acid citraconic acid and methylenemalonic acid carboxymethyloxymalonate carboxymethyloxysuccinate ciscyclohexanehexacarboxylate ciscyclopentanetetracarboxylate phloroglucinol trisulfonate polyacetal carboxylates suitably polycarboxylic acids may be selected from polyacrylate polymaleate maleic methylvinylethercopolymers maleic acrylic copolymers maleic olefinecopolymers polyvinylpyrrolidone polyoxymethylcarboxylates poly o hydroxy acrylate poly 3 hydroxymethyl hexamethylene 1 3 5 tricarboxyl poly 3 oxymethyl hexamethylene 1 3 5 tricarboxyl poly 4 methoxy tetramethylene 1 2 dicarboxylate poly tetramethylene 1 2 dicarboxylate poly vinyl methyl ether maleic anhydride mw 20 000 80 000 carboxymethyloxymalonate carboxymethyloxysuccinate or 1 2 3 4 cyclopentane tetracarboxylic acid it has been experimentally established that particularly advantageous results may be obtained in the process of the invention when the wash liquor additionally comprises a buffer the buffer may suitably be a phosphate borate citrate acetate adipate triethanolamine monoethanolamine diethanolamine carbonate especially alkali metal or alkaline earth metal in particular sodium or potassium carbonate or ammonium and hcl salts diamine especially diaminoethane imidazole or amino acid buffer likewise it has been experimentally established that particularly favourable results may be obtained in the process of the invention when the wash liquor additionally comprises a dispersing agent the dispersing agent may suitably be selected from nonionic anionic cationic ampholytic or zwitterionic surfactants more specifically the dispersing agent may be selected from carboxymethylcellulose hydroxypropylcellulose alkyl aryl sulphonates long chain alcohol sulphates primary and secondary alkyl sulphates sulphonated olefins sulphated monoglycerides sulphated ethers sulphosuccinates sulphonated methyl ethers alkane.