Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-06-01
2001-10-02
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S565000
Reexamination Certificate
active
06297397
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing high purity tetrasodium salt of ethylenediaminetetraacetic acid(EDTA-4Na). More particularly, it relates to a method for producing high purity and uniform crystals of EDTA-4Na from an aqueous solution of crude EDTA-4Na by crystallization using a mixed solvent.
2. Description of the Related Art
Tetrasodium salt of ethylenediaminetetraacetic acid (‘Tetrasodium EDTA’ or EDTA-4Na) is a white crystalline powder of no taste and no odor. It has a good solubility in water (103 g/100 ml water), although it is rarely soluble in organic solvents such as alcohols or ethers, etc.
EDTA-4Na and its derivatives, which act as sequestering agents or chelating agents, have a wide range of applications in dyeing, photographs, cosmetics, cleaning agents and synthetic resin industries, and the like. They are employed to avoid problems associated with metal ions in water or aqueous solutions by forming chelate complexes of the metal ions.
It may be prepared by reacting ethylene diamine, NaCN and formalin in the presence of an alkali at 60-150° C. as shown in Reaction Scheme 1 below to yield an aqueous solution of crude EDTA-4Na. When by-product ammonia is removed under reduced pressure, a solution of crude EDTA-4Na of about 40-50% purity is obtained. The solution still contains a large amount of impurities such as unreacted reactants and by-products (JP 57-45425A, JP 56-3930 A, U.S. Pat. No. 2,860,164).
These impurities limit the applications of the product EDTA-4Na and it is required to recover purified EDTA-4Na by various purification method in order to use it in foodstuffs or cosmetic industries, etc.
Crystallization based on solubility difference over temperature range is not efficient for purifying EDTA-4Na because the difference is not significant as shown in FIG.
1
.
Four typical methods for recovering crystals of EDTA-4Na have been known.
In one method so called ‘total drying’, the aqueous solution of crude EDTA-4Na of about 40% purity is dried by using spray dryer. This method yields only low purity EDTA-4Na product and cannot obtain high purity product.
In a second method, the aqueous solution of crude EDTA-4Na (about 40% purity) was adjusted to lower than pH 2 with HCI to precipitate an insoluble acid form of EDTA (U.S. Pat. No. 2,860,164). The acid form of EDTA obtained is washed with a large amount of water to completely remove by-product salts and four(4) equivalents of NaOH is added thereto to yield high purity EDTA-4Na (Reaction Scheme 2).
Although this method produces high purity EDTA-4Na, it still has several problems in association with the steps of crude EDTA-4Na→EDTA→EDTA-4Na. Thus, the reaction of crude EDTA-4Na and HCl gives a reaction waste containing a large amount of by-product salts and the addition of NaOH to Na-free EDTA to obtain EDTA-4Na increases production cost.
In a third method, 50% NaOH is used to salt out EDTA-4Na crystals (JP 62-61022B). This method produces an excessive NaOH-containing waste which causes environmental pollution, making it commercially disadvantageous.
In a fourth method, a single organic solvent, in particular methanol is used to precipitate high purity EDTA-4Na crystals. This method still has several problems in that the yield is not satisfactory and that the crystals of EDTA-4Na obtained are distributed in a wide range of particle diameters and the microparticles having a diameter less than about 30 &mgr;m occupy a large portion, requiring a long filtration time for recovery, that eventually contributes to the decrease in productivity (Comparative Examples 2-9).
Therefore, a new effective method for recovering high purity EDTA-4Na is needed.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for producing highly purified EDTA-4Na with a higher yield in an economic manner.
Another object of the present invention is to provide a method for recovering highly purified uniform crystals of EDTA-4Na from a solution containing EDTA-4Na and impurities, wherein said solution is subjected to a crystallization using a mixed solvent of methanol and an alcohol other than methanol.
The present invention provides a method for producing highly purified crystals of tetrasodium salt of ethylenediaminetetraacetic acid (EDTA-4Na), which comprises the steps of
reacting ethylene diamine, NaCN and formalin in the presence of an alkali to yield a solution containing EDTA-4Na and impurities;
subjecting the solution to crystallization over a solvent to give EDTA-4Na crystals; and
recovering the EDTA-4Na crystals,
wherein said crystallization solvent is a mixture of methanol and an alcohol other than methanol.
The above and other objects, features and applications of the present invention will be apparent to those of ordinary skill by the following detailed description.
REFERENCES:
patent: 2407645 (1946-09-01), Bersworth et al.
patent: 2845457 (1958-07-01), Kroll et al.
patent: 2860164 (1958-11-01), Kroll et al.
Derwent Abstract (1981-06727D) of JP 55151537 A. Nanbu et al. Crystallising sodium EDTA salt—by concentrating under reduced pressure to ppte. pure crystals. Nov. 1980.*
Derwent Abstract (1982-34298E) of JP 57048949 A. Wada et al. Ethylene diamine tetraacetic acid alkali metal salt prepn.—by adding caustic alkali to aq. soln to EDTA alkali metal and sepg. out crystals pptd. Mar. 1982.*
Ault, Addison. Techniques and Experiments for Organic Chemistry, 1987. pp 55-56.
Chaudhry Mahreen
Harrison & Egbert
Killos Paul J.
Tong Suh Petrochemical Corp., Ltd.
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