Fine grinding in a ball mill working in closed circuit with a forced vortex air classifier has been investigated in continuous laboratory scale, The ball mill was tested as a grate discharge mill and as an air-swept mill. Feldspar was chosen for the tests, The grinding process was also studied in connection with production of materials in different size classes. The final products had top sizes of 40, 20 and 10 microns. The air-swept mill configuration provided the best results both on energy and capacity basis. During production of material with top size of 40 microns, it was possible to increase the grinding capacity up to 28% when compared to the grate discharge mill configuration. This difference dropped to 20% during production of 10 micron size material. A corresponding decrease in power consumption was observed. The grate discharge mill could hardly produce such a fine material because the fine particles coated the grinding media and the lining of the mill. The internal classification in the air-swept mill made it possible to remove fines from the mill to prevent a slowing-down effect of the grinding. This internal classification furthermore resulted in good dispersion of the material discharged from the mill, reduced circulating load and lowered mill temperature.