De-inking toner-printed paper by selective agglomeration

Abstract

Laser printers and copy machines produce an increasing quantity of difficult-to-recycle paper. In the last several years, the need for an effective process to de-ink the toner has prompted a number of facilities to try toner agglomeration. The method requires the use of small amounts of an appropriate oil which serves to collect the toner into agglomerates that can be removed from paper fibers by screens or centrifugal cleaners. The technique shows promise by being simple, economical, and often very effective; however, it currently requires high-temperature and performs inconsistently, preventing significant use. A fundamental study of agglomeration is needed to determine how it operates, the key variables and their interactions. Therefore, the present study provides a fundamental understanding of agglomeration, laying the groundwork for process enhancement.We identify the key factors in agglomeration: concentration of cationic starch released from sheets in the repulper, oil composition, agitation rate, time and temperature. Furthermore, we examine how and why each factor influences agglomeration, and a population balance model is developed to help determine the quantitative relationships between the key variables. Cationic starch adsorbs to toner and oil interfaces, rendering them electrosterically stable to coalescence, thus preventing oil drops from reaching toner particle surfaces. However, when the oil contains an oil-soluble surfactant, the cationic starch adlayer is engulfed and some agglomeration resumes. Direct and novel use of the atomic force microscope provides insight into the cationic starch-induced reduction in oil-toner collision efficiency. Agitation rate and temperature influence the particle breakup rate, the latter by increasing aggregate strength by fusing toners above 35$\sp\circ$C. The model effectively predicts the experimental results, and shows how the competition between aggregate growth and breakup determines both the rate and degree of agglomeration. The results yield the agglomeration mechanism, quantitative expressions relating key variables and performance, and practical suggestions.