HK1038004A - Method for recovery of fluorinated alkanoic acids from waste waters - Google Patents

Description

Process for recovering fluorinated alkanoic acids from wastewater

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Fluorinated monomers are used as emulsifiers in the polymerization of fluorinated monomers in aqueous dispersions because fluorinated alkanoic acids have no telogenic properties. In particular, salts, preferably alkali metal or ammonium salts, of perfluorinated or partially fluorinated alkanecarboxylic or alkanesulfonic acids are used. These compounds are prepared by electrofluorination or by telomerization of fluorinated monomers, and these compounds are expensive. Therefore, many efforts have been made to recover these valuable materials from wastewater.

US-A-5442097 discloses A process for recovering fluorinated carboxylic acids from contaminated feedstocks in A usable form. In this process, the fluorinated carboxylic acids are released from these materials, if desired, in an aqueous medium with a sufficiently strong acid, the fluorinated carboxylic acids are then reacted with a suitable alcohol and the resulting ester is distilled off. The starting material here may be a polymerization mother liquor, in particular a polymerization mother liquor obtained from an emulsion polymerization, in which the fluorinated polymer is prepared in the form of colloidal particles with the aid of relatively large amounts of emulsifier. This process has proven to be very useful, but it requires a certain concentration of fluorinated carboxylic acid in the waste.

DE-a-2044986 discloses a process for recovering perfluorocarboxylic acid from a dilute solution, in which a dilute solution of perfluorocarboxylic acid is brought into adsorptive contact with a weakly basic anion exchange resin, whereby perfluorocarboxylic acid present in the solution is adsorbed on the anion exchange resin, the anion exchange resin is eluted with an aqueous ammonia solution, whereby adsorbed perfluorocarboxylic acid is transferred to an eluent, and the perfluorocarboxylic acid is finally separated from the eluent. However, relatively large amounts of dilute aqueous ammonia are required to complete the elution, and this process is also time consuming. These disadvantages are overcome by the process known from US-A-4282162 for elution of fluorinated emulsifier acids adsorbed on basic anion exchangers, in which the elution of the adsorbed fluorinated emulsifier acids from the anion exchanger is carried out with A mixture of dilute inorganic acids and organic solvents. In this process, the ion exchange resin is regenerated simultaneously with this acid.

It has been found that the last-mentioned process has problems in industrial practice, particularly when the treated waste water contains very fine solids, which in the past have not been regarded as a problem or at least not. In this case, the equipment containing the anion exchange resin is rapidly clogged with these solids to various degrees, and this problem becomes very noticeable due to the increase in flow resistance and the decrease in performance. Here, the upstream filters or frits typically used are ineffective.

It has also been found that these difficulties are caused by the emulsifier acid keeping the fine solids in a relatively stable colloidal suspension. When these acids are subsequently removed from the system by the anion exchange resin, this relatively stable dispersion is broken down, allowing solids to settle and plug the ion exchange resin. It has thus also been found that the performance of the process known from US-A-4282162 can be considerably improved if these solids are removed from the waste water before it is contacted with the anion exchange resin, thereby making it suitable for use also in waste water containing fine solids.

In a further aspect of the invention, it is possible to remove not only the solids present but also other interfering components which can be converted into solids. Such interfering components may be other acids or salts thereof which are also bound to the ion exchange resin and therefore not only occupy the ion exchange capacity, but may also require special precautions during and/or after elution of the emulsifier acid.

An example of such an interfering acid is oxalic acid, which is often used as a buffer. For example, the addition of calcium ions as chloride or hydroxide, either in stoichiometric amounts or in excess or in deficiency, enables all or a portion of the oxalic acid to be deposited as sparingly soluble oxalate, preferably with any other finely divided interfering solids present.

Accordingly, the present invention provides a process for recovering fluorinated emulsifier acid from wastewater, which process comprises first removing fine solids and/or materials convertible to fine solids from the wastewater, subsequently binding the fluorinated emulsifier acid to an anion exchange resin, and eluting the fluorinated emulsifier acid from the anion exchange resin. Further aspects of the invention and preferred embodiments thereof are described in detail below.

Suitable wastewater for treatment are process wastewaters in which surface-active fluorinated alkanoic acids are present. This process is particularly suitable for the waste water from the emulsion polymerization of fluorinated monomers, in which the fluorinated monomers are converted into finely divided polymers in the presence of relatively high concentrations of fluorinated emulsifier acids and with gentle stirring, and in which the latex obtained is coagulated, for example by vigorous stirring, after the desired solids concentration has been reached, so that the polymers are deposited as a fine powder.

It has been found that in known processes, relatively low molecular weight polymeric materials cause various difficulties; the adverse effects of these low molecular weight polymers become particularly pronounced when the polymerization process results in a broad molecular weight distribution. In the case of such "difficult" waste streams, the process of the present invention demonstrates its capabilities.

The method of removing the fine solids depends on these particular circumstances:

in the case of acidic waste water, neutralization may be carried out sufficiently (possibly partially) with a suitable base, such as calcium hydroxide, to cause the precipitation of colloids and any precipitable substances, such as oxalate ions present, while the emulsifier acid or its salt remains in solution.

Another possible way of interfering with colloidal deposition is to add suitable metal salts, such as aluminum salts (e.g. aluminum fluoride and aluminum sulfate), calcium salts (e.g. calcium chloride), magnesium salts (e.g. magnesium chloride and magnesium sulfate), iron salts (e.g. iron (ii) chloride or iron (iii) chloride and iron sulfate). In the case of acidic waste waters, it is also possible to add corresponding metals, for example aluminum, iron or magnesium. To improve flocculation, small amounts of flocculant may also be added.

Another possible method of causing the interfering colloids to deposit is electrocoagulation. Here, an electric field is applied to the wastewater to coagulate the colloidal particles. In the case of an inert electrode (e.g., titanium), the particles deposit on the surface. In the case of soluble electrodes (e.g. iron and/or aluminium), there is a high charge: the metal cations of the diameter ratio enter the solution, which causes coagulation as in the case of the addition of metal salts. The advantage of electrocoagulation is that it does not require the addition of further anions, such as chloride or sulphate ions. To improve coagulation, small amounts of coagulant may be added.

Suitable mechanical methods for removing fine solids are cross-flow filtration (e.g. using membranes, centrifugation), deep bed filtration (e.g. sand bed filters) or coating filtration with added filter aids (e.g. cellulose, perlite, diatomaceous earth).

The settled solids can be separated off in a manner known per se, for example by filtration (if desired with filter aids), decantation, flotation or sedimentation.

The adsorption of the emulsifier acids on the ion exchange resins can be carried out in a manner known per se. Suitable resins are, in particular, strongly basic anion exchange resins, for example under the trade name^AMBERLITEIRA-402、^AMBERJET 4200 (both manufactured by Rohm)&Haas offer),^PUROLITE A845 (supplied by Purolite GmbH) or^LEWATITMP-500 (supplied by Bayer AG).

The adsorption can be carried out in a manner known per se using ion exchange resins which are accommodated in conventional apparatus, such as tubes or columns, through which the waste water flows.

Elution of the bound emulsifier acid is likewise carried out in A manner known per se, preferably as disclosed in US-A-4282162.

Suitable methods for separating the emulsifier acids in the high purity required in polymerization applications are, for example, those disclosed in the abovementioned US-A-5442097 or in US-A-5312935, in which the eluate is first rendered substantially free of water and then treated with an oxidizing agent.

The waste water remaining after adsorption of the emulsifier acids is treated in a known manner, depending on the content of other materials or whether it is returned to the process. If desired, the residual fluorinated emulsifier acid can also be removed with conventional adsorbents such as activated carbon.

The invention is illustrated by the following examples.

Example 1

The raw material used was waste water from the copolymerization of Tetrafluoroethylene (TFE) and perfluoro (n-propyl vinyl ether) (PPVE), in which ammonium salts of perfluoro-n-octanoic acid and perfluoro-iso-octanoic acid (PFOA) in a molar ratio of 9: 1 were used as emulsifiers. The concentration of PFOA in the mother liquor was 1200 mg/l and the oxalic acid concentration was 1600 mg/l.

In a stirred vessel, 14 l of the mother liquor are mixed with 1.5 g/l of a 10% strength by weight aluminum chloride solution and stirred vigorously. The resulting precipitate was filtered off.

About 50 ml of commercial, strongly basic ion exchange resin (A)^AMBERLITE IRA-402,Rohm &Haas; styrene-divinylbenzene type, anion: chloride, gel, total capacity: 1.3 gram equivalent/liter, bulk density: 710 g/l) was charged into a cylindrical glass column (length: 25 cm, diameter: 16 mm) and the glass column was filled with frit and washed with water. To feed the ion exchanger, the pretreated mother liquor was pumped in at a linear speed of 1 m/h. The water leaving the column is collected by passing it upwards through the column and the concentration of PFOA is determined by mass balance. After feeding, the column was washed with 100 ml of water.

To regenerate the ion exchanger, 150 ml of a mixture of 89% by weight of methanol, 7% by weight of concentrated sulfuric acid and 4% by weight of water were passed through the column at a linear speed of 0.5 m/h and the eluate was collected. The column was then washed with 100 ml of water.

The eluent contained emulsifier acid present in 85% of the wastewater and 3900 mg/l oxalic acid.

Example 2

In a stirred vessel, 14 l of the mother liquor from example 1 are mixed with 1.5 g/l of a 10% strength by weight aluminum chloride solution and stirred vigorously. The pH is subsequently adjusted to 7.5 with a lime emulsion having a concentration of 10% by weight. The resulting precipitate was filtered off and the pH of the solution was adjusted to 4 with dilute sulfuric acid.

The protocol and procedure for the feeding and regeneration of the ion exchanger are similar to those of example 1.

Here, the eluent contained 95% of the emulsifier acid present in the wastewater and 1 mg/l oxalic acid.

Example 3

16 liters of waste water obtained from the treatment of fluorinated polymers were placed in a stirred vessel. The polymerization was carried out using an ammonium salt of PFOA as emulsifier at a PFOA concentration of 1200 mg/l. 2 g of a 10% strength by weight aluminum chloride concentration are added to this solution and the mixture is stirred vigorously. A lime emulsion with a concentration of 10% by weight was subsequently added to bring the pH to 7.5, and 3 mg/l of flocculant ( PRAESTOL A3015L, Slockhausen GmbH & Co. KG; polyacrylamide) were added. The resulting precipitate was filtered off and the pH was then adjusted to 4 with sulfuric acid.

The ion exchanger was fed and regenerated as in example 1.

Here, the eluent contained 91% of the emulsifier acid present in the wastewater.

Comparative example

The starting material used was the mother liquor resulting from the copolymerization of TFE and PPVE, in which the ammonium salt of PFOA was used as emulsifier. The concentration of PFOA is 1200 mg/l.

Approximately 50 ml of the strongly basic ion exchanger specified in example 1 were loaded onto a cylindrical glass column (length: 25 cm, diameter 16 mm), loaded with frit and washed with water. To feed the ion exchanger, the untreated mother liquor was pumped up through the bed with a pump. The pressure drop of the ion exchanger bed was measured with a pressure gauge. The feeding experiment had to be stopped after the passage of 400 ml of mother liquor, since the precipitated polymer lumps the resin.

Claims (10)

1. A process for recovering fluorinated emulsifier acid from waste water, which comprises first removing fine solids and/or materials convertible to fine solids from the waste water, subsequently binding the fluorinated emulsifier acid to an anion exchange resin, and eluting the fluorinated emulsifier acid from the anion exchange resin.

2. The process according to claim 1, wherein waste water resulting from the polymerization of fluorinated monomers is used.

3. A method according to claim 1 or 2, wherein fine solids are deposited.

4. A process according to claim 1 or 2, wherein fine solids are removed mechanically.

5. A method according to claim 1 or 2, wherein a material convertible to a solid is deposited.

6. A method according to one or more of claims 1, 2, 3 and 5, wherein the deposited material is separated by means of a deposition method.

7. A method according to one or more of claims 1, 2, 3 and 5, wherein the deposited material is separated by flotation.

8. A process for the recovery of pure fluorinated emulsifier acids having a very low oxalic acid content, which comprises treating the waste water with an aluminium salt solution while mixing slowly; then adjusting the pH value to 6-7.5 by using lime emulsion; filtering out the resulting sediment; after adjusting the pH of the solution to below 7 with sulfuric acid, the solution was passed through an ion exchanger.

9. The process according to one or more of the preceding claims, wherein the anion exchange resin used is a strongly basic anion exchange resin.

10. The process according to one or more of the preceding claims, wherein the elution of the fluorinated emulsifier acid from the anion exchange resin is carried out with a mixture of dilute inorganic acid and organic solvent.