Recycling of fluoropolymer residues

Abstract

The present invention describes a process for the utilization of partially fluorinated polymers. In a first process step, the polymer residues are heated under oxygen-free conditions at temperatures up to 900°C; this yields high returns of fluorine-containing monomers / products as well as anhydrous hydrofluoric acid (aHF). In a further process step, residues / byproducts from the first step can be combusted in a controlled manner with oxygen at temperatures up to 1000°C, producing HF, CO2, and water. This gas stream can be utilized via further process variants, generating aHF and / or fluorspar (CaF2). This process creates a closed fluorine cycle.

Description

Background of the invention

[0001] Fluoropolymers are irreplaceable in numerous areas of industry.

[0002] Recycling this class of polymers presents a challenge.

[0003] For perfluorinated polymers such as PTFE, PFA, or FEP, elegant recycling technologies exist in which the input monomers, such as tetrafluoroethylene (TFE) and hexafluoropropylene (HFP), are recovered from residual materials and can be reused for the production of new polymers. Such processes are described in WO 2021 / 165923 and WO 2010 / 039820.

[0004] For partially fluorinated polymers such as THV, ETFE, PVDF, elastomers (made from VDF, HFP, TFE), there are various recycling approaches in the patent literature.

[0005] WO 01 / 58840 describes an arc / plasma process in which THV polymers are decomposed into TFE and HFP; the yields are low and the energy input is extremely high.

[0006] EP 3246 289 describes a process for the combustion of polymers and fluorinated liquids / gases; the resulting hydrofluoric acid (HF) is captured as CaF2. The process generates a large amount of CO2, and the product fluorspar (CaF2) must be treated by reaction at high temperatures in the presence of oleum to obtain usable aHF again.

[0007] WO 2022 / 130212 describes a process for the combustion of partially fluorinated polymers in which the combustion gases are passed over hot carbon, producing aHF and syngas (CO / H2). The process is almost CO2-free, but consumes a lot of energy and usually fossil carbon.

[0008] In a DBU final report (https: / / www.dbu.de / OPAC / ab / DBU-Abschlussbericht-AZ-25198.pdf)

[0009] The pyrolysis of THV / ETFE polymers in a stirred bed reactor in the presence of steam or argon is described; although fluorine-containing monomers are produced, the yields are very low and numerous byproducts and residues are found.

[0010] The recycling of partially fluorinated residues in the sense of a sustainable and energy-efficient circular economy has not yet been satisfactorily resolved. Description

[0011] The inventors have developed a process by which valuable fluorinated monomers / intermediate products, such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), vinylidene fluoride (VDF), perfluorocyclobutane (cC4F8) and anhydrous HF (aHF), are obtained from partially fluorinated polymers in a first process step through a pyrolysis process.

[0012] In a further process step, additional hydrated HF can be produced from the residues in the presence of oxygen; this hydrofluoric acid can be captured as CaF₂ or directly converted to aHF. The residues comprise unusable pyrolysis byproducts, blown-out polymer dust, or repolymers.

[0013] Starting materials for the process include, for example, partially fluorinated polymers comprising the following monomers: TFE, HFP, VDF, ethylene (ET), propylene (Prop), and vinyl fluoride (VF). Typical examples are the semi-crystalline polymer classes such as THV (TFE, HFP, VDF), PVDF (VDF), ETFE (TFE, ET, HFP), and copolymers of chlorotrifluoroethylene (CTFE) and ethylene, PVF (VF). These polymers exhibit melting points in DSC ranging from 70°C to 300°C, preferably from 90°C to 280°C. The molecular weights, measured, for example, via the MFI, are not critical and can cover a wide range. Thus, the MFI values ​​can range from 1 to 10,000 g / 10 min (MFI at 297°C / 5 kg).

[0014] Another class of starting materials consists of partially fluorinated amorphous polymers such as elastomer copolymers (VDF, HFP), elastomer terpolymers (VDF, HFP, TFE), or polymers of TFE and propylene. Amorphous elastomers can also be cross-linked, for example, by cross-linking with polyols such as bisphenol A (BFA), or they can be peroxide-cross-linked, for example, with trially isocyanurate (TAIC). In the latter case, the polymers exhibit iodine- or bromine-containing cross-linking sites. Typically, cross-linked elastomers also contain fillers such as carbon black, silicates, etc., in concentrations of 1–40 wt%.

[0015] All of the above-mentioned partially fluorinated polymers can be modified with further comonomers. These include perfluorinated vinyl ethers such as PMVE (CF30CF=CF2), PPVE (C3F7OCF=CF2), or perfluorinated allyl ethers.

[0016] The CH2 content of the partially fluorinated polymer classes is 0.5-45 wt.%, preferably 1-35 wt.%, based on the fluoropolymer.

[0017] The partially fluorinated polymers can also be present as laminates / composite materials or mixtures with non-fluorinated polymers such as PP, PE, PS, polyamides, etc.

[0018] Examples include co-extruded articles such as hoses or coated fabrics. The proportion of non-fluorinated polymers compared to the partially fluorinated polymer is in the range of 1–90 wt.%, preferably 3–50 wt.%.

[0019] In a first process step, the partially fluorinated polymers are subjected to pyrolysis. For this purpose, the polymers are heated under largely exclusion of water and oxygen at temperatures up to 900°C, preferably up to 800°C.

[0020] Pyrolysis in the first process step is carried out, for example, in fluidized beds, stirred-bed reactors, rotary kilns, or extruders. Fluidized-bed or stirred-bed reactors equipped with mineral, ceramic, carbon-containing, or metal-containing bed materials are preferred; microwave-active bed materials can also be used, which, in conjunction with microwave input, lead to improved reaction control. Suitable bed materials include sand, pyrolysis coke, ZrO₂, SiC, metal chlorides / fluorides, and metal spheres. The particle sizes range from 0.2 to 10 mm.

[0021] Such reactors are described in WO 2021 / 165923 ; WO 2010 / 039820.

[0022] Typically, the reactors are filled with bedding material up to 50% of their volume, preferably up to 30%.

[0023] The polymer materials are comminuted and conveyed into the reactors with particle sizes ranging from 0.1 to 10 mm. Fluidizing / transport gases such as N₂, Ar, and CO₂ are used; it must be ensured that the fluidizing / transport gas contains less than 1 wt% oxygen and less than 2 wt% water, otherwise the resulting HF will contain too much water. The combined oxygen and water content should be below 2%.

[0024] Fluorinated gases, possibly as a mixture with N2, Ar, CO2, can also be used as fluidizing / transport gases. Suitable fluorine gases are substances with a boiling point of -140°C to 100°C, such as CF4, C2F6, cC4F8, CF2H2, CF3H, C3F7OCH3, and CF2ClH. These fluorine gases can partially decompose into fluorinated olefins and aHF under pyrolysis conditions, which is advantageous for the overall yield. Any unreacted organic matter is then decomposed in the second part of the reaction.

[0025] In the first part of the process, the polymer particles and the bed material are heated to 500–900°C, primarily producing TFE, HFP, VDF, cC4F8, and aHF. Depending on the residence time, between 2–20% VDF, 5–15% TFE, 1–5% HFP, 1–8% cC4F8, and 10–35% aHF are formed in a fluidized bed reactor. Residues range from 20–40%. It is noteworthy that this process produces monomers and cC4F8 in high yields; the production of VDF is surprising, as it was not found in any of the aforementioned literature.

[0026] The residence times are controlled via the fluidizing / transport gas quantities and range between 0.4 and 5 seconds for the fluidized bed reactor.

[0027] In a stirred bed reactor, residence times are longer, ranging from 2 to 10 seconds.

[0028] The yields of monomers and aHF are sometimes 50% lower, and the residues are correspondingly higher. The preferred reactor type for the process according to the invention is therefore the fluidized bed reactor. Both reactor types can be operated continuously.

[0029] During the pyrolysis process in the fluidized bed reactor, the smallest polymer particles / fine dust or repolymers and, if applicable, fillers are carried away; these are separated by filtration units or can be isolated in the quenched gas; these residues are collected for further recycling.

[0030] The pyrolysis raw gas is then quenched, during which aHF condenses out. The temperature in the condensation zone is typically 0 to -25°C, preferably -5 to -20°C. The remaining monomer mixture, cC4F8, and any fluorine gases used are separated by distillation. The monomers VDF, TFE, and HFP can be used for the production of new polymers; cC4F8 can be further decomposed into TFE / HFP in a special cleavage step or used as a fluidizing gas.

[0031] During continuous operation, the reactors can become filled with unusable pyrolysis byproducts. This often occurs when the partially fluorinated polymers do not contain fluorine-containing polymers. In a further process step, oxygen, preferably air, is then introduced as a fluidizing / transport gas. The introduced air can contain up to 5% water. The reactor temperatures can then be raised to 1000°C.

[0032] To avoid corrosive stress on the pyrolysis reactors, parallel-connected reactors / regenerators can be used, in which the residues, along with a portion of the bed material, are thermally treated with oxygen. The hot bed material is then pumped back into the pyrolysis reactors by the regenerators; it is evident that carbon-containing bed material is unsuitable.

[0033] The collected, blown-out residues can be fed back into the reactors, preferably into the regenerators, and burned there, or they can be treated in a separate, corrosion-protected standard furnace with oxygen / air.

[0034] The addition of oxygen causes the residues to be burned, producing water, HF and CO2.

[0035] The combustion gases, consisting of water, HF, and CO2, can be passed through a cascade of CaCO3-filled columns, producing CaF2. Alternatively, the combustion gases are passed through water and fixed as CaF2 using Ca(OH)2 or CaCO3.

[0036] Such methods are described in EP 2952478 and EP 3246289.

[0037] In this way, the remaining fluorine in the various residues is recovered as CaF2. To generate anhydrous HF from CaF2, the fluorspar is reacted with SO3 in an energy-intensive process at temperatures of approximately 400°C.

[0038] An alternative method for producing aHF involves passing the combustion gases over hot carbon, producing aHF and syngas (CO / H2). This method is described in WO 2022 / 130212. Examples Example 1

[0039] In a 5L fluidized bed reactor with external heating, filled with 0.3kg SiC spheres (d50 approx. 0.3mm) and operated with heated N2, THV agglomerate (sm. 165°C; composition: TFE 56mol%, %, VDF 32 mol%), MFI (265 / 5kg) 22.5g / 10min) is pyrolyzed under the following conditions, yielding the following results: temperature N2 current Length of stay THV amount THV TFE HFP VDF aHF rest °C g / min sec g g / min % % % % % 550 30 1,2 900 5,5 4 1 4 14 60 600 30 1,2 850 4,0 9 2 10 25 35 700 20 2,0 600 3,3 12 2 13 28 28 Example 2

[0040] Collected polymer residues from the fluidized bed reactor and filters are treated with air at 800°C in a 3L stirred bed reactor filled with 300g of ZrO₂ spheres. The residual fluorine content is 5%. The hot combustion gases are passed into water, which is kept at pH 9 by adding a Ca(OH)₂ suspension. Very fine fluorspar precipitates. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2021 / 165923 [0003, 0021] WO 2010 / 039820 [0003, 0021] WO 01 / 58840

[0005] EP 3246 289 [0006, 0036] WO 2022 / 130212 [0007, 0038] EP 2952478

[0036] Cited non-patent literature

[0000] DBU Final Report (https: / / www.dbu.de / OPAC / ab / DBU-Abschlussbericht-AZ-25198.pdf

[0008]

Claims

[1] Process for the pyrolysis of partially fluorinated polymers in the absence of oxygen and water at temperatures of 550 to 900°C [2] Method according to claim 1, wherein the partially fluorinated polymers may consist of TFE, HFP, VDF, ET, propylene and the CH2 unit is present in the polymer at 1-50 wt.%. [3] Method according to claim 1-2, wherein the oxygen and water content during pyrolysis is a maximum of 3%. [4] Method according to claims 1-3, wherein the partially fluorinated polymers are present as laminates or composites with non-fluorinated polymers, with a proportion of up to 50 wt% [5] Method according to one or more of the preceding claims, wherein the pyrolysis is carried out in a fluidized bed reactor. [6] Method according to one or more of claims 1-4, wherein the pyrolysis is carried out in a stirred bed reactor. [7] Method according to one or more of the preceding claims, wherein N2, Ar, CO2 is used as the fluidizing / transport gas, optionally with the addition of fluorinated gases. [8] Method according to one or more of the preceding claims, wherein the resulting product gas stream consisting of VDF, TFE, HFP, cC4F8 and aHF is quenched, the aHF is condensed and the fluorinated valuables are subjected to distillation. [9] Method according to one or more of the preceding claims wherein residues are burned in an oxygen-containing stream at temperatures up to 1000°C. [10] Method according to claim 9, wherein the combustion gases are passed over a cascade of CaCO3 or introduced into an aqueous CaCO3-, Ca(OH)2 suspension, in both cases CaF2 is produced. [11] Method according to claim 9, wherein the combustion gases are passed over hot carbon and the resulting aHF is condensed.