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Refining and Pulp Characterization
REFINING AND PULP CHARACTERIZATION 6.1 D^TRODUCTION TO REFINING Introduction Pulp refining is a mechanical treatment of pulp fibers to develop their optimum papermaking properties. The optmium paper properties, of course, depend on the product being made. Furthermore, there is always a tradeoff between various properties. Refining of fibers is very important before making paper from them. Refining increases the strength of fiber to fiber bonds by increasing the surface area of the fibers and making the fibers more pliable to conform around each other, which increases the bonding surface area and leads to a denser sheet. During refining, however, individual fibers are weakened and made shorter due to cutting action. With very long fibers of a few species this cutting action improves the formation of the sheet on the paper machine, but in most cases, it is an undesired effect of refining (even if formation is improved somewhat); consequently, refining is generally a tradeoff between improving fiber to fiber bonding and decreasing the strength of individual fibers. Most strength properties of paper increase with pulp refining, since they rely on fiber to fiber bonding. The tear strength, which depends highly on the strength of the individual fibers, actually decreases with refining. After a certain point the limiting factor of strength is not fiber to fiber bonding, but the strength of the individual fibers. Refining beyond this point begins to decrease other strength properties besides tear. Refining of pulp increases their flexibility and leads to denser paper. This means bulk, opacity, and porosity values decrease with refining. Fig. 6-1 shows four paper types made from increasingly refined fibers. Two of these samples are glassine papers which are typically refined to a much higher degree than most printing and packaging papers. It is apparent from these figures that fiber to fiber bonding increases with refining. Also, the volume of space between the fibers decreases; hence, refining increases the density of the sheet.
Fiber Brushing Refining at high consistency with a relatively large distance between the refiner plates increases fiber-fiber interactions that are termed fiber brushing. This tends to roughen the fiber surface, with minimal fiber cutting for improved fiber-fiber bonding. Fiber Cutting Operating refiners at low consistencies with a minimal distance between the refining surfaces increases fiber-bar contact, resulting in fiber cutting. This is desired with certain woods (especially redwood with 7 nmi fibers and cotton with long fibers) and other materials containing long fibers to increase the quality of formation on the paper machine. In most cases, however, it is desirable to minimize fiber cutting to maintain high paper strength. Drainage Drainage is the ease of removing water from pulp fibers, either by gravity or mechanical means. CSF is a measure of drainage and a useful means for determining the level of refining. Fibrillation Fibrillation is the production of rough surfaces on fibers by mechanical action; refiners break the outer layer of fibers, i. e., the primary cell wall, causing the fibrils from the secondary cell wall to protrude from the fiber surfaces. Average fiber length The average fiber length is a statistical average length of fibers in pulp. Fiber length is measured microscopically (nimiber average), by classification with screens (weight average) or by optical scanners (number average). Two such averages are expressed here, akin to molecular weight averages of polymers. L,. is the length of the fraction of fibers with a length of /, N,- is the number of fibers of length /, and W/ is the weight of the fraction of fibers with length z. The weight average fiber length is equal to or larger than the number average fiber length.
137
138
6. REFINING AND PULP CHARACTERIZATION
Fig. 6-1. Paper of increasingly refined pulps. Bleached kraft softwood fibers (top left); same pulp with refining (top right); machine glazed florist tissue (bottom left); and glassine weighing paper.
INTRODUCTION TO REFINING
nimiber average length
weight average length =
EiNT.xL.
TW.xL^
Theory One action of refining is the "pumping" of water into the cell wall making it much more flexible. This is sometimes compared to the softening of spaghetti upon cooking, which is relevant in that in both cases internal hydrogen bonds are broken with the addition of water molecules. While this would occur whenever fibers are thoroughly wetted, to the extent that refining disrupts the crystallinity of fibers, more water can be added to cellulose fibers. A second action of refining is fibrillation, that is, exposure of cellulose fibrils to increase the surface area of fibers, thereby improving fiber-fiber bonding in the final sheet. For example, the surface area of kraft, softwood fibers is on the order of 1 m^/g at 750 CSF, but it increases to about 5 mVg at 350 CSF. A third action in refining is delamination of the cell wall, such as between the primary and secondary layers, which increases the fiber flexibility. The analogy of greased leaf springs, which are much more flexible than the equivalent amount of solid metal, has been used to explain this effect. Refining decreases the pulp freeness, the rate at which water will drain through the pulp. Refined pulp, therefore, has a low freeness. The extent of refining is monitored by measuring the pulp freeness and the strength properties of handsheets made from the refined pulp. Pulp used to be treated in beaters, such as the Hollander beater that is explained below, but now refiners are used. The terms beating and refining are often used interchangeably, but refining is applicable to most modem equipment. Refining variables Pulps low in lignin (low yield pulps) and high in hemicelluloses are relatively easy to refine in terms of development of strength properties. High
139
temperature and high pH during refining decrease the refining energy requirements. Refining at high pH offers the advantages of an increase in the rate of hydration due to fiber swelling (Scallan, 1983), an increase in the ultimate strength of the paper, and an increase in sheet bulk. It also produces a lower stock freeness, reduces equipment corrosion, and produces a softer sheet. Refining at low pH hardens the fibers, leads to a denser sheet that is hard and snappy, and runs better on the paper machine. Refiner plate speed, plate clearance, and type of tackle are other important variables. Major changes in refining have resulted in the development of equipment that allows refining to occur at high consistency. High consistency refining leads to more fiber fibrillation and less fiber cutting. Refining at high consistency, however, may lead to fiber curling. Fig. 6-2 shows two pulps, one refined at low consistency (3%) in a Jordan refiner and one refined at high consistency in a double disk refiner. The fibers refined at high consistency have much more fibrillation and less fiber cutting. Canadian Standardfireeness,CSF Refining is easily monitored by the drainage rate of water through the pulp. A high drainage rate also means a high freeness. Obviously freeness is of utmost importance in the operation of a paper machine. A low freeness means that the paper machine will have to operate relatively slowly, a condition that is usually intolerable. The CSF test measures the drainage of 1 liter of 0.3 % consistency pulp slurry through a calibrated screen. The test is shown in Fig. 6-3. The device is shown in Fig. 6-4. The 1992 TAPPI Standard T 227 revision describes a change in design that was made in 1967, where the angle and shape of the side orifice was changed. The CSF test was developed for use with groundwood pulps and was not intended for use with chemical pulps; nevertheless, it is the standard test for monitoring refining in North American mills. It tends to be most influenced by the fines content of the pulp and to a smaller extent by the degree of fibrillation and other fiber properties. The water is diverted around a spreader cone where, if it drains quickly, some of it overflows to a side orifice and escapes collection from the bottom
140
6. REFINING AND PULP CHARACTERIZATION
Fig. 6-2. Refining at 3% consistency (left) shows more fiber cutting with less fibrillation than at 30% consistency. From Making Pulp and Paper, ©1967 Crown Zellerbach, with permission. orifice. The water from the side orifice is collected and measured in a graduate cylinder and reported in ml. Unrefined softwood might be 700 ml CSF, whereas mechanical pulp is about 100-200 ml CSF (Fig. 6-5). AIR COCK
I r ^ I TOP LID HINGES BOTTOM LID CALIBRATED SCREEN PLATE
SPREADER CONE SIDE ORIFICE 1000 mL GRADUATE CYLINDER
CALIBRATED ORIFICE
Fig. 6-3. Schematic diagram of the Canadian Standard freeness test in progress.
Fig. 6-4. The Canadian Standard freeness device.
INTRODUCTION TO REFINING
141
650
en o d Q
8
40
80
100 120 140 CSFFreeness,ml for 3 grams
CO
o
g
P
100
200
300 400 500 CSF Freeness, ml for 3 grams
600
700
Fig. 6-5. Comparison of freeness scales for mechanical and chemical pulps. Data from US 0809.
142
^- REFINING AND PULP CHARACTERIZATION
There are other freeness tests that are used around the world. Perhaps the most common one is the Schopper Reigler test, which is similar in concept to the CSF test. TAPPI TIS 0809-01 gives inter-conversions of CSF, Schopper Reigler units, Williams Precision, and Drainage Factor for various types of pulp. Fig. 6-5 has two graphical presentations from the first table of TIS 0809. Since the conversions are not exact, the two lines of each set represent the bounds of the conversion. See Section 17.5 for CSF correction equations for temperature and consistency. 6.2 REFINING Beater A beater is an early device (the Hollander Beater was invented in the 1700s) used to treat pulp to improve the papermaking properties. Beating is a batch process where the pulp slurry circulates through an oval tank around a midsection and passes between a revolving roll with bars and a bedplate with bars. The pulp is at about 6% consistency and emphasizes fiber brushing. The use of these had been phased out at most mills by the late 1970s because they are slow and expensive
to operate. Fig. 6-6 shows a Hollander beater that was in use until the mid 1970s. Some mills retain these for use in mixing stock for small paper machines. Refiners Refiners are machines that mechanically macerate and/or cut pulp fibers before they are made into paper. There are two principal types: disk and conical refiners. Disk refiners have superseded the conical refiners for many purposes, as they offer many advantages. The operation of a conical refiner (Fig. 6-7) is similar to the operation of a disk refiners, except for the geometry of the refiners. In conical refiners, the refining surfaces are on a tapered plug. These surfaces consist of a rotor that turns against the housing and the stator, both of which contain metal bars mounted perpendicularly to rotation. The Jordan refiner, patented by Joseph Jordan in 1858, is one type of conical refiner with a 12° angle on the rotor (with respect to the longitudinal axis); it is suited to low consistency refining (2% consistency) with much fiber cutting. The Claflin refiner uses a rotor with a 45° angle with respect to the longitudinal axis that revolves
Fig. 6-6. The Hollander beater for mill beating of pulp.
REFINING
143
Jordan
Claflin
Fig. 6-7. The Jordan and Claflin conical refiners. Reprinted from Making Pulp and Paper^ ©1967 Crown Zellerbach Corp., with permission. inside the mating shell. The inlet is at the small end of the taper. The Claflin refiner is intermediate in operation to the Jordan disk refiners. These two refiners are shown in Fig. 6-7. Disk refiners became available for papermaking in the 1930s, after the conical refiners. Disk refiners had enjoyed widespread use in
food processing. For example, they are used to process peanuts into peanut butter and corn into cornstarch and flour. The pulp slurry makes one pass between rotating plates equipped with teeth or bars. There are three common configurations: one fixed plate with one rotating plate (Fig. 6-8), two rotating plates that turn in opposite directions.
144
^. REFINING AND PULP CHARACTERIZATION
or a set of two pairs of plates formed by a double sided rotating disk between two stationary 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 indication of energy that does not contribute to refining), are more compact, and are easier to maintain. Disk refiners are also used for production of me- Fig, 6-8. Single-disk refiner (notice the plate segments). The ribbon chanical pulp from feeder (for pulping or high consistency refining) is observed through the wood chips. Tackle right disk. Courtesy of Andritz Sprout-Bauer. (the plates) is easily replaced; a wide variety of tackle metals (Table rotation. The Hollander beater has an angle of 0°, 6-1) and designs (Fig. 6-9) exist for pulping and the early conical refiners have 12° angles, the Claflin refiner has a 45° angle, and disk refiners refining. It is interesting to consider the historical have 90° angles. Generally, the higher the angle, development of beating and refining in terms of the higher the consistency at which refining can the angle of the bars with respect to the axis of occur, leading to lower fiber cutting. Table 6-1. Typical industry refiner plate metallurgies. Courtesy of Andritz Sprout-Bauer. 1 MetaUurgy 1
Hardness (Re)
Corrosion Resistance
Abrasion Resistance
Impact Resistance
Elongation
Fluidity
Cost
I Ni-Hard II White Iron
55-62
> Carbon and < SS
Good
Extremely brittle
None
Good
IX
50-55
Lower than
Good
Britfle
None
Fair
1.5 X
MCK&K-Alloys II White Iron
50-55
> Ni-Hard
Good
> Ni-Hard & X-C
None
Fair
1.5 X
440-C High-Carbon 1 Stainless Steel
55-60
Better than white iron
Less than white iron
Tougher than white iron
l%-2%
Poor
3X
SAl High-Carbon Stainless Steel
50-55
Same as 440-C
l%-2%
j Poor
3X
17-4 PH Stainless II steel (SS)
32-40
Excellent
Very poor
4X
X-C(Hi-C) White Iron
ss
, Same as f 440-C Less than others
j Same as 440-C Best resistance of all
10%15%
REFINING
Low Consistency Refiner Plates for Pulp & Paper Stock Preparation
High Consistency Refiner Plates for Pulp, Paper, Board & industrial
(Twin Flo Refiners)
Mechanical Pulping
Wet Processing
Cutting — Coarse Bar
SIngla-Dlsc/TwIn Refiner
Cornstarch
Fiber Development — Medium Bar
Double-Disc Refiner
Chemical Processing
145
FIberboard
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 consistency refining for fiber approach or hot stock refining. The consistency should be 3.5-5%; consistencies 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 refining, but also increases the refining power required. 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
A 1 1 U
2
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0.
Applied power
,' ->
Cutting
Fiberizing
—>
Fibrillizing
Hydraulic capacity
Plate Clearance
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Stock Acidity, pH y ' ^
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Consistency " ^
Multi-pass_ !
2 /
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^
Cleanliness
A 1
Applied power,"^^ intensity
/ Plate Life
Refining Action —>
—>
10
9H 8
• 001/002 Cutting • 079/080 Stren. Dev.
• 011/012 Cutting -^le 143/144 Stren. Dev. • 123/124 High Dev. - ^ 007/008 Max. Dev.
I 1 CO
2-
n 15
20
25
30
Refiner Size (Inches)
—T"
35
40
45
Fig. 6-10. The e^ect of refining variables on the refining process (Calderon, Sharpe, and Rodarmel, 1987). Courtesy of Andritz Sprout-Bauer.
REFINING
Fig. 6-11. Valley beater used for laboratory renning of pulp.
147
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 competition 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 commercial 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 nonstandardized 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) /
98-
Breaking length (km)
CSF(lOOsmL)
I
7-
2
65-
c3
4-
z"
^—.,3^
Tear (lOOs)
2" 1-
0m 0
"lOO^
loo
300
400
500" "600 PFI Count
TOO"
800
900
1000
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
Fig, 6-15. Standard pulp disintegrator. insert shows the mixing blade.
The
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 decreases 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 of 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 Hpday/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
discussed in more appropriate sections. Additional specialized tests that are not included here are available in the TAPPI Standards or similar resources. 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 ± 5 T ) 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 successive 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). Mechanical pulps such as PGW, RMP, IMP, and CTMP
have significant fiber curling. These pulps are prepared according to TAPPI Standard T 262, circulating 2% consistency stock at 90-95 °C (194203 °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
diameter holes) and the sample is 500 ml of 0.5% consistency stock. Pulp viscosity The pulp viscosity is a measure of the average chain length (degree ofpolymerization, DP) of cellulose. It is determined after dissolving the pulp in a suitable solvent such as cupriethylenediamine 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 pulping and bleaching operations and to a certain extent are unavoidable. Loss of pulp viscosity must be minimized by proper attention to important 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.
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 measuring 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
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 Standard 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:^':;:::?;;;|l^^^^
20
100
r
^^
/\
80
16
[
1 Y 60
12
[ 1
§
\
BliiiliiiWIBfe
|8
L /
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•«
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6.0
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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.
154
^. REFINING AND PULP CHARACTERIZATION
more important than the cell wall thickness; however, all of these fiber properties are interrelated, and no single variable means much alone. Many studies (for example, Kellogg and Gonzalez, 1976) show that mature boles, i.e., outer growth rings that are 10-50 rings from the center, have fibers with the longest length, the largest radial and tangential diameters, and the smallest fibril angles. Once again, these facts indicate that short rotation, plantation grown wood may have decreased strength properties. 6.5 ANNOTATED BIBLIOGRAPHY
Fig. 6-21. Britt jar test. having high tear resistance. The authors cite the work of Runkel who says that if the thickness of the cell wall is less than the radius of the lumen of fibers in wood, the wood makes a good paper. The relative thickness of cell walls is reflected in the fiber coarseness (mg/100 m of fiber). These workers minimized the importance of the fiber length/diameter ratio that many workers feel is
CCA.-26
TAPPI T- 2 3 0
CCA:20 NitraU
Intrinsic WscMity in c e o
OP
PipctU Viscosity 03%eenc in c e o
TAPPI T- 2 3 0
CCA: 16 TAPPI
Foiling bol 'Pip«tt« Viscosity Viscosity llconc. IXcone. in c e o wt euoKom
1200-
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90-1
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1200700-
WOO-
Intrinsic Viscosity In CEO
30030-
lOOO1000-
vxxr-.
*o-
Numerous chapters in this book relate to refining including Chapter 8, Fibrillation and fiber bonding, pp 160-180, not a look at the refining process, but a fundamental look at fibrillation of fibers induced by various treatments and how this promotes fiber bonding; Chapter 7, Bonding of cellulose surfac-
CCA : 2 8 Intrinsic Viscosity kteoctoMn llOO-
1300-1
General aspects of refining (Also see references 14, 15 for the effect of pH on refining) 1. Clark, J. d'A., Pulp Technology and Treatment for Paper, Miller Freeman Pub. Co., San Francisco, 1978, 751 p. (The 2nd ed. 1985, 878 p., is updated for some of the introductory, basic material, but is otherwise very similar).
000-
700-^
U UJ
15
QL 1000000-
0-
000-
30*0-
300-
800-
7-
20-
300-
10-
30--
000-j
(/)
10
50oJ
5
400H
0
<0012-
000400000-
205-
8-
300-
J
Fig. 6-22. Conversion nomograph for pulp viscosities. ©1963 . Reprinted from Siholta et al, (1963) with permission.
\
800
1
700
1
600
1
500 CSF
1
1
400
300
200
Fig. 6-23. Specific surface area versus CSF for laboratory pulp. After Robertson and Mason (1949).
ANNOTATED BIBLIOGRAPHY
155
es, pp 145-159 and early refiners, pp 205215 are of related interest; Chapter 12, Nature and effects of beating, pp 257-280; Chapter 13, Mill beating and refining, pp 281-316; Chapter 14, Laboratory beating, pp 317-362; and Chapter 25, Control of beating and refining, pp 560-568.
3.
Root, E.M., Stock preparation, in Pulp and Paper Science and Technology, Vol. 2, Libby, C.E., Ed. McGraw-Hill, New York, 1962, pp 1-39. A lot of useful information on early refining methods such as beaters and the Jordan refiner. This also has a good overview on the physical properties of fillers.
Many chapters from the first edition relate to pulp and fiber quality and testing. Chapter 9, Properties of pulps, pp 181-200; Chapter 11, Moisture content of bales, pp 230-256; Chapter 15, Test sheet making, pp 363-379; Chapter 16, Hand sheet testing, pp 380-400; Chapter 17, Fiber length, pp 402-437; Chapter 18,fibercoarseness, pp 438-449; Chapter 19, Intrinsic fiber strength, pp 450-464; Chapter 20, Cohesiveness, pp 465-487; Chapter 21, Wet fiber compactability, pp 488-503; Chapter 22, Drainage of water from pulp, pp 504-532, Chapter 23, Surface measurements, pp 533-540; Chapter 24, Wet-web strength, pp 541-559; Chapter 27, Characterization and control of mechanical pulps, pp 579-602; Chapter 28, Chemical and microscopical analysis, pp 603-613; Chapter 29, Optical characteristics, dirt, and shives, pp 614-638; and Chapter 32, Formulas for pulp properties, pp 680-698.
4.
Spencer, H.S., N.G.M. Tuck, and R.W. Gordon, Beating and refining, in Pulp and Paper Manufacture, Vol 5, MacDonald, R. G., Ed., McGraw-Hill, New York, 1969, pp 131-185. Scanning electron micrographs, fundamental aspects of refining, equipment, and refiner plates are included in this article.
Clark describes the properties of fibers (discussed in chapters 17-21) for papermaking in terms of five of their principal properties: the length-average fiber length, the coarseness (mg per 100 m of fiber, measured by Tappi Standard T 234, with coarse fibers giving lower strength due to poorer bonding), wet compactability (measured by the apparent sheet density, for example, TAPPI Standards T 205 and T 220), the intrinsic fiber strength, and the fiber cohesiveness.
6.
Calderon, P., P.E. Sharpe, and J.L. Rodarmel, Low consistency refiner plate design and selection. Spectrum (SproutBauer), Summer, 1987, 7 p.
Pulp surface area 5. Casey, J.P., Ed., Pulp and Paper Chemistry and Chemical Technology, 2nd ed. (1960), volume 2, p 621 has a discussion on the surface area of pulps. The more sophisticated types of studies mentioned use silver deposition or gas adsorption to measure the total available surface area of pulps. These studies give reasonably close values to each other (within a factor of two) for surface area. Fig. 6-23 is an approximation of the results of Robertson and Mason (1949) cited in this work. Robertson, A.A. and S.G. Mason, Specific surface area of cellulose fibres by the liquid penetration method. Pulp Paper Mag. Can, 50(13): 103-110(1949).
Fiber morphology and performance 7. Dadswell, H.E. and A.J. Watson, Influence of the morphology of woodpulp fibres on paper properties. Transactions of the "Formation and Structure of Paper" Symposium, 25-29 Sept., 1961, pp 537-572 (1962). (See also Appita 14(5): 168-176(1961) and ibid. 17(6): 146-156(1964).) 8.
R.B. Brown, Paper Trade J. 95(13):2729(1932).
156
9.
<5- REFINING AND PULP CHARACTERIZATION
Kellogg, R.M., and J.S. Gonzalez, Relationship between anatomical and sheet properties in western hemlock kraft pulps. Part I. Anatomical relationships. Transactions Tech, Soc, 2(3):69-72(1976).
10. Smith, W.E. and Von L. Byrd, Fiber bonding and tensile stress-strain properties of earlywood and latewood handsheets, USDA, For. Res. Ser. Res. Pap. FPL 193(1972), 9 p. 11. Horn, R. A., Morphology of wood pulp fiber from softwoods and influence on paper strength, USDA, For. Res. Ser. Res. Pap. FPL 242(1974), 11 p. This study concluded that the fibril angle of fibers was the most important factor in stretch properties of paper made from unrefined, softwood pulp and an appreciable factor in stretch properties of paper made from refined, softwood pulp. 12. Dinwoodie, J.M., The relationship between fiber morphology and paper properties: A review of the literature, Tappi J, 48(8):440447(1965). This includes 115 references. 13. Megraw, R.A., Wood Quality Factors in Loblolly Pine, Tappi Press, 1985, 88 p. This book is subtitled "The influence of tree age, position in tree, and cultural practice on wood specific gravity, fiber length, and fibril angle". Fiber chemical properties and performance 14. Scallan, A.M., The effect of acidic groups on the swelling of pulps: a review, Tappi J. 66(11):73-75(1983). This has 31 references. Acidic groups of pulp, i.e., carboxylic acids from hemicelluloses and possibly sulfonate groups in sulfite-pretreated mechanical pulps or unbleached or semi-bleached sulfite chemical pulps, contribute to fiber swelling in water. Fiber swelling contributes to ease of refining and development of paper strength. At pH below 7 the carboxylate groups are
protonated and no longer dissociated, decreasing fiber swelling. At higher pH, the metal counter ion plays a big role in fiber swelling with M+ > M^"^ > M^+. Also, it is recognized that swelling increases with the series H+ < Ca^"' < Mg^^ < NH4+ < Na+. Pulps bleached with hydrogen peroxide are often treated magnesium. It may be usefiil to wash these pulps with some dilute alkali to improve their papermaking properties. Alum would have a detrimental effect on pulp swelling and should be kept out of the process until after refining. 15. Lindstrom, T. and G. Carlsson, The effect of chemical environment on fiber swelling, Svensk Papperstidn 85(3):R14-20(1982). Swelling of pulps under a variety of pH and metal ion conditions was determined by the water retention values (WRV). Bleached softwood sulfate pulp was fairly insensitive to changes in WRV under these conditions since it had only 2.5 meq/100 g (acidic groups on pulp). An unbleached softwood sulfate pulp (47% yield with 38 kappa number and 7.4 meq/100 g acidic groups on pulp) had larger changes in WRV going from a low of 200 to a high of 260. The maximum WRV occurred at pH 8-9.5 with NaCl concentration below about 0.01 M. 0.05 M NaCl decreased the WRV to 220 at pH 9, and a NaCl concentration of 0.5 M decreased it to 200, the base value. The tensile index was directly proportional to WRV. These results indicate that one should keep the ionic strength of pulp low during pulping and pulp storage before and during papermaking. Pulp viscosity 16. Siholta, H.,B. Kyrklund, L. Laamanen, and L Palenius, Comparison and conversion of viscosity and DP-values determined by different methods. This is the most extensive and about the only publication on the subject. Paperija Puu (4a):225-232(1963).
EXERCISES
157
EXERCISES Refining 1. Plot representative tensile strength, tear strength, CSF, and specific surface area of paper made from pulp as a function of refining energy (time) applied to the pulp. 2.
Describe how refining helps fiber to fiber bonding in the final sheet. Give three mechanisms for its action.
3.
Describe the effect of pulp consistency on the refining process in terms of bar-fiber contacts. Use Fig. 6-24 to help you.
4.
What is the major drawback of any laboratory refining process?
5.
Give two methods whereby the level and effectiveness of refining is measured.
6.
One is visiting a mill using redwood fiber and sees a Jordan refiner. Is this unusual considering very few mills use Jordan refiners anymore?
7.
Using the discussion following references 14 and 15, describe why refining and papermaking at elevated pH (say pH 8 versus pH 4) often leads to a significant increase in the strength of the paper. When is this especially true?
As the upper bar passes the lower, low consistency will favor either trapping single fibers and cutting them, as in B, or stretching and breaking them, as in Views C and D.
At higher consistencies, the fibers will tend to form a blanket or cushion on the bars, leading to crushing rather than a cutting action on the fibers. Fig. 6-24. Effects of consistency on refining. ®1991 James E. Kline. Reprinted from Paper and Paperboard with permission.
Pulp characterization 8.
How are fiber size distributions measured?
9.
What is the significance of fiber size distributions?
10. What is the purpose of making laboratory handsheets?
Fiber Brushing Refining at high consistency with a relatively large distance between the refiner plates increases fiber-fiber interactions that are termed fiber brushing. This tends to roughen the fiber surface, with minimal fiber cutting for improved fiber-fiber bonding. Fiber Cutting Operating refiners at low consistencies with a minimal distance between the refining surfaces increases fiber-bar contact, resulting in fiber cutting. This is desired with certain woods (especially redwood with 7 nmi fibers and cotton with long fibers) and other materials containing long fibers to increase the quality of formation on the paper machine. In most cases, however, it is desirable to minimize fiber cutting to maintain high paper strength. Drainage Drainage is the ease of removing water from pulp fibers, either by gravity or mechanical means. CSF is a measure of drainage and a useful means for determining the level of refining. Fibrillation Fibrillation is the production of rough surfaces on fibers by mechanical action; refiners break the outer layer of fibers, i. e., the primary cell wall, causing the fibrils from the secondary cell wall to protrude from the fiber surfaces. Average fiber length The average fiber length is a statistical average length of fibers in pulp. Fiber length is measured microscopically (nimiber average), by classification with screens (weight average) or by optical scanners (number average). Two such averages are expressed here, akin to molecular weight averages of polymers. L,. is the length of the fraction of fibers with a length of /, N,- is the number of fibers of length /, and W/ is the weight of the fraction of fibers with length z. The weight average fiber length is equal to or larger than the number average fiber length.
137
138
6. REFINING AND PULP CHARACTERIZATION
Fig. 6-1. Paper of increasingly refined pulps. Bleached kraft softwood fibers (top left); same pulp with refining (top right); machine glazed florist tissue (bottom left); and glassine weighing paper.
INTRODUCTION TO REFINING
nimiber average length
weight average length =
EiNT.xL.
TW.xL^
Theory One action of refining is the "pumping" of water into the cell wall making it much more flexible. This is sometimes compared to the softening of spaghetti upon cooking, which is relevant in that in both cases internal hydrogen bonds are broken with the addition of water molecules. While this would occur whenever fibers are thoroughly wetted, to the extent that refining disrupts the crystallinity of fibers, more water can be added to cellulose fibers. A second action of refining is fibrillation, that is, exposure of cellulose fibrils to increase the surface area of fibers, thereby improving fiber-fiber bonding in the final sheet. For example, the surface area of kraft, softwood fibers is on the order of 1 m^/g at 750 CSF, but it increases to about 5 mVg at 350 CSF. A third action in refining is delamination of the cell wall, such as between the primary and secondary layers, which increases the fiber flexibility. The analogy of greased leaf springs, which are much more flexible than the equivalent amount of solid metal, has been used to explain this effect. Refining decreases the pulp freeness, the rate at which water will drain through the pulp. Refined pulp, therefore, has a low freeness. The extent of refining is monitored by measuring the pulp freeness and the strength properties of handsheets made from the refined pulp. Pulp used to be treated in beaters, such as the Hollander beater that is explained below, but now refiners are used. The terms beating and refining are often used interchangeably, but refining is applicable to most modem equipment. Refining variables Pulps low in lignin (low yield pulps) and high in hemicelluloses are relatively easy to refine in terms of development of strength properties. High
139
temperature and high pH during refining decrease the refining energy requirements. Refining at high pH offers the advantages of an increase in the rate of hydration due to fiber swelling (Scallan, 1983), an increase in the ultimate strength of the paper, and an increase in sheet bulk. It also produces a lower stock freeness, reduces equipment corrosion, and produces a softer sheet. Refining at low pH hardens the fibers, leads to a denser sheet that is hard and snappy, and runs better on the paper machine. Refiner plate speed, plate clearance, and type of tackle are other important variables. Major changes in refining have resulted in the development of equipment that allows refining to occur at high consistency. High consistency refining leads to more fiber fibrillation and less fiber cutting. Refining at high consistency, however, may lead to fiber curling. Fig. 6-2 shows two pulps, one refined at low consistency (3%) in a Jordan refiner and one refined at high consistency in a double disk refiner. The fibers refined at high consistency have much more fibrillation and less fiber cutting. Canadian Standardfireeness,CSF Refining is easily monitored by the drainage rate of water through the pulp. A high drainage rate also means a high freeness. Obviously freeness is of utmost importance in the operation of a paper machine. A low freeness means that the paper machine will have to operate relatively slowly, a condition that is usually intolerable. The CSF test measures the drainage of 1 liter of 0.3 % consistency pulp slurry through a calibrated screen. The test is shown in Fig. 6-3. The device is shown in Fig. 6-4. The 1992 TAPPI Standard T 227 revision describes a change in design that was made in 1967, where the angle and shape of the side orifice was changed. The CSF test was developed for use with groundwood pulps and was not intended for use with chemical pulps; nevertheless, it is the standard test for monitoring refining in North American mills. It tends to be most influenced by the fines content of the pulp and to a smaller extent by the degree of fibrillation and other fiber properties. The water is diverted around a spreader cone where, if it drains quickly, some of it overflows to a side orifice and escapes collection from the bottom
140
6. REFINING AND PULP CHARACTERIZATION
Fig. 6-2. Refining at 3% consistency (left) shows more fiber cutting with less fibrillation than at 30% consistency. From Making Pulp and Paper, ©1967 Crown Zellerbach, with permission. orifice. The water from the side orifice is collected and measured in a graduate cylinder and reported in ml. Unrefined softwood might be 700 ml CSF, whereas mechanical pulp is about 100-200 ml CSF (Fig. 6-5). AIR COCK
I r ^ I TOP LID HINGES BOTTOM LID CALIBRATED SCREEN PLATE
SPREADER CONE SIDE ORIFICE 1000 mL GRADUATE CYLINDER
CALIBRATED ORIFICE
Fig. 6-3. Schematic diagram of the Canadian Standard freeness test in progress.
Fig. 6-4. The Canadian Standard freeness device.
INTRODUCTION TO REFINING
141
650
en o d Q
8
40
80
100 120 140 CSFFreeness,ml for 3 grams
CO
o
g
P
100
200
300 400 500 CSF Freeness, ml for 3 grams
600
700
Fig. 6-5. Comparison of freeness scales for mechanical and chemical pulps. Data from US 0809.
142
^- REFINING AND PULP CHARACTERIZATION
There are other freeness tests that are used around the world. Perhaps the most common one is the Schopper Reigler test, which is similar in concept to the CSF test. TAPPI TIS 0809-01 gives inter-conversions of CSF, Schopper Reigler units, Williams Precision, and Drainage Factor for various types of pulp. Fig. 6-5 has two graphical presentations from the first table of TIS 0809. Since the conversions are not exact, the two lines of each set represent the bounds of the conversion. See Section 17.5 for CSF correction equations for temperature and consistency. 6.2 REFINING Beater A beater is an early device (the Hollander Beater was invented in the 1700s) used to treat pulp to improve the papermaking properties. Beating is a batch process where the pulp slurry circulates through an oval tank around a midsection and passes between a revolving roll with bars and a bedplate with bars. The pulp is at about 6% consistency and emphasizes fiber brushing. The use of these had been phased out at most mills by the late 1970s because they are slow and expensive
to operate. Fig. 6-6 shows a Hollander beater that was in use until the mid 1970s. Some mills retain these for use in mixing stock for small paper machines. Refiners Refiners are machines that mechanically macerate and/or cut pulp fibers before they are made into paper. There are two principal types: disk and conical refiners. Disk refiners have superseded the conical refiners for many purposes, as they offer many advantages. The operation of a conical refiner (Fig. 6-7) is similar to the operation of a disk refiners, except for the geometry of the refiners. In conical refiners, the refining surfaces are on a tapered plug. These surfaces consist of a rotor that turns against the housing and the stator, both of which contain metal bars mounted perpendicularly to rotation. The Jordan refiner, patented by Joseph Jordan in 1858, is one type of conical refiner with a 12° angle on the rotor (with respect to the longitudinal axis); it is suited to low consistency refining (2% consistency) with much fiber cutting. The Claflin refiner uses a rotor with a 45° angle with respect to the longitudinal axis that revolves
Fig. 6-6. The Hollander beater for mill beating of pulp.
REFINING
143
Jordan
Claflin
Fig. 6-7. The Jordan and Claflin conical refiners. Reprinted from Making Pulp and Paper^ ©1967 Crown Zellerbach Corp., with permission. inside the mating shell. The inlet is at the small end of the taper. The Claflin refiner is intermediate in operation to the Jordan disk refiners. These two refiners are shown in Fig. 6-7. Disk refiners became available for papermaking in the 1930s, after the conical refiners. Disk refiners had enjoyed widespread use in
food processing. For example, they are used to process peanuts into peanut butter and corn into cornstarch and flour. The pulp slurry makes one pass between rotating plates equipped with teeth or bars. There are three common configurations: one fixed plate with one rotating plate (Fig. 6-8), two rotating plates that turn in opposite directions.
144
^. REFINING AND PULP CHARACTERIZATION
or a set of two pairs of plates formed by a double sided rotating disk between two stationary 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 indication of energy that does not contribute to refining), are more compact, and are easier to maintain. Disk refiners are also used for production of me- Fig, 6-8. Single-disk refiner (notice the plate segments). The ribbon chanical pulp from feeder (for pulping or high consistency refining) is observed through the wood chips. Tackle right disk. Courtesy of Andritz Sprout-Bauer. (the plates) is easily replaced; a wide variety of tackle metals (Table rotation. The Hollander beater has an angle of 0°, 6-1) and designs (Fig. 6-9) exist for pulping and the early conical refiners have 12° angles, the Claflin refiner has a 45° angle, and disk refiners refining. It is interesting to consider the historical have 90° angles. Generally, the higher the angle, development of beating and refining in terms of the higher the consistency at which refining can the angle of the bars with respect to the axis of occur, leading to lower fiber cutting. Table 6-1. Typical industry refiner plate metallurgies. Courtesy of Andritz Sprout-Bauer. 1 MetaUurgy 1
Hardness (Re)
Corrosion Resistance
Abrasion Resistance
Impact Resistance
Elongation
Fluidity
Cost
I Ni-Hard II White Iron
55-62
> Carbon and < SS
Good
Extremely brittle
None
Good
IX
50-55
Lower than
Good
Britfle
None
Fair
1.5 X
MCK&K-Alloys II White Iron
50-55
> Ni-Hard
Good
> Ni-Hard & X-C
None
Fair
1.5 X
440-C High-Carbon 1 Stainless Steel
55-60
Better than white iron
Less than white iron
Tougher than white iron
l%-2%
Poor
3X
SAl High-Carbon Stainless Steel
50-55
Same as 440-C
l%-2%
j Poor
3X
17-4 PH Stainless II steel (SS)
32-40
Excellent
Very poor
4X
X-C(Hi-C) White Iron
ss
, Same as f 440-C Less than others
j Same as 440-C Best resistance of all
10%15%
REFINING
Low Consistency Refiner Plates for Pulp & Paper Stock Preparation
High Consistency Refiner Plates for Pulp, Paper, Board & industrial
(Twin Flo Refiners)
Mechanical Pulping
Wet Processing
Cutting — Coarse Bar
SIngla-Dlsc/TwIn Refiner
Cornstarch
Fiber Development — Medium Bar
Double-Disc Refiner
Chemical Processing
145
FIberboard
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 consistency refining for fiber approach or hot stock refining. The consistency should be 3.5-5%; consistencies 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 refining, but also increases the refining power required. 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
A 1 1 U
2
^-^^^^^^^^^.^
0.
Applied power
,' ->
Cutting
Fiberizing
—>
Fibrillizing
Hydraulic capacity
Plate Clearance
\
Stock Acidity, pH y ' ^
^.^
A 1 1
-^ V^ ^"^X .'-^ ^^
fU^ o u
. ^€^-^
^^''
/ /
V-\ ^^
^^ . / ^ ^ - ^ ' x ^
J ^
Consistency " ^
Multi-pass_ !
2 /
\
^
Cleanliness
A 1
Applied power,"^^ intensity
/ Plate Life
Refining Action —>
—>
10
9H 8
• 001/002 Cutting • 079/080 Stren. Dev.
• 011/012 Cutting -^le 143/144 Stren. Dev. • 123/124 High Dev. - ^ 007/008 Max. Dev.
I 1 CO
2-
n 15
20
25
30
Refiner Size (Inches)
—T"
35
40
45
Fig. 6-10. The e^ect of refining variables on the refining process (Calderon, Sharpe, and Rodarmel, 1987). Courtesy of Andritz Sprout-Bauer.
REFINING
Fig. 6-11. Valley beater used for laboratory renning of pulp.
147
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 competition 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 commercial 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 nonstandardized 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) /
98-
Breaking length (km)
CSF(lOOsmL)
I
7-
2
65-
c3
4-
z"
^—.,3^
Tear (lOOs)
2" 1-
0m 0
"lOO^
loo
300
400
500" "600 PFI Count
TOO"
800
900
1000
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
Fig, 6-15. Standard pulp disintegrator. insert shows the mixing blade.
The
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 decreases 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 of 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 Hpday/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
discussed in more appropriate sections. Additional specialized tests that are not included here are available in the TAPPI Standards or similar resources. 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 ± 5 T ) 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 successive 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). Mechanical pulps such as PGW, RMP, IMP, and CTMP
have significant fiber curling. These pulps are prepared according to TAPPI Standard T 262, circulating 2% consistency stock at 90-95 °C (194203 °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
diameter holes) and the sample is 500 ml of 0.5% consistency stock. Pulp viscosity The pulp viscosity is a measure of the average chain length (degree ofpolymerization, DP) of cellulose. It is determined after dissolving the pulp in a suitable solvent such as cupriethylenediamine 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 pulping and bleaching operations and to a certain extent are unavoidable. Loss of pulp viscosity must be minimized by proper attention to important 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.
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 measuring 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
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 Standard 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:^':;:::?;;;|l^^^^
20
100
r
^^
/\
80
16
[
1 Y 60
12
[ 1
§
\
BliiiliiiWIBfe
|8
L /
/
\ «»^
0
^
0
»•
40
1
1.0
1
•
1
2.0
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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.
154
^. REFINING AND PULP CHARACTERIZATION
more important than the cell wall thickness; however, all of these fiber properties are interrelated, and no single variable means much alone. Many studies (for example, Kellogg and Gonzalez, 1976) show that mature boles, i.e., outer growth rings that are 10-50 rings from the center, have fibers with the longest length, the largest radial and tangential diameters, and the smallest fibril angles. Once again, these facts indicate that short rotation, plantation grown wood may have decreased strength properties. 6.5 ANNOTATED BIBLIOGRAPHY
Fig. 6-21. Britt jar test. having high tear resistance. The authors cite the work of Runkel who says that if the thickness of the cell wall is less than the radius of the lumen of fibers in wood, the wood makes a good paper. The relative thickness of cell walls is reflected in the fiber coarseness (mg/100 m of fiber). These workers minimized the importance of the fiber length/diameter ratio that many workers feel is
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General aspects of refining (Also see references 14, 15 for the effect of pH on refining) 1. Clark, J. d'A., Pulp Technology and Treatment for Paper, Miller Freeman Pub. Co., San Francisco, 1978, 751 p. (The 2nd ed. 1985, 878 p., is updated for some of the introductory, basic material, but is otherwise very similar).
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ANNOTATED BIBLIOGRAPHY
155
es, pp 145-159 and early refiners, pp 205215 are of related interest; Chapter 12, Nature and effects of beating, pp 257-280; Chapter 13, Mill beating and refining, pp 281-316; Chapter 14, Laboratory beating, pp 317-362; and Chapter 25, Control of beating and refining, pp 560-568.
3.
Root, E.M., Stock preparation, in Pulp and Paper Science and Technology, Vol. 2, Libby, C.E., Ed. McGraw-Hill, New York, 1962, pp 1-39. A lot of useful information on early refining methods such as beaters and the Jordan refiner. This also has a good overview on the physical properties of fillers.
Many chapters from the first edition relate to pulp and fiber quality and testing. Chapter 9, Properties of pulps, pp 181-200; Chapter 11, Moisture content of bales, pp 230-256; Chapter 15, Test sheet making, pp 363-379; Chapter 16, Hand sheet testing, pp 380-400; Chapter 17, Fiber length, pp 402-437; Chapter 18,fibercoarseness, pp 438-449; Chapter 19, Intrinsic fiber strength, pp 450-464; Chapter 20, Cohesiveness, pp 465-487; Chapter 21, Wet fiber compactability, pp 488-503; Chapter 22, Drainage of water from pulp, pp 504-532, Chapter 23, Surface measurements, pp 533-540; Chapter 24, Wet-web strength, pp 541-559; Chapter 27, Characterization and control of mechanical pulps, pp 579-602; Chapter 28, Chemical and microscopical analysis, pp 603-613; Chapter 29, Optical characteristics, dirt, and shives, pp 614-638; and Chapter 32, Formulas for pulp properties, pp 680-698.
4.
Spencer, H.S., N.G.M. Tuck, and R.W. Gordon, Beating and refining, in Pulp and Paper Manufacture, Vol 5, MacDonald, R. G., Ed., McGraw-Hill, New York, 1969, pp 131-185. Scanning electron micrographs, fundamental aspects of refining, equipment, and refiner plates are included in this article.
Clark describes the properties of fibers (discussed in chapters 17-21) for papermaking in terms of five of their principal properties: the length-average fiber length, the coarseness (mg per 100 m of fiber, measured by Tappi Standard T 234, with coarse fibers giving lower strength due to poorer bonding), wet compactability (measured by the apparent sheet density, for example, TAPPI Standards T 205 and T 220), the intrinsic fiber strength, and the fiber cohesiveness.
6.
Calderon, P., P.E. Sharpe, and J.L. Rodarmel, Low consistency refiner plate design and selection. Spectrum (SproutBauer), Summer, 1987, 7 p.
Pulp surface area 5. Casey, J.P., Ed., Pulp and Paper Chemistry and Chemical Technology, 2nd ed. (1960), volume 2, p 621 has a discussion on the surface area of pulps. The more sophisticated types of studies mentioned use silver deposition or gas adsorption to measure the total available surface area of pulps. These studies give reasonably close values to each other (within a factor of two) for surface area. Fig. 6-23 is an approximation of the results of Robertson and Mason (1949) cited in this work. Robertson, A.A. and S.G. Mason, Specific surface area of cellulose fibres by the liquid penetration method. Pulp Paper Mag. Can, 50(13): 103-110(1949).
Fiber morphology and performance 7. Dadswell, H.E. and A.J. Watson, Influence of the morphology of woodpulp fibres on paper properties. Transactions of the "Formation and Structure of Paper" Symposium, 25-29 Sept., 1961, pp 537-572 (1962). (See also Appita 14(5): 168-176(1961) and ibid. 17(6): 146-156(1964).) 8.
R.B. Brown, Paper Trade J. 95(13):2729(1932).
156
9.
<5- REFINING AND PULP CHARACTERIZATION
Kellogg, R.M., and J.S. Gonzalez, Relationship between anatomical and sheet properties in western hemlock kraft pulps. Part I. Anatomical relationships. Transactions Tech, Soc, 2(3):69-72(1976).
10. Smith, W.E. and Von L. Byrd, Fiber bonding and tensile stress-strain properties of earlywood and latewood handsheets, USDA, For. Res. Ser. Res. Pap. FPL 193(1972), 9 p. 11. Horn, R. A., Morphology of wood pulp fiber from softwoods and influence on paper strength, USDA, For. Res. Ser. Res. Pap. FPL 242(1974), 11 p. This study concluded that the fibril angle of fibers was the most important factor in stretch properties of paper made from unrefined, softwood pulp and an appreciable factor in stretch properties of paper made from refined, softwood pulp. 12. Dinwoodie, J.M., The relationship between fiber morphology and paper properties: A review of the literature, Tappi J, 48(8):440447(1965). This includes 115 references. 13. Megraw, R.A., Wood Quality Factors in Loblolly Pine, Tappi Press, 1985, 88 p. This book is subtitled "The influence of tree age, position in tree, and cultural practice on wood specific gravity, fiber length, and fibril angle". Fiber chemical properties and performance 14. Scallan, A.M., The effect of acidic groups on the swelling of pulps: a review, Tappi J. 66(11):73-75(1983). This has 31 references. Acidic groups of pulp, i.e., carboxylic acids from hemicelluloses and possibly sulfonate groups in sulfite-pretreated mechanical pulps or unbleached or semi-bleached sulfite chemical pulps, contribute to fiber swelling in water. Fiber swelling contributes to ease of refining and development of paper strength. At pH below 7 the carboxylate groups are
protonated and no longer dissociated, decreasing fiber swelling. At higher pH, the metal counter ion plays a big role in fiber swelling with M+ > M^"^ > M^+. Also, it is recognized that swelling increases with the series H+ < Ca^"' < Mg^^ < NH4+ < Na+. Pulps bleached with hydrogen peroxide are often treated magnesium. It may be usefiil to wash these pulps with some dilute alkali to improve their papermaking properties. Alum would have a detrimental effect on pulp swelling and should be kept out of the process until after refining. 15. Lindstrom, T. and G. Carlsson, The effect of chemical environment on fiber swelling, Svensk Papperstidn 85(3):R14-20(1982). Swelling of pulps under a variety of pH and metal ion conditions was determined by the water retention values (WRV). Bleached softwood sulfate pulp was fairly insensitive to changes in WRV under these conditions since it had only 2.5 meq/100 g (acidic groups on pulp). An unbleached softwood sulfate pulp (47% yield with 38 kappa number and 7.4 meq/100 g acidic groups on pulp) had larger changes in WRV going from a low of 200 to a high of 260. The maximum WRV occurred at pH 8-9.5 with NaCl concentration below about 0.01 M. 0.05 M NaCl decreased the WRV to 220 at pH 9, and a NaCl concentration of 0.5 M decreased it to 200, the base value. The tensile index was directly proportional to WRV. These results indicate that one should keep the ionic strength of pulp low during pulping and pulp storage before and during papermaking. Pulp viscosity 16. Siholta, H.,B. Kyrklund, L. Laamanen, and L Palenius, Comparison and conversion of viscosity and DP-values determined by different methods. This is the most extensive and about the only publication on the subject. Paperija Puu (4a):225-232(1963).
EXERCISES
157
EXERCISES Refining 1. Plot representative tensile strength, tear strength, CSF, and specific surface area of paper made from pulp as a function of refining energy (time) applied to the pulp. 2.
Describe how refining helps fiber to fiber bonding in the final sheet. Give three mechanisms for its action.
3.
Describe the effect of pulp consistency on the refining process in terms of bar-fiber contacts. Use Fig. 6-24 to help you.
4.
What is the major drawback of any laboratory refining process?
5.
Give two methods whereby the level and effectiveness of refining is measured.
6.
One is visiting a mill using redwood fiber and sees a Jordan refiner. Is this unusual considering very few mills use Jordan refiners anymore?
7.
Using the discussion following references 14 and 15, describe why refining and papermaking at elevated pH (say pH 8 versus pH 4) often leads to a significant increase in the strength of the paper. When is this especially true?
As the upper bar passes the lower, low consistency will favor either trapping single fibers and cutting them, as in B, or stretching and breaking them, as in Views C and D.
At higher consistencies, the fibers will tend to form a blanket or cushion on the bars, leading to crushing rather than a cutting action on the fibers. Fig. 6-24. Effects of consistency on refining. ®1991 James E. Kline. Reprinted from Paper and Paperboard with permission.
Pulp characterization 8.
How are fiber size distributions measured?
9.
What is the significance of fiber size distributions?
10. What is the purpose of making laboratory handsheets?