Organic compounds -- part of the class 532-570 series – Organic compounds – Fatty compounds having an acid moiety which contains the...
Reexamination Certificate
2001-07-31
2002-08-20
Carr, Deborah D (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Fatty compounds having an acid moiety which contains the...
C554S226000, C554S187000, C562S605000
Reexamination Certificate
active
06437159
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to a process for the purification of fluorinated alkanoic acids such as, for example, perfluorooctanoic acid, from aqueous solutions of their salts.
The ammonium or alkali metal salts of fluorinated alkanoic acids are generally used in the aqueous emulsion polymerization of fluorinated olefin monomers such as tetrafluoroethylene, chlorotrifluoroethylene or vinylidene fluoride because of their high surface activity and low rate of telogen formation. In the course of these polymerizations, some of the fluorinated olefins hydrolyze to produce fluoride ions. After polymerization is complete and the resulting fluoropolymer is isolated, the fluorinated alkanoic acid ammonium or alkali metal salt is present in a dilute aqueous solution, along with ionic fluorides and other impurities. It is highly desirable to recover these fluorinated alkanoic acids and their salts for reuse, both because of their high cost and the need to minimize pollution problems. For such reuse, any impurities in the recovered compounds that may inhibit or change the course of the subsequent polymerization must be removed. Of particular use in such applications are the ammonium or alkali metal salts of fluorinated alkanoic acids, such as, for example, perfluorooctanoic acid (PFOA).
A known method of purification is by acidifying a fluorinated alkanoic ammonium or alkali metal salt solution by adding sulfuric acid until a pH of 1 to 2 is obtained, and subjecting the acidified solution to steam distillation. This is an energy-inefficient process, and yields a dilute solution of about 10-16 wt % fluorinated alkanoic acid. This must be neutralized with ammonia or alkali metal and concentrated for reuse by a means such as reverse osmosis. In an additional difficulty, the fluorides in the solution can co-distill during the steam distillation and corrode the distillation equipment.
U.S. Pat. No. 4,609,497 discloses a process for the recovery of ammonium or alkali metal salt of PFOA from a solution containing a selected nonionic polyether by extracting the ammonium salt with an organic liquid, decanting the organic layer containing the PFOA, contacting it with alumina to adsorb the PFOA, contacting the alumina with ammonium hydroxide to remove the PFOA , acidifying and decanting the resulting solution, steam distilling the organic phase in the presence of an acid and an oxidizing agent, and neutralizing the PFOA with ammonium hydroxide. This process is cumbersome to carry out, and requires the addition and removal of an organic extractant and an alumina adsorbent.
U.S. Pat. No. 4,282,162 discloses a process wherein, after a fluorinated carboxylic acid has been recovered from an aqueous solution by adsorption on a basic ion exchanger; it is eluted using a mixture of a mineral acid and an organic solvent. The eluate is then decanted, the lower organic layer neutralized and acidified, and the precipitated fluorinated carboxylic acid filtered. The fluorinated carboxylic acid obtained by this process is not of polymerization grade. U.S. Pat. No. 5,312,935 discloses a process for purifying such acid by oxidizing it in a solution containing less than 9% by weight of water. This process requires special ion exchange equipment, frequent replacement of the ion exchange resin, the addition and removal of an organic solvent, and a further purification step.
U.S. Pat. Nos. 5,442,097 and 5,591,877 disclose a process for recovery of fluorinated carboxylic emulsifiers from an aqueous solution containing its salt by acidification of the salt, reacting the acid with an alcohol to form the ester, distilling the resulting ester mixture, and decanting the ester layer. The resulting ester may be reacted with aqueous ammonia to form the corresponding ammonium salt. This process requires introduction of an organic reactant, which must be recovered, and requires carrying out additional esterification and de-esterification reactions.
The above processes are complicated and expensive to carry out, requiring multiple conversion steps in which the fluorinated alkanoic compound is either transferred to other media, such as an organic solvent or adsorbing solid, or converted to an organic ester, followed by transfer or conversion back to the fluorinated alkanoic compound. There is a need to develop a simpler process in which the fluorinated alkanoic acid can be isolated by an efficient means, and without such transfers or conversions. An advantage of the invention is that the invention process does not require transferring the fluorinated alkanoic acid to an organic solvent or an adsorbent, or its conversion to an organic ester. Another advantage of the invention is it does not require concentration of the purified ammonium or alkali metal salt solution with reverse osmosis, and it minimizes the content of corrosion-causing fluorides.
SUMMARY OF INVENTION
A process that can be used for isolating a fluorinated alkanoic acid from an aqueous solution also containing inorganic fluorides is provided. This process comprises (A) acidifying an aqueous solution comprising an ammonium or alkali metal salt of a fluorinated alkanoic acid to produce an acidified solution, (B) heating the acidified solution to produce an organic layer and an aqueous layer in which the organic layer comprises the fluorinated alkanoic acid, (C) separating and recovering the organic layer from the aqueous layer, (D) optionally washing the organic layer with an acid solution, (E) optionally isolating the fluorinated alkanoic acid, and (F) further optionally converting the fluorinated alkanoic acid to its ammonium or alkali metal salt.
DETAILED DESCRIPTION
The present invention can be used to isolate any fluorinated alkanoic acid. The fluorinated alkanoic acid can have the general formula of X-R
f
—COOH in which X is hydrogen, fluorine, chlorine, or combinations thereof and R
f
is a fluorinated group that can be saturated or unsaturated, linear or branched such as methyl-branched alkylene group having 5 to 12, preferably 5 to 10 carbon atoms per group. Generally the fluorinated alkanoic acid can contain some inorganic fluorides, from about 5 to about 1200, more typically about 10 to about 1000 mg/kg of the fluorinated alkanoic acid (ppm). The fluorinated alkanoic acid can be present as an aqueous solution of a metal or, preferably, ammonium salt.
Examples of such fluorinated alkanoic acids include, but are not limited to, perchlorofluoroalkanoic acids and perfluoroalkanoic acids. One such perfluoroalkanoic acid is perfluorooctanoic acid, also called perfluorocaprylic acid.
Optionally, the aqueous solution can be, before or after step (A), treated with an amount of a soluble aluminum salt. Preferably the amount of aluminum salt is at least equivalent to the inorganic fluoride content present in the aqueous solution. The treatment can be carried out for an effective period of time such that the inorganic fluorides are reacted to form mixed aluminum fluorides. The mixed aluminum fluorides can then be substantially removed in the separation step (C). Preferably this treatment is done before step (A). Also preferably the aluminum salt is soluble over the pH range of the process, and contains the same counter-ion as the acid to be used in step (A). For example, if the acid used in step (A) is sulfuric acid, the preferred aluminum salt is aluminum sulfate. The temperature of treatment can be any convenient temperature. This optional step is particularly preferred if the initial aqueous solution is high in fluorides. Using this procedure, the fluoride content of the fluorinated alkanoic acid and the amount of distillation residue can be reduced to an acceptable level, which means that the residue can be easily removed, for the recovery process.
Any acid that does not interfere or react with the fluorinated alkanoic acid can be used in step (A). The preferred acid is a mineral acid to react with the fluoroalkanoate and form the free fluorinated alkanoic acid, which is only slightly soluble in the mineral acid under these conditions. Examples of
Carr Deborah D
E. I. du Pont de Nemours and Company
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