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US 5,352,332 AGranted
Process for recycling bleach plant filtrate
OwnerGerald E. Maples, Raman Ambadyinventors
Life of this patent
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Abstract
A process for bleaching wood pulp is provided comprising subjecting the wood pulp, after brown stock washing, to an oxygen delignification stage, a washing sequence, a first chlorine dioxide bleaching stage, an oxidative extraction stage, at least one final chlorine dioxide bleaching stage and then recycling the filtrate from the oxidative extraction stage countercurrently through the bleaching plant and brown stock washing. Additionally, and quite beneficially, the filtrate from the first chlorine dioxide bleaching stage is also recycled countercurrently through the brown stock washing thereby significantly reducing the environmental impact associated with the manufacture of bleach wood pulp.
Claims19
A process for bleaching Kraft wood pulp comprising subjecting the wood pulp after brown stock washing to the following steps conducted in sequence:
- oxygen delignification in the presence of oxygen and oxidized white liquor followed by one to two stages of post oxygen washing;
- subjecting the oxygen delignified pulp to an acid treatment at a pH of about 2 to 4 to substantially remove non-process metals and washing the treated pulp with fresh water and discharging at least a portion of the filtrate from said washing to the sewer;
- bleaching in a first chlorine dioxide stage where consistency is maintained from about 9% to 14% and pH is about 2 to 4;
- partially neutralizing pulp, from said first chlorine dioxide bleaching stage, to a pH of 5 to 7 in a neutralization tank with filtrate from a subsequent oxidative extraction stage and with oxidized white liquor;
- thickening pulp on a decker after which oxidized white liquor is added to raise the pH to about 10 to 13 and oxygen and hydrogen peroxide bleaching chemicals are added;
- subjecting the thickened pulp and bleaching chemicals to a hot extraction stage;
- thoroughly washing pulp from said extraction stage with fresh water on two sequential washing stages to minimize the wash volume;
- subjecting said washed pulp to one or two final chlorine dioxide bleaching stages and thoroughly washing pulp after each stage with either fresh water or paper machine white water or a combination of both;
- discharging to sewer the filtrate from said washing, after each said final chlorine dioxide stage;
- using filtrate from said two stage sequential washing stages, after said extraction stage, to dilute and neutralize pulp from said first chlorine dioxide stage of bleaching in said neutralization tank such that filtrate from the extraction stage and the first chlorine dioxide stage are combined with pulp;
- separating the combined filtrates from the pulp on said decker and at least a portion of the filtrate is recycled to the last washer of the post oxygen washing stage as wash water;
- recovering filtrate from said last washer of the post oxygen washing stage in the cooking liquor recovery process by recycling countercurrently through brownstock washing constituting a weak black liquor which is evaporated to produce a strong black liquor;
- combusting said strong black liquor in a recovery boiler to produce a smelt and a flue gas such that chloride from the recovered bleach plant filtrate is preferentially enriched in the flue gas from combustion;
- collecting particles containing chloride and sulfate in said flue gas as ash in an electrostatic precipitator;
- removing chloride and potassium from said ash by either leaching with water or by evaporation crystallization from a water solution to produce a solid stream of sodium sulfate and a salt solution rich in chloride and potassium;
- discharging the salt solution to the sewer or recovering it for its chlorine value;
- mixing said solid sodium sulfate with said strong black liquor prior to combustion in said recovery boiler;
- and
- dissolving said recovery boiler smelt in water to form green liquor and converting it to pulping liquor in the causticizing process.
16 more elements
A process for bleaching Kraft wood pulp comprising subjecting the wood pulp after brown stock washing to the following steps conducted in sequence:
- oxygen delignification in the presence of oxygen and oxidized white liquor followed by one to two stages of post oxygen washing;
- subjecting the oxygen delignified pulp to a first chlorine dioxide bleaching stage where consistency is maintained at 2% to 5% and pH is about 2 to 4;
- thickening pulp from said first chlorine dioxide tower on a decker, without washing with fresh water, after which oxidized white liquor is added to raise the pH to about 10 to 13 and adding oxygen and hydrogen peroxide bleaching chemicals;
- subjecting the thickened pulp and bleaching chemicals to a hot extraction stage;
- thoroughly washing pulp from said extraction stage with fresh water on two sequential washing stages to minimize the wash water volume;
- subjecting said washed pulp to one or two final chlorine dioxide bleaching stages and thoroughly washing pulp after each stage with either fresh water or paper machine white water or a combination of both;
- discharging to the sewer the filtrate from said washing, after each said final chlorine dioxide stage;
- using a portion of filtrate from said thickening process following said first chlorine dioxide bleaching stage to dilute pulp from said last washer of the post oxygen stage;
- treating a portion of said filtrate from said first chlorine dioxide stage washer with sodium hydroxide and sodium carbonate to effect precipitation of metals that enter the bleach plant with the wood fiber;
- separating the precipitated material from the aqueous phase by gravity settling or filtration;
- disposing the precipitated sludge and using the metal free clarified filtrate to wash pulp on the first chlorine dioxide stage washer to prevent the carryover of metals to the extraction stage whereby all dissolved solids entering or generated in the first chlorine dioxide bleach stage must either pass to the oxidative extraction stage with the pulp or be disposed of with the precipitated metal sludge;
- using filtrate from said at least two stage sequential washing stages as wash water to countercurrently wash pulp beginning with the last washer of the post oxygen stage and continuing through brownstock washing which constitutes a weak black liquor;
- evaporating said weak black liquor to produce a strong black liquor;
- combusting said strong black liquor in a recovery boiler to produce a smelt and a flue gas whereby chloride from the recovered bleach plant filtrate is preferentially enriched in the flue gas from combustion;
- collecting particles containing chloride and sulfate in a said flue gas as ash in an electrostatic precipitator;
- removing chloride and potassium from said ash by either leaching with water or by evaporation crystallization from a water solution to produce a solid stream of sodium sulfate and a salt solution rich in chloride and potassium;
- discharging the salt solution to the sewer or recovered for its chlorine value;
- mixing said solid sodium sulfate with said strong black liquor prior to combustion in said recovery boiler;
- and
- dissolving said recovery boiler smelt in water to form green liquor and converting it to pulping liquor in the causticizing process.
17 more elements
Dependencies are inferred from the claim text.