The influence of starch on drainage and retention in paperboard mill systems

Transcription

1 The influence of starch on drainage and retention in paperboard mill systems ABSTRACT Paper and paperboard mills have been closing their whitewater systems for various reasons. Such closure has resulted in the buildup of organic contaminants which lead to reduced effectiveness of polymeric additives. Starch, as an organic contaminant, can be present at levels up to 10% by weight of fiber when secondary fibers are used as raw materials. This study determined the effects that nonionic, anionic, and cationic starches have, either alone or in combination with polymeric additives, on drainage, retention, web vacuum dewatering response, and wet pressing response. In combination with polymeric additives, starches did not have a measured adverse effect on polymeric additive performance. However, anionic starches alone can affect drainage either positively or negatively, depending upon fiber furnish. Cationic starch at 1% by weight improved drainage and retention. At higher levels, drainage was adversely affected. Nonionic starch had no measured effect. KEYWORDS Drainage Retention Starch Cationic compounds Anionic compounds Water reclamation Water removal White water A. M. Springer Associate Professor, Miami University, Oxford, Ohio S. Chandrasekaran Process Engineer, ContainerCorp. ofamerica, Chicago, T. H. Wegner Chemical Engineer, Forest Products Laboratory, Forest Service, U.S. Dept. of Agriculture, Madison, Wis In recent years paper and board mills have experienced increasing pressure to close their whitewater systems out of energy and environmental considerations. The dissolved/suspended solids level in the whitewater systems builds up if the effluent discharge is below 33 m 3 /metric ton and if these materials are not appreciably retained in the sheet (1). Strutz reported that for a fine paper mill system, the organic contaminants had a much more detrimental effect on retention than did the inorganic material (2). These organic materials, often polymeric in nature, exhibit steric hindrance effects as well as charge effects. The charge effects have usually been considered to be dominant and are the reason that measurement of zeta potential has gained importance recently (3). This has not, however, led to the development of a definitive control parameter, probably because the steric factors or the protective colloid actions were also important. Board mills using recycled paper and paperboard as a fiber source have large amounts of dissolved and sus 104 pended organic materials, so-called anionic trash, in their furnish. A major source of organic materials is starch. Based on a 1977 survey, 0.82 million metric tons of starches were used in papermaking alone, including anionic, nonionic, and cationic types (4). Most starch is added at the size press. In addition, nonionic starches are used as adhesives in corrugated containers. The total starch content of the sheet can be as much as 8% by weight. Analyses of whitewater samples from paperboard mills show starch concentrations ranging from 1.5% to 10.1% (5). As whitewater systems become more closed in paperboard mills, the polymeric additives used to promote drainage and retention are less effective. Much of this is probably the result of a system buildup of dissolved and suspended material, of which starches are a major constituent. Our study was undertaken to determine (a) the effects of starches (cationic, anionic, nonionic) on drainage, fines retention, web vacuum dewatering response, and wet pressing response, and (b) the interaction between the starches and the two major types of polymeric additives generally used to improve drainage and retention. Resultsand discussion Because the starches by themselves can affect measured parameters, the results are presented separately for the effect of starches used alone and those used in combination with polymeric additives. In addition, a qualitative model for the optimal use of polymeric additives in a paperboard mill is presented. Effect of starches alone Nonionic starches had no deleterious effect on drainage, fines retention, or web vacuum dewatering response with either the top-liner or filler-ply fiber furnish (Table I). Results show that anionic starches can affect drainage (Fig. 1). With the top-liner furnish, the strongly anionic starch (Anionic B) hindered drainage while having little effect on the filler-ply furnish. The opposite was true with the less strongly anionic starch (Anionic A). Web vacuum dewatering response, wet pressing re- February 1984 / Tappi Journal

2 sponse, and fines retention were not greatly affected by the anionic starches. Cationic starch improved drainage at the 1% level. The improvement in drainage was quite large with the filler-ply furnish and only slight with the top-liner furnish (Fig. 1). At the higherlevelsof5%and10%,thecationic starch adversely affected drainage, probably because of dispersion of the system. Fines retention also improved at the 1% cationic starch level. Web vacuum dewatering response was unaffected with the tap-liner furnish. Compared with the control, higher solids contents were measured with the filler-ply furnish, however. Cationic starch had little effect on wet pressing response. Interaction of starch and polymeric additives Under the conditions of this study, addition of starches (cationic, anionic, and nonionic) did not adversely affect polymeric additive performance. Results for cationic starches are given in Table I. Those for the anionic and nonionic starches have been published by Chandrasekaran (5). For any given starch level, increasing the polymeric additive level increased drainage and retention while decreasing web vacuum dewatering response. Wet pressing response was unaffected except in one isolated case, with 10% cationic starch, I. Effect of starches alone and cationic starch Tappi Journal / Vol. 67, No

3 1. Drainage time for top-liner and filler-ply furnishes as a function of starch type and concentration. 2. Web dewatering response to vacuum at various concentrations of starch for different levels of PAM and PEI. where a lower solids content was measured. Such a case would not likely be encountered in actual mill operation because cationic starches are not used at high levels. Effect of polymeric additives alone With no starch added, both polymers increased drainage and retention as the polymer level increased (Table I). The flocs were large with polyacrylamide additive (PAM), while with polyethyleneimine additive (PEI) the floc size was much smaller. The polymeric additives reduced the vacuum dewatering response (lower solids content) on the top liner furnish, but they had less effect on the filler-ply furnish. This may be the result of the higher fines content and the nature of the fines in the filler-ply furnish. Generally, however, web vacuum dewatering response is reduced by flocculation, which gives a web with larger pores. These pores allow air to break through more easily, resulting in a web that does not seal well. Wet pressing response was not affected by the polymeric additives, however. This is because water removal in wet pressing at high dryness levels is ultimately controlled by the rate of water movement through the fiber cell wall, not by web structure (6-8). A qualitative model for drainage and retention We observed that the drainage rate improved with both PAM and PEI polymeric additives. This suggests that floc size is of lesser importance than pore volume, because the drainage rate increased with macro flocs as well as with micro flocs. Although our study was not conducted at varying shear rates, we can speculate that the drainage rate is a function of resultant pore volume and that the polymer is important only in its effect on pore volume. Both polymers reduced the web dewatering response to vacuum, but the magnitude of this effect was generally greater with the PAM (Table I; Fig. 2). Flocculation with a polymeric additive is associated with some air leakage through the sheet, and this inhibits web vacuum dewatering response. A wellflocculated sheet would be expected to have poor vacuum dewatering response: conversely, a sheet containing no flocs or uniformly distributed tiny flocs should respond better. Retention improved with both polymers and was noticeable in spite of the thick mat formed in our tester. Retention seemed to be governed by the amount of flocculation and not the type offlocs. In summary, an ideal polymer can be defined as one that can offer the following benefits in any system: High drainage rate Good retention Good response to vacuum (higher solids content) No adverse effect on wet press dewatering. This study indicates that for a paper 106 February 1984 / Tappi Journal

4 board mill system with a high degree of closure, a cationic, low-molecularweight, high-charge-density polymer (PEI)will be more desirable than a high-molecular-weight, low-charge-density polymer (PAM) if anionic trash, other than starches, can be neutralized. Conclusions Effects of starches alone Nonionic starches had no measured deleterious effects on drainage, fines retention, or web vacuum dewatering response. Anionicstarchescanaffectmeasured furnish drainage either positively or negatively, depending on the fiber furnish. Web vacuum dewatering response,finesretention,andwetpressing are little affected by anionic starches. Cationic starches at 1% by weight improve drainage and fines retention. At higher levels, drainage is adversely affected. Interaction of starches with polymeric additives Under the conditions of this study, starches did not have a measured adverse effect on polymeric (PAM and PEI) additive performance. For any given starch level, increasing the polymeric addition increased drainage and retention but decreased web vacuum dewatering response. Wet pressing response was generally unaffetted by polymeric additives and starches, The ideal polymer An ideal polymer for a board mill system at low shear appears to be a cationic, low-molecular-weight, highcharge-density polymer (PEI). Experimental Furnishes Fiber furnishes. Two furnishes were used. One was a top-liner furnish prepared from 50% ledger cuttings and 50%sp;odb;eacjedsi;fatecittomgs.Tjos furnish had a CSF of 320 ml and a ph of 6.8, with a fines content of 39.5%. The other was a filler-ply furnish, consisting of 20% old corrugated cuttings, 30% news, and 50% carton stock. The ph was 6.9 and the CSF was 260 ml. The fines content was 47.5%. Fines contents were determined by TAPPI procedure T 261 pm-80. Furnishes were made up twice: the data in the first two sections of Table I are from the first batch of furnishes, while that in the third is from the second batch of furnishes. The freeness and fines contents were essentially the same, but other contaminant levels led to slightly different results. The trends are consistent although the absolute values differ slightly. Starches. Four commercial starches Tappi Journal / Vol. 67, No. 2 RETENTION were used-onecationic, two anionic, Test procedure and one nonionic. The starches were virgin and notrecycled, for convenience Because a single adequate test pro of experimentation. Starch concentra- cedure for measuring drainage, reten tions of1%,5%,and 10%byweightwere tion (fines and total), web vacuum de used, based on measurements from mill watering response, and wet pressing whitewater samples that showed starch response was not available, a new pro- concentrations ranging from 1.5% to cedure was developed (5, 9). The test 10.1% by weight (5). Six mill samples procedure is based on a modified Britt from two different mills, three ma- water release analyzer (WRA) (10). chines, and differing fiber furnishes Modificatiom are aimed at making the were tested using TAPPI procedure T formed web the principal resistance to 419 om-80. water removal, rather than the screen, Polymeric additives, Two polymeric piping, or valve. The screen was readditives were used, representing two placed with a 19.2%-open-area cylinder differing types generally used as drain- machine wire (52 38 mesh). The botage and retention aids. One additive tom of the WRA jar was opened up to was a cationic, high-molecular-weight, accept a 19-mm-ID pipe with a 19-mmlow-charge-density PAM. Three con- ID solenoid valve. Two electrodes were centrationsofadditivewereused:0.03%, located in the jar to function as liquid 0.09%,and0.13%byweight.Thesecond level detectors controlling a timer. A additivewasacationic,low-molecular- selector switch was designed to supply weight, high-charge-density PEI. The power to either an agitator or timer and three concentrations of additive used to the electrodes. A schematic of the test were 0.1%, 0.3%, and 0.5% by weight. apparatus is shown in Fig. 3. The step- Both polymeric additives were used at by-step procedure is as follows: manufacturer recommended additions 1. Fill the jar with water fom the and ph range. solenoid valve to the wire so that the wire is just submerged. Experimental design 2. Run the agitator at 6.67 rps and A factorial experimental design was pour 400 g of stock at 0.7% consistency. used with the two fiber furnishes, four 3. Agitate the stock for 15 s. starches at three starch concentrations each, and PAM polymeric additive at 4. Change the selector switch position three concentrations. With the PEI polyto the timer so that the agitator stops meric additive, only cationic and strongand the timer begins to run. Simul ly anionic (Anionic B) starches were taneously, open the solenoid valve, with evaluated at three polymer levels, three the vacuum in the suction flask at 5.1 starch levels, and two fiber furnishes. kpa. The timer automatically stops In addition, controls were run far the when the Liquid loses contact with the two fiber furnishes without any starch electrodes. This is recorded as the drain- or polymeric additive. The large quan- age time. tity of data generated precludes pre- 5. Increase the vacuum in the flask sentation of all data here. A complete to 30.5 kpa for 13 s. set of experimental data is available in 6. Close the solenoid valve; remove the thesis by Chandrasekaran (5). and weigh the pad. 3. Drainage, vacuum response, and retention tester. 107

5 RETENTION 7. Filter the filtrate through a Whatman No. 40 filter paper and determine the retention (total and fines). 8. Press the pad in a Noble and Wood press to find the response to wet press dewatering. Pressing conditions were selected to give a web dryness between 40% and 60% solids. These are the same conditions usually applied to produce handsheets in the Noble and Wood sheet machine. 9. Weigh the pad, dry in an oven, and reweigh. Calculate web dryness in response to the 30.5-kPa vacuum and wet pressing. This procedure allows us to measure drainage, retention (fines and total), response to vacuum, and response to wet pressing. The test models, as closely as possible, what happens on a modern board former with suction boxes. At the same time, it also represents what occurs on a fourdrinier machine with foils followed by suction boxes. In this work, the vacuum levels were chosen on the basis of the range encountered in paperboard mills. The 0.7% consistency was chosen to represent the typical vat consistency. The basis weight of the pad formed was 344 g/m 2, representing a board-grade basis weight. Literature cited 1. Heller, P., Scott, W., and Springer, A., Tappi 62(12): 79(1979). 2. Strutz, M., The Effect of Total Water Reuse and Alum Control on First Pass Retention, M.S. thesis, Miami University, Penniman, J., Optimization of the Electro-kinetics of the Papermaking Systems, TAPPI Retention and Drainage Short Course, Seminar Notes, May 20-27, 1979, Cincinnati, Ohio. 4. Turner, C.W., Pulp & Paper 42(12): 170(1978). 5. Chandrasekaran. S., The Importance of Starch as an Organic Contaminant on Retention and Drainage: Board Mill Systems, M.S. thesis, Miami University, Wahlstrom, B., Tappi 61(1): 75(1981). 7. Wahlstrom, B., Svensk Papperstid. 18: 32(1981). 8. Young, T.L., Caulfield, D.F., and Wegner, T.H., Tappi J. 66(4): 85(1983). 9. Wegner, T.H., Springer, A.M., and Chandrasekaran, S., Procedure for Measuring Drainage of Pulp Slurries, unpublished report, Britt, K.W., and Unbehend, J.E., 1980 Papermakers Conference Proceedings, Atlanta. April 13-16, TAPPI PRESS, Atlanta, p. 5. This work was completed as part of the requirement for the M.S. degree at Miami University. Partial funding for the project was provided by the USDA Forest Products Laboratory. Technical data on planning information was provided by the Forest Products Laboratory and the Box Board Research and Development Association. Received for review April 25, Accepted August 11, February 1984 / Tappi Journal