Deposits (Inorganic, Organic, Pitch, Stickies)

DEPOSITS first need to be analyzed to determine their composition. The composition often holds the key to solving deposit problems.

INORGANIC DEPOSITS are those that are composed mainly of material that becomes converted to ash when the deposit is incinerated at high temperature. In a typical procedure a sample of the deposit is placed on a tared "ash-free" filter paper, weighed, and then incinerated under standardized conditions, i.e. 900 oC and 30 minutes. X-ray fluorescence analysis can be used to determine the elemental composition of the metals in the ash.

Most inorganic deposits result from the combination of a divalent or multivalent cation with a divalent or multivalent anion, forming an insoluble precipitate. That means that once the main identity of the inorganic deposit is known, it can be attacked from either side; in principle, one can take measures to reduce the concentration of either the cation or the anion. Alternatively, scale control additives are available from chemical suppliers to control various forms of inorganic deposit.

Calcium carbonate deposits fizz when exposed to concentrated hydrochloric acid. Since the solubility of calcium carbonate decreases with increasing temperature, deposits may form on heat-exchanging equipment. Surprisingly, there seldom are calcium carbonate deposits in paper machine systems that use calcium carbonate filler; it appears likely that any excess calcium or carbonate ions, above the solubility product, simply adsorb onto the surfaces of the calcium carbonate particles already present.

Calcium oxalate deposits can be a serious problem in systems where the oxalate ion is formed during oxidative bleaching of kraft pulp. Should these deposits occur on the paper machine, it is likely that better pulp washing could solve the problem, or at least prevent the problem from reaching the paper machine. Though the problem might be attacked by changing either the bleaching chemistry or water hardness, it is more common for papermakers to use a scale-control additive [Richardson, Hipolit, 1990; May, 1991]. Another approach is to add an optimum amount of magnesium ion to change the deposition characteristics of the precipitate [LeFevre, Moran, 1996; Froass et al. 1997].

Barium sulfate deposits occur when the barium divalent cation is released from wood, especially in the case of unbleached kraft. The release occurs especially when the pulp first encounters acidic papermaking conditions, converting the insoluble barium hydroxide in the wood to soluble barium ions. Most paper machine systems have a relatively high content of sulfate ions due to the use of such additives as sulfuric acid and aluminum sulfate. The solubility product of barium sulfate is low, so it is common that the formation of barium sulfate is thermodynamically favored in paper mill systems. However, the ions tend to remain in supersaturated condition until they encounter intense hydrodynamic shear, which nucleates precipitation. Barium sulfate problem sometimes occur downstream of alum addition, since alum lowers the pH and also supplies additional sulfate ions. One way to minimize barium sulfate scale is to use polyaluminum chloride (PAC) in place of the alum. Another way is to acidify the pulp at an earlier, less vulnerable point in the process, perhaps in the first chest after the main refiners. Yet another way is to add scale-control chemicals.

Aluminum hydroxide deposits sometimes occur immediately downstream of the point of addition for aluminum sulfate (papermaker's alum). Factors that make such deposits more likely include (a) pH values above about 6, (b) inadequate hydrodynamic shear at the point of addition of the alum, and (c) excessive concentration of the alum at the point of addition. Though pre-dilution of alum usually is a good idea, it is not a good idea to dilute alum with alkaline water or to store diluted alum before its use. Some paper machine systems and furnishes may be susceptible to aluminum hydroxide deposits, and in such cases it may be possible to replace the alum with either poly-aluminum chloride (PAC) or highly charged cationic polymers such as polyamines.

ORGANIC DEPOSITS

The main components of organic deposits often can be judged from infra-red light absorption spectra. The best approach to overcoming an organic deposit problem is likely to be different, depending on whether the main binding agent in the deposit can be classed as pitch, stickies, slime bacteria or fungi, or a papermaking additive such as a defoamer, retention polymer, or sizing agent, etc. Keep in mind that the most problematic substance(s) in a deposit may not be those that are present in the highest concentrations. For instance, many deposits on paper machine equipment may be composed mainly of cellulose fibers.

Though direct analysis of deposited material with infrared (IR) spectrophotometry can go a long way to identifying key components of an organic deposit, usually it is worth considering other procedures. Let's start with some procedures that can make IR analysis itself more effective. The first of these is to combine IR analysis with microscopy. Ordinary light microscopy often can reveal obvious components such as fibers. But what about other "grains" or "flakes" in a deposit? Many IR spectrophotometers now include the option of microscopic analysis. As in any IR work, the strategy is to find a spectrum of pure, known material that can account for the light absorption maxima in the spectrum. You can start by obtaining spectra of various known materials that are in the input streams to the papermaking operation. In addition, absorption peaks can be compared to the wavelength maxima recorded in tables or "libraries" of IR data.

We'll come back to the subject of microscopy, but let's assume that we've already done what we can with observations of the as-received sample. One of the next important steps can be to perform an extraction. Depending on the choice of solvent, the extraction procedure can divide the sample into two or more components that differ according to their water-loving or hating nature. Relative to the initial sample, each fraction is likely to be either purified or concentrated, making subsequent analysis less difficult. To take one example, 20g of the sample may be placed in 50ml of methylethylketone (MEK) and some water in a separatory flask. After shaking the flask for several minutes, the organic phase is allowed to rise. The water-soluble portion and any fine solids can be withdrawn from the lower portion. The organic-soluble portion can be concentrated by evaporation of the MEK. The most common IR methods applied to organic-soluble, concentrated samples obtained in this way involve incorporation of the material into a pellet composed mainly of potassium bromide (KBr).

Much more detailed information about the molecular nature of an organic-soluble extract usually can be obtained by injecting some of it, in solution form, into a chromatography column. In particular it is worth considering the gas-liquid chromatography (GC) method. The method works because different molecular substances have differing affinities (partition coefficients) for the packing materials used in GC columns. The sample is injected into a carrier stream of solvent. The elution time can be compared relative to the elution times for various substances that might be suspected to be in the deposit. As an add-on, many leading laboratories that service the paper industry combine GC analysis with mass spectroscopy (MS). The MS procedure usually starts by ionizing the material with a flame or plasma. The combined GC-MS analysis can reveal the exact molecular structure of ionic breakdown products associated with each of the elution peaks obtained from the GC procedure. With modern computer analysis it is possible, in most cases, to make exact identifications of the components based on the molecular masses of the observed ionic byproducts.

Other methods to consider once you have concentrated the organic-soluble fraction of a deposit include nuclear magnetic resonance (NMR, to confirm the chemical structure), high performance liquid chromatography (HPLC, another method based on elution times), and thermogravimetric analysis (TGA, to determine melting points and glass-transition temperatures). All of these are well described in standard analytical chemistry textbooks.

It's unlikely that you have GC-MS capability in a particular paper plant (though your chemical supplier certainly has access to one), so lets get back to some methods that can be applied "right on the spot." Many of these fall within the category of "wet chemistry," i.e. tests that can be done with little more than a test tube and a few reagents. Some of the most widely used spot methods involve (a) color-forming reactions, (b) stains that have affinity for various substances, such as oily materials, (c) acid to reveal the likely presence of calcium carbonate by generation of CO2 bubbles, (d) iodine solution to reveal the likely presence of starch due to a black complex with the helical amylose chains, and (e) tests for slime bacteria such as the adenozenetriphosphate (ATP) test.

Every paper mill ought to have several microscopes and people should be encouraged to use them. One of them should be a low-power stereo microscope. Another of them ought to be equipped with options such as polarized light and phase-contrast illumination. Polarized light has the unique ability to reveal crystalline material that has different refractive indices according to the orientation. One such material consists of uncooked starch granules. Such particles will show up with a characteristic "maltese cross" pattern when viewed with polarized, transmitted light. It is also a good idea to have a calibrated scale on the eyepiece of one of the scopes so the user can estimate sizes. Especially if you are on the chemical sales end of things, then you also ought to consider having a camera attachment in order to make your points more persuasively to others in the plant.

PITCH

Pitch is such an important category of organic deposits that it rates its own page of information. But first, let's be clear on what we mean by the word. A deposit will be considered to be pitch-related if the key binding agent is derived from wood. The most common ingredients of wood pitch are resin acids (from softwood), fatty acids (from all kinds of wood), triglyceride fats, and various unsaponifiable materials such as beta-sitosterol. The tacky properties of pitch can change greatly, depending on whether it has become air-oxidized, polymerized, when the temperature changes, or when it is mixed with other materials. More details about pitch are given elsewhere in this guide.

STICKIES

Though the word is used to cover a variety of problems, most usually stickies are understood to involve adhesive materials coming from the reuse of waste paper pulp. Go to the page labeled "stickies" if it seems that this is what you are dealing with.

The main culprit in many problems with stickies is the polyvinylacetate (PVA) and other binders in the "pressure-sensitive" labels that have become so common in mail and throughout our society over the past couple of decades. The problem with stickies is that they cling together and tend to build up into globs or strings. They can adhere to papermaking equipment, they can fill felts, and they can make spots in the product. The best ways to deal with stickies include avoiding them (by selecting the kind of pulp source), removing them during deinking of the wastepaper (easier said than done), or adding enough talc to the system to overcome their tackiness. For more information about stickies, please click on this link to information about stickies.

References

Allen, L. H., Cavanagh, W. A., Holton, J. E., and Williams, G. R., "New Understanding of Talc Addition May Help Improve Control of Pitch," Pulp Paper 67 (13): 89 (1993).

Anon., "A Primer on Pitch Problems," Tappi 62 (4): 20 (1979).

Colman, I., Duckworth, S., and Stork, G., "Selection of Products for Deposit Prevention," Proc. 1996 Intl. Paper Coating and Chem. Symp., 221 (1996).

Conners, T. E., and Banerjee, S., Surface Analysis of Paper, CRC Press, Boca Raton, 1995.

Doshi, M. R., "Properties and Control of Stickies," Prog. Paper Recycling 1 (1): 54 (1991).

Douek, M., Guo, X.-Y., and Ing, J., "An Overview of the Chemical Nature of Deposits/Stickies in Mills Using Recycled Fiber," Proc. TAPPI 1997 Recycling Symp., 313 (1997).

Dreisbach, D. D., and Michalopoulos, D. L., "Understanding the Behavior of Pitch in Pulp and Paper Mills," Tappi J. 7 (9): 19 (1989).

Dunlop-Jones, N., and Allen, L. H., "The Influences of Washing, Defoamers, and Dispersants on Pitch Deposition from Unbleached Kraft Pulps," J. Pulp Paper Sci. 15 (6): J235 (1989).

Fogarty, T. J., "Cost-Effective, Common Sense Approach to Stickies Control," Tappi J. 76 (3): 161 (1993).

Froass, W. C., Francis, R. C., Dence, C. W., and LeFevre, G., "The Interactions of Calcium, Magnesium and Silicate Ions under Alkaline Conditions," J. Pulp Paper Sci. 3 (7): J318 (1997).

Gill, R. I. S., "Chemical Control of Deposits - Scopes and Limitations," Paper Technol. 37 (6): 23 (1996).

Kowalski, A., Bouchard, D., Allen, L., Larin, Y., and Vadas, O., "Pitch Expert. A Problem-Solving System for Kraft Mills," AI Mag. Fall 1993, 81.

Le Fevre, G., and Moran, J. R., "Silicate Chemistry Key to Solving Mill Scale Problems," TAPPI J. 79 (11): 77 (1996).

Petander, L., Ahlskog, T., and Juppo, A. J., "Strategies to Reduce AKD Deposits on Paper Machines," Paperi Puu 80 (2): 100 (1998).

Rende, D. S., "Wire Deposit Control and Barrier Chemistry," PIMA 76 (2): 32 (1994).

Richardson, P. F., and Hipolit, K. J., "Inorganics and their Impact on Deposit Formation in Neutral and Alkaline Papermaking Systems," TAPPI 1990 Neutral/Alkaline Papermaking Short Course Notes, 05 (1990).

Rudie, A., "Barium Sulfate Scale in the Fiberline," TAPPI J. 83 (10): 50 (2000).

Venditti, R. A., Chang, H. M., and Jameel, H., "Overview of Stickies Research at North Carolina State University," PaperAge 1999 (11): 18 (1999).

Wilhelm, D., K., Makis, S. P., and Banerjee, S., "Signature of Recalcitrant Stickies in Recycled Newsprint Mills," TAPPI J. 8 (12): 63 (1999).

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 (m_hubbe@ncsu.edu). 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.


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