Mech 450 – Pulping and Topic 3 – Mechanical Pulping

James A. Olson

Pulp and Centre, Department of Mechanical Engineering, University of British Columbia

Mechanical Pulping

. Comparison of Mechanical and Chemical Pulps

. Debarking

. Stone Groundwood

. Refiner Mechanical

. Thermo mechanical pulping (TMP)

. Chemi thermo mechanical pulping (CTMP) . Brightening

1 Mechanical Pulping

. Fibres mechanically removed from matrix

Chemical Pulping

. Lignin holding fibres together is dissolved

Lignin

Fibres

. In addition to fibre removal, fibres are broken and fines (fibres < 0.5mm) are created

. About 1/3 of pulp mass is in form of fines

2 . In contrast, chemical pulping produces intact fibres

General Parameters

Chemical Mechanical

Yield Fibre/Wood - Low 40-70% - High 90-98%

Cellulose Purity - High - lignin - Low - lignin dissolved remains

End Uses - Dissolving pulp - Low quality - High quality paper - High volume paper (e.g. book) (e.g. ) - Reinforcement pkg. - Molded products

Raw Material Sensitivity - Low - High

3 Quality Parameters

Chemical Mechanical

Strength - High - fibres intact - Low - fibres damaged

Bulk - Low - more and - High - few and less flexible fibres flexible fibres

Optical - Dark but bleachable - Bright but hard - Poor light scattering to bleach high - Good light scattering

Drainability - Good - long fibres, - Poor - short fibres, few fines many fines

Permanence - Good - Poor (optical)

Cost Parameters Chemical Mechanical

Raw Material - High - low yield - Low - high yield

Capital - High - Low

Operating - High - Low - becoming (chemicals, energy etc) lower - high for electrical energy

Auxiliary - High - Low - for slush pulp (pollution recovery etc) Mechanical pulps are generally used for short-life, inexpensive products, e.g. newsprint

4 History

. Pre-mid 1800’s paper made of rags. . 1841, Friedrick Keller “inventor” . 1848 Johan Voith in Heidenheim made first commercial grinder. . 1859 Voith developed “Raffineur” to break up any course material not properly ground. First success. . 1867 Full plant powered by steam. Paper made with 70% wood (Worlds fair Paris) . 1868 Tampella (finish company) started making grinders.

5 Debarking Drum

Ring Debarking

6 Debarking resistance

. Factors: 12 . Species

. Moisture content

. Felling season

. Storage duration 4 . Temperature 0 Debarking resistance N/cm^2 Jan May Sept Dec

Stone Groundwood (SGW)

. Pulp produced by pressing logs against rotating grindstone

. Unchanged for 150 years.

7 Action of grinder

. Circumferential speed 30 m/s . Grinding pressure 250kPa . Grits deform fibre-lignin matrix . Repeated visco-elastic deformation creates heat . Increased heat in wood . Heat softens lignin that’s found in between fibres and helps to release the fibres

Action of grinder

. Fibres are peeled back in layers

. Grits pass over partially removed fibres

. Develops surface and flexibility of fibres … paper strength.

. Fibres are released

. Next layer peeled off

8 Operating Parameters

. Species and property of wood

. Amount of spray water

. Temperature of spray water

. Rate of wood feed

. Pressure applied

. Speed of grinder

. Structure of stone

Pulp Constituents

. Shives: fibre bundles (3%)

. Long, intact fibres (20%)

. Short, broken fibres (35%)

. Fines (45%)

. Flour 30x30

. Fibrils 30x1

. Dust 1x1

9 Pulp Properties

. Higher strength as more energy applied

. CSF drops 150-50 ml as energy applied

. Brightest of unbleached pulps up to 65 ISO

Stone Sharpening

. Stones wear due to constant high- speed abrasion

. Ceramic stones

. Sharpening every 6-14 days

. Sharpness affects energy and production

grits

2.5mm

10 Continuous Grinding

Pressure Ground Wood (PGW)

. Higher pressure leads to higher temperatures

. Softer lignin, easier to detach whole fibres

. Stronger pulp

11 Example

. The quality of the pulp produced during grinding is dependent on the temperature in the grinding zone. Fibre can be liberated largely intact if the lignin has been softened by temperature, however, if the temperature is too low the fibres will be largely broken or if the temperature is too high the wood will start to darken. . Since virtually all of the grinding power is dissipated as heat in the grinding zone, it follows that temperature in that zone is controlled by the addition of shower water. . For a given grinding operation, wood, F, and dilution, D, (kg/s) enter the

grinder at Tin degrees C. The suspension leaving the grinder at Tout and at a consistency, C. Assume that the steam is not formed. Determine the electrical energy applied, E (J/kg), to maintain these outlet conditions.

E

F

C

D

12 13 Mech 450 – Pulping and Papermaking Topic 3b – Refiner Mechanical Pulping

James A. Olson

Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia

Refiner Mechanical Pulp (RMP)

. Wood chips are comminuted into fibres by bars on rotating and stationary discs

14 History

. 1957 Stora (Sweden) installed a Defibrator “raffinator”. Bauer shortly after . 1963 Both companies modified to operate under pressure to make Thermo-mechanical pulp . 1970’s First 100% TMP newsprint . 1980’s 2-stage refining and heat recovery . 1985 Large refiners 15MW. . Chemicals added to further soften lignin (CTMP). Mechanical pulps are replacing chemical pulps

15 Chip Handling

. Wood is typically chipped in a disc chipper . Goal is to have a high proportion of acceptable chips . 3-16 knives on a disc . 4 m diameter . 450 m^3 / hr of solid wood . Low cutting speed (20 m/s) as pin chips increase with speed

Effect of chip size

. Over size chips . Uneven feed in refiner . Reduces quality . Over thick fraction . Contains most of the knots . Decreases fibre length and long fibre portion . Decreases strength and brightness . Fines Fraction . Lowers energy consumption . Decreases strength, sheet density, brightness and light scattering . Creates linting problems and increases shive content

16 Chip washing

. Immersed in a tank fed by a paddle wheel (Sunds). . Removes: Rocks, metal, sawdust, bark . Adds moisture . Raises temperature

Chip Screening

. Chips are passed through a series of screens . Oversize: left on screen with 45 mm holes . Overthick: left on screen with 7 mm slots . Accept: left on screen with 7 mm holes . Pin chips: left on screen with 3 mm holes . Fines: pass through last screen . Overthick chips don’t react well to pre-treatments, lower yield . Fines and pin chips produce too many shives (not refined)

17 Chip Steaming/Preheating

. Atmospheric type . Steam to 80 - 95 C . Most are pressurized (50kPa to 110kPa over pressure) . Objective is to warm chip and equalize the moisture content . Can optimize a bit: . Higher temperature gives longer fibres, higher tensile . Lower temperatures give better optical properties . Chip impregnation systems . Used in CTMP Processes . Compresses chips • Water is removed and is high in extractives… fed to effluent • 4:1 compression ratio or higher . Passes chips into a pool liquor containing chemicals . Increase moisture content by 6-7%

Refining Equipment

Disc Refiner

18 Self Pressurization

. Refining imposes cyclic compression of visco-elastic material

. Generates tremendous amount of heat and steam

. Dilution required to maintain approx 30% consistency

. Steam pressure reaches max and flows both ways

. Can cause blow-back

Types of Refiners

. Single disc, . Moving rotor staionary stator . 1.7m Dia. 15 MW . Double Disc . Two counter-rotating discs . More power delivered . Less energy required per ton • Higher shives, less long fibres, (similar to SGW) . Twin refiner . One rotor, two stators… more refining surface • Low intensity refining possible

19 Refiner size over time

Conical Disc Refiners

. Flat disc section and conical section

. Increases grinding surface without increasing diameter

. Power: CD70, 76, 82 uses 15, 24 32 MW

20 Refining Action

. Chips are preheated to soften lignin . Chips hit breaker bars and undergo a series of normal and shear forces . Rapid Breakdown in screw feeder, entrance zone and breaker bars section. . Fractures along grains, mostly along fracture planes initiated in chipping . Match stick size fragments accumulate in refining zone with major axis along tangential direction . Match sticks defibred by longitudinal grinding and brooming . Fibres form flocs and flow out by steam drag and inertial forces . Flocs caught on bar edges and repeatedly compresssed by passing bars. Breakerbars

Refining action

. Fibre development step

. Fibres undergo cyclic compressions between bars

. Internally and externally delaminates the fibres

. Increases flexibility and surface area

21 Refiner Segment Design Parameters . Width of Grooves and Bars . Traditionally the main parameter . Wide grooves - narrow bars • reduce specific energy consumption in refiner • Open volume allows gap to be narrower and can result in lower pulp quality . Wide bars / narrower grooves • Increase specific energy consumption and improve quality • When Volume in groove is reduced steam flow is impeded and axial load is higher and infeed of fibres is more difficult. This can lead to unstable feed . Height of the bars . Higher the more open the groove volume, the better steam removal . Low bar height forces fibres to the plate gap an pulp quality improves. . Dam number, height, and placement . Forces pulp from the grooves to the plate gap . Residence time increases. . Hinders steam removal . Bar taper and angle . When bars form a pumping angle fibre are forced through, lower residence time which reduces energy consumption

Thermo-mechanical Pulp (TMP)

. Pulping carried out in two refiners in tandem

. First refiner - pressurized with steam (along with pre-steamer)

. Second refiner is atmospheric

. Produces longer fibre (stronger paper) and fewer shives (small bundles of fibres)

22 Theory PP E  No Load QC . Specific Energy E . Intensity: e  . Number of impacts 

. Intensity of each impact: “High Intensity”

Specific energy per EA impact I “Low Intensity”

EB

N

How do we calculate residence time?

. Force balance on element of pulp

FCF rr12 F bS

2 dv r 4()()rmrP r c r b2 c() r  UrCAr()fp () dr v m 2 v

r2 dr    v r1

23 Operating parameters

. Refiner speed (increase) . Increase intensity at same power . Lower energy at same freeness, lower length, and tear . Inlet Consistency (increase) . Increase moisture content and fibre length . Production rate (increase) . Reduce energy and lower length and strength . Preheating and steaming temperature . Not too critical . Plate Gap . Increases intensity . Lead to pad collapse

Effect of refining on coarseness

. Coarseness:

. Decreasing coarseness support delamination theory

. Lower coarseness of small fraction indicate they are created from fragments of cell wall

. Not always evident if we measure coarseness of whole pulp

. Difficult to measure coarseness of pulp with fines

24 Effect of refining on long fibres

Effect of refining on fibre width

. Refining reduces fibre width by removing outer wall material.

25 Effect of refining on wall thickness

. High intensity refining reduces wall thickness more at same energy

. Outer part of fibre wall is being peeled away

Effect of refining on fibre collapse

. X-section measured by CLSM . Collapse index is an indication of fibres ability to form ribbons . High intensity process creates more collapsed fibres at same energy . Wall stiffness about the same . Therefore wall thickness is less for high intensity

26 Effect of refining on fibre flexibility

. Effect of increasing energy plateaus at moderate energies

. Fibre development is mostly through removal of outer wall

. Not through internal delamination

Comparison of Pulp Properties

SGW RMP TMP

Energy required (GJ/ton) 5.0 6.4 7.0

Freeness 100 130 100-150

Burst index 1.2 1.6 1.8-2.4

Tear index 3.5 6.8 7.5-9.0

Breaking length (km) 3.2 3.5 3.9-4.3

Shive content (%) 3 2 0.5

Long fibre content (R48) 28 50 55

Fines content (P100) 50 38 35

Brightness (unbleached) 61.5 59 58.5

27 Miscellaneous Other Data

Typical Production Rate 300 Bdt/d (of one refiner) 800 Bdt/d - modern

Typical gap between plates 0.5-1 mm

Typical Specific Energy 7 GJ/t

Typical Power to Refiners 20-30 MW

(27,000 – 42,000 horsepower

 10-15 train diesel locomotive)

Latency Removal

. After refining fibres are kinked and curled and not suitable for papermaking

. Lignin cools and holds kinked shape

. Latency removal straightens fibres

. Low consistency

. 30 minutes

. 90 degrees C

28 Latency removal

. Latency removal result in:

Chemi Thermo Mechanical Pulping (CTMP)

29 Chemi-Mechanical Pulps

• To decrease energy cost or to improve pulp quality, chemical treatments are often added to mechanical pulping • Pretreatment of chips • to lower energy • Interstage treatment • lower energy, fibre flexibilization • Post-treatment • fibre flexibilization

Sulphonation reactions

30 Usual means is sulphonation using sodium sulphite or sodium bisulphite

Low sulphonate High sulphonate content (0-1%) content (1-2%)

Softening of Softening of middle lamella fibre wall lignin lignin

Improved fibre separation Increase in fibre flexibility Increased Decreased and conformability long fibre shive content content Decrease in freeness Increase in breaking length 1. increase in tear index Decrease in specific scattering 2. increase in freeness

Pulp Properties

. RMP fibres broken

. TMP separated at primary wall, some fibre broken

. CTMP Middle lamella very soft, almost all fibres separated at M.L.

31 Pulp Properties

. Light scattering reflects fines content

. Tensile reflects surface area and flexibility of long fibres.

Pulp Properties Changes during Refining

. Strength increase

. Corresponds to energy increase without cutting

32 “Alphabet” Pulps

. Many combinations of treatment and pulping processes are possible

SGW PGW RMP PURE TRMP MECHANICAL PRMP TMP

LFCMP HEAVY CTLF FRACTIONAL

PRINTING PULPS TCMP CRMP LIGHT

MONO PULPS MONO CHEMICALLY CTMP MODIFIED OPCO SCMP HEAVY

PULPS BCMP UHYBS UHYS REINFORCEMENT

Effect of sulphonation on Lignin softening temperature

33 Effect of yield on fibre stiffness

Effect of sulphonation on fibre length

34 Effect of sulphonation on tensile

Effect of Sulphonation Energy required

35 Effect of sulphonation on light scattering

Scattering vs Energy

36 Mechanical Pulp Brightening

. Often desirable to make pulp brighter (whiter)

. Do not want to remove lignin to keep yield high

. Use “brightening” chemicals, e.g. hydrogen peroxide

. Problem: If lignin not removed, brightness not permanent (reversion, yellowing)

. Example: BCTMP (Bleached Chemi-Thermo-Mechanical Pulp)

37 Screening and Cleaning

. Pulping process imperfect

. Small bundles of fibres (shives) remain

. These must be removed and further refined

. Mechanical pulping is therefore follows by an elaborate screening system . Subject of next lecture (after LC-refining)

TMP System

38 Process may also include “cleaners” (hydrocyclones)

Energy Recovery

. Enormous volume of steam produced from heat created in mechanical pulping

. This steam can be recovered and used for mill process steam, e.g. for paper drying

39 Energy balance

. About 65% of electrical energy can be recovered in this manner

RTS results

. Retention: Short retention in pre- heater (10-20s).

. The short time at elevated temperature reduces the brightness losses . Temperature: increase pressure to 5.5-6.0 bar

. Speed: Increase speed to 2000- 2500 RPM. Decreases specific energy to get same ‘quality of pulp’. 15% energy reduction.

40 Conclusions

. Refining characterized by specific energy and intensity . Refining removes outer wall material

. Thin wall, collapsible fibres . Smoother, stronger paper . Heat softens lignin

. More long fibres and less fines . CTMP softens lignin in fibre wall

. Even more long fibres, less fines . Makes fibres more collapsible at same wall thickness . Less fines

The end

41