An excerpt from "Alkaline Papermaking: A PIMA Magazine Roundtable," PIMA Magazine May 1990, p. 23-25, 28-30. Reprinted here with permission.
Participants in the alkaline roundtable are:
Terry Gallagher, product specialist, Nalco Chemical Co., Naperville, Ill.
T. Anthony Henle, product supervisor, paper technology group, Hercules Inc., Wilmington, Del.
C.A. "Kasy" King, research fellow, James River Corp., Neenah, Wisc.
Chuck Kunesh, director of research, Pfizer Specialty Minerals Research Center, Bethlehem, Pa.
Jerome Pflieger, papermaker, Omya Inc., Procter, Vt.
Barbara Wortley, senior technical manager, General Chemical Corporation Technical Center, Syracuse, N.Y.
PIMA Magazine: There is a great deal of pressure on the industry to produce more recycled paper. What kinds of problems can makers of recycled paper expect from mixed acid and alkaline waste paper and what technical solutions are available?
GALLAGHER: When an acid papermaker is faced with utilizing alkaline recycled broke he may experience foaming problems from the carbonate being exposed to an acid environment. If alum is being used, it can react with calcium carbonate to produce gypsum (calcium sulfate) which can cause deposition problem .
HENLE: Higher levels of alkaline broke in an acid system demand alum or acid addition levels well above the nom to achieve consistent pH levels of 4.0-4.5. Using a cationic rosin emulsion size rather than a rosin soap size can minimize the problem due to the emulsion's tolerance of higher pH. In a carbonate loaded alkaline system relatively moderate levels (15-20 percent) of acid broke can be tolerated. However, higher levels may contribute to press picking and foam.
KING: Because of the shade and brightness shift between alkaline and acid papers, the ability of an end user to mix these papers could lead to difficulties in these areas. All solutions are both costly and difficult. There is also a concern in the blending of alum/rosin containing broke with alkaline broke because of deposit and foam generation. I'm not as concerned about this as I used to be. The industry started out saying that alum in an alkaline wet end was not possible, then it moved to 3 pounds per ton for drainage and retention then to 5 to 10 pounds and now some are using 10 to 15 pounds per ton very successfully. I have recently seen high levels of acid broke on an alkaline paper machine. There were no negatives 3.n machine efficiency. We will learn how to run these blends.
KUNESH: The simplest solution is for makers of recycled paper to convert to alkaline. With the rapid conversion to alkaline by fine paper makers, recycled paper producers will, in the near future, find more alkaline waste paper to recycle than acid paper. Today, a recycled paper producer may insist on dissolving out the PCC from alkaline paper, but before long it will be much more economical to just run alkaline and use its PCC fillers.
PENNARTZ: If you look at the grades that currently use recycled fiber and the types of waste fiber used, the difference between acid and alkaline waste paper is the least of their problems. But as recycled fiber begins to be used in fine paper grades, we may see some problems for carbonate broke in acid wet ends. Most alkaline mills will probably have no problem with acid broke. However, for an acid mill, acid washing of alkaline broke is a proven way to eliminate calcium carbonate before it reaches the paper machine.
PFLIEGER: Foam is definitely a problem, particularly with carbonate broke in an acid system, but can also be a problem when using acid broke in an alkaline system. Adjustment of the pH in the pulper will eliminate a potential foam problem.
ROSTEK: Arid broke presents very few problems to a correctly set AKD sizing system. Problems will occur at recycled mills continuing to size with rosin/alum, when encountering recycled fiber. Either converting to a true alkaline process or to a neutral rosin system with the capacity to tolerate moderate levels of PCC will be necessary.
WORTLEY: Until the last few years, recycled paper users have encountered only minimal amounts of carbonate when purchasing uncoated secondary fiber. If no accelerations were made to handle it, variations in pH and foaming were encountered. To counter these problems, the carbonate was solubilized through alum and sometimes acid addition during stock preparation. With greater amounts of carbonate now being encountered, it is economical in terms of yield and chemical cost to continue in this mode. Secondary fiber users can operate effectively at neutral or alkaline pH to accommodate the carbonate entering their systems. To change from acid to neutral or alkaline papermaking, retention, sizing, slime control and effluent control all unit be considered.