Recovery of IDA and Glauber's salt from waste crystal liquors

The separation and recovery of iminodiacetic acid and sodium sulfate decahydrate from sodium sulfate solutions such as the liquor generated in the production of iminodiacetic acids are disclosed. The separation is accomplished by adjusting the temperature of the sodium sulfate solutions to crystallize the iminodiacetic acid and sodium sulfate decahydrate. Nitrilotriacetic acid optionally can be isolated prior to the crystallization of the IDA and sodium sulfate decahydrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the recovery of iminodiacetic acid and Glauber's 
Salt (Na.sub.2 SO.sub.4.lOH.sub.2 O) from sodium sulfate solutions such as 
the liquor generated in the process of producing iminodiacetic acid. 
2. Description of the Prior Art 
Typical prior art processes for the recovery of iminodiacetic acid from 
sodium sulfate solutions are disclosed in U.S. Pat. Nos. 3,808,269 and 
4,299,978. 
U.S. Pat. No. 3,808,269, the disclosure of which is herein incorporated by 
reference, discloses a process of recovering iminodiacetic acid (IDA) from 
a starting aqueous solution of sodium sulfate and the amino acid having a 
temperature above about 33.degree. C. and containing at least 5% amino 
acid. The process comprises adjusting the pH of the starting solution to 
1.5-3 to form an IDA precipitate and a first mother liquor; separating the 
IDA precipitate from the first mother liquor; and recovering the separated 
IDA. Sodium sulfate can then be precipitated from the first mother liquor 
by adjusting the temperature so as to prevent precipitation of IDA. 
U.S. Pat. No. 4,299,978, the disclosure of which is herein incorporated by 
reference, discloses a process for separating an "iminodiacetic acid 
component" from an aqueous glycine solution including IDA. The process 
comprises adding sulfuric acid in the presence of a sodium salt to the 
aqueous solution so as to lower the pH to 1.5 or less, whereby an 
"iminodiacetic acid component" is crystallized from the solution, and 
separating the precipitated IDA component. Glycine can thus be efficiently 
recovered with minimal levels of IDA. Glauber's salt is not generated. 
The foregoing references use processes where the precipitation of sodium 
sulfate with the amino acid is avoided. These processes generate waste 
liquor streams which include a substantial amount of product. Streams such 
as this have heretofore been discarded. 
Other approaches to the recovery of amino acids include U.S. Pat. No. 
3,510,575 where glycine is separated from NH.sub.4 Cl, and U.S. Pat. No. 
4,691,054 where amino acids are isolated by ion exchange from systems that 
are substantially free of inorganic ions (such as sodium sulfate). 
SUMMARY OF THE INVENTION 
The problems of the prior art have been overcome by the present invention 
which provides a process for separating IDA and Glauber's Salt from amino 
carboxylate containing solutions such as the waste liquors generated from 
the production of IDA. 
It is therefore an object of the present invention to provide a process to 
minimize generation of waste from the production of IDA. 
It is a further object of the present invention to provide a process for 
the recovery of value from the waste generated from the production of IDA. 
It is a still further object of the present invention to provide a process 
which reduces disposal costs in the production of IDA. 
According to the present invention, these and other objects which will 
become more apparent, are accomplished by providing a process for 
separating and recovering IDA and sodium sulfate decahydrate from a liquor 
containing IDA and sodium sulfate, which entails forming a slurry of 
precipitated IDA, sodium sulfate decahydrate and mother liquor by, for 
example, adjusting the temperature of the liquor to a level sufficient to 
crystallize the IDA and Glauber's Salt, followed by separation of the 
mixed crystals from the mother liquor. The mixed crystals can be recycled 
to a point in the IDA Production process. NTA optionally can be isolated 
prior to crystallizing the IDA and Glauber's Salt. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of preparing IDA from the corresponding nitrile can be 
accomplished according to the following sequence of reactions: 
EQU HN(CH.sub.2 CN).sub.2 +2H.sub.2 O+2NaOH.fwdarw.HN(CH.sub.2 COONa).sub.2 
+2NH.sub.3 
EQU HN(CH.sub.2 COONa).sub.2 +H.sub.2 SO.sub.4 .fwdarw.HN(CH.sub.2 COOH).sub.2 
+Na.sub.2 SO.sub.4 
Much of the amino acid now sent to waste in the IDA purge liquor from IDA 
production processes can be recovered, and optionally recycled, in the 
process of the present invention. This can be accomplished either 
batchwise or in a continuous process by forming a slurry comprising 
precipitated IDA and sodium sulfate decahydrate, and mother liquor. In one 
example of the batch process, aqueous solutions containing IDA and sodium 
sulfate, such as waste liquor and recycled liquor produced in the process 
for the production of IDA, are charged to a cooling crystallizer. The 
mixture is cooled to a temperature effective for precipitating the amino 
acid and Glauber's Salt. IDA seed crystals can be added at about the 
saturation temperature of IDA in the solution. Glauber's Salt seed 
crystals can be added at about the saturation temperature of sodium 
sulfate decahydrate in the solution. The solid, which is a mixed wet cake 
comprising IDA and Glauber's Salt, is separated from the mother liquor by, 
for example, centrifugation. The solid can be recycled to an earlier point 
in the production process. A portion of the mother liquor (e.g., 50%) can 
be recycled to the crystallizer, to reduce the slurry density, for 
example. 
In a second embodiment of the batch process, NTA is isolated prior to the 
cooling crystallization of IDA and sodium sulfate decahydrate. Thus, a two 
step crystallization is used; first, acidification followed by 
crystallization and separation of NTA from the resulting slurry comprising 
NTA and mother liquor, and second, neutralization back to about the 
original pH followed by forming a slurry comprising precipitated IDA and 
sodium sulfate decahydrate, then separation of the resulting crystals. The 
NTA crystallization can be accomplished by lowering the pH of the solution 
from about 2.6 to about 2.1 with, for example, sulfuric acid, to decrease 
the solubility of NTA. NTA seed crystals can be added to stimulate NTA 
crystallization. A typical starting solution obtained from the production 
process of IDA has a composition of about 2.9% NTA, 6.3% IDA and 22.6% 
Na.sub.2 SO.sub.4. The saturation temperature of NTA in such a solution 
after acidification to about pH 2.1 is greater than 40.degree. C. Sodium 
sulfate decahydrate precipitates from the mother liquor after NTA 
separation and after neutralization with, for example, sodium hydroxide 
back to a pH of about 2.6, at a temperature of about 27.degree. C. The 
mother liquor can be cooled to a temperature of about 5.degree. C. 
In another embodiment, a continuous crystallization can be used. A slurry 
of IDA, Glauber's Salt and liquor is prepared at the operating temperature 
(e.g., about 5.degree. C.). Fresh waste liquor is fed into the stirring 
slurry while cooling to maintain the operating temperature. The preferred 
residence time is about 2 hours. Both the IDA and Glauber's Salt 
crystallize since the crystallizer operates at a temperature below the 
saturation temperature of both. Slurry is constantly withdrawn and 
subjected to separation. A portion of the liquor can be returned to the 
crystallizer. 
Separation is preferably accomplished by centrifugation, although other 
forms of separation such as filtration or decantation could be used. 
Suitable centrifuges include the traditional vertical perforated bowl 
centrifuge, which provides excellent separation of entrained liquor. A 
speed setting corresponding to a centrifugal force of about 500 g-1000 g 
can be used, with a force of 900-1000 g being most preferable. 
In the IDA production process, wash water can be used to wash the cake 
generated in the IDA production step free of sodium sulfate. However, this 
wash causes some of the IDA in the cake to redissolve. Further, this water 
dilutes the liquor. By excluding the wash water, this redissolution and 
dilution are minimized. 
The temperature at which the IDA and Glauber's Salt are precipitated is a 
function of the concentration of the amino acid and the sodium sulfate in 
the solution. The typical waste purge stream from the process for the 
production of IDA has a composition of about 6% IDA and about 23% sodium 
sulfate. The preferred temperature to which such a solution should be 
cooled is about 5.degree. C. Those skilled in the art will be able to 
determine the necessary temperature to which the particular stream must be 
cooled to precipitate IDA and Glauber's Salt. Separation of precipitate 
can be carried out at more than one temperature during the cooling, to 
maintain a workable slurry density, for example. The resulting liquor can 
be recycled to the crystallizer. 
An IDA stream having the aforementioned composition precipitates because of 
the decreased solubility at about 5.degree. C. as compared to its 
solubility in the starting solution having a temperature of about 
40.degree. C. Simultaneously, solvent (i.e., water) is being removed as 
Na.sub.2 SO.sub.4 crystallizes as Na.sub.2 SO.sub.4.lOH.sub.2 O. Because 
this water becomes part of the solids in the slurry, the slurry density 
becomes high. In the continuous system, the slurry density can be adjusted 
appropriately by continuously recycling saturated 5.degree. C. mother 
liquor back to the crystallizer. 
The recovered solid, which is a mixture of IDA, Glauber's salt and some 
entrained liquor, can be recycled to the mix tank that contains the feed 
to a Na.sub.2 SO.sub.4 crystallizer in the IDA production process. Water 
is added to the solid to create a pumpable stream.

The instant invention will be better understood by referring to the 
following specific but non-limiting examples. It is understood that said 
invention is not limited by these procedures which are offered merely as 
illustrations; it is also understood that modifications can be made 
without departing from the spirit and scope of the invention. 
EXAMPLE 1 
1250 g of IDA liquor containing 6.4% IDA, 2.7% NTA, and 23.0% Na.sub.2 
SO.sub.4 was charged to a 1 liter crystallizer and equilibrated at 
40.degree. C. Five g of IDA seed crystals was added, then the mixture was 
linearly cooled at a rate of 5.8.degree. C./hr. Five grams of Na.sub.2 
SO.sub.4.lO H.sub.2 O (Glauber's Salt) seed crystals was added at the 
saturation temperature of 27.degree. C. At 13.degree. C. about two thirds 
of the very heavy crystal slurry was centrifuged. The liquor was returned 
to the crystallizer and the mixture of this liquor and the remaining one 
third of the slurry was cooled to 5.degree. C. The 5.degree. C. slurry was 
stirred for 1 hour and then centrifuged. A total of 525 grams of liquor 
and 695 grams of wet cake was recovered. After air-drying (during which 
most of the water of crystallization was lost), the cake was found to 
contain 18.5% IDA, 0.7% NTA, and 67.8% Na.sub.2 SO.sub.4 ; thus, 85% of 
the IDA and 90% of the Na.sub.2 SO.sub.4 contained in the liquor were 
recovered in the cake. 
EXAMPLE 2 
The process of Example 1 was repeated, but 45 grams of 96% H.sub.2 SO.sub.4 
was added to the liquor to reduce the pH from the original 2.7 down to 
2.0. The air-dried recovered cake contained 15.0% IDA and 6.1% NTA. The 
heavy contamination of the recovered solid demonstrated that this mode of 
operation was unsatisfactory. 
EXAMPLE 3 
The process of Example 2 was repeated except that after acidification to pH 
2.0 the liquor was seeded with NTA, stirred 2 hours at 40.degree. C., and 
then centrifuged to remove the crystallized NTA. The liquor was returned 
to the crystallizer, seeded with IDA, and cooled as described in Example 
1. The NTA crop contained 83% NTA, 4.1% IDA, and 5.0% Na.sub.2 SO.sub.4. 
The IDA crop was contaminated with 3.2% NTA which was considered 
unsatisfactory. 
EXAMPLE 4 
2500 grams of a new lot of IDA liquor containing 6.3% IDA, 3.0% NTA, and 
23.3% Na.sub.2 SO.sub.4 was charged to a 2 liter crystallizer and 
equilibrated at 40.degree. C. 72 grams of 96% H.sub.2 SO.sub.4 was added 
to pH 2.1, 10 grams of NTA seed was added, then the mixture was stirred 
for 2 hours. Additional H.sub.2 SO.sub.4 was added as needed to maintain 
this pH. The precipitated NTA was separated on a centrifuge and washed. 
The liquor was returned to the crystallizer and re-equilibrated to 
40.degree. C. 82.8 grams of 50% NaOH was added to bring the liquor back to 
the original pH of 2.7. 10 grams of IDA seed was added and the batch was 
linearly program cooled to 5.degree. C. over 6 hr. At 27.degree. C. 5.0 
grams of Glauber's Salt seed was added to initiate crystallization of 
Glauber's Salt. To maintain a workable slurry density, at both 25.degree. 
C. and 15.degree. C. about two thirds of the slurry was centrifuged, with 
the centrate being immediately returned to the crystallizer. The remaining 
slurry was centrifuged at 5.degree. C. None of these crops was washed. The 
recoveries were: 
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Analytical Data 
Grams % NTA % IDA % Na.sub.2 SO.sub.4 
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Air dried NTA crop 
63 95.0 4.4 0.4 
Mixed crops 735 0.2 13.8 82.7 
Liquor 1026 2.3 5.9 6.8 
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% Recovered in 
Solids from Original Liquor 
NTA IDA Na.sub.2 SO.sub.4 
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NTA Crop 67% 
IDA/Glauber's Salt Mixed Crop 
58% 89% 
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