U.S. patent application number 09/682172 was filed with the patent office on 2002-07-04 for fluorinated alkanoic acid purification process.
Invention is credited to Schultz, James Arnold.
Application Number | 20020087016 09/682172 |
Document ID | / |
Family ID | 24583720 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020087016 |
Kind Code |
A1 |
Schultz, James Arnold |
July 4, 2002 |
FLUORINATED ALKANOIC ACID PURIFICATION PROCESS
Abstract
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 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, (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.
Inventors: |
Schultz, James Arnold;
(Swedesboro, NJ) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
24583720 |
Appl. No.: |
09/682172 |
Filed: |
July 31, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09682172 |
Jul 31, 2001 |
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09644165 |
Aug 23, 2000 |
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6281374 |
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Current U.S.
Class: |
554/188 ;
554/175; 554/187 |
Current CPC
Class: |
C07C 53/21 20130101;
C07C 53/21 20130101; C07C 51/412 20130101; C07C 51/48 20130101;
C07C 51/412 20130101; C07C 51/48 20130101 |
Class at
Publication: |
554/188 ;
554/175; 554/187 |
International
Class: |
C11B 003/00 |
Claims
1. A process comprising (A) acidifying an aqueous solution
comprising an alkali metal salt of a fluorinated alkanoic acid with
an acid to produce an acidified solution, (B) heating said
acidified solution to produce an organic layer, which comprises
said fluorinated alkanoic acid, and an aqueous layer, (C)
separating and recovering said organic layer from the aqueous
layer, (D) washing said organic layer with an acid solution, (E)
isolating said fluorinated alkanoic acid, and (F) optionally
converting said fluorinated alkanoic acid to ammonium or alkali
metal salt wherein said acid used in step (A) contains less than
about 50 volume % water; said acid solution used in step (D)
contains less than about 10 volume % acid; and the pH of said
acidified solution is about 2 or less.
2. The process of claim 1 wherein said fluorinated alkanoic acid
has the formula of X-R.sub.f-COOH; X is hydrogen, fluorine,
chlorine, or combinations thereof; R.sub.f is a fluorinated group
having 5 to 12 carbon atoms per group; and said acidified aqueous
solution has a pH of about 2 or less.
3. The process of claims 1 wherein said fluorinated alkanoic acid
is perfluorinated.
4. The process of claim 1 wherein said fluorinated alkanoic acid is
perfluorooctanoic acid.
5. The process of claim 1, 2, 3, or 4 wherein said aqueous solution
is treated, prior to the step (A), with a soluble aluminum
salt.
6. The process of claim 1, 2, 3, or 4 wherein said acidified
aqueous solution produced in step (A) is treated, prior to step
(B), with a soluble aluminum salt.
7. The process of claim 1, 2, 3, or 4 wherein the acid is sulfuric
acid.
8. The process of claim 5 wherein the acid is sulfuric acid.
9. The process of claim 6 wherein the acid is sulfuric acid.
10. The process of claim 7 wherein said heating in step (B) is
carried out at a temperature of about 35.degree. C. to about
60.degree. C.
11. The process of claim 8 wherein the soluble aluminum salt is
aluminum sulfate.
12. The process of claim 9 wherein the soluble aluminum salt is
aluminum sulfate.
13. The process of claim 11 wherein said heating in step (B) is
carried out at a temperature of about 35.degree. C. to about
60.degree. C.
14. The process of claim 12 wherein said heating in step (B) is
carried out at a temperature of about 35.degree. C. to about
60.degree. C.
15. The process of claim 1 wherein the alkali metal is sodium.
16. The process of claim 1 wherein the process is a batch
process.
17. The process of claim 1 wherein the process is continuous.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 09/644,165 filed Aug. 23, 2000 now allowed
BACKGROUND OF INVENTION
[0002] 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.
[0003] 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).
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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
[0009] 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
[0010] The present invention can be used to isolate any fluorinated
alkanoic acid. The fluorinated alkanoic acid can have the general
formula of X-R.sub.f-COOH in which X is hydrogen, fluorine,
chlorine, or combinations thereof and R.sub.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.
[0011] 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.
[0012] 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).
[0013] 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.
[0014] 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 preferred
acids are those well-known mineral acids. Also preferred is that
the mineral acid contains less than about 50 volume % water because
above 50 volume % can increase the amount of water present in the
resulting mixture and therefore the solubility losses for the
fluorinated alkanoic acid. If the final purification is
distillation, the preferred acid is a relatively nonvolatile acid
that does not co-distill during distillation, such as sulfuric
acid, phosphoric acid, or nitric acid. The preferred acid is
sulfuric acid because of its low volatility. If the final
purification step is by crystallization or other means, any mineral
acid is satisfactory.
[0015] The amount of acid is preferably an amount sufficient or
effective to neutralize the ammonium or metal content of the
fluorinated alkanoic acid salt and to provide an acid environment
sufficient to minimize the solubility of the resulting fluorinated
alkanoic acid. Generally the amount can be determined by the pH of
the acidified aqueous solution, which can be any pH that can
facilitate the isolation or separation of the fluorinated alkanoic
acid. The pH can be as low as about 2 or lower. Excess acid,
however, can be difficult to remove completely in later steps and
is preferably avoided. For example, with sulfuric acid, the
preferred amount is about 1 to about 4, preferably about 1 to about
3, and most preferably about 1.3 times the theoretical amount
required for neutralizing the ammonium or metal ions, based on only
one proton of the sulfuric acid being used.
[0016] In step (B), the acidified solution can be heated to a
temperature and for a period of time sufficient or effective to
produce an organic (lower) layer and an aqueous (upper) layer.
Generally the temperature is preferably above the melting point of
the wet fluorinated alkanoic acid. The wet melting point is
appreciably lower than that of the dry or anhydrous fluorinated
alkanoic acid because of the solubilizing effect of the contained
water. For example, it was found the water content for
perfluorooctanoic acid to be about 12-15% at this point in the
process, and that it is a liquid at temperatures above
35-40.degree. C. in the process even though pure perfluorooctanoic
acid has a melting point of 55-56.degree. C. The separation
temperature may be between about 35 and 60.degree. C. A separation
temperature of 45 to 50.degree. C. is preferred. At temperatures
above about 60.degree. C., the solubility losses of
perfluorooctanoic acid in the aqueous layer may become excessive.
For other fluorinated alkanoic acids, the separation temperature is
preferably only slightly above the melting point for the wet
fluorinated alkanoic acid, and below the point where excessive
solubility losses occur.
[0017] Wishing not to be bound by theory, if the heating is carried
out at temperatures below the melting point of the wet fluorinated
alkanoic acid, the fluorinated alkanoic acid may be initially
present as small particles dispersed throughout the total liquid,
making isolation difficult. However, upon raising the temperature
above the fluorinated alkanoic acid's melting point, these
particles soon coalesce into a separate liquid phase. This phase
can then be easily separated from the aqueous phase by simple
decantation, since the phases form a relatively clean separation
with a minimum interfacial layer.
[0018] Thereafter, in step (C) the aqueous upper layer is separated
from the organic lower layer. Any means known to one skilled in the
art can be used in the separation. Presently decantation is
preferred for it is simple and easy to use.
[0019] Separation removes most of the water and with it a major
portion of the water-soluble impurities.
[0020] The recovered organic layer can be washed with a diluted
acid solution. The acid can be the same as that used in step (A).
The amount of the diluted acid is preferably effective to remove
most of the dissolved acid. It is preferred that the diluted acid
contains less than about 10 volume % acid to produce a purified
fluorinated alkanoic acid. For example, about 5 volume % sulfuric
acid can be satisfactory for the diluted or weak acid wash. One or
more acid washes can be carried out if desired to minimize
distillation residue. Excessive washing such as, for example, more
than 3 washes, is preferably avoided because of solubility losses
for the fluorinated alkanoic acid. The wash can be conducted at any
temperature, preferably at about 10 to about 60.degree. C. more
preferably about 35 to about 60.degree. C., and, most preferably 35
to 50.degree. C. A water wash is not recommended since it may
create emulsion problems due to ionization of the fluorinated
alkanoic acid.
[0021] The purified fluorinated alkanoic acid can be recovered from
the washed organic layer by any means known to one skilled in the
art such as, for example, distillation and crystallization.
Distillation is generally preferred. For example, in a batch
distillation, the first material overhead is generally the fraction
that co-distills with the small amount of dissolved water by a
steam distillation effect. This fraction can also contain most of
any fluorides that remain, in the form of the volatile hydrogen
fluoride. This fraction can be combined with the main product
fractions or recycled back to the start of the purification
process, depending on its amount and quality. After the dissolved
water is removed, the temperature of the distillation generally
rises to that of the boiling point of the fluorinated alkanoic
acid, which can be then readily vacuum distilled to a receiving
vessel. For fluorinated alkanoic acids, which are waxy solids at
ambient temperature, such as perfluorooctanoic acid, it is
preferred to keep tempered water on the condenser to keep them from
freezing out during distillation and plugging the condenser. If
desired, the molten product can be mixed with water to depress its
freezing point, reacted with ammonia to a pH about 5.5, and the
solids adjusted to a standard level for reuse as an emulsifier. The
amount of distillation residue remaining is relatively small,
depending on the efficiency of washing. It can be removed by
refluxing with a sodium hydroxide solution.
[0022] The resulting purified fluorinated alkanoic acid from step
(E) can readily be converted to the ammonium or alkali metal salt
and reused in a polymerization reaction. For convenience, it can be
neutralized to a pH of about 5.5 and standardized with water to an
appropriate concentration of ammonium or alkali metal fluorinated
alkanoate for subsequent use.
[0023] The process of the invention can be used for substantially
increasing the concentration of a fluorinated alkanoic acid alkali
metal or ammonium salt from a dilute aqueous solution of the
fluorinated alkanoic acid ammonium or alkali metal salt as low as
about 10 weight %. At the same time, the fluorinated alkanoic acid
ammonium salt can be separated from nearly all impurities such as
low molecular weight organic fluorides and inorganic fluorides,
making it suitable for reuse in polymerization reactions.
[0024] The process can be run as a batch or continuous operation.
Steps prior to distillation could be carried out in a single
contacting device.
[0025] The efficacy of this simple process is surprising, since
previously referenced U.S. Pat. Nos. 5,442,097 and 5,591,877 both
disclose that, if the original aqueous solution of the salt is
acidified with sulfuric acid, and attempts are made to distill the
resulting fluorinated carboxylic acids, they fail since these acids
form crystallizing hydrates which can block up the distillation
equipment. Moreover, distillation residues are produced in this
case which still contain considerable amounts of fluorine, the
disposal of which is very complex. The inventive process avoids
these difficulties.
[0026] Preferably the fluorinated alkanoic acid recovered in the
invention process is a perfluorinated alkanoic acid, and still more
preferably is perfluorooctanoic acid.
[0027] The following examples are intended to illustrate the
inventive process, but should not be interpreted as limiting the
scope of the invention in any way.
EXAMPLE 1
[0028] To a 1 liter-sample of 20 wt % ammonium PFOA having 780
parts per million by weight (ppm) of fluoride ion was added 200 ml
of saturated aluminum sulfate ((Al.sub.2(SO.sub.4).sub.3) solution.
The mixture was heated to 50.degree. C. and stirred for two hours.
A white precipitate was present. Then 100 g of concentrated
sulfuric acid was added slowly, with a 10.degree. C. heat rise. The
mixture was stirred at 50.degree. C. for one hour, and the phases
were partially separated to simulate a single reactor dip tube
method of separation. That is, about 80% of the upper aqueous phase
was withdrawn, and the bottom organic phase was washed with 300 ml
of 5 volume % sulfuric acid. After 5 minutes, the entire phase was
separated and the organic phase re-washed with 5 volume % sulfuric
acid. The organic phase, containing the PFOA, weighed 363 g. This
was then distilled at atmospheric pressure. The first cut (the
steam distillation cut) was taken with a still-pot temperature of
106.degree. C. and a head temperature of 102.degree. C., and
weighed 103 g.
[0029] The fluoride content was only 17 ppm. The second cut was
taken at atmospheric pressure with a still-pot temperature of
145.degree. C. and a head temperature of 142.degree. C., and
weighed 48 g. This was considered transition material. The third
cut was taken at a vacuum of 27 inches (68.6 cm) of mercury, a
still-pot temperature of 145.degree. C., and a head temperature of
142.degree. C. One hundred ninety (190) g of PFOA product was
recovered. The fluoride content was 7 ppm. The still-pot residue
weighed only 3 g. It was removed by refluxing 50 ml of 6 weight %
NaOH solution for three hours and a second wash of 14 weight % NaOH
refluxed for two hours.
[0030] By comparison, a similar run without the dilute acid washes,
i.e., using only the initial phase cut separation step, gave a
still-pot residue weighing 44 g.
EXAMPLE 2
[0031] A test was carried out on PFOA that had been processed to
remove fluorides by passing through a packed column containing
solid aluminum oxide granules. An 8,420 g solution of 20 weight %
PFOA ammonium salt containing 17 ppm of fluorides was treated with
768 gm of concentrated sulfuric acid.
[0032] Decantation and distillation were carried out as in Example
1, but without the treatment with aluminum sulfate. After
decantation, the aqueous layer weighed 6507 g and contained 77% of
the original fluorides. The organic layer weighed 2681 g. After
distillation, the steam distillation cut weighed 495 g and
contained 4 weight % of PFOA and 81 ppm fluorides. The transition
cut weighed 270 gm and contained 26 wt % PFOA and 33 ppm fluorides.
The first vacuum distillation cut weighed 164 g and contained 70
wt% PFOA and only 7 ppm fluorides, and the second cut weighed 1240
g of pure PFOA. The pot residue was 116 g.
[0033] This example shows that the fluoride content of the vacuum
distillation cuts was excellent using the inventive process even
without aluminum sulfate pretreatment. However, the concentration
of fluorides in the steam distillate was higher than the initial
overall concentration, since fluorides tend to concentrate in the
initial steam distillate. Even when starting with relatively
low-fluoride concentration material, it may be desirable to use the
aluminum sulfate pretreatment option to minimize the fluoride
content during the steam distillation.
EXAMPLE 3
[0034] A 1-liter solution containing 20 weight % of PFOA sodium
salt (0.459 moles) was treated with 2 molar equivalents of
concentrated sulfuric acid (89.9 g). The mixture was warmed to
50.degree. C., and the phases were separated. The bottom organic
layer weighed 231.2 g. This solution was distilled at atmospheric
pressure , using a still-pot temperature of 102.degree. C. to 106
.degree.C. and a head temperature of 102.degree. C., collecting
18.3 g of a 15 weight % solution of PFOA in water. The system was
then cooled and placed under vacuum and distilled, using 50.degree.
C. water on the condensor to prevent freezing and plugging. The
yield was 160 g of PFOA. The still heel was 49.5 g. Yield of PFOA
from the combined distillate fractions was 84%.
Comparative Example 1
[0035] A test was carried out using the previously known steam
distillation process. An 860 g solution of 20 wt % PFOA ammonium
salt was treated with 82.6 g of concentrated sulfuric acid. It was
then steam distilled to remove the PFOA, adding water to the
still-pot to replace the distillate removed as the steam
distillation progressed. The maximum concentration of PFOA in the
distillate was 16 wt %. A process of reverse osmosis would be
required to bring the concentration up to the desired 20%. Even
after 1200 ml of distillate had been recovered, there was still
0.25 wt % of PFOA in the new distillate, showing that the steam
distillation process does not have a well-defined end-point. The
total amount of distillate at this point was about 7 times the
weight of PFOA recovered, showing the very poor energy efficiency
of this process.
[0036] The inventive examples are superior to the above Comparative
Example in terms of space-time efficiency, avoidance of
re-concentrating the PFOA by reverse osmosis, and in energy
consumption.
* * * * *