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Ind. Eng. Chem. Rod. Res. Dev. 1982, 21, 150-153
directional cross section of the web (Marton, 1974). The bonding efficiency is strongly affected by the accumulation of pigments in the felt side layers. The finer the particle size, the higher the debonding effect. On the other hand, the filler distribution is relatively uniform in handsheets made on standard laboratory sheet mold. If pick resistance on the felt side of a 10% ash containing rawstock is studied, it is advisable to prepare a 1618% ash containing handsheet for comparison (Figure 4). The actual ratios naturally vary with furnish composition and basis weight. Better understanding of the interactions where fines play a major role can guide the papermaker to better control of machine stability, to utilize more economically the papermaking additives, and to optimize desired sheet properties. Literature Cited Brecht, W.; Erfurt, H. Tapp/ 1959, 42(12). 959. Brltt, K. W.; Unbehend, J. Tappi 1978, 59(2), 67. Corson, S. R.; Lobben, T. H. Pulp Pap. Can. 1980, 8 1 , T324.
Glertz, H. W. Das Papier 1974, 28, 137. Gossena. J. W. S.; Luner, P. Tapp/ 1978, 59(2). 89. Htun, M.; deRuvo, A. Svensk Papperstldn. 1978, 78, 507 Iwamida, T.; Sumi, Y.; Nakano, J. purp Pap. Can. 1980. 8 1 , T226. Jaycock, M. J.; Pearson. J. L. Svensk PappersrMn. 1975, 78, 167. Marton, J. Tapp/ 1974, 57(12), 90. Marton, J.; Marton, T. Tapp/ 1978, 59(12), 121. Marton, J. Tappl 1980a, 63(4), 87. Marton, J. Tapp/ 198Ob, 63(2), 121. Mohlin, U.-B. Swflsk Pappstldn. 1980, 83, 513. Schurz. J.; Schempp, W. Ce//ul.Cbem. Techno/. 1972. 6 , 99. Steenberg, B.; Sandgren, B.; Wahren, D. Svensk PapperstMn. 1980, 63, 395. Stradzins, E. Tappl 1972, 55(12), 1691. Stradzins, E. Tap@ 1974, 57(12), 76. Stradzins, E. Tappl 1981, 64(1), 31. Stratton, R. A.; Swanson, J. W. Tapp/ 1981, 64(1), 79.
Received for review April 20, 1981 Accepted June 12,1981
Paper presented at the 181st National Meeting of the American Chemical Society, Atlanta, GA, Mar 29-Apr 3,1981; Colloid and Surface Science Division; Symposium on Colloid Science and Papermaking.
Retention, Drainage, and Sheet Consolidation John E. Unbehend’ and Kenneth W. BrM State Univers/ty of New York, College of Envlronmental Science and FW8Sby, Syracuse, New York 13210
The topics of retention and water removal mechanisms on Fourdrlnler paper machines are covered. Concepts of “hard” and “soft” floc systems in paper stock systems are characterized as to their resistance to turbulence and redispersion. Water removal on the Fourdrinier is d i v i i into two parts: gravity/low pressure drainage and removal by vacuum. Different fiber fumishes and flocculants can cause the two zones to react in opposite directions: flocculation may Improve gravity drainage (freeness) but have poor response to vacuum.
Introduction The manufacture of paper may be considered to be the separation of fibrous and particulate solids from a liquid phase to form a coherent network on a moving fabric. Due to the large volumes of water used in this process, some of the particulate material (fine fibrillar matter derived from the mechanical refining of the fibers plus added mineral pigments) may not deposit on the fabric as readily as the fibrous components. This leads to low one pass retention as these fine materials pass through the fibrous mat and fabric into the white water system. These materials also hinder the drainage of the liquid phase by blocking the tortuous fiber interstices, slowing the flow from the network. This paper addresses the possible mechanisms involved in fines retention and water removal in the forming and vacuum zones of the Fourdrinier paper machine. Basic Considerations The removal of the liquid phase takes place in four distinct zones. (1) Forming Zone. Separation is by a filtration diffusion mechanism and is characterized by large volume flow of water, low consistencies, and relatively low head. Due to the greater degree of hydrodynamic shear, a large amount of the particulate material can be dislodged from the forming mat and carried through into the white water. (2) Vacuum Zone. Here the water removal mechanism is that of one fluid (water) being displaced by another fluid 0196-4321 18211221-0150$01.25/0
(air) in the porous network of the sheet. Because of the progressive consolidation of the sheet, much of the fine material has been immobilized and mechanical entrapment prevents further loss. (3) Press Zone. Water is squeezed out of a compressible body (wet sheet) into a less compressible, dryer body (felt). (4) Dryer Section. The remaining water must be evaporated by application of heat. The cost of removal of water at each of these points increases from the forming zone to the reel. More water removed in the vacuum zone leads to better press efficiency and each percent increase in solids in the presses yields 4 4 % energy savings in the dryer. The adsorption and retention of fines also relates to cost effectiveness. Low first pass retention of fines results in a buildup in the headbox system. During recirculation, this extra load of fines hinders the liquid drainage and reduces water removal rate, thereby slowing the machine down. In addition to the fines inherent tendency not to be retained, changes in machine design tend to accentuate retention problems. Faster water removal, increased speed, twin wire formers, greater vacuums, and reduced basis weights all tend to generate greater hydraulic shear, reducing the degree of fines retention. It would appear that the steps in water removal are essentially hydromechanical and are accomplished by mechanical devices that combine to form the paper ma@ 1982 American Chemical Society
chine. The major effort in seeking improvement of manufacturing efficiency has been in machine design. There is little doubt that this will continue to be of major importance; however, recent developments in the concepts of surface and colloid chemistry may bring more attention to improvements in this direction. A dramatic innovation that has come into the picture in recent years has been the utilization of synthetic polyelectrolytes of a wide range of molecular weights. These give the ability to flocculate fines and filler increasing retention and modifying water removal. These polymeric additives have met a real need in the paper industry and show even greater promise for the future. Fines Retention The ability of a fibrous network to retain the fines materials during the forming process is very dependent upon the degree to which the fines are coagulated or flocculated (either with other fines or with the fibers themselves), and the resistance of these aggregates to break down upon exposure to high levels of shear. In this section, the mechanisms and types of coagulation/flocculation that take place in the headbox system will be reviewed. The flocculated systems resistance to the types of redispersion and high shear environments encountered in the headbox and on the fabric during sheet formation will also be discussed. Experimental Section Retention and fiies fractionation have been carried out with the Dynamic Retention/Drainage Jar (DDJ) (Unbehend, 1977; Britt and Unbehend, 1976). Fines are defined as that material passing a 76-rm screen and are determined by successive washings on the DDJ (TAPPI, 1979). Dynamic retention (Unbehend, 1977) is measured by exposing paper stock to graduated levels of turbulence and comparing the amount of fines free in the liquid phase to those adsorbed on the fibrous fraction. A plot of percent fines retained vs. turbulence allows comparison of various stocks and additives as to their fines retention index. For further description of the apparatus and procedures, see the references noted above.
Hard and Soft Flocculation The sheet-forming environment in papermaking is distinguished in a number of ways from the classical systems traditionally used to study coagulation and flocculation. First, the suspension of particles has a broad range of particle shapes and sizes, chemical composition, and surface properties. Second, the high degree of turbulence encountered during the initial sheet forming stages is in sharp contrast to the static conditions employed to observe most colloidal systems. Because of this combination of unknown and uncontrolled factors, it has been necessary to depend on empirical experimental observations and less on recognized theory. These observations came through the paper machine or work with laboratory devices capable of delivering graduated turbulence. Experiments of this nature led to the conclusion that retention of the fines fraction was closely related to the degree of turbulence it was exposed to. Also, the flocculating and coagulating additives or “retention aids” added to the stock suspension could be classified over a wide range as to their ability to withstand exposure to and increasing levels of turbulence. In previous publications these have been classified “soft” and “hard” flocs (Unbehend, 1976). (1) Soft Floc. This is a combination of fiber and fines that exhibits good fines retention at low turbulence levels. Upon exposure to more turbulence, the retention decreases
20-
TURBULENCE, rpm
Figure 1. Effect of mechanical redispersion on hard floc (bridging mechanism) and soft floc (charge neutralization) systems.
substantially, but when allowed to reflocculate, it will return to original fines retention levels. (2) Hard Floc. This is a fines/fiber floc that shows good fines retention over a wide range of turbulence for brief period of exposure. Subsequent exposure will cause a breakdown in retention that cannot be reestablished after the system is allowed to reflocculate. The explanation of the mechanism for the action of these two characteristic flocculations can be taken from some of the theoretical work done on flocculation with salts (Overbeek, 1979)and polyelectrolytes (Gregory, 1973). The soft floc acts in the manner described in the definition by proceeding via a charge neutralization mechanism. With the addition of multivalent cations ( A P , Mg2+)or low molecular weight PEI (polyethylene imine), the overall charge of the surface will be reduced allowing flocculation to take place due to attractive forces between the particles. When the particles encounter shear, the forces of attraction are not sufficient to hold them together. As the turbulence subsides, the particles are free to readsorb on collision and flocculate to the same degree. The curve for flocculation index with MgClz in Figure 1 gives a good example of the soft floc mechanism. The solid dots are the points for initial flocculation. The X’s are the levels of retention after the flocced suspension was exposed to propeller agitation at 1000 rpm for 30 s in a vaned jar. The retentions are equivalent because the surfaces are not changed after exposure to turbulence. The hard floc seems to be the result of the establishment of bridges of polyelectrolytes between two particle surfaces (Britt and Unbehend, 1980). Here the establishment of a physical link beyond the double layer results in a strong bond between the two particles (Lamer and Healy, 1963). When a high molecular weight polyelectrolyte with an appreciable charge density adsorbs on a surface, it does so in a series of loops and trains (Silberberg, 1962). If these are long enough to extend beyond the double layer, they will be available for bonding with another surface (Figure 2a). When collision occurs, the particles flocculate (Figure 2b). When these systems are exposed to shear for a long period of time, the flocs tend to break down (decrease in retention on stirring) and do not tend to completely reestablish on standing. Part of the explanation may be that as the floc breaks down and the particles separate, the bridging molecules either stay with one particle or break apart. The remaining or partially freed polymer, being subject to strong hydrodynamic forces, will undergo reconformation on the surface. Areas of opposite charge are accessible and may cause the polymer to “lie down”
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Table I. Effect of Refining tNITlAL ADSORPTION
INITIAL FLOCCULATION b
refining time, min
5 25 35 45
freeness, mLCSF
water retention value, WRV,%
vacuum,%
575 191 156 46
42.4 32.5 29.8 25.0
18.4 22.4 25.1 20.2
dryness
after
Table 11. Vacuum Response of Various Pulps RECONFORMATION C
-
dryness
-- _ _ -- _-- - - - -- - _ sample
REFLOCCULATION d
Figure 2. Reconformation of adsorbed polyelectrolyte on exposure to shear and reflocculation.
(Figure 2c). As turbulence subsides, the flocs that reform will tend to be more like that of the charge neutralization type formed by the low molecular weight patch of the soft floc (Figure 2d). This mechanism is suggested by the action of the system flocculated with cationic polyacrylamide in Figure 1. The initial flocculation gives a high degree of retention. If the initial flocculation is exposed to a redispersion (1000 rpm for 30 s, vaned jar), and then tested for retention, the overall level drops (curve with the open circles), indicating reconformation of the adsorbed polymer. If a flocculated system is exposed to two redispersion cycles, there is a further drop (more reconformation). A three-cycle redispersion changes little from two; therefore, the polymer has reached a stable conformation on the surface and is acting like a charge neutralization induced soft floc. These classifications should not be judged “better” or “worse” or “effective” or “ineffective”. These only have meaning when applied to a particular situation. Basis weight and machine speed determine the severity of the retention problem. Low basis weights and high machine speeds place greater stress on a retention aid’s capacity to produce a tenacious floc. On a high basis weight product like board, a soft floc may be all that is required. Use of a polyelectrolyte that produces a hard floc may also produce over-flocculation resulting in poor formation and reduced dewatering efficiency.
Water Removal The work with retention has reinforced our awareness that retention and drainage and/or water removal are closely related phenomena. I t has been widely observed that the addition of a flocculant to a headbox stock, especially one with high fines content, greatly increases the freeness. This led to the marketing of polyelectrolytes as “drainage aids”. What was not recognized until recently was that this may produce adverse effects in the second zone of water removal which utilizes application of relatively high vacuum. Observation of this contradictory relationship between increased drainage and reduced vacuum efficiency was made on our pilot plant paper machine. Subsequent modification of the Dynamic Drainage Jar was effected to allow determination of response of water removal by vacuum of a variety of pulps and stocks with various additives. Experimental Section The response of paper stock to water removal by vacuum was measured using Vacuum Water Release Analyzer (Britt and Unbehend, 1980). A pad is formed under re-
(1)unbl SW kraft,
after vacuum,
vacuum,
%
in.Hg
freeness, mL CSF
final
unbeaten ( 2 ) unbl. SW kraft, beaten
9.6
2
700
16.0
6
250
(3) groundwood
21.1
16
85
Table 111. Effect of Polymer Additives on Vacuum Response dryness
additive none dual polymer, 0.03% dual polymer, 0.06% dual polymer, 0.11% dual polymer, 0.22% dual polymer, 0.22% redispersed
after vacuum, %
freeness, mL CSF
24.6 20.6 19.7 16.4 12.3 21.5
230 335 410 54 0 650 560
duced vacuum (8 in.Hg) and then exposed to full available house vacuum for a fixed period of time. The dryness attained is used as a measure of the vacuum necessary to pull water from a given composition of paper making stock or pulp. Vacuum Release The initial observations on the pilot paper machine showed that the application of a very strong flocculating system which moved the dry line on the Fourdrinier (also showing a great freeness increase in the lab), suggesting much better drainage, resulted in a wetter sheet at the couch than when no additives were present. This observation was reenforced by those of machine operators who had reported trials with drainage aids produced wetter sheets at the couch when compared to a control. Examination of the sheet showed obvious loss in formation. The corresponding loss in sheet consolidation and uniformity probably accounts for the decrease in water removal by the flatboxes, accompanied by a drop in flatbox vacuum. With this evidence in hand, the Dynamic Drainage Jar was modified to allow analysis of the water release properties of the paper stocks in response to vacuum (Britt and Unbehend, 1980). The data generated with this apparatus appear in Tables 1-111. If is felt that the desired action of vacuum applied to a wet sheet is the displacement of one fluid (water) by another fluid (air). To achieve maximum efficiency in this operation, sheet consolidation and uniformity is of vital importance. Lack of uniformity leads to channeling of the air through the areas of least resistance resulting in the inefficient displacement of water. This uniformity is a function of the pulp furnish, stock preparation, and chemicals added. The sheet structure that affects the response may be macro in character as to be visible to the eye as formation. There does not appear to be a micro
Ind. Eng. Chem. Prod. Res. Dev., Vol.
structure that is not notably visible but still affects air channeling and response to vaccum. The following give specific examples. The effect of refining on drainage and dryness response is shown in Table I. The unbeaten stock, although exhibiting a high freeness, shows poor response to vacuum. This response improves rapidly with refining and is reflected by an increased dryness. This is probably a result of improved sheet consolidation and uniformity due to increaeed flexibility of the pulp fibers. After going through a peak in vacuum response, a decrease is noted. The water retention value increases throughout this refining cycle and is a good indication of the degree of swelling (extent of water of hydration). It is believed that the vacuum response is a function of both sheet structure and water held by fiber swelling. As long as the structure is becoming more homogeneous and consolidation effects predominate, the vacuum response will improve. At a later point in the refining this effect levels off and the swelling then predominates; hence, vacuum response declines. Table I1 summarizes the effect of vacuum on different kinds of pulps with varying degrees of refining and different levels of freeness. The unbleached southern pine kraft in the unbeaten condition exhibits a high freeness (good gravity drainage) but a low dryness. This, combined with the low final vacuum, indicates the stiff unbeaten fibers are forming an open, porous structure with poor formation (the latter is obvious by appearance of the pad). Beating drops the freeness (probably due to development of fines), but the dryness and final vacuum go up, indicating better consolidation which promoted the dewatering action of the vacuum. In a majority of the situations tested, the most effective structure is indicated by a high final vacuum during the test (a measure of the resistance of the sheet to the passage of air). Groundwood is notorious for slow drainage on the freeness tester, but a newsprint furnish can be run at high speeds. The information in line 3 of Table I1 suggests a reason for the ability to run at higher speeds. Even though the water drains slowly under gravity (low freeness), the response to vacuum is excellent due to the homogeneous pad that is formed. Because of the even formation and compact consolidation, the vacuum can act to maximum effect resulting in high dryness. Finally, Table I11 shows the effect of a polymer flocculant on dryness/drainage. The dual polymer addition of an intermediate molecular weight cationic followed by a high molecular weight anionic produces a tenacious, hard floc (Britt, 1973). Without any applied redispersion (as in this case), the formation becomes less uniform as the dosage is increased. The increasing dosage is characterized by increasing freeness (drainage) and decreasing vacuum response. This is precisely why many drainage aids fall flat during machine trials. The water removal in the first zone of the Fourdrinier is improved (freeness increases), but there is such a negative effect on vacuum response that any increased drainage is counteracted resulting in a wetter sheet at the couch. The free drainage of the stock system and good vacuum dryness response are maintained when the flocced suspension is exposed to a measured amount of shear that disperses the macroflocs (improving formation), but that is not enough to degrade the microflocculation (as to decrease fines retention). Good drainage on
21, No. 2, 1982
153
the first section of the Fourdrinier table is possible in addition to high dryness after the vacuum flatboxes. The adverse effect on dryness seen in some cases does not mean that most retention aids lead to losses in vacuum response;however, both suppliers and papermakers should be aware of possible problems and guard against them. There are retention aids and physical conditions that are quite safe. The data do show again, however, that freeness is not a good indication of overall water removal on the paper machine. This approach to vacuum water removal is empirical in nature but does allow differentiation among effects of pulp, levels of refining, chemical additives, etc. Theoretical treatmenta of thistopic have been attempted, but the point should be made that understanding of this area is far from complete. Summary Flocculation can be described as being soft or hard, depending on its ability to withstand various degrees of turbulence. Generally soft flocs are produced by low molecular weight salts or polyelectrolytes and act via a charge neutralization mechanism. Hard flocs hold up well under turbulence and are produced by high molecular weight polyelectrolytes via a bridging mechanism. The hard floc can be degraded to a soft floc/charge neutralization mechanism when exposed to excess degrees or periods of turbulence. Water removal on the Fourdrinier is broken down into two parts-free drainage (as modeled by the freeness test) and vacuum drainage (as modeled by the vacuum water release analyzer). Many times the two operate contrary to one another with the freeness decreasing as the vacuum response increases. The vacuum response seems to be enhanced by refining as the fibers become more flexible and fines are produced. This leads to more homogeneous consolidation, allowing optimum action of vacuum on the fibrous network. Poor formation and/or a porous mat allow the vacuum to be satisfied by air passing through thin spots or open areas in the sheet instead of compressing the sheet and removing the maximum amount of water for each individual furnish. The wet end of a paper machine combines a number of interrelated colloidal phenomenon that cannot be treated independently. When the retention of fines solids is being considered, the resulting effect on the free drainage of liquid and the vacuum response must be considered. Literature Cited BrM, K. W. Tappl1878, 56, 83. B r t , K. W.; Unbehend, J. E. Tappl 1876, 59, 67. BrM, K. W.; Unbehend, J. E. Tappi 1880, 63,67. Gregory, J. J . COlloM Interface Sci. 1873, 42. 448. La Mer, V. K., Heely, T. W. Rev. Pure Appl. Cham. 1883, 13, 112. Overbwk, J. Th. (3. J . C o l M Interface Sci. 1878, 52, 408. Sllberberg, A. J . Chem. phvs. 1862, 66, 1872. TAPPI Standard 261. pm-1979. “TAPPI Offlclal Standards”, Technlcal Association of the Pulp and Paper Industry, Atlanta, GA, 1979. Unbehend, J. E. Tappi 1876, 59, 74. Unbehend, J. E. Tappi 1977. 60, 110.
Received for review August 3, 1981 Accepted December 14, 1981 This paper was presented at the 181st National Meeting of the American Chemical Society, Atlanta, GA, April 1981, as part of the S y ” on Phpiochemical Properties of Colloid Particles: Colloid Science and Papermaking.
