If it's "broke," fix it with coagulants.
The term "white pitch" was coined at this time to refer to deposits formed by the interaction of these broke materials and wet end chemicals. These insoluble deposits were seldom white, frequently caused downtime, and almost always showed up in combination with foam and entrained air. All of these "bad actors" were (are) negatively charged and consumers of cationic retention aids, starches, sizes, dyes, and other specialty wet end additives.
Our technical group developed the application of low molecular weight (LMW) and high charge density (HCD) chemicals to neutralize broke contaminants and measurement techniques to monitor their use and success. This paper summarizes the development of LMW and HCD coagulant applications--still used today--along with their measurement and control. The paper also includes several case studies and suggestions on chemical handling.
At the time this story begins, many retention packages and water treatment systems were using dual polymer systems comprised of LMW and HCD polymers followed by a much higher molecular weight cationic or anionic flocculant. Our group locked in on the highly negative character of the broke components and settled on trying to neutralize that negative nature with the cationic charge of LMW and HCD polymers and papermakers' alum. As these additives were applied, it was noted that the turbidity of the broke supernatant and of the broke/fiber blends was decreased significantly. Thus, turbidity became one of the measurement tools used to track success or failure. Surface charge or zeta potential was also found to be highly effective in following the success or failure of the cationic additives.
Working closely with polymer and alum suppliers, we found that some LMW and HCD coagulants neutralized the broke charge and cleared the broke turbidity, while others did not. Some worked here and some worked there, but we were always able to find one that did the job. The lab screening techniques of turbidity and charge used to select a particular polymer or alum would always extrapolate to the same results in the mills. Thus was born the concept of using LMW and HCD coagulants and/or alum to neutralize broke contaminants. These techniques are still widely used and have been extended into other areas of papermaking where charge neutralization is needed.
MEASUREMENT AND CONTROL
The surface charge measurement being used at the time was primarily surface charge or zeta potential. Streaming current detectors and color titration techniques were just getting started. Because zeta potential was a difficult number to obtain in the field, our group looked for another, easier measurement tool. Light transmission or turbidity was being used as a measure of white water solids--primarily as a result of the introduction of the on-line retention measurement systems. Hand held turbidity meters were widely used in the water treatment industry segments and so became our primary tool to measure coagulant success.
Applying zeta potential to the coagulated coated broke samples showed us that the best turbidity of filtrate water was obtained at slightly negative or zero charge. This does not imply that the machine ran better at zero charge, but that the coagulation of the coated broke contaminants was best obtained at near zero charge. The broke was generally only 15 to 25% of the total stock pull, so the broke charge impacted the headbox charge--but was not the determining factor in where the machine ran best. As noted above, charge measurement proved difficult to measure in the field, so turbidity came to be widely used as a quick and effective way to measure the broke coagulant's effectiveness.
The treated broke is filtered through a glass wool plug in a funnel and the filtrate turbidity is measured. If the coagulant is working, the turbidity is very clear, and if not, very turbid and opaque. Curves of turbidity can be generated to track coagulant effectiveness versus dose level. It is best to measure charge and/or turbidity after the coagulant is added and prior to the next additive. This allows the determination of the coagulant's effect without being confused by other additives. Turbidity of the broke filtrate should be taken towards 100% transmission for maximum impact. To reduce costs, it may be possible to move away from 100% transmission with lower coagulant doses.
The machine will tell you what dose works best for the cost of the coagulant. If one is using surface charge or zeta potential, the best target charge is slightly negative or near zero for the treated stream. If streaming current detectors are used for charge titration, the values obtained are not absolute or quantitative, so the titration for each set of conditions must be interpreted.
THE TECHNOLOGY TODAY
These broke treatment concepts grew out of existing retention aid polymers and continue to be used effectively today. Some of the benefits then and now for this treatment include reduced broke spots, minimizing broke deposits, lower retention aid costs, improved sizing efficiencies, better save all operation, improved biological control, fewer process charge swings, lower entrained air levels, and better dry strength additive efficiency. There are other likely benefits.
In addition to using LMW and HCD coagulants on highly charged brokes, this technology has come to be used in other situations where there is a high level of negatively charged contaminants. The coagulants are used to neutralize excess negative charge by collapsing that charged material onto fiber and fines surfaces and then taking them out of the system with the paper. The net result is a cleaner process that uses other specialty chemical additives more efficiently.
The coagulant is best added at the source of the contamination, such as poorly washed pulp, recycled fiber, contaminated river water, etc., thus treating the source of the problem and not the entire stock stream. An additional process benefit may be realized by adding 10 to 15 pounds per ton of talc right after the coagulant addition. Talc renders the coagulated latex and starch binders less sticky, increases the ash content, and lowers costs. Some examples of this coagulant concept being used in different parts of the industry include internal size promotion or fixation, efficiency gains for wet end starch and dry strength additives, optical whitener quenching, incoming water color removal, effluent neutralization prior to the clarifier, dual polymer retention systems, and drainage aids for save alls and forming fabrics.
Today, there are a variety of LMW and HCD coagulants on the market, from polyamides and polyamines, to polyethylene imines, alum, ferric chloride and so on. It may take you or your supplier a little up front time to screen the possibilities, but there is almost always a coagulant available that will work effectively to neutralize a particular contaminant stream, while improving efficiencies and lowering overall costs.
Additive Cost Reduction: A fourdrinier paper machine making offset printing papers with a sheet ash of 15 % was using a single component cationic flocculant for retention. Retentions were typically between 75 and 82% first pass with ash retention of 38 to 42%. The machine used a cationic corn starch, a promoted AKD internal size, ground calcium carbonate, biocides, defoamers, and dyes. The machine broke was a blend of uncoated and coated paper from the paper machine and off machine coaters that was fed at 10 to 50% of the total stock pull. The coagulant was added to the broke flow at 2.5 dry pounds per ton of dry broke flow. Retention aid flow was reduced by 15% and starch use was lowered by 30%, resulting in net savings of $0.50 per ton of good paper. This cost savings was coupled with an increase of time between wash ups and boil outs from 4 weeks to 5 weeks.
Internal Size: A fourdrinier machine producing lightweight specialty printing and writing papers was alkaline sized with AKD and used a bentonite microparticulate retention system. The HST sizing values for the paper produced averaged 30 to 55 seconds cured and 2 to 5 seconds uncured prior to the coating. This 10 to 20 % of sizing cure is unacceptable for a well-run alkaline sized paper machine. The objective of adding the LMW and HCD coagulant was to increase the HST% cure and not so much to raise the over-all HST level. The coagulant was added at 3.5 pounds per ton to the coater broke with no other machine changes. The uncured HST value increased to an average HST of 50 seconds while the cured value increased to an average of 58 seconds or an 86% cure rate. This higher level of % cure for the HST indicates a more efficient AKD-to-fiber attachment that resulted in less sheet slip and fewer breaks for hyrolyzate deposits. As an additional benefit, the AKD feed rate was lowered 30 % to get the machine-cured HST levels back to the target of 45 seconds.
STORAGE AND FEED
These LMW and HCD coagulants are almost always liquid products consisting of soluble polymer and water. The solids of the polymer can vary but is typically in the 40 to 50% range by weight. It is easier to compare one coagulant to another if the addition amount of polymer is in dry pounds of polymer compared to dry pounds or tons of stock. If water is being treated, the dry pounds of polymer per gallon or 1000 gallons of water is generally used. These polymers are typically stored in drums or bulk tanks and pumped neat to the process. It is always best to dilute the polymer with a minimum of 10 to 1 clean and warm water just prior to entering the process stream. The coagulant should be added away from other additives at a point of good shear or agitation to maximize its dispersion and adsorption onto the targeted contaminants. Proper storage times and temperatures should be observed based on your supplier's specifications.
Low molecular weight and high charge density coagulants have been shown to be effective in the treatment of contaminated machine broke systems and other contaminated process streams. These additives will change the cost structure but almost always result in more dollars saved than in their application costs. These technologies generally increase additive efficiencies and lower chemical consumption. Machines run cleaner with fewer breaks and longer times between wash ups and boil outs. Lab screening to select the best option is straightforward and usually extrapolates directly to machine operation. Monitoring and control is straightforward and simple to implement.
ABOUT THE AUTHOR
Kasy King is principal of Papermaking Process Consulting LLC, Appleton, Wisconsin, USA. Prior to founding his consultancy, King had a long career in the paper industry, including Scott Paper Co., SD Warren, Appleton Papers, and James River. He is a member of the Solutions! Editorial Board and the TAPPI Journal Editorial Board. King can be reached at +1 920 991-9102, or by email at firstname.lastname@example.org.
KASY KING, PROCESS CONSULTING LLC
WHAT YOU WILL LEARN
* How work on chemical treatment of broke led to the development of LMW and HCD coagulants.
* Other application areas for these treatments.
* Case studies on their successful use.
* Suggestions for effective chemical handling.
* "Synergistic effects from performance chemicals," Solutions!, by Kasy King, April 2004. To access this article, enter the following Product Code in the search field at www.tappi.org: 04APRS045. Or call 800 332-8686 (U.S.), 800 446-9431 (Canada), +1 (770) 446-1400 (Worldwide).
* "A marriage for performance sake" (performance chemicals), Solutions!, April 2003. Product Code: 03APRS029.
* "The ups and downs of specialty chemicals," by Kasy King, Solutions!, December 2002. Product Code: 02DECS037.
* "Wet End Chemistry: Making Pulp and Paper," TAPPI CD-ROM Series, by Jim Atkins. Product Code: MPP-11. ISBN: 1595100296. Member Price: $75.00. Non-Member Price: $115.00. Wet End Chemistry is the eleventh of 15 CD-ROMs in the Making Pulp and Paper CD-ROM Series. Through this interactive, self-paced CD-ROM, participants learn wet end chemistry terms, concepts, and processes.
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|Title Annotation:||SPECIALTY CHEMICALS|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Aug 1, 2005|
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