11e26567-f295-44c9-af0d-e8e510761066.doc Applying LDS to Monitor Flocculation in Papermaking M.G. Rasteiro and F.P. Garcia Chem Eng Dep, Coimbra University, Polo II, Pinhal de Marrocos, 3030-290 Coimbra, Portugal, email@example.com Flocculation is a dynamic process important to several industrial areas. In fact, inducing aggregation of particles can improve effluent treatment by sedimentation or can be used to separate a valuable product from the carrying fluid, among many other applications. In the case of papermaking, flocculation of the fine particles is essential to achieve adequate retention during sheet formation. Several additives have to be added to the stock suspension of fibres that feeds the paper machine, in order to produce a paper sheet with adequate characteristics (strength, whiteness, etc.). However, those additives, usually finely dispersed materials like calcium carbonate, bentonite, clay, etc, can be very easily lost during the sheet formation process if their flocculation is not induced. On the other hand, the extent of flocculation has to be controlled, since too much flocculation can lead to poor drainage, and thus higher energy costs, and also to irregular surfaces. This need for an accurate control of flocculation in papermaking was the motivation for this work. In this work we have decided to monitor the flocculation process by evaluating the particle size distribution as flocculation proceeds. The technique selected to control the flocculation process was LDS, since it is a particle size analysis technique easy to operate, fast and allowing repeated measurements. Moreover, LDS allows us to extract also information about the fractal dimension of the flocs. In addition, it is well known that LDS is being progressively adapted to on-line measurements, though no applications in papermaking have yet been reported. In this paper only results for the flocculation of the single components in the stock suspension will be reported (bentonite and calcium carbonate). The influence of the polymeric material used to promote flocculation and of its concentration will be studied. Since both bentonite and calcium carbonate are negatively charged only cationic and non-ionic polymers have been tested. However, polymers with different charge density and molecular weight were used, in order to identify the flocculation mechanisms that can better promote aggregation of the materials studied. The results obtained have shown that it is possible to use LDS to monitor the dynamics of flocculation processes. Moreover, LDS allowed the detection of different flocculation kinetics depending on the flocculant characteristics, dosage and pH. The LDS equipment used enabled also a qualitative evaluation of the flocs resistance by submitting the flocs to different shear stresses or ultrasound frequencies, once flocculation was considered terminated. In fact, resistance is one of the flocs characteristics of crucial importance in papermaking due to the high shears the stock suspensions have to stand.
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