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 logarithmic relationship between permeate flux and pigment concentration was demonstrated at ink concentrations above 0.4% with permeation rates becoming independent 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 concentration was limited by membrane fouling, which occurs because dilute ink dispersions have a low concentration of surface-active material. Increasing the concentration of surfactant in dilute ink dispersions decreased the degree of membrane fouling. However, excessive surfactant decreased permeation rate because of micelle formation. Clarification of concentrated ink dispersions (>0.4% ink solids) results in relatively stable permeation rates and less fouling. The inherently higher concentrations of surface-active materials in concentrated ink dispersions appear to retard fouling. Application : ultrafiltration shows promise as a means of clarifying ONP wash filtrates containing finely dispersed water-based flexographic inks. Control of surfactant concentration is key to prevention of membrane fouling.