Dr. Jeffrey Hsieh (left) shows pulp and paper student assistant Dana Beck a continuous de-inking tube reactor used to remove ink from recycled fibers.
THAT COMPLETED TO-DO LIST, newspaper, catalog or telephone book you just tossed into the appropriate recycling bin has a brighter future with the development of a patented technology for removing ink from recycled fibers.
Researchers at the Georgia Institute of Technology are applying a direct current electric field to recycled fiber slurry, thus removing more ink, saving chemicals and making the resulting fibers brighter than other de-inking processes do. The Georgia Tech process is especially effective for dirts (dark specks) reduction on laser-printed waste paper like that produced in most offices, says Dr. Jeffery Hsieh, director of Pulp and Paper Engineering.
Dr. Jeffrey Hsieh (left) shows pulp and paper student assistant Dana Beck a continuous de-inking tube reactor used to remove ink from recycled fibers.
"The process can be applied to all kinds of paper -- lightweight coated paper, newsprint and other kinds of fibers,"says Hsieh, professor of chemical engineering. "However, the most significant aspect of the process is that it gets ink out of office waste by almost reversing the activity of a laser printer."
In 1993, 8.4 million tons of printing and writing paper were recovered for recycling, according to the American Forest and Paper Association. The uniformity, availability and higher profits associated with this waste stream put it in high demand for tissue paper, new printing and writing paper, and exports. De-inking is one of the key operations performed in recycling such paper.
However, much office paper has been laser printed. The heat used in laser processing makes ink particularly hard to remove because it electronically deposits ink particles on the fibers.
Traditional recycling processes rely on chemical and mechanical actions to remove ink from such fibers and include several dispersion, flotation and washing steps. They thus reduce fiber strength, which must be compensated for with the addition of fortifying chemicals.
"These operations add to the cost of recycling and make it less economical, compared to virgin paper-making,"Hsieh explained. "There is a need for technology that de-inks fibers in fewer stages, to reduce fiber destruction and cost."
The de-inking process developed at Georgia Tech is based on a reactor comprising a central anode and a perimetal cathode, which can be retrofitted to existing recycled paper processing equipment. Ink particles carry charges ranging from high positive to weak negative; paper fibers usually carry a weak negative charge. Applying the direct current field to a reactor full of fiber slurry attracts the ink particles away from the fibers and causes the ink to coagulate. The massed particles float to the surface of the slurry with some help from gas bubbles generated by the electric field. The coagulated ink is then skimmed off the top of the slurry using rotation scoops, continuous conveyors, or other means.
This process removes twice as many of the ink specks from office paper, compared to traditional de-inking without the electric field. Paper made from the treated fibers has a significantly lower number of ink specks per recycled handsheet, when compared to a control sheet.
The current also helps remove dust particles from the fiber and creates oxygen in the reactor -- both actions improve fiber brightness and whiteness. Brightness evaluations showed an increase of two to six points on samples of paper ranging from newsprint to offset printing paper.
Finally, test results of tear, tensile and burst show that the strength of paper made from the Georgia Tech de-inked fibers is at least equal to those of control sheets. This results from the use of fewer mechanical steps and chemical additions that are known to weaken fibers.
If a reactor operator chose to use chemicals in the de-inking process, smaller amounts could be used because the electric current dissociates them through the reactor, increasing their efficiency, Hsieh says. The operator also could turn off the de-inking cell if he did not want to use the current on a particular batch of paper.
This process specifically addresses de-inking, while other patented electric processes focus on different objectives.
The patent was issued on Aug. 24, 1993 and a presentation on the work appears in the 1992 AIChE Forest Products Symposium Proceedings. Future research could address how the charge affects the metallurgy of reactor shafts, Hsieh says, as well as pilot plant trials and mill scale ups.
Hsieh is director of the Center for Excellence for High Yield Pulp Science at Georgia Tech, and is an adjunct professor at the Institute of Paper Science and Technology. He recently was named a Fellow of the Technical Association of the Pulp and Paper Industry for his service the association and the pulp and paper industry.
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