DE3841222A1 - Process for the parallel generation of silicon tetrafluoride and sulphur hexafluoride - Google Patents

Abstract

A process is provided for the parallel generation of silicon tetrafluoride and sulphur hexafluoride, in which elemental silicon is reacted with sulphur vapour at temperatures above 600@C to give silicon sulphide, this is purified by sublimation, finely milled and reacted at temperatures between 300 and about 550@C with depleted uranium hexafluoride, converted into the gas phase, in accordance with the equation SiS2 + 8 UF6 @ SiF4 + 2 SF6 + 8 UF4 The heterogeneous reactions, which are preferably to be carried out in a fluidised-bed furnace, give highly pure products because of the high purity of the starting material. An additional advantage arises from the fact that in the novel process using depleted uranium hexafluoride and waste silicon from semiconductor manufacture, two hitherto unutilised "waste materials" readily available in large amounts can be utilised (valorised).

Description

In order to produce nuclear fuel elements, the fissile isotope U, which is present only to a small extent in natural uranium deposits, must be enriched in a special process. This enrichment takes place at the stage of the gaseous uranium hexafluoride, UF ₆ . In addition to the UF ₆ enriched to a content of about 3 percent of cleavable isotopes, so-called depleted uranium hexafluoride remains as a by-product in a quantity 4 to 6 times greater than that of the enriched UF ₆ . It still contains about 0.20 to 0.25 percent of fissile isotope. Further depletion down to about 0.1 percent Ge of fissile uranium isotope is technically possible, but leads to increased costs and cannot be carried out economically in the current state of the art. Nevertheless, the depleted uranium material is a potential resource. Therefore, more than three hundred thousand tons of depleted uranium hexafluoride are already stored worldwide.

Because of the chemical aggressiveness of the UF ₆ , however, its storage is extremely complex and must be carried out in expensive stainless steel tanks with a high level of monitoring and control effort. In addition to uranium, there is another potential valuable substance in the UF ₆ with its high fluorine content. For the production of UF ₆ was according namely

UF₄ + F₂ → UF₆

expensive elemental fluorine used. A technical use of this high fluorination potential on a larger scale is not yet known. Fluorination reactions with UF ₆ that have already been described either lead to products for which there is very little need or are not to be carried out as technical processes.

The object of the present invention is therefore a simple one Process for the fluorochemical use of depleted uranium hexafluoride indicate which cheap to valuable products leads, which ver also on a correspondingly large scale are valuable.

This object is achieved according to the invention by a process for the parallel production of silicon tetrafluoride and sulfur hexafluoride, in which
elemental silicon with elemental sulfur at temperatures above 600 ° C to form silicon disulfide, SiS ₂ ,
cleaned the SiS ₂ ,
finely ground and
with depleted uranium hexafluoride converted to the gas phase at temperatures between 300 and approx. 550 ° C according to the equation

SiS₂ + 8 UF₆ → SiF₄ + 2 SF₆ + 8 UF₄

is implemented.

Furthermore, it is within the scope of the invention that small parts Waste silicon from semiconductor manufacturing is used, wel finely ground in a fluidized bed furnace with sulfur vapor is set.

Further embodiments of the invention are the subclaims refer to.

The method according to the invention shows a way of producing the technically valuable gases silicon tetrafluoride and sulfur hexafluoride, which is both inexpensive and leads to highly pure products. In particular, sulfur hexa fluoride is used as an insulation and arc extinguishing gas in electrical engineering in large quantities and is now indispensable. Because of its good dielectric properties, SF _{6 is used} in switchgear and disconnector systems, power and measurement transformers, for gas-filled power cables, fully enclosed indoor switchgear, X-ray generators, electrostatic particle accelerators and in many other medium and high voltage systems.

So far, sulfur hexafluoride has been produced from Sulfur and the expensive elementary fluorine gas

1/8 S₈ + 3 F₂ → SF₆

The silicon tetrafluoride can be converted back into elemental silicon be transferred, which one compared to the Sili used ciummaterial has increased purity and, for example, good for Solar cell purposes is suitable.

To convert silicon tetrafluoride into elemental silicon several methods are suitable. The reduction of silicon tetra fluoride in the arc on carbon electrodes delivers according to

SiF₄ + C → Si + CF₄

as a gaseous by-product, the very stable and because its inertia non-toxic carbon tetrafluoride, the is also used technically today. The formation of Si Silicon carbide is released by suitable reaction conditions closed.

The reduction in the arc under a hydrogen atmosphere at Wolf ram electrodes run according to the equation

SiF₄ + 2 H₂ → Si + 4 HF

The hydrogen fluoride produced as a gaseous by-product has a high purity and can by condensation or  Absorption in sodium or potassium hydroxide solution or milk of lime or alumi nium hydroxide in technically recyclable sodium, potassium, Calcium or aluminum fluoride can be transferred.

The reduction with high purity metallic sodium used above 150 ° C results in

SiF₄ + 4 Na → Si + 4 NaF

elemental silicon in the form of approximately 150 µm large particles, obtained by leaching the resulting sodium fluoride will. This by-product sodium fluoride can be used in the manufacture development of sodium hexafluorosilicate melt flow electrolytes chemical deposition of thin polycrystalline silicon layers ten can be used for solar cells.

However, the reduction of the silicon part is particularly advantageous trafluorids in plasma or in glow discharge under water atmosphere. With the help of this process, the silicon tetrafluoride directly for the production of amorphous silicon layers on suitable substrates according to

SiF₄ + 2 H₂ → Si _{F, H)} + 4 HF

be used. For the resulting hydrogen fluoride this applies already noted above.

For inexpensive silicon tetrafluoride gas another use follows after the American Ocrete process according to the implementation

2 Ca (OH) ₂ + SiF₄ → 2 CaF₂ + SiO₂ + 2 H₂O

With the help of this reaction, concrete structures can be strengthened and sealed. This also enables the large-scale production of synthetic cryolite, Na ₃ AlF ₆ for aluminum extraction by melt flow electrolysis.

Furthermore, high-purity and inexpensive silicon tetrafluoride  for the production of silicon fluoride as salts and electro lyte and for drinking water fluoridation to caries prophy lax of application engineering interest.

Waste Si serves as the raw material for the process according to the invention licium from semiconductor or solar cell silicon production which is used in the processing of silicon material in electronics technology or electronics in considerable quantities. Alone when cutting silicon wafers from single or polycrystalline NEN silicon rods, which mostly by means of diamond hole saws is performed, 30 to 50 percent and more silicon receive cut waste. Fal also when separating chips len the edge areas of the wafers as waste. An estimate shows that in 1987 about 5000 tons of waste silicate worldwide cium are incurred.

With the method according to the invention it is possible to remove foreign elements present in the waste silicon, for example dopants and impurities, by sulfidic separation. By reacting the waste silicon with sulfur, foreign elements are oxidized and bound away as sulfides. Because the metal sulfides in particular are very stable compounds that can easily be separated from the silicon sulfide themselves or as thiosilicates, a very pure SiS _{2 can be} obtained and used for the reaction with depleted uranium hexafluoride. The sulfur is also available in sufficient purity. Should it still be necessary for special impurities, the silicon sulfide can be sublimed for further cleaning even at 60 torr. It then occurs in colorless, silk-glossy crystal needles.

For the implementation with more than 600 ° hot sulfur vapor that becomes Silicon finely divided introduced into the reactor. The straight gene reaction proceeds all the more completely to silicon disulfide, the more finely divided the silicon is reacted. At the the brittle silicon is best finely ground. The powder can then be easily blown into the reactor, which preferably works as a fluidized bed furnace.  

The further reaction of the SiS ₂ with uranium hexafluoride vapor is again a heterogeneous reaction that can be carried out in a similar reactor. For this purpose, the silicon disulfide is also finely ground and placed in a fluidized bed furnace together with hot UF ₆ steam. Both the blowing in of the powdery solid and the introduction of the uranium hexafluoride vapor can be supported with a carrier gas stream, for example nitrogen. A prerequisite for this implementation is working in the absence of moisture in order to avoid hydrolysis of the uranium hexafluoride with the formation of hydrogen fluoride. Furthermore, the side walls of the reactor should be cooled during the reaction in order to dissipate the heat of reaction of the exothermic reaction. In addition, the deposition of the only solid uranium tetrafluoride formed on the cool reactor walls is facilitated. This uranium tetrafluoride layer also protects the reactor wall from a corrosive attack by the uranium hexafluoride.

The gaseous reaction products SiF ₄ and SF ₆ are continuously withdrawn from the reactor and can first of all be collected in liquid form by condensation at low temperatures. In one embodiment of the invention, the two components are then separated by fractional distillation, which is preferably carried out under a slight excess pressure.

Because of the different molecular size and the difference The molecular weight differs between the two substances also in their diffusion behavior, leading to an alternative Separation of the substances can be exploited. For example is separation via semipermeable membranes possible. A shit The two individual components can also be used in liquid form take place because the two substances cannot be mixed with each other form phases. The specifically heavier sulfur hexafluo rid can be pulled off the bottom of the condensation vessel, while the specifically lighter silicon tetrafluoride on top swims.

If the process according to the invention is limited to the pure preparation of sulfur hexafluoride, the SiF _{4 can} also be separated chemically by formation of an adduct with suitable reaction partners. Addition compounds with ammonia or other amines such as pyridine, from which the silicon tetrafluoride can easily be split off, are suitable for this purpose. With alcohols such as methanol, isopropanol or glycol, solvates can be formed in which the silicon tetrafluoride dissolves. If high-purity SiO _{2 is to be} obtained, the silicon tetrafluoride can also be hydrolyzed to form silica and hydrofluoric acid. A high-purity silicon dioxide is obtained from the silica by dewatering, which can be converted, for example by carbothermal reduction, into a high-purity silicon. Another possibility is the formation of silicofluorides by reaction with alkali metal or onium fluorides

2 MF + SiF₄ → M₂SiF₆

where M is a monovalent metal or onium ion. These Salts can be used as electrolytes for melt flow electrolysis be set.

With aluminum trichloride, silicon tetrafluoride forms into sili cium tetrachloride and solid aluminum trifluoride around:

3 SiF₄ + 4 AlCl₃ → 3 SiCl₄ + 4 AlF₃

The method according to the invention is suitable for recycling contaminated silicon from semiconductor production and at the same time provides another important technical gas, SF ₆ . Since the starting materials are available cheaply or almost free of charge (UF ₆ ), the process also represents an extremely cost-effective way of producing silicon tetrafluoride and sulfur hexafluoride. Impurities contained in sulfur or silicon can be successfully removed in the first process step. Since the uranium hexa fluoride also has a very high purity of greater than 99.99 percent due to the technically difficult process of isotope enrichment, highly pure products are obtained. The difficult to store aggressive uranium hexafluoride is converted into the almost inert uranium tetrafluoride, which can be stored cheaply and is still available for further depletion. The storage costs thus saved represent a further cost advantage of the method according to the invention.

Claims (9)

1. A process for the parallel production of silicon tetrafluoride and sulfur hexafluoride, in which
elemental silicon with elemental sulfur at temperatures above 600 ° C is converted to silicon disulfide, SiS ₂ ,
cleaned the silicon disulfide,
finely ground and
is reacted with depleted uranium hexafluoride in the gas phase at temperatures between 300 and about 550 ° C according to the equation SiS₂ + 8 UF₆ → SiF₄ + 2 SF₆ + 8 UF₄. 2. The method according to claim 1, characterized records that small-scale waste silicon from the Semiconductor manufacturing is used, which is finely ground in Fluidized bed furnace is implemented with sulfur vapor. 3. The method according to claim 1 or 2, characterized in that the reaction of SiS ₂ with uranium hexafluoride is carried out in a fluidized bed furnace. 4. The method according to any one of claims 1 to 3, characterized characterized in that the gas at room temperature shaped products silicon tetrafluoride and sulfur hexafluoride condensed at low temperatures and by means of fractional Distillation to be separated. 5. The method according to claim 4, characterized records that the fractional distillation under slight overpressure.   6. The method according to any one of claims 1 to 3, characterized in that the two product gases SiF ₄ and SF _{6 are separated} via their different diffusion properties. 7. The method according to claim 6, characterized in that the SiF ₄ and SF ₆ are separated from one another via a semipermeable membrane. 8. The method according to any one of claims 1 to 3, characterized characterized in that the gases silicon tetrafluo rid and sulfur hexafluoride together at low temperatures are condensed out and then the two are not together be separated from one another in the miscible liquid phases. 9. The method according to any one of claims 1 to 3, characterized in that the separation of the SiF ₄ from the low-reactivity sulfur hexafluoride is carried out chemically by adduct formation.