A vertical symmetrical vibrating mill that includes a top vibrating tube and a bottom vibrating tube.

The mixed powders are preferably ground by a vibration mill.

In the method, the fiber material refining (130) is conducted as compression refining performed by using a vibrating crusher.

The vertical symmetrical vibrating mill also includes a plurality of exciter elements including at least one exciter element connected to each of the top vibrating tube and the bottom vibrating tube.

The analytical fineness required for a subsequent spectroscopic examination is normally achieved with the aid of a disk vibration mill.

In operation, the vibrating mill is preferably arranged such that the common axis of the top vibrating tube and the bottom vibrating tube is oriented in a direction corresponding to the direction of gravity.