RU2019504C1 - Process for preparing silicon tetrafluoride - Google Patents

Abstract

FIELD: preparation of highly pure silicon compounds. SUBSTANCE: the silicon- and fluorine containing starting material reacts at 650-750 C. The silicon containing starting material includes rice or oat husk. The husk is first subjected to oxidizing roasting. The fluorine containing starting material includes uranium 235 isotope production waste containing uranium hexafluoride. EFFECT: improved method. 2 tbl

Description

 The invention relates to pyrochemical methods for processing production waste and can be used in the technology for producing high-purity silicon compounds for electronic devices, optical fibers, solar cells, semiconductor materials.

 The closest method to the proposed one is the method of producing silicon tetrafluoride by synthesizing it from silicon dioxide and fluorine-containing reagents, for example sodium fluoride, hydrogen fluoride, lead fluoride, etc. However, this method, as well as the above analogues, require the use of chemically obtained starting reagents, which leads to additional costs of material and energy resources, in addition, many of them are environmentally hazardous, such as lead fluoride, sulfuric, nitric and fluorosulfonic acids.

 The proposed demand is to increase environmental safety and reduce the cost of the process. The problem is solved by the fact that agricultural products are used as silicon-containing raw materials: rice (or oat) husk subjected to oxidative burning, and waste and dumps containing uranium hexafluoride are used as a fluorinating agent.

 After oxidative roasting, rice husk consists almost exclusively of silicon oxide, and uranium hexafluoride depleted in the 235 isotope is also practically a highly pure substance.

 As you know, uranium hexafluoride is a chemically active substance and is not only a volatile compound, but also a carrier of radioactivity. In case of depressurization of the containers in which it is stored and which undergo intensive corrosion, especially from the side of uranium hexafluoride, environmental pollution by highly toxic fluorides and radioactive compounds will occur. Therefore, the conversion of uranium hexafluoride to a non-volatile compound with the simultaneous use of fluorine in the national economy is a very urgent task. This problem is solved by this invention.

SiF₄+ U₃O₈+ O₂ . (1)
В сответствии с данной реакцией и заявляемым способом осуществляют следующие операции:
1) проводят окислительный обжиг шелухи в две стадии: при 400-450 и 700-750^оС по 2 ч;
2) шихтуют исходные материалы или дозируют в заданном соотношении;
3) проводят реакцию (I) при 650-750^оС;
4) улавливают тетрафторид кремния известными способами, а оксид урана складируют.A reaction occurs between the substances in question.
SiO ₂ + UF

SiF ₄ + U ₃ O ₈ + O ₂ . (1)
In accordance with this reaction and the claimed method carry out the following operations:
1) conduct oxidative roasting hull in two steps: at 400-450 and 700-750 ^{° C} for 2 hours;
2) charge raw materials or dose in a predetermined ratio;
3) carry out the reaction (I) at 650-750 ^about C;
4) silicon tetrafluoride is captured by known methods, and uranium oxide is stored.

The advantages of the proposed method are:
1) obtaining valuable tetrafluoride of silicon for the national economy;
2) the use of agricultural and industrial waste; Moreover, the waste is not only cheap raw materials, but requires the corresponding material and labor costs for their storage, occupying production and agricultural areas;
3) ensuring environmentally safe conditions for the use of silicon-containing agricultural waste (clogging of fertile land) and converting volatile, poisonous, radioactive uranium hexafluoride to a safe oxide-oxide for storage;
4) the release of areas occupied by the storage of waste for their useful use in agriculture and industrial sites.

PRI me R. The process of obtaining silicon tetrafluoride from rice husk and uranium hexafluoride was carried out on a plant consisting of the following main elements: a heated sealed container with uranium hexafluoride, the flow rate of which was regulated by a bellows valve and determined by a rotameter from leucosapphire; a tubular electric furnace, a nickel boat which is loaded with rice hulls, and in which the temperature was maintained at 650 ^{° C;} a container for collecting silicon tetrafluoride; chemical absorber column for trapping volatile fluoride residues; vacuum pump to create a vacuum in the working space of all units of the installation.

 The starting materials were rice husk and uranium hexafluoride. The composition of the fired rice and oat husks calcined in air is shown in Table 1. The silica content was about 98%.

 Uranium hexafluoride used "dump", i.e. after extraction of the isotope 235 of the main substance was not less than 99.9%.

 176 g of uranium hexafluoride was loaded into a chilled container, and a boat with fired rice husk in an amount of 100 g was placed in a tube furnace. 100 g of porous granules of sodium fluoride were passed into a container for trapping silicon tetrafluoride, through which gas passed, forming sodium silicofluoride, convenient for storage "and transportation of silicon tetrafluoride. A calcareous chemical absorber was poured into a control column for capturing fluorine-containing gases.

After achieving a furnace temperature of 650 ^{° C} to open the valve with uranium hexafluoride and the container is heated to a temperature of 60-80 ° ^C. Gaseous uranium hexafluoride by passing through the oven over a boat with rice husks at a temperature of 650 ^{° C,} react with it and converted to the oxide, and the resulting silicon tetrafluoride catches at 300 ^{° C} on the granules of sodium fluoride. At the end of the process, the products were weighed and the degree of reaction (I) was determined. The results are shown in table.2.

 According to calculations, reaction (I) passed from left to right in these experiments by 95%, and tetrafluoride was captured 91.8% of the resulting in the process.

It is noted that the process described by reaction (I) proceeds stepwise. At the first stage, non-volatile oxyfluorides UOF ₄ and UO ₂ F _{2 are formed} , which later on contact with an oxygen donor turn into trioxide, and it, in turn, decomposes at a temperature of 550-650 ^о С to uranium oxide and oxygen.

 The obtained silicon tetrafluoride can be a commercial product and can be purified by known methods.

 Thus, the use of the proposed method allows to obtain silicon tetrafluoride from agricultural and industrial waste, thereby using cheap feedstock and improving the environment, freeing up useful agricultural and industrial areas and giving the national economy valuable products.

Claims (1)

METHOD FOR PRODUCING SILICON TETRAFLUORIDE, including the interaction of silica and fluorine-containing raw materials at 650 - 750 ^o C followed by purification of the product, characterized in that rice or oat husk previously subjected to oxidative firing is used as a silica-containing raw material and the waste is used as a fluorine-containing raw material 235 containing uranium hexafluoride.

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