Application of membrane separation techniques to flexographic newsprint deinking

Inclusion of flexographic printed newspapers in the furnish to a recycling/deinking facility results in lowered brightness of the secondary fiber product. This is due to the hydrophilic surface chemistry of flexographic pigment particles dispersed upon pulping at alkaline pH, which are not effectively removed by flotation deinking. Although washing effectively removes flexographic ink residues from secondary fiber suspensions, the resulting filtrate contains substantial quantities of pigment necessitating clarification prior to recycling this water to the process or discharging it to the environment. The most common method of process water clarification is dissolved air flotation, which is not efficient at removing the flexographic pigments.; The inability of conventional clarification techniques to clarify flexographic pigment dispersions from wash deinking filtrate represents a significant barrier to closing the water loop of a deinking operation. This thesis describes the novel application of membrane separation techniques to clarify water based flexographic pigments from newsprint deinking effluent. Ultrafiltration and microfiltration were shown to completely reject flexographic pigments from simulated wash filtrate. Ultrafiltration was capable of higher clarification rates than microfiltration under most operating conditions. Data from a statistically designed experimental sequence were used to develop predictive algorithms which reveal interdependent effects of temperature, ink pigment concentration and surfactant content on ultrafiltration performance as measured by permeation rate and flux stability. A logarithmic relationship between permeate flux and pigment concentration was demonstrated, with permeation rates becoming independent of ink content at concentrations below.4%. The limit to permeate flux at low ink concentration was due to membrane fouling. Increasing the surfactant concentration decreased the degree of membrane fouling when clarifying dilute ink dispersions. However excessive surfactant resulted in decreased permeation rates due to micelle formation. Clarification of more concentrated ink dispersions ({dollar}>{dollar}.5%) resulted in more stable permeate flux and a lower degree of membrane fouling, presumably due to the inherently higher concentrations of surface active materials present. The pressure independent ultrafiltration permeation rates observed when clarifying the more concentrated ink dispersions were shown to be limited by mass transfer effects, as predicted by the concentration polarization model, rather than by osmotic pressure restrictions.