The research on silicon tetrafluoride as a raw material for the production of silane, crystalline silicon, amorphous silicon and silicon oxide has attracted more and more people's attention, especially with the emergence of new silane method, which has brought great benefits to the polycrystalline silicon industry. changes have attracted more and more people's attention. The source of silicon tetrafluoride in my country is mainly a by-product of the phosphate fertilizer industry. Chinese patent ZL201010529976. When the fluorine-containing gas is introduced into a reactor with sulfuric acid and silicon dioxide, the hydrogen fluoride is converted into silicon tetrafluoride gas, and then the resulting gas is passed through concentrated sulfuric acid or concentrated sulfuric acid containing hydrogen fluoride, activated carbon, diatomaceous earth, and low-temperature rectification. and other steps to prepare high-purity silicon tetrafluoride gas; Chinese patent application No. CN101973553A "Method for producing high-purity silicon tetrafluoride using fluosilicic acid" discloses the use of fluosilicic acid, a by-product of the phosphate fertilizer industry, and concentrated sulfuric acid mixed and heated to produce Silicon tetrafluoride gas is then filtered through concentrated sulfuric acid, concentrated sulfuric acid containing hydrogen fluoride, pure sulfuric acid, activated carbon, and diatomaceous earth, and is distilled at low temperature to obtain high-purity silicon tetrafluoride gas. According to measurements, the iodine content in phosphate rock in the phosphate fertilizer industry is about 0.0057% to 0.0076%. During the production process, the impurity iodine exists in the form of HI in the by-product fluosilicic acid of wet phosphoric acid concentration. The iodine content Around 115mg/L. When using fluosilicic acid and concentrated sulfuric acid to react to produce silicon tetrafluoride gas, part of HI reacts with concentrated sulfuric acid to form iodine elements. Therefore, iodine in silicon tetrafluoride gas exists in the form of HI and I2 elements. Industrial purification methods for silicon tetrafluoride fall into two categories: physical methods and chemical methods. Physical methods mainly refer to adsorption methods and freezing methods. The freezing method selectively removes some impurities based on the different melting and boiling points of substances; chemical methods mainly include the decomposition of concentrated sulfuric acid containing HF and the cobalt trifluoride method, which can remove tetrafluorofluoride. Hexafluorodimethylsilyl ether impurities in silicon gas. So far, there has been no report on a technical solution for purifying impurity iodine in silicon tetrafluoride gas.
Content of the invention
The present invention aims to provide a method for purifying impurity iodine in silicon tetrafluoride gas, so as to realize the iodine removal and purification of silicon tetrafluoride gas and make high-quality The purity of silicon tetrafluoride products provides a high-purity raw material for the production of silicon series products for the electronics, photovoltaic, and optical fiber industries. In order to effectively remove iodine in silicon tetrafluoride gas, the inventor, after repeated experiments, provided a purification method that uses a freezing method to remove the impurities HI and I2 in silicon tetrafluoride gas, including the following steps
The first step is to collect fluorine-containing gases including hydrogen fluoride and silicon tetrafluoride produced during the wet processing of phosphate rock, and introduce this fluorine-containing gas into a reactor containing sulfuric acid and silicon dioxide, so that the fluorine-containing gas contains The hydrogen fluoride in the fluorine gas is converted into silicon tetrafluoride gas; or fluorosilicic acid and concentrated sulfuric acid are mixed and heated to produce gaseous compounds, and then the gaseous compounds are introduced into a container with concentrated sulfuric acid to remove HF, water and other impurities. Obtain silicon tetrafluoride gas;
The second step is to introduce the silicon tetrafluoride gas obtained in the first step into the purification tank, and use concentrated sulfuric acid or a mixture of concentrated sulfuric acid and hydrofluoric acid to remove the moisture in the gas and oxygen-containing fluorosilicides;
In the third step, the silicon tetrafluoride gas enters a filter equipped with pre-dried activated carbon and diatomaceous earth to filter out impurities;
The fourth step is to introduce the SiF4 gas purified in the above steps into the freezing device, and freeze and remove HI and 12. In the first step of the above method, the wet treatment of phosphate rock is to use sulfuric acid or phosphoric acid or nitric acid or hydrochloric acid to decompose the phosphate rock. The mass fraction of sulfuric acid entering the collected fluorine-containing gas including hydrogen fluoride and silicon tetrafluoride is 85 % 98%, the mass ratio of the added silica and concentrated sulfuric acid is 1: 10 4 10; the silica is quartz sand, its silica content ≥ 95%, and its fineness is 0.3_ or less, The conversion temperature is 45°C to 130°C; the heating temperature for mixing the fluorosilicic acid and concentrated sulfuric acid is 80°C to 110°C. In the second step of the above method, the sulfuric acid is concentrated sulfuric acid with a mass fraction of 98%, and the hydrofluoric acid is anhydrous hydrogen fluoride; the reaction conditions are controlled at a temperature < 20°C.
In the third step of the above method, the activated carbon is pre-dried at -10°C -50°C, and the filtered impurities are S02, S03, H20 and some oxygen-containing fluorosilicon compounds; Diatomaceous earth is pre-dried at 200°C and 350°C; the filtered impurity is CO2.
In the fourth step of the above method, the freezing device is a tank with a vacuum layer and a sealed lid. The tank is a freezing chamber filled with refrigerant. There is a gas pipeline above the sealed lid, into which SiF4 gas is passed, freezing. The agent relies on the tube wall to absorb the heat of SiF to achieve the purpose of freezing and removing iodine; the freezing temperature is -85°C to 40°C, and the freezing time is 110min. The inventor pointed out that iodine element is solid at normal temperature, the boiling point of HI is -35.6°C, and the boiling point of SiF4 is -94.8°C. Therefore, the temperature for removing impurity iodine by freezing method must be controlled at -40°C to -85°C. within the range of C. The inventor also pointed out that the above-mentioned refrigeration device and freezing method are not limited, and may also include cold traps and other methods. The method of the invention utilizes the fluorine-containing gas or fluorosilicic acid and concentrated sulfuric acid produced during the wet treatment of phosphate rock to be mixed and heated, and then the impurity iodine is removed to produce gaseous compounds to prepare high-purity silicon tetrafluoride, which is useful for electronics, photovoltaics, The optical fiber industry provides high-purity raw materials for the production of silicon series products. It is suitable for chemical enterprises that process phosphate rock by wet method.