MXPA00011837A - Method for recovering fluorinated alkanoic acids from waste waters - Google Patents

Abstract

Fluorinated emulsifying agent acids can be isolated from waste waters by initially removing fine-particulate solids and/or fractions that can be converted into solids from the waste water, preferably by precipitation, by subsequently bringing the waste water into contact with an anion exchange resin and by eluting the adsorbed emulsifying agent acids from the latter.

Description

PROCEDURE FOR RECOVERING ALCOHOLIC ACIDS FLUORATED FROM WASTEWATER DESCRIPTIVE MEMORY In the polymerization of the fluorinated monomers in aqueous dispersion, use is made of fluoro alkanoic acids as emulsifiers, since they do not have telogenic properties. In particular, salts are used, preferably ammonium or alkali metal salts, perfluorinated or partially fluorinated alkanocarboxylic acids or alkanesulfonic acids. These compounds are prepared by electrofluorination or by telomerization of fluorinated monomers, which is expensive. Therefore, there have been many attempts to recover these valuable wastewater materials. US-A-5 442 097 describes a process for recovering fluorinated carboxylic acids in usable form from contaminated starting materials. In this process, the fluorinated carboxylic acid is released, if necessary, from these materials in an aqueous medium using a sufficiently strong acid; the fluorinated carboxylic acid is reacted with an appropriate alcohol and the formed ester is distilled. Here, the starting material can be a polymerization liquor; in particular, from an emulsion polymerization in which the fluoropolymer is prepared in the form of colloidal particles with the aid of relatively high amounts of emulsifier. This procedure has proved to be very useful, but requires a certain concentration of fluorinated carboxylic acid in the starting material. From US-A-4 369 266, it is known to pass a filtrate obtained from the ultrafiltration of fluoropolymer dispersions, containing stabilizing and fluorinated emulsifiers, by basic exchange resins, in which the fluorinated emulsifier is retained and recovers through a subsequent elution. DE-A-20 44 986 discloses a process for recovering perfluorocarboxylic acids from a diluted solution, wherein the dilute solution of the perfluorocarboxylic acids is brought into absorption contact with an anion exchange resin with a weak base and the perfluorocarboxylic acid present in the solution is thus absorbed in the anion exchange resin; The anion exchange resin is eluted with an aqueous solution of ammonia and the absorbed perfluorocarboxylic acid is transferred, therefore, to the eluent, and the acid is finally isolated from the eluted material. However, complete elution requires relatively large amounts of diluted ammonia solution and this procedure is also very time consuming. These disadvantages are overcome by the process known from US-A-4 282 162 for the elution of fluorinated emulsifier acids absorbed in basic anion exchangers, in which the elution of the fluorinated emulsifier acid absorbed from the anion exchanger is It is carried out using a mixture of dilute mineral acid and an organic solvent. In this process, the ion exchange resin is regenerated at the same time by the use of the acid. It has been found that the latter process presents problems in industrial practice when, in particular, the processed wastewater contains very fine solids, which in the past were often not recognized or at least not recognized as causing a problem. In this case, the devices containing the anion exchange resin are clogged with these solids more or less rapidly, which becomes evident as a result of the increase in the flow resistance and the reduction in performance. The filters or porous counter current glasses commonly used are ineffective here. It has also been found that these difficulties are caused by fine solids maintained in relatively stable colloidal suspension by the emulsifying acids. When these acids are removed after the system by the anion exchange resin, this relatively stable dispersion is destroyed and the solid precipitates and clogs the ion exchange resin. Therefore, it has also been found that the performance of the process known from US-A-4 282 162 can be considerably improved and also made suitable for waste waters containing fine solids if these solids are removed from the wastewater before that come in contact with the anion exchange resin.

- A further aspect of the invention is that it is possible to eliminate not only existing solids, but also other interfering constituents, which can become solids. Such interfering constituents may be other acids or their salts which bind in the same way to the ion exchange resin and, therefore, not only limit the ion exchange capacity, but may also require special precautions during and / or after the elution of the emulsifying acids. An example of such an interfering acid is oxalic acid, which is often used as a pH regulator. The addition of calcium ions in stoichiometric amounts or in excess or deficiency; for example, as chloride or hydroxide, it allows all the oxalic acid or a part to precipitate as sparingly soluble oxalate together with any additional solid present finely divided, According to this, the invention provides a process for the recovery of emulsifying acids fluorinated from wastewater, which consists, first, in removing fine solids and / or material that can be converted to fine solids from wastewater, in subsequently joining the fluorinated emulsifying acids in an anion exchange resin and in eluting the fluorinated emulsifying acids of the latter. In the following, additional aspects of the invention and its preferred embodiments are described in greater detail.

The wastewater suitable for the treatment is the wastewater of the process, in which surfactant fluorinated alkanoic acids are present. The process is particularly suitable for wastewater from the polymerization of fluorinated monomers by the emulsion method, in which the fluorinated monomer is converted in the presence of a relatively high concentration of fluorinated emulsifying acid and with slow stirring to a finely divided polymer, which is in a finely dispersed colloidal form and in which the obtained latex coagulates; for example, by intensive agitation, after the desired solids concentration has been achieved, so that the polymer is precipitated as a fine powder. It has been found that in the known treatment, it is especially relatively low molecular weight polymer material that causes difficulties; the adverse effect of these low molecular weight polymers becomes particularly noticeable when the polymerization process leads to a broad molecular weight distribution. Also, in the case of said 'difficult' wastewater, the process of the invention shows its capabilities. The method to remove fine solids depends on particular circumstances: In the case of acidic wastewater, it may be sufficient to carry out a neutralization - possibly partial - with appropriate bases, such as calcium hydroxide, resulting in colloid precipitation - and any precipitable substance, such as oxalate ions present - while the emulsifying acid or its salt remain in solution. Another possible way to precipitate the interfering colloids is the addition of appropriate metal salts; for example, aluminum salts, such as aluminum chloride and aluminum sulfate; calcium salts, such as calcium chloride; magnesium salts, such as magnesium chloride and magnesium sulfate; salts of iron, such as iron (II) chloride or iron chloride (III) and iron sulfate. In the case of acidic wastewater, the addition of corresponding metals, such as aluminum, iron or magnesium, is also possible. To improve flocculation, small amounts of flocculating agent can also be added. An additional possible way of precipitating interfering colloids is electrocoagulation. Here, an electric field is applied to the wastewater to coagulate the colloidal particles. In the case of inert electrodes (for example, titanium), the particles are deposited on the surfaces. In the case of the soluble electrodes (for example, iron and / or aluminum), the metal cations that have a high ratio of charge to diameters are introduced into the solution and they perform the coagulation as in the case of the addition of metal salts . An advantage of electrocoagulation is that it prevents the additional introduction of anions, such as chloride or sulfate. To improve flocculation, small amounts of flocculating agent can be added. . < - t Appropriate mechanical methods for removing fine solids are cross-flow filtration (eg, using membranes, centrifuges), deep-bed filtration (eg sand bed filters) or coating filtration with the addition of an auxiliary filtration (for example, cellulose, perlite, kieselguhr). The precipitated solids may be separated in a manner known per se; for example, by filtration, if it is necessary to use a filtering aid; by decanting, flotation or sedimentation. The absorption of emulsifying acids in ion exchange resins can be carried out in a manner known per se. Suitable resins are, in particular, strong base anion exchange resins such as those obtained, for example, under the trade names ®AMBERLITE IRA-402, ®AMBERJET 4200 (both Rohm &Haas), ®PUROLITE A845 ( Purolite GmbH) or ®LEWATIT MP-500 (Bayer AG). The absorption can be carried out in a manner known per se, with the ion exchange resins placed in conventional apparatuses, such as tubes or columns through which the waste water flows. Also, the elution of the bound emulsifying acids is carried out in a manner known per se, preferably from the method described in US-A-4 282 162. Appropriate methods for isolating the emulsifying acids in the high purity required for their use in the polymerization are, for example, those described in the aforementioned document US-A-5 442 097 or the one described in US-A-5 312 935, in which, first of all, the eluted material is substantially freed from water and then treated with oxidizing agents. Wastewater that remains after the absorption of the emulsifying acids is treated in a known manner, depending on the content of other materials or returned to the process. If desired, the residual fluorinated emulsifying acids can be removed using standard absorbers, such as activated carbon. The invention is illustrated by the following examples.

EXAMPLE 1 The starting material used is wastewater from the copolymerization of tetrafluoroethylene (TFE) and perfluoro (n-propylvinyl) ether (PPVE), in which the ammonium salt of n- and iso-perfluorooctanoic acid (PFOA) is used as an emulsifier. ) in a molar ratio of 9: 1. The concentration of PFOA in the liquor is 1200 mg / l and the concentration of oxalic acid is 1600 mg / l. In a stirred vessel, 14 liters of liquor are mixed with 1.5 g / l of a solution of aluminum chloride at a concentration of 10% by weight and vigorously stirred. The precipitate that forms is filtered. Approximately 50 ml of commercial ion exchange resin with a strong base (®AMBERLITE IRA-402, Rohm &Haas, styrene-divinylbenzene type, anion: chloride, gel, total capacity: 1.3 eq / l, volumetric density: 710 g / l) are introduced into a cylindrical glass column (length: 25 cm, diameter: 16 mm) provided with a porous glass and rinsed with water. To charge the ion exchanger, the pretreated liquor is pumped up through the column at a linear velocity of 1 m / h by means of a pump; the water leaving the column is collected and the concentration of PFOA is determined for the mass balance. After loading, the column is rinsed with 100 ml of water. To regenerate the ion exchanger, 150 ml of a mixture of 89% by weight methanol, 7% by weight concentrated sulfuric acid and 4% by weight water are passed through the column at a linear velocity of 0.5 m / hy the eluted material is collected. Subsequently, the column is rinsed with 100 ml of water. The eluate contains 85% of the emulsifying acid present in the wastewater and 3900 mg / l of oxalic acid.

EXAMPLE 2 In a stirred vessel, 14 liters of liquor, as in example 1, are mixed with 1.5 g / l of a solution of aluminum chloride at a concentration of 10% by weight and stirred vigorously. The pH is then adjusted to 7.5 using lime milk at a concentration of 10% by weight. The precipitate that forms is filtered and the pH of the solution is adjusted to 4 using dilute sulfuric acid. The arrangement and procedure for charging and regenerating the ion exchanger are similar to those of example 1. Here, the eluted material contains 95% of the emulsifying acid present in the wastewater and 1 mg / l of oxalic acid.

EXAMPLE 3 16 liters of wastewater from the treatment of fluorinated polymers are placed in a stirred container. The polymerization uses the ammonium salt of PFOA as an emulsifier, and the concentration of PFOA is 1200 mg per liter. 2 grams of a solution of aluminum chloride at a concentration of 10% by weight are added to this solution and the mixture is stirred vigorously. Lime slurry is then added at a concentration of 10% by weight to bring the pH to 7.5 and 3 mg / l of a flocculating agent is added (®PRAESTOL TO 3015 L, Stockhausen Gmbh &Co. KG, polyacrylamide). The precipitate that forms is filtered and the pH is adjusted to 4 using sulfuric acid. The charging and regeneration of the ion exchanger is carried out as in example 1. Here, the eluate contains 91% of the emulsifying acid present in the wastewater.

COMPARATIVE EXAMPLE The starting material used is a mother liquor from the copolymerization of TFE and PPVE, in which it is used as an ammonium salt emulsifier of PFOA. The concentration of PFOA is 1200 mg / l. Approximately 50 ml of the strong base ion exchange resin specified in example 1 is introduced into a cylindrical glass column (length: 25 cm, diameter: 16 mm) provided with a porous glass and rinsed with water. To charge the ion exchanger, the untreated liquor is pumped up through the bed by means of a pump. The pressure that falls on the bed of the ion exchanger is measured using a pressure gauge. The loading experiment had to stop after 400 ml of liquor had passed, since the resin was conglutinated as a result of the precipitated polymer.

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for recovering fluorinated emulsifier acids from wastewater, consisting, firstly, in removing fine solids and / or material that can be converted to fine solids from wastewater from the polymerization of fluorinated monomers, in subsequently binding the acids Fluorinated emulsifiers in an anion exchange resin and in eluting the fluorinated emulsifying acids of the latter.

2. The process according to claim 1, further characterized in that the fine solids are precipitated.

3. The process according to claim 1, further characterized in that the fine solids are mechanically removed.

4. The method according to claim 1, further characterized in that the material that is converted to solids is precipitated.

5. The process according to one or more of claims 1, 2 and 4, further characterized in that the precipitated materials are separated by sedimentation.

6. - The method according to one or more of claims 1, 2 and 4, further characterized in that the precipitated materials are separated by flotation.

7. A process according to one or more of the preceding claims, further characterized in that, for the recovery of acids of pure fluorinated emulsifiers having a very low content of oxalic acid, it consists of treating the waste water with a solution of aluminum salt while intensively mixing, then adjusting the pH to a value on the scale of 6 to 7.5 by means of 10 lime slurry, filtering the precipitate that forms and, after adjusting the pH of the solution to a value less than 7 using sulfuric acid, in passing the solution on an ion exchanger.

8. A method according to one or more of the preceding claims, further characterized in that the anion exchange resin used is a strong base anion exchange resin.

9. A process according to one or more of the preceding claims, further characterized in that the elution of the fluorinated emulsifying acid from the anion exchange resin is carried out using a mixture of dilute mineral acid and an organic solvent.