RU2387632C2 - Method of processing polytetrafluoroethylene wastes - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: described is a method of processing polytetrafluoroethylene wastes through continuous pyrolysis in the presence of water vapour at high temperature, distinguished by that pyrolysis is carried out in two successive steps, where pyrolysis temperature at the first step is 600-750°C, and 500-600°C at the second step. Temperature at the first step is higher than temperature at the second step.

EFFECT: owing to decomposition in two steps, the method enables simultaneous production of fluoro-monomers and fine-dispersed polytetrafluoroethylene.

2 cl, 4 ex, 1 tbl, 1 dwg

Description

The invention relates to the chemistry of macromolecular compounds, in particular to a method for processing fluoropolymers by thermal decomposition (pyrolysis) in the presence of water vapor.

The thermal decomposition of fluoropolymers such as polytetrafluoroethylene (PTFE) and a copolymer of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP), in the temperature range 400-700 ° C, leads to the predominant formation of the starting monomers that can be isolated from the gas mixture and returned to production after purification fluoroplastics and other organofluorine substances [Modorsky S. Thermal decomposition of polymers, M .: Mir, 1967, p.142, 174].

It is known that with thermal decomposition of PTFE in vacuum (residual pressure of 40-100 mm Hg) and a temperature of 600-800 ° C, TFE and HFP can be simultaneously obtained [US Pat. USA No. 2406153, NPK 260-653, publ. 08/20/1946].

The process of pyrolysis of fluoroplastics is carried out both in batch and continuous mode. Heat is supplied both by means of electric heating and by means of a high-frequency generator [US Pat. No. 6797913, IPC 7 B23K 9/23, NPK 219 / 121.41, publ. 09/28/2004]. The effective heat required for the decomposition of 1 kg of PTFE is about 1 kW. A significant drawback of these processes is that in order to increase the yield of tetrafluoroethylene it is necessary to carry out pyrolysis under reduced pressure (vacuum), while atmospheric air can get into gaseous pyrolysis products, which is unacceptable when working with fuel and prone to spontaneous polymerization of TFE. The formation of a toxic impurity, perfluoroisobutylene (PFIB), the presence of which complicates the purification of pyrolysis products, is also possible.

Recently, much attention has been paid to pyrolysis in the presence of water vapor, which eliminates the formation of PFIB and solves the problem of heating the fluoropolymer to the pyrolysis temperature without significant overheating of the reactor wall and, as a result, increased soot formation [US Pat. USA No. 3832411, IPC 7 C07C 21/18, NPK 260 / 653.3, publ. 08/27/1974]. The pyrolysis of PTFE is carried out at a temperature of 415-760 ° C with a ratio of water vapor and decomposition products equal to (4-40): 1. The pyrolysis products contain 85% TFE, 10% HFP and 5% octafluorocyclobutane (SFC). A significant disadvantage of this method is the periodic loading of PTFE and the low rate of decomposition of the polymer.

It is also known that in the process of pyrolysis of PTFE at a temperature of 480-530 ° C, low molecular weight finely dispersed fluoroplastic can be obtained (yield up to 50-75%), which is used as an additive to lubricating compositions [US Pat. RF №2133196, IPC 6 B29B 13/00, 17/00, publ. 07/20/1999; US Pat. RF №2100376, IPC 6 C08P 114/26, C08J 11/04, 11/10, C10M 169/04, publ. 12/27/1997; US Pat. RF №2035308, IPC 6 B29B 17/00, publ. 05/20/1995; US Pat. WO No. 2005/042629, IPC 7 C08J 11/12, C08F 14/26, B29B 17/00, publ. 05/12/2005]. The melting point of the obtained product is in the range of 220-320 ° C, the average particle size (0.1-10) microns. All methods for producing such a product are characterized by a serious disadvantage, namely, the need to neutralize the by-product gaseous organofluorine products. It should also be noted that the amount of generated and already accumulated PTFE wastes significantly exceeds the potential demand for finely dispersed fluoroplastic, which limits the possibility of processing a large amount of PTFE wastes.

The closest set of essential features to the proposed is a method for producing fluorinated monomers [US Pat. USA No. 5432259, IPC C07C 21/18, NPK 528/481, publ. 07/11/1995]. The method includes continuous pyrolysis of PTFE in the presence of water vapor at a temperature of 500-900 ° C (preferably 600-750 ° C) in a fluidized bed reactor, river sand with a particle size of 0.5 mm is used as the fluidized material. The ratio of water vapor and decomposition products is (1-2.5): 1. The polymer is fed into the reactor in the form of chips with a particle size of (0.3-0.4) mm. Thanks to the use of a fluidized bed reactor, a high yield of gaseous products - fluoromonomers is ensured.

The disadvantages of this method include the complexity of the hardware and its maintenance, the possibility of clogging with insufficient steam supply, the possible significant ablation of undecomposed PTFE at high linear gas velocities, as well as the need to supply a large amount of heat through the reactor wall or severe overheating of water vapor due to low mass ratio of water vapor and decomposition products. In addition, the disadvantages of the method include the lack of the possibility of obtaining, along with fluoromonomers, high-quality finely divided low molecular weight fluoroplastic.

The technical task of the present invention is the creation of a productive, economical method of decomposition of PFTE, ensuring the simultaneous production of fluoromonomers and finely divided fluoroplastic with the predominant production of fluoromonomers.

The technical problem is solved by continuous pyrolysis of polytetrafluoroethylene waste in the presence of water vapor at elevated temperature, according to the invention, the pyrolysis is carried out in two successive stages, the temperature in the first stage being 600-750 ° C, and the second 500-600 ° C. the temperature in the first stage is higher than the temperature in the second stage.

The steam supply for pyrolysis is 10-15 parts by weight. per 1 part by weight polytetrafluoroethylene waste.

Gases after the second pyrolysis stage, containing gaseous fluoromonomers and vaporized low molecular weight PTFE, are quenched with water to obtain a suspension containing finely dispersed fluoroplastic and a dust-free gas stream. Finely dispersed fluoroplastic after separation from water condensate and drying can be used as an additive to lubricating compositions. The gas stream, containing mainly TFE (80-85%), HFP (5-10%) and SFC (5-10%), as well as a small amount of carbon oxides, is sent for processing with the separation of these products by the method of step distillation. At the same time, the pyrogas is processed together with the pyrogas of the main production of TFE containing the same products as the pyrogas after decomposition of the fluoroplastic wastes.

The supply of water vapor for pyrolysis in an amount of 10-15 wt.h. per 1 part by weight polytetrafluoroethylene waste provides almost complete absence of external heat supply through the reactor wall. This leads to less soot formation and to obtain better finely dispersed fluoroplastic. The absence of a fluidizing agent (river sand) and the presence of the second stage of pyrolysis allows more complete utilization of PTFE wastes with the receipt of an additional product - finely divided PTFE. Thus, the proposed method is more environmentally friendly compared to the prototype. In this case, the gas stream is additionally cleaned of soot and foreign inclusions contained in the PTFE waste by changing the direction of the gas stream and reducing its linear velocity.

The installation for the disposal of PTFE waste is shown in the drawing.

PTFE waste enters the receiving hopper 1. From it, the waste is periodically dumped into the feed hopper 2, which is equipped with a coaxially mounted mixer for loosening crumbs, combined with a metering auger. The supply of PTFE to the pyrolysis is carried out at a given speed in the middle part of the pyrolysis reactor 3 (1st stage of pyrolysis). The reactor is a pipe mounted vertically and equipped with an external electric heater to compensate for heat loss. The material of the reactor is nichrome. The reactor is filled to a height of 500 mm with an inert nozzle. The temperature inside the reactor is measured in the nozzle layer, at a height of 100-200 mm above its layer and in the upper third. Steam with a temperature of 600-750 ° C is supplied to the lower part of the reactor from a superheater 11, which is a cascade of nichrome pipes.

Pyrogas containing gaseous decomposition products, water vapor, as well as undecomposed and finely dispersed fluoroplastic from the upper part of reactor 3, enters reactor 4 (2nd pyrolysis stage) for complete decomposition of fluoroplastic. The reactor 4 is a horizontal cylindrical heat-insulated apparatus made of steel 12X18H10T and equipped with a hatch for unloading soot and other mechanical impurities.

Pyrogas containing gaseous decomposition products and finely dispersed fluoroplastic enters the pipe-in-pipe type recuperation cooler 5 cooled by the initial water vapor. After the refrigerator 5, the pyrogas enters the quenching unit 6 to condense water vapor and separate the finely dispersed fluoroplastic from the pyrogas.

Node 6 consists of two crosses. Pyrolyzate enters the upper cross. The temperature of the pyrogas after quenching, controlled in the next crosspiece, is maintained at a predetermined level by the supply of water. Condensed water vapor and finely dispersed fluoropolymer enter through one of the two suction filters 7 through a water trap, working alternately to receive the suspension and to discharge the moist finely dispersed PTFE. The nutch filter operates under vacuum, which is created by the vacuum pump 8. The condensate after the nutch filter 7, freed from the finely dispersed fluoropolymer, enters the receiver 9. From the receiver 9, the condensate is pumped into the sewer by a centrifugal pump 10. The finely dispersed fluoropolymer unloaded from the filter is transferred for further drying.

Pyrogas from the upper cross of the hardening unit 6 is sent for processing together with the pyrolysis gases of difluorochloromethane containing the same products as pyrogas after decomposition of the fluoroplast waste.

The invention is illustrated by the following examples.

Example 1

The ground PTFE with a particle size of (3-6) mm is fed into reactor 3 (1st stage of pyrolysis), preheated to a temperature of 650 ° C. The feed rate of PTFE is 10 kg / h. Water vapor superheated to a temperature of 730-750 ° C is supplied to the reactor at a speed of 100 kg / h. The temperature at a height of 100-200 mm from the nozzle layer is 630-645 ° C, the temperature in the reactor 4 (2nd stage of pyrolysis) is 530-550 ° C. The total test time is 120 hours. After the tests, the reactors 3, 4 are disassembled to check the completeness of the decomposition of PTFE. The amount of PTFE in reactor 3 is 2 kg and in the reactor 4 to 4 kg. From the suction filter 7, after drying, 40 kg of fine PTFE were obtained. The product is a yellowish powder with a melting point of 294 ° C (determined by differential scanning calorimetry on a Setaram device) and an average particle size of 2.5 μm (determined by diffraction analysis of particle sizes on an Analizette 22 device). The yield of gaseous pyrolysis products was 96%. The results of this and subsequent experiments are shown in the table.

Examples 2-3

Carried out analogously to that described in example 1, varying the temperature of the reactors 3, 4 (1st and 2nd stages of pyrolysis), as well as the feed rate and temperature of superheated water vapor.

Example 4 (comparative)

The decomposition of PTFE is carried out similarly to that described in examples 1-3, but at a temperature in both stages of 500 ° C.

Thus, the proposed method due to the decomposition in two stages allows you to simultaneously obtain both fluoromonomers and fine PTFE. Thus, a decrease in the pyrolysis temperature to 500 ° C at both stages of the pyrolysis leads to an increase in the yield of finely divided PTFE to 44%. When carrying out the process under optimal conditions, the yield of fluoromonomers is 80-98%, the yield of finely divided PTFE is 1-15%.

The finely dispersed fluoroplast obtained by two-stage pyrolysis does not contain large particles of undecomposed fluoroplastic, is not contaminated with a fluidizing material, and can be used in lubricating compositions without further purification.

Claims (2)

1. A method of processing polytetrafluoroethylene waste by continuous pyrolysis in the presence of water vapor at elevated temperature, characterized in that the pyrolysis is carried out in two successive stages, the pyrolysis temperature in the first stage being 600-750 ° C, and the second - 500-600 ° C while the temperature in the first stage is higher than the temperature in the second stage. 2. The method according to claim 1, characterized in that the supply of water vapor for pyrolysis is 10-15 wt.h. per 1 part by weight polytetrafluoroethylene waste.

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