Deinking flexographic newsprint using ultrafiltration to close the water loop

Conventional flotation deinking technology is widely used for deinking secondary fiber furnish that has been printed by the letterpress or lithographic (offset) processes. This technology is not effective for removing water-based flexographic inks due to the hydrophilic nature of their pigment particles. Washing effectively removes flexographic ink dispersions from pulped secondary fiber; however, the resulting wash effluent contains significant quantities of pigment. The wash effluent must be clarified of pigment before it can be recycled within the mill without reducing pulp brightness. The difficulty in clarifying flexographic pigment dispersions from wash filtrate represents a significant barrier to closing the water loop of a newsprint deinking operation. Ultrafiltration was investigated as a means to remove dispersed water-based ink pigments from wash effluent. Ultrafiltration of high-concentration ink dispersions resulted in stable, high-production rates of pigment-free permeate. The use of high-concentration feeds prevented irreversible membrane fouling and actually demonstrated membrane regenerative properties. Inclusion of cellulose fines in the feed had variable effects on permeation rates.