Temperature and pH: Important variables for deresination in kraft ...

BROWNSTOCK WASHINGTemperature and pH:Important variables for deresinationin kraft brownstock washingBy L.H. Allen and C.L. LapointeAbstract: A Canadian softwood bleached kraft mill needed to reduce the dichloromethane extractivescontent of its pulp by half. This was achieved by raising the exit pH from the brownstockdecker and by increasing the temperature of brownstock washing. The results suggest that, on thebrownstock decker, a pulp exit pH less than 3 can harm deresination by precipitating the dissolvedand dispersed extractives onto the fibers. In addition, the results confirm the importance of usingthe highest temperature possible in unbleached washing to maximize deresination.SEVERAL YEARS AGO, staff in a Canadianbleached softwood kraft mill becameconcerned when one of their majorcustomers complained about pitch depositionproblems. The problems appearedto be peculiar to their pulp since they wereless evident when the customer used other pulps.Laboratory examination of the pulp in questionrevealed a higher than normal concentration ofextractives. A quick subsequent survey indicatedthat the extractives level of the pulp was approximatelytwice that from nearby competitors.The objective of this paper is to describe thesteps taken to successfully reduce the extractivesto an industry-average level.Our best understanding of what happens to theextractives or wood resin in kraft pulping and washingis found in the recent reviews of Ström [1] andBack [2]. There exists experimental evidence that,whereas the lignin is readily removed in the earlystages of washing, the wood resin is removed moretoward the end of washing [3,4]. The rationale forthis is as follows. In the kraft digester, the conditionsof high temperature (150-180°C) and alkaliconcentration cause saponification of glyceridesand esters of the wood resin to yield fatty acids. Thelatter tend to form liquid crystals, which are probablylamellar in shape and form separate droplets[1]. On cooling the cook below 100°C, because ofthe high ionic strength, the droplets have a tendencyto coagulate or “salt out” onto the fibers.Hence, they are relatively unavailable for removalby washing. It is only in the later stages of washing,when ionic strength has decreased sufficiently, thatthis wood resin is redispersed. In the redispersionprocess, the lamellar liquid crystalline phase willform micelles [5-9]. The micelles can further aid inderesination because some of the unsaponifiableportion of the wood resin tends to dissolve in theinteriors of the micelles through a process calledsolubilization. Once redispersed, the micelles canthen be washed out. Hence, thorough washing isespecially important for deresination.There is a pronounced effect of temperatureon washing [10]. In the older literature there is atemperature, the Krafft temperature, for a soap,above which its solubility rises sharply [11]. Nowadays,this rapid increase in solubility is recognizedas being due to the formation of micelles.The solubility of a molecular soap is low, whereasthat of the soap as micelles is high, once the Kraffttemperature has been exceeded. There are not alot of data available for the Krafft temperatures offatty acids of wood resin, but the range of knownvalues is 32-77°C, with the saturated acids tendingto have values toward the high end [1]. Hence theformation of micelles upon redispersion of liquidcrystals is highly temperature dependent andbrownstock washing of extractives from pulp isgreatly facilitated by higher temperature.The effect of temperature on washing appearsto be especially important in the pulping of pineand a little less so in spruce, presumably becauseof the larger size of the pits and larger relative pitarea in pine parenchyma cells, which allow easieroutward diffusion of resin components [2].The effect of pH at the last stage of washing,usually the brownstock decker in an older mill, hasreceived attention from several quarters. Acidificationof the brownstock screenroom is a well establishedtechnology for eliminating pitch deposit formation[12-14]. Acid is added to the unbleachedwhite water to bring the pH to a value of 6.3, ideally.Although the acidification is usually accomplishedwith SO 2 injection or addition of sulfuricacid (most commonly), carbon dioxide is also anattractive option in some mills. Carbon dioxideaddition has been observed in mills to not onlyreduce pitch deposition in the unbleached screenroom, but also to increase drainage rates on thewasher and hence improve washing efficiency[15]. Recent work by Luthe et al. [16] has demonstratedthat the increase in drainage at lower pH ismore due to deaeration of the stock than to anincrease in washing attributable to a more porousmat of less swollen fibers at lower pH.In measurements along the fiberline, the extractiveshave invariably been observed to decreasemonotonically in the unbleached part ofthe mill [17,4]. Such studies, if extended to thebleach plant, usually reveal a substantial (50-100%) increase in extractives during the first stageof bleaching. This has been attributed in part tothe coagulation of the dispersed resin particlesonto the pulp fibers [18]. The dispersed resindroplets are colloidally destabilized below pH 3,due to the protonation of carboxylate groups.Without sufficient electrostatic charge for stability,L.H. ALLENPapricanPointe-Claire,QCC.L. LAPOINTEPapricanPointe-Claire,QCPulp & Paper Canada T 294 104:12 (2003) ❘❘❘ 59

BROWNSTOCK WASHINGFIG. 1. Original extractives profile and results of trials inwhich fresh water was used on the brownstock deckershowers. In the original profile (black bars) the extractivescontent of the pulp was higher after the decker than afterthe last brownstock washer, which is unusual in kraftmills. Subsequent trials of using less acidic fresh water inthe decker showers raised the pH and lowered the extractivescontent of the pulp after the decker.the resin droplets collide with fibers and adhere to them [18,19].There are, of course, additional reasons that the extractives onfibers increase in the first stage of bleaching and these includethe addition of chlorine to double bonds of wood resin components(thus increasing molecular weight) and the precipitationof dissolved soaps onto the fibers with the low pH. The precipitationof dispersed extractives and soaps onto the fibers at low pHthrough an unusual circumstance became an important aspect ofincreasing the efficiency of washing on the brownstock decker ofthe mill in this study.EXPERIMENTALMillThe mill is a bleached softwood kraft market pulp mill located incentral Canada and employing a mixture of black spruce and jackpine. As is often the case with older mills, most of the washers in themill are operating at 20% above design capacity. Saltcake losses atthe brownstock decker run at 9-12 kg/t, which indicates brownstockwashing that is in the normal-to-poor range for kraft pulp mills[12]. Brownstock washing is accomplished with a pressure diffuserfollowed by two rotary drum washers. The final brownstock washingis done on a decker after the screen room. At the time of the work,the bleach plant employed a DcEoHD 1 E 2 D 2 sequence (10% Cl 2 infirst stage; mill has subsequently converted to ECF) and filtrates arecountercurrent (vs. split jump-stage) recycled. Prior to the work describedin this paper, the mill was employing a “cold blow” forgreater pulp strength [20] and this led to lower temperatures acrossthe fiberline. The final temperature of stock from the pressure diffuserwas usually in the range 75-90°C, but it sometimes dropped toas low as 50°C. Fresh water was used at the mill (seal water, coolingwater, and process water) at a rate of 100 m 3 /t. Throughout thework, no talc was used, but a total of 0.1 kg/t of a commercial pitchdispersant (Consperce VIIA, Constant Laboratories) was used inthe bleach plant at the Dc repulper and the decker white waterchest before the pulp machine.Before the trial, the DCM extractives content of the driedpulp typically ranged from 0.11-0.18%, depending on the woodsupplied to the digester. Benchmarking of the extractives contentof the bleached pulp with 16 other NBSK mills suggestedFIG. 2. Dispersed extractives concentration in a kraft millunbleached pulp suspension as a function of pH. Below pH3, the extractives particles are coagulated onto the fibresand fines, and cannot be washed from the pulp.that the extractives level was running at approximately twice theaverage value. This higher extractives content was felt to be alarge contributing factor to deposits in a customer paper millwhere the deposits were especially evident downstream of thekraft repulper, on the consistency sensor.Sampling at the MillSampling was done at eleven points along the fiberline, as indicatedin Figure 1. For drum washers, the samples were taken atthe doctor. Each sample was composed of equal amounts of pulptaken from the center and from each side of the drum to takeinto account the variation in washing across the width of thedrum. Care was taken to obtain the complete thickness of thefiber mat with each sample, because the pulp on top would bebetter washed than that on the bottom. At the blowline, the pulpwas sampled as blown from the digester and before dilution. Thepulp was then vacuum filtered on a Büchner funnel to eliminateas much black liquor as possible, before shipment to Paprican.An effort was made to sample along the fiberline taking intoconsideration the residence time of the pulp at each site. Threesets of samples were obtained with at least half an hour betweeneach set. At our laboratory, the three samples from each sitewere combined and the pH of the filtrate from each resultantpulp was measured. The pulps were centrifuged at our pilotplant to a consistency of approximately 30%. The thickenedpulps were then fluffed, frozen and freeze-dried.ExtractionsThe freeze-dried pulps were extracted with dichloromethane(DCM) in a Soxhlet extractor [21]. The volume of solvent wasreduced on a rotary evaporator and the remaining solvent andextracts were transferred to a pre-weighed vial. The rest of thesolvent was then evaporated under nitrogen and the dry extractswere freeze-dried for one night. Using these procedures, thestandard deviation for bleached softwood kraft pulp at 0.04% extractivesis ±5%.RESULTS AND DISCUSSION1. Extractives Profile Across MillWe began by determining the DCM extractives content of pulpsat various points along the fiberline. The results are shown withblack bars in Figure 1. It is evident that extractives start high atthe digester blowline (0.93%) and are greatly decreased by theend of the brownstock washers to 0.19%.60 ❘❘❘ 104:12 (2003) T 295 Pulp & Paper Canada

TABLE I. Extractives profiles and pH values along fibreline.BROWNSTOCK WASHINGTrials with fresh water used in decker showersOriginal profileDiffusion washer Diffusion washerFinal profilein operationdownExtractives* pH Extractives* pH Extractives* pH Extractives* pH(%) (%) (%) (%)Digester blowline 0.93 12.3 1.22 12.3 0.46 11.7 1.39 12.62nd brownstock washer 0.19 10.3 0.22 10.4 0.23 10.5 0.20 10.5Brownstock decker 0.28 2.4 0.18 9.2 0.20 9.8 0.17 9.9Dc washer 0.24 9.6 0.14 12.3Eo washer 0.20 9.4 0.13 11.9H washer 0.22 8.0 0.13 9.8D 1 washer 0.18 8.6 0.10 10.0E 2 washer 0.16 9.3 0.08 10.5D 2 washer 0.21 4.7 0.09 4.8Pulp machine headbox 0.21 4.4 ND** ND**Dried pulp 0.17 NA*** 0.10 NA*** 0.15 NA*** 0.08 NA****DCM extractives content of pulp ***Not determined ***Not applicableIt was startling to discover that the extractives content of thepulp actually increased at the brownstock decker. They rise from0.19% at the #2 brownstock washer to 0.28% at the decker. Webelieve this result to be significant and beyond experimentalerror and the random variation that occurs with time in theextractives measurement at a single point in a mill. This result isunusual, in that mill extractives profiles reported earlier havealways shown a decrease at the decker.Following the brownstock decker, the pulp proceeds to thebleach plant, where extractives values generally remained constant,except for small variations. The latter include slightdecreases in the extractives content after both E stages and smallincreases in extractives after the H and D 2 stages. The high pHvalues in the E stages make them opportunities for some deresination[18]. The minor increases in extractives in the H andD 2 stages may be due to the addition of chlorine to doublebonds in the resin molecules, which would result in a highermolecular weight and hence a higher resin density.It is evident from Figure 1 that most of the deresination isdone in the unbleached part of the mill and not much reductionis achieved in the bleach plant. Usually, a little more deresinationcan be achieved [18]. Unfortunately, the bleach plantin this mill was constructed with shorter-than-conventional droplegson all the washers, so that the internal vacuums in the washersare lower than industry norm.FIG. 3. Original and final extractives profiles. Black barsshow original profile. Gray bars give extractives contentsof pulps after both the pH changes on the decker and theincreases in brownstock washing temperatures.2. Brownstock Decker Trials (Fresh Water; Higher pH)The brownstock decker of this mill was operated with only Dc filtrategoing to the shower bars. As a result, the pH of the exit pulpwas 2.4 (Table I). Previous work at Paprican [18], shown in Figure2, has indicated that at pH values less than 3, the dispersedwood resin is coagulated onto the fibers and it drops to about aquarter of its original concentration in suspension at pH 2.4.We reasoned that this would account for the increased extractiveson the pulp at the brownstock decker and conducted several trialsin which fresh water at 60°C was used instead. Two such trials wereconducted and, during the second, the diffuser washer was down.The results are given in Table I and Figure 1. The pH valuesof the stock coming from the decker rose to 9.2 and 9.8 (TableI) and, as shown by the open and hatched bars in Figure 1, theextractives were lower (0.18 and 0.20 vs. 0.28%). Moreover, withthis drop in extractives, there is now a monotonic decrease alongthe unbleached fiberline, as observed in other mills in the past.To put the preceding a little more in perspective, it is probablyworth noting that Dc or D 0 filtrate can often be used in and aroundthe brownstock decker with benefits. In a conventional kraft milllacking oxygen delignification, it is common practice to use D 0 filtrateto lower pH at the exit of the unbleached high-density storagetank in preparation for the D 0 stage to save on ClO 2 costs. Furthermore,some mills use D 0 filtrate on the brownstock decker tolower pH in preparation for enzyme treatment in high-density storage.These practices are not likely to cause precipitation of the dispersedextractives onto the fibers, as seen in Figure 1 (original profile),unless the pH of the pulp pad falls to values less than 3.3. Higher TemperaturesThe next step undertaken was to increase the temperature ofbrownstock washing.To accomplish this, two approaches were taken.• Mill personnel replaced some brownstock washer shower waterwith clean evaporator condensate at 90°C.• The digester stock temperature at the blow valve wasincreased by about 10°C.Several days after the process had reached steady state, we tooksamples for a second extractives profile across the mill. The resultsare shown in Figure 3 (gray bars). For comparison, the original pro-Pulp & Paper Canada T 296 104:12 (2003) ❘❘❘ 61

BROWNSTOCK WASHINGfile results are shown with black bars. Overall,there was a significant improvement inthe DCM extractives across the fiberline.The most significant change was a smalldecrease in DCM extractives at the decker,instead of the pH-induced jump evident inthe original profile. Most importantly forcustomers, the extractives content of thedried, fully bleached pulp, was now reducedby over 50%. This, despite a significantlyhigher DCM extractives content in the blowline(1.39%, probably attributable to thewood) after the changes than in the originalextractives profile (0.93%).There was no noticeable change observedin pulp tear or tensile at the timeof the temperature increases and therehas been no detectable change sincethen. This observation is, of course, tentativesince there is always considerable variationin both the wood and the process.CONCLUSIONSElimination of excessive amounts of Dcstagefiltrate to the brownstock decker (toraise the unusually low pulp exit pH above3) and an increase in brownstock washingtemperatures have combined to produce asignificant decrease (~50%) in the extractivescontent of the fully bleached pulp.At the customer paper mill, thesechanges have alleviated the deposition prob-Résumé: Une usine canadienne de pâte kraft blanchie de résineux devait réduire de moitié lateneur en dichlorométhane (produit d’extraction du bois) de sa pâte. Pour ce faire, on a élevé lepH de sortie de l’épaississeur de pâte brune et augmenté la température du lavage de la pâtebrune. Les résultats suggèrent que, dans l’épaississeur de pâte brune, un pH de sortie inférieur à3 peut nuire à la dispersion de la résine en précipitant les matières extractives dissoutes et disperséessur les fibres. En outre, les résultats confirment l’importance d’employer la températurela plus élevée possible lors du lavage de la pâte écrue afin de maximiser la dispersion de la résine.Reference: ALLEN, L.H., LAPOINTE, C.L. Temperature and pH: Important variables forderesination in kraft brownstock washing. Pulp & Paper Canada 104(12): T294-297 (December,2003). Paper presented at the 89th Annual Meeting in Montreal, QC, on January 28-30, 2003. Notto be reproduced without permission of PAPTAC. Manuscript received on April 11, 2003. Revisedmanuscript approved for publication by the Review Panel on June 2, 2003.Keywords: KRAFT MILLS, KRAFT PULPS, BLEACHED PULPS, SOFTWOOD PULPS,EXTRACTIVES, METHANE, PH, TEMPERATURE, WASHING, UNBLEACHED PULPS.lem experienced earlier on the consistencysensor downstream of the kraft repulper.The results suggest that, on the brownstockdecker, a pulp exit pH less than 3 canharm deresination by precipitating theextractives onto the fibers. In addition, theyconfirm the importance [2,4] of using thehighest temperature possible in unbleachedwashing to achieve maximum deresination.LITERATURE1. STRÖM, G., “Physico-Chemical Properties and SurfactantBehavior”, in Pitch Control, Wood Resin andDeresination, E. Back and L.H. Allen, eds., ch. 5, TAP-PI Press, Atlanta, pp. 139-149 (2000).2. BACK, E.L., “Deresination in Pulping and Washing”,in Pitch Control, Wood Resin and Deresination, E.Back and L.H. Allen, eds., ch. 8, TAPPI Press, Atlanta,pp. 205-230 (2000).3. BERGSTRÖM, H. and CEDERQVIST, K.N., “Occurrenceof Resin Acids in Black Liquor and Wash Water”,Svensk Papperstid. 40(5):112 [in Swedish] (1937).4. TUFVESSON, M., BJÖRKLUND JANSSON, M., andBACK, E.L., SCAN Report 502, “Softwood Resin in KraftPulping and Subsequent Washing”, [in Swedish], (1987).5. STENIUS, P., PALONEN, H., STRÖM, G., andÖDBERG, L., “Micelle Formation and Phase Equilibriaof Surface Active Components of Wood”, in Surfactantsin Solution, K.L. Mittal and B. Lindman, eds.,Plenum Publishing, New York, Vol. I, p. 153 (1984).6. PALONEN, H., STENIUS, P., and STRÖM, G., “SurfactantBehavior of Wood Resin Components: TheSolubility of Rosin and Fatty Acid Soaps in Water andin Salt Solutions”, Svensk Papperstid. 85:R93 (1982).7. STRÖM, G., STENIUS, P., LINDSTRÖM, M., andÖDBERG, L., “Surface Chemical Aspects of theBehavior of Soaps in Pulp Washing”, Nordic Pulp Pap.Res. J., 5(1):44 (1990).8. LINDSTRÖM, M., ÖDBERG, L., and STENIUS, P.,“Resin and Fatty Acids in Kraft Pulp Washing. PhysicalState, Colloid Stability and Washability”, Nordic PulpPap. Res. J., 3(3):100 (1988).9. ÖDBERG, L., FORSBERG, S., MCBRIDE, G., PERS-SON, M., STENIUS, P., and STRÖM, G., “SurfactantBehavior of Wood Resin Components. Part 2. Solubilizationin Micelles of Rosin and Fatty Acids”, SvenskPapperstid, 88(12):R118 (1985).10. BJÖRKLUND JANSSON, M., BACK, E.L., andTUFVESSON, M.I., “A Review of Kraft Pulp Deresinationand Pitch Problems”, 1985 Trans. VIIIth FundamentalResearch Symposium Notes, Mech. Eng. Publ.,London, p. 729.11. KRAFFT, F. and WIGLOW, H., “On the Behaviorof Fatty Acid Alkalies and their Soaps in the Presenceof Water. IV. Soaps as Crystalloids”, Chem. Ber. 28:2566[in German] (1895).12. ALLEN, L.H., “Pitch Control in Pulp Mills”, in PitchControl, Wood Resin and Deresination, E. Back and L.H. Allen,eds., ch.11, TAPPI Press, Atlanta, pp. 269-291 (2000).13. AFFLECK, R.R., and RYAN, R.G., “Pitch Control in aKraft Pulp Mill”, Pulp Pap. Mag. Can., 70(24):107 (1969).14. ALLEN, L.H., “The Importance of pH in ControllingMetal-Soap Deposition”, Tappi J., 71(1):61-64 (1988).15. GIRARD, R., HO, C., GUTHRIE, P., and CAMPBELL,P., “The Effects of Carbon Dioxide on the Efficiency ofVarious Brownstock Washers”, in Proc. 1999 TAPPI PulpingConf., TAPPI Press, Atlanta, Vol. 3, p. 1225.16. LUTHE, C., BERRY, R., and NADEAU, L., “HowDoes Carbon Dioxide Improve Brownstock Washing?”Preprints CD, 2003 TAPPI Fall Tech Conf., Session 12,paper 3.17. JONASSON, R.G., FUHR, B., SITHOLÉ, B.B., andALLEN, L.H., “Studies on the Occurrence of Glycosidesin Aspen Wood and Kraft Pulp”, in preprints, 9thInternational Symposium on Wood and PulpingChemistry, CPPA, Montreal, pp. 43/1-6, June (1997).18. ALLEN, L.H. and LAPOINTE, C.L., “Physical Distributionof Resin in Bleached Kraft Pulp Mills”, Pulp Pap. Can.,88(12):T483-490 (1987); Pulp Pap. Can. 89(3):9 (1988).19. ALLEN, L.H., “Pitch in Wood Pulps”, Pulp Pap.Can., 76(5): T139-146 (1975).20. CYR, M.E., EMBLEY, D.F., and MACLEOD, J.M.,“Stronger Kraft Softwood Pulp -Achieved!”, Tappi J.,72(10):157-163 (1989).21. PAPTAC Standard Testing Methods G.13 and G.20.62 ❘❘❘ 104:12 (2003) T 297 Pulp & Paper Canada