Enhanced contaminants removal for paper recycling by adsorption deinking and new flotation methods
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The recent progress and increasing popularity in digital printing technology created new challenges in recycled paper quality and recycling process runnability due to the sub-micron size, strongly negative charge, and hydrophilic nature of ink particles. As a result, hydrophilic inks are not compatible with state of art flotation deinking that relies on surface hydrophobicity. The new hydrophilic ink would stay in the recirculating water, redeposit into the fiber lumen and decrease the fiber brightness. Adsorption deinking by chitosan and advanced flotation methods were studied for removal of hydrophilic inks from cellulose fiber and water. The ink behavior during pulping was investigated and it was shown that 53% of total ink stays in water phase, 31% redeposits into fiber lumen during pulping and 16% remains attached with cellulose fiber. Thus, redeposited ink is a major concern to control cellulose fiber quality and chitosan can adsorb ink particles during pulping which results in 5 points increase in ISO Brightness besides 90% water saving and 20% energy saving. For free inks in water, two different advanced flotation techniques were studied, including electroflotation and oily-bubble flotation. Firstly, electric treatment agglomerates ink particle from 60nm to 700nm and removes 25% ink through electroflotation mechanism. Secondly, the adsorption of ink particles on silicone oil/water interface was proved by dynamic interfacial tension, and the silicone oil coated bubble flotation shows advantages in both thermodynamics and kinetics. The usefulness of an oil layer outside air bubbles in reducing adsorption energy barrier was demonstrated by a model system and the root cause for the decrease in energy barrier is van der Waals interactions.