DEINKING OF PAPER
Most people practice some sort of recycling at home, and an increasing number of businesses are making it a point to recycle whenever possible. Among recycling programs, paper recycling is the most common and easiest in which to participate. You simply have to put the paper into the correct bin and forget about it. But what happens next?
How does that 30-page report you just shredded, or that brochure that came in the mail, or that newspaper that you read last night make it back into the recycling stream as a blank sheet of paper? Where does the ink go? How does it get removed?
Most types of paper intended for recycling will have some sort of printing on them and are subject to a deinking step in preparation for producing new paper. Depending on the paper type and it’s intended use, one of a few different deinking technologies may be used.
Before the paper can be deinked, it must be turned back into pulp. Pulping devices chop the paper into smaller pieces. Water and chemicals are then added to clean it and ensure the proper Ph values.
The pulp slurry is then sent to a centrifuge, which separates out the denser fiber material and other unwanted contaminates.
If you’ve ever done your own laundry (and kudos if you’ve never had to), you know that bleach and colored clothing don’t mix. Bleaching agents such as hydrogen peroxide and sodium dithionite are typically added during the pulping step. Bleaching destroys the colorants in inks and the brightens the remaining paper pulp, especially useful for recycled pulp intended to be used in higher quality graphic papers.
The most widely used deinking technology is flotation deinking. During this process ink is removed from the pulp by re-soaking it in a vat of water and applying certain chemicals called surfactants. Air is then introduced into the recovered pulp, and ink particles (along with other chemicals) will float and mix with the foam on the water surface. This foam is then removed from the vat.
Flotation deinking is used for the deinking of graphic papers such as newsprint and magazine papers and also commonly used as one of the steps for deinking of papers intended for use as hygiene papers (toilet paper, facial tissue, etc.).
Enzymatic deinking uses enzymes in conjunction with flotation deinking to augment the removal of inks. Recycling mills might use enzymatic deinking in place of bleach deinking.
Washing removes inks and other unwanted components (such as mineral fillers) by washing the water-soaked pulp on a wire screen. The pulp fibers are recovered from the screen and the filtered material is then further treated to remove the unwanted solid material. Washing is only effective in the removal of small particle size inks and not intended for use on highly, or even moderately, printed papers.
Washing is most commonly used in the production of hygiene papers because the mineral fillers found in most paper intended for recycling often lead the reduced quality of the hygiene products and must be removed. Washing is not effective for most papers due to the high yield loss during the process.