction is
• Sodium fluosilicate was precipitated by adding solid sodium fluoride directly to the as received commercial grade fluosilicic acid 18.
• the yield was a supernatant liquid containing mostly HF and some NaF and H 2 SiF 6 along with the sodium fluosilicate. HF is also given off (20).
• the supernatant fluid was removed and the sodium fluosilicate washed with cold distilled water to remove any remaining HF and H 2 SiF 6 . After filtering and drying in an oven at 200 degrees C, a minimum yield of 92% of pure sodium fluosilicate 22 (determined by x-ray diffraction) was obtained.
• the product sodium fluosilicate is a nonhygroscopic white powder that is very stable at room temperature and thus provides an excellent means for storing the silicon source before it is decomposed to silicon tetrafluoride.
• Precipitation under the just described conditions acts as a purification step, with most impurities in the original fluosilicic acid staying in solution. This effect is increased by adding suitable complexing agents to the fluosilicic acid solution previous to the precipitation.
• suitable complexing agents such as ammonia and organic agents such as EDTA (ethylenediaminetetraacetic acid) help to keep transition metal ions in solution during precipita tion of the fluosilicate.
• the fluosilicate is thermally decomposed 24, thus,
• SiF 4 gas prepared in this manner was determined by mass spectrometric analysis to be more pure than commercial grade SiF 4 , as shown in Table I. Ions formed from the sample gas were identified from the observed mass numbers, isotopic distribution and threshold appearance potentials. The detection limit was better than 0.005% . Positively identified gaseous impurities are listed in Table I; no metallic impurities were detected. Peaks corresponding to B compounds, such as BF 3 , were specially checked, but none were found.
• Mi, Co, K, and Cu were unchanged by precipitation of Na 2 SiF 6 whereas the elements Mg, Ca, Al, P, As, and Mo were diminished by a factor of 5-10. Some elements were concentrated into the Na 2 SiF 6 , namely Cr, Fe, and Ni.
• the fourth column in Table II is representative of the impurity content to be found in SiF 4 gas prepared on a commercial scale. The low content of P is of special significance for both semiconductor and solar cell applications. Elements known to reduce solar cell efficiency (V, Cr, Fe, Mo) are uniformly low in commercial grade SiF 4 . Only Mi, As, and Al are of comparable concentration in both Na 2 SiF 6 and SiF 4 at the 1 ppm or less level.
• SiF 4 /Na reaction the central operation of the pure Si process, (Fig. 1 ) is the reduction of SiF 4 by Na according to the reaction SiF 4 (g) + 4Na(l) - Si(s) + 4NaF(s)
• the kinetic behavior of the Na-SiP 4 reaction is complex because of the interplay of several factors, e.g., pressure of SiF 4 , vaporization of Na, local temperature, porosity of two solid products, and transport of SiF 4 and Na vapor through the product crust that forms on the liquid Na.
• reaction temperature 5 grams of Na were loaded in a Nl crucible (3 cm ID, 4 cm high) and heated in SiF 4 initially at 1 aim pressure. The Na surface tarnished at around 130 degrees C, with the formation of a thin brown film. As the temperature increased, the color of the surface film gradually changed from light brown to brown and finally to almost black.
• SiF 4 Na reaction became rapid at 160 degrees +/- 10 degrees C and liberated a large amount of heat, as indicated by a sudden rise in reaction temperature.
• the pressure in the reactor typically decreased slightly until the temperature increased sharply, with an associated rapid decrease in SiF 4 pressure.
• the reaction lasts for several seconds only.
• SiF 4 pressures below 0-3 aim the reaction mass was observed to glow at a dull red heat.
• a characteristic flame was observed.
• the shortest reaction time (20 sec) and the highest temperatures (about 1400 degrees C) were obtained when the initial pressure of SiF 4 was around 1 aim. In addition, complete consumption of Na was obtained for 1 aim SiF 4 .
• the silicon and sodium are removed (step 30) and combined with water (32) and a selected acid.
• the resultant silicon (34) and sodium fluoride (36) are then separated.
• the leaching and separation process is described in detail below in connection with the scaled up system.
• the reactor system is shown somewhat schematically in Fig. 3.
• the upper section 40 of the system illustrated constitutes a sodium dispenser and the lower section 42 is the reactor section where the reaction takes place between the Na and the SiF 4 .
• the sodium dispenser section 40 is a Pyrex glass vessel or hopper which is coated internally with epoxy resin on all glass surfaces that may contact Na.
• the sodium dispensing section 40 has a vertical cylindrical entry section 44 for receiving and holding the Na used in the process.
• the entry section 44 has its lower end open to, and sealed 46 to, one end (left in the drawing) of a horizontal cylindrical reactor feeding section 48.
• the upper end of the entry or storage section 44 is provided with a removably sealed end bell 50 which has a closable entry port 52 to receive Si 4 for the reaction and an agitator rod 54 which extends into the Na receiving entry chamber 44 for agitating its contents and forcing Na into the lower reactor feeding section 48.
• the reactor feeding section 48 is permanently sealed at the end adjacent the port 46 frcm the entry section 44 and has a sealing end bell 56 which accommodates a horizontal rod-like Na transferring "hoe" mechanism 58 or screw feeder (not shown).
• the upper wall has a closable entry port 60 to allow gas flow and the lower wall is open to a reactor communicating cylindrical port section 62.
• Sodium chips 63 were prepared by feeding 225g blocks of sodium (6 cm diameter rod, cut longitudinally) into a modified food processor with rotating cutting blades. The chips were fed into the entry section 44 of the Na dispenser 40 using a blanket of argon to minimize contact with atmospheric oxygen and moisture. While Na chips 63 were being introduced into the top of the storage chamber 44 of the dispenser 40 (2kg capacity), dry argon was introduced into the reactor feeding section 48 and flowed up through the chamber 44. The Na chips 63 were transfered from the storage chamber 44 to the reactor 42 by means of a horizontal "hoe" mechanism 58 or screw feeder (not shown). Downward flow of Na chips 63 in the storage chamber 44 was aided by agitation of the vertical rod 54. Notice that the throat-like cylindrical port section 62 of the Na dispenser 40 is surrounded by water cooling coils 64 effectively to keep the temperature in the sodium dispenser 40 so low that reaction does not take place between the Na and SiF 4 until they are in the reactor.
• the reduction reaction takes place in the lower reactor section 42 of the reactor system.
• the reaction chamber or vessel 66 of the reactor system is made of stainless steel or Inconel (20cm diameter and 90 cm high) and is fitted with a sheet nickel liner 68 (18cm diameter and 60cm high) and an inner liner of sheet Grafoil 70 (18cm diameter and 90cm high).
• the outside of the reactor chamber or vessel 66 is wrapped with four sets of heavy duty electrical heating tapes 72 (rated for use to 800 degrees C).
• the heating coils 72 are covered with Ka ⁇ wool insulation 74 (1.3 cm thick).
• reaction temperature When the reaction temperature is higjier than the melting point of NaF (988 degrees C), a clean phase segregation occurs, thus facilitating the extraction of Si.
• metal fluorides such as CaF 2 , BaF 2 , MgF 2 , et al. SiC, graphite and other noncontaminating ceramics are also candidates.
• any material selected from a powder of at least one fluoride or silicate of alkali or alkaline earth metals or an oxide of silicon or a powder reaction products of this reaction will perform the function.
• the system was evacuated, then filled with SiF 4 gas to a pressure of about 1 atm. Reaction was initiated as soon as Na chips were dropped to the bottom of the reactor, which was preheated to 400 degrees C. Reaction was sustained by manually adding Na chips 63 at a rate sufficient to maintain a given SiF 4 flow rate, as indicated by an electronic flowmeter (not shown). The maximum flow rate used was 380 liters SiF 4 /hr and a production rate of 0.5kg Si/hr.
• the temperature of the reactor walls in the region of the reaction products 78 rose to 600 degrees - 650 degrees C, as indicated by external thermocouples (not shown). Without a powder liner 76, the temperature of the nickel liner reached the melting temperature of NaF (998 degrees C), indicated by molten NaF observed on the outside of the nickel liner near seams as the reaction zone progressed upward.
• reaction products 78 completely occupied the cylindrical space inside the Ni Grafoil liner 68-70. Without the powder liner 76, the reaction products 78 are difficult to separate, however, they were pulverized with plastic equipment and routine checks were made for the presence of unreacted Na by acid titration.
• An important parameter regulating the rate and extent of the reaction is the surface-to-volume ratio of the Na feed. For the 18 cm diam Inconel reactor, no unreacted Na was observed in the reaction products even for the highest Na addition rate used (1.4kg/hr) when the Na chips had a surface to volume ratio of about 20 cm .
• the relative amounts of Si,NaF, and Na 2 SiF 6 were determined by x-ray diffraction using standard mixtures.
• the weight fraction of Na 2 SiF 6 was determined from the ratio of the peak intensities of Na 2 SiF 6 and KCl reference additive. The method is rapid and accurate to about +-5%.
• the presence of Na 2 SiF 6 was also cross-checked by thermo- gravimetry. The presence of Na 2 SiF 6 in the reaction product mixture is an indication of possible side reaction according to
• Figures 4, 5 and 6 show alternate structures for the reactor 42 of the system.
• the reactor structures all have common elements which, for simplicity of description and drawings, are given the same reference numerals in all Figures. That is, the reactor 42 of Figure 4 has all of the same elements as that of Figure 3 except it does not utilize a Grafoil liner (70 Fig. 3) between the powder liner 76 and the Ni (Inconel) lining 68.
• the use of the powder liner 76 simplifies the reactor construction considerably and the reactor 42 of Figure 5 is further simplified in that it utilizes the powder liner 76 adjacent the single walled reactor container 66 which is preferably of stainless steel but may be of materials such as Ni or high temperature
• the reactor 42 used with the liquid Na feed system of Figure 6 has the same elements as that of Figure 5 in that it has the outer cover of insulation 74, heater coils 72 between stainless steel reactor vessel 66 (inside) and the outer insulation, a Ni liner 68 lining the reactor container 66 and a powder liner 76 at the bottom and part way up the inner wall.
• the reason that it is only necessary to use the powder liner 76 near the bottom in the liquid feed system shown is that initially the reaction takes place only at the bottom of the reactor vessel 66 and the reaction product 78 builds up in a "stalagmitic" manner so that it does not touch the reactor vessel walls except in a limited area at the container bottom.
• the heater coils 72 surround only a limited part of the bottom of the reactor vessel. This is for the reason that once the reduction reaction starts at the bottom, the exothermic reaction supplies sufficient heat to keep the reaction going as the reaction product 78 builds up from the bottom.
• the system of Figure 6 is a liquid Na feed.
• the liquid Na feed head includes an end bell 80 that has a flange 77 secured and sealed to the upper flange 75 on the stainless steel reactor body 66 just as the solid Na delivery head of Figure 3.
• a valved Na feed tube 84 is centrally (and vertically) positioned in the top of the end bell 80 to introduce individual droplets 86 of Na which can drop directly into the reactor section 42.
• a valved SiF 4 feed tube 82 is positioned to deliver SiF 4 into the top of the end bell 80 so that it flows down around the Na droplets 86 into the reaction chamber.
• the final operation 14 in the flow diagram of Figure 1 is the separation of Si from the reaction products 78.
• the reaction product obtained by the SiF 4 -Na reaction is a porous, brown mass. This intimate mixture of NaF and Si is readily separated by aqueous leaching.
• the Si product obtained after leaching is a brown crystalline powder with particle sizes ranging from submicrometers up to 150 urn.
• Leaching is performed using 1.ON HCl in a polypropylene container although other acids such as H 2 SO 4 , CH 3 COOH, and HF are equally effective.
• the acid normality can vary in the range 0.1 - 1.ON without affecting the leaching process, which may be monitored by measuring the F- and Na+ concentrations in the leachant using ion selective electrodes. When the F- concentration is about 10 -5 mole/liter, leaching is stopped.
• the acidification of the leach solution is a precautionary measure to prevent increase in local pH due to reaction of H 2 O with Na, which could, in turn, result in Si loss by oxidation according to the reaction. Si + 2H 2 O SiO 2 + 2H 2
• Si can be oxidized at an initial rate of 15 weight percent per hour.
• the rate of oxidation increases with increased F- ion concentration in solutions with pH in the range of -0.8 to 10.
• the contact time may be minimized by using forced filtration, which yields a 98% complete recovery of Si.
• Differences in leaching rate due to particle size of the products, temperature of the leaching bath, and amount of stirring were found to be important only during the first minute of leaching.
• Fig.1 The process sequence shown in Fig.1 was selected because of the inherent simplicity of the steps and their independent and combined suitability for scale-up.
• Some purification occurs during precipitation (operation 1, Fig.1 ) for Mg, Ca, Al, P, and As due to the high solubility of their fluosilicates and fluosalts. Seme concentration takes place for Cr, Fe, and Ni, and this effect may be due to coprecipitation of these elements as fluorides since their fluosilicates are very soluble.
• Frcm Table II it is clear that most of the purification is accomplished as a result of the thermal decomposition in step 24 (Fig.1). Most transition metal fluorides are in very stable condensed phases at the decomposition temperature (650 degrees C) in step 24 (Fig.
• the Na feed, reactor materials, and possible contamination of the product during handling remain as possible sources of impurities in the Si.
• the impurities in Na can be divided roughly into three types according to their tendency to react with SiF 4 , as classified by the free energy of reaction.
• the first type of impurity includes aluminum and elements from the groups IA, IIA and IIIB.
• the free energy of reaction of SiF 4 with these impurities ranges from -100 to -200kcal/mole SiF 4 at room temperature and frcm -50 to -100kcal/mole SiF 4 at 1500 K. It is expected, therefore, that even when these impurities are present at the ppm level, they will react with the SiF 4 to form corresponding fluorides. Subsequently, the fluorides will be dissolved preferentially in the NaF phase.
• the second type impurity includes transition metals such as Mo, W, Fe, Co, Ni, and Cu, and the elements P, As, and Sb. These elements exhibit positive free energies of reaction in excess of 100 kcal/mole SiF 4 and are not expected to react with SiF 4 .
• the silicon resulting from the SiF 4 -Na reaction contains amounts of Fe, Ni, and Cr in proportion to the concentration of these elements in the Na feed.
• the mechanism by which these metals are transferred to the silicon has not yet been studied.
• the concentration of Fe, Cr, Ni, and also Ti can be decreased by a factor of about 10 ⁇ -10 for single-pass directional solidification or the Czochralski crystal-pulling procedures used presently for solar cell manufacture. At the resulting levels, these elements would not be detrimental to solar cell performance.
• Boron represents a third type of impurity. The free energy of reaction of this element with SiF 4 is positive but small
• Ni liner served primarily as a mechanical retainer for the Grafoil sheet 70 and did not contact the solid mixed reaction product 78. Both Ni and Inconel are selected for use in the reactor 42 (Fig. 3) because of their stability in the presence of fluoride compounds.
• Contamination during handling the reaction product 76 was a most important source of impurity pick up. Airborne dust could have contacted the products either during the removal from the reactor or during sampling and the physical separation of the reaction product 78 from the reactor vessel 66 required the use of hard contaminating tools.
• the sodium feed also requires careful attention as a possible source of impurity pick up and although electronic grade acid and deionized water were used for leaching the NaF, the large volume of liquid used could have contributed to the accumulation of impurity in the silicon. It is noted that the purity of the silicon produced by the SiF 4 -Na reaction, even without the protective powder container 76, is at least nominally appropriate for solar cell manufacture.

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