B. Bruce Sithole and Larry Allen
In this report, we review potential problems related to wood extractives that may be encountered when mills implement system closure processes. We explore some issues that may have to be addressed as mills close their process water circuits, and propose ways to avoid and combat wood resin deposition problems. The effects of wood resin on system closure will often depend on how the closure process is undertaken. To avoid excessive build-up of wood resin and other non-process elements in system closure, it may be necessary to minimise the amount of wood resin introduced into the process, or send bleach plant filtrates back to the recovery system. It might be helpful to have points of wood resin removal in the process, and to install effective metals removal processes. Since many of these options are available, system closure in pulp and paper mills may be instituted without too many problems being caused by wood resin.
Dissolved and colloidal substances (DCS) in pulp and paper manufacture include metal ions, inorganic anions, and organic compounds such as sugars, lignin's and wood resins (Table 1). Since experience shows that the concentrations of these materials can increase with system closure it is important to understand the impacts that they will have on system closure. Several reports have been written on the impact of compounds present in DCS on system closure1-4 but none that specifically target wood resin.
Table 1. Some dissolved and colloidal substances in pulp and papermaking
The purpose of this report is to review potential problems from wood resin and how they might be avoided. We feel that system closure should be possible if adequate precautions are taken in how it is done and if an effective pitch control program is implemented.
Wood extractives deserve serious consideration because of their potential adverse effect on various aspects of pulp and papermaking. Wood extractives comprise about 1-5% of the weight of wood but the problems they cause in open systems are disproportionately much larger than their concentrations. These problems are expected to be exacerbated by mill closure5. A recent presentation by Adams6 lists 18 papermaking issues that will be affected by system closure. A similar listing by Foster and Rende7 estimates that the issues will result in increases ranging from 13 to 110% in the cost of chemicals used to tackle the issues. A close look at these issues, reproduced in Table 2, reveal that the majority are strongly related to wood extractives, as will be detailed later in this report.
Table 2. Major issues in closing up paper making systems
An example of the flows and build-ups of DCS in a mill process is shown schematically in Figure 1 for a paper mill.
Figure 1. Schematic illustrating the flow of DCS in a paper mill
It is evident that if the paper mill were to be completely closed, the DCS either have to leave with the product or circulate and build-up in the paper mill. When some dissolved solids in the white water system reach a certain concentration, they begin to precipitate and interfere with paper making. Studies on closing water systems of various mills indicate that with increasing system closure, the concentration of DCS increases exponentially at high closure levels resulting in a very high increase of these elements in highly closed systems8. The increase is more rapid for recycled and wood–containing paper mills than for wood-free coated paper mills, as shown in Figure 2. Below 10 m3/t, the concentration of dissolved solids increases very rapidly. According to Adam6, while mill process conditions will determine the level of closure where solids begin to precipitate, most mills will experience precipitation when closure is in the 10 to 15 m3/t range. In practice, many mills may not reach the steep part of the curve and if they do (e.g., when operating at 2 m3/t) the low volume and high dissolved solids concentrations are often acceptable for combustion. As will be related later, data by Wearing and co-workers9 show that the steep part of the curve can be avoided, for example, by installation of inter-stage washing in a news mill.
Figure 2. Dissolved solids concentration as a function of system closure
Also, a recent report from Finland shows it should be possible to reduce 50-60% of a mill's fresh water without affecting productivity or product quality10. Nevertheless, it is important to understand the impact of DCS on system closure. In this report, we describe the potential impact of wood extractives on system closure.
PITCH DEPOSITION PROBLEMS
The problems of pitch deposition attributed to wood extractives have been well documented11-13. Suffice to say that pitch deposition problems cost the Canadian industry several hundred million dollars annually. Let us examine some of the factors causing problems.
Dispersed wood resin
Deposition due to metal soaps will be particularly problematic in the bleach plant. Brownstock washing can recover 95 -98% of dry solids in pulp. The remaining 2-5% carried with the pulp can contribute significantly toward mill BOD values and also result in increased bleach chemical demand17. In system closure the importance of good washing will be critical since carryover to the bleach plant and between stages can cause considerable process problems. Dissolved and colloidal substances such as wood resin and metals not removed in the washer prior to bleaching stages are more concentrated in a closed bleach plant than in conventional open bleach plant. For example, severe deposit problems were experienced during closed mill operation at Great Lakes Forest Products Ltd., Thunder Bay. Pitch, scale, and defoamer residue deposits plugged washer fabrics and wires, washer shower nozzles, and filtrate lines, forcing the mill to abandon the use of D/C stage filtrate that was used at the brownstock washers18. Reeve19, one of the pioneers in bleach plant closure, reported that experiences at the Thunder Bay mill pointed to several issues that had to be resolved before another attempt could be made at recovering bleach plant filtrates. One of the issues was effective removal from the system of DCS such as potassium, calcium, and wood resin19-20. Similar experiences occurred at the MoDo mill at Husum, Sweden, where system closure caused build-up of organic and inorganic deposits on some of the equipment21.
Sodium salts of wood resin are innocuous in kraft open systems due to their solubility in aqueous environments (exceptions are sodium salts of dextropimaric and levopimaric acids that are very insoluble22). However, the presence of high ionic concentrations dramatically reduces their solubility. Work done by Palonen et al.23, shows that the presence of 0.75 mmol/L NaCl makes sodium soaps of oleic and abietic acids insoluble (Table 3). This implies that a new kind of deposition problem due to sodium soaps of wood resin may be encountered in closed mills.
Table 3. Effect of salt concentration on solubility of sodium salts of wood resin components
Fortunately, the majority of sodium soaps are soluble in water and therefore will not cause deposition problems. The few that are tacky include sodium linolenate and potassium soaps of stearic and palmitic acids24. One may wonder how the NaCl concentrations used in the study by Palonen et al.23 compare with present mill values. The NaCl concentrations in open mills vary widely depending on the sampling point. For example, a survey at a Canadian mill showed that the concentration of sodium in the water used for recausticizing and bleaching was 6 x10-3 mol/L25. Corresponding values for whitewater and weak wash samples were 36 and 1,4200 x 10-3 mol/L, respectively.
IMPACT ON PULP WASHING
The behaviour of wood resin during pulp washing is important in kraft pulping. It is influenced by dispersed and dissolved forms of the wood resin. Highly alkaline black liquor contains fatty and resin acid soaps which are ionic surfactants. The ability of these surfactants to act as dispersants or solubilisers of wood resin components that survive the alkaline pulping process is dependent on ionic strength, temperature and pH32. Conditions in different stages of washing will determine the physical form of the wood resin. Studies by Swedish workers have shed some light on this23.
Figure 3. Effect of temperature and displacement washing on wood resin content of wash water (adapted from refs. 36, 37, 69)
Laboratory studies have shown that removal of wood resin by washing is impeded by high ionic strength, as shown in Figure 3; the high ionic strength (>0.5 M NaCl) decreases the solubility of wood resin soaps thereby causing them to adsorb onto fibers13. Lahdesmaki33 conducted a mass balance of wood resin in a brownstock washer line and found that the amount of wood resin sorbed on and/or entrapped in fibres was highest at the blow tank, decreased by almost 50% at the first washer, was almost negligible at the second washer, and then increased slightly at the third and fourth washers. Ström et al.32 took a closer look at the data and replotted them against the concentration of sodium ions in the system. This enabled them to conclude that the sodium ion concentration affected the distribution of wood resin as follows: In the blow tank the concentration of sodium ions was high (2 molar) and the solubility limit of the soap micelles was exceeded and the resin therefore precipitated onto the fibres. At the first washer, the amount of sorbed resin was strongly reduced because its solubility in 0.7 molar sodium ions is considerably higher than in 2 molar Na+ but, in any case, the solubility of the resin was still exceeded and therefore some resin remained precipitated and sorbed onto the fibres. The resin was completely soluble at the second washer, consequently, there was little resin on the fibres. The slight increase in the sorbed resin at the third and fourth washers was due to the critical micelle concentration (cmc) of the soaps which is higher than the actual concentration of ionised soaps.
These observations imply that efficient removal of wood resin in pulp washing requires maintaining a low cmc and a high stable salt concentration of 0.1-0.3 M NaCl. This can be achieved by choosing a proper soap composition which appears to be between 1:1 and 2:1 for a fatty acid/resin acid soap ratio. The ratio can be achieved by addition of tall oil in the digester, as is done in some mills that pulp birch and aspen. System closure will result in high concentrations of wood resin and metal ions, and these may result in poor washing because they affect the colloidal stability of resin particles. Conditions that will give high stability and low retention of resin on the fibres include low concentration of calcium ions, a high concentration of lignin that stabilises the colloidal particles and also reduces the concentration of free calcium ions, and a moderate ionic strength32,34. The metal ions may be highest in mills that pulp hardwoods since these woods contain four times more calcium ions than softwoods35.
Work by Assarsson36 and Laxén37 on laboratory and mill samples has shown that temperature and ionic strength affect removal of wood resin during pulp washing. A high wash water temperature and/or a hot wash displacement liquor maximises removal of wood resin from fibres (Figure 3). Lower temperatures (below 60oC) lead to recoagulation of wood resin onto fibres and the wood resin will not dissolve until the ionic strength has dropped to low levels in the latter part of the washer line.
IMPACT ON PULP BLEACHING AND MECHANICAL PULP BRIGHTENING OPERATIONS
The build-up of dissolved organic materials (wood resin) is said to profoundly affect the brightness of mechanical pulps. For example, Figure 4 shows that the presence of high extractives concentrations in pulp results in lower brightness levels before and after both hydrosulfite brightening and peroxide bleaching processes38. Later studies by Ekman et al.39, showed that alkaline peroxide bleaching affected reactive conjugated double bonds in softwood resin acids but was less reactive with fatty acids, including unsaturated acids. Other components such as triglycerides, sterols and steryl esters were also not affected40. Indeed, some additional dispersion of pulp extractives occurred resulting in higher concentrations of colloidal resin in a peroxide bleaching stage41.
It is of interest to note that wood resin is not affected by bleaching techniques using oxygen and peroxide42. For example, studies by Laamanen43 showed that wood resin components of kraft hardwood pulp were the same before and after peroxide bleaching. Ozone, on the other hand, caused considerable decrease in resin content of birch kraft pulp43. The author found that vigorous mixing in ozonation caused extensive surface oxidation and the extractives on the surface reacted with the ozone, resulting in a large number of polar substances. This is beneficial as it facilitates removal of resin in pulp washing.
CONSEQUENCES OF INCREASE IN TEMPERATURE IN SYSTEM CLOSURE
Depending on the process and the closure strategy employed, system closure can result in elevated process operating temperatures9 . For example one report shows that a closed recycled paperboard mill operates at temperatures of around 75oC44. How will this affect the behaviour of wood resin in system closure?
A couple of studies have shown that increases in temperature affect pitch deposition. Gustafsson, et al.45 demonstrated that the influence of temperature on the amount of pitch deposition on copper and stainless steel surfaces was dependent on the pH of the suspension: at pH 5, the deposition was highest at lower temperatures (10oC ) and rapidly decreased with increase in temperature stabilising at temperatures in excess of 30oC; at pH 7, maximum deposition was attained at temperatures between 30 and 50oC. Later studies by Hassler46 essentially confirmed the results: at pH 3 deposition was also highest at the lower temperatures and decreased with increase in temperature; at pH 8 the deposition was lowest at 25oC, increased to a maximum at 60oC and then decreased with increase in temperature beyond 60oC. Studies by Back12 showed that the effect of temperature on deposition was dependent on the surface of the material: increase in temperature from 12oC to about 60oC resulted in reduced deposition on metal surfaces (copper and stainless steel) and increased deposition on fabrics.
The effect of temperature on pitch deposition may be dependent on the chemistry of the wood components in the system. A study by Dreisbach and Michalopoulos47 showed that the deposition potential of an abietic acid pitch was minimal at neutral pH and low temperature, whereas the deposition potential of a fatty acid pitch was at its maximum under the same conditions. Pitch deposits with melting points or softening temperatures significantly greater than the process temperature remained hard and had little tendency to stick to surfaces. On the other hand, pitch deposits with melting points significantly lower than the process temperature remained fluid, and, while they could adhere to surfaces, they did not have sufficient cohesive character to accumulate or to bind other components to surfaces and create deposit problems.
The data from the preceding paragraphs seem to imply that system closure, with its consequent increase in temperature, could result in up to an 85% increase in pitch deposition in alkaline systems. Acid systems, on the other hand, will experience fewer deposition problems. However, the situation in alkaline systems may not be that bad because studies have shown that calcium carbonate in alkaline systems sorbs soaps and probably wood resin too34.
Impact on paper properties
Work by several researchers has shown that wood resin components can influence the coefficient of friction (COF) of paper and board surfaces. Inoue and co-workers52, showed that triglycerides increase the COF of linerboard whereas removal of wood extractives from linerboard by solvent extraction improved the friction properties. A study of the factors influencing the COF of newsprint paper showed that the COF is a function of the amount of extractives that are present on the sheet surface53. This study demonstrated that wood resin components affect COF differently; fatty acids, glycerides, and cholesteryl stearate acted as lubricants and reduced static friction of newsprint whereas ß -sitosterol and abietic acid increased friction. The authors postulated that the rheological properties of the various wood resin components may account for the differences in their friction properties.
System closure may result in more COF problems in wood-containing furnishes, if appropriate measures are not taken to remove or control the wood resin.
WOOD RESIN AS ALLERGENS
Resin acids are a common cause of contact allergy that results in dermatitis54,55. It is a specific allergy that is induced by very small amounts of the resin acids55 and has been estimated to affect 0.7% of the population in Denmark56. Dermatology clinics use patches containing resin acids when testing patients for allergic reactions57. Studies have shown that oxidation products of abietic and dehydroabietic acids are the main culprits of the allergy - the pure compounds themselves do not induce allergies58-60. Persons allergic to wood resin contract dermatitis by frequent handling of papers, as would happen, for example, in reading newspapers61,62. Wood-containing grades of paper cause the most problems since they contain more resin acids than chemical pulp grades. These problems will probably increase with system closure as there will be more resin acids circulating in the system. In addition, it is possible that the higher operating temperatures and constant recirculation of the process waters may accelerate oxidation of the wood resin thus resulting in even more contact dermatitis problems because the oxidised resin causes more allergenic problems62.
WOOD RESIN AS ODORANTS IN FOOD PACKAGING
In food packaging it is important to ensure that no materials will migrate from the paper products to cause odour and taste problems for the consumer. Autoxidation products of wood resin in paper products have been identified as the main sources of odour problems63,64. For example, a study of odour problems in food packaging paper from a Norwegian TCF bleached sulphite mill concluded that a major portion of the odour developed during paper storage was attributable to oxidation of wood resin and degradation of paper additives65. The study concluded by stating that the odour problems are expected to increase with increase in system closure. Glycerides, fatty acids and waxes are considered to be the major wood resin components that cause odour problems66,67. The mechanism of auto-oxidation of wood resin components is complex and is catalysed by metal ions such as Fe, Cu and Mn63,66. It is surprising to note that additives such as latexes and defoamers can also contribute to odour problems67.
Figure 5. The dissolved solids at the headbox can be greatly reduced by segregating the pulp and paper mill
whitewaters and by the incorporation of interstage washing
A detailed description of methods and procedures that can be used to reduce and/or control wood resin problems in pulping and papermaking can be found in a forthcoming book edited by Back and Allen69. Here we summarise a few of the techniques that can be used to reduce the impact of wood resin concentrations and to prevent them from being problematic.
Remove wood resin from the system
A technology for removing dispersed wood resin from pulp and paper mill process waters has been developed at Paprican85. The process entails removal of wood resin by centrifugation; mill trials have shown that colloidal and dispersed wood resin concentrations can be reduced typically by 33-70% in a single pass. Figure 6 illustrates removal of dispersed wood resin from a TMP mill plug screw feeder pressate. It is evident from the figure that an appreciable amount of the dispersed extractives was removed from all samples by centrifugation (shaded bars). Colloidal wood resin in process waters can also be removed by high-consistency thickening of paper stock between stock preparation and the paper or board machine86,87. The colloids in the filtrate should be removed by dissolved air flotation type clarifiers to avoid build-up of wood resin in closed systems.
Figure 6. Removal of dispersed extractives from a TMP mill plug screw feeder pressate. Open and hatched bars give concentrations of dispersed extractives of untreated and treated pressates, respectively. The shaded bars give percent reductions in dispersed extractives
A recent report on removal of extractives during bleaching of radiata pine bisulfite pulp shows that the most practical way of preparing a high brightness pulp with a low content of extractives was by the use of an alkaline extraction first stage, followed by washing and then steep bleaching with interstage washing88. The removal efficiencies for the extractives ranged from 75 to 94%.
Reduce sources of foaming
Foaming problems are often solved by eliminating the entrainment of air. This can be done by avoiding vortexes at pumps and by maintaining good seals in pumps. Unnecessary cascading of stock should be eliminated. Seals on washer downtags must be well maintained. Persistent foaming problems can be combated by using defoamers and/or deaerators that function in high temperature systems. Defoamers and deaerators facilitate coalescence of small bubbles which then rise to the surface and burst5. However, over-use of certain defoamers can result in deposition problems89. A review on defoamers for the pulp and paper industry is available89.
Laboratory studies indicate that removal of extractives by washing is dependent on washer design68. Subsequent mill studies have shown that washing mechanical pulps with a screw press removes wood resin more efficiently than twin wire or twin roll presses94.
Retain extractives in the sheet
Change physical properties of wood resin deposits
Periodic cleaning of mill system and equipment
The impact of wood resin on system closure will depend on how the mill closure process is undertaken. In order to avoid the steep part of the curve of the build-up of DCS in system closure it may be necessary to:
a) minimise the amount of wood resin introduced into the process, e.g., by wood seasoning, good barking, and using a less resinous wood furnish;
b) send bleach plant filtrates back to the recovery, for example, by using the BFR process;
c) have points of wood resin removal in the process, such as: high-consistency thickening, reverse osmosis, membrane technology, screw press, dissolved air flotation, and centrifugation;
d) install effective metals removal processes.
Since many of the above are available, system closure in pulp and paper mills may be implemented without too many problems due to wood resin. Further information on technologies for system closure may be found in a report by Panchapakesan112 and, for a Canadian perspective, in recent reports by Ramamurthy and Wearing113 and Francis and colleagues114.
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68. Mannisto, H., Mannisto, E., and Krogerus, M., Current environmental performance of the pulp and paper industry, Proceedings, 1996 TAPPI Minimum Effluent Mill Symposium, TAPPI PRESS, pp. 9-15 (1996).
69. Back, E. and Allen, L.H. (editors), "Pitch Control, Wood Resin, and Deresination", TAPPI PRESS, Atlanta (2000).
70. Cohen, W.E., The influence of resins on paper manufacture. In "Wood Extractives and their Significance to the Pulp and Paper Industry", W.E. Hillis (Ed.), Chapter 13, Academic Press, New York and London (1962).
71. Assarsson, A. and Akerlund, G., Studies on wood resin, especially changes during seasoning of wood. V. Changes in composition of non-volatile extractives during water seasoning of unbarked spruce, pine, birch, and aspen logs, Svensk Papperstidn., 70(6): 205-212 (1967).
72. Nugent, H.M., Allen, L.H., and Bolker, H.I., Effect of seasoning on the acetone extractives composition from Black Spruce, Jack Pine and Trembling Aspen, Trans. Tech. Sec. CPPA, 3(4): 103-109 (1977).
73. Allen, L.H., Sitholé, B.B., Macleod, J.M., Lapointe, C.L., and Mcphee, F.J., The importance of seasoning and barking in the kraft pulping of aspen, J. Pulp Pap. Sci., 17(3): J85-J91 (1991).
74. Blanchette, R.A., Farrell, R.L., Burns, T.A., Wendler, P.A., Zimmerman, W., Brush, T.S., and Snyder, R.A., Biological control of pitch in pulp and paper production by Ophiostoma piliferum, Tappi J., 75(12): 102-106 (1992).
75. Rocheleau, M.J., Sitholé, B.B., Allen, L.H., Iverson, S., Farrell, R., and Noël, Y., Fungal treatment of aspen chips for wood resin reduction: a laboratory evaluation, J. Pulp Pap. Sci., 24(2): 37-42 (1998).
76. Sharyo, M., Shimoto, H., Sakaguchi, H., Isaji, M., Fujita, Y., Awaji, H., Matsukura, M., and Hata, K., Recent progress and general status of the lipase pitch-control technology in Japan, Jpn. Tappi J., 47(10): 1223-1233 (1993).
77. Arseguel, D., Baeza, R., Delord, P., and Baboulene, M., Process for treating paper pulp and aqueous enzymatic preparation for carrying out the process, French patent # 2,709,765 (Talc de Luzenac) (1995).
78. Sarkar, J.M., Tseng, A.M., and Hartig, E.J., Applications of enzyme and polymers for controlling pitch in papermaking, Proceedings, TAPPI Papermakers Conference,TAPPI PRESS, pp. 175-182 (1995).
79. Mustranta, A., Fagernäs, L., and Viikari, L., Effects of lipases on birch extractives, Tappi J., 78(2): 140-146 (1995).
80. Dunlop-Jones, N., Douek, M., Jialing, H., Allen, L.H., and Dorris, G., The effects of certain chemical additives on the deresination of trembling aspen in kraft pulping, J. Wood Chem. Technol., 9(3): 365-386 (1989).
81. Roy-Arcand, L., Methot, M., and Archibald, F., Ozonation as a partial treatment for CTMP effluents, Proceedings, 1995 International Environ. Conf., TAPPI PRESS, pp. 1137-1154 (1995).
82. Jansson, K., Rampotas, C., and Terelius, H., Removal of extractives and heavy metals with the Netfloc system, Proceedings, 5th Internl. Conf. New Available Techn., SPCI, pp. 696-708 (1996).
83. Rampotas, C., Terelius, H., and Jansson, K., The Netfloc system - the tool to remove extractives and NPE, 1996 Minimum Effluent Symposium: Technical Challenges in Pulping and Papermaking, TAPPI PRESS, pp. 312-325 (1996).
84. Häggström, S., Lindqvist, B., and Sondell, B., Five years experience of closed-loop TCF-bleaching at Domsjö sulphite mill, Proceedings, 5th Internl. Conf. New Available Techn., SPCI, pp. 840-847 (1996).
85. Allen, L.H. and Lapointe, C.L., Centrifugal cleaning of pulp and paper mill process liquids, US patent 5,468,396 (1995).
86. Egenes, T.H. and Barbe, M.C., Pulp washing with screw presses, Proceedings, TAPPI Pulping Conf., TAPPI PRESS, Atlanta, pp. 551-560 (1990).
87. Meadows, D.G., An eye to the future: stock preparation, Tappi J., 81(2): 70-78 (1998).
88. Nelson, P.J., Chin, W.J., and Mulcahy, J.P., Removal of extractives during TCF bleaching of radiata pine bisulfite pulp, Proceedings, Appita '98, pp. 347-352 (1998).
89. Allen, S.L., Allen, L.H., and Flaherty, T.H., Defoaming in the pulp and paper industry, in "Defoaming: Theory and Industrial Applications", P.R. Garrett(Ed.), Ch. 3., Marcel Dekker Inc., New York, pp. 151-175 (1993).
90. Bihani, B.G., Goal of closed-cycle operation hinges on fiberline developments, Pulp Pap., 70(4): 87-90 (1996).
91. Tyrväinen, J., Law. K-N., and Valade, J.L., Alkaline-peroxide inter-stage treated mechanical pulp from Jack Pine (Pinus banksiana), Part II: Pulp optical properties, colour reversion, extractives content, and process implications, Pulp Pap. Can., 98(7): T223-T227 (1997).
92. Östberg, G.M.K., Öhrn, M.I., and Kvist, E.U., Use of carbon dioxide in the production of sulphate pulp, Proceedings, 5th Internl. Conf. New Available Techn., SPCI, pp. 508-515 (1996).
93. Ferweda, G.B., Washing improvement through brownstock acidification with carbon dioxide, Proceedings, Joint Spring Conf,, CPPA, Session 1, paper #3, 4 pages (1995).
94. Pöschil, K., Bräuer, P., and Kappel, J., Washing of board-grade spruce CTMP to low resin content, Proceedings, Appita '98, pp. 83-89 (1998).
95. Laleg, M. and Pikulik, I., Strengthening of mechanical pulp webs by chitosan, Nord. Pulp Pap. Res. J., 4(7): 174-181 (1992).
96. Laleg, M. and Pikulik, I., Unconventional strength additives, Nord. Pulp Pap. Res. J., 1(8): 41-47 (1993).
97. Laleg, M. and Pikulik, I., Improving retention and sheet strength with chitosan, PPR # 1073 (1994).
98. Allen, L.H., Polverari, M., Levesque, B., and Francis, W.D., Effect of system closure on the performance of retention aids in newsprint manufacture, PPR # 1273 (1997).
99. Tay, S., New enhancers to improve polyethylene oxide retention performance on deinked newsprint, Tappi J., 80(9): 149-156 (1997).
100. Ayukawa, B., Method for purification of waste water by treatment with zirconium salt, US patent 4,066,542 (Shikoku Paper Mfg. Co., Ltd.) (1978).
101. Allen, L.H. and Yaraskavitch, I., Effects of retention and drainage aids on paper machine drainage: a review, Tappi J., 74(7): 79-84 (1991).
102. Vihervaara, T. and Paakkanen, M., Raifix - new cationic polymers for controlling wet-end chemistry, Paper and Timber, 74(8): 631-633 (1992).
103. Vihervaara, T., A new generation of starch based cationic polymers for controlling wet-end chemistry, Proceedings, TAPPI Papermakers' Conference, TAPPI PRESS, pp. 529-533 (1994).
104. Douek, M. and Allen, L.H., Some aspects of pitch control with talc in unbleached kraft pulps, J. Pulp Pap. Sci., 17(5): J171-J177 (1991).
105. Allen, L.H., Cavanagh, W.A., Holton, J.E., and Williams, G.R., The use of talc for pitch and deposit control in the modern kraft pulp mill, Pulp Pap., 67(13): 89-91 (1993).
106. Richardson, P.E., New technology for pitch and stickies control, Proceedings, TAPPI Papermakers Conference, TAPPI PRESS, pp. 205-214 (1995).
107. Dunlop-Jones, N. and Allen, L.H., The influences of washing, defoamers and dispersants on pitch deposition from unbleached kraft pulps, J. Pulp Pap. Sci., 15(6): J235-J241 (1989).
108. Allen, L.H., Pitch control - optimization of alum usage in newsprint mills, Pulp Pap. Can., 82(11): T397-T404 (1981).
109. Allen, L.H., Pitch control with sodium aluminate and alum in newsprint mills, J. Pulp Pap. Sci., 8(3): TR85-TR93 (1982).
110. Mehes, D., Coordinated deposit control in alkaline paper making, Buckman Laboratories of Canada, Ltd. (1989).
111. Aston, D.A., Fryer, M.T., and Lambert, C.G., Improvement in newsprint sheet quality by effective pitch control, Preprints, 76th Annual Meeting, Tech. Section, CPPA, Montreal, pp. A113-A126 (1990).
112. Panchapakesan, B., Closed white water system designs, Proceedings, Papermakers Conference, TAPPI Press, pp. 219-225 (1993).
113. Ramamurthy, P. and Wearing, J., System closure: a Canadian perspective, Preprints, 84th Annual Meeting, Tech. Section, CPPA, Montreal pp. A215-A222 (1998).
114. Francis, D.W., Wearing, J.T., and Reside, D.A., Engineering assessment of system closure options for an older integrated newsprint mill, Pulp Pap. Can., 99(12): T437-T443 (1998).