A batch-type rotary grinding mill operable under seal to the atmosphere is provided with a discharge system which permits discharge of material from the mill under seal and without disturbing the seal in the mill. The discharge system comprises at least one discharge chute which is sealably secured to the shell wall, spiraling adjacent to the shell and traversing the shell wall from one end to the other. The material is discharged from the discharge chute via a hollow trunnion which, preferably, contains a conveyor to aid in passing the material out of the mill into an environmentally protected device.