THE EFFECTS OF WOOD EXTRACTIVES ON SYSTEM CLOSURE

B. Bruce Sithole and Larry Allen
Pulp & Paper Research Institute of Canada, 570 St John's Blvd, Pointe Claire,
Quebec, H9R 3J9, Canada

Abstract

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.

Introduction

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

Inorganic

Organic

Sodium
Calcium
Iron
Aluminium
Barium
Silica
Chloride
Sulphate
Oxalates
Silicone oils
Silicates
Fillers
Lime mud
Fly ash
Dregs

Wood extractives
Lignin
Lignans
Sugars
Hemicellulose
Defoamer oils
Coating latexes
Stickies
Plastics
Waxes

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

Drainage

Erosion

Scale

Colour

Foam

Temperature

Specks

Slime

Vacuum

Retention

Odour

Wire life

Plugging

Pitch

Felt life

Corrosion

Precipitation

Sizing

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

Figure 1

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
(adapted from Ref. 8.)

Figure 2

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
Work by Allen14 has shown that the concentration of dispersed wood resin in mill waters correlates well with pitch deposition problems, provided dispersed wood resin is the source of the deposition. The work showed that the potential for pitch deposition and related problems increased linearly with increasing wood resin particle concentration (provided dispersed resin was the single cause of the deposition). Different mills can tolerate different concentrations of dispersed wood resin. Depending on how it is done, system closure may increase this concentration many fold with a consequent increase in deposition problems if the wood extractives in dispersed form are allowed to increase unhindered.

Metal soaps
The free fatty and resin acids of wood resin can react with metal ions to form metal soaps. Sodium soaps are soluble in water but soaps from multivalent metal ions such as Mg, Ca, Al and Ba are insoluble in aqueous environments and are very tacky15. Their tackiness results in sticky deposition in pulp and paper processes. These soaps are formed above pH 616 where the acids are ionised. System closure will result in increased metal soaps deposition due to an increase in the concentration of multivalent metal ions and 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

NaCl, mol/L

Critical micelle concentrations

Na-oleate, mg/L

Na-abietate, mg/L

NaOleate:NaAbietate 1:1, mg/L

0

146

3.25x103

314

0.25

10.7

941

18

0.75

insoluble

Insoluble

8.8

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.

Polymerisation
Wood resin components are known to polymerise to form products that are difficult to dissolve in common solvents and in alkali26. A recent report implicates polymerised wood resin as a major component of pitch deposition in a fine paper mill27. Such polymerisation increases with time and temperature, as demonstrated in studies of polymerisation of linseed oils in repulping processes28,29. Since system closure usually results in higher operating temperatures in process systems it is conceivable that this will cause greater polymerisation of wood resin components and subsequently result in pitch deposition problems. Indeed, studies have shown that linseed oils in aqueous systems gel after three days at 25oC and the rate of gel formation increases with increase in temperature30. The mechanism is initial oxidation followed by polymerisation of the oils.

Foaming
Wood resin components are surface active agents that often cause foaming problems in pulping and papermaking processes. Their build-up with increasing system closure will lead to more foaming problems in closed mills31. Other causes of increased foaming include the use of narrower water circuit pipes that result in higher pressures on pulp slurries and whitewaters, higher water temperatures, wood pulps with higher amounts of wood extractives (TMP, PGW, CTMP), use of recycled fibres, higher machine speeds, and twin wires on paper machines5. The foaming can result in loss of vacuum, poor drainage, poor formation, holes and thin spots in the sheet, and over-use of defoamers (which can cause deposit formation).

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)

Figure 3

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.

Figure 4

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
Wood resin usually has the most effect on paper properties of mechanical grades because they contain more wood resin than kraft pulp furnishes. Mass balances of wood resin in a TMP mill and a groundwood specialities mill show that 70% of the resin that entered the mill exited with the dried product48. This wood resin attached to the fibres interferes with fibre-fibre bonding and is detrimental to the strength properties of paper49. Work by Wearing et al.,50 and, also, Francis and Ouchi1 has shown that handsheet properties of newsprint furnishes were adversely affected when the sheets were formed in water contaminated with dissolved and colloidal material derived from TMP and SCMP. Wet-web tensile strength was impaired due to lower surface tension forces and reduced fibre-fibre bonding whereas dry strength properties were impaired due to decreased bonded area and bond strength. Fatty acids were found to be particularly detrimental due to their high surface activity. A report by Wenzl38 also shows that the strength of paper and board is affected when larger amounts of wood extractives are present in the system. Wenzl suggests that the degree of impact of the extractives is dependent on the "reactivity" of the fibres which in turn is affected by the lignin content of the fibres and their fibrillation. He concludes that mechanical furnishes have more reactive fibres than chemical pulp furnishes. Wenzl38 and Woodward51 claim that coverage of fibre surfaces by the wood resin may also make sizing and the production of coloured grades more difficult. However, no evidence is given to support these statements.

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.

Solutions
Strategies for reducing the build-up of dissolved solids (DCS) in process waters to acceptable levels are presently available. For example, Wearing and co-workers9 have reported that the excessive build-up of dissolved solids in system closure in an integrated mill producing newsprint from 100% TMP furnish can be greatly reduced by segregating the pulp mill and paper mill white-waters and by the incorporation of inter-stage washing. Their data, reproduced in Figure 5, indicates that at constant headbox concentration, fresh water usage may be reduced from 10 to 3.7 m3/odt pulp. The addition of an efficient one-stage press washing would allow a further reduction of excess whitewater to 1.7 m3/odt pulp. The researchers, however, noted that extractives may be more poorly washed than other dissolved solids. Subsequent studies confirmed this, and showed extractives removal to be dependent on washer design68.

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
(adapted from Ref. 9)

Figure 5

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
Wood seasoning or storage is an effective way of reducing the wood resin load into a mill system70-73. In general, seasoning is desirable in mechanical pulping and in kraft pulping of aspen, birch and some tropical hardwoods, but not in kraft pulping of softwoods. Removal of wood resin in wood chips can be enhanced by spraying the chips with a fungus that thrives on wood resin74,75 whereas removal of wood resin on pulps can be enhanced a little by application of a lipase enzyme that breaks down wood resin components, specifically glycerides76-79. Tall oil or synthetic additives can be added in the digester to solubilize wood resin components of kraft hardwood furnishes that survive the alkaline pulping process80. Laboratory studies have shown that ozone can destroy some wood resin components in process waters81. Using chelating agents to remove metal ions that react with wood resin will help reduce deposition problems in system closure. A patented commercial system has been developed that can remove both extractives and heavy metals from process waters82. The process, Kemira Netfloc, consists of a flotation unit that employs polyethylene oxide (PEO): wood resin reacts with the PEO to form particles that are removed by flotation (the pitch particles can subsequently be disposed of by incineration). Removal efficiencies of wood resin and DCS of 90% have been reported83. Deposit control in closed-loop TCF bleaching at a MoDo mill (Domsjö) was achieved by controlled deresination with FROTAPULPER© screw defibrators and a system for recovering the liberated wood resin84.

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

Figure 6

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
Pulping wood species with less wood resin should help alleviate foaming problems caused by wood resin components . For example, kraft pulping of seasoned aspen results in lower foaming propensity of process liquors than pulping of fresh aspen73.

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.

Proper washing
Good washing of unbleached pulp will reduce the amount of wood resin that will be carried to the bleach plant and paper machine. This is very difficult to achieve in some mills that are recovery limited. The behaviour of wood resin soaps during pulp washing has been studied23,32. Application of wash aids in the bleach plant can improve washing efficiency by reducing carryover from stage to stage. This is especially beneficial for mills that are currently limited by their washer capacity90. A report on closed-loop technology at the Domsjö sulphite mill in Sweden mentions that improved control of washing was the most important aspect of the closure process84. Recently, interstage washing using a screw press has been shown to be an effective way of deresinating mechanical and aspen kraft pulps91. Also the use of carbon dioxide in brownstock washing in kraft mills without oxygen delignification has been reported to improve runnability due to increased dewatering capacity, decreased precipitation of calcium soaps, and decreased foaming92. A Canadian mill report shows that brownstock acidification with carbon dioxide improves pulp washing as a result of decreased soda and COD carryover into the bleach plant, reduced bleach plant chemical demand and lower pH of brownstock entering the bleach plant93. A lower pH, such as 6.3, has been shown to be useful in kraft mill brownstock screen rooms for reducing pitch deposition16.

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
An alternative to dealing with extractives that will accumulate on paper machines is to retain them in the sheet and thus sell them with sheet. This can be achieved by addition of retention aids that will make the wood resin substantive to fibres and also increase first-pass retention of fines and fillers. The feed rates of such additives should be carefully monitored and preferably automatically tied in with a production parameter. Additives that can be used include: chitosan, a highly cationic linear polysaccharide that performs well as a retention aid, strength agent, and scavenger of colloidal particles in 100% deinked paper mills and in LWC furnishes95-97; polyethylene oxide, a high molecular weight polymer that has been shown to be an effective retention aid and pitch control agent in system closure and recycled paper news mills98,99; zirconium oxychloride cationic polymer that reacts with dissolved and colloidal substances rapidly coagulating and attaching them to the sheet100,101; polyDADMAC, a workhorse polymer for many news mills101; and a new class of starch-based cationic polymers that, based on mill trials, have been claimed to have high affinities for DCS and fibres102,103. However, the use of some of these additives may not be cost effective in certain mills.

Change physical properties of wood resin deposits
Wood resin that precipitates or coagulates to form deposits can be controlled by using additives that change the nature of the deposits to make them less tacky. Such additives include talc that acts by coating the deposits thereby making them less apt to grow and cause problems104,105, surfactants that passivate pitch deposits106, or those that disperse and stabilise wood resin particles and prevent them from coagulating to cause deposits85,109, and alum that coagulates pitch particles and attaches them to fibres108,109.

Periodic cleaning of mill system and equipment
Periodic boil-outs will be needed to maintain cleanliness of equipment and mill systems. The boil-outs can be done using caustic soda, surfactants, and water at elevated temperatures110. Solvents can be used to clean fouled equipment. Mechanical dislodgement of deposits may also be required from time to time. Felt cleanliness can be maintained by continual spraying of chemicals that form a barrier on the felt to prevent deposition. For example, treatment of a machine returning wire ahead of the breast roll with a cationic polymer (trade name Darasperse) was shown to modify the surface of the forming wire preventing the build-up of pitch deposits111.

CONCLUSION

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.

REFERENCES

1. Francis, W. and Ouchi, M.D., Effects of dissolved and colloidal solids on newsprint properties, PPR # 1243 (1996).

2. Jemaa, N., Thompson, R., Paleologou, M., and Berry, R.M., Inorganic non-process elements in the kraft recovery cycle: sources, levels, process effects, and options for removal, MR # 348 (1997).

3. Thompson, C. and Garner, A., Paper machine corrosion and progressive closure of the white water system, Proceedings, TAPPI Papermakers' Conference, TAPPI PRESS, pp. 545-552 (1996).

4. Bryant, P.S., Transition metal profile monitoring and control in closed kraft mill fiberlines, Proceedings, TAPPI Environmental Conference, TAPPI Press, pp. 617-625 (1993).

5. Lorenčcak, P., Baumann, P., and Hughes, D., Deaeration in high temperature systems, Paper Tech., 38(9): 25-28 (1997).

6. Adams, T., Paper mill water, energy and non-process element balance, Minimum Impact Mill Monograph, TAPPI PRESS, pp. 45-54 (1994).

7. Foster, C. and Rende, D., How recycling, water reuse impact chemistry, PIMA, 79(1): 48-53 (1997).

8. Geller, A. and Göttsching, L., Closing water systems completely in the Federal Republic of Germany. Tappi J., 65(9): 97-101(1982).

9. Wearing, J.T., Huang, S., Piggott, A., Ouchi, M.D., and Wong, A., New white-water management strategies for integrated newsprint mills, Pulp Pap. Can., 86(5): T139-T145 (1985).

10. Lindholm, G., Reduction of fresh water consumption in pulp and paper production, Pap. Puu, 80(4): 260-263 (1998).

11. Allen, L.H., Mechanisms and control of pitch deposition in newsprint mills, Tappi J., 63(2): 82-87 (1980).

12. Back, E., The mechanism of pulp resin accumulation at solid surfaces - studies in pitch troubles, Svensk Papperstidn., 63(17): 556-564 (1960).

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