The present invention concerns a process and an implant for the manufacturing of polycrystalline silicon utilizing metallurgical silicon as starting material. In particular, the invention relates to a technological process of polycrystalline silicon manufacture with high degree of purity permitting its use in photovoltaic solar panels production.
By now, several technological processes for manufacturing polycrystalline silicon are known.
A first category of processes requires hydrogen reduction of trichlorosilane SiHCl3 in a Siemens core-type reactor. For example, such process is described in the patents DE2447691, DE1148217, JP2005336045, JP2005008430, RU2224715C1, RU2136950, U.S. Pat. No. 4,525,334. Alternatively, such processes require growing granulated silicon in a boiling-bed reactor with hydrogen reduction of trichlorosilane, as disclosed in the patents CA1218218 and U.S. Pat. No. 5,798,137.
A second category of processes, illustrated for example in the patents DE102005044328A1, U.S. Pat. No. 6,395,248, U.S. Pat. No. 6,623,801B2, U.S. Pat. No. 5,382,419, requires thermal decomposition of monosilane in a Siemens core-type reactor, and further growing granulated polycrystalline silicon on the seeds surface in a boiling-bed reactor as disclosed in the patents JP2000178028, U.S. Pat. No. 4,314,525, U.S. Pat. No. 4,786,477, U.S. Pat. No. 4,784,840, U.S. Pat. No. 4,868,013, U.S. Pat. No. 4,992,245.
A third productive category consists in a purification method of melted silicon through liquid and gas treatment, as illustrated in patents JP2007084398, JP60103015, JP11011925, and further recovery methods of silicon from initial high-purity quartzite as discloses in the patent DE3128979F1.
A common feature of the majority of such known processes is the prediction of polycrystalline silicon manufacture from gaseous silicone compounds, for example by recovery methods or pyrolysis methods of silicious compounds pyrolytic in a core-type or boiling-bed reactor. Alternatively, such known processes foresee of refining of the starting silicious compounds, such as melted silicon treatment and high-purity quartzite recovery, avoiding formation of intermediate gaseous silicones formation.
Furthermore, very few multiple-stage technological processes are known, wherein both polycrystalline silicon and intermediate silicones used in its manufacture are produced within a continuous technological cycle from metallurgical silicon without the need of purchasing intermediate gaseous silicones from relevant manufacturers. For example it is known a process for the production of solar-grade polycrystalline silicon by means of silicon tetrafluoride SiF4 decomposition in the inductively coupled argon-containing plasma. Such process involves the synthesis of silicon tetrafluoride through fluorinated gas feeding into the boiling bed of silicon pellets.
According to the Japanese patent JP2000178018, the production of polycrystalline silicon is based on a process comprising the reaction of metallurgical silicon with alcohol yielding trialkoxysilane; the trialkoxysilane disproportionation yielding monosilane; the thermal decomposition of monosilane in a boiling-bed reactor resulting in deposition of granulated silicon. The process involves recycling of gaseous reaction by-products and separation of high-purity quartz as one of the by-products. Similar technology is illustrated in the patent JP 2000178028.
Likewise, the patent RU 2 078 304 discloses a technological process for producing polycrystalline silicon by means of converting silicon tetrafluoride SiF4 into dioxide and then into monoxide silicon which can be recovered with the help of hydrogen at high temperatures. In such case, silicon tetrafluoride is the result of silicofluoride Na2SiF6 thermal decomposition.
Instead, the U.S. Pat. No. 4,084,024 discloses a closed process for the production of polycrystalline silicon, wherein halogen-containing silicon compounds are first obtained through a reaction of metallurgical silicon with an halogen and hydrogen halide in a single cycle, following which the purified gaseous compound undergoes a thermal decomposition yielding high-purity polycrystalline silicon.
The patent RU2122971 discloses the production of polycrystalline silicon in a closed technological cycle involving trichlorosilane hydrogen reduction followed by hydrogen reduction and obtaining of polycrystalline silicon; the by-products of the gaseous mixture (SiCl4, H2, HCl) are separated and reused in producing trichlorosilane SiHCl3, from metallurgical silicon.
The technology described in the U.S. Pat. No. 6,712,908 requires sequential steps of reacting metallurgical silicon with iodine yielding silicon tetraiodide followed by separation thereof from gaseous reaction by-products and deposition of silicon diiodide on the walls of the reactor providing polycrystalline silicon as a result.
The patent DE3311650 discloses the production of polycrystalline silicon from chlorosilane obtained from metallurgical silicon by means of reacting the latter with silicon tetrachloride and hydrogen with recycling of by-products for their reuse in production.
However, the above mentioned known solutions for producing polycrystalline silicon have significant disadvantages. Particularly, high energy consumption and low yielding of useful reaction products and the need for purchasing high-quality raw materials (high-purity metallurgical silicon) or siliceous gases for start-up production, are reported.
Another relevant drawback of the known solutions is the high production of wastes, difficult to be disposed from both an economic and environmental point of view.
The scope of the present invention is to overcome the cited problems, with the provision of a process allowing to operate the production of polycrystalline silicon starting from metallurgical silicon in optimal mode, to be employed in particular in the production of photovoltaic solar panels production or analogues thereof.
In view of the aforementioned aim of the invention, it is a further object of the present invention to provide a process enabling the production of polycrystalline silicon with a reduced energy consumption.
A further scope of the present invention is to provide a process enabling the production of polycrystalline silicon with an high yield in comparison to inlet raw materials.
Another objective of the present invention is to provide a plant for the production of polycrystalline silicon according to the aforementioned process by a structure endowed with a greater structural and functional simplicity and reliable operating conditions.
According to the present invention the aforementioned objectives are achieved by the process of claim 1.
The details of the invention will result more apparent from the detailed description of a preferred embodiment of the process for the production of polycrystalline silicon starting from metallurgical silicon, illustrated in the enclosed FIGURE, wherein:
The process for the production of polycrystalline silicon starting from metallurgical silicon, milled up to a predetermined granulometry, including the steps of:
a. reacting metallurgical silicon with anhydrous hydrogen fluoride (HF), under the pressure of substantially 1.1 bar at a temperature ranging between 250-600° C., and preferably at 500° C., to obtain silicon tetrafluoride (SiF4);
b. synthesis of monosilane (SiH4) through a reaction of hydrogenation of silicon tetrafluoride (SiF4) with alkaline or alkaline earth metals halide in fluid medium of organic solvent or melt salts;
c. carrying out the thermal decomposition of the monosilane (SiH4) in a boiling-pseudo fluidized bed reactor, under a pressure of substantially 2 bar and at a temperature of substantially 650° C. to obtain high purity granulated polycrystalline silicon.
The process for the production of silicon tetrafluoride (SiF4) is carried out by reacting metallurgical silicon with anhydrous hydrogen fluoride HF:
Si+4HF→SiF4+2H2
Preferably under a pressure of 1.1 bar and at a temperature comprised between the range 250-600° C.; the more effective temperature is about 500° C. The reaction is carried out in a boiling bed of metallurgical silicon pellets of 1 to 1.5 mm size, under a pressure not higher than 2 bar. The development of fluorocarbon gaseous compounds, like SiHF3,SiH2F2, is significantly inhibited while carrying out the preceding reaction in maximum excess hydrogen fluoride from 0.1 to 1.1%. The silicon tetrafluoride SiF4 undergoes purification in a recoverable absorber of HF traces and then is condensed in a low-temperature condenser-evaporator. The proceeding reaction is exothermic in nature (heat of reaction is 524.23 kJ/mole); thus heat supply from external heaters for material flow heating is only needed at the beginning of the process. Later on, the reaction heat is sufficient enough to maintaining the temperature of the reaction during the whole process. For excess heat removal from the reaction, the reactor contains extended surface heat-exchange elements.
The process for the production of silicon tetrafluoride SiF4 comprises the following stages:
The monosilane synthesis from silicon tetrafluoride SiF4 is carried out in lithium and potassium chlorides eutectic melt medium:
SiF4+2CaH2→SiH4+2CaF2
The above reaction is carried out in a bubbling reactor in ternary salt mixture ionic melt containing calcium hydride partly in the form of suspension and partly dissolved. Maximum pressure in reactor is 2.5 bar. The process temperature is conditional, on one side, on the salt mixture eutectic point, and on the other, on the necessity of preventing instant thermal decomposition of the monosilane directly produced in the reactor. Based on these considerations, the optimum process temperature range seems to be from 360 to 380° C.
The worked-out molten salt containing calcium fluoride undergoes recycling during which calcium fluoride CaF2 is separated by means of filtration, and the salt mixture is returned into the process.
Monosilane is refined by using absorbing agent or filtered in order to remove mechanical particles after which it is compressed into a gas holder with the help of a diaphragm-type compressor.
Calcium fluoride in the form of feldspar is supplied to the manufacturer of HF to carry out the reaction:
CaF2+H2SO4=2HF+CaSO4
The process for the production of monosilane according to the present method comprises the following steps:
The process is accompanied by precipitation of insoluble calcium fluoride CaF2. To separate it from the LiCl+KCl melt, a settling of precipitate is carried out within the calculated time and then calcium fluoride CaF2 is removed with the help of one of the below methods:
CaF2 is used for the abovementioned purposes as derived y-product.
The process for the production of granulated polycrystalline by means of thermal decomposition of monosilane-containing reaction mixture in a boiling-bed reactor is carried out according to the reaction:
SiH4→Si+2H2
The process corresponding to the preceding reaction is carried out in a boiling bed of silicon pellets dispersed in a monosilane-hydrogenous mixture. Reactor shell is made of quartz; in order to avoid deposition of siliferous products on the heated walls, reactor heating is performed by means of infrared radiation. The optimum process temperature is 650° C.; pressure in the reactor is maintained at 2 bar. In order to inhibit development of silicon agglomerates in gaseous phase, the monosilane which is fed into the reactor is diluted with hydrogen. The hydrogen generated during the process is then purified, compressed up to 3 bar and delivered for reuse in the production process.
The process for the production of granulated polycrystalline foresees the following steps:
where dãδ is current granule diameter, in mm; MSi is molar weight of silicon, in g/mole; d0 is starting granule diameter, in mm; S is bed diameter, in mm; H is bed height, in mm; ε is silicon pellets bed porosity; wSiH
When the calculated process time is elapsed according to equation, or the calculated finishing diameter of granules is reached, the process ends.
This process can also be carried out in a continuous reactor where constant withdrawal of produced polycrystalline silicon pellets and of core seeds is carried out.
For realization of the technological process in accordance with the described method, silicon seeds should be prepared for granulated polycrystalline silicon deposition, as well as starting silicon for etching in the course of SiF4 production. For these purposes, two separate ball crushers are used.
The process for producing monosilane SiH4 from silicon tetrafluoride SiF4 is carried out according to the reaction:
SiF4+4NaH→SiH4+4NaF
The reaction is carried out in a bubbling reactor analogous to the one used in the preceding example. The reactive medium of the organic solvent may be tetrahydrofuran, diethylene glycol, or some ethers; it is preferred the use of zinc chloride. Other zinc-containing materials to be applied for catalysts are metallic zinc, zinc oxide, zinc alkylates with the general formula R2Zn, wherein R is hydrogen radical with the general formula CnH2n+1, as well as zinc hydride. It is preferable to use zinc catalyst in a finely ground form and usually it may be stirred in the course of reaction and introduced into the reaction vessel after ether and solid reagent.
In accordance with invention, in the course of the reaction process an automatic viscosity control of reaction medium in reactor is conducted and its value maintained constant by means of adding liquid organic solvent as viscosity increases.
Because of the exothermic nature of the reaction, temperature control is carried out by means of reaction vessel cooling through circulation of refrigerating medium which can be liquid nitrogen or recycle water. It is preferable to avoid boiling of the liquid reaction mixture because otherwise it will pass into cavitation regime deteriorating interphase gas-liquid contact. Even if the reaction proceeds at temperatures below 0° C., effective production rate can be achieved in short time. Therefore, when using tetrahydrofuran, it is preferable to carry out the process in a temperature range from 0 to 5° C. The more high-boiling ether is used the more is the advantage of the improved catalytic effect achieved at high temperatures, depending on the characteristics of the reaction medium. In any event, reaction temperatures in the range between 5 and 35° C. are the most effective. Such temperatures are preferable for convenience and simplicity of the operation.
The process is self-initiating and exothermic in nature. The amount of reagents used is at least stoichiometric estimating the required hydride amount on the basis of the defined degree of hydrogenation of silicon tetrafluoride. The amount of ether should be sufficient to keep the reaction mixture in liquid form. The amount of catalyst may be chosen from a broad range of values; nevertheless, the molar ratio catalyst:silicon tetraflouride is comprised the range from 1:10 to 15:1. More preferably the range is from 1:8 to 2:1, and in particular is 1:2. The contact of reagents takes place during mixing; it is advisable that sodium hydride milling should be carried out with an abrasive agent, such as sodium chloride added to the reaction mixture in order to obtain the effect on the metal hydride to obtain a live reaction surface.
The process for the production of monosilane according to the present reaction is carried out in a two-chamber bubbling reactor, similar to the one described previously. The technological steps carried out during monosilane production according to this process are:
The process is accompanied by separation of sodium fluoride NaF precipitate. To separate it from organic solvent, the mixture of organic solvent and sodium fluoride is transferred into a recycling installation where centrifugal separation of solid precipitate takes place followed by organic solvent evaporization. The evaporated organic compound then enters a heat exchange installation where it liquates for possible reuse in the process.
In the monosilane synthesis process through the reaction of silicon tetrafluoride with calcium hydride, the calcium hydride used in the process is produced by calcium metal direct hydrogenation.
The reaction is carried out in a pseudo fluidized reactor through interaction of granulated calcium particles having a diameter from 1.5 to 5 mm, preferably at a temperature of 500° C. and under a pressure of 2.5 bar.
In accordance with the invention, recycling of unreacted hydrogen at the outlet of the reactor is contemplated for its reuse, after purification in different stages of the process cycle (calcium hydrogenation, monosilane thermal decomposition in a boiling-bed reactor).
The process for the production of calcium hydride contemplates the following operative steps:
Wherein the technical features mentioned in each claims are followed by reference sign, such reference sign have been included for better understanding of the claims and therefore they are not limiting for the scope of each element identified by such reference sign.
Number | Date | Country | Kind |
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BO2008A000646 | Oct 2008 | IT | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2009/007166 | 10/20/2009 | WO | 00 | 6/8/2011 |