The Kraft recovery process involves evaporating black liquor to concentrate it before burning it in a recovery furnace. This regenerates cooking chemicals and generates heat. In the recovery furnace, organics are burned while inorganics are converted to sodium carbonate and sulfide. The smelt from the furnace forms green liquor which is clarified and sent to a slaker where lime is added to precipitate calcium carbonate and form sodium hydroxide. The calcium carbonate slurry is treated in a lime kiln to regenerate lime for the causticizing process.

1. 1
Pulping and Bleaching
PSE 476/Chem E 471
Lecture #15
The Kraft Recovery Process

2. 2
Agenda
• Overview of process
• Black Liquor
• Evaporation
• Recovery Furnace
• Smelt
• Causticizing
• Lime Kiln

3. 3
Basics of the Kraft Recovery
System
This diagram (taken
from “Kraft Pulping:
A Compilation of
Notes” shows what
will be covered in
this lecture. The
object of this system
is to regenerate the
cooking chemicals
and to generate heat
= steam & power.
Cooking
&
Washing
Evaporation
&
Burning
Causticizing
BLACK LIQUOR
Alkali Lignin
Hydrolysis Salts
Sulfonation Products
GREEN LIQUOR
Na2CO3
Na2S
WHITE LIQUOR
NaOH
Na2S
Chips
Pulp
CaCO3
CaO
Makeup Chemicals
Heat Water

4. 4
Black liquor separation

5. 5
Basics of the Kraft Recovery
System II

6. 6
Black Liquor
Digester
Chips
NaOH
Na2S
Pulp
Black Liquor
14-18% Solids
pH 12+
65% Organics
35% Inorganics
Inorganics
NaOH (7%)
Na2S (19%)
Na2CO3 (36%)
Na2SO3 (9%)
Na2SO4 (13%)
Na2S2O3 (16%)
Notes

7. 7
Evaporation I
• In order to burn the black liquor, the solids content needs to
be raised to over 60%.
» This means that over 80% of the water needs to be removed.
• This is accomplished through the use of multiple effect
evaporators.

8. 8
Evaporation II
• Evaporation is accomplished through:
» Direst contact with flue gasses (old)
» Indirect contact with steam
- Falling film (new)
- Raising film (old)
• With indirect contact, steam is applied
countercurrent to the liquor (high temperature/pressure
steam is applied to thickest liquor). The steam produced
from the liquor is used to heat the next effect
(evaporator) at a lower temperature and pressure.
This continues to the first effect (most dilute liquor)
which is run under vacuum. (Please read the reading)

9. 9
Evaporation III

10. 10
Recovery Furnace
• Concentrated black liquor is burned in a
recovery furnace. The functions of the recovery
furnace are:
» Remove remaining water from black liquor.
» Burn the black liquor organic compounds
- Solves disposal issues.
- Produces steam (for heating and energy).
» Reduce sulfur species to sulfide.
» Generate Na2CO3 which will later be converted to
NaOH

11. 11
Recovery Furnace III
Smelt (molten Na2CO3, Na2S, etc)
Liquor
Air
Air
Air
System for recovery
of heat to generate power
Reduction
Zone
Combustion
Zone
Gasses from Pyrolysis
(A)
Notes
(B)
(C)
(D)

12. 12
Recovery Furnace IV
•Liquor is sprayed
to char bed (droplet
size 2-3mm)
•Particles should be
dry before landing
the char bed
•Char is required in
bed for effective
reduction of
sulfates

13. 13
Reduction of Inorganics
• Formation of sodium carbonate (Na2CO3)
» Combustion of organics results in the formation of
CO2 and CO. Sodium compounds are converted into
Na2O.
» Na2O + CO2  Na2CO3
» Na2S + H2O + CO2  Na2CO3 + H2S
• Reductive reactions in the char results in the
formation of sodium sulfide (Na2S)
» Na2SO4 + 2C  Na2S + 2CO2
» Na2SO4 + 4C  Na2S + 4CO

14. 14
Smelt
• The molten inorganics at the bottom of the
recovery furnace are known as the smelt. This
material at 1500°F leaves the furnace and is added
to water in the smelt tank.
» This results in the formation of green liquor
» This is also a dangerous operation. The extremely hot
smelt causes the water to instantly vaporize in a violent
reaction.
» Green liquor composition: NaOH (8%), Na2S (20%),
Na2CO3 (60%), Na2SO3 ( 3%), Na2SO4 (6%), Na2S2O3 (3%).

15. 15
Caustizing
• Caustizing is the process in which sodium
carbonate is converted into sodium
hydroxide.
• In the first step, green liquor is clarified to
remove insoluble materials.
» Calcium compounds
» Unburned carbon
» Metals
• Clarified green liquor is sent to a “slaker”
where lime (CaO/Ca(OH)2) is added .

16. 16
Slaker (Causticizing)
Chemistry
• Calcium carbonate (CaCO3) has extremely low
solubility in water while sodium carbonate (Na2CO3)
has fairly high solubility. When lime (CaO) is added to
a Na2CO3 solution, the following reaction occurs:
(important to note that CaO + H2O  Ca(OH)2 this is the slaking
reaction).
» Ca(OH)2 + Na2CO3  CaCO3 + 2 NaOH
» The calcium carbonate precipitates out of solution driving the
reaction to the right side of the above equation thus forming
sodium hydroxide.
» Calcium carbonate is removed through clarification forming
white liquor. This step completes the circle as the white liquor is
returned to the digester for pulping.

17. 17
Lime Kiln
• The wet calcium carbonate slurry is treated in
a lime kiln.
» This is a very long, refractory brick lined, slightly
tilted, rotating tube which is extremely hot (1500 to
2100°F).
» The calcium carbonate slurry is dried in the first
section of the kiln and then the calcium carbonate is
converted to CaO: CaCO3 + heat  CaO + CO2

18. 18
Black versus green liquor