Bleach plant filtrate recovery

In a pulp mill system having a pulping and recovery and regeneration cycle and a bleaching and purification operation including the steps of

• digesting raw cellulosic fibrous material in a pulping liquor including sodium hydroxide as at least one active pulping chemical,
• washing the digested cellulosic fibrous material substantially free from spent pulping liquor,
• forming an aqueous spent liquor containing spent pulping liquor and spent wash water from said washing step,
• subjecting said aqueous spent liquor to furnacing to form a smelt,
• subjecting said washed digested cellulosic fibrous material to said bleaching and purification operation using bleaching chemicals including chlorine atoms and sodium hydroxide extraction solutions,
• washing said bleached and purified pulp substantially free from residual bleaching and purification chemicals,
• recovering a washed, bleached and purified pulp from said bleaching and purification operation,
• separating from said bleaching and purification operation an acid aqueous filtrate and an alkaline aqueous filtrate, said latter filtrates in combination including all the aqueous material introduced to said bleaching and purification operation less any water present in said washed, bleached and purified pulp,
• introducing said latter filtrates to said pulping and recovery and regeneration cycle, whereby said smelt contains sodium chloride,
• regenerating pulping liquor from the components of said smelt by causticizing sodium carbonate in said smelt in aqueous solution by causticization reaction with lime to form sodium hydroxide from said sodium carbonate and precipitate calcium carbonate, separating said precipitated calcium carbonate from the resulting pulping liquor, kilning said separated calcium carbonate to form lime for recycle to said causticization reaction and scrubbing flue gases from said kilning to remove entrained particles,
• recycling said resulting pulping liquor as at least part of said pulping liquor in said digestion step, and
• separating solid sodium chloride from said pulping and recovery and regeneration cycle subsequent to said furnacing step and prior to said pulping liquor recycling step by concentrating said resulting pulping liquor after said separation of precipitated calcium carbonate to deposit sodium chloride therefrom, removing the deposited sodium chloride from the resulting concentrated pulping liquor and diluting the concentrated pulping liquor to the desired pulping liquor concentration for said pulping liquor recycling step,
• the improvement wherein said introduction of said filtrates to said pulping and recovery and regeneration cycle is carried out by:
• neutralizing part of said acid filtrate with fresh aqueous sodium hydroxide solution,
• using said neutralized filtrate as wash water for washing said digested cellulosic fibrous material in the portion of said washing step immediately prior to passage of washed pulp to said bleaching and purification operation,
• passing the remainder of said acid filtrate to said regeneration operation for use as an aqueous medium therein subsequent to said furnacing step and prior to said recycle step while excluding said remainder of said acid filtrate from said recycled pulping liquor,
• using part of said alkaline filtrate as wash water for washing said digested cellulosic fibrous material prior to washing with said neutralized filtrate in the remainder of said washing step, whereby said washing step is carried out wholly by said neutralized filtrate and said part of said alkaline filtrate, and
• using the remainder of said alkaline filtrate for said diluting of the concentrated pulping liquor to the desired pulping liquor concentration.

In a pulp mill having a pulping and recovery and regeneration cycle and a bleaching and purification operation including the steps of

• digesting raw cellulosic fibrous material in a pulping liquor,
• washing the digested cellulosic fibrous material substantially free from spent pulping liquor,
• forming an aqueous spent liquor containing spent pulping liquor and spent wash water from said washing step,
• subjecting said aqueous spent liquor to furnacing to form a smelt,
• subjecting said washed digested cellulosic fibrous material to said bleaching and purification operation using bleaching chemicals including chlorine atoms and sodium hydroxide extraction solutions,
• washing said bleached and purified pulp substantially free from residual bleaching and purification chemicals,
• recovering a washed, bleached and purified pulp from said bleaching and purification operation,
• separating from said bleaching and purification operation an acid aqueous filtrate and an alkaline aqueous filtrate, said latter filtrates in combination including all the aqueous material introduced to said bleaching and purification operation less any water present in said washed, bleached and purified pulp,
• introducing said latter filtrates to said pulping and recovery and regeneration cycle, whereby said smelt contains sodium chloride,
• regenerating pulping liquor from the components of said smelt,
• recycling said regenerated pulping liquor as at least part of said pulping liquor in said digestion step, and
• separating solid sodium chloride from said pulping and recovery and regeneration cycle subsequent to said furnacing step and prior to said recycling step,
• the improvement wherein said introduction of said filtrates to said pulping and recovery and regeneration cycle is carried out by:
• neutralizing part of said acid filtrate with fresh aqueous sodium hydroxide solution,
• using said neutralized filtrate as wash water for washing said digested cellulosic fibrous material in the portion of said washing step immediately prior to passage of washed pulp to said bleaching and purification operation,
• discharging the remainder of said acid filtrate from said pulp mill system,
• using part only of said alkaline filtrate as wash water for washing said digested cellulosic fibrous material prior to said washing with said neutralized filtrate in the remainder of said washing step, whereby said washing step is carried out wholly by said neutralized filtrate and said part of said alkaline filtrate, and
• passing the remainder of said alkaline filtrate directly to said pulping and recovery and regeneration cycle.

In a pulp mill system having a pulping and recovery and regeneration cycle and a bleaching and purification operation including the steps of

• digesting raw cellulosic fibrous material in a pulping liquor,
• washing the digested cellulosic fibrous material substantially free from spent pulping liquor,
• forming an aqueous spent liquor containing spent pulping liquor and spent wash water from said washing step,
• subjecting said aqueous spent liquor to furnacing to form a smelt,
• subjecting said washed digested cellulosic fibrous material to said bleaching and purification operation using bleaching chemicals including chlorine atoms and sodium hydroxide extraction solutions,
• washing said bleached and purified pulp substantially free from residual bleaching and purification chemicals,
• recovering a washed, bleached and purified pulp from said bleaching and purification operation,
• separating from said bleaching and purification operation an acid aqueous filtrate and an alkaline aqueous filtrate, said latter filtrates in combination including all the aqueous material introduced to said bleaching and purification operation less any water present in said washed, bleached and purified pulp,
• introducing said latter filtrates to said pulping and recovery and regeneration cycle, whereby said smelt contains sodium chloride,
• regenerating pulping liquor from the components of said smelt,
• recycling said regenerated pulping liquor as at least part of said pulping liquor in said digestion step, and
• separating solid sodium chloride from said pulping and recovery and regeneration cycle subsequent to said furnacing step and prior to said recycling step,
• the improvement wherein said introduction of said filtrates to said pulping and recovery and regeneration cycle is carried out by:
• neutralizing part of said acid filtrate with fresh aqueous sodium hydroxide solution,
• using said neutralized filtrate as wash water for washing said digested cellulosic fibrous material in the portion of said washing step immediately prior to passage of washed pulp to said bleaching and purification operation,
• passing the remainder of said acid filtrate to said pulping and recovery and regeneration cycle subsequent to said furnacing step and prior to said recycle step while excluding said remainder of said acid filtrate from said recycled pulping liquor,
• using part only of said alkaline filtrate as wash water for washing said digested cellulosic fibrous material prior to said washing with said neutralized filtrate in the remainder of said washing step, whereby said washing step is carried out wholly by said neutralized filtrate and said part of said alkaline filtrate, and
• passing the remainder of said alkaline filtrate directly to said pulping and recovery and regeneration cycle.