The invention relates to a method for processing dust when breaking rock, to a dust separator, and further to a rock-drilling rig. Rock material and air sucked from a borehole or some other operating location is separated from each other in the dust separator. The separated rock material falls into a discharge part of the dust separator, from which it can be dosed in batches into a discharging device. The discharging device comprises a closed compression space in which the rock material batch is compressed into a more compact state. This forms a solid matter cake that is removed from the dust separator.