Recovery of pulping liquors

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Recovery of Pulping liquors

Transcript of Recovery of pulping liquors

Page 1: Recovery of pulping liquors

Recovery of Pulping liquors

Page 2: Recovery of pulping liquors

Pulping Liquor:

Chemicals those are used in the pulp and paper making process called pulping

liquor. The active chemicals in the pulping especially in the kraft pulping are

NaOH and Na2S.

In kraft pulping process we generally get three liquors:

White liquor

Black liquor

Green liquor

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Definition of Liquors:

White LiquorStrongly alkaline solution mainly of sodium hydroxide and sodium sulfide. It is

used in the first stage of the Kraft process in which lignin and hemicellulose are

separated from cellulose fiber.

Green liquorGreen liquor is the dissolved smelt of sodium carbonate, sodium sulfide and

other compounds from the recovery boiler. The green liquor is usually reacted

with lime (CaO) to regenerate white liquor.

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Definition of Liquors:

Black liquor.The waste liquor from the kraft pulping

process. Contains most of the original

inorganic components (most in

different forms) and a high

concentration of dissolved organics.

Uses:• Energy source for the pulp mill

• Extraction of lignin

• Use as biofuel

Fig: Black liquor

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Pulping Liquor Cycle:

Digester

White Liquor

Black LiquorRecoveryFurnace

LimeKilm

Green Liquor

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Recovery Process of Pulping Liquors:

In pulping process wood chips are processed with white liquors.

About half of the wood is dissolved, and together with the spent

pulping chemicals, forms a liquid stream called weak black liquor.

The weak black liquor is separated from the pulp by washing, and is

sent to the kraft recovery system.

In kraft recovery system the inorganic pulping chemicals are

recovered for reuse.

The dissolved organics are used as a fuel to make steam and power.

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Recovery Process of Pulping Liquors:

Pulping liquors recovery involves mainly three steps:

Evaporation

Burning

Causticizing 

Fig: Recovery boiler

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Recovery Process of Pulping Liquors:

Evaporation:

Weak black liquor contains chemicals, wood organics such as lignin, and

water.

At this stage, the solids content is typically 12%~ 18%.

Before burning, water must be evaporated to raise the solids content to 65

%~ 70%.

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Recovery Cycle of Pulping Liquors:

Fig: Recovery Cycle of Pulping Liquors

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Recovery Process of Pulping Liquors:

Burning:

The recovery boiler is one of the largest and most expensive pieces of hardware in the mill. It is the heart of the chemical recovery process.

The heavy black liquor is sprayed into the furnace for combustion of organic

solids.

Heat liberated from burning serves to produce steam in the water circuit and

reduce sulfur compounds to sulfide.

The molten sodium compounds accumulate to form a smelt bed on the furnace

floor., consisting of Na2CO3 and Na2S

The molten smelt flow by gravity to the dissolving tank.

**This Na2CO3 and Na2S solution is called green liquor.

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Recovery Process of Pulping Liquors:

Causticizing :

Green liquor is sent to the causticizing area for transformation to white liquor for cooking. The process begins with clarification of the green liquor to remove impurities called dregs.

Clarified green liquor is then mixed with lime in the slaker to form white liquor.

The lime (CaO) activates the conversion of sodium carbonate (Na2CO3) in the

green liquor to form sodium hydroxide (NaOH) for white liquor.

To allow time for a complete reaction, the white liquor passes to a series of

agitated tanks called causticizers.

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Recovery of Heat from Combustion of Organics :

The organic compounds in black liquor serve as a fuel for the

production of steam which is used to generate electricity.

The heat released as a result of black liquor combustion is recovered

as high pressure/temperature.

Depending on the quality of the steam, and the type of the turbine, a

1000 t/d kraft pulp mill can generate 25 to 35 MW of electricity by

burning 1500 t/d black liquor dry solids in its recovery boiler.

Black liquor is the fifth most important fuel in the world, next to coal,

oil, natural gas, and gasoline.

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Operational Problems:

Frequent evaporator boil outs, high steam consumption, and low solids

in the product liquor.

Recovery boilers also have many problems including fouling of heat

transfer tubes.

Many problems can also occur in lime kilns including: low thermal

efficiency, high fuel consumption, dusting, etc.

**These operational problems result in poor pulping liquor quality, poor

pulp quality, increased mill energy consumption.

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Functions of Liquor Recovery Process:

The process has four main functions:

i) Minimizing the environmental impact of waste material (black

liquor) from the pulping process

ii) Recycling pulping chemicals, NaOH and Na2S

iii) Co-generating steam and power.

iv) Reducing the pulping cost.

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That’s All