RU1775419C - Method for processing polytetrafluorethelen - Google Patents

Abstract

Usage: disposal of polymer waste generated at various stages of its preparation and processing. SUMMARY OF THE INVENTION: A waste processing method consists in thermally degrading a polymer at 490-510 ° C in a gas stream. As a result of degradation, finely dispersed polytetrafluoroethylene and gaseous degradation products are obtained, the resulting mixture is constantly removed from the reaction zone, fine PTFE is separated, and gaseous products are returned to the reaction zone and used as a gas stream. Before returning to the reaction zone, gaseous products are cooled to room temperature. 1 il .. 1 tab.

Description

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The invention relates to a technology for processing polytetrafluoroethylene (PTFE) wastes, as a result of which finely dispersed polytetrafluoroethylene can be used as an additive to oils, antifriction composites, surface lyophilization, as well as tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) monomers, which can be used in the synthesis of polymers and freons or converted by known methods into hydrofluoric acid and other fluorine-containing products.

Of these products, finely divided PTFE is most preferred due to its high practical significance and a more rational method of preparation, since The currently used method for producing it by polymerization of a monomer (HFE) in an autoclave in a Freon-113 medium is complicated and unsafe.

A method for pyrolytic decomposition of PTFE wastes is known, which consists in thermally degrading PTFE chips in a stream of dry nitrogen at temperatures up to 500 ° C. The process is conducted in a quartz tube. The gas released during pyrolysis consists mainly of TFE and a small amount of HFP. A thin coating of paraffin-like material is formed on the walls of the reactor, which is low molecular weight, fine-dispersed PTFE.

The disadvantage of this method is the almost complete decomposition of polytetrafluoroethylene to gaseous monomers and, accordingly, a very low yield of finely dispersed PTFE.

There is also a method of pyrolysis of PTFE in a nickel reactor in a dry atmosphere

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nitrogen at temperatures above 450 ° C. In the result, up to 85% of TFE is formed, up to 5% of PTFE powder and the rest, mainly HFP. This method is the closest to the claimed technical essence and the achieved result.

The disadvantage of the prototype is the low yield of finely divided polytetrafluoroethylene.

The aim of the invention is to increase the yield of fine PTFE in the processing of PTFE waste.

The goal is achieved. that in the method of processing PTFE by thermal degradation in a gas stream, thermal degradation is carried out at 490-510 ° С with circulation of the gas flow in the installation until the complete decomposition of the feedstock, while gaseous thermal decomposition products are used as carrier gas, which are cooled before returning to the reaction zone to room temperature. In the event of an excess of gaseous products, they are disposed of by known methods. Prior to returning the gaseous reaction products back to the reaction zone, fine PTFE is isolated from them.

The method is carried out in the apparatus shown schematically in the drawing.

The installation includes a nickel reactor 1 placed in a tube furnace 2 and connected to a refrigerator 3 cooled by tap water to remove heat from gaseous thermal decomposition products. The installation also includes a vortex trap 4, where the condensed finely dispersed PTFE is separated from the gas phase, connected to a refrigerator 3 and a centrifugal fan 5, which ensures the circulation of the gas flow with thermal decomposition products in the installation. The installation is also equipped with a unit b for the disposal of gaseous products of thermal decomposition.

The method is carried out as follows.

Pulverized to a particle size of less than 10 mm, the PTFE shavings are placed in a reactor, closed and blown with dry inert gas or gaseous products of thermal degradation of the polymer after separation of the PTFE powder obtained in the previous cycles from the gas phase in order to remove air and moisture to prevent the formation of explosive during heat treatment fluoroperoxides. Then the reactor 1 is heated in a tube furnace 2 to a temperature of 490 ° C, at which the intensive decomposition of LTFE begins, water is supplied to the refrigerator 3 and the fan is turned on

5. In the future, the temperature in the reaction zone is maintained in the range of 490-510 ° C. The resulting thermal degradation products (finely divided and gaseous) are sent using a fan 5. to the refrigerator 3, where the gas mixture is cooled to room temperature, and then to trap 4, where the condensed polytetrafluoroethylene is condensed as a fine powder. After the fine PTFE has precipitated, the cooled gas stream, which is a mixture of tetrafluoroethylene and hexafluoropropylene, is returned to the reaction zone. Excess gaseous thermal decomposition products are fed to the disposal unit.

 Depending on the goal, gas utilization can be carried out as follows:

rectification gas separation for subsequent use in the synthesis of polymers and freons;

burning to soot and hydrogen fluoride followed by absorption of HF with water and

preparation of hydrofluoric acid and absorption of HF with sodium fluoride; and preparation of sodium bifluoride NaF HF.

The gas stream is circulated in the installation until the feedstock is completely decomposed, after which the installation is depressurized and fine-dispersed PTFE is unloaded from the vortex trap.

The proposed method allows to obtain fine PTFE with an output of up to

51% of the starting polymer. The increase in yield in comparison with the prototype is due to the use of gaseous products of thermal decomposition as a carrier gas, cooled before returning to the zone

reaction to room temperature. More than five-fold superiority in the molecular weight of gaseous products of thermal decomposition, the smallest of which belongs to TFE and is equal to 100, in comparison

with nitrogen with mass 14 used in the prototype, it allows more efficient absorption of thermal energy of PTFE polymer molecules at the molecular level and, thereby, prevents further

decay of PTFE to monomers. At the same time, there is no need to use an additional reagent - nitrogen, which increases the efficiency of the method.

Selected temperature range

thermal degradation due to the fact that at temperatures below 490 ° C the reaction rate is small, the process takes a long time and the yield of product is small. At temperatures above 510 ° C, the yield of the target product is cutting

falls because decomposition of PTFE to monomers and oligomers is in progress.

Example 1.1 kg of PTFE shavings obtained after processing the products on a lathe and crushed to a particle size of less than 10 mm are placed in the reactor and closed with a lid. The air is then displaced from the reactor with an inert gas, the furnace is turned on, and water is supplied to the cooling jacket of the refrigerator. After the temperature in the reaction zone reaches 490 ° С, the gas flow is circulated in the system using a centrifugal fan, which sends the finely dispersed gaseous products of thermal decomposition, first to the refrigerator, and then to the trap where the finely dispersed PTFE is deposited. The gaseous products of thermal decomposition are returned to the reactor, and their excess is sent to the recovery unit, where they are burned in the flame of an oxygen burner. The resulting hydrogen fluoride is absorbed in the column irrigated with water to form hydrofluoric acid. The process of thermal degradation lasts 3 hours. At the end of the process, as evidenced by the cessation of gas evolution in the reactor, the installation is depressurized and finely dispersed PTFE is unloaded from the trap.

The resulting product has a particle size determined using an electron microscope, equal to 0.1-1 microns. The yield of finely divided PTFE is 50%. The amount of hydrofluoric acid of 40% concentration is 0.95 kg, i.e. 95% of the estimated amount.

Examples 2-4. Carried out analogously to example 1 except for changing the temperature parameters.

The conditions of the examples and the results are shown in the table.

The use of the invention for the processing of PTFE wastes in comparison with the known methods makes it possible to almost completely return this valuable raw material to production and at the same time to obtain from 1 ton of waste up to 0.5 tonnes of finely dispersed PTFE and up to 1 ton of hydrofluoric acid, or up to 0.5 ton of tetraflu - ethylene and hexafluoropropylene. or 1.24 tons of NaF HF, depending on the method of disposal of gaseous products of thermal decomposition.

Thus, the advantage of the proposed method in comparison with the prototype is a significant increase in the yield of finely dispersed PTFE, which has high practical value, and in the simplification of the method due to the absence of the need to use an additional reagent, nitrogen, as the carrier gas.

Claims (1)

The claims A method of processing polytetrafluoroethylene by thermal decomposition of polytetrafluoroethylene waste in a gas stream to obtain polymer powder and gaseous decomposition products, characterized in that, in order to increase the yield of finely dispersed polytetrafluoroethylene and the economy of the method, thermal decomposition is carried out at 490-510 ° С, gaseous decomposition products are used as gas in this case, the decomposition products are continuously circulated and removed from the reaction zone with the separation of the finely dispersed polymer and the return of gas shaped degradation products, cooled to room temperature, the reaction zone. V b % l / H