Process for separating iminodiacetic acid from aqueous glycine solution

A novel separation process of an iminodiacetic acid component from an aqueous glycine solution including the same is provided. In this process, sulfuric acid is added, in the presence of sodium salt, to the aqueous glycine solution in such an amount that the pH of the aqueous glycine solution becomes 1.5 or less, whereby the iminodiacetic acid is crystallized from the solution and, then, the crystallized iminodiacetic acid component is separated from the mother liquor. Thus, glycine can be efficiently recovered without causing undesirable accumulation of the iminodiacetic acid component in the mother liquor.

The present invention relates to a process for separating an iminodiacetic 
acid component from an aqueous glycine solution including glycine or the 
salt thereof and iminodiacetic acid or the salt thereof. More 
specifically, it relates to a process for selectively crystallizing and 
separating the iminodiacetic acid component from the aqueous glycine 
solution by adding sulfuric acid, in the presence of a sodium salt, to the 
aqueous glycine solution in such an amount that the pH of the aqueous 
glycine solution becomes 1.5 or less. 
The effective separation of iminodiacetic acid from an aqueous glycine 
solution including glycine or the salt thereof and iminodiacetic acid or 
the salt thereof is a very important technique in the commercial 
production of glycine. That is, glycine is conventionally produced as 
follows. 
Glycinonitrile is hydrolyzed with an aqueous alkali metal hydroxide 
solution, whereby glycine in the form of the alkali metal salt is formed 
in the aqueous solution. Although only alkali metal hydroxide is 
specifically set forth herein for the sake of convenience, the term 
"alkali metal hydroxide" as used herein is intended to include the 
alkaline earth metal hydroxide, such as calcium hydroxide and magnesium 
hydroxide, together with the alkali metal hydroxide, such as sodium 
hydroxide, potassium hydroxide and the like. The resultant aqueous 
solution is then neutralized to the isoelectric point with an inorganic 
acid such as sulfuric acid or hydrochloric acid. Thus, an aqueous solution 
containing glycine and the neutral salt of the inorganic acid with the 
alkali metal hydroxide is formed. Glycine can be produced by the 
fractional crystallization of the glycine from the aqueous solution. 
However, a small amount of iminodiacetic acid is produced as a by-product 
during the hydrolysis of the glycinonitrile and is present, together with 
the inorganic salt, in the aqueous glycine solution at the pH of the 
isoelectric point of glycine (i.e. approximately 6) as the mono alkali 
metal salt thereof. Since the amount of the by-product (i.e. iminodiacetic 
acid) is relatively small and, also, since the solubility of the alkali 
metal salt thereof in water is much higher than that of the glycine and 
the inorganic salt, the alkali metal salt of the iminodiacetic acid 
remains in the mother liquor without causing the crystallization of the 
iminodiacetic acid when the glycine and the inorganic salt are 
fractionally crystallized. According to a typical example of the 
conventional glycine separation and recovery methods, as set forth in, for 
example, British Pat. No. 1,472,840, an aqueous glycine-containing 
solution, which is previously neutralized with an inorganic acid, is first 
heated and concentrated, whereby the neutral salt of the inorganic acid is 
crystallized. The heating temperature is generally within the range of 
105.degree. to 116.degree. C. The crystallized neutral salt of the 
inorganic acid is filtered out of the solution under a heated condition. 
The filtrate is then cooled to a temperature of 33.degree. to 40.degree. 
C. to crystallize the glycine. The crystallized glycine is thus 
fractionally recovered. Since the mother liquor still contains a large 
amount of glycine, together with the above-mentioned alkali metal salt of 
iminodiacetic acid, the mother liquor is recirculated to the 
above-mentioned glycine crystallization step, wherein the mother liquor is 
combined with a fresh aqueous glycine solution and is again subjected to 
the above-mentioned fractional crystallization operation. 
However, in the case where the mother liquor is directly circulated to the 
glycine separation step, the alkali metal salt of the iminodiacetic acid 
having a high solubility in water is gradually accumulated in the 
circulated mother liquor. This accumulation of the alkali metal salt of 
the iminodiacetic acid in the mother liquor results in the following 
problems. 
(1) The yield of the glycine obtained in one fractional crystallization 
operation is decreased. 
(2) The glycine product is contaminated with the alkali metal salt of 
iminodiacetic acid. (Since glycine is mainly used for animal feed and food 
additives, the contamination with such impurity should be avoided.) 
(3) The crystal size of the produced glycine becomes very fine and, as a 
result, the filtering operation thereof becomes difficult. 
Furthermore, since the iminodiacetic acid is useful as a chelate compound, 
the separation and the recovery of the iminodiacetic acid from the 
above-mentioned aqueous glycine solution is also desired. 
In order to prevent the excessive accumulation of the alkali metal salt of 
the iminodiacetic acid in the circulating mother liquor, a portion, or 
all, of the mother liquor may be continuously or intermittently withdrawn 
out of the circulating system (i.e. the renewal of the mother liquor). 
However, this results in unpreferable loss of the residual glycine in the 
mother liquor. In order to solve this problem, it has been proposed in 
British Pat. No. 1,472,840 and Japanese Patent Laid-Open Application No. 
118421/77 that the iminodiacetic acid be selectively isolated from the 
mother liquor by adding an inorganic acid to at least a portion of the 
circulating mother liquor to adjust the pH of the mother liquor within the 
range of 2.4.+-.0.5. This proposal is derived from the above-mentioned 
separation method of glycine, in which the alkali metal salt of glycine is 
neutralized to the vicinity of the isoelectric point thereof to 
crystallize glycine. That is, the above-mentioned proposal is based on the 
fact that the solubility of iminodiacetic acid in water is minimized in 
the vicinity of the isoelectric point thereof (i.e. pH=2.4.+-.0.5). 
Neverthless, this method involves a problem that one-pass recovery 
efficiency of the iminodiacetic acid is not satisfactory when the 
iminodiacetic acid is recovered from an aqueous solution containing, for 
example, glycine, sodium sulfate and mono sodium salt of iminodiacetic 
acid. 
Accordingly, an object of the present invention is to obviate the 
above-mentioned problems of the prior art and to provide a process for 
selectively and efficiently separating an iminodiacetic acid component 
from an aqueous solution including glycine (or the salt thereof) and 
iminodiacetic acid (or the salt thereof). 
Another object of the present invention is to prevent the accumulation of 
the iminodiacetic acid component in the circulating mother liquor without 
the renewal of the mother liquor in the above-mentioned recovery process 
of the glycine from an aqueous solution including glycine (or the salt 
thereof) and iminodiacetic acid (or the salt thereof). 
Other objects and advantages of the present invention will be apparent from 
the description set forth hereinbelow. 
In accordance with the present invention, there is provided a process for 
separating an iminodiacetic acid component from an aqueous glycine 
solution including the same comprising the steps of: 
(a) adding sulfuric acid, in the presence of a sodium salt, to said aqueous 
glycine solution in such an amount that the pH of the aqueous glycine 
solution becomes 1.5 or less, preferably 0.4 through 1.2, whereby the 
iminodiacetic acid is crystallized from the solution, and; 
(b) separating the crystallized iminodiacetic acid component from the 
mother liquor.

In the drawing, curve A represents the correlation in the case of three 
components, that is, iminodiacetic acid-sodium sulfate-glycine and curve B 
represents the correlation in the case of a single component, that is, 
iminodiacetic acid. 
For brevity's sake, in the description set forth below, the alkali metal 
(or alkaline earth metal) salt is exemplified by sodium salt. However, it 
should be noted that the sodium salt can be replaced by the other alkali 
metal salts and alkaline earth metal salts. 
As mentioned hereinabove, an aqueous solution containing sodium salt of 
glycine and, as a by-product, a small amount of disodium salt of 
iminodiacetic acid can be obtained from the alkaline hydrolysis of 
glycinonitride in an aqueous solution by the addition of a stoichiometric 
amount of, or a slightly excessive amount of, sodium hydroxide. The 
starting glycinonitrile can be prepared, for example, by reacting 
formaldehyde and hydrocyanic acid, followed by the amination of the 
resultant glycolonitrile with ammonia. 
The aqueous glycine solution prepared as mentioned above is then 
neutralized to a pH of approximately 6 through 7 by the addition of 
sulfuric acid, whereby an aqueous solution containing free glycine, sodium 
sulfate and mono sodium salt of iminodiacetic acid is obtained. As 
mentioned hereinabove, in order to recover glycine from this aqueous 
solution, it has been conventionally carried out that the aqueous glycine 
solution is first heated and concentrated to crystallize sodium sulfate in 
the solution and the crystallized sodium sulfate is filtered out of the 
solution under a heated condition. The glycine is fractionally 
crystallized from the resultant filtrate by cooling. Alternatively, 
glycine is first crystallized and separated from the aqueous glycine 
solution by cooling and, then, the sodium sulfate is crystallized by 
heating and filtered out of the solution under a heated condition. In both 
cases, the residual mother liquor still containing a large amount of 
glycine, together with mono sodium salt of iminodiacetic acid and sodium 
sulfate, is again returned to a fresh aqueous glycine solution for 
recovering the residual glycine, preferably after the removal of the 
iminodiacetic acid component. 
As mentioned hereinabove, since the direct circulation of the mother liquor 
is not preferable due to the gradual accumulation of the iminodiacetic 
acid component in the circulating mother liquor, it has been proposed in 
British Pat. No. 1,472,840 that iminodiacetic acid be crystallized and 
separated from the mother liquor containing the same by adjusting the pH 
of the mother liquor within the range of 2.4.+-.0.5. This method is based 
on the finding that, when an aqueous solution of monosodium salt of 
iminodiacetic salt is acidified by the addition of an inorganic acid such 
as sulfuric acid, the monosodium salt of iminodiacetic acid is converted 
to free iminodiacetic acid and the solubility of the iminodiacetic acid in 
water is lower than that of the monosodium salt and is minimized in the 
vicinity of a pH of 2.4. 
It is true that the solubility of iminodiacetic acid in an aqueous solution 
containing only iminodiacetic acid is minimized at the isoelectric point 
thereof, that is, in the vicinity of a pH of 2.4 and the solubility 
thereof is incleased in the case where the aqueous solution is made acidic 
or basic from the isoelectric point by the addition of, for example, 
sulfuric acid or sodium hydroxide. However, the present inventors have 
surprisingly found that, in the case of the three component system of 
iminodiacetic acid-sodium sulfate-glycine, the solubility of iminodiacetic 
acid in an aqueous solution is not minimized in the vicinity of the pH of 
2.4 of the isoelectric point, but minimized at a pH of 1.5 or less, 
especially within the range of 0.4 to 1.2. 
The solubility of iminodiacetic acid (which is sometimes referred to as 
"IDA" hereinbelow) in an aqueous solution containing three components, 
that is, IDA, sodium sulfate (1 mol per 1 mol of IDA) and glycine (1 mol 
per 1 mol of IDA) at a room temperature obtained from the experimental 
data is illustrated as curve A in the accompanying graphical drawing. 
Curve B in the graphical drawing illustrates the solubility of IDA in an 
aqueous solution containing only IDA. 
As is clear from the curves A and B in the accompanying drawing, the 
solubility of iminodiacetic acid in an aqueous solution containing the 
above-mentioned three components is remarkably small at a pH of 1.5 or 
less, preferably within the range of 0.4 to 1.2 and, therefore, the 
iminodiacetic acid can be selectively and efficiently separated from the 
above-mentioned mother liquor in the production of glycine. 
The mechanism of the above-mentioned behavior of the iminodiacetic acid in 
the three component system at a pH of 1.5 or less is not clearly 
understood, but it would seem that, without prejudice to the present 
invention, the eutectic crystal of the sulfuric acid salt of iminodiacetic 
acid and sodium sulfate or the double salt of iminodiacetic acid and 
sodium bisulfate is crystallized, from the analytical data of the 
crystallized substance. Furthermore, in order to cause the above-mentioned 
behavior of iminodiacetic acid at a pH of 1.5 or less, the presence of a 
sodium salt in the aqueous solution is essential. For instance, in a case 
where a potassium salt, an ammonium salt, a calcium salt, a magnesium salt 
or the like is present in lieu of the sodium salt, the crystallization of 
iminodiacetic acid cannot be effected. Accordingly, in the case where 
alkaline compounds other than sodium compounds are employed in the 
alkaline hydrolysis step of glycinonitrile, sodium salts such as sodium 
sulfate, sodium chloride, sodium carbonate sodium bicarbonate, sodium 
formate and the like, preferably sodium sulfate, should be added to the 
mother liquor of the glycine separation step, prior to the crystallizing 
separation of the iminodiacetic acid from the mother liquor. The sodium 
salt is preferably present in an amount of at least 0.5 mol, based on 1 
mol of the iminodiacetic acid and, more preferably, from 1 mol up to the 
saturated concentration. 
For instance, an aqueous iminodiacetic acid solution containing (i) 13.3 g 
(0.1 mol) of iminodiacetic acid, (ii) 14.2 g (0.1 mol) of sodium sulfate, 
17.4 g (0.1 mol) of potassium sulfate, a mixture of 7.1 g (0.05 mol) of 
sodium sulfate and 8.7 g (0.05 mol) of potassium sulfate or a mixture of 
14.2 g (0.1 mol) sodium sulfate and 17.4 g (0.1 mol) of potassium sulfate 
and (iii) 50 ml of water is prepared. The pH of each solution is adjusted 
to 0.5 by the addition of 8.0 g of sulfuric acid. The resultant solution 
is cooled to a room temperature to crystallize the iminodiacetic acid. The 
crystallized iminodiacetic acid is recovered and weighed. The recovery 
efficiency of the iminodiacetic acid is shown in the following Table. 
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Recovery Efficiency 
Salt (%) 
Na.sub.2 SO.sub.4 (0.1 mol) 
80 
K.sub.2 SO.sub.4 (0.1 mol) 
1 
Na.sub.2 SO.sub.4 (0.05 mol) + K.sub.2 SO.sub.4 (0.05 mol) 
56 
Na.sub.2 SO.sub.4 (0.1 mol) + K.sub.2 SO.sub.4 (0.1 mol) 
more than 80 
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As mentioned above, according to the present invention, since the 
iminodiacetic acid present in the mother liquor is crystallized and 
separated, for example, by filtration, from the mother liquor by 
acidifying a portion of, or all of the circulating mother liquor to a pH 
of 1.5 or less, the mother liquor can be advantageously combined with a 
fresh aqueous glycine solution, optionally after the mother liquor is 
decolored with activated carbon. Thus, the neutralization step and the 
fractional crystallization step of the glycine can be repeatedly and 
smoothly carried out without causing the unpreferable accumulation of 
iminodiacetic acid in the mother liquor. It should be noted that all of 
the circulating mother liquor is not subjected to the acidifying 
treatment, but a portion thereof can be subjected to the acidifying 
treatment in such a manner that the concentration of the iminodiacetic 
component does not become excessive (that is, the concentration is 
preferably kept below about 6% by weight). 
The foregoing specific embodiments have been disclosed only for the 
continuous operation in which the mother liquor is combined with a fresh 
aqueous glycine solution. However, it should be noted that the mother 
liquor is repeatedly used for selectively recovering glycine from the 
mother liquor in a manner as mentioned hereinabove. The iminodiacetic acid 
component (i.e. the above-mentioned eutectic crystal or double salt), 
which is separated from the aqueous glycine solution by the acidification 
of the solution to a pH of 1.5 or less, is again dissolved in water and 
the iminodiacetic acid can be recovered from this aqueous solution by 
adjusting a pH of the solution within the range of 1.9 to 2.9. 
As mentioned hereinabove, according to the present invention, an 
iminodiacetic acid component can be selectively and effectively recovered 
from an aqueous solution containing glycine and iminodiacetic acid in the 
form of a free acid and/or the salt thereof and glycine can be efficiently 
obtained from the aqueous glycine solution without the renewal of the 
mother liquor. 
The present invention is now illustrated by, but is by no means limited to, 
the following examples. 
EXAMPLES 1 TO 8 
An aqueous glycinonitrile solution was prepared from an aqueous 
glycolonitrile solution containing 48.9% by weight of glycolonitrile and 
0.2% by weight of hydrocyanic acid and ammonia in a conventional manner as 
described in J. Am. Chem. Soc., 56, 2197 (1934). From the resultant 
slightly brown aqueous solution, the unreacted ammonia was removed at 
atmospheric pressure and, then, a 48% by weight aqueous sodium hydroxide 
solution was added to the aqueous glycinonitrile solution in an amount of 
1.05 mol of sodium hydroxide per 1 mol of glycinonitrile and was allowed 
to react with the glycinonitrile at a temperature of 100.degree. C. for 1 
hour. 
An aqueous solution of the sodium salt of glycine having the following 
composition was obtained. 
______________________________________ 
Composition % by weight 
______________________________________ 
Sodium Salt of Glycine 
34.4 
Sodium Salt of Iminodiacetic Acid 
1.1 
Sodium Hydroxide 1.3 
Water 63.2 
______________________________________ 
This aqueous solution was subjected to the following fractional 
crystallization operation. 
A. First Step (Neutralization) 
The aqueous solution obtained above was neutralized to a pH of 6 through 7 
with 98% by weight sulfuric acid. 
B. Second Step (Fractional Crystallization) 
The aqueous solution neutralized in the first step was heated to the 
boiling point and concentrated, whereby sodium sulfate was crystallized in 
the solution. The concentration of the aqueous solution was stopped just 
before the glycine in the aqueous solution began to crystallize. The 
crystallized sodium sulfate was separated from the aqueous solution under 
the heated state by means of a centrifugal separator. The filtrate was 
cooled to a temperature of approximately 34.degree. C. to crystallize the 
glycine. The crystallized glycine was separated from the aqueous solution 
by means of a centrifugal separator. The filtrate was recovered as a 
mother liquor. The by-produced iminodiacetic acid remained in this 
filtrate in the form of the mono sodium salt thereof. 
C. Third Step (Fractional Crystallization) 
The mother liquor obtained in the second step was again heated to the 
boiling point and concentrated, whereby sodium sulfate was again 
crystallized. The heating was stopped just before the glycine in the 
solution began to crystallize. The crystallized sodium sulfate was 
separated from the aqueous solution under a heated state by means of a 
centrifugal separator. The filtrate was cooled to a temperature of 
approximately 34.degree. C., whereby the glycine in the aqueous solution 
was crystallized. The crystallized glycine was recovered by means of a 
centrifugal separator. The filtrate was also recovered as a mother liquor. 
The above-mentioned operation was further repeated twice. 
D. Fourth Step (Recovery of Iminodiacetic Acid) 
The mother liquor in which the content of the mono sodium salt of 
iminodiacetic acid was moderately concentrated was obtained after the 
fractional crystallization operation of sodium sulfate and glycine was 
repeated three times in the third step. The composition of the resultant 
mother liquor was as follows. 
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Composition % by weight 
______________________________________ 
Glycine 13.9 
Mono Sodium Salt of 
18.2 
Iminodiacetic Acid 
Sodium Sulfate 14.4 
Water 53.5 
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200 g each of the mother liquor was stirred at a room temperature 
(approximately 20.degree. C.) for 3 hours under various pH conditions 
listed in Table I below. The resultant crystallized precipitate was 
filtered with suction. Thus, cake and filtrate were obtained. The results 
are shown in the following Table I. 
TABLE I 
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Liquid 
pH at*.sup.1 Content of*.sup.3 
Recovery*.sup.4 
Example 
Crystal- Weight of*.sup.2 
in IDA Efficiency 
No. lization Cake (g) Cake (wt %) 
of IDA (%) 
______________________________________ 
1 0.0 79 34 86 
2 0.45 74 38 90 
3 0.8 80 36 92 
4 1.2 83 34 90 
5 1.5 74 36 85 
6 1.8 61 36 70 
7 2.3 47 36 54 
8 2.7 29 38 35 
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*.sup.1 The pH was adjusted with about 80% Sulfuric Acid. 
*.sup.2 The weight of the cake was weighed after 16 hours during at 
105.degree. C. 
*.sup.3 The weight of the iminodiacetic acid (IDA) was converted to a fre 
acid basis. 
##STR1## 
As is clear from the results shown in Table I, the recovery efficiency of 
the iminodiacetic acid was high within the range of a pH of 0.0 to 1.5. 
The recovery efficiency was decreased as the pH was increased. The purity 
of the recovered iminodiacetic acid was not affected by the pH condition. 
No substantial amount of glycine was crystallized together with the 
iminodiacetic acid and 90% or more of the glycine remained in the 
resultant mother liquor. 
The cake obtained in Example 3 was dissolved in water and the aqueous 
solution was decolored with a small amount of activated carbon. After the 
filtration, an aqueous sodium hydroxide solution was added to the aqueous 
solution in an amount such that the pH of the aqueous solution became 2.4. 
The aqueous solution was then heated and concentrated just before the 
sodium sulfate contained in the solution began to crystallize. The 
concentrated solution was allowed to cool with stirring, whereby the 
iminodiacetic acid was gradually crystallized. The crystallized 
iminodiacetic acid was filtered at a temperature of the aqueous solution 
of 34.degree. C. The filtered crystals were washed with water and then 
dried. Thus, 22.7 g of white crystals were obtained. The recovery 
efficiency of the iminodiacetic acid from the mother liquor was 73% and 
the recovery efficiency from the cake was 79%.