A 10 mm hydrocyclone was operated using a barite suspension with a maximum particle size of d(max) = 7 mu m. The test rig was equipped with a piston diaphragm pump for pressures up to 60 bar. At 40 bar and 20 degrees C, cut sizes d(50) were obtained down to 0.7 mu m; increasing the temperature to 50 degrees C resulted in d(50) values down to 0.5 mu m for a throughput of 0.6 m(3)/h. Another experiment was conducted at 40 bar using a batch hydrocyclone technology. Only the overflow was recirculated to the feed box, whereas the underflow was discharged via a collection box. Increasing the number of recirculations increased the separation of fines in the submicron range. The results showed that after 20 min particles with d(max) = 1 mu m were obtained in the hydrocyclone overflow. After 120 mm, the particles size distribution had a d(max) = 0.5 mu m and a mean size of d(50) = 0.2 mu m. This procedure requires high energy consumption and is thus suitable only for fractionating small quantities of particles in the submicron range. (C) 2014 The Authors. Published by Elsevier Ltd.