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1
TAPPI KRAFT RECOVERYSHORT COURSE
RECAUSTICIZING
CHAPTER 2.1 - PRINCIPLES AND PRACTICEBY
DALE SANCHEZVECTOR PROCESS EQUIPMENT INC.
TOPICS
• BASIC CHEMISTRY• FLOWSHEET OPTIONS• GREEN LIQUOR PREPARATION• WHITE LIQUOR PREPARATION• LIME MUD WASHING• CONTROLS
2
LIME KILNLIME MUDWASHING
WEAK WASH
LIMEMUD
WATER
WHITELIQUORCLARIFICATION
LIME MUD
WHITE LIQUOR
SMELTDISSOLVING
SMELTFROMRECOVERYBOILER
GREENLIQUORCLARIFICATION
DREGSSLAKER
/CAUSTICIZERS
LIME
GREENLIQUOR
GRIT
RECAUSTICIZING CYCLE
BASIC CHEMISTRY
GREENLIQUOR
+ LIME WHITELIQUOR
LIMEMUD
= +
Na2CO3 + H2O + CaO = 2NaOH + CaCO3
3
CHEMICAL REACTIONS
• SLAKINGCaO + H2O Ca(OH)2 + Heat
• CAUSTICIZINGCa(OH)2 + Na2CO3 2NaOH + CaCO3
OVERALL REACTION
Na2CO3 + H2O + CaO 2NaOH + CaCO3
• EQUILIBRIUM REACTION• REVERSIBLE• INCOMPLETE
4
REACTION AT SOLID/ LIQUID INTERFACE
UNREACTEDCORE
Na+
CO3=
OH-
CaO
CaCO3
DEFINITIONSTTA = TOTAL TITRATABLE ALKALI
NaOH + Na2S + Na2CO3
AA = ACTIVE ALKALINaOH + Na2S
EA = EFFECTIVE ALKALINaOH + ½ Na2S
SULFIDITY %Na2S / AA or Na2S / TTA
5
DEFINITIONSCE = CAUSTICITY %
NaOH x 100NaOH + Na2CO3
Lime Availability %CaO / Kiln Product
Note: All chemical concentrations are expressed on a Na2O basis e.g. TTA = 120 g/l as Na2O
Other units of measure are lb/ft3 & lb/gal
OPERATING CAUSTICIZING EFFICIENCY
75
80
85
90
95
100
105
0 50 100 150 200
TTA g/l
% CESULFIDITY=0%
SULFIDITY=15%
SULFIDITY=30%
OPERATING CE
5%
6
FREE CaO vs CAUSTICITY
01234567
70 75 80 85
% CAUSTICITY
% F
REE
LIM
E
FLOWSHEET WITH CLARIFIERS
Smelt Tank
Green LiquorClarifier
Dregs Filter Slaker /Causticizers
WhiteLiquor
Clarifier
Lime MudWasher
Lime Kiln
White Liquor
Weak Wash
Water
Lime Mud FilterLime
Green LiquorStabilization
7
Smelt Tank
GreenLiquorClarifier
Dregs Filter Slaker / Causticizers
Lime Kiln
White Liquor
Weak WashFLOWSHEET WITH PRESSURE FILTERS
Water
Lime Mud Filter
Pressure Filters
LimeGreen LiquorStabilization
Water
Smelt Tank
Green LiquorClarifier
Dregs Filter Slaker /Causticizers
Lime Kiln
WhiteLiquor
Weak WashFLOWSHEET WITH PRESSURE DISC FILTER
Water
Lime Mud Filter
Pressure Disc Filter
Lime
Green LiquorStabilization
Water
8
GREEN LIQUOR PREPARATIONSmelt
Weak Wash
Smelt DissolvingTank
StabilizationTank
Clarifier
Dregs Holding TankDregs Filter
ClarifiedGreen Liquor
Variation in Green Liquor Density or Temperature in Stabilization Tank
Time
Den
sity
or
Tem
pera
ture
Inlet
Outlet
9
OPTIONAL SIDEMOUNT AGITATOR
INLET PIPEDOWN COMER
RAW GREEN LIQUOR TOP MOUNTED AGITATOR
PUMP TO GREEN LIQUORCLARIFIER
RAW GREEN LIQUOR STABILIZATION TANK
GREEN LIQUOR CLARIFICATION OPTIONS
• SEDIMENTATION CLARIFIERS• PRESSURE FILTERS
– CROSS FLOW FILTERS– SOCK TYPE FILTERS
10
GREEN LIQUOR CLARIFIER
FEEDWELL
STORAGE ZONE
MAX LL
MIN LL
CLARIFIEDLIQUOR OUT
CLARIFIERFEED
UNDERFLOW OUT
RAKE DRIVE
CLARIFICATION ZONE
CLARIFIER INTERNALS
Photo courtesy of Dorr-Oliver Eimco
11
Clean Green Liquor!Clarifier Feed Clarified Green Liquor
Photo courtesy of Dorr-Oliver Eimco
GREEN LIQUOR CLARIFIER PROBLEMS
DIRTY GREEN LIQUOR• Clarifier too small (rise rate too high)• Properly designed internal components• Use of Contaminated condensate• Lack of Stabilization tank• Settling aid addition problem (Polymer)
12
DREGS PRECOAT FILTER
Photo courtesy of Dorr-Oliver Eimco
DREGS FILTER OPERATIONAL PROBLEMS
• Wet cake discharge• High soda loss• Difficulty forming a precoat
13
Clarifier
WHITE LIQUOR PREPARATIONGreen Liquor
LimeWhite Liquor
Lime Mud
Lime Slaker
Causticizers
Grit
LIME
GREEN LIQUOR
AGITATOR
TO VENT SCRUBBER
SECONDARY GREENLIQUOR FEED
OVERFLOW TROUGH
SLAKING COMPARTMENT
CLASSIFIER COMPARTMENT
GRIT
LIME SLAKER
TO CAUSTICIZERS
SCREWCLASSIFIER DRIVE
14
LIME SLAKER
Photo courtesy of Kadant Black Clawson Inc. – Goslin™ Products Group
LIME SLAKER
• Sized based on retention time in slaking compartment.– For Clarifiers – 15 to 20 mins– For Pressure filters – 20 to 25 mins
• Correct operation determines white liquor strength
15
LIME SLAKER PROBLEMS• OVERLIMING
– Lime feed rate and or GL flow and strength• BOILING
– GL Temperature control and or lime feed rate• GREEN LIQUOR TEMPERATURE
– Heating – Direct and indirect type heaters– Cooling – Indirect heat exchangers
• FRESH LIME MAKE-UP - the way it is added• GRIT – too much or too little grit can be an indicator
of a problem
FRESH LIME MAKE-UPSingle Silo
SLAKER From Kiln
FRESHLIME
SLAKER
Two SilosReburned Fresh
SLAKER
16
SLAKER GRIT HANDLING
• WASH GRIT TO pH LESS THAN 12.5 WITH GRIT WASHER
Photo courtesy of Kadant Black Clawson – Goslin™ Products Group
SLAKER GRIT HANDLING
• GRIND OR PULVERIZE GRIT AND FILTER ON DREGS FILTER OR RECYCLE TO LIME SLAKER
Photo courtesy of Kadant Black Clawson – Goslin™ Products Group
17
INLET
INLET
OUTLET
OUTLET
RISERPIPE
CAUSTICIZERS
WHITE LIQUOR CLARIFICATION
OPTIONS• SEDIMENTATION CLARIFIERS• VERTICAL TUBE TYPE
PRESSURE FILTERS• PRESSURE DISC FILTERS
18
WHITE LIQUOR CLARIFIER
FEEDWELL
STORAGE ZONE
MAX LL
MIN LL
CLARIFIEDLIQUOR OUT
CLARIFIERFEED
UNDERFLOW OUT
RAKE DRIVE
CLARIFICATION ZONE
WHITE LIQUOR CLARIFIER
For successful clarifier operation• Causticizer retention of 90 minutes• Good accurate torque indication on
rakes• Properly sized variable speed
underflow pumps with density and flow measurement
19
WHITE LIQUOR CLARIFIER PROBLEMS
• CLOUDY OVERFLOW– Over Liming– Fresh Lime
• LOW UNDERFLOW SOLIDS– Over Liming– Fresh Lime– Rat Holing– Pumping Rate
• Rat Holing
WHITE LIQUOR CLARIFIER PROBLEMS
DE FE
20
• Pumping Rate
WHITE LIQUOR CLARIFIER PROBLEMS
DE FE
FE
HOT WATER
UNDERFLOWPUMP
QD
QS+ QLQUF
QUF=QS+QL+QD
• Pumping Rate– Mass flow of solids = Slaker solids output.– Slaker solids output calculated from Green
Liquor flow and Lime rate.
WHITE LIQUOR CLARIFIER PROBLEMS
21
VERTICAL PRESSURE
FILTER
Photo courtesy of Kadant Black Clawson – Goslin™ Products Group
VERTICAL TUBE TYPE PRESSURE
FILTER
Photo courtesy of Kadant Black Clawson – Goslin™ Products Group
22
PRESSURE TUBE TYPE FILTERS
FEEDTANK
MUDTANK
D F
A B
WHITE LIQUOR TOSTORAGE TANK
FROMCAUSTICIZERS
TO LIME MUDWASHER (WEAKWASH FILTER)
Dp
WATER
FILTRATION CYCLEFilter 3-5 minsBackwash 3-5 secsRest 25-35 secs
PRESSURE TUBE TYPE FILTERS
Acid washing– Interval between washes 4 – 8 weeks– Acid types
• Sulphamic, Hydrochloric, Formic
– Alternative strategy• Change filter socks every 6 months or when needed.
23
PRESSURE TUBE TYPE FILTERS
For successful operation of pressure filters.• Clean green liquor < 100 mg/l• Causticizer retention time 150 -180 minutes.• Causticizing control system.
PRESSURE FILTER INTERNALS
Photo courtesy of Kadant Black Clawson – Goslin™ Products Group
24
PRESSURE FILTER PROBLEMS
• FREQUENT ACID WASHING– Green Liquor Dregs – contaminates filter socks– Overliming – temporary blinding of filter socks
• HIGH PRESSURE DROP– Overliming– Dregs– High Mud Level
• SHORT FILTER SOCK LIFE– Dregs contamination
WL PRESSURE DISC FILTER
Photo courtesy of Dorr-Oliver Eimco
25
FEEDTANK
WHITE LIQUOR PRESSURE DISC FILTER
COMPRESSOR
LIME MUDSLURRY TANK
TO LIME MUD STORAGE
WHITELIQUOR
WHITELIQUORSEPARATORFROM
CAUSTICIZERS
CAKEWASH WATER
WATER
LIQUOR YIELD100
17.9
3.20
82.196.8
0
20
40
60
80
100
CausticizedSlurry
GravitySystem (35%
Solids)
Disc FilterSystem(75%
Solids)
White Liquor with MudWhite Liquor to Digester
26
LIME MUD WASHING
Lime Mud Washer Lime MudStorage
Lime MudFilter
LimeMud Mixer
Weak Wash
Filtrate
WaterLime Mud
Water
LIME MUD WASHING
• SEDIMENTATION CLARIFIERS• VERTICAL PRESSURE FILTERS• NOT REQUIRED WITH
PRESSURE DISC FILTERS
27
LIME MUD WASHER PROBLEMS
• CLOUDY OVERFLOW– Over Liming, Fresh Lime, Scrubber Dust,
Contaminated Condensate.• LOW UNDERFLOW SOLIDS
– Over Liming, Fresh Lime, U/Flow Pumping• HIGH SODA IN WEAK WASH AND LIME
MUD– Water Balance, U/Flow Pumping
• TOO MUCH WEAK WASH – Water Balance
LIME MUD PRECOAT DRUM FILTER
Photo courtesy of Dorr-Oliver Eimco
28
LIME MUD PRECOAT DISC FILTER
Photo courtesy of Dorr-Oliver Eimco
LIME MUD FILTER PROBLEMS
• HIGH MOISTURE IN KILN FEED– Over Liming, Fresh Lime, Dregs, Overloaded
• FREQUENT PRECOAT CHANGES– Blinding of Precoat
• Over Liming, Fresh Lime, Dregs, Wash Water
• HIGH SODA IN KILN FEED– Cake Wash, High Moisture
• TRS EMISSIONS FROM KILN– Cake Wash, Overloaded
29
TRS Control viaMud Dewatering & Oxidation
TO KILN0.2% WaterSoluble Soda
WITHOXIDATIONNa2S = 0.005%
WITHOUTOXIDATION
Na2S = 0.04%
AIR
WashingZone
CONTROLS
• CAUSTICIZING CONTROLConductivityOn-Line TitrationNear Infrared Spectrometry
• GREEN LIQUOR TEMPERATURE AND DENSITY
• LIME MUD DENSITY CONTROL
30
SLAKER CONTROLCONTROLCONTROLSYSTEMSYSTEM
SCREW
C
C
CD
T
FGREEN LIQUOR
LIME SLAKER
CAUSTICIZERS
LIMELIME
CONDUCTIVITY BASED CONTROL SYSTEM
SLAKER CONTROLCONTROLCONTROLSYSTEMSYSTEM
SCREW
D
T
FGREEN LIQUOR
LIME SLAKER
CAUSTICIZERS
LIMELIME
TITRATION BASED CONTROL SYSTEM
SAMPLE LINE
AUTOMATICAUTOMATICTITRATORTITRATOR
WLFILTER
31
SLAKER CONTROLCONTROLCONTROLSYSTEMSYSTEM
SCREW
D
T
FGREEN LIQUOR
LIME SLAKER
CAUSTICIZERS
LIMELIME
NEAR-INFRARED BASED CONTROL SYSTEM
SAMPLE LINE
NIRNIRSENSOR(S)SENSOR(S)
WLFILTER
SLAKER CONTROL
CONTROLCONTROLSYSTEMSYSTEM
SCREW
T
T
FGREEN LIQUOR
LIME SLAKER
LIMELIME
STEAM CONTROL VALVE
HEATER
ΔT SLAKER CONTROL ANDGREEN LIQUOR DENSITY
D
WEAKWASH
32
Smelt Tank
Green LiquorClarifier
Dregs Filter Slaker /Causticizers
WhiteLiquor
Clarifier
Lime MudWasher
Lime Kiln
White Liquor
Weak Wash
Water
Lime Mud FilterLime
Green LiquorStabilization
DENSITY CONTROL LOCATIONS
EFFECT OF DENSITY CONTROLON WHITE LIQUOR CLARIFIER
UNDERFLOW BASED ON 1000 GPM OR LPM TO DIGESTER
ALL SOLIDS EXPRESSED AS SUSPENDED SOLIDS NOT TOTAL SOLIDS
12171135
400217
135
1000 1000 1000
1400
0
200
400
600
800
1000
1200
1400
1600
25% solids 35% solids 45% solids
gpm
or lp
m
Green Liquor
White Liquor withMudWhite Liquor toDigester
33
OTHER TECHNOLOGIES
• PRESSURE GREEN LIQUOR FILTERS• FILTER PRESS FOR DREGS
DEWATERING
SUMMARY
WHAT IS IMPORTANT?• GOOD GREEN LIQUOR
– Raw Green Liquor Stabilization Tank– Conservatively Sized Clarifier– Density and temperature control
• DO NOT OVERLIME– Causticizing Control– Fresh Lime Make-up.
34
CONCLUSIONFOR TROUBLE FREE OPERATION• ADEQUATELY SIZED EQUIPMENT• GOOD GREEN LIQUOR QUALITY• GOOD LIME QUALITY• EFFECTIVE CAUSTICIZING CONTROL
– Slaker and Density controls• LIME MUD WASHING