Process for preparing a dry particulate water-soluble polymeric colorant

A dry particulate water-soluble polymeric colorant having a molecular weight above 1000 daltons is prepared by (A) forming a starting solution which contains (a) an aqueous solvent and in each case from 0.1 to 20% by weight, based on the solution, of (b) polymeric, water-soluble colorant molecules having a molecular weight above 1000 daltons, (c) molecules of water-soluble polymeric colorants plus monomeric organic chromophore which have a molecular weight below 1000 daltons, and (d) inorganic salts; (B) contacting the starting solution with an anisotropic semipermeable polymeric membrane under a transmembrane pressure difference from 2.7 to 14.71 bar to form an aqueous ultrafiltrate which contains the components (a), (c) and (d) and a retentate which contains the components (a) and (b) and also not more than 1%, based on component (b), of component (c) and not more than 2% of inorganic salts; (C) isolating the retentate; and (D) spray-drying the isolated retentate to remove aqueous solvent as vapour phase and form a particulate solid phase which contains as its essential constituent a non-absorbable (non-resorbable) water-soluble polymeric colorant having a molecular weight above 1000 daltons.