Process for producing powdery water soluble cellulose derivatives

Process for production of water-soluble cellulose derivatives (I) in the form of powder involves: (a) swelling or dissolving (I) in 50-80 wt% water (based on total weight); (b) converting the solution or gel into finely-divided solid particles by vaporising the water in a high-speed rotary impact mill with the aid of a heat transfer/carrier gas; (c) separating the particles from the gas; and (d) optionally, drying. Independent claims are also included for: (i) powdered (I) obtained by this process, comprising less than 5 wt% with a particle size (PD) of less than 15 microns, less than 2 wt% with a PD below 10 microns and less than 1 wt% below 5 microns; (ii) powder (I) with less than 20 wt% below 15 microns, less than 10 wt% below 10 microns and less than 2 wt% below 5 microns; (iii) powder (I) with six different particle size distributions (A-F) as obtained by sieve analysis, e.g. distribution A with 98.5-100 wt% below 0.25 mm (cumulative through fraction), 95-100 wt% below 0.2 mm, 89-98 wt% below 0.16 mm, 79-92 wt% below 0.125 mm, 65-80 wt% below 0.1 mm and 35-45 wt% below 0.063 mm.