Effects of operating variables on ultrafiltration of flexographic pigments from wash deinking filtrate

The presence of flexographic printed newspapers in the furnish to a recycling-deinking plant results in lowered brightness of the secondary fibre product. This is due to the surface chemistry of flexographic pigment particles, which are hydrophilic and therefore not removed by conventional flotation deinking. Although washing effectively removes flexographic pigment particles from fibre suspensions, the resulting filtrate will contain substantial quantities of pigment necessitating clarification prior to recycling this water to the process or discharging it to the environment. Ultrafiltration has been demonstrated to be a viable technique for the removal of flexographic pigments from aqueous dispersions. This paper describes the results of a statistically designed experimental sequence tailored to optimize the ultrafiltration process for clarifying flexographic pigment dispersions. The operational parameters investigated include temperature, pigment concentration and surfactant concentration. Response variables used to assess the efficiency of the ultrafiltration process are permeation rate and stability of operation.