Process for recovering n-methyliminodiacetic acid from sodium sulfate
This is a Continuation-in-part of application Ser. No. 07/701,285, filed May 16, 1991. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention is directed to a process for preparing N-methyliminodiacetic acid, and in particular, a process for recovering the same from solutions of its disodium salt. 
     In the prior art, amino acids such as glycine and B-alanine have been prepared by hydrolyzing the corresponding nitrile with an aqueous alkaline earth metal hydroxide to form an alkaline earth metal salt of the amino acid, and treating the alkaline earth metal salt with carbon dioxide to form the free amino acid, and recovering the amino acid. 
     Another method of producing such amino acids includes treatment with sulfuric acid to convert the intermediate sodium salt (such as sodium glycinate in the case of glycine) to the free amino acid. However, such a process results in the formation of sodium sulfate, which is difficult to separate from the free amino acid. 
     U.S. Pat. Nos. 3,904,585 and 3,947,496 to Thunberg et al., the disclosures of which are incorporated by reference, disclose a process for separating glycine or B-alanine from such sodium sulfate solutions by fractional crystallization. Specifically, in the starting aqueous solution of sodium sulfate and the amino acid, which has a temperature above 33° C., a pH of 4.5-8.5, a mole ratio of amino acid to sodium sulfate of 1-5:1, and containing at least 5% of the amino acid, water is evaporated at a temperature of from 60° C. or 70° C. to about the normal boiling point in order to form a first slurry which is a mixture of precipitated sodium sulfate and a first mother liquor. The temperature is such that the precipitation of the amino acid is prevented. Upon separation of the precipitated sodium sulfate, the mother liquor is cooled to a temperature effective for precipitating the amino acid. The amino acid is separated and recovered, and the process is repeated. 
     U.S. Pat. No. 3,808,269 to Bragdon et al. discloses a similar process, except that iminodiacetic acid is separated from sodium sulfate solutions. 
     Methyliminodiacetic acid is of interest for photographic applications. In Chemical Abstracts 111(8):67804u, for example, it is disclosed that a bleaching solution for bleaching an exposed photographic material contains Fe(III) complex salts of organic acids including methyliminodiacetic acid. Similarly, in Chemical Abstracts 93(6):588196e, a bleaching solution comprised of FeCl 3 , KBr and methyliminodiacetic acid, adjusted to a pH of 6.0 with NH 3 , is disclosed. In Chemical Abstracts 77(4):24608v, the biodegradation of N-methyliminodiacetic acid in river water is disclosed. 
     Although other, perhaps more effective, chelating agents exist, N-methyliminodiacetic acid is biodegradable, thus making its use highly desirable in terms of environmental concerns However, where N-methyliminodiacetic acid is formed from its disodium salt by acidification with sulfuric acid, separation of the acid from the resulting sodium sulfate solution is problematic. None of the foregoing patents suggests the application of fractional crystallization for the separation of N-methyliminodiacetic acid from such solutions. 
     SUMMARY OF THE INVENTION 
     The problems of the prior art have been solved by the instant invention, which provides a process for separating and recovering N-methyliminodiacetic acid (&#34;MIDA&#34;) from sodium sulfate solutions by fractional crystallization, either batchwise or continuously. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     N-methyliminodiacetic acid can be formed from its disodium salt by acidification with sulfuric acid. The disodium salt can be formed by alkaline hydrolysis of N-methyliminodiacetonitrile (&#34;MIDAN&#34;) by saponification with sodium hydroxide. The synthesis of MIDAN is described by Eschweiler, Annelen, 279, 41, of which the disclosure is incorporated herein by reference, as follows: 
     
         CH.sub.3 NH.sub.2 +2HOCH.sub.2 CN→CH.sub.3 N(CH.sub.2 CN).sub.2 +2H.sub.2 O 
    
     Indeed, analogous chemistry has been applied to the preparation of N-ethyliminodiacetonitrile, as described in J.A.C.S. 80, 5944 (1958), the disclosure of which is herein incorporated by reference, and to the preparation of 2-hydroxyethyliminodiacetonitrile, as described in U.S. Pat. No. 3,864,378, the disclosure of which is herein incorporated by reference. 
     Upon alkaline hydrolysis of the MIDAN, the resulting disodium salt solution has a pH of greater than about 11. In the preferred embodiment of the present invention, a solution of the disodium salt of MIDA at a temperature of about 80° C. is acidified with sulfuric acid to a pH of approximately 2 (about the isoelectric point of MIDA). The acidified solution is then concentrated to remove water and to simultaneously crystallize anhydrous sodium sulfate. The concentration step is carried out at a temperature sufficient to prevent the co-precipitation of MIDA. (The saturation temperature of MIDA will depend upon the original concentration of MIDANa 2  in the starting solution). The precipitated anhydrous sodium sulfate is separated from the hot slurry and the resulting first mother liquor is cooled to a temperature effective for precipitating N-methyliminodiacetic acid in a second mother liquor while avoiding the precipitation of anhydrous sodium sulfate or sodium sulfate decahydrate. Preferably the temperature to which the first mother liquor is cooled is just above the transition temperature (approximately 32.4° C.) of sodium sulfate/sodium sulfate decahydrate. A temperature of about 35° C. has been found to be suitable. The precipitated acid is separated and recovered from the second mother liquor, and the latter may then be recycled to an appropriate earlier stage in the process such as the concentration step, and the process repeated. 
     The aforementioned separations of the precipitated sodium sulfate and MIDA are preferably accomplished by centrifugation, although other forms of separation such as filtration or decantation could be used. 
     The concentration of MIDANa2 in the starting solution must be in a range so that upon acidification, sodium sulfate is not co crystallized. Those skilled in the art will also appreciate that the temperature at which the acid precipitates is a function of the original concentration of the disodium salt, and thus either can be adjusted accordingly. Preferably the MIDANa 2  feedstock for the fractional crystallization is about a 30% MIDANa 2  solution. 
     In an additional embodiment, purge liquor from the fractional crystallization scheme can be cooled to a temperature low enough to precipitate, in the same mother liquor, both MIDA and sodium sulfate decahydrate. The resulting precipitate, which is a mixed wet cake of MIDA and sodium sulfate decahydrate (Glauber&#39;s salt), can be separated from its mother liquor by centrifugation, and recycled to an earlier point in the production process of MIDANa 2 . Water can be added to the cake to form a pumpable stream. All or a portion of the mother liquor can be recycled to the MIDA/sodium sulfate decahydrate crystallizer to reduce the slurry density. 
     Appropriate seeding may be carried out to initiate or enhance the various crystallizations. 
     The instant invention will be better understood by referring to the following specific but non-limiting examples. It is understood that modifications can be made without departing from the spirit and scope of the present invention. 
     EXAMPLE 1 
     Fractional Crystallization 
     1000 grams of MIDANa 2  solution (approximately 41.2%) was added to a stainless steel beaker and 280 grams of H 2  O was charged. Starting at ambient temperature, a total of 639.5 grams of 50% H 2  SO 4  was added to lower the pH to 2.0. The temperature rose during the addition to about 65° C. With vigorous stirring and a gas burner, the solution was concentrated by boiling to 1114 grams. The resulting 80°-90° C. material was placed into a plastic 2 liter graduate. 200 ml. of crystals out of 720 ml. settled to the bottom. The material was then centrifuged hot (the centrifuge was heated with steam). 755.7 grams of liquor and 203.2 grams of sodium sulfate were recovered. 
     The recovered liquor was cooled to 35° C. and allowed to stir at 35° C. overnight. During this time, 114.6g of water was lost by evaporation, thereby providing a slurry weighing 641.lg the next morning. The slurry was centrifuged at 35° C. and washed with water. 441.6 grams of liquor, 113.6 grams of MIDA and 172.6 grams of washings were recovered. 
     EXAMPLE 2 
     Fractional Crystallization 
     The feedstock was a 31.9% solution of MIDANa 2 . To a 2 liter graduated Pyrex beaker was added 1.00 kg of MIDANa 2  and sufficient water to give a 30% MIDANa 2  solution To this was added liquor from a previous cycle. 93% H 2  SO 4  was added to a pH of 2.0 while cooling to keep the temperature less than or equal to 60° C. This solution was boiled until the volume percent of settled crystals was approximately 20%. The centrifuge was heated with steam and then the hot slurry (greater than 80° C.) was separated. The Na 2  SO 4  cake was not washed before being dried. 
     The hot liquor was cooled to 35° C., placed in a 35° C. water bath, and stirred overnight. The slurry formed was centrifuged in the previously warmed centrifuge. The MIDA crystals were washed with approximately 10 ml. of water. The liquor was recycled to the next crystallization, and the process was repeated for a total of 15 cycles. No deterioration of crystallinity was observed and no solids/liquids separation problems arose. The liquor from the 15th cycle was carried through one more cycle in which no fresh MIDANa 2  was added. No crystallization problems occurred in this cycle. The absence of crystallization problems suggests that additional cycles would increase yield. Table 1 reports the data obtained from the foregoing. 
     
                                           TABLE I                                 
__________________________________________________________________________
Summary of Data from Cyclic MIDA Fractional Crystallization               
__________________________________________________________________________
Experiments                                                               
                                          Weight                          
Materials Charged          Weights        Gms                             
Gms                        Gms            Before                          
Cycle                 Liquor &amp;                                            
                           Before                                         
                                After                                     
                                     % Boiled                             
                                          Cool                            
No. MIDA No.sub.2                                                         
          H.sub.2 O Added                                                 
                93% H.sub.2 SO.sub.4                                      
                      Wash Boil-off                                       
                                Boil-off                                  
                                     off  Down                            
__________________________________________________________________________
 1  1000  383   186     0  1569 1251 20.3 1047                            
 2  1000  383   298    903 2584 1964 24   1524                            
 3  1000  383   233    936 2553 2029 20.5 1704                            
 4  1000  383   267   1384 3034 2391 21.2 2006                            
 5  1000  383   200   1475 3058 2458 19.6 2201                            
 6  1000  383   279   1994 3656 2916 20.2 2414                            
 7  1000  383   224   1652 3259 2578 21.1 2168                            
 8  1000  383   423   1593 3399 2718 20.1 2311                            
 9  1000  383   258   1683 3323 2425 27.0 1922                            
10  1000  383   235   1029 2647 2117 20.0 1787                            
11  1000  383   305   1340 3029 2324 23.3 1937                            
12  1000  383   224    657 2264 1754 22.5 1447                            
13  1000  383   250   1080 2713 2141 21.1 1793                            
14  1000  383   250   1240 2868 2224 22.5 1866                            
15  1000  383   278   1353 3014 2314 23.2 1907                            
16  1000  383   116   1220 1336 1051 21.4  860                            
17                                                                        
18                                                                        
__________________________________________________________________________
Contained                                                                 
MIDA                            Gms of Contained                          
Charged      Materials Recovered                                          
                                MIDA     Cumula-                          
Cycle                                                                     
    This                                                                  
        Cumula-                                                           
             MIDA,                                                        
                  Na.sub.2 SO.sub.4                                       
                       Liquid &amp;                                           
                             %  This                                      
                                    Cumula-                               
                                         tive                             
No. Cycle                                                                 
        tive gms  gms  Wash, gms                                          
                             A.I.                                         
                                Cycle                                     
                                    tive % Yield                          
__________________________________________________________________________
 1  319  319  0   154   903      0    0  0                                
 2  319  638 398  344  9362  94.0                                         
                                374  374 58.6                             
 3  319  957 193  248  1384  87.9                                         
                                169  543 56.7                             
 4  319 1276 309  291  1475  95.7                                         
                                296  839 65.8                             
 5  319 1595  70  253  1994  98.6                                         
                                 69  908 56.9                             
 6  319 1914 466  347  1652  91.1                                         
                                425 1333 69.6                             
 7  319 2233  28  319  1593  79.0                                         
                                226 1559 69.8                             
 8  319 2552 342  299  1683  95.4                                         
                                326 1885 73.9                             
 9  319 2871 511  384  1029  82.9                                         
                                424 2309 80.4                             
10  319 3190 253  241  1340  84.6                                         
                                214 2523 79.1                             
11  319 3509 592  292   657  66.0                                         
                                391 2914 83.0                             
12  319 3828 228  225  1080  77.3                                         
                                176 3090 80.7                             
13  319 4147 315  240  1240  82.1                                         
                                259 3349 80.8                             
14  319 4466 309  265  1353  86.3                                         
                                267 3616 81.0                             
15  319 4785 342  341  1277  94.3                                         
                                323 3939 82.3                             
16  319 4785 122  139   649  80.2                                         
                                 98 4037 84.4                             
17           4737 4382*                                                   
18                                                                        
__________________________________________________________________________
 *MIDA in composite sample = 2.2%