In: Proceedings of the 1995 International environmental conference; 1995 May 7-10; Atlanta, GA. Atlanta, GA: TAPPI PRESS: 869-881; 1995. Book 2. DEINKING OF FLEXOGRAPHIC NEWSPRINT: Conventional flotation technology is widely used for deinking USE OF ULTRAFILTRATION TO CLOSE THE secondary fiber furnish which has been printed by the WATER LOOP letterpress or lithographic (offset) processes, but is ineffective in removing flexographic ink residue1,3,4,5,6 which form fine Bradley H. Upton Gopal A. Krishnagopalan dispersions of pigment particles upon repulping7. The same Graduate Research Assistant Associate Professor properties of flexographic ink dispersions which render them Dept. of Chemical Eng. Dept. of Chemical Eng. difficult to remove by flotation deinking, specifically colloidal Auburn University Auburn University dimensions and hydrophilic surfaces, lend them to efficient Auburn, AL 36849 Auburn, AL 36849 removal by washing technique1,4,6,8. Washing is effective in USA USA the removal of flexographic ink residues from secondary fiber suspensions 2,9,10. However, the ink residues must be removed Said Abubakr from the wash filtrate before this stream can be recycled to the Project Leader process or discharged USDA-Forest Products Laboratory Madison, WI 53705-2398 It has been shown that froth flotation under acidic, or at least USA non-alkaline conditions improves the efficiency of this operation in removing water-based flexographic inks. A two ABSTRACT stage process consisting of non-alkaline flotation for removal of flexo inks, followed by alkaline flotation for oil-based ink removal has been proposed2. There is also interest in Conventional flotation deinking technology is widely used for developing flocculation aids (polymers) which can be added to deinking secondary fiber furnish which has been printed by the grey stock in order to render the flexographic pigment the letterpress or lithographic (offset) processes. This particles hydrophobic'. This would result in an increased technology is not effective for removing water based tendency for the flexo inks to be removed by conventional flexographic inks due to the hydrophilic nature of their flotation deinking. Membrane separation technology is a pigment particles, which form fine dispersions upon potentially attractive method for the removal of flexographic repulping. Washing is effective at removing flexographic ink ink residues from wash filtrate without further addition of dispersions from pulped secondary fiber, however, the chemicals". resulting wash effluent contains significant quantities of pigment. The wash effluent must be clarified of pigment A proposed flow diagram for the application of ultrafiltration before it can be recycled within the mill without reducing pulp techniques in a deinking process is shown in figure 1. brightness. The difficulty in clarifying flexographic pigment Deinking a mixed old newsprint (ONP) furnish would rely on dispersions from wash filtrate represents a significant barrier both froth flotation and washing. The wash filtrate would be to closing the water loop of a deinking operation. sent to a multiple stage ultrafiltration operation to remove the flexographic pigments prior to being recycled to the process. Ultrafiltration was investigated as a means to remove The concentrated ink residue, consisting primarily of carbon dispersed water-based pigments from wash effluent. black pigment, could be utilized in energy recovery. The Ultrafiltration of high concentration ink dispersions resulted in number of stages, membrane surface area of each stage, and stable, high production rates of pigment free permeate. The location in the deinking process to apply ultrafiltration must be use of high concentration feeds prevented irreversible optimized in order to maximize production rate and minimize membrane fouling, and actually demonstrated membrane capital (membrane) and pumping costs. regenerative properties. Inclusion of cellulose fines in the feed had mixed results on permeation rates. This study focuses on the feasibility of ultrafiltration techniques to remove dispersed water-based pigments from aqueous dispersions. An ultrafiltration apparatus was INTRODUCTION assembled and used to characterize the clarification of flexographic ink dispersions prepared from two commercially The most common printing techniques for the production of available water-based flexographic inks. The efficiency of the newspapers are letterpress and lithography (offset). ultrafiltration separation process is characterized by Flexographic printing is a relative newcomer in the newsprint parameters such as permeate flus, fouling rate, and cleaning publication field, but holds several advantages over letterpress requirements. and lithographic printing1,2. The primary disadvantage of flexographic newsprint publication are difficulties associated with recycling due to the water-dispersible flexographic inks. 1995 International Environmental Conference Proceedings / 869 EXPERIMENTAL measurement, the collected permeate was returned to the feed tank The ultrafiltration apparatus is equipped with adequate instrumentation and controls to manipulate important In some of the experiments cellulose fines were added to the operational parameters such as pressure drop across the feed Cellulose fines were generated by beating unprinted membrane surface (transmembrane pressure, newsprint stock, and collecting the fraction which passed temperature, and flowrate through the membrane module. through a 65 mesh screen and was retained by a 200 mesh Two identical polysulfone hollow fiber ultrafiltration screen Fines were added in amounts necessary to make membranes were used in this study. Each membrane module 0 025% and 0 05% (by weight) consistency dispersions is constructed in a shell and tube configuration, with a total of 1ft2 (0.0283m2) of surface area arranged into 68 hollow fibers The membrane module was cleaned between experimental of 0.043 inch (0.109 cm) ID. The molecular weight cut off runs. Immediately after an experiment in which flexographic (MWCO) of an ultrafiltration membrane is used as a measure ink dispersions were ultrafiltered the membrane module and of pore size, or retention ability, and is the approximate entire flow path were rinsed with tap water to remove as much molecular weight of the smallest compound which will be ink from the equipment as possible. A solution of tap water retained by the ultrafiltration membrane. The MWCO of the and a commercial detergent, with pH adjusted to 10 by membranes used in this study was 500,000. Figure 2 shows a addition of NaOH was then pumped through the membrane schematic diagram of the ultrafiltration apparatus. module and associated piping for several minutes. The dirty cleaning liquid was drained from the system, and a fresh Feed solutions for the ultrafiltration experiments were cleaning solution was pumped through the system for at least prepared using tap water, flexographic ink, surfactant, and 30 minutes. The third and final cleaning was performed using NaOH to adjust the pH to 9.7-10.7. A commercially available a fresh solution for as long as four hours. At this time the flux anionic and nonionic surfactant mixture was used, and the was measured using a fresh cleaning solution as the amount necessary to form a 0.01% solution was added (only 4 feedstream, at 25 psi 20°C, and 1 gpm. The resulting mL per 40 L of solution). Two formulations of flexographic value is the water flux, which was used to evaluate the ink were used in these studies. One was obtained from a effectiveness of the cleaning procedure on restoring regional newspaper publisher (ink #1), and was supplied as membrane performance. press ready ink. Another was obtained directly from an ink manufacturer (ink #2). Dispersions used in the ultrafiltration RESULTS experiments had ink concentrations ranging from 0.04% to 10%. The concentration of ink was determined by visible Flux Performance When Processing Flexographic Ink light spectroscopy. Absorption at 457 nm by a diluted feed Dispersions sample was compared to a calibration curve of light transmittance vs. ink concentration. Plots of permeate flus vs. for five dilute dispersions of ink #1 (less than 0.37%) are shown in figure 3. Ultrafiltration The feed solution was maintained at a constant ink achieved complete retention of the flexographic pigment, concentration during most of the experiments by recycling the producing clear permeate. All dispersions demonstrated pigment free permeate back into the feed tank However, linear flus vs. relationships, indicating that the system during some of the experiments the permeate was removed behaved as would be expected from the Hagen-Poiseuille law from the ultrafiltration loop as it was produced. This resulted for streamline flow through channels, which is believed to best in the feed solution (ink dispersion) becoming more describe the pressure controlled flow of fluid through concentrated as the ultrafiltration operation proceeded (the microporous membranes”. Flux vs. time data for batch total volume of feed also decreased during these experiments). ultrafiltration of a 0.05% dispersion of ink # 1 (25 psi) are The feed was kept well mixed by the action of the shown in figure 4. Flux stabilized at 48 1/m2 hr during the six recirculation pump. In most experiments the temperature was hour experiment, while the ink concentration increased from maintained constant by passing the feed through a constant 0.05%to 0.12%. The results indicate that flux was temperature