Retention (Low, Variable, High)
Before taking action to increase the retention efficiency of fine particles during the formation of paper, it is worth taking some time to consider the relative importance of this issue. None of your customers will ever call you up and complain that your first-pass retention was too low when you produced their order. Rather, your customers will focus on attributes such as brightness, strength, dust, wrinkles, and wavy edges - attributes that affect the end-use of the product. Furthermore, most modern paper machine systems are well equipped with save-alls, so relatively few fiber fines may leave with the wastewater, even if the retention efficiency in the forming section is very poor.
But there are some key advantages of maintaining moderately good to excellent retention efficiency. Perhaps the most critical cases are those in which a reactive additive, such as ASA size, has been added to the furnish. If such additives are not retained during the first pass, it can be expected that a measurable fraction will have decomposed by the time the same process water has been cycled back to the headbox. Moderate use of retention aids tends to keep the paper machine system cleaner by lowering the amounts of fines and pitch-like particles or emulsion droplets that are floating freely in the process water. By keeping many of the fines bound to the surface of fibers, the resulting paper tends to be more uniform in the thickness direction, especially in the case of Fourdrinier paper machines. Finally, a high level of fiber fines recirculated back to the headbox tends to make the basis weight harder to control; any small changes in retention efficiency can be expected to cause momentary maxima and minima in basis weight, requiring correction by the process control system. A moderate level of retention aid treatment helps keep such fine material bound to the fibers, eliminating some of the potential for short-term swings in basis weight.
The definition of first-pass retention (FPR) is 100% times the difference between the headbox consistency and the white water consistency, all divided by the headbox consistency. There is no such thing as "bad" retention, but sometimes papermakers decide to aim for a higher value of FPR for some of the reasons outlined above. Values of FPR higher than 90% are common during production of paperboard grades, while FPR values as low as 50% may be "normal" on a certain twin-wire former producing newsprint paper.
LOW FIRST-PASS RETENTION
Suppose that it has been decided to increase the first-pass retention on a certain paper machine. The most likely short-term answer is "increase the addition level of retention aid." A secondary answer may be to work with a retention aid supplier to identify a more effective additive or combination of additives, dependent on your furnish conditions and process equipment.
Let's briefly review some of the common materials used as retention aids and how they work. For simplicity, let's imagine a hypothetical paper machine system in which no chemical additives are being used initially. The first thing that one might consider adding is something like aluminum sulfate (papermakers' alum) or polyaluminum chloride (PAC). Such additives act by neutralizing the excess anionic charges at the surface of fibers and fines. In the absence of repulsive electrical forces, the solid particles can stick together by weak van-der-Waals forces (especially the London dispersion component). For a somewhat stronger effect one could treat the furnish with a high-charge cationic polymer. In addition to neutralizing some of the excess anionic charge of the solid surfaces in the furnish, such additives tend to form positively charged "patches" that can be attracted to negatively charged areas on other solid surfaces in the furnish. The next class of additives to consider are what are commonly known as retention aids. The most widely used retention aids are very-high-mass copolymers of acrylamide. They may be either cationic (positively charged) or anionic (negatively charged). Experimental results suggest that these retention aids act by forming molecular bridges between adjacent surfaces in the furnish. Finally, one can consider the use of microparticulate additives, which are typically added last to the system, downstream of the retention aid addition point, and usually right after the furnish has passed through a set of pressure screens to redisperse the fibers. The microparticles generally have a strong negative surface charge, high surface area, and tiny size, usually with the smallest dimension in the range of 1-20 nm. Evidence suggests that the microparticles function by interacting with the very-high-mass retention aid molecules or cationic starch molecules, causing the large molecules to contract.
Because there are so many choices of retention aid additives, dosage considerations, and addition point choices, it is recommended that trials be conducted with the help of a supplier of the chemicals. Initial tests can be carried out in the lab, though it is still most useful to run the tests with fresh, hot furnish from the paper machine.
Unexpectedly low values of first-pass retention can result from a variety of system changes. For instance, it is possible that the particle size of the mineral filler has decreased, so the total surface area that is available to take up retention aid molecules has increased. Since the surface area of fillers tends to be much higher than the same mass of cellulosic material, the effect may be equivalent to a reduction in the amount of retention aid being added. The same effect can happen due to other changes that increase the specific surface area of the furnish, i.e. increased refining or an increase in the ratio of hardwood to softwood fibers, etc.
Some things to check if the retention is unexpectedly low include (a) the flow of retention aid, including the accuracy of the meter, (b) the concentration of the additive in the preparation tank, (c) whether there are undissolved solids in the retention aid supply container, make-down system, or caught in a canister-type filter, (d) whether a canister-type filter is clogged. In the case of acidic papermaking conditions retention efficiency usually can be improved by increasing the pH at least as high as 4.5 or higher by NaOH or sodium aluminate and by using enough alum, e.g. 10 to 30 lb/ton in different cases.
Sometimes retention aid efficiency is hurt by the presence of interfering substances. For example, anionic colloidal materials in the furnish are known to have a very bad effect on the performance of cationic acrylamide copolymer retention aids. The anionic colloids can include such substances as wood pitch, carry-over of black liquor in the case of unbleached kraft paper production, carry-over of oxidized hemicellulose byproducts in the case of bleached kraft paper production, dispersants present in coated broke, anionic direct dyes in the case of deep color paper production, and dispersants in certain slimacides. This list is probably far from complete, but it includes some of the most common culprits. One approach to overcome some of these problems is to improve the efficiency or washing stages or avoid over-use of the offending materials. Another approach is to treat the furnish with a sufficient quantity of soluble aluminum product or high-charge cationic polymer to partly neutralize the negative charges in the system.
VARIABLE RETENTION EFFICIENCY
Additional measures may be appropriate if the retention efficiency varies over time. The best approach is to try to identify the root cause of the variation. For instance, does retention efficiency always get worse when the proportion of coated broke entering the system increases? Does it get worse only when the paper grade is changed to a lower basis weight? Do the cycles have anything to do with the preparation cycle of batches of retention aid copolymer? Is there a problem of unsteady water pressure, that may affect the delivery of retention aid polymer to the addition point? You also can review some of the factors mentioned in the previous paragraphs when considering what might be responsible for observed changes in retention. Once the root cause is identified, it is often possible to make changes to smooth out or reduce the variations.
A more aggressive approach is to practice online control of white water consistency. Devices are available from several suppliers for continuous measurement of white water consistency, usually based on optical principles. Frequent calibration of such equipment is required, and a lot of attention needs to be paid to what is happening at the point where the white water sample is being collected. The signals from the sensing device can be used to control the dosage of a very-high-mass acrylamide copolymer. Successful operation of an online retention control system can yield substantial benefits. In addition to keeping the retention level almost constant, it can be expected that the basis weight and paper properties also will remain within tighter limits.
An additional layer of online control can be beneficial if there are large variations in the cationic demand. Studies have shown cases in which in the efficiency of retention aid polymers were highly correlated with such variations. Online control of excess charge in the white water, or further back in the system, have the potential to overcome such effects.
RETENTION EFFICIENCY TOO HIGH
Papermakers almost never complain that the first-pass retention on their paper machine is too high, but they do complain about poor formation uniformity. If the paper has a higher than necessary degree of fiber flocculation, then it may be a good idea to cut back on the retention aid dosage. To avoid causing a web break it is usually a good idea to change retention aid dosages gradually over several minutes. As a courtesy, it is a good idea to alert the crew member responsible for adjusting the draws on the paper machine web, since large changes in retention efficiency have the potential to make the paper web go tight or slack.
Aloi, F. G., and Trsksak, R. M., "Retention in Neutral and Alkaline Papermaking," in J. M. Gess, Ed., Retention of Fines and Fillers during Papermaking, TAPPI Press, Atlanta, 1998, Ch. 5, p. 61.
Beck, M. W., "The Importance of Wet End Equipment and its Influence on Retention," in J. M. Gess, Ed., Retention of Fines and Fillers during Papermaking, TAPPI Press, Atlanta, 1998, Ch. 7, p. 129.
Doiron, B. E., "Retention Aid Systems," in J. M. Gess, Ed., Retention of Fines and Fillers during Papermaking, TAPPI Press, Atlanta, 1998, Ch. 8, p. 159.
Hubbe, M. A., "Retention and Hydrodynamic Shear," Tappi J. 69 (8): 116 (1986).
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Lancaster, E. P., "Retention: Definitions, Methods, and Calculations," in J. M. Gess, Ed., Retention of Fines and Fillers during Papermaking, TAPPI Press, Atlanta, 1998, Ch. 1, p. 3.
Maltesh, C., and Shing, J. B. W., "Effects of Water Chemistry on Flocculant Makedown and Subsequent Retention and Drainage Performance," Proc. TAPPI 1998 Intl. Environ. Conf., 227 (1998).
Strazdins, E., "Surface Chemical Aspects of Polymer Retention," Tappi 57 (12): 76 (1974).
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (firstname.lastname@example.org). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.