Whether substances come in with wood, process water, or recovered paper, papermakers should work to eliminate them from the overall paper cycle
Papermaking's Problem Substances Interfere with Machine Runnability
BY LESLIE WEBB There is a wide range of substances present in papermaking that can cause problems. These substances (Figure 1) might be loosely defined as "substances that impair the runnability of the papermaking process and/or the quality of the product." This rather wide definition would include substances that cause the range of conventional deposits at the wet end of the paper machine (Table 1) as well as "interfering substances."
The term "interfering (or disturbing or detrimental) substances" has been around for some time, but it begs the question-interfering with or disturbing what? All problematic substances interfere with the process/product in some way, but interfering substances do so through their physico-chemical interaction with chemical additives. This interaction is usually brought about by the charged groups present within the interfering substance.
These charges are usually negative and this has thus led to the common terminology "anionic trash." It should be noted that interfering substances are dissolved materials, although they may have originated from some particulate input to the system. They are thus quite distinct from the negative charges on particulate surfaces, which are usually beneficial because they provide attachment points for adsorption of cationic additives.
ORIGINS AND CHEMICAL CHARACTER. Interfering substances are not new to the papermaking process, but their significance has increased due to changes in the nature of some raw materials (e.g., more peroxide bleaching of mechanical pulps) and due to the increased efforts at water system closure. The latter is important because the interfering substances are part of the dissolved fraction with its inherently low single pass retention on the paper machine wire. When closing up, the buildup of dissolved anionic charges from interfering substances is thus much greater than that of charges on particulate surfaces, even charges on the fines fraction (Figure 2).
Interfering substances are not deliberately added to the wet end but are incidentally present in the following:
Virgin pulp from carryover of original or modified wood substances and pulping/bleaching chemicals
Recovered paper from carryover of original or modified paper substances and deinking/bleaching chemicals
Mill broke from surface-applied chemicals.
Typical substances from these different sources are summarized in Table 2. The anionic substances derived from virgin pulps should not be present at high levels on paper machines using recovered paper, unless the original paper machine was using chemicals to neutralize their effect (in which case their anionicity would probably no longer be measurable) or was itself operating a substantially closed water system (in which case they would be retained in their natural anionic state).
The nature of charged materials on paper machines using recovered paper depends on the grade of the paper and whether it has been deinked. By its nature, deinking removes much of the soluble material, including any potentially interfering substances, such as size press starches, but it can also introduce other interfering substances (e.g., silicates).
As in the case of virgin pulps, the actual carryover to the paper machine depends on the mode of water use and pulp washing efficiencies. In the particular case of mill broke, of course, all the materials (including those added to the paper surface) are returned to the paper machine, but at least in this case the nature of any interfering materials should be clearly known, and appropriate action can be planned more effectively.
There is one other raw material common to all paper machines-freshwater. Although this can be a source of organic substances (e.g., humic acids) with interfering anionic groups, its main contribution to the process chemistry is inorganic ions such as sodium, calcium, bicarbonate, and sulfate. These materials do influence wet-end chemistry, the most prevalent examples being calcium ions competing with cationic additives for adsorption on fibers and the ability of sulfates to accelerate corrosion through sustaining sulfate-reducing bacteria. However, they do not generally contribute to what is called anionic trash. The amount of freshwater used influences the microbiological conversion of nonionic substances, such as some starches, to species such as acetic acid with charged groups, but the molecular weight of such materials is too low to interfere with cationic additives.
EFFECTS ON PAPERMAKING. When pulps containing interfering materials enter the papermaking system, the interfering substances can do one of two things-remain associated with the particulate solids or dissolve in the liquid phase. Most of the substances listed in Table 2 are water-soluble, but their actual dissolution depends on a host of factors, some related to the pulp (e.g., its swelling and particle size distribution) and some to the process (e.g., temperature and chemistry).
Fiber swelling is influenced by the content of anionic substances, and the mutual repulsion between such groups is one of the main driving forces for their dissolution. Substances that can reduce this repulsion (e.g., cations such as calcium) play an important role in determining the overall extent of fiber swelling and the dissolution of interfering substances.
The quality of the freshwater and the degree of water closure of the papermaking circuit affect dissolution through their effect on the baseline chemistry and the buildup of simple electrolytes and heat. Given the interplay between these factors, predicting the overall dissolution of soluble materials is far from easy.
Anionic substances that do dissolve can interact with cationic additives forming insoluble polymer complexes (sometimes called symplexes) that do the following:
Inactivate the additive
Impair the additive's retention
Contribute to deposition
Impair drainage on the wire and in the press section.
Not all anionic substances interact in this way, with one important factor being the substance's molecular size. In the case of anionic hemicelluloses in mechanical pulps, it has been shown that enzymatic pre-treatment of the pulp lowers its measured cationic demand through hydrolysis of the polysaccharide to small oligosaccharides.1 One of the best-known effects from the presence of anionic interferences is the consumption of cationic retention aids and the consequent decline in fines retention. This is illustrated by the example in the sidebar, "Dissolved substances from coated broke,".
The stoichiometry of these interactions is not as easily predictable as it is for conventional chemical reactions between charged species, such as simple inorganic ions. It has been shown that the measured cationic demand of an anionic material depends on the nature (charge density and polymer size) of the cationic chemical, which is usually an organic polymer such as polyethyleneimine (PEI), and on the mixing conditions during the reaction. These factors determine how the two oppositely charged polymers conform as their polymer chains intermingle.
There is one substance listed in Table 2 that is conspicuous because it has a positive rather than a negative charge-calcium ions. Other cations will be present, but calcium is important because its level is not actively controlled, and it is therefore quite variable depending on water hardness and the vagaries of microbiological activity. It interferes indirectly with the performance of cationic additives through competition for adsorption sites on particulate surfaces, and it usually wins this battle since it is usually already present by the time the cationic chemical is added to the system.
Strictly speaking, this is not an interfering substance as previously defined since the adsorbed material causes the problem, not the dissolved calcium. But, of course, the latter provides the driving force for maintaining this equilibrium. An example of this effect is shown in the sidebar "Adverse effect of dissolved substances from recovered paper," . Conversely, cations such as calcium can improve the performance of anionic additives through acting as a link (sensitizer) between the negatively charged surface and anionic additive.
The overall retention of interfering substances that remain in the liquid phase depends on the degree of water closure of the paper machine. If retained, the interfering materials listed in Table 2 can be divided into those that would have some functional value in the product and those that would contribute little or nothing to the product's performance. The only substances falling into the former category are the pulp hemicelluloses and the size press starches, both of which would improve product strength. Retained lignin compounds would improve stiffness, but adversely affect brightness. The inorganic substances are unlikely to be present at levels where they could exert significant adverse effects.
MEASUREMENT AND MONITORING. All of the materials listed in Table 2 could be analyzed by specific chemical techniques but would take considerable time unless there was prior knowledge of their nature. Not surprisingly, the normal way of quantifying interfering substances is via their charge, since this property is an expression of how they interfere.
One method of controlling interfering substances (discussed below) is chemical treatment, which relies on the addition of chemicals that react preferentially with dissolved substances before they interact with particulate surfaces. In effect, the dissolved negative charges are titrated with a cationic chemical until the negative charges are neutralized and the cationic demand is close to zero.
The laboratory technique of polyelectrolyte or colloid titration is widely used by chemical suppliers to assay the charge of polymers in a similar manner and has been successfully applied to papermaking systems in the last few years. Initially, this was performed using a dye to indicate when the charges had been fully neutralized, but this has given way to the use of a streaming current detector (SCD) as the end-point indicator.
Of course, conventional electrokinetic techniques (e.g., electrophoresis for zeta potential measurement) cannot be applied directly to solutions, since there is no particle to track. Whereas these two methods of charge measurement used to be looked on as competitors, it is generally now accepted that they are complementary, with SCD being used for dissolved materials and the conceptually similar, but practically different, method of streaming potential used for particles. Although there are a number of SCD manufacturers (e.g., Milton Roy and Lechintec), the best-known is Mutek, with its well-established laboratory particle charge detector and its more recent online particle charge titrator.
Arguably, the most significant potential online application of this technique is the measurement of anionic trash originating from pulps and broke. This is best measured in the thick stock where it is most concentrated, but most SCD instruments cannot handle particulate solids due to the narrow passageway within the measurement cell. The obvious solution is filtration before measurement, and Mutek has now developed a high-consistency stock sampler that removes the solids before charge measurement on the filtrate.
The titrated cationic demand can be used to control the dosing of a charge-neutralizing chemical and thus regulate the charge character of the stock passing forward into the thin stock. In view of the inherent variability of anionic trash levels, these sensors will become important components in future real-time wet-end monitoring and control systems as a means of stabilizing charge through controlled dosing of charge-neutralizing chemicals.
CONTROLLING INTERFERING SUBSTANCES. An ideal hierarchy of control systems for interfering materials (Table 3) should be based on the principle that early prevention is better than later cure. Ideally, therefore, raw materials not containing interfering substances should be used preferentially, but this isn't too practical at the present time since it would make unavailable virtually all the fibrous raw materials used to make wood-containing papers (mechanical pulps) and recycled paper grades (recovered paper).
On the premise that the interfering substances present in recovered papers are principally derived from paper chemicals added in the previous cycle, papermakers producing grades that can be recovered and then recycled (i.e., most of them) should attempt not to use chemicals that will interfere the next time around. There should be some enlightened self-interest at work here since unless avoiding action is taken, the original paper machine will experience some interference effects from mill broke.
One of the important elements in the recyclability of papers is thus the content of problematic substances-stickies, white pitch, and interfering substances. The presence of problematic substances does not make the paper unrecyclable, but it does increase the resource inputs (energy, chemicals) required to produce a clean pulp with good machine runnability.
As noted in Table 2, there are two notable sources of surface-applied chemicals causing wet-end instability-size press starches and dispersants from coating pigments. Not all size press starches contribute to anionic trash-the worst offenders are oxidized starches-but most contribute to other wet-end difficulties such as slime growth. Cationic starches remain adsorbed to the recycled fiber and thus do not build up and cause problems. There are many published articles illustrating this approach and the benefits of a cleaner wet end and wastewater, plus superior paper quality.2
A similar approach is possible by replacing anionic with cationic coating dispersants. This approach is not fully developed, but superior coating coverage and printability have been demonstrated. A study by ECC International showed that, whereas coated broke with an anionically dispersed calcium carbonate lowered filler retention (from 74% to 50%), coated broke with a cationically dispersed carbonate lifted retention to 84% (all data with the same dose of a cationic polyacrylamide retention aid).3 The cationic dispersant is positively useful at the wet end when large quantities of anionic substances from other sources are present, since it provides "free" cationic material to help neutralize the interfering substances.
Once their intake is minimized, the papermaker can endeavor to optimize the distribution of interfering materials between the particulate and liquid phases to minimize any adverse effects and maximize any possible benefits. In addition to the benefits from minimizing interference effects, there is a strong natural inclination to minimize dissolution, since this maximizes the papermaking yield from raw materials and lowers wastewater loads.
However, harnessing the process conditions that minimize dissolution (e.g., low temperatures and high concentrations of multivalent cations) is not always practicable and needs to take into account the consequences of such changes on other aspects of wet-end chemistry (e.g., on deposit formation) and on productivity (e.g., on the ease of water removal).
Optimization of wet-end chemistry to eliminate the interference effects described above has mainly been undertaken by existing chemical suppliers, notably those selling the cationic chemicals (e.g., retention aids, starches) whose performance is adversely affected. Before describing the chemical techniques used to overcome this problem, it is worth mentioning an alternative approach. Whereas the chemical techniques are based on some form of on-machine charge neutralization, the alternatives are based on physical separation/treatment of the interfering materials, preferably as early as possible during stock preparation.
At its simplest, this would take the form of a pulp thickener similar to those used in off-machine pulping/deinking systems. These are sometimes used on paper machines prior to on-machine high-consistency dispersion processes or prior to high-consistency pulp storage, but the filtrate (and its load of dissolved materials) then tends to be recycled back or forward to the process.
The problem with diverting the filtrate elsewhere is that the wastewater loads are increased as less of the dissolved solids is retained in the paper. Ideally therefore, this technique of stock dewatering needs to be integrated with a process that recovers something of value from the filtrate's organic load. Instead of an extra thickening stage, the existing filtration taking place either on the machine wire or saveall could be harnessed to provide the feedstock for on-machine bio-treatment or ultrafiltration.
Work in Finland with the latter has shown substantial reductions (57% to 81%) in the cationic demand of various machine filtrates and whitewaters.4 These options are summarized schematically in Figure 3.
One possible technique to remove dissolved substances from the papermaking circuit in an environmentally sound way is anaerobic treatment to generate methane, a wastewater treatment system already practiced by many mills using recovered paper. A good example of this approach is provided by the zero-discharge KNP-BT mill at Zulpich, Germany, which has installed a combined anaerobic/aerobic treatment plant in parallel with the paper machines.5
The main motivation for this installation was the difficulty in satisfactorily controlling in-mill anaerobic breakdown of wastepaper-derived starches, leading to retention of odorous compounds in the linerboard/fluting products. The dissolved COD in the process water is thus reduced from 35,000 to 7,500 mg/l with a substantial drop in the levels of organic acids and a consequent rise in pH from 6.3 to 7.3. The conductivity declined from 9 to 4.5 mS/cm due to lower concentrations of calcium (down from 3.7 to 0.5 g/l) and sulfate (down from 1.5 to 0.5 g/l). The microbial content of the treated water is important, since it is fully returned to the process so the observed 95% reduction is a valuable additional benefit. The changes in chemical consumption have been small and, while there was an initial increase in biocide use, this subsequently declined below previous levels.
Having minimized the intake of interfering substances and considered the use of separation/removal techniques, the papermaker is probably still faced with the presence of interfering substances within the paper machine system. From the above discussion, it is evident that problems only occur if cationic additives are being used.
One way around the problem is to deliberately use noncationic additives, if they are available. An example is the use by newsprint mills of retention aid systems that are based on nonionic polyethyleneoxide (PEO). Since this approach is not always available, it is more normal to use cationic chemicals to neutralize the anionic interference.
Before looking at the chemical techniques in current use, it might be instructive to consider the charge density of papermaking materials-both the anionic interfering substances and the cationic additives (Table 4). It is evident that, although the anionic trash substances are only present as contaminants, their negative contributions (even at modest levels quoted in this example) are comparable with the positive charges from typical additions of some cationic additives.
It is worth pointing out that anionic carboxymethylcellulose (CMC) is deliberately added on machines making wet-strength papers to improve the retention of highly cationic wet-strength resin. CMC functions best when added prior to the resin. Its high molecular weight serves either to anchor the resin to the fiber surface or form a retentive, more weakly cationic complex.
It is hardly surprising that the cationic chemicals with high charge densities (PEI, polyDADMACS, and PAC) are the ones mainly being used for neutralization of anionic trash at paper mills. It should be noted that the charge of some cationic additives is pH dependent if the nitrogen (which is responsible for the cationic character) is only secondary or tertiary (as in unmodified PEIs) rather than quaternary (as in polyDADMACs). PAC provides a source of cationic aluminum that is more stable and more pH tolerant than is possible with the cationic complexes formed in situ by papermaker's alum. Polyaluminum silico-sulfate (PASS) should also be able to perform a similar role, but so far is little used in papermaking.
Much of the work in this area has been carried out on wood-containing furnishes, particularly coated grades such as lightweight coated (LWC) due to their high content of anionic trash and particulate fines from both pulp and broke. This work has highlighted the importance of both charge density and molecular size of the neutralizing chemical in determining its relative competitive affinity for negatively charged dissolved and particulate solids.
Marked differences in the efficacy of, for example, PAC and polyDADMAC, appear to be due to differences in system chemistry such as pH and conductivity, as well as differences in the nature of the anionic trash. There are obvious dangers in generalizing about the applicability of chemical control techniques between seemingly similar systems.
Much of the work has, not surprisingly, been directed at optimizing retention aid systems, but another strand of work relates to starch. Since the cationicity of starches, even at the upper end of the normal cationic range, is quite low, an obvious way to try to circumvent the problem of impaired starch retention in the presence of interfering substances is to increase the cationicity of the added starch.
Work in this area is being carried out by suppliers of both starches and of retention aid systems relying on the presence of cationic starch. Starches with cationic contents up to about ten times the normal level (i.e., up to 3 eq/kg) are being used as charge neutralizers. Whereas ordinary cationic starches are inactivated by anionic trash, the high cationicity of these new starches seems to be able not only to neutralize the anionic trash, but also to maintain their adsorption/flocculation characteristics. In some cases, however, it may still be more cost effective to use starches with a normal level of cationicity together with a separate charge neutralizer, such as PAC.
The use of cationic fillers has been investigated periodically as a means of improving filler retention rather than as means of controlling anionic trash. However, recent work has shown that filler pretreated with cationic polymer does neutralize the effect of anionic trash, but its utility is obviously limited to machines needing filler. There is also no evidence that using filler as a vehicle for adding cationic polymer is any more effective than adding a cationic polymer separately. This approach is also being developed for other minerals, such as precationized talc or zeolite and bentonite used in conjunction with a separate cationic polymer, such as a polyDADMAC.
PROBLEMATIC SUBSTANCES IN THE FUTURE. With the strong global trends of increasing use of recovered paper and decreasing use of water, ameliorative techniques relying on retaining problematic substances in the paper should be questioned. This continuous recycling of problematic substances perpetuates their presence in the paper cycle and undermines its sustainability.
The answer to maximizing runnability on paper machines using recovered paper is to ensure that all papers do not contain materials that are likely to cause problems either during deinking or papermaking (and other nonrecycling applications).
1. J.W. Thornton, Enzymatic Degradation of Polygalacturonic Acid Released from Mechanical Pulp During Peroxide Bleaching, TAPPI Journal, 1994, Vol. 77, No. 3, p. 161-167.
2. D. Glittenberg, Cationic Starch Reduces More Than Just COD, Pulp & Paper Europe, 1996, Vol. 1, No. 10, p. 3.
3. R. Bown, A review of the use of cationic pigments in papermaking, Proceedings of the Pira Conference, "The Chemistry of Papermaking," Stockport, 1992.
4. L. Habets, et al, Improved paper quality and runnability by biological process water recovery in closed water circuits of recycle mills, Proceedings of the 1996 TAPPI Environmental Conference, pp. 249-257.
5. J. Nuortila-Jokinen, et al, Removal of disturbing substances by ultrafiltration of makeup waters in the pulp and paper industry, Paperi ja Puu, 1994, Vol. 76, No. 4, pp. 256-261.