Flotation de-inking studies using model hydrophobic particles and non-ionic dispersants

A series of non-ionic alcohol ethoxylated surfactants (with HLB within the range of 11.1-12.5) were used as dispersants during flotation of mondispers e hydrophobised silica particles (representing ink particles) in de-inking Formulations. Laboratory scale flotation experiments, contact angle, dynami c surface tension and thin film drainage experiments were carried out. The reduction in dynamic surface tension at the air/solution interface (which i s dependent on the adsorption kinetics) followed the order C10E6 > C12E8 ap proximate to C12E6 > C14E6 and these values were lower than sodium oleate, which is commonly used in de-inking systems. In addition the non-ionics ads orbed on the hydrophobised silica particles reducing the contact angle. The se results indicated that the non-ionic surfactant with the highest CMC (C1 0E6) gave (a) the highest rate of adsorption at the air/solution interface (b) the froth with the greatest water content and higher froth volume (c) t he lowest reduction in contact angle and (d) the highest flotation efficien cy at concentrations above the CMC. It was also observed that flotation occ urred, in spite of the fact that thin-film measurements indicated that the adsorption of non-ionic at the air/solution and silica/solution interfaces reduced the hydrophobicity of the particles, as indicated by an increase in stability of the: aqueous thin film between the particle and air-bubble. T his result suggests that the bubble-ink particle captures mechanism (occurr ing through rupture of the thin aqueous film separating the interfaces) is not the only mechanism controlling the flotation efficiency and that other parameters (such as the kinetics of surfactant adsorption, foaming characte ristics, and bubble size) need to be taken into account. The kinetics is im portant with respect to the rate of adsorption of surfactant to both interf aces. Under equilibrium conditions, this may give rise to repulsive steric forces between the air-bubble and the particles (stable aqueous thin-films) . However, a lower amount of surfactant adsorbed at a freshly formed air bu bble or inkparticle (caused by slow adsorption rates) will produce a lower steric repulsive force allowing effective collection of particles by the bu bble. Also, it was suggested that the influence of alcohol ethoxylates on b ubble-size could effect the particle capture rate and mechanical entrainmen t of particles in an excessively buoyant froth, which will also play an imp ortant role in the Rotation recovery. (C) 2000 Elsevier Science B.V. All ri ghts reserved.