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US 6,569,284 B1Granted
Elemental-chlorine-free bleaching process having an initial Eo or Eop stage
AssigneeInternational Paper Company
InventorsCaifang Yin, Jasper H. Field
Life of this patent
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AssigneeInternational Paper Company
InventorsCaifang Yin, Jasper H. Field
Estimating the term…
The specification discloses an improved bleaching sequence which provides significant reduction in filtrate volumes and amounts of absorbable organic halide (AOX), color, and chemical oxygen demand (COD) in the effluent. These reductions are achieved without adversely affecting the brightness and viscosity properties of the bleached pulp and without a high capital cost and operating cost penalty or impact on existing mill processes. The bleaching sequence according to the invention for reducing filtrate volumes is an EOD1EOPD2 or EOPD1EOPD2 bleaching sequence wherein substantially all of the filtrates from the later bleaching stages are recycled to earlier bleaching stages and eventually treated in the initial EO or EOP stage prior to discharge from the plant. An advantage of the invention is that pulp having a Kappa number of greater than about 25 for softwood and greater than about 15 for hardwood may be fed to the bleaching process while still providing a bleached pulp having sufficient brightness, viscosity characteristics and good pulp yield.
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.
A process for bleaching a kraft brown stock pulp in a multi-stage elemental chlorine-free bleach sequence wherein the pulp has a consistency in the range of from about 25% to about 30% and a pH in the range of from about 3 to about 11 which comprises treating the brown stock pulp as the first bleaching stage in a bleaching sequence with an initial alkaline extraction stage assisted by oxygen or oxygen and peroxide (initial EO or initial EOP stage) 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 first chlorine dioxide stage D1 is washed in a pulp washer to produce a D1 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 at least a portion of the D1 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 stage filtrate to the initial EO or EOP stage and mixing the D1 stage filtrate with brown stock entering the initial EO or initial EOP stage so that the treated D1 stage filtrate is incorporated with the initial EO or initial EOP stage filtrate which is in turn incorporated within the bleach plant effluent and thereby not recycled to the recovery boiler, substantially all filtrates from later bleaching and extraction stages being recycled to earlier stages of the bleaching sequence and ultimately incorporated with the bleach plant effluent and thereby not recycled to the recovery boiler.
A pulp bleaching process for reducing total bleach plant effluent volume which comprises treating a kraft brown stock pulp having a consistency of from about 20 to about 30% by weight and a Kappa number greater than about 25 for softwood and greater than about 15 for hardwood as the first bleaching stage of an elemental chlorine-free bleaching bleach sequence in an initial alkaline extraction stage assisted by oxygen or oxygen and peroxide (initial EO or initial EOP stage) 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 is washed after the D1 stage, after the D2 stage, and after the EOP stage to provide filtrates, at least a portion of filtrate from the D2 stage contains chlorides and is recycled to a washer for the EOP stage for washing pulp after the EOP stage to produce an EOP stage filtrate, at least a portion of the EOP stage filtrate is recycled to the D1 stage for washing pulp after the D1 stage to produce a D1 stage filtrate containing chlorides, and at least a portion of the D1 stage filtrate is recycled to brown stock entering the initial EO or initial EOP stage and mixed with the brown stock to adjust the consistency of the brown stock entering the initial EO or initial EOP stage to from about 5 to about 10 wt. % and to effect treatment of the D1 stage filtrate, and wherein pulp is washed after the initial EO or initial EOP stage to produce an initial EO or initial EOP stage filtrate containing chlorides from at least the D1 stage filtrate mixed with brown stock entering the initial EO or initial EOP stage, the filtrate of which is in turn ultimately incorporated within the bleach plant effluent and thereby not recycled to recovery boilers.
Dependencies are inferred from the claim text.