intro hakonen

Chemical Pulping, introduction

Montes del Plata Supervisory Training 2011


2

THE CHEMICAL PULPING PROCESS

Target of today’s lecture

Give a general overview of the kraft pulping process

Get familiar with KnowPulp


3

Contents of the presentation

Introduction to the pulping process

Chemical pulping fiberlineWoodhandlingCookingWashing and ScreeningOxygen delignificationBleachingDrying and finishing

Chemical recovery systemBlack liquor evaporationRecovery boilerWhite liquor preparation


4

From Wood -> Pulp -> Paper products

Euca


5

From wood to pulp fibers


6


7

Paper and board consumption is growing mainly in developing countries

In 2009 consumption of paper was 54 kg/person

Consumption of paper

14.9.2011

LÄHDE: RISI Annual Review

Europe25 %

Asia42 %

Oceania1 %

North America21 %

Latin America7 %

Middle East2 %

Africa2 %


8

Wood Fiber


9


10

Wood components from the pulping point of view

Lignin binds the fibers togetherIn chemical pulping the main part of lignin shall be

dissolved in order to release fibersCellulose is the main chemical constituent of fibersIt is desired that cellulose is preserved to the

maximum extent without chemical damagesPart of the hemicelluloses are needed in the fibersMain part of the hemicelluloses is dissolved and

decomposed during the cooking process


11

Pulping processes

The purpose of pulping processes is to produce fibers for various end uses

The main classification of pulping processes:ChemicalSemi-chemicalChemi-mechanicalMechanical


12

Mechanical pulping methods

Rawmaterial

Process name Chemical handling Mechanicalhandling

Yield, %

Log Groundwood (GW) - Grinderstone

95…98

Chips Refiner mechanicalpulp (RMP)

- Plate refiner 94…97

Log Pressurizedgroundwood (PGW)

Steam, pressure Grinderstone

94…97

Chips Thermomechanicalpulp (TMP)

Steam, pressure Plate refiner 93…96


13

Chemi-mechanical pulping methods

Rawmaterial


Yield,%

Chips Chemi-mechanicalpulp (CTMP)

Many alternatives,for instancesodium sulfite(SW) and alkalineperoxide (HW)

Plate refiner 91…95

Chips Bleached chemimechanical pulp(BCTMP)

Bleached withalkaline peroxide and/or hydrosulfite



14

Semi-chemical pulping methods

Rawmaterial


Yield, %

Chips Neutral sulfite semichemical pulp (NSSC)

Na or NH4 sulfite Plate refiner 75…85

Chips Semi-chemical greenliquor

Na2S + Na2CO3 Plate refiner 65…85

Chips Semi-chemical soda NaOH Plate refiner 65…85

Chips Raw sulfate NaOH + Na2S Plate refiner 55…65

Chips Raw sulfite Acidic sulfite (Ca, Na,Mg)



15

Chemical pulping methods

Raw material

Process name Chemical handling Mechanical handling

Yield, %

Chips Neutral sulfite anthraquinone pulp

Na sulfite + carbonate + anthraquinone

None 50…60

Chips Sulfate (Kraft) NaOH + Na2S None 40…55

Chips Soda NaOH + anthraquinone

None 40…55

Chips Acidic sulfite Ca, Na, Mg, NH3 +SO2 None 40…55

Chips Bisulfite Na, Mg, NH3 + SO2 None 40…55


16

Production of papermaking fibers in the world 1970 - 2005


17

The main growth in:

Recycled fibres (RCF is also called ”waste paper”, DIP is Deinked pulp)

Bleached Hardwood Kraft Pulp (BHKP):Eucalyptus pulp from South AmericaMixed Tropical Wood and Acacia from Indonesia


18

Sulfate pulp mill for producing bleached pulp


19

Sulphate pulp mill: Fiberline


20

Woodhandling

Tasks:Receiving the logsDebarking the logsChippingScreening the chips (removing too big and too small

particles)Storing the chips in a controlled way(Handling the removed bark and wood waste from

screening)


21

Cooking

Tasks:

Chemical dissolving of a significant part of the wood components – particularly lignin - in order to release wood fibres

Maintaining the fiber yield and desired fiber properties as well as possible


22

Washing and screening

Tasks:

Remove and recover the dissolved components of wood and used cooking chemicals (in the form of black liquor) to energy production and chemical recovery

Removing dissolved organic material prior to further chemical treatment of pulp in oxygen delignification and bleaching

Removing uncooked wood material and solid impurities (screening)


23

Oxygen delignification

Tasks:

Further removal of lignin from fibers (better selectivity than in cooking)

Removing part of impurities

Removing part of extractives (pitch)


24

Bleaching

Tasks:Final removal of lignin from fibersIncreasing the whiteness (brightness) of fibersRemoving dirt and other impuritiesRemoving extractives (pitch)


25

Drying and Finishing

Tasks:

Water removal by means of dewatering, pressing and hot air blowing (drying machine)

Cutting the dried sheet to desired size (finishing)

Packing the pulp sheets into bales (finishing)

Wrapping the bales (finishing)


26

Sulfate pulp mill: Chemical Recovery

Not with euca


27

Evaporation

Tasks:Increasing the dissolved solids concentration of

black liquor before combustion in the recovery boiler

Separating certain by-products: methanol, talloil soap (in case of softwood pulp)


28

Recovery boiler

Tasks:

Combustion of the organic material of black liquor in order to produce energy (steam, electricity)

Producing an inorganic ”feedstock” (a molten mixture of Na2CO3 and Na2S) to be used for cooking chemical preparation


29

Causticizing

Tasks:

Preparing the cooking liquor (”white liquor”, a liquid with NaOH and Na2S as active components) from recovered cooking chemicals

Removing certain inorganic non-process elements (substances that mainly come with wood but may cause process problems if not removed)


30

Block diagram of the sulfate process

Fiberline

Chemical Recovery


31

Woodhandling – an overview

The main purpose of the woodhandling system of a pulp mill is to produce suitable wood chips to be used in the cooking process


32

Wood chip


33

The importance of the chip dimensions

Chip thickness:

The effectiveness of heat transfer (from steam and cooking liquor) is determined by chip thickness

The effectiveness of cooking liquor impregnation inside the chip particle is determined by the chip thicknessThe thinner the chips the betterIndustrial chipping produces a certain relation between the chip

length and thicknessPractical chip thickness max. limit is 5 – 8 mm


34

The importance of the chip dimensionsChip length:

The effectiveness of removing air from chips during steaming (before actual cooking) is determined by chip length

The effectiveness of cooking liquor penetration into the chip particle is determined by the chip lengthThe shorter the chips the betterHowever, making too short chips starts to cut also the fibers to

shorter which is not allowed due to pulp qualitythe practical chip length minimum limit with hardwood chips is

about 15 mm


35

Fiber length in chips as a function of chip length

ll


36

The main unit processes of woodhandling

TREECROSS

CUTTINGSTORAGECHIPPINGBARKING SCREENINGTREE

CROSSCUTTING

STORAGECHIPPINGBARKING SCREENING

SCREENING

CHIPPING

BARKING

LOGS

CHIPSTORAGE

BARKSTORAGE

SAWMILLCHIPS

SCREENING

CHIPPING

BARKING

LOGS

CHIPSTORAGE

BARKSTORAGE

SAWMILLCHIPS


37

Main parts of woodhandling


38

Wood intake

Logs are transported from the forest by means of various transportation methods (trucks, train, ship …)

The length of the logs is typically 3 – 4 meters

Part of the wood supply can in some cases be as ready chips from local sawmills

Logs are typically stored according to the species in the mill area (”woodyard”); sometimes also in the forest:

It is important to control the storage time accurately (eg. from quality reasons)


39

Wood intake


40

Debarking

Purpose of debarking:

The bark covering the log is not desired in the pulping process:

The fiber material in bark is not suitable compared to actual woodThe bark may contain such chemical substances that disturb the

pulping process (pitch, calcium compounds, silica …)=> Risk of additional chemical consumption in the process

The bark often contains small stones, sand and other mechanical impurities

=> Risk of mechanical wearing or breaking of process equipment


41

The debarking process

The removing of bark (debarking) can take place already in the forest

Manual debarking sometimes

Transportable debarking machines

The debarking at pulp mill woodyard takes place by means of debarking drums


42

Debarking


43

The operation principle of debarking drums

A debarking drum is an inclined rotating cylinder into which the logs are taken in the direction of the drum axis.

The rotating actions lifts the logs in the drum, the logs rub and wear against each other and the bark is thus removed

The bark is removed from the drum through holes and slots in the drum walls


44

Debarking drum


45

Debarking drum


46

Chipping


47

Industrial chipping by means of disc-type chippers


48

The operation principle of the chipper


49

The chipper …


50

The cutting speed is the main operating parameter determining the chipping results


51

The chipper produces also such particle material that is not of good quality

Target


52

Chip screening

Too big chip particles are removed(oversize and overthick chips)

Too small chip particles are removed(pin chips and fines)


53


54

Gyratory chip screen


55

Chip screening, transportation and storage


56

Andrés SuttnerAndrés GarcíaAndrés RamosClaudio Acosta

Conrado PonsEdgardo PiñeiroGabriel MachadoGonzalo Barcia

Gonzalo GascueGonzalo GodiñoGonzalo MarinelloIgnacio Caétano

José FreireJuan Daniel ConstantinLucía BouzaMarcel Arguiñarena

Marianella TorresMartín PereiroNéstor BenítezPablo Pirez

Romina RiccaSebastian SilveyraSebastián Laclau

Measuring, storing and pretreatment of round-wood. Measuring incoming raw material. Wood weighing.

Wood storage arrangements at the mill site

Debarking Chip storage and transportation

Chipping

Chip screening


57

Sulphate cooking process


58

The targets of cooking: cooking yield

Certain amount of wood material (mainly lignin) has to be dissolved in order to release the fibers

Depending of wood species 45 – 55 % of the wood material needs to be dissolved

Example: Weight of dry wood 100 kg Dissolved material 47 kg Fibers 53 kg Cooking yield 53 kg/100 kg * 100 = 53 %

With eucalyptus the cooking yield is 53 – 55 %


59


60


61

The targets of cooking: kappa number

80 – 90 % of the lignin in wood is dissolved during cooking

The extent of the dissolution of lignin is measured by kappa number:

• Kappa number gives the content of residual lignin in pulp (indirectly)

• Kappa number is determined by the consumption of KMnO4 (titration)

• The cooking kappa number of eucalyptus pulp is 15 - 18


62

The cooking chemicals dissolve lignin


63

The cooking conditions to dissolve wood lignin are very hard:

The cooking liquor (”white liquor”):very alkaline, pH ca. 14Active chemicals NaOH and Na2S

The cooking reaction temperature needs to be > 145 oCConventional cooking methods typically use a higher cooking temperature

(160 -170 oC)Modern cooking methods for easily processed hardwoods (like euca) can

use lower temperatures like 145 -155 oC


64

Typical composition of white liquor


65

White liquor terminology


66

White liquor terminology


67


68


69

Black liquor

Black liquor is formed during the cooking process from the dissolved wood material and reacted cooking chemicals

Composition of black liquor after cooking: Dissolved material 15 – 20 w-%

• ¾ organic, ¼ inorganic

Water 80 – 85 %


70


71

The main cooking variables

The degree of cooking is controlled by:The charge of Effective alkali per woodThe combination of cooking time and temperature (H-factor)


72


73

The industrial cooking process

There are two main types of cooking processes: Batch cooking Continuous cooking

Both the batch and continuous cooking processes have been further developed from the old (conventional) processes to modified modern processes

The modern processes have optimized alkali profiles and lower cooking temperatures


74

The main steps of continuous cooking

Chip steaming

Feeding the chips into the impregnation vessel (or directly to the digester if there is no separate impregnation vessel)

Impregnation of the chips with the cooking liquor

Feeding the impregnated chips into the digester

Heating the chips to the cooking temperature

Cooking phase

Displacement washing (washing & cooling of the chips)

Discharging the digester


75


76


77

Presteaming of chips

Purpose of presteamingHeating the chips to about 100 oC temperatureRemoving air from the inside of the chips (because air hinders the

penetration of the cooking liquor into the chips)

How presteaming is performed:Steam of 105 – 110 oC flows through the chip bed (normally this the

flash steam from black liquor flashing; quite seldom fresh steam is used)

Steaming takes place in the chip silo or a presteaming vesselResidence time in the presteaming is 15 - 20 min


78

The feeding of the chips into the impregnation vessel by means of pumping


79

Impregnation of chips

Purpose of impregnationPenetration of the cooking liquor into the chips by means of increased

pressureAllowing diffusion to drive the cooking chemicals into the wood

structure

Performing the impregnation in the cooking processChips and the cooking liquor are taken to the pressurized

impregnation vesselTemperature 100 – 110 oC, time 30 - 60 min


80

The steps of impregnation

Penetration through chip pores

Diffusion into the fibers


81


82

The digester process

The chip column moves downwards in the digester

There are various cooking zones in the digester depending on the process modification

Cooking liquors are withdrawn from the digester by means of screens between the cooking zones

Heating of the liquors outside the digester by means of steam Returning the liquors back to the digester by means of the

center-pipe system


83

Typical liquor circulation arrangementin a continuous digester


84

The displacement washing stage in the digester

The purpose of the digester washing stageCooling the cooked chips in order to stop the cooking reactions

and allowing gentle discharge of the digesterDisplacing the concentrated hot black liquor out of the digester

to be sent to evaporation

How the displacement washing stage is performed:The filtrate of the brown stock washing is used as the

wash/displacement liquorHeating the wash filtrateDisplacement of the filtrate through the cooked pulp bed


85

What is Displacement?

Digester contents having originallyhigh temperature described by the red colour

Cold liquid describedby the blue color istaken to the bottom of thechip bed, flow rate Vin

Displacementfrontier whichpushes the hot liquidahead like a piston

Hot (red) liquid is displaced fromthe top of the chip bed, flow rate Vout

Vin = Vout

Final situation:the colder liquid hasreplaced the hot liquid


86

What is displacement?


87

Flash steam to presteaming


88


89

Fiberline operations

Pulp washing

Knot separation and screening


Bleaching

Drying and baling


90

Pulp washing

Purpose pulp washing:

Removing the black liquor from the cooked pulp by using as little fresh water as possible

The removed black liquor needs to be as concentrated as possible in order to save steam in evaporation

The washed pulp has to be as clean (of the dissolved organic material of cooking) as required in order to save chemicals in oxygen delignification and bleaching


91

Pulp washing is an optimization task where different cost factors are compared. However, the limiting factor is certain demand for the washing loss

Operationalcosts, €/ton pulp

Wash water, m3/ton pulp

Chemical costs(bleaching, make-up chemicals)

Evaporation costs

Optimum operational costs

Demand for the washing loss(Bleaching, environmental load, Pulp quality factors)


92


93


94


95


96

Washing equipment and their operational principles

Digester washing:• Continuous cooking Hi-Heat washing• Displacement batch cooking (SuperBatch) terminal displacement

Diffuser washing:• Single or two phase atmospheric diffuser• Pressure diffuser

Drum filters:• Vacuum filters• Pressure filters

Multistage displacement washers:• DD-washers• Flat wire washers

Wash presses:• Dilution-thickener presses (screw or roll presses)• Displacement presses (Twinroll-, Compact Presses)


97

The operation principle of a 3-stage DD-washer


98

Screening of pulp

The purpose of screening

The main purpose of pulp screening is to separate harmful impurities from pulp with minimal fiber loss and acceptable cost level.

Screening system can also include reject treatment to process impurities into usable form.


99

The harmful impurities?

KnotsUncooked chips or ”real” knots

Shives, dirt specks• Shives = small partially cooked

wood pieces or fiber bundles• Dirt particles originate from bark or

can be sand or other non-wood material


100


101


102


103

The principle of screening


104


105


106


107


108


109


Oxygen delignification is a process, where the residual lignin left in pulp after cooking is removed using oxygen and alkali (NaOH). Investment in oxygen delignification stage can be considered for both environmental and economic reasons.

Oxygen delignification is more selective (saves better the pulp yield and preserves the pulp quality) than continuing the cooking process to the same lignin content of pulp

Typically 40 – 50 % of the residual lignin (measured by kappa number) after cooking is removed in oxygen delignification


110


111

Connection of the oxygen delignification with the chemical recovery

Oxygen delignification dissolves a significant part of the organic material in the pulp (typically 1,5 – 2,5 %)

The reaction chemicals contain valuable and recoverable chemicals – Na and S

Washing after the reaction stage is necessary in order to recover the dissolved organic material and the Na and S

Washing is also needed in order to save bleaching chemicals and reduce the environmental load in bleaching

Washing is connected with the brown stock washing system in order to arrange the recovery of the washed material


112

Connection of oxygen delignification with the washing process


113

Connecting pulp washing and O2-delignification with the fibreline

CookingChips

Black liquor to evaporation

White liquor

Screening

Rejects

Pulpwashing O2-

delig.

Pulpwashing

Wash water

Bleaching

DryingBaling

Pumping to the paper mill (integrated production)

Baled market pulp


114

The oxygen delignification process parameters

Chemicals:Oxygen gas

• Gives the ability to oxidize the residual lignin

NaOH• Gives the required alkalinity (pH > 11)

Reaction conditions:Temperature 80 – 110 oCResidence time 20 – 120 minPressure 4 – 10 barsPulp consistency 10 – 12 %


115


116

The chemical dosages of oxygen and NaOH dependon the kappa number of the entering pulp and on the required kappa number reduction

NaOH:Typically 2,5*(kappain – kappaout) as kg/ton pulp

Example: 2,5* (16 – 10) = 15 kg NaOH/ton pulp

O2:

Typically 0,8 … 1 * NaOH dosageExample: 0,8 * 15 = 12 kg O2/ton pulp


117

Using oxidized white liquor instead of fresh NaOH

The chemical balance of the pulp mill makes it necessary to recirculate Na and S as much as possible

For this reason, oxidized white liquor is used instead of fresh NaOH

Oxidized white liquor can be obtained from the normal white liquor (the cooking liquor) by oxidizing either with air or oxygen gas


118


119

The white liquor oxidizing process


120

Performing the oxygen delignification process

The oxygen delignification process can be either a single or double stage (two-stage)

The single stage process is the traditional process configuration

The modern processes are double stage processes• Possibilty to extend the delignification (have a larger kappa

reduction)• Possibilty to save chemicals and optimize the process in order

to preserve the yield and pulp properties


121


122

Pulp bleaching: the purpose of bleaching

The objective of bleaching is to improve the brightness and cleanliness of pulp.

This occurs either by removing or brightening the colored substances in the pulp.

Residual lignin is a major contributing factor in color in pulp, so it

must be removed or brightened.

Chemical pulps are typically bleached using lignin-removing bleaching and mechanical pulps by lignin-preserving bleaching.

Brightness with lignin removal bleaching lasts a lot better (i.e. there is a lot less yellowing).


123

The objectives of bleaching

Increased brightness Improving brightness stability Improved cleanliness

Reducing pitch (extractive material) content


124


125

The concept of pulp brightness

Brightness is defined as the amount of light of a certain wavelength that is reflected from a sheet of paper when the measurements aremade according to a standard method.

Brightness is defined by international agreements to be the reflectance measured at 457nm


126

Brightness of pulp in the process


127

Fully bleached pulp

The brightness target of the fully bleached pulp is > 88 % ISO brightness

The brightness measurement is standardized;various standard measurements for example according to the ISO- or GE- standards

Comparison of the light reflectance of the pulp surface with a standard MgO-surface


128


129


130

Bleaching sequences: combination of various bleaching chemicals and bleaching stages

Reaching high pulp brightness requires the using of various bleaching chemicals and several stages

Decomposing the residual lignin by means of a selective delignifying agent (chlorine dioxide in the D0-stage)

Dissolving the residual lignin in alkaline conditions (alkaline extraction, Eop-stage)

Final brightness increase by means of color-removing bleaching agents (chlorine dioxide and peroxide in the D- and P-stages)

Removing impurities (particularly by means of chlorine dioxide)


131


132

Bleaching sequences

Washing of pulp after each bleaching stage:Reaction products have to be removedOften the pH-level has to be shifted before the next stageFiltrates of bleaching need to be recirculated in order to

save fresh waterPart of the filtrates have to removed from the system:

the main environmental load from the pulp mill is in bleaching effluents (go to waste water treatment)


133


134


135

A – The Acid hydrolysis stage

Purpose:Removing hexenuronic acid (HexA) from the pulpHexA causes additional chlorine dioxide consumptionHexA causes post-coloration (yellowing) of the bleached

pulp

Chemicals used:Sulfuric acid (H2SO4)


136


137

D0 – Initial chlorine dioxide stage

Purpose:Removing residual lignin with better selectivity than

oxygen delignificationBrightness increase

Chemicals used:

• Chlorine dioxide, ClO2 (active delignifying chemical)

• Sulfuric acid, H2SO4 (pH-control)


138


139

Eop – Alkaline extraction stage

Purpose:Dissolving of the decomposed lignin fractions in the pulp after the

delignifying reactions of the D0-stage

Enhancing the oxidation reactions by means of a small dosage of oxygen and peroxide

Removing pitch (extractive material)Most of the environmental load of bleaching comes in the filtrates of

the Eop-stage

Chemicals:Sodium hydroxide, NaOH (giving the alkaline conditions)Oxygen, O2

Hydrogen peroxide, H2O2(oxidative chemicals)


140


141

D1 – chlorine dioxide stage for brightness increase

Purpose:The main brightening stageRemoving dirt particles (eg. bark specks and other organic

impurities)

Chemicals:Chlorine dioxide, ClO2 (brightening chemical)

NaOH (pH-control)


142


143

P –atmospheric peroxide stage

Purpose:Final brightness adjustmentIncreasing the brightness stability by means of oxidizing

some post-colorization causing components


144

Typical conditions of an atmospheric P-stage

Temperature 80 -90 oC Time 2 – 4 h Consistency 10 -12 % Final pH 10,5 - 11 H2O2 dosage 5 – 10 kg/ton pulp NaOH 5 – 10 kg/ton pulp (for pH-control) Additive chemicals:

Chelating agents (eg. EDTA) for removing Mn and Fe that cause peroxide decomposition and loss of pulp quality

MgSO4 to increase peroxide stability


145

Final acidification of pulp

Purpose:• Destroying residual chemicals (eg. peroxide)• Removing harmful metal ions (eg. Fe)

Chemicals:• The acidification pH is 4 – 5

• Alternative chemicals SO2-water, sodium bisulfite (NaHSO3) or sulfuric acid (H2SO4)


146


147

Environmental impact from bleaching

A significant part of the environmental load to the water recipient of a pulp mill comes in the bleaching effluents

The key environmental parameters of the bleaching effluent:

Chemical oxygen demand (COD)• Describes the amount of dissolved organic material

Suspended solids content (SSL)• Describes the amount of fibres and other solid material

Adsorbable organic chlorine (AOX)• Describes the amount of chlorinated organic material


148

Definitions of COD and AOX

COD (Chemical Oxygen Demand): Oxygen consumption caused by substances in wastewater when the substances oxidize as a result of effective chemical oxidants.

AOX (Adsorbable organic halogens): A method of measurement, which gives the total of organically bound chlorine of all sizes of molecules.


149


150

Bleach plant equipment


151

Unit operations of bleaching


152

Pulp Consistency

Consistency means how much fiber material there is in a suspension or pulp mat on the weight basis

The units: %: weight per cent of fibers kg/m3: c/kg/m3 = 10* c/%

The pulp suspension consistencies: Low consistency (LC): 0 – 6 % Medium consistency (MC): 8 – 15 % High consistency (HC): 25 – 35 %


153

Pumping of pulp and paper stock

Fiber rheology 1 – 2 %: like a fluid 6 – 10 %: like a viscous solid The increasing consistency

means increasing strength of the fiber network


154

MC-technology in bleaching

In principle the entire bleaching process takes place in the medium consistency range at the consistency level of 10 – 14 %

There are many MC-operations and MC-equipment in the bleaching process


155


156


157

Drying and finishing

The main unit processes:Bleached stock screening (post-screening)Drying machineBaling


158


159


160


161

Centricleaning system


162


163

Removing light

impurities, eg plastic particles


164

Drying machine

Purpose:Forming a fiber webRemoving water

Water removing mechanisms:Dewatering (filtration)PressingEvaporation (by means of hot air)


165


166


167


168

The operation principle of the headbox


169


170

The principle of wet pressing


171


172


173


174

Cutting the dried pulp into sheets


175

Baling line

Operations:Bale pressingApplying and folding the wrapping paperTyingStacking the bales


176

Bale press


177

Baling line operations


178

Chemical Recovery

The chemical recovery system of a pulp mill receives the black liquor from the fiberline and produces energy (steam and electricity) as well as cooking liquor (white liquor) to be used by the fiberline

The unit processes of the chemical recovery Black liquor evaporation Recovery boiler Causticizing Lime burning


179

Sulphate pulp mill: Chemical Recovery

Not with euca


180


181

Evaporation

Purpose:Concentrate the black liquor by evaporating waterUsing the minimum amount of fresh steamHandling the odorous gases released during the evaporation

The task of evaporationThe weak black liquor from the pulping process is in the DS-

concentration of 15 – 17 %The liquor needs to be concentrated to 75 – 85 % DS content

for firing in the recovery boilerSeparation of some by-products (in Fray Bentos the only by-

product is methanol to be used as a fuel in the lime kiln)


182


183


184

The principle of multistage evaporation


185


186

Multi-stage evaporation plant


187

Scaling of the heating surfaces is one of the operational problems in the evaporation plant: periodical washing of the most critical evaporation units is needed


188


189


190


191

Recovery boiler

The recovery boiler is a chemical reactor for the recovery of the cooking chemicals used and for the generation of heat from dissolved inorganic and organic material in the black liquor.

The recovery of cooking chemicals in a recovery boiler includes several steps:

combustion of the organic material in black liquor to generate heat

production of sodium fume to capture sulfur-containing combustion residues

reduction of the inorganic sulfur compounds to Na2S production of a smelt of molten Na2CO3 and Na2S recovery of inorganic dust from the flue gas recovery of the heat formed during combustion


192


193

Recovery boiler is the largest size unit process of a pulp mill


194

Schematic diagram of a recovery boiler


195


196


197


198

Spraying black liquor into the furnace


199


200


201


202

The reduction degree of Sulfur

Reduction reactions

Na2S+2O2 -> Na2SO4

Na2SO4+2C -> Na2S+2CO2

Na2SO4+4C -> Na2S+2CO

Degree of reduction (%) =

The reduction degree of a recovery boiler is > 90 %

100422

2 SONaSNa

SNa


203


204


205


206


207

Boiler water and steam system


208

Generation of steam and electricity

About 16 GJ/ADt thermal energy is generated by the burning of black liquor.

From this 3.2 GJ/ADt is used for the generation of electricity. The pulping process consumes 13 GJ/ADt.

A mill’s own electricity generation is 800 - 1300 kWh/ADt.

In a modern mill, electricity generation greatly exceeds pulp mill own consumption, so excess electricity can be sold.


209


210

White liquor preparation

The two main processes:Causticizing (called also Recausticizing)Lime kiln


211


212


213


214


215

Causticizing plant


216

From the smelt to green liquor

The smelt from the recovery boiler is a mixture of mainly Na2CO3 (sodium carbonate) and Na2S (sodium sulfide)

The smelt is dissolved into weak white liquor in order to form green liquor

Green liquor is processed into whiter liquor in the causticizing plant


217


218

The main unit processes of the recausticizing plant

Green liquor processingGreen liquor filtrationFiltration of dregsGreen liquor cooling

CausticizingSlakingCausticizersWhite liquor filtration and lime mud washing


219

Green liquor processing

Green liquor contains solid impurities (called dregs) which have to be separated

The dregs separation takes place either by means of green liquor clarification or filtration

The separated dregs contain a considerable amount of sodium. Thus, the dregs have to be washed

The washed dregs are dumped. Dregs contain typically coal, sand and certain non-process substances


220


221


222


223


224

Slaking: mixing the green liquor with CaO in order to form Ca(OH)2


225


226

Reaction Na2CO3 + Ca(OH)2 -> 2 NaOH + CaCO3 (s)

Lime mud


227

Causticizing degree (efficiency)


228


229

Lime mud

to the lime kiln

White liquor to

cooking


230


231

Water balance of causticizing


232

Lime burning in the lime kiln

The reaction:

CaCO3 + heat -> CaO + CO2

The reaction takes place in a rotating lime kiln

The lime burning requires auxiliary fuel (typically heavy oil, natural gas).


233


234

Lime kiln is one of biggest single machines of a pulp mill – length 100 – 150 m


235


236


237


238


239


240


241


242

Temperature profile in the lime kiln


243

Lime kiln brick lining zones


244

Part of the lime has to be periodically dumped and replaced by make-up lime or limestone addition


245

intro hakonen

Documents

pulping processchemical

chemical treatment of

chemical recoverynot

chemical dissolving

chemical damagespart

dissolved components

pulping point of viewlignin

wood waste