Short Communication Applying LDS to Monitor Flocculation in Papermaking M. G. RASTEIRO F. A. P. GARCIA Chemical Engineering Department, Coimbra University, Coimbra, Portugal M. DEL MAR PE ´ REZ Universidad de Murcia, Spain Flocculation of fines and fillers is important in the papermaking industry. To fulfill the need for accurate control of flocculation, laser diffraction spectroscopy (LDS) was selected to supply the floc size distribution. LDS allowed the detection of differ- ent flocculation mechanisms depending on the flocculant characteristics. The analy- sis relied mainly on the collection of two parameters: the size distribution of the flocs, characterized by its median, and their fractal dimension. Floc resistance to shear was assessed and related to the floc structure through the analysis of the frac- tal dimension. For the lower charge density, flocs grow faster, becoming less com- pact and, thus, less resistant. LDS proved to be a valuable technique to monitor flocculation processes. However, to obtain significant results, a close control of obscuration in the measuring cell is required. Keywords flocculation, fractal dimension, laser diffraction, papermaking Introduction Flocculation of the fine particles in papermaking is essential to achieve adequate retention during sheet formation. Chemical additives play a critical role in floccu- lation and thus on floc characteristics that are governed by different mechanisms: electrostatic, bridging, and patching. The predominant mechanism is mainly dependent on the charge density and molecular weight of the polymer. In this work cationic polyacrylamides (PAM) with different molecular weights and charge densities were used. In papermaking, flocculation has been traditionally monitored by hydrody- namic techniques that evaluate drainage as a function of flocculation conditions, like dynamic drainage jar tests (Bouyer et al., 2001). Flocculation can also be monitored by controlling the zeta-potential of the suspension if the electrostatic mechanism is We would like to acknowledge the financial support received from the European Project NODESZELOSS and the flocculant samples kindly given by Cytec. Address correspondence to M. G. Rasteiro, Chemical Engineering Dept., Coimbra University, Coimbra, Portugal. E-mail: mgr@eq.uc.pt Particulate Science and Technology, 25: 303–308, 2007 Copyright # Taylor & Francis Group, LLC ISSN: 0272-6351 print/1548-0046 online DOI: 10.1080/02726350701375956 303