Pulp Washing General Metso

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Pulp Washing

General



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Overview



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The First Washing Principle



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Why Washing?



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Pulp WashingBrown Stock Area

The primary objectives for washing the pulp in thebrown stock area are:

1. To clean the pulp, by removing the black liquor 2. To recover the chemicals in the black liquor 3. To recover the heat value in the wood

substances, e.g. lignin, in the black liquor sothat they can be used as fuel or can be

processed further 4. To decrease the amount of pollutants that aredischarged to the receiving waters

5.

To decrease the chemical demand in the bleachplant



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Composition of Wood



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Factors Affecting Pulp WashingBrown Stock Area

Pulp washing in the brown stock area is dependenton many different factors including:

1. The quality and the type of wood that is used2. The type of cooking stage that is used3. The temperature of the overall system



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Pulping



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Countercurrent WashingBrown Stock Area

In modern mills the wash liquor used in the brownstock and oxygen delignification washers flows

counter-current, i.e. in the opposite direction, to thatof the pulp.

The main advantage of countercurrent washing isthat the amount of wash liquor that is required to

wash the pulp is minimized.



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Sulfate Process



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Black Liquor

The composition of the black liquor that isgenerated in the digester area depends on many

independent factors, such as:

1. the raw material (species, age, etc.),2. the type of cooking chemicals that are used

(composition and concentration),

3. the cooking temperature,4. the cooking time.



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Black Liquor Composition

Organic Material

Lignin 500 kg/tonne dry pulp

Carbohydrates 500 “

Resin 45“

Inorganic Material

Cooking chemicals 555 kg/tonne dry pulp

Total Dry Solids 1600 kg/tonne dry pulp



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Washing Principles

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Displacement Washing Principle

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Wash LossBrown Stock Area

Wash loss is a measurement of the dissolvedorganic material that is present in the pulp streamand “carried over” from one stage to the next.

wash loss = kg COD in the liquor that leaves withthe pulp/ton pulp

where COD, chemical oxygen demand, is a

standard measurement used to determine theequivalent amount of oxygen that a water or effluent will consume when oxidized chemically.

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Dilution FactorBrown Stock Area

Dilution Factor, DF, is a method of describing theamount of washing that the pulp receives.

As the DF increases, more dissolved solids areremoved from the pulp, making it cleaner and easier tobleach.

When the dilution factor is controlled, the degree of

washing remains constant with changing production.The amount of wash liquor increases or decreasesautomatically.

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Washing Efficiency vsDilution Factor

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Dilution Factor ExampleBrown Stock Area

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Washing EfficiencyDisplacement Ratio with a dilution factor 2m3 /ton

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Washing Efficiency

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Diffuser

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Filter

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Press

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Dewatering Washing Principle

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Amount of liquor in the pulp

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Consistency

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Pulp Washing General Metso

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