Patent ID: 6569284
Filing Date: 2003-05-27
Classification: D21C

Abstract:
A method for bleaching a kraft brown stock pulp in a multi-stage elemental chlorine-free bleach sequence which comprises treating the brown stock as the first bleaching stage in the sequence with an initial alkaline extraction stage assisted by oxygen or oxygen and peroxide (EO or EOP) before a first chlorine dioxide stage (D1) in an EOD1EOPD2 or EOPD1EOPD2 bleaching sequence which produces a bleach plant effluent that is not recycled to a recovery boiler, wherein pulp from the D1 and D2 stages is washed after the stages to produce a D1 and D2 stage filtrate containing chlorides and pulp from the initial EO or initial EOP stage is washed after the stage to produce an initial EO or initial EOP stage filtrate and wherein at least a portion of the D1 and/or D2 stage filtrate is treated and the consistency of the brown stock entering the initial EO or initial EOP stage is reduced by recycling the D1 and/or D2 stage filtrate to the initial EO or initial EOP stage and mixing the recycled D1 and/or D2 stage filtrate with brown stock entering the initial EO or initial EOP stage so that the treated D1 and/or D2 stage filtrate is incorporated within the initial EO or initial EOP stage filtrate which in turn is ultimately incorporated into the bleach plant effluent and thereby not recycled to the recovery boiler, whereby the bleach plant effluent has a substantially reduced AOX, COD and color relative to such effluent in the absence of such recycling.