Influence of surfactant concentration on the ink removal selectivity in a laboratory flotation column

A laboratory flotation column equipped with Venturi aerators and an adjustable froth removal system was used to study the effect of an alkyl phenol ethoxylate surfactant on water, particle and surfactant transfer during flotation de-inking of recycled papers. Wash water was not used in order to emphasize the influence of surfactant concentration on water/particle transport in the froth.Surface tension, bubble size and contact angle measurements showed that the increase in surfactant concentration boosted pulp frothing, lowered bubble coalescence and ink/pulp contact angle, thus increasing water hold-up in the froth, gas hold-up in the aerated pulp and depressing ink floatability, respectively. Cross-linked phenomena determining the yield of the flotation process where interpreted using model equations describing particle removal in terms of flotation, entrainment and drainage in the froth. An exponential decrease in the flotation rate constant and in the drainage coefficient of ink, fillers, fine elements and in the fibre entrainment coefficient was observed when increasing the surfactant concentration. Whereas, the surfactant removal rate constant rose, in agreement with the increase in the surface area flux due to a reduction in bubble coalescence.