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Chemical Pulping, introduction
Montes del Plata Supervisory Training 2011
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THE CHEMICAL PULPING PROCESS
Target of today’s lecture
Give a general overview of the kraft pulping process
Get familiar with KnowPulp
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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
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From Wood -> Pulp -> Paper products
Euca
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From wood to pulp fibers
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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 %
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Wood Fiber
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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
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Pulping processes
The purpose of pulping processes is to produce fibers for various end uses
The main classification of pulping processes:ChemicalSemi-chemicalChemi-mechanicalMechanical
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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
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Chemi-mechanical pulping methods
Rawmaterial
Process name Chemical handling Mechanicalhandling
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
Plate refiner 90…94
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Semi-chemical pulping methods
Rawmaterial
Process name Chemical handling Mechanicalhandling
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)
Plate refiner 55…70
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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
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Production of papermaking fibers in the world 1970 - 2005
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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
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Sulfate pulp mill for producing bleached pulp
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Sulphate pulp mill: Fiberline
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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)
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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
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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)
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Oxygen delignification
Tasks:
Further removal of lignin from fibers (better selectivity than in cooking)
Removing part of impurities
Removing part of extractives (pitch)
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Bleaching
Tasks:Final removal of lignin from fibersIncreasing the whiteness (brightness) of fibersRemoving dirt and other impuritiesRemoving extractives (pitch)
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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)
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Sulfate pulp mill: Chemical Recovery
Not with euca
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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)
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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
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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)
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Block diagram of the sulfate process
Fiberline
Chemical Recovery
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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
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Wood chip
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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
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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
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Fiber length in chips as a function of chip length
ll
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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
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Main parts of woodhandling
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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)
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Wood intake
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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
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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
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Debarking
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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
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Debarking drum
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Debarking drum
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Chipping
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Industrial chipping by means of disc-type chippers
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The operation principle of the chipper
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The chipper …
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The cutting speed is the main operating parameter determining the chipping results
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The chipper produces also such particle material that is not of good quality
Target
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Chip screening
Too big chip particles are removed(oversize and overthick chips)
Too small chip particles are removed(pin chips and fines)
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Gyratory chip screen
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Chip screening, transportation and storage
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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
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Sulphate cooking process
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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 %
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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
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The cooking chemicals dissolve lignin
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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
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Typical composition of white liquor
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White liquor terminology
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White liquor terminology
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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 %
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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)
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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
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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
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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
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The feeding of the chips into the impregnation vessel by means of pumping
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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
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The steps of impregnation
Penetration through chip pores
Diffusion into the fibers
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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
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Typical liquor circulation arrangementin a continuous digester
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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
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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
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What is displacement?
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Flash steam to presteaming
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Fiberline operations
Pulp washing
Knot separation and screening
Oxygen delignification
Bleaching
Drying and baling
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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
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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)
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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)
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The operation principle of a 3-stage DD-washer
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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.
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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
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The principle of screening
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Oxygen delignification
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
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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
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Connection of oxygen delignification with the washing process
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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
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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 %
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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
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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
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The white liquor oxidizing process
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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
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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).
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The objectives of bleaching
Increased brightness Improving brightness stability Improved cleanliness
Reducing pitch (extractive material) content
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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
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Brightness of pulp in the process
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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
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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)
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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)
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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)
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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)
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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)
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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)
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P –atmospheric peroxide stage
Purpose:Final brightness adjustmentIncreasing the brightness stability by means of oxidizing
some post-colorization causing components
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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
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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)
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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
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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.
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Bleach plant equipment
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Unit operations of bleaching
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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 %
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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
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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
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Drying and finishing
The main unit processes:Bleached stock screening (post-screening)Drying machineBaling
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Centricleaning system
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Removing light
impurities, eg plastic particles
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Drying machine
Purpose:Forming a fiber webRemoving water
Water removing mechanisms:Dewatering (filtration)PressingEvaporation (by means of hot air)
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The operation principle of the headbox
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The principle of wet pressing
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Cutting the dried pulp into sheets
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Baling line
Operations:Bale pressingApplying and folding the wrapping paperTyingStacking the bales
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Bale press
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Baling line operations
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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
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Sulphate pulp mill: Chemical Recovery
Not with euca
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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)
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The principle of multistage evaporation
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Multi-stage evaporation plant
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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
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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
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Recovery boiler is the largest size unit process of a pulp mill
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Schematic diagram of a recovery boiler
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Spraying black liquor into the furnace
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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
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Boiler water and steam system
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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.
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White liquor preparation
The two main processes:Causticizing (called also Recausticizing)Lime kiln
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Causticizing plant
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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
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The main unit processes of the recausticizing plant
Green liquor processingGreen liquor filtrationFiltration of dregsGreen liquor cooling
CausticizingSlakingCausticizersWhite liquor filtration and lime mud washing
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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
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Slaking: mixing the green liquor with CaO in order to form Ca(OH)2
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Reaction Na2CO3 + Ca(OH)2 -> 2 NaOH + CaCO3 (s)
Lime mud
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Causticizing degree (efficiency)
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Lime mud
to the lime kiln
White liquor to
cooking
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Water balance of causticizing
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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).
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Lime kiln is one of biggest single machines of a pulp mill – length 100 – 150 m
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Temperature profile in the lime kiln
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Lime kiln brick lining zones
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Part of the lime has to be periodically dumped and replaced by make-up lime or limestone addition
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