Displacement washing deinking of water-based inks

The displacement of ink by water from pads prepared from aqueous mixtures of bleached kraft fibres and colloidally dispersed flexographic ink was investigated as a function of the concentration of calcium ion and sodium oleate. Washing efficiency was assessed from breakthrough curves based on filtrate analyses and on measurement of pad brightness after washing. During thickening, water-based ink retention in the pad was promoted by the presence of calcium soaps of fatty acids and by a high calcium ion concentration. Calcium soap/ink aggregates, whose sizes range between 0.5 and 3 μm, were found to be retained by adsorption on the fibres but no by mechanical entrapment in the porous network. Once deposited on the fibre surface, ink particles could not be fully desorbed with water. Removal of calcium ions by chelation, prior to thickening, eliminated ink deposition. Under condition where ink aggregates were trapped in the pad, displacement washing with a solution of chelating agent was not entirely effective.