DEINKING OF PAPER
Most people practice some recycling at home, and an increasing number of businesses are making it a point to recycle whenever possible. Paper recycling is the most common and most accessible recycling program. You have to put the paper into the correct bin and forget it. But what happens next?
How does that 30-page report you shredded, 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 printing on them and are subject to a deinking step in preparation for producing new paper. The choice of deinking technology depends on the paper type and its intended use.
Pulping
Before the paper can be deinked, you must turn it back into pulp. Pulping devices chop the paper into smaller pieces. Water and chemicals are added to clean it and ensure the proper pH values.
The pulp slurry goes to a centrifuge, separating the denser fiber material and unwanted contaminants.
Bleach Deinking
If you’ve ever done your 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 get added during the pulping step. Bleaching destroys the colorants in inks and brightens the remaining paper pulp, which is beneficial for recycled pulp used in higher-quality graphic papers.
Flotation Deinking
The most widely used deinking technology is flotation deinking. Ink is removed from the pulp during this process by re-soaking it in a vat of water and applying certain chemicals called surfactants. Air gets introduced into the recovered pulp, and ink particles (and other chemicals) will float and mix with the foam on the water surface. This foam is 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
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.
Wash 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 removing small particle size inks and not intended for use on high or even moderately printed papers.
Washing is most commonly used in the production of hygiene papers because the mineral fillers found in a majority of paper intended for recycling often lead to the reduced quality of the hygiene products and must be removed. Washing is not adequate for most papers due to the high yield loss during the process.
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