A method for grouping boxes, after stopping the head box of a train line, includes transversally displacing a last box of the train line with respect to the first box of the upstream line waiting for grouping and introducing a stop motion unit for holding the upstream line into the space cleared by the last box. The active stabilization targeting correction system works by separating the \u201csupport\u201d features of the handheld device from the \u201cprojectile launching\u201d features, and controlling their respective motion by electromechanical mechanisms. For that purpose there is provided after the dialyzer in or after the dialysing machine a measuring cell for spectrophotometric measurement. The wet pulp can be further processed by drying.