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

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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

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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

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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

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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

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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

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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

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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)

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DREGS PRECOAT FILTER

Photo courtesy of Dorr-Oliver Eimco

DREGS FILTER OPERATIONAL PROBLEMS

• Wet cake discharge• High soda loss• Difficulty forming a precoat

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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

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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

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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

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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

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INLET

INLET

OUTLET

OUTLET

RISERPIPE

CAUSTICIZERS

WHITE LIQUOR CLARIFICATION

OPTIONS• SEDIMENTATION CLARIFIERS• VERTICAL TUBE TYPE

PRESSURE FILTERS• PRESSURE DISC FILTERS

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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

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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

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• 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

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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

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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CONCLUSIONFOR TROUBLE FREE OPERATION• ADEQUATELY SIZED EQUIPMENT• GOOD GREEN LIQUOR QUALITY• GOOD LIME QUALITY• EFFECTIVE CAUSTICIZING CONTROL

– Slaker and Density controls• LIME MUD WASHING