Colloids and Surfaces A: Physicochem. Eng. Aspects 297 (2007) 79–83 The effect of calcium ions on the efficiency of polyethylene oxide–cofactor retention aid systems T.G.M. van de Ven a,b,∗ , M. Abdallah Qasaimeh a,c , C. Pigeon a,b , J. Paris d a Pulp & Paper Research Centre, McGill University, Montreal, Canada b Department of Chemistry, Pulp & Paper Research Centre, McGill University, Montreal, Canada c Department of Chemical Engineering, Pulp & Paper Research Centre, McGill University, Montreal, Canada d D´ epartement de G´ enie Chimique, Centre de Recherche en Ing´ enierie du Papier, ´ Ecole Polytechnique de Montr´ eal, Montreal, Canada Received 5 July 2006; received in revised form 7 September 2006; accepted 9 October 2006 Available online 13 October 2006 Abstract Polyethylene oxide (PEO)–cofactor retention aid systems are used in papermaking to improve fines retention in mechanical grade papers and in the control of pitch in process waters. Mill experience has shown that these retention aid systems work less well in furnishes containing a large amount of deinked pulps. Various dissolved and colloidal substances could possibly interfere with the PEO–cofactor complex and make it less effective. One of the compounds that could negatively affect the PEO–cofactor retention aid system is calcium, since deinked pulps can contain calcium carbonate fillers, which dissolve at neutral pH, thus increasing the calcium ion concentration in the whitewater. In this study the effects of calcium ions on fines deposition on fibers and on fines flocculation are examined. It is found that calcium ions can form colloidal-like complexes with the cofactor, which decrease the flocculation efficiency of PEO. The effects can be minimized by adding the cofactor just before the PEO addition. © 2006 Elsevier B.V. All rights reserved. Keywords: Fines flocculation; Deposition; Pulp fibers; Polyethylene oxide; Calcium ions; Cofactors; Retention aids 1. Introduction The use of recycled paper in paper making may have undesir- able side effects. Deinked pulps contain dissolved and colloidal substances (DCS), which might interfere with wet-end paper- making chemicals. Also, progress towards mill closure leads to a higher amount of suspended solids in the mill water [1–7]. Mills usually remove suspended solids from the whitewater by using filtration and clarification techniques [8]. However, even with the use of polymer based cationic fixation agents, DCS are difficult to remove from the whitewater [9]. A neutral polymer such as polyethylene oxide (PEO) is expected to be less affected by anionic compounds in the DCS than a cationic polyelectrolyte [10]. Therefore, the recycling of whitewater from recycled pulps is not expected to have such a detrimental effect on the efficiency with which PEO retains fines. ∗ Corresponding author at: Pulp & Paper Research Centre, McGill University, 3420 University Street, Montreal QC Canada H3A 2A7, Canada. E-mail address: theo.vandeven@mcgill.ca (T.G.M. van de Ven). However, mill experience shows that the efficiency of PEO diminishes when the content of deinked pulp in a furnish is too large. The decrease in PEO efficiency has also been seen in dynamic drainage jar experiments in which residual deinking chemicals were introduced [11]. Although the problem can be solved with the addition of bentonite [12–15], it would be of interest to know what causes the problem. Model experiments on fines deposition on fibers have shown that various compounds did not affect the deposition efficiency and thus cannot account for the detrimental effect of DCS in deinked pulps [16]. Com- pounds that were tested and found to have no effect are: oleates (a common fatty acid), silicates and DTPA (a chelating agent). Compounds that affected the PEO efficiency negatively were all of colloidal nature: oleate soaps, rosin particles (abietic acid), clay and fines. Their effect appeared to be related to surface area: PEO adsorption on colloidal material reduces the amount of PEO available for polymer bridging between fines and fibers. On a paper machine, PEO is usually added very close to the headbox, and little PEO adsorption on colloidal particles is expected, since the time scale of polymer adsorption on fibers is much shorter than that of PEO adsorption on colloids [17]. Thus, also colloidal 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.10.026