Patent GB-772726-A (GB)
Title: Improvements in or relating to alkylene polyamino polyacetic acid compounds
Filed: 1953-10-29
Abstract: Organic impurities present in alkylene polyamino poly-acetic acid compounds prepared by reacting together an alkylene polyamine, formaldehyde and an alkali-metal cyanide in an aqueous alkaline solution are rendered innocuous or eliminated by acidifying the solution constituting the reaction product thereby to convert the organic impurities present, especially formaldehyde polymers and hexamethylene tetramine, into volatile form, heating the resulting mixture to a temperature sufficiently high to volatilize these converted impurities and recovering the alkylene polyamino polyacetic acid compound present in the purified acidic solution.  The method may be used to obtain the amino-acetic acid compound as the purified free acid or salt, or when the compound exhibits chelating properties, in the form of a metal chelate by reaction of the acidified solution with a compound of the metal to be complexed, especially the lower valency form of a polyvalent metal such as Fe, Mn, Cr, Co, Zn and Cu.  The acidification may be effected by means of an acid such as H2SO4 or HCl or by a cationexchange resin which also removes alkali metal ions from the solution.  The acidified solution may then be spray- or drum-dried to volatilize the converted organic impurities, suitably at 250-400 DEG  F.  When the resulting powder contains alkali metal salts, the powder may be extracted with an alcohol in which these salts are insoluble.  The solutions of polyamino polyacetic acid compounds treated may be prepared as described in Specification 600,629 from any alkylene polyamine having at least 2 replaceable hydrogen atoms attached to different amino nitrogen atoms including ethylene diamine and its mono- and di-ethanol and -propanol derivatives and the chloro-phenyl derivatives described in U.S.A. Specification 2,489,363.  In examples: (1) a solution of tetra-sodium ethylene diamine tetracetate, prepared by the above method by reaction of ethylene diamine with formaldehyde and aqueous NaCN and NaOH, is acidified by H2SO4 to pH 4.5 yielding a solution of the di-sodium salt of the tetracetic acid which is spray dried; alternatively, acidification is effected by means of a cation-exchange resin, the final product then being the free tetracetic acid; (2) a similar solution of a sodium salt of the tetracetic acid is likewise acidified to pH 4.5, then ferrous sulphate is added to form a complex with the tetracetic acid and the solution is spray dried; (3) the solution of (1) is acidified to pH 2-3, then dried at 250-350 DEG  F. and the powder extracted with ethanol to give the pure diamino-tetracetic acid.  In a modification, a pure alkylene polyamino polyacetic acid compound exhibiting chelating properties is converted into a lower valency chelate complex of a polyvalent metal whose high valency state is the more stable, by adding formaldehyde and/or hexamethylene tetramine to an aqueous solution of the pure acid compound, acidifying the resulting solution and reacting it with a salt of the metal in lower valency form to yield the complex and then heating the solution to volatilize the formaldehyde present therein and to recover the complex in dry form.  Thus, formaldehyde and/or hexamethylene tetramine (2 per cent) may be added to the pure salt of (1) in water and this solution treated as in (2) for preparing the ferrous complex.