Abstract: A thickener for concentrating fiber suspensions is disclosed including a housing, a rotor arranged on the housing, a screen attached to the rotor for separating the fiber suspension into a first fraction passing through the screen substantially containing only fibers and a fraction not passing through the screen and containing coarse undesired particles, a stationary thickener surrounding the rotor and provided with through holes and supply conduits for supplying the first fraction to the stationary thickener for separating that fraction into a fraction passing through the holes in the stationary thickener and a separate fraction not passing through the holes in the stationary thickener. The through holes are dimensioned such that substantially only liquid is permitted to pass through the stationary thickener and the fraction which does not pass through the through holes comprises the thickened fiber pulp free of the undesired coarse particles.

Abstract: A method of bleaching chemical pulp, comprising two bleaching stages (D0 and, respectively, D1) with chlorine dioxide as dominating bleaching chemical and at least one intermediate alkaline bleaching stage (E). The ingoing pulp is dewatered to a concentration of 25-40% before the first chlorine dioxide stage (D0) and to 10-40% after the second chlorine dioxide stage (D1). Filtrate from the dewatering after the second chlorine dioxide stage (D1) is re-cycled and utilized for controlling the pulp concentration to 8-15% of the dewatered pulp to the first chlorine dioxide stage (D0).