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Chemistry for pollution free pulp & paper

Chemistry for pollution free pulp & paper

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  • 1. Opportunities in Wet-End Chemistry: FeatureEssay, Posted Oct. 2001Good Chemistry - Looking towards the Future ofPapermaking AdditivesMartin A. HubbeDept. Wood & Paper Sci., N.C. State Univ., Box 8005, Raleigh,NC 27695-8005Citation (public domain): an old story about a public hearing in which papercompany executives were describing their plans for a green-fieldmill. A spokesperson ended her presentation with a listing of themaximum levels of various substances in the liquid effluentfrom the proposed plant. "Our effluent water will have abiological oxygen demand of less than 10 parts per million, andit will have a pH of 7." At this point someone near the back ofthe room stood up and said, "I am a citizen of this town, and Iwill insist that the pH value be reduced to zero before the wateris discharged!"Part of our challenge as papermakers is to maximize theefficiency of our operations and make them increasingly eco-friendly. But, as illustrated by the story above, we also need tobe proactive in explaining the steps we are taking as an industry.Our challenge is to educate our fellow citizens that chemicals,used appropriately, are absolutely essential in this effort and thatthey also can be safe to use.
  • 2. What about the chemicals that one adds at the wet end of a papermachine? The public sometimes associates the word "chemical"with words like "pollutants," "emissions," "toxicity," or"hazard." As noted in an article by Reinbold (1994), "the publicno longer views technology as something beneficial." Someadvocates for the environment have described paper as a"chemical cocktail." The goal of this essay is to consider howwe, as papermakers, can do more in the years ahead to minimizeenvironmental impacts and also to achieve a more favorableimpression in the eyes of the public.Fig. 1. Full description given by Fig. 2. Full description given byGottsching (1993) Gottsching (1993)Where do we look for answers? In my opinion there arebasically three answers to our situation as suppliers and users ofpapermaking additives. I will spend the rest of this essayexpanding on each one of them in turn. The first answer is for usin the industry to show that each additive to a paper machine hasa clear and beneficial role. The second answer is to demonstrateprogress in understanding and minimizing environmental
  • 3. impacts of specific papermaking additives. The third answer isto envision the types of chemical additives and their uses in ahypothetical future paper mill. Our ideal paper mill of the futureshould be both profitable and as nearly "invisible" as possible interms of its impact on the environment.Part 1 - A Purpose for Each Wet-End ChemicalThink about your reaction when you see a really long list ofingredients on the container of a processed food item. Do youever read down through the list and wonder whether all of thoseodd-sounding chemical items are really needed? Its far worsefor those who happen to be allergic to one or more of thoseadditives. Unlike packaged food, paper products come with nolist of ingredients. Except for some factors that I will discussbelow, we are in a situation somewhat resembling the yearsbefore food labels. It is possible to list about 3000 differentkinds of chemicals that have been proposed for use inpapermaking (Reinbold 1994). Ingredient labels for paperproducts may or may not be a good idea; but it is also clear thatthere is an opportunity for the paper industry to tell the publicwhat is used in paper and why.Progress in explaining the environmental consequences ofpapermaking additives already has been achieved in a series ofpublications that appeared in the early 1990s. First, an article byReinbold (1994) clarifies just how few chemicals papermakersactually use. If one ignores brand designations, differences inconcentration, and minor variations in molecular mass orcomposition, then only about 200 individual chemicals arecommonly added to paper machines, not 3000. The relativelylow number of chemical additives used in papermaking is
  • 4. consistent with the fact that this industry mainly makes low-cost,high-volume products; we simply cant afford to use superfluouschemicals.An article by Göttsching (1993) makes the further point thatpapermaking practices are generally compatible with theenvironment. If one were to omit all chemical additives from apapermaking process, then the consequences would includelarger increases in emissions of solids, biological oxygendemand, and even of noxious gases - resulting from uncontrolledgrowth of slime in paper machine systems. This article, togetherwith a publication by Webb (1993) give an excellent run-downof the main types of chemical additives and the status of each ofthese additive relative to various environmental impacts.Saving Energy: Lets take a closer look at how wet-endadditives can reduce the energy required used in papermaking.Removal of water uses by far the largest component of thatenergy. Most water is removed by gravity drainage, applicationof vacuum, inertial effects, and pressing. However, most of theenergy is expended during a subsequent process, drying byevaporation (Hersh 1981; Specht 1992). Approximately 2 to 9million BTU are required per ton of product, to evaporate water.Substantial savings in energy can be achieved by shifting agreater proportion of the water removal to the preceding unitoperations of forming and pressing (Manson 1980; Nelson 1981;Manfield 1986; Marley 1990). One way to accomplish this goalis to accelerate dewatering with chemical additives. There hasbeen much work in this area (Auhorn 1982; Allen, Yaraskavitch1991; Litchfield 1994; Raisanen et al. 1995; McGregor, Knight1996). I it generally agreed that each 1% increase in solids
  • 5. content of a paper web should yield about a reduction of 4 to 5%in the net drying load (Shirley 1980; Nelson 1981; Auhorn 1982;Strawinksi 1985; Marley 1990). Pulp mills are often netproducers of energy in the form of steam or electricity, butsavings in the energy of drying has the potential to eitherdecrease the consumption of fossil fuels or decrease theproduction of greenhouse gases.The goal sounds great, but what chemicals are we talking aboutin terms of additives? Three classes of chemicals stand out asthe major drainage chemicals in current use (Allen,Yaraskavitch 1991; McGregor, Knight 1996; Scott 1996). Thesethree classes are often called "coagulants," "flocculants," and"microparticles." Coagulants used in papermaking are generallymultivalent or polymeric compounds of high positive chargedensity. Commonly used coagulants include aluminum sulfate("papermakers alum"), polyamines, and polyethyleneimine(PEI). The word "coagulate" implies that the negative surfacecharges of suspended solids, fibers, and colloidal material areneutralized, removing the electrical repulsion between thesesurfaces. Flocculants complete the process of bringing fineparticles together; the most widely used type of flocculants inthe paper industry are very high mass copolymers of acrylamide(Horn, Linhart 1991). Amounts typically less than 0.05% basedon product mass are sufficient to increase the retention of fineparticle in paper as it is being formed. Microparticles are tinynegatively charged particles such as colloidal silica, bentonite,or highly branched carboxyl compounds; they interact withcationic polyacrylamides or cationic starch to further promotedewatering (Langley, Litchfield 1986; Knudson 1993; Honig etal. 1993; Andersson, Lindgren 1996; Swerin et al. 1996). A
  • 6. common characteristic of all of these drainage-promotingchemicals is that, to perform their function, they adsorb onto thesurface of solids in the papermaking furnish. That means thatthese chemicals tend to be retained well in the paper; relativelylittle of it remains in liquid effluent from paper machines, evenbefore wastewater treatment.Defoamer chemicals affect many aspects of papermaking, inaddition to drainage, but it is the drainage benefits that have theclearest connection with environmental impact. A study byBrecht and Kirchner (1959) was among the first to clearly showthat air bubbles in a stock suspension can have an effect verysimilar to that of fiber fines in slowing the rate of drainage froma paper web. Especially in the case of heavier weights of paperor paperboard, higher levels of fines or bubbles can be expectedto clog the drainage channels in a wet sheet of paper (Gess 1989,1991). Defoamers are added to the wet end in the form ofemulsions; little droplets of oily material spread rapidly onbubble surfaces and cause the bubbles to coalesce. The result isless entrained air coming out of the headbox. In principle,improvements in drainage can be converted into dryer papergoing into the wet-press section. In turn, a dryer sheet cominginto the press section makes it possible to load the presses morewithout squashing the sheet. The happiest situation is whenincreased pressing results in a stronger, better-consolidatedsheet, with less water remaining to be evaporated. The wild cardin this situation is whether the resulting sheet still has enoughcaliper so that it can be calendared to meet a specifiedsmoothness.
  • 7. Decreasing Effluent Loads: A remarkable aspect of the "art" ofpapermaking is that paper is formed on a relatively coarse,continuous screen fabric; typically the openings in the fabric arelarge enough so that between about 5 and 50% of the solidsdelivered to the forming section are capable of passing throughthose openings. The small particulate material in paper, the"fines," may consist mostly of wood byproducts (Brecht, Klemm1953; Scott 1986; Gess 1991; Luukko, Paulapuro 1999; Rundlöfet al. 2000). Even before it is refined, a typical kraft pulpcontains about 5 to 10% by weight of such things as tinyparenchyma cells, used for food storage or conduction. Theprocess of refining pulp - passing the pulp slurry betweencounter-rotating metal plates or cones having raised bars - isnecessary to develop the bonding ability of fibers for mostgrades of paper, but refining also increases the level of fines inthe slurry. But all of these wood-derived fines can beoverwhelmed by fine material of a different type, the mineralfillers (Bown 1998). Calcium carbonate and clay are the majortypes of fillers used, and they make it possible to achieveopacity targets with less total materials.To understand how retention aid chemicals can impact theenvironment it is worthwhile to view papermaking operations asthe first step in a multi-step water clarification process (Leitz1993). Though there is a great deal of overlap between"retention chemicals" and the fore-mentioned "drainagechemicals," the emphasis of a retention program is to increasethe relative proportion of fine materials that stay with the wetpaper web as it is being formed (Jaycock, Swales 1994; Gess1998). The very-high-mass acrylamide copolymers,polyethylene oxide in combination with phenolic cofactors, and
  • 8. also high-mass acrylamides, in combination with microparticles,can be very effective retention systems, even in some caseswhere the surfaces of the suspended matter are far from beingneutral in charge. Higher retention efficiency implies that lesssolid material is present in the water that drains from the paper.The traditional name that papermakers used to describe thefiltrate water from papermaking is "white water." A generationago it used to be more common for white water to contain somuch clay, titanium dioxide, and air bubbles that it looked likemilk. Now, largely thanks to chemical additives, together withscreen devices called save-alls, solids levels of white water arekept under control and nearly all of the fine material eventuallyends up as paper.Avoiding Waste of Fibers: You may not think of strength aidsas fiber-saving chemicals, but you should. Consider the case ofrecycled office waste fibers. Such fibers tend to loose asignificant fraction of their bonding ability each time they aredried and reslurried (Lindström, Carlsson 1982; Klungness,Caulfield 1982; Howard, Bichard 1992; Nazhad, Paszner 1994;Zhang et al. 2001). The loss in bonding ability has beenattributed to essentially irreversible closure of pores in the cellwall (Stone, Scallan 1966), resulting in a loss of flexibility of thefiber surfaces (Paavlilainen, Luner 1986). Strengthspecifications become more difficult to achieve. One approach isto try to make up for the strength loss by increased refining.However, the furnish is likely to already have a relatively lowfreeness, so there comes a point where more refining is not theanswer. Rather, papermakers tend to use increased levels ofstrength additives, such as cationic starch or acrylamides
  • 9. (Marton 1980; Strazdins 1984; Howard, Jowsey 1989; Smith1992; Iwasa 1993; Glittenberg et al. 1994).Another situation in which strength additives can "save fiber"arises in the case of paper grades that are specified by strengthrather than basis weight. Such is the case for containerboardgrades made in accordance with the Rule 41 criteria (Gutmannet al. 1993). Briefly stated, the rule allows a producer todecrease the basis weight of a product as long as the combinedboard still meets various strength goals, such as crush resistance.In practice, papermakers use a combination of refining practices,dry-strength additives, and sometimes size-press addition tomake the premium-strength board and take advantage of Rule 41(Smith 1992).Can Chemicals Added Initially Benefit Recycling? It has beenshown that strength-enhancing chemicals added to never-driedkraft fiber can also have a beneficial effect after the same fibersare recycled (Higgins, McKenzie 1963; Grau et al. 1996;Laivins, Scallan 1996; Zhang, Hubbe 2000). Treatments foundto be effective included cationic starch and combinations ofcationic and anionic polymers. Results were consistent with theability of such chemicals to act as inter-fiber bonding agents -both in the initial paper and also in the recycled paper, evenwhen no additional polymeric material was added during thesecond generation of papermaking.Losses in fiber bonding ability due to drying, aging, andrecycling of paper made from kraft pulp may be minimized byalkaline papermaking conditions. Some benefit of alkalineconditions may result from reduced hydrolysis of cellulosemacromolecules (Wilson, Parks 1983; Nazhad, Paszner 1994).
  • 10. Further benefits may be associated with reduced closure of poresin the cell walls (Lindström, Carlsson 1982), and reducedstiffening of fibers. Though papermakers adjust pH values invarious different ways, one type of additive stands out in termsof adjusting the pH to minimize damage to fibers. Give up? Thatadditive is calcium carbonate filler. Recent recommendations forarchival papers require at least two percent calcium carbonate tomake sure that the paper remains buffered in a weakly alkalinepH range to make it resistant to gradual embrittlement(McComb, Williams 1981; Kelly, Weberg 1981; Anon. 1993).Work by Pycraft and Howarth (1980) shows further that over-drying of virgin paper is likely to harm the properties of thefibers, if they are to be used later for recycled paper.The Sludge Dewatering Press is Like a Little PaperMachine: Recycling of paper requires more fossil fuels orelectrical energy, compared to new pulp and paper from wood orsawmill waste. The recycling of paper also can produce a lot ofwaste sludge. Nevertheless, recycling usually is regarded ashaving a favorable net impact on the environment (Pajula, Kärnä1995; Jorling 2000). A key goal of increased recycling helpskeep the rate of tree harvesting below the growth rate of newtrees.Saving land-fill space is another motivation to recycle paper: itturns out that chemicals can play a beneficial role in helping toachieve this goal. The reason is that sludge from wastepaperrecycling can contain a lot of water (Dorica, Allen 1997;Kantardjieff 2000). The water content adds to the weight ofsludge to be discarded, and it also makes it more fluid-like, notthe ideal characteristic for building a stable landfill. Chemicals
  • 11. coagulants such as poly-aluminum chloride (PAC), essentiallythe same coagulants used in paper formation, can be used toassist pressing more water from sludge (Ghosh et al. 1985; Leitz1993; Pawlowska, Proverb 1996). Side benefits of sludgedewatering may include a) more stable, solid-like sludge, b) thecolloidal materials in the sludge will tend to be insolubilized inpolyelectrolyte complexes and precipitates, and c) the sludgewill be more valuable as a fuel source, if that option isconsidered (Harila, Kivilinna 1999). In principle well coagulatedwaste sludge is expected to have reduced rates of leaching.Part II - Minimizing the Environmental Impacts of EachType of Additive"You work for the paper industry? Then maybe you can explainthat smell when I drive into [you fill in the place name]." To putthe present discussion into context it is worth noting that mostrecent public concern has been directed at issues other thanpapermaking additives. Rather, greater attention has beendirected towards issues of pulping, tree harvesting practices,paper recycling, and, yes, air emissions (Vasara 2001). Another,possibly more authoritative measure of environmental concerncomes in the form of legislation. Pulping and bleaching havebeen center-stage in the so-called "cluster rule" regulations(Vice, Carroll 2001). While keeping this context in mind, westill have to seriously consider the potential impacts ofpapermaking additives, if and when they enter the environment.The good news is that substantial progress has beenaccomplished in the area of papermaking additives with respectto their toxicity, their biodegradability, and their ability to beremoved from the water phase during wastewater treatment
  • 12. (Jorling 2000; Hamm, Göttsching 1994; Swann 2000). Later inPart II we will consider various papermaking additives, focusingon their potential hazards.Fig. 3. See article by Goettsching Fig. 4. Full description given by(1993) Vasara (2001)A subtle, and often overlooked influence on chemical additivesfor papermaking comes in the form of Material Safety DataSheets. "MSDS" information often is kept in orange or yellowloose-leaf notebooks, adjacent to places where industrialchemicals are being used. As noted by Allen (1991), thesedocuments have encouraged a trend towards greater awarenessof what it being added to paper machines. Toxicity and safetyinformation in MSDS has provided a starting point for makingimprovements, and making substitutions toward less toxicmaterials.After toxicity, perhaps the second most serious issue isbiodegradability of chemical additives for papermaking.Essentially all excess water from US paper mills undergoes
  • 13. wastewater treatment before it is discharged. Bacterial actionduring the secondary wastewater treatment converts manyorganic chemicals into benign forms, and most of the biologicaloxygen demand (BOD) is consumed. Some approximate rules topredict biodegradability have been proposed. For instance,compounds that contain chlorine, nitrogen, sulfonic acid, or azo-groups are more likely to resist breakdown during watertreatment (Hamm, Göttsching 1994). Other factors that appear tohurt biodegradability include toxicity, long chain length ofpolymers, branching, and chemical substituents along polymerchains. Unsubstituted alkyl chains also resist biologicaldegradation (Swann 2000). The problem with persistentchemicals is that they might have the potential to accumulate inthe environment or in particular organisms.Wet-strength agents: Environmental concerns about wet-strength chemicals are often associated with their monomercomposition, possible residual monomers, and the possibility ofregenerating these monomers and releasing them into theenvironment. The traditional wet-strength resins most often usedfor acidic papermaking conditions are based on formaldehyde(Dulany 1989; Espy 1995; Spence 1999). Possibly in response tothese concerns, the usage of phenol-formaldehyde andmelamine-formaldehyde resins has decreased dramatically in theUS paper industry. Poly-amidoamine-epichlorohydrin (PAAE)resins have been replacing the formaldehyde resins in mostpaper applications requiring durable wet-strength (Espy 1995;Fischer 1996; Spence 1999).Besides the issues with biodegradability, users of wet-strengthagents face two additional concerns. First, difficulties in
  • 14. repulping wet-strength paper increase the likelihood that thefiber will be sent to landfills after its first use. Second, ifpapermakers decide to repulp the wet-strength paper, one needsto be concerned about the chemicals used as repulping aids.Hypochlorite bleach is sometimes used to repulp wet-strengthbroke (Espy 1992; Fischer 1997). Elevated pH or temperaturealso may be required to redisperse the fibers. At a minimum,recycling of wet-strength paper is likely to require higher energyinput in the repulping operation. That means that there is anenvironmental price to wet-strength treatment; sometimes theprice is paid in terms of increased landfilling, sometimes inincreased water treatment requirements, and sometimes inincreased energy expenditures. The ideal, in terms of wet-strength treatments, would be to find a non-toxic, biodegradablematerial that provided efficient, durable wet-strength underconditions of use, but which also repulped easily under slightlyhigher temperatures and hydrodynamic shear conditions in arepulping operation.Dyes: Papermaking colorants tend to have relatively poorbiodegradability (Webb 1993; Wahaab 2000). Fortunately therehas been a trend towards dyes with relatively high affinity forsolid surfaces. That means that the dyes tend to leave the papermachine as part of the product, not in the water to be treated. Inaddition, dyes entering the wastewater plant tend to be removedwith biological sludge (Webb 1993). High affinity onto solidsurfaces is generally achieved by development and use ofrelatively large, planar molecules - the so-called "direct" dyes.Affinity for fibers is further promoted by the trend for more useof cationic direct dyes, in cases where these are appropriate.Jackson (1993) noted that dye suppliers can minimize adverse
  • 15. environmental impacts by careful selection of adjunct materialsused to stabilize liquid dyes.Acrylamide copolymers: Considering their benefits in reducingthe waste of unretained fines, it is easy to love retention aids.Copolymers of acrylamide are the most widely used very-high-mass flocculants to promote fine-particle retention. On the onehand, acrylamide products are expected to contribute much lessto biological demand (BOD), compared to the amounts of starchproducts needed to render equivalent benefits in terms of eitherretention or dry strength (Iwasa 1993). On the other hand, theyare not easily biodegradable (Webb 1993), as is to be expected,based on their molecular mass (Hamm, Göttsching 1994). Themaximum permissible level of monomers present in acrylamidecopolymers is 750 ppm, compared to 100 ppm in the case ofother polymers (Swann 2000). Acrylamide products havereceived the more lenient limits due to their history of 40 yearsof use in the paper industry without evidence of harm.Another issue to consider is the use of mineral oil as thecontinuous phase of common retention aid emulsion products(Swann 2000). Oil introduced with retention aids probably ismostly adsorbed by fibers, with no adverse effects. However, itis possible to imagine a bad effect resulting from the followingsequence: a) a low-grade mineral oil, having a significantaromatic content, is used in the formulation; b) some of the samepaper is recycled in a batch that includes colored papers; and c)the paper is bleached with elemental chlorine (Fleming 1995;Lancaster et al. 1992). Fortunately, this combination ofcircumstances is probably rare these days due to the use ofpurified, alkyl mineral oils and the elimination of elemental
  • 16. chlorine from most pulp bleaching operations in the US(Deardorff 1997).Another way to address concerns about oils in retention aidproducts is to eliminate them from the formulation. One of theside-benefits of oil-free formulation can be a substantialreduction in shipping weight and transportation costs for a givenamount of active materials. Many water-in-oil retention aidemulsions have active solids contents in the range of 25 to 50%(Horn, Linhart 1991). Dry granular or "bead" acrylamide-typeflocculants, which have been available for many years, havenearly 100% active content. If it werent for the perceivedconvenience of pumpable liquid formulations it is likely that dryproducts would enjoy more widespread use. An especiallyelegant solution to this dilemma involves a dispersion ofacrylamide-copolymer particles in aqueous solutions (Feng et al.2001). Normally such copolymers would dissolve in water, butthe ion concentrations can be adjusted to prevent this fromhappening.Highly cationic copolymers: Efforts by papermakers toconserve fiber resources and water have led to increased usageof highly charged cationic polymers. One of the ways toconserve fiber resources is to use high-yield pulps, such asthermo-mechanical pulp (TMP). Wood pitch from TMP can be asource of tacky deposits on papermaking equipment, forcing themill to shut down often for cleanups (Back, Allen 2000).Another way to conserve fiber is through recycling. Wood pitchis less of a problem with recycled fibers, but the problem isreplaced by stickies from pressure-sensitive adhesives andcoating latex (Hsu 1997; Douek et. al. 1997; Venditti et al. 1999;
  • 17. Wilhelm et al. 1999). Some of the tackiness problems can beminimized by use of talc (Braitberg 1966; Allen et al. 1993).Also the furnish usually can be treated with highly chargedcationic materials such as polyethylene-imine (PEI),polyamines, or poly-diallyldimethyl-ammonium chloride (poly-DADMAC). Such cationic treatments can help to bind the tackymaterials to fibers so that they can be purged from the system(Gill, 1993; Fogarty 1993; Shetty et al. 1994; Magee, Taylor1994; Moormann-Schmitz et al. 1994). Highly cationicpolymers or soluble aluminum compounds are used for theneutralization of excess anionic colloidal charge in papermakingfurnish - often the first key step in optimization of drainage andretention systems (see references cited in Part I). Yet another useof highly cationic polymers is in the spraying or forming fabricsor press felts to inhibit deposition of tacky substances from thepaper (Allen 1991; Sawada 1997); here again, the use of theseagents is helping in the effort to use wastepaper and high-yieldpulps, both of which are worthy environmental goals.Highly substituted, synthetic polymers of the type used forprecipitation of tacky materials and the neutralization of excesscolloidal charge are not expected to be highly biodegradable(Hamm, Göttsching 1994). For example Wahaab (2000)observed very poor biodegradability in the case of a commercial,highly cationic polymer used for treatment of forming fabrics.One step towards addressing concerns about possibleenvironmental impacts of highly cationic polymers is to avoidusing more than is needed. For instance in the spraying offorming fabrics it is possible to minimize the chemical use byproper dilution and by use of a well-designed spray boom
  • 18. (Sawada 1997). When used to neutralize excess colloidal charge,it is possible to avoid overdose of highly cationic polymers bycarrying out online or laboratory charge titrations with streamingcurrent instruments (Bley 1992; Stitt 1998; Phipps 1999; Gill2000; Rantala, Koskela 2000; Chen et al. 2001). Charge controlto the neutral range has the advantage of tending to maximizeprecipitation of most polymers and fines onto fiber surfaces,reducing the amounts of polymeric and colloidal substances thatare sent to the wastewater treatment system.Recently there is yet another option to consider, the use ofhighly cationic polymers based on starch or other naturalproducts. Already a highly cationic polymer based on starch hasbeen used for charge neutralization and optimization of wet-endoperations (Vihervaara, Paakkanen 1992). Presumably suchmaterials might be more easily biodegraded, compared to theirsynthetic counterparts. "Not necessarily so," says Reinbold(1994). Rather, there is a wide range of variability in thebiodegradation of both natural and synthetic polymers.Biocides: Conventional slimacides are highly toxic. They haveto be to perform their function. Many do not break down readilyduring treatment of wastewater (Webb 1993). Concerns overthese types of biocides have resulted in pressure against biocideuse for papermaking in Sweden (Swann 2000). One of the goals,then, is to develop biocides that do their job and then self-destruct (Allen 1991).Enzymes are very good at self-destruction. The fragile nature ofenzymes is due to the fact that they consist of complex proteinswith many loops and coils that have to fit together in an exactway to perform some kind of function. Even moderate changes
  • 19. in pH or temperature can temporarily or permanently destroy theenzymes activity. Enzymes such as amylases are already usedfor cleaning up deposits on starch-preparation equipment andpaper machine wet-ends (Swann 2000).Another way to minimize the need for toxic agents to controlslime involves biodispersants (Crill 1993). Biodispersants makesense because bacteria attached to surfaces, the so-called"sessile" bacteria, tend to cause more problems than freelyfloating bacteria in paper mill systems. Although it is prematureto expect that biodispersants can eliminate the need for toxicbiocides, or of oxidizers such as chlorine dioxide, it isreasonable to expect the dependency on such materials to bereduced.Starch: Starch products probably wouldnt even be included inthe present discussion, but for the fact that the paper industryuses so much of them. The largest proportion of starch is addedto the surface of paper at the size press or in coatingformulations. Additional starch is commonly added at the wetend in levels up to about 1% on paper mass. Native,underivatized starch is close to ideal in terms of itsbiodegradability (Hijiya 1999). In addition to providing strengthand helping certain retention aid programs, starch products alsoare based on a renewable resource. The most common grade ofstarch used in the US is a byproduct of processing cornsweetener for soft drinks and other processed foods. The troubleis, size-press starch often makes up 1 to 5% of the mass ofvarious paper products. This is certainly true of printing papers.Since the kinds of starch most often used at the size press arepoorly attached to fibers, large amounts of starch can become
  • 20. solubilized through the repulping of dry-end broke. Such starchis likely to be a major contributor to BOD of liquid effluent fromthe mill. In other words, the problem is in the large amount ofstarch products in the effluent water, not in their rate ofdegradation in a well-run biological wastewater treatmentsystem.Work carried out by Roberts et al. (1987) showed a veryeffective way to minimize BOD contribution of starch ineffluent from paper machine systems. The answer is to usecationic starch (Webb 1994). Roberts showed a case in whichabout 85% of cationic starch was retained at neutral pH, whereasonly about 10% was retained when the experiments wererepeated with unmodified starch. It should come as little surprisethat most starch now added at the wet-end of paper machines iseither cationic or amphoteric (i.e. having both positive andnegative charged groups attached to the chain). The down side isthat cationization of starch appears to make it less biodegradable(Hamm, Göttsching 1994). In summary, the higher retention ofcationic starch and its good, though not perfect biodegradabilitymake it highly beneficial in terms of overall environmentalimpact of paper mills.Sizing Agents: Internal sizing agents are truly remarkable intheir ability to transform the nature of paper, even when theadded dosages are typically well below 1% of the dry mass ofproduct. The chemical composition of wood-derived fibersmakes them highly water-loving. Paper uses for cups, bags,cartons, and various printing applications can require that itresist water absorption and penetration.
  • 21. Rosin size has been criticized for its toxicity and for the fact thatrosin sizing usually requires the use of aluminum compounds(Webb 1993). But rosin products can claim a positive attributenot shared by the common alternative sizing agents; rosin is abyproduct of wood pulping. Rosin is a renewable, biodegradablematerial (Webb 1993). There is an interesting balance betweenrosins efficiency and its biodegradability; most rosin is reactedwith maleic or fumaric anhydride to produce "fortified" rosinsize. The fortified size is more storage-stable and more efficientin use. However, it also is less biodegradable than natural rosin(Webb 1993).Though it still is worth considering environmental implicationsof rosin size products, there has been a strong trend over the past20 years towards alkaline papermaking conditions and the use ofcalcium carbonate filler (Gill, Scott 1987; Laufmann et al.2000). Values of pH higher than about 7 make it increasinglyharder to size paper with conventional rosin products (Liu 1993;Schultz, Franke 1996; Wang et al. 2000). Fortunately, twowidely used "alkaline sizing agents" are available.Alkenylsuccinic anhydride (ASA), which is very popular forproduction of printing papers and gypsum board liner, is abyproduct of petroleum (Webb 1993). By contrast, alkylketenedimer (AKD) is made from fatty acids, a renewable resource. Ineither case, alkaline sizing agents tend to be much more efficientthan rosin in terms of the amounts needed to reach equivalentlevels of resistance to fluids.Surfactants: Some surface-active materials are added to paperintentionally, whereas others come along for the ride asstabilizers for other chemicals or as residuals from de-inking. If
  • 22. we use a broad definition, then the list of intentionally addedsurfactants would include sizing agents (e.g. rosin soap size),components of certain defoamers (i.e. water-insolublesurfactants), certain deposit-control additives, and debondingagents used in certain tissue products. Various nonionic andfatty-acid-based surfactants are used in flotation de-inking(Johansson, Ström 1998; Rao, Stenius 1998) and for theagglomeration of xerographic toners (Darlington 1989;Heitmann 1994; Bast-Kammerer, Salzburger 1995). Nonionicsurfactants also are used to stabilize such additives as retentionaid emulsions, dyes, and certain sizing agents.Probably the most obvious adverse environmental impact ofsurfactants would be cases of visible foam. But the more seriousimpacts should be evident to anyone who has opened their eyesin soapy water, or when shampooing. Has anyone interviewed afish on this subject?Issues of toxicity and persistence have been raised in the case ofnon-ionic surfactants (Hamm, Göttsching 1994). Nonylphenol-ethyoxylate products have been replaced, especially in Europe,due to concerns about their toxicity (Swann 2000). Linear alkyl(or alcohol) ethoxylates have taken their place in manyapplications. Though the latter are not regarded as toxic, thesaturated alkyl chains tends to make them poorly biodegradable(Swann 2000). Perhaps the next logical extension is to useunsaturated aliphatic (alkenyl) poly-ethers. Alternatively,perhaps the most economical solution is to do a better job atremoving surfactants before effluent water is discharged.Chelating Agents: The most common function of chelatingagents such as diethylenetriaminepentaacetate (DTPA) in
  • 23. papermaking is to keep certain metal ions from interfering withperoxide bleaching of mechanically defibered pulps. Strictlyspeaking this is not an issue of wet-end chemistry; usually thepulping and bleaching operations are regarded as separate frompapermaking. That matter aside, the problem with chelatingagents is that they resist biodegradation (Göttsching 1993;Hamm, Göttsching 1994; Reinbold 1994). The potential adverseeffect of persistent chelating agents follows from their likelytendency to interfere with natural uses of calcium and othermetals in aquatic organisms. Since peroxide bleaching is oftenused for recycled pulp, especially when it contains mechanicalfibers, there is active interest in finding biodegradablealternatives to chelating agents. One approach is to usesequesterants such as silicates. In laymans terms, a sequesterantis something that binds objectionable metal ions less efficientlythan a chelating agent, but enough to permit peroxide bleaching.Since the byproducts of peroxide bleaching tend to be non-toxic,it would be highly beneficial to find other ways of increasing itsefficient use in pulp mixtures that are likely to containmanganese, iron, and other divalent transition metal ions.While on the subject of metals, it is worth considering theenvironmental consequences of heavy metals in effluent frompaper mills. In the past there were concerns about heavy metalsin various printing inks. As noted by Göttsching (1993),papermakers have to work with their associates in publishingand converting companies to avoid contaminating the wastefiber supply with persistent hazardous materials. DSouza et al.(1998) observed that between 75% and 100% of various metalsentering a paper mill system by way of waste paper wereremoved as a component of sludge. However, the levels of metal
  • 24. in the sludge, and also in the product, were both below the levelof concern.Part 3 - A Vision for the Future"I dont know what they do in those buildings next door. Theyseem to do a lot of business and process a lot of waste materials.They seem to ship a lot of product. They always keep their lawnmowed and the people are always polite." My vision for thefuture paper mill is that it should be "invisible" in terms of itseffects on the environment. Neighbors, from urban people torural fish, ought to hardly notice its presence. The goal of Part 3is to consider what kinds of wet-end additives and relatedprocesses are likely to take place in that paper mill.Fig. 5. Paper technologist thinking of Fig. 6. Using the other dictionaryword "chemistry." definition of "chemistry""Fiber-Friendly"A lot of effort and capital goes into the production of fibers fromwood, as well as from alternative fiber sources such as sugar
  • 25. cane residues (bagasse), straw, and cotton. These are renewableresources. When managed properly, every tree that getsconverted into pulp for paper products gets replaced by newplanting and new growth of trees or other fibrous materials.Actually, the situation is even a bit more complicated than that.Rather than using all trees cut from the forest, the paper industrygets much of its wood fiber in the form of used paper an wastefrom lumber mills and related operations (Smith 1984; Kramer,Jurgen 1998).Even before one considers the effect of papermaking additives,plant fibers already have the following highly desirableattributes: a) they easily bond to each other without needing anyglue; b) they can easily be redispersed in water and formed intorecycled paper; c) they do not originally contain toxic materials;and d) after they have become too degraded or contaminated tobe worth recycling, they still can be used for energy generation(Göttsching 1993; Delefosse 1993; Norris 1998; Weigard 2001).By using fibrous waste products to fuel power boilers at thepaper mill it is possible to displace some of the need for fossilfuels and also reduce landfill requirements. Landfilling of paperproducts can result in production of greenhouse gases such asmethane (Wiegard 2001), so it makes more sense to use wastewood products for fuel and leave more petroleum, natural gas,and coal reserves in the ground.Having said all these nice things about plant fibers, especiallythose from wood, one of our high priorities as an industry oughtto be aimed at preserving their quality and in continuing to userenewable plant fibers as the main component in our products.Calcium carbonate is known to inhibit aging of paper by
  • 26. buffering the pH in the alkaline range. By contrast, acidic papertends to become embrittled during drying and storage, and thecellulose molecules gradually suffer hydrolysis (McComb,Williams 1981). In addition to its beneficial buffering ability,calcium carbonate may be preferred over clay products due to arelatively high purity of its deposits, so that mining of CaCO3generates less volume of "pits" in the ground and "piles" oftailings (Webb 1993). In cases where sludge from treatment ofpaper mill wastes is used for compost, the calcium carbonateprovides useful pH buffering.Mineral fillers, though abundant, are non-renewable, so thereseems little point in trying to load up paper with highpercentages of calcium carbonate, beyond what is needed toachieve opacity and smoothness specifications; rather it has beensuggested that papermakers ought to concentrate on achievinghigh smoothness and covering the paper with relatively thinlayers of mineral-based coatings (Lindström 1994; Swann2000). In that way any printing inks are likely to adhere to thecoating materials and the fibers can be more readily recovered"clean" when the resulting wastepaper is de-inked and recycled.A recent project at North Carolina State University has involvedefforts to minimize or compensate for loss of bonding ability ofkraft fibers when they are dried (Zhang et al. 2000). The visionthat comes out of this type of work is that fibers ought to betreated gently during each cycle of papermaking. One of the keystrategies in this regard is to avoid excessive dryingtemperatures or very low moisture contents, i.e. "over-drying"(Pycraft 1980). It also is recommended to avoid excessiveenergy or intensity of pulp refining (Baker 1995). In this regard,
  • 27. dry-strength chemicals such as cationic starch can help toachieve strength objectives with moderate savings in refiningenergy. Our recent work indicates that the proportional effect ofdry-strength additives added to never-dried pulp may be greaterwhen the fibers are recycled, compared to their effect on theinitial paper."Water-Friendly"In a paper mill of the future I envision that not only fiber, butalso water, is handled as a precious resource. Future mills arelikely to be choosing between the following two alternatives: a)continue the gradual trend of many years towards operation withless and less fresh water per unit of product (Springer 1978;Swann 1999); and b) operate with zero or very little discharge ofliquid effluent - in a so-called "closed water cycle" mode(Pietschker 1996; Wiseman, Ogden 1996). In either case Ienvision that paper mill operations will increasingly turn to theirown wastewater treatment systems as a source of "fresh" water.The logic is as follows: Some level of treatment is required evenof "clean" water from rivers or springs to remove sand, humicacids, and to control microbes. On the other hand, a paper millwill already have expended considerable effort in purifying thewastewater; it may be cleaner in some respects than untreated"fresh" water. In fact, some system already in place to conditionwhite-water for internal re-use are very similar to conventionalprimary clarification of wastewater (Sugi 1997).Not only are future paper mills likely to reuse some of theirwastewater, but it appears likely that some of them willessentially "bring the wastewater treatment plant into the papermill." The motivation for this trend is a need to control the
  • 28. build-up of biological oxygen demand and colloidal materials - aprobable consequence of increased recycling of both fibers andwater (Pietschker 1996; Zhang 1999). Successful applications ofthis type of technology have been reported (Delefosse 1993;Norris 1998). In some cases it is possible to justify the cost ofsuch processes as ultrafiltration (Norris 1998) and ozonization topurify water to be reused in papermaking, whereas the sametreatments would be considered too expensive if the wastewaterwere to be discharged (Demel, Kappen 1999). Combinations ofaerobic and anaerobic treatment have been recommended tominimize the volume of sludge (Göttsching 1993; Demel,Kappen 1999). Compact reactors for biological treatment maymake sense in terms of minimizing the volume of water as papermills begin to incorporate these operations as part of theirsystem (Tenno, Paulapuro 1999; Gubelt et al. 2000). The mostimportant attribute of paper chemicals, in order to be compatiblewith the biological treatment systems just described, isbiodegradability. Some progress has been made in this area(Hamm, Göttsching 1994; Wahaab 2000), but much more workis needed.Another essential part of efforts to reduce water usage is toselect combinations of additives that tend to "self-purge"themselves from the system by becoming retained on fibers. Inprinciple that implies avoiding substances like simple salts,sugars, and oils that have little affinity for fibers, even in thepresence of coagulants or retention aids. In isolated cases it maymake sense to remove excess salts by evaporation or reverseosmosis (Wigsten 1995; Norris 1998; Tenno, Paulapuro 1999).In principle it is possible to maximize the retention of bothcolloidal and fibrous materials in paper by control of highly
  • 29. cationic additives to achieve near neutral zeta potential (Bley1992; Moormann-Schmitz 1994), followed by very-high-massflocculants to collect primary particles into a particles largeenough to be mechanically retained (Horn, Linhart 1991). It isremarkable the extent to which these principles parallel thoseused in treatment of fresh water and wastewater (Leitz 1993)."Frugal of Energy and Raw Materials"I envision the ideal paper mill of the future as being frugal interms of energy and raw materials. Losses of fine materials canbe reduced to very low percentages in the paper forming processby use of an effective retention aid program on a paper machine,plus the use of a saveall to recover fine material from whitewater. Closing up the water system it is possible to conserveheat (Springer 1978; Wigsten 1995). Hot water promotes morerapid drainage, and extra heat energy has to be supplied to theextent that fresh water is used. In these respects the paperindustry already seems to be doing a very good job.Though the paper machine tends to be frugal, relatively largeamounts of fiber fines, fillers, and fibers can be lost when paperis recycled (Paula, Kärnä 1995; Dorica, Allen 1997; Kantardjieff2000). It is likely that much of such waste consists of ink,colloidal materials, and fiber fines too small to be of much valuein papermaking. However there may be opportunities to recyclethe mineral content of waste paper or of sediment in theclarifiers at paper mills. Studies have shown that it is possible to"burn off" various organic materials and recover gray fillerparticles that are useful for paper products with intermediatebrightness targets (Sohara, Westwood 1997; Johnston et al.2000; Moilanen et al. 2000; Wiseman et al. 2000).
  • 30. Further savings in energy, per ton of paper, are likely to come intwo areas. The use of chemicals to promote dewatering andreduce the need to evaporate water was discussed already in anearlier section. It is possible that further savings will be achievedby reducing the amount of water that needs to be pumped. Saidanother way, it will be possible to save electrical energyexpended at the fan pump by increasing the typical consistencyof headbox furnish. Higher-consistency forming has beenconsidered in various publications (Case 1990; Waris 1990).Already, modern headbox designs have been helpful in beingable to still form uniform paper with slightly less water(Kiviranta, Paulapuro 1990). But it seems that chemicaladditives will be needed to that minimize fiber flocculation atthe higher solids levels. Conventional "formation aid" strategiescan have a devastating effect on drainage (Wasser 1978; Lee,Lindström 1989). This is an area of wet-end chemistry that maybecome important in the future."High-Tech"It seems that no vision of the future ought to be completewithout the words "high tech." In terms of papermakingchemicals, the key "high tech" trends to look out for will includeautomation, new sensors, bio-engineered processes or additives,and nano-technology. Recently it seems that nano-technology isa growth area for research. In fact, papermakers have beeninvolved in nano-technology for many years. How big are thecolloidal silica "microparticles" used in drainage-enhancingprograms? The answer is "usually about 1 to 5 nm" (Moffett1994; Andersson, Lindgren 1996; Swerin et al. 1996). So, infact, we already use nano-technology.
  • 31. Bio-tech solutions are recently becoming important in the use ofenzymes for deposit control and slime control (Webb 1994).Enzymes also can be used to reduce the cationic demand ofprocess water, especially in cases involving thermomechanicalfiber (Buchert et al. 1996). In the future we can expect to seemore progress in the use of enzymes to assist with strengthdevelopment and to promote more rapid drainage (Eriksson etal. 1997).The large-scale, continuous, capital-intensive nature ofpapermaking operations make them attractive subjects forimproved process control strategies. The last couple of decadeshave brought substantial progress in the development andimplementation of tray-water solids sensors (Bernier, Begin1994; Artama, Nokelainen 1997). These have made it possibleto even out swings in first-pass retention by varying the additionrates of retention aids. However, in at least one case it wasshown that the demand for retention aid was strongly correlatedwith variations in cationic demand of the furnish (Tomney et al.1997). Therefore it makes sense to control cationic demand aswell, with a goal of getting closer to the root cause of thevariations. Significant progress has been achieved in onlinecharge control, especially with automated streaming currenttitrating devices (Tomney et al. 1997; Gill 2000; Rantala,Koskela 2000). In principle the same type of data can beobtained more accurately and reliably by a new streamingpotential titration method (Hubbe 1999). Other devices that arelikely to become more common, especially in large papermakingfacilities, include automated freeness testers (Lehtikoski 1991),online evaluation of fiber flocculation (Wågberg 1985; Alfano etal. 1998; Hubbe 2000), and automated monitoring of bio-films
  • 32. related to the growth of slime deposits (Robertson, Rice 1998;Dickinson 1999; Flemming et al. 2000).Earning the "Good Chemistry" LabelThe goal of "good chemistry" with the public, with our clients,and with our investors will require a long-term approach. Wehave to face the fact that papermaking is a highly capitalintensive enterprise with long lead times for new constructionand replacement of existing facilities. We cannot expect to keepup with every new change in focus of environmental issues(Vasara 2001). The challenge will be continue to makemeaningful, practical improvements in our practices affectingthe environmental even through changes of issues and economiccycles.So what about the "chemistry" between the paper industry andthe public? We need to encourage an atmosphere of "workingtogether" on environmental issues for the sake of long termprogress is exemplified by a Wisconsin initiative to create aprivate-public partnership (Schmidt 1998). A focus on headlinessometimes can lead to a view that our society is highly polarizedaround issues of environmentalism versus profitability(Reinbold 1994). However, a more cautious analysis of publicopinions reveals that the bulk of the American public tends tosee issues in a much more balanced light, compared to theirpoliticians (Wolfe 1998). Technical people in the paper industryhave a responsibility to be environmental advocates. Someworthy environmental goals for papermakers include a)continuing to rely mainly on a renewable, recyclable resource -wood fibers, b) taking steps to avoid deforestation - byreplanting, recycling, and avoiding waste, c) minimizing water
  • 33. pollution by careful development, selection, and use of wet-endchemicals, and d) minimizing energy use. As noted by Siekman(1998), some environmentally sound practices can be profitable,in addition to their intrinsic benefits.No, it probably wouldnt do much good to place an ingredientslabel on each sheet of paper, or even on each ream wrap, carton,or jumbo roll. But already there have been proposals to labelcertain products as "eco-friendly" (Rogers 1993). For instance,such labels could be awarded by an independent agency basedon a point system, with part of the score coming from suchissues as wet-end chemical practices, bleaching practices, or theamount of energy used in the life-cycle of a product. Ideally thisought to be a voluntary system, something like the ISOcertifications of paper mill practices. In that way, papercompanies will have the incentive to get their products certifiedso that they have the right to label their products as "eco-friendly."Achieving "good chemistry" on the paper machine and with thepublic will require more than good intentions. It will requiresignificant technological input and a long-term commitment onthe part of us in the industry to continue to make the neededprogress.Literature CitedAlfano, J. C., Carter, P. W., and Gerli, A., "Characterization ofthe Flocculation Dynamics in a Papermaking System by Non-Imaging Reflectance Scanning Laser Microscopy (SLM),"Nordic Pulp Paper Res. J. 13 (2): 159 (1998).
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