On Quantification of Residual Ink Content and Deinking Efficiency in Recycling of Mixed Office Waste Paper

Although (flotation) deinking has been a common industry practice for several decades, true residual ink content and deinking efficiency have never been quantified. Paper brightness and ERIC (Effective Residual Ink Concentration), based on measurements of the absorption coefficient of deinked pulp, have been used to determine performance of flotation deinking processes and the quality of deinked pulps. This study demonstrated a new method for quantifying true residual ink content of mixed office waste paper by determining the trace iron content in pulp samples. Two sets of laboratory studies were conducted, one using mixed pulp samples of different ink contents and the other using samples from flotation deinking experiments with different surfactant dosages. All pulp samples including flotation feeds and accepts were characterized by ERIC, brightness, and iron-content-derived ink content. The results suggest that ERIC and brightness are less suitable than iron content for evaluating the residual ink in very clean papers. In flotation studies, both ERIC and brightness predict lower deinking efficiencies than are obtained from iron-content measurements. However, brightness is suitable for evaluating the final quality of very clean deinked pulp because of its high accuracy at high brightness levels.