Extracting metal ions with diphosphonic acid, or derivative thereof

A method of removing metal ions from a non-aqueous medium comprising

• (a) reacting the metal ions with a thermodynamically-unstable water soluble complexing agent in an aqueous solution to form a water-soluble metal complex, thereby removing metal ions from the non-aqueous medium into the aqueous solution;
• and thereafter
• (b) decomposing the complexing agent into inorganic compounds thereby to destroy the water-soluble complex and release the metal ions in a substantially organic free form;
• and
• wherein the thermodynamically-unstable complexing agent has the formula:
• ##STR25## wherein the X and Y substituents are each selected from the group consisting of --H, halogen, --Z"H, --CO.sub.2 H, --CH.sub.2 Z"H, --CH.sub.3, --CH.sub.2 CH.sub.3, --CH.sub.2 NH.sub.2, --CONH.sub.2, --CH.sub.2 Cl, --CH.sub.2 Br, --CH.sub.2 F, --CH.sub.2 I, --CH.sub.2 NO.sub.2, --CH.sub.12 CN, --CH.sub.2 OR, --CH.sub.2 SR, ##STR26## R is an alkyl group with one to three carbon atoms;
• Z is oxygen or sulphur;
• Z' is oxygen or sulphur;
• and
• Z" is oxygen or sulphur;
• or a water soluble salt thereof.