Biermann Ch. Handbook of Pulping and Papermaking

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144 ^. REFINING AND PULP CHARACTERIZATION

or a set of two pairs of

plates formed by a

double sided rotating

disk between two sta-

tionary plates (Plate

23.)

Disk refiners are

able to operate at high

consistency, which

favors fiber fibrillation

with minimal fiber

cutting. They have

lower no-load energy

requirements (an indi-

cation of energy that

does not contribute to

refining), are more

compact, and are easi-

er to maintain. Disk

refiners are also used

for production of me-

chanical pulp from

wood chips. Tackle

(the plates) is easily

replaced; a wide variety of tackle metals (Table

6-1) and designs (Fig. 6-9) exist for pulping and

refining.

It is interesting to consider the historical

development of beating and refining in terms of

the angle of the bars with respect to the axis of

Fig, 6-8. Single-disk refiner (notice the plate segments). The ribbon

feeder (for pulping or high consistency refining) is observed through the

right

disk.

Courtesy of Andritz Sprout-Bauer.

rotation. The Hollander beater has an angle of 0°,

the early conical refiners have 12° angles, the

Claflin refiner has a 45° angle, and disk refiners

have 90° angles. Generally, the higher the angle,

the higher the consistency at which refining can

occur, leading to lower fiber cutting.

Table 6-1. Typical industry refiner plate metallurgies. Courtesy of Andritz Sprout-Bauer.

1 MetaUurgy

I Ni-Hard

White Iron

X-C(Hi-C)

White Iron

MCK&K-Alloys

II White Iron

440-C High-Carbon

1 Stainless Steel

SAl High-Carbon

Stainless Steel

17-4 PH Stainless

II steel (SS)

Hardness

(Re)

55-62

50-55

55-60

50-55

32-40

Corrosion

Resistance

> Carbon

and < SS

Lower than

> Ni-Hard

<Hi-C

Better than

white iron

Same as

440-C

Excellent

Abrasion

Resistance

Good

Less than

white iron

, Same as

f 440-C

Less than

others

Impact

Resistance

Extremely

brittle

Britfle

> Ni-Hard &

X-C

Tougher than

white iron

j Same as

440-C

Best resis-

tance of all

Elonga-

tion

None

l%-2%

10%-

15%

Fluidity

Good

Fair

Poor

j Poor

Very

poor

Cost

1.5

1.5 X

REFINING 145

Low Consistency Refiner

Plates for Pulp & Paper Stock

Preparation

(Twin Flo Refiners)

High Consistency

Refiner Plates for Pulp,

Paper, Board & industrial

Mechanical Pulping

Wet Processing

Cutting — Coarse Bar

SIngla-Dlsc/TwIn Refiner Cornstarch

Fiber Development — Medium Bar

Double-Disc Refiner

FIberboard

Chemical Processing

Maximum Development — Fine Bar

SIngle-Dlsc/TwIn Refiner

Food Processing

Double-Disc Refiner

Fig. 6-9. Representative refiner plate designs. Courtesy of Andritz Sprout-Bauer.

Disk refiner plates

Calderon, Sharpe, and Rodarmel (1987)

provide an informative summary of low consisten-

cy refining for fiber approach or hot stock refin-

ing. The consistency should be 3.5-5%; consis-

tencies below 2.5% causes undue wear and short

plate life. Fibrillation (separation of the S-1 cell

wall layer) occurs at the bar edge, so more,

narrow bars means higher fibrillation. Smaller

volume in the groves promotes refining action, but

decreases the volume that flows through the

refiner. Increasing the bar angle increases refin-

ing, but also increases the refining power re-

quired. Dams are used to inhibit channeling of the

pulp,

but are not required at low consistencies

with the proper selection of

the

plate pattern. Fig.

6-10 shows the effect of many refining variables

on the refining process.

146 6- REFINING AND PULP CHARACTERIZATION

^-^^^^^^^^^.^ Applied power

Cutting

-> Fiberizing —> Fibrillizing

Hydraulic capacity

Plate Clearance

^"^X

.'-^

^€^-^

^^''

^.^

V- ^

Refining Action —>

Stock Acidity, pH

y'^

Cleanliness

/ ^^ . /^^-^ ' Consistency

/ x^ J^ "^ Multi-pass_ !

/ Applied power,"^^

/ intensity

Plate Life —>

• 001/002 Cutting

• 079/080 Stren. Dev.

•

011/012 Cutting -^le 143/144 Stren. Dev.

•

123/124 High Dev. -^ 007/008 Max. Dev.

—T"

15 20 25

Refiner Size (Inches)

Fig. 6-10. The

e^ect

of refining variables on the refining process (Calderon, Sharpe, and

Rodarmel, 1987). Courtesy of Andritz Sprout-Bauer.

REFINING 147

Fig. 6-11. Valley beater used for laboratory

renning of pulp.

Laboratory refining

The valley beater (Fig. 6-11) is a laboratory

version of the beater used to evaluate pulps on a

small scale (TAPPI Standard T 200). It requires

frequent calibration and adjustments to maintain

standardization. For this reason it is facing com-

petition from the PFImill (Fig. 6-12), a laboratory

refiner using bars on the edge of a rotating disk

against a smooth bed (TAPPI Standard T 248).

The amount of refining on a PFI mill may be

reported in revolutions or PFI counts. One PFI

count is 10 revolutions. Neither method gives

results which are directly comparable to commer-

cial scale refining, though relative results can be

obtained. Other standard laboratory refiners are

(or have been) also used such as the Kollergang

(TAPPI UM 258), Lampen, and Jokro. For non-

standardized refining, double disk refiners as small

as 12 in. in diameter are available.

Fig. 6-13 shows a refining curve from PFI

mill refining of an unbleached commercial pulp.

Fig. 6-14 shows a beating curve of a commercial

pulp using the Valley beater. Both figures are the

Fig. 6-12. PFI mill used for laboratory refining of pulp. The insert shows the refining area.

148 <>• REFINING AND PULP CHARACTERIZATION

10-

"S"

Double folds,/'

(100s) /

CSF(lOOsmL)

Breaking length (km)

^—.,-

Tear (lOOs)

"lOO^

loo 300 400 500" "600 TOO"

800 900 1000

PFI Count

Fig. 6-13. PFI

miU

refining of commercial, unbleached Douglas-fir. (TAPPI standard handsheets.)

Breaking length (km)

Beating Time, minutes

Fig. 6-14. Valley beater curve for commercial softwood fiber. (TAPPI standard handsheets.)

REFINING 149

test results of laboratory handsheets. These

figures represent the strength qualities of paper

made from pulp refined at various levels, although

it is preferable to have more levels of refining to

give smoother curves. Also, one usually plots

strength versus CSF, which gives smoother

curves.

The tradeoff between pulp properties is

demonstrated by looking at the burst and tear

strengths with refining. Increased refining de-

creases the tear strength but increases the burst

strength. In brown paper bags both the burst and

tear strengths are important, although increasing

refining to raise the burst strength leads to lower

tear strength; hence, the tear strength of brown

paper grocery sacks tends to be low (as most

people have experienced at one time or another),

but they can hold heavy objects without bursting.

Refining

power

Refining power is a measure of the power

input to the motors of the refiner based on amount

pulp

processed. It is an indirect measure of the

energy expended in cutting and fibrillating the pulp

fibers, although only a small percentage of the

power actually is consumed by these processes.

Obviously, these values are important in design

and economic calculations and so forth. Refining

power is commonly expressed in units of

kilowatt-hours per ton or horsepower-days per ton

of pulp processed. Smook, Handbook for Pulp &

Paper

Technologists,

gives typical refining energy

requirements for different paper grades on page

189.

For example, tissue, and toweling (lightly

refined paper using low yield, bleached pulp)

require about 100-120 kWh/t (5-6 hp-day/ton)

pulp,

fine papers require about 230 kWh/t (12 Hp-

day/ton) pulp, while greaseproof glassine (a very

highly refined, very dense sheet) uses 400-500

kWh/t (20-25 Hp-day/ton) pulp. (There are

17.904 kWh per HP-day and 1.1 ton per t.)

6.3 PULP CHARACTERIZATION

Pulp characterization is very important to

determine the effects pulping, bleaching, refining,

etc.

on the properties of the pulp and, therefore,

on the final paper properties. Some of these

methods such as Canadian Standard freeness,

cellulose viscosity, and lignin content have been

Fig, 6-15. Standard pulp disintegrator. The

insert shows the mixing blade.

discussed in more appropriate sections. Additional

specialized tests that are not included here are

available in the TAPPI Standards or similar re-

sources. In order to disperse pulp fibers into

solution a standard disintegrator is often used as

shown in Fig. 6-15, which is used in a wide

variety of TAPPI Standard methods.

Moisture content

The moisture content of pulps is determined

by drying a weighed portion of pulp in an oven at

105 ± 3°C (221 ± 5T) to constant weight

(TAPPI Standard T 210). The sample is weighed

in the oven or cooled in a desiccator or other

closed container before weighing. The sample is

reheated for at least three hours until two succes-

sive weights show a variation of 0.1

or less.

Physical properties

To determine what effects pulp modifications

will have on the final paper product, laboratory

handsheets are made. Fig. 6-16 shows some of

the steps involved in making laboratory hand-

ISO 6- REFINING AND PULP CHARACTERIZATION

sheets on a square mold. Round handsheets 15.9

cm (6.25 in.) in diameter are made according to

TAPPI Standard T 205 and tested by methods in

TAPPI Standard T 220. This handsheet former is

called a British sheet mold (Fig. 6-17). Mechani-

cal pulps such as PGW, RMP, IMP, and CTMP

have significant fiber curling. These pulps are

prepared according to TAPPI Standard T 262,

circulating

consistency stock at 90-95 °C (194-

203

°F),

to fully develop their strength properties

by removing the curl. Other handsheet formers

are also used.

Fig. 6-16. Preparation of laboratory handsheets for pulp characterization. The pulp slurry is

added; the pulp is mixed; the sheet is pressed after draining; and the sheet is dried.

PULP CHARACTERIZATION

151

Fig. 6-17. British sheet mold for preparing

laboratory handsheets.

Fiber length and fines content

The fiber length of pulp is traditionally

measured by projection, which is a very tedious

procedure (TAPPI Standard T 232). Fiber lengths

can also be determined by fiber classification using

a series of at least four screens of increasingly

smaller openings (TAPPI Standard T 233); two

common instruments are the Clark type (Fig. 6-18)

or Bauer-McNett type (Fig. 6-19). More recently

it is measured automatically in dilute solutions

using optical methods. One common instrument

for optical analysis is the Kajaani (Fig. 6-20).

TAPPI Standard T 261 is a means of mea-

suring the fines content of pulps with a single

screen classifier, the so-called Brittjar test (Fig.

6-21). Usually the screen is 200 mesh (76 ptm

diameter holes) and the sample is 500 ml of 0.5%

consistency stock.

Pulp viscosity

The pulp viscosity is a measure of the aver-

age chain length

(degree

of polymerization, DP) of

cellulose. It is determined after dissolving the

pulp in a suitable solvent such as cupriethylene-

diamine solution (Plate 24). Higher viscosity

indicates a higher average cellulose DP that, in

turn, usually indicates stronger pulp and paper.

Decreases in viscosity result from chemical pulp-

ing and bleaching operations and to a certain

extent are unavoidable. Loss of pulp viscosity

must be minimized by proper attention to impor-

tant process parameters. Cellulose viscosity has

little merit in mechanical pulps since the cellulose

chains are not appreciably degraded by pulping or

bleaching processes that form these. Fig. 6-22 is

a comparison of pulp viscosities determined from

a variety of methods and cellulose DP.

Bleached

chemical

pulp

and

paper deterioration

Brightness reversion of bleached chemical

pulps is determined by TAPPI Standard T 260 by

measuring the brightness before and after exposing

the pulp to 100% humidity at 100°C (212°F) for

one hour. TAPPI Standard T 430, the copper

number of bleached pulp, paper, and paperboard,

measures the amount of CUSO4 reduced by the

fiber source. This is an indication of oxycellulose,

hydrocellulose, lignin, and sugars that reduce the

copper sulfate. These materials may be markers

for deterioration and may be an indicator for paper

permanence. Papers containing ZnS, CaSOj,

melamine resins, and other materials that reduce

CUSO4 may be used, provided the total reducing

power of the additives is known and no more than

about 75% of the total reducing power of the

sheet.

Miscellaneous tests

Dirt (foreign matter in a sheet that has a

marked contrasting color to the rest of the sheet)

in pulp is numerically quantified by TAPPI Stan-

dard T 213 (Section 34.18). Foreign particulate

matter (particles and unbleached fiber bundles

which are embedded in wood pulp as viewed by

transmitted light) is measured by TAPPI Standard

152 6. REFINING AND PULP CHARACTERIZATION

Fig. 6-18. Clark pulp fiber classifier.

A, Constant level funnel; B, first tank; C, screen; D, outlet; F, stopper; G, drainage

cup;

H, midfeather.

Fig. 6-19, Bauer-McNett pulp fiber classifier. Courtesy of Andritz Sprout-Bauer.

PULP CHARACTERIZATION I53

r /\ ^^

[

1 Y

[

1 \

/ / \

^ »• 1 1 • 1

BliiiliiiWIBfe

«»^

** ** 4K ^ « \m m! 1^'mmtm mmmi «>*; .*-»»•:!•

•« "1 > —• 1 1 1 =

:>:r:^':;:::?;;;|l^^^^

/•^';:'^::f:::;'^

"•"i-

' 1 • '• 1 t' 1

100

1.0

2.0 3.0

0 5. 0

6.0

7.0

Length, mm

Fig. 6-20. Kajaani optical fiber classifier with analysis of 19,060 fibers (60% hardwood, 40%

softwood by weight). Averages are number, 0.55 nun; length, 1.16 mm; and, mass, 1.88 mm.

T 246. The specific surface area of lightly beaten

pulps can be measured by the method of Clark

using silver deposition (TAPPI Standard T 226).

Fig. 6-23 shows the relative surface area of kraft

and sulfite pulps as a function of freeness.

6.4 PULP PROPERTIES VERSUS

PERFORMANCE

The morphology of paper fibers is important

to the properties of the final sheet. Dadswell and

Watson

(1961,

1962) reviewed the results of 35

references in their discussion of the influence of

the morphology of wood pulp fibers on paper

properties. They have many interesting points.

For example, when determining the effect of fiber

length on paper properties, one should not attempt

to fractionate pulp. With hardwoods one might

get vessels in one fraction that differ by more than

just fiber length from the other fibers; there may

be differences in the chemical composition of

various fractions too. An often used method they

cite (Brown, 1932) is to cut paper into narrow

strips,

repulp the cut paper, and form handsheets.

The main difference in such new hand-sheets is the

average fiber length as determined by the width of

the cut (or uncut) strips.

Another point they make is that wood fibers

with thin cell walls, which are from woods of low

specific gravity, tend to make dense well-formed

paper (with well bonded fibers) having high burst,

fold, and breaking length, compared to fibers with

thick cell walls from dense woods. The thickness

of fibers is also dependent on the ratio of latewood

to earlywood, the ratio of compression to normal

wood, and other factors. Softwoods with large

amotmts of latewood, on the other hand, give

bulky, porous sheets, with poorly bonded fibers.