1. Field of the Invention
The present invention relates to a process for preparing an amino-polycarboxylic acid or salts thereof.
2. Description of the Related Art
Amino-polycarboxylic acids possess a chelating ability and thus enable to chelate metals in water. For this property, amino-polycarboxylic acids are employed as chemicals for fibers and dyeing, builders for soap and detergents, complexing agents for metal surface treatment, chemicals for photography, bleaching agents for paper pulp, etc.
Sodium tripolyphosphate conventionally used as builders for detergents has an excellent chelating ability but is known to cause over-nourishment of, rivers, lakes and marshes, when the phosphorus contained therein is released into the environment. For this reason, sodium tripolyphosphate is not employed now.
Zeolite which is currently used as a builder for detergents has a weak chelating ability. Furthermore, zeolite is not biodegradable because of its inorganic nature and thus encounters a problem of environmental pollution. In addition, zeolite is water-insoluble and cannot be used for liquid, especially transparent liquid detergent, which is a restriction from an aspect of application. Furthermore, the water-insoluble nature of zeolite causes problems that zeolite adheres and fix onto the inner wall of a conduit or the like, or deposits at the bottom of rivers to cause sludge.
On the other hand, it is known that amino-polycarboxylic acids have been used as builders for detergents. However, in these known amino-polycarboxylic acids, ethylenediaminetetraacetic acid (EDTA) has an excellent chelating ability but is not biodegradable. Nitrilotriacetic acid (NTA) is somewhat biodegradable but reportedly has a teratogenic effect.
Even other known amino-polycarboxylic acids which have an excellent chelating ability, because of their poor biodegradation property, involve a problem of possible environmental accumulation of heavy metals harmful to the living body, when released to the environment.
A variety of amino-polycarboxylic acids have been investigated but no report has been made on any advantageous process for preparing amino-polycarboxylic acids having an excellent chelating ability and an excellent biodegradation property in an industrial scale.
As processes for preparing amino-polycarboxylic acids which are expected to be biodegradable, there are known a process by reacting an amino acid with a glycidylic acid ester (Japanese Patent Application KOKAI (Laid-Open) No. 63-267750), a process by reacting a sulfonic acid derivative with an amino-polycarboxylic acid (Japanese Patent Application KOKAI No. 5-186416), etc. The amino-polycarboxylic acids prepared by these processes contain a hydroxyl group or a sulfonic acid residue.
As processes for preparing amino-polycarboxylic acids containing a hydroxyl group, there are known a process by reacting a sodium iminodiacetate with glycidylamide (Japanese Patent Application KOKAI No. 2-115152), a process by reacting epoxysuccinic acid with glycine (Japanese Patent Application KOKAI No. 5-70795), etc.
However, these amino-polycarboxylic acids prepared by these processes involve problems that the hydroxyl group is not sufficient for imparting biodegradable property to the acids and the sulfonic acid residue is inferior to a carboxyl group in chelating ability.