Ultrafiltrative deinking of flexographic ONP: The role of surfactants

Ultrafiltration is a potentially viable method of removing finely dispersed flexographic Pigments from the deinking water loop. This work examines the effects of surface-active materials on ultrafiltration efficiency. A logar ithmic relationship between Permeate pur and pigment concentration was demo nstrated at ink concentrations above 0.4%, with permeation rates becoming i ndependent of ink content at lower concentrations. The pressure-independent ultrafiltration permeation rates observed at more highly concentrated ink dispersions were shown to be limited by mass-transfer effects, as predicted by the concentration polarization model. Permeate flux at low ink concentr ation was limited by membrane fouling which occurs because dilute ink dispe rsions have a low concentration of surface-active materials. Increasing the concentration of surfactant in dilute ink dispersions decreased the degree of membrane fouling. However, excessive surfactant decreased permeation ra tes because of micelle formation. Clarification of concentrated ink dispers ions (>0.4% ink solids) resulted in relatively stable permeation rates and less fouling. The inherently higher concentrations of surface-active materi als in concentrated ink dispersions appear to retard fouling.