Recovery of pulping liquors

1. Recovery of Pulping liquors

2. 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

3. Definition of Liquors:
White Liquor
Strongly 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 liquor
Green 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.

4. 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

5. Pulping Liquor Cycle:
Digester
White Liquor
Black Liquor
Recovery
Furnace
Lime
Kilm
Green Liquor

6. 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.

7. Recovery Process of Pulping Liquors:
Pulping liquors recovery involves mainly three steps:
 Evaporation
 Burning
 Causticizing
Fig: Recovery boiler

8. 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%.

9. Recovery Cycle of Pulping Liquors:
Fig: Recovery Cycle of Pulping Liquors

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.