A precise method to measure the sclereid content of kraft pulps

Transcription

1 A precise method to measure the sclereid content of kraft pulps By J. Hoffmann and D. Ouellet Abstract: Sclereids are cells from the bark of most trees that form large clusters. These bond poorly in a sheet and cause defects. There is currently no standard method to quantify pulp sclereid content. A variety of methods are used and some have serious limitations. A new sclereid measurement method was developed, based on three steps: sclereid concentration, fibre screening, and microscopical analysis. Results show that this method gives significantly higher precision and lower variability than other techniques. S CLEREIDS are thick-walled cells that form in the bark of most softwood and hardwood trees [1]. They are often referred to as stone cells, but this term is somewhat ambiguous because it is also sometimes used to identify clusters of phellem cells from outer bark. Because sclereids are found in clusters that are typically much larger than wood fibres, they can cause significant pulp quality problems. Sclereids are undesirable, and their content in pulp should be minimized. Obviously, the bark content of the incoming wood supply should be kept as low as possible to prevent sclereids from entering the process in the first place. When they do enter the process, effective removal strategies are needed to keep them out of the finished pulp. Research on sclereid removal is currently underway at Paprican. To compare the effectiveness of various removal strategies, an objective and precise method for measuring sclereid content in pulp was clearly needed. Existing methods were reviewed and tested. A detailed evaluation of the two most widely used methods was carried out and revealed that these methods were imprecise. In view of this, a new method was developed to improve the precision of sclereid content determinations. This report details this review and development process and demonstrates the advantages of the new method. EXISTING SCLEREID MEASUREMENT TECHNIQUES Despite the serious problems they can cause, and the recognized need by many mills to quantify sclereid content for quality control purposes, there is currently no standard method to characterize pulp sclereid content. A variety of methods are used by different mills and laboratories [2,3]. These methods are surveyed in this section, and can be divided into three broad categories: handsheet methods, methods based on visual examination of pulp samples, and methods based on separation of sclereids. It should be noted that many of the techniques listed here have never been reported in the literature. Many of the observations that follow are the result of experiments carried out in this laboratory. Note also that for the sake of sim- plicity, we will use the term sclereids to refer to sclereid clusters in this report. Handsheet Methods: Pressed handsheet method: One of the most widely used methods for sclereid measurement in industry involves calendering or heavy pressing of handsheets [2]. In this procedure, one or more standard handsheets are made. After pressing and drying, the handsheets are passed through a roll press one or more times. This heavy pressing makes sclereids appear as translucent spots ( windows ) in the sheet, and these can readily be identified and counted on a light box. Although this method is widely used, there is no accepted standard procedure for testing, and practice varies from mill to mill. Some mills reduce the oven-dry target weight of the handsheets from 1.2 grams to 1.0 g, 0.8 g, or even less in an attempt to enhance the formation of sclereid windows. In some cases, windows are only counted if they exceed a certain minimum size. Nip loading on the roll press, handsheet moisture content, and the propensity of the sheet to wrinkle, determine how many calendering passes are required to develop windows, and procedures vary widely between mills. The pressed handsheet method gives a measurement of the number of window producing particles in the handsheet. One of its drawbacks is that windows can be produced by a variety of other contaminants [2], and results must therefore be interpreted with care. Handsheet protrusion count: In this method, standard handsheets are removed from the press plates after drying, and the surface not in contact with the plate is inspected for protrusions or bumps caused by bulky particles like sclereids. This inspection can be performed visually or by touch. Tactile inspection is conducted by systematically sweeping the fingertips over the sheet surface to feel protrusions. Protrusions can also be detected visually under reflected light, and often a combination of visual and tactile inspection is used. In some cases, protrusions are only included in the total count if they exceed a certain minimum size. Protrusions, which are obviously produced by non-sclereid contaminants such as large shives, can be excluded from the total count, but in most cases it is difficult to determine the cause J. HOFFMANN, Paprican, Vancouver, BC D. OUELLET, Paprican, Vancouver, BC douellet@paprican.ca :7/8 (2005) T 168 Pulp & Paper Canada

2 FIG. 1. Comparison of pressed handsheet and black tray sclereid measurement methods. Sclereid counts with the two methods differ markedly, and the 95% confidence limits show that precision is relatively poor. FIG. 2. Schematic view of sclereid concentrator. of the protrusion. This is probably the biggest limitation of this technique, since the material responsible for the protrusions is not positively identified. Fibre knots and nodules, fibre strings, knife and blade knits, and shives may all create protrusions that are not easily distinguished from those caused by sclereids. Visual Examination Methods: Black tray method: Another widely used method to count sclereids is to disperse a small amount of pulp, usually one gram, in a black tray or over a blue glass plate. The tray is then scanned to count the number of sclereid particles. The counts are usually performed with the naked eye, or at best with the aide of a magnifying glass. There are many variants of the tray method. Differing background colours, illuminating methods, and stains have seen use in various laboratories. A major drawback of this method is the difficulty in finding and counting all the sclereids, because kraft pulps naturally form dense flocs, which can easily mask sclereids. Another issue is that it can be difficult with the naked eye to distinguish sclereids from other white particles. Accurate sclereid identification requires the use of a stereo-microscope with a magnification range of at least thirty times. Unfortunately, the field of view of such a microscope is limited, making it impractical for scanning the large area of a tray. Positive identification of sclereids with the tray method can therefore be a problem. Microscopical count: In this procedure, a small pulp sample (typically 0.5 to 1 gram) is diluted, and a portion of it is placed in a Petri dish and examined under a low power stereomicroscope. The sclereids are counted, and the Petri dish is repeatedly emptied, refilled, and examined until the whole pulp sample has been inspected. The sample must be dilute enough to limit the presence of flocs, which hinder the counting procedure and may cause the inspector to miss some sclereids. On the other hand, diluting the sample increases its volume, thus lengthening the over-all sample processing time. Performing multiple tests and averaging results is desirable in this case, but would obviously increase testing time. This procedure is time consuming and tedious. The sample mass must be minimized to make this method practical. It is therefore critical to obtain a representative sample, but variability between samples is an issue with such small amounts of pulp. A variation of this method is to manually pick suspected sclereids from the pulp by examining the sample with the naked eye, and place this material under a microscope for final identification and counting. Testing remains tedious in this case, and great care must be exercised when picking out sclereids since they can fragment into several pieces. Separation Methods: In general, separation methods aim to remove fibres and retain sclereid particles for analysis. The sclereids collected after separation are usually counted manually, a procedure that is greatly facilitated if the sample is relatively free of fibres, but in some cases the sclereids can simply be dried and weighed. In the latter case, the sample must be completely free of residual fibres to accurately reflect the sclereid content of the pulp. Flat screens: Flat screens such as the Valley or Somerville are commonly used to remove shives, and in principle they are also suitable for sclereid removal, although their use for this application presents a number of problems. First, the manipulations involved to collect sclereids after screening can easily lead to the loss or breakdown of some sclereids, thus skew- Pulp & Paper Canada T :7/8 (2005) 49

3 FIG. 3. Comparison of sclereid content measurements with the new Paprican method, the pressed handsheet method, and the black tray method. The 95% confidence limits show that the Paprican method is the most precise. FIG. 4. Number of tests required to ensure a maximum error of 2 sclereids per gram when determining the sclereid content of a population with a given standard deviation. ing the results. Also, the minimum standard slot size available for these screens is 0.15 mm. Many sclereids are small enough to pass through these openings but large enough to cause problems. Our tests have indicated that a 0.1-mm slot size is needed to retain most sclereids. It may be possible to obtain a custom made 0.1-mm plate, and this would certainly be recommended for sclereid removal. Pulmac screens: The Pulmac Shive Analyzer and the newer Pulmac Masterscreen are automated devices used to screen pulp samples to collect contaminants. They use screen plates with slots as narrow as mm and can process kraft pulp samples of the order of 10 grams or more. Contaminants are transferred to a collection cup by an automatic rinse cycle. The bottom of the collection cup is fitted with a fine mesh screen to let water out while retaining the rejects. A significant problem with these screens is that other contaminants such as shives and fibre knits are retained and thus remain as part of the sample. Depending on their abundance, these contaminants can pose significant difficulties for sclereid identification and counting. Fibre knits are particularly troublesome in market pulps reslushed from dried sheets. With softwood kraft pulps, our tests with the Pulmac Shive Analyzer, using a sample mass of 10 grams and a 0.1- mm screen plate, have shown that too much fibrous material generally remains in the screening chamber. This frequently caused the sample cup to overflow during the rinse sequence, with the associated loss of the sclereid sample. If the sample cup is removed and a pail is placed below the cup holder bracket, the entire sample and rinse water can be collected. This sample can be re-run through the analyzer, and in most cases the residual fibre will be almost completely eliminated. The collected rejects can then be placed on a black filter paper to count sclereids under a microscope. The newer Pulmac Master Screen is better equipped to provide extensive fibre removal in a single pass, yet we frequently found that too many residual fibres were present to easily perform a sclereid count, even when the device was run in the extended B mode. Again, as for the Pulmac Shive Analyzer, the sample can be collected and re-run through the screen to eliminate residual fibres. Flotation separation technique: A further method of separating fibres from sclereids, discussed by Forster [2], is to repeatedly dilute a pulp sample and decant off fibres that remain at the top. Sclereids tend to settle more quickly than fibres, and thus remain in the bottom of the container. When enough fibres have been floated off, the remaining sclereids can be placed in a container with a black finish and counted. However, Forster commented that the method was time-consuming, technique-sensitive, and lacked reproducibility. A variation of this method was described by Thair and Corcoran [3], who proposed the use of toluidine blue stain in sample preparation to differentiate sclereids from regular fibres. In this case, sclereids are counted in a dish with a transparent bottom placed over a white background. Forster [2] tested the technique and indicated that while staining improved the identification of sclereids, the method was still technique-sensitive and therefore subject to operator bias. Another way to remove fibres by flotation is to place the sample in a cylindrical container and inject a small tangential stream of water near the top. Once the container fills, water and fibres start to wash over the top, in theory leaving the sclereids deposited on the bottom. In experimenting with this method, we found no way to prevent some sclereid carryover in the overflow, in part because sclereids can be entangled in fibre flocs. It was also difficult to produce near complete fibre separation, even after extending the separation time to over 30 minutes, because some coarse fibres tended to settle out with the sclereids. The unpredictable loss of sclereids in the overflow and the poor separation efficiency made this method impractical. EVALUATION OF EXISTING SCLEREID MEASUREMENT TECHNIQUES The above review has highlighted the pros and cons of a variety of methods to measure sclereid content. Many of these methods are used for routine control in different mills, but their relative capacity to detect small changes in sclereid content is unknown. Therefore, it was decided to carry out a series of trials with the techniques that were considered most promising in terms of giving fast, accurate, and reliable results. Two methods were selected for evaluation: the pressed handsheet method and the black tray method. Both are widely used by mills because sample handling is easy and because they are relatively fast. Comparison of the Pressed Handsheet and Black Tray Methods: To compare the two methods, a series of pulp blends were prepared from two bleached softwood kraft pulps: one with relatively high sclereid content, and one with low sclereid content that had been screened through a Pulmac Shive Analyzer fitted with a 0.1-mm screen plate. The two pulps were combined to yield blends with 0%, 25%, 50%, 75% and 100% high sclereid pulp on a dry mass basis. For each blend, eight samples were prepared for testing with each method. The one exception is the low sclereid pulp (0% sample), which was not tested with the pressed handsheet method. For the handsheet method, eight standard 1.2-g handsheets were prepared for each blend. The finished handsheets were sent to a member company mill for calendering and sclereid counting. Calendering was done in a roller press with five passes at 6.9 MPa (1,000 psi), followed by five passes at 13.8 MPa (2,000 psi). Translucent spots with a minimum size of 0.2 mm 2 were counted as sclereids. For the black tray method, a total of :7/8 (2005) T 170 Pulp & Paper Canada

4 eight 1.2-g samples were processed for each blend. Each sample was split into two portions that were diluted to one litre, mixed, and transferred to the black tray. With the aide of a magnifying glass/ring lamp and dissecting probe, the entire area of the tray was systematically examined to count all white sclereid-like particles. The dissecting probe was used to sweep through the dilute pulp suspension and break apart flocs that could hide sclereids, and to turn over sclereids that were positioned on their side. The sclereid counts for each of the two sample portions were added and the total was normalized to the sample mass to give a count of the number of sclereids per gram of pulp. It should be noted that the analysis time and effort expended for each sample in this procedure were well beyond what is practical for routine control in a mill environment. The black tray test results presented here are therefore believed to be more precise than what routine practice would typically yield. The results of all tests are summarized in Fig. 1. Each data point in this graph is an average of the eight samples processed in each case, and the error bars represent the 95% confidence limits. A linear regression line is shown in each case, along with the R 2 value of the fit. For both methods, there is a good linear correlation between the sclereid counts and the percentage of high-sclereid pulp. The fit is somewhat better for the pressed handsheet method (higher R 2 ), although the results are more variable with this method, as evidenced by the wider confidence limits for the mean. The sclereid counts for the pressed handsheet method are systematically higher than those of the black-tray method. This could be explained by the fact that particles or contaminants other than sclereids can cause windows in the pressed handsheets. However, counts performed with the black tray method can also include non-sclereid particles that appear as white specks under low magnification. Another factor that may contribute to the discrepancy between the two methods is that some sclereids are translucent when suspended in water, making them much harder to detect with the black tray method. The biggest concern with these two methods is the variability in the results from sample to sample, as evidenced by the wide 95% confidence limits. This high variability makes it difficult to detect small changes between the mean sclereid content of two pulps at a statistically significant level. In the results shown here for the pressed handsheet method, the difference between the means of the 25% and 100% mixtures and those of the 50% and 100% mixtures are significant at the 99% level of confidence, but all other differences are not significant even at the 95% level. For the black tray method, the mean of the 0% mixture is significantly different from all others at the 99% level, and the 25% mixture has a significantly different mean than the 75% and 100% mixtures at the 95% level of confidence. All other differences are not statistically significant. NEW SCLEREID MEASUREMENT METHOD The precision of the results obtained with the pressed handsheet and black tray methods can only be improved by increasing the number of samples measured. The above results have shown that the precision is still fairly poor even when eight repeat samples are processed, and many more samples would need to be counted in each case to improve the situation. Since our interest was ultimately to track small differences in sclereid levels due to changes in operating parameters of industrial pressure screens and cleaners, it was clear that a more precise method for sclereid measurements would be required. The following sections describe the improved method developed at Paprican and the associated equipment. This new sclereid measurement method relies on three steps. A pulp sample is first run through a specially designed concentrator to remove most shives, knife edge knits, and fibres. The leftover material is then run through a standard Pulmac screen to wash away any remaining fibres, leaving a sample that consists almost exclusively of sclereid clusters. Finally, the sample is deposited on a dark filter paper and sclereids are counted under a microscope. Sclereid Concentration: A special concentrator apparatus, shown schematically in Fig. 2, was designed to accept a 10-gram (O.D.) sample of pulp at approximately 0.3% consistency and slowly meter this pulp into the feed flow of a small hydrocyclone. The system is automated and driven only by water pressure. The pulp sample is placed into the concentrator column and the system is turned on. The flow of fresh water through the eductor draws in a small flow from the bottom of the column, thus feeding the hydrocyclone with very dilute pulp. The low feed consistency in the hydrocyclone allows it to operate at extremely high sclereid removal efficiency. The sclereids thus end up in the reject flow, which returns to the column, and fibres go mostly to the accept flow, which is purged to sewer. The system runs automatically for five minutes, during which time the entire pulp sample circulates through the hydrocyclone about six times. At the end of the cycle, the content of the column is transferred to a collection cup with a fine mesh bottom. About 95% of the fibrous material from the original 10 gram sample has then been removed, leaving about half a gram of fibre and practically all the sclereids. In experiments with samples that KRAFT PULPS were seeded with a known number of dyed sclereids, at least 98% of the dyed sclereids were collected at the end. Screening: The sample collected in the sclereid concentrator cup is transferred into a Pulmac Shive Analyzer and screened for approximately five minutes using a 0.1-mm (0.004-in.) screen plate. The function of this screening step is to remove any fibres and small debris that remained at the end of the concentration step. Small particles like ray cells and tiny sclereid fragments are also washed away by passing through the screen. A 0.1 mm screen plate was selected in this case because it was the smallest practical slot size that could be used. The smaller mm (0.003-in.) slots tended to retain a small amount of fibres that could interfere with the microscopical count. Larger slots, like the 0.15-mm (0.006-in.), allowed many sclereids to pass through. In fact, an evaluation we conducted with kraft pulp showed that over 85% of the sclereids retained on the 0.1-mm screen plate will actually pass through the 0.15-mm plate. There is obviously a minimum size below which sclereids are not problematic, but this minimum size is largely productdependent. A 0.15-mm screen plate may therefore be suitable in some cases, but the 0.1-mm plate will retain a wider range of sclereid particles. It is worth noting that for this work, the original collection cup for the Pulmac Analyzer was replaced with a custom aluminium one fitted with a 150-mesh stainless steel screen. The new cup design ensures that no sclereids are lost through the screen or caught in the annular groove that holds the screen in place. The wall of the new cup was also raised to prevent splash-out during the sample transfer/rinse mode. Microscopical Count: After fibres have been removed from the original sample using the two steps described above, the remaining material can be deposited on a filter paper for examination under a stereomicroscope. A black filter paper provides better contrast for viewing bleached sclereids. To speed up the microscopical analysis, the filter paper is ruled with lines spaced 4 to 5 mm apart to create clearly defined bands that can quickly be scanned to count sclereids. A custom-machined die is used to emboss such lines in the filter paper. The sclereid sample can be evenly deposited on the filter paper using a vacuum funnel. The filter paper is then placed under a stereomicroscope and the sclereids are identified and counted. This step can take 5 to 20 minutes depending on sclereid contamination levels. The final count is expressed as sclereids per gram by normalizing the total count to the mass of the original pulp sample. Comparison of Results: The new method described above was put through the Pulp & Paper Canada T :7/8 (2005) 51

5 same evaluation as the handsheet and black tray methods. The same reference pulps with high and low sclereid concentration were used to produce blends for this evaluation. For each blend, eight samples (10 grams O.D.) were prepared and tested. The results are plotted in Fig. 3, along with those shown earlier for the handsheet and black tray methods. It can readily be seen from Fig. 3 that the results from the Paprican method fit very closely to the expected linear trend. The R 2 value for the regression line is greater than 0.99, the highest of the three methods. The precision of the new method is also superiour to the other two methods, as shown by the narrow confidence limits on the data points. Statistical tests of hypotheses show that the difference between the means of any two mixtures is highly significant (99% confidence level). It is also interesting to note that the sclereid counts with the new method fall somewhere between those of the pressed handsheet and the black tray methods. This supports the idea that for the pressed handsheet method, some windows may be caused by contaminants other than sclereids. Another possibility is that some of these windows are caused by sclereids that are small enough to pass through the Pulmac screen openings in the Paprican method. The black tray method systematically gave lower sclereid counts, most likely because small sclereids or sclereids that lie on their side are missed, since they are very hard to see without significant magnification. The idea that sclereids are missed from the counts in this case is supported by the fact that the Paprican method yields dry samples in which sclereids are positively identified before they are counted, thus establishing beyond any doubt that the true sclereid counts are greater than those obtained with the black tray method, even when the latter is carried out with great care. To appreciate the benefits of increased precision from this new test method, it is useful to determine the number of samples required to limit the uncertainty on the mean to a fixed amount. This uncertainty results from the fact that we are using the mean from a number of small samples to estimate the mean sclereid content of a large shipment of pulp, which we can refer to as our population. For a population with known standard deviation, we can be (1 )*100% confident that the difference between the sample mean and the true mean of the population will be less than a specified amount e, if the sample size is: z /2 2 n = ( ) e where z /2 is the value of the standard normal distribution leaving an area of /2 to the right [4]. A curve of n as a function of is plotted in Fig.4 for e = 2 (± 2 sclereids per gram) at the 95% confidence level ( /2 = 0.025). This curve shows, for example, that if the standard deviation of the population is 4, then about 15 repeat samples are required to ensure (with 95% confidence) that the sampling mean is within 2 sclereids per gram of the population mean. The symbols on the curve represent the results from the tests we carried out with the various methods. Since the standard deviation of the population is unknown, we used the standard deviation of the samples to locate these points on the graph. The horizontal error bars around each point represent the 95% confidence limits, i.e. the interval within which the standard deviation of the population lies with 95% certainty. These intervals are calculated from the chi-square distribution [4]. For the pressed handsheet method, given the large standard deviation, we can see that in the best of cases, at least 13 samples are required to narrow down the mean to within 2 sclereids per gram. In the worst case, this number reaches 57. The black tray method fares somewhat better, with 2 to 29 samples required depending on the case, although these numbers would likely be larger if the counts with this method had not been carried out so carefully. With the Paprican method, the results show that only 1 to 4 samples are needed to reach the same degree of certainty. If a decision has to be made about shipping pulp to a customer based on a specification for the maximum number of sclereids per gram, one can readily understand that an imprecise method of characterization of the sclereid content is more likely to lead to returned shipments. Another situation where a precise characterization method is desirable is when conducting trials for process optimization. Sclereid removal efficiency is undoubtedly affected by operating conditions for screens and cleaners, but it may be difficult or even impossible to measure any improvement if the test method used lacks precision. The new sclereid measurement method proposed here represents a marked improvement in this respect. CONCLUSION An evaluation of the two most common sclereid measurement techniques, the pressed handsheet and the black tray methods, has shown that their results are fairly imprecise. The usefulness of these methods to quantify the sclereid content of pulp is thus questionable except as a means to detect large outbreaks. A new sclereid measurement method has been developed. This new method is based on three steps: sclereid concentration, fibre screening, and microscopic analysis. Test results show that this new method has a significantly higher precision than the other measurement techniques. This implies that fewer repeat tests are required with the new method to precisely determine the mean sclereid content of a pulp. The new method also provides an objective means to evaluate the efficiency of sclereid removal strategies. ACKNOWLEDGEMENTS The authors would like to thank Robert Gooding, who contributed many useful ideas as project leader in the early days of this research effort. We also wish to thank James Drummond and Paul Watson for their thorough review of the manuscript. Murray Webster and Peter Lawrie of Skeena Cellulose Inc, Prince Rupert also deserve credit for useful discussions and support in evaluating sclereid levels in handsheets. Joe Aspler is acknowledged for useful discussions, as is Phil Allen for his comments on an early draft of the manuscript. LITERATURE 1. PARHAM, R.A. Pulp and Paper Manufacture, 3rd ed. Vol.1. Properties of fibrous raw materials and their preparation for pulping. Ed. M. Kocurek. Atlanta: Joint Textbook Committee of the Paper Industry, TAPPI (1983). 2. FORSTER, S. Sclereid control at Canfor Prince George. Preprints, 1987 Spring Conference, TS, CPPA, Whistler (1987). 3. THAIR, B.W., CORCORAN, P.J. A quantitative method for the measurement of sclereid clusters in bleached softwood kraft pulp. Preprints, 1979 Spring Conference, TS, CPPA, Harrison Hot Springs (1979). 4. WALPOLE, R.E., MYERS, R.H. Probability and Statistics for Engineers and Scientists. 2nd Ed. New York: MacMillan Publishing Co., 580 p. (1978). Résumé: Les sclérites sont des cellules provenant de l écorce de la plupart des arbres et qui se présentent en gros amas. Ceux-ci sont faiblement liés à la surface d une feuille et causent des défauts. Il n y a présentement pas de méthode normalisée pour mesurer la teneur en sclérites de la pâte. Un éventail de méthodes sont utilisées et certaines ont de sérieuses limitations. Une nouvelle méthode de mesure a été développée, basée sur trois étapes: concentration des sclérites, tamisage des fibres, et analyse au microscope. Les résultats montrent que cette méthode est beaucoup plus précise que les autres techniques. Reference: HOFFMANN, J., OUELLET, D. A precise method to measure the sclereid content of kraft pulps. Pulp & Paper Canada 106(7/8): T (July/August, 2005). Paper presented at the 90th Annual Meeting in Montreal, QC, on January 26 to 29, Not to be reproduced without permission of PAPTAC. Manuscript received on November 17, Revised manuscript approved for publication by the Review Panel on November 2, Keywords: KRAFT PULPS, SCLERENCHYMA, MEASUREMENT, TEST METHODS :7/8 (2005) T 172 Pulp & Paper Canada