How is ink removed from recycled paper?
There are two ways to remove ink from paper - by washing, by flotation, or a combination.
In the washing process, soaps are added to paper as it is being pulped. The ink dissolves in the water which then can be cleaned and re-used. Ink removed by flotation requires air to be passed through the paper pulp. This produces foam that captures the ink; the foam is then skimmed off.
After de-inking, paper pulp may be bleached to increase brightness. Hydrogen peroxide is the most commonly used bleach as it decomposes into water and hydrogen. Another method is to use inert, harmless brightening compounds.
Not all paper pulp needs to be bleached, and not all recovered paper needs to be de-inked. If ink is not removed from recycled paper during pulping, the ink will disperse in the pulp, discoloring it slightly. For some uses, this discoloration is immaterial.
Ink removed from recycled pulp can be burned to generate energy to run the paper mill, or can be used to make compost or gravel for roads.