Surface and colloid chemistry of flotation deinking

Abstract: This study has been concentrated on explaining the surfacechemical phenomena that occur during the agglomeration andflotation of ink particles. The mechanism of agglomeration ofink particles in a calcium-fatty acid collector system has beenstudied. The mechanism suggested involves the formation ofcalcium soap particles and a co-agglomeration of soap and inkparticles, which is strongly influenced by the electrolyteconcentration. Particles that remain in the pulp afterflotation did not differ in surface chemical character from theparticles that had been flotated. The main difference was thesize, which indicates that the residual particles in the pulpwere not left due to chemical reasons. The explanation isinstead related to factors governing the collection of smallink particles: collision frequency (hydrodynamics) and thenumber of soap particles in the system.It has been suggested that the problem in reality is theagglomeration and not the flotation. Inhibition ofagglomeration leads to a decreased flotation. Large particlesfloat while too small particles do not (<10µm). Thefatty acid soap chemistry used to agglomerate small inkparticles is insufficient due to the presence of surface activesubstances in or added to the paper raw material. Thesesubstances increase the colloidal stability of ink particlesand the calcium soap precipitates formed.It has been shown that the effect of non-ionic dispersionagent on ink agglomeration using fatty acid as principalcollector is highly temperature dependent. At temperaturesbelow the cloud point of the surfactant, the agglomerationefficiency was reduced but at temperatures above the cloudpoint the non-ionic surfactant contributed to the agglomerationprocess. Systems containing a mixture of fatty acid and anon-ionic surfactant at temperatures above the cloud point haveactually been shown to have a higher agglomeration efficiencyfor very small ink particles than a system without non-ionicsurfactant.A direct correlation between flotation and dynamic surfacetension was observed. The flotation efficiency was increased bya slower surface tension reduction. It was also observed thatadsorption of the surfactant to both the particle/liquid andthe air/liquid interface was needed to reduce the floatabilityof the particles.